Hydrophobic silica aerogel and method for its preparation

Abstract

The application relates to the technical field of chemical material preparation, in particular to a hydrophobic silica aerogel and a preparation method thereof. The preparation method comprises the following steps: preparing a silicate aqueous solution, performing ion exchange on the silicate aqueous solution by an ion exchange resin to make the solution pH reach 6.0-7.5, and obtaining a silica sol; mixing the silica sol with tert-butyl alcohol to obtain a mixed sol, and aging to obtain a gel; mixing tert-butyl alcohol and a modifier to obtain a modified liquid; mixing the gel and the modified liquid to perform a modification reaction, and obtaining a modified gel; and performing freeze drying on the modified gel to obtain the hydrophobic silica aerogel. The preparation method uses inexpensive silicate as raw material, and a hydrophobic silica aerogel is prepared by a freeze drying method. The preparation method directly modifies the gel under aqueous conditions, does not need to perform solvent replacement, significantly shortens the freeze drying time while maintaining the mesoporous structure of the gel, and improves the drying efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of chemical material preparation, and particularly relates to a hydrophobic silica aerogel and a preparation method thereof. BACKGROUND

[0002] Silica aerogel is a highly porous mesoporous material with ultra-low density, high porosity, high specific surface area and low thermal conductivity. Due to these excellent properties, silica aerogel has great application potential in various fields such as thermal insulation, catalysis and adsorption. At present, the industrial production of silica aerogel is usually prepared by supercritical drying with tetraethyl orthosilicate (TEOS) and tetramethyl orthosilicate (TMOS) as raw materials. The high cost of raw materials, huge equipment investment and dangerous and complex manufacturing process limit its wide industrial application.

[0003] In order to reduce the cost, in recent years, a preparation method with cheap sodium silicate as a precursor has been developed. However, in order to promote the hydrolysis gelation process, a catalyst is often introduced. The additional impurities and the sodium ions themselves will affect the performance of the product. Another way to reduce the production cost is to change the drying method, such as atmospheric drying, freeze drying, etc. Atmospheric drying has been widely concerned in recent years due to its simple operation and low cost. However, in order to maintain the mesoporous framework structure of the gel, a large amount of solvent replacement and surface methylation modification of the gel are needed to reduce the shrinkage effect of capillary force in the drying process, and a large amount of organic solvent is still needed in this process.

[0004] Currently, the hydrophobic silica aerogel with inorganic silica source as raw material is mostly prepared by normal pressure drying or supercritical drying after modification by silane coupling agent, or by making hydrophilic silica through supercritical drying and then making it hydrophobic through chemical vapor modification. In the article published in Journal of Non-Crystalline Solids, 481 (2018) 267-275, Zhi Li et al. synthesized alcohol gel through two-step acid-base catalysis, sol-gel process, and then prepared hydrophobic silica aerogel through aging, solvent exchange, surface modification and normal pressure drying. The patent with publication number CN111484021A discloses a preparation method for preparing hydrophobic product by modifying hydrophilic silica aerogel prepared by supercritical drying of carbon dioxide with sodium silicate as raw material. Both of the above methods have the disadvantages of solvent replacement, long cycle, and use of large amount of organic solvents. In comparison, the freeze-drying process is simple and can be carried out in aqueous conditions. Its principle is to convert the pore liquid in the gel into solid, and then remove the solid by sublimation, avoiding the damage of surface tension to the pore structure in the gas-liquid phase change process. However, due to the high expansion coefficient of pure water in nanoscale space, the ice will grow and expand during the freezing stage, which will destroy the skeleton structure of the gel and cause the collapse of the mesoporous structure of the gel. Therefore, only microporous silica xerogel can be obtained by freeze-drying. SUMMARY

[0005] The present application aims to at least partially solve one of the technical problems in the related art. To this end, the present application provides a hydrophobic silica aerogel and a preparation method thereof. The preparation method uses inexpensive silicate as raw material and prepares a silica aerogel with excellent hydrophobicity by freeze-drying method. The preparation method directly modifies the gel in aqueous conditions without solvent replacement, significantly shortens the freeze-drying time while maintaining the mesoporous structure of the gel, and improves the drying efficiency. The preparation method has low raw material cost, simple and environmentally friendly process, short preparation cycle, and is conducive to industrial scale production.

