Infrared spectroscopy study of aldehydes adsorbed on Rh-Sn bimetallic systems: Selective activation of aldehydes by tin

Infrared spectra of propionaldehyde was studied over silica-supported Rh-Sn bimetallic catalysts. Two absorption bands of the carbonyl group were obse rved at 1670 and 1720 cm(-1), and the aldehyde hydrogen (-CH=O) was also ob served at 2748 and 2848 cm(-1) over the Sn/Rh/SiO2 catalysts on which propi onaldehyde was preadsorbed. One of the absorption bands of carbonyl groups at 1670 cm(-1) readily disappeared by contact with H-2, whereas the other b and at 1720 cm(-1) remained. The band of the aldehyde hydrogen also disappe ared by the contact with H-2. The absorption band at 1670 cm(-1) was assign ed to a donating-on-top eta(1) adsorbed species that were bound to tin atom s with oxygen atoms of carbonyl groups. The band observed at 1720 cm(-1) wa s assigned to a species weakly adsorbed on the catalyst surface. The intens ity of the band at 1670 cm(-1) was increased with an Sn/Rh ratio up to unit y and then was gradually decreased with the Sn/Rh ratio of the catalysts. T he reduction temperature at 573 K yielded a maximum intensity ratio of I-16 70/I-1720 The activity for selective hydrogenation of unsaturated aldehydes to unsaturated alcohols indicated a maximum at 573 K of reduction temperat ure. These results clearly indicate that the adsorbed species observed at 1 670 cm(-1) in the infrared spectra is one of the most effective surface ads orption states in the selective hydrogenation reaction of carbonyl groups i n unsaturated compounds. Adsorption experiments indicated that the tin addi tion increased the O-2-adsorption capacity of the catalysts. Tin showed a h igh affinity for oxygen. These results can conclude that one of the roles o f tin is selective activation of carbonyl groups by binding to the Sn atoms with oxygen atoms of carbonyl groups in unsaturated carbonyl compounds, wh ich can be reduced to unsaturated alcohol by hydrogen atoms spilt over from Rh sites, in the hydrogenation of unsaturated aldehydes.