Preparation method and application of nitrogen-doped lignin carbon-based nonmetal catalyst

Abstract

The invention relates to a preparation method and application of a nitrogen-doped lignin carbon-based nonmetal catalyst, which comprises the following steps: carrying out high-speed ball milling treatment on industrial lignin of which the metal element content is not less than 20wt% and a nitrogen source, and carrying out high-temperature pyrolysis in an inert atmosphere to obtain the nitrogen-doped lignin carbon-based nonmetal catalyst. The pyrolysis temperature of the high-temperature pyrolysis is 750-900 DEG C, the ball milling rotating speed in the ball milling is 150-250 r / min, the ball milling time is more than 8 h, the agate balls comprise large agate balls, medium agate balls and small agate balls, the mass ratio of the large agate balls to the medium agate balls to the small agate balls is 1: 4: 1-1: 4: 4, the diameter of the large agate balls is 10-15 mm, the diameter of the medium agate balls is 4-6 mm, and the diameter of the small agate balls is 2-3 mm. The nitrogen-doped lignin carbon-based nonmetal catalyst is used as a solid base catalyst, the problem that lignin oxidative depolymerization depends on additional alkali at present is solved, mechanical force is adopted in the lignin depolymerization process to drive catalytic depolymerization of C-O / C-C bonds, high-temperature and high-pressure operation is avoided, and environmental protection and economical efficiency are achieved.

Description

Technical Field

[0001] The present invention relates to the field of preparation of carbon-based non-metallic catalysts and catalytic depolymerization of lignin, and in particular to a preparation method and application of a nitrogen-doped lignin carbon-based non-metallic catalyst, wherein the catalyst drives catalytic depolymerization of lignin under the action of mechanical force to obtain oxygen-containing aromatic chemicals. Background Art

[0002] Lignin is a three-dimensional network aromatic compound composed of a series of aromatic units linked by CO and CC bonds. Selective conversion of lignin into oxygenated aromatic monomers can replace petroleum-based chemical products. Oxidative depolymerization of lignin is an effective strategy for high-value conversion of lignin into oxygenated aromatic chemicals. The key lies in the development of a catalytic system that can controllably tailor CO / CC bonds. At present, the depolymerization of lignin is mainly focused on heterogeneous metal catalysts. For example, the authorized invention patent (CN115970732B) publicly reports that lignin-derived carbon-supported iron-based catalysts can effectively depolymerize lignin when the iron metal mass content is 2-10%. However, the preparation of heterogeneous metal catalysts generally requires the addition of exogenous metal sources, and heterogeneous metal catalysts generally have the problem of low metal atom utilization. In addition, in the current research on lignin depolymerization, due to the stubborn chemical properties of lignin, the depolymerization conditions are usually harsh. Even if the oxidative depolymerization strategy with milder operating conditions is adopted, it still requires higher temperatures (~ 120°C) and higher pressures (~ 1 MPa). In summary, it is necessary to develop non-metallic and mild catalytic systems for efficient depolymerization of lignin. This strategy is in line with the development direction of green chemical industry and is economical. Summary of the invention

[0003] The present invention aims to develop a nitrogen-doped carbon-based non-metallic catalyst using industrial lignin as a carbon source, and apply it to the field of lignin catalytic depolymerization. The catalyst efficiently breaks the CO / CC bonds of lignin under mechanical force to generate aromatic monomer products.

[0004] To achieve the above object, the technical solution adopted by the present invention is: In a first aspect, the present invention provides a method for preparing a nitrogen-doped lignin carbon-based non-metallic catalyst, wherein industrial lignin having a metal element content of not less than 20 wt% and a nitrogen source are subjected to high-speed ball milling and then subjected to high-temperature pyrolysis under an inert atmosphere to obtain a nitrogen-doped lignin carbon-based non-metallic catalyst; The pyrolysis temperature of the high temperature pyrolysis is 750-900°C. The ball milling speed is 150-250 r / min, the ball milling time is greater than 8 hours, and the total mass of the agate balls used in the ball milling is 5-10 times the mass of the object to be ball milled; the agate balls include large, medium and small agate balls, the mass ratio of the large, medium and small agate balls is 1:4:1-1:4:4, the diameter of the large agate ball is 10-15 mm, the diameter of the medium agate ball is 4-6 mm, and the diameter of the small agate ball is 2-3 mm.

