CN114380301B - Nano-sheet MFI molecular sieve and preparation method thereof - Google Patents
Nano-sheet MFI molecular sieve and preparation method thereof Download PDFInfo
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Abstract
The invention provides a nano flaky MFI molecular sieve and a preparation method thereof, and relates to the technical field of molecular sieves. Mixing a silicon source, tetrapropylammonium hydroxide and water, carrying out microwave heat treatment, and then dialyzing and purifying to obtain an MFI (MFI) crystal solution; and carrying out hydrothermal treatment on the MFI crystal solution to obtain the nano flaky MFI molecular sieve. The invention adopts a microwave heating auxiliary synthesis method, has the advantages of rapid reaction and uniform heating, and can efficiently synthesize MFI zeolite crystal with uniform structure; the MFI coarse crystal molecular sieve with high purity and uniform structure is obtained through dialysis and purification, and the MFI coarse crystal molecular sieve is used as a raw material, so that the production of a nano flaky MFI molecular sieve with stable morphology is facilitated, and the nano flaky MFI molecular sieve with uniform particle size, high purity and uniform structure is obtained. Moreover, the preparation method provided by the invention is simple to operate and simple in process, and can efficiently customize the nano-sheet zeolite material with the size required by practical application.
Description
Technical Field
The invention relates to the technical field of molecular sieves, in particular to a nano flaky MFI molecular sieve and a preparation method thereof.
Background
The nano zeolite generally refers to zeolite crystals with the particle size of several nanometers to several hundred nanometers, and has the same regular structure and microporous pore channels as the traditional zeolite molecular sieve (the size is 0.5-10 mu m), the reduction of the grain size can improve the external specific surface area and obviously shorten the pore channels, and the mass and heat transfer efficiency and the exposure degree of active centers of the zeolite in the adsorption and catalysis processes are greatly improved, so that the adsorption and catalysis performance of the zeolite is improved.
The special crystal face orientation morphology of the nano zeolite corresponds to three-dimensional pore channels with different average lengths, so that the nano zeolite is endowed with a specific diffusion advantage, for example, when the nano flaky MFI molecular sieve is used for preparing a gas sensing device, the thinner thickness of the crystal in the b-axis direction corresponds to the shorter straight pore channel, the selectivity is ensured, and the diffusion efficiency of gas in the nano zeolite is improved. The traditional nano zeolite synthesis method generally adopts low-temperature (below 100 ℃) hydrothermal treatment for several days or even weeks, and the stable product is generally nano spherical crystal particles and lacks of crystal face selectivity.
Disclosure of Invention
In view of the above, the present invention aims to provide a nano-sheet MFI molecular sieve and a preparation method thereof. The nano flaky MFI molecular sieve prepared by the preparation method provided by the invention has good particle size uniformity.
In order to achieve the above object, the present invention provides the following technical solutions:
the invention provides a preparation method of a nano flaky MFI molecular sieve, which comprises the following steps:
mixing a silicon source, tetrapropylammonium hydroxide and water, carrying out microwave heat treatment, and then dialyzing and purifying to obtain an MFI (MFI) crystal solution;
and carrying out hydrothermal treatment on the MFI crystal solution to obtain the nano flaky MFI molecular sieve.
Preferably, the molar ratio of silicon dioxide to tetrapropylammonium hydroxide in the silicon source is 1 (0.3 to 0.42).
Preferably, the microwave heat treatment comprises sequentially performing low-temperature microwave heat treatment and high-temperature microwave heat treatment; the temperature of the low-temperature microwave heat treatment is 70-90 ℃, and the time is 90-150 min; the temperature of the high-temperature microwave heat treatment is 100-130 ℃, and the time is 50-90 min.
Preferably, the dialysis purification comprises sequentially carrying out the dialysis purification of tetrapropylammonium hydroxide aqueous solution and the dialysis purification of water; the concentration of the tetrapropyl ammonium hydroxide aqueous solution for dialysis is 4-8 mmol/L.
Preferably, the hydrothermal treatment comprises static hydrothermal treatment or microwave hydrothermal treatment; the temperature of the static hydrothermal treatment is 60-90 ℃, and the time is 16-72 h; the temperature of the microwave hydrothermal treatment is 90-130 ℃, and the time is 0.5-3 h.
Preferably, the step of performing the hydrothermal treatment further comprises performing acid-base treatment on the MFI embryonic crystal solution or mixing the MFI embryonic crystal solution with a growth additive.
