CN116328749A - Modified magnesium-aluminum hydrotalcite catalyst and preparation method and application thereof - Google Patents

Abstract

The application relates to the technical field of methane cracking catalyst preparation, in particular to a preparation method of a modified magnesium aluminum hydrotalcite catalyst, which comprises the following steps: (1) raw material mixing: mixing a salt solution and a polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer solution, and uniformly stirring to obtain a reaction solution, wherein the salt solution is formed by mixing aluminum salt, magnesium salt and water; (2) precipitation crystallization: adding a precipitant into the reaction solution to perform coprecipitation reaction, performing hydrothermal crystallization after the reaction is finished, and performing suction filtration, washing, drying and calcination to obtain modified hydrotalcite; (3) dipping: and adding a mixed solution formed by nickel salt, lanthanum salt, citric acid and water into the modified hydrotalcite, soaking and stirring, and drying and calcining to obtain the lanthanum nickelate loaded modified hydrotalcite catalyst. The preparation method of the modified magnesium aluminum hydrotalcite catalyst can slow down the coking of the catalyst and promote the catalytic activity of the catalyst.

Description

A kind of modified magnesium aluminum hydrotalcite catalyst and its preparation method and application

technical field

The present application relates to the technical field of methane cracking catalyst preparation, in particular to a modified magnesium aluminum hydrotalcite catalyst and its preparation method and application.

Background technique

As a one-dimensional quantum material, carbon nanotubes are widely used in lithium-ion battery conductive agents, polymer composite materials, and catalyst supports due to their low density, excellent mechanical properties, electrical conductivity, and good physical and chemical stability. and other fields have broad application prospects. In addition, carbon nanotubes also have other good properties such as optical properties and hydrogen storage, making carbon nanotubes considered to be ideal reinforcement materials for polymer composites. The methane catalytic cracking process can simultaneously obtain carbon nanotubes and hydrogen without carbon oxides, and its atomic utilization rate reaches the maximum value, so this technical route has attracted widespread attention. In order to improve the catalytic efficiency of methane cracking, there are more and more researches on related reaction catalysts.

Currently, methane cracking catalysts mainly use noble metals (Pt, Pd) and a small amount of non-noble metals (Ni, Cu, Fe) as active sites. Noble metal catalysts have good catalytic activity, but cannot be mass-produced due to their high price. Non-noble metal catalysts reduce the reaction cost and at the same time can obtain a relatively ideal reactivity, so they have been widely used. Existing catalysts are prepared by using silicon-aluminum-titanium composite oxide as a carrier, precious metal palladium as an active component, and glass fiber as a reinforcing agent, and then extruding a honeycomb catalyst and drying and calcining. The catalyst not only has a strong mechanical Intensity and good methane hydrogen production activity, but the use of noble metal catalysts greatly increases the process cost. In addition, with fly ash as carrier, metal Ni and Fe as active components, CeO2 as metal additive, Ni-Fe-CeO2 fly ash catalyst was prepared by impregnation after co-precipitation and applied in methane cracking. Although it has good cycle performance, its catalytic activity is not very high. There is also a method to improve the production of hydrogen and carbon nanotubes by changing the carbon source. The catalyst used is a nickel-based and iron-based catalyst prepared by the sol-gel method. The selected carbon source includes methane and multi-carbon alkanes. Through multi-carbon The effect of alkanes on the cracking of the reaction system can significantly promote the cracking of methane, increase the production efficiency of carbon nanotubes, and reduce the production cost, but the conversion rate of methane is low, which is always about 49%. Therefore, the development of highly active, highly stable, and non-sintering-resistant carbon deposition catalysts is still a key issue for large-scale catalytic cracking of methane for hydrogen production.

Contents of the invention

In order to slow down the coking of the catalyst and improve the catalytic activity of the catalyst, the present application provides a modified magnesium aluminum hydrotalcite catalyst and its preparation method and application.

