A new use for modified sugarcane bagasse containing adsorbed Co2+and Cr3+ : catalytic oxidation of terpenes.

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ContentslistsavailableatScienceDirect

Industrial

Crops

and

Products

j o u r n al ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / i n d c r o p

A

new

use

for

modiﬁed

sugarcane

bagasse

containing

adsorbed

Co

2+

and

Cr

3+

:

Catalytic

oxidation

of

terpenes

Anderson

Gabriel

Marques

da

Silva,

Thenner

Silva

Rodrigues,

Leandro

Vinícius

Alves

Gurgel,

Patrícia

Aparecida

de

Assis,

Laurent

Frédéric

Gil

∗

,

Patricia

Alejandra

Robles-Dutenhefner

∗

DepartamentodeQuímica,InstitutodeCiênciasExataseBiológicas,UniversidadeFederaldeOuroPreto,CampusUniversitárioMorrodoCruzeiro, 35400-000OuroPreto,MinasGerais,Brazil

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received19April2013

Receivedinrevisedform20July2013 Accepted26July2013

Keywords:

Modiﬁedsugarcanebagasse Cobalt

Chromium Catalyticoxidation Monoterpenes

a

b

s

t

r

a

c

t

Thisstudydescribestheapplicabilityoftwochemicallymodiﬁedsugarcanebagassescontainingeither adsorbedCo2+_or_Cr3+_ions_as_{heterogeneous}_catalysts_for_the_{autooxidation}_of_{monoterpenes.}_The_main

objectivewastoinvestigatenewusesfortheseadsorbentmaterialswhichhadbeenpreviouslyemployed fortreatmentofaqueoussolutionsorefﬂuentscontainingmetalssuchasCo2+_and_Cr3+_._The_adsorption

efﬁciencyofCo2+_and_Cr3+_on_SCB2_and_EB_was_evaluated_by_adsorption_isotherms_and_other_techniques

suchasXRD,ICP-AESandTGA.Catalyticactivityofthefournewcatalysts,SCB2-Co,SCB2-Cr,EB-Co,and EB-Cr,wereassessedintheoxidationreactionof␤-citronellol(1),(+)-limonene(2),and(−)-␤-pinene(3) inafreesolventsystem.Resultsobtaineddemonstratedthatthesematerialswerepromisingcatalystsfor theoxidationofmonoterpenes.Reactantconversionrangedfrom49to78%asdeterminedbyGCanalysis andacombinedselectivityupto59%fortheoxidationproductswasachieved.

1. Introduction

Inrecentyears,theuseoflignocellulosicmaterialsassolid sup-portsfor the adsorptionof metallic cations hasreceivedmuch attention due to the low costs associated with these promis-ingalternativematerials(Gurgeletal.,2008a;Sud etal.,2008). Thenceforward,variousresearchgroupshavedeveloped adsorp-tionmaterialsbasedonlignocellulosicmatricesassolidsupports withgoodchemicalafﬁnityformetalliccations.Noteworthyisthe factthatamongstthebiomasssourcesavailable,those originat-ingfromagriculturalresiduessuchassugarcanebagasse, wood bark,and woodsawdusthave beenthemostintensively inves-tigatedforthedevelopmentofnewadsorbent materials(Ajmal et al., 1998; Gurgel et al., 2008b; Karnitz et al., 2009; Naiya et al., 2009; Pereira et al., 2009; Sud et al., 2008; Yu et al., 2008).IntheBrazilian scenario,theuseofagriculturalresidues fornovelapplicationsis a particularlyattractiveand promising sourceofnewmaterials,giventhecontinuallygrowinginvestments in recent years by local industry in the production of renew-ablefuels.AccordingtotheBrazilianMinistryofAgriculture,the

∗ Correspondingauthors.Tel.:+553135591229/3135591717; fax:+553135591707.

E-mailaddresses:[email protected](L.F.Gil), [email protected](P.A.Robles-Dutenhefner).

sugarcaneharvestinthe2011/2012seasonisexpectedtoreach 571.4milliontons,whichwillgenerateabout142.9milliontonsof bagasseand116.6milliontonsofstraw(CONAB,2011).Orinother words,onetonofsugarcaneproducesabout250kgofbagasseand 204kgofstrawwithanestimatedmoisturecontentof50%,andthus highlightingthepotentialofbagassederivativesforotherindustrial applications.

Sugarcanebagasse ismainly composedby40–50%cellulose, 25–30%polyoses, and 20–25%lignin (Karnitzet al., 2007).The chemical structuresof suchmacromoleculesare endowed with hydroxyl groups that can be transformed by modifying agents toincorporatecertainfunctional groupsabletoadsorbmetallic cationswithhighselectivity.Oneexample ofa compound that canbe“grafted”intothesugarcanebagassestructureis ethylene-diaminetetraaceticdianhydride(EDTAD).Inthiscase,amineand carboxylicacidfunctionsarereleasedproducingaveryefﬁcient complexingsubstructureintothelignocellulosicmatrixformetal adsorption(Karnitzetal., 2009).Otheragents suchassuccinic, maleicorphthalicanhydridescanbeusedtomodifythesurfaceof sugarcanebagassethroughanesteriﬁcationreactionresultingina surfacecontainingcarboxylicacidfunctionalgroups( COOH).This kindoffunctionalizationclearlyincreasestheadsorptioncapacity ofthelignocellulosicmaterials,whichhasbeendemonstratedin theliterature(Karnitzetal.,2007;Liuetal.,2007,2008;Naiyaetal., 2009;Srivastavaetal.,2006;Sudetal.,2008;Sunetal.,2003;Xiao etal.,2001).

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Modified sugarcane bagasse can efficiently adsorb metallic cationspresentinwaterbodies(Ajmaletal.,1998;Gurgeletal., 2008a;Karnitzetal.,2009;Naiyaetal.,2009)andeffluents mak-ingthisstrategyofadsorptionveryinterestingfromaneconomic andenvironmentalpointofview(Pereiraetal.,2009,2010). Nowa-days,sugarcanebagassegeneratedafterthecrushingofsugarcane, isburnedtoproducesteamforthegenerationofelectricenergyat sugarandethanolplants.

