FI59241C - CRYSTALIZATION OF XYLITOL IN VAT - Google Patents

Description

-1 M <11, ANNOUNCEMENT.su C Q O A A

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Patent aeddelat - * - * <51, Kv.ik.Wci.3 c 07 C 31/18, 29/78 FINLAND —FINLAND (21) —hK «nt« n «eknln | 312 ^ / 73 <22, H «k« n> tap * lvl - Amttkningfdaf 09 * 10.73 (23) AlkuptM —GiMgh «t * dt | 10/9/73

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Patent and Registry Office NihUvtluip ^ kuL * uik * un pvm.-

Fetent- oeh registerstyreisen 'Amöktn utiifd och uti. * Krtfun pubUnrid 31.03.81 (32) (33) (31) ^ 37 ^ * «y utuolkug * —Bujird prloritut 10.10.72 US A (US) 296U01 + (71) Suomen Sokeri Limited Liability Company, Mannerheimintie 15, 00260 Helsinki 26,

Suomi-Finl an d (FI) (72) Gerald Myer Jaffe, Verona, JJev Jersey, Peter Hans Weinert, Wayne,

The present invention relates to a process for the preparation of crystalline, substantially xylose-free xylitol with about 30% xylitol and about 0% by volume of crystalline xylitol-free xylitol. 3-0.5% of xylose and the rest of the aqueous mixture obtained by hydrogenating an aqueous solution of xylose.

In recent years, several interesting methods for the preparation of xylitol have been developed. In these methods, xylose is extracted from naturally occurring hemicellulose-containing plant raw materials and then reduced to xylitol.

Particularly interesting sources of xylose have been identified as hemicellulose-containing raw materials such as corn cobs, corn, cottonseed husks and husks, sunflower seed husks, oat husks, peanut husks, rice husks and husks, and bagasse, especially oat husks. Xylose is extracted from such hemicellulose-containing materials, preferably in one or more hydrolysis steps, after which it is separated and crystallized as a highly pure product. The xylose thus obtained is preferably reduced to xylitol in the usual manner chemically or catalytically in aqueous solution.

So far, however, it has not been resolved how to obtain virtually pure xylitol from an aqueous solution in which xylose reduction has been performed. In addition to the xylitol formed, the reaction mixture contains small amounts of about 30% by weight of unreduced xylose and other impurities derived from the hemicellulose feedstock, such as polysaccharides. In general, the most worrying impurity has been unreacted xylose, which has usually been present in an amount of about 0.3 to 0.5% by weight. Ingestion of xylose has been found to be associated with adverse physiological effects in mammals, e.g., ocular diseases. Therefore, the xylitol content of food for xylitol should not exceed about 0.10% by weight.

So far, xylitol has been separated by purifying the reaction solution with ion exchange resins, after which the solvent is evaporated to leave xylitol, or the xylitol is crystallized from an alcoholic water mixture. However, it has been found that separation of xylitol by evaporation of the solvent gives a product which contains significant amounts of unreduced xylose. On the other hand, after virtually pure xylitol has been separated by fractional crystallization, the alcohol-water mixture remains. This alcoholic by-product has contained relatively large amounts of xylose, xylitol, and alcohol, so it has been too valuable to discard. In addition, this alcoholic by-product is a potentially useful source of nutrients. However, the recovery of alcohol and the conversion of this by-product into a usable non-alcoholic nutrient has required costly further processing.

It is known from U.S. Pat. No. 2,989,569 to hydrolyze a cellulosic material and convert the resulting sugars to polyols, including xylitol. According to the patent, an aqueous solution containing xylose and hexoses is reduced with hydrogen in the presence of a hydrogenation catalyst at a temperature of 0 ° C and a hydrogen pressure of 60 kp / cm 2, after which the reaction mixture is passed to another reaction zone with a hydrogen pressure of 1 + 0 kp / cm and a temperature of 110 ° C. The resulting polyol solution can be purified by passing it through ion exchangers and removing dyes with a suitable resin. Crystalline xylitol is obtained by crystallizing the thus purified polyol solution.

However, the process according to said U.S. Pat.

Therefore, there is a need for a process which can advantageously separate substantially pure xylitol from the aqueous reaction mixture in which it is prepared and which provides a useful xylose and xylitol-containing by-product which does not require further treatment.

The process of the present invention is characterized in that a) said mixture is concentrated by heating to 30 ° C to 50 ° C under reduced pressure to obtain an aqueous solution containing about 60 to 70% by weight, preferably about 68% by weight of xylitol, and less than 5% by weight of xylose, preferably less than 1% by weight of xylose, b) crystallizing xylitol from said aqueous solution by cooling the solution to a temperature below 15 ° C, preferably to about 0 ° C, c) separating the crystallized xylitol from said solution, and d) forming the mother liquor is collected and combined with the next batch of hydrogenated xylose solution.

3 59241 This method also produces a valuable by-product containing xylose and xylitol and water, which can be used as such as livestock feed.

The mixture of xylose and xylitol used as a starting material in the present invention can be prepared by reducing xylose in a conventional manner to xylitol in aqueous solution. The reduction can be carried out by any conventional method of reducing an aldehyde or keto group to an alcohol in aqueous solution. The reduction can be carried out chemically, for example, by sodium malting or with a metal hydride complex such as lithium borohydride or sodium borohydride. Preferably, catalytic reduction with hydrogen is used in the presence of a noble metal catalyst such as platinum or palladium. Particularly preferred catalysts are nickel catalysts such as Raney nickel.

