Preparation method of high-performance binder-free chromium-doped molecular sieve

Abstract

The invention provides a high-performance binder-free chromium-doped molecular sieve and a preparation method thereof, and belongs to the technical field of metal-doped molecular sieve block catalysts. The preparation method comprises the following steps: modifying cotton fibers through nano titanium dioxide, carrying out silanization reaction, and loading transition metal to obtain modified cotton fibers; preparing a ZSM-5 crystal seed solution; and mixing the modified cotton fiber, ZSM-5 seed crystal liquid, hydroxypropyl methyl cellulose and Cr (NO3) 3.9 H2O, heating and stirring in a water bath, and carrying out ball milling to obtain the 3D printing ink. Performing 3D printing on the 3D printing ink to obtain an aluminosilicate primary blank; and carrying out dry glue conversion and roasting treatment on the aluminosilicate primary blank to obtain the chromium-doped molecular sieve. According to the invention, a binder is not adopted, and the modified cotton fiber is adopted as a framework, so that the 3D requirement can be met, and the material has relatively high mechanical strength, catalytic activity and hydrothermal stability and long service life.

Description

Technical Field

[0001] The invention belongs to the technical field of metal-doped molecular sieve bulk catalysts, and particularly relates to a method for preparing a high-performance, binder-free chromium-doped molecular sieve. Background Art

[0002] Chlorinated volatile organic compounds (CVOCs) pose significant risks to the ecological environment and human society. Metal-doped molecular sieves are highly effective catalysts for addressing CVOC pollution. However, conventional pelletized catalysts commonly suffer from issues such as high bed pressure drop, poor heat transfer, and localized thermal runaway, which limit their industrial application. 3D printing strategies for constructing honeycomb-structured catalyst blocks can effectively address this issue. In practical industrial applications, the mechanical strength of catalyst blocks is a key parameter affecting both application and performance. Introducing single or multiple binders into 3D printing inks can improve the mechanical strength of honeycomb blocks. However, the addition of binders can reduce the active component content and clog the catalyst micropores, thereby reducing the catalytic activity of the blocks. Furthermore, diffusion within the blocks is a key parameter influencing catalytic performance, especially when catalyzing macromolecular reactants. However, improving the diffusion performance of the blocks often results in a decrease in mechanical strength. Therefore, overcoming the trade-off between mechanical strength, active site concentration, and diffusion performance in honeycomb block catalysts is a key issue that needs to be addressed for the industrial application of 3D-printed honeycomb block catalysts. Chinese patent ZL202111360834.X uses an organic polymer as a binder to construct a molecular sieve block structure through 3D printing. Hydroxyl radicals are then used to promote condensation between the molecular sieve particles, enhancing the block's mechanical strength. However, the interparticle interaction is limited, and mechanical strength and diffusion performance still need to be improved.

[0003] Although the use of binders can be reduced or eliminated at present, the catalytic materials produced still have problems such as low catalytic activity, short service life, and poor thermal stability. Usually, the existing technology uses functional particles to modify the catalytic materials. However, for catalytic materials rich in voids, the loaded functional particles may cause the voids to be blocked, thereby reducing the contact area or active sites, and even causing problems such as active site poisoning.

[0004] Therefore, there is an urgent need for a molecular sieve that is both binder-free and has excellent performance. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a method for preparing a high-performance, binder-free chromium-doped molecular sieve. The present invention does not use a binder and uses modified cotton fiber as a skeleton to meet 3D requirements, and has high mechanical strength, catalytic activity, hydrothermal stability and a long service life.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] The present invention provides a method for preparing a high-performance, binder-free chromium-doped molecular sieve, comprising the following steps:

[0008] S1. The cotton fiber, the precursor solution, and the catalyst are mixed and stirred, filtered, and dried to obtain a first modified cotton fiber;

[0009] S2. The modified cotton fiber, toluene, and 3-chloropropyltrimethoxysilane are mixed to perform a silanization reaction; after the reaction is completed, the mixture is cooled, centrifuged, washed, and dried to obtain a second modified cotton fiber;

[0010] S3. The second modified cotton fiber, transition metal salt solution, and chelating agent are mixed and sequentially heated, filtered, washed, and dried to obtain a third modified cotton fiber;

[0011] S4. Aqueous tetrapropylammonium hydroxide (TPAOH), tetraethyl orthosilicate (TEOS), Al(NO3)3 and water were mixed and stirred and then pre-crystallized to obtain a ZSM-5 seed solution;

[0012] S5. The third modified cotton fiber, ZSM-5 seed solution, hydroxypropyl methylcellulose and Cr(NO3)3·9H2O were mixed and then heated in a water bath with stirring and ball milled to obtain a 3D printing ink;

[0013] S6. The 3D printing ink is 3D printed to obtain an aluminosilicate preform;

[0014] S7. The aluminosilicate preform is subjected to dry gel conversion and calcination treatment to obtain a chromium-doped molecular sieve.

