of as made zsm 5 and their catalytic in the methanolity of zsm‐5 by utilizing as‐made zsm‐5...

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Article

Externalsurfacemodificationofas‐madeZSM‐5andtheircatalyticperformanceinthemethanoltopropylenereaction

XuebinZhaoa,b,c,YangHongb,c,LinyingWangb,DongFanb,NanaYanb,c,XiaonaLiub,c, PengTianb,#,XinwenGuoa,*,ZhongminLiub,$aStateKeyLaboratoryofFineChemicals,PSU‐DUTJointCenterforEnergyResearch,SchoolofChemicalEngineering,DalianUniversityofTechnology,Dalian116024,Liaoning,China

bNationalEngineeringLaboratoryforMethanoltoOlefins,StateEnergyLowCarbonCatalysisandEngineeringR&DCenter,DalianNationalLaboratoryforCleanEnergy,CollaborativeInnovationCenterofChemistryforEnergyMaterials (iChEM),DalianInstituteofChemicalPhysics,ChineseAcademyofSciences,Dalian116023,Liaoning,China

cUniversityofChineseAcademyofSciences,Beijing100049,China

A R T I C L E I N F O

A B S T R A C T

Articlehistory:Received31May2018Accepted6June2018Published5xxxxxx2018

Post‐synthetictreatmentofhigh‐silicaas‐madeZSM‐5withorganictemplateinthemicroporeswasexploredtoreduce/removetheexternalsurfaceaciddensityofZSM‐5.ItisfoundthatNa2H2EDTAtreatmentcanselectivelyremovethesurfaceAlatoms,butgeneratesnewacidsites(likelysilanolnests)ontheexternalsurface.H3PO4treatmentisunabletoremovesurfaceAlatoms,whilesmallamountofPisleftontheexternalsurface,whicheffectivelydecreasestheaciddensity.Thecatalyticperformance of the resultantmaterials is evaluated in themethanol conversion reaction. H3PO4treatmentcaneffectivelyimproveboththecatalyticlifetimeandthestabilityofpropeneselectivity.Thisoccursduetoacombinationoftheincreasedtolerancetotheexternalcokedepositionandthedepressedcokingrate(reducedsidereactions).Na2H2EDTAtreatmentonlyprolongsthecatalyticlifetime,resultingfromtheimprovedtolerancetotheexternalcokedeposition.UndertheoptimizedH3PO4treatmentcondition,theresultantZSM‐5givesacatalyticlifetimeofabout1.5timeslongerthantheprecursor.Moreover,thepropeneselectivityisimproved,showingaslightincreasingtrenduntilthedeactivation.

©2018,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences.PublishedbyElsevierB.V.Allrightsreserved.

Keywords:MethanoltopropeneZSM‐5zeoliteModificationPhosphoricacidEthylenediaminetetraaceticacid

1. Introduction

Withthedepletionofoilresource,themethanoltopropene(MTP) process provides an alternative route to produce pro‐penebeyondtheconventional steamcrackingofnaphthaandfluid catalytic cracking process [1–4]. The MTP reaction hasarousedsignificantinterestinboththeacademicandindustrial

fieldssinceitwasoriginallydevelopedbyLurgicompany[1].Ithasbeenrecognizedthatthemethanolconversionoverzeolitecatalystsisatypicalacidcatalyzedprocessandcokedepositionisthemaincauseofthecatalystdeactivation[5–8].ZSM‐5zeo‐lite, with a 3D interconnected pore system composed of10‐membered rings, is currently the most effective catalyst,which has found application in the industrial fixed bed MTP

*Correspondingauthor.Tel:+86‐411‐84986133;Fax:+86‐411‐84986134;E‐mail:[email protected]#Correspondingauthor.Tel:+86‐411‐84379218;Fax:+86‐411‐84379289;E‐mail:[email protected]$Correspondingauthor.Tel:+86‐411‐84379998;Fax:+86‐411‐84379289;E‐mail:liuzm@dicp.ac.cnThisworkwassupportedbytheNationalNaturalScienceFoundationofChina(21676262),andtheKeyResearchProgramofFrontierSciences,CAS(QYZDB‐SSW‐JSC040).DOI:10.1016/S1872‐2067(18)63117‐1|http://www.sciencedirect.com/science/journal/18722067|Chin.J.Catal.,Vol.39,No.0,xxxxxx2018

2 XuebinZhaoetal./ChineseJournalofCatalysis39(2018)0–0

process. DuetothespacelimitationinthemicroporesofZSM‐5(0.51

×0.55nm,0.53×0.56nm),thecokedepositioninthecrystalinterior canbe greatly avoidedduring theMTP reaction.AndthedeactivationonZSM‐5iscloselyrelatedtotheexternalsur‐faceacidity[9].Therefore,attemptshavebeenmadetoreducetheexternalsurfaceacidityofZSM‐5,aimingtoretardthecokedepositionrateandimprovethecatalyticstability.Thestrate‐gies developed to passivate the external surface acid sites ofzeolites canbemainlyclassifiedasmacromoleculedeposition[10–19],synthesisofzeoliteswithcore‐shellstructure[20–23],metalornon‐metalmodification[24–28]andsoon.

