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Original citation:
Alexiadis, V. I., Chaar, M., van Veen, Andre C., Muhler, M., Thybaut, J. W. and Marin, G. B..
(2016) Quantitative screening of an extended oxidative coupling of methane catalyst library.
Applied Catalysis B: Environmental, 199 . pp. 252-259.
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ContentslistsavailableatScienceDirect
Applied
Catalysis
B:
Environmental
jo u r n al ho m e p ag e :w w w . e l s e v i e r . c o m / l o c a t e / a p c a t b
Quantitative
screening
of
an
extended
oxidative
coupling
of
methane
catalyst
library
V.I.
Alexiadis
a,
M.
Chaar
b,
A.
van
Veen
b,1,
M.
Muhler
b,
J.W.
Thybaut
a,∗,
G.B.
Marin
aaLaboratoryforChemicalTechnology,GhentUniversity,Technologiepark914,B-9052,Ghent,Belgium
bLaboratoryofIndustrialChemistry,DepartmentofChemistry,Ruhr-UniversityBochum,D-44780,Bochum,Germany
a
r
t
i
c
l
e
i
n
f
o
Articlehistory: Received6March2016
Receivedinrevisedform19May2016 Accepted4June2016
Availableonline6June2016
Keywords: Ethylene
Homogeneous-heterogeneousreaction network
Alkalineearthcatalyst NaMnWcatalyst Microkineticmodelling LiMgOcatalyst
a
b
s
t
r
a
c
t
Acomprehensivemicrokineticmodel,includingcatalystdescriptors,thataccountsforthehomogeneous aswellasheterogeneouslycatalyzedreactionstepsinOxidativeCouplingofMethane(OCM)wasusedin theassessmentoflargekineticdatasetsacquiredonfivedifferentcatalyticmaterials.Theapplicabilityof themodelwasextendedfromalkalimagnesiacatalystsrepresentedbyLi/MgOandSn-Li/MgOand alka-lineearthlanthanacatalystsrepresentedbySr/La2O3torareearth-promotedalkalineearthcalciumoxide
catalysts,representedbyLaSr/CaO,andtoaNa-Mn-W/SiO2catalyst.Themodelsucceededinadequately
simulatingtheperformanceofallfiveinvestigatedcatalystsintermsofreactantconversionandproduct selectivitiesintheentirerangeofexperimentalconditions.ItwasfoundthattheactivityofSr/La2O3,
intermsofmethaneconversion,isapproximately2,5,30and33timeshigherthanovertheLa-Sr/CaO, Sn-Li/MgO,Na-Mn-W/SiO2andLi/MgOcatalysts,respectively,underidenticaloperatingconditions.This
wasattributedmainlytothehighstabilityofadsorbedhydroxyls,thehighstabilityofadsorbedoxygen andthehighconcentrationofactivesitesofSr/La2O3.TheselectivitytowardsC2productswasfoundto
dependonthemethylradicalstickingcoefficientandthestabilityoftheadsorbedoxygenandwasthe highestontheNa-W-Mn/SiO2catalyst,thatis75%atabout1%methaneconversionand1023K,190kPa
andinletmolarCH4/O2ratioof4.
©2016TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCC BY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Oxidativecoupling of methane (OCM) toethylene hasbeen studiedfor morethan30 years[1]and offerssignificant indus-trialpotential,sinceitcouldbroadenthefeedstockbasisforthe chemicalindustry.Theproductionofethylenefromnaturalgas rep-resentsasignificantopportunitybecauseofitsmassive,worldwide useasamonomerinplasticsproduction.Furthermore,ethylene canbeoligomerizedintoliquidhydrocarbons,suchasalphaolefins orgasoline, thereby enabling theefficientutilization of natural gasinso-calledstrandedareas[2].Atpresent,ethyleneismainly producedbysteamcrackingofnaphtha,acrudeoilproduct.The importanceofalternativecarbonsourcesbecomesgreaterasoil priceincreasesaswellasinviewofsustainability[3].
OCMcanbedescribedasagasphasereactionwhichisinitiated viaheterogeneouscatalysis[4].Thecatalystisactivatedthrough
∗Correspondingauthor.
E-mailaddress:[email protected](J.W.Thybaut).
1 Presentaddress:UniversityofWarwick,SchoolofEngineering,Coventry,CV
7AL,UK.
reversible,dissociativeoxygenchemisorptionresultinginsurface oxygenspeciesthatprovokegasphasemethylradicalformationby hydrogenabstractionfrommethane.Theformedmethylradicals eithercouple in thegasphase to formC2 productsor are oxi-dizedtowardscarbonoxides[5,6].Severalmetaloxideshavebeen investigatedthoroughlyforthisreaction,someofwhichprovedto beeffectivecatalysts,suchaslithiummagnesiacatalysts,alkaline earthlanthanacatalysts,rareearth-promotedalkalineearth cal-ciumoxidecatalystsandNa-Mn-W/SiO2catalysts.However,none ofthemhasreachedthestageofcommercialimplementationyet, sincetheirperformanceintermsofC2yieldremainsrelativelylow
[5].
