ChineseJournalofCatalysis35(2014)1727–1739 

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Article   

SynthesisofmesoporousZSM‐5usinganewgeminisurfactantasamesoporousdirectingagent:Acrystallizationtransformationprocess

QuanyiWanga,b,e,YingxuWeia,*,ShutaoXua,MozhiZhanga,e,ShuangheMenga,DongFana,e, YueQia,JinzheLia,ZhengxiYua,CuiyuYuana,YanliHea,ShuliangXua,JingrunChena,e,JinbangWanga,BaolianSub,c,#,ZhongminLiua,d,§aDalianNationalLaboratoryforCleanEnergy,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences,Dalian116023,Liaoning,China bLaboratoryofInorganicMaterialsChemistry(CMI),UniversityofNamur(FUNDP),61ruedeBruxelles,B‐5000Namur,Belgium cStatekeyLaboratoryofAdvancedTechnologyforMaterialsSynthesisandProcessing,WuhanUniversityofTechnology,Wuhan430070,Hubei,China dStateKeyLaboratoryofCatalysis,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences,Dalian116023,Liaoning,China eUniversityofChineseAcademyofSciences,Beijing100049,China

A R T I C L E I N F O  

A B S T R A C T

Articlehistory:Received9April2014Accepted28May2014Published20October2014

  Anewgeminisurfactant, [C18H37(CH3)2–N+–(CH2)3–N+–(CH3)2C18H37]Cl2(C18‐3‐18),hasbeensuccess‐fully used as themesopore directing agent in the hydrothermal synthesis ofmesoporous ZSM‐5(MZSM‐5).ThesynthesisofMZSM‐5wasrealizedwithalowtemperaturecrystallizationprocessat130°C.TheamountofC18‐3‐18usedinthesynthesisaffectedtherelativecrystallinityandthetexturalpropertiesof theobtainedMZSM‐5.Detailed investigationshowed that the formationofMZSM‐5followed a crystallization transformation process. The use of C18‐3‐18 resulted in the formation ofmesoporousmaterial during the early stage of the synthesis,whichwas converted intoMZSM‐5crystalstemplatedbytetrapropylammoniumbromidetoformtheMFIphase.Asthesynthesispro‐ceeded,theMZSM‐5crystalsaggregatedintoparticlesbyweakinteractions.ThisworkshowsthatC18‐3‐18canbeusedasamesoporedirectingagent,whichcouldprovidearouteforthesynthesisofothermesoporouszeolites.

©2014,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences.PublishedbyElsevierB.V.Allrightsreserved.

Keywords:MesoporousZSM‐5Dual‐templatingGeminisurfactantInter‐crystallinemesoporeCrystallizationtransformation

 

 

1. Introduction

MesoporousZSM‐5 (MZSM‐5), normally referred toZSM‐5with both micropore and mesopore systems, is supposed tocombinethebenefitsofeachseparateporesizeregimeandhasgreat potential to improve the efficiency of zeolite catalysis,enhance the accessibility to active sites, and reduce thediffu‐sion obstacle [1–4]. Therefore, considerable efforts have fo‐

cusedonthesynthesisofMZSM‐5.Awidevarietyofsynthesisstrategieshavebeenproposedto

create mesopores in ZSM‐5 crystals. Various post‐treatmentmethods, including heat treatment [5], acid leaching [6–8],steaming treatment [6,7], alkaline leaching [9,10], and otherchemicaltreatments[11],haveproventobeefficientincreat‐ingmesoporesinZSM‐5crystals.Anotherstrategyforthesyn‐thesisofZSM‐5crystalscontainingmesoporesiscrystallization

*Correspondingauthor.Tel:86‐411‐84379335;Fax:86‐411‐84691570;E‐mail:weiyx@dicp.ac.cn #CorrespondingAuthor.E‐mail:bao‐lian.su@unamur.be,baoliansu@whut.edu.cn§Correspondingauthor.Tel:86‐411‐84379335;Fax:86‐411‐84691570;E‐mail:liuzm@dicp.ac.cnThisworkwassupportedbytheNationalNaturalScienceFoundationofChina(21273230,21273005,21103176,21103180). DOI:10.1016/S1872‐2067(14)60128‐5|http://www.sciencedirect.com/science/journal/18722067|Chin.J.Catal.,Vol.35,No.10,October2014

