Exploring the interplay of adsorption and diffusion for separation of CO 2 /CH 4 in zeolite-like metal–organic frameworks by a molecular dynamics simulation

Abstract

Atomically detailed simulations were performed to explore the coupled effects of adsorption and diffusion on the structural and transport properties of CO₂, CH₄, and their equimolar mixture in Na⁺ ion-exchanged rho zeolite-like metal–organic framework (Na-rho-ZMOF). It is observed that the adsorption of CO₂ in rho-ZMOF leads to slight slowing-down and speeding-up of the drift motion of Na⁺ ions, depending on loading of CO₂ molecules. The simulations of the diffusivities of single-component CO₂ and CH₄ indicate that a maximum diffusivity with varying loadings exists for both components, and for CO₂ the maximum diffusivity arises mainly from the electrostatic interactions between the CO₂ quadrupole moment and framework charges. According to our simulation results of the equimolar mixture, with an increase in loading, a diffusion selectivity reversal of CO₂ over CH₄ is observed; adsorption is the main factor for the separation process of CO₂ and CH₄ in Na-rho-ZMOF; and the permselectivity of CO₂ with respect to CH₄ as a function of total pressure also shows a maximum and finally approaches a plateau with an overall value of about 40.0, which is higher than many other commonly used MOF materials such as Cu-BTC and isoreticular MOFs at the same pressure.