Catalytic methyl mercaptan coupling to ethylene in chabazite: DFT study of the first C-C bond formation

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J. Baltrusaitis

a,∗

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_T.

_Buˇcko

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_,

_W.

_Michaels

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_,

_M.

_Makkee

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_G.

_Mul

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a_Department_of_Chemical_and_Biomolecular_Engineering,_Lehigh_University,_B336_Iacocca_Hall,₁₁₁_Research_Drive,_Bethlehem,_PA_18015,_USA

b_Department_of_Physical_and_Theoretical_Chemistry,_Faculty_of_Natural_Sciences,_Comenius_University_in_Bratislava,_Ilkoviˇcova_6,₈₄₂₁₅_Bratislava,_Slovak

Republic

c_Institute_of_Inorganic_Chemistry,_Slovak_Academy_of_Sciences,_Dubravska_cesta_9,₈₄₅₃₆_Bratislava,_Slovak_Republic,_Slovak_Republic d_Catalysis_Engineering,_Chemical_Engineering,_Delft_University_of_Technology,_Julianalaan_136,_NL₂₆₂₈_BL_Delft,_The_Netherlands e_Faculty_of_Science_&_Technology,_University_of_Twente,_PO_Box_217,_Meander_225,_NL₇₅₀₀_AE,_Enschede,_The_Netherlands

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Articlehistory:

Received14September2015

Receivedinrevisedform6January2016 Accepted10January2016

Availableonline13January2016 Keywords: Methylmercaptan Chabazite DFT Ethylene

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Methylmercaptan,CH3SH,isanindustrialwasteaswellasthereactiveproductofseveralH2andH2S

inducedcatalytichydrogenationprocessesofCOSandCS2.Itscouplingintovalueaddedproductsis

ofgreatimportanceinmonetizingsournaturalgas.Inthepresentwork,thefulltheoreticalcycleof catalyticCH3SHcouplingtoformethenewasinvestigatedbymeansofdensityfunctionaltheory(DFT)

usingchabaziteasamodelcatalystwithemphasisontheﬁrstC Cbondformation.Calculated thermo-dynamicswerecomparedwiththoseofanalogousandwellestablishedCH3OHprocessestoidentifythe

similaritiesanddifferencesinthereactivepathways.Withfewexceptions,CH3SHcatalytic

transforma-tionsareofhigherfreeenergywhencomparedtothoseofCH3OH.Thetrimethylsulfoniumion,TMS,

isostructuralwiththatofthetrimethyloxoniumion,TMO,isshowntobeakeyreactiveintermediateand athermodynamicallystablespeciesleadingtoetheneformation.

1. Introduction

Productionofvalueaddedchemicalsandfuels,suchasoleﬁns (ethene,propene),benzenederivatives(benzene,toluene, xylene-BTX),andC5+liquidfuelsisofmajorindustrialimportance.When

naturalgasisusedasarawmaterial,syngas(CO+H2)ormethanol

(CH3OH)routesaretypicallyused[1–3].CH3OHisaconvenient

platform molecule[4,5] to obtainhigher hydrocarbons since it is liquidin ambientconditions andreacts viawell-deﬁned and exploredcatalyticpathways,thusallowingforahighdesired prod-uctselectivity[6,7].FundamentalmechanismsofCH3OHcatalytic

conversionintooleﬁnsareimportantandhavebeenextensively researchedduetotheindustrialandsocietalimpactoftheprocess [8].Surfacerelatedoxoniumylide[9,10],carbene[8,9],dimethyl [8],carbocation[9],andradical[9]routeswereconsideredas mech-anisticstepsoftheﬁrstC Cbondformationanddimethylether (DME)wasfoundtobethemainintermediate.Notably,“carbon pool”hasbeenestablished[11–14]asacurrentlyconsensus

mech-∗ Correspondingauthor.

E-mailaddress:job314@lehigh.edu(J.Baltrusaitis).

anismforoleﬁnproduction,butitdoesnotaccountfortheﬁrstC C bondformation.

Farlessexploredandinvestigatedarethecatalyticprocesses that enablemethanethiolor methylmercaptan,CH3SH,catalytic

transformationsintovalueaddedproducts.CH3SHhasattracted

signiﬁcantattentionasindustrialwastegasinthepaperindustry, related to the Kraft sulfate pulp process [15]. It also is invari-ablypresentinfossilresources,suchasnaturalgas,andneedsto beremovedtolevelsbelow20ppmw[16].Conventional indus-trialprocesses for CH3SHremoval rely onabsorptionby liquid

amines and/orcatalyticoxidation releasinghighlyoxidized sul-furcompounds,suchasSOx[17].Veryfewattemptshave been madetocatalyticallyconvertCH3SHintovalueadded

hydrocar-bonsandoxygenates,includingCH4[18,19],formaldehyde[20,21],

BTX[22–24],and oleﬁns [25]. BTXandoleﬁns fromCH3SHare

ofparticularinterestandtheconceptsofnewMTG (Mercaptan-to-Gasoline)[26]andMTH(Mercaptan-to-Hydrocarbons)[22]were veryrecentlyintroduced.Thisapproachcanbeespecially power-fulasfarasstrongly“sour”naturalgascontainingCO2andH2Sis

concerned.TheSocietyofPetroleumEngineers(SPE)estimatesthat about40%oftheWorld’stotalaccessiblenaturalgasreservesare consideredsour,totalingto350Tcfwithover10%H2S[27].The

concentrationsofacidicgasescanrangeupto90%byvolumeand http://dx.doi.org/10.1016/j.apcatb.2016.01.021

