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

Abstract

The application is a preparation method and application of a nitrogen-doped lignin carbon-based non-metallic catalyst, which comprises the following steps: treating industrial lignin with a metal element content of no less than 20 wt% and a nitrogen source through high-speed ball milling, and then pyrolyzing under an inert atmosphere at a high temperature to obtain a nitrogen-doped lignin carbon-based non-metallic catalyst; the pyrolysis temperature of the high-temperature pyrolysis is 750-900 DEG C, the ball milling speed in the ball milling is 150-250 r / min, the ball milling time is greater than 8 h, the agate balls comprise 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. The nitrogen-doped lignin carbon-based non-metallic catalyst used in the application itself serves as a solid base catalyst, avoiding the problem that the current lignin oxidative depolymerization relies on an external base, and the lignin depolymerization process adopts a mechanical force-driven catalytic depolymerization of C-O / C-C bonds, avoiding high-temperature and high-pressure operation, and having environmental protection and economic nature.

Description

Technical Field

[0001] This invention relates to the field of carbon-based nonmetallic catalyst preparation and lignin catalytic depolymerization, specifically to a method for preparing a nitrogen-doped lignin carbon-based nonmetallic catalyst and its application. This catalyst drives lignin catalytic depolymerization under mechanical force to obtain oxygen-containing aromatic chemicals. Background Technology

[0002] Lignin is a three-dimensional network of aromatic compounds composed of a series of aromatic units linked by CO and C-C bonds. Its selective conversion into oxygen-containing aromatic monomers can replace petroleum-based chemical products. Oxidative depolymerization of lignin is an effective strategy for converting high-value lignin into oxygen-containing aromatic chemicals, with the key being the development of a catalytic system capable of controllably trimming CO / C-C bonds. Currently, lignin depolymerization research mainly focuses on heterogeneous metal catalysts. For example, a patented invention (CN115970732B) discloses that lignin-derived carbon-supported iron-based catalysts can effectively depolymerize lignin when the iron metal content is 2-10%. However, the preparation of heterogeneous metal catalysts generally requires the addition of exogenous metal sources, and heterogeneous metal catalysts generally suffer from low metal atom utilization. Furthermore, current lignin depolymerization research, due to the stubborn chemical properties of lignin, usually employs relatively harsh depolymerization conditions. Even with milder oxidative depolymerization strategies, high temperatures (~120℃) and high pressures (~1 MPa) are still required. In summary, it is necessary to develop efficient depolymerization of lignin using non-metallic and mild catalytic systems. This strategy aligns with the development direction of green chemistry and is also economical. Summary of the Invention

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

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0005] In a first aspect, the present invention provides a method for preparing a nitrogen-doped lignin carbon-based non-metallic catalyst, wherein 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 under an inert atmosphere to obtain a nitrogen-doped lignin carbon-based non-metallic catalyst.

[0006] The pyrolysis temperature of the high-temperature pyrolysis is 750-900℃.

[0007] The ball milling speed is 150-250 r / min, the milling time is greater than 8 h, and the total mass of the agate balls used in the milling is 5-10 times the mass of the object to be milled. The agate balls consist of large, medium and small agate balls, with a mass ratio of 1:4:1 to 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.

[0008] Furthermore, the large agate ball has a diameter of 10 mm, the medium agate ball has a diameter of 5 mm, and the small agate ball has a diameter of 3 mm, with a mass ratio of 1:4:2 to 1:4:3.

[0009] The mass ratio of lignin to nitrogen source is 1:2 to 2:1;

[0010] 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.

[0011] Furthermore, the pyrolysis temperature of the high-temperature pyrolysis is 800-900℃, and the pyrolysis time is 0.5-2 h.

[0012] Furthermore, the total mass of the agate balls used is 150-250 g, preferably 200-230 g.

[0013] Secondly, the present invention protects a nitrogen-doped lignin carbon-based nonmetallic catalyst obtained by the preparation method described above.

[0014] Thirdly, the present invention protects the 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 the catalyst, and the catalyst, lignin reaction substrate, and solvent are placed in a ball mill jar and mechanically ball-milled at a speed of 150-250 r / min to drive the catalytic depolymerization of lignin.

[0015] The ball milling time in the lignin catalytic depolymerization 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.

