Preparation And Characterization Of Metal Fluorides With High Specific Surface Areas

Gas-phase fluorinstion is a mainly industrial reaction for the production of refrigerants,foaming agents,aerosols,and cleaning agents,in which gaseous HF is often used as fluoride reagent while metal fluorides are used as fluorination catalysts.In general,metal oxides are often as precursors for fluorination catalysts,and these metal oxides are fluorinated into metal fluorides using gaseous HF or fluorine-containing organics as fluorination reagents.However,metal fluorides(mainly CrF3,AIF3,and MgF2,etc.)currently applied in the industry have rather low specific surface areas,leading to their low catalytic activities in fluorination reactions.Therefore,research and development on metal fluorides with high specific surface areas are of utmost importance in terms of fundmental research and industrial application.The main objective of the current research is to prepare metal fluorides with high surface areas by means of optimising synthesis conditions.Regarding the preparation of MgF2 with high specific surface area,the main results are summarized as follows:1)A co-precipitation method was used to prepare S1 and Mg composite oxides and/or hydrates,and then the prepared composites were reacted with hydrofluoric acid to convert into MgF2.The effects of the type and dosage of magnesium salt and silicon source as well as of metal doping on the textural properties of resultant MgF2 were systematically investigated.The results show that the prepared MgF2 would have the largest surface area when magnesium nitrate and sodium silicate are used as raw materials.In addition,the surface areaof the synthesized sample increases first and then decreases as the molar ratio of Si to Mg increases,and the BET surface area of MgF2 is 75 m2/g with a mesopore structure when the molar ratio of Si to Mg is 1:6,Furthermore,the surface area of MgF2increases first and then decreases when the amount of Cr doping increases,the pore size of the resultant sample becomes smaller but still possesses the characteristic of mesoporous materials,and the highest BET specific surface area is up to 155 m2/g.Finally,the trend of A1 doping into MgF2 is very similar to that of Cr doping,indicating that metal doping would enhance the surface area of MgF2.2)The synthesis method via the reaction of Mg-based metal-organic framework Mg-MOF-74 asMg source and hydrofluoric acid to prepare MgF2 was investigated,during whichthe effects of different wash solutions and calcination temperature on the textural properties of MgF2 samples were studied.The results show that the specific surface area of MgF2 decreases with increasing either the polarity of the wash solution or the calcination temperature.When the synthesized samples are washed with methanol and then calcined at 300?,the BET specific surface area of MgF2 can be up to 138 m2/g.In addition,the preparation of Cr2O3 with high specific surface area was carried out via a precipitation method,in which chromium nitrate and ureawere used as Cr source and as pH controlled medium and a small dosage of silicon was added into the synthesis solution.The effects of the amounts of urea,reaction temperature,sodium silicate and aceticacid on the textural properties of Cr2O3were investigated.The results show that under the optimized synthesis conditions,the BET specific surface areaofthe prepared Cr2O3 is as high as 350 m2/g.