PHOTOPHYSICAL PROPERTIES OF PYRENE IN ZEOLITES - A DIRECT TIME-RESOLVED DIFFUSE-REFLECTANCE STUDY OF PYRENE ANION-RADICALS IN ZEOLITE-X AND ZEOLITE-Y

The formation of pyrene anion radicals (Py(.-)) in the supercage of different alkali-ion-exchanged zeolites X and Y was studied using direct time-resolved diffuse reflectance techniques. Many factors such as the Si/Al ratio, the nature of charge balancing cations, the preactivation temperature, the pyrene loading, the state of hydration, and the nature of the surfaces (external versus internal) were examined in order to understand the formation and stabilization of Py(.-) in these samples, and also the mechanism of the photoinduced electron transfer processes. The results show that photoinduced electron transfer does not occur from pyrene to pyrene in the zeolites but occurs between pyrene molecule and the acidic and basic sites of the zeolites. The basic sites of the zeolites, responsible for the formation of Py(.-), are framework oxygen. Stabilization of Py(.-) requires the special environment of the zeolite supercage; it is noteworthy that Py(.-) cannot be formed on the external surface of a zeolite. The formation of Py(.-) in the different alkali-ion-exchanged zeolites X and Y follows the order of basicity of these samples, which is calculated using the Sanderson electronegativity equalization principle. Preactivation of the samples at temperatures of 350, 550, and 750 degrees C does not affect the ratio of the anion to cation, Py(.-)/Py(.+), yields. Posthydration of the samples alters the photophysical processes in the zeolites and gives rise to an increase in the yield of Py(.-). At low light intensities, the photoinduced electron transfer follows a single-photon mechanism.