DK173088B1 - Process for the preparation of chitosan by deacetylation of chitin, using the one used in the deacetylation - Google Patents

Description

in DK 173088 B1

The invention relates to a process for the preparation of chitosan by deacetylation of chitin using the hydroxide solution used in the deacetylation, in which 5 a) is treated with chitin 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 to achieve the desired degree of deacetylation, 10 b) separating the formed chitosan from the reaction medium which is the now acetate-containing sodium hydroxide solution, c) washing it separating the chitosan one or more times with water and separating the washed chitosan from the wash water; d) neutralizing the washed chitosan with a strong mineral acid; and e) washing the neutralized chitosan with water and drying to obtain a finished product.

Chitosan has a number of industrial applications, e.g. as thickening agents, gelling agents, film-forming agents, ion exchangers and flocculants, and as heavy metal binding agents, suture materials and as wound healing agents.

Chitin forms part of the outer skeleton of the invertebrate as a structural material and constitutes approx. 25% by weight of shrimp shells. Remains are mainly CaCO 2, but proteins and fats are also present.

The raw material in the preparation of chitosan is available in large quantities, e.g. in the form of shells from shrimp, lobsters and crabs, which appear as waste products in the fishing industry.

The shells are pre-treated with a mineral acid, usually hydrochloric acid, to remove CaCO 3 before further processing. Regarding this, reference is made to Danish Patent No. 157,452 B.

Chitin is a unbranched polysaccharide consisting essentially of repeat units of acetyl-D-glucosamine bound together by β-1,4-glycosidic bonds, according to the formula:

CH.OH

-Q- -l \? H%

A NH

c = o * CH,

The molecular weight of non-degraded chitin is in the range 10 from 1,000,000 to 2,000,000.

The acetyl group in the chitin repeat unit can be cleaved off by treating chitin with a hot, 30-60% by weight aqueous sodium hydroxide solution to form acetate ions and chitosan: 15

CH, OH

20 Η NH *

A competing reaction in the deacetylation is a cleavage of / 3-1,4 bonds, thereby reducing the molecular weight. This competitive cleavage is promoted by long treatment times which are required when sodium hydroxide concentrations are used at the lower end of said range, and furthermore by even relatively low concentrations of acetate arising from prior deacetylations.

Chitin and chitosan are both very voluminous products, so a process as defined above should be used in relation to chitin / chitosan large amounts of aqueous sodium hydroxide.

The reaction medium separated from this chitosan in step b) consists essentially of an aqueous solution of sodium hydroxide and acetate, as well as proteins, degradation products and soaps. This reaction medium cannot be used immediately for repeated reactions of new amounts of chitin, even if the sodium hydroxide concentration should have the desired value, as, as mentioned above, acetate promotes the competitive / 3-1,4 cleavage.

Thus, a considerable part of the reaction medium has to be discarded which, as mentioned above, is present in large quantities relative to the amount of chitosan produced. For environmental reasons, it is not possible to direct the highly alkaline reaction medium to a recipient, and so far the medium has been neutralized with a strong acid, especially hydrochloric acid and the highly saline neutralization product disposed of. This method, of course, increases the cost of carrying out the process and raises the cost of production of chitosan, as fresh sodium hydroxide and acid must be obtained for each production charge to neutralize excess thereof.

In order to reduce the occurrence of degradation products during chitosan production, U.S. Patent No. 4,195,175 to treat chitin under milder conditions, thereby treating finely ground chitin with a 35-50 wt% sodium hydroxide solution in w / w ratio 1: 2 to 1: 7 at 40 to 80 ° C for 40 to 160 hours for a closed container from which air is expelled and replaced with nitrogen. The acetate formed by the deacetylation of chitin, whose influence on the β-3,4-cleavage is not mentioned, is proposed to be removed from the used sodium hydroxide solution by crystallization under cooling. For example, from a spent boiling liquor containing 27% NaOH and 20%

NaAc, when cooled to 15 ° C for 16 hours, obtain a solution containing 33% NaOH and 9.5% NaAc which was recycled by deacetylation of chitin.

However, in practice, the acetate content of a recycled liquor for that purpose should be considerably lower, not more than 3-4%, if chitosan of uniform, good quality is to be obtained.

It is therefore an object of the invention to provide a process as set forth above, in which the reaction medium can be regenerated in the acid-free acetate-free form and then used for repeated reactions of new amounts of chitin, thereby reducing the molecular weight of the reaction medium. in the repeated reactions, chitosan formed does not still have to decrease.

