Protolignin Catalytic Depolymerization to Aromatic Chemicals via an Oxidation–Isolation–Hydrogenation Strategy

For the catalytic hydrogenation depolymerization of protolignin, simultaneously achieving the hydrogenation cleavage of ether bonds and restraining the potential condensation of the formed fragments is a critical point of the process. Meanwhile, efficient lignin isolation and β-O-4 structure preservation are also crucial. Herein, an oxidation–isolation–hydrogenation (OIH) strategy was proposed to achieve this goal. Unlike the traditional isolation-conversion pattern, the selective oxidation of lignin components in wood powders was first carried out before lignin isolation and subsequent hydrogenation conversion. The optimized O2/TEMPO//KCl system could selectively oxidize Cα–OH to Cα═O within the exposed β-O-4 structure, which can restrain the potential condensation reactions caused by the Cα+ intermediate after the dehydroxylation of Cα–OH in the acid-mediated lignin isolation from the lignocellulose substrate and subsequent conversion. Meanwhile, the preoxidation of Cα–OH to Cα═O could weaken the Cβ–OAr ether bond, which could be selectively cleaved over a Ni/MoS2 hydrogenation system to form aromatic monomers. Compared with the isolation–oxidation–hydrogenation strategy with a 4.9 wt % yield of phenolic monomers, the OIH strategy could give a 10.4 wt % yield of phenolic monomers from birch lignin, and the almost 110% monomer yield increase showed the importance of this lignin valorization pattern.