Impact of the extra-framework aluminum species on the properties of Brønsted acid sites in HFAU zeolites by using thiophene probe molecule: A periodic DFT study

The effects of extra-aluminum species (Al(OH)²⁺and Al³⁺) on the properties of Brønsted acid sites (BAS) were studied using thiophene as the model probe and HFAU as model zeolite by using a periodic DFT study. It is found that the Lewis acid strength of Al³⁺ species is higher than that of Al(OH)²⁺ species, and the hydroxyl species Al(OH)²⁺ in both directions have similar charge properties, that is, similar acid strength. The results of electronic properties combined with deprotonation energies (DPE) show that the BAS in 1-HFAU/Al(OH)²⁺, 2-HFAU/Al(OH)²⁺and HFAU/Al³⁺ zeolites have similar acid strength (1045 ± 11 kJ/mol). It can be concluded that the difference of hydroxyl direction and Lewis acid strength of Al(OH)²⁺and Al³⁺ species hardly affect the acid strength of BAS. By simulating the adsorption of thiophene, the adsorption energies and the changes of electronic properties and geometric structure during the adsorption process were obtained. The results show that thiophene in HFAU/Al³⁺ zeolite is easy to be adsorbed on Al³⁺ site, which is due to the strong Lewis acid strength of Al³⁺ adsorption site. The thiophene in HFAU/Al(OH)²⁺ zeolite is preferentially adsorbed on the BAS rather than the Al(OH)²⁺ adsorption site. In addition, Al(OH)²⁺ species can exert a weak interaction (dispersion interaction) on thiophene adsorbed on BAS to promote the adsorption of thiophene. The adsorption modes depend on the structure of Al(OH)²⁺ species and the direction of hydroxyl groups. This work explored the intrinsic properties of host-HFAU zeolite (containing extra-framework aluminum species) and guest-thiophene molecule from an electronic-level, and revealed the synergistic mechanism between acid sites.