EP0091221B1

EP0091221B1 - Solubilisation and hydrolysis of carbohydrates

Info

Publication number: EP0091221B1
Application number: EP19830301472
Authority: EP
Inventors: Pudens Leonard Ragg
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1982-04-05
Filing date: 1983-03-16
Publication date: 1987-09-09
Also published as: EP0091221A2; JPS58190400A; EP0091221A3; DE3373493D1

Description

This invention relates to the solubilisation and hydrolysis of glycosidically linked carbohydrates having reducing groups and in particular to the solubilisation and hydrolysis of starch and cellulose to glucose.
Cellulose is a polysaccharide which forms the main component of the cell walls of most plants. It is a polymer of (3-D-glucose units which are linked together with elimination of water to form chains of 2000―4000 units. In plants it occurs together with other polysaccharides and hemicelluloses derived from other sugars such as xylose, arabinose and mannose. In the woody parts of plants cellulose is intimately mixed and sometimes covalently linked with lignin. Wood, for example, normally contains on a dry weight basis 40-50% cellulose, 20-30% lignin and 10-30% hemicelluloses together with mineral salts, waxes, resins and proteins.
The solubilisation and hydrolysis of cellulose may be brought about by various treatments, including treatment with acids and with enzymes present in certain bacteria, fungi and protozoa. Such treatments result mainly in cleavage of the glycosidic links in the cellulose chain with a consequent reduction in molecular weight. Partial hydrolysis with acids produces a variety of products, often termed "hydrocelluloses", whose properties are determined by the hydrolysis conditions employed. Complete acid hydrolysis of cellulose yields glucose. Treatment with acid by solution and reprecipitation often increases the accessibility and susceptibility of cellulose to attack by enzymes, microbes and chemical reagents. Solubilisation and hydrolysis of cellulose by enzymes leads to various intermediate products depending upon the enzyme employed. The final product of enzymic treatment of cellulose is usually glucose but rigorous treatment may produce a further breakdown to ethanol, carbon dioxide and water.
In EP-A-0.044.622 we describe and claim a process for the modification, solubilisation and/or hydrolysis of a glycosidically linked carbohydrate by treatment with a mixture comprising an aqueous inorganic acid and a halide of lithium, magnesium and/or calcium or a precursor of said halide. The process of EP-A-0.044.622 is very successful. However easy separation of the metallic species from the organic products of the reaction cannot always be achieved by this process. British Patent Specification No. 625688 proposes the use of aluminium chloride as a starch modifying agent and British Patent Specification No. 718174 proposes a process for the enzymic modification of starch in which aluminium chloride is used.
According to the present invention we provide a process for the solubilisation and hydrolysis of a glycosidically linked carbohydrate to produce soluble oligosaccharides and/or glucose, in which the carbohydrate is contacted. with a mixture comprising an aqueous inorganic acid and a hydrated metal halide or a precursor of said halide at a temperature within the range 50° to 100°C, a reaction product is separated and metal ions are recovered from the separated product characterised in that the metal halide is an aluminium halide and the inorganic acid has a concentration within the range 0.5 to 5 molar.
Whilst the process of the invention is generally applicable to glycosidically linked carbohydrates, it is particularly applicable to starch and cellulose.
The products of solubilisation and/or hydrolysis include oligosaccharides, as well as tri-, di- and monosaccharides. Specifically the products from cellulose include cellodextrins, cellotriose, cellobiose and glucose. When the process is used to produce carbohydrate of enhanced susceptibility to enzymic hydrolysis, the susceptible carbohydrate may be treated with an enzyme in which case the exact nature of the final products will depend upon the enzyme employed and upon the reaction conditions. In the case of cellulose treatment with cellulases the product under appropriate conditions will be glucose.
The glycosidically linked carbohydrate can be present in any suitable state. It can be present as free or combined carbohydrate, in its natural state or in a processed or converted form. The process is particularly useful when applied to the conversion of the following types of carbohydrate-containing feedstocks:-

(a) silvicultural products such as wood, wood residues, mechanical and chemical wood pulps, various grades of recycled paper and other wood based products, and
(b) agricultural products and residues such as straw, bagasse, corn stover and other pulp and grain by-products. The process is also applicable to carbohydrates which exist in highly oriented forms such as crystalline cellulose, cotton and other ordered structures which are normally inaccessible to enzymes and other catalysts. Such inaccessibility may be compounded by the occurrence of a polysaccharide with other polymers such as the cellulose with lignin. The process of the invention is applicable to the modification or solubilisation of cellulose without prior delignification.

