IONIC LIQUIDS. J.D. HOLBREY, K.R. SEDDON.pdf

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Review Clean Products and Processes 1 (1999) 223236 Q Springer-Verlag 1999

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Ionic Liquids

J.D. Holbrey, K.R. Seddon

Received: 14 May 1999 / Accepted: 10 July 1999

J.D. Holbrey, K.R. SeddonThe QUILL Centre, The Queens University of BelfastStranmillis Road, Belfast BT9 5AG UKe-mail: J.Holbrey6qub.ac.ukhttp://www.ch.qub.ac.uk/staff/personal/krse-mail: K.Seddon6qub.ac.uk

The authors would like to thank the ERDF Technology

Development Programme and the QUESTOR Centre (JDH) forfinancial support, and the EPSRC and Royal Academy ofEngineering for the award of a Clean Technology Fellowship(to K.R.S.).

Abstract Ionic liquids are receiving an upsurge ofinterest as green solvents; primarily as replacements forconventional media in chemical processes. This reviewpresents an overview of the chemistry that has beendeveloped utilising ionic liquids as either catalyst and/orsolvent, with particular emphasis on processes that have

been taken beyond the pre-competetive laboratory stageand represent clean industrial technology with significantcost and enviromental benefits.

IntroductionSo what are ionic liquids, and what have they to do withgreen chemistry? The answer to the first question hasbeen addressed in a number of reviews and articlesrecently (Chauvin and Olivier-Bourbigou 1995, Chauvin1996, Freemantle 1998a, Freemantle 1998b, Freemantle1999, Hussey 1983, Hussey 1988, Seddon 1996, Seddon1997, Seddon 1998, Welton 1999), and will not bediscussed in detail here. The answer to the second ques-

tion will become the main text of this article. Clean tech-nology concerns the reduction of waste from an indus-trial chemical process to a minimum: it requires therethinking and redesign of many current chemical proc-esses. As defined by Roger Sheldon, the E-factor of aprocess is the ratio (by weight) of the by-products to thedesired product(s) (Sheldon 1993, 1997).

The Table illustrates that the dirty end of the chem-ical industry, oil refining and bulk chemicals, is remark-ably waste conscious: it is the fine chemicals and phar-maceutical companies who are using inefficient, dirty,processes, albeit on a much smaller scale. Volatileorganic solvents are the normal media for the industrial

synthesis of organics (petrochemical and pharmaceutical),with a current world-wide usage of ca. 4,000,000,000

Table 1. The Sheldon E-Factor (Sheldon 1993, 1997)

Industry Production / tons pa E-factor

Oil Refining 106108 0.1

Bulk Chemicals 104106 15Fine Chemicals 102104 550Pharmaceuticals 101103 25100

p.a. However, the Montreal Protocol has resulted in acompelling need to re-evaluate many chemical processesthat have proved otherwise satisfactory for much of thiscentury. There are four main alternate strategies:1. solvent-free synthesis,2. the use of water as a solvent,

3. the use of supercritical fluids as solvents, and4. the use of ionic liquids as solvents.

It is the purpose of this review to explore option (4),to allow it to be evaluated against the other strategies,and to demonstrate its viability for commercial develop-ment in all sectors of the chemical industry.

As discussed elsewhere recently, (Chauvin and Olivier-Bourbigou 1995, Chauvin 1996, Freemantle 1998a, Free-mantle 1998b, Freemantle 1999, Hussey 1983, Hussey1988, Seddon 1996, Seddon 1997, Seddon 1998), the pastfifty years have seen the melting point of ionic liquidsdrop from c800 7C to 96 7C from high-temperaturecorrosive environments to low-temperature benign

solvents. Although the archetypal ionic liquids are the N-butylpyridinium chloride-aluminium(III) chloride,[NBupy]Cl-AlCl3, and 1-ethyl-3-methylimidazolium chlo-ride-aluminium(III) chloride, [emim]Cl-AlCl3, systems(for phase diagram, see Fig. 1), the range of availableanions and cations has expanded enormously in the pastdecade. Indeed, it is our best estimate that, if binary andternary mixtures are included (and there are very goodpractical and economic reasons for doing that), there areapproximately one trillion (1018) accessible room temper-ature ionic liquids.

Take, for example, the standard cations [NBupy]c

and [emim]c, (see Fig. 2). If the butyl and ethyl groups,

respectively, are exchanged for a generic linear alkylfunction, R (where RpCnH2nc1; n B I [1, 18]), then aseries of cations is generated, all of which can be used asthe basis of ionic liquids. But is this just a case ofmethyl, ethyl, butyl, futyl? No. Figs. 3 and 4 illustrate thesignificant variation in properties which can be inducedby this simple change for the [Rmim][PF6] and


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Fig. 1. The experimental phase diagram for the [emim]Cl-AlCl3system, showing the formation of a compound, [emim][AlCl4],with a congruent melting point (Fannin et al. 1984)

Fig. 2. The aromatic heterocyclic N-butylpyridinium and 1-ethyl-3-methylimidazolium cations

Fig. 3. Melting point phase diagram for [Rmim][PF6] ionic

liquids as a function of alkyl chain length n showing themelting transitions from crystalline (closed square) and glassy(open square) materials and the clearing transition (closedcircle) of the liquid crystalline (LC) terms

Fig. 4. Melting point phase diagram for [Rmim][BF4] ionicliquids as a function of alkyl chain length n showing themelting transitions from crystalline (closed square) and glassy

(open square) materials and the clearing transition (closedcircle) of the liquid crystalline terms

[Rmim][BF4] ionic liquids, with data taken from our ownlaboratories (Bowlas et al. 1996, Gordon et al. 1998,Holbrey and Seddon 1999).

