A method for preparing mesoporous ultra-stable alumina

Abstract

The present invention belongs to the technical field of catalyst carrier materials, and specifically relates to a method for preparing mesoporous ultra-stable alumina. The present invention adopts a double aluminum method to prepare pseudo-boehmite, obtains alumina powder after calcination, and prepares a layer of fibrous pseudo-boehmite in the pore structure of the alumina powder. The pore structure and specific surface area of the mesoporous ultra-stable alumina are controlled by controlling the crystallization pH value and crystallization temperature during the reaction process, and sodium silicate and hexamethyldisilazane are added during the preparation process, which effectively improves the high-temperature stability of the mesoporous ultra-stable alumina, so that it still has good specific surface area and pore volume under hydrothermal conditions. The present invention uses sodium metaaluminate and aluminum sulfate as the main raw materials to prepare mesoporous stable alumina. No organic solvent is added as a template during the preparation process, which effectively avoids the volatilization of the organic solvent and is green and environmentally friendly.

Description

Technical Field

[0001] The invention belongs to the technical field of catalyst carrier materials, and particularly relates to a preparation method of mesoporous ultra-stable alumina. Background Art

[0002] In the modern petrochemical and chemical industries, over 90% of chemical reactions are carried out using catalysts. Activated alumina supports account for approximately 57% of solid catalyst carriers used in oil refining and petrochemicals. The expanding application of catalysts in the chemical and environmental fields is placing higher demands on the performance of activated alumina powder, and the preparation method is closely related to its performance.

[0003] During the catalytic cracking reaction, the oil and gas adsorbed on the surface of the regenerated catalyst need to be purged with high-temperature steam during the stripping stage. In high-temperature steam, the specific surface area and pore volume of the cracking catalyst will decrease, and the catalyst reaction performance will be reduced. In particular, the traditional alumina matrix does not have high hydrothermal stability and is prone to crystal phase transformation and pore structure collapse in a high-temperature steam atmosphere, which will cause the heavy oil processing capacity of the regenerated catalyst to be significantly reduced. Therefore, making alumina have high hydrothermal stability and maintaining the high porosity of the alumina matrix in a hydrothermal environment are important guarantees for improving the heavy oil processing capacity of the cracking catalyst. In addition, compared with macroporous alumina powder, although the specific surface area of alumina powder with a mesoporous structure may be relatively small, its pore size distribution is more uniform, which is conducive to the adsorption of substances and the occurrence of catalytic reactions, making it an ideal carrier material for catalysts.

[0004] The Chinese patent application document with publication number CN115594207A discloses a macroporous and large specific surface area boehmite and a preparation method thereof. The specific preparation method is as follows: (1) reacting sodium hydroxide, aluminum hydroxide and water to obtain a sodium aluminate solution; (2) dissolving aluminum sulfate solid to prepare an aluminum sulfate solution, and mixing the aluminum sulfate solution with the sodium aluminate solution, and crystallizing the mixture to obtain a slurry; (3) filtering the slurry to obtain a filter cake and a filtrate; (4) washing and drying the filter cake in sequence to obtain boehmite; wherein the filtrate in (3) is returned to the aluminum sulfate solution for dissolving the aluminum sulfate solid. This method uses the filtrate containing sodium sulfate to prepare the aluminum sulfate solution, which can make the boehmite obtained have a larger specific surface area and pore volume. However, the crystallization temperature in step (2) is 80-140°C and the pH value is 8-10. Under these conditions, the boehmite obtained will undergo crystal phase transformation and pore structure collapse at high temperature, resulting in changes in the pore structure, a decrease in specific surface area, and poor high-temperature stability, which will affect the reaction performance of the catalyst.

[0005] It can be seen that the main problems faced in the development of alumina are adjusting the pore volume and specific surface area of the material and improving the high-temperature stability of the alumina material. How to synthesize stable alumina with a mesoporous structure in a green and low-cost manner has become an urgent problem that researchers need to solve. Summary of the Invention

[0006] In order to solve the technical problems in the prior art of small pore volume and specific surface area of alumina materials and poor high-temperature stability of alumina materials, the present invention provides a method for preparing mesoporous ultra-stable alumina.

