Pyrolysis mechanism of a β-O-4 type lignin dimer model compoundA joint theoretical and experimental study

To help understand the pyrolysis mechanism of lignin, a non-phenolic lignin dimer model compound with β-O-4 linkage, namely 1-methoxy-2-(4-methoxyphenethoxy)benzene, was prepared. Its pyrolysis mechanism was investigated by density functional theory calculations and confirmed by the analytical pyrolysis–gas chromatography/mass spectrometry experiments. Possible pyrolytic pathways were proposed and analyzed based on three initial pyrolysis mechanisms of the model compound, including the Cβ–O homolytic mechanism, the Cα–Cβ homolytic mechanism and the Cβ–O concerted decomposition mechanism. The results indicate that the lignin dimer model compound is thermally stable at low pyrolysis temperature (300 °C), whereas at moderate pyrolysis temperature (500 °C), four major pyrolytic products will be generated via the initial Cβ–O concerted decomposition and Cβ–O homolysis, including the 1-methoxy-4-vinylbenzene, 2-hydroxybenzaldehyde, 1-ethyl-4-methoxybenzene and 2-methoxyphenol. Products from the Cα–Cβ homolysis can hardly be formed, due to the high-energy barriers and limitation of the available free hydrogen atoms. Secondary cracking of the primary pyrolytic products will take place to form some light compounds, which will be enhanced with the increase in pyrolysis temperature.