NL1010341C2 - Carbohydrates are oxidized by treatment with a Fe(III) complex as oxidizing agent in the presence of a di-tertiary-alkyl-nitroxyl compound as catalyst - Google Patents

Abstract

Carbohydrates are oxidized with a Fe(III) complex in the presence of a di-tertiary-alkyl-nitroxyl compound as catalyst. A process for the oxidation of carbohydrates by treatment with a Fe(III) complex as oxidizing agent in the presence of a catalytic amount of a di-tert.-alkyl-nitroxyl compound.

Description

Process for the oxidation of carbohydrates

The invention relates to a method of oxidizing carbohydrates containing a primary hydroxyl group.

It is known to oxidize carbohydrates such as starch and inulin and other primary alcohols with a di-tertiary-alkyl-nitroxyl compound such as TEMPO (WO 95/07303). TEMPO is used in catalytic quantity and regenerated continuously with hypochlorite. This oxidation leads to selective oxidation of the primary hydroxymethyl group to a carboxyl group (uronic acid). A drawback of this known method is that a high chlorine load is created and that a significant chain shortening occurs with polymeric carbohydrates.

It has now been found that the oxidation of carbohydrates with TEMPO can be improved, especially in selectivity to undesirable chain breakdown, by using an iron (III) complex, for example a hexacyanoferrate, as an oxidizing agent.

The method according to the invention is particularly suitable for the selective oxidation of alcohols containing both primary and secondary hydroxyl functions, such as 1,9-octadecane diol. This mainly includes carbohydrates and derivatives. Mono-, di- and oligosaccharides are non-reducing carbohydrates such as alkyl glysosides, trehalose, sucrose, raffinose, kestose and other fructans. For 20 reducing carbohydrates, the chain length should be at least 5, preferably at least 10 monosaccharide units. Polymeric, whether or not reducing, carbohydrates which can be selectively oxidized by the process according to the invention are, for example, α-glucans (starch, amylose, amylopectin), β-glucans (cellulose, chitin, xanthan, 1,3-glucans), glucomannans and glactomannans (guar, carob flour, etc.) and other bacterial or vegetable gums. Derivatives of such carbohydrates, such as alkylated, acylated, oxidized, cationic carbohydrates are also eligible. In the case of hydroxyalkylated carbohydrates (such as hydroxyethyl starch), the method according to the invention leads to the introduction of carboxymethyl groups, which can be an attractive alternative for the reaction with chloroacetic acid. Preference is given to starch, cellulose, chitin, gums and derivatives thereof. Starch is here understood to mean both natural starch and amylopectin or amylose enriched 1010341 2 as otherwise modified starch.

The oxidation of the primary hydroxyl group proceeds with a di-tertiary-alkyl-nitroxyl compound such as TEMPO (2,2,6,6-tetramethylipiperidin-1-oxyl). Also other di-tert-alkyl-nitroxyl compounds such as 4,4-dimethyloxazolidine-N-oxyl (DOXYL), 2,2,5,5-tetramethylpyrrolidine-N-oxyl (PROXYL) and 4-hydroxy-TEMPO and analogs and derivatives thereof as described in WO 95/07303 can be used in place of TEMPO. The catalytic amount of TEMPO is preferably 0.05-5 mol%, in particular 0.1-2 mol%, relative to the primary alcohol.

The iron (III) complex serves to selectively reoxidize TEMPO. As an iron (III) complex, especially a hexacyanoferrate, for example potassium hexacyanoferrate, can be used. Since the oxidation reaction is very selective, an amount of the iron complex corresponding to the desired degree of oxidation in the reaction product can be used. The cyanopherrate can, if desired, be regenerated with ozone, hydrogen peroxide and acid, with chlorine or by electrochemical means. Methods for regenerating potassium hexacyanoferrate have been described, for example, by Paweck and Hirsch, Z. Electrochem. 34 (1928) 684-696, and in DE-A-511898 and GB-A-365983.

