Vapor-Phase Fluorination Perchloroethylene To Synthesis Pentafluoroethane Over CR-Based Catalysts

1,1,1,2,2-pentafluoroethane(R-125, HCFC-125) has been considered as a promising alternative of ozone depletion substances (ODS). Because of its stable chemical property, non-toxicity, non-flammability, suitable critical point and boiling point, and the relatively low freezing point, it widely used in air conditioners, refrigerators, car air conditioning, refrigeration, industrial refrigeration equipment and fire extinguishing agent, etc. It is a new type of high efficient green refrigerants. In this thesis, vapor-phase hydrofluorination of perchloroethylene (PCE) to synthesis pentafluoroethane (HFC-125) was conducted. This process is divided into two steps, the first step is vapor phase fluorination of perchloroethylene (PCE) to synthesize2,2-dichloro-1,1,1-trifluoroethane (HCFC-123) and1-chloro-1,2,2,2-tetrafluoroethane (HCFC-124), the second step is fluorination HCFC-123to synthesize HFC-125. Compared with other processes, this process has many advantages such as low cost of reagents, less equipment corrosion and suitable for large scale production. In this work, fluorination of PCE to synthesize HCFC-123and HCFC-124was carried out over Al-based catalysts and Cr-based catalysts. Characterization techniques such as XRD, BET, Raman, H2-TPR, NH3-TPD, XPS were employed in order to obtain catalyst information such as phase structure, surface acidity and surface species.A series of ZnO/Al2O3catalysts were prepared by an impregnation method and tested for the vapor phase fluorination of PCE to HCFC-123and HCFC-124. It was found that activity of ZnF2/Al2O3catalyst and selectivity to HCFC-123, HCFC-124depended on phase structure of Al2O3and surface acidity of the catalyst. The highest activity was obtained on ZnF2/Al2O3catalyst.A series of M-Cr2O3catalysts with different promoters M (M=Y, Co, La, Zn) were prepared by an co-precipitation method and tested for vapor-phase fluorination of PCE to synthesize HCFC-123, HCFC-124and HFC-125. Experimental results showed that the promoter M improved the PCE conversion, and the doping of Y, Co, La and Zn in the Cr2O3catalysts were beneficial to the total selectivity. The highest activity was obtained on a La-Cr2O3(F) catalyst with90.6%of PCE conversion and93.7%to total selectivity (HCFC-123+HCFC-124+HFC-125) at300℃. Decrease of surface acid sites density in the catalyst could improve the specific reaction rate, and the formation of surface CrOxFy species could enhance the selectivity to HCFC-124and HFC-125for gas phase fluorination of PCE.A series of La2O3-Cr2O3catalysts with different La loadings were prepared by an co-precipitation method and tested for vapor-phase fluorination of PCE to synthesize HCFC-123, HCFC-124and HFC-125. It was found that PCE conversion gradually increased with increasing La contents, and the total selectivity first increased and then decreased. The highest total selectivity (93.7%) was obtained on a lLaF3-Cr2O3(F) catalyst. Furthermore, the catalyst was stable during the reaction. In these LaF3-Cr2O3catalysts, LaF3played an important role in regulating surface acidity of the catalyst, with small amount of La inhibiting carbon deposit on the catalyst surface.Fluorination of HCFC-123to synthesize HFC-125was performed on various Cr2O3catalysts calcined at different temperatures. It was found that calcination process could result in catalyst structure change as well as the surface acidity. The catalytic activity first increased and then decreased with increasing calcination temperature. The highest activity and total selectivity were obtained on a catalyst calcined at600℃. At reaction temperature of350℃, the conversion of HCFC-123was99%, selectivity to HFC-125was80%and the total selectivities to HCFC-124and HFC-125was92%over this catalyst.