【作者】 张帆；

【导师】 乐英红； 华伟明；

【作者基本信息】 复旦大学 ， 物理化学， 2012， 博士

【摘要】 丙烯是仅次于乙烯最重要的基本有机化工原料之一,可用于合成聚丙烯、丙烯醛、丙烯酸和环氧丙烷等材料,还可生产一系列衍生物,如塑料、丙纶、有机玻璃和环氧树脂等。全球范围内对丙烯的需求量非常大,并且呈现逐年递增的趋势,我国的丙烯供应严重依赖进口,而且国内市场供不应求。目前,世界上丙烯的来源主要有三方面：蒸汽裂解、炼油厂流化床的催化裂化和丙烷脱氢。由于前两种方法消耗石油,且反应温度高,能耗巨大,随着石油资源的日益枯竭和石油价格的飞涨,人们开始对由来源丰富的丙烷脱氢制丙烯感兴趣。丙烷纯脱氢制丙烯工艺已经实现了工业化,该工艺的主要缺点是由于受热力学条件的限制,需要高温的反应条件,能耗大,同时催化剂失活很快。CO2的引入可以提高平衡转化率,使反应温度降低；同时二氧化碳可以消除表面积炭,提高催化剂的稳定性；并且二氧化碳作为弱氧化剂可以防止深度氧化,保证目标产物丙烯的高选择性；另外,作为主要的温室气体,二氧化碳的利用在环保理念上也有一定的积极意义。因此二氧化碳气氛下丙烷脱氢制丙烯的新工艺逐渐受到人们的广泛关注。有关二氧化碳气氛下丙烷脱氢制丙烯催化剂的研究,活性组分主要集中在Cr、Ga和Zn的金属氧化物,载体主要选择γ-Al2O3、SiO2、介孔SiO2(MCM-41和SBA-15等)和ZrO2,但催化剂失活仍然较快,而且介孔氧化硅材料的水热稳定性不高。ZSM-5沸石是具有MFI结构的微孔晶体,拥有较大的比表面和较高的水热稳定性,被广泛用作催化剂和催化剂载体。我们实验室以前的研究工作表明,通过提高Si/Al和P修饰等方法减少HZSM-5沸石的酸性位和减弱其酸强度,进一步负载Ga2O3或ZnO制成的负载型催化剂对二氧化碳气氛下丙烷脱氢制丙烯反应具有很高的稳定性和较高的丙烯得率,但是目标产物丙烯的选择性较低。本论文在实验室已有工作基础上,致力于催化剂具有较好稳定性的前提下提高丙烯选择性和丙烯得率,并根据催化剂表征结果进行构效关系的研究。论文的具体内容如下：一、高温水蒸气处理的HZSM-5负载氧化锌催化剂的研究对Si/Al为25的HZSM-5沸石分别进行600℃、650℃和700℃高温水蒸气处理,并以此为载体制备负载型ZnO催化剂。XRD结果表明高温水蒸气处理后HZSM-5的MFI结构没有遭到破坏,27AlMAS NMR结果表明高温水蒸气处理使HZSM-5发生脱铝,部分四配位骨架铝脱出形成六配位非骨架铝。吡啶吸附红外和异丙苯裂解反应数据表明高温水蒸气处理使HZSM-5沸石载体的Br(?)nsted酸性位明显减小,从而大大提高了负载型ZnO催化剂对二氧化碳气氛下丙烷脱氢制丙烯反应的稳定性和丙烯得率。随着水蒸气处理温度从600℃升高至700℃,HZSM-5沸石载体的Br(?)nsted酸性位逐渐减少,而ZnO在载体表面的分散度逐渐提高。两者相反的变化趋势导致650℃水蒸气处理的HZSM-5负载ZnO催化剂表现出最佳的催化效果,当Zn含量为4%时,600℃反应的初始丙烷转化率、丙烯选择性和得率分别为54.3%、54.7%和29.7%,反应30小时后的数据分别为30.9%、65.6%和20.3%。