Selective hydrogenation of methyl oleate into unsaturated alcohols Relationships between catalytic properties and composition of cobalt-tin catalysts

The selective hydrogenation of methyl oleate into oleyl alcohol, intermediate in the formulation of surfactant agents, is carried out over bimetallic catalysts containing, cobalt and tin. However the tin content modifies the rates of the hydrogenation of the esters (methyl oleate and oleyl oleate) and of the side reaction of the transesterification between the methyl oleate and the alcohol formed. Indeed, the maximum of the unsaturated alcohol yield depends on the Sn/metal ratio. The analyses of the catalyst surface by XPS have shown that there are coexistence of metallic particles (Co-0) and/or oxides (CoO) with two different tin oxides (SnOx, SnOy), the SnOx species being close to zerovalent metallic particles. It is suggested that (I) [Co-0. . . (SnOx)(2)] should be the active sites for the selective hydrogenation of ester, (2) the SnOy species favor the reaction of transesterification and (3) the free metallic particles favor the hydrogenation of ethylenic bonds. On the other hand, the preparation method of the catalyst and the nature of the support modify the surface composition of the solid. The use of alumina or zinc oxide lead to the same yield of unsaturated alcohol whereas silica which has much more metallic Co-0 species gives a low activity due to a large tin enrichment. Over CoSn catalysts reduced with NaBH4 owing to the formation of cobalt aluminate, a small content of metallic cobalt is observed. The preparation of CoSn catalyst via a sol-gel method improves the homogeneity and the dispersion of the active sites. But the strong interactions between cobalt and the support do not allow the formation of both metallic cobalt and mixed active centers (Co-0 SnOx). Furthermore, some kinetic studies obtained with cobalt catalysts show that (1) the transesterification reaction is faster than the hydrogenation of esters, (2) the presence of methanol issued from the hydrogenation and from the transesterification strongly modifies the oleyl oleate hydrogenation. (C) 2000 Elsevier Science B.V. All rights reserved.