Study On The Catalysts Used ZSM-5Zeolite As Support For Dehydrogenation Of Propane To Propylene In The Presence Of Carbon Dioxide

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, Ga and Zn. The main catalyst supports are γ-Al2O3, SiO2, mesoporous SiO2(MCM-41, SBA-15and so on) and ZrO2. The catalyst deactivation is still faster, and mesoporous silica materials are poor in hydrothermal stability. ZSM-5zeolite with MFI structure is a kind of microporous crystal, which has a larger surface area and high hydrothermal stability. It is widely used as catalysts or catalyst supports. Previous work in our laboratory showed that the supported Ga2O3or ZnO catalyst on HZSM-5zeolite which was reduced the acidic sites and weakened the acid strength by increasing the Si/Al ratio and P modification exhibited high stability and high propylene yield for dehydrogenation of propane to propylene in the presence of carbon dioxide. However, the selectivity of the target product propylene is lower. On the basis of the previous work of our laboratory, the purpose of this dissertation is to improve the propylene selectivity and propylene yield in the premise of good stability, and study the structure-activity relationship according to the characterization results. The main content of this dissertation is summarized as follows.Part I:Steaming-treated HZSM-5supported zinc oxide catalystHZSM-5with molar Si/Al ratio of25was steaming treated at600℃,650℃and700℃, and used as the support to prepare supported ZnO catalysts. The XRD results indicate that MFI structure of HZSM-5is not damaged after high-temperature steaming treatment. The27Al MAS NMR results show that HZSM-5is dealuminated by steaming treatment, and a part of the four-coordinate framework aluminums are transformed to the six-coordinated non-framework ones. Pyridine adsorption IR spectra and cumene cracking reaction data reveal that Br(?)nsted acid sites of HZSM-5zeolite are significantly reduced by the steaming treatment, leading to greatly improving the propylene yield and stability of the supported ZnO catalysts for propane dehydrogenation to propylene in the presence of carbon dioxide. As the steaming temperature is increased from600℃to700℃, the Br(?)nsted acid sites of HZSM-5are gradually decreased, whereas the dispersion of ZnO on support surface is gradually increased. The two opposite variation trends lead to the fact that the ZnO catalyst supported on HZSM-5being steamed at650℃shows the best catalytic performance. When the Zn content is4%, the initial propane conversion, propylene selectivity and propylene yield at600℃are54.3%,54.7%and29.7%, while the data become30.9%,65.6%and20.3%after30h on stream. After the regeneration of the catalyst by burning coke in air, the initial propylene yield is restored completely, indicating that coking is the major cause of the deactivation. CO2improves the stability of the catalyst by eliminating part of the coke on the catalyst surface through the Boudouard reaction during the propane dehydrogenation.Part Ⅱ:Small crystalline NaZSM-5supported zinc oxide catalystNaZSM-5zeolites with the size of200-400nm and the molar Si/Al ratio of60,120,160and200were synthesized via the hydrothermal method, and used as the support to prepare supported ZnO catalysts. The n-propanol dehydration data prove that there are still a small number of acid sites present on the surface of NaZSM-5, which is reduced with an increase in the Si/Al ratio. The XPS results show that the dispersion of ZnO on NaZSM-5surface becomes better with an increase in the Si/Al ratio. The two opposite variation trends lead to the fact that the best catalytic performance is achieved on the supported ZnO catalyst with a Zn content of3%and molar Si/Al ratio of160for dehydrogenation of propane to propylene in the presence of CO2. The initial propane conversion, propylene selectivity and propylene yield at600℃are47.7%,92.8%and44.3%, while the data become42.0%,93.9%and39.4%after8h on stream. In contrast, the ZnO catalyst supported on HZSM-5with a molar Si/Al ratio of160(Zn content of3%) gives a high propane conversion. The initial propane conversion at600℃is93.7%, and decreases to75.5%after8h on stream. However, the propylene selectivity and propylene yield decrease significantly, being12.2%and11.4%at the initial stage as well as34.9and26.4%after8h on steam. The reason is that HZSM-5zeolite has more acid sites, leading to a large number of aromatics generated. The crystal size of NaZSM-5zeolite has a great influence on the activity of supported ZnO catalyst. The ZnO catalyst supported on NaZSM-5zeolite with a size of2-3μm and a molar Si/Al ratio of160(Zn content of3%) gives a propylene yield as low as about2%. The reason is that the surface silanols and the hydrogen bonded silanol nests are much more abundant for small crystalline NaZSM-5than large crystalline NaZSM-5, leading to better dispersion of ZnO on the zeolite surface. The regeneration experimental results show that coking is the major cause of catalyst deactivation. CO2improves the stability of the catalyst significantly by eliminating part of the coke on the catalyst surface through the Boudouard reaction during the propane dehydrogenation.Part Ⅲ:Small crystalline NaZSM-5supported gallium oxide