木质素在自然界中含量非常丰富，且含有大量的苯环结构，是自然界中唯一能提供可再生芳基化合物的资源。木质素的利用前景广阔，热解是木质素利用过程中最基础的一类反应，研究木质素热解机理对木质素的高效利用具有重要意义。现有实验技术难以检测复杂的中间产物，因此对木质素热解机理的认识受到限制。反应分子动力学（ReaxFF MD）模拟方法在模拟复杂体系的反应机理方面极具潜力。本论文采用ReaxFF MD方法研究木质素的热解机理。借助本课题组研发的两个特色工具：GMD-Reax和VARxMD，本论文得以开展规模为~104个原子的大规模木质素热解的反应模拟。本论文基于Adler木质素模型化合物，构建了包含9种类型连接键、含15920个原子的大规模木质素模型，进行了温度范围为300?2100 K的热解模拟。提出了木质素热解的三个阶段。第一阶段发生在较低温度，表现为原始木质素反应物分子因α-O-4和β-O-4连接键断裂而分解。第二阶段发生的温度较高，木质素分子中其他的连接键均开始参与反应，同时取代基丙基链和甲氧基也开始参与到反应中。第三阶段发生在高温条件，开始生产重碳产物。本论文中观察到的小分子的生成路径与文献基本一致。模拟结果进一步揭示了活泼的α-O-4和β-O-4连接键对反应活性较低的4-O-5和β-5连接键断裂的促进作用，还揭示了苯氧基在苯环开环过程中发挥的重要作用。本论文还利用只含β-O-4型连接键的模型体系的ReaxFF MD模拟，结合基于Adler木质素模型的模拟结果，揭示了甲氧基对小分子产物（H2、CH2O、CH4、CO2等）生成的促进作用、及对芳族产物（尤其是芳族类酚、醛产物）生成的抑制作用。讨论了简单模型化合物对木质素的热解行为认识的局限性。 

Lignin is abundant in nature and contains a large number of benzene ring structures, which is the only viable renewable resource to produce aromatic compounds. Lignin is promising for wide applications in chemical industry, among which pyrolysis is the most fundamental operation in the process of lignin utilization. Deep understanding on the reaction mechanisms of lignin pyrolysis has significant impact for efficient utilization of lignin.It is still difficult to detect complex reaction intermediates experimentally, which limits the understanding the mechanisms of lignin pyrolysis from experiments. The reactive molecular dynamics (ReaxFF MD) is a promising method to study the complex and diverse reaction pathways at molecular level. Initial reaction mechanisms of lignin pyrolysis are studied in this paper by large-scale ReaxFF molecular dynamics simulations (ReaxFF MD) facilitated by the unique tools, GMD-Reax and VARxMD, which are developed by the author’s research group.Simulations were performed over wide temperature range of 300?2100 K with a large lignin model, which contained 15920 atoms and was constructed based on Adler’s softwood lignin model. Three stages for lignin pyrolysis are proposed by pyrolysate fractions. Stage I is characterized with the complete decomposition of source lignin molecules at low temperatures dominated by breaking of α-O-4 and β-O-4 linkages. The temperature in stage II is relatively high where cracking of all the linkages occurs, accompanied by conversion of propyl chains and methoxy substituents. Stage III mapping to high temperature shows the formation of heavy pyrolysates. The pathways for small molecule formation observed in this work are broadly in agreement with the literature. In addition, it is revealed that the thermal breaking of weak linkages of α-O-4 and β-O-4 in lignin can promote the breaking of 4-O-5 and β-5 linkages that have relatively low reactivity. The observations in this thesis indicate that the oxygen substituents play extremely important roles in activating the aryl rings of lignin leading to the ring opening.ReaxFF MD simulations of lignin models containing single linkage of β-O-4 are performed. Combined with the simulation results of Adler lignin model, the effects of the methoxy group on lignin pyrolysis products (small molecules and aromatic compounds) are revealed. In addition, the limitations of lignin models with simple chemical structure in studying the lignin pyrolysis behavior are discussed.