Study with density functional theory method on methane dehydro-aromatization over Mo/HZSM-5 catalysts I: Optimization of active Mo species bonded to ZSM-5 zeolite

A study of the structure of active Mo species on HZSM-5 zeolite was carried out by applying the density functional theory (DFT). The interaction of the MoO2+ unit with the zeolite framework had been verified by an optimization procedure based on the DFT. The result of calculation reveals a dicoordinated bridging geometry in which the molybdenum coordinated simultaneously to both of the two lattice oxygen atoms, leading to a quadrangular geometry with an O(24)–Al(12)–O(20) bridge structure. The Mo ions having O(24)–Al(12)–O(20) bridges exist in identical planes, with equal bond distances of 1.72 Å to both the O(24) and O(20) atoms and an interatomic distance of 2.86 Å to lattice Al. The molybdenum has a tetrahedral geometry and a formal charge of +5. This kind of active Mo center possesses a typical C2v symmetry. Approximation vibrational analysis indicates that the vibrations of Mo–O (framework oxygen) bonds occur at the frequencies between 580 and 928 cm−1, while the vibrations of the Mo–O (extra-framework oxygen) bonds appear between 956 and 975 cm−1. Population analysis confirmed that the bonds between extra-framework oxygen atoms and the central Mo atom are of a typical covalent double bond in nature, whereas the coupling between the lattice oxygen atoms and the central Mo atom shows a sizable ionic contribution. Theoretical predictions from the present model of the Mo/HZSM-5 active centers agree well with our experimental results from magic angle spinning nuclear magnetic resonance (MAS NMR), electron spin resonance (ESR), and Fourier-transform infrared (FTIR) experiments.