Rate-limiting step of the iron porphyrin-catalysed oxidation of cyclohexane to adipic acid by DFT method

Deep understanding of the rate-limiting step in the oxidation process of cyclohexane to adipic acid would be useful for improving the activity of catalysts and selectivity of goal products. The rate-limiting step lied in high-valent species generation or C–H bond oxidation remains a controversial topic. In this paper, the mechanism of high-valent species generation and C–H bond activation was investigated by density functional theory. It was observed that the activation barrier of the high-valent species was lower than that of C–H bond activation; thereby the C–H bond oxidation was determined as the rate-limiting step. Calculated geometries and energies were in close agreement with the experimental observations. Furthermore, frontier molecular orbital analysis revealed that the C–H bond interacted with the high-valent species in different orientation and it showed how the reaction was manipulated and controlled by the iron–porphyrin in the catalytic process. Given the calculation correction, experiments were designed to reveal the rate-limiting step. This work provides a clear view of the debut on the rate-limiting step of the alkane oxidation. It should be a significant step forward for understanding the relationship between the porphyrin molecular structures and catalytic activity accurately and for predicting and designing high-activity catalysts.