Non-hydrolytic sol-gel synthesis of amine-functionalized silica: Template- and catalyst-free preparation of mesoporous catalysts for CO2 valorization

Carbon dioxide utilization presents an important and topical research topic. However, the performance of catalysts needed for CO 2 transformations does not achieve the necessary levels for their widespread application. To this end, we decided to study non-aqueous condensations providing amine-functionalized silica catalysts, possibly active in CO 2 -epoxide cycloaddition reaction. While non-hydrolytic sol-gel method is well-known for its efficiency in providing highly porous Lewis and Brønsted acid metallosilicates, here we show for the first time its application for the preparation of silica-based catalysts containing basic groups. First, the reaction conditions were screened to reproducibly obtain porous materials with preserved amine moieties. These were identified as follows: silicon tetraacetate and bridging tertiary amine silanes as precursors, toluene as a solvent, and temperature between 160 and 180 °C. In such a way, materials with up to 776 m 2 g −1 and 1.58 cm 3 g −1 were obtained in one-step process, without any template, after conventional drying step. Next, the amine-functionalized materials were tested in CO 2 -epoxide coupling providing cyclic organic carbonates with high selectivity (>99 %) and moderate activity (up to 86 % epichlorohydrin conversion after 1 h at 120 °C and 10 bar CO 2 ). The characterization of spent catalysts revealed a presence of cyclic organic carbonates at the catalyst surface as well as conversion of tertiary amine groups to quaternary ammonium moieties. • Amine functionalized silica with up to 776 m 2 g −1 and 1.58 cm 3 g −1 by non-hydrolytic sol-gel • Template-free and catalyst-free one-step synthetic process • Materials active in the production of cyclic organic carbonates from CO 2 and epichlorohydrin, styrene oxide, and 1,2-butylene oxide • Quaternization of amine sites observed in spent catalysts