Ionic liquids and catalysis: Recent progress from knowledge to applications

56 Pages·2.604 MB·English

by  Olivier-Bourbigou H., Magna L., Morvan D.

AppliedCatalysisA:General373(2010)1–56
ContentslistsavailableatScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Review
Ionic liquids and catalysis: Recent progress from knowledge to applications
H. Olivier-Bourbigou*, L. Magna1, D. Morvan2
IFPLYON,De´partementCatalyseMole´culaire,Rond-pointdel’e´changeurdeSolaize,BP3,69360Solaize,France
AR TI CLE I NFO ABS TRA CT
Articlehistory: Thisreviewgivesasurveyonthelatestmostrepresentativedevelopmentsandprogressconcerningionic
Received20May2009 liquids,fromtheirfundamentalpropertiestotheirapplicationsincatalyticprocesses.Italsohighlights
Receivedinrevisedform11September2009 theiremerginguseforbiomasstreatmentandtransformation.
Accepted6October2009
(cid:2)2009ElsevierB.V.Allrightsreserved.
Availableonline12October2009
Keywords:
Ionicliquids
Biphasiccatalysis
SupportedIonicLiquidCatalysis(SILC)
Taskspecificionicliquids(TSIL)
Proticionicliquids(PILs)
Thermoregulatedionicliquids
Biomass
Lignocellulose
Cellulose
Contents
1. Generalintroduction................................................................................................ 3
2. Ionicliquids:properties,evolutionandnextgenerations................................................................... 3
2.1. Propertiesofionicliquids....................................................................................... 3
2.2. Awideningrangeofionicliquidsavailable......................................................................... 5
2.2.1. Generalremarks ...................................................................................... 5
2.2.2. Proticionicliquids(PILs)................................................................................ 7
2.2.3. (Multi)-functionalionicliquids........................................................................... 8
2.2.4. ChiralILs ........................................................................................... 10
2.2.5. Switchable-polaritysolvents(SPS)....................................................................... 11
2.2.6. ILsatthefrontierbetweenorganicandinorganicmaterials................................................... 11
Abbreviations: IL(s),ionicliquid(s);[BMI]+,1-butyl-3-methylimidazolium;[BMMI]+,1-butyl-2,3-dimethylimidazolium;[MMI]+,1-methyl-3-methylimidazolium;[HMI]+,1-
hexyl-3-methylimidazolium; [OMI]+, 1-octyl-3-methylimidazolium; [AMI]+, 1-allyl-3-methylimidazolium; [AEI]+, 1-allyl-3-ethylimidazolium; [MI]+, 1-methyl-3-H-
imidazolium; [BMPy]+, N-butyl-3-methylpyridinium; [BPy]+, N-butylpyridinium; [PrMI]+, 1-propyl-3-methylimidazolium; [BMP]+, N-butyl-N-methylpyrrolydinium;
[PMP]+,N-propyl-N-methylpyrrolidonium;[NTf2](cid:2),bis(trifluoromethylsulfonyl)amide(CF3SO2)2N(cid:2);[OTf](cid:2),trifluoromethylsulfonateCF3SO3(cid:2);[OMs](cid:2),mesylateCH3SO3(cid:2);
[Fm](cid:2),formateHCOO(cid:2);[Ac](cid:2),acetateCH3COO(cid:2);TPPTS,triphenylphosphinetrisulfonatesodiumsalt;TPP,triphenyphosphine;PEG,poly(ethyleneglycol);LAB,linearalkyl
benzene;COD,1,3-cyclooctadiene;SWNT,singlewallcarbonnanotube;TSIL,taskspecificionicliquid;SILP,supportedionicliquidphasecatalysis;PSIL,polystyrene
supportedionicliquids;SPS,switchablepolaritysolvent;PTC,phasetransfercatalysis;LSER,linearsolvationenergyrelationship;MD,moleculardynamics;DFT,density
functionaltheory;QSAR,quantitativestructure–analysisrelationship;HDS,hydrodesulphurationprocess;MAO,methylaluminoxane;CIL,chiralionicliquid;PIL,proticionic
liquid;DMSO,dimethylsulfoxide;DMAc,dimethylacetamide;PPN,bis(triphenylphosphorylidine)ammoniumcation;DBU,1,8-diazabicyclo-[5.4.0]-undec-7-ene;DABCO,
1,4-diazabicyclo[2.2.2]octane;REACH,registration,evaluationandauthorisationofchemicals;EINECS,europeaninventoryofexistingcommercialchemicalsubstances;
EXAFS,extendedX-rayabsorptionfinestructure;ESI-MS,electrosprayionization-massspectrometric;NOESY,nuclearoverhauserenhancementspectroscopy;HOESY,
heteronuclearversionoftheNOESYexperiment;ROESY,rotatingframeoverhausereffectspectroscopy.
* Correspondingauthor.Tel.:[phone];fax:[phone].
E-mailaddresses:[email protected](H.Olivier-Bourbigou),
[email protected](L.Magna),[email protected](D.Morvan).
