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Mechanistic understandingof methanolcarbonylation: Interfacing homogeneous andheterogeneous catalysisvia carbonsupported IrALaAlyssa J.R.Hensley a , JianghaoZhang a , IlkaVinçon b , XavierPereira Hernandez a , DianaTranca b

Gotthard Seifert

b,? , Jean-SabinMcEwen a,c,d,e,? , YongWang a,e,? a

The Geneand LindaVoiland Schoolof ChemicalEngineering andBioengineering, WashingtonState University,Pullman, WA99164, UnitedStates

b c Department ofPhysics andAstronomy, WashingtonState University,Pullman, WA99164, UnitedStates d Department ofChemistry, WashingtonState University,Pullman, WA99164, UnitedStates e

Institute forIntegrated Catalysis,Pacific NorthwestNational Laboratory,Richland, WA99352, UnitedStates

articleinfo

Article history:

Received 17July 2017

Revised 21February 2018

Accepted 22February 2018

Available online5 April2018

Keywords:

Single-site heterogeneouscatalyst

Methanol carbonylation

IrALa complex

Promoter effects

Density functionaltheory

Attenuated totalreflectance-Fourier

transform infraredspectroscopy abstract The creationof heterogeneousanalogs tohomogeneous catalystsis ofgreat importance tomany indus-

trial processes.Acetic acidsynthesis viathe carbonylationof methanolis onesuch processand itrelies on

a difficult-to-separatehomogeneou sIr-basedcatalyst.Using acombination ofdensity functionaltheory (DFT) andattenuated totalreflect ance-Fouriertransform infrared(ATR-FTIR)spectroscopy,we determine the structureand mechanismfor methanolcarbonylation overa promisingsingle-site IrALa/C heteroge- neous catalystreplacement. Here,the Ircenter isthe activesite withthe acetyl-Ircomplex beinga rate controllingintermediat e.Furthermore,theLa bothatom icallydisperses theIr andacts asaLewisacid site. Infa ct,theLapromoter inthe IrALa/C catalystwas foundto behavesimilarly tohomogeneous pro- moters byabstracting aniodine fromthe Ircenter andaccelerating theCO insertionstep. Overall,this work provideskey insight intotheatomisticnature ofthe IrALa/C single-sitecatalyst andallows for the furtherdesign andoptimization ofsingle-site heterogeneouscatalysts. ?2018 ElsevierInc. Allrights reserved.

1. Introduction

Single-site heterogeneouscatalysts area newclass ofmaterials that interfacethe heterogeneousand homogeneous paradigms, combining thehigh activityand selectivityof homogeneous cata- lysts withthe highseparability ofheterogeneous catalysts.Exam- ples ofsuch catalystsinclude bimetallic surfaceswith an atomically dispersednoble metalsupported ona basemetal or oxide surface,as wellas beingexchanged intozeolites [1-9]. One industrially homogeneouscatalytic processcurrently withouta viable heterogeneousreplacement [10-12]is theproduction of acetic acidwhich isaccomplis hedvia thecarbonylationof metha- nol througheither theMonsant oprocess withanRhcatalyst [10,13,14]or theCativa processwith anIr catalystwith aRu pro- moter[14-18]. Themechanism forthe catalysishas thesame

major stepsfor bothprocesses [14,15], withthe keysteps beingthe oxidativeaddition ofiodomethane (MeI)to thereduced metalcenter ([M(CO)2

I 2 ) andthe migratoryinsertion ofCO intothe methyl-metalbond followedby thereductive eliminationof acetyl iodide (AcI)as theproduct [10,19-21]. Inour recentwork, we reported apromising heterogeneouscatalyst forthe carbonylation of methanolbased onIr anda Lapromoter [11]. Characterizationof this IrALa/C systemshowed thatthe catalystwas amolecular spe- cies withdistinct sitesformed fromtwo metalatoms [11], essen- tially creatinga single-sitecatalyst onan activatedcarbon support. Overall,the heterogeneous IrALa/C catalystshowed com- parable reactivityand selectivityto theanalogous homogeneous catalyst[11]. Furthermore,using Laas apromoter asopposed to Ru canalso reducethe costof thecatalyst. Thus,the IrALa/C heterogeneouscatalyst isa promisingalternative tothe conven- tional homogeneoussystem forthe carbonylationof methanolto acetic acid. Here, weestablish thestructure andmethanol carbonylation mechanismof theheterogeneous IrALa/C catalystusing acombi- nation ofdensity functionaltheory (DFT)and attenuatedtotal reflectance-Fouriertransform infrared(ATR-FTIR) spectroscopy. Additionally,we haveelucidate dthe roleoftheLa promoterin the carbonylationreaction. Overall,this workshows thatthe https://doi.org/10.1016/j.jcat.2018.02.022

0021-9517/?2018 ElsevierInc. Allrights reserved.

Corresponding authorsat: TheGene andLinda VoilandSchool ofChemical Engineering andBioengineering, WashingtonState University,Pullman, WA99164,

United States(J.-S. McEwenand Y.Wang).

E-mail addresses:gotthard.seifert@chemie.tu-dresden.de(G. Seifert), js.mcewen@wsu.edu(J.-S. McEwen),yong.wang@pnnl.gov(Y. Wang).

