Metal-Organic Frameworks in Germany: from Synthesis to Function
Jack D. Evans, Bikash Garai, Helge Reinsch, Weijin Li, Stefano Dissegna, Volodymyr Bon, Irena Senkovska, Roland A. Fischer, Stefan Kaskel, Christoph Janiak, Norbert Stock, Dirk Volkmer
MMetal–Organic Frameworks in Germany: from Synthesis to Function
Jack D. Evans a , Bikash Garai a , Helge Reinsch b , Weijin Li c , Stefano Dissegna c , Volodymyr Bon a ,Irena Senkovska a , Roland A. Fischer c , Stefan Kaskel a, ∗ , Christoph Janiak d , Norbert Stock b , DirkVolkmer e a Department of Inorganic Chemistry, Technische Universität Dresden, Bergstraße 66, 01062 Dresden, Germany b Institut für Anorganische Chemie der CAU Kiel, Max-Eyth-Straße 2, 24118 Kiel, Germany c Fakultät für Chemie–Anorganische und Metallorganische Chemie, Technische Universität München, 85748, Garching,Germany d Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1,40225 Düsseldorf, Germany e Chair of Solid State and Materials Chemistry, Institute of Physics, University of Augsburg, Universitätsstraße 1, 86159Augsburg, Germany
Abstract
Metal–organic frameworks (MOFs) are constructed from a combination of inorganic and organic unitsto produce materials which display high porosity, among other unique and exciting properties. MOFshave shown promise in many wide ranging applications, such as catalysis and gas separations. In thisreview, we highlight MOF research conducted by Germany-based research groups. Specifically, we featureapproaches for the synthesis of new MOFs, high-throughput MOF production, advanced characterizationmethods and examples of advanced functions and properties.
Keywords:
Metal–organic frameworks, Applications, Porous materials, Germany
1. Introduction
Metal–organic frameworks (MOFs) are coordination polymers composed of mono- or multinuclearcomplexes (clusters) and organic multifunctional ligands (linkers) forming extended, regular networkstructures generating defined pore spaces in a crystalline lattice with a high degree of functionality [1].
Since the first MOFs appeared in the 1990s, this research field has experienced tremendous growth andattention from a wide interdisciplinary community going far beyond coordination chemistry, includingsolid state, organic, physical, colloid, and, more recently, electrochemistry. Today, MOF research groupsare in engineering, environmental science, earth science and many other fields and the community isstill growing, as reflected by the increasing number of participants at the international MOF conference[2]. Industrial efforts in major industrial enterprises, small and medium-sized enterprises and startupsare spread all over the world. MOFs are record materials in terms of porosity achieving specific surfaceareas up to g − and methane storage capacity of ≈
240 cm cm − [3, 4]. Moreover, rationalfunctionalization at the molecular level allows for the integration of a wide range functionalities such asoptical, magnetic, electronic, chiral, catalytic, or sensor functions stemming either from the ligand orcluster moieties. Early MOF discoveries, such as MOF-2 [5], HKUST-1 [6] or CuSiF (4,4’-bipyridine) [7] had relatively low specific surface area, while important milestones, such as MOF-177 [8], MIL-101[9], and NU-110 [3] may be considered as important benchmark materials.In our present review we describe the contribution of Germany-based research teams towards progressin metal–organic framework research in various areas from fundamental research towards applications. In Germany, an early promoter of MOF research was BASF (U. Müller), who initiated industrialresearch, upscaling and tested the early catalytic applications of MOFs. BASF also filed several patentsquite early in the field, discouraging competing enterprises to enter the field. F. Schüth promoted MOF ∗ [email protected] a r X i v : . [ c ond - m a t . m t r l - s c i ] A p r esearch by including an early review chapter on MOF structures in 2002 in the Handbook of PorousSolids [10]. Early German reports in the field of porous coordination polymers appeared by R. Kempe,mostly focused on the structural aspects, C. Janiak, and N. Stock [11, 12, 13, 14]. A few groups stressedquite early the potential of these highly functional materials for applications in catalysis, for example themetal@MOF concept or Lewis acid catalyzed reactions [15, 16]. An important driver for the promotion ofGerman MOF research was the DFG priority program SPP 1362 (“Porous Metal-Organic Frameworks”)initiated in 2006 by S. Kaskel (coordinator), T. Bein, S. Ernst, R. A. Fischer, J. Kärger, E. Klemm, J.Lercher, J. Sauer, and F. Schüth [17]. In May 2006, 17 principal investigators presented their ongoingresearch in an informal meeting to identify new directions in MOF research in Dresden (Figure 1). Figure 1: (a) Participants of the first round table discussion and initiators of the German MOF research program SPP 1362(2006, TU Dresden). (b) Participants of the German MOF Program (2008, Dresden).
At that early stage the highly interdisciplinary character of this research field was expeditiously iden-tified ranging from high throughput synthesis (N. Stock), advanced characterization methods (A. Pöppl,J. Kärger), simulations (R. Schmid, G. Seifert), nanomorphologies (T. Bein) towards applications inhydrogen storage (M. Hirscher), catalysis (E. Klemm), and technical chemistry (M, Hartmann). How-ever, it took until 2008 to get 13 consortia funded, in total involving 39 investigators distributed all overGermany. The highly interdisciplinary character of the consortia was an important driver to promotecross-disciplinary cooperation at an early stage of MOF research in Germany. SPP 1362 was in forcefrom 2008-2014 supporting the increasing interest in MOF research, organizing network meetings andpromoting cooperative MOF projects and supporting more than 100 PhD students financially, in totalover the 6 years.More than 160 scientists from 14 different countries (Europe, Asia, North- and South America) at-tended the “International MOF Symposium” organized by SPP 1362 in 2013 [18]. The most importanthighlights generated either within this program or achieved by German groups outside the program arediscussed in this review. Some of the outcomes are also reflected in a special issue in
Microporous andMesoporous Materials dedicated to the “New Generation of Metal–Organic Frameworks” published in 2015[19]. Leading research groups from the program also contributed significantly to a recent monograph onthe “Chemistry of Metal-Organic Frameworks” [20]. Two large cooperative EU projects (Macademia andNanoMOF [21, 22]) further promoted the development of industrial applications of MOFs in various fields,also involving partners in Germany. The European training network DEFNET [23] further interlinkedPhD students in Germany in MOF research with those in other European countries. F. Schüth initiatedthe first discussions with G. Férey for the first international MOF conference organized by DECHEMAin Augsburg 2008. He motivated G. Férey to act as the chair of this conference with M. Hartmann and S.2askel in the local organizing committee. In recent years, the need to integrate MOFs into electronic orphotonic device architectures motivated efforts towards MOF thin film preparation and characterizationtargeting electrically conductive or semiconducting MOFs. To this end, a new DFG priority program(SPP 1928, COORNETs, 2016–2021) was initiated by R. A. Fischer in 2014 and the first projects werefunded in 2016 [24]. Over more than one decade, MOF research in Germany has significantly benefitedfrom excellent international cooperation with groups widespread all over the world in an amazingly di-verse and interdisciplinary field. In this regard, we hope our current review and summary of MOF scienceachievements in Germany may serve as fruitful source and guidance for partnering, initiation of futureinternational cooperation and prospective scientific exchange.
2. Chemistry and Materials
MOFs based on the tetravalent Zr cation are probably the most intensively investigated subclass ofthis group of materials in recent years. The main reasons for this are their outstanding stability due tostrong metal-ligand bonding [25] and their chemical tunability using chemically different, but topologicallyidentical ligands [26]. These advantages combined with substantial porosity were demonstrated for thefirst zirconium carboxylate MOF denoted as UiO-66 ([Zr O (OH) (BDC) ]) reported in 2008, where UiOstands for University of Oslo and BDC is benzenedicarboxylate [27]. The hexanuclear inorganic structuralbuilding unit (SBU) observed in this MOF has subsequently been encountered in most of the other Zr-MOFs reported. The contribution from the German community to this nevertheless diverse structuralchemistry is summarized below. It should be noted that several of the MOFs mentioned herein can beas well prepared employing hafnium instead of zirconium salts due to the similar chemistry of these twoelements.This topic emerged in the German research community more clearly after the group of P. Behrens elab-orated the concept of modulated synthesis for zirconium MOFs [28]. Initially the synthesis attempts forZr-MOFs only yielded microcrystalline compounds and the use of linker molecules other than terephthalicacid was often hampered by a low degree of reproducibility. Using monocarboxylic acids like benzoic,acetic or formic acid in excess and accurately controlling the water content of the synthesis mixture al-lowed for the reproducible synthesis of Zr-MOFs based on benzene-, biphenyl- and terphenyldicarboxylicacids with controllable crystal size. Eventually, crystals even suitable for single crystal X-ray diffractioncould be prepared. Today it is believed that modulators induce the formation of the SBU in the synthe-sis solution, making the preformed SBU readily accessible. However, modulators also act as competingligands for the linker molecules and thus the nucleation process is sufficiently slowed to allow the growthof large single crystals, or at least highly ordered microcrystals.This concept was further extended by the P. Behrens group using fumaric acid [29] or azobenzenedi-carboxylic acid [30] as linker molecules and resulted in the discovery of a series of compounds denotedas PIZOFs (porous interpenetrated zirconium–organic frameworks) [31]. PIZOFs are formed when thelength of the linear dicarboxylic acid exceeds the size of terphenyl building units. Hence, molecules withalternating phenylene (P) and ethynylene (E) fragments, like a dicarboxylic acid with PEPEP sequence,but also other linker molecules like quaterphenyldicarboxylic acid and others [32] are able to form PIZOFstructures. The topology of these remarkable frameworks is based on two interpenetrating fcu nets, theprototypical UiO-66 and its derivatives exhibit a non-interpenetrated fcu topology. Using modulatorassisted synthesis, developed in Hannover, the field of research dealing with new Zr-MOFs has prospered,mostly since the formation of large single crystals sufficient for conventional X-ray diffraction allows fora comparably facile way of structure determination. Consequently, following this study several dozens ofZr-MOFs have been reported, of which the vast majority were obtained using modulating monocarboxylicacids as additives [33].Particularly in Germany this topic was further extended by the group of S. Kaskel. In very thoroughstudies the influence of the amount and nature of the modulating agent was investigated, using predom-inantly bent dicarboxylic acids as linker molecules in which the angle between the acid groups is smallerthan °. This led to the discovery of a remarkable number of new Zr-MOFs with differing and thithertounreported topologies, different than the archetypical fcu framework. As a result, the first Zr-MOF witha slightly bent linker molecule (dithienothiophenedicarboxylic acid) exhibiting the reo topology couldbe obtained, denoted as DUT-51, where DUT stands for Dresden University of Technology [34]. Em-ploying 2,5-thiophenedicarboxylic acid as the linker, three new compounds (DUT-67, -68 and -69) weresynthesized, all crystallizing with different framework topologies ( reo , bon and bct , respectively) [35]. Thedifferent framework topologies exhibit very different pore systems with a pore diameter of Å for the bct . Å mesopore for the reo (DUT-68) topology. Very recently, the compoundDUT-126 was added to this series of 2,5-thiophenedicarboxylates, exhibiting a hbr topology [36]. Employ-ing 9-fluoreneone-2,7-dicarboxylic acid as linker, the typical fcu structure commonly occurring for linearlinker molecules was observed for a bent organic building block in DUT-122 [37] and the MOFs DUT-80and DUT-98 were reported which incorporate 9-(4-carboxyphenyl)-9H-carbazole-3,6-dicarboxylate as thelinker.Other than bent linker molecules, the linear 2,6-naphthalenedicarboxylic acid was as well investigated.This study yielded an analogue of UiO-66 denoted as DUT-52 ( fcu ), a compound with bcu topologydenoted as DUT-53 and a layered MOF named DUT-84. Notably, these frameworks are based on identicalbuilding units and synthesized by varying only the concentration of modulator. It is worth mentioningthat at least some of the aforementioned observed topologies ( bcu , reo , bct ) can be constructed startingfrom the fcu topology by systematically removing inorganic and/or organic building units while mostlypreserving the packing mode of the SBUs. Hence the face centered cubic arrangement, resembling the fcu topology, can be effectively manipulated into other, related structures by adjustment of linker geometryand the use of modulators.In further studies, MOFs exhibiting specific functionalities were generated to induce a certain propertyof the solids. For example using enantiopure proline-functionalized linker molecules, Zr-MOFs with fcu framework topology could be obtained, which are highly active catalysts for diastereoselective aldoladditions [38]. Similarly, the use of a tetrazine-based analogue of terephthalic acid resulted in a UiO-66analogue which showed a clear color change upon reaction with oxidizing probe molecules [39]. Figure 2: Some of the described MOFs based on Zr , Hf or Ce that were discovered or first characterized in Germany. Recently, the group of N. Stock further extended the diversity of Zr-MOFs. For example, the first Zr-MOF exhibiting scu topology based on 1,2,4,5-tetrakis(4-carboxyphenyl)benzene and denoted as CAU-24 was reported [40]. In efforts to broaden the structural chemistry of Zr-MOFs further, the use ofwater instead of the most often employed solvent dimethylformamide (DMF) was investigated. This wasinspired by the report of the synthesis of zirconium fumarate in mixtures of water and modulator [41] bythe group of P. Behrens. As a consequence, the new compound CAU-28 based on 2,5-furandicarboxylicacid was reported in Kiel [42]. While the linker molecule is very similar to the 2,5-thiophenedicarboxylicacid used for the DUT-series, the resulting structure is not similar to those reported for the DUT-compounds. This is possibly a consequence of the different solvent employed. Furthermore, CAU-22based on 2,5-pyrazinedicarboxylate could be obtained in water as solvent in combination with formicacid [43]. In contrast to all aforementioned Zr-MOFs, this framework does not incorporate the typical4Zr O (OH) (O CR) ] building unit, but a condensed version which represents a 1D polymer of edge-sharing hexanuclear clusters. Under very similar conditions, 2,5-pyridinedicarboxylic acid allows for thesynthesis of analogues of UiO-66 [44].In addition to compounds based on Zr , the synthesis of Ce -based MOF was also investigated inthe N. Stock group. The first reported examples were analogues of the UiO-66 structure incorporating(functionalized) terephthalate, fumarate or 4,4’-biphenyldicarboxylate as linker molecules [45]. Similarly,isoreticular versions of Zr-MOFs with 5,5’-bipyridinedicarboxylate forming the UiO-66 analogue, withtrimesate forming the MOF-808 analogue and with 2,5-thiophenedicarboxylate or 3,5-pyrazoledicarboxylateforming the DUT-67 analogue were reported using cerium [46]. The compounds CAU-24 and CAU-28 mentioned above are also accessible as Ce-MOFs. In general the reactions are carried out in wa-ter/DMF/modulator mixtures and in order to obtain Ce(IV)-MOFs synthesis times must be restrictedto minutes. The reactive tetravalent cation tends to be reduced under these conditions upon prolongedheating, eventually forming trivalent cerium formate. The Ce -based compounds are often catalyticallyactive due the redox behavior of Ce [47]. Unfortunately this also leads to a high reactivity and thuslower stability compared with their Zr-counterparts. In order to achieve higher stability without compro-mising the redox activity, mixed metal MOFs based on Zr and Ce were also synthesized, exhibitingthe UiO-66 or MOF-808 structure [48]. The modular approach to the synthesis of MOFs clearly allows for a variety of inorganic and or-ganic building blocks to be combined in many different topologies. This unique approach allows forMOFs to exhibit pore size distributions and textural properties almost without limitations [49]. Notably,MOFs have the unique potential of having a hierarchical pore structure ranging from the microporousto mesoporous size regime or singly microporous or mesoporous [50, 51]. Though most of the MOFsare microporous materials there are numerous mesoporous examples with extremely high specific surfaceareas above g − and Germany-based researchers have reported several examples and providedimportant insight into their synthesis.One strategy to extend the pores of a MOF to the mesoporous range is using the isoreticular prin-ciple, systematic enlargement of the ligand in a fixed framework topology [52]. The first mesoporousMOFs were introduced by O. M. Yaghi et al. using this approach [53]. However, there are two mainproblems with using this method to produce mesoporous materials, interpenetration and polymorphism,which often appear with increased ligand length. The group of S. Kaskel reported a route of avoidinginterpenetration in MOF-14 [54] by adjusting the synthesis conditions to produce DUT-34 [55]. Thoughthis did require screening a large number of alternative synthetic routes in order to obtain a phase purenon-interpenetrated sample. Alternatively, a similar non-interpenetrated structure can be obtained byovercoming the energetic stabilization provided by interpenetration. This was also demonstrated forMOF-14 by inserting a linear neutral ligand (bipyridine (bipy)) between the the SBU paddlewheels toproduce DUT-23 possessing an impressive pore volume of .
03 cm g − [55].In a similar strategy, by using two or more ligands during the synthesis a number of mesoporousmaterials with impressive porosity can be obtained. In particular the use of tritopic and ditopic carboxylicligands has been most successful. For example, UMCM-1 and DUT-6 (MOF-205) were synthesized usingthis approach [56, 57]. Most examples employ benzene-1,3,5-tribenzoate (btb) as the tritopic ligand andZn O as the metal node. For successful copolymerization of MOFs, there are geometric considerationsfor the length ratio of the ligands and their connectivity to the inorganic unit. Analysis suggests forMOFs constructed using di- and tritopic ligands ( L D and L T ) the L D L T ratio must be within the range of0.44 to 0.66 [58].Alternatively ligands with a bent shape can be employed to produce large mesoporous cavities. H.-C.Zhou et al. successfully demonstrated this concept to produce the porous framework PCN-21 with cavities and a surface area of g − , which was constructed using angular tetratopic ligands andCu paddlewheels [59]. The group of S. Kaskel also used this approach by combining a bent thiophenedicarboxylic ligands (2,5-thiophenedicarboxylic) and Zr-clusters to produce the mesoporous frameworkDUT-68 [35]. The complex hierarchical pore structure of DUT-68 contains: a rhombicuboctahedralmesopore, a cuboctahedral cage, a square antiprism and small octahedral pores. Notably this frameworkis also observed to be robust, hydrophilic, chemically, and thermally stable.Finally, using discrete metal–organic polyhedra (MOPs) as building units to construct highly porousthree-dimensional materials is an excellent approach as it provides a high degree of control over theresulting pore structure and topology [60]. The polyhedra act as supramolecular building blocks that cansubsequently be linked using multi-topic ligands to form a three-dimensional structure as demonstrated5n Figure 3. The smallest pore of the resulting framework can be straightforwardly tuned by changingthe size of the initial polyhedra. In addition by designing the shape, size and symmetry of the ligandconnecting the polyhedra the number of resulting pores and their size and shape can be controlled. BothS. Kaskel and H.-C. Zhou groups used this approach to produce mesoporous MOFs from cuboctahedralMOPs comprising of copper paddlewheels and carbazole-3,6-dicarboxylate ligands [61, 62]. For examplea linear moiety (biphenylene) is used to connect the MOPs in DUT-49 to produce a framework. whichcan be described as the extended cubic closed packing of cuboctahedral building units, with exceptionalgravimetric methane adsorption capacity [62]. Figure 3: (a) A carbazol-based MOP is linked by linear moieties to form (b), a three-dimensional framework. This is typifiedby DUT-49. [49] - Reproduced by permission of The Royal Society of Chemistry.
Another interesting aspect of porous MOFs consists of their chiral nature. On a compulsory prereq-uisite, chiral environment of a MOF is reflected from its crystal packing. Thus packing in a chiral spacegroup is the essential condition for exhibiting chiral property of the structure.Introduction of chirality has been made possible through four different approaches. The first approachbegins with achiral constituents, however self-resolution is induced to the framework structure duringcrystallization, thus bringing chirality to the final MOF structure. This chiral environment can also beinduced into the framework composed of chiral components, through use of a chiral template, therebydirecting the framework to crystallize in chiral environment as a second approach. The third approachbegins with chiral ligands and thereby forming a chiral framework with a stereocenter. Additionally afourth approach consists of post synthetic modification to introduce a chiral moiety into an otherwiseachiral MOF [63]. The chirality of the MOF structures has been proven useful in multiple aspects ofasymmetric catalysis and chiral separation [64].Of these approaches, starting with chiral ligand provides more control of the resulting framework chi-rality and thus German researchers have preferred this approach over the others for producing chiral MOFsfor desired purposes. The BDC linker was replaced with a chiral linker (enantiopure (S)-oxazolidinone)decorated BDC in the synthesis of UMCM-1 [56]; a chiral version of the MOF was obtained [65]. Theobtained chiral MOFs (iPr-Chir-UMCM-1 and Bn-Chir-UMCM-1) differ by the absence of an inversioncenter in the framework structures, which was present for UMCM-1. This generated chirality in the newframeworks allows for chiral separation applications. A stationary bed formed with the chiral MOFs wasemployed as a HPLC column and different chiral analogues were passed through, an immediate change inthe retention time was observed over the non-functionalized achiral MOF. An effective enanatioselectiveseparation was observed for 1-phenylethanol, with selectivity ( α ) and resolution (R S ) for the enantiomeras 1.6 and 0.65, respectively. Interestingly, the porous nature of the MOF was retained, even after 200injections of analytes through the MOF made HPLC column.Another family of chiral MOFs, bearing a chiral oxazolidinone group, was strategically synthesized byS. Kaskel and coworkers. The basic design of the chiral linkers consists of a tripodal unit suitable for a largepore aperture a with distinct topological feature [66]. The chiral oxazolidinone group was then attachedto the linker through chemical functionalization in an attempt to develop chiral environment near to themetal-linker junction. This functionalization put the chiral oxazolodinone moiety in close proximity to6he paddlewheel (SBU), which is known for catalytic activity through possible open metal site generation.On reaction of these chiral linkers with Zn(II) centers, two different frameworks (Zn (ChirBTB – 1) andZn (ChirBTB – 2) ) were observed as displayed in Figure 4. The resulting frameworks were observed tobe structurally different, owing to the difference on their steric bulk near to the coordinating positions.Thus, the less hindered moiety containing linker (ChirBTB-1) gives rise to a highly porous form with 3different types of pores in the structure with pore diameter ranging from . Å to . Å. The MOFformed from the other linker (ChirBTB-2) also bears the chiral characteristic but with a smaller pore sizeof Å. The observed difference in pore size has been used to encapsulate large sized dye molecules intothe wider pores of the MOF. Being located near to the catalytically active sites, the chiral moiety hasclearly influenced the tested Mukaiyama aldol reaction. This leads to an elevation of the enantiomericexcess of the product formed by catalysis from the neighboring open metal sites of the paddlewheel SBU.
Figure 4: a) chirBTB-n linker containing the chiral moieties and representative framework structures of the chiral MOFs(b: Zn (ChirBTB – 1) and c: Zn (ChirBTB – 2) ). d) Scheme of the enantioselective Mukaiyama aldol reaction reactionperformed using the chiral MOFs as catalyst. Adapted from [66], with permission. Insertion of such chiral functionality into the linker for MOF synthesis has proven beneficial in bringingin chirality into the MOF framework. However, such approaches are often difficult to apply when theligand of the pristine MOF structure is smaller in size. An alternative approach for functionalizingthe terephthalic acid linker requires a new methodology that has been presented in a recent report[67]. A three-dimensional analogue of the BDC linker was chosen for the purpose and the subsequentfunctionalization leads to equivalent distribution of chiral moieties in its vicinity. This makes it possiblefor chiral functionalities of the ligand to be evenly distributed along the structure of DUT-129, indicatinga possible enhancement in the interaction with other chiral components.Apart from the asymmetric linker induced chirality, a recent approach from D. Volkmer and coworkershighlights self-resolution during MOF formation to obtain the chiral MOF CFA-1 [68]. Here the achiraltriazole ligand, discussed in more detail in the subsequent section, on coordination with Zn(II) gives rise toa chiral MOF in the chiral space group P . Å, accessible through two different pore windows, having effective diameters of3.43 and . Å. Thus, there is potential for chiral MOFs to be successfully applied in related applicationsthrough an easy, and feasible, synthetic approach.Furthermore, in an attempt to achieve thin films for more practical application of the chiral MOFsfor separation purpose, a MOF thin film (SURMOF) was developed by the C. Wöll group [69]. Thepillar layered structure consisting of camphoric acid, as the chiral moiety, and DABCO as pillars wasgrown on a quartz crystal microbalance (QCM) substrate to have the SURMOF grown in the desired(110) and (001) directions. Expected enantioselectivity was observed when the vapor of R and S form ofa chiral probe (2,5-hexanediol) was passed over the resulting SURMOFs. This serves as an example forapplication of such materials towards stationary phases to separate racemates by GC.7 .4. Nitrogen containing ligands Metal carboxylate frameworks are ubiquitous in the chemistry of metal–organic frameworks. However,nitrogen containing ligands which form metal–azolate frameworks (MAFs) represent an important andstructurally diverse subfamily of (crystalline) coordination polymers [70], which have been comprehen-sively reviewed by J.P. Zhang et al. in 2012 [71] and for which Germany-based researchers have made asignificant contribution.In a broad sense the term MAFs refers to coordination polymers which are constructed from one-, two-,or three-dimensional chains of metal ions linked by anionic N-heterocyclic aromatic ligands, most of whichare derived from simple five-membered ring systems, such as those displayed in Figure 5 (Note that the listis non-comprehensive and does not include fused aromatic ring systems that will be highlighted later in thissection). The term “MAF”, as yet, has not received wide acceptance in scientific literature, presumablybecause the compound family itself contains distinguished members of coordination polymers such asthe zeolitic imidazolate frameworks (ZIFs) [72, 73], derived from imidazolate (im − ) ligands (Figure 5),a first member of which was described by F. Seel and J. Rodrian in 1969 [74]. ZIFs receive constantlygrowing attention within the scientific community owing to their ease of synthesis and exceptional chemicalrobustness [75]. This chapter on MAFs is devoted to the seminal contributions from German researchgroups to triazolates, with emphasis on continuous developments of the structural chemistry of thesecompounds rather than their (potential) use in technical applications. Figure 5: Structures of prototypical azoles and their corresponding azolates.
The structural chemistry of triazolate frameworks based on the most simple basic aromatic heterocyclicring systems Htz (1H-1,2,4-triazole) and Hvtz (1H-1,2,3-triazole) have been thoroughly investigated.Systematic investigations on tz − derived framework compounds have been conducted, amongst others,by J. Zubieta and coworkers [76, 77, 78]. The synthesis of 1,2,4-triazoles (Figure 6) has previously beenreviewed [79, 80], as well as aspects of their structurally diverse coordination chemistry [81, 82].In Germany, systematic crystallographic work on coordination polymers derived from 1,2,4-triazolateligands has been conducted by the H. Krautscheid group at the University of Leipzig in close collaborationwith K. V. Domasevitch and coworkers from Kiev University. However, the group’s work has focusedon N1-functionalized Htz linkers, which lack the ability to form an azolate, i.e. the anionic form of thelinker. Hence, their coordination chemistry formally resembles that of neutral 1,2-diazole ligands andshall not be reviewed here any further. Illustrative examples of frameworks derived from this approachinclude bitopic linkers such as 4-(4H-1,2,4-triazol-4-yl)benzoic acid [83, 84, 85] or polytopic triazole linkerscontaining an adamantyl backbone [86, 87].The coordination chemistry of rare earth (RE) element 1,2,4-triazolates has been explored by thegroup of K. Müller-Buschbaum at Cologne University [88], who presented the first solvent-free synthesisof this linker-type showing exclusive N-coordination of RE metal ions (Figure 7).The synthesis of 1,2,3-triazoles (Figure 8) has been reviewed previously [89] and aspects of the generalcoordination chemistry of these ligands are covered in [82]. The structural chemistry of metal-organicframeworks derived from 1,2,3-triazolate-type ligands has only recently been investigated. D. Volkmer andcoworkers at Ulm University have pioneered the field with systematic work on the development of MFU-4-type metal–organic framework compounds (MFU-4 being an acronym for metal–organic framework UlmUniversity), reported in 2009 [90]. The design of MFU-4-type framework compounds rests on uniquepentanuclear structural building units (SBUs), for which the term “Kuratowski-type” SBUs has beensuggested by the authors (Figure 9) [91]. 8 igure 6: (a) Naming and numbering scheme, tautomerism and (b) coordination properties of 1,2,4-triazole and its corre-sponding anion.Figure 7: The crystal structure of the framework [Yb(tz) ], showing the µ - η bridging mode of the 1,2,4-triazolato ligand.Reproduced from [88], with permission. igure 8: (a) Naming and numbering scheme, tautomerism and (b) coordination properties of 1,2,3-triazole and its corre-sponding anion.Figure 9: (a) Ball-and-stick representation of “Kuratowski-type” coordination units, highlighting the central octahedralcoordination site that results from six N-donor atoms situated at the centers of the edges of an imaginary tetrahedron,which is spanned by the four Zn(II) ions at the corners. M represents Zn, Fe, Co, Ni, or Cu and X represents Cl. (b)Simplified representation of metal centers and ligand donor atoms. (c and d) Derivation of a rational graphical schemerepresenting the connectivity of the coordination units. Reprinted with permission from [91]. Copyright 2010 AmericanChemical Society. Figure 10: Structural formulas of bis-(1,2,3-triazole) linkers prepared by D. Volkmer et al.
In clear distinction to the SBU of pyrazolate-based MFU-1 (Figure 11 left) [93], which seems tobe limited to the presence of a Co O core, the development of the structurally more flexible 1,2,3-triazolate-based MFU-4 family, which contains Kuratowski-type pentanuclear SBUs of the general formula[M II Zn X (L) ], where M represents Zn, Fe, Co, Ni, or Cu and X represents Cl, (Figure 11 right) providessynthetic access to a wide variety of metal–organic frameworks. Kuratowski-type SBUs feature a centralmetal ion coordinated to six triazolate ligands (L) that span a Cartesian coordinate system. Figure 11: Structural features of SBUs found in MFU-1 and MFU-4. Reprinted with permission from [91]. Copyright 2010American Chemical Society.
