WO1999038893A1

WO1999038893A1 - A method of producing chitosan by deacetylation of chitin under re-use of the hydroxide solution used for said deacetylation

Info

Publication number: WO1999038893A1
Application number: PCT/DK1999/000048
Authority: WO
Inventors: Jon Olavur Joensen
Original assignee: Matcon Rådgivende Ingeniørfirma A/S
Priority date: 1998-02-02
Filing date: 1999-02-01
Publication date: 1999-08-05
Also published as: DK173088B1; NO20003831D0; DK13998A; NO20003831L; AU2265399A

Abstract

In a conventional manufacture of chitosan from chitin by deacetylation thereof with a hot sodium hydroxide solution, separation of the reaction medium and washing, neutralization and drying of the resulting chitosan, the used natrium hydroxide solution, the reaction medium, is regenerated by addition of a low-molecular, polar organic compound for the attainment of two phases, the organic phase of which contains a substantial part of the acetate formed at the deacetylation and other impurities, and the aqueous phase, which substantially consists of sodium hydroxide solution freed of acetate and other impurities, is recycled for deacetylation of a new amount of chitin. A chitosan product resulting from the use of a lye regenerated in this manner does not substantially change the deacetylation degree and molecular weight at subsequent regenerations.

Description

A METHOD OF PRODUCING CHITOSAN BY DEACETYLATION OF CHITIN UNDER RE-USE OF THE HYDROXIDE SOLUTION USED FOR SAID DEACETYLATION

The present invention relates to a method of producing chitosan by deacetylation of chitin under reuse of the hydroxide solution used for said deacetylation, in which a) chitin is treated with a 30-60% by weight, prefer- ably 40-55% by weight, sodium hydroxide solution at a temperature of 50-110°C, preferably 70-110°C, for a period of 1.5 - 5 hours for the attainment of the desired degree of deacetylation, b) the resulting chitosan is separated from the reaction medium which is the now acetate-containing sodium hydroxide solution, c) the separated chitosan is washed one or more times with water and the washed chitosan is separated from the washings, d) the washed chitosan is neutralized with a strong mineral acid, and e) the neutralized chitosan is washed with water and dried for the attainment of a finished product.

Chitosan is widely used industrially, for instance as thickeners, chelating agents, film forming agents, ion exchangers and flocculating agents as well as heavy metal binding agents, suture materials and as wound healing agents.

Chitin is a constituent part of the exterior skeleton of invertebrata as a structural material and constitutes approximately 25% by weight of the shells of Crustacea. The rest is substantially constituted of CaCO-., but also proteins and fatty substances are present . The raw material for the production of chitosan is available in big quantitites, for instance in the form of shells from shrimps, lobsters and crabs which are found as waste products in the fishing industry.

The shells are pre-treated with mineral acid, usually hydrochloric acid for removal of CaCO., prior to the subsequent processing. In this respect reference is made to Danish patent no. 157 452 B.

Chitin is a polysaccharide with unbranched chain consisting substantially of repetition units of acetyl- D-glucose amine tied together by jδ-l, 4-glycosidic bindings, cf. the formula

—O

The molecular weight of not degraded chitin lies within the range of 1,000,000 to 2,000,000.

The acetyl group in the repetition unit for chitin may be cleaved off by treating the chitin with a hot, 30-60% by weight aqueous sodium hydroxide solution under formation of acetate ions and chitosan:

—O

H NH_a A competing reaction during the deacetylation is a cleavage of β-1 , 4 -bindings, whereby the molecular- weight is reduced. This competing cleavage is furthered by long treatment periods which are necessary when sodium hydroxide concentrations in the lower end of said interval are used, and further by even comparatively low concentrations of acetate originating from preceding deacetylations .

Chitin and chitosan are both very voluminous products, for which reason, in a method as defined above, big amounts of aqueous sodium hydroxide relative to chitin/chitosan are to be used.

The reaction medium separated from chitosan by this method in step b) consists substantially of an aqueous solution of sodium hydroxide and acetate as well as proteins, degradation products hereof and soaps. This reaction medium cannot immediately be used for repeated degradations of new amounts of chitin, not even if the sodium hydroxide concentration might have the desired value, as, as mentioned above, said acetate furthers the competing β- 1 , 4 -cleavage .

