Method for selective cleavage of lignin beta-o-4 linkages mediated by small molecule thiols

Abstract

The present application belongs to the technical field of lignin depolymerization, and particularly relates to a method for selectively breaking lignin beta-O-4 connection mediated by small molecule mercaptan. In an H-type electrolytic cell, the anode electrolytic cell uses a buffer as an electrolyte, the cathode electrolytic cell uses a lignin beta-O-4 model compound as a reaction substrate, and an acetonitrile / buffer mixed solution of small molecule mercaptan is used as an electrolyte to perform an electrochemical cleavage reaction. Among them, the small molecule mercaptan medium acts as a single electron transfer medium, and under the catalysis of electric current, the key C β -O bond in the lignin beta-O-4 model compound can be efficiently and selectively broken at room temperature and normal pressure to obtain corresponding phenolic and ketone monomers, solving the problems of high temperature and high pressure, expensive catalysts and the like in the lignin reduction pathway depolymerization.

Description

Technical Field

[0001] The invention belongs to the technical field of lignin depolymerization, and particularly relates to a method for selectively breaking lignin β-O-4 connections mediated by small molecule thiol. Background Art

[0002] Lignocellulosic biomass is the most abundant renewable resource on Earth, typically composed of 40-45 wt% cellulose, 25-35 wt% hemicellulose, 15-30 wt% lignin, and 10 wt% inorganic components. Lignin, rich in carbon and energy, is the most abundant natural polymer composed of aromatic monomers and is considered the most promising biofuel and an ideal phenol substitute. However, currently only 2% of lignin is converted to high-value-added forms; the vast majority is burned for energy generation, resulting in an extremely low utilization rate. Therefore, the high-value utilization of lignin is of great significance to the comprehensive utilization of biomass.

[0003] Lignin has a three-dimensional, network-like structure, a high molecular weight, poor solubility, and low reactivity. Therefore, degradation generally requires high temperatures, high pressures, or the assistance of various metal catalysts, and the degradation reaction easily produces a large amount of chemical waste. Therefore, rapidly degrading high-molecular-weight lignin into small aromatic monomers under mild conditions remains a major challenge. Summary of the Invention

[0004] The present invention aims to solve at least one of the problems existing in the above-mentioned prior art. To this end, the present invention provides a method for selective cleavage of lignin β-O-4 linkages mediated by small molecule thiol, and designs a small molecule thiol-mediated electrocatalytic reduction system that can efficiently and selectively cleave the key C in lignin β-O-4 model compounds at room temperature and pressure. β -O bond to obtain the corresponding phenolic and ketone monomers, which solves the problems that the depolymerization of lignin by reduction pathway usually requires high temperature, high pressure, and expensive catalysts.

[0005] In order to achieve the above objectives, the specific technical solutions are as follows:

[0006] The present invention provides a method for selective cleavage of lignin β-O-4 linkage mediated by small molecule thiol, which realizes the selective cleavage of key C in lignin β-O-4 model compound through electrocatalytic reduction mediated by small molecule thiol. βThe selective cleavage of the -O bond specifically includes the following steps: in an H-type electrolytic cell, the cathode electrolytic cell uses a lignin β-O-4 model compound as a reaction substrate and a small molecule thiol acetonitrile / buffer mixed solution as an electrolyte to perform an electrochemical cleavage reaction. The pH of the buffer solution is 5-10, the volume ratio of the acetonitrile to the buffer solution is 1:1-1:4, and the small molecule thiol medium acts as a single electron transfer medium. Under the catalytic action of the current, the key C in the model compound can be selectively cleaved. β -O bond, producing the target aromatic monomer.

[0007] Furthermore, in the H-type electrolytic cell, the anode electrolytic cell uses a buffer solution as the electrolyte, and the pH of the buffer solution is 5-10.

