Mild Synthesis of Cyclohexanol and Cyclohexanone Via Electrocatalytic Reduction of Phenols Using Dispersed Metal Catalysts

Abstract

Cyclohexanol and cyclohexanone are industrially important chemicals for the synthesis of Nylon polymers. On the industrial scale, they are produced by thermocatalytic processes, either phenol hydrogenation (140–170 oC, 1 atm) or cyclohexane oxidation (140–180 oC, 0.8–2 MPa), which require high temperatures and external gas supply. This work demonstrates that electrosynthesis of cyclohexanol and cyclohexanone can be done at mild conditions using diverse aqueous electrolytes and dispersed metal catalysts (Pt/C, Ru/C, and Pd/C).Electrocatalytic hydrogenation (ECH) of lignin-derived phenols (e.g., guaiacol and phenol) is performed in a stirred slurry H-cell under potentiostatic or galvanostatic control. Dilute mineral acid (H2SO4, HClO4), organic acid (CH4SO3), and inorganic salt (NaCl) worked efficiently as the electrolyte solutions. In the ECH of guaiacol at low temperatures (35–40 oC) and low Pt/C loading, high guaiacol conversions (83–96%) and Faradaic efficiencies (40–70%) were achieved with significant cyclohexanol selectivities (45–53%) after 4 h reactions. In the ECH of phenol, full conversion to cyclohexanol was achieved after 2 h with high current efficiency (90%). Remarkably, by pairing the NaCl catholyte and H2SO4 anolyte, the activity of Ru/C and Pd/C can be dramatically improved, showing the importance of electrocatalyst and electrolyte synergy in the ECH of phenolic compounds.The stirred slurry catalyst allows the cell to operate at the industrially relevant current densities (in this work, between 100–300 mA cm-2). Dispersion of the negatively charged catalyst particles in the solution also facilitates the mass transfer between the organic molecules and chemisorbed hydrogens through physical collisions during the reaction, thereby improving the ECH efficiency. Mild electrocatalytic reduction of phenolic compounds represents a promising route for selective and efficient production of renewable chemicals from biomass.