Formation and Chemical Structure of Carbon-13 Tracer Lignin-Carbohydrate Complexes (LCCs) During Kraft Pulping

In this study, a modified synthetic method for labeling a lignin dimer (guaiacylglycerol-beta-guaiacyl ether-[alpha-13C]) was developed. The chemical structure of the target compound was analyzed using 1H-NMR, 13C-NMR, and other analytical techniques. Then, the 13C-labeled phenolic lignin model compound was subjected to kraft pulping in the presence of xylose. Finally, the resulting reaction products were fractionated using acid precipitation and ethyl acetate extraction, and each fraction was analyzed by carbon-13 nuclear magnetic resonance (13C-NMR) and two-dimensional heteronuclear multiple quantum coherence (HMQC) spectroscopy. This aimed to investigate the occurrence of lignin-carbohydrate complexes (LCCs) during the conventional kraft pulping process. Employing ethanol as the reaction medium facilitated the bromination of 4-acetylguaiacol-[alpha-13C], resulting in a homogeneous reaction and significantly improving the yield of the brominated product to over 90%. Additionally, kraft pulping of the phenolic lignin model compound in the presence of xylose led to the occurrence of minor quantities of benzyl ether-type lignin-carbohydrate complex (LCC) structures, which were predominantly detected in the ethyl acetate extractive.