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 the geometric parameters of the SiO(H)Al site and the harmonic frequencies of the characteristics vibrational modes of the bridging surface hydroxyl group. The performance of the B3LYP variant of the density functional theory (DFT) method is examined and compared with both SCF HF and MP2 approximations. In order to find the best molecular model for the Bronsted site, two series of derivatives based on the prototype model cores SiO(H)Al and HAlO4 were considered. In the HAlO4 series, H3SiO(H)Al(OH)2OSiH3 and H3SiO(H)Al(OSiH3)3 were investigated. Among the models in the SiO(H)Al series were H3SiO(H)AlH3, H3SiO(H)Al(OH)3, H2(OH)SiO(H)Al(OH)3, (OH)3SiO(H)Al(OH)3, and (H3SiO)3SiO(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 models predicted at the B3LYP/D95(d,p) level are in good agreement with the experiments, 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 models are not suitable as models for the SiO(H)Al site due to the strong dipole moment of the OH group, which strongly affects the acidic H.