Preparation method of hydroxyl nano-aluminum oxide with controllable hydroxyl content

Abstract

The application discloses a preparation method of hydroxyl nano-aluminum oxide with controllable hydroxyl content, and relates to the technical field of inorganic functional material preparation. The method comprises the following steps: performing online gasification purification of an organic aluminum precursor in a closed integrated reaction system; guiding the online gasification purified organic aluminum precursor into a hydrolysis-hydrothermal crystallization reaction zone through a gas phase transportation channel, performing a controlled hydrolysis reaction, and generating amorphous aluminum hydroxide sol; sealing the hydrolysis-hydrothermal crystallization reaction zone, performing hydrothermal treatment on the amorphous aluminum hydroxide sol, and crystallizing the amorphous aluminum hydroxide into hydroxyl nano-aluminum oxide; and recovering and drying the hydroxyl nano-aluminum oxide. The application solves the fundamental contradiction between the improvement of material chemical purity and the maintenance of surface functional hydroxyl in the prior art, avoids the damage of traditional acid washing or calcination to the hydroxyl, and significantly improves the dispersibility, interface bonding force and electrolyte wettability of the material in the field of lithium ion batteries.

Description

Technical Field

[0001] This invention relates to the field of inorganic functional material preparation technology, and in particular to a method for preparing hydroxyl nano-alumina with controllable hydroxyl content. Background Technology

[0002] In the field of advanced inorganic non-metallic materials technology, the preparation and application of nanoscale functional materials is one of the core driving forces for the development of many high-tech industries. Among them, hydroxyl nano-alumina, especially materials with boehmite crystal structures, plays a crucial role in catalyst supports, structural ceramics, and microelectronic packaging due to its unique physicochemical properties. In recent years, with the transformation and upgrading of the global energy structure, new energy technologies represented by lithium-ion batteries have developed rapidly, which has set unprecedentedly high standards for the performance of key battery materials. Against this backdrop, hydroxyl nano-alumina, with its high specific surface area, excellent thermal stability, good electrical insulation, and controllable nanoscale morphology, has increasingly demonstrated its application value in lithium-ion battery separator coatings and cathode material surface modification, becoming one of the key functional materials for improving the overall performance of batteries.

[0003] Specifically, existing technologies commonly employ the method of coating hydroxyl-containing nano-alumina particles onto the surface of polyolefin separators to construct composite ceramic separators. The design principle of this structure lies in the fact that the nano-alumina coating significantly improves the separator's heat resistance. In extreme cases of thermal runaway within the battery, it effectively suppresses the shrinkage and deformation of the separator due to high temperatures, thereby preventing internal short circuits caused by direct contact between the positive and negative electrodes and greatly enhancing battery safety. Simultaneously, the polar hydroxyl groups on the surface of the hydroxyl-containing nano-alumina improve the separator's wettability to the electrolyte, reduce interfacial impedance, and facilitate rapid and uniform lithium-ion transport, thus improving the battery's rate performance and cycle stability. Similarly, applying it as a coating agent to the surface of the positive electrode material can effectively suppress side reactions between the positive electrode active material and the electrolyte, slow down the deterioration of the interfacial structure, and thus extend the battery's cycle life. These applications have successfully solved some of the technical bottlenecks faced by early lithium batteries in terms of safety and cycle durability, constituting the current mainstream technical solution.

[0004] However, with the continuous improvement of energy density and safety standards for lithium-ion batteries, the industry's performance requirements for key materials have entered a more refined level, prompting engineers to re-examine some inherent technical bottlenecks in existing preparation processes. Currently, although there are various process routes for the preparation of hydroxyl nano-alumina, they generally face a core challenge: the purity of the material is difficult to meet the stringent standards for high-end lithium battery applications. Trace metal impurity ions, especially electrochemically active impurities such as iron, sodium, and calcium, introduced from raw materials or the preparation process, will undergo irreversible redox reactions in the complex electrochemical environment inside the battery, catalyzing electrolyte decomposition, destroying the stable passivation film on the electrode surface, leading to a continuous increase in interfacial impedance and irreversible loss of active lithium. More seriously, the presence of magnetic foreign impurities can cause them to aggregate during the coating process due to the magnetic field, piercing the separator substrate and forming physical micro-short circuits, becoming a major hidden danger that could induce battery safety accidents. To solve this problem, existing technologies typically supplement the preparation of hydroxyl nano-alumina with post-treatment purification steps such as acid washing and high-temperature calcination.

[0005] Building upon this, a deeper and less obvious technical contradiction gradually emerges: there is an inherent conflict between the pursuit of ultimate chemical purity in materials and maintaining the integrity of their core functional groups. The reason why hydroxyl-containing nano-alumina exhibits excellent performance in battery systems lies fundamentally in the abundant and controllable activity of hydroxyl groups on its crystal surface. These hydroxyl groups are not only key anchors for the material to achieve good dispersion in the slurry and form a firm bond with the separator substrate or cathode particles, but also the core functional groups that determine its surface electrochemical properties. However, the strong acid treatment process used to remove metallic impurities inevitably changes the protonation state and distribution density of surface hydroxyl groups, and may even destroy its layered crystal structure; while the high-temperature calcination process removes organic residues or achieves specific crystal phase transformations, it also triggers violent surface dehydroxylation reactions, leading to a large loss of active sites. Existing technologies often treat purity improvement and surface function regulation as two separate or even mutually restrictive technical steps. The "high-purity" hydroxyl nano-alumina obtained through rigorous purification may have severely degraded surface functionality, resulting in a series of secondary problems such as poor dispersibility in slurry, decreased coating adhesion, and weakened wetting effect on electrolyte, ultimately greatly reducing the overall benefits of material application. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing hydroxyl nano-alumina with controllable hydroxyl content, so as to solve the inherent defects of the existing hydroxyl nano-alumina preparation process, namely the fundamental contradiction between improving the chemical purity of the material and maintaining the surface functional hydroxyl groups.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0008] A method for preparing hydroxyl-containing nano-alumina with controllable hydroxyl content specifically includes the following steps:

[0009] Step S1, online vaporization and purification of precursor: In the precursor vaporization and purification reaction zone of the closed integrated reaction system, the organoaluminum precursor is heated and vaporized, so that the non-volatile metal impurities are retained in solid or liquid phase. The purified gaseous precursor is transported to the hydrolysis-hydrothermal crystallization reaction zone through the gas phase transport channel.

[0010] First, a closed, integrated reaction system is provided, consisting of a precursor vaporization and purification reaction zone, a gas phase transport channel, and a hydrolysis-hydrothermal crystallization reaction zone. The system is then protected with an inert atmosphere. Specifically, the inert atmosphere protection involves introducing argon gas with a purity of not less than 99.999% into the closed, integrated reaction system and maintaining a slight positive pressure until the oxygen concentration inside the system is below 10 ppm, ensuring that the entire preparation process is carried out in an oxygen- and anhydrous (except for the reaction medium) ultra-clean environment.

