Chemical reactivity and sulfo-functionalization response of enzymatically produced lignin

Abstract

This study focuses on the physicochemical characterization of lignin produced via enzymatic hydrolysis of hardwood chips. Enzymatically produced lignin (HL) was water-insoluble particles that contained carbohydrates that were chemically bound to lignin. The X-ray photoelectron spectroscopy (XPS), 1H-NMR, 13C-NMR, 31P NMR and two-dimensional 1H-13C HSQC, 1H-13C HSQC-TOCSY, 1H-13C HMBC NMR analyses confirmed that the HL samples with the lower internal ether linkages had fewer β-O-4 aryl ether and lignin-carbohydrate (LCC) bonds but more phenolic hydroxyl and carboxylate groups. Moreover, the samples with the lower sugar content were smaller and more porous. The chemical reactivity analysis of HL samples in various sulfo-functionalization reactions (sulfonation S, sulfomethylation SM, and sulfoethylation SE) revealed a direct correlation between the degree of substitution (DS) and phenolic guaiacyl (G) and the condensed hydroxyl group of HL. Also, the carbohydrate content and particle size of HL were inversely related to the degree of substitution of sulfo-functionalization of HL. The SM- and S- modified HL samples with adequate water solubility (9−10 g/L), charge density (≥1.9 mmol/g), and small particle size (5 μm) were shown to be effective stabilizers for clay suspensions via reducing the instability index (TSI∼10) and settling velocity (−4 %/h) of clay particles in the suspension. The results obtained in this study shed light on the significant role of residual carbohydrates and internal ether linkages of HL on the physicochemical properties and sulfo-functionalization of HL.