NO140666B - PROCEDURES FOR THE PREPARATION OF CRYSTALLINIC, ESSENTIALLY XYLOSE - FREE, XYLITOL FROM A WATER SOLUTION CONTAINING XYLITOL AND XYLOSE - Google Patents

Description

The present invention relates to a method for producing

ling of crystalline, essentially xolysis-free xylitol from a mixture containing up to 30% xylitol, 0.3 - 0.5% xylose and the rest water, which mixture is obtained by hydration of a

dig solution of xylose.

In recent years, several processes have been developed for the production of xylitol. These processes have included extracting xylose from hemicellulose growth material, which occurs in nature, and then reducing the xylose to xylitol.

From US patent no. 2,989,569 it is known that wood sugars, i.e. sugars obtained by hydrolysis of cellulose-containing materials

is, can be further converted into polyols, including xylitol.

According to the patent, a sugar mixture containing xylose and hexoses is reduced, in the form of an approx. 40% aqueous solution, by passing the solution through a hydrogenation

zone in the presence of a hydrogenation catalyst at a temperature of approx. 70°C under a hydrogen pressure of approx. 60 kp/cm<2>, after which the reaction mixture is fed into a second reaction zone under a hydrogen pressure of approx. 40 kp/cm" at a temperature of 110 C.

According to example 1 in the patent document, the resulting polyol solution can be purified by passing the solution through an ion

change and are then de-coloured by passing them over a suitable resin, after which xylitol, which was the main component of the present

end polyols were recovered in high yield by a simple crystallization from the concentrated solution. In this case, xylose accounted for over 82% of the sugars present which also

led to xylitol being a significant component of the er-

held polyols.

Hemicellulose material such as maize cobs, maize, cotton seed husks and husks, sunflower seed husks, oat husks, peanut husks, rice husks and husks as well as bagasse, especially oat husks, have been found to be particularly useful sources of xylose. Extraction of xylose from such hemicellulosic materials has usually been accomplished by one or more hydrolysis steps followed by separation and crystallization of a high purity xylose. Reduction of the thereby is-

keeping the xylose into xylitol has been carried out in a conventional way by chemical or catalytic reduction in an aqueous solution.

In connection with the aforementioned method, however, one has a significant problem, namely obtaining a substantially pure xylitol from the aqueous solution in which the reduction of the xylose has been carried out. In the form the xylitol is formed in the aqueous reaction mixture, the xylitol, which has occurred in up to 30% by weight of the mixture, has

also contained small amounts of unreduced xylose as well as with other residual compounds from the hemicellulose material, such as polysaccharides. It is significant that the unreacted xylose, which has usually occurred in amounts of 0.3 - 0.5% by weight, has constituted the most serious contamination. It has been shown that negative physiological effects are connected with the consumption of xylose by mammals, such as visual disturbances. For this reason, the xylitol gets

as it is intended for human use, contain only an insignificant amount of xylose, i.e. not more than 0.10

% by weight

Until now, xylitol has been isolated by purifying the aqueous reaction mixture with ion exchange resin, after which either evaporation of the solvent has been carried out in order to recover the xylitol as a residue or by fractional crystallization of the xylitol from an alcohol-water mixture. However, it has been found that isolation of xylitol by evaporation gives a product containing significant amounts of unreduced xylose. On the other hand, by isolating essentially pure xylitol by fractional crystallization, an alcohol-water mixture has been obtained. This alcohol by-product has contained relatively large amounts of xylose and xylitol as well as alcohol, and for this reason said by-product has been far too valuable to be thrown away. Moreover, this alcohol by-product has constituted a potentially useful source of nutrients. However, expensive processes have been necessary to recover the alcohol and transform this by-product into a usable, i.e. alcohol-free, nutrient. Accordingly, there has been an unmet need for a method for rapidly producing substantially pure xylitol from the aqueous reaction mixture in which the xylitol is formed, and then a method by which a usable xylose-xylitol by-product is obtained that does not require further processing.

The method according to the present invention for obtaining crystalline xylitol, which is essentially free of xylose, from a mixture containing xylitol and xylose, is characterized by the following steps: a) Concentrate the mixture by heating to a temperature in the range of 30 - 50 °C under reduced pressure to give an aqueous solution containing 60 - 70% xylitol and 1 - 5% xylose, b) crystallize the xylitol from the aqueous solution by cooling to below 15°C,

c) separating the crystallized xylitol from the solution and,

d) collecting the formed mother liquor and combining it with a subsequent batch of hydrated xylose solution.

