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Biorenewable Resources No. 5

a special supplement to

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Surfactants from Biorenewable SourcesSurfactants from Biorenewable Sources

SD supplement14.indd 1 3/17/2008 9:52:32 AM

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The increasing number of biodiesel plants in southeast ■Asia will make available, in the near future, large quantities of C16 methyl ester fractions suitable for MES production at competitive cost (in spite of the sharp cost fluctuationsofpalmoilrecordedintheselastmonths).

Thecontinuousincreaseinfossiloilcostisreflected ■in a comparable increase in oil-derived raw material cost (i.e.,LABprice),makingtheuseofMESmoreattractiveeconomically.

The technological progress achieved in MES processing ■into powder form has led to a substantial improvement in MES quality, production safety, and a reduction in processingcost.

For these reasons, important new MES production plants arebeinginstalledbyDesmetBallestrainSoutheastAsiaand South America for an overall capacity close to 150,000 tons/yearofdryMES.Thesenewplantsareexpectedtostartindustrialproductionbytheendof2008.

The increased availability on the market of large quantities of MES meeting stringent quality standards should make it more attractive for detergent manufacturers to invest in adapting their formulations and production processes to incorporateMESasapartialortotalsubstituteforLAB.

This in turn should create new demand in the market, convince other surfactant manufacturers to invest in MES, and setupthebasisfortheaffirmationofMESasakeyworldwideplayer among the anionic surfactants applied to the detergent industry.

As a world-leading company in the design and supply of chemicalplantsforsurfactantsproduction,DesmetBallestrais committed to make available to surfactants manufacturers anefficient,safe,reliable,andproventechnologytomeetthefuturechallengesofthesurfactantindustry.■

IntroductionThe application of methyl ester sulfonate (MES) in the anionic detergent industry as a partial or total substitute for linear alkyl benzene (LAB)-based surfactants has been debated for several years. In spite of the appeal of MES as an anionic surfactant obtained from renewable sources and its more than satisfactory performance, until now its application in the surfactant and detergent industry has been limited to a few cases in which specifi c economic conditions have justifi ed its use in a large industrial scale.

Introduction / Surfactants from Biorenewable Sources

Contents

1 Introduction

2 Chemistry of Methyl Ester Sulfonates

10 The Challenge of the Anionic Surfactant Industry

inform StaffAreaManagerofPublications:JackWolowiec([email protected])ManagingEditor:JeremyCoulter([email protected])TechnicalProjectsEditor:MargueriteTorrey([email protected])AssociateEditor:CatherineWatkins([email protected])PublicationsDepartmentEditor:WilliamGillespie([email protected])AdvertisingSales:ValorieDeichman([email protected]) AOCS Offi cersPresident:PhilipBollheimer,Bollheimer&AssociatesInc.,Memphis,TN,

USAVice President: Casimir Akoh, University of Georgia, Athens, GA, USASecretary: Keith Grime, JKG Consulting, Cincinnati, OH, USATreasurer:StevenHill,KraftFoods,Inc.,Glenview,IL,USAExecutiveVicePresident:JeanWills,AOCS,Urbana,IL,USA AOCS Mission StatementTobeaglobalforumtopromotetheexchangeofideas,information,andexperience,toenhancepersonalexcellence,andtoprovidehighstandardsofquality among those with a professional interest in the science and technology offats,oils,surfactants,andrelatedmaterials.

sponsored by

by Corrado Mazzanti, Sales Director, Desmet Ballestra S.p.A., Milano, Italy

LionCorporation(Japan)hasbeenusingMESextensivelyin its laundry powder formulations for the domestic market since the early1990s. In theUnitedStates a coupleofsurfactants manufacturers are producing MES to a more limited extentandusingitasaco-surfactantintheirdetergentpowderformulations or for sales to third-party soap and detergent producers.

Inbothcasestheeconomicjustificationbehindtheuseof MES lies in the adoption of ME as the basic chemical available from largely integrated processes for oleochemical andsurfactantproduction.

The big players in the detergent industry have adopted amorecautiousapproachtowardMES,justifiedinpartbyadifferent evaluation of the ratio between the economic advantage of the use of MES and the problems and costs connected to its application on a large scale in detergent powder production (i.e.,modificationofexistingformulationsandmodificationoftheclassicaldetergentpowderproductionprocesses),aswellastheuncertainavailabilityinthemarketofsuitablefeedstocks.

