WO2019076977A1

WO2019076977A1 - Method of preparing a silica aerogel

Info

Publication number: WO2019076977A1
Application number: PCT/EP2018/078389
Authority: WO
Inventors: Shanyu ZHAO; Ana STOJANOVIC; Matthias Koebel
Original assignee: Prtc Nv; Empa
Priority date: 2017-10-18
Filing date: 2018-10-17
Publication date: 2019-04-25

Abstract

The invention relates to a method of preparing a silica aerogel starting from a dispersion of an aqueous water glass solution, an organosilicon and a nonpolar organic solvent. The gelation process is initiated by adding an acid to the dispersion. A phase transfer agent is added before, during or after the addition of said acid. Subsequently, the aqueous phase is separated from the organic phase and the organic phase comprising the gel is dried. The invention further relates to an aerogel obtainable by such method.

Description

Method of preparing a silica aerogel Field of the invention

[0001] The present invention relates to a method of preparing a silica aerogel starting from an aqueous water glass solution. The invention further relates to a silica aerogel obtainable by such method.

Background art

[0002] Aerogels such as silica aerogels are highly porous materials having remarkable properties such as high specific surface area and low thermal conductivity. These properties render aerogels a promising candidate for different applications. Aerogels are for example used as thermal insulating material or as catalyst support.

[0003] A common technique used for producing silica aerogels involves the reaction of a silicon alkoxide with water in a solvent in the presence of a catalyst. Commonly used silicon alkoxides comprise for example tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS). Although the formation of gels starting from silicon alkoxides as precursor is straightforward, the method has several drawbacks. Firstly, alkoxides of silicon are hazardous. Secondly, silicon alkoxides are expensive materials making the manufacturing process of aerogels expensive. Thirdly, the process requires supercritical drying of the wet gel. Supercritical drying is a complex and energy intensive process restricting the upscaling and commercialisation of the production process of aerogels.

[0004] An alternative technique to produce silica aerogels starts from an aqueous water glass solution as precursor. This technique is less expensive compared to the alkoxide technique as water glass is less expensive than silicon alkoxide but has the drawback that drying is extremely sensitive to residual water in the pores of the gel. Residual water that is present in the gel before drying may have a dramatic influence on the properties or quality of the aerogel. The presence of water in the gel may for example result in a partial collapse and densification of the gel and may cause a decrease of the fraction of mesopores of the aerogel during drying and therefore impact its thermal conductivity.

Gels produced by the water glass technique furthermore have the drawback that sodium ions present in the hydrogel should be removed from the gel, for example previously at the precursor stage through ion exchange or later on in the process after gelation by means of extraction with salt free wash solvents.

US2012/0225003 describes a method of preparing silica aerogel by preparing a dispersion starting from an aqueous water glass solution, a solvent and an organosilicon and by further adding an inorganic acid to cause simultaneously a gelation and a solvent exchange to extract the Na⁺ ions. In the solvent-exchange step, water contained in the network structure of the silica hydrogel is substituted by the solvent and Na⁺ ions are left behind in the water phase. The solvent-exchange process described in US2012/0225003 is however asking long process times to completely remove the water from the pores of the hydrogel and to avoid partial pore collapse during subsequent drying.

Summary of the invention

[0005] It is an object of the present invention to provide a method of preparing a silica aerogel avoiding the problems known in the art.

It is another object of the present invention to provide a method of preparing a silica aerogel having a reduced production time.

It is a further object of the present invention to provide a method of preparing a silica aerogel having improved properties in terms of mesopore fraction, density, surface area, pore size and thermal conductivity.

Furthermore, it is an object of the present invention to provide a method of preparing a silica aerogel using ambient drying.

