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Lilis

Hermida

a,b

_,

_Ahmad

_Zuhairi

_Abdullah

a,∗

_,

_Abdul

_Rahman

_Mohamed

a

a_School_of_Chemical_Engineering,_Universiti_Sains_Malaysia,₁₄₃₀₀_Nibong_Tebal,_Penang,_Malaysia b_Department_of_Chemical_Engineering,_Universitas_Lampung,_Bandar_Lampung_35145,_Lampung,_Indonesia

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received11May2011 Receivedinrevisedform 12September2011 Accepted14September2011

Keywords: HSO3SBA-15

Postsynthesisgrafting Glycerolmonolaurate Reusablecatalyst Reactionmechanism Kinetic

a

b

s

t

r

a

c

t

Monoglyceridesynthesisthroughglycerolesterificationwithlauricacidoverapropylsulfonicacid functionalizedSBA-15mesoporouscatalyst(HSO3SBA-15)wasstudiedunderreducedpressureto

con-tinuouslyremovewaterformed.Effectsofvariousreactionparameterssuchasreactiontemperature (413–433K),catalystloading(1–5%)andglycerol/lauricacidmolarratio(2:1and4:1)onlauricacid conversionandproductsselectivityweresuccessfullyelucidatedandcorrelatedwithreactionscheme andkineticmodel.TheparentandfunctionalizedSBA-15werecharacterizedusingFT-IR,ammonia chemisorptions,surfaceanalysis,TEMandSEM.Reusabilitybehaviorofthecatalystwasalso demon-strated.Glycerolesterificationcouldbemodeledasirreversibleparallelreactionsandthekineticdata weresuccessfullyfittedtoasecondorderkineticmodel.Theapparentactivationenergyfor monoglyc-erideformationusingacatalystloadingof5%andglycerol/lauricacidmolarratioof4:1wasfoundtobe 42kJ/mol.TheHSO3SBA-15catalystcouldbereuseduptofourtimeswithoutsignificantlossofcatalytic

activity.

1. Introduction

Monoglyceridehasvariousapplicationsinfoodproducts, cos-metics,pharmaceutical formulations,drug deliverysystems, oil well drilling operations, etc. [1,2]. Synthesis of monoglyceride throughglyceroltransesterificationwitholeicacidmethylester usingsolidbasiccatalystsinsolventfreeconditionwasfoundto befavourableatanelevatedtemperature(473K)overrehydrated Al–Mg mixed oxides and Al–Li mixed oxides catalyst that had BronstedbasicsitesandLewisbasicsites,respectively[3]. Mean-while, glyceroltransesterificationwith lauric acidmethyl ester overmontmorilloniteintercalatedwithlithiumhydroxide(LiK10) catalystandinthepresenceoftetramethylammoniumhydroxide (TMAOH)wasrecentlyreportedtoshowexcellentperformance atarelativelylowertemperature(403K)[4].However,theuseof organicsolventwillresultinanextrastepforitsseparationduring productpurificationprocess.Residualsolventcontentinthe prod-uctwillalsolimittheapplicationofthemonoglyceride,especially whenfoodproductsareconsidered[5–7].Glycerolesterifications withfattyacidsatlowtemperatureinsolventfreeconditionusing solidacidcatalystssuchasacidicresins,conventionalzeolites,and sulfatedironoxidewerereportedtobeunfavourableforthe reac-tionduetosmallporediametersoflessthan8 ˚A[8,9].

∗_{Corresponding}_author._Tel.:₊₆₀₄₅₉₉_6411;_fax:₊₆₀₄₅₉₄_1013.

E-mailaddresses:chzuhairi@eng.usm.my,azuhairi@yahoo.com(A.Z.Abdullah).

Toacceleratethereactioninvolvingbulkymoleculessuchas fattyacids,effectivesolidacidshavingporesizesbetween20and 100 ˚Aarerequired[10,11].HSO3SBA-15catalystwithaporesize of67 ˚Asynthesizedviapostsynthesisroutewasfoundtobean activesolidacidcatalystformonoglycerideformationthroughthe glycerolesterificationwithlauricacid[12].However,nosufficient informationhasbeenreportedontheinfluenceofimportant pro-cessvariables,reactionpathwayandkineticmodelontheproduct formationsintheesterificationprocess.Assuch,astudyonthese aspectsisdeemednecessaryforthepurposeofadvancementof knowledgeinthisresearcharea.

Reaction pathway is essential to beused for constructing a kineticmodelthatcanbeappliedtopredictthechangesinamount of specificproductobtainedduring areaction process.In order toobtainabetterunderstandingofthereactionpathway,several researchershaveundertakenvariousstudiesonglycerol esterifica-tionwithdifferentfattyacidsandsolidacidcatalyststoproduce monoglycerides[9,13–15].Basedontheirobservations,itcanbe concludedthatthereactionpathwaysofglycerolesterificationwith fattyacidcouldbemodeledeitherasconsecutivereactions(Fig.1) followingthefirstorderkineticmodelwithrespecttoglyceroloras irreversibleparallelreactions(Fig.2)followingsecondorderkinetic modelwithrespecttofattyacidandglycerol.Thereactionorders reportedintheopenliteraturescanbesummarizedinTable1.

