Mechanism of generation of substituted β-O-4 lignin dimer CH4 based on bimolecular pyrolysis study

Abstract

Reactants, radicals, intermediates and products during lignin pyrolysis coexisted in a complex pyrolysis system. They interacted with each other and dominate the lignin pyrolysis process. Different from the traditional unimolecular decomposition mechanism, this paper focuses on the bimolecular reaction pyrolysis mechanism. The density functional theory (DFT) GGA/RPBE method was used. A total of 27 β-O-4 type lignin dimer model compounds with different substituents (-OCH3, –OH, –CH2OH) were selected for the study, and 36 reaction pathways were simulated and calculated. The reaction of intermolecular interactions between methyl radicals and lignin was used to explore the effect law of different positions of substituents on intermolecular interactions. The results show that, There were strong interactions between the lignin molecules and methyl radicals, which could promote the pyrolysis of lignin and the stabilization of free radicals. The lignin molecules acted as the hydrogen donor and donated hydrogen to the methyl radical to generate CH4. The hydrogen donation capability of Cα position was better than that of Cβ position, which was more easily influenced by the OCH3 substituent on the A-ring (The benzene ring far from the O atom was the A-ring). Moreover, the OH substituent at the Cα position will facilitate the reaction. Both the A and B rings had an effect on the hydrogen donation capability of Cβ position. Furthermore, the CH2OH substitution at the Cβ position will inhibit the reaction. When the H atoms at the R1 and R2 positions were replaced by OCH3 substituents, the SH-type (S for syringyl unit, H for p-hydroxyphenyl unit) lignin, which has the strongest hydrogen donation capability. SH-type lignin has the strongest interaction with methyl radicals among the types and is the model compound most likely to produce CH4.