Microporous niobia–silica membranes: Influence of sol composition and structure on gas transport properties

Three different microporous niobia–silica gas separation membranes with an exceptionally low permeability for CO2 were prepared and their permeabilities for different gases were compared. The Nb:Si molar ratios of the sols varied between 0.33 and 0.8, and the radii of gyration varied between 2.4 and 3.2 nm, respectively. All three sols showed narrow particle size distributions, and no particles with hydrodynamic diameters above 60 nm were detected. The hydrogen and helium permeabilities of membranes could be correlated with the degree of structural evolution of the sols. The structurally least developed sol yielded more resistive membrane films. The increase in the loading of Nb5+ ions in the silica framework led to an increase of the H2/CO2 separation factor from ∼40 to 70, but also caused densification of the material, leading to a much more resistive network. Single gas permeation measurements showed a preferential permeability towards helium at 200 °C for all the membranes. X-ray photoelectron spectroscopy analysis of a niobia–silica thin film showed that it had a homogeneous distribution of Si and Nb atoms, and that this material exhibited a different interaction with CO2 than pure silica and pure niobia surfaces do.