Thermal Decomposition Mechanism of 1-ethyl-3-methylimidazolium Bromide Ionic Liquid (Preprint)

摘要

In order to better understand the volatilization process for ionic liquids, the vapor evolved from heating the ionic liquid 1-ethyl-3- methylimidazolium bromide was analyzed via tunable vacuum ultraviolet photoionization time of flight mass spectrometry (VUV-PI-TOFMS) and thermal gravimetric analysis mass spectrometry (TGA-MS). For this ionic liquid, the experimental results preclude the possible vaporization of intact ion-pairs or the formation of carbenes, but instead indicate the evolution of alkyl bromides and alkylimidazoles, presumably through alkyl abstraction via an SN2 type mechanism. Activation energies for the formation of the methyl and ethyl bromides were evaluated experimentally, and found to be in agreement with calculated values for the SN2 reactions. Comparisons of product photoionization efficiency (PIE) curves with literature data are in good agreement, and ab initio thermodynamics calculations are presented as further evidence for the proposed thermal decomposition mechanism. Estimates for the heat of vaporization of 1-ethyl-3-methylimidazolium bromide and 1-butyl-3- methylimidazolium bromide from molecular dynamics calculations and their gas phase heats of formation by G2MP2 calculations yield estimates for the ionic liquids' heats of formation in the liquid phase.