With the method according to the invention, xylitol is obtained, which contains at most 0.10% xylose, in yields of up to 95% from the aqueous reaction mixture in which the xylitol is formed from xylose. Likewise, this method yields a valuable and water-containing by-product, which contains xylose and xylitol, and which can be directly used as nutrients for animals.

The mixture of xylose and xylitol used as starting material according to the present invention can be obtained by reducing xylose to xylitol. This reduction of the xylose can be carried out in a conventional manner in an aqueous solution containing 1-60% by weight xylose, preferably 25-40% by weight xylose, whereby at least 95%, preferably 99%, of the xylose is reduced to xylitol. When carrying out this reduction of xylose to xylitol to thereby produce a mixture containing xylose and xylitol, one can use one or another conventional method for reducing an aldehyde or keto group in an aqueous solution to an alcohol. This reduction can be carried out chemically or catalytically, for example chemically using sodium amalgam or a complex metal hydride such as lithium borohydride or sodium borohydride. Preferably, one uses catalytic reduction by hydrogenation with 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 using conventional hydration conditions. In this reaction, neither temperature, pressure nor pH value are critical, and the reaction can conveniently be carried out at a temperature of 70°C - 120°C, and at a hydrogen

pressure of 10 atm - 50 atm as well as a water solution pH value of

3 - 10. The hydrogenation is preferably carried out at a hydrogen pressure of approx. 30 atm and at a temperature of R5°C - 10 5°C. The hydration continues quantitatively. The solid catalyst system can be readily removed from the aqueous reaction mixture containing the xylitol by conventional methods such as filtration.

The water-containing xylose and xylitol-containing mixture, which is obtained by the hydration of the xylose, can, if desired, be purified by means of one or more ion exchange resins. The solution may, for example, be allowed to pass through a layer of cationic ion exchange resin, and then, if desired, pass through a layer of anionic ion exchange resin. According to the present invention, one can use any conventional cationic ion exchange resin, such as cationic ion exchange resin based on crosslinked polystyrene sulfonic acid (for example, polystyrene sulfonic acid type resins), and any conventional anionic ion exchange resin, such as a crosslinked polystyrene containing quaternary ammonium groups or substituted amines. By allowing the hydration reaction medium to pass successively through a layer of cationic ion exchange resin and then through a layer of anionic ion exchange resin, all accompanying contaminants and residual contaminants can be removed from the xylitol. The remainder of these resins is made up of a colorless aqueous solution which essentially contains xylose and xylitol.

The aqueous reaction mixture obtained by the reduction

of the xylose, and which can optionally be treated with one or more ion exchange resins, is concentrated to contain 60 - 70% xylitol, 1-5% xylose and the rest water. The concentration of the xylose- and xylitol-containing reaction mixture can be carried out on con-

conventional way. The concentration of the solution is carried out by heating under reduced pressure at a temperature of 30°C - 50°C

for evaporation of the water.

According to the present invention, it has surprisingly been found that fractional crystallization of xylitol from an aqueous solution, which contains 60-70% xylitol and 1-5% xylose, gives substantially pure and crystalline xylitol, which does not contain more than 0.10% xylose. It has even been found that from a water solution, which contains 60 - 75% xylose and approx. 1% or less xylose, can obtain crystalline xylitol containing less than 0.05% xylose. It has further been found that at a xylitol concentration of

about. 68% is a maximum yield of xylitol which essentially does not contain xylose.

The crystallization of xylitol can, according to the presence

end of the invention is carried out in a conventional manner by cooling the solution from an initial temperature, whereby the solution is saturated with xylitol, to a lower temperature, whereby the xylitol precipitates. During this crystallization, the applied pressure and temperature are not critical, and room temperature (22°C) and atmospheric pressure are usually used as a starting temperature. The crystallization is preferably carried out by slowly cooling the solution to a temperature below 15°C, and a cooling to approx. 0°C.

The xylitol precipitate can be isolated by filtration, washing and drying in a conventional manner. The remaining mother liquor, son in-

holding xylitol and xylose, can be appropriately circulated to other crystallization batches.