This situation is rapidly changing owing to a number of factors that can be summarized as follows:


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Sulfonationofmethylesters(ME)toproduceMESisarathermorecomplexprocessthansulfonationofothermajorfeedstocks.Itisnowcommon,withmodernreactortechnology and present-day feedstock quality, to produce linear alkylbenzenesulfonates(LAS),primaryalcoholsulfates(PAS),alcoholethoxysulfates(AES),andalphaolefinsulfonates(AOS)without theneed forbleaching (3). In contrast,ME sulfonation leads to very dark colored products (Klett valueswellinexcessof1000)(4).Consequentlyallcurrentcommercial ME sulfonation processes require a bleaching step.OtherdistinguishingfeaturesofMEsulfonationaretheneedforasignificantlygreaterthanstoichiometricmoleratioof SO3 to feedstock and the need for a high-temperature aging step.

For MES manufacture at least three stages are essential:

1. An ME/SO3 contacting stage, in which SO3 is chemisorbed by the ME to give intermediate species. Ifthe mole ratio of SO3toMEissignificantlylowerthan1.2,fullconversionoftheMEcannotbeachieved.Thisstageisusuallycarriedoutcontinuouslyinafallingfilmreactor.Ifthereactionmixtureisneutralizedatthispoint,muchof the ME is recovered unconverted, with conversion ofMEtosulfonatedproductsintherangeof60–75%.The neutralized sulfonated products at this stage contain very little MES, being mainly composed of the “di-salt”

RCH(CO2Na)SO3Natogetherwithsodiummethylsulfate(SMS)MeOSO3Na.

2. An aging stage in which the intermediate species react, and the conversion of ME to sulfonated products goes to completion. This aging step is much more severe thanintheagingstepforlinearalkylbenzene(LAB)sulfonation,requiringtemperaturesofatleast80°C.Theresidence time required depends on the temperature, the mole ratio of SO3 to ME, the target conversion level, and thereactorcharacteristics.Thus,withabatchreactororanidealplugflowreactor(PFR)andamoleratioof1.2,45minutesat90°Cor3.5minutesat120°Cshouldgiveabout98%conversion.Withanidealcontinuouslystirredtankreactor(CSTR),theseagingtimeswouldneedtobedoubled.Usuallythisstageiscarriedoutcontinuouslyinreactors whose characteristics are intermediate between idealPFRandidealCSTR.

3. A neutralization stage.Iftheacidicreactionmixtureisnot neutralized, it deteriorates in color and, particularly for C16 and higher ME feedstocks, becomes very viscous andcanevensolidifyunlessheated.Neutralizationona commercial or pilot plant scale is usually carried out continuouslyinaloopreactor.ItisimportanttoavoidextremesofpHinneutralizationsoastoavoidhydrolysisofMEStodi-salt.Neutralizationisusuallycarriedouttogive a ca. 60% AM paste (AM = Active Matter, in this caseconsistingofMES+di-salt).

The neutralized product from a process involving just these three stages would be a very dark colored pasteorsolutionwithaKlettvaluewellabove1000.ThesulfonationproductwouldconsistofamixtureofMESand the di-salt, RCH(CO2Na)SO3Na,inproportionsofca.80:20.Sodiummethylsulfate,MeOSO3Na,isalsopresent,approximatelyequimolarwiththedi-salt.TheoverallreactionisshowninScheme1.

Depending on the formulation in which the MES is to be used, the presence of ca. 20% of the di-salt may or may notbeacceptable.ItisgenerallyregardedasaninferiorsurfactantcomparedwithMES.Usually,therefore,twofurther steps are involved:

4. Because of the high level of color produced, a bleaching step is necessary if the product is to be used for laundry detergents or other consumer products.

Methyl ester sulfonates (MES) are anionic surfactants with the general structure RCH(CO2Me)SO3Na. They can be made by sulfonation of saturated fatty acid methyl esters, RCH2CO2Me derived from natural fats and oils (1).

Interest in MES dates back to at least the early 1960s, when there were numerous publications by Stirton et al. from the U.S. Department of Agriculture and numerous patents from detergent manufacturers. Since that time interest has continued to grow, and developments in sulfonation technology have enabled MES to become an important part of the formulators’ repertoire. Currently there is major interest in MES because of the increasing availability of MES feedstocks with the C16 derivative (methyl hexadecanoate) as the major component, as a by-product of biodiesel production (2).

Biorenewable Resources No. 5 / March 2008

Chemistry of Methyl Ester SulfonatesDavid W. Roberts, Liverpool John Moores University, Liverpool, United Kingdom

Luigi Giusti and Alessandro Forcella, Desmet Ballestra S.p.A., Milano, Italy


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5. Depending on the specification required, a “re-esterification” step may be included in the process, to convert the di-salt precursor to an MES precursor. Thisconsistsoftreatingtheacidicreactionmixturewithmethanol before neutralization, and this step can reduce thedi-saltcontentoftheneutralizedfinalproducttowellbelow10%(basedon100%active).