Preferably, the acid-base treatment is to adjust the pH value of the MFI embryonic crystal solution to 5.7 to 12.7.
Preferably, the growth additive comprises one or more of a polyol, tetraalkylammonium bromide, urea, and an amino acid;
the mass of the growth additive is 1-20% of that of the MFI crystal solution.
The invention provides the nano flaky MFI molecular sieve prepared by the preparation method in the technical scheme, the length is 100-500 nm, the width is 100-200 nm, and the thickness is 10-100 nm.
The invention also provides application of the nano sheet MFI molecular sieve in … ….
The invention provides a preparation method of a nano flaky MFI molecular sieve, which comprises the following steps: mixing a silicon source, tetrapropylammonium hydroxide and water, carrying out microwave heat treatment, and then dialyzing and purifying to obtain an MFI (MFI) crystal solution; and carrying out hydrothermal treatment on the MFI crystal solution to obtain the nano flaky MFI molecular sieve. The invention adopts a microwave heating auxiliary synthesis method, has the advantages of rapid reaction and uniform heating, and can efficiently synthesize MFI zeolite crystal with uniform structure; the MFI rudiment crystal molecular sieve with high purity and uniform structure is obtained through dialysis and purification, the MFI rudiment crystal molecular sieve is used as a raw material, the MFI rudiment crystal molecular sieve is subjected to directional aggregation, the rudiment crystal molecular sieve is aggregated and crystallized in a layer-by-layer assembly mode, and then a nano flaky MFI molecular sieve crystal with stable morphology and high crystal face selectivity in the short b axis direction is obtained, and the obtained nano flaky MFI molecular sieve has uniform particle size, high purity and uniform structure. Moreover, the preparation method provided by the invention is simple to operate, simple in process, low in cost and low in production cost, and raw materials are cheap and easy to obtain, so that the preparation method is suitable for industrial production.
Furthermore, by controlling the temperature of the hydrothermal treatment, or carrying out acid-base treatment and then hydrothermal treatment on the MFI primary crystal solution, or mixing the MFI primary crystal solution with a growth additive and then carrying out hydrothermal treatment, the nano flaky MFI molecular sieve with the crystal length of 100-500 nm, the width of 100-200 nm and the thickness of 10-100 nm, which can be adjusted, can be obtained, and the size customization of the nano flaky MFI molecular sieve can be realized very conveniently.
As shown in the test results of the examples, the temperature of the hydrothermal treatment is controlled to be 80 ℃ to obtain the nano flaky MFI molecular sieve with the size of 120nm multiplied by 40nm, glycerol is used as a growth regulator to obtain the nano flaky MFI molecular sieve with the size of 200nm multiplied by 100nm multiplied by 20nm, and the pH value of the MFI crystal solution is adjusted to be 5.7 to obtain the nano flaky MFI molecular sieve with the size of 500nm multiplied by 200nm multiplied by 30 nm.
The invention provides the nano flaky MFI molecular sieve prepared by the preparation method in the technical scheme, the length is 100-500 nm, the width is 100-200 nm, and the thickness is 10-100 nm. The nano flaky MFI molecular sieve provided by the invention has the advantages of good size uniformity, high crystallinity and high purity.
Drawings
Fig. 1 is an SEM image of the nano-platelet MFI molecular sieve prepared in example 1.
Fig. 2 is a TEM image of the nano-platelet MFI molecular sieve prepared in example 1.
Fig. 3 is an XRD pattern of the nano-platelet MFI molecular sieve prepared in example 1.
Fig. 4 is an SEM image of the nano-platelet MFI molecular sieve prepared in example 2.
Fig. 5 is a TEM image of the nano-platelet MFI molecular sieve prepared in example 2.
Fig. 6 is a TEM image of the nano-platelet MFI molecular sieve prepared in example 3.
Fig. 7 is an XRD pattern of the nano-platelet MFI molecular sieve prepared in example 3.
Fig. 8 is a TEM image of the nano-platelet MFI molecular sieve prepared in example 4.
Fig. 9 is a TEM image of the nano-platelet MFI molecular sieve prepared in example 5.
Fig. 10 is a TEM image of the nano-platelet MFI molecular sieve prepared in example 6.
