REPOSITORIO INSTITUCIONAL DA UFOP: Conversion of fatty acids into hydrocarbon fuels based on a sodiumcarboxylate intermediate.

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Catalysis

Today

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / c a t t o d

Conversion

of

fatty

acids

into

hydrocarbon

fuels

based

on

a

sodium

carboxylate

intermediate

Deise

Morone

Perígolo

,

Fabiano

Gomes

Ferreira

de

Paula

,

Marcelo

Gonc¸

alves

Rosmaninho

_,

_Patterson

_Patrício

_de

_Souza

_,

_Rochel

_Montero

_Lago

_,

Maria

Helena

Araujo

a_{DepartamentodeQuímica,UniversidadeFederaldeMinasGerais,BeloHorizonte,MG31270-901,Brazil} b_{DepartamentodeQuímica,UniversidadeFederaldeOuroPreto,OuroPreto,MG,35400-000,Brazil}

c_{DepartamentodeQuímica,CentroFederaldeEducac¸ãoTecnológicadeMinasGerais,BeloHorizonte,MG,30421-169,Brazil} d_{InstitutoFederaldoMatoGrosso,CampusAvanc¸adodeDiamantino,Diamantino,MT,78402-000,Brazil}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received16December2015 Receivedinrevisedform27April2016 Accepted30April2016

Availableonlinexxx

Keywords: Freefattyacids Carboxylate Fuels

Sodiumhydroxide

a

b

s

t

r

a

c

t

Inthiswork,itwasinvestigatedtheconversionoffattyacidsintohydrocarbonbasedonthereactionwith

NaOHfollowedbyacontrolledthermaldecomposition.FTIR,Raman,UV–vis,XRD,TG-MS,SEM/TEM,CHN,

GC–MSshowedthatprecursorsbasedonNaOH/oleicacid(molarratios0.7,1.0,1.5and2.0)decomposed

at550◦_{Ctoproducethreefractions,i.e.liquid(5–37wt%),gas(52–70wt%)andsolid(10–31wt%).The}

liquidfractionwascomposedofacomplexmixturecontainingmainlyaromaticcompounds.Ontheother

hand,themajorgasfractionshowedaremarkableselectivityforpropane(56–61wt%)withsomeC1,C2,

C4,H2andCOx.ThesolidfractionshowedthepresenceofNa2CO3,Na2Oandparticlesofamorphous

andgraphenelikecarbon.Upontreatmentat800◦_{CthecarbonatedecomposestoCO}

2,oxidizesthe

carbonandregeneratedtheNa2Owhichcanpotentiallybeusedforanewreactioncycle.Theseresults

arepreliminarydiscussedintermsofacatalyticeffectofthebasicsodiumoxidetopromotecracking,

dehydrogenationandH-transferreactions.

©2016ElsevierB.V.Allrightsreserved.

1. Introduction

Theproduction offuels fromrenewable substrateshasbeen intensivelyinvestigated inthelastdecades[1].Theuseof veg-etableoilstoproducebiodiesel iscurrentlythemostimportant route[2–4].Thebiodieselproductionisusuallyperformedusing homogeneous(NaandKhydroxideoralkoxide)orheterogeneous basiccatalystsforthetransesterification[5–7].

Averyimportantcommoncontaminationinvegetableoilsis freefattyacids(FFA),forinstance,palm(Elaeisguineensis),macauba (Acrocomiaaculeata),pinhãomanso(Jatrophacurcas),usuallyhave highFFAcontents,e.g.20–70%[8].Soybeanusedoilwhichisavery importantwastecanalsohavefairlyhighconcentrationsofFFA, e.g.2–10%[9].ThepresenceoftheseFFAinconcentrationshigher than2%completelyhindersthebasiccatalysedbiodiesel produc-tionduetothealkalinecatalystsdeactivation,withtheformation ofsoap(fattyacidsalts),stableemulsionsandcomplicationsinthe

∗_{Correspondingauthor.}

E-mailaddress:[email protected](M.H.Araujo).

purificationstep[6,10,11].Analternativeroutetodealwithacidic oilsconsistsinesterificationinthepresenceofhomogeneous[12] andheterogeneousacidiccatalyst[5–7].Insomecases,the veg-etableoilisfurtherhydrolyzedtoproduceFFAandthenesterified usingacidcatalysis[13].

