Modified fly ash-based molecular sieve, preparation method thereof and application in gas targeted adsorption

Abstract

The present invention belongs to the technical field of resource recycling and environmental protection, and relates to a method for preparing gas adsorption materials from industrial solid wastes, specifically to a modified fly ash-based molecular sieve, a preparation method thereof, and an application in gas targeted adsorption. The preparation method is as follows: Mix fly ash with a solution of alkali metal hydroxide, and then carry out hydrothermal melting and melt crystallization to obtain a fly ash-based molecular sieve; Mix the fly ash-based molecular sieve with a solution containing phosphoric acid for reaction to graft phosphate groups onto the surface of the molecular sieve, obtaining a fly ash-based molecular sieve grafted with phosphate groups; Add the fly ash-based molecular sieve grafted with phosphate groups to a solution containing alkaline earth metal salts, and carry out stirring reaction to replace the alkali metal cations in the fly ash-based molecular sieve grafted with phosphate groups with alkaline earth metal cations. The modified fly ash-based molecular sieve provided by the present invention not only has high CO2 adsorption selectivity, but also improves the hydrophobicity of the molecular sieve, thereby improving the targeted adsorption ability of CO2.

Description

Technical Field

[0001] The present invention belongs to the technical field of resource recycling and environmental protection, and relates to a method for preparing gas adsorption materials from industrial solid waste, specifically to a modified fly ash-based molecular sieve, a preparation method thereof, and an application in gas targeted adsorption. Background Art

[0002] Disclosing the information of this background art section is only intended to increase the understanding of the overall background of the present invention, and is not necessarily regarded as an admission or an implication in any form that this information constitutes the prior art already known to those of ordinary skill in the art.

[0003] Currently, the mainstream resource utilization ways of fly ash mostly stay in the stage of large-scale and low added-value utilization, such as the preparation of building and filling materials, soil fertilization and pH adjustment, etc. There are few high-value and functional utilization schemes for fly ash. Si and Al in fly ash are the main elements of the molecular sieve framework, and its specific surface area can reach 300 - 500m 2 / kg. Therefore, fly ash can be used as a raw material for preparing molecular sieves to achieve high-value functional utilization.

[0004] Molecular sieves have excellent adsorption properties. However, due to the existence of compensating cations and Si-OH outside the framework, strong adsorption of polar molecules occurs on the surface of the molecular sieve, resulting in strong hydrophilic characteristics of the molecular sieve. There is competitive adsorption between molecules during the adsorption process. The stronger the hydrophilicity, the easier it is to adsorb water molecules. The adsorption sites on the surface of the molecular sieve are occupied by water molecules adsorbed, which will lead to a decrease in the ability of the molecular sieve to adsorb other gases. According to the research and understanding of the inventor, currently, the in-situ dealumination is mainly achieved through methods such as acid-base modification and hydrothermal treatment, and then the hydrophobicity of the molecular sieve is improved by increasing the silicon-aluminum ratio. However, the increase in the silicon-aluminum ratio often induces an increase in the pore channels of the molecular sieve framework, resulting in a mismatch between the effective diameter of the molecular sieve pore channels and the molecular dynamic diameter of CO2. Therefore, maintaining the CO2 shape selectivity of the molecular sieve while improving the hydrophobicity of the molecular sieve is a difficult problem in the field of preparing molecular sieves for targeted adsorption of CO2. Summary of the Invention

[0005] In order to solve the deficiencies of the prior art, the purpose of the present invention is to provide a modified fly ash-based molecular sieve, a preparation method thereof, and an application in gas targeted adsorption. The modified fly ash-based molecular sieve provided by the present invention not only has high CO2 adsorption selectivity, but also improves the hydrophobicity of the molecular sieve, thereby being able to improve the targeted adsorption ability of CO2.

