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Sulfonation of methyl esters (ME) to produce MES is a rather more complex process than sulfonation of other major feedstocks. It is now common, with modern reactor technology and present-day feedstock quality, to produce linear alkylbenzene sulfonates (LAS), primary alcohol sulfates (PAS), alcohol ethoxysulfates (AES), and alpha olefin sulfonates (AOS) without the need for bleaching (3). In contrast, ME sulfonation leads to very dark colored products (Klett values well in excess of 1000) (4). Consequently all current commercial ME sulfonation processes require a bleaching step. Other distinguishing features of ME sulfonation are the need for a significantly greater than stoichiometric mole ratio of 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. If the mole ratio of SO3 to ME is significantly lower than 1.2, full conversion of the ME cannot be achieved. This stage is usually carried out continuously in a falling film reactor. If the reaction mixture is neutralized at this point, much of the ME is recovered unconverted, with conversion of ME to sulfonated products in the range of 60–75%. The neutralized sulfonated products at this stage contain very little MES, being mainly composed of the “di-salt”

RCH(CO2Na)SO3Na together with sodium methyl sulfate (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 than in the aging step for linear alkylbenzene (LAB) sulfonation, requiring temperatures of at least 80°C. The residence time required depends on the temperature, the mole ratio of SO3 to ME, the target conversion level, and the reactor characteristics. Thus, with a batch reactor or an ideal plug flow reactor (PFR) and a mole ratio of 1.2, 45 minutes at 90°C or 3.5 minutes at 120°C should give about 98% conversion. With an ideal continuously stirred tank reactor (CSTR), these aging times would need to be doubled. Usually this stage is carried out continuously in reactors whose characteristics are intermediate between ideal PFR and ideal CSTR.

3. A neutralization stage. If the acidic reaction mixture is not neutralized, it deteriorates in color and, particularly for C16 and higher ME feedstocks, becomes very viscous and can even solidify unless heated. Neutralization on a commercial or pilot plant scale is usually carried out continuously in a loop reactor. It is important to avoid extremes of pH in neutralization so as to avoid hydrolysis of MES to di-salt. Neutralization is usually carried out to give a ca. 60% AM paste (AM = Active Matter, in this case consisting of MES + di-salt).

The neutralized product from a process involving just these three stages would be a very dark colored paste or solution with a Klett value well above 1000. The sulfonation product would consist of a mixture of MES and the di-salt, RCH(CO2Na)SO3Na, in proportions of ca. 80:20. Sodium methyl sulfate, MeOSO3Na, is also present, approximately equimolar with the di-salt. The overall reaction is shown in Scheme 1.

Depending on the formulation in which the MES is to be used, the presence of ca. 20% of the di-salt may or may not be acceptable. It is generally regarded as an inferior surfactant compared with MES. Usually, therefore, two further 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. This consists of treating the acidic reaction mixture with methanol before neutralization, and this step can reduce the di-salt content of the neutralized final product to well below 10% (based on 100% active).

REACTION CHEMISTRYInitial reactions and aging reactionsThe initial reaction steps in ester sulfonation occur during the ME/SO3 contacting stage. Probably via complexes formed reversibly between SO3 and the oxygen atoms of the ester, an α-sulfonated intermediate with 2:1 SO3/ME stoichiometry is formed. This intermediate is believed to have the structure RCH(SO3H)COOSO3Me (4–6). In the aging step, it reacts with the remaining ME, as shown in Scheme 2. The traditional interpretation (4) is that this is the only reaction occurring during aging (excluding color formation, which, despite the intense color, consumes only trace amounts of material), and that the di-salt and SMS that are found in the neutralized material come from residual RCH(SO3H)COOSO3Me. However, this interpretation is inconsistent with the facts:

1. If this were the only reaction, it should be possible to achieve 100% conversion with an SO3/ME mole ratio of 1:1. This is not the case: With 1:1 mole ratio, conversion does not increase beyond about 85% (6).

2. Kinetic plots show that there are at least two conversion-increasing reactions taking place in the aging step, with different rate constants (6).

Aging kineticsA kinetic model has been developed based on the proposal that two major intermediates are involved in aging (Scheme 3). One is RCH(SO3H)COOSO3Me and the other is a 3:1 adduct (or mixture of compounds with overall 3:1 SO3/ME stoichiometry) (6). Although this model is almost certainly simpler 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/ME mole ratios.

