science/technology
DESIGNER SOLVENTS Ionic liquids may boost clean technology development Michael Freemantle C&EN London
I
onic systems consisting of salts that are liquid at ambient temperatures can act as solvents for a broad spectrum of chemical processes. These ionic liquids, which in some cases can serve as both catalyst and solvent, are attracting increasing attention from industry because they promise significant environmental benefits, according to British and French researchers. "Probably the most important advantage is that they have no measurable vapor pressure," says chemistry professor Kenneth R. Seddon who leads a group of 18 researchers working on ionic liquids at Queen's University of Belfast, Northern Ireland. "Unlike conventional solvents used in the industrial syntheses of organic chemicals, they are nonvolatile and therefore do not emit vapors." Paul T. Anastas, chief of the industrial chemistry branch at the Environmental Protection Agency, Washington D.C., is convinced that ionic liquids can contribute significantly to green chemistry and the development of clean technology. "Green chemistry aims to design the hazards out of chemical products and processes, including solvents," he says. "With ionic liquids, you do not have the same concerns as you have with, for example, volatile organic solvents, which can contribute to air pollution. Ionic liquid chemistry is a very new area that is not only extremely interesting from a fundamental chemistry point of view, but could also have a very large impact on industry." Research on ionic liquids in Europe is attracting increasing interest and support from industry. For example, the Belfast group is working on various ionic liquids projects in collaboration with British companies such as BP Chemicals based in Sunbury-on-Thames, ICI Chemicals & Polymers in Runcorn, and British Nuclear Fuels (BNFL) in Sellafield. The group is also participating in a European consortium of industrial and academic partners that aims to develop and commercially exploit ionic liquids technology within the next five years. 32 MARCH 30, 1998 C&EN
At the French Petroleum Institute (TFP) at Rueil-Malmaison, near Paris, a group led by senior research scientist Hélène Olivier, has developed a butène dimerization process that uses an ionic liquid as a catalyst support. The process offers economic benefits over an existing IFP butène dimerization process widely used in the petrochemical industry, according to Olivier. "Most people think of ionic liquids as molten salts like molten sodium chloride or molten cryolite that is electrolyzed to produce aluminum," says Seddon. "These salts have very high melting points and are highly corrosive when molten. "To call an ionic liquid a molten salt is a bit like calling a car a horseless carriage," he continues. "It's a perfectly accurate description, but it gives you the wrong mental image." The ionic liquids that the British and French groups are working on are lowmelting-point molten salts that are liquid around room temperature. They typically have a liquid range of about 300 °C, far in excess of the 100 °C range for water or 44 °C for ammonia. "A liquid range of 300 °C is an amazing parameter to have for chemical engineering of these systems," says Seddon. The archetypal ionic liquid system is 1 -ethyl-3-methylimidazolium chloridealuminum chloride, known in abbreviated form as [emim]Cl-AlCl3. It is liquid and thermally stable from almost -100 °C to around 200 °C, depending on the molar ratio of [emim]Cl and A1C13 added to the system. For example, when the molar proportions are equal, the system is a neutral stoichiometric compound, [emim]+-
Seddon: remarkable new properties
[AICI4]", which melts at its congruent melting point of about 6 °C. The lowest melting point, -96 °C, is achieved when the molar percentages of the system are 35% [emim]Cl and 65% A1C13. Low-melting-point ionic liquids typically exhibit mixed organic and inorganic character. The cation is usually a heterocyclic cation such as [emim]+, l-butyl-3-methylimidazolium— [bmim]+—or N-butylpyridinium. These organic cations, which are relatively large compared with simple inorganic cations, account for the low melting points of the salts. The anions, on the other hand, determine to a large extent the chemical properties of the system. For example, the main anions present in cWoroaluminate ionic liquid systems such as [emim]ClAICI3 are CI", which is a Lewis base; [A1C14]", which is neither acidic nor basic; and the Lewis acid [A12C17]". The concentration of each anion, and therefore the Lewis acidity of the system, varies depending on the relative amounts of A1C13 and [emim]Cl added to the system. Tetrafluoroborate, [BF4]", and hexafluorophosphate, [PF6]", are among other types of anions that are ^-^—^— attracting the interest of ionic liquids research Low-melting-point ionic liquids groups. These ions do have organic cations not combine with their corresponding Lewis acAICI4][N0 3 ]ids and are therefore not potentially acidic. S N^N N * CH2CH3 Hocr Seddon refers to ionic C H2C H2C H2 C H3 liquids as "neoteric"— groundbreaking—sol1-Butylpyridinium -Ethyl-3-methylimidazolium vents. "They are a class tetrachloroaluminate nitrate of novel solvents with re-
