Method for preparing silica aerogel and aerogel prepared thereby

Abstract

The present invention relates to an ambient pressure method for the synthesis of silica aerogels, and more particularly to an ambient pressure method for the synthesis of silica aerogels. In an aspect, the present invention relates to a method for preparing controlled shaped fiber reinforced silica aerogel composites.

Description

[Technical field]

[0001] The present disclosure relates to a method for preparing an aerogel, and the aerogel prepared thereby. More particularly, the present disclosure relates to a method for preparing a silica aerogel using rapid ambient pressure drying. [Background technology]

[0002] Aerogels are porous materials with high specific surface areas and have a variety of commercial applications in diverse sectors such as construction, insulating, catalysis, and drug delivery. In particular, aerogels can be used as aggregates in cement applications to provide lightweight and insulating properties.

[0003] Aerogels are typically produced by a sol-gel process, in which a three-dimensional "wet-gel" scaffold is obtained before solvent exchange and / or drying to yield a porous aerogel structure.

[0004] The main obstacle to large-scale commercialization of aerogel is the drying process.

[0005] Freeze-drying processes are known for the production of aerogels (Klvana et al., "A New Method of Preparation of Aerogel-like Materials using a Freeze-Drying Process", Journal De Physique (1989): 50; C429-432). However, freeze-drying methods rely on sublimation of the freeze-drying solvent under vacuum and are energy intensive. Supercritical drying is another known technique that uses high pressure to reach the supercritical point of the drying solvent and requires high pressure and high temperature (Anderson et al., "Hydrophobic silica aerogels prepared via rapid supercritical extraction", Journal of Sol-Gel Science and Technology (2010); 53; 199-207). The high energy requirements of these processes limit their commercial viability.

[0006] Thus, ambient pressure drying (APD) methods provide a low energy consumption route to reach aerogels. Conventional APD methods rely on exchanging the original solvent used in wet gel preparation with an organic solvent with lower surface tension, such as hexane, heptane, or octane. This method also often includes additional surface modification to replace the -OH groups on the silica sulfate with more lipophilic groups to facilitate drying. Trimethylchlorosilane (TMCS) has been used for surface modification of silica gel, but this results in the generation of HCl, which typically must be removed. WO2016 / 132117 describes that the reaction of HCl generated during surface modification with TMCS with carbonate or bicarbonate ions in the silica wet gel generates CO2 gas from the inside, inside the pores of the wet gel. This allows for more rapid drying of the wet gel with ambient pressure drying. However, improvements are still needed in terms of economy, efficiency, and the morphology of the aerogels produced.

[0007] It is an object of the present invention to obviate or mitigate one or more of the disadvantages associated with the prior art. A scalable method of preparing silica aerogel would be advantageous. A method that is less energy consuming would be useful, which could be a method that can reduce the use of solvents. A method of preparing silica aerogel with reduced processing / drying times would be particularly advantageous, which could be a cost-effective method for producing the same. A method that can be used to form controlled-shaped aerogels would be particularly useful. Summary of the Invention

[0008] The present invention relates to a method for preparing aerogels, such as silica aerogels, which facilitate rapid drying under ambient pressure conditions due to a chemically driven self-pressurizing reaction that occurs both internally and externally during production. In embodiments, the aerogels can be prepared with fibers to allow for the preparation of controlled shaped products. Advantageously, the aerogels can be rapidly prepared under ambient pressure conditions, resulting in a reduction in time costs and energy consumption, thus facilitating the scale-up of the process.

[0009] Thus, in a first aspect of the present invention, there is provided a method for preparing a silica aerogel, the method comprising providing a precursor solution comprising a solution of a silicate and optionally a carbonate; reacting the precursor solution with a bicarbonate and with a silylating agent, the bicarbonate being in solid form.

[0010] In this process, initial gelation is achieved by reaction of silicate with bicarbonate to give a silica wet gel shell, carbonate and water, and the silylating agent reacts with the silicate to bring about further gelation and modification of the wet gel shell surface to give HCl.

