Irena Senkovska

A Mesoporous Metal–Organic Framework

A new class of porous materials namely metal–organic frameworks (MOFs) has set records in recent years regarding specific surface area and pore volume. Nevertheless, the search for compounds with very large pores and higher specific surface area remains a key challenge in the rapidly expanding field of MOFs, especially for applications in catalysis, drug delivery, and high-pressure gas storage. Compounds containing small windows or pores that are inaccessible for anchoring molecular catalysts, impregnation with catalyst precursors, or larger drug molecules pose limitations for MOFs in fine chemical transformation, nanoparticle formation, or drug delivery. For energy-storage applications at 200 bar, larger pores (2-3 nm) are essential to achieve a shift of the excess adsorption maximum towards higher pressure. Despite a somewhat reduced heat of adsorption, in practice such large pore MOFs outperform small pore MOFs as a result of the higher pore volume. A common concept to enhance the pore size in MOFs uses a linear extension of the linker in a given network topology. In such MOFs the pore diameter achievable is limited by interpenetration. A prominent example is the IRMOF series (isoreticular MOFs). Other examples of increasing pore size through linker extensions are [Cu3(btc)2] [11] (btc = benzene-1,3,5-tricarboxylate; tbo-topology), PCN-6 ([Cu3(tatb)2], tatb = 4,4’,4’’-s-triazine-2,4,6-triyltribenzoate; tbo), or MOF-14 ([Cu3(btb)2], btb = benzene1,3,5-tribenzoate; pto), built from paddle wheel clusters and tritopic linkers. However, in PCN-6 and MOF-14, which have the btb linker (a longer version of the btc linker), the porosity is reduced because of the presence of two interwoven 3D nets in the structure. The non-interpenetrated analogue of PCN-6 (PCN-6’) is obtained using a templating strategy, while a non-interpenetrated analogue of MOF-14 is unknown. Herein, we report an approach that avoids interpenetration by using a secondary linker to stabilize a highly open framework structure by crosslinking an extended Pt3O4topology. The resulting new mesoporous MOF material, DUT-6 (DUT= Dresden University of Technology), shows no interpenetration and has an extremely high gas adsorption capacity for n-butane, hydrogen, and methane. Single crystals of [Zn4O(2,6-ndc)(btb)4/3(def)16(H2O)9/2] (DUT-6; def = N,N-diethylformamide, 2,6-ndc = 2,6-naphthalenedicarboxylate) suitable for X-ray diffraction analysis were obtained from the reaction of H3(btb), H2(2,6-ndc), and zinc nitrate in a ratio of 3:2:14. The compound crystallizes in the cubic space group Pm3̄n. Dodecahedral mesoporous cages 2.5–3 nm in diameter are formed by twelve Zn4O 6+

Capture of Nerve Agents and Mustard Gas Analogues by Hydrophobic Robust MOF-5 Type Metal–Organic Frameworks

In this communication, a series of observations and data analyses coherently confirms the suitability of the novel metal-organic framework (MOF) [Zn(4)(μ(4)-O)(μ(4)-4-carboxy-3,5-dimethyl-4-carboxy-pyrazolato)(3)] (1) in the capture of harmful volatile organic compounds (VOCs). It is worthy of attention that 1, whose crystal structure resembles that of MOF-5, exhibits remarkable thermal, mechanical, and chemical stability, as required if practical applications are sought. In addition, it selectively captures harmful VOCs (including models of Sarin and mustard gas, which are chemical warfare agents), even in competition with ambient moisture (i.e., under conditions mimicking operative ones). The results can be rationalized on the basis of Henry constant and adsorption heat values for the different essayed adsorbates as well as H(2)O/VOC partition coefficients as obtained from variable-temperature reverse gas chromatography experiments. To further strengthen the importance of 1, its performance in the capture of harmful VOCs has been compared with those of well-known materials, namely, a MOF with coordinatively unsaturated metal sites, [Cu(3)(btc)(2)] and the molecular sieve active carbon Carboxen. The results of this comparison show that coordinatively unsaturated metal sites (preferential guest-binding sites) are ineffective for the capture of VOCs in the presence of ambient moisture. Consequently, we propose that the driving force of the VOC-MOF recognition process is mainly dictated by pore size and surface hydrophobicity.

Highly Hydrophobic Isoreticular Porous Metal-Organic Frameworks for the Capture of Harmful Volatile Organic Compounds