"Materials—even rocks, coal, and almarkable new properties," he says. "We 600 ionic liquid systems can, in princialso apply the term neoteric solvent to ple, be generated from around 10 sim- most anything organic—dissolve phenomsupercritical fluids because these solvents ple anions such as [BF4]~ and [PF6]" and enally well in ionic liquids," continues Sedalso show immense promise for clean the l-alkyl-3-methylimidazolium cation don. "Ionic liquids can therefore lead to synthesis." substituted with various alkyl groups in process intensification. We are working Olivier uses the acronym NAIL for the 2-, 4-, or 5- position, or N-alkylpyri- on one process where there would probanonaqueous (room-temperature) ionic dinium substituted in the 3- or 4- posi- bly be a physical reduction in plant size of liquids. "NAIL are a nonconventional but tion. When heteropolyanions, such as a factor of 20 because the materials are so increasingly important new class of sol- [PMo12O40]3", and tetralkylammonium much more soluble in ionic liquids than in vents," she says. "The spectrum of phys- and tetralkylphosphonium cations are conventional solvents." ical and chemical properties of NAIL is added to the list, a grand total of more In various collaborative projects with much larger than that of classical organic than a quarter of a million ionic liquid industry, the Belfast group has shown that or inorganic solvents." As well as wide systems is possible. a wide range of catalyzed organic reacliquid ranges, they offer attractive ranges "And that's without even trying," says tions can be carried out in room-temperaof physical properties such as density Seddon. "The anions and cations are al- ture ionic liquids. These include alkylaand viscosity and also have high heat most infinitely variable. You can't have tions, acylations, reductions, oxidations, conductivities, she notes. In addition, the any combination of cation and anion, oligomerizations, and polymerizations. miscibilities of ionic liquids with organic but for a given anion, you can tailor the In some cases, the ionic liquid acts as compounds can be varied extensively by cations to make an ionic liquid. And for a both solvent and catalyst. For example, altering the chain lengths of the [emim]Cl-AlCl3 system the alkyl substituents on the can be used as a solvent and cations. catalyst for Friedel-Crafts reactions. "A typical FriedelA good quality ionic liqCrafts reaction takes six or uid in a stoppered flask or seven hours to give you sealed tube looks exactly like about 80% yield of a mixwater, observes Thomas ture of isomers," says SedWelton, lecturer in inorganic don. "In a room-temperature chemistry at London's Impeionic liquid, the reaction is rial College of Science, Techcomplete in about 30 secnology, and Medicine. "Some onds with essentially 100% of the more viscous ones conversion." look like glycerol, but generally they are free-flowing colWelton and graduate stuorless liquids," he says. dent Narmatha Srinivasan have recently carried out "The chloroaluminate the Friedel-Crafts acylation ionic liquids are waterof ferrocene in [bmim]Clsensitive," continues Welton. A1C13 using both acetic an"When you expose them to hydride and acetyl chloride air, they produce a lot of as acylating agents. "The HCl. The tetrafluoroborates Olivier with sealed tube containing a hydrocarbon liquid above an [bmim]+[AICI4]~, containing blue nickel complex ions number of reactions that and hexafluorophosphates, ionic liquid, 2 involve aluminum chloride on the other hand, are air [NICIJ -. as a catalyst is enormous," and water stable. That makes them really exciting. You can work with particular cation, you [can] choose your says Welton. "The chloroaluminate ionic liquids are therefore clearly very them in an open beaker. Because they are anions." nonvolatile, there is no smell, and they can With this vast variety it is possible, in important." be used in high-vacuum systems." Much of the research in both Britain theory at least, to tailor the solvents to Ionic liquids with organic cations specific chemical reactions. "Ionic liq- and France is focusing on the developsuch as [emim]+ or [bmim]+ generally uids have a range of physical and chemi- ment of ionic liquids that are immiscible have high thermal and chemical stabili- cal