Encapsulation of [bmim+][Tf2N−] in different ZIF-8 metal analogues and evaluation of their CO2 selectivity over CH4 and N2 using molecular simulation

Abstract

Zeolitic imidazolate frameworks (ZIFs), a subfamily of metal–organic frameworks (MOFs), are considered as candidates for the development of energy-efficient and high-performing gas separation processes based on nanoporous materials. A recently reported class of hybrid materials, which consist of ionic liquid (IL) pairs encapsulated in the cages of ZIFs (namely IL@ZIFs), has revealed exceptional CO₂ selectivity. Herein, we investigate the effect of the metal center type of the framework on the performance of IL@ZIFs, exclusively using computational methods. We use the highly studied ZIF-8 and prepare metal variants, by replacing the original metal Zn²⁺ with Co²⁺, Be²⁺ and Cd²⁺. For each ZIF-8 metal analogue, we prepare IL@ZIFs of varying IL composition, by introducing methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim⁺][Tf₂N⁻]) in the ZIF cages. With the use of Monte Carlo simulations, we evaluate the selectivity of both the pristine ZIF-8 metal analogues and the IL@ZIF-8 metal analogues for CO₂/CH₄ and CO₂/N₂ mixtures. Our results show that metal variation affects the ZIF performance. Moreover, the IL composition affects the performance; for each IL@ZIF-8 case, there is an optimum IL composition that ensures maximum selectivity for the two mixtures. In an effort to facilitate the evaluation of the IL composition-related performance, we use the available pore volume (APV) parameter. Our analysis reveals that there is a common APV value among all IL@ZIF-8 analogues that dictates the optimum separation performance for both CO₂/CH₄ and CO₂/N₂ mixtures. This finding can help towards a better understanding of the design and preparation of these materials.