Microstructure and Hydrothermal Stability of Microporous Niobia-Silica Membranes: Effect of Niobium Doping Contents

Methyl-modified niobium-doped silica (Nb/SiO₂) materials with various Nb/Si molar ratios (n_Nb) were fabricated using tetraethoxysilane and methyltriethoxysilane as the silica source and niobium pentachloride as the niobium source by the sol-gel method, and the Nb/SiO₂ membranes were prepared thereof by the dip-coating process under an N₂ calcining atmosphere. Their microstructures were characterized and gas permeances tested. The results showed that the niobium element existed in the formation of the Nb-O groups in the Nb/SiO₂ materials. When the niobium doping content and the calcining temperature were large enough, the Nb₂O₅ crystals could be formed in the SiO₂ frameworks. With the increase of n_Nb and calcination temperature, the formed particle sizes increased. The doping of Nb could enhance the H₂/CO₂ and H₂/N₂ permselectivities of SiO₂ membranes. When n_Nb was equal to 0.08, the Nb/SiO₂ membrane achieved a maximal H₂ permeance of 4.83 × 10^-6 mol·m^-2·Pa^-1·s^-1 and H₂/CO₂ permselectivity of 15.49 at 200 °C and 0.1 MPa, which also exhibited great hydrothermal stability and thermal reproducibility.