Fe-Beta molecular sieve for N2O catalytic decomposition and preparation method thereof

Abstract

The invention discloses a Fe-Beta molecular sieve for N2O catalytic decomposition and a preparation method of the Fe-Beta molecular sieve. According to the method, chitosan and metal Fe < 3 + > ions are complexed to form a chitosan Fe < 3 + > complex, and then the complex, a silicon source, an aluminum source and a template agent are used as raw materials to prepare the Fe-Beta molecular sieve in one step through a hydrothermal method. Hydroxyl, amino and other groups rich in a chitosan molecular chain can effectively complex metal Fe < 3 + > ions to form a stable chitosan Fe < 3 + > complex, and the stable chitosan Fe < 3 + > complex is used as a Fe source to realize uniform dispersion of Fe species in the molecular sieve and effectively reduce formation of FexOy clusters and Fe2O3 nanoparticles, so that the N2O decomposition catalytic performance of the Fe-Beta molecular sieve is improved. Inorganic alkali metal ions are not introduced, so that a large amount of ammonia-nitrogen wastewater generated in a subsequent tedious ammonium exchange process can be avoided, the synthesis period is effectively shortened, and energy consumption, material consumption and environmental pollution are reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of molecular sieve catalysts, and particularly relates to an Fe-Beta molecular sieve for N2O catalytic decomposition and a preparation method thereof. Background Art

[0002] For a quite long period in the past, N2O has not received sufficient attention from the scientific research community and related fields, and was once considered a gas harmless to the environment and human body. However, in recent decades, with the in-depth exploration of N2O by researchers and the continuous increase in the attention to environmental issues, N2O has been identified as the third key greenhouse gas after CO2 and CH4. The global warming potential of N2O is significant, being 310 times that of CO2 and 21 times that of CH4. At the same time, the average lifespan of N2O in the atmosphere is about 116±9 years. After it enters the stratosphere, it will consume a large amount of ozone in the stratosphere through a series of complex photochemical reactions, thereby damaging the ozone layer and causing serious environmental problems such as ozone layer holes. Therefore, it is recognized as the main stratospheric ozone-depleting substance in the 21st century. In view of this, the elimination of N2O has become a research hotspot in the field of environmental governance in recent years.

[0003] Among many N2O elimination technologies, the direct catalytic decomposition method is considered to be one of the most promising methods, and its core is a highly active catalyst. Currently, the developed catalysts mainly include noble metal catalysts, metal oxide catalysts, and molecular sieve catalysts. Among them, Fe-Beta molecular sieve, due to its unique three-dimensional twelve-membered ring pore structure and large specific surface area, is beneficial to the dispersion of metal Fe species, forming rich reaction active centers. At the same time, its large pore size is conducive to the diffusion of N2O molecules, so it shows excellent reaction activity in N2O catalytic decomposition. The preparation methods of Fe-Beta molecular sieve can be summarized into ion exchange method, impregnation method, chemical vapor deposition method, and direct synthesis method. The Fe-based molecular sieve prepared by the ion exchange method has a low exchange degree. It is necessary to perform ammonium exchange on the Na-type molecular sieve matrix and then perform metal ion exchange. The pH needs to be strictly controlled during the metal exchange process, and its process is relatively cumbersome. Moreover, the active component distribution in the prepared Fe-based molecular sieve is uneven, resulting in unsatisfactory activity during the N2O catalytic decomposition process; most of the Fe species in the Fe-based molecular sieve prepared by the impregnation method are in the form of oligomeric Fe x O yExisting in the form of Fe2O3 nanoparticles in zeolites is not conducive to the catalytic decomposition of N2O. The chemical vapor deposition method generally uses FeCl3 as the iron source precursor, and the subsequent tail gas is corrosive to equipment. If not properly treated during the reaction, it may be discharged into the environment, causing environmental pollution. Therefore, special tail gas treatment equipment and measures need to be equipped, which increases the environmental protection cost and the complexity of operation. Moreover, the Fe species introduced into the zeolite by this method also have problems such as uneven distribution and easy agglomeration.

