Catalytic and reaction engineering studies on organic liquid phase oxidation of Cyclohexane

Liquid phase oxidation of hydrocarbons has been used for decades in converting the available feedstock to commercially viable products of importance in petrochemicals and polymer industry. Cyclohexane oxidation is among the ones that have been studied extensively and is of interest to researchers because of the challenges involved due to various side reactions which lower the selectivity to the desired compound. Oxidation of cyclohexane yields cyclohexanol (A) and cyclohexanone (K); commonly referred to as KA oil. It is an important chemical intermediate and is the starting materials in the preparation of adipic acid and caprolactam that are in turn used in the manufacture of nylon-6 and nylon-66 polymers. Conventionally, oxidation of cyclohexane is carried out on an industrial scale worldwide with air as an oxidant either without catalyst or with dissolved cobalt salts (cobalt acetate or cobalt napthenate) as the catalyst. In view of the decreasing selectivity of KA oil by various side reactions, the process is restricted to 4-5% conversion. KA oil selectivity is maintained 75-80% in the process. With the available knowledge in the literature, present research was focused towards developing a better catalyst that gives a better conversion, maintaining high selectivity to KA oil. Transition metals, cobalt and molybdenum are used for the study because of their proven activity in hydrocarbon oxidations; however, they are not explored before as mixed oxide for cyclohexane oxidations. Thesis is divided into three parts; the first part focuses on the catalytic oxidation of cyclohexane using molecular oxygen over cobalt molybdenum oxide (CoMoO4) catalysts in solvent free conditions. In addition to the pure oxides, the mixed oxides with different Co:Mo ratio are synthesized and evaluated for cyclohexane oxidation. These catalysts were characterized using different techniques including; surface area analysis (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effect of different parameters like oxygen pressure, reaction temperature, reaction time and catalyst loading on the performance of the catalyst is evaluated. These oxide catalysts were observed to deactivate during the course of the reaction and the reaction terminates after some time. The causes for the deactivation were traced and studied in the second part. Along with experimental verification supported with characterization techniques of thermo gravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FT-IR) on the spent and fresh catalyst, the underlying deactivation phenomenon was studied. Activity of the catalyst was regained after re-calcination and was comparable to the fresh catalyst. In third part, CoMoO₄ mixed oxide was supported on three different mesoporous silica, namely SBA-15, KIT-6, and LPFDU-12. The use of support was to increase the dispersion of oxide and provide high surface area. The oxide supported catalyst were studied for change in pore size of support, change in calcination temperature, and change in loading of oxides/g silica support using KIT-6 as support for study. The performance of supported catalysts is reported for solvent free oxidation of cyclohexane in the oxygen environment. The study finds its application in designing the catalyst by optimizing the synthesis parameters. The study concludes with modeling of reaction kinetics for cyclohexane oxidation based on the scheme proposed by Suresh et al (1988). The results have shown good agreement between the experimental data and model prediction for all the supported and unsupported catalyst. The values of the rate constants and the activation energies were evaluated as per the Arrhenius plot. The study has come out with a transition metal based Co-Mo oxide catalyst that has shown a potential in organic liquid phase oxidation of cyclohexane and finds as an alternative to the conventional counterparts. Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy of the Indian Institute of Technology Bombay, India and Monash University, Australia.