Spectroscopy on Supported Ionic Liquids

The immobilization of ionic liquids (ILs) is intimately connected with spectroscopy, with the goal to characterize the support, the IL film, or the catalyst. Besides the characterization by BET surface methods and catalytic activity, the most powerful tools are nuclear magnetic resonance (NMR) spectroscopy and infrared (IR) spectroscopy, which are discussed separately in the following. NMR spectroscopy is mostly performed in the solid state, where either the support itself or the IL film with the dissolved catalyst therein is characterized. For the latter case, some liquid-state NMR experiments are also applicable. Besides the characterization of the SILP system itself, solubility effects of the reactants using liquid-state NMR are well established to understand the kinetics. Those examinations are beyond the theme of this chapter and are therefore not discussed here. 

Further spectroscopic methods such as, for example. X-ray diffraction (XRD), were successfully apphed on the supported ionic liquid phase (SILP) in some cases. The results are rare to date, and are not discussed in this chapter. X-ray photoelectron spectroscopy (XPS) measurements are discussed in Chapter 7. 

Supported Ionic Liquids Fundamentals and Applications, First Edition. 

Edited by Rasmus Fehrmann, Anders Riisager, and Marco Haumann. 

---

The very first report on the use of ionic liquids as soluble supports was presented by Fraga-Dubreuil and Bazureau in 2001 [102]. The efficacy of a microwave-induced solvent-free Knoevenagel condensation of a formyl group on the ionic liquid (IL) phase with malonate derivatives (E1CH2E2) catalyzed by 2 mol% of piperidine was studied (Scheme 7.89). The progress of the reaction could be easily monitored by 1H and 13C NMR spectroscopy, and the final products could be cleaved from the IL... 

The immobilised ionic liquids have been extensively analysed. FT-IR and MAS-NMR spectroscopy show the disappearance of the hydroxyl groups on the surface of the supports. In FT-IR spectra this can be seen by a band at 3741 cm"1 assigned to terminal OH-groups which disappears after addition of an ionic liquid (spectra not shown). In crosspolarised MAS-NMR spectra the signals at -91 ppm and -101 ppm assigned to (SiO)2Si(OH)2 and (SiO Si-OH groups respectively cannot be detected after immobilisation (Figure 12). 

One important use of SFG vibrational spectroscopy is the orientational analysis of ionic liquids at gas-liquid interfaces. For example, the study of the structural orientation ofionic liquids using common cation types, that is, [BMIM], combined with different anions, gives information on the effects of both cation and anion types [3, 22, 26-28]. Additional surface analytical work includes SFG studies under vacuum conditions for probing the second-order susceptibility tensor that depends on the polar orientation of the molecule and can be correlated to the measured SFG signal intensities. Supporting information is frequently obtained by complementary bulk spectroscopic techniques, such as Raman and Fourier transform infrared (FTIR) analysis, for the analysis of the pure ionic liquids. 

---