Abstract
Titanium dioxide is the most extensively used heterogeneous catalyst for the photooxidation of toluene and other hydrocarbons, but it has low utility for the synthesis of benzyl alcohol, of which little is produced, or benzaldehyde, due to further oxidation to benzoic acid and cresol, among other oxidation products, and eventually complete mineralization to CO2. Et4N[FeCl4] functions as a photocatalyst through the dissociation of chlorine atoms, which abstract hydrogen from toluene, and the photooxidation of toluene proceeds only as far as benzyl alcohol and benzaldehyde. Unlike TiO2, which requires ultraviolet (UV) irradiation, Et4N[FeCl4] catalyzes the photooxidation of toluene with visible light alone. Even under predominantly UV irradiation, the yield of benzyl alcohol plus benzaldehyde is greater with Et4N[FeCl4] than with TiO2. Et4N[FeCl4] photocatalysis yields benzyl chloride as a side product, but it can be minimized by restricting irradiation to wavelengths above 360 nm and by the use of long irradiation times. The photonic efficiency of oxidation in one experiment was found to be 0.042 mol/einstein at 365 nm. The use of sunlight as the irradiation source was explored.
Highlights
While the photooxidation of toluene in the gas phase has been studied extensively [1,2,3], much less has been published on photooxidation in the liquid phase [4,5]
Photoinduced holes in TiO2 surfaces are very efficient at hydrogen abstraction and good results can be obtained with TiO2 catalysis of hydrocarbon photooxidation, but TiO2 is not well suited to applications intended to use sunlight
One of the objectives of the research presented in this paper was to demonstrate that the photooxidation of toluene could be effected by means of visible light
Summary
While the photooxidation of toluene in the gas phase has been studied extensively [1,2,3], much less has been published on photooxidation in the liquid phase [4,5]. In either the gas phase or the liquid phase, catalyzed photooxidation of toluene has relied almost exclusively on titanium dioxide. Photoinduced holes in TiO2 surfaces are very efficient at hydrogen abstraction and good results can be obtained with TiO2 catalysis of hydrocarbon photooxidation, but TiO2 is not well suited to applications intended to use sunlight. Its visible light absorptivity is virtually nil and most researchers use wavelengths reaching well down into the ultraviolet (UV) in order to get acceptable photoreaction rates or make extensive surface modifications by doping or thermal processing [6,7].
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