A kind of modified magnesium aluminum hydrotalcite catalyst provided by the application and its preparation method and application adopt the following technical scheme:

In a first aspect, the application provides a method for preparing a modified magnesium aluminum hydrotalcite catalyst, comprising the following steps:

(1) Mixing of raw materials: mix the salt solution, polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer solution, and stir evenly to obtain a reaction solution. The salt solution is composed of aluminum salt, magnesium salt mixed with water to form;

(2) Precipitation and crystallization: adding a precipitating agent to the reaction solution for co-precipitation reaction, hydrothermal crystallization after the reaction, and then suction filtration, washing, drying, and calcination to obtain modified hydrotalcite;

(3) Impregnation: adding a mixed solution of nickel salt, lanthanum salt, citric acid and water to the modified hydrotalcite, drying and calcining after impregnating and stirring to obtain a modified hydrotalcite catalyst supported by lanthanum nickelate.

This application adopts polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer solution as a template agent, so that it can be inserted into the laminate space of the precursor hydrotalcite during the co-precipitation process. The thermal process ensures the stable forming process of the crystal, and then the template agent is removed by calcination to obtain a composite oxide carrier with a porous structure, which fundamentally improves the structure of the catalyst, exposes more reduction sites, and improves the surface loading of nickel and lanthanum. The dispersion of the metal improves the catalytic activity. The average pore size of the catalyst is large, which is conducive to the diffusion of reactant molecules, slows down the coking of the catalyst, greatly improves the adsorption capacity of methane, and improves the reactivity of the catalyst; at the same time, the catalyst preparation method Simple and operable, the carbon nanotubes prepared by using the catalyst to catalyze the cracking of methane have uniform diameter and high yield

Preferably, in the step (1), the molar ratio of aluminum ions to magnesium ions in the salt solution is 1:(2-4), the aluminum salt is aluminum nitrate, and the magnesium salt is magnesium nitrate.

Preferably, the concentration of the polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer solution is 2 to 5 g/L, and the polyethylene oxide-polypropylene oxide-polyethylene oxide The volume ratio of the ethylene oxide triblock copolymer solution to the salt solution is 1:(1-3).

By adjusting the volume ratio of the polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer solution to the salt solution, the modified hydrotalcite can be further promoted to form a composite oxide carrier with a porous structure, from Improve the internal structure of the catalyst fundamentally.

Preferably, in the step (2), the precipitation agent is a sodium carbonate solution, and the concentration of the sodium carbonate solution is 0.5-1.5 mol/L.

Preferably, in the step (2), the hydrothermal crystallization temperature is 200-220°C, and the hydrothermal crystallization time is 10-20h; the drying temperature is 90-150°C, and the drying time is 8 ~12 hours; the calcination temperature is 600-750° C., and the calcination time is 4-5 hours.

Preferably, in the step (3), the nickel salt is nickel nitrate, the lanthanum salt is lanthanum nitrate, and the molar ratio of nickel ions to lanthanum ions in the mixed solution is 4: (5-6); The total ion molar ratio of said citric acid to nickel ions and lanthanum ions is 1:1.

Preferably, in the step (3), the immersion stirring time is 8-12 hours, the immersion temperature is 35-55° C.; the drying temperature is 80-120° C., and the drying time is 18-24 hours.

Preferably, in the step (3), the calcination includes: first decomposing at 450-500°C for 1-2 hours at a heating rate of 2-3°C/min, and then raising the temperature to 650-750°C at the same heating rate , Calcined 4 ~ 8h.

In order to increase the loading degree of metal nickel and lanthanum in the catalyst, it is necessary to control the impregnation time, and at the same time adopt the method of programmed segmental calcination, so that a strong force is generated between the metal nickel, lanthanum and the oxides of magnesium and aluminum, thereby reducing the The agglomeration among the metal particles improves the overall catalytic activity.

The second aspect of the present invention provides a modified magnesium aluminum hydrotalcite catalyst prepared by the above preparation method.

In the third aspect of the present invention, the prepared modified magnesium aluminum hydrotalcite catalyst is placed in a fluidized bed and reduced under a reducing atmosphere, the reducing atmosphere is hydrogen, the reducing space velocity is 0.2 to 0.5/h, and the reducing temperature The temperature of the fluidized bed is 600-650°C; then the temperature of the fluidized bed is raised to 650-700°C, and methane is introduced for cracking reaction. The flow rate of methane is 0.3-0.5m ³ /h, and the cracking reaction time is 180-210min to obtain hydrogen and carbon nanotube solid materials.

