TECHNICALFIELD

This invention relates to a process for preparing high purity sulfonated fatty acid alkyl ester surfactants. The process comprises a batch neutralization step wherein a sulfonated fatty acid alkyl ester acid mix is neutralized with an alkoxide in a substantially anhydrous medium of a lower alcohol in the presence of a buffering agent having a specified pKa value. Preferably, the surfactants are subjected to a separate color improvement process prior to use in detergent compositions including laundry detergent compositions.

BACKGROUND OF THE INVENTION

Sulfonated fatty acid alkyl ester surfactants (alternatively referred to as a-sulfo fatty acid alkyl ester surfactants, alkyl ester sulfonate surfactants, etc.) are well-known in the detergent field and have been disclosed in, e.g., U.S. Patent Numbers 5,118,440 (Cutler et al) and 4,438,025 (Satsuki et al), Japanese Laid Open Patent Publication Number 60-133097 (Application No. Showa 58-240021), Japanese Laid Open Patent Publication Number Sho 63-12466 (Patent Application No. Sho 61-151030), Japanese Laid Open Patent Publication Number Sho 59-105099 (Patent Application No.: Sho 57-215962), Japanese Laid Open Patent Publication Hei 2-173196 (Patent Application No. Sho 63-330479), Japanese Laid Open Patent Publication Number Sho 62-43500 (Patent Application No.: Sho 60-183729), and Japanese Laid Open Patent Publication Number Sho 50-151905 (Patent Application No.: 49-60284). Several processes for the manufacture of these sulfonated fatty acid alkyl ester surfactants have been disclosed in, e.g., U.S. Patent Numbers 4,695,409 (Piorr et al) and 4,820,451 (Piorr et al), German Patent Application 3 535 184 (Imamura et al), Japanese Laid Open Patent Publication Number 290842/90 (Application Number 113423/89), and "The Journal of the American Oil Chemists Society", Vol. 52 (1975), pp. 323-329.

The processes for making the sulfonated fatty acid alkyl ester surfactants described in the technical literature, though, disclose the practicability and desirability of performing the

neutralization step in aqueous media. The art has recognized certain problems inherent to such process steps, particularly handling difficulties and hydrolysis reactions. Certain of the processes yield undesirable levels of impurities such as sulfonated fatty acids salts (disalt), thereby producing a low-purity sulfonated fatty acid alkyl ester surfactant. These impurities deteriorate the desirable cleaning and viscosity characteristics of the sulfonated fatty acid alkyl ester surfactant.

Other processes described in the technical literature have observed the desirability of performing the neutralization of a sulfonated fatty acid alkyl ester acid mix in a substantially anhydrous, lower alcohol solvent medium. See, e.g., Japanese Patent OPI Publication 90- 290842. Significant problems associated with anhydrous neutralization, though, include the inability to control the neutralization step to avoid over- or under- neutralization of the acid mix, and the production of the highly undesirable impurity, dimethyl sulfate (DMS).

Now, however, a process for making a sulfonated fatty acid alkyl ester surfactant exhibiting good stability and having low levels of undesirable impurities has been discovered. By conducting a batch neutralization process step in a non-aqueous, lower alcohol medium in the presence of a buffering agent, the resultant product exhibits good stability and has a lower level of impurities. The reaction mixture exhibits good handling and in-process flow properties. Therefore, it is an object of this invention to provide a process for making sulfonated fatty acid alkyl ester surfactants containing minimal amounts of undesirable impurities. It is a further object of this invention to provide high purity sulfonated fatty acid alkyl ester surfactants having good flow properties during processing and which, following a ' separate color improvement process, are useful in detergent products. It is also an object of the invention to provide a process which prevents significant shifts in the pH of the acid mix as a result of over- or under-neutralization with the alkoxide solution and prevents hydrolysis of the resultant product.

SUMMARY OF THE INVENTION

The present invention encompasses a novel process for preparing a sulfonated fatty acid alkyl ester surfactant, said process comprising:

a) sulfonating fatty acid alkyl esters; and

b) batch neutralizing the product of step a) to a pH of from about 5 to 9 with an

n+ alkoxide of the formula (RO ) M in a substantially anhydrous medium of a n C to C alcohol and in the presence of a buffering agent having a pKa value between about 5 and about 9; wherein R is a C to C alkyl, M is an alkali

1 8 metal or alkaline earth metal cation, and n is 1 when M is an alkali metal cation and n is 2 when M is an alkaline earth metal cation;

wherein the molar ratio of the alkoxide to buffering agent is from about 5:1 to 100:1. In a preferred process, step b) comprises over-neutralizing the product of step a) to a pH of at least about 10 followed by re-neutralizing to a pH of about 6 to 8. This preferred process facilitates the degradation of the highly undesirable DMS impurity and results in a high purity alkyl ester sulfonate surfactant.

