DFT Study on the Effect of Aluminum Position in Zn-Exchanged MFI on Methane Activation

The position of aluminum atoms in ion-exchanged zeolites is known to affect the reactivity of active sites. In this work, we used density functional theory (DFT) calculations to systematically quantify the effect of the Al-atom position within the α-ring of Zn-exchanged MFI (Zn-MFI) on the activation of methane. Our DFT results indicate that the most stable configuration for the Zn-exchanged cluster of the α-ring is obtained when the Al atoms are located at the T11-T2 crystallographic sites. For each Al-atom configuration, we analyzed the reaction pathways for methane activation. Our results suggest that the activation of methane yields the formation of a Brønsted acid site, which can be formed at an oxygen atom within the α-ring or at an oxygen atom that lies outside the α-ring, and that the lowest reaction energy for methane activation is obtained when the Brønsted acid site is formed at the oxygen atom in which the highest occupied molecular orbital of the isolated cluster is located. Furthermore, our results indicate that the partial atomic charge of the Zn atom within the α-ring of MFI can be correlated with the transition-state energy of methane activation, which ranges from 87 to 131 kJ/mol depending on the location of Al atoms. The fundamental studies conducted in this work contribute to the elucidation of essential parameters and correlations, based on electrostatic and electron density, for the activation of methane on Zn-MFI zeolites.