Method for preparing high-purity spherical nano-alumina by alcoholate hydrolysis process

Abstract

The application discloses a method for preparing high-purity spherical nano-alumina by alcoholate hydrolysis process. The method comprises the following steps: mixing alkane and n-butanol according to a volume ratio to form a mixed solvent, wherein the volume fraction of the alkane is 10%-50%; dissolving aluminum alcoholate in the mixed solvent, adding deionized water under stirring to form a water-in-oil reverse micelle dispersion system; controlling the reaction temperature at 0-20 DEG C to carry out a confined hydrolysis reaction; demulsifying, separating, washing, drying and calcining to obtain high-purity spherical nano-alumina. The volume fraction of the alkane is larger, and the particle size of the obtained product is smaller, so that the particle size can be controlled in the range of 50-300 nm. The application does not need to add exogenous surfactants, does not need to use high-toxicity solvents such as acetonitrile, and does not need complex equipment such as a micro-channel reactor, and the process is simple, green and environment-friendly. The obtained product has high sphericity, controllable particle size and a purity of more than 99.99%, and is suitable for lithium battery diaphragm coating, precision polishing, transparent ceramic and other fields.

Description

Technical Field

[0001] This invention relates to the field of nanomaterial preparation technology, specifically to a method for preparing high-purity spherical nano-alumina using an alkoxide hydrolysis process. Background Technology

[0002] High-purity spherical nano-alumina, due to its excellent hardness, high melting point, chemical stability, and bioinertness, has broad application prospects in fields such as lithium-ion battery separator coatings, precision polishing, transparent ceramics, catalyst supports, and LED substrates. The alkoxide hydrolysis method, with its advantages of high raw material purity, mild reaction conditions, and high product purity, has become one of the important methods for preparing high-purity alumina.

[0003] However, the preparation of spherical nano-alumina by alkoxide hydrolysis has long faced the following technical challenges: the hydrolysis reaction of aluminum alkoxides is extremely fast and uncontrollable, and the aluminum hydroxide nanoparticles produced by hydrolysis are prone to agglomeration, resulting in a wide particle size distribution and poor sphericity of the product. To solve these problems, various improvement schemes have been proposed in the prior art. For example, Chinese invention patent CN117819584A uses a microchannel reactor for stepwise hydrolysis combined with ultrasonic-assisted dispersion to successfully prepare high-purity, large-pore-volume, and high-strength spherical alumina. However, the product obtained by this method is millimeter-sized alumina spheres, which belong to the catalyst carrier level and cannot meet the application requirements of nanoscale powders. Moreover, the microchannel reactor is difficult to scale up and requires high equipment investment, limiting its industrial application. Chinese invention patent CN115057457A obtains spherical nano-alumina through alkoxide hydrolysis combined with alkaline hydrothermal reaction, jet-assisted atomization, and spray drying as post-treatment steps. However, this process route is lengthy and energy-intensive, and the post-treatment spheroidization step increases the process complexity and the risk of impurity introduction.

[0004] Furthermore, academic literature (Sun Ziting et al., *Bulletin of the Chinese Ceramic Society*, Vol. 39, No. 3, 2020) reported a method for preparing submicron spherical alumina via alkoxide hydrolysis in an acetonitrile-n-butanol mixed solvent, finding that the sphericity of the product improved with increasing acetonitrile to n-butanol volume ratio. However, this study still has the following limitations: First, acetonitrile is costly and has a certain degree of toxicity, and solvent recovery is difficult, which is not conducive to industrial application; second, the product particle size is mainly controlled by adjusting the water volume, with limited control sensitivity, and the products are mainly submicron in size (200-500 nm), making it difficult to stably obtain nano-alumina below 100 nm; third, this study did not reveal the quantitative control law between solvent composition and product particle size, and could not achieve precise predictive control of particle size.

[0005] Another common technical approach involves using reverse micelles or microemulsions, employing surfactants (such as AOT, CTAB, Triton X-100, etc.) to form water-in-oil droplets as nanoreactors, synthesizing nanoparticles within a water core. However, the introduction of surfactants introduces serious contamination problems. Even after multiple washes, residual surfactants still affect the purity of alumina, especially for high-end applications requiring purity above 4N5 (99.995%). Furthermore, surfactant removal increases process costs and wastewater treatment burden.

