Oxidation of cyclohexanol to adipic acid with molecular oxygen catalyzed by ZnO nanoparticles immobilized on hydroxyapatite

One of the most important aliphatic diacids is adipic acid, which is produced industrially because of its application in the manufacture of nylon-6 and nylon-66. In the present work, a green methodology for the direct oxidation of cyclohexanol to adipic acid in a one-step reaction using ZnO nanoparticles supported on hydroxyapatite was developed. This work could be one of the very few articles on the use of ZnO nanoparticles for preparing adipic acid by using molecular oxygen as the greenest oxidant. The ZnO nanoparticles were prepared using different methods, such as reverse micelle, osmosis, alkaline hydrolysis, and impregnation, to compare their catalytic performances. We believe that oxygen deficiency in the ZnO nanocatalyst plays the main role in this oxidation reaction. The catalysts were characterized by different methods, such as FESEM, XRD, TEM, BET, and ICP. Among all the investigated catalysts, ZnO nanoparticles prepared by the reverse micelle method exhibited superior activity over all the other tested catalysts. To obtain the optimized conditions for the maximum conversion and selectivity, the reaction parameters, such as temperature, time, oxygen pressure, the amount of catalyst, amount of ZnO loaded on hydroxyapatite, and the method of preparation of the catalyst, were optimized. Under the found optimized conditions, a maximum conversion of 86% and selectivity of 96% was achieved for the production of adipic acid with ZnO/hydroxyapatite catalyst prepared by the reverse micelle method. The optimum conditions for the oxidation of cyclohexanol were 25 mg of catalyst, 120 degrees C, 15 bar oxygen pressure, and 8 h. Furthermore, this catalyst retained its catalytic activity with 84% conversion and 70% selectivity for adipic acid production after four cycles.