【作者】 李庆；

【导师】 周兴贵；

【作者基本信息】 华东理工大学 ， 化学工程， 2012， 博士

【摘要】 当今世界对丙烯的需求量呈稳步增长的趋势,而丙烷催化脱氢是一个增产丙烯的有效途径。因此,对丙烷脱氢过程的动力学以及结焦过程等方面的研究具有十分重要的意义。就面向工业化应用而言,本文分别研究了工业化双金属PtSn催化剂上丙烷脱氢过程中的脱氢动力学、结焦动力学、烧焦动力学,以及催化剂上生成的焦的性质。这些研究有利于工业反应器的设计以及丙烯生产、催化剂烧焦再生过程的操作条件的优化、控制。就面向基础研究而言,本文则研究了单金属Pt催化剂上Pt颗粒粒径和Sn的添加对丙烷脱氢动力学以及结焦行为的影响,建立了丙烷脱氢过程中Pt催化剂结构与催化性能之间的关系；探索了借助微观动力学分析来建立动力学模型的可行性。这些研究则有助于更好地认识丙烷脱氢反应过程,从而有助于方便快速地建立更加可靠的动力学模型,同时还有助于高性能工业催化剂的设计、开发。本文首先建立了工业化的PtSn催化剂上的全面的丙烷脱氢动力学模型,该模型包括脱氢模型、裂解模型、结焦模型以及失活模型。通过假设不同的反应机理和速率控制步骤,得到数个脱氢模型,并从拟合精度和参数个数两方面考虑,辨别出最优的模型。此外,通过数学推导,本文还将催化剂失活函数与结焦速率相关联。该处理方法在基于催化剂失活机理的同时,还避免了在线测量催化剂结焦量的难题。丙烷脱氢过程中,催化剂失活主要是由结焦引起。因此,有效避免催化剂快速失活和充分认识焦的性质分别对反应和催化剂再生过程有重要的现实意义。为此,本文在工业化的PtSn催化剂上研究了反应条件对结焦速率以及焦的性质的影响。研究发现,PtSn催化剂上生成了两种形式的焦,分别为金属上的焦和载体上的焦。丙烯浓度的增加导致焦的石墨化程度的升高；氢气浓度的增加导致焦的石墨化程度的降低。催化剂金属上结焦反应对丙烷的反应级数是1.7,而对丙烯和氢气的反应级数均是零级；载体上的结焦反应对丙烷的反应级数是1.4级,对丙烯的反应级数是1.0级,而对氢气的反应级数则为-0.7级。此外,根据动力学分析,本文得到了催化剂上的结焦机理。并分别建立了金属上和载体上的结焦动力学模型。烧焦动力学对催化剂烧焦操作条件的优化具有十分重要的意义。本文采用程序升温氧化的方法对PtSn催化剂的烧焦动力学进行了研究,分别建立了金属上的焦和载体上的焦的燃烧动力学。该动力学较好地预测了不同条件下的实验结果。两种焦的燃烧活化能分别约为86 kJ/mol和218 kJ/mol。为加深对Pt催化剂上丙烷脱氢过程的理解,以便建立更加全面、可靠的动力学模型,本文从单金属Pt催化剂出发,研究了金属Pt颗粒粒径对催化剂脱氢动力学以及结焦性能的影响。研究发现,丙烷脱氢是结构敏感的反应,丙烷的反应速率随着催化剂金属Pt颗粒粒径的增大而减小,但丙烯选择性则随着金属Pt颗粒粒径的增大而增大。当催化剂Pt颗粒粒径为4.6nm时,丙烯生成速率最高。对所有催化剂而言,丙烷的表观反应级数始终为一级,而氢气的表观反应级数则随着Pt颗粒粒径的增大逐渐从零级减小至-0.51级。同时脱氢反应的表观活化能随着Pt颗粒粒径的增大而增大。不同大小的Pt颗粒上的结焦行为也不同：催化剂的结焦速率随着金属Pt颗粒的增大而减小；焦的芳香性以及石墨化程度也随着金属Pt颗粒粒径的增加而变弱。传统的动力学研究方法是根据相关理论、文献,假设反应机理和速率控制步骤,推导出动力学模型,然后用实验数据对动力学参数进行回归。然而在此过程中,为确保模型参数的可靠性,需要大量的实验数据,实验量巨大。本文则尝试了一种新的动力学研究方法,即用微观动力学分析的手段建立动力学模型。首先用密度泛函理论(Density Functional Theory, DFT)计算模型中涉及到的动力学参数,然后根据微观动力学分析结果建立宏观动力学模型。微观动力学模拟结果显示,模型对单金属Pt催化剂上的动力学行为有较好的预测。如果用部分实验数据对个别参数进行优化调整,可以使模型对实验数据预测的准确程度大大提高。与传统的动力学研究方法相比,该方法可以得到较为可靠的动力学参数,同时又可大大减少动力学研究的实验量。工业上(Oleflex工艺)采用的是PtSn双金属催化剂,是因为Sn的添加有利于提高Pt催化剂对产物丙烯的选择性以及催化剂自身的稳定性。然而Sn的添加对Pt催化剂结焦行为,尤其是动力学行为的影响尚缺乏深入认识。因此,本文在对单金属Pt催化剂进行活性、稳定性等方面研究的基础上,研究了Sn的添加对Pt催化剂动力学和结焦行为的影响。实验研究发现,Pt/Al2O3和PtSn/Al2O3催化剂上脱氢反应对丙烷和氢气的表观反应级数相同,表观反应活化能相近。该结果表明,我们可以通过对Pt催化剂上丙烷脱氢反应过程的研究来在一定程度上认识PtSn催化剂上丙烷脱氢反应过程。本文在比较Pt催化剂和PtSn催化剂动力学行为的基础上,借助于Pt催化剂上的研究成果建立了PtSn催化剂上的微观动力学模型,并得到,PtSn催化剂上的速率控制步骤是第一步脱氢,吸附的氢原子是表面最丰富物种。该微观动力学模型可以较好地解释PtSn催化剂上的动力学现象。此外,Sn的加入提高了Pt催化剂的结焦速率,增强了生成的焦的石墨化程度。同时,Sn的加入促进了焦前体从金属到载体的迁移,从而可以有效防止Pt金属被生成的焦迅速覆盖。更多还原

