固体超强酸

EffectiveSO422/TiO2–ZrO2forpreparationandhydrolysisof1,3-propanediolacetals

JinboNi•MinWu•ZhaohuiYang•ChangfeiBuQinHe

•

Received:4November2009/Accepted:22March2010/Publishedonline:14April2010

´miaiKiado´,Budapest,Hungary2010ÓAkade

AbstractSO42-/TiO2–MxOy(M=Zr,Ce,La)werepreparedbytheprecipita-tion-impregnationmethodandcharacterizedbyX-raypowderdiffraction(XRD),Fouriertransforminfrared(FT-IR),andtemperature-programmeddesorption(NH3-TPD).Catalyticactivitieswereevaluatedintheacetalizationof1,3-propanediol(1,3-PD)withacetaldehydeandhydrolysisof2-methyl-1,3-dioxane(2MD).SO42-/TiO2–ZrO2(STZ)exhibitedthebestcatalyticactivitybothintheacetalizationandhydrolysis.WiththemolarratioofZr4?/Ti4?=1:4,thehighestyieldswere96.45%in3hand93.68%of2MDhydrolyzedin18h,incontrasttotheyieldslowerthan60%byusingothersuperacids.TheseresultsareconsistentwiththestrongestacidityofthesuperacidcontainingZr4?amongpreparedsuperacidscontainingothercations.Keywords

1,3-propanediolÁSolidsuperacidÁAcetalizationÁHydrolysis

Introduction

1,3-propanediol(1,3-PD)isapromisingchemicalintermediateusedinorganicsynthesisandasamonomerfortheproductionofbiodegradablepolymerspolytrimethyleneterephthalate(PTT),whichisanovelpolyesterwithgoodstretchandrecovery,andgooddyeabilityintextileapplications[1,2].Nowadays1,3-PDisproducedbyachemicalrouteandabiotechnologicalmethodthroughthemetabolismprocessoccurringinglycerolbymicroorganismssuchasCitrobacter,

J.NiÁM.Wu(&)ÁC.BuÁQ.He

SchoolofChemistryandChemicalEngineering,SoutheastUniversity,Nanjing2111,People’sRepublicofChinae-mail:wuminnj@163.com

Z.Yang

XinAnjiangVocationSchool,Hangzhou311600,People’sRepublicofChina

123

338J.Nietal.

Clostridia,Enterobacter,andKlebsiella[3,4].Recently,attentionhasbeenfocusedonthebiotechnologicalrouteduetotheenvironmentalbenefitsandutilizationofarenewablefeedstock.Inthebio-process,however,awell-knownproblemisthatthefinalconcentrationof1,3-PDinthefermentationbrothislow(50–130g/L)[5].Bearingthelowvolatilityandhydrophilicpropertiesof1,3-PD,recoveryofthisdiolfromthedilutesolutionsiscriticalforthedevelopmentofacommerciallyviableprocess.Conventionaldistillationresultsinalargeconsumptionofenergy[6],butmoreseriousexpenseinenergyforchromatographybecauseintheprocessofchromatography,thesolutionof1,3-PDisdilutedratherthanconcentrated,owingtothelowselectivityandcapacityofresinsoradsorbents[7].Alternatively,solventextractionmaysignificantlyreducethecostoftheproductseparation.However,thedistributionof1,3-PDintoextractionsolventsdoesnotappeartobegoodenoughtomakethisapproachefficient[8].Anotherwaytotacklethisproblemistoconvert1,3-PDintoasubstancewithouthydroxylgroupsandthentorecoveritbymeansofliquidextraction.Asuitablereactionispresentedbelow(Scheme1)usinganacidiccatalyst.Itincludesareversiblereactionbetweenacetaldehydeand1,3-PDtoformsubstituted2MD[9].Asimilarprocessingstrategyhasalreadybeensuccessfullyusedintherecoveryof1,2-propanediolfromdilutesolutions[10].

