Selection of Ionic Liquids for Green Chemical Applications - ACS

Chapter 1

Selection of Ionic Liquids for Green Chemical Applications

Downloaded by NEW YORK UNIV on October 16, 2014 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch001

John D. Holbrey, Megan B. Turner, and Robin D. Rogers Center for Green Manufacturing and Department of Chemistry, The University of Alabama, Tuscaloosa, AL 35487

Ionic liquids (ILs) are proving to be increasingly promising as viable media for not only potentially 'green' synthesis and separations operations, but also for novel applications, where the unique property set of the IL materials provides new options based upon different chemical and physical properties. The range and variability in the properties between individual examples within the class of solvents that are known as ILs, however, are both challenges, and opportunities for developing new and improved processes. Some of the challenges in understanding IL behavior and in selecting specific IL media for applications is presented in the context of research from The University of Alabama.

Introduction The 'greening* of chemical technology is based on opportunities, both real and perceived, to improve processes by using 'green principles' (/) that translate to improvements in production processes. These can be achieved by reducing or eliminating waste, improving chemical syntheses, extractions and/or separations, and reusability of reaction solvent. One of the core aspects of green chemistry that is attractive for both industrial and academic research teams to tackle is the

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© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


3 redesign of chemical processes to reduce, or eliminate, losses of solvents, particularly in situations where these are volatile organic compounds (VOCs). A number of approaches can be made, including incremental modification to reaction design (process optimization), increased efforts in recovery and recycling, and implementing a switch to solvents that are more environmentally benign. This can include both examples that are intrinsically biodegradable, or 'naturally occurring', or by design, utilizing properties of the solvents to ensure that they are not inadvertently released to the environment. The properties of ILs, and their uses as solvents for chemical reactions, have been reviewed extensively in recent years (2,3). "Ideal" solvent requirements may include low toxicity, low cost, high solute selectivity, inertness to materials, non-flammability, high capacity for solutes, low carrier selectivity, and moderate interfacial tension. Ionic liquids (ILs) can meet some of these requirements now, but it remains necessary to address issues of cost, availability, toxicity, and recycling.

Ionic Liquids - An Endless Trail of Possibilities The term 'Ionic Liquid' is used loosely to describe organic salts that melt below about 100 °C (4) and have an appreciable liquid range. ILs define a class of fluids rather than a small group of individual examples - the implications of this, with respect to the choice of ILs for particular, specific, processes will be developed later. The most commonly studied systems contain ammonium, phosphonium, pyridinium, or imidazolium cations, with varying heteroatom functionality. Seddon (5) has remarked that over 10 simple organic salts that might be potential ionic liquids could be prepared by varying the substitution patterns and anion choices, even just within imidazolium and pyridinium systems. In fact, over 30,000 1,3-functionalized imidazolium entries are recorded in the CAS database. Further scope for derivatization beyond ramification of linear alkylsubstituents, for example with branched, chiral, fluorinated, or an activefunctionality, can yield further useful materials. The degree and type of substitution renders the salts low melting, largely by reducing cation-anion Coulombic interactions and disrupting ion-ion packing. This results in low melting salts with reduced lattice energy and a marked tendency to form glasses on cooling, rather than crystalline solids (6,7). Common anions that yield useful ILs include hexafluorophosphate [PF ]\ tetrafluoroborate, [BF ]\ bis(trifyl)imide, [NTf ]\ and chloride, CI*. Although high symmetry pseudo-spherical, non-coordinating anions are commonly regarded as optimal for formation of ILs, the existence of low melting ILs containing anions such as methylsulfate (S), dicyanamide (9), and bis(trifyl)imide (10), show that shape and ion-interaction factors are far from clear-cut. Anions can control the solvent's reactivity with water, coordinating ability, and hydrophobicity. The [PF ]" and [NTfJ" anions produce hydrophobic 18

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In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