AnelaboratemicrokineticmodelforOCM,thatincludescatalyst descriptorsandaccountsforthermal,homogeneousandcatalytic, heterogeneousreactionstepshasbeendevelopedaimingatfine tuningthebestcatalyticmaterials[5,7].Modelparameterssuchas reactionenthalpiesandactivationenergieshavebeenrelatedto selectedchemisorptionenthalpies.Thelatteraredenotedas cat-alystdescriptorsandtheestablishedrelationshipsbetweenthese descriptorsandthemodelparametershavebeenconstructedto bevalidforanentirecatalystlibrary.Amoredetaileddescription oftheOCMmicrokineticmodelis presentedbyKechagiopoulos
http://dx.doi.org/10.1016/j.apcatb.2016.06.019
Nomenclature
Roman
E0 Reference activation energy of a reaction family, kJ/mol
F◦reactants Inletflowrateofreactants(CH4andO2),mol/s
rfi Reactionfamilyi
Si Selectivitytowardsproducti,%
W Catalystmass,kg
Xi Conversionofreactanti,%
Ffeed Totalflowrateofthereactorfeed,mol/s
p Pressure,kPa
T Temperature,K
Greek
˛ Transfercoefficientofareactionfamily
etal.[7],offeringaninsightintotheactualmodelconstruction,the OCMreactionpathwaysonwhichthemodelisbasedandthe intra-particleconcentrationprofilesofthesurfacespeciesincludedinthe model.TherecentstudyofAlexiadisetal.[5]focusesonthe abil-ityofthemodeltoassessOCMkineticdataacquiredoncatalysts thatarerepresentativeoftwodifferentclassesofmaterials,thatare lithiummagnesiaandalkalineearth-promotedlanthanacatalysts, andassociatetheirperformanceintermsofmethaneactivationand C2selectivitytoselectedcatalystdescriptors.
Inthepresentwork,themodel’sapplicabilitybeyondthe pre-viously used catalyst classes is demonstrated, that is for rare earth-promotedalkalineearthcalciumoxidecatalysts,represented byLaSr/CaO,andNa-Mn-W/SiO2 catalysts.Thankstothemodel, anunequivocalandquantifiedcomparisonoftheseOCMcatalyst classescomeswithinreach.Theimplementationofthe microki-neticmodelforaseriesofcatalystsbelongingtothesameclass beingratherstraightforward,asimilareffortforcatalystsbelonging todifferentclassesissignificantlymorechallenging.
2. Procedures
2.1. Experimental
Extensive OCM experimentaldatasets wereacquired over a 10%La-20%Sr/CaOanda1wt%Na-3%W-2%Mn/SiO2catalyst cover-ingawiderange ofoperatingconditionscomplementarytothe previouslyacquireddataontheLi/MgO,Sn-Li/MgOandSr/La2O3 catalysts.Perovskitecatalystshavealsobeenconsidered[8,9]but appearedtobeseverelyaffectedbyCrdiffusionwhendepositedon astainlesssteelmicroreactor[10]and,hence,havebeendiscarded inthepresentwork.Inwhatfollows,abriefdescriptionisgiven oftheLaSr/CaOandNaWMn/SiO2catalyticmaterials,the experi-mentalsetupandtheemployedoperatingconditionsduringthe OCMexperiments.Materialsandproceduresfortheacquisitionof literaturedataonLi/MgO,Sn-Li/MgOandSr/La2O3performances
[5,11,12]areprovidedalongsideforreference.
2.1.1. Catalysts
Rareearth-promotedalkalineearthcalciumoxidecatalytic sys-tems have only been investigated to a limited extent [13,14]
althoughtheyarerecognizedashighlyperformingandstableOCM catalysts.IthasbeenreportedthatCaOisahightemperature sta-ble,basiccatalystsupportwithLabeinginvolvedintheoxygen activationandSrenhancingtheC2selecticity.Withinthepresent study,thecatalyst10%La−20%Sr/CaOwasselectedtobe inves-tigatedmoreelaborately giventhepromisingperspectivesfrom preliminarydata[13].
Na-W-Mn/SiO2catalystsconstituteoneofthefewclassesthat havebeeninvestigatedextensivelyintheOCMreaction,butfor which nofundamental modelhad beendeveloped yet [15–21]. Theircatalyticperformanceatmoderatetemperatureand stabil-ityover extendedtime onstreamhasbeenreportedbyseveral researchgroups.
Thelatterisfurtherenhancedwhentheyhavebeenprepared viathewetimpregnationmethod[15,16].Despitetheextensive research on this catalyst class, a detailed interpretationof the fundamentalphenomena attheoriginof itsperformanceisnot available.Withinthecurrentinvestigation,a catalystwith com-position1wt%Na-3%W-2%Mn/SiO2wasselected,whichisidentical toaliteraturereportedcatalystintermsofpreparationmethodand composition[16].
