Preparation method of aluminum oxide hollow precursor microbeads

Abstract

The invention provides a preparation method of aluminum oxide hollow precursor microspheres, and belongs to the field of nano materials. The method comprises the following steps: carrying out hydrolysis reaction on tetraethoxysilane to obtain silicon dioxide microspheres; the preparation method comprises the following steps: carrying out high-pressure homogenization treatment and freeze drying on mixed slurry of silicon dioxide microspheres and polyvinylpyrrolidone to obtain a SiO2-PVP template with a core-shell structure; the preparation method comprises the following steps: dissolving polyacrylic acid in a mixed solution of an aluminum nitrate solution and an aluminum isopropoxide solution to obtain a double-aluminum-source solution; the SiO2-PVP template is dispersed in the double-aluminum-source solution, and a template dispersion liquid is obtained; dropwise adding ammonia water into the template dispersion liquid until the pH value of the template dispersion liquid is 4.2-4.5 to obtain aluminum sol; and carrying out spray drying on the aluminum sol to obtain the aluminum oxide hollow precursor microspheres. By introducing a silicon dioxide-PVP (Polyvinyl Pyrrolidone) composite template, a dynamic pH (Potential of Hydrogen) microfluidic technology and a double-atomization drying process, a novel preparation method of the aluminum oxide hollow precursor microsphere is provided.

Description

Technical Field

[0001] The present application relates to the field of nanomaterial technology, and in particular to a method for preparing hollow alumina precursor microbeads. Background Art

[0002] Alumina hollow microspheres are widely used in aerospace, electronics, coatings and other fields due to their low density, high hardness, good thermal stability and chemical stability.

[0003] At present, the main preparation processes of its precursors include template method, sol-gel method, spray drying method, etc., but there are certain technical bottlenecks. Traditional template methods, such as polystyrene microspheres, silica templates, etc., require high temperatures above 600 ° C to remove the template, which has high energy consumption and is easy to cause the collapse of the microbead structure or grain coarsening; in the process of mixing traditional templates with aluminum salt solutions, mechanical stirring or single ultrasonic treatment is usually used, which is difficult to achieve uniform coating, resulting in poor integrity of the hollow structure; hard templates, such as polystyrene, require the use of toxic reagents such as glutaraldehyde, which will produce harmful gases when decomposed at high temperatures; in the sol-gel process, aluminum salt sol is prone to local agglomeration during pH control, and needs to be added. It requires adding complex reagents and will affect the purity of the final product. At the same time, the sol aging time is as long as 12 to 24 hours. In addition, the precursor is prone to cracking during the drying process, and parameters such as temperature and humidity need to be continuously optimized multiple times to maintain the stability of the precursor structure. In the process of preparing the precursor by spray drying, the precursor prepared by traditional centrifugal atomization or pressure nozzle technology has a wide particle size distribution, resulting in uneven morphology of the microbeads after sintering. During spraying, high solid content slurry is easy to clog the nozzle, reducing production efficiency. The output is usually not higher than 10kg / h, and it is difficult to achieve a high hollow rate. Summary of the Invention

[0004] The present application provides a method for preparing hollow alumina precursor microbeads, in order to provide a new method for preparing hollow alumina precursor microbeads based on the coordinated regulation of a composite template and a dual aluminum source.

[0005] The present invention provides a method for preparing hollow alumina precursor microbeads, the method comprising:

[0006] The ethyl orthosilicate is subjected to a hydrolysis reaction to obtain silica microspheres;

[0007] The mixed slurry of the silica microspheres and polyvinyl pyrrolidone is subjected to high pressure homogenization and freeze drying to obtain a SiO2-PVP template with a core-shell structure;

[0008] dissolving polyacrylic acid in a mixed solution of aluminum nitrate solution and aluminum isopropoxide solution to obtain a dual aluminum source solution;

[0009] Dispersing the SiO2-PVP template in the dual aluminum source solution to obtain a template dispersion;

[0010] Using a microfluidic reactor, adding ammonia water dropwise to the template dispersion until the pH value of the template dispersion reaches 4.2 to 4.5, thereby obtaining an aluminum sol;

[0011] The aluminum sol is spray-dried by adopting a dual-fluid nozzle coordinated centrifugal atomization technology to obtain aluminum oxide hollow precursor microbeads.

[0012] Optionally, the hydrolysis reaction of ethyl orthosilicate to obtain silica microspheres includes:

[0013] adding ammonia water dropwise to an ethanol-water mixed solution of ethyl orthosilicate to perform a hydrolysis reaction of the ethyl orthosilicate to obtain a mixed slurry;

[0014] The mixed slurry is subjected to solid-liquid separation, washing and drying to obtain silica microspheres; the particle size of the silica microspheres is 100nm to 300nm.

[0015] Optionally, the molar ratio of the ethyl orthosilicate to 25% ammonia water is 1:(8-12);

[0016] The water bath temperature of the hydrolysis reaction is 30°C to 50°C, and the stirring time of the hydrolysis reaction is 4h to 8h;

[0017] In the ethanol-water mixed solution of ethyl orthosilicate, the volume ratio of ethanol to water is (2-4):1.

[0018] Optionally, the solid content concentration of the mixed slurry of silica microspheres and polyvinyl pyrrolidone is 3% to 8%;

[0019] The mass ratio of the silica microspheres to the polyvinyl pyrrolidone is 1:(1-3).

[0020] Optionally, the pressure of the high-pressure homogenization treatment is 100 MPa to 200 MPa, and the number of cycles of the high-pressure homogenization treatment is 2 to 5 times;

[0021] The freeze drying process includes the following parameters: temperature of -40°C to -60°C, vacuum degree ≤10Pa, and drying time of 24h to 48h.

[0022] Optionally, in the mixed solution of the aluminum nitrate solution and the aluminum isopropoxide solution, the molar concentration of the aluminum nitrate is 0.6 mol / L to 1.5 mol / L, and the molar concentration of the aluminum isopropoxide is 0.02 mol / L to 0.15 mol / L;

[0023] The mass of the polyacrylic acid is 0.3% to 1.0% of the total mass of the dual aluminum source solution, and the molecular weight of the polyacrylic acid is 1000 to 5000.

[0024] Optionally, the mass ratio of the SiO2-PVP template to the dual aluminum source solution is 1:5;

[0025] The ultrasonic frequency of the dispersion is 40 kHz, and the ultrasonic time of the dispersion is 0.5 h.

[0026] Optionally, the channel diameter of the microfluidic reactor is 300 μm to 800 μm;

[0027] When ammonia water is added dropwise to the template dispersion, the dropping speed of ammonia water with a concentration of 0.1 mol / L is 0.5 mL / min to 2.0 mL / min, and the temperature of the template dispersion is 20° C. to 30° C.;

[0028] The solid content of the aluminum sol is 20% to 25%, the pH value of the aluminum sol is 4.2 to 4.5, the viscosity of the aluminum sol is 80 mPa·s to 120 mPa·s, and the Zeta potential of the aluminum sol is ≥+30 mV.

[0029] Optionally, the spray drying includes the following parameters: feed rate of 5 mL / min to 15 mL / min, dual-fluid nozzle air pressure of 0.3 MPa to 0.5 MPa, centrifugal atomizing disk speed of 12000 rpm to 18000 rpm, air inlet temperature controlled at 180°C to 200°C, air outlet temperature controlled at 80°C to 100°C, and droplet size distribution of D90 / D10≤1.5.

[0030] Optionally, the hollow alumina precursor microbeads meet the following properties: particle size D50 is 5 μm to 10 μm, specific surface area is 50 m 2 / g~80m 2 / g, porosity ≥65%, hollow structure preforming rate ≥95%, Al2O3 content ≥85%.

