Revealing the thermal sensitivity of lignin during glycerol thermal processing through structural analysis

Woody biomass was treated in glycerol between 200 and 240 °C in an anhydrous environment to denature the biomass for biopolymer fractionation. After glycerol thermal processing (GTP), up to 41% of the initial Klason lignin of the starting biomass was recovered in a powdered form through a room temperature dioxane extraction followed by precipitation. 31P-nuclear magnetic resonance (NMR) of the GTP lignin revealed the syringyl phenolic functionality increased linearly with the log of the severity parameter establishing the impact of the thermal processing on structure. Further structural analysis via thioacidolysis and two-dimensional (2D) 13C–1H heteronuclear single quantum coherence (HSQC) NMR of the isolated lignin indicated GTP caused extensive β-O-4 bond decomposition and the liberated phenolic OH did not undergo further coupling. At the same time, condensation occurred on the aromatic C5 position of the phenylpropane units to yield GTP lignin with a relatively high molecular weight, comparable to that of enzymatic mild acidolysis lignin from non-thermally treated fibers. The recovered GTP lignin was more thermally stable compared to nearly all other lignin found in the literature. Additionally, the glass transition temperature was invariant to the processing severity parameters. These structural changes indicate lignin is highly sensitive to moderately high temperatures common to thermoplastic polymer processing conditions.