[0006] According to a first aspect of the present invention a method of preparing a silica aerogel is provided. The method comprises the steps of

preparing a dispersion comprising an aqueous phase and an organic phase starting from an aqueous water glass solution, an organosilicon and a nonpolar organic solvent;

adding an acid to the dispersion to prepare a gel (either a partially or fully gelled gel);

adding a phase transfer agent before, during or after the addition of the acid to the dispersion and/or to the gel (to the partially or fully gelated gel), the phase transfer agent is miscible both with water and with the nonpolar organic solvent;

separating the organic phase from the aqueous phase, the organic phase comprising the gel, the nonpolar organic solvent, part of the phase transfer agent and possibly further comprising unreacted organosilicon;

- drying said gel to remove the nonpolar organic solvent from the gel's pores, to remove the phase transfer agent from the gel and if the gel comprises unreacted organosilicon or impurities to remove the unreacted organosilicon or impurities from the gel.

The gel (either a partially or fully gelled gel) may have a mechanical appearance of a wobbly to hard.

[0007] By mixing the organosilicon and the aqueous water glass solution in a nonpolar organic solvent the polar groups (-SiOH groups) are replaced with nonpolar organic groups (-S-R groups) resulting in a hydrophobic aerogel. [0008] For the purpose of this invention water glass also called sodium silicate or soluble waterglass refers to a compound comprising sodium oxide (Na₂0) or other alkali oxides and silica or silicon dioxide (Si0₂). Water glass is soluble in water and forms an aqueous water glass solution. The aqueous water glass solution is preferably prepared by adding water (preferably distilled water) to water glass. [0009] The water glass solution used according to the present invention preferably comprises silica in an amount ranging between 0.1 wt% and 20 wt%, more preferably in an amount ranging between 3 wt% and 10 wt% and even more preferably in an amount ranging between 4 wt% and 7 wt%.

The molar ratio Na₂0:Si0₂ ranges preferably between 2: 1 and 1 :5 and more preferably between 1 :2 and 1 :4.

[0010] As organosilicon any organic derivative of a silicone containing at least one covalent silicon-carbon bond can be considered. Specifically, the organosilicon may be a silane-based compound, a siloxane-based compound, a silanol-based compound and a silazane-based compound. Examples of organosilicones comprises any one selected from the group consisting of hexamethyldisilazane (HMDS), hexamethyldisiloxane (HMDSO), trimethylchlorosilane (TMCS), trimethoxymethylsilane (TMMS) , methoxitrimethylsilane (MTMS), dimethylmethoxysilane (DMMS), dimethyldiethoxysilane (DIVIDES), tetraethoxysilane (TEOS), methyltrimethoxysilane (MTM), vinyltrimethoxysilane (VTMS), phenyltrimethoxysilane (PTMS), dimethylchlorosilane (DMCS) and mixtures of two or more thereof.

[0011] The nonpolar organic solvent is preferably an organic solvent immiscible with water selected from the group consisting of hexane (for example n-hexane), heptane (for example n- heptane), toluene, xylene, their fluorinated equivalents (for example fluorohexane or fluoroheptane) and mixtures comprising one or more of thereof.

[0012] In the method according to the present invention a substantial volume of the nonpolar organic solvent is used to prepare the dispersion comprising an aqueous phase and an organic phase as specified in the first step of the method of the present invention.

[0013] Preferably, the volume ratio of the volume of the nonpolar organic solvent and the volume of the organosilicon to prepare the dispersion ranges between 1 : 10 and 25: 1. More preferably, the volume of the nonpolar organic solvent is higher than the volume of the organosilicon. The volume ratio of the volume of the nonpolar organic solvent and the volume of the organosilicon ranges for example between 1 and 25, or between 2 and 20 as for example 3, 4, 5, 6, 8, 10, 12, 15, 18 or 20.

[0014] Preferably, the volume ratio of the volume of the nonpolar organic solvent and the volume of the water glass to prepare the dispersion ranges between 1 : 10 and 25: 1. More preferably, the volume of the nonpolar organic solvent is higher than the volume of the water glass. The volume ratio of the volume of the nonpolar organic solvent and the volume of the water glass ranges for example between 1 and 25, or between 2 and 20 as for example 3, 4, 5, 6, 8, 10, 12, 15, 18 or 20. [0015] In preferred embodiment the volume ratio of the volume of the nonpolar organic solvent and the volume of the organosilicon to prepare the dispersion as well as the volume ratio of the volume of the nonpolar organic solvent and the volume of the water glass to prepare the dispersion ranges between 1 : 10 and 25: 1. More preferably, the volume of the nonpolar organic solvent is higher than the volume of the organosilicon and higher than the volume of the water glass.

