CataToday2014-54-MIL 100-101-烷烃氧化

CatalysisToday238(2014)54–61

ContentslistsavailableatScienceDirect

CatalysisToday

journalhomepage:http://wendang.chazidian.com/locate/catto

d

HydrocarbonoxidationoverFe-andCr-containingmetal-organicframeworksMIL-100andMIL-101–acomparativestudy

OxanaA.Kholdeevaa,b,?,IgorY.Skobeleva,IrinaD.Ivanchikovaa,

KonstantinA.Kovalenkob,c,VladimirP.Fedinb,c,AlexanderB.Sorokind

BoreskovInstituteofCatalysis,Lavrentievave.5,Novosibirsk630090,RussiaNovosibirskStateUniversity,Pirogovast.2,Novosibirsk630090,Russiac

NikolaevInstituteofInorganicChemistry,Lavrentievave.3,Novosibirsk630090,Russiad

InstitutdeRecherchessurlaCatalyseetl’EnvironnementdeLyon(IRCELYON),UMR5256,CNRS-UniversitéLyon1,2,av.A.Einstein,69626VilleurbanneCedex,France

a

b

article

info

abstract

Articlehistory:

Received1December2013Accepted14January2014

Availableonline13February2014

Keywords:MIL-101MIL-100

HeterogeneouscatalysisAlkenesAnthracene

Selectiveoxidation

CatalyticpropertiesofFe-andCr-basedmetal-organicframeworks(MOFs)MIL-100andMIL-101havebeenassessedintwoliquid-phasereactions:solvent-freeallylicoxidationofalkenes(cyclohexene,?-and?-pinenes)withmolecularoxygenandoxidationofanthracene(AN)withtert-butylhydroperoxide(TBHP).Intheoxidationofalkenes,theproductselectivitystronglydependsonthenatureofmetal(FeorCr)but,forthesamemetal,onlyslightlydiffersfortheMIL-100andMIL-101structures.TheFe-containingMOFsaffordtheformationofunsaturatedalcoholswhileCr-basedMOFsgivemainlyunsaturatedketones.BothCr-MIL-100andCr-MIL-101favordecompositionofcyclohexenylhydroperoxidetoproduce2-cyclohexen-1-onewith67–69%selectivity.StabilitytowarddestructionreducedintheorderCr-MIL-101,Cr-MIL-100>Fe-MIL-100>Fe-MIL-101.IntheoxidationofanthraceneoverbothCr-MOFsandFe-MIL-101,theselectivitytoward9,10-anthraquinone(AQ)attained100%at92–100%ANconversion.Theturnoverfrequency(TOF)decreasedintheorderCr-MIL-101>Fe-MIL-101>Cr-MIL-100>Fe-MIL-100.Cr-MIL-101revealedsuperiorcatalyticperformanceintermsofANconversion,AQselectivityandTOF.NearlyquantitativeyieldofAQwasobtainedafter1.5hat100?Cinchlorobenzeneassolvent.NoleachingofactivemetaloccurredunderoptimalreactionconditionsandtheMOFscouldberecycledseveraltimeswithoutdeteriorationofthecatalyticproperties.

©2014ElsevierB.V.Allrightsreserved.

1.Introduction

Metal-organicframeworks(MOFs)haveattractedconsider-ableattentionowingtoauniquecombinationofproperties,suchascrystallineopenstructures,extremelyhighsurfaceareasandporevolumes,tunableporesizeandfunctionality.AlltheseallowconsiderationofMOFsasprospectivematerialsformolecularrecognition,separation,gasstorageandcatalysis[1–10].Thesematerialspotentiallycombineadvantagesofbothhomogeneousandheterogeneouscatalystsbecausetheirstructureconsistsofiso-latedmetalclustersoratomsconnectedbypolydentateorganicligandstoformarigidporousframework.TheimportantfeatureofsuchframeworkisahighnumberofregularlydistributeduniformmetalcentersthatareaccessibleforreagentsthesizeofwhichiscomparableorsmallerthantheMOFcageentrances.

?Correspondingauthor.Tel.:+73833269433;fax:+73833309573.

E-mailaddresses:khold@catalysis.nsk.su,khold@catalysis.ru(O.A.Kholdeeva).

