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AppliedCatalysisA:General392 (2011) 57–68 ContentslistsavailableatScienceDirect Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata Liquid phase aldol condensation reactions with MgO–ZrO and shape-selective 2 nitrogen-substituted NaY WenqinShena,GeoffreyA.Tompsetta,KarlD.Hammonda,RongXinga,FulyaDoganb,ClareP.Greyb, W.CurtisConnerJr.a,ScottM.Auerbacha,c,GeorgeW.Hubera,∗ aDepartmentofChemicalEngineering,UniversityofMassachusetts,686N.PleasantSt,Amherst,MA01003-3110,UnitedStates bDepartmentofChemistry,StateUniversityofNewYorkatStonyBrook,StonyBrook,NY11794-3400,UnitedStates cDepartmentofChemistry,UniversityofMassachusetts,Amherst,MA01003-9336,UnitedStates a r t i c l e i n f o a b s t r a c t Articlehistory: Thealdolcondensationreactionsoffurfural/hydroxymethylfurfural(furfurals)withacetone/propanal Received20July2010 inwater–methanolsolventswerestudiedoverthesolidbasecatalystsMgO–ZrO2,NaYandnitrogen- Receivedinrevisedform13October2010 substitutedNaY(Nit-NaY).Thereactionswereconductedat120◦Cand750psigofHeinbatchreactors. Accepted20October2010 Available online 28 October 2010 Nit-NaYexhibitedcatalyticactivityforaldolcondensationcomparabletoMgO–ZrO2andmuchhigher thanthatofNaY,indicativeoftheincreasedbasestrengthafterreplacingthebridgingoxygenwiththe lowerelectronegativitynitrogenoverNit-NaY.Thealdolcondensationoffurfuralswithacetoneproduces Keywords: twodifferentproducts,themonomerandthedimer.Themonomerisformedfromreactionoffurfurals Nitrogen-substitutedzeolite MgO–ZrO2 withacetone.Thedimerisformedfromreactionofthemonomerwithfurfurals.MgO–ZrO2hadahigher Aldolcondensation selectivitytowardsdimerformation.Incontrast,Nit-NaYwasmoreselectivetowardsthemonomer High-throughputreactor productduetothecagesizeintheFAUstructure,indicatingthatNit-NaYisashapeselectivebasecatalyst. Shape-selectivebasecatalyst Increasingthewaterconcentrationinthefeedsolutionorincreasingthefeedconcentrationledtoboth increasedcatalyticactivityanddimerselectivity.TheNit-NaYcatalystwasnotstableandlostcatalytic activitywhenrecycledduetoleachingoftheframeworknitrogen.Differentcharacterizationtechniques, includingXRD,highresolutionAradsorptionisotherm,basicsitestitration,CO2TPD-MS,TGAand29SiSP MASNMR,wereusedheretocharacterizethefreshandspentcatalysts.TheresultsshowthatNit-NaY maintainsonlypartoftheFAU-typecrystalstructure.Furthermore,thebasestrengthoverNit-NaYwas foundtobebetweenthatofMg2+–O2− pairandMg(OH)2.ThereactionmechanismoverNit-NaYwas discussed. © 2010 Elsevier B.V. All rights reserved. 1. Introduction occlusionwithbasicsaltsormetals[8–11],organicbasegrafting [12–15]andnitridation[16–23].Thesemethodsallhavetheirlim- Solidbasecatalystshaveattractedmuchattentionbecauseof itationsforpreparingshape-selectivebasiczeolites.Forexample, their potential to replace corrosive homogeneous base catalysts ion-exchangemethodsproduceonlyweakbases[24].Impregna- in industrially important reactions such as isomerizations, alky- tion or occlusion may cause the blockage of zeolite pores [25]. lations,condensations,additions,cyclizations,transesterifications Organic base grafting may only modify the basicity of the sur- and oxidations [1–5]. The use of solid catalysts provides a more faceorcausetheblockageofthemicroporesofzeolitesduetothe environmentallybenignprocessandreducesthecostofprocess- large organic groups, and thereby does not produce the desired ing due to the ease of separation and recycling. Base strength, shapeselectivity[26–28].Nitridationofzeoliteshasbeendiscussed stability,andshapeorsizeselectivityarethemostdesiredprop- as a promising option for the generation of truly shape selec- erties for solid base catalysts. Shape selective acidic zeolites are tive base catalysts [29,30]. Nitrided zeolites have been prepared thebackboneofthepetroleumindustry[6].However,shapeselec- bytreatingzeoliteswithammoniaatelevatedtemperatures.This tive basic catalysts have not been used commercially yet. Basic resultsinanincreasedbasestrengthoftheframeworkduetothe nanoporousmaterialscanbepreparedbyvariousmethodsinclud- substitution of bridging oxygen with the lower electronegativity ingion-exchangewithalkalimetalcations[5,7],impregnationor nitrogen [29,30]. Nitridation has been performed on a range of porousmaterialsincludingmesoporousMCM-41[31–33]andSBA- 15[34,35],aluminophosphates(AlPOs)[17,36],siliconsubstituted ∗ aluminophosphates(SAPOs)[37,38]andzeolites[17,18,39,40].The Correspondingauthor.Tel.:+14135450267;fax:+14135451647. E-mailaddress:[email protected](G.W.Huber). nitrogen-substituted materials have been evaluated by Knoeve- 0926-860X/$–seefrontmatter© 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.10.023 58 W.Shenetal./AppliedCatalysisA:General392 (2011) 57–68 Scheme1. Basecatalyzedaldolcondensationoffurfuralswithacetoneorpropanal. nagel condensations of benzaldehyde with molecules containing asMgO,CaO,Mg–Al-oxide,andMgO/ZrO preparedbyimpregna- 2 active methylenic groups with different pKa values and showed tion.However,thereisnopreferencetothemonomerorthedimer basicactivityandstability[17,18,32,36,38,40].However,thereare productbyusingeitherhomogeneousNaOHortheheterogeneous noreportsontheshapeselectivityoverthesenitrogen-substituted MgO–ZrO catalyst.Thispaperwillpresentthefirstreporttoapply 2 materials. nitrogen-substituted NaY for the reactions from bio-renewable A sustainable biomass economy requires the development of sourcestogeneratevalue-addedchemicalsandfuels.Theobjec- different integrated steps for the conversion of biomass-derived tivesofthisresearchare(1)todemonstratetheincreasedcatalytic feedstocksintoarangeoffuelsandchemicals[41].Manyofthese activitybynitrogensubstitutionoverNaYusingaldolcondensation steps involve basic catalysts. One key step in a biorefinery con- of furfurals with acetone or propanal as probe reactions and (2) cepttoproducedieselandjetfuelrangealkanesinvolvesC–Cbond todemonstratethatthenitrogensubstitutedzeolitescanbeshape formingreactionsbyaldolcondensations.Biomasscanbedepoly- selectivebasecatalysts.TheNit-NaYcatalystwillbecomparedwith