[0004] Based on the above problems, some research scholars have proposed to prepare Fe-Beta zeolite by a one-step direct synthesis method. For example, CN106745030A discloses a method for in-situ synthesizing Fe / Al-Beta zeolite. However, the synthesis system of Beta zeolite is an alkaline environment, and Fe 3+ ions are extremely easy to form Fe(OH)3 precipitation under alkaline conditions, resulting in difficult uniform dispersion of Fe species in the zeolite. And this method adds inorganic alkali metal NaOH, and subsequent ammonium exchange is required, increasing environmental pollution, and its process is relatively cumbersome. Therefore, preparing Fe-Beta zeolite based on the direct synthesis method still faces problems such as uneven dispersion of Fe species, cumbersome synthesis process, and environmental pollution. Summary of the Invention

[0005] Aiming at the deficiencies existing in the prior art, the purpose of the present invention is to provide an Fe-Beta zeolite for N2O catalytic decomposition and its preparation method. The present invention uses chitosan Fe 3+ complex as the iron source precursor, without introducing inorganic alkali metal ions, and prepares Fe-Beta zeolite by a one-step hydrothermal synthesis method. This method can not only simplify the synthesis process, but also achieve uniform distribution of Fe species in the zeolite, making the Fe-Beta zeolite have high reaction activity in the catalytic decomposition of N2O.

[0006] In order to achieve the above purpose, the technical scheme adopted by the present invention is as follows: A preparation method of an Fe-Beta zeolite for N2O catalytic decomposition, comprising the following steps: (1) Dissolve chitosan in an appropriate amount of deionized water, then slowly add the Fe source, and stir and react at 30-80 o °C for 2-24 h to fully complex chitosan with Fe 3+ . After the reaction is completed, dry the product to obtain chitosan Fe 3+ complex; (2) Mix the silicon source, aluminum source and template agent evenly, and stir at 30-70 o °C for 0.5-6 h to obtain a mixed gel; (3) The chitosan Fe obtained in step (1)3+ The complex is slowly added to the mixed gel prepared in step (2), and stirred at 30-70 o °C for 2-6 h to obtain a homogeneous gel; (4) Transfer the above homogeneous gel to a hydrothermal reaction kettle with a polytetrafluoroethylene liner, then place it in an oven for crystallization. After the crystallization is completed, cool, centrifuge, wash, dry, and calcine the crystallization product to obtain Fe-Beta molecular sieve.

[0007] Furthermore, the chitosan in step (1) is any one or more of carboxymethyl chitosan, chitosan oligosaccharide, chitosan hydrochloride, and chitosan nitrate; the Fe source is any one or more of ferric nitrate, ferric sulfate, and ferric chloride. The mass ratio of chitosan, deionized water, and Fe source is 1:80-150:0.3-1.12.

[0008] Furthermore, the silicon source in step (2) is any one or more of tetraethyl orthosilicate, fumed silica, silica sol, and industrial silica gel; the aluminum source is any one or more of aluminum chloride, pseudo-boehmite, aluminum sulfate, and aluminum isopropoxide; the template agent is any one or more of tetraethylammonium hydroxide, tetraethylammonium bromide, and trimethylcyclohexylammonium hydroxide.

[0009] Furthermore, the molar ratio of the homogeneous gel system in step (3) is: 1 SiO2:0.0125-0.0333 Al2O3:0.00125-0.0075 Fe2O3:0.25 template agent.

[0010] Furthermore, the crystallization temperature in step (4) is 120-180 o °C, the crystallization time is 24-96 h, the drying temperature is 80-120 o °C, the drying time is 12-24 h, the calcination temperature is 400-700 o °C, and the calcination time is 2-6 h.

[0011] The Fe-Beta molecular sieve prepared by the present invention has an Fe content of 0.25-2 wt% (mass percentage), and the proportion of isolated Fe 3+ is 80-90%.

[0012] The Fe-Beta molecular sieve described in the present invention can be used as a catalyst in the catalytic decomposition reaction of N2O.

