Effect of catalytic conditions on the synthesis of new aconitate esters

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Effect of catalytic conditions on the synthesis of new

aconitate esters

William Piang-Siong, Pascale de Caro, Corinne Lacaze-Dufaure, Alain Shum

Cheong Sing, William Hoareau

To cite this version:

William Piang-Siong, Pascale de Caro, Corinne Lacaze-Dufaure, Alain Shum Cheong Sing, William

Hoareau. Effect of catalytic conditions on the synthesis of new aconitate esters. Industrial Crops and

Products, Elsevier, 2011, vol. 35, pp. 203-210. �hal-00717594�

To link to this article:

DOI: 10.1016/j.indcrop.2011.06.031

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http://dx.doi.org/

10.1016/j.indcrop.2011.06.031

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http://oatao.univ-toulouse.fr/

Eprints ID:

5439

To cite this version:

Piang-Siong, William and De Caro, Pascale and Lacaze-Dufaure, Corinne

and Shum Cheong Sing, Alain and Hoareau, William Effect of catalytic

conditions on the synthesis of new aconitate esters. (2012) Industrial

Crops and Products, vol. 35 . pp. 203-210. ISSN 0926-6690

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Effect

of

catalytic

conditions

on

the

synthesis

of

new

aconitate

esters

William

Piang-Siong

_{, Pascale}

_de

_Caro

_{, Corinne}

_{Lacaze-Dufaure}

_{, Alain}

_Shum

_Cheong

_Sing

_,

William

Hoareau

a_{UniversitédeToulouse,INP,LCA(LaboratoiredeChimieAgro-Industrielle),ENSIACET,4alléeEmileMonso,F-31030Toulouse,France} b_{INRA,UMR1010CAI,F-31030Toulouse,France}

c_{UniversitédeToulouse,INPT,CIRIMATUMRCNRS5085,ENSIACET,4alléeEmileMonso,F-31030ToulouseCedex4,France}

d_{UniversitédelaRéunion,FacultédesSciencesetTechnologies,LCSNSA(LaboratoiredeChimiedesSubstancesNaturellesetdesSciencesdesAliments),15avenueRenéCassin,B.P.}

7151,97715Saint-DenisCedex9,LaRéunion,France

e_{eRcane,40BoulevardGabrielMacéBP315,97494Sainte-ClotildeCedex,LaRéunion,France}

Keywords: Aconiticacid Isoamylalcohol Esterification Heterogeneouscatalysis Ionicliquid Greenindicators

a

b

s

t

r

a

c

t

Sugarcaneisacropwhichgenerateslargeamountsofbiomassandajuicerichinhigh-valuenatural molecules.Afterextractingsugarfromthejuice,therecoveringofvariouscompoundssuchasorganic acidscontainedinmolassescouldcontributetoincreasethecompetivityofthesugarindustry.Therefore, accordingtothebiorefineryapproach,weproposetostudythechemicalconversionofoneoftheseacids, theaconiticacid,byesterificationreactions.Anewseriesofaconitateestershavebeensynthesizedby combiningaconiticacidandalcoholsfromnaturalorigin.Theeffectsofexperimentalconditionshave beeninvestigatedandhaveshownthatthetypeofcatalysishasasignificanteffectontheselectivity. Kinecticshavethusbeenperformedtodeterminethebestconditionstosynthetizeenriched composi-tionsinesters.Homogeneouscatalysisgeneratesthehighestyieldintriester.Heterogeneouscatalysis (macroporousresins)ispreferedfortheproductionofmonoesterswhilecatalysisassistedbyionicliquid isadaptedtopreparemainlydiesters.Greenindicatorshavebeendiscussedaccordingtothe calcula-tionsperformed.Theresultingpolyfunctionalestersaretotallybiosourcedmoleculesandhaveagreat potentialasbioproductsfordifferentapplications.

1. Introduction

Thereisanever-increasinginterest fortheuseofrenewable resources for the production of bioproducts based on natural ingredients.Therisingcrudeoilpriceduetoitsrarefactionincites to diversify the feedstocks towards renewable raw materials. Therearedifferentpurposestoreplaceapetroleumbasedproduct bya biobasedcompound, accordingtothespecificationsofthe products:

-acleanprocessofproductionwhichgeneratesalower environ-mentalimpactsuchasagaininVOCemission,

-alowtoxicitywhichmakesthemadaptedtobenignandgreen formulations,

-ahighbiodegradabilitywhichisnecessaryincaseofcontactwith theenvironmentattheirendoflife,

∗ Correspondingauthorat:UniversitédeToulouse,INP,LCA(Laboratoirede Chimie Agro-Industrielle), ENSIACET, 4alléeEmile Monso, F-31030Toulouse, France.Tel.:+330534323505;fax:+330534323597.

E-mailaddress:pascale.decaro@ensiacet.fr(P.deCaro).

-innovativepropertieswhichmakethemattractiveforusers.

