DFT study on the accommodation and role of la species in ZSM-5 zeolite

Theoretical investigations on the location and structure of lanthanum species in lanthanum-modified ZSM-5 zeolites were carried out. The mechanism of change in the acid properties of the ZSM-5 zeolite after addition of lanthanum cations, and the influence of lanthanum species on the hydrothermal stability of the ZSM-5 zeolite were also studied. Based on optimized cluster models and thermodynamic analyses it is demonstrated that the introduced lanthanum cations lie in the symmetrical six-membered oxygen rings with Al situated at T11 sites in the straight channels and exist as La(OH)2+ up to a typical hydrocarbon cracking temperature of 950 K. This structure is further confirmed by the existence of weak intramolecular hydrogen bonds between the hydroxyls in La(OH)2+ species and the lattice oxygens. Similarly, we found that the introduced lanthanum cations transformed the strong Brønsted acid sites (Si-OH-Al) into the weak Brønsted acid sites. Accordingly, the weak Brønsted acid sites have O-H stretching frequencies of 3742 and 3762 cm^-1 and are in the form Si-OLa(OH)₂-Al. Our results also suggest that the hydrothermal stability of the ZSM-5 zeolite is improved by introducing lanthanum cations due to the strengthened Al-O bond, increased steric hindrances, weakened acidity and weak intramolecular hydrogen bonds. Finally, we extended our investigation by considering the influence of the lanthanum species on the deactivation rate of the ZSM-5 zeolite.