It is thus necessary to dispose of a considerable part of the reaction medium which is, as mentioned above, present in big amounts relative to the amount of produced chitosan. For environmental reasons it is out of the question to conduct the strongly alkaline reaction medium to a recipient, for which reason it has up till now been necessary to neutralize the medium with a strong acid, in particular hydrochloric acid, and to dispose of the strongly saline neutralization product. This procedure makes of course the use of the method more expensive and increases the production price of chitosan, as for each production charge fresh sodium hydroxide as well as acid for neutralization of surplus thereof is to be provided. In view of reducing the amount of degradation products during the production of chitosan, US patent no. 4,195,175 suggests treatment of chitin under more mild conditions, finely ground chitin being thereby treated with a 35-50% by weight sodium hydroxide solution of in weight/weight ratio of 1:2 to 1:7 at 40 to 80°C for 40 to 160 hours in a closed container, from which air has been expelled and replaced by nitrogen. It is suggested to remove the acetate (the influence of which on the 3-1, 4-cleavage is not mentioned) resulting from the deacetylation of chitin from the used sodium hydroxide solution by crystallization under cooling. It was for instance possible from a used boiling stock containing 27% NaOH and 20% NaAc by cooling to 15°C for 16 hours to obtain a solution containing 33% NaOH and 9.5% NaAc which was re-used at the deacetylation of chitin.

However, for said purpose the acetate contents in a recycled stock should as experience has shown be considerably lower, not more than 3-4%, if chitosan of a homogenous, good quality is to be obtained.

It is thus the object of the invention to provide a method as stated above, by which method the reaction medium may be regenerated on a form substantially free of acetate and then re-used for repeated reactions of new amounts of chitin, the molecular weight for the chitosan resulting from the repeated reactions not progressively decreasing thereby.

It has now surprisingly turned out that by adding a sufficient amount of a low-molecular, polar organic compound to the reaction medium separated after a reaction with chitin (plus washings from wash of the chitosan produced) for the attainment of a two-phase system consisting of a substantially aqueous phase and a substantially organic phase (in which the major amount is constituted of said low-molecular, polar organic compound) , acetate for the major part transgresses to the organic phase, whereas the major part of sodium hydroxide remains in the aqueous phase . In accordance herewith, the method according to the invention is characterized in that bl) a sufficient amount of a low-molecular, polar organic compound is added to the reaction medium freed of chitosan in step b) for the attainment of a two- phase system, b2) the two-phase system is separated into a first phase containing the major amount of the added low- molecular, polar organic compound, some water and sodium hydroxide, and a substantial part of the acetate formed at the deacetylation and other impurities, and a second phase in form of an aqueous sodium hydroxide solution which has been substantially freed of acetate and said other impurities, and b3) the second phase, in which the concentration of sodium hydroxide is adjusted to a value of 30-60% by weight, is recycled to step a) .

After separation of the reaction medium from the voluminous chitosan in step b) , the latter is preferably pressed to remove as much of the reaction medium as possible.

In step c) the pressed chitosan is washed one or more times with water to remove undesirable residuals of in particular sodium hydroxide and acetate from the product. The washings thereof are united, optionally after evaporation, with the reaction medium.

It has turned out to be advantageous to wash the chitosan with the low-molecular, polar organic compound, optionally mixed with water, following which it is again appropriate to wash with water. Such a com- pound has namely turned out to be substantially more effective than water for washing out acetate from chitosan, and a thorough removal of acetate therefrom is important, as acetate in a subsequent wash with mineral acid would develop acetic acid which partially dissolves chitosan and entails filtration troubles.

In accordance herewith a preferred embodiment of the method according to the invention is characteristic in that in step c) after one or several washes of the chitosan with water, the chitosan is washed with the low-molecular, polar organic composition, optionally mixed with water, and that the washings therefrom are used in step bl) as a source for the low-molecular, polar organic composition.