Untilrecently,lowthermal,chemicalandsolvolyticstabilities(incomparisonwithinorganiczeolites)limitedtheapplicationofMOFsincatalysis.In2005,Féreyandco-workersreportedthesynthesisofamesoporouschromiumterephthalate,Cr-MIL-101,whichdemonstratedagoodresistancetoair,water,commonsol-ventsandthermaltreatment(upto300?C)[11].Taylor-Pashowetal.synthesizedaniron-containinganalog,Fe-MIL-101[12].TheMIL-101materialhasazeotypecrystalstructureconsistingoftwo

?(thecorrespondingtypesofcageswithadiameterof34and29A

?Coordinativelyunsaturatedmetalsiteswindowsare16and12A).

(CUS)canbeeasilygeneratedinthestructureofMIL-101byheat-ingthematerialinvacuum[13].Changingthelinkermoleculefrom1,4-benzenedicarboxylicacidfor1,3,5-benzenetricarboxylicacidresultedintheformationofanotherMOFstructure,MIL-100,which

?accessiblethroughmicroporouscontainscagesof27and24A

?respectively.BothCr-MIL-100[14]andwindowsof9and6A,

Fe-MIL-100[15]havebeenreportedbyFérey’sgroup.SchematicrepresentationoftheMIL-101andMIL-100structuresisshowninFig.1.

0920-5861/$–seefrontmatter©2014ElsevierB.V.Allrightsreserved.http://wendang.chazidian.com/10.1016/j.cattod.2014.01.010

O.A.Kholdeevaetal./CatalysisToday238(2014)54–61

55

Fig.1.SchematicrepresentationoftheMIL-101andMIL-100structures:M3O-carboxylatetrimer—primarybuildingunit,AandD—supertetrahedra,secondarybuildingunits,BandE—smallcages,CandF—largecagesofMIL-101andMIL-100,respectively.

Fe-MIL-100revealedcatalyticactivityintheoxidationofdiphenylmethaneandtriphenylmethanewithtert-butylhydroper-oxide(TBHP)[16]andFriedel–Craftsbenzylationofbenzene[15].Ironcoordinatedto1,3,5-benzenetricarboxylate(BTC),theso-called[Fe(BTC)]metal-organicframework,hasbeencommer-cializedbyBASFunderthetradenameofBasoliteF300.AlthoughtheexactcrystalstructureofFe(BTC)isstillunknown,itislikelythatithasastructuresimilartothatofFe-MIL-100.[Fe(BTC)]isaneffec-tivecatalystforaerobicoxidationofcyclooctane[17]andstyrene[18].Inturn,Cr-MIL-100operatesaseffectiveLewisacidcatalystinvariousorganictransformations[19].Cr-MIL-101isabletocatalyzecyanosilylationofbenzaldehyde[20],carboxylationofepoxides[21],benzilycoxidationoftetralin[22],sulfoxidationofthioetherswithH2O2[13],oxidationofcyclohexane[23]andallylicoxida-tionofalkeneswithTBHP[24]orO2[25,26].Recently,wereportedtheuseofFe-MIL-101intheallylicoxidationofalkeneswithmolecularoxygenandfoundaproductdistributionsigni?cantlydifferentfromCr-MIL-101[25,26].Sofar,nodirectcomparisonofthecatalyticbehaviorofMIL-100andMIL-101materialsinselec-tiveoxidationreactionshasbeenreported.Inthepresentwork,weassessedandcomparedcatalyticpropertiesofFe-andCr-basedMIL-100andMIL-101intwoliquid-phasereactions:solvent-freeallylicoxidationofalkeneswithmolecularoxygenandoxidationofanthracene(AN)to9,10-anthraquinone(AQ)usingTBHPasoxi-dant.

withoutadditionalpuri?cation.TBHPwasusedasasolutionindecane(4.7M;Aldrich)orasa70wt%aqueoussolution.Cyclo-hexenylhydroperoxidewaspreparedaccordingtotheliterature[27].Theexactconcentrationofhydroperoxideswasdeterminediodometricallypriortouse.

