A review of catalytic upgrading of bio oil to engine fuels

Jointly,HDOand zeolite crackingare referredtoas catalytic bio-oilupgradingandthesecouldbecomeroutesforproductionof secondgenerationbio-fuelsinthefuture,butbothroutesarestill farfromindus

jo u r n al h om ep a g 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 a

Review

a Department of Chemical and Biochemical Engineering, Technical University of Denmark, Søltofts Plads, Building 229, DK-2800 Lyngby, Denmark

b Institute of Chemical Technology and Polymer Science, Karlsruhe Institute of Technology (KIT), Engesserstrasse 20, D-79131 Karlsruhe, Denmark

c Haldor Topsø A/S, Nymøllevej 55, DK-2800 Lyngby, Denmark

Article history:

Received 13 May 2011

Received in revised form 30 August 2011

Accepted 31 August 2011

Available online 7 September 2011

Keywords:

Bio-oil

Biocrudeoil

Biofuels

Catalyst

HDO

Hydrodeoxygenation

Pyrolysis oil

Synthetic fuels

Zeolite cracking

Astheoilreservesaredepletingtheneedofanalternativefuelsourceisbecomingincreasinglyapparent Oneprospectivemethodforproducingfuelsinthefutureisconversionofbiomassintobio-oilandthen upgradingthebio-oiloveracatalyst,thismethodisthefocusofthisreviewarticle.Bio-oilproductioncan

befacilitatedthroughflashpyrolysis,whichhasbeenidentifiedasoneofthemostfeasibleroutes.The bio-oilhasahighoxygencontentandthereforelowstabilityovertimeandalowheatingvalue.Upgrading

isdesirabletoremovetheoxygenandinthiswaymakeitresemblecrudeoil.Twogeneralroutesfor bio-oilupgradinghavebeenconsidered:hydrodeoxygenation(HDO)andzeolitecracking.HDOisahigh pressureoperationwherehydrogenisusedtoexcludeoxygenfromthebio-oil,givingahighgradeoil productequivalenttocrudeoil.Catalystsforthereactionaretraditionalhydrodesulphurization(HDS) catalysts,suchasCo–MoS2/Al2O3,ormetalcatalysts,asforexamplePd/C.However,catalystlifetimesof muchmorethan200hhavenotbeenachievedwithanycurrentcatalystduetocarbondeposition.Zeolite crackingisanalternativepath,wherezeolites,e.g.HZSM-5,areusedascatalystsforthedeoxygenation reaction.Inthesesystemshydrogenisnotarequirement,sooperationisperformedatatmospheric pressure.However,extensivecarbondepositionresultsinveryshortcatalystlifetimes.Furthermorea generalrestrictioninthehydrogencontentofthebio-oilresultsinalowH/Cratiooftheoilproductasno additionalhydrogenissupplied.Overall,oilfromzeolitecrackingisofalowgrade,withheatingvalues approximately25%lowerthanthatofcrudeoil.Ofthetwomentionedroutes,HDOappearstohavethe bestpotential,aszeolitecrackingcannotproducefuelsofacceptablegradeforthecurrentinfrastructure HDOisevaluatedasbeingapathtofuelsinagradeandatapriceequivalenttopresentfossilfuels, butseveraltasksstillhavetobeaddressedwithinthisprocess.Catalystdevelopment,understanding

ofthecarbonformingmechanisms,understandingofthekinetics,elucidationofsulphurasasourceof deactivation,evaluationoftherequirementforhighpressure,andsustainablesourcesforhydrogenare allareaswhichhavetobeelucidatedbeforecommercialisationoftheprocess

© 2011 Elsevier B.V All rights reserved

Contents

1 Introduction 2

2 Bio-oil 2

3 Bio-oilupgrading—generalconsiderations 3

4 Hydrodeoxygenation 4

4.1 Catalystsandreactionmechanisms 6

4.1.1 Sulphide/oxidecatalysts 6

4.1.2 Transitionmetalcatalysts 7

4.1.3 Supports 8

4.2 Kineticmodels 9

4.3 Deactivation 9

5 Zeolitecracking 10

5.1 Catalystsandreactionmechanisms 10

∗ Corresponding author Tel.: +45 4525 2841; fax: +45 4588 2258.

E-mail address: aj@kt.dtu.dk (A.D Jensen).

0926-860X/$ – see front matter © 2011 Elsevier B.V All rights reserved.

5.2 Kineticmodels 11

5.3 Deactivation 12

6 Generalaspects 13

7 Prospectofcatalyticbio-oilupgrading 14

8 Discussion 16

9 Conclusionandfuturetasks 17

Acknowledgements 17

References 17

1 Introduction

Energyconsumptionhasneverbeenhigherworldwidethanitis

today,duetoourwayoflivingandthegeneralfactthattheWorld’s

populationisincreasing[1,2].Oneofthemainfieldsofenergy

con-sumptionisthetransportationsector,constitutingaboutonefifth

ofthetotal[3].AstheWorld’spopulationgrowsandmeansof

trans-portationbecomesmorereadilyavailable,itisunavoidablethatthe

needforfuelswillbecomelargerinthefuture[4].Thisrequirement

constitutesoneofthemajorchallengesofthenearfuture,aspresent

fuelsprimarilyareproducedfromcrudeoilandthesereservesare

depleting[5]

Substantialresearch is being carried out within thefield of

energyinordertofindalternativefuelstoreplacegasoline and

diesel The optimal solution would be an alternative which is

equivalenttotheconventionalfuels,i.e.compatiblewiththe

infras-tructureasweknowit,butalsoafuelwhichissustainableandwill

decreasetheCO2emissionandtherebydecreasetheenvironmental

man-madefootprint[6]

Biomassderivedfuelscouldbetheprospectivefuelof

tomor-rowasthesecanbeproducedwithinarelativelyshortcycleand

areconsideredbenignfortheenvironment[4,7].Sofarfirst

gener-ationbio-fuels(bio-ethanolandbiodiesel)havebeenimplemented

indifferentpartsoftheWorld[8,9].However,thesetechnologies

relyonfoodgradebiomass;firstgenerationbio-ethanolisproduced

fromthefermentationofsugarorstarchandbiodieselisproduced

onthebasisoffats[10–12].Thisisaproblemastherequirement

forfoodaroundtheWorldisaconstraintandtheenergyefficiency

perunitlandoftherequiredcropsisrelativelylow(comparedto

energycrops)[13].Forthisreasonnewresearchfocuseson

devel-opingsecondgenerationbio-fuels, whichcanbeproducedfrom

otherbiomasssourcessuchasagriculturalwaste,wood,etc.Table1

summarizesdifferentpathsforproducingfuelsfrombiomassand

displaywhichtypeofbiomasssourceisrequired,showingthata

seriesofpathsexistswhichcanutiliseanysourceofbiomass

Of the second generation biofuel paths,a lot of efforts are

presentlyspentonthebiomasstoliquidrouteviasyngasto

opti-mizetheefficiency[14–17]andalsosynthesisofhigheralcohols

fromsyngasor hydrocarbonsfrommethanol [16,18–22] Asan

alternative,theestimatedproductionpricesshowninTable1

indi-catethatHDO constitutea feasible routefor theproduction of

syntheticfuels.Thecompetivenessofthisrouteisachieveddue

toagoodeconomywhenusingbio-oilasplatformchemical(lower

transportcostforlargescaleplants)andtheflexibilitywithrespect

tothebiomassfeed[10,23–25].Furthermorethisroutealso

consti-tuteapathtofuelsapplicableinthecurrentinfrastructure[10]

