Electrocatalytic Hydrogenation of Benzaldehyde: Unexpected Enhancing Effect By Co-Adsorbed Proton Donor

Abstract

Electrocatalytic hydrogenation (ECH) is a promising route for low-temperature bio-oil treatment, wherein renewable electricity can be used to store hydrogen in hydrocarbon energy carriers used as transportation fuels.1 ECH of phenol and benzaldehyde (model compounds) shows rates and selectivity to hydrogenation that depend on the metal.2 As bio-oil is a mixture of compounds with different functionalities, development of a sustainable process towards bio-oil stabilization often requires the fundamental understanding of competitive hydrogenation pathways in presence of mixtures of model compounds. Thus, the present study aims to explore the hydrogenation kinetics and reaction pathways of simultaneous conversion of phenol and aromatic carbonyls on carbon supported metal catalysts. Under ECH, all catalysts tested (Rh/C, Pd/C, Ru/C, Cu/C) were active for benzaldehyde hydrogenation to benzyl alcohol, however, only Rh/C was active for the hydrogenation of phenol which yielded cyclohexanone and cyclohexanol. The hydrogenation activity of benzaldehyde increases in the following order Ru/C < Rh/C < Pd/C < Cu/C. H2 evolution reaction (HER) is the prevalent side reaction. Therefore, an important variable to study is the selectivity (Faradaic efficiency) defined as fraction of total electron used for hydrogenation. Hydrogenation selectivity was ~90% for both Ru/C and Pd/C, however, Rh/C and Cu/C showed ~50% selectivity. When phenol was co-fed alongside benzaldehyde, no phenol hydrogenation was noted on all catalysts tested. However, an enhancement in ECH rate of benzaldehyde (~2-4 fold depending on metal catalyst) was observed in case of Pd/C, Ru/C and Cu/C (Figure 1a). ECH of benzaldehyde was very similar on Rh/C in presence and absence of phenol. The presence of phenol in the feed increased Faradaic efficiency in case of Cu/C and decreased it on Rh/C and Ru/C. We hypothesize that phenol, co-adsorbed on the metal, affects the rates of HER hindering the reaction of hydronium ions with the metal. On the other hand, phenol interacts with benzaldehyde and thus, facilitates the hydrogenation and results in an enhancement in rate. Indeed, when the benzaldehyde hydrogenation was carried out on Pd/C with different phenolic compounds with varying pKa, increasing hydrogenation rates were noted with decreasing pKa of the phenolic compounds (Figure 1b). Although, an enhancement (x2) in benzaldehyde hydrogenation was noted with 20 mM of phenol, increasing phenol concentration further (until 200 mM) barely have an impact on the hydrogenation rate. However, no such enhancement was observed when the hydrogenation was studied with molecular H2 as the reduction equivalent instead of cathodic potential. This suggests that benzaldehyde hydrogenation might proceeds through an electrochemical step with direct involvement of the phenolic compounds. The results of the detail kinetic studies, and the corresponding reaction mechanisms will be addressed in this presentation. Figure 1