A carbon-coated manganese dioxide catalyst and its preparation method and application

Abstract

The present invention discloses a carbon-coated manganese dioxide catalyst, a preparation method, and an application thereof, and relates to the technical field of catalytic material preparation. The method comprises the following steps: pre-treating manganese dioxide, adding deionized water, stirring evenly, then adding a dispersant and a carbon source, emulsifying, and obtaining a mixture; sand milling, then spray drying, to obtain a carbon-coated manganese dioxide catalyst precursor; and placing the mixture in an activation gas for activation to obtain a carbon-coated manganese dioxide catalyst. The present invention uses manganese dioxide as the main catalytic active center and coats the surface of the manganese dioxide with a layer of carbon material, thereby reducing manganese dissolution and improving the activity and stability of the catalyst. This method solves the metal dissolution and secondary pollution problems of residual hydrogen peroxide in the decomposition of nitrogen-methyl morpholine oxide by conventional manganese dioxide, is environmentally friendly, and is conducive to application and promotion.

Description

Technical Field

[0001] The present invention relates to the technical field of catalytic material preparation, and in particular to a carbon-coated manganese dioxide catalyst and a preparation method and application thereof. Background Art

[0002] As a green and environmentally friendly solvent, N-methylmorpholine oxide (NMMO) has been widely used in the processing and conversion of cellulose. Its excellent solubility and low toxicity have made it highly sought after by the industry. Using NMMO as a solvent to dissolve wood pulp is an essential and critical step in the production of the highly sought-after lyocell fiber. This step requires a huge amount of NMMO and extremely high purity. Therefore, obtaining high-quality NMMO is crucial to the development of lyocell fibers. Therefore, improving NMMO quality is crucial to meeting the development needs of the lyocell fiber industry. Currently, the mainstream industrial route for synthesizing N-methylmorpholine oxide involves the oxidation of tertiary amines using N-methylmorpholine (NMM) as the raw material and H₂O₂ as the oxidant. To ensure a complete reaction, hydrogen peroxide is often used in excess, resulting in residual hydrogen peroxide in the NMMO after the reaction. This residual hydrogen peroxide can impair spinning performance during lyocell fiber production and consume antioxidants, leading to NMMO decomposition. Therefore, effectively removing residual hydrogen peroxide from NMMO products is crucial for improving their quality.

[0003] Catalysts used to decompose hydrogen peroxide are primarily divided into metal-based and non-metal-based catalysts. Metal-based catalysts offer high catalytic activity, but they inevitably suffer from metal dissolution, which can lead to secondary pollution risks. Non-metal-based catalysts, while less active than metal-based catalysts, are also widely used in catalytic reactions due to their low cost, availability, and environmental friendliness. Therefore, key challenges in improving NMMO quality remain: removing residual hydrogen peroxide from NMMO products, innovating catalyst preparation methods, and enhancing catalyst activity and stability. Summary of the Invention

[0004] In order to solve the above technical problems, the purpose of the present invention is to provide a carbon-coated manganese dioxide catalyst and its preparation method and application, so as to solve the problems of metal dissolution and low activity in residual hydrogen peroxide in the decomposition of N-methylmorpholine oxide in the prior art.

[0005] The present invention solves the above technical problems with the following technical solution: a method for preparing a carbon-coated manganese dioxide catalyst is provided, comprising the following steps:

[0006] (1) Pre-treating manganese dioxide, adding deionized water, stirring evenly, and then adding a dispersant and a carbon source, emulsifying, and obtaining a mixture;

[0007] (2) sand-milling the mixture obtained in step (1), and then spray-drying the mixture to obtain a carbon-coated manganese dioxide catalyst precursor;

[0008] (3) The carbon-coated manganese dioxide catalyst precursor prepared in step (2) is placed in an activation gas for activation to obtain a carbon-coated manganese dioxide catalyst.

[0009] On the basis of the above technical solution, the present invention can also be improved as follows:

[0010] Furthermore, in step (1), the manganese dioxide is placed in an acidic solution for pickling, and then washed and dried to complete the manganese dioxide pretreatment process.

[0011] The beneficial effect of adopting the above further technical solution is: pickling to remove impurities on the surface of manganese dioxide.

[0012] Furthermore, the mass ratio of manganese dioxide to the acidic solution is 1:10-20.

