High temperature steam investigation of cobalt oxide silica membranes for gas separation

Abstract

In this work we investigate the effect of high temperature steam on cobalt and cobalt oxide derived silica with an aim to providing an understanding of the permeation and gas separation performance of cobalt silica membranes exposed to simulated industrial wet gas streams. Cobalt silica (CoSi) and cobalt oxide silica (CoO x Si) xerogels were synthesized and exposed to steam at 500 °C for varying time periods. Subsequent characterization with FTIR and N 2 adsorption revealed that CoO x Si xerogels were significantly more hydrostable than CoSi xerogels, with few structural changes observed and only moderate densification (∼43%) of the CoO x Si matrix experienced even after the longest steam exposure. In comparison the CoSi matrix experienced severe densification (∼89%) after only short term steam exposure. CoO x Si was therefore selected as the optimal material for further membrane tests and CoO x Si membranes were subsequently synthesized using sol–gel techniques and exposed to steam in both high temperature long term stability studies and during temperature cycling tests. Exposure to steam had an adverse effect on membrane performance with the largest effect occurring during the initial stages where He permeance dropped from 3.95 × 10 −8 to 2.05 × 10 −10 mol m −2 s −1 Pa −1 and He/N 2 ideal selectivities from 123 to 45, respectively after 135 h of testing. However, following this initial period of steam conditioning, these membranes were able to oppose further deterioration and maintain acceptable steady state performance. Further tests with an increased steam rate showed that nitrogen permeation decreased more significantly than helium, leading to a rise in membrane ideal selectivity, suggesting that steam was competitively blocking the pores available to nitrogen adsorption and/or reducing the diffusion of nitrogen. Steam testing under cycling temperature conditions showed that below 400 °C, the membrane ideal selectivity was low. However, above 400 °C the membrane consistently exhibited a molecular sieving mechanism. In addition water permeation through the membrane varied with temperature suggesting that the membrane matrix continuously underwent reversible structural modification by the combined effect of water and temperature.