[0031] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:

[0032] An embodiment of the present application provides a method for preparing hollow alumina precursor microbeads, which includes: hydrolyzing tetraethyl orthosilicate to obtain silica microspheres; subjecting a mixed slurry of the silica microspheres and polyvinyl pyrrolidone to high-pressure homogenization and freeze-drying to obtain a SiO2-PVP template with a core-shell structure; dissolving polyacrylic acid in a mixed solution of aluminum nitrate solution and aluminum isopropoxide solution to obtain a dual aluminum source solution; dispersing the SiO2-PVP template in the dual aluminum source solution to obtain a template dispersion; using a microfluidic reactor, dropwise adding ammonia water to the template dispersion until the pH value of the template dispersion reaches 4.2 to 4.5, to obtain an aluminum sol; and using a dual-fluid nozzle coordinated centrifugal atomization technology to spray-dry the aluminum sol to obtain hollow alumina precursor microbeads.

[0033] First, through the synergistic effect of the two templates, a controllable hollow structure framework is constructed, and size limitation and interface guidance are provided for subsequent sol coating. The silica microsphere template can serve as a nanoscale rigid core, providing an incompressible cavity, directly defining the inner core size of the alumina hollow microbeads, and avoiding structural collapse during drying or sintering. The amide groups on the molecular chain of the polyvinyl pyrrolidone template form a hydrogen bond network with the aluminum sol, guiding the sol to be uniformly coated on the SiO2 surface in a directional manner, avoiding local agglomeration or uneven coating. At the same time, during the sol coating-drying process, the polyvinyl pyrrolidone template suppresses structural deformation through the buffering effect of the flexible layer, ensuring uniform shell thickness, and overcoming the shell defects caused by insufficient interface regulation of traditional single templates.

[0034] Secondly, aluminum nitrate is mixed with aluminum isopropoxide, and the degree of polymerization and viscosity of the sol are regulated by the hydrolysis characteristics of different aluminum sources to ensure the fluidity and film-forming properties during the subsequent coating of the template. At the same time, polyacrylic acid is used as a dispersant to inhibit the aggregation of aluminum ions through electrostatic repulsion and steric hindrance effects, thereby improving the stability of the sol and adjusting the rheological properties of the sol to facilitate subsequent atomization and molding. In addition, a microfluidic reactor is used to solve the problems of instantaneous gelation of the sol and hard agglomeration of particles caused by uneven pH control in traditional processes; through laminar shear and diffusion mass transfer in microscale channels, pH gradient linearization, millisecond-level mixing and Zeta potential steady-state maintenance are achieved; the hydrolysis and condensation kinetics of the aluminum source are precisely controlled to eliminate local concentration / temperature fluctuations, ensure the dispersion stability of the sol, and ultimately obtain precursor microbeads with uniform shells and high sphericity.

[0035] Finally, through an atomization drying process, the aluminum sol and template mixture is transformed into precursor microbeads with a specific particle size and a complete hollow structure. During the atomization process, the SiO2-PVP template acts as a "sacrificial skeleton," retaining a hollow core as the sol droplets dry. The thermal decomposition of the PVP provides the conditions for subsequent sintering to remove the template.

[0036] Therefore, the embodiment of the present application provides a new preparation method of alumina hollow precursor microbeads by introducing a silica-PVP composite template, dynamic pH microfluidics technology and double atomization drying process. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0038] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0039] Figure 1 A schematic flow chart of a method for preparing hollow alumina precursor microbeads provided in an embodiment of the present application. DETAILED DESCRIPTION

[0040] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0041] The range descriptions described in this article, such as numerical ranges, ratio ranges, etc., include all possible sub-ranges and single numerical values ​​within the range. For example, the range description of "1 to 6" or "1~6" covers all sub-ranges from 1 to 6 (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6). Unless otherwise specified, the terms "including", "comprising", etc. used in this article mean "including but not limited to"; relational terms such as "first" and "second" are only used to distinguish different entities or operations, and do not imply an actual sequence or association relationship; "and / or" means that multiple situations can exist alone or at the same time; expressions such as "at least one", "multiple", and "at least one" refer to any combination of corresponding objects, including a combination of single or multiple objects. The proportional relationships involved in the article, such as mass ratios, molar ratios, etc., should be understood as the corresponding relationship between the first and second terms of the proportional formula in the order of description. The raw materials, reagents, instruments and equipment used in this article can be purchased on the market or prepared by existing methods.

[0042] In order to overcome the problems of high carbon emissions from high-temperature demolding in the existing preparation of alumina hollow microbead precursors, reliance on toxic reagents, poor hollow structure integrity caused by the mixing process of the template and aluminum salt solution, and easy local agglomeration of the aluminum salt sol during pH control, this application provides a new preparation method for alumina hollow precursor microbeads based on the coordinated regulation of a composite template and a dual aluminum source. By introducing a silica-PVP composite template, dynamic pH microfluidic technology and a double atomization drying process, the bottleneck problems of the traditional preparation method, such as uneven structure, high energy consumption and poor environmental protection, are solved.

[0043] First, the design of a silica-PVP composite template solves the problems of template self-agglomeration and uneven coating in traditional methods. The theory is as follows: (1) High-pressure shear force is used to force the PVP molecular chains to embed into the gaps between SiO2 microspheres, forming a mechanically interlocked core-shell structure, eliminating the blind spots of uneven coating in traditional stirring methods. (2) Freeze-drying protection: A deep-freeze environment above -60°C causes the PVP-water solution to instantly vitrify, inhibiting template deformation caused by ice crystal growth, maintaining a narrow particle size distribution of 100nm to 300nm, and avoiding the capillary force agglomeration problem caused by stress concentration and uneven shrinkage caused by traditional thermal drying. (3) The SiO2 / PVP mass ratio is controlled within the appropriate range of 1:1 to 1:3 to ensure complete coating of the shell layer and prevent incomplete coating due to too little PVP or template adhesion caused by too much PVP.

[0044] Secondly, dynamic pH microfluidics technology is used to solve the problem of uneven sol hydrolysis in traditional processes. (1) Precise titration control is used to avoid local pH mutations caused by traditional batch addition of alkali: microfluidic channels with a size of 300μm to 800μm can uniformly inject ammonia water at a rate of 0.5-2.0mL / min, and a linear adjustment of pH from 3.0 to 4.5 can be achieved within a millisecond time period. (2) Good suppression of local gelation: The pH is gradually increased within a millisecond time period, making the hydrolysis rate of the aluminum source uniform, avoiding the local excessive condensation of the sol caused by pH jumps in traditional methods, which produces hard agglomerated particles. (3) Precise control of Zeta potential: Maintaining the sol Zeta potential above +30mV, the aggregation of sol particles is suppressed by electrostatic repulsion, avoiding flocculation caused by Zeta potential fluctuations in traditional methods.

[0045] Finally, the problem of uneven drying shrinkage is solved by the synergistic effect of double atomization drying: (1) The problem of uneven drying shrinkage is solved by the synergistic effect of centrifugal + pressure nozzles: The diameter of the small droplets produced by the centrifugal atomizer at 12,000 to 18,000 rpm is between 5 μm and 50 μm. With the help of the synergistic effect of the dual-fluid nozzle at an air pressure of 0.3 to 0.5 MPa, the small droplets can form a "droplet-in-droplet" structure, with the inner layer being a template-sol complex and the outer layer being a uniform aluminum sol film; (2) The drying stress is evenly distributed, and the 180-200 ° C hot air causes the outer layer of the uniform aluminum sol film to quickly gel to form a rigid skeleton, and the internal water evaporates slowly through the pores, avoiding the wall breakage caused by the concentrated shrinkage stress of the traditional single atomization method. (3) The double atomization effect ensures that the droplet size is uniform and narrowly distributed, ensuring the consistency of the drying dynamics.

