Thermal Decomposition Mechanisms of Alkylimidazolium Ionic Liquids with Cyano-Functionalized Anions

Abstract

Because of the unusually high heats of vaporization of room-temperature ionic liquids (RTILs), volatilization of RTILs through thermal decomposition and vaporization of the decomposition products can be significant. Upon heating of cyano-functionalized anionic RTILs in vacuum, their gaseous products were detected experimentally via tunable vacuum ultraviolet photoionization mass spectrometry performed at the Chemical Dynamics Beamline 9.0.2 at the Advanced Light Source. Experimental evidence for di- and trialkylimidazolium cations and cyano-functionalized anionic RTILs confirms thermal decomposition occurs primarily through two pathways: deprotonation of the cation by the anion and dealkylation of the imidazolium cation by the anion. Secondary reactions include possible cyclization of the cation and C2 substitution on the imidazolium, and their proposed reaction mechanisms are introduced here. Additional evidence supporting these mechanisms was obtained using thermal gravimetric analysis-mass spectrometry, gas chromatography-mass spectrometry, and temperature-jump infrared spectroscopy. In order to predict the overall thermal stability in these ionic liquids, the ability to accurately calculate both the basicity of the anions and their nucleophilicity in the ionic liquid is critical. Both gas phase and condensed phase (generic ionic liquid (GIL) model) density functional theory calculations support the decomposition mechanisms, and the GIL model could provide a highly accurate means to determine thermal stabilities for ionic liquids in general.

Dynamics Simulations and Statistical Modeling of Thermal Decomposition of 1-Ethyl-3-methylimidazolium Dicyanamide and 1-Ethyl-2,3-dimethylimidazolium Dicyanamide

“…This is consistent with the experimental observation, i.e., VUV−PI−TOFMS revealed a significant loss of 1 amu from EMIM + , but loss of 1 amu from EMMIM + occurred to a lesser extent. 8 Overall, our trajectory results are in good agreement with literature and experimental results. We therefore feel confident in using trajectories as a guide to extract additional mechanistic insight.…”

“…48 Also, in the temperature jump FTIR measurement of a similar ionic liquid 1-butyl-3-methylimidazolium dicyanamide (BMIM + DCA − ), methylation of the terminal nitrogen on DCA better matches the experimental spectra. 8 Other Decomposition Paths. Remaining trajectories can be characterized as paths 4, 5, and 6, accounting for 10%, 8%, and 4% of all EMIM + DCA − trajectories, respectively.…”

“…The RRKM predicted that higher stability of EMMIM + DCA − compared to EMIM + DCA − at this low temperature is consistent with the stability trend detected by TGA of these compounds at similar temperatures. 8 At higher temperatures, ethyl abstraction increases surprisingly quickly for EMMIM + DCA − and accounts for more than 60% of its decomposition, resulting in a higher overall decomposition rate of EMMIM + DCA − than that of EMIM + DCA − . The fast increase in ethyl abstraction rate for EMMIM + DCA − can be attributed to the properties of the associated transition state TS2.…”

“…Note that the gas-phase kinetics would be an upper limit to the reaction rates. As concluded in the companion article, 8 using the generic ionic liquid (GIL) model would either keep the barriers the same based on the nucleophilicity analysis of anions, or it would increase the barriers by stabilizing the ion pairs (reactants) relative to the transition states.…”

Isoconversional kinetic analysis applied to five phosphonium cation-based ionic liquids

“…In addition, anion nucleophilicity is another possible approach which should prove useful in predicting quantitative thermal stability trends in ionic liquids [79,80]. The pathway with the lowest activation free energy would be expected to be the most nucleophilic [81]. …”