[0005] Furthermore, the diameter of the large agate ball is 10 mm, the diameter of the medium agate ball is 5 mm, and the diameter of the small agate ball is 3 mm, and the mass ratios of the three are 1:4:2-1:4:3 respectively.

[0006] The mass ratio of lignin to nitrogen source is 1:2-2:1; The industrial lignin is at least one of alkali lignin and sodium lignin sulfonate, and the total content of alkali metal elements and alkaline earth metal elements in the industrial lignin is not less than 20 wt %; the nitrogen source is at least one of dicyandiamide, melamine and urea.

[0007] Furthermore, the pyrolysis temperature of the high temperature pyrolysis is 800-900°C, and the pyrolysis time is 0.5-2 h.

[0008] Furthermore, the total mass of the agate balls used is 150-250 g, preferably 200-230 g.

[0009] In a second aspect, the present invention protects a nitrogen-doped lignin carbon-based non-metal catalyst obtained by the preparation method.

[0010] In the third aspect, the present invention protects an application of the nitrogen-doped lignin carbon-based non-metallic catalyst in the catalytic depolymerization of lignin. The nitrogen-doped lignin carbon-based non-metallic catalyst is used as a catalyst, and the catalyst, lignin reaction substrate, and solvent are placed in a ball mill and mechanically ball milled at a speed of 150-250 r / min to drive the catalytic depolymerization of lignin.

[0011] In the catalytic depolymerization of lignin, the ball milling time is 12-24 h, and the mass ratio of the solvent to the agate balls is 1:5-1:10, preferably 1:7-1:8.

[0012] The lignin reaction substrate is a treated oxidized β-O-4 model substrate, the conversion rate of the β-O-4 model substrate is above 80%, the lignin depolymerization products include phenol and esters, and the yields of phenol and esters are both above 60%; the esters include methyl benzoate and methyl benzoylformate.

[0013] Preferably, the conversion rate of the β-O-4 model substrate is above 90%.

[0014] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention uses industrial lignin containing metal salts (metal element content is not less than 20 wt%) as a carbon source to prepare a nitrogen-doped lignin carbon-based non-metal catalyst for lignin oxidative depolymerization. Compared with the existing invention patent (CN115970732B) that uses lignin carbon to support iron-based catalysts, the present application uses nitrogen-doped lignin carbon-based non-metal catalysts without the addition of exogenous metals. The carbon source and nitrogen source are milled at high speed by a planetary ball mill during the preparation process. The uniform dispersion of the nitrogen source and lignin can promote subsequent full pyrolysis, increase the surface area of ​​the obtained nitrogen-doped carbon material and the content of pyridinic nitrogen and pyrrolic nitrogen, and facilitate oxygen activation; the metal salts (sodium salts, magnesium salts, etc.) contained in the selected industrial lignin can be used as pore-forming agents to promote the formation of micro / mesopores and alkaline sites of carbon materials during the pyrolysis process, and promote the breaking of CO / CC bonds in the subsequent lignin depolymerization. More importantly, in the present invention, the nitrogen-doped carbon catalyst is depolymerized with the lignin reaction substrate and solvent by mechanical force to obtain the target monomer product, avoiding high temperature and high pressure conditions, and having environmental protection and economy.

[0015] 2. In the present invention, industrial lignin is a widely used solid waste source for papermaking. Alkali lignin or sodium lignin sulfonate contains metal salts (sodium salts, magnesium salts, etc.) which form alkaline metal oxides (MOx, M=Na / Mg, etc.) during pyrolysis and can be used as alkaline sites to activate C in lignin. β -H bonds and promote the cleavage of CO / CC bonds.

[0016] 3. The present invention adopts nitrogen-doped lignin carbon-based non-metallic catalyst (no need to add metal or other exogenous reagents) to form abundant pyridinic nitrogen and pyrrolic nitrogen on the carbon surface by nitrogen doping, which can be used as sites for activating O2; the metal salt in industrial lignin can also be used as a pore-forming agent to promote the formation of pores in carbon materials during pyrolysis, thereby increasing the specific surface area of ​​lignin carbon materials, fully exposing active sites, improving the utilization rate of active sites, and promoting the breaking of CO / CC bonds.

