Biocatalytic Process for Preparing Phenolic Resins Using Peroxidase or Oxidase Enzyme

The present invention relates to an improvement in a process for preparing phenolic resins through biocatalytic oxidation of phenols wherein the oxidation reaction is performed in an organic solvent and, more particularly, in an aqueous-organic solvent system.

Alberti and Klibanov, BIOLOGICAL DETOXICATION, Chapter 22, Peroxidase for Removal of Hazardous Aromatics from Industrial Wastewaters, (1982), discloses that phenols can be removed from wastewaters as high molecular weight polymers by the action of peroxidase enzymes. The disclosed method relies on the ability of peroxidase enzyme to catalyze, with hydrogen peroxide, the oxidation of a variety of phenols and aromatic amines. Phenolic and aromatic amine free radicals are generated, which diffuse from the active center of the enzyme into solution, and polymerize to polyaromatic products. These high molecular weight polymers are water-insoluble and can be readily separated by filtration.

An object of the present invention is to provide an improved process for preparing phenolic resins for commercial use.

Another object of the present invention is to provide an improved process for preparing phenolic resins in higher yields.

Still another object of the present invention is to provide a process for preparing phenolic resins which facilitates recovery of the peroxidase enzyme.

In accordance with the present invention, biocatalytic oxidation of the phenol is carried out in an organic solvent and, more particularly, in an organic-aqueous solvent medium. It has been found that the oxidative polymerization reaction proceeds in higher yields and is capable of providing a higher molecular weight product when it is performed in these solvent systems.

One process in accordance with the present invention comprises preparing separate solutions of the phenol, enzyme, and peroxide, and mixing them. The phenol is typically dissolved in an organic solvent, and the enzyme and peroxide are typically dissolved in water. The organic-aqueous solvent system is formed by mixing the solutions. The solutions may be gradually added to a common reaction vessel, or the solutions of the phenol and the enzyme may be pre-mixed and the peroxide, neat or in solution, gradually added thereto. Embodiments of the present invention are also envisioned in which the enzyme is provided on a solid support and the process is carried out on a batch or continuous basis. In any process it is important to limit the rate of addition of the peroxide since excess peroxide inhibits the reaction

The organic solvent may be water miscible or water immiscible. If the solvent is water miscible, the reaction proceeds in the solution phase. If the solvent is water immiscible, a dispersion is formed upon mixing the phenol and the enzyme solutions and the reaction proceeds at the interface. The resin is recovered from the solvent phase and the enzyme can be recovered from the aqueous phase and re-used.

Accordingly, the present invention provides a process for preparing a phenolic resin which comprises reacting a phenol with a peroxidase or an oxidase or an oxidase enzyme and a peroxide or an oxidase enzyme and oxygen in an organic solvent containing medium to generate phenolic radicals which react to form a phenolic resin.

More particularly, the present invention provides a process for preparing a phenolic resin which comprises preparing a solution of a phenol in a water miscible or a water immiscible solvent and an aqueous solution of an oxidase or peroxidase enzyme, mixing said solutions and adding oxygen if an oxidase enzyme is used or adding a peroxide if a peroxidase enzyme is used thereto, wherein said phenol reacts with said enzyme and said oxygen or peroxide and generates phenolic radicals and said radicals react to form a phenolic resin.

Other objects and advantages of the present invention will become apparent from the following description and the appended claims.

Peroxidase and oxidase enzymes are known in the art and are commercially available. The most typical example of a peroxidase enzyme useful in the present invention is horseradish peroxidase, but other peroxidases such as chloroperoxidase (and other haloperoxidases), lactoperoxidase, and bacterial peroxidases are also useful. In addition, oxidase enzymes such as fungul laccase and tyrosinase may be useful.

The amount of the enzyme used to make the phenolic resin will depend on its activity. The enzyme functions as a catalyst and is not consumed in the reaction. For commercially available enzymes, the enzyme can be reacted in an amount of about 10 milligrams to 5 grams per 100 grams phenol.

