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 H-2, N-2, CH4 and CO2 were determined as a function of permeation time at 25 degrees C. H-2, N-2 and CH4 permeances almost did not change with the permeation time. However, CO2 permeance decreased from 9.8 x 10(-8) to 1.7 x 10(-8) mol/s m(2) 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, H-2, N-2, 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 H-2/CO2, N-2/CO2 and CH4/CO2 were 32.2, 12.9, and 11.9, respectively; the H-2/CO2 separation factor was 7.1 for a H-2/CO2 binary mixture (45% H-2). 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. (C) 2011 Elsevier B.V. All rights reserved.