Jointly,HDOand zeolite crackingare referredtoas catalytic

bio-oilupgradingandthesecouldbecomeroutesforproductionof

secondgenerationbio-fuelsinthefuture,butbothroutesarestill

farfromindustrialapplication.Thisreviewwillgiveanoverview

onthepresentstatusofthetwoprocessesandalsodiscusswhich

aspectsneed furtherelucidation.Eachroute willbeconsidered

independently.Aspectsofoperatingconditions,choiceofcatalyst,

reactionmechanisms,anddeactivationmechanismswillbe

dis-cussed.Theseconsiderationswillbeusedtogiveanoverviewofthe

Table 1

Overview of potential routes for production of renewable fuels from biomass The prices are based on the lower heating value (LHV) Biomass as feed implies high flexibility with respect to feed source.

Technology Feed Platform chemical Price [$/toe a ]

a toe: tonne of oil equivalent, 1 toe = 42 GJ.

b Published price: 2.04$/gallon [167] , 1 gallon = 3.7854 l,  = 719 kg/m 3 , LHV = 42.5 MJ/kg.

c Published price: 20–27$/GJ [197]

d Published price: 9–17$/GJ [197,21]

e Expenses for distribution and storage are not considered.

f Published price: 13–14$/GJ [197]

g Published price: 31–36$/GJ [197]

h Published price: 0.2–0.5$/l [193] ,  = 789 kg/m 3 , LHV = 28.87 MJ/kg.

i Published price: 0.6–0.8$/l [193]

j Published price: 0.8–1.1$/l [193]

k Published price: 0.5–1$/l [193] ,  = 832 kg/m 3 , LHV = 43.1 MJ/kg.

l Published price: 0.5–0.8$/l [193]

m Published price in USA April 2011: 2.88$/gallon excluding distribution, market-ing, and taxes [179] Crude oil price April 2011: 113.23$/barrel [196]

twoprocessescomparedtoeachother,butalsorelativetocrude oilasthebenchmark.Ultimately,anindustrialperspectivewillbe given,discussingtheprospectiveofproductionofbio-fuelsthrough catalyticbio-oilupgradinginindustrialscale

Otherreviews withinthe samefield are that by Elliott[26] from2007wherethedevelopmentwithinHDOsincethe1980s

isdiscussed,andareviewin2000byFurimsky[27]where reac-tionmechanismsandkineticsofHDOarediscussed.Moregeneral reviews of utilisation of bio-oil have beenpublished by Zhang

etal.[28],Bridgwater[29],andCzernikandBridgwater[30],and reviewsaboutbio-oilandproductionthereofhavebeenpublished

byVenderboschandPrins[31]andMohanetal.[32]

2 Bio-oil

AsseenfromTable1,bothHDOandzeolitecrackingarebased

onbio-oilasplatformchemical.Flashpyrolysisisthemostwidely appliedprocessforproductionofbio-oil, asthishasbeenfound

asafeasibleroute[16,26,33].Inthisreview,onlythisroutewillbe discussedandbio-oilwillinthefollowingrefertoflashpyrolysisoil Forinformationaboutotherroutesreferenceismadeto[16,34–37] Flash pyrolysis is a densification technique where both the mass-andenergy-densityisincreasedbytreatingtherawbiomass

atintermediatetemperatures(300–600◦C)withhighheatingrates (103–104K/s)andatshortresidencetimes(1–2s)[28,31,38].Inthis way,anincreaseintheenergydensitybyroughlyafactorof7–8

Table 2

Bio-oil composition in wt% on the basis of different biomass sources and production methods.

canbeachieved[39,40].Virtuallyanytypeofbiomassiscompatible

withpyrolysis,rangingfrommoretraditionalsourcessuchascorn

andwoodtowasteproductssuchassewagesludgeandchicken

litter[38,41,42]

Morethan300differentcompoundshavebeenidentifiedin

bio-oil,wherethespecificcompositionoftheproductdependsonthe

feedand processconditionsused[28].In Table2arough

char-acterisationofbio-oilfromdifferentbiomasssourcesisseen.The

principlespeciesoftheproductiswater,constituting10–30wt%,

buttheoilalsocontains:hydroxyaldehydes,hydroxyketones,

sug-ars,carboxylicacids,esters,furans,guaiacols,andphenolics,where

manyofthephenolicsarepresentasoligomers[28,30,43,44]

Table3showsacomparisonbetweenbio-oilandcrudeoil.One

crucialdifferencebetweenthetwoistheelementalcomposition,

asbio-oilcontains10–40wt%oxygen[28,31,45].Thisaffectsthe

homogeneity,polarity,heatingvalue(HV),viscosity,andacidityof

theoil

Theoxygenatedmoleculesoflowermolecularweight,especially

alcoholsandaldehydes,ensurethehomogeneousappearanceof

theoil,astheseactasasortofsurfactantforthehigher

molecu-larweightcompounds,whichnormallyareconsideredapolarand

immisciblewithwater[166].Overallthismeansthatthebio-oil

hasapolarnatureduetothehighwatercontentandistherefore

immisciblewithcrudeoil.Thehighwatercontentandoxygen

con-tentfurtherresultinalowHVofthebio-oil,whichisabouthalf

thatofcrudeoil[28,31,30,46]

ThepHofbio-oilisusuallyintherangefrom2to4,which

pri-marilyisrelatedtothecontentofaceticacidandformicacid[47]

Theacidicnatureoftheoilconstitutesaproblem,asitwillentail

harshconditionsforequipmentusedforbothstorage,transport,

andprocessing Commonconstructionmaterialssuchascarbon

steelandaluminiumhaveprovenunsuitablewhenoperatingwith

bio-oil,duetocorrosion[28,46]

Apronouncedproblemwithbio-oilistheinstabilityduring

stor-age,whereviscosity,HV,anddensityallareaffected.Thisisdue

tothepresenceofhighlyreactiveorganiccompounds.Olefinsare

Table 3

Comparison between bio-oil and crude oil Data are from Refs [10,11,28]

Overalltheunfavourablecharacteristicsofthebio-oilare asso-ciatedwiththeoxygenatedcompounds.Carboxylicacids,ketones, and aldehydes constitutesome ofthe mostunfavourable com-pounds,bututilisationoftheoilrequiresageneraldecreaseinthe oxygencontentinordertoseparatetheorganicproductfromthe water,increasetheHV,andincreasethestability

3 Bio-oil upgrading—general considerations

Catalyticupgradingofbio-oilisacomplexreactionnetworkdue

tothehighdiversityofcompoundsinthefeed.Cracking, decar-bonylation,decarboxylation,hydrocracking,hydrodeoxygenation, hydrogenation, and polymerizationhave been reportedto take placeforbothzeolitecrackingandHDO[51–53].Examplesofthese reactionsaregiveninFig.1.Besidesthese,carbonformationisalso significantinbothprocesses