An interesting feature of these phase diagrams is theappearance of liquid crystalline phases with the longeralkyl chains, and this is confirmed when their opticaltextures are examined (see Fig. 5). The crystal structureof [C14-mim][PF6] is illustrated in Figs. 6 and 7. Theimplications of the existence of these stable phases has

still to be explored in terms of stereochemical control ofreactions.The wide range of reactions that have been under-

taken in low temperature ionic liquid solvents is quite

remarkable. The range is limited simply by imaginationand the time required for investigation. The specific andselectable solvent properties are a key feature of ionicliquids as solvents and have been utilised, especially incombination with the catalytic properties of the chloroa-luminate(III) ionic liquids, to develop a range of syntheti-cally important catalytic reactions, some of which arebeing investigated as economically and environmentallyviable alternatives to existing industrial processes. Room

temperature ionic liquids have developed, in less than 20years, from an adjunct to the US Star Wars research onbattery electrolytes into an industrial reality as media forcatalytic chemical processes.


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Fig. 5. The characteristic thermotropic (left) and lyotropic(right) textures displayed by [C14-mim]Cl

Fig. 6. Single crystal X-raystructure of [C14-mim][PF6]

Fig. 7. Packing for [C14-

mim][PF6] within the crystal,showing the typical layeredstructure with interdigitatedalkyl-chains


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Here, we will describe many of the catalytic processeswhich use low temperature ionic liquids as reactionmedia. In a number of indicated cases, these processeshave been taken through the development process to apoint of industrial commercialisation and represent first

generation ionic liquid processes, principally based onchloroaluminate(III) ionic liquids which are currentlyready for industrial uptake.

Following this line, second generation ionic liquidprocesses based on other, more benign, ionic liquids arecurrently under investigation and development in avariety of laboratories around the world. Many of theseprocesses utilise the ability of many ionic liquids to selec-tively immobilise transition metal catalysts for liquid-liquid two-phase catalysis while permitting easy, oftentrivial, extraction of products. Areas under active studyinclude alkylation reactions, Diels-Alder cyclisations,Heck coupling reactions, hydrogenation, hydroformyla-

tion, oligomerisation, dimerisation and polymerisation ofolefins, and Friedel-Crafts chemistry.

In the remainder of this paper, for the sake ofconciseness, the term ionic liquid should, in general, betaken to mean ambient, or close-to-ambient, tempera-ture ionic liquid containing organic cations in contrastto the high temperature simple molten salts, such asNaCl-AlCl3.

Many of the processes discussed (in this terminology,first generation) make use of the chloroaluminate(III)ionic liquids described above, in the acidic rgime (thatis, above 50 mol% AlCl3). Here, the ionic liquids containboth [AlCl4]

and [Al2Cl7] anions, and unsurprisingly

have been used as substitutes for conventional solid orsuspended sources of aluminium(III) chloride. However,ionic liquids, in ideal cases, have no waste associatedwith them, whereas supported aluminium(III) chloridecatalysts involve large (and annually rising) wastedisposal costs, and a very dirty process.

This review is conveniently separated into reactions ofalkenes and reactions of arenes with only a relativelysmall amount of overlap (for instance in the section onlinear alkylbenzene production, which is the reaction ofan alkene with an arene!).

Reactions of Alkenes

The Dimersol/Difasol processThe Dimersol process developed by the IFP (Commereucet al. 1982) is widely used industrially for the dimerisa-tion of alkenes, typically propene and butenes, to themore valuable branched hexenes and octenes, withtwenty-five plants in operation world-wide producing ca.3!106 tonnes per annum. The C8 olefins produced areusually hydroformylated to C9 alcohols for use in themanufacture of plasticisers. The dimerisation process iscommonly operated solvent-free with the active catalyst,a cationic nickel complex of the general form

[LNiCH2R][AlCl4] (where L is PR3). The catalyst hasbeen found to be soluble in aromatic and halogenatedhydrocarbon solvents, and shows greater catalytic activityin solution. Although this process is used widely, the

separation of products from the catalyst is a majorproblem and leads to increased operational costs andenvironmental impact. Chauvin and coworkers at the IFPin France (Chauvin et al. 1988, 1990a, 1990b, 1994, 1995a,1997, Einloft et al. 1996, Olivier et al. 1992) reasoned that

chloroaluminate ionic liquids would be good solvents forthe nickel catalyst, and discovered that by using a ternaryionic liquid system ([bmim]Cl-AlCl3-EtAlCl2) (bmimpI-butyl-3-methylimidazolium), it is possible to form theactive catalyst from a NiCl2L2 precursor and that mostimportantly, the ionic liquid solvent stabilises the activenickel species.

