Electrocatalytic Hydrogenation of Oxygenated Compounds in Aqueous Phase

Electrocatalytic hydrogenation is a strategy to hydrogenate biogenic compounds under ambient conditions by replacing the thermal and H2 inputs by cathodic potential. This work compares the performances of this approach (in aqueous phase at room temperature) for the conversion of a variety of model oxygenated compounds over a series of metals. The target functionalities were carbonyl groups, aromatic rings, and ether bonds. All of the metals explored (Pt, Rh, Pd, and Cu) are active for the reduction of carbonyl compounds to alcohols. The conversion rate of benzaldehyde increased as a function of the metal as Pt < Rh < Pd (Cu was tested under different conditions). In contrast, only Rh and Pt were active for hydrogenation of aromatic rings (Rh was more active than Pt). In a comparison of the target functionalities, carbonyl groups are more reactive than aromatic rings and ether bonds in phenolic compounds and diaryl ethers on all of the explored metals. This carbonyl reactivity, however, is enhanced by the aromaticity of the molecule. Hence, the reactivity trend of the examined molecules is butyraldehyde < furfural < acetophenone < benzaldehyde. For phenolic compounds, phenol is more reactive than cresol and methoxyphenol. Thus, the presence of substituent groups on the functionality being converted (either carbonyl or aromatic ring) decreases the conversion rate. Ether bonds are cleaved under electrocatalytic conditions, which opens two main pathways for the conversion of aryl ethers: hydrogenation of the aromatic ring and hydrogenolysis of the ether bonds, whereas hydrolysis occurs as a minor pathway. Electrocatalytic hydrogenation competes with the H2 evolution reaction under the conditions of the tests, and therefore, the Faradaic efficiency (the fraction of current utilized in hydrogenation) and hydrogenation rate are correlated. That is, within the potential range explored, increasing hydrogenation rates lead to higher Faradaic efficiencies. The slope of this correlation, however, depends on the potential and on the functionality being hydrogenated.