BACKGROUND AND SUMMARY This application is a continuation-in-part application of U.S. patent application. Serial No. 409,990 for Process for the Production of Mannitol and Sorbitol, filed August 20, 1982. This invention relates to the production of

D- annitol and D-sorbitol (hereafter mannitol and sorbitol, respectively) . More particularly, the invention relates to a method for the production of the sugar alcohols mannitol and sorbitol in predetermined relative proportions using D-glucosone (hereafter, glucosone) as a substrate.

The sugar alcohols, or alditols, mannitol and sorbitol are used in many diverse fields including food, pharmaceuticals, cosmetics and textiles. Mannitol is used extensively in pharmaceutical applications as a base for chewable, multi-layered tablets. Sorbitol is used in numerous foods as an additive to impart body, texture and some sweetness.

Although both mannitol and sorbitol are widespread in nature, they are produced in commercial quantities by synthetic processes. A common industrial process for producing mannitol and sorbitol is through the catalytic hydrogenation of D-glucose, D-fructose (hereafter, glucose and fructose, respectively), or a mixture of the two sugars. Catalytic hydrogenation of invert sugar (50% glucose, 50% fructose) , using a Raney nickel catalyst results in the production of sorbitol and mannitol in a ratio of about 3 moles of sorbitol per mole of mannitol. A similar ratio of products is obtained through nickel hydrogenation of high fructose corn syrup (50% glucose, 42% fructose). Nickel hydrogenation of pure D-fructose typically results in a 1:1 ratio of mannitol to sorbitol, whereas nickel hydrogenation of glucose typically results in only

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sorbitol. (Data obtained from Albert et al, Catalysis of Organic Reactions, 1981.)

Thus, although processes for the catalytic hydrogenation of glucose, fructose or a mixture of the two to mannitol and sorbitol are known, the amount of mannitol produced in such processes is substantially less than the amount of sorbitol, except where fructose is used as the starting material. However fructose is relatively expensive, ard therefore is not practical as a starting material for economically competitive commercial production of mannitol or sorbitol.

U.S. Patent No. 4,173,514 discloses a process for producing a mannitol-rich mixture of sorbitol and mannitol from glucose. The process involves the steps of epimerizing glucose in an acidic aσueous solution to obtain a mixture of glucose and mannose, and treating this mixture with a glucose iso erase enzyme to convert a portion of the glucose in the mixture to fructose, resulting in a mixture of m'annose, fructose and glucose in a ratio of about 3 to 2 to 5. The mixture is catalytically hydrogenated, using a nickel catalyst, to yield a reaction product containing about 40% by weight mannitol, which is reported to be a significantly higher yield of mannitol than is obtainable by other known processes which use either glucose or sucrose as a starting material. The process is not satisfactory however, where mannitol-rich mixtures containing greater than 50% mannitol are desired.

It is an object of the present invention to provide an improved method for producing mannitol and sorbitol.

A more specific object of the invention is to provide a commercially practical method for producing a mixture of mannitol and sorbitol having a mannitol to sorbitol ratio which is substantially greater than that

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obtainable with known prior art methods used in producing mannitol and sorbitol commercially.

Still another object of the invention is to provide a method for producing a mannitol-enriched mixture of products wherein the relative proportion of mannitol in the mixture may be controlled.

It is yet another object of the invention to provide a method for producing a mannose/fructose/ glucose mixture having a predominance of mannose. Other objects and features of the invention will become apparent from the following detailed description of the invention and the accompanying illustrative examples.

Detailed Description of the Indention Very generally, the method of the invention in one form involves the preparation of a solution of glucosone and reduction of the glucosone by catalytic hydrogenation with a nickel catalyst in the presence of hydrogen. The reaction is terminated when a desired combined concentration of products is reached. The products may include the "intermediate" reduction products mannose, glucose and fructose, or the final reduction products mannitol and sorbitol.

