Partial Oxidehydrogenation of Cyclohexane to Cyclohexene：环己烷的部分氧化脱氢制环己烯

PartialOxidehydrogenationofCyclohexanetoCyclohexeneoverNickelSupportedCatalystsModifiedbyRare-EarthMetalOxides

HanyM.AbdelDayem*1,2,3,MohamedA.Alomair1,SalwaS.Sadek3andHoshamSamir3

1DepartmentofChemistry,CollegeofScience,KingFaisalUniversity,Al-Hasa,P.O.Box380,Hofuf,31982,SaudiArabia(

monamohus@yahoo.ccom

)

2CenterofResearchExcellenceinPetroleumRefining&Petrochemicals(CoRE-PRP)KingFahdUniversityofPetroleum&Minerals,Dhahran,SaudiArabia

3ChemistryDepartment,FacultyofScience,AinShamsUniversity,Abassia,Cairo,Egypt

PartialoxidehydrogenationofcyclohexanetoproducecyclohexeneselectivelywasstudiedoverNiO/-Al2O3-doped–Ce,-La,-Gd,-Dycatalystswithaflowreactor.CatalystswerecharacterizedusingN2adsorption,X-raydiffractionandatomicabsorptionspectroscopy.Dopingofcatalystwithrare-earthmetaloxideswasfoundtohavehighpromotingeffect.NiO/Dy–modified--Al2O3exhibitedhigheractivityforcyclohexaneconversionandhigherselectivityforcyclohexene;cyclohexeneselectivityofup80%wasachieved.Modificationof-Al2O3supportednickeloxidewithCe,Gd,Dyinhibitedtheformationofbulknickelaluminateandleadstohighernickelcontentascomparedtounmodifiedcarrier.

KEYWORDS:Cyclohexane,gasphase,NiOcatalyst,rareearthmetaloxides,-Al2O3

Introduction

Cyclohexenerepresentsanimportantbasicmaterialformanyvaluableintermediatesproducts,cyclohexeneisusedinmanufacturingof

\o"Adipicacid"

adipicacid

,

\o"Hexahydrobenzoicacid(pagedoesnotexist)"

hexahydrobenzoicacid

,

\o"Maleicacid"

maleicacid

,

\o"Cyclohexanol"

cyclohexanol

and

\o"Cyclohexeneoxide(pagedoesnotexist)"

cyclohexeneoxide

[1].Productionofcyclohexeneusingcyclohexaneasfeedstockisanidealeconomicalreaction,wherecyclohexanehaslesspollutionproblemsandhiscostislowerby5–6timesthanotherfeedstocksuchascyclohexanol.Accordingtoliteraturefourtechniqueshavebeenusedintheoxidationofcyclohexane:liquidphase[2],photo-catalyticoxidation(inliquidorgasphase),anodicoxidationbysparkdischarge(ANOF)[4]andgasphaseusingoxygen(air,ozone)[5,6].

Theliquidphaseoxidationofcyclohexaneischaracterizedbyahighenergyconsumptionandexpensiveinvestments.Ontheotherhand,photo-catalyticoxidationandANOFtechniquesarenoteasilyhandleatthelargescale.Gas-phaseoxidativedehydrogenationofcyclohexane(ODH)overmetaloxidesopensnewreactionroutecapableofreplacingclassicalliquid-phaseoxidationprocessesfortechnicalproductionofcyclohexanol/cyclohexanone.Therearevarietyofformulations(viz.,organometalliccomplexes,zeolitesbasedcatalysts,MCMbasedcatalysts,organometalliccomplex/SBA-15,metaloxides/TiO2basedcatalysts)proposedfortheoxidationofcyclohexaneusingthefourtechniquesmentionedabove.Theproblemofdevelopingefficientcatalystremainsunsolvedduetorelativelylowselectivitytodesiredproduct.

Oxidativedehydrogenationofcyclohexanebymolecularoxygentocyclohexeneusingmetaloxidescanbeapromisingreactionpathwayforthetechnicalproductionofcyclohexanol/cyclohexanone.ThemostpromisingcatalystsamongthemetaloxidesthathavebeentestedforcyclohexaneoxidationingasphaseareNiO/Al2O3[5],reduced-NiMoO4[7],metalvanadates(V2O5/Al2O3)[8],andCuOx[9].InthecaseofNiO/Al2O3catalyst,itisclearthatthecatalystselectivitydependsonthefollowingparameters:thenatureofnickeloxidespecies,theoxidationstateofnickel,theformationofbulknickelaluminateandthenickelcontentonthesupportsurface[5].