[0011] Subsequently, precursor loading and pretreatment of the reaction medium are performed. In the precursor vaporization and purification reaction zone, an industrial-grade organoaluminum precursor is loaded. This organoaluminum precursor is a substance that can be vaporized at a preset operating temperature and has a significant vapor pressure difference between its gaseous and liquid or solid phases; specifically, it is aluminum isopropoxide. In the hydrolysis-hydrothermal crystallization reaction zone, ultrapure water with a resistivity of not less than 18.2 MΩ·cm is introduced as the hydrolysis reaction medium, and a hydroxyl modifier is added to the ultrapure water. The hydroxyl modifier is a water-soluble small organic molecule with weak coordination ability that can be completely decomposed into gaseous products at subsequent processing temperatures; specifically, glycolic acid. Simultaneously, the pH value of the hydrolysis reaction medium is precisely adjusted to a preset initial value using nitric acid or ammonia of trace metal purity.

[0012] Next, the organoaluminum precursor is subjected to online vaporization purification and gas-phase transport. The precursor vaporization purification reaction zone is heated to a first preset temperature. and maintain the first preset pressure. This allows the aluminum isopropoxide precursor to continuously and stably vaporize, forming saturated vapor. The first preset temperature... The temperature range is 150°C to 220°C, with a first preset pressure. The range is from 1.1 bar to 2.0 bar absolute pressure. The selection is based on the principle of being above the boiling point of aluminum isopropoxide but below the vaporization temperature of metallic impurities. The first preset temperature T1 is set above the boiling point of aluminum isopropoxide but below the vaporization temperature of most metallic impurities, thereby ensuring that the metallic impurities remain in the residue of the precursor vaporization purification reaction zone in solid or liquid phase form. The gas phase transport channel is then heated to the second preset temperature. The Higher than To prevent condensation of the precursor vapor during transport, a mass flow controller is used to deliver purified aluminum isopropoxide vapor, free of non-volatile metal impurities, at a constant mass flow rate. It is introduced into the hydrolysis-hydrothermal crystallization reaction zone via the gas phase transport channel.

[0013] Step S2, controlled hydrolysis reaction: The gaseous organoaluminum precursor is passed into a hydrolysis reaction medium preloaded with ultrapure water and hydroxyl modifier, and the hydrolysis reaction is carried out under dynamic pH control to generate amorphous aluminum hydroxide sol.

[0014] A controlled hydrolysis reaction is carried out in the hydrolysis-hydrothermal crystallization reaction zone, and the reaction medium in the hydrolysis-hydrothermal crystallization reaction zone is heated and maintained at a third preset temperature. Simultaneously, mechanical stirring is started to maintain a constant stirring rate. The introduced ultrapure aluminum isopropoxide vapor is uniformly dispersed in the reaction medium in the form of tiny bubbles through a microporous distributor set below the liquid surface, and rapidly undergoes a hydrolysis reaction to generate amorphous aluminum hydroxide sol.

[0015] The reaction equation is: Throughout the hydrolysis process, a high-precision online pH meter monitors the pH changes of the reaction system in real time. This is coordinated with an automated titration system based on a PID algorithm, which dynamically maintains the system's pH within a preset range ΔpH by supplementing with trace amounts of nitric acid or ammonia of metal-grade purity. This range optimizes the hydrolysis rate and particle uniformity, with a titration response time of less than 10 seconds.

[0016] Step S3, Hydroxyl-regulated hydrothermal crystallization: The amorphous aluminum hydroxide sol is subjected to hydrothermal treatment under closed conditions, and crystallized into hydroxyl nano-alumina under the action of the hydroxyl regulator;

[0017] After the hydrolysis reaction is completed, a hydrothermal crystallization process with surface hydroxyl groups is performed. All inlets and outlets of the hydrolysis-hydrothermal crystallization reaction zone are sealed, transforming it into a closed hydrothermal reactor. The amorphous aluminum hydroxide sol system within the reaction zone is heated to a fourth preset temperature using a programmed temperature increase procedure. At this point, the internal pressure of the system reaches the second preset pressure as the saturated water vapor pressure increases. Under these hydrothermal conditions, a constant holding time is maintained. During this process, the amorphous aluminum hydroxide undergoes a phase transformation and recrystallizes into nanocrystals with a specific crystal structure, namely boehmite (AlOOH). Simultaneously, the pre-added hydroxyl regulator molecules form weak coordination bonds with the aluminum atoms of the (010) crystal plane of the AlOOH crystal through carboxyl and hydroxyl functional groups, with an adsorption energy of approximately -20 kJ / mol. This generates a steric hindrance effect, regulating crystal growth kinetics, inhibiting excessive grain growth and aggregation, and stabilizing the surface hydroxyl functional groups through a hydrogen bond network, preventing dehydration condensation and preventing condensation dehydration under hydrothermal conditions. The initial concentration of the hydroxyl regulator... This directly determines the hydroxyl density on the surface of the final product.

[0018] Step S4, Recovery and Drying: The hydroxyl nano-alumina is recovered, washed, and dried under controlled temperature conditions. The drying temperature is sufficient to decompose and remove the adsorbed hydroxyl regulator, but does not cause the hydroxyl nano-alumina to undergo a dehydroxylation phase transition.

[0019] Finally, the product is recovered, washed, and dried. The hydrolysis-hydrothermal crystallization reaction zone is cooled to room temperature, and the hydroxyl nano-alumina, i.e., boehmite slurry, generated by the reaction is discharged. The slurry is washed using a multi-stage centrifugal washing-ultrapure water redispersibility method until the conductivity of the supernatant after washing is less than 1 μS / cm, in order to completely remove the reaction byproduct isopropanol and residual ions. The washed slurry undergoes a final drying process. The drying process is a two-step process: first, the slurry is converted into micron-sized hollow spherical aggregates with preliminary fluidity through spray drying; then, the aggregates are dried at a fifth preset temperature under a vacuum of not less than 10 Pa. Vacuum drying is performed under vacuum, and a constant drying time is maintained. The fifth preset temperature The set value is sufficient to remove physically adsorbed water and interlayer water, and to promote the complete decomposition of the surface-adsorbed hydroxyl regulator (glycolic acid) into gaseous products such as carbon dioxide and water vapor, which are then removed by the vacuum system. However, this temperature is below the level at which significant dehydroxylation occurs in boehmite. The phase transition temperature is determined, thereby leaving clean and high-density hydroxyl functional groups constructed by this method on the product surface.