2.2.MOFsynthesisandcharacterization

2.Experimental

2.1.Materials

Cyclohexene(CyH),?-and?-pinene(Sigma-AldrichorAlpha-Aesar)werepuri?edpriortousebypassingthroughacolumn?lledwithneutralaluminatoremovetracesofpossibleoxi-dationproducts.Anthracene(99%)andanthraquinone(99%)werepurchasedfromAldrichandusedwithoutadditionalpuri?cation.Chlorobenzene,toluene,dichloroethaneandacetoni-?moleculartrile(Fluka)weredriedandstoredoveractivated4A

sieves.Allotherreactantswereobtainedcommerciallyandused

Cr-MIL-101[11]andFe-MIL-101[12]werepreparedbythesolvothermalmethodfollowingprotocolsreportedelsewhere[25].Fe-MIL-100andCr-MIL-100weresynthesizedaccordingtotheliterature[14,15]withsomemodi?cations.Inatypicalsynthe-sisofFe-MIL-100,140mgmetaliron(2.5mmol),350mgtrimesicacid(1.65mmol),5mLHF(5mmol),1.2mLHNO3(1.2mmol)and13mLH2OwereputintoaTe?on-linedstainlesssteelbombandheatedupto150?Candthenkeptatthistemperaturefor24h.Theresultingorangesolidwas?lteredoffandwashedwithhot(ca.80?C)waterfor3h.Then,thematerialwasdriedatroomtem-peratureovernight.InatypicalsynthesisofCr-MIL-100,100mgmetalchromium(2mmol),300mgtrimesicacid(1.4mmol),1mLHF(4mmol)and9mLH2OwereplacedintoaTe?on-linedstain-lesssteelbombandheatedwiththerateof20?C/minupto220?Cduring96h.Coolingwasperformedwiththerateof10?C/min.Theresultinggreensolidwas?lteredoff,washedwithwaterandacetoneanddriedatroomtemperature.

TheMOFswerecharacterizedbyX-raydiffraction(XRD)tech-niqueandlowtemperatureN2adsorptionmeasurements.XRDpatternsofallthematerialswereinaccordancewiththeliterature[11,12,14,15].Theaveragesizeofcrystalliteswasca.0.7and7?mfortheMIL-101andMIL-100materials,respectively.Thespeci?cBETsurfaceareaswereintherangeof3200–3400(MIL-101)and2000–2200(MIL-100)m2/g.ThecontentofCr(Fe)wasca.23and26wt%inactivatedMIL-101andMIL-100,respectively.

2.3.Catalytictestsandproductanalysis

CatalyticoxidationsofalkeneswithO2,productanalysisandproductadsorptionmeasurementswereperformedasdescribed

56

O.A.Kholdeevaetal./CatalysisToday238(2014)54–61

Table1

CyHoxidationwithmolecularoxygenoverMIL-100andMIL-101.

EntryCatalyst(mg)

T(?C)

Selectivity(%)

CyHconversion(%)

Reference

1

1Fe-MIL-100(13)50212Fe-MIL-100(13)40113Fe-MIL-100(26)40144Fe-MIL-101(13)50225Fe-MIL-101(13)40156Fe-MIL-101(26)40187Cr-MIL-100(26)60528

Cr-MIL-101(26)

60

56

Reactionconditions:1mLCyH,1barO2,0.02–0.03mmolTBHP(asinitiator),16h.

elsewhere[25,26].CatalyticoxidationsofANwerecarriedoutinthermostatedglassvesselsundervigorousstirring(600rpm).Reac-tionswerestartedbytheadditionofTBHPtoamixturecontaininganthracene,MOF,internalstandard(biphenyl)andsolvent.Priortouse,MOFswereactivatedinvacuum:Cr-MIL-101–at120?Cfor2handthen180?Cfor4h,Fe-MIL-101—at120?Cfor6handCr(Fe)-MIL-100—at150?Cfor6h.Samplesofthereactionmix-turewerewithdrawnperiodicallyduringthereactioncoursebyasyringe.Theoxidationproductswereidenti?edbyGS–MS.YieldsofAQandconversionsofANwerequanti?edbyGC.Eachexperi-mentwasreproduced3–4times.Turnoverfrequency(TOF)valuesweredeterminedfrominitialratesofANconsumption.Catalystre-usabilitywasstudiedin4–6timescaledexperiments.Beforereuse,catalystswere?lteredoff,washedwithhotchlorobenzene(PhCl)andacetone,anddriedinairat30–40?Cfor1–2h.Thenatureofcatalysiswasveri?edbyhot?ltrationtest[28].

2.4.Instrumentation

GCanalyseswereperformedusingagaschromatographAgi-lent4890Dequippedwitha?ameionizationdetectoranda

30m×0.25mmVF-5MScapillarycolumnorgaschromatographChromosGC-1000equippedwitha?ameionizationdetectoranda30m×0.25mmBPX-5capillarycolumn.GC–MSanalyseswerecarriedoutusingagaschromatographHP6890(a50m×0.25mmDB-5MScapillarycolumn)equippedwithamass-selectivedetec-torAgilentMSD5973.XRDmeasurementswereperformedonShimadzuXRD7000SorDRON-3M.Nitrogenadsorptionat77KwasmeasuredusinganASAP-2020instrument(Micrometrics)orAutosorbiQ(Quantachrome)withinapartialpressurerangeof10?6–1.0.

