Novel insight into pyrolysis behaviors of lignin using in-situ pyrolysis-double ionization time-of-flight mass spectrometry combined with electron paramagnetic resonance spectroscopy

In-situ detection on primary volatiles and stable radicals is of great importance for better understanding of lignin pyrolysis mechanisms and utilization. In this study, a novel in-situ pyrolysis time-of-flight mass spectrometry with double ionization sources was taken to in-situ detect primary volatiles and gas products, and the evolution of stable radicals in lignin pyrolysis residues was explored by EPR spectroscopy. The results show that the cleavage of β-O-4 linkage is mainly responsible for lignin depolymerization at 100–300 °C, releasing the G-type compounds. And these G-type compounds can further undergo O-CH₃, C_ar-OCH₃ and C_ar-OH bonds cleavage to form biphenolic hydroxyl compounds, phenols and aromatic hydrocarbons. According to the EPR analysis, the radical concentration increased from 10¹⁷ to 10¹⁹ spins/g with the temperature, and stable free-radical species are mainly composed of the o-methoxy and hydroxyl substituted phenoxy radicals and carbon-centered aromatic radicals, which can well interpret the demethylation, demethoxylation and dehydroxylation mechanisms.