Oxidation catalysis of unsaturated hydrocarbons with molecular oxygen via single-electron transfer in thermally treated H zeolites

An electron spin resonance investigation demonstrates that radical cations in dehydroxylated zeolites are intermediates of a catalytic oxidation reaction that consumes molecular oxygen and involves radical cation transients of unsaturated hydrocarbons. Radical cation generation has often been attributed to the action of Lewis acid sites, generated by a thermal treatment of a proton-exchanged zeolite. The present study of the conditions for radical cation generation and stabilization shows that the active centers are nonacidic, which means that separate special single-electron redox-active catalytic sites are present in the zeolite. Simultaneous monitoring by ESR of the organic radical cation, the superoxide radical anion, and molecular oxygen in the reaction of 2,5-dimethylhexa-2,4-diene with O2 in dehydroxylated H-MOR revealed in one case more than 4000 cycles per active center, demonstrating their catalytic activity.