Molecular sieve for selective catalytic reduction of nitrogen oxide and application thereof

Abstract

The invention belongs to the field of denitration, and particularly relates to a molecular sieve for selective catalytic reduction of nitrogen oxide and application of the molecular sieve. The molecular sieve for selective catalytic reduction of nitrogen oxide comprises one or a mixture of a beta-type pure-phase molecular sieve, a beta-type mixed-phase molecular sieve or a tungsten-containing Beta-type molecular sieve. The pure-phase Beta molecular sieve disclosed by the invention has excellent medium and low temperature methanol-SCR catalytic performance, and meanwhile, the yield of N2 is relatively high. The mixed-phase molecular sieve and the tungsten-containing Beta molecular sieve, which are prepared by a seed crystal auxiliary method and mainly comprise a Bata phase, have excellent methanol-SCR activity, have a wide temperature window and a high N2 yield, and can realize green synthesis of the Beta molecular sieve. The invention provides the alcohol-SCR denitration catalyst with high low-temperature activity, high N2 selectivity and wide temperature window, so that the problems of catalyst deactivation, flue blockage, equipment corrosion and the like caused by ammonium sulfate generation when sulfur-containing moving source tail gas and industrial waste gas are treated by an NH3-SCR denitration technology at present are solved.

Description

Technical Field

[0001] The invention belongs to the field of denitration, and specifically relates to a molecular sieve for selective catalytic reduction of nitrogen oxides and application thereof. Background Art

[0002] Nitrogen oxides (NOx) produced by mobile and stationary sources are highly irritating and corrosive, and have caused serious harm to human health and the ecological environment. Selective catalytic reduction technology is a technology that uses a reducing agent (ammonia produced by the decomposition of urea) to selectively reduce nitrogen oxides to water and nitrogen under the action of a catalyst. It has become the most widely used and mature technology for NOx emissions. However, this technology also has some unavoidable problems: (1) Exhaust gas from fixed and mobile sources of sulfur-containing fuels and mobile source exhaust gas (such as marine diesel engines) always contains high concentrations of sulfur dioxide (SO2). SO2 inevitably reacts with NH3, O2 and H2O to form ammonium sulfate ((NH4)2SO4) or ammonium bisulfate (NH4HSO4). When they mix with water and dust in the flue gas, they become corrosive and sticky, adhering to the surface of catalysts, equipment and pipelines, causing catalyst deactivation and damaging flue gas pipelines; (2) Urea as an NH3 source requires additional storage tanks, which increases the cost of use; in addition, the problem of ammonia escape will also cause additional environmental problems.

[0003] Lower alcohols with fewer carbon atoms, such as methanol and ethanol, are not easy to react with sulfur oxides to form sulfuric acid (hydrogen) ammonia deposits, and alcohol-SCR (HCOs-SCR) technology, which uses it instead of ammonia to selectively catalytically reduce nitrogen oxides, came into being. Compared with NH3-SCR technology, this technology can avoid the problems of catalyst deactivation or pipeline blockage, and at the same time, it can reduce the investment in urea tanks in some scenarios where lower alcohols are used as fuel. Using ethanol as a reducing agent, non-patent literature [ACS Catal. 2014, 4, 2776-2784, ACS Cata l. 2018, 8, 2699-2708] selectively reduces NOx on a silver-based catalyst supported by alumina, and its optimal catalytic effect can achieve complete conversion of NOx at 380°C. Non-patent literature [Mi croporous Mesoporous Mater., 2015, 203, 163-169] studied the ethanol-SCR catalytic performance of BEA molecular sieve supported silver catalyst, and its maximum conversion temperature was around 300°C, but the NOx conversion efficiency was only 30%. Patent CN 111841532B coated the active component Ag on a honeycomb ceramic carrier and was able to achieve a 95% NOx conversion rate at 250°C. However, it introduced ethanol gas with a molar ratio of 5:1, and the cost of the reducing agent was relatively higher.

[0004] For methanol as a reducing agent, the Ag / Al2O3 catalyst prepared in the non-patent literature [App l.Cata lB-Envi ron., 2016, 180, 291–300, Cata l.Today., 2015, 258, 454–460] has a NOx conversion rate of 60% at 250°C, but it produces about 30% N2O byproduct. The oxygen vacancy-rich LaFe 0.8 Cu 0.2 O3 catalyst has a NOx conversion efficiency of more than 90% above 400°C, but its activity below 400°C is still relatively poor. Molecular sieves have also been widely studied in methanol reduction of NOx. The non-patent literature [Cata l.Today., 2020, 355, 443–449] calcined and modified commercial HZSM-5 and found that it had a NOx conversion rate of about 60% at 300°C, but its low conversion efficiency cannot meet the actual use of industrial flue gas. The FER molecular sieve catalyst and MEL molecular sieve catalyst prepared by patent CN 114206495B and patent CN 118022825 A both have a NOx conversion rate of more than 80% and N2 selectivity at 250-350°C, showing a good CH3OH-SCR catalytic effect. However, the temperature window of methanol-SCR of the two types of catalysts is still relatively narrow, which cannot meet the treatment requirements of fixed source exhaust gas and diesel engine exhaust gas in a wide temperature range. Therefore, it is of great significance to develop some other catalysts with excellent CH3OH-SCR effect and low cost. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a molecular sieve for selective catalytic reduction of nitrogen oxides and its application.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] A molecular sieve for selective catalytic reduction of nitrogen oxides comprises one or a mixture of a beta-type pure phase molecular sieve, a beta-type mixed phase molecular sieve or a tungsten-containing beta-type molecular sieve.

