Reaction and surface characterization studies of K- and Cs-promoted, Zn/Cr spinel, higher alcohol synthesis catalysts PDF
Preview Reaction and surface characterization studies of K- and Cs-promoted, Zn/Cr spinel, higher alcohol synthesis catalysts
REACTIONANDSURFACECHARACTERIZATIONSTUDIES OFK-ANDCs-PROMOTED,Zn/CrSPINEL, HIGHERALCOHOLSYNTHESISCATALYSTS By WILLIAMS.EPLING ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT OFTHEREQUIREMENTSFORTHEDEGREEOF DOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 1997 ACKNOWLEDGMENTS IwouldliketothankGarHoflundforhisguidanceandassistanceduringmy studiesattheUniversityofFloridaandmystayinGainesville. Hisintroductiontolife beyondchemicalengineeringhasbeeninspiring. IwouldalsoliketothankCharles MountandJohnWolanfortheirpatiencewithmeinthelab. Furthermore,Imust acknowledgeB.andB.fortheirhumorandsanity. Finally,Iwouldliketorecognizeand thankthepeoplemostspecialinmylife,myparentsandsister,fortheirsupportalthough theydidnotunderstandwhyIchosetostayinschool. AndtoTonyaKleinforher patience, support and understanding from 536 miles away, I extend my deepest appreciation. TABLEOFCONTENTS page ACKNOWLEDGMENTS ii ABSTRACT iv CHAPTERS 1 INTRODUCTION 1 Overview 1 LiteratureReview 4 Fischer-TropschTypeCatalysts 5 ModifiedLow-TemperatureMethanolSynthesisCatalysts 8 ModifiedHigh-TemperatureMethanolSynthesisCatalysts 11 2 STUDYOFAK-PROMOTEDCOMMERCIALZn/CrSPINEL FORHAS 13 Experimental 13 ReactionStudies 13 CharacterizationStudies 14 ResultsandDiscussion 15 ReactionStudies 15 CharacterizationStudies 17 Summary 22 3 STUDYOFACs-PROMOTEDCOMMERCIALZn/CrSPINEL FORHAS 35 Experimental 35 ResultsandDiscussion 35 ReactionStudies 35 CharacterizationStudies 37 Summary 42 4 STUDYOFAK-PROMOTEDZnOCATALYSTFORHAS 55 Experimental 55 ResultsandDiscussion 55 ReactionStudies 55 CharacterizationStudies 58 Summary 62 REFERENCES 73 BIOGRAPHICALSKETCH 77 iii AbstractofDissertationPresentedtotheGraduateSchool oftheUniversityofFloridainPartialFulfillmentofthe RequirementsfortheDegreeofDoctorofPhilosophy REACTIONANDSURFACECHARACTERIZATIONSTUDIES OFK-ANDCs-PROMOTED,Zn/CrSPINEL, HIGHERALCOHOLSYNTHESISCATALYSTS By WilliamS.Epling August,1997 Chairman: GarB.Hoflund MajorDepartment: ChemicalEngineering Anequimolarmixtureofmethanolandisobutanolproducedfromsyngaswould be ideal for downstream production of methyl tertiary-butyl ether (MTBE), an oxygenatedadditive usedin gasolinetoday. Currently, MTBE isproducedfrom petroleumfeedstockandinanefforttoreducethisdependencemanystudieshave investigatedtheuseofmodifiedmethanolsynthesiscatalyststoproducemixturesof isobutanolandmethanolfromcoal-derivedsyngas. Furthermore,thepropermixturesof methanolandhigheralcoholscanbeuseddirectlyasfuelinmodifiedinternalcombustion enginesorasanadditive(e.g.,gasohol). Unfortunately,thecatalystswhichhavebeen discoveredandtestedthusfarproducehighlevelsofmethanolrelativetoisobutanolwith 3-to-lmoleratiosbeingtypical. Theproductionrateofisobutanolisalsorelativelysmall tobecommerciallyattractive. InthepresentseriesofstudiestheinvestigationofCsand K-promoted Zn/Cr spinel catalysts has led to improvements in product stream compositions. Theeffectsofpromotorloadingweremonitoredaswellasfoursetsof reactorconditionsinordertooptimizetheisobutanolproductionrate. Productflowrates ofisobutanolgreaterthan100g/kg-hrandmethanol-to-isobutanolmoleratiosbelow1.0 