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Black plate (256,1) zeolites were used as hosts for most of the subsequent experiments too. Zeolite-Y has the faujasite structure (Figure 8) with a 3- dimensional network (honeycomb) of spherical supercages of diameter 13A, and connected by ‘‘windows’’ of 8A aperture. In contrast, zeolite-L has a uni-dimensional structure of channels with a diameter of 7A. Zeolites Y and L are very useful in studies of photoxidation reactions, because they can be prepared free of either Brønsted or Lewis acid sites when dehydrated under vacuum, the presence of which could otherwise complicate devising mechanisms for them. On average there were 1–2 molecules of hydrocarbon for each supercage of zeolite-Y, and one O2 molecule per 3–4 super- cages. It was found that irradiation of 2,3-dimethylbut-2-ene even at light wavelengths of 750nm resulted in oxidation of the alkene, yielding the product 2,3-dimethyl-3-hydroperoxyl-1-butene, along with a small quantity (2%) of acetone which originated from the thermal decomposition of the primary alkene hydroperoxide106,107. The optical spectrum of this system shows a long absorption tail which extends into the red region of the spectrum (to 750 nm) from the CT absorption108. When 2,3-dimethylbut-2-ene and O2 are present together in the liquid or the gas phase, the CT tail begins only at 400 nm, which implies there is a stabilisation of the excited CT complex by the zeolite surface electrostatic field by up to 350nm which amounts to 3.5eV. The formal negative charge per supercage wall in zeolite-Y is -7, and is located on the framework oxygen atoms, but counterbalanced by poorly shielded cations resulting in an electrostatic field strength of 0.3VyA and 0.9VyA for NaY and BaY, respectively105. Thus the large dipole of the CT ion pair of around 15 Debye units will interact strongly with the internal surface field, providing stabilisation for it and greatly assisting its formation in the zeolite matrix. The oxidation of propylene109 required rather shorter wavelengths (5520nm), in accord with the fact that the ionisation potential of propylene (9.7 eV) is greater than that (8.3 eV) for 2,3-dimethylbut- 2-ene, but product selectivity proved harder to achieve in this system. The initial product allyl hydroperoxide was stable when the zeolite was maintained at a temperature of 100C and was formed along with 13% of propylene oxide as a minor product. If the zeolite containing allyl hydroperoxide was allowed to warm with excess propylene still present, propylene oxide was produced, but only acrolein if the excess propylene was pumped-off before warming. A reasonable mechanism can be invoked if allyl hydro- peroxide is the primary product of photo-oxidation of propylene, but is converted to acrolein by dehydration. Propylene oxide is 256 Christopher J. RhodesPDF Image | Properties and applications of zeolites
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