Research On Selective Oxidation Of Propane To Acrolein And Acrylic Acid

Propane is a cheap chemical material and exists widely in the natural gas and crude oil. During the petroleum resources becoming exhausted today, using the relatively rich propane resources, which could be transformed into the higher additional value of compounds, has become a hotspot of research and has the extremely vital significance in the natural gas industry. As the stability of propane molecule and activity of product catalysts which are suitable for industry are still to be developed. In the reported literature for the selective oxidation of propane, the yield of acrolein and acrylic acid were in general about 10% and 30%, respectively. Therefore, the attention should be paid on the catalyst research and development.The kind of mixed metal oxide catalysts, such as Bi-Mo-V and Mo-V-Te system, is the best one for all catalysts from selective oxidation of propane to acrolein. The catalyst of structure (mainly including M1 and M2 phases) and mechanism, which used for selective oxidation of propane to acrylic acid system, have been widely researched. As a result of the more elements and active species, the process of the catalyst preparation and element ratio in the catalyst are more important for the reaction because of the synergistic action between the elements. It is likely to have the different effect when the element content or the preparation conditions is slightly different.In order to further select the optimum catalysts for the selective oxidation of propane to acrolein and acrylic acid, the supported catalyst, doping element and element ratio in the catalysts affecting on catalytic performance have been researched and relation between properties and structure of the catalysts has also been discussed.The first chapter of this thesis introduces the significance of the selective oxidation of propane to acrolein or acrylic acid and the research status, including the introduction of the catalysts, influence factors of catalyst reaction mechanism. and so on. This chapter also describes the stereo basis and research direction.The second chapter mainly gives a brief of the experimental method, including preparation method of the catalysts, catalyst evaluation conditions and data processing. characterization methods and conditions are described.The third chapter mainly studies the catalysts on selective oxidation of propane to acrolein. Research includes Bi-Mo-V catalysts and supported Mo-V-Te catalysts. Theα-Bi1/3(MoO4)3 catalyst was detailed studied by V element gradually replacing Mo element. With the V element increasing, conversion of propane and acrolein yield gradually increased. When the V content was 0.55, the highest yield of acrolein was got. The amount of V doping increased the acidity of the catalyst and improved the redox ability; thereby the propane conversion was improved. At the same time, the amount of cationic vacancy caused by V doped might be another reason for the high catalytic performance. The Mo-V-Te catalysts supported on SiC were also studied. Neutral SiC carrier might promote the active center dispersion, isolate from the active site and prevent deep oxidation. When the loading was 10%, MoV0.1Te0.15/SiC catalyst for propane oxidation was the best. When P was doped to the catalyst, it greatly improved the catalytic performance, and the yield of 10.7% of acrolein was reached until P content was 0.05. The results showed that the amount of P doping improved the catalyst surface acidity which might be caused by P-Mo heteropoly acid and (VO)2P2O7, thereby the conversion of propane was higher. The amount of P content can also promoted better dispersion of active component, which improved catalytic redox ability and resulted in better catalytic performance.The fourth chapter studies the catalysts on the selective oxidation of propane to acrylic acid. The performance of MoaTebVxNbyOn catalysts were evaluated through changing different element ratio and the catalysts were characterized by XRD, Raman. XPS, NH3-TPD, H2-TPR, TPRO and O2-TPO. In the MoaTebVxNbyOn (a/b= 8.32) catalysts, when V/Nb ratio was lower (0.58～0.82), the performance of the catalysts was improved, especially V/Nb ratio at 0.58 or 0.82. The yield of acrylic acid was above 40%. The catalysts regulated V/Nb= 0.58 or 0.82 showed the improvement of the redox ability, total acid and oxygen mobility of catalysts, and the catalysts surface has more M1 phase, higher V5+/V4+ ratio and Te content, which can be the reason of high propane conversion and yield of acrylic acid. Base on the MoaTebVxNbyOn (x/y= 0.82) catalyst, different Mo/Te ratio regulation was also studied. When Mo/Te was 5.95, the yield of acrylic acid further increased and the highest yield was 47.2%. The characterization results showed that the catalyst improved the total acid and oxygen mobility further, but the ratio of V5+/V4+ on the surface was somewhat less, which result in the reaction temperature (400℃) of MoaTebVxNbyOn (a/b= 5.95, x/y= 0.82) catalyst higher than MoaTebVxNbyOn (a/b= 8.32. x/y= 0.82) catalyst. In order to develop catalysts of higher catalytic performance, according to the results after the V/Nb ratio and Mo/Te ratio had been controlled already, we expanded the doping amount of V or Nb and the range of reaction temperature, further optimized V/Nb ratio. A series of outstanding catalysts for propane selective oxidation to acrylic acid were finally gotten through dozens of catalyst evaluation. The highest yield of acrylic acid was up to 50% or more, which exceeded the value of current domestic and foreign literature and patent reported. Base on the literature, P element can increase the selectivity of a target product and enhance the catalytic performance. We undertook the study on the MoaTebVxNbyOn (a/b= 8.32, a/x= 6.63, x/y= 1.18) catalyst by doping P in the range of 0.002-0.01. Although the P content was low, the catalytic performance was remarkably improved. When the P content was lower (0.002-0.006), acrylic acid yield increased gradually, and the yield of acrylic acid as high as 53% when the P is 0.006. The amount of P leads to increase the acidic amount and V5+ content on the catalyst surface and improve redox ability of the catalysts, which made the conversion of propane higher. While decrease of high-temperature acid center strength on the surface, increase of Mo=O vibration peak intensity and surface Te4+ proportion, and enhancement of oxidative dehydration capacity might be the main reasons for increasing selectivity of acrvlic acid.