[0008] The pure phase Beta molecular sieve XRD spectrum has three obvious diffraction peaks at 6.9-8.6°, 20.8-21.8° and 21.8-22.8°, and weak diffraction peaks at 13.2-13.7°, 24.9-25.9°, 26.1-26.9°, 27.1-28.3°, 28.7-29.6° and 43.1-44°.

[0009] The XRD spectrum of the beta mixed-phase molecular sieve includes diffraction peaks belonging to the pure phase Beta molecular sieve and at 6.3-6.8°, 9.1-10.2°, 15.2-15.6°, 15.5-19.9° and 30.8-31.2° belonging to the MOR molecular sieve.

[0010] The molar ratio of silicon atoms to aluminum atoms in the beta pure phase molecular sieve, beta mixed phase molecular sieve or tungsten-containing Beta molecular sieve is less than 15:1; preferably, the tungsten content of the tungsten-containing Beta molecular sieve is less than 1%; preferably less than 0.5%.

[0011] The beta-type pure phase molecular sieve is prepared by a template method;

[0012] Preferably, the following steps are adopted:

[0013] 1) Preparation of gel: dissolving an alkali metal source in deionized water, adding a template, and then sequentially adding a silicon source and an aluminum source to the mixed solution, stirring and mixing thoroughly to obtain an initial gel;

[0014] The molar ratio of silicon source, aluminum source, template, alkali metal source and deionized water is 1: (0.008-0.04): (0.3-0.8): (0.02-0.05): (10-20);

[0015] 2) Hydrothermal reaction: After the above-mentioned gel is aged, it is subjected to a hydrothermal reaction at a certain temperature; preferably, the aging temperature is below 60° C., and the time is 2h-24h; the hydrothermal reaction conditions are 100-200° C. in an oven for 12h-7 days;

[0016] 3) Washing, calcining and ion exchange: The hydrothermally synthesized molecular sieve is repeatedly washed with deionized water until the solution filtered through the molecular sieve is neutral; the molecular sieve is dried at 80-100°C, and then calcined to remove the template, the calcination temperature is above 400°C, preferably above 500°C, usually below 900°C, preferably below 800°C, and more preferably below 700°C; the calcination time is 2-12h, preferably 4-8h.

[0017] Ion exchange can further obtain NH4 type and H type Beta molecular sieves. The calcined molecular sieve and ammonium salt are subjected to ion exchange at 60°C-100°C to obtain NH4-Beta molecular sieve. The ion exchange time is 4-12h, and the number of exchanges is preferably 3 times. The ammonium salt is any one of ammonium chloride, ammonium nitrate, ammonium acetate, and ammonium sulfate. The concentration of the ammonium salt can be 0.1mol / L-5mol / L, preferably 0.5mol / L-1.5mol / L.

[0018] The NH4-Beta molecular sieve can be further calcined to obtain the H-Beta molecular sieve. The calcination temperature is 350°C-650°C, preferably 500°C-550°C.

[0019] The beta-type mixed-phase molecular sieve is prepared by a seed crystal method;

[0020] Preferably, the following steps are adopted:

[0021] 1) Preparation of gel: dissolving an alkali metal source in deionized water, then adding a silicon source and an aluminum source and stirring and mixing them thoroughly, and finally adding seed crystals and mixing them uniformly to obtain an initial gel;

[0022] The molar ratio of silicon source, aluminum source, alkali metal source and deionized water is 1: (0.008-0.18):

[0023] (0.2~0.35):(10~30), the amount of seed added is 1-30wt% of the mass of the added silicon source;

[0024] 2) Hydrothermal reaction: After the above-mentioned gel is aged, it is subjected to a hydrothermal reaction at a certain temperature; preferably, the aging temperature is below 60° C., and the time is 2h-24h; the hydrothermal reaction conditions are 100-200° C. in an oven for 12h-7 days;

[0025] 3) Washing, calcining and ion exchange: The hydrothermally synthesized molecular sieve is repeatedly washed with deionized water until the solution filtered through the molecular sieve is neutral; the molecular sieve is dried at 80-100°C, and then calcined to remove the template, the calcination temperature is above 400°C, preferably above 500°C, usually below 900°C, preferably below 800°C, and more preferably below 700°C; the calcination time is 2-12h, preferably 4-8h.

[0026] Ion exchange can further obtain NH4 type and H type Beta molecular sieves. The calcined molecular sieve and ammonium salt are subjected to ion exchange at 60°C-100°C to obtain NH4-Beta molecular sieve. The ion exchange time is 4-12h, and the number of exchanges is preferably 3 times. The ammonium salt is any one of ammonium chloride, ammonium nitrate, ammonium acetate, and ammonium sulfate. The concentration of the ammonium salt can be 0.1mol / L-5mol / L, preferably 0.5mol / L-1.5mol / L.