merized into C5 and C6 sugars by hydrolysis-based approaches. theknownnon-shapeselectiveMgO–ZrO catalystusingdifferent 2 However,tomakedieselandjetfuelrangealkanesthatarebetween reactionconditions. C8toC15,C–Cbondformingreactionsmustoccur.Oneapproach to form these larger alkanes involves first a dehydration step to 2. Experimental producefurfurals.Thefurfuralsarethenreactedwithanaldehyde orketonebyaldolcondensationtoformC8toC15alkaneprecur- 2.1. Catalystpreparation sors.Theselargercompoundsthenundergohydrodeoxygenation reactionstoproducethefinalalkanes[42–45].Basecatalysisisthe MgO–ZrO was prepared by precipitation of Mg(NO ) ·6H O 2 3 2 2 keystepinC–Cbondformingreactionstoproducethechemicals (Aldrich) and ZrO(NO ) ·6H O (Aldrich) aqueous solution with 3 2 2 orfuelswithtargetedmolecularweight. NaOHasdescribedelsewhere[44,47,48].Typically,50.9gofmag- The products from the condensation of furfural with ace- nesiumnitrateand4.04gzirconylnitrateweredissolvedin1Lof tone or propanal are shown in Scheme 1. 5-Hydroxymethyl deionizedwater.A25wt.%NaOHaqueoussolutioninaseparation furfural(HMF)willundergothesamechemistryasfurfural.Ace- funnel was dropped into the mixed solution with vigorous stir- tone reacts with one molecule of furfural (or HMF) to form ringuntilthepHwasequalto10asmonitoredbyapHmeter.The furfural (or HMF) acetone monomer 4-(2-furyl)-3-buten-2-one resultinggelwasagedatroomtemperaturefor72handseparated (or4-[5-(hydroxymethyl)-2-furanyl]-3-buten-2-one),followedby byvacuumfiltration.Theprecipitatewasthoroughlywashedwith condensationwithanotherfurfural(orHMF)toformfurfural(or deionizedwateratleastfourtimesanddriedinanovenat120◦C HMF)acetonedimer1,5-di-2-furanyl-1,4-pentadien-3-one(or1,5- overnight.TheMgO–ZrO catalystwasobtainedbycalcinationof 2 bis[(5-hydroxlmethyl)-2-furanyl]-1,4-pentadien-3-one).Propanal theresultingdryprecipitatein100mL/minairflowat600◦Cfor reactswithonefurfural(orHMF)toformfurfural(orHMF)propanal 3hwitha3hheatingramp.NaY(CBV100withaSi/Alratioof2.55, monomer3-(2-furyl)-2-methylacrolein(or3-[(5-hydroxymethyl)- ZeolystInternational)wasusedasastartingmaterialtoprepare 2-furanyl]-2-methylacrolein)andthemonomercanfurtherreact nitrogen-substitutedNaY.Inatypicalsynthesis,about5gNaYwas with another propanal molecule to produce furfural (or HMF) loadedintoaquartztubewitha1in.outerdiametersupportedby propanal dimer 5-(2-furyl)-2,4-dimethyl-2,4-pentadienal (or 5- aquartzfrit.Beforenitridation,NaYwasfirstdriedunderaflow [(5-hydroxlmethyl)-2-furyl]-2,4-dimethyl-2,4-pentadienal). of 100mL/min nitrogen at 110◦C for 2h with a 0.5◦C/min ramp Aldol condensation of furfurals with acetone has been stud- rate,andthendehydratedat400◦Cfor6hwitha0.5◦C/minramp iedinhomogeneousethanol/watersolutions[46]andwater/THF rate.Itwasfoundbyuspreviouslythathighammoniaflowrates biphasic system [42] catalyzed by NaOH. It has also been stud- are crucial for substantial nitrogen substitution while maintain- iedinaqueoussolutionscatalyzedbysolidbasecatalystsfollowed ingacceptablemicroporosityandcrystallinity[21].TheNaYwas byhydrogenation[43]andthecombinedaldolcondensationwith nitridedinaflowof1L/minammonia(Airgas,anhydrous,99.99%) hydrogenation over bi-functional catalyst 5wt.% Pd/MgO–ZrO2 at850◦Cwitha1.5◦C/minrampfor24h.Aslowheatingratewas [44]. MgO–ZrO2 showed the highest catalytic activity and excel- usedheretoremovewatergraduallyandavoiddealuminationof lentrecyclingabilitycomparedwithothersolidbasecatalystssuch thezeolite[22].Theresultingmaterialwascooledtoroomtem- W.Shenetal./AppliedCatalysisA:General392 (2011) 57–68 59 peratureundernitrogenflowandstoredinadesiccator,labeledas atedonlybasedonthedisappearanceoffurfural(orHMF)andthe Nit-NaY. selectivitytoaldolproducts.Thedefinitionsareasfollows: molesoffurfuralconsumed 2.2. Catalyticreactionandanalysismethods furfuraldisappearance(%)= ×100 molesoffurfuralinput Thecatalystswereevaluatedinahighthroughputparallelauto- molesofmonomer maticreactorsystem(CAT24,HELGroup).Thesystemtemperature, monomerselectivity(%)= ×100 molesoffurfuralconsumed pressure,andgasflowratewerecontrolledandmonitoredthrough WinISOE670software.Inatypicalrun,2mLreactantsolution,a 2×molesofdimer suitableamountofcatalyst,andastirbarwereputintoaglasstube dimerselectivity(%)= ×100 molesoffurfuralconsumed reactorwithatotalcapacityof3mL.Allofthepreparedglasstube reactors(maximum24)wereloadedintoahighpressurechamber. selectivityofaldolproduct(%) Insidethechamber,stainlesssteelcoolingtipswereinsertedinto thetopofeachglasstube,effectivelypreventingthemixingofvapor molesofmonomer+2×molesofdimer = ×100 betweenglassreactors.Beforestartingthereaction,thechamber molesoffurfuralconsumed was purged with helium for 10min and pressurized to 500psig. = monomerselectivity+dimerselectivity Next,thechamberwasheatedto120◦C.Moreheliumwasintro- ducedtomaintainthefinalpressureat750psig.Thereactionwas keptat120◦Cfor16hor24handstirredat850rpm.Afterreac- molesofmonomer tion,thewholechamberwasquenchedinanicebathimmediately monomeryield(%)= ×100 molesoffurfuralinput tostopthereaction.Theresultingmixtureinsideeachglasstube reactorwastransferredtoa25mLsamplebottle.Theglassreactor molesofdimer waswashedthreetimeswithmethanol(HPLCgrade)todissolve dimeryield(%)= ×100 molesoffurfuralinput theproductsdepositedontheglasswall.SamplesforHPLCanal- ysiswerefilteredbyasyringefilter(Waters0.45(cid:2)mPESsyringe Fortheproductdistribution: membranefilter). molesofmonomer ThestabilitytestwasconductedinaParrbatchreactor(model monomer(%)= ×100 molesofmonomer+molesofdimer #4566) with an external temperature and stirring controller (Model #4835). A 75mL reactant solution (5wt.