The modified magnesium aluminum hydrotalcite catalyst prepared by this application is applied to the preparation of hydrogen and carbon nanotubes by methane cracking. Through the adjustment of the preparation parameters, the catalyst can achieve the optimal catalytic effect, and effectively improve the efficiency of hydrogen and carbon nanotube solid materials. Yield.

The application has the following beneficial technical effects:

1. The lanthanum nickelate modified magnesium aluminum hydrotalcite catalyst prepared by the present application adopts the polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer solution as a template to make it co-precipitate During the process, the laminate space of the precursor hydrotalcite can be inserted to ensure the stable forming process of the crystal through the hydrothermal process, and then the template agent is removed by calcination to obtain a composite oxide carrier with a porous structure, which fundamentally improves the structure of the catalyst. It exposes more reduction sites, improves the dispersion of nickel and lanthanum metals on the surface, thereby improving the catalytic activity, and the average pore size of the catalyst is large, which is conducive to the diffusion of reactant molecules, slows down the coking of the catalyst, and greatly improves the resistance to methane. The adsorption capacity of the catalyst improves the reactivity of the catalyst; at the same time, the preparation method of the catalyst is simple and the operability is strong, and the carbon nanotubes prepared by using the catalyst to catalyze the cracking of methane have uniform pipe diameter and high yield.

2. In the impregnation and activation step, in order to increase the loading degree of metal nickel and lanthanum in the catalyst, it is necessary to control the impregnation time, and at the same time adopt the method of segmental calcination to make the metal nickel, lanthanum and magnesium, aluminum oxides. A strong force is generated, thereby reducing the agglomeration between metal particles and improving the overall catalytic activity.

3. Apply the modified magnesium aluminum hydrotalcite catalyst prepared by this application to the preparation of hydrogen and carbon nanotubes by methane cracking. By adjusting the preparation parameters, the catalyst can achieve the optimal catalytic effect and effectively improve the solid state of hydrogen and carbon nanotubes. material yield.

Description of drawings

Figure 1 is a comparison chart of specific surface area and average pore diameter curves of two catalysts, LaNiO ₃ and LaNiO ₃ @MgAl-LDO.

Detailed ways

As the research enthusiasm for carbon nanotubes continues to increase, the demand for carbon nanotubes continues to increase. The existing carbon nanotube preparation methods use the method of cracking methane to obtain carbon nanotubes and hydrogen. In order to improve the cracking of methane into carbon nanotubes conversion rate, more and more workers began to invest in the research of its reaction catalyst. The inventors found in the research that when the magnesium aluminum hydrotalcite modified by lanthanum nickelate is used as a methane cracking catalyst, it not only has high catalytic activity but also has low preparation cost, which can meet the needs of industrial production of carbon nanotubes.

The present application will be further described below in conjunction with the examples.

Example 1

A preparation method of a modified magnesium aluminum hydrotalcite catalyst, comprising the following steps:

(1) Mixing of raw materials: mix the salt solution, polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer solution, and stir evenly to obtain a reaction solution. The salt solution is composed of aluminum salt, magnesium salt Mix with water to form.

Specifically, aluminum salt, magnesium salt and water are mixed to form a salt solution. The aluminum salt can be aluminum nitrate or aluminum chloride, which is specifically selected as aluminum nitrate in this application; the magnesium salt can be magnesium nitrate or magnesium chloride, which is specifically selected in this application. to magnesium chloride. The molar ratio of Al ³⁺ and Mg ²⁺ in the salt solution is 1:2, the Al ³⁺ in the salt solution is 1mol/L, and the Mg ²⁺ is 2mol/L, and the salt solution and 2g/L polyethylene oxide - Polypropylene oxide-polyethylene oxide triblock copolymer (P123) solution and 50 mL of deionized water were mixed and stirred to obtain a reaction solution. The volume ratio of the P123 solution to the saline solution is 1:(1-3). In this application, the volume ratio of the P123 solution to the saline solution is specifically selected as 1:2, that is, the P123 solution is 25mL and the saline solution is 50mL.