[0027] The NH4-Beta molecular sieve can be further calcined to obtain the H-Beta molecular sieve. The calcination temperature is 350°C-650°C, preferably 500°C-550°C.

[0028] Tungsten-containing Beta molecular sieves are prepared by template method or ion exchange method;

[0029] a) The template method uses the following steps:

[0030] 1) Preparation of gel: dissolve the alkali metal source in deionized water, add the template, then add the silicon source, aluminum source and tungsten source to the mixed solution in sequence, and stir and mix thoroughly to obtain the initial gel;

[0031] The molar ratio of silicon source, aluminum source, tungsten source, template, alkali metal source and deionized water is 1: (0.008-0.04): (0.001-0.1) (0.3-0.8): (0.02-0.05): (10-20);

[0032] 2) Hydrothermal reaction: After the above-mentioned gel is aged, it is subjected to a hydrothermal reaction at a certain temperature; preferably, the aging temperature is below 60° C., and the time is 2h-24h; the hydrothermal reaction conditions are 100-200° C. in an oven for 12h-7 days;

[0033] 3) Washing, calcining and ion exchange: The hydrothermally synthesized molecular sieve is repeatedly washed with deionized water until the solution filtered through the molecular sieve is neutral; the molecular sieve is dried at 80-100°C, and then calcined to remove the template, the calcination temperature is above 400°C, preferably above 500°C, usually below 900°C, preferably below 800°C, and more preferably below 700°C; the calcination time is 2-12h, preferably 4-8h.

[0034] Ion exchange can further obtain NH4 type and H type Beta molecular sieves. The calcined molecular sieve and ammonium salt are subjected to ion exchange at 60°C-100°C to obtain NH4-Beta molecular sieve. The ion exchange time is 4-12h, and the number of exchanges is preferably 3 times. The ammonium salt is any one of ammonium chloride, ammonium nitrate, ammonium acetate, and ammonium sulfate. The concentration of the ammonium salt can be 0.1mol / L-5mol / L, preferably 0.5mol / L-1.5mol / L.

[0035] The NH4-Beta molecular sieve can be further calcined to obtain the H-Beta molecular sieve. The calcination temperature is 350°C-650°C, preferably 500°C-550°C; the calcination time is 2-12h, preferably 4-8h.

[0036] b) The ion exchange method uses the following steps:

[0037] The H-Beta molecular sieve and a tungsten source are ion exchanged or impregnated in an aqueous solution, followed by calcination to obtain the tungsten-containing Beta molecular sieve; the ion exchange or impregnation temperature is 60°C-100°C; preferably 80°C, and the time is 0.5h-3h; preferably 2h; the calcination temperature is 350°C-650°C, preferably 500°C-550°C, and the calcination time is 2-12h, preferably 4-8h;

[0038] The H-Beta molecular sieve is prepared in the following manner:

[0039] 1) Preparation of gel: dissolving an alkali metal source in deionized water, adding a template, and then sequentially adding a silicon source and an aluminum source to the mixed solution, stirring and mixing thoroughly to obtain an initial gel;

[0040] The molar ratio of silicon source, aluminum source, template, alkali metal source and deionized water is 1: (0.008-0.04): (0.3-0.8): (0.02-0.05): (10-20);

[0041] 2) Hydrothermal reaction: After the above-mentioned gel is aged, it is subjected to a hydrothermal reaction at a certain temperature; preferably, the aging temperature is below 60° C., and the time is 2h-24h; the hydrothermal reaction conditions are 100-200° C. in an oven for 12h-7 days;

[0042] 3) Washing, calcining and ion exchange: The hydrothermally synthesized molecular sieve is repeatedly washed with deionized water until the solution filtered through the molecular sieve is neutral; the molecular sieve is dried at 80-100°C, and then calcined to remove the template, the calcination temperature is above 400°C, preferably above 500°C, usually below 900°C, preferably below 800°C, and more preferably below 700°C; the calcination time is 2-12h, preferably 4-8h.

[0043] Ion exchange can further obtain NH4 type and H type Beta molecular sieves. The calcined molecular sieve and ammonium salt are subjected to ion exchange at 60°C-100°C to obtain NH4-Beta molecular sieve. The ion exchange time is 4-12h, and the number of exchanges is preferably 3 times. The ammonium salt is any one of ammonium chloride, ammonium nitrate, ammonium acetate, and ammonium sulfate. The concentration of the ammonium salt can be 0.1mol / L-5mol / L, preferably 0.5mol / L-1.5mol / L.

[0044] The NH4-Beta molecular sieve is further calcined to obtain the H-Beta molecular sieve, and the calcination temperature is 350°C-650°C, preferably 500°C-550°C.

[0045] The silicon source is any one of fumed silica, white carbon black, ethyl orthosilicate, fine silica gel, silica sol, sodium silicate or tetraethoxysilane, or a combination of at least two thereof;

[0046] The aluminum source is any one of aluminum isopropoxide, aluminum sulfate, boehmite, aluminum oxide, sodium aluminate, aluminum hydroxide, aluminum nitrate nonahydrate or aluminum sulfate octadecadihydrate, or a combination of at least two thereof;

[0047] The alkali metal source is one of sodium hydroxide and potassium hydroxide;

[0048] The tungsten source is any one of ammonium metatungstate, sodium metatungstate and potassium tungstate or a combination of at least two of them, preferably ammonium metatungstate;

[0049] The template agent is one or more of tetraethylammonium hydroxide, tetraethylammonium bromide, diethylamine, and 1,6-hexanediamine, preferably tetraethylammonium hydroxide and tetraethylammonium bromide;

[0050] The seed crystal is a beta-type pure phase molecular sieve, or a commercial Beta molecular sieve; preferably, the silicon-aluminum ratio of the seed crystal is 10-60, and the coordinated cations in the seed crystal can be NH3 type, H type or Na type, preferably H type Beta molecular sieve.

