JP2008163053A - Method for controlling depolymerization of polysaccharides - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for controlling the depolymerization of a polysaccharide in a non-aqueous system, concretely a method for controlling the depolymerization of the polysaccharide with heat and/or an acid, to provide a polysaccharide material to which stability against depolymerization in a non-aqueous system is imparted, and to provide a method for producing the same. <P>SOLUTION: This method for producing the polysaccharide to which depolymerization resistance is imparted is characterized by treating a polysaccharide with boric acid or its derivative in an amount of 0.01 to 2.0 mol equivalent per monosaccharide unit contained in the polysaccharide, and a polysaccharide to which the depolymerization resistance is imparted is obtained by the method for producing the polysaccharide. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for suppressing depolymerization of polysaccharides (particularly cellulose).

  Cellulose is a main component of wood together with hemicellulose and lignin, and is a polysaccharide contained in wood by 30 to 50%. Its structure is a linear polymer in which D-glucopyranose is linked by β-1,4 and consists of a crystalline region in which the molecules are arranged in a fixed sequence and other amorphous regions. The molecular structure of the crystal region is shown in FIG. The cellulose molecular chain is rigid due to intramolecular hydrogen bonding between glucose units, and the molecular chain has a strong crystal structure due to the intermolecular interaction including hydrogen bonding. According to the crystal structure analysis of cellulose by the X-ray diffraction method, it is said that the structure of the crystal region is that linear molecules are regularly arranged in parallel. Due to such a strong crystal structure, cellulose has extremely high thermal, chemical and biological stability compared to other polysaccharides.

  As for thermal decomposition of cellulose, studies using thermogravimetric analysis (TGA) and suggested thermal analysis (DTA) are widely performed (Non-Patent Documents 1 to 4). FIG. 2 shows an example of TGA and DTA analysis of cellulose. Cellulose does not show a significant weight loss up to 300 ° C, but begins to lose weight at high temperatures above 300 ° C. A large endothermic peak is exhibited at around 350 ° C., and the weight is rapidly reduced, producing a small amount of carbonized residue. Therefore, it is considered that the main decomposition reaction of cellulose proceeds in a temperature range of 300 to 400 ° C.

  In the thermal decomposition of cellulose, it is known that depolymerization proceeds first. It is also known that this depolymerization is significantly accelerated by the addition of an acidic catalyst such as sulfuric acid. For example, cellulose added with sulfuric acid undergoes depolymerization even at a low temperature of about room temperature, giving monomer levoglucosan (1,6-anhydro-β-D-glucopyranose) and other low molecular weight products. .

By the way, boric acid (H ₃ BO ₃ ) is originally a weak Lewis acid, but acts as a very weak Bronsted acid in an aqueous solution as shown by the following formula [B (OH) ₃ + 2H ₂ O → H ₃ O ⁺ + B (OH) ₄ ⁻ _, pK _a = 9.2].

It is well known that cellulose forms borate ester complexes in this aqueous boric acid solution. Boric acid is said to promote dehydration and carbonization, and there are many studies as a flame retardant for cellulosic materials and wood. For cellulose, there are reports of a decrease in decomposition temperature, an increase in carbides, and inhibition of levoglucosan formation. (Non-Patent Documents 5 and 6). This is thought to be due to the fact that boric acid promotes carbonization of cellulose and forms a carbonized layer on the surface, thereby suppressing internal combustion, but the mechanism of action is not clear.
J. Polym. Chem., 6, 3217 (1968) Advances Carbohyd. Chem., 23, 419 (1968) Forest Chem., 16, 461 (1970) J. Anal. Appl. Pyrolysis, 8, 41 (1985) Int. Eng. Prod. Res. Dev., 21, 97 (1982) Thermochimica Acta, 193, 99 (1991)

  As mentioned above, cellulose has a strong crystal structure due to intermolecular interactions in which the molecular chain contains hydrogen bonds, and is thermally, chemically and biologically stable compared to other polysaccharides. Due to its extremely high properties, it is used in a wide range of applications such as paper, fibers, clothing, filters, films, granulating agents, reverse osmosis membranes, fiber reinforced plastic materials, and capacitor insulating materials. However, since it is unstable with respect to heat and acid, improvement in heat resistance and acid resistance is a problem.