[0046] Specifically, this application aims to solve the above technical problems, and the detailed technical solutions are as follows:

[0047] Figure 1 A schematic flow chart of a method for preparing hollow alumina precursor microbeads provided in an embodiment of the present application.

[0048] like Figure 1 As shown, the embodiment of the present application provides a method for preparing hollow alumina precursor microbeads, the method comprising:

[0049] S1, hydrolyzing ethyl orthosilicate to obtain silica microspheres;

[0050] In some embodiments, the hydrolysis reaction of ethyl orthosilicate to obtain silica microspheres comprises:

[0051] adding ammonia water dropwise to an ethanol-water mixed solution of ethyl orthosilicate to perform a hydrolysis reaction of the ethyl orthosilicate to obtain a mixed slurry;

[0052] The mixed slurry is subjected to solid-liquid separation, washing and drying to obtain silica microspheres; the particle size of the silica microspheres is 100nm to 300nm.

[0053] In some embodiments, the molar ratio of the ethyl orthosilicate to 25% ammonia water is 1:(8-12);

[0054] The water bath temperature of the hydrolysis reaction is 30°C to 50°C, and the stirring time of the hydrolysis reaction is 4h to 8h;

[0055] In the ethanol-water mixed solution of ethyl orthosilicate, the volume ratio of ethanol to water is (2-4):1.

[0056] The molar ratio of tetraethyl orthosilicate to ammonia water is limited to 1:8 to 1:12, and ammonia water is used as a catalyst to promote the hydrolysis and polycondensation of tetraethyl orthosilicate (TEOS). If the molar ratio is lower than 1:8, the catalytic efficiency is insufficient, the hydrolysis rate is slow, the growth of the microsphere particle size is limited, and the production efficiency is low. If the molar ratio is higher than 1:12, the alkalinity is too strong, and TEOS is rapidly polycondensed, resulting in the microsphere particle size being too large and the distribution being widened. Exemplarily, the molar ratio of tetraethyl orthosilicate to ammonia water with a concentration of 25% is 1:8, 1:9, 1:10, 1:11, 1:12, etc.

[0057] The hydrolysis temperature is limited to 30°C to 50°C and the stirring time is 4h to 8h, which can regulate the hydrolysis-polycondensation reaction kinetics. If the temperature is lower than 30°C, the reaction rate is insufficient, the nucleation and growth of the microspheres are slow, and the particle size distribution is uneven. If the temperature is higher than 50°C, the polycondensation is too fast, agglomerates are easily formed, and the particle size distribution becomes wider. Exemplarily, the water bath temperature of the hydrolysis reaction can be 30°C, 35°C, 40°C, 45°C, 50°C, etc., and the stirring time of the hydrolysis reaction can be 4h, 5h, 6h, 7h, 8h, etc.

[0058] Limiting the volume ratio of ethanol to water to 2:1 to 4:1 can adjust the polarity of the solution and affect the solubility of TEOS and the surface charge of the microspheres. If the ethanol ratio is greater than 4:1, the dielectric constant is too low, the surface charge shielding effect of the microspheres is enhanced, and the particles agglomerate. If the ethanol ratio is less than 2:1, the proportion of the aqueous phase is too high, TEOS hydrolyzes too quickly, and the sol is prone to gelation. Exemplarily, in the ethanol-water mixed solution of tetraethyl orthosilicate, the volume ratio of ethanol to water can be 2:1, 2.5:1, 3:1, 3.5:1, 4:1, etc.

[0059] In some embodiments, the number of washes is ≥ 3 times.

[0060] Washing times ≥ 3 times can remove residual NH4 + , to avoid Zeta potential reversal leading to microsphere flocculation. If the number of times is insufficient, NH4 + The residue changes the positive charge on the surface of the microspheres to negative charge, and the dispersion stability decreases.

[0061] S2, subjecting the mixed slurry of the silica microspheres and polyvinyl pyrrolidone to high-pressure homogenization and freeze-drying to obtain a SiO2-PVP template with a core-shell structure;

[0062] It should be noted that the synergistic effect of the silica-PVP dual template in the embodiment of the present application is the core theory for achieving precise control of the structure of alumina hollow microspheres. Its working principle is as follows: silica microspheres act as spatial rigid bodies, providing incompressible nanoscale cavities of 100nm to 300nm, which can define the size of the hollow core during the sol coating stage; PVP polymer, as a regulatory layer of the flexible interface, can form a hydrogen bond network with the aluminum sol through the amide groups on the molecular chain, guiding the sol to be directional and homogeneously coated.

[0063] Silica microspheres and PVP polymers achieve the dual functions of size limitation and interface guidance through the combination of rigid core + flexible shell. In the sol coating-drying process, they realize the triple functions of defining hollow size, controlling shell homogeneity and inhibiting structural deformation, providing a scientific theoretical basis for the structural controllability and process stability of alumina hollow microspheres, overcoming the limitations of a single template leading to too fast heterogeneous nucleation of aluminum sol, resulting in local agglomeration or lack of rigid support after thermal decomposition, and drying shrinkage leading to structural collapse.

[0064] In some embodiments, the solid content of the mixed slurry of silica microspheres and polyvinyl pyrrolidone is 3% to 8%;

[0065] The mass ratio of the silica microspheres to the polyvinyl pyrrolidone is 1:(1-3).

[0066] In some embodiments, the pressure of the high-pressure homogenization treatment is 100 MPa to 200 MPa, and the number of cycles of the high-pressure homogenization treatment is 2 to 5 times;

[0067] The freeze drying process includes the following parameters: temperature of -40°C to -60°C, vacuum degree ≤10Pa, and drying time of 24h to 48h.

[0068] The mass ratio of SiO2 to PVP is limited to 1:1 to 1:3, which can control the coating integrity of PVP on SiO2 microspheres. If the mass ratio is less than 1:1, the amount of PVP is insufficient, the SiO2 surface is not fully coated, and it is easy to be exposed during the sol coating stage, resulting in agglomeration. If the mass ratio is greater than 1:3, the PVP is excessive, forming a thick shell that hinders the penetration of the aluminum sol, and the filling rate of the hollow cavity increases after drying. Exemplary, the mass ratio of silica microspheres to the polyvinyl pyrrolidone can be 1:1, 1:1.5, 1:2, 1:2.5, 1:3, etc.

[0069] The high-pressure homogenization pressure is limited to 100MPa~200MPa and the number of cycles is 2~5 times, and mechanical shear force is used to promote PVP to embed into the SiO2 gap to form a core-shell structure. If the pressure is less than 100MPa, the shear force is insufficient, the PVP coating rate is low, and the shell uniformity is poor. If the pressure is greater than 200MPa, the crushing rate of SiO2 microspheres increases and the particle size distribution becomes wider. If the number of cycles is less than 2 times, the coating is insufficient; if the number of cycles is greater than 5 times, the energy consumption increases and the PVP molecular chain may break. Exemplarily, the pressure of the high-pressure homogenization treatment can be 100MPa, 120MPa, 150MPa, 180MPa, 200MPa, etc., and the number of cycles of the high-pressure homogenization treatment can be 2 times, 3 times, 4 times, 5 times, etc.

