Development of microporous cobalt oxide silica membrane for CO2/H2 separation

Abstract

Hydrogen as a clean energy carrier is proposed as a long term solution to address issues related to global warming, energy security and air pollution. H2 production is currently fossil fuel based and this is expected to remain the case during the transition to cleaner sources. Hence, high performance H2 separation from CO2 is required to meet purity production targets, whilst the CO2 can be transported and stored in safe geological sites. Microporous silica membranes are a promising technology for processing syngas streams because of the simplicity of the sol–gel synthesis coupled with great pore size tailorability at molecular sieving dimensions. However, poor hydrothermal stability of silica membranes remains a contributing factor to the delay of the commercial deployment of this technology, particularly for processing wet syngas streams at high temperature (500-600 °C). Recent improvements have been attained by doping metal oxides in the silica matrix. Cobalt oxide silica membranes show excellent separation performance and improved hydrothermal stability. However, there are significant knowledge gaps associated with the fundamental understanding of the influence of cobalt dopant on the hydrothermal stability of silica matrices. To address these gaps, this thesis systematically investigated the physicochemical properties and hydrothermal stability of cobalt oxide silica materials and the performance of resultant membranes. The first key contribution of this thesis is the finding that the hydrothermal stability of cobalt doped silica materials is highly dependent on the cobalt phase, particularly Co3+ as cobalt tetroxide (Co3O4). The Co3O4 silica materials resulted in less than 25% surface area loss whilst maintaining microporous structures when exposed to harsh hydrothermal conditions of 75 mol% H2O(v) at 550 °C for 40 h. Contrary to this, silica samples without Co3O4 completely densified under the same testing conditions. It is postulated that Co3O4 nanoparticles ‘shielded’ the silica matrix and inhibited to a certain degree the hydrolysis and condensation of the silica in the pores walls, thus conferring improved hydrothermal stability. A second key contribution is evidenced by sol–gel conditioning, thus aiding the formation of the Co3O4 in the silica network. By increasing the water ratio and decreasing the ethanol ratio, Co3O4 was formed at a much lower loading (Co/Si=0.1) than previously reported. The cobalt silica materials synthesised from the new sol–gel conditions showed a lower surface area loss after hydrothermal treatment (HT). The local structures around cobalt atoms were probed by X-ray absorption spectroscopy (XAS). For the low cobalt loading sample (Co/Si=0.05), the XAS results showed that the cobalt was highly dispersed in the silica network in a tetrahedral coordination (Co2+) with oxygen and a small proportion of Co-Co interactions in the second shell. Co3O4 long range order was observed for higher cobalt loading samples (Co/Si=0.10, 0.25), which suggests that Co3O4 acts as a physical barrier during exposure the HT, opposing the densification of the silica network. Based on these results, two membranes with the same cobalt loading (Co/Si=0.10) were prepared. The membrane with cobalt in the form of Co3O4 was much more stable than the one with cobalt tetrahedrally coordinated with silica network (Co2+). The Co3O4 and Co2+ silica membranes displayed activated transport and delivered similar He permeance and He/N2 selectivities. Nevertheless, the high content Co3O4 silica membrane proved to have superior hydrothermal stability, showing only a marginal decrease in He/N2 selectivity from 50 to 39 (22% loss). By contrast, the Co2+ silica membrane had a dramatic decline in selectivity from 41 to only 11 (73% loss) after exposure to 75 mol% H2O(v) at 550 °C for 40 h. To provide more insights for industrial separations, a cobalt oxide silica membrane was tested for both single gas and He/CO2 gas mixtures. The influence of hydrothermal treatment (HT) (25 mol% H2O(v) at 550 °C for 100 h) was also investigated. The HT exposure had a moderate influence on the performance of the membranes, with He fluxes reducing, though He purity remained high reaching >95% for a He/CO2 50/50 feed concentration. The final contribution of this thesis is demonstrating for the first time the preparation of high performance interlayer-free membranes using a novel silica seeding sol–gel technique. The seeded sols consisted of large silica particles of ~70 nm synthesised from Stӧber process and polymeric cobalt silica sol produced from acid-catalysed sol–gel process. The large silica seeds effectively blocked the large pores of the ceramic support, thus inhibiting deep infiltration of polymeric cobalt silica sols. Therefore, the sol–gel seeding technique allows for the formation of defect-free microporous thin silica films, otherwise not attainable in interlayer-free membranes by conventional sol–gel processes.