The macromolecules reported for the pas‐sivation/modificationof external surface acid sites of zeolitesinclude bulky organophosphorus compounds [10,11], bulkynitrogencompounds[12]andvarioussiloxane[14–19].Loschetal.[29]onceinvestigatedtheeffectofsurfacepassivationofnano‐sized ZSM‐5 on the methanol conversion reaction by achemicalliquiddeposition(CLD)techniqueusingtetraethylor‐thosilicate.Theresultantcatalystexhibitedanenhancedselec‐tivitytolowerolefins,butasignificantdecreaseinthestability,whichwasduetothepartialblockingofmicropores(thecokingprocessexclusivelyoccurredwithintheporesandfastenedthecatalyst deactivation). Phosphorus modification of HZSM‐5zeolitebywetimpregnationhaslongbeenknowntodecreasearomaticformationandimprovecatalyticstabilityinmethanolconversionchemistry[26,30,31].ImpregnationofHZSM‐5withphosphoric acid leads to the hydrolysis of framework Al andthe condensationofBrønsted acid siteswithPOHgroups, re‐sulting in higher framework Si/Al ratio and the decrease ofstrongacidsites[31,32].However,phosphoricacidcandepositbothintheinnerporeandexternalsurfaceofZSM‐5duetoitssmaller size. Recently, Liu et al. [24] reported an effectivemethod to prepare P‐modifiedH‐ZSM‐5with the preferentialcovering of the external surface acid sites by adding ethanolduringtheimpregnation.Thisoccurredbecausebulkyorganicphosphateswerelikelyformedfromthereactionofphosphoricacid and ethanol, which deposited on the external surface ofHZSM‐5 crystals. The obtained material gave high p‐xyleneselectivityalongwitharelativelyhighcatalyticactivityintolu‐enedisproportionationreaction.

Zeoliteswithcore‐shellstructurearegenerallysynthesizedbyepitaxialgrowthofinertshelloutsidezeolitecore.Inprinci‐ple,thedegreeofsurfacecoverageandshellthicknessarethekeyfactorsinfluencingthecatalyticperformance.Luoetal.[33]successfully synthesized ZSM‐5@S‐1 material, which gavecomparablepropeneselectivitywiththeZSM‐5precursorandaprolongedlifetimefrom74to139hintheMTPreaction.Thechange in lifetime should result from the reduced number ofexternalacidsitesandtheattenuatedoccurrenceofsidereac‐tions. Yin et al. [34] also demonstrated that the coverage ofsurface acid sites as a result of contiguous silicalite‐1 coatingwasthereasonforthehighstabilityandselectivityofZSM‐5inCH3Brconversionreaction.

Recently,selectivedealuminationmethodswerereportedtoremove theexternal surfaceacidsitesofZSM‐5. Inagakiet al.[35]foundthatpost‐syntheticHNO3treatmentofZSM‐5zeolite

synthesizedintheabsenceoforganicstructure‐directingagentcan selectively remove frameworkAl on the external surface,producing a unique ZSM‐5 zeolite catalyst that has very fewacid sites on its external surface. The treated ZSM‐5 showedhighresistancetocokeformationduringthecrackingofhexaneorotherparaffinmolecules.

Inthiscontribution,afacilepost‐synthetictreatmentmeth‐odwasdevelopedtoremove/modifytheexternalsurfaceacid‐ity of ZSM‐5 by utilizing as‐made ZSM‐5 containing organicstructure directing agent (OSDA) as the precursor. The exist‐ence of OSDA in the as‐made ZSM‐5 crystals may effectivelyblockthechannelsandconfinethemodificationonthecrystalexterior without altering the internal acidity and integrity ofthe crystals. Two common reagents, Na2H2EDTA and H3PO4,herein were explored for the post‐synthetic treatment. It ismentioned that largemoleculeEDTA, as a powerful chelatingagent,hasbeenwidelyusedforthedealuminationofYzeolite[36].But itsuse for thedealuminationofZSM‐5 isquite rare.Thevariations of structure integrity, surface aciddensity andcatalyticperformanceofthemodifiedZSM‐5wereinvestigated.Moreover, the H3PO4 concentrationwas optimized to furtherprolongthelifetimeintheMTPreaction.

2. Experimental

2.1. Materials

ThechemicalreagentsusedinsynthesisandmodificationofZSM‐5 zeolites include: tetraethyl orthosilicate (TEOS, TianjinKemiou Chemical Reagent Co), aluminium iso‐propoxide(Al(OPri)3, Sinopharm Chemical Reagent Co.),tetrapropylammonium hydroxide (TPAOH, 10 wt% TianjinJingrun Chemical Co), urea (Tianjin Damao Chemical Co),ethylenediamine tetraacetic acid disodium salt (Na2H2EDTA,TiandaChemicalReagentCo.Ltd.),phosphoricacid(85.0wt%,SichuanXianfengChemicalCo.).

2.2. SynthesisandmodificationofZSM‐5zeolites

ThesynthesisofZSM‐5zeolitewasaccordingtothereport‐ed literature [37]. The gel molar composition was300SiO2:1Al2O3:90TPAOH:6000H2O:450urea.Detained synthe‐sis procedure was carried out as follows: 104.2 g TEOS wasadded dropwise into 210.5 g TPAOH solution under stirring.After12h,0.681gAl(OPri)3wasadded into themixtureandstirredfor12h.Afterfurtheradditionof45.0gureaandstirredfor1h,theresultingmixturewastransferredintoanautoclave.The crystallizationwas carried out at 180 °C for 48 h undertumbling 60 r/min. The as‐made sample was centrifuged,washedwithdistilledwater,driedat120°Covernight.