[0011] The silylating agent also reacts with the generated HCl and with carbonates, either formed as by-products or optionally present in the precursor solution, to form CO. In this way, external pressure is generated, i.e., at the outer surface of the wet gel shell, which facilitates rapid drying.

[0012] In addition, the unreacted silylating agent and generated HCl within the pores of the wet-gel diffuse into the wet-gel shell core and react with the bicarbonate to generate CO2, thereby pressurizing it from the inside and further facilitating the drying process.

[0013] The steps of reacting the precursor solution with the bicarbonate and the silylating agent may be carried out sequentially (or sequentially), i.e., the precursor solution may be reacted first with the bicarbonate and then with the silylating agent, or the steps may be carried out simultaneously.

[0014] Accordingly, an embodiment of the present invention relates to a method of preparing a silica aerogel, the method comprising providing a precursor solution comprising a solution of a silicate and optionally a carbonate, reacting the precursor solution with a bicarbonate and with a silylating agent, the bicarbonate being in solid form, thereby preparing a wet gel; and drying the wet gel to form a silica aerogel.

[0015] In some aspects of the invention, the step of reacting the precursor solution with bicarbonate is carried out in the presence of fibers. The fibers may be added to the precursor solution before the reaction occurs, or may be added to the bicarbonate. This is useful when the aerogel needs to have a controlled shape. However, in some aspects, the fibers are not present and the aerogel is produced as an uncontrolled solid. This may then be crushed or ground to form a powder or granules depending on the intended use.

[0016] In one embodiment, the fibers are ceramic fibers, organic fibers, or carbon fibers.

[0017] In one aspect, the fibers are ceramic fibers. Triton™ ceramic chopped fibers are an illustrative example of a fiber that can be used, although one of ordinary skill in the art will appreciate that other fibers may be used.

[0018] Advantageously, when fibers are used, the method allows for the rapid production of controlled shaped reinforced aerogel composites.

[0019] The term composite is used to describe aerogels that have one or more additional components, such as fibers, that may be incorporated into the aerogel structure.

[0020] In one embodiment, the fibers surround the bicarbonate powder. The fibers may form a shape around the bicarbonate powder. For example, the fibers may take the shape of a sphere or ball surrounding the bicarbonate powder. In this embodiment, the sphere may be immersed in the precursor solution. The fibers impart structural integrity to the resulting aerogel, allowing for the preparation of a controllable hollow aerogel composite, in this example, a fiber-reinforced hollow aerogel composite (FRHAC).

[0021] Additionally, in this embodiment, the CO2 generated at the outer surface of the wet-gel shell is retained inside the shaped structure until it is released through the pores of the silica wet-gel. The gas can displace the liquid components of the gel and drying can be achieved at lower temperatures and pressures, making the process energy efficient.

[0022] The present invention thus relates to a method for producing a silica aerogel, the method comprising providing a precursor solution comprising a solution of a silicate and optionally a carbonate, reacting the precursor solution with a bicarbonate and a silylating agent, wherein the bicarbonate is in solid form, and wherein the reacting of the precursor solution with the bicarbonate is carried out in the presence of fibers.

[0023] In one aspect, the present invention relates to a method for making silica aerogel, the method comprising: providing a precursor solution comprising a solution of a silicate and optionally a carbonate; providing fibers surrounding a bicarbonate core to form a shaped structure; and reacting the precursor solution with a bicarbonate and a silylating agent, wherein the bicarbonate is in a solid form.

[0024] In one embodiment, the step of reacting the precursor solution with the bicarbonate involves immersing the shaped structure in the precursor solution, such that the precursor solution permeates through the fibers and reacts with the bicarbonate powder cores.

[0025] The fibers conveniently form a shaped structure that holds the CO2 produced until it diffuses through the pores of the wet gel, and a similar effect can be achieved without fibers by preparing the wet gel on a substrate. When the precursor reacts with the bicarbonate powder, the wet gel is formed as a layer. In this embodiment, the CO2 is held between the substrate and the outer surface of the wet gel layer until it is released through the pores of the wet gel. Suitable substrates are known to those skilled in the art and include, for example, glass, ceramic, and polymeric substrates.

