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Aqueous solutions of iodide ion liberate iodine in a continuing reaction in the presence of a silica-alumina catalyst, suitably activated by heat treatment, provided molecular oxygen is present. A similar reaction occurs in benzene solutions of tetramethylammonium iodide. Pure alumina and pure silica are both inactive for the oxidation. Hydrogen Y zeolite has an activity comparable with that of the most active of the amorphous silica-aluminas. The catalytic activity of both the amorphous and crystalline silica-aluminas for the iodide ion oxidation is related to their Bronsted acidity. In the presence of water, the Bronsted sites on the oxide surface are continuously regenerated as the oxidation proceeds. Although silica-aluminas are active for the oxidation of methyl iodide to iodine, the reaction occurs much more readily on the surface of pure alumina. Alumina samples dehydrated at about 630 °C show maximum oxidizing activity and maximum Lewis acid activity. The production of methyl peroxy radicals shows that basic sites are also involved. Dissociative adsorption of methyl iodide probably occurs on the alumina surface and is followed by association of the halogen atoms to produce molecular iodine. Bronsted acid sites are not involved.

The reduction of iodine to iodide ion occurs readily on the surface of partially dehydrated catalytic aluminas and silica-aluminas. An increase in the alumina content of the latter results in an increase in reducing activity. Replacement of the hydroxyl ion on the alumina surface by the fluoride ion decreases the reducing power. Two different surface sites are responsible for the reducing properties. For alumina samples dehydrated at low temperatures (