AsshowninScheme1,liquidacidsandion-exchangeresinshavebeenreportedinthecyclicacetalizationof1,3-PD[11–13].Thecatalyticefficiencyofliquidacidsishigh.However,theyarehardtohandleinpracticeduetotheirtoxicandcorrosivenature[14].Ionexchangeaffordseasierseparationandrecovery,buttheefficiencyislower[9].Hence,itisessentialtodiscoveranewcatalystsuperiortothem.Promisingcatalystsforthisreactioncanbeasolidacid,astable,regenerableandactiveoneatmoderatetemperatures.Overthepastfewyears,solidsuperacidshaveattractedconsiderableattention.Amongthose,SO42-/TiO2(ST)ismostimportant,andalsoholdsgreatpromiseforanumberofreactionsofindustrialimportance[14–16].

Recently,sulfatedtitaniamodifiedbyMxOy(M=Ce,Zr,La)hasbeenreportedtobeanefficientcatalystinaldolcondensationforitshighstrengthofacidityandhighcatalysisactivityatmildtemperatureinadditiontonon-toxicity[17–19].However,toourknowledge,thereisnoreportontheapplicationoftheminthesynthesisandhydrolysisof2MD.Therefore,inordertofindasuperiorcatalystforthereactionextractionof1,3-PD,inpresentpaper,attractedbyitsnon-toxicity,weselectitforatrialtoinvestigatewhetheritworks.

OH3CHHOCH2CH2O+HOH+H3CO+H2OCH2Scheme1Reactionof1,3-PDwithacetaldehyde123

Acetalization1,3-propanediolbySO42-/TiO2–ZrO2339

ExperimentalCatalystpreparation

Allchemicalreagentsintheexperimentwereofanalyticalgradeandusedwithoutanyfurtherpurification.SO42-/TiO2–MxOy(SO42-/TiO2–La2O3=STL,SO42-/TiO2–CeO2=STC,SO42-/TiO2–ZrO2=STZ)werepreparedbyprecipitationandimpregnationmethods[20,21].

FormulatedMx(NO3)y(M=Ce,Zr,La)waschargedintoasolutionofalcohol/H2O(100mL/25mL)ataround80°Cinanoilbath,andthenthesolutionofTi[OCH(CH3)2]4(22.5gin20mLalcohol)wasgraduallyadded.Afterdropwiseadditionof28.5%ammoniaandcontinuingtorefluxfor4h,theprecipitateswerefilteredout,washedtwicewithdeionizedwater,andthendriedat100°Cfor24h.Passingthrough100#mesh,thepowdereddryprecipitateswerefurthertreatedbytheincipientwetnessimpregnationmethodinasolutionofH2SO4(1mol/L),16mL(2mLH2SO4/1gdriedprecursor),atambienttemperaturefor24h,andsubsequentlydryingat100°Cfor6handcalciningat550°Cfor3hinamufflefurnace.

Catalystcharacterization

FT-IRwascarriedoutbySpectrometers-750(Nicolet,USA)withKBrpellets.TheXRDwasmeasuredbyaXD-3AX-RayDiffractometer(Shimadzu,USA),employingCuKaradiation(40kV,30mA)and2hof10–90°.NH3-TPDexperimentswereperformedonaCHEMBET-3000(Quantachrome,USA),usingammoniaasadsorbate,andthethermalconductivitydetector(TCD)asdetectorforcomparingtheacidityofsomecatalysts.AsforTCD,0.2gofcatalystwasatfirstdriedat350°Cfor1.0hatatmosphereof20v/v%O2/Hewithaflowingrateof45mL/min,andthenat100°C,gaseousNH3wasintroducedatarateof30mL/min.TheadsorptiontimeofNH3wasabout60min.Thereafter,thegasflowwasswitchedtoHeat30mL/minandcontinuedtoflowfor20mininordertoflushouttheexcessNH3.Thereactortemperaturewasthenelevatedto850°Catarateof10°C/minintheflowofHe(45mL/min).Synthesisof2MD

