A "one-pot" synthesis method for hydrophobic silica aerogels from silica precursors

Abstract

This application describes a simple and inexpensive method for preparing hydrophobic silica aerogels using sodium silicate solution as the silica precursor.

Description

Technical Field

[0001] The present invention relates to the synthesis of silica aerogels using a "one-pot" type co-precursor.

Background Art

[0002] Silica aerogels are nanoporous materials with excellent properties such as low density, adjustable transparency, high porosity, large surface area, and low thermal conductivity, and have an open pore structure. These properties are specifically useful in many applications, particularly in the construction and insulation of buildings. However, the structure of silica aerogels deteriorates over time due to the interaction between the OH groups on the Si atoms and the hydrogen bonding of water in a humid environment. As a result, the structure weakens and even breaks.

[0003] In the field of insulation, it is important for silica aerogels to have hydrophobicity. There are three main techniques for producing hydrophobic silica aerogels: methoxylation, silylation, and the co-precursor method (Anderson et al., 2011, Aerogel Book, Advances in sol-gel derived materials and technology, pages 47-77). These methods include the step of substituting hydroxyl groups with Si-R groups that are unstable to hydrolysis to obtain a hydrophobic aerogel. Methoxylation consists of heating a hydrophilic aerogel in the presence of methanol vapor to convert Si-OH groups to Si-OCH3 groups. The main limitations of this technique relate to the problems associated with the required high temperature and the hazards of the operating conditions.

[0004] Silylation involves the modification of the surface of a wet gel before drying using various silylating agents. The wet gel is prepared using standard gel / sol techniques, then solvent-exchanged, and then impregnated with the silylating agent. The disadvantages of this technique include the time required and the resulting consumption of the silylating agent and solvent.

[0005] The method using a co-precursor involves replacing alkoxide precursors of silica, such as tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate (TMOS), and other precursors, etc., with a certain amount of organosilanes, such as methyltrimethoxysilane (MTMS), and trimethylethoxysilane (TMES), etc. This method is easy and requires only minimal preparation time, but the cost of alkoxide precursors and organosilanes has been an obstacle to its industrialization.

[0006] Therefore, it is necessary to implement a method for preparing inexpensive hydrophobic silica aerogel.

Prior Art Documents

Non-Patent Documents

[0007]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] The present invention provides such a method by co-precursor synthesis using an inexpensive silica precursor.

[0009] This method combines multiple advantages, replaces alkoxide precursors, and shortens the time required for synthesis, thereby enabling cost reduction that is substantially compatible with industrial-scale production.

Means for Solving the Problems

[0010] Therefore, according to a first object, the present invention relates to a method for preparing a hydrophobic silica aerogel by a "one-pot" synthesis using an aqueous silica precursor, comprising the following mixture and reagents added simultaneously or sequentially: - ii) The step of mixing an aqueous silica precursor solution containing 4 to 31% by mass of SiO₂ and one or more alcohols and adjusting the pH to 0 to 5 may be pre-treated by passing the aqueous silica precursor solution through an ion exchange resin (step i)). - iii) The step of adding an organosilane. - iv) The step of hydrophobic wet gelation in the presence of a basic solution as a catalyst, and - v) The step of drying to obtain a hydrophobic silica aerogel.

[0011] Preferably, the above steps are carried out in the order of ii)-iii)-iv)-v).

[0012] Here, the expression "one-pot" used indicates that the synthesis steps ii), iii), iv), and optionally step i) are carried out in the same reactor by simultaneous or continuous mixing of various raw materials. According to one embodiment, the method includes a preliminary step i) of passing an aqueous solution of a silica precursor containing 4 to 31% by mass, particularly 4 to 14% by mass, preferably 4 to 8% by mass of SiO₂ through an ion exchange resin.

[0013] According to one embodiment, the aqueous silica precursor is selected from a sodium silicate solution, a silica colloid solution, a silica solution extracted from silica-rich materials (e.g., waste materials of buildings and demolished buildings, waste materials of silica-based insulation materials, glass, and mixtures thereof, etc.). Typically, the precursor solution is a silica solution, such as a sodium silicate solution.

[0014] Typically, the silica precursor solution contains 4 to 8% by mass, typically about 6% by mass of SiO₂ before passing through the ion exchange resin.

[0015] Typically, the silicate solution contains about 6% by mass of SiO₂.

[0016] "Silicic acid" refers to orthosilicic acid of the formula H₄SiO₄.

[0017] As the ion exchange resin, it is a carrier resin of an ionizable functional group that is insoluble in the precursor aqueous solution in particular, and a part of their H + resins having the property of reversibly exchanging with the counter ions of the silicate derived from the precursor solution are mentioned. Therefore, as the ion exchange resin, cation exchange resins, particularly, Amberlite (registered trademark) type resins, for example, Amberlite (registered trademark) IR-120H+ resin and the like are mentioned.

