Relating Dicarboxylic Acid Yield to Residual Lignin Structural Features

This study focused on understanding the relationship between dicarboxylic acid (DCA) yields derived from lignin and the structural attributes of their solid residues and corresponding acid-soluble lignin fractions. It is a continuation of the study by the authors on DCA production from bagasse lignin via hydrothermal liquefaction. It characterized the residues derived from the use of H2O2/chalcopyrite and sodium percarbonate for DCA formation, at temperatures between 60 and 300 degrees C with a reaction time of 3 h. FTIR, GPC, and 2D NMR were used to unravel the functional group changes, molecular weight distribution, and lignin substructures and linkages. The DCA yield correlated well, in a linear fashion, to the aromatic to aliphatic functional group ratio (AAFGR) and the degree of aromatic condensation (DAC). Supporting evidence of lignin depolymerization and repolymerization to explain the DCA yields was provided by GPC. Interestingly, the proportion of the a-oxidized substructure S' was found to be an indicator of the extent of lignin depolymerization, and its ratio to the S substructure was found to correlate with DCA yield at reaction temperatures up to 200 degrees C.