[0016] The lignin reaction substrate is a treated oxidized β-O-4 model substrate with a conversion rate of over 80%. The lignin depolymerization products include phenol and esters, with yields of both phenol and esters exceeding 60%. The esters include methyl benzoate and methyl benzoylformate.

[0017] Preferably, the conversion rate of the β-O-4 model substrate is above 90%.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] 1. This invention uses industrial lignin containing metal salts (metal element content not less than 20 wt%) as a carbon source to prepare a nitrogen-doped lignin carbon-based non-metallic catalyst for lignin oxidative depolymerization. Compared with the existing invention patent (CN115970732B) which uses iron-based catalysts supported on lignin carbon, this application uses a nitrogen-doped lignin carbon-based non-metallic catalyst without the addition of exogenous metals. In the preparation process, the carbon source and nitrogen source are ball-milled at high speed using a planetary ball mill. The uniform dispersion of the nitrogen source and lignin promotes subsequent full pyrolysis, increases the surface area of ​​the obtained nitrogen-doped carbon material and the content of pyridine nitrogen and pyrrole nitrogen, which is beneficial for oxygen activation. The metal salts (sodium salts, magnesium salts, etc.) contained in the selected industrial lignin can act as pore-forming agents to promote the generation of micro / mesopores and alkaline sites in the carbon material during pyrolysis, and promote the breaking of CO / CC bonds in the subsequent lignin depolymerization. More importantly, in this invention, the nitrogen-doped carbon catalyst, lignin reaction substrate, and solvent are mechanically catalytically depolymerized to obtain the target monomer product, avoiding high temperature and high pressure conditions, which is environmentally friendly and economical.

[0020] 2. In this invention, industrial lignin is widely sourced as a solid waste from papermaking. Alkali lignin or sodium lignin sulfonate itself contains metal salts (sodium salts, magnesium salts, etc.), which form alkaline metal oxides (MOx, M=Na / Mg, etc.) during pyrolysis. These oxides can act as alkaline sites to activate the C in lignin. β -H bond and promote CO / CC bond breaking.

[0021] 3. This invention uses a nitrogen-doped lignin carbon-based non-metallic catalyst (without the need for additional metals or other exogenous reagents). The nitrogen doping forms abundant pyridine nitrogen and pyrrole nitrogen on the carbon surface, which can serve as sites for activating O2. Metal salts in industrial lignin can also act as pore-forming agents 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.

[0022] 4. The nitrogen-doped lignin carbon-based non-metallic catalyst used in this invention is itself a solid base catalyst, which avoids the current problem of lignin oxidation and depolymerization relying on external base. Moreover, 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. Attached Figure Description

[0023] Figure 1 This is a comparison chart of the CO2-programmed temperature-increase desorption effects of NC 800 and NC 800-S after acid washing in Example 1. Detailed Implementation

[0024] The present invention will be further explained below with reference to embodiments, but these are not intended to limit the scope of protection of this application.

[0025] The present invention discloses a method for preparing a nitrogen-doped lignin carbon-based non-metallic catalyst, wherein industrial lignin containing metal salts (such as sodium and magnesium salts) with a metal element content greater than 20 wt% and a nitrogen source are ball-milled at high speed in an agate jar, and the ball-milled particles are then subjected to high-temperature pyrolysis under an inert atmosphere to obtain a nitrogen-doped lignin carbon-based non-metallic catalyst, referred to as nitrogen-doped lignin carbon.

[0026] The pyrolysis temperature of the high-temperature pyrolysis is 750-900℃.

[0027] The ball milling speed is 150-250 r / min, the milling time is 6-30 h, and the total mass of the agate balls used in the milling is 5-10 times the mass of all objects to be milled. The agate balls consist of large, medium and small agate balls, with a mass ratio of 1:4:1 to 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.

[0028] Using the aforementioned nitrogen-doped lignin carbon-based non-metallic catalyst as the catalyst, the catalyst, lignin reaction substrate, and solvent were placed in a ball mill jar and mechanically ball-milled at a speed of 150-250 r / min to drive the catalytic depolymerization of lignin. Samples were taken after ball milling and analyzed by gas chromatography.

[0029] Furthermore, the industrial lignin is at least one of alkali lignin containing metal salts, sodium lignin sulfonate, etc., preferably alkali lignin and / or sodium lignin sulfonate with sodium and magnesium content greater than 20 wt%.

