A method for depolymerizing lignin

Abstract

The present invention discloses a method for depolymerizing lignin. The method is based on screening the types of Lewis acid catalysts and organic acids, allowing a lignin model to depolymerize under the synergistic action of the Lewis acid catalyst and the organic acid. During the depolymerization process, the organic acid not only acts as a solvent to dissolve the lignin, but also participates in the reaction as a nucleophile, capturing active intermediates during the reaction to generate a target compound. Compared with existing methods for degrading lignin model materials, the present invention depolymerizes lignin under normal pressure, with mild reaction conditions and a higher product yield. This provides a new technical route for effectively utilizing lignin resources to produce high-value-added chemicals.

Description

Technical Field

[0001] The invention belongs to the technical field of biomass resource utilization, and particularly relates to a lignin depolymerization method. Background Art

[0002] Lignin, the primary component of lignocellulose, is a vital biomass resource and the only non-fossil source of renewable aromatic compounds in nature. Its depolymerization products—guaiacol, aromatic aldehydes (acids), and cycloalkanes—are used in new biomaterials, high-value chemicals, and energy. The lignin content in herbaceous plants ranges from 8-15%, in softwoods from 25-38%, and in hardwoods from 20-30%. Lignin is primarily composed of three precursors (p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol) linked by C-H bonds (β-O-4, α-O-4, and 4-O-5') and C-H bonds (β-β, β-1, β-5, and 5-5'). C-H bonds such as α-O-4 and β-O-4 are the predominant structures in natural lignin, accounting for 43-65%. Therefore, developing green, efficient, and cost-effective lignin depolymerization strategies is of paramount importance. Summary of the Invention

[0003] In response to the shortcomings of existing technologies, the present invention provides a lignin depolymerization method. The lignin depolymerization method provided by the present invention improves the bond breaking efficiency of CO, which has a high content in lignin, and provides a new technical route for effectively utilizing lignin resources to produce high-value chemicals.

[0004] The technical solutions of the present invention are as follows:

[0005] A lignin depolymerization method comprises dissolving lignin in a solvent and carrying out a reaction using Lewis acid as a catalyst.

[0006] Furthermore, the lignin includes sodium lignin sulfonate or a lignin dimer model compound.

[0007] Furthermore, the Lewis acid is trifluoromethanesulfonate.

[0008] Furthermore, the trifluoromethanesulfonate includes one of Hf(OTf)4, Al(OTf)3, Fe(OTf)3, and Ce(OTf)4.

[0009] Furthermore, the solvent is an organic carboxylic acid.

[0010] Furthermore, the organic carboxylic acid includes one of formic acid, acetic acid, propionic acid, and butyric acid.

[0011] Furthermore, the mass volume ratio of the lignin to the solvent is 5-100 mg:1 mL.

[0012] Furthermore, the molar mass ratio of the catalyst to lignin is 1-8 μmol:1 mg.

[0013] Furthermore, the reaction temperature is 80-160° C. and the reaction time is 1-48 hours.

[0014] Furthermore, the reaction is carried out under normal pressure.

[0015] The beneficial technical effects of the present invention are:

[0016] The present invention selects the types of Lewis acid catalysts and organic acids to depolymerize the lignin model under the synergistic action of the Lewis acid catalyst and the organic acid. During the depolymerization process, the organic acid not only acts as a solvent to dissolve the lignin, but also participates in the reaction as a nucleophile, capturing active intermediates in the reaction process to generate the target compound. Compared with existing methods for degrading lignin model substances, the present invention has a higher product yield and provides a new technical route for the effective utilization of lignin resources to produce high-value-added chemicals.

[0017] The method of the invention is to depolymerize lignin under normal pressure, the operation method is simple, the reaction conditions are mild, and the obtained depolymerization products are important pharmaceutical intermediates and bulk chemicals.

[0018] The lignin depolymerization method of the present invention has good stability, high reaction activity and low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a reaction process diagram of the depolymerization of the lignin dimer model compound in Example 1 of the present invention.

[0020] Figure 2 Schematic diagram of the reaction mechanism of depolymerization of the lignin dimer model compound in Example 1 of the present invention.

