A new approach to elucidate lignin structure-properties-performance correlation

The correlation between lignin structure, its properties and performance is crucial for lignin engineering in high-value products. Currently, the most common approach is to compare different lignins which differ for more than one parameter (i.e. Kraft vs. organosolv vs. lignosulfonates) in various applications by attributing the changes in their properties/performance specifically to a certain variable (i.e. phenolic OH groups). Herein, we suggest a novel approach to overcome this issue by changing only one variable at the time, while keeping all others constant before investigating the lignin performance. Indulin, a softwood Kraft lignin, was chosen as model substrate for this study. Selective lignin modifications were used to mask/convert specific functionalities, such as aliphatic (AliphOH) including benzylic OH (BenzOH) and phenolic OH (PhOH) groups, carboxyl groups (COOH) and carbonyl groups (CO) via methylation, acetylation, and reduction. The selectivity and completeness of the reactions were verified by comprehensive NMR analysis (31P and 2D HSQC techniques) of the modified preparations together with state-of-the-art molar mass (MM) characterization. Methylene blue dye adsorption was used to demonstrate and compare the performance of the obtained modified lignins. Additionally, the effect of these lignin functionalities on important lignin properties like the antioxidant activity and the glass transition temperature (Tg) were investigated. Overall, for the first time we here provide a reliable approach for the engineering of lignin-based products in high value applications by disclosing the role of specific lignin functionalities.