【作者】 胡晓燕；

【导师】 杨朝合；

【作者基本信息】 中国石油大学 ， 化学工程与技术， 2011， 博士

【摘要】 相比于以热反应为基础的传统的蒸汽裂解工艺,石脑油催化裂解以其低能耗、低二氧化碳排放量和良好的高附加值产物选择性等方面的优势,成为低碳烯烃生产的重要“后备军”。烷烃作为直馏石脑油的主要组分,是蒸汽裂解工艺生产低碳烯烃的理想原料。然而,在催化裂解反应条件（较温和）下,它的反应活性很低,是石脑油催化裂解所面临的问题之一。本论文主要针对烷烃催化裂解存在的难题,以正庚烷为模型化合物,开展了一系列的研究工作。正庚烷在以HZSM-5为活性组分的催化剂上反应,以五配位正碳离子的形成为前提的质子化裂解路径发挥了重要作用,导致了产物中C₃/C₄摩尔比值大于1,且大于相近转化率下1-庚烯裂解的该比值;在新鲜催化剂上,高转化率的获得伴随着大量氢转移反应的发生,液化气中烯烃度低;HZSM-5催化剂经过水热处理后,其表面酸量减少,尤其是强B酸,正庚烷的转化率迅速下降,氢转移反应的减少,液化气中的烯烃度提高。本文将含有晶格氧的金属氧化物催化剂引入到HZSM-5平衡剂中,以提高正庚烷的裂解反应活性,这种通过晶格氧氧化活化来提高烷烃反应活性的办法,可以克服通过增加催化剂的酸性而提高烷烃反应活性时所带来的增加氢转移反应和降低液化气中烯烃度的弊端。选用V₂O₅/Al₂O₃为晶格氧催化剂,它的引入能够提高正庚烷的转化率、丙烯和丁烯相对于乙烯的选择性和丁烯混合物中异丁烯的含量;在固定床反应器中,正庚烷转化率能够提高30%,丙烯收率可以增加约4个百分点;在循环流化床反应器中,转化率可以提高90%左右,丙烯收率可增加约8个百分点。利用脉冲式固定床微反-色谱联用装置对V₂O₅/Al₂O₃的引入对于正庚烷初始反应的影响进行了研究。结果表明,引入V₂O₅/Al₂O₃可提高正庚烷在HZSM-5平衡剂上的转化速率,并改变了初始产物选择性,推测V₂O₅/Al₂O₃的存在为正庚烷催化裂解的初始反应提供了另一条路径。在固定床反应装置上,通过设计不同的反应方式和催化剂性质的表征,对V₂O₅/Al₂O₃发挥影响的活性位和可能的反应机理进行了探讨。反应过程中,V₂O₅/Al₂O₃表面部分V⁵⁺被还原,晶格氧参与了反应,产物中出现CO和H₂O等氧化物。连续反应中,V₂O₅/Al₂O₃表面晶格氧逐渐被消耗,它对于反应的促进作用逐渐消失,表面晶格氧的参与行为对于V₂O₅/Al₂O₃对反应体系的影响有着直接的关系,经过再氧化后,V₂O₅/Al₂O₃的促进作用得以恢复,晶格氧的作用被再次确认。在HZSM-5平衡剂和V₂O₅/Al₂O₃组成的混合催化剂体系中,前者提供了正庚烷裂解反应的酸性活性位,仍然是反应的主要活性来源,V₂O₅/Al₂O₃的存在,提供了晶格氧活性中心,促进了正庚烷的初始反应速率,提高了烷烃的反应活性。在二者的共同作用下,正庚烷分子倾向于先与V₂O₅/Al₂O₃相互作用,活化产生的某中间物种在HZSM-5催化剂的酸性中心上发生裂解,并通过链传递促进其他正庚烷分子的裂解反应。更多还原

【Abstract】 Compared with the conventional steam cracking process, which was dominated by thermal cracking reactions, catalytic cracking of naphtha is advantangeous in lower energy consumption and carbon dioxide emission, higher selectivity to high-valued-compound products. Consequently, it is a competitive alternative for light olefin production. Paraffin, as main constituent of straight-run naphtha, is a perfect feedstock for steam cracking process of light olefin production. However, it is difficult to crack under mild conditions, posing a difficulty for the catalytic cracking process. This thesis focuses on difficulty confronted by catalytic cracking of paraffin; n-heptane was selected as model compound and series of related research work were performed.Protolytic cracking route, based on formation of penta-coordinated carbonium, played an important role in the initiation step during catalytic cracking of n-heptane over HZSM-5 catalyst. It caused that C3/C4 molar ratio was higher than 1 and the ratio of 1-heptene cracking with similar conversion. Over fresh catalyst, high conversion was achieved, accompanied by low light olefin selectivity, due to occurrences of hydrogen transfer reactions. After hydrothermal treatment, surface acidity decreased, especially the strong Bronsted acid sites, thus conversion of n-heptane declined significantly, light olefin selectivity was improved.The paper proposed to introduce reducible metal oxide containing lattice oxygen into HZSM-5 equlibrium catalyst, to improve reactivity of paraffin. This method can overcome the contradiction between conversion and light olefin selectivity brought by adjusting catalyst acidity. V₂O₅/Al₂O₃ was selected as catalyst for providing lattice oxygen; conversion of n-heptane, relative selectivity of propylene plus butylene to ethylene, and content of i-butylene in butylenes could be improved by V₂O₅/Al₂O₃; in fixed bed reactor, conversion of n-heptane could be improved by 30%, and propylene yield was about 4 percentages higher; in circulating fluidized bed unit, conversion could be improved by about 90%, and propylene yield was about 8 percentages higher.Influences of V₂O₅/Al₂O₃ on initiation reaction of n-heptane were investigated in a micro fixed bed reactor chromatography unit by pulse injection. The results demonstrated that V₂O₅/Al₂O₃ introduction improved reaction rate of n-heptane over equilibrium HZSM-5 catalyst and changed initial product selectivities, its presence provided another route for initiation reaction of n-heptane cracking.In fixed bed reactor, active sites of V₂O₅/Al₂O₃ and possible reaction mechanism were studied by designing different reacton modes and catalyst characterization. During the reaction, part of the V⁵⁺ on the surface of V₂O₅/Al₂O₃ was reduced, and lattice oxygen participated into the reaction, leading to formation of CO and H2O product. In continuous reaction, surface lattice oxygen was consumed and the promotion effects disappeared progressively, its participation behaviors were directly responsible for the influences brought by V₂O₅/Al₂O₃; role of lattice oxygen was further confirmed by recovery of V₂O₅/Al₂O₃ activity after oxidation. In the catalyst system composed of HZSM-5 catalyst and V₂O₅/Al₂O₃, the former remained primary active sites for n-heptane cracking; V₂O₅/Al₂O₃ functioned as lattice oxygen supplier, it promoted initial reaction rate and improved reactivity of paraffin. Under coactions of the two catalysts, it was favorable for n-heptane to interact with V₂O₅/Al₂O₃ and generate some active intermediate species before cracking over acidic sites in HZSM-5 catalyst; the cracked products could further promote cracking of other n-heptane molecules by chain transfer reactions.更多还原