In Situ Attenuated Total Reflection Infrared Spectroscopy of Imidazolium-Based Room-Temperature Ionic Liquids under “Supercritical” CO2

Abstract

In situ high-pressure attenuated total reflection infrared (ATR-IR) spectroscopy has been applied to elucidate the molecular interactions between dissolved CO(2) and three different imidazolium-based room-temperature ionic liquids, 1-n-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF(4)], 1-n-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF(6)], and 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [bmim][Tf(2)N], and also for 1-n-butylpyridinium tetrafluoroborate [bpy][BF(4)] swollen under "supercritical" CO(2) conditions (50 degrees C, 12.0 MPa). The results show that cation species as well as anion species of the ionic liquids affect the molecular state of the dissolved CO(2), because the Lewis acid-base interaction between the ionic-liquid anion species and CO(2) generates new anion species [X-CO(2)](-) (X = anion species of ionic liquid) which are more basic than X(-). Good correlation was found between the extent of imidazolium-ring C-H stretching band shifts and that of the anion species B-F, P-F, C-F, and SO stretching band shifts after the dissolution of CO(2). Largest shift of the ring C-H bands as well as the B-F band was observed for [bmim][BF(4)], indicating that the newly formed anion species [BF(4)-CO(2)](-) is most basic, which seems to be related to the higher activity of [bmim][BF(4)] compared to [bmim][PF(6)] for the catalytic cycloaddition of CO(2) to propylene oxide. Thus, the spectroscopy was applied also for the [bmim][BF(4)]-propylene oxide-CO(2) system (3.0 MPa, room temperature and 80 degrees C) to evaluate the reactivity of [BF(4)-CO(2)](-) and to acquire mechanistic information on the cycloaddition. Based on the spectroscopic results, a new plausible catalytic cycle was proposed, in which [BF(4)-CO(2)](-) species first attacks the electrophilic epoxide carbon, followed by cyclization of the corresponding intermediate to give propylene carbonate product as well as [bmim][BF(4)]. On the other hand, the shift of B-F stretching band of [bpy][BF(4)] under identical supercritical-CO(2) conditions was less drastic compared to [bmim][BF(4)], indicating that the Lewis acid-base interaction between BF(4)(-) and the dissolved CO(2) can be tuned by the choice of the cation species of the ionic liquid. Finally, the studies revealed that the strong Lewis acid-base interaction between the ionic liquids and the dissolved CO(2) has no promotional effect on the solubility of CO(2), because [bmim][Tf(2)N] which showed very small shifts of CF(3) and SO(2) bands and no imidazolium ring C-H band shifts after the dissolution of CO(2) exhibited much higher solubilizing power for CO(2) than [bmim][BF(4)]. The solubility of CO(2) in the ionic liquids seems to be determined by other factors such as the affinity of fluorine atoms for CO(2) and free volumes.