[0006] In order to achieve the above purpose, the technical solution of the present invention is as follows:

[0007] On the one hand, a preparation method of a modified fly ash-based molecular sieve includes the following steps:

[0008] Mix the fly ash with a solution of alkali metal hydroxide, and then carry out hydrothermal melting and melt crystallization to obtain fly ash-based molecular sieve;

[0009] Mix the fly ash-based molecular sieve with a solution containing phosphoric acid for reaction to graft phosphoric acid groups onto the surface of the molecular sieve, and obtain the fly ash-based molecular sieve grafted with phosphoric acid groups;

[0010] Add the fly ash-based molecular sieve grafted with phosphoric acid groups to a solution containing alkaline earth metal salts, and carry out a stirring reaction to replace the alkali metal cations in the fly ash-based molecular sieve grafted with phosphoric acid groups with alkaline earth metal cations, thus obtaining the product.

[0011] First of all, the fly ash-based molecular sieve of the present invention is used as a raw material for molecular sieve modification. Because there are small amounts of Ca, Mn, Ti, etc. in the fly ash, it can play an important role in strengthening the lattice stability of the molecular sieve, regulating the surface oxidation-reduction property, constructing targeted adsorption sites, etc., so that the molecular sieve has better CO2 affinity.

[0012] Secondly, the present invention grafts phosphoric acid groups onto the fly ash-based molecular sieve. It can not only improve the hydrophobic property of the fly ash by the selective adsorption of phosphoric acid groups to water molecules, but also produce a regulatory effect on the direction / rate of electron transfer between metals and oxygen on the surface of the fly ash-based molecular sieve. Thus, without changing the silicon-aluminum ratio of the molecular sieve, the polarity of the adsorption sites on the surface of the molecular sieve can be adjusted, and the targeted adsorption ability of the fly ash-based molecular sieve to CO2 can be regulated directionally.

[0013] Thirdly, the present invention uses alkaline earth metal cations to replace alkali metal cations to reconstruct the adsorption sites. It can not only enhance the fly ash molecular sieve to target and capture CO2 molecules among molecules with the same kinetic diameter, but also the increase in valence state can accelerate the electrostatic behavior between the surface and polar molecules while promoting acid-base pairing, strengthening the specific capture effect of the molecular sieve surface on polar-acidic gases.

[0014] Therefore, the modified fly ash-based molecular sieve prepared by the present invention solves the contradiction problem between low silicon-aluminum ratio and high hydrophobicity, thereby improving the hydrophobicity of the molecular sieve and the ability to target and adsorb CO2 at the same time.

[0015] On the other hand, a modified fly ash-based molecular sieve is obtained by the above preparation method.

[0016] In the third aspect, an application of the above-mentioned modified fly ash-based molecular sieve in gas targeted adsorption, and the gas is carbon dioxide.

[0017] The beneficial effects of the present invention are as follows:

[0018] The present invention uses fly ash and alkali metal hydroxides to prepare fly ash-based molecular sieves as modified raw materials, which have higher carbon dioxide affinity. Then, through phosphoric acid group grafting and alkaline earth metal ion exchange, the hydrophobicity of the fly ash-based molecular sieve and the targeted adsorption ability of CO2 are improved simultaneously. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The specification drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention.

[0020] Figure 1 This is a fixed-bed reaction test device for adsorbing carbon dioxide by the modified fly ash-based molecular sieve adsorbent prepared in each embodiment of the present invention. 1. Gas cylinder, 2. Gas path valve, 3. Mass flow controller, 4. Steam generator, 5. Humidity meter, 6. Fixed-bed reactor, 7. Gas chromatograph, 8. Computer;

[0021] Figure 2 This is the adsorption breakthrough curve of the modified fly ash-based molecular sieve adsorbent prepared in Example 1 of the present invention for CO2;

[0022] Figure 3 This is the adsorption breakthrough curve of the modified fly ash-based molecular sieve adsorbent prepared in Example 2 of the present invention for CO2;

[0023] Figure 4 This is the adsorption breakthrough curve of the modified fly ash-based molecular sieve adsorbent prepared in Example 3 of the present invention for CO2;

[0024] Figure 5 This is the adsorption breakthrough curve of the modified fly ash-based molecular sieve adsorbent prepared in Example 4 of the present invention for CO2;

[0025] Figure 6 This is the adsorption breakthrough curve of the fly ash-based molecular sieve adsorbent prepared in Comparative Example 1 of the present invention for CO2. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0027] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular forms are also intended to include the plural forms. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they specify the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0028] The fly ash described in the present invention is collected from the electrostatic precipitator of a coal-fired flue gas treatment system.