Examples of a "low-molecular, polar organic compound" are C- to C. , and in particular C-. to C₃ mono or polyvalent alcohols, ethers, esters, aldehydes, ketones, amines, amides, halogenated derivatives of these compounds and halogenated carbon hydrides as well as mixtures thereof. Preferred is, however, as a low- molecular, polar organic compound to use an alcohol having 1-3 carbon atoms, and in particular ethyl alcohol, but also methyl, n-propyl and isopropyl alcohol may be used.

Ethanol forms with lyes having concentrations in the range of approximately 400-700 g NaOH/1 (approximately 30-47% by weight NaOH) a well-defined two-phase system. At lower concentrations phase separation is not obtained, and at higher concentrations there is a risk of formation of an emulsion which breaks up with diffi- culty.

As stated above, the organic phase contains the major part of the acetate formed by the deacetylation of chitin. Moreover, it contains water, and for instance by use of a low-molecular alcohol, consider- able amounts of water. Furthermore, quite an amount of sodium hydroxide will be present by use of such an alcohol, for instance in a concentration of about half of the concentration in the aqueous phase.

Prior to the addition of a low-molecular, polar organic compound to the mixture of the reaction medium and washings, this mixture is distinctly unclear and flocculent. After the addition, two phases form, and the substances causing the indistinctness are surprisingly substantially extracted into the organic phase, which becomes unclear, whereas the aqueous sodium hydroxide containing phase appears clear and free of flocculation. The substances causing the indistinctness are presumably soaps, insoluble proteins or degradation products thereof. After finishing of the wash of the resulting chitosan with water, optionally then with the low- molecular, polar organic compound and advantageously then again with water, the washings being, possibly after further concentration, united with the reaction medium, mineral acid is added to the chitosan for neutralization thereof. Finally, one or more washes with water is carried out for removal of surplus salt, and drying is carried out for the attainment of the finished product. The separated organic phase may be disposed of in any appropriate manner, for instance by burning. By choice of suitable conditions, the burning may be carried out in such manner that the low-molecular, polar organic compound, acetate and other organic compounds are burned off, whereas sodium hydroxide will be left as a salt which optionally may be re-used in the present method. Alternatively, for instance if the low-molecular, polar organic compound contains halogen, it may be completely or partially recovered by distil- lation, and sodium hydroxide is recovered from the 8 distillation residue.

In the method according to the invention the deacetylation of chitin is as mentioned carried out at a concentration of sodium hydroxide of 30-60% by weight, preferably 40-55% by weight. At low sodium hydroxide concentration, the cleavage of the β- 1 , 4 binding competing with the deacetylation will be favoured, so that a chitosan product with low or too low molecular weight results. At high sodium hydroxide concentrations, in particular above 55% by weight, the solution is to be kept heated in order to prevent precipitation of sodium hydroxide.

The reaction between chitin and the aqueous sodium hydroxide solution is, as mentioned above, carried out at a temperature of 40-110°C, preferably 70-110°C during a period of from 1.5 to 5 hours. A low reaction temperature entails an extension of the reaction time for the attainment of chitosan with the same deacetylation degree. Reversely, a high reaction temperature favours cleavage of β- 1 , 4 -bindings in the chitin.

If chitosan with a high molecular weight and a high degree of acetylation is desired, which is the case if the chitosan for instance is to be used in fibrous or suture materials, the reaction conditions therefore have to be chosen such that the concentration of sodium hydroxide is high, and the temperature low.

Chitosan with a low molecular weight is obtained by a comparatively low concentration of sodium hydroxide in the reaction medium. Such a chitosan may for instance be used as a hair-aviving agent in shampoos and conditioners, possibly after chemical modification of the chitosan, if a high viscosity is undesirable.

To be able to control the decomposition of chitin for the formation of a chitosan having a desired chain length and deacetylation degree, it is, as explained above, of importance that the reaction medium does not contain too high concentrations of free acetate and impurities .

By repeated use of the same sodium hydroxide solution, which is regenerated by use of the method according to the invention, it is possible at each new charge of chitin to work with the same concentration of sodium hydroxide and sodium acetate, such that it is possible to produce a substantially identical chitosan product in each repeated reaction.

