WO1992000947A1

WO1992000947A1 - Improved esterification of oxyhydrocarbon polyols and ethers thereof, and products therefrom

Info

Publication number: WO1992000947A1
Application number: PCT/US1991/004877
Authority: WO
Inventors: Norman Milstein
Original assignee: Henkel Corporation
Priority date: 1990-07-09
Filing date: 1991-07-09
Publication date: 1992-01-23

Abstract

Conducting esterifications of oxyhydrocarbon polyols and their ethers in the presence of a reducing agent, or pretreatment of these materials with a reducing agent allows the synthesis of light colored ester products without bleaching. This method is particularly useful in connection with preparation of fatty acid esters of sorbitan from sorbitol when cyclization of the sorbitol precedes esterification, and the yield in the latter process can be improved by esterification at atmospheric pressure. The method is also particularly useful in synthesis of fatty acid esters of alkyl glucosides, and the results of the latter process can be improved by transesterification with fatty acid esters of lower alcohols catalyzed at temperatures at or below 180 °C by hypophosphite ions.

Description

IMPROVED ESTERIFICATION OF OXYHYDROCARBON POLYOLS AND ETHERS THEREOF, AND PRODUCTS THEREFROM

Field of the Invention

This invention relates to processes for the esterification of oxyhydrocarbon polyols and ethers of these polyols. "Oxyhydrocarbon polyols" are defined for purposes of this application as compounds that contain only hydrogen, carbon, and oxygen as elements, with at least as many carbon-oxygen single bonds as carbon atoms and with at least half of the oxygen atoms attached to a hydrogen atom. The invention also relates to esters of oxyhydrocarbon polyols and their ethers, these esters having improved properties, particularly lighter color.

Statement of Related Art

Oxyhydrocarbon polyols are generally derived from natural sources. Common examples are cellulose and numerous sugars and polysaccharides. Commercial materials of this type almost always include a variety of molecular types, including some that may still be of unknown or inexactly known chemical structure.

The ethers and esters of oxyhydrocarbon polyols are generally not natural products, but some of them have been familiar commercial products for the last several decades and can be prepared by various well known methods, which generally require the use of elevated temperature and acid or basic catalysis. Under such conditions, even in the practical absence of oxygen, it is generally known that all or almost all readily and economically available mixtures of oxyhydrocarbon polyols, their ethers, and/or their esters are susceptible to development of color to various degrees. (In the presence of oxygen, color development is even more pronounced.) Development of color is generally undesirable, because for most uses colorless or at least very slightly colored oxyhydrocarbon polyol esters are preferred.

A variety of chemical bleaching processes, using reagents such as hydrogen peroxide, other peroxides, and hypochlorites, and/or sorptive processes, using sorbents such as activated carbon and diatomaceous earth, are known for eliminating or reducing the color developed in esterifying practical, economically available mixtures of oxyhydrocarbon polyols and/or their ethers with one another and/or with other naturally occurring impurities, but all of these methods have some disadvantages. Bleaching with hydrogen peroxide in particular favors the undesirable development of haziness upon heating in the commercially important products called "sorbitan monoesters", although they usually contain an average degree of esterification substantially greater than 1 and contain some esters of sorbitol and/ or of isosorbide as well as those of sorbitan. Some commonly used filtration aids and/or sorbents such as diatomaceous earth, although they lighten the color of freshly treated sorbitan esters, produce products that rapidly darken during storage after treatment. Thus one object of the present invention is to provide light colored esters of oxyhydrocarbon polyols and their ethers without any need for chemical bleaching and/or for treatment with sorbents that increase the tendency of the product esters to darken during storage. U. S. Patent 4,297,290 of Oct. 27, 1981 to Stockburger teaches that the darkening of sorbitan fatty acid esters can be reduced by acid-catalyzed anhydrization, then reacting the resulting anhydrosorbitol with a fatty acid in the presence of base at a temperature not exceeding about 215º C. Recommended conditions for the anhydrization are a temperature of about 110 to 150º C at a reduced pressure such as 5 mm of Hg absolute in the presence of p-toluenesulfonic acid catalyst. It is taught that such reaction conditions make it possible to avoid bleaching, reduce the amount of bleaching agent, or use milder bleaching conditions.

Despite the teachings of U. S. Patent 4,297,290 as noted above, common commercial practice in the preparation of sorbitan monoesters from sorbitol is to esterify sorbitol first with basic catalyst under relatively mild conditions, such as a temperature of about 180º C, then to cyclize the sorbitol ester, using a soap catalyst, at about 220 - 230º C. This method generally results in such a dark colored product that subsequent bleaching is required for commercial acceptability of the product.

