Selective oxidation of benzyl alcohol to benzaldehyde in water by TiO2/Cu(II)/UV solar system

jo u r n al h om 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 e j

Selective

oxidation

of

benzyl

alcohol

to

benzaldehyde

in

water

by

TiO

2

/Cu(II)/UV

solar

system

Raffaele

Marotta

,

Ilaria

Di

Somma,

Danilo

Spasiano,

Roberto

Andreozzi,

Vincenzo

Caprio

DepartmentofChemicalEngineering,FacultyofEngineering,UniversityofNaples“FedericoII”,p.leV.Tecchio,80–80125–Naples,Italy

a

r

t

i

c

l

e

i

n

f

o

Received11February2011

Receivedinrevisedform25May2011 Accepted25May2011

Keywords: Benzylicalcohol

Selectivephotocatalyticoxidation Benzaldehydeproduction Titaniumdioxide Cu(II)reduction

a

b

s

t

r

a

c

t

Selectiveoxidationofbenzylalcoholtobenzaldehydeinaqueoussolution,underacidicconditions, throughtheTiO2/Cu(II)/solarUVphotocatalyticsystem,wasinvestigated.DifferentcommercialTiO2

samplesweretested.

Thebestresultfound,intermsofyield,wasof35%forbenzaldehydewithrespecttotheinitialbenzyl alcoholconcentration.Duringasinglerun,apartialconversionofbenzaldehydetobenzoicacidwas alsoobserved.By-products,presentattracelevel,were2-hydroxy-benzyl-alcohol, 4-hydroxy-benzyl-alcohol,2-hydroxy-benzaldehydeand4-hydroxy-benzaldehyde.Onthebasisoftheformationofthese speciesaproductionofHOradicalscouldbethusinferred.

Thestudysuggestedthatdifferentoperativeparameters,suchasthecompositionandamountof pho-tocatalyst,pH,ioniccomponentsinwaterandtheinitialconcentrationofCu(II)ions,playedanimportant roleinthephotocatalyticselectiveoxidationofbenzylalcohol.

Moreover,theresultsofthepresentinvestigationindicatedthat,attheendoftheprocess,Cu(II)could beregeneratedandreused,throughare-oxidationofCu(0),producedduringthephotolyticrun,withair inthedark.

Ageneralmechanismofoxidation,supportedbytheexperimentalresultswasproposed.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Titaniumoxide(TiO2)photocatalysisisoneofthemost

stud-iedprocessestooxidizeorganicmoleculesinaqueoussolutionby meansofasolarUVsourceandoxygen(orair)[1–3].Ithasbeen extensivelyinvestigatedthereactionmechanismthatmakesthe processoccurr.Itincludes,asthekeystep,thephotochemical for-mationofanelectron–holepaironTiO2particleswhichstartsthe

processitself[4].Titaniumoxidephotocatalystshaveagreat poten-tialalsoasaversatiletoolin“green”organicsynthesis[5].Inrecent years,TiO2photocatalysishasbeensuccessfullyproposed–

replac-ingwaterwithCH3CN–asaselectiveoxidationprocessofaromatic

alcoholstoaldehydes,withyieldsapproachingto100%,relyingon therefractorinessofthelattertotheoxidationbypositiveholes [6,7].However,sincewaterremainsanelectivesolventfor envi-ronmentallyfriendlyprocesses,otherapproachesarenecessaryin ordertoavoidtheuseoforganicsolvents.Apossibilitycouldrely ontheuseofCu(II)ionsaselectronacceptorstoreplaceoxygen inaTiO2photocatalyticprocessinwater.Theideasupportingthis

choiceisthat,intheabsenceofoxygen,itisinhibitedoneofthe photochemicalpathwaysleadingtotheunselectiveOHradical

pro-∗ Correspondingauthor.Tel.:+390817682968;fax:+390815936936. E-mailaddress:rmarotta@unina.it(R.Marotta).

duction[8,9].Infact,intheabsenceofoxygen,itisnotpossiblethe formationofsuperoxideradical(O•−₂)orhydrogenperoxide(H2O2)

whosephotolysiscangenerateOHradicals;therefore,inthiscase, OHradicalsshouldformjustasresultofthereactionofwaterwith positiveholes.Itisinterestingtonotethatadifferentapproach, basedontheuseofnanostructuredTiO2 catalysts,leadstovery

interestingresultsalthough,atthemoment,anexplanationofthem iswellbeyondtheabilityoftheAuthors[10,11].

Itiswellknownthatthesubstitutionofoxygenwithaspecies capableofreducing,bytrappingtheelectronsintheconducting band,stillenablestheoxidationoftheorganicspeciespresentin thesolution.Particularlyinterestingisthecaseinwhichoxygenis replacedbyametaliondissolvedinthesolution.Thelatterreduces toaloweroxidationstatebycapturingthephoto-generated elec-trons onTiO2,whereas theorganic species oxidizes,through a

directreactionwiththepositiveholes(h+_{)orOHradicals.Insome}

cases,thereductionofthemetalresultsintoitsprecipitationfrom thesolutionthusenablingitsseparationandrecovery.Ifthisisthe properaimoftheprocess,theroleoftheorganicspeciesisthat ofasacrificialagentwhoseoxidation makespossiblethe reduc-tionandtherecoveryofthemetal.Thereductionofmanymetals, suchasCu(II),Ni(II),Pb(II)andZn(II),hasbeeninvestigatedinthe pastbysomeresearchers[12,13].Amongtheothers,somepapers havebeendevotedtothephotoreductionofCu(II)inthepresence oforganicspeciessuchasformicoroxalicacids[14,15],methanol, 1385-8947/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved.