Inthepresentwork,glycerolesterificationwithlauricacidover theHSO3SBA-15isreported.Themainfocusistoinvestigatethe effectoftemperature,glycerol/lauricacidmolarratioandcatalyst

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Fig.1.Consecutivereactionsoftheglycerolesterification:(a)asproposedbySzelagandZwierzykowski[13]and,(b)asproposedbyMacierzancaandSzelag[14].

Fig.2. Irreversibleparallelreactionsforglycerolesterificationwithfattyacidas adoptedfromSanchezetal.[15].

loadingontheproductsselectivitytogainbetterunderstandingof reactionpathwaytoeventuallybuildakineticmodelfor mono-glycerideformation.The workalso includesmodel verification, determinationoftheapparentactivationenergyofthereactionand reusabilitystudyofthecatalyst.

2. Experimental

2.1. Catalystpreparationandcharacterization

SBA-15mesoporousmaterialwassynthesizedinthepresence ofPluronicP123 which isa triblock co-polymer witha molec-ularformula of HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H

andamolecularweightof5800asthestructure-directingagent. Afterthat,theSBA-15mesoporousmaterialwasfunctinonalized withpropylsulfonicacidbyrefluxing2gofSBA-15materialswith 2mLof3-mercaptopropyltrimethoxysilane(MPTMS).Then, soxh-letextractionwascarriedoutfor20h,followedbymildoxidation usingH2O2toformHSO3SBA-15.Thepreparationmethodsof SBA-15materialandHSO3SBA-15catalystwerethoroughlydescribedin apreviouslypublishedarticle[15].TheparentSBA-15mesoporous materialandHSO3SBA-15catalystwerethencharacterizedusing FT-IRspectroscopy,pulsechemisorptionfollowedbytemperature programmeddesorption(TPD)ofNH3,surfaceareaanalysis,TEM analysisandSEManalysis.Meanwhile,Fouriertransforminfrared (FT-IR)spectrumwasusedtodetectthepresenceoforganosulfonic acidgroupwhich wasgrafted onSBA-15mesoporous material. TheFT-IRspectroscopywasperformedusingaPerkinElmer2000 Fouriertransformedinfrared(FT-IR)system.

Acidity characterization of the catalysts involved pulse chemisorption of ammonia followed by its temperature-programmed desorption (TPD) using micromeritics Autochem II2920instrument.Thesamplewasfirstactivatedbyheatingto 150◦_C_in_helium_for₂_h,_and_then_cooled_down_to₁₂₇◦_C._During theammoniachemisorptionstep,5injectionsofammoniawere dosedontothesample(toensurethesamplewassaturated).After saturation,thetemperaturewassubsequentlyincreasedfrom127 to427◦_C_to_perform_the_TPD_of_NH

3analysis.

Pore size distribution was obtained from N2

adsorption–desorption isotherms of the prepared SBA-15 and HSO3SBA-15 samples. The N2 adsorption–desorption mea-surements were performed using a Quantachrome Autosorb-1 equipment.Priortotheexperiment,thesamplesweredegassed (P<10−1_Pa)_at₂₇₀◦_C_for₆_h._Surface_area_was_calculated_using_the BETmethod(SBET)andporesizedistributionwasdeterminedusing theBarrett–Joyner–Halenda(BJH)modelappliedtothedesorption branchoftheisotherm.Themicroporearea(S␮)wasestimated usingthecorrelationoft-HarkinsandJura(t-plotmethod).

TheTEMimageswereobtainedusingPhilipsCM12 transmis-sion electron microscope.The sample of about 0.08gwas first dispersedin5mlethanol.Then,thesolutionwasvigorouslyshaken forawhile.Subsequently,asmallamountofthesolutionwastaken

Table1

Summaryofpublishedkineticmodelsforglycerolesterificationwithfattyacidsreactionovervarioussolidacidcatalysts.

Typeofcatalysts Fattyacids(FA) G/FAa_molar_ratio _Reaction_pathways _Rate_reaction_expressionb _Refs.

NaorKsoaps LauricacidMyristicacidPalmiticacidStearicacid 1:1 Consecutivereactions −rG=k1CGrMono=k1CG−k2CMono [13]

Zinccarboxylates LauricacidMyristicacidPalmiticacidStearicacid 1:1 Consecutivereactions −rG=k1CGrMono=k1CG−k2CMono [14]

Sulfatedironoxide Oleicacid 1:1 Parallelreactions −rFA=−(1/W)(dCFA/dt)=k1CGCFA [9]

Zeolite(faujasite) Oleicacid 1:3,1:1and3:1 Parallelreactions −rFA=−(1/W)(dCFA/dt)=k1CGCFA [15] a_G/FA₌_{glycerol/fatty}_acid_molar_ratio.