The concentration of unconverted ME in the reaction mixture decreases with time according to a model (6) based on two concurrent pseudo first order reactions, one (due to sulfonation by the 3:1 intermediate) being faster than the other (sulfonation by the 2:1 intermediate). The rate constants kf 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 (%) = 100 M (1/M100) − 0.25 exp (−kst) − 0.167 exp (−kft)

M100 is the mole ratio that is just sufficient to give a conversion of 100% after prolonged aging. Rounded to one decimal place, its value is 1.2. These equations, for aging in a batch reactor system or in a plug flow system, can be used as guidelines when setting initial conditions before fine-tuning plant operation to meet a required specification.

By-productsSignificant levels (ca. 5% each) of two other by-products can be detected in fresh carefully neutralized solutions of MES. These, shown in Scheme 4, are iso-MES, RCH(CO2Na)SO3Me, and dimethyl sulfoalkanoate (di-MES), which are easily

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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hydrolyzed to di-salt and MES, respectively (7). Formation of the iso-MES precursor predominates in the early part of the aging, and the formation of di-MES predominates toward the end of the aging period. Dimethyl sulfate also can be detected in fresh carefully neutralized solutions of MES, at up to 1%, but rapidly decays to undetectable levels (7). If the acidic reaction product is treated with methanol before mild

neutralization, extra di-MES, but no iso-MES, is observed. 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 which the major intermediate, the mixed anhydride, acts as a methylating agent toward sulfonate groups. In the early stages of aging, the major species with a sulfonate group is the mixed anhydride itself, and toward the end of the aging process, the major sulfonate species is MES in its acid form. Scheme 5 represents the overall stoichiometry of the disproportionation reactions. It is likely that the detailed mechanisms are more complex than shown, involving dimethyl sulfate as the

methylating species. Dimethyl sulfate could be formed by attack of ionized MeOSO3H on the methyl group of the mixed anhydride.

The proposed precursor of iso-MES is the methylated mixed anhydride (MMA). Since iso-MES is hydrolyzed to di-salt, MMA can be regarded as a precursor of the di-salt. The di-acid shown in Scheme 5 is also a precursor of the di-salt. It is important to note that, since MMA does not have an ionizable sulfonate group, it cannot undergo the reversible intramolecular reaction to a cyclic mixed anhydride, which is proposed as a key step in the release of SO3 during aging (see Scheme 6). Thus, the SO3 in the form of the OSO3Me group in MMA is not available as a sulfonating agent. The formation of MMA explains why a 1:1 mole ratio of SO3/ME is not enough to give complete conversion. MMA, di-acid, and di-MES are all final products in the aging process.

Color formationAs mentioned earlier, severe color formation is a characteristic of ester sulfonation. ME feedstocks containing unsaturated fatty acid ME yield particularly severe colors, and these have been attributed to the formation of poylsulfonated impurities with conjugated double bonds (8). An unsaturated ME is an internal olefin with a carboxymethyl group at one end of the hydrocarbon chain. Olefins are very readily sulfonated by SO3, much more so than saturated esters, so if a mixture of saturated and unsaturated ME is sulfonated under MES conditions, the unsaturated ester tends to react first, at the double bond, and subsequently oversulfonation and oxidation of the resulting carboxymethyl internal olefin sulfonate compete with sulfonation of the saturated ester. In practice it is very difficult to produce MES of good color quality, even

Scheme 3. Model for MES aging kinetics

Scheme 4. By-products of ester sulfonation

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after bleaching, from an ME with an iodine value (IV) greater than 1. Generally, the lower the IV the better, and feedstocks hydrogenated to IV values of ca. 0.1 are preferred (1).

The foregoing should not be taken to mean that unsaturated impurities in the feedstock are the only cause of the color produced in saturated ME sulfonation. Even with laboratory feedstocks having undetectable levels of unsaturation, color formation is very severe compared with what is experienced in, for example, LAB sulfonation. However, the color can be removed by bleaching much more easily than when the feedstock has IV > 1.

