A high-porosity pseudoboehmite and its solvent-free preparation method

Abstract

This invention discloses a high-porosity pseudoboehmite and its solvent-free preparation method. The preparation method specifically includes: first, heating and melting aluminum isopropoxide, then slowly adding it to deionized water under mechanical stirring to hydrolyze it, obtaining a semi-solid hydrolysis product; then, subjecting the hydrolysis product to a two-stage closed-loop semi-solid-phase aging treatment, i.e., first aging at a low temperature of 80-120℃, then aging at a high temperature of 140-180℃; finally, drying and pulverizing to obtain the high-porosity pseudoboehmite. This invention requires no additives or organic solvents, and achieves synergistic regulation of surface hydroxyl reconstruction and grain boundary anchoring through a two-stage aging process. The resulting product has a porosity retention rate ≥98% and a specific surface area of ​​300-400 m². 2 With a gel solubility index of ≥96%, it has the dual advantages of being green and environmentally friendly and having excellent performance, making it a promising candidate for industrial applications.

Description

Technical Field

[0001] This invention belongs to the field of inorganic material preparation technology, specifically relating to a high-porosity pseudoboehmite and its solvent-free preparation method. Background Technology

[0002] Boehmite (AlOOH·nH2O, n=0.08-0.62) is an incompletely crystalline hydrated alumina with high specific surface area, large pore volume, and excellent colloidal properties. It is widely used in petroleum refining catalyst supports, adsorbents, and functional materials. Aluminum alkoxide hydrolysis is one of the important methods for preparing high-purity boehmite; using aluminum isopropoxide as a raw material, a product with a purity of over 99.99% can be obtained.

[0003] However, the traditional aluminum alkoxide hydrolysis method suffers from a long-standing technical bottleneck: enormous solvent consumption. Aluminum isopropoxide is a solid and must be dissolved in a large amount of solvent (such as isopropanol) to react uniformly with water; the amount of solvent used is typically 5-20 times the molar ratio of the aluminum source. This not only leads to high energy consumption and large equipment investment for solvent recovery but also poses safety hazards (isopropanol and other solvents are flammable and explosive) and environmental pressures. For example, patent CN119976913A discloses a technology for preparing high-purity boehmite using the aluminum alkoxide method. While the product purity is high, it does not address the issue of solvent reduction. Patent CN112624167B discloses a method for preparing boehmite using aluminum ingots as raw materials and water vapor as a reaction component. Although this achieves the innovation of "no organic solvents," this method is based on the direct hydrolysis of metallic aluminum, resulting in a violent and difficult-to-control reaction, and it is not suitable for aluminum alkoxide systems. Furthermore, the "solvent-free crystal transformation" technology disclosed in patent CN118255372A achieves solvent-free transformation during the crystal transformation stage, but a large amount of solvent is still required during the synthesis stage.

[0004] To address the solvent reduction issue in the aluminum alkoxide synthesis stage, patent CN118125478A discloses a method for preparing highly colloidal alumina. This method involves spray-mixing molten aluminum isopropoxide with an aqueous solution of an additive to obtain a powdered solid, which is then dried and sintered at high temperature to obtain highly colloidal alumina. While this method achieves solvent reduction, it still has the following technical limitations: (1) Specific additives (such as acetylenol polyether and diethylene glycol monobutyl ether) must be added, increasing raw material costs and process complexity; (2) The spray-mixing reaction requires high-performance equipment, making industrial scale-up difficult; (3) The final product is sintered alumina, not boehmite, limiting its application scenarios; (4) It does not involve refined control of the aging process, making it difficult to guarantee pore volume retention.