Theinfluenceofalkalimetal(Na,K,Cs)dopingonthesurfaceandcatalyticpropertiesof-Al2O3supportednickeloxideinODHofcyclohexanewasinvestigated[5].However,itwasfoundthatdopingofthecatalystwithalkalihavenopromotingeffect.Itwasreportedthatalkalidopingofthealuminasupportpriortonickelleadstolowernickelcontentandtoenhancetheformationofbulknickelaluminate.WheretheformationofbulknickelaluminatehasanegativeeffectonbothNiO/Al2O3activityandselectivity.

Oxygenconductingoxidessuchasceria,whenusedasadopantofsupportedtransitionmetaloxidescatalysts,areknowntosignificantlymodifymetal-supportinteractiontoenhancecatalyticperformance,duetotheireasyformationofoxygenvacancies[10,11],improveddispersionofmetals[12,13],andexcellentcapabilitiesofoxygenstorage/transport[12-14].Dopingof-Al2O3supportednickeloxidewithceriummetalscanprovideawaytoinhibittheformationofnickelaluminateandtomodifythenatureofnickeloxidesactivespecies.

Inthiswork,theinfluenceofrareearthmetals(Ce,La,Gd,Dy)dopingonthesurfaceandcatalyticpropertiesof-Al2O3supportednickeloxideintheODHofcyclohexanewasinvestigated.Inaddition,theinfluenceofdopingof-Al2O3carrierwithdifferentpercentageofceriumoxidewasalsostudied.

2.ExperimentalSection

Twocatalystsserieswereprepared;firstseries(I)isceriummodified-Al2O3supported-NiO,withdifferentweightpercentagesofCeO2(1%,5%and10w/w%).Secondseries(II)isarareearthmetals(La,Gd,Dy)modified-Al2O3,with10%w/w.

Ceriummodified--Al2O3,(xCeAl)werepreparedbywetimpregnationof-Al2O3(thermallyactivated,porousparticle,specificsurfacearea;169.4,m2/g)withasaltsolutioncontainingappropriateamountsofCe(NO3)3.6H2O(Aldrichsigma,99.9%pure)toobtaindifferentweightpercentagesofCeO2(1%,5%and10w/w%).Inthiscase,theamountofceriumnitratewasdissolvedinthesmallestpossiblevolumeofdeionizedwater.Aslurryof-Al2O3powderin200mldeionizedwaterwasthenadded.Theresultingslurrywastakentodrynessbycontinuousstirringandheatingat70oC.Thesolidobtainedwasthenkeptinanovenovernightat120oC,crushedinanagatemortarandcalcinedfor4hat500oC.TheresultingsolidsdenotedasxCeAl;where(x=1.0,5.0and10.0,respectively).La-,Gd-andDy-modified-Al2O3with(10%w/w)werealsopreparedbytheimpregnationmethodfollowingthesamerouteasusedinpreparationCeAl.Theresultingsolidsaredenotedas10LaAl,10GdAland10DyAl.

Nickeloxidesupportedonrareearthmetal-modified-Al2O3werepreparedaccordingtoprocedurereportedinrefs.[5,15].Therareearthmetal-modified-Al2O3aswellasanunmodified--Al2O3wereimpregnatedinanaqueoussolutioncontainingamountoffreshlypreparednickelhexamminateat318Kfor24hundercontinuousstirring.Thenickelloadedsamplewaswashedwithmethanolforseveraltimes,filtered,driedat393Kfor12handcalcinedat873Kfor6h.seriesIcatalystwerelabeled(Ni1.0CeAl,Ni5CeAlandNi10CeAl).Ontheotherhand,seriesIIcatalystswerelabeled(Ni10LaAl,Ni10GdAlandNi10DyAl),wherenickeloxidesupportedunmodified-Al2O3,catalystdenotedNiAl.

X-raydiffractionmeasurementswereperformedemployingPhilipsX’PertMPD(multipurposeX-raydiffractometer)employingCuK1,2radiation(λ=1.5405Å)for2θanglesvaryingfrom10°to80°.BETsurfaceareasweremeasuredusingthesurfaceareaanalyzerQuantaChromeNova2200porosityandporesizedistributionwereobtainedaccordingtoBarrett-Joyner-Halenda(BJH)method.

