Investigation of the coupled reaction of methyl acetate ... · served, caused by the extremely high...
Transcript of Investigation of the coupled reaction of methyl acetate ... · served, caused by the extremely high...
ChineseJournalofCatalysis39(2018)1960–1970
a v a i l a b l e a t www . s c i e n c e d i r e c t . c om
j o u r n a l h omep a g e : www . e l s e v i e r . c om / l o c a t e / c h n j c
Article
Investigationofthecoupledreactionofmethylacetateandn‐hexaneoverHZSM‐5
KuoYanga,b,JinzheLia,c,XiaoZhangb,#,ZhongminLiua,*aNationalEngineeringLaboratoryforMethanoltoOlefins,DalianNationalLaboratoryforCleanEnergy,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences,Dalian116023,Liaoning,China
bCollegeofScience,StateKeyLaboratoryofHeavyOilProcessing,ChinaUniversityofPetroleum,Beijing102249,ChinacUniversityofChineseAcademyofSciences,Beijing100049,China
A R T I C L E I N F O
A B S T R A C T
Articlehistory:Received24July2018Accepted26July2018Published5December2018
Thecoupled reactionofmethylacetateandn‐hexanewascarriedoutoveraHZSM‐5catalyst. Inaddition toa thermal couplingeffect, systematicvariations in theproductdistributionwerealsoobservedinthecoupledsystem.Thebezene‐toluene‐xylene(BTX)selectivitywasremarkablyim‐provedwhiletheH2,CO,andCO2selectivitydecreased.Rapiddeactivationofthecatalystwasob‐served,causedbytheextremelyhighreactivityofmethylacetate,whichwasalleviatedafteraddingn‐hexane.Theseresultsindicatedthatacouplingeffectexistsinthissystem.Adetailedpathwayforthecoupledsystemissuggestedbasedontheanalysisofthesurfacespecies,carbonaceousspeciesdepositedonthecatalyst,aswellastheproductselectivitychanges.Thegoodmatchbetweenthe"hydrogendeficiency"ofmethylacetateandthe"hydrogenrichness"ofn‐hexaneisconsistentwiththeobservedcouplingeffect.
©2018,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences.PublishedbyElsevierB.V.Allrightsreserved.
Keywords:Methylacetaten‐HexaneCoupledreactionHZSM‐5
1. Introduction
Steamcracking isoneof themost importantprocesses forlight olefin production in the petrochemical industry. Lightalkanes and naphtha are widely used as raw feeds in steamcracking. After decades of development, steam cracking hasbecomeavastlyprovenprocess.However,steamcrackingisanendothermic reactionoperatedathigh temperature(>800°C)and thus, the large energy consumption remains its maindrawback[1,2].Asolutiontothisproblemiscouplinganexo‐thermicreactiontothishighlyendothermicprocessundercat‐alyticconditionstoimprovetheoverallenergyefficiency.
Nowaket al. [3,4] studied the couplingofC4hydrocarbonsandmethanoloverZSM‐5at600–700°C.Theresults showedthatcompletethermalneutralitycouldbeachievedatameth‐anol/n‐butanemolarratioof3:1,wheretheyieldoflowerole‐fins was much higher than that with methanol or n‐butanealone.ThecouplingcrackingofmethanolwithC6hydrocarbonsandnaphthaalsoshowedhighselectivityforlowerolefinsandbecameanalmostthermoneutralreactionwithimprovedheatcontrol [5].Shabalinaet et al. [6]combinedmethanol to olefin(MTO)withthecatalyticcrackingofalkanes.Theresults indi‐cated that thedecompositionofmethanolover thecatalyst inthe coupled systemwasmuch lower compared to that in the
*Correspondingauthor.Tel/Fax:+86‐411‐84379998;E‐mail:[email protected]#Correspondingauthor.Tel:+86‐10‐89734802;E‐mail:zhangxiao@cup.edu.cnThisworkwassupportedbytheNationalNaturalScienceFoundationofChina(21303264,21576256,21273005),theScienceFoundationResearchFundsProvidedtoNewRecruitmentsofChinaUniversityofPetroleum,Beijing(YJRC‐2013‐49),theBasicAcademicResearchFundsofChinaUniver‐sityofPetroleum,Beijing(2462015YQ0601).DOI:10.1016/S1872‐2067(18)63147‐X|http://www.sciencedirect.com/science/journal/18722067|Chin.J.Catal.,Vol.39,No.12,December2018
KuoYangetal./ChineseJournalofCatalysis39(2018)1960–1970 1961
individualfeedsystem.Wanetal.[7]investigatedthecoupledreactionofethanolandn‐hexaneoverHZSM‐5.Ahigherinitialconversionrateofn‐hexanewasobservedinthecoupledsys‐tem. A systematic investigation on the coupling of methanoland n‐hexane over a well‐designed pulse reaction systemshowed an increased initial conversion rate and decreasedstartingreactiontemperature[8].Theseeffectswereattributedto theactivationofn‐hexaneby interactionwith themethoxyspeciesformedonthezeolite.
Theexplorationofotherexothermicreactionsthatmaybecoupled to alkane crackingmayprovidenew insight into thisimportanttechnology.Theconversionofmethylacetate(MAc)over a zeolite catalyst is ahighly exothermic reaction.MAc, acompound bearing a C=O group with an effective hydrogenindex (EHI) of 0.67 [9], has a “hydrogen‐deficient” elementalcompositionandisthusagoodcandidateforcoupledsystems.Moreover,MAccanbeproducedbycarbonylationofdimethyletherwith CO over a zeolite in large‐scale,which is the corestep for theworld’s first coal‐based ethanol process that hasbeenrecentlycommercialized[10].