Different approaches to produce fuels from FFA have been describedintheliteraturesuchasreformtohydrogen[14], cat-alytichydrodeoxygenation[15],hydrotreating[16]andcatalytic pyrolysisofsoaps[17–20].

Inthiswork,itisinvestigatedtheconversionoffreefattyacids contaminantsdirectlyintohydrocarbonfuels.Inthisprocess,the fattyacidreactswithNaOHtoformasodiumcarboxylate interme-diateasshowninEq.(1).

CnHmCOOH+NaOH→CnHmCOO−Na++H2O (1)

Thesodiumcarboxylatecanthenbethermallytreatedto decom-poseduetothestrongR-COO-Na+ionicinteractionthesodium cationcanretaintheoxideanion,andadeoxygenationmighttake place.Thedeoxygenationprocessofthecarboxylatecanleadtothe http://dx.doi.org/10.1016/j.cattod.2016.04.035

fragmentationoftheFFAmoleculeproducinghydrocarbon deriva-tivesandlikelycarbonoxidesandsodiumoxide(Eq.(2)).

CnHmCOO−

Na+

→hydrocarbons+COx+Na2O (2)

Hereon,adetailedinvestigationoftheprocessesdescribedin Eqs.(1)and(2) usingoleicacid(CH3(CH2)7CH CH(CH2)7COOH) andNaOHwithdifferentmolarratiosfollowedbythermal decom-positionisdescribedwiththecharacterizationofthedifferentsolid, liquidandgasproducts.

2. Experimental

Theprecursorsweresynthetized fromthereaction ofNaOH and oleic acid (OA) in different molar ratios (0.7, 1.0, 1.5 and 2.0).Theresultantmixturewastreatedat80◦_{Cfor24handthen}

cooledinadesiccator.Thecarboxylatesaltswerecharacterizedby InfraredSpectroscopy(IR,Perkin-ElmerSpectrumGXFT-IR Sys-tem,4000–400cm−1_,4cm−1_{ofresolution,64scans,KBrpellets)} andThermogravimetricAnalysiscoupledtoMassEspectrometry inanargonfluxof20mLmin−1_{,temperaturerangeof40–900}◦_C

andheatingrateof5◦_Cmin−1_{(TG-MS, NETZSCHthermobalance} modelSTA449F3coupledwithmassspectrometerNETZSCH Aëo-losmodelQMS403CwithEIandquadrupoleanalyzer).

Forthethermaldecompositionexperiments,60–100mgofthe carboxylatesaltswereplacedin aclosedtubularquartzreactor (batch mode)connected with a condenser tocollectthe liquid productsandavolumetricsystemtomeasureandcollectthegas productsforGCanalysis.Thereactorwasheatedinaceramic fur-nacefromroomtemperatureto550and to900◦_{C,both witha}

heatingrateof10◦_Cmin−1_{.Thematerialswerekeptatthose} tem-peraturesfor20min.Fromthisexperiment,threefractionswere obtained:solid,liquidandgaseous.

Thesolidproductsofthethermaldecompositionexperiments werecollectedandcharacterizedbyRamanspectroscopy(Bruker Senterra,CCDdetector,633nmand2mWLASER),X-Ray Diffrac-tion(XRD,ShimadzuXRD-7000,Cu(K␣)radiation,scanningrange 10–80◦_{, 4}◦_min−1_{), Thermogravimetric Analysis (TG, Shimadzu,} modelDTG-60H,airornitrogenflowof50mLmin−1_,temperature rangeof25–900◦_{Candheatingrateof10}◦_Cmin−1_),Scanning

Elec-tronMicroscopy(SEM,Quanta200FEI)andTransmissionElectron Microscopy(TEM,TecnaiG2-20–SuperTwinFEI–200kV). More-over,thesolutionobtainedafterwashingthesolidwithwaterwas analyzedbyTotalOrganicCarbonAnalysis(TOC,Shimadzumodel TOC-VCPH,1000timesdilutionfactor).

Theliquid productscondensed in a trapduringthethermal decompositionexperiment werecollectedand characterizedby ElementalAnalysis(CHN,PerkinElmer),Infraredspectroscopyand GasChromatography coupledwithmass spectroscopy (GC–MS, AgilentmodelGC7890,HP-5column)coupledwithamass spec-trometermodel5975CwithEIandaquadrupoleanalyzer).