244 R.Marottaetal./ChemicalEngineeringJournal172 (2011) 243–249

Scheme1.Mechanism depictingthesimultaneousphotocatalyticoxidationof organicspeciesandthereductionofCu(II).

n-propanol,2-propanolandn-butylalcohol[16]inabsenceof oxy-geninthesystem.Ithasbeenreportedthatthesimpleadmissionof airintothesystem,attheendoftheprocess,allowsthere-oxidation ofprecipitatedcopperwhichis,inthisway,completelysolubilised [15].Thenatureoftheprecipitatehasneverbeencompletely elu-cidated,Cu(0) and/orCu2Obeingreported bythemost partof

researchers[12,15–19], although someof theAuthorsreported someresultsinfavouroftheformationofCu(0)[14].

Theoverallbehaviourofthesystem,includingthereoxidation ofcopper,isillustratedinScheme1.

Atwostagesoxidationprocessresultsfromthescheme:inthe firststage,thereductionofcopper(II)allowstheoxidationofthe organiccompound,inthesecondonetheairreoxidizesthereduced formofcopper.Itisevidentfromthisschemethatoncethe re-oxidationofreducedcoppertoCu(II)byairistakenintoaccount, therole ofcopper(inthepresenceofTiO2 andUV)isthatof a

catalytitcspecies(movingbetweentwo(orthree)different oxida-tionstates(0or+1and+2),whichmakespossibletheoxidationof theorganicsubstancesbyairatambientconditions.Onthebasisof thisconsideration,theuseofCu(II)/TiO2/UVasaphotocatalytic

sys-temtooxidizeorganicspeciesbyaircanbeproposed.Atthebest oftheAuthors’knowledgenopreviousinvestigationshavebeen reportedontheuseofthissystemforselectiveoxidationoforganic molecules.

Benzaldehydeisthesimplestandindustriallythemost impor-tantaromaticaldehyde.Benzaldehydeisusedinalargenumberof applications;amongtheothers,asanindustrialsolventand com-mercialfoodflavouringanditisaninterestingkeyintermediatefor variousperfumesanddyes.

Benzaldehydeisproducedprincipallybythehydrolysisof ben-zylidene chloride orthe partialoxidation of toluene [20].Both theseprocessesemployveryhardoperatingconditionsandcause wastewaterdisposalproblems.

Fattima Al-Zahra Gassima and coworkers [21] investigated thepossibilitytoconvertliquidbenzylalcoholtobenzaldehyde throughasuspensionoftitaniumdioxide(anatase)andsensitized anataseunderanoxygenatmospherebutusingp-xyleneassolvent forbenzylalcohol.

InthepresentworkthepossibilitytoemploytheCu(II)/TiO2/UV

systemfortheselectiveoxidationofbenzylalcoholto benzalde-hydeinwaterisstudiedatvaryingtheoperatingconditions(TiO2

type,TiO2load,natureoftheinorganicanions,Cu(II)concentration

andpH).

2. Materialsandmethods

Experimentshavebeencarriedoutinabatchcylindricalglass jacketedreactor(280ml)equippedwithhigh-pressureUVlamp (HeliosItalquartz)mainlyemittingat305,313and366nm.The reactorhasbeenthermostatedat298K.

ThepHhasbeenregulatedwithphosphoricacidandmonitored bymeansofanOrion420A+pH-meter(Thermo).Inallthe

exper-240 210 180 150 120 90 60 30 0 Time (min) 0.00 0.20 0.40 0.60 0.80 1.00 [Bz Alcoh o l]/[Bz Alc o hol ]o

Fig.1.Photooxydationofbenzylalcohol– effectofTiO2type.[Cu(II)]o=1.50mM.

[Benzylalcohol]o=1.50mM.pH=2.0.T=25◦C.[TiO2]o=200mg/l. Runswithout

oxygen:•Aldrich(pureanatase,SA=9.5m2_g−1_{),P25Degussa(80%anatase,}

SA=50m2_g−1_{),Aldrich(purerutile,SA=2.5m}2_g−1_{),Aldrich(prevalentlyrutile,}

SA=2.7m2_g−1_{).RunwithoxygenwithoutCu(II):Aldrich(pureanatase).}

imentsthesolutionhasbeenpreventivelypurgedwithnitrogen toremovethedissolvedoxygenthatcouldhavecompetedwith cupricionsforthereactionwiththeelectrons.Duringtherunsa gaseousstreamofnitrogenhasbeencontinuouslyfedtothe irra-diatedmagneticallystirredsolutiontopreventanycontactwith oxygen.

