Theoretical Study On The Gas-phase Catalytic Synthesis Mechanism For C₁-C₂ Perfluoroalkyl Iodide

Short-chain(C1-C2)perfluoroalkyl iodide(CF3I and C2F5I)has high reaction activity as well as the extraordinary physical chemistry function,which makes it suitable for various applications,such as an intermediate for organic fluorine compounds,telogen for the telomerization of tetrafluoroethylene to long-chain perfluoroalkyl iodides,a raw material of fluorinated surfactant and fluorocarbon finishing agents.In addition,C1-C2 perfluoroalkyl iodide has potential applications in replacement of refrigerants and Halon extinguishing agents due to its low GWP value and low ODP value.The traditional synthesis technology does not seem to be the candidate for large-scale production due to the high production cost,low yield,batch processes,poor safety and great toxicity of raw material.In recent years,a direct technic process for the synthesis of C1-C2 perfluoroalkyl iodide by the reaction of C1-C2 perfluoroalkyl hydride(CHF3 and C2HF5)and iodine(I2)has been developed successfully.But to the best of our knowledge,there are only a f-ew experimental studies on the synthesis mechism of C1-C2 perfluoroalkyl iodide,which is still controversial.In this thesis,density functional theory(DFT)has been employed to investigate the mechanisms for the formation of C1-C2 perfluoroalkyl iodine and byproducts catalyzed by activated carbon(AC)and potassium fluoride(KF)in detail.The main contents are summarized as follows:Catalytic behavior over AC:First,the dehydrofluorination of C1-C2 perfluoroalkyl hydride is catalyzed by-COOH groups of AC to afford perfluorocarbene.which possesses the highest barrier(57.94 kcal/mol for CHF3,73.21 kcal/mol for C2HF5)and is accordingly identified as the rate-determining step.Second,the C1-C2 perfluorocarbene combines with graphite(001)surface strongly and disproportionates over graphite(001)surface via barriers of 36.82 kcal/mol(CF2)and 34.09 kcal/mol(CFCF3)to afford CF,CCF3 and C1-C2 perfluoroalkyl,instead of dimerization.C1-C2 perfluoroalkyl and CF associate via barrier(less than 25 kcal/mol)to afford C2-C3 perfluorocarbene on graphite(001),leading to the carbon chain growth.Next,the fluorine abstractions between the above fluorocarbon intermediates proceed with barriers of 2 kcal/mol-39 kcal/mol to afford free C1-C3 perfluoroalkane and adsorbed C1-C3 perfluoroalkyl,C radical and CC radical on graphite(001).Finally,C1-C3 perfluoroalkyls desorb from graphite(001)via barriers between 10 kcal/mol and 12 kcal/mol,and abstract the iodine atom from molecular iodine spontaneously to afford C1-C3 perfluoroalkyl iodide.Catalytic behavior over KF(111):The reactions between the fluorocarbon intermediates and KF(111)surface follow fluorine exchange mechanism.First,C1-C2 perfluorocarbene and C1-C2 perfluoroalkyl are adsorbed to hcp side or fcc side of KF(111)-Kn surface(KF(111)with potassium atom exposed to the surface)and fluorocarbon bonds are actived at different degree.Second,the above fluorocarbon intermediates proceed multi-step defluorination via barriers of 2 kcal/mol-29 kcal/mol over KF(111)-Kn surface to produce C and CCF radicals adsorbed in hep site or fcc side.Next,the fluorine atom of CCF is abstracted by C1-C2 perfluorocarbene or C1-C2 perfluoroalkyl to afford adosrbed CC radical on KF(111)-Kn surface via barriers of 22 kcal/mol-33 kcal/mol.The procedures mentioned above result in fluorine covered KF(111)-Kn surface,namely KF(111)-Fn.C1-C2 perfluoroalkyl and C1-C2 perfluorocarbene could abstract the fluorine atom of KF(111)-Fn surface spontaneously or though a moderate barrier(9 kcal/mol-31 kcal/mol)to afford free C1-C2 perfluoroalkane and adsorbed C1-C2 perfluoroalkyl on the KF(111)-Fn surface respectively.Finally,C1-C2 perfluoroalkyl desorp from KF(111)-Fn surface through barriers of 4 kcal/mol-29 kcal/mol,and then reacts with iodine to afford C1-C2 perfluoroalkyl iodide.In addition,when C2F5 is adsorbed with fcc-C/hcp-2F adsorption mode,the process of carbon-carbon bond rupture could proceed with a moderate barrier(28.23 kcal/mol)to afford the adsorbed CF2 with fcc-C/hcp-F and the adsorbed CF3 with fcc-F/hcp-F.Since the above reactions proceed at high temperature,the enthalpy correction(including ZPVE-correction)for barriers and reaction energies are calculated under different temperatures(298.15k,500k,825k and 1000k).The above mechanism is in agreement with available experimental data.