Method for producing low molecular weight aromatic lignin-derived compounds

Abstract

The present invention relates to a method for producing one or more low molecular weight aromatic lignin-derived compounds. The method preferably comprises providing lignocellulosic material, subjecting the lignocellulosic material to a pulping process, separating pulp to provide a substantially pulp-free process stream comprising a modified lignin-derived component, isolating the modified lignin-derived component, subjecting the isolated modified lignin-derived component to a decomposition step comprising oxidative cracking (cracking and oxidizing) or reducing under the influence of a catalyst or electro-oxidation, and subjecting the resulting products to an isolation step, to provide a low molecular weight aromatic lignin-derived compound. Said compound may be further modified, e.g. by annulation. The inventive method preferably comprises further oxidizing said compound to a redox active compound. Additionally, the present invention relates to compounds obtainable by the inventive method and to an assembly for carrying out the inventive method. Furthermore, the present invention refers to a method for providing an existing pulp and/or paper manufacturing plant with said assembly.

Claims

1. A method for producing at least one low molecular weight aromatic lignin-derived compound, the method comprising the steps of (A) providing optionally chopped lignocellulosic material; (B) subjecting the lignocellulosic material to a pulping process; (C) separating cellulose obtained in step (B) in a pulp separating step from the process stream obtainable from step (B), to provide a substantially cellulose-free process stream, wherein the process stream comprises modified lignin-derived components, hemicellulose and/or fragments thereof; wherein the process stream is provided as one single process stream or as at least two partial process streams; (D) isolating a fraction of modified lignin-derived components being comprised (D.1) in the process stream of step (C) or, (D.2) in at least one of the at least two partial process streams in step (C) from either of these process streams; (E) subjecting the fraction of modified lignin-derived components of step (D) to a chemical decomposition step, wherein the chemical decomposition step comprises (E.1) oxidative cracking (cracking and oxidizing) of the modified lignin components in the presence of a heterogenous or homogeneous catalyst comprising a metal ion or a metalloid component; (E.2) reductive cracking (cracking and reducing) of the modified lignin components in the presence of a heterogeneous or homogeneous catalyst comprising a metal ion or metalloid component; or (E.3) subjecting the modified lignin components to electro-oxidation in alkaline or acidic solution; (F) subjecting resulting modified lignin-derived products obtained in step (E) to an isolation and optionally purification step, wherein low molecular weight aromatic lignin-derived compounds are isolated from higher molecular weight aromatic lignin-derived components and/or other non-lignin-derived residual components and optionally purified, and wherein the at least one low molecular weight aromatic lignin-derived compound provided by step (F) comprises one aromatic ring and is further processed in a step (G), wherein said low molecular weight aromatic lignin-derived compound comprising one aromatic ring is subjected to an annulation reaction, wherein the annulation reaction product is a low molecular weight aromatic bi- or tricyclic annulated aromatic lignin-derived compound, wherein said compound is characterized by Formula (II), (III) or (IV) ##STR00042## wherein each of R.sup.2, R.sup.3, R.sup.5-R.sup.8 of Formula (II) is independently selected from the group consisting of hydrogen, hydroxy, carboxy, linear or branched, optionally substituted, C.sub.1-6 alkyl, linear or branched, optionally substituted, C.sub.1-6 alkenyl, linear or branched, optionally substituted, C.sub.1-6 alcohol, linear or branched, optionally substituted, C.sub.1-6 aminoalkyl, linear or branched, optionally substituted, C.sub.1-6 carboxyalkyl, linear or branched, optionally substituted, C.sub.1-6 alkoxy, linear or branched, optionally substituted, C.sub.1-6 aldehyde, ester, oxo and carbonyl; and optionally at least one of R.sup.2, R.sup.3, R.sup.5-R.sup.8 is hydroxy or C.sub.1-3 alkoxy, R.sup.1 and R.sup.4 of Formula (II) is/are selected from the group consisting of hydrogen, hydroxy, linear or branched, optionally substituted, C.sub.1-6 carboxyl, linear or branched, optionally substituted, C.sub.1-6 aldehyde, and linear or branched, optionally substituted, C.sub.1-6 alcohol, each of R.sup.1-R.sup.10 of Formula (III) is independently selected from the group consisting of hydrogen, hydroxy, carboxy, linear or branched, optionally substituted, C.sub.1-6 alkyl, linear or branched, optionally substituted, C.sub.1-6 alkenyl, linear or branched, optionally substituted, C.sub.1-6 alcohol, linear or branched, optionally substituted, C.sub.1-6 aminoalkyl, linear or branched, optionally substituted, C.sub.1-6 carboxyalkyl, linear or branched, optionally substituted, C.sub.1-6 alkoxy, linear or branched, optionally substituted, C.sub.1-6 aldehyde, ester, oxo and carbonyl; and optionally at least one of R.sup.2, R.sup.4, R.sup.5, R.sup.6 and R.sup.8 is hydroxy or C.sub.1-3 alkoxy, each of R.sup.2, R.sup.3 and R.sup.7-R.sup.10 of Formula (IV) is independently selected from the group consisting of hydrogen, hydroxy, carboxy, linear or branched, optionally substituted, C.sub.1-6 alkyl, linear or branched, optionally substituted, C.sub.1-6 alkenyl, linear or branched, optionally substituted, C.sub.1-6 alcohol, linear or branched, optionally substituted, C.sub.1-6 aminoalkyl, linear or branched, optionally substituted, C.sub.1-6 carboxyalkyl, linear or branched, optionally substituted, C.sub.1-6 alkoxy, linear or branched, optionally substituted, C.sub.1-6 aldehyde, ester, oxo and carbonyl; and optionally at least one of R.sup.2, R.sup.3 and R.sup.7-R.sup.10 is hydroxy or C.sub.1-3 alkoxy, and R.sup.1, R.sup.4, R.sup.5 and R.sup.6 of Formula (IV) is selected from the group consisting of hydrogen, hydroxy, linear or branched, optionally substituted C.sub.1-6 carboxyl, linear or branched, optionally substituted, C.sub.1-6 aldehyde, and linear or branched, optionally substituted, C.sub.1-6 alcohol.