Table1apresentsthemainstructuralpropertiesofthesetwo catalystsincomparisontopreviouslyinvestigatedSr/La2O3andthe MgObasedcatalysts[6].Ascanbeeasilyobserved,theinvestigated catalystspresentroughlysimilarphysicalpropertiesallowingto focusonthedifferencesinchemicalproperties.Thelowvaluesof thesurfaceareaofthecatalystsareinlinewithpreviousfindings
[13,16,22].
2.1.2. Setup
TheexperimentaldataoverLaSr/CaO andNaMnW/SiO2 were obtainedinafixedbedreactor,operatingintheplug-flowregime withminimalaxialandradialtemperaturegradientsand negligi-blepressuredropoverthecatalystbed.Theseconditionsjustify theuseofaone-dimensionalreactormodelinthesimulationof theexperimentaldatausingtheOCMmicrokineticmodel.Further informationontheset-uphasbeenpreviouslyreported[5].
2.1.3. Operatingconditions
Thereproducibilityofthekineticmeasurementswasensuredby applyingastandardizedlining-outpretreatmentforbothcatalysts, identicaltotheonepreviouslyappliedfortheSr/La2O3catalyst[5]. Theconcentrationofreactivegaswasmaintainedbelow70%for safetyreasonsintheset-upusedinthepresentwork.
VariableparametersweretheinletmolarratioofCH4:O2,the spacetimewhichwascalculatedbasedonthesumofbothreactant inletflowrates,thatisCH4andO2,thedilutionofthereactivegas andthecatalysttemperature,seealsoTable1b.Highreactantflow rateswereusedtoavoidexternalmasstransferlimitationstothe largestpossibleextent,seefurther.Themaximumemployed cata-lystmasswassetat350mgtokeepoxygenconversionbelow90%. Thecorrespondingspacetimesweredefinedaccordingly.Catalyst temperatureswerekeptbelow900◦Ctoensurelongterm
stabil-Table1a
Physicalpropertiesoftheinvestigatedcatalysts.
Property Sr/La2O3 LaSr/CaO NaMnW/SiO2 Li/MgO Sn-Li/MgO
Radiusofcatalystpellet(mm) 0.1 0.1 0.1 0.125 0.125
Surfacearea(m2/g) 2 4.7 2.3 1 2.8
Porevolume(cm3/g) 0.024 0.04 0.004 – –
Porosity(mg3mc−3) 0.27 0.27 0.27 0.29 0.27
Table1b
Operatingconditionsemployedovertheinvestigatedcatalysts.
Operatingcondition aSr/La2O3 LaSr/CaO NaMnW/SiO2 a(Sn-)Li/MgO
Pressure(kPa) 190 190 120 108–130
Temperature(K) 980–1180 950–1180 1012–1085 947–1013
CH4/O2(molCH4molO2−1) 2–4 2–4 1–6 2–12
W/Freactants(kgsmolreactants−1) 0.2–1.2 0.3–0.8 3–25 2–12
N2dilution(%) 80 50–80 30–80 0
Catalystloading(mgc) 15–75 50 250–350 375
aaccordingtopreviouslyacquiredexperimentaldata[5,11,12].
ityinthekineticexperimentation.Extensivekineticdatasetswere compiledoverLaSr/CaOandNaMnW/SiO2catalystscomprising157 and228observationsrespectively.Theoperatingconditions previ-ouslyusedfortheacquisitionoftheOCMkineticsdataonSr/La2O3, Li/MgOandSn-Li/MgO[5,11,12]areprovidedinTable1balongside forreference.
2.2. Modelsimulations
TheOCMmicrokineticmodelwasinthefirstinstancevalidated againsttheexperimentaldataproducedovertheLaSr/CaOandthe NaMnW/SiO2 catalysts.Below isabriefdiscussionofthemodel characteristicsasrequiredfortheproperunderstandingofthe cur-rentwork.Amore elaboratedescriptionofthemodel hasbeen reportedelsewhere[5,7].
2.2.1. Reactionkinetics
Thegasphasereactionnetworkofthemicrokineticmodelhas beendevelopedintermsof39elementarysteps[5–7,11,12,14,23]. Thecatalyticreactionnetworkcomprises26elementarysteps[5,7]
whichcanbeclassifiedintothreetypes,thatareadsorptionsteps, Eley-Ridealreactionsandsurfacereactions[5].
2.2.2. Reactormodel
The microkinetic model was implemented in a
one-dimensional, heterogeneous reactor model, which explicitly accountsforthe irreducible masstransport limitations induced bythehighreactivity oftheproducedradicals. Italsoaccounts forthediluenteffectontherelativecontributiontotheoverall conversionbyheterogeneousandhomogeneousreactionsdueto achangingavailablegasphaseandcatalystsurfaceinthecatalyst bed.Amoredetaileddescriptionofthereactormodel hasbeen givenbyKechagiopoulosetal.[7].
2.2.3. Modelparameters
Theratecoefficientsofthecatalyticstepswerecalculatedfrom therespectivepre-exponentialfactorsandactivationenergies.The maximumvalueofthepre-exponentialfactorsofthesteps involv-inginteractionof a gasphase species withthecatalyst surface
[6,24],thatistheadsorptionandtheEley-Ridealsteps,was esti-mated by collision theory. The pre-exponential factors for the adsorptionstepswerecorrectedwithseparateinitialsticking prob-abilities(S0)whichwereconsideredascatalystdescriptorsinthe microkineticmodel.Thepre-exponentialfactorvaluesforthe sur-facereactionstepsweredeterminedbytransition-statetheory.
