LEV zeolite catalyst and preparation method and application thereof

Abstract

The present invention belongs to the field of catalysts, and specifically relates to a LEV zeolite catalyst, a preparation method and an application thereof. The preparation method adopts the following steps: (1) synthesizing an organic structure directing agent N,N-dimethylmorpholinium cation; (2) preparing LEV zeolite using N,N-dimethylmorpholinium cation as a directing agent; (3) subjecting the LEV zeolite to a two-step ion exchange to obtain a LEV zeolite catalyst. The preparation method provided by the present invention prepares LEV zeolite molecular sieves by using a new type of inexpensive organic structure directing agent, has a wide range of synthetic gels, reduces preparation costs, and has important significance in the field of actual chemical production.

Description

Technical Field

[0001] The invention belongs to the field of catalysts, and in particular relates to an LEV zeolite catalyst and a preparation method and application thereof. Background Art

[0002] Globally, the emission control of nitrogen oxides is becoming increasingly stringent. Whether it is a mobile source or a fixed source, the removal technology of nitrogen oxides in exhaust gas has become a research hotspot. Among them, due to the unstable operating conditions of diesel locomotives, the NOx in the exhaust gas is highly volatile and generated in large quantities, which is difficult to control technically. At present, the removal technology of nitrogen oxides in diesel vehicle exhaust mainly adopts ammonia-based nitrogen oxide selective catalytic reduction (NH3-SCR) technology, the principle of which is to use a reducing agent to reduce NOx in the exhaust gas to non-toxic N2 under the action of a copper-based zeolite catalyst.

[0003] Among them, Cu-SSZ-13 molecular sieve with CHA topology has been commercialized, but its synthesis requires expensive N,N,N-trimethyl-1-adamantane ammonium hydroxide organic template. Therefore, it is necessary to develop new zeolite molecular sieve catalysts to reduce their costs. Patents CN113387369 A, CN108557841 A, and CN103601211A use cheap copper amine complexes and choline cations as templates and seeds to prepare CHA zeolite molecular sieve catalysts, which perform well in SCR catalytic applications, but there is still room for improvement.

[0004] Other topological zeolite molecular sieves such as AEI, KFI, AFX, etc. have been evaluated for the removal of NOx from diesel vehicle exhaust. According to patents US 2018 / 0272279 A1, CN 111266132 A, and CN 112495429A, they have achieved good denitrification effects. However, the synthesis of these topological structures still requires expensive and difficult-to-obtain templates.

[0005] LEV is a type of zeolite molecular sieve with unique structure and good acidity, which has potential application value in ammonia selective catalytic reduction of NOx (NH3-SCR). Its structural characteristics are: (1) It has a large micropore volume (about 0.3cm 3 / g); (2) having small eight-membered ring (8MR) pore openings; (3) the framework structure can be aluminosilicate or silicoaluminophosphate.

[0006] Currently synthesized LEV zeolites usually use foreign organic templates as structure-directing agents. Patents CN103124694A, US 11478748 B2, and US 20170050857A1 provide methods for preparing LEV zeolites, but the complexity and high cost of structure-directing agents (such as quinine-based template agents) and the long synthesis cycle limit the further commercial development and utilization of LEV zeolites. Therefore, developing new inexpensive organic structure-directing agents and synthesizing LEV zeolite catalysts with a suitable Si / Al ratio are of great significance to protecting the environment and promoting the rapid development of green industries. Summary of the invention

[0007] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a LEV zeolite catalyst and a preparation method and application thereof.

[0008] To achieve the above object, the technical solution adopted by the present invention is:

[0009] A method for preparing a LEV zeolite catalyst comprises the following steps:

[0010] (1) Synthesis of organic structure-directing agent N,N-dimethylmorpholinium cation;

[0011] (2) Preparation of LEV zeolite using N,N-dimethylmorpholinium cation as directing agent;

[0012] (3) LEV zeolite is subjected to two-step ion exchange to obtain LEV zeolite catalyst.