% total organ- molesofdimer ics,furfural/acetone=1bymoles,methanol/water=2byvolume) dimer(%)= ×100 molesofmonomer+molesofdimer and 250mg catalyst were added to the reactor. The reactor was firstpressurizedto200psigwithheliumandthendepressurized, repeatingthreetimestoreplaceallairinthereactorwhilechecking 2.3. Characterization thereactorforleaks.Next,thereactorwaspressurizedto500psig, heatedto120◦C,andpurgedwithadditionalheliumtomaintainthe X-raydiffraction(XRD)patternsweremeasuredwithaPhilips reactionpressureat750psig.Thereactionwaskeptat120◦Cfor X’Pert professional diffractometer using a Cu K˛ source with a 48handwassampledevery6–8h(about1mLpersample)from wavelengthof1.54A˚.Anacceleratingvoltageof45kVandacurrent the sampling port during the run. The samples were diluted ten of40Awereused.Aslitwidthof0.5◦wasusedonthesource.Scans timesbymethanolandfilteredbyusingasyringefilterbeforeanal- werecollectedusinganX’Celeratordetector. ysis.Afterthefirstrun,thereactorwascooledinanicebath.The ThehighresolutionAradsorptionisothermsofNaYandNit-NaY spent catalysts were collected by vacuum filtration and washed wereconductedatQuantachromeLaboratoriesusingcommercially thoroughlywithmethanol.Afterdryinginair,thespentcatalyst availableAutosorbinstruments.Therawdatawasreducedbyusing wasplacedintothereactorforasecondrunundertheexactsame theAutosorbsoftware(version1.53,Quantachromeinstrument). operatingconditions. TheBETsurfaceareasandmicroporevolumesofNaYandNit-NaY Anadditionalexperimentwasdesignedtotestifanyhomoge- werecalculatedbymultipointBETmethodandVt-methodinthe neousreactionoccurredduetoleachedchemicalsfromthecatalyst. relativepressurerangeof0.02–0.3,respectively.TheBETsurface Thecatalystwasfirstrefluxedatthereactiontemperatureinwater areasofthefreshandspentMgO–ZrO catalystwerecalculatedby 2 for2handwasquicklyfilteredatthattemperature.Next,theexact usingmultipointBETmethodoftheN adsorptionisothermsinthe 2 amountofreactantsandmethanoltomaintainthemethanol/water relativepressurerangeof0.01–0.5.Theporesizedistributionsof volumetricratioof2aswedidinthestabilitytestwereadded.The NaYandNit-NaYwereobtainedbyDFTmethodusingabuilt-inDFT mixture underwent the reaction at 120◦C for 48h and sampled kernel(Arat87K-Zeolite/Silica:Cylinder,pore,NLDFTads.branch every4–8h. model). The reactants and their condensation products were identi- Carbon dioxide chemisorption and temperature programmed fiedbygaschromatography/massspectrometry(GC/MS;Shimadzu desorption (TPD) were conducted by a thermo-gravimetric GC/MS-2100withaDB-5columnfromAlltech)ifpracticalorby analyzer(TAInstruments,SDTQ600)combinedwithamassspec- 1H NMR and analyzed by HPLC (Shimadzu, with an SPD-20AV trometer (eXtorr Inc.) operating at 10−5Torr. In a typical run, UVdetectorandareversephaseC-18columnfromAgilent).The approximately30mgofthefreshorspentcatalystwasplacedinto monomeranddimerstandardswerepreparedbyNaOHcatalyzed analuminasamplecontainer,weighed,andequilibratedat50◦C. aldol condensation at room temperature and purified by chro- Thesamplewasthenheatedto300◦Cunder100mL/minhelium matography.FurfuralandHMFwereanalyzedatUVwavelength for1hwitha10◦C/minramptodegasandthencooledto50◦Cuntil of254nmandthemonomer/dimerproductswereanalyzedata steady weight was established. The second gas, CO , was intro- 2 UV wavelength of 330nm; acetone and propanal are transpar- ducedintothechamberwithaflowrateof60mL/minforabout enttoultravioletlight.Thesideproductsaredifficulttoidentify 3h.Afterchemisorptionequilibriumwasestablished,theCO flow 2 since both the reactant (furfural or HMF) and aldol products are wasstoppedandthephysicallyadsorbedCO wasremovedunder 2 bio-degradable.Someofthedegradationproductsareinsolublein helium flow until no further weight change was observed. Next, methanol.Therefore,theperformanceofthecatalystswasevalu- thesamplewasheatedfrom50◦Cto800◦Cwithaheatingrateof 60 W.Shenetal./AppliedCatalysisA:General392 (2011) 57–68 20◦C/minandmaintainedat800◦Cfor40–80min.TheCO mass 2 spectrumfortheeffluentwassimultaneouslymonitored.Theback- ground control experiments were conducted in helium (without theCO adsorptionstep)forcalculationoftheweightlossdueto 2 thedecompositionofthecatalystitself.ForthespentMgO–ZrO 2 catalyst, an additional run was conducted in which the catalyst wasfirstcalcinedinaflowofairat600◦Cfor1h,followedbyCO 2 chemisorptionandTPDwiththesameprocedureasabovetoeval- uatethechangeofbasicsitesaftertworunsofthealdolreactionat 120◦Cforatotal91h.TheCO uptakewascalculatedbasedonboth 2 theweightchangesduringchemisorptionandTPD.Theweightgain duetoCO chemisorptionwasequaltotheweightdifferencebefore 2 chemisorptionandafterthereleaseofthephysicallyadsorbedCO . 2 TheweightlossduetotheCO TPDwasequaltothedifferenceof 2 totalweightlossduringTPDbetweensampleswithandwithout theCO chemisorptionstep.TheCO uptakeswerealsocalculated Fig.2. ThehighresolutionAradsorptionisothermsandtheporesizedistribution 2 2 basedontheCO desorptionpeakfromthemassspectra.Thepeak ofNaYandNit-NaY.TheresultsindicatethattheNit-NaYcatalystmaintainspore 2 sizedistributionandpartofthecrystalstructurewiththelossofporevolumeand areaswerecalibratedbyusing1mLSTPCO2ofastandardvolume surfaceareatoadegree.TheunitofdV(w)iscm3/A˚/g. loop. Basicsitetitrationwasconductedunderinert(helium)atmo- sphereaccordingtothemethodusedbyVidruketal.[49].Typically, [50]. Here, the cystallinity of Nit-NaY was about 52% based on a catalystpowder(50mg)wasplacedintoaflaskcontaining50mL comparisonofthereflectionpeakareasof(533),(642),and(660) o-xylenewith0.1wt.