Technical field

The present invention relates to an improved method for the synthesis of disulfated cleaning agents, for use in detergent compositions.

Backoround to the invention

Specific use of dianionic surfactants, that is surfactants having two anionically charged groups, as detergent components has been previously described. Disulfated surfactants, in particular 1 ,4 disulfated surfactants and alkoxylated forms thereof, are specific examples of such dianionic surfactants.

Disulfated surfactants have not to date found common use in the detergent industry because of the difficulty associated in deriving a commercially viable route for their bulk preparation. Commercial viability is, in general terms, dictated by the need to employ inexpensive, readily available starting materials capable of translation into 'end product' via a synthetic route which is energy efficient, employs inexpensive, readily available reagents and provides high yields.

Known syntheses of disulfated surfactants, in general, use an alkyl or aikenyl succinic anhydride as the principal starting material. This is initially subjected to a reduction step from which a diol is obtained. Subsequently the diol is subjected to a sulfation step to give the disulfated product. Optionally, an alkoxylation step may be introduced prior to the sulfation step such that alkoxylated disulfate cleaning agents are obtained.

As an example, US-A-3,634,269 describes phosphate-free detergent compositions containing 2-alkyl or alkenyl-1 ,4-butanediol disulfates. Described therein, is their preparation by the reduction of aikenyl succinic anhydrides with lithium aluminium hydride to produce either aikenyl or alkyl diols which are then sulfated.

Additionally, US-A-3,959,334 and US-A-4,000,081 describe 2-hydrocarbyl- 1 ,4-butanediol disulfates said to be suitable as lime soap dispersants. Again, the method for synthesizing these disulfates involves the reduction of aikenyl succinic anhydrides with lithium aluminium hydride to produce either aikenyl or alkyl diols which are then sulfated.

US-A-3, 832,408 and US-A-3,860,625 describe phosphate-free detergent compositions containing 2-alkyl or alkenyl-1 ,4-butanediol ethoxylate disulfates. Their preparation by the reduction of aikenyl succinic anhydrides with lithium aluminium hydride to produce either aikenyl or alkyl diols which are then ethoxylated prior to sulfation is described.

A problem associated with the known synthetic routes to 1 ,4 disulfated surfactants is that the reduction step involves the use of pyrophoric and expensive lithium aluminum hydride (L1AIH4) to reduce the anhydride to the 1 ,4-diol. Safe handling of pyrophoric materials requires special process equipment which further adds to processing complexity and cost. Additionally, aluminium salts which are formed in the process require special care in their disposal.

The Applicants have now found an alternative method of carrying out the reduction step involving hydrogenation of the anhydride in the presence of a transition metal-containing hydrogenation catalyst. Thus use of lithium aluminium hydride, and the potential problems related thereto, may be avoided.

Summary of the Invention

According to the present invention there is provided a method of synthesis of a disulfated cleaning agent from a substituted cyclic anhydride having one or more carbon chain substituents comprising in total at least 5 carbon atoms comprising the following steps:

(i) reduction of said substituted cyclic anhydride to form a diol;

(ii) optionally, alkoxylation of said diol to form an alkoxylated diol; and

(iii) sulfation of said diol or alkoxylated diol to form a disulfate

wherein said reduction step comprises hydrogenation under pressure in the presence of a transition metal-containing hydrogenation catalyst.

Detailed description of the invention

There is provided a method for the synthesis of a disulfated cleaning agent from a substituted cyclic anhydride.

Cyclic anhydride starting material

The cyclic anhydride starting material has a ring structure and comprises an acid anhydride linkage. Cyclic anhydrides are generally formed by a ring forming condensation reaction of a single organic compound having a first carboxylic acid (-COOH) functional group and a second -COY functional group separated from the carboxylic acid functional group by at least two carbon atoms, wherein Y is usually an -OH, or halogen functionality.

A specific example of an organic compound which may be condensed to form a cyclic anhydride is maleic acid which on self-condensation provides maleic anhydride. Maleic anhydride is readily available commercially.