Typicalpost‐syntheticmodificationtreatmentprocesswithH3PO4solutionwasasfollows:1.5gas‐madesamplewasaddedto20.0mLsolution(1mol/L) inaTeflon‐linedstainlesssteelautoclave.Aftertreatmentat90°Cfor12hundertumbling60r/min,theslurrywasfiltered,washedwithdistilledwater forthreetimesuntilpH=7.Allproductsweredriedat120°Cover‐nightandcalcinedat550°Cfor8htoremovetheorganictem‐

XuebinZhaoetal./ChineseJournalofCatalysis39(2018)0–0 3

plate.InthecaseofNa2H2EDTA,thesolutionpHwasadjustedto 1–2 by HCl and the final slurry was filtrated, followed bywashingwithlargeamountofhotwaterduetothelowsolubil‐ityofNa2H2EDTA.Theammoniumexchangeofcalcinedsamplewascarriedoutat80°CwithNH4NO3solution(1.0mol/L)forthreetimestoremovethepossibleresidualsodiumduringtheNa2H2EDTA treatment. The calcined precursor and samplesmodified by Na2H2EDTA and H3PO4 were named as ZSM‐5,ZSM‐5‐E‐x,ZSM‐5‐P‐xrespectively,wherexrepresentthesolu‐tionconcentration.

2.3. Characterization

The powder XRD pattern was recorded on a PANalyticalX'Pert PRO X‐ray diffractometer with Cu‐Kα radiation (λ =0.15418 nm), operating at 40 kV and 40 mA. The chemicalcomposition of solid sampleswas determinedwith a Philips‐Magix‐601X‐ray fluorescence (XRF) spectrometer. XPSmeas‐urements were performed using a Thermo ESCALAB 250XispectrometerwithAlKαradiationastheexcitationsource.Thesurfacechargeof thesamplewascalibratedbyreferencingtotheAl2ppeakofAl2O3at74.7eV.Thecrystalmorphologywasobserved using a scanning electron microscopy (HitachiSU8020).N2adsorption‐desorptionisothermsat–196°CweredeterminedonaMicromeriticsASAP2020.Prior to themeas‐urement,samplesweredegassedat350°Cundervacuumfor4h.ThetotalsurfaceareawascalculatedbasedontheBETequa‐tion.Themicroporevolumeandmicroporesurfaceareawereevaluatedusingthet‐plotmethod.27AlsolidstateNMRexper‐imentswereperformedonaVarianInfinityplus600WBspec‐trometer with BBO MAS probe operating at magnetic fieldstrengthof14.1T.Theresonancefrequencieswere104.2MHzandchemicalshiftswerereferencedto1.0mol/LAl(NO3)3.Thespinning rate of the samples at the magic angle was 4 kHz.Temperature‐programmeddesorptionofammonia(NH3‐TPD)wasmeasuredonachemicaladsorption instrumentofMicro‐metric 2920. Each sample (40–60 mesh, 0.20 g) was loadedintoaquartzU‐shapedreactorandpretreatedat600°Cfor1hin flowingHe.After thepretreatment, the samplewas cooleddownto100°CandsaturatedwithNH3gas.ThenNH3‐TPDwascarriedoutinaconstantflowofHe(20mL/min)from100to600°Cataheatingrateof10°C/min.

Fourier transform infrared (FT‐IR) spectra after pyridineadsorptionwere obtained on aBruker Tensor 27 instrumentwith a resolution of 4 cm−1. Samples were pressed into aself‐supportingwafer(R=13mm,20mg)andevacuated(10−2Pa)inanIRcellat723Kfor30minpriortoeachmeasurement.Adsorptionofpyridinewasconductedatroomtemperaturefor1 min to ensure saturated loading and then evacuation wascarried out at 350 °C for 30 min and cooled down to roomtemperaturepriortoacquisitionofIRspectra.ThedensitiesofBrønstedandLewisacidsiteswerecalculatedfromtheIAval‐ues of the difference spectra at 1540 and 1450 cm−1, respec‐tively,usingtheextinctioncoefficientsreportedbyEmeis[38].

2.4. Catalyticreaction

0.3 g calcined catalystwaspressed, sieved to 40–60meshand loaded in a fixed‐bed quartz tubular reactor with innerdiameterof8mm.Priortothereaction,thecatalystwasacti‐vatedat550°Cfor60min,andthenthetemperaturewasad‐justed to reaction temperature.Methanolwas fed by passingthe carrier gas (40.0mL/min) througha saturator containingmethanol at 35 °C, which gave a WHSV of 4.0 h–1. Methanolconversion was performed under atmospheric pressure. Theeffluent products from reactorwere kept at 200 °C and ana‐lyzed by an online Agilent 7890A GC equippedwith a PONAcapillarycolumn(100m×0.25mm×0.5m)andaFIDdetec‐tor.TheconversionandselectivitywerecalculatedonCH2ba‐sis. Dimethyl ether (DME) was considered as reactant in thecalculation. For the1,3,5‐triisopropylbenzene (TIPB) crackingreaction,similarconditionwasemployedexceptthatthereac‐tiontemperaturewas360°C.

2.5. Cokeanalysis

Thecokepercentofdischargedcatalystswascalculatedbythermogravimetricanalysis(TGA)andthelossbetween300to700°Cwasusedtoestimatecokecontent.Consideringthatthedifferentamountsofmethanolprocessedoneachcatalyst,theaveragecokedepositionratearedefinedasfollowed:

Rcoke=10C/((1–C/100)×T×M)where:Rcoke (mg/(gcat·h)): theaveragecokedepositionrate,C(%): the coke percent in discharged catalysts determined byTGA,T(h):reactiontime,M(g):thecatalystamount.