However,scarcereportsexistonMAc,especiallyonitscon‐versionovermolecularsieves.Romotowskietal. [11] investi‐gated the infrared (IR) spectral features of the interaction ofMAcwithH/NaZSM‐5 under specific experimental conditionsandtheysuggestedthatthereactionofMAcwithzeolitespro‐ceedsasfollows:
CH3COOCH3+HOzeolite→ CH3COOH+CH3Ozeolite (1)CH3COOH+HOzeolite→ CH3COO+zeolite+H2O (2)CH3COOH+HOzeolite→ CH3CO++Ozeolite+H2O (3)
CH3CO++CH3COO→(CH3)2CO+CO2 (4)Firstly,MAcinteractswithH/NaZSM‐5affordingaceticacid
and methoxy species. Acetic acid further reacts withH/NaZSM‐5to formacetateandacyliumions. Inaddition, thehydrolysisofMAcmayalsoleadtotheformationofaceticacid[11].Finally, the reaction of the acylium and acetate speciesaffordsacetoneandcarbondioxide.
MAc reacts on the ZSM‐5 surface to produce a certainamountofhydrocarbonspecies.However, largeamountsofCand O species are released fromMAc in the form of CO2 viadecarboxylation;as such, thehydrocarbonyield is lower thanthatusingmethanol.Someresearchershavemanagedtocom‐binemethanol with MAc; the yield of hydrocarbons was im‐proved as the decarboxylationwas transformed into a dehy‐dration process [9,12]. This suggested the existence of someformofsynergywhenspecieswithEHI<1andEHI>1reacttogetheroverZSM‐5.Suchaneffectmaypromotetheconver‐sionandimprovetheproductdistribution[13].
Asn‐hexane(themodelcompoundfornaphtha)hasanEHIof2.3and is thusa typicalhydrogen‐richalkane, the coupledreactionofMAcandalkanesholdsgreatpotential.Herein, thecoupledreactionofMAcandn‐hexaneoverHZSM‐5wasinves‐tigated in detail and comparedwith the individual reactions.Theeffectsofthereactionconditions(suchasthetemperature,spacevelocity,andratiooffeedstock)andcatalystSi/Alratioon the reaction behavior were evaluated. To determine thereactionpathwayforsuchacoupledsystem,avarietyofchar‐acterization methods, such as temperature‐programmed sur‐
face reactions (TPSRs), Fourier transform infrared (FT‐IR)spectroscopy, and gas chromatography‐mass spectrometry(GC‐MS),wereappliedtoanalyzetheactivespeciesdepositedonthecatalystsurface.
2. Experimental
2.1. Catalystandreactionconditions
HZSM‐5 (Si/Al = 19, 50, and 200) was obtained from theCatalystPlantofNankaiUniversity(Tianjin,China).Thezeolitewaspressedintoplateletsandthensievedto20–40mesh.MAc(99%)andn‐hexane (n‐Hex,99%)as feedstockswerepur‐chasedwiththehighestpurityavailable.
The catalytic reactions were performed in an electricallyheated stainless steel fixed bed reactor in the temperaturerange300–500°Cunderatmosphericpressure.Acatalystsam‐pleof2.5gwasloadedintothereactorineachexperiment.Thecatalystwasfirstactivatedat500°CunderN2flowfor1hbe‐forecoolingto thedesiredreactiontemperatureandthenthefeedstockswerepumpedsteadilyintothereactor.ThereactioneffluentwasanalyzedbyonlineGC(Angilent7890A)equippedwithaFIDdetector(PONAcapillarycolumn)andaTCDdetec‐tor (TDX‐1 packed column). In thiswork, the conversion andselectivity were calculated based on the carbon content per‐centage.
2.2. Characterizationmethods
In‐situFT‐IR studieswere carriedoutonaBrukerTensor27 FT‐IR spectrometer. All spectra were recorded using 32scanswith a resolution of 4 cm–1. TheHZSM‐5 samplewaferwasplacedinahigh‐temperaturehigh‐pressurecellfittedwithZnSewindows. The catalyst samplewas first activated underHeflowat500°Cfor2handthentheIRspectraoftheactivat‐edsampleswererecordedatdifferenttemperatures(denotedAi).Afterthat,heliumwasusedtocarrythevaporoftheindi‐vidual reactants or pre‐mixed reactants (W% (MAc/n‐Hex) =10%) continuously through the sample, heating the cell at atemperature‐programmed heating rate of 10 °C/min. At thesame time, the infrared spectra were recorded at differenttemperatureintervals(denotedBi).Ateachtemperature,theAispectrumwassubtracted fromtheBispectrum,providingtheinfrareddifferencespectrumatthatparticulartemperature.
Inthetemperature‐programmedexperiments,afreshcata‐lystsamplewaspretreatedin‐situbyheatingto500°Cfor2hunderN2flowandthencooledto100°C.ThestreamcarriedbytheN2wascontinuouslypassedthroughthecatalystbedfor30min. Then, the reactor was heated from 100 to 500 °C at aheatingrateof4°C/min.TheproductsweremonitoredwithanonlineOmnistarmassspectrometer.