4 solvents due to their lack of hydrogen-bond accepting ability (11), though not all ILs containing these anions are hydrophobic. Control over hydrophobicity and other physical properties is governed by cation-anion pair interactions. The range of functionality and resulting properties of ILs appear to suggest that chemometric design and factorial approaches to developing new ILs and studying the properties and characteristics would be advantageous: however, as yet, this does not appear to have been exploited. ILs are curious materials to be posited as solvents for 'green chemistry'. ILs are advanced, technological solvents that can be designed to fit a particular application. One regularly suggested advantage of ILs over VOCs as solvents, for both synthetic chemistry and for electrochemistry, is the intrinsic lack of vapor pressure. However, it is important, when discussing potential benefits from green chemical approaches, to remember that the aim is for improvements in the overall process. Although replacement of a VOC solvent in a process with an IL solvent will, necessarily, reduce VOC emissions from that reaction, overall efficiency (atom efficiency or Ε-factor (12)) depends on consideration of the overall process, not just the solvent used. In particular, it is important to emphasize that, although ILs are chemicals that can be applied as solvents and catalysts in green chemistry processes, they are not necessarily green chemicals. ILs can be designed to be flammable, unstable, or even toxic. A general absence of data on toxicity, environmental fate (decomposition, BOD, bioinhibition data, etc.) and, it appears, a reluctance to collect some of these data, does not help to support the green platform. While acute toxicity determinations take both time and money, environmental fate and cellular inhibition measurements are relatively quick and easy to perform and should be made.

Choice of Ionic Liquid - The Case Against l-Butyl-3-methylîmidazolium Hexafluorophosphate In terms of investigating the applications of ILs as media for chemical reactions and separations, as distinct from electrochemistry, the most widely studied IL is 1-butyl-3-methylimidazolium hexafluorophosphate ([C mim][PF ]) (13). The desirable features of an ionic, yet essentially water-immiscible, solvent as a replacement of VOCs in processes is obvious and does not need restating here. Hexafluorophosphate-containing ILs have been used for a combination reasons, including historically wide use within the peer group, its hydrophobic and non-coordinating nature, and its ease of preparation (//) as shown in Figure 1. This is despite the well established instability towards hydrolysis in contact with moisture forming volatiles, including HF, POF , etc., which can dissolve glassware and damage steel autoclaves and reactors. Proper care should be exercised when using the [PF ]"-containing ILs, as with all compounds containing possibly harmful decomposition products. 4

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5 f=\

MPF /H 0

Ri-CI

e

cr

2

[PFU

(M=H,Na,NH) 4

Figure 1. Ready synthesis and separation of[C4mim][PF ], Metathesis of the halide salts in aqueous solution results in separation of the [PF ]-containing IL as a dense, separate phase allowing simple, rapid isolation. If a neutral salt (Le. Na[PF ] or [NH ][PF ]) is used for the metathesis step extensive washing and neutralization of acid from the IL product is not required. 6

