Confined rapid thermolysis/FTIR/ToF studies of energetic ionic liquids.

摘要

Current research is focused on the identification of the initiation and secondary reactions occurring during the thermal decomposition of energetic ionic liquids (EILs). The synergistic diagnostic tools, rapidscan Fourier transform infrared (FTIR) spectroscopy and time-of-flight mass spectrometry (ToFMS), were utilized in conjunction with confined rapid thermolysis. The thermal decomposition involved submilligram quantities of each compound subjected to heating rates of 2,000K/s and temperatures to 435°C in an ambient inert gas at one atm.The objective of this research is to gain a thorough understanding of the reaction pathways and their corresponding rate parameters that govern the pre-ignition thermal decomposition of EILs. Although particular focus is laid on isolation of the initial reaction pathways, a detailed understanding of the effects of ring structure and the type and location of substituent groups on the secondary reactions are also of interest.The imidazolium salts comprised of 1-ethyl-3-methyl-imidazolium as the cation, and NO3-, Cl-, and Br- as the anions, and are found to be thermally most stable among the three families of salts. Examination of the evolution of gas-phase species reveal that the most probable sites for nucleophilic transfer and subsequent secondary reactions are primarily the methyl group and secondarily the ethyl group. The triazolium family is formed by pairing the 4-amino-1,2,4-triazolium cation with chloride (4ATCl), and nitrate (4ATN) anions, and rank second in thermal stability. Whereas a proton transfer from the N1 position primarily initiates decomposition in 4ATCl, the amino group is found to primarily participate in the initiation reaction in case of 4ATN. The substituted triazolium salts formed by pairing the 1-methyl-4-amino-1,2,4-triazolium and 1-methyl-4-amino-1,2,3-triazolium cations with iodide and nitrate salts are also found to initiate decomposition through the amino group.The compounds studied from the tetrazolium family are 2-amino-4,5-dimethyl-tetrazolium iodide (2AdMTZI) and nitrate (2AdMTZN), and 1-amino-4,5-dimethyl-tetrazolium iodide (1AdMTZI) and nitrate (1AdMTZN). The major decomposition pathway involves a nucleophilic transfer to the anion leading to the formation of methyl iodide and methyl nitrate, from 2AdMTZI and 2AdMTZN, respectively. Unlike the 2AdMTZ salts, the 1AdMTZ salts are found to initiate decomposition through three major pathways---formation of the corresponding methylated anion and 1-amino-5-methyl-tetrazole, formation of ammonia by the amino group, and expulsion of nitrogen from the tetrazole cation itself.The Arrhenius-type reaction rate parameters for the initiation reactions governing the thermal decomposition of several EILs are obtained by an evolutionary genetic algorithm that compares the difference between the experimental and simulated species evolution profiles from the decomposition process. The processes governing the decomposition of these energetic compounds are found to be autocatalytic in nature, and the autocatalytic agents are the strong acids generated by the initial decomposition step. Additionally, the pre-ignition condensed phase chemistry during the hypergolic reaction between 1-ethyl-3-methyl-imidazolium dicyanamide and nitric acid is analyzed by a modified confined rapid thermolysis setup under isothermal conditions.