iv have been attained. Surface characterization studies using X-ray photoelectron spectroscopy(XPS)andionscatteringspectroscopyrevealthatthecatalystundergoes changesduringthereductivepretreatmentandwithtimeonstreaminthereactor. These changescanbequalitativelycorrelatedtothecatalyticperformance. Furthermore,the XPSdataconsistentlyindicatethattheonlyphaseofZninthenear-surfaceregionofall thesamplesexaminedisZnO. Therefore,K-promotedZnOwasalsotestedforhigher alcoholsynthesis(HAS). Althoughthiscatalystyieldedasmalleramountofisobutanol incomparisontothepromotedZn/Crspinelcatalystsonaweightbasis,onasurface-area basisthiscatalystprovestobesuperior. Basedontheseresultsithasbeendetermined thatanalkali-promotedZnOmaterialisacatalyticallyactivephaseinHAScatalysts. CHAPTER1 INTRODUCTION Overview Gasolineiscurrentlytheprimarytransportationfuelusedintheworld. Thehigh consumptionoffuelandthediminishingsupplyofdomesticoilreservesandoilbeds makeitnecessarytofocusattentiononimprovingtheefficiencyoffueluseandonthe useofalternativesourcesforfuelwhichareinmoreabundantsupply. Furthermore, growingconcernsovertheenvironmenthavestimulatedinterestintheuseoffuelswhich resultinloweremissions. Theinterestinpollutioncontrolintransportationisduetothe factthatthelargestsourceofhydrocarbonemissionsintheU.S.isautomobileexhaust Emittedhydrocarbonsinthepresenceofsunlightundergoaseriesofreactionswhich createground-levelozone,theprimaryingredientofsmog. EmissionscontainingCOand NOxfromcombustionenginesarealsopollutionproblems. Duringtherecentpast,laws havebeenimplementedwhichhaverequiredareductionintoxicemissions,andhigher fuelqualitiesandimprovedcatalyticconvertersprovidedthischange. Throughoutthe automobileandfuelindustries,however,thereisaconstantefforttomakefurther improvementsonfuelqualityandmorecompletecombustionofthefuel. Alcoholsandethersasfuelscanprovidesolutionsfortheseproblems. Most ethersandalcoholshavehighoctaneratingsandbumtoproduceprimarilywaterand carbondioxide. Theseoxygenatescanbeuseddirectlyastransportationfuels;however, extensive modification ofengines, fueldelivery systems and national and global distributionfacilitieswouldbenecessaryandquitecostly. Mostlikely,theuseofthe oxygenatesasfuelswillnotoccurinthenearfutureunlessenvironmentallegislation forcestheissue. Alcoholsandethers,however, alsocan be addedto gasolinein quantitiesof10to20%withouthavingtoadaptvehicleengines. Inthiscapacity,no 1 2 changes are necessary in the distribution facilities and the volume of gasoline consumptionisreduced. Alcoholmixturesblendedwithgasolineandetheradditivesin gasolinebothareusedcurrentlyintheU.S.fuelmarket. Methyltertiary-butylether (MTBE)isfoundinmostoftheunleadedgasolinetoday. Ethers,particularlyMTBEwill continuetobeusedasadditivesinthefutureandtheirroleisexpectedtoincrease. Furthermore, theuseoftheseadditiveshas beneficialeffectsbeyondreducingthe consumptionoffuelthroughdisplacementbytheadditive. Forexample,MTBEis solubleingasoline,requiringnocosolvents,actsasanoctaneenhancerpromotingmore completecombustionandthuscausesareductioninemissionsandcanbeaddeduptoas much as 15% in gasoline. The t-butyl fragmentofthe additive is derivedfrom isobutylenewhichiscurrentlyproducedfrompetroleum. TheproductionofMTBEfrom