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 950K. 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 3762cm−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.