The central metal ion in the octahedral coordination environment can be varied [91] for the opportunityto obtain redox-active SBUs that are also Lewis acidic, with chemical properties which can be fine-tunedby selecting appropriate metal ions [94]. A graph theoretical analysis proves that [M II Zn X (L) ] unitscontain the nonplanar K , graph. Accordingly, there is no way to draw [M II Zn X (L) ] as a planargraph and thus a pseudoperspective skeletal formula as derived in Figure 9 to represent Kuratowski-11ype coordination compounds was proposed. The first triazolate-based MOF featuring Kuratowski-typesecondary building units, MFU-4, was constructed from bbta − linkers (Figure 10) and Zn Cl inoganicclusters to produce a framework featuring a very high thermal and hydrolytic stability [90]. MFU-4 alsofeatures small pore apertures ( . Å) which are highly selective for the adsorption of atoms or smallmolecules such as H , and it can therefore be applied in molecular sieving applications including some ofwhich are difficult to achieve with other kinds of porous materials, such as the separation of H /D [95]or N /CO mixtures [96].In order to separate mixtures of larger molecular adsorbates, or for catalytic transformations, a porousframework featuring large pore apertures is required. This led to the development of MFU-4large (MFU-4l) [97], an isoreticular large-pore variant of MFU-4, constructed from larger bttd − ligands (Figure 10).Attempts to extend the isoreticular series of cubic MFU-4-type frameworks with larger ligands, suchas bibta − or tqpt − (Figure 10) met with little success. However, these investigations led to the discoveryof crystallographically distinct framework structures (Figure 12), in which the fundamental KuratowskiSBU is retained — clearly demonstrating the robustness of the building block approach. Employing theeasy-to-synthesize H -bibta linker, D. Volkmer and coworkers reported CFA-1 (CFA being an acronymfor coordination framework Augsburg University), the first chiral metal–organic framework containingKuratowski-type SBUs [68]. This was soon followed by CFA-7, an interpenetrated framework variant ofthe MFU-4 family [98]. Figure 12: Overview of currently known framework types comprising rigid or partly flexible bis-triazolato linkers andKuratowski-type SBUs. [98] - Published by The Royal Society of Chemistry.
Amongst the different MFU-4 frameworks, MFU-4l attracted particular attention owing to its versatil-ity with respect to post-synthetic metal- and ligand-exchange and its functional properties. In a commu-nication from 2012 the Co(II)-exchanged derivative of MFU-4l (Co-MFU-4l) was reported to demonstratereversible gas-phase oxidation properties [99]. Subsequently, in 2014, D. Volkmer et al. showed the re-markable characteristics of Cu(I)-MFU-4l toward binding and activation of small molecules, such as O ,N , and H , with corresponding isosteric heats of adsorption of 53, 42, and
32 kJ mol − , respectively, de-12ermined by gas-sorption measurements and confirmed by density functional theory (DFT) calculations[100]. The H complex of trigonal pyramidal coordinated Cu(I) metal ions in this framework ranks amongthe strongest molecular hydrogen complexes of a 3D transition metal ion described in literature to date,and it has been used in boosting the efficiency of H /D separation by means of quantum sieving [101].Exchanging chloride with hydride anions, MFU-4l exhibits hydride transfer to electrophiles [99], whichsuggests these novel, yet robust, metal–organic frameworks are promising single-site catalytic materials,comprising earth-abundant metal elements. The concept of single-site catalysts requires the peripheral po-sition in the SBU of MFU-4-type frameworks zinc ions (“scorpionate-type coordination units”; Figure 13,left) to be exchanged by coordinatively unsaturated (“open”) metal sites. A variety of such exchange re-actions has been demonstrated by the groups of M. Dincă [102] and D. Volkmer [103] (Figure 13, right).Structures based on hetero-metal derivatives of MFU-4l exhibit a range of interesting catalytic reactions,recently demonstrated by M. Dincă and coworkers. These encompass the dimerization of ethylene to 1-butene [104] and the dimerization of propylene [105], both reactions being catalyzed by Ni(II)-exchangedMFU-4l. Additionally, Co-MFU-4l displays highly stereoselective heterogeneous diene polymerization[106]. Ni(II)-MFU-4l, also is shown to be catalytically active in a gas phase cyclic process with an overallstoichiometry 2 NO + CO −−→ N O + CO , demonstrating a potential use of these frameworks for theremoval of highly toxic gases [107]. Figure 13: Post-synthetic modifications of MFU-4l (DMF=N,N-dimethylformamide, DMA=N,N-dimethylacetamide). Re-produced from [103], with permission
An interesting metal azolate framework, corresponding to the formula [M(II)(vtz) ], is derived fromthe commercially available 1H-1,2,3-triazole ligand and divalent metal cations. The first member ofthis series, [Cd(vtz) ], was described in 2009 [108] followed by a report from the D. Volkmer group on[Cu(vtz) ] in 2012 [109]. Later in the same year, O. M. Yaghi et al. reported the synthesis of the metaltriazolate series termed MET-1 to 6, corresponding to [M(vtz) ] with M(II) = Mg, Mn, Fe, Co, Cu, andZn [110], for which they elucidated a cubic structure based on X-ray powder diffraction investigationsof microcrystalline powders. For the Cu(II)-containing framework this assignment is at odds with thesingle crystal structure determination for [Cu(vtz) ], which was shown to crystallize in the tetragonalcrystal system, from which it reversibly transforms into the cubic phase within the temperature range of120– ◦ C [111].Among the MET frameworks, [Fe(II)(vtz) ] showed unexpectedly high electrical conductivity, with aconductivity value of . × − S cm − which the authors could increase to a value of . × − S cm − by exposing the sample to iodine vapor. Although the given explanation, namely the introduction of13e(III) sites by partial oxidation, seems unlikely with regard to commonly reported standard redox po-tentials of Fe(II)/Fe(III) vs. I /I – redox couples, the study had a seminal influence on the developmenton electrically conductive frameworks. A more detailed theoretical analysis for the high electrical con-ductivity of MET-3 ([Fe(II)(vtz) ]), based on accurate band gap calculations, was recently given by M.Dincă and coworkers. [112].It is noted that up to now the structures of intermediate oligomeric species are largely unknown if thesimple vtz − ligand is employed. However, using larger benzotriazolate ligands, discrete nonanuclear co-ordination compounds can be isolated representing oligomeric species from the fusion of Kuratowski-typebuilding units [113, 114]. Lastly, the review on metal–triazolate frameworks would be rather incom-plete without mentioning a series of europium-containing triazolate framework structures reported by K.Müller-Buschbaum et al. [115, 116, 117], among which [Eu(II)(bta) ], is topologically equivalent to thecubic metal–triazolates. In contrast to the well-developed chemistry of porous metal carboxylates and azolates, the numberof porous metal phosphonates is limited to a few dozens of compounds [118, 119, 120, 121]. This is dueto the larger coordination flexibility of the phosphonate groups and the possibility to form – PO H – and– PO groups, which in turn leads to more dense, often layered, structures and makes the formation ofsuitable and easily available SBUs, prevalent in carboxylate MOFs, more difficult. In comparison to thecarboxylate group, the higher number of coordinating atoms and the higher charge of the phosphonategroup often results in a more chemically stable MOF, which is highly desirable for possible applications[121]. On the other hand, these properties also explain the difficulty in formation of large single crystalssuitable for single crystal X-ray structure determination [122]. Hence, scientists working in the field ofmetal phosphonates have relied on the structure elucidation using powder X-ray diffraction (PXRD) ormore recently electron diffraction data, often in combination with structure modelling.The chemistry of porous metal phosphonates has, at the beginning of the 1970s, been dominated byzirconium phosphonates [120, 123]. Their structures can be derived from the ones of [Zr(HPO ) ] · H Oand [Zr(PO )(O POH) ] · H O by replacing the – OH by organic moieties [124, 125]. Mostly dense non-porous structures are formed and porosity was introduced using a mixed-ligand approach through a)post-synthetic hydrolysis, b) use of sterically demanding phosphonic acids or c) topotactic ligand ex-change. Metal phosphonates with defined regular pore structures, highly crystalline metal phospho-nates, have been reported by less than a dozen of groups worldwide including the groups of T. Bein, H.Krautscheid and N. Stock in Germany. The first porous metal phosphonate was obtained using smallmethyl phosphonic acid CH PO H , but subsequently such compounds were synthesized by employingpolyphosphonic acids with N-containing groups, such as secondary or tertiary amines or triazoles. Fromthe reported structures it can be deduced, that either coordination of the N atoms, the presence of pro-tonated amines or the formation of N – H ··· O –– P hydrogen bonds seem to be helpful to form porous 3Dframework structures.Nitrogen-containing polyphosphonic acid molecules with – NCH PO H or – N(CH PO H ) groupshave been mainly employed, since these can be easily prepared starting from primary or secondary amines.Thus the groups of T. Bein and N. Stock were able to hydrothermally synthesize two alkaline earth phos-phonates Ca[(HO PCH ) NHCH C H CH N (CH PO H) ] · O [14], and Ba [O PCH NH CH PO ] · O[126] starting from the respective phosphonic acids. Both compounds exhibit reversible water uptake andare rare cases, even for porous metal phosphonates, since they do not exhibit a one-dimensional SBU intheir crystal structure. In the Ca-based compound two-dimensional nets of 8-rings are observed which arecomposed of alternating corner-linked CaO - and PO C-polyhedra. These nets are connected to a three-dimensional structure through the organic part of the linker molecules leaving space for non-coordinatingwater molecules. The crystal structure of the Ba compound is even more exceptional as the BaO poly-hedra are connected to a three-dimensional BaO framework and the ligands “line” the inner walls of thepores.The most common structural motive in porous metal phosphonates is the presence of one-dimensionalSBUs (Figure 14).The connection of one-dimensional SBUs to porous metal phosphonates seems to be especially effectiveusing N,N-bis(phosphonmethyl)piperazine as the linker, which was extensively studied by the groups ofG. Férey, C. Serre, N. Stock and especially P. A. Wright [127, 128, 129]. Thus, two series of compoundshave been described M-MIL-91 ([MOH n (HO PCH – C H N – CH PO H)] with M = Al, Ti and n = 1,0) [127], the well-known STA-12 series ([M (H O) (O PCH NC H NCH PO )] · xH O with M = Mg,Mn, Fe, Co, Ni) [128, 129] and STA-16, which displays the same topology as STA-12 but contains a larger14 igure 14: Scheme representing the structures of the majority of porous metal phosphonates. Red hexagons: inorganicstructural building unit (SBU), grey rectangle, triangle and square: phosphonic acids. Examples for polyphosphonic acidswhich have been successfully employed are shown at the bottom. linker and thus is the only known example for isoreticular structures in porous metal phosphonates [130].Important structural data was obtained by single crystal X-ray diffraction of STA-12(Mg) [129] and highresolution PXRD of Al-MIL-91 [131].Using a linear tetraphosphonic acid containing two bisphosphonate groups, the group of T. Bein ob-tained the only known flexible open-framework metal phosphonate of composition [NaLa((HO P) CH – C H – CH(PO H) )],[NaLa(H L)], [132] in a high-throughput study. The one-dimensional SBUs are formed by alternatingbisphosphonate (HO P – CHR – PO H) and La ions, which result in the formation of an anionicframework which is highly selective towards monovalent metal cations. Coordination by the phospho-nate and the N-containing groups was also observed by H. Krautscheid et al. in metal phosphonatesof composition [La L (H O) ]Cl · xH O and [Co L ( µ – OH)(H O) ](SO ) · xH O which were synthe-sized using the ligand 4-(4H-1,2,4-triazol-4-yl)phenyl phosphonic acid (H L) (Figure 14) [133, 134]. Inthese compounds also one-dimensional SBUs are observed which are linked by the organic component ofthe ligand. The La phosphonate structure contains large pores with a diameter of . and has beenstudied for proton conductivity due to its high water stability. Diffusivities close to that of liquid waterwere observed and thus conductivity via the vehicle mechanism was proposed [134].The formation of porous structures can also be accomplished using tri- and tetraphosphonic acids withtrigonal or tetragonal planar or tetrahedral geometry, respectively: most porous metal phosphonates be-long to this class of compounds [118, 135, 136]. Especially triphosphonic acids containing aromatic ringshave been employed since this ensures both rigidity and robustness [118]. One-dimensional SBUs arepredominantly present in these compounds and their connection often leads to honeycomb-like arrange-ments.Notably, a structure containing one-dimensional channels between dense, corrugated, hydrogen bondedlayers has been reported [137]. Using this information of tetratopic phosphonic acid chemistry, the N.Stock group recently used a planar geometrically demanding tetraphosphonic acid, Ni(4-tetraphosphonophenyl)porphyrin(Ni-H TPPP), in combination with Co and Zr /Hf ions to construct porous metal phosphonateswith one-dimensional SBUs (Figure 14). Importantly, electron diffraction data was necessary to eluci-date their crystal structures. In [Co (Ni – H TPPP)] · · O even the guest molecules andindividual hydrogen atoms could be directly located from the electron diffraction data [138]. The crystalstructure of [M (Ni – H TPPP)(OH/F) ] · xH O (M= Zr, Hf), denoted M-CAU-30 where CAU standsfor Christian-Albrechts-University, was determined by combining electron diffraction tomography forstructure solution and PXRD data for subsequent structural refinement [139]. In addition to very highthermal and chemical stabilities, these compounds show exceptionally high specific surface areas for metalphosphonates.The chemistry of porous metal phosphonates still relies to a large extend on serendipity, althoughimportant structural trends have already been established. One-dimensional SBUs have been shown to15ead to porous metal phosphonates but a higher variability in framework structures is expected whenwell-defined M – O clusters are accessible. Unfortunately no robust synthesis conditions for such clustersare yet known, partially due to the lack of commercially available phosphonic acids including a variety ofadditional functional groups, which would allow systematic investigations and the possibility to establishadditional and crucial structural trends.
3. Towards an Industrial Scale
The synthesis of MOFs is the most important step in their possible commercialization, since there isno application without a suitable material. An important synthetic approach, which has proved to behighly valuable for MOF science, are high-throughput (HT) methods using reactors for the solvothermalsynthesis of materials. These were first developed in the 1990s [140] for the investigation of zeolites butreactor development by German groups led to their use for MOF discovery and synthesis optimizationin 2004 by the group of T. Bein [141] and later in 2008 by the groups of O. M. Yaghi [142] and N. Stock[143]. Subsequently, HT methodology has been established in many groups and is routinely employed inthe investigation of metal carboxylates, phosphonates, imidazolates and pyrazolates. Unfortunately, theuse of HT methods is often not mentioned in the main manuscript of a publication but “hidden” in thesupporting information, although it can contain a wealth of information on reaction trends.HT methodology is based on the concepts of parallelization and miniaturization (of reactors) andautomation (of synthesis and characterization). The combination of such concepts allows for the efficientand accelerated investigation of the complex parameter space frequently observed for solvothermal syn-thesis [144]. Parameters, such as compositional (molar ratios of starting materials, pH, solvent, etc.) andprocess parameters (reaction time, temperature and pressure), are varied and their influence on productformation is monitored. As a result, new compounds can be discovered, their synthesis optimized and, im-portantly, fields of formation and synthesis-structure trends are established. HT methods can be regardedas important tool for bringing MOFs to an industrial scale [145]. A typical HT workflow comprising thedesign of experiment, the solvothermal synthesis, the work-up and automated characterization followedby data evaluation is shown in Figure 15.
Figure 15: Typical HT workflow as used in the discovery of new crystalline materials from solvothermal synthesis. Repro-duced from [144], with permission.
For the synthesis of MOFs, parallel reactors, often based on the 96 well-plate format, have beenemployed which can hold reaction mixtures ranging from several µ L to a few mL (Figure 16, left).In addition to conventional heating of the reactors, the group of N. Stock has introduced microwaveirradiation [145, 146, 147] and ultrasound [148, 149] as energy sources and also a temperature gradientoven in the HT workflow [150]. In order to facilitate the synthesis and rapid characterization of products,at least a small degree of automation is necessary. Fully automated HT set-ups have been described16ncluding automated liquid handling and solid dosing in the mg -range [151, 152]. HT characterizationis routinely carried out using PXRD, but also high throughput adsorption measurements have beendescribed, the latter being important for the fast screening of sample porosity [153, 154].The advantages of HT-methods in the discovery and synthesis optimization of CPs and MOFs hasbeen summarized in various publications [145, 155, 156]. The following three examples, by Germany-based researchers, demonstrate the advantages of this methodology and the resulting impact to MOFresearch.One of the first HT studies of MOFs in 2008, the functionalization of prototypical materials MIL-53, MIL-88 and MIL-101 was systematically studied for the first time and the importance of the aminofunctionalization for possible modification reactions and applications was stressed [143]. The fields offormation of all the studied functionalized materials were identified by solvothermal reactions of FeCl and2-aminoterephthalic acid (H BDC – NH ) in protic and aprotic reaction media, which allowed extractionof reaction trends. Surprisingly the molar ratio FeCl :H BDC – NH was found to be less important.The solvent was identified to have the most profound impact on the product formation and the otherkey parameters include reaction temperature and, remarkably, the overall concentration of the reactionmixture, an important consideration for synthesis scale-up.The limited amount of data obtained for Ni-based MOFs in 2010 inspired another HT-investigation[147]. Ni salts were reacted in DMF with different aromatic polycarboxylic acids, varying in size andgeometry, and a number of reaction parameters were varied including metal source (counter ion), reactiontemperature, time and addition of bases. In this HT study microwave-assisted heating was employedand isoreticular compounds of HKUST-1 and MOF-14 were obtained with Ni ions. Interestingly, bychanging the ligand from tricarboxylic acid to terephthalic acid and increasing the complexity of thesolvothermal system by the addition of a diamine, two framework polymorphs were obtained. One isbased on a square-grid of Ni-paddlewheel units and terephthalate ions, while the other is based on akagome-grid. The layers in these polymorphs are interconnected by diamine molecules to form highlyporous framework compounds.As a last example the results of the study on Al-MOFs is presented, which led to the discovery ofthe aluminium hydroxide isophthalate CAU-10-H [157]. Due to the interesting water sorption propertiesa green synthesis route was subsequently established employing HT methods and the synthesis scale-up was carried out which allowed the use of CAU-10 in adsorption driven chillers, as discussed laterin this review [158]. CAU-10-H was originally discovered employing dimethylformamide (DMF) as asolvent. The investigation of the system Al / isophthtalic acid (m-H BDC) / DMF-water mixture wascarried out in a 24-reactor HT set-up. The metal source, the molar ratio Al : linker and the solvent,a mixture of DMF and H O, were varied. After reaction at ◦ C for 12 h, the product was filteredoff and characterized by PXRD (Figure 16) [157]. The evaluation of the PXRD data resulted in thefollowing information and trends. As seen in the first three rows, under the given reaction conditionsusing aluminium chloride in the reaction mixture resulted in the formation of mixtures of crystallineCAU-10-H and m-H BDC or pure m-H BDC. In the last row the results using aluminium sulfate as themetal source is given. In almost all reactions CAU-10-H is detected. Starting from this discovery librarythe synthesis was further optimized and the decisive role of the solubility of m-H BDC was seen as a keyfactor since a mixture of DMF and water led to a phase pure product. This knowledge has been veryimportant in establishing a green synthesis route of CAU-10-H. In further studies, the DMF could bereplaced by using ethanol and the pH was adjusted using sodium aluminate (NaAlO ) and m-Na BDCas starting materials. Via high throughput screening suitable reaction conditions were identified at the level and the synthesis was scaled to at least the . -scale with a yield of up to 95% (based onthe amount of linker), using a
10 L round bottom flask.The speed of a HT investigation is determined by the time-limiting step in the workflow. The HTcharacterisation of new compounds is routinely carried out by PXRD measurements within a few minutes.While PXRD data only permits the identification of crystalline phases, no information on the porosityis obtained, which is a key property for many applications. An effective and rapid characterisationtool for the characterisation of up to 12 porous materials at ambient conditions has been reported bythe S. Kaskel group in 2011 [154]. The InfraSORP technique measures the thermal response uponadsorption of a test gas in real time using an optical sensor and it was used to screen various micro-and mesoporous materials for their adsorption capacity and specific surface area. It also allows forthe investigation adsorption kinetics, pore structure and isotherm characteristics within a few minutes.Up to now, mainly physisorption of test gases such as butane, propane or carbon dioxide were carriedout, but the InfraSORP was also used to investigate chemisorption of H S and NH . By carrying outtwo consecutive adsorption/desorption runs, the contributions of physisorbed and chemisorbed guest17 igure 16: Left 24 and 48 eight multiclaves typically used in the HT investigation under solvothermal reaction conditions.Right: HT screening of the system Al / isophthtalic acid (m-H BDC) / DMF-water that led to the discovery of CAU-10-H. In the first three rows AlCl · O was used as the metal source and in the last row Al (SO ) ·
18 H O was used.Product characterization was carried out using PXRD. Green: CAU-10-H; blue: m-H BDC; Red: mixture of CAU-10-Hand H BDC; Yellow: unknown phase. Reprinted with permission from [157]. Copyright 2012 American Chemical Society. molecules could also be distinguished [159].
The scale-up of common research laboratory parameters yielding amounts < of MOF up to condi-tions which allow the production at industrial scale can be considered a crucial challenge for the transferof MOFs to applications as adsorbents in real life. This was discussed by industrial researchers from BASFalready in 2006 along with the potential fields of applications for MOFs [160]. This challenge is closelyrelated to the development of synthesis conditions for the respective MOF that result in a reasonablepricing while being — if possible — environmentally benign and of course preserving performance. Hencesuch “green” synthesis conditions for MOFs were already postulated very early as a likely prerequisite forMOF production at large scale by BASF researchers [161] and therefore the topics “green” synthesis andindustrial development will be discussed side by side herein. Recently, M. R Hill and coworkers producedan excellent review detailing new and general synthetic routes for MOF production at a commercial scale[162].Green synthesis conditions comprise mild heating up to reflux only, and rely on employing mostlyor only water as solvent and have been recently described by H. Reinsch in more detail elsewhere [163].Already in 2006, researchers at BASF filed a patent in which the synthesis of the MOF aluminiumfumarate was described using DMF as solvent [164]. The precise structure of this compound could beelucidated only nine years later and the framework was identified as an analogue of the archetypical MIL-53 structure [165]. The compound shows particularly good performance for heat pumps [166] and otherapplications. Due to these remarkable properties and the commercially promising reactants, a truly greensynthesis using only sodium fumarate and aluminium sulfate as reactants under reflux was subsequentlydeveloped, again by the very same research group [167]. This currently serves as the benchmark forindustrially feasible, mild and cost-efficient synthesis conditions, as the synthesis uses only harmlessreactants at very mild conditions after short reaction times, producing minimal waste.Especially in the group of N. Stock, these synthesis conditions have served as inspiration for thedevelopment of similar processes yielding several other Al-MOFs under mild conditions. An interestingexample is the synthesis of the aluminium isophthalate CAU-10, which originally required the use ofDMF in closed autoclaves under pressure [157]. This potentially hazardous procedure could be optimizedinto an aqueous process, using only ethanol in small amounts as additive, and carried out under reflux[158], thus giving access to kg -scale amounts of CAU-10.In further studies by H. Reinsch the linker molecules mesaconic and citraconic acid were employed,separately, under conditions very similar to the production process for aluminium fumarate. This resultedin the synthesis of MOFs Al-MIL-68-Mes [168] and CAU-15-Cit [169]. These compounds are of particularinterest since the organic building units are derivatives of citric acid, which are readily available atindustrial scale. Furthermore, the “green” synthesis of Zr-MOFs were also investigated by the group inKiel. By using aqueous conditions suitable for the synthesis of zirconium fumarate, as reported by the P.18ehrens group, as a starting point several other compounds based on zirconium ions could be obtainedin water/acetic acid mixtures under reflux [170]. While this necessitates zirconylchloride as metal source,using the less corrosive sulphate allowed for the synthesis of zirconium trimellitate with UiO-66 frameworkstructure [171]. However, MOFs usually cannot be obtained under completely arbitrary conditions andthus the improvement of very harsh to milder synthesis conditions can be considered a relevant approachto “green” synthesis. In addition, the scalability is definitely not limited to mild conditions as variousproducts of interest can be only obtained under harsh conditions, or employing hazardous reactants.Examples of desired improvements for challenging synthesis conditions were recently outlined by thegroup of C. Janiak. The mesoporous chromium terephthalate MIL-101(Cr) is a versatile and extremelystable MOF, usually synthesized employing hydrofluoric acid as additive at very high temperatures,up to ◦ C [9]. In a different approach, the original synthesis of iron trimesate, MIL-100(Fe), wasimproved. This synthesis procedure also involves hydrofluoric acid and requires high temperatures, usingelemental iron as metal source [172]. Employing a mixture of water and dimethylsulfoxide, the C. Janiakgroup succeeded in the synthesis of MIL-100(Fe) at ambient pressure [173], again preserving the highporosity reported for the harsher synthesis conditions. Investigating the synthesis of another iconicMOF, zirconium terephthalate UiO-66, the amount of employed solvent could be drastically reduced bydry-gel conversion [174]. The respective reactants were employed as solid mixtures and kept spatiallyseparated from the comparably small amount of solvent used in a closed reactor. Under heating, thesolvent evaporates and induces the crystallization of UiO-66 in the solid mixture. This procedure is alsoapplicable for some derivatives of this MOF and the solvent is easily recycled and reused.Despite all these academic and industrial efforts, it must be stated that the availability of MOFsin amounts larger than few grams is still limited and the commercial exploitation of MOFs is still inits infancy. A few industrially prepared solids known by the “basolite” product name are commerciallyavailable in small quantities and marketed via Sigma Aldrich. While several start-ups in other countriestry to develop suitable procedures to make MOFs accessible in large amounts, BASF only producesAluminium fumarate at larger scale. Notably, shaping is an important requirement for any industrialtransfer [175] and shaped granules and spheres of MOFs are available from Materials Center Dresden[176].
4. Advanced Characterization
The traditional synthesis of MOFs usually occurs in solution, often at temperatures above the boilingpoint of the respective solvent. Under these conditions slight changes of reaction parameters, for examplethe replacement of the counter ion of the metal source, can have a strong impact on the product for-mation. As previously described, variation of reactant concentration, temperature or time may lead tointerpenetration and completely different framework structures. Thus the parameter space of a chemicalsystem is usually screened to find new MOFs.In situ techniques, which very often necessitate the use of synchrotron radiation, allow the non-invasiveobservation of chemical processes within a sealed reaction vessel without the need for quenching and exsitu characterization [155, 155, 177]. Crystalline intermediate phases may be observed and crystallizationkinetics can be deduced by applying physical models. An overview on in situ monitoring of the formationof crystalline solids in general and in situ studies on MOF syntheses have been summarized by N. Pienackand W. Bensch [178], N. Stock and S. Biswas [155], R. Walton and F. Millange [179] and I. Senkovskaand V. Bon [177]. In situ studies carried out by the groups of R. A. Fischer and C. Wöll on layer-by-layergrowth of SURMOFs [180] will be covered in Section 5.8.Early in situ XRD studies of microporous materials were carried out on zeolites and AlPOs. In 2010the first study on MOFs was reported by the groups of G. Férey and R. Walton [181] and this as wellas following work demonstrated the wealth of information obtained by such investigations. At this timeenergy dispersive X-ray diffraction (EDXRD) studies were employed and in addition to the evaluation ofcrystallization kinetics, “windows of stability” for certain MOF syntheses were established and crystallineintermediates were discovered that were further characterized after quenching. Shortly after, the Germangroups of P. Behrens, N. Stock and M. Wiebcke have made extensive use of the available equipment andsubsequent reactor development was carried out by the groups of R. E. Dinnebier, F. Emmerling and N.Stock. Some of the results of their work is summarized in the following paragraphs.The group of M. Wiebcke has used in situ EDXRD often in combination with other techniques, suchas time resolved static light scattering, to shed light on the role of modulators in the synthesis of ZIF-8and the formation and transformation of zinc imidazolate polymorphs [182, 183, 184]. While P. Behrens19nd coworkers studied the modulated synthesis of zirconium fumarate MOF with UiO-66 structure [185].They were able to elucidate the role of modulators as competitive ligands in the coordination equilibriaand bases in the deprotonation equilibria during nucleation and crystal growth. These important resultshas allowed for the fine adjustment of size and morphology of ZIFs and Zr-MOFs , an area which hasseen continued interest and investigation by researchers outside of Germany [186, 187].The chemistry of Al-MOFs has been extensively studied by the group of N. Stock using in situ EDXRDand high-energy monochromatic X-rays. This group also introduced energy input through MW-assistedheating [188, 189] and ultrasonication [149], for in situ crystallization studies. MW-assisted heating in thesynthesis of lanthanide phosphonates led to shorter crystallization times in comparison to conventionalheating, although no significant influence on induction time was observed. In these studies intermediatecrystalline phases were also discovered and in some cases these could be isolated by quenching. Furthercharacterization of the crystalline intermediates established the structural evolution of the final product[190]. In a study of the Al-MOF denoted CAU-1, the role of ligand functionalization on product formationwas elucidated and conventional and MW-assisted heating were compared (Figure 17). Here, MW-assistedheating led to shorter induction times and faster crystallization and the amino-modified compound (CAU-1-NH ) showed faster crystallization kinetics compared to the dihydroxy modified compound CAU-1-(OH) [189]. Surprisingly, using MW-assisted heating at higher temperatures no product formation ofCAU-1-(OH) was observed, while conventional heating resulted in this desired product. Figure 17: (a) Time-resolved in-situ EDXRD data measured during the crystallization of CAU-1-(OH) at 125 °C usingconventional heating. (b) Comparison of induction and reaction times between MW-assisted (blue) and conventional (black)heating for the crystal growth of CAU-1-(OH) in the range 120- ◦ C . Reproduced from [189], with permission. A combination of in situ infrared radiation (IR) and EDXRD measurements on the formation of anickel phosphonate under ultrasonication revealed faster crystallization upon increase of the ultrasoundamplitude under constant temperature [149]. Temporal evolution of the signals in the IR spectra wereattributed to enhanced crystallization kinetics for faster dissolution of the ligand.Development of third generation synchrotron sources and improved detectors have dramatically in-creased the scope of possible in situ analyses [177]. Far better time resolution and signal quality, incombination with new in situ reactors, has recently allowed the study of more complex reaction systemsin unprecedented detail. It was first demonstrated by the groups of T. Friščić and R. E. Dinnebier thatmechanochemical syntheses can be followed in situ within a ball mill [191]. Fast data collection and fullpattern-fitting allowed the study of sequential crystallization of zinc 2-ethylimidazolates and the role ofsmall amounts of liquids (liquid-assisted grinding) or additional salts (ion and liquid-assisted grinding).The group of F. Emmerling even combined in situ XRD analysis with Raman spectroscopic measurementsto investigate the formation of four model compounds and demonstrated information of crystallizationat the molecular and the crystallite scale [192].The combination of small- and wide-angle X-ray scattering (SAXS/WAXS) is also useful when shortacquisition times are necessitated, although such studies are still very rare. The synthesis of ZIFs [193, 194]and Al-MOFs [195] were investigated using these methods to provide insight to nucleation and crystalgrowth processes. The groups of M. Wiebcke and K. Huber studied the formation of ZIF-8 with one secondtime resolution. SAXS provided information on the presence and size of nano-sized clusters during the20ucleation and early growth of nanocrystals while WAXS followed the larger crystal growth. Based onthese results two possible pathways on the ZIF-8 formation at room temperature were established.Recently a new in situ reaction cell SynRAC (synchrotron-based reaction cell for the analysis ofchemical reactions) has been reported which was jointly developed by the groups of N. Stock, W. Benschand the beamline staff at P08, PETRA III, DESY [196]. In addition to using glass vessels that areroutinely employed in laboratory synthesis, the reaction cell also allows for precise control of reactiontemperature, including fast heating rates, which is important for rapid crystallization processes. Complexreaction schemes with multiple temperature ramps can be investigated, fully computer-controlled, andthe option to add liquids and solids at the beginning or during the synthesis can be chosen. Thus reactionscan be quenched or chemical parameters (concentration, pH, etc.) can be changed to study their influenceon product formation.