thisreactionpathwaycanthereforebelimitedbytheC4petroleumfeedstockavailability. Gasolineblendscontaininganhydrousfermentationethanol(gasohol)alsoare usedasmotorfuelstoday. Thesemixtureshaveanacceptablewatertolerance,an excellentoctanequalityandacceptablevolatilityandmaterialscompatibility. Although ethanolperformswellasagasolineextender,theproductionoffermentationethanolis notviablefromaneconomicstandpoint. Governmentexemptionsorassistanceare necessarytomakethisprocessindustriallycompetitive. Methanol-gasolineblendscan alsobeusedasafuelsourceforvehicles,butthemixturerequiresthepresenceofa higheralcoholinordertoachieveacceptablevolatilitiesandwatertolerancelevels. The performanceoftheseblendsisquitedependentontheconcentrationofthemethanolin theblendaswellastheratioofmethanoltohigheralcoholused. AnalternativerouteintheproductionofMTBEfrompetroleumfeedstockorin theproductionofalcoholmixtureswoulduseadomesticallyabundantsourceoffuel, namelycoal. Coal-derivedfuelscanbeproducedthroughtwomethods. Thedirect liquefaction method involves the high pressure hydrogenation of coal to liquid hydrocarbons. Theindirectliquefactionmethodconvertsthecoaltosyngas,amixtureof 3 COandH2,whichmaythenbeupgradedtomotorfuelsorfueladditives. Onesuch indirectprocessistheFischer-Tropschprocess. InthisprocessCOandH2arereacted overcatalyststoproducehydrocarbonswhichmaybereactedfurthertoformfuel. A broadrangeofproductsareproduced,however,andcarefuldownstreamseparation processesarerequired. Theheavierproductsofthisreactionprocess,suchaswaxes, mustbecrackedtobeusefulandalsomaysticktoandthereforedestroythecatalyst. Anotherinefficiencyistheformationoflighterproductsasgaseswhicharenotusefulas transportationfuels. Thisprocesshasbeeninvestigatedheavilyforover60yearsand moremajoradvancementsinthetechnologyarenotprobable. Anotherpossibleapproach usingcoalasanindirectfuelsourceisanalternativeroutetoformfueladditivessuchas MTBEoralcoholmixturesusingcoal-derivedsyngas. Aneconomicallyviablepathway whichusessyngastoproducetheappropriateamountsandratiosofmethanoland isobutanol could accomplish this task. For the production ofMBTE a separate intermediateprocessinvolvingthedehydrationoftheisobutanoltoisobutylenewould thenallowtheseproductstobeconvertedtothefueladditivethroughknowndownstream reactionprocesses. Thereactant,syngas,canbeproducedreadilyfromtheabundant domestic supplyofcoal, thusreducingthedependenceon adiminishingdomestic petroleumfeedstocktoproducetheadditiveorappropriatealcoholmixture. A process designed for higher alcohol synthesis (HAS) couldprovide the necessaryproductsfordownstreamsynthesistoMTBEorthepropermixturesforusein alcohol-gasolineblends. Higheralcohols,intheC3toC5range,canbeusedtoprovide volatilitycontrolandwatertoleranceinmethanol-gasolineblendsandapropermixtureof isobutanolandmethanolcouldbeusedtoproduceMTBE. Anidealdesignforthese processeswouldusealowH2/COratiosyngasandproducea1:1stoichiometricmixture ofmethanolandisobutanolfordownstreamMTBEsynthesisora5.4:1mixturefordirect useasthealcoholmixturefueladditive. Awell-designedcatalyticreactorprocessis necessary to achieve this goal and the selectivities and product flow rates must 4 necessarilybehigh. Extensiveresearchindevelopingacatalystwhichwouldprovidethe desiredproductstreamhasbeenaccomplished,andpreviousworkbyothergroupsis discussedbelow. Thisstudyfocusesonaseriesofcatalystswhichhavebeendeveloped