Hydrogenation of citral and carvone on Pt and PtSn supported metallic catalysts. A comparative study on the regioselectivity and chemoselectivity

Abstract

Pt and PtSn catalysts supported on oxide and carbonaceous materials have been studied for the regioselective and chemoselective hydrogenation of citral and carvone. The influence of the support properties, Sn addition, and reaction substrate structure were correlated with the catalytic behaviour. The catalysts were prepared by conventional impregnation. The results showed that adding Sn to the metallic phase strongly affects the selectivity to unsaturated alcohols (UA) in citral hydrogenation with bimetallic catalysts. At the same time, the structure of the carvone molecule plays an essential role in the formation of partially hydrogenated products derived from the regioselectivity of the reaction when both mono- and bimetallic catalysts are used. The highest selectivity towards UA obtained for the hydrogenation of citral on bimetallic PtSn catalysts can be explained by a strong Sn promoter effect, which allows a predominant atop adsorption mode of the carbonyl group. In the hydrogenation of carvone, the acid–base properties of the support and the Sn promoter scarcely influence the selectivity toward UA, but the bonding modes of the CO and CC groups, preferentially parallel to the catalytic surface, would explain the products formed and the lower selectivity toward UA compared to the citral molecule.