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2.2. Esterificationofglycerolwithlauricacid

Theglycerolesterificationprocesseswereusuallycarriedout undercontinuouswaterremoval.Waterremovalduring esterifica-tionisneededasitisformedasasideproductandwillinhibitthe activityofasolidacidcatalyst.Itcanalsopromotereversereaction. Besides,esterificationisanequilibriumlimitedreactioninwhich fullconversioncanonlybeachievedwhenoneoftheproductsis removed[16].Inthisstudy,esterificationofglycerolwithlauricacid usingdifferentHSO3SBA-15catalystloadings,lauricacid/glycerols molarratiosandreactiontemperatureswascarriedoutinabatch systemwithcontinuousremoval ofwater byworking ata low pressureof50.8cmHgandwithouttheuseofanysolvent.

Typically,theesterificationwascarriedoutinatwo-neckedflask reactorequippedwithastirrer,athermometer,atubeconnected withavacuumpumpandatubeforsampling.Thereactantsi.e. lauricacid,glycerolaswellasthecatalystwerefirstaddedtothe reactor.Beforeanyexperimentwasstarted,thesystemwasflushed withnitrogentocreateinertenvironment.Thereactionmixture wasthenheatedtoacertaintemperatureunderareduced pres-sureof50.8cmHg.Stirringat750rpmwasthenstartedandthe reactantswerestirredforupto7h.Summaryoftheexperimental conditionsisgiveninTable2.

2.3. Reusabilitystudy

InthereusabilitystudyoftheHSO3SBA-15catalyst,5wt.%of freshcatalyst withrespect totheinitialreactantswasusedfor thereactionperformedat433Kfor6handtheglycerol/lauricacid molarratiowassetat4:1.Aftertheexperiment,thecatalystwas filteredfromthecatalyticreactionmixture,successivelywashed withtolueneandacetoneandthendriedinanovenat373K.Then, thecatalystwasreusedforthesubsequentesterificationrunsusing thesamereactionconditions.Suchreusabilityexperimentswere repeatedfor4timesandtheactivitywasmeasuredbasedon con-versionoflauricacidandselectivitytomonoglyceride.

2.4. Productanalysis

Mono-, di- and triglycerides were analyzed based on gas chromatographymethodusingAgilent 7820Aand witha capil-larycolumn(15m×0.32mm×0.10␮mCPSil5CB).Theanalysis methodwasadoptedfroma publishedwork[17].Thedetector andinjector temperatureswere380◦_C _and ₂₅₀◦_C,_{respectively.} Thecolumntemperaturewassetat80◦_C_for₁_min_and_was_then programmedat15◦_C/min_to₃₃₀◦_C,_which_was_then_maintained constantfor2min.100␮Lofsamplewasthentransferredfromthe reactorintoasamplevialcontaining100␮Lofwaterand100␮Lof methylacetate.Next,themixturewasvortexedandtheorganic phasewasseparatedby meansof centrifugation.20␮L organic phasewasthendissolvedin480␮Lacetoneand100␮Lof0.2M pentadecanoicacidastheinternalstandard.Thesamplewasthen

Fig.3.NH3temperature-programmeddesorptionprofileofHSO3SBA-15catalyst.

directlyinjectedintothegaschromatograph.Conversionwas cal-culatedbasedontheamountoflauricacidconvertedinthereaction leadingtotheformationofmono-,di-,andtriglycerides. Mean-while,theselectivityofmonoglyceride,diglycerideandtriglyceride wereexpressedbasedontheratiooftheestertoallthereaction products(correctedbythereactioncoefficients),asgiveninEqs. (1),(2)and(3)[7];

SM(%)=

CMonoglyceride

CMonoglyceride+2CDiglyceride+3CTriglyceride

×100 (1)

SD(%)=

2CDiglyceride

×100 (2)

ST(%)=

3CTriglyceride

×100 (3)

3. Resultsanddiscussion

3.1. CharacterizationoftheSBA-15andHSO3SBA-15catalysts

TheevidencethatthefunctionalizedSBA-15materialindeed containedpropylsulfonicgroupwasprovidedbyFT-IRspectrum analysisasexplainedindetailinourarticle[12].Sulfonicacid sili-casareaclassofsolidBronstedacidcatalystthatcontainstethered organo-sulfonic acidgroups [18,19].Acidityanalysisofthe par-entSBA-15andthecatalystusingpulsechemisorptionsandTPD ofNH3intheregionof127–427◦CconfirmedthattheHSO3 SBA-15catalystcontainedweakandmediumBronstedacidsiteswith anoverallconcentrationof42␮mol/g.NH3-TPDprofileofthe cat-alystispresentedinFig.3.Thelocationsofthetwothepeaksare inagreementwithresultsreportedbyotherresearchers[19].They performedacidsitesanalysisinthetemperaturerangefrom50to 527◦_C_and_found_out_that_the_HSO

3SBA-15catalystshowedtwo peaksofacidsitesi.e.weakandmediumacidsitesintheNH3-TPD curvesrelatedtothedesorptionofNH3ontheregionof127–427◦C. At highertemperature(427–527◦_C),_the_propyl_sulfonic _groups begantodecompose.Meanwhile,theamountofacidsitesinthe SBA-15 withoutacidfunctionalizationwasnot detected,tothe confirmationthatSBA-15isindeedaneutralmaterial.

