SYNTHESIS OF HIGHER POLYOL FATTY ACID POLYESTERS USING HIGH SOAP:POLYOL RATIOS

Abstract: An improved solvent-free transesterification provess for producing higher polyol fatty acid polyesters is dis­ closed. In this process a mixture of a polyol, a fatty acid methyl, 2-methoxy ethyl or benzyl ester, an alkali metal fatty acid soap, and a basic catalyst, having a molar ratio of soap:polyol of from about 0.6:1 to about 1:1, preferably from about 0.75:1 to about 1:1, is heated to form a homogeneous melt. To this melt is subsequently added excess fatty acid methyl, 2-methoxy ethyl or benzyl ester, yielding the desired polyol fatty acid polyesters.

Description: This invention relates to an improved, high yield synthesis of higher polyol fatty acid polyesters, sucrose polyesters in particular, via transesterification. The food and pharmaceutical industries have recently focused attention on polyol polyesters for use as low calorie fats in food products and as pharmaceutical agents, e.g., for the lowering of blood cholesterol levels. U.S. Patent 3,600,186, Mattson and Volpenhein, issued August 17, 1971, describes low calorie food compositions formed by replacing at least a portion of the fat content of food products with higher polyol fatty acid polyesters. U.S. Patent 3,954,976, Mattson and Volpenhein, issued May 4, 1976, describes pharmaceutical compositions for inhibiting the absorption of cholesterol comprising effective unit dosage amounts of higher polyol fatty acid polyesters, as well as the method for treating hypercholesterolemia using these polyesters. Additional pharmaceutical uses are described in U.S. Patent 4,241,054, Volpenhein and Jandacek, issued December 23, 1980 (removal of halogenated toxins from the body), and U.S. Patent 4,264,583, Jandacek, April 28, 1981 (treatment of gallstones). As a result of these many uses for the higher polyol fatty acid polyesters, it would be desirable to have an efficient high yield synthesis for them. Historically, such syntheses have been conducted using a mutual solvent to solubilize a polyol and esters of long chain fatty acids, thus providing a homogeneous reaction medium suitable for catalytic transesterification. One variation of this process, known as the Snell synthesis, has been employed as a means for preparing both poly- and lower esters. However, the solvents employed in such processes are difficult to separate from the final product and are characteristically toxic, therefore limiting the usefulness of such syntheses in the food and pharmaceutical industries. Accordingly, efforts have been directed toward the discovery of high yield syntheses of polyol fatty acid polyesters which do not employ toxic solvents. US-A-3,963,699, Rizzi and Taylor describes the basic solvent-free transesterification process for synthesizing higher polyol fatty acid polyesters. In this three-step reaction, a mixture of a polyol (such as sucrose), a fatty acid lower alkyl ester (such as fatty acid methyl ester), an alkali metal fatty acid soap, and a basic catalyst is heated, forming a homogeneous melt, to which is added excess fatty acid lower alkyl ester to form the higher polyol fatty acid polyesters. The polyesters are then separated from the reaction mixture. The catalysts described in this patent as being useful include alkali metals, alloys of two or more alkali metals, alkali metal hydrides, and alkali metal alkoxides. The processes exemplified in this patent utilize sodium hydride, sodium hydroxide or dispersions of potassium as catalysts, and soap:sucrose mole ratios of about 0.3-0.4:1. Rizzi and Taylor, Journal of the American Oil Chemists ' Society 55 :398 (1978), further describe the reaction set forth in the Rizzi and Taylor patent. Advantages are demonstrated for catalyzed reactions versus uncatalyzed reactions; sodium hydride and sodium-potassium alloy are taught to be effective catalysts. At page 400, the paper teaches that alkali metal carbonates and alkali metal alkoxides are relatively ineffective as catalysts. US-A-4,334,061, Brossier, III, describes a method for separating and purifying the polyesters formed by the Rizzi and Taylor process. The procedure requires, in the separation step, an alkaline pH which is obtained by adding an alkali metal carbonate to the reaction mixture at the conclusion of the transesterification reaction. Thus, the carbonate compounds added do not function as catalysts for the transesterfication reaction. US-A- 2,893,990, Hass, et al describes a process for making carboxylic acid lower esters of sucrose and raffinose; generally, mono- or diesters are formed. In the process, a non-sucrose ester of