[0021] Figure 3 This is a two-dimensional nuclear magnetic resonance image of Example 25 of the present invention before and after depolymerization of sodium lignin sulfonate.

[0022] In the figure: a) 2D NMR image of sodium lignin sulfonate before depolymerization; b) 2D NMR image of the product after depolymerization of sodium lignin sulfonate. DETAILED DESCRIPTION

[0023] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0024] like Figure 2As shown, the mechanism of the method of the present invention is explained using trifluoromethanesulfonate Hf(OTf)4 as an example. The metal center of trifluoromethanesulfonate has strong electrophilicity and can attack the oxygen atom in the ether compound, forming an ether and hafnium (IV) complex. Electron transfer induces the intramolecular CO bond to break to form a carbocation. The generated carbocation reacts with acetic acid to produce benzyl acetate through a nucleophilic reaction, while the generated Ph-O-Hf(OTf)3 generates phenol under the mechanism of strong acid to weak acid reaction.

[0025] Example 1

[0026] A lignin depolymerization method, specifically:

[0027] 20 mg of the lignin dimer model compound raw material was dissolved in a pressure tube containing 1 mL of acetic acid, and 20 μmol of Hf(OTf)4 was added. The mixture was reacted at 80°C for 10 h to obtain a reaction product.

[0028] Examples 2-4

[0029] On the basis of Example 1, Al(OTf)3 was used instead of Hf(OTf)4 as the catalyst, and the other conditions and parameters remained unchanged, and Example 2 was set;

[0030] On the basis of Example 1, Fe(OTf)3 was used instead of Hf(OTf)4 as the catalyst, and the other conditions and parameters remained unchanged, and Example 3 was set;

[0031] On the basis of Example 1, Ce(OTf)4 was used instead of Hf(OTf)4 as the catalyst, and the other conditions and parameters remained unchanged, and Example 4 was set.

[0032] Examples 5-12

[0033] Based on Example 1, the reaction temperature was adjusted to 90° C., and the other conditions and parameters remained unchanged, and Example 5 was set;

[0034] On the basis of Example 1, the reaction temperature was adjusted to 100° C., and the other conditions and parameters remained unchanged, and Example 6 was set;

[0035] Based on Example 1, the reaction temperature was adjusted to 110° C., and the other conditions and parameters remained unchanged, and Example 7 was set;

[0036] Based on Example 1, the reaction temperature was adjusted to 120° C., and the other conditions and parameters remained unchanged, and Example 8 was set;

[0037] Based on Example 1, the reaction temperature was adjusted to 130° C., and the other conditions and parameters remained unchanged, and Example 9 was set;

[0038] Based on Example 1, the reaction temperature was adjusted to 140° C., and the other conditions and parameters remained unchanged, and Example 10 was set;

[0039] Based on Example 1, the reaction temperature was adjusted to 150° C., and the other conditions and parameters remained unchanged, and Example 11 was set;

[0040] On the basis of Example 1, the reaction temperature was adjusted to 160° C., and the other conditions and parameters remained unchanged, and Example 12 was set.

[0041] Examples 13-16

[0042] Based on Example 1, formic acid was used instead of acetic acid as the catalyst, and the other conditions and parameters remained unchanged, and Example 13 was set;

[0043] Based on Example 1, propionic acid was used instead of acetic acid as the catalyst, and the other conditions and parameters remained unchanged, and Example 14 was set;

[0044] Based on Example 1, butyric acid was used instead of acetic acid as the catalyst, and the other conditions and parameters remained unchanged, and Example 15 was set;

[0045] On the basis of Example 1, valeric acid was used instead of acetic acid as the catalyst, and other conditions and parameters remained unchanged, and Example 16 was set.

[0046] Example 17

[0047] A lignin depolymerization method, specifically:

[0048] 5 mg of the lignin dimer model compound raw material was dissolved in a pressure tube containing 1 mL of acetic acid, and then 5 μmol of Hf(OTf)4 was added and reacted at 80°C for 10 h to obtain a reaction product.

[0049] Example 18

[0050] A lignin depolymerization method, specifically:

[0051] 10 mg of the lignin dimer model compound raw material was dissolved in a pressure tube containing 1 mL of acetic acid, and 10 μmol of Hf(OTf)4 was added. The mixture was reacted at 80°C for 10 h to obtain a reaction product.

