Process for preparing alkyl polyglucosides

Abstract: A process is disclosed for preparing alkyl polyglucosides of general formula (I), by reacting an alcohol with a reducing sugar or with an equivalent of the latter, in the presence of a binary catalyst formed by a strong organic acid and a weak, also organic, base, having a Ka comprised within the range of from 10⁻⁸ to 10⁻¹.

Description: The present invention relates to a process for preparing alkyl polyglucosides. The field of surfactants has seen a considerable development during the past years; nowadays, a large share of world market is constituted by non-ionic surfactants, and in particular, by polyethoxylated alcohols and polyethoxylated alkylphenols. Such non-ionic surfactants have gained a considerable importance thanks to their good detergent properties, to their flexibility in the various formulations (compatibility with ionic surfactants) and to their low production cost. Besides these classes of non-ionic surfactants, new classes have been developed recently, which are characterized by hydrophilic moieties different from polyethoxy groups; among these, the esters of mono- and oligosaccharides seem to be particularly attractive, owing to their low cost and intrinsic biodegradability. However, this type of long chain esters, derived, e.g., from palmitic acid, are not satisfactory from the practical viewpoint, owing to the Limited stability of the esters group at pH values higher than 8, necessary in a large number of formulations. Such a limited chemical stability was overcome by introducing ether linkages resulting from the reaction of epoxides with a hydroxyethyl glucoside, as disclosed in Italian Patent N. 1,252,613 to the name of Enichem Augusta S.p.A.. Extremely good surfactants are also the alkyl-glucosides, whose preparations process has been widely disclosed in a large number of patents. The problems connected with their preparation essentially are as follows: (1) methods in order to prevent products from getting coloured; (2) search for optimal catalyst; (3) methods for neutralizing said catalyst; (4) methods in order to facilitate the removal of the excess alcohol by distillation. As regards the catalyst, the use of p-toluenesulfonic acid, sulfuric acid and sulfonic resins was already disclosed in patents filed prior to the Seventies. Recent developments are, for example, use of sulfosuccinic acid, as disclosed in WO 91/02742, and of dinonyl-naphthalene sulfonic acid, in WO 90/07516. As regards the problem of colour, a large number of patents exist which teach the use, besides the catalyst, of reducing acids (EP 77 167) or hydroxyacids (US 4,465,828). The resulting products, before being distilled in order to recover the excess alcohol, are neutralized with certain bases, as disclosed in 4,713,447 and EP 132 046. As regards distilling off the alcohol excess, in U.S. 4,510,306 and U.S. 4,889,925 the addition of fluidifier agents is disclosed in order to facilitate the stripping of the same alcohol. In any cases, the essential problem is the reduction of polyglucose, which, by causing an increase in product viscosity, makes it difficult the alcohol to be removed. In fact, in the presence of a large amount of polyglucose, the product is semisolid and makes it impossible thin-layer distillation techniques to be used. In WO 90/07516 patent a novel class of high-lipophilicity sulfonic acids is introduced, which considerably reduce the formation of polyglucose which, also considering the reaction economics, brings to a loss of yield of desired product. However, such catalysts have a high cost and must be purposely prepared. The present Applicant has found now that a glycosidation product can be obtained which is free from, or only contains extremely small amounts of, polyglucose, if the reaction is carried out in the presence of a particular catalytic system. Therefore, the subject-matter of the present invention is a process for preparing alkyl polyglucosides of general formula (I): H-(G) n -O-R in which: R is an alkyl of from 8 to 20 carbon atoms, which may be either linear or branched, saturated or unsaturated; G is a radical resulting from the removal of a H 2 O molecule of a monosaccharide commonly referred to as "reducing sugar", typically a hexose or a