Abstract
Tetraethylammonium tetrachloroferrate catalyzes the photooxidation of cyclohexane heterogeneously, exhibiting significant photocatalysis even in the visible portion of the spectrum. The photoproducts, cyclohexanol and cyclohexanone, initially develop at constant rates, implying that the ketone and the alcohol are both primary products. The yield is improved by the inclusion of 1% acetic acid in the cyclohexane. With small amounts of catalyst, the reaction rate increases with the amount of catalyst employed, but then passes through a maximum and decreases, due to increased reflection of the incident light. The reaction rate also passes through a maximum as the percentage of dioxygen above the sample is increased. This behavior is due to quenching by oxygen, which at the same time is a reactant. Under one set of reaction conditions, the photonic efficiency at 365 nm was 0.018 mol/Einstein. Compared to TiO2 as a catalyst, Et4N[FeCl4] generates lower yields at wavelengths below about 380 nm, but higher yields at longer wavelengths. Selectivity for cyclohexanol is considerably greater with Et4N[FeCl4], and oxidation does not proceed past cyclohexanone.
Highlights
The photochemical oxidation of hydrocarbons to alcohols, aldehydes, ketones, and carboxylic acids has been pursued as a compelling goal of green chemistry
A successful process would use only molecular oxygen as the oxidizing agent, would run at room temperature and ambient pressure, and would have a high photonic efficiency at wavelengths well into the visible range so that sunlight could be a viable source of photons
Titanium dioxide and polyoxometallate anions have in common the ability to create both oxidizing and reducing centers photochemically - holes and electrons on TiO2 and oxidized and reduced forms of the polyoxometallate
Summary
The photochemical oxidation of hydrocarbons to alcohols, aldehydes, ketones, and carboxylic acids has been pursued as a compelling goal of green chemistry. Polyoxometallate anions, and zeolites share the ability to generate active sites or ions (such as ·OH) that abstract hydrogen very efficiently, sometimes too efficiently for synthetic purposes, because oxidation often continues to complete mineralization, i.e., CO2 , with extended irradiation [2] Another disadvantage of these materials from the viewpoint of green chemistry is that all of them are white. Attempts have been made to dope these substances with compounds that absorb in the visible, generally with some improvement in the yield at higher wavelengths, but not always preserving efficiency in the UV Another approach to heterogeneously catalyzed photooxidation makes use of heterogenized tetrachloroferrate(III) ion, and was introduced by Maldotti et al [3]. Tetraethylammonium tetrachloroferrate, which is insoluble in hydrocarbons, was the best of these, and its properties as a photocatalyst were investigated in detail
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