Ionic Liquids - American Chemical Society

Chapter 3

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Are Ionic Liquids Green Solvents? William M. Nelson Waste Management and Research Center, 1 East Hazelwood Drive, Champaign, I L 61820

The examination of ionic liquids (ILs) as green solvents should include chemical and solvency characteristics, as well as environmental toxicological considerations. Ionic liquids can best be described as greener solvents, based upon current reactions and measured properties from the literature. Environmental impact and toxicity criteria can be used as measures to evaluate these compounds in terms of their environmental effects. These chemicals display a wide range of potential application as greener solvents.

Ionic Liquids as Greener Solvents Ionic liquids are a new class of solvents, which offer alternatives to conventional molecular solvents in many chemical applications, with both the thermodynamics and kinetics of reactions carried out in ionic liquids being different from those in conventional molecular solvents. These solvents are often fluid at room temperature, and commonly consist of organic cationic species and inorganic anionic species; they have no measurable vapor pressure, and hence can emit no Volatile Organic Compounds (VOCs).

30

© 2002 American Chemical Society

In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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One indication of the growth in interest in these compounds is shown by the increase in the number of articles on this topic during a ten-year period. (Figure 1) The current year is only half over! The questions, observations and guidelines we offer here will assist in rating ILs as greener solvents. (1)

Figure 1. Increase in number of articles on ionic liquids over a tenyear period

Green perspectives The principles of green chemistry provide a reference point for the chemical profession in trying to deal with the novel ethical considerations resulting from science and technology found in the 21 century. (2) The chemistry community must regard environmental protection as a sine qua non, offering opportunities to develop more cost effective processes and products.(3) This may be done in essentially two ways: Existing processes can be greened with existing technologies, or new technologies can emerge. Ionic liquids are a recent example of a hybrid of an existing technology (known solvents) that is maturing and being scientifically explored. As ILs develop, it becomes essential to critically evaluate their chemistry and environmental impacts. A set of considerations are needed to help assess st


32 their "green-ness".(3) To begin developing these, let us first enumerate the drivers influencing green chemistry.(Table 1)


Table 1. Major drivers affecting green chemistry

• • •

Major Drivers Human Welfare Quality of Life Environmental Protection

•

Sustainable Development

The challenge facing our world is to produce a sustainable civilization. This civilization should take from the earth only what is necessary for survival without compromising the potential of future generations. Chemistry has an important role to play in this effort.(4) In order to accomplish this three challenges must be met.(Table 2) Table 2. Challenges to sustainable chemistry Challenges •

Making renewable energy technologies a central component;

•

Developing reagents that are obtained from renewable sources;

•

Replacing polluting technologies by benign alternatives.

The obstacles to overcoming the challenges listed in Table 2 are shown in Table 3. Table 3. Obstacles to overcoming challenges to a sustainable chemistry Obstacles •

Incorporate environmental considerations into chemistry;

•

Emphasize only chemistry that is really green;

•

Avoid short-term and myopic thinking;

•

Understand ethics of sustainability.


33 What is a green solvent?


It will be difficult, i f not impossible, to find a purely green solvent. Standards are needed for quantitatively and systematically evaluating solvents to allow chemists to clearly assess whether or not resulting chemistries and chemical processes are 'greener.' The first step regarding evaluating solvents is the development of traits and general criteria. The following table lists representative considerations.(Table 4) Table 4. Traits and criteria for green solvents

• • • • • • •

General Traits and Criteria Generates less waste (in production, use, and disposal); Innocuous or more innocuous; Renewable resources in synthesis of solvents; Solve other environmental problems; Selectivity, reaction efficiency, efficient separation; Known hazards associated with solvents; Human & environment health effects are known.

It is not necessary for ionic liquids to possess all of these characteristics simultaneously to be considered more green (when compared to existing solvents). The results of this evaluation should indicate that effective use and reuse of ILs will contribute significantly to establishing them as a more environmentally benign solvent with reduced total impacts.(5) However, the traits and criteria should be used as a standard to rate the appropriateness of the use and/or development of these compounds in chemical applications.