The resultant product solution of the novel process herein may be used directly in formulating detergent products, but is preferably subjected to a woricing-up procedure wherein the dark-colored impurities formed during sulfonation of the fatty acid alkyl esters are separated from the resultant solution and, subsequently, the surfactant is recovered from the solution. The surfactant is useful in detergent compositions.

DETAILED DESCRIPTION OF THE INVENTION

Sulfonated fatty acid alkyl ester surfactants ("alkyl ester surfactants") are well-known in the art and are disclosed in the technical literature. These surfactants, when prepared according to the process of the present invention, comprise sulfonated fatty acid alkyl esters ("sulfonated alkyl esters") of the formula:

R - CH - COOR I M

SO ^" n

3 wherein R is on the average a C to C alkyl, R is on the average a C to C alkyl, M is

4 22 1 1 8 an alkali metal or alkaline earth metal cation, or a mixture thereof, and n is 1 when M is an alkali metal cation and n is 2 what M is an alkaline earth metal cation.

The sulfonated fatty acid alkyl esters ("alkyl ester sulfonates" or "sulfonated alkyl esters") constitute a major portion of the surfactant. Preferably, the sulfonated alkyl esters

amount to about 80% to 100%, preferably about 90% to 100%, by weight of the surfactant. The alkyl ester surfactant will likely also contain certain impurities including sulfonated fatty acid salts, fatty acid alkyl esters, and organic and inorganic sulfate salts at levels of 0 to about 20%.

The hydrophobic portion of these sulfonated alkyl esters have the sulfonate group at the a position, i.e., the sulfonate group is positioned at the carbon atom adjacent the carbonyl group. The alkyl portion of the hydrophobic portion, which corresponds to the R portion of the sulfonated fatty acid alkyl esters, is on the average a C to C alkyl. Preferably, the alkyl portion of this hydrophobic portion, R, is on the average a saturated straight- chain C to C hydrocarbon, particularly when R is -CH .

R , forming the ester portion of the sulfonated alkyl esters, is a on the average a C to C alkyl. Preferably, R is on the average a C to C alkyl, and most preferably a C alkyl, i.e., methyl.

When considered together, for heavy duty granular laundry detergent compositions, R and R preferably contain a total of about 15 to 17 carbons distributed between them.

Preferably the distribution is such that R is, on the average, a C to C alkyl

14 16

(approximately a 65% C , 35% C mix most preferably) and R is methyl. For heavy duty liquid laundry and light duty liquid dishwashing detergent compositions, R and R preferably contain a total of about 11 to 15 carbons, again with R preferably being CH .

The cationic portion, M, is an alkali metal or alkaline earth metal cation or mixture thereof. Preferably, M is selected from the group consisting of sodium, potassium, lithium, magnesium and calcium, and mixtures thereof. Most preferably, M is sodium or a mixture containing sodium. When M is an alkali metal cation (valence = 1) n is 1 and when M is an alkaline earth metal cation (valence = 2) n is 2.

The impurities contained in the alkyl ester surfactant preferably amount to less than about 10%, more preferably less than about 5%, by weight of the surfactant. Such impurities include sulfonated fatty acid disalts, fatty acid salts, and fatty acid alkyl esters. These impurities, when present in the surfactant, decrease the desirable cleaning characteristics for detergent compositions (when compared to compositions containing the surfactant without impurities) and worsen handling difficulties during processing of the surfactant.

The sulfonated fatty acid salt impurity comprises, e.g., sulfonated fatty acid salts of the formula:

R - CH - COO ^' M

' . ₊

SO M

3 when M is a monovalent cation. This impurity is commonly referred to as disalt. R is on the average a C to C alkyl, and M is an alkali metal or alkaline earth metal cation. It is

4 22 theorized, although not wishing to be bound by theory, that the acid form of disalts (di-acids), are formed in the presence of water via hydrolysis reactions. During sulfonation processes, a portion of the fatty acid alkyl esters reacts with sulfur trioxide, SO , to form what is

3 commonly called a mixed anhydride. The mixed anhydride reacts with water to form di-acids in one hydrolysis reaction. In the other hydrolysis reaction, un-neutralized sulfonated alkyl esters react with water to form di-acids. These di-acids form disalts upon neutralization. Disalt may also form via a hydrolysis reaction involving sulfonated alkyl esters and water at higher pH levels. The formation of higher levels of disalts have been observed during batch- type neutralization process steps.

The fatty acid salt impurity (commonly referred to as soaps) comprises fatty acid salts n+ of the formula (RCH COO ) M wherein R is on the average a C to C alkyl, M is an

2 n 4 22 alkali metal or alkaline earth metal cation, and n is 1 when M is an alkali metal cation and n is 2 when M is an alkaline earth metal cation. Although not wishing to be bound by theory, it is believed that soaps are formed via a hydrolysis reaction wherein un-sulfonated fatty acid alkyl esters react with water to form fatty acids. The fatty acids subsequently form soaps upon neutralization.