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Table3

SurfaceanalysisresultsofSBA-15andHSO3SBA-15catalyst.

Surfaceanalysis SBA-15 HSO3SBA-15

Totalsurfaceareaa_(m2_/g) ₅₄₂ ₆₀₇

Totalporevolume(cm3_/g) _0.795 _1.018

Microporeareab_(m2_/g) ₁₄₉ ₇₉

Mesoporearea(m2_/g) ₃₉₃ ₅₂₈

Averageporesize( ˚A) 59 67

a_Using_the_BET_equation_(S

BET),

b_Using_the_correlation_of_t-Harkins_and_Jura_(t-plot_method)

andanaverageporediameterof67 ˚Athatwerelargercompared tothoseoftheparentSBA-15material.Theincreaseinthepore sizeafterthefunctionalizationindicatedthatthepreparation con-ditionsused allowedtheenlargement of mesoporous structure oftheSBA-15andgraftingofsulfonicacidgroupsonthe meso-poroussurfaceatthesametime[20].Besides,themesoporearea ofthecatalyst(528m2_/g)_was_higher_than_that_of_the_parent SBA-15material(393m2_/g)._Meanwhile,_the_catalyst_had_a_micro_pore areaof79m2_/g_that_was_lower_than_that_of_the_parent_material i.e.149m2_/g._This_finding_suggested_that_the_catalyst_preparation conditionsallowedtherestructuringofthehexagonalstructuresin theHSO3SBA-15catalyst.Thisrestructuringresultedinsignificant disappearanceofmicroporestructures.

Furthermore,evidencesthatSBA-15andHSO3SBA-15catalyst mainlyhadmesoporousstructureswereconfirmedbyresultsof N2adsorption-desorptionmeasurementandanalysisofporesize distributionsoftheSBA-15andHSO3SBA-15catalystswiththe desorptionbranchusingBJHmethod.AsshowninFig.4.Fig.4(a), nitrogenadsorption–desorption isothermcurves for theparent SBA-15andmodifiedSBA-15exhibitedtypeIVisotherms,implying thattheywerematerialswithsufficientlyorderedmesostructures [21].Meanwhile,Fig.4(b)showstheirporesizedistributionsin whichporesizeatthemaximumpeakinSBA-15andHSO3SBA-15 intherangeofmesoporestructures.Inaddition,Fig.4(b)confirms thatthe HSO3SBA-15catalyst had more defined pore structure withlargerporesizeatthemaximumpeakthanthatoftheparent SBA-15.

TEMandSEMimagesofSBA-15andHSO3SBA-15catalystare presentedinFig.5.TEMimagesoftheparentSBA-15andthe cat-alystshowedthatin generaltheirstructureswerewell-ordered mesoporousmaterialasshowninFig.5(a)and(b),respectively.The SEMimagesofparentSBA-15andthecatalystpresentedinFig.5(c) and(d),respectively,revealthattheirstructureshadfibrousand porous-typemorphology.

3.2. Effectsofreactionparametersonthecatalystperformance

Thissectiondiscussestheeffectofreactiontemperature, cat-alyst loading and glycerol/lauric acid molar ratio on reaction products.Next,detailedexaminationonthereactionpathwaysof glycerolesterificationwithlauricacidovercatalystisattempted. Thisleadstotheidentificationofthesequenceofthereactionsteps. Inthiswork,experimentalerrorsinthemeasurementof concen-trationwerewithin±4%.However,particularfocuswasonlygiven togeneraltrendsintheresultsratherthanspecificdatapoints. Assuch,trendlinesweredeemedsufficienttoshowtheprocess behaviors.

3.2.1. Effectofreactiontemperature

ConsideringHSO3SBA-15catalysthadanaverageporediameter ofabout67 ˚A[12],diffusionlimitationhadbeenexcludedforthe esterificationofglycerolwithfattyacid[11].Severalcatalytictests werecarriedoutatvariousreactiontemperatures(413K,423K,and 433K)usingaconstantglycerol/lauricacidmolarratioof4:1anda constantcatalystloadingof5%.Theeffectsofreactiontemperature

Fig.4.N2adsorption–desorptionisothermcurves:(a)andporesizedistribution

curves:(b)ofSBA-15andHSO3SBA-15catalyst.