1728 QuanyiWangetal./ChineseJournalofCatalysis35(2014)1727–1739

using a dual‐templating method, which includes both ZSM‐5templatesandmesoporestructuredirectingagents.Inthedu‐al‐templating method, both hard templates, such as carbonnanoparticlesornanotubes[12,13],poly(methylmethacrylate)(PMMA) nanospheres [14], nano CaCO3 [15], and polymerbeads [16], and soft templates, such as cationic surfactants(CTABs) [17,18], nonionic alkyl poly(ethylene oxide) surfac‐tants [19], organosiliane [20], cationic polymer [21], silylatedpolymer [22], and natural products [23], have attracted con‐siderableattentionbecauseof theirhighefficiency increatingmesopores inZSM‐5.Recently,Ryooetal. [24–26]madepro‐gress in synthesizing MZSM‐5 using an organic surfactantequipped with a multi‐ammonium headgroup, which couldserve asboth a zeolitic templateandamesogenous structuredirectingagent.Synthesismethodsinvolvingthedevelopmentofnewsurfactantsformesoporouszeolitesynthesisarestillofinterestinthefieldofhierarchicalmaterials.

Inthisstudy,wereportafacileandhighlyefficientsynthesisroute to prepare MZSM‐5 using a new gemini surfactant,[C18H37(CH3)2–N+–(CH2)3–N+–(CH3)2C18H37]Cl2 (C18‐3‐18), as themesoporedirectingagent.Scheme1showsthestructuremodelof C18‐3‐18. To the best of our knowledge, the synthesis ofMZSM‐5usingC18‐3‐18asthemesoporedirectingagenthasnotbeenreported.TheformationmechanismofMZSM‐5templat‐edfromC18‐3‐18isalsodiscussedbasedonexperimentalobser‐vations.

2. Experimental

2.1. Chemicalsandsynthesis

Allofthechemicalreagentsinthisarticle(NaAlO2(41wt%Al2O3), tetraethyl orthosilicate (TEOS, 98.0%,), tetraprop‐ylammonium bromide (TPABr, 98.0%), NaOH (96.0%), andC18‐3‐18(95.0%))werecommercialproductsofanalyticalgrade,andwereusedasreceivedwithoutfurtherpurification.

The MZSM‐5 samples were synthesized with the hydro‐thermalmethodusingC18‐3‐18asthemesogenoustemplateandagelwiththemolarcompositionof40SiO2:1Al2O3:20NaOH:10 TPABr:x C18‐3‐18:7000 H2O with different crystallizationtemperaturesandtimes.Inatypicalsynthesisprocedure,0.26gNaAlO2,0.80gNaOH,and2.80gTPABrweredissolvedin135gH2O.Then,8.93gTEOSand3.33gC18‐3‐18(here,x=4)wereaddedintothesynthesisgelunderagitation.Aftermixing,thesynthesisgelwastransferredintoa200mLTeflon‐linedstain‐less steelpressurevessel, sealed, andheatedunderautogenicpressure with vigorous stirring. After crystallization, theas‐synthesized sample was centrifugally separated, washed,driedat120°Cfor12h,andcalcinedat600°Cfor6h.Herein,the sample synthesized at 150 °C for 36 h is denotedMZSM‐5‐A, the sample synthesizedat130 °C for120h isde‐

notedMZSM‐5‐B,andthesamplefirstsynthesizedat120°Cfor48handthenat175°Cfor6hisdenotedMZSM‐5‐C.Forcom‐parison, conventional ZSM‐5 was also synthesized startingfromthesynthesisgelwiththesamecompositionbutwithoutadding C18‐3‐18, and this sample is referred to as ZSM‐5. ThecrystallizationconditionsofZSM‐5were175°Cfor48h.

2.2. Characterization

X‐ray diffraction (XRD) patterns were obtained with aD/max‐rb X‐ray diffractometer, using Cu Kα radiation (λ =1.5405Å) at room temperaturewith instrumental settings of40 kV and 40 mA. The relative crystallinity was calculatedbasedontheintensityofthefivepeakswith2θ=22°–25°.

Scanningelectronmicroscopy(SEM)imageswereobtainedformorphological identification using a KYKY AMRAY‐1000Bscanningmicroscope.Transmissionelectronmicroscopy(TEM)images were obtained with a JEOL JEM‐2000Ex electron mi‐croscopeat120kV.