[0005] Therefore, how to achieve solvent-free preparation of pseudoboehmite without the addition of external additives while maintaining or even improving the pore structure and colloidal properties of the product remains a technical challenge that urgently needs to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to provide a high-porosity pseudoboehmite and its solvent-free preparation method. By combining melt hydrolysis and a two-stage semi-solid phase aging process, a solvent-free method is achieved while maintaining a large pore volume and improving the colloidal properties, thus overcoming the shortcomings of existing solvent-free preparation technologies that require the addition of additives, have high equipment requirements, and are difficult to maintain pore volume.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a high-porosity pseudoboehmite includes the following steps: (1) Preparation of molten aluminum source: Aluminum isopropoxide is heated to above its melting point to completely melt it and obtain molten aluminum source; the heating temperature is 120-160 ℃, preferably 130-150 ℃; (2) Melt dispersion hydrolysis: Heat the aqueous phase (deionized water) to 50-90 ℃, and slowly add the aqueous phase to the molten aluminum source in step (1) under mechanical stirring. Control the feeding rate to keep the system in a semi-solid paste state. After the feeding is completed, continue stirring for 5-30 min to obtain the semi-solid hydrolysis product. (3) Two-stage closed semi-solid phase aging: The semi-solid hydrolysis product of step (2) is subjected to two-stage solid phase aging under closed conditions: the first stage is aged at 80-120 ℃ for 2-6 h, and the surface hydroxyl groups are slowly reconstructed at low temperature; the second stage is aged at 140-180 ℃ for 1-4 h, and the grain boundary anchoring is strengthened. (4) Post-processing: Take out the aged product obtained in step (3), dry it at 80-120 ℃ for 6-24 h, pulverize it, and obtain the pseudoboehmite product.

[0008] Furthermore, the molar ratio of the aqueous phase to aluminum isopropoxide is 2-8:1, preferably 3-5:1; Furthermore, the aqueous phase added in step (2) is added by dripping or by a thin stream, and the feeding rate is controlled at 0.5-5 mL / min per 100 g aluminum isopropoxide to ensure that the aqueous phase is uniformly dispersed in the molten aluminum source and does not undergo local over-hydrolysis.

[0009] Furthermore, the mechanical stirring speed in step (2) is 200-1000 rpm, and the stirring paddle type is selected from anchor type, paddle type or ribbon type to meet the mixing requirements of high viscosity system.

[0010] Furthermore, the heating method for the two-stage closed semi-solid phase aging in step (3) can be programmed heating, with the temperature naturally rising to the second aging temperature after the first aging stage is completed, or segmented temperature control.

[0011] Furthermore, in the two-stage closed semi-solid phase aging process described in step (3), the pressure inside the reactor is self-generated pressure and controlled within the range of 0.1-1.2 MPa.

[0012] Furthermore, the two-stage closed semi-solid phase aging process described in step (3) can be carried out continuously in the same reactor without the need for intermediate material transfer.

[0013] Furthermore, the drying temperature in step (4) is 80-120 °C and the time is 6-24 h.

[0014] This invention also provides a pseudoboehmite prepared by any of the above-described methods, wherein the pseudoboehmite has a layered stacked structure with a layer thickness of 3-8 nm and a specific surface area of ​​300-400 m². 2 / g, pore volume 0.68-0.85 cm³ 3 / g, gel solubility index ≥96%.

[0015] Furthermore, the pore volume retention rate of the pseudoboehmite is ≥98% (compared to conventional solvent-based products).

[0016] The beneficial effects of this invention are as follows: This invention discloses a solvent-free method for preparing high-porosity pseudoboehmite, requiring no additives and using zero organic solvents. Through a two-stage aging process, it achieves synergistic regulation of surface hydroxyl reconstruction and grain boundary anchoring. The resulting product exhibits a porosity retention rate ≥98% and a specific surface area of ​​300-400 m². 2 With a gel solubility index of ≥96%, it has the dual advantages of being green and environmentally friendly and having excellent performance, making it a promising candidate for industrial applications. Attached Figure Description

[0017] Figure 1 The XRD test results are for the samples prepared in Examples 1-5.

[0018] Figure 2 The results are TEM test results of the sample prepared in Example 1. Detailed Implementation

[0019] To better understand the technical solution of the present invention, the following detailed description is provided in conjunction with specific embodiments and accompanying drawings, but this does not limit the scope of protection of the present invention.