[0013] The structure of the N-dimethylmorpholinium cation is shown in formula (I):

[0014]

[0015] The N-dimethylmorpholinium cation is prepared in the following manner: N-methylmorpholine is subjected to a methylation reaction to obtain an N,N-dimethylmorpholinium cation, which is then exchanged into a hydroxide form.

[0016] In step (2), the LEV zeolite is prepared in the following manner: the LEV zeolite is treated with raw materials having a molar ratio of SiO2:Al2O3:N,N-dimethylmorpholinium cation:M2O:H2O=1:0.02-0.1:0.2-0.4:0.1-0.3:8-15, and after crystallization reaction, the product is filtered, washed, dried, and calcined to obtain the LEV zeolite molecular sieve; wherein M is one or a combination of at least two of Na, K, and Cs.

[0017] The aluminum source and silicon source include an independent aluminum source, an independent silicon source or a zeolite different from the LEV zeolite;

[0018] The independent aluminum source includes one of aluminum isopropoxide, boehmite, sodium aluminate, aluminum hydroxide, aluminum nitrate nonahydrate, or aluminum sulfate octadecadihydrate, or a combination of at least two of them; the independent silicon source includes one of white carbon black, fine silica gel, fumed silica, silica sol, sodium silicate, or tetraethoxysilane, or a combination of at least two of them.

[0019] The zeolite different from LEV zeolite is Y zeolite with a silicon-aluminum atomic ratio of 5-30.

[0020] The conditions of the crystallization reaction are as follows: the crystallization reaction temperature is 140°C-170°C, the crystallization reaction time is determined according to the starting raw materials, the zeolite crystallization time is 12h-72h, and the independent silicon source and independent aluminum source crystallization time is 72h-168h.

[0021] The specific steps of the two-step ion exchange in step (3) are as follows:

[0022] (1) First step of ion exchange: ion exchange LEV zeolite with ammonium salt to obtain NH4-LEV zeolite; the concentration of LEV zeolite is 5g / L-20g / L, preferably 6-14g / L; the concentration of ammonium salt is 0.1mol / L-5mol / L, preferably 0.5-2mol / L; the ammonium salt can be one or a combination of at least two of ammonium chloride, ammonium nitrate, ammonium acetate and ammonium sulfate. Preferably, the ion exchange is carried out at 80°C.

[0023] (2) The second step of ion exchange: the NH4-LEV zeolite obtained in the previous step is subjected to ion exchange with a copper salt, and the LEV zeolite catalyst is obtained after calcination. The concentration of the NH4-LEV zeolite is 5g / L-20g / L, preferably 6-14g / L; the concentration of the copper salt is 0.001mol / L-0.5mol / L, preferably 0.005-0.05mol / L; the copper salt can be one or a combination of at least two of copper chloride, copper nitrate, copper acetate, and copper sulfate, preferably copper acetate. Preferably, the ion exchange is carried out at 80°C.

[0024] The present invention also includes an LEV zeolite catalyst obtained by the preparation method.

[0025] The present invention also includes an application of the LEV zeolite catalyst for selective catalytic reduction of nitrogen oxides.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The preparation method provided by the present invention prepares LEV zeolite molecular sieve by using a novel low-cost organic structure directing agent, has a wide range of synthetic gel, reduces the preparation cost, and has important significance in the field of actual chemical production.

[0028] The LEV zeolite catalyst obtained by the preparation method provided by the present invention has a two-dimensional topological structure ( Figure 4 ), excellent ion exchange performance, wide adjustable parameter range ( Figure 2 ), which can provide sufficient acidic sites for NH3-SCR reaction ( Figure 5-6 ), has excellent denitrification activity, and has broad application prospects in practical environmental catalysis, especially in diesel vehicle exhaust denitrification BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is the N,N-dimethylmorpholine cationic liquid synthesized by the present invention. 13 C NMR characterization results;

[0030] Figure 2 is the XRD pattern of the product obtained in Examples 1-4 of the present invention;

[0031] Figure 3 is the XRD pattern of the product obtained in Comparative Examples 1-4 of the present invention;

[0032] Figure 4 is a SEM image of the LEV zeolite obtained in Example 1-2 of the present invention;

[0033] Figure 5 is the NH3-SCR activity data of the LEV zeolite catalyst obtained in Example 1 of the present invention;

[0034] Figure 6 This is the NH3-SCR activity data of the LEV zeolite catalyst obtained in Example 2 of the present invention. DETAILED DESCRIPTION

[0035] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below in conjunction with the accompanying drawings and the best embodiments.

