苯基香豆满构型木质素热化学解聚的微观演变机制研究

In view of the complexity of lignin by thermochemical depolymerization and the difficulty of exploring its microreaction mechanism by laboratory experimental work, the phenylcoumarane lignin model connected with the characteristic linkages of α-O-4 and β-5 is adopted for the research, a new idea that combines with quantum chemical and experimental methods to illustrate micro transformation mechanism of phenylcoumarane lignin depolymerization is presented. Density functional theory (DFT) calculations are performed at B3LYP level 6-31++G(d,p) basis set for the phenylcoumarane compounds in order to optimize the equilibrium geometries of the model, find its thermochemical active sites. Based on the results of DFT calculations and the properties of depolymerized compounds, the main depolymerization reaction paths of phenylcoumarane lignin are come up with the transition state theory method, thoroughly investigating the thermodynamics and kinetics variables of these reactions to clarify the interaction with actual pyrolysis reaction. By means of probing the elementary reactions, the speed control step of lignin depolymerization, and the mechanism on competitive cleavage reactions of carbon-carbon bonds with aryl ether bonds, it is revealed at molecular level that the formation pathways of products and the microreaction mechanism of the phenylcoumarane lignin in the process of thermochemical depolymerization. Therefore, the present research will provide a theoretical basis to the molecular modeling of lignin, the control reaction of key products, and the promising development of value-added biochemicals.

针对木质素热化学解聚产物形成的复杂特性和实验过程中难以探索的微观演变途径的技术瓶颈，以具有α-O-4和β-5特征连接键的苯基香豆满木质素为研究对象，提出利用分子模拟和量子化学法构建苯基香豆满木质素模型、并与实验结合揭示其热化学解聚微观演变机理的构思。运用密度泛函理论B3LYP/6-31++G(d,p)方法对苯基香豆满木质素模型进行能量稳定构型优化和热化学反应活性位寻求。基于理论量化计算结果和实验解聚产物特性，揭示苯基香豆满木质素解聚的主要反应路径，在过渡态理论基础上计算反应路径的热力学与动力学变量，阐明特征连接键的热力学参数与实际反应的内在关系。在探索基元反应网络、速控步骤、以及芳基醚键与碳碳键的竞争反应机制的基础上，从分子水平揭示最终产物的演变途径和苯基香豆满构型木质素热化学解聚的微观反应机理。本项目的实施将为木质素结构构建、热解反应中关键产物调控及高附加值化学品开发利用提供理论依据。

针对木质素热化学解聚产物形成的复杂特性和实验过程中难以探索的微观演变途径的技术瓶颈，以具有α-O-4和β-5特征连接键的愈创木基(G-型)苯基香豆满构型木质素为研究对象，基于密度泛函理论(DFT)的量子化学法与实验结合揭示苯基香豆满木质素模型物的热化学解聚微观演变机理。一方面，运用DFT的B3LYP法结合全电子基组6-311+g*基于极化和弥散函数对分子结构进行优化，对G-型苯基香豆满木质素模型物进行势能面扫描，借助Gaussian程序Berny能量梯度法，对各分子在热解聚过程中可能生成的自由基进行几何构型的全优化，经频率计算获得298K下的标准热力学参数。另一方面，以G-苯基香豆满木质素模型物和木质素(G-型占主要地位)为实验对象，对其进行化学结构表征和热化学解聚实验研究，深入掌握解聚产物随影响因素的分布规律。综合理论量化计算和实验水平结果，具有G-型苯基香豆满构型木质素的化学结构稳定性较弱，低温时即发生热降解反应。G-苯基香豆满构型木质素二聚体在低温热解时生成一级产物苯并呋喃型产物，随着热解温度增加，因五元杂环不稳定性苯并呋喃型产物二次热解开环形成单酚类化合物。同时，低温时木质素发生降解反应，得到G-型化合物或苯并呋喃衍生物，模型物与木质素热解实验结果相一致，在此基础上设计了苯基香豆满木质素解聚的主要反应路径。木质素在低温下热解时，Cα-O裂解反应和Cα-Cβ脱氢反应之间发生竞争反应，DFT解离能表明苯基香豆满结构中Cα-Car发生裂解反应的同时，苯基呋喃环并不发生开环反应，故二氢苯并呋喃有较高产率。项目进一步阐明了特征连接键的热力学参数与实际反应的内在关系，从分子水平揭示最终产物的演变途径和苯基香豆满构型木质素热化学解聚的微观反应机理。本项目的实施将为木质素结构构建、热解反应中关键产物调控及高附加值化学品开发利用提供理论依据。