The activation energies of the catalytic steps were calcu-latedfromthesurfacethermodynamicsviaPolanyirelationships. Surface thermodynamics in turn were obtainedfrom the cata-lystdescriptorsbyapplyingthermodynamicconsistencythrough theconsideredcatalyticcycles.Thisallowedexpressingreaction enthalpiesforvariouselementarystepsasafunctionofalimited numberofunknowncatalystdescriptorswhicharesubsequently estimatedbyregression[6,25,26].
Alladsorptionstepswereconsideredtobenon-activated.As showninTable2,theEley-Ridealstepsweregroupedintoasingle
reactionfamily(rf1),whilethesurfacereactionstepswere subdi-videdintofourreactionfamilies,accountingforH-atomabstraction (rf2),recombinationofhydroxyls(rf3),catalyticoxidationofCO (rf4)andC–Cbondscission(rf5)respectively.Eachreactionfamily (rf1–rf5)hasaspecificsetofPolanyiparameters␣andE0.Values of␣weretakenfromliterature[27–30].Thereferenceactivation energies,E0,weredefinedaskineticdescriptorsandtheirvalues duringregressionanalysiswerefixedatthevaluesobtainedduring theparameteradjustmentovertheSr/La2O3data[5].
ThesetofcatalystdescriptorsintheOCMmicrokineticmodel canbeclassifiedasfollows:
a)ReactionenthalpyofhydrogenabstractionfromCH4(D1). b)ChemisorptionheatsofO2,CH2O,HCO,CO,CO2,H2O,C2H4O,
C2H3O(D2-D9).
c)Initialsticking probabilities of O2, CH3, CO, CO2, H2O, C2H4 (D10–D15).
d)Densityofactivesites(D16).
As shown in our previous work [5] a more pronounced
endothermicityofhydrogenabstractionfrommethane,thatisa higherD1,resultsinanequivalentdecreaseofthehydroxyl sta-bilityonthecatalystsurface.Thestabilityofthehydroxylspecies influencesthemethaneactivationandtheC2selectivity,mainlyvia theformationofadsorbedatomicoxygen.
2.2.4. Parameterestimation
Parameter estimation was performed using the Rosenbrock methodology[31]fortheinitialminimizationoftheobjective func-tion,thatistheweighedsumofthesquaredresiduals,followed byaLevenberg–Marquardtalgorithm[32]forthefinal optimiza-tion.Furtherinformation onparameterestimation procedureis givenelsewhere[5].AsdiscussedinSection2.2.3,theestimated parametersarethecatalystandkineticdescriptors.
Theinitialguessesforthecatalystdescriptorswerechosento bewithintherangeofphysicallyrealisticvalues[5].Forexample, theadsorptionheatofCO,rangesbetween60and180KJ/mol[33], whiletheadsorptionheatofwatercanreachupto65KJ/mol[34]. Themolarfractionsofmethane,oxygen,ethylene,ethane,carbon monoxideandcarbondioxideatthereactoroutletwereusedas responsesintheparameterestimationprocedure.
Table2
ReactionfamiliesandcorrespondingparametersemployedinthePolanyi relation-shipsconsideredinthemodel.Transfercoefficients,␣,wereobtainedfromthe literatureasindicatedwhilereferenceactivationenergieswereasdeterminedin ourpreviouswork[5].
Reactionfamily ␣ E0(kJ/mol)
rf1 HydrogenabstractionbyEley-Ridealreaction 0.75[27] 96.8 rf2 Hydrogenabstractionbysurfacereaction 0.50[28] 141.3
rf3 Recombinationofhydroxyls 0.65[29] 130.2
Fig. 1.Conversions and selectivities over La-Sr/CaO vs. catalyst temperature. Experimentalconditions:p=190kPa,CH4/O2=3,N2dilution=80%,W=50mgand Ffeed=3.0810−4mol/s.Symbols-experimentalpoints: :CH4conversion, :O2 conversion, :C2H6selectivity,:C2H4selectivity, :COselectivity, :CO2 selectivity.Experimentalpointspresentsamecolourwiththerespective calcula-tioncurves.Calculatedconversionsandselectivitieswereproducedwiththemodel describedinsection2andthevaluesofcatalystdescriptorslistedinTable4.
3. Resultsanddiscussion
3.1. Experimentalcatalystassessment
3.1.1. La-Sr/CaO
Anextensiveexperimentaldatasetcomprising157pointswas acquiredduringOCMtestingofthe10wt%La-20wt%Sr/CaOcatalyst. AspresentedinTable1b,duringtestingunderOCMconditions,the catalysttemperature,theinletCH4/O2 ratioandtheN2 dilution werevaried.Thelatterdeterminedthespacetimeofthereactants, sincethetotalgasfeedinletflowrateandthecatalystmasswere fixedat3.0810−4mol/sand50mgrespectively.Thestabilityofthe catalystwasverifiedwithinatimeonstreamof60hwithoutloss inactivityorselectivity.