The release of toxic pollutants into the environment, which includes oil spills, leaks of harmful industrial products, and the deliberate emission of chemical warfare agents is a risk of growing concern. Worthy of note, oil spill cleanups amount to over 10 billion dollars annually. Remediation of these environmental problems involves the use of large amounts of adsorbents such as sand, activated carbons, or zeolites. However, the effectiveness of such adsorbents is often limited by their affinity to moisture. Consequently, the search for highly hydrophobic porous materials to be used as suitable stopgap of harmful organics spills has become of paramount importance. In the past years, porous metal–organic frameworks (MOFs) have been extensively studied to explore their possible applications in near future technologies for the safe storage of energetically and environmentally relevant gases. The tunable nature of their pores might be beneficial also in cushioning environmental problems caused by the release of harmful volatile organic compounds (VOCs). A remarkable example of the design amenability of MOFs is the well-known isoreticular [Zn4OL3] series (L= arene-dicarboxylate), wherein the size and the functionality of the pores can be modulated in a highly rational and systematic way. Nevertheless, the advantageous structural features of this family of MOFs are readily hampered by its high sensitivity to moisture, which limits its practical applications. A similar size-scaling approach has been applied by Lillerud and coworkers on the isoreticular [Zr6O4(OH)4L6] series, [9] evidencing that a significant improvement in the stability of the material can be achieved with an appropriate combination of dicarboxylate linkers and oxophylic metal fragments. Alternately, it is possible to take advantage of the enhanced stability imparted by polyazolate-containing ligands in combination with borderline metal ions. Accordingly, we designed and isolated an isoreticular series of porous MOFs, the pore size and polarity of which was modulated by coupling stiff bi-pyrazolate or mixed pyrazolate/carboxylate linkers (Scheme 1) to Ni hydroxo clusters acting as 12-connected

A pressure-amplifying framework material with negative gas adsorption transitions

Adsorption-based phenomena are important in gas separations, such as the treatment of greenhouse-gas and toxic-gas pollutants, and in water-adsorption-based heat pumps for solar cooling systems. The ability to tune the pore size, shape and functionality of crystalline porous coordination polymers—or metal–organic frameworks (MOFs)—has made them attractive materials for such adsorption-based applications. The flexibility and guest-molecule-dependent response of MOFs give rise to unexpected and often desirable adsorption phenomena. Common to all isothermal gas adsorption phenomena, however, is increased gas uptake with increased pressure. Here we report adsorption transitions in the isotherms of a MOF (DUT-49) that exhibits a negative gas adsorption; that is, spontaneous desorption of gas (methane and n-butane) occurs during pressure increase in a defined temperature and pressure range. A combination of in situ powder X-ray diffraction, gas adsorption experiments and simulations shows that this adsorption behaviour is controlled by a sudden hysteretic structural deformation and pore contraction of the MOF, which releases guest molecules. These findings may enable technologies using frameworks capable of negative gas adsorption for pressure amplification in micro- and macroscopic system engineering. Negative gas adsorption extends the series of counterintuitive phenomena such as negative thermal expansion and negative refractive indices and may be interpreted as an adsorptive analogue of force-amplifying negative compressibility transitions proposed for metamaterials.

Application of a chiral metal–organic framework in enantioselective separation

A modular approach for the synthesis of highly ordered porous and chiral auxiliary (Evans auxiliary) decorated metal-organic frameworks is developed. Our synthesis strategy, which uses known porous structures as model materials for incorporation of chirality via linker modification, can provide access to a wide range of porous materials suitable for enantioselective separation and catalysis. Chiral analogues of UMCM-1 have been synthesized and investigated for the enantioseparation of chiral compounds in the liquid phase and first promising results are reported.

In Situ Synthesis of an Imidazolate‐4‐amide‐5‐imidate Ligand and Formation of a Microporous Zinc–Organic Framework with H2‐and CO2‐Storage Ability

In Situ Synthesis of an Imidazolate-4-amide-5-imidate Ligand and Formation of a Microporous Zinc-Organic Framework with H-2-and CO2-Storage Ability

Imine-linked polymer-derived nitrogen-doped microporous carbons with excellent CO2 capture properties.

A series of nitrogen-doped microporous carbons (NCs) was successfully prepared by direct pyrolysis of high-surface-area microporous imine-linked polymer (ILP, 744 m(2)/g) which was formed using commercial starting materials based on the Schiff base condensation under catalyst-free conditions. These NCs have moderate specific surface areas of up to 366 m(2)/g, pore volumes of 0.43 cm(3)/g, narrow micropore size distributions, and a high density of nitrogen functional groups (5.58-8.74%). The resulting NCs are highly suitable for CO2 capture adsorbents because of their microporous textural properties and large amount of Lewis basic sites. At 1 bar, NC-800 prepared by the pyrolysis of ILP at 800 °C showed the highest CO2 uptakes of 1.95 and 2.65 mmol/g at 25 and 0 °C, respectively. The calculated adsorption capacity for CO2 per m(2) (μmol of CO2/m(2)) of NC-800 is 7.41 μmol of CO2/m(2) at 1 bar and 25 °C, the highest ever reported for porous carbon adsorbents. The isosteric heats of CO2 adsorption (Qst) for these NCs are as high as 49 kJ/mol at low CO2 surface coverage, and still ~25 kJ/mol even at high CO2 uptake (2.0 mmol/g), respectively. Furthermore, these NCs also exhibit high stability, excellent adsorption selectivity for CO2 over N2, and easy regeneration and reuse without any evident loss of CO2 adsorption capacity.

