High temperature inorganic membranes may play an important role in the development of economical processes for pre-combustion and/or post-combustion CO2 capture. The CO2/N-2 selectivity of mesoporous silica membranes is enhanced by surface modification using APTS (3-aminopropyl-triethoxy silane). Unmodified silica membranes exhibit Knudsen diffusion behavior for most gases but also have some contribution from surface diffusion of heavier or interacting gases like CO2, CH4, etc. Gas separation experiments were performed on the modified membranes for both pure gas and mixed gases for a range of temperatures and feed gas compositions. Mixed gas separation factors as high as 10 for CO2 over N-2 were observed at 393 K and CO2 partial pressures of 15 kPa; whereas for pure gases (CO2 partial pressure of 303 kPa), no separation was seen. The hypothesized transport mechanism is the reaction of CO2 with surface amine groups to form a carbamate species and subsequent surface "hopping" of carbon dioxide. The CO2 binding at ambient conditions is sufficiently strong to greatly inhibit the surface diffusion of CO2, however, as the temperature increases, the CO2 permeance increases and selective transport of CO2 is observed. The permeance of CO2 is highly non-linear, and increases with decreasing CO2 partial pressure in the feed gas. NMR studies performed on the bare support and modified membrane shows the support has been modified by APTS. Pore size analysis on the unmodified and modified silica substrates show that the pore diameter of modified substrates are lower than that of the unmodified substrate indicating that the amine silanes are anchored on the pore walls of the substrate. The NMR measurement for an APTS/silica membrane with adsorbed CO2 shows the presence of a carbamate species and thus supports the hypothesized reaction mechanism or facilitated solid-state CO2 transport. The binding energy for CO2 adsorption is 15.5 kcal/mol and activation energy for CO2 diffusion/hopping from one amine group to another of APTS is 7.2 kcal/mol as computed using ab initio Density Functional Theory (DFT). (C) 2010 Elsevier B.V. All rights reserved.