Dehydrogenation Of Ethylbenzene To Styrene In The Presence Of Carbon Dioxide

Styrene is one of the most important raw materials in the chemical industry, widely used as the monomers producing synthetic rubber, resin and plastic. It's commercially produced mainly by the catalytic dehydrogenation of ethylbenzene under a large amount of superheated steam, using potassium-promoted iron oxide catalysts. However, this process is limited by the thermodynamic equilibrium and have low equilibrium conversions of ethylbenzene. At the same time this process consumes too much energy and leads to high cost. Searching for alternative processes of ethylbenzene dehydrogenation is necessary. As a kind of greenhouse gas, CO2 has been arousing widespread interest in the green chemistry field for its mild oxidative property. Introducing CO2 into the reaction of ethylbenzene dehydrogenation not only will breakup the limitation of the thermodynamic equilibrium and improve the equilibrium conversion, but also will depress the energy consumption and the cost. What's more, the consumption of greenhouse gas in this reaction coincides well with the resource-saving and environment-protecting requirement of green chemistry. Thus, ethylbenzene dehydrogenation with CO2 has been the paid much attention. The ever reported catalysts used in ethylbenzene dehydrogenation in the presence CO2 contain active compositions such as chromia, vanadia, Ceria and Zirconia, with carbon, Al2O3 and silica as the supports. The catalyst synthesis, selecting of the reaction conditions and the role of CO2 have been well documented. The possibility of dehydrogenation of ethylbenzene in the presence of CO2 instead of steam has been acknowledged. However, there are still some problems to be further studied.The emphases of this thesis are to select appropriate catalyst supports, and attempt to understand the role of CO2 and the reaction characteristics over various catalysts. The main contents of this thesis are as follows:1.Mesoporous MCF was employed as the support to prepare VOx/MCF catalysts with different vanadium loading. The catalytic behaviors of VOx/MCF catalysts in the ethylbenzene dehydrogenation in the presence of CO2 were investigated at 550℃and compared with that of VOx/MCM-41.The well-defined three-dimensional mesoporous structure with uniform pore diameter of The MCF support is retained when the vanadium loading is under 6%, getting catalysts with high suface area. The vanadium species in the VOx/MCF catalysts with V loading under 6% is highly dispersed on the surface of MCF support. Higher V loading in the catalyst leads to the formation of bulk-like V2O5 crystallites. The VOx/MCF catalyst with V loading of 6% exhibits the highest activity and EB conversion of 70.7% with styrene selectivity of 98.0% was achieved on this catalyst at 550℃with the partial pressure of CO2 is 14 kPa, the optimum partial pressure of CO2. The main promoting effort of CO2 in the dehydrogenation of ethylbenzene is behaved through following two ways:first, driving the oxidative dehydrogenation of ethylbenzene as a kind of mild oxidant; second, consuming the hydrogen formed in the sample dehydrogenation of ethylbenzene through reverse water-gas shift reaction, moving the reaction equilibrium to the productive direction. The activity of VOx/MCF catalyst is obviously higher than that of the VOx/MCM-41 catalyst, indicating that the MCF material with three-dimensional mesoporous structure and relatively lager pore diameter is better support than MCM-41 having one-dimensional mesoporous structure and relatively smaller pore diameter.2.Mesoporous HMS was employed as the support to prepare VOX/HMS catalysts with different vanadium loading. The catalytic behaviors of VOx/HMS catalysts in the ethylbenzene dehydrogenation in the presence of CO2 were investigated at 550℃and compared with that of VOx/MCF. The well-defined Hexagonal mesoporous with uniform pore diameter of The HMS support is retained when the vanadium loading is under 4%, getting catalysts with high surface area. The vanadium species in the VOx/HMS catalysts with V loading under 6% is highly dispersed on the surface of HMS support. Higher V loading in the catalyst leads to the formation of bulk-like V2O5 crystallites. The VOx/HMS catalyst with V loading of 4% exhibits the highest activity and EB conversion of 74.6% with styrene selectivity of 98.2% was achieved on this catalyst at 550℃with the partial pressure of CO2 is 14 kPa, the optimum partial pressure of CO2. The main promoting effort of CO2 in the dehydrogenation of ethylbenzene is behaved through following two ways:first, driving the oxidative dehydrogenation of ethylbenzene as a kind of mild oxidant; second, consuming the hydrogen formed in the sample dehydrogenation of ethylbenzene through reverse water-gas shift reaction, moving the reaction equilibrium to the productive direction. The activity of VOX/HMS catalyst is obviously higher than that of the VOX/MCF catalyst, which is maybe due to the shorter wormhole structure of HMS support, which is more favorable to the diffusion of reaction and production molecular.3.Mesoporous ZSM-5 (Marked as M-NaZ5) was employed as the support to prepare VOx/M-NaZ5 catalysts with different vanadium loading. The catalytic behaviors of VOx/M-NaZ5 catalysts in the ethylbenzene dehydrogenation in the presence of CO2 were investigated at 550℃and compared with that of VOx/NaZSM-5. The crystalline structure was well reserved and no bulk-like V2O5 crystalite was detected even when the V loading is as high as 8%, indicating the dispersing property of Vanadia on the surface of mesoporous ZSM-5 are better than that on the surface of microporous ZSM-5 supports.The catalysts showed preferable activities used in the dehydrogenation of ethylbenzene in the presence of CO2. The catalyst with the Si/Al ratio of 31 showed the highest activity when the V loading of which is 6%, on which the conversion of EB is 62.4% and the selectivity of styrene is 98.2%(0.5 h). CO2 behaved evident promotion in the dehydrogenation of ethylbenzene. The catalytic activity of the V loading mesoporous ZSM-5 catalyst on the dehydrogenation of ethylbenzene is evidently higher than that of V loading microporous ZSM-5 catalyst, and one of the reason could be that the inner-crystal mesopore is more favorable for the diffusion of reactants and products.