[0006] In summary, existing technologies for preparing high-purity spherical nano-alumina suffer from the following contradictions: they rely on complex equipment (microchannel reactors, spray drying towers) or high-cost, highly toxic solvents (acetonitrile), or require the introduction of exogenous surfactants that affect product purity, and they struggle to achieve precise control over product particle size from nanometer to submicron scales under simple process conditions. Therefore, developing a method for preparing high-purity spherical nano-alumina that requires no complex equipment, no exogenous surfactants, no highly toxic solvents, and can achieve controllable particle size has significant industrial value. Summary of the Invention

[0007] In view of this, this invention proposes a method for preparing high-purity spherical nano-alumina through alkoxide hydrolysis. The core technical concept of this invention lies in: utilizing a mixed solvent composed of alkanes and n-butanol to create a dispersion environment conducive to the formation of spherical nanoparticles without adding any exogenous surfactants. It was unexpectedly discovered that by adjusting the volume fraction of alkanes in the mixed solvent, the particle size of the final alumina product can be quantitatively controlled, achieving monotonic and precise control within the range of 50-300 nm. Simultaneously, conducting the hydrolysis reaction at low temperatures (0-20℃) further suppresses the intensity of the hydrolysis reaction, which is beneficial for obtaining smaller and more uniform product particles. This method achieves a simple, controllable, and green preparation of high-purity spherical nano-alumina without relying on complex methods such as acetonitrile, surfactants, and microchannel reactors.

[0008] The technical solution of this invention is implemented as follows: This invention provides a method for preparing high-purity spherical nano-alumina by alkoxide hydrolysis, the method comprising the following steps: (a) mixing alkanes and n-butanol in a volume ratio to form a mixed solvent, wherein the volume fraction of the alkanes in the mixed solvent is 10%~50%; (b) dissolving aluminum alkoxides in the mixed solvent, and adding deionized water under stirring to form a uniform dispersion system; (c) controlling the reaction temperature at 0~20℃ to allow the aluminum alkoxides to undergo a hydrolysis reaction; (d) demulsifying, separating, washing, drying, and calcining to obtain high-purity spherical nano-alumina; wherein, the larger the volume fraction of the alkanes, the smaller the particle size of the obtained product, thereby achieving controllable adjustment of the particle size in the range of 50~300 nm.

[0009] In the above technical solution, the alkane and n-butanol mixed solvent is the key medium of the present invention. n-Butanol possesses both hydrophilic and hydrophobic groups, and when mixed with water in the alkane continuous phase, it can spontaneously arrange itself at the oil-water interface, thereby forming a stable emulsion dispersion system. The alkane (such as n-decane, dodecane, etc.) as the continuous phase not only provides a low-polarity reaction environment but also adjusts the size of the dispersed phase droplets through its interaction with n-butanol, thus affecting the particle size of the hydrolysis products.

[0010] In some embodiments, the alkane in step (a) is selected from one or more of n-decane, undecane, or dodecane. Preferably, the alkane is n-decane or dodecane. These medium- to long-chain alkanes have suitable miscibility with n-butanol and moderate boiling points, facilitating recovery and recycling through distillation. Compared to acetonitrile, alkanes are non-toxic, low-cost, and immiscible with water, resulting in a more stable dispersion system.

[0011] In some embodiments, the volume fraction of the alkane in step (a) is 20% to 40%. Within this preferred range, the particle size distribution of the product is narrower (Span ≤ 0.5). When the alkane volume fraction is too low (<10%), the system is close to a homogeneous system of n-butanol-water, and the product particle size is large and widely distributed; when the alkane volume fraction is too high (>50%), the number of dispersed phase droplets decreases, the concentration of aluminum alkoxide in a single droplet is too low, the hydrolysis reaction is incomplete, and the yield decreases. Therefore, controlling the alkane volume fraction between 10% and 50% is a reasonable range that balances particle size adjustability, product morphology, and yield.

[0012] In some embodiments, the aluminum alkoxide described in step (b) is aluminum isopropoxide or aluminum sec-butoxide. These two alkoxides are the most commonly used aluminum alkoxide raw materials in industry, exhibiting high reactivity and solubility.

[0013] In some embodiments, the stirring speed in step (b) is 500-2000 rpm. The stirring speed directly affects the droplet size of the dispersion system. High-speed stirring is beneficial for forming smaller and more uniform droplets, thereby resulting in smaller particle size and narrower distribution of the final product.