The carbon specieswere analyzed and identified by Guis‐netetal'smethods [14,15]. The extracted oily liquidwas ana‐lyzedbyGC‐MS(Agilent7890A/5975CGC/MSDwithanHP‐5chromatographic column). Hexachloroethane was added todichloromethane as the internal standard and qualified usingtheNIST11database.
1962 KuoYangetal./ChineseJournalofCatalysis39(2018)1960–1970
3. Resultsanddiscussion
3.1. Comparisonofthecoupledreactionwiththelinear additionoftheindividualreactions
The initial conversion and product distribution of theco‐feeding system were compared to the individual reactionsystemsat350°C.AsshowninTable1,theinitialconversionofMAcwas100% indifferent reaction systemsoverHZSM‐5atdifferentSi/Alratiosandtheconversionofn‐Hexwasalsoverysimilarinthedifferentreactionsystems.
InFig.1,itcanbeclearlyseenthattheinitialproductselec‐tivitybyco‐feedingshowssystematicalvariationscomparedtothe linearadditionof the individualreagents (the linearaddi‐tionoftheindividualreactionsystemswasconsideredintermsofthemassfractionofthetworeactantsinthecoupledreactionsystem;theproductsintheindividualsystemsweremultipliedbythemass fractionof thecorrespondingreactants,andthencombinedfortheoverallcalculationoftheproductselectivity).
Forexample,atSi/Al=200, theoxygenatedspecies(DME,MeOH,HAc,andacetone)selectivitywas2.3%intheindividualsystem; however, theywere not detected in the coupled sys‐tem.Atthesametime,theselectivityofCOandCO2inthetwosystemswas also quite different. OverHZSM‐5 catalystswithdifferentSi/Alratios,theselectivityforCO+CO2inthecoupledsystemwaslowerthanthatintheindividualsystem.Inpartic‐ular,atSi/Al=200,COandCO2occupiedalargeproportionofthe product distribution in the individual reaction system. Inthe presence of n‐Hex in the coupled system, the selectivitytowardCOandCO2decreasedby6.4%.Thiscouldbeexplained
by changes in the reaction mechanism of MAc over HZSM‐5uponadditionofn‐Hex.Theseresultssuggestachemicalcou‐plingeffectinthecombinedsystem.
3.2. Effectofthereactiontemperatureonthecouplingbehavior
To further understand the observed coupling effects, thecombinedreactionofn‐HexandMAcwascarriedoutatdiffer‐entreactiontemperatures(from300to500°C).HZSM‐5withSi/Al=200wasusedas the catalyst inorder todiminish thesecondary reactions in the system. The results are shown inFigs.2and3.
Then‐HexconversionwasrelativelylowerthanthatofMAcbecause the reactivity of MAc ismuch higher. MAc can reactwith both the strong andweak acidic sites of HZSM‐5, whilen‐Hexcanonlyreactwiththestrongacidicsites.AsseeninFig.2,theMAcconversionwasimprovedinthecoupledsystematlowertemperatures,wherethedeactivationwaslessened.
Evidently,largeamountsofHAcandacetonewereformedatlowertemperaturesintheindividualsystem,asshowninFig.3(theHAcselectivitywas33.8%andacetoneselectivityof6.3%at300°C).Atthesametime,theCOxselectivitywasalsoveryhigh,especiallyat350°C,whiletheCOxselectivitywas24%intheindividualsystems.Theseresults indicatethatthedecom‐positionofMAcoverHZSM‐5isfavored.Unlikeintheindividu‐alsystems,thecouplingofn‐HexandMAcsignificantlyreducedtheselectivitytowardH2,CO,andCO2intherange300–500°C;moreover,HAc,MeOH,DME,andacetonewerenotfoundinthecoupledsystem.Mostimportantly,theselectivitytowardC2–C4olefins increased remarkably in the coupled system in therange300–400°C.Theseresultsdemonstratethattheadditionofn‐Hexpossibly changes themechanism ofMAc conversionoverHZSM‐5.
At 500 °C, the cracking of n‐Hexwas dominant and pro‐ducedH2and largeamountsofalkanes.Uponco‐feedingMAcwithn‐Hex, theH2andalkane selectivitywas significantly re‐duced,whiletheC5+(heavyaromatichydrocarbonscontainingmorethanfivecarbons)selectivitywassignificantlyimproved.The MAc capacity for “capturing hydrogens” from hydrocar‐bonswasthusconfirmed.
3.3. EffectoftheZSM‐5Si/Alratioonthecouplingbehavior
TocomparetheeffectofthecouplingbehavioratdifferentSi/Alratios,thereactionswerealsoperformedoverHZSM‐5atSi/Al=19and50.Duetotheincreaseddensityofacidicsites,theconversionofn‐HexoverSi/Al=19and50(Figs.4and6)wasevidentlyhigherthanatSi/Al=200.TheMAcconversionwasalsohigherthanthatatSi/Al=200andlowtemperature.
AsshowninFig.7,atSi/Al=50, itcouldbeobservedthat
Table1ConversionofMAcandn‐HexindifferentreactionsystemsoverdifferentSi/AlHZSM‐5catalystsat350°C,TOS=2.5min.