Preliminary Evaluation of Ionic Liquids A n examination of the benefits of room temperature ionic liquids systems reveals they act as solvents for a wide range of chemical processes. Their most important advantage is probably that they have no measurable vapor pressure. This is a two-edged sword: no adventitious emissions, but introduction of potential challenges to recovery of products and solvent purification. When ILs serve as both catalyst and solvent, questions regarding changes the solvent is undergoing must be addressed. Increasing attention from industry, as they promise significant environmental benefits, will make answering those questions imperative.(6) Even while chemical issues are being resolved, environmental/toxicological questions also need attention. The design of or the decision to use any solvent


34


involves an analysis that utilizes the chemical structure to identify what part of a molecule is providing the characteristic or property that is desired from the products and what part of the molecule might be responsible for toxicity or hazard.(l) The goal or end result of this can be achieved through several different strategies, the choice of which is largely dependent on the amount of information that already exists. The information that will be useful is shown in Table 5. The lack of information regarding ILs in many areas illuminates needed areas of research before ionic liquids are clearly regarded as green solvents. Table 5: Design and use of Green Solvents Considerations for the Design and Use of Green Solvents • Mechanism of action • Structure-activity relationships • Elimination of toxic functional group • Reduce bioavailability • Design for innocuous fate • Minimization of energy consumption

Greening of Ionic Liquids Ionic liquids, being composed entirely of ions, were once mainly of interest to electrochemists. Recently, however, it has become apparent that, inter alia, a wide range of chemical reactions can be performed in them.(7) As the interest in ionic liquids grows, there is a corresponding increase in the breadth of applications in which they are useful. It will be illuminative to briefly scan the current array of chemistries being conducted in this solvent system. This is not the complete list and the corresponding references have been selected.(Table 6) Interested readers are encouraged to search the literature. The important point in this is that, as the ionic liquids are used more, the considerations in Tables 1-5 become more imperative.

Discussions and Future Directions The present status of ionic liquids places them on the road to being a greener solvent. This is a healthy position not only for the segment of the chemical community which has a vested interest in them (researchers, suppliers and end-users) but for the chemical industry as a whole (as it provides an area that will be developed within green chemistry.) Let us now provide a preliminary evaluation of ILs as greener solvents.


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Table 6. Chemical applications of ionic liquids Name organic reactions


•

Diels-Alder reactions (19)

·

Weinreb amides

•

Friedel-Crafts (21)

·

Negishi cross-coupling

•

Heck reaction (22)

·

Claisen rearrangement

•

Trost-Tsuji C-C coupling

·

Stille coupling

•

Suzuki cross-coupling

·

Reformatsky Reactions

•

Proton donation

General Organic Reactions ·

Hydroformylation (28)

•

Dimerization

·

Stereoselective syntheses

•

Hydrogénation

·

Ether cleavage and epoxide

•

Reductive carbonylation

•

Polymer synthesis

·

Heterocyclic synthesis (29)

opening

•

Hydroesterification

·

Benzoylation

•

Allylation(26)

·

Arylation (30)

Physical Organic Chemistry •

Electron transfer

·

Analytical chemistry

•

Organometallic synthesis (8)

·

Dipolar cyclo-additions

Inorganic Chemistry Oxide ion transfer Synthesis of compounds

·

Supramolecular

·

(31) Electrochemistry (32), (33)

inorganic

syntheses

Catalysis Recycle catalysts

·

Homogeneous catalysis (34)

Immobilized catalysts (13)

·

Catalytic cracking (37)

Enzymatic catalysis (14)

·

Bioprocesses (35), (36)