[0018] At the outlet of the column, generally, a silicic acid solution having the same SiO2 concentration as the starting solution is obtained.

[0019] According to one embodiment, the alcohol added to the silicic acid is selected from the group consisting of ethanol, methanol, isopropyl alcohol, and mixtures thereof.

[0020] The amount of alcohol added generally depends on the properties desired for the aerogel, and typically, the alcohol is added in an amount of 10 to 40 mass / volume% with respect to the silicic acid solution.

[0021] Advantageously, the pH adjustment is performed by adding an acid, for example, an inorganic acid. According to one embodiment, the inorganic acid is selected from hydrochloric acid, nitric acid, sulfurous acid, and oxalic acid, and mixtures thereof. Typically, the concentration of the inorganic acid is from 0.1 to 2 mol·l -1 is.

[0022] According to one embodiment, the organosilane is of formula (I):

[0023]

Chemical formula

[0024] (wherein each R1 to R4 group, which may be the same or different, is independently selected from a linear or branched C1 to C12 alkyl group, or a linear or branched C2 to C12 alkenyl group) is selected from the compounds of.

[0025] Specifically, the organosilane is selected from methyltrimethoxysilane, methyltriethoxysilane, vinyltrimethoxysilane, isobutyltriethoxysilane, and isobutyltrimethoxysilane.

[0026] According to the present invention, the alkyl group represents a linear or branched saturated hydrocarbon group having 1 to 12 carbon atoms, preferably 1 to 5 carbon atoms.

[0027] When the alkyl group is linear, particularly methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl, nonyl, decyl, dodecyl, hexadecyl, and octadecyl groups are mentioned. When the alkyl group is branched or substituted with one or more alkyl groups, particularly isopropyl, tert-butyl, 2-ethylhexyl, 2-methylbutyl, 2-methylpentyl, 1-methylpentyl, and 3-methylheptyl groups are mentioned.

[0028] The alkenyl group is a linear or branched hydrocarbon group having C2 to C12, particularly C2 to C6, and contains one or more ethylenic unsaturations. Among the alkenyl groups, particularly allyl or vinyl groups are mentioned.

[0029] According to one embodiment, the wet gel contains 1 to 50% by mass, particularly 1 to 15% by mass, of the organosilane.

[0030] According to a particular embodiment, the synthesis and gelation are typically carried out at controlled temperatures and pressures in the ranges of 15 to 30 °C and 1 to 200 bar, respectively.

[0031] According to one embodiment, the basic aqueous solution is typically an ammonia solution having a concentration in the range of 0.1 to 2 mol·l -1 of.

[0032] Typically, after the final forming step of step iv), the wet gel is aged. Generally, this aging is carried out for a period in the range of 0 to 24 hours.

[0033] After aging, the gel can be washed. Typically, the washing is carried out using an organic solvent such as alcohol, more specifically ethanol.

[0034] Aging and / or washing are generally carried out under controlled temperature and pressure conditions, typically at a temperature in the range of 20 to 50 °C and a pressure in the range of 1 to 200 bar, respectively.

[0035] The wet gel can be prepared in any known form, such as in the form of a monolith, granules, or a composite containing organic or inorganic fibers.

[0036] Next, the gel is dried. The drying step (v) is typically carried out by reacting the reaction mixture obtained by evaporation at atmospheric pressure or by using one or more fluids under supercritical conditions to remove the organic solvent from the gel matrix in a manner that does not cause tension in the pore structure.

[0037] Generally, this drying is carried out by low-temperature supercritical drying (LTSCD) with carbon dioxide.

[0038] According to an exemplary embodiment, the present preparation method includes the following steps in simultaneous or sequential mixing: a) A step of preparing a silicic acid solution by passing a silicate solution through an ion exchange resin. b) A step of mixing the silicic acid solution with an organic alcohol. c) A step of adjusting the pH of the mixture to a range of 0 to 5 by adding an inorganic acid. d) A step of adding an organic silane having a concentration of 1 to 15% in the reaction mixture. e) 0.1 to 2 mol·l -1 A step of adding an ammonia solution having a concentration in the range of. f) A step of aging and washing the obtained gel at a controlled temperature and pressure. g) A step of performing low-temperature supercritical drying (LTSCD) with carbon dioxide. Brief Description of the Drawings

[0039]

Figure 1

Figure 2

BEST MODE FOR CARRYING OUT THE INVENTION

[0040] The following examples are given as examples of the present invention and are not limiting.

EXAMPLE

[0041] Synthesis of silica aerogel Dilute 30 ml of a sodium silicate solution (containing about 27% by mass of SiO2) with 143 ml of deionized water to obtain a sodium silicate solution containing about 6% by mass of SiO2. Next, pass this sodium silicate solution through an ion exchange resin (Amberlite IR-120H+) to remove Na + ions and obtain silicic acid. Mix 130 ml of this silicic acid with 52 ml of ethanol, then add 1 ml of hydrochloric acid (1N). Add 18 ml of a silylating agent (isobutyltriethoxysilane) and mix. After stirring for 1 hour, add 5 ml of an ammonia solution (1N) and carry out gelation for 10 minutes. After aging and washing, dry the silica hydrogel by a low-temperature supercritical drying method (LTSCD) with carbon dioxide.