[0006] To this end, the first aspect of the present application provides a preparation method of a hydrophobic silica aerogel, comprising the following steps:

[0007] Preparation of a silicate aqueous solution, ion exchange of the silicate aqueous solution by ion exchange resin to make the solution pH 6.0-7.5 to obtain a silica sol;

[0008] Mixing the silica sol with tert-butyl alcohol to obtain a mixed sol, and aging to obtain a gel;

[0009] Mixing tert-butyl alcohol aqueous solution and modifier to obtain a modification liquid;

[0010] Mixing the gel and the modification liquid to carry out modification reaction to obtain a modified gel;

[0011] Freeze-drying the modified gel to obtain the hydrophobic silica aerogel.

[0012] The preparation method provided by the application uses a cheap silicate as a raw material to prepare a silica aerogel with excellent hydrophobic performance by a freeze-drying method.

[0013] According to an embodiment of the application, the content of the silicate in the silicate aqueous solution is 0.45 mol / L-0.66 mol / L.

[0014] According to an embodiment of the application, the silicate in the silicate aqueous solution includes at least one of sodium silicate, potassium silicate and their hydrates.

[0015] According to an embodiment of the application, the ion exchange resin includes at least one of a strong acid cation exchange resin and a weak acid cation exchange resin.

[0016] According to an embodiment of the application, the content of the tert-butyl alcohol in the mixed sol is 10wt%-20wt%.

[0017] According to an embodiment of the application, the aging time is 24h-72h.

[0018] According to an embodiment of the application, the preparation method further includes: standing and aging the mixed sol to obtain the gel.

[0019] According to an embodiment of the application, the standing time is 2h-4h.

[0020] According to an embodiment of the application, the content of the tert-butyl alcohol in the tert-butyl alcohol aqueous solution is the same as the content of the tert-butyl alcohol in the mixed sol.

[0021] According to an embodiment of the application, the volume ratio of the modifier to the tert-butyl alcohol aqueous solution is (1.5-4.0):100.

[0022] According to an embodiment of the application, the modifier is hexamethyldisilazane.

[0023] According to an embodiment of the application, the gel and the modification liquid are mixed at a volume ratio of 1:1.

[0024] According to an embodiment of the application, the temperature of the modification reaction is 40℃-60℃.

[0025] According to an embodiment of the application, the time of the modification reaction is 2h-8h.

[0026] According to an embodiment of the present application, the freezing temperature and / or the drying temperature in the freeze-drying is -60℃ to -20℃.

[0027] According to an embodiment of the present application, the freezing time in the freeze-drying is 2h to 6h.

[0028] According to an embodiment of the present application, the drying time in the freeze-drying is 20h to 40h.

[0029] According to an embodiment of the present application, the drying process in the freeze-drying is carried out under vacuum condition, and the vacuum degree is < 5Pa.

[0030] The second aspect of the present application provides a hydrophobic silica aerogel prepared by the preparation method according to the first aspect.

[0031] The hydrophobic silica aerogel prepared by the preparation method provided by the present application has the advantages of high hydrophobicity, low apparent density, high porosity, high specific surface area, high pore volume, etc.

[0032] According to an embodiment of the present application, the contact angle of the hydrophobic silica aerogel with water is > 140°.

[0033] According to an embodiment of the present application, the apparent density of the hydrophobic silica aerogel is 0.04g / cm 3 ~ 0.05g / cm 3 .

[0034] According to an embodiment of the present application, the porosity of the hydrophobic silica aerogel is 97% to 98%.

[0035] According to an embodiment of the present application, the specific surface area of the hydrophobic silica aerogel is 600m 2 / g to 900m 2 / g.

[0036] According to an embodiment of the present application, the pore volume of the hydrophobic silica aerogel is 2.5cm 3 / g to 5.5cm 3 / g.

[0037] According to an embodiment of the present application, the average pore size of the hydrophobic silica aerogel is 10nm to 25nm.

[0038] According to an embodiment of the present application, the pore size distribution range of the hydrophobic silica aerogel is 1nm to 70nm.