1 Tel.:[phone];fax:[phone].
2 Tel.:[phone];fax:[phone].
0926-860X/$–seefrontmatter(cid:2)2009ElsevierB.V.Allrightsreserved.
doi:10.1016/j.apcata.2009.10.008
2 H.Olivier-Bourbigouetal./AppliedCatalysisA:General373(2010)1–56
2.3. LatestadvancesinthepreparationandpurificationofILs............................................................ 12
2.3.1. ThedifferentwaysofILspreparations.................................................................... 12
2.3.2. PurificationofILsandanalysisoftraceimpurities .......................................................... 13
3. Structureandself-organisationofILsatthesupramolecularlevel........................................................... 13
3.1. Solventpropertiesandsolventeffect............................................................................. 13
3.2. Structureandorganisation..................................................................................... 15
3.3. Towardamesoscopicorganisation .............................................................................. 16
3.4. Solute-ILsinteractions:whatimpactonorganicreactions?........................................................... 17
3.4.1. Interactionwithwater ................................................................................ 18
3.4.2. Interactionwitharomatichydrocarbon.Clathratebehaviour.................................................. 18
3.4.3. Interactionwithchiralsubstrates:inductionofchirality? .................................................... 19
3.4.4. Interactionwithacidandbase:towardnewscaleofacido-basicity ............................................ 19
3.5. Molecularmodelling.......................................................................................... 19
4. HowtheILscanaffectthecatalyticreactionspathway?................................................................... 20
4.1. Some‘‘unexpected’’effectsofILs................................................................................ 20
4.1.1. EffectofILsimpurities ................................................................................ 20
4.1.2. Effectofwaterandacidicprotons ....................................................................... 20
4.1.3. Effectofbases ....................................................................................... 21
4.1.4. ILsasadditives:surprisingeffect! ....................................................................... 21
4.2. Whenionicliquidsareinvolvedintheformationofmetalcomplexes.................................................. 22
4.2.1. Complexformationinvolvinganions..................................................................... 22
4.2.2. Complexformationinvolvingcations..................................................................... 23
4.3. ILsspeciallydesignedforcatalysis............................................................................... 25
4.3.1. Changeinmechanismpathwaybystabilisationofchargedtransitionstate,activespeciesorligands ................. 25
4.3.2. Solventfornon-chargedcatalysts........................................................................ 26
4.3.3. Solvent/stabiliserfornanoparticles....................................................................... 27
4.4. Ionicliquidsasmediumfor‘‘insitu’’spectroscopicinvestigations ..................................................... 28
4.5. Removingsulfurfromrefinerystreams........................................................................... 28
5. ConceptsforusingILsinhomogeneouscatalysis......................................................................... 28
5.1. MultiphasicILsystems........................................................................................ 29
5.1.1. Somechallengesandopportunitiesofmultiphasicsystems................................................... 29
5.1.2. UseofscCO asthetransportvectorforsubstratesandproducts .............................................. 29
2
5.1.3. Demonstrationofcontinuouscatlyticperformances......................................................... 30
5.2. Supportedionicliquidphasesystem(SILP)........................................................................ 31
5.2.1. ILssupportedonsolidinorganicsolid .................................................................... 31
5.2.2. ILssupportedonhybridorganic–inorganicmaterial......................................................... 32
5.2.3. ILssupportedonorganicpolymers....................................................................... 32
5.3. Switchablepolaritysolvents.................................................................................... 32
5.4. ThermoregulatedILs.......................................................................................... 34
5.5. Phasetransfercatalysis ....................................................................................... 34
6. Overviewofindustrialapplicationsandeconomicissues.................................................................. 35
6.1. Selectedexamplesofindustrial/pilotscaleapplicationsofILs......................................................... 36
6.1.1. Dimerizationandoligomerisationofolefins:ILassolventandNi-co-catalyst..................................... 36
6.1.2. Friedel-Craftsalkylationandacylationofaromatichydrocarbons:ILassolventandcatalyst......................... 37
6.1.3. Alkylationofolefinswithisobutane:ILassolventandacidcatalyst............................................ 37
6.1.4. Chlorinationandfluorinationreactions................................................................... 38
6.1.5. Ethercleavage....................................................................................... 39
6.1.6. Acidscavenging...................................................................................... 39
6.1.7. Hydrosilylation:ILassolventandnanoparticlestabiliser..................................................... 40
6.1.8. Isomerisation:ILasasolvent........................................................................... 40
6.1.9. Methanolcarbonylation ............................................................................... 40
6.1.10. Otherexamples...................................................................................... 41
6.2. Mainprocessengineeringchallengesandissues.................................................................... 41
6.2.1. ILstability,lifetimeandrecyclability..................................................................... 41
6.2.2. Safetyandenvironmentalissues:........................................................................ 42
7. ILsapplicationinthebiomasstransformationintofuelandchemicals ....................................................... 42
7.1. Processingoflignocellulosicandcellulosicmaterials................................................................ 43
7.1.1. Directsolventfordissolutionofcelluloseandsugars........................................................ 43
7.1.2. Treatmentoflignocellulosicmaterials.................................................................... 46
7.2. ApplicationsoftheuseofILsinthedissolutionofligno-cellulosicmaterials............................................. 48
7.2.1. Animprovementintheanalysisoflignocellulosicmaterial................................................... 48
7.2.2. Transformationofpoly-saccharidesinsugarsusingILs ...................................................... 48
7.2.3. Catalytictransformationofsugars....................................................................... 49
7.3. Transformationofvegetableoils................................................................................ 50
7.3.1. Transesterificationoftriglycerides:biodieselproduction ..................................................... 50
7.3.2. Methyloleatemetathesis............................................................................... 51
8. Generalconclusionandperspectives .................................................................................. 51
Acknowledgements................................................................................................ 51
References ....................................................................................................... 51
H.Olivier-Bourbigouetal./AppliedCatalysisA:General373(2010)1–56 3
1. Generalintroduction TheperformanceofanILwillstronglydependonthetechnologyin
which it is implemented. They can be utilised in very different
Ionic Liquids (ILs) have attracted rising interest in the last ways:homogeneous,multiphase,heterogeneous,inbiotransfor-
decadeswithadiversifiedrangeofapplications(Fig.1).Thetypes mationsorinorgano-catalysis.Theyplayaspecificroleinallthese
ofionicliquidavailablehavealsobeenextendedtoincludenew approaches.