By adjusting the volume ratio of the polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer solution to the salt solution, the modified hydrotalcite can be further promoted to form a composite oxide carrier with a porous structure, from The internal structure of the catalyst is fundamentally improved, thereby improving the catalytic efficiency of the catalyst.

(2) Precipitation and crystallization: add a precipitant to the reaction solution to carry out co-precipitation reaction, conduct hydrothermal crystallization after the reaction, and then undergo suction filtration, washing, drying, and calcination to obtain modified hydrotalcite.

Concrete, in the reaction solution that step (1) obtains, gradually add precipitant dropwise and carry out co-precipitation reaction, precipitant can be the mixed solution of sodium carbonate solution or sodium hydroxide and sodium carbonate, specifically select as sodium carbonate solution among the application . The concentration of the sodium carbonate solution can be 0.5-1.5 mol/L, specifically selected as 1 mol/L in this application. The sodium carbonate solution was added dropwise until the pH value of the solution was 10, the dropwise addition was stopped, and the temperature was stirred at 30° C. for 30 min, and a suspension was obtained after the reaction was completed. Transfer the suspension into a hydrothermal synthesis kettle, crystallize at 200°C for 10 hours, filter the crystallized suspension with suction, wash, and dry at 90-150°C for 8-12 hours, specifically selected in this application In order to dry at 120°C for 8h, then send it into a muffle furnace and calcinate at 600°C for 5h to obtain template-modified magnesium-aluminum hydrotalcite composite metal oxide (MgAl-LDO).

(3) Impregnation: adding a mixed solution of nickel salt, lanthanum salt, citric acid and water to the modified hydrotalcite, drying and calcining after impregnating and stirring to obtain a modified hydrotalcite catalyst supported by lanthanum nickelate.

Specifically, nickel salt, lanthanum salt, citric acid and water are mixed to form a mixed solution, the nickel salt can be nickel nitrate or nickel chloride, specifically selected as nickel nitrate in this application; the lanthanum salt can be lanthanum nitrate or lanthanum chloride, In this application, the specific choice is lanthanum nitrate; the molar ratio of nickel ions (Ni ³⁺ ) and lanthanum ions (La ³⁺ ) in the mixed solution is 4:6, that is, the Ni ³⁺ in the mixed solution is 4mol/L, and the La ³⁺ The molar ratio of citric acid to the sum of Ni ³⁺ and La ³⁺ is 1:1, that is, the citric acid in the mixed solution is 10 mol/L. Add the mixed solution to the MgAl-LDO prepared in step (2) and immerse and stir at 35-45°C for 8-12 hours. In this application, the specific choice is to immerse and stir at 45°C for 8 hours, and then dry at 80-120°C for 18-18 hours. 24h. In this application, the specific choice is to dry at 120°C for 18h, and then decompose at 500°C at a heating rate of 2°C/min for 1-2h. In this application, the specific choice is to decompose for 1h, and then use the same heating rate to 750 ℃, calcining for 4-8 hours, specifically selected as calcining for 6 hours in this application, to obtain lanthanum nickelate modified magnesium aluminum hydrotalcite catalyst LaNiO ₃ @MgAl-LDO.

In the impregnation activation step, in order to increase the loading degree of metal nickel and lanthanum in the catalyst, it is necessary to control the impregnation time, and at the same time adopt the method of programmed segmental calcination, so that the metal nickel, lanthanum and the oxides of magnesium and aluminum are strong. Force, thereby reducing the agglomeration between metal particles and improving the overall catalytic activity.

The LaNiO ₃ @MgAl-LDO catalyst prepared in Example 1 was applied to methane cracking reaction.

Specifically, take 2g of self-prepared LaNiO ₃ @MgAl-LDO and place it in a fluidized bed for reduction under a hydrogen atmosphere. 600-650°C, specifically selected as 600°C in this application, then raise the temperature of the fluidized bed to 700°C, feed methane to carry out the cracking reaction, the flow rate of methane is 0.3m ³ /h; the cracking reaction time is 180min , to obtain hydrogen and carbon nanotube solid materials.