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Fig.1. RepresentationoftheBrønstedacidsiteinthechabaziteframeworklocatedattheO1frameworkposition.

thisso-calledsub-qualitynaturalgas(SQNG)accountsfor approx-imately30%ofUSnaturalgasresources[28]withmostofthegas wellscappedandnotutilized[29].TheSPEexpectsgasdemandto growbyasmuchas2%peryearoverthenexttwodecades,andwith thedepletionofconventional(sweet)reservoirs,therequirement todeveloptechnologiesthatwouldenablethesafeandeconomic exploitationof thesour gasresourcesis of utmostimportance. Whilesourgasdirectprocessingisdifﬁcultduetothecorrosivityof H2S,catalyticrouteshavebeendevelopedtoselectivelyconvertit

intoCH3SH.Inparticular,hydrogensulﬁdemethanereforming[29]

via

2H2S+CH4= 4H2+CS2,H298K= 232.4kJmol−1 (1)

hasbeenproposedattemperaturesabove1000◦Cwith consider-ableCOSamountalsoformedvia

H2S+CO = COS+H2. (2)

CS2andCOShavebeenshowntoselectivelyreactwithH2to

formCH3SHoverNi,K,Co-promotedMoS2/SiO2[30–33]providing

forindirectroutesofsourgasprocessingtoCH3SH.Otherdirect

pathwaysofH2StransformationinthepresenceofCH4 andCO2

havebeenexplored.Baltrusaitisetal.[26] proposedconversion ofa CH4+H2SmixtureintoCH3SHand H2 using lightof a low

wavelength(205nm),potentiallyovercomingthelargebarrierfor H3C Hbondbreakingviaconicalintersectionrelatedrelaxation.

SyngasinthepresenceofH2ShasbeenconvertedtoCH3SH[34–36],

andthesamehasbeenachievedusingCO+H2Smixtures[37,38].

Finally,Barraultetal.[39]showedselectivetransformationsofboth COandCO2inthepresenceofH2SandH2toCH3SHoverKpromoted

WO3/Al2O3catalyst.Thelatterapproachcanunlockanestimated

over700TcfofsourgasreservesthatarebothCO2andH2Srich

[27].Thus,potentialsourgasprocessingtoyieldCH3SHasareactive

intermediatehasalreadybeenexplored.

Ontheotherhand,veryfewattemptsofCH3SHcatalytic

cou-plinghavebeenmadetoobtainloweroleﬁnsoranyvalueadded hydrocarbons,suchasBTX.ChangandSilvestri[25]reportedthat at755KusingH-ZSM-5catalyst,CH3SHwasconverted intoH2S

(whichlatercanbeconvertedintootherhighvaluehighvolume products,suchasH2SO4)andamixtureofhydrocarbonswithonly

7.0%selectivitytowardsC2=+C3=.Desulfurizationwasalsoonly

par-tial,with27.2%ofthecarbonfeedconvertedintodimethylsulﬁde (DMS).Butteretal.[18]claimedhighCH3SHconversiontoCH4at

531KonH-ZSM-5.Mashkinaetal.[19]identiﬁedthepresenceof CH4whenCH3SHwasprocessedonvariousacidcatalystsbetween

623and673K,whileatlowertemperaturesDMSwastheonly prod-uctatequilibriumconversion.Averyrecentwork(2013–2014)by CNRSandTOTALSA,France,focusedonthecatalyticconversionof

traceamountsofCH3SHoveracidiczeolites[22–24]andrevealed

absenceofanyloweroleﬁns.Inparticular,selectivitytowards(a) CH4,(b)DMSand(c)cokewasmainlyobserved,inadditiontothe

formationofBTXproducts.Thisissurprising,sinceoleﬁnsare typi-callyconsideredtobeprecursorsinBTXformation[40].Itisfeasible that,sinceCH3SHcatalyticcondensationwasattemptedat

temper-atureshigherthanthoseforCH3OH(823Kvs623K),anyethene

formedfurtherreactedonverystrongacidiczeolitesitestoyield cokeand CH4.Finally, Olahetal.[41] usedacidic WO3/␥-Al2O3

catalyststoformC2H4fromDMS,withthelatterbeingan

appar-entbottleneckinmostoftheliteratureworkwhenattemptingto convertCH3SH.

InthisworkweperformedacomparativeDFTcalculationsof CH3OHandCH3SHcatalyticcouplinginchabazitezeolitetoform

ethene,CH2=CH2.SinceCH3OHandCH3SHareisostructural,an

assumptioncanbemadethattheircatalytictransformationsinto ethene should follow the same fundamental mechanisms.This assumptionwillbeveriﬁedasoneoftheresearchobjectivesin try-ingtodeterminewhetherthereisacommonisostructuralreactive intermediateforbothoxygenandsulfurbasedspecies.Thus,we designedthisstudytodirectlycompareandcontrastCH3OHand

CH3SHfundamentalreactivestepsinordertoelucidatethelimiting

stepsofCH3SHconversiontoloweroleﬁns.