[0014] In some embodiments, the hydrolysis reaction described in steps (b) and (c) is carried out in two steps: first, a first portion of water is added at a water / aluminum molar ratio of 1:1 to 1.5:1, and then a second portion of water is added to bring the total water / aluminum molar ratio to 3:1 to 30:1 to complete the hydrolysis. The advantage of the two-step water addition strategy is that the first step involves adding less water, which is conducive to forming a stable dispersion system and allowing the aluminum alkoxide to partially hydrolyze at the interface; the second step involves slowly adding the remaining water under the condition that the dispersion system has stabilized, so that the hydrolysis reaction can continue and avoid the runaway hydrolysis and system destruction caused by adding a large amount of water at once.

[0015] In some embodiments, the reaction temperature in step (c) is controlled within the range of 0–20°C. The low temperature significantly reduces the rate constant of the hydrolysis reaction, resulting in a more gradual reaction rate, which is beneficial for obtaining product particles of uniform size. Simultaneously, the low temperature suppresses the localized temperature rise caused by the exothermic hydrolysis reaction, preventing the dispersed droplets from coalescing due to intensified thermal motion. This temperature range is one of the key process features of this invention.

[0016] In some embodiments, in step (b), before or simultaneously with dissolving the aluminum alkoxide, nano-alumina seeds are added to the mixed solvent. The seeds have a particle size of 5-20 nm and are added at an amount of 0.5%-2% of the aluminum source mass. The seeds are preferably γ-Al₂O₃ or α-Al₂O₃. The introduction of seeds can provide heterogeneous nucleation sites, which is beneficial for improving the particle size uniformity of the product. If the seed addition is too low (<0.5%), the effect is not significant; if it is too high (>2%), the seeds themselves may agglomerate, leading to a wider particle size distribution.

[0017] In some embodiments, in step (b), the aluminum alkoxide undergoes a pre-coordination reaction with acrylic acid before being dissolved in the mixed solvent to generate an aluminum alkoxide-acrylic acid complex, wherein the molar ratio of acrylic acid to aluminum alkoxide is 1% to 3%. The pre-coordination reaction is carried out at room temperature to 50°C. The polarity of this complex is lower than that of the original aluminum alkoxide, thus its hydrolytic reactivity is reduced, which helps to moderate the hydrolysis rate and protect the stability of the dispersion system. When the amount of acrylic acid is too low (<1%), the modification effect is not obvious; when it is too high (>3%), it may over-passivate the aluminum alkoxide, resulting in incomplete hydrolysis.

[0018] In some embodiments, the demulsification in step (d) is achieved by adding a small amount of methanol or ethanol. Methanol or ethanol can disrupt the structure of the dispersion system, releasing the solid product and facilitating subsequent separation. The washing process involves sequentially washing with ethanol and deionized water until neutral to remove residual organic matter and inorganic impurities.

[0019] In some embodiments, the calcination conditions in step (d) are: calcination at 500-600°C for 2-4 hours to obtain γ-Al₂O₃ phase spherical nano-alumina, or calcination at 1100-1200°C for 2-4 hours to obtain α-Al₂O₃ phase spherical nano-alumina. Different crystalline phases of alumina are suitable for different applications: γ-Al₂O₃ has a higher specific surface area and is suitable as a catalyst support or adsorbent; α-Al₂O₃ has higher hardness and thermal stability and is suitable as a precision polishing abrasive or structural ceramic raw material.

[0020] In some embodiments, the high-purity spherical nano-alumina has a purity ≥99.99% and a particle size distribution Span ≤0.5. Because the entire process of this invention does not use any exogenous surfactants or highly toxic, difficult-to-recover solvents such as acetonitrile, but only alkanes and n-butanol (both of which can be recovered and recycled through distillation), the final product has extremely low impurity content, consistently achieving a purity of 4N5 (99.995%) or higher. Simultaneously, through precise control of the alkane ratio to regulate the product particle size, a very narrow particle size distribution is achieved, with the Span value controlled below 0.5.

[0021] It should be noted that the possible dispersed droplet structures mentioned in this application are one possible explanation for observed phenomena (such as whitening of the reaction system and controllable particle size) based on existing colloid chemistry theories. The applicant is not limited to this theoretical explanation; the technical effects of this invention can be achieved through the specific operating conditions and product parameters in the embodiments, and do not depend on the correctness of the formation mechanism. As long as the steps and parameters described in this application are followed, the high-purity spherical nano-alumina can be obtained.

