羟甲基化可以显著提高碱木质素的羟基含量,是增强木质素反应活性的有效改性方法.为提高碱木质素的反应活性并分析碱木质素结构变化,笔者通过红外光谱(FTIR)、碳谱(DEPT 135)和异核单量子关系(HSQC)对羟甲基化木质素和原料木质素的结构分别进行表征,分析碱木质素在羟甲基化反应过程中结构变化,并对反应过程进行初步探索和验证.研究结果表明:在碱性条件下,木质素对羟基肉桂酸单元(PCA)、阿魏酸单元(FA)侧链上C7、C8双键,对羟基苯基单元(H)的C-3、C-5,愈创木基单元(G)的C-5和甲醛发生加成反应,形成羟甲基,而木质素的其他结构单元,如:紫丁香基单元(S)、β-O-4醚键结构(A)、苯基香豆满(C),并不能和甲醛发生反应.碱木质素改性的最优条件为:反应温度80℃,反应时间3h,pH为11,m(碱木质素)∶m(甲醛)为6∶1.在最优条件下得到的羟甲基化碱木质素的羟甲基质量分数为2.32％.本研究结果可为羟甲基化碱木质素基新材料的进一步研究和开发提供参考.%Lignin is a natural polymer that can be used as a raw material for the preparation of chemicals and materials.Hydroxymethylation is an effective modification method to increase the activity of the lignin and therefore improve the utilization of the lignin.This study compared the structure changes of lignin before and after the hydroxymethylation reaction to evaluate the improvement of the activity of lignin.The structures of the hydroxymethylated lignin and the raw lignin were characterized by FTIR,DEPT 135 and HSQC.In addition,the hydroxymethylation reaction was optimized by orthogonal design with altering the conditions including temperature,time,pH,and the mass ratio of lignin to formaldehyde.The results showed that the C7 and C8 in p-coumaric acid units (PCA) and ferulic acid units (FA),the C3 and C5 in p-hudroxyphenyl units,and the C5 inguaiacyl units (G) reacted with formaldehyde to formhydroxymethyl groups.This hydroxymethylation reactions mairdy included Prings reaction and Lederer-Manasse reaction.However,other units,such as syringyl units (S),β3-O-4 aryl ether linkages (A) and phenylcoumarane substructure (C),did not react with formaldehydeunder alkaline conditions.The optimal conditions of hydroxymethylation were under the reaction temperature of 80℃,reaction time of 3 h,the pH of 11 and the mass ratio of alkali lignin to formaldehyde of 6 ∶ 1,where the hydroxymethyl content reached 2.32％.This study explained the changes in the structure of the alkali lignin after hydroxymethylation,and the results could be helpful to promote the application of industrial alkali lignin.
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