Ionic Liquids Make Splash in Industry

Room-temperature ionic liquids have been used in industrial processes for 15 years or more, and their applications continue to expand. For example, they are being studied by industrial scientists as potential media for storing and transporting highly toxic or flammable gases.

The many current and potential industrial uses of ionic liquids--which typically consist of an inorganic anion and an organic or inorganic cation--was a major topic of discussion at the 1st International Congress on Ionic Liquids.

The congress, held in June in Salzburg, Austria, attracted more than 400 participants. It was the largest-ever meeting on ionic liquids, according to Kenneth R. Seddon, chemistry professor at Queen's University of Belfast, in Northern Ireland, and one of the congress organizers. He noted that participants came from 33 countries and six of the seven continents. (Antarctica was not represented.)

The first international meeting devoted to room-temperature ionic liquids was a North Atlantic Treaty Organization (NATO) advanced research workshop titled "Green Industrial Applications of Ionic Liquids," held five years ago in Heraklion, Crete (C&EN, May 15, 2000, page 37). "We struggled to get 40 to 50 people to attend that meeting," observed Robin D. Rogers, professor of chemistry and director of the Center for Green Manufacturing at the University of Alabama, Tuscaloosa. "Since then, there has been a groundswell of interest in ionic liquids. One of the outcomes of the Crete workshop was a new paradigm in thinking about chemical synthesis in general. The workshop also demonstrated a synergy between ionic liquids and green chemistry."

Rogers suggested, however, that the green credentials of ionic liquids have been overstated in some instances. For example, the low or zero volatility of ionic liquids is often seen as an environmental benefit in liquid/liquid separation schemes where the goal is to replace environmentally hazardous volatile organic solvents.

"Separations utilizing ionic liquids as the extracting phase are much more complex than those of simple molecular organic solvents," Rogers explained. "Because of the ionic nature of these liquids, coordination, speciation, and extraction mechanisms can and do vary from those in conventional solvent systems. The challenge is to exploit these different extraction mechanisms or to find combinations of ionic liquids and ligands that afford identical behavior to molecular solvents."

Rogers noted that the properties of ionic liquids, such as hydrophobicity and toxicity, vary immensely. "Ionic liquids are salts that melt below 100 °C. That is their only common property. Not all ionic liquids are green, and one should not make generic statements about the green benefits of ionic liquids."

EVEN SO, there is a plethora of potential applications, particularly as advanced functional materials, he added. "I'm pretty excited about the field in general, but not the solvent applications," Rogers told C&EN. "For me, the exciting developments come from the materials and other novel applications of ionic liquids. The materials are unique in being able to deliver two independent functionalities in one pure compound. This sort of flexibility has hardly been touched yet in ionic liquid applications.

"The key is to be able to use one ion to deliver one function and the second ion to deliver a different, completely independent function, or for one ion to be used to control the physical properties and the other for a given functionality," he continued. "If you tried to do this with a molecular compound, you would end up with all sorts of synthetic challenges."

In addition to hyped claims about the potential environmental benefits of ionic liquids, there has been widespread skepticism about their potential industrial applications. Some journal editors believe that published academic work on ionic liquids "is not truly industrially relevant and is surrounded by unjustified hype," observed Jason A. C. Clyburne, associate professor of materials chemistry at Simon Fraser University in British Columbia.

The wide variety of industrial applications of ionic liquids described in lectures and posters at the Salzburg congress should dispel many of the doubts.

Matthias Maase, a member of the global new business development team of BASF's intermediates division in Ludwigshafen, Germany, listed four commercial and seven pilot-scale industrial processes that employ ionic liquids as catalysts, performance additives, extractants, solvents, or liquid supports.

Ionic liquids may facilitate new ways of doing chemistry, with higher yields and fewer by-products, thanks to smaller, less expensive equipment, he said. "Apart from a few pioneering applications that are already established in industry, we feel that 2005 and 2006 will see the pilot phase for ionic liquid applications. We expect a number of large-scale processes to be launched in the following years."

Maase suggested that ionic liquids will be used industrially not only for classic applications such as chemical synthesis, catalysis, and electrochemistry, but also as performance chemicals, new materials, and engineering fluids for machinery and equipment and for use in the automotive, textile, construction, oil and gas, and energy industries.

BASF is already using ionic liquids in a process for the synthesis of alkoxyphenylphosphines (C&EN, March 31, 2003, page 9). The compounds are precursors for the synthesis of photoinitiators in UV-curable coatings. The process, known as BASIL (biphasic acid scavenging utilizing ionic liquids) was launched in October 2004. It uses the ionic liquid 1-methylimidazolium chloride to remove acids from the reaction mixture.