2. Theoreticalmethods

2.1. Electronicstructurecalculations

Periodic DFT calculations have been performed using the VASPcode[42–45].TheKohn–Shamequationshavebeensolved variationally in a plane-wave basis set using the projector-augmented-wave (PAW) method of Blochl [46], as adapted by Kresse and Joubert [46]. The exchange-correlation energy was described by the PBEgeneralized gradient approximation [47]. Brillouin-zonesamplingwasrestrictedtothe-point.The plane-wavecutoffwassetto400eV.Theconvergencecriterionforthe electronicself-consistencycycle,measuredbythechangeinthe totalenergybetweensuccessiveiterations,wassetto10−6eV/cell. Localandsemi-localdensityfunctionals,suchasPBEusedinthis work,failtodescribeweakmolecularinteractionaccurately[48]. Asasigniﬁcantpartoftheinteractionenergybetweenalkanesand alkenesandazeoliteisduetovanderWaalsinteractions, semi-empiricaldispersioncorrectionstotheDFTtotalenergiesandforces usingGrimmeapproach(DFT-D2)[49]implementedintheVASP code[50]wereused.DFT-D2combinedwithPBEfunctional pro-videsareasonableaccuracyforhydrocarbon–zeoliteinteractions,

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Structures[54]witha=13.675Å,b=13.675Å,c=14.767Å,_␣=90◦, ␤=90◦and␥=120◦wasconvertedintoaprimitiveunitcell (con-tainingaSi12O24unit)withlatticeparametersa,b,c=9.304Åand

␣,␤,␥=94.60◦,similarto9.319ÅobtainedusingperiodicHartree Fockmodels andGaussian basis sets [55,56]. Theuseof primi-tiveinsteadoftheconventionalunitcellallowsustoreducethe numberofatomspercellfrom108to36.Weemphasizethatthe primitivelatticevectorsgeneratethesameBravaislatticeasthe latticevectorsoftheconventionalcell.TocreateaBrønstedacid site,oneoftheSiatomsoftheframeworkwasreplacedbyanAl atomanda hydrogenatomwasplacedontotheO1oxygensite pointingtowardthecenterofthe8-memberedwindow(seeFig.1) almostintheplaneofthering[57].Accordingtoexperiment[58], thissettingrepresentsoneoutoftwohighlypopulatedacidsites presentin chabazite.Positionsofatoms, volumeaswellascell parameterswerefullyoptimizedusingplane-wavecutoffof520eV untilallforceswerelessthan0.005eV/Å.Resultingcell parame-tersa=9.372Å,b=9.369Å,c=9.407Å,␣=94.31◦,␤=94.44◦ and ␥=93.89◦ were ﬁxed,while allatomic positions of the zeolitic frameworkandoftheadsorbatemoleculeswererelaxedin sim-ulationspresentedinthiswork.

2.3. Structuraloptimizationcalculationsandmoleculardynamics TransitionstateshavebeenidentifiedusingtheDIMERmethod [59], as recently improved by Heyden et al. [60] and using a quasi-Newton method adapted for the saddle-pointrelaxation, implementedintheprogramGADGET[61].Atomicpositionswere consideredtoberelaxedifallforces actingontheatomswere smallerthan0.02eV/Å.Transitionstateswereproventobe first-ordersaddlepointsofthepotentialenergysurfaceusingvibrational analysis.Thepotentialenergyprofilesalongintrinsicreaction coor-dinates[62,63](IRCs)fortheforwardandbackwardreactionsteps wereidentifiedusingthedampedvelocityVerletalgorithm[64]. Thestructurescorrespondingtopotentialenergy minima along theIRCwerefurtherrelaxedusingaconjugate-gradientalgorithm suchastosatisfythesameoptimizationcriterionas for transi-tionstates.Vibrationalanalysiswasperformedtoensurethatthe relaxedstructurescorrespond totruepotentialenergy minima. Thisprocedureguarantees thatreactant and productstates are linkedtotransitionstates viaIRCs.Thefree-energycalculations havebeenperformedusingtheharmonic/rigidrotor approxima-tiontothetransitionstatetheory[65],theGibbsfreeenergiesare reportedforT=673K.Theuseofthisleveloftheoryisproblematic forentropy-drivenreactions,especiallyifsoftdegreesoffreedom (suchas hinderedtranslationsor rotations)are involved inthe reactioncoordinate[66].Therate-determiningstepsfor mecha-nismsdiscussedinthisworkare,however,enthalpy-driven,hence weexpectthatthisleveloftheoryprovidesareasonable descrip-tionof thermal effects. Born-Oppenheimer moleculardynamics (MD)wasperformedintheNVTensemble,temperaturewas

main-Fig.2. ProbabilitydistributionoftheR1 R2bondlengthdifferenceofCH3SHin

chabazite(Å)determinedusingmoleculardynamicsat600K.ThedistancesR1and R2aredeﬁnedintheinset.

tainedusingtheNosé–Hooverthermostat[67,68].Theequations ofmotionwereintegratedwithatime-stepof1fs.

