On the Heterogeneity of Iron-Oxo Species in Zeolites for the Oxidation of Methane to Methanol by Nitrous Oxide: A Theoretical Perspective

The conversion of methane to methanol (MTM) represents a pivotal objective in the C1 chemical industry. Transition metal, such as iron, exchanged zeolites are one category of the most active catalysts for direct conversion of MTM. One important topic in understanding the mechanism of Fe-zeolite catalyzed MTM is how the heterogeneity of catalytic (Fe) sites influences the system stability and reactivity. Employing DFT calculations and machine learning method, we herein studied the stability-reactivity relationship of a MTM catalytic cycle with N2O as the oxidant over Fe-exchanged zeolites. The Fe heterogeneity was introduced by using CHA and FER zeolites and looking at a number of related Fe species (FeII, FeO, and FeOH). A strong correlation was observed between the stability of such Fe species, which is primarily determined by the formation energy of FeII, and such a stability trend remains consistent throughout the MTM catalytic cycle. The reactivity analysis then demonstrated that less stable Fe species can exhibit higher reactivity when situated in specific sites. Further machine learning analysis validated the significant relevance of activation barriers with reaction energies in N2O decomposition step that is not sufficiently captured by the traditional one-dimensional Brønsted–Evans–Polanyi (BEP) relationship.