Evaluation of a new On-Stream Supercritical Fluid Deposition process for sol–gel preparation of silica-based membranes on tubular supports

A novel “On-Stream Supercritical Fluid Deposition” (OS-SFD) process has been investigated in this work coupling the sol–gel chemistry and a filtration/compression operation in supercritical CO2 (sc-CO2), for the production of uniform membranes on/in porous ceramic tubular supports. The versatility of this process allows both the direct formation of thin coatings on porous tubular membrane supports but also their internal modification. An attractive on-line control of the deposition process was operated by recording the transmembrane pressure evolution during membrane formation. Silica membranes were directly deposited on macroporous supports (155 mm long α-Al2O3, with 200 nm pore sizes) from TEOS derived sols dissolved in sc-CO2 and transported to the tubular support where the condensation/gelation and deposition occurred. The deposition mechanism has been correlated with the sol–gel transition in sc-CO2 conditions and the impact of the deposition temperature, sol formulation and sc-CO2 flow rate on the membrane characteristics (morphology, weight increase and single gas permeance) have been discussed. Supersaturation and precipitation of transported clusters followed by their condensation and gelation were found as key parameters controlling the silica-based membrane design and microstructure/compacity of the silica network.

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► Novel “On-Stream Supercritical Fluid Deposition” process has been developed.
► Addition of yttrium was considered for possible retarding the TEOS gelation.
► Silica membranes were deposited on macroporous supports from TEOS derived sols.
► Progress of the membrane formation was monitored “operando”.
► Sol composition and deposition temperature influenced the final membrane design.