SiO2-supported niobia catalyst with optimized acid concentration prepared via organic additive-assisted impregnation protocol for the vapor-phase aldol condensation of butanal

In this study, we prepared SiO 2 -supported niobium oxide (Nb/SiO 2 ) catalysts with optimized acid concentrations using an organic additive (OA)-assisted impregnation protocol for the vapor-phase aldol condensation of butanal to 2-ethyl-2-hexenal (2E2H). The inclusion of an OA, such as 18-crown-6-ether, during the impregnation process enhanced the acid concentration of the resulting Nb/SiO 2 catalyst. This increase in acid concentration proved to be crucial for the vapor-phase aldol condensation of butanal to 2E2H, as confirmed by a proportional correlation between the acid concentration and the formation rate of 2E2H. Additionally, the essential role of acid sites, in particular Brønsted acid sites, of the Nb/SiO 2 catalyst was confirmed through poisoning experiments using several probe molecules, including NH 3 , CO 2 , and 2,6- and 3,5-dimethylpyridine. Several other parameters, such as Nb content and molar ratio of OA to Nb, also affected the catalytic activity of Nb/SiO 2 catalyst. • Vapor-phase aldol condensation of butanal was catalyzed by SiO 2 -supported niobia. • Organic additive-assisted impregnation method dispersed Nb oxide on SiO 2 support. • 18-Crown-6-ether acted as the most effective organic additive. • The aldol condensation of butanal proceeded over Brønsted acid sites of Nb/SiO 2 . • The catalytic activity was proportional to the concentration of acid sites.