Cation location and migration in lanthanum-exchanged NaY zeolite

Abstract

LaNH4Y and LaY zeolites were prepared using a double-exchange double-calcination method by La exchange of NaY zeolite. The distribution of the La cations was determined by powder X-ray diffraction with Rietveld refinement. The results indicate that the La cations are initially located in supercages, and then their hydration shells are stripped off and they migrate to small sodalite structures located at SI′ during heating and dehydration. The changes in the T–O–T angles show significant distortion of the flexible framework and instability in LaNH4Y obtained using a one-exchange one-calcination process. For La cations located at SI′ and coordinated with O3, the T–O3 bond distance increased, which indicates that the rare-earth cations not only restrain framework dealumination, suppressing condensation of the unit cell, but also have an effect on the T–O3 bond distance, increasing the unit cell volume. The role of the rare-earth species is to ensure the hydrothermal stability of the zeolite in order to control the acid site density and catalytic activity. The effects of La cations and NH4+ cations on the zeolite acidity were studied using infrared spectroscopy and NH3 temperature-programmed desorption, and the mechanism of rare-earth stabilization of the Y zeolite is described.