| Propylene, only less important than ethylene, is one of the most important chemical raw materials, which can be used for synthesis of polypropylene, acrolein, acrylic acid, epoxy propane and so on. It can also produce a series of derivatives, such as plastic, polypropylene fiber, organic glass epoxy resin and so on. The worldwide demand for propylene is very large, and the trend has been increasing year by year. In China, the supply of propylene is strongly dependent on imports. Moreover, in domestic market demand exceeds supply. Currently, there are generally three ways to produce propylene industrially:the steam cracking, fluid catalytic cracking and propane dehydrogenation. The former two methods consume petroleum, which require high reaction temperature and huge energy consumption. With the petroleum resource exhausting and the price soaring, people start to be interested in dehydrogenation of propane with rich source to propylene.The process of propane pure dehydrogenation to propylene has been realized industrialization. However, this process has some inherent drawbacks, such as thermodynamic limitations for propane conversion, high energy requirements due to high reaction temperature and limited catalytic stability owing to coke formation. The utilization of carbon dioxide can improve the equilibrium conversion of dehydrogenation, and lower the reaction temperature. The coke can be eliminated by carbon dioxide to improve the catalyst stability. Moreover, CO2as a weak oxidant can give rather high propylene selectivity due to the lack of deep oxidation of propane. In addition, as the main greenhouse gase, the use of carbon dioxide also has positive significance on the aspect of environmental protection. The new process of the propane dehydrogenation to propylene in the presence of carbon dioxide has been gradually gained widespread concern.The active components of the catalysts for dehydrogenation of propane to propylene in the presence of carbon dioxide are mainly focused on the metal oxides of Cr and Ga. The main catalyst supports are y-Al2O3, SiO2, mesoporous SiO2(MCM-41, MSU-x, SBA-15and so on) ZSM-5and ZrO2. The preparation method of catalyst is basically a wet impregnation method, but the hydrothermal preparations of catalyst for the reaction are reported rarely. In the present work, we demonstrate that Cr2O3-ZrO2mixed oxide materials were prepared by a hydrothermal method, which was modified with CeO2, La2O3andY2O3to enhance the stability of catalysts.The purpose of this dissertation is to study in relation to their performance in the dehydrogenation of propane to propylene with CO2, and study the structure-activity relationship according to the characterization results. The main content of this dissertation is summarized as follows.Part I:Hydrothermally prepared Cr2O3-ZrO2catalystsCr2O3-ZrO2mixed oxides were prepared by a hydrothermal method. There is a good relationship between the initial activity and Cr6+content of the catalysts, showing that a high number of Cr6+species in the calcined catalyst is crucial for its high catalytic activity for dehydrogenation of propane to propylene in the presence of CO2. The highest activity is achieved on the catalyst with a Cr content of10%. In this case, the material prepared from a hydrothermal treatment at180℃the initial propane conversion and propylene yield at550℃are53.3%and42.1%, while the data become27.8%and25.2%after6h on stream, which exhibits obviously higher catalytic activity than the conventional Cr2O3-ZrO2, whose initial propane conversion and propylene yield of33.6%and28.4%respectively, response data were12.5%and11.9%respectively after6hours. The catalysts prepared by a hydrothermal method exhibit obviously higher activity than the conventional one (no hydrothermal treatment) due to the more Cr6+content in the former catalyst. The reason of catalyst deactivation is coking and reduction of Cr6+, the latter reason was proved by the H2pre-reduction experiments. The propylene yields of10CZ-180catalyst are76.5%and3.0%, respectively after the initial time and8h on stream in absence of CO2, which means the stability of the catalyst is obviously lower than those in the presence of CO2. One reason is that CO2can retain a higher surface concentration of Cr6+on the catalyst. The other reason is that CO2can eliminate part of the coke on the catalyst surface through the Boudouard reaction during the propane dehydrogenation. The regeneration performance of the catalyst is good, and the initial activity can be fully restored after deactivation regeneration by air. But the activity was recovered insufficiently after deactivation regeneration by CO2, because the amount of Cr6+cannot restore completely and carbon deposition cannot fully be eliminated.Part ⅡI:Using hydrothermal method for the highest reactivity Cr2O3-ZrO2catalyst (Cr content is10%, and hydrothermal treatment temperature is180℃) was modified with CeO2, La2O3and Y2O3; rare-earth oxides improved the catalyst stability in the propane dehydrogenation to propylene CO2. The catalyst showing the best activity in the last chapter was modified by CeO, La2O3andY2O3. For the catalyst modified by CeO2, when Ce content is5%, the highest propane conversion and propylene yield after6h on stream over this catalyst at550℃are31.1%and28.2%; For the catalyst modified by La2O3, when La content is3%, the highest propane conversion and propylene yield after6h on stream over this catalyst at550℃are32.8%and29.3%; For the catalyst modified by Y2O3, when Y content is5%, the highest propane conversion and propylene yield after6h on stream over this catalyst at550℃are31.6%and28.4%, which are higher than no-modified Cr2O3-ZrO2catalyst (propane conversion and propylene yield of27.8%and25.2%respectively). One cause for stability improvement is that introducing rare-earth oxides can retain a higher surface concentration of Cr6+on the catalyst.The other cause is that introducing rare-earth oxides can reduce the acidity of the catalyst, and decrease amount of carbon deposited on the catalyst surface. Rare-earth oxides can improve the regeneration of catalysts for carbon dioxide atmosphere. The catalyst activity recovery of Cr2O3-ZrO2modified with CeO2, La2O3and Y2O3after CO2regeneration was92%,92%and95%, respectively, were higher than non-doped Cr2O3-ZrO2catalyst (initial activity was returned to81%).This may be related to adding rare-earth oxide additives to improve the ability of catalyst for CO2activation. |
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