Reaction and surface characterization studies of K- and Cs-promoted, Zn/Cr spinel, higher alcohol synthesis catalysts

84 Pages·1997·3.3 MB·English

by  Epling, William S., 1970-

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