Selective fracture method of micromolecular thiol-mediated lignin beta-O-4 connection

Abstract

The invention belongs to the technical field of lignin depolymerization, and particularly relates to a selective fracture method of micromolecular mercaptan mediated lignin beta-O-4 connection. In an H-type electrolytic tank, an anode electrolytic tank takes a buffer solution as an electrolyte, a cathode electrolytic tank takes a lignin beta-O-4 model compound as a reaction substrate, and an acetonitrile / buffer solution mixed solution of small molecular mercaptan is taken as an electrolyte for electrochemical cracking reaction. Wherein a micromolecular mercaptan medium is used as a single electron transfer medium, and can efficiently and selectively break key C beta-O bonds in a lignin beta-O-4 model compound at normal temperature and normal pressure under the catalytic action of current to obtain corresponding phenol and ketone monomers, so that the problems that high temperature, high pressure and expensive catalysts are usually needed in depolymerization in a lignin reduction way are solved.

Description

Technical Field

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

[0002] Lignocellulosic biomass is the most abundant renewable resource on earth, generally composed of 40-45wt% cellulose, 25-35wt% hemicellulose, 15-30wt% lignin and 10wt% inorganic components. Lignin is rich in carbon and energy, and is the most abundant natural polymer compound composed of aromatic monomers in nature. It is considered to be the most promising biofuel and the most ideal substitute for phenol. However, only 2% of lignin is converted into high value-added, and the vast majority of lignin is used for combustion to generate energy, with 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 structure, large molecular weight, poor solubility and low reactivity. Therefore, the degradation of lignin generally requires high temperature, high pressure or various metal catalysts, and it is easy to produce a lot of chemical waste during the degradation reaction. Based on this, it is still an important challenge to quickly degrade high molecular weight lignin into small molecular aromatic monomers under mild conditions. 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 selectively breaking the β-O-4 connection of lignin mediated by small molecule thiol, and designs a small molecule thiol-mediated electrocatalytic reduction system, which can efficiently and selectively break the key C in the β-O-4 model compound of lignin at room temperature and pressure. β -O bond to obtain the corresponding phenolic and ketone monomers, which solves the problem that the depolymerization of lignin by reduction pathway usually requires high temperature and high pressure, expensive catalysts, etc.

[0005] In order to achieve the above objectives, the specific technical solutions are as follows: The present invention provides a method for selectively breaking the lignin β-O-4 connection mediated by small molecule thiol, and realizes the selective breaking of the key C in the lignin β-O-4 model compound through the 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 the lignin β-O-4 model compound as the reaction substrate and uses the acetonitrile / buffer mixed solution of small molecule thiol as the 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.

[0006] Furthermore, in the H-type electrolytic cell, the anode electrolytic cell uses a buffer solution as an electrolyte, and the pH value of the buffer solution is 5-10.

[0007] Further, 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.

[0008] 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.

[0009] Preferably, the addition amount of the small molecule thiol is 0.5-5 molar equivalents of the lignin β-O-4 model compound.

[0010] Furthermore, the electrochemical cleavage reaction time is 1-5 h, and the applied current is 1-20 mA.

[0011] 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.

[0012] Compared with the prior art, the present invention has the following beneficial effects: 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 C β-O bond cleavage rate, shortening the reaction time, while increasing 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 a new design idea for the sustainable synthesis of high value-added chemicals. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 Schematic diagram of the selective cleavage of lignin β-O-4 linkage mediated by small molecule thiol of the present invention; Figure 2 (i-iv) are the cleavage rate diagrams of 2-phenoxyacetophenone in Examples 1-4, respectively; Figure 3 (i-vi) are the cleavage rate diagrams of 2-phenoxyacetophenone in Examples 5-10, respectively. DETAILED DESCRIPTION

[0014] The principles and features of the present invention are described below with reference to examples. 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 are all commercially available unless otherwise specified. Figure 1 The schematic diagram of the selective cleavage of lignin β-O-4 linkage mediated by small molecule thiol of the present invention is shown in the figure. The following examples all use Shimadzu high performance liquid chromatograph (Japan) to track the cleavage rate of 2-phenoxyacetophenone (compound 1a) and the generation rate of aromatic monomer products phenol (compound 1b), acetophenone (compound 1c) and pinacol (compound 1d). Example 1