Fig. 1 depicts the effect of thecatalyst temperature onthe methaneandoxygenconversionandmajorproductselectivities ataN2dilutionof80%ofthetotalfeed.Thetotalpressureandinlet molarmethanetooxygenratiowerefixedat190kPaand3 respec-tively.Asexpected,methaneandoxygenconversionsincreasewith thetemperature.Inparticular,themethaneconversionreaches18% andthatofoxygenreaches50%at1128K.AsshowninFig.1,theC2 selectivity,thatisethaneandethylene,increaseswiththecatalyst temperature.Theethaneselectivityreachesamaximumof34%at 1100Kwhilethatofethyleneismoretemperaturedependentand increasesmonotonicallytoreach17.5%at1128K.Asinthecaseof Sr/La2O3describedelsewhere[5],theselectivitytowardscarbon oxidesdecreaseswithtemperature,withtheCOselectivitybeing moretemperature-dependentdroppingdownto15%at1128K.
3.1.2. Na-Mn-W/SiO2
A dataset comprising 228 observations was produced over 1wt%Na-3wt%W-2wt%Mn/SiO2 byvaryingthecatalyst temper-ature,theinertgasdilution,thespacetimeand theinletmolar CH4/O2ratio.Thetotalpressurewasagainfixedat120kPa. Cata-lyststabilitywasverifiedwithinatimeonstreamof60hasinthe caseofLaSr/CaO.
Fig.2presents theeffectof thecatalysttemperature onthe methaneandoxygenconversionandselectivitiestowardsC2and COxbykeepingN2dilution,spacetime,andinletCH4/O2ratio con-stantat75%,19.5kgsmol−1
reactantsand4respectively.Asexpected, methaneandoxygenconversionsincreasewiththetemperature, reachinga maximum of 9.5% and 30.3% respectively at 1071K undertheinvestigatedexperimentalconditions.AsshowninFig.2, theethaneselectivityslightlydecreaseswiththetemperatureat
Fig.2.ConversionsandselectivitiesoverNa-Mn-W/SiO2vs.catalyst tempera-ture.Experimentalconditions:p=120kPa,CH4/O2=4,N2dilution=75%andW/F
reactants=19.5kgsmol−1reactants.Symbols-experimentalpoints: :CH4conversion, :O2conversion, :C2H6selectivity,:C2H4selectivity, :COselectivity,
:CO2selectivity.Experimentalpointspresentsamecolourwiththerespective calculationcurves.Calculatedconversionsandselectivitieswereproducedwiththe modeldescribedinsection2andthevaluesofcatalystdescriptorslistedinTable4.
theseconditionswhiletheethyleneselectivityincreases monoton-icallywithincreasingtemperaturereaching15.1%at1071K.The CO2selectivitydecreasesconsiderablywiththetemperature,while thattowardsCOispracticallyzeroupto1020Kandsubsequently increases.
3.1.3. ComparisonoftheOCMbehaviouroftheinvestigated catalysts
Inourpreviouswork[5]extensiveOCMkineticdataof30,40 and 176experimentalpoints producedover Li/MgO, Sn-Li/MgO andSr/La2O3catalystrespectively,wereemployedforassessingthe performanceofanalkalimanganesecatalyst,itsSnpromoted ver-sionandanalkalineearth-promotedlanthanacatalyst.Notethat asignificantlywider rangeofspacetimeswasusedonthe (Sn-)Li/MgOandtheNaMnW/SiO2catalystscomparedtoSr/La2O3and LaSr/CaOcatalysts,seealsoTable1b.
Performances of the above five catalysts were compared in terms ofmethaneconversionand C2 selectivity.Representative experimentalresultsarepresentedinTable3.Alltheexperimental datahavebeenacquiredatacatalysttemperatureofapproximately 1020K.Theappliedmethanetooxygeninletratioof3.5wasslightly lowerinthecaseofSn-Li/MgO,comparedtotheotherfour cat-alysts,forwhich aratioof4wasemployed.Moreover,thetotal pressurewasclosetoatmosphericinthecasesofNaMnW/SiO2and Li-basedMgOcatalysts,whileitwasslightlyhigherinthecaseof Sr/La2O3andLaSr/CaOcatalyst,thatis190kPa.Duringthe exper-imentsovertheLi-basedMgOcatalysts,therewasnodilutionof thefeedreactivegas,whilefortheothercatalyststheexperiments wereconductedunderhighinertgas(N2)dilution,thatis75–80%. It is clear fromTable 3 that Li/MgOand Na-Mn-W/SiO2,by far,aretheleastactivecatalysts.Inparticular,thelattercatalyst exhibitsalowactivityevenwhenrelativelyhighspacetimesare employed.Sr/La2O3ismoreactiveandslightlylessselectivethan La-Sr/CaOatsimilaroperatingconditions.TheSn-Li/MgOcatalyst wasinvestigatedatmuch moreconcentratedconditions,higher reactantspacetimesandaslightlylowerCH4/O2 ratio.Allabove aspectsareexpectedtopromotemethaneconversion.Asseenin
Table3
TypicalOCMperformanceoftheinvestigatedcatalysts.