3.Resultsanddiscussion

3.1.Cyclohexeneoxidationwithmolecularoxygen

Previously,wefoundthatthepresenceofbothMOFcatalystandinitiator(TBHP)isrequiredtoaccomplishCyHoxidationatmildreactionconditions,viz.,1atmofO2and60?C[25].Hence,inallcatalyticexperiments,weaddedsmallamountsofTBHPtoinitiatethereaction.

3.1.1.Fe-MIL-100andMIL-101

Table1presentstheresultsonsolvent-freeCyHoxidationwithmolecularoxygenoverbothFe-andCr-containingMIL-100andMIL-101.Themainoxidationproductswere2-cyclohexen-1-one(1),2-cyclohexen-1-ol(2)andcyclohexenylhydroperoxide(3).Minoramountsofcyclohexeneoxidewerefoundinfew

2

3

364310Thiswork62277Thiswork60268Thiswork116327[25]58278[25]

57258Thiswork311712Thiswork38

6

16

[25]

experiments.

MIL-100 or MIL-10 140-60 oC, 1 bar О2

1

2

3

Inourpreviouswork,werevealedthattheproductdistributioninCyHoxidationoverFe-MIL-101stronglydependedonthereac-tiontemperature,which,inturn,determinedthenatureofcatalysis[25].Thusat50–60?C,Fe-MIL-101underwentironleachingandFespeciesinsolutionfavortheformationof3.Wedidnotinvesti-gatethestructureandcompositionofthesolubleironspecies.Itcouldbemonoordi(polynuclear)complexesofFe(III)withcar-boxylates(linker)orsomeotherligands(e.g.reactionproducts).Onthecontrary,theMOFwasstabletoleachingat40?Candpro-duced2asthemainoxidationproduct[25].ThesameregularitiesturnedouttooperatewithFe-MIL-100.Indeed,whenthereac-tionwasperformedat50?C,hydroperoxide3predominatedforbothFe-MIL-100(Table1,entry1)andFe-MIL-101(entry4).Whiledecreasingthereactiontemperaturedownto40?C,cyclohexenolbecamethemainproductoverbothFe–MOFsandformedwiththeselectivityof58–62%at7–8%conversion(entries2and5).Nofurthersubstrateconversionoccurreduponincreasingthereac-tiontime.Enlargingthecatalystamountproducednosigni?cantchangesinCyHconversionandproductselectivity(Table1,com-pareentries2and3orentries5and6).Hotcatalyst?ltrationtest(Fig.2a)provedthatthecatalysisoverFe-MIL-100at40?Cistrulyheterogeneous.Atsuchtemperature,bothFe–MOFscanbeusedrepeatedlywithoutdeteriorationofthecatalyticproperties.Table2presentstheresultsonCyHconversionandselectivitytowardthemainproduct2obtainedinfourconsecutivereuses.Importantly,studiesbyXRDrevealedthatFe-MIL-100preserveditsstructureafterfourreuses(Fig.3a).Thisdifferssigni?cantlyFe-MIL-100fromFe-MIL-101thatpartiallylosttheregularstructureunderthesameconditions[25].Therefore,wemayconcludethatFe-MIL-100andFe-MIL-101demonstratesimilarcatalyticpropertiesinCyH

Table2

RecyclingofMIL-101andMIL-100inCyHoxidation.

Catalyst

CyHconversiona(%)

Selectivityto2(%)

Fe-MIL-101[25]9(8,9,8)57(55,58,56)Fe-MIL-100

7(8,7,7)

60(63,61,62)Selectivityto1(%)Cr-MIL-101[25]16(15,17,16)56(58,56,57)Cr-MIL-100

11(13,12,11)

50(53,52,51)

Reactionconditions:1mLCyH,1barO2,0.02–0.03mmolTBHP(asinitiator),16h,40?CforFe-MILs(13mg)or60?CforCr-MILs(26mg).a

Inparentheses,2nd,3rdand4threuses.