Si/Alratio 19 50 200Reactionsystem Individual Coupled Individual Coupled Individual CoupledConversionofMAc(%) 100.00 100.00 100.00 100.00 100.00 100.00Conversionofn‐Hex(%) 99.95 99.75 33.95 33.96 1.12 1.03
CO+CO Oxy-compound0
5
10
15
20
25
2
Sel
ecti
vity
(%
)
Si/Al=19 Individual system Coupled system
2 CO+CO Oxy-compound
Si/Al=50
Product CO+CO Oxy-compound2
Si/Al=200
Fig.1.SelectivityofCO+CO2andoxygenatedcompounds(DME,MeOH,HAc, andacetone) in the coupledand individual systemsat the sameconversionofMac.Reactionconditions:T=350°C,WHSV(n‐Hex)=1h–1,WHSV(MAc)=0.1h–1,W%(MAc/n‐Hex)=10%,HZSM‐5catalystwithSi/Al=19,50,200.
KuoYangetal./ChineseJournalofCatalysis39(2018)1960–1970 1963
theH2selectivityinthecoupledsystemwaslowerthanthatintheindividualsystem.ThisindicatedthatHatomsfromn‐HexwerecapturedbyMAc.BycomparingtheC5+,C2–C4olefin,CO,CO2,andoxygenatedcompoundselectivity,itcanbeconcludedthat MAc captures hydrogens from n‐Hex producing large
amounts of olefins and aromatic hydrocarbons, and that thereactionofMAcoverHZSM‐5isthusinterrupted.Thereactionof MAc over HZSM‐5 generates large amounts of heavy aro‐maticsratherthanCO,CO2,orotheroxygenatedcompounds.
The coupling effect in the product selectivity was more
0 25 50 75 100
0
20
40
60
80
100
0 25 50 75 100 0 25 50 75 100 0 25 50 75 100 0 25 50 75 100
Con
vers
ion
(%)
Si/Al=200Conversion of n-hexane
Individual system Coupled system
Conversion of MAc Individual system Coupled system
300 oC 350 oC
400 oC
TOS(min)
450 oC
500 oC
Fig.2.ConversioninthedifferentreactionsystemsovertheHZSM‐5catalyst.Reactionconditions:Si/Al=200,T=300–500°C,WHSV(n‐Hex)=1h–1,WHSV(MAc)=0.1h–1,andatmosphericpressure.
300 350 400 450 5000.0
0.2
0.4
0.6
0.8
1.0
Si/Al=200 H2
individual system coupled system
300 350 400 450 5000
10
20
30
CO+CO2
300 350 400 450 5000
20
40
60
C1-C4 alkanes
300 350 400 450 5000
10
20
30
40
50
Oxygenated compounds (excepting for CO and CO2)
C5+
C2-C4 olefins
Sel
ecti
vity
(%
)
300 350 400 450 5000
10
20
30
40
50
Temperature (oC)300 350 400 450 500
0
10
20
30
40
50
60
Fig.3. Initialproductselectivity in thecoupledsystemand the linearadditionof the individual systems.Reactionconditions:Si/Al=200,WHSV(n‐Hex)=1h–1,WHSV(MAc)=0.1h–1.
1964 KuoYangetal./ChineseJournalofCatalysis39(2018)1960–1970
pronouncedoverHZSM‐5 at lowSi/Al ratios. For instance, atSi/Al = 19 (Fig. 5), theH2 and C1–C4 alkane selectivity in thecoupled systemwas lower than that in the individual system,and theC2–C4olefin andC5+ aromatics selectivitywashigher.TheselectivityofproductsisconsistentwiththeHatomsfromn‐Hex being captured byMAc. The reasonmight be the high
densityofacidicsites.Asaresult,theintermediatesofMAconHZSM‐5aremorepronetointeractwithn‐Hex.
TheselectivityofproductsoverHZSM‐5atSi/Al=200wassimilartothatatSi/Al=50.Insummary,thecouplingeffectofMAcandn‐Hexintheproductdistributionismainlyreflectedinthereductionof the formationofH2,CO,CO2,andoxygenated
300 350 400 450 5000
2
4
6
8
10
Si/Al=19 H2
individual system coupled system
300 350 400 450 5000.0
0.5
1.0
1.5
2.0
CO+CO2
300 350 400 450 5000
20
40
60
80
100
C1-C4 alkanes
300 350 400 450 5000
5
10
15
20
25
30
35
40
Sel
ecti
vity
(%
)
Oxygenated compound (excepting for CO and CO2)
C5+
C2-C4 olefins
Sel
ecti
vity
(%
)
300 350 400 450 5000
10
20
30
40
Temperature (oC)
300 350 400 450 5000
1
2
3
Fig.5.Initialproductselectivityinthecoupledsystemandthelinearadditionoftheindividualsystems.Reactionconditions:Si/Al=19,WHSV(n‐Hex=1h–1,WHSV(MAc)=0.1h–1,W%(MAc/n‐Hex)=10%.
0 25 50 75 100
0
20
40
60
80
100
0 25 50 75 100 0 25 50 75 100 0 25 50 75 100 0 25 50 75 100
Con
vers
ion
(%)
300 oC
Si/Al = 19
Conversion of n-hexane
Individual system
Coupled system
Conversion of MAc
Individual system
Coupled system
350 oC
400 oC
TOS (min)
450 oC
500 oC
Fig.4.ConversioninthedifferentreactionsystemsovertheHZSM‐5catalyst.Reactionconditions:Si/Al=19,T=300–500°C,atmosphericpressure,WHSV(n‐Hex)=1h–1,WHSV(MAc)=0.1h–1.
KuoYangetal./ChineseJournalofCatalysis39(2018)1960–1970 1965
compounds.
