Sites for CO 2 activation over amine-functionalized mesoporous Ti(Al)-SBA-15 catalysts

Abstract

Activation of CO 2 and its utilization in the synthesis of chloropropene and styrene carbonates over functionalized, mesoporous SBA-15 solids, have been investigated. The surface basicity of SBA-15 was modified with nitrogen-based organic molecules of varying basicity viz., alkyl amines (–NH 2), adenine (Ade), imidazole (Im) and guanine (Gua). The surface of SBA-15 was also functionalized with Ti 4+ and Al 3+ species. The acid–base properties of these modified SBA-15 materials were investigated by temperature-programmed desorption (TPD) and diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy. NH 3 and pyridine were used as probe molecules for acid sites, while CO 2 was used to characterize the basic sites. CO 2 was activated at the basic amine sites forming surface carbamate species (IR peaks: 1609 and 1446 cm −1). The latter reacted further with epoxides adsorbed on the acid sites forming cyclic carbonates. A correlation between the intensity of the IR peak at 1609 cm −1 and cyclic carbonate yield has been observed. The cyclic carbonate yields were higher when both the acid and base functionalities were present on the surface. The Ti- and Al-SBA-15 functionalized with adenine exhibited the highest catalytic activity and selectivity. There is an optimal dependence (“volcanic plot”) of the yield of cyclic carbonates on the desorption temperature, T max(CO 2) in the TPD experiments. These solid catalysts were structurally stable up to 473 K and could be recycled for repeated use. In addition to density, the strength and type of amine sites play a crucial role on CO 2 activation and utilization.