Surprisingly, it has now been found that by adding a sufficient amount of a low molecular weight polar organic compound to the reaction medium after reaction with chitin (plus washing water from washing the chitosan produced) to obtain a two-phase system consisting of a 10 in an essentially aqueous phase and a substantially organic phase (the bulk of which is said low molecular weight polar organic compound) can achieve acetate predominantly to the organic phase, whereas most of the sodium hydroxide remains in the aqueous phase.

Accordingly, the process of the invention is peculiar in that, if desired in step b), the chitosan-liberated reaction medium, if desired in admixture with the wash water of step c), provides a sufficient amount of a low molecular weight, polar organic compound to obtaining a two-phase system, b2) separating the two-phase system into a first phase comprising the major amount of the low molecular weight, polar organic compound, some water and sodium hydroxide, and a substantial portion of the acetate formed by the deacetylation and other impurities, and a a second phase in the form of an aqueous sodium hydroxide solution substantially free of acetate and said other impurities, and b) returning the second phase in which the concentration of sodium hydroxide is adjusted to a value of 30-60% by weight, to step a).

After separating the reaction medium from the bulky chitosan in step b), this is conveniently pressed to remove as much of the reaction medium as possible.

In step c), the pressed chitosan is washed once or more with water to remove unwanted residues of especially sodium hydroxide and acetate from the product. The wash water from this is combined, possibly after evaporation, with the reaction medium.

It has subsequently been found advantageous to wash the chitosan with the low molecular weight polar organic compound, optionally mixed with water, after which it is convenient to wash again with water. Namely, such a compound has been found to be significantly more effective than water for leaching acetate from chitosan, and a thorough acetate removal thereof is important, as acetate, upon subsequent washing with mineral acid, would form acetic acid that partially dissolves chitosan and entails filtering hassle.

Accordingly, a preferred embodiment of the process of the invention is peculiar in that, in step c), after one or more times of washing the chitosan with water, the chitosan with or the low molecular polar organic compound is followed or injected. if desired, in admixture with water, and in step bl) using the wash liquid therefrom as the source of the low molecular weight polar organic compound.

Examples of a "low molecular weight, polar organic compound" are for C 4, and especially C 1 to C 3 mono- and polyhydric alcohols, ethers, esters, aldehydes, ketones, amines, amides, halogenated derivatives of these compounds and halogenated hydrocarbons, as well as further mixtures thereof. Preferably, however, as a low molecular weight, polar organic compound, an alcohol of 1-3 carbon atoms is used, including especially ethyl alcohol, but also methyl, n-propyl and isopropyl alcohol can be used.

30 Ethanol forms with brine solutions with concentrations in the range of approx. 400-700 g NaOH / 1 (about 30-47 wt% NaOH) a well-defined two-phase system. At lower concentrations, phase separation is not achieved and at higher concentrations there is a risk of formation of a difficult fragile emulsion.

As mentioned, the organic phase contains most of the acetate formed by the deacetylation of chitin. In addition, it contains water and e.g. by using a significant amount of water in a low molecular weight alcohol. Further, some sodium hydroxide is contained herein; using such alcohol e.g. at a concentration of about half that of the aqueous phase.

Prior to adding low molecular weight polar organic compound to the reaction medium and wash water mixture, this mixture is clearly cloudy and fluffy. After the addition, two phases are formed and the cloud-emitting substances are essentially surprisingly extracted into the organic phase so that it becomes cloudy while the aqueous sodium hydroxide-containing phase appears clear and free from lint.

The clouding agents are probably soaps, insoluble proteins or their degradation products.

After washing of the prepared chitosan with water, if desired, then with the low molecular weight polar organic compound and preferably then again with water, whereupon the washing water, possibly after concentration, is combined with the reaction medium, mineral acid is added to the chitosan to neutralize this. . Finally, wash another 20 or more times with water to remove excess salts and dry to obtain the finished product.

The separated organic phase can be disposed of in any convenient manner, e.g. by burning. By selecting suitable conditions, the burning can be carried out in such a way that the low molecular weight, polar organic compound, acetate and other organic substances are burnt, whereas sodium hydroxide remains as a salt which, if desired, can again be used in the process according to the invention. Alternatively, for example, if the low molecular weight polar organic compound is halogen-containing, it can be recovered in whole or in part by distillation and sodium hydroxide can be recovered from the distillation residue.

In the process of the invention, the release of chitin as mentioned above is carried out at a concentration of sodium hydroxide of 30-60% by weight, preferably 40-55% by weight.

