Ionic Liquids as Green Solvents - ACS Publications - American

Chapter 3

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Challenges to the Commercial Production of Ionic Liquids Philip E. Rakita Director of Business Development, Ozark Fluorine Specialties, 1830 Columbia Avenue, Folcroft, PA 19032

Room temperature ionic liquids - particularly imidazolium hexafluorophosphates and tetrafluoroborates -- have been widely studied as substitutes for aromatic solvents in a broad range of chemical reactions and separation processes. In the absence of a reliable commercial supplier, researchers have had to either synthesize their own RTIL's or purchase them from research chemical supply houses in small amounts at high price. Ozark Fluorine Specialties has undertaken the pilot synthesis of a representative RTIL at the 200-kg scale. This paper outlines the issues that need to be addressed to achieve efficient commercial synthesis and make these compounds available on an industrial scale.

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

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Novel Solvents and Reagents for Diverse Applications As a novel class of solvents, room temperature ionic liquids (RTIL's), sometimes called simply ionic liquids (IL's), have properties that make them desirable alternatives for many industrial applications (such as synthesis and extraction processes). To date, researchers have had to either synthesize their own RTIL's or purchase them from research chemical supply houses in small amounts at high price. In order for these compounds to achieve their commercial potential, it must be demonstrated that they can be produced in bulk quantities (hundreds of kilograms at a time). Ozark Fluorine Specialties has undertaken the pilot synthesis of a representative RTIL at the 200-kg scale. This paper outlines the issues that need to be addressed to achieve efficient commercial synthesis and make these compounds available on an industrial scale.

What are ionic liquids? Different groups, depending of the focus of their particular interests, have described this large and growing class of novel solvents and reagents in various ways. Researchers have offered several definitions for this class of materials: • • • •

"Organic salts with melting points under 100 °C, often below room temperature" -Fluka product bulletin (i) "Liquids...composed entirely of anions and cations in contrast to molecular solvents" — Covalent Associates (2) "Liquids with a wide temperature range and no vapor pressure" —Covalent Associates (2) "Fused salts are liquids containing only ions" -Tom Welton (3)

The key features underlying most of these definitions are that ionic liquids have (A) a large liquid range and (B) no measurable vapor pressure because they are composed of ions rather than discrete molecules.

Why are they chemically interesting? RTIL's generally exhibit good solvent properties and can often facilitate and influence chemical reactions without being transformed in the process (5). Most RTIL's have negligible vapor pressure and miniscule flammability. They

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

34 frequently exhibit high thermal stability and wide working temperature ranges. For example, the TGA curve for l-butyl-3-methylimidazolium hexafluorophosphate (BMIPF ) shows stability in air up to 300 °C. (4) Owing to the multitude of possible combinations of cation and anion, they are susceptible to numerous permutations that allow the various physical and chemical properties to be adjusted almost at will. 6


Why are they commercially important? Considerable research (5,6) has shown that certain RTIL's are effective substitutes for common organic solvents (for example, benzene) where control of VOC emissions is critical. Some hydrophobic ionic liquids will form three liquid phase systems with water and hexane, offering interesting possibilities for extraction and separation technology as well as phase transfer chemistry. Other ionic liquids can act as liquid supports for re-usable catalyst systems. Ionic Liquids are being investigated as potential electrolytes in high performance energy storage systems such as batteries and double layer supercapacitors. Other researchers are investigating ionic liquids as heat transfer fluids in solar energy collectors. (5)

How are they obtained? Until recently, researchers were obliged to synthesize the ionic liquids they wished to study. Now an increasing number can be purchased from research chemical supply houses. Welton (5) lists three basic routes to the synthesis of IL's. These are: • • •

Metathesis of a halide salt with a salt of the desired anion Acid-base neutralization reactions Direct combination of a halide salt with a metal halide

For the limited quantity needed for laboratory study, the metathesis route provides the simplest and most direct route. Typically a Group I or ammonium or silver salt of the requisite anion is combined with a quaternary halide, often in a solvent. If the intended IL is not miscible with water, the by-product metal or ammonium halide can often be removed by extraction and separation, or in the case of the silver salts, by filtration of the insoluble precipitate. However, methods that work on the bench top can prove onerous in the chemical plant