催化剂经过空气烧炭再生,初始丙烯得率能完全恢复,说明积炭是催化剂失活的主要原因,CO2在丙烷脱氢反应中通过Boudouard反应消除催化剂的部分表面积炭,提高了催化剂的稳定性。二、小晶粒NaZSM-5负载氧化锌催化剂的研究用水热法合成了晶粒大小为200-400nm和Si/A1分别为60、120、160和200的NaZSM-5沸石,并以此为载体制备负载型ZnO催化剂。尽管是Na型ZSM-5,其表面仍然残留了少部分酸性位,正丙醇脱水数据表明,NaZSM-5沸石载体的酸性位随着硅铝比的升高而减少,XPS结果表明ZnO在NaZSM-5沸石表面的分散度随着硅铝比的增加而升高。两者相反的变化趋势导致硅铝比为160的NaZSM-5负载ZnO催化剂对二氧化碳气氛下丙烷脱氢制丙烯反应表现出最佳的催化效果,Zn的最佳含量为3%,600℃反应时该催化剂的初始丙烷转化率、丙烯选择性和得率分别为47.7%、92.8%和44.3%,反应8小时后的数据分别为42.0%、93.9%和39.4%。硅铝比为160的H型ZSM-5沸石负载ZnO催化剂(Zn含量3%)虽然具有很高的丙烷转化率,初始和反应8小时后的数据分别为93.7%和75.5%,但丙烯选择性和丙烯得率大幅度降低,初始和反应8小时后的数据分别为12.2%、11.4%%和34.9、26.4%,原因是HZSM-5沸石载体有较多的酸性位,导致了大量芳烃生成。NaZSM-5沸石载体晶粒大小对负载型ZnO催化剂的反应活性有很大影响,晶粒尺寸2-3μm和硅铝比为160的NaZSM-5负载ZnO催化剂(Zn含量3%)丙烯得率仅为2%左右,原因是小晶粒NaZSM-5的外表面硅羟基和硅羟基空穴远远多于大晶粒NaZSM-5,有利于ZnO在沸石表面的分散。再生实验结果表明积炭是催化剂失活的主要原因,CO2在丙烷脱氢反应中通过Boudouard反应消除催化剂的部分表面积炭,大大提高了催化剂的稳定性。三、小晶粒NaZSM-5负载氧化镓催化剂的研究用水热法合成了晶粒大小为200-400nm和Si/Al分别为100、160、200和300的NaZSM-5沸石,并以此为载体制备负载型Ga203催化剂。催化剂对二氧化碳气氛下丙烷脱氢制丙烯的反应活性随着NaZSM-5沸石载体硅铝比的升高先增加后下降,最佳硅铝比为200,Ga含量为3%时600℃反应的初始丙烷转化率、丙烯选择性和得率分别为62.0%、44.0%和27.3%,反应8小时后的数据分别为54.9%、48.5%和26.6%,反应8小时内丙烯得率始终保持在27%左右。NaZSM-5沸石载体晶粒大小对负载型Ga203催化剂的反应活性有很大影响,晶粒尺寸5μm左右和硅铝比为200的NaZSM-5负载Ga203催化剂(Ga含量3%)丙烯得率仅为2.5%左右,原因是小晶粒NaZSM-5的外表面硅羟基和硅羟基空穴远远多于大晶粒NaZSM-5,提高了Ga203在沸石表面的分散,分散度的提高由正丙醇脱水数据所证实。将少量CaO引入3%Ga/NaZSM-5(200)催化剂中,虽然降低了丙烷转化率,但可以有效降低催化剂的酸性,从而提高丙烯选择性,当Ca含量为0.1%时,丙烯选择性提高较大,而丙烯得率只是略有下降,反应8小时后的丙烯选择性和得率分别为70.9%和25.8%。