Development of inorganic-organic hybrid membranes for carbon dioxide/methane separation

Membrane separation processes are attractive for the removal of carbon dioxide from natural gas since they consume less energy than conventional methods such as amine absorption and pressure swing adsorption. In this work inorganic-organic hybrid membranes were prepared employing chemical vapor deposition (CVD) of tetraethylorthosilicate (TEOS) and 3-aminopropyltriethoxysilane (APTES) as silica and amino-silica precursors. They were deposited on the surface of a porous alumina support at high temperature using oxygen as a co-reagent. The objective was to enhance the permeance of CO₂ by placing amine groups on the surface of the membrane. The APTES/(TEOS+APTES) ratio R was varied from 0% to 100% in order to find an optimum composition for the separation of the CO₂ from CH₄. The best membrane was found to have a ratio R of 20% with a CO₂ permeance of 2.3×10^-7molm^-2s^-1Pa^-1 and an ideal CO₂/CH₄ selectivity of 40 at 393K and 0.10MPa of partial pressure difference.The transport mechanism for CO₂ permeation was surface diffusion and for CH₄ passage was gas-translation. The pore size of the membrane was evaluated by Tsuru[U+05F3]s method revealing a pore size of 0.44nm. The results are significant because the permeance level is above that necessary for commercial use, the selectivity is adequate to produce a pipeline quality natural gas (purity>2.4%), and the permeating gas is CO₂ which allows retention of methane at high pressure.