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Synthesis Modulation as a Tool To Increase the Catalytic Activity of Metal–Organic Frameworks: The Unique Case of UiO-66(Zr)

The catalytic activity of the zirconium terephthalate UiO-66(Zr) can be drastically increased by using a modulation approach. The combined use of trifluoroacetic acid and HCl during the synthesis results in a highly crystalline material, with partial substitution of terephthalates by trifluoroacetate. Thermal activation of the material leads not only to dehydroxylation of the hexanuclear Zr cluster but also to post-synthetic removal of the trifluoroacetate groups, resulting in a more open framework with a large number of open sites. Consequently, the material is a highly active catalyst for several Lewis acid catalyzed reactions.

Defect‐Engineered Metal–Organic Frameworks

Defect engineering in metal–organic frameworks (MOFs) is an exciting concept for tailoring material properties, which opens up novel opportunities not only in sorption and catalysis, but also in controlling more challenging physical characteristics such as band gap as well as magnetic and electrical/conductive properties. It is challenging to structurally characterize the inherent or intentionally created defects of various types, and there have so far been few efforts to comprehensively discuss these issues. Based on selected reports spanning the last decades, this Review closes that gap by providing both a concise overview of defects in MOFs, or more broadly coordination network compounds (CNCs), including their classification and characterization, together with the (potential) applications of defective CNCs/MOFs. Moreover, we will highlight important aspects of “defect-engineering” concepts applied for CNCs, also in comparison with relevant solid materials such as zeolites or COFs. Finally, we discuss the future potential of defect-engineered CNCs.

Modulated UiO-66-Based Mixed-Matrix Membranes for CO2 Separation

Mixed-matrix membranes (MMMs) composed of polyimide (PI) and metal-organic frameworks (MOFs) were synthesized using Matrimid as the polymer and zirconium terephthalate UiO-66 as the filler. The modulation approach, combined with the use of amine-functionalized linkers, was used for synthesis of the MOF fillers in order to enhance the intrinsic separation performance of the MOF and improve the particle-PI compatibility. The presence of amine groups on the MOF outer surface introduced either through the linker, through the modulator, or through both led to covalent linking between the fillers and Matrimid, which resulted in very stable membranes. In addition, the presence of amine groups inside the pores of the MOFs and the presence of linker vacancies inside the MOFs positively influenced CO2 transport. MMMs with 30 wt % loading showed excellent separation performance for CO2/CH4 mixtures. A significant increase in the mixed-gas selectivity (47.7) and permeability (19.4 barrer) compared to the unfilled Matrimid membrane (i.e., 50% more selective and 540% more permeable) was thus achieved for the MMM containing the MOF prepared from 2-aminoterephthalic acid and 4-aminobenzoic acid, respectively used as the linker and as the modulator.

The Structure of the Aluminum Fumarate Metal–Organic Framework A520

The synthesis of the commercially available aluminum fumarate sample A520 has been optimized and its structure analyzed through a combination of powder diffraction, solid-state NMR spectroscopy, molecular simulation, IR spectroscopy, and thermal analysis. A520 is an analogue of the MIL-53(Al)-BDC solid, but with a more rigid behavior. The differences between the commercial and the optimized samples in terms of defects have been investigated by inu2005situ IR spectroscopy and correlated to their catalytic activity for ethanol dehydration.

Automated Diffraction Tomography for the Structure Elucidation of Twinned, Sub‐micrometer Crystals of a Highly Porous, Catalytically Active Bismuth Metal–Organic Framework

