Heterogeneity of Brønsted Acidic Sites in Faujasite Type Zeolites due to Aluminum Content and Framework Structure

The effect of Al for Si substitutions in the tetrahedral sites of the faujasite framework with low and high aluminium content on the properties of Brønsted sites is investigated by computational techniques. A combined quantum mechanics−interatomic potential functions approach (QM-Pot.) is applied which uses periodic boundary conditions and treats the whole zeolite structure. Both the Hartree−Fock and the density functional (B3LYP) methods are employed. Energies of deprotonation, O−H stretching harmonic frequencies, and 1H NMR chemical shifts are calculated. In the first approximation, the acidity of Brønsted sites measured by the deprotonation energy is determined by the number of nearest neighbor Al atoms of the Si atom in the Al−O(H)−Si bridge, while the O−H stretching frequency and 1H chemical shift depend on their crystallographic position. Beyond this, the 1H NMR chemical shifts show a strong dependence on the overall lattice composition. There is no correlation between 1H NMR chemical shifts and deprotonation energies; however, a linear relation between 1H NMR chemical shifts and OH vibrational frequencies is supported.