Electrocatalytic Hydrogenation of Biogenic Molecules: Understanding Reactivity Trends

Abstract

The interest in storing electrical energy into chemical bonds has increased due to the increasing availability of sustainable electricity and its intermittent character. A promising approach is to couple electrolytic H2 production with transformations of organic compounds to chemicals and fuels. Thus, electrocatalytic hydrogenation (ECH) of phenolic compounds, di-aryl ethers and aromatic aldehydes, has been performed on Pt, Pd, Rh, and Ni supported on the working carbon electrodes of H electrolysis cells. At these conditions, H2 evolution (HER) is the prevalent side reaction, thus we define efficiency as the fraction of current used to reduce the organic compounds. The electrocatalytic hydrogenation of phenolic compounds and aromatic aldehydes readily occur on Pt and Rh, whereas Pd is highly efficient for reduction of aldehydes (Figure 1). The rates of ECH and HER increase with increasingly negative potentials but the efficiency is a strong function of the metal (e.g., Pt and Ni strongly favor HER). In the conversion of phenolic compounds and ethers, the dominant reaction pathway is hydrogenation to the corresponding alcohols and cycloalkyl ethers. Ether bonds undergo C-O bond cleavage (mainly via hydrogenolysis), which opens pathways to the formation of hydrocarbons. In contrast, aromatic and aldehydes and ketones undergo only reduction of the carbonyl groups without exhibiting any hydrogenation of the aromatic ring. Acidic media accelerates the rates of reduction and enables C-O bond cleavage of alcohols through a bifunctional (acid-metal) mechanism. We have further studied the effect of solvents in continuous flow systems. Similar to batch H-cells, the aromatic aldehydes are quantitatively converted to aromatic alcohols (Figure 1). The reactivity of these carbonyl compounds is strongly affected by decreasing polarity of the reaction media due to solvation effects. HER is less affected than ECH by increasing the C number of the co-solvent alcohol from methanol to i-propanol. This effect can be compensated by increasing cell potentials (see Figure 1 for the ECH of acetophenone) and the concentration of the reactant. This study shows 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. Figure 1