properties that can be tuned with a with reactants and products but dissolve ties and are relatively easy to make, ac- precision that is hard to imagine for a catalysts. Such systems, in which the cording to Welton. He cites the prepara- given reaction," says Seddon. products can be easily separated, offer tion of [bmim]Cl-AlCl3 as an example. Ionic liquids are good solvents for a immense potential for industrial develop"It's amazing to see," he exclaims. "You wide range of inorganic, organic, and ment, according to the researchers. just mix two white powders—l-butyl-3- polymeric materials, according to SedThese two-phase or "biphasic" catamethylimidazolium chloride and alumi- don. "For example, benzene is soluble lytic processes offer the benefits of num chloride—together in a beaker, and up to 50 volume % in tetrachloroalumi- both homogeneous and heterogeneous they just collapse into a liquid." nate-based ionic liquids," he says. "This catalysis, according to Olivier. For exThe imidazolium starting material is means that we have ionic liquids that ample, they combine the mild condireadily made by boiling commercially will dissolve covalent compounds. This tions and high efficiency and selectivity available methylimidazole with 1-chloro- was one of the biggest surprises when of homogeneous catalysis with the easy butane, followed by cooling. we started to work in this field in the separation of catalyst from reaction Seddon points out that more than early 1980s. products and optimal use of catalyst MARCH 30, 1998 C&EN 33
science/technology that are possible with heterogeneous catalysis. "Ionic liquids show great promise as solvents for two-phase catalysis," note Olivier and Yves Chauvin, former associate director at IFP. "Their chemical properties such as complexing ability and acidity can be tuned at will. It may be assumed that most of the known transitionmetal-catalyzed reactions could be carried out in organic-inorganic ionic liquids by fitting the liquid composition with the selected catalyst precursors." The IFP group in Paris is carrying out exploratory work on a range of biphasic catalytic reactions using ionic liquids. They include, for example, the use of [bmim]+[PF6r or [bmim]+[SbF6]- as ionic liquids and organorhodium complexes as catalysts for olefin hydrogénation and for the hydroformylation of olefins with carbon monoxide and hydrogen to yield aldehydes. In other work, they are examining the use of an organotungsten catalyst in [bmim]+[Al2Cl7r to carry out olefin metathesis. Using the same chloroaluminate system, they have also developed a process for dimerizing propene and butène using a Ziegler-Natta catalyst— nickelGD chloride and ethyl aluminum dichloride. "The use of ionic liquids in biphasic liquid-liquid catalysis is a real hot topic," comments Welton. "It is opening up a whole new world of chemistry." He points out that ionic liquids offer a highly polar but noncoordinating envi-
ronment for chemistry. It is difficult to dissolve catalysts in nonpolar, noncoordinating molecular solvents such as hexane, he explains. Polar solvents, such as acetonitrile, tend to coordinate metal complexes. "You often try to free up a site on a metal center because that is where catalysis occurs," he says. "But in many cases, when you can dissolve the catalyst in the polar solvent in sufficient concentration, the solvent coordinates on the active site and therefore blocks it. Ionic liquids such as the tetrafluoroborates offer a straightforward replacement of a solvent with a polar solvent that is noncoordinating. "I am also hoping to use ionic liquids, which are the ultimate polar solvents, with supercritical fluids, which are the ultimate nonpolar solvents, in biphasic liquid-liquid catalysis," says Welton. In such a system, the catalyst would be soluble in the ionic liquid but insoluble in the supercritical fluid that is the solvent for the reactants and products. According to Seddon, the reactions that have been observed so far in ionic liquids represent the tip of an iceberg. "It is possible to optimize such phenomena as the relative solubilities of the reactants and products, the reaction kinetics, the liquid range of the solvent, the cost of the solvent, the intrinsic catalytic behavior of the media, and the air stability of the system," he notes. "For the first time, it is possible to design a sol-
Postdoctoral researcher Yasmin Patell working with air-sensitive ionic liquids in inert atmosphere box at Queen's University of Belfast.
34 MARCH 30, 1998 C&EN
vent to optimize a reaction with control over both yield and selectivity, rather than to let the solvent dictate the course of the reaction. "All the indications are that room-temperature ionic liquids are the basis of a new industrial technology," he suggests.
