Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, School of Light Industrial and Food Engineering, Guangxi University, Nanning 530004, China.
Int J Mol Sci. 2023 Jul 22;24(14):11809. doi: 10.3390/ijms241411809.
Lignin is a polymer with a complex structure. It is widely present in lignocellulosic biomass, and it has a variety of functional group substituents and linkage forms. Especially during the oxidation reaction, the positioning effect of the different substituents of the benzene ring leads to differences in lignin reactivity. The position of the benzene ring branched chain with respect to methoxy is important. The study of the effect of benzene substituents on the oxidation reaction's activity is still an unfinished task. In this study, density functional theory (DFT) and the m062x/6-311+g (d) basis set were used. Differences in the processes of phenolic oxygen intermediates formed by phenolic lignin structures (with different substituents) with chlorine dioxide during the chlorine dioxide reaction were investigated. Six phenolic lignin model species with different structures were selected. Bond energies, electrostatic potentials, atomic charges, Fukui functions and double descriptors of lignin model substances and reaction energy barriers are compared. The effects of benzene ring branched chains and methoxy on the mechanism of chlorine dioxide oxidation of lignin were revealed systematically. The results showed that the substituents with shorter branched chains and strong electron-absorbing ability were more stable. Lignin is not easily susceptible to the effects of chlorine dioxide. The substituents with longer branched chains have a significant effect on the flow of electron clouds. The results demonstrate that chlorine dioxide can affect the electron arrangement around the molecule, which directly affects the electrophilic activity of the molecule. The electron-absorbing effect of methoxy leads to a low dissociation energy of the phenolic hydroxyl group. Electrophilic reagents are more likely to attack this reaction site. In addition, the stabilizing effect of methoxy on the molecular structure of lignin was also found.
木质素是一种具有复杂结构的聚合物。它广泛存在于木质纤维素生物质中，具有多种官能团取代基和键合形式。特别是在氧化反应中，苯环不同取代基的定位效应导致木质素反应性的差异。苯环支链相对于甲氧基的位置很重要。研究苯环取代基对氧化反应活性的影响仍然是一项未完成的任务。在这项研究中，使用密度泛函理论（DFT）和 m062x/6-311+g（d）基组研究了木质素结构（具有不同取代基）与二氧化氯反应过程中形成的酚氧中间体的差异。选择了六种具有不同结构的酚类木质素模型种。比较了木质素模型物质的键能、静电势、原子电荷、福井函数和双描述符以及反应能垒。系统揭示了苯环支链和甲氧基对木质素二氧化氯氧化反应机制的影响。结果表明，支链较短且吸电子能力较强的取代基更稳定。木质素不易受二氧化氯的影响。支链较长的取代基对电子云的流动有显著影响。结果表明，二氧化氯可以影响分子周围的电子排列，从而直接影响分子的亲电活性。甲氧基的吸电子效应导致酚羟基的离解能较低。亲电试剂更容易攻击这个反应位点。此外，还发现了甲氧基对木质素分子结构的稳定作用。
