Structural Characterization Of Lignin In The Pretreatment Process Of Agriculture Residues With Solid Alkali And Its Delignification Mechanism

The use of lignocellulosic resources instead of petroleum and other fossil fuels asrenewable materials for the production of high value-added chemical products is concerenmore and more by the human because of energy shortages and resource scarcity. Among thelignocellulosic resources, agricultural residues are a type of cellulose resources with richreserves for the potential utilization in biomass refinery industry. Therefore, how to efficientlymake use of them is of great significanceSolid alkali and active oxygen cooking is an environmentally friendly and efficienttechnology that developed by our laboratory, where MgO is used as a solid alkali, oxygen andhydrogen peroxide are used as active oxygen. In the cooking process, Mg²⁺is the onlyinorganic ion brought into the cooking system, not only as an alkali factor reacting with O^2-,but also as a protective agent for carbohydrate. Based on this theory, the delignificationmechanism of the solid alkali and active oxygen cooking is researched in the present work.Form the analysis of the cornstalk and bagasse lignins by3D HSQC-TOCSY NMR andother technologies, it was found that the water-soluble milled wood lignin （MWL） and MWLwere contained three types of basic structure units syringyl （S）, guaiacyl （G） andp-hydroxyphenyl （H）, main side-chain linkages β-O-4′（A/A′/A′′）, β-β′（B） and β-5′（C）structures, and esterified ferulic acid structure （FE） with a little content. The transformationstructures of syringyl （S′/S′′） were not found in the water-soluble MWL of bagasse.Comparing to the MWL, the molecular weight of the water-soluble MWL from the same rawmaterial was smaller. In addition, a great amount of the aliphatic hydroxyl group, and smallamounts of phenolic hydroxyl group in guaiacyl and condensed phenolic hydroxyl group werefound in the four types of lignins. The phenolic hydroxyl groups in syringyl andp-hydroxyphenyl, and carboxyl group were not found in the MWL of bagasse.The solid alkali and active oxygen cooking had a high delignification rate, which couldremove85.5%of lignin in cornstalk and95.4%of lignin in bagasse, and had a protectiveeffect for the carbohydrate especially for the cellulose. In the cooking process, the syringyl（S/S′/S′′） units had a high reactivity. A novel G′structure was formed byan oxidizing reactionof the guaiacyl （G） unit. However, the p-hydroxyphenyl unit was stable in the cooking. For the sid-chain linkages, the phenolic β-O-4′（A/A′/A′′） structures could be changed by analkaline autoxidation reaction, and their non-phenolic structures had different reactivities. Thenon-phenolic β-β′（B） and β-5′（C） structures were stable in the cooking process, but the β-1′（D） structure was completely destroyed in the cooking. Moreover, the removed esterifiedferulic acid structure （FE） was also broken in the cooking, while the esterifiedp-acetoxy-benzoic acid structure （P） was stable.In our research, it was found that only the MgO and Mg（OH）₂used in solid alkali andactive oxygen cooking had a high delignification rate and not occurred carbonizationphenomena. The solid alkali effectively could provide a weakly alkaline environment fordelignification, prevent the raw material carbonizing, and also have a protective effect for thecarbohydrate. The oxygen affected changes in the phenolic structure, leading to the oxidationof a benzylic alcohol group into a carbonyl group, and the facile attack of the ring-conjugatedstructure. The hydrogen peroxide could react with the carbonyl groups and double bonds onlignin side-chains, and further increasing the delignification rate.In the solid alkali and active oxygen cooking, the lignin structure units could be removedin different cooking time. The acid soluble lignin was mainly removed in the heating stage,with the temperature under100℃, the removal of the acid-insoluble lignin and thedegradation of the carbohydrate were occurred in the heating stage also, but with thetemperature above100℃. In the analysises of the lignin models, it was found that theguaiacol and partial p-hydroxybenzoic acid were stable in the solid alkali and active oxygencooking process. While ferulic acid, p-coumaric acid, ethyl4-hydroxybenzoate, eugenol,trans-cinnamic acid and ethyl benzoate were thoroughly broken. In addition, the molecularweight of the water-soluble MWL changed smaller compared to the MWL in the cooking,namely the MWL was destroyed more seriously in the solid alkali and active oxygen cooking.And the P structure could be formed in the cooking process.The pH value of the yellow liquors from the solid alkali and active oxygen cooking waslow, which was a weak alkaline environment, and a large amount of the formic acid and aceticacid were found in the yellow liquors. The yellow liquors mainly contained alkali lignin, withcertain Mg（OH）₂precipitate in them. Form the analysis of the yellow liquors in the cookingwith MgO/O₂, MgO/H₂O₂and MgO/H₂O₂/O₂, the main components in alkali and acid extracts were phthalic acid, mono （2-ethylhexyl） ester.