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

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 (T-g) 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 T-g increases in the sequence: pure SiO2 < TiO2-doped SiO2 < ZrO2-doped and Nb2O5-doped SiO2. Thus, the higher the T-g 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.