Vanadium Catalyst Mixed Ethylbenzene Dehydrogenation Process

Dehydrogenation of hydrocarbons is one of the challenges in the petroleum chemical industry, since it is often confined by the thermodynamic equilibrium. Styrene (ST) is one of the most important fundamental chemicals, which is used for the production of plastic, resin and synthetic rubber. The total output of styrene in the world is above 25 billion ton/a, and 90% of styrene is produced by catalytic dehydrogenation of ethylbenzene (EB) in the presence of steam, a high-energy-consuming and equilibrium-limited process.Oxidative dehydrogenation of EB has attracted much attention since the discovery of certain catalysts for the reaction in the early 1970s. Nevertheless, a considerable decrease in styrene selectivity owing to deep oxidation of hydrocarbons to carbon oxide makes it unpractical in economical point of view. The dehydrogenation process for the production of ST based on the selective oxidation of EB with the major global warming gas CO₂, has aroused widespread interest recently for its lower energy consumption and higher equilibrium yield of styrene. The usage of CO₂ instead of steam could provide several advantages such as reduction of the reaction temperature, remarkable energy saving in the distillation process of ST, restraining deactivation of catalysts to some degree, and so on. Numerous research works on this subject have been reported and the possibility of dehydrogenation of EB in the presence of CO₂ instead of O₂ has been acknowledged. Although the application of CO₂ is very effective, the catalysts deactivation mechanism and suitable measures to enhance their catalytic stability must be further investigated.Reaction coupling is one of the best substitutes of the commercial dehydrogenation, which can solve such problems and has been studied extensively in recent years. The mechanism of CO₂ oxidation is also discussed. It can be concluded that the remarkable promoting effect of CO₂ on the dehydrogenation of EB is due to both redox cycle of oxide catalyst and coupling of EB with reversed water gas shift reaction.In the present work, V₂O₅/γ-Al₂O₃ (V/Al) catalysts with different V loading, with different promoters were prepared by impregnation method. The catalysts were tested in the dehydrogenation of EB. The main contents of this thesis are as follows:1. V/Al catalysts were prepared by impregnation method, the catalysts were used in mild oxidative dehydrogenation of EB to ST in presence of CO₂. Effects of different catalysts on oxidative dehydrogenation were investigated, under reaction conditions: atmospheric pressure, 550℃, EB space velocity 20.4 mmol/g·cat·h. The results show that with increasing V loading the initial activity of V₂O₅/γ-Al₂O₃ catalysts in the dehydrogenation of ethylbenzene with CO₂ increases; whereas, the stability of these catalysts decreases. V(8%wt.)/Al has the best activity for dehydrogenation of EB. The addition of O₂ to the process improves the stability of the V/Al catalysts.2. The V/Al catalysts with different promoters were prepared by impregnation method. The results show that: alkali metal oxide and rare earth oxide were found being suitable promoters for V/Al catalysts. The V-Sb/Al catalyst affords the highest dehydrogenation activity, and good stability. The V(8%)Sb(9%)/Al catalyst has the highest ethylbenzene conversion and styrene selectivity. The initial activity of the catalysts in the dehydrogenation of ethylbenzene decreases by addition of O₂, whereas, the stability of these catalysts increase.3. The fresh and used catalysts were characterized using temperature-programmed reduction (TPR), X-ray photoelectron energy spectroscopy (XPS), X-ray diffraction (XRD), N₂ physical adsorption. The active center of dehydrogenation is belived to be related with V⁵⁺. Catalyst deactivation is mainly caused by coke deposition, which results in the decrease of the BET surface and pore volume. The addition of promoters can increase the BET surface and lead to higher dispersion of the active species and adjust the surface acidity and basicity. The V_1.1Sb_0.9O₄ phase was actually found to be the main component in the crystalline V-Sb bulk oxide system, this phase may be the active center of the V-Sb/Al catalysts. The addition of O₂ to the process improves the stability of the catalysts due to suppressing the coke deposition and keeping the vanadium species at high valence.