N2 adsorption‐desorption experiments were performed at–196 °C on a NOVA 4000 gas adsorption analyzer(QuantachromeCorp.). Each samplewas evacuated at 130 °Cfor1handthenat350°Cfor3hbeforeadsorption.

27Al magic angle spinning (MAS) nuclear magnetic reso‐nance (NMR) measurements were performed on a 600 MHzBrukerAvanceIIIequippedwitha4mmMASprobe.27AlMASNMRspectrawere recordedusingonepulse sequencewith aspinning rate of 12 kHz. 100 scanswere accumulatedwith aπ/8pulsewidthof0.75μsanda2srecycledelay.Thechemicalshiftswerereferencedto(NH4)Al(SO4)2∙12H2Oat−0.4ppm.

3. Resultsanddiscussion

3.1. Synthesisatdifferentcrystallizationtemperatures

TheXRDpatternsofthesamplessynthesizedusingdifferentcrystallization temperature routes are shown in Fig. 1 andcompared with the XRD pattern of conventional ZSM‐5. TheintrinsiclatticestructureoftheMFItopologywasobservedforallsamples,andnootherphaseswereformedduringthesyn‐

Scheme 1. Structure model of [C18H37(CH3)2–N+–(CH2)3–N+–(CH3)2‐C18H37]Cl2.

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2/( o )

ZSM-5

MZSM-5-A

MZSM-5-B

MZSM-5-C

Inte

nsity

Fig.1.XRDpatternsofZSM‐5andtheMZSM‐5samplessynthesizedatdifferentcrystallizationtemperatures.

QuanyiWangetal./ChineseJournalofCatalysis35(2014)1727–1739 1729

thesis.Furthermore,evenstarting fromthegelwiththesamecomposition using dual templates (TPABr and C18‐3‐18) therewasstillsomedifferenceintherelativecrystallinityofthesyn‐thesizedMZSM‐5samples,whichcanbeattributed to thedif‐ferent crystallization temperatures.Among the threeMZSM‐5samples,MZSM‐5‐AandMZSM‐5‐Chadlowrelativecrystallin‐ity. MZSM‐5‐B had the highest relative crystallinity and veryclosetothatofconventionalZSM‐5,indicatingthatthecrystal‐lization procedure forMZSM‐5‐B (130 °C for 120 h) had theoptimalconditionsforthesynthesisofMZSM‐5.

TheN2 adsorption‐desorption isothermsof ZSM‐5 and theMZSM‐5samplesareshowninFig.2(a).Thecurvesoftheporesize distribution, which were calculated from the adsorptionbranchesusingtheBJHmodel,areshowninFig.2(b).AsshowninFig.2(a),ZSM‐5 showsa representativeType I (Langmuir)isothermaccording to theclassificationof IUPAC,withnoob‐viousincreaseintheadsorbedN2amountandnodistincthys‐teresisloopathighrelativepressure,whichischaracteristicofmicroporousmaterialswith nomesoporosity. This is verifiedby the BJH pore size distribution curve of ZSM‐5 (Fig. 2(b)),wherethereisnoobviouspeakinthemesoporousrange,indi‐cating onlymicroporosity in ZSM‐5. As listed in Table 1, theBETsurfaceareaandtotalporevolumeofZSM‐5are328m2/gand0.16 cm3/g,while themesoporous surface areaandporevolumeareverylow,only59m2/gand0.03cm3/g,respective‐ly.

FortheMZSM‐5samples,theN2adsorption‐desorptioniso‐therms are greatlydifferent from that of conventional ZSM‐5.AsshowninFig.2(a),theN2adsorption‐desorptionisothermsof the MZSM‐5 samples are mixed Type I and Type IV iso‐therms, indicating the existence of both microporosity andmesoporosityinthethreesamples(MZSM‐5‐A,MZSM‐5‐B,andMZSM‐5‐C)synthesizedbythedual‐templatingmethodatdif‐ferent crystallization temperatures. The other important fea‐turesincludeadramaticincreaseintheadsorptionamountsathighrelativepressurecomparedwiththeconventionalZSM‐5isotherm.Inaddition,ahysteresisloopconfirmsthegenerationofmesopores. Because of the generationofmesopores in thethree samples, their mesoporous surface areas and mesopo‐