The coke formation insidemicropores (internal coke)wascalculated from the decrease in themicropore volume deter‐minedbyN2adsorptionat–196°C,assumingthecokedensity(dcoke)of1.22g/cm3.Thecokecontentdepositedontheexter‐nal surface was calculated by subtracting the internal cokecontent from the total coke content determined by TGA. Theamountsofinternalcokeandexternalcokewerecalculatedasfollows[39–41]:

Winternalcoke=(Vmicro,fresh–Vmicro,afterreaction× (1+Wtotalcoke))×dcoke

Wexternalcoke=Wtotalcoke–WinternalcokewhereWinternalcoke,Wexternalcoke, andWtotalcoke are the amounts(gcoke/gcat) of internal, external, and total coke, respectively.Vmicro,fresh and Vmicro, after reaction are the micropore volumes(cm3/g)ofthecatalystsbeforeandafterthereaction.dcokeisthecokedensity(1.22g/cm3).

3. Resultsanddiscussion

3.1. CharacterizationofZSM‐5zeolitesafterpost‐synthetictreatment

The XRD patterns of ZSM‐5 treated by Na2H2EDTA andH3PO4 solutions are shown in Fig. S1. It can be seen thatZSM‐5‐E‐1andZSM‐5‐P‐1haveatypicaldiffractionpatternofMFI topology, showing thehighstabilityofas‐madeZSM‐5 tothe post‐synthetic treatment. The relative crystallinity calcu‐latedbasedontheintensityofthecharacteristicpeaksat7.8o,8.8o and 22.4o are listed in Table S1. A decrease in relative


crystallinitywas observed for ZSM‐5‐E‐1. The SEM images ofthe samplesaredisplayed inFig. S2, showinga thin flakinessmorphologywith thickness of about 100 nm.No clear differ‐enceinthesurfacesmoothnessandmorphologycanbedistin‐guished for the crystals before and after post‐synthetic treat‐ment. The N2 adsorption‐desorption isotherms and pore dis‐tributionsofthesamplesaregiveninFig.S3.Thecorrespond‐ingtexturalpropertiesarelistedinTableS1.Itcanbeseenthatthe micropore surface area and micropore volume of thetreated samples display a little increase, while the externalsurfaceareaandmesoporevolumeslightlydecrease.Itisspec‐ulatedthattheremayexistsmallamountsofamorphousmate‐rials on the shell regionof theparent crystals,whichwas re‐movedbythepost‐synthetictreatment.

Solid‐state27AlMASNMRspectraweremeasuredtoinves‐tigatethechangeof localatomicenvironmentsinthesamplesand are displayed in Fig. 1.One strongbroad resonance cen‐teredaround56ppmisobservedforthecalcinedZSM‐5,whichisattributedtotetrahedralAlspecies[42].ItimpliesthatonlyframeworkAlatomsexistinthezeoliteprecursor.Noobviouschange can be found in the 27Al MAS NMR spectra afterpost‐synthetic treatment, indicating that the present strategycouldeffectivelyprotect the frameworkAl atoms fromdealu‐minationbypreventingaccessof themodifier intothecrystalinterior.

Table1liststhebulkandsurfaceSi/Alratiosofthecalcinedsamples. It can be seen that the bulk Si/Al ratio increasesslightly forZSM‐5‐E‐1 andZSM‐5‐P‐1.On the otherhand, theZSM‐5 precursor gives a surface Si/Al ratio of 60, which isobviously lower than the value of the bulk, implying an Alenrichment phenomenon on the crystal surface. The surfaceSi/Alratiosof thetreatedsamplesdiffer fromeachother; thevalueof ZSM‐5‐P‐1 is close to theprecursor,whereas a greatincrease is observed for ZSM‐5‐E‐1. It is speculated that theas‐madeZSM‐5hashighresistancetotheacidtreatment,andNa2H2EDTAismoreeffectivethanacidsforthedealuminationofhigh‐SiZSM‐5.Moreover,thereissmallamountsofPspeciesleftonZSM‐5‐P‐1(0.17wt%).ThebulkandsurfaceP/AlratiosofZSM‐5‐P‐1aregiveninTable1.AnobviouslyhighersurfaceP/Al ratio can be observed, implying that the P atoms aremainlylocatedontheexteriorofZSM‐5crystals.

The acid properties of the samples were examined byNH3‐TPD and pyridine‐adsorbed FT‐IR. The results aresummarized in Fig. 2 and Table 1. There exist two distinctdesorption peaks centered around 180 and 380 °C in theNH3‐TPDcurves(Fig.2(a)),correspondingtoweakandstrongacidsitesinthesamples.IncomparisonwithZSM‐5precursor,ZSM‐5‐E‐1 shows little change in thedesorption temperature

and peak height. However, the intensity of high‐temperaturedesorption peak of ZSM‐5‐P‐1 drops slightly, indicating thedecreaseofstrongacidsites.Theaciddensitycalculatedbasedon thehigh‐temperaturedesorption peak is listed inTable 1.Thepyridine‐adsorbedFT‐IRspectraaredisplayedinFig.2(b).The peaks at 1540 and 1455 cm–1 are attributed to thevibrations associated with pyridine adsorption on Brønstedacid sites and Lewis acid sites, respectively [43]. ThecorrespondingquantitativeresultsaregiveninTable1,whichindicate that the treated samples possess a slightly higherBrønsted acid sites and lower Lewis acid sites than theprecursor.Thevariationsoccurpossiblyduetotheremovalofthe amorphous materials on the external surface and areconsistent with the results in the part of texturalcharacterization. The acidity characterization results togetherwiththeaboveparagraphsimplythatthestructuralintegrityinthe crystal interior of ZSM‐5 remains intact after thepost‐synthetictreatment.

