jp020815p

Nitrogen-IncorporatedSAPO-11MolecularSieve:Synthesis,Characterization,andProperties

JianminXiong,†YunjieDing,*,†HejunZhu,†LiYan,†XiumeiLiu,‡andLiwuLin‡

NaturalGasUtilizationandAppliedCatalysisLaboratoryandStateKeyLaboratoryofCatalysis,DalianInstituteofChemistryandPhysics,TheChineseAcademyofSciences,Dalian,116023,ChinaReceiVed:March27,2002;InFinalForm:September23,2002

Nitrogen-incorporatedSAPO-11molecularsievesweresynthesizedbyasecondarysynthesistechniqueatlownitridationtemperature(400°C)withastartingmaterialofSAPO-11zeolitedopedwith2wt%Ru.TheobtainedpowdersamplesshowedhighactivityinKnoevenagelcondensation,whichactsasaprobingreactionforbasecatalysis.Thechangesinstructureofthenitrogen-incorporatedsampleswereinvestigatedbymeansof27Al-MASNMRand31P-MASNMRspectra,X-raydiffraction,andBETsurfaceareameasurements.Itwasfoundthatnitrogen-containingspecieshadincorporatedintotheframeworkofSAPO-11zeolite,inwhichsomeNatomsdirectlybondedtoAlandPatomsandformedAlN4,AlN2O2,andPN4groups.Thesespecieswithterminal-NH2groupsonthesurfacetogetherwereresponsibleforthebasicityofnitrogen-incorporatedSAPO-11zeolite.

Introduction

Thereplacementofliquidacidsandbasesbysolidsascatalystsinorganicreactionsisarequisiteforbetterenviron-mentalpreservation.Whiletheresearchonsolidacidcatalystswasboomedbyoilrefiningthroughthediscoveryofalargenumberofsystemsincludingzeolites,clays,mixedoxides,andsulfates,onlyafewsolidbasecatalystsareyetavailable.Amongthem,nitrogen-incorporatedmolecularsievesareofinterestduetotheirhighsurfaceareaandshape-selectivity.

TheSAPO-11molecularsievesarecomposedofPO4[x]AlO4andSiO4tetrahedron,andiftheoxygenatomsofthetetrahedronwerereplacedtosomelevelbycertainisoelectronicgroups,suchas-NH-species,asaresultofsuchsubstitution,itwillbeexpectedthattheLewisbasicityoftheframeworkincreasesduetothelowerelectronegativityofnitrogenwithrespecttooxygen.Nitrogen-incorporatedcompoundsmaythusbecomecandidatesofcatalystsforanewsetofbasecatalyticreactions.InrecentyearstherehavebeenmanyofreportsonthenitridationofamorphousAlPO4,inwhichamorphousAlPO4precursorswithhighsurfaceareaweresynthesizedthroughdifferentmethods1,2andthennitridizedinaflowofdryammoniaatabout800°C.Thebasicstrengthoftheoxynitridescouldbecontrolledbytheconditionsofnitridationduetothenitrogencontentoftheoxynitridesvaryingwiththetimeandtemperatureofnitridation.Thecontentofnitrogencouldachieveuptoabout20wt%,andtheBETsurfacearearemainedalmostunchangedafternitridation.3,4TheobtainedamorphousoxynitridesshowedhighactivityintheKnoevenagelcondensationbetweenbenz-aldehydeandmethylenecompounds.InsituIRandNMRspectraexhibitedthatthespeciesofAl-NH-P,Al-NH2andP-NH2onthesurfacewereresponsibleforthebasicityoftheamorphousoxynitrides.5,6However,fewreportsonnitrogen-incorporatedmolecularsievesappeared.Itisverydifficulttoavoidthecollapseofmolecularsieveframeworkwhennitri-dationofthemolecularsieveswasperformedinaflowofNH3