[0020] Example 1: Basic scheme (water / Al=4:1, two-stage aging)

[0021] (1) Preparation of molten aluminum source: Weigh 100 g of aluminum isopropoxide (purity ≥99.5%) and place it in a 500 mL reactor equipped with an anchor stirrer (stirring speed 300 rpm). Heat the oil bath to 140 ℃ to completely melt the aluminum isopropoxide into a transparent flowing liquid. (2) Melt dispersion hydrolysis: Measure 35 mL of deionized water (water / Al molar ratio = 4:1) and preheat it to 70 °C; under stirring conditions, slowly add it dropwise to the molten aluminum source obtained in step (1) at a rate of 1 mL / min. During the dropwise addition, the system gradually becomes a milky white semi-solid paste; after the dropwise addition is completed, continue stirring for 15 min to make the hydrolysis uniform. (3) Two-stage closed solid phase aging: The hydrolysis product of step (2) is immediately sealed in the reactor and placed in an oven. It is first aged at 100 °C for 4 h, and then the temperature is raised to 160 °C for 3 h. No solvent or additive is added during the aging process, and the pressure in the reactor is in the range of 0.3-1.0 Pa. (4) Post-processing: Take out the aged product, dry it at 100 °C for 12 h, crush and sieve it to obtain the pseudo-boehmite product, which is denoted as S1.

[0022] Example 2: Low water-to-aluminum ratio scheme (water / Al=3:1)

[0023] (1) Preparation of molten aluminum source: Same as step (1) in Example 1; (2) Melt dispersion hydrolysis: Measure 26 mL of deionized water (water / Al molar ratio = 3:1), preheat to 60 °C; add dropwise at a rate of 0.8 mL / min, the rest is the same as in Example 1; (3) Two-stage closed solid phase aging: The hydrolysis product of step (2) is immediately sealed in the reactor and placed in an oven. It is first aged at 90 °C for 5 h, and then the temperature is raised to 150 °C for 4 h. (4) Post-processing: Same as in Example 1, to obtain product S2.

[0024] Example 3: High water-to-aluminum ratio scheme (water / Al=5:1)

[0025] (1) Preparation of molten aluminum source: Same as step (1) in Example 1; (2) Melt dispersion hydrolysis: Measure 44 mL of deionized water (water / Al molar ratio = 5:1), preheat to 80 °C; add dropwise at a rate of 1.5 mL / min, the rest is the same as in Example 1; (3) Two-stage closed solid phase aging: The hydrolysis product of step (2) is immediately sealed in the reactor and placed in an oven. It is first aged at 110 °C for 3 h, and then the temperature is raised to 170 °C for 2 h. (4) Post-processing: Same as in Example 1, to obtain product S3.

[0026] Example 4: Two-stage aging temperature optimization scheme

[0027] (1) Preparation of molten aluminum source: Same as step (1) in Example 1; (2) Melt dispersion hydrolysis: Same as step (2) in Example 1; (3) Two-stage closed solid phase aging: The hydrolysis product of step (2) is immediately sealed in the reactor and placed in an oven. It is first aged at 80 °C for 6 h, and then the temperature is raised to 140 °C for 4 h. (4) Post-processing: Same as in Example 1, to obtain product S4.

[0028] Example 5: Optimization scheme for two-stage aging time

[0029] (1) Preparation of molten aluminum source: Same as step (1) in Example 1; (2) Melt dispersion hydrolysis: Same as step (2) in Example 1; (3) Two-stage closed solid phase aging: The hydrolysis product of step (2) is immediately sealed in the reactor and placed in an oven. It is first aged at 100 °C for 2 h, and then the temperature is raised to 160 °C for 4 h. (4) Post-processing: Same as in Example 1, to obtain product S5.

[0030] Comparative Example 1: Refer to method CN118125478A (including adjuvants + spraying)

[0031] (1) Preparation of molten aluminum source: Same as step (1) in Example 1; (2) Melt dispersion hydrolysis: Prepare an aqueous solution containing acetylenic diol polyether (additive / Al mass ratio = 0.02:1), and obtain powder by spray mixing reaction; (3) Post-processing: After drying at 100 ℃, the product is obtained and is denoted as D1.

[0032] Comparative Example 2: Melt hydrolysis + single-stage low-temperature aging

[0033] (1) Preparation of molten aluminum source: Same as step (1) in Example 1; (2) Melt dispersion hydrolysis: Same as step (2) in Example 1; (3) Single-stage aging: The hydrolysis product is immediately sealed in the reactor and placed in an oven for aging at 100 °C for 7 h; (4) Post-processing: Same as in Example 1, to obtain product D2.