The catalytic reduction can be carried out under ordinary hydrogenation conditions. In this reaction, temperature, pressure and pH are not critical, and the reaction can be preferably carried out at a temperature of about 70 to 120 ° C, a hydrogen pressure of about 10 atm to about 50 atm, and an aqueous solution having a pH of 3 to 10. Preferably, the hydrogenation is performed at a pressure of about 30 atm and a temperature of about 85-105 ° C. The reaction is quantitative. The solid catalyst can be easily removed from the xylitol-containing reaction mixture by conventional methods, for example, by filtration.

The mixture of xylose, xylitol and water formed as a result of the hydrogenation of xylose can be purified, if desired, with one or more ion exchange resins. For example, the solution may be passed through a cation exchange resin layer and then, if desired, through an anion exchange resin layer.

According to the present invention, any conventional cation-exchange resin can be used, such as a cationic crosslinked polystyrene-sulfonic acid ion exchange resin (e.g., polystyrene-sulfonic acid-type resins), and any conventional anion exchange resin or amine-crosslinked polystyrene resin, . By passing the reaction solution used in the hydrogenation first through the cation exchange resin layer and then through the anion exchange resin layer, all impurities are removed from the xylitol. The eluate from these resins is a colorless aqueous solution containing mainly xylose and xylitol.

11 59241

An aqueous solution containing 60-70% xylitol and less than about 5% xylose can be obtained by concentrating an aqueous solution formed by the reduction of xylose and optionally treated with one or more inone exchange resins. The concentration of the reaction mixture containing xylose and xylitol can be carried out in the usual manner. Preferably, the solution is concentrated by heating to evaporate the water in vacuo at a temperature of about 30 ° to about 50 ° C.

The process according to the invention gives crystalline xylitol with less than 0.05% xylose. Crystallization of xylitol from an aqueous solution with a xylitol content of about $ 68 gives a maximum yield of crystalline xylitol with virtually no xylose.

The fractional crystallization of xylitol according to the present invention can be carried out in the usual manner by cooling the solution from its initial temperature at which it is saturated with xylitol to the temperature at which the xylitol precipitates. The fractional crystallization is performed by slowly cooling the aqueous solution to a temperature below 15 ° C, most preferably to about 0 ° C.

Xylitol crystals can be isolated by filtration, washing and drying in the usual manner. The remaining mother liquor containing xylitol and xylose can be passed to other batches of crystallization.

The following examples further illustrate the invention. All temperatures are in degrees Celsius. Amterlite IR-120 is a crosslinked polystyrene-sulfonic acid cation exchange resin manufactured by Rohm & Haas, Philadelphia, Pa. Amberlite IRA-93 is a crosslinked polystyrene-tert-ammonium anion exchange resin manufactured by Rohm & Haas, Philadelphia, Pa.

Example 1 200 g of dried, ground oat hulls having the composition: 1. Water $ 7.8 2. Solids 92.2% a. Ash 5.1% b. Carbohydrates in their polysaccharide form 1) arabinose 2.2% 2) xylose 21 .5 $ 3) galactose 0.7 $ 1+) glucose 2, ¾% 5) mannose traces 6) other hemicellulose and proteins 8-10% 5 59241 was suspended in 800 ml of 0.02 molar hydrochloric acid. The pH of this suspension was 3> 5. The suspension was heated in a sealed pyrex tube at 120 ° C for one hour. The reaction mixture was then cooled to room temperature and the solid was filtered off. The filtered solid was then mixed with 800 mL of 0.175 M hydrochloric acid to give a pH of 1.1. This mixture was heated in a sealed pyrex tube at 120 ° C for one hour. The reaction mixture was then cooled to room temperature and the solid was filtered off. The filtered solid was washed with water and the washings were combined with the filtrate. The filtrate was deionized by passing it through a column of 110 ml of Amberlite IRA-93 (in its OH form). The column was washed twice with 50 ml of water (deionized) each time. The combined eluates were concentrated in vacuo to a syrupy product (water content 10%). To this was added 32 ml of methanol. The resulting mixture was cooled to -10 ° C, whereupon xylose precipitated as crystals. The resulting crystalline precipitate was filtered off, and the crystals were washed with 30 ml of methanol. After washing, the crystals were dried in vacuo to give 32.5 g of xylose (96% pure).

Example 2 15 g of xylose (prepared according to Example 1) was dissolved in 22.5 ml of water. To this solution were added 3.0 g of Raney nickel catalyst (50 wt% aqueous suspension) and 0.015 g of calcium carbonate.

This mixture was hydrogenated for about 1.5 hours at 100 ° C with a hydrogen pressure of 5050 psig. The mixture was then cooled and the catalyst was separated therefrom. The catalyst was washed with deionized water. The wash water was combined with the filtrate. The filtrate was passed through an H ml Amberlite IR-120 cation exchange resin layer (H +) and an 8 ml Amberlite IRA-93 anion exchange resin layer (OH). The columns were washed with water. The eluate and washings were combined to give an aqueous solution containing about 30 wt% xylitol and about 0.3-0.5 wt% xylose.

Example 3

The reaction solution of each batch containing xylose and xylitol prepared by the method of Example 2 was concentrated at 1 + 0 ° C with heating to give a total solids content of 70% by weight. The concentrated solution was then slowly cooled to 0-5 ° C over 2 hours. The precipitated xylitol from each batch was washed and dried. The mother liquor from each batch was recycled with the next batch of hydrogenation until the xylose content of the product from that batch was 0.05 wt%. The mother liquor was then concentrated until the solids content for each of the next two crystallizations was 70% by weight, and two more batches of xylitol were obtained.

The following table summarizes the results for each batch. The # number means weight percent.

6 59241

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