On the Efficient Separation of Gas Mixtures with the Mixed-Linker Zeolitic-Imidazolate Framework-7-8

A recently reported modification of the zeolitic-imidazolate framework-8 (ZIF-8) with partial replacement of the 2-methylimidazolate (mIm) linker with benzimidazolate (bIm), namely ZIF-7-8, is investigated with molecular simulations using a first-time reported force field. The size of the ZIF-7-8 aperture, which governs the gas-separation efficiency of this material and which has not been estimated before for this modification, is smaller than that of the original ZIF-8. The diffusivities of CO₂, N₂, and CH₄ estimated through transition state theory calculations result in remarkably high diffusion selectivities for CO₂/CH₄ and CO₂/N₂ mixtures. This performance enhancement is investigated in terms of structural flexibility in the form of the aperture motion through extensive estimation of the effective diameter, the total effective area, and the motion of the aperture linkers, of both ZIF-8 and ZIF-7-8. Both apertures exhibit an oscillation through the rotation of the linkers, which are adjusted according to the size of the penetrant molecules the moment they pass through it. Finally, a subsequent analysis reveals that there is strong dependency of the separation performance on the bIm-to-mIm ratio: below 33% bIm incorporation, the appearance of ZIF-8-alike wide apertures decreases dramatically the size-based selectivity of the mixtures in ZIF-7-8.