Propylene/propane membrane based separation is one of the most challenging tasks in modern chemical industry due to their similar chemical and physical properties. Current most welldeveloped method of separating propylene/propane is by applying multi-column distillation system which is extremely energy intensive and costly. Membrane-based separation method have provided a feasible way to solve this long existing problem but the performances (selectivity and permeance) of current most widely used polymeric membranes are still not satisfying. Zeolitic imidazolate framework (ZIFs), a kind of nanoporous material based on zeolitic topologies, has shown impressive potential in hydrocarbon gas separation attribute to its tunable structure and good thermal and chemical stability. Among all ZIF family members, ZIF-8, a promising candidate with effective aperture size of 4.0 ? has been proved to possess unprecedentedly high propylene/propane separation performance, well above the traditional polymeric upper limit. Its cobalt-substituted isostructural version, ZIF-67, have been predicted to have smaller effective aperture size (~3.9 ?) and therefore much better selectivity while relatively maintaining the permeance. However, syntheses of high-quality phase-pure ZIF-67 was challenging and no phase-pure ZIF-67 membrane have ever been reported of any propylene/propane selectivity.
Meanwhile, considering the high unit area cost of ZIF membrane, ultra-thin membrane with desired performance is highly demanded to improve the productivity and production intensity. In this research, I would like to demonstrate the successful syntheses of defect-free, ultrathin phase-pure ZIF-67 (thickness: 300 - 500 nm) membranes as well as ZIF-8 membranes via a novel seeding method named combined seeding. This combined seeding method consists of well-developed microwave-assisted seeding and electrophoretic nuclei assembly for crystallization of highly intergrown thin-films (ENACT) as the seeding procedure followed by a solvothermal secondary growth. The as-synthesized ZIF-67 showed a decent propylene/propane separation factor of 67 and a propylene permeance of ~ 80 GPUs. With an additional tertiary growth, the separation factor was increased to a record-high 298 with a propylene permeance of 30 GPUs. Furthermore, when the membranes were subjected upon post-synthetic ligand exchange (PSLE), the propylene permeance was enhanced to ~298 GPUs with a propylene/propane separation factor of 19. It should be noted that this is the first reported case of phase-pure ZIF-67 membranes showing propylene/propane separation.

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