[0007] In order to achieve the above object, the technical solution of the present invention is as follows:

[0008] A method for preparing mesoporous ultrastable alumina comprises the following steps:

[0009] S1: Mix aluminum hydroxide powder and sodium hydroxide solution evenly and react for 5-6 hours to prepare sodium aluminate solution;

[0010] S2: mixing the sodium aluminate solution prepared in step S1 with the aluminum sulfate solution, stirring and heating, adding the sodium aluminate solution to control the pH value of the reaction system to 7.8-8.3 until the reaction is complete, continuing to heat the reaction for crystallization, then adding sodium silicate, continuing the reaction, standing and cooling, filtering, washing, drying, and grinding to obtain a powdery material;

[0011] S3: calcining the powder obtained in step S2 and cooling it to obtain modified alumina powder;

[0012] S4: Add the modified alumina powder and hexamethyldisilazane prepared in step S3 to the aluminum sulfate solution, increase the temperature, add the sodium aluminate solution prepared in step S1 dropwise under stirring, add sodium carbonate to control the pH value of the reaction system to 9.7-10.0 until the reaction is completed, continue to increase the temperature for crystallization reaction, filter after cooling, wash, dry, and grind to obtain mesoporous ultra-stable alumina.

[0013] The present invention adopts a double aluminum method to prepare pseudo-boehmite, and controls the pore structure and specific surface area of the pseudo-boehmite by controlling the crystallization pH value and crystallization temperature during the reaction process. In step S2 of this preparation method, the crystallization pH value is controlled to be 7.8-8.3. Within this pH value range, the prepared pseudo-boehmite has a large specific surface area and pore volume. After the crystallization reaction is completed, the addition of sodium silicate can silicon-modify the prepared pseudo-boehmite. Studies have found that the addition of a small amount of silicate ions can be adsorbed on the surface of the pseudo-boehmite. During the roasting process, the silicon ions will occupy the vacancies of the aluminum oxide tetrahedron, reducing the total vacancies, thereby inhibiting the lattice vibration and phase transition of the aluminum oxide at high temperature, thereby improving the high-temperature stability of the aluminum oxide and effectively preventing the reduction of the specific surface area of the aluminum oxide during roasting. In addition, the present invention also prepares secondary pseudo-boehmite on the surface of the modified alumina powder obtained after calcination. By controlling the crystallization pH of the pseudo-boehmite to 9.7-10, the secondary pseudo-boehmite is given a fibrous morphology. The fibrous secondary pseudo-boehmite plays a supporting role in the pore size of the modified alumina powder, giving the resulting mesoporous ultra-stable alumina a relatively stable pore structure. Furthermore, hexamethyldisilazane is added during the preparation of the secondary pseudo-boehmite. Hexamethyldisilazane can introduce Si-CH3 groups on the surface of the alumina particles, converting them into SiO2 particles at high temperatures. This can inhibit the phase transition of the alumina, thereby improving the high-temperature stability of the alumina.

[0014] Furthermore, in step S1, the molar ratio of the aluminum hydroxide powder to the sodium hydroxide in the sodium hydroxide solution is 1:3.2-3.5; and the concentration of the sodium hydroxide solution is 3.5-4 mol / L.

[0015] Furthermore, the reaction temperature in step S1 is 125-130° C., and the reaction pressure is 0.3-0.4 MPa.

[0016] Furthermore, in step S2, the volume ratio of the sodium metaaluminate solution to the aluminum sulfate solution is 1.2-1.5:1; and the concentration of the aluminum sulfate solution is 0.2-0.4 mol / L.

[0017] Furthermore, the stirring and heating temperature in step S2 is 55-60° C., and the reaction time is 2-2.5 hours.

[0018] Furthermore, the temperature of the crystallization reaction in step S2 is 85-95° C., and the time of the crystallization reaction is 24-30 hours.