[0017] 4. The nitrogen-doped lignin carbon-based non-metallic catalyst used in the present invention is itself used as a solid base catalyst, which avoids the problem that the current lignin oxidative depolymerization relies on the addition of an alkali. In addition, the lignin depolymerization process uses mechanical force to drive the catalytic depolymerization of CO / CC bonds, avoiding high temperature and high pressure operation, and is environmentally friendly and economical. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a comparison chart of the CO2-programmed temperature desorption effects of NC 800 in Example 1 and NC 800-S after acid washing. DETAILED DESCRIPTION

[0019] The present invention is further explained below in conjunction with embodiments, but this is not intended to limit the scope of protection of the present application.

[0020] The preparation method of the nitrogen-doped lignin carbon-based non-metal catalyst of the present invention comprises the following steps: milling industrial lignin containing metal salts (such as sodium salts and magnesium salts) and having a metal element content greater than 20 wt% and a nitrogen source in an agate pot by a ball mill at high speed, and then pyrolyzing the milled particles at high temperature under an inert atmosphere to obtain a nitrogen-doped lignin carbon-based non-metal catalyst, referred to as nitrogen-doped lignin carbon; The pyrolysis temperature of the high temperature pyrolysis is 750-900°C. The ball milling speed is 150-250 r / min, the ball milling time is 6-30 h, and the total mass of the agate balls used in the ball milling is 5-10 times the mass of all the objects to be ball milled; the agate balls include large, medium and small agate balls, the mass ratio of the large, medium and small agate balls is 1:4:1-1:4:4, the diameter of the large agate balls is 10-15 mm, the diameter of the medium agate balls is 4-6 mm, and the diameter of the small agate balls is 2-3 mm.

[0021] The nitrogen-doped lignin carbon-based non-metal catalyst is used as a catalyst, and the catalyst, lignin reaction substrate, and solvent are placed in a ball milling tank and mechanically ball milled at a speed of 150-250 r / min to drive the catalytic depolymerization of lignin. After the ball milling is completed, samples are taken and analyzed by gas chromatography.

[0022] Furthermore, the industrial lignin is at least one of alkali lignin and sodium lignin sulfonate containing metal salts, preferably alkali lignin and / or sodium lignin sulfonate containing more than 20 wt % of sodium and magnesium.

[0023] Furthermore, the nitrogen source is at least one of dicyandiamide, melamine, urea, etc., preferably dicyandiamide.

[0024] Furthermore, the mass ratio of lignin to nitrogen source is 1:2-2:1; the pyrolysis temperature of the high temperature pyrolysis is 500-900°C, preferably 800-900°C, and the pyrolysis time is 0.5-2 h.

[0025] The ball milling speed, ball milling time and agate ball configuration used in the above two ball millings are the same.

[0026] The total mass of the agate balls used is 150-250 g, preferably 200-230 g.

[0027] Preferably, the diameter of the large agate ball is 10 mm, the diameter of the medium agate ball is 5 mm, the diameter of the small agate ball is 3 mm, and the mass ratio of the three is 1:4:2-1:4:3.

[0028] Preferably, the ball milling time is 12 h-24 h.

[0029] Furthermore, the mass ratio of the solvent to the agate balls is 1:5-1:10, preferably 1:7-1:8.

[0030] In the present invention, the lignin reaction substrate is an oxidized β-O-4 model substrate, the conversion rate of the β-O-4 model substrate is above 80%, the lignin depolymerization products include phenol and esters (methyl benzoate, methyl benzoylformate), and the yields of phenol and esters are both above 60%.

