Dehydrogenation Of Ethylbenzene To Styrene In The Presence Of Carbon Dioxide

Styrene is one of the most important monomers in the petrochemical industry, which is used for the production of plastic, resin and synthetic rubber. It is commercially produced mainly by the catalytic dehydrogenation of ethylbenzene using potassium-promoted iron oxide catalysts in the presence of a large amount of superheated steam. The main drawbacks of this present industrial process are equilibrium limitations regarding conversion, large energy consumption and high cost. Hence, a worldwide search for alternative processes is underway. Dehydrogenation of ethylbenzene in the presence of CO₂ has aroused widespread interest recently for its lower energy consumption and higher equilibrium yield of styrene. Major previous work of ethylbenzene dehydrogenation in the presence of CO₂ was focused on chromia and vanadia catalysts, with carbon and Al₂O₃ 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 MCM-41 was employed as the support to prepare VO_x/MCM-41 catalysts with different vanadium loading. The structural and physicochemical properties were well characterized. The catalytic behaviors of VO_x/MCM-41 catalysts were investigated at 550℃. The ordered hexagonal mesoporous structure with uniform pore diameter of the MCM-41 support is retained upon the vanadium incorporation, and the surface area, pore volume and pore diameter decrease with increasing the V loading. The vanadium species in the VO_x/MCM-41 catalysts with V loading < 1.0 mmol (g-MCM-41)^-1 exists mainly in a highly dispersed form of isolated and low-polymeric VO_x species on the MCM-41 support. Higher V loading in the catalyst leads to the formation of bulk-like V₂O₅ crystallites. The VO_x/MCM-41 catalyst with V loading of 1.5 mmol (g-MCM-41)^-1 exhibits the highest activity. EB conversion of 73.2% with styrene selectivity of 98.5% was achieved on this catalyst at 550℃and W/F = 180 (g-cat) h mol^-1. The optimum partial pressure of CO₂ is 14 kPa. The activity of VO_x/MCM-41 catalyst is obviously higher than that of the VO_x/SiO₂ catalyst prepared by using traditional Si_O2 as the support, which is due to high surface area and mesoporous channels with uniform size of the MCM-41 support. During ethylbenzene dehydrogenation in the presence of CO₂ over VO_x/MCM-41, both one-step pathway (i.e., direct oxidative dehydrogenation of ethylbenzene with CO₂) and two-step pathway (i.e., simple dehydrogenation of ethylbenzene thermodynamically assisted by reverse water-gas shift reaction) occurred. The former was confirmed by the TPR measurement, while the latter was evidenced by a separate experiment of single reverse water-gas shift reaction. 41% of styrene might be produced by the one-step pathway, 59% by the two-step pathway. Different reaction pathways in the presence and in the absence of CO₂ as well as a higher surface amount of active V⁵⁺ species under CO₂ atmosphere are responsible for the positive effect of CO₂ in the dehydrogenation of ethylbenzene.2. Mesoporous HMS was employed as the support to prepare Cr₂O₃/HMS catalysts with different Cr₂O₃ loading. The structural and physicochemical properties were well characterized. The catalytic behaviors of Cr₂O₃/HMS catalysts were investigated at 550℃. Moreover, the catalytic activity of Cr₂O₃/HMS was compared with Cr₂O₃/MCM-41. Hexagonal mesoporous structure with uniform pore diameter of the HMS support is retained upon the chromium incorporation, and therefore high surface areas were obtained on the final catalysts. Chromium species are highly dispersed on the surface of the HMS support, when the Cr₂O₃ loading is below 3%. Higher Cr₂O₃ loading in the catalyst leads to the formation of bulk-like crystallites. The activity of Cr₂O₃/HMS catalysts is strongly dependent on the loading. The Cr₂O₃/HMS catalyst with 3% Cr₂O₃ loading exhibits the highest activity. EB conversion of 61.1% with styrene selectivity of 98.6% was achieved on this catalyst at 550℃and W/F = 180 (g-cat) h mol^-1. Compared with Cr₂O₃/MCM-41, Cr₂O₃/HMS catalyst exhibits higher activity, mainly because the shorter channels of HMS facilitate the diffusion of reactants and products. During ethylbenzene dehydrogenation in the presence of CO₂ over Cr₂O₃/HMS, both one-step pathway (i.e., direct oxidative dehydrogenation of ethylbenzene with CO₂) and two-step pathway (i.e., simple dehydrogenation of ethylbenzene thermodynamically assisted by reverse water-gas shift reaction) occurred. The former was confirmed by the XPS measurement, while the latter was evidenced by a separate experiment of single reverse water-gas shift reaction. 66% of styrene might be produced by the one-step pathway, 34% by the two-step pathway. The percentage of one-step pathway is higher for the Cr₂O₃/HMS catalyst than Cr₂O₃/MCM-41. The reason could be that the oxidation ability of chromia is higher than that of vanadia. Different reaction pathways in the presence and in the absence of CO₂ as well as a higher surface amount of Cr⁶⁺ species under CO₂ atmosphere are responsible for the positive effect of CO₂ in the dehydrogenation of ethylbenzene.