a fatty acid (e.g., methyl stearate or methyl palmitate) is reacted with sucrose, preferably in a solvent. A wide range of alkaline catalysts, including sodium carbonate and potassium carbonate, are disclosed for use in the reaction. GB-A-2065634 also discloses the production of mono- and di-esters of sucrose as surfactants using a basic interesterification catalyst which can be any of the normal types, viz an alkali metal carbonate, alkoxide or hydride. The reaction system is solvent-free but incorporates mixed glycerides of a fatty acid of ≧ 8 carbon atoms as an essential component. It has now been found that by modifying the solvent-free transesterfication reaction described in the Rizzi and Taylor patent, discussed above, using significantly higher soap:sucrose mole ratios than those originally envisioned, shorter reaction times, more complete utilization of the polyol component, and improved yields of the higher polyol polyesters can be obtained. It is, therefore, an object of this invention to provide an improved solvent-free high yield synthesis of polyol fatty acid polyesters. The present invention encompasses an improved solvent-free transesterification process for synthesizing higher polyol fatty acid polyesters comprising the steps of: 1) heating a mixture consisting of (a) a polyol selected from monosaccharides, disaccharides and sugar alcohols, (b) a fatty acid ester wherein the fatty acid is selected from those fatty acids naturally occurring in fats and oils and the ester is selected from methyl esters, 2-methoxy ethyl esters, benzyl esters, and mixtures thereof, (c) an alkali metal C₈-C₁₈ fatty acid soap, and (d) a a basic catalyst selected from alkali metal hydrides and alkoxides, sodium carbonate and barium carbonate, to a temperature of from 110°C to 180°C at a pressure of from 13.3 Pa (0.1 mm mercury) to 101325 Pa (760 mm mercury) to form a homogeneous melt, the catalyst components (d) optionally being added stepwise to the reaction mixture; 2) adding to the reaction product of step (1) excess of said fatty acid ester component (b) and heating to a temperature of from 120°C to 160°C at a pressure of from 13.3 Pa (0.1 mm mercury) to 1330 Pa (10 mm mercury) to form the higher polyol fatty acid polyesters; and 3) separating the higher polyol fatty acid polyesters from the polyesters, by-product alcohol, and unreacted starting materials, wherein the molar ratio of soap:polyol in step (1) is from 0.6:1 to 1:1. Preferably the molar ratio is from 0.75:1 to 1:1, more preferably from 0.75:1 to 0.85:1, most preferably 0.75:1. The present invention encompasses improvements in the solvent-free transesterification reaction for forming higher polyol fatty acid polyesters described and claimed in U.S. Patent 3,963,699, Rizzi and Taylor, issued June 15, 1976. This process is characterized by a three-step reaction procedure, summarized below. By utilizing the improvements taught herein, the reaction described in the Rizzi and Taylor patent can be improved by reducing excessive foaming, shortening reaction times, increasing the yields of the higher polyol fatty acid polyesters, and yielding products having better (lighter) color characteristics. In fact, the improvements herein permit the reaction to be formulated as a single step solvent-free reaction for the production of higher polyol fatty acid polyesters. STEP 1 In the first step of the present process, a heterogeneous mixture of a polyol, fatty acid methyl, 2-methoxy ethyl or benzyl esters, an alkali metal fatty acid soap, and a basic catalyst is reacted to form a homogeneous melt comprising partially esterified polyol and unreacted starting materials. As used herein, the term "polyol" is intended to include any compound within the classes comprising monosaccharides, disaccharides and sugar alcohols. In practising the process disclosed herein, the selection of a suitable polyol is simply a matter of choice. The carbohydrates form the preferred classes of polyols. Monosaccharides suitable for use herein include, for example, mannose, galactose, arabinose, xylose, ribose, apiose, rhamnose, psicose, fructose, sorbose, tagitose, ribulose, xylulose, and erythrulose. Oligosaccharides suitable for use herein include, for example, maltose, kojibiose, nigerose, cellobiose, lactose, melibiose, gentiobiose, turanose, rutinose, trehalose, sucrose and raffinose. Although sugar alcohols are not carbohydrates in a strict sense, the naturally occurring sugar alcohols are so closely related to the carbohydrates that they are also preferred for use herein. The sugar alcohols most widely distributed in nature and suitable for use herein