Detailed Description
The invention provides a preparation method of a nano flaky MFI molecular sieve, which comprises the following steps:
mixing a silicon source, tetrapropylammonium hydroxide and water, carrying out microwave heat treatment, and then dialyzing and purifying to obtain an MFI (MFI) crystal solution;
and carrying out hydrothermal treatment on the MFI crystal solution to obtain the nano flaky MFI molecular sieve.
In the present invention, unless otherwise specified, all the raw material components are commercially available products well known to those skilled in the art.
According to the invention, a silicon source, tetrapropylammonium hydroxide and water are mixed, and after microwave heat treatment, dialysis and purification are carried out to obtain an MFI (MFI) crystal solution.
In the invention, the silicon source preferably comprises one or more of tetraethyl orthosilicate, silica sol and white carbon black, and more preferably tetraethyl orthosilicate or silica sol; the silica content in the silica sol is preferably 20 to 50wt%, more preferably 30 to 40%. The method takes tetrapropylammonium hydroxide as a template agent, and is favorable for obtaining the crystal with MFI structural characteristics; the MFI molecular sieve is prepared by taking a silicon source and tetrapropylammonium hydroxide as raw materials, and the raw materials are wide in source.
In the present invention, the molar ratio of silica to tetrapropylammonium hydroxide in the silicon source is preferably 1 (0.3 to 0.42), more preferably 1 (0.35 to 0.4). The invention controls the dosage ratio of the silicon source and the tetrapropylammonium hydroxide within the range, and is beneficial to obtaining more MFI crystal.
The mixing method is not particularly limited, and the raw materials can be uniformly mixed by adopting a mixing mode known to a person skilled in the art, such as stirring and mixing; the temperature of the stirring and mixing is preferably room temperature. In the present invention, the mixing is preferably performed in such a sequence that tetrapropylammonium hydroxide is dissolved in water, and the resulting aqueous tetrapropylammonium hydroxide solution is mixed with a silicon source and aged. In the present invention, the mass concentration of the tetrapropylammonium hydroxide aqueous solution is preferably 20 to 40%, more preferably 25 to 35%, and still more preferably 25 to 30%. In the present invention, the stirring speed of the mixing and aging is preferably 50 to 300rpm, more preferably 100 to 200rpm; the mixing and aging time is preferably 12-24 h, more preferably 15-20 h, the mixing is carried out under the above conditions, so that the silicon source can be fully hydrolyzed in the tetrapropyl ammonium hydroxide aqueous solution, and the subsequent nucleation and growth of the molecular sieve can be facilitated.
In the present invention, the microwave heat treatment preferably includes sequentially performing a low-temperature microwave heat treatment and a high-temperature microwave heat treatment; the microwave heat treatment is preferably carried out in a microwave reactor, and the microwave reactor is preferably an Antopa single-mode microwave reactor. In the invention, a large amount of primary crystals are formed in the mixed solution of the silicon source and the tetrapropylammonium hydroxide through low-temperature microwave heat treatment, and then the structure of the primary crystals is developed in an ordered way through high-temperature microwave heat treatment.
In the invention, the temperature of the low-temperature microwave heat treatment is preferably 80-90 ℃, and more preferably 85-90 ℃; the time for the low-temperature microwave heat treatment is preferably 90 to 150min, more preferably 90 to 130min, and further preferably 100 to 110min. The invention is beneficial to improving the efficiency of obtaining the crystal by controlling the temperature and the time of the low-temperature microwave heat treatment.
In the present invention, the temperature of the high-temperature microwave heat treatment is preferably 120 to 130 ℃, more preferably 122 to 128 ℃, and further preferably 124 to 125 ℃; the time for the high-temperature microwave heat treatment is preferably 50 to 80min, more preferably 50 to 70min, and even more preferably 50 to 60min. The invention is helpful to ensure the structure height order of the zeolite crystal by controlling the temperature and time of the high-temperature microwave heat treatment, and further ensures the stability of the structure and the size of the subsequently synthesized nano flaky MFI molecular sieve.
