METHOD FOR PRODUCING CHITOSAN

Abstract: A method for production of chitosan by the deacetylation of chitin, derived for example from crustacea shells or biomass residues, comprises forming a slurry of the chitin source with a water-miscible organic diluent, treating the slurry with aqueous alkali solution at an elevated temperature to deacetylate the chitin, and separating the chitosan reaction product.

Description: METHOD FOR PRODUCING CHITOSAN This invention relates to an improved method for the production of chitosan by the deacetylation of chitin. Conventionally, chitosan has been manufactured by the deacetylation of chitin derived from the exoskeletons of Crustacea, using hot concentrated alkali solutions. Chitin can be readily isolated from calcium carbonate, the only other major component present in the shells of Crustacea such as crabs, shrimps, prawns and lobsters, by treatment with a dilute mineral acid. The chitin residue can then be deacetylated with concentrated alkali solution, usually at the boiling point of the solution. In the production of chemicals by fermentation processes, residual products comprising mycelial mats are substantially all disposed of as waste products. It has recently been recognised that these by-products contain varying amounts of chitin, depending on the particular fungi used in the processes, comparable with the amount available from the crustacean shells, and may have an industrial use. However, the chitin in these products is associated with other polymeric materials from which it is not so easily separated. GB2026516 describes a method whereby residual products of fermentation processes involving particular filamentary fungi such as Allomyces, Aspergillus, Mucor and Penicillium, are treated with hot concentrated alkali solution in a manner analogous to previous methods using crab shells, to deacetylate the chitin present and the chitosan so produced is complexed with glucan, also found as a residual product of the fermentation processes involving these fungi, to provide a product having a direct end use without further treatment, exhibiting properties similar to those of chitosan alone in its more important applications. We have now found that by forming a slurry of the source of chitin, whether it * be from crustacean shells or biomass, with an organic diluent prior to treatment with a strong alkali solution, an efficient process for deacetylating chitin is obtained which requires much less alkali than previously used and thereby lessens the problems of recovery or disposal of strong alkali solutions associated with earlier methods. The organic diluent can be easily separated by ways known to those skilled in the art to recover it for further use. Generally distillation methods are preferred and the diluent recovered is re-cycled to the process. Accordingly, the present invention provides a method for the production of chitosan from chitin which comprises forming a slurry of chitin source with a water-miscible organic diluent which is essentially inert under the conditions of the reaction, treating the slurry at an elevated temperature with an aqueous alkali solution in a weight ratio of alkali:chitin source between 0.75:1 and 3.0:1, and separating chitosan reaction product from the liquid media. Preferably the organic diluent is an alcohol or ketone although other water-miscible diluents may be used. As with earlier methods, chitin for processing according to this invention can be economically derived from the shells of Crustacea such as crabs, prawns, lobsters or shrimps which are initially treated with a dilute mineral acid to remove calcium carbonate. Although available, chitin sources such as locusts and termites are not economical to process. Important sources of chitin which can be used in the process according to this invention, are the by-products of industrial fermentation processes. As well as chitin, biomass residues also contain variable amounts of other constituents such as other polysaccharides, pigments, proteinaceous material and lipids. A partial separation of chitin from other components found in biomass can be conveniently carried out by pre-treatment with dilute alkali solution which removes soluble polysaccharides, pigments, lipids and much of the protein. Normally a pre-treatment with about 2.5% w/v sodium hydroxide solution is satisfactory. Following deacetylation of the chitin, any residual insoluble polysaccharides if present may be separated from the chitosan product by dissolution of the chitosan in a dilute organic acid such as 0.5% acetic acid. All the chitosan products of the invention are soluble in dilute organic acids and suitably are at least 60% deacetylated, a degree of deacetylation which is typical of ccmnercially available chitosan used for water treatment, metal absorption and other applications. The alkali solution should be strong enough to effect the desired deacetylation of the chitin. An alkali