CN116920792A - Modified fly ash-based molecular sieve, preparation method and application in gas targeted adsorption - Google Patents

Abstract

The invention belongs to the technical field of resource recycling and environmental protection, relates to a method for preparing a gas adsorption material from industrial solid waste, and in particular relates to a modified fly ash-based molecular sieve, a preparation method and application thereof in gas targeted adsorption. The preparation method comprises the following steps: mixing fly ash with alkali metal hydroxide solution, and then carrying out hydrothermal melting and melt crystallization to obtain a fly ash-based molecular sieve; mixing the fly ash-based molecular sieve with a solution containing phosphoric acid for reaction, so that a phosphoric acid group is grafted on the surface of the molecular sieve, and obtaining the fly ash-based molecular sieve grafted with the phosphoric acid group; adding the fly ash-based molecular sieve grafted with the phosphate groups into a solution containing alkaline earth metal salt, and carrying out stirring reaction so that alkali metal cations in the fly ash-based molecular sieve grafted with the phosphate groups are replaced by alkaline earth metal cations. The invention provides a modified flyThe ash-based molecular sieve not only has higher CO ₂ Adsorption selectivity and increased hydrophobicity of molecular sieve, thereby increasing CO ₂ Is used for the target adsorption capacity of the (a).

Description

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

Technical field

The 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, it relates to modified fly ash-based molecular sieves, preparation methods and applications in gas targeted adsorption.

Background technique

The information in this Background section is disclosed solely for the purpose of increasing understanding of the general background of the invention and is not necessarily considered to be an admission or in any way implying that the information constitutes prior art that is already known to a person of ordinary skill in the art.

At present, most of the mainstream resource utilization methods of fly ash remain in the stage of large-scale and low-added value utilization, such as the preparation of construction and filling materials, soil fertilization and pH adjustment. There are few high-value and functional utilization plans for fly ash. . Si and Al in fly ash are the main elements of the molecular sieve skeleton, and their specific surface area can reach 300-500m ² /kg. Therefore, fly ash can be used as a raw material for preparing molecular sieves, and can be effectively utilized in high-value functional applications.

Molecular sieves have excellent adsorption properties, but the presence of compensating cations and Si-OH outside the skeleton causes polar molecules to strongly adsorb on the surface of the molecular sieve, resulting in strong hydrophilic properties of the molecular sieve. During the adsorption process, there is competitive adsorption between molecules. 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, which will lead to the weakening of the molecular sieve's ability to adsorb other gases. According to the inventor's research, in-situ dealumination is currently mainly achieved through acid-base modification, hydrothermal treatment and other methods, and then the silicon-to-aluminum ratio is increased to improve the hydrophobic performance of molecular sieves. However, increasing the silicon-to-aluminum ratio often induces an increase in the molecular sieve skeleton pores. This results in a mismatch between the effective diameter of the molecular sieve channels and the molecular dynamics diameter of _CO2 . Therefore, maintaining the CO ₂ shape-selective effect of molecular sieves while improving the hydrophobicity of molecular sieves is a difficult problem in this field to prepare molecular sieves for targeted adsorption of CO ₂ .

Contents of the invention

In order to solve the deficiencies of the existing technology, the purpose of the present invention is to provide modified fly ash-based molecular sieves, preparation methods and applications in gas targeted adsorption. The modified fly ash-based molecular sieves provided by the present invention not only have higher CO ₂ The adsorption selectivity is improved, and the hydrophobicity of the molecular sieve is improved, thereby improving the targeted adsorption capacity of CO ₂ .

In order to achieve the above objects, the technical solution of the present invention is:

On the one hand, a method for preparing modified fly ash-based molecular sieves includes the following steps:

Mix fly ash and a solution of alkali metal hydroxide, and then perform hydrothermal melting and melt crystallization to obtain fly ash-based molecular sieve;

The fly ash-based molecular sieve is mixed with a solution containing phosphoric acid for reaction, so that the phosphate group is grafted on the surface of the molecular sieve, and a fly ash-based molecular sieve grafted with the phosphate group is obtained;

The fly ash-based molecular sieve grafted with a phosphate group is added to a solution containing an alkaline earth metal salt, and a stirring reaction is carried out so that the alkali metal cations in the fly ash-based molecular sieve grafted with a phosphate group are replaced by alkaline earth metal cations.

