A manganese-based dual-ligand metal-organic framework low-temperature denitration catalyst and its preparation method

Abstract

This invention relates to a novel manganese-based dual-ligand metal-organic framework low-temperature denitration catalyst (TEOS&Mn-BTC) and its preparation method. The catalyst forms a hollow, sea urchin-like microsphere structure through layer-by-layer self-assembly of dual ligands. Dual-ligand coordination exposes more structural defects in the catalyst, improving electron transport efficiency, increasing electron transport channels and active sites, and also generating an electron-metal-support coordination structure that enhances denitration activity and thermal stability by constructing Si-O-Mn bonds. Simultaneously, the sea urchin-like nanostructure improves electron transfer efficiency, enhances effective contact between the adsorbate and active sites, shortens mass transfer distance, and promotes the formation of hierarchical porous structures. While dual-ligand coordination enhances denitration activity by regulating electronic structure and morphology, the oxidizing power is not enhanced enough to oxidize NH3 to N2O. This catalyst overcomes the seesaw effect between denitration activity and N2 selectivity, while also ensuring good water and sulfur resistance, showing broad prospects in the field of low-temperature selective catalytic denitration.

Description

Technical Field

[0001] This invention relates to a new material and its preparation method, specifically a manganese-based dual-ligand metal-organic framework material (TEOS&Mn-BTC) and its preparation method. This material can be used as a catalyst for selective catalytic reduction denitrification of flue gas at low temperatures (60~330℃), exhibiting excellent denitrification activity, high N2 selectivity, and outstanding water and sulfur resistance. Background Technology

[0002] Currently, coal energy remains a top priority for meeting people's livelihood needs in my country, but nitrogen oxides (NOx) produced in coal-fired flue gas are a significant concern. x It will cause serious harm to the human body and the natural world. NO x It causes environmental problems such as photochemical smog, ozone depletion, and acid rain, and also threatens human health. Therefore, treating the flue gas emitted from coal combustion to reduce NO in the exhaust gas is crucial. x Meeting or even lower emission standards is crucial.

[0003] Selective catalytic reduction (SCR) technology boasts high denitrification efficiency and low cost, making it the most effective method for NO removal. x The mainstream technology for NO treatment utilizes ammonia or urea as a reducing agent, and selectively removes NO under the action of a catalyst. x The reduction produces N2 and H2O, but this technology has limitations in its application. Currently, most denitrification processes are set after dust removal and desulfurization. After dust removal and desulfurization, the temperature of the coal-fired flue gas decreases, while the reaction temperature of traditional vanadium-based catalysts (V2O5-WO3 / TiO2) is above 300℃. Therefore, the exhaust gas needs to be reheated before denitrification, which leads to excessive energy consumption and increased operating costs. Furthermore, NO is also generated during the heating process. x The transformation converts NO into more harmful N2O or NO2. Simultaneously, during the SCR reaction, excessively high temperatures can cause NH3 to be oxidized, consuming the reducing gas NH3 while simultaneously producing NO. xIncreased NO production leads to decreased N2 selectivity. Therefore, developing low-temperature denitrification catalysts can avoid reheating and reduce NO conversion to N2O and NH3 consumption. However, most studies enhance activity by increasing the redox properties of active sites, which often enhances NH3 oxidation and reduces N2 selectivity, resulting in a seesaw effect between denitrification activity and N2 selectivity. Therefore, achieving both excellent low-temperature denitrification activity and high N2 selectivity is a key research direction. Catalysts used in denitrification include transition metal oxides, noble metals, and mixed metal oxides. Transition metal oxides (such as manganese-based catalysts) exhibit excellent low-temperature denitrification performance, but have poor sulfur resistance, and N2 selectivity decreases at higher temperatures. Commercially available vanadium-titanium catalysts are toxic and mainly used at medium to high temperatures, exhibiting low low-temperature denitrification activity. Noble metal catalysts are costly and unsuitable for denitrification processes with large catalyst usage. Therefore, developing novel, environmentally friendly catalysts that combine excellent low-temperature denitrification activity and high N2 selectivity is of great significance.

