Catalytic wet peroxide oxidation of vanillic acid as a lignin model compound towards the renewable production of dicarboxylic acids

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ContentslistsavailableatScienceDirect

Chemical

Engineering

Research

and

Design

jo u r n al ho 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 h e r d

Catalytic

wet

peroxide

oxidation

of

vanillic

acid

as

a

lignin

model

compound

towards

the

renewable

production

of

dicarboxylic

acids

Carlos

A. Vega-Aguilar

a,b

_,

_M.

_Filomena

_Barreiro

b,c

_,

_Alírio

_E.

_Rodrigues

a,∗ a_Laboratory_of_Separation_and_Reaction_Engineering_–_Laboratory_of_Catalysis_and_Materials_(LSRE-LCM),

FaculdadedeEngenharia,UniversidadedoPorto,RuaDr.RobertoFriass/n,4200-465Porto,Portugal

b_Laboratory_of_Separation_and_Reaction_Engineering_–_Laboratory_of_Catalysis_and_Materials_(LSRE-LCM),

PolytechnicInstituteofBraganc¸a,CampusSantaApolónia,5301-253Braganc¸a,Portugal

c_Centro_de_{Investigac¸ão}_de_Montanha_(CIMO),_Instituto_Politécnico_de_Braganc¸a,_Campus_de_Santa_Apolónia,

5300-253Braganc¸a,Portugal

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received28January2020

Receivedinrevisedform1April

2020

Accepted8April2020

Availableonline4May2020

Keywords:

Dicarboxylicacids

Titaniumsilicalite-1

Wetperoxideoxidation

Ligninoxidation

Succinicacid

a

b

s

t

r

a

c

t

Lignincanbedepolymerisedandusedasafeedstocktoobtainrenewableraw-materials,

providingagreenalternativetofossilcounterparts.Amongothers,C4dicarboxylicacids

(DCA),likesuccinic,malic,maleicandfumaricacids,whichcanfindapplicationsin

phar-maceuticals,foodindustry,andactassolvents,canbeobtainedfromligninoxidation.To

investigatetheirformation,theoxidationofvanillicacid(VA),aligninmodelcompound,was

studiedundercatalyticwetperoxideoxidation(CWPO)conditions,usingtitanium

silicalite-1(TS-1)asthecatalyst.Theeffectoftemperature,pH,andreactiontimewerestudied.In

asecondphase,catalystmodificationwithtransitionmetaloxides(Fe,Co,Cu)wastested.

ResultsshowedthatoxidationunderpH=10.5givesrisetocompleteVAconversionwith

hydroxylatedDCA,namelymalic(15mol%)andtartaric(5mol%)acids,asthemainproducts.

AtpH=4.0,theproductionofsuccinicacidwasimproved(7.4mol%),withVAconversion

achieving78%after2.0hofreaction.AtalkalinepH,H2O2reactivityishigher,leadingtoC4

-DCAdegradationtolow-molecularweightcompounds.Catalystdesilicationwasobserved,

pointedoutfortheconvenienceofusingneutralandacidicpH.InacidicpH,FeandCu

cat-alystsenhancedVAconversion,andFecatalystwasmoreselectivetowardssuccinicacid

production.

1. Introduction

Lignin is a three-dimensional heterogeneous biopolymer

providing plants with properties like rigidity,

water-impermeability, and resistance against microbial attack

(Kamm et al., 2008). Lignin structure can be envisaged as

basedinthreemonomericunits:p-hydroxyphenyl,guaiacyl,

andsyringyllinkedtogetherthroughetherandC-Cbonds

cre-∗ _{Corresponding}_author.

E-mailaddresses:[email protected](C.A.Vega-Aguilar),

[email protected](M.F.Barreiro),[email protected](A.E.Rodrigues).

atingacross-linkedpolymericstructure(Kammetal.,2008;Li

etal.,2015).Apromisingroutetowardsrenewablechemicals

andfuelsislignindepolymerisation.Nevertheless,thelignin

complex structure hindersits application, encouraging the

quest for innovative and more effective depolymerisation

strategies(Xuetal.,2014).

Lignincanbedepolymerisedbyseveralprocessesincluding

acid/base catalyseddepolymerisation,pyrolysis,

hydrotreat-ment, oxidation, reformingand gasification (Erdocia et al.,

2017;Lietal.,2015).Ligninoxidationcancausethecleavage

ofligninaromatic rings,and aryl etherbonds,among

oth-ers,beinganinterestingwaytoobtainlow-molecularweight

chemicalcompounds(Abdelazizetal.,2018;Kangetal.,2013;

https://doi.org/10.1016/j.cherd.2020.04.021

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PandeyandKim,2011).Inthiscontext,themoststudied

oxi-dantsaremolecularoxygen,nitrobenzene,metaloxidesand

hydrogen peroxide (H2O2)(Li et al., 2015;Pandey and Kim,

2011).Amongthem,oxidationwithH2O2isahighlypromising

strategy,especiallyinthepresenceofcatalysts,enablingmild

reactionconditionsandshorterreactiontimes(Chengetal.,

2017).Hydrogenperoxideismorereactivethanoxygen,even

atalkalineoracidicconditions,withthebenefitofbeing

con-sideredanenvironmentallybenignchemical(Maetal.,2015;

PandeyandKim,2011).