Figure 18: Left: picture of the recently developed fully computer-controlled SynRac reaction cell, which allows the precisecontrol of the reaction temperature, including fast heating and cooling as well as the possibility to add liquids and solidsduring the reaction. Right: complete setup with computer control and liquid dosing.
In proof of concept studies, this reactor has been employed to study the synthesis of Ce-UiO-66, whichforms rapidly and decomposes upon extended reaction times under the formation of cerium formate [45].Pre-heating of the solvent/linker solution to the targeted reaction temperature and injection of the linkersolutions allowed the reactor to reach the targeted reaction temperature within ≈
20 s , which allowedfor determination of kinetics of this fast crystallization process. In addition, a crystalline intermediatewas isolated and identified during the formation of [Bi(HIDC)(IDC)] [197] through quenching of thereaction mixture by direct cooling and subsequent characterization. The versatility of the SynRAC hasbeen proven in other in situ PXRD investigations and in a combined in situ XRD / XAS study of theevolution of palladium species in Pd@MOF catalysts during the Heck coupling reaction [198]. In oneof the first full scale studies using the SynRAC the linker conformation controlled flexibility of CAU-13 upon interaction of different pyrazines in solution at various temperatures was investigated [199].The results showed that stronger host-guest interactions required milder adsorption conditions whileharsher conditions nevertheless accelerated the conversion. Surprisingly the kinetic parameters for theintercalation of pyrazine indicated that dependent on intercalation temperature the rate limiting stepdiffers.As a final example, the green synthesis of Al-MIL-68-Mes is mentioned. This compound is readilyobtained upon heating a reaction slurry, which is rapidly formed directly after mixing the solutions of thestarting materials, after short reaction times and under mild aqueous reaction conditions. Surprisingly,the reaction temperature has only a slight influence on the induction time for crystallization, but the shapeof the crystallization curves are substantially different demonstrating that a temperature increase doesnot simply accelerate the reaction, as expected. To explain the crystallization kinetics, the homogeneityof the slurry and the dissolution properties of the intermediate amorphous phase were taken into accountand based on the extracted Avrami exponents, a plausible, but rather complex formation process waspostulated.The impressive developments at synchrotron sources in combination with newly developed reactordesigns open up a wide variety of experiments that will allow to study MOF formation with unprecedented21recision. Especially the combination of characterization methods (scattering, spectroscopic etc.), whichallow for the study of crystallisation processes at different length scales, will help us in the future tounderstand the formation and properties of MOFs. This may allow us to tune the reaction conditions inorder to obtain materials with desired properties. It should be considered that many reactors are wellestablished at different synchrotron facilities and scientists interested in such investigations are stronglyurged to contact beamline scientists or the reactor developers.
Adsorption-induced flexibility in MOFs, discovered in early 2000s by K. Kaneko, S. Kitagawa and G.Férey, required new characterization techniques, allowing for the detailed characterization of structuralchanges during the adsorption of guest molecules. The presence of long range order in MOFs make X-raydiffraction techniques preferential and most exact, for following the phase transitions. Since most offlexible MOFs could be synthesized as fine powders or show large amplitude of structural changes, PXRDis the appropriate technique for detection of the structural changes. Hence, R. Matsuda and coworkersused synchrotron X-ray powder diffraction for direct observation of the adsorbed hydrogen in the pores of[Cu (pzdc) (pyz)] n (pzdc = pyrazine-2,3-dicarboxylate, pyz = pyrazine) [200]. In Europe, P. L. Llewellynand coworkers designed a similar capillary-based setup for investigations of adsorption-induced transitionsin materials of the MIL-53 family [201, 202].In Germany, groups of R. A. Fischer, S. Kaskel, H. Krautscheid and N. Stock recognized early theboth academic and industrial potential of flexible MOFs and contributed not only to the synthesis of newflexible materials, but also to the development of new in situ PXRD instrumentation. In 2006, a flexible“gate pressure” pillared-layer MOF DUT-8(Ni) was synthesized in Dresden for the first time [203]. Atthat time, there was no experimental setup at German large scale facilities for measurement of PXRDpatterns under the required gas loading and temperature for DUT-8(Ni) flexibility. Therefore in orderto explain the structural behavior of DUT-8(Ni) during gas adsorption experiments, the group at TUDresden, with a support of the sample environment group of Helmholtz-Zentrum Berlin für Materialienund Energie (HZB), developed an automated instrumentation for in situ PXRD measurements duringadsorption and desorption for all non-corrosive gases [204]. The instrumentation is commissioned at theKMC-2 beamline of HZB and is available for the whole user community of the BESSY II synchrotron.This important instrumentation includes an adsorption chamber, mounted on a closed-cycle heliumcryostat, which ensures isothermal adsorption / desorption conditions within the temperature range 10–450 ± θ scans. The adsorption chamber is then connected to the adsorption instrument (BELSORP-max),allowing to measure low-pressure isotherms within the range − –1 bar. After reaching an adsorptionequilibrium conditions, PXRD patterns can be measured at each adsorption/desorption point. However,this setup allows the measurement of only low-pressure isotherms because of the limitation of the ad-sorption instrument and beryllium dome. Small modifications also allowed the use of this same setupfor in situ X-ray absorption spectroscopy measurements on the same beamline and temperature/pressureconditions.In order to test flexible MOFs under conditions of pressure and temperature swing adsorption anothercapillary-based setup was combined with a BELSORP-HP instrument at the same facility. The instru-mentation allows for the monitoring of adsorption in the pressure range of up to 80 bar and temperaturesbetween 200–300 K. Recently the setup was successfully used in the in situ study of methane hydrateformation in the confined pore space of porous carbons [205].One of the potential applications of chemically stable MOFs is adsorptive heat pumps. To shed thelight on the adsorption mechanism of water and other vapors, an in situ vapor cell is now constructed andwill be commissioned at the same facility. With three different types of in situ cells, KMC-2 beamline atBESSY II synchrotron became a multipurpose instrument for the study of crystalline porous materials atdefined gas / vapor loading in broad pressure and temperature ranges. In the recent five years, adsorptioninduced flexibility in a number of flexible MOFs namely, DUT-8(Ni) [206], ELM-11 [204], CAU-13 [207]solid solutions of [Zn (BME – bdc) x (DB – bdc) – xdabco] n [208], SNU-9 [209] and DUT-49 [210, 211], werestudied using these experimental setups.A similar experimental setup was described by R. A. Fischer and coworkers [212]. The setup was usedto investigate the multi-step breathing behavior in a pillared-layer system [Zn (BME – bdc) (dabco)] n atthe beamline of the BL9 of the synchrotron radiation facility DELTA. The cell uses a helium cryostat fortempering of the sample and the measurements proceed in reflection mode. However no isotherm can bemeasured with this setup, only PXRD patterns at defined CO loadings.22 igure 19: In situ PXRD cells for characterization of MOFs during gas and vapor adsorption: a) low-pressure cell; b)high-pressure cell; c) vapor cell. All above mentioned setups are available only at large scale facilities, which do not imply frequentuser access. Therefore H. Krautscheid and coworkers designed a setup, which can be mounted on thelaboratory powder diffractometer STOE STADI P. The cell consists of glass or quartz capillary, gluedon the VCR-support and connected to a “home-made” gas dosing system. The adsorption temperaturecan be controlled by nitrogen cryostream. Using this setup, PXRD patterns at different n -butane and1-butene loadings were measured on [Cu ( µ -O)( µ -OH) (Me trz – pba) ] at room temperature [213].In summary, development of in situ PXRD instrumentation for characterization of MOFs at variableguest loading is extremely important for elucidating framework dynamics and adsorption mechanisms.This was one of the crucial factors, which propelled the German flexible MOF community to the level ofthe leading groups in the field. Imperfections or defects naturally occur at temperatures above 0 K in every crystalline material andare responsible for several fascinating properties. The most stunning examples are precious gemstones likeEmerald Be Al (SiO ) or Sapphire Al O that acquire their impressive color from chromium impurities.For synthetic systems, only the precise control of defect concentration, allows for the fine tuning of certainproperties of the material, which has led to relevant discoveries and even novel technological applicationsof such materials. One example that originates from the precise control of defect chemistry is dopedsemiconductors that are used in many different devices. MOFs being crystalline porous materials are noexception. For this reason, it is exciting to see that defects are now recognized to be similarly useful intailoring MOFs properties.The first attempt to gain control over defect formation in MOFs was achieved with the modulationapproach. It is established that small amounts of monocarboxylic acids, or modulator, slow down thespeed of crystallization by impacting the equilibrium reaction (the formation of the framework). Incontrast, large modulator concentrations facilitate framework incorporation and in turn the formation ofdefects. The first report using this approach was given by U. Ravon et al. in 2010, using 2-toluic acid asmodulator in the synthesis of MOF-5 [214]. Since then, many research groups focused on the synthesisand characterization of defective MOFs [215, 216, 217]. R. A. Fischer [218], K. P. Lillerud [219] and A.L. Goodwin [220] were one of the first to strongly believe that defects can be exploited to enhance MOFsproperties.In 2014, R. A. Fischer and coworkers used the so-called mixed linker approach, where the linker ofthe parent framework is partially substituted by a linker having one different coordinating group, e.g.1,3,5-tricarboxylate (BTC ) by pyridine-3,5-dicarboxylate (pydc ) [218]. The material showed highactivity in the hydrogenation of 1-octene and CO adsorption.23n 2016, K. P. Lillerud and coworkers systematically studied defective UiO-66 (Figure 20) by applyingseveral different characterization techniques such as PXRD, BET (N physisorption), and high-resolutionnuclear magnetic resonance (NMR) after digesting the defective MOF [219]. Figure 20: Schematic representation of the modulation approach showing the generation of two types of defects, missinglinker on the right and missing cluster defects on left. Reprinted with permission from [219]. Copyright 2016 AmericanChemical Society.
In another study, the group of A. L. Goodwin achieved a milestone for defect characterization wherea combination of several techniques such as (anomalous) powder X-ray diffraction (PXRD) and pairdistribution function analysis (PDF) combined with computational modelling was used to access thedefect chemistry of UiO-66 [220]. These contributions are just few examples of the many other importantstudies and it is not surprising that this area of research has gained much attention during the past fewyears.In a recent review R. A. Fischer and coworkers outlined recent progress in the field [221]. Onehighlighted concept of the review is the intrinsic difficulty to characterize defects (such as the elucidation ofdefect concentration and spatial distribution). For instance, diffraction patterns of defective MOFs differfrom their respective pristine counterpart by minor changes which are difficult to detect by a laboratoryX-ray diffraction due to the instrument resolution limitations. Therefore, experimental techniques like X-ray absorption fine structure (EXAFS) which can probe the local structure should be used. This techniquerequires the use of synchrotron sources which are not readily accessible; therefore, the structural insightinto defects in MOFs is limited.Further insight from computational modeling of defective MOFs has appeared since 2015. So far thesystems of choice are mainly UiO-66 [222, 223] and its derivatives and some other examples HKUST-1[224]. Nevertheless, useful insight on how defects may affect MOFs mechanical properties have beenachieved. In particular, the group of V. Van Speybroeck provided a thermodynamic characterizationof the high-pressure behaviour of UiO-66 as a function of missing linker defects and linker expansionin the absence of guests. Indeed, for the defect-containing and/or expanded linker samples, a reducedmechanical stability is observed [223].It is clear that the in depth characterization of defective MOFs bear many problems which equallychallenges experimental and computational chemists. Presently, the community has demonstrated manyproperties which are closely linked to the defect concentration and chemistry. However, the defect struc-ture (including local and correlated defects) in MOFs is barely investigated. Recently some unconventionalcharacterization techniques were used in an attempt to solve this problem. Positron annihilation lifetimespectroscopy (PALS) [225, 226], ultra-high vacuum infrared spectroscopy (UHV-IR) with probe molecules[227], electron paramagnetic resonance (EPR) [228], acid-base titration [228] and water adsorption [229]are a few techniques recently applied to MOFs.One attempt to obtain a complete and clear picture of defects which are present was made by R.A. Fischer and coworkers in 2016. The authors combined a set of spectroscopic characterization data(XANES, XPS, UHV-FTIR with CO and CO ) to solve a complicated picture of the defects present inHKUST-1(Ru) [227]. In their work the authors identified two types of defects obtained with the usemixed-linker approach (Figure 21): Type A which corresponds to a structural and electronically modifiedRu paddlewheel and Type B which corresponds to a complete missing paddlewheel defect. In type Adefects, one out of the four bridging carboxylate groups of the parent 1,3,5-tricarboxylate (BTC) linkerin the regular paddlewheels was partly substituted by the (neutral) functional group of the defect linker(H, OH, NH and Br). Consequently, for charge compensation, the metal sites can be partly reduced(mixed-valent state < δ < ). According to XANES and XPS results the application of defect linkerswith coordinatively inactive functional groups like H is likely to induce missing-node type B defects whichare, accompanied by lower abundance of Ru and Ru δ + centers in the framework. On the contrary,24he material with coordinatively active functional groups like OH, NH and Br can induce preferentiallydefects of type A, which are accompanied by an increase of the Ru δ + centers, especially at moderatedegrees of incorporation. However, the authors could not exclude the simultaneous generation of bothdefects in these samples. Indeed, UHV-IR with CO as a probe molecule confirmed the presence of bothsites, in particular at high concentration of defects for the samples that have linkers with coordinativelyactive functional groups. Figure 21: Schematic description of the mixed linker approach used to produce type A and B defects in HKUST-1(Ru).Reproduced from [227], with permission.
A new fascinating technique that was recently used to characterize defects is PALS. This method candetect the release of gamma rays that are produced by the interaction of Positrons (Ps) (the antiparticlesof electron) with electrons. In a MOF, the main electron density is located at the framework, couplingthe Ps lifetime to the pore size. Already in 2010 PALS was used by M. Liu et al. for this purpose [230].However, in 2016 S. S. Mondal et al. [226] applied PALS to study mesopore formation in hydrogen-bonded imidazolate framework (HIF-3). The authors assigned missing building blocks in HIF-3 as reasonfor mesopores which are responsible for structural flexibility during gas uptake.In a more recent work the authors were able to detect the small differences in pore-sizes of imidazolateframework Potsdam (IFP) structures and variations of gas adsorptions properties, between microwave-assisted (MW) materials and conventional electrical (CE) heating conditions [225]. In particular MW-synthesized materials, due to the fast crystallization process, free linker molecules can be trapped andtherefore, reduce the pore sizes from micropores to ultramicropores. According to the authors theseultramicropores are responsible for the enhancement in the gas uptake capacities since the gas moleculescould interact more efficiently with the pore walls [225]. This example demonstrated that PALS has someinherent advantages over common N physisorption measurements. Most importantly, interconnectedpore space is not required to determine pore size from PALS. Therefore, this method can be applied foropen and closed-pore systems.EPR is also a useful technique to access the chemistry of defects in MOFs due to its high sensitivity.A. Pöppl and coworkers used this technique to precisely characterize the ligand environment of Ni ionsin flexible DUT-8(Ni) [231]. In their work they used the small nitric oxide (NO) as a magnetic probe toinvestigate the adsorption properties as well as the local adsorption sites of NO in DUT-8(Ni). The NOmolecule is accessible by EPR since it has one unpaired electron in an antibonding molecular Π state.In particular, the lowest rotational level of the Π / state of the free NO molecule allows the detectionof even small amounts of desorbed NO gas in adsorption experiments [231]. The authors were able todistinguish up to five different signals coming from defective site of the flexible and rigid material whichcan be attributed to NO molecules forming paramagnetic complexes with the Ni ions. These mightbe defective species since in principle defect free DUT-8(Ni) offers no open coordination sites for NO.25urthermore, the density of these species is one order of magnitude higher in the rigid material thanin the flexible material. In particular, only for the rigid defective DUT-8(Ni) material two Ni – NOadsorption species were observed. For these two species, the unpaired electron resides in a dz orbitalof the Ni ion instead of the dx y orbital, where it resides for the flexible material. A model whereone NO molecule bonds in the equatorial plane of a defective paddlewheel unit to the Ni ion canexplain the two species. This implies that at least one NDC linker (2,6-naphthalenedicarboxylate) doesnot coordinate to these units. The absence of some linkers in the rigid DUT-8(Ni) might decrease thetotal attractive force between the ligands originating from π - π stacking. This could also explain whyrigid DUT-8(Ni) stays porous even in the absence of adsorbates [231].Another useful technique to access the chemistry of defects is water adsorption measurements. Asdescribed by J. Canivet et al. [229], this technique can be rationalized using a simple set of parameters:the Henry constant (which is the slope of the adsorption pressure in the low pressure range), the pressureat which pore filling occurs, and the maximum water adsorption capacity. The first two parameters arecorrelated, both containing information related to the hydrophilicity of the material. These parameters,as shown by S. Dissegna et al., can be used to access the chemistry of coordinatively unsaturated sitesin UiO-66 [232]. As expected, an increase of defective sites in UiO-66 leads to an increase in bothhydrophilicity and the maximum water capacity. Both were then correlated with an increased activityfor a Lewis acid catalyzed reaction.The techniques described briefly in this section are selected to give the reader an overview of therapid evolving field in the defect characterization by a number of German and international groups. It isapparent, that to obtain in depth structural information of defects, standard analysis and unconventionaltechniques for MOFs, (such as PALS and EPR) should be combined in order to reach a higher level ofaccuracy. Nuclear magnetic resonance (NMR) has proven to be a powerful tool in characterizing adsorption ofporous materials. Two types of NMR techniques are popularly used for extracting required informationabout the MOF frameworks [233]. The first method uses regular solid state NMR, where chemicalshifts of the nuclei of interest are used to determine the chemical environment of constituents within theMOF lattice. The technique of magic angle spinning (MAS) is used generally to remove the spectralbroadening otherwise observed for solid samples. Further enhancement of the spectral sensitivity isgained from application of hyperpolarization techniques, such as cross polarization (CP) and dynamicnuclear polarization (DNP). Such solid state NMR spectra provide crucial information about the linkerconformation (mostly using C) and also possibly the metal cluster ( Zn, Al, etc.).The second and more beneficial application of NMR spectroscopy involves the use of NMR activeguest molecules (like Xe, CO , etc.) within the MOF pores. Chemical signal of the active nuclei differsignificantly with interaction of the environment, thus presenting a description of the local surroundingsof the guest molecules (Figure 22). The location of active gases into the pores and subsequently thepore environment can be easily detected by monitoring the change in its chemical shift. Subsequentlyinvestigation by the group of E. Brunner using Xe adsorption in MOFs has provided a deep insight tounderstand pore geometry, pore filling mechanism and most importantly kinetic features, such as thebreathing of flexible MOFs. The basic equation for Xe chemical shift from MOF pores is constructedfrom the addition of multiple factors: basic reference shift ( δ ), shift from interaction with the pore( δ S ), shift contributed from paramagnetic sites ( δ M ), shift contributed from the electric field caused bythe metal ions of the framework ( δ E ) and moreover, shift from intermolecular collision of the adsorbatemolecules ( δ Xe − Xe ), Equation 1. δ = δ + δ S + δ M + δ E + δ Xe − Xe (1)Thus the observation of the chemical shift in Xe NMR provides the location and interaction of theXe molecules with respect to the MOF pores [234]. Theoretical support to the observed difference inchemical shift for the Xe NMR in different MOFs has been established by K. Trepte et al. Theoreticalmodels constructed from the experimental observation during NMR spectroscopy has been applied fortwo MOFs (UiO-66 and UiO-67) with varying pore sizes [235]. It has been evidenced that at a givenpressure for larger pore sizes the inserted Xe molecules have less interaction with the surface, therebylowering the chemical shift. The predicted model has potential for predicting the chemical shift for similarscenario.Application of Xe NMR to study the pore environment of Cu (BTC) (H O) (Cu-BTC) MOF wasreported by W. Böhlmann et al. [236]. The presence of two different peaks confirmed the presence of two26 igure 22: Components of MOF structure and interaction with Xe atom to generate shift in NMR spectroscopy. Reproducedfrom [234]. types of pores within the framework. This concludes with lower value of δ S for larger sized pores, thus alower chemical shift indicated adsorption into the larger pore of Cu-BTC framework. The chemical shiftwas also found to vary in a non-linear fashion with change in Xe pressure. Furthermore, co-adsorptionof Xe with other species like water and ethylene provides an insight into the pore filling mechanism forsuch case. Thus, the smaller pores of Cu-BTC frameworks were observed to be adsorbed with waterand ethylene, preferentially over Xe. The Xe NMR for the ethylene coadsorption also revealed a stronginteraction between the gas and Cu sites.Application of Xe NMR for studying the kinetic behavior of flexible MOFs has been extensivelyinvestigated by the collaboration of the S. Kaskel and E. Brunner groups. The well-studied breathingbehavior of the DUT-8(Ni) has been examined using Xe NMR to measure and accurately deduce thegate opening pressure [203, 237]. In addition to the broad peak from adsorbed Xe into the pore, anothernarrow peak at lower chemical shift was also observed, which was attributed to the Xe molecules trappedinside the void between MOF particles. Retention of the peak intensity corresponding to the adsorbedN revealed the complete filling of the pore, in correlation with the adsorption isotherm. Isothermscollected at different temperature further demonstrated the kinetic behavior, with pronounced hysteresisas observed in the adsorption experiment. Precise determination of the gate opening pressure, as observedfrom the NMR analysis further supported the observations from other experimental techniques.A thorough investigation of the Xe NMR reveals an absence of Xe peak prior to the gate openingpressure for DUT-8(Ni) [237], with good correlation to both adsorption isotherm as well as XRD pattern.The pore opening, accompanied with a color change was observed beyond 12 bar, through the gradualappearance of a peak in the desired region. The observed chemical shift remained pressure independentbeyond the gate opening pressure, indicating a complete saturation of the pores. Presence of the peakcorresponding to Xe residing outside the MOF pore shows liquid like behavior under pressure. TheseXe atoms can undergo rapid exchange with the adsorbed atoms within a very short time span (in tensof milliseconds), characterized through cross-peaks at a mixing time of
25 ms in the 2D
Xe EXSYspectrum.Further studies for the breathing behavior were reported for a series of MOFs containing pendantgroups, and their solid solutions [208]. Application of in-situ C NMR spectroscopy to CO adsorptionin MOFs provide an insight to the process of gate opening (Figure 23). The narrow pore (np) form of thesynthesized and activated MOF allows for few CO molecules to be adsorbed within the pores, therebykeeping all the introduced CO outside and a single signal as that of Xe NMR. On application of thethreshold pressure required for phase transition and subsequent opening of the pores, the MOF switchesto the large pore ( lp ) form. Conversion into this lp form allows more and more CO molecules to beadsorbed, which causes a broadening of the peak. This chemical shift of the MOF with CO filled poresremains constant indicating a complete filling of the pores, immediate to the breathing phenomenon. Thecharacteristic desorption profile is clearly observed from the NMR as well, accounting for the hysteresisstep.The unique feature of the recently developed flexible MOF (DUT-49) with negative gas adsorption(NGA) property has also been studied by E. Brunner et al. using in-situ Xe NMR [238]. The phenomenonof NGA is unique with a feature of pressure amplification in the surrounding atmosphere. As discussedfurther in the flexible MOF section, the structural deformation of the carbazole biphenyl linker causesshrinkage in the unit cell of DUT-49 structure and release of the excess gas molecules resulting in NGA.27 igure 23: Different components their chemical compositions involved for formation of the flexible MOF and its pore openingin present of guest. b) C solid state NMR of the MOF in presence of different relative pressure of CO and c) its plot.Adapted from [208], with permission. Such a unique feature has made DUT-49 framework of acute interest and its study through in-situNMR provides excellent insight. At higher temperature, like 273 K, no flexibility was observed from theXe isotherm. Similar pattern was observed in the case of observed chemical shifts. However, adsorptionisotherms collected at lower temp (200 K) shows the signature change of NGA and the same was observedfrom drastic change in the difference of chemical shift. As the open pore ( op ) phase containing themesopores of the framework undergoes contraction to generate the closed pore ( cp ) phase with smallerpores, the interaction of the residual Xe atoms with the pore walls increases. This enhanced interactionthus causes a higher chemical shift in the NMR spectrum. This change in chemical shift thus can be usedto finely locate the conversion of the cp phase form to op phase form at higher pressure.
5. Advanced Function
The interdisciplinary approach of German MOF groups targeting specific functions by combiningsynthesis and characterization, as described earlier in this contribution, has produced many materialswith a number of advanced functions.
The research in the field of adsorptive gas storage was strongly motivated by increasing threat ofglobal warming, depletion of fossil fuels and interest to explore alternative renewable energy resources.Adsorption technology, in principle, could tackle this problem it two ways: improve the energy storagedensity for energy reach gases, such as hydrocarbons and hydrogen, capture the greenhouse gases, suchas carbon monoxide. But also storage of noble gases is considered as potential industrial application forMOFs. MOFs are predestinated for adsorption based applications because of their high specific surfacearea (as adsorption relies on surface upon which gas molecules can adsorb) as well as tunable nature ofsurface chemistry.The high potential of MOFs for gas storage materials was recognized quite early and the first reportsfor methane adsorption was presented by S. Kitagawa and coworkers in 1997–2000 [239, 240]. In 2003O. M. Yaghi et al. reported the first hydrogen adsorption measurement in MOF-5 and IRMOF-8 [241].These pioneering studies initiated great interest in MOFs as physisorption based gas storage materials inthe world and in Germany.In 2005–2006 B. Panella and M. Hirscher, based at MPI Stuttgart, presented hydrogen adsorptionmeasurements of MOF-5 and HKUST-1 over a wide range of pressures and at different temperatures28anging from
77 K to
298 K [242, 243]. Further studies were dedicated to the hydrogen storage capacityin other highly porous MOFs, such as MOF-177. Surprisingly the hydrogen uptake at 77 K, for allmaterials (both MOFs, and activated carbons), follows a linear dependence on the surface area with aslope of 1.9 wt% per g − (known as Chahine’s rule, Figure 24) [244, 245]. Figure 24: Hydrogen storage capacity at 77 K of different materials versus their specific BET specific surface area. Thesolid line is the hydrogen uptake as predicted from theory, the dashed line is a linear fit to all data points. Reproducedfrom [244] with permission of The Royal Society of Chemistry.