In accordance with the preferred embodiment of the invention, the enzyme is dissolved in water and added to a solution of the phenol. The enzyme solution concentration is not particularly critical. It typically ranges from about 10 to 365 mg per liter. It is generally desirable to prepare an enzyme solution at a concentration such that it can be added in an approximately equal volume to the solution of the phenol, but this may vary.

The oxidizing agent used in generating the free radicals is typically hydrogen peroxide, but other peroxides are useful. Examples of other potentially useful peroxides include methyl peroxide, ethyl peroxide, etc.

The peroxide is reacted in an amount of about 0.1 to 2.5 moles per 100 grams phenol and, more typically, about 0.1 to 0.5 moles per 100 grams phenol. Depending upon the nature of the oxidizing agent, it is reacted neat or as a solution. The preferred oxidizing agent, hydrogen peroxide, is dissolved in water. Its concentration may range from about 0.001 to 1.0 mol per liter.

The phenol can be reacted in a water-miscible or a water-immiscible solvent. Representative examples of useful water-immiscible solvents include hexane, trichloroethane, methyl ethyl ketone, ethyl acetate, and butanol. Examples of useful water-miscible solvents include ethanol, methanol, dioxane, tetrahydrofuran (THF), dimethyl formamide, methyl formate and acetone. The reaction is typically carried out at phenol concentrations of about 1 to 100 g per 100 ml solvent.

As indicated above, a number of different procedures may be used to react the phenol. Solutions of the phenol, enzyme, and peroxide may be individually prepared and metered into a reaction vessel, or solutions of the phenol and enzyme may be pre-mixed and the peroxide gradually added thereto. Alternatively, the enzyme and phenol may be dissolved in a common solvent and the peroxide added. Those skilled in the art will appreciate that a number of reaction/mixing sequences are useful provided that the peroxide is added at a controlled rate which is approximately equal to the rate at which it is consumed such that the concentration of the peroxide does not build to a level at which it undesirably inhibits the reaction.

The organic-aqueous system formed upon mixing the phenol, enzyme and peroxide may contain water and an organic solvent in a volumetric ratio (water:organic) in the range of about 1:10 to 10:1, more typically, 1:2 to 2:1, and most typically, about 1:1. The most preferred ratio will vary with the nature of the phenolic monomer(s) that is(are) polymerized.

The reaction of the phenol proceeds at room temperature, but temperatures of about 0.degree. to 40.degree. C. can be used. The enzymes are temperature sensitive and can lose their activity if the reaction temperature becomes too high. For example, temperatures in excess of about 60.degree. C. render horseradish peroxidase inactive. However, some latitude exists, depending upon the solvent system which is used. Certain solvents can stabilize the enzyme and thereby permit the use of higher temperatures. There is evidence in the literature that temperatures up to 100.degree. C. may be useful if the enzyme is immobilized or if appropriate solvents systems are used. See, for example, Zaks and Klibanov, "Enzymatic Catalysis in Organic Media at 100.degree. C.," Science, Vol. 224 pp 1249-1215, 15 June 1984.

The activity of the enzyme is pH dependent. The reaction is preferably carried out at a pH in the range of 4 to 12 and, more preferably, 4 to 9. Buffers can be used to maintain pH, but are not usually required. One example of a useful buffer is a potassium phosphate buffer.

A pH is selected at which the enzyme is highly active. This will vary with the nature of the enzyme and its source. The most preferred pH for Sigma Type I peroxidase (a product of Sigma Chemical Company) is about 6.0. Using this enzyme, deionized water can be used to form the solvent system.

While reference is herein made to the bulk oH of the reaction system, those skilled in the art will appreciate that it is the pH in the micro-environment of the enzyme that is critical. Thus, where the phenol is dissolved in a water immiscible solvent and the enzyme solution is dispersed in the solution of the phenol, it is the pH of the enzyme solution which is critical.

Phenolic resins prepared in accordance with the present invention are useful in a variety of applications depending on the nature of the phenol and the molecular weight of the resin. The resins are often mixtures of dimers, trimers, and very high molecular weight oligomers. Usually, the major part of the resin, i.e., greater than 50% by weight of the resin, is trimer or higher molecular weight compounds.