*Correspondingauthor.E-mail:dyj@dicp.ac.cn.Fax:86-411-4379143.†NaturalGasUtilizationandAppliedCatalysisLaboratory.‡StateKeyLaboratoryofCatalysis.

atatemperatureof800°C.Therefore,itisnoteasytoobtainnitrogen-incorporatedmolecularsieves.Meanwhile,finecrystal-linityandporestructuresofmolecularsieves,whichhaveconsiderablesignificanceinshape-selectivereactions,wouldbemaintainedwhennitridationwascarriedoutatlowtemperature.7Itiswell-knownthatonsometransitionmetals,suchasrutheniummetal,theammoniamoleculecanbeeasilydecom-posedintoNHx(x)1∼2)groups,whichareveryreactivespeciesfortheabovesubstitutionreaction.Inthisway,oxygenbridgingatomsinmolecularsievescouldbereplacedby-NH-speciesatlowtemperature.

Theobjectiveofthisworkistodevelopanimprovedmethodforpreparationofnitrogen-incorporatedSAPO-11zeolitebasecatalystatlowtemperatureandtocharacterizethestructurechangesduringnitridationbymeansofMASNMR,BET,andXRDmethods.ExperimentalSection

SamplePreparation.SAPO-11zeolitewassynthesizedbyasol-gelmethod,whichhasbeenpresentedelsewhere.8-10Theas-synthesizedSAPO-11wasfirstbakedatabout580°Cforabout4htoremovethetemplate,andthensoakedinRuCl3(purchasedfromJohnsonMattheycompany)solutiontogetaRuloadingof2.0wt%.Thenitwasdriedatambienttemperatureforseveraldays,anddriedfinallyinanovenat120°Cforabout4h(denotedasRu-SAPO-11).NitridationofSAPO-11molecularsievepowderwascarriedoutinatubularquartzreactorat400°Cinaflowofpureammonia.Theflowrateofammoniawascontrolledatca.30mL/min.Attheendofthenitridationprocess,thesamplewasslowlycooledtoroomtemperatureinanitrogenflow.Thepowdersobtainedafternitridizingfordifferenttimesof5,25,and50haredenotedasSAPO-11-5N,SAPO-11-25N,andSAPO-11-50N,respec-tively.TheMg-AlhydrotalciteusedinthispaperascontrastwasMg4Al2(OH)16CO3.Themethodofitssynthesisispresentedinref11.

BasicityEvaluation:TheContentofNitrogenandtheKnoevenagelCondensationReaction.Thecontentsofnitrogeninthenitrogen-incorporatedsamplesweredetectedbyalkaline

10.1021/jp020815pCCC:$25.00©2003AmericanChemicalSociety

PublishedonWeb01/17/2003

N-IncorporatedSAPO-11MolecularSieveJ.Phys.Chem.B,Vol.107,No.6,20031367

TABLE1:BETSurfaceAreaMeasurementsandNitrogenContentsforNitridedSAPO-11MolecularSieves

SAPO-11

SBET(m2/g)N(%,w/w)

a

SAPO-11-5N

1370.52

SAPO-11-25N

1260.77

SAPO-11-50N

1251.23

SAPO-11-5N-800a

281.25

1850

ThisisaSAPO-11withoutRunitridedat800°C.

Figure1.XRDpatternsfornitridedSAPO-11molecularsieves(a)calcinedSAPO-11;(b)Ru-SAPO-11;(c)SAPO-11-5N;(d)SAPO-11-25N;(e)SAPO-11-50N.

digestionwithmoltenNaOHat400°C,andtheresultingNH3wastitratedwith1NH2SO4;thedetailedprocedureisfoundedintheliterature.12Allreactionswerecarriedoutunderaninertatmosphere(N2),inaround-bottomflaskfittedwitharefluxcondenser.Theflaskwasimmersedinathermostatedwaterbathandthereactionmixturewasmagneticallystirred.Amixtureofpreviouslydistilledbenzaldehyde(4mmol)andethylcyanoacetate(4mmol)wasintroducedwithtolueneasasolvent(30mL)intotheflask.Oncethemixturereached50°C,0.2wt%ofthecatalystwasadded.Samplesofthereactionmixturewerethenperiodicallywithdrawnbya1µLmicrosyringeattimeintervalsof30,60,120,180,and240minandanalyzedbyShimadzugaschromatographGC-8A,equippedwithanFIDdetectorandanHP-5(cross-linked5%PHMEsiloxane)capillarycolumn.