The ring structure of the cyclic anhydride starting material contains from 4 to 7 carbon atoms, preferably from 4 to 6 carbon atoms in the ring structure. Most preferably the cyclic anhydride starting material is based on succinic anhydride which has a 5-membered ring structure containing 4 carbon atoms in the ring.

The cyclic anhydride starting material is substituted by one or more carbon containing substituents, such that in total, these substitutents contain at least 5 carbon atoms, preferably from 5 to 25 carbon atoms, more preferably from 7 to 21 carbon atoms.

In a preferred aspect all of the carbon chain substituent(s) comprise either alkyl or aikenyl chains, which may be branched or unbranched. In one preferred aspect they are essentially unbranched. In another preferred aspect the chains are primarily monobranched, that is more than 50% by weight of the chains are monobranched.

In one preferred aspect the substituted cyclic anhydride has a single carbon chain substituent. In another preferred aspect the substituted cyclic anhydride has two carbon chain substituents each having different points of attachment to the ring structure.

Substituted alkenylsuccinic and alkylsuccinic anhydrides are suitable starting materials herein. Preferred anhydrides of this type have the following structures:

where R and R2 are either H or an alkyl group. In one preferred aspect R2 is H.

Linear alkenylsuccinic anhydrides may be obtained in high yield from the single stage 'ene reaction' of maleic anhydride with an alpha-olefin. Branched alkenylsuccinic anhydrides may be obtained from the single stage 'ene reaction' of maleic anhydride with an intemal olefin, such as those obtainable from the familiar SHOP (tradename of the Shell Corporation) olefin making process. Preferably the substituted alkenylsuccinic or alkylsuccinic anhydride starting materials will be substantially pure and, in particular, dimeric impurities are preferably minimised. Thus, preferably the cyclic anhydride starting material contains less than 30% dimeric impurities, more preferably less than 10%, most preferably less than 5% or even less than 2.5 or 1 % by weight. Such impurities can be minimised by conventional techniques within the limit of the skilled person, for example

either by selection of appropriate reaction conditions in producing the cyclic anhydride starting material (such as low temperature) or by subsequent purification, for example by distillation.

Alkylsuccinic anhydride starting materials can be made by reducing alkenylsuccinic anhydrides. This reduction can be achieved under the conditions of the catalytic hydrogenation reduction step of the present invention.

The reduction step

The first step of the synthetic method of the invention is the reduction of the substituted cyclic anhydride to form a diol. The reduction step comprises hydrogenation under pressure in the presence of a transition metal- containing hydrogenation catalyst.

It is an advantage of the present invention that under the conditions of the catalytic hydrogenation reduction step any alkene linkages are also reduced to alkyl linkages. Thus, if an alkenylsuccinic anhydride is used as the starting material it is reduced via a (single) reduction step to the diol having alkyl chain substituents, as are desired. This contrasts with the situation where L1AIH4, which does not reduce alkene linkages, is used in the reduction step, wherein an extra step involving the reduction of the aikenyl succinic anhydride to the alkyl succinic anhydride (via e.g. Pd/hydrogen) must be employed to obtain the desired diol product.

(a) Hydrogenation catalyst

The hydrogenation catalyst acts functionally to enhance the efficiency of the reductive hydrogenation process. For use on a commercial scale it is desirable that the catalyst is easy to regenerate.

Preferably the catalyst contains a transition metal selected from the group consisting of the group VIA (particularly Cr), VIIA (particularly Mn), VIII (particularly Fe, Co, Ni, Ru, Rh) and IB (particularly Cu) elements. Catalysts containing mixtures of any of these transition metals are envisaged as are

catalysts containing other metals including the alkali and alkaline earth metals.

Copper-containing catalysts, particularly copper chromite (which is commercially available and relatively easy to regenerate) are most preferred.

The hydrogenation catalyst may advantageously be supported on an inert support material. The support material generally comprises an oxide salt comprising a metal selected from the group consisting of aluminium, silicon and any mixtures thereof. Supports comprising aluminium oxide or silicon dioxide are especially preferred. Clay materials are also suitable supports.