2. The method of claim 1, wherein separation of step (C) is carried out by blowing, sieving, centrifugation, filtration and/or washing, or any combination thereof.

3. The method of claim 1, wherein isolation of step (D) is carried out by extraction, countercurrent flow, stripping, ion-exchange, precipitation by a di- or multivalent cation, precipitation by CO.sub.2 in acidic solution, filtration or any combination thereof.

4. The method of claim 1, wherein in step (E.2) reductive cracking (cracking and reducing) of the modified lignin-derived components is carried out in the presence of a reducing agent and a heterogeneous catalyst comprising a metal selected from nickel, platinum, palladium, ruthenium, rhenium and gold.

5. The method of claim 4, wherein in step (E.2) reductive cracking (cracking and reducing) of the modified lignin-derived components is carried out on the surface of a support material selected from the group consisting of active carbon, silica, titaniumoxide and aluminumoxide.

6. The method of claim 4, wherein the reducing agent is hydrogen or a hydrogen donating alcohol.

7. The method of claim 4, wherein the heterogeneous catalyst comprises a metal selected from the group consisting of nickel and ruthenium, and the heterogeneous catalyst is optionally supported on activated carbon.

8. The method of claim 7, wherein the metal is nickel and the reductive cracking is carried out in an alcoholic solvent.

9. The method of claim 7, wherein the metal is ruthenium and the reductive cracking is carried out in an organic solvent.

10. The method of claim 1, wherein in step (E.3) electrooxidation is carried out galvanostatically and optionally at a pH from pH 1 to 14.

11. The method of claim 1, wherein isolation step (D) and/or isolation step (F) comprises filtration and/or extraction.

12. The method of claim 11, wherein the filtration is carried out in a ultrafiltration and/or nanofiltration cell comprising at least one molecular weight cut-off unit, wherein the at least one molecular weight cut-off unit has a cut-off level of 0.5 kDa to 2 kDa for step (D), and 1 kDa to 1.5 kDa for step (F).

