Prussian Blue Analogue-Derived CoO as Catalysts for Enhanced Selective Oxidation of Cyclohexane Using Molecular Oxygen.

Selective conversion of inert C-H bonds in alkanes into high-value-added functional groups (alcohols, ketones, carboxylic acids, etc.) plays a vital role in establishing a green and sustainable chemical industry. Catalytic selective oxidation of cyclohexane to KA oil (cyclohexanol and cyclohexanone) is a typical representative of alkane functionalization. In this work, hollow cage-like CoO (CoO-C) and particle CoO (CoO-P) were synthesized by calcining two types of Prussian blue analogues (PBAs), which were used to catalyze the selective oxidation of cyclohexane. The CoO-C predominantly exposed (311) crystal plane is easier to adsorb cyclohexane than CoO-P, which is beneficial to shorten the induction period, accelerate the reaction rate, and improve the conversion. Consequently, CoO-C displayed a 10% conversion of cyclohexane within 1 h, and the KA oil selectivity reached 90%. The CoO-P exposed (220) crystal plane has a higher molar percentage of oxygen vacancies and more active oxygen species, as well as a strong cyclohexanone adsorption capacity, which is conducive to the deep oxidation of cyclohexanone to adipic acid and other diacid products. The mechanism analysis of cyclohexane oxidation catalyzed by PBA-based CoO shows that it exemplifies the feasibility to tailor the surface of catalysts by modulating the PBAs, which ultimately influences their reaction performance for accelerating the reaction and maintaining high cyclohexane conversion and KA oil selectivity.