The photo-oxidation of cyclohexane on titanium dioxide: an investigation of competitive adsorption and its effects on product formation and selectivity

The photo-oxidation of cyclohexane on titanium dioxide was investigated in neat cyclohexane and in various solvents to determine the effect of the sol vent media on the cyclohexane oxidation rate and reaction selectivity to cy clohexanol and cyclohexanone. The solvents that were used in this study inc lude acetone, isopropanol, dichloromethane, chloroform, carbon tetrachlorid e, benzene, and n-hexane. It was found that the reaction rate and selectivi ty to the formation of cyclohexanol and cyclohexanone in various solvents d epend upon the relative strengths of adsorption of cyclohexane, cyclohexano ne, cyclohexanol, the solvent, and the partially oxidized solvent species o n the titanium dioxide particles. In non-polar solvents, cyclohexanol prefe rentially adsorbed onto the titanium dioxide particles and underwent deep o xidation, ultimately to carbon dioxide and water. Therefore, in non-polar s olvents, the selectivity of the reaction to cyclohexanol was very low. Howe ver, in polar solvents, cyclohexanol adsorbed to the titanium dioxide parti cles to a lesser extent due to the competition fur adsorption sites with th e solvent, and the selectivity of the reaction to cyclohexanol significantl y increased. Competitive adsorption. in part, determined the overall rate o f cpclohexane oxidation and selectivity to the desired products, cyclohexan one and cyclohexanol. The highest product-formation rate in this study was observed in dichloromethane, whereas chloroform and isopropanol significant ly inhibited the desired reactions. The ideal solvent for the photo-oxidati on of cyclohexane is one that minimizes the strengths of adsorption of the desired products on titanium dioxide and either does not compete with cyclo hexane and oxygen for adsorption sites or is strongly adsorbed but is non-r eactive with itself upon forming a radical on the illuminated titanium diox ide surface. (C) 2001 Elsevier Science B.V. All rights reserved.