Using the ionic liquid catalyst, the Dimersol reactioncan be performed as a two-phase liquid-liquid process atatmospheric pressure at between 15 7C and 5 7C. Underthese conditions, alkenes are dimerised with activitieswell in excess of that found in both solvent-free andconventional solvent systems. The products of the reac-

tion are not soluble in the ionic liquid, and form asecond less-dense phase that can be easily separated. Thenickel catalyst remains selectively dissolved in the ionicliquid phase, which permits both simple extraction ofpure products and efficient recycling of the liquid catalystphase. In addition to the ease of product/catalyst separa-tion, the key benefits from using the ionic liquid solventare the increased activity of the catalyst (1250 kg ofpropene dimerised per 1 g of Ni catalyst), better selec-tivity to desirable dimers (rather than higher oligomers),and the efficient use of valuable catalysts through simplerecycling of the ionic liquid.

This process, using the ionic liquid solvent system,

has recently been commercialised by the IFP, as theDifasol process. In this process butene is dimerised in acontinuous two-phase procedure with high conversion ofolefin and high selectivity to the dimer. Most impor-tantly, the Difasol system can be retro-fitted into existingDimersol plants to give improved yields, lower catalystconsumption and associated cost and environmentalbenefits.

Oligomerisation of buteneIn addition to the nickel-catalysed Difasol processesdeveloped by the IFP, it has been know for some timethat acidic ionic liquids themselves act as catalysts for the

dimerisation and oligomerisation of olefins, all be it, in asomewhat uncontrolled manner (Boon et al. 1986).Developments in our laboratories, in collaboration

with BP Chemicals, have shown that a wide range ofacidic chloroaluminate(III) and alkylchloroaluminate(III)ionic liquids catalyse the dimerisation and oligomerisa-tion of olefins (Abdul-Sada et al. 1995a, 1995b, Ambler etal. 1996). In what is an exceptionally simple system, theolefinic feedstock may be mixed with, or simply bubbledthrough, the ionic liquid catalyst to produce oligomericproducts which have low solubility in the ionic liquidcatalyst and separate as a less dense organic phase whichis readily removed by tapping off. Alternatively, in a

batch process (more commonly used for laboratorytesting), the ionic liquid is injected into a charged auto-clave batch reactor (see Fig. 8): again product extractionamounts to simply tapping off the reactor.


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Fig. 8. An ionic liquid autoclave rig from our laboratories, usedfor testing batch reactions

In addition to simple oligomerisation reactions, chlo-roaluminate(III) and alkylchloroaluminate(III) ionicliquids are particularly good catalysts for the polymerisa-tion of olefins. For example, isobutene can be polymer-ised in an acidic ionic liquid to poly(isobutene) with ahigher molecular weight than is formed using other poly-merisation processes (Ambler et al. 1996). The catalyticactivity of the ionic liquids increases towards higherdegrees of polymerisation from short-chain oligomers asthe alkylchain length of the 1-alkyl-3-methylimidazoliumor N-alkylpyridinium cation is increased, which providesa very effective mechanism for controlling the product

distribution of this process from oligomers to polymers.Poly(isobutene), traditionally prepared by the Cosdenprocess, is a valuable lubricant, and also a route tohigher value-added materials.

The ionic liquid polymerisation process has a numberof significant advantages over the industrial Cosdenprocess, which uses a supported or liquid phase alumi-nium(III) chloride catalyst (Weissermel and Arpe 1997).Using the ionic liquid process, the polymer forms a sepa-rate layer, which is substantially free of catalyst and ionicliquid solvent. This is readily removed by tapping off,and the absence of chloroaluminate(III) contamination inthe polymer removes the need for a subsequent aqueous

washing stage to remove the catalyst. The fact that thepolymer is insoluble in the ionic liquid greatly enhancesthe degree of control available to reduce undesirablesecondary reactions (i.e. isomerisations) withoutrequiring alkali quenching of the reaction. A secondarybenefit, in common with the Difasol process, is that theseparation of the products from the ionic liquid reactionsystem and elimination of aqueous washing steps enablesthe reuse rather than destruction of the catalyst, whichfurther reduces costs and wastage from the process.

This ionic liquid reaction system can be easily retro-fitted to existing Cosden processes, providing a contin-uous catalytic process with the elimination of costly

aqueous washing steps and associated destruction of thecatalyst.

Ziegler-Natta PolymerisationZiegler-Natta polymerisation of ethylene to linear a-olefins currently has a world capacity in excess of1.6!106 tonnes per year. The most common productionmethods involve the use of triethylaluminium catalysts at

ca. 100 7C and 100 atmospheres pressure. Other moremodern processes can utilise organometallic transitionmetal catalysts, typically nickel- or titanium-based, forexample, using a mixed alkylchloroaluminium(III) andtitanium(III) chloride catalyst in an organic solvent toproduce exceptionally pure a-olefins. Ziegler-Natta poly-merisation of ethylene has been reported in an ionicliquid solvent (Carlin and Wilkes 1990). Using dichloro-bis(h5-cyclopentadienyl)titanium(IV) with an alkyl-chlo-roaluminium(III) co-catalyst in an acidic [emim]Cl-AlCl3ionic liquid solvent, ethylene polymerisation wasreported. In comparison, analogous zirconium andhafnium complexes failed to show catalytic activity.