The process is applicable to glycosidically linked carbohydrates whether the glycosidic linkage is a (3-linkage as in cellulose, yeast glucan or laminarin, or an a-linkage as in starch, glycogen, dextran or nigeran. In particular it is applicable to starch which is converted to lower sugars including maltose and, as the main product, glucose. The process is also applicable to glycosidically linked carbohydrates with other constituent pentoses, hexoses, heptoses, amino sugars and uronic acids, as well as to the previously mentioned naturally occurring polymers of D-glucose. Such polymers with other constituents having industrial significance include wood hemicelluloses, yeast mannan, bacterial and seaweed alginates, industrial gums and mucilages and chitin. Carbohydrates containing 0- sulphate, N- sulphate, N- acetyl, O-acetyl and pyruvate groups can also be treated by the process of the invention as can carbohydrates derived by carboxymethylation, acylation, hydroxyethylation and other substitution processes provided that such carbohydrates contain glycosidic linkages. Acid labile substituents on carbohydrates may be lost during the process of the invention.
Preferred inorganic acids are hydrochloric, hydrobromic and hydriodic acids, hydrochloric acid being most economical and especially preferred.
The preferred hydrated aluminium halide is the hexahydrate AI 6)₃- 6H₂0. The hydrated aluminium halide can be present as the sole metal halide or in combination with other metal halides or precursors thereof. When more than one metal halide is to be present, the halide present together with the hydrated aluminium halide is preferably a halide (especially a chloride) of lithium, calcium and/or magnesium or a precursor of such a halide. It should be understood that in the reaction mixture the halide ions can be present as complex halide ions, these complex ions being generated within the mixture. In operation of the process of the invention, an aqueous solution of the hydrated aluminium halide is prepared and is thereafter acidified. Preferably the acid is added to the aqueous solution of the halide. However acidification can be achieved in the reverse manner, i.e. by adding the aqueous solution of the metallic salt to the acid. Preferably the acid employed has a concentration in the range 0.5 to 5 molar and is added to the aqueous solution of the metallic salt. The acidified aqueous solution of the metallic salt is used to treat the carbohydrate-preferably being added thereto. It should be noted that the extent to which aluminium chloride is soluble in aqueous hydrochloric acid varies inversely with the concentration of the acid.
In the process of the invention the treatment of the carbohydrate is preferably carried out at a temperature in the range 50° to 90°, especially in the range 65° to 75°. In general the conditions employed in the process of the present invention are similar to those employed in that of EP-A-0.044.622.
When the hydrated aluminium halide is used together with another metal halide, such as a lithium, magnesium and/or calcium halide, the relative proportions in which the two halides are present is preferably in the range 10:1 and 1:10 w/w.
The process of the present invention has the following advantages:-

1. Ability to dissolve and hydrolyse to soluble oligosaccharides as well as to tri-, di- and monosaccharides such as glucose without prior delignification.
2. Ability to handle high concentrations of difficultly soluble polysaccharides such as cellulose.
3. Ability to handle a wide range of potential sources of monosaccharides, especially glucose.
4. Easier separation and recovery of the metal ions.

Any suitable technique may be used to separate aluminium ions from the mixture produced by solubilisation and hydrolysis. Two suitable techniques are illustrated in Examples 2 and 3.
The invention is illustrated by the Examples given below in which Examples the analytical methods used are as described in EP-A-0.044.622.

Example 1

Starch hydrolysis using aqueous solutions of AlCl₃ · 6H₂0 and aqueous HCI

A series of starch hydrolysis reactions was performed using either technique 1 or technique 2, the technique used in any particular reaction depending upon convenience and upon the concentration of AlCl3 · 6H₂0 employed.

Technique 1:

To an aqueous solution of aluminium chloride AlCl3 · 6H₂0 (saturated at 20°C) was added concentrated (10 M) hydrochloric acid to give a final hydrochloric acid concentration of 2 M. Bottles were prepared containing hydrochloric acid saturated with aluminium chloride and starch and were allowed to stand at room temperature for 15 minutes. At the end of this time the bottles were placed in a water bath at 70°C. After one hour at 70°C the contents of the bottles were analysed for D-glucose by the glucose. oxidase method.

Technique 2:

