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Citethis:Chem.Soc.Rev.,2014,43,5594

WateradsorptioninMOFs:fundamentalsandapplications

abacd

Je´ro?meCanivet,AlexandraFateeva,YouminGuo,BenoitCoasneandDavidFarrusseng*a

ThisreviewarticlepresentsthefundamentalandpracticalaspectsofwateradsorptioninMetal–OrganicFrameworks(MOFs).ThestateoftheartofMOFstabilityinwater,acrucialissuetomanyapplicationsinwhichMOFsarepromisingcandidates,isdiscussedhere.Stabilityinbothgaseous(suchashumidgases)andaqueousmediaisconsidered.Byconsideringanon-exhaustiveyetrepresentativesetofMOFs,thedifferentmechanismsofwateradsorptioninthisclassofmaterialsarepresented:reversibleandcontinuousporefilling,irreversibleanddiscontinuousporefillingthroughcapillarycondensation,and

Received19thFebruary2014DOI:10.1039/c4cs00078a

http://wendang.chazidian.com/csr

irreversibilityarisingfromtheflexibilityandpossiblestructuralmodificationsofthehostmaterial.Wateradsorptionpropertiesofmorethan60MOFsamplesarereported.TheapplicationsofMOFsasmaterialsforheat-pumpsandadsorbent-basedchillersandprotonconductorsarealsoreviewed.Somedirectionsforfutureworkaresuggestedasconcludingremarks.

1.Introduction

Wateriseverywhere!Thedesignanddevelopmentofmoisture-stableporousmaterialsarecrucialforindustrialapplicationssuchasgasstorageandseparation,sensing,catalysis,andprotonconduction.Unfortunately,thefirstdiscoveredporousmetal–organicframeworksMOF-51andHKUST-1,2whichhavestronglycontributedtotheboostoftheMOFdomain,turnedouttobeparticularlymoisturesensitive.3–5Despitethegreatintereststirredbytheirextraordinaryspecificsurfaceandcalibratedporesize,thedegradationintheatmosphereofthesematerialshasobviouslylimitedinterestforindustrialapplications.Ontheotherhand,solubilitypropertiesofMOFsareespeciallyattractiveforinvivomedicalapplications.Water-unstableMOFscalledBio-MOFs(suchasFe-nicotinateBioMIL-1)weredesignedanddeveloped‘‘onpurpose’’asdrugcarriersunderphysiologicalconditions6,7andforimaging.8–10Theunderstandingofthedegradationmechanismsinthepresenceofwater,eitherinvapourorliquidphases,ishenceofutmostimportanceforthedesignanddevelopmentofthenextgenerationsofporous

a

´Lyon1,CNRS,UMR5256,2avenueAlbertEinstein,IRCELYON,Universite

F-69626Villeurbanne,France.E-mail:david.farrusseng@ircelyon.univ-lyon1.fr;Fax:+33472445399;Tel:+33472445365b

´riauxetInterfaces,Universite´Lyon1,UMR5615,LaboratoiredesMultimate

43Boulevarddu11Novembre1918,F-69622Villeurbanne,Francec

MultiScaleMaterialScienceforEnergyandEnvironment,CNRS/MIT,UMI3466,MassachusettsInstituteofTechnology,77MassachusettsAvenue,Cambridge,MA02139,USAd

DepartmentofCivilandEnvironmentalEngineering,MassachusettsInstituteofTechnology,77MassachusettsAvenue,Cambridge,MA02139,USA

coordinationpolymerswithappropriatewatersensitivityorinsensitivityinthecontextofrealapplications.ThefirstchapterofthisreviewdealswiththestabilityofMOFsinhumidatmo-spheresandinaqueousmedia,andpresentsthedifferentdegradation–dissolutionmechanisms.

