Unmasking radical-mediated lignin pyrolysis after benzyl hydroxyl shielding

Whether lignin benzyl hydroxyl shielding could promote its pyrolysis to phenolic compounds was investigated in this paper. Lignin benzyl hydroxyl was first preoxidized by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and stabilized by propionaldehyde respectively, then pyrolysis was conducted with milled wood lignin as a control. Organic stable radicals in pyrolytic chars were further detected to reveal lignin pyrolysis chemistry. Results showed that benzyl hydroxyl shielding process weakened lignin thermal stability, and decreased liquid yields regardless of the frequency of lignin β-O-4 linkages. In addition, char yield grew after benzyl hydroxyl shielding. Radical concentration was inversely proportional to β-O-4 content which indicated the non-negligible impact of shielded benzyl hydroxyl on lignin pyrolysis. Furthermore, gases from propionaldehyde stabilized lignin quenched its radicals. This work confirmed that lignin β-O-4 linkages and shielded benzyl hydroxyl both played the great role in radical-mediated pyrolysis, but the enhancement of liquid products could not be achieved via benzyl hydroxyl shielding.