The high diversity in the bio-oil and the span of potential reactionsmakeevaluationofbio-oilupgradingdifficultandsuch evaluationoftenrestrictedtomodelcompounds.Togetageneral thermodynamicoverviewoftheprocess,wehaveevaluatedthe followingreactionsthroughthermodynamiccalculations(basedon datafromBarin[54]):

ThisreactionpathofphenolhasbeenproposedbybothMassoth

etal.[55]andYunquanetal.[56].Calculatingthethermodynamic equilibriumforthetworeactionsshowsthatcompleteconversion

ofphenolcanbeachievedattemperaturesuptoatleast600◦C

atatmosphericpressureandstoichiometricconditions.Increasing eitherthepressureortheexcessofhydrogenwillshiftthe ther-modynamicseven furthertowardscompleteconversion.Similar calculationshavealsobeenmadewithfurfural,givingequivalent results.Thus,thermodynamicsdoesnotappeartoconstitutea con-straintfortheprocesses,whenevaluatingthesimplestreactionsof Fig.1formodelcompounds

Inpracticeitisdifficulttoevaluatetheconversionofeach indi-vidualcomponentinthebio-oil.Insteadtwoimportantparameters aretheoilyieldandthedegreeofdeoxygenation:

Yoil=



moil

mfeed



Fig 1.Examples of reactions associated with catalytic bio-oil upgrading The figure is drawn on the basis of information from Refs [51,53]

DOD=



1−wt%wtOinproduct

O in feed



HereYoilistheyieldofoil,moilisthemassofproducedoil,mfeed

isthemassofthefeed,DODisthedegreeofdeoxygenation,and

wt%Oistheweightpercentofoxygenintheoil.Thetwo

parame-terstogethercangivearoughoverviewoftheextentofreaction,

astheoilyielddescribestheselectivitytowardanoilproductand

thedegreeofdeoxygenationdescribeshoweffectivetheoxygen

removalhasbeenandthereforeindicatesthequalityofthe

pro-ducedoil.However,separatelytheparametersarelessdescriptive,

foritcanbeseenthata100%yieldcanbeachievedinthecase

ofnoreaction.Furthermore,noneoftheparametersrelatetothe

removalofspecifictroublesomespeciesandthesewouldhaveto

beanalyzedforindetail

Table4summarizesoperatingparameters,productyield,degree

ofdeoxygenation,andproductgradeforsomeofthework con-ductedwithinthefieldofbio-oilupgrading.Thereadercangetan ideaofhowthechoiceofcatalystandoperatingconditionsaffect theprocess.Itisseenthatawidevarietyofcatalystshavebeen tested.HDOandzeolitecrackingaresplitinseparatesectionsin thetable,whereitcanbeconcludedthattheprocessconditionsof HDOrelativetozeolitecrackingaresignificantlydifferent, partic-ularlywithrespecttooperatingpressure.Thetwoprocesseswill thereforebediscussedseparatelyinthefollowing

4 Hydrodeoxygenation

HDOiscloselyrelatedtothehydrodesulphurization(HDS) pro-cessfromtherefineryindustry,usedintheeliminationofsulphur fromorganiccompounds[43,57].BothHDOandHDSusehydrogen

Table 4

Overview of catalysts investigated for catalytic upgrading of bio-oil.

Hydrodeoxygenation

Zeolite cracking

a Calculated as the inverse of the WHSV.

b

H2S

AllthereactionsshowninFig.1arerelevantforHDO,butthe

principal reactionis hydrodeoxygenation, as thename implies,

andthereforetheoverallreactioncanbegenerallywrittenas(the

reactionisinspiredbyBridgwater[43,58]andcombinedwiththe

elementalcompositionofbio-oilspecifiedinTable3normalizedto

carbon):

CH1.4O0.4+0.7H2→1”CH2+0.4H2O (5)

Here “CH2” represent anunspecified hydrocarbon product.The

overallthermochemistryofthisreactionisexothermicandsimple

calculationshaveshownanaverageoverallheatofreactioninthe

orderof2.4MJ/kgwhenusingbio-oil[59]

Waterisformedintheconceptualreaction,so(atleast)two

liquidphaseswillbeobservedasproduct:oneorganicandone

aqueous The appearance of two organic phases has alsobeen

reported,which isduetotheproduction oforganiccompounds

withdensitieslessthanwater.Inthiscasealightoilphasewill

separateontopofthewaterandaheavyonebelow.The

forma-tionoftwoorganicphasesisusuallyobservedininstanceswith

highdegreesofdeoxygenation,whichwillresultinahighdegree

offractionationinthefeed[11]

InthecaseofcompletedeoxygenationthestoichiometryofEq

(5)predictsamaximumoilyieldof56–58wt%[43].However,the

completedeoxygenationindicatedbyEq.(5)israrelyachieveddue

tothespanofreactionstakingplace;insteadaproductwithresidual

oxygenwilloftenbeformed.Venderboschetal.[11]describedthe

stoichiometryofaspecificexperimentnormalizedwithrespectto

thefeedcarbonas(excludingthegasphase):

CH1.47O0.56 +0.39H2→0.74CH1.47O0.11+0.19CH3.02O1.09

HereCH1.47O0.11istheorganicphaseoftheproductandCH3.02O1.09

istheaqueousphaseoftheproduct.Someoxygenisincorporated

inthehydrocarbonsoftheorganicphase,buttheO/Cratiois

sig-nificantlylowerinthehydrotreatedorganicphase(0.11)compared

tothepyrolysisoil(0.56).IntheaqueousphaseahigherO/Cratio

thanintheparentoilisseen[11]

Regardingoperating conditions, a highpressure is generally

used,whichhasbeenreportedintherange from75to300bar

intheliterature[31,60,61].Patentliteraturedescribesoperating

pressuresintherangeof10–120bar[62,63].Thehighpressurehas

beendescribedasensuringahighersolubilityofhydrogeninthe

oilandtherebyahigheravailabilityofhydrogeninthevicinityof

thecatalyst.Thisincreasesthereactionrateandfurtherdecreases

cokinginthereactor[11,64].Elliottetal.[61]usedhydrogeninan

excessof35–420molH2perkgbio-oil,comparedtoarequirement

ofaround25mol/kgforcompletedeoxygenation[11]

Highdegreesofdeoxygenationarefavouredbyhighresidence

times[31].Inacontinuousflowreactor,Elliottetal.[61]showed

thattheoxygencontentoftheupgradedoildecreasedfrom21wt%

to10wt%whendecreasingtheLHSVfrom0.70h−1to0.25h−1over

aPd/Ccatalystat140barand340◦C.IngeneralLHSVshouldbein

theorderof0.1–1.5h−1[63].Thisresidencetimeisinanalogyto

batchreactortests,whichusuallyarecarriedoutovertimeframes

of3–4h[53,65,66]

