Olefin epoxidation method

Abstract

The invention belongs to the technical field of preparation of epoxy compounds, and relates to an olefin epoxidation method. According to the method, reaction raw materials containing olefin and hydrogen peroxide are subjected to a reaction on a fixed bed reactor using a forming catalyst formed by compounding a titanium silicalite molecular sieve and an organic matter carbonized product. The catalyst is obtained by extrusion molding, drying and carbonization of a mixture containing a titanium silicalite molecular sieve, a benzene ring-containing polymer and a dispersing agent, the obtained catalyst has high mechanical strength and is suitable for olefin epoxidation fixed bed reaction. The method provided by the invention can effectively inhibit formation of byproducts, and realizes efficient utilization of hydrogen peroxide and efficient proceeding of olefin epoxidation reaction.

Description

Technical Field

[0001] This invention relates to a method for epoxidation of olefins, belonging to the field of epoxide preparation technology. Background Technology

[0002] In 1983, Taramasso et al. first disclosed a method for synthesizing TS-1, a titanium-silicon molecular sieve with an MFI structure, in patent document US4410501. Subsequently, titanium-silicon molecular sieves with structures such as AFI, FER, MEL, MOR, MTW, MWW, and Beta were also developed. Titanium-silicon molecular sieves possess selective catalytic oxidation active centers and have broad application prospects in the manufacture of fine chemicals involving heterogeneous catalytic reactions.

[0003] To meet the operational requirements of continuous reactions in chemical production, titanium-silicon molecular sieves need to be formed into catalyst particles with specific sizes and shapes. In this process, chemical binders are required to improve the adhesion and aggregation properties of the titanium-silicon molecular sieves (Li Xiaotao. Research Progress in Molecular Sieves Forming Technology [J]. Industrial Catalysis, 2016, 24(3): 19-27). For example, patent document US6849570B2 discloses binders that can be used for forming titanium-silicon molecular sieves, including alumina, silica, hydrolyzable silicon-containing compounds and their hydrolysis products, boron-containing compounds, phosphorus-containing compounds, clay, etc. However, these chemical binders are dispersed between the titanium-silicon molecular sieve particles and may react with them, thus affecting the performance of the formed product. This phenomenon is particularly significant in the forming process of titanium-silicon molecular sieve catalysts used in olefin epoxidation reactions. For example, Li et al. found that TS-1 molecular sieve molding catalysts prepared using alumina as a binder have a large number of acidic sites, which easily leads to the formation of propylene glycol methyl ether as a byproduct in the propylene epoxidation reaction (LI G, WANG X, YAN H, et al. Epoxidation of propylene using supported titanium silicalite catalysts[J]. Applied Catalysis A: General, 2002, 236(1): 1-7).

[0004] Constructing zeolite molecular sieve extrusions using a self-supporting method can avoid the influence of chemical binders on the zeolite molecular sieves (Wang Deju, Liu Zhongneng, Yang Weimin, et al. Progress in the preparation and application of binder-free zeolite molecular sieves [J]. Petrochemical Technology, 2007, 36(10): 1061-1066). For example, Zuo et al. first bonded TS-1 molecular sieves with silica sol, and then hydrothermally treated the extrusions in tetrapropylammonium hydroxide solution; during this process, the binder crystallized around the original TS-1 particles to form molecular sieves and connect them to form a high-strength structure (ZUO Y, LIU M, HONG L, et al. Role of supports in the tetrapropylammonium hydroxide treated titanium silicalite-1 extrudates [J]. Industrial & Engineering Chemistry Research, 2015, 54(5): 1513-1519). The reaction results showed that this molding method can effectively suppress the formation of by-products. However, self-supporting molding requires the design of specific preparation routes for particular zeolite molecular sieves, and the process is often much more complex than bonded molding. Therefore, the application scope of this method is very limited.

[0005] Patent documents CN108479848B and CN108654683B disclose a highly stable monolithic titanium-silicon molecular sieve catalyst suitable for olefin epoxidation reactions. These catalysts utilize a stainless steel support with an internal pore structure formed by a stainless steel mesh filling. The active component, titanium-silicon molecular sieve, is loaded onto the surface of this stainless steel support through coating and calcination or in-situ crystallization. Using a stainless steel support not only suppresses side reactions but also facilitates catalyst loading and unloading. However, the specially designed stainless steel support is costly, the catalyst preparation process is complex, and the recycling and disposal of spent catalysts remain unresolved issues.

