【作者】 史雪君；

【导师】 季生福；

【作者基本信息】 北京化工大学 ， 化学工程与技术， 2009， 博士

【摘要】 乙烯、丙烯等低碳烯烃作为石油化工的重要基础原料,在石油化学工业中起着举足轻重的作用。目前乙烯和丙烯主要来自于各种碳氢化合物在高温条件下蒸汽热裂解和直接脱氢,由于反应温度高、能耗大、催化剂易结焦失活等原因,使其在工业生产中受到很大限制。与乙烷和丙烷直接脱氢制乙烯和丙烯相比,乙烷和丙烷的CO₂氧化脱氢制乙烯和丙烯在热力学上更为有利。采用CO₂作为温和氧化剂参与反应,不仅可以提高烯烃的选择性,还可以减少温室气体CO₂排放,实现碳资源的转化利用;SBA-15介孔材料具有较大的孔径分布、规则的孔道结构和较厚的孔壁以及良好的水热稳定性。因此,SBA-15分子筛有望作为乙烷和丙烷CO₂氧化脱氢制乙烯和丙烯高效催化剂的新型载体。整体式催化剂具有床层压降低、反应物径向分布均匀和良好的传递特性。这使得整体式催化剂已经成功的应用在环境污染控制领域。本论文分别制备了用于乙烷和丙烷CO₂氧化脱氢反应的负载型催化剂和金属基整体式催化剂,对催化剂的CO₂氧化乙烷或丙烷的脱氢性能性能进行了评价,并通过XRD、TEM、N₂-吸脱附、XPS、UV-vis和H₂-TPR等表征技术对催化剂的结构进行了表征。主要研究工作与结果如下:对于乙烷CO₂氧化脱氢反应,制备了Cr/SBA-15、Cr-Co/SBA-15、Cr-Ce/SBA-15负载型催化剂和Cr/SBA-15/Al₂O₃/FeCrAl、Ce/SBA-15/Al₂O₃/FeCrAl、Cr-Co/SBA-15/Al₂O₃/FeCrAl、Cr-Ce/SBA-15/Al₂O₃/FeCrAl金属基整体式催化剂。结果表明,Cr/SBA-15催化剂显示出较好的乙烷CO₂氧化脱氢活性。在Cr/SBA-15催化剂中,助剂Co的添加能够促进Cr物种在催化剂表面的分散,进而改变了催化剂的氧化还原特性和催化性能。Co的质量含量为1%的5Cr-1Co/SBA-15催化剂具有最好的脱氢活性,在T=700℃、GHSV=3600 mL·g^-1 ? h^-1和V_CO2/V_C2H6=3的反应条件下,乙烷的转化率和乙烯的选择性分别为53.9%和94.6%。Ce促进的Cr/SBA-15催化剂也具有很好的乙烷脱氢催化性能,Ce的添加也能够增加催化剂中Cr⁶⁺物种的相对含量,而Cr⁶⁺物种是催化剂具有较高乙烷脱氢反应性能的主要活性物种。在Ce的质量含量为10%的5Cr-10Ce/SBA-15催化剂上乙烷的转化率和乙烯的选择性可以分别达到55.0%和96.0%。对于Cr基Cr/SBA-15/Al₂O₃/FeCrAl整体式催化剂,由于FeCrAl金属载体具有良好的导热性,使得以FeCrAl为载体的催化剂床层上具有低的温度梯度,这可以避免或者降低在高温下进行的脱氢反应中催化剂活性相的烧结或者团聚。另外,介孔SBA-15分子筛的将Cr物种限制在SBA-15的介孔孔道内,降低或者避免了Cr物种的烧结和团聚,从而使得Cr/SBA-15/Al₂O₃/FeCrAl金属基整体式催化剂在CO₂氧化乙烷脱氢的过程中表现出较好的活性和稳定性。在1130h的乙烷CO₂氧化脱氢反应稳定性测试中,乙烷和CO₂的转化率分别从66.%和17.3%降到42.3%和10.6%。乙烯的选择性从95.5%降到90.1%。在Cr基整体式催化剂中,适量Co的添加能够改变催化剂中Cr氧化物的氧化还原能力和增加了Cr⁶⁺物种的含量,从而改变了Cr基整体式催化剂的氧化还原能力和乙烷脱氢催化性能。Co的质量含量为1%的5Cr-1Co/SBA-15/Al₂O₃/FeCrAl催化剂展示出最佳的乙烷氧化脱氢活性:在GHSV=6000 mL·g^-1 ? h^-1、V_CO2/V_C2H6=4和T=650℃的反应条件下,可以得到36.8%的乙烷转化率和99.4%的乙烯选择性。所有Ce修饰的Cr基整体式催化剂均显示出较好的CO₂氧化乙烷脱氢催化活性和稳定性。在整体式催化剂中,Ce的添加明显地提高了Cr基金属基整体式催化剂的乙烷脱氢性能和催化剂的稳定性。在1300h的CO₂氧化乙烷脱氢反应中,乙烷的转化率从51.1%降到45.2%,乙烯的选择性均高于96.4%。对于Ce/SBA-15/Al₂O₃/FeCrAl金属基整体式催化剂,高价态的Ce⁴⁺物种比低价态的Ce³⁺物种具有更高的乙烷脱氢活性。在脱氢反应中,Ce⁴⁺物种在脱氢反应中被还原为Ce³⁺物种,而Ce³⁺物种同时可以被CO₂氧化为Ce⁴⁺物种。Ce物种不同价态之间的氧化还原对于保持催化剂具有较高的乙烷脱氢催化性能起着重要的作用。对于丙烷CO₂氧化脱氢反应,制备了V/SBA-15、V-Cr/SBA-15负载型催化剂和V/SBA-15/Al₂O₃/FeCrAl、V-Cr/SBA-15/Al₂O₃/FeCrAl金属基整体式催化剂。在V/SBA-15催化剂中,V物种存在多种价态,其中V⁵⁺物种是主要的活性组分。Cr物种的添加能够促进V物种在催化剂表面的分散性能,增加V基催化剂中高价态V⁵⁺物种的含量,从而提高V基催化剂上的CO₂氧化丙烷脱氢的催化性能。对于以V为活性组分的V/SBA-15/Al₂O₃/FeCrAl整体式催化剂,催化剂中V物种以三种价态V⁵⁺、V⁴⁺和V³⁺物种的形式存在。其中具有四面体的V⁵⁺物种具有更高的丙烷氧化脱氢性能,而聚合的V⁵⁺物种则具有较低的脱氢性能。在V/SBA-15/Al₂O₃/FeCrAlCr金属基整体式催化剂中,添加适量的Cr能明显提高V基整体式催化剂的CO₂氧化丙烷脱氢催化性能,并在Cr的质量含量为10%的10V-10Cr/SBA-15/Al₂O₃/FeCrAl催化剂达到最大值:在GHSV=14400 mL·g^-1 ? h^-1、V_CO2/V_C3H8=3和700℃的反应条件下,可以得到61.3%的丙烷转化率和50.7%的丙烯收率。Cr物种的添加,增加了高分散的四面体的V物种的表面密度,并促进了四面体的V物种在催化剂表面的分散,从而提高了催化剂的活性。更多还原