O.A.Kholdeevaetal./CatalysisToday238(2014)54–61

57

CyH conve

rsion, %

CyH conversion, %

Time, h

Time, h

Fig.2.Hotcatalyst?ltrationtestfor(a)Fe-MIL-100(40?C)and(b)Cr-MIL-100(60?C).ReactionconditionsasinTable1,entries2and7.

oxidationbuttheformerhasahigherstabilitytowarddegradationunderthereactionconditions.

highselectivitytoward1realizedoverCr-MIL-101wasattributedtotheabilitytopromotedehydrationof3.

Cr-MIL-101(100)

3.1.2.Cr-MIL-100andMIL-101

IncontrasttoFe-MIL-100,themainproductofCyHoxidationoverCr-MIL-100is?,?-unsaturatedketone1,whichformswiththeselectivityof52%at12%CyHconversion(Table1,entry7).TheselectivityisclosetothatacquiredwithCr-MIL-101(entry8)althoughthesubstrateconversionoverMIL-101ishigher(16%versus12%).Hence,againtheproductdistributionisdeterminedmostlybythenatureofthetransitionmetalintheMOFframe-work.Hotcatalyst?ltrationtestcon?rmednoleachingofactivechromiumspeciesfromCr-MIL-100at60?C(Fig.2b),pointingoutthatCr-MIL-100ismorestabletowardmetalleachingthantheironanalogthatcannotsurviveatsuchtemperature.BothCr-MOFscouldbeusedatleastfourtimeswithoutlossofthecatalyticproperties(Table2).XRDshowednodegradationoftheCr-MIL-100structureafterfourconsecutivecycles(Fig.3b).Previously,thestructuralintegrityofCr-MIL-101wasalsocorroboratedbyXRD[25].Hence,incontrasttotheFe-MILs,nodifferenceinstabilityofCr-MIL-100andCr-MIL-101hasbeenfound.

ThustheproductselectivitiesfoundforCyHoxidationinthepresenceofFe-MIL-100andCr-MIL-100aresimilartothoseobtainedwithFe-MIL-101andCr-MIL-101[25],therebyindicat-ingthatthenatureoftransitionmetalratherthanthenatureoftheMOFlinkerdeterminatestheoxidationmechanism.LikeinthecaseofFe-MIL-101andCr-MIL-101[25],radicalscavengers(ionol)impedetheoxidationprocessesoverbothFe-MIL-100andCr-MIL-100,indicatingthatradicalchainstepsareinvolved.ToexplainthedifferenceintheproductdistributioninCyHoxidationoverFe-MIL-101andCr-MIL-101,wesuggesteddifferentpathwaysforthetransformationoftheprimaryoxidationproduct,3[25,26].The

60 oC

Earlier,Sheldonetal.demonstratedthepossibilityofsuchtransformationsforbothcyclohexenylhydroperoxide[27]andcyclohexylhydroperoxide[29]overCr-containingcatalysts.Indeed,wefoundthatbothCr-MIL-101andCr-MIL-100catalyzedecompositionof3toyield1.Fig.4showsplotsoftheaccumula-tionof1versustime.Cyclohexenoneselectivityremainsconstantduringthedecompositionprocess(67–69%).Theonlybyproductis2whichmightformalongwith1inthedecompositionof3via>Haber–Weissmechanism[30].Similarvaluesofcyclohexenoneselectivity(65–74%)werereportedbySheldonetal.forvariouschromiumcatalysts,includingCr(acac)3,Cr-APO5andCrS-1[27].PossibleroutesleadingtothepreferableformationofunsaturatedalcoholsoverFe-MIL-101havebeensuggestedearlier[25].SimilarproductdistributionsfoundforFe-MIL-100andFe-MIL-101implythatsimilarpathwaysmightoperatewithFe-MIL-100.

3.2.Terpeneoxidationwithmolecularoxygen

Oxidationoftworepresentativeterpenes,?-and?-pinenes,wasexploredundertheconditionsatwhichalltheMOFcatalystsremainstable,i.e.40and60?CforFe-andCr-MILs,respectively.TheresultsarepresentedinTable3.Intheoxidationof?-pinene,bothFe-MIL-100andFe-MIL-101producedasigni?cantamountofverbenolalongwithcampholenicaldehyde,hydroperoxidesandepoxide,butnoverbenonewasfoundinthereactionmixture

(a)(b)

A

AB

В

5

10

15 20 , degrees2θ

5 10

15 20 25

2θ, degree s

30 35

2530

Fig.3.XRDpatternof(a)Fe-MIL-100and(b)Cr-MIL-100:initialsample(A)andafterfourcatalyticcycles(B).ReactionconditionsasinTable2.