3gSO42-/TiO2–MxOy(M=Ce,Zr,La)and76.8g(1mol)99%1,3-PDwereaddedintoafour-neck500mLroundbottomflaskequippedwithastirrer,arefluxcondenser,adroppingfunnel,andathermometer,andthen53.3g(1.2mol)99%acetaldehydewasaddeddropwiseat50°C.Afterstandingfor3h,thecatalystwasfiltrated.Theorganicphasemainlycontaining2MDwaswashedwith20mLwater,neutralizationwithNaHCO3andthendriedwithanhydrousNa2SO4for30min.Thecolorlessdistillateoforganicspecies2MD,withacharacteristicodorwascollectedat80–84°C.Itcontained98.9%of2MD,analyzedbyaGC90Agaschromatograph(GC,NanJingRenHua,China).ThepHofthewaterlayerwas

123

340J.Nietal.

measuredbyPHS-3CpHmeter(ShanghaiPrecisionScientificInstrumentCo.,Ltd.,China).

Hydrolysisof2MD

Thehydrolysisreactionwaspresentedasfollows:20.0g2MD,4.0gH2Oand0.4gSO42-/TiO2–MxOy(M=Ce,Zr,La)weremixedat100°Cina50mLround-bottomflaskequippedwitharefluxcondenserunderstirringofamagneticstirrer.ThehydrolysisproductswereanalyzedbyaGCevery2h.

Resultsanddiscussion

TheFT-IRspectra(Fig.1)ofthesamplesweresimilartoeachother.Thestrongabsorptionpeakat3410cm-1andmildpeakat1625cm-1correspondingtotheflexionvibrationbandofH2Omoleculewereascribedtothephysisorbedwaterinthecatalyst.Asharpbandat1380cm-1wasallobservedinthecurveofSTC,STL,andSTZsampleswhichbelongedtothestretchingfrequencyofthefreesulfategroups[22].Moreover,thespectrumofSTC,STL,andSTZallcontainedthreeshouldersbutsplitbandsataround1210,1120,1070cm-1incurves,respectively,whichwerethecharacteristicfrequenciesofabidentateSO42-coordinatedtometalssuchasTi4?[23].SuchstructurescouldstronglywithdrawelectronsfromtheneighboringTications,resultinginlotsofelectron-deficientmetalcentersontheTicationsthatactedasstrongLewisacidsites.Theresultscorrespondedtothoseofotherworkers[24]andwerebelievedtobethesourceinthegenerationofalargeamountofsurfaceacidicsitesonsolidacidsofsulfatedmetaloxides.

3410STC121011201070Transmittance (%)STLSTZ1625138035003000250020001500-11000500Wavenumbers (cm)Fig.1FT-IRspectraofSO42-/TiO2–MxOy(M:Ti=1:4)calcinedat550°C123

Acetalization1,3-propanediolbySO42-/TiO2–ZrO2341

TheXRDofSTandSO42-/TiO2–MxOy(M=Ce,Zr,La)samplesbeforereactionareshowninFig.2.Thediffractogramsshowedthatthesamplesusedinthisstudywereallpresentinanatasephasesandwellcrystallized.Thetypicalfeaturesofthediffractogramwereobservedbetween20and40°(2h),withasharpandintensepeak(25.4°)dominantlyindexedtotheanatasephase(PDF-ICDD-4921),norutilephasewasdetectedintheseoxides,alltheseindicatingthatcalcinationat550°Cwassuitable.Moreover,nodiffractionpeakduetoMxOywasfound,suggestingthatnosolecrystallinephasecausedbyMxOywhichwerepresentinahighlydispersedmannerandhadstronginteractionwithTi4?[25].