【Abstract】 Ethylene and propylene are the important basic chemical feedstocks and play a decisive role in petrochemical industry. At the present time, the ethene and propylene are mainly come from the steam cracking and direct dehydrogenation of hydrocarbons. But the way is limited in the industrial production becaused of high reaction temperature, large energy consumption and deactivation of catalyst. Comparing with the steam cracking and direct dehydrogenation of hydrocarbons, the catalytic oxidative dehydrogenation （ODH） of ethane and propane with carbon dioxide is thermodynamically favorable and has been recognized as an attractive alternative. In addition, using CO₂ as a mild oxidant, could not only improve the alkene selectivity, but also reduce the emission of greenhouse gas and be considered as a source of carbon. The SBA-15 mesoporous material possesses high surface areas, hexagonal array of uniform tubular channels, thick walls, and higher thermal stability than many other mesoporous silica materials. So it is a promising new type of porous support for fabrication the catalyst for ODH of ethane and propane with carbon dioxide. The monolithic catalysts have many advantages such as low-pressure drop, favorable heat and mass transfer properties and minimum axial dispersion stem from the uniquely structured multi-channel configuration of monoliths. Therefore, monolithic catalysts have been extensively applied to the control of emissions. In this paper, the supported and metal based monolithic catalysts were prepared and their catalytic performances for the ODH of ethane or propane with carbon dioxde were tested. The structure of the catalysts was characterized by XRD, TEM, N₂ adsorption-desorption, XPS, UV-vis and H₂-TPR. The main research works and results are listed as follows:The Cr/SBA-15, Cr-Co/SBA-15, Cr-Ce/SBA-15 supported catalysts and Cr/SBA-15/Al₂O₃/FeCrAl、Ce/SBA-15/Al₂O₃/FeCrAl、Cr-Co/SBA-15/Al₂O₃ /FeCrAl、Cr-Ce/SBA-15/Al₂O₃/FeCrAl monolithic catalysts were prepared and their catalytic activity of ODH of ethane with CO₂ were tested. The results indicated that the Cr/SBA-15 catalyst showed excellent activity of ethane dehydrogenation. For the Cr-based catalyst, the addition of Co species promoted the dispersion of Cr species and also changed the redox properties and catalytic activity of the catalysts. The 5Cr-1Co/SBA-15 catalyst with 5%Cr and 1%Co exhibited the best catalytic activity in which 53.9% ethane conversion and 94.6% ethylene selectivity could be achieved under GHSV=3600 mL·g^-1 ? h^-1, V_CO2/V_C2H6=3 and 700℃reaction conditions. The Cr-based catalysts modified with Ce also exhibited excellent catalytic activity for the ODH of ethane with CO₂. Adding the Ce species to the 5Cr/SBA-15 catalyst could remarkably increase the relative content of Cr⁶⁺ species. The Cr⁶⁺ species is the main active species in the ODH of ethane. The 55.0% ethane conversion and 96.0% ethylene selectivity could be obtained over 5Cr-10Ce/SBA-15 catalyst at 700℃. The deactivation and regeneration reaction results showed that the addtion of Ce species to the Cr/SBA-15 catalyst could enhance the stability of the catalyst.For the Cr-based monolithic catalysts, the high thermal conductivity of