The molecular weight of the phenolic resin can be adjusted depending upon its particular end use. In one embodiment, the process of the present invention provides a phenolic resin which is useful as a developer in recording materials such as carbonless copy paper, heat-sensitive recording paper, electrothermographic recording paper and the like. The phenols used in developer resins are preferably para-substituted. The resins may range from about 500 to 5000 in molecular weight.

In another embodiment, the process of the present invention provides a phenolic resin which is useful as an adhesive. The phenols used in adhesives need not be para-substituted. The resins typically range from about 1000 to 15,000 in molecular weight but molecular weights up to at least 30,000 are attainable. Amoung other factors affecting molecular weight are solvent selection, phenol selection, and reaction conditions.

Phenols which are preferred for use in the present invention are represented by the Formula (I): ##STR1## wherein Y and Z are selected from the group consisting of a hydrogen atom, a halogen atom, an alkyl group, an alkoxy group, an aryl group, an allyl group, a phenylalkyl group, a --COOR group, a --NR.sup.1 R.sup.2 group, where R represents a hydrogen atom or a lower alkyl group, and R.sup.1 and R.sup.2 represent a hydrogen atom, an alkyl group, or a phenylalkyl group or Z in conjunction with the adjacent meta position forms a condensed benzene ring. Since polymerization proceeds via the ortho or para positions, when Y is at the ortho or para position, at least one of Y and Z must be a hydrogen atom or Z must form said condensed benzene ring. Y is preferably para to the phenolic hydroxyl group. Otherwise, the phenol adds as a terminal group as discussed below. At the para position, alkyl groups have a tendency to slow the reaction. The reaction appears to proceed best with a para-substituted aryl group.

A single phenol or a mixture of phenols may be used in the process of the present invention. In certain applications it may be desirable to produce phenolic resins having certain terminal groups. This can be accomplished by reacting certain phenols in which the Y substituent is in the para position and Y and Z are other than hydrogen or a condensed benzene ring. In this case the resin contains the Z substituent as a terminal group. Consequently, when the phenol is para-substituted, the latter Z-substituted phenols are typically used in a mixture with other phenols in which at least one of Y and Z is a hydrogen atom or Z is a condensed ring to provide copolymers. When the para position is unsubstituted, polymerization proceeds via the ortho and/or para position and the latter Z-substituted phenols can be incorporated mid-chain.

The alkyl group represented by Y and Z may contain up to 10 carbon atoms and include such alkyl groups as t-butyl, n-butyl, octyl, nonyl, etc. When R,R.sub.1, and R.sub.2 represent an alkyl group, it is typically a lower alkyl group having 1 to 4 carbon atoms.

Representative examples of alkoxy groups for Y and/or Z have 1 to 10 carbon atoms and include methoxy and ethoxy. When Y or Z is an aryl group, it is typically a phenyl group or substituted phenyl group such as a halogen-substituted phenyl group, an alkyl-substituted phenyl or a phenol group such as a 4'-phenol group.

Examples of a halogen atom include fluorine, chlorine, bromine and iodine.

Representative examples of phenylalkyl groups include benzyl, isopropylidene phenyl, butylidene phenyl, isopropylidene-4'-phenol, and butylidene-4'-phenol.

Specific examples of phenols which can be polymerized in accordance with the process of the present invention are phenol 4-t-butylphenol, 4-n-butylphenol, 4-ethylphenol, cresol, p-phenylphenol, p-octylphenol, p-nonylphenol, p-hydroxybenzoic acid, 4-hydroxynaphthoic acid, p,p'-biphenol, 4-aminosalicylic acid, salicylic acid, methyl salicylate, ethyl salicylate, 4,4'-isopropylidenediphenol, ethyl 4-hydroxybenzoate, etc.

In one embodiment, a phenolic developer resin capable of reacting with an electron-donating color precursor and producing a visible image is represented by the formula (II): ##STR2## where n is greater than 2, the phenolic units of the resin are directly bonded to one another through positions ortho or para to the hydroxyl group, Y is not hydrogen and is present at a position meta or para (preferably para) to the hydroxyl group.

In accordance with another embodiment, the phenolic developer resin is represented by the formula (III): ##STR3## where n, Y, and Z are defined as in formula (II).

