Gas-Phase Synthesis Of HFC-134a And HFC-125

Chlorofluorocarbons (CFCs) were widely used as refrigerants, foaming agents, solvents, cleaning agents and fire extinguishers, but they are ozone depletion substances (ODS) and contribute to global warming. To protect the ozone layer, the Montreal Protocol stipulates that CFCs must be phased out and substitutes for CFCs must be found. HFC-134a (1,1,1,2-tetrafluoroethane) has been generally accepted as a promising and environmentally-friendly CFC-12 (difluorodichloromethane) alternative.Co-precipitation and impregnation method were applied to prepare a Cr-Zn-Al catalyst precursor, which was used to fluorinate HCFC-133a (1,1,1-trifluoro-2-chloroethane) in the gas phase to synthesize HFC-134a. After being formed into columnar pellets, the precursor should be programmed sintered and activated. The catalyst was analyzed by BET, XRD and EDAX. The results show that the precursor is amorphous and has a surface area of 259m²/g. During the process of activation, it changes into partial crystal and CrF₃·3H₂O is found. In the catalyst solid chromium is present in many forms.Experiments of fluorinating HCFC-133a were carried out under different temperatures and feed rates. By-products were also investigated by GC-MS. The effects of reaction conditions on conversion of HCFC-133a and selectivity of HFC-134a were clarified. The results reveal that Cr-Zn-Al catalyst has a good catalytic activity and stability. When molar ratio of feeding HF to HCFC-133a changes from 8 to 10 at 350℃, conversion of HCFC-133a is above 30% and selectivity of HFC-134a is over 98%. The main by-products are HFC-143a (1,1,1-trifluoroethane) and HFC-125 (1,1,1,2,2-pentafluoroethane), and no HCFC-1122 (l,l-difluoro-2-chloroethylene) is found. Amorphous chromium oxide catalyst often deactivates within 120 hours due to coke deposition, but no big change on the Cr-Zn-Al catalytic activity is found after 360 hours' reaction.It is because a few of Zn and Al was supported that catalytic activity and stability of the Cr-Zn-Al catalyst is superior to amorphous Cr₂O₃ catalyst. The presence of oxygen is indispensable for the fluorination in the Cr-Zn-Al catalyst, which makes itdifficult to propose a reaction mechanism about the reaction of HCFC-133a and HF at the surface of catalyst.Furthermore, the Cr-Zn-Al catalyst was also applied to fluorinate HCFC-123a (l,l,l-trifluoro-2,2-dichloroethane) in the gas phase to synthesize HFC-125 at 350°C. The results show that total selectivity of HFC-125 and HCFC-124a (l,l,l,2-tetrafluoro-2-chloroethane) is above 98%. That means it will be practicable to co-produce HFC-134a and HFC-125 if HCFC-133a and HCFC-123a are fed together.