3. Resultsanddiscussion

3.1. Brønstedprotonmobilityinchabaziteandcharged CH3(SH2)+speciesformation

Webeginbyinvestigatingtheinteractionofthe SHmoiety inCH3SHwiththeBrønstedacidsiteformedasdescribedin

Sec-tion2.2.ThehydrogenatomofCH3SHismorepronetodissociate

inthepresenceofastrongBrønstedbasethanthatofCH3OH,as

evidencedbyitslowerpKa(10.3forCH3SHand15.5forCH3OH).

Atthesametime,thesulfuratominCH3SHiseasilypolarizable

andactsasagoodnucleophile:itcouldreactwiththeframework BrønstedacidprotontoformthechargedCH3(SH2)+ species.At

typicalreactiontemperaturesof600–700K[22,24]protontransfer fromitsprobablelow-temperaturelocation(aframeworkoxygen nexttoaluminumatom)totheother,possiblylessbasic frame-work oxygen atom, can be facilitated via thermal diffusion of thischargedcomplex,followedbyitsdissociation.Weperformed moleculardynamics(MD)calculationstoestimatetheprobability ofCH3(SH2)+formationinchabazite.Thesimulationtemperature

was set to that reported for CH3SH catalytic coupling[22–24]

(600K),closetoexpectedexperimentalreactiontemperature,and thetotallengthoftheMDtrajectorywas100ps.Multiple proton-exchangeeventsbetweenthemoleculeandtheacidsitehavebeen observed.Theprobabilitydensitywasdeterminedforthe geome-tryparameterR1-R2definedasthedifferenceindistancesbetween thesulfuratomandthehydrogenformingtheacidsiteinthe ini-tialconfiguration(R1),andbetweenthesulfurandhydrogenatoms formingtheSHgroupintheinitialconfiguration,seeFig.2.Itcan beseenthat thevalueoftheR1-R2differencewas±(0.2/1.5)Å for∼75% ofthesimulationtime (thechangeofsignindicatesa protonexchangebetweenzeolite andmolecule).These configu-rationscorrespondtothemoleculephysisorbedontheacidsite. However,formationofametastableCH3(SH2)+complexwasalso

observedwithsigniﬁcantlikelihood(∼10%)showingthatcharge separatedspeciescanbeformedthusallowingBrønstedprotons tobecomemobileatthereactivetemperatures.Thiswilllaterbe shownofcriticalimportancewherebothBrønstedacidformedona Si–O–AljunctionintheO1position,aswellastheoneformedonthe

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Fig.3. TheproposedmechanismsofCH3OHandCH3SHcatalytictransformationsonchabaziteintovalueaddedproducts.OnlyCH3OHisshownwithCH3SHfollowingthe

samefundamentalpathways.TransitionstatesdesignatedasTStogetherwiththeircorrespondingfreeenergyvaluesinkcal/molarefromthefree-energydiagramsshown inFigs.4–7.Blue(darkgray)valuesareforCH3SHpathwayswhereasred(lightgray)onesareforCH3OHpathways.(Forinterpretationofthereferencestocolorinthisﬁgure

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Fig.4.Free-energyreactionproﬁleforthedissociativeCH3OHandCH3SHtransformationpathway.CH3OHpathwaysareshowninredwhereasthoseforCH3SHinblue.The

pointsinvolvingSi–O–Silinkagearedenotedasv2andthepointsinvolvingAl–O–Silinkagearedenotedasv1,thelatterbeingshownindashedline.Molecularstructuresare showninFigs.S1–S4inSupportinginformation.Allenergieswerereferencedtotheenergyofcleanacidchabaziteandtwonon-interactingmoleculesofCH3OHorCH3SH.

TransitionstatesdesignatedasTSarealsoshownintheoverallproposedmechanisminFig.3withthecorrespondingTSfreeenergies.(Forinterpretationofthereferences tocolorinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

8-memberedringSi–O–Sijunctioncanbeinvolvedmechanistically inCH3OHandCH3SHcatalytictransformations,albeitatrather

dif-ferentenergetics.SimilarCH3OHandH2Oassistedprotonhopping

haspreviouslybeenobservedonpolyoxometallateBrønstedacids facilitatingmobilityofsurfacespecies,whilealsoallowingfor equi-libriumlocationsofprotonsatanelevatedtemperatureof433K [69].ThebarrierforunassistedH-transferinzeoliteisrelatively low (12–25kcal/mol) [70] compared tothehighest free energy barriersontheC Cbondformationroutes(videinfra)makingthe exactpositionoftheprotonontheframeworkunimportantforthe assessmentofthekineticlimitationsinvolved.