The phenolic resins can be homopolymers or copolymers, i.e., the individual Y or Z groups in a given phenolic developer resin may be the same or different and the Y groups may be located at different positions in accordance with the formula (II).

The phenolic developer resins may be metal-modified in a matter analogous to novolak developer resins to improve their reaction with color precursors and thereby improve the density and fastness of the image. For example, the phenolic developer resins can be modified by reaction with a salt of a metal selected from the group consisting of copper, zinc, cadmium, aluminum, indium, tin, chromium, cobalt, and nickel.

This modification can be made in an otherwise known manner. One method is by mixing and melting the resin with an alkanoate salt such as zinc propionate, zinc acetate, or zinc formate in the presence of an ammonium compound such as ammonium carbonate or ammonium acetate. The practice described in U.S. Pat. No. 4,173,684 can also be used.

The zinc-modified phenolic developer resins can also be formed by reacting zinc oxide or zinc carbonate and ammonium benzoate or ammonium formate with the resins in a manner analogous to the teachings in U.S. Pat. Nos. 4,165,102 and 4,165,103. Alternatively, the zinc-modified phenolic developer resins can be prepared by reaction with zinc chloride as shown in the examples below.

The metal content of the metal-modified phenolic developer resins should be more than 0.5 percent by weight and may range up to 15 percent by weight. Usually, a range of about 1.5 to 5 percent by weight is used.

In addition, to chemically modifying the phenolic developer resins as described above, other means conventionally used in the art to improve the developing ability of phenolic developer resins, can be used in conjunction with the phenolic developer resins of the present invention. For example, acidic metal salts can be incorporated into coatings of the phenolic developer resins as described in U.S. Pat. Nos. 3,516,845 and 3,723,156. The phenolic developer resins of the present invention can also be used in combination with other phenolic developer resins or compounds and need not be used alone.

Recording materials utilizing phenolic developer resins to produce colored images from colorless or substantially colorless materials are well known. Specific examples of such recording materials include pressure-sensitive carbonless copying paper, heat-sensitive recording paper, electrothermographic recording paper, and the like. They are described in more detail in U.S Pat. Nos. 2,712,507; 2,730,456; 2,730,457; 3,418,250; 3,432,327; 3,981,821; 3,993,831; 3,996,156; 3,996,405 and 4,000,087, etc. A photographic material has been developed which utilizes this method for forming colored images. See, for example, U.S. Pat. Nos. 4,399,209 and 4,440,846 to The Mead Corporation.

Recording materials can be prepared in a conventional manner. To provide a developer sheet, the phenolic developer resin may be dissolved in an appropriate solvent (typically acetone) and applied to the surface of the paper by blade or roll coating or the like. Alternatively, the developer resin may be used in the form of a resin grind analogous to the resin grinds described in U.S. Pat. No. 3,924,027 to Saito et al. For example, the resin may be pulverized and mixed with an organic high molecular compound such as starch or styrene-butadiene latex. This mixture is dispersed in water or a solvent that does not readily dissolve the phenolic developer resin or the high molecular compound and coated on an appropriate support.

The developer resin is usually applied in an amount of about 0.2 to 0.4 lbs. of resin/1300 sq. ft. (solids).

Where a self-contained recording material is desired, a mixture of the phenolic developer resin and microcapules containing the developer, can be coated upon a support as one layer or the developer and the microcapsules, can be applied in separate layers. For the preparation of photosensitive recording materials, see U.S. Pat. Nos. 4,399,209 and 4,440,846 which are incorporated herein by reference.

In addition to being useful as developer resins and as adhesives, the phenolic resins of the present invention are useful in other applications. In particular the lack of a methylene bridge imparts advantageous properties to the resins as counterparts. The resins of the present invention should exhibit greater resistance to photolytic and thermal degradation, greater rigidity and greater conductivity making the resins potentially attractive for plasma resistance in photoresists, as conductive polymers, antioxidants for plastics, rubbers and the like, and as molding materials for high temperature applications. The higher density of functional hydroxy groups is being investigated for use in epoxy resin systems where higher crosslink densities should yield higher thermal deformation temperatures.

