Electrocatalytic hydrogenation (ECH) of series of phenolic compounds, di-aryl ethers and aromatic aldehydes, has been performed in C-supported Pt, Pd, Rh, and Ni as working electrodes (cathodes), whereas a Pt mesh is used as counter electrode (anode) and an Ag/AgCl electrode is used as reference. H2 evolution (HER) is the prevalent side reaction on all metals, thus we define efficiency as the fraction of electrons used to reduce the organic compounds. Another series of experiments has been performed, where cathodic potential is replaced by a flow of H2 at atmospheric pressure. This is denoted as thermal catalytic hydrogenation (TCH). The electrocatalytic hydrogenation of aromatic rings and carbonyl groups readily occur on Pt, Rh, and Ni, whereas Pd is highly efficient for reduction in the reduction of carbonyl groups in aldehydes. The rates of carbonyl hydrogenation are higher than those of the saturation of aromatic rings. The rates of ECH of all tested compounds, as well as HER, increase with increasingly negative potentials. However, the efficiency is a strong function of the metal. Pt and Ni, for instance, strongly favor HER leading to low efficiencies. Other strategies to increase reaction rates at room temperature are increasing the concentration of hydronium ions and pressure. In the conversion of substituted phenols and diaryl ethers, the dominant reaction pathway is hydrogenation to cyclic alcohols and cycloalkyl ethers. Methoxy and benzyloxy groups undergo C-O bond cleavage via hydrolysis (minor route) and hydrogenolysis, which opens pathways to the formation of hydrocarbons. In stark contrast to the series of phenolic compounds, aromatic and pseudo-aromatic aldehydes and ketones undergo only reduction of the carbonyl groups without exhibiting any hydrogenation of the aromatic ring. For instance, benzaldehyde and furfural are selectively converted to benzyl alcohol and furfuryl alcohol, respectively. Increasing the concentration of hydronium ions enable C-O bond cleavage of the alcohols through a bifunctional (acid-metal) mechanism. The results show that electrocatalysis drives the desired transformations at mild conditions and allows a better control over the reaction pathways than thermal catalysis. Cathodic potentials leads to high rates through increasing the availability of surface H and by providing alternative pathways with low-energy barriers.

Reaction networks and mechanisms in electrocatalytic hydrogenation of oxygenated compounds

256th National Meeting (2018)

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