Hydrodimerisation of dienesThe commercially important hydrodimerisation of 1,3-butadiene to octa-2,7-dien-1-ol (Dullius et al. 1998, Silvaet al. 1998) has been demonstrated using palladium-basedcatalysts in [bmim][BF4], a neutral, non-acidic ionicliquid. The ionic liquid, in this case acts purely as a reac-tion solvent. The catalyst precursor [Pd(mim)2Cl2] wasprepared in situ from an imidazolium tetrachloropallada-te(II) salt, [bmim]2[PdCl4], dissolved in the ionic liquidsolvent. The reaction proceeds in a liquid-liquid two-phase system, the products separate from the catalyticreaction mixture as a separate layer on cooling, and are

removed by decantation.The cyclo-dimerisation of dienes via Diels-Alder

mechanisms have been reported, and are covered in alater section (see below).

Alkylation of olefinsOlefin alkylation is important as a route to producebranched iso-alkenes. Of particular importance has beenthe production of 2,3-dimethylbutene, which could bethen converted to a methoxy-ether for use as a fuel addi-tive to increase the octane rating. This has been achievedusing ionic liquid solvents at the IFP with a modificationto the dimersol/difasol process (changing the bulkiness of

the phosphine ligands on the nickel catalyst to favourformation of the desirable 2,3-dimethylbutenes frompropene) to optimise for high octane number fuel addi-tives (Chauvin et al. 1995a).

One of the advantages of an ionic liquid catalyst forthis reaction over conventional liquid acid catalysts isthat the catalytic activity can be controlled by adjustingthe concentration of the active catalyst ([Al2Cl7]

). Usinghighly acidic ionic liquids, described for olefin oligomer-isations, it is possible to alkylate isobutane (Abdul-Sadaet al. 1995b, Ambler et al. 1996, Chauvin et al. 1989) atlow temperatures (30c50 7C) with C2-C4 olefins. Thisis a reaction that is not effective using normal liquid

acidic catalysts (HF and H2SO4).These two ionic liquid based routes to optimised 2,3-dimethylbutene production from respectively propyleneand ethylene are very good homogeneous processes to


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Fig. 9. Ionic liquids permithigh yields and selectivity(endo:exo ratios) for the Diels-Alder reaction

compete with heterogeneous isomerisation and alkylationroutes currently used.

Diels-Alder reactionsIonic liquids have been demonstrated as effective solvents

for Diels-Alder reactions (Jaeger and Tucker 1989, Earleet al., 1999, Fischer et al., 1999) and show significant rateenhancements, high yields and selectivities (see Fig. 9)comparable with the best results obtained in conven-tional solvents. To date, the biggest developments inDiels-Alder chemistry have come through reactions inLi[ClO4]-Et2O, where the high electrolyte concentrationsare cited as beneficial through salt-effects and the highinternal pressure of the solvent. It is clear from thestudies cited that extending this concept through to theuse of ionic liquid solvents will lead to further extensionsin the scope of these reactions and eliminate the need forpotentially explosive perchlorate-based reaction media.

Also, Chauvin (Chauvin and Olivier-Bourbigou 1995)reports that Fe(NO)2 catalysts for the dimerisation ofbutadiene to 4vinylcyclohexene are effective in weaklycoordinating [bmim][AF6] (ApP or Sb) ionic liquids.The effect of solvent on the reaction rates in movingfrom conventional molecular solvents such as tolueneand THF (Huchette et al. 1978) to the ionic liquid isexceptional.

Hydrogenation and hydroformylationAs seen earlier, olefins are very reactive in acidic ionicliquids and dimerisation, polymerisation or oligomerisa-tion reactions readily take place. However, acidic

[emim]Cl-AlCl3 mixtures are not the only ionic liquids ofinterest. A significant advantage of using ionic liquids asreaction media over traditional molecular solvents is thewide range and almost infinite tunability of solvent andcatalytic properties.

The activity and properties of the liquid can readily becontrolled by changing composition or by changing thenature of either the anion or the cations present. Inneutral ionic liquids containing for example [BF4]

,[PF6]

, [SbF6] and [CuCl2]

anions, the reactive poly-merisation and oligomerisation reactions of olefins cata-lysed by acidic anions are not observed and morecontrolled, specific, reactions can be catalysed. These

ionic liquids are often generically referred to as non-chlo-roaluminates, though clearly this description is derivedfrom an immature view of the breadth of ionic liquiddevelopment, since ionic liquids containing chloroalumi-nate anions comprise only one of many possible anion

types. More useful, is the consideration of neutral ionicliquids (in terms of Franklin acidity), in which the anionexists as only a single species, in contrast to the equili-brium seen in the tetrachloroaluminate(III) systems:

2[AlCl4]a [Al2Cl7]

cCl

Since these ionic liquids can not support the existenceof reactive Lewis acid conjugate anions (such as [Al2Cl7]

i.e. there is no analogous mechanism to support2[BF4]

a [B2F7]

cF), they are much less reactive and

can be used as innocent solvents. Under these condi-tions, many conventional transition metal catalysts can

be utilised. Modification of the ionic liquid solventsallows the potential to immobilise catalysts, stabilisingthe active species and to optimise reactant/product solu-bilities to permit facile extraction of products.

Hydrogenation of olefins catalysed by transition metalcomplexes dissolved in ionic liquid solvents have beenreported using rhodium- (Suarez et al. 1996), and ruthe-nium- and cobalt-containing catalysts (Suarez et al. 1997).Hydrogenation rates have been shown to be up to fivetimes higher than the comparable reactions in propa-none, or in analogous two-phase aqueous-organicsystems. The solubilities of the alkene reagents, TOFs,and product distributions are strongly influenced by the

nature of the anion in the ionic liquid solvent, which canbe used to improve the selectivity of the hydrogenation(Chauvin et al. 1995b). Pentene has been hydrogenatedusing the Osborne rhodium catalyst [Rh(nbd)(PPh3)2][PF6] (where nbdpnorbornadiene) in ionic liquidscontaining [BF4]

, [PF6], [SbF6]

and [CuCl2] anions.