REACTION CHEMISTRYInitial reactions and aging reactionsThe initial reaction steps in ester sulfonation occur during the ME/SO3contactingstage.Probablyviacomplexesformedreversibly between SO3andtheoxygenatomsoftheester,anα-sulfonated intermediate with 2:1 SO3/ME stoichiometry is formed.ThisintermediateisbelievedtohavethestructureRCH(SO3H)COOSO3Me(4–6).Intheagingstep,itreactswiththeremainingME,asshowninScheme2.Thetraditionalinterpretation(4)isthatthisistheonlyreactionoccurringduringaging(excludingcolorformation,which,despitetheintensecolor,consumesonlytraceamountsofmaterial),andthatthedi-salt and SMS that are found in the neutralized material come from residual RCH(SO3H)COOSO3Me.However,thisinterpretation is inconsistent with the facts:

1. Ifthisweretheonlyreaction,itshouldbepossibletoachieve 100% conversion with an SO3/ME mole ratio of 1:1.Thisisnotthecase:With1:1moleratio,conversiondoesnotincreasebeyondabout85%(6).

2. Kineticplotsshowthatthereareatleasttwoconversion-increasing reactions taking place in the aging step, with differentrateconstants(6).

Aging kineticsA kinetic model has been developed based on the proposal that two major intermediates are involved in aging (Scheme 3).OneisRCH(SO3H)COOSO3Meandtheotherisa3:1adduct(ormixtureofcompoundswithoverall3:1SO3/ME stoichiometry)(6).Althoughthismodelisalmostcertainlysimpler than reality, it enables kinetic data to be interpreted so as to be able to calculate aging times required for different temperatures and different SO3/MEmoleratios.

The concentration of unconverted ME in the reaction mixturedecreaseswithtimeaccordingtoamodel(6)basedontwoconcurrentpseudofirstorderreactions,one(duetosulfonationbythe3:1intermediate)beingfasterthantheother(sulfonationbythe2:1intermediate).Therateconstantskf and ks for the faster and slower of these reactions, respectively, can be calculated from the temperature T (°K):

k = A exp(−B/T)for kf (sec−1)log A=12.10,B = 12,060for ks (sec−1)log A=11.52,B=12,130

The overall conversion C as a function of time t and mole ratio M of SO3/ME is given by

C(%)=100M (1/M100)−0.25exp(−kst)−0.167exp(−kft)

M100isthemoleratiothatisjustsufficienttogiveaconversionof100%afterprolongedaging.Roundedtoonedecimalplace,itsvalueis1.2.Theseequations,foraginginabatchreactorsystemorinaplugflowsystem,canbeusedasguidelineswhensettinginitialconditionsbeforefine-tuningplantoperationtomeetarequiredspecification.

By-productsSignificantlevels(ca.5%each)oftwootherby-productscanbedetectedinfreshcarefullyneutralizedsolutionsofMES.These,showninScheme4,areiso-MES,RCH(CO2Na)SO3Me, anddimethylsulfoalkanoate(di-MES),whichareeasily

Chemistry of Methyl Ester Sulfonates

Scheme 2. Traditional interpretation for ME sulfonation stoichiometry

Scheme 1. Overall chemistry of methyl ester sulfonation


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hydrolyzedtodi-saltandMES,respectively(7).Formationof the iso-MES precursor predominates in the early part of the aging, and the formation of di-MES predominates toward theendoftheagingperiod.Dimethylsulfatealsocanbedetected in fresh carefully neutralized solutions of MES, at upto1%,butrapidlydecaystoundetectablelevels(7).Iftheacidic reaction product is treated with methanol before mild

neutralization,extradi-MES,butnoiso-MES,isobserved.This suggests that the iso-MES precursor is very reactive to methanol.

The simplest interpretation for these by-products is that they are formed as a result of disproportionation reactions in whichthemajorintermediate,themixedanhydride,actsasamethylatingagenttowardsulfonategroups.Intheearlystagesofaging,themajorspecieswithasulfonategroupisthemixedanhydride itself, and toward the end of the aging process, the majorsulfonatespeciesisMESinitsacidform.Scheme5represents the overall stoichiometry of the disproportionation reactions.Itislikelythatthedetailedmechanismsaremorecomplex thanshown, involvingdimethyl sulfateas the

methylatingspecies.Dimethylsulfatecouldbeformedbyattack of ionized MeOSO3Honthemethylgroupofthemixedanhydride.