(b) Process details

The reductive hydrogenation step is carried out under pressure, and generally at elevated temperature. Usually a solvent is employed. This step can be carried out by a batch, continuous or vapor-phase process. A continuous process is preferred.

The pressure is typically from 1 x 10-5 to 1 x 10 ⁷ Pa, more preferably from 1 x 10 ⁶ to 5 x 1θ6 Pa. The temperature is generally from 150 to 350°C, more preferably from 200 to 300°C. The time of reaction is generally from 30 minutes to 10 hours.

Suitable solvents include alcohols, particularly methanol, ethanol, propanol and butanol.

It is to appreciated that the exact process conditions used for any particular synthesis will be varied to achieve optimum results in accord with the usual process optimization steps which will be within the remit of the skilled person. In particular the process conditions will be adjusted to minimize the occurence of any competing side-reactions.

One possible problem derives from the incomplete reduction of the cyclic anhydride, such that lactones are formed. These are however, convertible to

diols by further catalytic hydrogenation. It may be advantageous to carry out the hydrogenation in two steps, preferably as part of a continuous step- wise process, such that a lactone is formed in the first step followed by a second step in which the lactone is reduced to the diol. Conditions which favour lactone formation are high temperature (~300 °C) and low pressures (~ 1 x 10 ⁵ Pa). Any water formed during the hydrogenation will primarily be in the vapour phase, so that the anhydride is unlikely to be converted to a carboxylic acid which can inhibit the catalyst. The best conditions for diol formation from the lactone are lower temperatures (-220 °C) and high pressures (~ 1 x 10? Pa), both of which conditions minimize the production of furan by-product.

Furans can be formed by a ring closure reaction of the diol product. Generally any furan by-product should be present in an amount below 5% of the desired diol, preferably below 2%, most preferably below 1.5% or even below 1% by weight of the desired diol.

The tendency for such furans to form is greater at higher reaction temperatures and can be promoted by the transition-metal containing catalysts employed in the reduction step. The formation of furans may therefore be minimzed by the use of lower reaction temperatures and by designing the process such that once formed the diol is removed from the catalytic environment. The latter objective is met by the use of a continuous process whereby the reactants contact a high level of catalyst for a relatively short time and are then removed from the catalytic environment. By optimization of the time of contact with the catalyst the formation of the desired diol is maximized and that of the furan by-product minimized.

It has also been found that the presence of acids promotes furan formation. In particular, carboxylic acids which may be formed by certain ring-opening reactions of the cyclic anhydrides under the conditions of the reduction step can promote furan formation. This problem can be alleviated by first forming the lactone in a separate step as mentioned above or by the use of an additional esterification step in which the cyclic anhydride is first treated with an alcohol, particularly methanol, in the presence of an esterification catalyst to form a diester. The diester is then converted to the diol via the reduction step in accord with the invention.

Optional alkoxylation step

The diol may optionally be alkoxylated prior to the sulfation step, such that alkoxylated disulfate cleaning agents are obtained as the final product.

Suitable methods for the alkoxylation of diols are described in US-A- 3,832,408 and US-A-3, 860,625. The condensation products of the diols with from 1 to 25 moles, preferably from 2 to 10 moles of alkylene oxide, particularly ethylene oxide and/or propylene oxide, are preferred herein.

Sulfation step

The sulfation step may be carried out using any of the sulfation steps known in the art, including for example those described in US-A-3,634,269, US-A- 3,959,334 and US-A-4,000,081. In particular the sulfation may be carried out in two stages where the first stage involves treatment of the diol with a sulfation agent, generally selected from the group consisting of chlorosulfonic acid, sulfur trioxide, adducts of sulfur trioxide with amines and any mixtures thereof. The second stage involves neutralization, which is generally carried out using NaOH.