[0029] In view of the contradiction between the low silicon-aluminum ratio and high hydrophobicity in the molecular sieve as a gas-targeted adsorbent, which makes it difficult for the molecular sieve to simultaneously improve hydrophobicity and the ability to target-adsorb CO2, the present invention proposes a modified fly ash-based molecular sieve, its preparation method, and its application in gas-targeted adsorption.

[0030] A typical embodiment of the present invention provides a preparation method of a modified fly ash-based molecular sieve, which includes the following steps:

[0031] Mix fly ash with a solution of alkali metal hydroxide, and then carry out hydrothermal melting and melt crystallization to obtain a fly ash-based molecular sieve;

[0032] Mix the fly ash-based molecular sieve with a solution containing phosphoric acid for reaction, so that phosphoric acid groups are grafted onto the surface of the molecular sieve to obtain a fly ash-based molecular sieve grafted with phosphoric acid groups;

[0033] Add the fly ash-based molecular sieve grafted with phosphoric acid groups to a solution containing alkaline earth metal salts, and carry out stirring reaction, so that the alkali metal cations in the fly ash-based molecular sieve grafted with phosphoric acid groups are replaced by alkaline earth metal cations, thus obtaining the product.

[0034] When the fly ash-based molecular sieve is mixed with a solution containing phosphoric acid for reaction, the rate of grafting phosphoric acid groups onto the surface of the molecular sieve is slow. In order to accelerate the reaction, in some embodiments, during the reaction of mixing the fly ash-based molecular sieve with a solution containing phosphoric acid, heating is carried out, and the heating temperature is 40-80 °C. By increasing the temperature, the reaction can be accelerated. When the reaction temperature is too high, the solvent water evaporates relatively fast, and the reaction system can be sealed to avoid the loss of solvent evaporation.

[0035] In order to accelerate the rate of grafting phosphoric acid groups onto the surface of the molecular sieve, in some embodiments, methanol is added during the reaction of mixing the fly ash-based molecular sieve with a solution containing phosphoric acid. Using methanol as a reducing agent can increase the rate of grafting phosphoric acid groups.

[0036] In one or more embodiments, the volume ratio of phosphoric acid to methanol is 2-3:8-7. This is beneficial to further increase the rate of grafting phosphoric acid groups.

[0037] In one or more embodiments, during the process of mixing the fly ash-based molecular sieve with a solution containing phosphoric acid for reaction, the pH is 5 to 6, and the reaction time is 2 to 3 h.

[0038] The ways of grafting phosphoric acid groups on the surface of the molecular sieve include chemical bond connection and physical connection. Among them, the physical connection has poor stability, which affects the stability of the hydrophobic performance. In some embodiments, after the fly ash-based molecular sieve is mixed and reacted with a solution containing phosphoric acid, it is heated to 250 to 450 °C for calcination, which is beneficial to the transformation of the physical connection method into a chemical bond connection, so that the phosphoric acid groups on the surface of the fly ash-based molecular sieve are more firmly grafted on the surface of the molecular sieve, improving its stability.

[0039] In some embodiments, the fly ash-based molecular sieve grafted with phosphoric acid groups is added to a solution containing alkaline earth metal salts, and the temperature for stirring reaction is 60 to 90 °C. When the reaction temperature is too high, the solvent water evaporates quickly. The reaction system can be sealed to avoid solvent evaporation loss.

[0040] The alkali metal hydroxide can be potassium hydroxide, sodium hydroxide, etc., and the alkaline earth metal salt can be beryllium salt, magnesium salt, calcium salt, strontium salt, barium salt, etc. In some embodiments, the alkali metal hydroxide is sodium hydroxide, and the alkaline earth metal salt is magnesium salt or calcium salt. Research shows that the fly ash-based molecular sieve prepared with sodium hydroxide and then cation-exchanged with magnesium salt or calcium salt can better achieve the targeted adsorption of carbon dioxide.