[0052] Example 19

[0053] A lignin depolymerization method, specifically:

[0054] 100 mg of the lignin dimer model compound raw material was dissolved in a pressure tube containing 1 mL of acetic acid, and 100 μmol of Hf(OTf)4 was added, and the mixture was reacted at 80°C for 10 h to obtain a reaction product.

[0055] Example 20

[0056] A lignin depolymerization method, specifically:

[0057] 20 mg of the lignin dimer model compound raw material was dissolved in a pressure tube containing 1 mL of acetic acid, and then 40 μmol of Hf(OTf)4 was added and reacted at 80°C for 10 h to obtain a reaction product.

[0058] Example 21

[0059] A lignin depolymerization method, specifically:

[0060] 20 mg of the lignin dimer model compound raw material was dissolved in a pressure tube containing 1 mL of acetic acid, and 100 μmol of Hf(OTf)4 was added. The mixture was reacted at 80°C for 10 h to obtain a reaction product.

[0061] Example 22

[0062] A lignin depolymerization method, specifically:

[0063] 20 mg of the lignin dimer model compound raw material was dissolved in a pressure tube filled with 1 mL of acetic acid, and 160 μmol of Hf(OTf)4 was added. The mixture was reacted at 80°C for 10 hours to obtain a reaction product.

[0064] Examples 23-24

[0065] Based on Example 1, the reaction time was adjusted to 20 h, and the other conditions and parameters remained unchanged, and Example 23 was set;

[0066] Based on Example 1, the reaction time was adjusted to 48 h, and the other conditions and parameters remained unchanged, and Example 24 was set;

[0067] Example 25

[0068] A lignin depolymerization method, specifically:

[0069] 50 mg of sodium lignin sulfonate raw material was dissolved in a pressure tube containing 1 mL of acetic acid, and then 50 μmol of Hf(OTf)4 was added and reacted at 80°C for 10 h to obtain a reaction product.

[0070] Comparative Examples 1-7

[0071] On the basis of Example 1, NaOTf was used instead of Hf(OTf)4 as the catalyst, and the other conditions and parameters remained unchanged, setting Comparative Example 1;

[0072] On the basis of Example 1, AgOTf was used instead of Hf(OTf)4 as the catalyst, and the other conditions and parameters remained unchanged, setting Comparative Example 2;

[0073] On the basis of Example 1, Cu(OTf)3 was used instead of Hf(OTf)4 as the catalyst, and the other conditions and parameters remained unchanged, setting Comparative Example 3.

[0074] On the basis of Example 1, Zn(OTf)3 was used instead of Hf(OTf)4 as the catalyst, and the other conditions and parameters remained unchanged, setting Comparative Example 4;

[0075] On the basis of Example 1, Ce(OTf)3 was used instead of Hf(OTf)4 as the catalyst, and the other conditions and parameters remained unchanged, setting Comparative Example 5;

[0076] On the basis of Example 1, La(OTf)3 was used instead of Hf(OTf)4 as the catalyst, and the other conditions and parameters remained unchanged, and comparative example 6 was set;

[0077] On the basis of Example 1, Sm(OTf)3 was used instead of Hf(OTf)4 as the catalyst, and the other conditions and parameters remained unchanged, setting Comparative Example 7.

[0078] Comparative Examples 8-10

[0079] Based on Example 1, the reaction temperature was adjusted to 50° C., and the other conditions and parameters remained unchanged, and Comparative Example 8 was set;

[0080] Based on Example 1, the reaction temperature was adjusted to 60° C., and the other conditions and parameters remained unchanged, setting Comparative Example 9;

[0081] Based on Example 1, the reaction temperature was adjusted to 70° C., and the other conditions and parameters remained unchanged, and Comparative Example 10 was set;

[0082] On the basis of Example 1, the reaction temperature was adjusted to 200° C., and the other conditions and parameters remained unchanged, and Comparative Example 11 was set.

[0083] Comparative Example 12

[0084] A lignin depolymerization method, specifically:

[0085] 200 mg of the lignin dimer model compound raw material was dissolved in a pressure tube filled with 1 mL of acetic acid, and 200 μmol of Hf(OTf)4 was added. The mixture was reacted at 80°C for 10 h to obtain a reaction product.