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t

One situation where this is common is in the published procedures for preparing these ILs from aqueous solution, followed by extended drying under vacuum at high temperature to remove the residual water. Many novice researchers, preparing these ILs, may not be aware of the white fumes rising from the DL. This white fume may contain HF, among other species, which is very toxic and corrosive. It is important to emphasize that considerable care should be taken when handling possible HF-containing compounds. The use of the [PF ]" anion is probably not entirely acceptable within the *green paradigm'. It is worth noting that hydrolysis followed by phosphate analysis by ion chromatography is a standard method for the assay of [PF ]" anions. Although these ILs do not conform to the green principals discussed earlier, they will continue to have a use mostly in primary research rather than commercial applications. [C mim][PF ] has been widely studied, is well characterized, and readily available either commercially or via the synthesis indicated in Figure 1. Choosing the best IL, however, will still depend on many factors including reactivity, chemical reaction or separation process, physical properties, and non-chemical factors like cost. We would suggest that greater consideration of end-of-life factors be made in the development of 'green* solvents, as potentially full-scale commercial applications of ELs come to fruition. From this perspective, environmentally acceptable disposal of ILs will require the ILs to contain only elements and functionalities that are amenable to either biodégradation or incineration. From both these perspectives, the hexafluorophosphate anion is undesirable, despite the attractive properties that are introduced into ILs with fluorine-containing anion. Thus, there are plenty of opportunities still available for fundamental development of new IL types. Of interest are both new anions and cations, for example, hydrophobic ILs containing new hydrolytically stable [PF (R ) ]' and npn-toxic anions including octylsulfate and docusate (dioctylsulfosuccinate). Within the pharmaceutical and fopd-additive industries, the concepts of non-toxic pharmaceutical^ acceptable ions and GRAS (generally regarded as safe) materials are well understood, in terms of providing guidelines to chemical (and ion) types for which the toxicological and environmental hazards are established and considered to be acceptably low. The list of non-toxic pharmaceutical^ acceptable anions includes inorganic anions such as chloride, bromide, sulfate, phosphate, nitrate, and organic anions such as acetate, propionate, succinate, glycolate, stéarate, lactate, malate, tartrate, citrate, 6

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6 ascorbate, glutamate, benzoate, salicylate, methanesulfonate, and toluenesulfonate. It will immediately be recognized that many of the anions from this (incomplete) list support the formation of ILs with many organic cations. But it may also be obvious that many of these anions are used for preparation of crystalline organic salts. Consideration of both structural and chemical properties of both the anion and cation must be made in order to (i) prepare ionic liquids and (ii) obtain the desired properties of the IL as a solvent.


Linear Solvent Energy Relationships and Property Contributions Because of the large variability within the class of solvents known as ILs, the ability to develop IL mixtures, and the need to select an IL for a particular application, it will be increasingly important to be able to characterize the solvent properties of many different types of ILs. In contrast to many molecular solvents, that have relatively simple solvation interactions, ILs are complex solvents that can support many types of solvent-solute interactions (hydrogenbond donation and accepting, π-π, dipolar, ionic, etc.). In any IL, many different interaction types will be simultaneously present, and the resulting properties of the IL will depend on which interactions are dominant for the particular cationanion combination(s) and solute present. Effective use of the solvent characteristics of ILs can only come about with increased understanding of the properties and behavior of these fluids. Developing methods to characterize IL properties in terms of the different contributions to their solvent characteristics would greatly increase this understanding. In our research, we have looked at methods of developing core fundamental understanding of IL properties. One of these approaches is modeling of the solvent properties of ILs through a Linear Solvent Energy Relationship (LSER) in which the partitioning of organic molecules between ILs and a second phase such as water (14) is related to physio-chemical properties of the solvents through a multiple linear regression using Abraham's generalized solvent equation (75) shown in eq. 1: H

Η

Log D = c + rR + s7t + αΣα 2

2

Η

+ &Σβ + vV

2

2

(1)

x

Η

where R is the excess molar refraction, π is the dipolarity/polarizability, Σ α is the overall and effective hydrogen bonding acidity, Σ β is the overall and effective hydrogen bonding basicity, and V is the McGowan characteristic volume of the probe solutes. The corresponding terms r, s, a, b, and ν which relate the solute descriptors to the properties of the solvent system are extracted from the multiple regression. Distribution ratios (D = ([solute]n/[solute]org)) for a representative set of 20-30 organic solutes with varying parameters are determined using radiotracers. Since the tracers are introduced at extremely low concentrations, the data obtained can be related to partitioning at infinite dilution. 2