rous volumes are larger than those of ZSM‐5. However, theincrease inmesoporosityof theMZSM‐5samples isaccompa‐niedwithadecreaseofmicroporosity(Table1).Theporesizedistributions of theMZSM‐5 samples calculated from the ad‐sorptionbranchesusing theBJHmodel showawiderangeofpore sizes, which suggests the possible generation of in‐ter‐crystallinemesopores.AsshowninFig.2(b),themesoporesizes ofMZSM‐5‐A,MZSM‐5‐B, andMZSM‐5‐C vary from2 to100 nm.Most of themesopores inMZSM‐5‐A andMZSM‐5‐Carearound50nm,whilethepeakofmesoporesaround50nminMZSM‐5‐Bisrelatively lowintensity.However, thenumberofmesoporesintherange2to10nmissignificantlylargerforMZSM‐5‐BthanforMZSM‐5‐BandMZSM‐5‐C.

The representative SEM imagesofZSM‐5and theMZSM‐5samplesaregiveninFig.3.MicroporousZSM‐5appearstobecomposed of crystals with different sizes from 0.6 to 1 μm.MZSM‐5‐A is also composedof crystals ofdifferent sizes.Thelargercrystalswithasizeof∼10μmseemtobetheaggrega‐tion of smaller crystals of ∼1 μm. As shown in Fig. 3(e),MZSM‐5‐Biscomposedofdifferentsizedparticles,anditsen‐larged image (Fig. 3(f)) shows that the grainy surface ofMZSM‐5‐BistheaggregationofmanysmallZSM‐5crystals.Inaddition,itshouldbementionedthattheaggregationofZSM‐5crystals into particles can induce the formation of some in‐ter‐crystallinemesopores inMZSM‐5‐B,which canbe seen inFig.3(f).Thisresult isconsistentwiththeN2physicaladsorp‐tionmeasurement.Correspondingly,Fig.3(g)and(h) indicatethatMZSM‐5‐C,likeMZSM‐5‐AandMZSM‐5‐B,iscomposedofdifferent sized particles,which are composed of small ZSM‐5crystals. Among the threeMZSM‐5 samples, MZSM‐5‐B has a

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Mesopore size (nm)

ZSM-5 MZSM-5-A MZSM-5-B MZSM-5-C

(b)

Fig.2.N2adsorption‐desorptionisotherms(a)andporesizedistributions(b)ofconventionalZSM‐5andtheMZSM‐5samplessynthesizedatdiffer‐entcrystallizationtemperatures.

Table1 Textural properties of conventional ZSM‐5 and the MZSM‐5 samplessynthesizedatdifferentcrystallizationtemperatures.

SampleSurfacearea(m2/g) Porevolume(cm3/g)

SBET SMeso SMicro Vtotal VMeso VMicroZSM‐5 328 59 269 0.16 0.03 0.13MZSM‐5‐A 254 133 121 0.55 0.49 0.06MZSM‐5‐B 418 268 150 0.73 0.66 0.07MZSM‐5‐C 279 141 138 0.38 0.32 0.06

1730 QuanyiWangetal./ChineseJournalofCatalysis35(2014)1727–1739

moreregularmorphology,whichmayresultfromtherelativelylowcrystallizationtemperature.

Basedon thecharacterizationsmentionedabove, it canbeconcludedthatthedifferentcrystallizationroutescarriedoutatdifferenttemperaturesaffect themorphologyandthetexturalpropertiesofsynthesizedMZSM‐5.Arelatively lowcrystallinetemperature,suchas130°C,seemstobetheoptimalcrystalli‐zation temperature for the synthesis of MZSM‐5 with highmesoporoussurfacearea,highmesoporousvolume,andregu‐larmorphology,aswellasgoodrelativecrystallinity fromtheformationoftheMFIphaseduringthesynthesis.TherelativelylowcrystallizationtemperaturerequiredtosynthesizeMZSM‐5usingC18‐3‐18asamesoporedirectingagentmayoriginatefromits unique –N+–(CH2)3–N+– group, which still needs furtherinvestigation.

3.2. EffectoftheamountofC18‐3‐18

Theamountofmesoporousstructuredirectingagentusual‐ly affects the relative crystallinity, morphology, and texturalproperties of the synthesizedMZSM‐5. Thus, theeffect of theamountofC18‐3‐18wasalsoinvestigatedintheMZSM‐5synthe‐sis following the optimized crystallization route confirmed inSection3.1(crystallizationat130°Cfor96h).Themolarcom‐position of the starting gel was 40 SiO2:1 Al2O3:20 NaOH:10TPABr:xC18‐3‐18:7000H2O,inwhichxrepresentstheamountofC18‐3‐18usedinthesynthesisgel.

