Preparation And Permeation Of ZnO Induced Zeolitic Imidazolate Framwork-8Membranes

Metal organic frameworks (MOFs) membranes become a hot topic as they exhibit the superiority of both MOFs and membrane, while zeolitic imidazolate framework-8(ZIF-8) membrane is representative of this research area. The methodologies of the membrane synthesis process are generally derived from the synthesis of inorganic zeolite membrane modes, such as in situ synthesis without or with substrate modification and secondary seeded growth method. It is often difficult to prepare MOF membranes by the in situ growth method due to the slow and uneven nucleation of MOFs on the substrate. The secondary seeded approach requires the preparation of MOF seeds of high quality and a subsequent method for the steady attachment between the seeds and the substrate since the differences of the property between the support and ZIFs seeds lead the weak adhesion which could not be sintered to enhance the power just like zeolite seeds. Although there have been enormous progress in making MOF membranes, most of the membranes reported so far have been on flat discs with limited size. It is still a challenge and a need of new concepts and techniques for the synthesis of stable and high permeability MOFs membranes on tubular supports.Here we have designed a kind of intermediate layer acting as conjunctive and inducing layer, and this intermediate layer consists of zinc dioxide which induces and guides the formation of ZIF-8membrane. Different kinds of ZnO are introduced to the porous surface including ZnO nanoparticles (NPs) and ZnO nanorods (NRs). Through adjusting the micro-architecture, the stability of ZIF-8membrane was well resolved. The main contents are as follows:(1) Continuous ZIF-8membranes were prepared on7.5cm hollow fiber α-Al2O3tubes (HFT) by secondary-growth method. A uniform ZIF-8seeds layer was introduced by dip-coating method using an optimized seeds for different times. And this makes a good ground work for the well-defined ZnO induced ZIF-8membrane synthesis.(2) ZnO NPs layer was introduced on the inner tubular substrate by sol-gel technique. ZIF-8membranes were induced by activated ZnO layer. The activation of the ZnO NPs is crucial for the growth of ZIF-8membrane. The activated sites were obtained through a gently pretreated ZnO NPs with Hmim methanol solution. Through the high resolution TOF-SIMS analysis, ZIF-8SBU (secondary building unit) species were detected including Zn(Hmim), Zn(Hmim)2and Zn(Hmim)4et al. A kind of low-defect of ZIF-8membrane was formed by the induction of ZIF-8SBU species. The permeation properties of the ZIF-8membrane were tested. It is indicated that the ideal separation factor of H2/SF6reaches200with H2permeance of6×10-7mol·m-2s-1Pa-1. And this method shows well reproducibility and scalability. (3) ZIF-8@ZnO hybrid membrane was directly prepared on the ZnO nanorods modified inner surface of Al2O3support. A new hybrid membrane was obtained by growing ZIF-8crystals within the well-aligned ZnO nanorods. The H2permeance performance is4.1×10-8mol·m-2s-1Pa-1while the H2/CH4ideal separation factor is15.7. The ZIF-8@ZnO hybrid membrane showed good thermal stability at150℃.(4) To balance the permeance and separation factor, the "strut-like" ZIF-8membrane was induced by activated ZnO NRs. Different kinds of solvents were tested to get the optimal condition of ZnO NRs surface. After the treatment with Hmim methanol solution, the activated species on the surface of ZnO NRs consisted of ZIF-8SBU were still detected by TOF-SIMS."Strut-like" ZIF-8membranes were synthesized on the top of ZnO NRs by solvothermal synthesis. The H2permeance was1.6×10-7mol·m-2·s-1·Pa-1at30℃, which is fourfold of the ZIF-8@ZnO hybrid membrane, and the permselectivity of H2/CH4was12.6. While the H2permeance felled to5.6×10-8mol·m-2·s-1·Pa-1at200℃, H2/CO2permselectivity rose from5.2at30℃to10.5, showing that the membrane has high thermal stability. Owing to the flexibility of ZIF-8framework with the temperature, the pore may deform, leading to increasing the resistance of CO2permeance whose kinetic diameter was closed to the pore size. This also indicates the integrality of the "strut-like" membrane.