The fatty acid alkyl ester impurity comprise fatty acid esters of the formula

RCH COOR wherein R is on the average a C to C alkyl and R is on the average a C 2 1 4 22 1 ^ 1 to C alkyl. The source of this impurity is believed to be the unreacted (unsulfonated) fatty o acid alkyl esters. It is desirable to keep the level of this component as low as possible due to loss of yield, purity, performance and good in-process handleability.

Other impurities which are undesirable may exist as a component of the sulfonated fatty acid alkyl ester surfactant. In methyl ester surfactants, di-methyl sulfate ("DMS") having the formula CH -OSO O-CH is a highly undesirable component in the surfactant since it is a severe irritant of the eyes, respiratory tract, and skin and can be absorbed into the body through the skin. It has been observed that processes comprising a step conducted in aqueous media result in a surfactant product containing a very much reduced level of DMS. The production of DMS is more prevalent in processes conducted in non-aqueous, anhydrous

media.

DMS can be produced during the sulfonation of fatty acid methyl esters. Additionally, though, higher levels of DMS have been observed when the neutralization step is conducted in a normal batch method. Very low levels of DMS impurity can be achieved by over- neutralizing the sulfonated methyl ester acid mix, i.e., neutralizing the acid mix with a stoichiometric excess of alkoxide solution to a pH of at lease about 10. Therefore, a preferred means to eliminating DMS in the finished product comprises over-neutralizing the acid mix of step a) to a pH of at least about 10 followed by re-neutralizing the product to a pH of about 7. It has been found that DMS decomposes more quickly and completely during neutralization when the pH of the acid mix is raised to this higher pH. The subsequent neutralization process step is made easier with the help of the buffering agent.

In addition to the impurities set forth above, other impurities may be present in the neutralized paste including: sodium methyl sulfate; sodium sulfate; and color bodies. The color body impurities result from the harsh and complex sulfonation reaction required for alkyl esters, as well as minor side reactions of SO with impurities in the alkyl ester starting material (mono-, di- or tri-glycerides for example), or ---saturation in the methyl esters. Even very small quantities of certain light-absorbing chemicals can create a dark visual appearance.

It is desirable to maintain the levels of impurities including disalt, soap, fatty acid alkyl ester, and DMS impurities to a minimum when considering the production of the alkyl ester surfacant. The reduction in impurity contents in the surfactant improves the performance and formulatability of detergent compositions. The level of certain impurities, particularly DMS and disalt impurities, in the end-product is minimized by over-neutralizing the product stream of step a) with an alkoxide in a substantially anhydrous medium of a C to C alcohol and in the presence of a buffering agent.

STARTING MATERIALS

The starting material for the process of this invention include fatty acid alkyl esters of the formula:

RCH COOR

2 1 wherein R is on the average a C to C alkyl, and R is on the average a C to C alkyl.

Normally the alkyl chain, R, is a mixture of alkyl chains ranging in length, on the average, from about 4 carbons to 22 carbons. Preferably R is, on the average, a C to C alkyl,

10 16 and R is on the average a C to C alkyl. R is most preferably C (methyl) particularly when R is, on the average, a saturated C to C hydrocarbon. The R in the fatty acid

14 16 alkyl ester starting material will correspond to the R for the sulfonated fatty acid alkyl esters in the resultant surfactant product since the fatty acid alkyl esters directly react with the reactants in steps a) and b) to form the sulfonated alkyl esters.

Preferably the R in the fatty acid alkyl ester starting material is the same as the R in the sulfonated alkyl ester. To obtain this result, the number of carbon atoms in the alcohol of step b), the alkoxide in step b) and R of the fatty acid alkyl ester starting material are the same. Since R is most preferably methyl, the alkoxide solution of step b) preferably comprises methoxide and methanol.

The fatty acid alkyl ester starting material can be derived from unbranched C -C

6 24 carboxylic acids and C -C alcohols. From an economic standpoint, the methyl esters of commercial fatty acids are preferred. Methyl esters from palm kernel oil, coconut oil or tallow oil may be used. Since, during the sulfonation step, undesirable color bodies are formed due, in part, to unsaturated chain lengths in the fatty acid alkyl ester, the original fatty acid esters should be hydrogenated to such an extent that their IN. (iodine value) number is less than about 0.5.

Sulfur trioxide, SO , which may be used during the sulfonation step a) can be derived from passing a mixture of SO and oxygen over a heated catalyst such as platinum or vanadium pentoxide.

The alcohol utilized in step b) and utilized in an optional, intermediate alcohol reaction step is preferably a linear primary aliphatic C to C alcohol. Methanol, the preferred

1 o alcohol, can be derived from: (a) high-pressure catalytic synthesis from carbon monoxide and hydrogen, (b) partial oxidation of natural gas hydrocarbons, (c) gasification of wood, peat, and lignite or (dj methane with molybdenum catalyst (experimental). Ethanol can be derived from: (a) ethylene by direct catalytic hydration or with ethyl sulfate as an intermediate, (b) fermentation of biomass, especially agricultural wastes, or (c) enzymatic hydrolysis of cellulose. Propyl alcohol can be derived from the oxidation of natural gas hydrocarbons, also from fusel oil. Butyl alcohol can be derived from the hydrogenation of butyraldehyde, obtained in the Oxo process or condensation of acetaldehyde to form crotonaldehyde, which is then hydrogenated (aldol condensation). Other alcohols can be derived from the hydrogenation of fatty acids.

