7. Functionalized Inorganic Membranes for High-Temperature CO₂/N₂ Separation

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.