TIPB cracking reaction was employed to investigate theexternalsurfaceacidityofthesamples(Fig.3).ZSM‐5‐P‐1givesthe lowest TIPB conversion among the samples. This isreasonable because the strong interactions betweenP andAlatoms on the P‐rich surface can effectively decrease thenumberandstrengthofacidsites[44].However,unexpectedly,ZSM‐5‐E‐1 shows the highest catalytic activity. The TIPBconversionsonZSM‐5‐E‐1aremorethantwiceashighasthoseof the precursor (the TIPB cracking experiment has beenrepeated over repeatedly prepared ZSM‐5‐E‐1 and almostsimilar results are obtained). It seems contrary to the high

100 80 60 40 20 0 -20 -40

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ZSM-5

Fig.1.27AlMASNMRspectraofcalcinedsamples.

Table1TheSi/AlratioandacidpropertiesofZSM‐5beforeandafterpost‐synthetictreatment.

CatalystBulkphasea Surfacephaseb AciddensitybyPy‐IR(μmol/g) AciddensitybyNH3‐TPD

(μmol/g)Si/Al P/Al Si/Al P/Al B L B+LZSM‐5 124 – 60 – 112.2 15.5 127.6 112.6ZSM‐5‐E‐1 126 – 220 – 122.3 7.3 129.6 115.1ZSM‐5‐P‐1 130 0.44 78 2.17 119.6 5.8 125.4 104.3aDeterminedbyXRF.bDeterminedbyXPS.


surface Si/Al ratio of ZSM‐5‐E‐1 (Table 1). Possibly, silanolnests [45] are formed on the external surface of ZSM‐5‐E‐1crystals following the removal of frameworkAl atomsduringthe post‐synthetic treatment. The silanol groups interactingthrough extended hydrogen bonding have moderate acidityandcontributetotheenhancedTIPBcrackingactivity[47].

3.2. CatalyticperformanceinMTPreaction

Catalytic tests of methanol to olefins reaction over thesampleswerecarriedoutat480°CwithamethanolWHSVof4.0 h–1. The methanol conversion as a function of TOS ispresented in Fig. 4. Both ZSM‐5‐E‐1 and ZSM‐5‐P‐1 show animproved catalytic lifetime as compared with the ZSM‐5precursor. Given that the crystal interiors of ZSM‐5‐E‐1 andZSM‐5‐P‐1 are little influenced as revealed by the abovecharacterizations,theimprovementofcatalyticlifetimeshouldbecloselyrelatedtothechangeofthesurfaceproperties.

ThecokecontentofdischargedcatalystswasdeterminedbyTGanalysis(Fig.S4).Thecokedistributionsinthemicroporesand on the external surfacewere estimated according to theresultsofN2physisorptionandTGanalysis.AsshowninFig.4,thecokeismainlydistributedontheexternalsurfaceofZSM‐5(at least 75% of the total coke content), which is consistentwith the previous reports [9,39–41]. The treated samplespossessmore external coke than the precursor, indicating anincreased tolerance to the external coke deposition. Thecorresponding coke deposition rate is shown in Fig. 4.ZSM‐5‐P‐1 possesses lower external coke deposition rate,whereasZSM‐5‐E‐1giveshigherexternalcokedepositionratethantheprecursor.Thevariationsofexternalcokedepositionrate on the treated samples are consistent with the TIPBcracking results. On the other hand, both ZSM‐5‐E‐1 andZSM‐5‐P‐1givelowerinternalcokecontentanddepositionratethantheprecursor,whichevidencesanenhancedmasstransferin both samples (the micropores are more open after thetreatment).ItisinferredthattheimprovedcatalyticlifetimeofZSM‐5‐P‐1 is due to a combination of the increased externaltolerance to the external coke deposition and the depressedcoking rate (reduced side reactions); but for ZSM‐5‐E‐1, theincreased tolerance to the external coke should be themajorreason. Likely, the post‐synthetic treatment removes theamorphous materials around the pore openings and helpsimprovethetolerancetotheexternalcokedeposition.

The products selectivities on the samples as a function ofTOS are shown in Fig. 5. Moreover, detailed productdistributionsatTOS=60harelistedinTable2.ItcanbeseenthattheZSM‐5precursorgivespropeneasthemostabundantproduct,followedbybutene,C5+Nandethene.Theselectivitiestoethene,propeneandbutenedropas thereactionproceeds,while those of C5+N and aromatics increase. It is generallyaccepted that the reaction on ZSM‐5 follows a dual‐cyclemechanism: ethene mainly origins from the aromatics‐basedcycle, while propene and higher alkenes are formed to aconsiderableextent fromtheolefins‐basedcycle [46–48].The

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depression of lower olefins as the increase of TOS implies adroppingactivityofbothcycles.Theevolutiontrendofproductselectivity on ZSM‐5‐E‐1 resembles that on the ZSM‐5precursor,whichsuggestssimilarsurfacepropertiesofthetwosamples despite their different surface Si/Al ratios. However,forZSM‐5‐P‐1,theselectivityofpropeneismaintainedathighleveluntilthedeactivation;theselectivitiesofbuteneandC5+Nare relatively stable at the first dozens of hours, while theethene selectivity displays a continuous dropping trend. Thehigherpropene selectivityonZSM‐5‐P‐1 shouldbe attributedto the decreased external acid density, which abates theconsecutivereactionoflightolefinsontheexternalsurfaceandconsistswithitslowerselectivitiestoC9+aromatics(formedonthe external acid sites) and alkanes (C2‐40). These resultsdemonstrate that the reduction of external surface acid sites

can effectively improve both the catalytic lifetime and thestabilityofpropeneselectivity.