So, we intended toinvestigatethepotentialof aconitic acid (1,2,3-propentricarboxylicacid)asarawmaterialforthe prepara-tionofbioproducts.Aconiticacidisatricarboxylicacidindustrially producedbydehydrationofcitricacid.Aconiticacidcanalsobe extractedfromco-productsgeneratedbysugarindustry.

The up-grading of the natural aconitic acid represents an opportunity to use an available feedstock and to increase the competivinessofsugarindustry.Thisapproachcorrespondstothe biorefineryconceptwhichisbeingdevelopedinnumerous agroin-dustrychainsin ordertominimise thewasteanddiversifythe outlets.

Weproposetostudytheesterificationofaconiticacidwith nat-uralalcoholssuchasisoamylalcoholandlaurylalcohol.Isoamyl alcoholisoneoftheshortalcoholspresentinfuseloilsobtainedin thepotresidueafterdistillationofethanolfromfermentedsugar.

ThisC5alcohol representsbetween60and 75%offuseloils, accordingtotheoriginofbioethanol(beet,wheat,etc.).

Laurylalcohol(dodecanol)isobtainedfromlauricacidwhich is themain fattyacidpresent in coconutoil. It is used forthe

Fig.1.Thetwoisomersofaconiticacid.

preparationofsurfaceactiveagentsforcosmeticandhealthcare industry.

Thefirstobjectiveconsistsinpreparingesterswhichcouldhave interestingpropertiestoreplacepetroleumbasedproductswhich arebannedbyregulations.Estersfromnaturalorganicacidsare mentionedtobepotentialcandidates.Forexample,citrateesters areusedasplasticizersinformulationforpluralcomponent latex-foam(Olangetal.,2010)orforliquidcoatingcompositions(Jonn etal.,2008).Glycerolestersareusedaslubricantorsurfactantin formulationforcleaningapplications(Grossetal.,2008)andfor enginelubricants(Patiletal.,2010;Seemeyeretal.,2008).

Thesecondobjectiveistoselectasyntheticroutewhichis com-patiblewiththeprinciplesofgreenchemistry.Theimprovementof esterificationmethodstotakeintoaccounttheenvironmentaland sanitaryaspectsisstillachallengeinindustry.

Moreover, thestudy of new experimentalconditions is the opportunitytoorientatethereactiontowardsaspecificester cate-gory.

Aconiticacidhastwoforms,cisandtrans(Fig.1).Inthe sugar-cane,thetransisomerwhichisthemoststable,isthedominant one.

Acidaconitichasthusthreenon-equivalentacidfunctionsthat is tosay doted of a differentreactivity. In particular,two acid functionstakeparttothedelocalisationoftheelectron.Thus,the carbonylsitewhichisnotconjugatedshouldbethemostreactive. Esterificationreactionsfromaconiticacid(AA)leadtothree suc-cessivereactionstopreparemonoesters,diestersandonetriester (Fig.2).

Thesynthesisofaconitateestershasbeenpreviouslyperformed inbatchorcontinuousstirredreactorwithahomogeneous cata-lyst.Forexample,BruinsandCanapary(1956)haveusedsulfuric acidtosynthesizetributylaconitatewithahighmolarratio alco-hol:acid12:1at175◦_{C.Robertsetal.(1954)havesynthetisedesters}

ofaconiticacidusingp-toluenesulfonicacidwithamolarratio3.3:1 for3.5hbycodistillatingwaterwithtoluenewithayieldof tri-isoamylaconitateof 85.6%.Guthrieetal.(1976)have usedacid halidetopreparemethylaconitatewithamolarratioof65:1for 3.5h.

Theequilibriumisusuallyshiftcontinuouslybyadsorptionon dryingagentsorbycodistillationwith“entrainers”suchasbenzene ortoluene(Cockremetal.,1993;CoxandCarruthers,1936;Frappier

etal.,2002;WeibergandStimpson,1942).

Themajor drawbacksof homogeneouscatalyst arethe final neutralizationofthehomogeneouscatalyst,andtheextractionof reactionproductsbyavolatilesolvent.Moreover,theseconditions requirehightemperatures,aheavytreatmentandalargeexcessof alcohol.

Therefore, heterogeneous catalysisbecomes more and more attractiveforthechemicalindustry.Forinstance,zeolitesandmetal oxides,acidtreatedclays orcation-exchangeresins(Choudhary etal.,2001;Maetal.,1996)areusedtoperformclean esterifica-tions.

Macroporousresinsarementionedasaperformingcatalystfor esterification.Thus,theyconstituteanalternativetohomogeneous catalystssincetheyarenon-corrosive.Theycanbeeasilyremoved fromthereactionmixtureandcanbereusedafterthereaction.

Petrini etal. (1988)have synthetised monomethylaconitate

usingAmberlyst15withamolarratioof100:1forreagents,atroom temperaturefor10hwithayieldof80%andatotalselectivity.AG 50W-X4resinswereusedbyGiletal.(2006)topreparetributyl aconitatewithanexcessof50%ofalcoholat140◦_Cfor1.5h,to

provideafinalcompositionof83%intriester.