%phenolphthaleinandstirredatroomtem- betweenNaYandNit-NaY.Inaddition,asignificantreductionof perature for 4h in helium. The mixture was then titrated with theintensityratioofthereflectionscorrespondingto(220)and 0.01Mbenzoicacidino-xyleneuntilthecolorchangedfromcolor- (311)wasobserved,whichiscausedbythesubstitutionofnitrogen lesstopink.Theconcentrationofbasicsiteswascalculatedbased intothezeoliteframework[22].Therewerenofurtherstructural ontheamountofacidconsumed.Theinertatmospherewasused changes in the spent Nit-NaY after reaction at 120◦C for 75h in hereinordertoavoidtheerrorintroducedbyCO inair. amethanol–watersolvent.Fig.2showstheArisothermsofNaY 2 The29Sisingle-pulse(SP)magicanglespinning(MAS)nuclear and Nit-NaY and the corresponding pore size distributions. NaY magneticresonance(NMR)analysisofthefreshNit-NaYandspent andNit-NaYpresentsimilarisotherms.Theporesizeof7.2A˚ for Nit-NaYwasperformedonaVarian360MHzspectrometerwith NaYwasmaintainedoverNit-NaY.However,theAradsorptionof 4mmprobeusing14kHzMAS.A90◦ pulselengthof2.4(cid:2)swas Nit-NaY is less than that of NaY, indicating the reduction of the used with a recycle delay of 30s, as established in our previous porosity.Microporevolumes(Vt-method)ofNaY,Nit-NaYandthe experiments[22].Thechemicalshiftsreportedarewithrespectto spentNit-NaYare0.36mL/g,0.13mL/gand0.17mL/g(N2adsorp- neatliquidtetramethylsilane(TMS)at0ppm. tion),respectively.TheBETsurfaceareasofNaY,Nit-NaYandthe spentNit-NaYare954m2/g(C =−45),337m2/g(C =−4.3)and BET BET 368m2/g (C =−41, N adsorption), respectively. Although the 3. Results BET 2 BET equation is not valid for these microporous materials, this shows the relative reduction of surface area of ∼2/3, similar to 3.1. Catalystcharacterization the micropore volume. The results from the high resolution Ar adsorption isotherm and XRD are consistent, showing that Nit- 3.1.1. PowderXRDandhighresolutionadsorptionisotherm NaYmaintainsonlypartofthecrystalstructure,contributingtothe Fig.1showstheXRDpatternsofNaY,Nit-NaYandthespentNit- reductionofthesurfacearea,microporevolume,andcrystallinity. NaY.Nit-NaYretainedallthereflectionpeaksofNaY,indicatingthe The XRD pattern of the calcined MgO–ZrO catalyst (as seen FAUstructurewasmaintainedaftertreatmentinammoniaflowat 2 850◦Cfor24h.However,theintensitiesofthereflectionsoverNit- in the supplementary materials Fig. S1) shows two phases, cor- respondingtocubicMgOandtetragonalZrO ,respectively.After NaYweresignificantlyreducedcomparedtothatofNaY,indicating 2 reaction, the catalysts underwent a phase transition, where the reducedcrystallinity,consistentwithourpreviousreports[21,22]. spentMgO–ZrO containedMg(OH) andZrO .ZrO retainedhigh The percentage of crystalline material remaining in Nit-NaY can 2 2 2 2 crystallinitywithoutnoticeablechangesinthespentcatalyst,while beestimatedaccordingtothemethodintroducedbySolacheetal. the Mg(OH) showed broad XRD peaks, indicative of poor crys- 2 tallinity.TheBETsurfaceareaofthefreshMgO–ZrO is102m2/g 2 andincreasedto194m2/gforthespentMgO–ZrO ,whichmightbe 2 attributedtoaphasetransition(MgOtoMg(OH) )oraroughening 2 fromthefreshcatalysttothespentcatalyst. 3.1.2. CharacterizationofbasicsitesbyCO -TPDandtitration 2 Fig.3showstheCO TPD-MSprofilesoverthefreshandspent 2 MgO–ZrO catalysts.ThespentMgO–ZrO wascollectedaftertwo 2 2 runsoffurfuralcondensationwithacetoneat120◦Cfor91hand washedwithexcessmethanol.ItwasseenthatthespentMgO–ZrO 2 after regeneration has almost the same amount of CO uptake 2 at190◦CcomparedwiththefreshMgO–ZrO catalyst.Thespent 2 MgO–ZrO showedonlyaveryweakCO uptakepeakat120◦C, 2 2 but a sharp CO peak at 400◦C related to the decomposition of 2 adsorbedfurfuralanditsdegradationproducts,andabroadpeak Fig.1. ThepowderXRDpatternsofNaY,Nit-NaYandthespentNit-NaY. at800◦Cprobablyduetocoke.TherewasaverysmallCO peakat 2 W.Shenetal./AppliedCatalysisA:General392 (2011) 57–68 61 900 900 400 °C 550 °C 740 °C 800 800 6 700 800 °C 700 5 600 ressure (a.u.) 120 °C5 64 456000000 perature (°C) Pressure (A.U.) 3 800 °C 4 345000000 mperature (°C) PCO 2 190 °C 740 °C 300 Tem CO2 150 °C 2 200 Te 3 200 100 2 1 1 100 0 0 20 40 60 80 Time (min) 0 0 20 40 60 80 Time (min) Fig.4. CO2TPD-MSprofilesoverNaY,Nit-NaYandspentNit-NaY.Key:1.FreshNit- NaY,backgroundcontrolexperiment;2.FreshNaY,backgroundcontrolexperiment; 3.FreshNit-NaY;4.FreshNaY;5.SpentNit-NaY;6.SpentNit-NaY,background Fig.3. CO2TPD-MSprofilesoverthefreshandspentMgO–ZrO2catalyst.Key:1. controlexperiment. FreshMgO–ZrO2,backgroundcontrolexperiment;2.FreshMgO–ZrO2;3.Spent MgO–ZrO2calcinedat600◦Cfor1h;4.SpentMgO–ZrO2;5.SpentMgO–ZrO2,sig- nalatlowtemperature;6.SpentMgO–ZrO2,backgroundcontrolexperiment.The amountofadsorbedspeciesoverthespentNit-NaYwasmuchless signalsinSpectra1,2,3and5weremultipliedbyten. thanthatadsorbedbythespentMgO–ZrO . 2 TheresultsfromCO -TPD–TGA(Figs.S2andS3)areconsistent 2 withthosefromCO -TPD-MSexperiments.Inaddition,thefresh 2 740◦C(inThermograms1–3),whichmightbeduetothedecom- Nit-NaYshowedgoodstabilitytowardheatwiththeweightloss positionofmagnesiumorzirconiumcarbonatecompoundsformed similartoNaYduringthebackgroundcontrolexperiment. afterexposingthecatalysttoair. TheCO uptakescalculatedfromtheTGAweightchangesdur- 2 Fig. 4 shows the CO TPD-MS profiles of NaY, Nit-NaY and ingCO chemisorptionandTPDandtheCO uptakesfromCO -MS 2 2 2 2 thespentNit-NaY.ThespentNit-NaYcatalystwascollectedafter peaksoverthefreshandspentcatalystarelistedinTable1,respec- tworunsoffurfuralcondensationwithacetoneat120◦Cfor75h tively. The CO uptake over the fresh MgO–ZrO and the spent 2 2 andwashedwithexcessmethanol.ThefreshNit-NaY(inThermo- MgO–ZrO aftercalcinationat600◦Cfor1hareveryclose,indicat- 2 gram Curve 3) shows two types of CO desorption sites: one at ingthatthecatalystisregenerable.ThespentMgO–ZrO without 2 2 low temperature near 150◦C and the other at high temperature calcination shows only about 10% CO uptake. The CO uptake 2 2 above800◦C.SincethehightemperatureCO desorptionwasalso capacitiesbasedonboththeadsorptionanddesorptionfromTGA 2 observedonthefreshNaY,itmightberelatedtothedecomposition resultsareverycloseforallthreezeolitesamples,probablydueto ofcarbonatecompounds,forinstance,Na CO ,formedduringCO theabilityofthezeolitecagetocaptureCO .Theamountofbasic 2 3 2 2 chemisorption.ThelowtemperatureCO peaksat150◦Coverfresh sitesoverNit-NaYwasalsocalculatedbasedontheCO TPD-MS, 2 2 NaYandspentNit-NaYwereverybroadorabsent,indicatingthe whichwasaboutoneorderofmagnitudesmallerthanthatofthe lackofthesamekindofbasicsites.InadditiontotheCO desorp- freshMgO–ZrO .However,duetothedesorptionofthecaptured 2 2 tionpeaks,theCO partialpressureincreasedcontinuously(almost CO overabroadtemperaturerange,thebaselineofTPD-MSover 2 2 linearly)from200◦Cto800◦CduringCO TPD.ThespentNit-NaY thezeolitesamplesshifteddramatically,thusmakingthemethod 2 catalystshowedtwoCO uptakepeaksat550◦Cand740◦C.Again, lessaccurateforquantificationofbasicsitesofNit-NaY. 2 thesetwopeaksareassociatedwithadsorbedfurfurals,reaction Theconcentrationsofbasicsitescalculatedbytitrationarealso intermediates,and/orthedegradationproducts,respectively.The showninTable1.Thevaluesarecomparabletotheresultsfrom Table1 Theconcentrationofbasicsitesforfreshandspentcatalystsfrom(1)CO2adsorptioncalculatedfromtheTGAweightchange,(2)C2desorptioncalculatedbyTGAweight changeandTPD-MSareaand(3)benzoicacidtitration. Catalyst CO2adsorption CO2uptake(basicsitesconcentration)×104(mmol/mgcatalyst) Temperature Time Adsorption Desorption Titration Byweightchange Byweightchange ByTPD-MS MgO–ZrO2 Fresh 50◦C 3h 5.92 5.27 6.31 4.33 Spent 50◦C 3h 0.68 0.43 0.64 0.65 Spenta 50◦C 3h 6.47 5.76 6.05 – Nit-NaY Fresh 50◦C 3h 3.22 3.26 0.45 0.42 Spent 50◦C 3h 3.62 3.61 – – NaY Fresh 50◦C 3h 3.64 3.82 – – a ThespentMgO-ZrO2catalystcalcinedinairat600◦Cfor1hbeforeCO2adsorption. 