ThenickelcontentinthecatalystswasdeterminedbyAASonaUnicam939Englandsystem.Thecatalystsweredissolvedinnitricacid(68%)anddilutedwithdistilledwatertoconcentrationwithinthedetectionrangeoftheinstrument.AnattemptwascarriedouttodeterminethefreeNiparticlesonthesurfaceofsupportbymilddissolutionof0.2gcatalystin4NHNO3for2handthenanalysisofthesolutionbyAAS.

Catalyticactivitymeasurementswereperformedemployingaconventionalfixed-bedreactorsystemusingairasthecarriergasforthecyclohexanefeed.Thefollowingreactionconditionswereemployed:catalystweight,0.2g;flowrateofair,100ml/min.temperaturewasadjustedforeachcatalysttohavecyclohexaneconversion(5%),tocompareselectivityforcyclohexaneoxidativedehydrogenationproducts.Analysisofreactantsandproductswasperformedbyanon-lineShimadzuGC-17AwithFIDandTCDdetectorsusingtwocolumns;fusedsilicaFFAP-capillary(50m×0.32mi.d.,AD:0.46)(forcyclohexane,cyclohexene,cyclohexadieneandbenzene)andHaySepD(80/100)(forCOx).Furtherdetailscanbefoundelsewhere[7].Nohomogeneousgasphasereactionforconversionofcyclohexanewasobservedatusedreactiontemperatures.

3.ResultsandDiscussion

Nickeloxidecontentofthecatalystmeasuredbyatomicabsorptionspectroscopy(AAS)aresummarizedinTable1.Itisclearthat,therareearthoxides(Ce,Gd,Dy)-dopedcatalystsshowamuchhigherNiOcontentthantheunmodified-Al2O3carrier.InthecaseofNi10DyAlandNi10CeAlcatalyst,theNiOisnearbymonolayerdistribution,wherethetheoreticalmonolayernickeloxidecorrespondingtonickeloxideloading(12.17%w/w).Consideringthatashapeprojectionspaceof9.09ÅcorrespondingtoaNiO{100}unit[16]andmeasuredspecificsurfaceareaofrareearthoxides-modified--Al2O3samplesareintherange(90-95m2/g).Thissuggestedthatmostofthenickelprecursorisdistributedmainlyonthesurfaceandnotinthebulkofthesupportoxide.Thissuggestionconfirmedfromdissolutiondataresults;ThefreefractionsofNiparticleswhichcouldbeextractedin2NHNO3solution,areincludedintheTable1.ItisclearthatmildHNO3dissolvedmorethan70%oftotalNiOcontentinthecaseof10NiCeAl,10NiDyAland10NiGdAlcatalysts.However,InthecaseofparentNiAlcatalyst,itisclearthatthemajorfractionofnickelparticlesremainscombinedstronglywithorinthesupport.Thiscouldbeexplainedbyapartialblockingofthe-Al2O3porousbyrare-earthmetaloxidesespeciallyinthecaseofDy-,Ce-,Gd-modified-Al2O3catalysts.

Table1:Atomicabsorption(AAS)anddissolutiondata

Catalyst

TotalNi(w/w%)

Dissolutiondata

DissolvedNiw/w%

SurfacefreeNi(%)

NiAl

6.3

2.0

31.7

Ni5.0CeAl

8.1

5.2

64.2

Ni10CeAl

11.3

8.6

76.1

Ni10LaAl

5.0

1.9

38.0

Ni10DyAl

12.1

9.7

80.1

Ni10GdAl

8.3

6.2

74.8

Allthemeasuredadsorption/desorptionisothermsovervarioussampleswereoftypeIIofBrunauer’sclassification[17].VarioussurfaceparametersderivedfromtheobtainedisothermsaresummarizedinTable2.whichincludedthespecificsurfacearea[SBET(m2/g)],theBET-Cconstant,thetotalporevolumeasmeasuredat0.95P/Po[Vp(ml/g)]andtheaverageporeradiusassumingthatalltheporeswerecylindrical[r(Å)].Adecreaseinthespecificsurfaceareaof-Al2O3wasobservedafterimpregnationeitherwithNiOorrareearthmetaloxides,thisdecreaseismorepronouncedinthecaseofNiAlcatalyst.Whichwasaccompaniedbyadecreaseintheporevolume(Vp).Inaddition,asignificantincreaseintheaverageporeradius(r)wasobservedforallcatalysts.Thisincreaseinrmaybeinterpretedasarisingfromthepenetrationofnickeloxideand/orrareearthoxidesmoleculesinthealuminapores,therebycausingsomeexpansiontoslightlywiderporesize.

