Exploring the interplay of adsorption and diffusion for separation of CO2/CH4 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.