3.4. Effectofthefeedratioonthecouplingbehavior
Different feedratioswere tested in the temperature range300–500 °C. Themass fractionofMAc in the coupled system
increasedto43%.Comparedtothepreviousexperiments(Fig.6), the inactivation of HZSM‐5 becamemore severe, and theinitial conversion of n‐Hex in the individual reaction systemwasmuchhigherthanthatinthecoupledsystem(Fig.8).ThissuggeststhatanexcessiveamountofMAcwouldpreferentiallyoccupytheacidicsitesandresultintherapidinactivationofthe
300 350 400 450 5000
1
2
3
4
Si/Al = 50 H2
individual system coupled system
300 350 400 450 5000
2
4
6
8
10
CO+CO2
300 350 400 450 5000
10
20
30
40
50
60
70
80
C1-C4 alkanes
300 350 400 450 5000
5
10
15
20
25
30
Sel
ecti
vity
(%
)
C2-C4 olefins
Sel
ecti
vity
(%
)
300 350 400 450 5000
1
2
3
4
5
Oxygenated compounds (excepting for CO and CO2)
300 350 400 450 5000
10
20
30
40
50
C5+
BT
X+
C5+
Temperature (oC)
Fig.7.Initialproductselectivityinthecoupledsystemandthelinearadditionoftheindividualsystems.Reactionconditions:Si/Al=50,WHSV(n‐Hex=1h–1,WHSV(MAc)=0.1h–1,W%(MAc/n‐Hex)=10%.
0 25 50 75 100
0
20
40
60
80
100
0 25 50 75 100 0 25 50 75 100 0 25 50 75 100 0 25 50 75 100
Con
vers
ion
(%)
Si/Al = 50
Conversion of n-hexane
Individual system
Coupled system
Conversion of MAc
Individual system
Coupled system
300 oC 350 oC
400 oC
TOS (min)
450 oC
500 oC
Fig.6.ConversioninthedifferentreactionsystemsovertheHZSM‐5catalyst.Reactionconditions:Si/Al=50,T=300–500°C,W%(MAc/n‐Hex)=10%,atmosphericpressure,WHSV(n‐Hex)=1h–1,WHSV(MAc)=0.1h–1.
1966 KuoYangetal./ChineseJournalofCatalysis39(2018)1960–1970
catalyst.Inotherwords,thepresenceofexcessMAcresultsincompetitionwithn‐Hexoverthecatalyticsites.
3.5. CokespeciesonZSM‐5inthedifferentreactionsystems
Compared to those inFig.9(B)and (C), a smallamountofheavy aromatic compounds is observed in Fig. 9(A),which isconsistentwiththeslowerinactivationofthecatalyticreactionin the individual n‐Hex system. The reaction systems in Fig.9(B)and(C)produceaparticularlylargenumberofheavyar‐omaticspecies.Theacidicsitesarecoveredandtheporesareblocked by these heavy aromatic species, which is the mainreasonfortherapiddeactivationofthecatalyst.Comparingthetwospectra(Fig.9(B)and(C)),thecompoundpeakpositionisbasicallythesame,exceptthoseat250and300°C.At250°C,4,6‐dimethyl‐2H‐pyran‐2‐one(α‐pyrone) is only found in theindividualMAc system.With the increase in temperature, theamountofα‐pyronedecreasesandonlyasmallamountisob‐servedat300°C.Inviewofthecatalystdeactivationtrends,theadditionofn‐Hextothecoupledsysteminterruptsthepathforα‐pyroneandthecatalystdeactivationiscorrespondinglymit‐igatedcomparedtothecaseoftheindividualMAcsystem.Thisphenomenon implies a possible process of carbon deposition
and catalyst deactivation by MAc; that is, the formation ofα‐pyronefirstlyonthecatalyst,followedbyitsfurtherconver‐sion to other oxygen‐containing heavy aromatics compoundsand, finally, decomposition into aromatics and polycyclic aro‐maticscontaininglargeamountsofsidechains.
3.6. TPSRwithon‐linemassanalysisundercontinuousflow
AsshowninFig.4,aremarkabledifferencecanbeobservedat300°C:inthecoupledsystem,theinitialn‐Hexconversionis100% while, in the individual system, it is only 60%. Theonline mass was determined to clarify this observation. Theevolutionofthesignalintensityatm/e=86,i.e.,themolecularionpeakofn‐Hex,isshowninFig.10.Comparedtothatoftheindividualn‐Hexsystem,thestartingtemperatureofthen‐Hexcrackingreactiondecreases from208to174°Cdue toacou‐plingeffect.Intherangeof300to400°C,severalincreasesareobserved in curve (b). This indicates that isomerization reac‐tionsofn‐Hexpossiblyoccurinthecoupledsystem.
3.7. FT‐IRspectroscopicstudies
TheinfrareddifferencespectraofHZSM‐5inthev(OH)re‐
0 25 50 75 100
0
20
40
60
80
100
0 25 50 75 100 0 25 50 75 100 0 25 50 75 100 0 25 50 75 100
Con
vers
ion
(%)
Conversion of n-hexaneIndividual systemCoupled system
Conversion of MAcIndividual systemCoupled system
300 oC
350 oC
400 oC
TOS (min)
450 oC
500 oC
Fig.8.ConversioninthedifferentreactionsystemsovertheHZSM‐5catalyst.Reactionconditions:Si/Al=50,T=300–500°C,W%(MAc/n‐Hex)=43%,atmosphericpressure,WHSV(n‐Hex)=5h–1,WHSV(MAc)=2.16h–1.