Thegas productsformedduring the thermal decomposition experimentwerecharacterizedbyGasChromatography(GC, Shi-madzuGC-2014ATFequippedwithmethanator,TCDandFID).

3. Resultsanddiscussion

3.1. Synthesisandcharacterizationoftheprecursors

TheprecursorsweresynthetizedfromthereactionofNaOHand oleicacid(OA)indifferentmolarratios(0.7,1.0,1.5and2.0)named hereonas0.7Na,1.0Na,1.5Naand2.0Na,respectively.

IRspectraoftheNaoleateprecursorsshowedthatthecarbonyl bandoftheoleicacidat1710cm−1_{stronglydecreasedwiththe} appearanceofa newbandat1560cm−1 _{relatedtotheNa}+ car-boxylatewhichsuggeststhatmostoftheoleicacidhasbeenreacted (Fig.1)[21].

2000 1800 1600 1400 1200 1000

Oleic Acid

COOH

COO-Na+

Wavenumber /cm

Precursor

Tr

an

smit

an

ce

/ u.a.

Fig.1. FT-IRspectraobtainedforprecursorsandoleicacid.

0 100 200 300 400 500 600 700 800 900 0

20 40 60 80 100

3rd loss 2nd loss

1st loss

W

ei

g

ht

l

o

ss

/

%

Temperature / °C

OA 2.0Na 1.5Na 1.0Na 0.7Na

OA Evaporation

Fig.2.TGanalysesobtainedfortheprecursorsandpureoleicacid.

Thetemperatureinwhichthecarboxylateswoulddecomposeto producehydrocarbonswasdeterminedbyaTGstudyunderargon atmosphere(Fig.2).

Thepureoleicacidpresentedasingleweightlossinthe temper-aturerangeofca.200–300◦_{Cduetoevaporation.Ontheotherhand,}

theprecursorsshowedthreemainweightlossesintemperature rangesof100–400◦_C,400–500◦_{Cand700–900}◦_{C.Theprecursor}

0.7Nashowedasignificantgradualweightlossbetween100and 350◦_{C,likely relatedtopartialoleic acidevaporationduetoits}

highconcentrationandlowNa+_{content.Ontheotherhand,forthe} 2.0Naprecursoraweightlossofca.10%(100–150◦_{C)wasobserved,}

whichisprobablyrelatedtowatermoleculesduetohighNa+ con-tentonthesample.Thiseventwasfollowedbyasmallandgradual weightdecreaseofca.10%,upto400◦_{C.Fortheprecursors1.0Na,}

1.5Naand2.0Na,asignificantweightlossofca.60%wasobserved between400and500◦_{C.Theseexothermicweightlosses(seeDTA}

inSupplementarymaterial)arelikelyrelatedtothedecomposition oftheprecursors.

Athirdweightlosscanbeobservedattemperatureshigherthan 700◦_{CwhichcanberelatedtothecarbonatedecompositiontoCO2}

andalsotoareported[22]reactionofsodiumcarbonatewith car-bon(Eq.(3))[22].Asexpected,thisweightlossincreaseswiththe increaseofNa+_{contentinthesamplee.g.3%for0.7Naand12%for} 2.0Na.

Na2CO3(s)+2C(s)→ 3CO(g)+2Na(s) (3)

3.2. Investigationofthethermaldecompositionoftheprecursors

BasedontheTGresults,thethermaldecompositionofthe pre-cursorswasstudiedin atubularreactorintemperaturesof550 and900◦_{C.Theexperimentswerecarriedoutunderstaticargon}

OA 0.7Na 1.0Na 1.5Na 2.0Na 0

20 40 60 80 100

Pr

odu

ct

s (% w

/w

)

Na:OA

Liquid Gas Solid

Fig.3.Productdistribution(wt%)forthethermaldecompositionofoleicacidand thedifferentprecursorsat550◦_C.