  An object of the present invention is to provide a method for suppressing depolymerization of polysaccharides such as cellulose in a non-aqueous system, specifically, a method for suppressing depolymerization of polysaccharides by heat and / or acid. It is another object of the present invention to provide a polysaccharide material imparted with stability against depolymerization in a non-aqueous system and a method for producing the same.

  As a result of intensive studies in view of the above-mentioned problems, the present invention was conducted by adding a predetermined amount of boric acid to acidic sulfolane containing cellulose and performing heat treatment. It was found that the molecular weight reduction and carbonization due to transglycosylation of cellulose are remarkably suppressed. As a result of further research based on this knowledge, the present invention has been completed.

  That is, the present invention provides the following method for inhibiting the depolymerization of polysaccharides and polysaccharides imparted with depolymerization resistance.

  Item 1. A method for producing a polysaccharide imparted with depolymerization resistance, characterized in that the polysaccharide is treated with 0.01 to 2.0 molar equivalents of boric acid or a derivative thereof per monosaccharide unit contained therein.

  Item 2. A polysaccharide with anti-depolymerization resistance produced by the production method according to Item 1.

  Item 3. A method for treating a polysaccharide, comprising subjecting the polysaccharide to which anti-depolymerization resistance according to Item 2 is given to heat treatment and / or acid treatment at about 300 ° C. or less in a non-aqueous system.

  Item 4. A method for imparting depolymerization resistance to a polysaccharide, wherein the polysaccharide is treated with 0.01 to 2.0 molar equivalents of boric acid or a derivative thereof per monosaccharide unit contained therein.

  Item 5. A method for suppressing the depolymerization of a polysaccharide, wherein the polysaccharide is treated with 0.01 to 2.0 molar equivalents of boric acid or a derivative thereof per unit of monosaccharide contained therein, and heat treatment at about 300 ° C. or less in a non-aqueous system. And / or acid treatment.

  In the present invention, by adding a predetermined amount of boric acid or a derivative thereof to a polysaccharide (particularly a cellulose-containing material) in a non-aqueous system, depolymerization of the polysaccharide due to heat treatment or acid treatment can be remarkably suppressed.

  The present invention provides a polysaccharide in which depolymerization due to heat and acid is suppressed, and this comprises a polysaccharide (particularly a cellulose-containing material) at 0.01 to 2.0 per monosaccharide unit (particularly a glucose unit in cellulose) contained therein. It can be produced by treatment with a molar equivalent of boric acid or a derivative thereof. In the present specification, suppression of depolymerization may be referred to as “depolymerization resistance”.

  The polysaccharide typically includes a cellulose-containing material, but other than that, starch (amylose, amylopectin), hemicellulose (xylan, glucomannan, galactan), mannan, chitin, chitosan, and the like can be given. In particular, cellulose or a cellulose-containing material is suitable. The cellulose-containing material means a material containing cellulose. The form of cellulose may be any of powder, fiber, granule, film and the like.

Boric acid means H ₃ BO ₃ (orthoboric acid). Examples of derivatives of boric acid include organic boric acid compounds such as methyl boric acid, phenyl boric acid, 4-O-methylphenyl boric acid, and salts thereof, among which boric acid is preferred from the viewpoint of price.

  As a method of treating the polysaccharide with boric acid or a derivative thereof, the polysaccharide is treated with 0.01 to 2.0 molar equivalents of boric acid or a derivative thereof per monosaccharide unit contained therein. More specifically, the following methods can be mentioned.

  A polysaccharide is dispersed or suspended in an aprotic solvent, and boric acid or a derivative thereof is added and dissolved therein. The aprotic solvent is used to promote the reaction between boric acid and the hydroxyl group in the polysaccharide. Specific examples of the aprotic solvent include aprotic polar solvents such as sulfolane, dimethyl sulfoxide, N, N-dimethylformamide, and N-methylpyrrolidone. The amount of boric acid or its derivative added is 0.01 to 2.0 molar equivalents, preferably 0.5 to 1.0 molar equivalents per monosaccharide unit contained in the polysaccharide. With such an added amount, most of boric acid or a derivative thereof is consumed on the surface of the polysaccharide to form a hydroxyl group and an ester of the polysaccharide, and a boric acid ester film that is stable against heat and acid is formed. There is almost no excess boric acid or its derivative in the solvent.

