Conversion of isopropyl alcohol to acetone on irradiated zinc oxide

Abstract

Irradiated suspensions of zinc oxide containing isopropyl alcohol form acetone and peroxide provided an adequate concentration of oxygen is present. Initial quantum yields of these products range from 0.25 to 1.50 depending on concentration of alcohol, specific catalyst surface, light intensity, and temperature. The peroxide concentration reaches a limiting value under all conditions, but the acetone accumulates indefinitely. Detailed investigation shows that the products obtained are the result of reactions of radicals in the liquid phase as well as direct surface reactions. The former are particularly important in pure organic liquids. The data obtained are best explained on the assumption that the energy of radiation at 3650 Å, approximately 80 kcal absorbed by zinc oxide, gives rise to an active form of oxygen which initiates both the surface and liquid phase reactions. Consideration of the various possible reactive forms of oxygen which might be produced under these conditions leads to the conclusion that an excited molecular oxygen anion is the most probable active intermediate. This interpretation is consistent with observations on irradiated zinc oxide reported by other investigators and also previous studies carried out in this laboratory. Lack of sufficient oxygen in the reaction mixture, or the instability of the excited molecular oxygen anion in the gas phase, may account for earlier reports of failure to observe vapor phase conversion of isopropyl alcohol to acetone on irradiated zinc oxide. These systems may not have practical value for the formation of peroxide, but the photocatalyzed conversion of isopropyl alcohol to acetone in zinc oxide suspensions is a very efficient process. An attractive feature is the fact that it can be carried out at low temperatures without the intervention of undesirable side reactions.