Industrial potential "Industrial exploitation of ionic liquids is in its early days," according to Neil Winterton, research associate at ICI Chemicals & Polymers. He explains that ICI is only in the second year of an exploratory program on ionic liquids, and patents are filed but not yet published. "The use of ionic liquids is seen as being highly speculative," he tells C&EN. Winterton points out that roomtemperature ionic liquids have been studied actively since their discovery in the 1970s, predominantly for their possible application as battery electrolytes. Work on the exploitation of ionic liquids as reaction media started in the early 1980s, but only in the past five or six years has such work attracted industrial interest. "Our interest has been stimulated by the novel and potentially interesting properties exhibited by ionic liquids," continues Winterton. "With the infinite range of ionic liquids available in principle, considerable opportunities exist for process optimization once a lead system has been identified." Winterton points to three important economic drivers for the development of chemical processes that can also bring significant potential benefits for the environmental impact of chemical processes. The first is greater reaction selectivity and therefore less by-product or waste formation. Second, enhanced reaction rates with more active catalysts can lead to the reduction of plant size and cost. Finally, the operation of processes with fewer processing steps and milder conditions of temperature and pressure can result not only in reduced energy consumption and costs but also in an enhancement in safety. Industry is keen to explore ways in which ionic liquids can deliver some of these benefits, suggests Winterton. But there are as-yet-unanswered and critical questions relevant to the large-scale use of ionic liquids. "These include their cost, robustness in use, readiness with which products can be separated from them, heat- and mass-transfer questions, the degree to which ionic liquids can be recycled and reused, their ease of
manipulation on the industrial scale, materials compatibility, and safely issues," he says. A few patents associated with the chemical applications of ionic liquids have emerged over the past couple of years, revealing the beginnings of industrial interest, according to Winterton. "These have been concerned with olefin oligomerization and hydrogénation. I suspect others have been filed, are in the process of completion, and will be published in due course," he says. "As far as I am aware, little has entered the public domain concerning the use of ionic liquids on an industrial scale, and nothing has been claimed, even in general terms, to suggest that room-temperature ionic liquids are being used in a viable chemical process [being] operated commercially." Earlier this month, however, IFP in Paris revealed to C&EN that it has just launched a commercial process based on ionic liquids that is now available for licensing. The Difasol process for dimerizing butènes to isooctenes was developed at Rueil-Malmaison, and pilot-scale trials were carried out at IFP's pilot facilities at the Industrial Research & Development Center at Solaize, near Lyon. Jean-Alain Chodorge, senior process engineer at IFP, explains that the Difasol process provides significant benefits over IFP's existing Dimersol X process. The latter is a liquid-phase dimerization process that produces isooctenes from nbutenes using a Ziegler-Natta-type homogeneous catalyst based on a nickel component activated by an organoaluminum reduction compound. The Dimersol X process is currently in operation in five industrial plants with a total production of nearly 200,000 tons of isooctene per year. Isooctenes are used as a raw material for isononanol manufacture. According to IFP, C8 to C10 alcohols such as 2-ethylhexanol, isononanols, and isodecanol are in strong demand. Esterification of these alcohols yields dialkyl phthalates that are used as PVC plasticizers. The new Difasol process for manufacturing isooctenes consumes less catalyst. The process dimerizes n-butene in a continuous two-phase operation that uses the industrial Dimersol nickel catalyst dissolved in a chloroaluminate ionic liquid. The n-butenes are introduced continuously into the reactor, and the products, which are only poorly miscible with the ionic liquid, are separated in a settler.