pentose of formula C 6 H 12 O 6 or C 5 H 10 O 5 ; n is an integer comprised within the range of from 1 to 5; said process comprising the reaction of an alcohol with a reducing sugar or an equivalent thereof, which may be an alkyl glucoside or a compound capable of generating, in situ said reducing sugar, with said reaction being carried out in the presence of a binary catalyst formed by a strong organic acid, selected from the group consisting of alkylbenzenesulfonic acids and secondary and tertiary alkylsulfonic acids, and a weak organic base having a Ka value within the range of from 10 -8 to 10 -1 , selected from the group consisting of pyridine, picolines, lutidines, collidines, quinolines, isoquinoline, quinaldine, pyrazine, pteridine and tetramethylurea. Examples of strong organic acids are: hindered alkylbenzenesulfonic acids, as 2,4,6-trimethylbenzenesulfonic acid and 2,4,6-triisopropylbenzenesulfonic acid, secondary and tertiary alkylsulfonic acids, such as secondary alkyl sulfonic acids containing from 14 to 17 carbon atoms (SASA), cyclohexanecarboxy-1-sulfonic acid, those 2-hydroxyalkylsulfonic acids which can be obtained, for example, from the internal epoxides by reaction with NaHSO 3 or the corresponding alcoxy derivatives which can be obtained from the same 2-hydroxyalkylsulfonic acids by alkylation. The catalyst can be easily prepared either separately or in situ by mixing equivalent amounts of said acids and bases. Preferred catalysts are the salts of pyridine or quinoline with alkylbenzenesulfonic acids or secondary alkylsulfonic acids; the latter can be obtained, for example, according to EP-A-0 402 978 to the name of Enichem Augusta S.p.A., the content of which is incorporated thereto by reference. Among all, preferred are the quinoline salts of mesitylene sulfonic (or 2,4,6-trimethylbenzenesulfonic) acid and of the secondary alkylsulfonic acids of from 14 to 17 carbon atoms (SASA). The catalyst can be used in amounts comprised within the range of from 0.001 to 0.1 mol per mol of reducing sugar or of an equivalent thereof, and preferably of from 0.01 to 0.05 mol. The unique character of this catalyst is demonstrated by a comparison test disclosed in Example 3, in which the reaction is carried out in the presence of other catalysts, among which also a conventional acid, as p-toluenesulfonic acid. By operating under the same conditions of temperature, pressure and reaction mass stirring speed as of the test of Example 2 and collecting, every hour, identical samples of reaction product until the solution turns into homogeneous and glucose disappears, meaningful differences may be observed, as evidenced by Chart 1, in the kinetics of formation of polyglucose as a reaction byproduct. The end product, obtained with the binary catalyst according to the present invention, proves hence to be nearly completely free from polyglucose and that constitutes a typical feature of the present process, as compared to the prior art. By using the catalysts disclosed hereinabove, also a kinetic control of the reaction is more easily performed because, for example, products of formula (I) with a high level of alkyl-monoglucosides [(e.g., mixtures containing >70% of product with n = 1 in formula (I)] can be obtained more easily. Furthermore, when such binary catalysts are used, products with a much lighter colour are obtained at reaction end, than of the corresponding products as obtained with conventional acidic catalysts such as, e.g., p-toluenesulfonic acid and consequently, when the process is carried out under optimal conditions, decolorizing the end product is no longer necessary. The basic component of the binary catalyst is easily liberated at the end of the reaction by adding at least one equivalent of a strong base which neutralizes the acid component of the binary catalyst. The base is completely recovered during the step of distillation of alcohol excess. The free base containing alcoholic phase, after the addition of one equivalent of a strong acid, can be used again for a subsequent glycosidation, yielding the same results. More particularly, the process according to the present invention comprises the reaction of a reducing sugar or of an equivalent thereof, which may be an alkyl glucoside or a compound capable of