Toward the effective design of steam-stable silica-based membranes

• Thermal and hydrothermal evolution of unsupported silica-based membranes was studied.
• A good correlation was found between membrane steam-stability and the glass transition temperature of the membrane material.
• Results indicate that Nb(V), Ti(IV) and Zr(IV) ions act as network formers.

Pure silica and 5 mol% Nb2O5-, TiO2- and Zr2O-doped silica nanoparticles were synthetized via sol-gel method and used for preparing porous and dense unsupported membranes. After annealing at 723 K, samples of the porous membranes were hydrothermally treated for 2 days at 393 K. The materials were characterized by low temperature nitrogen sorption and calorimetric analysis before and after steam-exposure. The specific surface area loss after hydrothermal treatment for the porous pure SiO2 sample and the TiO2-, the ZrO2-, and the Nb2O5-doped SiO2 samples was respectively 51%, 40%, 29% and 28%, confirming a higher steam-stability for the doped silica membranes. The glass transition temperature (Tg) for membrane materials with different pore structure and thermal history was determined from their heat capacity curves, after densification at a temperature ⩾1373 K. The Tg increases in the sequence: pure SiO2 < TiO2-doped SiO2 < ZrO2-doped and Nb2O5-doped SiO2. Thus, the higher the Tg is, the lower is the specific surface area loss after hydrothermal treatment. These results support the hypothesis that Nb(V), Ti(IV), and Zr(IV) ions act as network formers in silica-based membranes and stabilize their porous structure by enhancing their network connectivity. Moreover, these results indicate that calorimetric analysis is a useful tool for comparing the structure and the stability of silica-based membranes with different composition but identical thermal history, and thus for developing basic knowledge for the effective design of steam-stable silica membranes.

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