Abstract
Thermal chemical hydrogenation (TCH) is a key reaction for converting petroleum and biomass feedstocks to value-added fuels and chemicals. Electrochemical hydrogenation (ECH) is a mild alternative where applied potential drives the hydrogenation of hydrocarbons with protons sourced from the aqueous electrolyte near ambient conditions. Lignocellulose is an abundant renewable feedstock comprised of complex phenolic compounds that can be valorized via ECH. Phenol and benzaldehyde are simple model compounds for lignocellulose that have been the focus of recent ECH studies. When both phenol and benzaldehyde are present as reactants, phenol has been shown to act as a proton shuttle, enhancing benzaldehyde ECH rates but with negligible phenol turnover. [1] The study of the role of electrolyte on the ECH of these aromatic hydrocarbons is limited to a few buffer systems (typically acetate, phosphate, or sulfate) and in a pH range of 1-5. However, it well-known that the competing hydrogen evolution reaction (HER) kinetics are slower in base, and that molecules like phenol dissociate within a pH range of 1-14. Thus, a complete understanding of the role of electrolyte pH, especially in alkaline media where slowed HER rates may improve faradaic efficiency of ECH, is desirable.In this work, we show using cyclic voltammetry and chronoamperometry with product analysis that there are contrasting pH trends between platinum and rhodium for phenol ECH, with the highest ECH rates on platinum at pH 1 and the highest ECH rates on rhodium at pH 10. In-situ electrochemical FTIR and electrochemical impedance spectroscopy reveal that this is the result of differences in hydrogen adsorption kinetics, phenol/H coverage, and competition with HER between the two metals. For phenol ECH on rhodium, we show that pH-driven changes dictate the balance between a hydrogen atom transfer (LH) mechanism and a phenol-mediated proton coupled electron transfer mechanism. We also discuss phenol’s role as a proton shuttle towards benzaldehyde hydrogenation in alkaline pH, near the pka of phenol. With this insight, we show that the unique properties of rhodium and strategic tuning of the acid-base chemistry of phenol and the electrolyte can be utilized to yield elevated ECH rates with enhanced FE through slowed HER.[1] Sanyal, U., et al., Hydrogen Bonding Enhances the Electrochemical Hydrogenation of Benzaldehyde in the Aqueous Phase. Angew Chem Int Ed 2021, 60 (1), 290–296. https://doi.org/10.1002/anie.202008178.
Published Version
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