[0022] The present invention has the following advantages over the prior art: This invention is the first to discover that, in an alkane-n-butanol mixed solvent system, the particle size of the final alumina product can be monotonically and precisely controlled within the range of 50-300 nm by adjusting the volume fraction of the alkane. This allows for the simple, green, and low-cost preparation of high-purity spherical nano-alumina without relying on complex equipment such as microchannel reactors or ultrasound, without adding any surfactants, or using highly toxic solvents such as acetonitrile. Furthermore, by introducing seed crystals and carboxylic acid pre-coordination modification, the particle size uniformity and morphology controllability are further improved, providing a versatile and scalable technical solution for the industrial production of high-purity spherical nano-alumina of different specifications. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a scanning electron microscope image of the high-purity spherical nano-alumina obtained in Example 6 of the present invention.

[0025] As can be seen from the image, the particles are regular spherical in shape and uniform in size. Detailed Implementation

[0026] The present invention will be further described in detail below with reference to specific embodiments and comparative examples. Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods in the art, and the materials and reagents used are commercially available. The aluminum isopropoxide and aluminum sec-butoxide used in the embodiments of the present invention were provided by Yangzhou Zhongtianli New Material Co., Ltd., with a purity ≥99.99%; n-butanol, n-decane, and dodecane were all analytical grade reagents purchased from Sinopharm Chemical Reagent Co., Ltd.; the resistivity of deionized water was ≥18 MΩ·cm.

[0027] Detection methods and instruments Particle size and particle size distribution: Approximately 20 mg of calcined alumina powder was added to 10 mL of anhydrous ethanol and ultrasonically dispersed for 10 minutes (40 kHz, 200 W) to form a homogeneous suspension. The particle size distribution was determined using a Malvern Panaco Zetasizer Nano ZS90 dynamic light scattering particle size analyzer. Test conditions: temperature 25℃, scattering angle 90°, equilibration time 120 seconds. Each sample was measured in triplicate. The volume average particle sizes D10, D50, and D90 were recorded, and the particle size distribution width index was calculated using Span = (D90-D10) / D50. The instrument was calibrated before measurement using 100 nm polystyrene standard spheres of known particle size.

[0028] Purity analysis: The impurity element content (Fe, Na, Si, Ca, Mg, etc.) of the final calcined product was determined using an Agilent 7800 inductively coupled plasma mass spectrometer (ICP-MS), and the Al2O3 content was calculated by difference method. Sample digestion was performed using a microwave digester with a concentrated nitric acid and hydrofluoric acid system at 200℃ for 30 minutes, followed by volume adjustment and analysis.

[0029] Crystal phase analysis: A Panaco Empyrean X-ray diffractometer (XRD) was used with Cu Kα radiation (λ=0.15406nm), tube voltage 40 kV, tube current 40 mA, scanning range 10°~80°, and step size 0.02°. The calcined alumina powder was pressed into a flat sample tray, and the diffraction pattern was recorded and compared with a standard PDF card to determine the crystal phase.

[0030] Morphological characterization: A small amount of calcined alumina powder was dispersed in anhydrous ethanol and dropped onto a copper mesh carbon support film. After drying, the particle morphology was observed using a field emission scanning electron microscope (SEM, Hitachi SU8010) with an accelerating voltage of 5 kV and a magnification of 50,000×. The SEM images (see...) Figure 1 The particles can be confirmed to be spherical.

[0031] Infrared spectroscopy analysis (used in examples involving carboxylic acids): A PerkinElmer SpectrumTwo Fourier transform infrared spectrometer was used, KBr pellet method, scanning range 4000~400 cm⁻¹ -1 Aluminum alkoxide-carboxylic acid complex samples were mixed with dry KBr and pressed into pellets. Infrared spectra were recorded, focusing on the asymmetric stretching vibration peak of the carboxylate ion (approximately 1580 cm⁻¹). -1 ) and symmetrical stretching vibration peak (approximately 1410 cm⁻¹) -1 The presence of ) indicates the occurrence of a coordination reaction.

[0032] Example 1 This embodiment is used to specifically illustrate the method of the present invention for preparing high-purity spherical nano-alumina. The operation steps are as follows: (1) Preparation of mixed solvent: Mix n-decane and n-butanol at a volume ratio of 1:9 (10% volume fraction of n-decane) and stir for 5 minutes at room temperature to obtain mixed solvent.

