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

0854425斯科特，苏珊娜L. 该项目的目标是开发一个统一的框架，用于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型金属氧化物催化剂在基材存在下自活化的能力。明确定义的模型催化剂的活性分布和产物分布将与明确定义的几何和电子结构的位点相关。最终，活性位点结构的知识将通过允许选择位点的组合以产生所需的产物分布（例如，聚合物分子量和分支含量）来促进这些催化剂的再设计。此外，催化剂活化机制的阐明将表明策略，使活化更加efficient.Intellectual MeritThe该项目的目标是解决一个长期存在的和复杂的问题，在一个非常成功的一类工业催化剂。用于烯烃聚合的菲利普斯催化剂（CrOx/SiO2）在半个世纪前被发现，并且它们在烯烃存在下的显著自活化能力一直是许多推测的主题。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。含有负载的Mo和W的第6族金属氧化物的相关催化剂也是自活化的，但它们促进烯烃复分解而不是聚合。所提出的方法，以确定如何发生自活化相结合的个人，明确定义的活性位点候选人的合成与分析的多个活性位点上存在的非均相催化剂。实验方法将与反应机制的计算建模相结合，以确定关键的过渡态。PI提出了探索引发机制和匹配的活性配置文件和产品分布的组分活性位点的非均相系统与那些结构明确的sites.合成方法的活性位点调查是互补的组合方法，催化剂发现。一旦感兴趣的催化剂配方被鉴定并通过筛选（平行或以其他方式）优化，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确定控制反应性的结构特征一直是均相催化的成功策略。通过将有机金属合成、表面科学、复杂体系的动力学/机理分析以及催化剂建模和评估等方面的专业知识相结合，可以实现该策略在多相催化领域的扩展。更广泛的影响多相聚烯烃催化剂是多功能材料，具有低聚、聚合、共聚、终止和再活化的能力，可生成具有理想物理性能组合的聚合物树脂。与此同时，最近在单活性中心催化剂合成方面的进展已经导致了具有非常低的多分散性的聚合物，以及对聚合机理的优雅的机理研究。多相催化剂的复杂性使其灵活性和多样性成为可能，因此值得进行同样的系统研究，以充分实现其优势。执行该项目需要在化学工程和化学方面具有很强的技能。研究生将作为跨学科研究团队的成员深入了解这两个领域。他们将学会欣赏和实施每个学科的解决问题的方法的优势。学生们还将体验到活跃网站的计算和实验建模之间的密切联系。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强有力的理由。

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.