Study On Cr-based Catalysts For Vapor-phase Fluorination To Synthesis CF₄ And Preparation Of β-AlF₃ With High Surface Area

Chlorofluorocarbons (CFCs) are considered as stratosphere ozone depletion substances (ODS), which were classified as controlled substances in the Montreal Protocol. For the removal technology of CFCs at present, trans-utilization of CFCs or developing CFCs substitutes is a very significant work.In the past, once widely CCl2F2 (CFC-12) was used as industrial and home refrigerants and propellants etc, which has been found to be the main culprit for the destruction of the stratospheric ozone layer. One of the main research objectives of this study is how to convert CCl2F2 to valuable chemical compounds of tetrafluoromethane (CF4). In this paper, it adopts the vapor phase Cl/F exchange reaction over Cr-based catalysts to synthsis CF4, which using CCl2F2 and HF as the reactants. And the surface Cr species and phase structures for catalysts were purposely investigated in detail by the kinds of characterization technologies. Beacause of the significant influence of AIF3 surpporter on the Cr-based catalyst as the Cl/F exchange reaction over the Cr/AlF3 catalyst, the higher surface area could cause the higher reactivity, so in this work, we report a novel method for convertingγ-Al2O3 to high surface areaβ-AIF3 by the gas-solid fluorination reaction with gaseous HF, just it showed the perfect catalystic activity for Cl/F exchange reation and a series of characterization was conducted to the prepared materials. In short, our works contain the three parts as follows:1. A series of CrOx-Y2O3 catalysts calcined at different temperatures have been prepared using a deposition-precipitation method and tested for vapor phase fluorination dichlorodifluoromethane (CCl2F2) for synthesis of tetrafluoromethane (CF4). A comparative study using CCl2F2 and CCIF3 as the reactants for the fluorination reaction reveals that the conversion of CCl2F2 to CCIF3 is very fast, while that of CCIF3 to CF4 is a rate-determining step. Pre-fluorinated CrOX-Y2O3 catalyst calcined at 600℃(CrYF-6) is found to be the most active for the reaction, CF4 yeild of 95% is stable during the testing period for the fluorination reaction. The characterization results indicate that the catalytic activity depends on the coordination of active Cr species and Lewis acid sites.2. Also, a series of Cr2O3-AlF3 catalysts with different Cr2O3 content were prepared by a deposition-precipitation method for the fluorination of dichlorodifluoromethane (CCl2F2) to synthesize tetrafluoromethane (CF4). The highest activity was obtained on the Cr2O3-AlF3 catalyst containing 61.2 wt% of Cr2O3, with the CF4 yield of 94% at 400℃. X-ray diffraction (XRD) and Raman spectroscopy results indicated the presence of bulk Cr2O3 and a-AlF3 phases in the catalysts. Ammonia temperature programmed desorption (NH3-TPD) and Fourier transformation infrared spectroscopy (FTIR) of pyridine chemisorption revealed strong Lewis acidity of the catalysts. The Cr species and Lewis acid in the Cr2O3-AlF3 catalysts were the active sites of fluorination and dismutation reaction of CCl2F2, respectively. This indicated bi-function catalytic performance of Cr2O3-AlF3 catalysts for fluorination and dismutation reaction.3. Well-crystallizedβ-AIF3 with high surface area was synthesized by a carbon template method. Porousγ-Al2O3 was filled with carbon and transformed to AIF3 by fluorination. After removing the carbon template by thermal combustion, the resultingβ-AIF3 had a surface area of 114 m2 g-1. Temperatures for fluorination and thermal combustion were crucial for the phase composition of the resulting sample. The high surface areaβ-AlF3 was very active for dismutation of CF2Cl2 due to its large amount of surface acid sites.