Akineticstudywasperformedfortheesterificationofcitricacid withethanolcatalyzedbyAmberlyst15(Kolahetal.,2006)witha yieldabove90%.Similarconditionswereappliedtothe esterifica-tionofsuccinicacidwithethanol(Kolahetal.,2008).

Room temperatureionicliquids (RTILs)called “designer sol-vent”do notcontributetovolatileorganiccompoundemissions thankstotheirlowvaporpressure.Theyrepresentanalternative mediawhichcouldbeinterestingfortheesterificationofasolid compound,sinceitactsbothassolvent-catalystandcanintervene intheshiftofequilibrium.

Coleetal.(2002)havereportedtheuseofBronstedacidicionic liquidasadualsolvent-catalystinesterificationreaction.Another

workbyZhuetal.(2003)hasshownthattheBronstedionic

liq-uid1-methylimidazoliumtetrafluoroborate[hmim][BF4]isalsoa

suitablecatalystfortheesterificationofcarboxylicacids(C2–C11) with a primary alcohol. Joseph et al. (2005) have shown that [bmim][PTSA]cancatalysethesynthesisofbenzylacetateingood yield(100%).Fraga-Dubreuiletal.(2002)synthesizebenzylacetate using[bmim][HSO4]withayieldof95%.[mimps]3[PW12O40]was

usedbyLengetal.(2009)topreparetri-n-butylcitratewithan

excessofalcohol(1:5)at130◦_{Cduring3htoobtainayieldof98%}

andaselectivityof98%.

Foranesterification,thechoiceofRTILsmusttakeintoaccount theseparationbetweentheestersandwaterformedduringthe reaction.

Accordingtothestateoftheart,fewstudiesonthe esterifica-tionofaconiticacidwerecarriedoutandfewdataaboutyieldsand selectivityareavailable.Mostofthemdealwiththesynthesesof triestersathightemperature(above140◦_{C)orinpresenceof}

venttogetanazeotropicmixture.Exchangerionresinsarecited tobemoreadequateforthepreparationofmonoestersatroom temperaturewithahighexcessofalcohol.

Weproposetocarryoutthesynthesisofaconitateesterswitha bettercontrolofselectivitybydevelopingnewexperimental con-ditionsforcleanerprocesses.Theemphasiswasplacedtocompare performancesofaconventionalhomogeneouscatalysiswithtwo othercatalyses:heterogeneousandionicliquid.

2. Materialsandmethods 2.1. Materials

Trans-aconiticacid(98%),isoamylalcohol (98%),sulfuricacid (95–98%), ionic liquid: 1-methylimidazolium hydrogensulfate [mim][HSO4](Basionics®AC39,BASF,≥95%)werepurchasedfrom

Sigma–Aldrich.Amberlyst®_{15(Fluka)wasusedinH}+_formwithout

modification.

2.2. Esterificationprocedure

Esterification reactions were performed in a batch reactor (50mL).Fortheconditionscorrespondingtohomogeneous(H2SO4)

andheterogenouscatalyses(resins),adevicetoremovewaterby distillationwasconnectedtothereactor.Masspercentagesof cat-alystareexpressedrelativetoisoamylalcoholweight.

2.2.1. Homogeneouscatalysis(method1)

Isoamyl alcohol (172.2mmol) and aconitic acid (28.7mmol) weremixedandheatedunderstirringuntilthesolubilizationof thesolidaconiticacid,before2.7wt.%ofsulfuricacidwasadded. Thereactionwasstirredat100◦_{Cduring540min.Attheendofthe}

reaction,20mLofethylacetatewasaddedtothereactionmedium andtheorganicphasewaswashedfourtimeswith10mLofwater. Theorganicphase wasdriedwith9gofsodiumsulphate.Ethyl acetateandisoamylalcoholwereremovedin thesametimeby evaporation.

2.2.2. Heterogeneouscatalysis(method2)

Isoamyl alcohol (172.2mmol) and aconitic acid (28.7mmol) weremixedandheatedunderstirringuntilthesolubilizationof thesolidaconiticacid,before3wt.%ofcationexchangeresinH+

Amberlyst®_{15(orenzyme)wasadded.Thereactionwasstirredat}

85◦_{Cduring90min.Attheendofthereaction,thecatalystwas}

separated throughfiltrationand washed two timeswith20mL ofethylacetate.Organicphasewasthenconcentratedbysolvent evaporation.

2.2.3. Catalysisbyionicliquid(method3)

Isoamyl alcohol (86.1mmol) and ionic liquid (9.50g) were mixedandheatedunderstirring.Whenthemixturereached100◦_C,

aconiticacid(28.7mmol)wasaddedandthemediumwasstirred during540min.Attheendofthereaction,themediumwascooled atroomtemperatureuntiltheformationoftwodistinctphases. 10mLofethylacetatewasaddedandtheorganicphasewas sep-aratedbysettling.Ionicphase waswashedwith20mLofethyl acetate.Theorganicphasewasdriedonsodiumsulphate(6g)and ethylacetatewasevaporated.