Example Set I - Synthesis of C14 alkyl-1.4-disulfate

Cyclic anhydride starting material

Decyl succinic anhydride as shown in the reaction scheme below (R = a heptyl group) was employed as the starting material. This material was obtained by hydrogenation in the presence of a Pd catalyst of the aikenyl succinic anhydride product obtained from the 'ene' reaction of maleic (acid) anhydride with dec-1-ene.

Reaction scheme - reduction step

The general reaction scheme for the reduction step is as outlined below:

Cu Chromite (cat)

Alcohol solvent R'OH

Alkyl 1 ,4-Diol

It should be noted from the above that both furan and half ester by-products can also be formed in the reaction.

Process details

The reactor utilized was an electrically heated 500 ml (39 mm internal diameter x 432 mm internal length) Autoclave Engineers type 316 (tradename) stainless steel rocking autoclave fitted with an internal thermocouple and valving for periodic sampling of reaction mixtures.

The reactor was charged with 50 ml of alcohol solvent and 5 grams of copper chromite catalyst, as sold by Engelhardt under the tradename CU- 1885P, that had been washed several times with high purity water then several times with alcohol solvent. The reactor and contents were then

heated to 250°C at a hydrogen pressure of 2.4 x 10 ⁶ Pa and held for 1 hour. The reactor was then cooled and charged (without exposing the catalyst to air) with 20 grams of the cyclic anhydride starting material and an additional 50 ml of alcohol solvent. The process was carried out under different conditions of pressure and temperature, and with varying reaction times. Details of the different reaction conditions and of the yields obtained are summarized in the table below:

Example Pressure Temp. Time Solvent 1. 4 Furan Other by¬ No. (10 ⁶ Pa) (°C) diol Yield product Yield

1 2.8 235 2.1 hr 1-butanol 74 9 17

2 2.1 210 48 hr 1-butanol 61 15 24

3 2.85 250 2.5 hr 1-butanol 62 9 26

4 2.1 250 15 hr methanol 24 41 35

5 2.1 300 15 hr methanol 0 76 24

6 2.1 200 15 hr 1 -octanol <5

7 2.1 192 4.5 days isobutanol 52 33 15

8 2.1 187 2.5 days ethylene <5 86 glycol

Analytical method details

Samples of the product from the reduction step were periodically taken. To determine the yield of 1 ,4 alkyl diol product and of the by-products, as given above, these samples were analyzed by Capillary Gas Chromatography, as now described:

The samples were filtered and then injected (280°C injection temperature and 100:1 split ratio) into a HP 5880 (tradename) GC utilizing a FID detector (detector at 320°C) and a J&W Scientific (tradename) DB-1 column (15 metres x 0.257 mm internal diameter; 0.25 film thickness). The following temperature program was utilized: 170°C initial temperature with no hold period; increasing to 180°C at a 1 °C/minute ramp; no hold period; increasing to 320 °C at 10 °C/minute ramp.

Sulfation step

The sulfation step was carried out, in each case, on the 1 ,4-alkyl diol product obtained from the reduction step. Chlorosulfonic acid was used which resulted in a high yield (typically > 90%) of the required C-14 alkyl 1 ,4 disulfate end-product as shown below:

Example Set II - Synthesis of C14 alkyl-1.4-disulfate

The aikenyl succinic anhydride product obtained from the 'ene' reaction of maleic (acid) anhydride with dec-1-ene (i.e. R = a heptyl group) is used directly as the cyclic anhydride starting material. The need for the additional 'pre-step' of reduction of the aikenyl succinic anhydride to an alkyl succinic anhydride is thus avoided. All other method steps are as in Example Set I.

The reaction scheme for the reduction step is thus as shown below:

yl

Alkyl-1 ,4-Diol OH

Example Set III - Synthesis of C14 alkyl-1.4-ethoxylate disulfate

The method steps of Example Set I are followed to give the 1 ,4 alkyl diol. This is then treated with an excess of ethylene oxide to give the ethoxylated diol. The sulfation step of Example Set I is then repeated to give a C14 alkyl-1 ,4-ethoxylate disulfate end-product.