forHAStoproducetheproperproductstreamforMTBEproduction. Surfacescience studieswereemployedaswelltohelpintheunderstandingofthecatalystsurfacesand ultimatelyleadtofurthercatalystimprovements. Theseexperimentshaveledtothe developmentofcatalystswhichaesuperiortothosepresentintheliteratureexaminedand leadtoasolidbasisforattainingaprocesswhichwouldbeeconomicallyviableineither theproductionofMTBEoralcoholadditivesintransportationfuels. LiteratureReview Higheralcoholsynthesisfromsyngas(CO/H2)hasbeenstudiedformanyyears. Thepossibilitiesofforminghydrocarbonsforfuel,alcoholsfordirectuseasfueland alcoholsasprecursorsforfuelorfueladditiveshavebeenthemotivatingfactorsbehind theinvestigations. Someoftheearlieststudieswereperformedinordertoelucidatethe reactionpathwayinHAS. FrolichandCryder(1)in 1930reportedtheformationof alcoholshigherthanmethanolbypassingsyngasoveraZn:Mn:Cr,1:1.1:1.03,catalyst Itwasreportedthatmethanolformsfromaformaldehydeintermediateandthatthehigher alcoholsformfromthemethanolthroughstepwisecondensationreactions. Oneyear later,astudypresentedbyGraves(2)supportedthisreactionsequence. Sincetheseearly studies, threeprimarygroups ofcatalystshave been adapted andtestedforHAS. ModificationsinFischer-Tropsch(FT)typecatalystsandinbothlow-temperature,low- pressureandhigh-temperature,high-pressuremethanolsynthesiscatalystsresultinthe conversionofCOandH2mixturestoalcohols. 5 Fischer-TropschTypeCatalysts Fischerandcoworkersinvestigatedtheproductionofhydrocarbonfuelsfrom syngasbeginninginthe1920sanddevelopedironandcobaltbasedcatalystsforthis operation. InlaterstudiesbyAndersonetal.(3,4),alkalipromotedornitridedFT catalystswereusedforHAS. Studieshaveshownthatthroughtemperatureandcontact timevariationsthehydrocarbonproductioncanbereducedinfavorofalcoholformation over Fe- and Co-containing FT catalysts. Specifically, decreases in operating temperaturesandreactantstreamcontacttimeincreasesthealcoholselectivitywith ethanolbeingtheprimaryproduct(5-7). ThemodificationofCo-containingFTcatalysts throughReandSrpromotionforHAShasbeenstudiedalso(8). Thesedoublypromoted CocatalystsproduceC2oxygenates,primarilyethanol. TheadditionofRepromotesthe activitywhileSrimprovestheselectivityofthecatalyst. Thetotalproductdistribution, however, follows an Anderson-Schulz-Flory (ASF) distribution. This product distributionischaracteristicofthelinearcondensationpolymerizationmechanism. A varietyofproductswerethereforeobserved,andlargehydrocarbonproductionswere notedaswell. TheuseofMocontainingcatalystshavebeenexaminedinHAS. Fujimotoand Oba(9)studiedaCo-Mocatalystwhichwaspromotedwithpotassiumcarbonate. Again thealcoholandthehydrocarbonproductstreamcompositionsfollowanASFdistribution. Thepresenceofthepotassiumcarbonateiseffectiveinincreasingtheselectivityto alcoholproduction through the reduction in hydrocarbon formation. Itwas also demonstratedthattheComustbeinareducedformforalcoholsynthesis. Kiennemann etal.(10)studiedtheeffectsofMoadditiontomodifiediron-basedcatalystsonHAS. TheyreportedthatcoprecipitatedFe-Cu-Mocatalystsproducehigherlinearalcoholsand theactivitiesandtheselectivitiestoalcoholformationofthesecatalystsisgreaterthan thosemadebythemoredirectimpregnationofasilicasupport. Thealcoholchain growthfollowsanASFproductdistribution. Theauthorsconcludedthattherateof
The list of books you might like