generating in situ said reducing sugar, with a monohydroxy alcohol of from 8 to 20 carbon atoms, in the presence of the binary catalyst as disclosed hereinabove, carried out at 90-130°C and with water being continuously removed as formed. Reducing sugars suitable for the intended purpose are hexoses or pentoses, as: glucose, mannose, galactose, arabinose, xylose, ribose and the like. Also higher sugars or substituted saccharides can be used which can be hydrolysed yielding monosaccharides: among these starch, maltose, saccharose, lactose, maltotriose, methyl-, ethyl-, or butyl-glucosides, and so forth, may be reminded here. Thanks to its low cost and large availability, the preferred monosaccharide is glucose. Suitable alcohols for this reaction are the linear or branched, saturated or unsaturated primary or secondary alcohols. Alcohol examples are octyl alcohol, decyl alcohol, dodecyl alcohol, tetradecyl alcohol, hexadecyl alcohol, oleyl alcohol and alcohols from oxosynthesis with a linear/branched ratio of 45:55, such as LIAL 111®, LIAL 123®, LIAL 145® or fractions of linear alcohols obtained from the latter by crystallization (ALCHEM 111®, 123®, 145®). The alcohol is used in an amount equal to, or larger than, the stechiometric amount of the reducing sugar or of its equivalent, and namely comprised within the range of from 1:2 to 1:10 and preferably of from 1:3 to 1:6. The selected value of the molar ratio conditions the characteristics of the end product of formula (I); so, if a product with a high polymerization degree (n) is desired, the reaction will be carried out at such low ratios as 1:2, whereas if a product prevailingly containing alkyl-monoglucoside (n = 1) is desired, such high ratios as 1:6 will be used. The alcohol performs furthermore the additional function of reaction solvent. The reaction temperature is comprised within the range of from 90 to 130°C; it is preferably comprised within the range of from 110 to 120°C. In order to obtain large amounts of alkylmonoglucosides [formula (I) with n = 1], discontinuing the reaction when carbohydrate conversion is not complete may prove advantageous. In this case, in order that the unreacted carbohydrate can be more easily recovered, diluting the reaction mixture with a solvent in which said carbohydrate is insoluble, such as hexane, heptane or toluene may prove suitable. Such an operation also offers the additional advantage of yielding a more fluid reaction mixture, hence more easily separable by carbohydrate filtration. At the end of the reaction, at least one mol of a strong base is added to the reaction mixture per each mol of catalyst used. By "strong bases" the alkali metal or alkaline-earth earth metal hydroxides and alkali metal alkoxides are understood. Preferred bases are sodium hydroxide and sodium methoxide. During this step, the weak organic base used, such as quinoline, is liberated from the strong acid and during the subsequent distillation step, it is recovered together with the alcohol and can hence be used again in another cycle. Any unreacted alcohol is then distilled off under a residual pressure of 13,33-66,65 Pa (0.1-0.5 mmHg), with a kettle temperature of 160-180°C. Such a distillation can be carried out with a traditional apparatus or, preferably, with a thin layer evaporator. The residue obtained from the above said distillation can be used as such or it can possibly be dissolved in water, for example by using an equal volume of water as compared to said residue, in order to obtain a solution at 50% by weight/weight (w/w). The following examples are supplied for merely illustrative purposes, and in no way they shall be construed as being limitative of the present invention. EXAMPLE 1 500 g of 1-dodecanol and 90 g of anhydrous glucose are charged to a flask of 1 litre of capacity equipped with stirrer, thermometer and distillation head. The mixture is heated up to 118-1200C and then 1.95 g is added of quinoline salt of mesitylene sulfonic acid (Mes.Q). The reaction equipment is connected with a vacuum pump and the internal system pressure is reduced down to approximately 3,33 KPa (25 mmHg). Heating is continued until the formation of H 2 O ends with, after about 4 hours, a slightly opalescent solution being obtained. An aliquot of the raw reaction mixture, dissolved in isopropanol: H 2 O = 1:1, has an extintion at 470 nm of 0.6 (E 470 = 0.6). 