[0026] In one embodiment, the silicate is selected from sodium silicate, potassium silicate, lithium silicate, or potassium silicate.

[0027] In one embodiment, the silicate is sodium silicate.

[0028] Sodium silicate, also known as "water glass," is particularly suitable for use in the present invention.

[0029] In some embodiments, the carbonate salt is selected from sodium carbonate, potassium carbonate, calcium carbonate, magnesium carbonate, iron carbonate, and ammonium carbonate.

[0030] In one embodiment, the carbonate is sodium carbonate.

[0031] In some embodiments, the bicarbonate salt is selected from sodium bicarbonate, potassium bicarbonate, calcium bicarbonate, magnesium bicarbonate, ferrous bicarbonate, and ammonium bicarbonate.

[0032] In one embodiment, the bicarbonate is sodium bicarbonate.

[0033] In some embodiments, the silylating agent has the general formula R3SiX, where R is a C1-C4 alkyl or halide and X is a halide, sulfate, or sulfonate group.

[0034] The halide may be chlorine, bromine or iodine.

[0035] The sulfonate may be a methyl sulfonate, a trifluoromethyl sulfonate, a benzyl sulfonate, or a toluene sulfonate.

[0036] The sulfate may be -OS(O)2-SiR3.

[0037] In some embodiments, the silylating agent is trimethylchlorosilane (TMCS), dimethyldichlorosilane, methyltrichlorosilane, and bis(trimethylsilyl) sulfate.

[0038] In some embodiments, the silylating agent is trimethylchlorosilane (TMCS).

[0039] The methods of the present invention produce a partially dried aerogel, which may be the final product, however, in some embodiments, one or more washing and / or drying steps may be performed to obtain the final aerogel product.

[0040] In certain embodiments, the method includes one or more washing steps.

[0041] In some embodiments, the method includes one or more drying steps. The drying steps may be carried out by conventional means, for example in an oven or a heat plate. Suitable heating means will be apparent to those skilled in the art.

[0042] The drying step involves heating under ambient pressure.

[0043] The heating may be carried out at a temperature of from 60°C to 500°C. In one embodiment, the heating is carried out at a temperature of from 60°C to 150°C.

[0044] A temperature of around 100°C (for example, 80°C to 120°C) may be preferable because the liquid phase has a boiling point of 100°C or lower.

[0045] The drying step may be carried out for 15 minutes to 24 hours. As will be appreciated by those skilled in the art, the length of the drying step will depend on the heating temperature, with lower temperatures requiring longer drying times. In some embodiments, the drying step is carried out for 15 minutes to 12 hours, or 15 minutes to 6 hours. In some embodiments, the drying step is carried out for 20 minutes to 3 hours, or 30 minutes to 2 hours. When the drying temperature is in the range of 60°C to 150°C, the drying time may be 30 minutes to 12 hours, 30 minutes to 8 hours, 30 minutes to 6 hours, or 30 minutes to 2 hours. [Brief description of the drawings]

[0046] [Figure 1] FIG. 1 shows a schematic diagram of a method according to an embodiment of the present invention. [Diagram 2] FIG. 2 shows an X-ray tomography image of a FRHAC prepared according to an embodiment of the present invention. [Diagram 3] FIG. 3 shows the mass change of a fiber-reinforced aerogel composite prepared according to an embodiment of the present invention. [Figure 4] FIG. 4 shows SEM images of (a) and (b) fiber-reinforced aerogel composites and (c) and (d) aerogels prepared according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] The invention will now be described, by way of example only, with reference to the accompanying drawings.

[0048] An embodiment of the present invention will be described with reference to Figure 1 and with reference to the reactions described below. In Figure 1(a), a ball of short ceramic fibers is shaped to surround a core of bicarbonate powder. The dimensions of the composite aerogels prepared can be adjusted by varying the amount of short fibers used.

[0049] As shown in Figure 1(b), gelation occurs through the reaction of sodium silicate with the sodium bicarbonate core, resulting in the formation of a silica gel shell.