HDOisnormallycarriedoutattemperaturesbetween250and

450◦C[11,57].Asthereactionisexothermicandcalculationsof

theequilibriumpredictspotentialfullconversionofrepresentative

modelcompoundsuptoatleast600◦C,itappearsthatthechoiceof

operatingtemperatureshouldmainlybebasedonkineticaspects

TheeffectoftemperaturewasinvestigatedbyElliottandHart[61]

forHDOofwoodbasedbio-oiloveraPd/Ccatalystinafixedbed

Table 5

Activation energy (E A ), iso-reactive temperature (T iso ), and hydrogen consump-tion for the deoxygenation of different functional groups or molecules over a Co–MoS 2 /Al 2 O 3 catalyst Data are obtained from Grange et al [23]

Molecule/group E A [kJ/mol] T Iso [ ◦ C] Hydrogen consumption

Carboxylic acid 109 283 3 H 2 /group Methoxy phenol 113 301 ≈6 H 2 /molecule 4-Methylphenol 141 340 ≈4 H 2 /molecule 2-Ethylphenol 150 367 ≈4 H 2 /molecule Dibenzofuran 143 417 ≈8 H 2 /molecule

thedifferenttypesoffunctionalgroupsinthebio-oil[23,67].Table5 summarizesactivationenergies,iso-reactivitytemperatures(the temperaturerequiredforareactiontotakeplace),andhydrogen consumptionfordifferentfunctionalgroupsandmoleculesovera Co–MoS2/Al2O3catalyst.Onthiscatalysttheactivationenergyfor deoxygenationofketonesisrelativelylow,sothesemoleculescan

bedeoxygenatedattemperaturescloseto200◦C.However,forthe morecomplexboundorstericallyhinderedoxygen,asinfurans

ororthosubstitutedphenols,asignificantlyhighertemperatureis requiredforthereactiontoproceed.On thisbasistheapparent reactivityofdifferentcompoundshasbeensummarizedas[27]: alcohol >ketone>alkylether>carboxylicacid

≈M-/p-phenol≈naphtol>phenol>diarylether

≈O-phenol≈alkylfuran>benzofuran>dibenzofuran

(7)

An important aspect of the HDO reaction is the consump-tionofhydrogen.Venderboschetal.[11]investigatedhydrogen consumption for bio-oilupgradingas a function of deoxygena-tionrateover aRu/Ccatalystinafixedbedreactor.Theresults are summarized in Fig.2 The hydrogenconsumption becomes increasinglysteepasa functionofthedegreeofdeoxygenation

Fig 2. Consumption of hydrogen for HDO as a function of degree of deoxygenation compared to the stoichiometric requirement 100% deoxygenation has been extrap-olated on the basis of the other points The stoichiometric requirement has been calculated on the basis of an organic bound oxygen content of 31 wt% in the bio-oil and a hydrogen consumption of 1 mol H 2 per mol oxygen Experiments were per-formed with a Ru/C catalyst at 175–400◦C and 200–250 bar in a fixed bed reactor fed with bio-oil The high temperatures were used in order to achieve high degrees

Fig 3. Yields of oil, water, and gas from a HDO process as a function of the degree

of deoxygenation Experiments were performed with eucalyptus bio-oil over a

Co–MoS 2 /Al 2 O 3 catalyst in a fixed bed reactor Data are from Samolada et al [81]

Thisdevelopmentwaspresumedtobeduetothedifferent

reac-tivity values of the compounds in the bio-oil Highly reactive

oxygenates,likeketones,areeasilyconvertedwithlowhydrogen

consumption,butsomeoxygenisboundinthemorestable

com-pounds.Thus,themorecomplexmoleculesareaccompaniedbyan

initialhydrogenation/saturationofthemoleculeandthereforethe

hydrogenconsumption exceedsthestoichiometricpredictionat

thehighdegreesofdeoxygenation[27].Thesetendenciesarealso

illustratedin Table5.Obviously, thehydrogenrequirement for

HDOofaketoneissignificantlylowerthanthatforafuran.Overall

thismeansthatinordertoachieve50%deoxygenation(ca.25wt%

oxygenin theupgradedoil)8molH2 perkgbio-oilis required

according to Fig 2 In contrast, complete deoxygenation (and

accompaniedsaturation)hasapredictedhydrogenrequirementof

ca.25mol/kg,i.e.anincreasebyafactorofca.3

Thediscussionaboveshowsthattheuseofhydrogenfor

upgrad-ingbio-oilhastwoeffectswithrespecttothemechanism:removing

oxygenand saturatingdouble bounds.Thisresultsindecreased

O/CratiosandincreasedH/Cratios,bothofwhichincreasethefuel

gradeoftheoilbyincreasingtheheatingvalue(HV).Mercaderetal

[60]foundthatthehigherheatingvalue(HHV)ofthefinalproduct

wasapproximatelyproportionaltothehydrogenconsumedinthe process,withanincrease intheHHVof 1MJ/kgpermol/kg H2

consumed

InFig.3theproductionofoil,water,andgasfromaHDOprocess usingaCo–MoS2/Al2O3catalystisseenasafunctionofthedegreeof deoxygenation.Theoilyielddecreasesasafunctionofthedegreeof deoxygenation,whichisduetoincreasedwaterandgasyields.This showsthatwhenharshconditionsareusedtoremovetheoxygen,a significantdecreaseintheoilyieldoccurs;itdropsfrom55%to30% whenincreasingthedegreeofdeoxygenationfrom78%to100%.It

isthereforeanimportantaspecttoevaluatetowhichextentthe oxygenshouldberemoved[68]

4.1 Catalystsandreactionmechanisms

AsseenfromTable4,avarietyofdifferentcatalystshasbeen testedfortheHDOprocess.Inthefollowing,thesewillbediscussed

aseithersulphide/oxidetypecatalystsortransitionmetalcatalysts,

asitappearsthatthemechanismsforthesetwogroupsofcatalysts aredifferent

4.1.1 Sulphide/oxidecatalysts Co–MoS2andNi–MoS2havebeensomeofthemostfrequently testedcatalystsfortheHDOreaction,asthesearealsousedinthe traditionalhydrotreatingprocess[26,27,64,67,69–83]

Inthesecatalysts,CoorNiservesaspromoters,donating elec-tronstothemolybdenumatoms.Thisweakensthebondbetween molybdenumandsulphurandtherebygeneratesasulphurvacancy site.ThesesitesaretheactivesitesinbothHDSandHDOreactions [55,80,84–86]

Romeroetal.[85]studiedHDOof2-ethylphenolonMoS2-based catalystsandproposedthereactionmechanismdepictedinFig.4 Theoxygenofthemoleculeisbelievedtoadsorbonavacancysiteof

aMoS2slabedge,activatingthecompound.S–Hspecieswillalsobe presentalongtheedgeofthecatalystasthesearegeneratedfrom theH2inthefeed.Thisenablesprotondonationfromthesulphurto theattachedmolecule,whichformsacarbocation.Thiscanundergo directC–Obondcleavage,formingthedeoxygenatedcompound, andoxygenishereafterremovedintheformationofwater

