Dealumination of zeolites II. Kinetic study of the dealumination by hydrothermal treatment of a NH 4NaY zeolite

Abstract

The hydrothermal treatment of a (NH 4) 0.8Na 0.2Y 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 dealumination and to characterize the variations of the crystallinity. The enrichment of the outer surface in Al 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 4 + (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 Al of the outer surface.