Thecontrolofwateradsorptioninmicroporoussolidsiscrucialforthedevelopmentofindustrialprocesses.Forinstance,thetemperaturerequiredfortheregenerationofadsorptionorchromatographycolumnsmadeupofmolecularsievesisgovernedbytheirwateradsorptionproperties.Ontheotherhand,hyperhydrophobiczeolitescanbeappliedformolecularspringsuponwaterintrusion.11,12Ofparticularimportanceforenvironmentalapplications,wateradsorptionisoftendetrimentalforCO2captureusinghydrophilicmaterialssincewateractsasastrongcompetitor.13Nevertheless,itwasdemonstratedthatcontrolledwateradsorptioncanenhanceCO2captureinMOFs14–16suchasMOF-100,17HKUST-1,18MIL-10119andMIL-53.20Theinventoryofwatereffectsondiverseapplica-tionsisvastandcomplex,andgoesthereforewellbeyondthisreview.WehavechosentofocusthisreviewontwoapplicationswherewateradsorptionpropertiesaredirectlyinvolvedintheperformanceoftheMOF.TheapplicationofMOFsasmaterialsfor(1)heat-pumpsandadsorbent-basedchillersand(2)protonconductorsisdescribedinthethirdchapterwithemphasis,wherepossible,ontherelationshipbetweenstructureandwateradsorption.OtherapplicationsforwhichwateradsorptioninMOFsisrelevantincludedehumidification,21waterpurifica-tion,22,23thermalbatteries,andproductionanddeliveryofdrinkingwaterinremoteareas(foraveryrecentstudyontheseapplications,seeref.24).

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Kaskelandco-workersfirstreportedthatwateradsorption–desorptionisothermsonaseriesofdiverseMOFsdisplayabroadvarietyofbehavioursfromhyperhydrophobicforZIF-8toexceptionalwatercapacityforMIL-100/-101.25ThediversityofwateradsorptionpropertiesofMOFsisregularlyconfirmedwithnovelMOFs.Theeffectsofporesizeandsurfacefunction-alizationbyorganicgroupshavebeenthoroughlyinvestigatedinthecaseofH2,CH4,andCO2adsorption.26–29AfewrulesofthumbhavebeenestablishedandpredictivemodelshavebeendevelopedtoguidetherationaldesignofMOFs.Incontrast,lessefforthasbeendevotedtounravelthediversemechanismsofwateradsorptioninMOFs.Theeffectsofporesize,poremorphology,andflexibilityonwateradsorptionhavenotbeeninvestigatedinasystematicfashion.ThelackofcomprehensiveandpredictivemodelsofwateradsorptionisobviouslylimitingthedesignofMOFsforapplicationswherewaterispresentordirectlyinvolved.ThesecondchapterofthisreviewprovidesadescriptionofthedifferentadsorptionmechanismsthatoccurinMOFs.Then,acomprehensivereviewoftheliteratureisillustratedwithspecificexamples.

2.StabilityofMOFsinthepresenceofwater

BeyondtheobservationofMOFdegradationinthepresenceofwaterbymeansofpowderX-raydi?raction(PXRD)andnitrogenadsorption,therationaldesignofwater-stablematerialsimpliesthestudyofthecomplexbehaviourofMOFsuponwaterexposure.Boththee?ectsofexposuretohumidvapours(likeinfluegases)andaqueousphasesmustbeconsidered.2.1.

Stabilityinpurewater

Stabilityinwatervapour.InaseminalstudyLowandco-workersstudiedbyX-RayDi?raction(PXRD)thestabilityofaseriesoftenMOFshavingdi?erentorganiclinkers,porestructures,metalnodenatureandcoordination,andmonitoredtheirstabilityafterexposureto1mol%steamforafewhours.30Theenergyassessedusingmolecularmodellingoftheliganddisplacementbywatermoleculeswascomparedwithexperi-mentalresultsforeachMOFtomaptheirsteamstability(Fig.1).Throughsuchacombinedvirtualandexperimentalscreening,theseauthorsinvestigatedhowtheframeworknature(metalcoordination,ligandcomposition)anddimensionalitygoverntherelativestabilitiesofMOFsinwater.Theyconcludedontheimportanceofmetal–ligandbondstrengthasakeycriterionofthewater-stabilityofthematerials,moreimportantthanthemetalgeometryorvalenceinthecaseofMOFscontainingtrivalentmetalliccations.Intheirstudy,themoststablematerialswereMIL-110/-101,CPO-27(alsoknownasMOF-74)andZIF-8.Itisnoteworthythatthisstudydidnottakeintoaccountthekineticsofthewater-inducedframeworkdecompositionsincethemolecularmodelingstrategywasbasedontheequilibriumgroundandtransitionstateconfigurations(thermodynamics).Whilesuchstabilitystudiesareveryuseful,itshouldbeemphasizedthatcontradictingresultsaresometimes

