Investigation into the correlation between the chemical structure of lignin and its temperature-dependent pyrolytic product evolution

Abstract

The complex and diverse chemical structures of lignins pose huge challenges in understanding their pyrolysis behavior and mechanism. Herein, five representative lignins, including three organosolv lignins (corncob lignin, pine lignin, eucalyptus lignin), alkali lignin, and dealkalized lignin, were used as feedstocks in this study. Comparison of their pyrolysis behaviors, product evolutions, and mechanisms at different temperatures was investigated by using thermogravimetric analyzer coupled with mass spectrometer (TG-MS) and pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS). The chemical structure of lignin before and after pyrolysis at different temperatures was characterized by Fourier transform infrared spectrometry (FTIR). The results show that alkali lignin and dealkalized lignins provide relatively lower maximum weight loss rates and relatively higher amounts of char, CH4, H2O, CO, and CO2 due to the presence of alkali and alkaline earth metals. The distributions of phenolic compounds from lignin pyrolysis are related to the chemical structure of lignin, especially its S/G ratio. Moreover, the pyrolysis temperature also exerts significant effects on the distribution of phenolic compounds. Pyrolysis of corncob lignin at 300 °C achieves the effective breakage of esterlinkage between p-Coumarates/ferulates and phenylpropane moieties of S units to form 4-vinylphenol and 2-methoxy-4-vinylphenol. The optimal temperature for the breakage of interunit linkages of S and/or G units in lignin is 500 °C. The syringol-type compounds can be converted into guaiacol-type compounds via demethoxylation reactions at 500 °C. The syringol-type and guaiacol-type compounds can be further converted into phenol-type compounds and aromatics by demethoxylation and dehydroxylation reactions at 900 °C. These findings can guide the development of novel pyrolysis processes of lignin for the selective production of specific phenolic compounds.