| Imidazolium ionic liquids are recognized as novel green solvents since its appearance.The excellent physicochemical properties of imidazolium ionic liquids,including the preeminent thermal stability,outstanding electroconductivity,and low toxicity are consistent with the intrinsically safe chemicals advocated by green chemistry,which made imidazolium ionic liquids a rising star in modern solvent chemistry.For the longest time,imidazolium ionic liquids were considered as non-flammable substance when considering its fairly-low vapor pressure.The main cause of the phenomena is that the force from the ionic bond is far greater than intermolecular forces Nonetheless,our experimental results have overturned this deduction and indicate that imidazolium ionic liquids may also lead to a fire or deflagration accident due to the rapid chemical reaction of its decomposition products when imidazolium ionic liquids are placed near the fire source or under a high-temperature environment.Hitherto,previous researches mainly focused on the industrial application of imidazolium ionic liquids,little information was known about its combustion and explosion risks.To ensure the intrinsic safety in the application of imidazolium ionic liquids,the investigation of thermal stability and flammability is thus inevitable.Three typical imidazolium ionic liquids including 1,3-dimethylimidazolium nitrate([Mmim][NO3]),1-butyl-3-methylimidazolium nitrate([C4mim][NO3]),and 1-butyl-2,3-dimethylimidazolium nitrate([C4mmim][NO3]),were chosen as examples to illustrate the methods to investigate the thermal decomposition and flame behaviors of imidazolium ionic liquids.The specific methods and relevant conclusions are as follows:Simultaneous thermogravimetric analyzer(STA)with a real-view sample observation unit and a differential scanning calorimetry(DSC)were used to preliminarily evaluate the thermal stability of three imidazolium ionic liquids.The results indicated that the order of thermal stability was[C4mmim]NO3>[C4mim]NO3>[Mmim]NO3.Methyl group substitution in the C2-position blocked the substitution nucleophilic decomposition mechanism,thus increasing the thermal stability.In addition,the replacement of ring hydrogen with a substituent group could increase the steric hindrance and van der Waals interactions,which was not favorable for the thermal degradation through the SN1 or SN2 mechanism,thus causing higher thermal degradation temperature of[C4mmim]NO3 Comparatively analyzing the Ea obtained by FWO and KAS methods,the difficulty in the decomposition of three imidazolium ionic liquids was:[Mmim]NO3<[C4mim]NO3≈[C4mmim]NO3,which showed the worst thermal stability of[Mmim]NO3 among three imidazolium ionic liquids from the side.Accelerating rate calorimeter(ARC)was used to simulate the thermal runaway reactions of three imidazolium ionic liquids under adiabatic conditions.The results indicated that[Mmim]NO3 presented the lowest stability and would decompose at a very low temperature.The "thermal inertia" was used to modify the temperature parameters,and it was found that the thermal hazard of the three imidazole ionic liquids was far greater than the test value under actual adiabatic conditions.At the same time,the safety risk assessment was performed on three imidazolium ionic liquids.The results indicated that all three imidazole ionic liquids belong to level III unacceptable risks.Therefore,it is necessary to take corresponding measures to reduce the risk level and ensure production safety.Thermogravimetry coupled with Fourier transform infrared spectroscopy(TG-FTIR)was used to analyze the thermal decomposition products and decomposition mechanisms of the three imidazole ionic liquids.It was found that the same gaseous product of the three imidazole ionic liquids was CO,which had a low content.The gaseous products of[Mmim]NO3 also contained N2O,NO2,and 1-methylimidazole.On the other hand,the gaseous products all contained CH3NCO,CH3CN,CH3OH and HCHO for[C4mim]NO3 and[C4mmim]NO3.The difference was that the gaseous products of[C4mim]NO3 also included N2O and 1-methylimidazole,while the gaseous products of[C4mmim]NO3 were NO2 and 1,2-dimethylimidazole.It could found that[Mmim]NO3 had two different decomposition pathways,but the main products were N2O and NO2.[C4mim]NO3 and[C4mmim]NO3 had the same decomposition mechanism,however,the main decomposition pathway was different.[C4mim]NO3 mainly decomposed to produce N2O and CH3OH,while the main products of[C4mmim]NO3 were CH3NCO,CH3OH and HCHO.A self-designed combustion test device and a high-speed camera were used to evaluate the flammability of four imidazolium ionic liquids.Expectedly,all three imidazolium ionic liquids will spontaneously combust during the heating process,and the combustion degree is tightly associated with the decomposition rates.With the increase of decomposition rates,the combustion phenomenon will be more obvious.TG and DSC results revealed the fairly-low vapor pressure of three imidazolium ionic liquids and the FTIR confirmed the existence of combustible products.Therefore,it was reasonable to conclude that the flammability of three imidazolium ionic liquids came from the combustible gaseous decomposition products rather than their own evaporation.Figure[47]table[22]reference[111]... |
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