The modified magnesium aluminum hydrotalcite catalyst prepared by this application is applied to the preparation of hydrogen and carbon nanotubes by methane cracking. Through the adjustment of the preparation parameters, the catalyst can achieve the optimal catalytic effect, and effectively improve the efficiency of hydrogen and carbon nanotube solid materials. Yield.

Example 2

(1) Raw material mixing:

Mix aluminum nitrate, magnesium nitrate and water to form a salt solution, the molar ratio of Al ³⁺ and Mg ²⁺ in the salt solution is 1:3, the concentration of Al ³⁺ in the salt solution is 1mol/L, and the concentration of Mg ²⁺ is 3mol/L. The salt solution was mixed and stirred with 3 g/L P123 solution and 50 mL deionized water to obtain a reaction solution.

(2) Precipitation and crystallization:

In the reaction solution obtained in step (1), sodium carbonate solution is gradually added dropwise to carry out co-precipitation reaction. The sodium carbonate solution was added dropwise until the pH value of the solution was 10, the dropwise addition was stopped, and the temperature was stirred at 30° C. for 30 min, and a suspension was obtained after the reaction was completed. The suspension was transferred to a hydrothermal synthesis kettle, and crystallized at 180°C for 12 hours. The crystallized suspension was suction filtered, washed, and dried at 120°C for 8 hours, and then sent to a muffle furnace. Calcined at 700° C. for 4 hours to obtain template-modified magnesium-aluminum hydrotalcite composite metal oxide (MgAl-LDO).

(3) Dipping:

Mix nickel nitrate, lanthanum nitrate, citric acid and water to form a mixed solution. The molar ratio of nickel ions (Ni ³⁺ ) to lanthanum ions (La ³⁺ ) in the mixed solution is 4:5, that is, the Ni ³⁺ in the mixed solution is 4mol/L, La ³⁺ is 5mol/L, and the molar ratio of citric acid to the sum of Ni ³⁺ and La ³⁺ is 1:1, that is, the citric acid in the mixture is 9mol/L. Add the mixed solution to the MgAl-LDO prepared in step (2), impregnate and stir at 45 °C for 8 h, then dry at 120 °C for 18 h, then decompose at 500 °C for 1 h at a heating rate of 3 °C/min, and then use the same The heating rate was raised to 750°C and calcined for 6 hours to obtain the lanthanum nickelate modified magnesium aluminum hydrotalcite catalyst LaNiO ₃ @MgAl-LDO.

The LaNiO ₃ @MgAl-LDO catalyst prepared in Example 2 was applied to methane cracking reaction.

Specifically, 3 g of self-prepared LaNiO ₃ @MgAl-LDO was placed in a fluidized bed, and reduced under a hydrogen atmosphere with a reduction space velocity of 0.2/h and a reduction temperature of 600°C, and then the temperature of the fluidized bed was raised to To 650°C, methane was introduced to carry out cracking reaction, the flow rate of methane was 0.3m ³ /h; the cracking reaction time was 210min, and hydrogen gas and carbon nanotube solid material were obtained.

Example 3

(1) Raw material mixing:

Mix aluminum nitrate, magnesium nitrate and water to form a salt solution, the molar ratio of Al ³⁺ and Mg ²⁺ in the salt solution is 1:4, the concentration of Al ³⁺ in the salt solution is 1mol/L, and the concentration of Mg ²⁺ is 4mol/L. The salt solution was mixed and stirred with 5 g/L P123 solution and 50 mL deionized water to obtain a reaction solution.

(2) Precipitation and crystallization:

In the reaction solution obtained in step (1), sodium carbonate solution is gradually added dropwise to carry out co-precipitation reaction. The sodium carbonate solution was added dropwise until the pH value of the solution was 10, the dropwise addition was stopped, and the temperature was stirred at 30° C. for 30 min, and a suspension was obtained after the reaction was completed. The suspension was transferred to a hydrothermal synthesis kettle, and crystallized at 220°C for 20 hours. The crystallized suspension was suction filtered, washed, and dried at 120°C for 8 hours, and then sent to a muffle furnace. Calcined at 750° C. for 4 h to obtain template-modified magnesium-aluminum hydrotalcite composite metal oxide (MgAl-LDO).