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Η

Η

2

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x



7 In a recent paper, Armstrong and co-workers (16) have also applied an LSER approach using Abraham's generalized solvent equation to characterize the properties of ILs from their interactions with organic solutes when used as stationary phases for gas-liquid chromatography. This provided the authors the ability to rank ionic liquids according to their usefulness in specific applications. Other approaches include using solvatochromatic probes such as Reichardt's dye (17) or Nile Red (18), and fluorescent probes (10,19) in order to determine the 'polarity' of DLs using empirical polarity scales (20). In all cases, the *polarity' values fall within a relatively narrow range, similar to short chain alcohols (10,16-18), even though ILs with similar 'polarities' can have remarkably different properties to each other, and to traditional molecular solvents. Studies using a range of different dyes, which respond to differing molecular interactions have been used to examine different polarity contributions. Another approach is the QSPR/CODESSA work of Katritzky and co-workers (21) which has been used to model the melting-points of imidazolium and pyridinium bromide salts. Partitioning of organic molecules in IL/water systems have been shown to follow traditional octanol/water distributions (22), which has useful implications for applying hydrophobic ILs as direct replacements for solvents such as benzene, toluene, dichloromethane, or chloroform in two-phase system separation schemes (21-24). The LSER analysis shows that the volume parameter, followed by the hydrogen-bond donating ability of the IL is the largest contributors to partitioning. The overall analysis indicates that the ILs studied are less polar than water (i.e. in terms of HBD and HBA) which is a reasonable conclusion, but has important ramifications for separations. Ions and ionic compounds will not, in the main, partition to an ionic liquid from water. To observe true solvent properties of ILs, partitioning studies of various solutes in IL/organic solvent systems have been conducted. Conclusions drawn from these studies are relevant, and effective, in predicting IL solvent properties due to the fact hexane has no polarizability or hydrogen bonding acidity or basicity leaving all effects as the result of the IL. The distribution ratios for the set of 21 probe solutes between toluene and [C mim][NTf ] are shown in Figure 2, plotted against their regression line. Table I lists the interaction parameters obtained for partitioning of organic solutes in a range of IL/organic biphasic systems and are compared to the descriptors calculated for JL/aqueous partitioning where available. These data indicate that the dominant interactions for transfer of a solute from an organic phase to the EL are hydrogen bond basicity (a) and dipolarity (s) of the IL. In contrast, for solute partitioning between ILs and aqueous phases, the volume (v) and hydrogen bond acidity (b) terms were most important. This approach will allow ILs to be classified on the basis of their interactions with solute probes. Compared to conventional molecular organic solvents, ILs are much more complex, both chemically, and in terms of the wide range of interactions that can take place. Characterization of ILs using simple 'polarity' scales fails to differentiate between the different types of interactions present, whereas LSER 4

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1 0 1 2 Log D [C mim][NTf ]/ToIuene Predicted 4

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Figure 2. Distribution ratios for organic solutes from the standard screening set between toluene and [C mim][NTf ] plotted against the predicted partition values from the LSER regression parameters shown in Table /. 4

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Table I. Interaction Parameters Obtained from the LSER Model for IL/Organic and IL/Water Partitioning of Organic Solutes (14,25). System [C mim][PF ]/hexane [C mim][PF ]/water [C mim][PF ]/toluene [Cmim][PF]/hexane [C mim][PF ]/water [C mim][PF ]/toluene [Cmim]tNTf2]/hexane [C mim][NTf ]/toluene 4

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a 1.82 -1.82 1.3 2.11 -1.48 1.26 2.56 1.13

Parameters V b s 1.27 2.25 -1.78 -1.63 -0.004 2.14 0.88 0.90 -1.23 1.06 2.02 -1.36 -2.15 0.27 2.31 0.53 0.93 -0.74 0.80 2.13 -1.11 0.89 0.49 -0.60

r -0.09 0.63 0.20 0.03 0.14 -0.08 -0.36 0.11

analysis using a range of solute probes provides a route to obtain direct determination of the magnitude and importance of the different interaction contributions. However, partitioning between ILs and aromatic solvents presents a second consideration that needs to be understood.