[0016] By adding an acid as for example nitric acid to the dispersion the gelation process is initiated and the silylation process is activated. By adding an acid to the dispersion a gel (hydrogel) either partially gelated or fully gelated is obtained. The gelation process is preferably carried out at a temperature ranging between 40 and 100 °C, more preferably at a temperature between 60 and 80°C, as for example at a temperature of 65 °C.

[0017] For the purpose of this invention the term 'hydrogel' refers to a gel (partially or fully gelated) with predominantly water in its pores. The term 'organogel' refers to a gel (partially or fully gelated) with predominantly solvent in its pores.

[0018] The acid comprises preferably a strong acid, for example an acid selected from the group consisting of nitric acid, hydrochloric acid, sulfuric acid, acetic acid, hydrofluoric acid or reactive compounds that release strong acids upon hydrolysis such as trimethylchlorosilane or other chlorosilanes and mixtures of two or more thereof.

[0019] Preferably the dispersion is stirred during or after the addition of the acid, for example with a speed ranging between 100 and 600 rpm, more preferably at a speed ranging between 200 rpm and 500 rpm, as for example between 300 rpm or 500 rpm. In preferred method the dispersion is stirred at a speed ranging between 300 rpm and 600 rpm, for example at a speed of 400 rpm or 500 rpm during the addition of the acid and at a speed ranging between 100 rpm and 300 rpm, for example at a speed ranging between 200 rpm and 300 rpm after the addition of the acid. [0020] By adding the phase transfer agent before, during or after the addition of the acid to the dispersion, the organic phase infiltrates the gel more efficiently and the amount of water present in the gel, i.e. in the pores of the gel, is reduced. By adding the phase transfer agent an organogel is obtained. In preferred embodiments the phase transfer agent is added once the acid is added to the dispersion, i.e. after the addition of the acid to the dispersion.

[0021] For the purpose of this invention "phase transfer agent" is defined as an agent that facilitates the migration of water from the gel (hydrogel) to the aqueous phase.

[0022] The phase transfer agent used according to the present invention is miscible in both the aqueous phase and the organic phase. Preferred phase transfer agents comprise alcohols, in particular propanol for example isopropanol and ethanol. Alternatively, the phase transfer agent may comprise a surfactant. With a surfactant is meant a compound comprising (a) hydrophobic group(s) as well as (a) hydrophilic group, such as a chemical compound or molecule that contains a hydrophobic (nonpolar organic) part covalently connected to a polar hydrophilic one. Surfactants may comprise cationic surfactants such as cetyltrimethylammonium chloride, anionic surfactants, non ionic surfactants or zwitterionic surfactants .

[0023] Preferably, the phase transfer agent has a boiling point between 65 °C and 200 °C, more preferably between 80 °C and 120 °C, for compatibility with the gelation (lower limit) and drying process (upper limit).

[0024] Preferably, the phase transfer agent has a dielectric constant lower than 22, for example lower than 21 or lower than 20. [0025] Preferably, the phase transfer agent comprises a compound that is not present in the dispersion comprising the aqueous phase and the organic phase which are made up of an aqueous water glass solution, an organosilicon and a nonpolar organic solvent. In particular it is preferred that the phase transfer agent is not yet present in the dispersion or in the gel at the moment of its addition to the dispersion and/or to the gel.

[0026] The phase transfer agent can be added before the addition of the acid, such as shortly before the addition of the acid, during the addition of the acid or after the addition of the acid, such as immediately or soon after the addition of the acid. It is also possible that the phase transfer agent is added partially before the addition of the acid and/or partially during the addition of the acid and/or partially after the addition of the acid. In preferred methods the phase transfer agent is added after the addition of the acid.