An abstract of published PCT Application WO 89 12714 teaches that cellulosic textiles may be crosslinked and esterified by heat treatment with a polycarboxylic acid in the presence of a catalyst which may be an alkali metal hypophosphite. U. S. Patent 4,820,307, according to an abstract of it, also has very similar teachings.

Abstracts of U. S. Patents 4,788,009, 4,693,847, 4,650,607, and 4,643,848 teach esterification of rosin or rosin esters with polyols, using salts or esters of hypophosphorous acid as a catalyst. Improved color of the products is asserted as a benefit of using this type of catalyst.

An abstract of published German patent application 3 541 813 teaches that a mixture of hypophosphorous acid and p-toluene sulfonic acid is useful as a catalyst for esterifying ethoxylated fatty alcohols with fatty acids. An abstract of Romanian patent document 87381 teaches that sorbitol may be intramolecularly dehydrated at 130 - 140º C using sulfuric, hypophosphorous, or p-toluene sulfonic acid as a catalyst, and the product then esterified at 220 - 250º C, using acidic or basic catalysts.

Abstracts of Romanian patent documents 84808, 84807, and 84806 teach that use of hypophosphorous acid at various stages in the manufacture of esters of glycerin and pentaerythritol produces products of better color. An abstract of Japanese patent document 88 040209 teaches a similar benefit for polymers produced by reacting terephthalic acid and 1,4-butanediol.

An abstract of U. S. Patent 4,844,924 teaches that the color of dietary fiber material may be reduced by esterifying, oxidatively bleaching, then reductively bleaching. The reductive bleach may be sodium borohydride.

An abstract of Japanese patent document 82 025597 teaches that a fatty acid product more resistant to discoloration on standing may be made by distillation in the presence of an antioxidant and a reducing agent; the latter may be sodium borohydride.

H. Wolff and W. Hill, "Fatty Acid Esters of Methylglucoside Prepared by the Alcoholysis Reaction", Journal of the American Oil Chemists Society. July 1948, p. 258 - 260 teaches the transesterification of methyl glucoside with methyl esters of fatty acids.

Description of the Invention

Except in the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or reaction conditions are to be understood as modified by the word "about" in defining the broadest scope of the invention. Practice of the invention within the boundaries corresponding to the exact quantities stated is preferable, however.

In one embodiment of the invention, it has now been found that the development of product color during the esterification of mixtures including at least 60, preferably at least 75, more preferably at least 85, percent by weight (hereinafter "w/o") of material selected from oxyhydrocarbon polyols and ethers thereof, said mixtures also including materials (which can be the oxyhydrocarbon polyols and/ or ethers thereof themselves, or can be other materials) that tend to develop color during esterification, can be substantially reduced by (i) pre-treatment of the mixture of oxyhydrocarbon polyols and/or ethers thereof with an effective amount of a reducing agent prior to performing the esterification, (ii) including an effective amount of a reducing agent in the reaction mixture during the esterification process, or (iii) both.

Preferably the reducing agent is a water-compatible one, i.e., one which retains its reducing power when in contact with or in solution in water and does not undergo any dangerous reaction, such as evolution of hydrogen or other flammable gas(es), with water when the reducing agent is in contact with or in solution in water.

In the broadest scope of this embodiment of the invention, an effective amount of reducing agent is defined to be any amount that causes a detectable reduction in product color, compared to a product made under esterification conditions otherwise identical except for the absence of reducing agent.

Preferably, the amount of reducing agent is correlated with the amount of "reducing sugar" present in the mixture including oxyhydrocarbon polyol(s) and/or ether(s) thereof to be esterified. The amount of "reducing sugar" can be measured by methods known in the art, preferably by the method described in connection with the working examples below. The amount of reducing agent to be used during a process according to this invention is preferably at least sufficient stoichiometrically to reduce all the "reducing sugar" present in the mixture to be esterified, or, increasingly more preferably, a 9, 14, or 25 % excess of reducing agent over this stoichiometric amount is used. Also preferably, the amount of reducing agent used is not more than the amount which will provide a 50 % excess, or more preferably a 35 % excess, over the εtoichiometric amount noted above.

While not wishing to be bound by any theory, it is believed that the better results obtained with excess reducing agent may occur because the conditions of the esterification reaction are capable of converting a small fraction of non-reducing oxyhydrocarbon polyol(s) and/or their ether(s), such as sorbitan or alpha methyl glucosides, into reducing forms of polyols.