0.4 g of CH 3 ONa is added and the reaction mixture is distilled under the reduced pressure of 13,33 Pa (0.1 mmHg) in a thin-layer evaporator Leybold-Herans Mod KDL1 heating at 170-180°C. 425 g of 1-dodecanol are recovered, which contain the amount of quinoline corresponding to the initially used catalyst, which hence can be used again in subsequent tests. The residue is a glass-like, slightly coloured product weighing 150 g; when analyzed by HPLC (high pressure Liquid chromatography) and by GC (gas chromatography) after preliminary silanation, it results to have the following composition: dodecyl monoglucoside = 75.0% dodecyl diglucoside = 14.8% dodecyl triglucoside = 4.9% dodecyl tetraglucoside = 2.1% dodecyl pentaglucoside = 1.3% glucose = absent polyglucose = 0.7% dodecanol = 0.5% EXAMPLE 2 500 g of alcohols LIAL 123 (linear and branched C 12 and C 13 oxo alcohols with linear:branched ratio = 45:55) and 90 g of anhydrous glucose are charged to the same equipment as of Example 1. The mixture is heated up to 118-120°C and then 1.95 g of quinoline salt of mesitylene sulfonic acid are added. By operating as in Example 1, after 6 hours a slightly opalescent solution is obtained. The extintion of an aliquot of the raw reaction mixture dissolved in isopropanol:H 2 O = 1:1 is of 1.0 (E 470 = 1). 0.4 g of CH 3 ONa is added and the reaction mixture is distilled under the reduced pressure of 13,33 Pa (0.1 mmHg), in a thin-layer evaporator Leybold-Herans Mod KDL1, heating at 180-190°C. The distillate is constituted by 428 g of alcohols LIAL 123® (containing quinoline) and the residue is constituted by 151 g of alkyl polyglucosides, as a glass-like, slightly coloured, solid product. When this residue, when analyzed by HPLC (high-pressure liquid chromatography) and GC (gas chromatography) after preliminary silanation, results to have the following composition: alkyl monoglucosides = 72.2% alkyl diglucosides = 16.2% alkyl triglucosides = 5.9% alkyl tetraglucosides = 2.6% alkyl pentaglucosides = 1.3% glucose = absent polyglucose = 2.3% free alcohols = 0.5% EXAMPLE 3 Comparison test with different catalysts Inside the same equipment of the Example 1 and by operating with the same amounts of reactants and according to the same modalities, instead of the quinoline salt of mesitylenic acid (Mes.Q), equivalent amounts (0.0059 mols) were used of the following catalysts: p-toluenesulfonic acid (PTS); pyridinium p-toluenesulfonate (PTS.Py); pyridinium 2,4,6-trimethylbenzenesulfonate (Mes.Py); pyridine salt of secondary alkyl sulfonic acids (SA-SA.Py). This latter catalyst was prepared as follows: 25 g of technical SASA, obtained by operating as disclosed in EP-A-0 402 978 to Enichem Augusta S.p.A. and constituted by 68% of secondary alkyl sulfonic acid with average molecular weight (MW ave ) 293, 8% of secondary alkyl disulfonic acid with MW ave 373, 8% of H 2 SO 4 , balance to 100% water, is treated with an excess of pyridine. A syrup-like solution is obtained which is thoroughly dehydrated under a reduced pressure at 1000C in Rotavapor. The residue is diluted with 100 cc of ethyl ether. The formed precipitate, constituted by pyridinum sulfate, is filtered off, and the ethereal solution is concentrated again to dryness. 23 g are obtained of a light yellow paste which is used as such in the glucosidation reaction. During the tests, samples consisting of equal amounts of reaction mixture were collected at constant time intervals of 1 hour. Such samples were diluted with ethyl ether and extracted three times with water. All the aqueous extracts were adjusted at a same volume and were analyzed, under the same conditions, by HPLC (high pressure liquid chromatography), using a Hypersil C 18 column with eluent H 2 O:CH 3 CN, gradient from 100% to 0% and Light Scattering detector. The results, expressed as relative polyglucose surface areas as a function of time (hours) are reported in the accompaning chart. From the latter, it can be clearly seen that by associating pyridine with p-TSA, the amount of polyglucose sharply decreases and such a decrease is even larger if the sulfonic group shielding action by the sterically hindering moieties increases.