Thisjournalis©TheRoyalSocietyofChemistry2014Fig.1SteamstabilitymapofMOFs.ThepositionofthestructureforagivenMOFrepresentsitsmaximumstructuralstabilityasprobedbyXRDmeasure-ments,whiletheenergyofactivationforliganddisplacementbyawatermoleculeasdeterminedbymolecularmodelingisrepresentedbythemagentanumbers(inkcalmolÀ1).Reprintedwithpermissionfromref.30.

reportedintheliterature.Forinstance,whileHKUST-1wasfoundtobehighlystableinwatervapourinref.28,otherauthorshaveobservedasignificantdecreaseintheirspecificsurfaceareaorwatercapacityafterwateradsorption.25,31

Morerecently,thestabilityofaseriesofsixMOFswasestimatedafterwateradsorptionmeasurementsunder80%RelativeHumidity(RH)atroomtemperature.5Thesurfacearealossestimatedfromnitrogenadsorptionat77Kwasusedtorankthesolidstabilityafterhumidityexposure.Onthetimeoftheexperiment,thestabilityfollowstheorder:UiO-66-NH24CPO-27,HKUST-1dDMOF-1,UMCM-1.Thecrystalstructureswerereportedtobepreservedforalmostallthesamples,exceptforDMOF-1andUMCM-1whoseinstabilitywasattributedtothelowcoordinationoftetracoordinatedzinc-carboxylateclusternodes.AlthoughZr-MOFssuchasUiO-66werefoundtobeextremelystableinthepresenceofwater,isostructuralUiO-67,MOF-805andMOF-806madeofbiphenyldicarboxylateorbipyridinedi-carboxylateligands(insteadofbenzenedicarboxylateinUiO-66)areunstableinwatervapor.24,32Theinstabilityoftheextendedbiaryl-basedZr-MOFderivativeswasattributedtothetorsionalstrainundergonebythecrystalleadingtoitsstructuralcollapse.SimilaradsorptionisothermsfoundforAl-MIL-100seemtoindicateinstabilityoratleastpartialcollapseofthestructure.24UsingPXRD,Dietzelandco-workerspointedoutthatexposuretooxygenfromambientairduringwaterdesorption/adsorptioninitiatesthedegradationofNi/Mg-CPO-27.33

Stabilityinliquidwater.ThestabilityofvariousMOFsimmersedinpurewaterorinwetdimethylformamide(DMF)hasbeeninvestigatedbyMatzgerandco-workersontimescalesfromhourstomonths.34ThestabilitywasassessedaccordingtoPXRDrecordedbeforeandafterexposureatroomtempera-ture.ThezinccarboxylatesMOF-5andMOF-177werefoundtobeunstableinwater:DMFmixtureswithratioshigherthan1:4.MOF-5,whichiscomposedofZn4O(COO)6secondarybuildingunits(SBUs),isindeedknowntobeunstableunderexposuretowatervapor4orliquidwater.3Thestabilityof

MOF-5

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orimidazolatesdissolveinacidicmedia,eitheraqueoushydro-chloricorhydrofluoricacidsolution.44Thisisduetotheprotonationoftheirorganiclinkerinsuchmedia.MOFstabilityinstrongalkalinesolutionshasbeenevaluatedforafewmaterials.Inanearlystudy,Yaghiandco-workersreportedthatthePXRDpatternofZIF-8remainsunchangedafter24hoursinan0.1and8Maqueoussodiumhydroxidesolutionat1001C.39TheseauthorsconcludedthatthehydrothermalstabilityofZIF-8issuperiortothoseoforiginalMCMandSBAmesoporoussilica,evencompetingwiththeultrastablederivativesofthesematerials.However,hydrolysisunderhydrothermalconditionsofthesamematerialswasreportedelsewhere.45OtherstudieshavereportedthatstrongaqueousbasessuchasKOHorNaOHdissolvethestructureof2014 02:53:32.