[0013] The present invention adopts the above technical solutions, with chitosan-Fe 3+The complex is used as an iron source precursor, without introducing inorganic alkali metal ions. The Fe-Beta molecular sieve is prepared in one step by hydrothermal synthesis. The preparation process of this molecular sieve is simple, and most of the Fe species are uniformly distributed in the Beta molecular sieve in the form of Fe³⁺, which is beneficial to improving the N2O catalytic decomposition performance of the Fe-Beta molecular sieve.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The hydroxyl groups, amino groups and other groups rich in the chitosan molecular chain can effectively complex with metal Fe 3+ ions to form a stable chitosan-Fe3+ complex. Using it as an Fe source can achieve uniform dispersion of Fe species in the molecular sieve, effectively reducing the formation of Fe x O y clusters and Fe2O3 nanoparticles, thereby improving the N2O decomposition catalytic performance of the Fe-Beta molecular sieve.

[0015] (2) The natural polymer chitosan reacts with the Fe source to form a chitosan-Fe 3+ complex, avoiding the use of organic complexing agents and reducing environmental pollution.

[0016] (3) The application of the chitosan-Fe³⁺ complex helps the Fe species to be uniformly distributed in the molecular sieve, effectively avoiding the formation of Fe(OH)3 precipitation and agglomeration of Fe species in the molecular sieve synthesis system, so that the Fe-Beta molecular sieve prepared by the present invention shows good catalytic activity in N2O catalytic decomposition.

[0017] (4) The Fe-Beta molecular sieve is prepared in one step by hydrothermal synthesis, effectively avoiding the problems of cumbersome post-treatment process and uneven distribution of Fe species.

[0018] (5) No inorganic alkali metal ions are added to the synthesis system, avoiding the problem of environmental pollution caused by a large amount of ammonia-nitrogen wastewater generated in the subsequent cumbersome ammonium exchange process, simplifying the catalyst preparation process, reducing energy consumption and material consumption, and at the same time reducing environmental pollution. Therefore, the preparation method of the present invention has important practical significance in the field of environmental governance and shows good application prospects. Description of the Drawings

[0019] Figure 1 It is the X-ray diffraction (XRD) pattern of the Fe-Beta molecular sieves obtained in Example 1, Example 3, Example 5 and the comparative example of the present invention.

[0020] Figure 2 It is the ultraviolet-visible diffuse reflection spectrum (UV-vis) pattern of the Fe-Beta molecular sieves obtained in Example 1, Example 3, Example 5 and the comparative example of the present invention.

[0021] Figure 3 The reaction results of the Fe-Beta zeolites obtained in Example 1, Example 3, Example 5 and the comparative example of the present invention for the catalytic decomposition of N2O. Detailed implementation manners

[0022] In order to more clearly illustrate the purpose and beneficial effects of the present invention, the following further elaborates the present invention in conjunction with the accompanying drawings and specific embodiments, aiming to help better understand the essence and characteristics of the present invention, and is not used to limit the present invention. Example

[0023] A preparation method of an Fe-Beta zeolite for the catalytic decomposition of N2O includes the following steps: (1) Add 20 g of deionized water, 0.2074 g of chitosan nitrate and 0.1212 g of iron nitrate to a 100 mL round-bottom flask in sequence. After mixing and stirring evenly, react at 30 o °C for 24 h. After the reaction is completed, dry the product to obtain a chitosan Fe 3+ complex.

[0024] (2) Add 0.6586 g of pseudoboehmite to 12.6362 g of tetraethylammonium hydroxide solution (mass fraction 35 wt%), mix and stir evenly. While stirring continuously, slowly add 18.0150 g of silica sol (mass fraction 40 wt%). Stir at 30 o °C for 2 h to obtain a mixed gel, and then add the above-obtained chitosan Fe 3+ complex, and continue to stir at 30 o °C for 6 h to obtain a homogeneous gel.