62 W.Shenetal./AppliedCatalysisA:General392 (2011) 57–68 100 90 80 70 % e 60 g a nt 50 e erc 40 P 30 20 10 0 MgO-ZrO2 NaY Nit-NaY Catalysts Furfural Disappearance Selec(cid:2)vity to Aldol Product Monomer Dimer Fig.5. 29SiMASNMRoverthefreshandspentNit-NaYcatalyst.ThespentNit-NaY Fig.6. Furfuraldisappearance,selectivitytoaldolproducts,andproductdistribution aftertworunsoffurfuralcondensationwithacetoneat120◦Cfor75hlost90%of foraldolcondensationoffurfuralwithacetoneoverMgO–ZrO2,NaYandNit-NaY. nitrogeninzeoliteframework. Reactionconditions:temperature:120◦C;pressure:750psigofhelium;reaction time:24h;totalorganicinsolvent:5wt.%;furfural:acetone=1bymoles;catalyst: 15mg;solvent:water/methanol=1byvolume. eitherTGAorTPD-MS.However,themethodcouldnotbeapplied toquantifythebasicsitesofNaYorthespentNit-NaYeither,since genoccurredaftertworunsoffurfuralcondensationwithacetone thepHindicatorwasinsufficienttoshowthecolorchange. inamethanol–watersolution. 3.1.3. 29SiMASNMR 3.2. Reactions Fig. 5 shows the 29Si SP MAS NMR spectra of the fresh and the spent Nit-NaY catalyst. The TO units in the zeolite become 3.2.1. Aldolcondensationreactions 4 TO4−xNx after nitrogen substitution (T: Si or Al). The symbol x MgO–ZrO2,Nit-NaYandNaYwereevaluatedforthecondensa- refers the number of nitrogen atoms bonded to a central silicon tionsoffurfuralandHMFwithacetoneorpropanal.Thereactions (oraluminum)atom.Thedetailedquantumchemicalcalculations werecarriedoutintheliquidphaseat120◦Cin750psigofHefor ofchemicalshiftsforallpossibleSi–NenvironmentsforNaYand 16hor24hundervigorousstirringinahighthroughputbatchreac- HYaregiveninourpreviouspublication[22].Theresonanceswith tor. The furfural (or HMF) and acetone (or propanal) molar ratio chemicalshiftsat−73and−82ppmcouldbeassignedto1Nenvi- was kept at 1. The total organic content of the feed was 5wt.%. ronments(i.e.,x=1)andtheresonancewithchemicalshiftsat−58 Thesolventwasa1:1volumetricmixtureofwaterandmethanol. and−64ppmcouldbeassignedtothe2Nenvironments(i.e.,x=2). Theamountofcatalystusedwasabout1/6ofthetotalorganicsby No3NenvironmentswereobservedforthefreshNit-NaYsample. weight(about15mg). Afterreaction,theresonanceassignedtothe2Nenvironmentsdis- Fig.6showsthecatalyticperformanceofthealdolcondensa- appeared and the intensity of the 1N resonance was decreased. tion of furfural with acetone over MgO–ZrO , NaY and Nit-NaY. 2 The spectra were deconvoluted so as to extract the intensities Theresultsoffurfural/propanal,HMF/acetone,andHMF/propanal of the different resonances, and thus the concentrations of the condensationreactionsoverthesecatalystsarelistedinTable2. different local environments of TO4−xNx. The deconvolution was The catalytic performance was evaluated by the disappearance performedbyconsideringthecalculatedandexperimentalchem- of furfural, the selectivity to aldol products, and monomer or icalshiftvaluesfornitrogensubstitutedTO unitsreported.TheN dimerdistribution.Inallexperiments,therewasnoacetoneself 4 content of the sample could then be estimated from these con- condensation product observed. There was also no furfural (or centrations, as discussed in our earlier paper [22]. The analysis HMF)/propanal dimer observed as products. MgO–ZrO had the 2 revealedthatthenitrogencontentdecreasedfrom10%to1%for highestactivityandselectivitytothealdolproducts.NaYexhibited thespentNit-NaY.Therefore,asignificantlossofsubstitutednitro- weak basicity with the lowest conversion. Nit-NaY showed dra- Table2 Theperformanceofaldolcondensationreactionsoffurfural(orHMF)withacetone(orpropanal)overMgO–ZrO2,NaYandNit-NaY.Reactionconditions:temperature:120◦C; pressure:750psigofhelium;totalorganicsinsolvent:5wt.%;furfural:acetone=1bymoles;catalyst:15mg;solvent:water/methanol=1byvolume. Reaction Reactant(1:1bymolar) Catalyst Reactiontime Disappearanceof Selectivitytoaldol Distribution(%) (h) furfurals(%) products(%) Monomer Dimer 1 HMF/acetone MgO–ZrO2 16 51.4 91.9 56.5 43.5 2 HMF/acetone NaY 16 14.1 89.7 85.9 14.1 3 HMF/acetone Nit-NaY 16 40.1 80.9 72.7 27.3 4 Furfural/propanal MgO–ZrO2 16 60.4 90.0 100 – 5 Furfural/propanal NaY 16 6.7 38.6 100 – 6 Furfural/propanal Nit-NaY 16 55.5 104.7 100 – 7 HMF/propanal MgO–ZrO2 16 86.8 57.0 100 – 8 HMF/propanal NaY 16 26.4 39.6 100 – 9 HMF/propanal Nit-NaY 16 86.4 53.7 100 – Note:TheperformanceoffurfuralcondensationwithacetoneisshowninFig.9.Theexperimenterrorisabout5%. W.Shenetal./AppliedCatalysisA:General392 (2011) 57–68 63 matically increased activity and selectivity to the aldol products MgO-ZrO2 100 comparedtoNaY.Inthecondensationoffurfuralwithacetoneas 90 showninFig.6,thefurfuraldisappearancesoverMgO–ZrO ,NaY 2 andNit-NaYwere59.9%,7.2%and35.1%,respectively.Amongthe 80 consumedfurfural,therewereabout100%,58.3%and84.3%reacted % 70 toformaldolproducts.Theunbalancedfurfuralwasprobablydue e 60 g a toundetectedintermediates[46],thedegradationproductsfrom nt 50 e eitherfurfuralormonomeranddimer,orthechemicalspeciesthat erc 40 adsorbedonthecatalystsurfaceincludingcoke.Itwasseenthat P 30 NaYhadthehighestratioofmonomertodimerofabout9.How- 20 ever,thetotalyieldofmonomerisverylow(3.5%)duetothelow 10 conversionandaldolproductselectivity.Theratioofmonomerto 0 