The process of the present invention also may be useful in preparing lower molecular weight phenolic compounds such as dimers or trimers and is presently being investigated for this purpose. Accordingly, in formula (II) and (III) above, the process should be useful in preparing compounds for which n is 1 or 2 as well as higher molecular weight compounds in which n is greater than 2.

The invention is illustrated in more detail by the following non-limiting examples.

30 mg of Sigma Type I horseradish peroxidase dissolved in 30 ml of 0.1 N potassium phosphate buffer, pH 9.0, 1.5 g of p-phenylphenol dissolved in 40 ml of ethanol and 12 ml of 3% hydrogen peroxide were each added to a dropping funnel. The dropping funnels were connected to a three-necked 300 ml round-bottom flask containing 100 ml of the phosphate buffer. While stirring constantly, the reagents were added drop-wise into the reaction flask. The reaction was allowed to proceed for ten minutes after the addition of the reagents. The product was collected by filtration and examined by thin layer chromatography.

300 milligrams of Sigma Type I horseradish peroxidase dissolved in 200 ml of 0.1 N potassium phosphate buffer, pH 9.0 and 8.3 g of p-phenylphenol with 6.7 g of salicylic acid dissolved in 200 ml of ethanol were each added to a dropping funnel. The dropping funnels were connected to a two-liter three-necked round-bottom flask containing 30 ml of 3% hydrogen peroxide dissolved in one liter of phosphate buffer. While stirring constantly, the reagents were added drop-wise to the reaction flask. The reaction was allowed to proceed for 15 mins. after the addition of the reagents. The product was collected by filtration and examined by thin layer chromatography.

The above product was air dried and re-dissolved in 100 ml ethanol containing 18.5 g of 1 N sodium hydroxide. To this, a solution of 100 ml water, 2 g zinc chloride and 1 ml of concentrated hydrochloric acid was added drop-wise. The pH was readjusted with 1 N sodium hydroxide until a precipitate formed and the product was collected by filtration.

30 milligrams of Sigma Type I horseradish peroxidase dissolved in 20 ml of 0.1 N potassium phosphate buffer, pH 9.0 and 1.5 g of 1-naphthol dissolved in 20 ml of ethanol were each added to a dropping funnel. The dropping funnels were connected to a 300 ml three-necked roundbottom flask containing 3 ml of 3% hydrogen peroxide dissolved in 100 ml of the phosphate buffer. While stirring constantly, the reagents were added drop-wise to the reaction flask. The reaction was allowed to proceed for 10 mins. after the addition of the reagents. The product was collected by filtration and analyzed by thin layer chromatography.

30 milligrams of Sigma Type I horseradish peroxidase dissolved in 30 ml of 0.1 N potassium phosphate buffer, pH 9.0, 1.2 g of p-phenylphenol with 0.3 g of 4-aminosalicylic acid dissolved in 40 ml of ethanol and 12 ml of 3% hydrogen peroxide were each added to a drooping funnel. The dropping funnels were connected to a 300 ml three-necked round-bottom flask containing 100 ml of phosphate buffer. While stirring constantly, the reagents were added drop-wise to the reaction flask. The reaction was allowed to proceed for 15 mins. after the addition of the reagents. The product was collected by filtration and examined by thin layer chromatography.

30 milligrams of Sigma Type I horseradish peroxidase dissolved in 30 ml of 0.1 N potassium phosphate buffer, pH 9.0 and 12 ml of 3% hydrogen peroxide were each added to a dropping funnel. The dropping funnels were connected to a 500 ml three-necked round-bottom flask containing 6.67 g of bisphenol A dissolved in 100 ml ethanol and 174 ml of the phosphate buffer. While stirring constantly, the reagents were added drop-wise to the reaction flask. The reaction was allowed to proceed for 30 mins. after the addition of the reagents. The viscous organic phase containing product was collected from a separatory funnel and was examined by thin layer chromatography.