3.3. Influenceofphosphoricacidconcentration

Given the higher propene selectivity and longer lifetimeobserved on ZSM‐5‐P‐1, the H3PO4 concentration in thepost‐synthetic treatmentwas furtheradjustedtooptimizethecatalyticperformance.TheproductSi/AlratiosandPcontentsofthesamplesaresummarizedinTable3.NoregularchangeintheSi/AlratiocanbeobservedfollowingtheincreaseofH3PO4concentration,andthesampleshavesimilarbulkandsurfacePcontents. The insignificant effect of H3PO4 solution on thedealumination of high‐Si as‐made ZSM‐5 is understandable,since high‐Si zeolites are generally acknowledged to be

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(b) Fig.4.ThemethanolconversionasafunctionofTOS,thecokecontent,cokedistributionandcokedepositionrateonthedischargedZSM‐5catalysts.(a)ZSM‐5,(b)ZSM‐5‐E‐1,(c)ZSM‐5‐P‐1.Reactionconditions:480°C,methanolWHSV=4.0h–1.

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(%)

TOS (h)

Fig.5.ProductsselectivityasafunctionofTOSoverZSM‐5beforeandafterpost‐synthetictreatment.Reactionconditions:480°C,methanolWHSV=4.0h–1.ThedatafluctuationintheselectivityofaromaticsoverZSM‐5‐P‐1isduetotheinterruptionbyaddingmethanoltothesaturator.


resistant to the acid solution. Indeed, we have carried outpost‐syntheticHCltreatmentofthesameprecursorandalmostunchangedSi/Alratiosareobtained(TableS2).

TheTIPBconversionsofthesamplesarealsolistedinTable3. A gradual drop of the TIPB conversion can be observedfollowing the increase of H3PO4 concentration, which impliesthe reduction of external acid density from ZSM‐5‐P‐1 toZSM‐5‐P‐5. The TIPB cracking on pure Si silicalite‐1 is alsotestedunderthesameconditions,whichgivesaconversionof1.8%andconfirmstheexistenceofsmallamountofacidsiteson the H3PO4‐treated samples. The reduction in the externalsurfaceaciddensityastheincreasingH3PO4concentrationmaybe caused by the slight degradation of surface structuralintegritywhenZSM‐5wastreatedunderhigherconcentrationacidsolution.

Fig. 6 shows the methanol conversion results over theH3PO4‐treated samples. The catalytic lifetime reaches amaximum of 118 h on ZSM‐5‐P‐3 and reduces to 92 h onZSM‐5‐P‐5.TheimprovementincatalyticlifetimeofZSM‐5‐P‐3isapparentlybiggerthanthatofhigh‐SiZSM‐5withsimilarPloading (obtainedby impregnationofHZSM‐5withH3PO4) inthe previous report [49]. The coke content in the dischargedcatalystswas evaluatedbyTGanalysis (Fig. S4)and the cokedistributionsinthecrystalinteriorandontheexternalsurfacearealsopresentedinFig.6.Clearly,ZSM‐5‐P‐1andZSM‐5‐P‐3have similar external coke content,which is obviously higherthan that on ZSM‐5‐P‐5. The external coke deposition rateshows a reducing trend from ZSM‐5‐P‐1 to ZSM‐5‐P‐5, inaccordance with the external acid density of the samplesrevealedbyTIPBcracking.Ontheotherhand,theinternalcokecontent and deposition rate showa sequence of ZSM‐5‐P‐3<ZSM‐5‐P‐5<ZSM‐5‐P‐1<ZSM‐5.TheabnormalorderbetweenZSM‐5‐P‐3andZSM‐5‐P‐5suggestsareducedmasstransferinZSM‐5‐P‐5,whichmaybecausedbythedegradationofsurfacestructural integrity of ZSM‐5‐P‐5. This speculation is also

consistentwiththelowerexternalaciddensity(TIBPcrackingability)andlowertolerancetotheexternalcokedepositiononZSM‐5‐P‐5. It implies that suitableH3PO4concentration in thepost‐synthetic treatment of as‐made ZSM‐5 is important tomaximizethecatalyticlifetime.

The product selectivities as a function of TOS over thesamples are shown in Fig. S5. Detailed product distributions(TOS=60h) arepresented inTable 2. Similar to ZSM‐5‐P‐1,ZSM‐5‐P‐3 and ZSM‐5‐P‐5 display similar evolution trend ofproductselectivityasthereactionproceeds.Theselectivitiesofpropene, butene and C5+N are relatively stable at the firstdozensofhours,whiletheselectivitytoethenedropsgradually.From Table 2, the selectivities towards heavy aromatic andalkanes decrease gradually from ZSM‐5‐P‐1 to ZSM‐5‐P‐5,which implies the restrained side reactions and consist withtheirdroppingexternalaciddensity.ThisalsoexplainswellthehighestpropeneselectivityobservedoverZSM‐5‐P‐5withthelowestexternalaciddensity.

