Computational Study of Brønsted Acidity of Faujasite. Effect of the Al Content on the Infrared OH Stretching Frequencies

In this study we use the experimental data from a commercial sample of faujasite (Si/Al ∼ 2.5) and we compare the infrared OH stretching bands with periodic density functional calculations of a faujasite unit cell of composition Si₃₄Al₁₄O₉₆H₁₄ (Si/Al = 2.4). Assuming that O4H is not populated, the computational results show the following bands for O1H, O2H, and O3H: 3588−3689, 3555−3588, and 3517−3560 cm⁻¹, respectively, which are in good agreement with the experimental values of 3648, 3571, and 3526 cm⁻¹, respectively. We have studied how the OxH frequencies (x = 1−4) change due to the presence of two neighbor Al centers (Si₄₆Al₂O₉₆H₂, Si/Al = 23), with respect to the isolated case (Si₄₇Al₁O₉₆H₁, Si/Al = 47). The experimental infrared bands cannot be explained by the high silica models (Si/Al = 47, 23) and can only be explained when taking into account (i) the experimental Al content, by using a Si₃₄Al₁₄O₉₆H₁₄ model, (ii) the experimental OH population, and (iii) the experimental Si(nAl) (n = 0−4) population.