[0020] The precursor vaporization and purification reaction zone is a 10 L Hastelloy C-276 reactor equipped with a 3 kW resistance wire heating jacket, a temperature controller based on a proportional-integral-derivative (PID) algorithm (temperature control accuracy ±0.5°C), a magnetic stir bar with a PTFE coating, a DN15 pneumatic angle seat valve for introducing argon gas, a DN50 quick-opening flange for loading the precursor, and a DN25 pneumatic ball valve located at the bottom of the reactor for discharging residue.

[0021] The gas phase transport channel is a Hastelloy C-276 pipe with an inner diameter of 25.4 mm and a length of 500 mm, which is filled with a 200 mm high layer of gas. The ceramic Raschig ring packing layer is used to increase the gas-liquid interface and residence time to capture droplets that may be entrained by the airflow; the outer wall of the pipe is wrapped with a 500 W heating belt, and its temperature is controlled by an independent PID controller.

[0022] The hydrolysis-hydrothermal crystallization reaction zone is a 20 L volume 316L stainless steel high-pressure reactor, lined with a 5 mm thick fusible polytetrafluoroethylene (PFA) layer to prevent metal ion contamination. The reactor is equipped with a top-entry high-torque magnetically driven stirrer with a three-bladed swept-back PFA impeller. Heating and cooling are achieved through heat transfer oil circulation within the reactor jacket, with the temperature of the heat transfer oil precisely controlled by a high-low temperature integrated circulation machine. The inlet for the ultrapure aluminum isopropoxide vapor is a sintered titanium bubbler with an average pore size of 20 μm, submerged 100 mm below the liquid surface. The reactor also integrates a high-temperature and high-pressure resistant Mettler Toledo InPro 3250i online pH electrode, a four-electrode conductivity sensor, a sheathed K-type thermocouple, and a piezoresistive pressure transmitter.

[0023] Furthermore, the process parameters in the preparation method are defined, including the first preset temperature. The specific values ​​are 150°C to 220°C. First preset pressure. The specific values ​​are 1.1 bar to 2.0 bar absolute pressure. Second preset temperature. The specific numerical settings are as follows +20°C. Mass flow rate of the introduced aluminum isopropoxide vapor. The specific values ​​are 40.85 g / min to 204.25 g / min. The third preset temperature... The specific values ​​are 70°C to 95°C. The stirring rate... The specific values ​​are 300 rpm to 800 rpm. The preset pH range ΔpH during the hydrolysis process is specifically 6.5 ± 0.2. The fourth preset temperature... The specific values ​​are 160°C to 240°C. The heat preservation time... The specific values ​​range from 2 hours to 12 hours. The initial concentration of the hydroxyl modifier in the hydrolysis reaction medium... The specific values ​​are from 0.005 mol / L to 0.1 mol / L. The fifth preset temperature... The specific values ​​are 120°C to 150°C. The vacuum drying time... The specific values ​​range from 8 to 24 hours. This is achieved by systematically adjusting the above parameters, especially the initial concentration of the hydroxyl modifier. Hydrothermal temperature and hydrothermal time It allows for precise and predictable customized control of the particle size distribution, specific surface area, and surface hydroxyl content of the final product, hydroxyl nano-alumina.

[0024] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0025] This invention, through the design of precursor gasification and gas-phase fractionation transport, completes the physical separation of non-volatile metal impurities at the initial stage of the reaction, ensuring that the aluminum source entering the hydrolysis reaction zone has extremely high purity from the source. The content of key metal impurities such as iron, sodium, and calcium in the final product can be controlled to below 10 ppb. The subsequent hydroxyl-regulated hydrothermal crystallization process is carried out in a clean and closed system. Through the action of chemical regulators, the hydroxyl functional groups on the material surface are actively and precisely constructed and protected.

[0026] This invention enables precise and quantifiable control of the hydroxyl content on the product surface. The introduced hydroxyl regulator and its concentration become a direct and effective process control parameter. By changing the initial concentration of the hydroxyl regulator, its adsorption coverage at the crystal growth interface can be directly modulated, thereby linearly controlling the specific surface area and surface hydroxyl density of the final product.

[0027] The hydroxyl nano-alumina prepared by this invention not only has extremely high chemical purity, but also exhibits excellent dispersion stability when applied to lithium-ion battery separator coating slurry due to its surface rich in effectively protected active hydroxyl groups, forming a uniform and defect-free coating. Its highly polar surface enhances the wetting and retention capacity of the electrolyte, significantly reducing interfacial impedance. At the same time, the abundant hydroxyl groups act as chemical anchors, greatly enhancing the bonding force between the coating and the separator substrate and cathode particles, thereby improving the mechanical stability and cycle durability of the composite structure. Attached Figure Description

[0028] Figure 1 This is a schematic flowchart of a method for preparing hydroxyl nano-alumina with controllable hydroxyl content provided in an embodiment of the present invention;

[0029] Figure 2 X-ray diffraction patterns: (a) X-ray diffraction pattern of product 1 prepared in Example 1; (b) X-ray diffraction pattern of boehmite standard card;

[0030] Figure 3 This is a particle size distribution curve of product 1 prepared in Example 1 of the present invention;

[0031] Figure 4 This is a graph showing the specific surface area measurement results of product 1 prepared in Example 1 of the present invention. Detailed Implementation

[0032] The features and exemplary embodiments of various aspects of the present invention will now be described in detail. To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely intended to explain the present invention and not to limit the present invention. For those skilled in the art, the present invention can be practiced without some of these specific details. The following description of the embodiments is merely to provide a better understanding of the present invention by illustrating examples of the invention.

[0033] Reference Figure 1 The technical solution disclosed in this invention relies on a specially constructed closed integrated reaction system. This system achieves seamless integration of multiple functional units in its physical structure, fundamentally ensuring the continuity, closedness, and ultra-clean environment of the entire preparation process. Specifically, the system mainly consists of three core regions: a precursor vaporization and purification reaction zone, a gas phase transport channel, and a hydrolysis-hydrothermal crystallization reaction zone.

[0034] In one specific embodiment, the precursor vaporization and purification reaction zone is a 10L reactor. The reactor body and all liquid-contacting and gas-contacting components are made of Hastelloy C-276 to ensure excellent corrosion resistance in high-temperature and organoaluminum compound environments. The reactor is externally equipped with a 3kW precision resistance wire heating jacket, and a high-precision temperature controller based on a proportional-integral-derivative (PID) algorithm provides closed-loop control, enabling stable temperature maintenance within the reactor with a control accuracy of ±0.5°C. A PTFE-coated magnetic stir bar is installed at the bottom of the reactor, driven by external magnetic force, to promote uniform heating of the material during precursor loading and the initial heating phase. To achieve inert atmosphere protection for the system, a pneumatic angle seat valve with a nominal diameter of DN15 is installed at the top of the vessel body for introducing high-purity argon gas; a quick-opening flange with a nominal diameter of DN50 is used for safely and conveniently loading solid or liquid precursors; and a pneumatic ball valve with a nominal diameter of DN25 is installed at the bottom of the vessel body for discharging non-volatile residues after the process is completed.