The oxidation can be carried out in aqueous medium, at a pH between 8 and 13, preferably between 8 and 12, and at mild temperatures, for example between 5 and 50 ° C. The carbohydrate can be dissolved or suspended in a high concentration, for example 5-50% by weight, if desired. It is not necessary to completely dissolve the carbohydrate. In the case of starch, the oxidation can also take place on the grain, the conditions being chosen (low temperature, not too high pH) so that no gelatinization occurs if this is not desired. Optionally, an auxiliary solvent such as isopropyl or tert-butyl alcohol or dioxane can be used. The oxidation process according to the invention yields carboxylic acids which have undergone little or no further reaction. In the case of polymeric carbohydrates, C6 carboxy carbohydrates (carbohydrates the 6-position of which has been oxidized to a carboxyl group) are obtained, which have not been significantly reduced in chain length and have not undergone any significant side reactions (ring opening). The carboxylic acid functions lead to a greatly increased solubility in water, which, for example, greatly increases the applicability for cellulose and chitinc. Thus, stable solutions or dispersions of carbohydrates in water can be prepared without retrogradation.

1010341 3

The carboxy carbohydrates can be used as inexpensive and environmentally friendly substitutes for non-degradable polymers such as polyacrylic acid, polyacrylic acid amide or relatively expensive degradable polymers such as dextrans and carboxy methyl cellulose. In concentrations of, for example, 1-5%, the C6 carboxy-5 carbohydrates can be used as papermaking auxiliaries, at higher oxidation rates (for example, 10-30%) they are useful as flocculants, polyelectrolytes, co-builders and antistatics.

The oxidized starches offer a special advantage. These have a favorable combination of good solubility, low viscosity and high molecular weight. The invention also relates to C6 carboxy starch with at least 1, preferably at least 2.5, rising to, for example, 10, carboxyl groups at the 6 position per 100 anhydroglucose units, a molecular weight of at least 1,000. 000 (1 MDa) and a viscosity of a 25% solution of at most 2000 (top), 100 (trough, 90 ° C), respectively. 500 (end, 40 ° C) Brabcnder units. The invention also relates to C6 carboxy starch, the product of the number of the carboxyl groups in the 6-position per 100 anhydroglucose units and the weight-average molecular weight in MDa is at least 10, in particular at least 15. This includes, for example, an oxidized starch with a molecular weight of at least 1 MDa and an oxidation degree (of the primary hydroxyl group) of at least 15% (20%), or an oxidized starch with a molecular weight of at least 5 MDa and an oxidation degree (of the primary hydroxyl group) of at least 3% (4%).

Example 1

Potato starch (208 g = 1.28 mol, moisture content 15%, average chain length 30-40 MDa) was suspended in 433 ml demineralized water. 169.1 g (0.513 mol = 0.128 eq) of K3Fe (CN) 6 (Merck,> 99%) were added and the pH was adjusted to 10.5 with 1N NaOH. Then 1.0 g (6.4 mmol) of TEMPO was added. The reaction was carried out at room temperature, keeping the pH at 10.5 with a pH stat (IN NaOH). At the end of the reaction (no further addition of caustic by the pH stat, after 2 hours), the reaction mixture was filtered off and washed three times with demineralized water (total 1 L). Then it was washed twice with acid water (pH 5, citric acid buffer, total 1 L), and finally 1 L of demineralin was again 1010341 4

Ionized water rinsed. The solid was freeze dried. Yield 91-98%.

Molecular weights were determined by High Performance Size Exclusion Chromatography with Multi-Angle Laser Light Scattering detector and a differential refractometer (HPSEC-MALLS-RI). Eluent: K-H phosphate buffer pH 8, injection 20 μΐ; column TSK 5 PWH-PE prep-guard 7.5x7.5 mm, Beekman, 6 following columns: Spherogel-TSK 1000 PW, 2000 PW, 3000 PW, 5000 PWHR and 6000 PW, 30x7.5 mm; analysis eluent: Dawn-F multi-angle laser photometer, analysis samples: RI detector Waters 410 at 50 ° C. The viscosity of a 25% solution was measured in a Brabender viscometer Viskograph E [40 ° C - ► 90 ° C 1.5 ° / min; 90 ° C 30 min; 90 ° C - * 40 ° C 10 1.5 ° / min; 75 rpm]. The results for weight average (Mw) and number average (Mn) molecular weight and viscosity (in Brabender units) are shown in Table 1.