13. The method according to claim 1, wherein the annulation reaction is a Friedel Crafts acylation.

14. The method of claim 1, wherein the low molecular weight aromatic bi- or tricyclic annulated compound provided by step (G) is further modified in a step (H) by oxidizing the at least one low molecular weight aromatic bi- or tricyclic annulated compound in the presence of (i.) an oxidizing agent selected from the group consisting of H.sub.2O.sub.2, O.sub.2 and air, and (ii.) a heterogeneous or homogeneous catalyst optionally comprising a metal ion or a metalloid component, to provide at least one quinone and/or hydroquinone compound, wherein said compound corresponds in structure to any of Formula (VII), (VIII) and/or (IX): ##STR00043## wherein each of R.sup.1-R.sup.8 with regard to Formula (VII) and/or each of R.sup.1-R.sup.10 with regard to Formula (VII) and (IX) is independently selected from the group consisting of hydrogen, hydroxy, carboxy, linear or branched, optionally substituted, C.sub.1-6 alkyl, linear or branched, optionally substituted, C.sub.1-6 alkenyl, linear or branched, optionally substituted, C.sub.1-6 alcohol, linear or branched, optionally substituted, C.sub.1-6 aminoalkyl, linear or branched, optionally substituted, C.sub.1-6 carboxyalkyl, linear or branched, optionally substituted, C.sub.1-6 alkoxy, linear or branched, optionally substituted, C.sub.1-6 aldehyde, ester, oxo and carbonyl, wherein at least one of R.sup.8 and R.sup.5 or R.sup.1 and R.sup.4 of Formula (VII) are hydroxy or oxo, or at least one of R.sup.9 and R.sup.6, R.sup.10 and R.sup.5, or R.sup.1 and R.sup.4 of Formula (VIII) are hydroxy or oxo, or at least one of R.sup.10 and R.sup.7 or R.sup.1 and R.sup.4 of Formula (IX) are hydroxy or oxo.

15. The method of claim 14, wherein the at least one quinone and/or hydroquinone compound is anthraquinone.

16. The method of claim 14, wherein the at least one quinone and/or hydroquinone compound is ##STR00044##

17. The method of claim 1, wherein in step (E.1) oxidative cracking (cracking and oxidizing) of the modified lignin-derived components is carried out in the presence of an oxidizing agent and a heterogeneous or homogeneous catalyst comprising a. a metal ion selected from Co(II), Cu(II) and Fe(III); or b. a metalloid component selected from B(III), Si(IV) and Al(III); optionally at a temperature of 30-400° C.

18. The method of claim 1, wherein in step (E.1) the homogeneous catalyst is selected from the group consisting of a salt, a coordination complex, a zeolite and a polyoxometalate comprising a metal ion selected from Co(II), Cu(II) and Fe(III).

19. The method of claim 17, wherein step (B) is carried out according to the acidic sulfite process (B.2) of step (B) and/or wherein step (E) is carried out in the presence of a catalyst according to step (E.1).

20. The method of claim 1, wherein step (B) comprises a process selected from (B.1) a Kraft process comprising the steps of (a) optionally pre-steaming the optionally chopped lignocellulosic material, wherein the preferably chopped lignocellulosic material is wetted and preheated with steam, (b) adding the optionally chopped lignocellulosic material to an aqueous alkaline solution comprising a Kraft pulping reactive agent selected from the group consisting of a sulfide agent, a sulfhydryl agent, a polysulfide agent and a sulfate salt, (c) cooking the optionally chopped lignocellulosic material in said aqueous alkaline solution, and (d) optionally conducting sulfonation in the presence of a sulfuric acid solution or sulfur trioxide; or (B.2) a sulfite process comprising the steps of (a) optionally pre-steaming the optionally chopped lignocellulosic material, wherein the optionally chopped lignocellulosic material is wetted and preheated with steam, (b) adding optionally chopped lignocellulosic material to an aqueous, optionally acidic solution comprising a sulfite or bisulfite agent, and (c) cooking the optionally chopped lignocellulosic material in said aqueous optionally acidic solution.

21. The method of claim 20, wherein the pH of the aqueous alkaline solution in sub-step (b) of step (B.1) is >10 and/or the temperature of the aqueous alkaline solution in sub-step (b) of step (B.1) is less than 100° C.; or wherein the pH of the aqueous optionally acidic solution in sub-step (b) of step (B.2) is 1 to 5 and/or the temperature of the aqueous optionally acidic solution in sub-step (b) of step (B.2) is less than 100° C.

22. The method of claim 20, wherein the pH of the aqueous alkaline solution in sub-step (b) of step (B.1) is >10 and/or the temperature of the aqueous alkaline solution in sub-step (b) of step (B.1) is less than 100° C.; or wherein the pH of the aqueous, optionally acidic, solution in sub-step (b) of step (B.2) is 1 to 5 and/or the temperature of the aqueous, optionally acidic, solution in sub-step (b) of step (B.2) is less than 100° C.

23. The method of claim 20, wherein cooking in sub-step (c) of step (B.1) is carried out in a pressurized vessel for at least 2 hours and optionally at a temperature of at least 150° C.; or wherein cooking in sub-step (c) of step (B.2) is carried out in a pressurized vessel for at least 3 hours at a temperature of at least 120° C.

