Influence of sol–gel conditioning on the cobalt phase and the hydrothermal stability of cobalt oxide silica membranes

• Hydrothermal stability of silica membranes is dependent on the cobalt oxide phase.
• Xerogels and membranes are exposed to 75 mol% water vapour and 500 oC.
• Tetrahedrally coordinated cobalt (Co2+) silica matrices are not hydrothermally stable.
• Octahedrally coordinated cobalt (Co3+) silica opposed hydrothermal densification.
• He/N2 selectivity loss of 22% (Co3+) and 73% (Co2+) due to water vapour exposure.

This work shows for the first time that the hydrothermal stability of cobalt oxide silica membranes is very dependent upon the cobalt oxide phase. Xerogels with the same cobalt loading (Co/Si=0.1) were characterised by N2 sorption, CP/MAS 29Si NMR, FTIR, Raman, XPS and DR UV–vis spectroscopy. It was found that the xerogels containing tetrahedrally coordinated cobalt (Co2+) in the silica matrix were hydrothermally unstable leading to a sharp loss of pore volume within 10 h when exposed to 75 mol% water vapour and 550 °C, followed by complete densification after 40 h. However, silica xerogels containing a high content of octahedrally coordinated cobalt (Co3+) in the form of Co3O4 were able to oppose structural densification and were consequently much more hydrothermally stable. The Co3O4 and Co2+ silica membranes were tested for single gas permeation using He, H2, CO2 and N2, delivering similar He permeance (2.48 × 10−7 and 2.85 × 10−7 mol m−2 s−1 Pa−1) and a He/N2 selectivity of 50 and 41 at 500 °C, respectively. Upon exposure to the same harsh hydrothermal conditions as the xerogels, the membranes were tested again for single gas permeation. The high content Co3O4 silica membrane saw only a marginal decrease in He/N2 selectivity to 39 (22% loss) whilst the tetrahedral cobalt coordination silica membrane had a dramatic decline in selectivity to only 11 (73% loss).

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