Selective adsorption of CO2 and SF6 on mixed-linker ZIF-7–8s : The effect of linker substitution on uptake capacity and kinetics

A series of mixed-linker Zeolitic Imidazolate Framework(ZIF)-7–8s constructed from different amounts of benzimidazolate (bIm) and 2-methylimidazolate (mIm) linkers were synthesized in this study. We demonstrated that by carefully controlling the ratios between bIm and mIm linkers, the particle morphology and pore size of ZIF-7–8 can be tailored to selectively adsorb CO2 or SF6 with significantly enhanced uptake capacity. ZIF-7–8 with 90% bIm and 10% mIm linkers showed a CO2 uptake of 1.44 mmol g−1 at 293 K (1 bar) and a CO2/N2 selectivity of over 30. On the other hand, ZIF-7–8 with 26% bIm and 74% mIm linkers had a high SF6 uptake of 2.08 mmol g−1 at 293 K (1 bar) as well as a high SF6/N2 selectivity of over 40. Isosteric enthalpy of adsorption calculations and cyclic adsorption experiments confirmed that both CO2 and SF6 were physisorbed on ZIF-7–8. CO2 was found to adsorb rapidly on ZIF-7–8, with over 80% of the total uptake capacity reached within 30 s. Detailed kinetics analysis concluded that the adsorption of CO2 and SF6 on these ZIF-7–8s could be described by pseudo-second order kinetics and the diffusion was found to be governed by a mixture of different mechanisms. The highly tunable sorption properties, high uptake capacity and high selectivity of ZIF-7–8 render them as interesting candidate sorbents for different greenhouse gases