Role of the support in the hydrogenation of citronellal on ruthenium catalysts

The hydrogenation of citronellal (3,7-dimethyl-6-octen-1-al) was performed on Ru supported on silica and carbon. The influence of the surface area and of the nature of the support on the products distribution was investigated. On the supported Ru catalysts it was found that the isomerisation of citronellal to unsaturated cyclic alcohols (isopulegols) occurs on the surface of the support. In the presence of the noble metal, the isopulegols are further hydrogenated to the corresponding saturated cyclic alcohols (menthols). The formation of the open chain hydrogenated products (citronellol, 3,7-dimethyloctanal and 3,7-dimethyloctanol) mainly occurs on the noble metal. The products distribution was found to depend strongly on the surface area and on the nature of the support. On the high surface area Ru/SiO2 catalysts, the main path of reaction is the isomerisation of citronellal to isopulegols. On Ru/C the main reaction products are the open chain hydrogenated products. This has been attributed to the low activity of the carbon towards the isomerisation of citronellal. (C) 1999 Elsevier Science B.V. All rights reserved.