dimeroverNit-NaYwaslowerthanthatobservedforNaYatabout 100% 75% 67% 50% 33% 25% 2.2. However, it was significantly higher than that observed for MgO–ZrO at0.8. Nit-NaY 2 100 The catalytic performance of HMF and acetone condensation 90 overMgO–ZrO ,NaYandNit-NaYwassimilartothatoffurfural 2 80 andacetonecondensationasseeninTable2.Again,MgO–ZrO had 2 70 thehighestcatalyticactivityandaldolproductselectivity.Ninety % twopercentoftheconsumedHMFformedthealdolproductswith ge 60 a 56.5% of monomer and 43.5% of dimer. NaY showed the lowest nt 50 e activitywith14.1%ofHMFdisappearance,butthiscatalystselec- erc 40 tivelyproducedmonomeroverdimer.TheactivityofNit-NaYwas P 30 comparabletoMgO–ZrO2withabout40.1%ofHMFconversionand 20 80.9%aldolproductselectivity.Theratioofmonomertodimerwas 10 2.7,higherthanthatofMgO–ZrO2whichwas1.3.Inthealdolcon- 0 densationoffurfural/HMFwithpropanal,thecatalyticbehaviorof 100% 75% 67% 50% 33% 25% MgO–ZrO andNit-NaYwassimilar,showingmuchgreateractivity Water Concentra(cid:2)on vol.% 2 andselectivitytowardsaldolproductthanNaY. Furfural Disappearance Selec(cid:2)vity to Aldol Product Insummary,incorporationofnitrogenintotheframeworkof Monomer Dimer NaY increased the catalytic activity of Nit-NaY dramatically. The Fig.7. Solventeffectonfurfuraldisappearanceandtheproductdistributionover catalytic activities over Nit-NaY for the aldol condensation reac- tions of furfural/HMF with acetone/propanal were comparable MgO–ZrO2andNit-NaY.Reactionconditions:temperature:120◦C;reactiontime: 24h;totalorganicinsolvent:5wt.%,furfural/acetone=1bymolar;catalyst:1/6 to MgO–ZrO2, but more selective towards the furfurals acetone totalorganicsbyweight(15mg);solvent:variationofmethanoltowatervolumetric monomer.Therelatedcatalyticactivityoverthestudiedcatalysts ratios. followstheorder:MgO–ZrO >Nit-NaY>NaY. 2 3.2.2. Effectofwater tionscarriedoutoverNit-NaY.Waterchangesnotonlythecatalytic Fig. 7 shows the solvent effect on the aldol condensations of activity,butalsotheproductdistribution. furfuralwithacetoneoverMgO–ZrO andNit-NaYwithwatercon- 2 tentvaryingfrom100vol.%to25vol.%inmethanol–watersolvents. Withdecreasingwatercontentfrom100vol.%to67vol.%,thefur- 3.2.3. Effectoffeedconcentration fural disappearance decreased from 91% to 50% over MgO–ZrO Fig. 8 shows the effect of feed concentration on the catalytic 2 catalyst.However,theoverallaldolproductselectivityincreased performanceoffurfuralandacetonecondensationoverMgO–ZrO2 andtheratioofmonomertodimerslightlyincreasedfrom0.55to and Nit-NaY. The reactions were conducted at a constant fur- ∼0.8.Furtherdecreasingthewatercontenttolessthan67vol.%pro- fural/acetonemolarratioof1,totalorganics/catalystweightratio ducednoobviouschangeofeitherfurfuraldisappearance(≈50%) of 6, and methanol/water volumetric ratio of 1. Increasing the or the aldol product selectivity (100%), while, the selectivity of feed concentration increased the furfural disappearance and the monomer to dimer varied slightly from 0.7 to 1.1. The average tendency to generate more dimer product over MgO–ZrO2. The monomer and dimer yields were approximately 15% and 20%, furfural disappearance increased from 23% to 40% to 57% to 65% respectively. However, the catalytic performance over Nit-NaY withfeedconcentrationsof1,5,8,and15wt.%,respectively.Mean- varied significantly with water content in the solvent. The fur- while,theratioofmonomertodimerdecreasedfrom5.7to0.96to fural disappearance decreased dramatically from 75% to 20% by 0.67to0.35,respectively.Increasingthefeedconcentrationfrom decreasingthewatercontentfrom100vol.%to25vol.%.Theratio 1wt.%to15wt.%alsograduallyincreasedthefurfuraldisappear- of monomer to dimer varied from 0.65 in pure water to 6.5 in ance over Nit-NaY from 14% to 56%. However, the selectivity of 25vol.%water.Afterdecreasingwatercontenttolessthan50vol.%, monomertodimerexhibitedquiteadifferenttrendfromthatover the monomer became the predominant product. Over the entire MgO–ZrO2.Ataverylowfeedconcentrationof1wt.%overNit-NaY, watertomethanolrangesstudied,thealdolproductselectivityover themonomerwasthepredominantaldolproductandtheratioof Nit-NaYwasmaintainedat90%. monomertodimerinthealdolproductswasashighas24.Atfeed The effect of water on aldol condensations of HMF with ace- concentrationsabove5wt.%,thechangeintheratioofmonomerto tone was also studied over MgO–ZrO and Nit-NaY. The results dimerdecreased,varyingfrom3to2.5to2atconcentrationsof5,8, 2 weresummarizedinTable3.Similarly,therewasnopreferenceto and15wt.%,respectively.Therefore,increasingthefeedconcentra- HMFacetonemonomerordimeroverMgO–ZrO ,but,decreasing tionincreasedthedisappearanceoffurfuralandtheselectivityto 2 watercontentto67vol.%generatedmoreHMFacetonemonomer dimeroverbothcatalysts.However,Nit-NaYmaintainedahigher thandimeroverNit-NaY.Therefore,waterplaysasignificantrolein selectivitytomonomerproductatallfeedconcentrationsthrough theliquidphasealdolcondensationreactions,especiallythereac- shapeselectivity. 64 W.Shenetal./AppliedCatalysisA:General392 (2011) 57–68 Table3 SummaryofsolventeffectandcatalysttofeedratioeffectoverMgO–ZrO2andNit-NaY.Reactionconditions:temperature:120◦C;pressure:750psigofhelium;totalorganics insolvent:5wt.