The following explanation for color formation from saturated ME has been proposed (9). The major reaction in the aging step is the conversion of the intermediate RCH(SO3H)COOSO2OMe to RCH(SO3H)CO2Me and SO3, which reacts with more of the ester. The proposed mechanism is via reversible formation of a cyclic β-anhydride and MeOSO3H, as shown in Scheme 6. As a minor side reaction, this β-anhydride may undergo reversible unimolecular ring

opening to a zwitterion, which could lose carbon monoxide to give an alkene sulfonic acid (Scheme 6). Alkene sulfonic acids are formed as major products in alpha olefin sulfonation, and alpha olefin sulfonates are very susceptible to color formation if aged in the acid form.

To suppress the unimolecular ring opening of the cyclic β-anhydride, and hence to suppress color formation, an extra competing reaction leading to release of SO3 should be provided. Inorganic sulfates should serve this purpose (Scheme 6) and indeed have been shown to reduce the extent of color formation (10, 11).

BleachingAll MES processes require a bleaching step. Usually hydrogen peroxide is used as the bleaching agent, and it can give good results when used either before or after neutralization.

Acid bleaching can be carried out on the acid after the re-esterification step, or simultaneously with re-esterification by addition of methanol at the same time. Hydrogen peroxide,

Chemistry of Methyl Ester Sulfonates

Scheme 5. Disproportionation reactions of the mixed anhydride

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usually added at 2–3%, is used as an aqueous solution (35 or 50%). The water introduced at this stage tends to hydrolyze the MES acid, which would lead to substantially increased levels of di-salt after neutralization. Residual methanol from the re-esterification, or methanol added at the bleaching stage, can suppress this hydrolysis and also reduces the viscosity of the reaction mixture (1). Without addition of methanol, the di-salt level would be too high for many applications.

Paste bleaching is in many ways simpler than acid bleaching—for example, hydrolysis during bleaching is not a major issue—but has sometimes been considered to be less reliable. This is probably because the implications of the chemistry of the by-product di-MES have not been fully appreciated. As discussed earlier, di-MES, which has

both the sulfonate and the carboxylate groups as their methyl esters (Scheme 4), is formed at about 5% level during the acid aging process, and a further 5% is formed from the iso-MES precursor if methanol is added in a re-esterification step. Although di-MES is easily hydrolyzed, the MES neutralization step is carried out under mild conditions, to avoid hydrolysis of the MES, and consequently some di-MES can survive. Di-MES is electrophilic and reacts with the nucleophilic hydroperoxide anion, leading to depletion of the bleach. To eliminate this source of inconsistency, the neutralized paste can be aged before bleaching (11) to allow the residual di-MES to be hydrolyzed. After paste aging the bleaching step gives consistently good results that are comparable with acid bleaching.

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

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OCCUPATIONAL AND CONSUMER SAFETY ISSUES FOR MES Like all sulfonate and sulfate surfactants in their acid forms, MES acid is corrosive. This in itself is no more of a problem than with well-established surfactants such as LAS acid. However, there are some special features uniquely associated with ester sulfonates.

Firstly, the methanol injected for re-esterification and/or acid bleaching is flammable, having a flash point of 10°C and being explosive in the range of 5–44% in air. Appropriate safety 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, precautions must be taken against explosions in the recovery step, which can happen if peroxides formed in the bleaching step build up. This is potentially more of a problem with acid bleaching than with paste bleaching.

Thirdly, the by-products in MES acid are hazardous by skin contact and, in the case of dimethyl sulfate, by inhalation. Dimethyl sulfate, detectable at 1% (100% AM basis) in fresh carefully neutralized MES solutions, penetrates skin readily and is a carcinogen in rats. However, it does not survive long after neutralization and is not a cause for concern regarding consumer safety. Di-MES, present in MES acid at ca. 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 consumer safety grounds. The 100 ppm level may be exceeded in the freshly neutralized paste, but not in bleached paste or in dried MES.

HYDROLYIC STABILITYThe CO2Me group of MES can undergo hydrolysis to CO2H (acid hydrolyis) or CO2Na (alkaline hydrolysis). Thus, MES can be hydrolyzed to di-salt or the corresponding acid. However, due to a combination of steric and electronic effects of the α-sulfo group, the hydrolysis is slower than for nonsulfonated esters. In the pH range 3–9.5 the hydrolysis is very slow (12). Under conditions of acid bleaching with aqueous hydrogen peroxide the pH will be below this range, which explains why hydrolysis during acid bleaching can be quite extensive if methanol is not added. In a neutralization loop the pH can be controlled below 9.5 so hydrolysis can be minimized at that stage.