四、小晶粒NaZSM-5负载氧化铬催化剂的研究用水热法合成了晶粒大小为200-600nm和Si/Al分别为30、60、120和200的ZSM-5沸石,并以此为载体制备负载型氧化铬催化剂。催化剂中的Cr6+含量和初始反应活性具有良好的对应关系,表明高Cr6+含量对催化剂表现出高的丙烷脱氢活性至关重要。最佳Cr含量和最佳硅铝比分别为3%和60,550℃该催化剂对二氧化碳气氛下丙烷脱氢制丙烯反应的初始丙烷转化率、丙烯选择性和得率分别为48.3%、86.0%和41.5%,反应8小时后的数据分别为30.1%、91.8%和27.6%。硅铝比为60的H型ZSM-5沸石负载氧化铬催化剂(Cr含量3%)不但活性明显低于3%Cr/NaZSM-5(60)催化剂,丙烯选择性也明显小于后者,初始丙烷转化率、丙烯选择性和得率分别为36.4%、76.0%和28.0%,反应8小时后的数据分别为24.9%、79.9%和19.9%,活性低的原因是催化剂中Cr6+含量较少,丙烯选择性小的原因是HZSM-5载体的酸性位多于NaZSM-5,导致裂解副产物增多oNaZSM-5沸石载体晶粒大小对负载型氧化铬催化剂的反应活性有很大影响,晶粒尺寸2-3gm和硅铝比为60的NaZSM-5负载氧化铬催化剂(Cr含量3%)丙烯得率低于11%,原因是小晶粒NaZSM-5的外表面硅羟基和硅羟基空穴远远多于大晶粒NaZSM-5,提高了氧化铬在沸石表面的分散和催化剂中Cr6+的含量。催化剂失活的原因是积炭和Cr6+的还原,H2预还原实验证实了Cr6+还原也是催化剂失活的一个原因。3%Cr/NaZSM-5(60)催化剂在没有CO2时的初始和反应8小时后的丙烯得率分别为37.5%和23.1%,明显低于有CO2时的丙烯得率,说明该催化剂上CO2对丙烷脱氢反应具有促进作用,原因之一是CO2通过逆水煤气变换反应消除丙烷脱氢生成的H2,推动了脱氢反应,原因之二是CO2气氛下催化剂表面保持更多量的Cr6+。CO2在丙烷脱氢反应中通过Boudouard反应消除催化剂的部分表面积炭,使得催化剂的稳定性有所提高。催化剂的再生性能良好,经两次失活再生后,催化剂的初始活性可以完全恢复。五、氧化镓修饰的小晶粒NaZSM-5负载氧化铬催化剂的研究采用浸渍法对在上一章中得到的反应活性最高的3%Cr/NaZSM-5(60)催化剂进行氧化镓修饰。当Ga含量为1.5%时修饰效果最好,对二氧化碳气氛下丙烷脱氢制丙烯反应的初始丙烷转化率、丙烯选择性和得率分别为50.8%、87.6%和44.5%,反应8小时后的数据分别为35.8%、93.0%和33.3%。氧化镓修饰不但提高了反应活性,而且提高了丙烯选择性和稳定性,反应活性提高的原因是氧化镓也具有丙烷脱氢性能,丙烯选择性提高的原因是氧化镓修饰降低了催化剂的酸性,增加了催化剂的碱性,从而加快了丙烯产物从催化剂表面的脱附。1.5%Ga3%Cr/NaZSM-5(60)催化剂在没有CO2时的初始和反应8小时后的丙烯得率分别为41.7%和27.7%,明显低于有CO2时的丙烯得率,说明该催化剂上CO2对丙烷脱氢反应具有促进作用,这是由于CO2通过逆水煤气变换反应消除丙烷脱氢生成的H2以及CO2气氛下催化剂表面有更多量的Cr6+。更多还原

【Abstract】 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.更多还原