Journal ofCatalysis 361(2018) 414-422

Contents listsavailable atScienceDirect

Journalof Catalysis

journal homepage:www.else vier.com/locate/jcat dispersion andactivity ofsingle-site heterogeneouscatalysts can be tunedvia thechoice ofpromoter (i.e.altering thepromoter"s oxygen affinityand electronegativity/Lewis acidity),thereby allowing forthe greaterdesign andoptimization ofheterogeneous catalysts forthe carbonylationof methanol.

2. Methodsand materials

2.1. Densityfunctional theory

DFT calculationswerecarried outusing theVienna Ab Initio Simulation Package(VASP) [22-24]. Theprojector-augmen ted wave (PAW)method [25,26]with aplane-wave basisset and an energycutoff of450 eVwere used.To modelthe electron exchange andcorrelation, thePerdew-Burke-Er nzerhof(PBE) functional[27]has beenapplied. Spinpolarization hasbeen included inall calculations. TheGaussiansmearing[28]method was usedwith asmearing widthof 0.2eV toimprove conver- gence, andthe totalenergy wasextrapolated tozero Kelvin.All gas phaseground stateoptimizati onsused theconjugategradi- ent methodand wereconsidered converged whenthe inter- atomic forceswere smallerthan 0.01eV/Å, whilesurface relaxations wereconsidered convergedwhen theforces wereless than 0.025eV/Å. Theenergy tolerancewas setto 10 ?7 eV. Calcu- lations formolecules inthe gasphase wereperformed usingan

18?19?20 Åbox, andone singlek-point, theGamma point,

was sufficientto spanthe Brillouinzone. Ab InitioMolecular Dynamics (AIMD)simulations havebeen computedfor aNVT ensemble at513 K,which isthe reactiontemperature. Asthe spin-polarizedground stateoptimizations resultedin anet zero magnetic momentfor thecomplexes examined here,the AIMD calculations werenot spinpolarized andthe energytolerance was setto 10 ?5 eV. Thetransition statescalculated herewere obtained usingthe ClimbingImage NudgedElastic Band(CINEB) method[29]. Theoptimizations alongthe minimumenergy path- ways (MEPs)were performedwith thefast inertialrelaxation engine (FIRE)optimizer withforce andenergy tolerancesof

0.05 eV/Åand 10

?7 eV, respectively[30,31]. Eachtransition state was foundto haveone imaginaryvibrational modealong agiven reaction pathway[32]. In orderto makea strongerconnection betweenthe theoryand experiment inthis work,we calculatedthe Gibbsenergy forour tested methanolcarbonylationreaction pathwaysusing standard statistical mechanicsprinciples [33]. TheGibbs energyfor each configurationwas calculatedas: G i ¼E i ?k B Tln q trans q rot q vib

ðÞð1Þ

whereE i is theDFT-calculated energyfor theith configuration; q trans ,q rot , andq vib are thetranslation, rotational,and vibrational partition functions;and k B andTare Boltzmann"sconstant and the reactiontemperature ,respectively.In thisstudy,thegas phase IrALa complexeswere treatedas themodel surfacein eachcase, and thereforeonly thevibrational entropyassociated witheach complex wasincluded inthe Gibbsenergies. However,the transla- tional, rotational,andvibrational entropycontributions fromthe gas phaseCO, MeI,and AcIspecies wereincluded forthe relevant ground stateconfigura tions.Thepartitionfunctions werecalcu- lated accordingto [34]: q trans 2pmk B

TðÞ

3=2 h 3 k B T

Pð2Þ

q rot;linear 8p 2 I linear k B T h 2

ð3Þq

rot;non?linear 8p 2 2pk B

TðÞ

3=2 ffiffiffiffiffiffiffiffiffiffiffiffiI A I B I C p rh 3

ð4Þ

q vib ¼Y i e ?ht i =2k B T 1?e ?ht i =k B T

ð5Þ

wheremandPare themass andpartial pressureof themole- cule ofinterest; I linear is themoment ofinertia fora linearmolecule; I A ,I B , andI C are theprincipal momentsof inertiaof themolecule of interest (non-linearcase); ris thesymmetry numberfor themole- cule ofinterest; and m i is theith vibrational mode.AllGibbsener- gies werecalculated ata temperatureof 513K, atotal pressureof

17 bar,a COconcentration of17.3 mol%,a MeIconcentration of1.8

mol%, andan AcIconcentratio nof 1.8mol%,consistentwith the catalytic experimentsperformedin ourprevious work(see Supple- mentary Materialfor moredetails) [11]. For thesurface calculations, asinglelayercarbon sheetwas simulated torepresent theactivated carbonsupport. Thesingle layer sheetwas foundto besufficient asonly weakinteractions exist betweendifferent layersof graphiteand theinfluence ofmul- tiple graphenelayers onadsorptio nenergies isnegligible[35]. Two different adsorptionsites havebeen modeledusing VASPand are shown inFig. S1;a pristinegraphene sheetto representthe basal planes andan OH-passivated armchair-graphenenanoribon (AGNR-OH)to simulatean adsorptionedge (Fig.S1). Anoptimized lattice constantof 2.467Å wasfound forgraphene whichis consis-quotesdbs_dbs21.pdfusesText_27

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