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Past and present The early efforts (up until around 1992) with ionic liquids were based upon their desirable chemical properties. These properties were quite appealing in the area of electrochemistry, and afforded opportunities in basic and applied research. This has continued. The green aspects of these solvents may have been noticed, but certainly not emphasized. Up till this point the demand for environmental accountability was not fully appreciated. The appearance of regulatory drivers in the early 1990's forced a réévaluation of the roles solvents play in chemistry and what constitutes desirable solvents. Some simple physical properties of the ionic liquids that make them interesting as potential solvents for synthesis are listed below.(12) These properties result in more "green" quality when compared to other solvents.(Table 7) Table 7. Properties and benefits in ionic liquids

•

• • •

Physical Property Good solvents for a wide range of both inorganic and organic materials Often composed of poorly conducting ions Immiscible with a number of organic solvents Non volatile

->

->

-»

-»

Benefit Unusual combinations of reagents can be brought into the same phase Highly polar yet noncoordinating solvents Provide a nonaqueous, polar alternative Used in high vacuum systems

Even i f one considers that the newer ionic liquids are free from many of the hydrolysis problems that make the halogenoaluminates(III) so difficult to handle, the breakdown products will include mineral acids (HX) and other acids from the breakdown of the anions (PF , B F , etc.). Even though most ammonium and imidazolium salts are hygroscopic and i f used open vessels, hydration will almost certainly occur; these new ionic liquids are much easier to handle than the halogenoalurninate(III) systems. Recent work has led to the development of a better physical understanding of ILs. (see Table 8) As good as these properties may seem to be, the evaluation of "green-ness" must continue, as valuable information is lacking. The information will come from several sources: expanded IL use and application in chemistry (even now an increasing range of reactions benefit from use of ionic liquids), measurement and systemization of physical properties, human and environmental toxicity tests (also taking advantage of molecular modeling) and the effects resulting from increased usage. 6

4


37 Table 8. Refined chemical properties of ionic liquids


Solvent Properties •

Polar phases (16)

•

Solvent strength and polarity acetonitrile and methanol

between

•

Forms strong hydrogen bonds

•

Shows conventional aromatic stacking

•

CUoroaluminate ionic liquids can be both neutralized and buffered by the addition of an alkali halide (17)

•

Liquid crystals are good solvents providing a partially ordered reaction environment

Evaluation: Preparation Presently, the most common ILs in use are those with alkylammonium, alkylphosphonium, N-alkylpyridinium, and Ν,Ν'-dialkylimidazolium cations. (Figure 2)

[PR H . ] X

4

+

X

(b)

00

-©

ν ι

R (d)

(c) X = BF

4>

P F , AICI4, N 0 , O T f 6

3

Figure 2. (a) Alkylammonium, (b) alkylphosphonium, (c) Ν , Ν ' dialkylimidazolium, and (d) N-alkylpyridinium cations and common anions.



38 Many alkylammonium halides are commercially available, or they can be prepared by the reaction of the appropriate halogenoalkane and amine. Preparation of the pyridinium and imidazolium halides may be similarly done. Other ionic liquids are made by the quarternerization of the appropriate amine. The synthesis procedures and source of raw materials should be scrutinized. Examples of the typical methods for the preparation of ionic liquids contain areas that might be made more green. (Table 9, note: those shown are edited and should not be used in the lab!) The areas for improvement concern the use of elevated temperatures or pressures, use of halogenated reagents, excessive purification procedures and hazardous reagents. The goal is to make the IL progressively greener, from "cradle to grave."(25) Table 9. Example of preparation of ionic liquids (edited) l-Butyl-3-methylimidazolium chloride, [bmimJCl 1-methylimidazole (freshly distilled from CaH ) and 1-chlorobutane and heated at 75 °C for 48 h under 3 bar pressure of dinitrogen. Excess chlorobutane was removed under reduced pressure with heating. The pale yellow molten product was crystallized on cooling as an off-white solid. 2