24. The method of claim 20, wherein sub-step (c) of the Kraft process (B.1) is carried out for 2 to 24 hours, or wherein sub-step (c) of the sulfite process (B.2) is carried out for 4 to 24 hours.

25. The method of claim 20, wherein Kraft process sub-step (c) of step (B.1) is carried out at a temperature of 150 to 190° C., or wherein sulfite process sub-step (c) of step (B.2) is carried out at a temperature of 120 to 170° C.

26. The method of claim 20, wherein sub-step (c) of step (B.1) or (B.2) is carried out at a pressure of at least 4 bar in the pressurized vessel.

27. The method of claim 20, wherein in sub-step (c) of step (B.1) or (B.2) is carried out in a batch mode or in a continuous mode.

Description

EXAMPLES

(1) The examples shown in the following are merely illustrative and shall describe the present invention in a further way. These examples shall not be construed to limit the present invention thereto. The following preparations and examples are given to enable those skilled in the art to more clearly understand and to practice the present invention. The present invention, however, is not limited in scope by the exemplified embodiments, which are intended as illustrations of single aspects of the invention only, and methods, which are functionally equivalent are within the scope of the invention. Indeed, various modifications of the invention in addition to those described herein will become readily apparent to those skilled in the art from the foregoing description, accompanying figures and the examples below. All such modifications fall within the scope of the appended claims.

Example 1: Preparation of Low Molecular Weight Aromatic Lignin-Derived Compounds by Cracking and Reduction by a Nickel Catalyst

(2) Reductive cracking (cracking and reducing) of a modified lignin-derived component according to step (E.2) of the inventive method may for example be carried out by means of a catalyst comprising nickel, e.g. supported on activated carbon (Ni/C). The catalysts are typically prepared by an incipient-wetness impregnation method and further treated by a carbothermal reduction method known in the art.

(3) Herein, nickel nitrate(II) hexahydrate [Ni(NO.sub.3).sub.2 6H.sub.2O] is used and optionally added into water in a beaker known in the art. The solution is then stirred, e.g. for at least 30 min, to prepare an impregnation stock solution. Activated carbon having a water absorption capacity of typically above 1.8 mL g.sup.−1 is added into the solution and the beaker may then covered by a culture dish to keep the sample wet for a prescribed time, preferably more than 12 h, more preferably 24 h. The sample is then dried at a temperature above 80° C., e.g. 120° C. overnight. The actual reduction is carried out in a container such as a preferably horizontal furnace in a flow of inert gas such as N.sub.2. The flow is, e.g., 10 mL min.sup.−1 or more, preferably 30 mL min.sup.−1 or more. The reduction temperature preferably reaches at least 400° C., preferably 450° C., e.g. over set time period such as at least 30 min, preferably at least 60 min. The temperature for conducting the reduction is maintained at 450° C. for at least 1 h, more preferably for at least 2 h. The Ni/SBA-15 catalysts are reduced at 550° C. for 2 h. The Ni/Al.sub.2O.sub.3 catalyst is reduced at 700° C. for 2 h. The metal loading for each nickel- and copper-based catalyst is 10% (w/w) relative to the support. Herein, birch sawdust serves as lignocellulosic material and is treated with the ethanol-benzene mixture (v/v ratio 1:2) for 12 h. The treated birch sawdust, solvent (m/v 1:20), and catalyst (w/w 20:1) are placed in an autoclave reactor. The reactor is sealed and purged with Ar 4 to 6 times to expel air. Then, the reducing reaction is conducted at 200° C. at a stirring speed of at least 300 rpm, preferably 500 rpm. When the desired reaction time (usually 2 to 10 h) is reached, the reactor is cooled to ambient temperature before sampling.

(4) Typically, the reaction generates 4-propylguaiacol and 4-propylsyringol as major products, together with minor alkene-substituted 4-propylguaiacol and 4-propylsyringol, as determined by standard gas chromatography. The compounds are isolated according to step (F), preferably by extraction.

Example 2: Preparation of Monomeric Aromatic Lignin-Derived Molecules from Lignosulfonate of a Sulfite Process by Electrooxidation

(5) Lignosulfonate is provided by step (D) according to the present invention. Thereof, a 1 M aqueous NaOH solution is prepared, comprising 1% (W/W) lignosulfonate. Said solution is subjected to an electrooxidation according to step (E.3). Therein, the solution is employed as anolyte. A 1 M aqueous solution is employed as katalyte. A flow cell with a flow rate of 250 ml/min is used. Electrolysis is allowed to take place galvanostatically for 8 h applying current of 1 mA/cm.sup.2. A typical resulting voltage is 1,4 V. The voltage curve typically is asymptotic and the solution changes preferably color from brown to dark brown.

