Regenerable solid CO2 sorbents prepared by supercritical grafting of aminoalkoxysilane into low-cost mesoporous silica

The present work examines the functionalization of silica supports via supercritical CO2 grafting of aminosilanes, which is an important step in the preparation of materials used as solid sorbents in CO2 capture. Four materials have been considered as solid supports: two commercially available silica gels (4.1 and 8.8 nm pore diameter), the mesoporous silica MCM-41 (3.8 nm pore diameter) and a microporous faujasite of the Y type. Mono- and di-aminotrialkoxysilane were chosen for this study. The optimal operating conditions required to have free aminosilane in solution were first evaluated by studying the phase behavior of the system scCO2/aminosilane at different pressures and temperatures. FTIR spectroscopy was used to determine the chemical structure of the grafted species. Aminosilane uptake was estimated by thermogravimetric and elemental analysis. Densities up to 3–4 molecules of monoaminosilane per nm−2 were reached by using a small amount of a cosolvent together with the supercritical CO2. The samples were characterized in regards of thermal stability, showing that aminosilane groups were covalently attached to the amorphous silica surface in the mesoporous supports, but not in the microporous zeolite. Low temperature N2 and ambient temperature CO2 isotherms were recorded to establish the adsorptive behavior of the prepared hybrid materials. The amine functionalized MCM-41 and the 8.8 nm silica gel exhibited a significant higher uptake of CO2 at low pressures compared with the bare supports. On the contrary, for the 4 nm silica gel and the zeolite the adsorption decreased after impregnation.