Aromatic-enriched bio-oil from lignin pyrolysis catalyzed by HZSM-5: Insights into product selectivity and in-situ deoxygenation mechanism

Abstract

The selectivity and catalytic mechanism of HZSM-5 in the pyrolysis of lignin model compounds at 600 °C were investigated using Py-GC/MS and density functional theory (DFT). The findings indicate that HZSM-5 elevated the relative content of aromatic hydrocarbons in the pyrolysis products of phenol, vanillin, and syringol by 12.3 %, 11.7 %, and 7.5 %, respectively. Meanwhile, the relative content of oxygen-containing products, including aldehydes and acids, was reduced by 15.7–52.0 %. Throughout the pyrolysis of lignin model compounds, various side chain reactions take place, such as dissociation, decarbonylation, isomerization rearrangement, alkylation, and aromatization. The selective catalytic decomposition mechanism of HZSM-5 on oxygen-containing functional groups involves both the physical selection imparted by the pore size structure and the selection of acid sites that form hydrogen bonds with hydroxyl, carbonyl, and methoxy groups. This mechanism was corroborated by DFT calculations, which showed that the energy barriers for the removal of most oxygen-containing functional groups were lowered by 1.7–72.4 kJ/mol. These results provide an in-depth understanding of the catalytic pyrolysis of lignin and pave the way for future research on the high-value utilization of waste lignin.