[0008] Furthermore, the lignin β-O-4 model compound is a mixture of one or more of 2-phenoxyacetophenone, 2-(2,6-dimethoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethane-1-one, 1-(3,4-dimethoxyphenyl)-2-(4-methoxyphenoxy)ethylene ketone, 2-(1-methoxyphenoxy)-1-phenylethanone, 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)ethane-1-one, and 1-(3,4-dimethoxyphenyl)-3-hydroxy-2-(2-methoxyphenoxy)propan-1-one, and the concentration of the lignin β-O-4 model compound is 0.5-5 mM.

[0009] Preferably, the small molecule thiol is a mixture of one or more of β-mercaptoethanol, 1-pentanethiol, mercaptopropionic acid, D-cysteine, cyclohexyl mercaptan, furan-2-methyl mercaptan, n-butyl mercaptan, n-heptyl mercaptan, cyclopentanethiol, allyl mercaptan, benzyl mercaptan, 1,2-ethanedithiol, 1,4-butanedithiol, 1,2-butanedithiol, and 1,4-benzenedithiol.

[0010] Preferably, the amount of the small molecule thiol added is 0.5-5 molar equivalents of the lignin β-O-4 model compound.

[0011] Furthermore, the electrochemical cleavage reaction time is 1-5 h, and the applied current is 1-20 mA.

[0012] Preferably, in the H-type electrolytic cell, the cathode is a reticulated glassy carbon electrode (RVC), the anode is a platinum electrode (Pt), and the H-type electrolytic cell is separated by a Nifion 117 proton exchange membrane.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] The present invention uses lignin β-O-4 model compound as the base, and adding different types of small molecule thiols can significantly accelerate the Cβ -O bond cleavage rate, shortening the reaction time, and improving the yield of the target product; the small molecule thiol medium required by the present invention has low cost and mild reaction conditions, which can effectively fill the technical gap of depolymerizing lignin by reduction pathway under green and mild conditions; the invention perfectly combines biomass conversion with organic chemistry and electrochemistry, providing new design ideas for the sustainable synthesis of high value-added chemicals. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Schematic diagram of the selective cleavage of lignin β-O-4 linkage mediated by small molecule thiols of the present invention;

[0016] Figure 2 (i-iv) are the cleavage rate graphs of 2-phenoxyacetophenone in Examples 1-4, respectively;

[0017] Figure 3 (i-vi) are the cleavage rate diagrams of 2-phenoxyacetophenone in Examples 5-10, respectively. DETAILED DESCRIPTION

[0018] The following examples describe the principles and features of the present invention. The examples are only used to explain the present invention and are not used to limit the scope of the present invention. The experimental methods used in the following examples are conventional methods unless otherwise specified. The materials, reagents, etc. used in the following examples can be obtained from commercial channels unless otherwise specified. Figure 1 Schematic diagram of the selective cleavage of lignin β-O-4 linkages mediated by small molecule thiols of the present invention. In the following examples, a Shimadzu high-performance liquid chromatograph (Japan) was used to track the cleavage rate of 2-phenoxyacetophenone (compound 1a) and the formation rates of the aromatic monomer products phenol (compound 1b), acetophenone (compound 1c), and pinacol (compound 1d). Example 1

[0019] A method for selectively cleaving lignin β-O-4 linkages mediated by small molecule thiols comprises the following steps:

[0020] In the cathode compartment of an H-type electrolytic cell, 20 mg of 2-phenoxyacetophenone (compound 1a) was dissolved in 10 mL of acetonitrile and 10 mL of pH 8 buffer solution. 20 mL of pH 8 buffer solution was added to the anodic compartment, and the reaction system was reacted at 5 mA for 2.5 hours. Example 2

[0021] A method for selectively cleaving lignin β-O-4 linkages mediated by small molecule thiols comprises the following steps:

