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

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 (CoOxSi) xerogels were synthesized and exposed to steam at 500 °C for varying time periods. Subsequent characterization with FTIR and N2 adsorption revealed that CoOxSi xerogels were significantly more hydrostable than CoSi xerogels, with few structural changes observed and only moderate densification (∼43%) of the CoOxSi matrix experienced even after the longest steam exposure. In comparison the CoSi matrix experienced severe densification (∼89%) after only short term steam exposure. CoOxSi was therefore selected as the optimal material for further membrane tests and CoOxSi 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/N2 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.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights▶ Cobalt oxide silica materials are more hydrostable than cobalt silica. ▶ Water exposure reduced gas ideal selectivity of CoOxSi membranes at 500 °C. ▶ Steam effect was more pronounced on nitrogen than helium diffusion. ▶ The membrane matrix underwent reversible structural modification by the combined effect of water and temperature.