Isosteric heat of adsorption is an important parameter required to describe the thermal performance ofadsorptive storage systems. It can be calculated from adsorption isotherms measured over wide ranges ofpressure and temperature or directly estimated using the coupled calorimetric–volumetric methods. TheM. Hirscher group studied the isosteric heats of hydrogen on many prototypical metal–organic frameworksusing both methods and find a good agreement between them [246]. M. Schlichtenmayer and M. Hirscheralso determined the storage capacity as a function of operating temperature for the isothermal operationof a storage system. It shows a maximum at an optimum operating temperature which is at highertemperature for materials with higher enthalpy of adsorption. The fraction of the total hydrogen storedthat can be released at the optimum operating temperature is higher for materials with lower enthalpyof adsorption than for those with higher enthalpy [247].In 2008, I. Senkovska and S. Kaskel reported excess methane storage capacities, and release curves,obtained from a mini-tank prototype operating up to 200 bar at room temperature, for three representa-tive MOFs: HKUST-1, Zn (bdc) dabco, and MIL-101(Cr). At higher pressure, the specific pore volumedictates the gravimetric storage capacity of a porous material, so mesoporous materials with high porevolume show higher uptake because of higher density of gas compressed inside the pores. Moreover, toincrease the usable capacity of the high pressure tank, where the usable capacity is the amount of gas thatcan be released at a certain operating temperature between the maximum pressure and the minimumpressure possible for the storage tank [209], mesoporous materials (such as MIL-101, Figure 25), whichadsorb weakly in the low pressure region show better performance.In the search of such materials with a high pore volume, a plethora of mesoporous MOFs (DUT-6 [57],DUT-23 [55], DUT-9 [248], DUT-32 [249], DUT-49 [62], DUT-76 [250]) were reported. These materialsdisplay excellent adsorption characteristics and were developed by the Kaskel group using rational designstrategy in 2009–2015. At the time of the invention, the DUT-23(Co) and DUT-49 were record holders interms of maximum methane excess adsorption capacity, with
266 mg g − (100 bar, 298 K) and
308 mg g − (110 bar, 298 K), and still belong to the top ten compounds worldwide [251].As previously mentioned, in 2000 the German chemical company BASF started to explore the com-mercial potential of MOFs. BASF is focused on commercializing MOF solutions [160, 252] for the trans-portation industry [253] and is partnering with several technology, development and original equipmentmanufacturers to pilot the use of MOF materials for enhanced natural gas storage in vehicles in Europe,Asia and North America. Vehicles equipped with natural gas fuel systems, containing MOF materials,were introduced by BASF in 2013 [254].Not only the gas storage performance but also the understanding of factors influencing the adsorp-tion behavior has been in the focus of research. Theoretical studies of adsorption properties using grandcanonical Monte Carlo (GCMC) simulations and DFT calculations were performed by the M. Fröba group29 igure 25: Methane desorption isotherms at 293 K for MIL-101(Cr) (green), HKUST-1 (blue), and MIL-101(Cr)–HKUST-1mixture (orange). The red line represents the amount delivered by a filled empty cylinder. [49] - Reproduced by permissionof The Royal Society of Chemistry. [255, 256, 257], showing that simple force field methods provide a reliable prediction of hydrogen adsorp-tion isotherms in MOFs. Futher examination of the calculated density fields permits the identification ofstructural features that create preferential adsorption regions [255]. The same group has also synthesizedand measured hydrogen adsorption at 77 K for a number of MOFs, such as PCN-12-Si [258], and UHM-6(UHM: University of Hamburg Materials) [259], containing silylene groups.The group of H. Krautscheid studied the adsorption properties of series of isomorphous MOFs inclose collaboration with J. Möllmer and R. Staudt [260, 261]. For example, Cu paddlewheel basedtriazolyl isophthalate containing MOFs demonstrated small changes within the MOF structure causedistinct differences in the thermal, adsorptive and catalytic properties, showing the possibility to designthe adsorbents with specific gas adsorption properties [262]. A number of the advanced functions demonstrated by MOFs are caused by dynamic features of theframework structure, such MOFs and similar materials are labelled as “soft porous crystals” [263]. Dy-namic features can include flexible moieties extending into the pore network or large volume changes of theframework. Germany-based researchers have played an important role in the discovery and investigationof flexible MOFs [264].S. Kaskel and coworkers reported the structure and properties of DUT-8 a series of paddlewheel basedpillared-layered frameworks containing Cu , Zn , Co and Ni based metal nodes [265, 203]. Inter-estingly, the Cu based framework, DUT-8(Cu) demonstrates rigid behavior with no structural changesupon solvent removal. However, DUT-8 frameworks, constructed with Ni, Zn, and Co paddlewheels,indicate structural changes during evacuation. Following the adsorption of a number of gases, such as N and Xe, the DUT-8(Ni) structure transforms between closed and open phases, completely, leading to aunique, and large reversible “breathing” transition. The transition in DUT-8(Ni) was investigated usingadvanced in situ experiments, such as Xe NMR [203, 237] and in situ PXRD [206], detailed previously.The R. A. Fischer group also reported a family of similar pillared-layered frameworks that show a tran-sition between a closed and open phase [266]. The functionalized metal–organic frameworks (fu-MOFs)have the general formula [Zn (fu – L) dabco] n and the pores of this material are filled the dangling sidechains (L). In contrast to DUT-8, phase transitions in fu-MOFs can be controlled by temperature. Uponan increase in temperature, thermal motion of the dangling side chains increase inducing a phase transi-tion from closed pore to open pore, illustrated in Figure 26. Interestingly, the temperature of this phasetransition is dependent on the type of linker functionalization. The material based on the ligand BME-bdc (2,5-bis(2-methoxyethoxy)benzenedicarboxylate) shows a transition temperature of
493 K and whenexchanged by a larger ligand DB-bdc (2,5-dibutoxybenzenedicarboxylate), a lower transition temperatureof
398 K is observed [266]. Materials which contain mixtures of BME-bdc and DB-bdc show transitiontemperatures between
493 K and
398 K , demonstrating this transition is tunable. Further studies investi-gated the influence of chain length, polarity, and grade of saturation of functionalization on the resultingflexibility of fu-MOFs, demonstrating adjustable flexibility of this family of materials [267]. Recently, R.A. Fischer and coworkers also reported different CO induced breathing mechanisms demonstrated byfu-MOF materials constructed with different paddlewheel composition [268].30 igure 26: Temperature dependence of crystallographic specific volume (Vspec) of fu-MOFs constructed from BME-bdc (a)and DB-bdc (b). Reproduced from [266], with permission. O (OH) SBUs [269]. This material wassynthesized using rational supermolecular building block approach to produce a one-dimensional porearchitecture. DUT-98 shows reversible structural transformations upon gas adsorption of specific gasesand vapors (N , CO , n -butane and alcohols) at characteristic pressures. This was investigated using insitu PXRD experiments to elucidate the multiple steps in the adsorption isotherm and hysteretic behaviorupon desorption.In 2016, the S. Kaskel group reported the behavior of the flexible MOF DUT-49, a material whichshows an unexpected negative gas adsorption (NGA) phenomenon [210], Figure 27. As DUT-49 fills withgas, it suddenly contracts and releases a significant portion of gas. This negative gas adsorption process isextremely unique and was not observed in porous materials previously. Using a combination of advancedin situ experiments this processes was discovered and explained with respect to the disparate relativeenthalpies of adsorption of the two phases. Figure 27: The experimental isotherm for DUT-49 for methane adsorption at
111 K and the type 1 isotherm, calculated bysimulation, for the static DUT-49 structure (blue and red, respectively). Adapted from [270], with permission from Elsevier.
Finally, while this review has focused on the research of experimental groups the work of Germansimulation groups has provided valuable insight into flexible MOF materials. The T. Heine group has usedBorn–Oppenheimer molecular dynamics simulations to investigate the low-frequency lattice vibrationalmodes of DUT-8(Ni) [271] and illustrated that they correspond to the oscillation of the breathing mode.This group has also reported an extension to the universal forcefield [272] for treatment of MOFs structuresthat subsequently has been used to describe the flexibility present in these systems [273]. The group ofR. Schmid at Ruhr-University Bochum has also reported a forcefield constructed from first principles todescribe the dynamics of MOF structures (MOF-FF) [274]. This forcefield has played an important rolein elucidating the microscopic mechanism of NGA in DUT-49 [270].
In industrial chemical processes membranes are frequently used, where membranes refer to thin layersof a material with a resistance and selectivity to the passage of different substances thus membranesenable the separation of substances [275] Important examples for the application of membranes includethe purification of gases, seawater desalination, the purification of sewage and fuel vapor recovery [276].Membrane separation processes are usually advantageous to conventional separation methods, such asdistillation, crystallization, adsorption or absorption, as membrane separation processes incur lower costs,lower energy consumption and have simpler process conditions [277]. Notably, energy savings of up to 50%could be reached with membrane separation processes over other separation technologies [278, 279]. Whilenot detailed here MOF technologies have also demonstrated potential for the separation of industriallyrelevant liquid mixtures, such as hydrocarbon mixtures [280].Organic polymers are often the material of choice for commercial membranes since they are inex-pensive, easy to manufacture, flexible and sufficiently mechanically stable. Yet, a major disadvantage ofpolymeric membranes is their inverse relation, that is, a trade-off between gas permeation and selectiv-ity. Permeability and selectivity (permselectivity) are the most important membrane parameters that32etermine the economics of separation processes [281]. Permeability ( P ) is the gas flow rate multipliedby the thickness of the membrane, divided by the area and by the pressure difference across the mem-brane. Selectivity S is the separation factor for gas mixtures and is calculated from mole fractions of thecomponents produced in the permeate, divided by corresponding mole fractions in the feed [282]. Ideallya membrane should have both a high permeability and a high selectivity. A low permeability necessitatesa larger membrane area or more membrane modules for the given gas volume and time. A low selectivityrequires more steps in the separation with more complex operations, typically leading to higher costs.Large volume separations, such as O /N for oxy-combustion or CO /CH for natural gas treatmentsrequire highly permeable membranes.High permeability is often the aim in membrane development as optimum membrane selectivity de-pends on the process and the operating conditions, especially the pressure ratio. The electricity cost forthe compressor to build up the pressure is usually the largest operating expense so an affordably lowpressure ratio determines the preferred membrane selectivity [283]. Calculations of the optimal designfor a membrane post-combustion CO capture process showed a trade-off between membrane area andpermeate CO concentration for a given pressure ratio, which results in a narrow optimum selectivityrange [284].For polymer membranes high permeability goes together with low selectivity and vice versa. Thisinverse relation between permeation and selectivity is illustrated by the Robeson upper bound displayedin Figure 28 [281, 285]. A possible way to overcome this trade-off is by using new porous materials orembedding porous fillers as additives into conventional polymer matrices to give mixed-matrix membranes(MMMs) [286, 287, 288].Zeolitic imidazolate framework (ZIF) based membranes have been studied extensively studied byJ. Caro and coworkers from Leibniz University Hannover [289]. These MOF based membranes havebeen prepared by growing ZIF nanocrystals on a suitable porous support. The pore window of the ZIFstructure thus selectively allows suitably sized gases to pass through, providing molecular sieving. Ithas been established that proper orientation of ZIF-8 nanocrystals [290] on the support plays a crucialrole in attaining the best separation performance. Further improvement in separation efficiency has beenachieved when the support has been modified with layer double hydroxides (LDH) such as ZnAl – CO [291] or ZnAl – NO [292]. LDHs provide more sites for heterogeneous nucleation for ZIF-8, resulting in amore intergrown membrane. Additionally, Zn in the LDH undergoes “partial self-conversion” to form athin continuous layer of ZIF-8 membrane. Such laminated ZIF-8 membranes have shown high selectivitytowards H with a separation factor of 54.1 over CH [292]. Further studies explored the selectivity forH over CO and ZIF-7 was shown to perform better than ZIF-8 [293, 294]. The window size of ZIF-8( . Å) allows few CO molecules (kinetic diameter of . Å) to pass. However, ZIF-7 has a smaller poresize ( . Å) which selectively allows H diffusion over CO , providing an efficient separation (separationfactor of 6.5 for ZIF-7 increased from 4.7 for ZIF-8, which follows Knudsen separation behavior).A second approach using the favorable interaction between the NH groups and CO has also beenshown to be useful in reducing CO permeance and thereby enhancing the selectivity for H in themixture. Post functionalization of Mg-MOF-74 with NH functionality thus increases the performanceof the membrane, demonstrated by an increase in room temperature H /CO selectivity from 10.5 to 28[295].ZIF-22, with similar pore diameter to ZIF-7, also demonstrates a similar enhanced selectivity (separa-tion factor of 7.2) for H /CO [296]. Here, ZIF-22 nanocrystals were anchored to the porous TiO supportthrough chemical functionalization with 3-aminopropyltriethoxysilane (APTES). The 3-aminopropylsilylgroups from APTES readily coordinate to the available Zn sites creating a covalent attachment withthe support. This covalent attachment was demonstrated between the amine group of APTES func-tionality of the modified surface and – CHO group of the ZIF linker from ZIF-90 [297]. The resultingimine condensation thus developed a strong and efficient interaction of the nanocrystal with the aluminasupport, providing high thermal and hydrothermal stability to the membrane. However, when APTESfunctionalization is employed as post-synthetic modification to the pristine membrane of ZIF-90 [298],the pore window was narrowed and the defects in the intercrystallite spaces are repaired to produce aless defective membrane. These changes are observed for the selectivity of H over other tested gases,such as CO , CH , C H and C H .Additionally, the separation of hydrocarbons, such as ethene/ethane has also been achieved usingZIF-8 based membranes [299]. Recent reports from J. Caro and coworkers has demonstrated the effectof photoisomerization of azobenzene as guests within a membrane based on UiO-67 [300].MMMs are comprised of micro- or nano-sized additives or filler components dispersed within a con-tinuous organic polymer phase. Various porous or non-porous inorganic additives have been tested as33 igure 28: Schematic representation of the inverse relation between permeability and selectivity for organic polymer mem-branes with the 1991 and 2008 Robeson upper bounds[281, 285]. *The distance or position or the commercially interestingarea relative to upper bound depends on the separation problem. filler materials [282, 301, 302, 303, 304, 305]. MMMs attempt to combine the excellent flexibility andprocessability of polymers and the high gas separation performance of other porous materials, such aszeolites [306] or MOFs [307], in order to overcome the Robeson upper bound [285]. The addition offillers to polymer materials can also address other challenges of commercially used polymer materials,like membrane plasticization, low contaminant resistance or biofouling.MOFs are widely studied as fillers in mixed matrix membranes and compared for their impact onthe separation performance of pure polymer materials and such MOF-MMMs have been the subject ofrecent reviews [282, 308, 309, 310, 311]. Most reported studies on the preparation and characterization ofMOF-based MMMs are still often fundamental and work is still ongoing to develop an understanding ofthe separation mechanisms, the role of the MOFs and the effect of the polymer/MOF particle interface[312]. This knowledge is needed to develop MOF–MMMs, which are suitable for industrial applications.In Germany, the C. Janiak group found water-stable MIL-101 microcrystals adhere well to polysulfone(PSF) and yield a very robust mixed-matrix MIL-101-PSF membranes (Figure 29) [313]. The PSFpolymer does not appear impressive in terms of its separation performance. However, PSF is employedon the scale of several thousand tons per year for membranes for dialysis and water treatment andis one of the most important glassy polymers used in industrial membrane gas separation [314, 315].PSF is characterized by high temperature stability and good mechanical properties. Furthermore, it isindustrially available with high molecular weights and consistent specifications so that deviations, whichare observed from different polymer batches or impurities can therefore be eliminated. Figure 29: Mesoporous cage in MIL-101(Cr) as part of a MTN zeolite network with a pore diameter of Å, the polysulfonerepeating unit and MIL-101-PSF membrane. Adapted from [313] with permission from The Royal Society of Chemistry.
For O /N separation the MIL-101-PSF membrane exhibited a remarkable four-fold increase in thepermeability of O to required values of above 6 barrer for MIL-101 loadings of 24 wt%, thereby, retainingthe PSF selectivity for O over N (Figure 30). Also, the presence of some agglomeration of the MIL-101particles in the polymer at 24 wt% loading does not alter these properties. The essentially unchangedO /N selectivity is consistent with literature results where permeability enhancements for MOF–MMMs34ould be found, but selectivity enhancements were less pronounced. High loading of up to 24 wt% of MIL-101 in PSF could be achieved with the MIL-101 particles showing very good adhesion with polysulfonein the MMM and excellent long term stability. The comparison of these results with a compilation fromother MOF-containing MMMs, previously reported, show that the MIL-101-PSF membranes exhibitedmuch higher O permeabilities ( > barrer) than any other MOF-based mixed-matrix membranes [313]. Figure 30: O /N permeability and separation performance of pure polysulfone (PSF) and MIL-101-PSF membranes withdifferent MIL-101 wt% loadings. Reproduced from [316]. Further single gas (N , CO and CH ) permeation tests with MIL-101-PSF MMMs showed a significantincrease of gas permeabilities of the MMM without any loss in selectivity. Isochorus, single gas N ,CO and CH , permeation experiments of MIL-101-PSF membranes show an almost linear increase inpermeability for the fastest studied gas CO and at the most slight increases in ideal CO /N andCO /CH selectivities for increasing MIL-101 content [316]. Figure 31: CO /N (left) and CO /CH (right) permeability and separation performance of pure PSF and MIL-101/PSFmembranes with different MIL wt% loadings. Reproduced from [316]. Notably, the CO /N separation performance of MIL-101/PSF with 19 wt% filler was higher thanthose of most other known MOF–MMMs [316]. The Maxwell model was able to reproduce approximateeffective permeabilities and the ideal selectivity for the O /N and CO /N separation performance ofthe MIL-101(Cr)/PSF membranes with different filler loadings, under the assumption of a higher gaspermeability P in the dispersed MOF phase ( P d ) than in the continuous polymer phase ( P c ), thatis, P d >> P c . Positron annihilation lifetime spectroscopy (PALS) indicated that the increased gaspermeability is not due to free volume changes in the PSF polymer but due to the added large freevolume inside the MIL-101 filler particles [316].A number of other MOFs have also been investigated for use in MMMs. As described previously, ZIF-8has a pore aperture with a diameter of . Å allowing for preferential adsorption of small molecules [73].Combination of ZIF-8 and MIL-101(Cr) in a 1:1 ratio in PSF yielded a MMM with higher selectivity for35O in mixed-gas CO /CH permeation tests for an MMM with combined 16 wt% MIL-101(Cr)/ZIF-8(in 1:1 mass ratio, that is 8+8wt%) compared to analogous 16 wt% single-filler MMMs (either MIL-101(Cr) or ZIF-8) (Figure 9). The CO /CH selectivity also increased significantly from 23 for pure PSFto 40 for the 16 wt% combined MIL-101(Cr)/ZIF-8 membrane [317]. SEM images reveals the breakingof polymer chain packing and linking due to the presence of fillers leading to an increase in polymermatrix free volume which, together with the large porosity of MIL-101(Cr), can explain the permeabilityimprovement in agreement with previous publications related to ZIF-8 containing MMMs [318]. Figure 32: CO /CH permeability (given as bottom bars) and mixed-gas separation performance (as top lines) of purePSF (0 wt% MOF loading), MIL-101(Cr)@PSF (green), ZIF-8@PSF (red) and combined or mixed-MOF MIL-101(Cr)/ZIF-8@PSF MMMs (blue), for different MOF loadings. The loading of the combined MIL-101(Cr)/ZIF-8@PSF MMMs comesthrough MIL-101/ZIF-8 in a 1:1 ratio. Good quality membranes with 35 wt% MOF loading were only possible in the caseof the filler combination with 17.5 wt% MIL-101 (Cr) + 17.5 wt% ZIF-8. Reproduced from [317], with permission. Pure MOF-5 membranes and MOF-5 MMMs with different polymer materials have shown good sep-aration properties for dry CO /CH gas mixtures [319, 320]. However, the low water stability of MOF-5 is an insurmountable problem for possible real-world applications. The decomposed material is nolonger porous towards N adsorption, which is seen as a fundamental prerequisite for MOF fillers inMMMs in order to have an effect on the separation process. An isostructural MOF-5 analogue, 3D-[Co ( µ -O)(Me pzba) ] with the bi- or mixed-functional pyrazolate-carboxylate ligand, allowed for thepreparation of moisture-tolerant [Co ( µ -O)(Me pzba) ]/Matrimid mixed-matrix membranes by the C.Janiak group [321]. Synthesis of 3D-[Co ( µ -O)(Me pzba) ] by microwave-assisted heating gave smallparticle size and less aggregation compared to the conventional solvothermal synthesis. The control ofthe particle size and shape of MOF-fillers is crucial for the preparation of MOF MMMs with improvedseparation properties [322]. The small MOF particles were uniformly embedded in the polymer withoutaggregation and with good MOF-polymer compatibility as shown through the SEM images in combina-tion with EDX cobalt element mapping (Figure 33). The good MOF-polymer compatibility was furtheraffirmed by the improved CO /CH separation performance of the MMMs over neat Matrimid mem-branes. This improvement corresponds to more than 46 % in CO /CH selectivity for 24 wt% fillerloading and 3 bar transmembrane pressure, relative to the pure Matrimid membrane, together with anenhanced permeability of 49 % for CO .More investigations and considerable research effort is required for better understanding of the per-meation mechanisms and selectivity properties, the roles of filler loading, geometry, pore size and thenature of the molecular interactions on preferential adsorption when preparing single-MOF-MMMs andmixed-MOF-MMMs. It is evident that the combinations of different types of fillers may lead to substan-tial improvements in gas separation using MMMs. The efforts of German researchers and other talentedresearchers from the rest of the world have produced a number of membrane materials that provideexceptional separation performance. Tunable emission property of suitable metal–organic frameworks has made them an attractive candi-date for selective sensing of hazardous chemicals, such as solvents, ions and biomolecules. Contributionfor this application comprises of the development of suitable MOFs and the processes for real time ap-plication. [323] As discussed in several literature reports, the emission property of a MOF arises from 536 igure 33: SEM images of cross-section of Matrimid with different loadings of [Co ( µ -O)(Me pzba) ] as filler (a) 8 wt%,(b) 24 wt%. EDX-mapping of cobalt (blue) in MMM cross-sections (c) 8 wt%, (d) 24 wt% [Co ( µ -O)(Me pzba) ]). Somedetection of Co is found outside of the membrane cross section due to reflection of the electron beam on the sample holder.The bottom of the images corresponds to the bottom of the membrane when casted. Adapted with permission from [321].Copyright 2017 American Chemical Society. major pathways: a) ligand based emission, b) metal based emission, c) metal to ligand charge transfer(MLCT), d) ligand to metal charge transfer (LMCT) and e) metal to metal charge transfer (MMCT)[324]. Of these methods, the ligand and metal based emissive MOFs are most straightforward and can beeffectively achieved by choosing suitable components. Thus, the change in the emission property directlyreflects their change from inside the framework, in the presence of an analyte. A comprehensive reviewon the application of MOFs for sensing and a global overview has been previously addressed [325].Using the emissive property of a Zn based MOF (DUT-25), the S. Kaskel group have demonstrated asuccessful sensor to detect different solvents [326]. The MOF has nbo-b topology, cross linked with secondlinker, and contains a [Zn O(CO ) ] SBUs. DUT-25 shows intense emission and the emission intensity ofthe MOF is observed to decrease in the presence polar solvents such as MeOH and acetonitrile. Impor-tantly, a sequential bathochromic shift of the emission maxima corresponds to the presence of solvents inincreasing polarity. This distinct change in emission maxima and intensity adds to the appropriateness ofDUT-25 for optical sensing, as the material is transparent in nature. This transparency allowed for visualmonitoring as key sensor feature and subsequently a responsive molecule (Nile Blue, a solvatochromicdye) was loaded into the framework. The dye loaded MOF showed a prominent visual change underinfluence of different solvents with prompt action. The interaction of the pore encapsulated dye with lesspolar solvents such as n-heptane, diethyl ether, 1,4-dioxane, and acetone produces a light blue color. Thisblue color changed to turquoise to sense the presence of acetonitrile and light blue for MeOH. Moreover,identical color change has been shown on exposure of the respective vapor, demonstrating the usefulnessof the ordered pores of the structure. Apart from the pure solvatochromic property, the encapsulated dyehas been deprotonated on interaction with NMP, producing a clear and distinct color change to purple.On protonation from protic solvent, such as MeOH, it shows a slow and hypsochromic transformation toa green color.Similar emission enhancement to detect different solvents was reported by K. Müller-Buschbaum etal. [327]. This report demonstrated the applicability of a Ce based MOF for rapid detection of waterand oxygen through “turn-off” of emission. While exposure to CH CN causes a “turn-on” emissiondemonstrating dual performance for multiple analyte detection.On a further illustration, chemical stability of Zr based MOFs has been applied to develop a newsolvatochromic MOF (DUT-122) [37] from 9-fluorenone-2,7-dicarboxylic acid, based on the well-knownsolvatochromic behavior of fluorene derivatives. Formation of the porous framework leads to blue shiftingof the emission maxima of the linker from inside the framework by
21 nm . Expected solvatochromic37 igure 34: (a) Structural representation of DUT-25 MOF and (b) its emission spectra under influence of different solvents.c) Solvatochromic color change of Nile-Blue loaded DUT-25. Adapted from [326], with permission. behavior was promptly observed for the MOF with a variation of emission maxima from
477 nm forTHF to
521 nm for MeOH. Here the incapsulation of the solvent into the MOF pores and the effectiveinteraction plays a crucial role in the resulting color of the MOF. This shows the effectiveness of solventinteraction with pore walls leading to a shift of the emission maxima upon a change of solvent polarity.Non-radiative transitions occur when OH groups from the analyte induce vibrational coupling, accountingfor a red shift in such cases. The inherent stability of the Zr cluster makes DUT-122 suitable for detectingthe presence of water. DUT-122 shows a sharp change in color (darkening of the yellow color) as wellas emission property (red shift of emission maxima to
536 nm ) in water and the kinetic analysis of theinteraction revealed a fast response profile.The challenge for detection of trace amount of water in organic solvent has been recently addressedby K. Müller-Buschbaum and coworkers. They have reported the development of a composite materialfor facile detection of water from organic solvents, using a luminescent MOF as the active sensor. Thechosen lanthanide MOF [328] shows intense fluorescence because of the lanthanide SBU, sensitized bythe bipyridine containing linkers. The luminescent MOF was then applied to superparamagnetic mi-croparticles of Fe O /SiO , forming a core/shell structure having the MOF layer as the active shell. Thiscomposite provides the capability to interact with applied magnetic field, a prerequisite for easy andefficient separation from any medium. Notably, the oxophilic nature of the lanthanides makes the MOFsusceptible towards hydrolysis and the hydrolyzed product was found to lose the luminescence property.Thus, the loss in emission property has a direct correlation with the subsequent water concentrationresponsible for the hydrolysis. A detectable lowering of emission intensity was observed by the presenceof a mere 0.1% water in organic solvents like toluene and hexane. The emission continued to quenchefficiently in proportion with the water concentration, demonstrating possible quantitative estimation.Theoretical detection limits of 0.01% ( µ g ) and 0.03% ( µ g ) water in toluene and hexane, respectively,was achieved.In further work, a red emitting Eu based MOF (Eu-bipy or Eu-BDC) was added in addition to agreen emitting Tb-bipy MOF, forming a tri-component microparticle [329]. The complex microparticlescontain a shell comprising of two differently emissive MOFs, and a superparamagnetic core. Yellowemission of these composites is a sum of the red and green emission from individual MOFs and is suitableas a ratiometric sensor. Loss of emission from hydrolysis of the MOFs, the key working principle for thesensors, shows different emission at different concentration based on the difference in hydrolysis kinetics38f the individual luminescent MOFs. A gradual shift of the emission from yellow towards red region ofthe spectra thus indicates the water content in hexane from anhydrous to contaminated. Furthermore, adetection limit of 0.03% (
20 mg ) water in anhydrous hexane was observed, comparable to the well-knownKarl Fischer titration.
Figure 35: Scheme for generation of microparticle with dual emission, featuring a luminescent and superparamagneticcore/shell. Reproduced from [329] with permission of The Royal Society of Chemistry.
Detection of fluoride in water [330] has been demonstrated by the group of T. Bein using a hybridcomposite of the metal-organic framework NH -MIL-101(Al) and fluorescein. The reporter moiety con-taining fluorescein as fluorophore remains covalently attached to the linker and doesn’t show any emissionwhen inserted in the MOF. However, on treatment with water, the MOF structure decomposes, leading tothe release of the reporter molecules with a possibility for complex formation with the incoming analyte.On selective complexation with fluoride ions in water, it shows a “turn-on” of the emission. The abilityto selectively detect fluoride from a mixture of other anions and with a sensitivity of 15 ppb makes thissensor remarkable.Fabrication of thin films, a compulsory prerequisite for device based application has been attemptedby the R. A. Fischer group [331]. They demonstrated the development of quartz crystal microbalance(QCM) substrates for the application of sensing many volatile organic compounds, such as benzene,toluene, ethylbenzene, isomers of xylene and hexane and alcohols. Growth of ZIF thin films on the QCMsubstrate allowed for analytes to pass through the ZIF pores at different rates, based on the pore sizeand surface functionality. Selectivity of ZIFs towards different analytes on such devices has potential tobe beneficial in industrial purposes including biofuel recovery and isomer separation at near future. Owing to the unique property of chemical tunability from inside a defined framework MOFs are con-sidered as promising candidates for catalysis. Being heterogenous phase, MOFs also bear advantages, interms of recyclability and ease of catalyst removal, over conventional homogenous catalysts. These highlyattractive features of MOFs led to many German chemists to investigate their capability in catalyzingnumerous important reactions. For the ease of discussion we classify the MOF based catalysts into 2 ma-jor categories, based on the chemical form of the active catalyst: MOFs having metal center as catalyticactive site and composite materials using the MOF as support and/or active form of catalyst.The metal ions present in the structure often act as structural nodes, providing structural integrityand rigidity to the framework. However, based on the coordination environment, the metal ion containingSBUs have high potential to serve as catalytically active centers. A chromium(III) cluster, the buildingunit for MIL-101, has proven highly effective for the cyanosilylation reaction of aldehydes [332]. The roleof unsaturated sites for catalysis has been studied by N. Stock and coworkers [333], using the Co-basedmicroporous framework (Co-BTT), catalyzing ring opening reaction of styrene oxide along with oxidationof cycloalkanes and benzyl compounds. Similar strategies incorporating a Rh paddlewheel during MOFconstruction has proven valid in achieving good performance for hydrogenation reactions; achieving highconversion of styrene into ethylbenzene without any side products [334].Taking advantage from the well known redox active property of cerium oxide, the N. Stock groupalso has shown the catalysis property of the UiO-66 architecture, when constructed using Ce node. Thethermal and chemical stability of the MOF allowed for catalytic aerobic oxidation of benzyl alcohol39hrough a redox pathway [45]. They also demonstrated the effect of – NH groups on the ligand using anAl-based MOF towards Knoevenagel condensation reaction, an important reaction for industrial processes[335]. Likewise decoration of the organic linker with – SO H functionalities led to an increase in theacidic character. The ordered presence of functional groups in the MOF framework made it capable todehydrating ethanol vapor towards formation of ethylene with an enhancement from 7% conversion fornon-functionalized linker containing MOF up to 91% for the sulfonate functionalized MOF [336]. Solvent-free hydroxymethylation has been shown to be catalyzed using a Bi based MOF [337]. The acidity fromthe MOF structure is found to prevent any consecutive condensations and polymerization, showing theadvantage of the designed structure of MOFs.Another important strategy to improve the metal center based catalytic activity of MOFs has beendeveloped by R. A. Fischer and coworkers through selective defect formation [227]. Formation of defectsinside the framework causes more sites to be exposed and available as an active site. This improves thealready active Ru based MOFs for catalyzing the ethylene dimerization reaction and Paal–Knorr pyrrolesynthesis, a reaction of acute industrial importance.Liquid phase oxidation of cyclohexene using tert-butyl hydroperoxide as oxidant has been thoroughlystudied by D. Volkmer and coworkers. Co(II) complexes are known to be active for such catalysis underhomogenous conditions thus incorporation of catalytically active Co(II) sites, inside a MOF, demonstratesa good turnover number (TON) with recyclability as achieved using [Co(II)(BPB)] · as a reducing agent proceeds at an extremely fast rate when the Pd contentinside the framework is significantly higher. This highlights a novel method to overload Pd for providingan improved catalytic activity. Microporous materials with high water stability, high water uptake capacity at low but not toolow relative humidity and with tunable hydrophilicity, such as MOFs, are currently in the focus forreversible cycling water adsorption in order to achieve low temperature heat transformation applications[345, 346, 347]. Heat transformations in thermally driven adsorption chillers (TDCs) or adsorption heatpumps (AHPs) are an alternative to traditional air conditioners or heat pumps operating on electricityor fossil fuels. Thermally driven adsorption chillers and heat pumps feature a two-step process, which isdepicted in Figure 36.By using solar or waste heat as the driving energy, TDCs or AHPs can significantly help to minimizeprimary energy consumption and greenhouse gas emissions generated by industrial or domestic heating40 igure 36: Principle of adsorption chilling or adsorption heat pump. Working cycle: A working fluid (typically H O) isevaporated at low pressure by application of evaporation heat Q evap (useful cold), and adsorbed at a microporous material,releasing adsorption heat Q ads . Regeneration cycle: When the adsorbent is saturated, driving heat Q des is applied fordesorption of the working fluid. The vapor then condenses in a cooler, and condensation heat Q cond is released. [345] -Published by The Royal Society of Chemistry (RSC) on behalf of the Centre National de la Recherche Scientifique (CNRS)and the RSC. and cooling processes, especially when run on electricity. TDCs and AHPs are based on the evaporationand consecutive adsorption of coolant liquids, preferably water, under specific conditions. TDCs andAHPs are established technologies and are commercially available with small scale capacities of 5–
25 kW produced by InvenSor (Germany), Fahrenheit (ex SorTech, Germany), Mitsubishi Plastics (Japan), Solab-Cool (Netherlands) and Jiangsu Huineng (China) and large scale capacities of 50– manufacturedby Power Partners (USA), GBU mbH (Germany), HIJC (ex-Nishiyodo, USA) and Mayekawa (Japan)[348]. The ranges of application of TDCs and AHPs, as well as their efficiencies, power densities and totalcosts, are substantially influenced by the microporosity and hydrophilicity of the employed adsorptionmaterials. At present TDCs and AHPs use either silica gel or zeolites. Here, we briefly summarize cur-rent investigations by Germany-based researchers for development and possibilities of MOFs comparedto classical materials.Several classes of materials are potentially promising for adsorption heat transformation applications[349]. These materials include metal aluminophosphates (AlPOs, SAPOs, MeAPOs), ordered poroussolids, porous carbons and various composites (SWSs, AlPO-Al foil). With their large water uptakes,MOFs surpass those materials (Figure 37), at the same time, the variability of the MOF building blocks(metal node and organic linker) allows for tuning the microporosity and hydrophobicity/hydrophilicity,depending on the specific application.