Laundry powders have traditionally been made by spray drying an aqueous slurry of the surfactants and the inorganic salts in a spray drying tower to form the bulk of the formulated powder. Spray drying is still widely used, but nontower routes are now also used. Under spray-drying conditions MES undergoes partial hydrolysis to di-salt. Thus, MES is not suitable for formulation by spray drying. However, it can be used as the dried solid for nontower production of powders.

SURFACTANT PROPERTIES AND APPLICATIONS OF MESIn Table 1 critical micelle concentrations (CMC) and Krafft points (TK) are tabulated for MES homologs and the corresponding PAS homologs. For the sodium salts and the calcium salts, the CMC values of the two surfactants are similar when the same carbon numbers (excluding the methyl carbon of the ester group in MES) are compared. However, the TK values for sodium PAS are much higher (by over 20°C) than for the corresponding sodium MES. The difference is even greater (by over 40°C) when TK values of the calcium salts are compared. The di-salt that can result from hydrolysis of MES has a much higher TK value than the corresponding MES, as illustrated in Table 1 for the C16 homolog.

Table 1. CMC and TK for MES and linear PASa

Surfactant CMC (mmolar) TK (°C) ReferenceSodium salts C12 MES 5.3 <0 4,12C12 PAS 8 8 14C14 MES 2.8 6 4,12C14 PAS 2 30 14C16 MES 0.4 17 4,12C16 PAS 0.4–0.6 45 14C18 MES 0.08–0.16 30 4,12C18 PAS 0.2 56 14C16 Di-salt 65 4Calcium salts C14 MES 0.66 28 12C14 PAS 0.68 71 12C16 MES 0.19 41 12C16 PAS 85 12C18 MES 0.04 49 12

a Abbreviations: CMC, critical micelle concentration; TK, Krafft temperature; MES, methyl ester sulfonate; PAS, primary alcohol sulfates.

The figures in Table 1 demonstrate that for MES the higher homologs, which are more abundant in vegetable oils and have better detergency, are sufficiently water soluble to be useful in low-temperature laundry products. For PAS the higher homologs are too insoluble and the less abundant lower homologs (C12–C14) are more suitable for low-temperature laundry products. It is also clear that MES is much more calcium tolerant than PAS. The di-salt, however, is less water soluble and less calcium tolerant.

Various detergency measurements comparing different homologs of MES against each other and against other surfactants have been reported (e.g., 4, 13). Such comparisons do not always translate well into realistic consumer use situations, but the following general conclusions can be drawn:

Chemistry of Methyl Ester Sulfonates

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1. The optimal detergency with MES is for the C16 homolog (whose parent ME will be the most abundant if the feedstock is sourced as a by-product from biodiesel).

2. MES detergency is more resistant to water hardness than detergency of other anionic surfactants.

3. In enzyme detergent formulations, enzyme activity is less affected by MES than by other surfactants.

4. MES is a good lime-soap dispersant, i.e., when used as a co-surfactant with soap in hard water it prevents the precipitation of calcium soap.

These surfactant properties of MES are all consistent with the concept that the polar but uncharged ester linkage, being in close proximity to the negatively charged sulfonate group, reduces the charge density of the latter so that electrostatic binding to cations is weaker than in simple sulfonates and sulfates. A similar effect, in this case the interaction between the ether linkage and the sulfate group, underlies the differences between PAS and AES.

Because of these properties and because of the perceived potential 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-based products are extensively used, in combination with soap (4). For these applications a carbon number distribution with C16 dominant is the optimum. Because of its mildness to skin and mucous membranes, MES is also of interest for dishwashing and shampoo applications, the C12/C14 carbon numbers being preferred (4).

ENVIRONMENTAL CHARACTERISTICSIn aquatic toxicity studies, MES has been shown to behave, like other anionic surfactants, as a polar narcotic (15). Table 2 shows EC50 values to Daphnia for C12 to C16 homologs. C18 MES is too insoluble to be tested alone. Intrinsically its toxicity is greater than that of the C16 homolog, and for

tallow-based MES (C16/C18) fish toxicity EC50 values of 0.4–0.9 have been reported (16). However, the primary degradation of ester sulfonates (see below) is fast and would prevent an accumulation of toxicity.