[0041] In some embodiments, the fly ash-based molecular sieve substituted with alkaline earth metal cations is heated to 500 °C for calcination, which can make the metal ions on its surface more firmly embedded in the fly ash-based molecular sieve, thereby improving the stability of the modified fly ash-based molecular sieve.

[0042] Another embodiment of the present invention provides a modified fly ash-based molecular sieve obtained by the above preparation method.

[0043] A third embodiment of the present invention provides an application of the above modified fly ash-based molecular sieve in gas targeted adsorption, and the gas is carbon dioxide.

[0044] Specifically, the modified fly ash-based molecular sieve is used as a carbon dioxide gas targeted adsorbent.

[0045] Specifically, the gas containing carbon dioxide is introduced into the modified fly ash-based molecular sieve for adsorption. The adsorption temperature is 20 to 30 °C. Before adsorption, the modified fly ash-based molecular sieve is pretreated with nitrogen. The pretreatment temperature is 200 to 300 °C.

[0046] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in combination with specific examples and comparative examples.

[0047] The adsorption performance of the modified fly ash-based molecular sieve adsorbent prepared in the following examples was tested. The fixed-bed reaction test device used in the test is as Figure 1 shown, and it is composed of a gas cylinder 1, a gas path valve 2, a mass flow controller 3, a steam generator 4, a hygrometer 5, a fixed-bed reactor 6, a gas chromatograph 7 and a computer 8.

[0048] The adsorbent was first pretreated under nitrogen purge at a pretreatment temperature of 250 °C for 2 h, and then the adsorption test was carried out at 25 °C and 1 atmospheric pressure. The reaction inlet was CO2, and the concentrations at the inlet and outlet of the fixed-bed reactor were measured respectively to calculate the adsorption capacity. The calculation formula for the CO2 adsorption capacity is as follows:

[0049]

[0050] where, v: adsorption capacity of the adsorbent, mg / g;

[0051] F: gas volume flow rate, mL / min;

[0052] M: molar mass of the adsorbate, g / mol;

[0053] m: mass of the adsorbent, g;

[0054] C0: gas inlet concentration, ppm;

[0055] C t : gas outlet concentration at time t, ppm.

[0056] Judging from the calculation formula, when the CO2 flow rate is the same and the gas concentration is the same, the longer the adsorption breakthrough time, the more the adsorption capacity.

[0057] Example 1

[0058] Preparation of fly ash-based molecular sieve: a. Grind the fly ash sample into powder and perform particle size screening to select a sample with a particle size of 200 mesh. Mix the fly ash with NaOH solution (fly ash: NaOH particles: water = 1 g: 0.5 g: 5 ml), and then put the mixture into a hydrothermal autoclave and heat it to 80 °C for hydrothermal reaction for 12 h to carry out hydrothermal melting and melt crystallization to obtain fly ash-based molecular sieve. b. At room temperature, suspend the molecular sieve powder in deionized water, treat it with ultrasonic waves for 10 h, and continue to stir for 2-3 hours to solidify it. d. Wash the obtained molecular sieve sample to neutrality, and then dry it in a constant temperature drying oven to a constant mass.

[0059] Grafting phosphate groups: a. Prepare a phosphoric acid solution. Slowly drip 80 ml of methanol and 20 ml of (10 mol / L) concentrated H3PO4 into the stirred molecular sieve suspension to maintain the pH between 5 and 6, and continue stirring for 2 hours. b. Filter the reaction solution and wash it three times with methanol. Dry the sample and calcine the treated molecular sieve at 500 °C for 2 h.

[0060] Perform metal ion exchange: a. Prepare a metal ion solution. Dissolve 10 g of MgCl2 in 100 ml of deionized water. b. Mix the grafted molecular sieve with the metal ion solution, keep stirring and heat to 80 °C for 24 hours. c. Filter the reaction solution, wash it three times with deionized water, dry the sample, and calcine the treated molecular sieve at 500 °C for 2 h to obtain the modified fly ash-based molecular sieve. The adsorption breakthrough curve is as Figure 2 shown, and adsorption breakthrough occurs near 4000 s.

