Lignin Depolymerization to Guaiacol and Vanillin Derivatives via Catalytic Transfer Hydrogenolysis using Pd-Lewis Metal Oxide Supported on Activated Carbon Catalysts

Lignin valorization is the key to sustainability of second-generation bioethanol refineries. In this study, the catalytic activities of Pd-Lewis acid metal oxides (Mo, Zr) supported on commercial activated carbon (AC) and activated biochar (ABC) were evaluated for the selective production of monomeric phenols from alkali lignin through transfer hydrogenolysis. The catalysts were thoroughly characterized for phase, degree of graphitization, textural properties, and metal dispersion. The Pd crystallite size was 2.1-2.5 Å in the case of AC-supported catalysts, while it was smaller (1.4-1.5 Å) for ABC-supported catalysts. AC exhibited better degree of graphitization than ABC. 2%Pd-5%Mo supported on AC showed good catalytic activity for the production of guaiacol, alkyl guaiacols, alkene guaiacols, and vanillin derivatives, and their respective yields were 4.2, 10.3, 3.4, and 12 wt.%. Compared to other alcohols like methanol and ethanol, the use of isopropyl alcohol as solvent resulted in better yields of guaiacol, alkyl guaiacols and alkene guaiacols. Condensation reactions of the guaiacyl unit and syringyl unit were confirmed through Fourier transform infrared spectroscopic analysis of the residue. Dimerization of monomeric phenols was observed with Pd-metal oxide supported on ABC than with AC-based catalyst.