Preparation Of Cr₂O₃ And Catalytic Performance For The Dehydrofluorination Of HFC-152a To Vinyl Fluoride

VF?vinyl fluoride?is one of the most important raw materials for the production of polyvinyl fluoride which is a polymer material mainly used in the flammability-lowering coatings of airplane interiors and photovoltaic module backsheets.The dehydrofluorination of 1,1-difluoroethane to vinyl fluoride is the most promising method.Chrome oxides show high catalytic activity and posibility for regeneration follwing deactivation and therefore are widely adopted as catalysts in the reactions of dehydrofluorination.Deactivation is one of the main problems of this kind of chemical reactions.This thesis mainly investigates the preparation of chrome oxide and the performances of catalysts prepared with different methods are compared.The results are as follow.1.Preparation of Cr₂O₃ hexagonal prism.Uniform Cr₂O₃ hexagonal prism structure with a prism length of 285 nm and width of 233 nm was successfully synthesized via the reaction of CrCl₃ solution with NaBH₄ solution at room temperature.The fresh Cr₂O₃ catalyst has a surface area of 139 m²/g,a pore volume of0.19cm³/g which is significantly higher than that of porous single-crystal Cr₂O₃synthesized by hard template method and commercial Cr₂O₃.A significant improvement in surface acidity is observed for Cr₂O₃ hexagonal prisms compared with commercial one as well as the existence of relatively strong acidic sites.Cr₂O₃hexagonal prism catalyst show much higher activity and stability for HFC-152a producing CH₂=CHF.At time on stream of 70 h,a loss of activity by 16%was recorded for commercial catalyst.Nevertheless,with the same reaction time,only2.4%decrease in activity was detected for hexagonal prisms.No significant morphology changes or sintering of the catalyst are observed after 70 h reaction.2.Preparation of Cr₂O₃ by solution combustion.With glycine as the feul,the catalyst particles are smaller and more uniform compared with urea and ethylene.[?gly?₂Cr?OH?₂Cr?gly?₂]⁰ was formed via the reaction of Cr³⁺with glycine in the preparation that lead to the a stable precursor which contributed to the uniform particles.The feeding ratio of chrome nitrate to glycine affects the morphology and particle size as well as catalytic performace significantly.With low glycine/chrome nitrate feeding,combustion takes palce under lean-burn conditions.With the increase in glycine feul,combustion temperature is enhaced which favors the formation of Cr₂O₃ crystaline.With the ratio of 10/3,Cr₂O₃ catalyst reveals high surface area and high activity for the dehydrofluoriation of HFC-152a.In addition,it shows high stability as its activity drops only 0.9%after reaction of 70 h.3.Cr₂O₃ synthesized by template method.NaF was used as a soft template during the solution combustion synthesis of Cr₂O₃.Following the doping of NaF,Cr₂O₃ particles aggregated significantly as NaF destroys the stability of the complex state[?gly?₂Cr?OH?₂Cr?gly?₂]⁰ during the preparation of Cr₂O₃ by solution combustion,probably forming[Cr?H₂O?F]^2-.As a result,lower Lewis acid and lower dehydrofluoriation activity was observed.In the last attempt,pollen was selected as a hard template during the precipation synthesis method.The pH of the solution influences the species of chrome during the preparation of Cr₂O₃ by pollen template.Under high pH conditons,Cr³⁺precipates readily in the solution rather than the surface of pollen.Under controlled pH values,the amount of precipitation can be limited and most precipitation takes place on the surface of pollen resulting a porous,linked network structure which is similar to the original pollen structure.