[0013] Furthermore, the mass ratio of manganese dioxide to the acidic solution is 1:10.

[0014] Furthermore, the concentration of the acidic solution is 0.01-0.5 mol / L.

[0015] The beneficial effect of adopting the above further technical solution is that: using hydrochloric acid with too high a concentration for pickling may react with manganese dioxide to produce toxic chlorine gas.

[0016] Furthermore, the concentration of the acidic solution is 0.1 mol / L.

[0017] Furthermore, the acidic solution is a hydrochloric acid solution, a sulfuric acid solution or a nitric acid solution.

[0018] Furthermore, the acidic solution is a hydrochloric acid solution.

[0019] Further, pickling is performed for 2-8 hours.

[0020] Further, pickling was performed for 4 hours.

[0021] Furthermore, in step (1), the mass ratio of manganese dioxide, deionized water, dispersant and carbon source is 10-50:50-200:1:20-80.

[0022] Furthermore, in step (1), the dispersant is citric acid, polyvinyl pyrrolidone, polyoxyethylene polyoxypropylene ether block copolymer, alkylphenol polyoxyethylene polyoxypropylene ether, hexadecyltrimethylammonium bromide or dodecyldimethylbetaine.

[0023] Furthermore, in step (1), the dispersant is polyvinyl pyrrolidone.

[0024] Furthermore, in step (1), the carbon source is carboxymethyl cellulose, glucose, sucrose ester, trehalose, polyethylene glycol, white sugar or xanthan gum.

[0025] Furthermore, in step (1), the carbon source is polyethylene glycol.

[0026] Furthermore, in step (1), the emulsification time is 0.5-2h.

[0027] Furthermore, in step (2), the sanding particle size D 50 0.5-10μm.

[0028] The beneficial effect of adopting the above further technical solution is: after the carbon source and manganese dioxide are emulsified evenly and then sand-milled, the uniformity of the existing carbon coating can be improved, and uneven carbon coating or excessive free carbon generation in the subsequent process can be avoided, thereby affecting the activity and stability of the catalyst.

[0029] Furthermore, in step (2), the inlet air temperature of the spray drying is 230-250°C, and the outlet air temperature is 90-100°C.

[0030] Furthermore, in step (2), the inlet air temperature of the spray drying is 240°C and the outlet air temperature is 95°C.

[0031] Furthermore, in step (3), the activation gas is H2, CO2, NH3 or water vapor.

[0032] The beneficial effect of adopting the above-mentioned further technical solution is that by spray drying it at an inlet air temperature of 240°C and an outlet air temperature of 95°C, a spherical particle precursor containing a carbon source can be obtained in a relatively short time. However, the spray drying temperature is not enough to carbonize the carbon source to form a carbon coating layer, so high-temperature activated gas is still required to achieve carbon coating on the surface of manganese dioxide.

[0033] Furthermore, in step (3), the activation gas is NH3.

[0034] The beneficial effect of adopting the above-mentioned further technical solution is that the use of ammonia as the activation atmosphere can effectively achieve nitrogen doping of the carbon coating layer, which is beneficial to improving the electron transfer speed and increasing the catalytic activity.

[0035] Further, activation is performed at 400-900° C. for 1-5 hours.

[0036] Furthermore, the catalyst was activated at 700°C for 3 h.

[0037] The present invention also provides a carbon-coated manganese dioxide catalyst prepared by the method.

[0038] The present invention also provides application of the catalyst in decomposing residual hydrogen peroxide in N-methylmorpholine oxide products.

[0039] The present invention has the following beneficial effects:

[0040] 1. The present invention combines the advantages of catalytically active metal-based catalysts and non-metal-based catalysts, reducing manganese dissolution while ensuring catalytic activity;

[0041] 2. The carbon network formed by the carbon material coated on the surface of manganese dioxide can increase the electron transfer rate during the catalytic reaction, and the carbon network can effectively inhibit the dissolution of manganese, thereby improving the catalytic activity and stability, and effectively avoiding secondary pollution after the use of the catalyst.

