| Pentafluoroethyl iodide (C2F5I) has higher reaction activity as well as the special physical chemistry function, which makes it for various applications, such as telogen for the telomerization of tetrafluoroethylene to long-chain perfluoroalkyl iodides, a raw material of resins, functional materials and an intermediate for medicines and agrochemicals. However the traditional methods for the preparation of C2F5I have poor safety, high production cost and great toxicity of raw material. In recent years, a direct method process for the synthesis of C2F5I by the reaction of pentafluoroethane (C2F5H) and iodine (I2) has been developed successfully, which makes the green production become possible. But to the best of our knowledge, in the open literature, little information about it was reported. In this dissertation, a vapor-phase catalytic process for preparation of C2F5I from C2F5H was studied, especially the synthetic routes, reaction mechanism, catalytic technique and technological conditions.Studies confirmed that Joback’s group contribution was the best method to estimate the thermodynamic data of ideal gas C2F5I, and then the group bond contribution method. The thermodynamic data such as ΔGf,298(?),ΔAf,298(?) and Sf,298(?) is-1060.31kJ·mol-1,-1005.79kJ·mol-1and388.34J·mol-1·K-1respectively. According to the Gibbs free energy of the six different synthetic routes, the reaction between C2F5H with I2is an appropriate synthetic route. Thermodynamic calculation also showed that the equilibrium constants increase with the temperature increasing, and temperature rising was beneficial to the synthesis of C2F5I within a proper range. Over the experimental line system, the gas-phase catalytic reaction for the synthesis of C2F5I by the reaction of C2F5H and I2was conducted. The GC and GC-MS showed that C2F5I could be synthesis by this synthetic route.The thermal decomposition properties of C2F5H were studied in a tubular reactor. The decomposed gas was characterized by gas chromatography-mass spectrometry (GC-MS), gas chromatography (GC). The results showed that the hydrogen fluoride eliminated to produce C2F4from C2F5H was the main reaction between600～1000℃, while in the pyrolysis of C2F5H over catalyst Rb-K/AC at550℃, the main products contained C4F10and C3F8,but no C2F4was detected. Nine primary reaction pathways were proposed based on the chemical bond types of the hemolytic cleavage. Using the DFT-(U)B3LYP/6-31G*method, the bond dissociation energies for all kinds of bonds were calculated. It was found that the activation energy of the H-transfer reaction and the C-C bond fission reaction, which are211.73kJ/mol and380.10kJ/mol, the activation energy of the production of CF3CF:and CF2:carbine were426.61kJ/mol and943.21kJ/mol. The result illustrated that the hydrogen fluoride elimination was the most feasible reaction in pyro lysis of C2F5H through empty reactor, but over catalyst Rb-K/AC, the formation of CF3CF: carbine was. H2as capture agent, was fed into reactor with C2F5H over catalysts or non-catalyst. It was found that in pyrolysis of C2F5H through empty reactor in the presence of H2, C2H2F4was obtained via the reaction of C2F4carbene with H2. But over catalyst, the formed CF3CF: carbene was adsorbed on the surface of AC. Moreover, the X-ray photoelectron spectroscopy (XPS) studies confirmed that the Cls signal around282.1eV could be assigned to C. It is proposed that in high temperature, catalyst Rb-K/AC promotes the dehydrofluorination of C2F5H to form CF3CF:carbene, and CF3CF: disproportionates to generate CF3CF2-radical and a new carbonaceous carbon which has different structure to graphite, at last CF3CF2-radical reacts with I-to form C2F5I.The catalytic technique, including the active component and the carrier in the catalyst was investigated in detail. It was found that alkali metal salts as the active component showed a higher catalytic activity, especially the combined active substance having20wt%loading and2:1mass ratio of RbNO3and KF. The Rb-K/AC catalyst was characterized by TG-DTA, FTIR, XPS and XRD, and the results indicated that the species of the active component RbNO3before the reaction was Rb2O, and during the reaction process were Rb2O and RbF due to the reaction of Rb2O with the by-product HF. Activated carbon (AC), as the catalytic carrier, showed higher activity due to its high surface area, low ash content and slightly alkaline. And porous metal fluoride, such as porous aluminum fluoride and porous magnesium fluoride, as carriers showed medium activity and stable properties in the presence of O2and HF. So, Porous metal fluoride processed industrial potential application as carrier for this reaction.Modification of AC with different acid solution might result in various surface oxygen groups, which can enhance the anchoring interaction between AC supports and metal precursors and change the dispersion and basicity of the catalyst. The influence of HCl, HNO3, and HF treatments on AC used as a support for Rb-K catalysts in the synthesis of C2F5I by reacting C2F5H with I2was studied. It was found that after the HCl treatment, the Rb-KF/AC-HCl catalyst with a high dispersion and moderately basicity was helpful for the enhancement of catalytic activity for C2F5I synthesis. The order of catalyst dispersion was as follows:Rb-K/AC-HNO3> Rb-K/AC-HF> Rb-K/AC-HCl>Rb-K/AC. The same sequence was also observed for the amount of the acid surface oxygen groups on AC, but not for the basicity of the catalyst. The key role of acid treatment on AC surface chemistry and the basic sites, which are closely related to catalyst dispersion and basicity, is examined to rationalize these findings. Based on the thermodynamic analysis and catalytic mechanism, the reaction conditions such as reaction temperature, space velocity and the molar ratio were optimized after eliminating the influence of internal and external diffusion. The appropriate reaction temperature was550～600℃, space velocity was110～130h-1and the molar ratio of C2F5H and O2was21.7:4. Under the optimum conditions mentioned above, the catalyst life of Rb-K/AC was enhanced remarkably.To improve further the catalyst life, the variations of catalysts after reaction and the deposited carbon over the catalysts were analyzed and characterized by XPS, TEM and TG-DTG etc. The results of the experiment indicated that the deactivation was caused mainly by the deposition of coke formation which led the pore blocking and active site coverage. Further investigation revealed that the coke species over Rb-K/AC catalyst include CF, CF2, CF3and carbonaceous carbon, and the mechanism of the coke formation is the disproportionation of C2HF5and CF3CF:carbene. The addition of O2to the reaction system could prevent the catalyst deactivation by burning off the coke during150～350℃. |
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