[0039] The beneficial effects of the present application relative to the prior art are:

[0040] (1) tert-butyl alcohol as anti-solvent promotes sol-gel in the mixed sol, without additional catalyst;

[0041] (2) By controlling the proportion of tert-butyl alcohol in the mixed sol, the mesopore size distribution of the aerogel can be adjusted, without the need for a large amount of solvent replacement of the gel as in the prior art in order to maintain the mesoporous framework structure of the gel, thereby saving a large amount of organic solvent, reducing cost and simplifying the preparation process;

[0042] (3) The modification of the silica gel in the prior art is usually carried out in a non-polar solvent system, for example, the gel is modified after mixing the modifier with n-hexane, and then subsequent drying steps are carried out, and there are few reports of direct modification in an aqueous solution. The non-polar solvent system such as n-hexane needs to replace the liquid in the pore of the gel with a non-polar solvent before modification, which takes a long replacement time and a large amount of solvent. Moreover, the non-polar solvent such as n-hexane needs a very low freezing temperature as a freeze-drying medium, which causes a large energy consumption of freeze-drying. In order to improve the freeze-drying effect, the n-hexane needs to be replaced with a polar solvent after modification, which consumes time and solvent. The preparation method provided by the present application directly mixes and reacts the aged gel with a modification liquid in an aqueous solvent system, which can complete the hydrophobic modification without changing the concentration of the freezing solvent in the subsequent freeze-drying process, without the need for solvent replacement. In addition, by controlling the volume content of the modifier in the modification liquid, the modification reaction does not destroy the mesoporous structure of the gel, and the modification process significantly improves the drying efficiency of freeze-drying and shortens the drying period;

[0043] (4) The hydrophobic silica aerogel provided by the present application has excellent performance, high hydrophobicity, low apparent density, high porosity, high specific surface area, high pore volume, and uniform pore size distribution;

[0044] (5) The preparation method provided by the present application has simple production process, short cycle, easy operation, less consumption of organic solvent and less waste liquid in the production process, and is suitable for industrial production and application.

[0045] Additional aspects and advantages of the present application will be in part apparent and in part pointed out hereinafter in the description. BRIEF DESCRIPTION OF DRAWINGS

[0046] The above and / or additional aspects and advantages of the present application will become apparent and be readily appreciated from the description of the embodiments, taken in conjunction with the following drawings in which:

[0047] Figure 1 A physical diagram of the hydrophobic silica aerogel prepared in Example 1 of the present application is shown;

[0048] Figure 2A transmission electron microscope image of the hydrophobic silica aerogel prepared in Example 1 of the present application is shown.

[0049] Figure 3 A hydrophobic angle test image of the hydrophobic silica aerogel prepared in Example 1 of the present application is shown. DETAILED DESCRIPTION

[0050] Embodiments of the present application are described in detail below. The embodiments described below are examples for explaining the present application and are not to be understood as limiting the present application.

[0051] It should be noted that the terms "first", "second" are used only for descriptive purposes and should not be construed as indicating or implying relative importance or a specific order of the technical features indicated. Thus, features defined with "first", "second" can include one or more of the features implicitly or explicitly. Further, in the description of the present application, the meaning of "a plurality of" is two or more unless otherwise specified.

[0052] The endpoints of the ranges and any values disclosed herein are not limited to the precise values stated. The ranges or values should be construed to be approximations that allow for significant variation. Within the ranges or values, endpoints are not required to be included.

[0053] In order that the present application can be more readily understood, certain technical and scientific terms are specifically defined below. Unless specifically defined herein, all other technical and scientific terms used in this document concur in meaning with the general use of those terms by those skilled in the art to which the present application pertains.

[0054] In this document, the terms "comprising" or "including" are open-ended terms, i.e. the inclusion of the recited elements, but not the exclusion of other elements.

[0055] In this document, the terms "optionally", "optional" or "option" generally mean that the subsequently described event or circumstance can or can not occur, and that the description includes instances where the event or circumstance occurs and instances where it does not.

[0056] According to an embodiment of the present application, the present application provides a method for preparing a hydrophobic silica aerogel, comprising the following steps:

[0057] (1) preparing an aqueous silicate solution, and ion exchanging the aqueous silicate solution through an ion exchange resin to a solution pH of 6.0-7.5 to obtain a silica sol.

[0058] Specifically, the cations (e.g. sodium ions) in the silicate are removed by ion exchange resin.

[0059] According to a specific embodiment of the present application, the content of the silicate in the aqueous silicate solution is 0.45 mol / L to 0.66 mol / L, and as some specific examples, the content of the silicate in the aqueous silicate solution can be 0.45 mol / L, 0.5 mol / L, 0.55 mol / L, 0.6 mol / L, 0.66 mol / L, etc.

[0060] According to a specific embodiment of the present application, the type of the silicate in the aqueous silicate solution is not particularly limited, and as some specific examples, the silicate in the aqueous silicate solution includes at least one of sodium silicate, potassium silicate and their hydrates. Specifically, the type of the hydrate is not particularly limited, and for example, it can be sodium metasilicate nonahydrate. The aqueous silicate solution can also be a solution obtained by leaching rice husk ash with sodium hydroxide or potassium hydroxide solution, so as to realize resource utilization.