The proposed precursor of iso-MES is the methylated mixedanhydride(MMA).Sinceiso-MESishydrolyzedtodi-salt,MMAcanberegardedasaprecursorofthedi-salt.The di-acid shown in Scheme 5 is also a precursor of the di-salt.Itisimportanttonotethat,sinceMMAdoesnothaveanionizable sulfonate group, it cannot undergo the reversible intramolecularreactiontoacyclicmixedanhydride,whichisproposed as a key step in the release of SO3 during aging (see Scheme6).Thus,theSO3 in the form of the OSO3Me group in MMAisnotavailableasasulfonatingagent.TheformationofMMAexplainswhya1:1moleratioofSO3/ME is not enough togivecompleteconversion.MMA,di-acid,anddi-MESareallfinalproductsintheagingprocess.

Color formationAs mentioned earlier, severe color formation is a characteristic ofestersulfonation.MEfeedstockscontainingunsaturatedfatty acid ME yield particularly severe colors, and these have been attributed to the formation of poylsulfonated impurities withconjugateddoublebonds(8).AnunsaturatedMEisaninternalolefinwithacarboxymethylgroupatoneendofthehydrocarbonchain.Olefinsareveryreadilysulfonatedby SO3,muchmoresothansaturatedesters,soifamixtureof saturated and unsaturated ME is sulfonated under MES conditions,theunsaturatedestertendstoreactfirst,atthedoublebond,andsubsequentlyoversulfonationandoxidationof theresultingcarboxymethyl internalolefinsulfonatecompetewithsulfonationofthesaturatedester.InpracticeitisverydifficulttoproduceMESofgoodcolorquality,even

Scheme 3. Model for MES aging kinetics

Scheme 4. By-products of ester sulfonation



afterbleaching,fromanMEwithaniodinevalue(IV)greaterthan1.Generally,thelowertheIVthebetter,andfeedstockshydrogenatedtoIVvaluesofca.0.1arepreferred(1).

The foregoing should not be taken to mean that unsaturated impurities in the feedstock are the only cause of the color producedinsaturatedMEsulfonation.Evenwithlaboratoryfeedstocks having undetectable levels of unsaturation, color formationisveryseverecomparedwithwhatisexperiencedin,forexample,LABsulfonation.However,thecolorcanbe removed by bleaching much more easily than when the feedstockhasIV>1.

ThefollowingexplanationforcolorformationfromsaturatedMEhasbeenproposed(9).Themajorreactionin the aging step is the conversion of the intermediate RCH(SO3H)COOSO2OMe to RCH(SO3H)CO2Me and SO3, whichreactswithmoreoftheester.Theproposedmechanismis via reversible formation of a cyclic β-anhydride and MeOSO3H,asshowninScheme6.Asaminorsidereaction,this β-anhydride may undergo reversible unimolecular ring

openingtoazwitterion,whichcouldlosecarbonmonoxidetogiveanalkenesulfonicacid(Scheme6).Alkenesulfonicacidsareformedasmajorproductsinalphaolefinsulfonation,andalphaolefinsulfonatesareverysusceptibletocolorformationifagedintheacidform.

To suppress the unimolecular ring opening of the cyclic β-anhydride, and hence to suppress color formation, an extracompetingreactionleadingtoreleaseofSO3 should be provided.Inorganicsulfatesshouldservethispurpose(Scheme6)andindeedhavebeenshowntoreducetheextentofcolorformation(10,11).

BleachingAllMESprocessesrequireableachingstep.Usuallyhydrogenperoxideisusedasthebleachingagent,anditcangivegoodresultswhenusedeitherbeforeorafterneutralization.

Acid bleaching can be carried out on the acid after the re-esterificationstep,orsimultaneouslywithre-esterificationbyadditionofmethanolatthesametime.Hydrogenperoxide,


Scheme 5. Disproportionation reactions of the mixed anhydride


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usuallyaddedat2–3%,isusedasanaqueoussolution(35or50%).ThewaterintroducedatthisstagetendstohydrolyzetheMES acid, which would lead to substantially increased levels ofdi-saltafterneutralization.Residualmethanolfromthere-esterification,ormethanoladdedatthebleachingstage,cansuppress this hydrolysis and also reduces the viscosity of the reactionmixture(1).Withoutadditionofmethanol,thedi-saltlevelwouldbetoohighformanyapplications.