Chromiumis present intheearth’scrustatan average con-centrationof10mg/kgandintheoresitmayreachuptomore than 50%in weight. Afterprocessing of theore,the metalcan beconvertedtoseveralcompoundsandusedinmanyindustrial processes suchas the production of alloys, electrolytic plating, refractorybricks, pigmentsand tannic agents for leather. Toxic chromiumcompounds occurnaturallyatlow concentrations in the earth’scrust and it is through industrial activities suchas fabricationofcement,electroplating,foundries,welding,mining, industrialandmunicipalwaste,incineration,fertilizers,and spe-ciallyinitsuseintanneriesthatcontaminationcanoccurathigher levels(Gurgeletal.,2009).Inthiscontext,analternativeusefor sugarcanebagasseisdescribed,inprinciple,asverypromisingin theremovalofheavymetalionssuchasCr3+_from_contaminated

watersandefﬂuents.However,evenaftersolvingtheproblemof watercontamination,thedisposalofadsorbentmaterials contam-inatedwithmetalionsaftertheadsorptionprocessdoesnothave anobvioussolutionnoraspeciﬁcdestinationand/orsubsequent use,whichcouldthereforemakethisprocesseconomically unfea-sible.

Solidmaterialscontainingtransitionmetalssuchaschromium andcobaltarewidelyusedasheterogeneouscatalystsinvarious reactionsofinterestforchemical,petrochemical,pharmaceutical andfoodindustries(Boor,1979;Keii,2004;Robles-Dutenhefner etal.,2004,2011).Anexampleisthefunctionalizationof monoter-penes to obtain compounds of interest to the ﬁne chemicals industryaswellasothersubstancescategorizedasessentialoils (Robles-Dutenhefneretal.,2009;Salesetal.,2003).

Terpenes constitute a class of natural products that can be transformedintocompoundscommerciallyimportantfor indus-trialproductionof fragrances, perfumesand ﬂavors(Pybus and Sell,1999).Theapplicationofcobaltandchromiumascatalysts forhomogeneousand heterogeneousoxidationofterpeneswas alreadyreportedintheliteraturebyvariousresearchgroups(da Silvaetal.,2003;Robles-Dutenhefneretal.,2004,2005;Rochaetal., 2009;Spezialietal.,2005,2009).

Thus,thisstudyaimstodevelopaprocessthatusesagricultural by-productssuchassugarcanebagasse,modiﬁedwithorganic lig-andssuchassuccinicanhydrideandEDTAdianhydride,andusethe adsorbentmaterialsproducedtoremoveCo2+_and_Cr3+_from_single

metalionaqueoussolutions.Finally,theapplicationofthese adsor-bentmaterialscontainingadsorbedCo2+_and_Cr3+_as_{heterogeneous}

catalystsforthechemicaltransformationofnaturalterpenic sub-stratesinvaluableoxygenatedderivativesispresented.

2. Experimental

2.1. Chemicals

Succinicanhydride andpyridine werepurchasedfromVetec (Brazil). EDTA (disodium salt), acetic anhydride, and N,N -dimethylformamide (DMF)werepurchased fromSynth(Brazil). The monoterpenes (␤)-citronellol (1), (+)-limonene (2), ( −)-␤-pinene (3) were purchased from Sigma-Aldrich. Pyridine was reﬂuxedovernightwithNaOHanddistilledpriortouse.DMFwas treatedwithmolecularsieve (4 ˚A)prior touse. CoCl2·6H2Oand

CrCl3·6H2OwerepurchasedfromMerck.

2.2. PreparationofthecatalystsSCB2-CoandSCB2-Crfrom succinylatedsugarcanebagasse

Thechemicalmodificationofsugarcanebagasse (B)was per-formedaccordingtothesyntheticrouteshowninFig.1.Tothis end,sugarcanebagassewasinitiallymilledusingatungstenring millandsievedusingasievesystemcomposedbyscreensof1.68, 0.251,0.152,and0.075mm.Subsequently,thefractionof0.075mm waswashedwithhexane:ethanol1:1(v/v)inaSoxhlet appara-tusfor24handdriedinanovenat80◦C.Sugarcanebagasse(3g), drypowder(0.075mm),succinicanhydride(9g),andanhydrous pyridine (30mL) wereaddedtoa round-bottomflaskequipped witha refluxcondenser.Thesuspensionwasheatedatpyridine refluxfor24h.Attheendofthereaction,thesuccinylated sugar-canebagasse(SCB1)wastransferredtoasinteredglassfunnelwith porosity4andsuccessivelywashedwith1.0mol/Lofaceticacid indichloromethane,95%ethanol,0.01mol/Lhydrochloridricacid solution,distilledwater,95%ethanolandthenwithacetone.SCB1 wasdriedat80◦Cinanovenfor2h.Next,SCB1wascooledina desiccatorandweighed(Gurgeletal.,2008a).

SCB1wastreatedwithasaturatedsodiumbicarbonatesolution inordertoreleasethecarboxylategroups,producingthesorbent materialSCB2.Then,SCB2waswashedwithdistilledwater,95% ethanol,acetone,anddriedat80◦Cinanovenfor2h.Afterthat, SCB2wasseparatelytreatedwithaqueousspikedsolutionsofCo2+

(0.2mol/L)andCr3+_(0.2_mol/L)_for₁_h,_and_the_solid_and_liquid

frac-tionswereseparatedusingasinteredglassfunnelwithporosity4, washedwithdistilledwater,anddriedat80◦Cinanovenfor2h. SCB2treatedwithCo2+_and_Cr3+_aqueous_solutions_was_referred_to

asSCB2-CoandSCB2-Cr,respectively,ascanbeseeninFig.1. 2.3. PreparationofthecatalystsEB-CoandEB-Crfromsugarcane bagassemodiﬁedwithEDTAdianhydride(EDTAD)