Catalyst Spacetime,kgsmol−1
reactants CH4/O2 Feeddilution,% CatalystT,K p,kPa XCH4,% SC2,%
Sr/La2O3 0.8 4 80 1023 190 7 23.4
La-Sr/CaO 0.8 4 80 1020 190 3.2 26.7
Na-Mn-W/SiO2 19.5 4 75 1021 120 2.1 52.5
Li/MgO 4 4 0 1023 112 0.9 49
Sn-Li/MgO 2 3.5 0 1023 117 12.4 58.1
efficientlyaccommodatingthesedifferencesin operating condi-tionsandquantifiesthecatalystperformanceintermsofcatalyst descriptors,whichallowanunequivocalcomparisonofthe consid-eredcatalysts.
3.2. ExtendingthemicrokineticmodelvaliditytoLaSr/CaOand NaMnW/SiO2
Themicrokineticmodelusedinthepresentworkhadalready beenvalidatedagainstthedataacquiredontheSr/La2O3andthe MgO-based catalysts[5].The experimental dataproduced over LaSr/CaOand NaMnW/SiO2 catalystshavenow alsobeen simu-latedmakinguseofthismodeltoquantifythedistinctactivityand selectivitypatternsintheperformanceofthesetwoinvestigated catalysts,seeFig.1and2comparedtothepreviouslyinvestigated ones.Asreportedinourpreviouswork[5],themicrokineticmodel ismostsensitivetothefollowingfourcatalystdescriptors:the
H-atomabstractionenthalpyfrommethane(D1),thechemisorption heatofoxygen(D2),theinitialstickingprobabilityofCH3radicals onthesurface(D11)andtheactivesitedensity(D16).Duringthe regressionanalysis,onlythefouraforementioneddescriptors,D1, D2,D11andD16,seeTable4,wereallowedtovary.Thevaluesof theremainingcatalystandkineticdescriptors,werefixedatthe valuesobtained,duringtheparameterestimationperformedfor theSr/La2O3data[5].
Reduceddatasetsof36and43experimentsfortheLaSr/CaO andNaMnW/SiO2 catalystrespectivelywereusedinthe param-eterestimationprocedureinordertoreducethecomputational time.Theestimatedcatalystdescriptorvalueswerethenvalidated againstall157and228observationsoneachoftheinvestigated catalysts.Fig.3aandbpresenttheparitydiagramsforCH4,O2and C2products.
AscanbeclearlyseenfromFig.3aandb,themicrokineticmodel yieldedaverygoodagreementwiththeexperimentaldataforthe
Table4
CatalystdescriptorestimatesasobtainedbyregressionofthemicrokineticmodelreportedbyAlexiadisetal.[5]totheexperimentaldataobtainedonLa-Sr/CaOand Na-Mn-W/SiO2.PreviouslyreportedvaluesforSr/La2O3,Li/MgOandSn-Li/MgO[5]areincludedforeasycomparison.
Catalystdescriptor Unit La-Sr/CaO Na-Mn-W/SiO2 Sr/La2O3 Li/MgO Sn-Li/MgO
D1 Reactionenthalpyofhydrogen abstractionfromCH4
(kJmol−1) 65.1±1.6 81.4±0.002 44.4±0.2 91.2±0.2 56.6±0.8
D2 ChemisorptionheatofO2 (kJmol−1) 139.6±5.3 44±1.6 119.5±3.5 73.6±2.2 60.54±2.6 D11 Initialstickingprobability
ofCH3·
1.14±0.0310−4 1±0.210−5 6.49±0.510−4 1.19±0.000210−4 6.22±0.110−5
D16 Densityofactivesites (molm−2) 6.4±1.510−6 4.57±0.0510−7 9.84±110−6 4.33±0.00910−7 1.33±0.0310−6
Value±95%confidenceinterval.F-valueforsignificanceoftheregression=871.1;Ftabulatedvalue=3.36.
completerangeofexperimentalconditionsovereachofthetwo catalysts,despitethelimitednumberofadjustableparameters. Fur-thermore,theoutletmolarfractionsofeachoftheC2productsand carbonoxidesaswellasthoseofwater,notshowninFig.3,were adequatelysimulatedbythemodel.
Inlinewiththepreviouswork[5]themaximumabsolutevalue amongthebinarycorrelationcoefficientsamountedto0.85and occurred between the reaction enthalpy of methane hydrogen abstractionD1andtheactivesiteconcentrationD16.Suchavalue allowsconcludingthatthereisnosignificantcorrelationbetween theadjustableparameters.ThehighFvaluefortheglobal signifi-canceoftheregression,amountingto870,illustratesthemodel’s capabilityofadaptingitselftotheexperimentaldata.