ReferencingtheNH3-TPDtechnique,theacidityofsuperacidspreparedinthispaperwasevaluatedbecausethestrengthofaciditywasimportanttothecatalyticactivityaccordingtotheacidiccatalyticmechanismofacetalizationandhydroly-zation.TheresultsaresummarizedinFig.3.Thehightemperaturepeakat770°CforSTZ,thepeakat810°CforSTLandthatat800°CforSTCwereduetothedecompositionofSO42-inthesecatalysts[26].Inadditiontothedecompositionpeak,awelldefinedplateauarea(478–602°C)andanasymmetricpeakat210°CwereobservedforSTZ.TheformerwasattributedtodesorptionofNH3fromthestrongacidsitesandthelatterfromtheweaklyadsorbedNH3.Betweenthesetwopeaks,therewasaplateauarea(271–478°C)whichwasattributedtodesorptionofNH3fromthemedium-strongacidsites.AsimilardistributionofacidsiteswasfoundinSTL,NH3desorptionpeakofstrongacidsitesinSTLwaslocatedinthearea(474–580°C)whichwassmaller,andtheplateauarea(271–474°C)arisedfromthemedium-strongacid.Moreover,asforSTC,theplateauarea(271–470°C)andlittlepeak(470–575°C)correspondedtoNH3releasedfrommedium-strongandstrongacidsites.InthepreviousstudyanincreaseintheaciditymightbeduetotheformationofmixedoxideswithinteractionbetweenTiandZrorLaorCeatoms

Intensity(a.u.)STCSTLSTZSO4/TiO22-1020304050607080902theta/degreeFig.2X-raypowderdiffractionspectraofSO42-/TiO2,SO42-/TiO2–MxOy(M:Ti=1:4)calcinedat550°C123

342

77081028000800J.Nietal.

30000Intensity(a.u.)2600024000220002000018000100210271STZSTLSTC478602580575600700800474470500200300400Temperature/°CFig.3ThedistributionofthenumberandthestrengthofacidsitesintheSO42-/TiO2–MxOy(M:Ti=1:4)measuredbyNH3-TPDthroughoxygenlinkages(Ti–O–ZrorTi–O–LaorTi–O–Ce)[27].AspresentedinFig.3,theintensityofthepeakcorrespondingtostrongacidsiteswasmoreintenseinthecaseofSTZsampleincomparisontoSTLandSTC.TheseresultsprovidedanimpressionthattheincorporatedsupportoxideplayedavitalroleindeterminingthestrengthandnatureoftheacidsitesofSTcatalysts.InFig.3,wecouldconcludethatincorporationofzirconiatotitanialedtostrongeracidsites,lanthaniaconducedtomediumstrengthacidsites,ceriainducedtheweakestacidsites.

Theacetalizationreactionswerecarriedoutat50°Cfor3h.AsshowninFig.4,byusingSTZ,theconversionof1,3-PDwasthehighest,andtheorderofconversionmagnitudewasfound,STZ[STC[STL.ForSTZ(Zr4?:Ti4?=1:4),theyieldof2MDreached96.45%after3h,whilecatalyzedbySTL(La3?:Ti4?=1:4)andSTC(Ce4?:Ti4?=1:4)theyieldswereonly57.79%and41.35%after3h,respectively.Theseresultsindicatedthattheactivityofcatalystwashighlydependentonitsacidstrengthdistribution.Astheacetalizationwasatypicalacidcatalyzedreaction[28],manyresearchershadreportedontherelationshipbetweentheactivityandtheacidityofcatalysts[29].Itwasreportedthatthesurfacesitesofstrongacidappearedtoberesponsiblefortheconversionof2MD[30].Inthepresentstudy,asdiscussedabove,theorderofactivityofSTC,STLandSTZwascoincidenttothatofthestrengthofthestrongacidsitesofthesesuperacids.Therefore,weconsideredthattheacidityofsuperacidsplayedakeyroleinthecatalysis.