The alkoxide utilized in die neutralization step b) of the invention is a C to C n+ 1 8 alkoxide of die formula (RO ) M wherein n is 1 when M is an alkali metal cation and n is n 2 when M is an alkaline earth metal cation. The alkoxide can be derived by dissolving a metal (corresponding to the M in the alkoxide) in the alcohol. Solutions containing the desired alkoxide and alcohol are also commercially available, e.g., 25% concentration methoxide in methanol solution commercially available from Occidental Chemical.

Alternatively, the alkoxide solution can be derived by chemically reacting an alcohol with a

50% hydroxide solution in a column, continuously removing alkoxide in alcohol solution at the bottom and dilute alcohol/water solution from the top of the column. See U.S. Patent No.

2,877,274 (Kramis). Since the process herein requires an anhydrous medium, a major portion of water is preferably removed from the solution prior to use in the process. The alkoxide solution is preferably not be derived by dissolving, e.g., sodium or potassium hydroxide in an alcohol. Such reactions yield one mole of water for every mole of alkoxide produced.

In the instant process, the neutralization step is conducted in the presence of a buffering agent which facilitates maintaining the pH of the neutralized reaction product of step b) within the desired pH range. As referred to in the process of the invention hereof, pH is defined as the pH measured from a 1-2% (by weight of the surfactant) solution of the product of step b) in deionized water with a pH meter. It is in the pH range of about 5 to 9 that optimum performance and chemical stability of the resultant surfactant are realized. Targeting the pH of the neutralized reaction product of step b) to between about 5 and about 9, preferably between about 6 and about 8, minimizes undesirable degradation or hydrolysis of the resultant surfactant.

The buffering agent is generally present in step b) at a level such that the molar ratio of the alkoxide to buffering agent is from about 5:1 to about 100:1, preferably from about 10:1 to about 50:1. Any compatible material or mixture of materials which has the effect of maintaining the pH of the reaction product of step b) within the pH range of about 5 to about 9, can be utilized as the buffering agent in the instant invention. Such materials have a pKa value between about 5 and 9, preferably between about 6 and 8. The buffering agent can include, for example, citric acid, succinic acid, phosphoric acid, and pyrophosphoric acid. Such materials can be used either alone or in combination as the buffering agent.

Preferably, the buffering agent is selected from the group consisting of citric acid and mixtures containing citric acid.

THE PROCESS

Numerous descriptions of processes for the manufacture of the sulfonated fatty acid alkyl ester surfactants are disclosed in the technical literature. The process of this invention comprises two essential steps:

a) sulfonating fatty acid alkyl esters, and b) batch neutralizing with an alkoxide in a substantially anhydrous medium of a

C to C alcohol and in the presence of a buffering agent. 1 o

This process results in a high purity sulfonated fatty acid alkyl ester surfactant. Typically, the resultant product solution will contain some color bodies. If desired, the resultant product solution may be subjected to a color-body removal process before the surfactant is incorporated into a detergent composition.

SULFONATTON OF FATTY ACID ALKYL ESTERS

The sulfonation of the fatty acid alkyl esters carried out in step a) of this invention 'can be carried out by any known sulfonation process. For example, alkyl esters of C -C carboxylic acids can be sulfonated with gaseous SO in a falling film reactor. The alkyl esters and gaseous SO will not completely react to form sulfonated fatty acid alkyl esters at ambient temperatures and pressures. Therefore, this sulfonation process generally includes a mixing step wherein the alkyl esters are brought into contact with a SO /air mixture (about 5% SO in air, by volume) at a molar ratio of SO :alkyl ester of about 1.1:1 to 1.4:1 followed by heating of the mixture to about 75-95oC for approximately 20-90 minutes. Preferably, the dew point of the air used for mixing with the SO is about -40oC or lower.

Descriptions of acceptable sulfonation processes are described in "a-Sulfonated Fatty Acids and Esters: Manufacturing Process, Properties, and Applications" by W. Stein and H. Baumann, The Journal of the American Oil Chemists Society. Volume 52 (1975), pp 323- 325; and U.S. Patent 3,485,856 incorporated herein by reference. See also Surfactants in Consumer Products. J. Falbe (Editor), pp. 75-80.

The fatty acid alkyl ester starting material should contain a minimum amount of unsaturated carbon double bonds, i.e., hydrogenated to such an extent that their IN. number

is less than about 0.5. During this sulfonating step, color bodies are produced due to the harsh reaction conditions (highly acidic SO , high temperature, etc.). The color quality of the product of step a) is typically poor. Preferably, this product is not subjected to any intermediate process step that proceeds in aqueous media, e.g., bleaching. Hydrolysis reactions of intermediate reactants produce the acid form of sulfonated fatty acids which, upon neutralization form the disalt impurity.

