基于密度泛函理论的β-O-4型木质素模型物亚临界水热解聚机理研究

针对木质素亚临界水热反应的复杂特性和实验室试验难以探索其微观反应历程的技术瓶颈问题，提出利用分子模拟和量子化学计算方法构建其模型物结构模型及分析热化学性能，并与实验结合揭示其水热解聚机理的构思。通过4-羟基-3-甲氧基苯乙酮溴代后碱催化聚合、还原等步骤合成具有β-O-4型连接的木质素模型物愈创木基丙三醇-β-愈创木基醚，运用密度泛函理论B3LYP/6-31++G(d,p)方法，进行几何构型优化，寻找最低能量稳定构型和热化学活性位；基于模型物水热反应产物分析设计可能的反应路径，采用过渡态理论寻找过渡态，计算反应路径的热力学、动力学变量，探索模型物水热解聚为酚类化合物的均裂、β键断裂、自由基重排等基元反应网络、速控步骤；结合水热转化实验结果，揭示模型物亚临界水热解聚机理、主要产物形成的微观演化过程。本项目的实施将为木质素结构构建、亚临界水热反应调控及高附加值化学品开发利用提供理论和技术支持。

In view of the complexity of lignin depolymerization under subcritical water conditions and the difficulty of exploring its microscopic reaction mechanism by laboratory experimental work, a creative idea that combined with quantum chemical and experiment method to illustrate lignin hydrothermal depolymerization mechanism is presented. Density functional theory methods will be used to build structural models of lignin model compounds, find its thermochemical active sites and calculate the thermodynamic parameters etc. Experimental methods will be mainly used to study the depolymerization pathway of lignin and the product distribution. A typical β-O-4 type lignin dimer model compound, guaiacyl glycerol-β-guaiacyl ether, will be synthesized by bromine replace with copper bromide, catalytic polymerization under alkaline conditions, and then reduction with sodium borohydride. Density functional theory (DFT) calculations were performed at B3LYP level 6-31++G(d,p) basis set for the title compounds in order to construction of the structure model, geometry optimization, equilibrium geometries, and calculation of bond energy and thermodynamic parameters. The transition states will be located by transition state theory method and were confirmed by visual inspection of the imaginary frequency. The possible reaction pathways will be designed according to hydrothermal conversion results of the model compound. The thermodynamics and kinetics of these reactions will be thoroughly investigated based on the optimization and quantum chemistry calculation results of reactants, intermediates, transition states and products. By these means, we can explore the elementary reactions and the speed control step of lignin converted to phenolic compounds in subcritical water, such as homolytic cleavage, β-O bond cleavage, free radical rearrangement. Finally, combining of the theoretical analysis and experimental results, we will explain the depolymerization mechanism of lignin model compound in subcritical water conditions, and reveal formation process of the main phenolic products. The present research will provide theoretical and technical support to molecular modeling of lignin structure, key regulatory of subcritical hydrothermal conversion, and development and utilization of value-added chemicals.