[0035] Immediately following the outlet of the precursor vaporization and purification reaction zone is a gas phase transport channel. This channel is a Hastelloy C-276 pipe with an inner diameter of 25.4 mm and a total length of 500 mm. To prevent condensation or backflow of the precursor vapor during transport, a 500-watt heating element is evenly wound around the outer wall of the pipe, and its wall temperature is precisely controlled by an independent PID controller. More importantly, a 200 mm high section is filled in the middle of the pipe's interior. Ceramic Raschig ring packing layer. The main function of this packing layer is not catalysis or reaction, but to effectively capture any tiny droplets or solid particles that may be entrained by the high-speed gas flow by increasing the tortuosity and mass transfer resistance of the gas phase flow. It plays a role in physical gas phase distillation and purification, ensuring that only pure precursor vapor can enter the next reaction zone.

[0036] The end of the gas phase transport channel connects to the hydrolysis-hydrothermal crystallization reaction zone. This reaction zone is a more complex 20L 316L stainless steel high-pressure reactor with a design pressure resistance of no less than 25 bar. To fundamentally eliminate trace ion contamination from the reactor's metal materials, the inner wall of the reactor and all parts in contact with the reactants are lined with a 5mm thick layer of fusible polytetrafluoroethylene (PFA). The PFA lining not only provides extreme chemical inertness, but its smooth surface also helps prevent product particles from adhering to the reactor wall. The reactor's stirring system uses a top-entry high-torque magnetically driven stirrer, ensuring complete sealing within the reactor and avoiding the leakage and contamination problems that may arise from traditional mechanical seals. The impeller is a three-bladed swept-back PFA impeller, whose special design provides efficient macro-mixing while generating moderate shear force, which is beneficial for the uniform dispersion of initial sol particles. Temperature control of the reactor body is achieved through the circulation of heat transfer oil within the reactor jacket. The temperature of the heat transfer oil is precisely programmed and controlled by an external high-low temperature integrated circulation machine, enabling rapid heating, temperature control, and cooling. Purified precursor vapor is introduced into the reactor through a sintered titanium bubbler with an average pore size of 20 micrometers. This bubbler is precisely positioned 100 mm below the surface of the reaction medium to ensure that the vapor is uniformly dispersed throughout the liquid phase in the form of tiny bubbles, significantly increasing the gas-liquid contact area and mass transfer efficiency. Furthermore, to achieve precise monitoring and control of the reaction process, multiple online sensors are integrated into the reactor body, including a high-temperature and high-pressure resistant Mettler Toledo InPro 3250i online pH electrode, a four-electrode conductivity sensor, a sheathed K-type thermocouple, and a high-precision piezoresistive pressure transmitter. All sensor signals are connected to the central control system, enabling real-time data acquisition and automated control of the reaction process.

[0037] Based on the above-mentioned integrated reaction system, the preparation method disclosed in this invention is specifically carried out in the following steps in sequence.

[0038] Step S1, online vaporization and purification of precursor: In the precursor vaporization and purification reaction zone of the closed integrated reaction system, the organoaluminum precursor is heated and vaporized, so that the non-volatile metal impurities are retained in solid or liquid phase. The purified gaseous precursor is transported to the hydrolysis-hydrothermal crystallization reaction zone through the gas phase transport channel.

[0039] First, the entire system is initialized and the environment is set up. Before loading any materials, the entire closed system, consisting of the precursor vaporization and purification reaction zone, the gas phase transport channel, and the hydrolysis-hydrothermal crystallization reaction zone, must be strictly protected with an inert atmosphere. Specifically, high-purity argon gas (99.999% purity) is continuously introduced into the system through a DN15 pneumatic angle seat valve, while the system's exhaust valve is slightly opened for purging. Once the oxygen concentration monitoring value inside the system is below 10 ppm, the exhaust valve is closed, and the argon gas flow rate is adjusted through a proportional valve to maintain a slightly positive pressure of 0.1 ± 0.02 bar, ensuring an oxygen-free environment. This ensures that the entire preparation process, from precursor purification to final product formation, is carried out in an oxygen-free and non-reactive moisture-free ultra-clean environment, eliminating any external factors that may lead to side reactions or introduce impurities.

[0040] Subsequently, precursor loading and reaction medium pretreatment are performed. In the precursor vaporization and purification reaction zone, a measured amount of industrial-grade organoaluminum precursor, such as aluminum isopropoxide, is loaded through a DN50 quick-opening flange. In the hydrolysis-hydrothermal crystallization reaction zone, a predetermined volume of ultrapure water with a resistivity of not less than 18.2 MΩ·cm is introduced as the core medium for the hydrolysis reaction. Next, a hydroxyl group regulator, a key technical aspect of this invention, is added to the ultrapure water. In a preferred embodiment, this hydroxyl group regulator is glycolic acid. As a water-soluble small organic molecule, it possesses the ability to undergo weak coordination with aluminum ions or the surface of newly formed alumina, and it can completely decompose into gaseous products such as carbon dioxide and water at subsequent drying temperatures, without introducing any solid residue. Simultaneously, using trace amounts of nitric acid or ammonia solution of metal-grade purity, the pH value of the reaction medium in the hydrolysis-hydrothermal crystallization reaction zone is precisely adjusted to a preset initial value, such as 6.50, via a high-precision titration pump.

[0041] Next, the online vaporization purification and gas-phase transport process of the organoaluminum precursor is initiated. The precursor vaporization purification reaction zone is heated to a first preset temperature through an external heating jacket. The specific temperature range is set between 150°C and 220°C. Simultaneously, by adjusting the argon gas replenishment rate, the absolute pressure in this area is maintained at the first preset pressure. The temperature and pressure range is 1.1 bar to 2.0 bar. Under these conditions, the aluminum isopropoxide precursor can be continuously and stably vaporized to form saturated vapor. The choice of temperature is precisely calculated; its value is significantly higher than the boiling point of aluminum isopropoxide, but far lower than the vaporization temperature of most metallic impurities existing in the form of salts, oxides, or alkoxides. Therefore, these non-volatile metallic impurities will remain in the solid or liquid phase in the residue at the bottom of the precursor vaporization purification reaction zone, achieving atomic-level physical purification of the precursor. Simultaneously, the gas phase transport channel is heated to a second preset temperature via an external heating belt. , The value is usually set to be greater than The temperature is kept approximately 20°C higher to absolutely ensure that the purified precursor vapor will not condense due to temperature differences during transport. A high-precision mass flow controller ensures that the purified aluminum isopropoxide vapor, free of non-volatile metal impurities, is transported at a constant mass flow rate. For example, 40.85 g / min to 204.25 g / min is stably introduced into the hydrolysis-hydrothermal crystallization reaction zone via a gas phase transport channel.