[0015] In the present invention, in order to avoid the use of binders, silanized cotton fibers are used as a support for the block dry glue conversion process, thereby preventing the block from collapsing and serving as a template to form an additional pore structure. In addition, after the silanized cotton fibers are hydrolyzed, the Si-OH formed can not only fix the molecular sieve nuclei, but also stabilize the active site Cr 6+ , ultimately forming a high Cr 6+ The catalyst is a chromium-doped molecular sieve bulk catalyst with high concentration and acidic site concentration, high diffusion performance, high mechanical strength and no binder; and in this process, the ZSM-5 molecular sieve nucleates, grows and fuses to form a high-strength bulk structure.

[0016] Preferably, the mass ratio of the cotton fiber, the precursor solution and the catalyst in S1 is 1:(1-5):(0.05-0.2).

[0017] Preferably, the precursor solution is prepared by mixing tetrabutyl titanate and anhydrous ethanol in a mass ratio of 1:(1-3);

[0018] The catalyst is hydrochloric acid or nitric acid.

[0019] Preferably, the stirring time is 2 to 4 hours.

[0020] By adopting the above technical solution, the present invention modifies cotton before preparing silanized cotton fibers so that it is loaded with nano-titanium dioxide. By loading the nano-titanium dioxide, the molecular sieve has a certain self-purification ability, thereby improving the service life and effect; moreover, by loading the nano-titanium dioxide before silanization of the cotton fibers, the blockage of the gaps can be reduced, and the distribution is more uniform without adverse phenomena such as agglomeration, thereby avoiding the problem of conventional loaded functional particles reducing the catalytic activity.

[0021] Preferably, the mass ratio of the modified cotton fiber, toluene and 3-chloropropyltrimethoxysilane in S2 is 1:(100-500):(1-3).

[0022] Preferably, the silanization reaction temperature in S2 is 40-50° C., and the reaction time is 8-12 h.

[0023] Preferably, the mass ratio of the second modified cotton fiber, the transition metal salt solution, and the chelating agent in S3 is 1:(1-3):(0.1-0.5).

[0024] Preferably, the transition metal salt solution is a mixture of cobalt salt / nickel salt and water in a mass ratio of 1:(10-20).

[0025] Preferably, the cobalt salt is cobalt nitrate or cobalt chloride; and the nickel salt is nickel nitrate or nickel acetate.

[0026] Preferably, the chelating agent is disodium edetate or citric acid.

[0027] By adopting the above technical solution, the present invention introduces transition metals to improve the hydrothermal stability of the molecular sieve. The transition metal ions can enter the molecular sieve's crystal lattice, change the molecular sieve's crystal structure and the properties of the acidic sites, and enhance the molecular sieve's structural stability and resistance to hydrothermal aging under high temperature and high humidity conditions. In addition, the use of cobalt / nickel will not cause molecular sieve poisoning. At the same time, the use of chelating agents reduces the release rate of metal ions, prevents the transition metal ions from binding to active sites, and reduces clogging.

[0028] Preferably, the molar ratio of tetrapropylammonium hydroxide aqueous solution, ethyl orthosilicate, Al(NO3)3 and water in S4 is 1:0.01-0.02:0.25-0.36:150.

[0029] Preferably, the pre-crystallization temperature in S4 is 85-95° C., and the time is 3-6 hours.

[0030] Preferably, the mass ratio of the third modified cotton fiber, ZSM-5 seed solution, hydroxypropyl methylcellulose and Cr(NO3)3·9H2O in S5 is (1-2):(2-6):(0.1-1):(15-25).

[0031] Preferably, the water content of the 3D printing ink in S5 is 15-18 wt %.

[0032] Preferably, the 3D printing in S6 adopts an axial push 3D printer to complete layer-by-layer printing at room temperature.

[0033] Preferably, the temperature for the dry rubber conversion in S7 is 140-160° C. and the time is 3-5 days.