FeCrAl support could avoid the sinter of activity phase （Cr species）, which is expected for the oxidative dehydrogenation of ethane occurring at high temperature. In addition, the mesoporous structure of SBA-15 could depress Cr species sintering, and enhance the catalytic activity and stability. During the 1130 h of stability reaction, the conversion of C₂H₆ dropped from 66.5% to 42.3%, and the selectivity for C₂H₄ decreased from 95.5% to 90.1% on 5Cr/SBA-15/Al₂O₃/FeCrAl catalyst. The activity and the stability of the catalysts could be improved when 5Cr/SBA-15/Al₂O₃/FeCrAl was doped by Co and Ce. After modified by Co species, the redox ability and catalytic activity changed and the relative content of the Cr⁶⁺ species increased. The 5Cr-1Co/SBA-Al₂O₃/FeCrAl catalyst exhibited excellent activity with 36.8% ethane conversion and 99.4% ethylene selectivity under GHSV=6000mL·g^-1 ? h^-1, V_CO2/V_C2H6=4 and 650℃reaction conditions. All the catalysts modified by Ce species showed better activity than that of Cr-based monolithic catalysts. The ODH performance could reach maximum at 5Cr-10Ce/SBA-15/Al₂O₃/FeCrAl monolithic catalysts. In the 1300 h stability reaction, the ethane conversion decreases from 51.1 % to 45.2 % and the C₂H₄ selectivity is above the 96.4 %. For the Ce/SBA-15/Al₂O₃/FeCrAl catalysts, the high oxidation state Ce⁴⁺ species had higher catalytic activity than the Ce³⁺ species in the monolithic catalysts. The Ce⁴⁺ species was reduced to Ce³⁺ species in the ethane dehydrogenation process, and the reduced Ce species was reoxidized to the Ce⁴⁺ species by treatment with CO₂. The Ce redox cycle played an important role in the catalyst’s high activity.The V/SBA-15, V-Cr/SBA-15 supported catalysts and the V/SBA-15 /Al₂O₃/FeCrAl and Cr modified V-based monolithic catalysts were prepared and the catalytic activity of ODH of propane with CO₂ was tested. The results indicated that, in V-based catalysts, the vanadium exhibited multiple valences, including V⁵⁺ species which is main activit component for ODH of propane. The addition of Cr species in the V/SBA-15 catalyst could enhance the dispersion of the V species and increase the content of the high valence V speicies, especially the relative amount of the V⁵⁺ species. As a result, the catalytic activity of propane dehydrogenation had been enhanced.For the V/SBA-15/Al₂O₃/FeCrAl and Cr modified V-based monolithic catalysts, 10V-10Cr/SBA-15/Al₂O₃/FeCrAl catalyst had the best catalytic activity in which 61.3% propane conversion and 50.7% propylene yiled could be achieved under GHSV=14400 mL·g^-1 ? h^-1, V_CO2/V_C3H8=3 and 700℃reaction conditions. According to the results of characterization, the good catalytic performance of the V-based monolithic catalyst could be related to the higher surface concentration of dispersed tetrahedral V⁵⁺ species and the synergistic action of V and Cr species.更多还原