Microwave-assisted selective cleavage of CαCβ bond for lignin depolymerization

Lignin is an alternative renewable feedstock for the production of low molecular weight aromatics. The cleavage of phenyl side-chain C α C β bond is a valid depolymerization pathway to obtain aromatic monomers . The selective cleavage of the C α C β bonds in two lignin model dimers and lignin with ferric sulfate as the catalyst was investigated to narrow the distribution of aromatic monomers. The C α C β bonds in phenolic and non-phenolic dimers were selectively cleaved under microwave irradiation . The acidity promoted the etherification of C α -hydroxyl groups with methanol solvent. Both the etherification and the decomposition of alkenyl products occurred in fast reaction steps, but the formation of alkenyl products was a decomposition-determining step. Density functional theory (DFT) calculations indicated that the phenolic dimer was more thermodynamically favorable to form alkenyl product than non-phenolic dimer, which resulted in the C α C β bond cleavage reactivity of phenolic dimer being higher than that of non-phenolic dimer. Microwave heating contributed to 96.3% of C α C β bond cleavage of organosolv lignin (OL), and a narrow distribution of aromatic monomers which mainly contained vanillin , syringaldehyde , methyl vanillate , and methyl syringate. This work highlights the prospect of microwave-assisted selective cleavage of the C α C β bond to achieve the selective production of aromatic monomers from lignin.