[0019] The temperature of the crystallization reaction has a great influence on the pore size distribution of pseudo-boehmite. The higher the temperature, the higher the crystallinity of the pseudo-boehmite and the larger the pore size. However, at high temperatures, the crystallinity and order of the pseudo-boehmite will decrease, resulting in an uneven distribution of the pore structure. The crystallization reaction temperature in step S2 of the present invention is selected to be 85-95°C. The pseudo-boehmite obtained within this temperature range not only has a larger pore size, but also has a moderate grain size and a uniform pore size distribution.

[0020] Furthermore, the molar mass of the sodium silicate in step S2 is 0.5%-0.8% of the total molar mass of aluminum in the sodium metaaluminate solution and the aluminum sulfate solution.

[0021] The present invention modifies pseudo-boehmite with sodium silicate by controlling the molar ratio of sodium silicate to aluminum in the reaction system. Adding a small amount of sodium silicate effectively increases the specific surface area and pore volume of the pseudo-boehmite. This is because sulfate desorbs from the pseudo-boehmite surface at high temperatures, while silicate adsorbs on the surface, inhibiting the growth of pseudo-boehmite grains and increasing the specific surface area and pore volume. However, if an excessive amount of sodium silicate is added during the calcination process, the excess silica generated can clog the pore structure of the alumina, resulting in a decrease in the specific surface area and pore volume.

[0022] Furthermore, in step S3, the calcination temperature is 820-830° C., the calcination time is 4-5 h, the calcination heating rate is 20-30° C. / min; and the cooling rate is 40-50° C. / min.

[0023] Research has found that the calcination temperature of pseudo-boehmite affects the pore structure of the resulting alumina. After low-temperature calcination, the specific surface area and pore volume of the alumina are significantly reduced. This is because the pseudo-boehmite grains continue to grow and the grain size increases during the low-temperature calcination process. High-temperature calcination causes the pore structure of the alumina to collapse or become clogged, resulting in a decrease in the specific surface area. The calcination temperature selected by the present invention is 820-830°C. When calcined within this temperature range, the specific surface area of the alumina obtained increases slightly, while the pore volume and pore diameter do not change much. This is because within this temperature range, only a small amount of pseudo-boehmite undergoes crystal transformation, generating boehmite. The aluminum atoms migrate to the boehmite, causing pitting of the alumina phase, resulting in a slight increase in the specific surface area.

[0024] Furthermore, in step S4, the volume ratio of the aluminum sulfate solution to the sodium metaaluminate solution is 1:1.2-1.5; and the concentration of the aluminum sulfate solution is 0.2-0.4 mol / L.

[0025] Furthermore, in step S4, the mass ratio of the modified alumina powder to hexamethyldisilazane is 10-13:1; and the mass ratio of the modified alumina powder to sodium aluminate in the sodium aluminate solution is 7-10:2-3.

[0026] Furthermore, the temperature of the heating in step S4 is 50-55°C.

[0027] Furthermore, the temperature of the crystallization reaction in step S4 is 140-150° C., and the time of the crystallization reaction is 20-24 hours.

[0028] The present invention adjusts the growth degree of fibrous pseudo-boehmite in the pores of modified alumina powder by adjusting the crystallization reaction temperature of secondary pseudo-boehmite. The higher the temperature, the longer the crystallization time. However, the pores of the pseudo-boehmite itself will also be destroyed under hydrothermal conditions. Therefore, it is necessary to strictly control the growth of fibrous pseudo-boehmite in the pores of alumina.