It is also preferable, particularly when an excess over the stoichiometric amount of reducing agent as described above is used, that the reducing agent used be "reaction stable", which is defined to mean herein that if the reducing agent is mixed with the materials normally used for a esterification reaction, except for any polyol mixture, and is exposed to the temperatures used for a normal esterification for the time normally required, the reducing power of the reducing agent will not diminish to less than one half of its original value.

For convenience, it is generally preferred to utilize all of the reducing agent at the same time as the esterification is carried out, but it is equally effective to treat the mixture to be esterified with reducing agent before esterifying and allow the excess reducing agent to remain in place during the esterification process.

Preferable reducing agents for use in the process include sodium, potassium, and lithium borohydrides; calcium, barium, and magnesium borohydrides; lithium aluminum borohydride; and quaternary ammonium and tertiary amine borohydrides. Sodium borohydride is most preferred, because it is the most water-compatible of the preferred group. Mixed as well as single types of reducing agents may be used.

A process according to this invention may be combined with conventional chemical bleaching and/or sorptive color reduction processes applied to the crude ester product, if needed or desired for further improvements. However, one of the advantages of a process according to this invention is that the need for such post synthesis color reduction treatment is often eliminated by use of a process according to this invention, and avoidance of any such post treatment often results in a more stable product, as has been generally noted above and will be specifically illustrated below.

One particularly preferred specific embodiment of the invention is the manufacture of sorbitan esters, starting with commercial grade sorbitol as a raw material. This process requires two distinct steps: (i) cyclizing the straight carbon chain of sorbitol to the carbocyclic ring of sorbitan and (ii) replacing some of the hydrogen atoms from the -OH groups in either sorbitan or sorbitol with acyl groups. These two steps can be performed in either order, but it has been generally preferred commercially in the prior art to esterify first under alkaline conditions, then cyclize, and finally neutralize the products. Alternatively, as noted above, U. S. Patent 4,297,290 teaches cyclizing first under partial vacuum, then esterifying the resulting sorbitan. It has now been found that improved yields, compared with either of these prior art processes, are possible by cyclizing first at atmospheric pressure, preferably using a catalyst including hypophosphite ions, more preferably hypophosphorous acid, then esterifying. Reduced color may be and preferably is achieved by treating the sorbitol before or during reaction with a reducing agent as described generally above. When the esterification of sorbitol is performed first, reduced color may be independently promoted by neutralizing the mixture including sorbitol esters thus produced to reduce its pH to not more than 7.5, preferably not more than 7.0, or more preferably not more than 6.5, before cyclizing.

In this particular embodiment of the invention, it is preferred to utilize hypophosphorous acid and/or hypophosphite ions as the catalyst during the cyclization step of the process, rather than the soap type catalysts generally preferred commercially in the prior art. In a process according to this invention for making sorbitan esters, sodium or potassium hydroxide is preferred as catalyst for the esterification stage, with sodium hydroxide generally more preferred, except that when the product esters are to be ethoxylated, potassium hydroxide catalyst for the esterification stage is preferred.

One advantageous characteristic of the process of cyclizing sorbitol at atmospheric pressure is that the degree of cyclization can be readily controlled by monitoring the amount of water distilled from the reaction mixture. It is preferred in operating according to this process to select a particular degree of cyclization or dehydration of the sorbitol starting material that is desired, to carry out the cyclization reaction in apparatus which causes the water formed during the reaction to distill from the reaction mixture and be readily measurable by volume and/or mass as the reaction progresses, and then to observe the amount of water distilled during the cyclization and discontinue the cyclization reaction when the desired amount of water, including any amount originally present in or added along with the principal reagents, has been collected.

Preferred temperatures are 200 - 220º C for the esterification reaction and 160 - 170º C for the cyclization reaction when the latter is performed first. If the cyclization is performed predominantly after esterifying sorbitol, it is preferred that the temperature during cyclization not exceed 235º C.

Under economically practical conditions, the fatty acid used to manufacture sorbitan esters will almost always contain some acids other than those which are most common and give their name to the mixture. While most such impurities cause little difficulty in processing, it has been found that acyl groups that contain three or more carbon-carbon double or triple bonds have much greater likelihoods of producing a dark color than do acyl groups with two or fewer such unsaturated bonds. Accordingly, it is increas ingly more preferred that the acid mixtures used in a process according to this invention for making sorbitan esters contain not more than 8, 3, or 1 number percent of acyl groups with three or more carbon-carbon double or triple bonds.

