Oxidation of adamantane with 1 atm molecular oxygen by vanadium-substituted polyoxometalates

Abstract

The oxidation of adamantane with molecular oxygen as a sole oxidant was efficiently promoted by catalyst precursors of vanadium-substituted Keggin-type phosphomolybdates such as H 4PVMo 11O 40, H 5PV 2Mo 10O 40, and H 6PV 3Mo 9O 40 in butyronitrile. The major product was a tertiary C H bond oxygenated product of 1-adamantanol, and secondary C H bond oxygenated products were also formed. The total yield of oxygenated products for the oxidation of adamantane in the presence of H 5PV 2Mo 10O 40 reached 84%. NMR and IR data show that the vanadium-substituted phosphomolybdates, such as H 4PVMo 11O 40 and H 5PV 2Mo 10O 40, decompose to form the monomeric vanadium species ( V V O 2 + (main) and V IVO 2+) and PMo 12 O 40 3 − Keggin anion. The reaction mechanism involving a radical species was proposed from ESR and kinetic data. The catalysts initially abstract the hydrogen of adamantane to form the adamantyl radical and reduced catalysts. This step would be promoted mainly by the vanadium species, such as V V O 2 + , and the phosphomolybdates, PMo 12 O 40 n − , enhance the activity. The adamantyl radical formed promotes the successive formation of the key intermediates, such as adamantyl radical and hydroperoxide species.