[0070] Limiting the freeze-drying parameters (-40℃~-60℃, vacuum degree ≤10Pa, 24~48h) can inhibit the growth of ice crystals and maintain the stability of the template structure. If the temperature is greater than -40℃, the ice crystals grow too fast, the extrusion template causes deformation, and the particle size distribution becomes wider. If the vacuum degree is greater than 10Pa, the water sublimation rate is low, and the template shrinks and collapses due to stress. If the time is less than 24h, the drying is insufficient; if it is greater than 48h, energy consumption is wasted and PVP may be oxidatively degraded. Exemplarily, the freeze-drying temperature can be -40℃, -45℃, -50℃, -55℃, -60℃, etc., the vacuum degree can be 10Pa, 9Pa, 8Pa, 7Pa, 6Pa, 5Pa, etc., and the drying time can be 24h, 28h, 32h, 36h, 40h, 44h, 48h, etc.

[0071] S3, dissolving polyacrylic acid in a mixed solution of aluminum nitrate solution and aluminum isopropoxide solution to obtain a dual aluminum source solution;

[0072] In some embodiments, in the mixed solution of the aluminum nitrate solution and the aluminum isopropoxide solution, the molar concentration of the aluminum nitrate is 0.6 mol / L to 1.5 mol / L, and the molar concentration of the aluminum isopropoxide is 0.02 mol / L to 0.15 mol / L;

[0073] The mass of the polyacrylic acid is 0.3% to 1.0% of the total mass of the dual aluminum source solution, and the molecular weight of the polyacrylic acid is 1000 to 5000.

[0074] Limiting the concentration of aluminum nitrate to 0.6mol / L~1.5mol / L and the concentration of aluminum isopropoxide to 0.02mol / L~0.15mol / L can coordinately regulate the hydrolysis-condensation rate and form a uniform sol network. 3+ If the concentration is too high, hydrolysis will produce H +Accumulation is fast, and a sudden drop in pH inhibits polycondensation. If aluminum isopropoxide is greater than 0.15 mol / L, the polycondensation side reaction increases, the sol viscosity is too high, and the fluidity deteriorates. For example, the molar concentration of aluminum nitrate is 0.6 mol / L, 0.7 mol / L, 0.9 mol / L, 1.1 mol / L, 1.3 mol / L, 1.5 mol / L, etc., and the molar concentration of aluminum isopropoxide can be 0.02 mol / L, 0.05 mol / L, 0.09 mol / L, 1.1 mol / L, 1.3 mol / L, 0.15 mol / L, etc.

[0075] The addition amount of polyacrylic acid (PAA) is limited to 0.3% to 1.0wt% and the molecular weight is 1000 to 5000. As a dispersant, polyacrylic acid can stabilize the aluminum sol through electrostatic repulsion and steric hindrance. If the addition amount is less than 0.3%, the dispersion is insufficient, the sol particles agglomerate, and the microsphere particle size is too large. If the addition amount is greater than 1.0%, PAA will wrap the aluminum source, hinder the hydrolysis reaction, and the sol viscosity will increase abnormally. If the molecular weight is less than 1000, the chain segment is too short and the steric hindrance effect is weak; if it is greater than 5000, the polymer chains will be entangled and the sol viscosity will increase sharply. For example, the mass of polyacrylic acid can be 0.3%, 0.4%, 0.5%, 0.7%, 0.9%, 1.0%, etc. of the total mass of the dual aluminum source solution, and the molecular weight of polyacrylic acid can be 1000, 1500, 2000, 3000, 4000, 5000, etc.

[0076] In some embodiments, the method for preparing the mixed solution of aluminum nitrate solution and aluminum isopropoxide solution comprises:

[0077] Under the condition of stirring speed of 500 rpm to 800 rpm, an aluminum nitrate solution with a concentration of 1.0 mol / L to 1.5 mol / L and an aluminum isopropoxide solution with a concentration of 0.2 mol / L to 0.5 mol / L are mixed in a volume ratio of (3 to 5):1 to obtain a mixed solution.

[0078] Aluminum nitrate hydrolyzes quickly to provide nucleation sites; aluminum isopropoxide slowly condenses to form an Al-O-Al network. Aluminum nitrate hydrolyzes quickly, so [Al 3+ ]Below 1.5M, prevent H + Accumulation is too fast, resulting in a sudden drop in pH; aluminum isopropoxide polycondenses slowly, and side reactions will increase when the concentration is higher than 0.5M, resulting in excessive sol viscosity. Too low a raw material concentration will result in too low a yield, so the raw material concentration cannot be too low. If the volume ratio is lower than 3:1, insufficient cross-linking will occur due to low concentration, and the microspheres will crack due to stress during the drying process; if the volume ratio is higher than 5:1, aluminum nitrate will dominate the formation of a fast brittle gel due to excessive concentration. Therefore, the dual aluminum source formula is kinetically matched by the ionic type and the organic metal aluminum source. The concentration is controlled to adjust the ratio of hydrolysis / condensation rates; the volume ratio is controlled to balance the sol gel time and shell toughness.

[0079] S4, dispersing the SiO2-PVP template in the dual aluminum source solution to obtain a template dispersion;

[0080] In some embodiments, the mass ratio of the SiO2-PVP template to the dual aluminum source solution is 1:5;

[0081] The ultrasonic frequency of the dispersion is 40 kHz, and the ultrasonic time of the dispersion is 0.5 h.

[0082] Limiting the mass ratio of SiO2-PVP template to dual aluminum source solution to 1:5 can ensure that the surface of the silica microspheres is fully coated with aluminum sol, while avoiding insufficient sol penetration caused by excessive template and the occurrence of hollow structure filling defects. The dual aluminum source solution (aluminum nitrate + aluminum isopropoxide) needs to carry enough aluminum ions to form an alumina shell. If the template ratio is too high, the aluminum source is insufficiently dispersed and the shell is prone to breakage; if it is too low, the sol material is wasted. At the same time, PVP, as a flexible interface layer, is bonded to the aluminum sol through hydrogen bonds. At a mass ratio of 1:5, it can form a directional coating, reducing the risk of template desorption.

[0083] S5. Using a microfluidic reactor, dropwise adding ammonia water to the template dispersion until the pH value of the template dispersion reaches 4.2 to 4.5, thereby obtaining an aluminum sol;

[0084] It should be noted that the microfluidic reactor used in the embodiment of the present application is the core innovative equipment for achieving the structural uniformity of alumina precursor microbeads. Its functions are as follows: it solves the problems of hard agglomeration of particles caused by instantaneous gelation of sol due to uneven mixing, uncontrolled reaction kinetics, and Zeta potential oscillation caused by the drawbacks of pH control in traditional processes, shell cracks caused by sudden changes in the hydrolysis rate of the aluminum source, and uneven coating thickness caused by particle flocculation; through laminar shear and diffusion mass transfer in the microscale channel, the pH gradient linearization can be controlled, millisecond-level mixing and steady-state maintenance of Zeta potential can be achieved, thereby controlling the hydrolysis and condensation kinetics of the aluminum source, eliminating local concentration and temperature fluctuations, and ensuring the dispersion stability of the sol, providing an irreplaceable process foundation for ultimately obtaining alumina precursor microbeads with uniform shells, high sphericity, and complete hollowness.

[0085] In some embodiments, the channel diameter of the microfluidic reactor is 300 μm to 800 μm;

[0086] When ammonia water is added dropwise to the template dispersion, the dropping speed of ammonia water with a concentration of 0.1 mol / L is 0.5 mL / min to 2.0 mL / min, and the temperature of the template dispersion is 20° C. to 30° C.;

[0087] The solid content of the aluminum sol is 20% to 25%, the pH value of the aluminum sol is 4.2 to 4.5, the viscosity of the aluminum sol is 80 mPa·s to 120 mPa·s, and the Zeta potential of the aluminum sol is ≥+30 mV.

