JP2007006827A - Method for synthesis of lignin-polyester copolymer using enzyme - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for the effective utilization of lignin and the synthesis of a biodegradable polyester copolymer. <P>SOLUTION: A lignin-polyester copolymer is produced by polymerizing lignin and a cyclic ester in the presence of an electron acceptor and a phenol oxidase. Preferably, the electron acceptor is a peroxide, the phenol oxidase is a peroxidase, especially horseradish peroxidase, and the cyclic ester is a lactide or ε-caprolactone. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to effective utilization of lignin. More specifically, the present invention relates to a method of synthesizing a useful copolymer by reacting lignin and a cyclic ester.

  In recent years, interest in environmentally friendly materials has increased, and as one of them, great expectations are placed on biomass, which is a renewable resource for preserving the global environment. Biomass is defined as "all organic substances produced by conversion of solar energy by photosynthesis". That is, it can be said that biomass is produced from carbon dioxide in the atmosphere. Therefore, using biomass does not increase the amount of carbon dioxide emission on the earth, and therefore it is less likely to destroy the environment.

  Most of the biomass on the earth is woody biomass. This woody biomass can be mainly classified into cellulose, hemicellulose, and lignin. Among these, lignin occupies about 20 to 30% of the wood material, but is discharged as a waste liquid in a pulp manufacturing process in a paper mill. Conventionally, the lignin discharged in this way could not be used as a material, and most of it was directly burned as a heat source in a factory or the like. However, since the amount of emission is enormous, effective utilization of new lignin is expected from the viewpoint of effective utilization of natural resources.

  Lignin is a kind of polymer constituting a plant body, and has a complex structure having an aromatic ring structure in the structure. Such lignin is characterized by having extremely superior heat resistance compared to other natural polymers and biodegradable plastics. However, on the other hand, it is rigid and brittle, and as it is, it is difficult to mold it on a plate or film. Although chemical modification of lignin to improve molding processability is also being studied, lignin is resistant to various chemical reactions, and in order to change its quality, environmentally intensive reaction conditions are required. There are many cases. For this reason, there is still room for study on effective use of lignin.

Patent Document 1 discusses the effective use of such lignin. There is disclosed a method for obtaining a polymer by polymerizing lignin and an organic compound such as glucose, vanillic acid, sorbit, and acrylamide. However, these are merely to obtain a polymer, and there is still room for improvement in consideration of the environment.
JP-A-8-505780

  According to the present invention, lignin, which has been difficult to use effectively in the past, can be converted into a material that is friendly to the environment and easy to use effectively by an environment-friendly method.

  A method for synthesizing a lignin-polyester copolymer according to the present invention is characterized in that a lignin-polyester copolymer is formed by polymerizing a lignin and a cyclic ester in the presence of a peroxide and a peroxidase. It is.

  The lignin-polyester copolymer according to the present invention is synthesized by the above-described method.

  According to the present invention, lignin can be effectively used. Specifically, a lignin-polyester copolymer imparted with elasticity and moldability can be obtained by modifying lignin under environmentally friendly reaction conditions. This lignin-polyester copolymer has heat resistance and chemical reaction resistance derived from lignin, and also has biodegradability.

Lignin that can be used in the present invention is a main component of a plant such as wood, and is a network polymer compound formed by condensation of structural units having a skeleton of phenylpropane. Here, the structural unit specifically has the following structure.

  The structure of lignin is complex and has various molecular weights, and its structure is difficult to identify, but generally the above structural units are bonded to each other to form a network polymer compound. Further, depending on the type of plant body, the constituent ratio of the structural unit body and the overall molecular weight change. In the present invention, any lignin can be used.

  The lignin is classified into an intermediate layer lignin having an action of adhering plant cells to each other and a cell membrane lignin contained in the cell membrane, and any lignin can be used in the present invention. In addition, lignin as it is in the plant body is called protolignin. In addition to protolignin, there are various isolated lignins obtained by separating protolignin with inorganic or organic reagents. You can also.

  Moreover, although lignin is marketed as a chemical reagent, it can also be utilized. Further, it can be purified prior to the reaction. As a purification method, any method such as washing with water or ion exchange can be used.

  The cyclic ester that can be used in the present invention has a cyclic structure and has a —C (O) —O— bond in the ring. Examples of such cyclic esters include cyclic monoesters and cyclic diesters. Preferred cyclic esters are ε-caprolactone and lactide. These are preferable because they give a biodegradable polyester by ring-opening polymerization, and an environmentally friendly material can be obtained.