Highly porous nitrogen-doped polyimine-based carbons with adjustable microstructures for CO2 capture

A series of highly porous nitrogen doped porous carbons (NPCs) have been successfully prepared using a novel porous polyimine as the precursor. The resulting NPCs have a high specific surface area of up to 3195 m2 g−1, high pore volume and micropore volume (up to 1.58 and 1.38 cm3 g−1, respectively), narrow micropore size distributions, and adjustable nitrogen (1.52–5.05 wt%) depending on the activation temperatures (600–750 °C). The CO2 uptakes of the NPCs prepared at higher temperatures (700–750 °C) are lower than those prepared at milder conditions (600–650 °C). At 1 bar, NPC-650 demonstrates the best CO2 capture performance and could efficiently adsorb CO2 molecules of 3.10 mmol g−1 (136 mg g−1) and 5.26 mmol g−1 (231.3 mg g−1), at 25 and 0 °C, respectively. The NPCs also show good a initial CO2/N2 adsorption selectivity of up to 23.4 and an adsorption ratio of CO2/N2 (6.6) at 1 bar. Meanwhile, these NPCs exhibit a high stability and facile regeneration/recyclability without evident loss of the CO2 capture capacities.

Carbide-Derived Carbon Monoliths with Hierarchical Pore Architectures†

Porous carbon materials are crucial components in catalysis, gas storage, electronics, and biochemistry. A hierarchical pore architecture in these materials is essential to achieve high surface areas combined with advanced mass transport kinetics. Widely used approaches for the generation of microor mesopores are activation and nanocasting. In contrast, macroporous carbon materials are primarily obtained by carbonization of polymeric precursor gels or replication of larger templates. A relatively new class of microand mesoporous carbon material with tunable porosity are carbide-derived carbon materials (CDCs). High-temperature chlorination of carbides leads to selective removal of metalor semi-metal atoms and allows control over the pore size of the resulting CDCs in a subngstrcm range by changing synthesis conditions or the carbide precursor. These materials have been studied for applications in gas storage and as electrode materials in supercapacitors because of their high specific surface areas. Recently, metal etching from pyrolyzed pre-ceramic components (polysilsesquioxanes or polysilazanes) was found to be a useful route towards carbide-derived carbon materials with enhanced porosity and gas-storage properties. A significant step towards ultrahigh specific surface area combined with a hierarchical mesoporous–microporous system was achieved using nanocasting of silica templates (SBA-15 or KIT-6) with polycarbosilane precursors and subsequent chlorine treatment of the resulting ordered mesoporous silicon carbides. These ordered mesoporous CDCs offer specific surface areas as high as 2800 mg 1 and total pore volumes of up to 2 cmg . Their mesostructure can be easily controlled by changing the silica hard template, resulting in excellent performance in protein adsorption, gas storage, and as electrodes for supercapacitors. However, such carbon materials are available only as nonstructured micrometer-sized powders and cannot be shaped into films without the addition of binders or the use of high mechanical stress, leading to structural deformation. Chlorine treatment of mechanically mixed Si/SiC precursors was found to be a useful route towards monolithic CDC with a hierarchical pore system. The presence of a free metal phase in the precursor system provides the opportunity to introduce a secondary macroporosity of 3 mm sized channels with a volume of 0.23 cmg 1 along with the microporous carbide-derived carbon material system. The introduction of large transport pores in polymerbased CDCs might be an alternative way to form materials that combine high surface areas with efficient fluid transport. The current literature describes a variety of routes for the production of highly macroporous ceramics from precursor polymers with controllable cell and window sizes. In particular, direct blowing of polycarbosilanes was found to be a useful approach for the generation of silicon carbide foams that might be suitable materials for the production of hierarchical CDCs. In the following, we describe a novel synthesis route for monolithic carbide-derived carbon materials, including micro-, meso-, and macroporous structures with extremely high specific surface area. They can be obtained by hightemperature chlorination of macroporous polymer-derived silicon carbide (SiC-PolyHIPE). A soft-templating approach starting from a high internal phase emulsion (HIPE) was used with an external oil phase consisting of liquid polycarbosilane SMP-10 and the cross-linker paradivinylbenzene. Using Span-80 as surfactant to stabilize the internal water phase, the application of oxidic or carbon hard templates and the corresponding template removal under harsh conditions is no longer necessary. After cross-linking the polymer chains, the resulting PolyHIPEs were pyrolyzed to silicon carbides at maximum temperatures of 700, 800, and 1000 8C and subsequently converted into CDCs by chlorine treatment at the maximum pyrolysis temperature (Supporting [*] M. Oschatz, L. Borchardt, Dr. I. Senkovska, N. Klein, Dr. R. Frind, Prof. Dr. S. Kaskel Department of Inorganic Chemistry Dresden University of Technology Bergstrasse 66, 01062 Dresden (Germany) E-mail: stefan.kaskel@chemie.tu-dresden.de

Solid-State NMR Spectroscopy of Metal-Organic Framework Compounds (MOFs)

Nuclear Magnetic Resonance (NMR) spectroscopy is a well-established method for the investigation of various types of porous materials. During the past decade, metal–organic frameworks have attracted increasing research interest. Solid-state NMR spectroscopy has rapidly evolved into an important tool for the study of the structure, dynamics and flexibility of these materials, as well as for the characterization of host–guest interactions with adsorbed species such as xenon, carbon dioxide, water, and many others. The present review introduces and highlights recent developments in this rapidly growing field.