Currently, dicarboxylic acids (DCA) are produced from

petrochemical routes or edible biomass fermentation, and

findusesinthepharmaceuticalindustry,polymersynthesis,

asafoodadditive,or usedaschemicalprecursorsofother

compounds (Höfer, 2015).In the actual context ofbiomass

valorisation,C4-DCA,whichincludesuccinic,fumaric,maleic

andmalicacids,werelistedamongthe12buildingblockstobe

producedviathissyntheticroute(WerpyandPetersen,2004).

DCAareamongtheproductsthatcanbeobtainedfrom

lignincatalyticwetperoxideoxidation(CWPO),aftercleavage

ofthearomaticring.Theproductselectivitydependsonthe

oxidisingagent,catalysttype,processconditionsandreactor

type(Kangetal.,2013;Maetal.,2015).InthecontextofDCA

fromlignin,severalstudieshavebeencarriedoutusingwet

peroxideoxidation,withandwithoutcatalyst(Croninetal.,

2017;Hasegawaetal.,2011;Kangetal.,2019;Maetal.,2015; Suetal.,2014;Yinetal.,2015;Zengetal.,2015).However,many

oftheligninoxidationprocesses,includingthecatalysedones,

arenotselectiveenoughtofavourtheproductionofaspecific

DCA,generatingamixtureofDCA,introducingtheneedof

additionalstepsofseparationandpurification.

Titaniumsilicalite-1(TS-1)catalyst,asyntheticzeolitewith

aMFIframework structure,currentlyusedin thechemical

industry,hasshowngoodcatalyticactivityforoxidation

reac-tionsusingH2O2(Pˇrech,2018).TheincreasedH2O2reactivityis

relatedtoseveralTS-1characteristics,including

microporos-ity,hydrophobicnature,andthepresenceoftitaniumatoms,

whichreducestheelectrondensityoftheO-Obonds,making

theoxidantmoresusceptibletonucleophilicattack(Clerici,

2015;Pˇrech,2018).TS-1catalysthasbeenalreadytested in

guaiacolperoxideoxidationundermildalkalineconditions,

producing different DCA, mainly maleic, malic and oxalic

acids(Suetal.,2014).Theseresultspointedouttheinterest

toproceedwiththetestingofTS-1withotherligninmodel

compounds,namelywiththeoneswithhighercomplexity.

Catalystmodification withtransitionmetals(Fe,Mn,Co,

Cu, Ni) isreported as a strategy toenhance the oxidation

activitybyincreasinghydroxylradical’sformation(Schutyser

etal.,2018;Védrine,2017).Comparativelywithprecious

met-als(Pd,Au),transitionmetalshavelowerprices,provideeasy

reactionconditions,andpromisingresultsforlignin

depoly-merisation(Zengetal.,2015).Transitionmetalscanbeused

bothashomogeneousandheterogeneouscatalysts,

constitut-inganinterestingstrategytomodifytheTS-1catalyst.

Theobjectiveofthepresentworkwastostudythe

oxida-tionofvanillicacid(VA)toproduceC4-DCA,usingH2O2asthe

oxidisingagentinthepresenceofTS-1catalyst.Comparatively

withotherligninmodelcompoundsfoundinprevious

oxida-tionstudieswithTS-1,VAsharesasimilararomaticstructure

toguaiacyl,whichisthemostcommonstructuralunitin

soft-woodlignins,andcanrepresentthebehaviourofthearomatic

ring-openingreactionstowardstheproductionofC4-DCA.VA

canalsobefoundasaproductofligninoxidationwithO2,

whichisanimportantprocesstoachievearomaticcompounds

(Rodrigueset al.,2018).Inasecondphaseofthework,

TS-1catalyst wasmodified withtransitionmetals(Fe, Co,Cu)

and tested inthe oxidationreaction. The effectofpH and

reactiontimeonmaleic,fumaric,tartaric,succinicandmalic

acidsproductionyieldwasstudied.Moreover,theoccurrence

ofdegradationproductswaschecked.

2. Materials

and

methods

2.1. Materials

Vanillic acid (97%), DL-malic acid (≥99.0%), fumaric acid

(≥99.0%), maleic acid (>99%), succinic acid (≥99.0%),

L-(+)-tartaricacid(≥99.5%),malonicacid(>99%),oxalicacid

dihy-drate(≥99.0%),lacticacidsolution(85%,p.a.),FeCl3·6H2O(97%)

andCo(NO3)2·6H2O(99%)werepurchasedfromSigma–Aldrich

Co.LLC.Otherreagentswerepurchasedfromdifferent

suppli-ers:formicacid(Chem-labs,>99%),acetonitrile(VWR,HPLC

grade),sulfuricacid(Chem-labs,95–97%),sodiumhydroxide

(Merck,p.a.),hydrogen peroxidesolution(Fluka, >30%p.a.),

Cu(NO3)2·3H2O(Merck,p.a.).CatalystTS-1wasboughttoACS

Materials,LLC.All reactantswere usedasreceivedwithout

furtherpurification.