There are significant solvent effects in this reaction; inthe best case (the [SbF6]

ionic liquid), hydrogenationrates were five times higher than the comparable reactionin propanone. In contrast, using a chloride-containingionic liquid as solvent resulted in only isomerisation ofpent-1-ene to pent-2-ene, presumably through chloridecoordination to the metal-centre deactivating the catalyst.


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Conjugated dienes are more soluble in the ionic liquidsthan simple olefins, which allows for selective hydrogena-tion. For example, cyclohexadiene is five times moresoluble than cyclohexene in [bmim][SbF6] ionic liquid,and is selectively hydrogenated (98% selectivity, 96%

conversion) to cyclohexene, which separates from theionic liquid. Cyclohexene has also been hydrogenatedusing Wilkinsons catalyst [RhCl(PPh3)3] or[Rh(cod)2][BF4] (codp1,4-cyclooctadiene) in ionic liquidscontaining [BF4]

and [PF6] anions.

Asymmetric hydrogenations of a-acetamidocinnamicacid (Chauvin et al. 1995b) to (S)-phenylalanine with acationic chiral rhodium catalyst in [bmim][SbF6] ionicliquid, and 2-arylacrylic acids (Monteiro et al. 1997) withchiral ruthenium catalysts in [bmim][BF4] ionic liquids,have been reported with a reasonable 64% ee. Palladiumcatalysts (Carlin and Fuller 1997) immobilised in an ionicliquid-polymer gel membrane (Fuller et al. 1997)

containing either [emim][OTf] or [emim][BF4] have alsobeen reported as catalysts for heterogeneous hydrogena-tion reactions.

Hydroformylation (Fuller et al. 1997) of pent-1-ene in[bmim][PF6] with rhodium catalysts shows high catalyticactivity; again, the products separate as a second organicphase. It was noted that a small part of the neutral cata-lyst leached into the organic phase. In general, charged,especially cationic transition metal complexes are mosteffectively immobilised in the ionic liquid solvents.

Hydrogenation reactions occur readily using ionicliquids as the catalyst-containing components of a two-phase systems. The key feature is that the transition

metal catalysts can be immobilised in the polar ionicliquid phase and are not preferentially extracted intoorganic solvents. The activity may be higher thanconventional homogeneous hydrogenations in, forexample, propanone or as two-phase aqueous-organicsystems where expensive, often synthetically challenging,modified ligands are often required.

In the communications described, it is not onlydemonstrated that hydrogenation reactions occur inneutral (or, more correctly, non-acidic) ionic liquids, butalso more generally that many catalytically-active transi-tion metal complexes can be immobilised in ionic liquidsolvents without the need for specially modified ligands.

The reaction rates and selectivities depend on the relativesolubilities of reactants and products in the ionic liquidphase. This is demonstrated by the selective and specifichydrogenation of cyclohexadiene to cyclohexene, andillustrates the potential to tailor reactions by modifyingthe properties of the ionic liquid solvent.

Miscellaneous reactions of alkenesAlkylammonium and phosphonium salts have been usedas solvents for the Heck coupling of aromatic halideswith activated olefins using palladium catalysts (Kauf-mann et al. 1996): the excess of PPh3 required in conven-tional solvents is not required to stabilise the palladium

catalyst in ionic liquid solvents. Although Heck couplingreactions are not utilised on an industrial scale, and ingeneral produce one equivalent of salt, they are widelyused in the pharmaceutical and fine chemical sectors. In

a recent example, the Heck coupling of benzoic acidanhydride with linear a-olefins has been demonstrated ina process which produces no salt by-products (Stephan etal. 1998) and presents an alternative route to the forma-tion of linear alkylbenzenes (LABs) described later. The

benefits of ionic liquid solvents (reactivity, productextraction, etc.) could provide further enhancements tothese series of reactions. Nucleophilic aromatic substitu-tion reactions have also been studied in molten dodecyl-tributylphosphonium salts (Fry and Pienta 1985).

Reactions of ArenesUsing the acidic, chloroaluminate(III) ionic liquids ascatalytic solvents, aromatic rings show a high reactivityto olefins and undergo a wide range of Lewis acid cata-lysed chemistry, including clean electrochemical andphotochemical polymerisation to form conducting polym-eric films, a commercially important alkylation of

benzene with olefins, and Friedel-Crafts alkylation andacylation reactions.

Formation of poly(p-phenylenes)Poly(p-phenylenes) have attracted a lot of attention ashighly stable conducting polymers for the development ofconductive polymeric films and electrolytes (Kovacic andJones 1987). Polymers can be obtained by polymerisationof benzene using many different chemical methods, butthe polymers produced are often of poor quality(contaminated with catalyst and chlorinated and oxygen-ated residues) and are usually obtained as powders withlow relative molecular mass. Electrochemical polymerisa-

tion of arenes (Abuabdoun 1989) in acidic chloroalumi-nate(III) ionic liquids with either N-alkylpyridinium or1,3dialkylimidazolium cations has been shown toproduce conducting polymers with superior purity,conductivity and greater molecular mass than can beobtained by chemical methods. In addition, electrochem-ical polymerisation allows the preparation of desirableconducting films. For example, benzene can be electropo-lymerised to condensed ring conducting polymers(Trivedi 1989), and more importantly, to linear poly(p-phenylene) (Arnautov 1997, Kobryanskii and Arnautov1992b, 1993b) with superior chain lengths.

