NL9301172A - Method for oxidizing carbohydrates. - Google Patents

Abstract

Method for oxidizing carbohydrates with a primary hydroxyl group, such as starch, cellulose, inulin and fractions and derivatives thereof, by treatment with nitric acid or a salt thereof in the presence of a catalytic amount of nitrite in the presence of phosphoric acid. The catalytic amount of nitrite is in particular 1-25% by wt relative to the carbohydrate. The phosphoric acid preferably has a content between 75 and 92%. The oxidation leads to products with a high content of carboxyl groups, without significant chain degradation.

Description

Werkwi.ize for oxidizing carbohydrates.

The invention relates to a method of oxidizing carbohydrates containing a primary hydroxyl group by treatment with nitrogen oxides in the presence of phosphoric acid.

Such a method, in which an excess of sodium nitrite is used, is known from the work of T.J. Painter et al. (Carbohydrate Bes. 55, 95-103 (1977), ibid. 140, 61-68 (1985)).

The oxidation of carbohydrates, such as starch and cellulose, is important because it allows the properties of the carbohydrates to be changed in a desired direction. Oxidized carbohydrates, for example, can be used as thickeners, gelling agents, binders, swelling agents, stabilizers and complexing agents (phosphate substitutes). Most processes for the oxidation of polymeric carbohydrates are to a greater or lesser extent accompanied by an undesired depolymerization (hydrolysis). Furthermore, such oxidation is not always specific either: for example, starch can be oxidized both at the 6-position primary hydroxyl group, resulting in a carboxy starch with an intact carbon skeleton, and at the 2,3-position secondary hydroxyl groups. leading to a disruption of the carbon-carbon chain in the glucose units ("dicarboxy starch").

Oxidized carbohydrates with an intact carbon skeleton, i.e., carbohydrates oxidized to the primary hydroxyl function, commonly referred to as polyuronic acids, often benefit for certain applications, such as as a complexing agent or as a stabilizer.

According to the above-described known method according to T.J. Painter et al., Cellulose or amylose is oxidized with sodium nitrite in phosphoric acid. Thereby a product is obtained which, in the case of the oxidation of cellulose, has a content of 87.5% of glucuronic acid and, in the case of oxidation of amylose, in a yield of 66 resp. 86% a content of glucuronic acid of 67-75% resp. Has $ 2- ^ 6%.

A drawback of this known method is the high oxidant consumption and the long reaction time (24 hours or more). In addition, the reaction mixture must meet certain viscosity and foaming requirements. Furthermore, a higher effective yield is desired.

A method has now been found which does not have the above-mentioned drawbacks.

The process according to the invention for oxidizing carbohydrates with a primary hydroxyl group by treatment with nitrogen oxides in the presence of phosphoric acid is characterized in that the carbohydrate is oxidized with nitric acid or a salt thereof in the presence of a catalytic amount of nitrite. Preferably, a salt of nitric acid is used.

Thanks to the process according to the invention, an oxidized carbohydrate which is at least equivalent in terms of degree of oxidation, selectivity of the oxidation and avoidance of depolymerization can be obtained in a much shorter time (approximately 7 hours), with a reduced consumption of oxidant. to the products of the known processes. The method according to the invention also appears to be independent of the stability of a foam and of a certain viscosity, so that the method is more widely applicable. ·

A catalytic amount of nitrite means an amount that is less than an amount required for oxidation of all primary hydroxyl groups to carboxyl groups, in particular less than 50 # of the amount required for that complete oxidation, according to the following gross reaction equation: RCH20H + 4 NO2 '-> RCOO * + 4 NO + 3 OH'

Expressed in moles #, the catalytic amount of nitrite is thus less than 200% relative to the carbohydrate monomer, preferably 1-100 moles #, more preferably 2-50 moles #, and most preferably 10-20 moles #. Preferably, the catalytic amount of nitrite, expressed as sodium nitrite, is 1-25 wt.%, More preferably 5-10 wt.%, Relative to the carbohydrate. The nitrite is preferably a salt of nitrous acid, such as sodium or potassium nitrite.