families and generations of ionic liquids with more specific and Evenmorethandiversity,anotherkeywordfortheend-useris
targetedproperties.Thisexpandinginteresthasledtoanumberof prediction.Whenwillitbepossibletomoveaheadtorationaldesign
reviews on their physico-chemicalproperties,the design of new ofionic liquids?Isit possibletopredict whichioniccombination
families of ionic liquids, the chemical engineering and the wide results in a given set of properties? Most work towards under-
range of arrangements in which ILs have been utilised (liquid standing and knowledge has been achieved on imidazolium
phase, multiphase, immobilized on supports, ...) and pilot or cations, certainly the most popular cation but not the only one.
industrialdevelopments[1]. NewfamiliesofILswithvariousothercationshavebeendeveloped
theselastdecades.ILsarenottrivial.Theyaregenerallycomposed
WhyILshaveattractedsomuchattentioninthelastfewdecades? of asymmetric and flexible ions, with components of highly
differentsizesandshapes,andinvolvedifferenttypesofdominant
Inadditiontothefactthattheyarenowcommerciallyavailable, interactions. Theoretical treatment and interpretations are com-
there is a better understanding of the effect of ionic liquids plicated.However,itisimportanttohaveabetterunderstandingof
(chemical and physical properties as well as engineering fluids). neat IL’s properties, and their properties and interactions with
Consequently, ionic liquids have been used more widely and other species such as molecular species or metal complexes to
efficiently, with better control over the overall process. The betterunderstandtheirroleincatalysis.
introductionofstructuralfunctionalitiesonthecationicoranionic Theaimofthisreviewisnottoprovideanexhaustivelist(orstate
part has made it possible to design new ILs with targeted oftheart)ofthewiderangeofcatalyticreactionsoccurringinILs.
properties [2]. More recently, ILs appear to be the subject of Several good, recent reviews have already illustrated that point
fundamentalpublicationsaimedatimprovingtheunderstanding (Table1).Thisreviewfocusesmainlyonrecentlypublishedmaterial.
ofthesesolvents,predictingtheirphysico-chemicalpropertiesand Wehaverestrictedourselvestogiveasurveyonthelatest,most
publicationsdescribingtheir use inincreasingly diverseapplica- representative developments and progress on ionic liquids and
tions such as sensors, fuel cells, batteries, capacitors, thermal catalysis.ThisreviewalsocoversthedifferentaspectsofILs,fromthe
fluids,plasticizers,lubricants,ionogels,extractantsandsolventsin knowledgewehaveofthesemediatotheuseoftheirpropertiesfor
analysis, synthesis, catalysis and separation, to name just a few. catalysis,catalyticprocessesandengineering.Moreparticularly,the
Somenewapplications,suchasenergeticcompoundsorpharma- followingarereported:(i)thedesignofnewgenerationsofILs:the
ceuticalILs,arestillemerging.ILscanbeusedasmorethanjusta evolutionsandkeyevents(ageneralhistoryoftheILsisdescribedby
alternative‘‘green’’solvents.Theydifferfrommolecularsolvents J. Wilkes [3]). (ii) fundamental properties of ILs: structure and
bytheiruniqueioniccharacterandtheir‘‘structureandorganisa- organisation,IL-soluteinteractions,(iii)theIL’seffect:howILscan
tion’’ which can lead to specific effects. They are tuneable, impacttheoutcomeofthereactionandhowitispossibletocontrol
multipurposematerials. thereactionprocess,(iv)thediversityofILuseincatalyticprocesses:
WhenreadingpapersonILs,oneofthekeywordsisdiversity. homogeneous, multiphase, heterogeneous, (v) comments on
Diversity of anion–cation combinations, diversity of modes of industrial applications and commercial aspects of ILs: barriers to
preparation, modes of purification and nature of impurities overcome? (vi) key events in environmental catalysis: this last
(quality),diversityofproperties,diversityofmodeofuse,diversity chapterfocusesontherolethatILscanplayinthetreatmentand
ofapplications.Thisisoneofthereasonswhyitissodifficultto transformationofbio-resourcesandinbio-processes.
makegeneralisationsabouttheirphysicalpropertiesortheiruse.
ThecontributionILsmaketohomogeneouscatalysishasmoreto 2. Ionicliquids:properties,evolutionandnextgenerations
do with the enhancement of catalytic performances (activity,
selectivity or new chemistry) and the possibility of catalyst 2.1. Propertiesofionicliquids
separation and recycling by immobilizationin the IL-phase than
with environmental concerns. They can act as solvents, as Consideringthebroadrange ofILs andapplications[46],itis
multifunctional compounds like solvents and ligands, solvents difficult to generalise their properties and to report general
andcatalysts,stabilisingagentsforthecatalystsorintermediates. tendencies. Sometimes the authors emphasise their differences
Fig.1.EvolutionofILgenerations.
4 H.Olivier-Bourbigouetal./AppliedCatalysisA:General373(2010)1–56
Table1 described in open literature and can be easily found in a good
GeneralreviewsonILs(from2003to2008). database (e.g. ‘‘ILThermo—managed by the US National Institute of
Year Ref. Standards and Technologies)’’ [48]. They will not be reported in
detailinthisreview—onlyalistofcriticalremarksisgivenbelow.
General(catalysis)
Task-specificILs 2004 [4]
ILsincatalysis 2004 [5] (cid:3) Melting point: Data must be considered with caution as the
CatalysisinILs 2006 [6] meltingpointofmanyILsmaybeuncertainastheycanundergo
HomogeneouscatalysisinILs 2007 [7] supercoolingandbecauseofthepotentialpresenceofimpurities.
