Ru-Catalyzed Hydrogenolysis of Lignin: Base-Dependent Tunability of Monomeric Phenols and Mechanistic Study

Substantial attention has been given to depolymerization of lignin into monomeric phenols in recent years because lignin is a renewable and CO2-netural aromatic resource. Recent results indicated that the base can shift the selectivity from C3-fragmented phenols to C2-fragmented phenols partially in transition metal-catalyzed lignin hydogenolysis, while reaction mechanisms have remained elusive. Using a series of dimeric, trimeric, and polymeric β-O-4 lignin mimics, as well as their deuterated analogues, we now report an in-depth experimental study on the mechanism of Ru/C-catalyzed hydogenolysis lignin. Experimental evidence based on substrate probes, reactivity examination of possible intermediates, and isotopic labeling experiments confirmed that the reported pathways, such as enol ether generated via α,β-dehydration reaction or Cα carbonyl compounds generated via dehydrogenation or consecutive Cβ–O and Cγ–OH bonds hydrogenolysis, are irrelevant to current reactions. For C3-fragmented phenols with Ru/C catalyst under neutral condition, we deduced that the monolignol such as coniferyl alcohol is formed primarily through a concerted hydrogenolysis process, where Cα–O and Cβ–O bonds are ruptured synchronously. For C2-fragmented phenols generated by the combination of Ru/C and Cs2CO3, the reaction should start from quinone methide specie generated from the dehydration (or demethanolization) reaction between phenolic proton and Cα–OH (or Cα–OMe). The followed deprotonation of Cγ–OH and the coordination of oxygen with Ru results in a Ru specie, which undergoes Cβ–H, Cβ–O, and Cβ–Cγ bonds cleavage to release 4-vinylphenol. In the case of Ru/C-catalyzed hydrogenolysis of an enzymatic mild acidolysis lignin (EMAL) derived from birch tree, the effects of some key parameters such as temperature, reaction time, as well as the type and dosage of base were also examined in terms of monomer yields and selectivity. We found the formation of C2-phenols is a base-dependent process, which is in line with the proposed mechanism. Under optimized conditions, a high proportion of C2-phenols (44%) could be obtained with 26.6 wt % total monomers yield.