[0004] Manganese's excellent redox properties and environmentally friendly nature have led to its widespread application in denitrification. However, its poor sulfur resistance and reduced selectivity at higher temperatures limit the development of manganese-based denitrification catalysts. Some studies have explored adding another element to manganese, such as transition metals like iron (Fe), nickel (Ni), and cobalt (Co), or rare earth elements like cerium (Ce), samarium (Sm), gadolinium (Gd), and europium (Eu), to create a bimetallic synergistic effect and improve sulfur resistance and low-temperature denitrification activity. While the addition of transition metals or rare earth elements increases activity, it usually comes at the cost of reduced N2 selectivity, making it difficult to maintain a balance between denitrification activity and N2 selectivity.

[0005] Metal-organic frameworks (MOFs), formed by the self-assembly of metal cations and organic ligands, have attracted much attention in the field of denitrification catalysis due to their tunable pore size, numerous tunable active sites, and high specific surface area. Our team utilized the structural characteristics of quasi-MOFs to construct quasi-Mn-BTC, based on improved thermal stability, to prepare a highly sulfur-resistant low-temperature denitrification catalyst, overcoming the bottleneck of poor sulfur resistance in manganese-based denitrification catalysts (ACS Catal. 2023, 13, 5020−5032). However, a drawback of this catalyst is the decrease in N2 selectivity at higher reaction temperatures (>180℃). Increasing the tunability of the MOF structure, through the regulation of electronic and morphological structures, can expose more highly efficient active sites while suppressing the oxidative properties of the catalyst, making it difficult for NH3 to be oxidized to N2O. This could potentially break the seesaw effect between denitrification activity and N2 selectivity, improving N2 selectivity while ensuring both denitrification activity and sulfur resistance. Therefore, this invention attempts to introduce a second ligand into the MOF structure based on quasi-Mn-BTC to construct a dual-ligand MOF, TEOS&Mn-BTC, thereby improving its overall catalytic performance in low-temperature denitrification and promoting its industrial application. In summary, this invention prepares the TEOS&Mn-BTC dual-ligand MOF low-temperature denitrification catalyst via a solvothermal method and two-stage calcination, exhibiting excellent low-temperature denitrification activity, high N2 selectivity, and outstanding resistance to water and sulfur. Summary of the Invention

[0006] One objective of this invention is to provide a manganese-based dual-ligand metal-organic framework low-temperature denitrification catalyst, specifically TEOS & Mn-BTC, primarily for the removal of NO from flue gas. x Selective catalytic reduction removal in a lower temperature range.

[0007] A second objective of this invention is to provide a method for preparing a manganese-based dual-ligand metal-organic framework low-temperature denitration catalyst. This method uses tetraethyl orthosilicate (TEOS) and trimesic acid (BTC) as ligands to form a manganese-based dual-ligand coordination structure under solvothermal reaction. The specific preparation steps are as follows:

[0008] (1) Place 3-7 ml of tetraethyl orthosilicate (TEOS) and 2-3 g of triterpenoid (BTC) in 50-100 ml of ethanol solution and stir magnetically at room temperature until completely dissolved to form solution A.

[0009] (2) Dissolve 1-3 grams of manganese acetate tetrahydrate in 30-50 ml of ultrapure water to form solution B.

[0010] (3) After mixing solution A and solution B, stir magnetically for 2 hours at room temperature, then transfer to a 150-300 ml reaction vessel and seal it, and carry out a solvothermal reaction at 110-150℃ for 15-20 hours.

[0011] (4) After the solvothermal reaction is completed and the temperature of the reactor drops to room temperature, the solid product is collected by centrifugation, washed with ethanol 4 to 6 times, and then dried under vacuum at 60 to 100°C for 8 to 12 hours. After grinding, a white solid powder is obtained.

