Alkali metal cation control of oxidation reactions of radicals in zeolites

The dynamics of the xanthyl radical in a series of alkali-metal cation exchanged (Li+, Na+, K+, Rb+, and Cs+)Y faujasites in the absence and presence of molecular oxygen are examined by using laser flash photolysis. Upon laser photolysis of xanthene-9-carboxylate incorporated within the supercages of NaY in the absence of oxygen, prompt formation of the xanthyl radical and the xanthylium cation is observed. The xanthyl radical is formed by photoionization of xanthene-9-carboxylate to the corresponding acyloxy radical that then rapidly decarboxylates. The prompt xanthylium cation is produced by photoionization of the xanthyl radical. This mechanism for the prompt xanthylium cation is supported by results from two-color laser and photoinduced electron transfer experiments. When oxygen is introduced into the sample, the radical is almost completely quenched. In addition, a slow growth of the xanthylium cation at 375 nm is observed. This growth is due to heterolytic cleavage of the peroxyl radical formed upon reaction of the xanthyl radical with molecular oxygen. The formation and resulting growth of the xanthylium cation in the presence of oxygen is found to be highly dependent on the zeolite counterion with significant carbocation formation occurring in both LiY and NaY, little carbocation formation in KY, and no carbocation formation in RbY and CsY. These are the first results showing how the oxidation of radicals to carbocations within zeolites can be controlled by simple alkali metal exchange.