2.3. Characterization

2.3.1. High-performanceliquidchromatography(HPLC)

Aconitic acid and the aconitate esters were identified with DionexHPLCusing a reversed phaseC18 column(Omnisphere, 4.6mm×250mm).Themobilephaseiscomposedofwaterwith 0.1%H3PO4 andCH3CN(1.0mL/min) accordingtothefollowing

gradient:50%CH3CN(t=0–5min)to100%CH3CN(t=15–20min)

to50%CH3CN(t=25–30min).

TheUVdetection(Hewelett-Packard1100)wasperformedat awavelengthof210nm.Aconiticacidandtri-isoamylaconitate wereidentifiedandquantifiedbycomparingHPLCretentiontime andpeakareawiththeirrespectivecalibrationstandards.Mono anddiesterswereidentifiedbyHPLC–MS.

Therelativepercentagesofcompounds(aconiticacidandesters) weredeterminedbytheratiobetweenproductpeakareaandthe sumofcompoundspeakarea.Aconiticacidwasquantifiedthrough acalibrationperformedwithacommercialstandard(98%). 2.3.2. Nuclearmagneticresonance(NMR)

1_{H and}13_{CNMR spectrawerecollected ona BrukerAvance}

300spectrometerwitha5mmBBFOATMAprobe.Allspectrawere acquiredat298.0KusingCDCl3orDMSO-d6assolvent.Chemical

shiftsarereportedaspartspermillionfromtetramethylsilanewith anabsolutefrequency300.13MHz.

1_Hand13_{CNMRoftrans-aconiticacid(AA)(DMSO-d}

6).1HNMR:

ı6.92(s,1H,C CH),and3.95ppm(s,2H,CH2).13CNMR:ı171.76

(s,C O),167.84(s,C O),167.84(s,C O),140.60(s,C CH),129.32 (s,C CH),and33.17ppm(s,CH2).

1_{H and} 13_{C NMR of mono-isoamyl aconitate (MIA) (CDCl} 3). 1_{H NMR: ı 7.07 (m, 1H, C CH), 4.13–4.18 (m, 2H, O–CH} 2), 3.95 (m, 2H, CH2), 1.71–1.62 (m,1H, CH2–CH–),1.57–1.46 (m, 2H, CH2–CH–), and 0.95–0.90ppm (m, 6H, CH3). 13C NMR: ı 170.87–170.38 (d, C O), 169.80–169.58 (d, C O), 165.19 (d, C O),141.99–140.95–138.93(t,C CH),131.04–130.07(t,C CH), 64.08–63.96(d,CH2–O),37.11(s,O C–CH2),33.07–32.86(m,CH2), 24.95–24.65(m,CH)and22.55–22.37ppm(m,CH3). 1_Hand13_{CNMRofdi-isoamylaconitate(DIA)(CDCl}

3).1HNMR:

ı7.04–6.96(d,1H, C CH),4.28–4.10(m,4H,O–CH2),3.95–3.94

(d, 2H, CH2), 1.71–1.69 (m, 2H, CH2–CH–), 1.60–1.50 (m, 4H,

CH2–CH–),and0.95–0.90ppm(t,12H,CH3).13C NMR:ı170.82

(s, C O), 169.92 (d, C O), 165.17 (d, C O), 141.99–138.95 (d, C CH),130.97–128.22(d,C CH),64.80–63.91(d,CH2–O),37.14

(s,O C–CH2),33.34–32.85(d,CH2),25.02(s,CH)and22.39ppm

(d,CH3).

1_Hand13_{CNMRoftri-isoamylaconitate(TIA)(CDCl}

3).1HNMR: ı6.92(s,1H,C CH),4.28–4.10(m,6H,O–CH2),3.96(s,2H,CH2), 1.72–1.62 (m,3H, CH2–CH–),1.60–1.50(m, 6H, CH2–CH–),and 0.95–0.90ppm(t,18H,CH3).13CNMR:ı169.89(s,C O),166.06 (s, C O), 165.43 (s, C O), 139.98 (s, C CH), 129.06 (s, C CH), 64.55–63.71(d,CH2–O),37.15(s,O C–CH2),33.15(s,CH2),25.02 (d,CH)and22.40ppm(d,CH3).

3. Resultsanddiscussion

Threecatalysesconditionshavebeenselected:homogeneous catalyse (H2SO4)as thestandardconditions, Amberlyst 15as a

cationexchangerand1-methylimidazoliumhydrogensulfateasan ionicliquid(Fig.3).

Withmacroporousresins,theminimumalcohol/aconiticacid ratiowhich waspossibletouseis equalto6. Theliquid phase constitutedbythealcoholmustcoverthesolidphase(resin).

Theamountofionicliquidwasdeterminedastheminimum volumetoensurethesolubilizationoftheaconiticacid.Withionic liquid,thestartingmediumwashomogeneousandbecame bipha-sicduringtheformationofesters.

Fig.4.Chargesontheatoms(NBOpopulationanalysis).A:aconiticacid;B:isoamyl alcohol.

Theeffectsofseveralexperimentalparametershavebeen stud-iedinordertooptimizetheconversionofaconiticacidandtoenrich themediumwithoneoftheesters.