CatalysisinILs 2007 [8]
(cid:3) Volatility:FortypicalILs,normalboilingtemperatures(T ),which
Transitionmetal-catalysedreactionsin 2007 [9] b
non-conventionalmedia correlatewiththeirvapourpressureat1atmosphere,cannotbe
ThepathaheadforILs 2007 [10] experimentally determined as ILs decompose at a lower
ApplicationsofILsinthechemicalindustry 2008 [11] temperature. It has nevertheless been reported that ILs can be
CatalystswithionictagandtheiruseinILs 2008 [12]
distilledat200–3008Cbutundersignificantlyreducedpressure
Specificreaction/topic(catalysis) andatverylowdistillationrate(<0.01gh(cid:2)1)[47].Thequestion
PolymerizationprocessesinILs 2004 [13] is how ionic are ILs? The ionic nature (or ionicity) of ILs can
Supportedionicliquidphase(SILP)catalysis 2006 [14]
partiallyexplaintheirnegligeablevapourpressureintheliquid
OxidationsoforganiccompoundsinILs 2006 [15]
FunctionalisedimidazoliumsaltsforTSILs 2006 [16] state, which distinguishes them from molecular solvents.
andtheirapplications Quantitative descriptions of the ionicity would be a useful
OlefinhydroformylationinILs 2007 [17] indicator for characterizing ILs. This has tentatively been done
BrønstedacidsinILs 2007 [18] usingtheeffectiveconcentrationofions[49].
EnantioselectivecatalysisinILs 2007 [19]
(cid:3) Nonflammability:MuchoftheinterestforILshasbeencentredon
AsymmetricsynthesisinILs 2007 [20]
LanthanidesandactinidesinILs 2007 [21] their possible use as ‘‘green’’ alternatives to volatile organic
ILsinseparations 2007 [22] solvents,mainlybecauseILsareconsideredasnon-volatileand
OlefinmetathesisinILs 2008 [23] consequently non-flammable at ambient and higher tempera-
Palladium-catalysedreactionsinILs 2008 [24]
tures. However there are many other potential solvents that
ILstowardssupercriticalfluidapplications 2007 [25]
ILsinheterocyclicsynthesis 2008 [26] meet these criteria but that have not been subject to such
ApplicationsofchiralILs 2008 [27] interest. It is worth mentioning that it is not because they are
SynthesisandapplicationofchiralILs 2008 [28] non-flammablethatILscanbeusednearaheatsource.ILsare
ElectrochemicalreactionsinILs 2008 [29] combustible.Theyevencanbefine-tunedforenergeticcontent
Bio-catalysis/biomass andreplacehydrazineanditsderivatives[50,51].
BiocatalysisinILs—advantagesbeyondgreentechnology 2003 [30] (cid:3) Thermalandchemicalstability:Theonsetofthermaldecomposi-
BiocatalytictransformationsinILs 2003 [31]
tion calculated from fast thermogravimetric analysis (TGA)
ILs:Greensolventsfornonaqueousbiocatalysis 2005 [32]
indicateshighthermalstabilityformanyILs,generally>3508C.
ChemicalandbiochemicaltransformationsinILs 2005 [33]
Biocatalysisinnon-conventionalmedia(ILs,scFluids.) 2007 [34] However,lowervaluesarefoundforlong-termstabilitywhichis
BiocatalysisinILs 2007 [35] importanttoconsiderwhenILsareusedincatalyticprocesses.
Ionicgreensolventsfromrenewableresources 2007 [36] PhosphoniumILs with[NTf ](cid:2)or [N(CN) ](cid:2)anionsdecompose
2 2
OxidoreductasebehaviourinILs 2008 [37]
completelytovolatileproductsinasinglestep.Thedegradation
BiotransformationsandorganocatalysiswithILs 2008 [38]
DissolutionandfunctionalmodificationofcelluloseinILs 2008 [39] products indicate that Hofmann elimination process and/or
dealkylation reactions occur. On the contrary, ILs based on
Synthesis(inorganic&organic)
nitrogencationsdonotdecomposecompletelyandgeneratechar
Metal-containingILsandILscrystalsbasedon 2005 [40]
imidazoliummoiety residue(cyanogroupsarepronetopolymerization)[52].
ILssolventpropertiesandorganicreactivity 2005 [41] (cid:3) Conductivity and electrochemical window: ILs conductivity is an
ApplicationofzeolitesinsupercriticalfluidsandILs 2007 [42] interestingpropertytoconsiderasILscanplaytheroleofboth
Thephosphorusaspectsofgreenchemistry 2007 [43]
solventsandelectrolytesinelectrochemicalreactions.ILsexhibit
ApplicationofILsinpolymerscience 2009 [44]
broadrangeofconductivitiesspanningfrom0.1to20mScm(cid:2)1.In
Analysis generalhigherconductivitiesarefoundforimidazolium-basedILs
ILsinchromatographicandelectromigrationtechniques 2008 [45]
incomparisonwiththeammoniumones.Manyfactorscanaffect
their conductivity, such as viscosity, density, ion size, anionic
andnottheirsimilarities.Someofthepropertiesdescribedsome chargedelocalization,aggregationsandionicmotions[29].Strong
years ago are now subject to controversy: e.g. electrochemical ion-pair associations have been invoked in the case of [NTf2](cid:2)
window; long-term thermal stability (thermal stability was basedILs,tounderstandtheirlowerconductivityincomparison
certainly overestimated in the past); polarity; volatility (some with [BF4](cid:2) based ILs [53]. Concerning their electrochemical
ILsaredistillableundercertainconditions[47]).Whyallofthese window,itistypicallyfoundintherange4.5–5V,whichissimilar
conflictingresults?Becauseanevolutiontowarda betterunder- to or slightly larger than that found in conventional organic
standing of these media, better characterization with improved solvents,butlargerthanthatofaqueouselectrolytes.Quaternary
knowledge and quantificationof theirimpurities(ionchromato- ammonium is generally more stable toward reduction than
graphy, ICP-MS) which are well-known to affect the thermo- imidazoliumwhichcan lead tothe formationof N-heterocyclic
physicalpropertiesofILs,havebeenachievedinrecentyears.The carbenes. The challenge here is still to design ILs with wide
differentexperimentaltechniquesusedandtheestimationofdata electrochemicalwindowalongwithgoodelectricalconductivity.