Inaddition,asshowninFig.4,theeffectofdifferentratiosofZr4?/Ti4?on2MDconversionwasobvious.Withamolarratioof1:4(Zr4?/Ti4?),theconversionof2MDquicklyattained94.09%in1h,96.45%for3handthereafterleveledoffindicatingtheequilibriumstateofreactionsystem,whereaswithfurtherdecreaseofZr4?,theconversiondecreasedslightly.Forexample,astheratiosofZr4?toTi4?were1:2,1:8and1:16,theconversionof2MDwere83.56%,90.06%and76.65%at1h.Inareference[30],itwasreportedthatthesample(Zr4?/Ti4?=1:4)had

123

Acetalization1,3-propanediolbySO42-/TiO2–ZrO2343

100 Zr:Ti=1:2 Zr:Ti=1:8 Zr:Ti=1:4 Zr:Ti=1:16 STZ(washed with water) STL STC Sulfuric AcidConversion of 1,3-PD(%)8060402000.00.51.01.52.02.53.0Reaction time/hFig.4EffectsofSO42-/TiO2–ZrO2andSO42-/TiO2–CeO2(Ce:Ti=1:4),SO42-/TiO2–La2O3(La:Ti=1:4)onacetalizationof1,3-PDandacetaldehydeat50°Cmaximumacidityandimpartedhighestsurfacearea.Thismaybeoneofthereasonsfortheseresults.Furthermore,thehighcatalyticactivitymaycomefromthesolidaciditselforthesulfateleachingfromthesolidacid.Inordertomakeitclear,twoexperimentswerecarriedout.3gfreshSTZ(Zr4?:Ti4?=1:4)wasfirstwashedinalargeamountofwaterunderultrasonicfor0.5h,andthenusedforacetalization.ThecatalyticactivityisshowninFig.4.AsshowninFig.4,theconversionof1,3-PDwas85.46%at0.5hwhileitwas96.12%at3h.Theconversionchangedappreciablyafterwashing.Meanwhile,inordertodetecthowmuchsulfuricacidleachedfromtheSTZ,theamountofsulfuricacidwasmeasuredinthewashingsolutioninaccordancewiththestandardofAssociationofAnalyticalCommunities(AOAC990.08).Itwasfoundthattherewasabout2.0mgfreeSO42-leachingfrom1gSTZinthesolution.Next,weevaluatedthecontributionofleachingsulfuricacidtothecatalyticactivityandtheacetalizationwascarriedoutwiththesameconcentrationoftheleachingsulfuricacid.AsshowninFig.4,theconversionof1,3-PDwasonly12.32%at0.5h,whilst25.82%at3h,farlowerthanthatofSTZ.TheseresultsindicatedthatthecontributionofleachingsulfuricacidwassmallandSTZdominatedthecatalyticactivity.Anyway,thecatalystsofdifferentratioofZr4?/Ti4?wereinvestigatedinourlaterwork.

Theefficiencyofsolidsuperacidinthehydrolysisof2MDwasdemonstratedat100°Cfor18h.Haoetal.[31]reportedthattheequilibriumconstantoftheacetalizationwasdecreasedwhenthetemperatureincreased.Therefore,theequilibriumconstantof2MDhydrolysiswasraisedwiththetemperatureascending.Wecouldinferthatthecatalyticefficiencyofthecatalystswassimilartothatinacetalization.Theconversionof2MDvs.thehydrolysistimeisshowninFig.5.AsshowninFig.5,a93.68%1,3-PDconversionwasachievedin18hwhenSTZused

