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 N-2 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 H-2/CO2 and H-2/N-2 permselectivities of SiO2 membranes. When n(Nb) was equal to 0.08, the Nb/SiO2 membrane achieved a maximal H-2 permeance of 4.83 x 10(-6) mol center dot m(-2)center dot Pa-1 center dot s(-1) and H-2/CO2 permselectivity of 15.49 at 200 degrees C and 0.1 MPa, which also exhibited great hydrothermal stability and thermal reproducibility.