[0012] (5) The white solid powder is pretreated at 300-350°C for 2-4 hours under a nitrogen atmosphere, and then calcined at 300-350°C for 4-6 hours under an air atmosphere to obtain the final TEOS&Mn-BTC catalyst.

[0013] The manganese-based dual-ligand metal-organic framework low-temperature denitration catalyst of the present invention is characterized in that: the catalyst is a metal-organic framework material (TEOS&Mn-BTC) constructed by coordinating tetraethyl orthosilicate (TEOS) and pyromellitic acid (BTC) as dual ligands, with manganese as the metal coordination center, and exhibits a hollow sea urchin-like microsphere structure; TEOS&Mn-BTC generates a large number of structural defects that are beneficial to the denitration reaction during the competitive coordination process of the dual ligands, and these defects are observed in the Raman spectrum at 620~700 cm⁻¹. -1 The presence of Raman peaks associated with oxygen vacancies is observed. Simultaneously, the dual ligands construct silicon (Si)-oxygen (O)-manganese (Mn) bonds, resulting in an electron-metal-support coordination structure that enhances thermal stability and catalytic activity. The electronegativity difference between Si and Mn causes valence electrons to transfer from Mn to Si via O, placing Mn in an electron-deficient state, thus generating Lewis acids beneficial for denitration reactions. The dual ligands undergo layer-by-layer self-assembly with manganese to form hollow urchin-like microspheres. This structure exposes more active sites, improves electron transfer efficiency, enhances effective contact between the adsorbate and active sites, shortens mass transfer distance, avoids the detachment of intermediate products, and also contributes to the formation of hierarchical porous structures. TEOS&-Mn-BTC contains uniformly distributed Mn, C, H, and O, where Mn is composed of Mn... 2+ Mn 3+ Mn 4+ Composed of three valence states, with Mn 3+ The design of the two ligands also introduced ethyl hydrophobic functional groups into the structure, which are visible in the 500–800 cm⁻¹ region of the FTIR spectrum. -1 An ethyl peak appears at [location missing]; TEOS&Mn-BTC exhibits a distinct hierarchical porous structure and a large specific surface area, ranging from 150 to 200 m². 2 / g.

[0014] The catalyst preparation method of the present invention is characterized by: using tetraethyl orthosilicate (TEOS) and pyromellitic acid (BTC) as ligands to form a manganese-based dual-ligand coordination structure under solvothermal reaction.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] The catalyst prepared in this invention exposes numerous structural defects beneficial to catalytic reactions during the self-assembly of the two ligands, and generates a large number of oxygen vacancies in situ during the two-stage calcination process. The unique structural design and special activation treatment result in abundant structural defects in the catalyst, thus ensuring excellent low-temperature denitrification activity. At 36,000 h... -1 At a space velocity of 500 ppm NO, the denitrification efficiency reaches over 90% at 60℃ and over 108,000 h⁻¹. -1 At high air velocity, when 500 ppm NO is introduced, the denitrification efficiency remains above 80% within a temperature range of 150-330℃.

[0017] The design of dual ligands enhances catalytic activity by modulating the electronic structure, thereby increasing electron delocalization, accelerating charge redistribution and transport, and generating more electron transport channels. Furthermore, the layer-by-layer self-assembly of dual ligands leads to the formation of hollow, urchin-like microspheres. This structure enhances the contact between the adsorbate and active sites, shortens the mass transfer distance, prevents the detachment of intermediate products, and reduces adsorbate loss during mass transfer, thus improving catalytic activity. Although the activity enhancement is achieved through the modulation of electronic structure and morphology, the catalyst's oxidizing power is not increased to the point of oxidizing NH3 to N2O. Therefore, the dual-ligand coordination design overcomes the seesaw effect between denitrification activity and N2 selectivity, ensuring high N2 selectivity for the catalyst while improving low-temperature denitrification activity.