123

344

1009080 Zr:Ti=1:2 Zr:Ti=1:4 Zr:Ti=1:8 Zr:Ti=1:16J.Nietal.

STL STC Resin 732#Conversion of 2MD (%)706050403020100012345671011121314151617181920Reaction time/hFig.5EffectsofSO42-/TiO2–ZrO2andSO42-/TiO2–CeO2(Ce:Ti=1:4),SO42-/TiO2–La2O3(La:Ti=1:4)onthehydrolysisof2MDat100°Cinreactionsystem.Differently,underthesameconditions,theconversionof1,3-PDofSTL(La3?:Ti4?=1:4)andSTC(Ce4?:Ti4?=1:4)wereonly44.93%and35.32%,respectively.Asacomparison,weconductedthehydrolysisof2MDbyemployingthestrongacidion-exchangeresin732#(SinopharmChemicalReagent).Theresultsindicatedthatthehydrolysisratewasmuchlower.Itmeantsuperacidsweremoreefficientforthehydrolysisof2MD.InFig.5,whentheratiowas1:4,theconversionof1,3-PDcouldarrive93.68%in18handthereactionratewasmuchhigherthan1:2,1:8and1:16.Theseresultscorrespondedwiththatinacetalizationandaccordedwithourinferenceaforesaid.

Asmentionedabove,oneoftheadvantagesofsolidacidcatalystsisthefacilityofseparationandreusability.Accordingly,wedesignedanexperimenttoinvestigatecatalyticstabilities.STZ(Zr4?:Ti4?=1:4)wasselectedasanexample.Aftera3-hreaction,STZwasseparatedanddriedat70°C,andthenreused.ThevariationofpHatdifferentreactiontimeisshowninFig.6aandtheconversionof1,3-PDisillustratedinFig.6b.AsshowninFig.6a,thevariationofpHinaqueoussolutioncanbedividedintotwostages.Intherangeof0to0.5h,thepHdecreaseddramaticallywhenthecatalystwasadded,irrespectiveofthetimesofSTZused,thenthepHslightlydecreased.ThefirststagecorrespondedtothenormalLewisacidpropertiesinwater,theLewisacidsitesbeingconvertedtoBrønstedacidsitesviaprotontransferinthepresenceofwater[32–34].However,theslightdecreaseofpHduringthereactionmightbealsorelatedtothevolumedecreaseofaqueousphaseastheoilphaseproduced.Ontheotherhand,asshowninFig.6a,thepHincreasedwiththerecycletimesincreasingwhichwaspossiblyresultedfromthelossofsulfate.Meanwhile,asshowninFig.6b,theconversiongraduallydecreased

123

Acetalization1,3-propanediolbySO42-/TiO2–ZrO23.53.02.5 First time Third time Fifth time10080345

Second time Fourth timeaConversion(%)bpH2.01.51.00.50.00.00.51.01.52.02.53.0604020012345Reaction time/hNumber of runsFig.6aThepHvariationtrendsofacetalizationinfivereactioncycles.bThecatalystactivityofacetalizationof1,3-PDandacetaldehydeinfivereactioncyclesasthereactioncyclesincreased.Theseresultsindicatedthattheleachingofsulfateacidactuallyoccurredinthereactionsmentionedabove.What’smore,withthenumberofreusetimesincreasing,thecatalyticactivitydecreasedobviously.Theseresultsindicatedthatthecatalyticactivitydecayedafterseveraltimesofcatalystsreuse.However,evenafterfiveruns,STZstillexhibitedarelativelyhighconversion(86.32%)whichindicatedthatSTZstillhadahighstability.

Asaresult,comprehensivelytakingtheresultofwashingexperiment(Fig.4)intoaccount,wethoughtthattheleachingofsulfuricaciddidaffectthecatalyticactivityofSTZ,butthemaincontributionofactivitywasfromtheacidsitesonSTZ.