[0042] 3. The present invention uses carbon sources with different functional groups to coat the surface of manganese dioxide, which can effectively reduce the dissolution of manganese ions in the manganese dioxide during the catalytic reaction and effectively improve the stability of the catalyst; in addition, the carbon layer can promote electron transfer and effectively improve the catalytic activity. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 The graphs are of hydrogen peroxide removal efficiency and manganese dissolution amount of the catalysts of Examples 1-4;

[0044] Figure 2 The graphs of hydrogen peroxide removal efficiency and manganese dissolution amount of the catalysts of Examples 5, 3 and 6 are as follows;

[0045] Figure 3 Graphs showing hydrogen peroxide removal rates and manganese dissolution rates for the catalysts of Examples 3, 7, and 8;

[0046] Figure 4 These are the stability test results of the C@MnO2-NH3 catalyst of Example 8. DETAILED DESCRIPTION

[0047] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples are only used to explain the present invention and are not intended to limit the scope of the invention. In the embodiments, if specific conditions are not specified, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be purchased commercially.

[0048] Example 1:

[0049] A carbon-coated manganese dioxide catalyst, the preparation method of which comprises the following steps:

[0050] (1) placing the manganese dioxide in an acidic solution (hydrochloric acid solution) with a concentration of 0.1 mol / L for pickling for 4 hours, then washing and drying to remove surface ash and impurities, thereby completing the manganese dioxide pretreatment; wherein the mass ratio of manganese dioxide to acidic solution is 1:10;

[0051] Deionized water was added and stirred evenly, followed by addition of a dispersant (polyvinyl pyrrolidone) and a carbon source (carboxymethyl cellulose), and the mixture was placed in a basket mill and emulsified for 1 hour to obtain a mixture; wherein the mass ratio of manganese dioxide, deionized water, dispersant, and carbon source was 20:100:1:40;

[0052] (2) Place the mixture obtained in step (1) in a sand mill and sand grind it until the particle size is D 50 The particle size was 0.6 μm, and then spray-dried, with the inlet air temperature of the spray-drying being 240°C and the outlet air temperature being 95°C, to obtain a carbon-coated manganese dioxide catalyst precursor;

[0053] (3) The carbon-coated manganese dioxide catalyst precursor prepared in step (2) was placed in an activation gas (H2) and activated at 700°C for 3 hours to obtain a carbon-coated manganese dioxide catalyst (C@MnO2-CMC).

[0054] Example 2:

[0055] A carbon-coated manganese dioxide catalyst, the preparation method of which comprises the following steps:

[0056] In step (1), the carbon source is glucose, and finally a carbon-coated manganese dioxide catalyst (C@MnO2-GLU) is obtained. The rest is the same as in Example 1.

[0057] Example 3:

[0058] A carbon-coated manganese dioxide catalyst, the preparation method of which comprises the following steps:

[0059] In step (1), the carbon source is polyethylene glycol, and finally a carbon-coated manganese dioxide catalyst (C@MnO2-PEG or C@MnO2-PEG700 or C@MnO2-H2) is obtained. The rest is the same as in Example 1.

[0060] Example 4:

[0061] A carbon-coated manganese dioxide catalyst, the preparation method of which comprises the following steps:

[0062] In step (1), the carbon source is white sugar, and a carbon-coated manganese dioxide catalyst (C@MnO2-SUC) is finally obtained. The rest is the same as in Example 1.

[0063] Example 5:

[0064] A carbon-coated manganese dioxide catalyst, the preparation method of which comprises the following steps:

[0065] In step (1), the carbon source is polyethylene glycol;

[0066] In step (3), the activation temperature is 600 °C;

[0067] Finally, a carbon-coated manganese dioxide catalyst (C@MnO2-PEG600) was obtained, and the rest was the same as in Example 1.

[0068] Example 6:

[0069] A carbon-coated manganese dioxide catalyst, the preparation method of which comprises the following steps:

[0070] In step (1), the carbon source is polyethylene glycol;

[0071] In step (3), the activation temperature is 800 °C;

[0072] Finally, a carbon-coated manganese dioxide catalyst (C@MnO2-PEG800) was obtained, and the rest was the same as in Example 1.

[0073] Example 7:

[0074] A carbon-coated manganese dioxide catalyst, the preparation method of which comprises the following steps:

[0075] In step (1), the carbon source is polyethylene glycol;

[0076] In step (3), the activation gas is CO2;

[0077] Finally, a carbon-coated manganese dioxide catalyst (C@MnO2-CO2) was obtained, and the rest was the same as in Example 1.