The number of metal–organic framework (MOF) compounds has increased almost exponentially over the last decade as a consequence of their fascinating structures and potential applications. They are composed of inorganic building units, such as metal ions or clusters, which are connected through organic linker molecules to form a porous three-dimensional network. Most of the MOFs are based on rigid polycarboxylate linker molecules, but a large variety of metal ions, mainly transition-metal ions, have also been incorporated. The chemical and thermal stability of metal carboxylate based MOFs is crucial for potential applications and depends on the metal ions incorporated. In general, metal ions in higher oxidation states lead to more stable structures. While the use of divalent metal ions often results in the formation of single crystals, whose structures can be routinely determined by single-crystal X-ray diffraction, triand tetravalent metal carboxylates are mostly obtained as microcrystalline powders and the determination of their structures poses immense challenges. 4c,5] Direct methods have been successfully employed, but complicated structures with large unit cells necessitate the use of nonstandard approaches. Thus, computational assisted structure determination, namely, the AASBU approach (assembling of secondary building units), the ligand-replacement strategy, and DFT calculations have been applied. Recently automated diffraction tomography (ADT) has been introduced as a new method for collecting three-dimensional electron diffraction data from single nanosized crystals, thus allowing singlecrystal analysis even for porous and organic sub-microcrystalline samples. A trivalent metal that exhibits interesting catalytic properties is bismuth. It is nontoxic, noncarcinogenic, and for a rare metal relatively inexpensive, and thus bismuth compounds are used as green catalysts. Despite these characteristics, the number of bismuth-based MOFs is rather limited and only a few compounds with limited porosity have been described. This is in contrast to the many known bismuth-oxo clusters, which could possibly be used for the construction of new MOFs. Here, we present the synthesis of the first highly crystalline, porous, and catalytically active bismuth-based MOF Bi(BTB) (BTB = 1,3,5-benzenetrisbenzoate), whose structure was elucidated by a combination of electron diffraction, Rietveld refinement, and DFT calculations. Bi(BTB), denoted as CAU-7 (CAU = ChristianAlbrechts-Universit t) was synthesized by using conventional as well as microwave (MW) assisted heating. The reaction of Bi(NO3)3·5 H2O with H3BTB in methanol at 120 8C led to phase-pure CAU-7 (for a detailed synthesis procedure see the Supporting Information). The reaction time can be reduced from 12 h to 20 min by using MW-assisted instead of conventional heating, but this leads to the formation of 10–20 mm large agglomerates of strongly intergrown elongated crystals of about 100 nm (see Figures S2–S4 in the Supporting Information). The addition of DMF in the conventional synthesis results in the formation of larger rodlike crystals ranging from 200 to 300 nm in length. Transmission electron microscopy confirmed that isolated CAU-7 crystals have a typical rodlike shape with different length/diameter ratios (see Figure S5 in the Supporting Information). Such isolated rods were used to collect electron diffraction data by automated diffraction tomography (ATD). Single-crystal ADT electron diffraction datasets were collected using a cryo holder cooled to 120 K and mild illumination conditions. To prevent beam damage and improve the signal intensity, the diffraction data were acquired in the precession mode. The three-dimensional diffraction space reconstruction leads to lattice parameters a = 32 , b = 28 , c = 4 , a = b = g = 908, and extinction group Pb-a. The reconstructed reciprocal space is shown in Figure 1. [*] M. Feyand, T. Reimer, Prof. Dr. N. Stock Institut f r Anorganische Chemie Christian Albrechts Universit t zu Kiel Max-Eyth Strasse 2, 24118 Kiel (Germany) E-mail: stock@ac.uni-kiel.de

Improving the mechanical stability of zirconium-based metal–organic frameworks by incorporation of acidic modulators

The ability to retain structural integrity under processing conditions which involve mechanical stress, is essential if metal–organic frameworks (MOFs) are to fulfil their potential as serious candidates for use in gas sorption, separation, catalysis and energy conversion applications. A series of zirconium dicarboxylates, predicted to be amongst the more mechanically robust MOFs, have been found to undergo rapid collapse upon ball-milling, resulting in catastrophic losses of porosity. An inverse relationship between collapse time and framework porosity has been found. Addition of acidic modulator ligands (e.g. trifluoroacetic acid) to UiO-66 provided a striking increase in mechanical robustness, the degree of which is inversely related to modulator pKa. This effect, caused by an increased strength of the zirconium–carboxylate bond, provides an important concept to design microporous hybrid frameworks capable of retaining their structure under harsh processing conditions.