[0036] Example 1: A method for preparing a LEV zeolite catalyst, the steps are as follows:

[0037] (1) Preparation of N,N-dimethylmorpholinium cation: 50.6 g of N-methylmorpholine and 350 mL of ether were stirred thoroughly to dissolve, and the device was placed in an ice bath. 71 g of methyl iodide was placed in a constant pressure burette, and methyl iodide was slowly added dropwise under strong stirring. After the addition was completed, the mixture was reacted at room temperature for 3 days, and then dried by rotary evaporation to obtain N,N-dimethylmorpholinium iodide powder. The liquid 13 CNMR spectrum Figure 1 The N,N-dimethylmorpholine iodide powder is ion-exchanged with an anion exchange resin to a hydroxide form, and concentrated at 80°C for later use.

[0038] (2) Preparation of LEV zeolite. The molar ratio of each raw material for catalyst preparation is SiO2: Al2O3: N, N-dimethylmorpholinium cation: Na2O: H2O = 1: 0.033: 0.2: 0.2: 12.6; first dissolve sodium hydroxide in deionized water, add N, N-dimethylmorpholinium cation after cooling to room temperature, stir thoroughly for more than 30 minutes, then add Y zeolite (silicon-aluminum atomic ratio is 15), stir and age at room temperature for 2 hours and then load into the reactor. Place the reactor in a 160°C forced air drying oven for reaction for 24 hours. After the obtained product is filtered, washed and dried, it is calcined in a muffle furnace at 560°C for 8 hours to obtain LEV zeolite molecular sieve. The XRD spectrum is as follows Figure 2 SEM as shown. Figure 4 (a) shown.

[0039] (3) Preparation of LEV zeolite catalyst. Weigh 1 g of LEV zeolite and 5.35 g of ammonium chloride and place them in 100 g of deionized water, stir at 80°C for 8 h, centrifuge and dry to obtain NH4-LEV zeolite molecular sieve; weigh 0.3 g of copper acetate and 1.0 g of NH4-LEV zeolite and place them in 100 g of deionized water, stir at 80°C for 1 h, centrifuge and dry, and calcine at 550°C for 4 h to obtain a LEV zeolite catalyst sample.

[0040] The NH3-SCR activity of Cu_LEV zeolite catalyst is shown in Figure 5 The T90 temperature window of fresh samples is 250-600°C, and that of aged samples is 250-480°C, which are better than other reported LEV zeolite catalysts and comparable to commercial Cu-SSZ-13 zeolite catalysts.

[0041] Example 2: The N,N-dimethylmorpholinium cation used in Example 2 and the process for preparing the LEV zeolite catalyst are the same as those in Example 1, except that the method for preparing the LEV zeolite is different.

[0042] The preparation of LEV zeolite specifically includes the following steps: the molar ratio of each raw material for catalyst preparation is SiO2: Al2O3: N, N-dimethylmorpholinium cation: Na2O: H2O = 1: 0.033: 0.2: 0.2: 9.62. Sodium hydroxide is first dissolved in deionized water, and N, N-dimethylmorpholinium cation and aluminum hydroxide are added in sequence after cooling to room temperature, and then silica sol (SiO2 content is 40wt%) is slowly added after being fully stirred for more than 30 minutes, and then stirred and aged for 2 hours at room temperature and then loaded into a reactor. The reactor is placed in a 150°C forced air drying oven for reaction for 120 hours, and the obtained product is filtered, washed, and dried, and then calcined in a 560°C muffle furnace for 8 hours to obtain LEV zeolite molecular sieve.