It is also preferred that all steps in a process for making sorbitan esters according to this invention be carried out in the substantial absence of oxygen whenever the temperature of the reagents exceeds 90, or more preferably, 25º C. Commonly a protective atmosphere of nitrogen or other inert gas is used to achieve such absence of oxygen. It is increasingly more preferable for the concentration of oxygen in any gas in contact with the hot reagents to be no greater than 0.9, 0.2, 0.03, and 0.005 w/o.

It is normally preferable not to bleach sorbitan ester products according to, or made by a process according to, this invention, because it has been found that conventional bleaching increases the tendency of the product to develop an undesirable haziness upon heating. Conventionally bleached products also tend to develop rancid odors.

Filtration of a sorbitan ester product according to, or made by a process according to, this invention, through diatoroaceous earth, although conventional for other sorbitan esters, also is preferably avoided, inasmuch as it has been found that contact with diatomaceous earth under filtration conditions decreases the color stability of the (unbleached) product. On the other hand, filtration through Dicalite™ 476, an amorphous, mineral, sodium potassium aluminum silicate of volcanic origin, known geologically as perlite, available commercially from Grefco, Inc., Torrance, California, USA, has no adverse effect on color stability and is thus preferred for a process according to this invention whenever filtration is needed, as it usually is.

It is preferred to treat sorbitan esters after making and filtering them with a high surface area absorbent, preferably an alkaline earth metal and/or alkali metal sili cate material with a specific surface area of at least 125, more preferably at least 270, still more preferably at least 400, square meters per gram. The most preferred absorbent is Kagnesol™ 30/40, a magnesium silicate sol that is commercially available from Reagent Chemical and Research, Inc., Houston, Texas, USA. Typical, non-limiting conditions of treatment, e.g., could be stirring the ester product together with 0.5 w/o of Magnesol™ 30/40 for one hour at 100º C. By this type of treatment, the haze temperature of the product can often be raised by 20, 40, or even as much as 75º C. Products treated in this way are superior in haze resistance to all commercial sorbitan monoester products compared against them. For example, sorbitan monooleate with a haze point greater than 200º C, sorbitan monostearate with a haze point greater than 140º C, and sorbitan monolaurate with a haze point greater than 150º C have all been prepared in this way. (All of these nominally "mono" esters of sorbitan are actually mixtures including substantial amounts of di- or even higher esters, along with some unesterified polyol.)

It is generally highly preferred to add an antioxidant to sorbitan esters to avoid darkening upon storage, particularly when the esters have been made by initially cyclizing sorbitol at normal atmospheric pressure and/or have not been bleached. Suitable and preferred antioxidants include butylated hydroxy toluene ("BHT"), butylated hydroxy anisole ("BHA"), vitamin E and other tocopherols, and tertiary butyl hydroquinone ("TBHQ"), with BHA most preferred. As a non-limiting example, addition of 0.02 w/o BHA, in the form of 20 w/o solution of the BHA in food grade oleic acid, to so-called "sorbitan mono-oleate" and "sorbitan mono-laurate", resulted in products with color stability for more than one year, whereas the same products without the addition of BHA darkened noticeably in only a few days.

A second particularly preferred embodiment of the invention is represented by the transesterification of fatty acid esters of low molecular weight alcohols with α- and/or β-etherified glucosides, for example reaction of α-methyl glucoside with methyl oleate to form α-methyl glucoside dioleate.

In this particularly preferred embodiment, it is preferred that a combination of a base, preferably a relatively weak base such as an alkali metal carbonate salt, more preferably potassium carbonate because of its greater solubility than sodium carbonate, and hypophosphite ion from hypophosphorous acid or one of its salts, preferably from sodium dihydrogen hypophosphite, be used to promote the reaction. While not wishing to be bound by theory, it is believed that the main function of the base is to saponify a small fraction of the fatty acid ester used to a soap that helps disperse all the reagents intimately with one another. Hypophosphite ion is believed to catalyze the transesterification reaction itself and is preferred to most other catalysts known for this purpose because of its ability to act as a reducing agent, supplementing the other reducing agents that may be, and preferably are, used in accordance with the embodiment of this invention involving treatment with reducing agents to purify starting materials from reducing sugars.