[0006] Similar to the self-supporting zeolite molecular sieve forming process described above, in the preparation of carbon materials, high-molecular organic compounds such as pitch are often used as chemical binders. The forming purpose is achieved by creating a carbon-carbon composite structure between the carbon material powder to be formed and the carbonization product of the binder (Yang Yongbin, Dong Yinrui, Zhong Qiang, et al. Application and research progress of high-temperature coal tar pitch binder carbonization and consolidation in carbon profiles [J]. Chemical Industry and Engineering Progress, 2022, 41(12): 6419-6429). However, it is traditionally believed that organic binders are not suitable for the chemical bonding of zeolite molecular sieves. The reasons are, firstly, that organic binders will oxidize and decompose during high-temperature calcination; secondly, zeolite molecular sieves are difficult to link with carbon through covalent bonds.

[0007] By altering the heat treatment method of powders, some high-molecular-weight organic compounds can also act as binders for inorganic powders. Patent document CN1181917C discloses a mixed metal catalyst containing a flammable binder. This flammable binder is formed by the pyrolysis of organic polymers in an inert atmosphere at 300-600℃. The organic polymers include polyacrylonitrile, phenolic plastics, polyamides, polyurethanes, cellulose and its derivatives, hemicellulose materials, polyfurfuryl alcohol, styrene-divinylbenzene copolymers, phenolic resins, furan resins, polyimide resins, polyphenylene resins, phenolic foams, and polyurethane foams. Patent document CN107029668B discloses a honeycomb molecular sieve-activated carbon composite adsorbent. This adsorbent is prepared by mixing Y-type molecular sieves, activated carbon powder, expanded graphite, silica sol, organic binder, and deionized water through steps such as mixing, kneading, vacuum kneading, aging, honeycomb extrusion molding, low-temperature microwave shaping, and microwave vacuum sintering. The aforementioned organic binders are sodium carboxymethyl cellulose, sodium hydroxymethyl cellulose, polyvinyl alcohol, phenolic resin emulsion, and acrylic resin emulsion. Patent document CN111115631B discloses a method for preparing coffee grounds-based molded porous carbon material. This method uses coffee grounds as raw material, mixing them with solvents, extrusion aids, binders, and structural reinforcing agents, followed by kneading, extrusion molding, drying, carbonization, and activation to obtain porous carbon. The aforementioned binders are starch, sodium carboxymethyl starch or montmorillonite, sepiolite, silicates, silica sol, sodium silicate, dilute nitric acid, and phosphoric acid.

[0008] The high-molecular-weight organic compounds capable of forming carbonization products with specific morphologies are not limited to the substances mentioned above. For example, patent document CN116272874B discloses a method for preparing plastic carbon materials. This method involves mixing treated waste plastic particles with ferric chloride, adding water, freeze-drying, and pyrolyzing to obtain plastic carbon materials. The aforementioned waste plastic particles are polyethylene, polypropylene, polyethylene terephthalate, and polymethyl methacrylate. Zhang Wenjun, on the other hand, used polyphenylene sulfide nonwoven fabric as a carbon source to design and prepare carbon materials for supercapacitors with different morphologies and structures (Zhang Wenjun. Preparation and Electrochemical Energy Storage Research of Polyphenylene Sulfide Derivative Carbon-Based Composite Materials [D]. Tianjin University of Technology, 2023). Summary of the Invention

[0009] This invention aims to provide a method for olefin epoxidation that overcomes the shortcomings of existing technologies. To achieve this objective, the invention employs the following technical solution:

[0010] The reactants containing olefins and hydrogen peroxide are reacted in a fixed-bed reactor using a molded catalyst composed of titanium-silicon molecular sieves and organic carbonization products.

[0011] The above-mentioned shaped catalyst, composed of titanium-silicon molecular sieves and organic carbonization products, is prepared through the following steps:

[0012] Step a: Prepare a mixture containing titanium silicate molecular sieve, benzene ring polymer, and dispersant;

[0013] Step b: The mixture obtained in step a is extruded into strips and dried to obtain the precursor.

[0014] Step c: Carbonize the precursor obtained in step b in a non-oxidizing atmosphere.

