Preparation of ZIF-8 membranes supported on ceramic hollow fibers from a concentrated synthesis gel

This paper presents the synthesis of zeolitic imidazolate framework (ZIF)-8 membranes on the outer surface of alumina hollow fibers using a concentrated synthesis gel. The resulting membranes were characterized by X-ray diffraction, scanning electron microscopy, focused ion beam/scanning electron microscopy and high-resolution transmission electron microscopy. The results showed that the crystalline ZIF-8 membranes were compact and continuous, and contained micro-cavities. Single gas permeances of H2, N2, CH4 and CO2 were determined as a function of permeation time at 25 °C. H2, N2 and CH4 permeances almost did not change with the permeation time. However, CO2 permeance decreased from 9.8 × 10−8 to 1.7 × 10−8 mol/s m2 Pa within 12 h, and CO2 permeation led to an unrecoverable reduction in gas permeation rates in the further permeation experiments. After the first round tests, H2, N2, CH4 and CO2 permeances in the second round permeation tests dropped by 40%, 24%, 11%, and 6%, and remained constant in the further repeated measurements; the equilibrium ideal selectivities of H2/CO2, N2/CO2 and CH4/CO2 were 32.2, 12.9, and 11.9, respectively; the H2/CO2 separation factor was 7.1 for a H2/CO2 binary mixture (45% H2). The small CO2 permeance should arise from the strong CO2 adsorption in ZIF-8 membranes with micro-cavities, and the adsorbed CO2 partially blocked the gas permeation.