It is believed, although not wishing to be bound by theory, that the reaction between the alkyl esters and SO in step a) occurs in two stages. First, SO reacts with the alkyl ester forming an intermediate complex and activating the carbon at the alpha position (*) as follows:

H H

I l

R - C - C = 0 + SO — > R - C - C = 0

I I ³ I I

H OR H OSO R

1 3 1

In the second stage, another molecule of SO attaches to the activated alpha carbon (*) generating what is commonly referred to as a mixed anhydride:

H SO H

I , ³

1 . 1

R - C - C = 0 + SO — > R - C - C = 0

1 1 1 1

H OSO R H OSO R

3 1 3 1

The reaction is best carried out in a falling-film reactor using very dilute SO in an inert gas (e.g., 5% SO in dry air, by volume). The reaction should be carried out with not more than about 40% excess SO to avoid charring. A significant amount of unreacted fatty acid alkyl ester remains in the product stream leaving the falling film reactor. Therefore, the sulfonating step preferably includes an additional process step wherein the SO /alkyl ester mix is allowed to react at elevated temperatures (80 to 90oC), commonly referred to as digestion. Upon heating in the digestion step, most of the mixed anhydride reacts with fatty acid alkyl esters to form the acid form of sulfonated fatty acid alkyl esters.

NEUTRALIZATIONTOAPHOFFROMABOUT5TO9

The product of step a) is substantially all in the acid form of sulfonated fatty acid alkyl esters. In accordance with step b) of the process herein, this acid mix is neutralized to a pH of about 5 to 9 with an alkoxide in a substantially anhydrous medium of a C to C alcohol, e.g., with an alkoxide solution which comprises an alkoxide of the formula (RO n+ ) M and a lower alcohol. R is a C to C , preferably C to C , most preferably C alkyl, n 1 8 1 6 1

M is an alkali metal or alkaline earth metal cation, preferably sodium, potassium, lithium, magnesium, or calcium, or mixtures thereof, and n is 1 when M is an alkali metal cation and n is 2 when M is an alkaline earth metal cation. The alkoxide solution comprises a C to C

1 8 alcohol. The neutralization of the acid mix is also performed in the presence of a buffering agent having a pKa value between about 5 and about 9. The molar ratio of alkoxide to buffering agent in the neutralization step is from about 5:1 to about 100:1, preferably from about 10:1 to about 50:1.

The primary reaction taking place during step b) (when n for the alkoxide is 1) is:

(I) R OH

H OR H OR

1 1

The amount of alkoxide solution required for Reaction (I) is considered to be within the experimental ability of one having ordinary skill in the art. The amount of alkoxide present in the alkoxide solution utilized in step b) is generally the amount required to neutralize the product of step a) to a pH of about 5 to 9.

The process of the invention herein provides an improved method for batch neutralizing the acid form of sulfonated alkyl esters. Without a buffering agent, neutralization of the product of step a) to a pH of about 5 to 9 is difficult to perform. Under typical neutralization processes which do not incorporate a buffering agent, the exact stoichiometric equivalent of alkoxide relative to acid mix is determined. Then during neutralization, the pH must be continually monitored to avoid under- or over-neutralization of the acid mix. Even so, over- or under-neutralization of the acid mix is common. With the addition of a buffering agent, such neutralization is accomplished more easily and more

accurately to the desired pH of about 5 to about 9. One can estimate the quantity of alkoxide required to neutralize the acid mix, then perform the neutralization to the desired pH level without excessive monitoring of the pH level of the reaction mixture.

Additionally, some amount of buffering agent may remain in the finished surfactant product which will serve as a hydrolysis inhibitor. Dry surfactant powder can absorb water from the atmosphere, particularly in humid climates. Such water may react with the sulfonated alkyl esters according to certain hydrolysis reactions to produce disalt impurity. The buffering agent herein inhibits such reactions and provides a chemical stability to the surfactant product.

A preferred embodiment of the process herein comprises batch neutralizing the product of step a) to a pH of about 3 as described hereinabove without the buffering agent, followed by further neutralization to a pH of from about 5 to 9 in the presence of a buffering agent. When the buffering agent comprises a carboxylic acid functionality which may form esters with the alcohol at low pH, this process reduces the amount of buffering agent required since at low pH levels a large portion of such buffering agent is degraded; at pH levels above about 3 the buffering agent provides the desired benefit.