The method according to the invention will now be elucidated in detail by means of examples of embodiments .

EXAMPLE 1

200 kg chitin from shrimp shells, which by treatment with acid and fish viscera silage had been freed of calcium carbonate and proteinaceous substances, as described in DK patent no. 157 452 B, was reacted with 6000 1 of an aqueous 53% by weight sodium hydroxide solution (820 g NaOH/1) at a temperature of 105°C for 5 hours .

The reaction medium was filtered off and the resulting chitosan was pressed and washed twice with a total of 2000 1 water, which was filtered off. Then 200 1 ethanol and 500 1 water were added under stirring, and filtering and pressing were carried out again. 3 washes with 2000 1 water were carried out, following which the chitosan was neutralized by carefully adjusting the pH-value by means of a HCl to approx. 9 in a cold, aqueous suspension. The neutralized chitosan was washed again with water and dried for the attainment of 140 kg of a chitosan product with a molecular weight of 1,200,000 and a deacetylation degree of 95%. 10

The reaction medium was united with washings (from before the ethanol treatment) and evaporated until the concentration of NaOH therein was 650 g/1. The concentration of sodium acetate was approx. 1.2% by weight. ater/ethanol, which had been filtered off from the chitosan, was added to the evaporated liquid. When left standing, two phases formed which were separated.

The upper ("organic") phase contained the major part of the ethanol and the major part of the acetate formed. Moreover, this phase contained some sodium hydroxide, water, fatty substances and proteins as well as degradation products thereof. This phase was burned.

The second ("aqueous") phase was a clear solution containing sodium hydroxide in a concentration of approx. 650 g/1 and sodium acetate in a concentration of less than 0.3% by weight. This aqueous sodium hydroxide solution, to which sodium hydroxide from the above burning of the upper phase had been added, was used once more for reaction of chitin as described above .

This resulted in a chitosan product with a molecular weight of 1 500 000 and a deacetylation degree of 95%.

The same sodium hydroxide solution was regenerated as described above and re-used further 5 times for reaction of chitin. The chitosan product resulting after the 5th reaction had a molecular weight of 1,500,000 and a deacetylation degree of 95%.

EXAMPLE 2

370 kg 30% hydrochloride (HCl) was added under stirring to 4000 kg drained shrimp shells with 20% dry matter (TS) in an aqueous suspension. When the bubbling was finished, and when all 11 shells had effectively settled, the shells were drained by pressing or centrifuging in a decanter. The shrimp shells were thereby freed of the major part of the ash after the acid treatment. Proteins, oils and other organic compounds in addition to chitin were made water-soluble by boiling of the shells in a 1.5 M NaOH solution for 2-3 hours. Following this, the shells were drained and washed again 4 times with hot water. In view of removing the remaining part of the ash, the shells were treated under stirring by hydrochloric acid with a pH-value of approx. 1. Then the shells were drained, washed and pressed to yield 525 kg of a product with 40% TS which was substantially constituted of chitin.

The wet chitin was mixed in a reactor with 6000 1 water and 5500 kg NaOH corresponding to 695 g NaOH/1 or a sodium hydroxide concentration of 46.6% by weight, followed by stirring for 3 1/2 hours at a temperature of 80°C.

The reaction medium was filtered off, and the resulting chitin pressed. 2000 1 hot water was added and stirring was carried out for 10 minutes, following which washings were filtered off and the chitosan pressed. Washing and pressing were repeated 4-6 times. The chitosan was washed with cold water, and pH in the washings was carefully adjusted by addition of a strong mineral acid to a value of approx. 9.

The chitosan was drained, washed twice with hot water and dried to yield a chitosan product with a molecular weight of approx. 800 000 and a deacetylation degree of 78%.

150 ml methanol was added under stirring to the separated reaction medium. When left standing, two phases resulted which were then separated. 12

The upper phase contained the major part of the methanol and the major part of the acetate resulting from the deacetylation. Moreover, this phase contained some sodium hydroxide, water, fatty substances and proteins as well as degradation products thereof . This phase was burned.

