Hydroxyl-terminated carbon dots for efficient conversion of cyclohexane to adipic acid

Abstract

Adipic acid (HOOC(CH2)4COOH, AA) is a crucial chemical in industrial manufactures and mainly produced by the oxidation of cyclohexane with air and nitric acid via a homogeneous two step path in industry. However, the conventional industrial method inevitably results in the generation of nitrous oxide (N2O, etc.), which is the main cause of the greenhouse effect and ozone depletion. Herein, we engineered five kinds of carbon dots (CDs) with different oxygen-containing functional groups on their surfaces, which were directly used as catalysts over the selective catalytic oxidation of cyclohexane. Among the five CDs, the hydroxyl-terminated CDs show the best catalytic activity with the conversion of cyclohexane ~ 29.43% and selectivity to AA ~ 95.89% under 130 °C, 20 atm with oxygen as oxidant. With the CDs directly as catalyst, the one-step efficient catalytic oxidation reaction for cyclohexane to AA was realized to replace the two-step routes. In addition, the density functional theory (DFT) computational results were performed to further prove that the more quantity of hydroxyl on CDs and the higher O2 pressure can boost the higher catalytic activity of CDs over oxidation of cyclohexane.