Preparation and application of a new nitrogen-doped carbon desulfurization catalyst

Abstract

The present invention discloses a method for preparing a novel nitrogen-doped carbon desulfurization catalyst with a multi-level pore structure. The nitrogen-doped carbon catalyst is prepared by the following method: first, a carbon-nitrogen precursor and a molten salt template are dissolved in deionized water in a certain proportion, and then the mixture is transferred to a freeze drying oven and dried to obtain a white powder, and finally the obtained white powder is carbonized under an inert atmosphere, washed, centrifuged, and dried to obtain a sample. The nitrogen-doped carbon catalyst prepared by this method has a rich multi-level pore structure, a high specific surface area and a nitrogen content. The nitrogen-doped carbon catalyst prepared by the present invention has a high hydrogen sulfide conversion rate and sulfur selectivity when used for the selective oxidation reaction of hydrogen sulfide, and still has high catalytic activity and stability under reaction conditions rich in water vapor and a high oxygen / sulfur ratio. Its desulfurization performance is higher than that of catalysts of the same type, and it has broad industrial application prospects.

Description

Technical Field

[0001] The present invention belongs to the field of novel functional material preparation and environmental catalysis, and specifically relates to a preparation method of a novel nitrogen-doped carbon catalyst and its application in the field of desulfurization. Background Art

[0002] Hydrogen sulfide (H2S) is a colorless, highly toxic gas with a rotten-egg odor. It is widely present in the extraction and purification of fossil fuels such as oil, natural gas, and coal. In industrial production, it not only corrodes production equipment but also poisons and deactivates industrial catalysts. In environmental protection, if untreated H2S gas is directly released into the atmosphere, it can form acid rain, causing severe environmental pollution and serious impacts on human health. Therefore, H2S gas generated by industrial production must be purified and treated to meet national standards before it can be discharged. Various strategies are used in industry to remove H2S, including separation, adsorption, and the Claus process. Currently, the most commonly used process in industry is the Claus process, which involves first absorbing and concentrating the H2S, then partially burning the H2S, and then allowing the resulting sulfur dioxide (SO2) to react with the H2S to produce elemental sulfur. This process effectively removes H2S and recycles sulfur. However, due to thermodynamic constraints, the Claus process exhaust still contains 3% to 5% H2S that has not been fully converted into elemental sulfur. Therefore, there is an urgent need to develop a process for treating H2S in Claus exhaust that is not constrained by thermodynamic equilibrium and can completely remove H2S.

[0003] Because the selective oxidation process is not constrained by thermodynamic equilibrium and can completely convert H2S into elemental sulfur, it is also an advanced, simple, and energy-efficient process. Therefore, it is the preferred catalyst for removing incompletely removed H2S from Claus tail gas. Currently, common H2S selective oxidation catalysts are primarily metal-based, including metal oxide catalysts such as γ-Al2O3, TiO2, Fe2O3, and CeO2. While metal oxide catalysts are highly active, they have a low specific surface area, resulting in sulfur coating the active centers. Furthermore, the metal oxides readily react with H2S to form metal sulfides and sulfates, leading to catalyst poisoning and deactivation, resulting in decreased performance. Furthermore, the difficulty in secondary recycling also limits the further development of metal oxide catalysts in this reaction. Therefore, there is an urgent need to develop environmentally friendly, highly active, and stable non-metallic desulfurization catalysts that can replace metal oxide catalysts.

[0004] Porous carbon materials have been widely studied and applied in fields such as gas adsorption and catalytic conversion due to their chemical properties such as high specific surface area and pore volume, abundant surface acid and base sites, excellent electrical conductivity, and good thermal stability. Generally speaking, after doping porous carbon materials with nitrogen atoms or nitrogen-containing basic groups, the surface area, pore structure, and surface chemical properties of the porous carbon materials can be greatly adjusted, which will greatly improve their catalytic performance. Compared with traditional desulfurization catalysts such as metal oxides and oxide supports, nitrogen-doped porous carbon catalyst materials have a large specific surface area, ultra-high porosity, and abundant catalytic active sites. They do not require further modification such as loading and impregnation, and are a more ideal desulfurization catalyst to be developed in the future. Summary of the Invention

[0005] The purpose of this invention is to design and develop a novel nitrogen-doped porous carbon catalyst for the selective oxidation of H2S with high catalytic activity and stability. The nitrogen-doped porous carbon prepared in this invention has a large specific surface area and abundant structural nitrogen sites, demonstrating high catalytic activity and stability in the H2S selective oxidation reaction.