3.1. Quantumcalculations

We haveperformed quantumcalculations toinvestigatethe reactivityofaconiticacidandisoamylalcohol.Wehavefirstlooked forthemoststableconformationsofbothcisandtransisomers.The structureswereoptimisedattheB3LYP/pVTZandMP2/pVTZlevel oftheoryusingtheGaussian03package(Frischetal.,2004).

Thetransisomerismorestableof2.6kcal/mol(MP2/pVTZ)than thecis isomer and isthus thermodynamically favoured.This is inagreementwiththeisomerisationofthecistothetrans iso-mer observedexperimentally. The bond lengths aresimilar for thetwo cisandtransisomers.Weestimatedamolarvolumeof 120.5cm3_{/molforthetransisomer,withanaverageradiusof4.49 ˚A}

andamolarvolumeof106.0cm3_{/molforthecisisomer,withan}

averageradiusof4.33 ˚A.

Inordertocalculatethechargesontheatomsfortheaconitic acidandtheisoamylalcohol,weperformedaNBOandMulliken populationanalysis.TheresultsarepresentedinFig.4.

Thefirststepofthereactionofesterificationisaprotonationof theoxygenatomofthecarbonylfunctionoftheaconiticacid.Itis followedbyanucleophilicadditionofthealcoholonthecarbonof thecarbonylfunction.Thecalculatedchargesonthethreeoxygen atomsoftheaconiticacidmoleculearesimilarandshouldexhibit asimilarreactivity.

ThecalculatedchargesontheCatoms(Fig.5)ofthecarbonyl functionsshowthatC6hasaslightlymorepositivecharge.This differencemayexplainthepreferentialreactivityofthissiteforthe additionofthealcoholmolecule.Thisdataisinagreementwiththe experimentalpartsincewehaveobservedthatthechemicalshift

Fig.5. Trans-aconiticacid.

ofC6onaconiticacid(172ppm)hasmovedafteresterification.The mono-isoamylaconitatethuspresentsachemicalshiftat170ppm. Thecalculatedchargesonethyleniccarbon,C2andC3atoms showthatC2hasahigherelectronicdensity,meaningthatin13_C

NMR,C3ismoreunshieldedthanC2.Itisthuspossibletodetermine thefollowingchemicalshifts:ıC2=129ppmandıC3=140ppm.

3.2. Effectoftemperature

Theeffectoftemperaturefrom85to120◦_Conthe

esterifica-tionofaconiticacidwithisoamylalcoholwasstudied.Resultsare presentedinTable1.Anincreasingtemperaturedoesnotimprove theconversionofaconiticacidbutactspositivelyontheyieldof triester.

Whateverthecatalyst,theproportionoftriesterinthemedium isthusincreasedwithtemperature.Wecannotethat heteroge-neouscatalysisisverysensitivetotemperaturesinceanincrease from85◦_Cto100◦_{C,isenoughtoenhancethepercentageoftriester}

from6to46%.

Thediesterscontentsdependontherelativeconversionratesof monoesterintodiestersandofdiestersintothetriester.

Thetemperatureconditionswhichleadtothehighestcontents oftriester(66%),diesters(59%)andmonoesters(54%)willbekept forthefollowingstudies.

3.3. Effectofcatalystloading

TheeffectsofthecatalystloadingarepresentedinTable2.For homogeneouscatalysis,thecatalystloadingvariesfrom1.7to3.7% wtandforheterogeneouscatalysisbetween3and7wt.%.

Itappearsthatcatalystloadinghasfeweffectsontheconversion ofaconiticacid.But,theyieldoftriesterincreaseswithcatalyst loadingforhomogeneouscatalysisleadingtoanenrichedmedium intriester.Moreover,itisnotusefultoincreasetheloadingabove 3%inheterogeneouscatalysttopreparemonoesters.

Weobservethattheyieldsofdiestersandtriesterarebetter whentheionic liquidis usedas acosolvent.In thelatter case, theionicliquidbrings12.5timesmoremilli-equivalentsH+_than

Table1

Influenceoftemperature.

Catalysis T(◦_{C) Acidconversion(%) Yieldoftriester(%)}* _{Relativepercentageofesters(%)}

Monoesters Diesters Triester

85 88.4 34.0 10 50 40 Homogeneousa _{100 91.2 59.1 4 30 66} 120 87.3 65.2 3 26 71 85 86.1 6.9 54 40 6 Heterogeneousb _{100 86.6 43.3 8 46 46} 120 86.3 56.5 5 41 54 85 88.1 7.4 38 52 10 Ionicliquidc _{100 87.6 19.7 15 59 26} 120 89.3 34.1 5 51 44

Reactionconditions:5h,a_{molarratioisoamylalcohol/aconiticacid,6:1,catalystloading,2.7wt.%;}b_{molarratioisoamylalcohol/aconiticacid,6:1,catalystloading,5wt.%;} c_{molarratioisoamylalcohol/aconiticacid,3:1,ionicliquidasco-solvent.}

Table2

Influenceofcatalystloading.