Table2:Surfacecharacteristicparametersof-Al2O3support,pureNiO/-Al2O3catalystandrareearthmetalsmodified-NiO/-Al2O3catalysts.

Sample

CBET

SBET

(m2/g)

Vp

(ml/g)

r(Å)

BJH

SCum

(m2/g)

method

VCum

(cc/g)

-Al2O3

115.6

169.4

0.350

31

224.1

0.36

NiAl

109

102.2

0.195

38

136.7

0.21

Ni10CeAl

116

105

0.213

41

144.7

0.23

Ni10CeAl

105

108.2

0.216

40

147

0.23

Ni10LaAl

80

108.2

0.217

40

145.2

0.23

Ni10DyAl

126

129.9

0.241

37

168

0.25

Ni10GdAl

275

134.3

0.241

36

166.7

0.25

TheX-raydiffractionpatternsoftheparentNiAl,CeAlandvariousNixCeAlsamplesarepresentedinFigure1.TheXRDpatternoforiginalNiAlindicatesthepresenceof-Al2O3(JCPDSfile,10-0452),NiOofdspacing2.40,2.11and1.47Å(JCPDSfile,78-0643)andNiAl2O4ofdspacing2.85,2.43and1.55Å(JCPDSfile,78-1601).InthecaseofNixCeAlcatalysts(Figure2)-Al2O3andNiOphaseswerealsodetected.Inaddition,thepeakscharacteristicofCeO2ofdspacing3.12,2.28,1.90and1.63weredetectedinthepatternsofNi5.0CeAlandNi10CeAl.However,thefollowingdifferenceswereobservedi)thepeakscharacteristicsofNiOinNixCeAlcatalystshaveahigherintensitythanthatobservedinthepatternofNiAlsample,ii)asignificantincreaseintheintensityofthepeakcharacteristicofNiOat2θ=43.25o(dspacing=2.11)withincreasingceriumcontentinthesesamples;andiii)anewpeakcharacteristicofNiOwasappearedat2θ=75.54o(dspacing=1.25Å)inthediffractionpatternofNi10CeAlcatalyst.

Figure1:PowderX-raydiffractionpatternofNiO/-Al2O3catalyst.Peaksmarkedbythesymbols"●","◊"and"‡"indicatethosepeaksassignedtoAl2O3,NiO,andNiAl2O4,respectively.

Figure3displaysthediffractionpatternsofNi10LaAl,Ni10GdAl,Ni10DyAl;besidethatofNiAlandNi10CeAlforcomparison.InthecaseofNi10LaAlcatalystonly-Al2O3phasewasobserved.Moreover,nopeakscharacteristicsofnickeloxideorlanthanumoxidephasesweredetected.TheseresultscanberelatedtothelowernickelcontentobservedinNiLaAlsampleanalyzedbyAAS.Onthecontrary,Gd2O3phaseofdspacing3.12,2.69,1.90,1.64and1.35Å(JCPDSfile,11-0608)andDy2O3phaseofdspacing3.34,3.03and1.79Å(JCPDSfile,19-0436)weredetectedinthediffractionpatternsofNi10GdAlandNi10DyAl,respectively.Ontheotherhand,theintensityofthepeakscharacteristicofNiOaremorepronouncedinthepatternofNi10DyAl.Nofeaturesofcrystallinenickelaluminateandrareearthmetalsaluminatecanbeobservedinthediffractionpatternsofrareearthmetals(Ce,La,Gd,Dy)modified-Al2O3understudy.

Figure2:PowderX-raydiffractionpatternofcerium-modified--Al2O3supportednickeloxidecatalyst.Peaksmarkedbythesymbols"●","◊"and"□"indicatethosepeaksassignedtoAl2O3,NiO,andCeO2,respectively.

.

Figure3:PowderX-raydiffractionpatternofLa,Dy,Gd-modifed--Al2O3supportednickeloxidecatalyst.Peaksmarkedbythesymbols"●","◊","*"and"+"indicatethosepeaksassignedtoAl2O3,NiO,Dy2O3andGd2O3,respectively.