uncertaintymayalsohaveinfluencedthediscrepanciesintermsof (cid:3) Density:AconsiderableamountofdataonthedensityofILsare
physico-chemical properties. However, ILs have widely accepted available in the literature [54]. ILs are generally denser than
generic properties. They consist entirely of ions (Scheme 1). For either organic solvents or water, with typical density values
example,in[BMI][PF ]whichmeltsat128C,theionicconcentra- rangingfrom1to1.6gcm(cid:2)3.Thedensityofionicliquidsversus
6
tion is 4.8mol/L. The melting point of ILs should be less than pressureandtemperaturehasalsobeenmodelled[55].
1008C, even if this is an arbitrary temperature limit, and their (cid:3) Viscosity: From the engineering aspect, the viscosity of ILs can
ionicity should be >99%. All these generic properties have been affecttransportpropertiessuchasdiffusionandmaybeanissue
H.Olivier-Bourbigouetal./AppliedCatalysisA:General373(2010)1–56 5
Scheme1.Maincationsandanionsdescribedinliterature.
inpracticalcatalyticapplications.Itplaysamajorroleinstirring, ‘‘chemical’’properties.ProticILsandBrønstedand/orLewisILs
mixing and pumping operations. The viscosity of many ILs is canbeusedasacidcatalystsandsolvents.BasicILshavealso
relativelyhighcomparedtoconventionalsolvents,onetothree recently been reported as playing a dual role of solvent and
orders of magnitude higher. For a variety of ILs it has been base-catalyst with a particular interest and potential for
reportedtorangefrom66to1110cPat20–258C.Thedesignof celluloseacetylation[63].ILsbearingafunction(phosphorous,
lessviscousILsisstillachallengeformanyapplications[56]. nitrile,imine,amine,alkyne)havebeenappliedasbothligands
(cid:3) Polarity:Thepolarityisoneofthemostimportantpropertiesfor andsupportsforimmobilizingandrecyclingtransitionmetal
characterizingthesolventeffectinchemicalreactions[57].Itis homogeneous catalysts [64] or as protective agents and
also the property which has probably been the most widely solvents for the stabilisation of metal nanoparticles. ILs
discussed in the case of ILs. Why? Because there is no single supportedorgano-catalysts(suchasprolineasachiralcatalyst
parameter and direct measurement that can characterize IL inasymmetricsynthesis)havebeendevelopedtoimprovethe
polarity.Solvatochromicdyescanbeusedtodetermineempirical recovery of the catalyst which is often used in substantial
polarityparametersbuttheseparameters(Kamlet-Taftequation) quantities[65].ChiralILs,suchassolventsandchiralinducting
areprobablynottrulyindependentontheprobemoleculeused. agents,havebeenmodifiedinvariousways,thechiralitybeing
ThedifficultyinthecaseofILsistofindasuitablesolubleprobe incorporatedonthecationicoranionicpartoftheILs.
which measures the polarity parameters as independently as (2) Thetuneabiliyofcombinationsofcationsandanionsandthe
possibleoftheotherinfluencesofthesolvent[58,59]. possibility to achieve modification of the cation and/or the
(cid:3) Toxicityandbiodegradability[50,60]:Theearlyclaimsofthelow anion part offer access to ILs with targeted properties. For
toxicity and biodegradability of ILs has often been reduced to examples, the hydrophilicity/hydrophobicity flexibility, the
theirnegligiblevapourpressurewhich,ofcourse,isnotrealistic. decreaseofILsviscosity,andtheincreaseofILsstabilityarestill
Ithasbeenconfirmedthatcommonlyusedionicliquidsarenot challenging targets. [NTf ](cid:2) and [N(CN) ](cid:2) anions already
2 2
easily biodegradable. But should this be a major limitation to appearasgoodcandidatestogetILswithlowerviscosity.The
theiruseonindustrialcases? replacement of alkyl group on the imidazolium by more
(cid:3) Surfacetension:Ithasbeenthetopicofarelativelyminornumber flexibleethergroupwasalsoawaytodecreasebothviscosity
of studies. ILs have relatively moderate surface tensions andmeltingpoint(Scheme2).Thereplacementofalkylgroups
comparedtoorganicsolvents[61]. by oligoether groups has been shown to decrease the ionic
liquid’s viscosity significantly. This effect has been demon-
Forindustrialimplementation,someILpropertiesmustbe investi- stratedbothforsubstituentsattheanion(suchassulfate)[66]
gatedunderrealprocessconditions.Ascreeningofsomeproperties
such as compressibility has been examined under long-term
conditionsandunderhighpressure[62].Howcantheybecompared
toconventionalsolvents?Fig.2givesatentativequalitativedescription
of ILs compared to alternative solvents, in terms of polarity and
volatility.
2.2. Awideningrangeofionicliquidsavailable
2.2.1. Generalremarks
ThenumberofILshasexpandedexponentiallyrecentyears.A
compilationofallthedescribedcationsandanionsisnotpossible.