[0033] (2) Aluminum alkoxide dissolution: Under dry nitrogen protection, 5.0 g of aluminum sec-butoxide was dissolved in 50 mL of the above mixed solvent and stirred in a 30°C water bath until completely dissolved to obtain an aluminum alkoxide solution with a concentration of 10 wt%.

[0034] (3) Hydrolysis reaction: The above solution was transferred to a 250 mL jacketed reactor. The temperature of the reactor jacket water bath was set to 15℃, and stirring was started (1500 rpm). The first portion of deionized water was added dropwise to the system at a flow rate of 0.5 mL / min, controlling the water / aluminum molar ratio of the first portion of deionized water to be 1.2:1. During the dropwise addition, the reaction system gradually changed from transparent to milky white, indicating the formation of a stable emulsion dispersion system. After the first portion of water was added, stirring was continued for 30 minutes to allow the system to stabilize fully. Then, the second portion of deionized water was added at a flow rate of 0.5 mL / min until the total water / aluminum molar ratio was 12:1, and the reaction was continued with stirring for 2 hours. During the reaction, the system temperature was controlled at 15±2℃ through a cooling water circulation system.

[0035] (4) Demulsification and separation: Add 5 mL of anhydrous ethanol to the reaction system and stir for 5 minutes to break up the dispersion. Transfer the mixture to a centrifuge tube and centrifuge at 8000 rpm for 10 minutes. Discard the upper organic phase and collect the lower white precipitate. Wash the precipitate twice with ethanol and then three times with deionized water until the conductivity of the washing solution drops below 10 μS / cm.

[0036] (5) Drying and calcination: The washed product was dried in an oven at 105℃ for 6 hours. The dried white powder was placed in a muffle furnace and heated to 550℃ at a heating rate of 5℃ / min. It was then calcined at this temperature for 3 hours and naturally cooled to room temperature to obtain γ-Al2O3 phase spherical nano-alumina.

[0037] Example 2 This embodiment is basically the same as that of embodiment 1, except that the volume ratio of n-decane to n-butanol in step (1) is adjusted to 3:7 (30% volume fraction of n-decane), and the other operating conditions are exactly the same.

[0038] Example 3 This embodiment is basically the same as that of embodiment 1, except that the volume ratio of n-decane to n-butanol in step (1) is adjusted to 5:5 (n-decane volume fraction 50%), and the other operating conditions are exactly the same.

[0039] Example 4 This embodiment further introduces seed crystals based on embodiment 2.

[0040] (1) Preparation of mixed solvent: Same as in Example 2.

[0041] (2) Preparation of seed suspension: Take γ-Al2O3 nanopowder with an average particle size of 5 nm (purchased from Sigma-Aldrich, catalog number 934054, purity 99.95%, crystal phase is γ phase), add it to 5 mL n-butanol, and disperse it ultrasonically (40 kHz, 200 W) for 15 minutes under ice-water bath conditions to obtain a seed suspension with a concentration of 2 g / L.

[0042] (3) Aluminum alkoxide dissolution and seed crystal addition: Under dry nitrogen protection, 5.0 g of aluminum sec-butoxide was dissolved in 50 mL of mixed solvent and stirred in a 30°C water bath until completely dissolved. 0.5 mL of the seed crystal suspension prepared in step (2) was added to the solution, so that the amount of seed crystal added was 1.0% of the mass of the aluminum source. Stirring was continued for 5 minutes to ensure uniform dispersion of the seed crystals.

[0043] (4) to (6) are the same as steps (3) to (5) in Example 1.

[0044] Example 5 This embodiment further introduces carboxylic acid pre-coordination modification based on Example 2.

[0045] (1) Preparation of aluminum alkoxide-carboxylic acid complex: 5.0 g of aluminum sec-butoxide and 0.10 g of acrylic acid (acrylic acid / aluminum molar ratio of approximately 2.5%) were mixed and stirred in a 40°C water bath for 45 minutes to allow the acrylic acid and aluminum sec-butoxide to undergo a complete pre-coordination reaction, generating an aluminum sec-butoxide-acrylic acid complex. Infrared spectroscopy analysis of the prepared complex showed that the acrylic acid monomer was at approximately 1700 cm⁻¹. -1 The -C=O stretching vibration peak at this location is significantly weakened, while the peak at approximately 1585 cm⁻¹ is also significantly reduced. -1 Approximately 1415 cm -1 Carboxylate ions (COO) appear at the location - The asymmetric and symmetric stretching vibration absorption peaks indicate that acrylic acid has undergone a coordination reaction with aluminum alkoxide to form an aluminum alkoxide-carboxylic acid complex.

