Understanding the Reactivity and Tunability of MOF-Supported Cluster Catalysts and MOF Nodes towards Alkane Partial Oxidation Using Density Functional TheoryPublic
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Natural gas is likely to become one of the main sources of carbon-based chemicals in the next century due to rapidly increasing natural gas production levels. This has created new incentives to find materials that are active and selective towards alkane partial oxidation reactions that are relevant for natural gas upgrading. Among the reactions of interest to exploit natural gas resources are the direct conversion of methane to methanol as a way to bypass the current energy-intensive two-step process that goes through a syngas intermediate, and the oxidative dehydrogenation of propane to propylene to meet current increasing demand for propylene. Metal-organic frameworks (MOFs) are promising heterogeneous catalysts for alkane partial oxidation reactions. They exhibit high porosity that allows the rapid diffusion of reactants and products, and their well-dispersed and well-defined metal centers can act as catalytic active sites or serve as a support for the growth of single-atom or cluster catalysts. The well-known structure and composition of MOFs allow us to model these materials using first-principles computational methods and creates an opportunity to understand their catalytic behavior at the molecular level. The modular structure of MOFs further allows us to identify structure-function relationships that relate their chemical properties to their performance and ultimately to explore ways in which their catalytic activity can be tuned and improved. In this work we investigate the activity and electronic structure of MOF-supported cluster catalysts and open-metal sites in MOF nodes using first-principles methods. Our objective is to understand their performance towards light alkane partial oxidation and identify relevant properties that can be tuned to improve their catalytic activity towards these reactions to ultimately guide experimental design of promising materials for alkane partial oxidation. Using density functional theory (DFT), we studied the catalytic activity of diiron oxide clusters that can be grown via atomic layer deposition (ALD) on a MOF support towards the partial oxidation of methane to methanol. We studied the activity of the clusters and examined the differences in activity of its available active sites. Using population analysis and molecular orbital analysis, we studied the change in the electronic structure of the clusters as the reaction progresses. This further allowed us to identify important properties of the catalyst that can be tuned for future screening of MOF-supported, ALD-grown metal oxide clusters for partial oxidation of methane to methanol. In a second study we investigated the catalytic activity of the MOF PCN-250 towards alkane oxidative dehydrogenation. The Fe2M node of PCN-250 consists of two Fe sites in a +3 oxidation state and an M site in a +2 oxidation state. To study the tunability of PCN-250 towards alkane oxidative dehydrogenation and develop structure-function relationships, we varied the composition of the Fe2M node by replacing the M site with metals from Ti to Zn and studied the reaction on both the Fe and M sites of the node. We identified potential descriptors to predict the catalytic performance of PCN-250 and through collaboration experimentally validated our computationally predicted catalytic trends. This work demonstrated the potential of using first-principles methods and metal-organic frameworks to design catalysts for difficult reactions such as oxidative dehydrogenation. To expand on our previous work on PCN-250, we explored further the tunability of the trimetallic (M1)2(M2) node, which is found in MOFs such as PCN-250, MIL-100, and MIL-101, by replacing both M1 and M2 sites with metals from V to Ni. We used DFT to model the partial oxidation of methane to methanol on both sites of the node and assessed the structure-function relationships identified in our previous work for this larger space of node compositions. This allowed us to study in detail the effect of the metals that act as spectators on the most important reaction barriers and on the structure-function relationships when the reaction is observed on a particular metal site of the trimetallic node. Through this screening, we also identified top-performing node compositions with barriers lower than those obtained for the experimentally tested MOFs with Fe2M nodes.
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