Pt/TiO2/ZSM-5催化剂的制备及其催化转化正丁烷

摘要

采用溶胶-凝胶法和等体积浸渍法，分别对ZSM-5分子筛进行TiO2改性和Pt负载，获得了具有脱氢-裂解双功能的Pt/TiO2/ZSM-5催化剂，采用XRD、N2吸附-脱附、TEM、XPS和NH3-TPD对样品的晶体结构，孔结构、形貌、活性金属价态和酸性质等进行了表征，并研究了正丁烷在此催化剂上催化转化制备低碳烯烃的反应规律。研究结果表明，TiO2的引入，一方面使得改性后的ZSM-5分子筛获得了额外的酸性中心，特别是强酸性位含量的增加，有助于促进正丁烷的活化；另一方面Pt与TiO2之间存在“金属-载体”强相互作用（SMSI），在H2还原气氛下， Pt能够促进TiO2的还原，生成Ti3+物种，而Ti3+的存在增加了Pt周围的电荷密度,降低了Pt对低碳烯烃（ C2=～C3=）的吸附能力，抑制了深度脱氢和生焦反应，从而提高双功能催化剂对烯烃的选择性。当 H2还原温度为450℃时, Pt/10TiO2/ZSM-5催化剂在625℃下的正丁烷转化率为76.1%，低碳烯烃（C2=～C3=）收率为50.9%，分别比Pt/ZSM-5催化剂提高了16.7%和12.6%。%Pt/TiO2/ZSM-5 catalyst with dehydrogenation-cracking bifunctionality catalytic activity was prepared by modifying ZSM-5 zeolite with titanium dioxide (TiO2)via sol-gel method and then loading Pt to the titanium modified ZSM-5 zeolite by incipient impregnation method. The as-prepared catalyst was characterized by means of XRD, N2 adsorption-desorption, TEM, XPS and NH3-TPD to analyze crystal structure, pore properties, morphology, valence states of active metal and acid properties of the catalyst, and the catalytic performance for the cracking ofn-butane into light olefins was investigated. The results showed that the introduction of titanium dioxideprovided additional acid sites to ZSM-5 zeolite, especially increasing the strong acid centers and enhancing the activation ofn-butane. In addition, after reduction in hydrogen atmosphere, the partially reduced Ti3+ species was generated which was catalyzed by platinum due to the strong metal-support interaction (SMSI) between Pt and TiO2. The formation of appropriate amount of Ti3+ species enhanced the electron density around Pt, and thus weakened the adsorption of ethene and propene on Pt atoms. After reduction by hydrogen at 450℃, n-butane conversion of 76.1% and yield of light olefins (C2=—C3=) of 50.9% were achieved at the reaction temperature of 625℃ over Pt/10TiO2/ZSM-5 catalyst, which was 16.7% and 12.6% higher than those of Pt/ZSM-5 catalyst, respectively.