[0051] The present invention also includes an application of the molecular sieve for selective catalytic reduction of nitrogen oxides, using lower alcohol as a reducing agent; preferably, the lower alcohol is methanol or ethanol.

[0052] Compared with the prior art, the present invention has the following beneficial effects:

[0053] The pure phase Beta molecular sieve of the present invention has excellent medium and low temperature methanol-SCR catalytic performance, and the yield of N2 is also high. The mixed phase molecular sieve mainly composed of Beta phase and tungsten-containing Beta molecular sieve prepared by the seed-assisted method exhibits excellent methanol-SCR activity, has a wide temperature window and a high N2 yield, and can realize the green synthesis of Beta molecular sieve. The present invention provides an alcohol-SCR denitration catalyst with high low-temperature activity, high N2 selectivity and a wide temperature window, thereby solving the problems of catalyst deactivation, flue blockage, equipment corrosion, etc. caused by the production of ammonium sulfate when the current NH3-SCR denitration technology treats sulfur-containing mobile source tail gas and industrial waste gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 It is the XRD spectrum of Beta molecular sieve with different configurations;

[0055] Figure 2 is the XRD pattern of the product obtained in Examples 1-7 of the present invention;

[0056] Figure 3 It is the XRD spectrum of the products obtained in Examples 8-9 of the present invention and Comparative Examples 1-3. DETAILED DESCRIPTION

[0057] 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.

[0058] Molecular sieve

[0059] The Beta molecular sieve involved in the present invention refers to a eutectic molecular sieve specified in the Database of Zeolite Structures of the International Molecular Sieve Association. Molecular sieves are a type of inorganic microporous crystalline material with a three-dimensional ordered structure, which is composed of TO4 (T = Si, Al, P, etc.) tetrahedrons as basic structural units and connected by bridging oxygen atoms at the same vertices. For Beta molecular sieves, its structural characteristics are: (1) It has three different body configurations of ABC, usually composed of A-body and B-body in different proportions; (2) It has twelve-membered ring channels with diameters of 6.3×7.5 and 6.9×6.0; according to different AB body ratios, its different X-ray diffraction patterns are as follows: Figure 1 As shown, if a substance has Figure 1 If the molecular sieve has similar spectral information, it can be determined that the molecular sieve is a Beta molecular sieve.

[0060] The pure phase Beta molecular sieve described in the present invention has the following characteristics when analyzed by XRD spectrum: it has three obvious diffraction peaks at 6.9-8.6°, 20.8-21.8° and 21.8-22.8°, and has multiple weak diffraction peaks at 13.2-13.7°, 24.9-25.9°, 26.1-26.9°, 27.1-28.3°, 28.7-29.6° and 43.1-44°.

[0061] For the Beta-based mixed phase molecular sieve, in addition to the above diffraction peaks, there are diffraction peaks belonging to the MOR molecular sieve at 6.3-6.8°, 9.1-10.2°, 15.2-15.6°, 15.5-19.9° and 30.8-31.2°, among which the diffraction peak at 9.1-10.2° is the most obvious. For tungsten-containing Beta molecular sieves, since the introduction of a small amount of tungsten does not cause changes in the Beta crystal form, but only causes its diffraction peak to weaken, the XRD spectrum characteristics described above are still applicable to them.

[0062] Example 1

[0063] Preparation of beta pure phase molecular sieve:

[0064] 1) Preparation of gel: 15 g of silica gel (40 wt%) was slowly added to 21.2 g of tetraethylammonium hydroxide solution (40 wt%), and stirred thoroughly. Then, 0.656 g of sodium aluminate was added, and the mixture was stirred and mixed to obtain a uniform initial gel.

[0065] The molar ratio of the gel is SiO2 (silicon source): Al2O3 (aluminum source): tetraethylammonium hydroxide (template): Na2O (alkali metal source): H2O = 1:0.04:0.36:0.04:13.8.

[0066] 2) Hydrothermal reaction: The gel was then aged at room temperature for 24 h with stirring and then transferred to a pressure vessel and hydrothermally heated at 140 °C for 4 days.

[0067] 3) Washing, calcination and ion exchange: After the reaction solution is cooled, the white powder material is recovered by suction filtration, and the hydrothermally synthesized molecular sieve is repeatedly washed with deionized water until the solution filtered through the molecular sieve is neutral; then it is dried at 80°C for 6 hours to obtain molecular sieve A1. It is then calcined at 550°C for 6 hours to remove the template to obtain molecular sieve A2.

