Synthesis and characterization of zeolitic imidazolate framework ZIF-7 for CO₂ and CH₄ separation

There are surging demands to separate carbon dioxide from methane in upgrading low-quality natural gas including biogas with an energy efficient process. In this work, adsorption equilibria and kinetics of CO₂ and CH₄ on a zeolitic imidazolate framework ZIF-7 were determined at various temperatures and gas pressures up to 1 bar, to evaluate the feasibility of removing CO₂ from CH₄ in a vacuum swing adsorption process using ZIF-7 as adsorbent. The as-synthesized ZIF-7 sample was characterized with scanning electron microscopy for crystal structure, powder X-ray diffraction for phase structure, thermal gravimetric analysis for thermal stability, and carbon dioxide adsorption at 0 C for pore textural properties. To study the gate-opening of ZIF-7, CO₂ adsorption was measured at five different temperatures (273, 298, 323, 348 and 373 K) and pressures up to 10 bar. A thermodynamic model was employed to estimate the free energy of phase transition between narrow pore and large pore, and a modified dual-site Sips equation could fit the S shape CO₂ isotherms well. Adsorption equilibrium selectivity (α) and adsorbent selection parameter for pressure swing adsorption processes (S) were calculated from the adsorption equilibrium data. The relatively high values of adsorption selectivities suggest that ZIF-7 is a promising adsorbent candidate for CO₂/CH₄ separation in a pressure swing adsorption process.