[0061] Example 2

[0062] Prepare fly ash-based molecular sieve: a. Grind the fly ash sample into powder and perform particle size screening to select the sample with the target particle size. Mix the fly ash with NaOH solution (fly ash: NaOH particles: water = 1 g: 0.5 g: 5 ml), then put the mixture into a hydrothermal autoclave and heat to 80 °C for hydrothermal reaction for 12 h to perform hydrothermal melting and melt crystallization to obtain fly ash-based molecular sieve. b. At room temperature, suspend the molecular sieve powder in deionized water and treat it with ultrasonic waves for 10 h, and continue stirring for 2 - 3 hours to solidify it. d. Wash the obtained molecular sieve sample to neutrality, and then dry it in a constant temperature drying oven to a constant mass.

[0063] Grafting phosphate groups: a. Prepare a phosphoric acid solution. Slowly drip 70 ml of methanol and 30 ml of (10 mol / L) H3PO4 into the stirred molecular sieve suspension to maintain the pH between 5 and 6, and continue stirring for 2 hours. b. Filter the reaction solution and wash it three times with methanol. Dry the sample and calcine the treated molecular sieve at 500 °C for 2 h.

[0064] Perform metal ion exchange: a. Prepare a metal ion solution. Dissolve 10 g of MgC12 in 100 ml of deionized water. b. Mix the grafted molecular sieve with the metal ion solution, keep stirring and heat to 80 °C for 24 hours. c. Filter the reaction solution, wash it three times with deionized water, dry the sample, and calcine the treated molecular sieve at 500 °C for 2 h to obtain the modified fly ash-based molecular sieve. The adsorption breakthrough curve is as Figure 3 shown, and adsorption breakthrough occurs near 3000 s.

[0065] Example 3

[0066] Preparation of fly ash-based molecular sieve: a. Grind the fly ash sample into powder, conduct particle size screening, and select the sample with the target particle size. Mix the fly ash with NaOH solution (fly ash: NaOH particles: water = 1 g: 0.5 g: 5 ml), then put the mixture into a hydrothermal autoclave and heat it to 80 °C for hydrothermal reaction for 12 h to carry out hydrothermal melting and melt crystallization to obtain the fly ash-based molecular sieve. b. At room temperature, suspend the molecular sieve powder in deionized water, treat it with ultrasonic waves for 10 h, and continue stirring for 2 - 3 hours to solidify it. d. Wash the obtained molecular sieve sample to neutrality, and then dry it in a constant temperature drying oven until a constant mass is obtained.

[0067] Grafting of phosphate groups: a. Prepare phosphoric acid solution. Slowly drop 80 ml of methanol and 20 ml of (10 mol / L) concentrated H3PO4 into the stirred molecular sieve suspension to maintain the pH between 5 - 6, and continue stirring and reacting for 2 hours. b. Filter the reaction solution and wash it 3 times with methanol. Dry the sample, and calcine the treated molecular sieve at 500 °C for 2 h.

[0068] Carry out metal ion exchange: a. Prepare metal ion solution. Dissolve 10 g of CaCl2 in 100 ml of deionized water. b. Mix the grafted molecular sieve with the metal ion solution, keep stirring and heat it to 80 °C for reaction for 24 hours. c. Filter the reaction solution, wash it 3 times with deionized water, dry the sample, and calcine the treated molecular sieve at 500 °C for 2 h to obtain the modified fly ash-based molecular sieve. The adsorption breakthrough curve is as Figure 4 shown, and adsorption breakthrough is achieved near 3400 s.

[0069] Example 4

[0070] Preparation of fly ash-based molecular sieve: a. Grind the fly ash sample into powder, conduct particle size screening, and select the sample with the target particle size. Mix the fly ash with NaOH solution (fly ash: NaOH particles: water = 1 g: 0.5 g: 5 ml), then put the mixture into a hydrothermal autoclave and heat it to 80 °C for hydrothermal reaction for 12 h to carry out hydrothermal melting and melt crystallization to obtain the fly ash-based molecular sieve. b. At room temperature, suspend the molecular sieve powder in deionized water, treat it with ultrasonic waves for 106, and continue stirring for 2 - 3 hours to solidify it. d. Wash the obtained molecular sieve sample to neutrality, and then dry it in a constant temperature drying oven until a constant mass is obtained.

