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The formation and analysis of ten microporous triazolyl isophthalate based MOFs, including nine isomorphous and one isostructural compound is presented. The compounds 1 M – 3 M with the general formula [ M ( R 1 - R 2 - trz - ia ) ] ∞ 3 ·x H 2 O (M 2+ = Co 2+ , Cu 2+ , Zn 2+ , Cd 2+ ; R 1 = H, Me; R 2 = 2py, 2pym, prz (2py = 2-pyridinyle; 2pym = 2-pyrimidinyle; prz = pyrazinyle)) crystallize with rtl topology. They are available as single crystals and also easily accessible in a multi-gram scale via refluxing the metal salts and the protonated ligands in a solvent. Their isomorphous structures facilitate the synthesis of heteronuclear MOFs; in case of 2 M , Co 2+ ions could be gradually substituted by Cu 2+ ions. The Co 2+ :Cu 2+ ratios were determined by ICP-OES spectroscopy, the distribution of Co 2+ and Cu 2+ in the crystalline samples are investigated by SEM-EDX analysis leading to the conclusions that Cu 2+ is more favorably incorporated into the framework compared to Co 2+ and, moreover, that the distribution of the two metal ions between the crystals and within the crystals is inhomogeneous if the crystals were grown slowly. The various compositions of the heteronuclear materials lead to different colors and the sorption properties for CO 2 and N 2 are dependent on the integrated metal ions.
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 Tòth 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 Henry's law constants. From the experimental selectivities the potential of both MOFs in gas separation of CH4 from N2 can be derived.
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.
High pressure adsorption phenomena are discussed for different gases on HKUST-1 (Cu3(BTC)2, commercially available product BasoliteTM C300). Sorption isotherms for hydrogen, nitrogen, methane and carbon dioxide on HKUST-1 were measured in the temperature range of 273–343 K and at pressures up to 50 MPa. The calculated surface excess adsorption capacities for all four adsorptive are one of the highest reported in the literature for HKUST-1 samples. All surface excess data were further calculated from the experimental data by using the helium buoyancy correction. A detailed description was given.
Also a procedure to calculate the absolute amount adsorbed from the surface excess amount by using two different models is shown. Using one model, the density and the volume of the adsorbed phase can be calculated. The density of the adsorbed phase ρads corresponds to the liquid density of the adsorptive at its boiling point ρliq,BP. In case of hydrogen no excess maximum was found up to 50 MPa, so that one model could not be applied. Finally, the isosteric heat of adsorption for each gas was calculated by using the Clausius–Clapeyron equation.
Synthesis and crystal structure of a novel copper-based MOF material are presented. The tetragonal crystal structure of [ ∞ 3 ( Cu 4 ( μ 4 -O ) ( μ 2 -OH ) 2 ( Me 2 trz p ba ) 4 ] possesses a calculated solvent-accessible pore volume of 57%. Besides the preparation of single crystals, synthesis routes to microcrystalline materials are reported. While PXRD measurements ensure the phase purity of the as-synthesized material, TD-PXRD measurements and coupled DTA–TG–MS analysis confirm the stability of the network up to 230 °C. The pore volume of the microcrystalline material determined by nitrogen adsorption at 77 K depends on the synthetic conditions applied. After synthesis in DMF/H2O/MeOH the pores are blocked for nitrogen, whereas they are accessible for nitrogen after synthesis in H2O/EtOH and subsequent MeOH Soxhleth extraction. The corresponding experimental pore volume was determined by nitrogen adsorption to be V Pore = 0.58 cm 3 g - 1 . In order to characterize the new material and to show its adsorption potential, comprehensive adsorption studies with different adsorptives such as nitrogen, argon, carbon dioxide, methanol and methane at different temperatures were carried out. Unusual adsorption–desorption isotherms with one or two hysteresis loops are found – a remarkable feature of the new flexible MOF material.
Uptakes of 9.2 mmol g−1 (40.5 wt %) for CO2 at 273 K/0.1 MPa and 15.23 mmol g−1 (3.07 wt %) for H2 at 77 K/0.1 MPa are among the highest reported for metal–organic frameworks (MOFs) and are found for a novel, highly microporous copper‐based MOF (see picture; Cu turquoise, O red, N blue). Thermal analyses show a stability of the flexible framework up to 250 °C.
Metal–organic frameworks (MOFs) as highly porous materials have gained increasing interest because of their distinct adsorption properties.1–3 They exhibit a high potential for applications in gas separation and storage,4 as sensors5 as well as in heterogeneous catalysis.6 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.11, 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.13 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).14 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).15, 16 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.17
Herein, we present a novel microporous copper‐based MOF equation image[Cu(Me‐4py‐trz‐ia)] (1; Me‐4py‐trz‐ia2−=5‐(3‐methyl‐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‐ia2−, 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,18 for which up to now only a few coordination polymers are known.
This work provides a series of methane adsorption isotherms and breakthrough curves on one 5A zeolite and one activated carbon. Breakthrough curves of CH4 were obtained from dynamic column measurements at different temperature and pressure conditions for concentrations of 4.4 – 17.3 mol.‐% in H2/CH4 mixtures. A simple model was developed to simulate the curves using measured and calculated data inputs. The results show that the model predictions agree very well with the experiments.
Regarding the importance of adsorptive removal of carbon monoxide from hydrogen-rich mixtures for novel applications (e.g. fuel cells), this work provides a series of experimental data on adsorption isotherms and breakthrough curves of carbon monoxide. Three recently developed 5A zeolites and one commercial activated carbon were used as adsorbents. Isotherms were measured gravimetrically at temperatures of 278–313 K and pressures up to 0.85 MPa. Breakthrough curves of CO were obtained from dynamic column measurements at temperatures of 298–301 K, pressures ranging from 0.1 MPa to ca. 6 MPa and concentrations of CO in H2/CO mixtures of 5–17.5 mol%. A simple mathematical model was developed to simulate breakthrough curves on adsorbent beds using measured and calculated data as inputs. The number of parameters and the use of correlations to evaluate them were restricted in order to focus the importance of measured values. For the given assumptions and simplifications, the results show that the model predictions agree satisfactorily with the experimental data at the different operating conditions applied.
The separation of nitrogen and methane from hydrogen-rich mixtures is systematically investigated on a recently developed binder-free zeolite 5A. For this adsorbent, the present work provides a series of experimental data on adsorption isotherms and breakthrough curves of nitrogen and methane, as well as their mixtures in hydrogen. Isotherms were measured at temperatures of 283–313 K and pressures of up to 1.0 MPa. Breakthrough curves of CH4, N2, and CH4/N2 in H2 were obtained at temperatures of 300–305 K and pressures ranging from 0.1 to 6.05 MPa with different feed concentrations. An LDF-based model was developed to predict breakthrough curves using measured and calculated data as inputs. The number of parameters and the use of correlations were restricted to focus on the importance of measured values. For the given assumptions, the results show that the model predictions agree satisfactorily with the experiments under the different operating conditions applied.