Chemical oxidative polymerisation of benzene to

poly(p-phenylenes) (Kobryanskii and Arnautov 1992a,1993a, 1993c) using [N-butylpyridinium][AlCl 4] ionicliquid solvent has also been achieved, the relative molec-ular mass of the polymers being higher than in conven-tional solvents. This is attributed to the much highersolubility of the polymer in ionic liquid solvents, whichallows extended polymerisation.

Electrochemical studies of anthracene (Carlin et al.1992), methylanthracene (Lee et al. 1996), tetrathiaful-valene (Carter and Osteryoung 1994) and 9,10-anthraqui-none (Cheek and Osteryoung 1982, Cheek and Spencer1994), photoelectrochemical oxidation of aromatic hydro-carbons and decamethylferrocene (Thapar and Rajeshwar

1982), and photochemical oligomerisation of anthracene(Hondrogiannis et al. 1993) have all been reported inionic liquids. In addition, the synthesis of silane polymerfilms for electrodes (Carlin and Osteryoung 1994), and


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the electrochemical oxidation and polymerisation ofethylbenzene (Kobryanskii and Arnautov 1993b) havealso been reported.

Reduction of aromatic ringsLewis acid/redox chemistry of 1,2-diarylethanes(Buchanan et al. 1985), hydrogenation of polycyclicaromatics (Buchanan et al. 1981a, 1982), hydride extrac-tion from polycyclic aromatics (Buchanan et al. 1981b,Smith et al. 1980, Zingg et al. 1984), bond cleavage inphenylalkanes (Buchanan et al. 1983), chlorination vs. C-C coupling of anthracene radicals (Chapman et al. 1985a,1985b) and reduction of aromatic ketones (Cheek 1987,1990, 1991, 1992a, 1992b, Cheek and Herzog 1984a,1984b) and phenazine and perylene (Coffield et al. 1990,1991, 1992) have all been reported in the literature usinghigher temperature ionic liquid systems. No studies havebeen published concerning room-temperature ionicliquids, but work from our own laboratories (Adams etal. 1999) reports the facile reduction of arenes to cyclichydrocarbons with a variety of reducing agents: e.g.

Perylene is similarly reduced:

Both of these reduction sequences is stepwise, each

isomer representing a thermodynamic minimum.Electophilic substitution (Skrzynecki-Cooke andLander 1987) and other reactions of naphthalenes (alkyla-tion, acylation, condensation and migration) in acidic

ionic liquids (Ota 1987a, 1987b) (including a one-potsynthesis of anthraquinone from benzene in 94% yield)have been reported. Anthracene undergoes photochemical[4c4] cycloaddition reactions (Hondrogiannis et al. 1993,Pagni et al. 1994) in acidic chloroaluminate(III) ionic

liquids. A much wider range of redox products areformed than occur in conventional solvents; the strongBrnsted acidity of the ionic liquid induces protonationof anthracene, by residual traces of HCl, to form ananthracenium species which couples readily via photo-chemically- driven electron-transfer mechanisms.

Synthesis of linear alkylbenzenes (LABs)The alkylation of arenes with long-chain linear olefins isan example of Friedel-Crafts alkylation which has parti-cular global industrial importance for the synthesis oflinear alkylbenzenes (LABs) (Almeida et al. 1994), whichare used in the formulation of detergents. The industrial

scale of this reaction, coupled with the implications thatswitching to an ionic liquid process may entail, merits itsdiscussion as a particular example.

LAB was first introduced in the 1960s as a precursorto alkylbenzenesulfonates, which are widely used asdetergents, and also as emulsifiers, wetting agents, dry-cleaning additives, lubricants and greases. The globalmarket for LAB is in excess of 2.5!106 tonnes per year.

Although LABs are produced by alkylation of benzenewith chloroalkane feeds, the principal industrial processesfor the formation of LAB are alkylation of benzene withdo decene over liquid HF or AlCl3 catalysts. A fixed-bedheterogeneous non-corrosive acid catalyst system (Detal

from UOP) (Imai et al. 1995) has also recently beenintroduced. In the liquid acid catalyst processes, espe-cially with HF, the handling of corrosive catalysts leads toincreases in the capital cost of the plants and has impli-cations for the disposal of neutralisation products gener-ated within the process.

The production of LAB using chloroaluminate(III)ionic liquids as the acid catalyst has been recentlydescribed in the patent literature (Abdul-Sada et al.1995b, Ambler et al. 1996, Lacroix et al. 1998); this is aspecific example of the general activation and alkylationwith olefins in ionic liquids under acidic conditions. Theapplication of an acidic ionic liquid catalyst as a direct

replacement for solid AlCl3 promises significant improve-ment in the reaction selectivity combined with ease ofproduct separation and elimination of catalyst leaching.Again, as with the Difasol process, the potential toretrofit existing plants will lead to massively reducedcatalyst consumption and simplify the production processthrough the elimination of caustic quenching steps asso-ciated with catalyst leaching.

