STRUCTURES, ENERGETICS AND VIBRATIONAL FREQUENCIES OF ZEOLITIC CATALYSTS - A COMPARISON BETWEEN DENSITY-FUNCTIONAL AND POST-HARTREE-FOCK APPROACHES

Structures, energetics and vibrational frequencies of zeolitic cluster models have been investigated with an ab initio method at the correla ted level involving the second-order Moller-Plesset (MP2) and with the density functional theory (DFT) method including local and non-local spin density functions. Full optimization of structures has been carri ed out with 3-21G, 6-31G, 6-311G*, DZVP basis sets. The comparison of geometries of zeolite clusters between the DFT (Becke-Lee-Yang-Parr a nd Vosko-Wilk-Nusair) and MP2 results agrees with 1 pm for Si-O and O- H, while the weaker Al-O bond length agrees with 2-4pm depending on th e exchange-correlation potential employed. The SiO(H)-Al, Si-O-H, and Si-O-Si bond angles are in good agreement with MP2. The flexible Si-O- Si angle is well represented by BLYP but not by VWN, the latter yieldi ng angles 12 degrees and 18 degrees smaller than the MP2 and coupled p air functional results, respectively. This suggests that BLYP should b e used. The acidity of zeolites at the BLYP/G-311G level is evaluated by proton affinity; it is virtually identical to that from MP2/DZP an d is also close to the result for G1 theory within the desired 10 kJ m ol(-1) accuracy. The DFT OH stretching frequencies of zeolite clusters are predicted to within 4% of the experimental value. The DFT methods are computationally efficient and appear to provide results that are generally of comparable quality to MP2.