The urge to discover alternatives to substitute fossil fuels arouse the interest in the methane use as feedstock to several reactions due its abundance, versatility, and low cost. However, the reactions with methane usually require high pressure and temperatures due its chemical and physical properties as low reactivity, high bond energy, high simmetry, low polarizability, etc. Therefore, its use as feedstock is still a great challenge. Methanol is one of the compounds that can be produced through the methane oxidation to methanol (MTM) which is a very desirable valorization of methane. In this context, the exchanged zeolites have gained great prominence in the last few years as a good candidate to catalyze the MTM reaction. Combined with the features of zeolites and inspired in the active site of methane monooxygenase (MMO) enzyme (which oxidize methane at room temperature with high selectivity), the metal (particularly Cu) exchanged zeolites was successfully applied to catalyze the MTM reaction. In order to improve the exchanged zeolites reactivity on MTM reaction is essential to know its mechanism, the roles of the active site and the zeolite framework in the MTM mechanism. In this way, theoretical calculations have been demonstrating great importance to describe the reaction mechanism, reactivity insights and its applicability on rational design of catalytic systems. Thus, in this work we applied a δ-cluster method to study the methane oxidation to methanol at [Cu2(μ-O)]2+-SSZ-13 zeolite.[1-5].
The zeolite cluster was cut-off from a CHA crystallographic information file using a Python automated method with size 3Å and loaded with two Al atoms, two Cu atoms and a μ-O bonded between Cu atoms and besides that the atoms in the zeolite cluster edge have been their valences completed with H atoms (Cu2Al2Si14O36H27). All geometry optimization calculations were performed at Gaussian09 package with ωB97XD functional and def2-SVP basis set. In this study the [Cu2(μ-O)]2+-SSZ-13 zeolite was calculated with neutral charge and multiplicity triplet. The analysis of the cluster structure showed that the structure integrity was maintained. The calculations presented an activation enthalpy energy for methane CH bond cleavage of 11.50 kcal/mol that was considered suitable compared with theoretical results present by Mahyuddin and co-workers.[1] However, this activation energy is still underestimated compared with experimental energy presented by Dinh and co-workers.[2] Our results also demonstrated a great stabilization on radical intermediate which have being further investigated. 

[1] M. H. Mahyuddin et. al., Acc. Chem. Res., 2018, 51, 2382.
[2] K. T. Dinh et. al., J. Am. Chem. Soc. 2019, 141, 11641.
[3] B. Ipek et. al., J. Phys. Chem. C., 2018, 122, 1, 540.
[4] E. Borfecchia et. al., Chem. Sci., 2015, 6, 548.
[5] A. N. Migues, et. al., ACS Catal. 2015, 5, 2859.

DFT study of methane oxidation by [Cu2(μ-O)]2+ exchanged SSZ-13 zeolite

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