Threefractionswereobtained:solid,liquidandgas.Itcanbe observedthatthepureoleicacidalmostcompletelyevaporates (about90%)withtheformationofverysmallamountofgas, prob-ablydue tosomedecomposition process.Foralltheprecursors investigated,thegasfractionwasthemainproduct,ca.52–70wt%. Theliquidfractionwasgenerallysmallexceptforthesample0.7Na oranylowersodiumcontent,whichisprobablyduetothe evapo-rationofoleicacid.Previousliteratureworksonthepyrolysisof differentNaand Casoapsin thepresence ofcatalystsorunder extremeconditions(heatingrateof1000◦_Cs−1_{)showedmainly} theformationofliquidproducts[17–20].

3.3. Characterizationofthesolidfraction

Thecompositionofthesolidfractionwasinvestigatedby dif-ferenttechniquessuchasXRD,Ramanspectroscopy,TG,SEMand TEM.

Fig.4. Ramanspectraforthesolidobtainedbythermaldecompositionat550◦

Cof theprecursorsandselectedTEMshowingcarbonandNa2CO3structures.

TheXRDpatternsobtainedforthegrey-blacksolidindicatedthe presenceofsodiumcarbonate(Na2CO3JCDPS37-451,see Supple-mentarymaterial).

TheRamanspectra(Fig.4)ofthesolidsprecursorsshowedthree bands at1068,1350 and1588cm−1_{.The bandat1068cm}−1 _is relatedtothesymmetricstretchingofCObandfromNa2CO3[23]. TEMimages(seedetailinFig.4)showedmanysharpedgeddense structureslikelyrelatedtocrystallineNa2CO3.Thebandsat1350 and1588cm−1_{arereferredasDandGbands,respectively,typical} forcarbonaceousmaterials[24].TheDbandisrelatedtothe pres-enceofdefectivecarbonstructureswhereastheGbandisrelatedto sp2_{moreorganizedgraphenecarbonstructures[25].Infact,TEM}

Fig.5. SEMimagesforthecrudesolidobtainedbythermaldecompositionofallprecursorsat550◦

150 300 450 600 750 900 50

55 60 65 70 75 80 85 90 95 100

2.0Na 1.5Na

0.7Na

Wei

gh

t l

o

ss

/

%

Temperature / °C

1.0Na C + O2 -> CO2

Na2CO3 -> Na2O + CO2 2NaHCO3 -> Na2CO3 + CO2 + H2O

Fig.6.TGanalysesforthesolidfractionobtainedbythermaldecompositionofthe precursorsat550◦_C.

images(seedetailinFig.4)clearlyshowedamorphousstructures andgraphenelikeparticlesrelatedtocarbon.

Afterwashingthesolidwithdilutedacid,thecarbonatebandin 1068cm−1_{wasremovedandonlytheDandGbandsareobserved} (seeSupplementarymaterial).TEManalysesofthiswashedsample showedthedifferentcarbonamorphousandgraphenelike struc-tures(seeSupplementarymaterial).

SEMimages(Fig.5)showedneedle-shapedagglomerates struc-tureswhichareinagreementwithNa2CO3crystals[26–28].

Theamountofcarbonpresentinthesolidfractionwas deter-minedbytheweightlossintheTG(Fig.6)duetooxidationasshown inEq.(4).

C(s)+O2(g)→ CO2(g) (4)

Thecarbonoxidationwasobservedatrelativelylow temper-atures,i.e. 300–500◦_{C (Fig.6)compared to othercarbon based}

materials[29],which suggestsavery reactivedefective carbon. Accordingtotheobservedweightlosses,thecarboncontentswere 17,25, 9 and 5%for theprecursors0.7Na, 1.0Na,1.5Na,2.0Na, respectively.

Itcanalsobeobservedaweightlossattemperatureshigherthan 750◦_{Crelatedtocarbonatedecomposition[22].Thecarbonate}

con-tentwasanalyzedbyTCafterdissolutionoftheNa2CO3inwater. Fig.7showsthecompositionofthesolidfractionestimatedbyTG andTC.Inallcases,theNa2CO3isthemainproductandthecarbon contentdecreasedasexcessofsodiumisusedinthereaction.Itis interestingtoobservethattheNa2Odoesnotseemtoincreasefor higherNacontentintheprecursor.Thisislikelyrelatedtothe cat-alyticeffectofexcesssodiumontheformationofmoresolidcarbon andalsomorecarbonate.