[0043] XRD spectrum Figure 2 SEM as shown. Figure 4 (b) shows the activity Figure 6 , the T90 temperature window of fresh samples is 250-600℃, and the T90 temperature window of aged samples is 250-450℃.

[0044] Example 3: The N,N-dimethylmorpholinium cation used in Example 3 and the process for preparing the LEV zeolite catalyst are the same as those in Example 1, except that the method for preparing the LEV zeolite is different.

[0045] The preparation of LEV zeolite specifically includes the following steps: the molar ratio of each raw material for catalyst preparation is SiO2: Al2O3: N, N-dimethylmorpholinium cation: Na2O: H2O = 1: 0.033: 0.2: 0.2: 15. First, CBV712 zeolite (SiO2 / Al2O3 = 12) is added to deionized water to provide all Al sources and part of Si sources, and N, N-dimethylmorpholinium cation is added after stirring for 20 minutes, and the mixture is fully stirred for more than 30 minutes, and then water glass (Na2O·2SiO2 aqueous solution) is slowly added as a supplementary Si source, and the mixture is stirred and aged for 2 hours at room temperature and then loaded into a reactor. The reactor is placed in a 170°C forced air drying oven for reaction for 72 hours, and the obtained product is filtered, washed, and dried, and then calcined in a 560°C muffle furnace for 8 hours to obtain the LEV zeolite molecular sieve.

[0046] XRD spectrum Figure 2 As shown in the figure, the T90 temperature window of fresh samples is 250-600℃, and the T90 temperature window of aged samples is 250-450℃.

[0047] Example 4: In order to prove that zeolite has a wider gel ratio range, the molar ratio of each raw material is changed to SiO2:Al2O3:N,N-dimethylmorpholinium cation:Na2O:H2O=1:0.02:0.3:0.1:15 relative to Example 2. The synthesis steps are the same, and the XRD spectrum of the product is as follows Figure 2 As shown. The T90 temperature window of fresh samples is 250-550°C, and the T90 temperature window of aged samples is 250-420°C. The present application tests SiO2:Al2O3:N,N-dimethylmorpholinium cation:Na2O:H2O:0.1:0.2-0.4:0.1-0.3:8-15, and the T90 temperature window of fresh samples is at least 250-550°C, and the T90 temperature window of aged samples is at least 250-420°C. The results show that different raw material molar ratios can achieve good implementation effects within the range, among which the molar ratio SiO2:Al2O3:N,N-dimethylmorpholinium cation:Na2O:H2O=1:0.033:0.2:0.2:9.62-15 has the best effect.

[0048] Comparative Example 1: The only difference from Example 1 is that the amount of N,N-dimethylmorpholinium cation added is 1 / 3 of the original amount, and the product is amorphous. Figure 3 As shown, the experiment shows that good NH3-SCR performance can be achieved when the molar ratio of N,N-dimethylmorpholinium cation to SiO2 addition is in the range of 0.2-0.4.

[0049] Comparative Example 2: The only difference from Example 1 is that the amount of NaOH added is doubled, and the product is ANA zeolite. Figure 3 The experiment shows that good NH3-SCR performance can be achieved when the molar ratio of NaOH to SiO2 is within the range of 0.1-0.3.

[0050] Comparative Example 3: The difference from Example 1 is that the amount of water added is 3 times the original amount, and the product is amorphous. Figure 3 shown.

[0051] Comparative Example 4: The difference from Example 1 is that the reaction temperature is 110°C and the product is amorphous. Figure 3 As shown, the experiment shows that good NH3-SCR performance can be achieved when the reaction temperature is in the range of 140℃-170℃.

[0052] Hydrothermal aging method and activity test:

[0053] (1) Hydrothermal aging of LEV catalyst. The LEV zeolite catalyst was hydrothermally aged at 750°C for 16 h at a volume space velocity of 100,000 h -1 The total gas flow rate is 30 mL / min, the gas distribution is O2 10%, water vapor 10% (carried out with N2), and N2 is the balance gas.

