Electrocatalytic Hydrogenation of Guaiacol Using a Slurry Reactor with Carbon-Supported Platinum Catalyst in Different Aqueous Electrolytes

Abstract

An ideal upgrading process of biomass derivatives into renewable chemicals/fuels aims for energy densification and carbon emission neutrality. This work focuses on the reductive upgrading of lignin model compounds, namely guaiacol and phenol, using electrocatalytic hydrogenation (ECH). Compared to the classic thermal hydrogenation process, ECH offers some advantages, in terms of greener hydrogen gas (internally supplied from water oxidation), milder operating conditions (lower temperature and pressure, elimination of typical catalyst deactivation due to coking), cleaner process (possibility of integration with solar and/or wind energy, which are renewable and becoming more economical sources of electricity), and simpler operation (controlled by current/voltage input). In this work, ECH of guaiacol was performed in a stirred slurry reactor (SSR) configuration of an H-type cell using Pt/C catalyst. Different pairs of aqueous electrolytes (as catholyte-anolyte combinations), including acid (0.2 M H2SO4), neutral (0.5 M NaCl), and base (0.2 M NaOH) were tested using chronocoulometry, showing that the choice of anolyte has a significant impact on the electron transfer and the reduction rates. Acidic anolyte is preferable for the ECH of phenolics due to the highest proton concentration and specific conductance, whereby the acid-acid and neutral-acid electrolytes were found to be the most effective pairs, resulting in high guaiacol conversion (80–92%) at reasonable current efficiencies (40–62%) after 4 h reaction. The two major products were cyclohexanol (yield: 34–44%) and 2-methoxycyclohexanol (yield: 23–37%) indicating the effective aromatic ring saturation. The hydrogenation rates can be increased by increasing the cathode potential and/or by using higher acid concentration. Increasing the applied voltage contributed to faster reduction at the expense of current efficiency. High conversion of guaiacol (91%) was achieved using HClO4 (0.5 M) as both anolyte and catholyte at -1.0 V (vs. Ag/AgCl) and 34 oC, resulting in 54% cyclohexanol selectivity at moderate current efficiency (53%). A parametric study of guaiacol ECH was carried out using a jacketed cell and the variables were: acid concentration (0.2–1.0 M), superficial current density (110–255 mA cm-2), and temperature (30–60 oC). The current efficiency increased with the acid concentration and temperature; but decreased at the higher superficial current densities due to the enhanced hydrogen evolution reaction. Interestingly, at higher temperature (60 oC), the deoxygenation route was more favored than the ring saturation route, as indicated by the higher cyclohexanone and the lower 2-methoxycyclohexanol yields compared to those obtained at lower temperature (30–40 oC). The present study aims to contribute to the development of a mild electrochemical reduction process for catalytic conversion of lignin-derived substrates (e.g., phenolic compounds). Figure 1