Journal of Alloys and Compounds, 194 (1993) 167-172 167 JALCOM 559 An investigation by EXAFS of the thermal dehydration and rehydration of cerium- and erbium-exchanged Y-zeolite Frank J. Berry and Jos6 F. Marco School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT (UK) Andrew T. Steel Unilever Research, Port Sunlight Laboratory, Bebington, Wirral, Merseyside L63 3JW (UK) (Received November 3, 1992) Abstract The changes in the local environments of cerium and erbium in rare-earth exchanged Y-zeolites which are induced by dehydration and rehydration processes have been investigated by extended X-ray absorption fine structure (EXAFS). The cerium L3-edge EXAFS recorded from ca. 6 wt.% cerium-exchanged Y-zeolite show the hydrated cerium ion to be located in the large spaces in the supercages of the zeolite framework. Treatment in vacuo at 300 °C induces limited dehydration but the EXAFS study shows a closer approach of cerium to the zeolite framework. Washing of the material in ammonium chloride removes most of the cerium ions which are weakly held within the zeolite supercages but does not affect the ions located within the small cages. Treatment of the sample in vacuo at 300 °C induces the migration of most of the cerium into multiple site occupation of the small cages of the zeolite structure. Treatment in steam causes further changes in site occupancy and the migration of the cerium ions from the supercages in the zeolite framework. The erbium L3-edge EXAFS data recorded from ca. 8 wt.% erbium-exchanged Y-zeolite following identical treatments are essentially similar to those recorded from cerium-exchanged Y-zeolite. I. Introduction Lanthanide-ion exchanged Y-zeolites have been used for many years as hydrocarbon cracking catalysts in the petroleum industry [1--4]. The catalytic properties ap- pear to depend on the nature, number, and location of the exchanged lanthanide cations [3] and the acidity of the zeolite framework [3]. Although infrared spec- troscopy has been used successfully for the examination of the acidic hydroxyl groups in calcined zeolites [5] some difficulty remains in distinguishing between the hydroxyl groups which are attached to the lanthanide metal and those which are attached to the zeolite framework. Although X-ray diffraction, sometimes in conjunction with infrared spectroscopy, has been used to identify the different stages of dehydration of the hydrated lanthanide species at various temperatures [6-13] (and has also indicated the presence of adjacent lanthanide ions connected to bridging hydroxyl groups [6]) the partial occupation by the lanthanide ions of the exchange sites and the presence of coordinated water molecules has resulted in a less than unequivocal interpretation of the results. Furthermore, despite the success of recent neutron diffraction studies [14] of lanthanum-exchanged Y-zeolite which have described the hydrolysed lanthanum ions in terms of terminal rather than bridging hydroxyl groups, the exact de- scription of the local environment of the rare-earth ions and the changes which are induced by thermal treatment remains the subject of considerable uncer- tainty. The clarification of these matters is important since the presence of lanthanide ions is known to enhance the acidity and also preserve the structure of zeolites during their use as hydrocarbon cracking catalysts. Hence a description of the sites in the zeolite framework which are occupied by different lanthanide ions as a result of their different sizes and an understanding of how their local coordination changes as a result of dehydration and rehydration processes is crucial to an appreciation of their influence on the structural and catalytic chemistry of the materials. In this respect it is relevant to note the element specificity of extended X-ray absorption fine structure (EXAFS) and its su- perior sensitivity for elucidating the local coordination of the lanthanide ion as compared with that of con- ventional diffraction techniques. Given that EXAFS has recently been shown [15, 16] to be sensitive to the 0925-8388/93/$6.00 © 1993- Elsevier Sequoia. All rights reserved