Chapter 7
In Situ IR Spectroscopic Study of the CO Induced Swelling of Ionic Liquid Media 2
Nikolaos I. Sakellarios and Sergei G. Kazarian*
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Department of Chemical Engineering and Chemical Technology, Imperial College, London SW7 2AZ, United Kingdom
In situ ATR-IR spectroscopy has been used to investigate the effect of high pressure C O on ionic liquid media, since it allows for the simultaneous measurement of two important phenomena (swelling and gas sorption) while it also provides molecular level insight into the behavior of each constituent of the mixture in question. More specifically, in this report, data are presented relating CO pressure with the swelling of room temperature ionic liquids [RTILs] based on the 1-butyl3-methyl-imidazolium cation combined with the hexafluorophosphate anion. The absorbance of the IR bands of [bmim][PF ] was used to calculate the ionic liquid swelling and data up to 72 bar at 25°C were recorded and consecutively compared with literature data, while the V band (2335 cm ) of dissolved C O was used to determine quantitatively its sorption into the ionic liquid medium. 2
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© 2005 American Chemical Society
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In Ionic Liquids III A: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
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90 The realization that the ecosystem is deteriorating due to the use and generation of hazardous substances (e.g. volatile organic compounds) by the plethora of the industrial processes currently in operation, has led to the advent of "green chemistry". A comparatively new concept, "green chemistry" represents a fundamental shift in the approach to chemical production and process plant designing. Furthermore, the collaboration between engineers and chemists involved in "green chemistry" aims at devising new, environmentally friendly, synthesis roots for existing chemicals, which utilize novel alternatives to ordinary solvents. According to Professor Seddon at Queen's University Belfast (2) there are four alternatives, with the most promising being supercritical fluids and ionic liquids, without that meaning that solventfree synthesis or the use of water as a solvent would not be more appropriate in some cases (3). In the past few years, supercritical fluids (SCFs), have earned significant attention from the scientific community involved in the development of cleaner production processes, due to reduced catalyst contamination as well as improved selectivity and efficiency compared with conventional solvents (4). On the other hand, ionic liquids (ILs) are currently attracting significant attention as novel solvents for the development of new "green" technologies due to their fascinating properties; including the ability to tailor them for specific applications, their excellent chemical and thermal stability (J) and their extended liquid range [approx. 300°C for commercially available ionic liquids] (J, 6). However the most intriguing features of ionic liquids is their non-volatile and highly solvating nature, allowing them to be used in deep vacuum systems without any mass loss and also to readily solvate a wide range of organic, inorganic and organometalic compounds (7, 8). Examples of technologies that could benefit from the unique combination of properties presented by ionic liquids are organic synthesis (P), separations (10) and electrochemistry (//) to mention a few. Nevertheless, prior to making the transition between laboratory and industrial scale applications, it must be ensured that sufficient studies have been conducted to address the effect of ionic liquids on living organisms. The reason for this is that although utilizing ionic liquids ameliorates air pollution, it is almost inevitable that at some stage there could be release of small amounts into environment, including the water supplies. It is also possible that pollution may be involved in the manufacturing of ionic liquids. However, the full potential of utilizing these two ''innovative" alternatives to common solvents was realized only when scientist decided to combine them. By combining ionic liquids and supercritical fluids, one can take advantage of the intriguing properties of both media, hence making the resulting chemical processes even more efficient and environmentally friendly. Recent work has demonstrated that C 0 is highly soluble in certain ionic liquids (12-16), an observation extending the potential applications of IL in separating C0 -soluble 2
2
In Ionic Liquids III A: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
91 reaction products from IL by the use of SCF/liquid extraction (17). Additionally, recent work has also reported on liquid phase volume expansion of ionic liquids with the introduction of large amounts of C0 (/). The in situ spectroscopic approach developed by our group provides effective means to investigate these phenomena in more detail and hence try to provide insight at the molecular level. 2
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Theoretical Background In single reflection ATR-IR spectroscopy, IR light is caused to be internally reflected at the interface between a spectroscopic crystal and the sample. Hovever, radiation does in fact penetrate a short distance into the rarer medium, as shown in Figure 1. This penetrating radiation is characterized as an evanescent electric field and its intensity subsides exponentially as it penetrates deeper into the sample, according to the equation below (18): zld
E~E,-e >
(1)
where Ε is the electric field amplitude at a penetration distance ζ into the sample, E is the electric field amplitude at the interface and d is the penetration depth given by: 0
p
P
2
2
ΐη
2π(ηΐ*ΐη θ-η ) 2
where λ is the wavelength of the incident beam, θ is the incident angle and nj and n are the refractive indices of the ATR-IR crystal and the sample respectively. 2
Figure 1: Schematic view of the ionic liquid on the ATR-IR
In Ionic Liquids III A: Fundamentals, Progress, Challenges, and Opportunities; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.
92 However since all quantitative calculations are consistently based upon the molar absorptivity used in transmission spectroscopy, in order to apply the Beer-Lambert law [Equation 6] for analyzing the data obtained, a pathlength giving the same absorbance in transmission as that obtained by ATR-IR spectroscopy is required. This pathlength is labeled effective thickness, d& and it is dependent on the polarization of the incident light (/
