Selective hydrogenation of cinnamaldehyde over Pd/SiO2 catalysts: selectivity promotion by alloyed Sn

The effect of added Sn on the catalytic behavior of Pd/SiO2 sol-gel catalysts in the liquid-phase hydrogenation of cinnamaldehyde at 25 °C and 3 atm H2 pressure was studied for Sn-loadings ranging from 0 to 3%. Sn–Pd alloying could be confirmed by means of X-ray diffraction (XRD)-measurements. Although no significant amounts of cinnamyl alcohol could be detected, added Sn was found to have a positive effect on selectivity by increasing the rate constant ratio (k2/k1) of the hydrogenation route to cinnamyl alcohol to that producing phenylpropanal. Produced cinnamyl alcohol reacts, however, consecutively and very rapidly to phenylpropanol. The observed promotion of selectivity (however, at the expense of activity) is discussed and attributed to both geometric and electronic effects of alloying Pd with Sn, resulting in a strong suppression of the rate of forming the saturated aldehyde rather than enhancing the rate of forming cinnamyl alcohol.