In addition, Maase noted that BASF is operating a pilot-scale extractive-distillation process that uses an ionic liquid as the extractant and a pilot-scale chlorination process that employs an ionic liquid as a solvent.

He also discussed the role of purity and color in the commercial use of ionic liquids. "The definition of purity strongly depends on the specific requirements of the targeted application," he said. "In the end, we do not sell purity--we sell performance." Maase added that ionic liquids tend to get colored during prolonged thermal treatment, but this usually does not affect the processes for which they are used.

It came as a surprise to many participants at the Salzburg congress that the use of ionic liquids in an industrial process for organic synthesis dates back to 1990. That's when BASF launched a chlorination process that employed a separation liquid phase consisting of low-melting (chloromethylene)dialkylammonium chloride salts--known as Vilsmeier salts--as catalysts in phosgenation reactions.

In 1996, Eastman Chemical launched a cocatalyst ionic liquid system for the synthesis of 2,5-dihydrofuran. This compound is an intermediate used in the production of many commodity, specialty, and fine chemicals, according to Stephen N. Falling, a researcher at Eastman. "It provides access to tetrahydrofuran and other members of the furan family."

THE CONTINUOUS liquid-phase process uses a trialkyltin iodide Lewis acid and a tetraalkylphosphonium iodide ionic liquid to catalyze the rearrangement of 3,4-epoxy-1-butene to 2,5-dihydrofuran.

"The continuous rearrangement process requires that the catalysts are essentially nonvolatile so that they do not codistill with the product," Falling said. "The cocatalyst system gives high selectivity for 2,5-dihydrofuran and provides an efficient means for catalyst recovery."

The process was operated by Texas Eastman Division in a plant with a capacity of 1,400 metric tons per year at Longview. "Although the process was a technical success, the plant shut down in December last year for business reasons when Eastman exited from fine chemicals," Falling said. "The 3,4-epoxy-1-butene technology is now available for sale or licensing."

One of the most exciting and impressive potential industrial applications of ionic liquids is their use for the storage and delivery of gases that are highly toxic, flammable, and/or reactive. Hazardous gases are commonly packaged in high-pressure cylinders, either pure or mixed with an inert gas, observed Air Products & Chemicals chemist Daniel J. Tempel. "An unintended or uncontrollable release can result in significant injury or death."

He noted that the electronics industry uses toxic gases such as phosphine (PH₃), boron trifluoride (BF₃), and arsine (AsH₃) to dope silicon with phosphorus, boron, and arsenic ions. Doped silicon is a key component of field-effect transistors.

"An ion-implant tool operates at very high voltages, and the process requires that the gas sources be at the same voltage as the tool," Tempel explained. "The gas sources must be installed within the ion implanter in close proximity to the operators. The ion implant segment of the industry has therefore adopted the use of less hazardous gas sources in which gases are stored in standard cylinders containing a zeolite or activated carbon adsorbent. The cylinders contain subatmospheric pressures of physically adsorbed toxic gases that are delivered to a process under vacuum on demand."

Air Products has developed a subatmospheric ionic-liquid-based technology for storing and delivering gases that offers a number of advantages over the solid physical-adsorption technology.

"We recognized that chemical complexation could be a superior alternative to physical adsorption and realized that ionic liquids have properties that are ideally suited for this application," Tempel said. "Ionic liquids can be selected or tuned through choice of anion and/or cation to provide the desired reactivity for a given gas. They are also easy to agitate mechanically and pump, and they are better at transferring heat than porous solids."

The Air Products chemists tested several Lewis-acidic ionic liquids for reversible complexation with PH₃, a relatively weak Lewis base, and obtained the best results using an alkylmethylimidazolium chlorocuprate(I) salt. The team showed that 2 mol of PH₃ can react with 1 mol of the ionic liquid.

The chemists then investigated BF₃ to demonstrate that the concept could be extended to a strongly Lewis-acidic gas. "Molecular modeling predicted that dialkylimidazolium tetrafluoroborate salts are well-suited for reversible complexation with BF₃," Tempel said. "Laboratory-scale experiments confirmed this, and we selected an alkylmethylimidazolium tetrafluoroborate ionic liquid for commercial-scale development."

Laboratory testing of commercial-scale prototypes of the PH₃ and BF₃ systems showed "excellent" gas evolution rates and long-term gas/liquid stability, according to Tempel. In addition, the ionic liquids can be recycled numerous times without loss of capacity. Air Products' first patent application covering the new ionic-liquid-based technology, commercially named GASGUARD Sub-Atmospheric Systems, was published in the U.S. last October. The storage and delivery systems are now being tested at several customer sites.