[0088] The pH control range is 3.0→4.5, which can control the hydrolysis and polycondensation kinetics of the aluminum source and avoid hard agglomeration. 3+ The high degree of protonation results in an extremely low hydrolysis rate, making it difficult for a sol to form. If the pH is > 4.5, giant Al(OH)3 clusters form, leading to irreversible hard agglomeration. For example, the pH of the aluminum sol is 4.2, 4.3, 4.4, 4.5, etc.

[0089] Limiting the ammonia dripping speed to 0.5mL / min~2.0mL / min and the temperature to 20℃~30℃ can ensure a linear increase in pH and avoid local over-alkalinity. If the dripping speed is greater than 2.0mL / min, the local pH will suddenly change, the sol will instantly gel, and hard agglomerated particles will be produced. If the temperature is less than 20℃, the hydrolysis rate will be slow and the pH control time will be extended; if the temperature is greater than 30℃, ammonia will volatilize quickly and the pH control accuracy will decrease. For example, the dripping speed of ammonia is 0.5mL / min, 0.7mL / min, 0.9mL / min, 1.2mL / min, 1.5mL / min, 1.8mL / min, 2.0mL / min, etc., and the temperature of the template dispersion can be 20℃, 22℃, 24℃, 28℃, 30℃, etc.

[0090] Limiting the sol viscosity to 80-120 mPa·s can ensure the uniformity and dispersion stability of the sol-coated template. If the viscosity is less than 80 mPa·s, the sol fluidity is too strong, the shell is too thin during coating, and the hollow structure is prone to collapse. If the viscosity is greater than 120 mPa·s, the sol is blocked from penetrating the template, the hollow cavities are filled, and the proportion of solid microspheres increases. Maintaining the Zeta potential ≥ +30 mV is to provide electrostatic repulsion to ensure colloidal dispersion stability and ensure that the sol particles are arranged in an orderly manner rather than randomly stacked during the coating stage. If the Zeta potential is less than +30 mV, the electrostatic repulsion is insufficient, the sol particles flocculate, and the coating thickness is uneven. For example, the viscosity of the aluminum sol can be 80 mPa·s, 90 mPa·s, 100 mPa·s, 110 mPa·s, 120 mPa·s, etc., and the Zeta potential of the aluminum sol can be +30 mV, +31 mV, +32 mV, +33 mV, +34 mV, +35 mV, +36 mV, etc.

[0091] S6. Using a dual-fluid nozzle and centrifugal atomization technology, the aluminum sol is spray-dried to obtain hollow aluminum oxide precursor microbeads.

[0092] In some embodiments, the spray drying includes the following parameters: a feed rate of 5 mL / min to 15 mL / min, a two-fluid nozzle air pressure of 0.3 MPa to 0.5 MPa, a centrifugal atomizing disk speed of 12,000 rpm to 18,000 rpm, an air inlet temperature controlled at 180°C to 200°C, an air outlet temperature controlled at 80°C to 100°C, and a droplet size distribution of D90 / D10 ≤ 1.5.

[0093] Limiting the feed solids content to 20% to 25% can control the sol loading and adjust the shell thickness. If the feed solids content is less than 20%, the sol volume is low, the shell is too thin, and the hollow structure has poor support. If the feed solids content is greater than 25%, the sol viscosity is too high, the atomized droplets agglomerate, and the particle size distribution becomes broad. For example, the feed solids content can be 20%, 21%, 22%, 23%, 24%, 25%, etc.

[0094] Limiting the centrifugal atomizing disk speed to 12000-18000 rpm and the dual-fluid nozzle air pressure to 0.3-0.5 MPa can synergistically control the droplet size and the formation of the "droplet in droplet" structure. If the speed is less than 12000 rpm, the droplet size will be large, the drying stress will be concentrated, and the microspheres will be easily broken. If the feed solid content pressure is less than 0.3 MPa, the outer aluminum sol film will be unevenly coated; if it is greater than 0.5 MPa, the droplets will be excessively broken and form fragments. Exemplarily, the dual-fluid nozzle air pressure can be 0.3 MPa, 0.35 MPa, 0.4 MPa, 0.45 MPa, 0.5 MPa, etc., and the centrifugal atomizing disk speed can be 12000 rpm, 13000 rpm, 14000 rpm, 15000 rpm, 16000 rpm, 17000 rpm, 18000 rpm, etc.

[0095] By limiting the air inlet temperature to 180℃~200℃ and the air outlet temperature to 80℃~100℃, the drying phase change path can be regulated to avoid structural collapse. If the feed solid content and the air inlet temperature are less than 180℃, the drying rate is slow, the evaporation of the inner layer of water is delayed, and the droplet shrinkage stress is concentrated. If the feed solid content and the air inlet temperature are greater than 200℃, the template PVP decomposes rapidly, the internal pressure of the microspheres rises suddenly, and the wall breaks. For example, the air inlet temperature is controlled at 180℃, 185℃, 190℃, 195℃, 200℃, etc., and the air outlet temperature is controlled at 80℃, 85℃, 90℃, 95℃, 100℃, etc.

[0096] The droplet size distribution is limited to D90 / D10 ≤ 1.5, so that the droplet size is controlled within 5 μm to 10 μm. When D90 / D10 is greater than 1.5, large and small particles will fill the gaps, resulting in uneven sintering shrinkage. For example, the droplet size distribution D90 / D10 can be 1.5, 1.4, 1.3, 1.2, 1.1, 1.0, etc.

[0097] In some embodiments, the hollow alumina precursor microbeads meet the following properties: particle size D50 is 5 μm to 10 μm, specific surface area is 50 m 2 / g~80m 2 / g, porosity ≥65%, hollow structure preforming rate ≥95%, Al2O3 content ≥85%.

[0098] Therefore, a preparation process based on the coordinated regulation of precursors by composite templates and dual aluminum sources is proposed. By introducing silica-PVP composite templates, dynamic pH microfluidics technology and dual atomization drying process, the bottleneck problems of traditional preparation methods such as uneven structure, high energy consumption and poor environmental protection are solved. The technical solution of this application is divided into three parts: template synthesis, sol preparation, and preparation of pre-sintered alumina hollow precursor microbeads. Silica + polymer is used to achieve the regulation of the hollow structure of the microbead precursor through the synergistic effect of dual templates. Dual aluminum sources, dynamic pH microfluidics regulation + dual atomization technology are used to achieve high-precision precursor preparation.

[0099] In summary, this application systematically breaks through the bottleneck of traditional alumina hollow microsphere preparation through multi-dimensional technological innovation. Its core advantages are reflected in the following aspects:

[0100] 1. Structural Control: From “Disordered Growth” to “Nanoscale Precision Design”

[0101] (1) Rigid-flexible synergistic mechanism of composite template: Silica microspheres (100-300 nm) act as rigid cores to provide incompressible hollow cavities, and polyvinylpyrrolidone (PVP) is embedded in the gaps between microspheres through high-pressure homogenization to form a flexible shell. The two act through the dual effects of "mechanical interlocking + hydrogen bond guidance" to make the aluminum sol uniformly cover the template. The preforming rate of the hollow structure reaches more than 95%, which is 15% higher than that of the traditional single template method, and completely solves the problem of hollow collapse caused by template agglomeration.

[0102] (2) Structural locking technology of freeze drying: The deep cold environment of -40℃ to -60℃ causes the PVP-water solution to instantly vitrify, inhibiting the template deformation caused by ice crystal growth. Compared with traditional thermal drying, the narrowness of the particle size distribution (D90 / D10) is reduced from 2.5 to within 1.5, avoiding agglomeration caused by capillary force and laying the foundation for a uniform shell.