Liquid Clathrate Formation with IL/Aromatic Mixtures The remarkable solubility, but rarely complete miscibility, of benzene with ILs (26) may be a result of liquid clathrate formation, first described by Atwood


9 (27) for highly reactive air-sensitive alkylaluminum salts and benzene or toluene. Since this discovery, an expanded range of organic salts, for example [A1C1 ]*, [HX„]X\ [X ]\ and also [BF ]" anions (28) have been shown to support liquid clathrate formation (29). It has been recognized that ILs can support liquid clathrate formation (30) and Zaworotko and co-workers (31) have suggested that an approach to developing further liquid clathrate sustaining systems would be to investigate organic salts with low melting points, that is, ionic liquids. Liquid clathrate formation between conventional 1-alky1-3methylimidazolium ILs and aromatic hydrocarbons (benzene, toluene, and xylenes) have been investigated (32). Liquid clathrate phases form spontaneously under ambient conditions on mixing of the aromatic solvent with the IL. The liquid clathrates phases obtained exhibited typical behavior characteristics, namely, low viscosity (especially relative to the initial neat ILs), immiscibility with excess aromatic solvents, and non-stoichiometric, but reproducible, compositions. The molar composition of the lower liquid clathrate phase formed on contacting ILs with a range of aromatic hydrocarbons are shown in Figure 3, as determined by proton NMR. EL concentration in the upper, aromatic phase was below the NMR detection limit in all the systems examined. 4


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Figure 3. Ratio of aromatic to ionic liquid in the lower phase of the liquid clathrate biphasic systems, data from ref32. Connecting lines are a visual guide to changes in the liquid clathrate phase composition. Organic components are benzene (circle), toluene (square), and o-xylene(diamond), m-xylene (down triangle), and p-xylene (up triangle) repectively.


10 In all the examples studied, miscibility with benzene was greatest, and the maximum aromatic content in the lower phase of the liquid clathrate biphase decreases following the order benzene > toluene > xylenes. The aromatic content of the liquid clathrate phase is somewhat lower than that found in [A^RÔI]" systems (28), but largely comparable with the values observed in [(HX)„X]"-eontaining liquid clathrates (28), and with the 0.66 mole fraction solubility of benzene in [C4inim][PF ] reported by Blanchard and Brennecke (33). Note that the examples described here all contain between 1.5-3.5 moles of benzene for every mole of IL in the lower phase. This has important consequences for interpreting IL/aromatic biphasic reactions and separations, in that both phases may be *aromatic-rich\ leading to significant differences in solubilities, reaction kinetics, or extraction behavior, when compared to non-clathrate forming IL-biphases such as with alkanes or ethers. Much of the chemistry developed in ILs utilizes IL-organic biphasic systems as reaction and extraction media. Recent examples include hydrogénation (34), Friedel-Crafts alkylation (35), ring-closing metathesis (36), and ring-opening metathesis polymerization (37) reactions, and comparisons of results using different solvents may need to be interpreted in the context of liquid clathrate formation.


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Summary As is reflected in the contributions to this Symposium Series volume, significant efforts are being made in utilizing ILs in green chemistry. It is clear that progress is indeed being made, and that the prospects for the future are good, based on the continuing commitment to excellent and innovative research from both industry and academia. Choosing the right ionic liquid for a given task can be based upon a number of factors: performance, availability, or cost, and correct selection can be vital to the overall success of the process. Models to understand contributions to the solvent properties of ILs are being developed and will act as yet another tool to aid in choosing the appropriate IL(s) for a given task. Both fundamental and applied research into the properties of a broad range of ILs continue to be needed to better understand their potential in replacing traditional organic solvents and operating in the 'green' paradigm.

Acknowledgements The ionic liquids research at The University of Alabama is supported by the U.S. Environmental Protection Agency STAR program through grant number R82825701-0 (Although the research described in this article has been funded in part by EPA, it has not been subjected to the Agency's required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred).


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References 1 2


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Selection of Ionic Liquids for Green Chemical Applications - ACS

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