Thechemicalmodificationofsugarcanebagasse(B)withEDTA dianhydridewascarriedoutusingthesyntheticrouteshownin Fig. 2. Sugarcane bagasse (3g), dry powder (0.075mm), EDTA dianhydride(15g),andanhydrousN,N-dimethylformamide(DMF) (126mL) wereadded toa round-bottomflask equippedwitha refluxcondenser.Themixturewasheatedat75◦Cunderconstant magneticstirringfor24h. Attheendofthereaction,sugarcane bagassemodifiedwithEDTA(EB)wastransferredtoasinteredglass funnelwithporosity4andwashedwithDMF,95%ethanol,distilled water,saturatedsodiumbicarbonatesolution,distilledwater,95% ethanol,andthenwithacetone.EBwasdriedat80◦Cinanoven for2h,andthencooledinadesiccatorandweighed(Karnitzetal., 2009).Afterthat,EBwassubsequentlytreatedwithaqueousspiked solutionsofCo2+_(0.2_mol/L)_and_Cr3+_(0.2_mol/L)_for₁_h,_and_then

solidandliquidfractionswereseparatedasdescribedinSection 2.2.EBtreatedwithCo2+_and_Cr3+_aqueous_solutions_was_referred

toasEB-CoandEB-Cr,respectively(Fig.2). 2.4. Biobasedcatalystscharacterization

2.4.1. Fouriertransforminfraredspectroscopy(FTIR)

Sugarcanebagasse(B),succinylatedsugarcanebagasse(SCB1) andmodiﬁedbagassewithEDTAdianhydride(EB)were charac-terizedbyFTIRspectroscopy.Infraredspectrawererecordedona NicoletImpact410FTIRspectrometerbyusingKBrdiskfrom4000 to400cm−1with32scansand4cm−1ofresolution.Thesamples werepreparedbymixing1mgofpreviousdriedB,SCB1,and/orEB with100mgofspectroscopicgradeKBrpowder.

2.4.2. Speciﬁcsurfaceareaandporesizedistribution

The speciﬁcsurface area(BET method) ofSCB2 and EBwas measured by the N2 adsorption/desorption isotherms at liquid

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Fig.1.SyntheticrouteusedtopreparethecatalystsSCB2-CoandSCB2-Cr.

nitrogen temperature (77K) on an Autosorb-Quantachromium (NOVA-1200). Thepore size distributions werecalculated from thedesorptionisothermsusingtheBarrett–Joyner–Halenda(BJH) method.

2.4.3. X-raydiffraction

X-ray diffraction (XRD) patterns of adsorbents containing adsorbedCo2+_and_Cr3+_(SCB2-Co,_EB-Co,_SCB2-Cr_and_EB-Cr)_were

recordedonaShimadzudiffractometermodelXRP-6000using Mg-ﬁlteredFeK_␣radiation(=1.9374 ˚A)generatedatavoltageof40kV andcurrentof30mAwithascanrateof2◦/minfrom7◦to70◦.

2.4.4. Thermogravimetricanalysis

Thermogravimetryanalyses(TGA)ofcatalystswerecarriedout inasimultaneousTGA-DTAequipmentmodelSDT2960(TA Instru-ments).The sampleswereheatedfrom25 to900◦C ata linear heatingrateof10◦C/minundersyntheticairatmosphereataﬂow rateof50mL/min.

2.4.5. Co2+_and_Cr3+_contents_in_the_adsorbents

TheamountofCo2+_and _Cr3+ _adsorbed_on_SCB2_and _EB_was

determined using inductively coupled plasma atomic emission spectrometry(ICP-AESSpectromodelCirosCDD).Forthis,a sam-pleof0.2500gofcatalystwasaccuratelyweighedandtransferred

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toa250mLErlenmeyerﬂask.Afterthat,10.0mLofhydrochloric acid/nitricacidaqueoussolution,3:1(v:v),preparedfrom concen-tratedacidswereadded.Thesuspensionwastransferredtoahot plateandheatedto80◦Candkeptinthistemperaturefor15min. Thesuspensionwascooledandthesolidandliquidfractionswere separatedbysingleﬁltration.TheconcentrationofCo2+_and_Cr3+

wasdeterminedbyICP-AES.

2.5. AdsorptionstudyofCo2+_and_Cr3+_on_SCB2_and_EB

TheadsorptionstudiesofCo2+_and_Cr3+_on_SCB2_and_EB_were

dividedintothreesteps:(1)adsorptionasafunctionofcontact time;(2)adsorptionasafunctionofpHofthemetalionsolution; and(3)effectofinitialmetalionconcentrationonadsorption. 2.5.1. AdsorptionstudyofCo2+_and_Cr3+_as_a_function_of_contact

time

AdsorptionexperimentsofCo2+_and_Cr3+_on_SCB2_and_EB_were

performedforthepurposeof determiningtheadsorption equi-libriumtime.100mgsamplesofSCB2and/orEBwereaccurately weighed and transferred to 250-mL Erlenmeyer ﬂasks with a 100.0mLofaqueousCo2+_and/or_Cr3+_solutions_of_known

concen-tration(190mg/LforCo2+_and₁₈₀_mg/L_for_Cr3+_)._The_Erlenmeyer

ﬂasksweremechanicallystirredfordifferentperiodsoftimeona rotaryshakerat100rpmat25◦C.ThepHofthesuspensionswas adjustedbyaddingafewdropsofaqueousNaOHand/orHCl solu-tions(0.1–5.0mol/L)inordertoachievevaluesof6.4and6.0for adsorptionofCo2+_on_SCB2_and_EB_and_3.7_and_3.2_for_adsorption

ofCr3+_on_SCB2_and_EB._At_the_end_of_each_period_of_time,_adsorbate

andadsorbentwereseparatedbysingleﬁltrationusing Whatman-41quantitativeﬁlterpaperforinstrumentalanalysis.Threealiquots of10.0mLofeachmetalionsolutionwereusedtodeterminemetal ionequilibriumconcentration.Co2+_equilibrium_{concentration}_was

determinedbydirecttitrationwithaqueousEDTAstandard solu-tion(3mmol/L)atpH10usingmurexideasanindicatorandCr3+

equilibriumconcentrationwasdeterminedbybacktitrationofan excessofaqueousEDTAstandardsolution(3mmol/L)with aque-ousMg2+_standard_solution₍₂_mmol/L)_at_pH₁₀_using_Eriochrome

BlackTasanindicator.AbuffersolutioncomprisedofNH3/NH4+

wasusedtoadjustthepHto10inthebothtitrations(Gurgeletal., 2008b).