  Or after adding polysaccharide to the aqueous solution containing a boric acid or its derivative (s), this is dried and the polysaccharide which impregnated boric acid or its derivative (s) is obtained. Also in this case, the addition amount of boric acid or a derivative thereof is 0.01 to 2.0 molar equivalents, preferably 0.5 to 1.0 molar equivalents per monosaccharide unit contained in the polysaccharide. The aqueous solution containing the polysaccharide may be usually dried at 100 to 120 ° C. for about 10 to 40 hours. By this drying treatment, most of boric acid or its derivatives is consumed for ester formation with hydroxyl groups on the surface of the polysaccharide.

  The number of moles of monosaccharide units contained in the polysaccharide can be usually determined by dividing the weight of the contained polysaccharide (for example, cellulose) by the molecular weight of the monosaccharide (for example, the molecular weight of glucose unit: 162). . As described above, the obtained monosaccharide unit is treated with 0.01 to 2.0 molar equivalents of boric acid or a derivative thereof as described above. Outside this range, the excellent depolymerization resistance of the polysaccharide is not exhibited. In this case, excess boric acid is present in the solvent or solid on the polysaccharide, which is thought to promote depolymerization.

  The polysaccharide treated with boric acid or a derivative thereof is stabilized with little progress of depolymerization even under heat treatment and / or acid treatment under non-aqueous conditions. As the heat treatment, when the treatment is performed at a temperature of about 300 ° C. or lower, depolymerization is effectively suppressed. Examples of the acid treatment include inorganic acids such as sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid and hydrobromic acid, and organic acids such as acetic acid and trifluoroacetic acid. Also in the case of acid treatment, depolymerization is effectively suppressed if the temperature is about 300 ° C. or lower.

  For example, as in Example 1, when a cellulose suspension of sulfolane to which boric acid and sulfuric acid are added is heat-treated at 200 ° C. for 6 minutes, 95% by weight or more of the cellulose is recovered.

  The heat treatment and / or acid treatment conditions are not particularly limited as long as they are non-aqueous conditions. For example, polysaccharides are dispersed or suspended in an aprotic solvent in which boric acid or a derivative thereof is dissolved. Examples include heating the product as it is, adding an acid, and adding an acid to the heat treatment. Alternatively, the polysaccharide impregnated with boric acid or its derivative obtained by drying is heat-treated as it is, an acid is added, and an acid is added and heat-treated. Furthermore, when the polysaccharide is mixed and kneaded with other materials (resins and the like), it can be heat-treated. The atmosphere may be either in air or in an inert gas, but it is preferable to treat in an inert gas such as nitrogen.

  According to the polysaccharide material of the present invention, depolymerization by heat treatment and / or acid treatment at about 300 ° C. or less can be effectively suppressed. Thereby, for example, when cellulose is used as a polysaccharide, the physical properties such as the original strength of cellulose are preferably maintained.

  In the present specification, depolymerization means a chemical reaction in which a polymer is decomposed into monomers by heat and acid. For example, when cellulose is used as the polysaccharide, it means a reaction in which cellulose is decomposed into low molecular monomers such as glucose, levoglucosan, levoglucosenone, furfural and the like.

  The polysaccharide obtained by the treatment of the present invention has significantly higher thermal stability and acid stability than the untreated polysaccharide. This is considered to be because the hydroxyl group of the polysaccharide interfacial molecule and boric acid form an ester, and the monosaccharide units and the polysaccharide molecules are firmly linked. For example, when cellulose is used as the polysaccharide, it is considered that the hydroxyl group of an interfacial molecule of crystalline cellulose and boric acid form an ester to firmly connect glucose units and cellulose molecules. Cellulose surface molecules are fixed from both sides by hydrogen bonds from the inside and borate esters from the outside, and the conformational change required for transglycosylation (chair type → half-chassis type) is remarkably suppressed. Therefore, it is considered that low molecular weight and thermal decomposition were suppressed. For example, a schematic diagram is shown in FIG.

  The resulting polysaccharide is used in a wide range of applications. For example, heat stabilization of cellulose and starch-based films used in separation membranes, reactors, etc., strength reduction suppression in FRP (fiber reinforced plastic) production using cellulose as reinforced fibers, and anti-coloring agents for cellulosic materials at high temperatures Can be used. Moreover, long-term stable preservation | save of the material (for example, paper, a film, etc.) containing cellulose is possible.

