HETEROGENEITY OF BRONSTED ACIDIC SITES IN FAUJASITE-TYPE ZEOLITES DUETO 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 propert ies of Bronsted sites is investigated by computational techniques. A c ombined quantum mechanics-interatomic potential functions approach (QM -Pot.) is applied which uses periodic boundary conditions and treats t he whole zeolite structure. Both the Hartree-Fock and the density func tional (B3LYP) methods are employed. Energies of deprotonation, O-H st retching harmonic frequencies, and H-1 NMR chemical shifts are calcula ted. In the first approximation, the acidity of Bronsted sites measure d by the deprotonation energy is determined by the number of nearest n eighbor Al atoms of the Si atom in the Al-O(H)-Si bridge, while the O- H stretching frequency and H-1 chemical shift depend on their crystall ographic position. Beyond this, the H-1 NMR chemical shifts show a str ong dependence on the overall lattice composition, There is no correla tion between H-1 NMR chemical shifts and deprotonation energies; howev er, a linear relation between H-1 NMR chemical shifts and OH vibration al frequencies is supported.