Aluminum-silicon hybrid precursor sol and preparation method thereof, and aluminum oxide aerogel

Abstract

This application relates to an aluminum-silicon hybrid precursor sol and its preparation method, as well as an alumina aerogel, belonging to the field of inorganic nanoporous material preparation technology. The method includes: mixing an aminoalkoxysilane, deionized water, and an acid catalyst in anhydrous ethanol, and then performing a pre-hydrolysis reaction at a pre-hydrolysis temperature to obtain an activated solution containing silanol oligomers; mixing and dissolving an inorganic aluminum salt and a carboxylic acid reagent in anhydrous ethanol, so that the carboxylic acid reagent reacts with the Al in the inorganic aluminum salt. 3+ A complex is formed to obtain an aluminum-carboxylic acid complex solution; under continuous stirring, the aluminum-carboxylic acid complex solution is added dropwise to the activation solution to obtain a hybrid reaction solution; the hybrid reaction solution is allowed to stand and age at an aging temperature to form a homogeneous and stable sol with rheological properties, thus obtaining an aluminum-silicon hybrid precursor sol.

Description

Technical Field

[0001] This application relates to the field of inorganic nanoporous material preparation technology, and in particular to an aluminum-silicon hybrid precursor sol and its preparation method, and an alumina aerogel. Background Technology

[0002] Alumina aerogels, due to their high porosity, low thermal conductivity, and excellent high-temperature potential, have broad application prospects in extreme thermal insulation fields such as aerospace thermal protection, energy-saving linings for industrial kilns, and high-temperature catalyst supports. However, alumina aerogels face severe stability problems when operating above 1000℃: the metastable γ-Al₂O₃ or θ-Al₂O₃ undergoes an uncontrollable crystal transformation to the thermodynamically stable α-Al₂O₃, accompanied by intense grain growth and sintering densification, leading to the collapse of the aerogel's nanoporous structure, a sharp decrease in specific surface area, and a severe deterioration in thermal insulation performance. Suppressing high-temperature phase transformation and sintering is the core bottleneck for the practical application of alumina aerogels.

[0003] To suppress the high-temperature phase transformation of alumina, existing technologies generally employ a strategy of introducing a silicon-aluminum composite source. This involves forming thermally more thermally stable Al-O-Si bonds or an amorphous SiO2 coating layer within the alumina framework, thereby increasing the nucleation energy barrier of α-Al2O3 and delaying or preventing the crystal transformation. For example, patent US20250091884A1 discloses a method for preparing aluminum-silicon composite aerogels using organic aluminum alkoxides (such as aluminum isopropoxide) and tetraethyl orthosilicate as raw materials. This method leverages the mild and controllable hydrolysis characteristics of organic aluminum alkoxides, enabling uniform hybridization of aluminum and silicon at the molecular level, resulting in materials with improved high-temperature stability. However, organic aluminum alkoxides are expensive and highly sensitive to air and moisture, requiring stringent operating conditions, which severely restricts their industrial application.

[0004] To reduce costs, researchers have attempted to replace organic alcohol aluminum salts with inexpensive inorganic aluminum salts (such as AlCl3·6H2O and Al(NO3)3·9H2O). However, Al... 3+ The hydrolysis kinetics in an aqueous environment are extremely fast, instantly generating amorphous aluminum hydroxide precipitate, making it extremely difficult to form a stable and uniform sol, and even more impossible to achieve fine and controllable molecular-level hybridization with silicon sources. Summary of the Invention

[0005] This application provides an aluminum-silicon hybrid precursor sol and its preparation method, as well as an alumina aerogel, to solve the following technical problem: how to develop a precursor sol preparation method that can overcome the problem of instantaneous hydrolysis and precipitation of inorganic aluminum salts and achieve uniform hybridization of aluminum-silicon components at the molecular level. In a first aspect, embodiments of this application provide a method for preparing an aluminum-silicon hybrid precursor sol, the method comprising: After mixing aminoalkoxysilane, deionized water and acid catalyst in anhydrous ethanol, a pre-hydrolysis reaction was carried out at a pre-hydrolysis temperature to obtain an activated solution containing silanol oligomers. Inorganic aluminum salts and carboxylic acid reagents are mixed and dissolved in anhydrous ethanol, so that the carboxylic acid reagent reacts with the Al in the inorganic aluminum salt. 3+ A complex is formed, yielding an aluminum-carboxylic acid complex solution; Under continuous stirring, the aluminum-carboxylic acid complex solution was added dropwise to the activation solution to obtain a hybrid reaction solution; The hybrid reaction solution is allowed to stand and age at an aging temperature to form a homogeneous and stable sol with rheological properties, thus obtaining an aluminum-silicon hybrid precursor sol.

[0006] Optionally, the molar ratio of the deionized water to the total molar amount of all hydrolyzable alkoxy groups in the aminoalkoxysilane is (0.1–0.8):1; the inorganic aluminum salt is in the form of Al 3+ The molar ratio of the inorganic aluminum salt to the carboxylic acid reagent is 1:(0.5-1.5); the inorganic aluminum salt is in the form of Al 3+ The molar ratio of the total molar amount to the molar amount of the aminoalkoxysilane is 1:(0.08~0.25).

[0007] Optionally, the molar ratio of the deionized water to the total molar amount of all hydrolyzable alkoxy groups in the aminoalkoxysilane is 0.25:1; the inorganic aluminum salt is in the form of Al 3+ The molar ratio of the total molar amount to the molar amount of the aminoalkoxysilane is 1:0.12.

[0008] Optionally, the pre-hydrolysis temperature is 40℃~70℃, and the pre-hydrolysis reaction time is 0.5h~3h.

[0009] Optionally, after adding the aluminum-carboxylic acid complex solution dropwise to the activation solution to obtain the hybrid reaction solution, the method further includes: subjecting the hybrid reaction solution to a hybrid reaction at a temperature of 50°C to 75°C for 2 to 6 hours.