[0068] Weigh 1g A2 molecular sieve and 5.35g ammonium chloride and add them to 100g deionized water, stir at 80℃ for 8h, centrifuge and dry, repeat this operation 3 times, and finally calcine in air at 550℃ for 4h to obtain molecular sieve A. The XRD spectrum of molecular sieve A is shown in Figure 2 The results of the methanol-SCR activity of molecular sieve A are shown in Table 1.

[0069] Example 2

[0070] The difference between Example 2 and Example 1 is that the gel ratio is changed. The molar ratio of the gel is SiO2 (silicon source): Al2O3 (aluminum source): tetraethylammonium hydroxide (template): Na2O (alkali metal source): H2O = 1: 0.02: 0.36: 0.015: 13.8. Molecular sieve B is prepared, and its XRD spectrum measurement results are as follows: Figure 2 The results of the methanol-SCR activity of molecular sieve B are shown in Table 1.

[0071] Example 3

[0072] Preparation of beta type mixed phase molecular sieve:

[0073] 1) Preparation of gel: Dissolve 2.65g sodium hydroxide in 32g water, slowly add 8g fumed silica under stirring, and age at room temperature for 1h; dissolve 0.54g sodium aluminate in water under stirring, add the above solution and stir for 1h; add 0.8g seed crystals to the above gel and stir evenly. The molar ratio of the gel is SiO2 (silicon source): Al2O3 (aluminum source): Na2O (alkali metal source): H2O = 1: 0.025: 0.25: 16. The mass of the added seed crystal is 10% of the mass of SiO2.

[0074] 2) Hydrothermal reaction: The gel was then aged at room temperature for 2 h with stirring, and then transferred to a pressure vessel and hydrothermally heated at 140 °C for 48 h.

[0075] 3) Washing, calcination and ion exchange: After the reaction solution is cooled, the white powder material is recovered by filtration, and the hydrothermally synthesized molecular sieve is repeatedly washed with deionized water until the solution filtered through the molecular sieve is neutral; then it is dried at 80°C for 6 hours to obtain molecular sieve C1. It is then calcined at 650°C for 6 hours to obtain molecular sieve C2.

[0076] Weigh 1g C2 molecular sieve and 5.35g ammonium chloride and add them to 100g deionized water, stir at 80℃ for 8h, centrifuge and dry, repeat this operation 3 times, and finally calcine in air at 550℃ for 4h to obtain molecular sieve C. The XRD spectrum measurement results of molecular sieve C are shown in Figure 2 The results of the methanol-SCR activity of molecular sieve C are shown in Table 1.

[0077] Example 4

[0078] The difference between Example 4 and Example 3 is that the gel ratio is changed. The gel molar ratio is SiO2 (silicon source): Al2O3 (aluminum source): Na2O (alkali metal source): H2O = 1:0.033:0.25:16. Molecular sieve D is prepared, and its XRD spectrum measurement results are as follows: Figure 2 The results of the methanol-SCR activity of molecular sieve D are shown in Table 1.

[0079] Example 5

[0080] The only difference between Example 5 and Example 3 is the calcination temperature. The molecular sieve C1 obtained by filtration and drying is calcined in air at 550°C for 6 hours. After ion exchange, molecular sieve E is obtained. The XRD spectrum measurement results are as follows: Figure 2 The results of the methanol-SCR activity of molecular sieve E are shown in Table 1.

[0081] Example 6

[0082] The difference between Example 6 and Example 3 is that the content of added seed crystals is the same, and the mass of added seed crystals is 20% of the mass of SiO2. Molecular sieve F is prepared, and its XRD spectrum measurement results are shown as follows: Figure 2 The results of the methanol-SCR activity of molecular sieve F are shown in Table 1.

[0083] Example 7

[0084] The difference between Example 7 and Example 3 is only the hydrothermal temperature. In this example, molecular sieve G is prepared by hydrothermal treatment at 130°C for 48 hours. The XRD spectrum of the prepared molecular sieve G is shown in FIG. Figure 2The results of the methanol-SCR activity of molecular sieve G are shown in Table 1.

[0085] Example 8

[0086] Preparation of tungsten-containing Beta molecular sieves by template method;

[0087] 1) Gel preparation: 15g silica gel (40wt%) was slowly added to 21.2g tetraethylammonium hydroxide solution (40wt%), and stirred thoroughly. Then 0.656g sodium aluminate and 0.124g ammonium metatungstate were added, and a uniform initial gel was obtained after stirring and mixing. The molar ratio of the gel was SiO2 (silicon source): Al2O3 (aluminum source): tetraethylammonium hydroxide (template): tungsten source: Na2O: H2O = 1: 0.04: 0.36: 0.005: 0.04: 13.8.

[0088] 2) Hydrothermal reaction: The gel was then aged at room temperature for 24 h with stirring and then transferred to a pressure vessel and hydrothermally heated at 140 °C for 4 days.

[0089] 3) Washing, calcination and ion exchange: After the reaction solution is cooled, the white powder material is recovered by filtration, and the hydrothermally synthesized molecular sieve is repeatedly washed with deionized water until the solution filtered through the molecular sieve is neutral; then it is dried at 80°C for 6 hours to obtain molecular sieve H1. It is then calcined at 550°C for 6 hours to remove the template agent to obtain molecular sieve H2. Molecular sieve H2 is subjected to ion exchange to obtain molecular sieve H. The XRD spectrum measurement results of molecular sieve H are shown as follows: Figure 3 As shown, it is confirmed that it is a pure phase Beta type molecular sieve with a tungsten content of 0.5wt%. The methanol-SCR activity results of molecular sieve H are shown in Table 1.