Theconcentrations of benzylalcohol, benzaldehyde,benzoic acid, 2-hydroxy-benzyl alcohol, 4-hydroxy-benzyl alcohol, 2-hydroxy-benzaldehydeand4-hydroxy-benzaldehydeatdifferent reactiontimeshavebeenevaluatedbyHPLCanalysis.Forthis pur-pose,theHPLCapparatus(Agilent1100)hasbeenequippedwith adiodearrayUV/Visdetector(=220,230,250nm)andasinergy 4␮Hydro-RP80A(Phenomenex)column,usingamobilephaseof 50%buffer,30%H2Oand20%CH3CN,flowingat1.0mlmin−1.One

litreofbufferhasbeenmadeby10mlofphosphoricacidsolution (5.05M),50mlofmethylalcoholandwaterforHPLC.

Theconcentrationofcupricionshasbeenmeasuredbymeans ofacolorimetricmethodusingananalyticalkit(basedonoxalic acidbis-cyclohexylidenehydrazide,cuprizone®_{)purchasedfrom}

Macherey-Nagel.AnUV/Visspectrometer(Unicam)hasbeenused forthemeasurementsatawavelengthof585nm.

Four commercial microcrystalline TiO2 powders have been

studied:(1)TiO2DegussaP25(80%anatase,20%rutile,BETspecific

surfacearea50m2_g−1_{),(2)TiO2Aldrich(pureanatasephase,BET}

specificsurfacearea9.5m2_g−1_{),(3)TiO2Aldrich(purerutilephase,}

BETspecificsurfacearea2.5m2_g−1_{),(4)TiO2Aldrich(rutilephase}

withsmallamountofanatase,BETspecificsurfacearea2.7m2_g−1_).

BETspecificsurfaceareashavebeenmeasuredbythe single-pointBETmethodusingaMicrometricsFlowSorb2020apparatus. Cu(II) ions have been introduced in the system as cupric sulphate.Benzylalcohol,benzoicacid,phosphoricacid,cupric sul-phate,sodiumsulphateandsodiumdihydrogenphosphate,with apurity>99.0%(w/w),havebeenpurchasedfromSigmaAldrich aswellasbenzaldehydewithapurity>90% (w/w)andusedas received.

Fig.2. EffectofTiO2type:benzaldehyde(a)andbenzoicacid(b)production.[Benzylalcohol]o=1.50mM.[Cu(II)]o=1.50mM.pH=2.0.T=25◦C.[TiO2]o=200mg/l.Only

strippingwithnitrogenatdark().Runswithoutoxygen:•Aldrich(pureanatase,SA=9.5m2_g−1_{),P25Degussa(80%anatase,SA=50m}2_g−1_{),Aldrich(purerutile,}

SA=2.5m2_g−1_{),Aldrich(prevalentlyrutile,SA=2.7m}2_g−1_{).RuninpresenceofoxygenwithoutCu(II):Aldrich(pureanatase).}

3. Resultsanddiscussion

Preliminaryphotolyticruns(datanot shown)inpresence of thesubstrateand Cu(II)ionswithoutTiO2 or thesubstrateand

TiO2withoutCu(II)additiontothesolutionundernitrogengaseous

stream,didnotresultintoanyconsumptionofbenzylalcoholeven forlongreactiontimes(morethan12h).

3.1. EffectofTiO2type

Theresultsobtainedduringsomerunsofphotoxidationof ben-zylalcoholatpH=2.0,withdifferentTiO2commercialsamples,at

aloadequalto200mg/l,areshowninFig.1.Thediagramsshow thatthereactivityofbenzylalcoholisstronglyinfluencedbythe typeofTiO2usedintheexperiment,whereasthespecificsurface

areacouldnotbetakenasapredictorofthesamplereactivity.In particular,thebestresultsintermsofconversion,atafixed react-ingtime,areobservedwhenAldrichTiO2(pureanatase)isusedin

therun.Inthiscase,after120minofreaction,theconcentrationof Cu(II)approachestozerowithaconversionofthealcoholofabout 75%,despiteofaninitialratio[benzylalcohol/Cu(II)]=1.

Asimilarreactivity,butwithalowerconversionofbenzyl alco-hol(65%), isshown byaP25 Degussasample inwhich anatase formis present at 80% witha specific surface area(50m2_g−1₎

higherthantheAldrichsample(pureanatase,9.5m2_g−1_).The

cat-alystsinwhichTiO2ispresentasrutileform,eitherprevalentlyor

totally,showloweractivitiesthanthesamplescontaining preva-lentlyanatase.

Theobservedresultscanbeexplainedthroughthefactthatfor bothcrystallographicformsofTiO2(anataseandrutile),thevalence

band(VB)redoxpotentialsaremorepositive(2.96and2.85Vvs NHErespectively)[27]thanthe•OH/−OHand•OH/H2Oredox

cou-ples(1.89and2.72VvsNHErespectively)[28].Consequently,both adsorbedwaterandhydroxylgroupscanbeoxidizedtoreactive hydroxylradicalsonbothirradiatedTiO2typessurfaces[1]:

TiIV−OH−+h+→TiIV−OH• (r1)

TiIV−H2O+h+→TiIV−OH·+H+ (r2)

Nevertheless the more negative redox potential(−0.27V vs NHE)oftheanataseconductionband(CB),makesitmore competi-tivethantherutileone(−0.15VvsNHE)forreductionreactions [27]. In thissense,taking intoaccount thatthe standardredox potentialofCu2+_{/Cu(0)coupleis 0.337V [9],cupricionscanbe}

reducedtometalcopperbyanataseCBelectronsmoreeasilythan byrutileCBelectrons.

