Research On The Catalysis Used For Selective Oxidation Of Propane

Propylene and acrylic acid are important raw materials and intermediates in the modern chemical industry. Propylene is produced commercially from oil cracking gas, then after partial oxidation, acrylic acid can be obtained. Because of the growing shortage of conventional oil and coal, how to transform the relatively rich propane resources into higher additional value of compounds has became a hot-spot of research and also has the extremely vital significant in the natural gas industry. As the stability of propane molecule and activity of products, the development of catalysts which can used in mild conditions for industry should be paid more attention.In researches, complex metal oxides have been mostly studied in the catalytic system of propane oxidative dehydrogenation to propylene, but the yield of propylene reported is still unsatisfied. It is because that deep oxidation of intermediates and products is favorable thermodynamically to form signficant amount by-products of COx, which leads directly to lower utilization of propane resources. Costine discovered that, in catalytic reactions for selective oxidation of low carbon alkanes, the selectivity of product was obedient to the empirical relations to the value of Dlimit (Dlimit=D0-H-reactant-D0-H-product) under certain conversion. The Dlimit of propane to propylene reaches40KJ/mol which is greater than the critical value (30KJ/mol) proposed by Costine. So it is almost impossible to obtain high propylene selectivity with high conversion. Therefore, it is necessary to explore a new reaction which is suitable for selective oxidation of low carbon alkanes.Based on the above considerations, if the deep oxidation products are CO rather than CO2, the rational use of propane resources and reduction of by-products will certainly lead to considerable economic benefits and higher energy utilization. Therefore, it is necessary for us to develop new catalysts in the reaction.The Mo-V-Te-Nb-Ox system is considered to be the best kind of catalysts for selective oxidation of propane to acrylic acid. And now its preparation methods, chemical compositions and surface characteristics are still in research. With large-scale trial on this issue, our group has obtained two kinds of catalysts with optimal catalytic performance. They are Mo-V-Te-Nb-Ox catalysts with Mo/V=8.33, Mo/Te=5.95, Te/Nb=0.98and Mo/V=11.92, Mo/Te=6.15, Te/Nb=0.98.As we all know, appropriate redox ability and acid properties, especially effective oxygen species and acid sites have played important roles on the conversion of propane and selectivity of target oxygenates. Firstly we doped transition metals with various oxidation states or acidic elements into Mo-V-Te-Nb-Ox catalysts. Thus it can improve redox ability of the catalysts, which could have better catalytic performances. Then with the same crystalline structures, we added an appropriate material in order to increase productivity and decrease cost.According the former considerations, this paper mainly discussed as follows.The first chapter of this thesis introduces the situations of the selective oxidation of propane to propylene and acrylic acid, including the introduction of the catalysts, reaction mechanisms and essential factors and so on. Furthermore, the topic and scope of this paper are also introduced.The second chapter mainly gives a brief introduction of the experimental methods, including preparation of the catalysts, reaction conditions, characterization methods and data processmg.The third chapter mainly includes two parts, firstly, double component Fe-O-M (M=V, Cu and Zn) catalysts was used for propane selective oxidative reaction in order to obtain better selectivity of propylene and CO. The experinental results showed that only V/Fe2O3catalysts effectively inhibited the generation of CO2, and the sum of selectivity for propylene and CO was high Secondly, the performance of the FeVxOy catalysts with different amounts of vanadium and different reaction conditions in the selective oxidation of propane to propylene and CO was measured, and the sum of them can be as high as90%. The catalysts have been characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Raman, H2-TPR and NH3-TPD. The relationship between the structure and catalytic properties was preliminarily discussed. The results indicated that chemical interaction had taken place between the V and Fe, which can be referred to V-O-Fe bonds and the formation of Fe (VO4), and that isolated VO4species played an important role for augmenting the selectivity of propylene and CO and the FeVO4could be recognized to be effective crystal phase for the selectivity of propylene and CO. Meanwhile, with the increase of vanadium content, the distribution of all the elements proportion and valence state on the surface of the catalysts as well as the acid amount and acid sites has been changed immensely. All of these affect the catalytic performance and improve the selectivity of CO and inhibit that of CO2.The last chapter focuses on the catalysts for selective oxidation of propane to acrylic acid. In order to develop catalysts with good redox ability and acid properties, we doped various amounts of Fe and P elements into the optimal catalyst B. As a result, the yield of more than60%was achieved. The results of the catalysts characterized by XRD, Raman, H2-TPR, NH3-TPD and O2-TPD demonstrated that although Fe or P doping did not significantly change the phase structures, it helped to regulate the contents of different metal ions with different valences on the catalyst surface. If the Mo6+, V5+and Te4+species were in appropriate contents, the catalysts could have good catalytic performance. Meanwhile, the changes of ammonia desorption amount in strong acid sites and oxygen desorption amount in higher temperature regions could have great influence on catalysts activity.The Mo-V-Te-Nb-Ox catalysts added with SiO2were also studied on the selective oxidation of propane to acrylic acid. The results showed that the catalytic performance was closely to the dosage and particle size of SiO2, the less doping amount and the smaller particle size, the higher yield of of acrylic acid achieved. From the preliminary characterization of this kind of catalysts, it can be concluded that although the average particle size of the catalysts decreases and the specific surface area increased, but the crystal phase structure did not change after addition of SiO2, and the exposed SiO2on catalyst surface should be the primary reason for the decline of catalytic activities. Therefore, the idea to dope SiO2in the preparation of Mo-V-Te-Nb-Ox catalysts may be suitable, but the improvement of methods and techniques in the catalyst preparative process is needed.