Fig.2PhasediagrammappingthestabilityofzinccarboxylateSBUsinvariouswater–DEFmixtures.Reprintedwithpermissionfromref.36.

underdi?erentrelativehumiditieswasalsodemonstratedinastudyonsamplingformaldehydefromair.35ThekineticsofMOF-5synthesisindiethylformamide(DEF)inthepresenceofwaterwasstudiedbyMertensandco-workers.36Hydratedzincnitrateactsasabu?erandstabilizesthepHensuringslowaciddeprotonationandleadingtoafairlyconstantprecipitationrateandwell-orderedcrystals.ThestudyshowsthatMOF-5isobtainedasaninter-mediatesolidwhichistransformedtothermodynamicallyfavour-ableMOF-69C.ThelatterisbuiltfrominorganicZn3(OH)2(COO)4monodimensionalinfinitechains.Inasystematicstudy,astabilityphasediagramindiethylformamidewasproposedasafunctionoftemperatureandwaterconcentration.ThisphasediagramshowsthatthestabilitydomainofMOF-5isrestrictedtolowwaterconcentration(Fig.2).

http://wendang.chazidian.completecollapseoccursathigherwaterloadingthroughthereplacementoftheligandoxygenatomsbywateroxygenatomsintheZncoordinationsphere.37Matzgerandco-workersalsoshowedthatUMCM-150isstableinawater:DMFratio9:2forhoursandalsoformonthswithaloweramountofwater(water:DMF=3:40).34Incontrast,copperpaddlewheelMOFssuchasMOF-505andHKUST-1werefoundtobestableformonthsinaqueoussolvent(water:DMF=5:1).However,HKUST-1startstodecomposeinpurewaterafter24hours.FinallythechromiumcarboxylateMIL-100andzincimidazolateZIF-8werestableformonthsinpurewater.Inlinewiththelatterresult,PXRDshowedthatimidazolateMOFssuchasZIF-8orSIM-1arestableaftertreatmentinboilingwaterforatleast24hours.38,39Bellatandco-workersshowedthatAl-MIL-53undergoesdegradationinboilingwatertoformcore–shellstructures.40ThesurfaceofMIL-53crystalsistransformedintogammaaluminaundertheseconditionswhileligandsareinter-calatedintotheMOFpores.2.2.

Stabilityinaqueousacid/base

WhilesomeMOFssuchasZr-porphyrinPCN-222,224,and225arestableundersomeextremeaqueousconditions,41–43itisgenerallyacknowledgedthatmostMOFsformedbycarboxylates

5596|Chem.Soc.Rev.,2014,43,5594--5617UiO-66andUiO-66-NH246aswellasthatofAl-MIL-53.40

Ontheotherhand,materialssuchasBioMOFs,whicharedesignedformedicalapplications,werestudiedinsimulatedphysiologicalmedia.Serreandco-workersrankedthestabilityofaseriesofsevencarboxylate-basedMOFsinaphosphatebu?eraqueoussolutionatpH=7.4and371C.47ThestabilityfollowstheorderFe-MIL-100/-1274Fe-MIL-53,UiO-66-NH24Fe-MIL-53-BrcUiO-664UiO-66-Br.ThisseriesshowsthatthestabilityoftheUiOseriesisrelatedtothedonore?ectoftheligandsubstituentasindicatedbytheHammetconstant(themostelectronrichbeingthemoststable).48Theligandreleaseisfacilitatedbyitsdisplacementbyphosphatesfromthebu?ersolutionandtheformationofmetaloxide.IndeedahigherstabilityofthetestedMOFswasfoundinpurewaterthaninphosphatebu?er.

Beyondsimpledegradationorcollapseoftheframeworkstructure,thestabilityofMOFscanberelatedtoanassembly–disassemblyequilibrium.Cohenandco-workersstudiedtheabilityofMOFstoexchangeligandsaswellasmetalcationsinwatersuspension.49,50Asanexample,whentheUiO-66solidmadefrom1,4-benzenedicarboxylicacidissuspendedinanaqueoussolutioncontainingthesubstitutedlinker2-amino-1,4-benzenedicarboxylicacid,thelatterisintroducedinthesolidbyanexchangeprocessyieldinganisoreticularUiOsolidcontainingbothlinkers.Althoughthemechanismofligandexchangehasnotbeeninvestigatedindepth,itisobviousthatthesolubilitydifferenceofthelinkersisanimportantdrivingforceoftheprocess.Similarly,isoreticularMIL-68-(Br/NH2)solidswereobtained.51TheauthorshaveextendedthisprincipletocationexchangetoAl-MIL-53withanaqueoussolutionofFe3+leadingto(Fe/Al)-MIL-53.492.3.