The oxidation can be carried out with nitric acid or a salt thereof, such as lithium, sodium, potassium or calcium nitrate. Preferably, the oxidant uses 0.8-2 equivalents, more preferably 1-1.5 equivalents, ie 1.33-2 moles nitrate or nitric acid per mole carbohydrate monomer, or about 50-75 wt% nitric acid or the corresponding amount of nitrate to the carbohydrate. The oxidation with nitrate can be represented in the following gross reaction equation: 3 RCH20H + 4 NO3 “- 3 RC00” + 4 NO + OH '+ 4 H20 The process is autocatalytic, which is evidenced by the fact that there is an induction period, which is drastic is shortened, but does not disappear completely, by adding a small amount of nitrite It is assumed that nitrite is not itself the catalyst, but directly or indirectly initiates the oxidation reaction to form a catalyst (eg nitric oxide) which then oxidizes the reaction by nitrate going on.

The method according to the invention is useful for oxidizing carbohydrates of very different nature and origin (vegetable, animal, microbial, synthetic). Both monomeric carbohydrates (mono-saccharides, sugar alcohols) and dimer, oligomeric and polymeric carbohydrates can be oxidized, especially when they have a primary alcohol function. Examples of polymeric carbohydrates are β-glucans, in particular cellulose and its fractions, derivatives and hydrolysis products thereof, α-glucans, in particular starch and fractions, derivatives and hydrolysis products - such as amylose and amylo-dextrin - thereof, and cyclic equivalents thereof such as cyclo-dextrin, further other polysaccharides, such as inulin, and natural or artificial gums, such as xanthan, pectin, guar, johannis bread meal, pectin, algin, gum arabic, dragant, carrageenan, agar, ghatti etc. In particular, the method is suitable for the oxidation of oligosaccharides and polysaccharides, such as starch, cellulose or inulin, or a fraction, a hydrolysis product or a derivative thereof.

The process according to the invention can be used for the preparation of fully carboxylated carbohydrates, i.e. of polyuronic acids. However, the process can also be advantageously used to prepare partially carboxylated carbohydrates in which only part of the primary hydroxyl groups of the carbohydrate have been oxidized. Carbohydrates with a carboxyl content of at least 60%, in particular of at least 70%, are preferably prepared.

The process according to the invention is carried out in an acidic reaction medium, which acid preferably also has water-extracting properties. In particular, phosphoric acid is used for this, in which the concentration of phosphoric acid in the medium is preferably at least 50% by weight, more preferably at least 60% by weight and in particular 75 "92% by weight. completely anhydrous, but the reaction mixture contains 5 ~ 25 wt.% water.

It has been found important that the carbohydrate dissolves well in the reaction medium. For sparingly soluble carbohydrates such as potato starch, it may be necessary to stir well for several hours (e.g. 2 hours) before the starch has completely dissolved. Dissolving usually takes about 1 hour for water-soluble potato starch and for amylose, and less than half an hour for low molecular weight carbohydrates such as cyclodextrin.

The oxidation of polymeric carbohydrates is generally slower than that of low-molecular carbohydrates, because in the first case the high viscosity leads to foaming and thus to a lower concentration of oxidant in the liquid phase.

The reaction temperature can range from about 0 ° C to about 50eC. Preferably, the reaction is conducted at temperatures of 30 ° C or less, and more preferably at room temperature or lower temperatures.

After the reaction, the mixture can be worked up and the oxidized carbohydrate can be isolated by adding a solvent in which the phosphoric acid and the other inorganic substances dissolve and the product does not dissolve, for example an alcohol. Further purification can be carried out in a manner known per se. The yields are generally above 80-90 #.

Examples

General

Potato starch (water soluble, 21.5 # amylose, 10 # water) and β-cyclodextrin (13.5 # water) were from Avebe. Amylodextrin (degree of polymerization 25) was obtained from waxy maize starch with pullulanase (see Dutch patent 165 500). Microcrystalline cellulose (Avicel) was from Merck. H3PO4 (85 #), HNO3 (65 #), NaNO2, NaNO3, NaOH, and NaH2PO4.2H20 were all analytical grade (Merck). Polygalacturonic Acid (98 #) was from Sigma; D20 (99.9 #) from Isotec Ine. and ethanol (96 #) from Gist-Brocades.