[0029] Compared with the prior art, the preparation method of mesoporous stable alumina provided by the present invention has the following technical advantages:

[0030] (1) The present invention adopts a double aluminum method to prepare pseudo-boehmite, obtains alumina powder after calcination, and prepares a layer of fibrous pseudo-boehmite in the pore structure of the alumina powder. The pore structure and specific surface area of the mesoporous ultra-stable alumina are controlled by controlling the crystallization pH value and crystallization temperature during the reaction process;

[0031] (2) The present invention adds sodium silicate and hexamethyldisilazane during the preparation process, which effectively improves the high-temperature stability of the mesoporous ultrastable alumina, so that it still has a good specific surface area and pore volume under hydrothermal conditions;

[0032] (3) In the present invention, mesoporous stable alumina is prepared using sodium metaaluminate and aluminum sulfate as main raw materials. No organic solvent is added as a template during the preparation process, which effectively avoids the volatilization of the organic solvent and is green and environmentally friendly. DETAILED DESCRIPTION

[0033] The following will be further described in conjunction with specific embodiments, but the present invention is not limited to the following embodiments. Those skilled in the art can make various modifications based on the basic concept of the present invention, but as long as they do not deviate from the basic concept of the present invention, they are all within the scope of the present invention.

[0034] Example 1

[0035] A method for preparing mesoporous ultrastable alumina comprises the following steps:

[0036] S1: Aluminum hydroxide powder and a 3.5 mol / L sodium hydroxide solution were mixed evenly (the molar ratio of aluminum hydroxide powder to sodium hydroxide in the sodium hydroxide solution was 1:3.2), pressurized to 0.3 MPa, heated to 125°C, and reacted for 6 hours to obtain a sodium aluminate solution;

[0037] S2: The sodium aluminate solution prepared in step S1 is mixed with a 0.2 mol / L aluminum sulfate solution in a volume ratio of 1.2:1, and the mixture is heated to 55° C. with stirring. The sodium aluminate solution is added to control the pH value of the reaction system to 7.8 until the reaction is completed. The mixture is heated to 85° C. and crystallized for 24 hours. Then, sodium silicate is added (the molar mass of sodium silicate is 0.5% of the total molar mass of aluminum in the sodium aluminate solution and the aluminum sulfate solution). The mixture is reacted for 2 hours, allowed to stand and cool, filtered, washed with deionized water until the washing liquid is free of sulfate ions, dried, and ground to obtain a powdery material.

[0038] S3: heating the powder obtained in step S2 to 820°C at a rate of 20°C / min and calcining for 5 hours, and then cooling the temperature to 25°C at a cooling rate of 40°C / min to obtain modified alumina powder;

[0039] S4: The modified alumina powder and hexamethyldisilazane obtained in step S3 are added to a 0.2 mol / L aluminum sulfate solution in a mass ratio of 10:1, the temperature is raised to 50°C, and the sodium aluminate solution obtained in step S1 is added dropwise under stirring (the volume ratio of sodium aluminate solution to aluminum sulfate solution is 1.2:1, and the mass ratio of modified alumina powder to sodium aluminate in the sodium aluminate solution is 7:2). Sodium carbonate is added to control the pH value of the reaction system to 9.7 until the reaction is completed, and the temperature is continued to be raised to 140°C. The crystallization reaction is carried out for 20 hours. After cooling, the mixture is filtered, washed, dried, and ground to obtain mesoporous ultra-stable alumina.

[0040] Example 2

[0041] A method for preparing mesoporous ultrastable alumina comprises the following steps:

[0042] S1: Aluminum hydroxide powder and a 4 mol / L sodium hydroxide solution were mixed evenly (the molar ratio of aluminum hydroxide powder to sodium hydroxide in the sodium hydroxide solution was 1:3.5), pressurized to 0.4 MPa, heated to 130°C, and reacted for 5 h to obtain a sodium aluminate solution;

[0043] S2: The sodium aluminate solution prepared in step S1 is mixed with a 0.4 mol / L aluminum sulfate solution in a volume ratio of 1.5:1, and the mixture is stirred and heated to 60°C. The sodium aluminate solution is added to control the pH value of the reaction system to 8.3 until the reaction is completed. The mixture is heated to 95°C and crystallized for 30 hours. Then, sodium silicate is added (the molar mass of sodium silicate is 0.8% of the total molar mass of aluminum in the sodium aluminate solution and the aluminum sulfate solution). The mixture is reacted for 2.5 hours, allowed to stand and cool, filtered, washed with deionized water until the washing liquid is free of sulfate ions, dried, and ground to obtain a powdery material.