35 where tons of products need to be made efficiently and cheaply. Questions of raw material cost, by product removal and disposal and the recovery and reuse of solvents dominate the equation. A Specific Example—1 -butyl-3-methylimidazolium hexafluorophosphate, BMIPF , is one of the most widely studied IL's to date (7). There have been literally hundreds of R&D publications on its preparation, properties and use. Numerous reactions have been studied using BMIPF as solvent. Its physical properties have teen extensively examined. Table I gives some representative values. It is possibly one of the best-evaluated IL's known. The compound has certain attractive attributes from an industrial perspective. The cost of the raw materials for its manufacture is modest. Several synthetic routes are known. For these reasons, Ozark Fluorine Specialties has chosen this compound as the first to scale up to production at the >100 kg. batch size. The remainder of this paper will address the specific issues and challenges of that scale-up. 6


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Table L Typical Properties of B M I P F Form: Specific gravity: Melting point: Boiling point: Solubility in water: Viscosity:

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Liquid 1.36 ~8°C dec. >350 °C Insoluble 3.12g/cm-secat30°C

SOURCE: Ozark Fluorine Specialties, product data sheet, April 2002.

Selecting the Synthetic Route Raw Material Issues The industrial preparation of BMIPF , involves first the synthesis of the quaternary ammonium cation, ΒΜΓ, as a halide salt, followed by replacement of the halide with the PF anion, either through metathesis or via an acid-base neutralization. Either route requires a quaternization using butyl chloride and 1-methylimidazole, both of which are commercially available. The anion is also commercially available, either as the potassium salt or as the free acid, HPF6. Since the potassium (or other cationic salt) is typically made from the free acid, 6

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36 the latter route (neutralization) is favored from a cost perspective as one less step is required. There are a number of other factors, including process parameters, which must be considered. These are summarized in Table II.

Table II. Process Parameters to be considered in the synthesis of BMIPF6: Comparison of the metathesis and neutralization routes


Metathesis route Neutralization route Materials of construction

Glass OK

Monel or other metal resistant to HF

Thermodynamic considerations

Negligible ΔΗ

ΔΗ of neutralization (heat removal)

Handling safety

Non-hazardous reagents

Treat like HF

By-products to dispose of

KC1 (aq) or other halide salt

HC1 (aqueous)

Generally speaking, when an IL containing the PF or BF anion is prepared, the process parameters such as materials of construction of the reactor, dealing with the enthalpy of reaction and the handling safety of strong acid starting materials favor the metathesis route over neutralization. Thus, when a few grams or a few hundred grams are needed for bench top studies, metathesis is the route of choice. By contrast, where the synthetic infrastructure is already in place for the preparation and handling of HPF or HBF , the subsequent conversion of the acid to the "quat salt" IL involves little additional effort. Because of its extensive experience with handling HF in bulk, Ozark Fluorine Specialties has elected the inherently simpler and less costly neutralization route to BMIPF using HPF . 6

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Other Issues As mentioned above, using HPF as a reagent for synthesis is inherently more hazardous than using its salts. The other components of the cation, butyl chloride and 1-methylimidazole also require respect, owing to their own 6


37 specific hazard classifications (see Table III). The byproduct of the final coupling reaction of the cation and anion leaves only an aqueous solution of hydrochloric acid, a byproduct readily handled by a modern chemical synthesis plant.

Table ΠΙ. Hazard Classifications of Raw Materials for BMDPF Synthesis


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Raw Material 1-Butyl chloride 1-Methyl imidazole Hexafluorophosphoric acid Potassium Hexafluorophosphate

Hazard Classification Flammable liquid, irritant Corrosive, hygroscopic liquid Corrosive, toxic Liquid None

SOURCE: Supplier Material Safety Data Sheets

Although the product has been extensively studied as a chemical substance, both as a reagent and solvent, little is known about its toxicology. Efforts to extrapolate from the properties of "similar" substances may not be valid. Certainly a substance without a measurable vapor pressure poses minimal risks from inhalation. The lack of vapor also reduces the risk of flammability to negligible. As a result, for transportation purposes, BMIPF does not require special regulation or packaging. 6

To date B M P F has not been submitted to the U.S. Environmental Protection Agency for inclusion on the directory of chemical substances known as the TSCA inventory. EPA regulations permit the manufacture and use of new substances not registered, provided "suitably qualified individuals" use them for "R&D purposes" and not entered into commerce. Certain exemptions apply which could allow the commercial use of limited volumes under the so-called "low volume exemption." Limited to a specific time period, a specific volume, and a specific application, this exemption could provide a suitable mechanism for commercial trials using BMIPF . Ozark Fluorine Specialties is prepared to work with commercial partners to identify and support such an application for exemption. 6