[0010] Optionally, the aging temperature is 15℃~30℃, and the static aging time is 12h~48h.

[0011] Optionally, the aminoalkoxysilane is 3-aminopropyltriethoxysilane or 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane; The inorganic aluminum salt is aluminum nitrate nonahydrate or aluminum chloride hexahydrate; The carboxylic acid reagent is citric acid or tartaric acid.

[0012] Optionally, the acid catalyst is concentrated hydrochloric acid or concentrated nitric acid, and the pH value of the pre-hydrolysis reaction system is 4.0 to 6.5.

[0013] Secondly, embodiments of this application provide an aluminum-silicon hybrid precursor sol, which is prepared by the preparation method described in the first aspect.

[0014] Thirdly, embodiments of this application provide an alumina aerogel, which is prepared from the aluminum-silicon hybrid precursor sol described in the second aspect. After heat treatment at 1300°C for 2 hours, the alumina aerogel exhibits a linear shrinkage rate ≤10% and a specific surface area ≥90 m². 2 / g.

[0015] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a method for preparing an aluminum-silicon hybrid precursor sol, the method comprising: mixing an aminoalkoxysilane, deionized water, and an acid catalyst in anhydrous ethanol, and then performing a pre-hydrolysis reaction at a pre-hydrolysis temperature to obtain an activated solution containing silanol oligomers; and mixing and dissolving an inorganic aluminum salt and a carboxylic acid reagent in anhydrous ethanol to allow the carboxylic acid reagent to react with the Al in the inorganic aluminum salt. 3+ A complex is formed to obtain an aluminum-carboxylic acid complex solution. Under continuous stirring, the aluminum-carboxylic acid complex solution is added dropwise to the activation solution to obtain a hybrid reaction solution. The hybrid reaction solution is aged at an aging temperature to form a rheologically stable homogeneous sol, yielding an aluminum-silicon hybrid precursor sol. First, an aminoalkoxysilane is partially pre-hydrolyzed with a limited amount of water to generate an activation solution containing silanol oligomers, pre-establishing bonding sites that can react with aluminum. Simultaneously, a carboxylic acid reagent is used to react with Al in the inorganic aluminum salt. 3+ Forming stable aluminum Carboxylic acid complexes, blocking Al 3+ The coordination hydration sites fundamentally inhibit Al 3+ The kinetic tendency of instantaneous hydrolysis upon contact with water; finally, aluminum under continuous stirring conditions... The carboxylic acid complex solution was slowly added dropwise to the activation solution, so that each aluminum complex molecule was immediately surrounded by surrounding silanol groups upon entering the system and preferentially underwent a condensation reaction, forcing the formation of Al. O Si bonds instead of Al OH precipitation is used to achieve molecular-level homogeneous hybridization of aluminum and silicon components using inexpensive inorganic aluminum salts as raw materials, thus obtaining a precursor sol that can be used to prepare high-temperature stable alumina aerogels. Attached Figure Description

[0016] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0017] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic flowchart illustrating a method for preparing an aluminum-silicon hybrid precursor sol provided in an embodiment of this application. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0021] Figure 1 This is a schematic flowchart illustrating a method for preparing an aluminum-silicon hybrid precursor sol provided in an embodiment of this application.

[0022] Please see Figure 1 In a first aspect, embodiments of this application provide a method for preparing an aluminum-silicon hybrid precursor sol, the method comprising: S1. After mixing aminoalkoxysilane, deionized water and acid catalyst in anhydrous ethanol, a pre-hydrolysis reaction is carried out at a pre-hydrolysis temperature to obtain an activated solution containing silanol oligomers. S2. Dissolve the inorganic aluminum salt and carboxylic acid reagent in anhydrous ethanol, so that the carboxylic acid reagent reacts with the Al in the inorganic aluminum salt. 3+ A complex is formed, yielding an aluminum-carboxylic acid complex solution; S3. Under continuous stirring, the aluminum-carboxylic acid complex solution is added dropwise to the activation solution to obtain a hybrid reaction solution; S4. The hybrid reaction solution is allowed to stand and age at an aging temperature to form a homogeneous and stable sol with rheological properties, thereby obtaining an aluminum-silicon hybrid precursor sol.

[0023] In step S1, aminoalkoxysilane, deionized water, and an acid catalyst are mixed in anhydrous ethanol and subjected to a pre-hydrolysis reaction at a pre-hydrolysis temperature to obtain an activated solution containing silanol oligomers. The aminoalkoxysilane serves as a silicon source, providing hydrolyzable alkoxy groups. The amino group (-NH2) in the aminoalkoxysilane molecule can assist in anchoring the aluminum-carboxylic acid complex in subsequent hybridization reactions, promoting the formation of Al-O-Si bonds. Deionized water, as a reactant in the pre-hydrolysis reaction, hydrolyzes the alkoxy groups in the aminoalkoxysilane, converting them into silanol groups (-Si-OH). The acid catalyst provides hydrogen ions, catalyzing the hydrolysis of alkoxy groups and accelerating the formation of silanol groups. Simultaneously, the hydrolysis rate and degree of polymerization are controlled by adjusting the pH value. Anhydrous ethanol serves as a reaction solvent, dispersing the reactants and providing a homogeneous reaction environment, avoiding uneven reactions caused by excessively high local concentrations. The essence of the pre-hydrolysis reaction: In the presence of an acid catalyst, deionized water attacks the silicon atoms in the aminoalkoxysilane, causing the alkoxy group (-OR) to detach and be replaced by a hydroxyl group (-OH), generating silanols and the corresponding alcohols. The newly generated silanols can then undergo moderate condensation polymerization to form oligomers linked by silicon-oxygen bonds (Si-O-Si).

