Selective C-C Bond Cleavage of Methylene-Linked Lignin Models and Kraft Lignin

Biorefinery and paper pulping lignins, referred hereto as technical lignins, contain condensed C-C interunit linkages. These robust C-C linkages with higher bond dissociation energies are difficult to disrupt under hydrogenolysis conditions, which are generally used for cleaving C-O bonds of native lignin in biomass or model C-O linked compounds. Thus, selective interunit C-C cleavage to release aromatic monomers for high-value applications is a challenge. We report an effective catalytic system to cleave such C-C bonds selectively under mild conditions. A representative methylene-linked C-C model dimer achieves 88% yield of mainly two aromatic monomers within 1.S h at a reasonably low temperature (250 degrees C) using a commercial CoS2 catalyst. Aromatic monomers convert to nonaromatic products upon the reaction for a prolonged time. The interunit C-C bond of the dimer become unreactive to cleavage upon dehydroxylation of aromatic rings, while the methoxyl group has little effect on the cleavage. beta-1 and 5-5 C-C linked model dimers formed demethoxylated hydroxyl dimers as the major products. Depolymerization of a purified kraft lignin fraction yields five aromatic monomers and lower-molecular-weight soluble products. This study opens up the possibility of valorization of technical lignins using inexpensive catalysts.