Lignin Carboxymethylation: Probing Fundamental Insights into Structure–Reactivity Relationships

Abstract

Amidst declining fossil-based resources and environmental challenges, the focus on biobased materials has intensified. Carboxymethylation is one way to introduce reactive functionality to enhance the reactivity of lignin for a specified application. This research investigates the carboxymethylation of four lignin sources: eucalyptus kraft lignin, spruce kraft lignin, birch cyclic extracted organosolv lignin, and spruce cyclic extracted organosolv lignin. Our aim is to elucidate the role of the lignin structure in its reactivity. Using the advanced analytical techniques NMR spectroscopy, Fourier transform infrared spectroscopy, density functional theory, and size-exclusion chromatography, we provide a comprehensive characterization of the modified lignin. The findings offer valuable insights into how the chemical and physical properties of molecular lignin affect the selectivity and efficiency of the carboxymethylation reaction. These fundamental findings hold great potential for guiding considerations on the selection of lignin sources for specific applications based on their molecular properties.

Oxidative Carboxylation of Lignin: Exploring Reactivity of Different Lignin Types

“…All EtOAc-soluble fractions of oxidized lignin samples from different sources exhibited a similar increase in the carboxylic acid content of approximately 3 mmol/g. These results were comparable with the carboxylic acid content of lignin obtained through carboxymethylation modification, as reported in our previous research …”

“…These findings demonstrated an increased presence of G units in hardwood lignin following oxidation, suggesting that S units are more prone to oxidation and subsequent ring opening. The higher reactivity of hardwood lignin was recently explored by Andriani et al, 47 who proposed that the molecular structure of S-rich lignin enhanced the accessibility of reactive sites due to the greater exposure of hydroxyl groups in S units, as opposed to the more compact and less accessible structure in G units.…”

“…These results were comparable with the carboxylic acid content of lignin obtained through carboxymethylation modification, as reported in our previous research. 47 …”