The main reviews can provide an overview. Many diverse
motivations can explain the design of new IL families. Some of
themaredescribedinthefollowingpoints.
(1) Agreatdealofattentionhasbeendevotedto(multi)functional
ILs (often termed task-specific ILs) aimed at using synergic Fig.2.Typicalpolarityandvolatilitycharacteristicsofalternativesolvents.
6 H.Olivier-Bourbigouetal./AppliedCatalysisA:General373(2010)1–56
Scheme2.ILswithtargetedproperties(decreaseviscosityanddensity).
Scheme3.NewmethimazolebasedILs.
andforsubstituentsatthecation(suchasPEG-functionalised is replaced with ILs based on dicyanamide anions, have been
imidazolium dialkylphosphates) [67]. Novel ILs with Si proposed.ItisexpectedthattheseILscanbefine-tunedforbetter
substitutedcationswerealsoreportedandpresentareduction energy content and physical properties [51]. ILs such as
ofviscositythankstoamoreflexiblesidechainthananether dialkylimidazolium formate were produced as liquids having
[68]. This may be important since mass transfer may be strong hydrogen bond accessibility. They are good solvents for
important, reaction rate can be increased by reducing the polysaccharidesdissolution[77] undermildconditionsand high
viscosityoftheILs[69]. concentrations(Section7.1.1).
TherehasbeenanincreasedinterestforILsthatpresentbetter (3) Costandbiodegradabilityhavealsobeenamainconcernand
inertness under reaction conditions. The hydrolysis of [PF ](cid:2) or newfamiliesofILsderivedfromrenewablefeedstockorfrom
6
[BF ](cid:2) anions to generate HF in situ has been the object of ‘‘lowcost’’startingmaterialshavebeendescribed(Scheme5).
4
numerousreports.Reactionscatalysedbyproticacidshaveoften These‘‘Bio-ILs’’areentirelycomposedofbiomaterials[78].An
beendescribedin[PF ](cid:2)basedILs,probablythankstothepresence example is given by the development of the ‘‘deep eutectic
6
of HF. The formation of transition metal fluoride under certain mixtures’’ liquid systems based on chloline chloride [79] for
conditionshasalsobeenobserved[70,71].Theionicliquidbased whichthequalificationof‘‘ionicliquids’’isstillthesubjectof
onthe[(C F ) PF ](cid:2)anionhasbeenrecentlyproposedasamore controversies. Choline can be used as alternative cation in
2 5 3 3
chemicallystablealternativeto[PF ](cid:2)[72]. combination with suitable anion to generate ILs (choline
6
TheactivationoftheC(2)-Hoftheimidazoliumtoleadtothe salicylate melts at 508C and was described in 1960). The
N-heterocycliccarbene(NHC)inpresenceofbaseisalsolargely physicalproperties(viscosity,meltingpoint,thermalstability,
described[73].Consequently,increasedinteresthasbeenfound polarity)ofdifferentcarboxylateanionssuchasacetate,tartrate,
in phosphonium ILs because of their higher stability under lactate,succinate,glycolate,maleatecoupledwithcholinecation
basic conditions, such as in Grignard reactions [74]. To protect have been described. Surprisingly glycolate presents a Tm of
the acidic C(2)-H, 2-methylimidazole based ILs are often used. 388C. The thermal stability range of the series is 183–2238C
By analogy, methimazole based ILs have been described in [78].Themaleategivesmoderateviscosity.OtherinterestingILs
whichtheC(2)protonisreplacedbyathiollinkage(Scheme3) based on choline cations have been prepared by direct
[75]. neutralisationofcholinehydroxidewithdifferentaromaticor
The latest applications of ILs concern ILs with biological cyclicaliphaticcarboxylicacids(Scheme6).Surprisingly,some
properties (Scheme 4) [76]. Hypergolic fuels in which hydrazine oftheseILsshowlowTgandTm.Thebiodegradabilityproperties
H.Olivier-Bourbigouetal./AppliedCatalysisA:General373(2010)1–56 7
Scheme4.ExamplesofILswithtargetedfunctions.
Scheme5.Cost-effectiveILs.
oftheseILshavebeenreported[80].Veryrecently,itwasshown (5) ILs recyclability [84] becomes one of the main issues when
thattheincorporationofestersidechainmoietyonpyridinium process developments are envisioned: distillable ILs (under
ornicotiniumcationcouldleadtobiodegradableILscontraryto relativelynormalpressureandtemperatureconditions)orILs
thepyridiniumanaloguesILs[81]. presentinglowthermalstabilityhavebeendesigned.TheseILs
(4) New materials have been developed using imidazolium as cancontainaweaklybasicanionandacationformedfroma
backbonetoaccesstofunctionalsilicagelsorcarbonnanotubes tertiary amine and an exchangeable proton (Scheme 7). By
withflexibleproperties[82].TheILisimmobilizedonthesolid distillation,theneutralacidorbase(ifvolatileenough)canbe
supportbycovalentbonds generallybetweenthesilylgroup separatedfromtheionizedspecies.Theycansubsequentlybe
andtheimidazoliumcation.Theimmobilizationofmetalions recombinedtoreformtheIL.Thereisavastnumberofcation–
on silica surface offers a novel class of materials where the anioncombinationofsuchproticILs.CarbamatebasedILsform
environmentofthemetaliscomparabletothatfoundinofthe anotherclassofdistillableILs(seeswitchablesolvents).Each
type[BMI] [MX ][83]. applicationrequiresspecificproperties,thereisnoILsthatcan
2 4
satisfyallofthem.Wewillfocushereonthelastdevelopments.