[0061] According to a specific embodiment of the present application, the type of the ion exchange resin is not particularly limited, and as some specific examples, the ion exchange resin includes at least one of strong acid cation exchange resin and weak acid cation exchange resin.

[0062] (2) mixing the silica sol with tert-butyl alcohol to obtain a mixed sol, and aging to obtain a gel.

[0063] The tert-butyl alcohol as an anti-solvent can promote the gelation process of the mixed sol, without the need for additional addition of a catalyst.

[0064] Specifically, the judging method of the gel in the present application is that the gel is not flowing when it is inclined at 45°.

[0065] According to a specific embodiment of the present application, the content of the tert-butyl alcohol in the mixed sol is 10 wt% to 20 wt%, and as some specific examples, the content of the tert-butyl alcohol in the mixed sol can be 10 wt%, 15 wt%, 20 wt%, etc.

[0066] By controlling the proportion of the tert-butyl alcohol in the mixed sol, the mesopore size distribution of the aerogel can be adjusted, without the need for a large amount of solvent replacement of the gel as in the prior art in order to maintain the mesoporous framework structure of the gel, thereby saving a large amount of organic solvent, reducing the cost and simplifying the preparation process.

[0067] According to a specific embodiment of the present application, the aging time is 24 h to 72 h, and as some specific examples, the aging time can be 24 h, 48 h, 72 h, etc.

[0068] According to a specific embodiment of the present application, the preparation method further comprises: standing and aging the mixed sol to obtain the gel.

[0069] According to a specific embodiment of the present application, the standing time is 2h-4h, and as some specific examples, the standing time can be 2h, 3h, 4h, etc.

[0070] (3) mixing the aqueous tert-butyl alcohol solution and the modifier to obtain a modified liquid.

[0071] According to a specific embodiment of the present application, the content of tert-butyl alcohol in the aqueous tert-butyl alcohol solution is the same as the content of tert-butyl alcohol in the mixed sol. Controlling the content of tert-butyl alcohol in both to be the same can keep the content of tert-butyl alcohol in the gel unchanged, and subsequent solvent replacement is not needed.

[0072] According to a specific embodiment of the present application, the volume ratio of the modifier to the aqueous tert-butyl alcohol solution is (1.5-4.0):100, and as some specific examples, the volume ratio of the modifier to the aqueous tert-butyl alcohol solution can be 1.5:100, 2.0:100, 2.5:100, 3.0:100, 3.5:100, 4.0:100, etc.

[0073] According to a specific embodiment of the present application, the type of the modifier is not particularly limited, and as some specific examples, the modifier is hexamethyldisilazane.

[0074] (4) mixing the gel and the modified liquid to perform a modification reaction to obtain a modified gel.

[0075] The preparation method provided by the present application directly mixes and reacts the aged gel with the modified liquid under aqueous conditions, which can complete the hydrophobic modification while not changing the concentration of the freezing solvent in the subsequent freeze-drying process, and solvent replacement is not needed. In addition, by controlling the volume content of the modifier in the modified liquid, the modification reaction does not destroy the mesoporous structure of the gel, and the modification process significantly improves the drying efficiency of freeze-drying and shortens the drying period.

[0076] According to a specific embodiment of the present application, the gel and the modified liquid are mixed at a volume ratio of 1:1.

[0077] According to a specific embodiment of the present application, the temperature of the modification reaction is 40℃-60℃, and as some specific examples, the temperature of the modification reaction can be 40℃, 50℃, 60℃, etc.

[0078] According to a specific embodiment of the present application, the time of the modification reaction is 2h-8h, and as some specific examples, the time of the modification reaction can be 2h, 4h, 6h, 8h, etc.

[0079] (5) freeze-drying the modified gel to obtain the hydrophobic silica aerogel.

[0080] According to a specific embodiment of the present application, the freezing temperature and / or the drying temperature in the freeze-drying is -60℃ to -20℃, and as some specific examples, the freezing temperature and / or the drying temperature in the freeze-drying can be -60℃, -50℃, -40℃, -30℃, -20℃, etc.

[0081] According to a specific embodiment of the present application, the freezing time in the freeze-drying is 2h to 6h, and as some specific examples, the freezing time in the freeze-drying can be 2h, 4h, 6h, etc.

