Co-based zeolitic imidazolate framework ZIF-9 membranes prepared on α-Al₂O₃ tubes through covalent modification for hydrogen separation

Hydrogen has been regarded as the most promising clean and renewable energy. Beside the production of the hydrogen, the separation of hydrogen is also an import issue before it can be used in fuel cells. Membrane-based separation technologies have gained considerable attentions due to its high efficiency and low energy consumption. Zeolite imidazolate framework (ZIF) membranes have drawn intense interest due to their zeolite-like properties such as permanent porosity, uniform pore size and exceptional thermal and chemical stability. It is rather challenged to prepare well-intergrown Co-based zeolitic imidazolate frameworks (ZIFs) membranes on porous α-Al₂O₃ tubes since Co-based ZIFs prefer to form crystals in the synthesis solution rather than grow as membrane layer on the support surface. In this work, we report the preparation of high-quality ZIF-9 membrane with high H₂/CO₂ selectivity and excellent thermal stability by using 3-aminopropyltriethoxysilane (APTES) as a covalent linker to modify the α-Al₂O₃ tube. Due to the formation of covalent bonds between APTES and ZIF-9, ZIF-9 nutrients are bound to the support surface, thus promoting the growth of dense and phase-pure ZIF-9 membrane with a thin thickness of about 4.0 μm. The gas separation performances of the ZIF-9 membrane were evaluated by single gas permeation and mixture gas separation of H₂/CO₂, H₂/N₂ and H₂/CH₄, respectively. The mixture separation factors of H₂/CO₂, H₂/CH₄, and H₂/N₂ of the ZIF-9 membrane are 21.5, 8.2 and 14.7, respectively, which by far exceeds corresponding Knudsen coefficients. Moreover, the as-prepared ZIF-9 membrane exhibits excellent stability at a relatively broad range of operating temperature, which is beneficial for the industrial application of hydrogen separation or further membrane reactor.