[0025] (3) Transfer the above homogeneous gel to a hydrothermal reaction kettle with a polytetrafluoroethylene lining, crystallize at 120 o °C for 96 h. After the crystallization is completed, cool the solid product, centrifuge, wash it with deionized water until neutral, dry it at 100 o °C for 18 h, and calcine it at 550 o °C for 4 h to obtain an Fe-Beta zeolite. Example

[0026] A preparation method of an Fe-Beta zeolite for the catalytic decomposition of N2O includes the following steps: (1) Add 26 g of deionized water, 0.3250 g of chitosan oligosaccharide and 0.0975 g of ferric chloride to a 100 mL round-bottom flask in sequence. After mixing and stirring evenly, react at 45 o °C for 12 h. After the reaction is completed, dry the product to obtain a chitosan Fe3+ Complex

[0027] (2) Add 0.4391 g of pseudo-boehmite to 12.6362 g of tetraethylammonium hydroxide solution (mass fraction 35 wt%), mix and stir evenly. While continuously stirring, slowly add 18.0150 g of silica sol (mass fraction 40 wt%). Stir at 40 o °C for 2 h to obtain a mixed gel. Then add the chitosan Fe 3+ complex obtained above and continue to stir at 40 o °C for 4 h to obtain a homogeneous gel.

[0028] (3) Transfer the above homogeneous gel to a high-temperature hydrothermal reaction kettle with a polytetrafluoroethylene lining, crystallize at 140 o °C for 72 h. After the crystallization is completed, cool the solid product, centrifuge it, wash it with deionized water until neutral, dry it at 100 o °C for 18 h, and calcine it at 550 o °C for 4 h to obtain Fe-Beta molecular sieve. Example

[0029] A preparation method of Fe-Beta molecular sieve for N2O catalytic decomposition, comprising the following steps: (1) Add 50 g of deionized water, 0.5027 g of chitosan hydrochloride and 0.3636 g of iron nitrate to a 100 mL round-bottom flask in sequence. After mixing and stirring evenly, react at 60 o °C for 6 h. After the reaction is completed, dry the product to obtain chitosan Fe 3+ complex.

[0030] (2) Add 0.3039 g of pseudo-boehmite to 12.6362 g of tetraethylammonium hydroxide (mass fraction 35 wt%) solution, mix and stir evenly. While continuously stirring, slowly add 18.0150 g of silica sol (mass fraction 40 wt%). Stir at 40 o °C for 2 h to obtain a mixed gel. Then add the chitosan Fe 3+ complex obtained above and continue to stir at 40 o °C for 4 h to obtain a homogeneous initial gel.

[0031] (3) Transfer the above homogeneous gel to a high-temperature hydrothermal reaction kettle with a polytetrafluoroethylene lining, crystallize at 160 o °C for 60 h. After the crystallization is completed, cool the solid product, centrifuge it, wash it with deionized water until neutral, dry it at 100 o °C for 18 h, and calcine it at 550 oCalcined at 4 h to obtain Fe-Beta zeolite. Example

[0032] A preparation method of Fe-Beta zeolite for N2O catalytic decomposition, comprising the following steps: (1) Add 97 g of deionized water, 0.6445 g of carboxymethyl chitosan and 0.7200 g of ferric sulfate to a 150 mL round-bottom flask in sequence. After mixing and stirring evenly, react at 45 o °C for 12 h. After the reaction, dry the product to obtain chitosan Fe 3+ complex.

[0033] (2) Add 0.2469 g of pseudoboehmite to 12.6362 g of tetraethylammonium hydroxide (mass fraction 35 wt%) solution, mix and stir evenly. While stirring continuously, slowly add 18.0150 g of silica sol (mass fraction 40 wt%). Stir at 40 o °C for 2 h to obtain a mixed gel. Then add the above-obtained chitosan Fe 3+ complex and continue to stir at 40 o °C for 4 h to obtain a homogeneous initial gel.

[0034] (3) Transfer the above homogeneous gel to a high-temperature hydrothermal reaction kettle with a polytetrafluoroethylene lining, crystallize at 180 o °C for 24 h. After the crystallization is completed, cool the solid product, centrifuge, wash it with deionized water until neutral, dry it at 100 o °C for 18 h, and calcine it at 550 o °C for 4 h to obtain Fe-Beta zeolite. Example

[0035] A preparation method of Fe-Beta zeolite for N2O catalytic decomposition, comprising the following steps: (1) Add 65 g of deionized water, 0.6027 g of chitosan nitrate and 0.4848 g of ferric nitrate to a 150 mL round-bottom flask in sequence. After mixing and stirring evenly, react at 45 o °C for 12 h. After the reaction, dry the product to obtain chitosan Fe 3+ complex.