The present invention will be described in more detail with reference to examples, but is not limited thereto.
[Cellulose sample]
Cellulose powder (made by Toyo Roshi, 200-300 mesh) was used as the cellulose sample. The water content was 4.66% (absolutely dry standard), and the water content was corrected when calculating the yield.
[Basic heat treatment operations]
Method I: Heat treatment in flask (Figure 4)
A predetermined amount of cellulose, sulfolane containing a predetermined amount of sulfuric acid, and a stirrer were added to a 30 ml eggplant-shaped flask, and the mixture was stirred with a stirrer to suspend the cellulose. A three-way cock with a cooling tube and a nitrogen balloon is attached to the flask. After the inside of the flask and the cooling tube is replaced with nitrogen, it is heated by being immersed in an oil bath adjusted to a predetermined temperature under normal pressure while stirring with a stirrer. Went. After reacting for a certain period of time, it was air-cooled for 15 seconds with a dryer and cooled for 1 minute in ice water. The post-process which neutralized by adding 50 mg of sodium hydrogencarbonate was performed.

Method II: Heat treatment in ampoule (Figure 5)
Ampoule (length: 6.0cm) with one side closed, washed (1N HClaq 1.0ml x 2, distilled water 1.0ml x 5), a predetermined amount of cellulose was added to a dry Pyrex (registered trademark) glass tube, and the inside of the system was purged with nitrogen × Inner diameter 1.0 cm). The prepared ampule was suspended with a wire (0.45 mmφ), inserted from the upper part of the muffle furnace set to a predetermined temperature, and heat-treated for a predetermined time. As the heat treatment temperature, a value indicated by a mercury thermometer inserted from the upper part of the muffle furnace was used. The measurement part of the mercury thermometer was the same height as the sample part of the ampoule (6.5 cm from the muffle furnace upper side wall). After the heat treatment, it was air-cooled with a dryer for 30 seconds and cooled in ice water for 1 minute.
[Analysis of sulfolane soluble part after heat treatment]
(1) GPC analysis The sulfolane solution after heat treatment was diluted 4 times with THF and subjected to gel filtration chromatography (GPC analysis) to obtain a molecular weight distribution in RID (suggested refractive index detector). In addition, since it is difficult to analyze the molecular weight distribution due to the peak due to impurities in sulfolane (3,3 ', 4,4'-tetrahydroxybiphenyl), the sample subjected to GPC analysis is subjected to vacuum distillation twice to remove impurities. Purified sulfolane from which was removed was used. The analysis conditions are as follows: Apparatus: Shimadz LC-10A, column: Shodex KF801 + KF802, solvent: THF, flow rate: 1.0 ml / min, detector: suggested refractive index detector (RID), column chamber temperature: 40 ° C.
(2) ¹ H-NMR analysis To 1.0 ml of the obtained sulfolane-soluble part, 100 μl of 7.48 mg / ml p-dibromobenzene / sulfolane solution was added as an internal standard, of which 0.15 ml was added to 0.45 ml of deuterated chloroform and DMSO-d. Those dissolved in ₆ and filtered through cotton candy were subjected to proton nuclear magnetic resonance ( ¹ H-NMR) analysis. BruckerAC-400 (400 MHz) was used for the analysis.
(3) ¹¹ B-NMR analysis 0.45 ml of DMSO-d ₆ was added to 0.15 ml of the sulfolane solution after the heat treatment, and filtered using a cotton candy, and ¹¹ B nuclear magnetic resonance spectrum ( ¹¹ B-NMR was measured using an external standard method). )analyzed. BruckerAC-500 (500 MHz) was used for the analysis. As an external standard sample, 0.45 ml of DMSO-d ₆ was added to 0.15 ml of sulfolane containing boric acid (0.6 wt%) and filtered with cotton candy.
[Example 1] Heat treatment of cellulose in sulfolane in the presence of boric acid 10.0 mg of cellulose (62 µmol in terms of glucose unit) was suspended in 1.0 ml of sulfolane to which boric acid (0.9 wt%) and sulfuric acid (0.1 wt%) were added. The mixture was made turbid and heat-treated at 200 ° C. for 2 to 6 minutes by the method I described above. The amount of boric acid added to cellulose is 3.0 molar equivalents in terms of glucose units.