2.5.2. AdsorptionstudyofCo2+_and_Cr3+_as_a_function_of_pH

carriedouttodeterminetheeffectofpHonthemetalionadsorption process.Samplesof100mgwereaccuratelyweighedand trans-ferredto250-mLErlenmeyerﬂaskswith100.0mLofaqueousCo2+

and/orCr3+_solutions_of_known_{concentration}₍₁₉₀_mg/L_for_Co2+

and180mg/LforCr3+_)._The_Erlenmeyer_ﬂasks_were_mechanically

stirredfor30minonarotaryshakerat100rpmat25◦C.Thecontact timeusedinthisstudywasthatobtainedfromstudyofadsorption asafunctionofcontacttime(Section2.5.1).ThepHofthe suspen-sionswasadjustedbyaddingafewdropsofaqueousNaOHand/or HClsolutions(0.1–5.0mol/L).ThepHrangeusedinthestudieswas from2.4to7.1forSCB2-Co,from1.2to6.6forEB-Co,from2.0to4.5 forSCB2-Cr,andfrom2.0to4.6forEB-Cr.Throughoutthe exper-imentsthepHwaschecked,andifanyvariationtookplace,the pHwascorrectedtotheinitialvaluechosenforeachsetof exper-iments.Themetalionequilibriumconcentrationwasdetermined asdescribedinSection2.5.1.

2.5.3. AdsorptionstudyofCo2+_and_Cr3+_as_a_function_initial

metalionconcentration–adsorptionisotherms

performedtodeterminetheeffectofinitialmetalionconcentration andtoobtaintheadsorptionisotherms.Samplesof100mgwere

accuratelyweighedandtransferredto250-mLErlenmeyerﬂasks with100.0mLof aqueousCo2+_and/or _Cr3+ _solutions _of _known

concentrationfrom120to290mg/Landfrom130to290mg/Lfor Co2+_and_SCB2/EB_adsorbent_materials,_and_from₁₀₅_to₂₈₀_mg/L

forCr3+_and_SCB2/EB_adsorbent_materials,_{respectively.}_An

equilib-riumtimeof30minwaschosenforalladsorptionsystemsbased onthestudyofadsorptionasafunctionofcontacttime.ThepHof suspensionsofSCB2-Co,SCB2-Cr,EB-Co,andEB-Crwasadjusted for6.5,5.8,5.9,and4.5,respectively.SomevariationsofthepH ofsuspensionswerecorrectedalongthetimebyaddingdropsof aqueousNaOHand/orHClsolutions(0.1–1mol/L).Theequilibrium concentrationofCo2+_and_Cr3+_was_also_determined_by_titration_as

describedinSection2.5.1.Equilibriumadsorptioncapacitieswere calculatedasfollows:

qe= (Ci−Ce

)×Vm

mads

(1) whereqe(mg/g)istheequilibriumadsorptioncapacity,CiandCe

(mg/L)initialandequilibriummetalionconcentration,Vm(L)the

volumeofthemetalionsolution,andmads(g)themassofadsorbent

material.

2.6. Freeenergyofadsorption

AccordingtoLiu (2009),theGibbsfreeenergy change(G◦) indicatesthedegreeofspontaneityofanadsorptionprocess.The G◦ofadsorptioniscalculatedasfollows:

G◦=−RTlnKa (2)

whereRisthegasconstant8.314J/Kmol,T(K)theabsolute temper-ature,andKaisthethermodynamicequilibriumconstantwithout

units.TheLangmuirequilibriumconstanthasbeenoftenusedfor calculationofG◦usingEq.(2)inadsorptionstudies(Liu,2009). However,itshouldbepointedoutthatthethermodynamic equi-libriumconstantinEq.(2)isunitless,whilsttheLangmuirconstant hasunitsofL/mol.Liu(2009)hasdemonstratedindetailthatthe relationshipbetweentheLangmuirequilibriumconstant,KL,and

thethermodynamicequilibriumconstant,Ka,canbegivenbythe

followingequation. Ka=

_K L e × (1 mol/L)

(3) whereeisafunctionoftheionicstrength(Ie)(loge=−Az2Ie1/2)

ofthesoluteatadsorptionequilibriumandthechargecarriedby thesolute(z)(Debye–Hückellaw).AccordingtoLiu(2009),inthe caseofneutraladsorbates,adsorbateswithweakchargesorfora dilutesolutionofchargedadsorbate,thethermodynamic equilib-riumconstantofadsorptioncanbereasonablyapproximatedbythe Langmuirequilibriumconstant.Inthiscase,theLangmuir equilib-riumconstantcanbeappliedfordeterminationofG◦,andEq.(3) turnsto:

G◦≈−RTln

KL×(1mol/L)

=−RTlnKL (4)

AdetailedreadingofLiu’sdiscussionisstronglyrecommended byauthorstoavoiderroneoususeofthisapproachinthecalculation ofG◦.

2.7. Catalyticevaluationofadsorbentmaterials

Incatalyticoxidationexperiments,monoterpenes␤-citronellol (1),(+)-limonene(2),and(−)-␤-pinene(3)wereused.Reactions werecarriedoutinaglassreactorequippedwithamagneticstirrer. Theglassreactorwasconnectedtoagasburetcontainingmolecular oxygentomeasurethegasuptake.Inatypicalrun,amixtureof ␤-citronellol(14mmol),dodecane(4.4mmol)andthecatalyst(0.1g,

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4000 3500 3000 2500 2000 1500 1000 500 35 40 45 50 55 60 65 70 75 80 85 B 2854 2922 1736

Transmittanc

e(%

)

Wavenu

mber

(c

m

-1

)

SCB1 1215 1653 1163 1419 1747 1734 2852 2929 2970 1739 (a) 4000 3500 3000 2500 2000 1500 1000 500 30 35 40 45 50 55 60 65 70 75 80 1736 2854 2852 2929 2970 2922 B EB

Tr

an

sm

ittan

ce

(%

)

