Amorphous SiO2 catalyst for vapor-phase aldol condensation of butanal.

Graphical abstract Highlights • SiO 2 with weak acidity exhibited stable catalytic activity in the aldol condensation of butanal. • Both large pore size and large surface area of SiO 2 efficiently promoted the catalytic activity. • 2-Ethyl-2-hexenal selectivity of 90% with 48.3% conversion was obtaiend over SiO 2 at 240 °C. • Silanol groups on SiO 2 surface were suggested to be the active sites for the reaction. Abstract Vapor-phase aldol condensation of butanal to form 2-ethyl-2-hexenal was carried out over several oxide catalysts such as SiO 2 -Al 2 O 3 , Al 2 O 3 , ZrO 2 , and SiO 2. Catalysts with moderate and strong acid sites such as Al 2 O 3 and SiO 2 -Al 2 O 3 were active for the reaction in the initial period, whereas they deactivated rapidly. In contrast, SiO 2 with weak acidity showed a low but a stable catalytic activity for the formation of 2-ethyl-2-hexenal. Thermogravimetric analyses of the samples used after the reactions indicate that SiO 2 has the smallest amount of carbonaceous species that contributed to its stable activity among the tested catalysts. SiO 2 catalysts with different pore sizes and specific surface areas were examined: SiO 2 with a mean pore diameter of 10 nm and a surface area of 295 m2 g−1 showed the best catalytic performance and gave a 2-ethyl-2-hexenal selectivity of 90% at a conversion of 48% at 240 °C. In the catalytic test using deuterated SiO 2 , which was prepared by contacting SiO 2 with deuterated water before the reaction, it was confirmed by a mass spectrometer that the deuterium atom of SiOD was transferred to a 2-ethyl-2-hexenal molecule during the reaction. It is indicated that silanol groups on the SiO 2 surface played a role as an active site. [ABSTRACT FROM AUTHOR]