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 ZIF7-8 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.

Automated summary

Mixed-linker ZIF-7-8s constructed from varying amounts of bIm and mIm linkers were synthesized to selectively adsorb CO2 or SF6 with enhanced uptake capacity. Both CO2 and SF6 were found to be physisorbed on the ZIF-7-8 materials, with adsorption kinetics described by pseudo-second order kinetics and diffusion controlled by a mixture of mechanisms.