Catalytic hydrogenolysis of lignin under a syngas atmosphere: the enhancement of Cβ–O bond cleavage with CO

Abstract

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