  Heat treatment of cellulose in sulfolane to which only sulfuric acid was added was also performed.

  The sulfolane-soluble part was directly used for each analysis, and the cellulose residue insoluble in sulfolane was filtered and washed with a Kiriyama funnel (saturated sodium hydrogen carbonate aqueous solution 1.0 ml × 2, distilled water 1.0 ml × 5), and then the entire filter paper was 105 It dried at 24 degreeC for 24 hours, and measured the weight.

  FIG. 6 collectively shows the sulfolane-soluble part and the residue insoluble in sulfolane after 6 minutes of heat treatment, the recovery rate of the residue, and the amount of levoglucosenone and furfurals produced in the sulfolane-soluble part.

  In the system to which only sulfuric acid was added, the cellulose was completely solubilized, and the sulfolane solution turned dark brown. In contrast, in the boric acid addition system, cellulose was not solubilized at all, 95% was recovered as a light brown residue, and the sulfolane solution remained colorless and transparent. Focusing on the amount of levoglucosenone and furfurals produced in the sulfolane-soluble part, the total yield of these systems reached 35.5% in the system containing only sulfuric acid, but the production was significantly suppressed to 0.5% in the system containing boric acid. You can see that.

Moreover, the GPC analysis result of a sulfolane soluble part is shown in FIG. In the sulfuric acid addition system, low molecular products such as levoglucosenone and furfural and peaks considered to be polymerized were confirmed, whereas in the boric acid addition system, none of them was confirmed. Also, in ¹ H-NMR analysis of the sulfolane-soluble part [sulfolane-soluble part / DMSO-d ₆ = 1/3 (v / v)], no product was observed, and the signal of unreacted boric acid Only (boric acid recovery rate: 81.2%) was confirmed. That is, it became clear that cellulose was not solubilized at all in the boric acid addition system.

  Since the amount of boric acid recovered is 81.2%, the amount of boric acid acting on the cellulose is 0.564 molar equivalent [3.0 × (1-0.812)] in terms of glucose unit of cellulose.

  As described above, in the sulfuric acid addition system, cellulose is quickly solubilized even at a low temperature of 200 ° C. due to the acid catalytic effect on the transglycosylation of protonic acid, to give levoglucosenone, furfurals and polymers thereof. In the system where boric acid is present, no solubilizate was observed despite the presence of protonic acid, and cellulose was recovered as an unreacted residue, so that cellulose was solubilized by the presence of boric acid. It became clear that it was remarkably stabilized.

The reason why boric acid suppresses solubilization of cellulose is considered to be because cellulose surface molecules are stabilized by the reaction of boric acid and the progress of solubilization is suppressed. The fact that stabilization with boric acid is an effect on cellulose surface molecules is supported by the fact that 80% or more of boric acid is not consumed and recovered without being reacted. The mechanism of action of boric acid on the cellulose surface is thought to be due to borate ester formation with hydroxyl groups on the cellulose surface.
[Example 2] Heat treatment of cellulose impregnated with boric acid (1) 200 → 340 ° C., 5 ° C./min A predetermined amount of boric acid and 150 μl of distilled water were added to 10.0 mg of cellulose (62 μmol in terms of glucose unit), A boric acid impregnated cellulose sample was prepared by drying at 105 ° C. for 24 hours. The amount of boric acid added was 0.2 molar equivalent (0.77 mg) or 3.0 molar equivalent (11.5 mg) with respect to the glucose unit of cellulose. The prepared sample was heat-treated in the ampoule from 180 ° C. to 340 ° C. at a heating rate of 5 ° C./min in the ampoule. When the temperature reached 200, 220, 240, 260, 280, 300, 320, 340 ° C., the ampule was taken out from the upper part of the muffle furnace for 10 seconds, and the state of the sample was photographed (FIG. 8).

  According to FIG. 8, in the non-added cellulose, yellowing starts from 240 to 260 ° C., and blackening (carbonization) proceeds rapidly from around 300 ° C. On the other hand, it can be seen that in the cellulose to which 0.2 molar equivalent of boric acid was added, carbonization was remarkably suppressed particularly in a temperature range of 300 ° C. or lower. On the other hand, in the cellulose added with 3.0 molar equivalents of boric acid, on the contrary, the carbonization temperature shifted to a low temperature, and an interesting result was obtained that blackening (carbonization) started from around 220 ° C.

