Thermo-catalytic depolymerization of lignin over Pd-based catalysts: Role of catalyst support on monoaromatics selectivity

The condensed structure of technical lignin presents significant challenges in converting it into valuable functionalized building blocks, such as single-ring aromatics. In this study, we explored how palladium-based catalysts influence the selectivity of monoaromatics during the fast pyrolysis depolymerization of lignin. A systematic study on the influence of the pyrolysis temperature (500 < T < 700 °C) and catalyst support nature (SiO₂, Al₂O₃ and Carbon) was carried out by analytical pyrolysis. Creosol and guaiacol were the predominant species after non-catalytic and Pd/SiO₂ catalyzed pyrolysis of Kraft lignin. However, when Pd/C and Pd/Al₂O₃ were applied, the selectivity to monoaromatics increased drastically, to 50.9 % and 34.4 %, respectively. The catalytic activity of Pd/C and Pd/Al₂O₃ was ascribed to the presence of medium and weak acid sites in the supports, which provides active sites for phenolics adsorption and deoxygenation. Additionally, metallic Pd sites activated the C-O and H₂ bonds to further assist deoxygenation reactions, thus inducing a bifunctionality to these materials. This study offers valuable insights into the bifunctional effects of supported catalyst during the thermo-catalytic depolymerization of lignin.