[0031] Example 1 (1) Preparation of nitrogen-doped lignin carbon-based non-metallic catalyst (NC): Alkali lignin (sodium and magnesium content of 20 wt%) and dicyandiamide were ball milled in an agate jar at a high speed of 200 r / min for 12 h. The total mass of the agate balls was 230 g. The diameters of the agate balls used were 10 mm, 5 mm, and 3 mm, respectively, and the mass ratio of large, medium, and small agate balls was 1:4:2. The ball-milled sample was then placed in a quartz boat and heated at a rate of 5 °C / min to the final temperature under a N2 atmosphere and maintained for 1 h. After cooling, it was taken out of the pyrolysis furnace, ball-milled and washed with deionized water, and dried to obtain the NC catalyst.

[0032] The other conditions remained unchanged, only the final temperature (500℃, 650℃, 800℃) was changed to obtain different catalysts, which were respectively denoted as NC 500, NC 650, and NC 800. In order to study the importance of alkaline position in lignin oxidative depolymerization, NC 800 was acid-washed to obtain acid-washed NC 800-S.

[0033] (2) Catalytic depolymerization experiments of β-O-4 model substrate were carried out on the catalysts obtained at different final temperatures. 30 mg of lignin model substrate, 30 mg of catalyst and 30 ml of methanol were placed in a ball mill for mechanical catalytic reaction for 12 h (200 r / min). After the ball milling, samples were taken and analyzed by gas chromatography.

[0034]

[0035] As can be seen from Table 1, NC 800 prepared at a pyrolysis temperature of 800°C can achieve a conversion rate of lignin β-O-4 model substrate of more than 80% (97.8% in the table), and the yield of phenol and esters is more than 60%. This is because sufficient pyridinic nitrogen / pyrrolic nitrogen and alkaline sites can be generated at a sufficient pyrolysis temperature, thereby promoting oxygen activation and the cleavage of CO / CC bonds in lignin, while NC 800-S after acid washing lacks alkaline sites (see Figure 1), resulting in a significant decrease in the conversion rate of the β-O-4 model substrate and the yield of the product.

[0036] Example 2 The steps of this example are the same as those of Example 1, except that the carbon sources used are alkali lignin, sodium lignin sulfonate, dealkalized lignin, and acid-treated alkali lignin, and the pyrolysis temperature used is fixed at 800° C. to obtain NC 800 (alkali lignin / sodium sulfonate / dealkalized lignin / acid treatment).

[0037] The purpose of acid treatment of alkali lignin is to remove the metal salts in alkali lignin. The specific process is as follows: hydrofluoric acid and nitric acid are mixed in a ratio of 1:2 to prepare 45 mL of mixed acid, 15 g of alkali lignin is weighed and slowly added to the mixed acid while stirring until no bubbles are generated, and then filtered. During the filtration process, it is rinsed with deionized water, and finally dried in an oven at 60 °C to obtain acid-treated desalinated alkali lignin.

[0038]

[0039] As can be seen from Table 2, using alkali lignin and sodium lignin sulfonate containing metal salts and having a metal element (Na and Mg) content of not less than 20wt% as carbon sources, the prepared nitrogen-doped lignin carbon can catalytically convert more than 80% of the lignin β-O-4 model substrate, and the yield of phenol and esters is more than 60%. Dealkalized lignin or acid-washed alkali lignin cannot promote pore formation during the pyrolysis process due to the lack of sufficient metal salts. The resulting catalytic material has a small specific surface area and lacks sufficient alkaline sites, resulting in low conversion rate of the β-O-4 model substrate and low yield of the product.

[0040] Example 3 The steps in this example are the same as those in Example 1, except that the ball milling time in the catalyst preparation process of this example is 2 h, 6 h, 12 h, and 24 h, respectively, the pyrolysis temperature is fixed at 800° C., and the carbon source is fixed at alkali lignin with a sodium and magnesium content of 20 wt %, to obtain NC 800-2 / 6 / 12 / 24.

[0041] The results are shown in Table 3:

[0042] It can be seen from Table 3 that when the ball milling time is 12 hours or more, the nitrogen-doped lignin carbon-based non-metallic catalyst obtained by pyrolysis can efficiently convert lignin, the conversion rate of the lignin β-O-4 model substrate reaches more than 90%, and the yield of phenol alone and the yield of esters are more than 60%. This is because a certain ball milling time can achieve uniform mixing and doping of nitrogen source and lignin, and the obtained nitrogen-doped lignin carbon has a higher specific surface area and active nitrogen species content, which can effectively break the CO / CC bond in lignin.