[0034] Preferably, the calcination temperature in S7 is 500-600° C. and the calcination time is 3-8 hours.

[0035] Contains at least the following beneficial technical effects:

[0036] (1) The present invention uses 3D printing technology to prepare molecular sieves and prepares specific 3D printing inks. The ink does not contain a binder. The silanized cotton fiber can be used as a support in the block dry gel conversion process to prevent the block from collapsing and as a template to form an additional pore structure. After the silanized cotton fiber is hydrolyzed, the Si-OH formed can fix the molecular sieve nucleus and stabilize the active site Cr 6+ , first constructed with high Cr 6+ A binderless chromium-doped molecular sieve bulk catalyst with high concentration and acidic site concentration, high diffusion performance, high mechanical strength, and comprehensive improvement in its catalytic activity and stability in CVOC combustion. This invention constructs a Cr-doped aluminosilicate bulk through 3D printing, and then uses a dry gel conversion method to convert the amorphous aluminosilicate into a uniform, compact ZSM-5 crystalline phase. The ZSM-5 crystals fuse together, significantly improving the bulk's mechanical strength.

[0037] (2) The present invention reduces the problem of clogging of the gaps caused by the subsequent loading by loading nano-titanium dioxide before silanizing the cotton fibers, reduces agglomeration, and improves the uniformity of distribution; after silanizing the cotton fibers, the transition metal is loaded and a chelating agent is used to control the ion release to reduce clogging; the self-purification ability and thermal stability are improved by double loading without affecting the performance. DETAILED DESCRIPTION

[0038] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0039] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also encompassed by the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0040] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0041] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the present invention. The present description and examples are intended to be illustrative only.

[0042] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0043] Unless otherwise specified, the "room temperature" and "normal temperature" mentioned in the present invention are all calculated as 25±2°C.

[0044] Unless otherwise specified, all raw materials and instruments used in the following examples of the present invention are commercially available.

[0045] Example 1

[0046] This embodiment provides a method for preparing a high-performance, binder-free chromium-doped molecular sieve, which comprises the following steps:

[0047] (1) Tetrabutyl titanate and anhydrous ethanol are mixed in a mass ratio of 1:1 to obtain a precursor solution;

[0048] Then, the cotton fiber, the precursor solution, and nitric acid were mixed in a mass ratio of 1:1:0.1, stirred for 3 hours, filtered, and dried to obtain the first modified cotton fiber;

[0049] (2) 2 g of the first modified cotton fiber was added to 200 mL of toluene, followed by the addition of 4 g of 3-chloropropyltrimethoxysilane; the mixture was then heated and stirred in a 45°C water bath for 10 h, cooled to room temperature, and centrifuged. The lower precipitate was rinsed with anhydrous ethanol and dried in an oven at 100°C overnight to obtain the second modified cotton fiber;

[0050] (3) mixing the second modified cotton fiber, cobalt nitrate solution (30 wt%), and disodium ethylenediaminetetraacetic acid in a mass ratio of 1:1:0.3, heating the mixture at 70° C. for 2 h, filtering, washing, and drying the mixture to obtain a third modified cotton fiber;

[0051] (4) Tetrapropylammonium hydroxide aqueous solution (25 wt% aqueous solution), Al(NO3)3·9H2O and H2O were added to a 100 mL flask, followed by slow dropwise addition of ethyl orthosilicate, stirred at room temperature for 24 h, and then placed in a stainless steel crystallization kettle and pre-crystallized at 90°C for 5 h to obtain a ZSM-5 seed solution; wherein the molar ratio of tetrapropylammonium hydroxide aqueous solution, ethyl orthosilicate, Al(NO3)3 and water was 1:0.01:0.3:150.

[0052] (5) The third modified cotton fiber was added to the ZSM-5 seed solution, and then HPMC and Cr(NO3)3·9H2O were added and then placed in a 50°C water bath for heating and stirring. When the water content remained at 17 wt%, the mixed system was ball milled to obtain a 3D printing ink; wherein the mass ratio of the third modified cotton fiber, ZSM-5 seed solution, hydroxypropyl methylcellulose and Cr(NO3)3·9H2O was 1:4:0.5:20.