[0034] Comparative Example 3: Melt hydrolysis + single-stage high-temperature aging

[0035] (1) Preparation of molten aluminum source: Same as step (1) in Example 1; (2) Melt dispersion hydrolysis: Same as step (2) in Example 1; (3) Single-stage aging: The hydrolysis product is immediately sealed in the reactor and placed in an oven for aging at 160 °C for 7 h; (4) Post-processing: Same as in Example 1, to obtain product D3.

[0036] Comparative Example 4: Melt hydrolysis + out-of-range low-temperature aging + normal high-temperature aging

[0037] (1) Preparation of molten aluminum source: Same as step (1) in Example 1; (2) Melt dispersion hydrolysis: Same as step (2) in Example 1; (3) Low-temperature aging beyond the range: The hydrolysis product is immediately sealed in the reactor and placed in an oven for aging at 60 °C for 4 h; (4) Normal high temperature aging: Reheat to 160 ℃ and age for 3 h; (5) Post-processing: Same as in Example 1, to obtain product D4.

[0038] Comparative Example 5: Melt hydrolysis + normal low-temperature aging + out-of-range high-temperature aging

[0039] (1) Preparation of molten aluminum source: Same as step (1) in Example 1; (2) Melt dispersion hydrolysis: Same as step (2) in Example 1; (3) Normal low temperature aging: The hydrolysis product is immediately sealed in the reactor and placed in an oven for aging at 100 °C for 4 hours; (4) Over-range high temperature aging: Reheat to 200 ℃ and age for 3 h; (5) Post-processing: Same as in Example 1, to obtain product D5.

[0040] Comparative Example 6: Traditional solvent-based aluminum alkoxide method

[0041] (1) Dissolution of aluminum isopropoxide: 50 g of aluminum isopropoxide was dissolved in 83 mL of isopropanol (aluminum concentration 1.0 mol / L). (2) Hydrolysis of aluminum isopropoxide: Hydrolyze for 3 h by slowly adding 66 mL of deionized water at 85 °C (water / Al molar ratio = 15:1); (3) Single-stage aging: The hydrolyzed product was aged at 100 °C for 8 h; (4) Post-processing: washing and drying to obtain pseudoboehmite product, denoted as D6.

[0042] The following performance tests were performed on the samples obtained from the above embodiments and comparative examples: 1. Phase analysis: X-ray diffraction (XRD) was used to analyze the phase composition of the sample.

[0043] 2. Morphological observation: The morphology of the sample was observed using a transmission electron microscope (TEM), and the thickness of the lamellar layers was calculated.

[0044] 3. Pore structure analysis: Specific surface area (BET method) and pore volume (BJH method) were determined by nitrogen adsorption-desorption method.

[0045] 4. Colloidal Index Test: A sample containing Al2O3 at mass m1 is dispersed in a mixed solution of 20 wt.% dilute nitric acid and deionized water to form a suspension with an Al2O3 mass fraction of 10% and an acid-to-aluminum ratio of n(acid):n(Al2O3) = 1:10. The suspension is stirred for 15 min, and after colloidal dissolution, it is centrifuged at 5000 r / min for 10 min. The bottom precipitate is dried, calcined at 1000 ℃, cooled, and weighed (m2). The fraction of alumina in the colloidal portion relative to the total alumina content in the sample [100% × (m1 - m2) / m1] is the colloidal index.

[0046] 5. Solvent usage calculation: Calculate the molar ratio of the total amount of organic solvent used in each method to aluminum isopropoxide.

[0047] 6. Pore volume retention rate calculation: The pore volume retention rate of each sample is calculated based on the pore volume of the traditional aluminum alkoxide solvent method (D4) (100%).

[0048] The test results are shown in Table 1 and Figures 1-2 As shown.

[0049] Depend on Figure 1 It can be seen that the samples prepared in the examples are all pseudoboehmite phases.

