Thermal reactivities of catechols/pyrogallols and cresols/xylenols as lignin pyrolysis intermediates

Thermal reactivities of lignin pyrolysis intermediates, catechols/pyrogallols (O-CH₃ homolysis products) and cresols/xylenols (OCH₃ rearrangement products), were studied in a closed ampoule reactor (N₂/600 °C/40-600 s) to understand their roles in the secondary reactions step. Reactivity tends to be enhanced by increasing the number of substituent groups on phenol and this effect was greater for -OH than for -CH₃. Thus, catechols/pyrogallols were more reactive than cresols/xylenols and syringol-derived products were more reactive than corresponding guaiacol-derived products. Catechols/pyrogallols were effectively converted into CO (additionally CO₂ in the case of pyrogallols) in the early stage of pyrolysis. In contrast, cresols/xylenols were comparatively stable and produced H₂, CH₄ and demethylation products (cresols and phenol) after prolonged heating. All intermediates except phenol and 2-ethylphenol formed coke during a long heating time of 600 s (second stage coking). Based on the present results, the roles of intermediates in tar, coke and gas formation from guaiacol and syringol are discussed at the molecular level, focusing on their differences. Molecular mechanisms of gas formation from pyrogallols and demethylation of cresols/xylenols are also discussed.