[0050] In the figures, sodium carbonate is shown included in the precursor, however, one skilled in the art will recognize that sodium carbonate is formed as a product of the reaction between sodium silicate and sodium bicarbonate and that sodium carbonate is not required in the precursor solution, i.e., it is an optional component.

[0051] Then, a silylating agent is dropped onto the surface of the silica gel (Fig. 1(c)), resulting in surface modification and further gel formation during the formation of HCl. The silylating agent and the generated HCl also react with the carbonate to form CO2. In addition, the unreacted silylating agent and the generated HCl inside the gel pores diffuse to the core (Fig. 1(d)) and react with the solid bicarbonate to generate CO2. The silica gel shell is mechanically reinforced by the short fibers, so that the silica gel shell does not expand during the rapid gas generation process (Fig. 1(e)), and retains the increased pressure caused by gas generation until the CO2 is released through the shell pores.

[0052] The reactions that occur during this process are: JPEG2024533595000002.jpg85151

[0053] At the beginning of the process, a sodium silicate (Na2SiO3) solution is mixed with the short fibers and reacted with a sodium bicarbonate (NaHCO3) powder core via (reaction 1) to form a silica gel shell.

[0054] After addition of a silylating agent, in this embodiment trimethylchlorosilane (TMCS), to the silica gel shell, further silica gel is formed by reaction between TMCS and the remaining sodium silicate (reaction 2), and the surface of the silica gel is modified with TMCS. As a result of the surface modification with TMCS, the FRHAC has hydrophobic properties. The reaction also produces hydrogen chloride (HCl) as a result of (reaction 3). TMCS and the resulting HCl react with sodium carbonate (Na2CO3) to produce carbon dioxide (CO2) gas (reactions 4 and 5).

[0055] The unreacted TMCS and the generated HCl in the pores of the resulting silica gel shell diffuse into the sodium bicarbonate core. CO2 gas is generated, causing a sudden pressure increase on the silica gel shell (reactions 6 and 7). Due to the non-Newtonian behavior of silica gel and the mechanical reinforcement of the silica gel shell by the short fibers, the silica gel shell does not expand during the rapid gas generation stage and retains the sudden pressure increase until the generated CO2 gas is slowly released through the pores of the silica gel shell. Thus, a hollow structure is obtained, as shown in the X-ray tomography image (Figure 2).

[0056] Reaction 7 is the overall reaction between TMCS and sodium bicarbonate solution. Thus, this chemical process not only produces CO2 gas but also consumes water, which provides a great advantage for thermal drying of wet gels. EXAMPLES

[0057] The present invention will now be more fully described with reference to the following illustrative examples.

[0058] Example 1 material and method Sodium carbonate (≧99%), sodium bicarbonate (≧99.7%) and trimethylchlorosilicate (TMCS, ≧97%) were purchased from Sigma-Aldrich™ and used without further purification. Short ceramic fiber Triton™ and sodium silicate (water glass) were purchased from Fisher Scientific™.

[0059] Samples were imaged in high vacuum mode with 10 keV accelerating voltage using an FEI XL30 ESEM-FEG (Environmental Scanning Electron Microscope-Field Emission Gun) at Newcastle University. All samples were coated with gold to increase electrical conductivity prior to SEM imaging. The specific surface area and porosity of the samples were characterized by nitrogen adsorption-desorption method on a Thermo Scientific™ SURFER at Newcastle University. A μCT, Xradia 410 Versa (4 μm isotropic voxel size) at Durham University was used for X-ray microtomography scanning. The software Avizo9 was used to process the resulting tomography datasets.

[0060] 1.1 Fiber-reinforced hollow aerogel composite (FRHAC) To prepare fiber reinforced hollow aerogel composites (FRHAC), first the precursor was prepared by a mixture of water glass, deionized water and sodium carbonate solution (molar ratio Si:H2O:Na2CO3=5:167:1). A shaped ball of 0.03g of short ceramic fibers covering a core of 0.1g of sodium bicarbonate was prepared by shaping the fibers by hand. Then, the shaped ball was immersed in 1ml of precursor, after which 1ml of trimethylchlorosilane was dropped on its surface. After 10min, it was washed with deionized water three times. Finally, the FRHAC sample was dried at 100℃ for 24h.