In the present invention, the dialysis membrane for dialysis purification is preferably a cellulose semipermeable membrane; the cellulose semipermeable membrane preferably has a molecular weight cut-off of 1.5 to 8kDa, more preferably 3.5 to 6kDa. In the present invention, the dialysis purification is preferably performed under low-speed stirring, and the stirring speed is preferably 20 to 50rpm, more preferably 30 to 50rpm. In the present invention, the dialysis purification preferably comprises sequentially carrying out a dialysis purification of an aqueous tetrapropylammonium hydroxide solution and a dialysis purification of water. In the present invention, the concentration of the tetrapropylammonium hydroxide aqueous solution for dialysis is preferably 4 to 8mmol/L, more preferably 5 to 6mmol/L; the temperature for dialysis purification of the tetrapropylammonium hydroxide aqueous solution is preferably room temperature, and the time is preferably 12 to 48 hours, and more preferably 24 to 30 hours. According to the method, the concentration of the tetrapropyl ammonium hydroxide aqueous solution is controlled, so that the removal of redundant raw materials (silicon source and tetrapropyl ammonium hydroxide) is facilitated while the yield of the MFI crystal is ensured, and the MFI crystal is reduced due to the fact that part of the MFI crystal is dissolved when the concentration of the tetrapropyl ammonium hydroxide aqueous solution is too high. In the invention, the water dialysis purification is preferably deionized water dialysis purification, the temperature of the deionized water dialysis purification is preferably room temperature, and the time is preferably 12-48 h, and more preferably 24-30 h; the water dialysis purification process preferably comprises water replacement, the replacement interval time is preferably 3 to 12 hours, more preferably 5 to 10 hours, and the water dialysis purification is preferably carried out until the pH of the dialysate is 7.0 to 8.0. In the present invention, the dialysis membrane for dialysis purification is preferably a cellulose semipermeable membrane; the cellulose semipermeable membrane preferably has a molecular weight cut-off of 1.5 to 8kDa, more preferably 3.5 to 6kDa. In the present invention, the dialysis purification is preferably performed under low-speed stirring, and the stirring speed is preferably 20 to 50rpm, more preferably 30 to 50rpm.
The invention adopts a microwave heating-assisted synthesis method, has the advantages of rapid reaction and uniform heating, can efficiently synthesize high-purity MFI zeolite with a uniform structure, and then synthesize the nano flaky MFI molecular sieve from the MFI zeolite crystal with high purity and a uniform structure, and is beneficial to the generation of products with stable morphology. The method adopts a dialysis purification method, has the advantages of accurate particle screening and more reserved effective components, can obtain high-purity MFI (MFI) microcrystal aqueous solution, and provides a high-quality raw material for further synthesizing the MFI molecular sieve of the nanosheets.
After obtaining the MFI crystallite solution, the MFI crystallite solution is subjected to hydrothermal treatment to obtain the nano flaky MFI molecular sieve.
In the invention, before the hydrothermal treatment, the MFI embryonic crystal solution is subjected to acid-base treatment or mixed with a growth additive to obtain a pretreated MFI embryonic crystal solution. In the present invention, the acid-base treatment is preferably performed to adjust the pH of the MFI raw crystal solution to 5.7 to 12.7, more preferably 6 to 12, still more preferably 7 to 10, and most preferably 8 to 9. In the present invention, the acid-base treatment is preferably performed using an acid or a base; the acid preferably comprises hydrochloric acid and/or sulfuric acid; the acid is preferably used in the form of an aqueous acid solution, and the concentration of the aqueous acid solution is preferably 0.5 to 2mol/L, more preferably 0.8 to 1.5mol/L, and further preferably 0.9 to 1.1mol/L; the base preferably comprises sodium hydroxide and/or potassium hydroxide; the base is preferably used in the form of an aqueous base solution, and the concentration of the aqueous base solution is preferably 0.5 to 2mol/L, more preferably 0.8 to 1.5mol/L, and further preferably 0.9 to 1.1mol/L. In the present invention, the growth additive preferably includes one or more of a polyol, tetraalkylammonium bromide, urea, and an amino acid; the polyol preferably comprises ethylene glycol and/or glycerol; the tetraalkylammonium bromide preferably comprises tetrabutylammonium bromide and/or tetraethylammonium bromide; the amino acids preferably include lysine and/or arginine. In the present invention, the mass of the growth additive is preferably 1 to 20%, preferably 1 to 15%, and more preferably 1 to 10% of the mass of the MFI green crystal solution. The mixing method is not particularly limited, and the raw materials can be uniformly mixed by adopting a mixing mode known by the technical personnel in the field, such as stirring and mixing; the temperature of the mixing is preferably room temperature.