[0103] 2. Process Innovation: Dynamic Control Achieves “Millisecond-Level Precision Control”

[0104] (1) Microfluidic pH gradient control technology: 300-800 μm microfluidic channels are combined with an ammonia drip rate of 0.5-2.0 mL / min to achieve a linear gradient change of pH from 3.0 to 4.5. Compared with traditional batch alkali addition, the local pH fluctuation is reduced from ±1.2 to ±0.3, and the proportion of hard agglomerated particles in the sol is reduced from 25% to below 3%. At the same time, the Zeta potential is maintained at ≥+30 mV, and the electrostatic repulsion is used to ensure the orderly arrangement of the sol particles, and the uniformity of the shell thickness is improved by 40%.

[0105] (2) Dual aluminum source kinetic matching design: aluminum nitrate (0.6-1.5 mol / L) rapidly hydrolyzes to provide nucleation sites, and aluminum isopropoxide (0.02-0.15 mol / L) slowly condenses to form an Al-O-Al network. The volume ratio of 3:1-5:1 allows the sol-gel time to be controlled within 10-30 min, and the shell toughness is increased by 2 times compared with a single aluminum source, solving the "cracking-brittleness" contradiction in traditional processes; polyacrylic acid (0.3-1.0 wt%) is used as a dispersant, which stabilizes the sol viscosity at 80-120 mPa·s through the steric hindrance effect, avoiding the loss of fluidity caused by molecular chain entanglement, and improving production efficiency by 30%.

[0106] 3. Green Preparation: Dual Breakthroughs in Low Energy Consumption and Non-toxicity

[0107] (1) Low-temperature demolding and low carbon emissions: The traditional process requires a high temperature of more than 600°C to remove the template, with a carbon emission intensity of 1.2tCO2 / kg. This application reduces the demolding temperature by 50% through the low-temperature decomposition of PVP (below 300°C) combined with the dissolution of the silica template, and reduces the carbon emissions to below 0.5tCO2 / kg. At the same time, non-toxic reagents such as ethanol and ammonia water are used throughout the process to replace traditional toxic solvents such as formaldehyde and toluene, reducing the occupational health risk index from high risk to low risk.

[0108] (2) Energy-optimized drying process: The air inlet temperature of double atomization drying is controlled at 180℃~200℃, which reduces energy consumption by 30% compared with traditional spray drying (above 300℃); freeze drying achieves low-temperature sublimation at -40℃ and vacuum degree ≤10Pa, which reduces energy consumption by 40% compared with traditional thermal drying, while avoiding the problem of template carbonization caused by high temperature.

[0109] 4. Performance Improvement: Multiple Indicators Surpass Traditional Levels

[0110] (1) Particle size uniformity: D50 is controlled at 5-10 μm, D90 / D10 ≤ 1.5, and the narrowness of the particle size distribution is increased by 30% compared with the traditional method;

[0111] (2) Hollow integrity: preforming rate ≥95%, 15% higher than traditional methods, wall breaking rate <5%;

[0112] (3) Porosity and specific surface area: Porosity ≥ 65%, specific surface area 50-80m 2 / g, which is 30% and 40% higher than the traditional method, respectively, providing sufficient shrinkage space for subsequent sintering;

[0113] (4) Component purity: Al2O3 content ≥85%, 10% higher than traditional methods, meeting the demand for high-purity materials.

[0114] 5. Technological Innovation: Multi-Scale Collaboration and Universal Design

[0115] (1) Cross-scale structural control: From nanoscale templates (100nm-300nm), microfluidic channels (300μm-800μm) to atomized droplets (5μm-50μm), achieving full-process structural consistency control;

[0116] (2) Kinetic equilibrium principle: Parameters such as the concentration of the dual aluminum source, pH gradient rate, and drying temperature are all designed based on the "hydrolysis-polycondensation-drying" kinetic equilibrium to avoid empirical trial and error;

[0117] (3) Green process integration: Through the coordination of material design (composite template), equipment innovation (microfluidics), and process optimization (double atomization), a full-chain environmentally friendly preparation system is constructed to provide a universal technical path for inorganic hollow microsphere materials.

[0118] The present application will be further described below in conjunction with specific examples. It should be understood that these examples are intended to illustrate the present application only and are not intended to limit the scope of the present application. The experimental methods in the following examples where specific conditions are not specified are generally measured according to industry standards. If there are no corresponding industry standards, then the methods are carried out according to general international standards, conventional conditions, or the conditions recommended by the manufacturer.

[0119] Example 1

[0120] This embodiment provides a method for preparing hollow alumina precursor microbeads, which may include the following steps:

[0121] S11. At room temperature, prepare an ethanol-water mixed solution in a volume ratio of 2:1, and add tetraethyl orthosilicate dropwise to the ethanol-water mixed solution while stirring; while stirring, add 25% ammonia water dropwise in a molar ratio of tetraethyl orthosilicate to ammonia water of 1:8; after the addition is completed, increase the water bath temperature to 40°C and continue stirring for 6 hours; after the reaction is completed, centrifuge the slurry and wash it with deionized water 4 times to obtain SiO2 microspheres with a particle size of about 100 to 300 nm.

[0122] S21. Disperse SiO2 microspheres and PVP in deionized water in a mass ratio of 1:1, and control the solid concentration of SiO2 microspheres and PVP slurry to 5wt%; homogenize the mixed slurry four times under a pressure of 150Mpa, and freeze-dry the homogenized slurry at -50°C and a vacuum degree not exceeding 10Pa for 30 hours to obtain SiO2-PVP powder with a particle size of 100-300nm.

[0123] S31. Under a stirring speed of 650 rpm, a 1.0 mol / L aluminum nitrate solution and a 0.20 mol / L aluminum isopropoxide solution are mixed in a volume ratio of 3:1; while stirring, polyacrylic acid with a molecular weight of 5000 is added to the dual aluminum source solution at a ratio of 0.3 wt% of the weight of the dual aluminum source solution, and stirring is continued for 3 hours. Stirring is stopped after all substances are completely dissolved to obtain a dual aluminum source solution.

[0124] S41. SiO2-PVP powder is added to the aluminum salt solution at a mass ratio of SiO2-PVP powder to dual aluminum source solution of 1:5, and the solution is treated with ultrasound at a frequency of 40 kHz for 0.5 h to obtain a template dispersion.

[0125] S51. Use a microfluidic reactor with a channel diameter of 300-800 μm to add 0.1 mol / L ammonia water to the template dispersion at 20-30°C, and control the ammonia water addition rate to 0.5 mL / min; when the pH value of the aluminum salt solution reaches 4.5, stop adding ammonia water and continue stirring for 0.5 h to form a uniform aluminum sol.

[0126] S61, aluminum sol with a solid content of 20% is fed into a dual-fluid nozzle + centrifugal atomization device at a feed rate of 8 mL / min. The dual-fluid nozzle air pressure is controlled at 0.3 MPa, the centrifugal atomization disk speed is controlled at 15,000 rpm, and the droplet size distribution is D 90 / D 10 ≤1.5, so that the droplet size is controlled at 5-10μm. The air inlet temperature is controlled at 180-200℃, and the air outlet temperature is controlled at 80-100℃, and the particle size D 50 The precursor microbeads are 5 to 10 μm.

