Photoelectrochemical Synthesis of Adipic Acid by Selective Oxidation of Cyclohexanone

A (photoelectrochemical) PEC strategy is first reported to achieve the selective oxidation of cyclohexanone (CYC) to adipic acid using a cooperative Ni(OH)₂/TiO₂ photoanode with H₂O as an oxygen source under mild conditions, achieving high adipic acid selectivity (>90%) in a wide potential window (from 0.3 to 1.3 V versus RHE). Moreover, a new Ni^{2+ δ} ─OH^* reactive center with moderate oxidation capacity is in situ generated on Ni(OH)₂/TiO₂ photoanode under illumination, which abstracts H atoms from C_α─H bonds in CYC to obtain 2−hydroxycyclohexanone intermediate, and thereby facilitates subsequent C─C cleavage to produce adipic acid.

Abstract

Adipic acid, an essential building stock for polymers, is conventionally synthesized through thermal oxidation of ketone−alcohol oil. However, this process requires excessive nitric acid as oxidants, causing the emission of ozone−depleting greenhouse gas nitrous oxide. Herein, a photoelectrochemical (PEC) strategy is reported for the efficient synthesize adipic acid by selective oxidation of cyclohexanone (CYC). High adipic acid selectivity (>90%) in a wide potential window (from 0.3 to 1.3 V versus RHE) is achieved under ambient conditions based on TiO₂ nanorods array photoanode modified with nickel hydroxide nanosheets (Ni(OH)₂/TiO₂). Experimental and theoretical data reveal that a new Ni^{2+ δ} ─OH^* reactive center with moderate oxidation capacity is in situ generated on Ni(OH)₂/TiO₂ photoanode under illumination, which abstracts H atoms from C_α─H bonds in CYC to obtain 2−hydroxycyclohexanone intermediate, and thereby facilitates subsequent C─C cleavage to produce adipic acid. Moreover, the PEC synthesis of adipic acid from industrial raw material of phenol is achieved by coupling cathodic phenol reduction to CYC and photoanodic CYC oxidation to adipic acid, demonstrating a new and sustainable approach for adipic acid synthesis by directly using solar energy.