Paste bleaching is in many ways simpler than acid bleaching—forexample,hydrolysisduringbleaching isnot a major issue—but has sometimes been considered to belessreliable.Thisisprobablybecausetheimplicationsof the chemistry of the by-product di-MES have not been fullyappreciated.Asdiscussedearlier,di-MES,whichhas

boththesulfonateandthecarboxylategroupsastheirmethylesters(Scheme4),isformedatabout5%levelduringtheacid aging process, and a further 5% is formed from the iso-MESprecursorifmethanolisaddedinare-esterificationstep.Although di-MES is easily hydrolyzed, the MES neutralization step is carried out under mild conditions, to avoid hydrolysis oftheMES,andconsequentlysomedi-MEScansurvive.Di-MES is electrophilic and reacts with the nucleophilic hydroperoxideanion,leadingtodepletionofthebleach.Toeliminate this source of inconsistency, the neutralized paste canbeagedbeforebleaching(11)toallowtheresidualdi-MEStobehydrolyzed.Afterpasteagingthebleachingstepgives consistently good results that are comparable with acid bleaching.

Scheme 6. Main aging reaction mechanism, proposed color formation mechanism



OCCUPATIONAL AND CONSUMER SAFETY ISSUES FOR MES Likeallsulfonateandsulfatesurfactantsintheiracidforms,MESacidiscorrosive.Thisinitselfisnomoreofaproblemthanwithwell-establishedsurfactantssuchasLASacid.However, there are some special features uniquely associated withestersulfonates.

Firstly, the methanol injected for re-esterification and/or acid bleaching is flammable, having a flash point of 10°C andbeingexplosiveintherangeof5–44%inair.Appropriatesafety procedures for storage and handling therefore need to be followed when methanol addition is a part of the MES process.

Secondly, if methanol recovery is part of the process, precautionsmustbetakenagainstexplosionsintherecoverystep,whichcanhappenifperoxidesformedinthebleachingstepbuildup.Thisispotentiallymoreofaproblemwithacidbleachingthanwithpastebleaching.

Thirdly, the by-products in MES acid are hazardous by skincontactand,inthecaseofdimethylsulfate,byinhalation.Dimethylsulfate,detectableat1%(100%AMbasis)infreshcarefully neutralized MES solutions, penetrates skin readily andisacarcinogeninrats.However,itdoesnotsurvivelongafter neutralization and is not a cause for concern regarding consumersafety.Di-MES,presentinMESacidatca. 5–10%, is a strong skin sensitizer in guinea pigs, but at levels below 100 ppm is not considered to give cause for concern on consumersafetygrounds.The100ppmlevelmaybeexceededin the freshly neutralized paste, but not in bleached paste or indriedMES.

HYDROLYIC STABILITYThe CO2Me group of MES can undergo hydrolysis to CO2H (acidhydrolyis)orCO2Na(alkalinehydrolysis).Thus,MEScanbehydrolyzed todi-saltor thecorrespondingacid.However, due to a combination of steric and electronic effects of the α-sulfo group, the hydrolysis is slower than for nonsulfonatedesters.InthepHrange3–9.5thehydrolysisisveryslow(12).UnderconditionsofacidbleachingwithaqueoushydrogenperoxidethepHwillbebelowthisrange,whichexplainswhyhydrolysisduringacidbleachingcanbequiteextensiveifmethanolisnotadded.InaneutralizationloopthepHcanbecontrolledbelow9.5sohydrolysiscanbeminimizedatthatstage.

Laundrypowdershavetraditionallybeenmadebyspraydrying an aqueous slurry of the surfactants and the inorganic salts in a spray drying tower to form the bulk of the formulated powder.Spraydryingisstillwidelyused,butnontowerroutesarenowalsoused.Underspray-dryingconditionsMESundergoespartialhydrolysistodi-salt.Thus,MESisnotsuitableforformulationbyspraydrying.However,itcanbeusedasthedriedsolidfornontowerproductionofpowders.

SURFACTANT PROPERTIES AND APPLICATIONS OF MESInTable 1 criticalmicelle concentrations (CMC) andKrafft points (TK)aretabulatedforMEShomologsandthecorrespondingPAShomologs.Forthesodiumsaltsandthecalcium salts, the CMC values of the two surfactants are similarwhenthesamecarbonnumbers(excludingthemethylcarbonoftheestergroupinMES)arecompared.However,the TKvaluesforsodiumPASaremuchhigher(byover20°C)thanforthecorrespondingsodiumMES.Thedifferenceisevengreater(byover40°C)whenTK values of the calcium saltsarecompared.Thedi-saltthatcanresultfromhydrolysisof MES has a much higher TK value than the corresponding MES, as illustrated in Table 1 for the C16homolog.

Table 1. CMC and TK for MES and linear PASa

Surfactant CMC (mmolar) TK (°C) ReferenceSodium salts C12MES 5.3 <0 4,12C12PAS 8 8 14C14MES 2.8 6 4,12C14PAS 2 30 14C16MES 0.4 17 4,12C16PAS 0.4–0.6 45 14C18MES 0.08–0.16 30 4,12C18PAS 0.2 56 14C16Di-salt 65 4Calcium salts C14MES 0.66 28 12C14PAS 0.68 71 12C16MES 0.19 41 12C16 PAS 85 12C18MES 0.04 49 12

a Abbreviations: CMC, critical micelle concentration; TK, Krafft temperature;MES,methylestersulfonate;PAS,primaryalcoholsulfates.

