Pyrolysis mechanism of β-O-4 type lignin model polymers with different oxygen functional groups on Cα

Abstract

Two kinds of linear lignin model polymers composed of only β-O-4 linkages with the Cα connected with a carbonyl group and a hydroxyl group, respectively, were synthesized to investigate the mechanism of lignin pyrolysis and the effects of different oxygen functional groups on Cα during lignin pyrolysis. A thermogravimetric analyzer coupled with a flourier transform infrared spectrometer (TG-FTIR) was employed to examine the evolution of main gaseous compounds during pyrolysis with a heating rate of 15 K/min from room temperature to 900 °C. The results showed that the carbonyl groups were the source of CO2 emission at low temperatures, rather than the ether bonds derived from β-O-4. Decarbonylation of the carbonyl group leads to CO2 emission, and CH4 was generated by the dealkylation effect of the alkyl side chains connected to the benzene rings. Analytical pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was introduced to reveal the distribution of reaction products at 500, 650, and 800 °C. In contrast to the hydroxyl group on Cα, which promoted polymer pyrolysis predominantly through β-O-4 bond breakage, the carbonyl on Cα enhanced the activity of the polymer, significantly increased the proportion of Cα-Cβ disruption mode during pyrolysis, and substantially reduced the breakage temperature of the β-O-4 bond from 366 to 328 °C. Finally, the pyrolysis mechanism and reaction pathway of both the linear lignin model polymers composed of only β-O-4 linkages were elucidated in this study.