Mechanistic study of the selective hydrogenation of carboxylic acid derivatives over supported rhenium catalysts

Abstract

The structure and performance of TiO2-supported Re (Re/TiO2) catalysts for selective hydrogenation of carboxylic acid derivatives have been investigated.

Recent Developments of Heterogeneous Catalysts for Hydrogenation of Carboxylic Acids to their Corresponding Alcohols

“…Moreover, the same group further investigated the active site of Re/ TiO 2 catalyst and the reaction mechanism with characterization and kinetics. [45] The oxidation state of Re species is estimated to be between 0 and + 4 by XPS and XAS analyses, which results are similar to some bimetallic ReO x -Pd catalyst systems as Catalyst stability: * is described if the activity and selectivity were not almost changed by reuse tests.…”

“…The high selectivity was derived from the weaker adsorption of benzene ring than the carboxylic aids on Re clusters, which was determined by DFT calculation. Moreover, the same group further investigated the active site of Re/TiO 2 catalyst and the reaction mechanism with characterization and kinetics . The oxidation state of Re species is estimated to be between 0 and +4 by XPS and XAS analyses, which results are similar to some bimetallic ReO x ‐Pd catalyst systems as shown below, however the detailed reaction mechanism of Re/TiO 2 was not discussed in these catalyst systems.…”

Rhenium – A Tuneable Player in Tailored Hydrogenation Catalysis

“…Supported rhenium materials tend to have larger particle sizes when synthesized via CVD or thermal decomposition of Re 2 (CO) 10 , whereas microwave‐assisted decomposition can form small NPs of around 5 nm, and aqueous impregnation of Re water‐soluble precursors usually attains nanoclusters under 2 nm. The larger NP size observed with CVD or Re 2 (CO) 10 decomposition could also be related to the use of carbon‐based support common to those synthesis methods since, even with aqueous impregnation, Re/C presented particles averaging 5 nm in size, as opposed to Re/TiO 2 , which presented particles <2 nm, [45] as seen on Figure 3. The presence of XRD peaks related to metallic rhenium, on the other hand, does seem to be more common on materials synthesized via CVD and Re 2 (CO) 10 decomposition, which could indicate that these methods lead to more crystalline particles of rhenium.…”

“…Regarding monometallic rhenium, Toyao et al. reported Re/TiO 2 as an effective catalyst for the hydrogenation of carboxylic groups, such as the hydrogenation of triglycerides to alcohols reaching 90 % yield depending on substrate, [45] aromatic carboxylic acids, [40] esters and amines [31] to their respective alcohols with preserved aromatic rings and in very high yields, as long as a pre‐reduction step was performed, preferably at 500 °C, which decreases the average oxidation state of rhenium in ReO x , despite not completely reducing it to metallic Re [45] . These authors also performed mechanistic studies in order to determine that the selectivity of Re/TiO 2 is due to a higher affinity for adsorbing carboxyl groups rather than benzene rings, [31,40] as will be further discussed in the “Mechanistic insights” section.…”

Hydrogenation of benzoic acid to benzyl alcohol over Pt/SnO2

“…This was attributed to rapid reaction of the C=C group to yield phenylpropanoic acid which as for the non-conjugated phenylacetic acid gave only reaction of the carboxylic acid group. Unsaturated carboxylic acid hydrogenation has also been reported over Re/TiO2 at 140°C/50 bar H2 where acid hydrogenations proceeded without reaction of the aromatic ring for substrates such as phenylpropanoic acid [4,6]. Interestingly, it was found that Re/TiO2 did have an ability to hydrogenate an aromatic ring as was demonstrated with the reaction of propylbenzene to propyl cyclohexane; however, this reaction did not occur when in the presence of an acid or alcohol.…”