PREDICTING ABSOLUTE AND SITE-SPECIFIC ACIDITIES FOR ZEOLITE CATALYSTSBY A COMBINED QUANTUM-MECHANICS INTERATOMIC POTENTIAL FUNCTION-APPROACH

The approach used describes the Bronsted site by the Hartree-Fock meth od and a T(O)DZP basis set, while the periodic zeolite framework and t he 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 t he crystallographic position and of different framework structures on the properties and reactivity of zeolitic Bronsted sites, For H-faujas ite (Si/Al = 47) protonation of all four crystallographically differen t oxygen positons is considered. In agreement with experiment protonat ion 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 st able than protonation at the Al(12)-O(24)-Si(12) site located at the c hannel 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 H-1 NMR chemical shift to increase in the same seq uence. The method also yields absolute and site specific acidity value s. The deprotonation energy-a measure of acidity-obtained by this comb ined scheme is decomposed into the quantum mechanical contribution for the cluster itself and the long-range contribution. The former reflec ts the structural constraints imposed on the active site by the framew ork and the latter the influence of the crystal potential. With increa sing cluster size the long-range correction decreases slowly, while th e total energy stays remarkably stable within a few kJ/mol. For H-ZSM- 5 and H-faujasite heats of deprotonation (proton affinities) of 1205 a nd 1169 kJ/mol, respectively, are calculated. Hence, for the same larg e Si/Al ratio Bronsted sites in the faujasite lattice are predicted to be more acidic than in the ZSM-5 lattice. This difference is due to d ifferences of both the local structures (including the structure relax ation) and the crystal potentials. No correlation is found between T-O -T bond angles or H-1 NMR chemical shifts and heats of deprotonation.