[0022] In the cathode compartment of an H-type electrolytic cell, 20 mg of 2-phenoxyacetophenone (Compound 1a) and β-mercaptoethanol (Compound 2a) were dissolved in a solvent consisting of 10 mL of acetonitrile and 10 mL of pH 8 buffer solution, with the β-mercaptoethanol added at a 0.5 molar equivalent of 2-phenoxyacetophenone. 20 mL of pH 8 buffer solution was added to the anodic compartment, and the reaction system was allowed to react at 5 mA for 2.5 hours. Example 3

[0023] Refer to Example 2, the difference from Example 2 is that the added amount of β-mercaptoethanol (Compound 2a) is 1 molar equivalent of 2-phenoxyacetophenone. Example 4

[0024] Refer to Example 2, the difference from Example 2 is that the added amount of β-mercaptoethanol (Compound 2a) is 2 molar equivalents of 2-phenoxyacetophenone. Example 5

[0025] Referring to Example 2, the difference from Example 2 is that the small molecule thiol is n-butyl mercaptan (Compound 2b), and the added amount is 1 molar equivalent of 2-phenoxyacetophenone. Example 6

[0026] Referring to Example 2, the difference from Example 2 is that the small molecule thiol is 1-pentanethiol (Compound 2c), and the added amount is 1 molar equivalent of 2-phenoxyacetophenone. Example 7

[0027] Referring to Example 2, the difference from Example 2 is that the small molecule thiol is mercaptopropionic acid (Compound 2d), and the added amount is 1 molar equivalent of 2-phenoxyacetophenone. Example 8

[0028] Refer to Example 2, the difference from Example 2 is that the small molecule thiol is D-cysteine ​​(compound 2e), and the added amount is 1 molar equivalent of 2-phenoxyacetophenone. Example 9

[0029] Referring to Example 2, the difference from Example 2 is that the small molecule thiol is cyclohexylthiol (Compound 2f), and the added amount is 1 molar equivalent of 2-phenoxyacetophenone. Example 10

[0030] Referring to Example 2, the difference from Example 2 is that the small molecule thiol is furan-2-methylthiol (compound 2g), and the added amount is 1 molar equivalent of 2-phenoxyacetophenone.

[0031] Table 1 is a comparison of the yields of aromatic monomer products from Examples 1 to 10 (the yield of each compound is the data after 2.5 hours of electrolysis reaction). Figure 2 (i-iv) are the cracking rate diagrams of 2-phenoxyacetophenone in Examples 1-4, Figure 3 (i-vi) is the cleavage rate diagram of 2-phenoxyacetophenone in Examples 5-10, wherein ● represents phenol, ▲ represents acetophenone, ▼ represents pinacol, and the rest is 2-phenoxyacetophenone.

[0032]

[0033] The above data show that the cleavage rate of 2-phenoxyacetophenone (compound 1a) is slow under the action of 5 mA current alone, with more than 80% remaining after 2.5 h of reaction. The yields of the target products 1b and 1c are only about 20% (Entry 1). Figure 2 (i-iv) show that with the addition of 0.5 eq of β-mercaptoethanol, the cleavage rate of 2-phenoxyacetophenone (compound 1a) increased significantly, with a residual rate of only about 25%. The yields of phenol (compound 1b) and acetophenone (compound 1c) increased significantly, reaching 67% and 56%, respectively (Entry 2). When the addition amount of β-mercaptoethanol was 1 eq, the yields of phenol (compound 1b) and acetophenone (compound 1c) further increased to 93% and 76% (Entry 3). Continuing to increase the amount of β-mercaptoethanol (2 eq, Entry 4) can further increase the substrate cleavage rate to a certain extent, and the yields of the electrolysis products phenol (compound 1b) and acetophenone (compound 1c) also change slightly. As the reaction proceeds, the product acetophenone (compound 1c) is gradually consumed, generating the by-product pinacol (compound 1d) (yield 15%). Therefore, when the current was 5 mA and the molar ratio of β-mercaptoethanol to substrate 2-phenoxyacetophenone (compound 1a) was 1:1, the raw material 2-phenoxyacetophenone (compound 1a) was almost completely consumed, and the yields of the products phenol (compound 1b) and acetophenone (compound 1c) could reach 93% and 76%, respectively. Under these conditions, the electrolysis effect was the best.