Figure 37: Comparison of typical water uptake capacities as loading lifts (in g water/g dry sorption mate-rial) within an adsorption/desorption cycle for traditional porous materials and MOFs (SAPO/AlPO = silica-aluminophosphate/aluminophosphate). Reproduced from [346], with permission.
Water stability is, however, a problem for MOFs. Many are not stable at all and decompose uponprolonged contact with water even at room temperature [350, 351]. At present MIL-101, CAU-10-H, Al-fum and UiO-66 appear to be the most water stable MOFs [351]. That is why a considerable amount oftesting of MOFs for water adsorption for heat transformation is for inital stability investigation. It shouldbe noted, water is the obvious fluid of choice for thermally driven chillers and adsorption heat pumps,however, alcohols, such as ethanol, could be an alternative in the context of the hydrolytic instability of41OFs [345, 352].The C. Janiak group began testing MOFs for heat-transformation applications in 2009. This groupinitially reported water cycling of the mixed-ligand MOF [Ni ( µ -btc) ( µ -btre) ( µ -H O) ] (btre = 1,2-bis(1,2,4-triazol-4-yl)ethane), also named ISE-1. This MOF was synthesized from water with an initialwater content of ca. 30 wt% [353]. This water content can be reversibly desorbed and adsorbed overseveral cycles (Figure 38) [354]. Figure 38: Top: Space-filling presentation of the mixed-ligand MOF ISE-1 with 52% water-filled volume. The crystal waterin the channels is not shown. [353] - Reproduced by permission of The Royal Society of Chemistry. Bottom: Water loadinglift over 10 different cycles with four of them shown for stability test. Reprinted (adapted) with permission from [354].Copyright 2009 American Chemical Society.
The adsorption of water vapor in the highly porous MIL-101(Cr) of over − (between ◦ C and ◦ C under a water vapor pressure of . ) together with the stability over several cycles makes MIL-101(Cr) the record holder for water uptake [355]. MIL-101(Cr) is synthesized in quite harsh acidic aqueousreaction conditions ( ◦ C in water) [9]. Thus, fundamental water stability of this material can beexpected and was verified over 40 water adsorption cycles. Unfortunately, MIL-101(Cr) is too hydrophobicas the main uptake of the otherwise advantageous s-shaped isotherm occurs only at p/p ≈ . [355].There are many MOFs that demonstrate improved water adsorption compared to ISE-1. For example,the water uptake of .
75 g g − for MIL-100(Fe) and MIL-100(Cr), at small relative pressures of p / p < . , with a steep s-shaped isotherm and comparatively small hysteresis, including very good cyclestability, make these MOFs ideal candidates for cycling water adsorption [356, 357, 358]. The isostructuralMIL-100(Al) features a similar isotherm, albeit with a smaller loading lift of slightly less than . − [356]. Notably, MIL-101(Cr) can be functionalized with amino, nitro or other groups on the benzene-1,4-dicarboxylate ligand through time-controlled postsynthetic modification of the parent material [359].Hydrophilic nitro ( – NO ) or amino ( – NH ) functionalities were introduced into MIL-101(Cr) in orderto achieve increased water loading at lower p / p values. Fully and partially (p) aminated MIL-101(Cr)-(p)NH showed an unchanged water uptake capacity with respect to parent MIL-101(Cr) (about − )and very high water stability over 40 adsorption-desorption cycles [360].Further, the feasibility of the metal-organic frameworks UiO-66, UiO-67, H N-UiO-66 and H N-MIL-4225(Ti) as adsorption materials in heat transformations were investigated, since at the start of the projectzirconium MOFs of the UiO-66 family were regarded as rather water stable. The amino-modified com-pounds H N-UiO-66 and H N-MIL-125 feature indeed a high heat of adsorption (89.5 and . − ,respectively) and a very promising H O adsorption isotherm due to their enhanced hydrophilicity (Fig-ure 39). For H N-MIL-125 the very steep rise of the H O adsorption isotherm in the . < p/p < . region is especially beneficial for the intended heat pump application [361]. Yet, H N-UiO-66 appears oflimited stability during a multi-cycle hydrothermal stress test while the water uptake of H N-MIL-125remains unchanged during short, non-equilibrium cycles. H N-MIL-125 appears to show hydrothermalstability comparable to MIL-100(Fe) or MIL-101(Cr) [361].
Figure 39: Water adsorption/desorption isotherms of UiO-66, H N-UiO-66, UiO-67 and H N-MIL-125, acquired at T = ◦ C . Adsorption: filled symbols, desorption: empty symbols. [361] - Reproduced by permission of The Royal Society ofChemistry. The distinctive water adsorption properties of microporous aluminum fumarate (Al-fumarate, BasoliteA520) with an s-shaped isotherm, a narrow hysteresis and a loading > . − at relative pressures as lowas p/p = 0 . under realistic working conditions represented a significant advancement of MOF-based ad-sorption heat transformation processes [166]. Furthermore, the C. Janiak group demonstrated aluminumfumarate shows favorable and unprecedented cyclic hydrothermal stability [166]. For the application ofheat transformation, unhindered heat and mass transfer are crucial for fast adsorption/desorption cyclesand high power density. A µ m thick, polycrystalline, thermally well coupled and highly accessiblecoating of microporous aluminum fumarate was deposited onto an aluminum metal substrate via thethermal gradient approach. This was found to be stable for the first 4500 adsorption/desorption cycleswith water vapor, as judged by unchanged crystallinity by PXRD [166]. The maximum water exchangeof .
35 g g − of Al-fumarate is lower than for other MOFs, however, it can be fully utilized under realistic,isobaric working conditions due to the hydrophilicity of Al-fumarate. The desired, steep s-shape of theisotherm in a relative pressure band of . < p/p < . had not been observed for other water-stableMOFs before. In this context, Al-fumarate compares well with hydrophilic, zeolitic adsorbents such asAlPO-18 or AlPO-4 [362]. Importantly, Al-fumarate can be produced relatively cost-efficiently via a sim-ple precipitation reaction in a continuous reactor or even by extrusion. This is a strong advantage andin contrast to, for example, the template-based AlPO-18 synthesis, or various other hydro/solvothermalsynthesis routes required for other MOFs. Al-fumarate also does not contain heavy metals, fluorine orcritical organic compounds.Another aluminium based MOF, aluminium isophthalate MOF CAU-10-H, reported by N. Stock andcoworkers [157] exhibits promising water adsorption characteristics for the application in heat-exchangeprocesses with water as working fluid (Figure 40)[157, 363, 364]. CAU-10-H coating with Silikophen wasevaluated for its long-term stability under closed-cycle conditions for 10,000 water adsorption/desorptioncycles, which are approaching typical expected lifetimes for adsorptive heat transformation applications[364]. No degradation of the adsorption capacity could be observed which makes CAU-10-H the moststable MOF under these humid cycling conditions reported. While the water uptake of .
26 g g − forthe coated sample is lower compared to other stable MOFs, the successful coating procedure and highstability up to 10,000 cycles under working conditions make CAU-10-H the current top performing MOFfor heat pump applications [364]. Since CAU-10-H is the second MOF demonstrating such long-termstability and since the first MOF with such property (aluminum fumarate) is also based on Al , we43xpect that Al-based MOFs are the most promising MOF adsorbents for application in water based heatpumps [364]. Figure 40: The three-dimensional centrosymmetric tetragonal framework structure of CAU-10-H with its square shapedone-dimensional channels. [364] - Published by The Royal Society of Chemistry.
It is evident that shaping MOFs, which are normally obtained as powders or microcrystals, is one in-dispensable factor for potential applications of MOFs [345]. In order to make use of MOFs for cyclic wateradsorption applications, shaping into monoliths, granules or coatings could be envisioned. Thereby, theessential porosity of the MOF should not be lost and the amount of MOF in the shaped formed should beas high as possible. MIL 101(Cr) was successfully embedded into a macroporous and monolithic oil-water(o/w) high internal phase emulsion (HIPE) foam, based on crosslinked poly(2 hydroxyethyl methacrylate)(HEMA) [365] or cross-linked poly(N-isopropyl acrylamide) (NIPAM) [366]. These hierarchical and me-chanically stable monolithic composite materials with up to 59 wt% of MIL-101(Cr) for the HEMA-HIPEand a realistic 71 wt% for the NIPAM-HIPE show higher methanol and water vapor uptake capacitiescompared to the pure HIPE. Pre-polymerization of the HIPE emulsions was shown to be an indispensablefactor for synthesizing highly porous composites where the micro- and mesopores of MIL 101(Cr) remainpartially unblocked. The maximum vapor exchange of the MIL-101@HEMA-HIPE composite and theNIPAM-HIPE composite are certainly lower than in pure MIL-101(Cr) but most can be utilized underrealistic working conditions up to p/p = 0 . [365, 366].Monolithic MOF composites were also synthesized using micro-to-mesoporous MIL-100(Fe,Cr) andMIL 101(Cr) with a pre-polymerized mesoporous resorcinol-formaldehyde based xerogel as binding agent[367]. The monolithic bodies could be loaded with up to 77 wt% of powdery MOF material with retentionof the MOF surface area and porosities (from N adsorption) by pre-polymerization of the xerogel solution.These MIL-101(Cr)@xerogel-H O composites matched the wt%-correlated BET values and water uptakeswithin experimental error. As an indication of the hierarchical nature the 77 wt% MIL-101(Cr)@xerogel-H O composite achieved .
79 g g − water uptake at p/p = 0 . while for bulk MIL-101(Cr) only .
57 g g − water uptake could be achieved [367].Recently, a step towards application was made by the group of S. K. Henninger at the FraunhoferInstitute for Solar Energy Systems by fabricating a functional, full-scale heat exchanger coated with
493 g of the microporous aluminium fumarate MOF (Basolite A520) using a polysiloxane-based binding agent(Figure 41) [368]. The function of the heat exchanger was evaluated to a gross cooling power of (at the beginning of the adsorption cycle) or, respectively an average cooling power of
690 W (up to a limitof 90% equilibrium loading in 7 minutes) under the working conditions of a realistic adsorption chiller of ◦ C – ◦ C – ◦ C (temperature level of heat source, heat rejection/condenser and evaporator). Withthe inherent multi-cycle stability of microporous aluminum fumarate and the excellent long-term stabilityof polysiloxane coatings, reported in the literature, these results clearly suggest that the technology hasthe potential for industrial application and can significantly advance adsorption-based chilling [368].44 igure 41: Heat-exchanger (a) before and (b) after coating with
493 g of the aluminum fumarate MOF and drying. (c)Gross cooling power P cool of the aluminum fumarate-coated heat exchange for operating conditions ◦ C – ◦ C – ◦ C (red line, lower part), calculated from the integral heat of evaporation Q int , evap (blue line, upper part). The green lineindicates P cool = for a half cycle time of t = 74 s. Adapted with permission from [368]. Copyright 2017 AmericanChemical Society. .7. Biomedical applications Another advanced application of porous MOFs includes biomedical applications which has been show-cased by S. Wuttke and coworkers. The porous nature with tunable pore environment has made MOFsattractive candidate for host-guest chemistry [369, 370]. The responsiveness of bare or modified frame-works helps to produce the desired control of guest encapsulation and their release. This triggered releaseprocess has proven useful in release of important biologically active cargo, towards successful applicationof MOFs as drug delivery vehicles.Nanoparticles of mesoporous MOFs, MIL-100(Fe) and MIL-101(Cr), posses significant porosity (3205and g − , respectively) and thus were chosen for developing a delivery vehicle system [371]. Ontreatment with dispersion of lipids (1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC), a lipid bilayer wasobserved to develop around the MOF nanoparticles. Thus, a system for dye loaded MOF nanoparticleswith lipid bilayer coating was easily achieved for loading into target cancer cells. The active dyes fromsuch nanoparticles were then shown to release from their core on triggering with suitable stimuli like anonionic surfactant (triton X-100). Application of such trigger causes an immediate release of the loadedcargo, ready for action inside the cancer cell. This approach serves as a backbone for later studies toinclude other dyes like fluorescein [372].Over the time, systematic development has been made with these model systems, including investi-gations of biocompatibility. Studies for such lipid bilayer coated MOF nanoparticles have been carriedout to investigate any potential health risk, using human endothelial cells and mouse lung cells as modelsystem [373]. As discussed, MOFs have invoked continuous interest for academic and industrial applications inthe last decade due to their intrinsic porous characteristics, unique structural diversities and tunablefunctionalities [252]. There is great demand for fabricating powdered porous materials as films becauseof the obvious advantages of surfaces and interfaces for use as smart films, chemical sensors, electronicdevices (Figure 42) [374, 375, 376]. In this prospect, MOF thin films were developed with the pioneeringwork of R. A. Fischer, C. Wöll and their coworkers in 2005. The conventional solvothermal method wasused to deposit patterned crystalline MOF-5 films on COOH/CF-terminated self-assembled monolayers(SAMs) on Au(111) surfaces [377].In 2007, MOF systems were extended to HKUST-1 on SAMs terminated with a different functionalgroup under solvothermal crystallization conditions by T. Bein and coworkers [378]. They also studiedthe water adsorption behavior of HKUST-1 films at different temperatures monitored by a quartz crystalmicrobalance (QCM) instrument [379], which offers an alternative strategy for researchers to study gasadsorption behavior. Also, as described previously, using the conventional solvothermal method J. Caroand coworkers further developed a seeded secondary growth method to obtain dense MOF membranes,such as ZIF-7, ZIF-8 and ZIF-90 membranes, with high multi-gas selectivity [293]. This work thereforeprovides an alternative way to fabricate solid support-MOF membrane, which is likely superior to gasseparation via molecular sieves.
Figure 42: Scheme of typical deposition techniques and applications that were developed in Germany in last decade. (930 ±
15) m cm − , which is smaller than twice the Kr BET value for the interpenetrated MOF-508a, (627 ±
15) m cm − . This implies that MOF-508a with a small Kr BET adsorption value behaves like aninterpenetrated structure, whereas the MOF-508a with a large Kr BET adsorption value behaves like anon-interpenetrated structure. This discovery shows an alternative way to improve the tunability of theporosity and surface areas as well as the flexibility of MOF films, facilitating the application of SURMOFsin gas adsorption, drug storage and release and guest molecules encapsulation [383].The advantages of the LPE technique enable fabricated MOF films to be employed for the applicationof selective adsorption, gas/vapor sensors, catalysis, luminescence sensors and photovoltaic device [381,384, 385]. For example, from 2012–2017, R. A. Fischer and coworkers integrated SURMOFs on SAMfunctionalized gold substrates for enantioselective gas/volatile organic compound adsorption [69, 386,331]. Here the uptake of guest molecules can be directly monitored by QCM. The chosen MOFs forSURMOFs were developed from single MOFs to multi-variant heteroepitaxially grown MOFs (MOF-on-MOF concept) [387, 388, 389]. The heteroepitaxial growth of MOF-on-MOF can greatly improvethe adsorption kinetics of the SURMOF composites [388, 389, 390]. The integration of different MOFsonto one SURMOF system can inherit their own characteristics, imparting synergetic functions for theSURMOF. Thus, the MOF-on-MOF concept can expand the potential application of SURMOFs in variousresearch areas.C. Wöll and coworkers investigated the biocompatibility of SURMOFs by encapsulating bioactivemolecules into the pores of SURMOFs using the LPE technique. SURMOF-2 ([Cu(bdc) ] n ) was foundto be remarkably stable in water and artificial seawater, however, SURMOF-2 is found to degrade veryfast in typical cell culture media. The results suggest that SURMOFs can act as a release matrix forpharmaceuticals and other drugs. This study opens up the possibility of applying SURMOFs in thebiochemical field [391, 392].Moreover, C. Wickleder, C. Wöll and coworkers fabricated HKUST-1 SURMOFs loaded with europium β -diketonate complexes via LPE. Luminescence spectra of these SURMOFs before and after the loadingEu-based compounds suggest that MOF thin films are promising as materials for photonic antennae [393].Similarly, SURMOFs can also encapsulate other fluorescent molecules, which also show interesting opticalproperties [394]. D. Schlettwein, C. Wöll and coworkers immobilized ferrocene (Fc) as a redox mediatorinto the lattices of SURMOFs [395]. Because Fc is easily oxidized to Fc + when applying a voltage, Fcmolecules in proximity of an electrode will be quickly oxidized. The charges transfer through the film bya hopping transport mechanism between adjacent loaded Fc molecules in the SURMOF lattices. Thus,the Fc molecules in the SURMOFs will be oxidized. This study suggests that SURMOFs loaded withredox mediators are promising in the field of catalysis and electronic devices [395].Furthermore, C. Wöll and coworkers also reported the post-synthetic modification of LPE depositedSURMOFs for standard click chemistry with high conversion yields [396]. For this, [Zn (N – bdc) (dabco)]was first deposited on SAM modified Au substrates via LPE method and a strain-promoted metal-freeclick reaction was used to functionalize the SURMOF with different reactants containing strained triplebonds. Considering the advantages of strain-promoted azide-alkyne cycloaddition (SPAAC), the alkyneproducts were obtained with a yield of up to 100%.In 2015, C. Wöll and coworkers reported that SURMOFs fabricated on magnetic nanoparticles via theLPE method showed promising applications in catalysis, chromatographic separation and drug deliverysystems [397]. Additionally, they also developed SURMOFs with promising dielectric, optical and sensingproperties from 2013–2016 [385, 398]. In 2013, the optical constant (dielectric constant) of HKUST-147URMOFs was measured before and after removing the water/EtOH trapped in the structure [398]. Itwas observed that the optical constant of HKUST-1 SURMOFs decreases to 1.39 after the removal ofthe solvent in the voids of the SURMOFs. However, the optical constant of HKUST-1 SURMOFs in-creases again to 1.55-1.6 after re-exposure to water/EtOH atmosphere. The dependence of the opticalproperties on water/EtOH adsorption reveals the potential application of SURMOFs for optical sensingdevices. In 2015, J. Liu et al. fabricated crystalline porphyrin MOFs on SAM modified fluorine-dopedtin oxide FTO via the spray LPE method [399]. FTO supported SURMOFs were assembled as a pho-tovoltaic device with conductive FTO as the bottom electrode and iodine/triiodide as the top electrode.As studied by current-voltage (I-V) curves, the assembled porphyrin Zn-SURMOF based photovoltaicdevice shows a photocurrent efficiency of 0.45% with an open-circuit voltage of . , a short circuitcurrent density of .
71 mA cm − and a fill factor of 0.65. These values demonstrate the efficiency of por-phyrin Zn-SURMOF based photovoltaic devices is more than a factor of two compared to photovoltaicdevices without porphyrin Zn-SURMOFs (open circuit voltage of .
57 V , short-circuit current density of .
45 mA cm − , fill factor of 0.55 and photocurrent efficiency of 0.2%). Porphyrin-based SURMOFs showpromising application for photovoltaic devices, which play a crucial role in energy storage and conversion.To summarize, Germany-based researchers have pioneered the development of liquid phase epitaxytechnique for the fabrication of MOF thin films ranging from understanding the growth mechanism tovarious applications in the last decade. The SURMOFs fabricated by liquid phase epitaxy approach havedemonstrated promising applications in various area, especially in nanotechnology.Additionally, T. Bein and coworkers demonstrated approaches for the growth of MOF thin films viacrystallization at room temperature and thin gel-layer synthesis method [400]. Highly orientated MOFthin films, such as MIL-88B, MIL-53, HKUST-1 and UiO-68-NH , were obtained on various SAM modifiedsolid surfaces by crystallization at room temperature [401]. Fluorescent dye molecules were covalentlymodified inside the pores of MOF thin films by post-synthesis methods, which demonstrate size-selectivefluorescence quenching behavior. MOF thin films of MIL-88B also show high water/gas adsorption ability[402]. Furthermore, A. Terfort and coworkers, based at Goethe University Frankfurt, developed a simpleprecipitation route, an evaporation method to deposit MOF thin films at room temperature, allowing forthe precise localization of MOF particles on a solid surface [403]. The A. Terfort group also developedevaporation method and inkjet-printing approaches for integrating patterned MOFs onto solid surfaces[404]. These contributions have potential for different micro-printing and nanotechnological areas.Apart from the above mentioned methods, a variety of advanced deposition techniques, such asLangmuir-Blodgett layer-by-layer deposition [405], electrochemical deposition [406], microwave-inducedthermal deposition [407], and chemical vapor deposition [408], have also been developed to fabricate MOFthin films with well-defined morphology and structures.MOF thin films fabricated by different techniques will perform different functions due to the differencesin the resulting particles sizes, surface roughness, film thickness and film stabilities obtained by thedifferent deposition approaches. This exciting research ranging from various deposition techniques toapplications of MOF thin films has been carried out all over the world. Despite these encouraging reports,the study of MOF thin films is still at an emerging stage of vigorous development and considerable effortis required before MOF thin films can be industrialized and used commercially.
6. Conclusion
MOFs are clearly a promising class of materials displaying many advanced functions and Germany-based researchers have invested significant effort into understanding these materials. In this review, wehave particularly outlined the reports of German research groups for the synthesis of new MOFs, ap-proaches for producing MOFs on an industrial scale, the development of advanced measurement methodsfor characterization and finally examples of potential functions and properties.We believe the collaborative nature of MOF research in Germany will continue to flourish and enablethe discovery of new materials and significant scientific advancement. For example, MOF thin filmpreparation and characterization of conductive or semiconducting MOFs is being pioneered by researchersacross Germany and this work is poised to revolutionize electronic and photonic devices.While this work has highlighted the work of German investigators we note these studies have bene-fited tremendously from fundamental and groundbreaking studies from researchers all around the world.Moreover, much of this work is the product of excellent international collaboration with other Europeanand international researchers which we hope will continue into the future.48 . Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft [grant number FOR2433]; Eu-ropean Research Council (ERC) under the European Union’s Horizon 2020 research and innovationprogramme [grant number 742743]; Bundesministerium für Wirtschaft und Energie (BMWi) [grant num-ber 0327851B]; Bundesministerium für Bildung und Forschung in the project Optimat [grant number03SF0492C].
