The photo-oxidation of cyclohexane on titanium dioxide was investigated in neat cyclohexane and in various solvents to determine the effect of the solvent media on the cyclohexane oxidation rate and reaction selectivity to cyclohexanol and cyclohexanone. The solvents that were used in this study include acetone, isopropanol, dichloromethane, chloroform, carbon tetrachloride, benzene, and n-hexane. It was found that the reaction rate and selectivity to the formation of cyclohexanol and cyclohexanone in various solvents depend upon the relative strengths of adsorption of cyclohexane, cyclohexanone, cyclohexanol, the solvent, and the partially oxidized solvent species on the titanium dioxide particles. In non-polar solvents, cyclohexanol preferentially adsorbed onto the titanium dioxide particles and underwent deep oxidation, ultimately to carbon dioxide and water. Therefore, in non-polar solvents, the selectivity of the reaction to cyclohexanol was very low. However, in polar solvents, cyclohexanol adsorbed to the titanium dioxide particles to a lesser extent due to the competition for adsorption sites with the solvent, and the selectivity of the reaction to cyclohexanol significantly increased. Competitive adsorption, in part, determined the overall rate of cyclohexane oxidation and selectivity to the desired products, cyclohexanone and cyclohexanol. The highest product-formation rate in this study was observed in dichloromethane, whereas chloroform and isopropanol significantly inhibited the desired reactions. The ideal solvent for the photo-oxidation of cyclohexane is one that minimizes the strengths of adsorption of the desired products on titanium dioxide and either does not compete with cyclohexane and oxygen for adsorption sites or is strongly adsorbed but is non-reactive with itself upon forming a radical on the illuminated titanium dioxide surface.
- Partial oxidation
- Titanium dioxide
ASJC Scopus subject areas
- Process Chemistry and Technology