2.2. Oxidationprocedure

VAwasoxidisedusingclosedsteelreactors(20mL)

compris-ing an inner PTFE vial,where5.00mLofa 10.0g/Lvanillic

acidsolution,0.500mLofa30wt%H2O2solutionand5.0mg

ofTS-1catalyst wereplaced(10wt%,VA-basis). Theadded

oxidant amount was estimated based on oxygen

stoichio-metric demandforcompleteVAoxidationtoCO2 andH2O.

Afteradding allreactantsandcatalyst,pHwasadjustedto

the desiredvalueusingNaOH2.0mol/LorH2SO4 2.0mol/L,

forthealkalineandacidmedium,respectively.Thereactors

were then closed andheated to145±1◦C,usingaheating

plate equippedwithathermocouple,and themixturekept

understirring(600rpm),fortherequiredreactiontime.After

thedesiredreactiontime,thereactorswerequenchedinan

icewaterbathandsamplesrecoveredforanalysis.Theeffect

ofpH(4.0–12.0)andreactiontime(0–6h)onselectedDCAwas

studied. Moreover,theformationofDCAdegradation

prod-uctswaschecked.Experimentsweredoneintriplicate,except

whenindicated.The20mL-reactorwasusedasafirststepto

study theprocess.By usingthislow-volumesystem, itwas

possibletoaccuratelycontrolthereactionconditions,

espe-cially temperature. A145◦C temperature was chosenafter

validating the dependence of temperature against C4-DCA

yieldandVAconversion.

TheconductedexperimentscoveredapHrangewhere

sev-eral ligninsare solubleinaqueoussolution, avoidingmass

transferproblems.Moreover,thecatalystdoesnotactinthe

ligninstructure,butintheH2O2O-Obond,toformhydroxyl

radicals.Theseradicalsaresolubleinaqueoussolutionand

areresponsibleforthering-openingreaction.

2.3. QuantificationofVA,C4-DCA

Quantification of VA, DCAs (maleic, fumaric, tartaric,

suc-cinic and malicacids), and degradation products(malonic,

oxalic, formic, acetic and lactic acids) was done by

high-performance liquidchromatography (HPLC). Theapparatus

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Detec-tor (210, 260nm) and a Phenomenex® _RezexTM _ROA _H+

column (300mm×7.8mm) conditioned at 50◦C. The

sol-ventprogramming (flowrateof0.5mL/min) wasas follows:

0.0–10.0minH2SO44mmol/L;20.0–42.5min:15%acetonitrile

inH2SO44mmol/L;47.5–100.0min:H2SO44mmol/L.For

injec-tion (injection volume 20␮L), samples from the oxidation

reactionwereacidifiedwithH2SO42mol/LtopH∼2,dilutedas

neededandfilteredthrougha0.22␮mpore-sizefilter.The

tar-getDCA,degradationproducts,andVAwerequantifiedusing

calibrationcurvespreparedfromavailablecommercial

stan-dards.

Gelpermeationchromatography(GPC)analysiswasused

toevaluatemolecularweightchangesduetotheused

oxida-tionprocedureswiththemodifiedcatalysts.Calibrationwas

doneusingpolystyrene(PS)standards.Moredetailsaboutthe

appliedmethodcanbefoundelsewhere(Costaetal.,2018),

andtheHPLCequipmentcorrespondstotheonementioned

above.

2.4. CarboxylicacidsyieldandVAconversion

TheinformationobtainedinHPLCquantificationwasusedto

determinetheindividualcarboxylicacid(CAi)yields(Eq.(1)),

andVAconversion(Eq.(2)).TotalC4-DCAyieldwasobtained

asthesumoftheindividualC4-DCAyields(succinic,fumaric,

maleic,malicandtartaric).

VAconversion(mol%)= [VA]O−[VA]f

[VA]_O ×100 (1)

where[VA]0istheinitialVAconcentration,and[VA]fistheVA

finalconcentration.

CAiYield(mol%)=

[CAi]f

[VA]_O ×100 (2)

where[CAi]fisthemolarconcentrationoftheindividualC4

-DCA(CAi=maleic,fumaric, tartaric, succinic ormalicacid)

ordegradationproduct(CAi=malonic,oxalic,formicorlactic

acids).

VA conversion and DCA yield were expressed as

aver-age±standarddeviation.

2.5. Catalystmodificationprocedure

Catalystmodificationwithtransitionmetals(Fe,Cu,andCo)

wasdoneusingamodifiedwetimpregnationmethod

accord-ingtothemethodologydescribedelsewhere(Prasetyokoetal.,

2010).Briefly,theTS-1catalystwasplacedinasolution

con-taininga sufficient amount of the cation salt (FeCl3·6H2O,

Cu(NO3)2·3H2O,Co(NO3)2·6H2O),toyieldmaterialswitha

load-ingof2wt%ofthecorrespondingoxide(Fe2O3,CuO,andCoO,

respectively).Thesuspensionwasheatedat80◦Cfor3hunder

stirring, followedbywater evaporationovernight at100◦C.

Afterthat,theobtainedproductwascalcinedat550◦Cfor3h

toobtainthemodifiedcatalyst.