【Abstract】 The world demand for propylene is growing steadily in recently years. Propane dehydrogenation is an on-purpose and effective technique to increase the production of propylene. Therefore, in this thesis, the studies on the kinetics and coke formation during propane dehydrogenation are studied. In view of industrial application, the kinetics of dehydrogenation, coke formation and coke combustion as well as the nature of coke were investigated on the industrialized PtSn bimetallic catalyst. The results of the research would contribute to the design of the reactor and the optimal control of the operation during industrial production and catalyst regeneration. In view of fundamental research, the size effects of the kinetics of propane dehydrogenation as well as coke formation are studied; a new approach for the establishment of kinetics is proposed. Besides, the effects of Sn addition on the kinetics of propane dehydrogenation as well as coke formation are also studied. The results from fundamental research could deepen the understanding of the reaction on the Pt catalysts, help to establish more reliable kinetics in shorter periods and provide essential information for the design and exploration of high-performation catalysts.In this thesis, the complete kinetics of propane dehydrogenation including dehydrogenation, cracking, coking and deactivation are established. The best model was selected from models assuming different mechanisms and rate-determining steps by standards of fitting accuracy and the number of parameters. Besides, the deactivation function is related to the coking rate, which could reflect the real mechanism of catalyst deactivation and avoid the problem of the transient measure of deposited coke on the catalyst.The catalyst is deactivated mainly by coke formation during propane dehydrogenation. In order to avoid fast deactivation and to recognize the nature of the coke formed on the catalyst, the coking mechanism and the effects of reaction conditions on the nature of coke are investigated. Two categories of coke are identified, coke on the metal and coke on the support. The degree of graphitization of the coke is enhanced by propylene but weakened by hydrogen. On the metal, the coking reaction orders to propane, propylene and hydrogen are 1.7,0.0 and 0.0, respectively; while on the support, the coking reaction orders to propane, propylene and hydrogen are 1.4,1.0 and -0.7, respectively. A mechanism of coke formation is then obtained based on the kinetic analysis and the kinetic models for coke formation on the metal and the support are established and can well describe the experiments.The activity of the deactivated catalyst is usually restored by coke combustion. The kinetics of coke combustion play an important role in controlling the coke-burning conditions and preventing the sintering of the active metal. The coke-burning kinetics of the two categories of coke formed on the catalyst are both established through a temperature programmed approach. The apparent activation energies for the combustion of the two categories of coke are 86 kJ/mol and 218 kJ/mol, respectively.The correct and detailed understanding of the mechanism for propane dehydrogenation concerns the accuracy and reliability of the kinetic model. However, the mechanism for propane dehydrogenation may be different on catalysts with different structures. The studies on the effects of Pt particle size on the performances of Pt/AlO3 catalysts during propane dehydrogenation are helpful for the further understanding of the reaction, which is also very useful for catalyst design. It is obtained that the reaction rate decreases but the selectivity to propylene increases with the Pt particle size. The yield of propylene reaches the maximum when the size of the Pt particle is 4.6 nm. For all the catalysts, the apparent reaction orders to propane are all the first, but the orders to hydrogen are decreasing from zero to minus half as the size of the Pt particle increases. The apparent activation energy is also found increasing with the size of the Pt particle. Besides, it is also found that the coking rate decreases obviously with the Pt particles size and more hydrogen-deficient coke can be formed on smaller Pt particles.The complete, accurate and reliable kinetic model can be then obtained on the basis of the good understanding of the reaction. Traditionally, the kinetic model is established by the sequence of assuming mechanism and RDS, deriving kinetic model and fitting kinetic parameters by experimental data. However, the reliability of the mechanism is not validated and that of the kinetic parameters has to be guaranteed by fitting a lot of experiments. In our study, the kinetic model is established based on the micro-kinetic analysis and the parameters are obtained by DFT calculation. The model can correctly predict the kinetic behaviors of Pt catalysts. Furthermore, the prediction can be greatly improved after the optimization of two specific parameters. This approach of kinetic study can bring about more reliable parameters and greatly reduce the kinetic experiments.However, the bimetallic PtSn catalyst, instead of monometallic Pt catalyst, is actually employed in the industry (Oleflex process) because Sn can improve the selectivity to propylene and retain the activity of the catalyst. However, the understanding of the effects of Sn on the kinetics and coking properties of the Pt catalysts are still not clear although it is important. In this thesis, the effects of Sn addition on the kinetics as well as the coking properties of Pt catalysts are presented. The experimental results indicate that the reaction orders to propane and hydrogen are both the same on the Pt and PtSn catalysts. The apparent activation energies are also close to each other. This fact also indicates that the knowledge about the kinetics on the monometallic Pt catalyst can be extended to the bimetallic PtSn catalyst to a certain degree, which significantly facilities the further related research on bimetallic catalysts. Through employing the mechanism on the Pt catalyst, the microkinetics on the PtSn catalyst is established and the rate determining step as well as the most abundant surface intermediate is identified. Nevertheless, the effects of Sn on the coking properties of the Pt catalysts are apparent. The addition of Sn has increased the coking rate and meanwhile promoted the migration of the coke precursor from metal to the support. The degree of graphitization of the coke formed is also enhanced after Sn addition.更多还原