Mild electrocatalytic hydrogenation and hydrodeoxygenation of bio-oil derived phenolic compounds using ruthenium supported on activated carbon cloth

Electrocatalytic hydrogenation (ECH) is an option for stabilizing or upgrading bio-oil that employs mild conditions (≤80 °C and ambient pressure) compared to hydrotreatment. In this study, phenol, guaiacol (2-methoxyphenol), and syringol (2,6-dimethoxyphenol) were chosen as model lignin-like substrates because of their abundance in bio-oil and their high resistance to hydrogenation relative to the carbonyl compounds. Cathodic reduction was catalyzed by ruthenium supported on activated carbon cloth (Ru/ACC), a novel electrocatalyst. Incipient wetness impregnation and cation exchange methods were employed to prepare the electrocatalyst using three different ruthenium precursors. Scanning electron microscopy revealed that ruthenium nanoparticles within the range of 10 to 20 nm were deposited on the support by both catalyst synthesis methods. Catalysts prepared by cation exchange were more active than those prepared using incipient wetness impregnation, presumably because of support surface functionalization by the oxidation pretreatment. When using incipient wetness impregnation, catalysts synthesized with precursor hexaammineruthenium(III) chloride showed the best activity and electrochemical efficiency, followed by catalysts prepared with ruthenium(III) chloride and ruthenium(III) nitrosyl nitrate. The Ru/ACC electrocatalyst reduced guaiacol, phenol and syringol with similar electrochemical efficiencies, but temperature was an important variable; the electrochemical efficiency for guaiacol reduction increased from 8% at 25 °C to 17% at 50 °C, but then dropped back to 10% at 80 °C. Solution pH also affected catalyst activity and product selectivity, with acidic conditions favoring guaiacol conversion, electrochemical efficiency and cyclohexanol selectivity.