Predicting Absolute and Site Specific Acidities for Zeolite Catalysts by a Combined Quantum Mechanics/Interatomic Potential Function Approach

The approach used describes the Brønsted site by the Hartree−Fock method and a T(O)DZP basis set, while the periodic zeolite framework and the interaction between the active site and the framework are described by a shell model potential parametrized on the same type of ab initio data for cluster mo...

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Published in:The journal of physical chemistry. B 1997-11, Vol.101 (48), p.10035-10050
Main Authors: Eichler, Uwe, Brändle, Martin, Sauer, Joachim
Format: Article
Language:English
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Summary:The approach used describes the Brønsted site by the Hartree−Fock method and a T(O)DZP basis set, while the periodic zeolite framework and the interaction between the active site and the framework are described by a shell model potential parametrized on the same type of ab initio data for cluster models. It is capable of reproducing the effect of the crystallographic position and of different framework structures on the properties and reactivity of zeolitic Brønsted sites. For H-faujasite (Si/Al = 47) protonation of all four crystallographically different oxygen positons is considered. In agreement with experiment protonation on O(1) and O(3) is preferred. For the orthorhombic form of H-ZSM-5 (Si/Al = 95) protonation of the Al(7)−O(17)H−Si(4) site proves more stable than protonation at the Al(12)−O(24)−Si(12) site located at the channel intersection. In agreement with experiments, the OH vibrational frequency is predicted to decrease according to O(1)H-FAU > H-ZSM-5 > O(3)H-FAU, and the 1H NMR chemical shift to increase in the same sequence. The method also yields absolute and site specific acidity values. The deprotonation energya measure of acidityobtained by this combined scheme is decomposed into the quantum mechanical contribution for the cluster itself and the long-range contribution. The former reflects the structural constraints imposed on the active site by the framework and the latter the influence of the crystal potential. With increasing cluster size the long-range correction decreases slowly, while the total energy stays remarkably stable within a few kJ/mol. For H-ZSM-5 and H-faujasite heats of deprotonation (proton affinities) of 1205 and 1169 kJ/mol, respectively, are calculated. Hence, for the same large Si/Al ratio Brønsted sites in the faujasite lattice are predicted to be more acidic than in the ZSM-5 lattice. This difference is due to differences of both the local structures (including the structure relaxation) and the crystal potentials. No correlation is found between T−O−T bond angles or 1H NMR chemical shifts and heats of deprotonation.
ISSN:1520-6106
1520-5207
DOI:10.1021/jp971779a