Mechanistic insight into hydroxy-methylation of hardwood Kraft lignin

IF 3.1 2区 农林科学 Q1 FORESTRY
Micaela B. Peralta, Nicolò Pajer, Claudia Crestini, Verónica V. Nicolau
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Abstract

In view of developing upcycling strategies for hardwood Kraft lignin, hydroxy-methylation of Eucalyptus Kraft lignin under alkaline conditions (pH 9 and 11) at different temperatures (50 °C and 70 °C) was studied in the present effort with the double objective of optimizing the reaction conditions and understanding the functionalization mechanism of C5 in either terminal or internal guaiacyl units during hydroxy-methylation. Formaldehyde consumption was estimated via titration of the oximated free formaldehyde; the hydroxy-methylation degree under the reaction was estimated by calculating the ratio in Condensed hydroxyl/Guaiacyl (Condensed OH/G-OH) via a new difference UV-spectroscopy. The reliability of the difference UV-method results for the analyses of the hydroxy-methylated lignins was statistically analysed and compared with that of vacuum-dried and sonicated samples. Hydroxy-methylated samples were then fully characterised by NMR (31P and HSQC) and GPC. The reaction temperature of 50 °C, pH 11, and period time of one hour resulted as the optimal conditions for the hydroxy-methylation, preventing the side-reactions leading to the formation of dimethylene-glycol addition products. The 31P and 1H–13C HSQC NMR revealed the absence of undesirable formaldehyde Cannizzaro by-products and the lack of hydroxymethyl groups in the aliphatic side chain under the studied conditions. GPC analyses, comparing two methodologies, revealed increases in molar mass of the hydroxy-methylated samples upon the formaldehyde addition. The selective hydroxy-methylation at the C5 guaiacyl site demonstrates that Eucalyptus Kraft lignin is as a promising candidate for resol production.

Abstract Image

硬木牛皮纸木质素羟基甲基化的机理研究
为了开发硬木牛皮纸木质素的升级再循环策略,本研究在不同温度(50 °C和70 °C)、碱性条件(pH值为9和11)下对桉树牛皮纸木质素进行了羟基甲基化,其双重目的是优化反应条件,并了解羟基甲基化过程中端部或内部愈创木酰基单元中C5的官能化机制。甲醛消耗量是通过滴定氧化游离甲醛来估算的;反应中的羟甲基化程度是通过新的紫外光谱差异法计算缩合羟基/愈创木酰基(缩合 OH/G-OH)的比率来估算的。经统计分析,差分紫外法分析羟甲基化木质素结果的可靠性与真空干燥和超声处理样品的可靠性进行了比较。然后通过核磁共振(31P 和 HSQC)和气相色谱对羟甲基化样品进行了全面表征。反应温度为 50 °C、pH 值为 11、反应时间为一小时是羟基甲基化的最佳条件,可防止副反应导致二亚甲基乙二醇加成产物的形成。31P 和 1H-13C HSQC NMR 显示,在所研究的条件下,不存在不良的甲醛 Cannizzaro 副产物,脂肪族侧链中也没有羟甲基。通过比较两种方法进行的 GPC 分析表明,添加甲醛后,羟甲基化样品的摩尔质量增加。C5 愈创木酰基位点的选择性羟甲基化表明,桉树牛皮纸木质素是一种很有希望用于生产 resol 的候选物质。
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来源期刊
Wood Science and Technology
Wood Science and Technology 工程技术-材料科学:纸与木材
CiteScore
5.90
自引率
5.90%
发文量
75
审稿时长
3 months
期刊介绍: Wood Science and Technology publishes original scientific research results and review papers covering the entire field of wood material science, wood components and wood based products. Subjects are wood biology and wood quality, wood physics and physical technologies, wood chemistry and chemical technologies. Latest advances in areas such as cell wall and wood formation; structural and chemical composition of wood and wood composites and their property relations; physical, mechanical and chemical characterization and relevant methodological developments, and microbiological degradation of wood and wood based products are reported. Topics related to wood technology include machining, gluing, and finishing, composite technology, wood modification, wood mechanics, creep and rheology, and the conversion of wood into pulp and biorefinery products.
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