Inanycase,asreportedbyothers[22,23],severalother prop-erties of the tested photocatalysts such as particle geometry, crystallinity, density of surface functional groups, and defects, shouldbeconsideredtoforeseethebehaviouroftheadoptedTiO2.

Moreover,sinceforeachCu(II)ionreducedtoCu(0),two pho-togeneratedelectronsareconsumedandtwopositiveholeshave tobesaturated,theexperimentaldatainvariablyindicatethe exis-tenceofsecondaryreactionsinwhichotherspecies,incompetition withbenzylalcohol molecules,consumethepositiveholesthus reducingtheconsumptionratio[benzylalcohol]/[Cu(II)],toavalue lowerthan1.0.

AsitcanbeinferredfromFig.2a,duringtheprocessthesubstrate ismainlyconvertedintobenzaldehydethatpartiallyundergoesto afurtheroxidationtobenzoicacid(Fig.2b).Whenthehighest con-version ofthesubstrateis achieved(72%), 35%of initialbenzyl alcoholresultedtobeconverted intobenzaldehydeandonly8% intobenzoicacidforpureanataseTiO2.

Inordertobetterunderstandtheimportanceofreplacing oxy-gen with Cu(II) ions, the data related to a run in which the alcoholiscontactedwithoxygen,withoutanyadditionofCu(II), in thepresence ofAldrichTiO2 (pureanatase) sample are

pre-sented in Figs. 1 and 2a (emptycircles). Although the system resultstobecapableofpromptlyconvertingbenzylalcohol(Fig.1), the yield in benzaldehyde is very low (Fig.2a). Similar results have been alsoobtainedwith differentTiO2 samples (datanot

shown).

Theslightdecreaseoftheconcentrationprofilesof benzalde-hydeforreactiontimeshigherthan90min(Fig.2a:fulldiamonds, full circles),when Cu(II)is totally converted toCu(0), couldbe ascribedtoitslossfromthesolutionduetoastrippingeffectof theinlet nitrogengaseous stream bubbling.Anexample of the importanceofthiseffectisgiveninthesamefigure(Fig.2a:empty squares)reporting theresults collectedby bubblinga nitrogen

246 R.Marottaetal./ChemicalEngineeringJournal172 (2011) 243–249 240 210 180 150 120 90 60 30 0 Time (min) 0.00 0.20 0.40 0.60 0.80 1.00 [C u( II )] /[C u (I I) ]o

Fig.3. Photoreductionofcopper(II)–effectofTiO2type.[Cu(II)]o=1.50mM.

[Ben-zylalcohol]o=1.50mM.pH=2.0.T=25◦_{C.[TiO2]o=200mg/l.}

Runswithoutoxygen:•Aldrich(pureanatase,SA=9.5m2_g−1_{),P25Degussa(80%}

anatase,SA=50m2_g−1_{),Aldrich(purerutile,SA=2.5m}2_g−1_),Aldrich

(preva-lentlyrutile,SA=2.7m2_g−1_).

gaseousstreaminasolutioninitiallycontainingonlybenzaldehyde (atdarkandinabsenceofTiO2andCu(II)ions).

Small amounts of undesired by-products, such as 2-hydroxy-benzyl-alcohol,4-hydroxy-benzyl-alcohol, 2-hydroxy-benzaldehydeand4-hydroxy-benzaldehyde,havebeendetected duringtherunsinthereactingsolutions,asaresultofhydroxyl rad-icals(HO._{)attacktobenzylalcoholandbenzaldehydemolecules.In}

fact,despitetheeliminationofoxygenfromthesysteminhibited HO._{formationfromH}

2O2photolysis,theproductionofhydroxyl

radicalsthroughtheadsorbedwaterand/orhydroxylgroupswith positiveholes(r1andr2)cannotberuledout.

Duringeachrun,thedecreaseoftheconcentrationofcupricions (Fig.3)isaccompaniedbytheprecipitationofapurplesolidwhich mixedwithTiO2 particles.Accordingtowhatreportedbysome

oftheAuthorsinapreviouspaper[14],thissolidcouldbeCu(0). WhenCu(II)istotallyconvertedtoCu(0),i.e.at120minforP25 DegussasampleandTiO2Aldrichpureanatase(Fig.3,fulldiamonds

andcircles),nofurtherconsumptionofbenzylalcohol(Fig.1,full diamondsandcircles)neitherproductionofbothbenzaldehydeand benzoicacidisobserved(Figs.2aandb,fulldiamondsandcircles). Thepossibilitytoreuseacertaincopperamountformoreruns hasbeendirectlyinvestigated(Fig.4).After90minofoxidationrun, thelamphasbeenswitchedoffandanoxygenstreamhasbeenfed tothereactorfor110min.Afterabout60min–theCu(II) concentra-tionreachesapproximatelythesamevalueasthatatthebeginning oftherunwhereasduringthisstepnofurtherconsumptionofthe substrateisrecorded.Atthistime,thepurplecolourdisappeared andthesolutionreturnedtobewhite.Thesystemthusobtained (at200min),hasbeenusedforanewoxidationexperimentaftera nitrogengasbubblingtopurgeoxygenandswitchingonthelamp.

