Study on the pyrolysis mechanism of three guaiacyl-type lignin monomeric model compounds

• Vanillin, vanillic acid and vanillyl alcohol were selected as lignin models.
• Pyrolysis mechanism was revealed via combined Py-GC/MS and DFT calculations.
• Two different competing degradation reactions were recognized in the three models.
• Priority of common and specific degradation was different depending on the models.
• The occurrence conditions of synergy and radical-induced pathway was studied.

In this work, vanillin, vanillic acid and vanillyl alcohol were selected as guaiacyl-type monomeric model compounds to study the secondary pyrolysis mechanism of lignin. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was used to characterize products derived from the pyrolysis of lignin model compounds. Based on the free-radical theory, it was speculated that the pyrolysis of lignin model compounds concerned two processes: common degradation and specific degradation. By applying the density functional theory (DFT) method to study the detailed reaction pathways and energy changes, it revealed that the potential energy of the synergy process was lower than that of the radical-induced process during the functional group removal. Thereby CO, CO2, and HCHO were preferentially released along the synergy route. The enthalpy changes and experimental yields of products from the further degradation of guaiacol were basically consistent. Furthermore, the distinction of the potential energy between common degradation and specific degradation of each lignin model compound under various temperatures contributed to the different priority of the two degradation processes, as well as the different yields and species of pyrolytic products.