Wavenumber (cm

-1

₎

(b) 1741 1406 1633

Fig.3. ComparativeFTIRspectraof(a)BandSCB1and(b)BandEB.

ca.6.5–12wt.%)wasintensivelystirredat80◦Candoxygen pres-sureof1atm.Thereactionswerefollowedbymeasuringtheuptake ofoxygenandbygaschromatography(GC)usingdodecaneasan internalstandard(Shimadzu2014instrument,Carbowax20M cap-illarycolumn,FIDdetector).Thereactorswereplacedinanoilbath; then,thesolutionswereintensivelystirredat60◦Cforthereported time.Structuresofproductswereconﬁrmedbygas chromatogra-phycoupledtomassspectrometry(ShimadzuQP5000instrument, DB5capillarycolumn,70eV).Uponcompletionofthereaction,the catalystwasﬁlteredoff,washedwithdistilledwater,EtOH,and reused.Tocontrolmetalleaching,thecatalystwasremovedatthe reactiontemperatureafter2handthesolutionwasallowedtoreact further.

3. Resultsanddiscussion

3.1. Biobasedcatalystscharacterization

Fig.3aandbshowsFTIRspectraforrawsugarcanebagasse(B), succinylatedbagasse(SCB1)andsugarcanebagassemodiﬁedwith EDTAdianhydride(EB).AscanbeseeninFig.3aandb,the com-parativeFTIRspectraofbothmaterialsshowabroadandintense bandat3440cm−1assignedtothevibrational(OH)modesofthe

Table1

TexturalpropertiesofSCB2andEBmaterials.

Material BETsurfacearea (m2_/g)

Totalporevolume

(cm3_g−1₎ Average_diameterpore_(Å)

SCB2 2.7 10.0 155.0

EB 1.5 4.0 108.0

hydroxylgroupspresentinthepolysaccharidefractionof sugar-cane bagasse. In Fig.3a, it is noteworthythat functionalization ofsugarcanebagassesurface isevidencedbytheappearanceof bands2970,2929,and2852cm−1,correspondingtoasymmetric andsymmetricstretchingofthemethylenegroups(␥CH2)present

insuccinylgroup(Gurgeletal.,2008a).Thebands at1419and 1215cm−1 arerelatedtooutofplanebendingvibrationsforthe carboxylicacidhydroxylgroups(␦OH).Bands1653and1655cm−1 correspondtocarbonylgroupsstretching(␥C O)alsopresentin thesuccinylgroup.InFig.3b,itshouldbementionedthatinthe comparativespectraofBandEB,theappearanceofthestrongbands at1743cm−1areattributedtoaxialdeformationofestergroups ( O C O)and thepeak observedat1631cm−1 is attributedto axialsymmetricdeformationofcarboxylategroups,bothpresent inthemoleculeofEDTAwhichischemicallybondedtosugarcane bagasse.ThesebandsconﬁrmtheintroductionofEDTAdianhydride inthesolidsupportwithconsequentreleaseofcarboxylategroups (Karnitzetal.,2009).

Table1 summarizesthe textural properties of SCB2and EB beforeadsorptionexperiments.TheirBETsurfaceareasarevery smallwithlargetotalporevolumevaluesindicatingthat function-alizationofthesugarcanematrixfortheadsorptionexperiments was effective. Moreover, it also suggests that a chemisorption phenomenonmaybeoccurbetweentheorganicmatrixandthe selectedmetallicions.

X-raydiffractionpatternsforSCB2-Co,EB-Co,SCB2-Cr,and EB-Cr(ﬁgure not shown)indicate that the crystalline structure of celluloseIwasmaintainedafterchemicalmodiﬁcationofsugarcane bagassewithsuccinicanhydrideandEDTAdianhydride.Inaddition, nodiffractionpeakscharacteristicforchromiumandcobaltphases werenoticedinthediffractograms.

Thermogravimetric curves for SCB2-Co, EB-Co, SCB2-Cr, and EB-CrareshowninSupplementaryFig.1.Ascanbeseenin Sup-plementaryFig.1a–dandinTable2,theinitialsmallweightloss, whichoccurredattemperaturesbetween49and60◦C, represent-ingapproximately10–14%ofthetotalweightlosswasattributed tothevaporizationofboundwaterinthesamples.Forallsamples, theinitialweightlosswasfollowedbyaplateauthatextendedto thestartofthemaindecomposition eventsand causingweight losses around 70–83%. According to Ouajai and Shanks(2005) andBilbaandOuensanga(1996)depolymerization,deacetylation anddehydrationofhemicellulosesanddehydrationandbreakof

Table2

ThermogravimetricdataforthermaldecompositionofSCB2-Co,SCB2-Cr,EB-Co,and EB-Cr.

Rawsugarcane bagasse

SCB2-Co SCB2-Cr EB-Co EB-Cr

TD,1(◦C) 70 55 53 60 49 M1(%) 6.3 14 10 12 10 TD,2(◦C) 320 324 318 310 284 TD,3(◦C) 376 366 349 349 334 TD,4(◦C) – 392 366 371 359 M2,3,4(%) 83 70 79 80 83 Residue(MxOy,%) 10.7 16 11 8 7 Co(%)a _– _12.6 _– _6.3 _– Cr(%)a _– _– _7.5 _– _4.8

a_In_the_calculation_of_the_percentages_of_Co_and_Cr_on_SCB2_and_EB_the_formation

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␣-and␤-aryl-alkyl-etherbondsinligninoccurredatabout320◦C. Ascanbe seenin Table2, theseconddecomposition tempera-ture(TD,2)forthesamplesSCB2-Co,EB-Co,andEB-Criscloserto

320◦C,which is ingood agreementwiththeobservationmade by Ouajaiand Shanks(2005) and Bilbaand Ouensanga (1996). OuajaiandShanks(2005)andBilbaandOuensanga(1996)also noticedthatthedecompositioneventaround∼376◦_C,_third_and

fourthdecompositionevents(TD,3andTD,4)couldbeattributedto

cellulosedecompositionandbreakingofC Cbondsbetween struc-turalunitsoflignin.Thiswouldallowforthedecarbonylationand decarboxylationreactionstotakeplaceandresultinlargeweight losses.