[0082] According to a specific embodiment of the present application, the drying time in the freeze-drying is 20h to 40h, and as some specific examples, the drying time in the freeze-drying can be 20h, 30h, 40h, etc.

[0083] According to a specific embodiment of the present application, the drying process in the freeze-drying is carried out under vacuum condition, and the vacuum degree is < 5Pa.

[0084] According to a specific embodiment of the present application, the preparation method further comprises: filtering and washing the modified gel, and freeze-drying to obtain the hydrophobic silica aerogel.

[0085] According to a specific embodiment of the present application, the washing agent for the washing is a tert-butyl alcohol aqueous solution, and the concentration thereof is the same as the concentration of the tert-butyl alcohol in the mixed sol.

[0086] According to an embodiment of the present application, the second aspect of the present application provides a hydrophobic silica aerogel obtained by the preparation method according to the first aspect.

[0087] The hydrophobic silica aerogel can be prepared by the preparation method provided by the present application, and has the advantages of high hydrophobicity, low apparent density, high porosity, high specific surface area, high pore volume, etc.

[0088] According to a specific embodiment of the present application, the contact angle of the hydrophobic silica aerogel with water is > 140°, and as some specific examples, the contact angle of the hydrophobic silica aerogel with water can be 141°, 142°, 143°, 144°, etc.

[0089] According to a specific embodiment of the present application, the apparent density of the hydrophobic silica aerogel is 0.04g / cm 3 ~ 0.05g / cm 3 , and as some specific examples, the apparent density of the hydrophobic silica aerogel can be 0.04g / cm 30.045 g / cm 3 0.05 g / cm 3 , etc.

[0090] According to a specific embodiment of the present application, the hydrophobic silica aerogel has a porosity of 97% to 98%, and as some specific examples, the hydrophobic silica aerogel can have a porosity of 97%, 97.5%, 98%, etc.

[0091] According to a specific embodiment of the present application, the hydrophobic silica aerogel has a specific surface area of 600 m 2 / g to 900 m 2 / g, and as some specific examples, the hydrophobic silica aerogel can have a specific surface area of 600 m 2 / g, 700 m 2 / g, 800 m 2 / g, 900 m 2 / g, etc.

[0092] According to a specific embodiment of the present application, the hydrophobic silica aerogel has a pore volume of 2.5 cm 3 / g to 5.5 cm 3 / g, and as some specific examples, the hydrophobic silica aerogel can have a pore volume of 2.5 cm 3 / g, 3.5 cm 3 / g, 4.5 cm 3 / g, 5.5 cm 3 / g, etc.

[0093] According to a specific embodiment of the present application, the hydrophobic silica aerogel has an average pore diameter of 10 nm to 25 nm, and as some specific examples, the hydrophobic silica aerogel can have an average pore diameter of 10 nm, 15 nm, 20 nm, 25 nm, etc.

[0094] According to a specific embodiment of the present application, the hydrophobic silica aerogel has a pore size distribution range of 1 nm to 70 nm, and as some specific examples, the hydrophobic silica aerogel can have a pore size distribution range of 1 nm, 5 nm, 10 nm, 20 nm, 50 nm, 70 nm, etc.

[0095] The scheme of the present application will be explained below in connection with examples. Those skilled in the art will understand that the following examples are only for illustrating the present application, and should not be considered as limiting the scope of the present application. If a specific technique or condition is not mentioned in the examples, the technique or condition described in the literature in the art or according to the product manual is used. If the manufacturer of a reagent or instrument is not mentioned, it is a conventional product that can be obtained commercially.

[0096] Example 1

[0097] The embodiment provides a hydrophobic silica aerogel and a preparation method thereof, and the preparation method comprises the following steps:

[0098] Na2SiO3·9H2O is weighed and added into deionized water, a magnetic stirrer is added to uniformly stir, and a 0.64 mol / L sodium silicate solution is prepared;

[0099] H-type weak-acid cation exchange resin is intermittently added into the sodium silicate solution under stirring, the pH change of the solution is detected by using a pH meter, the pH of the solution is adjusted to about 7, and the silica sol after ion exchange is separated by vacuum filtration through a Buchner funnel;

[0100] 100 mL of the silica sol is taken into a beaker, tert-butyl alcohol is added, the content of the tert-butyl alcohol is 18 wt%, uniform stirring is conducted, sealing is performed, and standing is conducted at room temperature for 2 h, then sealed aging is continuously conducted at room temperature for 24 h, and a gel is obtained;