[0053] (6) Quickly transfer the printing ink to 5cm 3 The syringe was placed in a push-type 3D printer. A computer program was used to match the slurry advance speed with the x-, y-, and z-axis movement speeds, and layer-by-layer printing was completed at room temperature. Printing parameters were as follows: layer height 0.65 mm, needle diameter 12.2 mm, extrusion rate 105%, and nozzle aperture 0.85 mm. After printing, the block was dried at room temperature for 12 hours, resulting in a 10 mm × 10 mm × 10 mm chromium-doped aluminosilicate block.

[0054] (7) The aluminosilicate block was placed in a polytetrafluoroethylene beaker, which was then placed in a polytetrafluoroethylene-lined stainless steel autoclave containing 3 g of deionized water. The catalyst block was not in direct contact with the deionized water in the autoclave. The polytetrafluoroethylene-lined autoclave was placed in a stainless steel crystallization autoclave, which was then transferred to a 150°C oven for crystallization for 4 days. Finally, the catalyst block was removed and calcined at 550°C for 6 hours to obtain a chromium-doped molecular sieve.

[0055] Example 2

[0056] This embodiment provides a method for preparing a high-performance, binder-free chromium-doped molecular sieve, which comprises the following steps:

[0057] (1) Tetrabutyl titanate and anhydrous ethanol are mixed in a mass ratio of 1:3 to obtain a precursor solution;

[0058] Then, the cotton fiber, the precursor solution, and the catalyst (hydrochloric acid or nitric acid) are mixed in a mass ratio of 1:5:0.2, stirred for 2 to 4 hours, filtered, and dried to obtain the first modified cotton fiber;

[0059] (2) 1 g of the first modified cotton fiber was added to 500 mL of toluene, followed by the addition of 3 g of 3-chloropropyltrimethoxysilane; the mixture was then heated and stirred in a 45°C water bath for 10 h, cooled to room temperature, and centrifuged. The lower precipitate was rinsed with anhydrous ethanol and dried in an oven at 100°C overnight to obtain the second modified cotton fiber;

[0060] (3) mixing the second modified cotton fiber, cobalt chloride solution (30 wt%), and citric acid in a mass ratio of 1:3:0.5, heating the mixture at 70° C. for 2 h, filtering, washing, and drying the mixture to obtain a third modified cotton fiber;

[0061] (4) Tetrapropylammonium hydroxide aqueous solution (25 wt% aqueous solution), Al(NO3)3·9H2O and H2O were added to a 100 mL flask, followed by slow dropwise addition of ethyl orthosilicate, stirred at room temperature for 24 h, and then placed in a stainless steel crystallization kettle and pre-crystallized at 95°C for 3 h to obtain a ZSM-5 seed solution; wherein the molar ratio of tetrapropylammonium hydroxide aqueous solution, ethyl orthosilicate, Al(NO3)3 and water was 1:0.02:0.25:150.

[0062] (5) The third modified cotton fiber was added to the ZSM-5 seed solution, and then HPMC and Cr(NO3)3·9H2O were added and then placed in a 50°C water bath for heating and stirring. When the water content remained at 15 wt%, the mixed system was ball milled to obtain a 3D printing ink; wherein the mass ratio of the third modified cotton fiber, ZSM-5 seed solution, hydroxypropyl methylcellulose and Cr(NO3)3·9H2O was 1:6:1:25.

[0063] (6) Quickly transfer the printing ink to 5cm 3 The syringe was placed in a push-type 3D printer. A computer program was used to match the slurry advance speed with the x-, y-, and z-axis movement speeds, and layer-by-layer printing was completed at room temperature. Printing parameters were as follows: layer height 0.65 mm, needle diameter 12.2 mm, extrusion rate 105%, and nozzle aperture 0.85 mm. After printing, the block was dried at room temperature for 12 hours, resulting in a 10 mm × 10 mm × 10 mm chromium-doped aluminosilicate block.

[0064] (7) The aluminosilicate block was placed in a polytetrafluoroethylene beaker, which was then placed in a polytetrafluoroethylene-lined stainless steel autoclave containing 3 g of deionized water. The catalyst block was not in direct contact with the deionized water in the autoclave. The polytetrafluoroethylene-lined autoclave was placed in a stainless steel crystallization autoclave, which was then transferred to a 160°C oven for crystallization for 5 days. Finally, the catalyst block was removed and calcined at 600°C for 3 hours to obtain a chromium-doped molecular sieve.

[0065] Comparative Example 1

[0066] The preparation method of this comparative example is the same as that of Example 1, except that modified cotton fiber is not prepared and used, and only seed solution is used for 3D printing, and the printing steps are the same.