[0086] Comparative Example 13

[0087] A lignin depolymerization method, specifically:

[0088] 20 mg of the lignin dimer model compound raw material was dissolved in a pressure tube containing 1 mL of acetic acid, and then 2 μmol of Hf(OTf)4 was added and reacted at 80°C for 10 h to obtain a reaction product.

[0089] Comparative Example 14

[0090] On the basis of Example 1, the reaction time was adjusted to 1 h, and the other conditions and parameters remained unchanged, and Comparative Example 14 was set to obtain a reaction product.

[0091] Test Case

[0092] The reaction products obtained in Examples 1-24 of the present invention and Comparative Examples 1-14 were centrifuged, and the supernatant was taken for qualitative and quantitative analysis by high performance liquid chromatography. The conversion rate of the raw material, the reaction product, and the yield were calculated. The results are shown in Table 1.

[0093]

[0094]

[0095] The amount of substance of the product is the amount of substance of phenol or benzyl acetate.

[0096] Table 1

[0097]

[0098]

[0099] (1) Effect of trifluoromethanesulfonate type on raw material conversion rate and product yield:

[0100] According to the results of Examples 1-4 and Comparative Examples 1-7 in Table 1, the trifluoromethanesulfonates Hf(OTf)4, Al(OTf)3, Fe(OTf)3, and Ce(OTf)4 selected in the present invention have a raw material conversion rate of 92-99% when catalyzing the lignin dimer model compound, and the yields of phenol and benzyl acetate in the obtained product are 97-92% and 90-96%, respectively; when NaOTf, AgOTf, Cu(OTf)3, Zn(OTf)3, Ce(OTf)3, La(OTf)3, and Sm(OTf)3 are used as catalysts to catalyze the lignin dimer model compound, the raw material conversion rate decreases significantly, the conversion rate is 27-62%, and the yields of phenol and benzyl acetate in the obtained product also decrease significantly, being 22-58% and 26-61%, respectively. This may be due to the varying effective charge densities of the metal centers of different trifluoromethanesulfonates, which in turn lead to varying Lewis acid strengths in the catalysts. The greater the effective charge density of the metal center, the stronger the Lewis acidity and the higher the catalytic activity. Therefore, the type of trifluoromethanesulfonate significantly influences the feedstock conversion and product yield of the depolymerization reaction of the lignin dimer model compound.

[0101] (2) Effect of reaction temperature on raw material conversion rate and product yield:

[0102] According to the results of Examples 1, 5-12 and Comparative Examples 8-11 in Table 1, when the reaction temperature is 80-160°C, the raw material conversion rate of the depolymerization reaction of the lignin dimer model compound can reach 99%, and the yields of phenol and benzyl acetate in the obtained product are 62-92% and 94-97%, respectively; when the temperature is lower than 80°C, the raw material conversion rate decreases significantly, the conversion rate is 12-67%, and the yields of phenol and benzyl acetate in the obtained product also decrease significantly, 11-65% and 12-66%, respectively. Obviously, when the temperature is too low, the reaction activity of the catalyst is low; when the temperature is higher than 160°C and reaches 200°C, although the conversion rate is high, the yield of phenol decreases significantly. This is because the temperature is too high and the product phenol will further undergo esterification reaction under the action of the catalyst, thereby affecting the selectivity of the product. Therefore, the reaction temperature has a significant effect on the raw material conversion rate and product yield of the depolymerization reaction of the lignin dimer model compound.

[0103] (3) Effect of the relative amount of lignin and organic carboxylic acid on raw material conversion rate and product yield:

[0104] According to the results of Examples 1, 17-19 and Comparative Example 12 in Table 1, when the relative amount of lignin and organic carboxylic acid is 5-100 mg: 1 mL, the raw material conversion rate of the depolymerization reaction of the lignin dimer model compound is 86-99%, and the yields of phenol and benzyl acetate in the resulting product are 80-93% and 85-97%, respectively; and when the relative amount of lignin and organic carboxylic acid exceeds the scope of the present invention, due to the high concentration of lignin, the reaction activity may be affected by mass transfer problems, resulting in a significant decrease in the raw material conversion rate, the conversion rate is 69%, and the yields of phenol and benzyl acetate in the resulting product are also significantly decreased, 67% and 68%, respectively. Therefore, the relative amount of lignin and organic carboxylic acid has a significant effect on the raw material conversion rate and product yield of the depolymerization reaction of the lignin dimer model compound.

