Study toward a More Reliable Approach to Elucidate the Lignin Structure–Property–Performance Correlation

Abstract

The correlation between lignin structure, its properties, and performance is crucial for lignin engineering in high-value products. Currently, a widespread approach is to compare lignins which differ by 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 a time while keeping all others constant before investigating the lignin properties/performance. Indulin AT (Ind-AT), a softwood Kraft lignin, was chosen as the model substrate for this study. Selective (analytical) 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 ( 31 P and 2D HSQC) of the modified preparations together with state-of-the-art molar mass (MM) characterization. Methylene blue (MB) adsorption, antioxidant activity, and glass transition temperature (T g ) were used to demonstrate and compare the properties/performance of the obtained modified lignins. We found that the contribution of different functionalities in the adsorption of MB follows the trend BenzOH > −COOH > AlipOH > PhOH. Noteworthy, benzylic −OH contributes ca. 3 and 2.3 times more than phenolic and aliphatic −OH, respectively. An 11% and 17% increase of T g was observed with respect to the unmodified Indulin by methylating benzylic −OH groups and through reduction, respectively, while full acetylation/methylation of aliphatic and phenolic −OH groups resulted in lower T g . nRSI experiments revealed that phenolic −OH play a crucial role in increasing the antioxidant activity of lignin, while both aliphatic −OH groups and −COOHs possess a detrimental effect, most likely due to H-bonding. Overall, for the first time, we provide here a reliable approach for the engineering of lignin-based products in high value applications by disclosing the role of specific lignin functionalities.

Unveiling the Nature of lignin’s Interaction with Molecules: A Mechanistic Understanding of Adsorption of Methylene Blue Dye

“…The very weak band at 2713 cm –1 , attributed to H-bonded ν(-N-(CH 3 ) 2 , was observed for Ind and Ind-R lignins with the MB adsorbed. The absence of this band in the spectra of the other lignin derivatives loaded with MB could either be due to the lower concentration of MB adsorbed on these samples or masking of their aliphatic (including benzylic) groups …”

“…Since the role of −COOH groups was assumed to be negligible, an ∼25% decrease in the adsorption of MB by Ind-Me was attributed predominantly to the conversion of AlipOH groups (particularly BenzOH) into methoxy. Overall, the contribution and the activity of each functional group to the adsorption of MB could be represented by the following trend: BenzOH> −COOH > AlipOH > PhOH, which was presented and discussed in our previous work . Interestingly, there is practically no adsorption of MB on Ind-DMS , where there is almost complete blocking of all of the functional groups.…”

“…Detailed information about the preparation procedures and characterization of the modified lignin samples was presented in our previous work. 19 Here, the most important details are briefly given in Figure 1 .…”

“… 18 However, as of the current time, there has not been a comprehensive investigation to establish the adsorption mechanism of organic molecules on lignin. In our recent study, 19 we proposed an approach to elucidate the correlation between lignin structure, properties, and performance by selectively masking one specific lignin functionality at a time. This approach allowed for quantitative estimation of the adsorption activity of each functional lignin group.…”

Upgrading AquaSolv Omni (AqSO) biorefinery: access to highly ethoxylated lignins in high yields through reactive extraction (REx)

“…However, the large variations in applied lignin substrates and adhesive formulations do not allow a reliable (numerical) comparison of the adhesive performance between studies. 74 Consequently, the discussion will mainly focus on the differences in used additives, solvents, and catalysts as well as the advantages/disadvantages of each study.…”