Catalysis Catalystloading(wt.%) Acidconversion(%) Yieldoftriester(%) Relativepercentageofesters(%)

Monoesters Diesters Triester

1.7 90.3 47.1 6 41 53 Homogeneousa _{2.7 91.2 59.1 4 30 66} 3.7 89.5 64.5 3 27 70 3 87.0 3.7 59 36 5 Heterogeneousb _{5 86.1 6.9 54 40 6} 7 88.5 5.3 53 40 7 Ionicliquidc _{5 84.6 11.1 35 51 14} d _{Cosolvent 87.6 19.7 15 59 26}

Reactionconditions:5h,a_{molarratioisoamylalcohol/aconiticacid6:1,100}◦_C;b_{molarratio6:1,85}◦_C;c_{molarratio6:1,100}◦_C;d_{molarratio3:1,100}◦_C.

itdoesincatalyticconditions,whichenhancestheconversionof monoestersintodiestersandintotriester.

Intheabsenceofdevicetoremovewater,theuseofahydrophilic ionicliquidseemsefficienttotrapwaterandtoshiftthe equilib-rium.Furthermore,asthesolubilizationofaconiticacidisensured byionicliquid,itispossibletoreducetheexcessofalcoholtothe ratio3:1.

Eventually,forhomogeneouscatalysis,theconditionsallowing tominimisetheamountofacidiceffluentsandtogetanenriched mediaintriestercorrespondstoacatalystloadingequalto2.7%.

For heterogeneouscatalysis,we observedthat 3%ofcatalyst loadingisenoughtogetanenrichedmediuminmonoesters.

3.4. Kineticstudies

3.4.1. Freecatalyticesterification

Thereactionofaconiticacidwithisoamylalcoholwastested underfreecatalystunderthestandard,100◦_{Candamolarratio}

“isoamylalcohol:aconiticacid”of6:1.

Fig.6 represents therelativepercentage of compounds dur-ingthereaction,correspondingtotheself-catalysiscapacityofthe reaction.

Spontaneously,thereactionofesterificationgeneratesthe for-mation of monoesters which are converted progressively into diestersandinmorelimitedproportionsintotriester.

Thisexperimentconfirmsthatmonoestersandthendiestersare intermediatesforthesynthesisofthefinaltriester.

Finally,thecontributionof self-catalysismustbeconsidered especiallyforlowcatalystloadingsunderhightemperatures.

Fig.6. Self-catalyzedesterification ofaconiticacid: molarratioisoamyl alco-hol/aconiticacid,6:1,100◦_{C(:AA;N:MIA;:DIA;×:TIA; :conversion).}

3.4.2. Kineticprofilesaccordingtocatalyst

Thepreliminarytestsallowedtodefinetheexperimental con-ditionssummarizedinTable3toperformthekineticstudies.

Figs.7–9showtheplotsoftheconversionofaconiticacidas

limitingreactant.Therelativepercentagesinthereactionmedia (aconiticacid,monoesterMIA,diesterDIAandtriesterTIA)arealso represented.

Itwasfoundthattheuseofsulphuricacidleadstothemaximum conversioncloseto100%before2h.

Thecation-exchangeresinalsoprovidesahighconversionbut witha slowerkinetic:almost100%ofaconiticacidisconverted after4h.

With the ionic liquid, a lower conversion close to 70% is observed,thatisstillacceptable.

Wecannotethatthetypeofcatalysthasaninfluenceonthe selectivityofreactionsincethecompositionsofestersare differ-entaccordingtotheconditions.Table4indicatesthemajoresters presentinthefinalreactionmedia;homogeneouscatalystmainly providesthetriesterwhereasheterogeneouscatalystorientatesthe reactiontowardstheproductionofmonoestersanddiesters.The ionicliquidratherfavourstheformationofdiesters.

Asitisnotpossibletogetatotalselectivityforthemonoesters andthediesters,wethoughtthatitcouldbeinterestingto pre-parecompositionsenrichedwiththedesiredester.Wehavethus selectedspecifictimesofreactioncorrespondingtothehighest con-tentinesters(Table4).Unreactedaconiticacidofconditions2*and 3,wasremovedbyextractionwithwater.

Itisthuspossibletopreparefiveenrichedcompositionswith oneoftheesters.Methods1and2improved performancesthat werementioned in previousworks as wegot almost complete conversionofacidandhigherestercontents.

Moreover,theproductionofthesemixturesavoidsan expen-sivesteptoseparateestersandtheirresultingpropertiesmaybe adaptedtosomeapplications.

3.4.3. Effectofthehydrocarbonchainlength

Thepreviousconditionshavebeenadaptedtothesynthesisof aconitateestersfromdodecylalcohol.Experimentalconditionsand resultsarepresentedinTable5.

Dodecylalcoholpresentsagoodreactivitytowardsaconiticacid, leadingtoahighconversionoftheacidintotriester,under homo-geneouscatalyst(method1′_).

Table3

Conditionsforkineticreaction.