3.2. EffectofinitialCu(II)concentration

Figs. 5a and b show the resultsobtained in oxidation runs at pH=2.0 with the same TiO2 sample (Aldrich, pureanatase,

200mg/l)butatdifferentinitialconcentrationsofCu(II),addedas CuSO4. 300 270 240 210 180 150 120 90 60 30 0 Time (min) 0.00 0.20 0.40 0.60 0.80 1.00 Normalized concentration

Nitrogen bubbling

Light on Oxygen bubbli Light off ng Nitrogen bubbl Light on ing

Fig.4. NormalizedconcentrationprofilesforCu(II)(circles)andbenzylalcohol (squares)withlightonandnitrogenpurgeorlightoffandoxygenpurge. [Ben-zylalcohol]o=1.50mM.[Cu(II)]o=1.16mM.TiO2(Aldrich,pureanatase)=200mg/l.

pH=2.0.T=25◦_C.

AhigherinitialconcentrationofCu(II)resultsintoadecrease ofthesystemreactivityascanbeeasilyverifiedbycomparingthe half-lifetimeforthesubstratewhichchangedfrom50to90minfor [Cu(II)]oequalto1.12mMand2.30mMrespectively.Moreover,for

bothruns,theselectivitiestobenzaldehyde,evaluatedat15min (72%)and90min(57%),donotchangesignificantlyfromthe val-uescalculatedbyusingthedatashowninFigs.1and2a.Apossible explanationforthesefindingscouldbefoundsupposingachange ofthelightabsorptionpropertiesofthereactingsolutions,withan increasinginnerfiltereffectatincreasingCu(II)initial concentra-tions.UVabsorptionmeasurementshavebeenthusperformedon thereactingsolutionsinordertoevaluatethecapabilityofcupric solutionstoabsorbthelampradiationattheadoptedwavelengths (305, 313and 366nm).However, thevalues, estimated forthe molarextinctioncoefficientsofcupricaquocomplexesatpH=2.0, allowedtoruleoutthepossibilityofexistenceofanyinnerfilter effectduetothesespecies(datanotreported).

Thesearchofanexplanationoftheobservedreducedreactivity oftheoxidationofthealcoholatincreasingCu(II)concentrations revealedsomedifficultieswhichforcedtheattentiononthefact thatfortherunsconsidereda parallelincreaseofsulphateions hadtobetakenintoaccount.Thatis,sincethesaltusedtoprepare thesolutionsiscupricsulphate,itisevidentthatanyincreaseof Cu(II)resultedintooneofsulphatesandthatithasbeennecessary tounderstandtheeffectonthesystemreactivityexertedbythese species.

3.3. Effectofinitialsulphateconcentration

Somephotocatalytictestshavebeenthuscarriedoutvaryingthe initialsulphateconcentrationwithdifferentadditionsofNa2SO4

salt(Fig.6).

Theresultsobtainedintheseexperimentsindicatethatsulphate ionsexertamarkedinhibitingeffectonthephotoactivityofTiO2,by

decreasingtheoxidationrateofbenzylalcoholanditsconversion atincreasingtheinitialsulphateconcentration(fullsymbols).

Thisbehaviourcanbeascribed,asreportedbyothers[24,29,30], bothforthereactionbetweensulphatespeciesandthepositive

180 150 120 90 60 30 0 Time (min) 0.00 0.10 0.20 [Benzald ehyd e ]/ [Benzyl alco 180 150 120 90 60 30 0 Time (min) 0.00 0.20 0.40 0.60 [Bz Al cohol ]/ [Bz Alc ohol

Fig.5. EffectofinitialCu(II)concentration:Benzylalcoholconsumption(a)andbenzaldehydeproduction(b).TiO2(Aldrich,pureanatase)=200mg/l,pH=2.0,T=25◦C.

[Benzylalcohol]o=1.50mM.[Cu(II)]o:•1.12mM,1.42mM,1.84mM,2.30mM.

holes(h+_{)withtheformationofsulphateionradicals(SO}− 4•)onthe

illuminatedtitaniumdioxidesurface:

SO2₄−+h+→SO−₄• (r3)

andforaradicalscavengingeffectofsulphateions:

SO2₄−+HO•→SO−₄•+HO− (r4)

SO−₄•speciesarereportedtobelessreactivethanthehydroxyl radicalstowardsorganicmolecules[24].

Inanycase,asreportedbyAbdullahandcoworkers[31],a cat-alystdeactivation,bytheadsorbedsulphateionswhichcanblock theTiO2activesites,cannotberuledout.

Theselectivitytobenzaldehydewhichincreasedatincreasing sulphateconcentration(emptysymbols)confirmsthecapabilityof sulphateionstoscavengeveryreactiveandunselectivehydroxyl radicals. 150 120 90 60 30 0 Time (min) 0.00 0.20 0.40 0.60 0.80 1.00 [Bz Alcoho l]/[ Bz Alco hol]o 0.00 0.20 0.40 0.60 0.80 1.00 [B en zald e hyde ]/[Bz A lc oh ol ]o

Fig. 6. Effect of initial sulphate concentration: [Benzyl alcohol]o=1.50mM.