[0090] Example 9

[0091] Preparation of tungsten-containing Beta molecular sieve by ion exchange method;

[0092] 0.246 g of ammonium metatungstate was added to 200 ml of aqueous solution, and then 2 g of molecular sieve A prepared in Example 1 was added. The reaction was carried out at 80°C for 1 hour to perform ion exchange. After the solution was cooled, solid powder was recovered by filtration and dried at 80°C for 6 hours. The obtained powder was calcined at 550°C in air for 8 hours to obtain molecular sieve I. The XRD spectrum measurement results are shown as follows: Figure 2 As shown, it is confirmed that it is a pure phase Beta molecular sieve with a tungsten content of 0.8wt%; the alcohol-SCR activity results of molecular sieve I are shown in Table 1.

[0093] The mass percentage of tungsten in the molecular sieve is 0.05-10%, such as 0.05%, 0.1%, 0.2%, 0.5%, 1%, 2%, 4%, 6%, 8% or 10%. When the content of tungsten is above 1%, the high temperature catalytic performance tends to be reduced, and when it is below 1%, more preferably below 0.5%, the temperature window of the Beta molecular sieve is significantly improved.

[0094] Comparative Example 1

[0095] The H-type Beta molecular sieve with Si / Al ratio of 15 produced by the catalyst plant of Nankai University was calcined at 550℃ in air for 4h to remove the possible uncalcined template agent, thereby obtaining molecular sieve J. Its XRD shows Figure 3 , which is confirmed to be a pure phase Beta molecular sieve. The methanol-SCR activity results of molecular sieve I are shown in Table J, which shows that the low silicon aluminum synthesized by the present invention has better low-temperature catalytic activity than the pure phase Beta molecular sieve.

[0096] Comparative Example 2

[0097] The difference between Comparative Example 2 and Example 3 is only the hydrothermal temperature. In this comparative example, molecular sieve K is prepared by hydrothermal treatment at 160°C for 48 hours. Figure 3 , confirming that it is a molecular sieve with MOR molecular sieve as the main phase. The methanol-SCR activity results of molecular sieve K are shown in Table 1.

[0098] Comparative Example 3

[0099] Molecular sieve A and molecular sieve K were mixed in a mass ratio of 9:1, ball-milled at 200 rpm for 2 h, and then calcined at 550 °C in air for 4 h. Molecular sieve L was obtained, and its XRD showed Figure 3 , confirming that it is a molecular sieve with Beta molecular sieve as the main phase. The methanol-SCR activity results of molecular sieve L are shown in Table 1.

[0100] Methods for evaluating catalyst activity

[0101] The prepared catalyst was pressed into shape, crushed, and 20-40 mesh particles were screened and filled into a fixed bed flow quartz tube. -1 ) The gas composition was tested for activity at 150-650℃. The gas composition was: 500ppm NO; 1000ppm CH3OH; 10vol% O2; 5vol% H2O; N2 was the balance gas. The removal activity of nitrogen oxides was calculated based on the concentrations of outlet NOx (NO, NO2) and N2O at different temperatures;

[0102] [NOx] conversion rate = {inlet [NOx] - outlet [NOx]} / inlet [NOx] * 100%;

[0103] [N2] yield = {inlet [NOx] - outlet [NOx] - 2 outlet [N2O]} / inlet [NOx] * 100%;

[0105] Table 1

[0106]

[0107]

[0108] It can be seen from the results of the above embodiments and comparative examples that the pure phase Beta molecular sieve prepared by the present invention has excellent medium and low temperature methanol-SCR catalytic performance, and the yield of N2 is also relatively high. The mixed phase molecular sieve mainly composed of Beta phase and tungsten-containing Beta molecular sieve prepared by the seed-assisted method exhibits excellent methanol-SCR activity, has a wide temperature window and a high N2 yield, and can realize the green synthesis of Beta molecular sieve. The above invention solves the problems of poor low temperature activity of catalysts, low N2 yield, and narrow temperature window in the existing CH3OH-SCR technology, and has great practical significance for environmental catalysis, especially in the field of sulfur-containing fixed and mobile source nitrogen oxide control.

[0109] 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 molecular sieve for selective catalytic reduction of nitrogen oxides, characterized in that: It includes one or a mixture of beta pure phase molecular sieve, beta mixed phase molecular sieve or tungsten-containing beta molecular sieve.

2. The molecular sieve for selective catalytic reduction of nitrogen oxides according to claim 1, characterized in that: The pure phase Beta molecular sieve XRD spectrum has three obvious diffraction peaks at 6.9-8.6°, 20.8-21.8° and 21.8-22.8°, and weak diffraction peaks at 13.2-13.7°, 24.9-25.9°, 26.1-26.9°, 27.1-28.3°, 28.7-29.6° and 43.1-44°.

3. The molecular sieve for selective catalytic reduction of nitrogen oxides according to claim 1, characterized in that: The XRD spectrum of the beta mixed-phase molecular sieve includes diffraction peaks belonging to the pure phase Beta molecular sieve and at 6.3-6.8°, 9.1-10.2°, 15.2-15.6°, 15.5-19.9° and 30.8-31.2° belonging to the MOR molecular sieve.