2.6. Catalystcharacterisation

Modified catalysts were analysed by Scanning Electron

Microscopy(SEM),Energy-dispersiveX-raySpectroscopy(EDS)

andElectron BackscatteredDiffractionanalysis(XRD), after

beingproducedandappliedintheoxidationprocess.The

cat-alystsusedinthe oxidationprocesswere washedwithhot

water,anddriedat80◦Covernight,beforecharacterisation.

Fig.1–EffectofpHinC4dicarboxylicacids(DCA)yieldand

vanillicacid(VA)conversion,withTS-1(145◦C,2.0h).

SEM and EDS were performed using a High Resolution

(Schottky) Environmental Scanning Microscope with X-Ray

MicroanalysisandElectronBackscatteredDiffractionanalysis

(Quanta400FEGESEM/EDAXGenesisX4M).EDSwasdoneat

highvacuum,15keV,10mmworkingdistance,anda50Lsec

collectiontime.Forthat,sampleswerecoatedwithAu/Pdby

sputteringusing the SPIModule SputterCoaterequipment

(15mA,80s).

XRDwasdoneinadiffractometerPANalyticalEMPYREAN,

usingCuK␣1,2(1.5406 ˚A)radiation.Diffractionpatternswere

collectedoverarangeof4◦<2<70◦,usinga0.5◦diverging

andantidivergingslits,0.04radSollerslits(receivingand

scat-tering),stepsizeof0.0167◦anda30mintotaltime.TheX-ray

tube worked at 45kV and 40mA. Crystallinity percentages

werecalculatedbasedonpeakintensityat2=23.04◦.TS-1

non-modifiedcatalystwasgivena100%crystallinity.

In addition to morphological characterisations, atomic

absorption(AA)wasusedtocheckleachingofthetransition

metalsfromthemodifiedcatalysts.Forthat,thesampleswere

centrifuged and digestedusing 10%ofconcentrated HNO3.

TheanalysiswasdoneusingaGBC932plusequipment,

fol-lowingtheFlameMethodsManualforAtomicAbsorptionby

GBCScientificEquipmentPTYLtd.

3. Results

and

discussion

3.1. OxidationwithTS-1catalyst

3.1.1. EffectoftemperatureandpHconditions

ConcerningpHconditions,thestudiedrangewas4.0–12.0at

145◦C,during2h,followingpreliminaryresultspointedout

the higher DCA yield at this temperature. Theproduction

yieldofC4-DCA(maleic,fumaric,tartaric,succinicandmalic

acids),aswellastheVAconversion,areregisteredinFig.1.

ThehighestreactivitywasachievedforbasicpHs,with

com-plete VAconversion over pH10.5.Atacidic medium,afull

VAconversionwasnotreachedforthetestedtime.WhenpH

increased,thestrongeroxidationlevelleadstotheruptureof

thearomaticring,anditsconversiontohydroxylatedDCAs,

likemalicandtartaricacids.InpH4.0,nohydroxylatedacids

wereobtained,andVAconversionwaslower.

Nocatalystwasrecoveredafterthe reactionatpHof12

indicatingthatveryhighpHsarenotrecommendedtobeused

withTS-1catalysts.

Considering temperature, VA conversion increasedwith

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temper-Fig.2–(a)Vanillicacid(VA)and(b)C4-DCAconversionat

pH10.5()andpH4.0(),forcatalysed(—)and non-catalyzed(—)oxidations(3.0h,byduplicate).

ature range of 80–145◦C (Fig. 2). Moreover, catalysed and

non-catalysedreactionswerecomparedatpHconditionsof

10.5and4.0,confirmingthattheuseofTS-1catalystimproved

conversion,especiallyunderalkalineconditionsatlower

tem-peratures.VAconversionforthenon-catalysedreactionrise

withtemperatureincrease,startingfrom32%at80◦Ctofull

conversion at 145◦C. The catalysed reaction gives rise to

higherconversionatlower temperatures(80◦C and100◦C).

Athighertemperatures(120◦C and 145◦C),thereisno

dif-ference betweencatalysed and non-catalysed reactions for

alkalinepH,whileforacidic pHthenon-catalysedreaction

ledtobetterVAconversion.Also,forthenon-catalysed

reac-tions,afinalsolutionwithintensedarkcolour,togetherwith

thepresenceofsediments,resultingfromthecondensation

ofdifferentcompounds,wasobserved.Thisobservation

indi-catesthatVAwasoxidisedtoquinones,moleculeswithhigh

colourintensity,correspondingtothefirststagedescribedin

Fig.3. Theseoxidisedstructuresreacted witheachother to

producecondensationproducts,instead ofpromoting

ring-openingreactionstoformDCAs.AsseeninFig.2b,inallcases,

C4-DCAproductionwashigherforthecatalysedreaction,in

comparisonwiththenon-catalysedone.

Thehigher reactivity ofthe catalysed reaction was due

tothe improvement inthe H2O2 nucleophilic attack

capa-bility. This effect is achieved when H2O2 is adsorbed on

catalyst’stetrahedralTiactivesites,formingTi-OOHspecies

and increasing the partial negative charges of the oxygen

atoms(Xiaetal.,2017).Themicroporosityandthe

hydropho-bicnatureoftheporesarealsoimportantfactors,enhancing

Ti-OOHformation,avoidingsolvationofTiatomsandactive

species(Clerici,2015).

