A Synthetic Approach to Active Site Deconvolution in Supported Cr Catalysts for Olefin Polymerization

该项目的目标是建立一个统一的框架，以理解和控制用于烯烃转化，特别是烯烃复分解和聚合的负载金属氧化物催化剂在底物存在下的自激活能力。定义良好的模型催化剂的活性分布和产物分布将与定义良好的几何和电子结构的位置相关。最终，对活性位点结构的了解将有助于这些催化剂的重新设计，通过选择位点的组合来产生所需的产品分布，例如聚合物分子量和分支含量。此外，催化剂活化机制的阐明将为提高活化效率提供策略。这个项目的目标是在一类非常成功的工业催化剂中解决一个长期存在的复杂问题。用于烯烃聚合的菲利普斯催化剂（CrOx/SiO2）早在半个世纪前就被发现了。自那以后，这种催化剂在烯烃存在下的自激活能力一直是人们猜测的主题。在知道活性位点的性质之前，我们不太可能确定它们是如何形成的，或者为什么这种催化剂在恢复失活位点方面如此有效。含有Mo和W的负载6族金属氧化物的相关催化剂也具有自活化性，但它们促进烯烃的复分解而不是聚合。所提出的确定自活化如何发生的方法结合了对存在于非均相催化剂上的多个活性位点的分析来合成单独的、定义良好的活性位点候选物。实验方法将与反应机制的计算模型相结合，以确定关键的过渡态。PIs提出探索启动机制，并将异构系统中组分活性位点的活性谱和产物分布与结构明确的位点的活性谱和产物分布相匹配。活性位点研究的合成方法是催化剂发现的组合方法的补充。一旦通过筛选（平行或其他）确定并优化了有趣的催化剂配方，进一步的改进取决于操纵活性位点结构的能力。通过合成模型化合物来识别控制反应性的结构特征一直是均相催化的成功策略。将有机金属合成、表面科学、复杂系统动力学/机理分析以及催化剂建模和评价等方面的专业知识结合起来，可以将这一策略扩展到多相催化领域。更广泛的影响非均相聚烯烃催化剂是一种多功能材料，具有低聚、聚合、共聚、终止和再活化的能力，可以产生具有理想物理性能组合的聚合物树脂。与此同时，单位点催化剂合成的最新进展导致聚合物具有非常低的多分散性，以及对聚合机制的精细机理研究。多相催化剂的复杂性使其具有灵活性和多样性，因此值得进行同样的系统研究，以充分发挥其优势。执行这个项目需要很强的化学工程和化学技能。作为跨学科研究团队的成员，研究生将对这两个领域有深入的了解。他们将学会欣赏和实施每个学科解决问题的方法的优势。学生还将体验到活性位点的计算和实验建模之间的紧密联系。这项工作的技术成功为催化研究人员的跨学科培训提供了有力的理由。

0854425 Scott, Susannah L. The goal of the project is to develop a unified framework for understanding and controlling the ability of supported metal oxide catalysts used in olefin transformations, specifically olefin metathesis and polymerization, to self-activate in the presence of the substrate. The activity profiles and product distributions for well-defined model catalysts will be correlated with sites of well-defined geometrical and electronic structure. Ultimately, knowledge of active site structures will facilitate the re-engineering of these catalysts by allowing the selection of combinations of sites to generate a desired product distribution, for example, polymer molecular weight and branch content. Furthermore, elucidation of catalyst activation mechanisms will indicate strategies to make the activation more efficient.Intellectual MeritThe goal of this project is to solve a longstanding and complex problem in a very successful class of industrial catalysts. Phillips catalysts for olefin polymerization (CrOx/SiO2) were discovered half a century ago, and their remarkable ability to self-activate in the presence of olefin has been the subject of much speculation ever since. Until the nature of the active sites is known, it is unlikely that we will be able to determine how they are formed or why this catalyst is so effective at reviving deactivated sites. Related catalysts containing supported group 6 metal oxides of Mo and W are also self-activating, but they promote olefin metathesis rather than polymerization. The proposed approach to determining how self-activation occurs combines synthesis of individual, well-defined active site candidates with analysis of the multiple active sites present on the heterogeneous catalysts. Experimental approaches will be integrated with computational modeling of reaction mechanisms to identify key transition states. The PIs propose to probe initiation mechanisms and match activity profiles and product distributions of the component active sites in the heterogeneous systems with those of structurally well-defined sites.The synthetic approach to active site investigation is complementary to the combinatorial approach to catalyst discovery. Once interesting catalyst formulations are identified and optimized by screening (parallel or otherwise), further improvements depend on the ability to manipulate active site structure. Identifying the structural features that control reactivity by synthesizing model compounds has long been a successful strategy in homogeneous catalysis. The extension of this strategy in heterogeneous catalysis can be accomplished by combining expertise in organometallic synthesis, surface science, kinetics/mechanistic analysis of complex systems and catalyst modeling and evaluation.Broader ImpactsHeterogeneous polyolefin catalysts are multifunctional materials with the ability to oligomerize, polymerize, copolymerize, terminate and reactivate, creating polymer resins with desirable combinations of physical properties. In parallel, recent advances in the synthesis of single-site catalysts have led to polymers with very low polydispersities, as well as elegant mechanistic studies of polymerization mechanisms. The complexity of the heterogeneous catalysts is enabling in both its flexibility and its diversity, and therefore deserving of the same systematic investigation in order to fully realize its benefits.Execution of this project requires strong skills in both chemical engineering and chemistry. Graduate students will develop a deep understanding of both fields as members of an interdisciplinary research team. They will learn to appreciate and implement the strengths of each discipline's approach to problem-solving. Students will also experience the close connection between computational and experimental modeling of active sites. Technical success in this work provides a powerful justification for interdisciplinary training of catalysis researchers.