Examples 2-4

Example 1 was repeated, but using 84.6 g (0.257 mol = 0.064 eq), 67.4 g (0.205 mol = 0.051 eq) and 42.3 g (0.128 mol = 0.032 eq) K3Fe (CN) 6, respectively. . The yields were 94-97.97-99 and 91-98%, respectively. The molecular weight and viscosity results are shown in Table 1.

Table 1 pre-oxidation Mw M „viscos. viscos. viscos.

image medium (MDa) (MDa) top trough end 20 1 10% Fe 1.77 0.654__0__0__0 2 5% Fe 4.33__U0__44__0__0 3 4% Fe 5.16__MO__60__10__80 4 2.5% Fe 22.2__6.96 3180 1440> 10000 compare 4% Cl 0.771 0.170__440 70> 440 25 Comparative example

Example 1 was repeated, but using 61.7 ml (1.6 mol / l) NaOCl (0.098 mol = 0.049 eq). The yield was 90-95%. The molecular weight and viscosity results are shown in Table 1.

1010341 5

Example 5

Examples 1-4 were repeated with corn starch and with flour (starch + protein). The results are shown in Table 2.

Table 2 5 starch oxidation Mw Mn viscos. viscos. viscos.

medium (MDa) (MDa) top valley end 10% Fe 1.2__032__40__0__53_ maize 5% Fe 2 '° 0 8__55__0 259 4% Fe 4.5__15__Γ71__20__400 __2% Fe 12,1__4j6__328__55__383 10% Fe 1,2__041__20__0 20 flour 5% Fe 2 '4 °> 8 ____ 40 180 4% Fe__08__0.9 150 70 190 __2% Fe__06__5.3 580 160 640

Example 6

Cellulose was treated as in Example 1. It was thereby oxidized to 6-10 carboxy cellulose. However, the product did not dissolve in water, so that no viscosity measurements could be performed.

1010341

Claims (9)

A process for oxidizing carbohydrates by treatment with an oxidizing agent in the presence of a catalytic amount of di-tertiary-alkyl-nitroxyl compound, characterized in that an iron (HI) complex · 5 is used as the oxidizing agent. The process according to claim 1, wherein a hexacyano-ferrate is used as the oxidizing agent. The process according to claim 1 or 2, wherein the oxidation is carried out at a pH of 8-12 and / or a temperature of 10-40 ° C. The process according to any one of claims 1-3, wherein the cyanoferrate is regenerated electrochemically. Process according to any one of claims 1-3, wherein the cyanopherrate is regenerated with hydrogen peroxide or chlorine. 6. A method according to any one of claims 1-5, wherein the carbohydrate is starch, cellulose, chitin or a polysaccharide gum. A process according to any one of claims 1-6, wherein at least 0.025 mol (0.10 mol%) iron complex is used relative to the carbohydrate. 8. Carboxy starch having a weight average molecular weight of at least 1 MDa and a cindviscositcit of a 25% solution of at most 500 Brabender units. 9. Carboxy starch of which the product of the number of the carboxyl groups in the 6-position per 100 anhydroglucose units and the weight-average molecular weight in MDa is at least 10, in particular at least 15. 1010341 COOPERATION TREATY (PCT) REPORT ON NEWNESS RESEARCH OF INTERNATIONAL TYPE IDENTIFICATION OF THE NATIONAL APPLICATION Characteristic of the applicant or of the authorized representative NO 41984 Dutch application no. Date of the request for an international type study Granted by the International Research Inquiry (ISA) to the request for an international type study No SN 32174 EN I. CLASSIFICATION OF THE BOTTOM PIECE using different classifications, indicate all classification symbols) Volgem the International Classification (IPC) Int.Cl.6: C 08 B 31/18, C 08 B 15/02 II. FIELDS OF TECHNIQUE EXAMINED Minimum documentation examined _ Classification system Classification symbols _ Int.Cl.6: C 08 B Examined documentation other than minimum documentation to the extent that such documents are included in the areas examined III. [[NO RESEARCH POSSIBLE FOR CERTAIN CONCLUSIONS (comments on supplementary sheet) IV. | | LACK OF UNIT OF INVENTION (Notes on Supplementary Sheet) PC-'lSA / rOKil C 7 ï 9? N