3.4. Characterizationoftheliquidproduct

TheliquidproductswereanalyzedbyCHNwhichshowed sim-ilarresultsforallsamples,i.e.65–76%Cand6–7%H.Thisgeneral compositionclearlysuggestsaveryhighC/Hratiowhichindicates thepresenceof aromaticcompounds.In fact IRspectra(Fig.8) showedbandsintherangeof3040–3290cm−1 _{relatedto C H} stretching,1400and1650cm−1_{possiblyduetotheC Cstretching} ofaromaticcompounds.Someofthespectrashowedasmallband at1718cm−1_{indicatingthepresenceoflowamountsofcarboxylic} acids[30].Moreover,UV–visspectra(seeSupplementaryMaterial) showedbandsat340nmrelatedtoaromatics.

0.7Na 1.0Na 1.5Na 2.0Na

0 20 40 60 80

Precursor

Per

c

en

tag

e

% C % _Na2CO3

%_Na2O

Fig.7. Compositionofthesolidfraction.

4000

3500

3000

2500

2000

1500

1000

500

Tr

an

sm

it

an

ce

/ u

.a

.

0.7Na

C=C

OA

=CH

1.0Na

1.5Na

2.0Na

Wavenumber /cm

-1

Fig.8.FT-IRspectrafortheliquidfractionsobtainedbythethermaldecomposition ofthedifferentprecursors.

PreliminaryGC–MSshowed,besidesthepresenceofoleicacid, the formation of a complex mixture of differenthydrocarbons whichneedsamoredetailedcharacterization.

3.5. Characterizationofthegasfraction

TG-MS analyses were carried out in order to detect the moleculesformedinthegasproductduringthethermal decompo-sition.Also,attheendoftheexperiments,thegaseswerecollected andanalyzedbyGC.TheTGMSobtainedresultsareshowninFig.9. TheobtainedspectrashowedtheformationofH2,CO,CO2and hydrocarbonsC1–C4forallprecursors(seetheotherTGMSprofiles inSupplementarymaterial).

TheGCanalysisshowedthat,at550◦_{C,thegasesformedwere}

essentiallyhydrocarbons(60–70mol%),H2(20–32mol%)and car-bonoxides(Fig.10).Moreover,theselectivityforC3 washigher than90mol%amongthehydrocarbons.

Itisinterestingtoobservethatthegascompositionwerevery similarforalltheprecursorswithaslightincreaseinH2forhigher Nacontent.

Afterdecomposition at 550◦_{C thegasfrom thereactor was}

0 20 40 60 80 100

0

200

400

600

800

Lo

ss

Ma

ss

/ %

TG

MS Signal

C4H10

C3H8

C

₃

H

₆

C2H6

Io

n

Cu

rre

nt

/

u.

a

.

Temperature / °C

H

CH

CO/C2H4

Fig.9.TG-MSspectrumoftheprecursor1.0Na.

0.7 Na 1.0 Na 1.5 Na 2.0 Na 0

10 20 30 40 50 60 70

Prec

ursor

Mo

l /

%

H₂ H2

H₂

H₂

CO/

CO₂ CO/ CO₂

CO/

CO₂ CO/ CO₂ C3 _C3

C3 C3

Fig.10.Gasfractioncomposition(mol%)at550◦_C.

smallamountofgaswasproducedbetween550and900◦_Cwhich

isinagreementwiththeTGweightlossobservedathigher tem-peratures.GCanalysisofthisgasshowedthepresenceofhydrogen, hydrocarbonsandcarbonoxides.

3.6. Generalconsiderations

Consideringtheinformationgatheredfromthemassbalance,GC andTGanalysesofthedifferentfractionsageneralideaofproduct distributionfortheprecursor2.0NaispresentedinFig.11.

Itcanbeobservedthatthesolidproductca.31wt%iscomposed mainlyofsodiumcarbonate,sodiumoxideandcarbon.TGresults showedthatattemperaturesnear800◦_{C thesodiumcarbonate}

decomposesandoxidizesthecarbontoproduceCOandCO2and awhitepowdercomposedofNa2O.Themostimportantaspectof thisdecomposition/carbonoxidationistheregenerationofNa2O whichcanbereusedforanewreactionwithmorefattyacid.