[0018] The structural design of dual-ligand coordination generates Si-O-Mn bonds, leading to the formation of an electron-metal-support coordination structure. This structure can enhance the thermal stability and catalytic activity of the catalyst.

[0019] TEOS & Mn-BTC catalysts can be prepared simply through solvothermal synthesis and two-stage calcination. The method is easy to implement and safe and environmentally friendly. Attached Figure Description

[0020] [1] Figure 1 This is a scanning electron microscope image of the TEOS&Mn-BTC low-temperature denitrification catalyst.

[0021] [2] Figure 2 The results are the denitrification performance and N2 selectivity test results of the TEOS&Mn-BTC catalyst obtained according to Example 1. Detailed Implementation Example 1:

[0022] TEOS & Mn-BTC dual-ligand low-temperature denitration catalyst was prepared according to the preparation method proposed in this invention:

[0023] (1) Place 3 mL of tetraethyl orthosilicate (TEOS) and 2 g of triterpenoid (BTC) in 50 mL of ethanol solution and stir magnetically at room temperature until completely dissolved to form solution A.

[0024] (2) Dissolve 1 gram of manganese acetate tetrahydrate in 30 ml of ultrapure water to form solution B.

[0025] (3) After mixing solution A and solution B, stir magnetically for 2 hours at room temperature, then transfer to a 150 mL reaction vessel and seal it, and carry out a solvothermal reaction at 110 °C for 15 hours.

[0026] (4) After the solvothermal reaction is completed and the temperature of the reactor drops to room temperature, the solid product is collected by centrifugation, washed 4 times with ethanol, dried at 60°C under vacuum for 8 hours, and then ground to obtain a white solid powder.

[0027] (5) The white solid powder was pretreated at 300°C for 2 hours under nitrogen atmosphere and then calcined at 300°C for 4 hours under air atmosphere to obtain the final TEOS&Mn-BTC catalyst.

[0028] This catalyst is a manganese-based dual-ligand metal-organic framework low-temperature denitration catalyst, constructed with tetraethyl orthosilicate (TEOS) and pyromellitic acid (BTC) as dual ligands. The manganese element at the coordination center is highly uniformly distributed on the hollow sea urchin-shaped microsphere structure. During the competitive coordination process of the dual ligands, the catalyst generates numerous structural defects beneficial to the denitration reaction, and a large number of oxygen vacancies are generated in situ during calcination. Simultaneously, the dual-ligand coordination forms an electron-metal-support coordination structure through Si-O-Mn bond formation. This structure improves the catalyst's activity and thermal stability, while also generating more Lewis acid sites. Furthermore, the hollow sea urchin-shaped microsphere structure exposes more active sites, improving electron transfer efficiency, enhancing the effective contact between the adsorbate and active sites, shortening the mass transfer distance, and preventing the detachment of intermediate products, thereby improving denitration activity, while the oxidizing power is not enhanced enough to oxidize NH3 to N2O. The abundant structural defects and active sites, along with the unique hollow sea urchin-shaped microsphere structure, give this catalyst excellent low-temperature denitration performance and high N2 selectivity. At a space velocity of 36,000 h⁻¹, the catalyst exhibits good performance. -1 Under reaction conditions of 500 ppm NO and 500 ppm NH3, the denitrification performance remained above 90% at 60~330℃, and the N2 selectivity remained above 90%. Example 2:

[0029] TEOS & Mn-BTC dual-ligand low-temperature denitration catalyst was prepared according to the preparation method proposed in this invention:

[0030] (1) Place 5 mL of tetraethyl orthosilicate (TEOS) and 2.5 g of triterpenoid (BTC) in 80 mL of ethanol solution and stir magnetically at room temperature until completely dissolved to form solution A.

[0031] (2) Dissolve 2 grams of manganese acetate tetrahydrate in 40 ml of ultrapure water to form solution B.