Structure–stabilityrelationships

Metal–ligandbond.ThestabilityofMOFsinwatercanbeattributedtoboththeelectronicandstericeffectsoftheligandonthemetalnode.Indeed,thestrengthofmetal–oxygen/metal–nitrogenbonds,30combinedwiththeshieldingabilityoftheligandtoprotecttheinorganicnodeagainstwatercoordination,drivethewaterresistanceofthematerials.Asdemonstratedforpyrazolate52andimidazolate,39thestabilityoftheframeworkformedbyitscoordinationtoacationincreasesuponincreasingthepKaoftheligand;themetalligandbondisstrongerwhenveryacidicmetalsorverybasicligandsareused.This

property

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explainswhyMOFsmadeupofstrongLewisacids(Al(III),Cr(III))and/orazolates(pKa=14vs.carboxylatepKa=3.5)arethemoststablesuchasAl-MIL-53,Cr-MIL-101andZIF-8(Fig.1).30

Inadditiontothermodynamics,kineticsplaysanimportantroleinthedegradationofMOFsthroughhydrolysis.Moreover,hydrophobicligandssuchasmethylatedlinkersimprovethestabilityofthestructureonashorttimescaleonly(thepositionofthesubstituentsbeingcrucial).Indeed,forDMOFseries,thefullytetramethylatedterephthalateligandallowstheframeworktobestableuponwateradsorption.53,54Giventhefactthathighwateruptakeisobservedinthiscase,thestabilitycannotbeattributedtowaterexclusionbuttotheshieldingofC(O)–OandZnclusterssurroundedbythenumerousmethylgroupswhich2014 02:53:32.

Theseauthorshaveproposedarankingaccordingtotheinertness(i.e.,lowlability)ofthemetalcation.

BycomparingcationsintheisoreticularseriesofMOFs,Chabalandco-workersdescribedthebehaviouroftheM2(bdc)2(dabco)seriescontainingeitherCu,Zn,CoorNicationsunderhumidconditions.63Thewater-induceddegradationmechanismwasreportedtobedi?erentdependingonthecationinvolved.Accordingtodensityfunctionalcalculationscombinedwithexperimentalwatervaporadsorption,theCu–ObondswerefoundtoundergohydrolysistoformhydroxidespecieswhiletheliganddisplacementoccurstobreakZn/Co–Obonds.Finally,undertheconditionsdescribedabove,theNi-dabcoMOFshowsaverylowreactivityi.e.highstabilityinthepresenceofwater.makethenodehardlyaccessibletowater.SimilarconclusionswerereportedforMOF-508havingMe-bipypillars,55Banasorb-22withtrifluoromethoxygroups,56andframeworksmadeupofphosphonatelinkers.57

Inadditiontotheligandstructure,themetalcoordinationintheframeworknodeswasfoundtogovernthewaterstabilityofMOFstructures.IRMOF-1-typestructureswiththethreemetalsZn,Mg,andBewerestudiedusingBorn–OppenheimerMolecularDynamicsinordertodeterminetheirbehaviourinliquidwater.58ThefullyhydratedBebasedcompoundswerefoundtobemorestablethantheiranaloguescontainingMgorZn.ThereasongivenbytheauthorstoexplaintherelativeinstabilityofMgandZn-basedMOFscomparedtotheBeanaloguesisthetendencyofthemetaltoformpenta-andhexa-coordinationspherescombinedwiththeflexibilityoftheM4Ocoreandtheweakermetal-oxidebonds.Furthermore,MgstructureswerefoundtobreakdifferentlyfromtheZnanaloguesduetothelargerrigidityoftheircoreandthestrongMg–Ocoordination.Thiswouldpreventfurtherwatercoordinationtothemetal,incontrasttoZn-IRMOF-1inwhichZnclustersaremoreflexibleandopenupmoreeasily.Moreover,theseauthorsalsopointedoutthatkineticeffectswerebehindthehydrothermalresistanceofBe-IRMOF-1structures;theactivationenergyofthemetal–liganddissociationleadingtothehydratedBe–terephthalatecompoundfromBe-IRMOF-1isindeedverysteep.Similarly,inthecaseofUiO-type5,59andMIL-100/-101MOFs,60,61thehighstabilityofthesematerialswasrelatedtothehighcoordinationofthemetalcentres.Indeed,uponwateradsorptionupto90%RH,the8-coordinatedzirconium-basedUiO-66wastheonlyonetoretainbothitscrystallinityandporositycomparedtoMg-MOF-74,DMOF-1,HKUST-1,andUMCM-1.5

