Potential of sub- and supercritical CO2 reaction media for sol–gel deposition of silica-based molecular sieve membranes

• Potential of supercritical CO2-based deposition method was investigated.
• Deposition process was characterized by specific “deposition signature” curves.
• Derived silica membranes were deposited directly on macroporous alumina support.
• Molecular sieve membranes with activated gas transport for helium were obtained.

A new eco-friendly method recently developed in our group has been further investigated for the preparation of gas selective silica-based molecular sieve membranes on/in macroporous tubular ceramic supports without any intermediate layer. The synthesis protocol under sub- and supercritical conditions was based on an “On-Stream Supercritical Fluid Deposition method” (OS-SFD) applying supercritical carbon dioxide (scCO2) as an attractive “green” solvent with easily adjustable properties enabling a controlled solubilisation/reaction of precursors and their transport to the ceramic support. Parameters influencing the final membrane characteristics such as permeates flow rate, calcination treatment and deposition steps have been examined for a selected reaction mixture, transmembrane pressure and defined deposition temperatures. On-line monitoring of the membrane formation process (deposition signature curve) was used in this process. Membrane characteristics are discussed in correlation with their gas permeation properties. The optimized crack-free silica membranes prepared at 50 °C have a compact microstructure but a thermal stability limited to 400 °C. A second deposition run allowed a recovery of the molecular sieving behaviour with a thermally activated transport for He up to 350 °C. These promising results demonstrate the potential of this novel method for the preparation of uniform molecular sieve membranes deposited directly on macroporous supports with virtually zero waste.

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