Atkins: replace mineral acids
"The process not only makes much better use of the catalyst and therefore leads to greatly reduced catalyst disposal and costs," points out Chodorge, but "the process also has other advantages over the homogeneous Dimersol X process. For example, the conversion is between 70 and 80% on dilute feed, and the dimer selectivity is 90 to 95%. This is very high." According to IFP, current Dimersol units can be fitted with the two-phase Difasol system. "This improves isooctene yield and provides significant benefits associated with catalyst consumption," the institute states. "Therefore, the overall economy of the process is improved." Used in combination with the Dimersol X process, the process has the potential of producing isooctenes on a scale of 20,000 to 90,000 tons per year per unit, Chodorge tells C&EN. "There is a very good possibility that this will be the first large-scale application of room-temperature ionic liquids," he says. Since the early 1980s, BP Chemicals in Sunbury-on-Thames, originally through the BP Venture Research Unit, has been collaborating with Seddon and his group to explore the potential industrial uses of ionic liquids as solvents and catalysts. Initial collaboration focused on the ability of ionic liquids to dissolve kerogen—a fossilized, insoluble organic material present in a sediment. "We had a problem characterizing the kerogen fractions of rocks for oil exploration," explains Martin P. Atkins, strategic alliances program manager at BP Chemicals. "We needed the dates [the ages of the rocks]
to give us an idea of where the oil was residing in various formations. The only way we could [date the rocks] accurately was to dissolve the kerogen out of these rocks." Just one reagent could do this— hydrofluoric acid. "This presented a very difficult problem for the analysts in our laboratories," says Atkins. "We tried sulfuric acid and nitric acid, but they did not work as well. So we asked Seddon to investigate the use of ionic liquids to dissolve kerogen. They worked. Overnight, we dispensed with hydrofluoric acid and started using ionic liquids with great success." That was around 15 years ago. Now the focus of collaboration between BP Chemicals and the Belfast group is on the use of ionic liquids for refinery processes such as alkylations of aromatic hydrocarbons. These alkylations are typically carried out by the reaction of an alkyl halide with an aromatic hydrocarbon using an acid catalyst. Zeolites, aluminum halides, and alkyl aluminum halides are also used in refinery alkylations. "We are looking to replace mineral acids like sulfuric acid or hydrofluoric acid," says Atkins."These are very volatile and corrosive, so there is a real problem if there is a release of acid at any stage. "Traditionally, the acid goes out with the product, so it has to be recovered in distillation and concentration processes before being used again, or [the acid must be] disposed of by neutralization," he explains. "Ionic liquids are nonvolatile and totally immiscible with the product, so they form a complete separate layer. They can therefore be used in a recycle mode." One possible refinery use of ionic liquids is in the manufacture of ethylbenzene, which is then converted by catalytic dehydrogenation to styrene, the precursor to polystyrene. "We have been looking at a process to make ethylbenzene that uses an imidazolium-aluminum chloride ionic liquid system as solvent and catalyst," Atkins tells C&EN. "The ethylbenzene is made by the direct alkylation of benzene with ethylene without using ethyl chloride as an alkylating agent. "It is not a foregone conclusion that we are going to develop the process," he says. "We still need to see if we have a viable process and at the same time compare the detailed economics with the new generation of zeolite-based technologies." BP Chemicals is also investigating the MARCH 30, 1998 C&EN 35
science/technology use of the same class of ionic liquids for the oligomerization of olefins, such as butènes, to produce synthetic lubricants or fuel additives containing 20 to 60 carbon atoms. "We are now taking stock of the entire system, looking at the whole flow sheet," says Atkins. "Before we can make a decision to go forward, two things have to be in place. First, the final product has to meet the performance required in the marketplace. Second, the cost of production has to be in the right ballpark." The recycling of the ionic liquids is a critical factor, according to Atkins. "The engineering is lagging behind the chemistry," he says. "For example, no one, as far as I am aware, has ever taken an ionic liquid to exhaustion and tried to recover its activity." Mixing of the reactants with the ionic liquids is also critical. "The reaction rate depends on the interfacial area between the feed and the catalyst," Atkins explains. "You need high shear. We have therefore developed a loop reactor that provides counterflow to get good mixing." The loop reactor forces the ionic liquid up through the liquid feeds. Then the products and ionic liquid separate by gravity in another part of the loop. The ionic liquid, which is more dense, sinks to the bottom of the separator and is pumped back into the reactor. "We ran the ethylbenzene process using a 1-alkyl3-methylimidazolium aluminum chloride ionic liquid in a pilot-scale loop reactor for 300 loops without any deactivation," Atkins says. The development of ionic liquids at BP Chemicals for commercially viable processes still requires a lot of effort and attention, Atkins suggests. "This is a very speculative area, and it may still be some time before these processes become industrially or commercially important," he says. "The engineering and the economics, rather than the chemistry, are driving the development now."