3.2. FormationoftheC CbondinCH3OHandCH3SHon

chabaziteviaadissociativemechanism

TheproposedmechanisticnetworkshowninFig.3involvesC C bondformationviadissociativeandassociative/oxonium mecha-nisticsteps,commonlyproposedtobeinvolvedinCH3OHcoupling

reactions [8–10,69,71–73]. The key intermediates involved are surfacemethoxy( OCH3)andylide( CH2)inadissociative

mech-anism,aswellasdimethylether(DME),trimethyloxonium(TMO), ethanol(EtOH),methylethylether(EtOMe),ethyldimethyl oxo-niumion (EDMO)in associativeand oxoniummechanisms.The adsorption of CH3OH (CH3SH) on a Brønsted proton to form

monomers or dimers at higher concentrations is a commonly accepted initial step in C C bond formation [69,71]. The dis-sociative mechanism was ﬁrstlyconsidered, and thecalculated free-energy reaction proﬁleis shown in Fig. 4 with the corre-sponding optimized structural models shown in Figs. S1–S4in Supportinginformation.MinimainvolvingSi–O–Silinkagesasa binding site are denoted as v2 and minima involving Al–O–Si linkages are denotedas v1. Thelatter is shown witha dashed line. Theenergies showninFig.4and discussedinthetextare referenced tothe energyof clean acidchabaziteand two non-interactingmoleculesofCH3OHorCH3SH.OurcalculatedGibbs

free-energies of adsorption for CH3OH and CH3SH are positive

withthevaluesof 1.86and21.76kcal/mol,respectively.Thisis incontrastwithzero-temperaturevalues reportedin the litera-turefor CH3OH in chabaziteof −19to−13kcal/molcalculated

usingB3LYPand asplitvalencebasisset[74].Thethermal cor-rections totheGibbsfree energy usedin thepresentworkare evidentlyneededtorealisticallyassesstheinteractionenergies. WhenachargeseparatedCH3(OH2)+moleculewasconsideredto

beformedawayfromtheO1Brønstedbasesite(routev1), adsorp-tionfree-energiesof13.94and21.06kcal/molwereobtainedfor CH3OHandCH3SH,respectively.Notably,bothCHA CH3SH

struc-tures(corresponding toroutesv1and v2,seeFig.S3(IIv1)and S4(IIv2))wereobtainedfromIRCstartingfromthecorresponding

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Fig.5.Free-energyreactionproﬁlefortheassociativeCH3OHandCH3SHtransformationpathway.CH3OHpathwaysareshowninredwhereasthoseforCH3SHinblue.

CHA-CH3+DME(DMS)toCHA–TMO(TMS)pathwayisshownintheinset.MolecularstructuresareshowninFigs.S5–S8inSupportinginformation.Allenergieswerereferenced

totheenergyofacidchabaziteandfournon-interactingmoleculesofCH3OHorCH3SH.TransitionstatesdesignatedasTSarealsoshownintheoverallproposedmechanism

inFig.3withthecorrespondingTSfreeenergies.(Forinterpretationofthereferencestocolorinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

TSs(seeFigs.S3(IIIv1)andS4(IIIv2))andresultedinthecharged, almostenergy-degeneratecomplexes.Incontrast,CH3OHdoesnot

necessarilyformthemetastableintermediateCH3(OH2)+complex

at the Brønsted site. This reaction step is completed by form-ingaH2Oor H2SmoleculewiththeCH3 groupattachedtothe

zeoliteframework (Figs.S1–S4(IV)).Relativelyhighstability of CHA-H+CH3OHv2conﬁgurationascomparedtoCHA-H+CH3OH

(IIv1)canbeobservedfromthefreeenergyvalues.Furthermore, thebarrierfor CH3 transfer fromSi–O–Sito Al–O–Si (seeFig.

S2(VIv2TS))isratherlarge,butequilibriumisshiftedtowardsa methoxygrouplinkedtoamorebasicAl–O–Sisitewiththe cor-respondingenergyof0.84kcal/mol.MethoxyformedonAl–O–Si isverystablethermodynamicallyandneedstoundergoaseriesof furtherreactionstobecomeactivated.Inparticular,anapparent ratelimitingstepforthismechanismisencountered duringthe TS2(seeFig.3)toformylideincorporatedintotheCHAstructure. AsshowninFigs.S1(VII–IX)andS2(V,VIII,IX),thisreactionstepis linkedwithalargestructuralrearrangementofthezeolite frame-work.Calculatedforwardreactionbarriersof 60–70kcal/molin combinationwitharatherfacilereversionoftheylideintoadsorbed methoxyeffectively precludesthis pathway,inagreementwith someliteraturedatareportingareactionbarrierof78kcal/molin ZSM-5[71].Importantly,iftheylide species canbeformedvia differentpathways, suchas aradical mechanism[75],it would rapidlyconvertintotheethoxy CHA-CH2CH3+H2Ointhe

pres-enceofasecond,sequentiallyaddedCH3OHmolecule(seeFigs.

