The effects of combining guaiacol and syringol on their pyrolysis

Abstract

Pyrolysis of guaiacol/syringol mixtures was studied in an ampoule reactor (N₂/600°C/40–600 s) to understand the reactivities of the aromatic nuclei in hardwood lignins. By comparing the results with those of pure guaiacol or syringol, significant effects of their combination were observed during early stage pyrolysis: charring and gas formation were markedly suppressed. Yields for the tar fractions (MeOH-soluble) were increased by combination but the contents of GC/MS-detectable products with low molecular weight were dramatically decreased in these fractions, especially catechol-type compounds diminished, which are formed through the O-CH₃ homolysis reaction and subsequent donation of hydrogen atoms. These observations are discussed at the molecular level. Inhibition of the charring reaction, which can act as a strong hydrogen donor, would increase the probability of radical coupling reactions of the O-CH₃ homolysis products instead of the formation of catechol and pyrogallol and their derivatives. The latter compounds are important intermediates for gas formation, especially for CO. Based on these lines of evidence, a hypothesis is proposed according to which polyxadaromatization during carbonization activates the pyrolytic reactions of guaiacol and syringol.