NMR spectra were recorded with a VARIAN UNITY-400 spectrometer (H resonance frequency 400 MHz, 13-C resonance frequency 100 MHZ). 13 C NMR spectra were gated decoupled, allowing quantitative determination. All samples were dissolved in D20. HPLC analysis was performed with a 7.5 x 600 mm Ultrapac TSK G5000PW column coupled to an R1 detector (Spectra Physics, SP 8430). As eluent, phosphate buffer (0.1M NaH2 PO4.2H2 O with 1 M NaOH adjusted to pH 7) was used. The absorbance at 520 nm was measured with a Perkin-Elmer Lambda 5 UV / VIS spectrophotometer. Centrifuge in a Sorvall RC-5B device.

For the oxidation reactions, the indicated amount (dry weight) of carbohydrate was dissolved in phosphoric acid (85 #) at 4 ° C and then the oxidizing agents (e.g., finely ground sodium nitrate and sodium nitrite) were added. Potato starch was slowly added to the phosphoric acid to prevent gelation and lump formation. The reactions were run at 4 ° C (reaction time about 30 hours, little depolymerization) or at room temperature (reaction time about 3 hours, more depolymerization).

The potato starch reaction mixtures were worked up by adding ethanol / water 80/20 to precipitate the product. The liquid was filtered and the residue was washed with ethanol / water 80/20 until the liquid was neutral. The reaction mixtures with amylodextrin or β-cyclodextrin were worked up by adding ethanol / water 80/20 and centrifuged. The product was then dissolved in water, precipitated with ethanol and centrifuged again. This was repeated two more times.

The products were dried under reduced pressure at 50 ° C for 4 hours. The glucuronic acid content was determined by the uronic acid determination of Blumenkrantz and Abdoe-Hansen, Anal. Biochem. 54, 484 (1973) A calibration curve was made from polygalacturonic acid (5, 10, 15, 20 µg). 20 µg samples of the reaction product were always measured, the percentage of which was determined on the basis of the calibration line. The carboxyl content of the product was determined by titration of 0.2 g of product with a 0.10 M NaOH solution, or by adding an excess of 0.1 M calcium acetate solution and back titration of the released acetic acid with 0, 10 M NaOH. The products were further characterized using NMR.

Example I

2.5 g of potato starch (water-soluble) was dissolved in 25 ml of 85 # phosphoric acid. 2.0 g of sodium nitrate and 0.2 g of sodium nitrite (finely ground) were added to the solution. After 6 hours of reaction at room temperature, the reaction mixture was worked up. Yield: 2.44 g; percentage of uronic acid: 85% Example II

Example I was repeated except that the reaction was carried out at 4 ° C for 24 hours. Yield: 2.36 g; percentage of uronic acid: J0%; less depolymerization occurred than at room temperature (Example I).

Example III

Potato starch (10.0 g) was dissolved in 50 ml of 85% phosphoric acid. 7.5 E sodium nitrate and 0.3 g sodium nitrite (finely ground) were added to the solution. After reaction for 3 hours at room temperature, the reaction mixture was worked up. Yield: 10.3 E! percentage of uronic acid: 59% - The lower uronic acid content is probably due to the reaction time being too short.

Example IV

Oxidation of β-cyclodextrin.

The procedure of Example 11 was repeated with 3 ml of water and a variable amount of sodium nitrate: 1) 0.5 g; 2) 1.0 g and 3) 1.5 E · For work-up, the following percentages of glucuronic acid were measured: 1) 62%; 2) 74% and 3) 82%. After work-up, the percentages were 62%, 61% and 58%, respectively. The yield of 1) was not quantitative, the yield of 2) and 3) was.

Example V

Oxidation of B-cyclodextrin.

To 0.70 g of sodium nitrate (8.2 mmol) in 5.0 ml of 85% phosphoric acid, 0.61 (dry weight) of β-cyclodextrin (= 3.7 mmol of anhydroglucose units. Part of the nitrate only dissolved during the reaction After half an hour, 40 mg (0.58 mmol) of sodium nitrite was added.After another 80 minutes, a percentage of glucuronic acid of about 80 was obtained, which did not increase much after that.

Example VI

Example V was repeated, however, instead of 40 mg of sodium nitrite, 60, 20 and 0 mg (0.87, 0.29 and 0 mmol) of sodium nitrite were added, respectively. The results, along with those of Example V, are shown in FIG. 1. It shows that the amount of nitrite does not have a major influence on the conversion rate and the conversion speed, but it does influence the induction time. The line for 0 g nitrite does not slope until after about three hours.