Green synthesis of zirconium-MOFs

The synthesis of Zr-MOFs under green, industrially feasible conditions was investigated. Two new compounds with bcu-topology and the fluorinated analogue of UiO-66 exhibiting fcu-topology were obtained and characterised. All products exhibit permanent porosity. In the bcu-frameworks the interaction with sulfate anions apparently induces an unusual eightfold connectivity of the Zr cluster.

A Flexible Photoactive Titanium Metal–Organic Framework Based on a [TiIV3(μ3-O)(O)2(COO)6] Cluster

The synthesis of titanium-carboxylate metal-organic frameworks (MOFs) is hampered by the high reactivity of the commonly employed alkoxide precursors. Herein, we present an innovative approach to titanium-based MOFs by the use of titanocene dichloride to synthesize COK-69, the first breathing Tiu2005MOF, which is built up from trans-1,4-cyclohexanedicarboxylate linkers and an unprecedented [Ti(IV)3(μ3-O)(O)2(COO)6] cluster. The photoactive properties of COK-69 were investigated in depth by proton-coupled electron-transfer experiments, which revealed that up to one Ti(IV) center per cluster can be photoreduced to Ti(III) while preserving the structural integrity of the framework. The electronic structure of COK-69 was determined by molecular modeling, and a band gap of 3.77 eV was found.

First examples of aliphatic zirconium MOFs and the influence of inorganic anions on their crystal structures

Utilizing the aliphatic linker molecule adipic acid (1,6-hexanedioic acid, HO2C–C4H8–CO2H) or 3-methyladipic acid (racemic mixture, HO2C–C4H7CH3–CO2H), the first crystalline zirconium adipates were synthesized under aqueous conditions. Their structures were deduced from powder X-ray diffraction data and were confirmed by Rietveld refinements. For all three compounds, the inorganic nodes are related to the well-known Zr6O4(OH)4 cluster frequently observed in aromatic zirconium MOFs. Employing ZrOCl2·8H2O and 3-methyladipic acid, a framework with bcu topology was obtained. Starting from adipic acid and Zr(SO4)2·4H2O, we observed the incorporation of sulfate into the crystal structure. Four sulfate anions are coordinated to each Zr–oxo cluster in a bidentate fashion. In this complex structure, square grids formed by Zr–oxo clusters and adipate anions and furthermore a hydrogen-bonded inorganic dia net can be observed. The third compound presented here is structurally related to the zirconium methyladipate. Using adipic acid and adding CrO42− under strongly acidic conditions leads to the incorporation of Cr2O72− into the bcu net. The dichromate anions are coordinated twofold to two different Zr–oxo clusters in a monodentate fashion and thus serve as inorganic connectors between the frameworks nodes.

Three Series of Sulfo‐Functionalized Mixed‐Linker CAU‐10 Analogues: Sorption Properties, Proton Conductivity, and Catalytic Activity

Ten mixed-linker metal-organic frameworks [Al(OH)(m-BDC-X)(1-y)(m-BDC-SO3H)y] (H2BDC = 1,3-benzenedicarboxylic acid; X = H, NO2, OH) exhibiting the CAU-10-type structure were synthesized. The compounds can be grouped into three series according to the combination of ligands employed. The three series of compounds were obtained by employing different ratios of m-H2 BDC-X and m-H2BDC-SO3Li. The resulting compounds, which are denoted CAU-10-H/Sx, -N/Sx and -O/Sx, show exceptionally high thermal stability for sulfonated materials of up to 350u2009°C. Detailed characterization with special focus on polarity and acidity was performed, and the impact of the additional SO3H groups is clearly demonstrated by changes in the sorption affinities/capacities towards several gases and water vapor. In addition, selected samples were evaluated for proton conductivity and as catalysts for the gas-phase dehydration of ethanol to ethylene. While only very low proton conductivities were observed, a pronounced increase in catalytic activity was achieved. Although reactions were performed at temperatures of 250 and 300u2009°C for more than 40u2005h, no desulfonation and no loss of crystallinity were observed, and stable ethanol conversion resulted. This demonstrates the high stability of this material.