At low sodium hydroxide concentrations, the cleavage of the β-1,4 bond competing with the deasylation will be favored to obtain a low or too low molecular weight chitosan product. At high sodium hydroxide concentrations, especially above 55% by weight, the solution should be kept heated to avoid precipitation of sodium hydroxide.

The reaction between chitin and the aqueous sodium hydroxide solution is carried out as mentioned at a temperature of 50-110 ° C, preferably 70-110 ° C for a period of 1.5 to 5 hours. A low reaction temperature results in an extension of the reaction time to obtain chitosan of the same degree of deacetylation. Conversely, a high reaction temperature favors cleavage of / 3-1,4 bonds in chitin.

If chitosan with a high molecular weight and a high degree of dilatation is desired, which is the case if the chitosan e.g. should be used for fibrous or suture materials, the reaction conditions must therefore be chosen such that the concentration of sodium hydroxide is high and the temperature is low.

Low molecular weight chitosan is obtained at a relatively low concentration of sodium hydroxide in the reaction medium.

Such a chitosan can be used, for example, as a hair-wetting agent in shampoos and conditioners, possibly after the chitosan has been chemically modified if a high viscosity is undesirable.

In order to control the reaction of chitin to form a chitosan having a desired chain length and degree of deacetylation, it is substantially explained, as above, that the reaction medium does not contain too high concentrations of free acetate and of impurities.

By repeatedly using the same sodium hydroxide solution regenerated using the process of the invention, with each new batch of chitin, the same concentration of sodium hydroxide and acetate can be employed so that a substantially identical chitosan product can be prepared. each repeated turnover.

The process of the invention will now be further elucidated by means of exemplary embodiments.

EXAMPLE 1 200 kg of chitin from shrimp shells, which, upon treatment with acid and fish embryo silage, were liberated from calcium carbonate and protein substances as described in DK Patent No. 157,452 B is reacted with 6000 l of an aqueous 53 wt% sodium hydroxide solution (820 g NaOH / 1) at a temperature of 105 ° C for 5 hours.

The reaction medium is filtered off and the resulting chitosan 10 is pressed and washed twice with a total of 2000 l of water which is filtered off. Then, with stirring, 200 l of ethanol and 500 l of water are added and filtered and compressed again. Wash 3 times with 2000 liters of water, then neutralize the chitosan by gently adjusting the pH to about 15 with the aid of HCl. 9 in a cold, watery slurry. The neutralized chitosan is washed again with water and dried to obtain 140 kg of a chitosan product having a molecular weight of 1 200 000 and a degree of deacetylation of 95%.

Reaction medium is combined with wash water (from before the ethanol treatment) and evaporated until the concentration of NaOH herein is 650 g / l. The sodium acetate concentration is approx. 1.2% by weight. To the evaporated liquid, water / ethanol filtered from the chitosan is added with stirring. Upon standing, two phases are formed which are separated.

The upper ("organic") phase contains the majority of the ethanol and the majority of the acetate formed by the deacetylation. In addition, this phase contains some sodium hydroxide, water, fats and proteins and their degradation products. This phase is incinerated.

The second ("aqueous") phase is a clear solution containing sodium hydroxide at a concentration of approx. 650 g / l and sodium acetate at a concentration of less than 0.3% by weight. This aqueous sodium hydroxide solution, added sodium hydroxide from the above burning of the upper phase, is again used for the reaction of chitin as described above.

This results in a chitosan product having a molecular weight of 1,500,000 and a degree of deacetylation of 95%.

9 DK 173088 B1

The same sodium hydroxide solution is regenerated as indicated above and recycled an additional 5 times to react with chitin. The chitosan product obtained after the 5th reaction has a molecular weight of 1,500,000 and a degree of deacetylation of 95%.

EXAMPLE 2

To 4000 kg of drained shrimp shells with 20% dry matter (TS) 10 in an aqueous slurry 370 kg of 30% hydrochloric acid (HCl) are added with stirring.

When finished shaking and when all shells are effectively settled, the shells are dewatered by pressing or centrifuging in a decanter. The shrimp shells are hereby released 15 for most of the ash after the acid treatment.

Proteins, oils and other organics besides chitin are rendered water soluble by boiling the shells in a 1.5 M NaOH solution for 2-3 hours. Then the shells are dewatered and washed again 4 times with warm water.

For the purpose of removing the last portion of the ash, the shells are treated with stirring with hydrochloric acid at a pH of approx. 1. Thereafter, the shells are dewatered, washed and pressed to obtain 525 kg of a product with 40% TS which is essentially chitin.