Friedel-Crafts chemistryThe Franklin acidic chloroaluminate(III) ionic liquids(containingX(AlCl3)10.5) are very aggressive solvents most organic materials are dissolved at high concentra-

tions. However, the acidic nature of the solvent causesready reaction of the solutes usually via hydride abstrac-tion reactions leading to oligomerisation or isomerisa-tion, and Friedel-Crafts alkylation or acylation reactions.


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Although several Friedel-Crafts reactions have beencovered in isolation in earlier sections (i.e. alkylation ofarenes), a generalisation of Friedel-Crafts reactions, cata-lysed in acidic ionic liquids, warrants further considera-tion.

For a general discussion of Friedel-Crafts chemistry,readers are recommended to look at the classic textbook (Olah 1964). Friedel-Crafts alkylations and acyla-tions are of great commercial importance, although thereare considerable problems, especially with misnamedcatalytic Friedel-Crafts acylation reactions, which areactually stoicheometric, consuming 1 mole of AlCl3 permole of reactant. The nett result is massive usage ofaluminium(III) chloride and associated problems withdisposal of salt and oxide by-products. Typically, Friedel-Crafts alkylation and acylation reactions are run in aninert solvent with suspended or dissolved aluminium(III)chloride as a catalyst, and may take six hours and go

only to 80% completion to give a mixture of isomericproducts.

Both alkylation and acylation reactions under Friedel-Crafts conditions have been demonstrated using chloroa-luminate(III) ionic liquids as both solvent and catalysts(Adams et al. 1998, Boon et al. 1987a, 1987b, 1986, Joneset al. 1985, Levisky et al. 1984, Ota 1987a, 1987b, Piersmaand Merchant 1990, Wilkes 1987). Reaction rates aremuch faster (often essentially instantaneous) with totalreagent conversion, and often with surprising specificityto a single product.

Alkylation

The alkylation of benzene with a wide number of alkylhalides in acidic chloroaluminate(III) ionic liquids (Boonet al. 1986) leads to rapid and largely uncontrolledpolyalkylation with evolution of hydrogen chloride. Forexample, the alkylation of benzene with chloromethane inan acidic ionic liquid gives a mixture of mono- to hexa-substituted products. The ionic liquid solvent/catalystactivates the reaction and the alkylation can beperformed even at temperatures as low as 20 7C in theionic liquid solvent. The products have a low solubility inthe ionic liquid and are easily separated.

General organic reactions in low melting chloroalumi-nate ionic liquids have been described (Levisky et al.

1983, 1984) including Friedel-Craft alkylations, acylations,chlorinations and nitrations in acidic ionic liquids (Boonet al. 1987a, 1987b, 1986, Piersma and Merchant 1990).

The alkylation of arenes using an olefin (Abdul-Sadaet al. 1995b, Ambler et al. 1996) rather that alkylhalidehas been described for the LAB process. In a typical,generalised procedure, the reaction is performed between80200 7C using 0.5% ionic liquid catalyst. For example,benzene is efficiently alkylated with ethylene to ethylben-zene, which can then be dehydrogenated as a source ofstyrene. This forms the basis of a competetive procedurefor alkylation of benzene without using ethyl chloride.

AcylationClassical Friedel-Crafts acylation reactions typically useAlCl3 catalysts for the acylation of arenes with acylchlo-rides. However, these are not truly catalytic reactions and

actually consume one molar equivalent of AlCl3 throughreaction with the acyl group (see Fig. 10). Friedel-Craftsacylation reactions are industrially important, despite thelack of a true catalytic process, and the associatedmassive consumption of aluminium(III) chloride. Many

acylation reactions have been demonstrated in acidicchloroaluminate(III) ionic liquids liquids (Adams et al.1998, Boon et al. 1986). As with the conventional proc-esses, difficulties remain from the reaction being non-catalytic in aluminium(III) chloride which necessitatesdestroying the ionic liquid catalyst by quenching withwater to extract the products. However, regioselectivityand reaction rates observed from acylation reactions inionic liquids are equal to the best published results.

Friedel-Crafts acylation of benzene is promoted byFranklin acidic chloroaluminate(III) ionic liquids (Boonet al. 1986): as is typical for acylation reactions, selec-tively monoacylated products are formed through deacti-

vation of the aromatic ring by the first acyl substituent.The acylated products of these reactions show high selec-tivities to a single isomer: for example toluene, chloro-benzene and anisole are acylated in the 4-position with98% specificity. Naphthalene is acylated in the 1-positionwhich is the thermodynamically unfavoured productunder conventional Friedel-Crafts acylation conditionscompared to the derivatisation at the 2-position, thenormal product (Adams et al. 1998).

This apparent reversal of selectivity in the ionic liquidsystem is because the acylating species is a fully ionisedlinear acylium ion ([RCO]c), which is small (see Fig. 11)compared to the (acyl chloride)-AlCl3 aducts responsible

for acylation in conventional systems. The acylium ion isstable in the ionic liquid and has been crystallised as asalt from a chloroaluminate(III) ionic liquid. However, sofar the greatest problems with Friedel-Crafts acylations,namely the non-catalytic reactivity with respect to AlCl3has not been adequately addressed. In addition tobenzene and other simple aromatic rings, a range oforganic and organometallic substrates (e.g. ferrocene)(Dyson et al. 1997, Surette et al. 1996) have been acylatedin acidic chloroaluminate(III) ionic liquids. Predomi-nantly mono-acylated products were prepared in goodyields.

