Investigating adsorption and selectivity of CO₂ on zeolitic imidazolate framework-78 using molecular dynamic simulation

Abstract

The removal of CO₂ from natural gas that mainly contributed by CH₄ is most important elementary step in up-steam petroleum industry. ZIF-78 is a promising material for the separation of CO₂ from a gas mixture. In this work, the molecular behavior of CO₂ and CH₄ in ZIF-78 as well as the adsorption selectivity and diffusion selectivity of CO₂ in CO₂/CH₄ mixtures were investigated using Grand Canonical Monte Carlo (GCMC) and molecular dynamic (MD) simulations. The RDF results show that the CO₂ is strongly adsorbed at O atoms of the –NO₂ group of ZIF-78 whereas the CH₄ is mostly observed at H atoms of the nitroimidazolate linker of ZIF-78, respectively. The self diffusivity of CO₂ does not depend on the concentration but it is the case for CH₄. From both kinds of selectivity, the significantly higher of adsorption selectivity (8.08) than diffusion selectivity (~0.3) indicates the main contribution of adsorption selectivity in membrane permeability. It demonstrates that using ZIF-78 in equilibrium-based separation will be more efficient than using them in membrane-based separation applications.