2.2.2. Proticionicliquids(PILs)
WhileoneofthefirstIL,describedin1914byWalden[85],wasof
‘‘the protictype’’[EtNH ][NO ](withamp=12.58C,describedin
3 3
nearly all reviewson ILs!), aproticILs largely dominatethe open
literature due to their inertness relative to organometallic com-
poundsandtheirpotentialofapplications,particularlyincatalysis.
However,therehasbeenaresurgenceofinterestfortheseProticILs
essentially because of their great potential for proton transfer
applicationsinfuelcelltechnologies.AreviewwaswrittenbyPoole
includingtheuseoftheseProticILsinchromatography[86].Someof
theseILspresentlowmeltingpoints(wellbelow1008C)andhigh
conductivities(over10(cid:2)2Scm(cid:2)1at1308C)[87].Mostofthenon-
proticILsaresynthesisedbytransferringanalkylgrouptothebasic
nitrogensitethroughS 2reactions.ProticILsareformedthrough
N
Scheme6.‘‘Bio’’ILs. directprotontransferfromaBrønstedacidtoabase(oraBrønsted
8 H.Olivier-Bourbigouetal./AppliedCatalysisA:General373(2010)1–56
Scheme7.ProticILssynthesisedbydirectprotonation(X(cid:2)=[NTf2](cid:2),[CF3SO3](cid:2),[CF3CO2](cid:2),[CH3SO3](cid:2),[HCOO](cid:2),[HSO4](cid:2),[H2PO3](cid:2)).
base).Theypresenttheadvantageofbeingcost-effectiveandeasily
prepared as their formation does not involve the formation of
residualby-products.ExamplesofProticILsaregiveninScheme7
[88,89].ManyoftheseProticILsinvolveverystrongacids,suchas
HNTf ,andhencetheequilibriumisheavilyshiftedtotherightthus
2 Scheme8.N,N-dimethylethanolammoniumformateILs.
producingILsascompletelyionicsalts.TheseILsaregenerallyliquid
atroomtemperature.
NMR measurements show that the N–H proton is not labile, low temperatures compared with their alkylated homologues
whichtendstoindicatethattheseILscannotbereallyconsidered while this property has been advanced as an advantage for
as Brønsted acids [90]. The acidic properties of these ILs could recycling.
ratherbeascribedtothepresenceofresidualacidinthemedium AnotherinterestingexampleofProticILsisbasedontheuseof
comingfromthesynthesis.Thepurityhastobecheckedproperly hydrophilic monodispersed and hyperbranched dendrimer poly-
byamoresensitivemeanthanNMR.Inthecaseofweakacid,such mers base such as polyamidoamine (Fig. 3). Protonation of this
as acetic acid, the neutralisation reaction will reach a point of polymerwithBrønstedacid,followedbymetatheticexchangeof
equilibrium.The‘‘completeornot’’ionicityofthesemixtureshave anionwith[NTf ](cid:2)leadstotheformationofanhydrophobicILthe
2
been discussed by different groups and the challenge is still to low T of which being ascribed to the flexible nature of the
m
provideanunambiguousmeasurementofthisdegreeofionization, dendritic backbone (T =(cid:2)2.58C). Conductivity and thermal
m
since the values for equilibrium constants are not known under degradation (near 3508C) were determined. This IL, beside its
non-aqueousconditions[91].Theseliquidscanprobablybebest use as proton conductive electrolytes, could be well suited for
describedas‘‘liquidmixtures’’ofionicandneutralspecies.Ithas particlesencapsulation[95].
beensuggestedthattobeclassifiedasILs,accordingtoaformal BrønstedILscanbeclassifiedbesidesthePILs.Anoverviewon
definition,theproductsmustbe>99%ionized,andthusacareful theseILsincorporatingcarboxylicestersandacidgroupsandtheir
selectionofacidsandbases(basedonpK )isrequired[92]. zwitterionic counterpart has been written in 2004 [96]. Another
a
Thecaseofproticbases(suchasdialkylamines)hasalsobeen recentreviewgivesanoverviewonthedifferentBrønstedILsand
studied (Scheme 8). In 1:1 mixtures, the boiling point is usually theirapplicationsinorganicsynthesisandcatalysis[18].
much higher than the average value of the acid and the base
precursors. This may suggest that significant and fast proton 2.2.3. (Multi)-functionalionicliquids
transferbetweenacidandbasemoleculesoccurs[93]. Recently, ILs based on different cations and anions bearing
One can predict a growing interest in near future in ILs with functionalgroupshavebeentheobjectofseveralrecentreviews
‘‘dissociable protons’’ not only as potential solvents but also for [12,16,96–100].
theirdifferentpropertiesandbehaviours,theirabilitytoformH-
bonds (proton donor and acceptor) and their use to build a 2.2.3.1. Solvent and acid or base function. Acidic and basic ILs
hydrogen-bond network [94]. However, a limitation of these representnewclassesofILs(Scheme9).Theacidorbasicfunction
protonatedimidazoliumsaltsisthattheydecomposeatrelatively canbeattachedeitherontheanion,eitheronthecation[63].ILs
Fig.3.ILsbaseddendrimerpolymersbase(reprintedwithpermissionfrom[95].Copyright2009AmericanChemicalSociety).