Methods Catalysis T(◦_{C) Catalyst}

loading(wt.%)

Molarratioisoamyl alcohol:aconiticacid

1 Homogeneous 100 2.7 6:1

2 Heterogeneous 85 3 6:1

Table4

Compositionofestersinselectedmedia.

Methods Catalysis Selectedtime(min) Percentageofthemajorester(%) Acidconversion(%) 1a Homogeneous 40 Diesters(57) 98 1 540 Triester(79) 99 2a Heterogeneous 90 Monoesters(67) 73 2 540 Diesters(59) 98

3 Ionicliquid 540 Diesters(69) 64

a_{Derivedfrommethods1or2bymodifyingthereactiontime.}

Fig.7.Esterificationofaconiticacidcatalyzedbysulfuricacid,accordingtomethod 1(:AA;N:MIA;:DIA;×:TIA; :conversion).

Fig.8. Esterificationofaconiticacidcatalyzedbyion-exchangeresin,accordingto method2(:AA;N:MIA;:DIA;×:TIA; :conversion).

Forheterogeneousconditions,theconversionofaconiticacidis lowerwithdodecylalcoholcomparedtoisoamylalcohol(methods 2and2′_{).Thisresultcanbeexplainedbythelimitingeffectdueto}

themorerestrictedaccessibilityofthecatalyticsitesbythelong chainalcoholandtoaloweralcohol:acidratio.

Fig.9.Esterificationofaconiticacidwithionicliquid,accordingtomethod3(:AA; N:MIA;:DIA;×:TIA; :conversion).

Therefore,thechainlengtheffectismainlyobservedinthe pres-enceofmacroporouscatalysis.

Moreover,theresultsconfirmthatmacroporouscatalyticsites alsolimittheconversionofmonoestersanddiestersintotriester.In fact,macroporousresinsrepresentefficientconditionstoprepare preferentiallymonoestersanddiesterswithbothalcohols.

Thecompositionsinmonoesters/diestersobtainedwith meth-ods2and2′_{showthatthetemperatureisasignificantparameterto}

enhancetheconversionofmonoesterintodiester,asitwasnoted withTable1.

Finally,heterogeneouscatalysisoffersperformingconditionsto reachagood selectivitytowardsmono/diestersbutalsoletthe possibilitytoobtaincompositionrichintriesterbyactingonthe temperature.

3.5. Environmentalfactors

AconvenienttoolisproposedbyEissenandMetzgertocompare alternativechemicalsynthesesregardingtheirpotential environ-mental impact.TheEATOS(EnvironmentalAssessment Toolfor Organic Synthesis) procedure(Eissen and Hungerbühler, 2003;

EissenandMetzger,2002)allowstocalculateenvironmental

per-formancesmetricswhenthesystematicdesignofmoresustainable processesisundertakenonalaboratoryscale.Metricswhichare consideredarethefollowingones:

Table5

Comparisonbetweenthereactivityofisoamylalcoholanddodecylalcohol(for5h).

Methods Catalysis Percentageofthemajorester(%) Aconitic conversion(%) Temperature(◦_{C) Catalyst} loading(%) Molarratio alcohol:acid 1

Homogenous Tri-isoamylaconitate(66) 91 100 2.7 6:1

1′ _{Tridodecylaconitate(75) 96 100 2.7 3:1}

2

Heterogeneous Mono-/di-isoamylaconitate(59/36) 87 85 3 6:1

Fig.10.Preparationoftri-isoamylaconitate.

-Themassindex,S−1_{,whichisthemassofallrawmaterials[kg]}

usedforthesynthesis,permassunitofthepurifiedproduct(raw materials,solvents,catalysts,auxiliaries,etc.).

- Theenvironmentalfactor,E,whichrepresentsthemassofwastes [kg]permassunitoftheproduct.

ForthedeterminationofEandS−1_{,thefollowingmaterialshave}

beenintegratedforthecalculationoftheamountofwastes: -Thenon-reactantalcoholandacidaccordingtotheyield; -Theco-productssuchaswaterandotheresters;

-Thehomogeneouscatalyst(H2SO4);

-Theamountofwaternecessarytoremovethehomogeneous cat-alyst;

-Theamountofsodiumsulphatetodryorganiclayer;

-Aquarterofthemacroporousresinandionicliquidweight,since thetestshaveshownthattheycouldbeusedfourtimeswithout significantlossofperformances;

-Theextractionsolvent.

Thesehypothesesallowedtocomparethedifferentroutesto obtainthetargetedproduct.

Calculationswerecarriedoutwithtwodifferentpurposes: -Comparisonofdifferentsynthesesofthetriester;

-Comparisonofsynthesesforthreedifferentenrichedmedia. 3.5.1. CalculationofE-factorandS−1_{forsynthesesoftriester}

Thecalculationsdealwiththesynthesisoftri-isoamylaconitate accordingtothemethodsI,IIandIIIwhichhavebeenadaptedfrom methods1,2and3(Fig.10andTable6).

