Electron Transfers Induced by t-Stilbene Sorption in Acidic Aluminum, Gallium, and Boron Beta (BEA) Zeolites

Abstract

The adsorption of trans-stilbene (t-St) on the acidic aluminum-, gallium-, and boron-containing zeolite beta (H-AlBEA, H-GaBEA, and H-BBEA, respectively) is investigated using UV/vis and time-resolved UV/vis spectroscopy, electron paramagnetic resonance spectroscopy, and Raman spectroscopy. On H-AlBEA, the results show a fast and spontaneous one-electron oxidation of t-St, quickly followed by the recapture of an electron from the zeolite framework by the t-St•+ radical cation and the formation of a long-lived charge-transfer complex (t-St H-AlBEA•–•+). This charge-transfer complex (I) evolves over a period of months into a spectroscopically distinct charge-transfer complex (II). Evidence for the (undetected) intermediate t-St•+ radical cation is obtained using time-resolved UV/vis spectroscopy. Similar electron-transfer processes have been observed in the acidic ferrierite, ZSM-5, and mordenite aluminosilicate zeolites. The key difference is that the rate of electron recapture by the t-St•+ radical cation is much faster than that in all of the other zeolites. This is in agreement with the trend observed in the ferrierite, ZSM-5, and mordenite zeolites: looser fit leads to a lower stability of the radical cation intermediate and leads to a rapid hole transfer to the zeolite framework to form a charge-transfer complex. In the case of boron-containing beta, the rate of formation of the radical cation is slow, and the yield is small. Gallium-containing zeolite beta shows intermediate behavior when compared with the Al and B forms of zeolite beta.