[0024] Step S2 involves dissolving the inorganic aluminum salt and carboxylic acid reagent in anhydrous ethanol to form a complex with the trivalent aluminum ions in the inorganic aluminum salt, yielding an aluminum-carboxylic acid complex solution. The inorganic aluminum salt provides trivalent aluminum ions (Al... 3 +The inorganic aluminum salt is selected from aluminum nitrate nonahydrate or aluminum chloride hexahydrate, which is much cheaper than organic aluminum alkoxides. A carboxylic acid reagent acts as a ligand, forming a stable chelate complex with trivalent aluminum ions. The carboxylic acid reagent is selected from citric acid or tartaric acid, both of which are polydentate carboxylic acids containing multiple carboxyl groups (-COOH), capable of forming five- or six-membered ring chelate structures with trivalent aluminum ions. Anhydrous ethanol is used as a solvent to dissolve the inorganic aluminum salt and carboxylic acid reagent, providing a homogeneous reaction environment. In step S2, the carboxyl groups (-COOH) in the carboxylic acid reagent partially dissociate in anhydrous ethanol, and the carboxyl group (-COO) dissociates into the carboxyl group. - The oxygen atom of the aluminum ion coordinates with the trivalent aluminum ion to form a stable aluminum-carboxylic acid complex.

[0025] In step S3, under continuous stirring, the aluminum-carboxylic acid complex solution is added dropwise to the activation solution to obtain a hybrid reaction solution. It is crucial that the aluminum-carboxylic acid complex solution be added dropwise to the activation solution in step S3, rather than in reverse or simultaneously. This order is mandatory to ensure that the aluminum-carboxylic acid complex preferentially undergoes a hybrid reaction rather than reacting with water when it encounters a large amount of silanol oligomers. Stirring ensures that the added aluminum-carboxylic acid complex solution is rapidly dispersed in the activation solution, preventing excessively high local concentrations that could lead to the hydrolysis and precipitation of trivalent aluminum ions. The stirring speed should be moderate, ensuring thorough mixing while avoiding excessive bubble formation. In step S3, when the aluminum-carboxylic acid complex approaches the silanol oligomers, the oxygen atom in the carboxylic acid ligand interacts with the hydrogen atom in the silanol, or through a ligand exchange reaction, causing the trivalent aluminum ions to form an Al-O-Si bond with the silanol. The formation rate of this bond is faster than the rate of spontaneous hydrolysis of trivalent aluminum ions to form aluminum hydroxide precipitate; therefore, the reaction is forced towards a preferential hybridization pathway. After the addition is complete, the resulting hybrid reaction solution is a homogeneous mixture without precipitate, in which an aluminum-silicon hybrid network structure has been initially formed.

[0026] Step S4 involves allowing the hybrid reaction solution to stand at an aging temperature to form a homogeneous and stable sol with rheological properties, yielding an aluminum-silicon hybrid precursor sol. Under static conditions, unreacted active groups in the hybrid reaction solution continue to undergo condensation reactions, gradually expanding and perfecting the aluminum-silicon hybrid network. Simultaneously, small molecules (such as alcohols and water) in the system diffuse and redistribute, making the sol composition more homogeneous.

[0027] In some embodiments, the aminoalkoxysilane is 3-aminopropyltriethoxysilane or 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane.

[0028] Aminoalkoxysilanes refer to silane coupling agents that simultaneously contain an amino group and at least one hydrolyzable alkoxy group in their molecule. In the preparation method of aluminum-silicon hybrid precursor sol, the aminoalkoxysilane is 3-aminopropyltriethoxysilane or 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane. The chemical formula of 3-aminopropyltriethoxysilane is H2N(CH2)3Si(OC2H5)3, and the molecule contains one amino group (located at the end of the propyl chain) and three hydrolyzable ethoxy groups (-OC2H5). The chemical formula of 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane is H2N(CH2)2NH(CH2)2NH(CH2)3Si(OCH3)3, and the molecule contains one primary amino group, two secondary amino groups (i.e., a polyamine segment containing three nitrogen atoms), and three hydrolyzable methoxy groups (-OCH3).

[0029] In some embodiments, the molar ratio of the deionized water to the total molar amount of all hydrolyzable alkoxy groups in the aminoalkoxysilane is (0.1 to 0.8):1.

[0030] Deionized water refers to water that has undergone ion exchange treatment to remove impurity ions. In the preparation of aluminum-silicon hybrid precursor sol, it is used as a reactant in the hydrolysis reaction to initiate the hydrolysis reaction of alkoxy groups in aminoalkoxysilanes.

[0031] The term "total hydrolyzable alkoxy groups" refers to the total number of alkoxy groups in an aminoalkoxysilane molecule that can be hydrolyzed by deionized water and converted into silanol groups.

[0032] In the pre-hydrolysis step, the molar amount of deionized water added is only 0.1 to 0.8 times the molar amount of all hydrolyzable alkoxy groups in the aminoalkoxysilane. Since this ratio is less than 1, it means that the molar amount of deionized water is less than the theoretical stoichiometric amount of water required to completely hydrolyze all alkoxy groups. Therefore, the pre-hydrolysis reaction is partial hydrolysis, meaning that only a portion of the alkoxy groups are converted to silanol groups, while the remaining alkoxy groups remain on silicon atoms. In the presence of an acid catalyst, deionized water hydrolyzes the alkoxy groups in the aminoalkoxysilane, with the general reaction formula: Si-OR + H2O → Si-OH + ROH. The generated silanol groups are the active sites for subsequent hybridization reactions with aluminum-carboxylic acid complexes to form Al-O-Si bonds. The amount of deionized water directly determines the extent of the hydrolysis reaction, thereby controlling the concentration, molecular weight distribution, and stability of the silanol oligomers in the activation solution.