This wet chitin is mixed in a reactor with 6000 l of water and 5500 kg of NaOH, corresponding to 695 g of NaOH / 1 or a sodium hydroxide concentration of 46.6 wt%, then stirred for 3 1/2 hours at a temperature of 80 ° C. .

The reaction medium is filtered off and the obtained chitosan 30 is pressed. To this add 2000 1 hot water and stir for 10 minutes. , then wash water is filtered off and the chitosan is pressed. Washing and pressing are repeated 4-6 times.

The chitosan is washed with cold water and the pH of the wash water is carefully adjusted by adding strong mineral acid 35 to a value of approx. 9th

The chitosan is dewatered, washed twice with warm water and dried to give a chitosan product having a molecular weight of approx. 800,000 and a deacetylation rate of 78%.

To the separated reaction medium is added with stirring 150 l of methanol. Upon standing, two phases are formed which are separated.

5 The upper phase contains the majority of the methanol and the majority of the acetate formed by the deacetylation. In addition, this phase contains some sodium hydroxide, water, fats and proteins, and their degradation products.

This phase is incinerated.

The lower phase is a clear solution containing sodium hydroxide at a concentration of 700 g NaOH / 1. The sodium acetate concentration is below 0.3% by weight. This phase is used to reactivate chitin as described above, adding to it NaOH / Na 2 O obtained from burning the top phase, optionally together with fresh NaOH and optionally water, to restore fluid quantity and concentration.

This results in a chitosan product having a molecular weight of 1,000,000 and a deacetylation degree of 78%.

20 Sodium hydroxide solution regenerated in this way and used 5 times gives, on the 5th reaction, a chitosan product which is essentially identical to the chitosan product obtained after the first regeneration.

It will be seen that using the process of the invention can greatly recycle NaOH and thus save on the disposal of spent, strong basic reaction medium and furthermore obtain less expense on acquiring fresh NaOH.

For the chitosan product obtained by the process according to the present invention, the degree of deacetylation is substantially constant. The molecular weight is greater in the product from the first regeneration than that of the product obtained using fresh sodium hydroxide solution, but remains substantially constant at the subsequent regenerations.

35

Claims (8)

A process for the preparation of chitosan by deacetylation of chitin using the hydroxide solution used in the acetylation using 5 a) treating chitin 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 to achieve the desired degree of deacetylation, 10 b) separating the formed chitosan from the reaction medium which is the now acetate-containing sodium hydroxide solution, c) washing the separated chitosan one or more times with water and separates the washed chitosan from the wash water; d) neutralizes the washed chitosan with a strong mineral acid; by adding, to the reaction medium liberated in step b) for chitosan, if desired, in admixture with the washing water of step c), a sufficient amount of a low molecular weight po learn organic compound to obtain a two-phase system, b2) separates the two-phase system in a first phase comprising the bulk of the low molecular weight, polar organic compound, some water and sodium hydroxide, and a significant portion of the acetate formed by deacetyl and other impurities, and a second phase in the form of an aqueous sodium hydroxide solution substantially liberated from acetate and said other impurities, and b3) returning the second phase in which the concentration of sodium hydroxide is adjusted to a value. of 30-60% by weight, to step a). 35 12 DK 173088 B1 Process according to claim 1, characterized in that, in step c), after washing the chitosan with water one or more times, washing or injecting the chitosan with the low molecular weight polar organic compound, if desired in admixture with water, is followed. and that in step b1), the wash liquid is used as the source of the low molecular weight polar organic compound. 3. A process according to claim 1 or 2, characterized in that in step b1) the low molecular weight polar organic compound is used in an amount as calculated on the amount of reaction medium and water resulting from washing of chitosan before the acid treatment. is 1-15% by volume, preferably 2-5% by volume. Process according to claims 1-3, characterized in that the low molecular weight polar organic compound is a C 13 aliphatic alcohol. Process according to claim 4, characterized in that the alcohol is ethanol. 6. A process according to claims 1-4, characterized in that the adjustment of the concentration of sodium hydroxide in the second phase is carried out by evaporation or by addition of NaOH in solid form or in the form of a concentrated aqueous solution thereof. Process according to claims 1-4, characterized in that by adding sodium hydroxide to the mixture of the chitosan-liberated reaction medium and washing water before or during its contact with the low molecular weight polar organic compound, the second phase upon return to step a) will contain 30-60 30% by weight sodium hydroxide. Process according to claims 1-4, characterized in that the first phase of step b2) is subjected to a distillation for the recovery of the low molecular weight polar organic compound or that it is disposed of by burning.