More particuarly, the method of the invention in its simplest form utilizes a solution of glucosone as the starting material. The glucosone may be obtained from any suitable source, and in a preferred form of the invention, is produced by the enzymatic oxidation of glucose, as taught in U.S. Patent No. 4,246,347. The glucose-oxidizing enzyme employed includes a carbohydrate oxidase, such as glucose-2-oxidase or pyranose-2-oxidase. Sources of these enzymes include the microorganisms Polyporous obtusus and Aspergillus oryzae. The concentration of the glucosone used may be

as low as 1% or less, or as high as about 30% or greater. Examples I-III below are concerned with the conversion of a 3% glucosone solution. Example IV describes a reaction using a 30% glucosone solution. High-concentration glucosone solutions can be prepared, for example from enzymatically-produced, lyophilized glucosone.

According to an important feature of the present invention, glucosone is reduced by catalytic hydrogenation with a nickel catalyst to produce a mixture of mannitol and sorbitol having a molar ratio of mannitol to sorbitol ranging from between about 1.5:1 to 3:1, depending upon reaction conditions, as discussed below. The production of a mannitol-enriched mixture of the two alditols from the catalytic hydrogenation of glucosone is unexpected for several reasons.

First, in the method described in the above-cited U.S. Patent No. 4,246,347, catalytic hydrogenation of glucosone derived from the enzymatic oxidation of glucose produces fructose having a purity greater than 90%. The preferred catalyst used in the glucosone to fructose conversion includes 5% palladium on carbon. The above patent also reports the catalytic hydrogenation of glucosone to fructose using Raney nickel catalyst, although the amount of fructose produced in the hydrogenation reaction was found to be low. There is no teaching nor suggestion in the patent that Raney nickel would be effective in catalytically hydrogenating glucosone to a mannitol-enriched mannitol and sorbitol mixture.

Other teachings in the prior art also suggest the use of Raney nickel for producing fructose from glucosone by catalytic dehydrogenation. U.S. Patent No. 4,321,324 discloses a method for converting glucose to fructose by catalytically hydrogenating glucosone at a

preferred temperature of between about 100° and 150°C to produce fructose. The patent does not show any data relating to the yield of fructose from glucosone under the reaction conditions described. However, experiments performed in support of the present application have revealed the following: In the temperature range between about 20° and 80°C, the amount of fructose produced, if any, by hydrogenating glucosone in the presence of Raney nickel is quite small. Secondly, glucosone itself is quite unstable at temperatures above 100°C, and therefore would not be expected to produce a pure reaction product under the elevated-temperature reaction conditions disclosed in the '324 patent. In any event, the cited patent does not show or suggest catalytically hydrogenating glucosone with a Raney nickel catalyst, under the reaction conditions in the method of the present invention, to yield a mannitol-enriched mixture of mannitol plus sorbitol. Finally, as discussed above, the prior art teaches catalytic hydrogenation of glucose, fructose or a mixture of the two sugars to produce a reaction mixture composed of mannitol plus sorbitol, and having a mannitol to sorbitol ratio of at most 1:1. None of these reactions suggest that glucosone would be a suitable starting material in a hydrogenation reaction to produce mannitol plus sorbitol, nor that the mixture obtained would be mannitol-enriched.

Experiments conducted in support of the present application indicate that the reduction of glucosone to a mannitol/sorbitol mixture is a two-step reaction. The first step involves reduction of glucosone to a mixture of mannose, glucose and fructose which predominates in mannose, i.e., contains more than about 50% mannose. In the second step, each of the intermediate sugars is further reduced to mannitol or to a mannitol/sorbitol

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ixture. The tv/o-step nature of the reaction can be best observed and exploited by using a high concentration-glucosone solution in the reaction. As will be seen in Example IV below, a 30% solution of glucosone is reduced quite slowly in the reaction, yielding predominantly mannose, fructose and glucose intermediates after about 7 days, and the final mannitol and sorbitol products after between about 14 to 19 da s _^ The invention thus can be practiced, by terminating the reaction at the intermediate stage, to produce a mixture of mannose, glucose and fructose which predominates in mannose.