Table 2. Daphnia toxicity of MES

MES EC50a (mg/L)

C12 184

C14 28

C16 7

a EC50, effective concentration at which a 50% response is observed.

The biodegradation characteristics of MES are rather similar to those of LAS. Although it is resistant to anaerobic biodegradation, in aerobic systems MES undergoes rapid biodegradation of the alkyl chain to a slower-degrading residue, which is ultimately completely mineralized (16). The biodegradation pathway is shown in Scheme 7. The initial ω-oxidation step at the end of the alkyl chain is followed by a sequence of β-oxidation cycles to arrive at monomethyl α-sulfosuccinate. This undergoes desulfonation to succinic acid, which features naturally in cell metabolism.

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. It is conceivable that a future economic situation could arise, for example government incentives to combine bioethanol technology with biodiesel technology to produce ethyl ester biodiesel, where ethyl esters become attractive as sulfonation feedstocks. The sulfonation characteristics of ethyl esters are very similar to those of ME, although higher temperatures or longer reaction times are required for the aging and transesterification stages. It should be quite straightforward to adapt an MES production operation to produce ethyl ester sulfonates.

Scheme 7. MES biodegradation

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REFERENCES1. Roberts, D.W., Manufacture of Anionic Surfactants, in

F.D. Gunstone and R.J. Hamilton, eds., Oleochemical Manufacture and Applications, Sheffield Academic Press, Sheffield, U.K., 2001, pp. 55–73.

2. Ahmad, S., P. Siwayanan, Z. Abd Murad, H. Abd Aziz, and H. Seng Soi, Beyond Biodiesel. Methyl Esters as the Route for the Production of Surfactants Feedstock, inform 18:216–220, 2007.

3. Roberts, D.W., Sulfonation Technology for Anionic Surfactant Manufacture, Org. Proc. Res. Devel. 2:194–202, 1998.

4. Schwuger, M.J., and H. Lewandowski, α-Sulfomono-carboxylic Esters, in H.W. Stache, ed., Anionic Surfactants, Organic Chemistry, Vol. 56 in Surfactant Science Series, Marcel Dekker, New York, 1995, pp. 461–500.

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

6. Roberts, D.W., P.S. Jackson, C.D. Saul, C.J. Clemett, and K. Jones, A Kinetic and Mechanistic Investigation of Ester Sulphonation, Proc. 2nd World Surfactants Congress, Paris, Vol. II, 38–41, 1988.

7. Roberts, D.W., C.J. Clemett, C.D. Saul, A. Allan, and R.A. Hodge, Intermediate By-products in Methyl Ester Sulphonation, Jorn. Com. Esp. Deterg. 26:27–33, 1995.

8. Yamada, K., and S. Matsutani, Analysis of the Dark Colored Impurities in Sulfonated Fatty Acid Methyl Ester, J. Am. Oil Chem. Soc. 73:121–125, 1996.

9. Roberts, D.W., The Origin of Colour Formation in Methyl Ester Sulphonation, Jorn. Com. Esp. Deterg., 37:153–159, 2007.

10. United States Patent 6,657,071, to Lion Corporation. December 2, 2003.

11. United States Patent Application USSN 61/026,174, Desmet Ballestra S.p.A., February 8, 2008.

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

13. Satsuki, T., Applications of MES in Detergents, inform 3: 1099–1108, 1992.

14. Domingo, X., Alcohol and Alcohol Ether Sulfates, in H.W. Stache, ed., Anionic Surfactants, Organic Chemistry, Vol. 56 in Surfactant Science Series, Marcel Dekker, New York, 1995, pp. 223–312.

15. Roberts, D.W., S.J. Marshall, and G. Hodges, Quantitative Structure-Activity Relationships for Acute Aquatic Toxicity of Surfactants, World Surfact. Congr., 4th, 4: 340–351, 1996.

16. Gode, P., W. Guhl, and J. Steber, Ökologische Bewurtung von α-Sulfofettsauremethylestern, Fat Sci. Technol. 89:548–552, 1987. ■

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