solution containing not less than 38%, especially 38% to 40%, sodium hydroxide is preferred as this has the added advantage of being stable on storage. The amount of alkali solution required relative to the quantity of chitin source on a weight basis is preferably no higher than 1:1. Larger amounts of alkali may be present but there is no significant advantage in incorporating these amounts into the reaction medium. In some cases less alkali can be used where there is sufficient organic diluent present to ensure that the reaction mixture can be stirred efficiently. Generally a weight ratio of alkali solution to chitin source between 0.75 and 3.0:1 is used. This compares favourably with prior methods which require weight ratios of alkali solution to chitin source of up to about 10:1. According to the practice of this invention, the water miscible organic diluent is present in a quantity sufficient to form a mobile slurry with the chitin source. Generally at least 4 parts by weight but no more than 20 parts by weight diluent to 1 part by weight chitin source are required. Organic materials having boiling points between 40°C and 120°C are preferred as diluents. Typically these may be selected from alcohols and ketones but in principle any water miscible organic diluent essentially inert under the conditions of the reaction and having a boiling point within the preferred range can be used. The diluent functions as a distribution medium for the alkali and is preferably not miscible with it. A small solubility of the alkali in the diluent can however be tolerated and may even be beneficial in ensuring the uniform distribution of the alkali through the solid material. If moulds, yeasts or fungi which are residues of fermentation processes, are wet with an organic solvent after extraction of the desired product, the residues are conveniently further diluted with the same solvent in a suitable amount to form a slurry as required for the process of the invention. When using organic diluents having relatively low boiling points, it is preferable, though not essential, to carry out the reaction under pressure so as to permit the use of higher reaction temperatures. In general the higher the temperature, the shorter the reaction time required. To avoid degradation of chitin within the reaction vessel it is preferable to carry out the process in an inert atmosphere; otherwise products having undesirably low molecular weights may be obtained. Generally for commercial applications, average molecular weights in the order of 1 x 10^ to 2 x lO^ are required, depending on the intended use. Slurries containing an alcohol as a diluent, such as iso-propanol, which reflux at higher temperatures than say acetone, have the advantage that the process can be carried out without using a pressure vessel. However, these diluents are generally not so easily recovered for further use because of the formation of azeotropes containing about 12-20% water. The invention is further illustrated by reference to the following examples wherein parts are all parts by weight. Example 1 Samples of finely divided chitin (1 part) prepared from prawn shells, were slurried in t-butanol (15 parts) and varying amounts of sodium hydroxide solution (50% w/w) ranging from 0.75-1.5 parts, were added. The mixtures were stirred for 15 minutes at room temperature and were then heated for three hours at 75°C. The products were filtered, washed to remove residual alkali and dried. All products were found to be soluble in 1% aqueous acetic acid. Example 2 Four samples of chitin (1 part) prepared as in Example 1, were slurried in iso-propanol (16 parts) and sodium hydroxide solutions of varying concentrations (1.5 parts) were added. The mixtures were stirred for 15 minutes at room temperature and were then heated for 3 hours at 80°C. The products were filtered, washed until free from residual alkali and dried. The products, three of which were soluble in 1% aqueous acetic acid solution, had the following properties: Viscosity of 1% solution Sample %NaOH (w/w) % residual (amide) Pas (cps) *1 35 - 0.021 ( 21 ) 2 40 43 0 .3 85 (385) 45 39 ( 90) 0.090 4 50 34 0.064 ( 64 ) * Comparative - mainly insoluble product Example 3 Chitin (1 part) prepared from prawn shells, was slurried in iso-propanol (16 parts) and 50% w/w sodium hydroxide solution (1.5 parts) was added. The mixture was stirred for 15 minutes at room temperature and then heated for four hours at 80°C. The product was filtered, washed to remove residual alkali and dried. The product was soluble in 1% aqueous acid solution (0.24% insoluble matter) and a 1% solution in this solvent had a viscosity of 0.037 Pas (37 cps). On analysis the product was shown to have 32.5% residual amide groups. Example 4 Chitin (1 part) prepared from prawn shells, was slurried in iso-propanol (16 parts) and 50% w/w sodium hydroxide (1.5 parts) was added. After stirring for 15 minutes at room temperature, the