Daniel Lässig

A Microporous Copper Metal–Organic Framework with High H2 and CO2 Adsorption Capacity at Ambient Pressure

Metal–organic frameworks (MOFs) as highly porous materials have gained increasing interest because of their distinct adsorption properties. They exhibit a high potential for applications in gas separation and storage, as sensors as well as in heterogeneous catalysis. In the last few years, the H2 storage capacity of MOFs has been considerably increased. Mesoporous MOFs show high adsorption capacities for CH4, CO2, and H2 at high pressures. [2, 3, 7–10] To increase the uptake of H2 and CO2 by physisorption at ambient pressure, adsorbents with small micropores as well as high specific surface areas and micropore volumes are required. 12] Such microporous materials seem to be more appropriate for gas-mixture separation by physisorption than mesoporous materials. For gas separation in MOFs the interactions between the fluid adsorptive and “open metal sites” (coordinatively unsaturated binding sites) or the ligands are regarded as important. Industrial processes, such as natural-gas purification or biogas upgrading, can be improved with those materials during a vapor-pressure swing adsorption cycle (VPSA cycle) or a temperature swing adsorption cycle (TSA cycle). The microporous MOF series CPO-27-M (M = Mg, Co, Ni, Zn), for example, shows very high CO2 uptakes at low pressures (< 0.1 MPa). Concerning H2 adsorption, the microporous MOF PCN-12 offers with 3.05 wt % the highest uptake at ambient pressure and 77 K reported to date. Herein, we present a novel microporous copper-based MOF 3 1[Cu(Me-4py-trz-ia)] (1; Me-4py-trz-ia 2 = 5-(3methyl-5-(pyridin-4-yl)-4H-1,2,4-triazol-4-yl)isophthalate) with extraordinarily high CO2 and H2 uptakes at ambient pressure, the H2 uptake being similar to that in PCN-12. The ligand Me-4py-trz-ia (Figure 1 a), which can be obtained from cheap starting materials by a three-step synthesis in good yield, combines carboxylate, triazole, and pyridine functions and is adopted from a recently presented series of linkers, for which up to now only a few coordination polymers are known. Single crystals of 1 that are suitable for X-ray crystal structure analysis were prepared by diffusion of copper sulfate and H2(Me-4py-trz-ia). Larger quantities of microcrystalline 1 are obtained not only by solvothermal synthesis, but also in multigram scale by simple reflux of the starting materials in water/acetonitrile (see Supporting Information). According to the single crystal X-ray structure analysis, 1 crystallizes in the monoclinic space group P21/c (no. 14) with four formula units per unit cell. The asymmetric unit contains one linker anion and two crystallographically independent Cu ions residing on inversion centers. The copper ion Cu1 is coordinated in a square-planar fashion by two monodentate carboxylate and two pyridine functions in trans position leaving two accessible open metal sites per Cu1 atom (Figure 1b), whereas the second copper ion Cu2 is coordinated by monodentate triazole and chelating carboxylate groups forming a distorted octahedron (Figure 1 c). For this reason, both copper ions represent planar fourfold nodal points and the ligands act as tetradentate linkers in a 3D network with pts topology and a 3D pore system (Figure 1d). With narrow channels of about 250 600 pm in crystallographic c direction connecting the micropores with a diameter of approximately 550 pm, the structure has a calculated porosity of about 55% according to PLATON. Powder X-ray diffraction (PXRD) studies on the assynthesized microcrystalline sample 1a confirm both, the agreement with the simulated powder pattern of 1 based on [*] D. L ssig, J. Lincke, Prof. Dr. R. Gl ser, Prof. Dr. H. Krautscheid Universit t Leipzig, Fak. f r Chemie und Mineralogie Johannisallee 29, 04103 Leipzig (Germany) E-mail: krautscheid@rz.uni-leipzig.de

Pure and mixed gas adsorption of CH4 and N2 on the metal–organic framework Basolite® A100 and a novel copper-based 1,2,4-triazolyl isophthalate MOF

Pure gas adsorption isotherms of CH4 and N2 and their binary mixtures were measured at 273 K, 298 K and 323 K and up to 2 MPa on two different microporous metal–organic frameworks (MOFs), i.e. the commercially available Basolite® A100 and the recently reported copper-based triazolyl benzoate MOF 3∞[Cu(Me-4py-trz-ia)] (1). The Toth isotherm model and the vacancy solution model were used to describe the experimentally determined isotherms and proved to be well suited for this purpose. While 1 shows a more homogeneous surface with a nearly constant isosteric heat of adsorption of 18–18.5 kJ mol−1 for CH4 and 12–15 kJ mol−1 for N2, the isosteric heat of adsorption at zero coverage for Basolite® A100 is 19 kJ mol−1 for CH4 and 16.2 kJ mol−1 for N2, decreasing significantly with increasing loading. Binary adsorption isotherms were measured gravimetrically to determine the total adsorbed mass of CH4 and N2. The van Ness method was successfully applied to calculate partial loadings from gravimetrically measured binary adsorption isotherms. Further studies by volumetric–chromatographic experiments support the good correlation between experimental data and predictions by the vacancy solution model (VSM-Wilson) and the ideal adsorbed solution theory (IAST) from pure gas isotherms. The experimental selectivities were determined to be αCH4/N2 = 4.0–5.0 for 1, slightly higher than for Basolite® A100 with αCH4/N2 = 3.4–4.5. These values are in good agreement with predictions for ideal selectivities based on Henrys law constants. From the experimental selectivities the potential of both MOFs in gas separation of CH4 from N2 can be derived.