First of all, the fly ash-based molecular sieve of the present invention is used as a raw material for molecular sieve modification because fly ash contains a small amount of Ca, Mn, Ti, etc., which can enhance the stability of the molecular sieve lattice, regulate surface redox properties, and build targeted adsorption sites. Play an important role in other aspects, so that the molecular sieve has better CO ₂ affinity.

Secondly, the present invention uses phosphate groups grafted on fly ash-based molecular sieves, which not only can selectively adsorb water molecules through the phosphate groups and improve the hydrophobic properties of fly ash, but also can absorb electrons between metal and oxygen on the surface of fly ash-based molecular sieves. The transfer direction/rate produces a regulatory effect, thereby modulating the polarity of the adsorption sites on the surface of the molecular sieve without changing the silicon-aluminum ratio of the molecular sieve. It can also directionally regulate the targeted adsorption capacity of fly ash-based molecular sieves for CO ₂ .

Thirdly, the present invention uses alkaline earth metal cations instead of alkali metal cations to reconstruct the adsorption sites. Not only can fly ash molecular sieves be added to target capture _CO2 molecules in molecules with the same dynamic diameter, but the increase in valence can also accelerate the surface and The electrostatic behavior between polar molecules simultaneously promotes acid-base pairing and strengthens the specific capture effect of polar-acidic gases on the surface of the molecular sieve.

Thus, the modified fly ash-based molecular sieve prepared in the present invention solves the conflicting problem between low silicon-to-aluminum ratio and high hydrophobicity, thereby simultaneously improving the hydrophobicity of the molecular sieve and its ability to target CO ₂ adsorption.

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

The third aspect is an application of the above-mentioned modified fly ash-based molecular sieve in targeted adsorption of gas, where the gas is carbon dioxide.

The beneficial effects of the present invention are:

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 phosphate group grafting and alkaline earth metal ion exchange, the hydrophobicity of the fly ash-based molecular sieves and Targeted adsorption capacity of CO ₂ .

Description of the drawings

The description and drawings that constitute a part of the present invention are used to provide a further understanding of the present invention. The illustrative 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.

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

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

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

Figure 4 is the adsorption breakthrough curve for CO ₂ of the modified fly ash-based molecular sieve adsorbent prepared in Example 3 of the present invention;

Figure 5 is the adsorption breakthrough curve for CO ₂ of the modified fly ash-based molecular sieve adsorbent prepared in Example 4 of the present invention;

Figure 6 is the adsorption breakthrough curve for CO ₂ of the fly ash-based molecular sieve adsorbent prepared in Comparative Example 1 of the present invention.

Detailed ways

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

It should be noted that the terms used herein are for the purpose of describing specific embodiments only, and are not intended to limit the exemplary embodiments according to the present invention. As used herein, the singular forms are also intended to include the plural forms unless the context clearly indicates otherwise. Furthermore, it will be understood that when the terms "comprises" and/or "includes" are used in this specification, they indicate There are features, steps, operations, means, components and/or combinations thereof.

The fly ash of the present invention is collected from the electrostatic precipitator of the coal-fired flue gas treatment system.

In view of the contradiction between low silicon-to-aluminum ratio and high hydrophobicity in molecular sieves as gas targeted adsorbents, which makes it difficult for molecular sieves to simultaneously improve hydrophobicity and targeted adsorption of _CO2 , the present invention proposes modified fly ash-based molecular sieves and Preparation method and application in gas targeted adsorption.