[0067] Comparative Example 2

[0068] The preparation method of this comparative example is the same as that of Example 1, except that unmodified cotton fibers are mixed with the seed solution for 3D printing, and the printing steps are the same.

[0069] Comparative Example 3

[0070] This comparative example provides a method for preparing a high-performance, binder-free chromium-doped molecular sieve, which comprises the following steps:

[0071] (1) 2 g of the first modified cotton fiber was added to 200 mL of toluene, and then 4 g of 3-chloropropyltrimethoxysilane was added; the mixture was then heated and stirred in a water bath at 45°C for 10 h, and then cooled to room temperature and centrifuged. The lower precipitate was washed with anhydrous ethanol and dried in an oven at 100°C overnight to obtain the modified cotton fiber;

[0072] (2) Tetrapropylammonium hydroxide aqueous solution (25 wt% aqueous solution), Al(NO3)3·9H2O and H2O were added to a 100 mL flask, followed by slow dropwise addition of ethyl orthosilicate, stirred at room temperature for 24 h, and then placed in a stainless steel crystallization kettle and pre-crystallized at 90°C for 5 h to obtain a ZSM-5 seed solution; wherein the molar ratio of tetrapropylammonium hydroxide aqueous solution, ethyl orthosilicate, Al(NO3)3 and water was 1:0.01:0.3:150.

[0073] (3) The modified cotton fiber was added to the ZSM-5 seed solution, and then HPMC and Cr(NO3)3·9H2O were added and then placed in a 50°C water bath for heating and stirring. When the water content remained at 17 wt%, the mixed system was ball milled to obtain a 3D printing ink; wherein the mass ratio of the third modified cotton fiber, ZSM-5 seed solution, hydroxypropyl methylcellulose and Cr(NO3)3·9H2O was 1:4:0.5:20.

[0074] (4) Quickly transfer the printing ink to 5cm 3 The syringe was placed in a push-type 3D printer. A computer program was used to match the slurry advance speed with the x-, y-, and z-axis movement speeds, and layer-by-layer printing was completed at room temperature. Printing parameters were as follows: layer height 0.65 mm, needle diameter 12.2 mm, extrusion rate 105%, and nozzle aperture 0.85 mm. After printing, the block was dried at room temperature for 12 hours, resulting in a 10 mm × 10 mm × 10 mm chromium-doped aluminosilicate block.

[0075] (5) The aluminosilicate block was placed in a polytetrafluoroethylene beaker, which was then placed in a polytetrafluoroethylene-lined stainless steel autoclave containing 3 g of deionized water. The catalyst block was not in direct contact with the deionized water in the autoclave. The polytetrafluoroethylene-lined autoclave was placed in a stainless steel crystallization autoclave, which was then transferred to a 150°C oven for crystallization for 4 days. Finally, the catalyst block was removed and calcined at 550°C for 6 hours to obtain a chromium-doped molecular sieve.

[0076] Comparative Example 4

[0077] This comparative example provides a method for preparing a high-performance, binder-free chromium-doped molecular sieve, which comprises the following steps:

[0078] (1) Tetrabutyl titanate and anhydrous ethanol are mixed in a mass ratio of 1:1 to obtain a precursor solution;

[0079] Then, the cotton fiber, the precursor solution, and nitric acid were mixed in a mass ratio of 1:1:0.1, stirred for 3 hours, filtered, and dried to obtain the first modified cotton fiber;

[0080] (2) 2 g of the first modified cotton fiber was added to 200 mL of toluene, followed by the addition of 4 g of 3-chloropropyltrimethoxysilane; the mixture was then heated and stirred in a 45°C water bath for 10 h, cooled to room temperature, and centrifuged. The lower precipitate was rinsed with anhydrous ethanol and dried in an oven at 100°C overnight to obtain the second modified cotton fiber;

[0081] (3) Tetrapropylammonium hydroxide aqueous solution (25 wt% aqueous solution), Al(NO3)3·9H2O and H2O were added to a 100 mL flask, followed by slow dropwise addition of ethyl orthosilicate, stirred at room temperature for 24 h, and then placed in a stainless steel crystallization kettle and pre-crystallized at 90°C for 5 h to obtain a ZSM-5 seed solution; wherein the molar ratio of tetrapropylammonium hydroxide aqueous solution, ethyl orthosilicate, Al(NO3)3 and water was 1:0.01:0.3:150.