Theliquidproductsareformedinrelativelysmallamountsand initialIR, UV and CG-MS analysessuggest thesignificant pres-enceofaromaticcompounds(seeSupplementarymaterial).Amore detailed characterization of this liquid fraction is necessary to envisagepotentialapplications,e.g.fuel,solvent,etc.

Themajorgasfraction,58wt%,wascomposedofpropaneand smallamountsofH2,C1,C2andC4.Thecarbondistributioninthe differentreactionproductsisshowninFig.12.

Itcanbeclearlyobservedthatalmost70%ofthecarbonatoms fromtheoleateendedupaspropaneandasignificantfractionof theCispresentintheliquidproducts.

Na

_Oleate

-Prec

ursor

(2.0Na)

550

_C

Soli

d

produ

cts

31 wt%

liquid

products

11 wt%

Gas

products

58 wt%

C

: 50 wt%

C

-C

: 3 wt%

CO

: 4 wt%

H

: 1 wt%

Mainly

aromati

c

organics

Na

CO

: 26 wt%

Carbo

n: 2 wt%

Na

O: 3 wt%

Na

O

CO

900

_C

Liquid

Solid

Gas

0 10 20 30 40 50 60 70

Li

qu

id

fr

ac

ti

o

n

CO /

CO

2

CH

4

, C

2

and

C4 C3

Car

b

on m

o

l /

%

Products

Ca

rb

o

n

Na

2

CO

3

Fig.12. Carbonatomsdistribution(mol%)inthedifferentproductsof2.0Na decom-positionat550◦

C.

Althoughthereactionmechanismisnotclear,apparentlya com-plexreactioninvolvingdifferentprocessesistakingplaceduring thedecompositionofthesodiumoleateprecursor.Fig.13shows somepossiblereactions.

Thehydrocarbonchainisfragmentingtoproducepropaneas themainproduct.Asimplecalculationshowsthatfromthe18C atoms,12Cwillresultin4propanemolecules.Theformationof highamountsofpropanefromC17H33chaininvolvesasignificant hydrogentransferamongthecarbons.Asaresultofthisprocess, partofthecarbonswillbeconvertedtoaromaticcompounds(as observedintheliquidfractionwithaC/Hrationear1/1)and a significantamountof solid carbon(char). Therefore, thesedata suggeststhatthesodiumoleateprecursordecomposes/cracks, pro-duces/transfershydrogenandaromatizetosmallmoleculesand solidcarbon

Itisinterestingtoobservethatsodiumhasafundamentalrole inthesereactions,sincepureoleicaciddoesnotdecomposesand onlyevaporates.ThestrongbasiccharacterofNa2Oformedinthe reactionlikelypromotescatalyticcrackingreactionsandalsotheH transferprocesses[31].

Thereasonfortheremarkableselectivityobservedforpropane is not clear and further investigation is necessary in order to

Fig.14. Formationanddecompositioncycleofsodiumoleate.

understandthepossiblereactionsteps,theinteractionofthealkyl chainwiththeNa+ion,thedehydrogenationandH-transfer pro-cesses.

Theobtainedresultsalsopointtoapossiblecyclicprocesswhere thesodiumhydroxideoroxidecanberegeneratedandreusedfor anewreaction(Fig.14).

4. Conclusions

Thisworkshowsthatfreefattyacid,acommonand undesir-ablecontaminationpresentinvegetableoils,canbeconvertedto hydrocarbonsbyasimpleprocessofreactionwithNaOHfollowed byacontrolledthermaldecomposition.Inthisdecomposition pro-cessthesodiumoleateisconvertedwitharemarkableselectivityto propaneandinsmallamountsC1,C2,C4andH2,whichissimilarin severalaspectstoliquefiedpetroleumgas.Thisprocessalsoresults inaliquidmixturewitharomaticcompoundswithpotentialfor useasfuels.Thesodiumoxidecanberegeneratedattemperatures higherthan800◦_{Candreusedforanewreactioncycle.}

Acknowledgements

TheauthorsacknowledgefinancialsupportfromCNPq,CAPES andFAPEMIG.Theauthorsalsowouldliketoacknowledgethe Cen-terofMicroscopyatUFMG.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound, intheonlineversion,athttp://dx.doi.org/10.1016/j.cattod.2016.04. 035.

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