Thermal reactions of guaiacol and syringol as lignin model aromatic nuclei

Abstract

Thermal reactions of guaiacol (2-methoxyphenol) and syringol (2,6-dimethoxyphenol) were compared in a closed ampoule reactor (N 2/400–600 °C/40–600 s) to obtain information on the thermal reactivities of lignin aromatic nuclei, guaiacyl and syringyl types. For both compounds, the O–CH 3 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 OCH 3 to aromatic CH 3 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 CH 4 and CO 2) formation was more extensive in syringol. This effective coking can be explained by the influence of the additional OCH 3 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.