In the present invention, the above lignin is oxidized to react with a cyclic ester. This oxidation reaction proceeds by the coexisting phenol oxidase and the electron acceptor. In this invention, arbitrary phenol oxidase can be used in the range which does not impair the effect of this invention. Such enzymes include laccase and peroxidase. Various peroxidases are known, but particularly preferred in the present invention is horseradish peroxidaze (hereinafter sometimes referred to as HRP). This peroxidase acts on a hydroxyl group bonded to the aromatic ring of lignin in the presence of an electron acceptor, particularly a peroxide, to cause its oxidation reaction. In this reaction, after oxidizing lignin, peroxidase returns to the initial state, and thus acts as a catalyst (enzyme). An example of this reaction mode is as follows.

  As an example, lignin having only a hydroxyl group in an aromatic ring, hydrogen peroxide as a peroxide, and HRP as a peroxidase are shown, but similar reactions occur with other lignin structures and peroxidases. That is, first, peroxidase is oxidized by peroxide, and the oxidized peroxidase oxidizes the hydroxyl group bonded to the aromatic ring of lignin.

  When laccase is used, oxygen coexisting in the reaction system can be contributed as an electron acceptor.

A method for synthesizing a copolymer according to the present invention comprises the step of converting a lignin and a cyclic ester into a peroxide and a peroxidase selected from, for example, hydrogen peroxide or an organic peroxide in the presence of an electron acceptor and a phenol oxidase. In the presence of By this reaction, the hydroxyl group bonded to the aromatic ring of lignin is oxidized, and the cyclic ester is polymerized using the hydroxyl group as a base point to form a lignin-polyester copolymer. An example of the reaction is as follows.

  The above formula shows an example, and lignin may have various substituents. n is a number indicating the degree of polymerization and can be any number of 1 or more.

  According to the present invention, it is possible to synthesize a lignin-polyester copolymer in which a polyester is bonded to a lignin hydroxyl group.

  The reaction is generally performed in a solvent. Examples of the solvent that can be used include water, hydrocarbons, alcohols, esters, ethers, ketones, and mixed solvents thereof. These can be appropriately selected from the solubility of lignin and cyclic esters, the reaction rate of the polymerization reaction, and the like. Of these, a mixed solvent of ketones such as acetone or cyclic ethers such as dioxane and water is preferable.

  In addition, since the polymerization reaction in the present invention is performed by an enzyme such as peroxidase, the pH of the reaction mixture is preferably controlled. The pH can be controlled by any method. Specifically, there are a method of controlling by adding acid or alkali while measuring pH, a method of adding a buffer, and the like. In particular, the method using a buffer solution is preferable because a special device for controlling pH is unnecessary. The pH of the reaction mixture varies depending on the raw material, solvent, temperature, required copolymer properties, etc., but generally the pH is 5.0 to 9.0, and is particularly controlled to 6.0 to 8.0. It is preferred that

  The reaction temperature and reaction time also vary depending on the raw material, solvent, temperature, or desired properties of the copolymer, etc., but generally 20 to 40 ° C., preferably 25 to 30 ° C., for 0.5 to 72 hours, Preferably it is 0.5 to 24 hours. In the present invention, it is not particularly necessary to raise the temperature, and the reaction can be performed near room temperature without special temperature control.

  As the reaction proceeds, the produced copolymer precipitates due to its low solubility. For this reason, the copolymer can be recovered as a precipitate. The recovered copolymer can be purified as necessary to obtain the desired lignin-polyester copolymer.

  The obtained lignin-polyester copolymer is not constant depending on the molecular weight and structure of the lignin used as a raw material and the polymerization reaction. For example, a copolymer having a weight average molecular weight Mw of 30,000 to 120,000 is synthesized. Can do. Since the resulting copolymer is composed of lignin, which is a natural polymer, and biodegradable polyester, it has a low environmental impact. Furthermore, it combines the heat resistance and chemical reaction resistance derived from lignin and the processability provided by the polyester portion.

  The present invention will be described below with examples.

  1 g of alkaline lignin (manufactured by Aldrich, Mw: 60,000, Mn: 10,000) was dissolved in 50 ml of a saturated aqueous sodium chloride solution, 50 ml of 1N sulfuric acid was added, and the mixture was allowed to stand at room temperature for 12 hours. The resulting precipitate was separated by centrifugation, washed with water three times, and further dried in a window oven set at 105 ° C. for 24 hours to obtain purified alkali lignin.

