Microwave-assisted selective cleavage of CαCβ bond for lignin depolymerization

- The CαCβ bonds in phenolic and non-phenolic dimers were selectively cleaved using microwave heating (MH).
- The formation of alkenyl products was a decomposition-determining step.
- The phenolic dimer was more thermodynamically favorable to form alkenyl product than non-phenolic dimer.
- MH contributed to 96.3% of CαCβ bond cleavage of organosolv lignin and a narrow distribution of aromatic monomers.

Lignin is an alternative renewable feedstock for the production of low molecular weight aromatics. The cleavage of phenyl side-chain CαCβ bond is a valid depolymerization pathway to obtain aromatic monomers. The selective cleavage of the CαCβ bonds in two lignin model dimers and lignin with ferric sulfate as the catalyst was investigated to narrow the distribution of aromatic monomers. The CαCβ bonds in phenolic and non-phenolic dimers were selectively cleaved under microwave irradiation. The acidity promoted the etherification of Cα-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 CαCβ bond cleavage reactivity of phenolic dimer being higher than that of non-phenolic dimer. Microwave heating contributed to 96.3% of CαCβ 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 CαCβ bond to achieve the selective production of aromatic monomers from lignin.