Fig.9.Depositedcarbonspeciesinthethreedifferentreactionsystems.(A)Individualn‐Hexsystem;(B)IndividualMAcsystem;(C)Coupledreactionsystem.Reactionconditions:T=250–500°C,WHSV(n‐Hex)=5h–1,WHSV(MAc)=2.16h–1,TOS=110min.
KuoYangetal./ChineseJournalofCatalysis39(2018)1960–1970 1967
gion is shown in Fig. 11(A). In the individual n‐Hex reactionsystem, two types of hydroxyl groups are observed from thenegative nature of the bands at 3750 and 3610 cm–1, whichcorrespond to terminal hydroxyl Si(OH) and bridge hydroxylSi(OH)Algroups[16–20].Withtheincreaseintemperature,theintensity of these negative features gradually decreased. Abroadbandat3480cm–1wasobservedat200°C,attributedtothe interactionof thebridgehydroxylsof the zeolitewithhy‐drocarbon compounds and the corresponding formation ofH‐bonds [18,19], which disappeared at temperatures above300°C.IntheindividualMAcsystem,thebandat3480cm–1isnot observed, and the bands at 3610 and 3750 cm–1 do notchange significantly with the temperature variation. Further‐more,inthecoupledsystem,thespectrumissimilartothatforthe individual MAc reaction system. These observations sug‐gestthat,inthecoupledsystem,MAcisadsorbedontheacidicsitesbeforen‐Hex.
Intheindividualn‐Hexreactionsystem,thebandsat2964,2935,and2874cm–1areattributedtothephysicaladsorptionofn‐Hex and other hydrocarbons over the zeolite [21]. Com‐pared to that in the individualn‐Hexreaction system, theap‐pearance of a new band at 2850 cm–1 in the coupled system
couldbeassignedtothesymmetricstretchingvibrationofthe–CH3 group of adsorbedMAc. In the individualMAc reactionsystem,thebandat3013cm–1canbeassignedtotheOHvibra‐tionsof themethylcarboxonium ion (CH3OH2+), formedbyat‐tractionofaskeletalproton[22].At200°C,asmallshoulderat2980cm–1appearsnexttothe2964cm–1band,whichcouldbeassigned to surfacemethoxy groups formed at Brönsted acidsites[16].Withtheincreaseintemperature,theintensityofthe3013cm–1bandgraduallyweakens,whilethatofthe2980cm–1bandgraduallyincreasesinthetemperaturerange200–400°C.At 500 °C, the intensity of the 2980 cm–1 band slightly de‐creased. These observations suggest that the methylcarboxo‐nium ion is gradually converted into surfacemethoxy specieswith the increase in temperature. In view of the decreasingtrendofthebandat2850cm–1,itcanbeinferredthattheC–ObondofMAchasbeencleaved.The IRdifference spectrumofthe coupled system is similar to that of the individual n‐Hexsystem.ThissuggeststhatMAcfirstformsanintermediatewithHZSM‐5,which then rapidly interactswithn‐Hex to generateotherhydrocarbons.
In thecoupledsystem, the initialhydride transfer reactionstartswiththeattackofn‐HextothesurfacemethoxyspeciesformedfromtheMAcreaction.Thepossibleprocess isshowninScheme1.
3.8. Proposedreactionpathwayforthecoupledn‐HexandMAcreaction
Fig.12showsthechangesintheinitialproductdistributionfor the different reaction systems in the temperature range125–500°C.At lowtemperatures,MAcismainlydecomposedintoHAc,DME,andMeOH.Above200°C, thedecarboxylationreaction of HAc is dominant, the CO and CO2 selectivity is
1.00E-009
2.00E-009
3.00E-009
4.00E-009
5.00E-009
6.00E-009
7.00E-009
(b)
(a)
100 200 300 400 500
208
Ion
abun
danc
e (a
.u)
Temperature (oC)
174
Fig.10.Signalevolutionunderaflowofthereactantmixtureandindi‐vidualn‐HexpassingthroughtheHZSM‐5zeolite(Si/Al=19,functionaltemperature:m/e=86;(a)individualn‐Hex,(b)mixtureofreactants).
4000 3600 32004000 3600 3200 4000 3600 3200
200 oC
Wavenumber (cm-1)
300 oC
400 oC
(A)
3610
3750 500 oC
MAc
200 oC
300 oC
n-Hex
3480
3610
3750
400 oC
500 oC
A
bsor
banc
e(A
.U)
200 oC
300 oC
400 oC
3610
3750
500 oC
n-Hex+MAc
3000 28003000 2800 3000 2800
200 oC
Wavenumber (cm-1)
300 oC
400 oC
2850
2874
(B)
2935
2980
2964
3013
500 oC
MAc
200 oC
300 oC
n-Hex
287429
3529
64
400 oC
500 oC
A
bsor
banc
e(A
.U)
200 oC
300 oC
400 oC
2850
2874
2935
2964
500 oC
n-Hex+MAc
Fig.11.IRdifferencespectrarecordedfortheHZSM‐5zeolite(Si/Al=50)undercontinuousflowofastreamofn‐Hex,MAc,andamixtureofn‐HexandMAc.
Scheme1.FormationofmethoxyspeciesonHZSM‐5.