An undesirable side reaction of process step a) produces the highly undesirable dimethyl sulfate (DMS) impurity. In processes comprising a step conducted in aqueous media, e.g., aqueous bleaching step, DMS is not a significant problem because DMS readily degrades in aqueous media. However, when the process is conducted in substantially anhydrous media, as in the process of the invention herein, DMS impurity is particularly troublesome. In anhydrous media, the DMS impurity does not readily degrade unless special precautions or process variations are incorporated. In accordance with a preferred embodiment of die invention herein, DMS impurity, which would otherwise remain in the surfactant, can be minimized by over-neutralizing the acid mix to a pH at least about 10, followed by re-neutralization to a pH of about 6 to about 8. It has been found that the DMS impurity is readily degraded (more completely and quickly) at this higher pH range in non- aqueous media.

Therefore, a preferred process comprises, after sulfonating fatty acid alkyl esters, over-neutralizing the reaction product of step a) to a pH of at least about 10, followed by re- neutralizing die product to a pH of about 5 to 9, preferably about 6 to 8. The preferred process would be difficult to perform without a buffering agent since the pH of the reaction mixtures would be difficult to control. With the addition of a buffering agent in the alkoxide solution, the over-neutralization and the re-neutralization of the product of step a) is made

easier and results in low levels of DMS impurity.

The method used to over-neutralize the product of step a) to a pH of at least about 10 is the same as used for neutralizing to a pH of about 5 to about 9. The step is performed with an alkoxide in a medium of a C to C alcohol that contains less than 0.1 wt% water.

1 8 One method of acquiring this low level of water in the alkoxide solution involves combining the alkoxide solution with a sacrificial ester of the formula R COOR wherein R is a C to

2 1

C alkyl or benzoate, preferably CH or benzoate, and R is a C to C alkyl, preferably

CH and the molar ratio of sacrificial ester to water is from about 0.5:1 to about 2.0:1,

3 preferably from about 0.75:1 to about 1.5:1. The sacrificial ester is preferably selected from the group consisting of methyl acetate, methyl benzoate, and mixtures thereof. This step need not be performed, though, in the presence of a buffering agent. When the buffering agent comprises a carboxylic acid functionality which may form esters with the alcohol at low pH, a large portion of such buffering agent would be degraded. Therefore, the buffering agent is preferably incorporated in the re-neutralizing step.

The method used to re-neutralize the over-neutralized mixture can be any known method for neutralizing a basic solution to a substantially neutral pH. A preferred method involves reacting the over-neutralized mixture with an anhydrous weak acid having a pKa between about 5 and 9, e.g., citric acid. The amount of this neutralizing component required to re-neutralize the over-neutralized mixture to a pH of about 5 to 9 is considered to be within the experimental ability of one having ordinary skill in the art. This step is conducted in die presence of the buffering agent which provides die same benefits as in the general process , described above.

In addition to Reaction II above, other reactions may take place during step b), but these are for the most part undesirable. The hydrolysis reactions of most concern involve the hydrolysis ^" of un-neutralized sulfonated fatty acid alkyl esters (from the acid mix) and neutralized sulfonated fatty acid alkyl esters. Under either reaction, the species reacts with water to form undesirable disalt impurity. The hydrolysis reactions are particularly troublesome when process b) is conducted at a final pH of at least about 10. The hydrolysis reactions heavily favor formation of the disalt impurity at these high pH levels.

A dilemma encountered herein involves attempting to eliminate or reduce the amount of DMS impurity while attempting to inhibit or prevent the formation of disalt impurity. In order to eliminate the DMS impurity the process preferably comprises elevating the pH of the reaction mixture in step b) to at least about 10. Yet, at this higher pH range, the reaction mixture favors certain hydrolysis reactions which produce undesirable disalt impurity.

Therefore, when conducting the neutralization reaction, care should be taken to conduct such reactions in substantially anhydrous media with components that are substantially anhydrous. By neutralizing the acid mix with the substantially anhydrous alkoxide solution to a pH level of at least about 10, DMS impurity is allowed to degrade while die formation of disalt impurity is inhibited and, therefore, a high purity sulfonated fatty acid alkyl ester surfactant is produced which contains low levels of disalt and DMS impurities.

As used herein, the term "substantially anhydrous" requires that the level of water in a solution or mixture be less than about 2% by weight of the solution or mixture. Preferably the solution or mixture contains less than about 0.5% by weight of water. Most preferably, the solution or mixture is essentially water-free. A specific advantage in conducting the process of the invention herein via substantially anhydrous media is die ease of processability of the reactant and product solutions. The technical literature recognizes the problems encountered with sulfonated fatty acid alkyl ester surfactant solutions containing water. It seems that the surfactant forms viscous pastes in water which can require special handling equipment, e.g., special pumps, heat exchangers, etc. An advantage of conducting the process of the invention herein via anhydrous alcoholic media is diat the process does not require the special equipment that may be required for processes involving an aqueous media. The process of the invention herein involves solutions which are relatively fluid and non- viscous which do not require special pumps to process. These advantages are in addition to inhibiting or preventing the formation of disalt or otirer impurities. Moreover, the substantially anhydrous alcoholic medium allows for the effective separation of dark colored impurities during post-neutralization purification steps, i.e., a no-bleach color-body removal process described below.

