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

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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.