Conclusion

Efficientandenvironmentalfriendlyproceduresforthereactiveextractionof1,3-propanediolcatalyzedbysolidacidswereinvestigated.SO42-/TiO2–MxOy(M=Zr,Ce,La)werepreparedbyacoprecipitationmethodandimpregnatingwithsulfuricacid.TheresultsofFT-IR,XRD,NH3-TPDclearlydemonstratethatSO42-/TiO2–MxOyhadhighacidstrengthandallthesampleswereallinanatasephase.OurworkshowedthatSTZexhibitedhighestcatalystactivitywhenthemolarratioofZr4?toTi4?was1:4,theconversionof2MDinacetalizationand1,3-PDinhydrolysisreactionwas96.45%in3hand93.68%in18h,respectively.Meanwhile,STZshowedgoodcatalyticstabilities,thesulfateleachingwasonlyabout2mgfrom1gSTZ.AlthoughthelossofsulfatedidaffectthecatalyticactivityofSTZ,theconversionof2MDinacetalizationcatalyzedbySTZafterwashinginwaterfor3hwas96.12%whichchangedappreciablycomparedwithfreshSTZ.What’smore,thecatalyststillhadhighconversion(86.32%)afterfiveruns.Accordingtotheirhighcatalyticactivitiesandgoodstabilitiesinthereversibleacetalizationreaction,thecatalystSTZ(Zr4?/Ti4?=1:4)wasfoundtobeapromisingcatalystintheseparationof1,3-PDfromfermentationbroth.

123

346J.Nietal.

References

1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.

WillkeT,VorlopK(2008)EurJLipidSciTechnol110:831GuptaMN,RaghavaS(2007)ChemCentJ1:17

NakamuraCE,WhitedGM(2003)CurrOpinBiotechnol14:454

PapanikolaouS,FickM,AggelisG(2004)JChemTechnolBiotechnol79:11MaBB,XuXL,ZhangGL,WuM,LiC(2009)ApplBiochemBiotechnol152:127AmesTT(2002)USPatent6,361,983

WilkinsAE,LoweDJ(2004)USPatent6,812,000

XiuZL,ZengAP(2008)ApplMicrobiolBiotectnol78:917

HaoJ,XuF,LiuHJ,LiuDH(2006)JChemTechnolBiotechnol81:102BroekhuisRR,LynnS,KingCJ(1994)IndEngChemRes33:3230MalinowskiJJ(2000)BiotechnolProg16:76

SchwenkE,FleischerG,WhitmanB(1938)JAmChemSoc160:1702GuptaN,Sonu,KadGL,SinghJ(2007)CatalCommun8:1323

YinHL,TanZY,LiaoYT,FengYJ(2006)JEnvironRadioact87:227

NodaLK,AlmeidaRM,GoncalvesNS,ProbstLFD,SalaO(2003)CatalToday85:69ReddyBM,SreekanthPM,YamadaY,KobayashiT(2005)JMolCatalA:Chem227:81SugunanS,RadhikaT,SujaH(2003)ReactKinetCatalLett79:27DevulapelliVG,WengHS(2009)CatalCommun10:1711

NakajimaA,ObataH,KameshimaY,OkadaK(2005)CatalCommun6:716FurutaS,MatsuhashiH,ArataK(2004)CatalCommun5:721BastowTJ,WithfieldHJ(1999)ChemMater11:3518

SaurO,BensitelM,SaadABM,LavalleyJC,TrippCP,MorrowBA(1986)JCatal99:104RoscoeJM,AbbattJPD(2005)JPhysChemA109:9028MaoW,MaHZ,WangB(2009)JHazardMater167:707

ZhaoH,BenniciS,ShenJ,AurouxA(2009)ApplCatalA:Gen356:121LinCH,LinSD,YangYH,LinTP(2001)CatalLett73:121

TanabeK,SumiyoshiT,ShibataK,KiyouraT,KitagawaJ(1974)BullChemSocJpn47:10KawabataT,MizugakiT,EbitaniK,KanedaK(2001)TetrahedronLett42:8329KumarR,KumarD,ChakrabortiAK(2007)Synthesis2:299

´F,BachelierJ,LavaheyJC(1993)JMolCatalA:Chem84:283LahousseC,AboulaytA,Mauge

HaoJ,LiuHJ,LiuDH(2005)IndEngChemRes44:4380

KhderAS,El-SharkawyEA,El-HakamSA,AhmedAI(2008)CatalCommun9:769ComaA(1995)ChemRev95:559

MishraMK,TyagiB,JasraRV(2003)IndEngChemRes42:5727

123