4. The molecular sieve for selective catalytic reduction of nitrogen oxides according to claim 1, characterized in that: The molar ratio of silicon atoms to aluminum atoms in the beta pure phase molecular sieve, beta mixed phase molecular sieve or tungsten-containing Beta molecular sieve is less than 15:1; preferably, the tungsten content of the tungsten-containing Beta molecular sieve is less than 1%; preferably less than 0.5%.

5. The molecular sieve for selective catalytic reduction of nitrogen oxides according to claim 1, characterized in that: The beta-type pure phase molecular sieve is prepared by a template method; Preferably, the following steps are adopted: 1) Preparation of gel: dissolving an alkali metal source in deionized water, adding a template, and then sequentially adding a silicon source and an aluminum source to the mixed solution, stirring and mixing thoroughly to obtain an initial gel; The molar ratio of silicon source, aluminum source, template, alkali metal source and deionized water is 1: (0.008-0.04): (0.3-0.8): (0.02-0.05): (10-20); 2) Hydrothermal reaction: After the above-mentioned gel is aged, it is subjected to a hydrothermal reaction at a certain temperature; preferably, the aging temperature is below 60° C., and the time is 2h-24h; the hydrothermal reaction conditions are 100-200° C. in an oven for 12h-7 days; 3) Washing, calcination and ion exchange: The hydrothermally synthesized molecular sieve is repeatedly washed with deionized water until the solution filtered through the molecular sieve becomes neutral; The molecular sieve is dried at 80-100°C and then calcined to remove the template. The calcination temperature is above 400°C, preferably above 500°C, usually below 900°C, preferably below 800°C, and more preferably below 700°C. The calcination time is 2-12h, preferably 4-8h. Ion exchange can further obtain NH4 type and H type Beta molecular sieves. The calcined molecular sieve and ammonium salt are subjected to ion exchange at 60°C-100°C to obtain NH4-Beta molecular sieve. The ion exchange time is 4-12h, and the number of exchanges is preferably 3 times. The ammonium salt is any one of ammonium chloride, ammonium nitrate, ammonium acetate, and ammonium sulfate. The concentration of the ammonium salt can be 0.1mol / L-5mol / L, preferably 0.5mol / L-1.5mol / L. The NH4-Beta molecular sieve can be further calcined to obtain the H-Beta molecular sieve, and the calcination temperature is 350°C-650°C, preferably 500°C-550°C.

6. The molecular sieve for selective catalytic reduction of nitrogen oxides according to claim 1, characterized in that: The beta-type mixed-phase molecular sieve is prepared by a seed crystal method; Preferably, the following steps are adopted: 1) Preparation of gel: dissolving an alkali metal source in deionized water, then adding a silicon source and an aluminum source and stirring and mixing them thoroughly, and finally adding seed crystals and mixing them uniformly to obtain an initial gel; The molar ratio of silicon source, aluminum source, alkali metal source and deionized water is 1: (0.008-0.18): (0.2~0.35):(10~30), the amount of seed added is 1-30wt% of the mass of the added silicon source; 2) Hydrothermal reaction: After the above-mentioned gel is aged, it is subjected to a hydrothermal reaction at a certain temperature; preferably, the aging temperature is below 60° C., and the time is 2h-24h; the hydrothermal reaction conditions are 100-200° C. in an oven for 12h-7 days; 3) Washing, calcination and ion exchange: The hydrothermally synthesized molecular sieve is repeatedly washed with deionized water until the solution filtered through the molecular sieve becomes neutral; The molecular sieve is dried at 80-100°C and then calcined to remove the template. The calcination temperature is above 400°C, preferably above 500°C, usually below 900°C, preferably below 800°C, and more preferably below 700°C. The calcination time is 2-12h, preferably 4-8h. Ion exchange can further obtain NH4 type and H type Beta molecular sieves. The calcined molecular sieve and ammonium salt are subjected to ion exchange at 60°C-100°C to obtain NH4-Beta molecular sieve. The ion exchange time is 4-12h, and the number of exchanges is preferably 3 times. The ammonium salt is any one of ammonium chloride, ammonium nitrate, ammonium acetate, and ammonium sulfate. The concentration of the ammonium salt can be 0.1mol / L-5mol / L, preferably 0.5mol / L-1.5mol / L. The NH4-Beta molecular sieve can be further calcined to obtain the H-Beta molecular sieve, and the calcination temperature is 350°C-650°C, preferably 500°C-550°C.

