ImprovingO2N2SelectiveFiltrationUsingCarbonNanotub

ImprovingO2/N2SelectiveFiltrationUsingCarbonNanotube-ModifiedMixed-MatrixMembranesAmodifiedmixed-matrixmembrane(MMM)wasdesignedinordertoimproveO2/N2selectivefiltration.ThefabricatedMMMconsistofpurifiedandfunctional-izedmulti-walledcarbonnanotubes(MWCNT)andpolyethersulfone.Character-izationofthepreparedmembranesandthefunctionalizedMWCNTwasdonebyscanningelectronmicroscopy,Fouriertransforminfraredspectroscopyandther-mogravimetricanalysismethods.Thepermeabilitiesandselectivitiesofthemem-braneswereimprovedusingmodifiedMWCNTasMMMfiller.Theeffectsofthesolvent,thecarbonnanotubetypesanddosage,thepolymerdosage,thecastingmethod,andtheappliedpressureinboththemembranefabricationandgassepa-rationprocesseswereinvestigated.Keywords:Gasselectivity,Gasseparation,Mixed-matrixmembrane,Multi-walledcarbonnanotubesReceived:September9,2014;revised:April20,2015;accepted:July20,2015DOI:10.1002/ceat.2014005381IntroductionGasseparationisanimportantissuethathasattractedtheat-tentionofmanyscientists.Asisknown,greenhousegases(suchasCH4andCO2)aresoharmfultotheenvironmentthattheirhazardouseffectsonlifecannotbeignored.Forinstance,oneofthemostimportantdestructiveeffectsofthesegasesisthedepletionoftheozonelayer.Hence,muchresearchhasbeenconductedontheseparationandenrichmentofgasmixtures[1].Oxygen-enrichedcombustionisconsideredtobeanener-gy-efficientprocessinpowerplants;however,itneedsatre-mendousamountofoxygen-enrichedair.Inairseparationapplications,membraneprocesseshavebecomecompetitiveintermsofbothcostandperformance,andthereforeofferout-standingadvantagesoveralternativeoxygensupplytechnolo-gies[2].Gasadsorption,pressureswingadsorption,cryogenicdistillation,andmembraneseparationaresomeofthesepara-tionmethodsthathavebeenusedfordecades.Allofthesemethodshavetheirownlimitationsanddifficulties[3–6].Membraneseparationisapromisingmethodandhasattractedmuchattentioninthelastdecades.Membrane-basedsepara-tionsarehighlyrecommendedbecauseoftheirsimplicity,lowcapitalandoperationalcost,andhighefficiency[7].Mem-branesareclassifiedintothreegroups:polymeric,ceramic,andmetallicmembranes.Polymericmembraneshavebeenem-ployedinmanyareasofresearchowingtotheirsimplicityandlowcapitalcostascomparedtoothermembranes.However,thesemembraneshavetheshortcomingoflowefficiency,whichcanbeeliminatedbyusingfillers.Thus,itisnecessarytoimprovetheirperformancethroughusingsuitablefillers[8,9].Mixed-matrixmembranes(MMM)areanewtypeofpolymericmembranescomprisingapolymericmatrixandaninorganicfiller.ThemostattractivecharacteristicsofMMMaretheirgoodmechanicalstrengthduetotheuseofapolymermatrixandtheirgoodpermeabilityandselectivitybecauseoftheinor-ganicfillers[10–15].OneofthemostchallengesinusingMMMasaseparationdeviceistheestablishmentofagoodin-terfacialinteractionbetweenthepolymerandtheinorganicfill-er.Thecompatibilityoffillersandpolymershasbeeninvesti-gatedbymanyresearchers.Hudionoetal.[16]usedzeoliteasfillerinMMM.Theyconcludedthattheselectivityandperme-abilitycharacteristicsofthemembraneareenhancedbyusinginorganiczeolitematerials.Therecentlyemployedfillersmayhavesomedisadvantagesandshortcomings.Forinstance,zeoliteshaveanadhesiveexternalsurface,whichreducestheapplicabilityofzeolite-poly-mermembranes[17,18].ThedifficultiesincontrollingtheadhesionbetweenthepolymerphaseandtheexternalsurfaceofthezeolitehaveresultedintheuseofnewmaterialsforthepreparationofMMM.Recently,metal-organicframeworks(MOF)havebeenstudiedforMMMingasseparationpro-cesses[19,20].Theseorganiclinkagesprovideausefulplat-formforchemistriesthatmayimproveadhesiontopolymermatrices[21].However,experimentalevidencefromPerezetal.[19]hasshownthat,despitethehighsurfaceareaoftheactivatedMOF-5,nosignificantchangesintheidealselectivityforanygaspairswasobserved,evenifhigherloadingsofMOFparticleswereembeddedinthepolysulfone(PSF)matrix[22].Chem.Eng.Technol.2015,38,No.11,2079–2086ª2015WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim–Correspondence:Dr.SedighehZeinali([e-mail]),ShirazUniversity–NanochemicalEngineering,MollasadraStreet,CollegeofEngineeringNo.2,Shiraz,Fars7193615511,Iran.ResearchArticle2079DespitethepredominantfocusonMOF,thereisalsoanin-creasingresearchinterestinusingcarbonnanotubes(CNT)forMMMfabrication.CNTbelongtothefullerenestructuralfam-ily,whichhaveasix-memberedcarbonring(hexagon)inthehoneycomblatticerelativetotheaxisofthenanotubes.CNTcanbesynthesizedassingulartubes,calledsingle-walledcar-bonnanotubes(SWCNT),orasaseriesofshellsofdifferentdiametersspacedaroundacommonaxis,calledmulti-wallednanotubes(MWCNT).SinceCNThavebeendiscoveredbyIjima[23],theywereusedforseveralpurposes.ThespecificcharacteristicsofCNT,suchastheirhighselectivity,mechanicalstrength,aspectratio,andtherapidtransportrate,resultedintheirextensiveapplica-tionasefficientinorganicfillers[24].MWCNTaremoreappropriatefo