MASNMRMeasurement.MASNMRexperimentswereperformedonaBrukerDRX-400spectrometer.Dataof31Pwereobtainedbymeasuringthesamplesusingafrequencyof161.97MHz,pulsewidthofπ/8,delayof2.0s,and100scans.For27Al-MASNMRexperiments,afrequencyof104.26MHandz400scansinsteadwereused.

XRDAnalysis.XRDpatternsweremeasuredinaironRigakuD/Max-rbdiffractometerusingCuKRradiationat40kVand50mA.The2θangleswerescannedfrom5°to50°atarateof2°min-1.

BETMeasurement.Thesurfaceareasofthedifferentnitrogen-incorporatedSAPO-11sampleswereobtainedbynitrogenadsorptionusinganASAP2010(Micromeritics)afteroutgassingthesamplesundervacuumat250°C.ResultsandDiscussion

NitrogenContent.Thenitrogencontentsinthenitrogen-incorporatedsamplesaresummedinTable1.ThenitrogencontentincreasedwiththetimeofnitridationforRu-dopedsamples.ItisnotablethatthecontentofnitrogenforSAPO-11-50Nthatwasnitridedatlowtemperatureof400°CwasalmostcomparabletothatofthesamplewithoutRuthatwasnitridedat800°C.

BETMeasurements.FromthedatashowninTable1,itisfoundthatthesurfaceareasofSAPO-11-5N,SAPO-11-25N,andSAPO-11-50Nsamplesdecreasedverylittleafternitrida-tion,whichiscomparabletothatoftheSAPO-11precursor,butthesamplelostmostsurfaceareaifnitrindationisconductedat800°C.

XRDAnalysis.TheXRDpatternsofsamplesarepresentedinFigure1.Fromthespectraitwasfoundthattheintensityofdiffractionpeakat2θ)8.09increased,therestofdiffractionpeaksatlower-angleremainedunchanged,andthepeakat2θ)19.89disappearedwhileanewpeakat2θ)20.65appearedafternitridation.Theseresultsshowthatsomechangesinchemicalcomposition(thiswasfurtherconfirmedbytheresultsof27Aland31PMASNMRmeasurements)andcrystal-linestructureoccurredduringnitridation.

BasicityEvaluationofNitrogen-IncorporatedSAPO-11Zeolites.TheKnoevenagelcondensationisusuallyusedinorganicsynthesistoproducealkenesfrommoleculescontainingcarbonylgroups.Thisreactioniscatalyzedbybasesundermildreactionconditions.Knoevenagelcondensationisoftenusedasaprobingreactiontoevaluatethebasicityofoxynitrides.Indeed,byreactingonemolealdehydewithonemolemoleculecontainingactivatedmethylenegroupswithdifferentpKavalueinthepresenceofabasecatalyst,itispossibletofindtheminimumpKavalueofthereactantfromwhichthecatalystbecomesactive.Thus,inthiswaycatalystswithdifferentbasicitiescanbeconvenientlycompared.5,13

HeretheKnoevenagelcondensationwasreaction1.Thatlittle

1368J.Phys.Chem.B,Vol.107,No.6,2003Figure2.ConversionvsreactiontimefornitridedSAPO-11andMg-Alhydrotalcite.

activitywasfoundforallsampleswithoutdopingRu,eventhroughnitridationcarriedoutinawholerangeofnitridationtemperaturesfrom400to800°C,meansthattherewerenobasicsitesinthesesamplesatall.

