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

► A concentrated synthesis gel was used to synthesize ZIF-8 membranes. ► Porous alumina hollow fibers were used as the membrane support. ► The ZIF-8 layer was polycrystalline and possessed micro-cavities. ► The ZIF-8 membranes had strong CO 2 adsorption property and thus low CO 2 permeance. 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 H 2, N 2, CH 4 and CO 2 were determined as a function of permeation time at 25 °C. H 2, N 2 and CH 4 permeances almost did not change with the permeation time. However, CO 2 permeance decreased from 9.8 × 10 −8 to 1.7 × 10 −8 mol/s m 2 Pa within 12 h, and CO 2 permeation led to an unrecoverable reduction in gas permeation rates in the further permeation experiments. After the first round tests, H 2, N 2, CH 4 and CO 2 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 H 2/CO 2, N 2/CO 2 and CH 4/CO 2 were 32.2, 12.9, and 11.9, respectively; the H 2/CO 2 separation factor was 7.1 for a H 2/CO 2 binary mixture (45% H 2). The small CO 2 permeance should arise from the strong CO 2 adsorption in ZIF-8 membranes with micro-cavities, and the adsorbed CO 2 partially blocked the gas permeation.