8. ReferencesReferences [1] H. Furukawa, K. E. Cordova, M. O’Keeffe, O. M. Yaghi, The chemistry and applications of metal–organic frameworks,Science 341 (2013) 1230444–1230444.[2] MOF2018 Conference Committee, MOF2018 6th International Conference on Metal–Organic Frameworks & OpenFramework Compounds, http://mof2018.com/, 2018. (accessed 5 July 2018).[3] O. K. Farha, I. Eryazici, N. C. Jeong, B. G. Hauser, C. E. Wilmer, A. A. Sarjeant, R. Q. Snurr, S. T. Nguyen,A. Özgür Yazaydın, J. T. Hupp, Metal–organic framework materials with ultrahigh surface areas: is the sky thelimit?, J. Am. Chem. Soc. 134 (2012) 15016–15021.[4] T. Tian, Z. Zeng, D. Vulpe, M. E. Casco, G. Divitini, P. A. Midgley, J. Silvestre-Albero, J.-C. Tan, P. Z. Moghadam,D. Fairen-Jimenez, A sol–gel monolithic metal–organic framework with enhanced methane uptake, Nat. Mater. 17(2017) 174–179.[5] H. Li, M. Eddaoudi, T. L. Groy, O. M. Yaghi, Establishing microporosity in open metal–organic frameworks: gassorption isotherms for Zn(BDC) (BDC = 1,4-benzenedicarboxylate), J. Am. Chem. Soc. 120 (1998) 8571–8572.[6] S. S.-Y. Chui, S. M.-F. Lo, J. P. H. Charmant, A. G. Orpen, I. D. Williams, A chemically functionalizable nanoporousmaterial [Cu (TMA) (H O) ] n , Science 283 (1999) 1148–1150.[7] S. Noro, S. Kitagawa, M. Kondo, K. Seki, A new, methane adsorbent, porous coordination polymer [ { CuSiF6(4,4 (cid:48) -bipyridine)2 } n], Angew. Chem. Int. Ed. 39 (2000) 2081–2084.[8] H. K. Chae, D. Y. Siberio-Pérez, J. Kim, Y. Go, M. Eddaoudi, A. J. Matzger, M. O’Keeffe, O. M. Yaghi, A route tohigh surface area, porosity and inclusion of large molecules in crystals, Nature 427 (2004) 523–527.[9] G. Férey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surblé, I. Margiolaki, A chromium terephthalate-based solid with unusually large pore volumes and surface area, Science 309 (2005) 2040–2042.[10] S. Kaskel, Porous metal-organic frameworks, in: F. Schüth, K. S. Sing, J. Weitkamp (Eds.), Handbook of PorousSolids, Wiley-VCH Verlag GmbH, 2008, pp. 1190–1249.[11] T. Schareina, C. Schick, B. F. Abrahams, R. Kempe, Coordination polymers of bipyridyldicarboxylates - a cobaltcontaining 12, 3-net with potential reactive sites, Z. Anorg. Allg. Chem. 627 (2001) 1711–1713.[12] K. Abu-Shandi, H. Winkler, B. Wu, C. Janiak, Open-framework iron phosphates: Syntheses, structures, sorptionstudies and oxidation catalysis, CrystEngComm 5 (2003) 180.[13] C. Janiak, Engineering coordination polymers towards applications, Dalton Trans. 0 (2003) 2781.[14] N. Stock, A. Stoll, T. Bein, A new calcium tetraphosphonate containing small pores,Ca[(HO PCH ) N(H)–CH C H CH –N(H)(CH PO H) ] · O, Microporous Mesoporous Mater. 69 (2004)65–69.[15] S. Hermes, M.-K. Schröter, R. Schmid, L. Khodeir, M. Muhler, A. Tissler, R. W. Fischer, R. A. Fischer, Metal@MOF:Loading of highly porous coordination polymers host lattices by metal organic chemical vapor deposition, Angew.Chem. Int. Ed. 44 (2005) 6237–6241.[16] K. Schlichte, T. Kratzke, S. Kaskel, Improved synthesis, thermal stability and catalytic properties of the metal-organicframework compound Cu (BTC)
29] G. Wißmann, A. Schaate, S. Lilienthal, I. Bremer, A. M. Schneider, P. Behrens, Modulated synthesis of zr-fumarateMOF, Microporous Mesoporous Mater. 152 (2012) 64–70.[30] A. Schaate, S. Dühnen, G. Platz, S. Lilienthal, A. M. Schneider, P. Behrens, A novel zr-based porous coordinationpolymer containing azobenzenedicarboxylate as a linker, Eur. J. Inorg. Chem 2012 (2012) 790–796.[31] A. Schaate, P. Roy, T. Preuße, S. J. Lohmeier, A. Godt, P. Behrens, Porous interpenetrated zirconium-organicframeworks (PIZOFs): A chemically versatile family of metal-organic frameworks, Chem. Eur. J. 17 (2011) 9320–9325.[32] J. Lippke, B. Brosent, T. von Zons, E. Virmani, S. Lilienthal, T. Preuße, M. Hülsmann, A. M. Schneider, S. Wuttke,P. Behrens, A. Godt, Expanding the group of porous interpenetrated zr-organic frameworks (PIZOFs) with linkersof different lengths, Inorg. Chem. 56 (2016) 748–761.[33] Y. Bai, Y. Dou, L.-H. Xie, W. Rutledge, J.-R. Li, H.-C. Zhou, Zr-based metal–organic frameworks: design, synthesis,structure, and applications, Chem. Soc. Rev. 45 (2016) 2327–2367.[34] V. Bon, V. Senkovskyy, I. Senkovska, S. Kaskel, Zr(IV) and Hf(IV) based metal–organic frameworks with reo-topology,Chem. Commun. 48 (2012) 8407.[35] V. Bon, I. Senkovska, I. A. Baburin, S. Kaskel, Zr- and Hf-based metal–organic frameworks: Tracking down thepolymorphism, Cryst. Growth Des. 13 (2013) 1231–1237.[36] F. Drache, V. Bon, I. Senkovska, J. Getzschmann, S. Kaskel, The modulator driven polymorphism of Zr(IV) basedmetal–organic frameworks, Philos. Trans. Royal Soc. A 375 (2016) 20160027.[37] F. Drache, V. Bon, I. Senkovska, M. Adam, A. Eychmüller, S. Kaskel, Vapochromic luminescence of a zirconium-basedmetal-organic framework for sensing applications, Eur. J. Inorg. Chem 2016 (2016) 4483–4489.[38] C. Kutzscher, G. Nickerl, I. Senkovska, V. Bon, S. Kaskel, Proline functionalized UiO-67 and UiO-68 typemetal–organic frameworks showing reversed diastereoselectivity in aldol addition reactions, Chem. Mater. 28 (2016)2573–2580.[39] G. Nickerl, I. Senkovska, S. Kaskel, Tetrazine functionalized zirconium MOF as an optical sensor for oxidizing gases,Chem. Commun. 51 (2015) 2280–2282.[40] M. Lammert, H. Reinsch, C. A. Murray, M. T. Wharmby, H. Terraschke, N. Stock, Synthesis and structure of Zr(IV)-and Ce(IV)-based CAU-24 with 1,2,4,5-tetrakis(4-carboxyphenyl)benzene, Dalton Trans. 45 (2016) 18822–18826.[41] G. Zahn, H. A. Schulze, J. Lippke, S. König, U. Sazama, M. Fröba, P. Behrens, A water-born Zr-based porouscoordination polymer: Modulated synthesis of Zr-fumarate MOF, Microporous Mesoporous Mater. 203 (2015) 186–194.[42] A. C. Dreischarf, M. Lammert, N. Stock, H. Reinsch, Green synthesis of Zr-CAU-28: Structure and properties of thefirst Zr-MOF based on 2,5-furandicarboxylic acid, Inorg. Chem. 56 (2017) 2270–2277.[43] S. Waitschat, H. Reinsch, N. Stock, Water-based synthesis and characterisation of a new Zr-MOF with a uniqueinorganic building unit, Chem. Commun. 52 (2016) 12698–12701.[44] S. Waitschat, D. Fröhlich, H. Reinsch, H. Terraschke, K. A. Lomachenko, C. Lamberti, H. Kummer, T. Helling,M. Baumgartner, S. Henninger, N. Stock, Synthesis of M-UiO-66 (M = Zr, Ce or Hf) employing 2,5-pyridinedicarboxylic acid as a linker: defect chemistry, framework hydrophilisation and sorption properties, DaltonTrans. 47 (2018) 1062–1070.[45] M. Lammert, M. T. Wharmby, S. Smolders, B. Bueken, A. Lieb, K. A. Lomachenko, D. D. Vos, N. Stock, Cerium-based metal organic frameworks with UiO-66 architecture: synthesis, properties and redox catalytic activity, Chem.Commun. 51 (2015) 12578–12581.[46] M. Lammert, C. Glißmann, H. Reinsch, N. Stock, Synthesis and characterization of new Ce(IV)-MOFs exhibitingvarious framework topologies, Cryst. Growth Des. 17 (2017) 1125–1131.[47] S. Smolders, K. A. Lomachenko, B. Bueken, A. Struyf, A. L. Bugaev, C. Atzori, N. Stock, C. Lamberti, M. B. J. Roef-faers, D. E. De Vos, Unravelling the redox-catalytic behavior of Ce metal-organic frameworks by x-ray absorptionspectroscopy, ChemPhysChem 19 (2017) 373–378.[48] M. Lammert, C. Glißmann, N. Stock, Tuning the stability of bimetallic Ce(IV)/Zr(IV)-based MOFs with UiO-66 andMOF-808 structures, Dalton Trans. 46 (2017) 2425–2429.[49] I. Senkovska, S. Kaskel, Ultrahigh porosity in mesoporous MOFs: promises and limitations, Chem. Commun. 50(2014) 7089.[50] W. Xuan, C. Zhu, Y. Liu, Y. Cui, Mesoporous metal–organic framework materials, Chem. Soc. Rev. 41 (2012)1677–1695.[51] H.-Y. Guan, R. J. LeBlanc, S.-Y. Xie, Y. Yue, Recent progress in the syntheses of mesoporous metal–organicframework materials, Coord. Chem. Rev. 369 (2018) 76–90.[52] O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi, J. Kim, Reticular synthesis and the design ofnew materials, Nature 423 (2003) 705–714.[53] M. Eddaoudi, Systematic design of pore size and functionality in isoreticular MOFs and their application in methanestorage, Science 295 (2002) 469–472.[54] B. Chen, Interwoven metal-organic framework on a periodic minimal surface with extra-large pores, Science 291(2001) 1021–1023.[55] N. Klein, I. Senkovska, I. A. Baburin, R. Grünker, U. Stoeck, M. Schlichtenmayer, B. Streppel, U. Mueller, S. Leoni,M. Hirscher, S. Kaskel, Route to a family of robust, non-interpenetrated metal-organic frameworks with pto-liketopology, Chem. Eur. J. 17 (2011) 13007–13016.[56] K. Koh, A. Wong-Foy, A. Matzger, A crystalline mesoporous coordination copolymer with high microporosity, Angew.Chem. Int. Ed. 47 (2008) 677–680.[57] N. Klein, I. Senkovska, K. Gedrich, U. Stoeck, A. Henschel, U. Mueller, S. Kaskel, A mesoporous metal-organicframework, Angew. Chem. Int. Ed. 48 (2009) 9954–9957.[58] K. Koh, A. G. Wong-Foy, A. J. Matzger, Coordination copolymerization mediated by Zn O(CO R) metal clusters:a balancing act between statistics and geometry, J. Am. Chem. Soc. 132 (2010) 15005–15010.[59] W. Zhuang, S. Ma, X.-S. Wang, D. Yuan, J.-R. Li, D. Zhao, H.-C. Zhou, Introduction of cavities up to 4 nm intoa hierarchically-assembled metal–organic framework using an angular, tetratopic ligand, Chem. Commun. 46 (2010)5223.[60] J. J. P. IV, J. A. Perman, M. J. Zaworotko, Design and synthesis of metal–organic frameworks using metal–organic olyhedra as supermolecular building blocks, Chem. Soc. Rev. 38 (2009) 1400.[61] W. Lu, D. Yuan, T. A. Makal, Z. Wei, J.-R. Li, H.-C. Zhou, Highly porous metal–organic framework sustained with12-connected nanoscopic octahedra, Dalton Trans. 42 (2013) 1708–1714.[62] U. Stoeck, S. Krause, V. Bon, I. Senkovska, S. Kaskel, A highly porous metal–organic framework, constructed from acuboctahedral super-molecular building block, with exceptionally high methane uptake, Chem. Commun. 48 (2012)10841.[63] Z. Wang, S. M. Cohen, Postsynthetic modification of metal–organic frameworks, Chem. Soc. Rev. 38 (2009) 1315.[64] G. Nickerl, A. Henschel, R. Grünker, K. Gedrich, S. Kaskel, Chiral metal-organic frameworks and their applicationin asymmetric catalysis and stereoselective separation, Chem. Ing. Tech. 83 (2011) 90–103.[65] M. Padmanaban, P. Müller, C. Lieder, K. Gedrich, R. Grünker, V. Bon, I. Senkovska, S. Baumgärtner, S. Opelt,S. Paasch, E. Brunner, F. Glorius, E. Klemm, S. Kaskel, Application of a chiral metal–organic framework in enan-tioselective separation, Chem. Commun. 47 (2011) 12089.[66] K. Gedrich, M. Heitbaum, A. Notzon, I. Senkovska, R. Fröhlich, J. Getzschmann, U. Mueller, F. Glorius, S. Kaskel,A family of chiral metal-organic frameworks, Chem. Eur. J. 17 (2011) 2099–2106.[67] C. Kutzscher, D. Janssen-Müller, A. Notzon, U. Stoeck, V. Bon, I. Senkovska, S. Kaskel, F. Glorius, Synthesisof the homochiral metal–organic framework DUT-129 based on a chiral dicarboxylate linker with 6 stereocenters,CrystEngComm 19 (2017) 2494–2499.[68] P. Schmieder, D. Denysenko, M. Grzywa, B. Baumgärtner, I. Senkovska, S. Kaskel, G. Sastre, L. van Wüllen,D. Volkmer, CFA-1: the first chiral metal–organic framework containing kuratowski-type secondary building units,Dalton Trans. 42 (2013) 10786.[69] B. Liu, O. Shekhah, H. K. Arslan, J. Liu, C. Wöll, R. A. Fischer, Enantiopure metal-organic framework thin films:Oriented SURMOF growth and enantioselective adsorption, Angew. Chem. Int. Ed. 51 (2011) 807–810.[70] S. R. Batten, N. R. Champness, X.-M. Chen, J. Garcia-Martinez, S. Kitagawa, L. Öhrström, M. O’Keeffe, M. P. Suh,J. Reedijk, Terminology of metal–organic frameworks and coordination polymers (IUPAC recommendations 2013),Pure and Applied Chemistry 85 (2013) 1715–1724.[71] J.-P. Zhang, Y.-B. Zhang, J.-B. Lin, X.-M. Chen, Metal azolate frameworks: From crystal engineering to functionalmaterials, Chem. Rev. 112 (2011) 1001–1033.[72] Y.-Q. Tian, C.-X. Cai, X.-M. Ren, C.-Y. Duan, Y. Xu, S. Gao, X.-Z. You, The silica-like extended polymorphism ofcobalt(II) imidazolate three-dimensional frameworks: X-ray single-crystal structures and magnetic properties, Chem.Eur. J. 9 (2003) 5673–5685.[73] K. S. Park, Z. Ni, A. P. Cote, J. Y. Choi, R. Huang, F. J. Uribe-Romo, H. K. Chae, M. O’Keeffe, O. M. Yaghi,Exceptional chemical and thermal stability of zeolitic imidazolate frameworks, Proc. Natl. Acad. Sci. U.S.A 103(2006) 10186–10191.[74] F. Seel, J. Rodrian, Über die umsetzung von bis(tetracarbonylcobalt)quecksilber mit imidazol, J. Organomet. Chem.16 (1969) 479–484.[75] A. Phan, C. J. Doonan, F. J. Uribe-Romo, C. B. Knobler, M. O’Keeffe, O. M. Yaghi, Synthesis, structure, and carbondioxide capture properties of zeolitic imidazolate frameworks, Acc. Chem. Res. 43 (2010) 58–67.[76] W. Ouellette, A. V. Prosvirin, J. Valeich, K. R. Dunbar, J. Zubieta, Hydrothermal synthesis, structural chemistry,and magnetic properties of materials of the MII/triazolate/anion family, where MII= Mn, Fe, and Ni, Inorg. Chem.46 (2007) 9067–9082.[77] W. Ouellette, A. V. Prosvirin, V. Chieffo, K. R. Dunbar, B. Hudson, J. Zubieta, Solid-state coordination chemistryof the Cu/triazolate/X system (X = F − , Cl − , Br − , I − , OH − , and SO − ), Inorg. Chem. 45 (2006) 9346–9366.[78] W. Ouellette, B. S. Hudson, J. Zubieta, Hydrothermal and structural chemistry of the zinc(II)- and cadmium(II)-1,2,4-triazolate systems, Inorg. Chem. 46 (2007) 4887–4904.[79] S. Maddila, R. Pagadala, S. B. Jonnalagadda, 1,2,4-triazoles: A review of synthetic approaches and the biologicalactivity, Letters in Organic Chemistry 10 (2013) 693–714.[80] K. T. Potts, The chemistry of 1,2,4-triazoles., Chem. Rev. 61 (1961) 87–127.[81] J. G. Haasnoot, Mononuclear, oligonuclear and polynuclear metal coordination compounds with 1,2,4-triazole deriva-tives as ligands, Coord. Chem. Rev. 200-202 (2000) 131–185.[82] G. Aromí, L. A. Barrios, O. Roubeau, P. Gamez, Triazoles and tetrazoles: Prime ligands to generate remarkablecoordination materials, Coord. Chem. Rev. 255 (2011) 485–546.[83] D. Lässig, J. Lincke, H. Krautscheid, Highly functionalised 3,4,5-trisubstituted 1,2,4-triazoles for future use as ligandsin coordination polymers, Tetrahedron Lett. 51 (2010) 653–656.[84] D. Lässig, J. Lincke, R. Gerhardt, H. Krautscheid, Solid-state syntheses of coordination polymers by thermal conver-sion of molecular building blocks and polymeric precursors, Inorg. Chem. 51 (2012) 6180–6189.[85] J. Lincke, D. Lässig, J. Moellmer, C. Reichenbach, A. Puls, A. Moeller, R. Gläser, G. Kalies, R. Staudt, H. Krautscheid,A novel copper-based MOF material: Synthesis, characterization and adsorption studies, Microporous MesoporousMater. 142 (2011) 62–69.[86] G. A. Senchyk, A. B. Lysenko, I. Boldog, E. B. Rusanov, A. N. Chernega, H. Krautscheid, K. V. Domasevitch, 1,2,4-triazole functionalized adamantanes: a new library of polydentate tectons for designing structures of coordinationpolymers, Dalton Trans. 41 (2012) 8675.[87] G. A. Senchyk, A. B. Lysenko, A. A. Babaryk, E. B. Rusanov, H. Krautscheid, P. Neves, A. A. Valente, I. S.Gonçalves, K. W. Krämer, S.-X. Liu, S. Decurtins, K. V. Domasevitch, Triazolyl–based copper–molybdate hybrids:From composition space diagram to magnetism and catalytic performance, Inorg. Chem. 53 (2014) 10112–10121.[88] K. Müller-Buschbaum, Y. Mokaddem, Three-dimensional networks of lanthanide 1,2,4-triazolates: 3 ∞ [yb(tz)3] and3 ∞ [eu2(tz)5(TzH)2], the first 4f networks with complete nitrogen coordination, Chem. Commun. (2006) 2060–2062.[89] E. I. Tsyganova, L. M. Dyagileva, The reactivity of metal β -diketonates in the thermal decomposition reaction,Russian Chem. Rev. 65 (1996) 315.[90] S. Biswas, M. Grzywa, H. P. Nayek, S. Dehnen, I. Senkovska, S. Kaskel, D. Volkmer, A cubic coordination frameworkconstructed from benzobistriazolate ligands and zinc ions having selective gas sorption properties, Dalton Trans.(2009) 6487.[91] S. Biswas, M. Tonigold, M. Speldrich, P. KoÌ^gerler, M. Weil, D. Volkmer, Syntheses and magnetostructural in-vestigations on Kuratowski-type homo- and heteropentanuclear coordination compounds [MZn Cl (L) ] (M II = Zn, e, Co, Ni, or Cu; l = 5,6-dimethyl-1,2,3-benzotriazolate) represented by the k , graph, Inorg. Chem. 49 (2010)7424–7434.[92] V. L. Himes, A. D. Mighell, A. R. Siedle, Synthesis and structure of Cu (BTA) (t-C H NC) , a mixed-valentcopper-nitrogen cluster containing η -benzotriazolate, J. Am. Chem. Soc. 103 (1981) 211–212.[93] M. Tonigold, Y. Lu, B. Bredenkötter, B. Rieger, S. Bahnmüller, J. Hitzbleck, G. Langstein, D. Volkmer, Heterogeneouscatalytic oxidation by MFU-1: A cobalt(II)-containing metal-organic framework, Angew. Chem. Int. Ed. 48 (2009)7546–7550.[94] Y.-Y. Liu, M. Grzywa, M. Tonigold, G. Sastre, T. Schüttrigkeit, N. S. Leeson, D. Volkmer, Photophysical propertiesof kuratowski -type coordination compounds [M II Zn Cl (Me bta) ] (M II = Zn or Ru) featuring long-lived excitedelectronic states, Dalton Trans. 40 (2011) 5926.[95] J. Teufel, H. Oh, M. Hirscher, M. Wahiduzzaman, L. Zhechkov, A. Kuc, T. Heine, D. Denysenko, D. Volkmer, MFU-4- a metal-organic framework for highly effective h2/d2separation, Adv. Mater. 25 (2012) 635–639.[96] G. Sastre, J. van den Bergh, F. Kapteijn, D. Denysenko, D. Volkmer, Unveiling the mechanism of selective gate-drivendiffusion of CO over N in MFU-4 metal–organic framework, Dalton Trans. 43 (2014) 9612–9619.[97] D. Denysenko, M. Grzywa, M. Tonigold, B. Streppel, I. Krkljus, M. Hirscher, E. Mugnaioli, U. Kolb, J. Hanss,D. Volkmer, Elucidating gating effects for hydrogen sorption in MFU-4-type triazolate-based metal-organic frame-works featuring different pore sizes, Chem. Eur. J. 17 (2011) 1837–1848.[98] P. Schmieder, M. Grzywa, D. Denysenko, M. Hambach, D. Volkmer, CFA-7: an interpenetrated metal–organicframework of the MFU-4 family, Dalton Trans. 44 (2015) 13060–13070.[99] D. Denysenko, T. Werner, M. Grzywa, A. Puls, V. Hagen, G. Eickerling, J. Jelic, K. Reuter, D. Volkmer, Reversiblegas-phase redox processes catalyzed by Co-exchanged MFU-4l(arge), Chem. Commun. 48 (2012) 1236–1238.[100] D. Denysenko, M. Grzywa, J. Jelic, K. Reuter, D. Volkmer, Scorpionate-type coordination in MFU-4lmetal-organicframeworks: Small-molecule binding and activation upon the thermally activated formation of open metal sites,Angew. Chem. Int. Ed. 53 (2014) 5832–5836.[101] I. Weinrauch, I. Savchenko, D. Denysenko, S. M. Souliou, H.-H. Kim, M. L. Tacon, L. L. Daemen, Y. Cheng,A. Mavrandonakis, A. J. Ramirez-Cuesta, D. Volkmer, G. Schütz, M. Hirscher, T. Heine, Capture of heavy hydrogenisotopes in a metal-organic framework with active Cu(I) sites, Nat. Commun. 8 (2017) 14496.[102] C. K. Brozek, L. Bellarosa, T. Soejima, T. V. Clark, N. López, M. Dincă, Solvent-dependent cation exchange inmetal-organic frameworks, Chem. Eur. J. 20 (2014) 6871–6874.[103] D. Denysenko, J. Jelic, K. Reuter, D. Volkmer, Postsynthetic metal and ligand exchange in MFU-4l: A screeningapproach toward functional metal-organic frameworks comprising single-site active centers, Chem. Eur. J. 21 (2015)8188–8199.[104] E. D. Metzger, C. K. Brozek, R. J. Comito, M. Dincă, Selective dimerization of ethylene to 1-butene with a porouscatalyst, ACS Cent. Sci. 2 (2016) 148–153.[105] R. J. Comito, E. D. Metzger, Z. Wu, G. Zhang, C. H. Hendon, J. T. Miller, M. Dincă, Selective dimerization ofpropylene with Ni-MFU-4l, Organometallics 36 (2017) 1681–1683.[106] R. J.-C. Dubey, R. J. Comito, Z. Wu, G. Zhang, A. J. Rieth, C. H. Hendon, J. T. Miller, M. Dincă, Highlystereoselective heterogeneous diene polymerization by Co-MFU-4l: A single-site catalyst prepared by cation exchange,J. Am. Chem. Soc. 139 (2017) 12664–12669.[107] D. Denysenko, D. Volkmer, Cyclic gas-phase heterogeneous process in a metal–organic framework involving a nickelnitrosyl complex, Faraday Discuss. 201 (2017) 101–112.[108] X.-H. Zhou, Y.-H. Peng, X.-D. Du, J.-L. Zuo, X.-Z. You, Hydrothermal syntheses and structures of three novelcoordination polymers assembled from 1,2,3-triazolate ligands, CrystEngComm 11 (2009) 1964.[109] M. Grzywa, D. Denysenko, J. Hanss, E.-W. Scheidt, W. Scherer, M. Weil, D. Volkmer, CuN Jahn–Teller centersin coordination frameworks comprising fully condensed Kuratowski-type secondary building units: phase transitionsand magneto-structural correlations, Dalton Trans. 41 (2012) 4239.[110] F. Gándara, F. J. Uribe-Romo, D. K. Britt, H. Furukawa, L. Lei, R. Cheng, X. Duan, M. O’Keeffe, O. M. Yaghi,Porous, conductive metal-triazolates and their structural elucidation by the charge-flipping method, Chem. Eur. J.18 (2012) 10595–10601.[111] X.-L. Wang, C. Qin, S.-X. Wu, K.-Z. Shao, Y.-Q. Lan, S. Wang, D.-X. Zhu, Z.-M. Su, E.-B. Wang, Bottom-upsynthesis of porous coordination frameworks: Apical substitution of a pentanuclear tetrahedral precursor, Angew.Chem. Int. Ed. 48 (2009) 5291–5295.[112] L. Sun, C. H. Hendon, S. S. Park, Y. Tulchinsky, R. Wan, F. Wang, A. Walsh, M. Dincă, Is iron unique in promotingelectrical conductivity in MOFs?, Chem. Sci. 8 (2017) 4450–4457.[113] S. Biswas, M. Tonigold, M. Speldrich, P. Kögerler, D. Volkmer, Nonanuclear coordination compounds featuring { M L } cores (m = Ni II , Co II , or Zn II ; L = 1,2,3-benzotriazolate), Eur. J. Inorg. Chem 2009 (2009) 3094–3101.[114] S. Biswas, M. Tonigold, H. Kelm, H.-J. Krüger, D. Volkmer, Thermal spin-crossover in the [M Zn Cl L ] (M= Zn, Fe(II); L = 5,6-dimethoxy-1,2,3-benzotriazolate) system: structural, electrochemical, Mössbauer, and UV-visspectroscopic studies, Dalton Trans. 39 (2010) 9851.[115] K. Müller-Buschbaum, Y. Mokaddem, MOFs by solvent free high temperature synthesis exemplified by ∞ [Eu (Tz ∗ ) (Tz ∗ H) ], Solid State Sci. 10 (2008) 416–420.[116] J.-C. Rybak, I. Schellenberg, R. Pöttgen, K. Müller-Buschbaum, MOFs by transformation of 1D-coordination polymersII: The homoleptic divalent rare earth 3D benzotriazolate ∞ [Eu(Btz) ] initiating from ∞ [Eu(Btz) (BtzH) ], Z. Anorg.Allg. Chem. 636 (2010) 1720–1725.[117] K. Müller-Buschbaum, Y. Mokaddem, F. Schappacher, R. Pöttgen, [Eu(Tzpy) ]: A homoleptic framework containing { Eu II N } icosahedra, Angew. Chem. Int. Ed. 46 (2007) 4385–4387.[118] M. Taddei, F. Costantino, R. Vivani, Robust metal-organic frameworks based on tritopic phosphonoaromatic ligands,Eur. J. Inorg. Chem 2016 (2016) 4300–4309.[119] K. J. Gagnon, H. P. Perry, A. Clearfield, Conventional and unconventional metal–organic frameworks based onphosphonate ligands: MOFs and UMOFs, Chem. Rev. 112 (2011) 1034–1054.[120] A. Clearfield, K. Demadis (Eds.), Metal Phosphonate Chemistry, Royal Society of Chemistry, 2011.[121] G. K. H. Shimizu, R. Vaidhyanathan, J. M. Taylor, Phosphonate and sulfonate metal organic frameworks, Chem.Soc. Rev. 38 (2009) 1430. ) and Zr(R-OPO ) compounds (R =organic radical), J. Inorg. Nucl. Chem. 40 (1978) 1113–1117.[124] A. Clearfield, G. D. Smith, Crystallography and structure of α -zirconium bis(monohydrogen orthophosphate) mono-hydrate, Inorg. Chem. 8 (1969) 431–436.[125] M. B. Dines, R. E. Cooksey, P. C. Griffith, R. H. Lane, Mixed-component layered tetravalent metal phospho-nates/phosphates as precursors for microporous materials, Inorg. Chem. 22 (1983) 1003–1004.[126] S. Bauer, H. Müller, T. Bein, N. Stock, Synthesis and characterization of the open-framework barium bisphosphonate[Ba (O PCH NH CH PO ) (H O) ] · O, Inorg. Chem. 44 (2005) 9464–9470.[127] C. Serre, J. A. Groves, P. Lightfoot, A. M. Z. Slawin, P. A. Wright, N. Stock, T. Bein, M. Haouas, F. Taulelle,G. Férey, Synthesis, structure and properties of related microporous N,N‘-piperazinebismethylenephosphonates ofaluminum and titanium, Chem. Mater. 18 (2006) 1451–1457.[128] J. A. Groves, S. R. Miller, S. J. Warrender, C. Mellot-Draznieks, P. Lightfoot, P. A. Wright, The first route to largepore metal phosphonates, Chem. Commun. (2006) 3305.[129] M. T. Wharmby, G. M. Pearce, J. P. Mowat, J. M. Griffin, S. E. Ashbrook, P. A. Wright, L.-H. Schilling, A. Lieb,N. Stock, S. Chavan, S. Bordiga, E. Garcia, G. D. Pirngruber, M. Vreeke, L. Gora, Synthesis and crystal chemistryof the STA-12 family of metal N,N (cid:48) -piperazinebis(methylenephosphonate)s and applications of STA-12(Ni) in theseparation of gases, Microporous Mesoporous Mater. 157 (2012) 3–17.[130] M. T. Wharmby, J. P. S. Mowat, S. P. Thompson, P. A. Wright, Extending the pore size of crystalline metalphosphonates toward the mesoporous regime by isoreticular synthesis, J. Am. Chem. Soc. 133 (2011) 1266–1269.[131] N. Hermer, M. T. Wharmby, N. Stock, Re-determination of the crystal structure of MIL-91(Al), Z. Anorg. Allg.Chem. 643 (2016) 137–140.[132] M. Plabst, T. Bein, 1,4-phenylenebis(methylidyne)tetrakis(phosphonic acid): A new building block in metal organicframework synthesis, Inorg. Chem. 48 (2009) 4331–4341.[133] S. Begum, S. Horike, S. Kitagawa, H. Krautscheid, Water stable triazolyl phosphonate MOFs: steep water uptakeand facile regeneration, Dalton Trans. 44 (2015) 18727–18730.[134] S. Begum, Z. Wang, A. Donnadio, F. Costantino, M. Casciola, R. Valiullin, C. Chmelik, M. Bertmer, J. Kärger,J. Haase, H. Krautscheid, Water-mediated proton conduction in a robust triazolyl phosphonate metal-organic frame-work with hydrophilic nanochannels, Chem. Eur. J. (2014) 8862–8866.[135] T. Zheng, Z. Yang, D. Gui, Z. Liu, X. Wang, X. Dai, S. Liu, L. Zhang, Y. Gao, L. Chen, D. Sheng, Y. Wang, J. Diwu,J. Wang, R. Zhou, Z. Chai, T. E. Albrecht-Schmitt, S. Wang, Overcoming the crystallization and designability issuesin the ultrastable zirconium phosphonate framework system, Nat. Commun. 8 (2017) 15369.[136] A. Schütrumpf, A. Bulut, N. Hermer, Y. Zorlu, E. Kirpi, N. Stock, A. Özgür Yazaydın, G. Yücesan, J. Beckmann,From tetrahedral tetraphosphonic acids E[p-C H P(O)(OH) ] (E=C, Si) to porous Cu- and Zn-mofs with largesurface areas, ChemistrySelect 2 (2017) 3035–3038.[137] N. Hermer, H. Reinsch, P. Mayer, N. Stock, Synthesis and characterisation of the porous zinc phosphonate[Zn (H PPB)(H O) ] · xH O, CrystEngComm 18 (2016) 8147–8150.[138] B. Wang, T. Rhauderwiek, A. K. Inge, H. Xu, T. Yang, Z. Huang, N. Stock, X. Zou, A porous cobalt tetraphosphonatemetal-organic framework: Accurate structure and guest molecule location determined by continuous-rotation electrondiffraction, Chem.: Eur. J 24 (2018) 17429–17433.