[0050] Table 1 Performance test results of the samples prepared in the examples and comparative examples

[0051] As shown in Table 1, compared with Comparative Example 1, the present invention does not add any additives during the preparation process, resulting in lower raw material costs and a simpler process, while D1 requires the addition of additives such as acetylenol polyether. Furthermore, the present invention uses mechanical stirring dispersion + two-stage semi-solid phase aging, which has low equipment requirements and is easy to scale up, while D1 uses spray mixing, which requires high equipment investment.

[0052] Due to excessive grain growth and agglomeration at high temperatures, single-stage high-temperature aging (D3) resulted in a significant decrease in pore volume (0.54 cm³). 3 / g); while the pore volume of single-stage low-temperature aging (D2) remained relatively good (0.76 cm). 3 / g), but due to insufficient surface hydroxyl reconstruction, the colloidal index is low (80%); the pore volume retention and colloidal index of the product aged at ultra-low temperature + normal high temperature (D4) and the product aged at normal low temperature + ultra-high temperature (D5) are both reduced; the two-stage aging at suitable temperature (S1-S5) of this invention achieves the synergy of high pore volume retention and high colloidal solubility, that is, the surface hydroxyl reconstruction is completed in the low temperature stage, and the grain boundary anchoring is strengthened in the high temperature stage.

[0053] The water-to-aluminum ratio also affects the performance of pseudo-boehmite products. When water / Al = 3:1 (S2), hydrolysis is incomplete and the colloidal properties are poor. When water / Al = 4-5:1 (S1, S3), the pore volume and colloidal properties are optimal, indicating that the water-to-aluminum ratio should be controlled at 4-5:1.

[0054] The pore volume retention rates of S1, S3, and S5 are all ≥100%, proving that the solvent-free + two-stage aging process of this invention can not only maintain the pore volume, but is even superior to the traditional solvent method, breaking through the traditional technical understanding that solvent-free methods inevitably sacrifice pore structure.

[0055] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made within the scope of the claims of the present invention should be included in the scope of the present invention.

Claims

1. A method for preparing a high-porosity pseudoboehmite, characterized in that: Includes the following steps: (1) Aluminum isopropoxide is heated to above its melting point to obtain a molten aluminum source; (2) After heating the aqueous phase, add it to the molten aluminum source obtained in step (1) under mechanical stirring. Control the feeding rate to keep the system in a semi-solid paste state to obtain a semi-solid hydrolysis product. (3) The semi-solid hydrolysis product obtained in step (2) is subjected to a two-stage semi-solid phase aging treatment under closed conditions. First, it is aged at 80-120 ℃ for 2-6 h, and then aged at 140-180 ℃ for 1-4 h. (4) The aged product obtained in step (3) is dried and pulverized to obtain boehmite.

2. The preparation method according to claim 1, characterized in that: The heating temperature in step (1) is 120-160 ℃.

3. The preparation method according to claim 1, characterized in that: In step (2), the temperature for heating the water phase is 50-90℃.

4. The preparation method according to claim 1, characterized in that: The aqueous phase in step (2) is added by dripping or by a thin stream, and the addition rate is controlled at 0.5-5 mL / min per 100 g of aluminum isopropoxide; the mechanical stirring speed is 200-1000 rpm.

5. The preparation method according to claim 1, characterized in that: The molar ratio of the aqueous phase to aluminum isopropoxide in step (2) is 3-5:

1.

6. The preparation method according to claim 1, characterized in that: In the two-stage semi-solid phase aging process described in step (3), the pressure inside the reactor is self-generated pressure and is controlled within the range of 0.1-1.2 MPa.

7. The preparation method according to claim 1, characterized in that: The drying temperature in step (4) is 80-120℃ and the time is 6-24 h.

8. Boehmite prepared by the preparation method according to any one of claims 1-7.

9. The pseudoboehmite according to claim 8, characterized in that: The pseudoboehmite exhibits a layered stacked structure with a layer thickness of 3-8 nm and a specific surface area of ​​300-400 m². 2 / g, pore volume 0.68-0.85 cm³ 3 / g, gel solubility index ≥96%.

10. The pseudoboehmite according to claim 8, characterized in that: Its pore volume retention rate is ≥98%.

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