(3) Dipping:

Mix nickel nitrate, lanthanum nitrate, citric acid and water to form a mixed solution. The molar ratio of nickel ions (Ni ³⁺ ) and lanthanum ions (La ³⁺ ) in the mixed solution is 4:6, that is, the Ni ³⁺ in the mixed solution is 4mol/L, La ³⁺ is 6mol/L, and the molar ratio of citric acid to the sum of Ni ³⁺ and La ³⁺ is 1:1, that is, the citric acid in the mixture is 10mol/L. Add the mixed solution to the MgAl-LDO prepared in step (2), impregnate and stir at 45 °C for 8 h, then dry at 120 °C for 18 h, then decompose at 450 °C for 1 h at a heating rate of 3 °C/min, and then use the same The heating rate was raised to 650°C and calcined for 6 hours to obtain the lanthanum nickelate modified magnesium aluminum hydrotalcite catalyst LaNiO ₃ @MgAl-LDO.

The LaNiO ₃ @MgAl-LDO catalyst prepared in Example 3 was applied to methane cracking reaction.

Specifically, take 5g of self-prepared LaNiO ₃ @MgAl-LDO and place it in a fluidized bed for reduction in a hydrogen atmosphere with a reduction space velocity of 0.2/h and a reduction temperature of 600°C, and then raise the temperature of the fluidized bed to To 650°C, methane was introduced to carry out the cracking reaction, the flow rate of methane was 0.5m ³ /h; the cracking reaction time was 210min, and hydrogen gas and carbon nanotube solid material were obtained.

Example 4

(1) Raw material mixing:

Mix aluminum nitrate, magnesium nitrate and water to form a salt solution. The molar ratio of Al ³⁺ and Mg ²⁺ in the salt solution is 1:4, the concentration of Al ³⁺ in the salt solution is 1mol/L, and the concentration of Mg ²⁺ is 4mol/L. The salt solution was mixed and stirred with 5 g/L P123 solution and 50 mL deionized water to obtain a reaction solution.

(2) Precipitation and crystallization:

In the reaction solution obtained in step (1), sodium carbonate solution is gradually added dropwise to carry out co-precipitation reaction. The sodium carbonate solution was added dropwise until the pH value of the solution was 10, the dropwise addition was stopped, and the temperature was stirred at 30° C. for 30 min, and a suspension was obtained after the reaction was completed. The suspension was transferred to a hydrothermal synthesis kettle, and crystallized at 220°C for 20 hours. The crystallized suspension was suction filtered, washed, and dried at 120°C for 8 hours, and then sent to a muffle furnace. Calcined at 750° C. for 4 h to obtain template-modified magnesium-aluminum hydrotalcite composite metal oxide (MgAl-LDO).

(3) Dipping:

Mix nickel nitrate, lanthanum nitrate, citric acid and water to form a mixed solution. The molar ratio of nickel ions (Ni ³⁺ ) and lanthanum ions (La ³⁺ ) in the mixed solution is 4:6, that is, the Ni ³⁺ in the mixed solution is 4mol/L, La ³⁺ is 6mol/L, and the molar ratio of citric acid to the sum of Ni ³⁺ and La ³⁺ is 1:1, that is, the citric acid in the mixture is 10mol/L. Add the mixed solution to the MgAl-LDO prepared in step (2), impregnate and stir at 45 °C for 8 h, then dry at 120 °C for 18 h, then decompose at 450 °C for 1 h at a heating rate of 3 °C/min, and then use the same The heating rate was raised to 700°C and calcined for 6 hours to obtain the lanthanum nickelate modified magnesium aluminum hydrotalcite catalyst LaNiO ₃ @MgAl-LDO.

The LaNiO ₃ @MgAl-LDO catalyst prepared in Example 4 was applied to methane cracking reaction.