ThesamereactionwasperformedwiththeSAPO-11-5N,SAPO-11-25N,SAPO-11-50N,andMg-Alhydrotalcitecatalysts.FromtheresultsshowninFigure2,wecanseethatthenitridesweremuchmoreactivethanMg-AlhydrotalciteinKnoevenagelcondensation.Thenitrogen-incorporatedSAPO-11samplesgave8.3%,7.9%,and9.5%conversion,respectively,dependingonthetimeofnitridation.TheconversionofbenzaldehydeforSAPO-11-5NwashigherthanthatforSAPO-11-25N,duetoasignificantnumberof-NH2terminalgroupsexistinginthesamplesatthebeginningofnitridation.4Theselectivityofthiscondensationproduct(malononitrileben-zylidene)wasabout100%forallsamples,nofurtherreactionfollowingaMichael-typeadditionwasobserved.

AccordingtotheobservationfromBenitez,12freshlypreparedamorphousaluminophosphateoxynitride(AlPON)catalystsreadilyunderwenthydrationandhydrolysisunderatmosphericconditions.Hydrationandhydrolysiswereobservedfornitrogen-incorporatedSAPO-11molecularsievecatalysts,butthelossofbasicitywasnotobvious.FortheSAPO-11-50N,theconversionofbenzaldehydestillremainedatabout8%,whichwascomparabletothefreshcounterpart,eventhoughthesamplewasplacedunderatmosphericconditionsformorethantwomonths,implyingthatoxygenatomsintheframeworkofSAPO-11zeolitescouldbesubstitutedtosomelevelsbynitrogen-containinggroups.ThissubstitutionoccurringduringnitridationwouldbeconfirmedbythefollowingNMRmeasurements.MASNMRSpectrumCharacterization.High-resolutionmagic-anglespinningsolid-stateNMRisapowerfultooltoelucidatedetailsofmolecularsievestructure.Inthecaseofthisstudy,thecombinedresultsof27Al(Figure3)and31P(Figure4)MASNMRdataprovidedmoreinformationaboutthechangesthatoccurredaftertreatmentofSAPO-11witham-monia.

(1)27Al-MASNMRSpectra.The27Al-MASNMRspectraofthecalcinedSAPO-11andthenitrogen-incorporatedSAPO-11samplesareshowninFigure3.Theassignmentsofchemicalshifts,foundintheliterature,aresummedinTable2.Thepeakat38.83ppmwastypicallytheAlO4tetrahedronwithneighbor-ingPO4tetrahedron.14Thereweremanyopinionsabouttheassignmentof30ppm:oneascribedittofour-coordinatedAl,15-19anotherrelateditwiththefive-coordinatedAl,20-22andChenandco-workersconsidereditsassignmentaseitherfour-

Xiongetal.

Figure3.27AlMASNMR(a)calcinedSAPO-11;(b)SAPO-11-25N;(c)SAPO-11-50N.

Figure4.31PMASNMR(a)calcinedSAPO-11;(b)SAPO-11-25N;(c)SAPO-11-50N.

coordinatedAlorfive-coordinatedAlbasedonthecalcinationtemperature.23Thepeakat-14ppmwasascribedtooctahedralAlcoordinatedwith4PO4tetrahedronand2H2Omolecules.14The27Al-MASNMRspectraofSAPO-11-25N(Figure3b)andSAPO-11-50N(Figure3c)samplesindicatedthatthepeakat-9ppmisAlhexacoordinatedto4PO4and2NH3,implyingtheH2OofoctahedroninthecalcinedSAPO-11samplewassubstitutedbyNH3duringnitridation.Anewpeakat12.48ppmoccurredinthespectraofSAPO-11-25NandSAPO-11-50NandwasascribedtoAlpentacoordinatedwith4PO4and1NH3.4Itisemphasizedtonotethatthesignalsbetween84and126ppm,whichbelongedtothestructuralnitrogenAl(NxO4-x)-(x)1∼4),swelledupaftertheSAPO-11hadbeennitridedfor25h.After50hnitridation,twowidenewsignalsat108

