Optimal Surface Amino-Functionalization Following Thermo-Alkaline Treatment of Nanostructured Silica Adsorbents for Enhanced CO₂ Adsorption

Special preparation of Santa Barbara Amorphous (SBA)-15, mesoporous silica with highly hexagonal ordered, these materials have been carried out for creating adsorbents exhibiting an enhanced and partially selective adsorption toward CO₂. This creation starts from an adequate conditioning of the silica surface, via a thermo-alkaline treatment to increase the population of silanol species on it. CO₂ adsorption is only reasonably achieved when the SiO₂ surface becomes aminated after put in contact with a solution of an amino alkoxide compound in the right solvent. Unfunctionalized and amine-functionalized substrates were characterized through X-ray diffraction, N₂ sorption, Raman spectroscopy, electron microscopy, ²⁹Si solid-state Nuclear Magnetic Resonance (NMR), and NH₃ thermal programmed desorption. These analyses proved that the thermo-alkaline procedure desilicates the substrate and eliminates the micropores (without affecting the SBA-15 capillaries), present in the original solid. NMR analysis confirms that the hydroxylated solid anchors more amino functionalizing molecules than the unhydroxylated material. The SBA-15 sample subjected to hydroxylation and amino-functionalization displays a high enthalpy of interaction, a reason why this solid is suitable for a strong deposition of CO₂ but with the possibility of observing a low-pressure hysteresis phenomenon. Contrastingly, CH₄ adsorption on amino-functionalized, hydroxylated SBA-15 substrates becomes almost five times lower than the CO₂ one, thus giving proof of their selectivity toward CO₂. Although the amount of retained CO₂ is not yet similar to or higher than those determined in other investigations, the methodology herein described is still susceptible to optimization.