[0036] (2) Add 0.4391 g of pseudoboehmite to 12.6362 g of tetraethylammonium hydroxide (mass fraction 35 wt%) solution, mix and stir evenly. While stirring continuously, slowly add 18.0150 g of silica sol (mass fraction 40 wt%). Stir at 40 oStir at [Celsius degree] for 2 h to obtain a mixed gel, and then add the chitosan-Fe complex obtained above. 3+ Continue to stir at 40 o °C for 4 h to obtain a homogeneous initial gel.

[0037] (3) Transfer the above homogeneous gel to a high-temperature hydrothermal reaction kettle with a polytetrafluoroethylene liner, and crystallize at 160 o °C for 60 h. After the crystallization is completed, cool, centrifuge the solid product, wash it with deionized water until neutral, and dry it at 100 o °C for 18 h, and calcine it at 550 o °C for 4 h to obtain Fe-Beta molecular sieve.

[0038] Comparative example Prepare Fe / Beta molecular sieve by impregnation method (1) Add 0.3044 g of pseudo-boehmite to 12.6362 g of tetraethylammonium hydroxide (mass fraction 35 wt%) solution, mix and stir evenly, and slowly add 18.0150 g of silica sol (mass fraction 40 wt%) under continuous stirring. Continue to stir at 40 o °C for 4 h to obtain a homogeneous gel.

[0039] (2) Transfer the above homogeneous initial gel to a high-temperature hydrothermal reaction kettle with a polytetrafluoroethylene liner, and crystallize at 160 °C for 36 h. After the crystallization is completed, cool, centrifuge the solid product, wash it with deionized water until neutral, and dry it at 100 o °C for 18 h, and calcine it at 550 o °C for 4 h to obtain Beta molecular sieve.

[0040] (3) Weigh 3 g of the above Beta molecular sieve in a crucible and measure the saturated water absorption rate. Weigh 0.22 g of ferric nitrate and dissolve it in 1.4 g of deionized water to obtain a ferric nitrate solution. Then, add this ferric nitrate solution to the Beta molecular sieve prepared above in batches and multiple times. After sealing overnight, drying at 100 o °C for 18 h, and calcine it at 550 o °C for 4 h to obtain Fe / Beta molecular sieve with a theoretical Fe content of 1 wt%.

[0041] Perform XRD tests on the molecular sieves prepared in Example 1, Example 3, Example 5 and the comparative example. The test results are as Figure 1 shown. As can be seen from the figure, the synthetic products are all Beta molecular sieves.

[0042] Perform UV-vis tests on the molecular sieves prepared in Example 1, Example 3, Example 5 and the comparative example. The test results are as Figure 2 shown. AsFigure 2 It can be seen that the Fe species in the Fe-Beta zeolite prepared by the present invention are uniformly dispersed, and the isolated Fe 3+ accounts for 80.8%, 86.4% and 90.3% respectively, and no obvious agglomeration phenomenon appears. However, in the Fe / Beta zeolite prepared by the traditional impregnation method (comparative example), the isolated Fe 3+ accounts for only 45.8%, and the Fe species mainly exist in the form of Fe x O y clusters and Fe2O3 nanoparticles, and serious agglomeration phenomenon appears.

[0043] Application Example The zeolites prepared in Example 1, Example 3, Example 5 and the comparative example were used as catalysts, and their N2O catalytic decomposition performance was tested.

[0044] The test conditions are as follows: The catalyst was fixed on a fixed-bed reactor with quartz wool for the evaluation of N2O catalytic decomposition activity. The N2O content at the outlet of the fixed-bed reactor was detected by Thermo Fisher IGS Gas. The reaction gas composition was 4000 ppm N2O, with N2 as the balance gas, the total gas flow rate was 400 mL / min, the catalyst dosage was 0.6 g, and the space velocity was 40000 h -1 , and the activity test results are as Figure 3 shown.