Supplementary data associated with this article can be found,intheonlineversion,athttp://dx.doi.org/10.1016/j.indcrop. 2013.07.047.

ChromiumandcobaltcontentsinSCB2-Cr,EB-Cr,SCB2-Co,and EB-CowerequantiﬁedbyICP-AESafteracidicdigestionofcatalysts togivevaluesof6.5%,5%,12%,and9wt%,respectively.These val-uesmayvaryaccordingtotheadsorptionexperimentperformed. Thisstudyaimsmainlytoassessthereactivityofthematerialafter theadsorptionexperiment.Theinﬂuenceofoptimizationofthe adsorptionprocessonthecatalytic activityof materialwasnot studiedinthisinvestigation,butinstead,thefeasibilityofobtaining promisingnewapplicationsforadsorbentscontainingmetalions wasthemainfocushere.

3.2. AdsorptionstudyofCo2+_and_Cr3+_on_SCB2_and_EB

3.2.1. Effectofcontacttime

Experimentstoinvestigatetheeffectofcontacttimeon adsorp-tionofCo2+_and_Cr3+_on_SCB2_and_EB_were_carried_out_to_determine

theadsorptionequilibriumtime(te).AscanbeseeninFig.4a,te

wasattainedafter5minforbothmetalionsandadsorbent materi-als.Therefore,acontacttimeof30minwasusedfortheadsorption studiesasafunctionofpH.Thistimeisenoughtohandlethose pHvariationsthatinevitablyoccurandcorrectionsweremadeto ensurethatinitialpHwassimilartoendpH.

3.2.2. EffectofpH

Oneofthemostimportantparametersthatdirectlyaffectthe adsorptionofCo2+_and_Cr3+_is_the_pH._The_dependence_of_metal

ionsuptakeonpHisrelatedtopKaofthefunctionalgroupsonthe

surfaceoftheadsorbentmaterialandtheformofthemetalionin thesolution,i.e.presenceofthespeciessuchasMn+_,_M(OH)(n−1)+_,

and M(OH)n.Solubilityproduct constants(Lide,2012)(Ksp)for

Co2+_(5.92_×₁₀−15₎_and_Cr3+_(3.0_×₁₀−29₎_were_used_to_calculate

themaximum pHat which thesemetal ionsmaynot occuras hydrolyzedspecies.Based onCo2+_and _Cr3+_{concentrations} _and

theirKsp,themaximumpHvaluesthatcanbeusedforadsorption studieswerefoundtobe8.1and5.3,respectively.SCB2isexpected tobeaweaklybasicionexchanger,whileEBisanamphotericand containsbothacidicandbasicfunctionalgroupsascanbeseenin Figs.1and2.AscanbeseeninFig.4b,atpHlowerthan3.5, func-tionalgroupsofSCB2areessentiallyprotonatedandtheadsorption capacity(q)ofCo2+_and_Cr3+_is_very_low_at_this_pH._Increasing_the

pH,thecarboxylicacidgroupsaredeprotonatedandtheadsorption capacityconsequentlyincreasedand reacheda plateaufor SCB-Co2+_._For_the_adsorption_system_SCB-Cr3+_,_increasing_pH_increased

theadsorptioncapacity,howevernoplateauwasnoticeddueto restrictionofthemaximumpHforCr3+_adsorption._As_can_be_seen

inFig.4b,atlowpH1–2,functionalgroupsofEBarepartially pro-tonatedandtheadsorptioncapacity(q)ofCo2+_and_Cr3+_is_low,_but

largerthanforSCB2atthesamepHrange.IncreasingthepH,the adsorptioncapacityisalsoincreased.ForCo2+_,_q_increases_as_pH

wasincreasedandreachedaplateau,whileforCr3+_q_increases_as

pHwasincreased,butnoplateauwasattained.Thisobservationis

0 5 10 15 20 25 30 3540 45 50 55 6065 70 75 80 85 90 95100 0 10 20 30 40 50 60 70 80 90 100 110 q (m g/ g) Time (min) SCB2-Co EB-Co SCB2-Cr EB-Cr

(a)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 0 10 20 30 40 50 60 70 80 90 100

(b)

q (m g/ g) pH SCB2-Co EB-Co SCB2-Cr EB-Cr 0 20 40 60 80 100 120 140 160 180 200 220 240 260 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

_(c)

_SCB2-Co EB-Co SCB2-Cr EB-Cr Ce /q (g /L ) C_e (mg/L)

Fig.4.StudiesofadsorptionofCo2+_and_Cr3+_on_SCB2_and_EB:_(a)_effect_of_contact

time,(b)effectofpH,and(c)effectofinitialmetalionconcentration–adsorption isotherms.

similartothatnoticedforCo2+_and_Cr3+_adsorption_on_SCB2._The

largeradsorptioncapacityofEBinrelationtoSCB2atlowpH val-uesisattributedtothechemicalpropertiesofiminoacetategroups fromEDTA,whichhavelowerpKavaluesthansuccinylgroups.The

lowpKavaluesoftheiminoacetategroupsmakeitpossibleforEB

toadsorbmetalionsatlowpHvalues,whichgivestoEBadsorbent materialspecialproperties.

(7)

Table3

LangmuirparametersforadsorptionofCo2+_and_Cr3+_on_SCB2_and_EB.

