Mechanism Of Thermal Decomposition Of HFO-1234yf By DFT Study

In recent years,with the intensification of global warming,the European Union（EU）has carried out a series of legislation to control emissions from fluorinated greenhouse gases（F-gases）with global warming potential（GWP）higher than 150.Thus,presently used third-generation refrigerants like R134 a will be replaced.The global warming potential（GWP100）of 2,3,3,3-Tetrafluoro-1-propene（HFO-1234 yf,CF₃CF=CH₂）is only 4 and its lifetime in the atmosphere is only 11 days.HFO-1234 yf also has a low toxicity and a mild flammability and is regarded as one of the most promising alternative working fluids.As a kind of working fluids,HFO-1234 yf has good physical and chemical properties which make it have bright prospects in ORC cycle and has earned much concern from many researchers.With the difference of refrigeration cycle,the operation of ORC cycle often occurs over-temperature phenomenon which may lead to pyrolysis of working fluid.However,the thermal stability and pyrolysis properties of HFO-1234 yf have not been studied by scholars.Density functional theory is employed to investigate the reaction pathways and products in the thermal decomposition of HFO-1234 yf,the detailed contents are as follow.（1）Density functional theory is employed to study the preliminary pyrolysis of HFO-1234 yf.The reaction pathway of CF₃CF=CH₂ exciting to triple excited state CF₃CF-CH₂ and direct homogeneous reaction pathway are compared.The results show that the reaction pathway of CF₃ CF = CH₂ exciting to triple excited state CF₃CF-CH₂ needs lower activation energy.（2）By analyzing the reaction pathways of CF₃CF-CF₂,activation energies of generating CF₃,H and F radical are compared.In the initial stage of thermal decomposition of HFO-1234 yf,the activation energy required to produce F radicals is much higher.In contrast,activation energy required to generate CF₃ radicals and H radicals is low.（3）The chain reactions between HFO-1234 yf and CF₃ radicals,H radicals,F radicals are investigated.The activation energies of reactions that CF₃CF=CH₂ is abstracted by CF₃,H and F radicals are calculated.Among these reactions,CF₃ radical forms CF₃ H and CF₄,H radical forms HF and H₂,F radical forms HF.The results reveal that CF₃ H,CF₄,HF and H₂ is likely produced in chain reactions.（4）The pyrolysis pathways of the intermediates IM1-IM5 produced by the first two reaction stages were analyzed.Among these pathways,IM1,IM2 and IM3 forms CF₃C≡CH.IM4 forms CF₂=C=CH₂.IM5 forms CF≡CH The results show that the activation energy required for generating CF₂=C=CH is much higher,so CF₂=C=CH is not likely produced.The activation energies required for generating CF₃C≡CH and CF≡CH is much lower,CF₃C≡CH and CF≡CH should be generated.