The preferred hydrogenation catalyst in the reaction of the invention is Raney nickel. Other catalysts which may be successfully employed in the invention include 5% ruthenium on carbon and 5% rhodium on carbon. Catalytic hydrogenation processes with Raney nickel are described in the prior art and are well known to those skilled in the art. The hydrogenation reaction is preferably carried out within a temperature range of between about 20° and 80°C. As will be seen in Example II, the ratio of mannitol to sorbitol produced in the reaction may be increased or decreased selectively by performing the hydrogenation reaction at the upper or lower end of the specified temperature range, respectively.

The hydrogenation reaction in the method of the invention is preferably performed in a pressure range of between about 5 and 50psi. The pH of the reaction is preferably maintained between about 3 and 6. Reactions performed accordingly to the method of the invention and reported in Example II indicate that the greatest mannitol to sorbitol product ratios are achieved at a pH of around 4.5. The hydrogenation reaction is performed under

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the above-selected temperature, pH, and pressure conditions until a desired combined concentration of mannitol and sorbitol is reached. If the reaction is allowed to run to completion, where substantially all of the glucosone has been utilized, the combined concentration of mannitol and sorbitol may, under suitable reaction conditions, constitute more than 90% by weight of the products formed in the mixture. The time required to complete the reaction of the invention using different types of catalysts will be discussed below in Example III.

The process of the present invention will now be illustrated more particularly with reference to the following examples. ^' Example I

A 3% aqueous solution (pH 4.5) of glucosone was prepared from glucose according to the teaching of above-cited U.S. Patent No. 4,246,347. To 4ml of this solution, placed in a micro-hydrogenator apparatus (Supelco Inc., Belleforte, PA), 250 mg of Raney nickel (Pfaltz and Bauer Co., Stamford, CN) was added. The mixture was stirred under 8psi hydrogen (h ₂ ) gas at 25°C for 10 hours. The mixture was then analyzed by high performance liquid chromotography (HPLC) , using a BioRad Aminex Carbohydrate column, where the mobile phase was water at 85 ^β C, and the solvent flow through the column was set at 0.6 ml/ in. Detection was by refractive index. Known sugars and sugar alcohols have the following retention times on this HPLC system: glucosone- (12.3 min.), fructose (14.2 min.), mannitol (18.3 min.) and sorbitol (22.4 min.). The analysis showed complete conversion of glucosone to a product mixture containing mannitol and sorbitol in a ratio of 3.2:1. The possibility that the conversion of

glucosone to a mannitol-rich mixture of mannitol and sorbitol proceeds solely through a fructose intermediate was examined. A 3% aqueous solution (pH 4.5) of fructose was catalytically hydrogenated under conditions identical to those just described for the hydrogenation of glucosone. Analysis of the reaction mixture showed complete conversion of fructose and a product mixture containing mannitol and sorbitol in a ratio of 1.2 to 1. This result is similar to above-noted results obtained in the prior art, where catalytic hydrogenation of fructose is found to produce a mixture of mannitol and sorbitol in an approximately 1:1 ratio. The result indicates that the reaction of the invention cannot be explained solely on the basis of a fructose intermediate. Example II

This example examines the production of a mannitol enriched mixture of mannitol and sorbitol from glucosone under different pH, pressure, and temperature reaction conditions. The general reaction procedure described in Example I was followed. The different reaction conditions and the results obtained for each are summarized in TABLE I.

TABLE I

D- -Mannitol:

Reaction Temperature Pressure pH D- -Sorbitol Product

A 25°C 30 psi 4.5 2.5:1

B 25 30 3.0 2.0:1

C 25 30 6.0 2.2:1

D 78 8 4.5 3.3:1

E 78 30 4.5 3.2:1

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A comparison of reactions A and E in Table I shows that the ratio of mannitol to sorbitol formed in the product, mixture is increased significantly by raising the temperature at which the hydrogenation reaction is performed from 25 ^β C to 78°C. A comparison of reaction A with reactions B and C indicates that the ratio of mannitol to sorbitol in the reaction mixture is optimized by performing the reaction at a pH of around 4.5. A slight effect of reduced pressure on increasing the ratio of mannitol to sorbitol in the product mixture may be suggested by a comparison of reactions D and E.

