基于基团贡献法的碱木质素结构及其热解机理分子模拟与实验研究

It is commonly accepted that alkali lignin structure is as a group of single-ring aromatics with two attributes of propanoid side chain and phenolic or methoxyphenol substituent connected by handful types of interunit linkage and pyrolysis process leaves the rings intact. The most popular three substituents are hydroxyl-phenyl(H), guaiacy(G) and syringyl(S). Based on the structural and reactional characteristics of alkali lignin, the research on its structure model constructing, thermal stability and pyrolysis mechanism by molecular simulation and quantum chemical method, combined with experimental results, is proposed by this application. CP/MAS13CNMR, ultimate analysis and XPS are used to study the structural characteristics of alkali lignin. Parsing the structural characteristics provides quantitative values for the statistical distribution of the attributes. By viewing each unit of the alkali lignin model as the statistical probability of the contribution of the attributes , the function to optimize composition is set up to generate a best fit molecular assembles , which match with the available analytical results . The high-poly model compounds of β-O-4 linked alkali lignin is synthesized using acetophenone-derived regent as initiator, followed by brominating, polymerizing by alkali catalyzed oxygen anion nucleophilic addition reaction and reducing. Molecular simulation and quantum chemistry calculation QM/QM(MM) are adopted to simulate the minimization geometry of alkali lignin structural model, active centers, to scan the free radical-based pyrolysis reaction network from high-poly model compounds and model compounds in molecular assembles to phenol. The rate-determining step and phenol production pathway are explored. Combining the quantum calculation with the experimental results from TG-IR-MS, Py-GC-MS and pyrolysis in tubular reactor of alkali lignin and high-poly model, the target-directed alkali lignin pyrolysis mechanism will be formed, which have important significance for determining alkali lignin structure, conversion process and new product development at present and in the future.

针对碱木质素由对羟苯基、愈创木基和紫丁香基三种单环苯丙基团单元、以有限方式连接的结构特点和苯环不参与热解的反应特性，提出用分子模拟和量子化学方法构建其结构模型及分析热化学性能，并与实验结合揭示其热解机理的构思。通过成分分析和交叉极化/魔角旋转核磁共振的测定，运用基团贡献法思想，建立表征碱木质素结构参数的概率密度函数，构造其特征分子系综；通过苯乙酮衍生物溴代后碱催化聚合、还原等步骤合成具有β-O-4型连接的碱木质素高聚模型物，运用分子力学和量子化学QM/QM(MM)优化方法，寻找碱木质素最低能量构型、几何优化构型和热化学活性位，探索热解自由基片段、高聚模型物和分子系综模型物等不同层次模型物热解为苯酚的基元反应网络、速控步骤；结合热重、裂解气相质谱、管式炉实验结果，揭示碱木质素定向热解规律、形成生成苯酚的反应机理，为碱木质素结构构建、热化学反应调控及新产品开发利用提供理论和技术支撑。

本项目针对木质素由对羟苯基、愈创木基和紫丁香基三种单环苯丙基团单元、以有限方式连接的结构特点和苯环不参与热解的反应特性，以分子模拟和量子化学方法构建木质素结构和分析热化学性能，并结合实验探讨其解聚和小分子解聚产物催化加氢的反应途径和机理。.构建了以10个C9单元为基础的木质素结构模型，探索了碱木质素特征官能团和连接键对其结构影响。碱木质素中愈创木基型结构单体含量约占70%，每C9单元环中含有1.26个甲氧基，碱木质素中酰基和羧基含量高。C9单体之间的连接主要以β-O-4连接键为主，羟基主要是其制备过程中β-O-4连接键水解所致。木质素结构模型的QM/QM和QM/MM (oniom) 分子模拟和量化计算表明，木质素热解首先是聚合度降低过程，甲氧基O-C键是化学反应最易发生的部位，而酚羟基键能最大，难以断裂。.富含木质素的玉米芯水解残渣在移动床热解-固定床催化裂化实验中， HZSM-5（25）较强Brönsted酸性中心和较大孔径，有利于木质素热解有机蒸汽中含氧物种甲氧基脱除和转甲基化作用，改质生物油中单酚类和芳烃物种选择性分别为53.2%和29.5%。木质素解聚及解聚小分子加氢脱氧过程及机理研究中发现，Pt/SiO2- ZrO2、Ni/TiO2-ZrO2、Ru/γ-Al2O3的金属-载体相互作用及较强的载体酸性有利于加氢脱氧反应进行。载体酸性中心的存在，使木质素降解小分子以加氢反应为主，强金属-载体相互作用则有利于脱羰基反应。.本研究将量子化学理论研究与催化热解实验结合，研究了木质素类废弃物结构及解聚过程重要化学键的断裂及含氧基团的加氢机理，完善了木质素结构构建和热解为烃类和酚类物质的量子化学理论及热裂解过程控制手段，可为木质素应用开发及热解产物分布调控途径提供理论指导。项目执行期间共发表研究论文18篇，SCI论文13篇，EI论文1篇，国际会议论文3篇。