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Abstract: A magnesium-based metal organic framework (MOF), also known as Mg-MOF-74, was successfully synthesized, characterized, and evaluated for adsorption equilibria and kinetics of CO2 and CH4. The Mg-MOF-74 crystals were characterized with scanning electron microscopy for crystal structure, powder X-ray diffraction for phase structure, and nitrogen adsorption for pore textural properties. Adsorption equilibrium and kinetics of CO2 and CH4 on the Mg-MOF-74 adsorbent were measured in a volumetric adsorption unit at 278, 298, and 318K and pressures up to 1bar. It was found that the Mg-MOF-74 adsorbent prepared in this work has a median pore width of 10.2Å, a BET specific surface area of 1174m2/g, CO2 and CH4 adsorption capacities of 8.61mmolg−1 (37.8wt.%) and 1.05mmolg−1 (1.7wt.%), respectively, at 298K and 1bar. Both CO2 and CH4 adsorption capacities are significantly higher than those of zeolite 13X under similar conditions. The pressure-dependent equilibrium selectivity of CO2 over CH4 (q CO2/q CH4) in the Mg-MOF-74 adsorbent showed a trend similar to that of zeolite 13X and the intrinsic selectivity of Mg-MOF-74 at zero adsorption loading is 283 at 298K. The initial heats of adsorption of CO2 and CH4 on the Mg-MOF-74 adsorbent were found to be 73.0 and 18.5kJmol−1, respectively. The adsorption kinetic measurements suggest that the diffusivities of CO2 and CH4 on Mg-MOF-74 were comparable to those on zeolite 13X. CH4 showed relatively faster adsorption kinetics than CO2 in both adsorbents. The diffusion time constants of CO2 and CH4 in the Mg-MOF-74 adsorbent at 298K were estimated to be 8.11×10−3 and 4.05×10−2 s−1, respectively, showing a modest kinetic selectivity of about 5 for the separation CH4 from CO2. [Copyright &y& Elsevier]

Abstract: Separation of carbon dioxide and methane is an important issue in upgrading low-quality natural gas. Adsorption equilibria and kinetics of CO2 and CH4 on a copper metal–organic framework (MOF), Cu(hfipbb)(H2hfipbb)0.5 [H2hfipbb=4,4′-(hexafluoroisopropylidene) bis(benzoic acid)], were investigated to evaluate the feasibility of removing CO2 from CH4 in a pressure swing adsorption process using this new MOF adsorbent. The heat of adsorption of CO2 on the Cu–MOF at zero-coverage (29.7kJ/mol) is much lower than those on a carbon molecular sieve and a zeolite 5A adsorbent; and the heat of adsorption of CH4 on the Cu–MOF (21.4kJ/mol) is similar to that on the zeolite 5A adsorbent and smaller than that on a carbon molecular sieve. The Cu–MOF being investigated has apertures of (∼3.5×3.5Å), which favors the kinetically controlled separation of CO2 and CH4. The kinetic selectivity is found to be 26 at 298K, and the overall selectivity (combining the equilibrium and kinetic effects) is about 25 for an adsorption separation process. These results suggest that the Cu–MOF adsorbent is an attractive alternative adsorbent for the CO2/CH4 separation. [Copyright &y& Elsevier]