%;furfural:acetone=1bymoles. Run# Catalyst Org/Cat Solvent(vol.) Reactiontime Reaction Furfural(orHMF) Selectivityto Productdistribution(%) (wt.) temperature(◦C) disappearance(%) aldolproduct(%) Monomer Dimer Reactant:HMF/acetone 1 MgO–ZrO2 6 Water 16h 120 93.3 55.9 50.4 49.6 2 MgO–ZrO2 6 Water/methanol=2:1 16h 120 68.5 84.5 48.6 51.4 3 MgO–ZrO2 6 Water/methanol=1:1 16h 120 41.2 82.2 55.9 44.1 4 MgO–ZrO2 6 Water/methanol=1:2 16h 120 38.6 81.1 67.8 32.2 5 Nit-NaY 6 Water 16h 120 92.0 74.8 48.2 51.8 6 Nit-NaY 6 Water/methanol=2:1 16h 120 51.4 82.5 63.6 36.4 7 Nit-NaY 6 Water/methanol=1:1 16h 120 34.3 75.0 72.0 28.0 8 Nit-NaY 6 Water/methanol=1:2 16h 120 34.1 75.1 77.2 22.8 Reactant:furfural/acetone 9 MgO–ZrO2 60 Water/methanol=1:1 24h 120 32.9 106.1 54.1 45.9 10 MgO–ZrO2 30 Water/methanol=1:1 24h 120 36.4 102.3 50.7 49.3 11 MgO–ZrO2 18 Water/methanol=1:1 24h 120 47.8 100.5 45.0 55.0 12 MgO–ZrO2 12 Water/methanol=1:1 24h 120 51.8 102.3 41.9 58.1 13 MgO–ZrO2 6 Water/methanol=1:1 24h 120 53.9 103.5 44.8 55.2 14 MgO–ZrO2 3 Water/methanol=1:1 24h 120 56.7 95.4 45.6 54.4 15 MgO–ZrO2 2 Water/methanol=1:1 24h 120 66.0 75.7 42.4 57.9 16 Nit-NaY 12 Water/methanol=1:1 24h 120 33.5 57.5 82.1 17.9 17 Nit-NaY 6 Water/methanol=1:1 24h 120 48.9 55.7 69.9 30.1 18 Nit-NaY 4 Water/methanol=1:1 24h 120 59.8 68.7 65.1 34.9 19 Nit-NaY 3 Water/methanol=1:1 24h 120 70.6 72.1 55.6 44.4 20 Nit-NaY 2 Water/methanol=1:1 24h 120 82.8 75.7 47.1 52.9 21 Nit-NaY 1 Water/methanol=1:1 24h 120 97.5 59.6 58.4 41.6 3.2.4. Effectofcatalysttofeedratio TheresultsofaldolcondensationofHMFwithacetonebyvaria- tionofcatalysttofeedratiooverMgO–ZrO andNit-NaYareshown 2 inFig.9.Inaddition,tabulateddataonfurfuralacetonecondensa- MgO-ZrO 2 100 90 MgO-ZrO 80 100 2 70 90 % e 60 80 g enta 50 e % 6700 Perc 3400 entag 50 20 erc 40 P 30 10 20 0 10 1% 5% 8% 15% 0 Nit-NaY 1/60 1/30 1/18 1/12 1/6 100 90 Nit-NaY 100 80 90 70 % 80 ge 60 % 70 Percenta 345000 ercentage 456000 20 P 30 10 20 0 10 1% 5% 8% 15% 0 Feed Concentra(cid:2)on 1/60 1/30 1/12 1/6 1/4 Catalyst to feed ra(cid:2)o Furfural Disappearance Selec(cid:2)vity to Aldol Product Monomer Dimer HMF Disappearance Selec(cid:2)vity to Aldol Product Monomer Dimer Fig.8. Effectoffeedconcentrationonfurfuraldisappearance,aldolproductselectiv- ityandtheproductdistributionoverMgO–ZrO2andNit-NaY.Reactionconditions: Fig.9. EffectofcatalysttofeedratiobyweightontheHMFdisappearance,the temperature:120◦C;reactiontime:24h;totalorganics/catalystratioof6,fur- selectivitytoaldolproductsandtheproductdistributionoverMgO–ZrO2andNit- fural/acetone=1bymoles;solvent:water/methanol=1byvolume. NaY.Reactionconditions:temperature:120◦C;reactiontime:16h;totalorganicsin solvent:5wt.%,HMF/acetone=1bymoles;solvent:methanol/water=1byvolume. W.Shenetal./AppliedCatalysisA:General392 (2011) 57–68 65 tionoverthesetwocatalystsislistedinTable3forcomparison.The 100 100 acalytsatlyttoicfepeedrfroartmioa(nccaetaolvyestr/MtogtaOl–oZrrgOa2niwcsasrainndgienpgenfrdoemnt1o/6f0thteoc1a/t6- nce % 8900 MgO-ZrO2 8900 a byweight).TheHMFdisappearanceslightlyincreasedfrom31%to ar 70 70 % 4adatp1etpc%hreaeeanarlsdeaevntdehcleweoitifnoth8ctar0ieln%aaclstrdoeeoda9ls0dpe%rriao.nmTdchuaaecttitamcslayeolsllneytocafmtrmioveomirtuytn9ow%t.daHtisomor3wee8lrea%rvtaibevtyrieo,cltywhhueaannHsgasMfilnfieFggchdtttihesldye- ural Disappe 34560000 34560000 Selec(cid:2)vity catalysttofeedratiofrom1/60to1/4overNit-NaY.Theselectivity urf 20 20 F ofmonomerincreasedatthelowerconversiononbothcatalysts. 10 10 Nit-NaYdisplayedmoreselectivitytothemonomerproductover 0 0 0 10 20 30 40 50 theentirerangethanthatoftheMgO–ZrO catalyst.Forfurfural 2 Reac(cid:2)on Time (h) condensationwithacetoneasshowninTable3,furtherincreasing thecatalysttofeedratioto1resultedinalmost97.5%furfuraldisap- 100 100 pearanceoverNit-NaY.But,thehigherthefurfuraldisappearance, % 90 90 tctm1hoo/o4euthnloldeoovmwuenrnoeeNbtrrabitptlhe-arNeonddaciaseYulsdd.coTotflhulvaerestfeddutlhierifnaecfelt)miir.nveTeicnthrtycheeeaar(sessnooewodmblasacesenarnotdvoafehlotdyhebsinevtncitdtoeoheupegfsearapaercrtddteiiaafvrtleailirtyotyeinocnaocpnonerdftofrasodiebbrulotuechvtcteees- al Disappearance 345678000000 345678000000 Selec(cid:2)vity % tivitybetweenthetwocatalystsduetocatalysttofeedratiomight ur 20 20 indicatethedifferentreactionmechanismsoverthesecatalysts.It urf 10 10 F shouldalsobenotedthattheratioofmonomertodimerforHMF 0 0 0 10 20 30 40 50 acetonecondensationwashigherthanthatforfurfuralacetonecon- Reac(cid:2)on Time (h) densationoverMgO–ZrO .Thismaybeduetotherelativelyshort 2 reactiontimeforHMFacetonecondensationreaction(16h),which Run 1-Furfural Disappearance Run 1-Monomer Selec(cid:2)vity isinsufficienttoreachthereactionequilibriumasobservedduring Run 1-Dimer Selec(cid:2)vity Run 2-Furfural Disappearance thestabilitytesting(Section3.2.5). Run 2-Monomer Selec(cid:2)vity Run 2-Dimer Selec(cid:2)vity 3.2.5. Catalyststability Fig.10. TOSfurfuraldisappearanceandtheselectivitiestomonomeranddimer Fig.10showsthetime-on-steam(TOS)catalyticperformance overMgO–ZrO2andNit-NaY.Thesecondrunwasconductedbyusingthecatalyst recycledfromthefirstrun.Reactionconditions:reactionvolume:75mL;tempera- ofMgO–ZrO2andNit-NaYandtheirstabilitytowardsrecyclingfor ture:120◦C;pressure:750psigofhelium;totalorganics5wt.%,furfural/acetone=1 furfural/acetonecondensation.Thereactionswereconductedina bymoles;solvent:methanol/water=2;catalysts:250mg,about1/12totalorganics ParrbatchreactorasdescribedinSection2.3.Thefurfuraldisap- byweight. pearanceincreasedfrom12%at1hto80%afterabouttwodaysof reactionovertheMgO–ZrO catalyst.Theselectivitytomonomer 2 offurfuralontothecatalystsurfaceand/orthebasestrengthofthe increasedinitiallyfrom13%at1hgraduallytoamaximumof40% spentNit-NaYcatalystwhichwasinsufficienttoinitiatethereac- at19h,andthen,slowlydecreasedto24%at44h.Theselectivity tionintermediatestowardsthealdolproduct(reversiblereactions todimerincreasedcontinuouslyfrom4%at1hto76%at44h.The fromfurfuraltothereactionintermediates)atthebeginningofthe selectivitytothealdolproductsandtheproductdistributiondur- reaction.ThelossofthecatalyticactivityoverthespentNit-NaYin ingthetime-on-stream(TOS)reactionaresummarizedandlisted thelongrunwaspresumablyduetothelossofbasicsites(nitro- inTable4.Itwasseenthattheinitialaldolproductsselectivitywas genintheframework).Thisisconsistentwiththecharacterization aslowas17.3%andincreasedtoabout100%withTOSat19h.The resultsfromCO TPD,benzoicacidtitrationand29SiSPMASNMR unbalancedfurfuralatthebeginningofthereactionwaspresum- 2 inSections3.1.2and3.1.3. ably caused by undetected reaction intermediates. After catalyst recycling,theMgO–ZrO catalystretainedmostoftheactivitywith 2 onlyaslightdecreaseindimerselectivity. 