[0015] In the olefin epoxidation method provided by the present invention, the olefin is one or more of propylene, chloropropylene, butene or pentene.

[0016] In the olefin epoxidation method provided by this invention, hydrogen peroxide is an aqueous solution with a mass fraction of 1%-30%.

[0017] In the olefin epoxidation method provided by the present invention, the reaction raw materials also contain one or more of methanol, ethanol, propanol, and tert-butanol, and the molar ratio of the reactants to hydrogen peroxide is (5-50):1.

[0018] In the olefin epoxidation method provided by the present invention, the molar ratio of olefin to hydrogen peroxide is (1-20):(0.1-2).

[0019] In the olefin epoxidation method provided by the present invention, the reaction temperature is 30-150℃, preferably 30-100℃.

[0020] In the olefin epoxidation method provided by the present invention, the reaction pressure is 0.1-8 MPa, preferably 0.1-5 MPa.

[0021] In the olefin epoxidation method provided by this invention, the olefin liquid hourly space velocity is 0.1-15 h⁻¹. -1 Preferred time: 0.1-10h -1 .

[0022] In the olefin epoxidation method provided by this invention, the reaction raw materials may contain alkaline substances.

[0023] In the olefin epoxidation method provided by the present invention, the framework type of the titanium-silicon molecular sieve in catalyst preparation step a is one or more of AFI, *BEA, FER, MEL, MFI, MOR, MTW, and MWW, and the titanium-silicon molar ratio is (0.001-0.05):1, preferably (0.005-0.04):1, and more preferably (0.01-0.03):1.

[0024] In the olefin epoxidation method provided by this invention, the benzene ring-containing polymer in catalyst preparation step a is specifically one or more of polystyrene, polyphenylene ether, lignin, or phenolic resin. Differences in production processes can lead to differences in the physicochemical properties of the aforementioned benzene ring-containing polymers. For example, currently industrially produced polystyrene includes general-purpose polystyrene, high-impact polystyrene, expandable polystyrene, metallocene polystyrene, and other subcategories; lignin is derived from alkaline pulping or extracted using organic solvents; and commercially available phenolic resins exist in various forms such as aqueous solutions, alcoholic solutions, and solids. The olefin epoxidation catalyst preparation method provided by this invention does not limit the specific subcategories of polystyrene, polyphenylene ether, lignin, or phenolic resin used; the benzene ring-containing polymer used can be in the form of powder, fluid, or solution dissolved in a dispersant during feeding.

[0025] In the olefin epoxidation method provided by this invention, the dispersant in catalyst preparation step a is one or more of water, methanol, ethanol, benzene, toluene, xylene, acetone, dichloromethane, trichloromethane, or carbon tetrachloride. The above-mentioned dispersant does not chemically react with the titanium-silicon molecular sieve during mixing and molding, and is removed during the drying process of the molded body. Adding a dispersant allows the benzene-ring-containing polymer to dissolve and swell, thereby better contacting and mixing with the titanium-silicon molecular sieve. Simultaneously, adding a dispersant also facilitates smooth molding operations. Therefore, the choice of dispersant type is determined by the physicochemical properties of the benzene-ring-containing polymer used. For example, polystyrene is soluble in benzene, toluene, xylene, dichloromethane, trichloromethane, or carbon tetrachloride; polyphenylene ether is soluble in benzene, toluene, or trichloromethane; lignin is partially soluble in methanol, ethanol, and acetone; and phenolic resin is soluble in water or methanol, ethanol, and acetone depending on the curing stage. The olefin epoxidation catalyst preparation method provided by this invention does not limit the amount of dispersant used. In practice, the determination should be based on factors such as the absorption capacity of titanium-silicon molecular sieves, the solubility properties of benzene-containing polymers, and the requirements of extrusion molding operations.

[0026] In the olefin epoxidation method provided by the present invention, the mass ratio of titanium silicate molecular sieve to benzene ring-containing polymer in catalyst preparation step a is 0.6-3, preferably 0.8-3.

[0027] In the olefin epoxidation method provided by this invention, the extrusion molding process in catalyst preparation step b is well known to those skilled in the art, as described by Zhang Jiguang (Zhang Jiguang. Catalyst Preparation Process Technology [M]. Beijing: China Petrochemical Press, 2004), Ertl et al. (ERTL G, H, The monograph by F, et al., Handbook of Heterogeneous Catalysis [M]. 2nd ed. Weinheim: Wiley, 2008, elaborates on this topic in detail.