[0105] (4) Effect of the relative amount of catalyst and lignin on raw material conversion rate and product yield:

[0106] According to the results of Examples 1, 20-22 and Comparative Example 13 in Table 1, when the relative amount of catalyst to lignin is 1-8 μmol: 1 mg, the raw material conversion rate of the depolymerization reaction of the lignin dimer model compound can reach 99%, and the yields of phenol and benzyl acetate in the resulting product are 69-92% and 93-96%, respectively; and when the relative amount of catalyst to lignin is lower than the scope of the present invention, the raw material conversion rate decreases significantly, the conversion rate is 68%, and the yields of phenol and benzyl acetate in the resulting product also decrease significantly, 66% and 65%, respectively. Obviously, when the catalyst concentration is low, its conversion efficiency is significantly reduced. Therefore, the relative amount of catalyst to lignin has a significant effect on the raw material conversion rate and product yield of the depolymerization reaction of the lignin dimer model compound.

[0107] (5) Effect of reaction time on raw material conversion rate and product yield:

[0108] According to the results of Examples 1, 23-24, and Comparative Example 14 in Table 1, when the reaction time is 10-48 h, the raw material conversion rate of the depolymerization reaction of the lignin dimer model compound can reach 99%, and the yields of phenol and benzyl acetate in the resulting product are 78-92% and 92-95%, respectively. When the reaction time is shorter, such as 1 h, the raw material conversion rate decreases significantly, reaching 44%, and the yields of phenol and benzyl acetate in the resulting product also decrease significantly, reaching 39% and 43%, respectively. Obviously, this is because the reaction time is too short, resulting in a significant decrease in the conversion rate, which also leads to a decrease in the yields of phenol and benzyl acetate in the product.

[0109] (6) Depolymerization effect of the depolymerization method of the present invention on the real lignin system:

[0110] The reaction product obtained in Example 25 of the present invention was centrifuged and the supernatant was discarded to obtain a residue. The obtained residue and the sodium lignin sulfonate raw material were subjected to two-dimensional nuclear magnetic resonance (2D HSQC NMR) analysis and detection. The results were as follows: Figure 3 As shown. Figure 3 The results show that after the depolymerization reaction of sodium lignin sulfonate, the CO bond signal peak representing the connection between the benzene ring units is significantly weakened or disappears, indicating that the method of the present invention can be effectively used for breaking the CO bond in the real lignin system, and the method of the present invention can be used for the depolymerization of lignin.

[0111] The above description is merely a preferred embodiment of the present invention, and the present invention is not limited to the above embodiment. It is understood that other improvements and variations directly derived or imagined by those skilled in the art without departing from the spirit and concept of the present invention should be considered to be included in the scope of protection of the present invention.

Claims

1. A method for depolymerizing lignin, characterized in that: The lignin is dissolved in a solvent and reacted with Lewis acid as a catalyst; The Lewis acid is trifluoromethanesulfonate; The trifluoromethanesulfonate includes one of Hf(OTf)4, Al(OTf)3, Fe(OTf)3, and Ce(OTf)4; The solvent is an organic carboxylic acid.

2. The method according to claim 1, characterized in that The lignin includes sodium lignin sulfonate or a lignin dimer model compound.

3. The method according to claim 1, characterized in that The organic carboxylic acid includes one of formic acid, acetic acid, propionic acid and butyric acid.

4. The method according to claim 1, wherein The mass volume ratio of the lignin to the solvent is 5-100 mg:1 mL.

5. The method according to claim 1, wherein The molar mass ratio of the catalyst to lignin is 1-8 μmol:1 mg.

6. The method according to claim 1, characterized in that The reaction temperature is 80-160° C. and the reaction time is 1-48 hours.

7. The method according to claim 1, characterized in that The reaction is carried out under normal pressure.

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