Catalytic oxidation of cyclohexanol to cyclohexanone using a combination of rhodium (111), iron (111) and molecular oxygen
Creators: name: Abbot, John
Publisher: name: University of British Columbia
Year: 2010

description: The catalytic conversion of cyclohexanol to cyclohexanone using a combination of rhodium trichloride trihydrate and ferric chloride in the presence of molecular oxygen was investigated. No conversion to cyclohexanone occurred in the absence of rhodium trichloride trihydrate, but some degree of conversion was found in the absence of ferric chloride. The optimum conditions for catalytic oxidation were produced by using a combination of rhodium trichloride trihydrate and ferric chloride, and under these conditions the rate of conversion to the ketone declined steadily, until the mixture contained approximately U0% cyclohexanone. For a fixed amount of cyclohexanol and rhodium trichloride trihydrate it was found that there was an optimum amount of ferric chloride necessary to produce the maximum yield in the shortest possible time. Addition of ferric chloride in excess of this optimum amount tended to suppress the rate of conversion to the ketone. This can probably be explained by the additional production of water and cyclohexene (see below). Using a cyclohexanol/ferric chloride ratio in the optimum range at a given temperature, increasing the rhodium trichloride trihydrate concentration beyond a certain level did not significantly increase the final yield, or the reaction rate. The oxidation reaction occurred under acidic conditions, this acidity being the result of interaction between ferric chloride and cyclohexanol (and cyclohexanone) , The acidity of a typical system was found to decline rapidly as the reaction progressed. Cyclohexene was produced in a side reaction, together with water. This is presumably the result of cyclohexanol undergoing an elimination reaction under acidic conditions. Using the optimum cyclohexanol/ferric chloride ratio at 100deg, the the cyclohexene content remained at less than 10%, during the course of the reaction. Introduction of cyclohexene in amounts in excess of 20 % greatly suppressed the conversion to cyclohexanone, presumably due to strong complexation of the olefin with a rhodium species. water was produced during the catalytic oxidation in amounts greater than could be accounted for by production of cyclohexene. This additional water content of the reaction mixture in a closed system was in good agreement with that predicted on the basis of the equation: [ ] The presence of water in the reaction mixture tended to suppress the oxidation of cyclohexanol to cyclohexanone. Using the optimum ratio of components, very little conversion to the ketone occurred in the presence of oxygen, at temperatures below 50deg. Increasing the temperature from 100deg to 150deg increased the rate of oxidation but had little effect on the final yield of cyclohexanone. , Oxygen was found to be necessary for catalytic oxidation to occur. The measured oxygen absorption for a reaction mixture containing an optimum ratio of components, was found to be in good agreement with that predicted by the above equation. Using an optimum ratio of components in the presence of oxygen, the conversion to cyclohexanone was limited to approximately 40%. This limit was probably due to an interaction between cyclohexanone and some active rhodium species essential for catalytic activity.