Fig 4. Proposed mechanism of HDO of 2-ethylphenol over a Co–MoS 2 catalyst The dotted circle indicates the catalytically active vacancy site The figure is drawn on the

For the mechanism towork, it is a necessity that the

oxy-gengroupformedonthemetalsitefromthedeoxygenationstep

is eliminatedaswater During prolongedoperationithasbeen

observedthatadecreaseinactivitycanoccurdueto

transforma-tionofthecatalystfromasulphideformtowardanoxideform.In

ordertoavoidthis,ithasbeenfoundthatco-feedingH2Stothe

systemwillregeneratethesulphidesitesandstabilizethecatalyst

[79,84,87,88].However,thestudyofSenoletal.[87,88]showedthat

traceamountsofthiolsandsulphideswasformedduringtheHDO

of3wt%methylheptanoateinm-xyleneat15barand250◦Cina

fixedbedreactorwithCo–MoS2/Al2O3co-fedwithupto1000ppm

H2S.Thus,thesestudiesindicatethatsulphurcontaminationofthe

otherwisesulphurfreeoilcanoccurwhenusingsulphidetype

cat-alysts.AninterestingperspectiveinthisisthatCo–MoS2/Al2O3is

usedasindustrialHDScatalystwhereitremovessulphurfromoils

downtoalevelofafewppm[89].Ontheotherhand,Christensen

etal.[19]showedthat,whensynthesizinghigheralcoholsfrom

synthesisgaswithCo–MoS2/Cco-fedwithH2S,thiolsandsulfides

wereproducedaswell.Thus,theinfluenceofthesulphuronthis

catalystisdifficulttoevaluateandneedsfurtherattention

On thebasis of densityfunctional theory(DFT) calculations,

Mobergetal.[90]proposedMoO3ascatalystforHDO.These

cal-culationsshowedthatthedeoxygenationonMoO3 occursimilar

tothepathinFig.4,i.e.chemisorptiononacoordinatevely

unsat-uratedmetalsite,protondonation,anddesorption.Forbothoxide

and sulphide type catalysts the activityrelies onthe presence

ofacidsites.Theinitialchemisorptionstepisa Lewisacid/base

interaction,wheretheoxygenlonepairofthetargetmoleculeis

attractedtotheunsaturatedmetalsite.Forthisreasonitcanbe

speculatedthat thereactivityofthesystemmust partlyrelyon

theavailabilityandstrengthoftheLewisacidsitesonthecatalyst

GervasiniandAuroux[91]reportedthattherelativeLewisacidsite

surfaceconcentrationondifferentoxidesare:

Cr2O3>WO3>Nb2O5>Ta2O5>V2O5≈MoO3 (8)

Thisshould be matched against therelative Lewisacid site

strengthofthedifferentoxides.ThiswasinvestigatedbyLiand

Dixon[92],wheretherelativestrengthswerefoundas:

The subsequent step of the mechanism is proton donation

Thisreliesonhydrogenavailableonthecatalyst,which forthe

oxideswillbepresentashydroxylgroups.Tohaveprotondonating

capabilities,Brønstedacidhydroxylgroupsmustbepresentonthe

catalystsurface.InthiscontexttheworkofBuscashowedthatthe

relativeBrønstedhydroxylacidityofdifferentoxidesis[90]:

ThetrendsofEqs.(8)–(10)incomparisontothereactionpath

ofdeoxygenationrevealsthatMoO3functionsasacatalystdueto

thepresenceofbothstrongLewisacidsitesandstrongBrønsted

acidhydroxylsites.However,WhiffenandSmith[93]investigated

HDOof4-methylphenolover unsupportedMoO3 and MoS2 ina

batchreactorat41–48barand325–375◦C,andfoundthatthe

activ-ityofMoO3waslowerthanthatforMoS2andthattheactivation

energywashigheronMoO3thanonMoS2forthisreaction.Thus,

MoO3 mightnotbethebestchoiceofanoxidetypecatalyst,but

onthebasisofEqs.(8)–(10)otheroxidesseeminterestingforHDO

SpecificallyWO3isindicatedtohaveahighavailabilityofacidsites

Echeandiaetal.[94]investigatedoxidesofWandNi–Wonactive

carbonforHDOof1wt%phenolinn-octaneinafixedbedreactor

at150–300◦Cand15bar.Thesecatalystswereallprovenactivefor

HDOandespeciallytheNi–Wsystemhadpotentialforcomplete

conversionofthemodelcompound Furthermore,a lowaffinity

forcarbonwasobservedduringthe6hofexperiments.Thislow

Fig 5.HDO mechanism over transition metal catalysts The mechanism drawn on the basis of information from Refs [95,96]

valuewasascribedtoabeneficialeffectfromthenon-acidiccarbon support(cf.Section4.1.3)

4.1.2 Transitionmetalcatalysts Selectivecatalytichydrogenationcanalsobecarriedoutwith transitionmetalcatalysts.Mechanisticspeculationsforthese sys-temshaveindicatedthatthecatalystsshouldbebifunctional,which canbeachievedinotherwaysthanthesystemdiscussedinSection 4.1.1.Thebifunctionalityofthecatalystimpliestwoaspects.On onethehand,activationofoxy-compoundsisneeded,whichlikely couldbeachievedthroughthevalenceofanoxideformofa tran-sitionmetalor onan exposedcation, oftenassociatedwiththe catalystsupport.Thisshouldbecombinedwithapossibilityfor hydrogendonationtotheoxy-compound,whichcouldtakeplace

ontransition metals, as theyhave thepotential toactivate H2 [95–98].ThecombinedmechanismisexemplifiedinFig.5,where theadsorptionandactivationoftheoxy-compoundareillustrated

totakeplaceonthesupport

Themechanismofhydrogenationoversupportednoblemetal systems is still debated Generally it is acknowledgedthat the metalsconstitutethehydrogendonatingsites,butoxy-compound activationhasbeenproposedtoeitherbefacilitatedonthemetal sites[99–101]oratthemetal-supportinterface(asillustratedin Fig 5)[102,99,103].This indicatesthat these catalytic systems potentiallycouldhavetheaffinityfortwodifferentreactionpaths, sincemanyofthenoblemetalcatalystsareactiveforHDO

AstudybyGutierrezetal.[66]investigatedtheactivityofRh,

Pd,andPtsupportedonZrO2forHDOof3wt%guaiacolin hexade-caneinabatchreactorat80barand100◦C.Theyreportedthatthe apparentactivityofthethreewas:

Rh/ZrO2>Co–MoS2/Al2O3>Pd/ZrO2>Pt/ZrO2 (11) Fig.6showstheresultsfromanotherstudyofnoblemetal cat-alystsbyWildschutetal.[53,104].HereRu/C,Pd/C,andPt/Cwere investigatedforHDOofbeechbio-oilinabatchreactorat350◦C and200barover4h.Ru/CandPd/Cappearedtobegoodcatalysts fortheprocessastheydisplayedhighdegreesofdeoxygenation andhighoilyields,relativetoCo–MoS2/Al2O3andNi–MoS2/Al2O3

asbenchmarks

Throughexperimentsinabatchreactorsetupwithsynthetic bio-oil(mixtureofcompoundsrepresentativeoftherealbio-oil)at

350◦Candca.10barofnitrogen,Fisketal.[105]foundthatPt/Al2O3 displayedcatalyticactivityforbothHDOandsteamreformingand thereforecouldproduceH2insitu.Thisapproachisattractiveasthe expenseforhydrogensupplyisconsideredasoneofthe disadvan-tagesoftheHDOtechnology.However,thecatalystwasreported

tosufferfromsignificantdeactivationduetocarbonformation

Fig 6. Comparison of Ru/C, Pd/C, Pt/C, Co–MoS 2 /Al 2 O 3 and Ni–MoS 2 /Al 2 O 3 as

cat-alysts for HDO, evaluated on the basis of the degree of deoxygenation and oil yield.