Fig.11showsaquantitativeassessmentofmethodsI,IIandIII. TheprotocolIIisthemosteffectiveprocedurewithregardtoits massefficiency(S−1_=10.3kgkg−1_{),andtoitsenvironmentalfactor}

(E=9.3kgkg−1_{).TheeffectivenessofmethodIIisduetoaneasy}

treatmentstep(recoveringofcatalyst,lowamountofauxillaries, nosewage).Moreover,weknowthattheyieldoftriestercanstill beimprovedbyrisingthetemperature.

WecanconfirmthattheweakpointofthemethodIisthe pro-ductionofsewage.MethodIIIispenalizedbyseveralfactors:the lowerconversionintriester,theuseofanextractionsolventand theamountofionicliquid.

Table6

Datausedforthecalculationofgreenunitsforthesynthesesoftri-isoamylaconitate forareactiontimeof300min.

Conditions Methods

I II III

Temperature 100 120 120

Catalystloading(%) 3.7 5 Cosolvent Acidconversion(%) 89.5 86.3 89.3 Yieldoftriester(%) 64.5 56.5 34.1

Fig.11.CalculationofmassindexS−1_{andenvironmentalfactorE(softwareEATOS)}

forthetri-isoamylaconitatesynthesis–methodsI(H2SO4),II(Amberlyst15)and

III(Ionicliquid).

3.5.2. CalculationofEandS−1_{forsynthesisofenrichedmedia}

Thecalculationsconcernthesynthesesofthethreeenriched media,withtheconditionspresentedinTable7.

Fig.12showsthecomparisonofmassindexS−1_{andE-factorfor}

themethods1,2and3.

Theproductionofmonoesterwithmethod2leadstothelowest massefficiency(S−1_=18.5kgkg−1_{)andthelowestenvironmental}

factor(E=17.5kgkg−1_{)duetotheadvantagesdescribedfor}

hetero-geneouscatalyst.

Formethod3,wecanthusidentifytwolimitingfactors,theyield andtheamountofauxiliaries.Bymodifyingthecharacteristicsof ionicliquid,wemightaffectpositivelythemassefficiencyandthe environmentalfactor.Thepresentresultsshowtheuseofanionic liquidcanbejustifiedwhenaspecificselectivityistargeted. Table7

Datausedforthecalculationofgreenunitsforthesynthesisofenrichedmedia accordingtothedescribedprotocols.

Conditions Methods

1 2 3

Temperature(◦_{C) 100 85 100}

Catalystloading(%) 2.7 3 Cosolvent

Reactiontime(min) 540 90 540

Acidconversion(%) 99 73 64

Fig.12. CalculationsofmassindexS−1_{andenvironmentalfactorE(softwareEATOS)}

forthepreparationofenrichedcompositions–methods1(H2SO4),2(Amberlyst15)

and3(Ionicliquid).

Thesedatashouldbetakenintoconsiderationwhenascale-up isplaned.

4. Conclusion

Thisstudyshowsthepotentialitiesofaco-productofthe sug-arcaneindustryasrawmaterialtoprepareseveralesters.

Newconditionshavebeenstudiedtosynthesizeaconiticesters inastirredbatchreactorandtoorientatethereactiontowardsan estercategory(monoesters,diestersortriester).

Quantumcalculations have giveninsight in thereactivity of thethreesitesandhaveconfirmedthesitewhichispreferentially esterified.

Intheexperimentalpart,theeffectsofparametershavebeen studiedtoprepareenrichedcompositionsavoidingafurtherstep ofpurificationandleadingtoimprovedyieldsforspecificesters.

Moreover,thepreparationofesterswithdifferenthydrocarbon chainsallowstomodulatethephysico-chemicalpropertiesofthe moleculestomeetseveralspecifications.

Fortheproductionoftriesterswithlongorshortchains, homo-geneouscatalysisleadstothebestselectivity,butheterogeneous catalystrepresentsthebestconditionstomeetgreen chemistry criteria.

For the production of enriched media,high conversions are obtained to produce compositions with major esters contents above65%.Theseenrichedcompositionshavevariablehydrophilic propertiesaccordingtothenumberofesterifiedsites,which mod-ifiesthepolarhead.

Amongthethreeenrichedcompositions,thecalculatedgreen indicators(massindexand environmentalfactor)werethebest fortheheterogeneousconditions.Despitethelackofbenefits con-cerninggreenindicators,conditionsassistedbyanionicliquidare performingtoshifttheequilibriumwithoutthecostofadistillation.

Thefeaturesofresinmacroporoussitesfostertheproduction ofmonoestersanddiesters,bylimitingtheconversionofdiesters intotriester.Inthiscase,wehaveshownthatanincreasingtriester proportioncanbereachedbyactingontemperature.

Finally,macroporousresinsstillremainthebestwaytoimprove conventional methods into more ecofriendly routes. Moreover, suchconditionsoffertheopportunitytoactontheselectivityand thustopreparenewbioproductsofgreatinterest.

Acknowledgements

WewouldliketothanktheRegionalCouncilofReunionIsland foritsfinancialsupport.eRcaneisalsogratefullyacknowledgedfor itsfinancialcontributionanditscooperation.

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