[0030] Furthermore, the nitrogen source is at least one of dicyandiamide, melamine, urea, etc., preferably dicyandiamide.

[0031] 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℃, preferably 800-900℃, and the pyrolysis time is 0.5-2 h.

[0032] The ball milling speed and milling time were the same in both of the above two milling processes, and the configuration of the agate balls was the same.

[0033] The total mass of the agate balls used is 150-250 g, preferably 200-230 g.

[0034] Preferably, the large agate ball has a diameter of 10 mm, the medium agate ball has a diameter of 5 mm, and the small agate ball has a diameter of 3 mm, with a mass ratio of 1:4:2 to 1:4:3.

[0035] Preferably, the ball milling time is 12 h-24 h.

[0036] Furthermore, the mass ratio of the solvent to the agate ball is 1:5-1:10, preferably 1:7-1:8.

[0037] In this invention, the lignin reaction substrate is an oxidized β-O-4 model substrate with a conversion rate of over 80%. The lignin depolymerization products include phenol and esters (methyl benzoate, methyl benzoylformate), with yields of phenol and esters both exceeding 60%.

[0038] Example 1

[0039] (1) Preparation of nitrogen-doped lignin carbon-based nonmetallic catalyst (NC): Alkali lignin (sodium and magnesium content of 20 wt%) and dicyandiamide were ball-milled in an agate jar at 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, and the mass ratio of large, medium, and small agate balls was 1:4:2. The uniformly milled sample was then placed in a quartz boat and heated to the final temperature at a rate of 5 °C / min under a N2 atmosphere and held for 1 h. After cooling, the sample was removed from the pyrolysis furnace, ball-milled uniformly, washed with deionized water, and dried to obtain the NC catalyst.

[0040] With other conditions unchanged, different catalysts were obtained by changing only the final temperature (500℃, 650℃, 800℃), which were designated as NC 500, NC 650, and NC 800, respectively. To study the importance of the base site in the oxidative depolymerization of lignin, NC 800 was acid-washed to obtain acid-washed NC 800-S.

[0041] (2) Catalytic depolymerization experiments of β-O-4 model substrate were carried out on 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 jar and mechanically catalyzed for 12 h (200 r / min). After ball milling, samples were taken and analyzed by gas chromatography.

[0042]

[0043] As shown in Table 1, NC 800 prepared at a pyrolysis temperature of 800℃ can achieve a lignin β-O-4 model substrate conversion rate of over 80% (reaching 97.8% in the table), and a phenol and ester yield of over 60%. This is because sufficient pyrolysis temperature can generate enough pyridine nitrogen / pyrrole nitrogen and base sites, thereby promoting oxygen activation and CO / CC bond breaking in lignin. In contrast, NC 800-S after acid washing lacks base sites (see Table 1). Figure 1 This leads to a significant decrease in substrate conversion and product yield for the β-O-4 model.

[0044] Example 2

[0045] The steps in this embodiment are the same as in Embodiment 1, except that the carbon source used is alkali lignin, sodium lignin sulfonate, dealkalized lignin, and acid-treated alkali lignin, and the pyrolysis temperature is fixed at 800℃ to obtain NC 800 (alkali lignin / sodium sulfonate / dealkalized lignin / acid-treated).

[0046] The purpose of acid treatment of alkali lignin is to remove metal salts from alkali lignin. The specific process is as follows: 45 mL of mixed acid is prepared by mixing hydrofluoric acid and nitric acid in a ratio of 1:2. 15 g of alkali lignin is weighed and slowly added to the mixed acid while stirring until no more bubbles are produced. Then, the mixture is filtered. During the filtration process, the mixture is rinsed with deionized water. Finally, it is dried in an oven at 60 °C to obtain acid-treated desalted alkali lignin.

[0047]

[0048] As shown in Table 2, using alkali lignin and sodium lignin sulfonate, which contain metal salts and have a metal element (Na and Mg) content of not less than 20 wt%, as carbon sources, the prepared nitrogen-doped lignin carbon can catalyze the conversion of lignin β-O-4 model substrates by more than 80%, and the yield of phenols and esters by more than 60%. Dealkali-treated lignin or acid-washed alkali lignin lacks sufficient metal salts to promote pore formation during pyrolysis, resulting in a smaller specific surface area and a lack of sufficient base sites in the catalytic material, leading to lower conversion rates of β-O-4 model substrates and lower product yields.

