Catalysts For Vapor-phase Dehydrofluorination Of HFC-245fa To 1,3,3,3-tetrafluoropropene

Up to now,1,1,1,2-tetrafluoroethane(HFC-134a)is the most commonly used as refrigerant,even though the global warming potential(GWP)of this refrigerant relative to CO2 on a 100-year time horizon is as high as 1430.In response to the challenge of global warming and depletion of the atmospheric ozone,the EU's automotive air conditioning directive stipulates that from 2017 onwards,all new vehicles prohibit the use of GWP greater than 150 refrigerant.In addition,the European Union's new F-gas specifies that after January 1,2022,the placing on the market of refrigerators and freezers for commercial use that contain HFCs with GWP of 150 or more will be prohibited.Thus,there is a need for compositions that meet both low ozone depletion potential as well as having low GWP.1,3,3,3-tetrafluoropropene(HFO-1234ze)decomposes rapidly in the atmosphere and has very low GWP(less than 1),zero ozone depletion potential and low flammability,is now useful in numerous applications including as a foaming agent,aerosol propellant,refrigerant,solvent,and also as a monomer for homopolymers and copolymers,etc.Accodingly,HFO-1234ze has been considered as a future substitute for HFC-134a as a new generation coolant,HFO-1234ze is also thought to be promising as a next generation foaming agent.In this paper,first of all the enthalpy change and Gibbs free energy change during the syntheses of HFO-1234ze via gas-phase dehydrofluorination of 1,1,1,3,3-Pentafluoropropane(HFC-245fa)was estimated based on density functional theory(DFT),then the vapor-phase catalytic dehydrofluorination of HFC-245fa was conducted to synthesize HFO-1234ze over chromium-based catalysts,and the catalysts were characterized by various techniques such as BET,XRD,NH3-TPD,Raman,H2-TPR,TG-DTA,XPS and so on,to investigate the phase structures,surface acid properties,surface species,and deactivation reasons of the catalysts.The detailed contents of this work are as follows:1.The changes of Gibbs free energy and enthalpy during the syntheses of HFO-1234ze via gas-phase dehydrofluorination of HFC-245fa were calculated by density functional theory(DFT)with DMol3 in Materials Studio.The value of Gibbs free energy indicated that the dehydrofluorination of HFC-245fa is non-spontaneous and endothermic,and proper increase of reaction temperature is benefit for the dehydrofluorination.2.A series of NiO/Cr2O3 catalysts were prepared by an impregnation method with different Ni content,and tested for the synthesis of HFO-1234ze from 1,1,1,3,3-pentafluoropropane via gas-phase dehydrofluorination,and the effect of reaction temperature was also investigated.It was found that NiO/Cr2O3 catalyst has better reaction performance than Cr2O3,the HFC-245fa conversion of 93%was obtained on the 15NiO/Cr2O3 catalyst at 350 ?(GHSV=7800 mL g-1 h-1),which was much higher than that of Cr2O3 catalyst(70%).Furthermore,the conversion of HFC-245fa increased with the increase of reaction temperature.The characterization results revealed that with increasing Ni contents in the catalysts,the special surface area increased gradually,but the surface acidity increased at first and decreased later on.In addtion,NiO/Cr2O3 has higher valent Cr species on the surface of Cr2O3 catalyst.The stability test showed that the selectivity of HFO-1234ze remained unchanged,15NiO/Cr2O3 catalyst had better stability,Cr2O3 was easy to be inactive at 320 ?.Raman spectroscopic and thermal-gravimetric analysis results revealed that carbon deposition on catalyst may lead to catalyst deactivation.3.The NiN/Cr2O3 and NiA/Cr2O3 catalysts were prepared by the impregnation method with Ni(NO3)2 and Ni(CH3COO)2 as precursors,respectively,and the effects of different precursors on the gas-phase catalytic dehydrofluorination of HFC-245fa to synthesize HFO-1234ze were studied.For both catalysts,the selectivity for HFO-1234ze was very high,but the conversion of HFC-245fa was varied with different catalysts,and the catalytic activity of the NiN/Cr2O3 catalyst was much lower than that of NiA/Cr2O3 catalyst,even lower than that of Cr2O3 catalyst.With the addition of Ni(NO3)2,the specific surface area of the NiN/Cr2O3 catalyst also decreased,and the surface acidity of the NiN/Cr2O3 catalyst was reduced sharply.However,unlike the NiN/Cr2O3 catalyst,the NiA/Cr2O3 catalyst had larger specific surface area and higher total surface acidity.The characterization results indicated that the reactivity may mainly be affected by the surface structure and surface acidity of the catalyst.