1968 KuoYangetal./ChineseJournalofCatalysis39(2018)1960–1970
proved,andtheHAcselectivitydecreases.BycomparingBandC, the addition of MAc changes the product distribution ofn‐Hex,especiallyatlowtemperatures;theselectivityofalkanesin the coupled system significantly decreases, while the C5+aromaticsselectivityincreases.
Weproposeaseriesofpossiblereactionroutesforthiscou‐pled reaction system (Scheme 2). MAc is preferentially ab‐sorbedonthecatalyst.At150°C, thereactionconsistsmainlyofthereversecarbonylationofDME.At250°C,MAcreactswiththe catalyst to form 4,6‐dimethyl‐2H‐pyran‐2‐one, which isthenmainly decomposed to CO, CO2, and C6+. At 300 °C,MAcinteractswithHZSM‐5producingmainlymethoxyspeciesandacetic acid. With the increase in temperature, the methoxyroutepredominates.
Twotypesofreactionroutesexistforn‐Hexinthecoupledsystem. One fraction ofn‐Hex interacts directlywithHZSM‐5undergoingasinglemolecularmechanismtogeneratealkanesandH2,whiletheotherlargerfractioninteractswiththemeth‐oxyspeciesviaabimolecularmechanismtogeneratealkanes,olefins,andBTX.
4. Conclusions
Couplingthereactionsofn‐HexandMAcoverazeolitecat‐alystwas found to improve then‐Hex conversion. The initialcracking temperature ofn‐Hexwas reducedupon additionofMAc, asprovenbyTPSRmeasurements.Undermost reactionconditions,theselectivityforalkanesandH2decreasedandtheselectivitytowardolefinsandheavyaromaticsincreasedinthecoupled reactionsystem.Theseresults suggested that thehy‐drogen atoms in n‐Hex are captured by MAc. Based on theGC‐MSanalysisofcarbonspecies,wesuggestedthatα‐pyrone
might be themain active intermediate species in the catalystdeactivation induced byMAc. Methoxy species, the active in‐termediates,weredetectedby insituFT‐IR spectroscopyandwerepresumed to serve as the active sites forn‐Hexconver‐sion.Amechanismwasalsoproposed to explain thepossiblereactionroutespresentinthiscoupledsystem.
References
[1] S.M.Alipour,Chin.J.Catal.,2016,37,671–680.[2] C.Qi,Y.Wang,X.Ding,Chin.J.Catal.,2016,37,1747–1754.[3] S.Nowak,H.Günschel,A.Martin,K.Anders,B.Lücke,M.J.Philips,
M.Ternan,ProceedingsoftheNinthInternationalCongressonCa‐talysiseds.,1988,4,1735–1742.
[4] A.Martin,S.Nowak,B.Lücke,H.Günschel,Appl.Catal.A,1989,50,149–155.
[5] B.Lücke,A.Martin,H.Günschel,S.Nowak,MicroporousMesopo‐rousMater.,1999,29,145–157.
[6] L.B. Shabalina, V. I. Erofeev, T. S.Minakova, L.M.Koval,Russ. J.Appl.Chem.,2002,75,1979–1983.
[7] J. Wan, F. Chang, Y. Wei, Q. Xia, Z. Liu, Catal. Lett., 2009, 127,348–353.
[8] F.Chang,Y.Wei,X.Liu,Y.Zhao,L.Xu,Y.Sun,D.Zhang,Y.He,Z.Liu,Appl.Catal.A,2007,328,163–173.
[9] N.Y.Chen,D.E.Walsh,L.R.Koening,Preprints‐AmericanChemicalSociety,DivisionofPetroleumChemistry, 1987,32,277–289.
[10] H.Zhou,W.Zhu,L.Shi,H.Liu,S.Xu,Y.Ni,Y.Liu,L.Li,Z.Liu,Catal.Sci.Technol.,2015,5,1961–1968.
[11] T.Romotowski,J.Komorek,Zeolites,1991,11,35–41.[12] C. D. Chang, Ν. Υ. Chen, L. R. Koenig, D. E. Walsh, Pre‐
prints‐AmericanChemicalSociety,DivisionofPetroleumChemistry,1983,28,146–152.
[13] C.D.Chang,W.H.Lang,A.J.Silvestri,USPatent3998898,1976.[14] M.Guisnet,J.Mol.Catal.A,2002,182–183,367–382.
150 200 250 300 350 400 450 5000
10
20
30
40
50
60
70
80
90
100(A)Si/Al = 50
Individual MAc
Alkanes
C5+
OlefinsMeOH
DME
CO+CO2
HAc
Sel
ecti
vity
(%
)
Temperature (oC)150 200 250 300 350 400 450 500
0
10
20
30
40
50
60
70
80
90
100
(B)Si/Al = 50Individual n-hexane
H2
Olefins
C5+
Alkanes
Sel
ecti
vity
(%
)
Temperature (oC)
150 200 250 300 350 400 450 5000
10
20
30
40
50
60
70
80
90
100
(C)Si/Al = 50Coupled system
CO+CO2
Olefins
C5+
AlkanesSele
ctiv
ity
(%)
Temperature (oC)
Fig.12.ChangesintheproductdistributionoverHZSM‐5fortheindividualMAcreactionsystem(A),individualn‐Hexreactionsystem(B),andcou‐pledsystem(C)(T=125–500°C).
Scheme2.Transformationofn‐HexandMAcinthecoupledsystemoverHZSM‐5.