[0032] (3) After mixing solution A and solution B, stir magnetically for 2 hours at room temperature, then transfer to a 200 mL reaction vessel and seal it, and carry out a solvothermal reaction at 130 °C for 18 hours.

[0033] (4) After the solvothermal reaction is completed and the temperature of the reactor drops to room temperature, the solid product is collected by centrifugation, washed 5 times with ethanol, dried at 80°C under vacuum for 10 hours, and then ground to obtain a white solid powder.

[0034] (5) The white solid powder was pretreated at 310°C for 3 hours under a nitrogen atmosphere and then calcined at 320°C for 5 hours under an air atmosphere to obtain the final TEOS&Mn-BTC catalyst.

[0035] This catalyst is a hollow sea urchin-shaped microsphere metal-organic framework denitration catalyst (TEOS&Mn-BTC) coordinated with tetraethyl orthosilicate (TEOS) and pyromellitic acid (BTC) dual ligands. The unique structure of the hollow sea urchin-shaped microspheres exposes more active sites, improving electron transfer efficiency, enhancing effective contact between the adsorbate and active sites, shortening mass transfer distance, and preventing the detachment of intermediate products. It also contributes to the formation of a hierarchical porous structure, thereby improving denitration activity. TEOS&Mn-BTC generates numerous structural defects beneficial to improving denitration activity during the dual-ligand competitive coordination process. The dual ligands also construct Si-O-Mn bonds, leading to the generation of an electron-metal-support coordination structure that improves thermal stability and catalytic activity. The electronegativity difference between Si and Mn also causes valence electrons to transfer from Mn to Si via O, placing Mn in an electron-deficient state, thus generating Lewis acids favorable for denitration. Simultaneously, because the oxidizing power is not enhanced enough to oxidize NH3 to N2O, high N2 selectivity is ensured. At a space velocity of 36,000 h⁻¹, [the catalyst exhibits high performance]. -1 Under reaction conditions of 500 ppm NO, the denitrification performance remained above 90% at temperatures ranging from 90 to 330°C, with a space velocity of 108,000 h⁻¹. -1At this temperature, the denitrification performance remains above 80% at 150~300℃. Simultaneously, the catalyst exhibits excellent water and sulfur resistance; when 6% H2O is introduced into the reaction mixture and the reaction continues for 10 hours, the performance decreases by only 1~2%, and when 100 ppm SO2 is introduced and the reaction continues for 10 hours, the performance decreases by only 5~6%.

[0036] Example 3:

[0037] TEOS & Mn-BTC dual-ligand low-temperature denitration catalyst was prepared according to the preparation method proposed in this invention:

[0038] (1) Place 7 mL of tetraethyl orthosilicate (TEOS) and 3 g of triterpenoid (BTC) in 100 mL of ethanol solution and stir magnetically at room temperature until completely dissolved to form solution A.

[0039] (2) Dissolve 3 grams of manganese acetate tetrahydrate in 50 ml of ultrapure water to form solution B.

[0040] (3) After mixing solution A and solution B, stir magnetically for 2 hours at room temperature, then transfer to a 200 mL reaction vessel and seal it, and carry out a solvothermal reaction at 150 °C for 20 hours.

[0041] (4) After the solvothermal reaction is completed and the temperature of the reactor drops to room temperature, the solid product is collected by centrifugation, washed with ethanol 6 times, dried at 100°C under vacuum for 12 hours, and then ground to obtain a white solid powder.

[0042] (5) The white solid powder was pretreated at 330°C for 4 hours under a nitrogen atmosphere and then calcined at 350°C for 6 hours under an air atmosphere to obtain the final TEOS&Mn-BTC catalyst.