[0046] (2) Preparation of mixed solvent: Same as in Example 2.

[0047] (3) Complex dissolution and hydrolysis: Dissolve the complex prepared in step (1) in 50 mL of mixed solvent, and the remaining steps are the same as steps (2) to (4) in Example 1.

[0048] Example 6 This embodiment is a combination of Embodiment 4 and Embodiment 5, with the introduction of seed crystals and carboxylic acid pre-coordination modification.

[0049] (1) Preparation of aluminum alkoxide-carboxylic acid complex: Same as step (1) in Example 5.

[0050] (2) Preparation of mixed solvent: Same as in Example 2.

[0051] (3) Preparation of seed suspension: Same as step (2) in Example 4.

[0052] (4) Complex dissolution and seed addition: Dissolve the aluminum sec-butoxide-acrylic acid complex prepared in step (1) in 50 mL of mixed solvent, add 0.5 mL of the seed suspension prepared in step (3) (1.0% of the seed addition amount), and stir for 5 minutes.

[0053] (5) Hydrolysis reaction: Same as step (3) in Example 1.

[0054] (6) Separation and calcination: Same as steps (4) to (5) in Example 1.

[0055] Example 7 In this embodiment, aluminum isopropoxide is used instead of aluminum sec-butoxide as the aluminum source, and the other conditions are the same as in Example 6.

[0056] (1) Preparation of aluminum alkoxide-carboxylic acid complex: 5.0 g aluminum isopropoxide and 0.10 g acrylic acid were mixed and stirred in a water bath at 40°C for 45 minutes to carry out pre-coordination reaction.

[0057] (2) to (6) are the same as steps (2) to (6) in Example 6.

[0058] Example 8 In this embodiment, dodecane is used instead of n-decane as the continuous alkane phase, and the other conditions are the same as in Example 2.

[0059] (1) Preparation of mixed solvent: Mix dodecane and n-butanol at a volume ratio of 3:7 (dodecane volume fraction 30%).

[0060] (2) to (5) are the same as steps (2) to (5) in Example 1.

[0061] Example 9 (calcination temperature adjusted to obtain α-Al2O3 phase) This embodiment uses the process conditions of Example 2 to prepare hydrated alumina precursor, but increases the calcination temperature to obtain α-Al2O3 phase.

[0062] The precursor preparation steps are the same as steps (1) to (4) in Example 1. After drying, the white powder is placed in a muffle furnace and heated to 1150°C at a heating rate of 5°C / min. It is then calcined at this temperature for 3 hours and naturally cooled to room temperature to obtain α-Al2O3 phase spherical nano-alumina.

[0063] Comparative Example 1 (Alkane-free control) This comparative example illustrates the crucial role of the presence of a continuous alkane phase in controlling product particle size.

[0064] Except for step (1), in which only pure n-butanol (0% n-decane volume fraction) is used as the mixed solvent, the other operating conditions are exactly the same as in Example 1.

[0065] Comparative Example 2 (Acetonitrile-n-Butanol Solvent Control) This comparative example is used to compare the differences between the acetonitrile-n-butanol system in the prior art and the alkane-n-butanol system of the present invention. The acetonitrile / n-butanol volume ratio of 3:1 described in the D3 paper (Sun Ziting et al., *Bulletin of the Chinese Ceramic Society*, Vol. 39, No. 3, 2020) was used, with a mixed solvent of acetonitrile and n-butanol used instead of the n-decane-n-butanol mixed solvent of the present invention. Other conditions, such as hydrolysis temperature and water addition method, were kept consistent with Example 1 (15°C, two-step water addition) to compare the influence of the solvent system under the same reaction conditions. It should be noted that the interfacial structure of the acetonitrile-n-butanol-water system is fundamentally different from the alkane-n-butanol system of the present invention, which does not contain acetonitrile, due to the miscibility of acetonitrile and water. This comparative example is only used to illustrate the comparative effect of different solvent systems under the same reaction framework.

[0066] Comparative Example 3 (Room Temperature Hydrolysis Control) This comparative example illustrates the effect of low temperature conditions on the particle size distribution of the product.

[0067] The hydrolysis temperature in step (3) of Example 1 was adjusted to 25°C (room temperature), and the remaining operating conditions were exactly the same as in Example 1.