[139] T. Rhauderwiek, H. Zhao, P. Hirschle, M. Döblinger, B. Bueken, H. Reinsch, D. D. Vos, S. Wuttke, U. Kolb,N. Stock, Highly stable and porous porphyrin-based zirconium and hafnium phosphonates – electron crystallographyas an important tool for structure elucidation, Chem. Sci. 9 (2018) 5467–5478.[140] J. Klein, C. W. Lehmann, H.-W. Schmidt, W. F. Maier, Combinatorial material libraries on the microgram scalewith an example of hydrothermal synthesis, Angew. Chem. Int. Ed. 37 (1998) 3369–3372.[141] N. Stock, T. Bein, High-throughput synthesis of phosphonate-based inorganic–organic hybrid compounds underhydrothermal conditions, Angew. Chem. Int. Ed. 43 (2004) 749–752.[142] R. Banerjee, A. Phan, B. Wang, C. Knobler, H. Furukawa, M. O’Keeffe, O. M. Yaghi, High-throughput synthesis ofzeolitic imidazolate frameworks and application to CO capture, Science 319 (2008) 939–943.[143] S. Bauer, C. Serre, T. Devic, P. Horcajada, J. Marrot, G. Férey, N. Stock, High-throughput assisted rationalizationof the formation of metal organic frameworks in the iron(III) aminoterephthalate solvothermal system, Inorg. Chem.47 (2008) 7568–7576.[144] N. Stock, High-throughput investigations employing solvothermal syntheses, Microporous Mesoporous Mater. 129(2010) 287–295.[145] H. Reinsch, N. Stock, High-throughput studies of highly porous Al-based MOFs, Microporous Mesoporous Mater.171 (2013) 156–165.[146] M. Feyand, C. F. Seidler, C. Deiter, A. Rothkirch, A. Lieb, M. Wark, N. Stock, High-throughput microwave-assisteddiscovery of new metal phosphonates, Dalton Trans. 42 (2013) 8761.[147] P. Maniam, N. Stock, Investigation of porous Ni-based metal–organic frameworks containing paddle-wheel typeinorganic building units via high-throughput methods, Inorg. Chem. 50 (2011) 5085–5097.[148] F. Niekiel, N. Stock, Discovery of new calcium etidronates employing ultrasound adapted high-throughput methods,Cryst. Growth Des. 14 (2013) 599–606.[149] L.-H. Schilling, N. Stock, High-throughput ultrasonic synthesis and in situ crystallisation investigation of metalphosphonocarboxylates, Dalton Trans. 43 (2014) 414–422.[150] S. Bauer, N. Stock, Implementation of a temperature-gradient reactor system for high-throughput investigation ofphosphonate-based inorganic–organic hybrid compounds, Angew. Chem. Int. Ed. 46 (2007) 6857–6860.[151] M. Moliner, J. Serra, A. Corma, E. Argente, S. Valero, V. Botti, Application of artificial neural networks to high-throughput synthesis of zeolites, Microporous Mesoporous Mater. 78 (2005) 73–81.[152] K. Sumida, S. Horike, S. S. Kaye, Z. R. Herm, W. L. Queen, C. M. Brown, F. Grandjean, G. J. Long, A. Dailly,J. R. Long, Hydrogen storage and carbon dioxide capture in an iron-based sodalite-type metal–organic framework(Fe-BTT) discovered via high-throughput methods, Chem. Sci. 1 (2010) 184.[153] A. D. Wiersum, C. Giovannangeli, D. Vincent, E. Bloch, H. Reinsch, N. Stock, J. S. Lee, J.-S. Chang, P. L. Llewellyn, xperimental screening of porous materials for high pressure gas adsorption and evaluation in gas separations: Ap-plication to MOFs (MIL-100 and CAU-10), ACS Comb. Sci. 15 (2013) 111–119.[154] P. Wollmann, M. Leistner, U. Stoeck, R. Grünker, K. Gedrich, N. Klein, O. Throl, W. Grählert, I. Senkovska,F. Dreisbach, S. Kaskel, High-throughput screening: speeding up porous materials discovery, Chem. Commun. 47(2011) 5151.[155] N. Stock, S. Biswas, Synthesis of metal-organic frameworks (MOFs): Routes to various MOF topologies, morphologies,and composites, Chem. Rev. 112 (2011) 933–969.[156] P. Maniam, N. Stock, Chapter 3. high-throughput methods for the systematic investigation of metal phosphonatesynthesis fields, in: Metal Phosphonate Chemistry, Royal Society of Chemistry, 2011, pp. 87–106.[157] H. Reinsch, M. A. van der Veen, B. Gil, B. Marszalek, T. Verbiest, D. de Vos, N. Stock, Structures, sorptioncharacteristics, and nonlinear optical properties of a new series of highly stable aluminum MOFs, Chem. Mater. 25(2012) 17–26.[158] D. Lenzen, P. Bendix, H. Reinsch, D. Fröhlich, H. Kummer, M. Möllers, P. P. C. Hügenell, R. Gläser, S. Henninger,N. Stock, Scalable green synthesis and full-scale test of the metal-organic framework CAU-10-h for use in adsorption-driven chillers, Adv. Mater. 30 (2017) 1705869.[159] A. Werner, M. Wöllner, P. Bludovsky, M. Leistner, C. Selzer, S. Kaskel, Rapid screening of zeolite acidity by thermalresponse measurements using InfraSORP technology, Microporous Mesoporous Mater. 268 (2018) 46–49.[160] U. Mueller, M. Schubert, F. Teich, H. Puetter, K. Schierle-Arndt, J. Pastré, Metal–organic frameworks—prospectiveindustrial applications, J. Mater. Chem. 16 (2006) 626–636.[161] M. Gaab, N. Trukhan, S. Maurer, R. Gummaraju, U. Müller, The progression of al-based metal-organic frameworks– from academic research to industrial production and applications, Microporous Mesoporous Mater. 157 (2012)131–136.[162] M. Rubio-Martinez, C. Avci-Camur, A. W. Thornton, I. Imaz, D. Maspoch, M. R. Hill, New synthetic routes towardsMOF production at scale, Chem. Soc. Rev. 46 (2017) 3453–3480.[163] H. Reinsch, “green” synthesis of metal-organic frameworks, Eur. J. Inorg. Chem 2016 (2016) 4290–4299.[164] K. Christoph, M. Ulrich, S. Markus, Metal organic frameworks based on aluminum fumarate, the preparation thereof,shaped bodies comprising such frameworks, and uses therefor, 2014. US 8734652 B2.[165] E. Alvarez, N. Guillou, C. Martineau, B. Bueken, B. Van de Voorde, C. Le Guillouzer, P. Fabry, F. Nouar, F. Taulelle,D. de Vos, J.-S. Chang, K. H. Cho, N. Ramsahye, T. Devic, M. Daturi, G. Maurin, C. Serre, The structure of thealuminum fumarate metal-organic framework A520, Angew. Chem. Int. Ed. 54 (2015) 3664–3668.[166] F. Jeremias, D. Fröhlich, C. Janiak, S. K. Henninger, Advancement of sorption-based heat transformation by a metalcoating of highly-stable, hydrophilic aluminium fumarate MOF, RSC Adv. 4 (2014) 24073–24082.[167] L. Emi, M. Ulrich, T. Natalia, M. Hendrick, C. Gerhard, B. Stefan, Process for preparing porous metal-organicframeworks based on aluminum fumarate, 2013. US 8524932 B2.[168] H. Reinsch, T. Homburg, N. Heidenreich, D. Fröhlich, S. Hennninger, M. Wark, N. Stock, Green synthesis of a newAl-MOF based on the aliphatic linker mesaconic acid: Structure, properties and in situ crystallisation studies ofAl-MIL-68-mes, Chem. Eur. J. 24 (2018) 2173–2181.[169] N. Heidenreich, A. Lieb, N. Stock, H. Reinsch, Green synthesis of a new layered aluminium citraconate: crystalstructures, intercalation behaviour towards H O and in situ PXRD studies of its crystallisation, Dalton Trans. 47(2018) 215–223.[170] H. Reinsch, S. Waitschat, S. M. Chavan, K. P. Lillerud, N. Stock, A facile “green” route for scalable batch productionand continuous synthesis of zirconium MOFs, Eur. J. Inorg. Chem 2016 (2016) 4490–4498.[171] H. Reinsch, D. Fröhlich, S. Waitschat, S. Chavan, K.-P. Lillerud, S. Henninger, N. Stock, Optimisation of synthesisconditions for UiO-66-CO P-C H -SO )(H O) (Ln = La-Gd), Inorg. Chem.49 (2010) 11158–11163.[189] T. Ahnfeldt, J. Moellmer, V. Guillerm, R. Staudt, C. Serre, N. Stock, High-throughput and time-resolved energy-dispersive x-ray diffraction (EDXRD) study of the formation of CAU-1-(OH) : Microwave and conventional heating,Chem. Eur. J. 17 (2011) 6462–6468.[190] M. Feyand, A. Hübner, A. Rothkirch, D. S. Wragg, N. Stock, Copper phosphonatoethanesulfonates: Temperaturedependent in situ energy dispersive x-ray diffraction study and influence of the pH on the crystal structures, Inorg.Chem. 51 (2012) 12540–12547.[191] T. Friščić, I. Halasz, P. J. Beldon, A. M. Belenguer, F. Adams, S. A. Kimber, V. Honkimäki, R. E. Dinnebier,Real-time and in situ monitoring of mechanochemical milling reactions, Nat. Chem. 5 (2013) 145–145.[192] L. Batzdorf, F. Fischer, M. Wilke, K.-J. Wenzel, F. Emmerling, Direct in situ investigation of milling reactions usingcombined x-ray diffraction and raman spectroscopy, Angew. Chem. Int. Ed. 54 (2014) 1799–1802.[193] J. Cravillon, C. A. Schröder, R. Nayuk, J. Gummel, K. Huber, M. Wiebcke, Fast nucleation and growth of ZIF-8nanocrystals monitored by time-resolved in situ small-angle and wide-angle x-ray scattering, Angew. Chem. Int. Ed.50 (2011) 8067–8071.[194] M. Goesten, E. Stavitski, E. A. Pidko, C. Gücüyener, B. Boshuizen, S. N. Ehrlich, E. J. M. Hensen, F. Kapteijn,J. Gascon, The molecular pathway to ZIF-7 microrods revealed by in situ time-resolved small- and wide-angle x-rayscattering, quick-scanning extended x-ray absorption spectroscopy, and DFT calculations, Chem. Eur. J. 19 (2013)7809–7816.[195] E. Stavitski, M. Goesten, J. Juan-Alcañiz, A. Martinez-Joaristi, P. Serra-Crespo, A. V. Petukhov, J. Gascon,F. Kapteijn, Kinetic control of metal-organic framework crystallization investigated by time-resolved in situ x-rayscattering, Angew. Chem. Int. Ed. 50 (2011) 9624–9628.[196] N. Heidenreich, U. Rütt, M. Köppen, A. K. Inge, S. Beier, A.-C. Dippel, R. Suren, N. Stock, A multi-purpose reactioncell for the investigation of reactions under solvothermal conditions, Rev. Sci. Instrum 88 (2017) 104102.[197] S. R. Sushrutha, S. Natarajan, Bismuth carboxylates with brucite- and fluorite-related structures: Synthesis structureand properties, Cryst. Growth Des. 13 (2013) 1743–1751.[198] N. Yuan, V. Pascanu, Z. Huang, A. Valiente, N. Heidenreich, S. Leubner, A. K. Inge, J. Gaar, N. Stock, I. Persson,B. Martín-Matute, X. Zou (2018) Under revision.[199] H. Reinsch, J. Benecke, M. Etter, N. Heidenreich, N. Stock, Combined in- and ex situ studies of pyrazine adsorptioninto the aliphatic MOF Al-CAU-13: structures, dynamics and correlations, Dalton Trans. 46 (2017) 1397–1405.[200] Y. Kubota, M. Takata, R. Matsuda, R. Kitaura, S. Kitagawa, K. Kato, M. Sakata, T. C. Kobayashi, Direct observationof hydrogen molecules adsorbed onto a microporous coordination polymer, Angew. Chem. Int. Ed. 44 (2005) 920–923.[201] P. L. Llewellyn, G. Maurin, T. Devic, S. Loera-Serna, N. Rosenbach, C. Serre, S. Bourrelly, P. Horcajada, Y. Filinchuk,G. Férey, Prediction of the conditions for breathing of metal organic framework materials using a combination ofx-ray powder diffraction, microcalorimetry, and molecular simulation, J. Am. Chem. Soc. 130 (2008) 12808–12814.[202] L. Hamon, P. L. Llewellyn, T. Devic, A. Ghoufi, G. Clet, V. Guillerm, G. D. Pirngruber, G. Maurin, C. Serre,G. Driver, W. van Beek, E. Jolimaître, A. Vimont, M. Daturi, G. Férey, Co-adsorption and separation of CO -CH mixtures in the highly flexible MIL-53(Cr) MOF, J. Am. Chem. Soc. 131 (2009) 17490–17499.[203] N. Klein, C. Herzog, M. Sabo, I. Senkovska, J. Getzschmann, S. Paasch, M. R. Lohe, E. Brunner, S. Kaskel, Monitoringadsorption-induced switching by 129xe NMR spectroscopy in a new metal–organic framework Ni (2,6-ndc) (dabco),Phys. Chem. Chem. Phys. 12 (2010) 11778.[204] V. Bon, I. Senkovska, D. Wallacher, A. Heerwig, N. Klein, I. Zizak, R. Feyerherm, E. Dudzik, S. Kaskel, In situmonitoring of structural changes during the adsorption on flexible porous coordination polymers by x-ray powderdiffraction: Instrumentation and experimental results, Microporous Mesoporous Mater. 188 (2014) 190–195.[205] L. Borchardt, W. Nickel, M. Casco, I. Senkovska, V. Bon, D. Wallacher, N. Grimm, S. Krause, J. Silvestre-Albero,Illuminating solid gas storage in confined spaces – methane hydrate formation in porous model carbons, Phys. Chem.Chem. Phys. 18 (2016) 20607–20614.[206] V. Bon, N. Klein, I. Senkovska, A. Heerwig, J. Getzschmann, D. Wallacher, I. Zizak, M. Brzhezinskaya, U. Mueller,S. Kaskel, Exceptional adsorption-induced cluster and network deformation in the flexible metal–organic frameworkDUT-8(Ni) observed by in situ x-ray diffraction and EXAFS, Phys. Chem. Chem. Phys. 17 (2015) 17471–17479.[207] F. Niekiel, J. Lannoeye, H. Reinsch, A. S. Munn, A. Heerwig, I. Zizak, S. Kaskel, R. I. Walton, D. de Vos, P. Llewellyn,A. Lieb, G. Maurin, N. Stock, Conformation-controlled sorption properties and breathing of the aliphatic Al-MOF[Al(OH)(CDC)], Inorg. Chem. 53 (2014) 4610–4620.[208] V. Bon, J. Pallmann, E. Eisbein, H. C. Hoffmann, I. Senkovska, I. Schwedler, A. Schneemann, S. Henke, D. Wallacher,R. A. Fischer, G. Seifert, E. Brunner, S. Kaskel, Characteristics of flexibility in metal-organic framework solid solutionsof composition [Zn (BME-bdc) x (DB-bdc) -xdabco] n : In situ powder x-ray diffraction, in situ NMR spectroscopy, andmolecular dynamics simulations, Microporous Mesoporous Mater. 216 (2015) 64–74.[209] V. Bon, I. Senkovska, D. Wallacher, D. M. Többens, I. Zizak, R. Feyerherm, U. Mueller, S. Kaskel, In situ obser-vation of gating phenomena in the flexible porous coordination polymer Zn (BPnDC) (bpy) (SNU-9) in a combineddiffraction and gas adsorption experiment, Inorg. Chem. 53 (2014) 1513–1520.[210] S. Krause, V. Bon, I. Senkovska, U. Stoeck, D. Wallacher, D. M. Többens, S. Zander, R. S. Pillai, G. Maurin, F.-X. oudert, S. Kaskel, A pressure-amplifying framework material with negative gas adsorption transitions, Nature 532(2016) 348–352.[211] S. Krause, V. Bon, I. Senkovska, D. M. Többens, D. Wallacher, R. S. Pillai, G. Maurin, S. Kaskel, The effect ofcrystallite size on pressure amplification in switchable porous solids, Nat. Commun. 9 (2018).[212] S. Henke, D. C. F. Wieland, M. Meilikhov, M. Paulus, C. Sternemann, K. Yusenko, R. A. Fischer, Multiple phase-transitions upon selective CO adsorption in an alkyl ether functionalized metal–organic framework—an in situ x-raydiffraction study, CrystEngComm 13 (2011) 6399.[213] M. Lange, M. Kobalz, J. Bergmann, D. Lässig, J. Lincke, J. Möllmer, A. Möller, J. Hofmann, H. Krautscheid,R. Staudt, R. Gläser, Structural flexibility of a copper-based metal–organic framework: sorption of c4-hydrocarbonsand in situ XRD, J. Mater. Chem. A 2 (2014) 8075–8085.[214] U. Ravon, M. Savonnet, S. Aguado, M. E. Domine, E. Janneau, D. Farrusseng, Engineering of coordination polymersfor shape selective alkylation of large aromatics and the role of defects, Microporous Mesoporous Mater. 129 (2010)319–329.[215] F. Vermoortele, B. Bueken, G. L. Bars, B. V. de Voorde, M. Vandichel, K. Houthoofd, A. Vimont, M. Daturi,M. Waroquier, V. V. Speybroeck, C. Kirschhock, D. E. D. Vos, Synthesis modulation as a tool to increase thecatalytic activity of metal–organic frameworks: The unique case of UiO-66(Zr), J. Am. Chem. Soc. 135 (2013)11465–11468.[216] O. Karagiaridi, M. B. Lalonde, W. Bury, A. A. Sarjeant, O. K. Farha, J. T. Hupp, Opening ZIF-8: A catalyticallyactive zeolitic imidazolate framework of sodalite topology with unsubstituted linkers, J. Am. Chem. Soc. 134 (2012)18790–18796.[217] F. Cirujano, A. Corma, F. L. i Xamena, Conversion of levulinic acid into chemicals: Synthesis of biomass derivedlevulinate esters over Zr-containing MOFs, Chem. Eng. Sci. 124 (2015) 52–60.[218] O. Kozachuk, I. Luz, F. X. Llabrés i Xamena, H. Noei, M. Kauer, H. B. Albada, E. D. Bloch, B. Marler, Y. Wang,M. Muhler, R. A. Fischer, Multifunctional, defect-engineered metal-organic frameworks with ruthenium centers:Sorption and catalytic properties, Angew. Chem. Int. Ed. 53 (2014) 7058–7062.[219] G. C. Shearer, S. Chavan, S. Bordiga, S. Svelle, U. Olsbye, K. P. Lillerud, Defect engineering: Tuning the porosity andcomposition of the metal–organic framework UiO-66 via modulated synthesis, Chem. Mater. 28 (2016) 3749–3761.[220] M. J. Cliffe, W. Wan, X. Zou, P. A. Chater, A. K. Kleppe, M. G. Tucker, H. Wilhelm, N. P. Funnell, F.-X. Coudert,A. L. Goodwin, Correlated defect nanoregions in a metal–organic framework, Nat. Commun. 5 (2014).[221] S. Dissegna, K. Epp, W. R. Heinz, G. Kieslich, R. A. Fischer, Defective metal-organic frameworks, Adv. Mater.(2018) 1704501.[222] A. W. Thornton, R. Babarao, A. Jain, F. Trousselet, F.-X. Coudert, Defects in metal–organic frameworks: acompromise between adsorption and stability?, Dalton Trans. 45 (2016) 4352–4359.[223] S. M. J. Rogge, J. Wieme, L. Vanduyfhuys, S. Vandenbrande, G. Maurin, T. Verstraelen, M. Waroquier, V. V.Speybroeck, Thermodynamic insight in the high-pressure behavior of UiO-66: Effect of linker defects and linkerexpansion, Chem. Mater. 28 (2016) 5721–5732.[224] J. P. Dürholt, J. Keupp, , R. Schmid, The impact of mesopores on the mechanical stability of HKUST-1: A multiscaleinvestigation, Eur. J. Inorg. Chem 2016 (2016) 4517–4523.[225] S. S. Mondal, S. Dey, A. G. Attallah, R. Krause-Rehberg, C. Janiak, H.-J. Holdt, Insights into the pores of microwave-assisted metal–imidazolate frameworks showing enhanced gas sorption, Dalton Trans. 46 (2017) 4824–4833.[226] S. S. Mondal, S. Dey, A. G. Attallah, A. Bhunia, A. Kelling, U. Schilde, R. Krause-Rehberg, C. Janiak, H.-J.Holdt, Missing building blocks defects in a porous hydrogen-bonded amide-imidazolate network proven by positronannihilation lifetime spectroscopy, ChemistrySelect 1 (2016) 4320–4325.[227] W. Zhang, M. Kauer, O. Halbherr, K. Epp, P. Guo, M. I. Gonzalez, D. J. Xiao, C. Wiktor, F. X. LIabrés i Xamena,C. Wöll, Y. Wang, M. Muhler, R. A. Fischer, Ruthenium metal-organic frameworks with different defect types:Influence on porosity, sorption, and catalytic properties, Chem. Eur. J. 22 (2016) 14297–14307.[228] R. C. Klet, Y. Liu, T. C. Wang, J. T. Hupp, O. K. Farha, Evaluation of brønsted acidity and proton topology inZr- and Hf-based metal–organic frameworks using potentiometric acid–base titration, J. Mater. Chem. A 4 (2016)1479–1485.[229] J. Canivet, A. Fateeva, Y. Guo, B. Coasne, D. Farrusseng, Water adsorption in MOFs: fundamentals and applications,Chem. Soc. Rev. 43 (2014) 5594–5617.[230] M. Liu, A. G. Wong-Foy, R. S. Vallery, W. E. Frieze, J. K. Schnobrich, D. W. Gidley, A. J. Matzger, Evolutionof nanoscale pore structure in coordination polymers during thermal and chemical exposure revealed by positronannihilation, Adv. Mater. 22 (2010) 1598–1601.[231] M. Mendt, F. Gutt, N. Kavoosi, V. Bon, I. Senkovska, S. Kaskel, A. Pöppl, EPR insights into switchable and rigidderivatives of the metal–organic framework DUT-8(Ni) by NO adsorption, J. Phys. Chem. C 120 (2016) 14246–14259.[232] S. Dissegna, R. Hardian, K. Epp, G. Kieslich, M.-V. Coulet, P. Llewellyn, R. A. Fischer, Using water adsorptionmeasurements to access the chemistry of defects in the metal–organic framework UiO-66, CrystEngComm 19 (2017)4137–4141.[233] S. I. Brückner, J. Pallmann, E. Brunner, Nuclear magnetic resonance of metal-organic frameworks (MOFs), in: TheChemistry of Metal-Organic Frameworks: Synthesis, Characterization, and Applications, Wiley-VCH Verlag GmbH& Co. KGaA, 2016, pp. 607–628.[234] H. Hoffmann, M. Debowski, P. Müller, S. Paasch, I. Senkovska, S. Kaskel, E. Brunner, Solid-state NMR spectroscopyof metal–organic framework compounds (MOFs), Materials 5 (2012) 2537–2572.[235] K. Trepte, J. Schaber, S. Schwalbe, F. Drache, I. Senkovska, S. Kaskel, J. Kortus, E. Brunner, G. Seifert, The originof the measured chemical shift of Xe in UiO-66 and UiO-67 revealed by DFT investigations, Phys. Chem. Chem.Phys. 19 (2017) 10020–10027.[236] W. Böhlmann, A. Pöppl, M. Sabo, S. Kaskel, Characterization of the metal-organic framework compoundCu (benzene 1,3,5-tricarboxylate) by means of Xe nuclear magnetic and electron paramagnetic resonance spec-troscopy, J. Phys. Chem. B 110 (2006) 20177–20181.[237] H. C. Hoffmann, B. Assfour, F. Epperlein, N. Klein, S. Paasch, I. Senkovska, S. Kaskel, G. Seifert, E. Brunner, High-pressure in situ129xe NMR spectroscopy and computer simulations of breathing transitions in the metal–organicframework Ni (2,6-ndc) (dabco) (DUT-8(Ni)), J. Am. Chem. Soc. 133 (2011) 8681–8690. Xe NMR spectroscopy, J. Phys. Chem. C 121 (2017) 5195–5200.[239] M. Kondo, T. Okubo, A. Asami, S. ichiro Noro, T. Yoshitomi, S. Kitagawa, T. Ishii, H. Matsuzaka, K. Seki, Rationalsynthesis of stable channel-like cavities with methane gas adsorption properties: [ { Cu (pzdc) (L) } n ] (pzdc=pyrazine-2,3-dicarboxylate, L=a pillar ligand), Angew. Chem. Int. Ed. 38 (1999) 140–143.[240] M. Kondo, T. Yoshitomi, H. Matsuzaka, S. Kitagawa, K. Seki, Three-dimensional framework with channeling cavitiesfor small molecules: { [M (4, 4 (cid:48) -bpy) (NO ) ] · xH O } n (M = Co, Ni, Zn), Angew. Chem. Int. Ed. 36 (1997) 1725–1727.[241] N. L. Rosi, Hydrogen storage in microporous metal-organic frameworks, Science 300 (2003) 1127–1129.[242] B. Panella, M. Hirscher, H. Pütter, U. Müller, Hydrogen adsorption in metal–organic frameworks: Cu-MOFs andZn-MOFs compared, Adv. Funct. Mater. 16 (2006) 520–524.[243] B. Panella, M. Hirscher, Hydrogen physisorption in metal-organic porous crystals, Adv. Mater. 17 (2005) 538–541.[244] M. Schlichtenmayer, M. Hirscher, Nanosponges for hydrogen storage, J. Mater. Chem. 22 (2012) 10134.[245] M. Hirscher, Hydrogen storage by cryoadsorption in ultrahigh-porosity metal-organic frameworks, Angew. Chem.Int. Ed. 50 (2010) 581–582.[246] A. F. Kloutse, R. Zacharia, D. Cossement, R. Chahine, R. Balderas-Xicohténcatl, H. Oh, B. Streppel, M. Schlicht-enmayer, M. Hirscher, Isosteric heat of hydrogen adsorption on MOFs: comparison between adsorption calorimetry,sorption isosteric method, and analytical models, Appl. Phys. A 121 (2015) 1417–1424.[247] M. Schlichtenmayer, M. Hirscher, The usable capacity of porous materials for hydrogen storage, Appl. Phys. A 122(2016).[248] K. Gedrich, I. Senkovska, N. Klein, U. Stoeck, A. Henschel, M. R. Lohe, I. A. Baburin, U. Mueller, S. Kaskel, Ahighly porous metal-organic framework with open nickel sites, Angew. Chem. Int. Ed. 49 (2010) 8489–8492.[249] R. Grünker, V. Bon, P. Müller, U. Stoeck, S. Krause, U. Mueller, I. Senkovska, S. Kaskel, A new metal–organicframework with ultra-high surface area, Chem. Commun. 50 (2014) 3450.[250] U. Stoeck, I. Senkovska, V. Bon, S. Krause, S. Kaskel, Assembly of metal–organic polyhedra into highly porousframeworks for ethene delivery, Chem. Commun. 51 (2015) 1046–1049.[251] Y. He, F. Chen, B. Li, G. Qian, W. Zhou, B. Chen, Porous metal–organic frameworks for fuel storage, Coord. Chem.Rev. (2017).[252] A. U. Czaja, N. Trukhan, U. Müller, Industrial applications of metal–organic frameworks, Chem. Soc. Rev. 38 (2009)1284.[253] H. Oh, S. Maurer, R. Balderas-Xicohtencatl, L. Arnold, O. V. Magdysyuk, G. Schütz, U. Müller, M. Hirscher, Efficientsynthesis for large-scale production and characterization for hydrogen storage of ligand exchanged MOF-74/174/184-M (M = Mg , Ni ), Int. J. Hydrog. Energy 42 (2017) 1027–1035.[254] H. Furukawa, U. Müller, O. M. Yaghi, “heterogeneity within order” in metal-organic frameworks, Angew. Chem. Int.Ed. 54 (2015) 3417–3430.[255] M. Fischer, F. Hoffmann, M. Fröba, Preferred hydrogen adsorption sites in various MOFs-a comparative computationalstudy, ChemPhysChem 10 (2009) 2647–2657.[256] M. Fischer, B. Kuchta, L. Firlej, F. Hoffmann, M. Fröba, Accurate prediction of hydrogen adsorption in metal-organicframeworks with unsaturated metal sites via a combined density-functional theory and molecular mechanics approach,J. Phys. Chem. C 114 (2010) 19116–19126.[257] M. Fischer, F. Hoffmann, M. Fröba, Molecular simulation of hydrogen adsorption in metal-organic frameworks,Colloids Surf. A 357 (2010) 35–42.[258] S. E. Wenzel, M. Fischer, F. Hoffmann, M. Fröba, Highly porous metal-organic framework containing a novelorganosilicon linker - a promising material for hydrogen storage, Inorg. Chem. 48 (2009) 6559–6565.[259] D. Frahm, M. Fischer, F. Hoffmann, M. Fröba, An interpenetrated metal–organic framework and its gas storagebehavior: Simulation and experiment, Inorg. Chem. 50 (2011) 11055–11063.[260] J. Bergmann, K. Stein, M. Kobalz, M. Handke, M. Lange, J. Möllmer, F. Heinke, O. Oeckler, R. Gläser, R. Staudt,H. Krautscheid, A series of isomorphous metal-organic frameworks with rtl topology – metal distribution and tunablesorption capacity via substitution of metal ions, Microporous Mesoporous Mater. 216 (2015) 56–63.[261] M. Kobalz, J. Lincke, K. Kobalz, O. Erhart, J. Bergmann, D. Lässig, M. Lange, J. Möllmer, R. Gläser, R. Staudt,H. Krautscheid, Paddle wheel based triazolyl isophthalate MOFs: Impact of linker modification on crystal structureand gas sorption properties, Inorg. Chem. 55 (2016) 3030–3039.[262] U. Junghans, M. Kobalz, O. Erhart, H. Preißler, J. Lincke, J. Möllmer, H. Krautscheid, R. Gläser, A series of robustcopper-based triazolyl isophthalate MOFs: Impact of linker functionalization on gas sorption and catalytic activity †,Materials 10 (2017) 338.[263] S. Horike, S. Shimomura, S. Kitagawa, Soft porous crystals, Nat. Chem. 1 (2009) 695–704.[264] A. Schneemann, V. Bon, I. Schwedler, I. Senkovska, S. Kaskel, R. A. Fischer, Flexible metal–organic frameworks,Chem. Soc. Rev. 43 (2014) 6062–6096.[265] N. Klein, H. C. Hoffmann, A. Cadiau, J. Getzschmann, M. R. Lohe, S. Paasch, T. Heydenreich, K. Adil, I. Senkovska,E. Brunner, S. Kaskel, Structural flexibility and intrinsic dynamics in the M (2,6-ndc) (dabco) (M = Ni, Cu, Co,Zn) metal–organic frameworks, J. Mater. Chem. 22 (2012) 10303.[266] S. Henke, A. Schneemann, R. A. Fischer, Massive anisotropic thermal expansion and thermo-responsive breathing inmetal-organic frameworks modulated by linker functionalization, Adv. Funct. Mater. 23 (2013) 5990–5996.[267] S. Henke, A. Schneemann, A. Wütscher, R. A. Fischer, Directing the breathing behavior of pillared-layeredmetal–organic frameworks via a systematic library of functionalized linkers bearing flexible substituents, J. Am.Chem. Soc. 134 (2012) 9464–9474.[268] A. Schneemann, P. Vervoorts, I. Hante, M. Tu, S. Wannapaiboon, C. Sternemann, M. Paulus, D. F. Wieland, S. Henke,R. A. Fischer, Different breathing mechanisms in flexible pillared-layered metal–organic frameworks: Impact of themetal center, Chem. Mater. 30 (2018) 1667–1676.[269] S. Krause, V. Bon, U. Stoeck, I. Senkovska, D. M. Többens, D. Wallacher, S. Kaskel, A stimuli-responsive zirconiummetal-organic framework based on supermolecular design, Angew. Chem. Int. Ed. 56 (2017) 10676–10680.[270] J. D. Evans, L. Bocquet, F.-X. Coudert, Origins of negative gas adsorption, Chem 1 (2016) 873–886.[271] A. Krylov, A. Vtyurin, P. Petkov, I. Senkovska, M. Maliuta, V. Bon, T. Heine, S. Kaskel, E. Slyusareva, Raman pectroscopy studies of the terahertz vibrational modes of a DUT-8(Ni) metal–organic framework, Phys. Chem.Chem. Phys. 19 (2017) 32099–32104.