[0101] An 18 wt% tert-butyl alcohol aqueous solution is prepared, hexamethyldisilazane (HMDS) is added, and the volume ratio of the HMDS to the tert-butyl alcohol aqueous solution is 1.5:100, uniform mixing is conducted, and a modified liquid is obtained;

[0102] 100 mL of the modified liquid and 100 mL of the gel are mixed, modification reaction is conducted at 50 DEG C for 6 h after sealing, a modified gel is obtained, and the modified gel is washed with 18 wt% tert-butyl alcohol aqueous solution three times to remove residual modified liquid;

[0103] The modified gel is frozen in a freeze dryer (cold trap temperature -60 DEG C to -20 DEG C) for 2 h, dried at -40 DEG C under vacuum (vacuum degree < 5 Pa) for 20 h, and a light blue hydrophobic silica aerogel is obtained, a physical object diagram of which is shown in Figure 1 , and a transmission electron microscope (TEM) diagram is shown in Figure 2 .

[0104] The contact angle of the hydrophobic silica aerogel with water is 143 DEG (a hydrophobic angle test diagram is shown in Figure 3 ), the apparent density is 0.045 g / cm 3 , the porosity is 97.9%, the specific surface area is 873 m 2 / g, the pore volume is 5.3 cm 3 / g, the average pore size is 18.0 nm, and the pore size distribution range is 1 nm to 70 nm.

[0105] Embodiment 2

[0106] The embodiment provides a hydrophobic silica aerogel and a preparation method thereof, and the preparation method comprises the following steps:

[0107] Weigh Na2SiO3·9H2O into deionized water, add a magnetic stirrer to stir uniformly, and prepare a 0.45 mol / L sodium silicate solution;

[0108] Add H-type weakly acidic cation exchange resin to the sodium silicate solution intermittently while stirring, detect the pH change of the solution with a pH meter, adjust the pH of the solution to about 6, and separate the ion-exchanged silica sol by vacuum filtration with a Buchner funnel;

[0109] Take 100 mL of the silica sol in a beaker, add t-butanol, and make the content of t-butanol 10 wt%, stir uniformly, seal, and stand still at room temperature for 2 h, then continue to seal and age at room temperature for 24 h to obtain a gel;

[0110] Prepare a 10 wt% t-butanol aqueous solution, add hexamethyldisilazane (HMDS), and make the volume ratio of HMDS to the t-butanol aqueous solution 3:100, mix uniformly to obtain a modification liquid;

[0111] Mix 100 mL of the modification liquid and 100 mL of the gel, seal, and modify at 40℃ for 8 h to obtain a modified gel, and wash with 10 wt% t-butanol aqueous solution for three times to remove residual modification liquid;

[0112] Freeze the modified gel in a freeze dryer (cold trap temperature -60℃ to -20℃) for 2 h, and dry at -40℃ under vacuum (vacuum degree < 5 Pa) for 20 h to obtain a hydrophobic silica aerogel.

[0113] The contact angle of the hydrophobic silica aerogel with water is 141°, the apparent density is 0.042 g / cm 3 , the porosity is 98.0%, the specific surface area is 624 m 2 / g, the pore volume is 2.53 cm 3 / g, the average pore size is 11.4 nm, and the pore size distribution range is 1 nm to 70 nm.

[0114] Example 3

[0115] The present embodiment provides a hydrophobic silica aerogel and a preparation method thereof, the preparation method comprising the following steps:

[0116] Weigh Na2SiO3·9H2O into deionized water, add a magnetic stirrer to stir uniformly, and prepare a 0.45 mol / L sodium silicate solution;

[0117] Add H-type weakly acidic cation exchange resin to the sodium silicate solution intermittently while stirring, detect the pH change of the solution with a pH meter, adjust the pH of the solution to about 6, and separate the ion-exchanged silica sol by vacuum filtration with a Buchner funnel;

[0118] Take 100 mL of silica sol in a beaker, add t-butyl alcohol, and make the content of t-butyl alcohol 20 wt%, stir uniformly, seal, stand for 2 h at room temperature, then continue to seal aging at room temperature for 24 h, to obtain a gel;

[0119] Prepare a 20 wt% t-butyl alcohol aqueous solution, add hexamethyldisilazane (HMDS), and make the volume ratio of HMDS to the t-butyl alcohol aqueous solution 4:100, mix uniformly, to obtain a modified liquid;

[0120] Mix 100 mL of the modified liquid and 100 mL of the gel, after sealing, modify at 60℃ for 2 h, to obtain a modified gel, wash with 20 wt% t-butyl alcohol aqueous solution for three times, to remove residual modified liquid;

[0121] Freeze the modified gel in a freeze dryer (cold trap temperature -60℃ to -20℃) for 2 h, and dry at -40℃ under vacuum (vacuum degree < 5 Pa) for 20 h, to obtain a hydrophobic silica aerogel.