Specifically, take 5g of self-prepared LaNiO ₃ @MgAl-LDO and place it in a fluidized bed for reduction in a hydrogen atmosphere with a reduction space velocity of 0.2/h and a reduction temperature of 600°C, and then raise the temperature of the fluidized bed to To 700°C, methane was introduced to carry out cracking reaction, the flow rate of methane was 0.3m ³ /h; the cracking reaction time was 210min, and hydrogen gas and carbon nanotube solid material were obtained.

Comparative Example 1: Preparation of Catalyst LaNiO ₃

Mix nickel nitrate, lanthanum nitrate, citric acid and water to form a mixed solution. The molar ratio of nickel ions (Ni ³⁺ ) and lanthanum ions (La ³⁺ ) in the mixed solution is 4:6, that is, the Ni ³⁺ in the mixed solution is 4mol/L, La ³⁺ is 6mol/L, and the molar ratio of citric acid to the sum of Ni ³⁺ and La ³⁺ is 1:1, that is, the citric acid in the mixture is 10mol/L. The mixed solution was impregnated and stirred at 45°C for 8 hours, dried at 120°C for 18 hours, then decomposed at 500°C at a heating rate of 2°C/min for 1 hour, and then calcined at 750°C at the same heating rate for 6 hours to obtain the catalyst LaNiO ₃ .

Take 2g of LaNiO3 prepared by the above method and place it in a fluidized bed, and carry out reduction under a hydrogen atmosphere, the reduction space velocity is 0.2/h, and the reduction temperature is 600°C, then the temperature of the fluidized bed is raised to 700°C, and methane is introduced The cracking reaction is carried out, the flow rate of methane is 0.3m ³ /h; the cracking reaction time is 180min, and hydrogen gas and carbon nanotube solid material are obtained.

performance testing

The catalysts prepared in Examples 1-4 and Comparative Example 1 were applied to methane cracking, and performance testing was performed on relevant parameters in the cracking reaction. The results are shown in Table 1.

The methane cracking related parameter result of table 1 embodiment and comparative example

Example 1 Example 2 Example 3 Example 4 Comparative example 1 Catalyst dosage (g) 2.0 3.0 3.0 5.0 2.0 Methane reaction volume (m ³ /h) 0.3 0.3 0.5 0.3 0.3 Reaction temperature (°C) 700 650 650 700 700 Response time (min) 180 210 210 210 180 Carbon nanotube yield (g) 160.5 158.9 267.5 154.1 94.3 Carbon nanotube purity (%) 99.8 99.3 99.6 99.2 72.6 Methane conversion rate (%) 98.3 97.6 98.0 95.3 80.2

As can be seen from Table 1, the catalysts prepared by the examples of the present application can effectively promote the cracking of methane in the methane cracking reaction, and the conversion rate of methane and the output of carbon nanotubes in all the examples are much higher than the methane in the comparative examples The conversion rate shows that the catalytic efficiency of using lanthanum nickelate modified magnesium aluminum hydrotalcite catalyst in the examples of the present application is higher than that of using lanthanum nickelate catalyst alone.

The specific surface area (S _BET ) and average pore diameter of the LaNiO ₃ @MgAl-LDO catalyst in Example 1 and the LaNiO ₃ catalyst in Comparative Example 1 are shown in Table 2, and their pressure and adsorption capacity curves are shown in Figure 1 .

Specific surface area and average pore diameter detection result of table 2 embodiment 1 and comparative example 1

catalyst S _BET (m ² ·g ^-1 ) Average pore size (nm) LaNiO ₃ 10.2 15.3 LaNiO ₃ @MgAl-LDO 27.3 26.7

It can be seen from Table 2 that the specific surface area and average pore diameter of the LaNiO ₃ @MgAl-LDO catalyst prepared in Example 1 of the present application are much higher than that of the LaNiO ₃ catalyst prepared in Comparative Example 1, and the higher surface area and average pore diameter can improve the catalyst The amount of adsorption of methane enables methane to react on the surface of the catalyst, thereby improving the catalytic efficiency and reactivity of the catalyst, thereby increasing the conversion rate of methane cracking and the production of carbon nanotubes. It can be seen from Figure 1 that under the same relative pressure, the adsorption capacity of the LaNiO ₃ @MgAl-LDO catalyst prepared by the present application is higher than that of the LaNiO ₃ catalyst, and the results are consistent with the aforementioned results, further illustrating that the LaNiO ₃ prepared by the present application The @MgAl-LDO catalyst can effectively adsorb methane gas and improve the catalytic efficiency of methane cracking.