Preferably the amount of carbonate ion used is from 0.2 to 5, more preferably from 0.8 to 1.7, w/o of the amount of oxyhydrocarbon polyol ester reacted, and the amount of hypophosphite ions is from 0.04 to 1.3, more preferably from 0.2 to 0.7, w/o of the amount of oxyhydrocarbon polyol ether reacted.

Preferably the temperature during transesterification is not allowed to exceed 190, or more preferably 180, degrees C. In order to achieve a practically sufficient reaction rate, this usually means that a partial vacuum during transesterification is needed to drive the reaction by distilling off methanol or other lower alcohol from the fatty acid ester used. Reaction under adequate vacuum is substantially complete, so that the amount of fatty acid ester with a lower alcohol to be used should be determined by stoichiometric calculation to produce the desired average degree of esterification of the glucoside ether reacted. As already noted, generally a mixture of molecules with different degrees of esterification, including some unesterified glucoside ether, will be the actual reaction product.

The practice of the invention may be further appreciated from the following examples and illustrations.

EXAMPLES

Preferred Method for Determininc Amount of "Reducing Sugar" in the Oxyhydrocarbon Polyol (s) and/or Etherfs) Thereof to be Esterified

This method is adapted from one given in the British Pharmacopoeia. In the method, the reducing sugars or other reducing impurities present react with the Benedict solution to produce cuprous oxide. The cuprous oxide is reacted with an excess of standard iodine solution. The remaining iodine is titrated with sodium thiosulfate solution to an end point with starch indicator. All reagents specified should be of conventional purity for analytical work.

Preparation of sodium cupri-citrate ("Benedict") solution

Dissolve 25 grams (hereinafter "g") of cupric sulfate in 700 milliliters (hereinafter "ml") of distilled water and then add 50 g of citric acid. Carefully and slowly add 144 g of anhydrous sodium carbonate. Transfer the solution to a 1000 ml volumetric flask and dilute to the mark with distilled water. Store the solution in a dark brown bottle.

Procedure for Analysis

1. Weigh, to the nearest 0.1 milligram, about 10 g of the polyol and/or polyol ether mixture to be analyzed into a 500 ml Erlenmeyer flask and add 25 ml of distilled water and about ten glass beads. Also prepare a blank flask containing the water and beads but none of the mixture to be analyzed. Use both test flask and blank in all subsequent steps.

2. Add 25 ml of the sodium cupri-citrate solution prepared as described above to each flask, insert a short stemmed funnel into each flask to serve as a reflux condenser, and place each flask so prepared on a hot plate at a sufficiently high temperature to cause the flask contents to boil within 4 minutes (hereinafter "min") and allow to continue boiling for 3 min. Remove flasks from hot plate, remove funnels from flasks, and cool flasks in an ice bath until the contents are cold.

3. Add 10 ml of glacial acetic acid to each flask while swirling and return flask to ice bath until the contents have again cooled. Then add an accurately measured 25 ml of 0.1 E iodine solution to each flask while swirling. After the contents have become uniform in color, add 10 ml of 3 % HCl solution in water and 1 ml of starch indicator solution to each flask. Titrate with 0.1 N sodium thiosulfate solution to the starch endpoint (color change from blue-black to blue-green) .

4. Calculate w/o "reducing sugars" as (0.180) (B - S)/W, where B is the ml of sodium thiosulfate to titrate the blank, S is the ml of sodium thiosulfate to titrate the flask contents with the sample, and W is the weight of the sample in grams. If S is less than (0.8) «B, possible incomplete reaction is indicated; in that case, repeat the procedure using a smaller sample.

Example 1; Preparation of α-Methγl Glucoside Dioleate

The raw materials and quantities used are as follows: α-methyl glucoside 145.5 g Water 145.5 g Potassium carbonate sesquihydrate 3.8 g Sodium borohydride 0.075 g Water 15 g Sodium hypophosphite 0.84 g

Methyl oleate 448 g

The α-methyl glucoside used is Horizon™ STA MEG 106 brand from Staley Chemical Co.; the methyl oleate is Emery™ type 2303. The amount of sodium borohydride is calculated to give a 25 % excess over the amount needed to reduce all the "reducing sugar" in the quantity of α-methyl glucoside used, as determined by the analytical method noted above on a sample of the same lot of α-methyl glucoside. For use in the synthesis, the amounts of potassium carbonate and sodium borohydride used are dissolved and/or suspended in the separately listed 15 g of water, immediately before being added to the reaction mixture.