Assessment of hydrogen storage by physisorption in porous materials

As a basis for the evaluation of hydrogen storage by physisorption, adsorption isotherms of H2 were experimentally determined for several porous materials at 77 K and 298 K at pressures up to 15 MPa. Activated carbons and MOFs were studied as the most promising materials for this purpose. A noble focus was given on how to determine whether a material is feasible for hydrogen storage or not, dealing with an assessment method and the pitfalls and problems of determining the viability. For a quantitative evaluation of the feasibility of sorptive hydrogen storage in a general analysis, it is suggested to compare the stored amount in a theoretical tank filled with adsorbents to the amount of hydrogen stored in the same tank without adsorbents. According to our results, an “ideal” sorbent for hydrogen storage at 77 K is calculated to exhibit a specific surface area of >2580 m2 g−1 and a micropore volume of >1.58 cm3 g−1.

An isomorphous series of cubic, copper-based triazolyl isophthalate MOFs: linker substitution and adsorption properties.

An isomorphous series of 10 microporous copper-based metal-organic frameworks (MOFs) with the general formulas (∞)(3)[{Cu(3)(μ(3)-OH)(X)}(4){Cu(2)(H(2)O)(2)}(3)(H-R-trz-ia)(12)] (R = H, CH(3), Ph; X(2-) = SO(4)(2-), SeO(4)(2-), 2 NO(3)(2-) (1-8)) and (∞)(3)[{Cu(3)(μ(3)-OH)(X)}(8){Cu(2)(H(2)O)(2)}(6)(H-3py-trz-ia)(24)Cu(6)]X(3) (R = 3py; X(2-) = SO(4)(2-), SeO(4)(2-) (9, 10)) is presented together with the closely related compounds (∞)(3)[Cu(6)(μ(4)-O)(μ(3)-OH)(2)(H-Metrz-ia)(4)][Cu(H(2)O)(6)](NO(3))(2)·10H(2)O (11) and (∞)(3)[Cu(2)(H-3py-trz-ia)(2)(H(2)O)(3)] (12(Cu)), which are obtained under similar reaction conditions. The porosity of the series of cubic MOFs with twf-d topology reaches up to 66%. While the diameters of the spherical pores remain unaffected, adsorption measurements show that the pore volume can be fine-tuned by the substituents of the triazolyl isophthalate ligand and choice of the respective copper salt, that is, copper sulfate, selenate, or nitrate.

Solid-State Syntheses of Coordination Polymers by Thermal Conversion of Molecular Building Blocks and Polymeric Precursors

The syntheses and crystal structures of a mononuclear cadmium complex and five novel coordination polymers based on 1,2,4-triazolyl benzoates are presented. The compounds (∞)(3)[Cd(H-Me-trz-pba)(2)] (2), (∞)(3)[Cd(Me-3py-trz-pba)(2)] (4), and (∞)(3)[Zn(H-Me-trz-pba)(2)] (6) can be obtained by solvothermal synthesis or simple heating of the starting materials in appropriate solvents, and are also accessible by thermal conversion of the complex [Cd(H-Me-trz-pba)(2)(H(2)O)(4)] (1), the one-dimensional (1D) coordination polymer (∞)(1)[Cd(Me-3py-trz-pba)(2)(H(2)O)(2)]·H(2)O (3), and the porous three-dimensional (3D) framework (∞)(3)[Zn(H-Me-trz-pba)2]·4H(2)O (5), respectively. The driving force for these conversions is the formation of thermally stable, nonporous, crystalline 3D coordination polymers. The structural transformations are accompanied by the loss of water and reveal significant changes of the coordination spheres of the metal ions caused by a rearrangement of the triazolyl benzoate ligands. Compounds 2, 4, 5, and 6 exhibit 4- and 5-fold interpenetration of diamondoid networks (dia) and are thermally stable up to 380 °C.

113Cd solid-state NMR for probing the coordination sphere in metal-organic frameworks.