are sorbitol, xylitol mannitol and galactitol. Preferred carbohydrates and sugar alcohols include sucrose, xylitol, sorbitol and mixtures thereof. As used herein, the term "fatty acid esters" is intended to include the methyl, 2-methoxy ethyl and benzyl esters of fatty acids containing about eight or more carbon atoms, and mixtures of such esters. Such esters can be prepared by the reaction of diazoalkanes with, or derived by alcoholysis from, the fatty acids naturally occurring in fats and oils. Fatty acid esters can be derived from either saturated or unsaturated fatty acids. Suitable preferred saturated fatty acids include, for example, capric, lauric, palmitic, stearic, behenic, isomyristic, isomargaric, myristic, caprylic, and anteisoarachadic. Suitable preferred unsaturated fatty acids include, for example, maleic, linoleic, licanic, oleic, linolenic, and erythrogenic acids. Fatty acids derived from soybean oil, palm oil, sunflower oil, safflower oil, and corn oil and mixtures thereof are especially preferred for use herein. The methyl esters of these fatty acids are preferred for use herein, since their use in the process herein tends to result in unusually high yields of polyol fatty acid polyesters. As used herein, the term "alkali metal fatty acid soap" is intended to include the alkali metal salts of saturated and unsaturated fatty acids having from eight to eighteen carbon atoms. Accordingly, suitable alkali metal fatty acid soaps include, for example, the lithium, sodium, potassium, rubidium, and cesium salts of fatty acids such as capric, lauric, myristic, palmitic, licanic, parinaric, and stearic acids, as well as mixtures thereof. Mixtures of fatty acids derived from soybean oil, sunflower oil, safflower oil, and corn oil are preferred for use herein. Accordingly, preferred alkali metal fatty acid soaps include, for example, the potassium soap made from soybean oil fatty acids and the sodium soap made from sunflower oil fatty acids. The basic catalysts generally suitable for use herein are those selected from alkali metal hydrides, such as sodium, lithium and potassium hydride; alkali metal alkoxides, such as potassium t-butoxide and sodium methoxide, sodium carbonate, barium carbonate, or mixtures of these compounds. The use of Potassium Carbonate as a catalyst in processes of the present type is disclosed and claimed in the commonly assigned European Patent Application Publication No. 0132941. In a preferred embodiment of this invention, the catalyst is dispersed in a suitable carrier so as to insure uniform distribution of the catalyst throughout the reaction mass. Suitable carriers or dispersing agents include, for example, methanol and fatty acid methyl esters. In carrying out step 1, the above-described reactants are combined to form a heterogeneous mixture. The precise ratio of reactants can be freely selected from within the guidelines set forth hereinafter. However, routine experimentation may be necessary in order to establish the optimum concentrations for a given set of reactants. In general, the heterogeneous mixture comprises from 10% to 50%, preferably from 15% to 30%, by weight of the polyol; from 40% to 80%, preferably from 55% to 75%, by weight of the fatty acid esters; from 1% to 30%, preferably from 5% to 20%, by weight of the alkali metal fatty acid soap; and from 0.05% to 5%, preferably from 0.1% to 0.5%, by weight of the basic catalyst component. It may be desirable to initiate the reaction by initially introducing from 0.1% to 1% by weight of catalyst and thereafter introducing additional catalyst as the reaction proceeds. It has surprisingly been found that when this mixture is formed so as to include relatively high molar ratios of soap:polyol, increased yields of the higher polyesters (e.g., the octaesters) are obtained when compared with similar reactions carried out using lower art-disclosed soap:polyol molar ratios (e.g., 0.3-0.4:1). Specifically, these higher soap:polyol ratios result in increased yields of the higher polyols, more complete utilization of the polyol reaction component, and/or faster disappearance of free polyol from the reaction mixture. The process of the present invention therefore uses soap:polyol molar ratios in step 1 of from 0.6:1 to 1:1. Preferred soap:polyol ratios fall in the range from 0.75:1 to 1:1, more preferably from 0.75:1 to 0.85:1, and most preferably 0.75:1. The heterogeneous mixture is heated to a temperature within the range of from 110°C to 180°C, preferably from 130°C to 145°C, under a pressure of from 13.3Pa(0.1 mm Hg) to 101325Pa(760 mm Hg), preferably from 66.5 Pa(0.5 mm Hg) to 3325Pa (25 mm