[0027] Also during or after the addition of the phase transfer agent to the dispersion or the at least partially gelled or fully gelled gel, the dispersion or at least the partially or fully gelated gel is preferably stirred, for example with a speed ranging between 100 and 600 rpm, as for example 300 rpm, 400 rpm or 500 rpm.

[0028] To remove the nonpolar solvent in the pores, the phase transfer agent and the unreacted organosilicon form the gel (organogel), the hydrophobized gel in the organic phase is preferably dried at a temperature ranging between room temperature and 250°C, more preferably at a temperature ranging between 100 and 170°C as for example at a temperature of 150 °C. The gel (organogel) is preferably dried at a pressure of ranging between 0.1 atm and 2 atm, more preferably at a pressure of 1 atm. [0029] The volume ratio of the water glass to the phase transfer agent ranges preferably between 10: 1 and 0.5: 1. In particular, the volume of the water glass is higher than the volume of the phase transfer agent. Preferably, the volume ratio of the volume of the water glass and the volume of the phase transfer agent ranges between 5:1 and 1 :1 , more preferably between 5: 1 and 1.1 :1 , most preferably between 5: 1 and 2: 1 , as for example about 3:1.

[0030] The volume ratio of the volume of the nonpolar organic solvent and the aqueous water glass solution ranges preferably between 3: 1 and 1 :3 and more preferably between 1.5: 1 and 1 : 1.5.

[0031] In preferred methods, the organic phase, in particular the gel, is washed with an acid, preferably an inorganic acid, before the drying of the organic phase. By washing the gel, Na⁺ ions are at least partially extracted from the gel. The organic phase can be washed once or can be washed repeatedly. By such additional washing step the thermal conductivity of the final aerogel may be reduced considerably. The thermal conductivity of the final aerogel can be reduced by 1 to 5 mW m^" K^"1 , for example by 2 to 3 mW m^" K^"1 by such additional washing step.

[0032] Compared to the method described in US2012/0225003, the organic phase infiltrates the gel (the hydrogel) more efficiently by the addition of the phase transfer agent and Na⁺ ions are extracted more efficiently from the organic phase. This efficiency is further improved by the addition of an acidic solution wash step of the organic phase.

[0033] The aerogel obtained by the method according to the present invention has improved characteristics in terms of mesoporous structure, density, surface area, pore size and thermal conductivity compared to aerogels obtained by methods known in the art. The method according to the present invention has furthermore a reduced production time compared to methods known in the art.

[0034] According to a second aspect of the present invention an aerogel obtained by the above described method is provided. The aerogel powder is characterized by a BET surface area (determined by the method of Brunauer, Emmet and Teller) ranging between 450 and 900 m²/g, and more typically between 600 and 900 m²/g, and a BJH pore size (determined by the method of Barrett, Joyner and Halenda) size ranging between 5 and 50 nm, and more typically between 7 and 20 nm. The aerogel powder has a packed bed thermal conductivity (lambda) lower than 20 mW m^" K^"1 for example 19.4 mW m^" K^"1.

[0035] The aerogel obtained by the method of the present invention has a BJH mesopore volume above 1 cm³/g, and more typically above 2 cm³/g. The BJH mesopore volume is the volume of pores with a diameter between 2 and 50 nm, as determined by BJH analysis. Brief description of the drawings

[0036] The present invention will be discussed in more detail below, with reference to the attached drawings, in which:

Fig. 1 depicts a flow chart of a method according to the present invention.

Description of embodiments

[0037] The present invention will be described in more detail and referring to the flowchart of Figure 1. Referring to Figure 1 , an aqueous water glass solution 102, an organosilicon 104 and a nonpolar organic solvent 106 are mixed to form a dispersion 108. By adding an inorganic acid 1 10 the dispersion 108 is at least partially gelling to form an at least partially gelled gel (hydrogel) 1 12. A phase transfer agent 1 14 is added before, during or after the addition of the acid to the dispersion 108 and/or to the at least partially gelated gel (hydrogel) 1 12. In a subsequent step a phase transfer agent 1 14 (for example isopropanol) is added to the at least partially gelated gel 1 12 to obtain a gel with a reduced water content 1 16. By adding the phase transfer agent 1 14 the migration of the water from the gel 1 12 to the aqueous phase is facilitated and an organogel is obtained. Preferably, the obtained gel 1 16 is dried in a subsequent step.