Metalion Material pH Langmuiradsorptiondata

Qmax(mg/g) Qmax(mmol/g) b(L/mg) R2 −G◦(kJ/mol) SSR

Co2+ _SCB2 _6.4 _113.12_±_3.32 _1.92_±_0.06 _0.184_±_0.101 _0.9957 _23.04_±_1.36 _0.004

EB 5.8 85.69±2.44 1.45±0.04 0.083±0.027 0.9960 21.06±0.82 0.007

Cr3+ _SCB2 _5.9 _61.92_±_6.90 _1.19_±_0.13 _0.009_±_0.002 _0.9522 _15.18_±_0.48 _0.167

EB 4.5 45.09±1.21 0.87±0.02 0.013±0.001 0.9964 16.13±0.16 0.028

3.2.3. Effectofinitialmetalionconcentrationandadsorption isotherms

Adsorption isotherms can be used to describe how adsor-batesinteractwithadsorbents.Inthissense,Langmuiradsorption isothermmodelhastraditionallybeenusedtoquantifyand con-trasttheperformanceofdifferentbiosorbentmaterials.Alinearized formofLangmuirequationisshownasfollows(Langmuir,1918): Ce qe = 1 Qmaxb+ Ce Qmax (5)

whereqe(mg/g)istheequilibriumadsorptioncapacity,Qmax(mg/g)

is themaximum amountof metalion per unit weightof SCB2 and/orEBtoformacompletemonolayercoverageonthesurface boundathighequilibriummetalionconcentration,Ce(mg/L)isthe

equilibriummetalionconcentration,andb(L/mg)istheLangmuir constantrelatedtotheafﬁnityofbindingsites.Qmaxcharacterizes

thepracticallimitingadsorptioncapacitywhenthesurfaceisfully coveredbymetalionsandbrepresentsthebondenergyrelatedto theadsorptionphenomenonbetweenmetalionsandbiosorbent materials.

AlinearizedplotofCe/qeagainstCeisobtainedfromthe

Lang-muirmodelandisshowninFig.4c.Qmaxandbwerecomputed

fromtheslopesandinterceptsofdifferentstraightlinesobtained fromlinearregressionanalysisoftheexperimentaldata(Fig.4c). Thehighcoefﬁcientsofdeterminationobtainedbyﬁtting exper-imentaldatawithLangmuirmodel indicatethattheadsorption phenomenonofCo2+_and_Cr3+_can_be_described_by_this_model.

TheresultsforCo2+_and_Cr3+_adsorption_on_SCB2_and_EB_using

LangmuirmodelarepresentedinTable3.Theseresultsarevery closetothoseobtainedbyICP-AES. Forexample, Qmax forCo2+

adsorptiononSCB2is113.1mg/g,correspondingto11.31wt%of Co2+_, _which_is _coherent_with_those_results_obtained_by_ICP-AES

(12wt%).Foralltheobtainedmaterials,EB-Cr,EB-Co,SCB2-Cr,and SCB2-Co,thevariationsbetweentheobtainedresultsfromICP-AES methodandadsorptionisothermswereinarangefrom5to10%, andthusconﬁrmingthecontentofCo2+_and_Cr3+_in_the_adsorbents.

SCB2 exhibited larger maximum adsorption capacity (Qmax)

andLangmuirconstant(b)thanEBforCo2+_and_Cr3+_adsorption.

Recapitulating,SCB2showedlargerQmaxvaluesforCo2+andCr3+

adsorptionin pHvalues closer to6, howeverSCB2 exhibited a smalleradsorptioncapacity(q)thanEBinpHvaluesfrom1to3. Therefore,SCB2maybeusedtotreatefﬂuentscontainingmetal ionsinpHvalueshigherthan3.5–4.0,whileEBmaybeusedto treatefﬂuentsinmoreacidicpHvalues.

Themajorityofthematerialstestedformetalliccationic adsorp-tionintheliteraturedonotpresentagoodcapacityofadsorption inacidicconditions(Karnitzetal.,2009).Inaddition,theresults obtainedinthisstudyshowedthatthereisthepossibilityof modu-latingtheperformanceoftheadsorbentmaterialstosuitethepHof theaqueoussystembyspeciﬁcsurfacemodiﬁcations.InacidicpH values,EDTAligandgraftedtobagasseprovidedgoodadsorption performancefortheadsorbentmaterialEB,whileinlessacidicpH, succinicanhydridegraftedtobagasseprovidedalargeradsorption capacityfortheadsorbentmaterialSCB2.Therefore,mixingboth

adsorbentmaterialscouldbeveryusefulfortreatingefﬂuentswith varyingpHrangesandthusaffordinghighlyversatilematerials.

3.3. Catalyticevaluation

Theaimofthepresentstudywastoinvestigatetheapplication ofthebiosorbentsSCB2andEBasheterogeneouscatalysts.These materialswereappliedinthecatalyticoxidationofterpenic sub-stratesas␤-citronellol(1),(+)-limonene(2)and(−)-␤-pinene(3). Citronellal(4)andcitronellolepoxide(5)arethemainproducts from_{␤-citronellol}(1) oxidation. The oxidation productsof (+)-limonene(2)arecarveol(6),carvona(7)andlimoneneepoxide(8). Pinocarveol(9),myrtenal(10)andmyrtenol(11)aretheoxidation productsof(−)-␤-pinene(3)withneitherepoxidenor correspond-ingglycolderivativesbeingdetectedinthereactionmedium.The obtainedresultsareshownindetailinTable4.

Theallylicoxidationandepoxidationproductswereobtained, despite the main mechanism being allylic oxidation according toliterature(Salesetal.,2003).Inthesereactionsafreeradical chainmechanismisusuallysuggestedacompetitionbetweenthe abstractionoftheallylichydrogentogiveallylicoxidation prod-uctsand the additionof thealkylperoxy radicalto the double bondresultinginepoxideproductsisexpected(SheldonandKochi, 1981).However,citronellal(4)isthemainproductofcitronellol oxidation correspondingto alcohol functionoxidation. Further-more,itisalsopossibletonoticethatthechromiumionadsorbed onbothmaterialspromotedthehighestconversionstocitronellal. Asexpected(Robles-Dutenhefneretal.,2009)thereisastrong effectofmonoterpenestructureontheproductnatureascanbe seeninTable4.(+)-Limonene(2)and␤-citronellol(1)yielded epox-idationproducts,whiletheoxidationproductsof(−)-␤-pinene(3) wereonlyformedbyallylicoxidation,whichalsooccurswith (+)-limonene(2).

Ablankreactionwithallsubstrates,inwhichnocatalystwas added showed virtually no activity (<10% conversion for 24h) (thesereactionsarenotshownintheTable4).Theeffectof mod-iﬁedmatricescontainingadsorbedCo2+_and_Cr3+_in_succinylated

bagasse(SCB2)andsugarcanebagassemodiﬁedwithEDTA dian-hydride(EB)wasevaluated.