The figures in Table 1 demonstrate that for MES the higher homologs, which are more abundant in vegetable oils andhavebetterdetergency,aresufficientlywatersolubletobeusefulinlow-temperaturelaundryproducts.ForPASthehigher homologs are too insoluble and the less abundant lower homologs (C12–C14)aremoresuitableforlow-temperaturelaundryproducts.ItisalsoclearthatMESismuchmorecalciumtolerantthanPAS.Thedi-salt,however,islesswatersolubleandlesscalciumtolerant.

Various detergency measurements comparing different homologs of MES against each other and against other surfactantshavebeenreported(e.g.,4,13).Suchcomparisonsdonot always translate well into realistic consumer use situations, but the following general conclusions can be drawn:



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1. TheoptimaldetergencywithMESisfortheC16 homolog (whose parent ME will be the most abundant if the feedstockissourcedasaby-productfrombiodiesel).

2. MESdetergencyismoreresistanttowaterhardnessthandetergencyofotheranionicsurfactants.

3. Inenzymedetergentformulations,enzymeactivityislessaffectedbyMESthanbyothersurfactants.

4. MESisagoodlime-soapdispersant,i.e.,whenusedasa co-surfactant with soap in hard water it prevents the precipitationofcalciumsoap.

These surfactant properties of MES are all consistent with the concept that the polar but uncharged ester linkage, being incloseproximitytothenegativelychargedsulfonategroup,reduces the charge density of the latter so that electrostatic binding to cations is weaker than in simple sulfonates and sulfates.Asimilareffect,inthiscasetheinteractionbetweenthe ether linkage and the sulfate group, underlies the differences betweenPASandAES.

Becauseofthesepropertiesandbecauseoftheperceivedpotential for cheap availability of the feedstocks, interest has grown in using MES, in combination with other anionic surfactants, in laundry powders and, in markets where soap-basedproductsareextensivelyused,incombinationwithsoap(4).Fortheseapplicationsacarbonnumberdistributionwith C16dominantistheoptimum.Becauseofitsmildnessto skin and mucous membranes, MES is also of interest for dishwashing and shampoo applications, the C12/C14 carbon numbersbeingpreferred(4).

ENVIRONMENTAL CHARACTERISTICSInaquatictoxicitystudies,MEShasbeenshowntobehave,likeotheranionicsurfactants,asapolarnarcotic(15).Table2 shows EC50 values to Daphnia for C12 to C16homologs.C18MESistooinsolubletobetestedalone.IntrinsicallyitstoxicityisgreaterthanthatoftheC16 homolog, and for

tallow-based MES (C16/C18)fishtoxicityEC50valuesof0.4–0.9havebeenreported(16).However,theprimarydegradationofestersulfonates(seebelow)isfastandwouldpreventanaccumulationoftoxicity.

Table 2. Daphnia toxicity of MES

MES EC50a (mg/L)

C12 184

C14 28

C16 7

a EC50,effectiveconcentrationatwhicha50%responseisobserved.

The biodegradation characteristics of MES are rather similartothoseofLAS.Althoughitisresistanttoanaerobicbiodegradation, in aerobic systems MES undergoes rapid biodegradation of the alkyl chain to a slower-degrading residue,whichisultimatelycompletelymineralized(16).ThebiodegradationpathwayisshowninScheme7.Theinitialω-oxidationstepattheendofthealkylchainisfollowedbya sequence of β-oxidationcyclestoarriveatmonomethylα-sulfosuccinate.Thisundergoesdesulfonationtosuccinicacid,whichfeaturesnaturallyincellmetabolism.

ESTER SULFONATES OTHER THAN METHYLSo far only ME sulfonates have been developed to the stage of production on a manufacturing scale and for use in consumer products.Itisconceivablethatafutureeconomicsituationcouldarise,forexamplegovernmentincentivestocombinebioethanol technology with biodiesel technology to produce ethyl ester biodiesel, where ethyl esters become attractive assulfonationfeedstocks.Thesulfonationcharacteristicsofethyl esters are very similar to those of ME, although higher temperatures or longer reaction times are required for the aging andtransesterificationstages.Itshouldbequitestraightforwardto adapt an MES production operation to produce ethyl ester sulfonates.