7. The molecular sieve for selective catalytic reduction of nitrogen oxides according to claim 5, characterized in that: Tungsten-containing Beta molecular sieves are prepared by template method or ion exchange method; a) The template method uses the following steps: 1) Preparation of gel: dissolve the alkali metal source in deionized water, add the template, then add the silicon source, aluminum source and tungsten source to the mixed solution in sequence, and stir and mix thoroughly to obtain the initial gel; The molar ratio of silicon source, aluminum source, tungsten source, template, alkali metal source and deionized water is 1: (0.008-0.04): (0.001-0.1) (0.3-0.8): (0.02-0.05): (10-20); 2) Hydrothermal reaction: After the above-mentioned gel is aged, it is subjected to a hydrothermal reaction at a certain temperature; preferably, the aging temperature is below 60° C., and the time is 2h-24h; the hydrothermal reaction conditions are 100-200° C. in an oven for 12h-7 days; 3) Washing, calcination and ion exchange: The hydrothermally synthesized molecular sieve is repeatedly washed with deionized water until the solution filtered through the molecular sieve becomes neutral; The molecular sieve is dried at 80-100°C and then calcined to remove the template. The calcination temperature is above 400°C, preferably above 500°C, usually below 900°C, preferably below 800°C, and more preferably below 700°C. The calcination time is 2-12h, preferably 4-8h. Ion exchange can further obtain NH4 type and H type Beta molecular sieves. The calcined molecular sieve and ammonium salt are subjected to ion exchange at 60°C-100°C to obtain NH4-Beta molecular sieve. The ion exchange time is 4-12h, and the number of exchanges is preferably 3 times. The ammonium salt is any one of ammonium chloride, ammonium nitrate, ammonium acetate, and ammonium sulfate. The concentration of the ammonium salt can be 0.1mol / L-5mol / L, preferably 0.5mol / L-1.5mol / L. The NH4-Beta molecular sieve can be further calcined to obtain the H-Beta molecular sieve. The calcination temperature is 350°C-650°C, preferably 500°C-550°C; the calcination time is 2-12h, preferably 4-8h. b) The ion exchange method uses the following steps: The H-Beta molecular sieve and a tungsten source are subjected to ion exchange or impregnation in an aqueous solution, followed by calcination to obtain the tungsten-containing Beta molecular sieve; the ion exchange or impregnation temperature is 60°C-100°C; preferably 80°C, and the time is 0.5h-3h; preferably 2h; the calcination temperature is 350°C-650°C, preferably 500°C-550°C, and the calcination time is 2-12h, preferably 4-8h; The H-Beta molecular sieve is prepared in the following manner: 1) Preparation of gel: dissolving an alkali metal source in deionized water, adding a template, and then sequentially adding a silicon source and an aluminum source to the mixed solution, stirring and mixing thoroughly to obtain an initial gel; The molar ratio of silicon source, aluminum source, template, alkali metal source and deionized water is 1: (0.008-0.04): (0.3-0.8): (0.02-0.05): (10-20); 2) Hydrothermal reaction: After the above-mentioned gel is aged, it is subjected to a hydrothermal reaction at a certain temperature; preferably, the aging temperature is below 60° C., and the time is 2h-24h; the hydrothermal reaction conditions are 100-200° C. in an oven for 12h-7 days; 3) Washing, calcination and ion exchange: The hydrothermally synthesized molecular sieve is repeatedly washed with deionized water until the solution filtered through the molecular sieve becomes neutral; The molecular sieve is dried at 80-100°C and then calcined to remove the template. The calcination temperature is above 400°C, preferably above 500°C, usually below 900°C, preferably below 800°C, and more preferably below 700°C. The calcination time is 2-12h, preferably 4-8h. Ion exchange can further obtain NH4 type and H type Beta molecular sieves. The calcined molecular sieve and ammonium salt are subjected to ion exchange at 60°C-100°C to obtain NH4-Beta molecular sieve. The ion exchange time is 4-12h, and the number of exchanges is preferably 3 times. The ammonium salt is any one of ammonium chloride, ammonium nitrate, ammonium acetate, and ammonium sulfate. The concentration of the ammonium salt can be 0.1mol / L-5mol / L, preferably 0.5mol / L-1.5mol / L. The NH4-Beta molecular sieve is further calcined to obtain the H-Beta molecular sieve, and the calcination temperature is 350°C-650°C, preferably 500°C-550°C.

8. The molecular sieve for selective catalytic reduction of nitrogen oxides according to any one of claims 5 to 7, characterized in that: The silicon source is any one of fumed silica, white carbon black, ethyl orthosilicate, fine silica gel, silica sol, sodium silicate or tetraethoxysilane, or a combination of at least two thereof; The aluminum source is any one of aluminum isopropoxide, aluminum sulfate, boehmite, aluminum oxide, sodium aluminate, aluminum hydroxide, aluminum nitrate nonahydrate or aluminum sulfate octadecadihydrate, or a combination of at least two thereof; The alkali metal source is one of sodium hydroxide and potassium hydroxide; The tungsten source is any one of ammonium metatungstate, sodium metatungstate and potassium tungstate or a combination of at least two of them, preferably ammonium metatungstate; The template agent is one or more of tetraethylammonium hydroxide, tetraethylammonium bromide, diethylamine, and 1,6-hexanediamine, preferably tetraethylammonium hydroxide and tetraethylammonium bromide; The seed crystal is a beta-type pure phase molecular sieve, or a commercial Beta molecular sieve; preferably, the silicon-aluminum ratio of the seed crystal is 10-60, and the coordinated cations in the seed crystal can be NH3 type, H type or Na type, preferably H type Beta molecular sieve.

9. Use of a molecular sieve for selective catalytic reduction of nitrogen oxides according to any one of claims 1 to 7, characterized in that: A lower alcohol is used as a reducing agent; preferably, the lower alcohol is methanol or ethanol.

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