VAPOR PHASE OXIDATION OF CYCLOHEXANE OVER SUPPORTED METAL-OXIDE CATALYSTS: In-Situ DRIFTS STUDIES

Abstract

Over the past two decades, various catalysts have been developed and tested for liquid phase selective oxidation of cyclohexane in batch operation using various oxidants with or without solvents. However, very little effort has been put in the development of suitable catalysts and their application for selective oxidation of cyclohexane in vapor phase and continuous operation. Selective oxidation of cyclohexane produces cyclohexanone (K) and cyclohexanol (A) and collectively known as KA oils. KA oil is an important feedstock for adipic acid and caprolactam which are essential raw materials to produce nylon 6 6 and nylon 6, respectively. Catalytic conversion of cyclohexane to KA oil via vapor phase oxidation is one of the most promising routes among all the proposed cyclohexane conversion in batch operations. In recent years, demand of KA oil is met by commercial production based on liquid phase oxidation at 8 – 15 atm pressure and temperature 150 oC with 4 – 7% conversion of cyclohexane and selectivity of KA oil above 85%. Leaching of catalysts’ active sites, product separation, harsh reaction conditions, very low conversion and yield, and uses of corrosive chemicals are some major drawbacks associated with the liquid phase oxidation reaction. Recovery and regeneration of catalyst used in the liquid phase cyclohexane oxidation reaction requires additional unit operations. Moreover, the effluent of the units possesses some toxic chemicals due to leaching of active sites of the catalysts, which has direct impact to the environment. In addition, much energy is required for the operation of the additional units. Several drawbacks associated with the liquid phase oxidation in batch process makes this technology cost inefficient and deleterious to environment. In recent years, oxidation of cyclohexane in vapor phase and continuous process has produced better and more environmentally friendly results. Furthermore, even under moderate reaction conditions, a very good conversion is obtained. Vapor phase oxidation is carried out at elevated temperature i.e., above the boiling point of cyclohexane and KA oils. Therefore, addition of oxy-functional group to cyclohexane by breaking C-H bond selectively and keeping the cyclic ring unaffected is a great challenge because bond enthalpy of C-H is higher than that of C-C bond. Conversion of cyclohexane is compromised in attempt of increasing the selectivity of KA oil. As a result, it is mandatory to develop such heterogeneous catalysts that could oxygenate the cyclohexane selectively in vapor phase oxidation reaction. In this thesis, series of Fe-Mn oxide catalyst supported on -Al2O3 with changing metal ratios and total metal loadings were developed by using different method of synthesis. Also, the alumina support was modified with addition of varying composition of MgO and CeO2. Performance of the prepared catalysts were tested for selective vapor phase oxidation of cyclohexane in a differential reactor (High Vacuum Chamber – Diffuse Reflection Measurement) (HVC-DRM-5) and a downward flow packed bed reactor operating in continuous process. Various characterization techniques such as specific surface area (BET), X-ray diffraction (XRD), hydrogen-temperature programmed reduction (H2-TPR), ammonia- temperature programmed desorption (NH3-TPD), caron di-oxide temperature programmed desorption (CO2-TPD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), FT-IR spectroscopy, Raman spectroscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were employed to characterize the prepared catalysts. Further, the reaction parameters were optimized by conducting the reaction at different temperature (180 – 300 oC), oxygen to nitrogen ratio (0.75 – 2.0), contact time (11.67 kg-cat.s mol-1 to 70 kg-cat.s mol-1) at atmospheric pressure to increase the cyclohexane conversion and product selectivity. The stability and durability of the optimized catalysts were also tested. A series of catalyst zFexMn100-x/Al2O3 (x = 0, 25, 50, 75, and 100%, z = 5, 10, 15, and 25%) was synthesized by incipient wetness impregnation method. Catalytic performances of the prepared catalyst were evaluated for vapor phase cyclohexane oxidation reaction. The activity result demonstrated that the catalyst 20Fe50Mn50/Al2O3 was the most active and selective towards KA oil amongst the prepared catalysts. This catalyst exhibited maximum cyclohexane conversion of 2.43% with 81% selectivity