[0043] This catalyst is a low-temperature denitration catalyst with a dual-ligand metal-organic framework and an electron-metal-support coordination structure, exhibiting a hollow sea urchin-like microsphere structure. This structure enhances the contact between the adsorbate and the active site, shortens the mass transfer distance, reduces mass transfer loss, and promotes the formation of a hierarchical porous structure, thereby leading to enhanced catalytic activity. During the competitive coordination process of the dual ligands, more structural defects beneficial to improving denitration activity are exposed, and a large number of oxygen vacancies are also generated in situ during calcination activation. However, the structural modulation by the dual ligands does not enhance the catalyst's oxidizing power, thus ensuring that NH3 is not oxidized to N2O, breaking the seesaw effect between denitration activity and N2 selectivity. Simultaneously, the introduction of ethyl groups enhances water resistance while maintaining activity and selectivity. At a space velocity of 36,000 h⁻¹, [the catalyst was successfully denitrated]. -1 Under reaction conditions of 500 ppm NO, the denitrification performance of the catalyst remained above 90% at 90~330℃, and the performance only decreased by 1~2% after 10 hours of continuous reaction with 6% H2O introduced into the reaction mixture.

Claims

1. A manganese-based dual-ligand metal-organic framework low-temperature denitration catalyst, characterized in that: The catalyst is a metal-organic framework material, TEOS&Mn-BTC, constructed with manganese as the metal coordination center, using tetraethyl orthosilicate (TEOS) and pyromellitic acid (BTC) as dual ligands. It exhibits a hollow, sea urchin-like microsphere structure. During the competitive coordination process of the dual ligands, TEOS&Mn-BTC generates numerous structural defects beneficial to the denitration reaction, which are observed in the Raman spectrum from 620 to 700 cm⁻¹. -1 The presence of Raman peaks associated with oxygen vacancies is observed. Simultaneously, the dual ligands construct silicon (Si)-oxygen (O)-manganese (Mn) bonds, resulting in an electron-metal-support coordination structure that enhances thermal stability and catalytic activity. The electronegativity difference between Si and Mn causes valence electrons to transfer from Mn to Si via O, placing Mn in an electron-deficient state, thus generating Lewis acids beneficial for denitration reactions. The dual ligands undergo layer-by-layer self-assembly with manganese to form hollow urchin-like microspheres. This structure exposes more active sites, improves electron transfer efficiency, enhances effective contact between the adsorbate and active sites, shortens mass transfer distance, avoids the detachment of intermediate products, and also contributes to the formation of hierarchical porous structures. TEOS&-Mn-BTC contains uniformly distributed Mn, C, H, and O, where Mn is composed of Mn... 2+ Mn 3+ Mn 4+ Composed of three valence states, with Mn 3+ The design of the two ligands also introduced ethyl hydrophobic functional groups into the structure, which are visible in the 500–800 cm⁻¹ region of the FTIR spectrum. -1 An ethyl peak appears at [location missing]; TEOS&Mn-BTC exhibits a distinct hierarchical porous structure and a large specific surface area, ranging from 150 to 200 m². 2 / g; The preparation steps are as follows: (1) Place 3-7 mL of tetraethyl orthosilicate (TEOS) and 2-3 g of triterpenoid (BTC) in 50-100 mL of ethanol solution and stir magnetically at room temperature until completely dissolved to form solution A; (2) Dissolve 1-3 grams of manganese acetate tetrahydrate in 30-50 ml of ultrapure water to form solution B; (3) After mixing solution A and solution B, stir magnetically for 2 hours at room temperature, then transfer to a 150-300 ml reaction vessel and seal it, and carry out a solvothermal reaction at 110-150℃ for 15-20 hours; (4) After the solvothermal reaction is over and the temperature of the reactor drops to room temperature, the solid product is collected by centrifugation, washed with ethanol 4 to 6 times, and then dried under vacuum at 60 to 100°C for 8 to 12 hours. After grinding, a white solid powder is obtained. (5) The white solid powder is pretreated at 300-350°C for 2-4 hours under a nitrogen atmosphere, and then calcined at 300-350°C for 4-6 hours under an air atmosphere to obtain the final TEOS&Mn-BTC catalyst.

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