[0049] Example 3

[0050] The steps in this embodiment are the same as in Embodiment 1, except that the ball milling time for the catalyst preparation process in this embodiment is 2 h, 6 h, 12 h, and 24 h, the pyrolysis temperature is fixed at 800℃, and the carbon source is fixed as alkali lignin with sodium and magnesium content of 20 wt%, to obtain NC 800-2 / 6 / 12 / 24.

[0051] The results are shown in Table 3:

[0052]

[0053] Table 3 shows 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, with a lignin β-O-4 model substrate conversion rate of over 90%, and phenol and ester yields of over 60%. This is because a certain ball milling time allows for uniform mixing and doping of the nitrogen source and lignin, resulting in nitrogen-doped lignin carbon with a high specific surface area and high content of active nitrogen species, effectively breaking the CO / CC bonds in lignin.

[0054] Example 4

[0055] The steps in this embodiment are the same as in Example 1, except that the ball milling speeds used in the catalyst preparation process in this embodiment are 100 r / min, 200 r / min, 300 r / min, and 400 r / min, the pyrolysis temperature is fixed at 800℃, and the carbon source is fixed as alkali lignin, resulting in NC 800-100 / 200 / 300 / 400. The results are shown in Table 4.

[0056]

[0057] As shown in Table 4, nitrogen-doped lignin carbon-based non-metallic catalysts obtained by pyrolysis at ball milling speeds of 150-250 r / min can efficiently convert lignin. This is because excessively low speeds result in larger ball milling particle sizes, while excessively high speeds cause the sample to adhere to the sidewall of the agate jar due to centrifugal action, affecting thorough ball milling. Consequently, nitrogen source and lignin cannot be fully doped, resulting in a lower specific surface area or a lack of basic active sites in the obtained catalyst, thus affecting its catalytic depolymerization activity of CO / CC bonds.

[0058] Where this invention does not cover, existing technologies apply.

Claims

1. The application of a nitrogen-doped lignin carbon-based non-metallic catalyst in the catalytic depolymerization of lignin, characterized in that, Industrial lignin with a metal element content of not less than 20 wt% and a nitrogen source are processed by 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℃. The ball milling speed is 150-250 r / min, the milling time is greater than 8 hours, and the total mass of the agate balls used in the milling is 5-10 times the mass of the object to be milled. The agate balls consist of large, medium and small agate balls, with a mass ratio of 1:4:1 to 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 mass ratio of industrial lignin to nitrogen source is 1:2-2:

1. The industrial lignin is at least one of alkali lignin and sodium lignin sulfonate. 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.

2. The application according to claim 1, characterized in that, The large agate ball has a diameter of 10mm, the medium agate ball has a diameter of 5mm, and the small agate ball has a diameter of 3mm. The mass ratio of the three is 1:4:2-1:4:

3.

3. The application according to claim 1, characterized in that, The pyrolysis temperature of the high-temperature pyrolysis is 800-900℃, and the pyrolysis time is 0.5-2h.

4. The application according to claim 1, characterized in that, The total mass of the agate balls used is 150-250g.

5. The application according to claim 1, characterized in that, The total mass of the agate balls used is 200-230g.

6. The application according to claim 1, characterized in that, Using the nitrogen-doped lignin carbon-based non-metallic catalyst as the catalyst, the catalyst, lignin reaction substrate, and solvent are placed in a ball mill jar and mechanically ball-milled at a speed of 150-250 r / min to drive the catalytic depolymerization of lignin.

7. The application according to claim 6, characterized in that, The ball milling time in the catalytic depolymerization of lignin is 12-24 h, and the mass ratio of the solvent to the agate balls is 1:5-1:

10.

8. The application according to claim 6, characterized in that, The mass ratio of the solvent to the agate ball is 1:7 to 1:

8.

9. The application according to claim 6, characterized in that, The lignin reaction substrate is an oxidized β-O-4 model substrate with a conversion rate of over 80%. The lignin depolymerization products include phenol and esters, with yields of both phenol and esters exceeding 60%. The esters include methyl benzoate and methyl benzoylformate.

10. The application according to claim 9, characterized in that, The conversion rate of the β-O-4 model substrate is over 90%.

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