[0006] The present invention provides a novel nitrogen-doped porous carbon catalyst, which adopts the following technical solution:

[0007] (1) mixing a nitrogen-carbon precursor and a template in a certain ratio and completely dissolving them in a solvent to obtain a clear solution;

[0008] (2) placing the clear solution obtained in step 1) in a freeze drying oven to obtain a white solid powder;

[0009] (3) The solid powder obtained in step 2) is heated to 600-1000° C. under an inert atmosphere and maintained at 900° C., more preferably 800° C., to obtain a solid product.

[0010] (4) The solid obtained in step 3) is washed, centrifuged, filtered, and dried to obtain a nitrogen-doped porous carbon catalyst.

[0011] Based on the above technical solution, preferably, the carbon source in step 1) is one or more of glucose, sucrose or chitosan, the nitrogen source is one or more of melamine, dicyandiamide or urea, and the template is one or more of sodium chloride, potassium chloride, sodium hydroxide or potassium hydroxide.

[0012] Based on the above technical solution, preferably, the freeze-drying temperature in step 2) is -50 to -30°C, and the drying time is 10 to 60 hours.

[0013] Based on the above technical solution, preferably, the inert atmosphere in step 3) is one or a combination of two or more of argon, helium, and nitrogen, and the heating rate of the temperature program is 1 to 10°C / min.

[0014] Based on the above technical solution, preferably, the washing condition in step 3) is water washing until the pH of the filtrate is neutral, and the drying condition is placing the washed sample in an oven at 60-100°C for 10-30 hours.

[0015] The nitrogen-doped carbon catalyst was used in the selective oxidation of H2S, specifically using 10,000 ppm H2S and 25,000 ppm O2 as feed gases, with Ar as the balance gas, and the prepared nitrogen-doped porous carbon as the catalyst. The feed gas flow rate was 100 mL / min, the catalyst mass was 0.1 g, and the reaction temperature was 130-210°C.

[0016] The significant advantages of the present invention compared with the prior art are:

[0017] (1) The present invention prepares a novel nitrogen-doped porous carbon non-metallic catalyst. Since the catalyst does not contain metal, it will not react to form sulfates that will cause inactivation of the active sites. In addition, the nitrogen atoms are well anchored in the carbon skeleton, which can also prevent the active sites from sintering during the reaction.

[0018] (2) In the present invention, the introduction of NaCl leads to an increase in the size of the mesopores and the introduction of additional macropores, resulting in a synthesized catalyst with a continuously enhanced open pore structure. Furthermore, NaCl, in its molten state at high temperatures, can act as a protective agent, allowing the nitrogen-carbon precursor to be uniformly heated during the carbonization process, preventing the collapse and denitrification of the carbon nanostructure. Compared to the commonly used silica template, this highly precise and convenient mesoporous template has significant advantages, such as fewer reaction steps, avoidance of hazardous chemicals, and scalability.

[0019] (3) The advantages of the catalyst prepared by the present invention are that it exhibits excellent catalytic activity, high sulfur selectivity and good activity stability in the H2S selective oxidation reaction, and still has high H2S catalytic oxidation activity under conditions containing water vapor, high oxygen / sulfur ratio and complex gas conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 These are scanning electron microscope images of the nitrogen-doped carbon catalysts prepared in Example 1 (a, b), Example 2 (c, d), and Example 3 (e, f).

[0021] Figure 2 N2 adsorption-desorption curves of the nitrogen-doped carbon catalysts prepared in Examples 1-3 and Comparative Example 1 for physical adsorption tests.

[0022] Figure 3 The pore size distribution spectra of the nitrogen-doped carbon catalysts prepared in Examples 1-3 and Comparative Example 1 for physical adsorption tests are shown.

[0023] Figure 4 XRD patterns of the nitrogen-doped carbon catalysts prepared in Examples 1-3 and Comparative Example 1.

[0024] Figure 5 These are the hydrogen sulfide oxidation stability test results of the nitrogen-doped carbon catalyst prepared in Example 2. DETAILED DESCRIPTION

[0025] The specific implementation method of the present invention is described in detail below. The specific implementation method described here is only used to illustrate and explain the present invention and is not used to limit the present invention.

