单不饱和脂肪酸甲酯中碳碳双键气相催化加氢原理与动力学调控

Biodiesel has been considered as a renewable source for transportation and mainly consists of fatty acid methyl esters (FAMEs). To increase oxidative stability of biodiesel, polyunsaturated FAMEs, which are prone to peroxidation, in biodiesel have to be partially hydrogenated to monounsaturated ones. Simultaneously, the hydrogenation of C=C bond in monounsaturated FAMEs should be avoided or minimized to preserve good cold flow properties of the biodiesel. Since the hydrogenation rates of polyunsaturated FAMEs are generally higher than those of monounsaturated ones, a key point in biodiesel hydrogenation process is the inhibition of hydrogenation of C=C bond in monounsaturated FAMEs. Considering the hydrogenation of C=C is thermodynamically favorable, kinetic control is required to achieve the above purpose. It is therefore desirable to understand the fundamental aspects of C=C bond hydrogenation. Traditionally, the typical model reaction for understanding FAME hydrogenation is liquid-phase hydrogenation of methyl oleate, which contains 19 carbon atoms and two oxygen atoms. In this case, the adsorption of methyl oleate on a metal surface is complex because of many adsorption configurations, and it could be even more complicated if possible solvent effects are included. Thus, this project for understanding C=C hydrogenation would start from gas-phase hydrogenation of methyl crotonate, a simple monounsaturated FAME, over Pt/Al2O3. By using various in-situ characterizations and intrinsic kinetics, together with theoretical calculations, it is expected to reveal the underlying mechanism of C=C bond hydrogenation, especially the nature of two rate regimes demonstrated in my previous study. At the same time, by comparative study the behaviors of hydrogenation of small olefins, this project will unravel the similarities and differences between methyl crotonate and olefin hydrogenations. It is also expected that a rational method for design of C=C bond hydrogenation will be constructed from theoretical micro-kinetics. This project will not only provide guidance for hydrogenation of monounsaturated FAMEs, but also for the conversion of C=C bond containing chemicals to high value-added products.

生物柴油是成分为脂肪酸甲酯的可再生资源。为提高其稳定性和保持低温流动性，需要对多级不饱和脂肪酸甲酯中的部分C=C键加氢，同时抑制单不饱和脂肪酸甲酯中C=C键饱和，后者是关键。但C=C键加氢是热力学易进行反应，因此需要掌握单不饱和脂肪酸甲酯中C=C键催化加氢原理，从动力学角度进行调控。传统研究使用的模型分子（油酸甲酯）在金属表面的空间吸附构型复杂，存在溶剂化效应，不利于上述目标的实现。因此，本项目以气相小分子2-丁烯酸甲酯中C=C键在Pt/Al2O3上的加氢为探针反应，通过各种原位表征和本征动力学研究，并结合理论计算，揭示C=C键加氢原理及动力学特点，特别是加氢双反应动力学区域的本质。同时通过研究比较不同尺寸烯烃分子的反应行为和本征动力学特点，深入挖掘不同分子间C=C键加氢的差异与共性规律，构建理论微反动力学模型，实现C=C键加氢动力学的理性调控。本项目也为C=C键的高质转化提供了理论指导。

C=C键加氢是化工行业最常见的反应之一，选择性是关键，这需要深入研究并理解C=C键加氢机理。本项目采用负载Pt为模型催化剂，以该反应的结构敏感性为突破口，通过不同尺寸Pt颗粒催化丁二烯加氢的原位谱学以及本征动力学，在此基础上结合理论计算，揭示了影响丁二烯加氢产物选择性的关键因素，理解了丁二烯加氢原理。具体来讲，丁二烯加氢生成正丁烯和丁烷所涉及到的氢原子类型不同，前者主要是顶位氢原子，二生成丁烷则需要处于三重洞位的氢原子。产物选择性与氢原子的覆盖度和分布密切相关，这也是C=C键加氢结构敏感性的本质。通过抑制氢原子从顶位向三重洞位的迁移，可以降低丁二烯饱和成丁烷的选择性，这可以通过金属掺杂Pt形成合金，从而实现上述目的。研究表明，Ag、Au、Zn、Sn四种金属与Pt形成合金能有效抑制C=C键的加氢饱和。本项目为含C=C键的选择性加氢提供了理论基础和指导。