Hg) Within these temperature and pressure ranges, a homogeneous melt of partially esterified polyol and unreacted starting materials will form in from 1 to 4 hours. STEP 2 In the second step of the instant process, excess fatty acid methyl, 2-methoxy ethyl, or benzyl esters are added to the homogeneous melt formed in step 1. As used herein, the term "excess" is intended to include sufficient fatty acid esters to raise the overall ester: polyol mole ratio above 8:1, preferably to 12:1. Although ratios beyond 12:1 can be used, as a general rule, such ratios do not noticeably decrease reaction time or improve the yield and, therefore, tend to be impractical. When fatty acid methyl esters are used, it is preferred that after the excess ester is added to the reaction mixture, the mixture be heated to a temperature of from 120°C to 160°C, preferably 135°C, at a pressure from 13.3Pa(0.1 mm Hg) to 1330Pa (10 mm Hg), preferably from 66.5Pa (0.5 mm Hg) to 266Pa (2 mm Hg), to form the polyol fatty acid polyester material. The reaction time for step 2 is preferably less than 10 hours, and generally is between 2 and 8 hours. It should be noted that as the transesterification reaction proceeds, a lower alcohol is formed as a by-product. In order to promote the reaction, the alcohol by-product is preferably removed. Many removal techniques are known in the art, any one of which can be used to effectively and efficiently remove the lower alcohol. Vacuum removal both with and without an inert gas sparging has been found to promote the reaction. In any event, the formation of a lower alcohol presents no significant obstacle to the use of the process in the food or pharmaceutical industries. The use of certain catalyst and the soap:polyol ratios, defined herein, permit the combination of steps 1 and 2 into a single reaction step. In this single step approach, a mixture of (a) a polyol selected from monosaccharides, disaccharides and sugar alcohols; (b) an alkali metal fatty acid soap; (c) a basic catalyst selected from sodium carbonate and barium carbonate; and (d) an excess of fatty acid methyl, 2-methoxy ethyl or benzyl ester (wherein the soap:polyol molar ratio is from 0.6:1 to 1:1, preferably from 0.75:1 to 1:1, more preferably from 0.75:1 to 0.85:1, most preferably 0.75:1), is heated to a temperature of from 100°C to 180°C at a pressure of from 13.3Pa (0.1 mm Hg) to 101325Pa(760 mmHg), thereby forming higher polyol fatty acid polyesters. STEP 3 In the third step of the process, the polyol fatty acid polyesters formed in step 2 are separated from the reaction mix containing polyesters, soap, and unreacted starting materials. Separation can be accomplished by any of the separation procedures routinely used in the art. Distillation, water washing, conventional refining techniques or solvent extraction are preferred due to their simplicity and economy and the most preferred technique is water addition to the reaction mixture followed by centrifugation to separate the higher polyol fatty acid polyesters from the remainder of the reaction mixture. The following example is intended to further clarify the invention. Each of the following reactions was carried out in a 1 liter 3-neck flask containing a stirrer, thermometer, reflux condensor, and vacuum outlet. EXAMPLE Higher sucrose polyesters were prepared by the following reaction sequence: Step (1) : 3.6 g 85% KOH pellets (0.055 moles) dissolved in some methanol and 103 g (0.347 moles) soybean oil fatty acid methyl esters (FAME) were heated and stirred at reflux for two hours. 25g sucrose (0.073 moles) and basic catalyst in the amounts shown below were added and the condenser removed. The soap:sucrose molar ratio of the mixture was 0.75:1. The methanol was evaporated from the mixture under a gentle stream of nitrogen. When the reaction reached 100°C, a vacuum was applied and the temperature brought to 135°C. Conditions were maintained for two hours. Step (2) : 174 g additional FAME (0.585 moles) was drawn into the reactor. The final molar ratio of FAME to sucrose was 12:1. The temperature was allowed to recover to 135°C and stirring under vacuum was continued for three hours. The vacuum slowly decreased in this time to from 665Pa (5.0 mm Hg) to 66.5Pa (0.5 mm Hg), as the methanol formed during the reactions was removed. The reaction was cooled to 90-100°C and 200 ml 80:20:2 (by weight) water:alcohol:salt added and the mix stirred for 10 minutes at 80°C. The reaction mix was transferred to a separatory funnel and the phases allowed to separate. The lower aqueous soap solution was discarded and the lipid phase returned to the reactor for additional