The lower phase was a clear solution containing sodium hydroxide in a concentration of 700 g NaOH/1. The concentration of sodium acetate was below 0.3% by weight. This phase was used for renewed reaction of chitin as described above, NaOH/Na.,0 originating from burning of the upper phase, possibly together with fresh NaOH and water, being added to restore the amount of liquid and the concentration. The resulting chitosan product had a molecular weight of 1 000 000 and a deacetylation degree of 78%.

A solution of sodium hydroxide regenerated in this manner and used 5 times, yielded at the 5th reaction a chitosan product which was substantially identical with the chitosan product resulting from the first regeneration.

It will be seen that by using the method according to the invention it is possible to a high extent to reuse NaOH and thus save costs for disposing of used, strongly basic reaction medium, and furthermore, costs for providing fresh NaOH are reduced.

For the chitosan product obtained by use of the method according to the invention the deacetylation degree is substantially constant. The molecular weight is higher in the product from the first regeneration than for the product resulting from the use of fresh sodium hydroxide solution, but the molecular weight remains substantially constant at the following regene- rations.

Claims

13

P A T E N T C L A I M S 1. A method of producing chitosan by deacetylation of chitin under re-use of the hydroxide solution used for said deacetylation, in which a) chitin is treated with a 30-60% by weight, preferably 40-55% by weight, sodium hydroxide solution at a temperature of 50-110°C, preferably 70-110°C, for a period of 1.5 - 5 hours for the attainment of the desired degree of deacetylation, b) the resulting chitosan is separated from the reaction medium which is the now acetate-containing sodium hydroxide solution, c) the separated chitosan is washed one or more times with water and the washed chitosan is separated from the washings, d) the washed chitosan is neutralized with a strong mineral acid, and e) the neutralized chitosan is washed with water and dried for the attainment of a finished product, c h a r a c t e r i z e d in that bl) a sufficient amount of a low-molecular, polar organic compound is added to the reaction medium freed of chitosan in step b) for the attainment of a two- phase system, b2) the two-phase system is separated into a first phase containing the major amount of the added low- molecular, polar organic compound, some water and sodium hydroxide, and a substantial part of the acetate formed at the deacetylation and other impurities, and a second phase in form of an aqueous sodium hydroxide solution which has been substantially freed of acetate and said other impurities, and b3) the second phase, in which the concentration of sodium hydroxide is adjusted to a value of 30-60% by weight, is recycled to step a) . 14

2. A method according to claim 1, c h a r a c t e r i z e d in that in step c) after one or several washes of the chitosan with water, the chitosan is washed with the low-molecular, polar organic composi- tion, optionally mixed with water, and that the washings therefrom is used in step bl) as a source for the low-molecular, polar organic composition.

3. A method according to claim l or 2, c h a r a c t e r i z e d in that in step bl) the low-molecu- lar, polar organic composition is used in an amount which, calculated on the amount of reaction medium and water originating from washing of chitosan prior to the acid treatment, amounts to 1-15% by volume, preferably 2-5% by volume.

4. A method according to claims 1-3, c h a r a c - t e r i z e d in that the low-molecular, polar organic composition is a C-._₃ aliphatic alcohol.

5. A method according to claim 4, c h a r a c t e r i z e d in that the alcohol is ethanol.

6. A method according to claims 1-4, c h a r a c - t e r i z e d in that the adjustment of the concentration of sodium hydroxide in the second phase is carried out by evaporation or by adding NaOH in solid form or in form of a concentrated aqueous solution thereof .

7. A method according to claims 1-4, c h a r a c - t e r i z e d in that by addition of sodium hydroxide to the mixture of the reaction medium freed of chitosan and washings before or during its contact with the low- molecular, polar organic compound it is ensured that the second phase at the recycling to step a) will contain 30-60% by weight of sodium hydroxide.

8. A method according to claims 1-4, c h a r a c - t e r i z e d in that the first phase from step b2) is subjected to distillation for recovering of the low- molecular, polar organic compound or that it is disposed of by burning.