Solid aluminium chloride AICI₃. 6H₂0 was added to concentrated hydrochloric acid and thereafter water was added to raise the volume and to give the required final concentrations. Bottles containing solution of hydrochloric acid, aluminium chloride and starch were prepared and were allowed to stand for 15 minutes at room temperature. After this time the bottles were placed in a water bath at 70°C for 1 hour and thereafter the contents were analysed for D-glucose by the glucose oxidase method.
The results are set out in Table 1. In the table the experiment marked with an asterisk is a comparative experiment with an alternative catalyst system using magnesium chloride Mg C1₂. 6H₂0.
Using technique 2 further experiments were carried out in which starch hydrolysis was continued for periods exceeding one hour, percentage yields of glucose being measured at intervals. The results are shown in Figures 1 and 2 of the drawings.
Figures 1 and 2 of the drawings are graphs of percentage yield of glucose against time in hours for starch solutions hydrolysed in the presence of hydrochloric acid and aluminium chloride AI Cl₃ · 6H₂0.
In Figure 1 the hydrochloric acid concentration used was 3 molar and the aluminium chloride concentration was 0.5 molar. Curve A in Figure 1 is for a 30% w/v starch solution showing a yield of 87% after 3 hours. Curve B in Figure 1 is for a 40% starch solution showing a yield of 70% after 3 hours.
In Figure 2 the hydrochloric acid concentration used was 2 molar and the aluminium chloride concentration was 0.5 molar. Curves C, D and E in Figure 2 are for starch solutions having concentrations of 26% w/v, 33% w/v and 40% w/v respectively. In curves C and D there are peaks at 93% yield after 4 hours and 81 % yield after 3 hours respectively in the two curves. In curve E the yield after 3 hours is 57% and after 6 hours 65%.

Example 2

Recovery of aluminium from reaction product

A solution of a mixture of 1 molar hydrochloric acid, 2 molar aluminium chloride (Al Cl₃ · 6H₂0) containing 20% w/v glucose was reduced in volume from 200 ml to 100 ml by heating to 60°C under reduced pressure. At this volume the crystals which formed were removed from the glucose solution by filtration. These crystals (37 g wet) contained 9.1% w/w aluminium (81 % w/w AI C13. 6H₂0) and 8.4% w/w glucose. This represents 35% of the original aluminium content of the hydrochloric acid/aluminium chloride/glucose mixture. The remaining syrup contained 28.9% w/w glucose and 4.6% w/w aluminium (i.e. the remainder or 65% of the original aluminium). This recovery procedure is set out in Table 2.
N.B.
87% of the original aluminium and 88% of the original glucose were accounted for by analysis.

Example 3

Recovery of aluminium from reaction product

A solution of a mixture of 2 molar hydrochloric acid, 2 molar aluminium chloride AI Cl₃ · 6H₂0 containing 20% w/v glucose was reduced in volume from approximately 400 ml to approximately 250 mls by heating to 60°C under reduced pressure. 92 g of crystals formed and were removed by filtration. These crystals contained 9.7% w/w aluminium (86.8% AI Cl₃ · 6H₂O) and 7.1% w/w glucose.
Hydrochloric acid (10 M strength) was then added to samples of the remaining glucose syrup solution in various proportions to give 7.4% w/w, 13% w/w and 22% w/w final concentrations of hydrochloric acid. This caused more crystals to form in proportion to the concentration of hydrochloric acid. Thus:-

(a) 13% of the aluminium was recovered as crystals from the 7.4% w/w HCI sample;
(b) 23% of the aluminium was recovered as crystals from the 13% w/w HCI sample; and
(c) 39% of the aluminium was recovered as crystals from the 22% w/w HCI sample.

These crystals were composed of 5.6 to 5.9% w/w aluminium but still contained 5 to 10% w/w glucose.
The first stage of this separation process gave a yield of aluminium of 41 % while the second stage gave a maximum yield of a further 20% of aluminium leaving 1.6% w/w aluminium in the glucose syrup.
This recovery procedure is set out in Table 3.

Claims

1. A process for the solubilisation and hydrolysis of a glycosidically linked carbohydrate to produce soluble oligosaccharides and/or glucose, in which the carbohydrate is contacted with a mixture comprising an aqueous inorganic acid and a hydrated metal halide or a precursor of said halide, at a temperature within the range 50° to 100°C, a reaction product is separated and metal ions are recovered from the separated product characterised in that the metal halide is an aluminium halide and the inorganic acid has a concentration within the range 0.5 to 5 molar.

2. A process according to claim 1 for the solubilisation and hydrolysis of cellulose to produce a cellodextrin, cellotriose, cellobiose and/or glucose.

3. A process according to claim 1 forthe solubilisation and hydrolysis of starch to produce D-glucose or a mixture of sugars containing D-glucose.

4. A process according to any one of the preceding claims wherein the inorganic acid is hydrochloric acid.

5. A process according to any one of the preceding claims wherein the hydrated halide of aluminium or precursor of said halide is aluminium chloride hexahydrate AIC13. 6H₂0.

6. A process according to any one of the preceding claims wherein there is present, in addition to the hydrated halide of aluminium or precursor thereof, a halide of lithium, calcium and/or magnesium or a precursor of such a halide.

7. A process according to any one of the preceding claims wherein the carbohydrate is contacted with the mixture at a temperature in the range 50° to 90°C.

8. A process according to claim 7 wherein the temperature is in the range 65° to 75°C.

9. A process according to claim 6 wherein the mixture contains two metal halides, a hydrated halide of aluminium and a halide of lithium, calcium or magnesium, the relative proportions of the two halides being in the range 10:1 to 1:10 w/w.