Sorbents and catalysts based on mesostructured silica for pollutant removal and Carbon Capture and Utilization technologies

In recent years, concerns about heavy metal pollution and rising CO₂ levels contributing to global warming have intensified. This study focuses on synthesizing and functionalizing mesostructured silicas (MCM-41, MCM-48) to develop amine–silica sorbents for potential applications in removing Cd²⁺ from aqueous solutions and in Carbon Capture and Utilization (CCU) technologies. Due to their ordered mesoporous structure, nitrogen's lone pairs donating to Cd²⁺, and the high affinity of amines for CO₂, amine–silica sorbents are suitable for these applications. Additionally, mesostructured silica-based composites were developed by impregnating MCM-41 with copper, zinc, and zirconium oxidic phases (CZZ) for potential use in the catalytic hydrogenation of CO₂ to methanol.
Mesostructured silica was synthesized using the sol–gel technique with a templating agent, a siliceous alkoxide precursor, and two distinct solvent systems—water (MCM-41) and water/ethanol (MCM-48). Amine–silica sorbents were obtained through post-synthesis grafting with aminopropyltriethoxysilane, and were tested for Cd²⁺ removal and CO₂ capture. To reduce the environmental impact of the grafting process, an eco-friendly solvent (butanol instead of toluene) was used. The CZZ@MCM-41 composites were prepared through auto-combustion. For developing Cd²⁺-removal sorbents and CZZ catalysts, mesostructured silica was also synthesized from industrial waste (hexafluorosilicic acid, FSA). All sorbents were characterized using XRD, TEM-EDX, TEM, N₂-physisorption, thermogravimetric analysis, and ATR-FTIR.
MCM-48 was obtained by adding ethanol as a co-solvent, maintaining other experimental conditions equal to MCM-41 synthesis. MCM-41 from both FSA and TEOS exhibited similar textural properties. The amino groups' concentration was similar (~2 mmol/g) for MCM-41 functionalized with either toluene or butanol; however, the use butanol instead of toluene resulted in a lower CO₂ adsorption performance (968 μmol CO₂/g sorbent vs. 480 μmol CO₂/g sorbent). Amine–silica sorbents showed a Cd²⁺ removal efficiency of 98-99% and an adsorbed cadmium amount (q_e) of ~40-45 mgCd(II)/g sorbent for MCM-41 from both TEOS and FSA.