With the method according to the present invention, xylitol can be obtained with a purity of 99% and higher, and which does not contain more than 0.10% xylose. Yields of up to 95% can be achieved by recycling the mother liquors at subsequent rates. The aqueous mother liquor by-products remaining after recycling contain xylose and xylitol, and can be mixed with animal feed.

The following examples will further illustrate the invention. All temperatures are given in °C. "Amberlite IR-120" is a cation exchange resin based on cross-linked polystyrene sulfonic acid, which is manufactured by Rohm & Haas, Philadelphia, Pa, U.S.A. "Amberlite IRA-93" is anion exchange resin based on crosslinked polystyrene tertiary ammonium manufactured by Rohm & Haas, Philadelphia, Pa, USA.

Production of xylose

200 g soaked oats with the following composition:

is suspended in 800 ml of a 0.02-m hydrochloric acid-water solution. The pH of this suspension was 3.5. The suspension was heated in a closed pyrex glass for one hour at 120°C. The reaction mixture was then cooled to room temperature, and the solids were filtered off. The filtered solids were later mixed in 800 mL of a 0.175 M hydrochloric acid-water solution to obtain a pH of 1.1. This mixture was heated in a closed pyrex glass at 120°C for one hour. The reaction mixture was then cooled to room temperature and the solids were filtered off. The filtered solids were washed with water, and the wash was combined with the filtrate. The filtrate was deionized by passing it through a column containing 110 ml "Amberlite IRA-93" (in 0H~ form). The column was washed twice with each time 50 ml of water (deionized). The combined eluates were concentrated under vacuum to a syrup (water content 10% by weight). To this syrup was added 32 ml of methanol. The resulting mixture was cooled to -10°C to thereby obtain xylose in form of a crystalline precipitate. The crystalline precipitate obtained was filtered, and the crystals were washed with 30 ml of methanol. After washing, the crystals were dried in vacuum, whereby the yield was 32.5 g of xylose (to 96% purity).

Reduction of xylose to xylitol

15 g of the xylose obtained was dissolved in 22.5 ml of water.

To this solution was added 3.0 g of Raney nickel catalyst (to a 50% by weight water suspension) and 0.015 g of calcium carbonate. This mixture was hydrated for approx. 1.5 hours at a temperature of 100°C and at a hydrogen pressure of approx. 30 atm. The mixture was then cooled and the catalyst was separated away. The catalyst was washed with deionized water. The washing water was combined with the filtrate. The filtrate was passed through 4 ml of "Amberlite IR-120" cation exchange resin (H<+>) and through 8 ml of "Amberlite IRA-93" anion exchange resin (0H~). The columns were washed with water. The eluate and wash water were combined, and

this resulted in a water solution containing approx. 30% xylitol by weight and approx. 0.3-0.5% xylose by weight.

Example

The aqueous solution for each batch, which was obtained according to the method stated above, and which contained xylose and xylitol, was concentrated to a total of 70% by weight solids by heating at 40°C. The concentrated solution was then cooled slowly over two hours to a temperature of 0 - 5°C. The xylitol obtained, which precipitated from each batch, was filtered, washed and dried. The mother liquor from each batch was recycled in subsequent batches of the hydration solutions until the xylose content of the xylitol, which was obtained from a certain batch, was 0.05% by weight. The mother liquor was then concentrated to a total of 70% by weight of solids for each of two consecutive crystallizations, thereby obtaining two further yields of xylitol.

The following table shows the results from each rate. % indicates % by weight.

Claims (3)

1. Procedure for the production of crystalline, essentially xylose-free xylitol from a mixture containing up to 30% xylitol, 0.3 - 0.5% xylose and the rest water, which mixture is obtained by hydrating an aqueous solution of xylose, characterized by the following steps: a) Concentrate the mixture by heating to a temperature in the range 30 - 50 °C under reduced pressure to give an aqueous solution containing 60 - 70% xylitol and 1 - 5% xylose, b) crystallize the xylitol from the aqueous solution by cooling to below 15°C, c) separating the crystallized xylitol from the solution and, d) collecting the formed mother liquor and combining it with a subsequent batch of hydrated xylose solution. 2. Method according to claim 1, characterized in that the mixture in step a is concentrated to 68% xylose by weight. 3. Method according to claim 2, characterized in that the xylitol is crystallized by cooling to 0°C.