A new insight of lignin pyrolysis characteristics based on dehydrogenation polymers (DHPs)

Abstract

Understanding the pyrolysis mechanism of native lignin is essential for improving the conversion efficiency of fast pyrolysis technology. However, due to the complex and various nature of lignin interunit linkages, the pyrolysis mechanism of lignin has not been clearly revealed so far, especially for native lignin. In this work, two types of guaiacyl-dehydrogenation polymers (G-DHPs) were synthesized using two different methods in vitro, namely endwise polymerization (G-DHPs-endwise) and bulk polymerization (G-DHPs-bulk), which were used as native lignin models. The chemical structures of the synthesized G-DHPs along with natural G-lignin (i.e., milled wood lignin, MWL) were analyzed by using 2D HSQC NMR, and their pyrolysis characteristics were investigated by using a fixed bed reactor. In comparison with G-DHPs-bulk, G-DHPs-endwise had more similar structure and pyrolysis characteristics with that of MWL, which suggested that G-DHPs-endwise was a more suitable model for G-type lignin. Further mechanical study revealed that the pyrolysis of G-DHPs-endwise underwent an end-breaking pathway, by which the branched carbon of the coniferyl alcohol structure ends were cleaved at the early stage to depolymerize into guaiacol. This study can thus enhance the understanding of the thermal depolymerization of native lignin and provide the evidence for revealing pyrolysis mechanism.