The synthesis is performed in a four necked flask equipped with an agitator/stirrer, thermometer, inlet for nitrogen, outlet to a vacuum pump, and a condenser set for distillation. To avoid contamination, the stirrer shaft is lubricated with α-methyl glucoside dioleate.

To begin reaction, the reagents are charged separately to the flask at room temperature after a nitrogen atmosphere had been established therein, in the order listed above, except that the already noted solution/suspension of potassium carbonate and sodium borohydride is added in one step. The reaction slurry is then heated with constant agitation at atmospheric pressure to 180º C. During heating, the reaction mixture becomes viscous and foamy, but at about 150º C, the viscosity begins to diminish rather sharply.

When the temperature reaches 180º C, vacuum is applied until the pressure inside the flask is reduced to about 66 millibars. Within fifteen minutes, methanol begins to distill vigorously. After ninety minutes at 180º C, the reaction mixture is cooled under vacuum to 80º C and the vacuum then filled with nitrogen. While still under a nitrogen blanket, the reaction mixture is neutralized with 1.75 g of 75 w/o H₃PO₄ solution in water. The neutralized reaction mixture is then opened to the ambient atmosphere and fil tered using perlite filter media and Whatman #2 filter paper. After filtration, the filtrate is cooled to room temperature and stored.

Example 2

This illustrates the manufacture of sorbitan esters from sorbitol by cyclizing first, then esterifying.

A three liter capacity flask equipped with a stirrer, heating mantle, gas exit and inlet means with a flow of nitrogen to maintain a nitrogen atmosphere in the flask, neck and condenser for distillation, and thermometers for measuring the temperatures of the flask liquid and of the condensing vapor if any is charged with 520 g of 70 % pure liquid sorbitol (Neosorb™ 70/02S, commercially available from Roquette Corporation, Gurnee, Illinois) = 2.00 moles of pure sorbitol; 4.2 g of 50 % aqueous hypophosphorous acid = 0.032 moles H₃PO₂; and 724 g of commercial grade "stearic acid" = about 1.3 moles each of stearic and palmitic acids (Grade E - 132 Stearic Acid, commercially available from Emery Chemical Division of Henkel Corp., Cincinnati, Ohio). After charging is complete and the nitrogen atmosphere is established, the temperature is raised over the course of about 65 min to 165º C. The amount of distillate, primarily water, is monitored at intervals during the reaction, in order to determine how much cyclization has occurred. During the first ninety minutes, with the temperature of the flask contents constant at 165' C once that temperature is reached, 143 g of distillate is collected. In the next 50 min, only 10 g of distillate is collected, and over the next 35 minutes only 2.5 g of distillate is collected. The vapor temperature reaches a high of 115º C at about the time that the flask contents temperature reaches 165º C but then falls, remaining within the range of 98 - 105º C for the last 85 minutes while the flask is at 165º C.

The temperature of the flask contents is then raised to 170º C over a period of 15 min and is maintained there for the next 200 min. The vapor temperature during this period falls slowly from 105º C to a final value of 93º C, under a slight vacuum maintained during the final 25 min, while the distillate collection rate falls from about 15 to about 8 g per hour. The flask contents are then allowed to cool.

The cooled reaction mixture as described above is reheated to 72º C while still under a nitrogen atmosphere, and after this 1.8 g of 50 aqueous H₃PO₂ and 13.2 g of aqueous KOH are added to the contents. The temperature of the flask contents is then raised over 45 min to 210º C and maintained there for 240 min, during which time 49 g of additional distillate is collected. An amount of 3.9 g of 75 % aqueous H₃PO₄ is then added to neutralize the remaining potassium hydroxide. The product has an Acid No. of 10.00, a Saponification No. of 149.2, a Hydroxyl No. of 238.1, and a melting point of 52.8 - 53.4º C. The yield is 1002 g of product.

Comparative Example 1

This is the same as Example 2, except that the initial cyclizing-anhydrization reaction is performed at 120º C and at a pressure of only 6 - 7 millibars for 110 min (without any measurement of the amount of water distilled off from the reaction mixture) in accordance with the general teachings of Stockburger, U. S. Patent 4,297,290 column 3 lines 13 - 19. The yield is less than 900 g.

Example 3

Example 2 is repeated, except that sodium borohydride is added to the initial mixture in an amount 25 % over the stoichiometric amount required to reduce all the "Reducing Sugars" in the sorbitol used. The product has a lighter color than that of the product from Experiment 2, but other results are essentially the same.