[0028] In the olefin epoxidation method provided by the present invention, the drying atmosphere in catalyst preparation step b is air, and the drying temperature is room temperature to 250°C.

[0029] The selection of the non-oxidizing atmosphere required for carbonization in step c of catalyst preparation in the olefin epoxidation method provided by this invention is well known to those skilled in the art. The non-oxidizing atmosphere may contain one or more of hydrogen, helium, methane, carbon monoxide, carbon dioxide, nitrogen, and argon.

[0030] In the olefin epoxidation method provided by the present invention, the carbonization temperature in catalyst preparation step c is 450-850℃, preferably 525-725℃; the carbonization time is 0.5-6h, preferably 1-5h.

[0031] In the olefin epoxidation method provided by the present invention, the catalyst preparation is limited by restricting the amount of titanium-silicon molecular sieve, the type and amount of benzene ring-containing polymer, and the carbonization conditions. The mass fraction of carbon in the catalyst can usually be controlled within the range of 5%-25%.

[0032] The most significant difference between the olefin epoxidation method provided by this invention and the prior art is that:

[0033] An olefin epoxidation catalyst profile was prepared by using specific amounts of titanium-silicon molecular sieves and benzene-ring-containing polymers, which are carbonized products of titanium-silicon molecular sieves and benzene-ring-containing polymers.

[0034] The resulting catalyst exhibits high mechanical strength and is suitable for fixed-bed olefin epoxidation reactions. This invention offers the following advantages:

[0035] The olefin epoxidation method provided by this invention can be carried out in a fixed-bed reactor, which can effectively suppress the formation of by-products and achieve efficient utilization of hydrogen peroxide and efficient olefin epoxidation reaction. Detailed Implementation

[0036] The following embodiments will further illustrate the present invention. However, the present invention is not limited to the following embodiments.

[0037] In the following embodiments, the radial compressive strength of the obtained samples was determined using a particle strength tester, and the determination and calculation were performed according to the chemical industry standard HG / T 2782-2011. The carbon content of the obtained samples was determined using an organic elemental analyzer, and the samples were vacuum dried at 150°C for 12 hours before weighing. The crystal phase of the obtained samples was determined using an X-ray powder diffractometer.

[0038] Example 1

[0039] Commercially available TS-1 type titanium-silicon molecular sieve with MFI structure (titanium-silicon molar ratio of 0.03:1) was calcined at 550℃. One part by weight of the above TS-1 type titanium-silicon molecular sieve and one part by weight of polyphenylene ether were mixed, ground, and passed through a 400-mesh sieve. The component passing through the sieve was collected. 1.5 parts by weight of toluene were added and stirred thoroughly. Using a catalyst forming machine equipped with a 3mm pore size circular perforated plate, the product was extruded into strips at an extrusion pressure of 9MPa and air-dried at room temperature. The air-dried extruded strips were kept at 80℃ for 24 hours to obtain the precursor. The precursor was carbonized at 650℃ in an Ar atmosphere for 4 hours to obtain the olefin epoxidation catalyst. The radial compressive strength of this catalyst was 72 N·cm. -1 The mass fraction of carbon in it is 17%.

[0040] The above-mentioned olefin epoxidation catalyst was crushed, and 20-40 mesh particles were sieved out and loaded into a fixed-bed reactor for propylene epoxidation. Propylene and a mixture of hydrogen peroxide (20% aqueous solution) and methanol were pumped separately into the reactor. The molar ratio of propylene, hydrogen peroxide, and methanol was 3:1:10. The reaction temperature was 50℃, the reaction pressure was 2 MPa, and the propylene liquid hourly space velocity was 1.2 h⁻¹. -1 Samples were collected 30 minutes after the reaction temperature stabilized. The reaction time was 6 hours. Product analysis showed that the effective utilization rate of hydrogen peroxide was 87%, and the selectivity for propylene oxide was 82%.

[0041] Compare with Example 1

[0042] The TS-1 type titanium-silicon molecular sieve described in Example 1 was formed using alumina as a binder, and the forming operation method was the same as in Example 1. The difference was that the extrusion raw material was a mixture of TS-1 type titanium-silicon molecular sieve, pseudoboehmite, guar gum powder, and dilute nitric acid (10% by mass) in a mass ratio of 20:8:1:29. After air drying, the extruded product was calcined at 550°C in air for 4 hours to obtain the olefin epoxidation catalyst. The radial compressive strength of this catalyst was 63 N·cm. -1 .