onlauricacidconversionandreactionproductsi.e.monoglyceride, diglycerideandtriglycerideyieldsareshowninFig.6.Lauricacid conversionsignificantlyincreasedwiththeincreaseinthe reac-tiontemperatureinwhichtheconversionincreased.Forexample, itincreasedfrom75%at413Kto85%at423Kthento95%at433K atareactiontimeof7h.Thesefiguresuggestedthehighpotential ofthemesoporouscatalystincatalyzingthereaction.This obser-vationalsosuggestedthatbyincreasingthetemperature,kinetic energy ofglyceroland lauricacidwashigher sothatthe num-berchemisorptionsontheactivesitesandsubsequentlyeffective interactionbetweenthereactantmoleculeswereenhanced[22].By thepresenceofacidsiteseitherintheinnerortheouterporesof HSO3SBA-15catalyst,thehighereffectivecollisionsledtothe sig-nificantincreaseinlauricacidconversion.Thecontinuousremoval ofwaterfromthereactingsystemalsohelpedinimprovingthe conversionasthereversereactionwassuppressed.

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Fig.5.TEMimagesof(a)SBA-15,(b)HSO3SBA-15,andSEMimagesof(c)SBA-15and(d)HSO3SBA-15.

earlierattachedfattyacidendofthemonoglycerideanddiglyceride molecules.

3.2.2. Effectofcatalystloading

Severalcatalytictestswerealsoconductedat433Kwith differ-entHSO3SBA-15catalystloadings(1wt.%,3wt.%and5wt.%with respecttothereactants)usingaconstanttemperatureof433Kand aconstantglycerol/lauricacidmolarratioof4:1.Effectsofcatalyst loading(W)onlauricacidconversionandproductsofthe reac-tionscanbeseeninFig.7.ResultsinFig.7(a)indicatethatwhen thecatalystloadingwasincreasedfrom1wt.%to3wt.%at reac-tiontimeof7h,lauricacidconversionincreasedfrom90%to95%. Thisobservationsuggested thattheincrease incatalyst loading from1wt.%to3wt.%increasedthenumberofacidsitesavailable forthereactionasthecatalysthadanacidsiteconcentrationof 42␮mol/gasconfirmedinthecharacterizationresults.However, withanincreaseinthecatalystloadingfrom3%to5wt.%,nofurther enhancementintheconversionwasobserved,especiallyatlonger reactiontime.Thiscouldbeattributedtothefactthatbeyonda certaincatalystloading,therewasanexcessofcatalystacidsites thanactuallyrequiredbythereactantmoleculessothatthe con-versionlevelledoff[23].Similartrendwasalsodemonstratedby glyceroltransesterificationreaction[4]tosuggestthatthe benefi-cialroleofhighercatalystloadingwasonlylimitedtolowreactant conversion.

Fig.7(b)showsthatata reactiontimeof7h, monoglyceride yieldsremained virtually stable when thecatalyst loading was increasedfrom1wt.%to3wt.%.Theseresultsindicated thatthe occurrenceofreactionintheinternalmesoporesofHSO3SBA-15

catalystdidnotincreasebyincreasingthecatalystloadingfrom 1wt.%to3wt.%.However,diglycerideyieldslightlyincreasedwith theincreaseinthecatalystloadingfrom1wt.%to3wt.%.Thisresult wasattributedtotheoccurrenceofreactionintheexternalporesof thecatalystsothatdiglycerideformationwasimproved.However, ifthecatalystloadingwasincreasedfrom3%to5wt.%, monoglyc-erideyieldwasfoundtoimprovefrom60%to68%whilediglyceride yielddecreasedfrom30%to27%andtriglycerideyieldwasrather stableatlowlevel.Thisobservationindicatedthatbyincreasing thecatalystloadingfrom3%to5wt.%,theoccurrenceofreactionin theinternalmesoporeofHSO3SBA-15catalystwasmorefrequent ratherthanintheexternalporeofthecatalyst.Assuch,the for-mationofmonoglyceridewasmorefavourableratherthanthatof diglycerideortriglyceride.

3.2.3. Effectofinitialglycerol/lauricacidmolarratio

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Fig.6.Effect of reactiontemperatureonglycerolesterification using a glyc-erol/lauricacidmolarratioof4:1andacatalystloadingof5wt.%.

The increase in monoglyceride yield at higher molar ratio ofglycerolto lauric acidwasattributedto a higherchance for lauricacidtoreactwithglycerolmainlyintheporesofHSO3 SBA-15 mesoporous catalyst to produce and allow the diffusion of monoglyceride.Thisobservationresultwasinagreementwiththe glycerolesterificationwithlauricacidoverSO3HMCM-41 meso-porouscatalyst[24].Meanwhile,Machadoetal.[5,6]reportedthat theincreaseinglycerol/lauricacidmolarratioresultedinadecrease inmonoglycerideyieldinglycerolesterificationreactionwith lau-ricacidcatalyzedbyzeolite.Thiswasbecausethesmallporesizeof zeolite(below20 ˚A)retardedtheformationanddiffusionofbulky moleculessuchasmonoglyceride[10].Assuch,thereactionmainly occurredontheexternalporeofthecatalystthatprovidedlarger opportunityfortheformationofthebulkiermolecules(diglyceride andsubsequentlytrigyceride).

