Characterization of residue from catalytic hydrothermal depolymerization of lignin
Ranen Roy, Balawanthrao Jadhav, Md Sajjadur Rahman, Douglas E. Raynie
Abstract
In this study, investigation of the conversion of lignin into its phenolic monomers using catalytic hydrothermal depolymerization was conducted. To reveal the reaction mechanism, both lignin and its residue were analyzed employing thermogravimetric analysis, infrared spectroscopy, 1H nuclear magnetic resonance spectroscopy, gel permeation chromatography, and CuO oxidation. Thermal analysis showed that higher percentages of residual carbon belongs to the residue (46%) than the lignin (26%) indicating higher condensation in the residue. NMR demonstrated the presence of β-O-4 and β-5 linkages in the residue, and a higher aliphatic/aromatic ratio in the residue (3.03) than the lignin (0.96). Gel permeation chromatography showed higher molecular weight distribution and polydispersity for the residue. Oxidative degradation followed by GC-MS analysis resulted in fewer monomeric products from the residue. Fewer ether bonds in the residue were confirmed. Overall, the occurrence of the condensation reaction during the catalytic hydrothermal depolymerization process was verified by thermal and chromatography studies. Higher aliphatic nature of the residue also indicated possible condensation side reactions. Therefore, this study advances a mechanistic understanding of catalytic hydrothermal depolymerization reaction by comparing the breakdown of the glycosidic ether, C–C linkages, and condensation of the lignin and its residue.