Catalytic hydrogenolysis of lignin under syngas: the enhancement on the Cβ-O bond cleavage with CO

Abstract

Catalytic hydrogenolysis of lignin has attracted steady attentions in regard of producing high-value aromatic chemicals from the renewable resource and achieving the “carbon neutrality” goal. However, current techniques for lignin hydrogenolysis are highly challengeable due to the necessity of pure H2 and relatively low yields in monomers. In this work, a novel and efficient approach has been proposed for the conversion of lignin to monophenols under readily available and cost-effective syngas. The results demonstrated that CO has the dramatic promotion on the bagasse lignin hydrogenolysis over commercialized Pd/C at 230 °C for 4 h, the yield of monophenols reaches 357.2 mg/g at 3.0 MPa syngas (CO/H2=2/1), which is approximately 1.7 times of that under pure H2. As evidenced by quantitative 31P NMR analysis, the promoting effects of CO on lignin depolymerization are positively correlated with the content of side-chain hydroxyl groups in lignin. Further investigations on lignin model conversion reveal that CO suppresses the dehydroxylation at Cα-OH, with concomitant enhancement of Cβ-O bond cleavage (increasing from 10% in H₂ to 64% in syngas). This inhibitory effect on side-chain hydroxyls elimination directly correlates with the 1.7-fold increase of monophenol yields observed during catalytic depolymerization. Therefore, this work provides a new insight in lignin depolymerization using industrial catalyst of Pd/C and the cost-effective syngas.