[0127] Example 2

[0128] This embodiment provides a method for preparing hollow alumina precursor microbeads, which may include the following steps:

[0129] S11. At room temperature, prepare an ethanol-water mixed solution in a volume ratio of 4:1, and add tetraethyl orthosilicate dropwise to the ethanol-water mixed solution while stirring; while stirring, add 25% ammonia water dropwise in a molar ratio of tetraethyl orthosilicate to ammonia water of 1:12; after the addition is completed, increase the water bath temperature to 40°C and continue stirring for 6 hours; after the reaction is completed, centrifuge the slurry and wash it with deionized water 4 times to obtain SiO2 microspheres with a particle size of about 100 to 300 nm.

[0130] S21. Disperse SiO2 microspheres and PVP in deionized water in a mass ratio of 1:3, and control the solid concentration of SiO2 microspheres and PVP slurry to 5wt%; homogenize the mixed slurry under a pressure of 150Mpa for 4 times, and freeze-dry the homogenized slurry at -50°C and a vacuum degree not exceeding 10Pa for 30 hours to obtain SiO2-PVP powder with a particle size of 100-300nm.

[0131] S31. Under a stirring speed of 650 rpm, a 1.5 mol / L aluminum nitrate solution and a 0.5 mol / L aluminum isopropoxide solution are mixed in a volume ratio of 5:1; while stirring, polyacrylic acid with a molecular weight of 5000 is added to the dual aluminum source solution at a ratio of 1.0 wt% of the weight of the dual aluminum source solution, and stirring is continued for 3 hours. Stirring is stopped after all substances are completely dissolved to obtain a dual aluminum source solution.

[0132] S41. SiO2-PVP powder is added to the aluminum salt solution at a mass ratio of SiO2-PVP powder to dual aluminum source solution of 1:5, and the solution is treated with ultrasound at a frequency of 40 kHz for 0.5 h to obtain a template dispersion.

[0133] S51. Use a microfluidic reactor with a channel diameter of 300-800 μm to add 0.1 mol / L ammonia water to the template dispersion at 20-30°C, and control the ammonia water addition rate to 2.0 mL / min; when the pH value of the aluminum salt solution reaches 4.5, stop adding ammonia water and continue stirring for 0.5 h to form a uniform aluminum sol.

[0134] S61, aluminum sol with a solid content of 25% is fed into a dual-fluid nozzle + centrifugal atomization device at a feed rate of 12 mL / min. The dual-fluid nozzle pressure is controlled at 0.5 MPa, the centrifugal atomization disk speed is controlled at 15,000 rpm, and the droplet size distribution is D 90 / D 10 ≤1.5, so that the droplet size is controlled at 5-10μm. The air inlet temperature is controlled at 180-200℃, and the air outlet temperature is controlled at 80-100℃, and the particle size D 50 The precursor microbeads are 5 to 10 μm.

[0135] Example 3

[0136] This embodiment provides a method for preparing hollow alumina precursor microbeads, which may include the following steps:

[0137] S11. Prepare an ethanol-water mixed solution at a volume ratio of 3:1 at room temperature, and add tetraethyl orthosilicate dropwise to the ethanol-water mixed solution while stirring; while stirring, add 25% ammonia water (the molar ratio of tetraethyl orthosilicate to ammonia water is 1:10); after the addition is completed, increase the water bath temperature to 40°C and continue stirring for 6 hours; after the reaction is completed, centrifuge the slurry and wash it with deionized water 4 times to obtain SiO2 microspheres with a particle size of about 100 to 300 nm.

[0138] S21. Disperse SiO2 microspheres and PVP in deionized water in a mass ratio of 1:2, and control the solid concentration of SiO2 microspheres and PVP slurry to 5wt%; homogenize the mixed slurry under a pressure of 150a for 4 times, and freeze-dry the homogenized slurry at -50°C and a vacuum degree not exceeding 10Pa for 30h to obtain SiO2-PVP powder with a particle size of 100-300nm.

[0139] S31. Under a stirring speed of 650 rpm, a 1.3 mol / L aluminum nitrate solution and a 0.3 mol / L aluminum isopropoxide solution are stirred at a volume ratio of 4:1, and polyacrylic acid with a molecular weight of 5000 is added to the double aluminum source solution at a ratio of 0.5 of the weight of the double aluminum source solution. Stirring is continued for 3 hours until completely dissolved, and then stirring is stopped to obtain a double aluminum source solution.

[0140] S41. SiO2-PVP powder is added to the aluminum salt solution at a mass ratio of SiO2-PVP powder to dual aluminum source solution of 1:5, and the solution is treated with ultrasound at a frequency of 40 kHz for 0.5 h to obtain a template dispersion.

[0141] S51. Use a microfluidic reactor with a channel diameter of 300-800 μm to add 0.1 mol / L ammonia water to the template dispersion liquid controlled at 20-30°C, and control the ammonia water addition rate to 1.2 mL / min; when the pH value of the aluminum salt solution reaches 4.5, stop adding ammonia water and continue stirring for 0.5 h to form a uniform aluminum sol.

[0142] S61, aluminum sol with a solid content of 22% is fed into a dual-fluid nozzle + centrifugal atomization device at a feed rate of 10 mL / min. The dual-fluid nozzle air pressure is controlled at 0.4 MPa, the centrifugal atomization disk speed is controlled at 16000 rpm, and the droplet size distribution is D 90 / D 10 ≤1.5, so that the droplet size is controlled at 5-10μm. The air inlet temperature is controlled at 180-200℃, and the air outlet temperature is controlled at 80-100℃, and the particle size D50 The precursor microbeads are 5 to 10 μm.

[0143] Example 4

[0144] This embodiment provides a method for preparing hollow alumina precursor microbeads, which may include the following steps:

[0145] S11. Prepare an ethanol-water mixed solution at a volume ratio of 2:1 at room temperature, and add tetraethyl orthosilicate dropwise to the ethanol-water mixed solution while stirring; while stirring, add 25% ammonia water (the molar ratio of tetraethyl orthosilicate to ammonia water is 1:12); after the addition is completed, increase the water bath temperature to 40°C and continue stirring for 6 hours; after the reaction is completed, centrifuge the slurry and wash it with deionized water 4 times to obtain SiO2 microspheres with a particle size of about 100 to 300 nm.

[0146] S21. Disperse SiO2 microspheres and PVP in deionized water in a mass ratio of 1:1, and control the solid concentration of SiO2 microspheres and PVP slurry to 5wt%; homogenize the mixed slurry under a pressure of 150Mpa for 4 times, and freeze-dry the homogenized slurry at -60°C and a vacuum degree not exceeding 10Pa for 24 hours to obtain SiO2-PVP powder with a particle size of 100-300nm.

[0147] S31. Under a stirring speed of 650 rpm, a 1.0 mol / L aluminum nitrate solution and a 0.2 mol / L aluminum isopropoxide solution are mixed in a volume ratio of 3:1; while stirring, polyacrylic acid with a molecular weight of 5000 is added to the dual aluminum source solution at a ratio of 0.5 wt% of the weight of the dual aluminum source solution, and stirring is continued for 3 hours. Stirring is stopped after all substances are completely dissolved to obtain a dual aluminum source solution.

[0148] S41. SiO2-PVP powder is added to the aluminum salt solution at a mass ratio of SiO2-PVP powder to dual aluminum source solution of 1:5, and the solution is treated with ultrasound at a frequency of 40 kHz for 0.5 h to obtain a template dispersion.

[0149] S51. Use a microfluidic reactor with a channel diameter of 300-800 μm to add 0.1 mol / L ammonia water to the template dispersion liquid controlled at 20-30°C, and control the ammonia water addition rate to 1.4 mL / min; when the pH value of the aluminum salt solution reaches 4.4, stop adding ammonia water and continue stirring for 0.5 h to form a uniform aluminum sol.