washes: first with a second 80:20:2 water: alcohol:salt wash and then with 2% aqueous acetic acid and two water washes (all at 80°C). The lipid was dried under vacuum, bleached with 1-5% Filtrol 105 (a bleaching earth), filtered and steam deodorized at 205°C to remove excess FAME. The sample was weighed and the yield calculated as percent sucrose recovered as octaester. The octaester content of the reaction product was determined by separating the mix on a silica gel column and weighing the relative amount of octaester and partial esters recovered. The product formed comprised a mixture of the higher polyesters of sucrose, having a high octaester content. This general procedure was used to compare the effectiveness of various catalysts. Those tested included sodium carbonate (Na₂CO₃), sodium methoxide (NaOMe), sodium hydride (Na.H) and potassium hydroxide (KOH), all at 10 mole percent of the sucrose in the reaction. NaOMe, and potassium hydride (K.H) were also compared at four weight percent of the sucrose. The results are summarized in the table below. To assess the effect of soap level on the reaction, the procedure as described above was utilized. The concentration of soap in the reaction mix was controlled by either varying the amount of KOH added at the beginning or by adding varying amounts of preformed potassium soaps. Two analytical methods were used to monitor the effect of soap on the reaction. In the first, varying amounts of KOH and radiolabelled sucrose were used. At the end of the first two hours, before the second addition of FAME, the reaction was stopped and partitioned between hot water and ethyl acetate. The amount of ¹⁴C activity found in the water relative to that added as free sucrose at the beginning of the reaction was a measure of unreacted sucrose. These results are summarized below. In the second test, varying amounts of pre-formed potassium soap were added to the mixture in place of KOH. The effect of the soap was judged by measuring yield of sucrose polyesters and octaester content. The results obtained are as follows.

Claims: A solvent-free transesterification process for synthesizing higher polyol fatty acid polyesters comprising the steps of: 1) heating a mixture consisting of (a) a polyol selected from monosaccharides, disaccharides and sugar alcohols, (b) a fatty acid ester wherein the fatty acid is selected from those fatty acids naturally occurring in fats and oils and the ester is selected from methyl esters, 2-methoxy ethyl esters, benzyl esters, and mixtures thereof, (c) an alkali metal C₈-C₁₈ fatty acid soap, and (d) a a basic catalyst selected from alkali metal hydrides and alkoxides, sodium carbonate and barium carbonate, to a temperature of from 110°C to 180°C at a pressure of from 13.3 Pa (0.1 mm mercury) to 101325 Pa (760 mm mercury) to form a homogeneous melt, the catalyst components (d) optionally being added stepwise to the reaction mixture; 2) adding to the reaction product of step (1) excess of said fatty acid ester component (b) and heating to a temperature of from 120°C to 160°C at a pressure of from 13.3 Pa (0.1 mm mercury) to 1330 Pa (10 mm mercury) to form the higher polyol fatty acid polyesters; and 3) separating the higher polyol fatty acid polyesters from the polyesters, by-product alcohol, and unreacted starting materials, characterised in that the molar ratio of soap:polyol in step (1) is from 0.6:1 to 1:1. A process according to claim 1 wherein the molar ratio of soap:polyol is from 0.75:1 to 1:1, preferably from 0.75:1 to 0.85:1. A process according to either one of claims 1 and 2 wherein the polyol is selected from sucrose, xylitol, sorbitol, and mixtures thereof. A process according to any one of claims 1-3 wherein the methyl esters are derived from materials selected from soybean oil, safflower oil, palm oil, sunflower oil, corn oil and mixtures thereof. A process according to any one of claims 1-4 wherein the mixture of step (1) is heated to a temperature of from 130°C to 145°C. A process according to any of claims 1-5 wherein the reaction mixture of step (1) comprises from 10% to 50% by weight of the polyol, from 40% to 80% by weight of the fatty acid ester, from 5% to 20% by weight of the alkali metal fatty acid soap, and from 0.05% to 5% by weight of the catalyst component, A process according to any one of claims 1-6 wherein water is added to the mixture of step (2), said mixture is centrifuged and the higher polyol fatty acid polyesters are separated therefrom.

Inventors: VOLPENHEIN, ROBERT ANTHONY
Applicants: THE PROCTER & GAMBLE COMPANY
Application year: 1984
Main classification: C07H13
IPC codes: C07H13