TheXRDpatternsofthesamplessynthesizedusingdifferentamounts of C18‐3‐18 are shown in Fig. 4. Even varying theamountofmesoporousstructuredirectingagent, the intrinsiclatticestructureoftheMFItopologywasidentifiedforallfoursamples,andnootherphaseswereformedduringthesynthe‐sis of the MZSM‐5 samples. Furthermore, when C18‐3‐18 wasaddedinamountsofx=2and4,therewasalmostnodifference

in the relative crystallinity of the two synthesized MZSM‐5samples. When more C18‐3‐18 was added (x = 6), the relativecrystallinity of the synthesizedMZSM‐5 structurewas higherthanwithx=2or4.FurtherincreasingtheamountofC18‐3‐18(x= 8) resulted in a decrease of the relative crystallinity of thesynthesizedMZSM‐5samplecomparedwithx=6.Thisisusu‐ally observed during the synthesis of MZSM‐5 using the du‐al‐templatingmethod.Undermostcircumstances, thetwodif‐ferent templating systems, i.e., the ZSM‐5 structure directingagent(TPABr)andthemesoporoustemplate(C18‐3‐18),workina competitive rather than a cooperativemanner. If toomuchC18‐3‐18 isadded intothesynthesisgel,eventhoughthemeso‐porousphasecanbegenerated,theveryslowformationoftheMFIphaseresultsinitbeingdifficulttotransformthegenerat‐edmesoporousphasetocrystallizedZSM‐5.Therefore,moreofthemesoporousphaseremains,whichresultsinalowrelativecrystallinity.

(a) (b) (c) (d)

(h)(g)(f)(e)

Fig.3.SEMimagesofZSM‐5andtheMZSM‐5samplessynthesizedatdifferentcrystallizationtemperatures.(a)ZSM‐5;(b)Enlargementof(a);(c)MZSM‐5‐A;(d)Enlargementof(c);(e)MZSM‐5‐B;(f)Enlargementof(e);(g)MZSM‐5‐C;(h)Enlargementof(g).

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Inte

nsity

2/( o )

x = 2

x = 4

x = 6

x = 8

Fig.4.XRDpatternsoftheMZSM‐5samplessynthesizedusingdifferentamountsofC18‐3‐18.

QuanyiWangetal./ChineseJournalofCatalysis35(2014)1727–1739 1731

TheN2adsorption‐desorptionisotherms,theporesizedis‐tributioncurves,andtextualpropertiesofthesamplessynthe‐sizedusingdifferentamountsofC18‐3‐18areshowninFig.5andTable2.As shown inFig.5a, all of theN2adsorptionandde‐sorption isotherms of theMZSM‐5 samples aremixed Type Iand Type IV isotherms, indicating the coexistence of mi‐cropores and mesopores. With increasing added amount ofC18‐3‐18 fromx =2 to6, the total surfacearea (SBET) increasedfrom325m2/gto515m2/g.Amongthethreesamples,thetotalporevolumeofthesamplesynthesizedwithx=4(0.82cm3/g)washigher than theother twosamples (0.52cm3/g forx=2and0.66cm3/gforx=6),whichmaystemfromitsrichmeso‐porosity.Therelativelylowporevolumeofthesamplesynthe‐sizedwith x = 6 can be explained by the generation ofmoremicropores, resulting in the loss of mesopores. Further in‐creasingtheamountofC18‐3‐18tox=8,resultedintheobtainedMZSM‐5 having the highest surface area and pore volumeamongthefoursamples,whichisbecauseofthegenerationofahighly mesoporous structure. However, considering its verylow relative crystallinity and microporous surface area, theaddition of toomuch of the mesopore directing agent is notbeneficialforthegenerationofMZSM‐5.

Theabovementionedresultsindicatethatunderthepresentexperimentalconditions,theadditionofamoderateamountofC18‐3‐18duringthesynthesis(x=4and6)favorsthesynthesisofMZSM‐5 with good relative crystallinity and relatively goodmesoporegeneration.

