Catalysts For Gas-phase Dehydrofluorination Of HFC-245fa To HFO-1234ze,HFO-1234yf And TFP

Hydrofluorocarbons（HFCs）sush as 1,1,1,2-tetrafluoroethane（HFC-134a）are widely used as the third generation automotive refrigerants due to their zero ozone depletion potential（ODP）.However,it has a high global warming potential（GWP=1300）and stays in the atmosphere for a long time,causing global warming when used in large quantities.Hydrofluorolefins（HFOs）such as1,3,3,3-tetrafluoropropylene（E\Z-HFO-1234ze）and 2,3,3,3-tetrafluoropropylene（HFO-1234yf）with less life cycle climate performance（LCCP）,zero ozone depletion potential,very low GWP,good compatibility with other materials and non-flammable safety are considered to be an ideal alternative to HFCs and are expected to be the new generation of environmentally friendlyrefrigerant.Besides,HFO-1234ze and HFO-1234yf can also be applied in foaming agent,cleaning agent and aerosol propellant.3,3,3-trifluoropyne（TFP）is suitable for foaming agents and refrigerants due to its GWP=0,ODP=0.In this paper,the catalytic performance of C-AlF₃,3V-xFe/MgF₂ and ZnO/Cr₂O₃catalysts for the gas-phase dehydrogenation of 1,1,1,3,3-pentafluoropropane（HFC-245fa）was studied.Various characterizations such as XRD,XPS,BET,Raman,NH3-TPD,pyredine FTIR were conducted to analyze structure and properties of the catalyst and discuss the catalytic reaction mechanism.Detailed contents as follows:1.A series of C-AlF3 composite catalysts were prepared using a sol-gel method and different aluminum precursors.These catalysts were very active in gas-phase dehydrofluorination of HFC-245fa.For example,a catalyst calcined at 400℃ with aluminum isopropoxide as the precursor（denoted as C-AlF3-AIP-400）gave a reaction rate of 8.30μmol s^-1g_cat^-11 and a selectivity to HFO-1234ze of above 99%at 340℃.More importantly,the C-AlF3 catalysts showed excellent stability during 100 reaction,while the pure AlF₃ catalyst suffered severe deactivation owing to the coke deposition on the surface.In contrast,pre-deposited carbonaceous species formed in the catalyst due to the incomplete decomposition of organic precursors during the preparation procedure,which blocked strong acidic sites in the AlF₃ and thus maintained the catalyst stability.2.A series of 3V-xFe/MgF2 catalysts with different V/Fe molar ratios were prepared by a sol-gel method,and the effect of Fe promotion on the catalytic performance of gas-phase dehydrofluorination of HFC-245fa was investigated.It was found that proper V/Fe ratio was helpful to improve the activity.The 3V-0.5Fe/MgF₂catalyst（with a V/Fe ratio of 6/1）gave a HFC-245fa conversion of 76.6%at 320℃,which was 13.4%higher than that on the 3V/MgF2（63.2%）.The synergy between V and Fe species resulted in enhanced surface acidity which accounts for the enhanced activity.The VO_x and FeO_x species in the fresh catalyst could be transformed to VOF_x and FeF₃ species during the reaction,which were regarded as the active sites.It was also found that the VOFx was more active than the FeF3.However,higher content of Fe in the catalyst resulted in the partial coverage of V species,which led to decline of surface acidity and thus the suppressed activity.Also,surface carbon deposition was responsible for the slight catalyst deactivation.3.The pure Cr₂O₃ catalyst was prepared by a precipitation method and a series of supported ZnO/Cr₂O₃ catalysts were prepared by a impregnation method.The promoted role of ZnO on the activity for gas-phase dehydrofluorination of HFC-245fa to E\Z-HFO-1234ze,HFO-1234yf and TFP was investigated.It is found that with the Zn load increasing,the crystallite size of the Cr₂O₃ and the surface area of catalysts both increased,while the surface acid of catalysts reached the highest(1.02μmol g^-1)with the 2%of ZnO load.Surface Cr species of Zn O/Cr₂O₃ catalysts were identified,including Cr₂O₃,Cr（OH）₃ and CrO₃.The relative content of Cr₂O₃ increased with the ZnO load increasing,while the relative content of Cr（OH）3 and CrO3 decreased.It was also found that the selectivities to four products（E-HFO-1234ze,Z-HFO-1234ze,HFO-1234yf and TFP）were greatly affected by the ZnO loading and reaction temperature.The lowest selectivies to E-and Z-HFO-1234ze is 61.2 and 15.7%,respectively,while the highest selectivities to HFO-1234yf（11.7%）and TFP（9.8%）were reached at 450℃on the 2ZnO/Cr₂O₃ catalyst,because of it had the highest surface acidity.