Thermal Stability And Decomposition Mechanism Of Four Imidazole Ionic Liquids

With the increasing awareness of safety,sustainability,and environmental-friendly chemistry,new green chemical process solutions are required.The use of organic solvents is one of the major factors affecting the risks and environmental impact of chemical processes.The emergence of ionic liquids provides the possibility of fundamentally solving the problem of environmental pollution caused by organic solvents in the chemical industry.However,the occurrence of explosion accidents during the production of ionic liquids and the gas generated by their decomposition can break people’s perception of the high thermal stability of ionic liquids.Therefore,it is necessary to explore the thermal stability and thermal decomposition mechanism of imidazolium-based ionic liquids to provide guarantees for achieving green,economic,sustainable,and safe industries.This article investigates the thermal stability,decomposition mechanism,and thermal hazards of imidazolium-based ionic liquids with different alkyl chain lengths of nitrate anions,including[Mmim]NO₃,[Emim]NO₃,[Bmim]NO₃,and[Hmim]NO₃.The specific research work and related conclusions are as follows:The decomposition characteristics and thermal stability of the four ionic liquids were studied using a simultaneous thermogravimetric analyzer.The results showed that the decomposition process of[Mmim]NO₃ and[Hmim]NO₃ occurs in two stages,but the reactants in the second stage are less,while[Emim]NO₃ and[Bmim]NO₃ have only one decomposition stage.When the heating rate was 2.0℃min^–1,their extrapolated onset temperatures were 158.0℃（[Mmim]NO₃）,279.6℃（[Emim]NO₃）,274.8℃（[Bmim]NO₃）,and 273.0℃（[Hmim]NO₃）.The thermal stability of[Mmim]NO₃ was much lower than that of the other three ionic liquids,and the stability of[Emim]NO₃,[Bmim]NO₃,and[Hmim]NO₃ gradually decreases but the difference was not significant.The thermal decomposition products of four ILs were measured using a thermogravimetric-fourier transform infrared spectrometer,and their decomposition mechanisms were analyzed.The results showed that[Emim]NO₃,[Bmim]NO₃,and[Hmim]NO₃ had the same thermal decomposition products,including gaseous water,carbon dioxide,nitrous oxide,methanol,formaldehyde,and nitrotoluene.The thermal decomposition products of[Mmim]NO₃ included gaseous water,carbon dioxide,and nitrous oxide,which were the same as those of the other three ILs,and also included nitrogen dioxide.The decomposition mechanism for all four ILs involves a bimolecular nucleophilic substitution reaction,followed by the formation of 1-methylimidazole and nitrate esters.An accelerated adiabatic calorimeter was used to simulate uncontrolled reactions of four ionic liquids and measure the adiabatic data during the reaction process.The results showed that the adiabatic temperature rise（corrected）ΔT_ad,s of[Mmim]NO₃,[Emim]NO₃,[Bmim]NO₃,and[Hmim]NO₃ were 332.7,240.7,254.2,and 266.4℃,respectively,and the time required to reach the maximum reaction rate（corrected）TMR_ad,s were 19.8,18.1,24.6,and 23.5 min,respectively.After conducting a risk assessment using the safety matrix,it was determined that all four types of ionic liquids pose an unacceptable H-level risk.Therefore,it is necessary to improve safety prevention measures and accident handling procedures.Figure[30]Table[18]Reference[86]...