[0078] Example 8:

[0079] A carbon-coated manganese dioxide catalyst, the preparation method of which comprises the following steps:

[0080] In step (1), the carbon source is polyethylene glycol;

[0081] In step (3), the activation gas is NH3;

[0082] Finally, a carbon-coated manganese dioxide catalyst (C@MnO2-NH3) was obtained, and the rest was the same as in Example 1.

[0083] Example 9:

[0084] A carbon-coated manganese dioxide catalyst, the preparation method of which comprises the following steps:

[0085] (1) placing the manganese dioxide in an acidic solution (sulfuric acid solution) with a concentration of 0.01 mol / L for pickling for 8 hours, then washing and drying to remove surface ash and impurities, thereby completing the manganese dioxide pretreatment; wherein the mass ratio of manganese dioxide to acidic solution is 1:15;

[0086] Deionized water was added and stirred evenly, followed by addition of a dispersant (citric acid) and a carbon source (glucose), and the mixture was placed in a basket mill and emulsified for 0.5 h to obtain a mixture; wherein the mass ratio of manganese dioxide, deionized water, dispersant, and carbon source was 10:50:1:20;

[0087] (2) Place the mixture obtained in step (1) in a sand mill and sand grind it until the particle size is D 50 The particle size was 0.5 μm, and then spray-dried, with the inlet air temperature of the spray-drying being 230°C and the outlet air temperature being 90°C, to obtain a carbon-coated manganese dioxide catalyst precursor;

[0088] (3) The carbon-coated manganese dioxide catalyst precursor prepared in step (2) was placed in an activation gas (CO2) and activated at 400°C for 5 h to obtain a carbon-coated manganese dioxide catalyst (C@MnO2-GLU[H2SO4]).

[0089] Example 10:

[0090] A carbon-coated manganese dioxide catalyst, the preparation method of which comprises the following steps:

[0091] (1) placing the manganese dioxide in an acidic solution (nitric acid solution) with a concentration of 0.5 mol / L for pickling for 2 h, then washing and drying to remove surface ash and impurities, thereby completing the manganese dioxide pretreatment; wherein the mass ratio of manganese dioxide to acidic solution is 1:20;

[0092] Deionized water was added and stirred evenly, followed by addition of a dispersant (polyvinyl pyrrolidone) and a carbon source (polyethylene glycol), and the mixture was placed in a basket mill and emulsified for 2 h to obtain a mixture; wherein the mass ratio of manganese dioxide, deionized water, dispersant, and carbon source was 50:200:1:80;

[0093] (2) Place the mixture obtained in step (1) in a sand mill and sand grind it until the particle size is D 50 The particle size was 10 μm, and then spray-dried, with the inlet air temperature of the spray-drying being 250°C and the outlet air temperature being 100°C, to obtain a carbon-coated manganese dioxide catalyst precursor;

[0094] (3) The carbon-coated manganese dioxide catalyst precursor prepared in step (2) was placed in an activation gas (NH3) and activated at 900°C for 1 h to obtain a carbon-coated manganese dioxide catalyst (C@MnO2-PEG[HNO3]).

[0095] Test example

[0096] The carbon-coated manganese dioxide catalyst prepared by the present invention was tested for its hydrogen peroxide decomposition performance, with manganese dioxide used as a comparison. The specific testing method was as follows: under process conditions of 70°C and a catalyst dosage of 0.2 g / L, the catalyst was used to decompose residual hydrogen peroxide during the preparation of N-methylmorpholine oxide.

[0097] 1. Different carbon sources

[0098] The carbon-coated manganese dioxide catalysts prepared in Examples 1-4 were tested for performance, and the results are shown in Table 1. Figure 1 .

[0099] Depend on Figure 1 It can be seen that compared with the manganese dioxide catalyst alone, the catalytic activity of manganese dioxide after carbon coating is not greatly improved, but manganese dissolution is effectively suppressed. The group of catalysts with the least manganese dissolution is C@MnO2-PEG (Example 3), which has a hydrogen peroxide removal rate of 98.2% and a manganese dissolution amount of 5 ppm.

[0100] 2. Different temperatures

[0101] The carbon-coated manganese dioxide catalysts prepared in Examples 5, 3 and 6 were tested for performance. The results are shown in Figure 2 .

