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.