Nuclear fuel reprocessing "Ionic liquids have potential as part of the development of new environmentally beneficial methods of spent nuclear fuel treatment," according to a statement issued by BNFL to C&EN last month. "One of the avenues being explored is to dissolve the spent nuclear fuel in the ionic liquid—for example, a substituted pyridinium nitrate—then carry out chemistry in this solution to separate the components of the fuel in a compact and 36 MARCH 30, 1998 C&EN
comfortably, and can satisfy current environmental regulations, according to Hutson. "However, we are performing extensive R&D to provide us, in the longer term, with alternative spent fuel management scenarios that will allow us to continue to economically process more chemically complex and more highly irradiated fuels and to meet even more demanding environmental constraints," he says. "From the limited studies so far undertaken, ionic liquids are currently showing considerable promise as a contributor to achieving this aim." In a patent published last month, BNFL reveal a method of dissolving the spent fuels in ionic liquids containing oxidants. The process converts a metal fuel that is insoluble in an ionic liquid into a soluble form by raising its oxidation state. For example, when uranium(TV) oxide, U0 2 , or plutonium(IV) oxide, Pu0 2 , is added to an oxi— ^ — dizing ionic liquid, they do not dissolve directly. RathIonic liquids and products er, the metals are convertseparate in catalytic loop reactor ed to complexes containing U(VT) and Pu(VT), and these oxidized species dissolve. Product The ionic liquid is typically a nitrate, such as 1-butylpyridinium nitrate, preReactor pared by mixing aqueous Gravity silver nitrate with 1-butylseparator pyridinium chloride. The oxidizing agents are Br0nsted acids, such as sulfuric Feed acid or nitric acid, or Lewis acids, such as the nitroniIonic liquids pump um cation, NO+. When the oxidant is mixed with the ionic liqThe irradiated fuel is first separated uid, another ionic liquid is formed. For from its cladding by dissolving it in hot example, when nitronium tetrafluorobo+ concentrated nitric acid. At the compa- rate salt, [NO] [BF4]~, is added as an oxiny's $3 billion Thermal Oxide Repro- dant provider to the ionic liquid 1-bucessing Plant (THORP), which came on- tylpyridinium nitrate, another ionic liqline at Sellafield in 1993, tri-n-butyl uid, 1-butylpyridinium tetrafluoroborate, phosphate diluted with kerosene is is formed. used as a solvent. After extraction and separation, uranyl and plutonium ni- European consortium Last April, a consortium of industrial trates are converted to oxides for reuse and academic partners embarked on a as fuels. "The process involves many solvent- three-year program to develop and exextraction cycles," says Hutson. "The ploit clean acid technology using ionic highly active liquid waste, which con- liquids. The project aims to manufacture tains 50 or 60 fission products such as ionic liquids and simultaneously develop cesium-137, remains in the nitric acid. It processes for their use as alternatives to is concentrated by evaporation and con- conventional organic solvents and acid verted into solid glass form in our vitrifi- catalysts. The consortium's partners include the cation plant." THORP can readily handle the spent School of Chemistry at Queen's Universifuel currently available for reprocessing ty of Belfast; two research institutes in
hence cost-effective manner and to minimize the waste produced." Collaboration with Seddon's group in Belfast is an essential factor in this exploratory work, stresses BNFL. "Spent fuel consists mainly of uranium or uranium oxide containing the fissile U-235 isotope," explains Graham V. Hutson, senior chemist and intellectual property rights manager at BNFL. "About 1% is plutonium-239, another valuable fissile material, and up to 3% is fission product waste. "All commercial reprocessing at the moment essentially uses the Purex process, a solvent-extraction process that separates the fuel into three streams," says Hutson. "The process separates uranium and plutonium for reuse and concentrates the fission product waste so that it can be stored safely and without excessive cost."