[0026] Example 1

[0027] First, 10g of sodium chloride (NaCl) was dissolved in 40mL of deionized water. Then, 1g of anhydrous glucose and 3g of dicyandiamide were added to the NaCl solution. The solution was stirred in a 70°C oil bath for 1 hour and then dried in a freeze-drying oven for 48 hours. The resulting white powder was placed in a tube furnace and heated to 700°C at a rate of 2°C / min under an inert atmosphere for 2 hours to obtain a black powder. The resulting black powder was washed with deionized water for 8 hours, then centrifuged and filtered to remove the NaCl in the sample. The sample was then transferred to a 60°C oven and dried for 24 hours to obtain Sample A.

[0028] Example 2

[0029] First, 10g of sodium chloride (NaCl) was dissolved in 40mL of deionized water. Then, 1g of anhydrous glucose and 3g of dicyandiamide were added to the NaCl solution. The solution was stirred in a 70°C oil bath for 1 hour and then dried in a freeze-drying oven for 48 hours. The resulting white powder was placed in a tube furnace and heated to 800°C at a rate of 2°C / min under an inert atmosphere for 2 hours to obtain a black powder. The resulting black powder was washed with deionized water for 8 hours, then centrifuged and filtered to remove the NaCl in the sample. The sample was then transferred to a 60°C oven and dried for 24 hours to obtain Sample B.

[0030] Example 3

[0031] First, 10g of sodium chloride (NaCl) was dissolved in 40mL of deionized water. Then, 1g of anhydrous glucose and 3g of dicyandiamide were added to the NaCl solution. The solution was stirred in a 70°C oil bath for 1 hour and then dried in a freeze-drying oven for 48 hours. The resulting white powder was placed in a tube furnace and heated to 900°C at a rate of 2°C / min under an inert atmosphere for 2 hours to obtain a black powder. The resulting black powder was washed with deionized water for 8 hours, then centrifuged and filtered to remove the NaCl in the sample. The sample was then transferred to a 60°C oven and dried for 24 hours to obtain Sample C.

[0032] Comparative Example 1

[0033] Add 1g of anhydrous glucose and 3g of dicyandiamide to 40mL of deionized water. Stir the solution in a 70°C oil bath for 1 hour and then freeze-dry it in a freeze-drying oven for 48 hours. The resulting white powder is placed in a tube furnace and heated to 800°C at a rate of 2°C / min under an inert atmosphere. Hold for 2 hours to obtain a black powder, designated Sample D.

[0034] Comparative Example 2

[0035] Dissolve 0.47 g of iron precursor salt hexahydrate and ferric nitrate in 5 ml of a mixture of deionized water and anhydrous ethanol. Add 3 g of the SiC support to the solution, dry at room temperature, and then dry in an oven at 110°C for 2 hours. The resulting solid is then programmed to 400°C and held at the maximum temperature for 120 minutes. This yields an Fe₃O₄ / SiC catalyst, designated Sample E.

[0036] Comparative Example 3

[0037] A commercial γ-Al2O3 catalyst (CAS No. 1344-28-1) purchased from Maclean was used and designated as Sample F.

[0038] Example 4

[0039] This example is used to illustrate the method for selective oxidation removal of hydrogen sulfide provided by the present invention.

[0040] The H2S selective oxidation catalyst prepared in Example 1 was used to carry out the H2S selective oxidation removal reaction. The catalyst loading was 100 mg, and the total reaction gas flow rate was 100 mL / min. The H2S concentration in the feed gas was 10,000 ppm, the O2 concentration was 25,000 ppm, and Ar gas was used as the balance gas. The reaction conditions and results for the selective oxidation removal of hydrogen sulfide are shown in Table 1.

[0041] The calculation formulas for H2S conversion (%), sulfur selectivity (%) and sulfur yield (%) are as follows:

[0042] H2S conversion rate (%) = (H2S concentration in feed gas - H2S concentration in discharge gas) / H2S concentration in feed gas × 100%;

[0043] Sulfur selectivity (%) = (H2S concentration in feed gas - H2S concentration in discharge gas - SO2 concentration in discharge gas) / (H2S concentration in feed gas - H2S concentration in discharge gas) × 100%;

[0044] Sulfur yield (%) = (hydrogen sulfide conversion (%)) × (sulfur selectivity (%)).

