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 318 K and pressures up to 1 bar. It was found that the Mg-MOF-74 adsorbent prepared in this work has a median pore width of 10.2 Å, a BET specific surface area of 1174 m2/g, CO2 and CH4 adsorption capacities of 8.61 mmol g−1 (37.8 wt.%) and 1.05 mmol g−1 (1.7 wt.%), respectively, at 298 K and 1 bar. 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 (qCO2/qCH4) 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 298 K. The initial heats of adsorption of CO2 and CH4 on the Mg-MOF-74 adsorbent were found to be 73.0 and 18.5 kJ mol−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 298 K 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.