[0033] The molar ratio of deionized water to the total molar amount of all hydrolyzable alkoxy groups in the aminoalkoxysilane is (0.1–0.8):1, achieving partial rather than complete hydrolysis. Partial hydrolysis avoids excessive cross-linking of the aminoalkoxysilane, preventing the formation of a three-dimensional network or precipitation, thus obtaining a stable activated solution containing an appropriate amount of silanol oligomers. Secondly, within this ratio range, the silanol groups generated by hydrolysis undergo moderate condensation reactions, forming oligomers of suitable molecular weight. These oligomers retain sufficient reactivity (silanol groups) while exhibiting good solubility and stability, facilitating subsequent uniform mixing with aluminum-carboxylic acid complexes. If the amount of deionized water is less than 0.1:1, the degree of hydrolysis is too low, resulting in insufficient silanol groups to provide enough active sites for subsequent hybridization. This leads to ineffective anchoring of the aluminum-carboxylic acid complex, increasing the risk of spontaneous hydrolysis and precipitation of trivalent aluminum ions. If the ratio of deionized water to siloxane exceeds 0.8:1, the degree of hydrolysis is too high, leading to excessive condensation between silanol groups. This may result in the formation of insoluble siloxane gels or large aggregates, causing the activation solution to become turbid or lose its fluidity, thus preventing the achievement of uniform molecular-level hybridization with the aluminum-carboxylic acid complex. In some embodiments, the molar ratio of the deionized water to the total molar amount of all hydrolyzable alkoxy groups in the aminoalkoxysilane is 0.25:1.

[0034] In some embodiments, the acid catalyst is concentrated hydrochloric acid or concentrated nitric acid, and the pH value of the pre-hydrolysis reaction system is 4.0 to 6.5.

[0035] An acid catalyst refers to an acidic substance added during the pre-hydrolysis step of the preparation method of aluminum-silicon hybrid precursor sol. The acid catalyst is used to accelerate the hydrolysis reaction of the alkoxy group in aminoalkoxysilanes and simultaneously adjust the pH value of the pre-hydrolysis reaction system. In the embodiments of this application, the acid catalyst is concentrated hydrochloric acid (commercially available reagent, mass fraction range 36%-38%) or concentrated nitric acid (commercially available reagent, mass fraction range 65%-68%).

[0036] The pre-hydrolysis reaction system refers to a mixed reaction system composed of aminoalkoxysilane, deionized water, acid catalyst, and anhydrous ethanol in the pre-hydrolysis step.

[0037] A pH value of 4.0–6.5 indicates that the hydrogen ion concentration in the pre-hydrolysis reaction system is within 10 ppm. -4 mol / L to 10 -6 · 5 Between mol / L, this pH range falls within the weakly acidic range. Within this pH range, the amino group (-NH2) in aminoalkoxysilanes is partially protonated to form -NH3. +This results in the silanol oligomers in the activated solution possessing both positively charged groups (protonated amino groups) and negatively charged groups (partially dissociated silanol groups). This amphoteric characteristic facilitates subsequent electrostatic attraction and coordination with aluminum-carboxylic acid complexes, promoting preferential hybridization reactions.

[0038] In some embodiments, the pre-hydrolysis temperature is 40°C to 70°C, and the pre-hydrolysis reaction time is 0.5h to 3h.

[0039] A pre-hydrolysis temperature of 40℃ to 70℃ provides sufficient activation energy to ensure the hydrolysis reaction completes within a reasonable time. Below 40℃ (e.g., room temperature, approximately 15–30℃), the hydrolysis rate of alkoxy groups in aminoalkoxysilanes is slow; even with an acid catalyst, it takes more than 3 hours to reach the desired silanol conversion. This results in an excessively long pre-hydrolysis reaction time, reducing preparation efficiency, and may also increase the risk of system contamination or side reactions. Above 70℃, the hydrolysis rate is too fast, leading to the rapid generation of a large number of silanol groups. These silanol groups rapidly undergo condensation reactions, easily forming excessively large siloxane aggregates or even insoluble gels. The gelled activation solution cannot achieve molecular-level homogeneous mixing with the aluminum-carboxylic acid complex, leading to the failure of subsequent hybridization reactions (resulting in precipitation or turbidity).

[0040] A pre-hydrolysis reaction time of 0.5 to 3 hours ensures that the pre-hydrolysis reaction reaches the required degree of conversion. Under pre-hydrolysis conditions of 40°C to 70°C and in the presence of an acid catalyst, after 0.5 hours of reaction, a sufficient proportion of alkoxy groups in the aminoalkoxysilane are hydrolyzed into silanol groups. The quantity and activity of the generated silanol oligomers are sufficient to meet the requirements of subsequent hybridization reactions. If the reaction time is less than 0.5 hours, the degree of hydrolysis is insufficient, and the concentration or molecular weight of the silanol oligomers in the activation solution is too low, failing to effectively anchor the aluminum-carboxylic acid complex and increasing the risk of aluminum salt hydrolysis precipitation. When the pre-hydrolysis reaction time exceeds 3 hours, even if the temperature is controlled at 40°C to 70°C, the already generated silanol groups will continue to undergo condensation reactions, leading to a continuous increase in the molecular weight of the oligomers, which may eventually form gels or precipitates. Therefore, the upper limit of 3 hours ensures that the silanol oligomers maintain a suitable molecular weight and good solubility, and that the activation solution remains clear, transparent, and has good flowability.

[0041] In some embodiments, the inorganic aluminum salt is aluminum nitrate nonahydrate or aluminum chloride hexahydrate.

[0042] Inorganic aluminum salts refer to those used in the preparation of aluminum-silicon hybrid precursor sols to provide trivalent aluminum ions (A... l3+The method involves dissolving inorganic aluminum salts in anhydrous ethanol with carboxylic acid reagents. The carboxylic acid reagents then form complexes with the trivalent aluminum ions in the inorganic aluminum salts, yielding an aluminum-carboxylic acid complex solution. In the embodiments of this application, the inorganic aluminum salt is aluminum nitrate nonahydrate or aluminum chloride hexahydrate. The chemical formula of aluminum nitrate nonahydrate is Al(NO3)3·9H2O, and the chemical formula of aluminum chloride hexahydrate is AlCl3·6H2O. Compared with organic aluminum alkoxides (such as aluminum isopropoxide and aluminum sec-butoxide), aluminum nitrate nonahydrate and aluminum chloride hexahydrate have market prices that are more than 50% lower, and their supply is stable and their purity is easy to control. Using these two inorganic aluminum salts allows for the low-cost advantage of this method.