  To 100 mg of purified alkaline lignin, 50 mg of lactide, 600 μl of 0.1 M acetate buffer (pH 5.0) and 1.8 ml of water were added and stirred, and heated to 40 ° C. to dissolve. Further, 7 ml of dioxane was added to make uniform, and then 500 μl of an HRP solution (1 mg / ml) and 100 μl of a hydrogen peroxide solution (1 mM) were added and stirred at room temperature for 1 hour.

  The purified alkali lignin as a raw material and the reaction product were measured for confirming the progress of the polymerization reaction using gel filtration chromatography. For the measurement, a combination of an LC-VP GPC system (manufactured by Shimadzu Corporation) and a photodiode array ultraviolet-visible spectrophotometer detector (SPD-M10A) was used. As column, Shodex Asahipak GF-7MHQ (manufactured by Showa Denko KK) was used, and a water / methanol mixture (95: 5, v / v) was used as an eluent. The flow rate was 0.8 ml / min and the column temperature was 40 ° C. The sample was injected into a syringe, filtered using a polytetrafluoroethylene membrane filter aqueous solvent (Millipore) having a pore size of 0.45 μm, and 20 μl was injected into a GPC apparatus.

  When the elution time was measured for each of the purified alkali lignin and the reaction product as a raw material, a peak was observed at 17.7 minutes for the alkaline lignin as a raw material, whereas the reaction product was 16.9 minutes. A peak was observed at 13.3 minutes. The peak at 16.9 minutes in the reaction product corresponds to the raw material, and the peak at 13.3 minutes indicates that a component having a higher molecular weight is present, indicating that the polymerization reaction is proceeding. .

  In order to measure the molecular weight of the copolymer obtained as a result of the progress of the polymerization reaction, the sample was acetylated and then measured using gel filtration chromatography.

  The sample was acetylated by the following method.

  To 100 mg of sample, 3 ml of pyridine and 3 ml of acetic anhydride were added and allowed to dissolve. The standing time was 24 hours to 2 months. After dissolution, it was dropped into about 60 ml of ice water and left for 30 minutes. Thereafter, the mixture was extracted three times with chloroform using a separatory funnel, and the obtained organic phases were combined, washed once with water, dried to dryness with an evaporator, and then dried under reduced pressure in a desiccator.

The molecular weight of the obtained acetylated product was measured by GPC. For the measurement, an LC10A GPC system (manufactured by Shimadzu Corporation) combined with a differential refractometer (RID-10A) was used. The column used was a serial column of Shodex K-806M and K-802 (both manufactured by Showa Denko KK), and chloroform was used as the eluent. The flow rate was 0.8 ml / min and the column temperature was 40 ° C. The sample was dissolved in chloroform for high-performance liquid chromatography, injected into a syringe, and filtered using a polytetrafluoroethylene membrane filter aqueous solvent (Millipore) having a pore size of 0.45 μm. The injection amount into the GPC apparatus was 50 μl. As a standard, a standard curve of polystyrene standard MW = 1.3 × 10 ^{3 to} 3.15 × 10 ⁶ was prepared using Shodex Standard SM-105 (manufactured by Showa Denko KK), and relative molecular weight in terms of polystyrene of each sample. It was determined.

  In the purified alkali lignin as a raw material, a peak corresponding to Mw of about 60,000 is recognized, and in the reaction product, a peak corresponding to Mw of about 120,000 is also included in addition to a peak corresponding to Mn of about 60,000. Admitted.

Furthermore, ¹ H-NMR measurement was performed on each of the purified alkali lignin as a raw material and the reaction product. The purified alkaline lignin was dissolved in heavy water and a small amount of sodium hydroxide as a sample, and the reaction product was measured by dissolving in deuterated chloroform. For the measurement, DRX-300 (manufactured by BRUKER) was used. In the ¹ H-NMR spectrum of the reaction product, a peak was observed near 1.2 ppm, but not in the raw material. This peak is considered to belong to the proton of —CH ₃ in the polylactic acid, and it was found that the reaction product had a polylactic acid moiety formed by ring opening of lactide.

  Further, transmission infrared absorption spectrum measurement was performed for each of the purified alkali lignin as a raw material and the reaction product. For the measurement, JASCO FT-IR615 (manufactured by JASCO Corporation) was used. Each sample was sufficiently dried on phosphorus pentoxide in a desiccator, 100 mg of KBr was added to 1 mg of the sample, and the mixture was crushed with an agate crucible to give a pellet. The measurement was performed in a vacuum.