[0071] Grafting of phosphate groups: a. Prepare phosphoric acid solution. Slowly drop 70 ml of methanol and 30 ml of (10 mol / L) concentrated H3PO4 into the stirred molecular sieve suspension to maintain the pH between 5 - 6, and continue stirring and reacting for 2 hours. b. Filter the reaction solution and wash it 3 times with methanol. Dry the sample, and calcine the treated molecular sieve at 500 °C for 2 h.

[0072] Perform metal ion exchange: a. Prepare a metal ion solution. Dissolve 10 g of CaC12 in 100 ml of deionized water. b. Mix the grafted molecular sieve with the metal ion solution, keep stirring and heat to 80 °C for 24 hours of reaction. c. Filter the reaction solution, wash it 3 times with deionized water, dry the sample, and calcine the treated molecular sieve at 500 °C for 2 h to obtain the modified fly ash-based molecular sieve. The adsorption breakthrough curve is as Figure 5 shown, and adsorption breakthrough is achieved near 2800 s.

[0073] Comparative Example 1

[0074] Prepare fly ash-based molecular sieve: a. Grind the fly ash sample into powder and perform particle size screening, select the sample with a particle size of 200 mesh. Mix the fly ash with NaOH solution (fly ash: NaOH particles: water = 1 g: 0.5 g: 5 ml), then put the mixture into a hydrothermal autoclave and heat to 80 °C for 12 h of hydrothermal reaction, perform hydrothermal melting and melt crystallization to obtain fly ash-based molecular sieve. b. At room temperature, suspend the molecular sieve powder in deionized water, treat it with ultrasonic waves for 10 h, and continue stirring for 2 - 3 hours to solidify it. d. Wash the obtained molecular sieve sample to neutral, and then dry it in a constant temperature drying oven to a constant mass to obtain fly ash-based molecular sieve. The adsorption breakthrough curve is as Figure 6 shown, and adsorption breakthrough is achieved near 2200 s.

[0075] The modified fly ash-based molecular sieves prepared in Examples 1 - 4 and the fly ash-based molecular sieve prepared in Comparative Example 1 both have the ability to target-adsorb carbon dioxide. Based on this, the adsorption breakthrough curve of carbon dioxide was further studied. It can be seen from the comparison of the adsorption breakthrough curves of carbon dioxide that after the modification of grafting phosphate groups and metal ion exchange in the present invention, the time of carbon dioxide adsorption breakthrough is significantly increased, and the flow rate of carbon dioxide is the same, indicating that the adsorption capacity of the modified fly ash-based molecular sieve for carbon dioxide is significantly increased, thus proving that the modification of grafting phosphate groups and metal ion exchange in the present invention can significantly improve the ability of the modified fly ash-based molecular sieve to target-adsorb carbon dioxide.

[0076] Detect the hydrophobic property of the modified fly ash-based molecular sieve through the static water adsorption performance test.

[0077] The specific steps are as follows:

[0078] (1) Weigh a certain mass of the sample and place it in a porcelain boat.

[0079] (2) Put the porcelain boat containing the sample into a forced-air drying oven and pretreat it at 250 °C for 2 h.

[0080] (3) Take out the porcelain boat, cool it at room temperature for 20 - 25 s, then transfer the sample to a pre - weighed weighing bottle, gently cover the bottle cap and immediately place it in a vacuum desiccator.

[0081] (4) Turn on the vacuum pump to make the air pressure in the vacuum desiccator less than 1.0×10 3 Pa, turn off the vacuum pump, and wait for the sample to cool to room temperature.

[0082] (5) Slowly rotate the piston on the lid of the vacuum desiccator to let the atmosphere into the desiccator.

[0083] (6) Open the vacuum desiccator, take out the weighing bottle, and immediately weigh it using an analytical balance.

[0084] (7) Gently shake the sample in the weighing bottle to make it form an even layer, then open the weighing bottle cap and place it in a desiccator containing saturated sodium chloride aqueous solution.