Detailed examples

Example 1 -15

[0038] Examples 1- 15 describe detailed examples of preparing an aerogel using the following starting materials

sodium silicate solution (26.5% w/w Si0₂, molar ratio Na₂0:Si0_{2 :} 1 :3.1 , pH=1 1.5) nitric acid (HN0₃, 65%) obtained from Sigma-Aldrich;

hexamethyldisilazane HMDS (98.5%, AB1 1 1 174) supplied by abcr, Germany;

- heptane (UN 1206) supplied by Brenntag, Switzerland and

isopropanol supplied from Thommen Furler AG, Switzerland

[0039] At room temperature, 8.5 ml water glass is diluted with 41.5 ml H₂0 to prepare an aqueous water glass solution. 6ml HMDS and 60 ml heptane are added to the aqueous water glass solution. After vigorous stirring with a mechanical stirrer for 5 minutes for example at a speed ranging between 200 and 800 rpm at room temperature 4 ml of concentrated nitric acid (65%) is added in a drop-wise manner. The temperature of the solution is set at 65°C under reflux conditions. When the solution reaches the temperature of 65 °C, a phase transfer agent is added. The phase transfer agent and the amount of phase transfer agent that is added is specified in Table 1. The solution is then kept at 65°C under reflux condenser for 30 minutes under constant stirring. Subsequently, the bottom aqueous phase is removed and the gel is washed twice by 50 ml 0.3M HN0₃ solution in 1 hour. After the complete removal of the aqueous phase, the wet gel is dried at 150°C for 40 minutes at 1 atm. Table 1

Example 16

[0040] Example 16 uses the same starting material as examples 1-15. At room temperature, 8.5 ml water glass is diluted with 41.5 ml H₂0 to prepare an aqueous water glass solution. 6ml HMDS and 60 ml heptane are added to the aqueous water glass solution. After vigorous stirring with a magnetic stirrer for 5 minutes for example at a speed ranging between 200 and 300 rpm at room temperature 4 ml of concentrated nitric acid (65%) is added in a drop-wise manner. The temperature of the solution is set at 65°C under reflux conditions. When the solution reaches the temperature of 65 °C, the chamber is opened shortly and a phase transfer agent (15 ml isopropanol) is added. The solution is then kept at 65°C for 1 hour under constant stirring and reflux conditions. After the solution is left for an hour, to settle and to have a clear phase separation, the upper phase being the organic phase, the lower phase being the aqueous phase. Subsequently, the bottom aqueous phase is removed (e.g. by suction with pipette) and the gel in the organic phase is washed twice with 50 ml 0.3M HN0₃ solution for 10 minutes each. After the complete removal of the aqueous phase, the wet gel is dried at 150°C for 2 hours at 1 atm.

[0041] Some of the examples 1-15 and example 16 are characterized in detail. The results are given in Table 2 (BET surface, BJH Pore volume, BJH pore size). The BET surface area, the BJH pore volume and BJH pore size are derived from N₂ adsorption-desorption isotherms.

[0042] The particle size distribution of the samples is measured is measured by a Coulter particle size analyzer in dry condition and in isopropanol suspension. The particle size distribution of the samples ranges between 4 and 200 μιη. [0043] The thermal conductivity was analyzed by using a custom built guarded hot plate (guarded zone : 50 * 50 mm2, measuring zone: 25 * 25 mm2) designed for small samples of low thermal conductivity materials with a 15 °C temperature difference. In order to be consistent with measurements according to European Standards, 10 calibration measurements were carried out using conventional expanded polystyrene samples measured once in a 50 * 50 cm2 calibrated and validated testing equipment. The small guarded hot plate measurement data was then calibrated using these known standards. Table 2

[0044] For sample 16 the packed bed density and the packed bed thermal conductivity were determined. The packed bade density of sample 16 was 0.106 g/cm³ and the packed bed thermal conductivity 19.4mW/(m.K).