(6) Samples of the solution are taken every hour over a time span of 8 h and subsequently examined photometrically. Thereof, an absorption profile typical for ortho-benzoquinone is determined. Hence, a lower molecular weight aromatic lignin-derived compound, quinone compound, is prepared by said method.

(7) Said compound is then isolated according to step (F) of the present invention. Therefore, said compound is extracted by dichloromethane and subsequently subjected to cycles of charging and discharging processes in a flow cell. The voltage curve shows that the compound is redox active, which may be reversibly electrolyzed.

Example 3: Preparation of an Annulated Quinone Compound by a Friedel-Crafts Acylation

(8) Vanillin as a low molecular weight aromatic lignin-derived compound is provided by step (F) according to the present invention. Said compound is further annulated according to step (G) and oxidized according to step (H) according to the present invention in five steps as follows:

(i) Synthesis of 4-(benzyloxy)-3-methoxybenzaldehyde (2)

(9) ##STR00030##

(10) Vanillin (1) (1.0 eq.) and benzyl chloride (1.2 eq.) are dissolved in N,N-dimethylformamide and potassium iodine (0.5 mol %) is added. Afterwards potassium carbonate is added and the reaction is stirred above 60° C., preferably between 60 to 120° C. for at least 1 h, preferably 1 to 8 h. After completion of the reaction, the solution is diluted with distilled water and extracted with an appropriate solvent. The organic phase is washed with brine and the product is then isolated from the organic phase.

(ii) Synthesis of 4-(benzyloxy)-3-methoxybenzoic acid (3)

(11) ##STR00031##

(12) A mixture of 1,2-dimethoxyethane and potassium hydroxide (5 to 20 eq.) is purged with oxygen and the calculated amount of isolated product 2 (1.0 eq.) is added. After the absorption of oxygen ceases, the mixture is diluted with distilled water and neutral organic products are extracted with an appropriate solvent. The aqueous layer is acidified and the acidic organic products are extracted with an appropriate solvent. Product 3 is isolated from the organic layer.

(iii) Synthesis of 4-(benzyloxy)-3-methoxybenzoyl chloride (4)

(13) ##STR00032##

(14) Isolated product 3 (1.0 eq.) is dissolved in thionyl chloride (5-20 eq.) and the mixture is stirred at 60 to 120° C. for 1 to 8 h. After completion of the reaction excess thionyl chloride is evaporated to yield desired acyl chloride 4.

(iv) Synthesis of Anthraquinones (5-7)

(15) ##STR00033##

(16) Aluminiumtrichloride (0.1 eq.) is added to the crude acyl chloride 4 and the mixture is stirred for 30 to 300 min at −20 to 60° C. After completion of the reaction the mixture is carefully quenched with bicarb solution. The product is extracted with an appropriate solvent and the organic layer is washed with brine. The product is then isolated from the organic phase.

(v) Synthesis of 2,6-dihydroxy-3,7-dimethoxyanthracene-9,10-dione 8 and 2,6-dihydroxy-1,7-dimethoxyanthracene-9,10-dione 9

(17) ##STR00034##

(18) ##STR00035##

(19) Anthraquinone 5 or 6 are dissolved in ethyl acetate, methanol or ethanol and palladium on charcoal (1 to 30 weight %) is added. The mixture is stirred at room temperature under hydrogen atmosphere (1-10 bar). The catalyst is filtered off and the product (9) is isolated from the mixture.

(20) The product is then characterized by spectrographic means, and provided as redox active compound according to the present invention.

Example 4: Derivatization of (Hydro-)quinones

Example 4.1 Reduction of Dimethoxy Benzoquinone

(21) ##STR00036##

(22) 23.2 g of sodium dithionite (0.134 mol, 1.32 eq.) was added to the suspension of 17.0 g (0.101 mol, 1.0 eq.) 2,6-dimethoxycyclohexa-2,5-diene-1,4-dione in 100 mL H.sub.2O. After 2 h stirring at room temperature the precipitate was filtered off and dried in the air to give 15.85 g (0.093 mol, 92% yield) of 2,6-dimethoxybenzene-1,4-diol as a white solid.

