CO2 Uptake and Adsorption Kinetics of Pore-Expanded SBA-15 Double-Functionalized with Amino Groups

The CO2 adsorption properties of amine double-functionalized SBA-15 adsorbents were investigated, focusing on the adsorption kinetics of these samples as compared to those prepared by grafting and impregnation. The double-functionalization method, recently developed to obtain amino-containing mesoporous solids as CO2 adsorbents with a high adsorption capacity and selectivity, consists of the impregnation of previously grafted materials. The adsorbents were prepared by grafting the pore-expanded SBA-15 with aminopropyl-trimethoxysilane (AP), followed by impregnation with either polyethyleneimine (PEI) or tetraethylenepentamine (TEPA). The influence of the amount of the organic molecule incorporated and the functionalization method on the CO2 adsorption results was studied, particularly on the kinetics and diffusion behavior. Equilibrium adsorption isotherms showed that the CO2 uptake was enhanced for materials prepared by the double-functionalization technique due to both the higher nitrogen content and the amino group efficiency. On the other hand, CO2 adsorption maximum rates were found to be significantly different for AP-grafted and PEI- or TEPA-impregnated materials, being higher for the latter adsorbents. The adsorption rate for impregnated materials depended on the type of organic molecule used and also on the amount loaded, especially for PEI. The adsorption rate was slightly lower for the double-functionalized samples than the simply impregnated sorbents, regardless of the organic compound used for impregnation (TEPA or PEI). However, the results showed that all double-functionalized sorbents exhibited a maximum adsorption rate of about 0.28 mol of CO2 per active nitrogen and minute, which was unchanged with the variation in the nitrogen content (9.8-15.3 %N) or the free pore volume of the sample after impregnation (0.05-0.47 cm(3)/g).