[0045] Table 1. Test results of catalytic oxidation of H2S by catalysts prepared in Examples 1-3 and Comparative Examples 1-5

[0046]

[0047] It can be seen from Table 1 that catalyst B has a -1 h -1 At a high mass space velocity of , it still has a sulfur yield of more than 90%, which is much higher than other comparative catalysts. Compared with other comparative examples, the nitrogen-doped carbon catalyst provided by this patent has a rich multi-level pore structure and a high specific surface area, which can expose more active sites, promote the mass transfer efficiency of the reactants on the catalyst, and make the catalyst have a higher conversion rate and stability; and the nitrogen-doped carbon catalyst does not contain any metal elements, and will not react with sulfur to form sulfates and metal sulfides to poison and deactivate the catalyst, and the secondary recovery rate is high. Therefore, it shows that the performance and recyclability of the catalyst prepared by this patent are higher than those of the existing comparative catalysts.

[0048] Example 5

[0049] This example illustrates the method for the selective oxidation of H2S provided by the present invention. The nitrogen-doped carbon catalyst B prepared in Example 2 was used to conduct the selective oxidation of H2S, and the effect of the water vapor content in the reaction gas on the catalytic results was investigated. The catalyst loading was 100 mg, and the total reaction gas flow rate was 100 mL / min. The H2S concentration in the feed gas was 10,000 ppm, the O2 concentration was 25,000 ppm, and Ar gas was used as the balance gas. The conditions for the H2S selective oxidation reaction and the test results are shown in Table 2.

[0050] Table 2. Test results of catalyst B for catalytic oxidation of H2S at different water vapor concentrations

[0051]

[0052] As can be seen from Table 2, the catalyst activity slightly decreased after water vapor was introduced into the reaction gas, but the reaction activity of the catalyst did not decrease with the increase of water vapor content. Therefore, it can be shown that the catalyst prepared in the present invention can be used for the selective oxidation of H2S under high water vapor conditions.

[0053] Example 6

[0054] This example illustrates the method for the selective oxidation of H2S provided by the present invention. The nitrogen-doped carbon catalyst B prepared in Example 2 was used to conduct the selective oxidation of H2S, and the effect of varying O2 content in the reaction gas on the catalytic results was investigated. The catalyst loading was 100 mg, and the total reaction gas flow rate was 100 mL / min. The H2S concentration in the feed gas was 10,000 ppm, with Ar gas serving as the balance gas. The conditions for the H2S selective oxidation reaction and the test results are shown in Table 3.

[0055] Table 3 Test results of catalyst B for catalytic oxidation of H2S at different O2 / H2S conditions

[0056]

Claims

1. A novel nitrogen-doped carbon catalyst for the selective oxidation of H2S. The synthesis method of the novel nitrogen-doped porous carbon catalyst comprises the following steps: (1) mixing a nitrogen-carbon precursor and a template agent in a certain ratio and completely dissolving them in a solvent to obtain a clear solution; the template agent is sodium chloride; the carbon source is glucose, and the nitrogen source is dicyandiamide; (2) placing the clear solution obtained in step 1) in a freeze drying oven to obtain a white solid powder; (3) Heating the solid powder obtained in step 2) to 800-900°C under an inert atmosphere and maintaining the temperature for 2-12 hours to obtain a solid product; the heating rate is 1-10°C / min (4) The solid obtained in step 3) is washed, centrifuged, filtered, and dried to obtain a nitrogen-doped porous carbon catalyst; the washing condition is water washing until the pH of the filtrate is neutral, and the drying condition is placing the washed sample in an oven at 60-100°C for 10-30 hours.

2. The use according to claim 1, characterized in that The freeze-drying temperature in step (2) is -50 to -30°C, and the drying time is 10 to 60 hours.

3. The use according to claim 1, characterized in that: The inert atmosphere in step (3) is one or a combination of two or more of argon, helium, and nitrogen.

4. The use according to claim 1, characterized in that: The catalyst is applied to the H2S selective oxidation reaction under conditions containing water vapor, a high oxygen / sulfur ratio and complex gases, and selectively oxidizes the H2S gas to generate sulfur.

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