4. Conclusions

Insummary,wehavepresentedafacilestrategytoreducetheexternalsurfaceacidityofhigh‐silicaZSM‐5byutilizingtheas‐made zeolite as a precursor. The existence ofOSDA in theas‐made ZSM‐5 crystals effectively blocks the channels andconfinesthemodificationonthecrystalexteriorwithoutalter‐ing the internal acidity and integrity of the crystals. H3PO4

Table2ProductsdistributionofthemethanolconversionoverZSM‐5beforeandafterpost‐synthetictreatmenta.

Sample Lifetime(h)Selectivityb(wt%)

CH4 C2‐40 C2H4 C3H6 C4H8 C5+N BTEX AC9+ZSM‐5 80 1.6 4.1 7.9 41.0 21.3 16.6 4.7 2.8ZSM‐5‐E‐1 98 1.3 4.1 7.9 41.0 22.1 16.7 4.5 2.4ZSM‐5‐P‐1 100 1.3 1.8 7.9 44.5 24.5 15.7 2.7 1.6ZSM‐5‐P‐3 118 1.0 1.9 8.7 44.1 25.4 14.6 2.9 1.3ZSM‐5‐P‐5 92 0.9 1.5 7.0 45.9 24.9 17.0 2.0 1.0aReactionconditions:480°C,WSHV=4.0h–1.Thelifetimeisdefinedasmethanolconversionofabove99%.bThedataarecollectedatTOS=60h.C5+N

non‐aromaticproductswithcarbonnumber>4,BTEX:lightaromaticswithcarbonnumber<9,AC9+:heavyaromaticswithcarbonnumber≥9.

Table3The product compositions and TIPB conversion (TOS = 5 min) overZSM‐5modifiedbyH3PO4solutionwithdifferentconcentrations.

Catalyst Si/Ala Si/AlbPcontent(wt%) TIPBconversion

(%)Bulka Surfaceb

ZSM‐5‐P‐1 130 78 0.17 1.38 6.5ZSM‐5‐P‐3 132 55 0.12 1.21 5.7ZSM‐5‐P‐5 128 79 0.13 1.30 4.5Silicalite‐1 – – – – 1.8aDeterminedbyXRF.bDeterminedbyXPS.

0.00

0.05

0.10

0.15

0.20

0.25

0.0

0.5

1.0

1.5

2.0

Internal External

C

oke

(wt%

)

ab

cc

a

Internal External

R

coke

(m

gg ca

t1h

1) b

Fig.6.ThemethanolconversionasafunctionofTOS,thecokecontent,cokedistributionandcokedepositionrateoverZSM‐5beforeandafterthe H3PO4 Treatment. (a) ZSM‐5‐P‐1, (b) ZSM‐5‐P‐3, (c) ZSM‐5‐P‐5.Reactionconditions:480°C,methanolWHSV=4.0h–1.


treatment cannot remove surface Al atoms, but the smallamount of P left on the external surface effectively decreasestheaciddensity.Na2H2EDTAtreatmentselectivelyremovesthesurfaceAlatoms.However,newacidsites(likelysilanolnests)aregeneratedon theexternal surface afterNa2H2EDTA treat‐ment.BothNa2H2EDTAandH3PO4treatmentscanprolongthecatalytic lifetime of ZSM‐5 in theMTP reaction. The reducedexternalsurfaceaciddensityontheH3PO4‐treatedZSM‐5hasalower external coke deposition rate (reduced side reactions),whichhelpsthesampleshowhigherpropeneselectivity.UndertheoptimizedH3PO4concentration,thetreatedZSM‐5extendsthecatalyticlifetimefrom80hoftheprecursorto118h.

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XuebinZhao,YangHong,LinyingWang,DongFan,NanaYan,XiaonaLiu,PengTian*,XinwenGuo*,ZhongminLiu*DalianUniversityofTechnology;DalianInstituteofChemicalPhysics,ChineseAcademyofSciences;UniversityofChineseAcademyofSciences

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高硅ZSM-5原粉的外表面改性及其在甲醇制丙烯反应中的应用

赵学斌a,b,c, 洪杨b,c, 王林英b, 樊栋b, 闫娜娜b,c, 刘晓娜b,c, 田鹏b,#, 郭新闻a,*, 刘中民b,$ a大连理工大学化工学院, 宾州州立大学-大连理工大学联合能源研究中心, 精细化工国家重点实验室, 辽宁大连116024

