A DFT–ONIOM study on the effect of extra-framework aluminum on USY zeolite acidity Original Research Article

The effect of Al(OH)2+ EFAL (extra-framework aluminum) species in the acid strength of USY zeolite was studied by the ONIOM scheme, at PBE1PBE/6-31G(d,p) level. The model consists of a hexagonal prism and the sodalite cage (T30), representing a real part of the zeolite Y system that allows calculation of different acid sites (O1 and O3). The acid strength was calculated by computing the deprotonating energy and comparing the results with the data obtained for an isolated acid site. The calculations indicated that there occurs an increase in acid strength, depending on the relative position between the EFAL and the acid site. However, the role of Al(OH)2+ EFAL is not to increase the energy of the acid form, through Brϕnsted/Lewis synergism, but to stabilize the conjugated base, formed upon deprotonation, by hydrogen bonding and nucleophilic interaction with the framework oxygen atom.