Effect of calcination temperature on carbon dioxide separation properties of a novel microporous hybrid silica membrane

A novel microporous hybrid silica membrane for the separation of carbon dioxide, fabricated through sol–gel deposition of a microporous Nb-doped ethylene-bridged silsesquioxane layer on a multilayer porous support, was reported. Effect of the calcination temperature on H2/CO2 separation properties of Nb–BTESE membrane was investigated. Low CO2 permeance was imparted by doping acidic niobium centers into the hybrid silica networks. Denser hybrid silica networks as well as more Lewis acid sites were generated as the calcination temperature elevated, which imparted very low CO2 permeance to the novel hybrid membrane while retaining its relative high H2 flux in the order of ∼10−7 mol m−2 s−1 Pa−1. Dominant densification occurred in the Nb-doped hybrid silica networks when the calcination temperature was lower than 400 °C. Meanwhile, the Nb–BTESE membrane showed relatively weak acidity which was induced by niobium doping. Dual effects are working when the heat-treated temperature was higher than 400 °C. On the one hand, the increased surface acidity reduced the number of sites and/or affinity for adsorption of CO2 as the calcination temperature elevated. On the other hand, membrane densification occurred during the calcination process. Therefore, the permselectivity of H2/CO2 for Nb–BTESE membrane could be tuned by altering the calcination temperature. The Nb–BTESE membrane calcined at 450 °C showed both relative high hydrogen permeance (∼9.7 × 10−8 mol m−2 s−1 Pa−1) and excellent H2/CO2 permselectivity (220), as compared with Nb–BTESE membranes calcined at other temperatures.