b中国科学院大连化学物理研究所, 甲醇制烯烃国家工程实验室, 国家能源低碳催化与工程研发中心,

洁净能源国家实验室, 能源材料化学协同创新中心, 辽宁大连116023 c中国科学院大学, 北京100049

摘要: 使用来源广泛的甲醇为原料制取丙烯, 是对石油路线生产丙烯的重要补充. 尽管以ZSM-5分子筛为催化剂的固定床

甲醇制丙烯(MTP)技术已经实现了工业应用, 但进一步提高催化剂的寿命和丙烯的选择性一直是学术界和工业界的研究热

点. MTP过程作为典型的酸催化反应, 积碳是催化剂失活的主要原因, 由于ZSM-5分子筛十元环孔道的空间限制作用, 积碳

主要分布在外表面. 因此, 消除外表面的酸性位点对延长催化剂的寿命至关重要, 但修饰外表面酸性位点的同时往往会改

变样品的其他性质, 如孔径、整体酸量等. 本文分别使用Na2H2EDTA和H3PO4处理高硅ZSM-5分子筛原粉(微孔内含有机模

板剂)来选择性地减小或消除外表面的酸密度, 而不影响分子筛内部的性质, 并考察了处理后样品的MTP性能. 使用N2物理

吸附、SEM和27Al MAS NMR表征样品的织构性质、形貌和Al原子的化学环境, XPS和三异丙苯裂解实验表征外表面的硅

铝比和酸密度. 表征结果表明, Na2H2EDTA处理虽然可以选择性的脱除表面Al原子, 但会在外表面产生新的酸点(可能是

硅巢). H3PO4处理虽然不能脱除表面的Al原子, 但外表面残留的P物种能够有效的减小酸密度. MTP评价结果表明, H3PO4

处理能够有效的延长催化剂的寿命和维持丙烯的选择性, 这是因为H3PO4处理既提高了外表面的容碳能力, 也抑制了积碳

沉积的速率. Na2H2EDTA处理仅能增加外表面的容碳能力, 所以其只能延长催化寿命. 通过进一步优化H3PO4后处理的条

件, 处理后的ZSM-5样品的催化寿命可以延长至前体的1.5倍, 同时, 丙烯的选择性也略有提高, 并且在失活前维持增加的趋

势.

关键词: 甲醇制丙烯; ZSM-5分子筛; 改性; 磷酸; 乙二胺四乙酸

收稿日期: 2018-05-31. 接受日期: 2018-06-06. 出版日期: 2018-00-05.

*通讯联系人. 电话: (0411)84986133; 传真: (0411)84986134; 电子信箱: [email protected] #通讯联系人. 电话: (0411)84379218; 传真: (0411)84379289; 电子信箱: [email protected] $通讯联系人. 电话: (0411)84379998; 传真: (0411)84379289; 电子信箱: [email protected]

基金来源: 国家自然科学基金(21676262); 中国科学院前沿科学重点研究计划(QYZDB-SSW-JSC040).

本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).

For Author Index:

ZHAO Xuebin, HONG Yang, WANG Linying, FAN Dong, YAN Nana, LIU Xiaona, TIAN Peng, GUO Xinwen, LIU Zhongmin


SupportingInformationfor


XuebinZhaoa,b,c,YangHongb,c,LinyingWangb,DongFanb,NanaYanb,c,XiaonaLiub,c, PengTianb,#,XinwenGuoa,*,ZhongminLiub,$aStateKeyLaboratoryofFineChemicals,PSU‐DUTJointCenterforEnergyResearch,SchoolofChemicalEngineering,DalianUniversityofTechnology,Dalian116024,Liaoning,China

bNationalEngineeringLaboratoryforMethanoltoOlefins,StateEnergyLowCarbonCatalysisandEngineeringR&DCenter,DalianNationalLaboratoryforCleanEnergy,CollaborativeInnovationCenterofChemistryforEnergyMaterials (iChEM),DalianInstituteofChemicalPhysics,ChineseAcademyofSciences,Dalian116023,Liaoning,China

cUniversityofChineseAcademyofSciences,Beijing100049,China

*Correspondingauthor.Tel:+86‐411‐84986133;Fax:+86‐411‐84986134;E‐mail:[email protected]*Correspondingauthor.Tel:+86‐411‐84379218;Fax:+86‐411‐84379289;E‐mail:[email protected]$Correspondingauthor.Tel:+86‐411‐84379998;Fax:+86‐411‐84379289;E‐mail:[email protected]

Fig.S1.XRDpatternsofas‐madeZSM‐5beforeandafterpost‐synthetictreatment.

TableS1RelativecrystallinityandtexturepropertiesofZSM‐5beforeandafterpost‐synthetictreatment.

Catalyst RaSurfaceareab(m2/g) Porevolumec(cm3/g)

SBET Smicro Sext Vmicro VextZSM‐5 100 377 318 60 0.151 0.078ZSM‐5‐E‐1 81.2 392 349 43 0.164 0.058

ZSM‐5‐P‐1 103.8 380 331 49 0.156 0.066aRelativecrystallinitycalculatedbasedontheintensityofthecharacteristicpeaksat7.8o ,8.8oand22.4o; bSBET:BETsurfacearea,Smicro:t‐plotmi‐croporoussurfacearea,Sext=SBET–Smicro;cVmicro:t‐plotmicroporousvolume,Vext=Vtotal–Vmicro(VtotalisevaluatedatP/P0=0.99).

Fig.S2.SEMimagesofZSM‐5zeolitesbeforeandafterpost‐synthetictreatment.


Fig. S3. N2 adsorption‐desorption isotherms and pore size distribution curve calculated from the adsorption branch of ZSM‐5 before and afterpost‐synthetictreatment.

Fig.S4.TGcurvesofdischargedZSM‐5catalystsafterthemethanolconversionreaction.

TableS2TheSi/AlratioofZSM‐5beforeandafterpost‐syntheticHCltreatment.a

Sample Si/AlBulkb Si/AlSurfacecZSM‐5 124 60ZSM‐5‐H‐1 124 44

aTreatmentconditionforZSM‐5‐H‐1:1mol/LHClsolution,90°C,12h.bDeterminedbyXRF. cDeterminedbyXPS.


Fig.S5.ProductselectivitiesasafunctionofTOSoverZSM‐5zeolitesmodifiedbyH3PO4solutionwithdifferentconcentration.

of as made zsm 5 and their catalytic in the methanolity of zsm‐5 by utilizing as‐made zsm‐5...

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