In the present invention, the hydrothermal treatment preferably includes a static hydrothermal treatment or a microwave hydrothermal treatment. In the present invention, the temperature of the static hydrothermal treatment is preferably 60 to 90 ℃, more preferably 70 to 90 ℃, and further preferably 80 to 90 ℃; the time of the static hydrothermal treatment is preferably 16 to 72 hours, more preferably 20 to 60 hours, and further preferably 24 to 48 hours; in a specific embodiment of the present invention, the static hydrothermal treatment is preferably performed by placing the MFI embryonic crystal solution or the pretreated MFI embryonic crystal solution in a reaction kettle, sealing the reaction kettle, and placing the reaction kettle in a static constant temperature oven for static hydrothermal treatment. In the invention, the temperature of the microwave hydrothermal treatment is preferably 90-130 ℃, more preferably 100-130 ℃, and further preferably 110-120 ℃; the time of the microwave hydrothermal treatment is preferably 0.5 to 3 hours, more preferably 1 to 2.5 hours, and further preferably 1 to 2 hours; in a specific embodiment of the present invention, the microwave hydrothermal treatment is preferably performed by placing the MFI green crystal solution or the pretreated MFI green crystal solution in a microwave reaction tube, and then placing the microwave reaction tube in a microwave synthesizer for microwave hydrothermal treatment.
According to the invention, by controlling the temperature of hydrothermal treatment, or carrying out acid-base treatment and then hydrothermal treatment on the MFI primary crystal solution, or mixing the MFI primary crystal solution with a growth additive and then carrying out hydrothermal treatment, the nano flaky MFI molecular sieve with the crystal length of 100-500 nm, the width of 100-200 nm and the thickness of 10-100 nm, which can be adjusted, can be obtained, and the size customization of the nano flaky MFI molecular sieve can be realized very conveniently. Specifically, the temperature of hydrothermal treatment is controlled to be 80 ℃ to directly obtain the nano-sheet MFI molecular sieve with the size of 120nm multiplied by 40nm, the pH value of MFI primary crystal solution is adjusted to be 11.41 by sodium hydroxide solution for pretreatment to obtain the nano-sheet MFI molecular sieve with the size of 160nm multiplied by 80nm, the pH value of MFI primary crystal solution is adjusted to be 5.7 by hydrochloric acid solution to obtain the nano-sheet MFI molecular sieve with the size of 500nm multiplied by 200nm multiplied by 30nm, glycerol is used as a growth regulator to obtain the nano-sheet MFI molecular sieve with the size of 200nm multiplied by 100nm multiplied by 20nm, lysine is used as a growth regulator to obtain the nano-sheet MFI molecular sieve with the size of 300nm multiplied by 150nm multiplied by 50nm, and lysine is used as a growth regulator to obtain the laminated nano-sheet MFI molecular sieve, and the thickness of a single crystal is 10-25 nm.
After the hydrothermal treatment is completed, the invention preferably further comprises the steps of cooling the suspension obtained by the hydrothermal treatment to room temperature, then carrying out solid-liquid separation, washing the obtained solid product with water, and then drying to obtain the nano flaky MFI molecular sieve. The solid-liquid separation mode is not particularly limited, and the solid-liquid separation mode known to a person skilled in the art can be adopted, such as centrifugal separation; the rotation speed of the centrifugal separation is preferably 12000-15000 rpm, more preferably 13000-14000 rpm; the time for the centrifugal separation is preferably 3 to 10min, and more preferably 3 to 5min. In the present invention, the water washing is preferably centrifugal water washing; the temperature of the centrifugal water washing is preferably-10 to 0 ℃, and more preferably-10 to-5 ℃; the rotating speed of the centrifugal water washing is preferably 12000-15000 rpm, more preferably 13000-14000 rpm; the number of the washing is preferably 5 to 7, more preferably 6 to 7; the time of single water washing is preferably 3-10 min, and more preferably 6-8 min; the mass ratio of the silicon source to the water for single water washing is preferably 1:2 to 3, more preferably 1:2 to 2.5; the centrifugal water washing is preferably carried out in a refrigerated centrifuge. In the present invention, after water is added in a single centrifugal water washing, it is preferable to perform ultrasonic treatment before performing the next centrifugal water washing, the ultrasonic treatment is preferably performed in an ultrasonic machine, and the purpose of the ultrasonic treatment is to re-disperse solids and ensure clean washing. In the present invention, the drying temperature is preferably 60 to 80 ℃, more preferably 65 to 75 ℃, and further preferably 70 ℃; the drying time is preferably 8 to 12 hours, more preferably 9 to 11 hours, and still more preferably 10 hours.