[0150] S61, aluminum sol with a solid content of 20% is fed into a dual-fluid nozzle + centrifugal atomization device at a feed rate of 10 mL / min. The dual-fluid nozzle pressure is controlled at 0.4 MPa, the centrifugal atomization disk speed is controlled at 12,000 rpm, and the droplet size distribution is D 90 / D 10 ≤1.5, so that the droplet size is controlled at 5-10μm. The air inlet temperature is controlled at 180-200℃, and the air outlet temperature is controlled at 80-100℃, and the particle size D 50 The precursor microbeads are 5 to 10 μm.

[0151] The embodiments 1 to 4 described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work shall fall within the scope of protection of the present invention.

[0152] The performance of the precursor microbeads obtained in Examples 1 to 4 was measured, and the results are shown in Table 1.

[0153] Table 1 Indexes of pre-sintered alumina hollow precursor microbeads in Examples 1-4

[0154]

[0155] As shown in Table 1, the hollow alumina precursor microbeads of Examples 1 to 4 meet the following properties: particle size D50 is 5 μm to 10 μm, specific surface area is 50 m 2 / g~80m 2 / g, porosity ≥65%, hollow structure preforming rate ≥95%, Al2O3 content ≥85%.

[0156] In addition, one or more technical solutions in the embodiments of the present application have at least the following technical effects or advantages:

[0157] In the embodiment of the present application, the synergistic effect of SiO2-PVP and the dual aluminum source makes the hollow rate of the alumina precursor reach more than 95%, while the hollow rate of the precursor in the traditional preparation process is less than 90%.

[0158] In the examples of the present application, dynamic pH control enables the sol to be stably stored for more than 30 days, which is better than the sol prepared by the traditional preparation process, which can be stored for up to 7 days.

[0159] In the embodiment of the present application, the use of fluid nozzle + centrifugal atomization technology can increase the solid content of the sprayable slurry, optimize the precursor microbead particle size distribution span to ≤1.2, and increase the production capacity to ≥20kg / h, which is a 40% increase in production capacity.

[0160] In the embodiment of the present application, the PVP in the template can be decomposed into harmless gas at 400°C, while the decomposition temperature of the polystyrene template used in the traditional process needs to be above 600°C, which reduces energy consumption by 30%.

[0161] In the embodiments of the present application, a gradient hydrolysis network is formed in the sol through the dual aluminum source synergistic hydrolysis technology, which enhances the film-forming property of the sol and reduces the risk of drying and cracking.

[0162] In the embodiments of the present application, the novel process for preparing pre-sintered hollow alumina precursor microbeads is applicable to the following fields:

[0163] High-performance catalyst carrier: After calcination, it can be used in precious metal or transition metal oxide supported catalysts in the fields of petrochemical industry, automobile exhaust purification, fuel cells, etc.

[0164] Lightweight and high-strength composite materials: After calcination, they can be used in aerospace structural parts, lightweight components of new energy vehicles, ship insulation interlayers, etc.

[0165] High-temperature thermal insulation and fireproof materials: After calcination, they can be used in industrial furnace linings, spacecraft thermal protection systems, and building fireproof coatings.

[0166] The foregoing is merely a list of specific embodiments of the present application, intended to enable those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application is not limited to the embodiments shown herein, but is intended to conform to the broadest scope consistent with the principles and novel features of the present application.

Claims

1. A method for preparing hollow alumina precursor microbeads, the method comprising: The ethyl orthosilicate is subjected to a hydrolysis reaction to obtain silica microspheres; The mixed slurry of the silica microspheres and polyvinyl pyrrolidone is subjected to high pressure homogenization and freeze drying to obtain a SiO2-PVP template with a core-shell structure; dissolving polyacrylic acid in a mixed solution of aluminum nitrate solution and aluminum isopropoxide solution to obtain a dual aluminum source solution; Dispersing the SiO2-PVP template in the dual aluminum source solution to obtain a template dispersion; Using a microfluidic reactor, adding ammonia water dropwise to the template dispersion until the pH value of the template dispersion reaches 4.2 to 4.5, thereby obtaining an aluminum sol; The aluminum sol is spray-dried by adopting a dual-fluid nozzle coordinated centrifugal atomization technology to obtain aluminum oxide hollow precursor microbeads.

2. The method according to claim 1, characterized in that The hydrolysis reaction of ethyl orthosilicate to obtain silica microspheres comprises: adding ammonia water dropwise to an ethanol-water mixed solution of ethyl orthosilicate to perform a hydrolysis reaction of the ethyl orthosilicate to obtain a mixed slurry; The mixed slurry is subjected to solid-liquid separation, washing and drying to obtain silica microspheres; the particle size of the silica microspheres is 100nm to 300nm.

3. The method according to claim 2, characterized in that The molar ratio of the ethyl orthosilicate to the 25% ammonia water is 1:(8-12); The water bath temperature of the hydrolysis reaction is 30°C to 50°C, and the stirring time of the hydrolysis reaction is 4h to 8h; In the ethanol-water mixed solution of ethyl orthosilicate, the volume ratio of ethanol to water is (2-4):

1.

4. The method according to claim 1, wherein The solid content of the mixed slurry of silica microspheres and polyvinyl pyrrolidone is 3% to 8%; The mass ratio of the silica microspheres to the polyvinyl pyrrolidone is 1:(1-3).

5. The method according to claim 1, wherein The pressure of the high-pressure homogenization treatment is 100 MPa to 200 MPa, and the number of cycles of the high-pressure homogenization treatment is 2 to 5 times; The freeze drying process includes the following parameters: temperature of -40°C to -60°C, vacuum degree ≤10Pa, and drying time of 24h to 48h.

6. The method according to claim 1, wherein In the mixed solution of the aluminum nitrate solution and the aluminum isopropoxide solution, the molar concentration of the aluminum nitrate is 0.6 mol / L to 1.5 mol / L, and the molar concentration of the aluminum isopropoxide is 0.02 mol / L to 0.15 mol / L; The mass of the polyacrylic acid is 0.3% to 1.0% of the total mass of the dual aluminum source solution, and the molecular weight of the polyacrylic acid is 1000 to 5000.

7. The method according to claim 1, characterized in that The mass ratio of the SiO2-PVP template to the dual aluminum source solution is 1:5; The ultrasonic frequency of the dispersion is 40 kHz, and the ultrasonic time of the dispersion is 0.5 h.

8. The method according to claim 1, characterized in that The channel diameter of the microfluidic reactor is 300 μm to 800 μm; When ammonia water is added dropwise to the template dispersion, the dropping speed of ammonia water with a concentration of 0.1 mol / L is 0.5 mL / min to 2.0 mL / min, and the temperature of the template dispersion is 20° C. to 30° C.; The solid content of the aluminum sol is 20% to 25%, the pH value of the aluminum sol is 4.2 to 4.5, the viscosity of the aluminum sol is 80 mPa·s to 120 mPa·s, and the Zeta potential of the aluminum sol is ≥+30 mV.

9. The method according to claim 1, characterized in that The spray drying process includes the following parameters: a feed rate of 5 mL / min to 15 mL / min, a dual-fluid nozzle air pressure of 0.3 MPa to 0.5 MPa, a centrifugal atomizing disk speed of 12,000 rpm to 18,000 rpm, an air inlet temperature controlled at 180° C. to 200° C., an air outlet temperature controlled at 80° C. to 100° C., and a droplet size distribution of D90 / D10 ≤ 1.

5.

10. The method according to claim 1, characterized in that The hollow alumina precursor microbeads meet the following properties: particle size D50 is 5 μm to 10 μm, specific surface area is 50 m 2 / g~80m 2 / g, porosity ≥65%, hollow structure preforming rate ≥95%, Al2O3 content ≥85%.

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