Molecular simulation for adsorption and separation of CH₄/H ₂ in zeolitic imidazolate frameworks

In this work, the grand canonical Monte Carlo method was employed to study the adsorption and separation characteristics of CH₄/H₂ on MOF-5 and five zeolitic imidazolate frameworks (ZIFs), including two sodalite (SOD), ZIF-8 and -67, two merlinoite (MER), ZIF-10 and -60, and one DFT, ZIF-3. Simulations show that more CH₄ molecules are adsorbed in all frameworks than H₂, which is consistent with a higher pure gas isosteric heat of adsorption of CH₄ as compared with that of H ₂. For both gases, adsorbed amounts primarily rely on the physical and chemical parameters of the adsorbent. Results of density distribution profiles and equilibrium snapshots of the ZIFs indicate that the most preferential gas adsorption sites for both CH₄ and H₂ are the positions near linkers. At high pressures, CH₄ begins to fill up in the center of the SOD cage. We also found that the selectivity for CH ₄ increased with the difference between the isosteric heats of adsorption of CH₄ and H₂, Δq_st, but decreased to some extent due to the packing effect. Both the isosteric heats of adsorption and the packing effect are mainly influenced by the topology of the framework.