Periodic DFT investigation of the effect of aluminium content on the properties of the acid zeolite H-FER
Abstract
Periodic DFT calculations were performed on H-FER models having Si/Al ratios of 71 : 1, 35 : 1 and 8 : 1, in order to investigate the effect of aluminium content on the properties of the zeolite Bronsted acid sites. Relative stability of these sites was found to be dependent on Si/Al ratio, which is the main factor dictating the relative concentration of Bronsted acid sites having different types of local configuration, to the point that some types of acid site are formed only when the aluminium content of the zeolite is relatively high.
The number of AlO4 tetrahedra sharing an oxygen with the SiO4 tetrahedron involved in the Bronsted acid site determines the Si-O(H)-Al angle, O-H stretching frequency and deprotonation energy (and hence acid strength). For Bronsted acid protons not involved in intra-zeolite H-bonding, a correlation was found between Si-O(H)-Al angle and O-H stretching frequency.