The method takes common silicon source and tetrapropylammonium hydroxide as raw materials to prepare the nano flaky MFI molecular sieve, and has the advantages of wide raw material source, low price and low production cost.
The invention provides the nano flaky MFI molecular sieve prepared by the preparation method in the technical scheme. In the present invention, the length of the nanosheet MFI molecular sieve is preferably 100 to 500nm, more preferably 120 to 500nm, and even more preferably 200 to 500nm; the width of the nano flaky MFI molecular sieve is preferably 100-200 nm, more preferably 120-200 nm, and further preferably 120-150 nm; the thickness of the nano flaky MFI molecular sieve is preferably 10 to 100nm, more preferably 20 to 80nm, and even more preferably 20 to 50nm.
The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. It is to be understood that the described embodiments are merely exemplary of the invention, and not restrictive of the full scope of the invention. All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments in the present invention, belong to the protection scope of the present invention.
Example 1
(1) Dissolving tetrapropylammonium hydroxide in water to prepare a tetrapropylammonium hydroxide aqueous solution with the concentration of 20 wt%; 3g of tetraethyl orthosilicate is dissolved in 6.1686g of tetrapropyl ammonium hydroxide aqueous solution, stirred and aged for 24h at room temperature and 150rpm, then transferred to a microwave reactor, and subjected to low-temperature microwave heat treatment for 90min at 90 ℃ and then high-temperature microwave heat treatment for 50min at 130 ℃ to obtain an MFI zeolite mixed solution. Transferring the MFI zeolite mixed solution into a cellulose semipermeable membrane with the molecular weight cutoff of 3.5kDa, then soaking the cellulose semipermeable membrane into 1L of tetrapropylammonium hydroxide aqueous solution with the concentration of 6mmol/L, dialyzing and purifying for 24h at room temperature and 30rpm, then placing the mixture into deionized water for dialysis and purification, and replacing the deionized water every 12h until the pH value of the dialyzed external liquid reaches 7.0 to obtain an MFI crystal solution. Wherein the molar ratio of the silicon source to the tetrapropylammonium hydroxide is 1.
(2) Placing the MFI crystal solution in a hydrothermal reaction kettle, standing in a preheated constant-temperature oven at 80 ℃ for static heat treatment for 24 hours, taking out the hydrothermal reaction kettle, immersing the hydrothermal reaction kettle in cold water at 15 ℃ for rapid cooling, transferring the obtained milky white turbid liquid into a centrifuge tube, placing the centrifuge tube in a refrigerated centrifuge, carrying out centrifugal washing at-10 ℃ and 12000rpm for 3min, pouring out supernatant after each centrifugal washing, adding 1.5g of deionized water, uniformly treating in an ultrasonic machine, and carrying out next centrifugal washing again, wherein the centrifugal washing times are 7 times. And then drying in an oven at 80 ℃ for 12h to obtain the nano flaky MFI molecular sieve crystal.
Fig. 1 is a Scanning Electron Microscope (SEM) image of the nano-platelet MFI molecular sieve prepared in this example. As can be seen from fig. 1, the nano-sheet MFI molecular sieve crystal obtained in this example has a length × width × thickness =120nm × 120nm × 40nm, is a nano-sheet MFI molecular sieve crystal having a high crystal plane selectivity in the short b-axis direction, and has good dimensional uniformity.
Fig. 2 is a Transmission Electron Microscope (TEM) image of the nano-sheet MFI molecular sieve prepared in this example. As can be seen from fig. 2, the nano-platelet MFI molecular sieve prepared in this example is a uniform well-crystallized defect-free crystalline material.
Fig. 3 is an X-ray diffraction pattern of the nano-sheet MFI molecular sieve prepared in this example. As can be seen from fig. 3, the nano flaky molecular sieve prepared in this example has a highly crystalline MFI structure.
Example 2
A nano-platelet MFI molecular sieve was prepared according to the method of example 1, differing from example 1 only in that: in the step (2), firstly, adjusting the pH value of the MFI embryonic crystal solution to 11.41 by using a sodium hydroxide aqueous solution with the concentration of 1mol/L, and then placing the MFI embryonic crystal solution in a hydrothermal reaction kettle for hydrothermal treatment of crystals in a constant-temperature oven for 18 hours; the centrifugation rate was 15000rpm and the centrifugation time was 5min.