4. Discussion IncontrasttoMgO–ZrO ,Nit-NaYdisplayedadifferentcatalytic 2 performanceandstabilityforthealdolcondensationreaction.In 4.1. Shapeselectivity thefirstrun,furfuraldisappearanceincreasedwithTOSfrom37% at0.5hto73%at47h.Theselectivitytomonomerincreasedfrom ShapeselectivezeolitecatalysiswasfirstreportedbyWeiszand 9%at0.5htoaround40%at10.5handremainedconstantuntilthe co-workersin1960[51]inthepetroleumindustryandwaslater end of the reaction. The selectivity to dimer increased at a rela- applied for the production of high-grade gasoline from biomass tivelyslowrate,startingat2%at0.5h,increasingto50%at20.5h, compounds [52,53]. Shape selective catalysis is defined as the and gradually increasing to 60% after 47h of reaction. The total transformationofreactantsintoproductsdependingonhowthe selectivitytoaldolproductsincreasedwithTOSfrom10%toabout processedmoleculesfitintotheporeofacatalyst[54].Here,we 100%intheperiodof30hreactionat120◦C.Inthesecondrun,the estimatedthekineticdiametersandcriticaldiametersofvarious furfuraldisappearancefirstdecreasedfrom33%to12%inthefirst moleculesaslistedinTable5inordertodetermineifshapeselec- 5hofreactionthenslowlyincreasedfrom11%to21%after30h.In tivityisexpectedtooccurwithindifferentsizesofzeolitepores. addition,theselectivitytodimerwasrelativelylow,lessthan2%in Themethodsofcalculationofkineticandcriticaldiametercanbe theentireTOSrange,while,theselectivitytomonomerincreased foundinthesupplementarymaterials. fromabout1%to14%.Duringthefirst5hofreaction,therewere The12-ringwidowsizeofzeoliteYis7.4A˚ [6].Thehighresolu- traceamountsofmonomeranddimerproductsobserveddespite tionAradsorptionisothermsshowthattheporesizesofNaYand thefactthattheinitialfurfuraldisappearancewasclosetothatof Nit-NaYareabout7.2A˚,intermediatetothesizesoffuraldehyde thefirstrun.Thisabnormalbehaviorwasreproducibleandtherea- acetonemonomeranddimer.Thesizeatthefaujasiteporeopen- sonisnotunderstoodatpresent.Itislikelycausedbyadsorption ingissufficienttoaccommodatethemonomerproductbutistoo 66 W.Shenetal./AppliedCatalysisA:General392 (2011) 57–68 Table4 SummaryofTOSfurfuraldisappearanceandaldolproductdistributionoverMgO–ZrO2andNit-NaY.ReactionwasconductedinaParrbatchreactorat120◦Cin750psigof helium.Organics:furfural/acetone=1,5wt.%;solvent:water/methanol=2byvolume;catalysts:1/12oftotalorganicsbyweight;reactionvolume:75mL.Thesecondrun wasconductedbyusingcatalystrecycledfromthefirstrun. Reactiontime(h) Furfural Selectivitytoaldol Distribution(%) Reaction Furfural Selectivitytoaldol Distribution(%) disappearance(%) products(%) time(h) disappearance(%) products(%) Monomer Dimer Monomer Dimer MgO–ZrO2:firstrun MgO–ZrO2:secondrun 1 12.2 17.3 86.7 13.3 1 23.1 6.0 84.1 15.9 2 11.4 36.1 85.9 14.1 3.5 29.1 10.6 87.8 12.2 4 20.4 44.5 81.8 18.2 7.5 27.8 34.4 82.4 17.6 6 24.9 56.7 76.7 23.4 11.5 34.8 49.5 76.0 24.0 10 30.9 68.5 69.6 30.1 18.5 47.6 73.7 61.1 38.9 19 40.7 97.6 58.5 41.5 28 59.9 84.0 51.2 48.8 26 52.9 105 50.2 49.8 34.5 62.8 99.0 48.3 51.7 34 66.9 99.9 44.4 55.6 44 71.6 94.7 42.8 57.2 44 77.6 100.4 38.9 61.1 47 79.6 98.1 42.7 57.3 Nit-NaY:firstrun Nit-NaY:secondrun 0.5 36.6 10.1 91.9 8.1 1 32.9 1.3 89.6 10.4 3 40.7 35.3 83.3 16.7 3 24.6 1.7 60.0 40.0 5 39.5 47.8 81.0 19.0 5 11.5 3.4 69.0 31.0 10.5 44.8 69.9 72.0 28 8 10.7 6.7 88.6 11.4 13 51.1 71.5 69.0 31.0 11 14.0 7.5 92.0 8.0 20.5 56.8 92.3 62.5 37.5 21 18.2 12.7 94.6 5.4 30 65.3 102.0 62.3 37.7 28 21.4 16.0 93.7 6.3 47 73.3 100.4 57.3 42.7 12 Fig.11summarizesthefurfuraldisappearanceasafunctionof themolarratioofmonomeroverdimeratthemethanol/watersol- o) 10 ventvolumetricratioof1and2.ItisclearlyseenthattheNit-NaY (cid:2) wasmoreselectivetowardsmonomerintheentirerangeoffur- a ar r 8 fural disappearance than that of MgO–ZrO2, thus evidence that ol theshapeselectivitydidoccuroverNit-NaY.Furtherexperiments m er ( 6 snhootwafefedctthedatbtyhfeeeshdacpoencseelnetcrtaitviiotny,tcoatmaloynstomtoefreeodverratNioit,-aNnadYrewaacs- m Di tiontime.However,thewatercontentinthesolventhasadramatic / er 4 Nit-NaY effectontheselectivityoverNit-NaY.SincetheNit-NaYcatalyst m o lostasignificantdegreeofitsFAU-typecrystallinityandmicrop- n Mo 2 MgO-ZrO2 orosityafternitrogensubstitution,thesignificantamountofdimer wespeculateisgeneratedbytheexternalsurfacereaction.Inthe future, the optimization of nitridation procedure should be per- 0 formed in order to minimize the aldol dimer formation due to 0 20 40 60 80 100 Furfural Disappearance (%) externalsurfacereaction. Fig.11. Furfuraldisappearanceasafunctionofmolarratiooffurfuralacetone 4.2. Effectofwater monomeroverdimer.Thedataarecomparedatmethanol/watersolventvolumetric ratioof1(solidsymbols)andmethanol/watervolumetricratioof2(opensymbols). Water content in the solvent plays a significant role on the performance of both catalysts, especially for Nit-NaY. Increasing water content leads to a higher conversion and increased dimer selectivityonbothMgO–ZrO andNit-NaY.Ontheonehand,the 2 smalltoallowtherapiddiffusionofthedimerproduct,thusleading furfural/acetonedimerisnotabletodissolveinthesolventwith toahighselectivitytowardsthemonomerproduct.Incontrast,the highwatercontent,thus,theprecipitationofdimerdrivesthereac- freshMgO–ZrO showsabroadporesizedistributionfrom5A˚ to tiontowardsthegenerationofmoredimer.Ontheotherhand,the 2 1000A˚,thushavingnopreferencetomonomerduetothecatalyst poresofthezeolitemightbecompletelyfilledwithwaterwhen texture.Here,NaYwasspecificallychosenasthestartingmaterial usingsolventswithhighwatercontentinmethanol.Therefore,the becauseNa+ mightleadtostrongerbasicstrengthafternitrogen reactiontakesplaceontheexternalzeolitesurface,allowingdimer substitutionthanHY[55]. tobeformedmoreeasily. Table5 Listofthekineticdiametersandcriticalsizesofthemoleculesofinterest. Molecule Kineticdiameter(A˚) Criticalsize(A˚) Molecule Kineticdiameter(A˚) Criticaldiameter(A˚) Width Length Width Length Furfural 5.7 4.56 5.99 HMF 6.2 5.42 8.64 FAmonomer 6.3 4.87 9.89 HAmonomer 6.8 4.97 12.15 FAdimer 7.4 6.03 13.75 HAdimer 8.0 6.66 17.95 FPmonomer 6.3 5.24 8.64 HPmonomer 6.8 5.22 10.90 FPdimer 6.9 5.54 10.81 HPdimer 7.2 6.71 12.91 Note:“FA”meansfurfuralacetone;“FP”meansfurfuralpropanal;“HA”meansHMFacetone;“HP”meansHMFpropanal.
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