Example 4

This example illustrates the preparation of a sorbitan ester from sorbitol by first esterifying under basic catalysis, then neutralizing, then cyclizing. The equipment used is the same as for Example 1. A nitrogen atmosphere is established in the flask, and 669 g of 70 % sorbitol (from Roquette as above), 541 g of iauric acid (Grade E - 626 from Emery as above), 3.0 g of 50 % aqueous H₃PO₂, and 3.0 g of 50 % aqueous NaOH are charged to the flask at room temperature in the order given above, with stirring. Heating is then begun, with stirring continuing, and the flask contents temperature reaches 126º C after about one hour and 215º C after another 130 min. The temperature is then maintained in the range from 215 - 220º C for another SO min, after which time the Acid No. of the contents has fallen to 6.2.

An amount of 2.40 g of 75 % aqueous H₃PO₄ is then added to the flask contents, which are then cooled to 100º C while the pressure in the flask is reduced to about 500 millibars. Heating is then resumed, and the flask contents reach the desired temperature range of 220 - 230º C within about one hour. Temperature and pressure are maintained at those levels for about 75 min, at which point the pressure is reduced to about 130 millibars. These conditions are maintained for about four additional hours, after which the reaction mixture is cooled. The yield is 826 g (unfiltered) of sorbitan "monolaurate", with a Gardner color of 1+, an Acid No. of 0.30, a Saponification No. of 168.5, a Hydroxyl No. of 321.6, a pH of 7.57, a viscosity of Z-4 (measured with a Gardner viscosimeter) and a haze point of 120º C.

Comparative Example 2.

This is the same as Example 4, except that the neutralization with H₃PO₄ is not performed at the point indicated in Example 4, but instead is performed after all other described process steps. The color of the product is much darker than in Example 4.

Example 5

This is the same as Example 2, except that (1) 820 g of oleic acid is used instead of the stearic acid of Examp le 2, (2) the temperature before addition of the potassium hydroxide is not allowed to rise above 165º C, with a slight vacuum to produce a vapor temperature of about 95º C being maintained for the last 200 sinutes before addition of the potassium hydroxide; (3) the temperature after addition of the potassium hydroxide is kept at or below 200º C, with vacuum after the first two hours of reaction to reduce the vapor temperature into the range of 60 - 80º C; and (4) the final product is 1087 g of sorbitan monooleate with a Gardner color of 2+, an Acid No. of 6.69, a Saponification No. of 150.63, and a Hydroxyl No. of 205.5.

What is claimed is:

Claims

1. A process for esterifying a mixture, at least 60 w/o of said mixture consisting of materials selected from the group consisting of oxyhydrocarbon polyols and ethers thereof, said mixture also comprising material that darkens in color during said esterifying, wherein the improvement comprises treating said mixture before, during, or both before and during the esterifying with an amount of reducing agent effective to reduce the degree of darkening that occurs during esterification of said mixture.

2. A process according to claim 1, wherein said at least 60 w/o of said mixture consists of material selected from the group consisting of α-alkyl glucosides and ,5-alkylglucosides, and said esterification is achieved by a transesterification reaction between the alkyl glucoside component of said mixture and material selected from the group of fatty acid esters of alcohols having no more than 4 carbon atoms.

3. A process according to claim 2, wherein said transesterification reaction occurs in the presence of hypophosphite ions in an amount effective for catalysis of the cyclization reaction and at a temperature not greater than about 180º C.

4. A process according to claim l, wherein said at least 60 w/o of said mixture consists of material selected from the group consisting of sorbitan and sorbitol.

5. A process according to claim 4, comprising an additional step of cyclizing sorbitol to sorbitan before esterifying the latter, said cyclizing being performed at atmospheric pressure and at a temperature in the range from about 150 to about 170º C in the presence of hypophosphite ions in an amount effective for catalysis of the cyclization reaction and said esterifying being performed at a temperature in the range from about 200 to about 220º C in the presence of a catalytically effective amount of sodium hydroxide, potassium hydroxide, or a mixture of sodium and potassium hydroxide.

6. A process according to claim 4, wherein said at least 60 w/o of the mixture esterified is sorbitol, the ester mixture produced is neutralized to a pH not greater than 7.5, and the neutralized mixture including sorbitol esters is cyclized at a temperature in not greater than about 235º C in the presence of hypophosphite ions in an amount effective for catalysis of the cyclization reaction to produce a mixture consisting predominantly of sorbitan esters.

7. A process according to claim 6, wherein said reducing agent is sodium borohydride and is used in an amount that is at least 14 % in excess over the stoichiometric amount required to reduce all the reducing sugars present in the mixture esterified.