[0061] 1.2 Non-reinforced aerogel (NRA) Aerogel samples without short ceramic fibers were prepared by adding 1 ml of precursor directly to 1 g of sodium bicarbonate powder without stirring, and then adding 1 ml of TMCS onto the surface. Bubbling was observed on the surface due to the displacement of the liquid while gas was released through the pores of the wet gel. After the bubbling stopped, the gel was washed three times with deionized water and finally dried on a hot plate at 100 °C for 24 h.

[0062] 1.3 Characterization In an attempt to mechanically open the blocked holes and measure the true surface area, the NRA samples were ground into powder. The ground powder was first washed twice with deionized water, dried, and characterized. In addition, the ground powder was washed three times with ethanol to remove any synthesis by-products, and repeatedly characterized.

[0063] 1.3.1 Minimum heat drying The minimum baking time for the FRHAC synthesized in Example 1.1 was determined by monitoring the mass loss from the beginning of drying. Each experiment was repeated three times. The results are shown in Figure 3, which shows that after 30 minutes of baking at 100°C, the mass loss stopped and the gel was completely dry.

[0064] 1.3.2 SEM Various pore sizes ranging from micropores to macropores were observed in the SEM images of FRHAC (Figures 4(a) and (b)). The porous surface of the non-fiber-reinforced aerogel sample (NRA) can be observed in Figures 4(c) and (d).

[0065] The bulk densities of FRHAC and NRA were determined by weight and volume and are shown in Table 1 below. The pores of FRHAC and NRA samples were analyzed by nitrogen adsorption-desorption isotherm, which showed that the specific surface area of ​​FRHAC was about 36 m 2 / g. The specific surface area was small (6 m 2Although short fibers having a specific surface area of ​​39 m / g can provide a smaller specific surface area to the FRHAC than the NRA without short fibers, the NRA specific surface area does not show a significant increase, being 39 m 2 To further study the properties of the aerogel component in the FRHAC, the NRA was ground into a powder, which had a specific surface area of ​​approximately 128 m 2 / g, indicating that macropores were dominant in the original NRA. Finally, the ground NRA powder was additionally washed with water. The specific surface area of ​​the washed NRA powder was more than 10 times higher than that of the original NRA, suggesting that the by-product salts generated from reactions 2, 4, 5, 6, and 7 had reduced the specific surface area of ​​the original NRA as analyzed by nitrogen adsorption. The grinding and washing procedure also reduced the pore volume to 0.55 cm 3 / g to 4.33cm 3 / g (Table 1), indicating that there are unidentified micropores and mesopores in the original FRHAC sample.

[0066] [Table 1]

[0067] Table 1: Bulk density, BET surface area and pore analysis of fiber reinforced hollow aerogel composites (FRHAC), non-reinforced aerogel samples (NRA), and short fibers.

[0068] The results show that the method of the present invention can be used to rapidly prepare aerogels with significantly reduced time and energy consumption. In particular, the results show that aerogels can be prepared and completely dried with drying times on the order of 30 minutes. In embodiments, short fiber reinforcement enables the production of controlled shaped aerogel products with a wide range of potential commercial applications.

[0069] All of the features and / or steps of the disclosed methods or processes disclosed herein (including the accompanying claims, abstract and drawings) may be combined in any combination, except combinations in which at least some of the features and / or steps are mutually exclusive. Each feature disclosed herein (including the accompanying claims, abstract and drawings) may be replaced with another feature serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features. The invention is not limited to the details of the above-described embodiments. The invention extends to any novel one or any novel combination of the features disclosed herein (including the accompanying claims, abstract and drawings), and to any novel one or any novel combination of the steps of the disclosed methods or processes.

[0070] With respect to the use of virtually any plural and / or singular term herein, those of skill in the art may translate from plural to singular and / or from singular to plural as appropriate to the context and / or application. The various singular / plural permutations may be expressly set forth herein for purposes of clarity.