[0045] It can be seen from the figure that the N2O catalytic decomposition performance of the Fe-Beta zeolites prepared in Example 1, Example 3 and Example 5 is higher than that of the Fe / Beta zeolite prepared in the comparative example, indicating that the method for preparing Fe-Beta zeolite by one-step hydrothermal method using chitosan Fe 3+ complex can effectively control the distribution and existence form of Fe species in the zeolite, so that the Fe species are mainly in the form of Fe 3+ and are uniformly dispersed in the zeolite, reducing the formation of Fe x O y clusters and Fe2O3 nanoparticles, thereby endowing the Fe-Beta zeolite with higher N2O catalytic decomposition activity.

[0046] The above are only the preferred embodiments of the present invention, but the present invention is not limited to the above specific embodiments. The above specific embodiments are only illustrative and not restrictive. Under the inspiration of the present invention, those of ordinary skill in the art can make several improvements and deformations without departing from the purpose of the present invention, and these all belong to the protection scope of the present invention.

Claims

1. A preparation method of Fe-Beta molecular sieve for catalytic decomposition of N2O, characterized in that, It includes the following steps: (1) Dissolve chitosan in water, then add the Fe source, and stir and react at 30~80 o °C for 2~24 h to fully complex chitosan with Fe 3+ . After the reaction, dry the product to obtain the chitosan-Fe 3+ complex; (2) Mix the silicon source, aluminum source and template agent evenly and stir at 30~70 o °C for 0.5~6 h to obtain a mixed gel; (3) Add the above chitosan Fe 3+ complex to the mixed gel, and stir at 30~70 o °C for 2~6 h to obtain a homogeneous gel; (4) Transfer the above homogeneous gel into a hydrothermal reaction kettle with a polytetrafluoroethylene liner, and then place it in an oven for crystallization. After crystallization, cool, centrifuge, wash, dry, and calcine the crystallization product to obtain Fe-Beta molecular sieve.

2. The preparation method of an Fe-Beta molecular sieve for N2O catalytic decomposition according to claim 1, characterized in that, The chitosan described in step (1) is any one or more of carboxymethyl chitosan, chitosan oligosaccharide, chitosan hydrochloride, and chitosan nitrate.

3. The preparation method of an Fe-Beta molecular sieve for N2O catalytic decomposition according to claim 1, characterized in that, The Fe source is any one or more of ferric nitrate, ferric sulfate, and ferric chloride.

4. The preparation method of an Fe-Beta molecular sieve for N2O catalytic decomposition according to claim 1, characterized in that, The mass ratio of the chitosan, water, and Fe source described in step (1) is 1:80 - 150:0.3 - 1.

12.

5. The preparation method of an Fe-Beta molecular sieve for N2O catalytic decomposition according to claim 1, characterized in that, The silicon source described in step (2) is any one or more of tetraethyl orthosilicate, fumed silica, silica sol, and industrial silica gel; the aluminum source is any one or more of aluminum chloride, pseudo-boehmite, aluminum sulfate, and aluminum isopropoxide; the template agent is any one or more of tetraethylammonium hydroxide, tetraethylammonium bromide, and trimethylcyclohexylammonium hydroxide.

6. The preparation method of an Fe-Beta molecular sieve for N2O catalytic decomposition according to claim 1, characterized in that, The system molar ratio of the homogeneous gel described in step (3) is: 1 SiO2:0.0125 - 0.0333 Al2O3:0.00125 - 0.0075 Fe2O3:0.25 template agent.

7. The preparation method of an Fe-Beta molecular sieve for N2O catalytic decomposition according to claim 1, characterized in that, The crystallization temperature described in step (4) is 120~180 o °C, the crystallization time is 24~96 h, the drying temperature is 80~120 o °C, the drying time is 12~24 h, the calcination temperature is 400~700 o °C, and the calcination time is 2~6 h.

8. Fe-Beta molecular sieve obtained by the preparation method according to any one of claims 1 to 7.

9. The Fe-Beta molecular sieve according to claim 8, wherein, The Fe content of the Fe-Beta molecular sieve is 0.25 to 2 wt%, and the proportion of isolated Fe 3+ is 80 to 90%.

10. Application of the Fe-Beta molecular sieve according to claim 8 in the N2O catalytic decomposition reaction.

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