Example 4.2: Oxidation of Methoxy Benzohydroquinone

(23) ##STR00037##

(24) 1.4 g of catalyst Cu/AlO(OH) was added to a solution of 8.2 g (0.059 mol) 2-methoxy-1,4-dihydroxybenzene in 250 mL ethyl acetate, and the reaction mixture was stirred at room temperature for 147 h under an O.sub.2 atmosphere. After the conversion determined by HPLC reached 99%, the reaction mixture was filtered, and the recovered catalyst was washed with ethyl acetate (100 mL×3). The filtrate was collected and solvent was removed in vacuo to give 7.66 g (0.055 mol, 95% yield) of 2-methoxycyclohexa-2,5-diene-1,4-dione as a yellow-brownish solid.

Example 4.3: Acetylation of Methoxy Benzohydroquinone

(25) 8.24 g (0.059 mol, 1.0 eq.) of 2-methoxybenzene-1,4-diol was weighed into a 250 mL reaction flask equipped with a reflux condenser. 60 mL of dichloroethane and 15 mL (0.159 mol, 2.7 eq.) of acetic anhydride were added. 12 ml (0.096 mol, 1.63 eq.) of boron trifluoride ether solution was then slowly added at room temperature with stirring. The reaction mixture was heated to 90° C. for 20 hours. The mixture was cooled to 60° C., 30 mL H.sub.2O was added followed by 10 mL HCl (6 M). The resulting mixture was heated to 100° C. for 30 min, cooled down and extracted with ethyl acetate (150 mL×3). The combined extracts were washed sequentially with H.sub.2O (100 mL), saturated sodium bicarbonate (100 mL) and H.sub.2O (100 mL) and then dried with anhydrous sodium sulfate. The solvent was removed in vacuo to give a brown solid residue, which was washed with methanol to give 7.49 g (0.041 mol, 70% yield) of 1-(2,5-dihydroxy-4-methoxyphenyl)ethan-1-one as a beige solid.

Example 4.4 Addition of Isonicotinic Acid to Benzoquinone

(26) 2.16 g (0.02 mol, 1.0 eq.) of p-benzoquinone was suspended in 6.4 mL of acetic acid. 2.46 g (0.02 mol, 1.0 eq.) of nicotinic acid was added and the mixture was stirred for 2 h at rt. The resulting dark mixture was diluted with 3 mL of water and treated with 6.6 mL of HCl (6 M). On cooling, solid precipitated which was filtered off and dried overnight at 60° C. to give 3.13 g (0.012 mol, 59% yield) of 3-carboxy-1-(2,5-dihydroxyphenyl)pyridin-1-ium chloride as an yellow solid.

Example 4.5 Sulfonation of Anthraquinone

(27) ##STR00038##

(28) A solution of anthraquinone was heated (180° C.) in a solution of 20%-40% SO.sub.3 in concentrated sulfuric acid (oleum), resulting in a mixture of sulfonated anthraquinones. The crude mixture was poured onto ice and partially neutralized with calcium hydroxide. Subsequently, the mixture was filtrated and concentrated to yield the final product.

Example 4.6: Sulfonation of hydroquinone (1,4-Dihydroxybenzene)

(29) ##STR00039##

(30) A solution of hydroquinone was heated (80° C.) in a solution of 20%-40% SO.sub.3 in concentrated sulfuric acid (oleum), resulting in a mixture of sulfonated hydroquinones. The crude mixture was poured onto ice and partially neutralized with calcium hydroxide. Subsequently, the mixture was filtrated and concentrated to yield the final product.

Example 4.7: Sulfonation of 1,4-Dihydroxy-2,6-dimethoxybenzene

(31) A solution of hydroquinone was heated (80° C.) in a solution of 20%-35% SO.sub.3 in concentrated

(32) ##STR00040##
sulfuric acid (oleum), resulting in a mixture of sulfonated 1,4-dihydroxy-2,6-dimethoxybenzenes. The crude mixture was poured onto ice and partially neutralized with calcium hydroxide. Subsequently, the mixture was filtrated and concentrated to yield the final product.

Example 4.8: Sulfonation of 2-Methoxyhydroquinone

(33) ##STR00041##

(34) A solution of 2-methoxyhydroquinone was heated (80° C.) in a solution of 20%-40% SO.sub.3 in concentrated sulfuric acid (oleum), resulting in a mixture of sulfonated 2-methoxyhydroquinones. The crude mixture was poured onto ice and partially neutralized with calcium hydroxide. Subsequently, the mixture was filtrated and concentrated to yield the final product.