Characterization of lignin residues derived from the alkaline hydrolysis of glycol lignin

Abstract

Lignin residues from the alkaline hydrolysis of solvolytic lignin have been characterized by elemental analysis, size exclusion chromatography, Fourier transform infrared spectroscopy, 13C NMR, and for methoxyl content. All lignin residues were found to have higher carbon and lower oxygen contents than the original glycol lignin. Methoxyl contents decreased with increasing severity of treatment. All treated lignins have lower molecular weights than the original, which is indicative of the extensive depolymerization of glycol lignin by alkaline hydrolysis. The appearance of new bands in the 3050–2800 cm−1 and 900–800 cm−1 regions in the infrared spectra along with increasing absorption intensity under more severe conditions demonstrates that, despite the tendency to depolymerization, new carbon–carbon bonds are formed due to repolymerization reactions. 13C NMR revealed that extensive cleavage of the side chains and β-O-4 interunit linkages had occurred during alkaline hydrolysis and that the new bonds formed due to condensation reactions at higher severities are of the methine, methylene, methyl and carboxyl types. Keywords: residual lignins, alkaline hydrolysis, characterization.