Increasing Selective CO2 Adsorption on Amine-Grafted SBA-15 by Increasing Silanol Density

Two template removal methods were employed to create porosity in mesoporous silica SBA-15: ethanol extraction versus conventional high-temperature calcination. The resulting silicas were subjected to amine (3-aminopropyl) grafting and studied for their CO2 adsorption properties. The goal was to significantly increase the surface silanol density, and hence the grafted amine loading, leading directly to increased CO2 adsorption capacity and CO2/N2 selectivity. Thus, the silanol density was increased from 3.4 OH/nm2 for the calcined SBA-15 to 8.5 OH/nm2 for the SBA-15 by solvent extraction. Correspondingly, for these two samples, the grafted amine loading was increased from 2.2 to 3.2 mmol/g, and the CO2 adsorption capacity was increased from 1.05 to 1.6 mmol/g at conditions relevant to CO2 capture (0.15 bar and 25 °C), or a 52% increase. The CO2/N2 selectivity was increased from 46 to 131. The isosteric heats of adsorption, the sorbent stability during cyclic adsorption–desorption, and the (positive) effects of moisture on CO2 adsorption were also investigated and compared.