The results of ␤-citronellol (1), (+)-limonene (2) and ( −)-␤-pinene (3) oxidation showed that the catalysts tested were promising foroxidation reactionsofthestudiedmonoterpenes, withconversionresultsrangingbetween49and78%andwith com-binedselectivityforoxidationproductsupto59%.Thedifferencein massbalance(Table4)wasattributedtotheformationofoligomers, whichwerenotidentiﬁedbygaschromatography.

Both chromium and cobalt promoted high conversions and selectivity values in succinylated bagasse matrix (SCB2). The adsorbentscontainingcobaltweremore activethanthose with chromium(Table4).Thedifferenceinactivitybetweencatalysts canbeattributedtodifferentoxidativemechanismsbetweencobalt andchromium.However,thehighestturnovernumber(TON) val-ues,whichconsistinthemolarratiobetweenconvertedsubstrate andmetalunderconsideration,wereobtainedforcatalystsinEBas aresultoflowermetalcontentadsorbedtothismatrix.

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Table4

Oxidationof␤-citronellol(1),(+)-limonene(2)and(−)-␤-pinene(3)catalyzedbyCoandCradsorbentsundersolvent-freeconditions.a

Substrate Catalyst Conversionb_(%) _Selectivityb_(%) _TONc

OH

(1)

O

(4)

OH O

(5)

Oxidation Epoxidation SCB2-Cr 58 23 21 125 EB-Cr 49 19 18 137 SCB2-Co 67 32 14 104 EB-Co 61 28 13 108

(2)

HO

(6)

O

(7)

O

(8)

Allylicoxidation Epoxidation

SCB2-Cr 75 15 19 13 180 EB-Cr 60 12 14 5 185 SCB2-Co 78 27 22 10 115 EB-Co 73 21 17 8 143

(3)

OH

(9)

CHO

(10)

OH

(11

)

Allylicoxidation SCB2-Cr 70 9 19 21 180 EB-Cr 58 7 16 19 192 SCB2-Co 76 9 22 27 119 EB-Co 69 8 19 22 144

a_Reaction_conditions:_0.100_g_of_catalyst,₃₅₃_K,_substrate₌₅_mL,_O

21atm,and12h. b_Determined_by_gas_{chromatography.}_The_other_products_were_not_identiﬁed. c _TON_–_mol_of_{substrate/mol}_of_Cr_or_Co.

Apreliminaryanalysisofdifferencesinresultsobtainedbythe studiedmatricesindicatedthatSCB2favorscobaltandchromium metalstopromotehigherconversionsandselectivitiesthanEB.The lowerconversionandselectivityresultsobtainedusingcatalystsin EBcanbeattributedtotheEDTAmoiety,whichisapolydentate ligandandformsastablecoordinationcomplexwiththemetalion. Forthisreason,onlyfewcoordinationsitesareavailableforthe catalyticprocess.

Inaddition,reportsontheautoxidationofanumberof monoter-penesincluding ␣-pinene(Gomes and Antunes, 1997; Lajunen, 2001; Lajunen et al., 2000; Rothenberg et al., 1998), ␤-pinene (GomesandAntunes,1997),andlimonene(GomesandAntunes, 1997)inthepresenceofcobaltcomplexeshavebeenpreviously published.Thesereportshavedemonstratedthatthesesubstrates yieldedcomplexmixturesofoxygenatedderivativeswithavery lowselectivityforallylicoxidationproducts.Furthermore, homo-geneousandheterogeneouscatalyticoxidationsofmonoterpenes (␣-pinene,␤-pinene,andlimonene)usingcobaltascatalystwere also previously studied (Gomes and Antunes, 1997; Lajunen, 2001; Lajunen et al., 2000; Robles-Dutenhefner et al., 2004; Rothenberget al., 1998). Thus, comparing the resultsreported abovewiththeresultsobtainedinthisstudyitispossibletonotice

that the oxidation of monoterpenes (␣-pinene, ␤-pinene, and limonene)alsoyieldedproductsfromallylicoxidation.However, ahigherconversionofthesubstrateswasobservedinthepresent study.

TheCoandCrcatalystscanberemovedfromthereaction mix-turebysimple centrifugationorfiltrationandwashed afterthe reactionwithamixtureofethanol/water.Therecoveredcatalysts after washing can be reused several times without a signifi-cantlossof activityand selectivity.Nosignificantleaching was observedsuggestingastronginteractionbetweenthematrixand the metallic ion, since the adsorption process occurs by com-plexationofacationbycarboxylate( COO−)andaminogroups ( NR1R2),resultingintheformationofchelateswithhigh

stabil-ity.Thisbehaviorisveryimportanttechnologically.Thus,asthe catalystdidnotreleasecobaltandchromiumtothemedium,it canbeeasilyrecoveredeitherbycentrifugationorﬁltrationand re-used.

Theuseofbiomass-based compounds,chromiumandcobalt immobilizedonasolidmatrixasthecatalysts,molecularoxygen astheﬁnaloxidantandfreesolventconditionsaresigniﬁcant prac-ticaladvantagesofthisenvironmentallyfriendlyprocessandadd valuetothemetalcontaminatedadsorbents.

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4. Conclusions

Inthis study, analternativeapplication for adsorbent mate-rialswasproposed. Tothebestof ourknowledge,this studyis thefirstcaseinwhichlignocellulosicadsorbentsareappliedina catalyticoxidationprocess.Theoxidationof␤-citronellol(1), (+)-limonene(2)and(−)-␤-pinene(3)resultedinhighlyvaluableallylic oxygenderivatives.Inaddition,theadsorbentsmaybereusedat leastthreetimesascatalystwithoutdetectableleachingand sig-nificantlossoftheiractivity.Furthermore,theadsorptionstudies alsodemonstratedthattheadsorbentscanbeusedtotreateffluents inalargepHrange,whichcanbeveryusefulinindustrialprocesses.

Acknowledgments

TheauthorsaregratefultoUniversidadeFederaldeOuroPreto, FAPEMIG,CNPq,andCAPES.AuthorswouldalsoliketothankDr. JasonG.Taylor(UFOP)forinsightfuldiscussionandforreviewing themanuscriptforitsEnglishusage.

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