Structures, energetics and vibrational frequencies of zeolitic catalysts: a comparison between density functional and post-Hartree-Fock approaches

Abstract

Structures, energetics and vibrational frequencies of zeolitic cluster models have been investigated with an ab initio method at the correlated level involving the second-order Møller-Plesset (MP2) and with the density functional theory (DFT) method including local and non-local spin density functions. Full optimization of structures has been carried 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 and Vosko-Wilk-Nusair) and MP2 results agrees with 1 pm for SiO and OH, while the weaker AlO bond length agrees with 2-4 pm depending on the exchange-correlation potential employed. The Si O(H)Al, SiOH, and SiOSi bond angles are in good agreement with MP2. The flexible SiOSi angle is well represented by BLYP but not by VWN, the latter yielding angles 12 ° and 18 ° smaller than the MP2 and coupled pair functional results, respectively. This suggests that BLYP should be used. The acidity of zeolites at the BLYP/6-311G*level is evaluated by proton affinity; it is virtually identical to that from MP2/DZP and is also close to the result for G l theory within the desired 10 kJ mol-1accuracy. 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. © 1995.