MIL-101 as Reusable Solid Catalyst for Autoxidation of Benzylic Hydrocarbons in the Absence of Additional Oxidizing Reagents

Materiaux de l'Institute Lavosier-101 (MIL-101) promotes benzylic oxidation of hydrocarbons exclusively by molecular oxygen in the absence of any other oxidizing reagent or initiator. Using indane as model compound, the selectivity toward the wanted ol/one mixture is higher for MIL-101(Cr) (87% selectivity at 30% conversion) than for MIL-101(Fe) (71% selectivity at 30% conversion), a fact that was associated with the preferential adsorption of indane within the pore system. Product distribution and quenching experiments with 2,2,6,6-tetramethyl-1-piperidinyloxy, benzoic acid, and dimethylformamide show that the reaction mechanism is a radical chain autoxidation of the benzylic positions by molecular oxygen, and the differences in selectivity have been attributed to the occurrence of the autoxidation process inside or outside the metal organic framework pores. MIL-101 is reusable, does not leach metals to the solution, and maintains the crystal structure during the reaction. The scope of the benzylic oxidation was expanded to other benzylic compounds including ethylbenzene, n-butylbenzene, iso-butylbenzene, 1-bromo-4-butylbenzene, sec-butylbenzene, and cumene.