Aachen, Germany; BP Chemicals, which is managing the project; the chemical company Akzo Nobel, based in Arnhem, the Netherlands; and Elementis Specialties in Durham, England. Elementis Specialties, known until last December as Harcros Durham Chemicals, is a division of the London-based chemical company Elementis. The ionic liquids project is funded by the European Community's Industrial & Materials Technologies program, also known as BRITE-EURAM, which is part of the EC's 4th Framework Program for Research & Development, 1994-98. "The consortium received a $5 million grant from the European Community to build three pilot plants for our ionic liquids technology by 1999 with the expectation of having the processes running by about 2002," Seddon tells C&EN. According to Atkins at BP Chemicals, the project aims to examine the technical and economic feasibility of using ionic liquids in comparison with other acid systems. He points out that acid catalyst technology is widely used by industry to manufacture detergent intermediates, fine chemicals, and specialty products, as well as bulk chemicals. However, the efficiency of these processes is severely limited by the high cost of effluent treatment. Furthermore, organic solvents, even when recycled efficiently, give rise to atmospheric emissions. The consortium hopes that the use of ionic liquids will reduce the amount of waste produced in these processes and therefore reduce process costs. The project has three specific objectives, explains Atkins. "The first is to manufacture ionic liquids and make them commercially available to smalland medium-sized enterprises who want to use or test them." Roger Wareing, product development manager at Elementis Specialties, notes that there are currently no commercial manufacturers of ionic liquids. Elementis Specialties and Akzo Nobel "are responsible for developing processes to manufacture the most promising ionic liquids discovered by the academic partners," he tells C&EN. Elementis Specialties has an anhydrous aluminum chloride manufacturing facility in Durham, and the company is involved because many ionic liquids incorporate aluminum chloride, says Wareing. "We are only at the end of the first year of a three-year project, so it's rela-
tively early days yet," he adds. "At the moment, we are engaged in laboratoryscale process development. At the end of the project, we shall be in a position to identify the most likely candidates for commercialization, go through with these on pilot-scale trials, and then finally build plants to commercialize their manufacture." The second consortium objective is to explore the use of ionic liquids as catalysts for aromatic alkylation reactions. Specifically, the goals are to achieve better than 90% conversion of the reactants, more than 95% product selectivity, and over 99.5% recyclability of the ionic liquids. The project is focusing on the alkylation of benzene with C8 to C l6 olefins to produce linear alkyl benzenes (LABs). According to Seddon, nearly all LABs are converted by sulfonation to linear alkyl sulfonates, a major surfactant used in household and industrial detergents. "There are currently several different operating systems," he says. "These use either alkene or chloroalkane feeds and hydrofluoric acid, sulfuric acid, aluminum chloride, or zeolite supported catalysts." All have their problems, continues Seddon. "For example, the HF and H2S04 processes produce huge amounts of acid waste," he says. "The alkene and aluminum chloride process generates a lot of aluminum chloride waste, which is expensive to dispose of. So the aim is to replace these catalysts with cWoroaluminate ionic liquids." Preliminary work suggests that the chloroaluminate ionic liquids perform better than the acid catalysts. For example, decene conversion is 998% in an ionic liquid compared with 99.2% with aluminum chloride and 92.1% with sulfuric acid. The ionic liquids have the added advantage of being 99.5% recoverable and are therefore highly recyclable. "The methodology of using ionic liquids for LAB production is being developed to offer a potential retrofit option for existing plants that currently use either aluminum chloride or hydrofluoric acid technologies," notes Seddon. "A rigorous economic assessment is now needed." The third consortium objective focuses on the use of ionic liquids as catalysts for producing fine chemicals intermediates. The aim is to achieve better efficiency and recyclability than existing catalysts and provide low-temperature processing routes. An example is the use of ionic liquids for the acylation of aromatic
compounds such as chlorobenzene with acetyl chloride to produce the para-substituted product.
Growing interest Michael J. Green, director of product development and technology at Courtaulds Coatings, Tyneside, England, suggests that interest in ionic liquids is about to take off exponentially. "Seddon has been working on the topic for a long time, generating data, and gathering knowledge and information," he says. "People are now coming on board, and I think we are at that point on an exponential curve where it begins to shoot upward." As evidence of the growing interest, Green cites a meeting on ionic liquids that was held at the Royal Academy of Engineering, London, on Jan. 16. The meeting was chaired by J. D. (Hamish) Rankin, senior science and technology associate of ICI Engineering Technology, Runcorn, and gathered together representatives of Seddon's industrial sponsors such as BP Chemicals, ICI, Unilever, and BNFL. "To sit around a table with a large number of industrialists from different companies and hear them talking collectively and enthusiastically with a single academic is something I have not come across before," says Green. "It shows that the commercial potential of ionic liquids is now being recognized by industry." The purpose of the meeting was to identify a path forward for ionic liquids. "The aim is to see if we can pull together a program of fundamental research on ionic liquids," Hutson tells C&EN. "What came out of the meeting was a desire for the companies to collaborate on research areas that are generic to everybody." An anecdote that Seddon tells about one of his early forays into this area of research serves as a footnote to this interest and enthusiasm. In 1982, he submitted a proposal to a research council in the U.K. to investigate the use of roomtemperature ionic liquids to study catalytic processes that might have industrial applications. The proposal was turned down. One referee thought the systems were too simple and therefore not worth pursuing. Another stated that the systems were too complicated for potential applications. A third obviously did not read the correct proposal. "He wondered why we wanted to study the neutron diffraction patterns of vanadium bronzes," according to Seddon.^ MARCH 30, 1998 C&EN 37
