Functionalized Inorganic Membranes for High-Temperature CO2/N2 Separation

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Inorganic membranes play an important role in the development of economical processes for pre-combustion and/or post-combustion capture of carbon dioxide (CO2) at high temperatures. Mesoporous silica, due to its chemical and mechanical properties, is considered as a candidate for the capture of CO2 at high temperatures. Bare silica membranes exhibit Knudsen diffusion behavior for most gases but also exhibit the contribution of surface diffusion for heavier or interacting gases such as CO2 and CH4. The CO2/N2 selectivity of mesoporous silica membranes can be enhanced by surface modification using aminosilanes such as APTS (3-aminopropyl-triethoxy silane). The important aspect of such modified membranes is that they can be operated at high temperatures typically encountered in post-combustion gas streams (flue gas). For modified silica membranes, mixed gas separation factors as high as 10 for CO2 over N2 were observed. The transport mechanism in such membranes is the reaction of CO2 with the amine groups (in aminosilanes) to form a carbamate species and subsequent surface “hopping” of carbon dioxide. Under ambient conditions, CO2 is strongly bounded to the amine groups and thus greatly inhibits the surface diffusion of CO2; however, as the temperature increases, the CO2 permeance increases and selective transport of CO2 is observed. Thus, in surface-modified facilitated transport membranes, economical CO2 separation is achieved through the combination of the chemical reaction of CO2 associated with amine absorption along with the simplicity and low operating costs of membrane processes.