Alkylation modification for lignin color reduction and molecular weight adjustment

The application of industrial kraft lignin is limited by its low molecular weight, dark color, and low solubility. In this work, an efficient crosslinking reaction with N,N-Dimethylformamide (DMF) and 1,6-dibromohexane was proposed for adjusting the molecular weight and color of lignin. The chemical structure of alkylation lignin was systematically investigated by gel permeation chromatography (GPC), ultraviolet spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, and 2D heteronuclear single quantum correlation nuclear magnetic resonance (HSQC NMR) spectra. After the alkylation modification, the molecular weights of the lignin were increased to 1643%. The resinol (13-13), 13-aryl ether (13-O-4), and phenylcoumaran (13-5) linkages were still the main types of the linkages. The formation of 13-13 linkage would be inhibited at high temperatures. The color reduction of lignin can be attributed to the low content of chromophores and low packing density. This alkylation lignin will be a new and general approach for developing molecular weight-controlled and light-colored lignins, which can find more applications in cosmetics, packing, and other fields.

Automated summary

An efficient crosslinking reaction using DMF and 1,6-dibromohexane was proposed to adjust the molecular weight and color of industrial kraft lignin. After alkylation modification, the molecular weights of lignin increased by 1643%, and the main types of linkages remained resinol, 13-aryl ether, and phenylcoumaran. The reduction in color was attributed to the low content of chromophores and low packing density of the alkylation lignin.