Molecular Simulations and Theoretical Predictions for Adsorption and Diffusion of CH4/H2 and CO2/CH4 Mixtures in ZIFs

Adsorption and diffusion of CO2/CH4 and CH4/H-2 mixtures were computed in zeolite imidazolate frameworks (ZIFs), ZIF-68 and ZIF-70, using atomically detailed simulations. Adsorption selectivity, diffusion selectivity, and membrane selectivity of ZIFs were calculated based on the results of atomistic simulations. Mixture adsorption isotherms predicted by the ideal adsorbed solution theory agree well with the results of molecular simulations for both ZIFs. Mixture diffusivity calculations indicate that diffusion of CH4 is increased with increasing concentration of H-2 in the CH4/H-2 mixture, while the diffusivity of H-2 decreases with increasing CH4 concentration. In contrast, the diffusivity of CH4 is essentially independent of the concentration of CO2 in the CO2/CH4 mixture, while CO2 diffusivity decreases with increased CH4 loading, even though the diffusivity of CH4 is substantially larger than that of CO2. This unusual behavior can be explained in terms of differences in adsorption site preferences due to charge-quadrupole interactions.