Hydrogen peroxide reactivity depends on pH since it

canactasanucleophileor electrophilespecies(Xiangand

Fig.3–Stepsforvanillicacidoxidation. (ModifiedwithpermissionfromCroninetal.(2017)

Lee, 2000). In wet peroxide oxidation, the active species

for compound’s oxidationare hydroxylradicals (HO•), and

hydroperoxyl anions (HOO−). These compounds can later

degradetomolecularoxygen,decreasingtheirreactivity(Yin

et al.,2015).Hydrogenperoxidecandecomposeduring

oxi-dationreactions;itisstableinacidicconditions,butquickly

decomposestoH2O,O2 andOH−abovepH=6.0(maximum

decompositionoccursatitspKa).Moreover,itissensitiveto

temperatureandthepresenceoftransitionmetalions(Xiang

andLee,2000).Phenolicunitsarestabletoalkalineperoxide

but becomereactive againsthydroxylradicalsderivedfrom

H2O2decomposition(Sunetal.,1999).MolecularO2wasnot

studiedinthisworkasanoxidantagentduetoitslow

reac-tivitycomparedtoH2O2,avoidingthering-openingreactions

toobtainDCA.

3.1.2. EffectofreactiontimeinC4-DCAproductionunder

alkalineandacidicconditions

Giventheresultsoftheprevioussection,pH4.0andpH10.5

werechosenforacidicandalkalineconditions,respectively,

together withthe temperature of145◦C,tostudy the

pro-ductionofC4-DCAfromVA.Foralkalineconditions,complete

VAconversionwasachievedinlessthan30min,andC4-DCA

productionreachedthehighestyieldat2h,beingmalicacid

theonepresentedathigheramount(Fig.4a).However,after

3h, the yieldof malic, fumaric, maleic and succinic acids

decreased, whichsuggestedthat theseDCAwere degraded

to low-molecularweightcompounds. Tartaric acid

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Fig.4–Effectoftimeinvanillicacid(VA)oxidation.(a)C4dicarboxylicacid(C4-DCA)yield,VAconversionand(b)

degradationproductsyieldatpH10.5;(c)C4-DCAyield,VAconversionand(d)degradationproductsyieldatpH4.0.(All

reactionsat145◦C).

Malonicacid,anintermediateproductintheC4-DCA

degrada-tionprocess,decreasedthroughtime;whilelactic,aceticand

formicacids,alsodegradation productsaccordingtoFig.3,

increasedtheirconcentrationuntil3h,but thendecreased,

confirmingthedegradationoftheC4-DCA(Fig.4b).This

obser-vationindicatesthatH2O2ishighlyreactiveatalkalinepHand

hightemperatures.Afterprolongedreactiontimes,an

over-oxidationofthealreadyproducedC4-DCAoccurs,originating

low-molecular weight degradation products, like malonic,

oxalic,lactic,aceticandformicacids.Therefore, the

oxida-tionreactionshouldbeperformedatshorttimestoavoidthe

lossofC4-DCAproducts.Also,thehigh nucleophilic

capac-ityofH2O2atalkalinepHcausesthehydroxylationofdouble

bonds,inbothmaleicandfumaricacids,leadingtothe

forma-tionofmalicandtartaricacids,butavoidingtheproductionof

succinicacidathigheryields.Inthiswork,succinicacidwas

identified(upto5.6mol%),whileSuetal.(2014)thatreported

guaiacolalkalineperoxideoxidationwithTS-1at80◦Cshown

noproductionofsuccinicacid,buthighermaleicacidyield

(upto27.7mol%).Lookingtoachievehighyieldsforsuccinic

acid,asaccordingtostepc)inFig.3,asourceofH+_should_be

included,whichisnotthecaseofalkalineoxidation.

Accordingto Fig. 3, vanillicacid is firstlyoxidised to

o-and p-quinones,which react with freeradicals to produce

ring-openingreactions,obtainingmaleicacid.Maleicacidis

isomerised to fumaric acid if heated at120◦C in aqueous

solution(Whelan,1994),producingmalicacidathigher

tem-peratures(Gaoet al., 2018).Thisobservation indicatesthat

duringthetransitionofmaleic tofumaricacid,the double

bondis morelabile tobe hydroxylated,explaining why at

higher temperaturesmalic and tartaric acids were present

athigheramounts,especiallyafterseveralminutesof

reac-tion.Thehydroxylated acids were atlower amounts(max.

2.9mol%)inSuetal.(2014)work,whichusedlower

tempera-tures.

AtacidicpH(pH=4.0),VAconversionwasslower

compar-ativelywithalkalinemedium(Fig.4c).TheH2O2reactivityat

alkalinepHishigherthaninacidicpH,since,forpH>9,

disso-ciationtohydroperoxideanionsoccurs(Yinetal.,2015).This

behaviourremainsevenwhentheTS-1catalystisused,afact

relatedtotheinferiorreactivityofH2O2 inacidicpH(Xiang

andLee,2000).