Fig. 4 is a scanning electron microscope image of the nano-sheet MFI molecular sieve prepared in this example. As can be seen from fig. 4, the nano-sheet MFI molecular sieve crystal obtained in this example has a length × width × thickness =160nm × 160nm × 80nm, is a nano-sheet MFI molecular sieve crystal having a high crystal plane selectivity in the short b-axis direction, and has good size uniformity.
Fig. 5 is a transmission electron microscope image of the nano-sheet MFI molecular sieve prepared in this example. As can be seen from fig. 5, the nano flaky MFI molecular sieve prepared in this example is a uniform crystalline material with good crystallization and no defects.
Example 3
A nano-platelet MFI molecular sieve was prepared according to the method of example 1, differing from example 1 only in that: in the step (2), firstly, the pH value of the MFI embryonic crystal solution is adjusted to 5.69 by using a hydrochloric acid aqueous solution with the concentration of 1.1mol/L, and then the MFI embryonic crystal solution is placed in a hydrothermal reaction kettle and subjected to crystal hydrothermal treatment for 72 hours in a constant-temperature oven; the centrifugation rate was 15000rpm and the centrifugation time was 5min.
Fig. 6 is a transmission electron microscope image of the nano-sheet MFI molecular sieve prepared in this example. As can be seen from fig. 6, the nano-sheet MFI molecular sieve prepared in this example is a uniform-sized sheet, and the nano-sheet MFI molecular sieve crystal has a length × width × thickness =500nm × 200nm × 30nm, and is a uniform crystal material with good crystallinity and no defects.
Fig. 7 is an X-ray diffraction pattern of the nano-flake MFI molecular sieve prepared in this example. As can be seen from fig. 7, the nano-flake zeolite prepared in this example has a highly crystalline MFI structure.
Example 4
(1) Dissolving tetrapropylammonium hydroxide in water to prepare 25wt% tetrapropylammonium hydroxide aqueous solution; 3.3g of tetraethyl orthosilicate is dissolved in 4.5686g of tetrapropyl ammonium hydroxide aqueous solution, stirred and aged for 24h at room temperature and 100rpm, then transferred to a microwave reactor, and subjected to low-temperature microwave heat treatment at 70 ℃ for 120min and high-temperature microwave heat treatment at 120 ℃ for 70min to obtain an MFI zeolite mixed solution. Transferring the MFI zeolite mixed solution into a cellulose semipermeable membrane with the molecular weight cutoff of 4.5kDa, then soaking the cellulose semipermeable membrane into 1L of tetrapropylammonium hydroxide aqueous solution with the concentration of 8mmol/L, dialyzing and purifying for 36h at room temperature and 40rpm, then placing the mixture into deionized water for dialysis and purification, and replacing the deionized water every 12h until the pH value of the dialyzed external liquid reaches 7.0 to obtain an MFI crystal solution. Wherein the molar ratio of the silicon source to the tetrapropylammonium hydroxide is 1.
(2) Adding glycerol (the mass of the glycerol is 10% of that of the MFI crystal solution) into the MFI crystal solution, stirring and mixing for 15min under the condition of 150rpm, transferring the mixture into a microwave reaction tube, placing the microwave reaction tube in a microwave synthesizer at 110 ℃ for microwave hydrothermal treatment for 3h, taking out the microwave reaction tube, immersing the microwave reaction tube in cold water at 15 ℃, quickly cooling, transferring the obtained milky white suspension into a centrifuge tube, placing the centrifuge tube in a refrigerated centrifuge, pouring out supernatant under the conditions of-5 ℃ and 12000rpm, adding 1.5g of deionized water, carrying out centrifugal water washing, wherein the time of single centrifugal water washing is 3min, pouring out the supernatant after each centrifugal treatment, adding 1.5g of deionized water, uniformly treating in an ultrasonic machine, and carrying out next centrifugal water washing for 7 times. And then drying in an oven at 80 ℃ for 12h to obtain the nano flaky MFI molecular sieve crystal.
Fig. 8 is a transmission electron microscope image of the nano-sheet MFI molecular sieve prepared in this example. As can be seen from fig. 8, the nano-flaky MFI molecular sieve crystal prepared in this example is a uniform-sized flaky crystal material having a length × width × thickness =200nm × 100nm × 20nm, and is a uniform crystal material with good crystallinity and no defects.