[0085] (8) Put the desiccator into a forced - air drying oven, set the temperature to 35 ± 1 °C, and adsorb water at a constant temperature for 24 h. Open the desiccator lid, immediately cover the weighing bottle cap, take out the weighing bottle, and weigh it. The calculation formula for the water absorption of the adsorbent is as follows:

[0086]

[0087] x: Static water adsorption capacity, %;

[0088] m1: Weight of the weighing bottle, g;

[0089] m2: Weight of the weighing bottle plus the weight of the pre - treated sample, g;

[0090] m3: Weight of the weighing bottle plus the weight of the sample after water absorption, g.

[0091] After the static water adsorption performance test, it shows that the hydrophobicity of the modified fly - ash - based molecular sieves prepared in Examples 1 - 4 of the present invention is increased by 20% compared with the initial unmodified molecular sieves. Thus, it is proved that the modified fly - ash - based molecular sieves of the present invention not only improve the hydrophobicity performance but also improve the carbon dioxide targeted adsorption performance.

[0092] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of a modified fly ash-based molecular sieve, characterized in that, It includes the following steps: Mix fly ash with a solution of alkali metal hydroxide, and then carry out hydrothermal melting and melt crystallization to obtain fly ash-based molecular sieve; Mix the fly ash-based molecular sieve with a solution containing phosphoric acid for reaction to graft phosphate groups on the surface of the molecular sieve, and obtain a fly ash-based molecular sieve grafted with phosphate groups; Add the fly ash-based molecular sieve grafted with phosphate groups to a solution containing alkaline earth metal salt, and carry out stirring reaction. The temperature of the stirring reaction is 60-90 °C, so that the alkali metal cations in the fly ash-based molecular sieve grafted with phosphate groups are replaced by alkaline earth metal cations, thus obtaining; During the reaction of mixing the fly ash-based molecular sieve with a solution containing phosphoric acid, heat is carried out, and the heating temperature is 40-80 °C; After the reaction of mixing the fly ash-based molecular sieve with a solution containing phosphoric acid, heat to 250-450 °C for roasting.

2. The preparation method of the modified fly ash-based molecular sieve according to claim 1, characterized in that During the reaction of mixing the fly ash-based molecular sieve with a solution containing phosphoric acid, methanol is added.

3. The preparation method of the modified fly ash-based molecular sieve according to claim 2, characterized in that, The volume ratio of phosphoric acid to methanol is 2-3:8-7.

4. The preparation method of the modified fly ash-based molecular sieve according to claim 1, characterized in that, During the reaction of mixing the fly ash-based molecular sieve with a solution containing phosphoric acid, the pH is 5-6.

5. The preparation method of the modified fly ash-based molecular sieve according to claim 1, characterized in that, The alkali metal hydroxide is sodium hydroxide, and the alkaline earth metal salt is magnesium salt or calcium salt.

6. The preparation method of the modified fly ash-based molecular sieve according to claim 1, characterized in that, Heat the fly ash-based molecular sieve after being replaced by alkaline earth metal cations to 500 °C for roasting.

7. A modified fly ash-based molecular sieve, characterized in that, Obtained by the preparation method according to any one of claims 1-6.

8. An application of the modified fly ash-based molecular sieve according to claim 7 in gas targeted adsorption, and the gas is carbon dioxide.

9. Use of the modified fly ash-based molecular sieve according to claim 8 in gas targeted adsorption, characterized in that, Pass the gas containing carbon dioxide into the modified fly ash-based molecular sieve for adsorption.

10. Use of the modified fly ash-based molecular sieve according to claim 9 in gas targeted adsorption, characterized in that, The adsorption temperature is 20-30 °C.

11. Use of the modified fly ash-based molecular sieve according to claim 9 in gas targeted adsorption, characterized in that, Before adsorption, the modified fly ash-based molecular sieve is pretreated with nitrogen.

12. Use of the modified fly ash-based molecular sieve according to claim 11 in gas targeted adsorption, characterized in that, The pretreatment temperature is 200-300 °C.

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