Pure, amine-derivatized and nickel-doped sol-gel silica membranes have been developed on tubular Membralox-type commercial ceramic supports for the purpose of carbon dioxide separation from nitrogen under coal-fired power plant flue gas conditions. An extensive synthetic and permeation test study was carried out in order to optimize membrane CO2 permeance, CO2:N-2 separation factor and resistance against densification. Pure silica membranes prepared under optimized conditions exhibited an attractive combination of CO2 permeance of 2.0 MPU (1 MPU = 1 cm(3)(STP) cm(-2) min(-1) atm(-1)) and CO2:N-2 separation factor of 80 with a dry 10:90 (V/V) CO2: N-2 feed at 25 degrees C. However, these membranes exhibited flux decline phenomena under prolonged exposure to humidified feeds, especially in the presence of trace SO2 gas in the feed. Doping the membranes with nickel (11) nitrate salt was effective in retarding densification, as manifested by combined higher permeance and higher separation factor of the doped membrane compared to the pure (undoped) silica membrane after 168 h exposure to simulated flue gas conditions. (C) 2009 Elsevier B.V. All rights reserved.