Controllable Preparation Of Aluminum Fluoride And Catalytic Performance For Dehydrofluorination Of Fluorinated Alkanes

As a strong solid acid,aluminum fluoride shows excellent Lewis acid property.It can be directly used as the catalyst or the carrier,and is widely adopted as the catalyst in reactions including F/Cl exchange,dehydrofluorination,and disproportionation.In addition,aluminum fluoride catalyst has attracted more and more attention in recent years because of its low cost,rich sources and environmental friendship.In this study,aluminum fluoride catalysts with different morphology,crystal structures and specific surface areas were prepared by three different methods.The morphology,surface properties and phase structure of the catalyst were studied by XRD,BET,FT-IR,XPS,SEM,TEM,NH₃-TPD and other characterization methods.The structure characteristics of the catalyst were correlated with the performance of dehydrofluorination.（1）Uniform AlF₃ spherical catalyst was prepared by PVDF mediated calcination method.PVDF not only plays a template role in the whole reaction,but also control the growth of crystals.By varying in mass ratio（1:1¹:6）between Al（NO₃）₃·9H₂O and PVDF,AlF₃ spherical catalysts with different crystal types（θ-AlF₃,β-AlF₃,andα-AlF₃）and different specific surface areas were obtained.The activity of AlF₃ spherical catalysts was evaluated by dehydrofluorination of 1,1,1,2-tetrafluoroethane（HFC-134a）.It was found thatθ-AlF₃ showed the highest catalytic activity,which was attributed to the large amounts of medium and strong acid on the surface ofθ-AlF₃.Compared with the aluminum fluoride prepared by precipitation method,it was found that aluminum fluoride prepared by PVDF mediated calcination method possesses higher stability without deactivation within 40 h of reaction,indicating that the presence of carbon in AlF₃ particle improves the stability of catalyst.（2）With ethanol,DMF and water as solvents,aluminum fluoride catalysts with different morphologies were prepared by hydrothermal or solvent-thermal methods.The dehydrofluorination of 1,1,1,3,3-pentafluoropropane（HFC-245fa）was adopted as the model reaction to evaluate the activity of these catalysts.The results showed that AlF₃ prepared with ethanol exhibits high catalytic performance and stability.It was found that certain amounts of surfactant CTAB increase the specific surface area of AlF₃ catalyst,and improves the activity of the catalyst.（3）AlF₃ catalyst was prepared by grinding with aluminum nitrate as aluminum source and PVDF as fluorine source.With F/Al molar ratio of 3:1,calcination at 600℃in N₂ atmosphere,the derived AlF₃ catalyst presents the highest activity.The effects of three aluminum precursors,namely aluminum nitrate,aluminum acetylacetone and aluminum hydroxide,on the dehydrofluorination of HFC-245fa were investigated.The results indicate that AlF₃ catalyst prepared with aluminum hydroxide as the aluminum precursors had the best performance and did not deactivate after 14 hours of reaction.In summary,α-AlF₃,β-AlF₃,θ-AlF₃ andκ-AlF₃ were prepared by three different methods.Various morphologies,such as sphere,red blood cell,cube and so on were obtained.The maximum specific surface area of AlF₃ prepared reached 309 m²/g.Following optimization,high catalytic activity for the dehydrofluorination reaction was achieved.