This specific embodiment is only an explanation of this application, and it is not a limitation of this application. Those skilled in the art can make modifications to this embodiment without creative contribution according to needs after reading this specification, but as long as the rights of this application All claims are protected by patent law.

Claims (10)

1. The preparation method of the modified magnesium aluminum hydrotalcite catalyst is characterized by comprising the following steps of:

(1) Mixing the raw materials: mixing a salt solution and a polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer solution, and uniformly stirring to obtain a reaction solution, wherein the salt solution is formed by mixing aluminum salt, magnesium salt and water;

(2) And (3) precipitation crystallization: adding a precipitant into the reaction solution to perform coprecipitation reaction, performing hydrothermal crystallization after the reaction is finished, and performing suction filtration, washing, drying and calcination to obtain modified hydrotalcite;

(3) Dipping: and adding a mixed solution formed by nickel salt, lanthanum salt, citric acid and water into the modified hydrotalcite, soaking and stirring, and drying and calcining to obtain the lanthanum nickelate loaded modified hydrotalcite catalyst.

2. The method according to claim 1, wherein in the step (1), the molar ratio of aluminum ions to magnesium ions in the salt solution is 1 (2-4), the aluminum salt is aluminum nitrate, and the magnesium salt is magnesium nitrate.

3. The preparation method according to claim 1 or 2, wherein the concentration of the polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer solution is 2-5 g/L, and the volume ratio of the polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer solution to the salt solution is 1 (1-3).

4. The method according to claim 1 or 2, wherein in the step (2), the precipitant is a sodium carbonate solution, and the concentration of the sodium carbonate solution is 0.5 to 1.5mol/L.

5. The method according to claim 4, wherein in the step (2), the hydrothermal crystallization is performed at a temperature of 200 to 220 ℃ for a time of 10 to 20 hours; the drying temperature is 90-150 ℃ and the drying time is 8-12 h; the calcination temperature is 600-750 ℃ and the calcination time is 4-5 h.

6. The method according to claim 1, wherein in the step (3), the nickel salt is nickel nitrate, the lanthanum salt is lanthanum nitrate, and the molar ratio of nickel ions to lanthanum ions in the mixed solution is 4: (5-6); the mole ratio of the citric acid to the total ions of nickel ions and lanthanum ions is 1:1.

7. The method according to claim 6, wherein in the step (3), the dipping and stirring time is 8 to 12 hours, and the dipping temperature is 35 to 55 ℃; the drying temperature is 80-120 ℃ and the drying time is 18-24 h.

8. The method according to claim 7, wherein in the step (3), the calcining comprises: decomposing for 1-2 h at 450-500 ℃ at a heating rate of 2-3 ℃/min, then raising the temperature to 650-750 ℃ at the same heating rate, and calcining for 4-8 h.

9. A modified magnesium aluminium hydrotalcite catalyst, characterised in that it is obtainable by a process according to any one of claims 1 to 8.

10. The use of the modified magnesium aluminum hydrotalcite catalyst according to claim 9, wherein the prepared modified magnesium aluminum hydrotalcite catalyst is placed in a fluidized bed and reduced in a reducing atmosphere, wherein the reducing atmosphere is hydrogen, the reducing space velocity is 0.2-0.5/h, and the reducing temperature is 600-650 ℃; then the temperature of the fluidized bed is raised to 650-700 ℃, methane is introduced to carry out cracking reaction, and the flow rate of the methane is 0.3-0.5 m ³ And/h, the time of the cracking reaction is 180-210 min, and the hydrogen and carbon nano tube solid material is obtained.

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