[0044] S3: heating the powder obtained in step S2 to 830°C at a rate of 30°C / min and calcining for 4 hours, and then cooling the temperature to 25°C at a cooling rate of 50°C / min to obtain modified alumina powder;

[0045] S4: The modified alumina powder and hexamethyldisilazane prepared in step S3 are added to a 0.4 mol / L aluminum sulfate solution in a mass ratio of 13:1, the temperature is raised to 55°C, and the sodium aluminate solution prepared in step S1 is added dropwise under stirring (the volume ratio of sodium aluminate solution to aluminum sulfate solution is 1.5:1, and the mass ratio of modified alumina powder to sodium aluminate in the sodium aluminate solution is 10:3). Sodium carbonate is added to control the pH value of the reaction system to 10.0 until the reaction is completed, and the temperature is continued to be raised to 150°C. The crystallization reaction is carried out for 24 hours. After cooling, it is filtered, washed, dried, and ground to obtain mesoporous ultra-stable alumina.

[0046] Example 3

[0047] A method for preparing mesoporous ultrastable alumina comprises the following steps:

[0048] S1: Mix the aluminum hydroxide powder and the sodium hydroxide solution with a concentration of 3.8 mol / L evenly (the molar ratio of sodium hydroxide in the aluminum hydroxide powder and sodium hydroxide solution is 1:3.4), pressurize to 0.35MPa, heat up to 128℃, and react for 5.3 hours to prepare a sodium metaaluminate solution;

[0049] S2: The sodium aluminate solution prepared in step S1 is mixed with a 0.31 mol / L aluminum sulfate solution in a volume ratio of 1.4:1, and the mixture is stirred and heated to 58°C. The sodium aluminate solution is added to control the pH value of the reaction system to 8.1 until the reaction is completed. The mixture is heated to 90°C and crystallized for 26 hours. Then, sodium silicate is added (the molar mass of sodium silicate is 0.7% of the total molar mass of aluminum in the sodium aluminate solution and the aluminum sulfate solution). The mixture is reacted for 2.3 hours, allowed to stand and cool, filtered, washed with deionized water until the washing liquid is free of sulfate ions, dried, and ground to obtain a powdery material.

[0050] S3: heating the powder obtained in step S2 to 825°C at a rate of 26°C / min and calcining it for 4.6 hours, and then cooling it to 25°C at a cooling rate of 45°C / min to obtain modified alumina powder;

[0051] S4: The modified alumina powder and hexamethyldisilazane obtained in step S3 are added to a 0.3 mol / L aluminum sulfate solution in a mass ratio of 12:1, the temperature is raised to 53°C, and the sodium aluminate solution obtained in step S1 is added dropwise under stirring (the volume ratio of sodium aluminate solution to aluminum sulfate solution is 1.4:1, and the mass ratio of modified alumina powder to sodium aluminate in the sodium aluminate solution is 9:2). Sodium carbonate is added to control the pH value of the reaction system to 9.8 until the reaction is completed, and the temperature is continued to be raised to 145°C. The crystallization reaction is carried out for 23 hours. After cooling, the mixture is filtered, washed, dried, and ground to obtain mesoporous ultra-stable alumina.

[0052] Comparative Example 1

[0053] The preparation method of aluminum oxide in this comparative example is similar to that in Example 3. The difference between this comparative example and Example 3 is that in step S2 of this comparative example, the pH value of the reaction system is controlled to be 9.0.

[0054] Comparative Example 2

[0055] The preparation method of aluminum oxide in this comparative example is similar to that in Example 3. The difference between this comparative example and Example 3 is that the temperature of the crystallization reaction in step S2 of this comparative example is 140°C.

[0056] Comparative Example 3

[0057] The preparation method of aluminum oxide in this comparative example is similar to that in Example 3. The difference between this comparative example and Example 3 is that an equal amount of deionized water is used instead of sodium silicate in step S2 of this comparative example.