[0043] In some embodiments, the carboxylic acid reagent is citric acid or tartaric acid.

[0044] When carboxylic acid reagents and inorganic aluminum salts are mixed and dissolved in anhydrous ethanol, the carboxyl groups (-COOH) in the carboxylic acid reagents react with the trivalent aluminum ions (Al) in the inorganic aluminum salts. 3+ A coordination reaction occurs, forming a stable aluminum-carboxylic acid complex. In the preparation method of aluminum-silicon hybrid precursor sol, the carboxylic acid reagent is citric acid or tartaric acid. The chemical formula of citric acid is C6H8O7, and the chemical formula of tartaric acid is C4H6O6. After inorganic aluminum salts (aluminum nitrate nonahydrate or aluminum chloride hexahydrate) are dissolved in anhydrous ethanol, trivalent aluminum ions tend to undergo solvation with water or alcohol, but free trivalent aluminum ions readily hydrolyze to form aluminum hydroxide precipitate. When citric acid or tartaric acid is present, the carboxyl oxygen atom in the carboxylic acid reagent acts as a coordinating atom, forming a coordinate bond with the trivalent aluminum ion to generate a stable aluminum-carboxylic acid complex. This complex encapsulates the trivalent aluminum ion inside the ligand, significantly reducing the concentration of free trivalent aluminum ions, thereby thermodynamically and kinetically inhibiting the hydrolysis precipitation reaction.

[0045] In some embodiments, the inorganic aluminum salt is in the form of Al 3+ The molar ratio of the total molar amount to the molar amount of the carboxylic acid reagent is 1:(0.5 to 1.5).

[0046] When the molar amount of carboxylic acid reagent is less than Al 3+ When the molar amount of the carboxylic acid reagent is 0.5 times that of Al (i.e., the ratio is less than 1:0.5), the amount of carboxylic acid reagent is insufficient to form a complex with most of the trivalent aluminum ions. A large number of free or partially hydrated trivalent aluminum ions exist in the system. These free trivalent aluminum ions react rapidly with water molecules when added to the activation solution to form aluminum hydroxide precipitate, leading to turbidity, unevenness, or even complete failure of the sol. When the molar amount of the carboxylic acid reagent is higher than that of Al... 3+When the molar amount is 1.5 times (i.e., the ratio is higher than 1:1.5), the excess carboxylic acid reagent will result in a large number of free carboxylic acid molecules or carboxylate ions in the system. These free carboxylic acids will compete with the aluminum-carboxylic acid complex for trivalent aluminum ions, making it difficult for the silanol oligomer to replace the carboxylic acid ligand in subsequent steps, thereby hindering the formation of Al-O-Si bonds.

[0047] In some embodiments, the inorganic aluminum salt is in the form of Al 3+ The molar ratio of the total molar amount to the molar amount of the aminoalkoxysilane is 1:(0.08~0.25).

[0048] In the preparation method of aluminum-silicon hybrid precursor sol, aminoalkoxysilane is converted into an activated solution containing silanol oligomers through a pre-hydrolysis reaction in step S1. The silanol oligomers are the active sites for subsequent hybridization reactions with aluminum-carboxylic acid complexes to form Al-O-Si bonds. When inorganic aluminum salts are in the form of A... l3+ When the molar ratio of the inorganic aluminum salt to the aminoalkoxysilane is 1:0.08–0.25, the total amount of silanol oligomers in the activation solution is sufficient to undergo hybridization reactions with most of the trivalent aluminum ions, ensuring that the aluminum element is uniformly dispersed in the silicon-oxygen network in the form of Al-O-Si bonds. In some embodiments, the inorganic aluminum salt is in the form of Al... 3+ The molar ratio of the total molar amount to the molar amount of the aminoalkoxysilane is 1:0.12.

[0049] In some embodiments, after adding the aluminum-carboxylic acid complex solution dropwise to the activation solution to obtain the hybrid reaction solution, the method further includes: subjecting the hybrid reaction solution to a hybrid reaction at a temperature of 50°C to 75°C for 2 to 6 hours.

[0050] The hybridization reaction refers to the process of adding an aluminum-carboxylic acid complex solution dropwise to an activation solution and then holding the resulting hybrid reaction solution at a specific temperature. During the hybridization reaction, trivalent aluminum ions in the aluminum-carboxylic acid complex undergo a condensation reaction with the silanol groups in the silanol oligomers to form Al-O-Si bonds. Simultaneously, the carboxylic acid ligands are gradually replaced and released by the silanol groups. The hybridization reaction is a crucial step in the formation of the aluminum-silicon hybrid network. In the embodiments of this application, after adding the aluminum-carboxylic acid complex solution dropwise to the activation solution to obtain the hybrid reaction solution, the hybrid reaction solution is subjected to a hybridization reaction at a temperature of 50°C to 75°C for 2 to 6 hours. This effectively promotes the formation of Al-O-Si bonds between the trivalent aluminum ions in the aluminum-carboxylic acid complex and the silanol groups in the silanol oligomers, ensuring that the hybridization reaction proceeds fully without causing precipitation or excessive cross-linking.

[0051] In some embodiments, the aging temperature is 15°C to 30°C, and the aging time is 12h to 48h.

