Oxidative Dehydrogenation Of Cyclohexane Over Mg-V-O Catalysts

The process of the oxidation of cyclohexane to cyclohexanone and cyclohexanol suffers from several problems, such as the lower cyclohexane conversion, lower product selectivity, higher energy consumption and the environmental pollution. Therefore, since the process of direct oxidation of cyclohexene to adipic acid proposed by Noyori et al. has been put forward and used in practice, cyclohexene is considered as the optimal raw material and the direct oxidation of cyclohexene process can be considered as the environmentally benign route in producing cyclohexanone, cyclohexanol and adipic acid. Cyclohexene can be obtained from two ways. One is the commercial process of partial hydrogenation of benzene to cyclohexene, which suffers from major problems of both low yield and low selectivity to cyclohexene. The other is the process of cyclohexane dehydrogenation. Since cyclohexane dehydrogenation to benzene is a thermodynamically favorable reaction, this process results in both high yield and high selectivity to benzene. To obtain cyclohexene as much as possible and to deal with cyclohexane from the partial hydrogenation of benzene, the oxidative dehydrogenation of cyclohexane to cyclohexene and benzene will be of critical importance in forming a new technology as indicated below.The oxidative dehydrogenation of cyclohexane over Mg-V-O catalysts was investigated in this thesis as the following aspects.Citric acid complexation under mild condition was proposed to prepare monophasic and well crystallized Mg3(VO4)2 particle to be used as an active catalyst for the oxidative dehydrogenation of cyclohexane. The catalyst characterization and the catalytic test results suggest the gel precursor might be (NH4)2[VO2(C6H6O7)]2-3MgC6H6O7, and the amount of citric acid and the calcination temperature are critical to the purity and the structure of Mg3(VO4)2. Among the catalysts tested, Mg3(VO4)2 prepared with the (Mg+V)/citric acid molar ratio of 1:1.2 and calcined at 823 K for 6 h exhibits the best catalytic performance with an excellent thermal stability.The catalytic active phase was identified among Mg3(VO4)2, Mg2V2O7 and MgV2O6 pure magnesium vanadates. The characterization and evaluation results show that Mg3(VO4)2 has the isolated active sites, weakly basic surface and lower reducibility of the metal cations, and could be recognized as the catalytic active phase.A series of mechanically mixed catalysts were studied in the reaction in attempting to investigate the biphasic synergetic effect. The finding reveals the purity and the composition of Mg-V-0 catalysts play an important effect on the overall catalytic performance in the oxidative dehydrogenation of cyclohexane.(1) MgO+Mg3(VO4)2 mixed catalysts system:When MgO content is less than 10 wt.%, the excess MgO might in a highly dispersion into Mg3(VO4)2. There is no synergetic effect between them. When MgO content is higher than 10 wt.%, the epitaxial phenomena might lead to the formation of coherent interface between MgO and Mg3(VO4)2, therefore, the catalytic activity is improved with a decreasing selectivity to cyclohexene.(2) Mg3(VO4)2+Mg2V2O7 mixed catalysts system:The biphasic synergetic effect might arise from the remote control mechanism between Mg3(VO4)2 and Mg2V2O7, which plays a positive effect on the catalytic performance in the oxidative dehydrogenation of cyclohexane. Mg3(VO4)2 acting as donor or accepter of the mobile oxygen is depended on the main phase in the mixed catalyst. A better catalytic behavior could be obtained over the Mg3(VO4)2+Mg2V2O7 mixed catalyst when the content of Mg3(VO4)2 was more than 70 wt.%.(3) MgV2O6+V2O5 mixed catalysts system:When V2O5 content was less than 60 wt.%, the synergetic effect might arise from the remote control mechanism, which plays a negative effect on the catalytic performance. When V2O5 content was higher than 60 wt.%, V2O5 would form a contamination layer on the entire catalyst to result in a similar catalytic performance with that of V2O5.In order to improve the selectivity and the yield of cyclohexene, the effects of alkali/alkaline earth metals (Li, Na, K and Ca) introduced to Mg3(VO4)2 and the introduction of the additives (steam, acetic acid and TCM) into the feedstream on the physicochemical properties of the catalyst and the catalytic behaviors in the oxidative dehydrogenation of cyclohexane were investigated.(1) The addition of alkali/alkaline earth metal to Mg3(VO4)2 affects the physicochemical nature and the catalytic behavior in the reaction. During the coordination of the additive with the surface active species, the additive blocked the active sites and hindered the reducibility of the active species thus decreasing the catalytic activity. Moreover, the additive could improve the selectivity to cyclohexene by changing the acid-basic nature, the redox property as well as the type and number of the oxygen species as investigated by H2-TPR and XPS characterization.(2) The effects of the addition of the additives on the catalytic behavior were also investigated. The addition of steam to the reacting mixture has a negative effect on the conversion due to significant decrease of active sites on the catalyst surface and the short of the retention time. The addition of TCM to the feedstream affects the removability of lattice oxygen in the catalyst, which results a high conversion of cyclohexane and a low selectivity to cyclohexene. The addition of acetic acid to cyclohexane plays the interesting effects. A lower addition amount of acetic acid only affects the isolation of the active sites and the acid-base property of catalyst surface, while a higher addition amount of acetic acid affects not only the acid-base property of the catalyst surface but also the degree of crystallinity and the grain size of Mg3(VO4)2 catalyst.Furthermore, the effects of the reaction temperature, W/F and molar ratio of cyclohexane to oxygen on the oxidative dehydrogenation of cyclohexane over Mg3(VO4)2 catalyst were obtained. The kinetic of the reaction was investigated using the fixed reactor in the case of eliminating external diffusion. A Power-Law type model was proposed and the kinetic parameters were estimated by nonlinear regression of the experimental data, and the predicted concentration profiles were in agreement with the experimental results.The research of this thesis exhibits that the application potentiality of Mg-V-0 catalyst. The the preparation method, catalytic active phase, and the biphasic synergetic effect between the different phases were investigated in order to obtain the optimal composition of Mg-V-0 catalysts. In order to obtain the higher selectivity and the yield of cyclohexene, the effects of the addition of alkali/alkaline earth metal to Mg3(VO4)2 and the introduction of the additive into the feedstream on the physicochemical properties of the catalyst and the catalytic behaviors in the oxidative dehydrogenation of cyclohexane were studied. Furthermore, the effects of the reaction temperature, W/F and molar ratio of cyclohexane to oxygen on the oxidative dehydrogenation of cyclohexane over Mg3(VO4)2 catalyst were obtained and the kinetic model of the reaction was proposed in order to give a better understanding of the oxidative dehydrogenation of cyclohexane over Mg-V-0 catalysts.