[0058] Comparative Example 4

[0059] The preparation method of alumina described in this Comparative Example is similar to Example 3. The difference between this Comparative Example and Example 3 is that the molar mass of sodium silicate in step S2 of this Comparative Example is 1.5% of the total molar mass of aluminum in the sodium metaaluminate solution and aluminum sulfate solution.

[0060] Comparative Example 5

[0061] The preparation method of aluminum oxide in this comparative example is similar to that in Example 3. The difference between this comparative example and Example 3 is that the calcination temperature in step S3 of this comparative example is 1300°C.

[0062] Comparative Example 6

[0063] The preparation method of aluminum oxide in this comparative example is similar to that in Example 3. The difference between this comparative example and Example 3 is that the pH value of the reaction system in step S4 of this comparative example is 8.5.

[0064] Comparative Example 7

[0065] The preparation method of aluminum oxide in this comparative example is similar to that in Example 3. The difference between this comparative example and Example 3 is that the temperature of the crystallization reaction in step S4 of this comparative example is 200°C.

[0066] Comparative Example 8

[0067] The preparation method of aluminum oxide in this comparative example is similar to that in Example 3. The difference between this comparative example and Example 3 is that hexamethyldisilazane is not added in step S4 of this comparative example.

[0068] Test example

[0069] Test samples: alumina prepared in Examples 1 to 3 and Comparative Examples 1 to 8;

[0070] Test Method: The specific surface area and pore volume of the test samples were measured using the BET method before thermal aging and after aging at 1200°C for 4 hours. Specifically, the samples were degassed under vacuum at 300°C for 8-12 hours, with the vacuum level at the degassing station below 45 mTorr. After degassing, isothermal adsorption / desorption was performed at 77.8 K. The specific surface area of the sample was calculated using the BET equation. The volume of nitrogen adsorbed in the sample was recorded at the maximum relative pressure P / P0, from which the pore volume was derived.

[0071] Test results: See Table 1.

[0072] Table 1

[0073]

[0074] As can be seen from Table 1, the pore volume and specific surface area of the mesoporous ultrastable alumina prepared in Examples 1 to 3 of the present invention are large, and the changes in the specific surface area and pore volume after thermal aging are relatively small, which shows that the mesoporous ultrastable alumina provided by the present invention has a good pore structure and good high-temperature stability, and still has good load performance after high-temperature aging.

[0075] Compared with Example 3, Comparative Example 1 changed the pH value of the reaction system in step S2, and Comparative Example 2 changed the crystallization reaction temperature in step S2, but the specific surface area and pore volume of the obtained alumina were reduced to a certain extent, which shows that the pH value of the reaction system and the temperature of the crystallization reaction are one of the key factors for adjusting the pore structure of alumina. As the pH value of the reaction system increases, the crystallinity of the pseudo-boehmite increases, and the particle size becomes larger, resulting in a decrease in specific surface area. The pore volume first increases with the increase in crystallinity, but as the pH value continues to increase, the crystal nucleus undergoes a dissolution-reformation process in the solution system, resulting in a decrease in pore volume; and an increase in the crystallization reaction temperature will promote the increase in the crystallinity of the pseudo-boehmite, but will reduce the uniformity of the pore size distribution, resulting in a decrease in specific surface area; sodium silicate was not added in step S2 of Comparative Example 3, the amount of sodium silicate added in step S2 of Comparative Example 4 was changed, and no sodium silicate was added in step S4 of Comparative Example 8 Hexamethyldisilazane, but the specific surface area and pore size of alumina after high-temperature aging are reduced, which shows that the addition of sodium silicate and hexamethyldisilazane can effectively improve the high-temperature stability of the pore structure of alumina, and excessive sodium silicate will cause the pores of alumina to be blocked, thereby reducing the specific surface area and pore volume; Comparative Example 5 changes the calcination temperature in step S3, but the specific surface area and pore volume of the obtained alumina are reduced, which shows that after exceeding the calcination temperature provided by the present invention, the pores of alumina will collapse, resulting in pore blockage, thereby reducing the specific surface area and pore volume; Comparative Example 6 changes the pH value of the reaction system in step S4, and Comparative Example 7 changes the crystallization reaction temperature in step S4, but the specific surface area and pore volume of the obtained alumina after high-temperature aging are slightly reduced, which shows that the pseudo-boehmite prepared in step S4 of the present invention can provide certain support for the pore structure of alumina, thereby improving the high-temperature resistance of alumina.