[0068] Comparative Example 4 (Conventional one-step hydrolysis with water as a control) This comparative example illustrates the effect of a two-step water addition strategy on particle size distribution.

[0069] The two-step water addition in step (3) of Example 1 is changed to a one-step water addition: deionized water with a total water / aluminum molar ratio of 12:1 is added directly to the reaction system at one time, and the dropping rate is still 0.5 mL / min. The other operating conditions are the same as in Example 1.

[0070] Comparative Example 5 (No seed crystals / No carboxylic acid / Room temperature / One-step water addition control—Simplest conditions) This comparative example is used to comprehensively evaluate the combined effect of the core process parameters of the present invention.

[0071] The following simplest process conditions are adopted: (1) n-decane and n-butanol are mixed in a volume ratio of 3:7 to form a mixed solvent; (2) aluminum sec-butoxide is dissolved in the mixed solvent without adding seed crystals and carboxylic acid; (3) at room temperature (25°C), all deionized water (water / aluminum molar ratio 12:1) is added at a rate of 0.5 mL / min; (4) the remaining steps are the same as steps (4) to (5) of Example 1.

[0072] Summary of conditions for examples and comparative examples Table 1 Summary of process parameters for each embodiment / comparative example

[0073] Note: The calcination temperature in Example 9 is 1150℃, which is different from 550℃ in Example 2, in order to obtain the α-Al2O3 phase.

[0074] Characterization results of the products of Examples 1-9 and Comparative Examples 1-5 Table 2 Characterization results of products from Examples 1-9

[0075] Table 3 Characterization results of products from Comparative Examples 1-5

[0076] Table 4. Impurity element content of the product in Example 6 (determined by ICP-MS)

[0077] Note: Al2O3 purity = 100% - (total content of each impurity element after conversion to oxides). Calculations show that the Al2O3 purity of the product in Example 6 is above 99.995%. The impurity spectra of other examples are similar to those of Example 6 and will not be listed individually.

[0078] Results Analysis (I) The regulation of particle size by alkane volume fraction Examples 1-3 investigated the effect of varying the n-decane volume fraction from 10% to 50% on the product particle size. Table 2 shows that as the n-decane volume fraction increased from 10% to 50%, the product particle size D50 decreased from 285 nm to 62 nm. Simultaneously, the Span value remained below 0.5 throughout the entire range, indicating that each example achieved a narrow particle size distribution.

[0079] (II) Comparison of alkane-free systems (Comparative Example 1) Comparative Example 1 used pure n-butanol as the solvent without adding an alkane continuous phase. The results showed that the product's D50 increased to 438 nm and the Span increased to 0.85.

[0080] (III) Comparison with the existing acetonitrile-n-butanol system (Comparative Example 2) Comparative Example 2 prepared the product using acetonitrile and n-butanol at a volume ratio of 3:1, following the conditions described in paper D3. The D50 was 226 nm, and the Span was 0.56. This was compared to Example 2 (30% n-decane, acetonitrile-free): Example 2 had a D50 of 142 nm and a Span of 0.42. Further comparison was made between Example 6 (with both seed crystals and carboxylic acid added, 30% n-decane) and Comparative Example 2: Example 6 had a D50 of 98 nm, a Span of 0.28, and a purity of 99.995%.

[0081] (iv) The effect of low temperature conditions Comparison of Example 2 (15°C) and Comparative Example 3 (25°C): When the reaction temperature increased from 15°C to 25°C, the product D50 increased from 142 nm to 178 nm, and the Span increased from 0.42 to 0.68.

[0082] (v) The function of adding water in two steps Comparison of Example 2 (two-step water addition) and Comparative Example 4 (one-step water addition): Under the one-step water addition condition, the product D50 was 196 nm and the Span was 0.72; under the two-step water addition condition, the D50 was 142 nm and the Span was 0.42.

[0083] (vi) The role of seed crystals in inducing heterogeneous nucleation Comparison of Example 2 (without seed crystals) and Example 4 (with 1.0% γ-Al2O3 seed crystals): After adding seed crystals, the product D50 decreased from 142 nm to 130 nm, and the Span decreased from 0.42 to 0.32.

[0084] (vii) The role of carboxylic acid pre-coordination modification Comparison of Example 2 (without carboxylic acid) and Example 5 (pre-coordinated with acrylic acid): After carboxylic acid modification, the D50 of the product decreased from 142 nm to 118 nm, and the Span decreased from 0.42 to 0.35.