ThenatureofthemetalionitselfalsoplaysacrucialroleinthestabilityofMOFsinwater,asdemonstratedbyJhungandco-workersforisotopicMIL-53/-47frameworks.62BasedonboththeBETsurfaceareaandPXRDanalysis,theseauthorsrankedthestabilityofthesematerialsafterexposureto7Â10À2MNaOH,7Â10À2MHClaqueoussolutionsatroomtemperatureandpurewaterat801C:Cr-MIL-534Al-MIL-534V-MIL-47.Theseauthorsdidnotattributetherelativestabilityoftheframeworktothemetal–ligandbondstrength(theM–ObondenergyfollowstheorderV4Al4Cr).Moreover,therelativestabilitycannotbeattributedeithertothemetaloxidationstateortothecoordina-tiongeometrywhicharethesameforthethreemetalcations.

Thisjournalis©TheRoyalSocietyofChemistry2014Degradationmechanisms.Lowandco-workersreportedtwomaindegradationmechanismsofMOFsexposedtowater:(1)liganddisplacementand(2)hydrolysis.Bothmechanismswereestablishedfromcomputationalchemistryandconfirmedexperimentally.30TheliganddisplacementreactioninvolvestheinsertionofawatermoleculeintotheM–Ometal–ligandbondoftheframework.Thisleadstotheformationofahydratedcationandtothereleaseofafreeligand:

Mn+—LnÀ+H2O-Mn+—(OH2)ÁÁÁLnÀ

(1)

Incontrast,duringthehydrolysisreaction,themetal–ligandbondisbrokenandwaterdissociatestoformahydroxylatedcationandafreeprotonatedligand:

Mn+—LnÀ+H2O-Mn+—(OH)À+HL(nÀ1)À

(2)

AliganddisplacementmechanismwasproposedtoexplainthestructuralbreakdownofZrMOFs(UiO-66)inthepresenceofeithersodiumhydroxideorwater(Fig.3).32

Usingfirst-principlescalculationswithvariouswaterloadings,theliganddisplacementwasalsoproposedasthemainmechanismfortheframeworkdecompositionofthehydrophobicIRMOF.64Itwasfoundthat,inadditiontothewatermoleculeinvolvedintheliganddisplacement,additionalwaterstabilizesboththehydratedmetalspeciesandtheligandbeingdisplaced.Further-more,Coudertandco-workersreportedthatZnOclustersactashydrophilicdefectsinhydrophobicMOFs.Theydemonstratedthat,athighwaterloading,themetaloxideclusterstabilizesawaterclusterinitsneighbourhoodwhichpromotestheliganddisplacement(seealsoref.65foratheoreticaldiscussionon

the

Fig.3Liganddisplacementmechanismproposedtoexplainthebreak-downofUiO-66inthepresenceofsodiumhydroxideorwater.Reprintedwithpermissionfromref.32.

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stabilityofMOFsinwater).SuchamechanismmayexplainthepresenceofZn–OHdefectswhichareactiveinacidcatalysis.66,67

Inadditiontothethermodynamicstabilityoftheframe-work,whichisgenerallyattributedtotheM–Obondstrengthasmentionedabove,morecomplexparametersarerequiredtoevaluatethekineticstabilityofMOFs.Waltonandco-workersreportedthatthebreakdownoffunctionalDMOFsuponexposuretowaterisgovernedbykinetics.54BoththeirexperimentalandcomputationalstudiesshownodirectcorrelationbetweenthestabilityofaseriesofverywatersensitiveDMOFsandthebasicityoftheirligands.Thewaterresistanceofsuchframeworksdependsthusontheactivationenergybarrierforthecorrespondinghydro-lysisreaction.SimilarconclusionswerereportedbyBellarosaetal.2014 02:53:32.

