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

• Published in: Molecular catalysis 2023-10, Vol.549, p.113474, Article 113474
• Main Authors: Kumar, Madambakam Midhun, Prabhudesai, Vallabh S., Vinu, Ravikrishnan
• Format: Article
• Language: English
• Subjects: Activated carbon; Guaiacol; Lewis acid metal oxide; Lignin; Palladium; Transfer hydrogenolysis; Vanillin
• ISSN: 2468-8231; 2468-8231
• DOI: 10.1016/j.mcat.2023.113474

•Effects of Lewis metal oxides, carbon supports and solvents on phenolic yields were evaluated.•Lignin monomers in the oil (∼36 wt.%) was high using isopropanol solvent at 240°C, 3 h.•High yields of alkyl guaiacols (10.3 wt.%) and vanillin derivatives (12 wt.%) were obtained.•The nature of carbon support is shown to determine the lignin conversion and product selectivity. 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. [Display omitted]