In the reaction product, a peak at 1717 cm ⁻¹ that was not observed in the raw material was observed. This peak is attributed to the carbonyl group, and the carbonyl group contained in the polylactic acid produced by the reaction was confirmed. In the spectrum of the raw material, a peak attributed to the hydroxyl group bonded to the aromatic ring of the lignin structural unit is observed near 3300 cm ⁻¹ , but this peak decreases in the reaction product, and the aromatic ring of the lignin structural unit is reduced. It was confirmed that the hydroxyl group bonded to was changed by the reaction, that is, it acted as a reactive group.

  The polymerization reaction was carried out by changing the reaction solvent in Example 1.

  To 100 mg of purified alkaline lignin obtained in the same manner as in Example 1, 50 mg of lactide, 1 ml of 0.1 M acetate buffer (pH 5.0) and 3.4 ml of water were added and stirred, and heated to 40 ° C. to dissolve. . Further, 5 ml of acetone was added to homogenize, 500 μl of HRP solution (1 mg / ml) and 100 μl of hydrogen peroxide solution (1 mM) were added, and the mixture was stirred at room temperature for 1 hour.

The obtained reaction product was subjected to transmission infrared absorption spectrum measurement in the same manner as in Example 1. Compared with the spectrum of the raw material generated alkali lignin, in the spectrum of the reaction product, a new peak attributed to the carbonyl group appears in the vicinity of 1718 cm ⁻¹ , and it also belongs to the hydroxyl group bonded to the aromatic ring of the lignin structural unit. The peak around 3300 cm ⁻¹ was low.

  The polymerization reaction was carried out by changing the lactide of Example 1 to ε-caprolactone.

  To 100 mg of purified alkaline lignin obtained in the same manner as in Example 1, 40 mg of ε-caprolactone, 600 μl of 0.1 M acetate buffer (pH 5.0) and 1.8 ml of water were added and stirred to dissolve. Further, 7 ml of dioxane was added to make uniform, and then 500 μl of an HRP solution (1 mg / ml) and 100 μl of a hydrogen peroxide solution (1 mM) were added and stirred at room temperature for 1 hour.

  The obtained reaction product was subjected to transmission infrared absorption spectrum measurement in the same manner as in Example 1.

Compared with the spectrum of the generated alkali lignin of the raw material, in the spectrum of the reaction product, a new peak attributed to the carbonyl group appears in the vicinity of 1717 cm ⁻¹ , and also attributed to the hydroxyl group bonded to the aromatic ring of the lignin structural unit. The peak around 3300 cm ⁻¹ was low. From this spectrum, it was confirmed that a copolymer in which polylactic acid was bonded to the hydroxyl group bonded to the aromatic ring of the lignin structural unit was formed.

  The polymerization reaction was carried out by changing the reaction solvent in Example 3.

  To 100 mg of purified alkaline lignin obtained in the same manner as in Example 1, 40 mg of ε-caprolactone, 1 ml of 0.1 M acetate buffer (pH 5.0), and 3.4 ml of water were added and stirred to dissolve. Further, 5 ml of acetone was added to homogenize, 500 μl of HRP solution (1 mg / ml) and 100 μl of hydrogen peroxide solution (1 mM) were added, and the mixture was stirred at room temperature for 1 hour.

  The obtained reaction product was subjected to transmission infrared absorption spectrum measurement in the same manner as in Example 1.

Compared with the spectrum of the generated alkali lignin of the raw material, in the spectrum of the reaction product, a new peak attributed to the carbonyl group appears in the vicinity of 1717 cm ⁻¹ , and also attributed to the hydroxyl group bonded to the aromatic ring of the lignin structural unit. The peak around 3300 cm ⁻¹ was low. From this spectrum, it was confirmed that a copolymer in which polycaprolactone was bonded to the hydroxyl group bonded to the aromatic ring of the lignin structural unit was formed.

Claims (7)

  A method for synthesizing a lignin-polyester copolymer, comprising polymerizing a lignin and a cyclic ester in the presence of an electron acceptor and a phenol oxidase to form a lignin-polyester copolymer.   The method for synthesizing a lignin-polyester copolymer according to claim 1, wherein the electron acceptor is a peroxide or oxygen.   The method for synthesizing a lignin-polyester copolymer according to claim 1 or 2, wherein the phenol oxidase is peroxidase or laccase.   The method for synthesizing a lignin-polyester copolymer according to claim 3, wherein the peroxidase is horseradish peroxidase.   The method for synthesizing a lignin-polyester copolymer according to any one of claims 1 to 4, wherein the cyclic ester is lactide or ε-caprolactone.   The method for synthesizing a lignin-polyester copolymer according to any one of claims 1 to 5, wherein the polymerization reaction is carried out at 20 to 40 ° C.   A lignin-polyester copolymer synthesized by the method according to claim 1.