[0052] Static aging refers to the process of keeping the hybrid reaction solution still at an aging temperature without stirring, heating, or other active intervention, allowing the system to undergo slow physicochemical changes under natural conditions. In the preparation method of the aluminum-silicon hybrid precursor sol, the hybrid reaction solution obtained in step S3 has formed a preliminary aluminum-silicon hybrid network through heating and hybridization. However, at this point, the aluminum-silicon hybrid network in the hybrid reaction solution is not yet complete, and unreacted active groups (such as residual silanol groups, incompletely replaced carboxylic acid coordination sites, free alcohol and water molecules, etc.) still exist in the system. At the aging temperature, slow condensation reactions occur between silanol groups in the hybrid reaction solution, between silanol groups and residual coordination sites of aluminum, and between unreacted aluminum ions and adjacent silanol groups in the aluminum-carboxylic acid complex. Although these reactions are at a low rate (due to the temperature dropping to 15℃~30℃), after aging for more than 12 hours, it is sufficient for the hybrid network to further expand and cross-link, forming a three-dimensional aluminum-silicon hybrid framework. In this embodiment, aging the hybrid reaction solution at a temperature of 15°C to 30°C for 12 to 48 hours allows the aluminum-silicon hybrid network to further develop and mature, forming a homogeneous and stable sol with rheological properties. 15°C to 30°C is a typical room temperature range, which can be achieved without additional heating or cooling equipment.

[0053] Secondly, embodiments of this application provide an aluminum-silicon hybrid precursor sol, which is prepared by the preparation method described in the first aspect.

[0054] Aluminum-silicon hybrid precursor sol is a precursor material used to prepare alumina aerogel. It contains an aluminum-silicon hybrid network precursor and can be stored stably for more than 30 days under closed conditions at room temperature (15℃~30℃).

[0055] The aluminum-silicon hybrid precursor sol is prepared based on the above-described method for preparing the aluminum-silicon hybrid precursor sol. The specific steps of the preparation method can be referred to the above embodiments. Since the aluminum-silicon hybrid precursor sol adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0056] Thirdly, embodiments of this application provide an alumina aerogel, which is prepared from the aluminum-silicon hybrid precursor sol described in the second aspect. After heat treatment at 1300°C for 2 hours, the alumina aerogel exhibits a linear shrinkage rate ≤10% and a specific surface area ≥90 m². 2 / g.

[0057] Alumina aerogel refers to a porous nanomaterial prepared from an aluminum-silicon hybrid precursor sol through gelation, aging, solvent exchange, and supercritical drying processes. Alumina aerogel is primarily composed of alumina, containing a small amount of silicon uniformly hybridized through Al-O-Si bonds.

[0058] The 1300℃ heat treatment for 2 hours refers to placing alumina aerogel in a high-temperature furnace, heating it to 1300℃ at a certain heating rate (e.g., 5℃ / min), holding it at 1300℃ for 2 hours, and then cooling it to room temperature with the furnace. This heat treatment condition is used to simulate the service conditions of alumina aerogel under extreme high-temperature environments and is an accelerated testing method for evaluating the high-temperature structural stability of alumina aerogel.

[0059] Linear shrinkage rate refers to the percentage change in size of alumina aerogel in a certain direction after heat treatment at 1300℃ for 2 hours, compared to the original size in that direction before heat treatment. The formula for calculating linear shrinkage rate is: Linear shrinkage rate = (Dimension before heat treatment - Dimension after heat treatment) / Dimension before heat treatment × 100%. Linear shrinkage rate is an important indicator for evaluating the high-temperature sintering resistance of alumina aerogel. The smaller the linear shrinkage rate, the stronger the aerogel's ability to maintain its original shape and pore structure at high temperatures. In the embodiments of this application, the linear shrinkage rate is ≤10%.

[0060] Specific surface area refers to the surface area per unit mass of alumina aerogel, expressed in square meters per gram (m²). 2 / g). Specific surface area was determined by nitrogen adsorption BET method, and the sample needed to be degassed at 200℃ for 4 hours before testing. Specific surface area is a core parameter for measuring the richness of the pore structure and nanoscale uniformity of alumina aerogel. The larger the specific surface area, the higher the porosity and the finer the skeleton of the aerogel, and the better the thermal insulation performance. In the examples of this application, the specific surface area is ≥90 m². 2 / g.

[0061] The alumina aerogel obtained in this application is particularly suitable for extreme high-temperature environments such as aerospace thermal protection, industrial kiln insulation, and high-temperature catalyst carriers.

[0062] The present application is further illustrated below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national / industry standards; if there is no corresponding national / industry standard, they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0063] Example 1 Weigh 1.33 g of 3-aminopropyltriethoxysilane (APTES), 0.081 g of deionized water, and 0.017 g of 68% concentrated nitric acid. Add them to a three-necked flask containing 5.53 g of anhydrous ethanol and carry out a pre-hydrolysis reaction for 1 hour under stirring in a water bath at 50°C to obtain an activated solution containing silanol oligomers (denoted as solution A).

[0064] Weigh out 18.76 g of aluminum nitrate nonahydrate and 10.51 g of citric acid monohydrate, add them to 34.55 g of anhydrous ethanol and dissolve at room temperature to obtain a transparent aluminum-carboxylic acid complex solution (denoted as solution B).

[0065] Under vigorous stirring, solution B was slowly added dropwise to solution A at a rate of approximately 1 mL / min. After the addition was complete, the system was heated to 60°C and the reaction was continued under reflux with stirring for 4 hours to obtain a hybrid reaction solution. The hybrid reaction solution was cooled to room temperature (25°C) and allowed to stand for 24 hours, followed by dilution with anhydrous ethanol to obtain an aluminum-silicon hybrid precursor sol. This sol showed no change after 40 days of storage at room temperature under sealed conditions.

