Lignin is the most abundant aromatic polymer in nature, occurring alongside cellulose and hemicellulose in wood and grass. Technical lignins, such as kraft lignin (KL) and lignosulfonate (LS), are produced globally in large quantities as byproducts of the pulp and paper industries. Hence, they represent promising renewable resources, and their valorization and widespread application are in high demand. However, their complex and heterogeneous chemical structures with various physicochemical properties hinder their widespread use. In this context, the accurate identification and quantification of their functional groups, such as hydroxy (OH) and carboxy (COOH) groups, are essential for reliable characterization and subsequent applications. 31P nuclear magnetic resonance (NMR) spectroscopy after lignin phosphitylation is the most commonly employed technique for this purpose. However, the conventional CDCl3/pyridine (pyr) (1:1.6, v/v) mixed solvent system is suitable for KL but not for LS due to their solubility differences, requiring time-consuming pretreatment steps to convert LS into the corresponding lignosulfonic acid and alter its solubility. Herein, we propose a facile and versatile 31P NMR analysis method using a new solvent system based on an ionic liquid, 1-allyl-3-methylimidazolium chloride, and pyr (2:1, w/v). The accuracy of this method was validated using lignin-OH/COOH model compounds and benchmarked against conventional solvent systems. Importantly, this approach enables systematic and quantitative analysis of the functionality of various technical lignins, including KL and LS, without the need for ion-exchange or purification pretreatments.
Negi et al. (Fri,) studied this question.