A typical embodiment of the present invention provides a method for preparing modified fly ash-based molecular sieves, which includes the following steps:

Mix fly ash and a solution of alkali metal hydroxide, and then perform hydrothermal melting and melt crystallization to obtain fly ash-based molecular sieve;

The fly ash-based molecular sieve is mixed with a solution containing phosphoric acid for reaction, so that the phosphate group is grafted on the surface of the molecular sieve, and a fly ash-based molecular sieve grafted with the phosphate group is obtained;

The fly ash-based molecular sieve grafted with a phosphate group is added to a solution containing an alkaline earth metal salt, and a stirring reaction is carried out so that the alkali metal cations in the fly ash-based molecular sieve grafted with a phosphate group are replaced by alkaline earth metal cations.

When the fly ash-based molecular sieve is mixed with a solution containing phosphoric acid for reaction, the phosphate groups are grafted on the surface of the molecular sieve slowly. In order to speed up the reaction, in some embodiments, the fly ash-based molecular sieve is mixed with a solution containing phosphoric acid for reaction. During the process, heating is performed, and the heating temperature reaches 40 to 80°C. The reaction can be accelerated by increasing the temperature. When the reaction temperature is too high, the solvent water evaporates quickly, and the reaction system can be sealed to avoid solvent evaporation losses.

In order to speed up the grafting of phosphate groups onto the surface of the molecular sieve, in some embodiments, methanol is added during the reaction between the fly ash-based molecular sieve and the solution containing phosphoric acid. Using methanol as a reducing agent can increase the grafting rate of phosphate groups.

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

In one or more embodiments, when the fly ash-based molecular sieve and the solution containing phosphoric acid are mixed and reacted, the pH is 5-6. The reaction time is 2 to 3 hours.

The way in which phosphate groups are grafted onto the surface of the molecular sieve includes chemical bonding and physical connection. The physical connection is less stable, thus affecting the stability of the hydrophobic performance. In some embodiments, fly ash-based molecular sieves and phosphoric acid-containing After the solution is mixed and reacted, it is heated to 250 to 450°C for roasting. It is conducive to changing the physical connection mode to chemical bond connection, so that the phosphate groups on the surface of the fly ash-based molecular sieve can be more firmly grafted on the surface of the molecular sieve and improve its stability.

In some embodiments, the fly ash-based molecular sieve grafted with phosphate groups is added to a solution containing an alkaline earth metal salt, and the temperature for the stirring reaction is 60 to 90°C. When the reaction temperature is too high, the solvent water evaporates quickly, and the reaction system can be sealed to avoid solvent evaporation losses.

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 hydrogen Sodium oxide, alkaline earth metal salt is magnesium salt or calcium salt. Studies have shown that using fly ash-based molecular sieves prepared with sodium hydroxide and then using magnesium salts or calcium salts for cation exchange can better achieve targeted carbon dioxide adsorption.

In some embodiments, the alkaline earth metal cation-substituted fly ash-based molecular sieve is heated to 500°C for roasting. The metal ions on the surface can be more firmly embedded in the fly ash-based molecular sieve, thereby improving the stability of the modified fly ash-based molecular sieve.

Another embodiment of the present invention provides a modified fly ash-based molecular sieve, which is obtained by the above preparation method.

The third embodiment of the present invention provides an application of the above-mentioned modified fly ash-based molecular sieve in targeted adsorption of gas, where the gas is carbon dioxide.

Specifically, modified fly ash-based molecular sieves were used as carbon dioxide gas targeted adsorbents.

Specifically, gas containing carbon dioxide is passed into the modified fly ash-based molecular sieve for adsorption. The adsorption temperature is 20~30℃. Before adsorption, the modified fly ash-based molecular sieve is pretreated with nitrogen. The pretreatment temperature is 200~300℃.

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

The adsorption performance test was performed on the modified fly ash-based molecular sieve adsorbent prepared in the following examples. The fixed bed reaction test device used in the test is shown in Figure 1, which consists of a gas cylinder 1, a gas valve 2, a mass flow controller 3, a water steam generator 4, a hygrometer 5, a fixed bed reactor 6, and a gas chromatograph. 7 and computer 8.