[Cu(II)]o=1.15mM.

TiO2(Aldrich, pureanatase)=200mg/l. pH=2.0.T=25◦C.Fullsymbols: benzyl

alcoholconsumption,emptysymbols:benzaldehydeproduction.[SO2−₄ ]o:(•,)

1.15mM;(,)1.90mM;(,)2.40mM.

Therefore,theresultsreportedinFig.5couldnotbeattributed onlytoaneffectofCu(II)concentrationbutalsotoacombination ofthelatterandtheconcentrationofsulphateions.Inparticular, thenegativeeffectofsulphateonthesystemreactivityprevailed overtheeffectexertedbyCu(II).

3.4. EffectofTiO2load

The effect of initial TiO2 loadwas successively investigated

bycarrying out someoxidation experimentsin which different amountsofthephotocatalystperlitrewereaddedtothereacting solutions(Figs.7aandb).

Asexpected,anincreaseofthecatalystloadfrom55mg/l to 200mg/lresultsintoamarkedincreaseofthesystemreactivity withthehalf-lifetimeforthesubstraterangingrespectivelyfrom 240minto60min.AnincreaseofTiO2load,forvalueshigherthan

200mg/l,doesnotleadtoanincreaseofsystemreactivityprobably duetoscatteringandscreeningofradiationbytheexcessparticles, whichmaskpartofthephotosensitivesurface[26,32].

3.5. EffectofpH

Fig.8aandbreporttheresultsobtainedvaryingthepHofthe solution,intherange2.0–4.0.Duringthetests,increasingthepH from2.0to4.0,adecreaseofbenzylalcoholconsumptionand ben-zaldehydeformationratesoccurred.Theseresultshavetwomain causes.Firstofall,TiO2surfacemaybecharacterizedbyan

ampho-tericcharacter,thatiseitherpositiveornegativechargecouldbe presentonitasfunctionofpH:

≡TiOH+₂  ≡TiOH+H+ (r5)

≡TiOH ≡TiO−+H+ (r6)

ThepH’svalueatwhichTiO2hasanetzerosurfacechargeis

definedpHzpc.ThesurfacehasanetpositivechargeforpH<pHzpc,

whereasforpH>pHzpc,thesurfacehasanetnegativecharge.

Itcouldbesupposed,accordingtothepHzpcequalto4.2forthe

adoptedTiO2Aldrichsample[22],thatanincreaseofthepHfrom

2.0to4.0reducestheconcentrationofpositivechargesonthe cat-alystsurfacepartiallyinhibitingtheadsorption(andthereactivity) ofbenzylalcohol(pKa=15.2[25]).

248 R.Marottaetal./ChemicalEngineeringJournal172 (2011) 243–249 300 270 240 210 180 150 120 90 60 30 0 Time (min) 0.00 0.20 0.40 0.60 0.80 1.00 [B z Alcohol]/[B z Alcoho l]o

(a)

(b)

Fig.7. EffectofinitialTiO2load.Benzylalcoholconsumption(a)andbenzaldehydeproduction(b).[Benzylalcohol]o=1.50mM.[Cu(II)]o=1.50mM.pH=2.0.T=25◦C.TiO2

(Aldrich,pureanatase):•55mg/l,100mg/l,150mg/l,200mg/l.

Ontheotherhand,sincephosphoricacidhasbeenusedtoadjust thepHofthereactingsolutions,inthepHrange2.0–4.0,the preva-lentspeciespresentisH2PO−4.SincethecapabilityofH2PO−4 ionto

reactwiththepositiveholesformedonthecatalystafterradiation iswellknown[23]:

H2PO−₄+h+→H2PO4• (r7)

itisclearthatforanyincreaseofpHresultingintoahigher concen-trationofthision,amarkedinhibitionofthedirectoxidationofthe substratecouldbeexpected.Inordertoconfirmthesehypothesis, somefurtherphoto-oxidationtestshavebeenperformed,withand withoutanadditionofNaH2PO4(2.44mM)salt,onbenzylalcohol

solutions(1.60mM),atpH=2.0(regulatedwithH3PO4),inthe

pres-enceofTiO2(200mg/l,Aldrich,pureanatase)andaninitial[Cu(II)]

equalto1.15mM(datanotshown).After120minofreaction,when Cu(II)hasbeencompletelyreducedtoCu(0),benzylalcoholtotal

conversionsof55.0%(withoutNaH2PO4)and45.8%(withNaH2PO4)

havebeenachievedrespectively.Moreover,thepercentageofthe substrateconvertedintobenzaldehydehasbeen25.7%(without NaH2PO4)and32.7%(withNaH2PO4),thusindicatingacapability

ofdihydrogenphosphateionstobehaveasaradicalscavenging towardshydroxylradicals[23]:

H2PO−4 +HO•→H2PO4•+HO− (r8)

Startingfrompreviouslydiscusseddata,collectedatdifferent experimentalconditions,thefollowingsimplifiedpictorialscheme ofthemechanismfortheselectivephotooxidationofbenzylalcohol tobenzaldehydeandbenzoicacidbyTiO2photocatalysiscouldbe

depicted(Scheme2).

