Hydrogenation of fatty esters over ruthenium-tin catalysts; Characterization and identification of active centers

The selective hydrogenation of methyl oleate into oleyl alcohol was performed over RuSn/alumina catalysts. A maximum yield of unsaturated alcohol was obtained for a bulk atomic ratio to Sn/Ru=4. The side reaction of the transesterification between methyl oleate and oleyl alcohol leading to oleyl oleate was strongly decreased. The ruthenium-tin catalysts prereduced with NaBH(4) have been characterized by TPR and XPS analysis. The addition of tin increased the content of Ru degrees on the catalyst surface without modification of the total Ru content. When the catalysts were reduced with NaBH(4), a decrease of the tin and ruthenium contents on the surface was observed. On the catalyst surface were found two separate tin oxide species which differed in the oxidation state of tin. The proportion and the repartition of each species was a function of the total tin content. Over such catalysts, the reaction scheme involved three steps: (1) the hydrogenation of the methyl oleate into the oleyl alcohol, (2) the transesterification reaction between the methyl oleate and the oleyl alcohol with the formation of the heavy oleyl oleate ester, (3) the hydrogenation of this heavy ester into oleyl alcohol. The first and the third steps could involve mixed ruthenium-tin sites while the second step could require tin species without an interaction with ruthenium. If it is assumed that the rate determining step is the oleyl oleate hydrogenation, the results of this study show that active centers corresponding to a stoechiometry Ru/SnOx = 2 could be involved in this reaction. The activation of esters (methyl oleate and oleyl oleate) would occur via a hemiacetal intermediate over these mixed sites where the more reduced tin species could be close to the ruthenium. (C) 1998 Academic Press.