Predicting structural change of lignin macromolecules before and after heat treatment using the pyrolysis-GC/MS technique

Milled wood lignin extracted from poplar wood was subjected to a heat treatment in the temperature range of 150–300 °C at 50 °C intervals. The results of several chemical characterizations (proton nuclear magnetic resonance (1H NMR), derivatization followed by reductive cleavage (DFRC), elemental composition) and thermogravimetric analysis revealed that cleavage of the β-O-4 linkage accompanied by condensation and a charring reaction occurred during the heat treatment, leading to the formation of a thermally stabilized lignin at 300 °C (300-L) with enrichment of the condensed carbon–carbon bond. Pyrolysis-GC/MS data showed that 41 kinds of pyrolysis products, mainly composed of monomeric phenols, were released from the heat-treated lignin, and the yield of such products was significantly dependent on the lignin sample and the pyrolysis temperature. The maximum yield of total pyrolysis products from 300-L was obtained at a pyrolysis temperature of 600 °C, whereas the maximum yield from the other samples was observed at 500 °C due to the high thermal stability and degree of condensation of 300-L. Moreover, the pyrolysis-GC/MS results suggested that the side chain in the phenol skeleton (C3C6) was considerably degraded at 300 °C, resulting in the accumulation of a C6/C1C6/C2C6-type lignin structure in the 300-L sample. With an increase in the pyrolysis temperature, the syringyl/guaiacyl (S/G) ratio of all lignin samples, as determined from the pyrolysis products, constantly decreased due to an increase in G-type compounds released in the high temperature region.