2.2.2 1 -Butyl-3-methylimidazoHum hexafluorophosphate, [bmim][PF6] [bmim]Cl in H 0 was cooled to 0 °C and hexafluorophosphoric acid was added. It was stirred for 2 h. The ionic liquid phase was washed with water, saturated aqueous N a H C 0 solution and extracted into dichloromethane . The organic phase was dried over M g S 0 , filtered, the solvent removed and the ionic liquid dried for 6 h at 70 °C in vacuo. 2

3

4

Evaluation: Reaction chemistries Two desirable characteristics of ionic liquids are apparent. First, they yield significant rate enhancement and high yield/selectivity. Second, they reduce two waste streams commonly associated with some traditional chemistries (through efficiency and catalysis). 1. The rate enhancement and high yield/selectivity must include improved and greener methods of isolation. Without this, the former is reduced. For example, a desirable discovery has been the ability to improve product isolation. This improved techniques results from extraction with supercritical carbon dioxide and it can further the 'greenness' of syntheses involving ILs.(20) Advances like these will be needed in the future.


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2.

Without a reliable and standard means for recycling the IL, it could become an undesireable wastestream. A n example of this is provided by the Heck coupling of aryl halides or benzoic anhydride with alkenes, which can be performed with excellent yields in room-temperature ionic liquids. It has been shown that ILs provide a medium that dissolves the palladium catalyst and allow the product and byproducts to be easily separated. Consequently, the catalyst and ionic liquid can be recycled and reused.(ll) This results in further increases in catalyst productivity.(23)

Evaluation: Environmental considerations It is necessary for usto think beyond the confines of the use of ionic liquids as merely a solvating medium. A recent review also claimed that l-Alkyl-3allcoxymemylimidazolium chlorides showed strong antimicrobial activity.(18) These must be substantiated, but they provide more evidence for the green-ness of ionic liquids. Most clearly, the lack of documented toxicity data is an area of necessary work. Anecdotal comments indicate safety in this regard, but the results from current work is critical. In the interim complete QSARS and modeling studies would help to assure favorable environmental profiles. Three points must be highlighted: 1. Breakdown products and environmental lifetimes for ILs are needed; 2. Human and environmental toxicity studies will show ILs to be safe within use, but not benign; 3. Increased use will reveal unknown problems.

Conclusions: Report Card Ionic liquids are becoming established as a potentially viable, environmentally benign alternative to existing solvents. They represent a greener alternative in many applications. As this maturation continues, green considerations for the evaluation of their development and use must also be continually raised. Let us give an initial rating of the ILs.(Table 10)


40 Table 10. Report card Greening of Ionic Liquids •


•

Does the solvent use lead to less energy expenditure? Does the solvated species react more efficaciously and selectively?

•

Does the solvent improve atom-economy?

•

What is solvent distribution into environment?

•

W i l l the solvent be absorbed by organisms and how will it affect them?

•

Is the solvent toxic? Can it be detoxified?

G

Ρ

υ

*

*

*

G: good; Ρ: Poor; U : unknown

References (1) Nelson, W. M. In Green Chemical Syntheses and Processes; Anastas, P. T., Heine, L . G., Williamson, T. C., Eds.; American Chemical Society: Washington, DC, 2000; V o l . 767, pp 313-328. (2) Jonas, H . The imperative of Responsibility: In Search of Ethics for the Technological Age; University of Chicago Press; Chicago, 1984. (3) Lancaster, M. Green Chemistry 2000, 2, G65-G67. (4) Collins, T. Science 2001, 291, 48-49. (5) Curzons, A . D.; Constable, D. J. C.; Mortimer, D. N.; Cunningham, V . L . Green Chemistry 2001, 3, 1-6. (6) Freemantle, M. Chemical & Engineering News 1998, 76, 32-37. (7) Earle, M. J.; Seddon, K . R. Pure Appl. Chem. 2000, 72, 1391-1398. (8) Dyson, P. J.; Grossel, M. C.; Srinivasan, N.; Vine, T.; Welton, T.; Williams, D. J.; White, A . J. P.; Zigras, T. Journal of the Chemical Society-Dalton Transactions 1997, 3465-3469. (9) Olivier, H . Aqueous-Phase Organometallic Catalysis : Concepts and Applications 1998, 555-563. (10) Blanchard, L . Α.; Hancu, D.; Beckman, E . J.; Brennecke, J. F. Nature 1999, 399, 28-29.