[0042] Step S2, controlled hydrolysis reaction: The gaseous organoaluminum precursor is passed into a hydrolysis reaction medium preloaded with ultrapure water and hydroxyl modifier, and the hydrolysis reaction is carried out under dynamic pH control to generate amorphous aluminum hydroxide sol.

[0043] Subsequently, a strictly controlled hydrolysis reaction is carried out in the hydrolysis-hydrothermal crystallization reaction zone. The reaction medium in this zone is heated and precisely maintained at a third preset temperature by circulating heat transfer oil within the jacket. The specific temperature range is 70°C to 95°C. Simultaneously, activate the top-loading magnetic stirrer to maintain a constant stirring rate. For example, 300 rpm to 800 rpm is used to ensure the macroscopic homogeneity of the reaction system. The introduced ultrapure aluminum isopropoxide vapor is uniformly dispersed in the reaction medium as tiny bubbles through a microporous distributor placed below the liquid surface, and undergoes a rapid and complete hydrolysis reaction with water molecules to generate aluminum hydroxide sol in its initial amorphous state. The chemical reaction equation for this process is: During the entire hydrolysis process, the pH value of the system will drift due to the consumption of water and the generation of alcohol. To strictly limit the nucleation and growth process to optimal conditions, an online pH meter installed on the reactor monitors the pH changes of the reaction system in real time and forms a closed-loop control with an automatic titration system. Once the pH value deviates from the preset range ΔpH, for example, 6.5±0.2, the automatic titration system will immediately replenish pre-prepared dilute nitric acid or dilute ammonia solution with trace metal purity, dynamically and accurately maintaining the pH value of the system within the target narrow range.

[0044] Step S3, Hydroxyl-regulated hydrothermal crystallization: Amorphous aluminum hydroxide sol is subjected to hydrothermal treatment under closed conditions, and crystallized into hydroxyl nano-alumina under the action of a hydroxyl regulator;

[0045] Once all the predetermined amounts of aluminum isopropoxide precursor have been transported and hydrolyzed, the process seamlessly switches to the surface hydroxyl-controlled hydrothermal crystallization stage. At this point, all inlet and outlet valves of the hydrolysis-hydrothermal crystallization reaction zone are sealed, transforming it into a completely closed high-pressure hydrothermal reactor. Using the programmed temperature rise function of the integrated high- and low-temperature circulating machine, the amorphous aluminum hydroxide sol system already formed inside the reactor is heated to the fourth preset temperature at a set rate. The specific value range is from 160°C to 240°C. As the temperature rises, the saturated vapor pressure of the water inside the vessel increases sharply, causing the total pressure inside the system to automatically reach the second preset pressure. Under these hydrothermal conditions, a constant heat preservation time is maintained. The specific numerical range is from 2 to 12 hours. In this high-temperature, high-pressure hydrothermal environment, two crucial synergistic transformation processes occur: First, the thermodynamically metastable amorphous aluminum hydroxide undergoes a phase transition, recrystallizing into nanocrystals with a specific crystal structure, typically boehmite (AlOOH), through a dissolution-reprecipitation mechanism. Second, hydroxyl modifier molecules, pre-added to the reaction medium, selectively adsorb onto specific crystal faces of the growing AlOOH crystals. This adsorption effectively regulates crystal growth kinetics through the steric hindrance effect of its molecular structure and the weak coordination between its functional groups and aluminum atoms on the crystal face, significantly inhibiting excessive crystal growth and irreversible hard agglomeration. More importantly, this adsorption layer protects the hydroxyl functional groups on the crystal surface under hydrothermal conditions, preventing intermolecular condensation dehydration and the formation of Al-O-Al bridges, thereby stably solidifying the high-density surface hydroxyl groups on the surface of the final product. Experimental data show that the initial concentration of the hydroxyl modifier in the hydrolysis reaction medium... The density of hydroxyl groups on the surface of the final product shows a clear positive correlation, thus providing quantifiable process parameters for the customized control of surface functionality.

[0046] Step S4, Recovery and Drying: The hydroxyl nano-alumina is recovered, washed, and dried under controlled temperature conditions. The drying temperature is sufficient to decompose and remove the adsorbed hydroxyl regulator, but does not cause the hydroxyl nano-alumina to undergo a dehydroxylation phase transition.

[0047] Finally, after the hydrothermal crystallization process, the product is recovered, washed, and dried. First, the hydrolysis-hydrothermal crystallization reaction zone is cooled through a heat transfer oil circulation system until the material in the reactor cools to room temperature and the pressure returns to atmospheric pressure. Then, the bottom valve is opened to discharge the slurry containing hydroxyl nano-alumina, i.e., boehmite. The slurry is purified using a multi-stage centrifugal washing-ultrapure water redispersion method. Specifically, a high-speed centrifuge is used to separate the slurry into solid and liquid phases, discarding the supernatant, which mainly contains the reaction byproduct isopropanol and residual ions. Then, an equal volume of ultrapure water is added, and the filter cake is redispersed into a uniform suspension using ultrasound or high shear. This washing-redispersion process is repeated several times until the supernatant collected after the final washing has a conductivity of less than 1 μS / cm, indicating that the reaction byproducts and residual ions have been completely removed. The finally washed slurry undergoes a meticulously designed two-step drying process. The first step involves using spray drying technology to rapidly atomize and dry the slurry in a high-temperature gas stream, forming micron-sized hollow spherical aggregates composed of nanoparticles with preliminary fluidity. This morphology is beneficial for heat and mass transfer during the subsequent vacuum drying process. The second step involves drying the spray-dried aggregate powder at a fifth preset temperature under a vacuum of not less than 10 Pa. Deep vacuum drying is performed at, for example, 120°C to 150°C, and the drying time is maintained at a constant temperature. For example, 8 hours to 24 hours. Fifth preset temperature. The choice of temperature setting is extremely crucial: this temperature setting provides sufficient energy to completely remove physically adsorbed water and interlayer water, and to promote the complete decomposition of surface-adsorbed hydroxyl modifiers into gaseous products such as carbon dioxide and water vapor, which are then completely removed by the vacuum system. Simultaneously, this temperature is strictly controlled to ensure significant dehydroxylation of the boehmite and its subsequent transformation into... Below the critical temperature of the phase transition. Through this step, what is ultimately left on the surface of the product are clean and high-density native hydroxyl functional groups carefully constructed by the method of this invention.