TheporesizedistributionsofthefourMZSM‐5samples(Fig.5(b))indicatethatthegeneratedmesoporeshaveawiderangeofsizes(2–30nm)whenaddingamoderateamountofC18‐3‐18(x=2,4,and6)intothesynthesisgel.Increasingtheamountof

C18‐3‐18 to x = 8 results in the generatedmesopores having anarrowsizerangecenteredat4nm.

3.3. FormationprocessofMZSM‐5

To investigate the formation process of MZSM‐5, the syn‐thesiswascarriedout fordifferentcrystallization timesusingthe same amount of C18‐3‐18 (x = 4) at 130 °C. Samples weresynthesizedandcharacterizedforcrystallizationtimesof8,24,48,72,96,and120h.Thesamplessynthesizedfor96and120hhavebeenmentionedabove,thesamplewithx=4inSection3.2andMZSM‐5‐BinSection3.1.

Thesmall‐angleandthewide‐anglepowderXRDpatternsofthesamples synthesized fordifferentcrystallization timesareshowninFig.6.Whenthesynthesiswasperformedfor8or24h, no diffraction peaks are present in thewide‐angle powderXRDpatternofthesolidsamples.However,apeakispresentintheir small‐angle powder XRD patterns, suggesting the two

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(b)

Fig.5.N2adsorption‐desorptionisotherms(a)andporesizedistributions(b)oftheMZSM‐5samplessynthesizedusingdifferentamountsofC18‐3‐18.

Table2 Texturalpropertiesof theMSM‐5samplessynthesizedusingdifferentamountsofC18‐3‐18.

C18‐3‐18 amount

Surfacearea(m2/g) Porevolume(cm3/g)SBET SMeso SMicro Vtotal VMeso VMicro

x=2 325 253 72 0.52 0.49 0.03x=4 491 389 102 0.82 0.78 0.04x=6 515 405 110 0.66 0.61 0.05x=8 720 698 22 1.16 1.15 0.01

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(a)

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nsity

t = 8 h

(b)

Fig. 6. Small‐angle (a) and wide‐angle (b) powder XRD patterns ofsamplessynthesizedfordifferentcrystallizationtimes.

1732 QuanyiWangetal./ChineseJournalofCatalysis35(2014)1727–1739

samples are mesoporous materials. For the samples synthe‐sizedformorethan24h,ZSM‐5isdetectedastheonlycrystal‐linephaseduringthecrystallizationprocess.ThisindicatestheslowcrystallizationcharacterofZSM‐5underthegivenexper‐imental conditions, inwhich an induction period in the earlystage is required for the crystallization process. Diffractionpeaks of ZSM‐5become evident after crystallization for 48h,andtheintensityofthepeaksgreatlyincreaseswhenincreas‐ingthecrystallizationtimeto72h.

ThecrystallizationcurvebasedontherelativecrystallinitycalculatedfromFig.6isshowninFig.7.Thereferencesampleused here is conventional ZSM‐5, whose relative crystallinitywasdefinedas100%.Duringtheinitial24h,thereisaninduc‐tion period and no ZSM‐5 phase forms. For a crystallizationtimeof48h,therelativecrystallinitysharplyincreasesto63%.Then,therelativecrystallinitygraduallyincreasesfrom63%to93%withincreasingcrystallizationtimefrom48to120h.

N2 adsorption‐desorption experiments were conducted toinvestigatethechangeofthetexturalpropertiesofthesynthe‐sized samples during the crystallization process. The N2 ad‐sorption‐desorption isotherms and the pore size distributioncurves of the samples synthesized at different crystallizationtimesareshowninFig.8.Allof theN2adsorption‐adsorptionisothermsofthesamplescrystallizedformorethan8hhaveatypical Type IV isotherm, indicating mesoporous structures.Thepore size is centeredat4.5nm.Mesoporeswithporedi‐

ametergreater than10nmarealsopresent.When increasingthecrystallizationtimeto24h, thesynthesizedsample isstillmostly mesoporous, although a small number of microporesareformed,asindicatedinFig.8(b)andTable3.Theporesizedistributionalso changes, andmoremesoporesof~20nm indiameterarepresent.As thecrystallization time is further in‐creased to 48 and 72 h, the corresponding N2 adsorp‐tion‐desorption isotherms change and pore size distributionshowaremarkabledecreaseinthenumberof∼4nmdiametermesopores,andanincreaseinthenumberof~30nmdiametermesopores.