Semiconductor wafers produced using the GASGUARD systems "have all met the specifications demanded by these customers," Tempel noted.

According to Clyburne, this application is particularly important. "The discovery hits a number of the major points people have been highlighting for years for ionic liquids, namely their nonvolatile nature, their resistance to fire, and their ease of handling," he said. "It also shows that ionic liquids have properties that have been promoted for and expected from porous solids."

At the University of Colorado, Boulder, chemical engineering professor Richard D. Noble and coworkers have been investigating the use of ionic liquids in composite membrane structures to obtain mechanically stable structures for gas separation applications. "I believe that gas and condensable vapor separations are more viable targets for ionic liquids than liquid-phase separations," he remarked. "One of our approaches involves using hydrophilic polymers to make a homogeneous composite membrane that has the stability of the polymer while maintaining some of the useful properties of the ionic liquid."

In a separate development, scientists at the Austrian Center of Competence for Tribology (AC²T), Wiener Neustadt, have been evaluating the potential of a range of ionic liquids as lubricants for steel/steel contacts. "The negligible vapor pressure, good viscosity-temperature behavior, low flammability, and thermal stabilities of ionic liquids are desirable properties for lubricants," explained AC²T senior researcher Nicole Dörr.

She observed that preliminary results were promising. "Ionic liquids offer potential as lubricants, although we identified some limitations, such as corrosiveness," she said. "In general, the tribological behavior of ionic liquids can be divided into two categories. Some exhibit low friction coefficients compared with conventional non-ionic-liquid reference samples, but higher wear. Others exhibit higher friction but low wear."

In Germany, Degussa is working on the use of ionic liquids for various applications. The company's Oligomers & Silicones business unit, for example, has successfully developed an ionic liquids hydrosilylation process for producing polyether siloxane products for use as stabilizers for polyurethane foams, antifoaming agents, dispersing agents, emulsifiers, and other industrial applications. In this multiphase process, the ionic liquid immobilizes the precious-metal catalyst, enabling it to be recovered and reused. The negligible vapor pressure of the ionic liquid also allows the volatile organic products to be separated by distillation without the formation of an azeotrope.

In another Degussa project, scientists at Creavis Technologies & Innovation are focusing on the use of ionic liquids to enhance the performance of lithium-ion batteries. "Lithium battery technology is by far the most advanced technology for battery cells, but because of cost and safety issues, it is not established for large-scale applications," said Anna Prodi-Schwab, R&D manager at Creavis.

Lithium-ion batteries typically contain a graphite negative electrode, a lithium transition-metal-oxide positive electrode, and an electrolyte mixture of an electrochemically stable lithium salt and ethylene carbonate dissolved in an organic solvent such as dimethyl carbonate. The ethylene carbonate forms a passivation film on the anode that protects the electrolyte against reduction while it is ionically conductive. The film ensures a high number of charge and discharge cycles during which lithium ions respectively intercalate into and deintercalate from the graphite electrode. A drawback of these batteries is the low boiling point and easy flammability of the solvent, which poses a safety risk and reduces the temperature range for applications.

"We have shown that an electrolyte containing the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide permits reversible lithium intercalation into standard graphite when vinylene carbonate is used in small amounts as an additive," Prodi-Schwab said.

In a recent paper, Prodi-Schwab and colleagues show that the cycling behavior of the graphite electrode in the ionic-liquid-based electrolyte is comparable with that of a graphite electrode in a standard electrolyte based on organic carbonates with respect to reversible charge capacity (Carbon 2005, 43, 1488).

A number of companies are now producing ionic liquids commercially. For example, Goldschmidt Industrial Specialties, which is part of Degussa's Oligomers & Silicones business unit, manufactures a range of ammonium ionic liquids known as the AMMOENG product line. The products are commercially available in R&D quantities from Solvent Innovation, a company based in Cologne, Germany.

BASF produces bulk quantities of a range of technical-grade imidazolium ionic liquids known as BASIONICS.

The phosphonium ionic liquids used for the Eastman 2,5-dihydrofuran process were supplied by Cytec. "We have been producing phosphonium ionic liquids for many years," said Brian J. McSwigan, Cytec's global marketing director for phosphine and phosphorus specialties.

In Darmstadt, Germany, Merck is developing new technologies for the production of ionic liquids. The synthesis of ionic liquids is typically based on the alkylation of amines, phosphines, or heterocyclic compounds such as imidazole or pyridine, with alkylating agents, some of which are toxic, Merck chemist Nikolai V. Ignat'ev explained. "For sustainable development, ionic liquids need to be produced without toxic compounds and volatile organic solvents," he said.