The reactivity of chloroaluminate(III) ionic liquids is

such that even coals and other carbonaceous materialscan be dissolved and reacted. Coals can be acylated inchloroaluminate(III) ionic liquids under much milderconditions than can be used with just AlCl3 as catalyst(Newman et al. 1987, 1980, 1984). Acylation of coals isused as a primary step in liquifaction and desulfurisationprocessing.

Photogalvanic cellsHydrophobic ionic liquids containing triflate and bis{(tri-fluoromethyl)sulfonyl}amide (bis(triflyl)amide;[N{SO2CF3}2]

) anions have been used as inert,conducting solvents for solar cells (Bonhte et al. 1996,

Papageorgiou et al. 1996, Bonhte and Dias 1997).Conventional solar cell technology utilises either organicsolvents containing dissolved electrolytes or conductingsolid electrolytes. Both models present a range of prob-


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Fig. 10. The (Benzoyl chloride)-AlCl3aduct formed from the reaction betweenbenzoyl chloride and aluminium(III)chloride (Nieuenhuyzen et al., unpub-lished results)

Fig. 11. The linear [CH3CO]c ion and

tetrahedral [AlCl4] ion, as isolated in

the ionic [CH3CO][AlCl4] salt (LeCarpentier and Weiss 1972)

lems, including difficulty maintaining good interfacecontacts with solid electrolytes, the volatility of organic

solvents which necessitates using sealed cells, electrolytesolubility and problems of incompatability of organicsolvents with the epoxy resins used in the cell construc-tion. Conducting polymer electrolytes have been devel-

oped to overcome some of these problems; ionic liquidelectrolytes combine the advantages of liquid, solvent-

based electrolytes with those of polymer-electrolytes.By using ionic liquids as the electrolytes, large electro-chemical windows (14 V), good conductivity, andthermal stability, are achieved with excellent resistance to


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oxidation which is caused by uv-excitation of the TiO2semiconductor supports. The negligible vapour pressureof the ionic liquids, hydrophobicity and compatabilitywith the epoxy resins used permits simple cell construc-tion. Although difficulties with the viscosity of some of

the ionic liquid electrolytes is still a feature, solar cellshave been prepared which are especially good under lowlight intensities with sensitiser turnovers in excess of 50million.

Miscellaneous reactionsA number of investigations of chlorination reactions havebeen made in Lewis acidic ionic liquids. In 1,3-dialkylim-idazolium-chloroaluminate ionic liquids, the imidazoliumcation undergoes electrophilic substitution at the C(4)and C(5) positions to yield 1,3dialkyl-4,5-dichloroimida-zolium ionic liquids (Donahue et al. 1985, Lipsztajn andOsteryoung 1984). The dichlorinated ionic liquids do not

appear to undergo further chlorination and are excellentsolvents for chlorination reactions (Boon et al. 1987a,1987b, Lipsztajn and Osteryoung 1984, Lipsztajn et al.1986). The chlorination of anthracene in [SbF6]

ionicliquids (Chapman et al. 1985a) has been reported, andother conventional Lewis acid catalysts have been used inionic liquids solvents, for example in a mixed tetraalky-lammonium nitrate/CuCl2 mixture, the difficult synthesisof a-chloroketones (Atlamsani and Bregeault 1991) hasbeen achieved.

Tetraalkylammonium and 1,3-dialkylimidazoliumbromides have been used as highly regioselective solventsfor O-alkylation of b-naphthoxide (Badri and Brunet

1992). Recent work from our own laboratories hasextended this to efficient generalised N- and O-alkylation(Earle et al. 1998) and has made the observation that theionic liquids mimic the behaviour of dipolar aproticsolvents. Consequently, they can be used as clean replace-ment solvents for DMF and DMSO in a wide range ofreactions. Importantly, the ionic liquids are non-volatile,easy to dry and can be quantitatively recovered at theend of the reaction; the products were easily extractedfrom the ionic liquid into an organic ether phase.

SummaryThese studies have shown that classical transition-metal-

catalysed hydrogenation, hydroformylation, isomerisa-tion, dimerisation and coupling reactions can beperformed in ionic liquid solvents. Using conventionalsolvents, selectivities, TOF and reaction rates are effec-tively uncontrolled: however by using ionic liquid mediafor the catalysis, it is possible to have a profoundinfluence on all these factors. A combination of subtle(i.e. changing cation substitution patterns) and gross(anion type) modifications to the ionic liquid solvent canpermit very precise tuning of reactions.

Ionic liquids have been used as effective solvents andcatalysts for clean chemical reactions; as replacements forvolatile organic and dipolar aprotic solvents (i.e. DMF,

DMSO) and solid acid catalysts in reactions ranging fromthe laboratory to industrial scale. They provide a mediumfor clean reactions with minimal waste and efficientproduct extraction, an area which is currently being

investigated (Huddleston et al. 1998, Blanchard et al.1999). The future directions for catalytic reactions inionic liquid solvents clearly rely on screening existingcatalysts and a greater depth of understanding of thefeatures influencing the specific solvent properties of the

different types of ionic liquids. It is anticipated thatcombinatorial techniques will have a significant role toplay.

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