(720a) Hydrogenolysis of Lignin Model Compounds over Transition Metal Catalysts in a Continuous Flow Reactor

Catalytic hydrogenolysis is a promising method for lignin valorization. However, due to the lack of information on reaction kinetics and mechanism of lignin hydrogenolysis, an efficient process to produce versatile lignin monomers cannot yet be established. β–O–4-linkages are dominant (50-60%) in native lignin, and are the target for hydrogenolysis reactions. We will present results exploring the reaction kinetics and mechanism of lignin hydrogenolysis using two β-ether authentic lignin model compounds; guaiacylglycerol-β-guaiacyl ether (GG) and veratrylglycerol-β-guaiacyl ether (VG), representing the phenolic end-units and internal units of the lignin polymer, respectively. Hydrogenolysis of lignin model compounds was carried out in a flow-through system using metal and bi-metallic catalysts. We found that the phenolic model compound GG underwent homolysis without catalyst under neutral conditions producing coniferyl alcohol, while the analogous reaction was not observed for the etherified model compound VG. These results suggest that lignin homolysis only occurs from the end-units through a quinone methide intermediate. The hydrogenolysis of GG and VG using a Pd/C catalyst resulted in the formation of dihydroconiferyl alcohol and dihydroveratryl alcohol with >90% selectivity, respectively. The rate of GG hydrogenolysis was 2.3 times faster than that of the VG reaction, indicating that a peeling reaction pathway is faster for lignin hydrogenolysis. Moreover, the conversion of lignin model compounds under argon is around 4 times slower than the hydrogenolysis reaction. The activation energy in the presence of H₂ (72 kJ/mol) is lower than the barrier in the absence of H₂ (120 kJ/mol) over Pd/C. Thus, when the reaction medium is saturated with H₂, the reaction proceeds through the hydrogenolysis pathway and the production of undesired compounds under Ar can be suppressed. In addition, the reaction kinetics and product distribution for lignin hydrogenolysis differ based on the metal catalyst, the promoter and the support material. We observed that the incorporation of promoters like Ag to the parent Pd catalyst decreases the activation energy by around 10 kJ/mol. Fundamental reaction kinetics studies on hydrogenolysis of relevant lignin model compounds allow modeling of reaction environments for the complex mechanisms of lignin conversion.