As used herein, the term "batch neutralize" means to react the product of step a) (acid mix) with an alkoxide solution by adding the alkoxide solution into an amount of acid mix (normal batch neutralization) or by adding die acid mix into an amount of alkoxide solution (reverse batch neutralization). Reverse batch neutralization may produce undesirable levels of disalt impurity. Therefore, normal batch neutralization is preferred over reverse-batch neutralization. Sufficient agitation and/or mixing should be provided to allow the reactants to intimately mix and completely react in the chamber. It has been found that a wide range of mixers provide adequate mixing of the reaction mixtures, i.e., the acid mix and alkoxide solution. For example, paddle mixers commercially available from (Charles Ross & Son Company, Greerco Company or IKA) as well as static motionless mixers (providing shear rates of about (5000) sec ) provide die required conditions for proper neutralization of the

reactants.

Because the process of the invention herein is conducted in non-aqueous media, i.e., substantially anhydrous alcoholic media, die reactant and product streams exhibit good handling and in-process flow properties. In aqueous media, sulfonated fatty acid alkyl ester surfactants form viscous pastes which are difficult to process. In anhydrous media of a C to C alcohol, these surfactants are fluid and do not require sophisticated or expensive designs o or equipment to process.

OPTIONAL PROCESS STEPS

Preferably, the process of the invention herein should not include any process step wherein bleaching of the reactants is conducted. Such bleaching steps are described in, e.g., U.S. Patent Numbers 4,695,409 and 4,820,451 which cite references describing acidic bleaching with hydrogen peroxide (U.S. Patent Numbers 3,142,691; 3,159,547; 3,251,868; and 3,354,187) and hypochlorite (U.S. Patent Number 3,452,064). Such bleaching steps are generally conducted in aqueous media and would raise the problems discussed above regarding impurities and in-process flow properties. Therefore, the process of the invention preferably does not include any bleaching process step, whether intermediate or in combination with the process steps.

Even though the product of step a) is substantially all in the acid form of sulfonated fatty acid alkyl esters, a significant amount of the mixed anhydride may remain in die reaction mixture after the sulfonation step a). The mixed anhydrides, if allowed to react with water, will form sulfonated fatty acids via a hydrolysis reaction. Upon neutralization, these fatty acids form the disalt impurity. It is desirable, therefore, to convert the mixed anhydrides remaining in die product stream of step a) to the acid form of sulfonated fatty acid alkyl esters. This is accomplished by reacting the mixed anhydrides with an alcohol.

Thus, an optional, but preferred, intermediate step involves the reaction of die sulfonated product of step a) with an alcohol. The alcohol reaction step comprises reacting the product of step a) with from about 3% to 25%, preferably about 10% to 20%, by weight ofthe product of step a), of an alcohol. The alcohol is a C to C alcohol, preferably a C to

1 8 1

C alcohol, most preferably methanol, particularly when die fatty acid alkyl ester starting

6 materials are C -C fatty acid metiiyl esters. Reacting the product of step a) with a C to

C alcohol and subsequently neutralizing this product with the alkoxide solution produces a high purity sulfonated fatty acid alkyl ester surfactant.

During this alcohol reaction step, the anhydride reacts with the alcohol to generate more desired product for neutralization in step b), i.e., the acid form of the sulfonated fatty acid alkyl ester, according to die following reaction:

SO H SO H ι ³ . i ³

(UI) R - C - C = O + ROH — > R - C - C = 0 + R O SO H I I

1 3

I I

H OSO R H OR

3 1

This reaction is relatively fast and most of the remaining mixed anhydrides are converted to the acid form of sulfonated fatty acid alkyl esters when die appropriate level of alcohol is used.

Therefore, a preferred embodiment of the invention herein pertains to a process for preparing the preferred sulfonated fatty acid methyl ester surfactant. Such process comprises: a). sulfonating fatty acid metiiyl esters of the formula:

RCH COOCH

2 3 wherein R is on die average a C to C alkyl;

10 16 b) reacting the product of step a) with from about 5% to 25%, by weight of the product of step a), of a C to C alcohol, preferably methanol;

1 6 c) over-neutralizing the product of step b) to a pH of at least about 10 with an

- ^* n+ alkoxide having die formula (CH 0 ) M in a substantially anhydrous

3 n medium of a C to C alcohol, preferably methanol, wherein M is an alkali metal or alkaline earth metal cation, and n is 1 when M is an alkali metal cation and n is 2 when M is an alkaline earth metal cation; and d) re-neutralizing die product of step c) to a pH of about 6 to 8; in the presence of a buffering agent having a pKa value between about 5 and 9; wherein the molar ratio of methoxide to buffering agent is from about 5: 1 to about 100: 1.

This process results in high-purity, high-yield surfactant solution containing sulfonated fatty acid metiryl esters and low levels of impurities including disalts, soaps, fatty acid methyl esters and DMS.