[0122] The contact angle of the hydrophobic silica aerogel with water is 154°, the apparent density is 0.049 g / cm 3 , the porosity is 97.7%, the specific surface area is 708 m 2 / g, the pore volume is 2.94 cm 3 / g, the average pore size is 13.1 nm, and the pore size distribution range is 1 nm to 70 nm.

[0123] Comparative Example 1

[0124] This comparative example provides a silica aerogel and a preparation method thereof, the preparation method comprising the following steps:

[0125] Weigh Na2SiO3·9H2O and add it to deionized water, add a magnetic stirrer to stir uniformly, and prepare a 0.64 mol / L sodium silicate solution;

[0126] Add H-type weakly acidic cation exchange resin to the sodium silicate solution intermittently while stirring, detect the pH change of the solution with a pH meter, adjust the pH of the solution to about 7, and separate the ion-exchanged silica sol by vacuum filtration with a Buchner funnel;

[0127] Take 100 mL of silica sol in a beaker, add t-butyl alcohol, and make the content of t-butyl alcohol 18 wt%, stir uniformly, seal, stand for 2 h at room temperature, then continue to seal aging at room temperature for 24 h, to obtain a gel;

[0128] Freeze the gel in a freeze dryer (cold trap temperature -60℃ to -20℃) for 2 h, and dry at -40℃ under vacuum (vacuum degree < 5 Pa) for 40 h, to obtain a silica aerogel.

[0129] The silica aerogel has a contact angle with water of 8° and an apparent density of 0.049 g / cm 3 , a porosity of 97.7%, a specific surface area of 672 m 2 / g, a pore volume of 2.9 cm 3 / g, and an average pore diameter of 15.9 nm, with a pore diameter distribution ranging from 1 nm to 70 nm. Since the gel surface was not methylated in this comparative example, the freeze-drying time required for this comparative example was 40 h, which is twice the freeze-drying time of Example 1.

[0130] Comparative Example 2

[0131] This comparative example provides a silica aerogel and a method for preparing the same, the method comprising the following steps:

[0132] Na2SiO3·9H2O was weighed into deionized water, and a magnetic stirrer was added to stir uniformly to prepare a 0.64 mol / L sodium silicate solution;

[0133] H-type weakly acidic cation exchange resin was added intermittently into the sodium silicate solution while stirring, and the pH change of the solution was detected by a pH meter to adjust the pH of the solution to about 7. The ion-exchanged silica sol was separated by vacuum filtration using a Buchner funnel;

[0134] 100 mL of the silica sol was taken in a beaker, and t-butanol was added to make the content of t-butanol 18 wt%, and stirred uniformly, sealed, and left to stand at room temperature for 2 h, and then continued to age at room temperature for 24 h to obtain a gel;

[0135] The 100 mL of the gel was subjected to solvent replacement with t-butanol and n-hexane respectively for one day, so that the pore liquid in the gel was replaced with n-hexane;

[0136] Hexamethyldisilazane was mixed uniformly with n-hexane at a volume ratio of 2.5:100 to obtain a modification liquid;

[0137] 100 mL of the modification liquid and 100 mL of the gel after solvent replacement were mixed, and after being sealed, the modification reaction was carried out at 50°C for 6 h to obtain a modified gel. The modified gel was subjected to solvent replacement with t-butanol and 18 wt% t-butanol aqueous solution respectively for one day, so that the pore liquid in the modified gel was replaced with 18 wt% t-butanol aqueous solution again. The modified gel was frozen in a freeze dryer (cold trap temperature -60°C to -20°C) for 2 h, and dried under vacuum (vacuum degree <5 Pa) at -40°C for 20 h to obtain a silica aerogel.

[0138] The silica aerogel has a contact angle with water of 127° and an apparent density of 0.048 g / cm 3It has a porosity of 97.8% and a specific surface area of ​​685 m². 2 / g, pore volume 6.9cm 3 / g, with an average pore size of 22.2nm and a pore size distribution range of 1nm to 70nm. Since the gel surface in this comparative example underwent methylation modification, the required freeze-drying time was still 20h. However, this process requires a large amount of organic reagents for prolonged solvent replacement.