The texturalpropertiesof the samples synthesized fordif‐ferentcrystallizationtimesarelistedinTable3.Withthegen‐eration of theMFI phase in the synthesized samples, themi‐croporoussurfaceareaandvolumeincreasewhilethesurfaceareaandporevolumeassociatedwiththemesoporoussurfacedecrease.SBETandSMesoofthesamplesdecreasewithincreasingcrystallizationtimewhileSMicrotendstoincrease.Thevariationoftheporevolumeshowsalmostthesametrendasthatofthesurfaceareasexceptfortheslightlylowertotalporevolumeat8h.FromtheXRDcharacterizations,boththesamplessynthe‐sized for8 and24h aremesoporous.The small difference inthe pore volumes indicates that the mesoporosity developedduringthisperiod.

Toclarifythestructuralchangeprocessofthesamplessyn‐thesized for different crystallization times, TEM imageswereobtained.Figure9showstheTEMimagesofthesamplessyn‐thesized for different crystallization times. As shown in Fig.9(a), only mesoporous material with an irregular lamellarstructurewas formed during the initial 8 h,which is in goodagreementwiththeXRDresults(Fig.6(a)).Aftersynthesisfor

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Fig.7.CrystallizationcurveofMZSM‐5synthesizedat130°C.

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Fig.8.N2adsorption‐desorptionisotherms(a)andporesizedistributions(b)ofsamplessynthesizedatdifferentcrystallizationtimes.

Table3 Texturalpropertiesofsamplessynthesizedfordifferentcrystallizationtimes.

Crystallizationtime(h)

Surfacearea(m2/g) Porevolume(cm3/g)SBET SMeso SMicro Vtotal VMeso VMicro

8 690 690 0 1.07 1.07 024 668 624 44 1.22 1.21 0.0148 548 508 40 1.16 1.15 0.0172 502 431 71 0.84 0.81 0.03

 

QuanyiWangetal./ChineseJournalofCatalysis35(2014)1727–1739 1733

24h, theobtainedsamplestill retains themesoporousphase,asindicatedintheXRDpattern,whilethecorrespondingTEMimage(Fig.9(b))indicatesthatthesamplegrowsintoahollowstructure. This change in the structure confirms the develop‐mentofmesoporesduringthesynthesisperiodfrom8to24h,which may lead to the difference in pore volume (Table 3).Further increasing the crystallization time to 48 h leads to arapidtransformationfromthemesoporousstructurewithhol‐lowmorphologytothecrystallinephase.AsshowninFig.9(c),alargenumberofcrystalsareobserved,althoughtheshellpartof thehollowstructureremainsuntransformed.Uponheatingthe synthesis gel for 72 h, there is no amorphous phase andonly crystals are observed (Fig. 9(d)). In addition, the ZSM‐5crystals aggregate to form particles, which is consistent withtheSEMresults.

To obtain more information about the zeolite frameworkformationandthelocalcoordinationenvironmentofAlspeciesduringthesynthesisprocess,27AlMASNMRexperimentswere

conducted. Since theMFI phasewas confirmed to form aftercrystallizationfor48h,onlythesamplessynthesizedfor8,48,and120hwereusedforthe27AlMASNMRinvestigation.

Figure10showsthe27AlMASNMRspectraofsamplessyn‐thesizedforcrystallizationtimesof8,48,and120h.Twopeaksarepresent in thespectraofall threesamples.Thepeakcen‐teredat~54ppmcorrespondstotetrahedralAlofframeworkAlspecies.Theotherpeak,centeredat~0ppmwithlowerin‐tensity, isascribed tooctahedralAl,which isnormallyassoci‐ated with non‐framework Al species. Furthermore, from Fig.10,thepeaksat~54ppmforallthreesampleshavehighinten‐sity,which indicates that large amounts of tetrahedral Al arepresentintheZSM‐5framework,evenattheearlystageofthesynthesis (8 h). Furthermore, the intensity of the peak at~0ppm slowlydecreasedwith the increaseof the crystallizationtimefrom8to120h.ThisindicatesthatanincreasingamountofoctahedralAlinthenon‐frameworkAlspeciesareconvertedintotetrahedralAlandlocatedintheframeworkoftheMZSM‐5withincreasingcrystallizationtime.

Basedontheanalysisoftheresultsabove,apossiblemech‐anism to explain the formation ofMZSM‐5 using C18‐3‐18 as amesoporedirectingagentforthecrystallizationtransformationprocessisshowninScheme2.Duringtheearlystage(StepI),a

(b)(a)

(d)(c)

Fig.9.TEMimagesofsamplessynthesizedforcrystallizationtimesof8(a),24(b),48(c),and72h(d).