The adsorbent is first pretreated under nitrogen purge at a pretreatment temperature of 250°C and a time of 2 hours, and then an adsorption test is performed at 25°C and 1 atmospheric pressure. The reaction inlet is CO ₂ and the inlet and outlet of the fixed bed reactor are tested respectively. concentration to calculate the adsorption capacity. The calculation formula for _CO2 adsorption capacity is as follows:

Among them, v: adsorption capacity of adsorbent, mg/g;

F: gas volume flow rate, mL/min;

M: Adsorbate molar mass, g/mol;

m: mass of adsorbent, g;

C ₀ : gas inlet concentration, ppm;

_Ct : gas outlet concentration at time t, ppm.

Judging from the calculation formula, the _CO2 flow rate is the same and the gas concentration is the same. The longer the adsorption breakthrough time, the greater the adsorption amount.

Example 1

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

Grafting phosphate groups: a Prepare phosphoric acid solution. Slowly drop 80 ml of methanol and 20 ml (10 mol/L) concentrated H ₃ PO ₄ 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 with methanol three times. The sample was dried, and the treated molecular sieve was calcined at a high temperature of 500°C for 2 h.

Perform metal ion exchange: a Prepare metal ion solution. Dissolve 10g _MgCl2 in 100ml 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 and wash it three times with deionized water, dry the sample, and roast the treated molecular sieve at a high temperature of 500°C for 2 hours to obtain a modified fly ash-based molecular sieve. The adsorption breakthrough curve is shown in Figure 2, and the adsorption breakthrough is achieved near 4000s.

Example 2

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

Grafting phosphate groups: a Prepare phosphoric acid solution. Slowly drop 70 ml of methanol and 30 ml (10 mol/L) H ₃ PO ₄ 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 with methanol three times. The sample was dried, and the treated molecular sieve was calcined at a high temperature of 500°C for 2 hours.

Perform metal ion exchange: a Prepare metal ion solution. Dissolve _10gMgCl2 in 100ml 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 and wash it three times with deionized water, dry the sample, and roast the treated molecular sieve at a high temperature of 500°C for 2 hours to obtain a modified fly ash-based molecular sieve. The adsorption breakthrough curve is shown in Figure 3, and the adsorption breakthrough is achieved near 3000s.

Example 3

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

Grafting phosphate groups: a Prepare phosphoric acid solution. Slowly drop 80 ml of methanol and 20 ml (10 mol/L) concentrated H ₃ PO ₄ 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 with methanol three times. The sample was dried, and the treated molecular sieve was calcined at a high temperature of 500°C for 2 h.

Perform metal ion exchange: a Prepare metal ion solution. Dissolve _10gCaCl2 in 100ml 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 and wash it three times with deionized water, dry the sample, and roast the treated molecular sieve at a high temperature of 500°C for 2 hours to obtain a modified fly ash-based molecular sieve. The adsorption breakthrough curve is shown in Figure 4, and the adsorption breakthrough is achieved near 3400s.

Example 4

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

Grafting phosphate groups: a Prepare phosphoric acid solution. Slowly drop 70 ml of methanol and 30 ml (10 mol/L) concentrated H ₃ PO ₄ 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 with methanol three times. The sample was dried, and the treated molecular sieve was calcined at a high temperature of 500°C for 2 h.

Perform metal ion exchange: a Prepare metal ion solution. Dissolve _10gCaCl2 in 100ml 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 and wash it three times with deionized water, dry the sample, and roast the treated molecular sieve at a high temperature of 500°C for 2 hours to obtain a modified fly ash-based molecular sieve. The adsorption breakthrough curve is shown in Figure 5, and the adsorption breakthrough is achieved near 2800s.

Comparative example 1

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

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

The hydrophobic properties of modified fly ash-based molecular sieves were tested through static adsorption water performance testing.

Specific steps are as follows:

(1) Weigh a certain mass of sample and place it in a porcelain ark.

(2) Place the porcelain ark containing the sample into a blast drying oven and pretreat it at 250°C for 2 hours.

(3) Take out the porcelain ark, cool it at room temperature for 20-25 seconds, then move the sample to the weighed weighing bottle, lightly cover the bottle cap and immediately place it in a vacuum desiccator.

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

(5) Slowly rotate the piston on the vacuum dryer cover to allow the atmosphere to flow into the dryer.

