Thermal reactions of guaiacol and syringol as lignin model aromatic nuclei

Thermal reactions of guaiacol (2-methoxyphenol) and syringol (2,6-dimethoxyphenol) were compared in a closed ampoule reactor (N2/400–600 °C/40–600 s) to obtain information on the thermal reactivities of lignin aromatic nuclei, guaiacyl and syringyl types. For both compounds, the O–CH3 bond homolysis, which was observed at >400 °C, initiated their decomposition. This homolysis was followed by several temperature-dependent reactions; radical-induced rearrangement to convert the aromatic OCH3 to aromatic CH3 structures and condensation into high molecular weight (MW) products were the next steps (≈400 °C); then, coke formation became extensive (≈450 °C); effective gas formation required higher temperature such as >550–600 °C. The syringol- and guaiacol-characteristic GC/MS-detectable low MW products were explained with the above mentioned reactions. As for the difference between guaiacol and syringol, coke and gas (especially CH4 and CO2) formation was more extensive in syringol. This effective coking can be explained by the influence of the additional OCH3 group in syringol, which doubles the opportunity for coke formation. This, in turn, reduces the yields of GC/MS-detectable low MW products. Demethoxylation to form guaiacol was also observed in syringol, even though the reactivity was not so high. These reactions are discussed at the molecular level.