[0082] (4) The second modified cotton fiber was added to the ZSM-5 seed solution, and then HPMC and Cr(NO3)3·9H2O were added and then placed in a 50°C water bath for heating and stirring. When the water content remained at 17 wt%, the mixed system was ball milled to obtain a 3D printing ink; wherein the mass ratio of the third modified cotton fiber, ZSM-5 seed solution, hydroxypropyl methylcellulose and Cr(NO3)3·9H2O was 1:4:0.5:20.

[0083] (5) Quickly transfer the printing ink to 5cm 3 The syringe was placed in a push-type 3D printer. A computer program was used to match the slurry advance speed with the x-, y-, and z-axis movement speeds, and layer-by-layer printing was completed at room temperature. Printing parameters were as follows: layer height 0.65 mm, needle diameter 12.2 mm, extrusion rate 105%, and nozzle aperture 0.85 mm. After printing, the block was dried at room temperature for 12 hours, resulting in a 10 mm × 10 mm × 10 mm chromium-doped aluminosilicate block.

[0084] (6) The aluminosilicate block was placed in a polytetrafluoroethylene beaker, which was then placed in a polytetrafluoroethylene-lined stainless steel autoclave containing 3 g of deionized water. The catalyst block was not in direct contact with the deionized water in the autoclave. The polytetrafluoroethylene-lined autoclave was placed in a stainless steel crystallization autoclave, which was then transferred to a 150°C oven for crystallization for 4 days. Finally, the catalyst block was removed and calcined at 550°C for 6 hours to obtain a chromium-doped molecular sieve.

[0085] Comparative Example 5

[0086] This embodiment provides a method for preparing a high-performance, binder-free chromium-doped molecular sieve, which comprises the following steps:

[0087] (2) 2 g of the first modified cotton fiber was added to 200 mL of toluene, followed by the addition of 4 g of 3-chloropropyltrimethoxysilane; the mixture was then heated and stirred in a 45°C water bath for 10 h, and then cooled to room temperature and centrifuged. The lower precipitate was rinsed with anhydrous ethanol and dried in an oven at 100°C overnight to obtain the first modified cotton fiber;

[0088] (3) mixing the second modified cotton fiber, cobalt nitrate solution (30 wt%), and disodium ethylenediaminetetraacetic acid in a mass ratio of 1:1:0.3, heating the mixture at 70° C. for 2 h, filtering, washing, and drying the mixture to obtain the second modified cotton fiber;

[0089] (4) Tetrapropylammonium hydroxide aqueous solution (25 wt% aqueous solution), Al(NO3)3·9H2O and H2O were added to a 100 mL flask, followed by slow dropwise addition of ethyl orthosilicate, stirred at room temperature for 24 h, and then placed in a stainless steel crystallization kettle and pre-crystallized at 90°C for 5 h to obtain a ZSM-5 seed solution; wherein the molar ratio of tetrapropylammonium hydroxide aqueous solution, ethyl orthosilicate, Al(NO3)3 and water was 1:0.01:0.3:150.

[0090] (5) The second modified cotton fiber was added to the ZSM-5 seed solution, and then HPMC and Cr(NO3)3·9H2O were added and then placed in a 50°C water bath for heating and stirring. When the water content remained at 17 wt%, the mixed system was ball milled to obtain a 3D printing ink; wherein the mass ratio of the third modified cotton fiber, ZSM-5 seed solution, hydroxypropyl methylcellulose and Cr(NO3)3·9H2O was 1:4:0.5:20.

[0091] (6) Quickly transfer the printing ink to 5cm 3 The syringe was placed in a push-type 3D printer. A computer program was used to match the slurry advance speed with the x-, y-, and z-axis movement speeds, and layer-by-layer printing was completed at room temperature. Printing parameters were as follows: layer height 0.65 mm, needle diameter 12.2 mm, extrusion rate 105%, and nozzle aperture 0.85 mm. After printing, the block was dried at room temperature for 12 hours, resulting in a 10 mm × 10 mm × 10 mm chromium-doped aluminosilicate block.

[0092] (7) The aluminosilicate block was placed in a polytetrafluoroethylene beaker, which was then placed in a polytetrafluoroethylene-lined stainless steel autoclave containing 3 g of deionized water. The catalyst block was not in direct contact with the deionized water in the autoclave. The polytetrafluoroethylene-lined autoclave was placed in a stainless steel crystallization autoclave, which was then transferred to a 150°C oven for crystallization for 4 days. Finally, the catalyst block was removed and calcined at 550°C for 6 hours to obtain a chromium-doped molecular sieve.