mixture was heated under reflux for three hours at its boiling point (approximately 83°C). The product was filtered, washed until neutral and dried. The product was soluble in 1% aqueous acetic acid solution and a 1% solution in this solvent had a viscosity of 0.28 Pas (280 cps). On analysis the product was shown to have 31% residual amide groups. Example 5 Chitin (1 part) prepared from prawn shells, was slurried in iso-propanol and 50% sodium hydroxide solution was added as in Example 4. The mixture was refluxed as described in Example 4 but in an atmosphere of nitrogen. The product was filtered, washed until neutral and dried. The product was soluble in 1% aqueous acetic acid solution and a 1% solution in this solvent had a viscosity of 0.44 Pas (440 cps). On analysis the product was shown to have 31% residual amide groups and a molecular weight of 1.2 x 10^. Example 6 Chitin (1 part) prepared from prawn shells, was slurried in acetone (16 parts) and 50% w/w sodium hydroxide (1.5 parts) was added. After stirring for 15 minutes at room temperature, the mixture was heated for five hours at 56°C. The product was filtered, washed until neutral and dried. On analysis the product had a residual amide content of 37.5% and a 1% solution in 1% aqueous acetic acid solution had a viscosity of 0.137 Pas (137 cps). Example 7 Fungal mycelium (1 part) was slurried in acetone (10 parts) and 40% w/w sodium hydroxide (1 part) was added. The mixture was heated for six hours at 83°C. The product was filtered, washed until free from residual alkali and dried. The product contained 72% chitosan extractable with 0.5% aqueous acetic acid. The extracted chitosan had a molecular weight of 2.73 x 10 5 . Example 8 Dried fungal mycel ium ( 1 part ) was slurried in acetone ( 8 parts) in an autoclave lined with polytetrafluoroethylene and having an operational volume of 10 litres. 40% soαium hydroxide ( 1 part) was added and the mixture was heated under a pressure of 0.165 MPa (24 p.s.i.) at 85 C for a period of one hour with constant stirring. The solid product was removed, washed, neutralised and then dried. The extractable chitosan had a residual amide content of 24.6% and a molecular weight of 5 x 10^. Example 9 Fungal mycelium (1 part) was slurried in acetone (10 parts) and 40% sodium hydroxide solution (1 part) was added. After stirring for 15 minutes at room temperature, the mixture was heated under pressure at 85°C for three hours. The recovered solid product was washed until neutral and dried. The product contained 60% chitosan extractable with 0.5% aqueous acetic acid. The extracted chitosan had a residual amide content of 23% and a molecular weight of 2.13 x 10 . Example 10 Dried fungal mycelium (1 part) was slurried in a mixture of acetone (16 parts) and 40% aqueous sodium hydroxide (2.8 part). The whole was heated in an autoclave with constant stirring for a period of three hours. The temperature reached 100°C and the pressure 3 bar gauge. The solid product was removed, washed, neutralised, rewashed and dried. On treating this product with 0.5% aqueous acetic acid the chitosan dissolved, and after separation from residual solid, was reprecipitated with alkali, washed and dried. The chitosan was found to have an average molecular weight of 1.1 x 10- 3 and a residual amide content of 36%.

Claims: CLAIMS 1. A method of producing chitosan from chitin, comprising forming a slurry of a chitin source with a water - miscible organic diluent which is essentially inert under conditions of reaction of chitin with alkali, deacetylating the chitin by treating the slurry at an elevated temperature with an aqueous alkali solution in a weight ratio of alkali:chitin source between 0.75:1 and 3.0:1, and separating chitosan reaction product from the liquid media. 2. A method according to claim 1 wherein the organic diluent is an aliphatic alcohol or ketone. 3_. A method according to claim 1 or 2 wherein the organic diluent has a boiling point between 40 C and 120°C. 4. A method according to claim 1, 2 or 3 wherein the chitin source is derived from shells of Crustacea. 5. A method according to claim 1, 2 or 3 wherein the chitin source is a by-product of an industrial fermentation process. 6. A method according to any one of the preceding claims wherein the aqueous alkali solution is a 38-40% by weight sodium hydroxide solution. 7. A method according to any one of the preceding claims wherein the weight ratio of diluent to chitin source is between 4/1 and 20/1. 8. A method according to any one of the preceding claims wherein the alkali treatment is performed at super-atmospheric pressure. 9. A method according to any one of the preceding claims wherein the alkali treatment is carried out under an inert (non-oxidising) atmosphere.

Inventors: ROBERTS GEORGE ANDREW FRANCIS, ROBERTS, GEORGE, ANDREW, FRANCIS
Applicants: RTZ CHEMICALS LTD, RTZ CHEMICALS LIMITED
Application year: 1989
Main classification: C08B37
IPC codes: C08B37