41 (11) Carmichael, A . J.; Earle, M. J.; Holbrey, J. D.; McCormac, P. B . ; Seddon, K . R. Organic Letters 1999, 1, 997-1000. (12) Welton, T. Chemical Reviews. 1999, 99, 2071-2083. (13) DeCastro, C.; Sauvage, Ε.; Valkenberg, M. H . ; Holderich, W . F. Journal of Catalysis 2000, 196, 86-94. (14) Erbeldinger, M.; Mesiano, A . J.; Russell, A . J. Biotechnology Progress 2000, 16, 1129-1131. (15) Larsen, A . S.; Holbrey, J. D.; Tham, F. S.; Reed, C. A . Journal of the American Chemical Society 2000, 122, 7264-7272. (16) Furton, K . G.; Morales, R. Analytical Chimi Acta 1991, 246, 171. (17) Koronaios, P.; Osteryoung, R. A . Journal of the Electrochemical Society. 1999, 146, 2995-2999. (18) Pernak, J. Przemysl Chemiczny 2000 X, 79, 150. (19) Zulfiqar, F.; Kitazume, T. Green Chemistry 2000, 2, 137-139. (20) Blanchard, L . Α.; Brennecke, J. F. Industrial & Engineering Chemistry Research 2001, 40, 287-292. (21) Stark, Α.; MacLean, B . L . ; Singer, R. D. Journal of the Chemical SocietyDalton Transactions 1999, 63-66. (22) Bohm, V . P. W.; Herrmann, W. A. Chemistry-Α European Journal 2000, 6, 1017-1025. (23) Howarth, J.; Dallas, A . Molecules 2000, 5, 851-855. (24) Mathews, C. J.; Smith, P. J.; Welton, T. Chemical Communications 2000, 14, 1249-1250. (25) Varma, R. S.; Namboodiri, VV. Chemical Communications 2001, 15, 643644. (26) Chen, W . P.; X u , L . J.; Chatterton, C.; Xiao, J. L . Chemical Communications. 1999, 1247-1248. (27) Brasse, C. C.; Englert, U.; Salzer, Α.; Waffenschmidt, H . ; Wasserscheid, P. Organometallics 2000, 19, 3818-3823. (28) Wasserscheid, P.; Waffenschmidt, H . Journal of Molecular Catalysis AChemical 2000, 164, 61-67. (29) Kitazume, T.; Zulfiqar, F.; Tanaka, G. Green Chemistry 2000, 2, 133-136. (30) X u , L . J.; Chen, W. P.; Ross, J.; Xiao, J. L . Organic Letters 2001, 3, 295297. (31) Scott, J. L.; MacFarlane, D . R.; Raston, C. L.; Teoh, C. M. Green Chemistry 2000, 2, 123-126. (32) Fung, Y. S.; Zhou, R. Q. Journal of Power Sources 1999, 82, 891-895. (33) Katayama, Y.; Dan, S.; Miura, T.; Kishi, T. Journal of the Electrochemical Society 2001, 148, C102-C105. (34) Olivier, H . Journal of Molecular Catalysis A-Chemical 1999, 146, 285-289. (35) Lau, R. M.; van Rantwijk, F.; Seddon, K . R.; Sheldon, R. A . Organic Letters 2000, 2, 4189-4191. (36) Schofer, S. H . ; Kaftzik, N.; Wasserscheid, P.; Kragl, U. Chem. Commun. 2001, 425-426. (37)Adams, C. J.; Earle, M. J.; Seddon, K . R. Green Chemistry 2000, 2, 21-23.


Ionic Liquids - American Chemical Society

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