catalystNaZSM-5zeolites with the size of200-400nm and the molar Si/Al ratio of100,160,200and300were synthesized via the hydrothermal method, and used as the support to prepare supported Ga2O3catalysts. The catalytic activity for dehydrogenation of propane to propylene in the presence of CO2first increases, and then decreases with an increase in the molar Si/Al ratio of NaZSM-5. The best Si/Al ratio is200. When the Ga content is3%, the initial propane conversion, propylene selectivity and propylene yield at600℃are62.0%,44.0%and27.3%, while the data become54.9%,48.5%and26.6%after8h on steam. The propylene yield keeps at about27%during the8h reaction. The crystal size of NaZSM-5zeolite has a great influence on the activity of supported Ga2O3catalyst. The Ga2O3catalyst supported on NaZSM-5zeolite with a size of ca.5μm and a molar Si/Al ratio of200(Ga content of3%) gives a propylene yield as low as2.5%. The reason is that the surface silanols and the hydrogen bonded silanol nests are much more abundant for small crystalline NaZSM-5than large crystalline NaZSM-5, leading to better dispersion of ZnO on the zeolite surface, which is confirmed by the results of n-propanol dehydration. Introducing a small amount of CaO into the3%Ga/NaZSM-5(200) catalyst reduces the propane conversion, but it also can effectively reduce the acidity of the catalyst, so as to improve the propylene selectivity. When the Ca content is0.1%, the propylene selectivity is improved greatly, and the propylene yield just declines slightly, which are70.9%and25.8%, respectively, after8h on stream.Part IV:Small crystalline NaZSM-5supported chromium oxide catalystNaZSM-5zeolites with the size of200-600nm and the molar Si/Al ratio of30,60,120and200were synthesized via the hydrothermal method, and used as the support to prepare supported chromium oxide catalysts. 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 of3%and Si/Al ratio of60. In this case, the initial propane conversion, propylene selective and propylene yield at550℃are48.3%,86.0%and41.5%, while the data become30.1%,91.8%and27.6%after8h on stream. In contrast, the chromim oxide catalyst supported HZSM-5zeolite with a molar Si/Al ratio of60(Cr content3%) displays not only lower activity but also lower propylene selectivity than the3%Cr/NaZSM-5(60) catalyst. The initial propane conversion, propylene selective and propylene yield are36.4%,76.0%and28.0%, while the data become24.9%,79.9%and19.9%after8h on stream. The lower activity is due to the less Cr6+content in the catalyst. The lower propylene selectivity is due to the more acid sites present on HZSM-5than on NaZSM-5, leading to the increased cracking by-products. The crystal size of NaZSM-5zeolite has a great influence on the activity of supported chromium oxide catalyst. The chromim oxide catalyst supported on NaZSM-5zeolite with a size of2-3μm and a molar Si/Al ratio of60(Cr content of3%) gives a propylene yield of less than11%. The reason is that the surface silanols and the hydrogen bonded silanol nests are much more abundant for small crystalline NaZSM-5than large crystalline NaZSM-5, leading to better dispersion of chromim oxide on the zeolite surface and a higher content of Cr6+in the 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 of3%Cr/NaZSM-5(60) catalyst are37.5%and23.1%, respectively after the initial time and8h on stream in absence of CO2, which are obviously lower than those in the presence of CO2. This indicates that CO2has a promoting effect on the propane dehydrogenation over this catalyst. One reason is that CO2can improve the equilibrium conversion of propane by eliminating H2generated in propane dehydrogenation via the reverse water-gas shift reaction. The other reason is that CO2can retain a higher surface concentration of Cr6+on the catalyst. CO2improves the stability of the catalyst by eliminating 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 the second time deactivation regeneration.Part Ⅴ:Gallium oxide modified small crystalline NaZSM-5supported chromium oxide catalystThe3%Cr/NaZSM-5(60) catalyst showing the best activity in the last chapter was modified by gallium oxide using the impregnation method. The best result is obtained when Ga content is1.5%. The initial propane conversion, propylene selective and propylene yield over this catalyst at550℃are50.8%,87.6%and44.5%, while the data become35.8%,93.0%and33.3%after8h on stream. Gallium oxide modification not only increases the activity, but also improves the propylene selectivity and stability. The reason for activity enhancement is that gallium oxide has also the ability to dehydrogenate propane. The cause for propylene selectivity improvement is that introducing a small amount of gallium oxide can reduce the acidity and increase the basicity of the catalyst, thus accelerating the desorption of propylene from the catalyst surface. The propylene yields of1.5%Ga3%Cr/NaZSM-5(60) catalyst are41.7%and27.7%, respectively, after the initial time and8h on stream in the absence of CO2, which are obviously lower than those in the presence of CO2. The reasons are that CO2can eliminate H2generated in propane dehydrogenation via the reverse water-gas shift reaction, and retain a higher surface concentration of Cr6+on the catalyst.