N-IncorporatedSAPO-11MolecularSieve

TABLE31Pand272:AlSummaryMASNMR

ofAssignmentofChemicalShiftsforchemicalshift

(ppm)

assignment

ref27Al

38.83Al(OP)4

1430(1)four-coordinatedAl15-19(2)pentacoordinatedAl

20-22-14(3)Al(4OP,2Hdecidedbythecalcinationtemp.23-9Al(4OP,2NH2O)143)412.48Al(4OP,1NH3)88AlO2N224108AlN431

P

--29.623.65P(OAl)P(3OAl,OH)4

25,2627-6.4

PN4

4,28

and88ppmappearedinFigure3c,whichareascribedtoAltetracoordinatedwith4nitrogens(AlN4)andAltetracoordinatedwith2nitrogensand2oxygens(AlO2N2)respectively.24ThisimpliedthattheNatomsweredirectlybondedtothestructuralAlatoms.

(2)31P-MASNMRSpectrum.The31P-MASNMRspectraofthecalcinedSAPO-11anditssamplestreatedwithNH3at400°Cfor25and50hareshowninFigure4.ThesummedassignmentsofchemicalshiftsarelistedinTable2.Inthe31P-MASNMRspectrumofcalcinedSAPO-11sample(Figure4a),aresonanceat-29.60ppmisattributedtothetypicaltetrahedralPsitewith4neighboringAlO4tetrahedrons,25,26andthepeakat-23.65ppmshouldbeassignedtotetrahedralP(3AlO4,OH).27IncomparingthespectrumofcalcinedSAPO-11withthatofSAPO-11-50N(Figure4c),itisworthnotingthatanewpeakat-6.40ppmappeared,whichwasascribedtothetetrahedralPN4.ThisshiftisconsistentwiththereplacementofObyNinPO4tetrahedron.4,28TheresultsprovideevidencefortheincorporationofnitrogenintotheSAPO-11framework.Therefore,wecanconcludethatthenitrogenhasdirectlybondedtoPatoms,justastoAlatoms.4Oxynitrideshavethecharacterofbeingliabletohydrolysis,14butthenitrogen-incorporatedSAPO-11zeoliteswereexposedtothemoistairfordifferenttimesandfoundthattheactivityinKnoevenagelcondensationwaslostverylittleevenaftertwomonths.TheseresultsfurtherprovedthatthenitrogenhadenteredintotheframeworkofSAPO-11,becausetheterminalgroupsof-PNH2orhydroxyl-aminephysicallyadsorbedareliabletohydrolysis.Conclusion

Anewfamilyofsolidbasecatalysts,nitrogen-incorporatedSAPO-11molecularsieves,hasbeensynthesizedwithastartingmaterialofSAPO-11zeolitedopedwith2wt%Rubynitridinginanammoniaflowatalowtemperatureof400°C.Thenitrogen-incorporatedSAPO-11zeoliteswerecharacterizedbyhighspecificsurfaceareas,finecrystallinity,shape-selectivity,andbeingactivecatalystsfortheKnoevenagelcondensation.Thechangesinstructureoftheobtainedsampleswerestudiedbymeansof27Al-MASNMRand31P-MASNMRspectra;it

J.Phys.Chem.B,Vol.107,No.6,20031369

canbeconcludedthatnitrogen-containingspeciesincorporatedintotheframeworkofSAPO-11zeolite,inwhichsomeNatomsdirectlybondedtoAlandPatomsandformedAlN4,AlN2O2,andPN4groups.Thesespecies,togetherwiththeterminal-NH2group,areresponsibleforthebasicityofnitrogen-incorporatedSAPO-11zeolites.

Acknowledgment.SupportedfinanciallybytheNationalNaturalScienceFoundationofChina(20043001).ReferencesandNotes

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