In situ attenuated total reflection infrared spectroscopy of imidazolium-based room-temperature ionic liquids under "supercritical" CO(2).
In situ attenuated total reflection infrared spectroscopy of imidazolium-based room-temperature ionic liquids under "supercritical" CO(2).
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DOI：
10.1021/jp800424d
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发表时间：
2009-01
期刊：
影响因子：
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通讯作者：
T. Seki;J. Grunwaldt;A. Baiker
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文献类型：
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作者：
T. Seki;J. Grunwaldt;A. Baiker
原位高压衰减全反射红外 (ATR-IR) 光谱已用于阐明溶解的 CO(2) 与三种不同的咪唑基室温离子液体 1-正丁基-3-甲基咪唑鎓四氟硼酸盐 [bmim][BF(4)] 之间的分子相互作用， 1-正丁基-3-甲基咪唑鎓六氟磷酸盐[bmim][PF(6)]和1-正丁基-3-甲基咪唑鎓双(三氟甲基磺酰基)亚胺[bmim][Tf(2)N]，以及1-正丁基吡啶鎓四氟硼酸盐[bpy][BF(4)] 在“超临界”CO(2) 条件（50 摄氏度，12.0 MPa）下膨胀。结果表明，离子液体的阳离子种类和阴离子种类都会影响溶解的CO(2)的分子状态，因为离子液体阴离子种类与CO(2)之间的路易斯酸碱相互作用产生新的阴离子种类[X-CO(2)](-)（X=离子液体的阴离子种类），其碱性比X(-)强。 CO(2) 溶解后，咪唑环 C-H 伸缩带移动程度与阴离子 B-F、P-F、C-F 和 SO 伸缩带移动程度之间存在良好的相关性。 [bmim][BF(4)] 观察到环 C-H 带以及 B-F 带的最大位移，表明新形成的阴离子种类 [BF(4)-CO(2)](-) 是最基本的，这似乎与 [bmim][BF(4)] 与 [bmim][PF(6)] 相比，在 CO(2) 催化环加成反应中具有更高的活性有关。 环氧丙烷。因此，该光谱也适用于[bmim][BF(4)]-环氧丙烷-CO(2)体系（3.0 MPa，室温和80℃），以评估[BF(4)-CO(2)](-)的反应性并获取环加成的机理信息。基于光谱结果，提出了一种新的催化循环，其中[BF(4)-CO(2)](-)物质首先攻击亲电子环氧化物碳，然后环化相应的中间体，得到碳酸丙烯酯产物以及[bmim][BF(4)]。另一方面，在相同的超临界-CO(2)条件下，[bpy][BF(4)]的B-F伸缩带的移动与[bmim][BF(4)]相比不太剧烈，表明BF(4)(-)和溶解的CO(2)之间的路易斯酸碱相互作用可以通过选择离子液体的阳离子种类来调节。最后，研究表明，离子液体与溶解的CO(2)之间强的路易斯酸碱相互作用对CO(2)的溶解度没有促进作用，因为[bmim][Tf(2)N]在CO(2)溶解后表现出非常小的CF(3)和SO(2)带移动并且没有咪唑环C-H带移动，对CO(2)表现出更高的溶解能力 比 [bmim][BF(4)]。 CO(2) 在离子液体中的溶解度似乎由其他因素决定，例如氟原子对CO(2) 的亲和力和自由体积。
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.