Electrocatalytic hydrogenation of catechol on Rh–Al₂O₃ in different media — pH-Dependent reduction mechanism for intermediate formation

Abstract

The electrocatalytic hydrogenation of catechol was carried out in aqueous media in different pH ranges with Rh-Al 2 O 3 powder catalyst. The reactions were conducted in an H-cell used as a dynamic cell, with a reticulated vitreous carbon electrode in contact with the catalyst powder as the working electrode. It was shown that the final product is 1,2-cyclohexanediol (cis and trans isomers) and that several intermediates are detected depending on the pH of the solution. Different media, from pH 5 to 13, were studied. One of the intermediates is 1,2-cyclohexanedione, detected at all pH values. The other is 2-hydroxycyclohexan-1-one, only observed at pH ≤ 7. To determine the mechanism of the reactions involved, the electrocatalytic hydrogenation of these intermediates to form the final 1,2-cyclohexanediol product was also conducted, and their UV spectroscopy and cyclic voltammetry data recorded. The influence of the nature of the solution was screened by measuring the Henry constant of each molecule.Résumé : L'hydrogénation électrocatalytique du catéchol a été menée en milieu aqueux à différents pH sur des poudres de Rh-Al 2 O 3 comme catalyseur. Les réactions ont été réalisées dans une cellule en H utilisée en mode dynamique avec des électrodes de carbone vitreux réticulé en contact avec le catalyseur comme électrodes de travail. Il a été montré que le produit final est le 1,2-cyclohexanediol (isomers cis et trans) et que plusieurs intermédiaires sont détectés dépendam-ment du pH de la solution. Divers milieux de pH 5 à 13 ont été étudiés. Un des intermédiaires est la 1,2-cyclohexanedione, formée quel que soit le pH. L'autre est la 2-hydroxycyclohexan-1-one, uniquement observée à pH ≤ 7. Afin de détermi-ner le mécanisme des réactions impliquées, l'hydrogénation électrocatalytique de ces intermédiaires en 1,2-cyclohexanediol a été réalisée ainsi que la spectroscopie UV-visible et la voltampérométrie cyclique. L'influence de la nature du milieu a été évaluée en mesurant les constantes de Henry pour chacune de ces molécules.Mots clés : hydrogénation électrocatalytique du catéchol, mécanisme de réduction dépendant du pH, spectroscopie UVvis, voltampérométrie cyclique, constantes de Henry.Brisach-Wittmeyer et al. 1647

Mild electrocatalytic hydrogenation and hydrodeoxygenation of bio-oil derived phenolic compounds using ruthenium supported on activated carbon cloth

“…Previous research [12][13][14][15][16][17] on ECH of phenolic compounds has shown that large cathode surface areas are usually necessary to achieve high reaction rates. Electrodes made with pressed metallic powder particles have been successfully used, 18,19 but their mechanical strength is weak without addition of a binding material 20 and much surface area is lost in the powder pressing step.…”

Selective electrochemical reactions of an alumina hydrate crystallization inhibitor

“…The detection of the two isomers (cis and trans) of the cyclohexanediol suggests the presence of intermediate products. This particular aspect is the subject of another article from our group [18]. For temperatures between 25°C and 65°C, no significant effect on the ECH of catechol was observed under the conditions tested, as shown in Fig.…”

Electrocatalytic Hydrogenation of Biomass-Derived Organics: A Review

“…55,88 However, on Pd and Rh, phenol hydrogenates faster, and the hydrogenation of catechol proceeds faster toward the final products at pH 5 than at higher or lower pH values. 88,160,162 In stark contrast, the ECH of phenolic compounds on Raney Ni is enhanced as the pH becomes more basic. 147,148…”

“…99,144,158 The contribution of Meńard and co-workers from the 1990s to the late 2010s must be highlighted because they mapped hydrogenation of phenol (and related organic compounds) on diverse catalysts (i.e., metal particles on carbon electrodes, carbon powders, and metal oxides) under several reaction conditions. 99,100,[144][145][146][160][161][162]173 ECH of phenolic compounds carried special momentum throughout the 2010s as low-temperature hydrogenation of biogenic feedstocks gained importance for the production of fuel and base chemicals. 19,339 Advances in the design of electrochemical reactors for organic transformations [152][153][154]175 have been paralleled by efforts to detail reaction networks and mechanisms and to explore the commonalities of electroand thermocatalysis.…”

“…The first kinds of electrocatalysts used for organic reduction were bulk Pt and platinized Pt electrodes. ,− The strategy of using bulk metals has been continuously applied for coinage metals, with Raney Ni and Devarda Cu being the most prominent examples. ,− However, a natural progression in catalyst design was to disperse the metals (especially expensive Pt-group metals) on carbon ,,,,− or in conductive polymers. − Dispersing the metals on carbon has been shown to enhance the conversion of phenol, keeping current densities low (calculated based on the exposed metal surface area) and, therefore, Faradaic efficiencies high . Pd and Rh have also been supported on oxides such as alumina and BaSO 4 , maintaining their hydrogenation activity under electrocatalytic operation. ,,− …”