The Composition of Saturated Vapor over 1-Butyl-3-methylimidazolium Tetrafluoroborate Ionic Liquid: A Multi-Technique Study of the Vaporization Process

Abstract

A multi-technique approach based on Knudsen effusion mass spectrometry, gas phase chromatography, mass spectrometry, NMR and IR spectroscopy, thermal analysis, and quantum-chemical calculations was used to study the evaporation of 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4). The saturated vapor over BMImBF4 was shown to have a complex composition which consisted of the neutral ion pairs (NIPs) [BMIm+][BF4−], imidazole-2-ylidene C8N2H14BF3, 1-methylimidazole C4N2H6, 1-butene C4H8, hydrogen fluoride HF, and boron trifluoride BF3. The vapor composition strongly depends on the evaporation conditions, shifting from congruent evaporation in the form of NIP under Langmuir conditions (open surface) to primary evaporation in the form of decomposition products under equilibrium conditions (Knudsen cell). Decomposition into imidazole-2-ylidene and HF is preferred. The vapor composition of BMImBF4 is temperature-depended as well: the fraction ratio of [BMIm+][BF4−] NIPs to decomposition products decreased by about a factor of three in the temperature range from 450 K to 510 K.

Evaporation/Decomposition Behavior of 1-Butyl-3-Methylimidazolium Chloride (BMImCL) Investigated through Effusion and Thermal Analysis Techniques

“…This evidence suggests that under effusion conditions, the thermal decomposition of BMImCl is strongly enhanced, whereas the release of gas is greatly reduced under open-surface evaporation. These findings confirm the outcomes of previous works on BMImPF 6 [17] and BMImBF 4 [19]. Moreover, the results of the KEML measurements reported above show that the KEMS-derived pressures, being obtained with an orifice of 1 mm diameter, are much more likely lower than the equilibrium values of processes displayed in Equations ( 4) and (5), which means that performing a thermodynamic analysis on such processes would be improper.…”

“…The occurrence of the thermal decomposition of BMImCl, suggested by the TG experiments, was further investigated by conducting Knudsen effusion mass loss experiments. As mentioned in the introduction, it has been recently shown that the competition between evaporation and decomposition in the BMImPF 6 [17] and BMImBF 4 [19] ILs is significantly affected by the degree of confinement in the sample: the greater the confinement is (e.g., effusion conditions with small orifices), the more thermal decomposition becomes important compared to evaporation, and vice versa. Therefore, under the so-called Langmuir or open-surface conditions, the decomposition is so disfavored that the simple congruent evaporation to NIPs can become the largely dominant process [19].…”

“…As mentioned in the introduction, it has been recently shown that the competition between evaporation and decomposition in the BMImPF 6 [17] and BMImBF 4 [19] ILs is significantly affected by the degree of confinement in the sample: the greater the confinement is (e.g., effusion conditions with small orifices), the more thermal decomposition becomes important compared to evaporation, and vice versa. Therefore, under the so-called Langmuir or open-surface conditions, the decomposition is so disfavored that the simple congruent evaporation to NIPs can become the largely dominant process [19]. In spite of the fact that this behavior was observed only recently for ILs, similar evidence was previously reported for some molecular substances [31].…”

“…Unlike TG, KEML, QCM, and other "blind" techniques, information on the composition of the vapor phase can be obtained by KEMS, provided that the ions detected in the mass spectrum are correctly assigned to the corresponding neutral precursors. In the field of ILs, this technique was successfully applied in the past decade for identifying either the NIP or other species produced by evaporation [5,24,25] and to study the co-occurrence of evaporation and thermal decomposition/dissociation phenomena [17,19].…”

“…Subsequently, License et al [18] succeeded in identifying the main decomposition products of 1-alkyl-3-methylimidazolium tetrafluoroborate and hexafluorophosphate, by combining ex situ experiments with mass spectrometry measurements. Finally, Kudin et al [19] showed that under free surface (Langmuir) evaporation, the decomposition of 1-butyl-3alkyl-methylimidazolium tetrafluoroborate was strongly reduced in favor of congruent evaporation into NIPs.…”