Claims: Process for preparing alkyl polyglucosides of general formula (I): H-(G) n -O-R in which: R is an alkyl of from 8 to 20 carbon atoms, which may be either linear or branched, saturated or unsaturated; G is a radical resulting from the removal of a H 2 O molecule of a monosaccharide commonly referred to as "reducing sugar", typically a hexose or a pentose of formula C 6 H 12 O 6 or C 5 H 10 O 5 ; n is an integer comprised within the range of from 1 to 5; said process comprising the reaction of an alcohol with a reducing sugar or an equivalent thereof, which may be an alkyl glucoside or a compound capable of generating, in situ said reducing sugar, characterized in that said reaction is carried out in the presence of a binary catalyst formed by a strong organic acid, selected from the group consisting of alkylbenzenesulfonic acids and secondary and tertiary alkylsulfonic acids, and a weak organic base having a Ka value within the range of from 10 -8 to 10 -1 , selected from the group consisting of pyridine, picolines, lutidines, collidines, quinolines, isoquinoline, quinaldine, pyrazine, pteridine and tetramethylurea. Process according to claim 1, in which the preferred alkylbenzenesulfonic acids are 2,4,6-trimethylbenzenesulfonic acid and 2,4,6-triisopropylbenzenesulfonic acid. Process according to claim 1, in which the preferred secondary and tertiary alkylsulfonic acids are secondary alkyl sulfonic acids containing from 14 to 17 carbon atoms (SASA), cyclohexanecarboxy-1-sulfonic acid, 2-hydroxy- or 2-alcoxy-alkylsulfonic acids. Process according to the preceding claims, in which the binary catalysts are selected from the pyridine or quinoline salts with alkylbenzenesulfonic acids or secondary alkylsulfonic acids. Process according to claim 4, in which the preferred binary catalysts are the quinoline salts of mesitylenesulfonic acid (or 2,4,6-trimethylbenzenesulfonic acid) and secondary alkylsulfonic acids containing from 14 to 17 carbon atoms (SASA). Process according to claim 5, in which the preferred binary catalyst is the quinoline salt of mesitylenesulfonic (or 2,4,6-trimethylbenzenesulfonic) acid. Process according to the preceding claims, in which the binary catalyst is prepared by mixing equivalent amount of a strong organic acid and of a weak organic base selected from among those as disclosed hereinabove. Process according to the preceding claims, in which the binary catalyst is used in amounts comprised within the range of from 0.001 to 0.1 mol per mol of reducing sugar or of an equivalent thereof. Process according to claim 8, in which the binary catalyst is used in amounts comprised within the range of from 0.01 to 0.05 mol per mol of reducing sugar or of an equivalent thereof. Process according to claim 1, in which the molar ratio of the reducing sugar or its equivalent to the alcohol is comprised within the range of from 1:2 to 1:10. Process according to claim 10, in which the molar ratio of the reducing sugar or its equivalent to the alcohol is comprised within the range of from 1:3 to 1:6. Process according to claim 1, in which the alcohol is used as the reaction solvent. Process according to claim 1, in which the reaction temperature used is comprised within the range of from 90 to 130°C. Process according to claim 13, in which the reaction temperature used is comprised within the range of from 110 to 120°C.

Inventors: BORSOTTI GIAMPIETRO, NATALONI LUIGI, PELLIZZON TULLIO, SANTINI CLAUDIO, BORSOTTI, GIAMPIETRO, NATALONI, LUIGI, PELLIZZON, TULLIO, SANTINI, CLAUDIO
Applicants: CONDEA AUGUSTA SPA, CONDEA AUGUSTA S.P.A.
Application year: 1993
Main classification: C07H15
IPC codes: C07H15, C07B61, B01J31