[0139] Comparative Example 3

[0140] The only difference between this comparative example and Example 1 is that the volume ratio of HMDS to tert-butanol aqueous solution in the modified solution is 1:100.

[0141] The silica aerogel prepared in this comparative example had a water contact angle of 14.8° and an apparent density of 0.045 g / cm³. 3 It has a porosity of 97.9% and a specific surface area of ​​807 m². 2 / g, pore volume 5.35cm 3 / g, with an average pore size of 24.9nm and a pore size distribution range of 1nm to 70nm. The prepared sample still exhibits aerogel characteristics. However, due to the low HMDS concentration leading to insufficient modification, the aerogel's hydrophobic angle is <90°, and it still exhibits hydrophilicity. The freeze-drying time was 40h.

[0142] Comparative Example 4

[0143] The only difference between this comparative example and Example 1 is that the volume ratio of HMDS to tert-butanol aqueous solution in the modified solution is 5:100.

[0144] The silica aerogel prepared in this comparative example had a contact angle of 151° with water and an apparent density of 0.163 g / cm³. 3 It has a porosity of 87.0% and a specific surface area of ​​272 m². 2 / g, pore volume 0.92cm 3 The aerogel has an average pore size of 5.3 nm and a pore size distribution range of 1 nm to 10 nm. Although the modified aerogel exhibits hydrophobicity with a hydrophobic angle >140° and a freeze-drying time of 20 h, the excessive HMDS results in an apparent density >0.1 g / cm³. 3 Specific surface area <300m² 2 / g, pore volume <1cm 3 / g, the average pore size and porosity are also reduced, which will significantly reduce the performance of the aerogel.

[0145] In the description of the specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are contained in at least one embodiment or example of the present application. In the specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any suitable manner in any one or more embodiments or examples. In addition, the person skilled in the art can combine and combine the different embodiments or examples described in the specification and the features of the different embodiments or examples without contradiction.

[0146] Although the embodiments of the present application have been shown and described above, it is understood that the above-described embodiments are exemplary and are not to be construed as limiting the present application, and the person skilled in the art can make changes, modifications, replacements and variations to the above-described embodiments within the scope of the present application.

Claims

1. A method for preparing a hydrophobic silica aerogel, characterized in that, Includes the following steps: An aqueous silicate solution is prepared, and the solution is subjected to ion exchange with an ion exchange resin until the pH of the solution is 6.0-7.5 to obtain a silica sol. The silica sol was mixed with tert-butanol to obtain a mixed sol, which was then aged to obtain a gel. A modified solution is obtained by mixing an aqueous solution of tert-butanol with a modifier. The gel and the modifying liquid are mixed to carry out a modification reaction, thereby obtaining a modified gel; The modified gel was freeze-dried to obtain the hydrophobic silica aerogel; The content of tert-butanol in the aqueous tert-butanol solution is the same as the content of tert-butanol in the mixed sol; The volume ratio of the modifier to the tert-butanol aqueous solution is (1.5~4.0):100; The modifier is hexamethyldisilazane.

2. The preparation method according to claim 1, characterized in that, The silicate content in the silicate aqueous solution is 0.45 mol / L to 0.66 mol / L; And / or, the silicate in the aqueous silicate solution includes at least one of sodium silicate, potassium silicate, and their hydrates; And / or, the ion exchange resin includes at least one of a strong acid cation exchange resin and a weak acid cation exchange resin.

3. The preparation method according to claim 1, characterized in that, The content of tert-butanol in the mixed sol is 10wt%~20wt%.

4. The preparation method according to claim 1, characterized in that, The aging time is 24 h to 72 h; And / or, the preparation method further includes: allowing the mixed sol to stand and age to obtain the gel, wherein the standing time is 2 h to 4 h.

5. The preparation method according to claim 1, characterized in that, The gel and the modified liquid are mixed at a volume ratio of 1:1; And / or, the temperature of the modification reaction is 40℃~60℃; And / or, the modification reaction takes 2 h to 8 h.

6. The preparation method according to claim 1, characterized in that, The freezing temperature and / or drying temperature in the freeze-drying process is -60℃ to -20℃. And / or, the freezing time in the freeze-drying process is 2 h to 6 h; And / or, the drying time in the freeze-drying process is 20 h to 40 h; And / or, the freeze-drying process is carried out under vacuum conditions, with a vacuum degree < 5 Pa.

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