Kinetic and mechanistic studies of cyclohexane oxidation with tert-butyl hydroperoxide over M–N4 catalysts†

Abstract

Oxidation of cyclohexane with tert-butyl hydroperoxide (TBHP) is a common probe reaction for molecular complexes, though the kinetics of this reaction are seldom reported. Here, we synthesize metal–nitrogen-doped carbons (M-N-Cs) and a series of zeolite-encapsulated metal phthalocyanine (MPC) catalysts and compare their reactivity in cyclohexane oxidation with TBHP. These materials all have primary binding sites that include square planar metals bound to four nitrogen atoms (M–N₄ sites). We measure the apparent activation energy for this reaction, and compare the reactivity of M-N-C and MPC catalysts with varied metal central atoms (M = Fe, Mn, Co, Cu, Cr, Ni), of which Fe-containing catalysts are the most reactive. Fe-N-C catalysts are more stable with reuse than FePC catalysts. Apparent reaction orders are less than one for both reactants, suggesting a surface mediated reaction. Cyclohexanol and cyclohexanone form in parallel at short reaction times, while cyclohexanol further reacts to cyclohexanone at longer reaction times. DFT calculations show that the reaction may follow a radical-mediated Eley–Rideal mechanism that is primarily mediated via tert-butoxy radicals formed at the metal site. Microkinetic modeling of the proposed mechanism reproduces experimental trends in product rates and selectivity of the main reaction products without requiring any parameter estimation. This study demonstrates that M-N-C and MPC catalysts perform cyclohexane oxidation with TBHP with similar per metal-atom initial rates. These results will enable judicious use of cyclohexane oxidation with TBHP as a probe reaction to compare reactivity of catalysts with M–N₄ active sites.