S1–S4(X–XII)).Thisisobviousfromtheverylargeratiobetween forwardandreversereactionbarriers,especiallyinthecasewhere ylideisformednexttotheframework Alatom(v1). A particu-larlylow energy is neededfor the activation of theCH3(SH2)+

moleculeinthev1pathwayreactingviaTS3(seeFig.3andthe cor-respondingstructuresinFig.S3(X–XII)).Consecutively,adsorbed CH2CH3canundergotransferbetweenSi–O–SiandAl–O–Si

link-agesafterdesorbingH2O(H2S)(Fig.S1(XIII-XV))withabarrierof

only14.25kcal/mol,whichismuchlowerthanthatinthe CH3

case(33.06kcal/mol).Theethoxyspeciesformedonthemorebasic Al–O–Siisthermodynamicallymorestableby15.15kcal/molthan thatlinkedtotheSi–O–Sisequence.Theethoxyspeciesundergoesa deprotonationreaction(Fig.S1(XV–XVII))viaTS4atrelativelylow barrier.Desorptionoftheﬁnalreactionproduct(ethene)isthen thermodynamicallyfavored,resultinginthecalculatedfree-energy changeof−22.11kcal/molfortheoverallprocess.

3.3. FormationoftheC Cbondviaassociativeandoxonium mechanisms

Theassociativemechanismissubsequentlyconsidered.The cal-culated free-energy reaction proﬁleis shown in Fig. 5 (energy ofcleanzeoliteandfournon-interactingmoleculesofCH3OHor

CH3SH is used as reference) and the corresponding optimized

structuralmodelsarepresentedinFigs.S5–S8inSupporting infor-mation. The initialconﬁguration contains two CH3OH (CH3SH)

moleculesinteractingviathe H-bondednetwork of OH( SH) moietiesofbothmoleculesandBrønstedacidsites(seeFigs.S5(II) andS7(II)inSupportinginformation).Thecomputedfree-energies ofadsorptionare9.51and17.33kcal/molforCH3OHandCH3SH,

respectively,withthelatterbeinglessstabilizedandthushigher in energydue to thelackof thestronghydrogenbonds inthe S H linkages.Thereactionofmoleculesintheadsorption com-plexoverthetransitionstateTS5(Figs.S5(III)andS7(III))results informationoftheprotonatedDME(DMS)species– dimethyloxo-nium(dimethylsulfonium)(Figs.S5(IV)andS7(IV))–withforward andreversebarriersbeingalmostidentical.Thisstepisfollowed bytheexergonicdesorptionanddeprotonationonAl–O–Si link-agestoformDME(DMS)shownin Figs.S5(V) andS7(V).Upon reaction with the second DME(DMS) molecule, a stable CHA-H+DMEcomplex transforms via TS6(Figs. S5(VII)and S7(VII)) intotrimethyloxonium(TMO)ortrimethylsulfonium(TMS)

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con-Fig.6.Free-energyreactionproﬁlefortheEDMO(EDMS)transformationpathwaysderivedfromTMO(TMS)structures.EDMOpathwaysareshowninredwhereasthose forEDMSinblue.MolecularstructuresareshowninFigs.S9andS10inSupportinginformation.Allenergieswerereferencedtotheenergyofcleanacidchabaziteandfour non-interactingmoleculesofCH3OHorCH3SH.TransitionstatesdesignatedasTSarealsoshownintheoverallproposedmechanisminFig.3withthecorrespondingTSfree

energies.(Forinterpretationofthereferencestocolorinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

Fig.7.Free-energyreactionproﬁlefortheCH3CH2OH(CH3CH2SH)transformationpathwaysderivedfromTMO(TMS)structures.ResultsforCH3CH2OHpathwaysareshown

inred,whereasthoseforCH3CH2SHinblue.MolecularstructuresareshowninFigs.S11andS12inSupportinginformation.Allenergieswerereferencedtotheenergyof

cleanacidchabaziteandfournon-interactingmoleculesofCH3OHorCH3SH.TransitionstatesdesignatedasTSarealsoshownintheoverallproposedmechanisminFig.3

withthecorrespondingTSfreeenergies.(Forinterpretationofthereferencestocolorinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

tainingaCHA+TMO+CH3OHorCHA–TMS+CH3SHcomplex(Figs.

S5(VIII)andS7(VIII)).Thebarrierforthereverseprocessforthis partofthereactionintheoxygenpathwayisonly7.09kcal/mol, whichismuch lowerthanthatfortheforwardreactionstepof 31.02kcal/mol.TMSformationfromtwoDMSmolecules,onthe otherhand,undergoesalargeractivationbarrierintheforward directionof39.56kcal/mol,butresultsinaCHA–TMScomplexthat ismuchmorestablethanitsoxygenatedcounterpart.Asan alter-nativerouteofTMO(TMS)formation,adsorbedmethoxyspecies canundergorelativelyfacilereactionwithDME(DMS)viaTS6a, asshown in Fig.5 inset(the correspondingstructuresare pre-sentedinFigs.S6andS8inSupportinginformation).Weconsidered two variants ofthis mechanism differing in theinitialposition oftheCH3 groupeitheronSi–O–SioronSi–O–Allinkages.The

reactionproceededwithratherlowactivationbarriers,beingmost thermodynamicallyfeasibleonAl–O–Silinkages.Notably,TMSin conﬁgurationIXaisthermodynamicallymorefavorablethanTMO. Asexpected,themethylgroupdetachmentfromtheSi–O–Si pro-ceedsevenfasterduetoitsweakerinteractionenergy(ascompared totheAl–O–Si)withthislinkage.Thisisoneofthesituations(IXa ofTMO vs TMS)where thetrendof thermodynamicstability is reversedbetweenoxygenandsulfurspecies,sinceoxygen-based compoundsappeartobemorethermodynamicallystable other-wise. The resultingTMO (TMS) species is physisorbed,i.e. very weaklyinteractingwiththeCHAframework, andthus the acti-vationbarriertoundergosurfacefacilitatedrearrangementtothe CHA-H-EtOMecomplexviaTS7(Figs.S5(X)andS7(X))israther high.Additionally,physisorbedCHA-H+TMScanreconﬁgurevia

