The Kinetics Of Propane Dehydrogenation And Coke Formation Over Pt Catalysts

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.