[0043] The performance of the olefin epoxidation catalyst obtained in Control Example 1 was evaluated using the reaction conditions described in Example 1. Product analysis results showed that the effective utilization rate of hydrogen peroxide was 72%, and the selectivity for propylene oxide was 67%.

[0044] The results of Example 1 and Comparative Example 1 show that the olefin epoxidation catalyst prepared by the method provided by the present invention can meet the requirements of fixed-bed olefin epoxidation reaction for catalyst mechanical strength and catalytic performance, and its mechanical strength and catalytic performance are superior to those of the catalyst prepared by the alumina bonding method.

[0045] Compare with Example 2

[0046] The preparation process of the olefin epoxidation catalyst described in Example 1 was repeated, except that the amounts of polyphenylene ether and toluene were changed to 4 parts and 3 parts, respectively. The radial compressive strength of this catalyst was 151 N·cm. -1 The mass fraction of carbon in it is 47%.

[0047] The performance of the olefin epoxidation catalyst obtained in Control Example 2 was evaluated using the reaction conditions described in Example 1. Product analysis results showed that the effective utilization rate of hydrogen peroxide was 14%, and the selectivity for propylene oxide was 53%.

[0048] The results of Example 1 and Comparative Example 2 show that the obtained olefin epoxidation catalyst can also be formed even with organic carbonization product content exceeding the range provided by this invention, and the formed catalyst has higher strength. However, compared to catalysts with titanium-silicon molecular sieves and benzene-ring-containing polymers within the range provided by this invention, its catalytic performance decreases sharply. In other words, controlling the amount of titanium-silicon molecular sieves and benzene-ring-containing polymers is a necessary means to achieve efficient utilization of hydrogen peroxide and efficient olefin epoxidation reaction.

[0049] Example 2

[0050] Ti-Beta molecular sieves with a *BEA structure were synthesized according to the method disclosed in Example 8 of patent document CN113443635B. The titanium-silicon molar ratio of the obtained Ti-Beta molecular sieve was 0.015:1. The obtained molecular sieve was ground and passed through a 200-mesh sieve, and the component that passed through the sieve was collected.

[0051] By weight, 1 part polystyrene foam was mixed with 3 parts xylene to form a paste, and then 2 parts of the aforementioned Ti-Beta molecular sieve were added and thoroughly mixed. Using a catalyst forming machine equipped with a 3mm pore size circular perforated plate, the mixture was extruded into strips at an extrusion pressure of 7 MPa and dried at 150°C to obtain the precursor. The precursor was carbonized at 550°C in a 10% / 90% H2 / N2 atmosphere for 3 hours to obtain the olefin epoxidation catalyst. The radial compressive strength of this catalyst was 59 N·cm. -1 The carbon mass fraction is 12%.

[0052] The above-mentioned olefin epoxidation catalyst was crushed, and 20-40 mesh particles were sieved and loaded into a fixed-bed reactor for the 1-butene epoxidation reaction. 1-Butene and a mixture of hydrogen peroxide (20% aqueous solution)-methanol-concentrated ammonia (26% aqueous solution) were pumped separately into the reactor. The molar ratio of 1-butene, hydrogen peroxide, and methanol was 3:1:10, and the pH of the hydrogen peroxide-methanol-ammonia mixture was 8. The reaction temperature was 45℃, the system pressure was 2.5 MPa, and the liquid hourly space velocity (LISH) of 1-butene was 0.65 h⁻¹. -1Samples were collected 30 minutes after the reaction temperature stabilized. The reaction time was 6 hours. Chromatographic analysis showed that the effective utilization rate of hydrogen peroxide was 85%, and the selectivity for epoxide was 87%.

[0053] The spent catalyst was discharged from the reactor and calcined in air at 500°C for 24 hours to obtain a white powdery solid. This sample was a pure phase with a Beta molecular sieve structure.

[0054] The results of Example 2 show that the olefin epoxidation catalyst prepared using the method provided by this invention can meet the requirements of fixed-bed olefin epoxidation reaction for catalyst mechanical strength and catalytic performance. The catalyst is also easy to recover after use.

