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Black plate (258,1) iso-butane and oxygen gas loaded into zeolite BaY were caused to react by irradiation with green or blue light to yield tert-butyl hydroperoxide in a single-photon process. The selectivity was 98%, even at 50% conversion of the reactants loaded into the zeolites113. When propane was similarly oxidised114, acetone and water were detected by reflectance IR as the final products when 150–300mm Hg of propane and 1 atm pressure of O2 were adsorbed into zeolite BaY and exposed to blue or green light from either a tungsten lamp or a laser. Some additional bands were ascribed to isopropyl hydroperoxide, a species that disappeared under dark conditions along with the concomitant increase in features from acetone and water. Thus isopropyl hydroperoxide appears to be the primary photo-product, as is confirmed from the results of identical experi- ments repeated using perdeuteropropane114. The possibility that thermal oxidations of alkanes might take place in zeolites in the absence of light was also explored and for propane in CaY, even at room temperature some oxidation was found to occur114. The effect of zeolite Y cation-exchanged with a series of alkaline- earth cations on selective propane oxidation at room temperature was studied using in situ infrared spectroscopy, from which isopropyl hydroperoxide was observed as a reaction intermediate which may decompose to form acetone and water. In contrast to previous reports, BaY was active at room temperature. The rate of reaction was seen to increase in the order BaY5MgY5SrY5CaY as gauged by the rate of formation of adsorbed acetone. The acetoneywater ratio was found to increase with cation size, with no other detectable products, while the acetoneyisopropyl hydro- peroxide ratio was found to decrease as the number of Brønsted acid sites decreased. Both observations provide evidence that Brønsted acid sites are involved in this reaction, in addition to the alkaline earth metal cations. A two-step mechanism is suggested in which the conversion of propane into isopropyl hydroperoxide takes place on the cations, while the decomposition into acetone occurs at Brønsted acid sites115. An investigation of the number of supercages involved in the visible light-induced reaction between 2,3-dimethylbut-2-ene and O2 in NaY zeolite was made in which the growth of the sole product, 2,3-dimethyl-3-hydroperoxy-but-1-ene was studied using rapid-scan FT-IR spectroscopy with a 150ms photolysis pulse. Since the pulse duration is short in comparison with the estimated timescale over which cage-to-cage diffusion occurs, it is possible to determine the number of cages involved. It was estimated that a minimum of 1% of all NaY supercages actually take part in the 258 Christopher J. RhodesPDF Image | Properties and applications of zeolites
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