TheabundanceofC4-DCAproductionunderacid

condi-tionswasdifferentfromalkalinepH,ascanbeseeninFig.4c;

the moreabundantacidwas succinic(7.4mol%),

accompa-niedbysmallquantitiesofmaleicandfumaricacids.Malic

andtartaricacidswerenotdetectedafter6hofreaction.As

seen inFig.3,whenacidic pHisselected, thealready

pro-ducedmaleic/fumaricacidsavoidthehydroxylationreaction

towardsmalic/tartaricacids.Still,theH+_source_promotes

sat-urationofthedoublebondstoproducesuccinicacidathigher

selectivity.Previouspublications(Bietal.,2017;Croninetal.,

2017;Maetal.,2014)showthatsuccinicacidcanbeobtained

fromligninandligninmodelcompounds(guaiacol,catechol,

vanillin)usingperoxideoxidationunderacidicpHinthe

pres-ence of differentcatalysts. Nevertheless, noneofthe cited

worksusedTS-1catalystinacidicmediumtoobtainsuccinic

acid.Inthiswork,thehighestsuccinicacidyieldwasachieved

after2.0hreactionat145◦C(7.4mol%;5.2wt%),similarlyto

theonesreportedbyMaetal.(2014),usingFeCuS2catalystto

oxidiseguaiacol(5.4wt%)andcatechol(4.5wt%).

Underacidicconditions,lowerlevelsofdegradation

prod-uctswere obtained,comparatively withalkalineconditions

(Fig.4d).Lacticandformicacidsconcentrationdidnotincrease

throughreactiontime,asithappenedinalkalineoxidation.

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Fig.5–SEMimagesfrommodifiedTS-1catalysts(50,000×).

Table1–Transitionmetal-modifiedTS-1catalyst’sphysicalproperties.

Reactioncondition Ti/Siratioa _{Crystallinity}_(%)b

Cu/TS-1 Co/TS-1 Fe/TS-1 Cu/TS-1 Co/TS-1 Fe/TS-1

Aftermodification 0.024 0.019 0.026 100 91 100

pH10.5 0.032 0.034 0.026 60 27 46

pH4.0 0.023 0.019 0.024 100 97 91

a _Non-modified_TS-1_Ti/Si_ratio:_0.023.

b _Based_on_peak_intensity_at₂₌_23.04◦_._TS-1_non-modified_catalyst₌_100%_{crystallinity.}

case,beingtheproducedC4-DCAbetterpreserved,avoiding

theextensivedegradationobservedinthealkalineoxidation.

Therefore,accordingtothedesiredC4-DCA,differentpH

con-ditionsshouldbeselected.If succinicacid isthetarget,VA

oxidation should beperformed atacidic medium to avoid

maleic/fumaricacidsoxidationtotheirhydroxylated

deriva-tives.

3.2. OxidationwithmodifiedTS-1catalyst

3.2.1. Catalystmodificationwithtransitionmetaloxides

Transitionmetals,likeCo,Fe,Cu,MnandNi,havebeenwidely

studiedbecauseoftheir capabilitytoactivate theperoxide

oxidation;ashomogeneous(solublesalts)orheterogeneous

(metaloxides, supported metaloxides, or as part of

com-pounds,suchasCuFeS2)catalysts(LearyandSchmidt,2010;

Maetal.,2014;Sunetal.,1999;Vangeeletal.,2018;Védrine, 2017;Zengetal.,2015).Thesemetalscanenhancethe

produc-tionoffreereactiveradicals,convertingVAtoquinones,but

alsocausingring-openingreactions,producingDCA(Bietal.,

2017).TheycanincreaseH2O2reactivity,whichisimportant

atacidicpH,likeinFenton’sreaction,whichusesFe2+_/Fe3+_ion

pairasacatalystwithareactivitypeakatpH=2.8–3.0(Wang

etal.,2012).

A2wt%metallicoxideloadingwasselectedbasedonthe

workofWidiartietal.(2012),whichreportedthathigherloads

wouldcovertheTS-1surface,blockingtheaccesstothepores.

Ifthisblockageoccurs,theH2O2willnotaccessTi4+sitesto

releasetheHO•andHO2−radicals,decreasingtheactivityof

themodifiedcatalyst,andenhancingH2O2dismutationtoH2O

andO2.Thisbehaviourwaspreviouslyobservedwith

(7)

Fig.6–XRDDiffractogramsformodifiedTS-1catalysts:(a)Fe/TS-1;(b)Co/TS-1;(c)Cu/TS-1.Eachfigureincludesthe modifiednon-usedcatalyst,thecatalystusedunderacidicandalkalineconditions,andtheoriginalTS-1diffractogramas comparison.

catalysts.Inthiswork,modificationswithCu,CoandFehave

beenchosen.

As seen in the SEM images (Fig. 5, Pre-oxidation), all

modifiedcatalystspresentednometaloxidecrystalsvisible

on the particle’s surface. XRD results (Fig. 6) showed that

theMFIstructurewasnotaffectedafterimpregnationwith

themetal,namelythe characteristicdiffractionlinesof

TS-1(2=23.04,23.23, 23.65,23.88, 24.36)werepresent,atthe

samepeakheight,inallmodifiedcatalysts.Crystallinity

per-centage,aftermodificationwiththeselectedmetals,didnot

change (Table 1).No diffractionlines were assigned tothe

metaloxidesinthemodifiedcatalysts,duetotheirlow

per-centageinthematrix.TheTi/Siratiooftheoriginalcatalyst

(Ti/Siratio=0.023)waskeptafterthemetalimpregnation,as

reportedinTable1.