[0093] Comparative Example 6

[0094] This embodiment provides a method for preparing a high-performance, binder-free chromium-doped molecular sieve, which comprises the following steps:

[0095] (2) 2 g of cotton fiber was added to 200 mL of toluene, followed by the addition of 4 g of 3-chloropropyltrimethoxysilane; the mixture was then heated and stirred in a 45°C water bath for 10 h, cooled to room temperature, and centrifuged. The lower precipitate was rinsed with anhydrous ethanol and dried in an oven at 100°C overnight to obtain modified cotton fiber;

[0096] (4) Tetrapropylammonium hydroxide aqueous solution (25 wt% aqueous solution), Al(NO3)3·9H2O and H2O were added to a 100 mL flask, followed by slow dropwise addition of ethyl orthosilicate, stirred at room temperature for 24 h, and then placed in a stainless steel crystallization kettle and pre-crystallized at 90°C for 5 h to obtain a ZSM-5 seed solution; wherein the molar ratio of tetrapropylammonium hydroxide aqueous solution, ethyl orthosilicate, Al(NO3)3 and water was 1:0.01:0.3:150.

[0097] (5) The third modified cotton fiber was added to the ZSM-5 seed solution, and then HPMC and Cr(NO3)3·9H2O were added and then placed in a 50°C water bath for heating and stirring. When the water content remained at 17 wt%, the mixed system was ball milled to obtain a 3D printing ink; wherein the mass ratio of the third modified cotton fiber, ZSM-5 seed solution, hydroxypropyl methylcellulose and Cr(NO3)3·9H2O was 1:4:0.5:20.

[0098] (6) Quickly transfer the printing ink to 5cm 3 The syringe was placed in a push-type 3D printer. A computer program was used to match the slurry advance speed with the x-, y-, and z-axis movement speeds, and layer-by-layer printing was completed at room temperature. Printing parameters were as follows: layer height 0.65 mm, needle diameter 12.2 mm, extrusion rate 105%, and nozzle aperture 0.85 mm. After printing, the block was dried at room temperature for 12 hours, resulting in a 10 mm × 10 mm × 10 mm chromium-doped aluminosilicate block.

[0099] (7) The aluminosilicate block was placed in a nano-titanium dioxide dispersion for vacuum-assisted adsorption, filtered, dried, and then transferred to a polytetrafluoroethylene-lined stainless steel autoclave containing 3 g of deionized water. The catalyst block was not in direct contact with the deionized water in the autoclave. The polytetrafluoroethylene-lined autoclave was placed in a stainless steel crystallization autoclave, and the entire body was transferred to a 150°C oven for crystallization for 4 days. Finally, the catalyst block was removed and calcined at 550°C for 6 hours to obtain a chromium-doped molecular sieve.

[0100] Experimental Example 1

[0101] The mechanical strength test was performed on the catalyst blocks obtained in each embodiment and comparative example. The results are shown in Table 1.

[0102] Table 1 Mechanical strength test table

[0103]

[0104]

[0105] As shown in Table 1, the mechanical strength of Comparative Examples 1 to 6 was lost to varying degrees. Although Comparative Example 6 used modified cotton fibers, its mechanical strength was significantly reduced due to the influence of the subsequent loading particles.

[0106] Experimental Example 2

[0107] Catalytic performance analysis

[0108] Using chlorobenzene (CB) as a model CVOCs pollutant, the catalytic CB combustion performance was measured in a fixed bed at 250°C for chlorobenzene conversion, with each test lasting 5 h. The service life was tested multiple times, as shown in Table 2.

[0109] Table 2

[0110]

[0111]

[0112] As shown in Table 2, the chlorobenzene conversion rate of Examples 1 to 2 after 50 cycles is still above 80%; Comparative Example 1 does not contain a fiber skeleton and its service life is difficult to meet the requirements; Comparative Example 2 uses ordinary cotton fiber, which has a certain manufacturing effect and voids, so its initial catalytic activity is high but the cycle life is insufficient; Comparative Example 3 uses silanized cotton fiber, but after long-term use, the active voids are blocked by pollutants, resulting in a significant decrease in catalytic activity; Comparative Example 4 uses nano-titanium dioxide and silanized dual-modified cotton fiber, which has self-purification ability and therefore has higher activity after repeated use. Although Comparative Example 6 also contains nano-titanium dioxide, the subsequent loading of nano-titanium dioxide will block the voids, resulting in a reduction in active sites, which also affects the strength and reuse rate; Comparative Example 5 does not use nano-titanium dioxide, but the transition metal doped therein has an improving effect on hydrothermal stability, so its catalytic activity is higher than that of Comparative Example 3.