Ignat'ev and coworkers at Merck have recently developed a synthesis of ionic liquids using alcohols instead of alkyl halides as alkylating agents. They have used the method to prepare ionic liquids with the bis(pentafluoroethyl)phosphinate anion. Ignat'ev noted that such ionic liquids are typically water soluble and can be used as starting materials for preparing ionic liquids with other anions.

The group also has prepared a range of ionic liquids by the direct alkylation of amines, phosphines, and heterocyclic compounds using alkyl trifluoromethanesulfonates (alkyl triflates). "Alkyl triflates are powerful alkylating agents," Ignat'ev observed. "We have recently developed a patented practical synthesis for these alkylating reagents in which the only by-product is CO₂."

Because of the growing usage of ionic liquids, the issue of toxicity and biodegradability of the materials is attracting increasing interest in the ionic liquids community. Considerable progress has been achieved in assessing the general biological activity of ionic liquids, noted professor of bioorganic chemistry Bernd Jastorff and coworkers at the Centre for Environmental Research & Technology at the University of Bremen, Germany, in a recent paper (Green Chem. 2005, 7, 362). "This progress has so far focused on the screening methods for larger sets of compounds, and on the class of imidazolium-based ionic liquids."

More toxicological data are required on the influence of ionic liquids on human health, according to the group. "Almost no information about carcinogenicity, genotoxicity, or teratological effects is available," the authors pointed out.

In addition, environmental exposure estimations are complicated because the ionic constituents of ionic liquids tend to be surface active. Conventional bulk-phase-oriented exposure models are therefore of limited value. The group pointed to the need for further risk assessment studies, more (eco)toxicological data, more data on the exposure pathways for selected technical applications, more information on (bio)transformation and sorption processes, and bioaccumulation studies.

At the Salzburg meeting, professor of chemical engineering Joan F. Brennecke outlined work carried out with professor of biological sciences Gary A. Lamberti and coworkers at the University of Notre Dame, in Indiana, on the environmental risks of imidazolium and pyridinium ionic liquids to aquatic organisms and ecosystems.

"Not all ionic liquids are toxic," she pointed out. "They span the range."

The team carried out toxicity bioassays to test for effects on water fleas, duckweed, snails, minnows, algae, and bacteria. "Across all organisms studied, ionic liquid toxicity increased with increasing chain length of alkyls substituted onto both imidazolium- and pyridinium-based cations," Brennecke noted. "Ionic liquids with octyl chains on the cation are more toxic than many volatile solvents currently in use. However, ionic liquids with butyl or hexyl chains are about as toxic or less toxic than many volatile solvents. We found that varying the anion did not significantly alter ionic liquid toxicity."

The group also assessed the ability of microbial communities from two wastewater treatment plants to biodegrade ionic liquids. "Over 28 days, ionic liquids with octyl substituents on the cations exhibited several indicators of biodegradability," Brennecke observed. "While less toxic, the butyl- and hexyl-substituted compounds were not readily biodegradable."

BASF's Maase presented a complete toxicological profile of two widely used ionic liquids: 1-ethyl-3-methylimidazolium ethylsulfate and 1-butyl-3-methylimidazolium chloride. The first compound is not harmful in terms of acute oral toxicity, is nonirritating to skin and eyes, and is nonsensitizing and nonmutagenic, Maase observed. The second compound turned out to have acute oral toxicity, he reported. "Neither liquid is readily biodegradable. The 'magic' ionic liquid that fits all toxicity, ecotoxicity, and performance criteria does not exist."

Maase also presented the results of an ecoefficiency analysis of the company's BASIL process. "The analysis clearly indicated that it is possible to make chemical processes much more sustainable with the help of ionic liquids," he said.

The burgeoning interest in, and range of current and potential applications of, room-temperature ionic liquids impressed many of the congress participants. "As a relative newcomer to ionic liquids, I was struck by the quality of research and the unusual chemistry that was presented," Clyburne remarked. "It was particularly exciting to hear that industry has been using ionic liquids for over a decade, giving evidence for those of us in academia that what we are studying is industrially relevant."

James H. Davis Jr., professor of chemistry at the University of South Alabama, Mobile, offered similar sentiments: "I am in absolute awe at the number of researchers who are now involved, the energy and imagination that are being brought to bear on developing new ionic liquids, the search for ever more diverse and unique applications, and the growing understanding of the links between ionic liquid structure, composition, and properties."