[0048] To illustrate the technical effects of the present invention more specifically, several embodiments and comparative examples are listed below.

[0049] Example 1

[0050] This invention aims to prepare an ultrapure hydroxyl nano-alumina with a high surface hydroxyl content. First, the closed integrated reaction system is constructed and initialized. 1.5 kg of industrial-grade aluminum isopropoxide is charged into the precursor vaporization and purification reaction zone; wherein the aluminum isopropoxide purity is 98.5%, and the main metal impurities are Fe: 25 ppm, Na: 38 ppm. 15 L of ultrapure water with a resistivity of 18.2 MΩ·cm is charged into the hydrolysis-hydrothermal crystallization reaction zone, and 60.85 g of analytical-grade glycolic acid is precisely added to achieve the initial concentration in the water. The concentration reached 0.05 mol / L. The pH of the mixed solution was precisely adjusted to 6.50 using ultrapure nitric acid. An argon purging procedure was then initiated until the oxygen analyzer reading dropped below 5 ppm and maintained at a slightly positive pressure. Next, the precursor vaporization and purification reaction zone was heated to the first preset temperature. The absolute pressure inside the vessel is controlled by supplementing it with argon gas. Simultaneously, the gas phase transport channel is heated to the second preset temperature. The hydrolysis-hydrothermal crystallization reaction zone is heated and maintained at a third preset temperature. and with Stirring at a rate of [missing information]. Once everything is ready, turn on the mass flow controller to dispense the purified aluminum isopropoxide vapor at [missing information]. A constant flow rate of ammonia solution was introduced into the hydrolysis-hydrothermal crystallization reaction zone. During the hydrolysis process, which lasted approximately 12.3 minutes, 0.1 mol / L ultrapure ammonia solution was added via an automatic titration system to dynamically maintain the pH value of the system within the range of 6.5 ± 0.1. After the hydrolysis reaction was completed, all valves in the hydrolysis-hydrothermal crystallization reaction zone were closed, and the system was heated to the fourth preset temperature using a programmed temperature increase. At this point, the internal pressure of the system automatically increases and stabilizes at approximately 15.5 bar. Hydrothermal treatment is then performed at this temperature, with a holding time... The process was set to 6 hours. After hydrothermal treatment, the reactor was cooled to room temperature, and the slurry was discharged. The mixture was washed three times using a high-speed centrifuge, each time with ultrapure water. The final washed filter cake was redispersed and spray-dried, then subjected to a vacuum of 5 Pa and a fifth preset temperature. Vacuum drying is carried out under the following conditions, and the drying time is... The extraction time was 16 hours. The final product was a white powder, product 1. Inductively coupled plasma mass spectrometry (ICP-MS) analysis showed that the Fe content in product 1 was less than 0.01 ppm and the Na content was less than 0.01 ppm, meeting the ultra-high purity standard. X-ray diffraction analysis showed that... Diffraction peaks appeared at the location corresponding to the (020), (120), (031), (131), and (151) crystal planes of boehmite (AlOOH), consistent with the boehmite standard card. No characteristic diffraction peaks of the η-Al2O3, γ-Al2O3, and Al(OH)3 phases were observed, proving that the product is a well-crystallized boehmite (AlOOH) crystal phase. X-ray diffraction analysis was performed on an X-ray diffractometer under the following conditions: Cu-Kα radiation. The scanning range was 5° to 80° (2θ), with a scanning step size of 0.02° (2θ). The surface hydroxyl content was determined to be 3.2 mmol / g by acid-base titration. The specific surface area was determined to be 155 m² / g by nitrogen adsorption (BET). The median particle size D50 was determined to be 350 nm by laser particle size analyzer.

[0051] Example 2

[0052] The purpose was to verify the effect of adjusting the concentration of the hydroxyl modulator on the product properties. The preparation process was essentially the same as in Example 1, except that the mass of glycolic acid added to the hydrolysis-hydrothermal crystallization reaction zone was increased to 108.3 g, thus increasing its initial concentration in water. The concentration was correspondingly increased to 0.095 mol / L. All other operating parameters, including temperature, pressure, time, flow rate, and pH control range at each stage, remained completely consistent with Example 1. A white powder product 2 was finally obtained. Analysis showed that Product 2 was also an ultra-high purity boehmite crystal phase, with Fe and Na contents both below 0.01 ppm. However, its surface hydroxyl content, measured by acid-base titration, was significantly increased to 4.5 mmol / g, and its specific surface area correspondingly increased to 182 m² / g, while its median particle size D50 decreased to 280 nm. The results of this example clearly demonstrate that increasing the concentration of the hydroxyl modifier can more effectively suppress grain growth during the hydrothermal process, thereby obtaining smaller grain size and a larger specific surface area, and thus stabilizing a higher density of surface hydroxyl groups.

[0053] Example 3

[0054] The purpose is to demonstrate the ability to control product particle size by adjusting hydrothermal process parameters. The preparation process is basically the same as in Example 1, with the main difference being the adjustment of process parameters in the hydrothermal crystallization stage: the fourth preset temperature... The temperature was set to a lower 180°C, at which point the corresponding system saturated vapor pressure was approximately 10.0 bar. Simultaneously, the holding time was... The holding time was extended to 10 hours. All other operating parameters, including precursor purification conditions, hydrolysis conditions, and hydroxyl modifier concentration (maintained at 0.05 mol / L), remained completely consistent with those in Example 1. A white powder product 3 was finally obtained. Analysis showed that product 3 was still a high-purity boehmite (AlOOH) crystal phase, with Fe and Na contents both below 0.01 ppm. Its surface hydroxyl content was 3.0 mmol / g, at the same level as the product in Example 1. However, its D50 particle size significantly increased to 520 nm, while its specific surface area correspondingly decreased to 110 m² / g. The results of this example indicate that extending the holding time at a lower hydrothermal temperature is beneficial for slow grain growth and the Ostwald ripening process, thereby obtaining customized products with larger particle sizes. Simultaneously, due to the presence of the hydroxyl modifier, its surface hydroxyl density can still be maintained at a high level.

