Activity and selectivity patterns in the oxidation of allyl iodide on the period IV metal oxides - The participation of lattice oxygen in selective and total oxidation reactions

The oxidation of allyl iodide was studied on the period IV metal oxides wit h O-18(2). In this reaction the selectivity and activity of the period IV m etal oxides were determined as a function of temperature. The activity patt ern of the allyl iodide oxidation reaction shows the same saw-tooth like pa ttern as the oxidation of H-2 and CH4; i.e. maxima at MnO2, Co3O4 and CuO. However, the differences in activity of the period IV metal oxides in the o xidation of allyl iodide are within one-order of magnitude, which is much s maller than for the oxidation of H-2 and CH4 Although in the oxidation of a llyl iodide the first step, i.e. abstraction of a hydrogen, is bypassed, th e activity is still determined by the average metal-oxygen bond strength: t he higher the M-O bond strength the lower the activity. The selectivity pat tern looks similar to the activity pattern, only the selectivity pattern va ries in anti-phase with the activity pattern: a high M-O bond strength resu lts into a high selectivity to acrolein. The oxidation of allyl iodide with O-18(2) and the oxidation of propylene with O-18(2) On Co3O4 revealed that the total as well as the selective oxidation products were formed with lat tice oxygen from the metal oxide. Reaction of an adsorbed intermediate with gas phase oxygen can only take place if the surface of the oxide is strong ly reduced. (C) 1999 Elsevier Science B.V. All rights reserved.