Reiner Staudt

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.

High‐Throughput and Time‐Resolved Energy‐Dispersive X‐Ray Diffraction (EDXRD) Study of the Formation of CAU‐1‐(OH)2: Microwave and Conventional Heating

Aluminium dihydroxyterephthalate [Al(8)(OH)(4)(OCH(3))(8)(BDC(OH)(2))(6)]⋅x H(2)O (denoted CAU-1-(OH)(2)) was synthesized under solvothermal conditions and characterized by X-ray powder diffraction, IR spectroscopy, sorption measurements, as well as thermogravimetric and elemental analysis. CAU-1-(OH)(2) is isoreticular to CAU-1 and its pores are lined with OH groups. It is stable under ambient conditions and in water, and it exhibits permanent porosity and two types of cavities with effective diameters of approximately 1 and 0.45 nm. The crystallization of CAU-1-(OH)(2) was studied by in situ energy-dispersive X-ray diffraction (EDXRD) experiments in the 120-145 °C temperature range. Two heating methods-conventional and microwave-were investigated. The latter leads to shorter induction periods as well as shorter reaction times. Whereas CAU-1-(OH)(2) is formed at all investigated temperatures using conventional heating, it is only observed below 130 °C using microwave heating. The calculation of the activation energy of the crystallization of CAU-1-(OH)(2) exhibits similar values for microwave and conventional synthesis.

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.

Mixed gas adsorption of carbon dioxide and methane on a series of isoreticular microporous metal–organic frameworks based on 2-substituted imidazolate-4-amide-5-imidates

In this work the adsorption of CO2 and CH4 on a series of isoreticular microporous metal–organic frameworks based on 2-substituted imidazolate-4-amide-5-imidates, IFP-1–IFP-6 (IFP = Imidazolate Framework Potsdam), is studied firstly by pure gas adsorption at 273 K. All experimental isotherms can be nicely described by using the Toth isotherm model and show the preferred adsorption of CO2 over CH4. At low pressures the Toth isotherm equation exhibits a Henry region, wherefore Henrys law constants for CO2 and CH4 uptake could be determined and ideal selectivity αCO2/CH4 has been calculated. Secondly, selectivities were calculated from mixture data by using nearly equimolar binary mixtures of both gases by a volumetric–chromatographic method to examine the IFPs. Results showed the reliability of the selectivity calculation. Values of αCO2/CH4 around 7.5 for IFP-5 indicate that this material shows much better selectivities than IFP-1, IFP-2, IFP-3, IFP-4 and IFP-6 with slightly lower selectivity αCO2/CH4 = 4–6. The preferred adsorption of CO2 over CH4 especially of IFP-5 and IFP-4 makes these materials suitable for gas separation application.

Network Flexibility: Control of Gate Opening in an Isostructural Series of Ag-MOFs by Linker Substitution

An isostructural series of 15 structurally flexible microporous silver metal-organic frameworks (MOFs) is presented. The compounds with a dinuclear silver core as secondary building unit (Ag2N4) can be obtained under solvothermal conditions from substituted triazolyl benzoate linkers and AgNO3 or Ag2SO4; they exhibit 2-fold network interpenetration with lvt topology. Besides the crystal structures, the calculated pore size distributions of the microporous MOFs are reported. Simultaneous thermal analyses confirm the stability of the compounds up to 250 °C. Interconnected pores result in a three-dimensional pore structure. Although the porosity of the novel coordination polymers is in the range of only 20-36%, this series can be regarded as a model system for investigation of network flexibility, since the pore diameters and volumes can be gradually adjusted by the substituents of the 3-(1,2,4-triazol-4-yl)-5-benzamidobenzoates. The pore volumes of selected materials are experimentally determined by nitrogen adsorption at 77 K and carbon dioxide adsorption at room temperature. On the basis of the flexible behavior of the linkers a reversible framework transformation of the 2-fold interpenetrated network is observed. The resulting adsorption isotherms with one or two hysteresis loops are interpreted by a gate-opening process. Due to external stimuli, namely, the adsorptive pressure, the materials undergo a phase transition confirming the structural flexibility of the porous coordination polymer.

Adsorption Equilibria of Natural Gas Components on Activated Carbon: Pure and Mixed Gas Isotherms

A typical Natural Gas (NG) composition in South America contains ca. 86% methane, 10% ethane, 1.5% carbon dioxide, 1% nitrogen and other heavier hydrocarbons. The composition plays an important role in studies of NG storage systems since heavier alkanes may accumulate in the adsorbent bed, saturate it in long-term use and affect the storage capacity. In this study, measurements of experimental data for the adsorption equilibria of binary mixtures of CH4 with C2H6, CO2, C3H8 and N2 — as obtained in a volumetric/chromatographic set-up — are presented and compared with mathematical models for multi-component adsorption based on pure component adsorption data. Experimental results were compared with predictions from the Extended Langmuir (EL) model and the Ideal Adsorbed Solution Theory (IAST) model. The IAST model was capable of providing a more accurate fit to most of the obtained data, other than those measured for the least adsorbed gases (CH4/N2).

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.