Ru-Sn catalysts for selective hydrogenation of crotonaldehyde: Effect of the Sn/(Ru+Sn) ratio

The effects of the Sn/(Ru + Sn) ratio and Ru loading on the properties of silica-supported RuSn catalysts were investigated. Ru-Sn/SiO2 bimetallic catalysts were prepared by the impregnation method and characterized in situ by XPS and Mossbauer spectroscopy. The XPS results indicated the presence of two Sn species on the surface of the catalysts, in different oxidation states-Sn-0 and Snn+, and Sn-119 Mossbauer spectra revealed Sm(II), Sn(IV), and intermetallic particles with structures such Ru3Sn7 1, Ru3Sn7 11, and oxide species (RuOSn). The contents of the various species depended on Sn loading. The reaction data showed that the Ru/SiO2 catalyst was active only for hydrogenation of the C=C bond, while the addition of a small amount of Sn (I wt.% of Sn) made the catalyst active for hydrogenation of both the C=C and C=O bonds. Upon increasing the Sn loading from I to around 3.6 wt.%, the specific rate for C=C hydrogenation is greatly reduced whereas the specific rate for C=O hydrogenation decreases more gradually. The activity for C=C hydrogenation remains unvarying for Sn loading higher than around 3.6 wt.%. This results in an optimum selectivity to crotyl alcohol at Sn/(Ru + Sn) molar ratio around 0.2-0.3. The abrupt change of activity toward C=O hydrogenation is observed as a lower binding energy than expected is observed for the Ru-0. This enhancement of the surface electron density of the Ru particle is crucial to improving the adsorption and reactivity of the C=O bond, but has the opposite effect on hydrogenation of the C=C bond. The geometric effect of formation of Sn-Ru intermetallic and SnOx, compounds on the Ru particle surface may play a part in the suppression of C=C bond hydrogenation. (c) 2006 Elsevier B.V. All rights reserved.