Improving propylene/propane separation performance of Zeolitic-Imidazolate framework ZIF-8 Membranes

• ZIF-8 membrane microstructures were controlled by changing synthesis variables.
• There exists an optimal catalyst amount for high quality ZIF-8 membranes.
• Nature of zinc salts determines the membrane microstructures.
• Microstructure optimization results in enhanced propylene/propane separation.
• Solvothermal activation resulted in further enhancement in propylene permeance.

Zeolitic-imidazolate framework ZIF-8 has shown great potential for high resolution separation of propylene/propane mixtures based on the size exclusion principle. Recently, we have successfully prepared highly propylene-permselective ZIF-8 membranes by developing a counter diffusion-based in situ method. As a follow-up study, here, we attempted to further improve the separation performance of ZIF-8 membranes prepared by the counter-diffusion method by controlling membrane microstructures via manipulating important synthesis variables as well as activation processes.Important synthesis variables investigated include the sodium formate to ligand ratios, the nature of zinc salts. With a sodium formate to ligand ratio of 0.12 and a mixture of zinc nitrate and zinc chloride, we have successfully synthesized ZIF-8 membranes with much improved propylene/propane separation performances with the separation factor of ~70 and the propylene permeance of ~268×10−10 mol/m2 s Pa. Furthermore, solvothermal activation processes were found to be effective in significantly increasing the propylene permeance while maintaining decent propylene/propane separation factors.

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