FeO_x/MgF₂ And V₂O₅/MgF₂ Catalysts For Vapor-phase Dehydrofluorination Of 1,1,1,3,3-pentafluoropropane

1,3,3,3-tetrafluoropropene?HFO-1234ze?has advantages such as zero ozone depletion potential?ODP = 0?,low global warning potential?GWP = 6?and shorter climatic performance?LCCP =14?,as well as outstanding performance parameters such as low toxicity,large refrigeration capacity,high energy efficiency,good compatibility with other materials,high efficiency and non-flammable safety.Thus,it is a promising candidate for the replacement of the currently used 1,1,1,2-tetrafluoroethane?HFC-134a?refrigerant,and recognized as the new generation of refrigerant.In addition,HFO-1234ze is widely applied in foaming,cleaning,aerosol propellants and so on.The synthesis of HFO-1234ze via catalytic dehydrofluorination of 1,1,1,3,3-pentafluoropropane?HFC-245fa?has good potential in practical industrial application.However,the loss of active species and the coke deposition on the catalysts are the main drawbacks of the currently employed catalyst systems.Therefore,the development of highly efficient catalysts for this reaction and the understanding of reaction mechanism are of great academic value and practical importance,which is the main goal of this research.In this thesis,gas phase catalytic dehydrofluorination of HFC-245fa was carried out over supported FeO_x/MgF₂ and V₂O₅/MgF₂.Various characterizations were conducted to illustrate the catalyst structure-catalytic performance relation and the reaction mechanism.The detailed contents of this work are as follows:1.supported FeO_x/MgF₂ catalysts with different Fe contents were prepared by an impregnation method and tested for the gas phase dehydrofluorination of 1,1,1,3,3-pentafluoropropane?HFC-245fa?.The results showed that the overall activities of the catalysts followed the order of 3FeO_x/MgF₂>6FeO_x/MgF₂>1FeO_x/MgF₂>MgF₂,and the highest HFC-245fa conversion 80.8%was obtained on the 3FeO_x/MgF₂ at 340 ?.The NH3-TPD results indicated that the surface acidity of the catalyst was closely related to the amount of Fe loading.The high surface acidity on the 3FeO_x/MgF₂ was due to the highly dispersed FeO_x species strongly interacting with the MgF₂ support.Meanwhile,the carbon deposited on the surface of 3FeO_x/MgF₂ catalyst did not affect the catalyst stability,which showed long-term stability in 100 h reaction.The enhanced activities on the FeO_x/MgF₂ catalysts were probably due to the formation of some active FeO_xFy species through the reaction between FeO_x and HF.2.A series of supported V₂O₅/MgF₂ catalysts were prepared and tested for dehydrofluorination of 1,1,1,3,3-pentafluoropropane?HFC-245fa?to synthesize 1,3,3,3-tetrafluoropropene?HFO-1234ze?.The addition of V₂O₅ in MgF₂ resulted in up to 5-fold increase in HFC-245fa conversion?from 19.2 to 95.2%?and much enhanced catalyst stability.Characterization results revealed that the dehydrofluorination initiated on the MgF₂ support triggered the transformation of V₂O₅ to vanadium oxyfluoride?VOFx?species via the reaction between V₂O₅ and HF,and such species were responsible for the improved activity as they had much higher turnover frequencies?TOFs?than the MgF₂?0.762 s-1 v.s.0.026 s-1 at 320 ??.The kinetic results indicated that the 4.3V₂O₅/MgF₂ had much lower activation energy?44.6 ±1.9 kJ mol-1?than the MgF₂?69.0 ± 0.8 kJ mol-1?.Accordingly,reaction mechanism on the V₂O₅/MgF₂ catalyst was proposed,which included slow dehydrofluorination on MgF₂ and fast dehydrofluorination on the VOFx species.