[0066] Gel formation, aging, solvent exchange, and supercritical drying: The above-mentioned aluminum-silicon hybrid precursor sol was injected into a mold and allowed to stand in a constant temperature oven at 40°C for 12 hours to form a wet gel. The wet gel was immersed in 200 mL of anhydrous ethanol and aged at 40°C for 24 hours (with the anhydrous ethanol replaced every 12 hours). After aging, the wet gel was transferred to an autoclave and solvent exchanged with liquid carbon dioxide (purity ≥99.9%) at 10°C and 6.0 MPa, with fresh liquid carbon dioxide replaced every 2 hours for a total of 6 exchanges. Then, the temperature was raised to 45°C and the pressure increased to 12.0 MPa, and maintained for 2 hours for supercritical drying. The pressure was then slowly released to atmospheric pressure at a rate of 0.5 MPa / min. Finally, the alumina aerogel was purged with high-purity nitrogen at 45°C for 30 minutes to obtain the alumina aerogel block.

[0067] Performance Testing: The alumina aerogel block was placed in a muffle furnace and heated to 1300℃ at a programmed rate of 5℃ / min, held for 2 hours, and then cooled with the furnace to obtain the heat-treated sample. The linear shrinkage rate of the heat-treated sample was tested to be 7.2%, and the specific surface area was 118 m² / g. X-ray diffraction patterns showed only weak transition phase alumina diffraction peaks, with no obvious α-Al₂O₃ crystallization characteristic peaks.

[0068] Example 2 This embodiment is basically the same as Embodiment 1, except that: the amount of deionized water used is 0.041 g; the temperature of the pre-hydrolysis reaction is 55°C; and the time of the pre-hydrolysis reaction is 1.5 hours. Everything else is the same as in Embodiment 1.

[0069] The obtained aluminum-silicon hybrid precursor sol showed no change after 40 days of storage at room temperature under sealed conditions. After heat treatment at 1300℃ for 2 hours, the alumina aerogel exhibited a linear shrinkage rate of 8.1% and a specific surface area of ​​105 m² / g.

[0070] Example 3 This embodiment is basically the same as Embodiment 1, except that: Weigh 1.66 g of 3-aminopropyltriethoxysilane (APTES), 0.101 g of deionized water, and 0.021 g of 68% concentrated nitric acid. Add these to a three-necked flask containing 6.91 g of anhydrous ethanol. Perform a pre-hydrolysis reaction in a water bath at 50°C with stirring for 1 hour to obtain an activated solution containing silanol oligomers. The rest is the same as in Example 1.

[0071] The obtained aluminum-silicon hybrid precursor sol showed no change after 40 days of storage at room temperature under sealed conditions. After heat treatment at 1300℃ for 2 hours, the alumina aerogel exhibited a linear shrinkage rate of 6.9% and a specific surface area of ​​122 m² / g.

[0072] Example 4 This embodiment is basically the same as Embodiment 1, except that: Weigh 12.07 g of aluminum chloride hexahydrate and 10.51 g of citric acid monohydrate, add them to 34.55 g of anhydrous ethanol and dissolve at room temperature to obtain a transparent aluminum-carboxylic acid complex solution. The rest is the same as in Example 1.

[0073] The obtained aluminum-silicon hybrid precursor sol showed no change after 40 days of storage at room temperature under sealed conditions. After heat treatment at 1300℃ for 2 hours, the alumina aerogel exhibited a linear shrinkage rate of 7.8% and a specific surface area of ​​110 m² / g.

[0074] Example 5 This embodiment is basically the same as Embodiment 1, except that: Weigh 18.76 g of aluminum nitrate nonahydrate and 7.51 g of tartaric acid, add them to 34.55 g of anhydrous ethanol and dissolve at room temperature to obtain a transparent aluminum-carboxylic acid complex solution. The rest is the same as in Example 1.

[0075] The obtained aluminum-silicon hybrid precursor sol showed no change after 40 days of storage at room temperature under sealed conditions. After heat treatment at 1300℃ for 2 hours, the alumina aerogel exhibited a linear shrinkage rate of 8.5% and a specific surface area of ​​98 m² / g.

[0076] Example 6 This embodiment is basically the same as Embodiment 1, except that: Weigh 1.59 g of 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane (DAMO), 0.081 g of deionized water, and 0.017 g of 68% concentrated nitric acid. Add these to a three-necked flask containing 5.53 g of anhydrous ethanol and carry out a pre-hydrolysis reaction for 1 hour with stirring in a water bath at 50°C to obtain an activated solution containing silanol oligomers. The rest is the same as in Example 1.

[0077] The obtained aluminum-silicon hybrid precursor sol showed no change after 40 days of storage at room temperature under sealed conditions. After heat treatment at 1300℃ for 2 hours, the alumina aerogel exhibited a linear shrinkage rate of 7.5% and a specific surface area of ​​115 m² / g.

[0078] Comparative Example 1 20.42 g of aluminum isopropoxide was mixed with 100 mL of anhydrous isopropanol (approximately 79 g) and heated to 80 °C under reflux for 1 hour to dissolve. 2.08 g of tetraethyl orthosilicate was mixed with 5 g of ethanol, 0.5 g of water, and 0.05 g of concentrated hydrochloric acid (37% by mass) and pre-hydrolyzed at room temperature for 30 minutes. The pre-hydrolyzed solution was added dropwise to the aluminum isopropoxide solution and refluxed at 80 °C for another 2 hours. After cooling, the solution was aged for 24 hours to obtain a sol. After undergoing the same gelation, aging, solvent exchange, supercritical drying, and heat treatment at 1300 °C as in Example 1, the linear shrinkage rate of the sample was measured to be 15.8%, the specific surface area was 75 m² / g, and the X-ray diffraction pattern showed obvious α-Al₂O₃ crystallization peaks.

[0079] Comparative Example 2 All the raw materials in equal amounts from Example 1—1.33 g APTES, 18.76 g aluminum nitrate nonahydrate, 10.51 g citric acid monohydrate, 0.081 g deionized water, 0.017 g concentrated nitric acid, and 40.08 g anhydrous ethanol—were mixed together in a flask at once and stirred at room temperature. After about 5 minutes, a large amount of white flocculent precipitate was produced. After stirring for another 30 minutes, the precipitation became irreversible, and a homogeneous sol could not be formed, making subsequent processes impossible.