[0076] The above embodiments are merely illustrative of the present invention and are not intended to limit the present invention. Persons skilled in the art should not modify the above embodiments without departing from the spirit and scope of the present invention. Any equivalent modifications or alterations made by persons skilled in the art without departing from the technical spirit of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A method for preparing mesoporous ultrastable alumina, characterized in that: The following steps are involved: S1: Mix aluminum hydroxide powder and sodium hydroxide solution evenly and react for 5-6 hours to prepare sodium aluminate solution; S2: mixing the sodium aluminate solution obtained in step S1 with the aluminum sulfate solution, stirring and heating, adding the sodium aluminate solution to control the pH value of the reaction system to 7.8-8.3 until the reaction is complete, continuing to heat the reaction for crystallization, then adding sodium silicate, continuing the reaction, standing and cooling, filtering, washing, drying, and grinding to obtain a powdery material; S3: calcining the powder obtained in step S2 and cooling it to obtain modified alumina powder; S4: adding the modified alumina powder and hexamethyldisilazane prepared in step S3 to the aluminum sulfate solution, raising the temperature, and adding dropwise the sodium metaaluminate solution prepared in step S1 while stirring. Sodium carbonate is added to control the pH value of the reaction system to 9.7-10.0 until the reaction is complete, and continuing to raise the temperature for crystallization reaction. After cooling, filtering, washing, drying, and grinding, to obtain mesoporous ultrastable alumina; The temperature of the crystallization reaction in step S2 is 85-95° C., and the crystallization reaction time is 24-30 hours; The calcination temperature in step S3 is 820-830°C; The temperature of the crystallization reaction in step S4 is 140-150° C., and the crystallization reaction time is 20-24 hours.

2. The method for preparing mesoporous ultrastable alumina according to claim 1, wherein: In step S1, the molar ratio of the aluminum hydroxide powder to the sodium hydroxide in the sodium hydroxide solution is 1:3.2-3.5; the concentration of the sodium hydroxide solution is 3.5-4 mol / L.

3. The method for preparing mesoporous ultrastable alumina according to claim 1, characterized in that: The reaction temperature in step S1 is 125-130° C., and the reaction pressure is 0.3-0.4 MPa.

4. The method for preparing mesoporous ultrastable alumina according to claim 1, wherein: In step S2, the volume ratio of the sodium metaaluminate solution to the aluminum sulfate solution is 1.2-1.5:1; the concentration of the aluminum sulfate solution is 0.2-0.4 mol / L; the stirring and heating temperature is 55-60° C., and the reaction time is 2-2.5 hours.

5. The method for preparing mesoporous ultrastable alumina according to claim 1, wherein: The molar mass of the sodium silicate in step S2 is 0.5%-0.8% of the total molar mass of aluminum in the sodium metaaluminate solution and the aluminum sulfate solution.

6. The method for preparing mesoporous ultrastable alumina according to claim 1, wherein: In step S3, the calcination time is 4-5 hours, the calcination heating rate is 20-30° C. / min; and the cooling rate is 40-50° C. / min.

7. The method for preparing mesoporous ultrastable alumina according to claim 1, wherein: In step S4, the volume ratio of the aluminum sulfate solution to the sodium metaaluminate solution is 1:1.2-1.5; the concentration of the aluminum sulfate solution is 0.2-0.4 mol / L.

8. The method for preparing mesoporous ultrastable alumina according to claim 1, wherein: In step S4, the mass ratio of the modified alumina powder to hexamethyldisilazane is 10-13:1; the mass ratio of the modified alumina powder to sodium metaaluminate in the sodium metaaluminate solution is 7-10:2-3.

9. The method for preparing mesoporous ultrastable alumina according to claim 1, wherein: The temperature of the heating in step S4 is 50-55°C.

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