DFT Study of the Catalytic Activity of Cu-MFI Zeolite and the Role of the Substitution Sites of Aluminum Atoms

Methane (CH₄) is readily available in gas reserves across the globe. However, its conversion into commodity chemicals, such as methanol or acetic acid is a challenge. Recent works have demonstrated that MFI zeolite modified with Cu²⁺ activates CH₄ at low to moderate temperatures.¹ It is well known that the ionic exchange of Cu²⁺ in zeolites requires aluminum atoms in the framework to compensate the charge difference. In this study, we employ quantum mechanical calculations to analyze how the positions of the aluminum atoms in the framework impacts the catalytic activity of the system.

It has been reported that only three substituted rings have catalytic and adsorption activity in Cu-MFI. These have been identified as a, b, g, and f rings, using the nomenclature proposed by Groothaert et al.^2,3 In this work, we systematically analyzed all the possible substitution sites of the aluminum atoms in the a, b, g and f rings of the MFI framework. Then the adsorption of methane and several probe molecules, such as NO and CO, on Cu-MFI were analyzed to determine the mechanism of adsorption, the position of adsorbates, as well as to quantify the effect of the position of the Al atoms on the energies of adsorption. The calculations were performed with wB97XD exchange-correlation functional in order to take into consideration the long range interactions of the zeolite. The basis sets used were 6-31+g** for O, Si, Al, and H atoms and SDD for Cu atoms.

Our results demonstrate that the stability of the rings’ structure might be related to the distance between the Cu²⁺ and the closest Al atom in the framework. It was observed that as that distance increases, the structure is less stable. Ongoing work includes the analysis of adsorption of molecules on the zeolite and the vibrational frequencies of the adsorbates.