Figs. 1 and 2compare thesimulated conversions and prod-uctselectivitiestotherespectiveexperimentalvalues measured atdifferentcatalysttemperatures.Themodelclearlyadequately reproducesthemethaneand oxygenconversions aswellasthe selectivitiestoC2productsandcarbonoxidesovertheLaSr/CaO andNaMnW/SiO2 catalystsgivingconfidenceintheconstructed mechanismandthecorrespondingmodelparameters.The param-eterestimatesobtainedforthecatalystdescriptorsarereportedin
Table4.Theirvaluesarephysicallyrealistic,sincetheyliewithin literature reportedconstraints, as mentioned in Section 3, and arealsostatisticallysignificant,seeTable4.Therespective cata-lystdescriptorvaluescorrespondingtoSr/La2O3and(Sn-)Li/MgO
[5]areprovidedalongsideforreference.Thevaluespresentedin
Table4highlightthedifferentphysicochemicalpropertiesofthe fiveinvestigatedcatalystsandexplaintheirdistinctOCM perfor-mances,asillustratedinTable3,seealsothenextsection.
3.3. Interpretationandquantificationoftheexperimental observations
Asmentioned in Section 3 and elaborately described in our previous work [5] the methane hydrogen abstraction reaction enthalpy,D1,determinesthehydroxylchemisorptionenthalpyto alargeextent.Thehydrogenabstractionfrommethaneappearsto behighlyendothermicinthecaseoftheLi-MgOaswellasofthe Na-Mn-W/SiO2 catalyst.Correspondingly,hydroxylsareless sta-bleonthesecatalysts,thatistheirchemisorptionheatisrelatively lessnegativeand,withintherangeofthecatalystdescriptorvalues presentedinTable4,thiswasfoundtoleadtoadecreaseof hetero-geneouslycatalyzedmethaneconversion.Thepositivedifference intheactivationenergiesbetweentheforwardandthebackward directionofthemethanehydrogenabstractionstepincreaseswith theendothermicity,slowingdownthemethaneconversion. Fur-thermore,theactivesiteconcentration(D16)onthesetwocatalysts is much lowerthan in thecase of Sr/La2O3,La-Sr/CaOand Sn-Li/MgOcatalysts,implyingthelimitedcontributionofthecatalytic reactionstotheoverallOCMreactionrateoverNa-Mn-W/SiO2and Li-MgO.Thesefindingsrevealtheintrinsicreasonforthelow activ-ityofNa-Mn-W/SiO2andLi-MgOcatalystsillustratedinTable4.
ThecatalystdescriptorvaluespresentedinTable4providean adequateexplanationforthelowactivityyetsimultaneouslyhigh
selectivitytoC2productsattainedwiththeNaMnW/SiO2catalyst. Indeed,thiscatalystexhibitstheleastnegativeoxygen chemisorp-tionheat,seeD2 inTable4,whichindicatesweakerbondingof atomic oxygentotheNaMnW/SiO2 catalystsurface and,hence, lowercontributionofthecatalyticreactionnetworktotheoverall OCMreaction,sinceatomicoxygenisinvolvedinalargenumber ofcatalyticsteps[5].Thiseffectivelyslowsdownmethane acti-vationonNaMnW/SiO2,yetitalsolimitssurfaceCO2formation. Thesurfaceproductionofcarbondioxideisfurtherlimitedbythe decreasedoxidationrateofmethylradicalsinducedbytheirlow stickingcoefficientonthesurfaceoftheNaMnW/SiO2catalyst,see D11,Table4.
Themethanehydrogenabstractionreactionenthalpy,thatis D1,seeTable4,allowsexplainingthehigheractivityofSr/La2O3 comparedtothatofLaSr/CaOundertheemployedOCMreaction conditions. As can beseen in Table 4, Sr/La2O3 exhibits a less endothermicEley-RidealmethaneH-atomabstractionstep,D1.The correspondinghydroxylchemisorptionheatontheSr/La2O3 sur-facewascalculatedequalto278kJ/mol[5],whileforLaSr/CaOthe samewaslimitedto257KJ/mol.Differencesinoxygen chemisorp-tionheat(D2)anddensityofactivesites(D16)arelesscriticalfor explainingthehigheractivityofSr/La2O3.
Ascanbereadilyseenin Table4 andreportedinour previ-ous work[5], thehigher activityof Sr/La2O3 compared tothat of Sn-Li/MgOis attributedto thehigher values forD2 and D16 andthelowerD1.Furthermore,Sn-Li/MgOexhibitsahigher selec-tivity toC2 productsthanLaSr/CaO due tothecorrespondingly lower oxygenchemisorption heat(D2)and thelower valuefor methylradicalstickingcoefficientonitssurface(D11).However, acomparisonbetweenLaSr/CaOandSn-Li/MgOcatalystinterms ofmethaneactivationisnot straightforward.Thelattercatalyst exhibitsalowermethaneH-atomabstractionreactionenthalpy, D1,yettheoxygenchemisorptionheat,D2,andthedensityofactive sites,D16,onitssurface arelowerthanthose oftheSn-Li/MgO catalysts.
