Photocatalytic air oxidation of cyclohexane in CH2Cl2-C6H12 mixtures over TiO2 particles -: An attempt to rationalize the positive effect of dichloromethane on the yields of valuable oxygenates

Product selectivity and yields of cyclohexanol and cyclohexanone formation in the photocatalytic air oxidation of cyclohexane on TiO2 particles in the presence of dichloromethane were determined at 303 +/- 5 nm and 4.35 nEinstein cm(-2)s(-1), as a function of cyclohexane molar fraction, x. Apparent initial photonic efficiencies, xi(ox), for total monooxigenated products (cyclohexanol+cyclohexanone) ranged from 19 to 51% depending on solvent composition, attaining a maximum for an equimolar mixture of CH2Cl2-C6H12. Singlet oxygen, O-1(2), formation was determined by the specific 2,2,6,6-tetramethyl-4-piperidone EPR assay. Yields of O-1(2), correlate with the amount of cyclohexanol detected in the mixtures, strongly suggesting that cyclohexanol is formed through the recombination of cyclohexylperoxy radicals which is favoured in the more polar media. Simultaneous determinations of chloride yields provide direct evidence of the active participation of dichloromethane in the photocatalytic system. A mechanism is proposed to account for the observed increment in the apparent efficiencies and the selectivity changes. (c) 2006 Elsevier B.V. All rights reserved.