New theoretical insight on the acid sites distribution, their local structures and acid strength of the SAPO-11 molecular sieve

ONIOM(DFT:PM3) calculations were carried out to investigate and characterize possible acid sites of SAPO-11 molecular sieve. Two functionals were employed: B3LYP and ωB97X-D. This last functional includes dispersion effects that are absent in the former. Benzene, pyridine, and ammonia interaction energies as well as the OH stretching frequencies of the POH, SiOH, and bridged Si(OH)Al groups were used to characterize the acid sites. This work shows that the adsorption of benzene on the surface is as strong as the adsorptions inside main channel of SAPO-11. Pyridine adsorption on the surface is weaker than the one corresponding to the main channel. NH3 molecule interacts strongly with all OH groups or acid sites present in SAPO-11. Moreover, the results reveal that it is possible to adsorb two NH3 molecules at only one Bronsted site. The adsorption of the second NH3 molecule is energetically favorable mainly due to the hydrogen bond formation between the NH3 molecules. In general the interaction energy increases with the type of functional used, according to the trend ωB97X-D > B3LYP. The results show that ONIOM methodology seems to be suited to investigate the acid sites in SAPO-11.