A DFT study of the models of the bronsted acid sites of zeolite catalysts

A systematic study of the different cluster models of the Bronsted site has been carried out in order to predict how the local structure influences th e geometric parameters of the SiO(H)Al site and the harmonic frequencies of the characteristics vibrational modes of the bridging surface hydroxyl gro up. The performance of the B3LYP variant of the density functional theory ( DFT) method is examined and compared with both SCF HF and MP2 approximation s. In order to find the best molecular model for the Bronsted site, two ser ies of derivatives based on the prototype model cores SiO(H)Al and HAlO4 we re considered. In the HAlO4 series, H3SiO(H)Al(OH)(2)OSiH3 and H3SiO(H)Al(O SiH3)(3) were investigated. Among the models in the SiO(H)Al series were H3 SiO(H)AlH3, H3SiO(H)Al(OH)(3), H-2(OH)SiO(H)Al(OH)(3), (OH)(3)SiO(H)Al(OH)( 3), and (H3SiO)(3)SiO(H)Al(OSiH3)(3). The research reveals that the acidic OH bond length and its stretching frequency are greatly influenced by the H bond formed between the acidic proton and its next-nearest neighbor oxygen on the aluminum atom. The geometry and the frequencies for the larger mode ls predicted at the B3LYP/D95(d,p) level are in good agreement with the exp eriments, suggesting that the B3LYP/D95(d,p) theory is reliable for a study of the Bronsted site. This study also suggests that the OH terminated mode ls are not suitable as models for the SiO(H)Al site due to the strong dipol e moment of the OH group, which strongly affects the acidic H.