EXAMPLE

[0042] Properties The hydrophobic silica aerogel obtained in Example 1 is characterized as follows. Bulk density: 100 kg / m 3 Contact angle: 130° Thermal conductivity: 0.0157 W / m·K

[0043] The bulk density is determined by the ratio of its mass to the volume of its geometric envelope.

[0044] The measurement of the contact angle consists of measuring the angle at which a water droplet forms a contact point with the surface of a solid (sample) and the gas phase (here the atmosphere). The apparatus used for the measurement of the contact angle is a Digidrop goniometer.

[0045] A method using a fluxmeter was used to measure the thermal conductivity. Two plates located on both sides of the sample can be heated or cooled, whereby the temperature difference between the heated plate and the cooled plate can be accurately known. The data acquisition system can track the changes in heat flux and temperature to determine the thermal conductivity.

[0046] The microscopic observation was carried out with a scanning electron microscope (Philips, XL30).

[0047] The nanostructure of the obtained hydrophobic silica aerogel is shown in Figure 2.

Claims

1. A method for preparing a hydrophobic silica aerogel by “one-pot” synthesis of hydrophobic silica using an aqueous silica precursor, comprising the following steps: - ii) An aqueous solution of a silica precursor containing 4 to 31% by mass of SiO 2 may be pretreated in advance by passing it through an ion exchange resin in a step (step i)), and mixing an aqueous silica precursor with one or more alcohols and adjusting the pH to 0 to 5. - iii) A step of adding an organosilane to the mixture of step ii), wherein the organosilane has the formula (I): 【Chemical 1】 (In the formula, each of the same or different R1 to R4 groups is independently selected from a linear or branched C1-C12 alkyl group or a linear or branched C2-C12 alkenyl group) A step selected from the compounds of - iv) A step of hydrophobic wet gelation in the presence of a basic solution as a catalyst, and - v) A step of drying to obtain a hydrophobic silica aerogel Including A preparation method, wherein steps (ii) to (iv) are carried out in the same reactor.

2. [[ID=,10]]The preparation method according to claim 1, wherein the aqueous silica precursor is selected from a sodium silicate solution, a silica colloid solution, and a silica solution extracted from a silica-rich material.

3. The aqueous solution of the aqueous silica precursor before the optional step i) contains 4% to 8% by mass of SiO 2 The preparation method according to claim 1 or 2, which contains 2 .

4. The preparation method according to any one of claims 1 to 3, wherein the alcohol is selected from the group consisting of ethanol, methanol, isopropyl alcohol, and mixtures thereof.

5. The preparation method according to any one of claims 1 to 4, wherein the pH adjustment is carried out by adding an acid selected from hydrochloric acid, nitric acid, sulfurous acid, oxalic acid, and mixtures thereof.

6. The concentration of the acid is in the range of 0.1 to 2 mol·l -1 The preparation method according to claim 5, wherein the range is as follows.

7. The preparation method according to any one of claims 1 to 6, wherein the organosilane is selected from methyltrimethoxysilane, methyltriethoxysilane, vinyltrimethoxysilane, isobutyltriethoxysilane, and isobutyltrimethoxysilane.

8. The preparation method according to any one of claims 1 to 7, wherein the amount of the organosilane is 1 to 50% by mass of the hydrophobic wet gel.

9. The preparation method according to any one of claims 1 to 8, wherein steps (iii) and (iv) are carried out at a controlled temperature and pressure in the ranges of 15 to 30 °C and 1 to 200 bar, respectively.

10. The basic solution is an ammonia solution having a concentration in the range of 0.1 to 2 mol·l -1 The preparation method according to any one of claims 1 to 9, wherein the basic solution is an ammonia solution having a concentration in the range of 0.1 to 2 mol·l

11. The preparation method according to any one of claims 1 to 10, wherein aging and / or washing is carried out before contacting the wet gel with a fluid in a supercritical stage.

12. The preparation method according to claim 11, wherein the washing is carried out with ethanol at a temperature and pressure in the ranges of 20 to 50 °C and 1 to 200 bar, respectively.

13. The preparation method according to any one of claims 1 to 12, wherein the hydrophobic wet gel is prepared in the form of a monolith, granules, or a composite containing organic or inorganic fibers.

14. The preparation method according to any one of claims 1 to 13, wherein the drying is carried out at atmospheric pressure or in one or more fluids under supercritical conditions.

15. The preparation method according to any one of claims 1 to 14, wherein the steps are carried out in the order of (ii)-(iii)-(iv)-(v).

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