Gate opening effect of zeolitic imidazolate framework ZIF-7 for adsorption of CH4 and CO2 from N2

Abstract

We report adsorption isotherms of CO₂ and CH₄ on the zeolitic imidazolate framework ZIF-7 that exhibit gate opening features associated with a flexible framework structure. This phenomenon has been reported by others for CO₂ and light alkanes (e.g. ethane, ethylene, propane), but our study presents for first time experimental data to show that CH₄ can also induce a gate opening effect in ZIF-7. Uptakes of CO₂, CH₄ and N₂ on ZIF-7 were measured by a gravimetric adsorption apparatus at temperatures of 303–323 K and pressures up to 4494 kPa. From the CH₄ isotherm measured at 303 K the transition pressure for the gate opening in ZIF-7 was estimated as 1245 kPa and the free-energy change associated with the structural phase change was 5.70 kJ mol⁻¹. At an adsorption temperature of 303 K the phase transition pressure for CO₂ in ZIF-F was 78 kPa and the free energy change was 2.43 kJ mol⁻¹. The gate opening behaviour observed in this study shows ZIF-7 may have a potential selectivity for CH₄ from N₂ of more than 10 from an equimolar CH₄ + N₂ mixture. The equilibrium selectivity of ZIF-7 at 303 K and pressures close to 100 kPa are predicted to be 24 for CO₂ from CH₄ and 101 for CO₂ from N₂.