Synergy between Ru and WO_x Enables Efficient Hydrodeoxygenation of Primary Amides to Amines

Abstract

The selective reduction of primary amides to the corresponding amines is a key reaction in both pharmaceutical chemistry and the polymer industry; yet, it requires a stoichiometric amount of expensive and environmentally unfriendly hydride reagents. Hydrodeoxygenation (HDO), on the other hand, is a highly desired environmentally benign alternative; yet, it presents significant challenges in activity and selectivity control. In this work, we have developed a bifunctional 2Ru1W/SiO2 catalyst, which afforded 93.6% conversion and 83.7% selectivity of the primary amine after reaction for 12 h for the HDO of cyclohexanecarboxamide at 433 K, and the initial reaction rate reached 48.2 h–1, 1 order of magnitude higher than other catalysts reported earlier. The catalyst also presented excellent reusability and substrate diversity. The performances of various Ru–W/SiO2 catalysts critically depend on the synergy between W and Ru, with the best performance accessible at the W/Ru atomic ratio of 0.27. Characterizations and reaction kinetics have revealed that Ru promoted the reduction of W6+ to W5+ via hydrogen spillover, and the resultant low-coordinated W5+ is responsible for the selective activation of the CO bond of the amide, leading to almost zero reaction order with respect to the amide and much lower apparent activation energy over the 2Ru1W/SiO2 bimetallic catalysts in contrast to the first order and higher activation energy over Ru/SiO2. Nevertheless, W/Ru ratios higher than 0.27 resulted in larger WO x domains and lower valence states of W, which adsorbed the amide substrate so strongly that the catalyst surface was poisoned, leading to a dramatic decrease in both activity and selectivity. The catalyst strategy as well as the mechanism understanding presented in this work will provide insights into the bifunctional catalyst design used for other demanding chemical transformations.

Selective hydrogenation of amides and imides over heterogeneous Pt-based catalysts

“…These results suggest that the existence of Pt nanoparticles could reduce their neighboring Mo species to a certain extend. In other reported bifunctional catalysts (Ru–WO x /SiO 2 16 and Pt–V 2 O 5 /HAP 15 ), the low-coordinated W 5+ and V 3+ were proposed to activate the CO bond in amides. Hence, we would naturally consider that in our case, these coordinately unsaturated Mo species might also contribute to the adsorption and activation of carbonyl group in the amide.…”

“…However, the toxicity of vanadium oxide limits its application potential. More recently, Wang and co-workers 16 developed a bifunctional RuW/SiO 2 catalyst which afforded the hydrogenation of primary amides at 160 °C and 50 bar H 2 . It is worth noting that molecularly-defined catalysts have been reported to be active for amide hydrogenations, too.…”

Production of 1,2-propanediol from fructose over a biochar-supported RuWCu catalyst

“…41 The Ru 3 W 24 /biochar also displayed two peaks at 185 and 321 °C, attributed to the reduction of Ru 4+ and W 6+ , respectively. 32 In comparison with the W 24 Cu 5 /biochar, the W 6+ reduction peak shied to a lower temperature in the Ru 3 W 24 /biochar, indicating that the presence of Ru species facilitated the reduction of W 6+ . Two peaks were also observed in the H 2 -TPR prole of the Ru 0.…”

“…Similar observations have been reported in previous studies. 6,32,33 In contrast, no signicant leaching of Ru and Cu species into the liquid phase was observed, indicating their stable features. Nonetheless, these observations highlight the need for improving the stability of W species for further applications.…”

“…Regarding the role of W species, which are primarily responsible for retro-aldol condensation reactions, 6,31,32 the effect of W content in the Ru 3 W y Cu 5 /biochar catalyst (y: 6% to 48%) on product distribution was investigated (entries 5 and 12-15, Table 2). The results demonstrated that the yield of 1,2-PDO increased from 38.3% for Ru 3 W 6 Cu 5 /biochar to 42.5% for Ru 3 W 36 Cu 5 /biochar, and slightly decreased to 41.5% for Ru 3 W 48 Cu 5 /biochar.…”