Characterizations Of CrO_x-Y₂O₃ Catalysts For Fluorination Of 2-chloro-1, 1, 1-trifluoroethane

1,1,1,2-tetrafluoethane has been considered as one of the promising alternative of ozone depletion substances(ODS).In this paper,the CrO_x-Y₂O₃ catalysts prepared by a deposition-precipitation method has been employed to test for the fluorination of 2-chloro-1,1,1-trifluoroethane (CF₃CH₂Cl)to synthesize 1,1,1,2-tetrafluoroethane(CF₃CH₂F).The catalysts were characterized by XRD,Raman spectroscopy,H₂-TPR, CO-TPR,BET,TEM and XPS techniques.1.A series of CrO_x-Y₂O₃ catalysts with different Cr loadings were prepared by a deposition-precipitation method and tested for the fluorination of 2-chloro-1,1,1-trifluoroethane(CF₃CH₂Cl)to synthesize 1,1,1,2-tetrafluoroethane(CF₃CH₂F).The highest activity was obtained on a pre-fluorinated catalyst with Cr 19.5 wt%,with 19%of CF₃CH₂Cl conversion at 320℃.The effect of Cr loading on the catalyst composition and structure was investigated.X-ray diffraction and Raman results indicated that the CrO_x species with high oxidation state(Cr(â…¥))were well dispersed on the catalyst surface when the Cr loading was lower than 19.5 %.With the increasing Cr loading,crystalline Cr₂O₃ and polymeric chromate were formed,which resulted in difficulty in pre-fluorination of the catalysts.It was also found that the CrF_x,CrO_xF_y or Cr(OH)_xF_y phase originated from high oxidation state Cr(â…¥)species were the active sites for the fluorination reaction.2.The fluorination of CF₃CH₂Cl to synthesize CF₃CH₂F)was performed on various CrO_x-Y₂O₃ catalysts calcined at different temperatures.The highest activity was obtained on a calcined at 400℃,with 19%of CF₃CH₂Cl conversion at 320℃.Then the catalytic activity decreased with the increasing calcination temperature.XRD and Raman results indicated that the CrO_x species(Cr(â…¥))were well dispersed on the catalyst surface when the catalyst was calcined at 400℃.With increasing calcination temperature,most of the CrO_x species changed from high oxidation state Cr(â…¥)to low oxidation state Cr(â…¤)or Cr(â…¢)species.It was found that the catalytic activity was related to the Cr species,that the high oxidaiton state Cr(â…¥)species were easily to be fluorinated to the active sites,while the Cr(â…¤)or Cr(â…¢)were difficultly to be fluorinated.3.The CrO_x-Y₂O₃ catalysts were doped by Zn or Mg,and the effect of the Zn or Mg on the catalyst sturture and catalytic activity were investaged. The catalytic activity first increased then decreased when the catalyst doped by Zn,the Zn2.5%-CrO_x-Y₂O₃ shows the best activity with a 27% of CF₃CH₂Cl conversion at 320℃.However,when doped by Mg,the catalytic activity decreased,with the increasing Mg content,the catalytic activity decreased.It was found that the CrO_x species with high oxidation state(Cr(â…¥))were well dispersed on the catalyst surface when the catalysts doped by Zn.However when the catalysts doped by Mg,the CrO_x species become polymeric,which resulted in difficulty in pre-fluorination of the catalysts.The low content Zn increased the catalytic activity for the decreasing of the activity energy,however,the high content Zn reduced the total activity site,then the catalytic activity decreased.