[0055] Example 4

[0056] The purpose is to demonstrate the universality of the method of the present invention, that is, it is not limited to a specific precursor or regulator. The preparation process is similar to that of Example 1, but the raw materials are replaced: 1.9 kg of industrial-grade aluminum sec-butoxide is charged into the precursor vaporization and purification reaction zone. In the hydrolysis-hydrothermal crystallization reaction zone, the added hydroxyl modifier was replaced with lactic acid. The amount added was 67.56 grams, which resulted in an initial concentration in the water. The concentration is 0.05 mol / L. Since aluminum sec-butoxide has a higher boiling point than aluminum isopropoxide, the first preset temperature is accordingly set... Increase to 220°C, second preset temperature The temperature was increased to 240°C to ensure effective vaporization and purification. All other operating parameters, including hydrolysis temperature, hydrothermal conditions, and pH, remained completely consistent with those in Example 1. A white powdery product 4 was finally obtained. Analysis showed that product 4 was a high-purity boehmite (AlOOH) crystalline phase with Fe and Na contents both below 0.01 ppm. Its surface hydroxyl content was 3.5 mmol / g, specific surface area was 168 m² / g, and D50 particle size was 320 nm. These results demonstrate that the method framework of this invention has good versatility; by replacing different organoaluminum precursors and hydroxyl modifiers with similar functional groups, the same efficient preparation of the target product can be achieved.

[0057] To further highlight the superiority and necessity of the method of the present invention, several comparative examples were set up for comparative analysis.

[0058] Comparative Example 1

[0059] A traditional sol-gel method combined with post-treatment acid washing purification process was employed. 1.5 kg of industrial-grade aluminum isopropoxide (identical to that in Example 1) was slowly added to 15 L of deionized water, and the mixture was vigorously stirred at 90°C for 4 hours to undergo hydrolysis, yielding boehmite slurry. The slurry was centrifuged, washed, and dried at 120°C to obtain a crude product. A portion of the crude product was tested; the Fe content was 23 ppm, the Na content was 35 ppm, and the surface hydroxyl content was 2.5 mmol / g. The remaining crude product was placed in a 1 mol / L nitric acid solution and acid-washed at 60°C for 2 hours to remove metallic impurities. After acid washing, the product was repeatedly washed with deionized water until neutral, and then dried at 120°C to obtain the final product D1. Testing showed that the Fe content of product D1 was reduced to 1.2 ppm and the Na content to 2.5 ppm. Although the chemical purity was improved, it was far from reaching the level of this invention. More seriously, the surface hydroxyl content dropped sharply to 0.8 mmol / g, confirming that while the strong acid treatment removed impurities, it also caused irreversible and severe damage to the functional hydroxyl structure on the material surface.

[0060] Comparative Example 2

[0061] The traditional sol-gel method combined with high-temperature calcination purification was employed. The steps for preparing the crude boehmite product were exactly the same as in Comparative Example 1. The obtained crude product was placed in a muffle furnace and calcined at 450°C for 4 hours. This temperature is generally considered to remove some organic residues and decompose some impurity salts, but it also causes the boehmite to... A phase transition occurs, triggering severe dehydroxylation. The final product is D2. Analysis shows that product D2 contains 22 ppm Fe and 33 ppm Na, indicating that high-temperature calcination is ineffective in removing these metallic impurities. X-ray diffraction analysis indicates that the product is boehmite and... The mixed phase has an almost undetectable surface hydroxyl content, below 0.1 mmol / g, indicating that the high-temperature treatment is completely contrary to the purpose of this invention to maintain a high hydroxyl content.

[0062] Comparative Example 3

[0063] This invention aims to directly demonstrate the absolute necessity of the online vaporization purification step. The preparation process basically adopts the apparatus and process of Example 1, but the precursor vaporization purification step is deliberately omitted. Specifically, 1.5 kg of industrial-grade aluminum isopropoxide, homologous to that of Example 1, is directly added to the hydrolysis-hydrothermal crystallization reaction zone of a 0.05 mol / L glycolic acid solution, followed by the same controlled hydrolysis and hydroxyl-controlled hydrothermal treatment process as in Example 1. The final product is D3. Analysis shows that product D3 has a surface hydroxyl content of 3.1 mmol / g, a specific surface area of ​​152 m² / g, and a particle size of 360 nm, which are very close to the physical and surface chemical parameters of the product of Example 1. However, its chemical purity is extremely poor, with Fe content as high as 24 ppm and Na content as high as 36 ppm, essentially equivalent to the impurity levels in the raw materials. This result irrefutably proves that without the crucial online vaporization purification step, even with subsequent fine-tuning techniques, it would be impossible to obtain the ultra-high purity product sought by this invention.

[0064] Comparative Example 4

[0065] The preparation process basically adopted the apparatus and process of Example 1, but the addition of the hydroxyl modifier was completely omitted. That is, only 15 L of ultrapure water was added in the hydrolysis-hydrothermal crystallization reaction zone, and the pH was adjusted to 6.5, without adding any glycolic acid. All other steps, including the online vaporization purification of the precursor, hydrolysis conditions, hydrothermal conditions, etc., were exactly the same as in Example 1. The final product D4 was obtained. The chemical purity of product D4 was found to be extremely high, with Fe and Na contents both below 0.01 ppm, which was entirely due to the effectiveness of the online purification step. However, due to the lack of crystal growth inhibition effect of the hydroxyl modifier in the hydrothermal process, significant growth and agglomeration of crystals occurred. Its D50 particle size increased sharply to 850 nm, and the specific surface area correspondingly dropped sharply to only 45 m² / g. Its surface hydroxyl content also decreased significantly to 1.1 mmol / g.

[0066] Comparative Example 5

[0067] The experiment of Comparative Example 4 was repeated, yielding highly consistent results: the D50 particle size of product D5 was 820 nm, the specific surface area was 48 m² / g, and the surface hydroxyl content was 1.2 mmol / g. These results from both comparative examples demonstrate that hydroxyl modifiers are crucial and indispensable components for controlling the nanostructure of the product and maintaining a high density of surface hydroxyl content.

[0068] Comparative Example 6

[0069] The ultrapure product prepared using the present invention was simulated, but subjected to a prior art acid washing treatment. 100 g of product 1 prepared in Example 1 was placed in 1 L of 1 mol / L nitric acid solution and stirred at 60°C for 2 hours. In product 1 prepared in Example 1, Fe < 0.01 ppm, Na < 0.01 ppm, and the surface hydroxyl content was 3.2 mmol / g. After treatment, the sample was repeatedly washed until neutral and then dried at 120°C to obtain product D6. The purity of product D6 remained unchanged, and the Fe and Na contents were still below the detection limit. However, its surface hydroxyl content decreased significantly to 0.9 mmol / g, and the specific surface area also decreased slightly to 140 m² / g due to particle surface etching.

[0070] This experiment demonstrates that even for materials that have reached ultrapure levels, any destructive or separating post-processing steps can still damage their critical surface functionality, thus indirectly confirming the inherent superiority of the integrated preparation method of this invention.

[0071] The table below summarizes the key process parameters and product performance data of the above embodiments and comparative examples for easy comparison.