[0102] Depend on Figure 2 The catalyst prepared at an activation temperature of 700°C (Example 3) showed higher activity, achieving a hydrogen peroxide removal rate of 98.2% and a manganese release of 5 ppm. This is primarily due to the fact that at lower activation temperatures, the carbon layer coating the manganese dioxide surface fails to form a stable graphitized structure, which affects electron transfer and reduces catalyst activity. At higher activation temperatures, the carbon layer collapses, reducing the contact surface between the catalytically active sites and the substance. Furthermore, the carbon coating in the collapsed areas is less effective, leading to increased manganese release.

[0103] 3. Different activation gases

[0104] The carbon-coated manganese dioxide catalysts prepared in Examples 3, 7 and 8 were tested for performance. The results are shown in Table 1. Figure 3 .

[0105] Depend on Figure 3The catalyst prepared in an NH3 activation atmosphere exhibited superior catalytic performance (Example 8), achieving a 99.4% removal rate for residual hydrogen peroxide in the NMMO product and a manganese dissolution rate of 4 ppm. This is primarily due to the sintering in an NH3 atmosphere, which introduces additional nitrogen doping and forms nitrogen-containing functional groups on the surface of the carbon-coated manganese dioxide catalyst. These groups, in turn, form Mn-Nx catalytically active centers with manganese. This structure facilitates the catalytic oxidative decomposition of hydrogen peroxide and effectively chelates manganese ions, preventing manganese dissolution and significantly enhancing catalytic activity.

[0106] 4. Stability

[0107] The carbon-coated manganese dioxide catalyst prepared in Example 8 was subjected to stability testing. The specific testing method was as follows: after the performance test, the catalyst was reused 4 times, the catalyst was separated using a solvent filter, dried, placed in a N2 atmosphere, and heat treated at 600°C for 2 h to test the stability of the catalyst. The results are shown in FIG. Figure 4 .

[0108] Depend on Figure 4 It can be seen that after four repeated experiments, the catalyst activity dropped to 90.1%, but after heat treatment in a nitrogen atmosphere, the activity of the catalyst was restored. The main reason for this is that after four repeated experiments, some active sites on the catalyst surface were blocked by organic matter, resulting in a decrease in catalytic activity. After sintering at 600°C, the organic matter was removed, and the catalytic active sites reappeared. This shows that the deactivation of this catalyst is reproducible, indicating the good stability of the catalyst.

[0109] 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 preparing a carbon-coated manganese dioxide catalyst, characterized in that: The following steps are involved: (1) Pre-treating manganese dioxide, adding deionized water, stirring evenly, and then adding a dispersant and a carbon source, emulsifying, and obtaining a mixture; (2) sand-milling the mixture obtained in step (1), and then spray-drying the mixture to obtain a carbon-coated manganese dioxide catalyst precursor; (3) placing the carbon-coated manganese dioxide catalyst precursor prepared in step (2) in an activation gas for activation to obtain a carbon-coated manganese dioxide catalyst; In step (1), the mass ratio of manganese dioxide, deionized water, dispersant and carbon source is 10-50:50-200:1:20-80; In step (1), the carbon source is carboxymethyl cellulose, glucose, sucrose ester, trehalose, polyethylene glycol, white sugar or xanthan gum; In step (3), the activation gas is H2, CO2, NH3 or water vapor; In step (3), activation is performed at 400-900°C for 1-5 hours.

2. The method for preparing the carbon-coated manganese dioxide catalyst according to claim 1, wherein In step (1), the manganese dioxide is placed in an acidic solution, pickled, and then washed and dried to complete the manganese dioxide pretreatment process.

3. The method for preparing the carbon-coated manganese dioxide catalyst according to claim 1, wherein In step (1), the dispersant is citric acid, polyvinyl pyrrolidone, polyoxyethylene polyoxypropylene ether block copolymer, alkylphenol polyoxyethylene polyoxypropylene ether, hexadecyltrimethylammonium bromide or dodecyldimethylbetaine.

4. The method for preparing the carbon-coated manganese dioxide catalyst according to claim 1, wherein In step (2), the sanding particle size D 50 0.5-10 μm.

5. A carbon-coated manganese dioxide catalyst prepared by the method for preparing a carbon-coated manganese dioxide catalyst according to any one of claims 1 to 4.

6. Use of the carbon-coated manganese dioxide catalyst according to claim 5 in decomposing residual hydrogen peroxide in N-methylmorpholine oxide products.

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