[0085] (viii) Synergistic effect of the three elements Example 6 (with simultaneous addition of seed crystals and carboxylic acid) further reduced D50 to 98 nm and Span to 0.28, and increased purity to 99.995% compared to Example 2. The Span (0.28) of Example 6 was less than that of Example 4 (0.32) and Example 5 (0.35). In contrast, Comparative Example 5 (simplest conditions) had D50 = 352 nm and Span = 0.94. From Comparative Example 5 to Example 6, each additional process element resulted in a stepwise improvement in product performance.

[0086] (ix) The influence of the type of aluminum alkoxide Comparison of Example 6 (aluminum sec-butoxide) and Example 7 (aluminum isopropoxide): The products obtained under the same conditions have D50 values ​​of 98 nm and 105 nm, respectively, and Span values ​​of 0.28 and 0.30, respectively, with similar indicators.

[0087] (x) The influence of the type of alkane Comparison of Example 2 (n-decane) and Example 8 (dodecane): Under the same volume fraction (30%), the product D50 of both was 142 nm and 138 nm, respectively, and the Span was 0.42 and 0.44, respectively, with no significant difference.

[0088] (xi) Effect of different calcination temperatures Comparison between Example 9 (calcined at 1150℃) and Example 2 (calcined at 550℃): The precursors are the same, but the calcination temperatures are different. The resulting products have crystalline phases of α-Al2O3 and γ-Al2O3, respectively, and the particle size and particle size distribution are basically the same.

[0089] (xii) Purity analysis of products from each example The purity of the products in all the above embodiments reached 99.99% or higher, with the purity of Example 6 reaching 99.995%.

[0090] (xiii) Morphological analysis SEM observation results (see) Figure 1 This indicates that the product of Example 6 consists of regular spherical particles.

[0091] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing high-purity spherical nano-alumina via alkoxide hydrolysis, characterized in that, Includes the following steps: (a) An alkane and n-butanol are mixed in a volume ratio to form a mixed solvent, wherein the volume fraction of the alkane in the mixed solvent is 10% to 50%; (b) Dissolve the aluminum alkoxide in the mixed solvent, and add deionized water while stirring to form a uniform dispersion system; (c) Control the reaction temperature at 0~20℃ to allow the aluminum alkoxide to undergo a hydrolysis reaction; (d) Demulsification, separation, washing, drying, and calcination were performed to obtain high-purity spherical nano-alumina; The higher the volume fraction of the alkane, the smaller the particle size of the resulting product, thus achieving controllable adjustment of the particle size within the range of 50~300 nm.

2. The method according to claim 1, characterized in that, The alkane mentioned in step (a) is one or more of n-decane, undecane, or dodecane.

3. The method according to claim 1, characterized in that, The volume fraction of the alkane mentioned in step (a) is 20% to 40%.

4. The method according to claim 1, characterized in that, The aluminum alkoxide mentioned in step (b) is aluminum isopropoxide or aluminum sec-butoxide.

5. The method according to claim 1, characterized in that, The stirring speed described in step (b) is 500~2000 rpm.

6. The method according to claim 1, characterized in that, The hydrolysis reaction described in steps (b) and (c) is carried out in two steps: first, a first portion of water is added at a water / aluminum molar ratio of 1:1 to 1.5:1, and then a second portion of water is added to bring the total water / aluminum molar ratio to 3:1 to 30:1 to complete the hydrolysis.

7. The method according to claim 1, characterized in that, In step (b), before or simultaneously with dissolving the aluminum alkoxide, nano-alumina seed crystals are added to the mixed solvent. The seed crystals have a particle size of 5-20 nm and are added in an amount of 0.5%-2% of the mass of the aluminum source.

8. The method according to claim 1, characterized in that, In step (b), the aluminum alkoxide undergoes a pre-coordination reaction with acrylic acid before being dissolved in the mixed solvent to generate an aluminum alkoxide-acrylic acid complex, wherein the molar ratio of acrylic acid to aluminum alkoxide is 1% to 3%.

9. The method according to claim 1, characterized in that, The calcination conditions described in step (d) are: calcination at 500~600℃ for 2~4 hours to obtain the γ-Al2O3 phase, or calcination at 1100~1200℃ for 2~4 hours to obtain the α-Al2O3 phase.

10. The method according to claim 1, characterized in that, The high-purity spherical nano-alumina has a purity of ≥99.99% and a particle size distribution Span≤0.5.

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