3.2.2. C4-DCAproductionunderalkalineandacidic

conditions

Tocomparetheeffectofthecatalystmodificationwith

metal-licions,reactionsweredoneat145◦Cfor2h,whichwasthe

timeandtemperaturewherethehighestyieldforsuccinicacid

wasobtained.Atalkalineconditions,allthemodifiedcatalysts

giverisetocompleteVAconversion,buttheproductionyield

ofthetargetedC4-DCAwasnotimproved(Fig.7a).Onlymaleic

andfumaricacidswereproducedatsimilaryields,

compara-tivelywiththenon-modifiedTS-1catalyst,andtartaricacid

wasnotdetected.Aprominentbubblingwasobservedwhen

the Co/TS-1 and Cu/TS-1 catalystwere added tothe

reac-tionmedium,indicatingthatH2O2isquicklydegradedtoO2,

skippingthereleaseoftheactiveradicalsneededtothe

ring-openingreactions.SinceTS-1canonlyactivateH2O2andnot

O2(Pˇrech,2018),lowerproductivityisexpectedwhenH2O2is

degradedtoO2.Alltheseoxidationsproduceddark-coloured

solutions, and organic sediments in the Co/TS-1 reaction,

whichmaybederivedfromVAoxidisedderivatives

condensa-tionreactions.Thecondensationreactionsgaverisetohigher

molecular-weightcompounds,whichwasconfirmedbyGPC

analyses(Fig.8).Lowerover-oxidationoccurred,becauselower

degradation productsyields were obtained (Fig. 7c,e),

con-firmingthatmodifiedcatalystsenhancedoxidantdegradation

insteadofpromotingtheoxidationofVAintoC4-DCA.

Under acidic conditions, VA conversion was improved

when using Cu/TS-1 and Fe/TS-1, achieving complete

con-version after2hat145◦C.WithCo/TS-1catalyst, only57%

conversionwasachieved,whileforTS-1non-modifiedcatalyst

78%conversionwasreached.Thehigheractivityfor

Cu/TS-1 catalystwasexpected dueto the intermediatereduction

potentialofCu2+₍_Schutyser_et_al.,₂₀₁₈_),_which_is_confirmed

(8)

prod-Fig.7–Yieldsachievedwithmodifiedcatalysts,andcomparisonwithnon-modifiedcatalyst,underdifferentpHs.C4-DCA

yieldsat(a)pH=10.5and(b)pH=4.0;VAconversionandacidyieldsat(c)pH=10.5and(d)pH=4.0;Degradationproductsat (e)pH=10.5and(f)pH=4.0(145◦C,2.0h).

Fig.8–GPCanalysisforvanillicacidoxidationusingmodifiedTS-1catalysts,underdifferentpH.Vanillicacidoxidation withnon-modifiedTS-1GPCcurveisshownasacomparisonforeachcurve.(a)pH4.0,(b)pH10.5.(Reactionconditions 145◦C,2.0h).

uctofVAoxidation(Fig.7f).Ahigheramountofmalicacid

wasalsoobserved,suggestingtheconversionoftheoriginal

maleic/fumaricacidsintothiscompound,avoidingthe

con-versiontosuccinicacid,evenatacidicpH.Fe/TS-1improved

VAoxidationduetoironcapacitytoactasaFenton’scatalyst,

enhancingHO•andHO2•radicalsformationthroughthe

mix-tureofFe2+_/Fe3+_ions₍_Wang_et_al.,₂₀₁₂_)._Malic_acid_was_also

producedwithFe/TS-1and Cu/TS-1catalysts(Fig.7b),

sug-gestingthat thisoxidationwasstronger thanthe onewith

non-modifiedTS-1.

TheFe/TS-1catalystwastheonlymodifiedcatalystform

providing a high yield of succinic acid (7.9mol%); slightly

higher than with the non-modified TS-1. Moreover, lower

amountsofdegradationproductsweredetected,showingthat

oxidationwasmoreselective.Ironhasbeenreportedasthe

lessreactiveofthethreetestedmetals(Schutyseretal.,2018).

Thislower reactivity lowers H2O2 degradation but

generat-ingtheneededradicalstoopenthearomaticring.However,

theappearanceofmalicacid,whichwasnotpresentinthe

(9)

reactivity,Fe catalystproducesmorefreeradicalsthan the

non-modified TS-1catalyst. Under acidic oxidation, higher

molecularweightcompoundswereproducedmainlywhenFe

andCucatalystswereused,but atloweramounts,in

com-parisontoalkalineoxidation(Fig.8b).Moreover,noorganic

sedimentsnorcolouredsolutionswere observedunderthis

pHcondition.

