Selective synthesis of transition metal ion nanoclusters embedded in zeolites for methane oxidation at low temperatures

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The crystalline nature of zeolite micropores provide a wide variety of sites able to host transition metals as cations, metal oxides or metal clusters. The stabilization of Cu and Fe cations or metal-oxo nanoclusters in zeolite frameworks has led to successful catalysts for the selective oxidation of alkanes at low temperatures, mimicking the metal center in enzymes. In particular, the challenging selective oxidation of methane to methanol with O2 can be successfully performed at 150-200 °C on Cu-zeolites. However, these materials have still limitations for practical applications, such as the necessity of an extraction step to release the strongly adsorbed methanol. For a knowledge-based design of oxidation catalysts, it is necessary to understand how the combination of nano-architectures of oxygen and metal cations in different oxidation states lead to a particular catalytic function. Cu has been found to be active in several zeolite topologies, but information on the structure of the active species is still lacking in most of the cases because of the large concentration of inactive Cu in the materials. The first reported Cu-oxo active site in zeolite-based methane oxidation catalysts was a [Cu2O]2+ cluster embedded in ZSM-5. We have recently identified the active site in CuMOR as a tricopper oxo complex, [Cu3O3]2+, and found that such active site can also be formed in CuZSM-5. We hypothesize that the radical character of O in the Cu-O-Cu bridges, a common feature found in both active Cu-oxo clusters, is crucial for the activation of methane.With this background, the main objective of this project is to generate materials consisting in metal ions embedded in zeolites with well-defined active sites for selective oxidation of light alkanes. This requires controlling those synthesis parameters that affect the final structure of active metal-oxo clusters. Thus, an important part of this project is directed to gain insight into the elementary steps involved in the formation of active metal-oxo clusters in zeolites (namely, the chemical processes in metal ion exchange, dehydration, thermal treatments and generation of active oxygen species) as well as into the mechanism of activation of the substrate. Furthermore, our experience with Cu-MOR and Cu-MFI have shown that identical Cu-oxo clusters exhibit different catalytic activity if stabilized in different frameworks. Thus, the role of the local environment and potential confinement effects in the ability to activate methane will be studied by exploring zeolites with different micropore size and connectivity. An additional objective of the project is to find strategies to release the strongly adsorbed methanol in order to close the catalytic cycle for methane oxidation on this type of catalysts. Finally, we will explore the activity in methane oxidation of Ni and Co cations in zeolites, so that the specificity of the metal cation for the oxidative C-H activation can be revealed.