Theirradiationofthephotocatalyticsurfaceleadstothe forma-tionofpositiveholes(h+_{)inthevalenceband(VB)andelectrons}

(e−)in theconductionband(CB).Firstof all,thepositive holes

180 150 120 90 60 30 0 Time (min) 0.00 0.20 0.40 0.60 0.80 1.00 [Bz Alcoho l]/[Bz Alcohol]o

(a)

(b)

Fig.8. EffectofpH.Benzylalcoholconsumption(a)andbenzaldehydeproduction(b).[Benzylalcohol]o=1.50mM.[Cu(II)]o=1.50mM.TiO2(Aldrich,pureanatase)=200mg/l.

Scheme2.MechanismofselectiveoxidationofbenzylalcoholbyTiO2/Cu(II)/UV.

reactwith benzyl alcohol (substrate) and benzaldehyde (inter-mediate)toproducebenzaldehydeandbenzoicacidrespectively. Surfacehydroxylradicalsarealsoformedbythereactionofwater moleculeswiththe holes.Hydroxyl radicals attackboth benzyl alcoholandbenzaldehydetoformundesiredby-products(2-and 4-hydroxybenzylalcohols,2-and4-hydroxybenzaldehyde).Finally, theholes canbetrappedbysulphateanddihydrogenphosphate ionstogeneratelessreactiveSO−₄•andH2PO4•species.The

elec-trons in the conduction band reactwith Cu(II) ions which are reducedtoCu(I)andCu(0).

4. Conclusion

Thepossibility tooxidizebenzylalcohol to benzaldehydein aqueous solution, under acidic conditions, using the photocat-alyticsystemTiO2/Cu(II)/solarUVhasbeenstudiedinthepresent

work.FoursamplesofTiO2 characterizedbydifferent

crystallo-graphicformsand specificsurfaceareashavebeenusedduring the experiments. The best result found was a yield of 35% of benzaldehyde with respect of the initial benzyl alcohol. Ben-zaldehydehasbeenalsopartiallyconvertedtobenzoicacid.The presence of undesired by-products, such as 2-hydroxy-benzyl-alcohol,4-hydroxy-benzyl-alcohol,2-hydroxy-benzaldehydeand 4-hydroxy-benzaldehyde,hasbeendemonstratedandindicatedan activeproductionofsurfaceHOradicals.Attheendoftheprocess copper(II)wastotallyreducedtocopper(0),whichcouldeasilybe reoxidizedtoCu(II),inthedark,inthepresenceofoxygen.

The effect of the catalyst load, the nature of the inorganic anions(SO2−₄ ,H2PO4−),theinitialCu(II)concentrationandpHof

the solution has been also investigated. The sulphate and di-hydrogen-phosphateanionsresultedtoexertanegativeeffecton thephotooxidationratesofbenzylalcoholandtobehaveas scav-engerstowardssurfaceHOradicals.Adecreaseofbenzylalcohol oxidationandbenzaldehydeformationrateshasbeenobservedby increasingthepHfrom2.0to4.0.

Acknowledgements

TheAuthorsaregratefultoIng.LucaMicoliforhisassistanceon BETmeasurements.

References

[1] K.Kabra,R.Chaudhary,R.L.Sawhney,Treatmentofhazardousorganicand inor-ganiccompoundsthroughaqueous-phasephotocatalysis:areview,Ind.Eng. Chem.Res.43(2004)7683–7696.

photocatalysts,J.Photochem.Photobiol.C:Photochem.Rev.9(2008)157–170. [6]S.Higashimoto,N.Kitao,N.Yoshida,T.Sakura,M.Azuma,H.Ohue,Y.Sakata, Selectivephotocatalyticoxidationofbenzylalcoholanditsderivativesinto cor-respondingaldehydesbymolecularoxygenontitaniumdioxideundervisible lightirradiation,J.Catal.266(2)(2009)279–285.

[7]S.Higashimoto,N.Suetsugu,M.Azuma,H.Ohue,Y.Sakata,Efficientand selec-tiveoxidationofbenzylicalcoholbyO2intocorrespondingaldehydesonaTiO2

photocatalystundervisiblelightirradiation:Effectofphenyl-ringsubstitution onthephotocatalyticactivity,J.Catal.274(2010)76–83.

[8]R.Gao,A.Safrany,J.Rabani,ReactionsofTiO2excesselectroninnanocrystallite

aqueoussolutionsstudiedinpulseandgamma-radiolyticsystems,Radiat.Phys. Chem.67(2003)25–39.

[9] M.I.Litter,Heterogeneousphotocatalysistransitionmetalionsin photocat-alyticsystems,Appl.Catal.B:Environ.23(1999)89–114.

[10] G.Palmisano,S.Yurdakal,V.Augugliaro,V.Loddo,L.Palmisano,Photocatalytic selectiveoxidationof4-methoxybenzylalcoholtoaldehydeinaqueous sus-pensionofhome-preparedtitaniumdioxidecatalyst,Adv.Synth.Catal.349 (2007)964–970.