[0080] Comparative Example 3 The difference between this comparative example and Example 1 is as follows: Weigh 0.44 g of 3-aminopropyltriethoxysilane (APTES), 0.027 g of deionized water, and 0.006 g of 68% concentrated nitric acid. Add these to a three-necked flask containing 1.84 g of anhydrous ethanol. Perform a pre-hydrolysis reaction in a water bath at 50°C with stirring for 1 hour to obtain an activated solution containing silanol oligomers. The rest is the same as in Example 1.

[0081] The resulting aerogel had a linear shrinkage rate of 18.5% and a specific surface area of ​​65 square meters per gram.

[0082] Comparative Example 4 The difference between this comparative example and Example 1 is as follows: Weigh 3.32 g of 3-aminopropyltriethoxysilane (APTES), 0.203 g of deionized water, and 0.042 g of 68% concentrated nitric acid. Add these to a three-necked flask containing 13.82 g of anhydrous ethanol. Perform a pre-hydrolysis reaction in a water bath at 50°C with stirring for 1 hour to obtain an activated solution containing silanol oligomers. The rest is the same as in Example 1.

[0083] The resulting sol had extremely high viscosity, the gelation time was shortened to 2 hours, the linear shrinkage rate of the aerogel was 25.3%, and the specific surface area was 40 square meters / gram.

[0084] Comparative Example 5 Equivalent amounts of methyltriethoxysilane (MTES) were used instead of APTES. 1.07 g of MTES was weighed, and 0.36 g of glacial acetic acid, 0.081 g of deionized water, and 0.017 g of concentrated nitric acid were added to a three-necked flask containing 5.53 g of anhydrous ethanol. The rest of the process was the same as in Example 1. The resulting sol became turbid on day 5 and produced flocculent precipitate on day 7; the aerogel exhibited a linear shrinkage rate of 14.2%, a specific surface area of ​​82 m² / g, and localized sintering and agglomeration.

[0085] Furthermore, one or more technical solutions in the embodiments of this application have at least the following technical effects or advantages: The aluminum-silicon hybrid precursor sol prepared in this application embodiment can be stably stored at room temperature under sealed conditions for more than 30 days without gelation or precipitation. This stability far exceeds that of ordinary commercial sols (which typically have a stability period of less than 7 days), and also provides great convenience and process window for industrial-scale storage, transportation and subsequent processing.

[0086] In this application, the embodiments use inexpensive inorganic aluminum salts to replace organic aluminum alcohol salts such as aluminum isopropoxide, thereby reducing the overall raw material cost.

[0087] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this 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 this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed in this application.

Claims

1. A method for preparing an aluminum-silicon hybrid precursor sol, characterized in that, The method includes: After mixing aminoalkoxysilane, deionized water and acid catalyst in anhydrous ethanol, a pre-hydrolysis reaction was carried out at a pre-hydrolysis temperature to obtain an activated solution containing silanol oligomers. Inorganic aluminum salts and carboxylic acid reagents are mixed and dissolved in anhydrous ethanol, so that the carboxylic acid reagent reacts with the Al in the inorganic aluminum salt. 3+ A complex is formed, yielding an aluminum-carboxylic acid complex solution; Under continuous stirring, the aluminum-carboxylic acid complex solution was added dropwise to the activation solution to obtain a hybrid reaction solution; The hybrid reaction solution is allowed to stand and age at an aging temperature to form a homogeneous and stable sol with rheological properties, thus obtaining an aluminum-silicon hybrid precursor sol.

2. The method according to claim 1, characterized in that, The molar ratio of the deionized water to the total molar amount of all hydrolyzable alkoxy groups in the aminoalkoxysilane is (0.1–0.8):1; the inorganic aluminum salt is in the form of Al 3+ The molar ratio of the inorganic aluminum salt to the carboxylic acid reagent is 1:(0.5-1.5); the inorganic aluminum salt is in the form of Al 3+ The molar ratio of the total molar amount to the molar amount of the aminoalkoxysilane is 1:(0.08~0.25).

3. The method according to claim 2, characterized in that, The molar ratio of the deionized water to the total molar amount of all hydrolyzable alkoxy groups in the aminoalkoxysilane is 0.25:1; the inorganic aluminum salt is in the form of Al 3+ The molar ratio of the total molar amount to the molar amount of the aminoalkoxysilane is 1:0.

12.

4. The method according to claim 1, characterized in that, The pre-hydrolysis temperature is 40℃~70℃, and the pre-hydrolysis reaction time is 0.5h~3h.

5. The method according to claim 1, characterized in that, After adding the aluminum-carboxylic acid complex solution dropwise to the activation solution to obtain the hybrid reaction solution, the process further includes: subjecting the hybrid reaction solution to a hybrid reaction at a temperature of 50°C to 75°C for 2 to 6 hours.

6. The method according to claim 1, characterized in that, The aging temperature is 15℃~30℃, and the static aging time is 12h~48h.

7. The method according to claim 1, characterized in that, The aminoalkoxysilane is 3-aminopropyltriethoxysilane or 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane; The inorganic aluminum salt is aluminum nitrate nonahydrate or aluminum chloride hexahydrate; The carboxylic acid reagent is citric acid or tartaric acid.

8. The method according to claim 1, characterized in that, The acid catalyst is concentrated hydrochloric acid or concentrated nitric acid, and the pH value of the pre-hydrolysis reaction system is 4.0 to 6.

5.

9. An aluminum-silicon hybrid precursor sol, characterized in that, The aluminum-silicon hybrid precursor sol is prepared by the preparation method according to any one of claims 1 to 8.

10. An alumina aerogel, characterized in that, The alumina aerogel is prepared from the aluminum-silicon hybrid precursor sol as described in claim 9. After heat treatment at 1300°C for 2 hours, the alumina aerogel exhibits a linear shrinkage rate ≤10% and a specific surface area ≥90 m². 2 / g.

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