Insights into the Solubility of Carbon Dioxide in Grafted Mesoporous Silica for the Catalytic Synthesis of Cyclic Carbonates by Nanoconfinement

Gas solubility can go beyond classical bulk-liquid Henry’s law saturation under the nanoconfinement of a liquid phase. This concept establishes the foundation of the current study for developing a novel catalytic system for transformation of carbon dioxide to cyclic carbonates at mild conditions with major emphasis on application for CO2 capture and utilization. A series of mesoporous silica-based supports of various pore sizes and shapes grafted with a quaternary ammonium salt is synthesized and characterized. CO2 sorption in styrene oxide, either in bulk or nanoconfined state, as well as catalytic reactivity for CO2 transformation into styrene carbonate, are experimentally evaluated. The family of mesoporous catalysts with aligned cylindrical pores (MCM-41 and SBA-15) with pore sizes ranging from 3.5 to 9 nm exhibit enhanced sorption of CO2 in nanoconfined styrene oxide with maximum sorption capacity taking place in MCM-41 with the smallest pore size. The catalysts with interconnected cylindrical pores (KIT-6) with pore sizes ranging from 4.5 to 8.7 nm showed CO2 solubilities almost equal to the bulk solubility of styrene oxide. Monte Carlo simulations revealed that the oversolubility in styrene oxide confined complex is directly related to the density of adsorbed solvent in the nanopore, which is less than its bulk density. Catalytic reactivities correlate with CO2 sorption enhancement, showing higher turnover frequencies for catalysts having higher CO2 sorption capacity. The turnover frequency is increased by a factor of 7.5 for grafted MCM-41 with the smallest pore size with nanoconfined styrene oxide in comparison to the homogeneous reaction implemented in bulk.