Identical Composition and Distinct Performance: How ZIF-8 Polymorphs’ Structures Affect the Adsorption/Separation of Ethane and Ethene

Zeolitic imidazolate frameworks (ZIFs) can separate olefins from paraffin with very little energy consumption, which is critical for the chemical industry. They can form a large diversity of polymorphs, which show identical chemical formulas but distinct olefin/paraffin separation performance. The adsorption and separation of ethane/ethene on several ZIF-8 polymorphs, some recently synthesized in experiments, were studied by grand canonical Monte Carlo and ideal adsorbed solution theory simulations in this work. These two methods yielded consistent results, which also agree with experiments. The adsorption mechanism, especially the relationship between the spatial arrangement of the metal and ligand in these polymorphs and the adsorption of olefins/paraffin, was revealedviain-depth investigation of the simulation data. The adsorption of ethane/ethene and propane/propene was compared to further elucidate that the low pressure uptake well correlates with ZIF-gas interaction, for example, the heat of adsorption and Henry coefficient, while high pressure uptake correlates with the ZIFs’ geometrical structures, such as porosity or accessible surface area. The adsorption competition between ethane and ethene in the mixture and the separation performance of the ZIF polymorphs were also investigated. Our work could help understand how ZIFs’ structures affect their olefin/paraffin adsorption behavior and develop nanoporous materials with improved separation performance.