Scheme 7. MES biodegradation


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REFERENCES1. Roberts,D.W.,ManufactureofAnionicSurfactants,in

F.D.GunstoneandR.J.Hamilton,eds.,Oleochemical Manufacture and Applications,SheffieldAcademicPress,Sheffield,U.K.,2001,pp.55–73.

2. Ahmad,S.,P.Siwayanan,Z.AbdMurad,H.AbdAziz,andH.SengSoi,BeyondBiodiesel.MethylEstersastheRoute for the Production of Surfactants Feedstock, inform 18:216–220,2007.

3. Roberts,D.W.,SulfonationTechnologyforAnionicSurfactant Manufacture, Org. Proc. Res. Devel. 2:194–202,1998.

4. Schwuger,M.J.,andH.Lewandowski,α-Sulfomono-carboxylicEsters,inH.W.Stache,ed.,Anionic Surfactants, Organic Chemistry, Vol.56inSurfactantScienceSeries,MarcelDekker,NewYork,1995,pp.461–500.

5. Schmid,K.,H.Baumann,W.Stein,andH.Dolhaine,Proc. 1st World Surfactants Congress, Munich, Vol. II, 105,1984.

6. Roberts,D.W.,P.S.Jackson,C.D.Saul,C.J.Clemett,andK.Jones,AKineticandMechanisticInvestigationofEsterSulphonation, Proc. 2nd World Surfactants Congress, Paris, Vol. II,38–41,1988.

7. Roberts,D.W.,C.J.Clemett,C.D.Saul,A.Allan,andR.A.Hodge,IntermediateBy-productsinMethylEsterSulphonation, Jorn. Com. Esp. Deterg. 26:27–33,1995.

8. Yamada,K.,andS.Matsutani,AnalysisoftheDarkColoredImpuritiesinSulfonatedFattyAcidMethylEster,J. Am. Oil Chem. Soc. 73:121–125,1996.

9. Roberts,D.W.,TheOriginofColourFormationinMethylEster Sulphonation, Jorn. Com. Esp. Deterg., 37:153–159,2007.

10. UnitedStatesPatent6,657,071,toLionCorporation.December2,2003.

11. UnitedStatesPatentApplicationUSSN61/026,174,DesmetBallestraS.p.A.,February8,2008.

12.Stein,W.,andH.Baumann,α-SulfonatedFattyAcidsand Esters: Manufacturing Process, Properties, and Applications, J. Am. Oil Chem. Soc. 52:323–329,1975.

13.Satsuki,T.,ApplicationsofMESinDetergents,inform 3: 1099–1108,1992.

14. Domingo,X.,AlcoholandAlcoholEtherSulfates,inH.W.Stache,ed.,Anionic Surfactants, Organic Chemistry,Vol.56inSurfactantScienceSeries,MarcelDekker,NewYork,1995,pp.223–312.

15. Roberts,D.W.,S.J.Marshall,andG.Hodges,QuantitativeStructure-Activity Relationships for Acute Aquatic ToxicityofSurfactants,World Surfact. Congr., 4th, 4: 340–351,1996.

16. Gode,P.,W.Guhl,andJ.Steber,ÖkologischeBewurtungvonα-Sulfofettsauremethylestern,Fat Sci. Technol. 89:548–552,1987.■


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The use of natural fats and oils as renewable raw material sources for the detergent industry has enjoyed a worldwide revival ofinterest,thanksinparttothelargeuseofpalmoilasrawmaterialforbiodieselproduction.Thishasmadeavailableonthemarketpalmoil-derivedmethylester(ME)fractions(mainlyC16)thathavelimitedappealforbiodieselapplicationbutareidealasrawmaterialforsulfonation.

Palm oil-derived ME indeed represents the ideal choice on which to base the production of detergent intermediates as well asformethylestersulfonate(MES)productionviasulfonationandneutralization.

The MES application in detergent production can be as a primary anionic surfactant or as a contributing surfactant to the packageformulation.

MES offers many advantages to detergent formulators: high biodegradability, high tolerance to water hardness (particularly vs.Ca2+ions),highcompatibilitywithotherdetergentingredients(includingenzymes),andagoodoveralldetergency.

ThestepsconstitutingtheDesmetBallestracontinuousME-productionprocess,startingfromneutraloils,areindicatedinFigure1.

Technical outline of dry-MES powder production technologyDesmetBallestrastartedresearchanddevelopment(R&D)ontheMESproductiontechnologyinthemid-1970swithextensivepilotplantactivitiesleadingtoindustrialplantsin1979.In1985thefirstfallingfilmreactorswereinstalled,andatechnicalcooperationwasimplementedwithqualifieddetergentproducers.

From1994on,theR&DhasbeenfocusedonupgradingMEcharacteristicsandonbleachingtechniques,aswellasondryingandphysicalshapemodificationofMES.

