JOURNAL OF CATALYSIS130, 459--470 (1991) Dealumination of Zeolites 1 II. Kinetic Study of the Dealumination by Hydrothermal Treatment of a NH4NaY Zeolite Q. L. WANG, G. GIANNETTO, M. TORREALBA, G. PEROT, C. KAPPENSTEIN, AND M. GUISNET 2 URA CNRS DO 350, Catalyse en Chimie Organique, Universit~ de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France Received February 13, 1990; revised January 22, 1991 The hydrothermal treatment of a (NH4)0.8Na0.2Y zeolite was carried out in a flow reactor in order to determine quantitatively the effect of the operating conditions (time, temperature, water pressure) on the dealumination of this zeolite. X-ray diffraction was used to follow the process of dealumina- tion and to characterize the variations of the crystallinity. The enrichment of the outer surface in AI was measured by X-ray photoelectron spectroscopy (XPS). First the temperature of the zeolite was raised rapidly to the temperature chosen for the hydrothermal treatment (260 to 820°C) under nitrogen flow. During this rise in temperature there was a significant dealumination caused by water physically adsorbed or liberated by dehydroxylation. This dealumination is accompanied by a partial collapse of the zeolite framework. After the ceiling temperature was reached, the zeolite was submitted to a flow of water vapor. Two periods could be distinguished, namely an initial one in which dealumination was rapid, and a second one in which it was slow. Only the aluminum atoms with which NH~- (and not Na ÷) are associated could be extracted from the framework. For both periods the kinetic orders with respect to these aluminum atoms and to water are equal to 1; the apparent activation energy is about 20 kJ mol-1 for the rapid dealumination and 70 for the slow process. The first period would correspond to the dealumination of the protonic zeolite and the second to that of the zeolite exchanged by cationic aluminum species. Under high water pressure (>50 kPa) there is a relationship between zeolite dealumination and subsequent migration of extra- framework aluminum species to the outer surface of the crystallites. However, these processes are independent. Indeed during the self-steaming of the zeolite, dealumination occurs without enrichment in AI of the outer surface. © 1991Academic Press.Inc. INTRODUCTION The thermal and the catalytic stability of Y zeolites is considerably improved by de- alumination (1-3). This dealumination is generally carried out through steaming at high temperatures of a NH4Y zeolite. Al- though this technique is currently used, there exist few quantitative data concerning the effect which the operating conditions (time of steaming, temperature, water pres- sure) have on the rate ofdealumination. This I Part I of this series is Ref. (15). 2 To whom correspondence should be addressed. technique causes the formation of extra- framework aluminum species which can mi- grate to the outer surface of the crystallites (4-8). We show here how the operating con- ditions determine the rate of dealumination of a (NH4)0.8Na0.2Y zeolite, the crystallinity of the dealuminated samples, and the rate of migration of the extra-framework species to the outer surface of the crystallites. The mechanism of dealumination is discussed on the basis of the kinetic data. EXPERIMENTAL The dealumination was carried out by hy- drothermal treatment of various samples of 459 0021-9517/91 $3.00 Copyright© 1991by Academic Press.Inc. All rightsof reproduction in any formreserved.