Example -III In the two examples above, the catalyst used was Raney nickel. The present example examines the effect of different catalyst on the hydrogenation reaction. The general reaction procedure for each catalyst was the same as that used in Example I, except that the period of reaction required for complete conversion of glucosone varied with different caalysts, as indicated in TABLE II. As seen in TABLE II, Raney nickel is a preferred catalyst in the present invention, producing a mannitol/sorbitol mixture having a ratio of mannitol to sorbitol about twice that produced by catalysis ^' with 5% ruthenium on carbon or 5% rhodium on carbon. The ruthenium and rhodium catalysts were obtained from Englehardt Industries (Newark, NJ) .

TABLE II

D- -Mannitol

Time for D- -Sorbitol

Reaction Catalyst (1) Compl. Con . Product

A Raney nickel lOhr 3.2:1

B 5% ruthenium 72 1.6:1 on carbon

C 5% rhodium ort 36 1.6:1 carbon

EXAMPLE IV

This example illustrates the application of the method of Example I to a more concentrated glucosone solution. The general procedure of Example I was used, but the starting solution was a 30% aqueous glucosone solution, prepared by lyophilizing enzymatically-produced glucosone and redissolving to a concentration of 300 mg/ l.

A small pressure reactor was charged with 3.5 ml of the glucosone solution and about 300 mg of Raney nickel. The reaction was carried at a room temperature at a hydrogen pressure of 40 psi, and was monitored by a HPLC, using conditions described in Example I. The reaction solution was monitored for glucosone, glucose, mannose, fructose, mannitol and sorbitol.

After 7 days of reaction, no glucosone remained in the solution which contained glucose, mannose, fructose, mannitol and sorbitol in a ratio of about 4:18:7:3:1, respectively. After 13 days the solution contains mannitol and sorbitol in a ratio of about 2.6:1, and small amounts of mannose and glucose, estimated to constitute about 12% of the reaction products.

At 19 days, the products consisted of about 97%

mannitol and sorbitol in a ratio of about 2.5:1, the balance of the products being glucose and mannose. The total products formed represented about 90% of the starting material in the reaction. From the foregoing description and examples, it can be seen how various objects of the present invention are achieved. According to one important feature of the invention, it has been found that catalytic hydrogenation of glucosone with a catalyst such as Raney nickel produces a mannitol-enriched mixture of mannitol and sorbitol which may have a mannitol to sorbitol ratio of 3:1 or greater and a total content of mannitol and sorbitol, in the final reaction mixture, of 90% or greater. This result represents a significant improvement over prior art methods of making mannitol. Where catalytic hydrogenation of glucose, fructose or a mixture of the two has been used in the commerical production of itfannitol, a mannitol-sorbitol mixture predominating in sorbitol is produced. The relative proportion of mannitol in a mixture produced from glucose may be increased by epimerizing and/or isomerizing glucose prior to hydrogenation but this approach is not entirely satisfactory due to the additional steps involved, and because sorbitol still predominates in the mixture produced.

It is known in the prior art that mannitol can be produced in an approximately 1:1 ratio with sorbitol by catalytic hydrogenation of fructose, and that essentially pure mannitol can be produced by catalytic hydrogenation of mannose. However, in both of these methods, the relatively high cost ^" όf the starting material, fructose or mannose, is a serious obstacle to commerical processes based on these reactions. In the present invention, the starting material, glucosone, may be produced by a simple commerically practical reaction

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whose starting material can be generated by the enzymatic oxidation of glucose.

It has also been shown herein that the process of the invention can be adapted by performing the hydrogenation reaction at a selected temperature and pH, to produce a mixture of mannitol and sorbitol having a desired ratio of mannitol to sorbitol. In particular, the method of the invention can be adapted by performing the reaction at a relatively high temperature to produce a mixture having a mannitol to sorbitol ratio of greater than 3:1. The reaction can be performed using high concentrations of glucosone, and may be terminated at an intermediate reaction stage to give a mixture of mannose, glucose and fructose which predominates in mannose.

While specific examples of the present invention have been described herein, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit of the invention.