3.3. ReusabilityofHSO3SBA-15catalysts

Catalystreusabilityis acrucialaspectinpractical monoglyc-erideproduction.Inthepresentstudy,theHSO3SBA-15catalyst wastestedwithrespecttothereusabilityintheglycerol esterifica-tionwithlauricacidat433Kfor6husingthecatalystamountof5% (withrespecttoweightofinitialreactant).Afterthecatalytic ester-ificationreaction,theusedcatalystwasrecoveredbyfilteringfrom thereactionmixture,washedwithsolventanddriedat100◦_C._The catalystwasthenreusedforanotherglycerolesterificationunder thesameoperatingconditions.

AsseeninFig.9,theHSO3SBA-15catalystshowedarathergood activityretentioninseveralcyclesofusage.Thecatalystretained upto90% fromits initialconversionof 93.7% and up to68.7%

Fig.7.Effectofcatalystloadingonglycerolesterificationusingaglycerol/lauricacid molarratioof4:1at433K.

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Fig.9.LauricacidconversionandmonoglycerideselectivityshownbyHSO3SBA-15

catalystwhenusedinfoursuccessiveruns.

fromitsinitialmonoglycerideselectivityof 70.2% inthefourth cyclewithoutexperiencingsignificantdeactivation.Theresultwas attributedtopostsynthesis-graftingmethodappliedinthis cata-lystpreparationmethodleadingtocovalentattachmentoforgano sulfonicacidgroupstoporoussilicaframeworkswithouttherisk ofporeblockageorleachingofthegraftedcomponent[25–29].A minordecreaseinlauricacidconversionandmonoglyceride selec-tivitycouldbeattributedtominordeactivationtosomeactivesites (sulfonicacidgroup)ordefectsinthemesoporusstructureofthe catalyst.Inthisrespect,thesignificantroleofsmalllossinthe cata-lystamountsusedfromrun1torun4couldberuledoutascatalyst amountsbetween3and5wt.%didnotposesignificantdifferent intheactivityasprovenbyresultsinFig.7.Theactivityretention ofHSO3SBA-15inpresentstudywascomparabletothatreported intheliterature[30]utilisingsimilarcatalystforthesynthesisof chromenesfromchromanol.

3.4. Reactionpathway

To get a better understanding of the reaction pathways of glycerolesterificationwithlauricacid,formationsofreaction prod-uctsi.e.monoglyceride,diglycerideandtriglyceride,asafunction of time at different temperatures, catalyst loadings and initial glycerol/lauricacidmolarratioswerescrutinized.Monoglyceride, diglycerideandtriglycerideyieldsasfunctionsofreactiontimeat differenttemperaturesareshowninFig.6.Theprofilessuggestthat thereactionbetweenlauricacidandglycerolmainlyformed mono-glyceride.Maximummonoglycerideyieldwiththedecreaseinthe monoglycerideyieldafterreachingacertainmaximumpointare notobservedinthefigure.Theseresultsindicatedthatdiglyceride wasdirectlyproducedfromtheesterificationreactionbetween lau-ricacidandglycerolandnotfromthetransesterificationreaction betweenmonoglycerideandglycerol.Thelatterreactionisknown tobeabase-catalyzedreaction[4].

Thesameexplanationwasapplicableinthecaseoftriglyceride formationinwhichtriglyceridewasalsodirectlysynthesizedfrom thereactionbetweenlauricacidand glycerolandnot fromthe reactionbetweendiglyceride and glycerolasthere wasno sig-nificantdecreaseindiglycerideyieldasnotedinthefigure.Thus, thereactionpathwaysoftheesterificationofglycerolwith lau-ricacidinthisstudyweredeemedtofollowirreversibleparallel reactionsinwhichreactionmechanisminvolvedaseriesof glyc-erolesterificationwithlauricacidreactionsthatoccurredatthe sametimetoformmonoglyceride,diglycerideandtriglyceride.It canbeconcludedfromFig.6thattheincreaseinreactiontimeat differenttemperaturescausedpositiveeffectsonmonoglyceride, diglycerideandtriglycerideformations.

andtriglycerideyieldsincreasedonlyforthefirst2h,subsequently theyieldswerefoundtobestableforreactiontimesbetween2 and7h.Theseobservationsindicatedthatlauricacidandglycerol weremainlyconsumedtoformmonoglyceride,suggestingthatthe esterificationtookplacemostlyintheinternalmesoporesrather thanintheexternalporesofthecatalyst.

