PHOTOPHYSICAL PROPERTIES OF PYRENE IN ZEOLITES .3. A DIRECT TIME-RESOLVED DIFFUSE REFLECTANCE STUDY IN ALKALI CATION-EXCHANGED ZEOLITE-X AND ZEOLITES-Y

Time-resolved diffuse reflectance spectroscopy is used to investigate the photolysis of pyrene (Py) impregnated alkali cation-exchanged zeolites X and Y. These studies suggest that pyrene molecules are distributed in at least two different sites (active and non-active) on the zeolites X and Y. Excitation of pyrene adsorbed in non-active sites produces singlet excited states of pyrene with characteristic spectrum and photoionized species by absorbing a second photon, and triplet excited states through inter-system crossing; however, pyrene adsorbed in active sites is photoionized through a single-photon process, The products of the photoionization are the cation radical of pyrene and the ionized electron that is captured by the zeolite framework cation (e.g. Na4(3+) for sodium-exchanged zeolite Y). The absorption spectrum of the captured electron in the zeolite framework cation depends on the alkali cation exchanged. Coadsorbed water blocks the oxygen quenching of 3Py* and trapped electron whereas the absorption spectrum of the trapped electron shifts to about 620 nm regardless of the alkali cation exchanged in the zeolite.