[0045] The addition of isopropanol, ethanol or surfactant leads to an increase in surface area and BJH pore volume compared to the absence of isopropanol, ethanol, or surfactant. However, at the highest ethanol and isopropanol concentrations, the mesopores volume decreases again, most likely due to a densification and overall loss of porosity. The gains in mesoporous volume fraction by using appropriate quantities of phase transfer agent will lead to a decrease in gas phase and overall thermal conductivity.

Claims

A method of preparing a silica aerogel, said method comprising the steps of

adding an acid to said dispersion to prepare a gel;

adding a phase transfer agent before, during or after the addition of said acid to said dispersion and/or to said gel, said phase transfer agent being miscible both with said water and with said nonpolar organic solvent;

separating the aqueous phase from the organic phase, said organic phase comprising said gel, said nonpolar organic solvent, at least part of said phase transfer agent and possibly unreacted organosilicon;

drying said gel to remove said nonpolar organic solvent, said phase transfer agent and said unreacted organosilicon from said gel.

A method according to claim 1 , wherein the volume ratio of the volume of said nonpolar organic solvent and the volume of said organosilicon to prepare said dispersion ranges between 1 : 10 and 25: 1.

A method according to claim 1 or claim 2, wherein the volume ratio of said nonpolar organic solvent to the volume of the water glass to prepare said dispersion ranges between 1 : 10 and 25: 1.

A method according to any one of the preceding claims, wherein said dispersion is prepared starting from a volume of said non-polar solvent that is larger than the volume of said organosilicon and that is larger than the volume of the water glass .

A method according to any one of the preceding claims, wherein the volume ratio of the volume of the water glass and the volume of said phase transfer agent ranges between 10: 1 and 0.5: 1.

A method according to any one of the preceding claims, wherein the volume of the water glass is higher than the volume of the phase transfer agent.

A method according to any one of the preceding claims, wherein the phase transfer agent is not yet present in the dispersion or in the gel at the moment of its addition to the dispersion and/or to the gel.

8. A method according to any one of the preceding claims, wherein said gel is dried at a temperature between 100 and 170 °C and at a pressure ranging between 0.1 and 2 atm.

9. A method according to any one of the preceding claims, wherein said method further comprises the step of washing said gel with an inorganic acid before said drying of said gel.

A method according to any one of the preceding claims, wherein said aqueous water gl solution has a silica content ranging between 0.1 wt% and 20 wt%.

1 1. A method according to any one of the preceding claims, wherein said organosilicon comprises any one selected from the group consisting of hexamethyldisilazane (HMDS), hexamethyldisiloxane (HMDSO), trimethylchlorosilane (TMCS), trimethoxymethylsilane

(TMMS), methoxitrimethylsilane (MTMS), dimethylmethoxysilane (DMMS), dimethyldiethoxysilane (DMDES), tetraethoxysilane (TEOS), methyltrimethoxysilane (MTM), vinylmethoxysilane (VMS), phenyltrimethoxysilane (PTMS), dimethylchlorosilane (DMCS) and mixtures of two or more thereof.

12. A method according to any one of the preceding claims, wherein said nonpolar organic solvent comprises any one selected from the group consisting of hexane, heptane, toluene, xylene , their fluorinated equivalents and mixture of two or more thereof.

13. A method according to any one of the preceding claims, wherein said acid comprises any one selected from the group consisting of nitric acid, hydrochloric acid, sulfuric acid, acetic acid, hydrofluoric acid, trimethylchlorosilane and mixtures of two or more thereof.

14. A method according to any one of the preceding claims, wherein said phase transfer agent comprises an alcohol or a surfactant.

15. A method according to claim 14, wherein said alcohol comprises isopropanol.

16. An aerogel obtainable by the method as defined in any one of claims 1 to 15.