The resultant product of the process herein is an essentially non-aqueous mixture comprising sulfonated fatty acid alkyl ester surfactant and alcohol. This product may be subjected to a working-up procedure depending on the end use desired. For example, simple separation of die resultant components can be accomplished in many ways including precipitation of die surfactant from the solution, evaporation of die alcohol or a combination thereof.

The known processes for sulfonating fatty acid alkyl esters in accordance with step a) of the invention will likely suffer from the formation of dark-colored impurities. In order to obtain high sulfonation yields, excess sulfonating agent in combination with greater processing times and/or temperatures is required. These conditions can result in undesirable side reactions including die formation of dark-colored impurities.

For aesthetic and other reasons, the dark-colored sulfonated fatty acid alkyl ester compositions are not suitable for use directly in washing or cleansing agents in detergent products. The dark-colored impurities can be separated from die solution comprising the sulfonated fatty acid alkyl ester surfactant and a suitable solvent, e.g., a C -C alcohol, by

1 o separation methods described hereinafter. Separation of the dark-colored impurities from die solution can be enhanced with an adsorbent material. After removal of the dark colored impurities, die sulfonated fatty acid alkyl ester surfactant can be recovered from die solvent to yield a product with improved, i.e. lighter, color.

In particular, a process for improving the color of the surfactant (containing dark- colored impurities formed during die preparation of die surfactant) comprises:

(1) forming a solution comprising:

(a) the sulfonated fatty acid alkyl ester surfactant and dark-colored impurities formed during die preparation of the surfactant; and

(b) a solvent in an amount sufficient to substantially dissolve die surfactant;

(2) separating said dark-colored impurities from die solution;

(3) recovering surfactant from the solution.

The step comprising separating dark-colored impurities from die solution of die surfactant in alcohol can be achieved by settling/clarification, centrifugation, filtration, adsorption, or a combination tiiereof. In a preferred embodiment, die solution is treated with an adsorbent material such as activated carbon, activated alumina, or silica gel.

After separation of the dark-colored impurities from die solution, die

surfactant having improved color can be recovered from die solvent solution by known methods. Such recovery methods include, e.g., precipitation of die sulfonated fatty acid alkyl ester from the solution, evaporation of the lower alcohol solvent from the solution or a combination thereof.

The process for making sulfonated fatty acid alkyl ester surfactant of die invention hereof is particularly suited to die process for improving the color of the surfactant since the surfactant is already substantially dissolved in a solvent (C -C alcohol). In order to improve the color thereof, one simply needs to separate the dark-colored bodies from die solution and recover the surfactant from the solvent. Having subjected die surfactant to the process for improving the color thereof, i.e., steps (1)- (3) above, the resultant product can be used directly in cleansing and washing agents and products.

As used herein, all percentages, parts, and ratios are by weight unless otherwise stated.

The following examples illustrate the process of the invention and facilitate its understanding.

EXAMPLE I

Ester sulfonic acid is produced by conventional sulfonation of palm stearin fatty acid methyl ester. The acid component of the metiiyl ester consists essentially of saturated fatty acids with an Iodine Value of 0.28 and die following chain length distribution (by weight percent):

C - 0.23

12 C - 1.55

14 C - 0.08

15 C - 66.75

16 C - 0.15

17 C - 31.28

18 C - 0.19

20 o o

The sulfonation reaction is carried out at 80 C to 95 C in an annular failing film

reactor using a mixture of sulfur trioxide and air (SO content: 3-4% by volume; SO excess: 15-30 mole percent). The sulfonated methyl ester acid mix is then digested in the presence of an added 25 parts of CH OH in a closed vessel for 35 to 40 minutes at a o o 3 temperature of 80 C to 95 C. The degree of sulfonation after digestion is 95%.

A substantially anhydrous solution comprising sodium metiioxide, methanol and citric acid is prepared by mixing 99 wt % of a commercially available metiioxide in methanol solution (from Occidental Chemical; concentration = 25% wt% metiioxide in methanol; containing 0.5 wt% water) with 1 wt % anhydrous citric acid pellets in a standard paddle blade mixer for 2 minutes. The molar ratio of metiioxide to citric acid is 88: 1.

The acid mix is then neutralized in a paddle blade mixer by adding sufficient anhydrous metiioxide solution to the acid mix to raise the pH of the neutralized acid mix to 7.0. The resulting solution is maintained at this pH level for about 2 minutes.

By preparing the ester sulfonate surfactant according to the above method, the neutralization is conducted more quickly and witii greater accuracy to the desired final pH than by methods in which no citric acid buffer is used.

The methyl ester sulfonate surfactant is isolated from die metiianol solution by chilling o the solution to 5 C and filtering off the precipitated surfactant. After drying die wet powder, die resultant surfactant is shown to contain no detectable DMS level and low level of disalt impurity.

Such surfactant demonstrates lower levels of impurities than similar surfactants prepared via other processes including processes conducted in aqueous media. The surfactant also has improved storage stability to ester hydrolysis over ester 'sulfonate surfactants having no buffer.