58

O.A.Kholdeevaetal./CatalysisToday238(2014)54–61

Concentration, MConcentration, M

Time, min

Time, m in

Fig.4.Formationofcyclohexenone(??)inthecourseofdecompositionofcyclohexenylhydroperoxide3(?)

overCr-MIL-100(a)andCr-MIL-101(b).Reactionconditions:

0.2mmolof3,26mgMOF,

1mLcyclohexane,60?C.

Table3

Solvent-freeoxidationof?-and?-pineneswithO2overMIL-101and

MIL-100.

Entry

Catalyst

Conversion(%)

Selectivity(%)

ROOHɑ

1234

Fe-MIL-100Cr-MIL-100

Fe-MIL-101[26]Cr-MIL-101[26]

9201226

321533–30

–39

99915

9338

44372514

ROOHɑ

78910

Fe-MIL-100Cr-MIL-100

Fe-MIL-101[26]Cr-MIL-101[26]

12181322

64205732

–2–23

1033–27

26454318

Reactionconditions:1.5mLofalkene,1barO2,0.02–0.03mmolTBHP(initiator),16h,40?CforFe-MILs(13mg)or60?CforCr-MILs(26mg).a

Sumofhydroperoxides.

(Table3,entries1and3).Theconversionof?-pinenewasslightlylowerforFe-MIL-100relativetoFe-MIL-101(9%versus12%).In?-pineneoxidation,bothFe-MILsgavetwounsaturatedalcohols(pinocarveolandmyrtenol)alongwithhydroperoxides,butprac-ticallynoketonesformed(Table3,entries7and9).ThesubstrateconversionwascloseforbothMOFs:12%and13%forFe-MIL-100andFe-MIL-101,respectively.

Cr-containingMIL-100andMIL-101transformed?-and?-pinenemainlytohydroperoxidesandunsaturatedketonesandalcohols,withketonespredominatingoveralcohols(Table3).Minoramountsofepoxideandcampholenicaldehydealsoformedinthereactionwith?-pinene.HighersubstrateconversionswereattainedforCr-MIL-101thanforCr-MIL-100(26%versus20%and22%versus18%for?-and?-pinene,respectively).AlthoughthecompositionofproductsismorecomplicatedintheoxidationofterpenesthanincaseofCyH,mostlikely,becauseofisomer-ization/rearrangementreactionstypicalofterpenoids,wemayconcludethat,ingeneral,Fe-MILsfavortheformationofunsatu-ratedalcoholswhileCr-MILsmediatetheformationofunsaturatedketones.

organiclinkernatureproduceslesseffectonthealkeneconversionandproductselectivity.Interstingly,thesituationisdifferentifwecompareadsorptionpropertiesoftheMOFswithregardstotheCyHoxidationproducts.Weevaluatedthediffusiontimeandproduct(1and2)adsorptionconstantsforFe-andCr-MIL-100followingthemethodologyreportedinourpreviouswork[25].TheresultsarepresentedinTable4incomparisonwiththeresultsacquiredealierfortheMIL-101analogues.Whilethediffusiontimedoesnotexceed1.5minforalltheMOFs,theadsorptionconstantsarehigherforMIL-100thanforMIL-101and,surprisingly,donotdependonthenatureofmetal.

Previouslywedemonstratedthattheadditionofboth1and2couldstopCyHoxidationoverCr-andFe-MIL-101[25].Wefoundsimilarrate-retardingeffectsuponadditionoftheseproductstothereactionwithFe-andCr-MIL-100(Fig.5aandb).Water,theproductwhichformsinastoichiometricamountalongwith1,canalsoactasaninhibitorofthealkeneoxidationcatalyzedbyMOFs,ascon?rmedbytheexperimentwithH2Oadditives(Fig.5band

3.3.Productadsorptionstudies

Table4

Adsorptionofcyclohexenone(1)andcyclohexenol(2)onMIL-100andMIL-101.

Material

K1(M?1)

K2,(M?1)

Diffusiontime(min)

FromthedatapresentedinTables1and3,onecanjudgethatthenatureofmetalstronglyaffectsnotonlytheproductdistributionbutalsotheattainablesubstrateconversionthatisconsiderablyhigherforCr-containingMOFsthanfortheironanalogues.The

Fe-MIL-100

Fe-MIL-101[25]Cr-MIL-100

Cr-MIL-101[25]

8±23±18±23±1

8±21±0.38±21±0.3

11.51.51.5