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.
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
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:
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.
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.
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.
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.
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
Fig.3. Initialproductselectivity in thecoupledsystemand the linearadditionof the individual systems.Reactionconditions:Si/Al=200,WHSV(n‐Hex)=1h–1,WHSV(MAc)=0.1h–1.
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
TheselectivityofproductsoverHZSM‐5atSi/Al=200wassimilartothatatSi/Al=50.Insummary,thecouplingeffectofMAcandn‐Hexintheproductdistributionismainlyreflectedinthereductionof the formationofH2,CO,CO2,andoxygenated
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
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.
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.
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.
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
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.
