Characterization of Structural Changes of Lignin in the Process of Cooking of Bagasse with Solid Alkali and Active Oxygen as a Pretreatment for Lignin Conversion

A high yield of lignin was generated when solid alkali (MgO) and active oxygen (O 2 and H 2 O 2 ) were used in bagasse cooking as a pretreatment for lignin conversion. In the present work, the lignin present in raw material, pulp, and yellow liquor was characterized by Fourier transform infrared spectroscopy (FTIR) and heteronuclear single-quantum coherence (HSQC) NMR. The results showed that 95.4% of the lignin was obtained from the raw material and present in the yellow liquor. The syringyl (S/S′/S″) units could be changed completely and removed from the raw material, and a novel G′ unit with a carbonyl group was generated through an oxidizing reaction. The phenolic structures had a high reactivity during the cooking process, and the β- O -4′ (A/A′/A″) structures had different levels of reactivity. Moreover, the H unit, P structure, and nonphenolic β-5′ and β–β′ structures were stable throughout the cooking process.