Selective synthesis of transition metal ion nanoclusters embedded in zeolites for methane oxidation at low temperatures
Solid State and Surface Chemistry, Material Synthesis
Physical Chemistry of Solids and Surfaces, Material Characterisation
Final Report Abstract
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 on Cu-zeolites. However, these materials have still limitations for practical applications. 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 when exchanged in several zeolite topologies, but information on the structure of the active species is still under debate because of the large concentration of inactive Cu in the materials. With this background, the main objective of this project was to generate and characterize materials consisting in metal ions embedded in zeolites with well-defined active sites for selective oxidation of light alkanes. For the determination of the structure of Cu-oxo nanoclusters it is necessary to combine X-ray absorption spectroscopy (in synchrotron facilities) with theory calculations and complementary in situ IR, UV-vis spectroscopies and NMR. On the other hand, the quantification of active species via the titration of sites with CH4 on a well-defined ion exchanged material provides information on the number of Cu ions per site. Such structural studies require therefore a coordinated effort of researchers with complementary expertise, which has somewhat delayed the publication date for the outcome of these studies. The role of the local environment and potential confinement effects in the ability to activate methane was studied by exploring zeolites with different micropore size and connectivity. Frameworks capable to stabilize Cu-oxo clusters in certain constrained environments related to 8- and 10MR pores are presenting the best catalytic performance (MOR, MEL). Conversely, a high fraction of inactive Cu is present in zeolites with abundant Al pairs in 6 MR such as CHA and FER. Here, it is still necessary further studies for a better understanding on the role of Al distribution and pairing in order to achieve the full potential of Cu zeolites in this reaction. Our studies on a highly active CuMOR series of catalysts have shown that extraframework Al in the micropores can enhance activity and our current working hypothesis is the formation of active Cu-Al-oxo clusters. We have studied the chemical processes in metal ion exchange, dehydration, thermal treatments and generation of active oxygen species in CuMOR and shown that the mobility of Cu ions, in particular Cu+ formed as reaction intermediate, is key in the (re)generation of active sites. On the other hand, the activation of CH4 at higher chemical potentials allows for the utilization of a second O active species in the Cu3O3 clusters stabilized in CuMOR, doubling the activity. The latter study was published in Chem Eur J. as a Very Important Paper and was selected for the front cover of the issue of 18th June 2020 and a cover profile. We have explored the activity in methane oxidation of Fe, Ni and Co cations in zeolites, in order to understand the specificity of the metal cation for CH4 activation. We decided not to perform structural studies on these materials, due to their poor catalytic performance. The last task in the project was to find strategies to release the strongly adsorbed methanol and close the catalytic cycle for methane oxidation. Reaction tests of surface CH3 on Cu-zeolites with ethanol and propanol to form ethers were planned, but they could not be performed due to accumulated delays in the working program.
Publications
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Oxidative Functionalization of Methane on Heterogeneous Catalysts. Alkane Functionalization, Wiley, 2018, Editors A.J.L. Pombeiro and M.F.C. Guedes da Silva, p 141
M. Sanchez‐Sanchez, J.A. Lercher
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Formation of Active Cu-oxo Clusters for Methane Oxidation in Cu-Exchanged Mordenite. Journal of Physical Chemistry C (2019), 123 (14), 8759-8769
T. Ikuno, S. Grundner, A. Jentys, G. Li, E. A. Pidko, J. Fulton, M. Sanchez-Sanchez, J. A. Lercher
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Selective oxidation of methane to methanol on Cu-based catalysts supported on microporous materials. Doctoral Thesis, Technische Universität München 2019
Takaaki Ikuno
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Cu-oxo nanoclusters for direct oxidation of methane to methanol: formation, structure and catalytic performance. Catalysis Science and Technology 2020
L. Tao, I. Lee, M. Sanchez-Sanchez
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Efficient formation of Cu-oxo active sites for direct methane oxidation to methanol. Abstracts of Papers, 259th ACS National Meeting & Exposition, Philadelphia, PA, United States, March 22-26, 2020
M. Sanchez-Sanchez, I. Lee, L. Tao, T. Ikuno, J. A. Lercher
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Importance of methane chemical potential for its conversion to methanol on Cu-exchanged mordenite. Chemistry A European Journal (2020), 26, 7563-7567 (Front Cover)
J. Zheng, I. Lee, E. Khramenkova, M. Wang, B. Peng, O. Gutierrez, J. L. Fulton, D. Camaioni, R. Khare, A. Jentys, G. Haller, E. Pidko, M. Sanchez-Sanchez and J. A. Lercher
