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 SiO and OH, while the weaker AlO bond length agrees with 2–4 pm depending on the exchange-correlation potential employed. The Si O(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 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 −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.