towards KA oil at 220 oC, 1 atm pressure, and O2:N2 ratio of 1.5 in a differential reactor using 65 mg of the catalyst. Various characterization results revealed the formation of solid solution of Fe-Mn oxide possessing highest dispersion of Mn while the Fe remained in crystalline phase to some extent in the catalyst 20Fe50Mn50/Al2O3. Highest dispersion of metal oxides enhanced the mobility of lattice oxygen in the catalyst surface which resulted in highest activity and selectivity. Further, the in-situ DRIFTS study of the reaction and adsorption of vapor cyclohexane and gaseous CO2 was carried out over the prepared catalysts using the differential reactor (HVC-DRM-5) to comprehend the mechanistic pathway of the reaction. Throughout the investigation, the cyclohexanolate, phenolate, and unidentate carbonate species were noted. Reactive-cyclohexanol is created when the catalyst's lattice oxygen activates the cyclohexane's C-H bond. Reactive-cyclohexanol is then further broken down by dehydration and dehydrogenation. In addition, magnesia modified alumina with varying content of MgO was synthesized by incipient wetness and wet impregnation technique and used as support for the synthesis of supported 20 wt% Fe-Mn (1:1) oxide catalysts. In the vapour phase selective oxidation of cyclohexane, the impact of synthesis methods such as incipient wetness impregnation, wet impregnation, and sequential impregnation of iron and manganese over the modified supports was examined. The inclusion of MgO caused the development of MgAl2O4 spinel, which changed the acidic-basic characteristics and altered how the Fe-Mn metal oxides interacted with the alumina support. When the catalyst synthesized by sequential impregnation (SI-I) (addition of Mn followed by Fe) method, the iron and manganese oxides were observed in highly dispersed phase, however the iron oxide was found in their crystalline phase when the catalyst synthesized by SI-II (addition of Fe followed by Mn) method. The catalyst prepared by SI-I method exhibited highest activity and yielded to KA oil and remained stable for more than 10 h. The formation of adsorbed cyclohexanolate (and phenolate) species were observed as intermediate species during the in-situ DRIFTS studies for the reaction. The catalytic performance was also examined by varying the catalyst weight. The distribution of the product and the percent conversion of cyclohexane were dramatically affected by increasing the weight of the catalyst in the reactor. The MgO-modified catalyst and sequential impregnation of iron and manganese showed high conversion and KA selectivity. Further, the -Al2O3 support was promoted with varying amount of CeO2 to obtain a modified support xCeAl (x = 1, 2, 3, 4, 5, and 6 wt%). 20 wt % of Fe: Mn (1:1) was impregnated to 3CeAl to obtain highly active catalyst 20FeMnOδ/3CeAl. Reaction mechanism and chemical kinetics of the vapor phase cyclohexane (Cy-H) oxidation were investigated over the catalyst 20FeMnOδ/(3CeAl) using a continuous downward flow packed bed reactor with varying contact time from 1.7 h to 10.3 h, partial pressure of vapor cyclohexane from 1.3 kPa to 2 kPa, and reaction temperature from 180 oC to 240 oC at 101.325 kPa pressure. Moreover, an in-situ DRIFTS (adsorption and reaction) studies of reactant and product were conducted separately. In addition, the kinetic approach based on Power-Law and modified Mars-Van Krevelen (MVK) models were developed and validated with the experimental results. The power law model considering single step oxidation reaction suggested a pseudo first order (n = 1.12) reaction kinetics with respect to Cy-H concentration with activation energy of ~21.22 kJ/mol. A more realistic heterogeneous kinetic approach considering modified-MVK model in association with the observations based on the in-situ DRIFT spectroscopic studies revealed that the reaction was initiated by lattice oxygen present in the oxide catalyst, and a quasi-equilibrium state existed between the vapor cyclohexane (Cy-H) and adsorbed cyclohexane (Cy-H•O*) onto lattice oxygen which rapidly transformed into cyclohexanol (Cy-ol) as product by its rearrangement. The chemisorbed species (Cy-H•O*) interacted with the adjacent lattice oxygen to form cyclohexanolate (Cy-olate*) an intermediate species which further converted into cyclohexanone (Cy-one), and cyclohexene (Cy-ene) as products. The activation energies from the kinetic study were reported as Eact: ~42 kJ/mol (formation of Cy-ol) ~126 kJ/mol (formation of Cy-ene), ~108 kJ/mol (formation of Cy-one).