[0043] Example 4 The steps of this example are the same as those of Example 1, except that the ball milling speeds used in the catalyst preparation process of this example are 100 r / min, 200 r / min, 300 r / min, and 400 r / min, respectively, the pyrolysis temperature used is fixed at 800°C, and the carbon source used is fixed at alkaline lignin, and NC 800-100 / 200 / 300 / 400 is obtained. The results are shown in Table 4:

[0044] As shown in Table 4, when the ball milling speed is 150-250 r / min, the carbon-based non-metal catalyst of nitrogen-doped lignin obtained by pyrolysis can efficiently convert lignin. This is because if the speed is too low, the ball milled particles will have a higher particle size, while if the speed is too high, the sample will stick to the side wall of the agate jar due to centrifugal effect, affecting sufficient ball milling, and then the nitrogen source and lignin cannot be fully doped, resulting in a low specific surface area of ​​the obtained catalyst or a lack of alkaline active sites, affecting its catalytic depolymerization of CO / CC bonds.

[0045] Matters not covered by the present invention are applicable to the prior art.

Claims

1. A method for preparing a nitrogen-doped lignin carbon-based non-metal catalyst, characterized in that: Industrial lignin with a metal element content of not less than 20 wt% and a nitrogen source are subjected to high-speed ball milling and then subjected to high-temperature pyrolysis in an inert atmosphere to obtain a nitrogen-doped lignin carbon-based non-metallic catalyst; The pyrolysis temperature of the high temperature pyrolysis is 750-900°C. The ball milling speed is 150-250 r / min, the ball milling time is greater than 8 hours, and the total mass of the agate balls used in the ball milling is 5-10 times the mass of the object to be ball milled; the agate balls include large, medium and small agate balls, the mass ratio of the large, medium and small agate balls is 1:4:1-1:4:4, the diameter of the large agate ball is 10-15 mm, the diameter of the medium agate ball is 4-6 mm, and the diameter of the small agate ball is 2-3 mm.

2. The preparation method according to claim 1, characterized in that: The diameter of the large agate ball is 10 mm, the diameter of the medium agate ball is 5 mm, and the diameter of the small agate ball is 3 mm. The mass ratios of the three are 1:4:2-1:4:3 respectively.

3. The preparation method according to claim 1, characterized in that: The mass ratio of lignin to nitrogen source is 1:2-2:1; The industrial lignin is at least one of alkali lignin and sodium lignin sulfonate, and the total content of alkali metal elements and alkaline earth metal elements in the industrial lignin is not less than 20 wt %; the nitrogen source is at least one of dicyandiamide, melamine and urea.

4. The preparation method according to claim 1, characterized in that: The high temperature pyrolysis has a pyrolysis temperature of 800-900° C. and a pyrolysis time of 0.5-2 h.

5. The preparation method according to claim 1, characterized in that: The total mass of the agate balls used is 150-250 g, preferably 200-230 g.

6. A nitrogen-doped lignin carbon-based non-metal catalyst obtained by the preparation method according to any one of claims 1 to 5.

7. Use of the nitrogen-doped lignin carbon-based non-metal catalyst according to claim 6 in catalytic depolymerization of lignin, characterized in that: The nitrogen-doped lignin carbon-based non-metal catalyst is used as a catalyst, and the catalyst, lignin reaction substrate and solvent are placed in a ball milling jar to perform mechanical ball milling at a speed of 150-250 r / min to drive the catalytic depolymerization of lignin.

8. The use according to claim 7, characterized in that: In the catalytic depolymerization of lignin, the ball milling time is 12-24 h, and the mass ratio of the solvent to the agate balls is 1:5-1:10, preferably 1:7-1:

8.

9. The use according to claim 7, characterized in that: The lignin reaction substrate is an oxidized β-O-4 model substrate, the conversion rate of the β-O-4 model substrate is above 80%, the lignin depolymerization products include phenol and esters, and the yields of phenol and esters are both above 60%; the esters include methyl benzoate and methyl benzoylformate.

10. The use according to claim 9, characterized in that: The conversion rate of β-O-4 model substrate was above 90%.

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