3.5. Kineticmodelofmonoglycerideformation

Asthereactionpathwaysfollowed irreversibleparallel reac-tions,thereactionratewascalculatedbasedonsecondorderkinetic modelwithrespecttoglycerolandfattyacid.Theamountsoflauric acidconsumedinthereactiontoformmonoglycerideatanytime wereconsideredtobeequaltotheamountsofglycerolconsumed asgiveninEq.(4);

CG0XG0=CFA0XFA0 (4)

M= CG0

CFA0

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IfXM=monoglycerideyield,then;

CG=CFA0−CFA0XM (6)

CG=glycerolconcentration,mol/L CFA =fattyacidconcentration,mol/L CG0 =initialglycerolconcentration,mol/L XG0 =initialglycerolconversiontomonoglyceride CFA0 =initialfattyacidconcentration,mol/L XFA0 =initialfattyacidconversiontomonoglyceride

M=molarratioofglyceroltofattyacid

k=overallrateconstant,L/(molh)

ks =specificrateconstant,L/(molh) t=time,h

BysubstitutingEqs.(4)–(7)inthesecondorderkineticmodel fromTable1,−rFA=−(1/W)(dCFA/dt)=k1CGCFA,anexpressioninEq.

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hadhigherkineticenergyatthehighertemperature.Asaresult, collisionsbetweenreactant moleculesin theinnerporesofthe catalystweremore frequent.Thissituationwouldspeed upthe rateofmonoglycerideformation[22].However,toohigh temper-ature(beyond200◦_C)_could_result_in_the_side_reactions_leading_to degradationoffattyacidsaswellasthemonoglyceride[4].

Theactivationenergyandpre-exponentialfactorfor monoglyc-erideformationwereobtainedusinganArrheniustypefunctionby plotting(lnks)vs(1/T).Inthisstudy,theexperimentwasrepeated severaltimessothatodddatacouldbeidentifiedanddiscarded. Finally,theln(k)vs(1/T)plotresultedinalinearlinewithanR2 valueof0.93whichwasregardedassufficientlyhightoindicatethe accuracyofthemodelused.Thevalueofactivationenergy(E)and pre-exponentialfactor(A)werefoundtobe10kcal/mol(42kJ/mol) and655L/(molgcath),respectively.Theactivationenergyobtained overtheHSO3SBA-15catalystformonoglycerideformationinthe glycerolesterificationwithlauricacidwaslowerthanthatobtained overzinccarboxylatescatalyst,i.e.51kJ/mol,asreportedinthe lit-erature[14].Thisresultindicatedthattheglycerolesterification catalyzedbyHSO3SBA-15catalystrequiredasignificantlylower energyformonoglycerideformationthanthatcatalyzedbyzinc carboxylates.Thereactioninthisstudywasalsomoreinfluenced byanincreaseinthereactiontemperature[4].Theloweractivation energycouldbeattributedtothehighsurfaceareaandmesoporous structuresofHSO3SBA-15catalystwhichwasaccessibletobulky moleculesoflauricacidtomoreselectivelyreactwithglycerolto producemonoglyceride.Besides,themacroporesontheexternal surfaceofthecatalystcouldassistthereaction.Itshouldbeborne inmindthatthesulfonicacidfunctionalizationwaspossibleonthe internalandexternalsurfaceasthisacidgroupwillattachtosurface silanols[24].

Furthermore,theactivationenergyobtainedinglycerol esterifi-cationwitholeicacidformonoglycerideformationwas26.5kJ/mol overultrastableNaY-zeolitecatalystcontainingmicroandmeso structures[15]. However,the selectivityto monoglyceridewas ratherpoor.Thisresultcouldbeattributedtothesmallporesize thatwouldposesomegeometricconstrainttothereactantaswell astheproduct[6].Meanwhile,superaciditycatalystshavingmuch smallparticlecharacteristicswithsizesbelow100nm,suchas sul-fatedironoxide,leadoxide,zincchloride,cobaltchlorideandtin chloridegavehigheractivationenergiesof68.2kJ/mol,66.8kJ/mol, 78.8kJ/mol, 74.0kJ/mol, and 78.3kJ/mol, respectively. Thesame valuewasreportedtobeat89.3kJ/molforthereactionwithout catalyst(fattyacidcanactasautocatalyst)[9].Itcouldthereforebe concludedthatthepresenceofsolidacidcatalystsreducedenergy barrierforthemonoglycerideproductionfromoleicacidand fur-therreductioncouldbeachievedusingacatalysthavingmesopore structuresasmonoglyceridewouldbefavourablyformedinthe internalporesofthecatalyst.Thiswassupportedbythe character-izationresultsinthisstudythatshowedpredominatelymesopores ratherthanmicroporesthatcontributedtotheoverallsurfacearea. Withthekineticparametersobtained,thekineticmodelofrate expressionformonoglycerideformationinglycerolesterification withlauricacidoverHSO3SBA-15catalystwasproposedasgiven inEq.(9).

Monoglyceride yields under various conditions as predicted usingthekineticmodelproposedwerefoundtosatisfactorilyfit theexperimental ones obtainedin this study. It clearly shows a good agreementbetween the model predictions and experi-mentalresultsundervariousconditions,withR2₌_0.96._The_result evidentlyshowedthesufficientvalidity oftheproposedsecond orderkineticmodeltorepresentthereaction.Thisresultindirectly

validatedtheassumptionthatthereactionfollowedanirreversible parallelreactionmechanismwithsecondorderofreaction.