3.2.3. Effectofreactionconditionsinthemodifiedcatalysts

The modified catalysts suffered degradation after alkaline

oxidation(Fig. 5), resulting inparticles with smaller

diam-eters and loss of organisation. EDS analysis showed that

theTi/Si ratiowashigherafteralkalineoxidation(Table1),

suggestingthat the internalzeolite structurewas affected,

andSiatomswereleached outfrom thecatalyststructure.

XRDresultsshowedthatalkalineoxidationleadsto

signifi-cantMFIcrystallinitylosses.Thiseffectcanbeobservedas

adecreaseinthe intensityoftheTS-1characteristicpeaks

(Fig.6),thusareductionofcrystallinitypercentage(Table1).

ThehighercrystallinitylosswasobservedfortheCo/TS-1

cat-alyst.Titanosilicateszeolites,includingTS-1,arereportedas

resistanttoconcentrated mineralacids,but nottoalkaline

pHsduetodesilicationdevelopment(Pˇrech,2018).Toavoid

catalystdegradation,therecommendedNaOHconcentration

shouldbelowerthan0.2mol/L,sincehigherlevelscancause

totaldisruptionoftheframework(Pˇrech,2018).

Afteracidicoxidation,thecatalystparticlesdidnotshow

anystructuralchanges(Fig.5),andXRDresultsconfirmedthat

crystallinitywasmaintained(Table1),pointingoutthatacidic

environmentissaferforTS-1catalyststructure.

Tocheckleachingofmetallicions,inacidicoralkaline

envi-ronment,solubleionswerequantifiedbyatomicabsorption

analysis.Fe/TS-1catalystshowednoleaching,whileCo/TS-1

andCu/TS-1showedleachinginacidicpH(5mg/Land13mg/L,

respectively),and Cu/TS-1 leachingatalkalinepH(5mg/L).

CuOhasthelowestsolubilityinalkalinemedium(pHaround

8–9)at145◦C,andhighsolubilityinbothacidicandalkaline

pH,explainingtheobservedleaching(Palmer,2005).

There-fore,Co/TS-1andCu/TS-1catalystsdidnotleadtoenhance

DCAyield,andshowedlowchemicalstability,duetoleachate

ofmetals,disablingtheirpossibleuseuntilnoprocedureto

avoidleachingcouldbecarriedout.OnlyFe/TS-1showedto

beastablecatalyst,inbothmedia.Stabilityincreasecouldbe

achievedifdifferentmetalsalts,selectedamongtheonesthat

havebeenproventobeinsolubleinalkalineoracidicpH,are

chosen,whichshouldbedoneindividuallyaccordingtoeach

metallicion.Itisnotrecommended toincreasecalcination

temperature,duetopossiblesinteringoftheTS-1catalyst.

4. Conclusions

TheuseofTS-1catalystinthecatalyticwetperoxide

oxida-tionofVAenhancedtheproductionofC4-DCA,comparatively

withthenon-catalysedreaction.Thetypeofproducedacids

dependsmainlyonthe usedtemperature, pHandreaction

time.Analkalinemediumcausedanover-oxidationofVA,

giv-ingrisetohydroxylatedacids,likemalicandtartaric,together

withhighdegradationlevels.InacidicpH,themainproduced

acidwassuccinic,andaloweramountofdegradation

prod-uctswasobserved.Therefore,VAoxidationactedsimilarlyto

otherligninmodel compounds,wheretheoxidationofthe

aromaticringisthefirststep,gettingopentoproduceC4-DCA.

However,theseC4-DCAcanquicklydegradetolow-molecular

weightcompounds,especiallyunderalkalineconditionsand

longreactiontimes.Ifsuccinicacidisthetargetcompound,

acidicoxidationshouldbeselected.

WhenTS-1catalystismodifiedwithFe,CoandCuoxides,

theproductivityofC4-DCAwasnotimprovedunderalkaline

pH, due tothe high degradation ofH2O2 to O2, associated

mainlywithCoandCuoxides.InacidicpH,Fe/TS-1and

Co/TS-1catalystsincreasedVA conversionandoxidativereaction,

but only Fe/TS-1showed a slightimprovement insuccinic

acid production and stability againstleaching. Leaching in

CuandCocatalystswasobserved,andtheiruseisnot

rec-ommended.Underalkalineoxidation,modifiedTS-1catalysts

suffereddesilicationandlossofexternalstructure,butnotin

acidconditions.Thus,itisrecommendedcarryingout

oxida-tionswithTS-1catalystatacidicorneutralpH.

Conflict

of

interest

Theauthorsdeclarenocompetingfinancialinterest.

Author

contributions

Themanuscriptwaswrittenthroughthecontributionsofall

authors.Allauthorshavegivenapprovaltothefinalversion

ofthemanuscript.Allauthorscontributedequally.

Acknowledgements

TheauthorsgratefullyacknowledgesupportfromFundac¸ão

para a Ciência e a Tecnologia (FCT), Portugal,Grant

num-bers: UID/EQU/50020/2019, UID/AGR/00690/2019; European

CooperationinScienceandTechnology,Grantnumbers:

Lig-noCOST(CA17128)andCostaRicanScience,Technologyand

TelecommunicationsMinistry,CostaRica.Scholarship

num-ber:MICITT-PINN-CON-2-1-4-17-1-002.

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