Study On The Hydrogenation Process Of Nitrocyclohexane: The Reaction Mechanism And Kinetics

As the intermediate of ε-caprolactam, cyclohexanone oxime has been playingan important role in the fiber industry. There have been considerable disadvantages ofthe conventional production route of cyclohexanone oxime, such as, huge energyconsumption, heavy pollution and low atom efficiency. So the need for thedevelopment of new environment friendly production routes has been urgent. Thecyclohexane nitrate reduction route poses many advantages over other emergingroutes. Our research group has achieved great progress in the partial hydrogenation ofnitrocyclohexane into cyclohexanone oxime catalyzed by Pd/CNTs, however, for nowwe have done little probe into the reaction mechanism and kinetics of the route.Firstly, this paper established the analytical method of the products of the partialhydrogenation of nitrocyclohexane into cyclohexanone oxime, secondly, the productsof the reaction were identified by GC-MS, and an unstable intermediate has beenidentified as cyclohexyl hydroxylamine.In order to make the measured kinetic data in the intrinsic dynamics zone, thisinvestigation designed experiments to exclude the influence of inter and externaldiffusion and used the time-sharing method for the collection of the kinetic data.Based on literature and reasonable assumptions, this study adopted the Langmuir-Hinshelwood adsorption model to establish the rate equation of products. This studyadopted the Runge-Kutta method to solve the parameters of the kinetic models andfound that the established model,which was based on schemeⅠand assumption (2),could calculate the results in good agreement with the collected kinetic data. All thesehad suggested that nitrocyclohexane was first hydrogenated into an unstableintermediate, and then the intermediate was further hydrogenated into cyclohexanoneoxime and cyclohexylamine; besides, we could also concluded that there were twoadsorption sites on the surface of Pd/SWNTs, the hydrogen was adsorbed on one ofthe two sites and dissociated.The atomic state of hydrogen did not compete withreaction products to adsorb on the same adsorption site. By observing the rateconstants obtained by calculating, we could notice that the intermediate produced bythe hydrogenation of nitrocyclohexane would convert into cyclohexone oxime fastly.That is to say, the accumulated intermediate was little. By using the calculatedconstants of reaction rate, through Arrhenius equation we also got the activation energy of each reaction was56.5836kJ/mol,83.4367kJ/mol and177.5196kJ/molrespectively.In this investigation, the catalyst recycle of the partial hydrogenation ofnitrocyclohexane into cyclohexone oxime catalyzed by Pd/C was preliminarilystudied. When the catalyst recycle was conducted five times, the catalyst activity ofPd/C decreased greatly. The content of Pd in every recycled catalyst was measured byAES, and found that with the times of recycle increase, the content of Pd decreasedaccordingly, which could partly explain the decreasing catalyst activity of Pd/C.