[272] A. K. Rappe, C. J. Casewit, K. S. Colwell, W. A. Goddard, W. M. Skiff, UFF, a full periodic table force field formolecular mechanics and molecular dynamics simulations, J. Am. Chem. Soc. 114 (1992) 10024–10035.[273] M. A. Addicoat, N. Vankova, I. F. Akter, T. Heine, Extension of the universal force field to metal–organic frameworks,J. Chem. Theory Comput. 10 (2014) 880–891.[274] S. Bureekaew, S. Amirjalayer, M. Tafipolsky, C. Spickermann, T. K. Roy, R. Schmid, MOF-FF - a flexible first-principles derived force field for metal-organic frameworks, Phys. Status Solidi B 250 (2013) 1128–1141.[275] P. Aptel, J. Armor, R. Audinos, R. Baker, R. Bakish, G. Belfort, B. Bikson, R. Brown, M. Bryk, J. Burke, I. Cabasso,R. Chern, M. Cheryan, E. Cussler, R. Davis, Terminology for membranes and membrane processes (IUPAC recom-mendation 1996), J. Memb. Sci. 120 (1996) 149–159.[276] V. Abetz, T. Brinkmann, M. Dijkstra, K. Ebert, D. Fritsch, K. Ohlrogge, D. Paul, K.-V. Peinemann, S. Pereira-Nunes, N. Scharnagl, M. Schossig, Developments in membrane research: from material via process design to industrialapplication, Adv. Eng. Mater. 8 (2006) 328–358.[277] W. J. Koros, R. Mahajan, Pushing the limits on possibilities for large scale gas separation: which strategies?, J.Memb. Sci. 175 (2000) 181–196.[278] J. C. Davis, R. J. Valus, R. Eshraghi, A. E. Velikoff, Facilitated transport membrane hybrid systems for olefinpurification, Sep. Sci. Technol. 28 (1993) 463–476.[279] X. He, M.-B. Hägg, Membranes for environmentally friendly energy processes, Membranes 2 (2012) 706–726.[280] S. Mukherjee, A. V. Desai, S. K. Ghosh, Potential of metal–organic frameworks for adsorptive separation of industriallyand environmentally relevant liquid mixtures, Coord. Chem. Rev. 367 (2018) 82–126.[281] L. M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Memb. Sci. 62(1991) 165–185.[282] H. B. T. Jeazet, C. Staudt, C. Janiak, Metal–organic frameworks in mixed-matrix membranes for gas separation,Dalton Trans. 41 (2012) 14003.[283] Y. Huang, T. C. Merkel, R. W. Baker, Pressure ratio and its impact on membrane gas separation processes, J. Memb.Sci. 463 (2014) 33–40.[284] T. C. Merkel, H. Lin, X. Wei, R. Baker, Power plant post-combustion carbon dioxide capture: An opportunity formembranes, J. Memb. Sci. 359 (2010) 126–139.[285] L. M. Robeson, The upper bound revisited, J. Memb. Sci. 320 (2008) 390–400.[286] C. M. Zimmerman, A. Singh, W. J. Koros, Tailoring mixed matrix composite membranes for gas separations, J.Memb. Sci. 137 (1997) 145–154.[287] B. Zornoza, A. Martinez-Joaristi, P. Serra-Crespo, C. Tellez, J. Coronas, J. Gascon, F. Kapteijn, Functionalizedflexible MOFs as fillers in mixed matrix membranes for highly selective separation of CO from CH at elevatedpressures, Chem. Commun. 47 (2011) 9522.[288] T. C. Merkel, Ultrapermeable, reverse-selective nanocomposite membranes, Science 296 (2002) 519–522.[289] H. Bux, F. Liang, Y. Li, J. Cravillon, M. Wiebcke, J. Caro, Zeolitic imidazolate framework membrane with molecularsieving properties by microwave-assisted solvothermal synthesis, J. Am. Chem. Soc. 131 (2009) 16000–16001.[290] H. Bux, A. Feldhoff, J. Cravillon, M. Wiebcke, Y.-S. Li, J. Caro, Oriented zeolitic imidazolate framework-8 membranewith sharp H /C H molecular sieve separation, Chem. Mater. 23 (2011) 2262–2269.[291] Y. Liu, N. Wang, J. H. Pan, F. Steinbach, J. Caro, In situ synthesis of MOF membranes on ZnAl-CO LDH bufferlayer-modified substrates, J. Am. Chem. Soc. 136 (2014) 14353–14356.[292] Y. Liu, J. H. Pan, N. Wang, F. Steinbach, X. Liu, J. Caro, Remarkably enhanced gas separation by partial self-conversion of a laminated membrane to metal-organic frameworks, Angew. Chem. Int. Ed. 54 (2015) 3028–3032.[293] Y.-S. Li, F.-Y. Liang, H. Bux, A. Feldhoff, W.-S. Yang, J. Caro, Molecular sieve membrane: Supported metal-organicframework with high hydrogen selectivity, Angew. Chem. Int. Ed. 49 (2009) 548–551.[294] Y. Li, F. Liang, H. Bux, W. Yang, J. Caro, Zeolitic imidazolate framework ZIF-7 based molecular sieve membranefor hydrogen separation, J. Memb. Sci. 354 (2010) 48–54.[295] N. Wang, A. Mundstock, Y. Liu, A. Huang, J. Caro, Amine-modified Mg-MOF-74/CPO-27-Mg membrane withenhanced H /CO separation, Chem. Eng. Sci. 124 (2015) 27–36.[296] A. Huang, H. Bux, F. Steinbach, J. Caro, Molecular-sieve membrane with hydrogen permselectivity: ZIF-22 in LTAtopology prepared with 3-aminopropyltriethoxysilane as covalent linker, Angew. Chem. Int. Ed. 49 (2010) 4958–4961.[297] A. Huang, W. Dou, J. Caro, Steam-stable zeolitic imidazolate framework ZIF-90 membrane with hydrogen selectivitythrough covalent functionalization, J. Am. Chem. Soc. 132 (2010) 15562–15564.[298] A. Huang, N. Wang, C. Kong, J. Caro, Organosilica-functionalized zeolitic imidazolate framework ZIF-90 membranewith high gas-separation performance, Angew. Chem. Int. Ed. 51 (2012) 10551–10555.[299] H. Bux, C. Chmelik, R. Krishna, J. Caro, Ethene/ethane separation by the MOF membrane ZIF-8: Molecularcorrelation of permeation, adsorption, diffusion, J. Memb. Sci. 369 (2011) 284–289.[300] A. Knebel, L. Sundermann, A. Mohmeyer, I. Strauß, S. Friebe, P. Behrens, J. Caro, Azobenzene guest molecules aslight-switchable CO valves in an ultrathin UiO-67 membrane, Chem. Mater. 29 (2017) 3111–3117.[301] D. Bastani, N. Esmaeili, M. Asadollahi, Polymeric mixed matrix membranes containing zeolites as a filler for gasseparation applications: A review, J. Ind. Eng. Chem. 19 (2013) 375–393.[302] T.-S. Chung, L. Y. Jiang, Y. Li, S. Kulprathipanja, Mixed matrix membranes (MMMs) comprising organic polymerswith dispersed inorganic fillers for gas separation, Prog. Polym. Sci. 32 (2007) 483–507.[303] G. Dong, H. Li, V. Chen, Challenges and opportunities for mixed-matrix membranes for gas separation, J. Mater.Chem. A 1 (2013) 4610.[304] T. J. H. B., J. Christoph, Metal-Organic Frameworks: Frameworks in Mixed-Matrix Membranes, American CancerSociety, pp. 1–15.[305] B. Zornoza, C. Tellez, J. Coronas, J. Gascon, F. Kapteijn, Metal organic framework based mixed matrix membranes:An increasingly important field of research with a large application potential, Microporous Mesoporous Mater. 166(2013) 67–78.[306] A. Julbe, Zeolite membranes – synthesis, characterization and application, in: Studies in Surface Science andCatalysis, Elsevier, 2007, pp. 181–219. capture?, Chem. Soc. Rev. 44 (2015)2421–2454.[310] J. Dechnik, C. J. Sumby, C. Janiak, Enhancing mixed-matrix membrane performance with metal–organic frameworkadditives, Cryst. Growth Des. 17 (2017) 4467–4488.[311] J. Dechnik, J. Gascon, C. J. Doonan, C. Janiak, C. J. Sumby, Mixed-matrix membranes, Angew. Chem. Int. Ed. 56(2017) 9292–9310.[312] R. Semino, J. C. Moreton, N. A. Ramsahye, S. M. Cohen, G. Maurin, Understanding the origins of metal–organicframework/polymer compatibility, Chem. Sci. 9 (2018) 315–324.[313] H. B. T. Jeazet, C. Staudt, C. Janiak, A method for increasing permeability in O /N separation with mixed-matrixmembranes made of water-stable MIL-101 and polysulfone, Chem. Commun. 48 (2012) 2140.[314] R. W. Baker, Future directions of membrane gas separation technology, Ind. Eng. Chem. Res. 41 (2002) 1393–1411.[315] P. Bernardo, E. Drioli, G. Golemme, Membrane gas separation: A review/state of the art, Ind. Eng. Chem. Res. 48(2009) 4638–4663.[316] H. Jeazet, T. Koschine, C. Staudt, K. Raetzke, C. Janiak, Correlation of gas permeability in a metal-organic frameworkMIL-101(Cr)–polysulfone mixed-matrix membrane with free volume measurements by positron annihilation lifetimespectroscopy (PALS), Membranes 3 (2013) 331–353.[317] H. B. T. Jeazet, S. Sorribas, J. M. Román-Marín, B. Zornoza, C. Téllez, J. Coronas, C. Janiak, Increased selectivityin CO /CH separation with mixed-matrix membranes of polysulfone and mixed-MOFs MIL-101(Cr) and ZIF-8, Eur.J. Inorg. Chem 2016 (2016) 4363–4367.[318] B. Zornoza, B. Seoane, J. M. Zamaro, C. Téllez, J. Coronas, Combination of MOFs and zeolites for mixed-matrixmembranes, ChemPhysChem 12 (2011) 2781–2785.[319] E. V. Perez, K. J. Balkus, J. P. Ferraris, I. H. Musselman, Mixed-matrix membranes containing MOF-5 for gasseparations, J. Memb. Sci. 328 (2009) 165–173.[320] M. Arjmandi, M. Pakizeh, Mixed matrix membranes incorporated with cubic-MOF-5 for improved polyetherimidegas separation membranes: Theory and experiment, J. Ind. Eng. Chem. 20 (2014) 3857–3868.[321] J. Dechnik, A. Nuhnen, C. Janiak, Mixed-matrix membranes of the air-stable MOF-5 analogue [Co ( µ -O)(Me pzba) ] with a mixed-functional pyrazolate-carboxylate linker for CO /CH separation, Cryst. Growth Des.17 (2017) 4090–4099.[322] A. Sabetghadam, B. Seoane, D. Keskin, N. Duim, T. Rodenas, S. Shahid, S. Sorribas, C. L. Guillouzer, G. Clet,C. Tellez, M. Daturi, J. Coronas, F. Kapteijn, J. Gascon, Metal organic framework crystals in mixed-matrix mem-branes: Impact of the filler morphology on the gas separation performance, Adv. Funct. Mater. 26 (2016) 3154–3163.[323] W. P. Lustig, S. Mukherjee, N. D. Rudd, A. V. Desai, J. Li, S. K. Ghosh, Metal–organic frameworks: functionalluminescent and photonic materials for sensing applications, Chem. Soc. Rev. 46 (2017) 3242–3285.[324] L. V. Meyer, F. Schönfeld, K. Müller-Buschbaum, Lanthanide based tuning of luminescence in MOFs and denseframeworks – from mono- and multimetal systems to sensors and films, Chem. Commun. 50 (2014) 8093.[325] K. Müller-Buschbaum, F. Beuerle, C. Feldmann, MOF based luminescence tuning and chemical/physical sensing,Microporous Mesoporous Mater. 216 (2015) 171–199.[326] R. Grünker, V. Bon, A. Heerwig, N. Klein, P. Müller, U. Stoeck, I. A. Baburin, U. Mueller, I. Senkovska, S. Kaskel,Dye encapsulation inside a new mesoporous metal-organic framework for multifunctional solvatochromic-responsefunction, Chem. Eur. J. 18 (2012) 13299–13303.[327] L. V. Meyer, F. Schönfeld, A. Zurawski, M. Mai, C. Feldmann, K. Müller-Buschbaum, A blue luminescent MOF asa rapid turn-off/turn-on detector for H O, O and CH Cl , MeCN: 3 ∞ [Ce(Im) ImH] · ImH, Dalton Trans. 44 (2015)4070–4079.[328] T. Wehner, K. Mandel, M. Schneider, G. Sextl, K. Müller-Buschbaum, Superparamagnetic luminescentMOF@Fe O /SiO composite particles for signal augmentation by magnetic harvesting as potential water detec-tors, ACS Appl. Mater. Interfaces 8 (2016) 5445–5452.[329] T. Wehner, M. T. Seuffert, J. R. Sorg, M. Schneider, K. Mandel, G. Sextl, K. Müller-Buschbaum, Composite materialscombining multiple luminescent MOFs and superparamagnetic microparticles for ratiometric water detection, J.Mater. Chem. C 5 (2017) 10133–10142.[330] F. M. Hinterholzinger, B. Rühle, S. Wuttke, K. Karaghiosoff, T. Bein, Highly sensitive and selective fluoride detectionin water through fluorophore release from a metal-organic framework, Sci. Rep. 3 (2013).[331] M. Tu, S. Wannapaiboon, K. Khaletskaya, R. A. Fischer, Engineering zeolitic-imidazolate framework (ZIF) thin filmdevices for selective detection of volatile organic compounds, Adv. Funct. Mater. 25 (2015) 4470–4479.[332] A. Henschel, K. Gedrich, R. Kraehnert, S. Kaskel, Catalytic properties of MIL-101, Chem. Commun. (2008) 4192.[333] S. Biswas, M. Maes, A. Dhakshinamoorthy, M. Feyand, D. E. D. Vos, H. Garcia, N. Stock, Fuel purification, lewisacid and aerobic oxidation catalysis performed by a microporous Co-BTT (BTT − = 1,3,5-benzenetristetrazolate)framework having coordinatively unsaturated sites, J. Mater. Chem. 22 (2012) 10200.[334] G. Nickerl, U. Stoeck, U. Burkhardt, I. Senkovska, S. Kaskel, A catalytically active porous coordination polymerbased on a dinuclear rhodium paddle-wheel unit, J. Mater. Chem. A 2 (2014) 144–148.[335] A. Dhakshinamoorthy, N. Heidenreich, D. Lenzen, N. Stock, Knoevenagel condensation reaction catalysed by Al-MOFs with CAU-1 and CAU-10-type structures, CrystEngComm 19 (2017) 4187–4193.[336] N. Reimer, B. Bueken, S. Leubner, C. Seidler, M. Wark, D. De Vos, N. Stock, Three series of sulfo-functionalizedmixed-linker CAU-10 analogues: Sorption properties, proton conductivity, and catalytic activity, Chem. Eur. J. 21(2015) 12517–12524.[337] M. Feyand, E. Mugnaioli, F. Vermoortele, B. Bueken, J. M. Dieterich, T. Reimer, U. Kolb, D. de Vos, N. Stock,Automated diffraction tomography for the structure elucidation of twinned, sub-micrometer crystals of a highlyporous, catalytically active bismuth metal-organic framework, Angew. Chem. Int. Ed. 51 (2012) 10373–10376.[338] Y. Lu, M. Tonigold, B. Bredenkötter, D. Volkmer, J. Hitzbleck, G. Langstein, A cobalt(II)-containing metal-organic ramework showing catalytic activity in oxidation reactions, Z. Anorg. Allg. Chem. 634 (2008) 2411–2417.[339] M. Tonigold, Y. Lu, A. Mavrandonakis, A. Puls, R. Staudt, J. Möllmer, J. Sauer, D. Volkmer, Pyrazolate-basedcobalt(II)-containing metal-organic frameworks in heterogeneous catalytic oxidation reactions: Elucidating the roleof entatic states for biomimetic oxidation processes, Chem. Eur. J. 17 (2011) 8671–8695.[340] Y.-Y. Liu, K. Leus, M. Grzywa, D. Weinberger, K. Strubbe, H. Vrielinck, R. V. Deun, D. Volkmer, V. V. Speybroeck,P. V. D. Voort, Synthesis, structural characterization, and catalytic performance of a vanadium-based metal-organicframework (COMOC-3), Eur. J. Inorg. Chem 2012 (2011) 2819–2827.[341] M. Grzywa, C. Geßner, B. Bredenkötter, D. Denysenko, J. van Leusen, P. Kögerler, E. Klemm, D. Volkmer, Coordi-nation frameworks assembled from CuII ions and H -1,3-bdpb ligands: X-ray and magneto structural investigations,and catalytic activity in the aerobic oxidation of tetralin, Dalton Trans. 43 (2014) 16846–16856.[342] I. Luz, C. Rösler, K. Epp, F. X. L. i Xamena, R. A. Fischer, Pd@UiO-66-type MOFs prepared by chemical vaporinfiltration as shape-selective hydrogenation catalysts, Eur. J. Inorg. Chem 2015 (2015) 3904–3912.[343] C. Rösler, S. Dissegna, V. L. Rechac, M. Kauer, P. Guo, S. Turner, K. Ollegott, H. Kobayashi, T. Yamamoto,D. Peeters, Y. Wang, S. Matsumura, G. Van Tendeloo, H. Kitagawa, M. Muhler, F. X. Llabrés i Xamena, R. A.Fischer, Encapsulation of bimetallic metal nanoparticles into robust zirconium-based metal-organic frameworks:Evaluation of the catalytic potential for size-selective hydrogenation, Chem. Eur. J. 23 (2017) 3583–3594.[344] W. Zhang, Z. Chen, M. Al-Naji, P. Guo, S. Cwik, O. Halbherr, Y. Wang, M. Muhler, N. Wilde, R. Gläser, R. A.Fischer, Simultaneous introduction of various palladium active sites into MOF via one-pot synthesis: Pd@[Cu -xPd x (BTC) ] n , Dalton Trans. 45 (2016) 14883–14887.[345] F. Jeremias, D. Fröhlich, C. Janiak, S. K. Henninger, Water and methanol adsorption on MOFs for cycling heattransformation processes, New J. Chem. 38 (2014) 1846–1852.[346] S. K. Henninger, F. Jeremias, H. Kummer, C. Janiak, MOFs for use in adsorption heat pump processes, Eur. J.Inorg. Chem 2012 (2011) 2625–2634.[347] C. Janiak, S. K. Henninger, Porous coordination polymers as novel sorption materials for heat transformationprocesses, CHIMIA International Journal for Chemistry 67 (2013) 419–424.[348] I. S. Girnik, Y. I. Aristov, Dynamic optimization of adsorptive chillers: The “AQSOA™-FAM-Z02 – water” workingpair, Energy 106 (2016) 13–22.[349] Y. I. Aristov, Challenging offers of material science for adsorption heat transformation: A review, Appl. Therm. Eng.50 (2013) 1610–1618.[350] J. J. Low, A. I. Benin, P. Jakubczak, J. F. Abrahamian, S. A. Faheem, R. R. Willis, Virtual high throughput screeningconfirmed experimentally: Porous coordination polymer hydration, J. Am. Chem. Soc. 131 (2009) 15834–15842.[351] K. Leus, T. Bogaerts, J. D. Decker, H. Depauw, K. Hendrickx, H. Vrielinck, V. V. Speybroeck, P. V. D. Voort, Sys-tematic study of the chemical and hydrothermal stability of selected “stable” metal organic frameworks, MicroporousMesoporous Mater. 226 (2016) 110–116.[352] B. B. Saha, I. I. El-Sharkawy, T. Miyazaki, S. Koyama, S. K. Henninger, A. Herbst, C. Janiak, Ethanol adsorptiononto metal organic framework: Theory and experiments, Energy 79 (2015) 363–370.[353] H. A. Habib, J. Sanchiz, C. Janiak, Mixed-ligand coordination polymers from 1,2-bis(1,2,4-triazol-4-yl)ethane andbenzene-1,3,5-tricarboxylate: Trinuclear nickel or zinc secondary building units for three-dimensional networks withcrystal-to-crystal transformation upon dehydration, Dalton Trans. (2008) 1734.[354] S. K. Henninger, H. A. Habib, C. Janiak, MOFs as adsorbents for low temperature heating and cooling applications,J. Am. Chem. Soc. 131 (2009) 2776–2777.[355] J. Ehrenmann, S. K. Henninger, C. Janiak, Water adsorption characteristics of MIL-101 for heat-transformationapplications of MOFs, Eur. J. Inorg. Chem 2011 (2010) 471–474.[356] F. Jeremias, A. Khutia, S. K. Henninger, C. Janiak, MIL-100(Al, Fe) as water adsorbents for heat transformationpurposes—a promising application, J. Mater. Chem. 22 (2012) 10148–10151.[357] P. Küsgens, M. Rose, I. Senkovska, H. Fröde, A. Henschel, S. Siegle, S. Kaskel, Characterization of metal-organicframeworks by water adsorption, Microporous Mesoporous Mater. 120 (2009) 325–330.[358] G. Akiyama, R. Matsuda, S. Kitagawa, Highly porous and stable coordination polymers as water sorption materials,Chem. Lett. 39 (2010) 360–361.[359] S. Bernt, V. Guillerm, C. Serre, N. Stock, Direct covalent post-synthetic chemical modification of Cr-MIL-101 usingnitrating acid, Chem. Commun. 47 (2011) 2838.[360] A. Khutia, H. U. Rammelberg, T. Schmidt, S. Henninger, C. Janiak, Water sorption cycle measurements on func-tionalized MIL-101(Cr) for heat transformation application, Chem. Mater. 25 (2013) 790–798.[361] F. Jeremias, V. Lozan, S. K. Henninger, C. Janiak, Programming MOFs for water sorption: amino-functionalizedMIL-125 and UiO-66 for heat transformation and heat storage applications, Dalton Trans. 42 (2013) 15967.[362] S. Henninger, F. Schmidt, H.-M. Henning, Water adsorption characteristics of novel materials for heat transformationapplications, Appl. Therm. Eng. 30 (2010) 1692–1702.[363] D. Fröhlich, S. K. Henninger, C. Janiak, Multicycle water vapour stability of microporous breathing MOF aluminiumisophthalate CAU-10-H, Dalton Trans. 43 (2014) 15300–15304.[364] D. Fröhlich, E. Pantatosaki, P. D. Kolokathis, K. Markey, H. Reinsch, M. Baumgartner, M. A. van der Veen, D. E. D.Vos, N. Stock, G. K. Papadopoulos, S. K. Henninger, C. Janiak, Water adsorption behaviour of CAU-10-H: a thoroughinvestigation of its structure–property relationships, J. Mater. Chem. A 4 (2016) 11859–11869.[365] M. Wickenheisser, C. Janiak, Hierarchical embedding of micro-mesoporous MIL-101(Cr) in macroporous poly(2-hydroxyethyl methacrylate) high internal phase emulsions with monolithic shape for vapor adsorption applications,Microporous Mesoporous Mater. 204 (2015) 242–250.[366] M. Wickenheisser, T. Paul, C. Janiak, Prospects of monolithic MIL-MOF@poly(NIPAM)HIPE composites as watersorption materials, Microporous Mesoporous Mater. 220 (2016) 258–269.[367] M. Wickenheisser, A. Herbst, R. Tannert, B. Milow, C. Janiak, Hierarchical MOF-xerogel monolith composites fromembedding MIL-100(Fe,Cr) and MIL-101(Cr) in resorcinol-formaldehyde xerogels for water adsorption applications,Microporous Mesoporous Mater. 215 (2015) 143–153.[368] H. Kummer, F. Jeremias, A. Warlo, G. Füldner, D. Fröhlich, C. Janiak, R. Gläser, S. K. Henninger, A functional full-scale heat exchanger coated with aluminum fumarate metal–organic framework for adsorption heat transformation,Ind. Eng. Chem. Res. 56 (2017) 8393–8398. -terminated self-assembled monolayers on Au(111), J. Am. Chem. Soc.127 (2005) 13744–13745.[378] E. Biemmi, C. Scherb, T. Bein, Oriented growth of the metal organic framework Cu (BTC) (H O) · xH O tunablewith functionalized self-assembled monolayers, J. Am. Chem. Soc. 129 (2007) 8054–8055.[379] E. Biemmi, A. Darga, N. Stock, T. Bein, Direct growth of Cu (BTC) (H O) · xH O thin films on modified QCM-goldelectrodes – water sorption isotherms, Microporous Mesoporous Mater. 114 (2008) 380–386.[380] O. Shekhah, H. Wang, S. Kowarik, F. Schreiber, M. Paulus, M. Tolan, C. Sternemann, F. Evers, D. Zacher, R. A.Fischer, C. Wöll, Step-by-step route for the synthesis of metal-organic frameworks, J. Am. Chem. Soc. 129 (2007)15118–15119.[381] A. Bétard, R. A. Fischer, Metal–organic framework thin films: From fundamentals to applications., Chem. Rev. 112(2011) 1055–1083.[382] A. Gölzhäuser, C. Wöll, Interfacial systems chemistry: Out of the vacuum-through the liquid-into the cell,ChemPhysChem 11 (2010) 3201–3213.[383] O. Shekhah, H. Wang, M. Paradinas, C. Ocal, B. Schüpbach, A. Terfort, D. Zacher, R. A. Fischer, C. Wöll, Controllinginterpenetration in metal–organic frameworks by liquid-phase epitaxy, Nat. Mater. 8 (2009) 481–484.[384] M. Allendorf, A. Bétard, R. A. Fischer, Deposition of thin films for sensor applications, in: Metal-Organic Frameworks,Wiley-VCH Verlag GmbH & Co. KGaA, 2011, pp. 309–335.[385] J. Liu, C. Wöll, Surface-supported metal–organic framework thin films: fabrication methods, applications, andchallenges, Chem. Soc. Rev. 46 (2017) 5730–5770.[386] B. Liu, M. Tu, R. A. Fischer, Metal-organic framework thin films: Crystallite orientation dependent adsorption,Angew. Chem. Int. Ed. 52 (2013) 3402–3405.[387] B. Liu, R. A. Fischer, Liquid-phase epitaxy of metal organic framework thin films, Science China Chemistry 54(2011) 1851–1866.[388] M. Tu, R. A. Fischer, Heteroepitaxial growth of surface mounted metal–organic framework thin films with hybridadsorption functionality, J. Mater. Chem. A 2 (2014) 2018–2022.[389] D. Zacher, K. Yusenko, A. Bétard, S. Henke, M. Molon, T. Ladnorg, O. Shekhah, B. Schüpbach, T. de los Arcos,M. Krasnopolski, M. Meilikhov, J. Winter, A. Terfort, C. Wöll, R. A. Fischer, Liquid-phase epitaxy of multicomponentlayer-based porous coordination polymer thin films of [M(L)(P) . ] type: Importance of deposition sequence on theoriented growth, Chem. Eur. J. 17 (2011) 1448–1455.[390] O. Shekhah, K. Hirai, H. Wang, H. Uehara, M. Kondo, S. Diring, D. Zacher, R. A. Fischer, O. Sakata, S. Kita-gawa, S. Furukawa, C. Wöll, MOF-on-MOF heteroepitaxy: perfectly oriented [Zn (ndc) (dabco)] n grown on[Cu (ndc) (dabco)] n thin films, Dalton Trans. 40 (2011) 4954.[391] M. Hanke, H. K. Arslan, S. Bauer, O. Zybaylo, C. Christophis, H. Gliemann, A. Rosenhahn, C. Wöll, The biocom-patibility of metal–organic framework coatings: An investigation on the stability of SURMOFs with regard to waterand selected cell culture media, Langmuir 28 (2012) 6877–6884.[392] M. P. A. Sancet, M. Hanke, Z. Wang, S. Bauer, C. Azucena, H. K. Arslan, M. Heinle, H. Gliemann, C. Wöll,A. Rosenhahn, Surface anchored metal-organic frameworks as stimulus responsive antifouling coatings, Biointerphases8 (2013) 29.[393] H. C. Streit, M. Adlung, O. Shekhah, X. Stammer, H. K. Arslan, O. Zybaylo, T. Ladnorg, H. Gliemann, M. Franzreb,C. Wöll, C. Wickleder, Surface-anchored MOF-based photonic antennae, ChemPhysChem 13 (2012) 2699–2702.[394] K. Müller, J. Wadhwa, J. S. Malhi, L. Schöttner, A. Welle, H. Schwartz, D. Hermann, U. Ruschewitz, L. Heinke,Photoswitchable nanoporous films by loading azobenzene in metal–organic frameworks of type HKUST-1, Chem.Commun. 53 (2017) 8070–8073.[395] A. Dragässer, O. Shekhah, O. Zybaylo, C. Shen, M. Buck, C. Wöll, D. Schlettwein, Redox mediation enabled byimmobilised centres in the pores of a metal–organic framework grown by liquid phase epitaxy, Chem. Commun. 48(2012) 663–665.[396] Z. Wang, J. Liu, H. K. Arslan, S. Grosjean, T. Hagendorn, H. Gliemann, S. Bräse, C. Wöll, Post-synthetic modificationof metal–organic framework thin films using click chemistry: The importance of strained C–C triple bonds, Langmuir29 (2013) 15958–15964.[397] S. Schmitt, M. Silvestre, M. Tsotsalas, A.-L. Winkler, A. Shahnas, S. Grosjean, F. Laye, H. Gliemann, J. Lahann,S. Bräse, M. Franzreb, C. Wöll, Hierarchically functionalized magnetic core/multishell particles and their postsyn-thetic conversion to polymer capsules, ACS Nano 9 (2015) 4219–4226.[398] E. Redel, Z. Wang, S. Walheim, J. Liu, H. Gliemann, C. Wöll, On the dielectric and optical properties of surface-anchored metal-organic frameworks: A study on epitaxially grown thin films, Appl. Phys. Lett. 103 (2013) 091903.399] J. Liu, W. Zhou, J. Liu, I. Howard, G. Kilibarda, S. Schlabach, D. Coupry, M. Addicoat, S. Yoneda, Y. Tsutsui,T. Sakurai, S. Seki, Z. Wang, P. Lindemann, E. Redel, T. Heine, C. Wöll, Photoinduced charge-carrier generation inepitaxial MOF thin films: High efficiency as a result of an indirect electronic band gap?, Angew. Chem. Int. Ed. 54(2015) 7441–7445.[400] A. Schoedel, C. Scherb, T. Bein, Oriented nanoscale films of metal-organic frameworks by room-temperature gel-layersynthesis, Angew. Chem. Int. Ed. 49 (2010) 7225–7228.[401] F. M. Hinterholzinger, S. Wuttke, P. Roy, T. Preuße, A. Schaate, P. Behrens, A. Godt, T. Bein, Highly orientedsurface-growth and covalent dye labeling of mesoporous metal–organic frameworks, Dalton Trans. 41 (2012) 3899.[402] C. Scherb, R. Koehn, T. Bein, Sorption behavior of an oriented surface-grown MOF-film studied by in situ x-raydiffraction, J. Mater. Chem. 20 (2010) 3046.[403] J.-L. Zhuang, D. Ceglarek, S. Pethuraj, A. Terfort, Rapid room-temperature synthesis of metal-organic frameworkHKUST-1 crystals in bulk and as oriented and patterned thin films, Adv. Funct. Mater. 21 (2011) 1442–1447.[404] J.-L. Zhuang, D. Ar, X.-J. Yu, J.-X. Liu, A. Terfort, Patterned deposition of metal-organic frameworks onto plastic,paper, and textile substrates by inkjet printing of a precursor solution, Adv. Mater. 25 (2013) 4631–4635.[405] R. Makiura, S. Motoyama, Y. Umemura, H. Yamanaka, O. Sakata, H. Kitagawa, Surface nano-architecture of ametal–organic framework, Nat. Mater. 9 (2010) 565–571.[406] R. Ameloot, L. Stappers, J. Fransaer, L. Alaerts, B. F. Sels, D. E. D. Vos, Patterned growth of metal-organicframework coatings by electrochemical synthesis, Chem. Mater. 21 (2009) 2580–2582.[407] Y. Yoo, H.-K. Jeong, Rapid fabrication of metal organic framework thin films using microwave-induced thermaldeposition, Chem. Commun. (2008) 2441.[408] I. Stassen, M. Styles, G. Grenci, H. V. Gorp, W. Vanderlinden, S. D. Feyter, P. Falcaro, D. D. Vos, P. Vereecken,R. Ameloot, Chemical vapour deposition of zeolitic imidazolate framework thin films, Nat. Mater. 15 (2015) 304–310.