3-Aminopropyltriethoxysilane functionalized inorganic membranes for high temperature CO₂/N₂ separation

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/N₂ 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 N₂ were observed at 393K and CO₂ partial pressures of 15kPa; whereas for pure gases (CO₂ partial pressure of 303kPa), 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 15.5kcal/mol and activation energy for CO₂ diffusion/hopping from one amine group to another of APTS is 7.2kcal/mol as computed using ab initio Density Functional Theory (DFT).