Mechanism of Rare Earth Cations on the Stability and Acidity of Y Zeolites

Abstract

The influences of rare earth (RE) cations on the stability and acidity of Y zeolites were studied by X-ray powder diffraction (XRD), solid-state magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR), temperature-programmed desorption (TPD) of ammonia, and Fourier transform infrared of pyridine (Py-FTIR). The results showed that the stability of Y zeolites was enhanced markedly and the medium acid amount increased, but the strong acid amount of Y zeolites decreased with the introduction of RE cations. Combined with the density functional theory (DFT) calculations, a comprehensive model was proposed to describe the mechanism of RE cations on the stability and acidity of Y zeolites. RE cations located at the sodalite β cage I' sites of Y zeolites bonded strongly with O atoms, which strengthened the interaction between framework Al and its neighbouring O atoms and then enhanced evidently the stability of Y zeolites. It was the enhanced stability of REHY zeolites that restrained the release of framework Al and the formation of extra-framework Al, hence led to weaker Bronsted acid strength of REHY than that of USY. Meanwhile, RE cations located at the sodalite β cage I' sites of Y zeolites made the negative charge of O1 reduce and the bond length of Al―O1 shorten, which resulted in stronger Bronsted acid strength of REHY than that of HY. Moreover, the Bronsted acid amount of REHY was more than that of USY, but less than that of HY.