Experiments were performed with beech bio-oil in a batch reactor at 350◦C and

200 bar over 4 h Data are from Wildschut et al [53,104]

Tosummarize,thenoblemetalcatalystsRu,Rh,Pd,andpossibly

alsoPtappeartobepotentialcatalystsfortheHDOsynthesis,but

thehighpriceofthemetalsmakethemunattractive

Asalternativestothenoblemetalcatalystsaseriesof

inves-tigations of base metal catalysts have been performed, as the

pricesofthesemetalsaresignificantlylower[106].Yakovlevetal

[98] investigated nickel based catalysts for HDO of anisole in

a fixed bed reactor at temperaturesin the range from 250 to

400◦C andpressuresintherangefrom5to20bar.InFig.7the

resultsoftheseexperimentsareshown,whereitcanbeseenthat

specificallyNi–Cuhadthepotentialtocompletelyeliminatethe

oxygencontentin anisole Unfortunately, this comparison only

givesavagueideaabouthowthenickelbasedcatalystscompare

toothercatalysts.Quantificationof theactivityand affinity for

carbonformation ofthesecatalystsrelative tonoble metal

cat-alystssuchasRu/C and Pd/C orrelative to Co–MoS2 would be

interesting

Zhaoetal.[107]measuredtheactivityforHDOinafixedbed

reactorwhereahydrogen/nitrogengaswassaturatedwithgaseous

guaiacol(H2/guaiacolmolarratioof33)overphosphidecatalysts

supportedonSiO2atatmosphericpressureand300◦C.Onthisbasis

thefollowingrelativeactivitywasfound:

Ni2P/SiO2>Co2P/SiO2>Fe2P/SiO2>WP/SiO2>MoP/SiO2

(12) AllthecatalystswerefoundlessactivethanPd/Al2O3,butmore

stablethanCo–MoS2/Al2O3.Thus,theattractivenessofthese

cat-Fig 7.Performance of nickel based catalysts for HDO HDO degree is the ratio

between the concentrations of oxygen free product relative to all products

Experi-ments performed with anisole in a fixed bed reactor at 300 ◦ C and 10 bar Data from

alystsisintheirhigheravailabilityandlowerprice,comparedto noblemetalcatalysts

AdifferentapproachforHDOwithtransitionmetalcatalysts waspublishedbyZhaoet al.[108–110].Inthesestudiesit was reportedthatphenolscouldbehydrogenatedbyusinga hetero-geneousaqueoussystemofametalcatalystmixedwithamineral acidinaphenol/water(0.01mol/4.4mol)solutionat200–300◦C and40baroveraperiodof2h.Inthesesystemshydrogen dona-tionproceedsfromthemetal,followedbywaterextractionwith themineralacid,wherebydeoxygenationcanbeachieved[109] BothPd/CandRaney®Ni(nickel-aluminaalloy)werefoundtobe effectivecatalystswhencombinedwithNafion/SiO2asmineralacid [110].However,thisconcepthassofaronlybeenshowninbatch experiments.Furthermoretheinfluenceofusingahigherphenol concentrationshouldbetestedtoevaluatethepotentialofthe sys-tem

Overallitisapparentthatalternativestoboththesulphur con-tainingtypecatalystsandnoblemetaltypecatalystsexist,butthese systemsstillneedadditionaldevelopmentinordertoevaluatetheir fullpotential

4.1.3 Supports Thechoiceofcarriermaterialisanimportantaspectofcatalyst formulationforHDO[98]

Al2O3hasbeenshowntobeanunsuitablesupport,asitinthe presenceof largeramounts ofwater it willconverttoboemite (AlO(OH)) [11,26,111] Aninvestigationof Laurent and Delmon [111]onNi–MoS2/␥-Al2O3showedthattheformationofboemite resultedin theoxidationof nickelonthecatalyst.These nickel oxideswereinactivewithrespecttoHDOandcouldfurtherblock otherMoorNisitesonthecatalyst.Bytreatingthecatalystina mixtureofdodecaneandwaterfor60h,adecreasebytwothirdsof theactivitywasseenrelativetoacasewherethecatalysthadbeen treatedindodecanealone[26,111]

Additionally,Popovetal.[112]foundthat2/3ofaluminawas coveredwithphenolicspecies whensaturating itat400◦Cin a phenol/argonflow.Theobservedsurfacespecieswerebelievedto

bepotentialcarbonprecursors,indicatingthatahighaffinityfor carbonformationexistsonthistypeofsupport.Thehighsurface coveragewaslinkedtotherelativehighacidityofAl2O3

AsanalternativetoAl2O3,carbonhasbeenfoundtobeamore promisingsupport[53,94,113–115].Theneutralnatureofcarbon

isadvantageous,asthisgivesalowertendencyforcarbon forma-tioncomparedtoAl2O3[94,114].AlsoSiO2hasbeenindicatedasa prospectivesupportforHDOasit,likecarbon,hasageneral neu-tralnatureandthereforehasarelativelylowaffinityforcarbon formation[107].Popovetal.[112]showedthattheconcentration

ofadsorbedphenolspeciesonSiO2wasonly12%relativetothe concentrationfoundonAl2O3at400◦C.SiO2onlyinteractedwith phenolthroughhydrogenbonds,butonAl2O3dissociationof phe-noltomorestronglyadsorbedsurfacespeciesontheacidsiteswas observed[116]

ZrO2 and CeO2 have alsobeenidentifiedaspotentialcarrier materialsforthesynthesis.ZrO2hassomeacidiccharacter,but sig-nificantlylessthanAl2O3[117].ZrO2andCeO2arethoughttohave thepotentialtoactivateoxy-compoundsontheirsurface,asshown

inFig.5,andtherebyincreaseactivity.Thus,theyseemattractive

intheformulationofnewcatalysts,seealsoFig.7[66,98,117,118] Overalltwoaspectsshouldbeconsideredinthechoiceof sup-port On one hand the affinityfor carbon formation shouldbe low, which to some extent is correlated tothe acidity (which shouldbelow).Secondly,itshouldhavetheabilitytoactivate oxy-compoundstofacilitatesufficientactivity.Thelatterisespecially importantwhendealingwithbasemetalcatalysts,asdiscussedin Section4.1.2