[0072]

[0073] Based on the results of all the above embodiments and comparative examples, it can be concluded that the method for preparing hydroxyl-containing nano-alumina with controllable hydroxyl content disclosed in this invention successfully solves the fundamental contradiction between improving the chemical purity of materials and maintaining the surface functional hydroxyl groups in the prior art through an innovative design that couples the online vaporization purification of precursors with the hydrothermal crystallization depth controlled by hydroxyl groups into an integrated reaction system. This method can stably, controllably, and predictably prepare hydroxyl-containing nano-alumina materials with both ultra-high chemical purity and highly tunable surface hydroxyl content. The comprehensive physicochemical properties of the product, including purity, surface hydroxyl density, specific surface area, and controllability of particle size distribution, significantly surpass those of materials obtained through existing separation-based technical routes.

[0074] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape, and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing hydroxyl nanooxide aluminum with controllable hydroxyl content, characterized in that, The method comprises the following steps: Step S1, precursor online gasification purification: in the precursor gasification purification reaction zone of the closed integrated reaction system, the organic aluminum precursor is heated and gasified, and the non-volatile metal impurities are retained in the form of solid or liquid phase, and the purified gas phase precursor is transported to the hydrolysis-hydrothermal crystallization reaction zone through the gas phase transport channel; the gas phase transport channel is heated to a second preset temperature which is higher than the gasification temperature of the organic aluminum precursor, so as to prevent the condensation of the precursor vapor during the transport process; Step S2, controlled hydrolysis reaction: the gas phase organic aluminum precursor is introduced into the hydrolysis reaction medium pre-loaded with ultrapure water and a hydroxyl regulating agent, and a hydrolysis reaction is carried out under the condition of dynamic control of pH value, to generate an amorphous aluminum hydroxide sol; In the hydrolysis-hydrothermal crystallization reaction zone, the reaction medium in the hydrolysis-hydrothermal crystallization reaction zone is heated and maintained at a third preset temperature, and mechanical stirring is started at the same time, and a constant stirring rate is maintained; the third preset temperature is 70°C to 95°C, and the stirring rate is 300 rpm to 800 rpm; the hydroxyl regulating agent is an organic small molecule which is completely decomposed into a gas phase product in the drying process; the hydroxyl regulating agent is selected from glycolic acid or lactic acid, which is a water-soluble organic small molecule with weak coordination ability and is completely decomposed into a gas phase product in the temperature condition of the drying process, and the initial concentration of the hydroxyl regulating agent in the hydrolysis reaction medium is 0.005 mol / L to 0.1 mol / L; Step S3, hydroxyl-regulated hydrothermal crystallization: the amorphous aluminum hydroxide sol is subjected to hydrothermal treatment under closed conditions, and is crystallized into hydroxyl nano-alumina under the action of the hydroxyl regulating agent; the hydrothermal treatment is carried out by programmed heating, the amorphous aluminum hydroxide sol system in the hydrolysis-hydrothermal crystallization reaction zone is heated to a fourth preset temperature, so that the internal pressure of the system reaches a second preset pressure as the saturation water vapor pressure rises; under the condition of the fourth preset temperature and the second preset pressure, a constant holding time is maintained, the fourth preset temperature is 160°C to 240°C, and the holding time is 2 hours to 12 hours; Step S4, recovery and drying: the hydroxyl nano-alumina is recovered, washed, and dried by a two-step drying method: first, the washed slurry is converted into micron-sized hollow spherical aggregates by spray drying; then, the aggregates are subjected to vacuum drying under vacuum conditions, the drying temperature is sufficient to decompose and remove the adsorbed hydroxyl regulating agent, but does not cause the dehydroxylation phase transition of the hydroxyl nano-alumina, the vacuum degree of the vacuum drying is not less than 10 Pa, the drying temperature is 120°C to 150°C, and the drying time is 8 hours to 24 hours.

2. The method of claim 1, wherein the hydroxyl group content of the hydroxyl group-containing nano-aluminum oxide is controlled. The method further comprises a step of inert atmosphere protection treatment inside the closed integrated reaction system before the on-line gasification purification of the organic aluminum precursor; the inert atmosphere protection treatment is specifically: introducing high-purity inert gas into the closed integrated reaction system to perform replacement purging until the oxygen concentration inside the system is lower than a preset threshold, and maintaining the system in a slightly positive pressure state thereafter. ​ 3. The method for preparing hydroxyl-containing nano-alumina with controllable hydroxyl content according to claim 1, characterized in that, The on-line gasification purification step of the organic aluminum precursor specifically comprises: heating the precursor gasification purification reaction zone to a first preset temperature and maintaining a first preset pressure, so that the organic aluminum precursor continuously gasifies to form precursor saturated vapor, and the non-volatile metal impurities contained in the organic aluminum precursor remain in the residue of the precursor gasification purification reaction zone in solid or liquid phase form; the purified precursor vapor is introduced into the hydrolysis-hydrothermal crystallization reaction zone at a constant mass flow rate through a mass flow controller.

4. The method for preparing hydroxyl-containing nano-alumina with controllable hydroxyl content according to claim 3, characterized in that: The organic aluminum precursor is a substance that can be gasified at the first preset temperature and has a significant vapor pressure difference between its gas phase and liquid phase or solid phase; the organic aluminum precursor is selected from aluminum isopropoxide or aluminum sec-butoxide.

5. The method of claim 1, wherein the hydroxyl nanooxide of controlled hydroxyl content is characterized by: The hydrolysis reaction medium is composed of ultrapure water and the hydroxyl regulator; the ultrapure water has a resistivity of not less than 18.2 MΩ·cm.

6. The method for preparing hydroxyl-containing nano-alumina with controllable hydroxyl content according to claim 1, characterized in that, The controlled hydrolysis reaction step specifically comprises: heating and maintaining the hydrolysis reaction medium in the hydrolysis-hydrothermal crystallization reaction zone at a third preset temperature while starting mechanical stirring to maintain a constant stirring rate; the introduced organic aluminum precursor in gas phase state is uniformly dispersed in the hydrolysis reaction medium in the form of bubbles through a microporous distributor arranged below the liquid surface; During the entire hydrolysis reaction process, the pH value change of the reaction system is monitored in real time through an on-line pH meter, and the system pH value is dynamically maintained within a preset narrow range ΔpH through the linkage of an automatic titration system by supplementing trace metal grade acid or base.

7. The method for preparing hydroxyl-containing nano-alumina with controllable hydroxyl content according to claim 1, characterized in that, In the recovery and drying step, the washing is performed by multi-stage centrifugal washing-ultrapure water redispersion of the slurry until the conductivity of the supernatant after washing is lower than a preset cleanliness threshold.

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