[0034] Figure 3(i-vi) demonstrate that Examples 5-10 all achieve electrocatalytic cleavage of model compounds. All thiols, under the synergistic effect of a 5 mA current, completely cleave the substrate 2-phenoxyacetophenone (Compound 1a) within 2.5 h, yielding phenol (Compound 1b) in 96%-99% yields. Acetophenone (Compound 1c) exhibits a generation-accumulation-consumption pattern, reaching its maximum yield around 2 h (although a small amount of starting material 1a remains in some reactions at this time). As the electrolysis proceeds, small thiols lead to the formation of the byproduct pinacol (Compound 1d) to varying degrees. D-cysteine ​​and cyclohexylthiol, respectively, contribute to the production of 61% and 56% of pinacol (Compound 1d) from the electrolysis of the substrate 2-phenoxyacetophenone (Compound 1a) (Entries 8 and 9), while the yields of acetophenone 1c are only 34% and 1%. It is particularly noteworthy that when the current was 5 mA, the content of n-butyl mercaptan was 1 eq, and the electrolysis time was 2.5 h, the electrolysis effect was good, the substrate 2-phenoxyacetophenone (compound 1a) was completely cracked, and the yields of the products phenol (compound 1b) and acetophenone (compound 1c) reached 96% and 73%, respectively, and the byproduct pinacol (compound 1d) was produced in a small amount, only 17%.

[0035] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for selective cleavage of lignin β-O-4 linkages mediated by small molecule thiols, characterized in that: The method comprises the following steps: in an H-type electrolytic cell, the cathode electrolytic cell uses a lignin β-O-4 model compound as a reaction substrate and an acetonitrile / buffer mixed solution of a small molecule thiol as an electrolyte to perform an electrochemical cracking reaction; The small molecule thiol is a mixture of one or more of β-mercaptoethanol, 1-pentanethiol, mercaptopropionic acid, D-cysteine, cyclohexyl mercaptan, furan-2-methyl mercaptan, n-butyl mercaptan, n-heptyl mercaptan, cyclopentanethiol, allyl mercaptan, benzyl mercaptan, 1,2-ethanedithiol, 1,4-butanedithiol, 1,2-butanedithiol, and 1,4-benzenedithiol.

2. The method according to claim 1, characterized in that The lignin β-O-4 model compound is a mixture of one or more of 2-phenoxyacetophenone, 2-(2,6-dimethoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethane-1-one, 1-(3,4-dimethoxyphenyl)-2-(4-methoxyphenoxy)ethylene ketone, 2-(1-methoxyphenoxy)-1-phenylethanone, 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)ethane-1-one, and 1-(3,4-dimethoxyphenyl)-3-hydroxy-2-(2-methoxyphenoxy)propan-1-one.

3. The method according to claim 1, characterized in that The concentration of the lignin β-O-4 model compound is 0.5-5 mM.

4. The method according to claim 1, wherein The addition amount of the small molecule thiol is 0.5-5 molar equivalents of the lignin β-O-4 model compound.

5. The method according to claim 1, wherein In the H-type electrolytic cell, the anode electrolytic cell uses a buffer solution as the electrolyte.

6. The method according to claim 1, characterized in that The electrochemical cleavage reaction time is 1-5 hours, and the applied current is 1-20 mA.

7. The method according to claim 1, characterized in that In the H-type electrolytic cell, the cathode is a mesh glassy carbon electrode RVC, the anode is a platinum electrode Pt, and the H-type electrolytic cell is separated by a Nifion 117 proton exchange membrane.

8. The method according to claim 5, characterized in that In the cathode electrolytic cell and the anode electrolytic cell, the pH value of the buffer solution is 5-10; in the cathode electrolytic cell, the volume ratio of acetonitrile to the buffer solution is 1:1-1:4.

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