Giventhedifferencesinoperatingconditionsusedinthe deter-minationoftheOCMkinetics,anunequivocalcomparisonamong thevariouscatalystsisonlyfeasibleviamicrokinetic modelling. Forthispurpose,microkineticmodelsimulationshavebeen per-formedattheoperatingconditionsinvestigatedfortheSr/La2O3 catalyst.AsshowninFig.4,themethaneconversionoverSr/La2O3 catalystissimulatedapproximately2,530and33timeshigher thanovertheLa-Sr/CaO,Sn-Li/MgO,Na-Mn-W/SiO2 andLi/MgO catalysts,respectively,underidenticaloperatingconditions.
Fig.4.Simulatedmethaneconversion(fulllines)andC2selectivity(dottedlines) vs.spacetime;(green)Sr/La2O3;(purple)La-Sr/CaO;(red)Sn-Li/MgO;(orange) Na-Mn-W/SiO2;(blue)Li/MgO.Operatingconditions:T=1023K,p=190kPa,CH4/O2=4, dilution:80%.Simulationresultswereobtainedwiththemodeldescribedinsection 2andthevaluesofcatalystdescriptorslistedinTable4.Notethatbecauseofthe differencesinoperatingconditionsusedduringexperimentaldataacquisition,no observedvaluesarereportedinthisfigure.(Forinterpretationofthereferencesto colourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)
Fig.5.Simulated C2selectivityvs. CH4 conversion.(green)Sr/La2O3;(purple) La-Sr/CaO;(red)Sn-Li/MgO;(orange)Na-Mn-W/SiO2;(blue)Li/MgO.Operating con-ditions:T=1023K,p=190kPa,CH4/O2=4,dilution:80%.Simulationresultswere obtainedwiththemodeldescribedinsection2andthevaluesofcatalyst descrip-torslistedinTable4.Notethatbecauseofthedifferencesinoperatingconditions usedduringexperimentaldataacquisition,noobservedvaluesarereportedinthis figure.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereader isreferredtothewebversionofthisarticle.)
TheC2 selectivityasafunctionofthemethaneconversionis presentedinFig.5toallowabettercomparisonamongthe inves-tigatedcatalystsasafunctionoftheC2selectivity.Asclearlyseen atOCMoperatingconditionsthatleadtosimilarCH4conversion, thatisaround1–2%,byemployingthemicrokineticmodel,thefive catalystsarerankedwithrespecttoC2selectivityasfollowsfrom hightolow:
Na-Mn-W/SiO2∼Sn-Li/MgO>La-Sr/CaO>Sr/La2O3>Li/MgO.
4. Conclusions
ThepresentOCMmicrokineticmodelincludingcatalyst descrip-torsenablesthegenericdescriptionofOCMreactionperformances on5differentcatalystsbelongingto 4families basedona sin-glereactionnetwork,irrespectiveoftheoperatingconditionsat whichthecatalyticperformancehasbeenevaluated.The applica-bilityofthemodelwasextendedfromalkalimagnesiacatalysts representedbyLi/MgOandSn-Li/MgOandalkalineearthlanthana
catalystsrepresentedbySr/La2O3totwomorecatalystfamilies, that is rare earth-promoted alkaline earth calcium oxide cata-lysts,representedbyLaSr/CaO,andaNa-Mn-W/SiO2catalyst.The agreementbetweensimulatedandexperimentalconversionsand selectivitiesonallfiveinvestigatedcatalystsforthecompleterange ofexperimentalconditionswasadequate.
TheSr/La2O3catalystwasfoundtobethemostactivewhile Na-Mn-W/SiO2 provedtobethemostselectivecatalysttowardsC2 products.TheaboveanalysisshowedthattheidealOCMcatalyst shouldexhibitalowreactionenthalpyofmethaneH-atom abstrac-tionandhighactivesitedensity,descriptorswhichleadtohigh activityasinthecaseofSr/La2O3,andlowmethylradicalsticking coefficient,thusfavouringtheC2selectivityasinthecaseof Na-Mn-W/SiO2.Regardingtheoxygenchemisorptionheat,itwasshown thatahighvalueofthisdescriptore.g.,onSr/La2O3promotes cat-alystactivitybutlimitsC2selectivity,whereastheoppositeeffect isinducedwithalow value,e.g.,onNa-Mn-W/SiO2.Becauseof theseconflictingtrendsanoptimalmediocrevalueisexpectedfor theidealcatalyst.Amicrokineticmodelincludingcatalyst descrip-torsisideallysuitedtoprovideguidelinesforthedevelopmentof optimalcatalyticsystems,theadequatetranslationoftheoptimal catalystdescriptorvaluesintoacorrespondingcatalystsynthesis recipebeingacriticalaspect.
Acknowledgements
Thispaperreportsworkundertakeninthecontextoftheproject “OCMOL,OxidativeCouplingof Methanefollowed by Oligomer-izationtoLiquids”.OCMOLisaLargeScaleCollaborativeProject supportedbytheEuropeanCommissioninthe7thFramework Pro-gramme(GAn◦228953).ForfurtherinformationaboutOCMOLsee:
http://www.ocmol.euorhttp://www.ocmol.com.
Theresearchleadingtotheseresultshasalsoreceived fund-ing from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grantagreementn◦615456i-CaD.
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