Highly hydrogen-permselective zeolitic imidazolate framework ZIF-8 membranes prepared on coarse and macroporous tubes through repeated synthesis

Highly hydrogen-permselective zeolitic imidazolate framework ZIF-8 membranes are prepared on coarse and macroporous Al₂O₃ tubes through a repeated synthesis strategy for the separation of H₂/CO_2, H₂/CH₄, and H₂/C₃H₈ mixtures. Since the amino-groups of 3-aminopropyltriethoxysilane (APTES) can coordinate to the free Zn²⁺ centers and thus bind the growing nano-crystals directly, the nucleation and growth of well intergrown ZIF-8 membranes could be promoted through a repeated synthesis of the ZIF-8 layer on APTES-modified macroporous Al₂O₃ tubes. The SEM and XRD characterizations indicate that a dense and phase-pure ZIF-8 membrane with a thickness of about 25.0 μm can be formed on the alumina tubes by microwave synthesis at 105 °C for 3 h, and no cracks, pinholes or other defects were observed in the membrane layer. The ZIF-8 membranes were evaluated in single gas permeation and mixed gas separation. Attributed to the preferential adsorption affinity and capacity as well as a highly porous structure with nano-sized channels, the ZIF-8 membrane displays a high hydrogen permselectivity. For the separation of equimolar H₂/CO₂, H₂/CH₄, H₂/C₃H₈ mixtures at 100 °C and 2 bar, the mixture separation factors of H₂/CO₂, H₂/CH₄, and H₂/C₃H₈ are 5.6, 44.0, 328.6, with a H₂ permeance of about 1.1 × 10^-7 mol m^-2 s^-1 Pa^-1, which is promising for potential applications in hydrogen purification and separation.