[0071] Those of skill in the art will understand that, in general, the terms used in this specification, and particularly in the appended claims, are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). Furthermore, if a specific number of introduced claim provisions is intended, such intent will be expressly set forth in the claim, and in the absence of such a setting forth, those of skill in the art will understand that no such intent exists. For example, as an aid to understanding, the appended claims may include the use of the introductory phrases "at least one" and "one or more" to introduce claim provisions. However, the use of such phrases should not be interpreted as an intent to limit a particular claim containing such introduced claim provision to embodiments containing only one such provision, even if the same claim contains the introductory phrases "one or more" and "at least one" and an indefinite article such as "a" or "an" (e.g., "a" and / or "an" should be interpreted to mean "at least one" or "one or more"); the same applies to the use of definite articles used to introduce claim provisions. In addition, even if a specific number of introduced claim provisions is explicitly specified, one skilled in the art will recognize that such provisions should be interpreted to mean at least the specified number (e.g., the explicit specification of "two provisions" means, without other qualification, at least two provisions or more than two provisions).

[0072] It will be appreciated that various aspects of the present disclosure are described herein for purposes of illustration, and that various modifications may be made without departing from the scope of the present disclosure, and, accordingly, the various aspects disclosed herein are not intended to limit the true scope thereof, which is set forth by the appended claims.

Claims

1. 1. A method for preparing silica aerogel, comprising: providing a precursor solution comprising a solution of a silicate, and optionally a carbonate; and reacting the precursor solution with bicarbonate; and reacting with a silylating agent; wherein the bicarbonate is in solid form.

2. 10. The method of claim 1, wherein reacting the precursor solution with bicarbonate is carried out in the presence of fibers.

3. The method of claim 2 , wherein the fibers are ceramic fibers, organic fibers, or carbon fibers.

4. The method of claim 3 wherein the fibers are ceramic fibers.

5. 3. The method of claim 2, wherein the method further comprises providing fibers surrounding a core of bicarbonate to form a shaped structure prior to reacting the precursor solution with the bicarbonate.

6. The method of claim 5 , wherein reacting the precursor solution with the bicarbonate comprises immersing the shaped structure in the precursor solution.

7. The method according to any one of claims 1 to 6, wherein the silicate is selected from sodium silicate, potassium silicate, lithium silicate or calcium silicate.

8. 8. The method of claim 7, wherein the silicate is sodium silicate.

9. 7. The method according to any one of claims 1 to 6, wherein the carbonate is selected from sodium carbonate, potassium carbonate, calcium carbonate, magnesium carbonate, iron carbonate, and ammonium carbonate.

10. 10. The method of claim 9, wherein the carbonate is sodium carbonate.

11. 7. The method of any one of claims 1 to 6, wherein the bicarbonate salt is selected from sodium bicarbonate, potassium bicarbonate, calcium bicarbonate, magnesium bicarbonate, iron bicarbonate, and ammonium bicarbonate.

12. 12. The method of claim 11, wherein the bicarbonate is sodium bicarbonate.

13. The silylating agent is a compound of the general formula R ３ SiX (wherein R is C １ ~C ４ The method of any one of claims 1 to 6, wherein X is an alkyl or halide, and X is a halide, sulfate, or sulfonate group.

14. 14. The method of claim 13, wherein the silylating agent is trimethylchlorosilane (TMCS), dimethyldichlorosilane, methyltrichlorosilane, or bis(trimethylsilyl) sulfate.

15. 15. The method of claim 14, wherein the silylating agent is trimethylchlorosilane (TMCS).

16. The method according to any one of claims 1 to 6, further comprising a drying step.

17. 17. The method of claim 16, wherein the drying step comprises heating under ambient pressure conditions.

18. The method of claim 17, wherein the heating is carried out at 60°C to 150°C.

19. The method of claim 16, wherein the drying step is carried out for 15 minutes to 24 hours.

20. The method of claim 1, wherein reacting the precursor solution with bicarbonate and reacting it with the silylating agent are carried out simultaneously.

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