TheDesmetBallestraapproachforMESproductionistomakeuseofexistingsulfonationplants,withminimummodificationsandequipmentadditions,ortodesignnewsulfonationplantsinaccordancewiththecustomer’srequests.

FIG 1. Desmet Ballestra continuous methyl ester (ME) production process.

The Challenge of the Anionic Surfactant Industryby Icilio Adami, R&D Director, Desmet Ballestra S.p.A., Milano, Italy



The Challenge of the Anionic Surfactant Industry

Inallcases,thedesignofMESplantisbasedoncharacteristicssuchas:

Plant operation ■ (safety) Optimized product ■ (quality) Favorable process ■ (economy)

ThetypicalMESplantconfiguration(Fig.2)makesuseofwellprovenprocessequipmentandoperationstepsallowingsmoothandsafeplantoperationfullyincompliancewiththeMESchemistry.

ThisapproachhasresultedinthedevelopmentofaMESprocess(Fig.3)offeringthefollowingadvantages:

Use of consolidated and well-proven ■sulfonation and neutralization process equipment

Noneedofsolventsforbleaching ■and neutralization

Noriskofhazardousby-products ■formation (as in the case of acidic bleaching)

Noneedofcostlycorrosion-proof ■material for equipment construction

Noriskofexplosivity(H ■ 2O2 added in presence of H2O and always at pH ≥6–6.5)

Low-demand ing i n s t a l l a t i on ■requirements

Possibilitytoretrofitexistingplant ■with MES-dedicated process sections withminimumimpactonmodificationcost

ME-sulfonation by last generation MTFR (multitube film sulfonation reactor)

The sulfonation of ME is accomplished bythelastgenerationMTFR(Figs.4,5),which offers the following outstanding advantages:

Use of consolidated and well-proven ■sulfonation process

MaximizedconversionofME→ ■MES

Possibilitytoretrofitexistingplant ■

Minimized reaction by-products ■

Optimized temperature control ■

MaximizedSO ■ 3 absorption

FIG. 2. MES production technology.


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ME-SO3H neutralization by double-step loop unitThesulfonicacidaftertheMTFRisagedandre-esterifiedtomaximizetheMES-acidyield.Itisthenneutralizedinthewell-consolidateddouble-stepneutralizationoperationthatisbasedonthe“forcedloop”principle.Thisunitensuresthefollowingadvantages:

Use of well-proven process equipment ■

Noneedofsolventsorviscosityaid ■

Highly active MES paste ■

Noneedforcostlycorrosion-proofequipment ■

Accurate pH and temperature control ■

Minimized di-salt content ■

MESNa neutral bleachingTheDesmetBallestraMEStechnologyisbasedonbleachingin the neutral phase, entailing the following features:

Nosolventorviscosityaidisneeded ■

Nohazardousby-products(asincaseofacidicbleaching) ■are produced

Noriskofexplosivityexists ■

ThepastecolorofMESislow(seeFig.6) ■

FIG. 3. Typical MES plant configuration.

FIG. 4. Multitube film sulfonation reactor.

FIG. 5. Schematic of multitube film reactor.

FIG. 6. Bleaching of MES paste in neutral phase. AM, active matter.



MESNa drying by wiped film evaporator (Fig. 7)The use of dry MES offers the following advantages for the manufacture of detergents: workability in processes where the liquid formcannotbeused,increasesinrangeofproductformulationsandperformance,andeaseinhandlinganddosing.Likewise,therearebenefitstobegainedinmanufacturingsurfactants:easyandsafeproductionprocesses;addedvaluefortheproduction;easystorage,handling,andtransportation;increasedproductionrange/variety;possibilityofadditionalmarketingopportunities.

Exhaust gas treatment by wet brink filterTheexhaustgasesfromthesulfonationaretreatedbythenewlydevelopedN.E.G.T.(newexhaustgastreatment)basedontheuseofawet-filter.Thistreatmentisdesignedtoeliminateacidicdrippingandtherelevantdisposalandcost.Thetreatmentprovidesthefollowingbenefits:

Elimination of maintenance demand and cost of traditional system ■

Recoverable acidic dripping production ■

Easyretrofittingintoexistingsulfonationplants ■

Gas cleaning accomplished by coupling of chemical reaction (organic + SO ■ 3)andphysicalfiltration(filteringsurfacethatiswettedbyorganicfeedstockitself)

Highandprovenefficiencywiththefullrangeofprocessableorganicfeedstocks ■

Easyoperationandcontrol■ ■

FIG. 7. Two-step process for drying MES.

The Challenge of the Anionic Surfactant Industry


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