[0055] Compare with Example 3

[0056] The preparation process of the olefin epoxidation catalyst described in Example 2 was repeated, except that polystyrene foam was replaced with polyphenylene sulfide. This extruded precursor crumbled upon light contact after drying.

[0057] The results of Example 2 and Comparative Example 3 show that although the catalyst obtained using the method for preparing the olefin epoxidation catalyst provided by the present invention is composed of titanium-silicon molecular sieves and carbonized products of benzene-ring polymers, the selection of organic raw materials is not arbitrary. When the selection of benzene-ring polymers exceeds the range provided by the present invention, the mechanical strength of the resulting olefin epoxidation catalyst decreases sharply, failing to meet the requirements of fixed-bed reactions.

[0058] Example 3

[0059] The preparation process of the olefin epoxidation catalyst described in Example 1 was repeated, except that 1 part polyphenylene ether and 1.5 parts toluene were replaced with 1.2 parts dealkalized lignin and 1.2 parts acetone, respectively. The radial compressive strength of the resulting olefin epoxidation catalyst was 55 N·cm. -1 The carbon mass fraction is 12%.

[0060] The performance of the olefin epoxidation catalyst obtained in Example 3 was evaluated using the reaction conditions described in Example 1. Product analysis results showed that the effective utilization rate of hydrogen peroxide was 80%, and the selectivity for propylene oxide was 85%.

Claims

1. A method for epoxidation of olefins, characterized in that: A reaction mixture containing an olefin in a molar ratio of (1-20):(0.1-2)hydrogen peroxide is reacted in a fixed-bed reactor at a temperature of 30-150℃, a pressure of 0.1-8 MPa, and a liquid hourly space velocity (LISH) of 0.1-15 h⁻¹. -1 The reaction uses a molded catalyst composed of titanium-silicon molecular sieves and organic carbonization products, which is prepared through the following steps. Step a: Weigh out titanium-silicon molecular sieve and benzene ring-containing polymer at a mass ratio of 0.6-3, add dispersant and mix evenly. Step b: Extrude the mixture obtained in step a into strips and dry them in air at room temperature to 250°C to obtain the precursor. Step c: Carbonize the precursor obtained in step b in a non-oxidizing atmosphere at 450-850℃ for 0.5-6 hours.

2. The method for olefin epoxidation according to claim 1, characterized in that: The olefin is one or more of propylene, chloropropylene, butene or pentene.

3. The method for olefin epoxidation according to claim 1, characterized in that: Hydrogen peroxide is an aqueous solution with a mass fraction of 1%-30%; the reaction raw materials also contain one or more of methanol, ethanol, propanol, and tert-butanol, and the molar ratio of these to hydrogen peroxide is (5-50):

1.

4. The method for olefin epoxidation according to claim 1, characterized in that: The reaction temperature is 30-100℃, the reaction pressure is 0.1-5MPa, and the olefin liquid hourly space velocity is 0.1-10h. -1 .

5. The method for olefin epoxidation according to claim 1, characterized in that: In catalyst preparation step a, the framework type of the titanium-silicon molecular sieve is one or more of AFI, *BEA, FER, MEL, MFI, MOR, MTW, and MWW, and the titanium-silicon molar ratio is (0.001-0.05):1, preferably (0.005-0.04):1, and more preferably (0.01-0.03):

1.

6. The method for olefin epoxidation according to claim 1, characterized in that: In catalyst preparation step a, the benzene ring-containing polymer is one or more of polystyrene, polyphenylene ether, lignin, or phenolic resin.

7. The method for olefin epoxidation according to claim 1, characterized in that: In catalyst preparation step a, the dispersant is one or more of the following: water, methanol, ethanol, benzene, toluene, xylene, acetone, dichloromethane, trichloromethane, or carbon tetrachloride.

8. The method for olefin epoxidation according to claim 1, characterized in that: In catalyst preparation step a, the mass ratio of titanium-silicon molecular sieve to benzene-containing polymer is 0.8-3.

9. The method for olefin epoxidation according to claim 1, characterized in that: The carbonization temperature in catalyst preparation step c is 525-725℃.

10. The method for olefin epoxidation according to claim 1, characterized in that: The carbonization time in step c of catalyst preparation is 1-5 hours.