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TS7toform DSMY(dimethylsulfonium methylide-structure IX inFig.S7)which, unlikeitsoxygencounterpart,correspondsto aminimumonPES.Thisintramolecularrearrangementishighly unfavorable thermodynamically andthus is identiﬁed asa rate limitingstepforthisTMO(TMS)transformation.However,ifthe intermediateCHA-H-EtOMeisformed,itisratherstableandcan undergodecompositionintoCHA−CH2CH3+CH3OH.This

unsta-bleintermediatequicklyrevertstotheinitialstructureviaTS8(Figs. S5(XII)andS7(XII)),asisobviousfromthelargefree-energy differ-encesbetweentheinitialandﬁnalreactionstates.Alternatively,a concertedmechanismtoyieldCHA-H+CH2CH2+CH3OHthrough

␤-eliminationviaTS9(Figs.S5(XVI) andS7(XVI))ismuch more favorableforanoxygenspecies,asobviousfromthecomparison offree-energiesofactivation.Thesulfurspecies,ontheotherhand, arenotstabilizedbytheCHAframeworkandrevertbackinto CHA-H-EtSMevialowbarrier.Mostimportantly,therelativestabilityof theproductsXIVandXVIIIsuggeststhattheethyleneformationvia theconcertedmechanismwithEtOMe(EtSMe)asintermediateis thermodynamicallymoreviable.

Fig.6showsthefree-energyreactiondiagramforan alterna-tivepathway for the CHA+TMO transformation (and its sulfur containinganalogTMS)proceedingviaformationofCHA+EDMO (CHA+EDMS),ratherthanundergoingintramolecular rearrange-mentintoEtOMe(EtSMe).Inthiscase,theenergyofcleanzeolite andfournon-interactingmoleculesofCH3OHorCH3SHwasusedto

deﬁnezeroontheGaxis.Thecorrespondingoptimizedstructural models are shown in Figs. S9 and S10 in Supporting informa-tion.TheformationofstableEDMO(EDMS)speciesviaalkylation ofTMO (TMS)withanadditionalCH3OH(CH3SH)moleculewas

foundtobeveryexergonic. Here, thealkylationbarrier forthe sulfurandoxygencontainingmoleculescanbeinferredtobe sim-ilarenergetically since thereaction doesnot involve new C O orC Sbondformation,butratherC Cbondformationvia sim-ilarpathwaysforboth oxygenandsulfurcompounds.Literature reportsthatdimethyloxoniummethylide(DOMY)shouldbean intermediatespeciesinCHA+EDMOformation,withthecomputed CHA-H+DOMYtoCHA+EDMOreactionproceedingviaavirtually barrierlessprocess[73].Ourdataconﬁrmtheabsenceofthe bar-riertoformCHA+EDMO;CHA-H+DOMYhasbeenlocatedonthe potentialenergysurfaceasatransientspeciesduetothestrong interactionbetweenits CH2moietyandframeworkSiatom(see

FigS9).Thisstep,however,isisostructuralbetweenbothDOMY andDSMYandwouldnotaffectthereactivepathway.The subse-quenttransformationsofCHA+EDMOproceedintwoscenarios, oneinvolvingaconcertedC HbondbreakingandBrønstedacid siteregenerationonAl–O–SiviaTS10,andtheotherinvolvingan extrastepof CH2CH3 formationviaTS11followedby

deproto-nationviathetransitionstateTS4thatwasalreadyshowninFig. S1(XVI).Intheconcertedmechanismforoxygenatedspecies,the reversebarrieris greaterthantheforward,whereasTS11leads toa CH2CH3 productthat ismore stablethanitsEDMO

reac-tant.Hence,forsulfur-containing species,thepathwayviaTS10 isslightlymorefavorableastheproductCHA-H+DMS+CH2CH2

islowerinfreeenergy.ThesituationisreversedintheEDMOcase andTS11mechanismismorefavorablethermodynamically. Never-theless,theoverallenergeticsforbothmechanismsisverysimilar andoneshouldexpectthatbothpathwaysareactive(atdifferent proportions).Theethoxyspeciescanfurtherbedeprotonatedvia mechanismdiscussedpreviouslyinconnectionwiththe dissocia-tivemechanism(seeSection:3.2)yieldingthegasphaseCH2=CH2

andregeneratedBrønstedacidsite.