4.2 Kineticmodels

Athoroughreviewof severalmodel compoundkinetic

stud-ieshasbeenmadebyFurimsky[27].However,sparseinformation

onthekineticsofHDOofbio-oilisavailable;heremainlylumped

kineticexpressionshavebeendeveloped,duetothediversityof

thefeed

Sheuetal.[119]investigatedthekineticsofHDOofpine

bio-oilbetweenca.300–400◦C over Pt/Al2O3/SiO2, Co–MoS2/Al2O3,

andNi–MoS2/Al2O3catalystsinapackedbedreactor.Thesewere

evaluatedonthebasisofakineticexpressionofthetype:

−dwoxy

dZ =k· wm

Herewoxyisthemassofoxygenintheproductrelativetothe

oxy-genintherawpyrolysisoil,Zistheaxialpositioninthereactor,k

istherateconstantgivenbyanArrheniusexpression,Pisthetotal

pressure(mainlyH2),misthereactionorderfortheoxygen,and

nisthereactionorderforthetotalpressure.Inthestudyitwas

assumedthatallthreetypesofcatalystcouldbedescribedbya

firstorderdependencywithrespecttotheoxygeninthepyrolysis

oil(i.e.m=1).Onthisbasisthepressuredependencyandactivation

energycouldbefound,whicharesummarizedinTable6.Generally

apositiveeffectofanincreasedpressurewasreportedasnwasin

therangefrom0.3to1.Theactivationenergieswerefoundinthe

rangefrom45.5to71.4kJ/mol,withPt/Al2O3/SiO2havingthe

low-estactivationenergy.TheloweractivationenergyforthePtcatalyst

wasinagreementwithanobservedhigherdegreeofdeoxygenation

comparedtothetwoother.Theresultsofthisstudyare

interest-ing,however,theratetermofEq.(13)hasanon-fundamentalform

astheuseofmassrelatedconcentrationsandespeciallyusingthe

axialpositioninthereactorastimedependencymakestheterm

veryspecificforthesystemused.Thus,correlatingtheresultsto

othersystemscouldbedifficult.Furthermore,theassumptionofa

generalfirstorderdependencyforwoxyisaveryroughassumption

whendevelopingakineticmodel

AsimilarapproachtothatofSheuetal.[119]wasmadeby

Su-Pingetal.[67],whereCo–MoS2/Al2O3wasinvestigatedforHDOof

bio-oilinabatchreactorbetween360and390◦C.Hereageneral

lowdependencyonthehydrogenpartialpressurewasfoundover

apressureintervalfrom15barto30bar,soitwaschosentoomit

thepressuredependency.Thisledtotheexpression:

−dCoxy

HereCoxy isthetotalconcentrationofalloxygenatedmolecules

Ahigherreactionorderof2.3wasfoundinthiscase,compared

totheassumptionofSheu etal.[119].Thequitehighapparent

reactionordermaybecorrelatedwiththeactivityofthedifferent

oxygen-containingspecies;theveryreactivespecieswillentaila

highreactionrate,butasthesedisappeararapiddecreaseintherate

willbeobserved(cf.discussioninSection4).Theactivationenergy

wasinthisstudyfoundtobe91.4kJ/mol,whichissomewhathigher

thanthatfoundbySheuetal.[119]

Table 6

Kinetic parameters for the kinetic model in Eq (13) of different catalysts

Experi-ments performed in a packed bed reactor between ca 300–400◦C and 45–105 bar.

Data are from Sheu et al [119]

oftheHDOofphenolonCo–MoS2/Al2O3inapackedbedreactor basedonaLangmuir–Hinshelwoodtypeexpression:

−dCPhe

d =k1·KAds·CPhe+k2·KAds·CPhe

(1+CPhe,0·KAds·CPhe)2 (15) HereCPheisthephenolconcentration,CPhe,0theinitialphenol con-centration,KAdstheequilibriumconstantforadsorptionofphenol

onthecatalyst,theresidencetime,andk1andk2rateconstantsfor respectivelyadirectdeoxygenationpath(cf.Eq.(1))anda hydro-genationpath(cf.Eq.(2)).Itisapparentthatinordertodescribe HDOindetailallcontributingreactionpathshavetoberegarded Thisispossiblewhenasinglemoleculeisinvestigated.However, expandingthisanalysistoabio-oilreactantwillbetoo compre-hensive,asallreactionpathswillhavetobeconsidered

OverallitcanbeconcludedthatdescribingthekineticsofHDO

iscomplexduetothenatureofarealbio-oilfeed

4.3 Deactivation

ApronouncedprobleminHDOisdeactivation.Thiscanoccur throughpoisoningbynitrogenspeciesorwater,sinteringofthe catalyst,metaldeposition(specificallyalkalimetals),orcoking[59] Theextentofthesephenomenaisdependentonthecatalyst,but carbondepositionhasproventobeageneralproblemandthemain pathofcatalystdeactivation[120]

Carbon is principally formed through polymerization and polycondensationreactionsonthecatalyticsurface,forming pol-yaromaticspecies.Thisresultsintheblockageoftheactivesites

onthecatalysts[120].SpecificallyforCo–MoS2/Al2O3,ithasbeen shownthatcarbonbuildsupquicklyduetostrongadsorptionof polyaromaticspecies.Thesefilluptheporevolumeofthe cata-lystduringthestartupofthesystem.InastudyofFonsecaetal [121,122],itwasreportedthataboutonethirdofthetotalpore vol-umeofaCo–MoS2/Al2O3catalystwasoccupiedwithcarbonduring thisinitialcarbondepositionstageandhereafterasteadystatewas observedwherefurthercarbondepositionwaslimited[120] Theratesofthecarbonformingreactionsaretoalargeextent controlledbythefeedtothesystem,butprocessconditionsalso playanimportantrole.Withrespecttohydrocarbonfeeds,alkenes and aromaticshavebeenreportedashavingthelargestaffinity forcarbon formation,due toa significantlystronger interaction withthecatalyticsurfacerelativetosaturatedhydrocarbons.The strongerbindingtothesurfacewillentailthattheconversionof thehydrocarbonstocarbonismorelikely.Foroxygencontaining hydrocarbonsithasbeenidentifiedthatcompounds withmore thanoneoxygenatomappearstohaveahigheraffinityfor car-bonformationbypolymerizationreactionsonthecatalystssurfaces [120]

Cokingincreaseswithincreasingacidityofthecatalyst; influ-encedbybothLewisandBrønstedacidsites.Theprinciplefunction

ofLewisacidsitesistobindspeciestothecatalystsurface.Brønsted sitesfunctionbydonatingprotonstothecompoundsofrelevance, formingcarbocationswhicharebelievedtoberesponsiblefor cok-ing[120].Thisconstituteaproblemasacidsitesarealsorequired

in themechanism ofHDO (cf.Fig.4).Furthermore,it hasbeen foundthatthepresenceoforganicacids(asaceticacid)inthefeed willincreasetheaffinityforcarbonformation,asthiscatalysesthe thermaldegradationpath[104]

Inordertominimizecarbonformation,measurescanbetakenin thechoiceofoperatingparameters.Hydrogenhasbeenidentifiedas efficientlydecreasingthecarbonformationonCo–MoS2/Al2O3asit willconvertcarbonprecursorsintostablemoleculesbysaturating surfaceadsorbedspecies,asforexamplealkenes[120,123]