(6) Open the vacuum dryer, take out the weighing bottle, and weigh it immediately with an analytical balance.

(7) Gently shake the sample in the weighing bottle to spread it into a uniform layer, then open the weighing bottle cap and place it in a desiccator filled with saturated sodium chloride aqueous solution.

(8) Place the dryer into the blast drying box, set the temperature to 35±1°C, and adsorb at a constant temperature for 24 hours. Open the dryer cover, immediately close the weighing bottle cap, take out the weighing bottle, and weigh. The formula for calculating the water absorption capacity of the adsorbent is as follows:

x: static water adsorption capacity, %;

m ₁ : Weighing bottle weight, g;

m ₂ : weight of the weighing bottle plus sample after pretreatment, g;

m ₃ : Weight of the sample after adding water to the weighing bottle, g.

Static adsorption water performance testing shows that the hydrophobicity of the modified fly ash-based molecular sieves prepared in Examples 1 to 4 of the present invention is 20% higher than that of the initial unmodified molecular sieves. This proves that the modified fly ash-based molecular sieves of the present invention not only improve Hydrophobic properties, and improved carbon dioxide targeted adsorption performance.

The above descriptions are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims (10)

1. A method for preparing modified fly ash-based molecular sieve, which is characterized by comprising the following steps: Mix fly ash and a solution of alkali metal hydroxide, and then perform hydrothermal melting and melt crystallization to obtain fly ash-based molecular sieve; The fly ash-based molecular sieve is mixed with a solution containing phosphoric acid for reaction, so that the phosphate group is grafted on the surface of the molecular sieve, and a fly ash-based molecular sieve grafted with the phosphate group is obtained; The fly ash-based molecular sieve grafted with a phosphate group is added to a solution containing an alkaline earth metal salt, and a stirring reaction is carried out so that the alkali metal cations in the fly ash-based molecular sieve grafted with a phosphate group are replaced by alkaline earth metal cations. 2. The preparation method of modified fly ash-based molecular sieve as claimed in claim 1, characterized in that, during the reaction process of the fly ash-based molecular sieve and the solution containing phosphoric acid being mixed, heating is performed, and the heating temperature is between 40 and 80°C; Or, in the process of mixing the fly ash-based molecular sieve with the solution containing phosphoric acid for reaction, methanol is added; Preferably, the volume ratio of phosphoric acid to methanol is 2-3:8-7; Preferably, when the fly ash-based molecular sieve and the solution containing phosphoric acid are mixed and reacted, the pH is 5 to 6. 3. The preparation method of modified fly ash-based molecular sieve according to claim 1, characterized in that, after the fly ash-based molecular sieve is mixed and reacted with a solution containing phosphoric acid, it is heated to 250-450°C for roasting. 4. The preparation method of modified fly ash-based molecular sieve as claimed in claim 1, characterized in that the fly ash-based molecular sieve grafted with phosphate groups is added to a solution containing alkaline earth metal salts, and the temperature for the stirring reaction is 60 to 90°C. ℃. 5. The preparation method of 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 modified fly ash-based molecular sieve according to claim 1, characterized in that the fly ash-based molecular sieve substituted by alkaline earth metal cations is heated to 500 °C for roasting. 7. A modified fly ash-based molecular sieve, characterized in that it is obtained by the preparation method described in any one of claims 1 to 6. 8. Application of the modified fly ash-based molecular sieve according to claim 7 in targeted adsorption of gas, where the gas is carbon dioxide. 9. The application of the modified fly ash-based molecular sieve in gas targeted adsorption as claimed in claim 8, characterized in that the modified fly ash-based molecular sieve is used as a carbon dioxide gas targeted adsorbent. 10. The application of the modified fly ash-based molecular sieve in gas targeted adsorption as claimed in claim 8, characterized in that gas containing carbon dioxide is passed into the modified fly ash-based molecular sieve for adsorption; preferably, The adsorption temperature is 20-30°C; preferably, the modified fly ash-based molecular sieve is pre-treated with nitrogen before adsorption; further preferably, the pre-treatment temperature is 200-300°C.