Fig.7describes aﬁnal mechanism considered withEtOHas anintermediatespecies[73].Thecorrespondingoptimized struc-turalmodelsareshowninFigs.S11andS12.Aratelimitingstep istheintramolecularrearrangementofCHA-H+DME(DMS)(Figs. S11(V)andS12(V))intoCHA-H+EtOH(EtSH)(Figs.S11(VII)and

S12(VII))withverylow reactionratescompared tothe mecha-nismsinvolvingTMOpathways.AnotablestabilityofCHA-H+DME species as opposed to CHA-H+DMS is due to the need in the deprotonation of the latter on the Brønsted site via transient methyl sulfonium methylide (MSMY) shown in Fig. S12 (V) to undergoCHA-H+EtSH formation.Ifformed,CHA-H+EtOH rear-ranges into stable CH2=CH2 via a concerted TS13 mechanism,

whereasadsorbed CH2CH3 formedviaTS14ismuch higherin

free energy than the initial state, and readily reverts back to CHA−H+EtOH.AccordingtotheGibbsfreeenergiesshowninFig.7, transformationsthroughTS13aremorelikelytoleadtostable prod-uctsthanthoseviaTS14.Similartransformationsproceedinsulfur pathways,althoughallofthespeciesarehigherinenergythantheir oxygenatedcounterpartsduetothelackofstabilizinghydrogen bondswiththezeoliteframework.

3.4. Kineticandthermodynamicaspectsofthekeyintermediates inCH3SHtransformationintoethene

Datapresentedsofarareconsistentwiththeexpectationthat TMOisakeyintermediateinCH3OHtransformationstoCH2CH2

[73].Similarly,reactionofTMSformedviareactionoftheadsorbed methoxywithDMSproceedswithalargereversebarrier,asshown ininsetofFig.5,andservesasathermodynamicsinkforthe over-allprocess.ThegreaterthermodynamicstabilityoftheCHA–TMS complexas opposed tothatof CHA–TMOin insetof Fig.5can beexplainedbythehighernucleophilicityofsulfuranditslower electronegativitywhilereverseorderinFig.6isdue tothefact that on a reactive landscape TMS is effectively reorganized as DSMY(Fig.S7). Combinationofthesefactors allowsthe forma-tionofarelativelystablechargeseparatedcomplexofCHA−–TMS+_.

CHA-H+DSMY was always formed as a highenergy precursor tothefurthermoleculartransformation viaEDMS intermediate basedroutes. Moreover,itcan beseenfromFig.6 thatDMEis anadditionalproductinCH2CH2formation,leadingtohigh

con-centrationsinthereactivemixtureand furtherfacilitating TMO formation,consistentwiththeliterature[8,10,25,69].DMSisalsoa thermodynamicallyfavoredproductofCH3SHcoupling,inaccord

withthe literature[22–24]. Since higher temperaturesneedto be maintained to overcome thermodynamic limitations in the equivalenttransformationsofsulfurproducts,secondaryreaction sequencesmayproceed,suchasCHA-H+TMStoCHA+DMS+CH4

orCHA+TMS+CH3SHtoCHA−H+DMS+CH4+CH2S,observedto

occurwithhighratesintheoxygenatedcounterparts[76].Thisis ingoodagreementwiththeexperimentalobservationofDMSand CH4asthemainproductsoftheCH3SHcouplingreactionon

zeo-lites[22–24].Intargetedolefinfuelproduction,suchsidereactions formingparaffinproductsareintuitivelyundesirable. Mediation oftheBrønstedacidstrengthviametaldopantcanbeproposed asapossiblesolutiontothisproblem.Themetal–Al–O–Silinkage couldpossiblypreventtheTMSmethanationreactiononstrong Brønstedacidsites,butthemechanisticdataonTMO(TMS)catalytic transformationsinzeolitesarenotreadilyavailable.Fromthedata presented,acrucialmechanisticstepenablingdirectCHA-TMSto CHA-EDMSreactionwouldproceedviaC Cbondformation– alky-lationwithanadditionalCH3SH.Efficientsp3 hybridizedcarbon

alkylationreactionsviaC Hbondactivationhavebeenrecently showntoproceedonPd[77],whileratherinactivemethaneC H bondswereactivatedinGa/ZSM5[78].

4. Conclusions

CH3SHcouplingintovalueaddedproductsand theﬁrstC C

bond formation in ethene wasinvestigated onchabazite Bron-sted acidic sites. Thermodynamics was compared withthat of

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Partialﬁnancial supportfromLehigh University is gratefully acknowledged.WethankSURFsara(www.surfsara.nl)forthe sup-portinusingtheCartesiusComputeCluster(theNetherlands).Part ofthecalculationswasperformedusingcomputationalresourcesof supercomputinginfrastructureofComputingCenteroftheSlovak AcademyofSciencesacquiredinprojectsITMS26230120002and 26210120002supportedbytheResearchandDevelopment Oper-ationalProgramfundedbytheERDF.T.B.acknowledgessupport fromtheprojectVEGA-1/0338/13.Dr.SteeveChrétienisgratefully acknowledgedforusefuldiscussions.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound, intheonline version,athttp://dx.doi.org/10.1016/j.apcatb.2016. 01.021.

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