Adsorption of CO₂ and CH₄ on a magnesium-based metal organic framework

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^-2s^-1, respectively, showing a modest kinetic selectivity of about 5 for the separation CH₄ from CO₂.