[0113] Experimental Example 3

[0114] Hydrothermal stability test

[0115] The obtained molecular sieve was hydrothermally treated. Water vapor with a concentration of 1000 ppm was introduced into the micro-fixed bed reactor at a flow rate of 125 mL / min at 700°C, and then the activity was tested. See Table 3.

[0116] Table 3 Hydrothermal stability test table

[0117]

[0118]

[0119] As can be seen from Table 3, Examples 1 to 2 have higher hydrothermal stability. Comparative Examples 3 to 5 show that different modification methods of cotton fibers have different effects on hydrothermal stability, among which transition metal modification is the most obvious.

[0120] 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 principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing a high-performance, binder-free chromium-doped molecular sieve, characterized in that: The following steps are involved: S1. The cotton fiber, the precursor solution, and the catalyst are mixed and stirred, filtered, and dried to obtain a first modified cotton fiber; S2. The modified cotton fiber, toluene, and 3-chloropropyltrimethoxysilane are mixed to perform a silanization reaction; after the reaction is completed, the mixture is cooled, centrifuged, washed, and dried to obtain a second modified cotton fiber; S3. The second modified cotton fiber, transition metal salt solution, and chelating agent are mixed and sequentially heated, filtered, washed, and dried to obtain a third modified cotton fiber; S4. Aqueous tetrapropylammonium hydroxide solution, ethyl orthosilicate, Al(NO3)3 and water were mixed and stirred and then pre-crystallized to obtain a ZSM-5 seed solution; S5. The third modified cotton fiber, ZSM-5 seed solution, hydroxypropyl methylcellulose and Cr(NO3)3·9H2O were mixed and then heated in a water bath with stirring and ball milled to obtain a 3D printing ink; S6. The 3D printing ink is 3D printed to obtain an aluminosilicate preform; S7. The aluminosilicate preform is subjected to dry gel conversion and calcination treatment to obtain a chromium-doped molecular sieve.

2. The preparation method according to claim 1, characterized in that The mass ratio of cotton fiber, precursor solution and catalyst in S1 is 1:(1-5):(0.05-0.2); The precursor solution is prepared by mixing tetrabutyl titanate and anhydrous ethanol in a mass ratio of 1: (1-3); The catalyst is hydrochloric acid or nitric acid.

3. The preparation method according to claim 1, characterized in that The mass ratio of the modified cotton fiber, toluene and 3-chloropropyltrimethoxysilane in S2 is 1:(100-500):(1-3).

4. The preparation method according to claim 1, characterized in that The silanization reaction temperature in S2 is 40-50° C., and the reaction time is 8-12 hours.

5. The preparation method according to claim 1, characterized in that The mass ratio of the second modified cotton fiber, the transition metal salt solution, and the chelating agent in S3 is 1:(1-3):(0.1-0.5); The transition metal salt solution is prepared by mixing cobalt salt / nickel salt and water in a mass ratio of 1: (10-20); The cobalt salt is cobalt nitrate or cobalt chloride; the nickel salt is nickel nitrate or nickel acetate; The chelating agent is disodium edetate or citric acid.

6. The preparation method according to claim 1, characterized in that The molar ratio of tetrapropylammonium hydroxide aqueous solution, ethyl orthosilicate, Al(NO3)3 and water in S4 is 1:(0.01-0.02):(0.25-0.36):

150.

7. The preparation method according to claim 1, characterized in that The pre-crystallization temperature in S4 is 85-95° C. and the time is 3-6 hours.

8. The preparation method according to claim 1, characterized in that The mass ratio of the third modified cotton fiber, ZSM-5 seed solution, hydroxypropyl methylcellulose and Cr(NO3)3·9H2O in the S5 is (1-2):(2-6):(0.1-1):(15-25).

9. The preparation method according to claim 1, characterized in that The water content of the 3D printing ink in S5 is 15-18 wt %.

10. A chromium-doped molecular sieve, characterized in that: The method is prepared by any one of claims 1 to 9.

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