Example VII

Example V was repeated, but instead of 0.7 g of sodium nitrate, 0.5, 0.4, 0.3, respectively. 0.2 and 0.1 g of sodium nitrate were used. The results, along with those of Example V, are shown in FIG. 2. This shows that when less than an equivalent amount of sodium nitrate (<0.4 g) is used, the reaction is considerably slower.

Example VIII

Example V was repeated, however with the phosphoric acid 0.2, 0.4, 0.7, 1.0 and 1.5 ml of water (resulting in 82 #, 79%, 75%, 71 # and 65, respectively). # phosphoric acid) was added. The results, along with those of Example V, are shown in FIG. 3 * This shows that more water leads to a slower reaction, especially when less than 75 # phosphoric acid is present. At levels below 75 #, the color of the reaction mixture changed from green to light blue. The result with 92 # phosphoric acid was almost the same as with 85 # phosphoric acid. At a phosphoric acid content of 95 #, the reaction rate dropped again and the reaction mixture turned yellow.

Example IX

Oxidation of β-cyclodextrin.

The oxidation was performed at room temperature with sodium nitrate as the oxidizing agent. 1.4 g of this was used in 15 ml of 85 # Η3Ρ0ή with a variable amount of water (0 ml, 2 ml and 4 ml, resulting in a phosphoric acid concentration of 85 #, 75 # and 67 #, respectively) and 50 mg of sodium nitrite . Each time a maximum percentage of uronic acid of 70-75 # was reached: with 85 # H3PO4 this maximum was reached after about 3 hours, with 75 # after about 7 hours and at 67 # after about 9 hours.

If the same reaction was carried out with 2 ml of water but without sodium nitrite, a noticeable start of the reaction was only observed after 11 hours.

Example X.

Oxidation of amylodextrin (degree of polymerization 25).

2 g of amylodextrin was oxidized in 15 ml of 85 # phosphoric acid and 3 ml of water with 1.5 g of NaNO 3 and 0.05 g of NaNO 2. After 8 hours, a percentage of uronic acid of 79 # was found. After work-up, the percentage of uronic acid was 74 #. Example XI

Potato starch (2 g, dry weight) was oxidized with 1 ml of nitric acid and 3 ml of water in 16 ml of phosphoric acid in the presence of 50 mg of sodium nitrite. Working up after 8 hours, 90 # glucuronic acid was found (uronic acid determination). The carboxyl content of the isolated product was 80%.

Example XII

Microcrystalline cellulose (7.0 g) was dissolved in 100 ml of 85 # phosphoric acid at 4 ° C. Finely ground sodium nitrate (6.0 g) and finely ground sodium nitrite (0.7 g) were added to the solution.

After 26 hours of reaction at 4 ° C, 400 ml of ethanol (-20 ° C) was added. The precipitate formed was washed and dried. Yield: 7.2 g. Uronic acid content: 86 #.

Example XIII

Amylose (10.0 g) was dissolved in 100 ml of 85 # phosphoric acid at 4®C. Finely ground sodium nitrate (8.0 g) and finely ground sodium nitrite (1.0 g) were added to the solution. After 28 hours of reaction at 4 ° C, 400 ml of ethanol (-20 * 0) was added. The precipitate formed was washed and dried. Yield: 10.1 g. Uronic acid content: 64 #.

Claims (7)

A process for oxidizing carbohydrates with a primary hydroxyl group by treatment with nitrite in the presence of phosphoric acid, characterized in that the carbohydrate is oxidized with nitric acid or a salt thereof in the presence of a catalytic amount of nitrite. The process according to claim 1, wherein the catalytic amount of nitrite is 1-25 wt. # Of the carbohydrate (2 - 50 moles # of the carbohydrate monomer). The process according to claim 2, wherein the catalytic amount of nitrite is 2-20 wt. # Of the carbohydrate (4-40 mol. # Of the carbohydrate monomer). Method according to any one of claims 1-3. the concentration of the phosphoric acid being 75 ~ 92 wt%. A process according to any one of claims 1-4, wherein the carbohydrate is oxidized in the presence of 5 "25 wt.% Water. The process according to any one of claims 1 to 5, wherein the carbohydrate is oxidized with a salt of nitric acid. A method according to any one of claims 1-6, wherein the carbohydrate is starch, cellulose or inulin, or a fraction, a hydrolysis product or a derivative thereof.