H.Olivier-Bourbigouetal./AppliedCatalysisA:General373(2010)1–56 9
Scheme9.AcidandbasicILs.
containingpolynuclearmetallicanionssuchaschloroaluminates, ILs,eitheronthecation,eitherontheanion(Scheme11).Byadding
havebeenknownforalongtimefortheirpotentialLewisacidity CO pressure to the solutions, the basicity can be significantly
2
andsuperacidityinpresenceofprotons.Theyhavebeenextended reduced.ThebasicitycanberepeatedlyrecoveredbyremovingCO
2
tootherpolynuclearanionsthatarestableinpresenceofwaterand bybubblingN totheILs.Thissimpleandreversiblemethodcould
2
oxygen such as chloroferrate or chlorozincate. The chloroferrate havepotentialapplicationsindifferentfields[113].Further,some
anionshavebeenassociatedwith[NEt H]+togeneratecheapand basic ILs containing biodegradable components, including ILs
3
easy to make acidic catalysts. Interestingly,Brønsted acidity can derivedfromnaturalaminoacidshavebeendeveloped[113].
alsobeintroducedbyadditionofBrønstedacidssuchasHForHCl Finally, ILs synthesised by the reaction of [RMI][OH] with
intohalidebasedILs.Thisisawaytoreducethevolatilityofthe differentpoly-acidssuchasoxalicacid,malicacid,phthalicacid,
acid bysupporting it intheILsthroughtheformationofX(HX) tartaricacid.canbementioned.Accordingtothequantityofacid
n
type anion ([X](cid:2)=[F](cid:2) or [Cl](cid:2)) [101]. The species [HCl ](cid:2), added,theseILscandisplayacertainaciditylevel.Theyhavebeen
2
[H Cl ](cid:2), and [H Cl ](cid:2) are also known to form when Brønsted usedasbuffersinpHsensitivecatalyticreactionforcontrollingthe
2 3 3 4
acidsaredissolvedinchloride-richchloroaluminateionicliquids acidity in non-aqueous media. The interest is that they may
[102,103].Butsomeleachingoftheacidinpresenceofanorganic presentsolubilisationpropertiesinorganicsolvents[114].
phasecanbeexpected. InthecaseoftheseacidorbasicILs,itisworthemphasisingthat
Alkanesulfonicorcarboxylateacidgroupshavebeencovalently thepresenceofimpurities,suchaswater,halide,organicbasesor
tethered to different cations such as imidazolium, benzimidazo- acidsortracesofsolvents,mainlycomingfromthesynthesisofthe
lium [104], pyridinium [105], ammonium, or phosphonium
tethered sulfonic acid tosylate [106]. An interesting IL has been
described with the acidity linked to a quaternary ammonium
([Me N-(CH ) -CO H][X]),associatedwiththe[NTf ](cid:2)anion,this
3 2 2 2 2
compoundhasameltingpointof578C.Itisknowntosolubilise
metaloxide[107].
ThehydrogenatomintheC(2)-positionofthedialkylimidazo-
liumcationcanalsobeproposedassourceofacidity.Forexample,N-
heterocyclic carbenes have been electrogenerated by cathodic
cleavageofthe C(2)-hydrogenbond of imidazolium-based room-
temperatureionicliquids.Thesecarbenesprovedtobequitestable
basesthatcanbeusedforthedeprotonationofbromoamines[108].
Basic ILs have been less developed than acidic ones. Amine
organicbaseshavebeentetheredtoILcations.ThesefunctionalILs Scheme10.ExamplesofbasicILs.
werefirstlysynthesisedtocaptureCO [109,110].Mono-charged
2
diamine based ILs which incorporate Lewis basicitysite (DABCO
type) on the cation with both thermal stability and low melting
pointcanbeobtainedwhenassociatedwith[NTf ](cid:2)anion(Scheme
2
10)[111,112].
One interesting concept has been described to switch the
basicityofILs.Thisisachievedbyusingamino-groupcontaining Scheme11.ExampleofswitchableLewisbasicILs.
10 H.Olivier-Bourbigouetal./AppliedCatalysisA:General373(2010)1–56
Scheme12.FunctionalILsasligandsandsupportsfortransitionmetalcomplexes.
ILs, can dramatically modify their acido-basic properties. The 2.2.4. ChiralILs
determinationofthelevelofacidityoftheseILshasmostoftime The number of publications dealing with chiral ILs (CILs)
not been determined which can sometimes lead to misunder- grew rapidly [28]. The source of chirality can be provided
standingintheroleoftheILs. either on the cation, on the anion or both on anion and cation
(Scheme 13).
2.2.3.2. Full-sizeimage. AlargerangeofCILshavebeenpreparedbasedonchiralamino-
Solventandligand. Inmultiphasecatalysis,themainchallengeisto acids anions and ammonium, imidazolium and phosphonium
recyclethecatalyst,tomaintainthetransitionmetalintheILphase cations[116,117].Besideimidazolium,guanidiniumcationshave
and to prevent its loss by leaching on workup. The number of also opened the opportunity to create a new family of chiral ILs
described tagged ligands is huge and their field of applications basedonnaturalchiralanions.TheapplicationsoftheseCILscanbe
covernearlyallcatalyticreactions.Greatprogress,especiallyinthe found in asymmetric catalysis, but also in spectroscopic and
organic synthesis of tagged ligands, has been achieved (Scheme chromatographicapplications.Inasymmetricsynthesis,itisoften
12).Ionicphosphorusligandsarethesubjectofongoingresearch believedthatCILscanbeusedaschiralsolventsandassoleinducer
fordifferentcatalyticreactions.ILsopenanewfieldfortheuseP–O ofchiralityduetotheirpolymer-likebehaviourandpotentialhigh
basedligands,rarelyusedinwaterbecauseoftheirsensitivityto degree of organisation. However, very few results are reported
hydrolysis.Almostadozenofcationicphosphiteligandshavebeen whichdemonstratesuchpotential.Thefirstresultwasreportedby
recently described, some of them could have been produced on the group Vo-Thanh in the Baylis-Hillman reaction [118] The IL
quitelargescale[115]. usedisbasedonthechiralephedriniumcation(Scheme14).The
Scheme13.ExamplesofchiralILs.