Spectroscopic techniques are a powerful tool for structure determination, especially if single-crystal material is unavailable. (113)Cd solid-state NMR is easy to measure and is a highly sensitive probe because the coordination number, the nature of coordinating groups, and the geometry around the metal ion is reflected by the isotropic chemical shift and the chemical-shift anisotropy. Here, a detailed investigation of a series of 27 cadmium coordination polymers by (113)Cd solid-state NMR is reported. The results obtained demonstrate that (113)Cd NMR is a very sensitive tool to characterize the cadmium environment, also in non-single-crystal materials. Furthermore, this method allows the observation of guest-induced phase transitions supporting understanding of the structural flexibility of coordination frameworks.

Structural flexibility of a copper-based metal–organic framework: sorption of C4-hydrocarbons and in situ XRD

Pure component sorption isotherms of n-butane, isobutane, 1-butene and isobutene on the metal–organic framework (MOF) 3∞[Cu4(μ4-O)(μ2-OH)2(Me2trz-pba)4] at various temperatures between 283 K and 343 K and pressures up to 300 kPa are presented. The isotherms show a stepwise pore filling which is typical for structurally flexible materials with broad adsorption–desorption hysteresis loops. Gate opening pressures in their endemic characteristic depend on the used hydrocarbon gases. From all investigated gases only the isotherms of 1-butene present a second step at a relative pressure above p/p0 = 0.55. As a consequence, only 1-butene can fully open the framework resulting in a pore volume of 0.54 cm3 g−1. This result is in good agreement with the value of 0.59 cm3 g−1 calculated based on single crystal structure data. The isosteric heat of adsorption was calculated from the experimental isotherms for all C4-isomers. At low loadings the isosteric heat is in a narrow region between 41 and 49 kJ mol−1. Moreover, in situ XRD measurements at different relative hydrocarbon pressures were performed at 298 K for the C4-isomers. The differences in the pressure-depending powder diffraction patterns indicate phase transitions as a result of adsorption. Similar diffraction patterns were observed for all C4-hydrocarbons, except 1-butene, where the second step at higher relative pressure (p/p0 > 0.55) is accompanied by an additional phase transition. This powder pattern resembles that of the as-synthesized MOF material containing solvent molecules in the pore system. The resulting structural changes of the material during guest and pressure induced external stimuli are evidenced by the new coupled XRD adsorption equipment.

Paddle Wheel Based Triazolyl Isophthalate MOFs: Impact of Linker Modification on Crystal Structure and Gas Sorption Properties

Syntheses and comprehensive characterization of two closely related series of isomorphous metal-organic frameworks (MOFs) based on triazolyl isophthalate linkers with the general formula ∞(3)[M2(R(1)-R(2)-trz-ia)2] (M = Cu, Zn) are presented. Using solvothermal synthesis and synthesis of microcrystalline materials on the gram scale by refluxing a solution of the starting materials, 11 MOFs are readily available for a systematic investigation of structure-property relationships. The networks of the two series are assigned to rutile (rtl) (1-4) and α-PbO2 (apo) (5-9) topology, respectively. Due to the orientation of the triazole substituents toward the cavities, both the pore volume and the pore diameter can be adjusted by choice of the alkyl substituents. Compounds 1-9 exhibit pronounced microporosity with calculated porosities of 31-53% and show thermal stability up to 390 °C as confirmed by simultaneous thermal analysis. Systematic investigation of adsorption properties by CO2 (298 K) and N2 (77 K) adsorption studies reveal remarkable network flexibility induced by alkyl substituents on the linker. Fine-tuning of the gate opening pressure and of the hysteresis shape is possible by adjusting the substitution pattern and by choice of the metal ion.

Halogeno(triazolyl)zinc complexes as molecular building blocks for metal-organic frameworks.

The isomorphous title complexes, dichlorido[4-(3,5-dimethyl-4H-1,2,4-triazol-4-yl)benzoic acid-kappaN(1)]zinc(II) dihydrate, [ZnCl(2)(C(11)H(11)N(3)O(2))(2)].2H(2)O, and dibromido[4-(3,5-dimethyl-4H-1,2,4-triazol-4-yl)benzoic acid-kappaN(1)]zinc(II) dihydrate, [ZnBr(2)(C(11)H(11)N(3)O(2))(2)].2H(2)O, were synthesized and crystallized by slow evaporation of the solvent from a solution of the ligand and either zinc chloride or zinc bromide, respectively, in water/ethanol. The Zn(II) ions occupy twofold axes in the noncentrosymmetric orthorhombic space group Fdd2. The metal ion is approximately tetrahedrally coordinated by two monodentate triazole groups of the ligands and additionally by two halide ions. The water molecules incorporate the complexes into a three-dimensional framework made up by hydrogen bonds. Furthermore, each complex possesses two hydrogen-bond-donor sites represented by the carboxy groups and two acceptor sites at the noncoordinating N atoms of the triazoles.