Adsorption, diffusion, and separation of CH4/H-2 mixtures in covalent organic frameworks: molecular simulations and theoretical predictions

Abstract

Grand canonical Monte Carlo and equilibrium molecular dynamics simulations were used to compute adsorption isotherms and self-diffusivities of CH4/H2 mixtures at various compositions in three representative covalent organic frameworks (COFs). Several properties of COFs such as adsorption selectivity, working capacity, diffusion selectivity, gas permeability, and membrane selectivity were evaluated and were compared with metal organic frameworks (MOFs), zeolites, zeolite imidazolate frameworks (ZIFs), and carbon nanotubes. Results showed that COF-6 outperforms traditional zeolites CHA, LTA, and ITQ-29 and MOFs IRMOF-1, CuBTT, and MOF-177 in adsorption-based CH4 selectivity. Membrane selectivities of COF-5, COF-6, and COF-10 were found to be higher than those of zeolites and similar to ZIFs and MOFs. Adsorption isotherms and diffusivities of CH4/H2 mixtures in the pores of COF-6 were computed using both atomically detailed simulations and theoretical correlations. Results showed that theoretical correlations based on single component adsorption and diffusion data can be used to accurately predict mixture adsorption and diffusion of gases in COFs.