Example 5
A nano-platelet MFI molecular sieve was prepared according to the method of example 1, differing from example 1 only in that: in the step (2), lysine (the mass of the lysine is 1% of the mass of the MFI primary crystal solution) is added into the MFI primary crystal solution and stirred for 15min at 200rpm, and then the mixture is placed in a hydrothermal reaction kettle and subjected to crystal hydrothermal treatment for 48h in a constant-temperature oven at 80 ℃.
In the figure, the length of the nano flaky MFI molecular sieve crystal prepared in the example is 300nm, the width is 150nm, and the thickness is 50nm.
Fig. 9 is a transmission electron microscope image of the nano-sheet MFI molecular sieve prepared in this example. As can be seen from fig. 9, the nano-flaky MFI molecular sieve crystals prepared in this example were uniformly sized platelets, and were uniform crystal materials with good crystallinity and no defects, with length × width × thickness =300nm × 150nm × 50nm.
Example 6
A nano-platelet MFI molecular sieve was prepared according to the method of example 1, differing from example 1 only in that: in the step (2), tetrabutylammonium bromide (the mass of the tetrabutylammonium bromide is 2.5 percent of the mass of the MFI primary crystal solution) is added into the MFI primary crystal solution and stirred for 30min at the condition of 150rpm, and then the mixture is placed in a hydrothermal reaction kettle and subjected to crystal hydrothermal treatment for 72h in a constant-temperature oven at the temperature of 100 ℃.
The nano-sheet MFI molecular sieve crystal obtained in the embodiment is in a laminated nano-sheet shape, the thickness of a single crystal sheet is about 18-25 nm, the length of the whole particle is 150-200 nm, and the width is 80-120 nm.
Fig. 10 is a transmission electron microscope image of the nano-sheet MFI molecular sieve prepared in this example. As can be seen from FIG. 10, the nano-sheet MFI molecular sieve prepared in this example has a morphology of a stacked nano-sheet, the thickness of a single crystal sheet is about 10-25 nm, the length of the whole particle is 150-200 nm, and the width is 80-120 nm.
The embodiment shows that the nano flaky MFI molecular sieve prepared by the invention has uniform particles, the length, the width and the thickness of the crystal of the nano flaky MFI molecular sieve can be regulated and synthesized according to needs, and the customized synthesis requirement of a nano zeolite material can be met.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (9)
1. A preparation method of a nano flaky MFI molecular sieve is characterized by comprising the following steps:
mixing a silicon source, tetrapropylammonium hydroxide and water, carrying out microwave heat treatment, and then dialyzing and purifying to obtain an MFI (melt flow index) crystal solution;
carrying out hydrothermal treatment on the MFI crystal solution to obtain a nano flaky MFI molecular sieve;
the microwave heat treatment comprises low-temperature microwave heat treatment and high-temperature microwave heat treatment which are sequentially carried out;
the dialysis purification comprises sequentially carrying out tetrapropylammonium hydroxide aqueous solution dialysis purification and water dialysis purification.
2. The method according to claim 1, wherein the molar ratio of silicon dioxide to tetrapropylammonium hydroxide in the silicon source is 1 (0.3-0.42).
3. The preparation method according to claim 1, wherein the temperature of the low-temperature microwave heat treatment is 70-90 ℃ and the time is 90-150 min; the temperature of the high-temperature microwave heat treatment is 100-130 ℃, and the time is 50-90 min.
4. The method according to claim 1, wherein the concentration of the aqueous tetrapropylammonium hydroxide solution for dialysis is 4 to 8mmol/L.
5. The method of claim 1, wherein the hydrothermal treatment comprises a static hydrothermal treatment or a microwave hydrothermal treatment; the temperature of the static hydrothermal treatment is 60-90 ℃, and the time is 16-72 h; the temperature of the microwave hydrothermal treatment is 90-130 ℃, and the time is 0.5-3 h.
6. The method as claimed in claim 1 or 5, wherein the hydrothermal treatment further comprises subjecting the MFI embryonic solution to an acid-base treatment or mixing with a growth additive.
7. The production method according to claim 6, wherein the acid-base treatment is to adjust the pH of the MFI crystallite solution to 5.7 to 12.7.
8. The method of claim 6, wherein the growth additive comprises one or more of a polyol, a tetraalkylammonium bromide, urea, and an amino acid;
the mass of the growth additive is 1-20% of that of the MFI crystal solution.
9. The nano-flaky MFI molecular sieve prepared by the preparation method of any one of claims 1 to 8, having a length of 100 to 500nm, a width of 100 to 200nm and a thickness of 10 to 100nm.
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