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

摘要

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 Angstrom to both the O(24) and O(20) atoms and an interatomic distance of 2.86 Angstrom to lattice Al. The molybdenum has a tetrahedral geometry and a formal charge of +5. This kind of active Mo center possesses a typical C-2v 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. (C) 2001 American Institute of Physics. [References: 34]