4. Conclusions

Behaviors of glycerol esterification with lauric acid over HSO3SBA-15in thetemperaturerange between413and 433K, catalystloadingfrom0to5%w/wwithrespecttothereactants, andglycerol/lauricacidmolarratioof2:1and4:1were success-fullyinvestigatedin astirredbatchreactorunderlow pressure. Monoglycerideyieldwasfoundtoincreasewithanincreaseinthe temperature,catalystloadingandmolarfeedratio.HSO3SBA-15 catalyst couldberepeatedlyusedfor uptofourcycleswithout experiencing significantdeactivation. Theglycerolesterification reaction pathways were considered to be irreversible parallel reactions for the formation of monoglyceride, diglyceride and triglyceride under these conditions. Thus, second order kinetic model with respect to glycerol and lauric acid gave a better representation of kinetic behavior. Theestimated rateconstant increased with the increase in temperature to correctly indi-cate that the reaction was more favourable with the increase intemperature.ByusingArrheniustypefunction,theactivation energy for monoglyceride formation through the esterification wasfound tobe 42kJ/mol (or10kcal/mol). Meanwhile, kinetic modelofrateofexpressionwas−rFA=654.64exp(−9702/RT)CGCFA mol/(Lgcath)whichcouldbesatisfactorilyusedtopredictthe pro-gressivechangesinamountofmonoglycerideformedduringthe reaction.

Acknowledgments

ThecontributionofProf.SubhashBhatiainshapingupcatalysis researchcapabilityatSchoolofChemicalEngineering,Universiti SainsMalaysianeedsspecialacknowledgement.AShortTermand aResearchUniversity(RU)grantfromUniversitiSainsMalaysiato supportthisworkarealsogratefullyacknowledged.

References

[1]L.Hermida,AcidfunctionalizationofSBA-15mesoporouscatalystfor glyc-erolesterificationtomonoglyceride,MScthesis,UniversitiSainsMalaysia, Malaysia,2010.

[2]R.Nakamura,K.Komura,Y.Sugi,Catal.Commun.9(2008)511–515. [3]A.Corma,S.B.A.Hamid,S.Iborra,Velty,J.Catal.234(2005)340–347. [4]A.Z.Abdullah,T.Y.Wibowo,R.Zakaria,Chem.Eng.J.167(2011)328–334. [5]M.S.Machado,J.Pérez-Pariente,E.Sastre,D.Cardoso,A.M.deGuere ˜nu,Appl.

Catal.A203(2000)321–328.

[6] M.S.Machado,D.Cardoso,J.Perez-Pariente,E.Sastre,A.M.deGuere ˜nu,Stud. Surf.Sci.Catal.130(2000)3417–3422.

[7]Y.Pouilloux,S.Abro,C.Vanhove,J.Barrault,J.Mol.Catal.A149(1999)243–254. [8]E.Heykants,W.H.Verrelst,R.F.Parton,P.A.Jacobs,Stud.Surf.Sci.Catal.105

(1997)1277–1284.

[9]F.S.Gurner,J.Am.Chem.Soc.73(1996)347–351.

[10]J.Perez-Pariente,I.Diaz,F.Mohino,E.Satre,Appl.Catal.A254(2003)173–188. [11]K.Wilson,J.H.Clark,PureAppl.Chem.72(2000)1313–1319.

[12]L.Hermida,A.Z.Abdullah,A.R.Mohamed,J.App.Sci.10(24)(2010)3199–3206. [13]H.Szelag,W.Zwierzykowski,Lipid100(1998)302–307.

[14]A.Macierzanka,H.Szelag,Ind.Eng.Chem.Res.43(2004)7744–7753. [15]N.Sanchez,M.Martınez,J.Aracil,Ind.Eng.Chem.Res.36(1997)1524–1528. [16]T.A.Nijhuis,A.E.W.Beers,F.Kapteijn,J.A.Moulijn,Chem.Eng.Sci.57(2002)

1627–1632.

[17]Y.C.Yang,S.R.Vali,Y.H.Ju,J.Chin.Inst.Chem.Eng.34(2003)617–623. [18]W.M.VanRhijn,D.E.DeVos,B.F.Sels,W.D.Bossaert,P.A.Jacobs,Chem.

Com-mun.(1998)317–318.

[19] I.Dıaz,C.Marquez-Alvarez,F.Mohino,K.Prez-Pariente,E.Sastre,J.Catal.193 (2000)283–295.

[20]Y.Zheng,X.Su,X.Zhang,W.Wei,Y.Sun,Stud.Surf.Sci.Catal.156(2005) 205–212.

[21]B.Rac,A.Molnar,P.Forgo,M.Mohai,I.Bertoti,J.Mol.Catal.A.Chem.244(2006) 46–57.

[22]K.A.Connors,ChemicalKinetics:TheStudyofReactionRateinSolution,VCH PublisherInc,NewYork,1990.

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