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

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 Mössbauer spectroscopy. The XPS results indicated the presence of two Sn species on the surface of the catalysts, in different oxidation states—Sn0 and Snn+, and 119Sn Mössbauer spectra revealed Sn(II), Sn(IV), and intermetallic particles with structures such Ru3Sn7 I, Ru3Sn7 II, 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 Cdouble bond; length as m-dashC bond, while the addition of a small amount of Sn (1 wt.% of Sn) made the catalyst active for hydrogenation of both the Cdouble bond; length as m-dashC and Cdouble bond; length as m-dashO bonds. Upon increasing the Sn loading from 1 to around 3.6 wt.%, the specific rate for Cdouble bond; length as m-dashC hydrogenation is greatly reduced whereas the specific rate for Cdouble bond; length as m-dashO hydrogenation decreases more gradually. The activity for Cdouble bond; length as m-dashC 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 Cdouble bond; length as m-dashO hydrogenation is observed as a lower binding energy than expected is observed for the Ru0. This enhancement of the surface electron density of the Ru particle is crucial to improving the adsorption and reactivity of the Cdouble bond; length as m-dashO bond, but has the opposite effect on hydrogenation of the Cdouble bond; length as m-dashC 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 Cdouble bond; length as m-dashC bond hydrogenation.