Infrared Spectroscopy Study of Aldehydes Adsorbed on Rh−Sn Bimetallic Systems:  Selective Activation of Aldehydes by Tin

Abstract

Infrared spectra of propionaldehyde was studied over silica-supported Rh−Sn bimetallic catalysts. Two absorption bands of the carbonyl group were observed at 1670 and 1720 cm-1, and the aldehyde hydrogen (−CHO) was also observed at 2748 and 2848 cm-1 over the Sn/Rh/SiO2 catalysts on which propionaldehyde was preadsorbed. One of the absorption bands of carbonyl groups at 1670 cm-1 readily disappeared by contact with H2, whereas the other band at 1720 cm-1 remained. The band of the aldehyde hydrogen also disappeared by the contact with H2. The absorption band at 1670 cm-1 was assigned to a donating-on-top η adsorbed species that were bound to tin atoms with oxygen atoms of carbonyl groups. The band observed at 1720 cm-1 was assigned to a species weakly adsorbed on the catalyst surface. The intensity of the band at 1670 cm-1 was increased with an Sn/Rh ratio up to unity and then was gradually decreased with the Sn/Rh ratio of the catalysts. The reduction temperature at 573 K yielded a maximum intensity ratio of I 1670/I 1720. The activity for selective hydrogenation of unsaturated aldehydes to unsaturated alcohols indicated a maximum at 573 K of reduction temperature. These results clearly indicate that the adsorbed species observed at 1670 cm-1 in the infrared spectra is one of the most effective surface adsorption states in the selective hydrogenation reaction of carbonyl groups in unsaturated compounds. Adsorption experiments indicated that the tin addition increased the O2-adsorption capacity of the catalysts. Tin showed a high affinity for oxygen. These results can conclude that one of the roles of tin is selective activation of carbonyl groups by binding to the Sn atoms with oxygen atoms of carbonyl groups in unsaturated carbonyl compounds, which can be reduced to unsaturated alcohol by hydrogen atoms spilt over from Rh sites, in the hydrogenation of unsaturated aldehydes.

Hydrogen Spillover in Encapsulated Metal Catalysts: New Opportunities for Designing Advanced Hydroprocessing Catalysts

“…[43][44][45] According to the earlier study,m odification of Sn to the group VIII metal catalysts can significantly enhance the selectivity toward unsaturated alcohols because Sn helps the selective adsorption and activation of C=Of unctional groups. [46] In this respect, the authors additionally supported SnO x on the surface of SP (Sn/SP) for the chemoselective hydrogenation of crotonaldehyde (Scheme 3). Analysis using EXAFS and XRD showedt hat Sn exists on the surfaceo fS iO 2 as forms of SnO x and does not form ab imetallic alloy with the embedded Pt particles (i.e.,S na nd Pt are physically separated).T he supported SnO x can be reduced at milder temperature (673 K) under H 2 compared with SnO x supported on Pt-free silica (773 K).…”

Hydrogenation of Citral Over IrAu/TiO2 Catalysts. Effect of the Preparation Method

“…The general behavior of the samples is typical of supported Group VIII metals; a rapid hydrogenation of citral takes place at the beginning, then the catalyst activity decreases after this earlier period. The explanation generally proposed is that a decomposition of the citral or unsaturated alcohols yielding chemisorbed CO and carbonaceous species accumulate on the catalyst surface block a portion of the active sites [7,[32][33][34][35]. Nevertheless, it is worth to note that this situation is less pronounced in the case of the IrAu-S catalyst compared to Ir/TiO 2 DPU.…”

Hydrogenation of Dimethyl Adipate to 1,6-Hexanediol on Supported Rh–Sn Catalysts

“…Three reaction steps are likely for the hydrogenation of carboxylic acids or esters 10), 17) ; adsorption and activation of the substrate on Sn, activation of hydrogen on Rh, and hydrogenation by the activated hydrogen. Considering these steps, with increased Sn/Rh ratio up to 1.0 adsorption of DMA will be facilitated to give higher conversion and selectivity, whereas further increase in Sn/Rh ratio above 1.0 will reduce adsorption and activation of hydrogen to give poor catalytic performance.…”

“…Although Sn has no or little hydrogenation activity, Sn combined with noble metals modifies the properties of single-component catalysts, resulting in dramatic improvement in the catalytic performance 11) . In the case of hydrogenation of esters with Ru _ Sn catalyst, Ru is believed to activate the hydrogen and Sn is considered to activate the substrate by adsorbing the carboxyl group 8),10), 17) . Such effects result in improvement of the performance of catalysts.…”