8. A process according to claim 5, wherein said reducing agent is sodium borohydride and is used in an amount that is at leas€ 14 % in excess over the stoichiometric amount required to reduce all the reducing sugars present in the mixture esterified.

9. A process according to claim 4, wherein said reducing agent is sodium borohydride and is used in an amount that is at least 14 % in excess over the stoichiometric amount required to reduce all the reducing sugars present in the mixture esterified.

10. A process according to claim 3, wherein said reducing agent is sodium borohydride and is used in an amount that is at least 14 % in excess over the stoichiometric amount required to reduce all the reducing sugars present in the mixture esterified.

11. A process according to claim 2, wherein said reducing agent is sodium borohydride and is used in an amount that is at least 14 % in excess over the stoichiometric amount required to reduce all the reducing sugars present in the mixture esterified.

12. A process according to claim 1, wherein said reducing agent is sodium borohydride and is used in an amount that is at least 14 % in excess over the stoichiometric amount required to reduce all the reducing sugars present in the mixture esterified.

13. A process for the preparation of a mixture consisting predominantly of material selected from the group consisting of esters of sorbitan with one or more fatty acids, said process comprising the steps of:

(A) cyclizing sorbitol to sorbitan at atmospheric pressure and at a temperature in the range from about 160 to about 170º C in the presence of hypophosphite ions in an amount effective for catalysis of the cyclization reaction; and

(B) esterifying the sorbitan formed in step (A) by reaction with one or more fatty acids at a temperature in the range from about 200 to about 220º C in the presence of a catalytically effective amount of sodium hydroxide, potassium hydroxide, or a mixture of sodium and potassium hydroxide.

14. A process according to claim 13, wherein a particular degree of dehydration of the sorbitol starting material is selected, the cyclization reaction is performed in apparatus which causes the water formed during the reaction to distill from the reaction mixture and be collected in a separate container as the reaction progresses, and the cyclization reaction is discontinued when the amount of water collected corresponds to the selected degree of dehydration.

15. A process according to claim 14, wherein the hypophosphite ions used as catalyst in step (A) are supplied from addition of hypophosphorous acid to the reaction mixture.

16. A process according to claim 13, wherein the hypophosphite ions used as catalyst in step (A) are supplied from addition of hypophosphorous acid to the reaction mixture.

17. A process for the preparation of a material consisting predominantly of one or more esters of sorbitan with one or more fatty acids, said process comprising the steps of: (A) esterifying a material consisting predominantly of sorbitol by reaction with one or more fatty acids at a temperature in the range from about 200 tc about 220º C in the presence of a catalytically effective amount of sodium hydroxide, potassium hydroxide, or a mixture of sodium and potassium hydroxide;

(B) neutralizing the mixture formed in step (A) to reduce its pH value to not more than about 7.5; and

(C) cyclizing the neutralized mixture formed in step (B) by reaction at a temperature not greater than 235º C in the presence of hypophosphite ions in an amount effective for catalysis of the cyclization reaction.

18. A process according to claim 17, wherein the pH value at the end of step (B) is not greater than about 6.5.

19. A composition of matter having a Gardner color value not greater than 2 and selected from the group consisting of (A) a mixture having a haze temperature greater than about 200º C and consisting essentially of materials selected from the group consisting of sorbitan and esters of sorbitan with oleic acid, (B; a mixture having a haze temperature greater than about 150º C and consisting essentially of materials selected from the group consisting of sorbitan and esters of sorhitar. with lauric acid, (C) a mixture having a haze temperature greater than about 140º

C and consisting essentially of materials selected from the group consisting of sorbitan and esters of sorbitan with stearic acid, and (D) a mixture consisting essentially of materials selected from the group consisting of α- and β-methyl glucosides and esters of α- and β-methyl glucosides with fatty acids.

20. A composition according to claim 19, additionally comprising a color stabilizing effective amount of an antioxidant and selected from the group consisting of (A) a mixture having a haze temperature greater than about 200º C and consisting essentially of materials selected from the group consisting of sorbitan and esters of sorbitan with oleic acid, (B) a mixture having a haze temperature greater than about 150º C and consisting essentially of materials selected from the group consisting of sorbitan and esters of sorbitan with lauric acid, and (C) a mixture having a haze temperature greater than about 140º C and consisting essentially of materials selected from the group consisting of sorbitan and esters of sorbitan with stearic acid.