CO2

13658|Chem.Commun.,2015,51,13658--13661Thisjournalis©TheRoyalSocietyofChemistry2015Citethis:Chem.Commun.,2015,51,13658Anaminoacidionicliquid-basedtoughiongelmembraneforCO2capture†FarhadMoghadam,EijiKamio,*AyumiYoshizumiandHidetoMatsuyama*Atoughandthindouble-networkgelmembranecontainingaminoacidionicliquidsasaCO2carrierexhibitedsuperiorCO2permeabilityandstabilityunderpressurizedconditions.ConcernsonCO2emissionsfromcoal-firedpowerplantsarerapidlyincreasingowingtotheirmajorcontributiontoglobalwarmingandclimatechange.Hence,thedevelopmentofaneconomicallyviableandenvironmentallyfriendlyCO2capturetechnologyisinevitable.Fromanengineeringpointofview,thelowpartialpressureofCO2andhugevolumeofemittedfluegasaretwoseriouschallengestoincreasingplantsizeandminimizingthecostofCO2capture.Duringthelasttwodecades,membranetechnologyhasdrawnconsiderableinterestowingtothepotentialforaCO2captureprocesswithlowenergyconsumptionandlowcapitalandoperatingcosts.AfeasibleanddesirableCO2separationmembranewithhighCO2fluxthatdoesnotsacrificeCO2/N2selectivityshouldbedevelopedtofulfillpracticalrequirements.Inthiscontext,roomtemperatureionicliquids(RTILs)havebeenproposedasattractiveCO2permeationmediaforsupportedionicliquidmembranes(SILMs)1owingtotheirdistinctpropertiessuchashighCO2absorptioncapacity,negligiblevaporpressure,hugechemicaldiversity,andhighthermalstability.However,‘‘blow-out’’oftheILsthroughtheporesofthesupportmem-branes,whichistheinherentproblemofSILMsduetotheweakcapillaryforcefortheholdingofILsintheporoussupportandbecomesseriousunderpressurizedconditions,limitstheapplica-tionofSILMstolowtrans-membranepressuresystems.2There-fore,thedevelopmentofamaterialwhichovercomesblow-outofILshadbeendesiredforIL-basedCO2separationmembranes.Incorporationofnon-polymerizableionicliquids(freeILs)intothematrixoffunctionalcross-linkableILs,calledpoly(RTIL)/RTILcom-positemembranes,hasbeenconsideredtobeapromisingstrategytoovercometheblow-outissueofSILMs.3However,despiteblow-outimprovement,thepoly(RTIL)/RTILcompositemembranessufferfromlowCO2permeabilityduetothelowCO2absorbabilityofRTILs.4Recently,theuseofamine-functionalizedtask-specificILs(TSILs)forCO2capture,whichhaveatleastaCO2-reactiveaminomoietyinthemolecule,hasbeenconsideredasanattractiveapproachtoovercomepoorCO2absorbability.4b,5Theamine-functionalizedTSILscanchemicallyandreversiblyreactwithCO2toformCO2complexes(carbamicacidand/orcarbamate)6andincreasethetrans-membraneconcentrationgradientoftheCO2complexes,whichrealizesfastandselectiveCO2permeation.BytakingadvantageofselectivereactionsbetweenCO2andTSILs,werecentlyreportedanewclassofSILMscontainingaminoacidILs(AAILs).7TheAAIL-basedSILMsdemonstrateremarkablyfastandselectiveCO2permeationbasedonafacilitatedtransportmechanism.TheadvantagesofAAILsoverwidelyusedconventionalnon-IL-typeCO2carriersmakethempromisingcandidatesasCO2carriersandasdiffusionmediaforfacilitatedtransportmembranes.However,similartoconventionalSILMs,theAAIL-basedSILMscannotbeusedunderpressurizedconditionsbecausetheAAILsareheldinaporoussupportsystemwithweakinteractionsandeasilyleakfromthesupport.ToresolvetheaforementionedissuerelatedtothepressurestabilityofAAIL-basedSILMs,weproposedastraightforwardapproachtofabricatepolymergelmembranescontainingAAILs(AAIL-gelmembranes).8ThedevelopedAAIL-basedmembranewith70wt%oftheAAILcontentpreventedtheblow-outofAAILsunderpressurizedconditionsupto1MPabecausethestronginteractionbetweenthepolymernetworkandtheAAILyieldedlargeosmoticpressureoftheAAIL-gels.Inaddition,theAAIL-gelmembranesexhibitedacceptableCO2separationpotential,nearlyanalogoustothatofAAIL-basedSILMs.However,toachievedesirableCO2separationperformance,alargeamountofAAILsmustbeincorporatedintotheAAIL-gelmembrane,andthisisassociatedwithadecreaseinmechanicalstrengthaswellasCenterforMembraneandFilmTechnology,DepartmentofChemicalScienceandEngineering,KobeUniversity,1-1Rokkodai-cho,Nada-ku,Kobe,Hyogo657-8501,Japan.E-mail:e-kamio@people.kobe-u.ac.jp,matuyama@kobe-u.ac.jp†Electronicsupplementaryinformation(ESI)available:MaterialsandgeneralproceduresforthepreparationofAAIL-basedDNiongelmembranes,measure-mentoftheAAILcontent,effectsoftheAAILcontent,CO2partialpressure,andmembranethicknessoftheDNiongelmembranesongaspermeabilities,fracturestressdataoftheDNiongels,andadurabilitytestoftheDNiongelmembranes.SeeDOI:10.1039/c5cc04841aReceived11thJune2015,Accepted20thJuly2015DOI:10.1039/c5cc04841awww.rsc.org/chemcommChemCommCOMMUNICATIONPublishedon21July2015.DownloadedbySichuanUniversityon19/05/201703:49:03.ViewArticleOnlineViewJournal|ViewIssueThisjournalis©TheRoyalSocietyofChemistry2015Chem.Commun.,2015,51,13658--13661|13659pressureresistanceoftheAAIL-gelmembranes.Thatis,thetrade-offbetweenmechanicalstrengthandCO2separationabilitystillremainsachallenge.Moreover,thelowmechanicalstrengthofthegelmembranehindersthepreparationofthingelmembraneswithhighCO2fluxforpracticalapplications.Here,weproposeanovel,toughAAIL-gelmembranewithexcellentmechanicalstrengthbasedonaspecificdouble-network(DN)gelmatrix9thatimprovesthepressureresistanceofthegelmembranecontainingalargeamountofAAILandalsoenablesthefabricationofathinmembranewithhighCO2permeance.Tothebestofourknowledge,thisisthefirstreportonthefabricationofAAIL-basedpolymericgelmembranesexhibitingexcellentCO2permeabilityandCO2/N2selectivityaswellasoutstandingstabilityunderpressurizedconditions.TheDNgelconceptwasappliedtodeveloptoughAAIL-basedgelmembranesinthisstudy.TheseDNgelswerefirstproposedtopreparetough,high-water-contenthydrogelsforuseasalternativestonaturaltissuesofthehumanbody.9DNhydrogelscontaintwoasymmetricpolymernetworks:thefirstisarigid,brittle,andtightlycross-linkedpolyelectrolyteandthesecondisasoft,ductile,andlooselycross-linkedpolymer.ThisuniquestructureoffersmanypolymernetworkoptionsfortheDNgels.ForthefabricationofAAIL-basedDNgels,themostimportantparameteristhecompatibilitybetweenAAILsandthepolymernetworks.Inapreviousreport,8wefoundthatpolyvinylpyrrolidone(PVP)andpolydimethylacrylamide(PDMAAm)havegoodcompatibilitywithseveralphosphonium-basedAAILs.BecausePVPandPDMAAmareneutralpolymers,weadoptedthemasthesecondnetworkoftheAAIL-basedDNiongels.AsthefirstnetworkoftheDNgel,poly(2-acrylamido-2-methyl-1-propanesulfonicacid)(PAMPS),whichisarigidpolyelectrolyte,wasusedbecauseitprovideshighosmoticpressureinsidethegelduringimmersioninwater.ThelargeosmoticpressurebetweentheinsideandoutsideofthegelledtofullswellingandtheincorporationofalargeamountofAAILsintothegelmatrix.Additionally,itcouldcontributetotheenhancementoftheholdingabilityofAAILsinthegelnetworksduetotheincreaseoftheosmoticpressureoftheAAIL-basedDNgelowingtothecounter-ionsofPAMPS.BasedontheexcellentCO2permeationpropertiesofprolinate-basedAAILs,10weselectedtetrabutylphosphoniumprolinate([P4444][Pro])andtriethyl(pentyl)-phosphoniumprolinate([P2225][Pro])astheAAIL.Althoughpoorretentionoflow-viscosity[P2225][Pro]insidetheporoussupportmakesitdifficulttofabricateSILMs,thesynthesizedDNcouldstronglyholdallAAILs,upto80wt%.AAIL-basedDNiongelmembraneswerepreparedviaamulti-steppreparationmethodasfollows:(1)DNhydrogelpreparation,(2)impregnationofanAAIL/watermixtureintotheDNmatrix,and(3)removalofwaterfromthegelbyevaporation.First,DNhydrogelfilmsconsistingofPAMPSandeitherPVPorPDMAAmwerepreparedaccordingtotheprocedureexplainedintheESI.†AlthoughthehydrogelwithPAMPS/PVPwasweak,thehydrogelcomposedofPAMPS/PDMAAmexhibitedextraordinarytoughness.ThePAMPS/PDMAAmDNhydrogelwasimmersedintheaqueoussolutionofAAILsfor24htoincorporateAAILsintothegelnetwork.Afterdryingat373Kinavacuumovenforatleast8htoremoveanypossiblewater,AAIL-basedDNiongelmembranesweresuccessfullyprepared.TakingtheeffectofAAILcontentintoconsideration,DNgelmembraneswithdifferentAAILcontentswerepreparedusingaqueoussolutionswithdifferentconcentrationsofAAILs.TheAAILcontentinthepreparedgelmembranesreachedupto80wt%andbecameconstantastheAAILconcentrationintheaqueoussolutionincreased(Fig.S1,ESI†).ItisworthpointingoutthatevenDNgelmembranescontainingupto80wt%AAILaremechanicallystrongenoughtobeeasilymanipulatedbyhandandtheirgastransportpropertiescanbeevaluatedunderpressurizedconditions.ThepreparedAAIL-basedDNiongelmembraneisshowninFig.1(a).Typicalphotographsdemon-stratingtheextraordinarymechanicalstrengthoftheAAIL-basedDNiongelsarealsoshowninFig.1(bandc).TheeffectofAAILcontentonCO2andN2permeationpropertiesispresentedinFig.S3(ESI†).AsshowninFig.S3(ESI†),theCO2andN2permeabilitiesdependedontheAAILcontent,i.e.,thepermeabilitiesincreasedastheAAILcontentincreased.TheincreaseinN2permeabilitylikelyresultedfromthedecreaseinthediffusionresistance(whichisafunctionofthepolymercontentandgelnetworkdensity).RegardingtheCO2permeability,whichwasmoresignificantlyaffectedbytheAAILcontentthanN2,thestrongdependencyisthecombinedeffectofthedecreaseindiffusionresistanceandtheincreaseintheCO2absorptionamount(resultingfromtheincreaseinavailableAAILs(carriers)intheDNgelnetwork).Thatis,theCO2permeationthroughtheAAIL-basedDNiongelmembraneswasbasedonafacilitatedtransportmechanism.ThefacilitatedCO2transportwasalsoconfirmedbasedontheCO2partialpressuredependencyofCO2permeabilitiesforthe[P4444][Pro]-basedand[P2225][Pro]-basedDNiongelmembraneswithvariousAAILcontents(seeESI†(5)andFig.S4andS5).OneofthemostoutstandingfeaturesoftheAAIL-basedDNgelsistheextraordinarymechanicalstrength.WemeasuredthefracturestressoftheDNiongelsconsistingof[P4444][Pro]and[P2225][Pro]withdifferentcontents.ItisworthmentioningthateventheDNiongelswithca.80wt%AAILcontentsdidnotbreakundercompressionstressofmorethan25MPa(Fig.S6(a),ESI†),whichismorethan25timeshigherthanthosepreviouslyreportedfor[P4444][Pro]-basediongelsconsistingofPVPandPDMAAmsinglenetworkspreparedbyourgroup.8Duringcompression,noAAILleakagefromthegelnetworkwasobservedasacon-sequenceofgoodcompatibilitybetweenPDMAAmandtheAAILs.Fig.1(a)TypicalAAIL-basedDNiongelmembrane,(b)AAIL-basedDNiongelbeforecompression(top),duringcompression(middle),andaftercompression(bottom),and(c)theAAIL-basedDNiongelmembranebeforestretching(left)andunderstretching(right).AAILs:aminoacidionicliquids.CommunicationChemCommPublishedon21July2015.DownloadedbySichuanUniversityon19/05/201703:49:03.ViewArticleOnline13660|Chem.Commun.,2015,51,13658--13661Thisjournalis©TheRoyalSocietyofChemistry2015Regardingthetensilestrength,theAAIL-basedDNiongelmem-branesexhibitedhightensilefracturestressofmorethan0.5MPa(Fig.S6(b–d),ESI†).Wealsoconfirmedthehysteresisofthestress–straincurvesandsofteningbehavioroftheAAIL-basedDNgelmembraneduringcyclicstretchingtests(Fig.S7,ESI†),whichisaspecificpropertyofDNgels.11Ingeneral,itishardtoconfirmtheDNstructuredirectly.However,thespecificsofteningbehaviorandhysteresisarethestrongevidenceoftherolesofthebrittle1stnetworkas‘‘sacrificialbonds’’andtheductile2ndnetworkas‘‘hiddenlength’’inthegel.12Thatis,thetougheningmechanismoftheAAIL-basedDNgelswasattributedtotheDNconcept.Basedontheexcellentmechanicalstrengthobserved,itwasexpectedthatthefabricatedAAIL-basedDNiongelmembranescouldbeusedunderpressurizedconditions.Fig.2demonstratesthepressureresistanceoftheAAIL-basedDNgelmembranes.TheAAILcontentsoftheDNiongelmembranesusedinthisinvestiga-tionwereca.80wt%.Thepressureresistancewasevaluatedatanelevatedtemperatureof373Kandaconstantpermeatesidepressure(atmosphericpressure)throughchangingthefeedsidepressurefrom100to500kPa.AsclearlyindicatedinFig.2,theCO2permeability,theN2barrierproperty,andCO2/N2selectivityforbothAAILsremainedconstantatahighlevelforthewholerangeoftrans-membranepressuredifferencesinvestigatedinthisstudy.Inparticular,theobservedCO2permeabilityandCO2/N2selectivitiesweremorethan4000Barrerand100athighpressuredifferenceconditions,respectively.TheseresultsindicatedthattheAAIL-basedDNiongelmembraneswithstoodpressurizedconditionsduetotheirtoughDNstructure.Inaddition,theAAIL-basedDNiongelmembranesshowedstableCO2andN2permeationfor5daysatelevatedtemperaturesunderpressurizedconditions(Fig.S8,ESI†).ThehighandselectiveCO2permeability,thesuperiorpressureresistance,andthelongtermstabilityofthefabricatedAAIL-basedDNiongelmembranesmakethemanattractiveoptionforabroadrangeofCO2captureapplications.AreductioninthethicknessofthefabricatedDNgelmem-branesstillremainedachallenge.Inpracticalapplications,ahighfluxandCO2-selectivemembraneisrequiredtoreducethesizeandcostoftheCO2separationunit.TheCO2fluxbasicallydependsonthemembranethicknesswhentheCO2permeationislimitedbyintra-membranediffusionofCO2.Inotherwords,fastCO2permeationcanbeachievedthroughmaintainingCO2permselectivitybyreducingmembranethickness.ThetoughAAIL-basedDNiongelhasthepotentialtobefabricatedinathinnerformwithhighCO2permselectivitybecausetherate-controllingstepofCO2permeationthroughtheAAIL-basedmembraneswouldbecontrolledbyintra-membranediffusionoftheCO2complex.7bTherefore,wefabricatedthinnerAAIL-basedDNiongelmembranesandmeasuredtheCO2andN2fluxes.Inthisinvestigation,[P4444][Pro]-basedDNiongelmembraneswereusedtocomparetheperformanceswiththatoftheAAIL-SILM.TheCO2partialpressuredependencyofCO2andN2permeances(GPU)forthethinnestAAIL-basediongelmembrane(58mm)preparedinthisstudyareshowninFig.3alongwiththoseforthethickAAIL-basedDNiongelmembrane(600mm)andtheAAIL-SILM(37mm).TheCO2permeanceofthethinnerAAIL-basedDNiongelmembranewasalmostidenticaltothatoftheAAIL-SILM,althoughtheiongelmembranewasthickerthantheAAIL-SILM.ThisresultindicatesthatthediffusionresistanceintheDNiongelnetworkissmallerthanthatoftheporoussupportusedforAAIL-SILMsowingtothelowpolymercontentintheDNiongelmembrane.Furthermore,basedontherelationshipbetweentheCO2permeanceandinvertedthicknessofthemembrane,1/d,showninFig.S9(ESI†),theCO2permeanceincreasedlinearlywiththedecreaseof1/d,whichindicatesthattheCO2permeationthroughthethinnerDNiongelmembranewasstillcontrolledbyintra-membranediffusion.ThisraisesthepossibilitythatfasterCO2permeationcanbeachievedthroughfurtherreductionsinmembranethickness.Inthisstudy,wedemonstratedtheadvancedpropertiesofAAIL-basedDNiongelmembraneswithrespecttoCO2separation.ThepreparedmembranesweretoughandhadhighpressureFig.2PressureresistanceoftheAAIL-basedDNgelmembranes(experi-mentalconditions:T=373K,Psweep=atmosphericpressure,PCO2,feed=10kPa,andrelativehumidity=0%,membraneproperties:thicknesswas600mmandAAILcontentwasca.80wt%).Fig.3CO2partialpressuredependencyofthethinAAIL-basedDNiongelmembranecontainingca.80wt%of[P4444][Pro]astheCO2separationmedium(experimentalconditions:T=373K,Pfeed=Psweep=atmosphericpressure,andrelativehumidity=0%).ChemCommCommunicationPublishedon21July2015.DownloadedbySichuanUniversityon19/05/201703:49:03.ViewArticleOnlineThisjournalis©TheRoyalSocietyofChemistry2015Chem.Commun.,2015,51,13658--13661|13661resistanceaswellashighCO2permeabilityandCO2/N2selectivity.ThisisthefirstreporttoutilizeaDNgelnetworkforiongels.ManyopportunitiestoimprovethemechanicalstrengthaswellastoreducethepolymercontentforhigherCO2permeabilitystillremain.OptimizationoftheAAIL-basedDNiongelmembraneswouldprovidefeasibleanddesirableCO2separationmembranesforvariouspracticalCO2separationprocesses.ThisworkwassupportedbyJSPSKAKENHIGrantNumber15K18256.Notesandreferences1(a)P.Scovazzo,J.Kieft,D.A.Finan,C.Koval,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