Oxidative coupling of Kraft lignin mediated with hypervalent iodine reagent (III) for enhanced removal of dye in water.

One of the most attractive routes to valorize lignin is to prepare lignin-based materials which can be used as high-value products, such as dye absorbents. Various processes have been investigated to improve the properties of lignin. In this study, a hypervalent iodine reagent (III) was employed to achieve oxidative coupling of Kraft lignin (KL). The structural changes were investigated by FTIR, quantitative 31P NMR, 2D-HSQC, and GPC. Oxidative coupling between lignin aromatic C(sp2) H bonds and aromatic C(sp2)-H functionalization with aliphatic hydroxyl group occurred via inter- or intramolecular modes, with resultant decreases of the aromatic protons and hydroxyl groups. In addition, selective modification of aliphatic OH groups in β-O-4′ linkages occurred leading to the formation of aromatic carbonyl groups. Increases in molecular weights with the appearance of conjugated C O groups were also observed. The oxidative crosslinked lignin (KL-2.0) exhibited better adsorptive features for brilliant green (251.2 mg/g) than that of unmodified KL (59.4 mg/g). Their adsorption kinetics were investigated by pseudo-first-order, pseudo-second-order and mixed-order models. Furthermore, the Langmuir and Freundlich models were employed to study the adsorption isotherms. Oxidative coupling of Kraft lignin between lignin C(sp2) H bonds and aromatic C(sp2) H functionalization with aliphatic hydroxyl groups was achieved by employing hypervalent iodine reagent (III), resulting an new oxidative cross-linked lignin with better adsorptive features for brilliant green (251.2 mg/g) than that of unmodified KL (59.4 mg/g). [Display omitted] • New oxidative coupling of lignin with high selectivity was achieved in the presence of hypervalent iodine reagent (PIFA). • The treatment of KL with PIFA appeared to selectively oxidize hydroxyl group in β-O-4′ linkages into carbonyl group. • The adsorption capacity of lignin for brilliant green has been significantly improved after oxidative coupling modification. [ABSTRACT FROM AUTHOR]