[11]V.Augugliaro,T.Caronna,V.Loddo,G.Marcì,L.Palmisano,S.Yurdakal, Oxi-dationofaromaticalcoholsinirradiatedaqueoussuspensionsofcommercial andhome-preparedrutileTiO2:aselectivitystudy,Chem.Eur.J.14(2008)

4640–4646.

[12]S.W.Zou,C.W.How,J.P.Chen,Photocatalytictreatmentofwastewater contam-inatedwithorganicwasteandcopperionfromsemiconductorindustry,Ind. Eng.Chem.Res.46(2007)6566–6571.

[13]K.Kabra,R.Chaudhary,R.L.Sawhney,EffectofpHonsolarphotocatalytic reduc-tionanddepositionofCu(II),Ni(II),Pb(II)andZn(II):speciationmodellingand reactionkinetics,J.Hazard.Mater.149(2007)680–685.

[14] M.Canterino,I.DiSomma,R.Marotta,R.Andreozzi,Kineticinvestigationof Cu(II)ionsphotoreductioninpresenceoftitaniumdioxideandformicacid, WaterRes.42(2008)4498–4506.

[15]N.S.Foster,R.D.Noble,C.A.Koval,Reversiblephotoreductivedepositionand oxidativedissolutionofcopperionsintitaniumdioxideaqueoussuspensions, Environ.Sci.Technol.27(2)(1993)350–356.

[16]N.S.Foster,A.N.Lancaster,R.D.Noble,C.A.Koval,Theeffectoforganichole scavengeronthephotodepositionofcopperintitaniumdioxidesuspensions, Ind.Eng.Chem.Res.34(1995)3865–3871.

[17] J.W.M.Jacobs,F.W.H.Kampers,J.M.G.Rikken,C.W.T.Bulle-Lieuwma,D.C. Kon-ingsberger,CopperphotodepositiononTiO2studiedwithHrEMandEXAFS,J.

Electrochem.Soc.136(1989)2914–2923.

[18] M.Bideau,B.Claudel,L.Faure,M.Rachimoellah,Photooxidationofformicacid byoxygeninthepresenceoftitaniumdioxideanddissolvedcopperions: oxy-gentransferandreactionkinetics,Chem.Eng.Commun.93(1990)167–179. [19]S.Morishita,PhotoelectrochemicaldepositionofcopperonTiO2particles.

Gen-erationofcopperpatternswithoutphotoresists,Chem.Lett.(1992)1979–1982. [20] Ullmann’sEncyclopediaofIndustrialChemistry,sixthed.Wiley-VCH,

Wein-heim,2005.

[21]G.FattimaAl-ZahraGassima,N.AhmedAlkhateebb,H.FalahHu,Photocatalytic oxidationofbenzylalcoholusingpureandsensitizedanatase,Desalination209 (2007)342–349.

[22]J.Ryu,W.Choi,Substrate-specificphotocatalyticactivitiesofTiO2and

multi-activitytestforwatertreatmentapplication,Environ.Sci.Technol.42(2008) 294–300.

[23]S.Ahmed,M.G.Rasul,N.MartensWayde,R.Brown,M.A.Hashib,Heterogeneous photocatalyticdegradationofphenolsinwastewater:areviewoncurrent sta-tusanddevelopments,Desalination261(2010)3–18.

[24] C.Hu,T.Yuchao,L.Lanyu,H.Zhengping,W.Yizhong,T.Hongxiao,Effectsof inorganicanionsonphotoactivityofvariousphotocatalystsunderdifferent conditions,J.Chem.Technol.Biotechnol.79(2004)247–252.

[25]F.M.Menger,M.Ladika,Originofrateaccelerationsinanenzymemodel:the p-nitrophenylestersyndrome,J.Am.Chem.Soc.109(1987)3145–3146. [26]J.M.Hermann,Heterogeneousphotocatalysis:stateoftheartandpresent

appli-cations,Top.Catal.34(1–4)(2005)49–65.

[27]R.Long,Y.Dai,B.Huang,Structuralandelectronicpropertiesofiodine-doped anataseandrutileTiO2,Comput.Mater.Sci.45(2009)223–228.

[28]H.A.Schwarz,R.W.Dodsonj,Equilibriumbetweenhydroxylradicalsand thal-lium(II)andtheoxidationpotential ofOH(aq), J. Phys.Chem.88(1984) 3643–3647.

[29]O.Carp,C.L.Huisman,A.Reller,Photoinducedreactivityoftitaniumdioxide, Prog.SolidStateChem.32(2004)33–177.

[30]M.Ziegmann,T.Doll,F.H.Frimmel,Matrixeffectsonthephotocatalytical degra-dationofdichloroaceticacidandatrazineinwater,ActaHydrochim.Hydrobiol. 34(2006)146–154.

[31]M.Abdullah,G.K.C.Low,R.W.Matthews,Effectsofcommoninorganicanionson ratesofphotocatalyticoxidationoforganiccarbonoverilluminatedtitanium dioxide,J.Phys.Chem.94(1990)6820–6825.

[32] D.Vione,C.Minero,V.Maurino,M.E.Carlotti,T.Picatonotto,E.Pelizzetti, DegradationofphenolandbenzoicacidinthepresenceofaTiO2-based