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Cost is an issue central to any commercial use of BMIPF . On the face of it, the compound, as available today in small volumes, is not cheap. Realistically, it may never be "cheap" when compared to other common solvents. However, 6


38 there are lots of ways to look at cost and purchase price is only part of the equation. With the increasing regulation of volatile organic compounds (VOC's) in the workplace and the environment, a solvent that has no "volatile" character has some advantages. The cost of more conventional solvents should also reflect the expenses and capital costs required for personal protective equipment (PPE) and emission control hardware and monitoring equipment. The example of benzene, once a common "aromatic" solvent for chemical synthesis should serve to put the potential for RTIL's in perspective. Table IV lists comparative prices for various solvents in use today. Although BMIPF may never approach the price of $1 per pound, it is reasonable to expect, based on economies of scale, that pricing at the $20 per pound level could be achieved.


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Table IV· Comparative Prices of Various Solvents Solvent

Benzene Dimethylformamid Dimethylsulfoxide Hexamethylphosphoramide Acetonitrile Sulfolane N,N-Dimethylacetamide

Cost of One Liter Catalog Supply House $27.30 22.90 29.67 200.00 18.04 44.80 23.70

Cost in Bulk (per pound or gallon) $0.75/gallon 1.20/pound 1.00/pound 0.65-0.75/pound 1.12/pound

Note: One-liter pricesfromAldrich or Lancaster catalogs. Bulk prices taken from Chemical Market Reporter (April 8,2002) or company price sheets.

And, if so, what does this mean for the commercial potential of BMIPF ? First of all, it means that the compound would have to be used in applications where it was recycled. No one today chooses to dispose of solvents or by-products if they have commercial value. For a high priced solvent like BMIPF , re-use is essential to economic viability. That means either applications such as a heat transfer fluid, where re-use is built into the application, or recycle. And this poses the next big unsolved challenge—how to purify IL's. 6

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BMIPF can be synthesized to high levels of purity. Multiple water washings of the reaction product, followed by vacuum drying or sparging with inert gas lead 6


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to low levels of water, free acid (measured as HF), and chloride impurities. Table V gives a typical analysis of product made via Ozark's pilot process. A perhaps more important question is what does the IL look like after it has been used in a synthetic procedure. Are there contaminants left behind in the solvent? What are they? Do they matter and if so how can they be removed? These questions need to be answered in the context of specific applications, not abstractly or in an absolute sense.

Table V. Typical Analysis of ΒΜΠΨ

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Property C Η Ν PF " F Chloride FreeHF Moisture 6

Observed Value 33.53% 5.50% 9.72% 51.3% 39.3% 20 ppm None detected. 0.42%.

Calculated value 33.81% 5.32% 9.86% 51.01% 40.11%

Methods are being developed to recover and reuse IL's via vacuum evaporation of volatiles and extraction. Extractions with VOC solvents obviate some of the advantages of using IL's in the first place, but supercritical C 0 extraction may offer promise. 2

In summary, the manufacturing technology for BMIPF and other IL's is substantially advanced. There are no significant impediments to the commercial production and use of these important new solvents. 6

References

1. 2. 3. 4.

Anon. Chem Files. 2001, vol.1, number 7; p 3, Fluka, Buchs Switzerland. Anon., "Ionic Liquids: Enabling Solvents", Covalent Associates, Woburn, M A , April 2001. Welton, T. Chem. Rev. 1999, vol.99, pp 2071-2083. Koel, M . Proc. EstonianAcad.Sci. Chem., 49 (3), 145 (2000).


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5. Ionic Liquids—Industrial Applications to Green Chemistry; Rogers, R. D.; Seddon, K. R., eds. ACS Symposium Series 818; American Chemical Society: Washington, DC, 2002. 6. See for example: Freemantle, M . , Chem. Eng. News, 15 May p. 37 (2000) and 1 January, p. 21 (2001). 7. Wilkes, J. S.; Zaworotko, M . J., J. Chem. Soc., Chem. Commun. 1992, 965.


Ionic Liquids as Green Solvents - ACS Publications - American

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