CATION LOCATION AND MIGRATION IN LANTHANUM-EXCHANGED ZEOLITE NAY STUDIED BY X-RAY-POWDER DIFFRACTION AND MAS NMR-SPECTROSCOPY

The population of cation sites in a lanthanum-exchanged zeolite NaY (L aNaY-73) in the course of calcination and at conditions similar to cat alysis (T = 623 K, continuous flow of m-xylene and nitrogen over calci ned zeolite LaNaY) has been studied by ex situ and in situ X-ray powde r diffraction. Ex situ magic-angle spinning (MAS) NMR spectroscopy at room temperature complemented the diffraction results. During calcinat ion of LaNaY, a cation migration process (lanthanum from supercage to sodalite cage, sodium from sodalite cage to supercage), after pretreat ment at 363 K, was observed. In the calcined sample all lanthanum cati ons are located at position Sl' in the sodalite cage while 10 of the 1 6 sodium cations per unit cell occupy position SII in the supercage. A sodium migration from the sodalite cages to position SII in the super cages was observed by ex situ temperature-dependent experiments on sam ples calcined between 298 and 623 K. X-Ray powder diffraction, as well as multinuclear solid-state NMR spectroscopy, has shown that adsorpti on of m-xylene induces sodium migration from the sodalite cages to pos ition Sll in the supercage, whereas the positions of lanthanum are not influenced. Furthermore, m-xylene could be located by in situ X-ray p owder diffraction at conditions similar to catalysis at the preferred adsorption site in front of the six-ring window of the supercage, as d etermined by ex situ diffraction experiments at low temperature. The p ositions and populations of the cations derived at 623 K agree with th e results obtained from ex situ diffraction experiments carried out at this temperature.