Fig 8. Yields of oil and gas compared to the elemental oxygen content in the oil from

a zeolite cracking process as a function of temperature Experiments were performed

with a HZSM-5 catalyst in a fixed bed reactor for bio-oil treatment Yields are given

relative to the initial biomass feed Data are from Williams and Horne [127]

Temperaturealsoaffectstheformationofcarbon.Atelevated

temperaturestherateofdehydrogenationincreases,whichgives

anincreaseintherateofpolycondensation.Generallyanincrease

inthereactiontemperaturewillleadtoincreasedcarbonformation

[120]

ThelossofactivityduetodepositionofcarbononCo–MoS2/

Al2O3hasbeencorrelatedwiththesimplemodel[124]:

Herekistheapparentrateconstant,k0istherateconstantofan

unpoisonedcatalyst,andCisthefractionalcoverageofcarbon

onthecatalyst’sactivesites.Thisexpressiondescribesthedirect

correlationbetweentheextentofcarbonblockingofthesurface

andtheextentofcatalystdeactivationandindicatesanapparent

proportionaleffect[120]

5 Zeolite cracking

Catalyticupgradingbyzeolitecrackingisrelatedtofluid

cat-alyticcracking(FCC),wherezeolitesarealsoused[57].Compared

toHDO,zeolitecrackingisnotaswelldevelopedatpresent,partly

becausethedevelopmentofHDOtoalargeextenthasbeen

extrap-olatedfromHDS.Itisnotpossibletoextrapolatezeolitecracking

fromFCCinthesamedegree[43,58,125]

Inzeolitecracking,allthereactionsofFig.1takeplacein

princi-ple,butthecrackingreactionsaretheprimaryones.Theconceptual

completedeoxygenationreactionforthesystemcanbe

character-izedas(thereactionisinspiredbyBridgwater[43,58]andcombined

withtheelementalcompositionofbio-oilspecifiedinTable3

nor-malizedtocarbon):

CH1.4O0.4→0.9“CH1.2+0.1CO2+0.2H2O (17)

With“CH1.2”beinganunspecifiedhydrocarbonproduct.Asfor

HDO,thebio-oilisconvertedintoatleastthreephasesinthe

pro-cess:oil,aqueous,andgas

Typically,reactiontemperaturesintherangefrom300to600◦C

areusedfortheprocess[51,126].Williamsetal.[127]investigated

theeffect oftemperatureonHZSM-5catalystsforupgradingof

bio-oilinafixedbedreactorinthetemperaturerangefrom400

to550◦C,illustratedinFig.8.Anincreasedtemperatureresulted

in a decrease in theoilyield and an increase in thegas yield

Thisisdue toan increasedrateof crackingreactionsat higher

temperatures,resultingintheproductionofthesmallervolatile

compounds.However,inordertodecreasetheoxygencontenttoa

significantdegreethehightemperatureswererequired.In

conclu-sion,itiscrucialtocontrolthedegreeofcracking.Acertainamount

ofcrackingisneededtoremoveoxygen,butiftherateofcracking becomestoohigh,atincreasedtemperatures,degradationofthe bio-oiltolightgasesandcarbonwilloccurinstead

IncontrasttotheHDOprocess,zeolitecrackingdoesnotrequire co-feedingofhydrogenandcanthereforebeoperatedat atmo-sphericpressure.Theprocessshouldbecarriedoutwitharelatively highresidencetimetoensureasatisfyingdegreeofdeoxygenation, i.e.LHSVaround2h−1[16].However,Vitoloetal.[128]observed thatbyincreasingtheresidencetime,theextentofcarbon for-mationalsoincreased.Onceagainthebestcompromisebetween deoxygenationandlimitedcarbonformationneedstobefound

InthecaseofcompletedeoxygenationthestoichiometryofEq (17)predicts a maximum oilyield of 42wt%, which is roughly

15wt%lowerthantheequivalentproductpredictedforHDO[43] ThereasonforthisloweryieldisbecausethelowH/Cratioofthe bio-oilimposesageneralrestrictioninthehydrocarbonyield[30] ThelowH/Cratioofthebio-oilalsoaffectsthequalityofthe prod-uct,astheeffectiveH/Cratio((H/C)eff)oftheproductfromaFCC unitcanbecalculatedas[57,129]:

Heretheelementalfractionsaregiven inmol%.Calculatingthis ratioonthebasisofarepresentativebio-oil(35mol%C,50mol%H, and15mol%O,cf.Table3)givesaratioof0.55.Thisvalueindicates thatahighaffinityforcarbonexistintheprocess,asanH/Cratio toward0impliesacarbonaceousproduct

Thecalculated(H/C)eff valuesshouldbecomparedtotheH/C ratioof1.47obtainedforHDOoilinEq.(6)andtheH/Cratioof1.5–2 forcrudeoil[10,11].Somezeolitecrackingstudieshaveobtained H/Cratiosof1.2,butthishasbeenaccompaniedwithoxygen con-tentsof20wt%[127,130]

ThelowH/Cratioofthezeolitecrackingoilimpliesthat hydro-carbonproductsfromthesereactionstypicallyarearomaticsand furtherhaveagenerallylowHVrelativetocrudeoil[28,43] Experimentalzeolitecrackingofbio-oilhasshownyieldsofoil

inthe14–23wt%range[131].Thisissignificantlylowerthanthe yieldspredictedfromEq.(17),thisdifferenceisduetopronounced carbon formation in the system during operation, constituting 26–39wt%oftheproduct[131]

5.1 Catalystsandreactionmechanisms Zeolites are three-dimensional porous structures Extensive workhasbeenconductedinelucidatingtheirstructureand cat-alyticproperties[132–137]

Themechanismforzeolitecrackingisbasedonaseriesof reac-tions.Hydrocarbonsareconvertedtosmallerfragmentsthrough generalcrackingreactions.Theactualoxygeneliminationis associ-atedwithdehydration,decarboxylation,anddecarbonylation,with dehydrationbeingthemainroute[138]

Themechanismforzeolitedehydrationofethanolwas inves-tigatedbyChiangandBhan[139]andisillustratedinFig.9.The reactionisinitiatedbyadsorptiononanacidsite.Afteradsorption, twodifferentpathswereevaluated,eitheradecompositionroute

orabimolecularmonomerdehydration(bothroutesareshownin Fig.9).Oxygeneliminationthroughdecompositionwasconcluded

tooccurwithacarbeniumionactingasatransitionstate.Onthis basisasurfaceethoxideisformed,whichcandesorbtoform ethy-leneandregeneratetheacidsite.Forthebimolecularmonomer dehydration,twoethanolmoleculesshouldbepresentonthe cat-alyst,wherebydiethylethercanbeformed.Preferenceforwhichof thetworoutesisfavouredwasconcludedbyChiangandBhan[139]

tobecontrolledbytheporestructureofthezeolite,withsmallpore structuresfavouringthelessbulkyethyleneproduct.Thus, prod-uctdistributionisalsoseentobecontrolledbytheporesize,where