Cation location and migration in lanthanum-exchanged zeolite NaY studied by X-ray powder diffraction and MAS NMR spectroscopy

Abstract

The population of cation sites in a lanthanum-exchanged zeolite NaY (LaNaY-73) in the course of calcination and at conditions similar to catalysis (T= 623 K, continuous flow of m-xylene and nitrogen over calcined zeolite LaNaY) has been studied by ex situ and in situ X-ray powder diffraction. Ex situ magic-angle spinning (MAS) NMR spectroscopy at room temperature complemented the diffraction results. During calcination of LaNaY, a cation migration process (lanthanum from supercage to sodalite cage, sodium from sodalite cage to supercage), after pretreatment at 363 K, was observed. In the calcined sample all lanthanum cations are located at position SI′ in the sodalite cage while 10 of the 16 sodium cations per unit cell occupy position SII in the supercage. A sodium migration from the sodalite cages to position SII in the supercages was observed by ex situ temperature-dependent experiments on samples calcined between 298 and 623 K. X-Ray powder diffraction, as well as multinuclear solid-state NMR spectroscopy, has shown that adsorption of m-xylene induces sodium migration from the sodalite cages to position SII in the supercage, whereas the positions of lanthanum are not influenced. Furthermore, m-xylene could be located by in situ X-ray powder diffraction at conditions similar to catalysis at the preferred adsorption site in front of the six-ring window of the supercage, as determined by ex situ diffraction experiments at low temperature. The positions and populations of the cations derived at 623 K agree with the results obtained from ex situ diffraction experiments carried out at this temperature.