[0054] (2) Catalyst test. Fresh and aged samples (about 50 mg) were loaded into the reactor and tested in the temperature range of 100-600°C. The gas distribution was NO 500ppm; NH3 500ppm; O2 5%; H2O 5%; N2 as the balance gas. The total flow rate was 30mL / min and the volume space velocity was 240,000h -1 The activity data are listed in Figure 5 and Figure 6 middle.

[0055] The results of the above examples show that the raw materials for preparing LEV zeolite molecular sieves are cheap and the method is simple, which solves the problems of high synthesis cost and complicated preparation process in the prior art. The present invention develops a new organic structure directing agent N,N-dimethylmorpholinium cation and uses it to synthesize LEV zeolite, and at the same time obtains LEV zeolite molecular sieve catalysts with excellent NH3-SCR activity, which has great practical significance in environmental catalysis, especially in the field of diesel vehicle exhaust denitrification treatment.

[0056] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a LEV zeolite catalyst, characterized in that: Use the following steps: (1) Synthesis of organic structure-directing agent N, N-dimethylmorpholinium cation; The structure of the N, N-dimethylmorpholinium cation is shown in formula (I): （I）; (2) Using N, N-dimethylmorpholinium cation as a directing agent to prepare LEV zeolite; the LEV zeolite is treated with raw materials having a molar ratio of SiO2:Al2O3:N, N-dimethylmorpholinium cation:M2O:H2O=1:0.02-0.1:0.2-0.4:0.1-0.3:8-15, and after crystallization, the product is filtered, washed, dried, and calcined to obtain the LEV zeolite molecular sieve; wherein M is one of Na, K, and Cs or a combination of at least two of them; The conditions of the crystallization reaction are as follows: the temperature of the crystallization reaction is 140°C-170°C, the time of the crystallization reaction is determined according to the starting raw materials, the time of zeolite crystallization is 12 h-72 h, and the time of independent silicon source and independent aluminum source crystallization is 72 h-168 h; (3) LEV zeolite is subjected to two-step ion exchange to obtain LEV zeolite catalyst.

2. The method for preparing the LEV zeolite catalyst according to claim 1, characterized in that: The N-dimethylmorpholinium cation is prepared in the following manner: N-methylmorpholine is subjected to a methylation reaction to obtain an N, N-dimethylmorpholinium cation, which is then exchanged into a hydroxide form.

3. The method for preparing the LEV zeolite catalyst according to claim 1, characterized in that: The aluminum source and the silicon source include an independent aluminum source, an independent silicon source or a zeolite different from LEV zeolite; the independent aluminum source includes one of aluminum isopropoxide, boehmite, sodium aluminate, aluminum hydroxide, aluminum nitrate nonahydrate, or aluminum sulfate octadecadihydrate, or a combination of at least two of them; the independent silicon source includes one of white carbon black, fine silica gel, fumed silica, silica sol, sodium silicate, or tetraethoxysilane, or a combination of at least two of them.

4. The method for preparing the LEV zeolite catalyst according to claim 3, characterized in that: The zeolite different from LEV zeolite is Y zeolite with a silicon-aluminum atomic ratio of 5-30.

5. The method for preparing the LEV zeolite catalyst according to claim 1, characterized in that: The specific steps of the two-step ion exchange in step (3) are as follows: (1) The first step of ion exchange: ion exchange LEV zeolite with ammonium salt to obtain NH4-LEV zeolite; (2) The second step of ion exchange: the NH4-LEV zeolite obtained in the previous step is subjected to ion exchange with copper salt, and the LEV zeolite catalyst is obtained after calcination.

6. A LEV zeolite catalyst obtained by the preparation method according to any one of claims 1 to 5.

7. Use of the LEV zeolite catalyst according to claim 6, characterized in that: Used for the selective catalytic reduction of nitrogen oxides.

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