Methyl Linoleate and Methyl Oleate Bond Dissociation Energies: Electronic Structure Fishing for Wise Crack Products

The world depends on petroleum for everything from the plastics that contain our food to the natural gas that heats our homes to the gasoline that feed our cars’ engines. With rising prices of petroleum reflecting demand for this finite resource, attention has been turned to alternative sources of energy. Biodiesel, which exhibits many of the same properties as conventional diesel but is derived from biological sources, is an attractive alternative. Fats and oils are converted to biodiesel, fatty acid methyl esters (FAMEs), by transesterification. FAMEs are subsequently thermally cracked to form more lightweight transportation fuels such as natural gas, kerosene, and possibly gasoline. We aim to further understand the thermal cracking procedure, at an atomic level, in hopes that this may aid in future engineering of viable fuels. We will present our study on the effective computational modeling of bond dissociations in the FAMEs methyl linoleate and methyl oleate, which are the most common biodiesel products of soybeans and rapeseeds (also known as canola seeds). We have employed quantum chemical methods, including the density functionals B3LYP, M06-2X, and B97D; the wave function-based MP2; and the composite CBS-QB3 method. Bond dissociation in a 44-reaction database set for which experimental energies are known is used to evaluate methods. We find that the M06-2X/6-31+G­(d,p) model chemistry provides results comparable to the composite CBS-QB3 method at a much reduced cost. Last, data are compiled for possible bond dissociations in FAMEs methyl oleate and methyl linoleate.当今世界，从包装食品的塑料、供暖住宅的天然气，到驱动汽车引擎的汽油，各类用品无不依赖石油。随着石油作为有限资源的需求攀升导致价格上涨，学界与业界的目光已转向替代能源。生物柴油（biodiesel）的理化性质与常规柴油颇为相近，却源自生物质资源，是极具吸引力的替代燃料。脂肪与油脂通过酯交换反应可转化为生物柴油，即脂肪酸甲酯（fatty acid methyl esters，FAMEs）。随后，FAMEs可通过热裂解反应生成更轻质的运输燃料，例如天然气、煤油，乃至汽油。本研究旨在从原子层面深入解析热裂解反应的机理，以期为未来可实用化运输燃料的工程开发提供理论支撑。本次研究将针对FAMEs中的亚油酸甲酯与油酸甲酯展开键解离的高效计算建模，这两种物质均为大豆与油菜籽（又称菜籽）制备生物柴油的常见产物。我们采用了多种量子化学计算方法，包括密度泛函B3LYP、M06-2X与B97D，基于波函数的MP2方法，以及复合CBS-QB3方法。我们选取了包含44个已知实验解离能的反应数据集，用以评估各计算方法的性能表现。研究发现，M06-2X/6-31+G(d,p)模型化学的计算结果可与复合CBS-QB3方法媲美，但计算成本大幅降低。最后，我们整理了油酸甲酯与亚油酸甲酯中所有可能发生的键解离相关数据。