[0015] A method for selectively cleaving lignin β-O-4 linkages mediated by small molecule thiol comprises the following steps: In the cathode cell of the H-type electrolytic cell, 20 mg of 2-phenoxyacetophenone (compound 1a) was dissolved in a solvent consisting of 10 mL of acetonitrile and 10 mL of a buffer solution at pH = 8. 20 mL of a buffer solution (pH = 8) was added to the anode electrolytic cell, and the reaction system was reacted at 5 mA for 2.5 hours. Example 2

[0016] A method for selectively cleaving lignin β-O-4 linkages mediated by small molecule thiol comprises the following steps: In the cathode cell of the 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, where the amount of β-mercaptoethanol added was 0.5 molar equivalent of 2-phenoxyacetophenone. 20 mL of buffer solution (pH=8) was added to the anode electrolytic cell, and the reaction system was reacted for 2.5 hours at 5 mA current. Example 3

[0017] Referring 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

[0018] Referring to Example 2, the difference from Example 2 is that the amount of β-mercaptoethanol (Compound 2a) added is 2 molar equivalents of 2-phenoxyacetophenone. Example 5

[0019] 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

[0020] 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

[0021] 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

[0022] Referring 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

[0023] 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

[0024] 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.

[0025] 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 for 2.5 hours of electrolysis reaction). Figure 2 (i-iv) are the cleavage rate diagrams of 2-phenoxyacetophenone in Examples 1-4, Figure 3 (i-vi) is the cleavage rate diagram of 2-phenoxyacetophenone in Example 5-10, wherein ● represents phenol, ▲ represents acetophenone, ▼ represents pinacol, and the rest is 2-phenoxyacetophenone.

[0026]

[0027] The above data show that the cleavage rate of 2-phenoxyacetophenone (compound 1a) is slow under the action of 5 mA current alone, and more than 80% of it still remains 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 β-mercaptoethanol, the cleavage rate of 2-phenoxyacetophenone (compound 1a) increased significantly, the residual rate was only about 25%, and 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) were further increased to 93% and 76% (Entry 3); further increasing 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 changed slightly. As the reaction proceeded, the product acetophenone (compound 1c) was gradually consumed to generate the by-product pinacol (compound 1d) (yield 15%). Therefore, when the current is 5 mA and the molar ratio of β-mercaptoethanol to substrate 2-phenoxyacetophenone (compound 1a) is 1:1, the raw material 2-phenoxyacetophenone (compound 1a) is almost completely consumed, and the yields of the products phenol (compound 1b) and acetophenone (compound 1c) can reach 93% and 76%, respectively. Under this condition, the electrolysis effect is the best.

[0028] Figure 3(i-vi) show that Examples 5-10 can achieve electrocatalytic cleavage of model compounds. All thiols can completely cleave the substrate 2-phenoxyacetophenone (compound 1a) under the synergistic effect of 5 mA current within 2.5 h, and the product phenol (compound 1b) has a yield of 96%-99%. Among them, acetophenone (compound 1c) shows a trend of generation-accumulation-consumption, and almost reaches the maximum yield around 2 h of reaction (but at this time, in some reactions, a small amount of raw material 1a still remains); as the electrolysis reaction proceeds, small molecule thiols all lead to the production of byproduct pinacol (compound 1d) to varying degrees, among which D-cysteine ​​and cyclohexylthiol cause the electrolysis reaction of the substrate 2-phenoxyacetophenone (compound 1a) to produce 61% and 56% of pinacol (compound 1d) (Entry 8 and Entry 9), respectively, and the yield of acetophenone 1c is 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 cleaved, and the yields of the products phenol (compound 1b) and acetophenone (compound 1c) reached 96% and 73%, respectively, and less by-product pinacol (compound 1d) was produced, only 17%.

[0029] 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 principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for selective cleavage of lignin β-O-4 linkages mediated by small molecule thiol, characterized in that: The method comprises the following steps: in an H-type electrolytic cell, a 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.

2. The method according to claim 1, characterized in that 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.

3. 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.

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

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

6. The method according to claim 1, characterized in that In the H-type electrolytic cell, the anode electrolytic cell uses a buffer solution as the electrolyte.

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

8. 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.

9. The method according to claim 6, characterized in that In the cathode electrolytic cell and the anode electrolytic cell, the pH 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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