Characterization And Preparation Of The High Surface Areaα-Alf₃, Cr₂O₃and Cr₂O₃-α-alf₃Catalyst

It is well known that aluminum fluoride is important inorganic materials, since it can be used both as commodity chemicals and as catalysts or support for the ozone-friendly alternatives to chlorofluorocarbons. It is reported that the chemistry of the aluminum-fluorine system is very rich through the existence of several forms of AlF3α,β,γ,ε, δ,η,θ,к-AIF3, the most common phase is the α-phase and β-phase, the β-phase posses the significantly catalytic activity and strong Lewis acid sites compared to the α-phase due to its the significantly high surface area. However, for some high temperature reactions, during reaction process β-AIF3has occurred crystalling phase change into stable α-AlF3, resulting in the specific surface area decreased dramatically. Therefore, it is necessary that synthetic high surface area of α-AIF3supports.Cr-AlF3catalysts are widely applied in various halogen containing organic reactions. It is well known that the catalytic performance of Cr-AIF3catalyst is correlated to its surface property such as surface acid sites. Since the surface acid amount depends on the surface area, the catalyst with high surface area is always desirable. Therefore, an improvement of the reactivity and stability of the Cr-AIF3catalyst may lie in the synthesis of stable Cr-AlF3catalyst with high surface area. In short, our works contain the three parts as follows:1. High surface area α-AIF3(HS-α-AIF3) has been prepared by a carbon hard template method. The synthesis procedure consisted of three consecutive steps:First, impregnation of sucrose (C12H22O11) aqueous solution with γ-Al2O3and a followed thermal treatment. Second, thermal treatment of the obtained solid with HF. Third, removal of the carbon template with high temperature combustion. Combined with XRD, BET, NH3-TPD、SEM-EDX and the FTIR spectra of pyridine adsorption results, it was found that the optimal conditions for the highest surface area of the synthesized α-AIF3(66m2·g-1) are as follows:The temperatures for fluorination, carbon calcination and carbon removal are400,450and425℃respectively, and the HF/N2volume ratio is1:4. In addition, the HS-α-AIF3catalyst was more active for the dismutation of CCl2F2than the catalyst prepared by a direct fluorination of the y-Al2O3, due to its higher amount of acid sites.2. A high surface area Cr2O3catalyst was prepared using a modified sol-gel method. It was found that the optimal conditions for the highest surface area of the synthesized Cr2O3(61m2·g-1) are as follows:The temperatures for carbon calcination and carbon removal are400℃respectively, and the molar ratio of sucrose and chromium ion is1:4. In addition, the high surface area Cr2O3catalyst was more active for the fluorination of CF3CH2Cl to synthesize CF3CH2F than the catalyst prepared by a direct synthesis, due to its higher amount of acid sites.3. A high surface area Cr2O3-a-AlF3catalyst was also prepared using a carbon hard template method. It was found that the fluorination process was crucial for the resulting Cr2O3-a-AlF3, with a high surface area of115m2·g-1at the optimal conditions. The high surface area Cr2O3-a-AlF3catalyst was more active for the decomposition of1,1-difluoroethane than the catalyst prepared by a direct fluorination of the Cr2O3-γ-Al2O3, due to its higher amount of acid sites.