10 g of p-phenylphenol with five grams of ethyl salicylate dissolved in 400 ml of ethanol, 120 ml of 3% hydrogen peroxide dissolved in 80 ml of 0.1 N potassium phosphate buffer, pH 9.0 and 120 mg of Sigma Type I horseradish peroxidase dissolved in 200 ml of the phosphate buffer were each added to Erlenmeyer flasks. While stirring constantly, the reagents were added drop-wise to a two-liter round-bottom flask using a peristaltic pump.

The reaction was allowed to proceed for 20 mins. after the addition of the reagents. The product was collected by filtration and examined by thin layer chromatography.

5 grams of product dissolved in 100 ml of acetone condenser and containing 100 ml of an aqueous 50% potassium hydroxide solution. This solution was heated at reflux for 3 hrs. and allowed to cool to room temperature. Two grams of zinc chloride were added and allowed to stir for 10 mins. in the reaction flask. The reaction mixture was adjusted to pH 4 and poured into 1.2 liters of vigorously stirring cold water. The product was collected by filtration and examined by thin layer chromatography.

7 milligrams of Sigma Type I horseradish peroxidase dissolved in 80 ml of 0.05 N potassium phosphate buffer, pH 5.0 was added to a 250 ml round-bottom flask containing 5 g of 4-t-butylphenol and 5 g of p-phenylphenol dissolved in 20 ml of ethyl acetate. While stirring constantly, 135 ml of 0.8% hydrogen peroxide was added dropwise to the reaction flask. The reaction was allowed to proceed for 20 mins. after the addition of the peroxide. The organic phase containing product was collected, concentrated on a steam bath and examined by thin layer chromatography.

7 milligrams of Sigma Type I horseradish peroxidase dissolved in 80 ml of 0.05 N potassium phosphate buffer, pH 5.0 was added to a 250 ml round-bottom flask containing 8 g of 4-t-butylphenol and 2 g of p-phenylphenol dissolved in 20 ml of ethyl acetate. While stirring constantly, 137 ml of 0.8% hydrogen peroxide was added dropwise to the reaction flask. The reaction was allowed to proceed for 20 mins. after the addition of the peroxide. The organic phase containing product was collected, concentrated on a steam bath and examined by thin layer chromatography.

7 milligrams of Sigma Type I horseradish peroxidase dissolved in 80 ml of 0.05 N potassium phosphate buffer, pH 5.0 was added to a 150 ml round-bottom flask containing 8 g of bisphenol A and 2 g of p-phenylphenol dissolved in 20 ml of ethyl acetate. While stirring constantly, 127 ml of 0.6% hydrogen peroxide was added dropwise to the reaction flask. The reaction was allowed to proceed for 20 mins. after the addition of the peroxide. The organic phase containing product was collected, concentrated on a steam bath and examined by thin layer chromatography. This resin was also zincated by a procedure described in U.S. Pat. No. 4,025,490.

The ability of the resins to develop leuco dyes was tested by gravure printing a 3% solution of the resin on a strip of paper, spraying with CF detector, and determining the optical density on a MacBeth densitometer. Table I compares the optical densities of the above-mentioned experimental resins.

50 grams of bisphenol A were dissolved in 70 ml of methanol and 30 ml of ethyl acetate. 60 milligrams of horseradish peroxidase were dissolved in 200 ml of deionized water. Both solutions were added to a one liter, three-necked round bottom flask and stirred at 250 rpm's. 250 ml of a 3% hydrogen peroxide solution were added over an approximately 4 hour period. Upon completion of the reaction, the aqueous and organic phases were allowed to separate and the product recovered by evaporating the solvent. GPC analysis shows approximately 50 % trimer/tetramer, 15% pentamer/hexamer, 30% heptamer and higher and 5% lower molecular weight material.

One kilogram of bisphenol A was dissolved in 1.4 liters of methanol and 600 ml of ethyl acetate. 500 milligrams of horseradish peroxidase were dissolved in 2 liters of deionized water. Both solutions were added to a 12 liter 3-necked round bottom flask and stirred at 250 rpm's. One liter of 15% hydrogen peroxide was added over an approximately 8 hour period. Upon completion of the reaction, the aqueous and organic phases were allowed to separate and the product recovered by evaporating the solvent. GPC analysis shows approximately 60% oligomer with Mn=1100, 25% dimer and 15% monomer.

Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.