Theconversion(%)ofcyclohexaneandselectivityofreactionproductsatisoconversion(5%)forallthestudiedcatalystsarelistedinTable3.InthecaseofCe-,Gd-,Dy-modified-NiO/Al2O3thereisadecreaseinreactiontemperature(T(~5%))requiredtokeepcyclohexaneconversionconstantatca~5%comparingwithNiAlcatalyst.However,anincreaseinT(~5%)wasobservedinthecaseofLa-modified-NiO/Al2O3.WhichindicatedthatdopingofcatalystsbyCe,orLaorGd,enhancedthecatalyticperformanceincyclohexaneconversion.SuchbehaviourcanindicatethattheobservedhighercyclohexaneconversionofrareearthoxidesmodifiedcatalystmightduetothesecatalysthaveahighersurfaceareathanNiAlcatalyst(Table2).AscanbealsoseeninTable3,atiso-conversion(~5%);Ni10DyAlexhibitedthehighestselectivitytowardscyclohexene.TheaboveresultsconfirmedthatdopingofNiO/Al2O3bytheserareearthoxides(Ce,Dy,Gd)leadstoasignificantmodificationsincatalyticperformanc.

Table3:ComparisonofperformancesofdifferentcatalystsforcyclohexaneODH

catalyst

Ta(~5%)

(oC)

Conversion

(%)

S(C6H10)b(%)

S(C6H6)c(%)

S(COx)d

(%)

NiAl

312

4.8

67.1

18.5

14.4

Ni5CeAl

308

5.2

72.3

12.4

15.3

Ni10CeAl

301

5.6

78.6

12.1

9.3

Ni10LaAl

317

4.9

60.6

17.9

21.5

Ni10DyAl

296

5.2

82.2

10.2

7.6

Ni10GdAl

305

5.1

75.3

14.0

10.7

Cyclohexanemolarfeedrate:8x10-3mol/hg,W=0.2g,flowrate=100ml/min.

aTemperatureatwhichthecyclohexaneconversionis~5%,bSelectivitytocyclohexene,cSelectivitytobenzene,dselectivitytoCOandCO2.N.b.traceamountsofcyclohexadiene(<0.3%)wereobservedforallstudiedcatalysts,inthistableyieldofcyclohexadienewasaddedtothatofbenzene.

RefereeingtoXRDresults,theformationofbulknickelaluminatewasonlyobservedinthenon-dopedcatalyst;NiAl.Itiswellknownintheliterature[5]thatthenickelfromnickelaluminatespeciesisverystabletoreductionitneedstemperatureabove800oC.whichmeansattheusedreactiontemperatures(280-330oC)bulknickelsitesfromnickelaluminatedonotparticipateinreduction/reoxidationcyclesaccordingtoMarsvanKrevelenmechanism(MVK)[18]andremaincatalyticinactive.Thepromotingeffectofrareearthoxides(Ce,GdandDy)canbeexplainedduetothefactthatearthmetalsinhibitstheformationofnickelaluminateandcausestheformationofNiOinmuchlargerproportion(seeAASresults)especiallyinthecaseDy-modifiedcatalyst(Ni10DyAl).AccordingtoliteraturethereductionofbulkNiOcausesattemperaturearound300oC[19]namely,thesesitecanobeyMVKreduction/oxidationmechanism.DissolutiondataindicatesthatNiOspeciesisweaklyinteractedordonotformanysignificantbondwithsupportintherareearthoxides(Ce,Gd,Dy)-modifiedcatalysts.Consequently,theirreductionissimilartothatofunsupportedNiO[20].However,basedonbothdissolutionandN2adsorptionresultsinthecaseofNiO/Al2O3itseemsthatmostofNiOspeciesarelocatedintheporesofthecatalyst,soitneedshighertemperaturetoreactivatedaccordingtoMVKmechanism.

4.Conclusions

Basedontheresultsobtainedinthisworkonecanconcludethat,comparedwithNiO/Al2O3theCe-,Dy–andGdmodifiedcatalystsaremoreactiveandefficientinselectiveproductionofcyclohexenebyODHofcyclohexane.Atsmallerconversionabout5%,theselectivitiesofcatalyststowardscyclohexeneincreaseintheorderNi10LaAl<NiAl<Ni10GdAl<Ni10CeAl<Ni10DyAl.ThehighercatalyticperformanceofrareearthoxidesmodifiedcatalystscanbeattributedtothatceriummetalsdopingofaluminacarrierpriortonickelimpregnationleadstohighernickelcontentascomparedtounmodifiedAl2O3carrier.Whiletheformationofbulknickelaluminateisinhibited,whichhasanegativeinfluenceonboththeactivity