KuoYangetal./ChineseJournalofCatalysis39(2018)1960–1970 1969
[15] M.Guisnet,L.Costa,F.R.Ribeiro,J.Mol.Catal.A,2009,305,69–83.[16] T. R. Forester, R. F. Howe, J. Am. Chem. Soc., 1987, 109,
5076–5082.[17] S. M. Campbell, X. Z. Jiang, R. F. Howe,MicroporousMesoporous
Mater.,1999,29,91–108.[18] L. Kubelkova, J. Novakova, K. Nedomova, J. Catal., 1990, 124,
441–450.
[19] J.Rakoczy,T.Romotowski,Zeolites,1993,13,256–260.
[20] M.Trombetta,T.Armaroli,A.GutierrezAlejandre,J.RamireSolis,
G.Busca,Appl.Catal.A,2000,192,125–136.
[21] S.Kotrel,M.P.Rosynek,J.H.Lunsford,J.Catal.,2000,191,55–61.
[22] P.L.Benito,A.G.Gayubo,A.T.Aguayo,M.Olazar,J.Bilbao,J.Chem.
Technol.Biotechnol.,2015,66,183–191.
乙酸甲酯和正己烷在HZSM-5上耦合反应的研究
杨 阔a,b, 李金哲a,c, 张 潇b,#, 刘中民a,* a中国科学院大连化学物理研究所, 洁净能源国家实验室(筹), 甲醇制烯烃国家工程实验室, 辽宁大连116023
b中国石油大学理学院, 重质油国家重点实验室, 北京102249 c中国科学院大学, 北京100049
摘要: 蒸汽裂解是石油化工行业生产低碳烯烃和芳烃的一种非常重要的手段. 石脑油和一些低碳烷烃在蒸汽裂解中被广
泛用作原料. 经过长时间的发展和改进, 蒸汽裂解技术已经有了长足的进步, 但它作为一个强吸热的反应过程, 需要非常
高的温度才能进行, 巨大的能耗是蒸汽裂解技术所要面临的最大问题. 虽然催化裂解能够显著降低裂解温度到650 oC左
右, 但若将强吸热的反应和强放热的反应进行耦合, 这将是一种全新的解决能源利用率问题的途径. 我们将强放热的乙
酸甲酯在HZSM-5上的反应和强吸热的正己烷的裂解反应结合, 使得热量得到耦合. 其次, 乙酸甲酯拥有碳氧双键, 是一
种“缺氢化合物”, 而正己烷作为一种“富氢”化合物, 其元素组成上具有大量的氢元素. 两种反应物的共同反应, 除了热量
耦合效应外, 必将具有某种形式的元素耦合效应.
在固定床反应器上考察了不同条件下乙酸甲酯和石脑油模型化合物正己烷在HZSM-5上的反应, 发现耦合体系中产
物分布的系统性变化是显而易见的. 在多数反应条件下, 耦合体系中烯烃和C5以上芳烃的选择性升高, 而烷烃, H2, CO和
CO2的选择性则明显降低, 表明元素耦合效应存在于耦合体系中, 且契合了“缺氢”的乙酸甲酯能够夺取“富氢”的正己烷中
氢元素的特性.
在Si/Al=19的HZSM-5分子筛上, 反应温度为300 oC时, 耦合体系的正己烷初始转化率达到100%, 而此时正己烷单独
GraphicalAbstract
Chin.J.Catal.,2018,39:1960–1970 doi:10.1016/S1872‐2067(18)63147‐XInvestigationofthecoupledreactionofmethylacetateandn‐hexaneoverHZSM‐5
KuoYang,JinzheLi,XiaoZhang*,ZhongminLiu* DalianInstituteofChemicalPhysics,ChineseAcademyofSciences;ChinaUniversityofPetroleum; UniversityofChineseAcademyofSciences
Thecouplingofmethylacetate(hydrogen‐deficient)andn‐hexane(hydrogen‐rich)notonlyexhibitsthermalcouplingbutalsochemicalcoupling,wherebytheproductdistributionisimprovedcomparedtothecatalyticcrackingofindividualn‐hexane.
1970 KuoYangetal./ChineseJournalofCatalysis39(2018)1960–1970
进料体系的初始转化率只有58.9%. 我们通过TPSR对两种体系的正己烷变化趋势进行考察, 发现乙酸甲酯的加入会显著
降低正己烷的初始裂解温度, 进而促进正己烷的转化, 同时加剧了正己烷的芳构化.
乙酸甲酯非常高的活性导致催化剂的迅速失活, 但随着正己烷的加入而得到缓解. 通过GC-MS对积碳物种的分析发
现, 正己烷的加入改变了乙酸甲酯在分子筛上形成的积炭物种前驱体, 这也为我们研究耦合体系的反应机理提供了证据.
结合低温乙酸甲酯的产物分布和原位红外对三种反应体系的研究, 我们提出了一种正己烷和乙酸甲酯耦合体系的反应机
理.
关键词: 乙酸甲酯; 正己烷; 耦合反应; HZSM-5
收稿日期: 2018-07-24. 接受日期: 2018-07-26. 出版日期: 2018-12-05.
*通讯联系人. 电话/传真: (0411)84379998; 电子信箱: [email protected] #通讯联系人. 电话: (010)89734802; 电子信箱: [email protected]
基金来源: 国家自然科学基金(21303264, 21576256, 21273005); 中国石油大学(北京)引进人才科研启动基金(YJRC-2013-49); 中国石油大学(北京)基础学科研究基金(2462015YQ0601).
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).