Fig.4ExcitedstatewavefunctioncalculatedbymeansofDFTforMg-CPO-27.Mgatomsareshowningreen,oxygenatomsinred,carbonatomsingray,forthe(Be)IRMOF-1framework.58

ThepredictionofwaterstabilityfromtheMOFcompositionandstructureremainsqualitative.MajortrendsinMOFstabilityversuswaterareacknowledged,evenifsomedataavailableintheliteraturearesomewhatcontradictory.Thesediscrepanciesmayfindtheirorigininthedi?erentmethodsusedtoassessthestabilityoftheframeworks.Theassessmentofcrystallinityofasampleaftermoistureexposureorhydrothermaltreatmentmaybemisleadingsincecrystallinitymightberetainedwhileapartofthesolidisdissolved.68ThemeasurementsofBETsurfaceareaandporousvolumefromN2adsorptionisothermsafterexposuremayalsobemisleadingforthesamereasons.Ontheotherhand,cyclingwateradsorptionisothermsismoreappropriatefortheevaluationofthestabilityundermoistureexposure.Typically,ifthedesorptionbranchinsuchwateradsorptionisothermscrossestheadsorptionbranch,suchasinthecaseofDUT-4,itcanbeconcludedthattheporousframeworkhascollapsedtotallyorpartially.25Thelimitationsofsuchatechniquearethattheoriginofthedegradationcannotbedetermined(amorphisation,hydrolysis,orother).Moreover,thesemeasurementsathighwaterpressure(higherthanPsatat701C),whicharerelevantforhydrothermalprocesses,aredi?culttoperform.Inadditiontoprobingstructuralstability,wateradsorption–desorptionisothermsprovideanappropriatemeansforthecharacterisationofhydrophilic–hydrophobicpropertiesoftheporoussolids.69

3.Wateradsorption–desorptioninMOFs

3.1.

Adsorption–desorptionmechanisms

Therearethreemaintypesofwateradsorptionmechanism:(i)adsorptiononmetallicclusterswhichmodifiesthefirstcoordinationsphereofthemetalion(chemisorption),(ii)layer/cluster(reversible)adsorption,and(iii)capillarycondensation(irreversible).Weillustratebelowthesemechanismsusingthreewell-knowncasestudies:microporousCPO-27(alsoknownasMOF-74),UiO-66,http://wendang.chazidian.comparisonwithwateradsorptionmechanismsinporoussilicaandcarbonsisalsomade.

ThecrystalstructureofCPO-27resemblesahoneycombandcanbeobtainedwithNi2+,Mg2+,Co2+,andZn2+asmetalions.

5598|Chem.Soc.Rev.,2014,43,5594--5617andhydrogenatomsinwhite.Thetwophasesofthewavefunctionareshowninyellowandteal.Reprintedwithpermissionfromref.

70.

Theintersectionsofthehoneycombareformedbyone-dimensionalhelicalchainsofcis-edge-connectedmetal–oxygencoordinationoctahedra.Thechannelshaveanaccessiblediameterofapproximately1.1nm.Initssolvatedstate,themetalatomiscoordinatedbyasinglewatermoleculewhiletheremainingcoordinationsitesareoccupiedbyoxygenatomsbelongingtotheorganiclinker.Uponheating,thesolvatingwatermoleculeisdesorbedwhichleadstoacoordinativelyunsaturatedmetalsite(referredtoas‘‘cus’’)inthedehydratedstructure.Suchadehydrationstepcorrespondstoatransformationofanoctahedralsix-coordinatedintoafive-coordinatedspeciesinasquare-pyramidalgeometry.Anillustrationoftheelectronwave-functioncalculatedbymeansofDensityFunctionalTheory(DFT)showsthelargeunoccupiedorbitalindehydratedMg-CPO-27(Fig.4).70

Thethermodi?ractogramofZn-CPO-27containsabruptchangeswhichrevealstructuredeformationoftheclusterupondehydration.However,theporoustopologyremainsidenticaland,forallmetalions,astructureanalogoustothedehydratedCPO-27isrecoveredonceallthewatermoleculesleavethecompound.ThedensityprobabilityofwaterwasdeterminedusingsinglecrystalX-raydi?ractionatroomtemperatureonahydratedsample(Fig.5).71Thethermaldisplacementellipsoidsdrawnata50%probabilitylevelshowthatthedensity

gets

Fig.5SinglecrystalX-raystructureofCPO-27-Znatroomtemperaturewithatomlabelling.Theellipsoidscorrespondtothermaldisplacementofwatermoleculeswithanoccupancyprobabilityequalto50%.Thesizeoftheellipsoidsindicatesthemobilityofwatermolecules.Reprintedwithpermissionfromref.71.

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