Such behavior can be significantly explained by the action mechanism of boric acid described above. Also in the heat treatment of cellulose impregnated with boric acid, it is considered that the surface molecules of cellulose react with boric acid to form an ester as in the sulfolane system. On the other hand, although boric acid is weak, it acts as an acid. In general, in the thermal decomposition of cellulose, it is known that the thermal decomposition temperature is remarkably shifted to low temperature by adding acidic substances such as sulfuric acid, hydrochloric acid, and zinc chloride, which is consistent with the behavior of the system with 3.0 equivalents of boric acid added. To do. Boric acid is a very weak Bronsted acid in aqueous solution (pK _a = 9.2), originally it is weak Lewis acids. Boric acid is considered to act as a Lewis acid in pyrolysis, but boric acid molecules that react with cellulose surface molecules to form esters become tetravalent or lose their action as acids by being immobilized. . When the amount of boric acid added is small, most of the boric acid in the system is consumed by the reaction with cellulose surface molecules, so it does not act as an acid catalyst and stabilizes by the formation of boric acid esters. It is considered that the thermal decomposition was suppressed by this. On the other hand, when the amount of boric acid added is excessive, boric acid present in a high concentration while remaining unreacted acts as an acid catalyst, which has an effect of suppressing thermal decomposition due to boric acid ester formation, resulting in carbonization. it is conceivable that.
(2) 300 ° C
A boric acid-impregnated cellulose sample prepared from 10.0 mg of cellulose was heat-treated in an ampoule at 300 ° C. for 2 to 10 minutes by the method (II). The amount of boric acid added was 0 to 3.0 molar equivalents (0 to 11.5 mg) based on cellulose glucose units (FIG. 9).

  From FIG. 9, it can be seen that the carbonization suppression effect is greatest when the amount of boric acid added is 0.5 to 1.0 equivalent. It is also clear that when the amount added is small (0.05, 0.2 molar equivalent), the carbonization-inhibiting action during long-time heating is relatively small, but a relatively large stabilizing action is shown at the beginning of heating. Became.

It is a schematic diagram of the molecular structure of the crystalline region of cellulose. An example of thermogravimetric analysis (TGA) and suggestion thermal analysis (DTA) analysis of cellulose is shown. The mechanism of stabilization by boric acid for cellulose depolymerization is shown. 2 is a schematic diagram of an apparatus used in Method I. FIG. It is a schematic diagram of the apparatus used by the method II. 2 is a table showing the thermal decomposition behavior of cellulose in Example 1. 2 is a graph showing the GPC analysis results of the sulfolane-soluble part in Example 1. In Example 2, the time-lapse photograph when each sample of cellulose, cellulose + boric acid (0.2 mol eq) and cellulose + boric acid (3.0 mol eq) is heated to 200 to 340 ° C. is shown. In Example 2, the time-lapse photograph when each sample in which the content of boric acid was changed (0.05 to 3.0 mol eq) was treated at 300 ° C. for 0 to 10 minutes is shown.

Claims (5)

  A method for producing a polysaccharide imparted with depolymerization resistance, characterized in that the polysaccharide is treated with 0.01 to 2.0 molar equivalents of boric acid or a derivative thereof per monosaccharide unit contained therein.   A polysaccharide with anti-depolymerization resistance produced by the production method according to claim 1.   The polysaccharide treatment method according to claim 2, wherein the polysaccharide is imparted with anti-polymerization resistance by non-aqueous heat treatment and / or acid treatment at about 300 ° C. or less.   A method for imparting depolymerization resistance to a polysaccharide, wherein the polysaccharide is treated with 0.01 to 2.0 molar equivalents of boric acid or a derivative thereof per monosaccharide unit contained therein.   A method for suppressing the depolymerization of a polysaccharide, wherein the polysaccharide is treated with 0.01 to 2.0 molar equivalents of boric acid or a derivative thereof per unit of monosaccharide contained therein, and heat treatment at about 300 ° C. or less in a non-aqueous system. And / or acid treatment.

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