Preparation And Performance Of Chromium-Based Nanorods Catalysts For Fluorination Of 2-Chloro-1,1,1-Trifluoroethane To 1,1,1,2-Tetrafluoroethane

Chlorofluorocarbons (CFCs) have been widely used in cooling agents, foaming agents, buffing abrasive agents, displacement drying agents, gaseous dielectrics and power cycle working fluids. As chlorine atom in chlorofluorocarbons can easily cleave to form Cl free radical under strong ultraviolet rays, and Cl free radical will deplete ozone, CFCs are considered as stratosphere ozone depletion substances (ODS). With the further implementation of Montreal Protocol on ozone depletion substances, the development of CFCs substitutes becomes particularly important.Chlorine/fluorine exchange reactions are the main reactions for the synthesis of CFCs alternatives such as 1,1,1,2-tetrafluoroethane (HFC-134a), difluoromethane (HFC-32),1,1,1,2,2-pentafluoroethane (HFC-125) and so on. As we all know, Cr2O3 is the main catalyst for the chlorine/fluorine exchange reactions. Many researches have been done on the Cr2O3 catalyst. However, the application of the Cr2O3 catalysts with different morphologies is rarely reported. Thus, this research tries to look for a breakthrough from the morphology of the Cr2O3 catalyst, in order to find a chromium-based catalyst with high catalytic performance.In this paper, ordered Cr2O3 nanorods, Cr2O3 nanorods containing microporous structure and Cr2O3-Y2O3 nanorods catalysts were synthesized by a vacuum assisted impregnation method, and had been tested for the vapor-phase fluorination of CF3CH2Cl to synthesize CF3CH2F. The catalysts were characterized by XRD, TEM, BET, H2-TPR, XPS, NH3-TPD, Raman spectroscopy techniques. The catalytic performance of prepared catalysts was investigated through the vapor-phase fluorination of CF3CH2Cl to synthesize CF3CH2F.The detailed contents of this work are as follows:1. Ordered Cr2O3 nanorods were synthesized by a vacuum assisted impregnation method under the ultrasound-assisted dispersion. For comparison, Cr2O3 nanoparticles were prepared by a deposition-precipitation method. The effects of different morphologies on the Cr2O3 catalyst were investigated in the vapor-phase fluorination of CF3CH2Cl to synthesize CF3CH2F. It was found that at 320 ℃, the HCFC-133a conversion of 24.0% was obtained on the Cr2O3 nanorods catalyst, which was higher than that on the Cr2O3 nanoparticles catalyst (18.5%). Moreover, the Cr2O3 nanorods catalyst had a reaction rate of 0.012 μmol m-2 s-1, which was twice as high as that of the Cr2O3 nanoparticles catalyst (0.005 μmol m-2 s-1). The high catalytic performance of the Cr2O3 nanorods catalyst was related to the oxidation states of the Cr species and surface acidity. The higher content of surface Cr(Ⅵ) species and the higher acid density on the Cr2O3 nanorods catalyst were the two important factors for the better performance.2. Cr2O3 nanorods catalyst containing microporous structure was synthesized by a vacuum assisted impregnation method under the ultrasound-assisted dispersion. Then it was tested for the vapor-phase fluorination of CF3CH2CI to synthesize CF3CH2F. It was found that at 320 ℃, the HCFC-133a conversion of 28.0% was obtained on the Cr2O3 nanorods catalyst containing microporous structure, which was higher than that of Cr2O3 nanorods catalyst (24.0%). Thus, the construction of microporous structure could improve the macro conversion of HCFC-133a. The main reason was that microporous structure could absorb reactant molecules on the surface, which could consequently accelerate the reaction. However, there were regular exposure planes exsiting in the Cr2O3 nanorods catalyst, which was beneficial for the improvement of catalytic performance. The construction of microporous structure destroyed the crystal surface effect, which deacreased the areal specific reaction rates of the Cr2O3 nanorods catalyst containing microporous structure.3. Cr2O3-Y2O3 nanorods catalyst was synthesized by a vacuum assisted impregnation method under the ultrasound-assisted dispersion. Then it was tested for the vapor-phase fluorination of CF3CH2Cl to synthesize CF3CH2F. It was found that at 320 ℃, the HCFC-133a conversion of 26.0% was obtained on the Cr2O3-Y2O3 nanorods catalyst, which was higher than that of Cr2O3 nanorods catalyst (24.0%). Moreover, the Cr2O3 nanorods catalyst had a reaction rate of 0.013 μmol m-2 s-1, which was a bit higher than that of the Cr2O3 nanorods catalyst (0.012 μmol m-2 s-1). It may be caused by the interaction of Cr2O3 and Y2O3.