Study of guaiacol pyrolysis mechanism based on density function theory

• Five pyrolytic pathways of guaiacol were proposed focusing on its methoxy group.
• Standard thermodynamic and kinetic parameters in each reaction were calculated.
• Both thermodynamic and kinetic analyses of each reaction pathway were proposed.
• Relatively optimal pyrolytic pathways of guaiacol were obtained.

Five possible pyrolytic pathways of guaiacol were proposed with an emphasis on the reactivity of the methoxy group. Pathway 1 is about the homolysis of OCH3. Pathways 2–4 focus on the demethoxylation of guaiacol. Pathway 5 concerns the OCH3 rearrangement. Standard thermodynamic and kinetic parameters of each reaction pathway were calculated at different temperatures based on density functional theory methods by using Gaussian 03 package at B3LYP/6–31G++(d,p) level. According to the calculation results, the five reaction pathways were ranked as Path 3, Path 1, Path 4, Path 2 and Path 5, in descending order of reactivity. Kinetic analyses results of the three demethoxylation reaction pathways (Path 2, Path 3 and Path 4) indicate that coupling a hydrogen radical to the carbon atom to which the methoxyl group bond can effectively lower the reaction energy barrier that existed in the process of demethoxylation. Pathway 5 demonstrates the possible formation mechanism of o-quinonemethide which is the key polymerization intermediate during lignin pyrolysis process.