150 100 50 0 -50 -100

Chemical shift (ppm)

8 h

48 h

120 h

0

54

Fig.10. 27AlMASNMRspectraof samplessynthesized forcrystalliza‐tiontimesof8,48,and120h.

Si

Al

C18-3-18

TPABr

ZSM-5 crystal

Step I Step II Step III Step IV

Scheme2.ProposedformationprocessforthesynthesisofMZSM‐5usingC18‐3‐18asamesoporousstructuredirectingagent.

1734 QuanyiWangetal./ChineseJournalofCatalysis35(2014)1727–1739

mesoporous irregular lamellar structure is formed from thestartingsynthesisgelunderthestructuredirectingfunctionofC18‐3‐18.Themesoporousphasethenchangesfromtheirregularlamellarstructuretoahollowstructure(StepII).Asthehydro‐thermal synthesisproceeds (Step III), themesoporoushollowstructurecrystallizesintoZSM‐5crystals(MFIphase)withtheinvolvement of the TPABrmicroporous template. Finally, thegeneratedZSM‐5crystalsgrowandaggregatetoformMZSM‐5particleswithlargediametermesopores(>10nm)betweenthecrystalparticles (Step IV),and the initially formedmesoporesof∼4nminsizearedestroyedwithcrystallization.

4. Conclusions

Insummary,MZSM‐5withinter‐crystallinemesoporeswassuccessfully synthesized by a dual‐templating hydrothermalmethodusinganewgeminisurfactant,C18‐3‐18,asamesoporedirecting agent. This synthesis method was confirmed to beeffective at different crystallization temperatures. A low crys‐tallizationtemperature,suchas130°C,wasbetterforthesyn‐thesis of MZSM‐5 with good crystallinity, high mesoporoussurfacearea,highmesoporousvolume,andregularmorpholo‐gy.Therelativelylowcrystallizationtemperaturerequiredmayoriginatefromtheunique–N+–(CH2)3–N+–groupoftheC18‐3‐18mesopore directing agent. The amount of C18‐3‐18 used in thesynthesis affected the relative crystallinity and the texturalproperties of the obtained MZSM‐5. Optimizing the ratio ofmesoporousdirectingagent(C18‐3‐18)tomicroporoustemplate

(TPABr)ensuredthesynthesisofMZSM‐5.Detailed investiga‐tionshowedthattheformationofMZSM‐5followedacrystalli‐zationtransformationprocess.C18‐3‐18causedtheformationofmesoporousmaterialduringtheearlystage,whichwassubse‐quently converted into MZSM‐5 crystals by the function ofTPABras thetemplate for the formationof theMFIphase.Asthe synthesis proceeded, the MZSM‐5 crystals aggregate intoparticlesbyweakinteractions.C18‐3‐18asamesoporedirectingagentcouldbeextendedtothesynthesisofothermesoporouszeolites.

Acknowledgment

B.L.SuacknowledgestheChineseCentralGovernment foran“ExpertoftheState”positionintheprogramof“ThousandsTalents”andtheChineseMinistryofEducationforaChangjiangScholarpositionattheWuhanUniversityofTechnology.

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GraphicalAbstract

Chin.J.Catal.,2014,35:1727–1739 doi:10.1016/S1872‐2067(14)60128‐5

SynthesisofmesoporousZSM‐5usinganewgeminisurfactantasamesoporedirectingagent:Acrystallizationtransformationprocess

QuanyiWang,YingxuWei*,ShutaoXu,MozhiZhang,ShuangheMeng,DongFan,YueQi,JinzheLi,ZhengxiYu,CuiyuYuan,YanliHe,ShuliangXu,JingrunChen,JinbangWang,BaolianSu*,ZhongminLiu*DalianInstituteofChemicalPhysics,ChineseAcademyofSciences,China;UniversityofNamur(FUNDP),Belgium; WuhanUniversityofTechnology,China;UniversityofChineseAcademyofSciences,China

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Pagenumbersrefertothecontentsintheprintversion,whichincludeboth theEnglishandChinese versions of thepaper.The online versiononlyhastheEnglishversion.ThepageswiththeChineseversionareonlyavailableintheprintversion.