Aerogels

Abstract

This invention relates to methods of producing aerogels and composites thereof. In particular, the invention relates to methods of producing silica aerogels and composites thereof. The invention also relates to doped aerogels and doped silica aerogels. The method involves the use of alkaline solutions, and particularly aqueous alkaline solutions, during the aerogel drying process. The method is more energy efficient and cheaper than prior art methods.

Claims

1. A method for the production of an oxide aerogel optionally comprising a dopant, the method comprising: reacting an oxide wet gel with an electrophile to generate CO.sub.2 gas, thereby providing a porous solid oxide containing the CO.sub.2 gas, the porous solid oxide optionally comprising a dopant; wherein a liquid component of the oxide wet gel comprises an aqueous alkaline solution that comprises carbonate ions, bicarbonate ions, or a mixture thereof.

2. A method of claim 1, wherein the oxide is silica.

3. A method of claim 1, wherein the electrophile is a reagent which reacts with the oxide wet gel to generate an acid.

4. A method of claim 3, wherein the electrophile is a silylating agent having the formula R.sub.3SiX, wherein R is independently at each occurrence selected from a C.sub.1-C.sub.4 alkyl group and halide; and X is independently selected from halide and sulfate groups.

5. A method of claim 4, wherein the electrophile is trimethylchlorosilane (TMCS).

6. A method of claim 5, wherein the volume ratio of the TMCS to the oxide wet gel is in the range of from 0.01 to 3.

7. A method of claim 1, wherein the aqueous alkaline solution is an aqueous sodium bicarbonate solution.

8. A method of claim 1, wherein the aqueous alkaline solution has a concentration in the range from 1% to 10% w/v.

9. A method of claim 1, wherein the method comprises the step of: treating a precursor oxide wet gel with an aqueous alkaline solution comprising carbonate ions, bicarbonate ions, or a mixture thereof to provide an oxide wet gel having a liquid component which comprises the aqueous alkaline solution.

10. A method of claim 9, wherein the method comprises the step of: forming the precursor oxide wet gel.

11. A method of claim 1, wherein the method comprises: reacting an oxide monomer, an electrophile and an aqueous alkaline solution comprising carbonate ions, bicarbonate ions, or a mixture thereof to generate a porous solid oxide comprising the CO.sub.2 gas, the porous solid oxide optionally comprising a dopant.

12. A method of claim 1, wherein the process comprises the step of: converting the porous solid oxide containing the CO.sub.2 gas into the oxide aerogel.

13. A method of claim 12, wherein the method comprises the step of: drying the porous solid oxide containing the CO.sub.2 gas to provide the aerogel.

14. A method of claim 13, wherein the drying step comprises heating the porous solid to a temperature of less than 100 C.

15. A method of claim 1, the method comprising the step of: powdering the oxide aerogel to form a powdered oxide aerogel.

16. A method of claim 1, wherein the method is a method of making a composite comprising an oxide aerogel.

17. A method of claim 16, wherein the composite is an oxide aerogel-fibre composite and both the oxide wet gel and the porous solid oxide comprise fibres supported in the oxide matrix.

18. A method of claim 1, wherein the method comprises the step of: reducing the amount of the dopant in the porous solid oxide to provide a porous solid oxide having a reduced amount of the dopant.

19. A method of claim 18, wherein the step of reducing the amount of the dopant comprises washing the porous solid oxide or the oxide aerogel with water.

20. A method of claim 1, wherein the method comprises the step of: reducing the amount of the dopant in the oxide aerogel to provide an oxide aerogel having a reduced amount of the dopant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

(2) FIG. 1 shows optical images of (a) silica aerogel sample A1; (b) silica aerogel sample A2; (c) silica aerogel sample A3.

(3) FIG. 2 shows scanning electron microscope (SEM) images of (a) silica aerogel sample A1; (b) silica aerogel sample A2; (c) silica aerogel sample A3.

(4) FIG. 3 provides a schematic explanation of comparison between (a) a conventional APD method with organic low surface tension solvent; (b) an APD method of the present invention, using am aqueous sodium bicarbonate solution.

(5) FIG. 4 shows the proposed net chemical reactions for the methods of the invention using sodium bicarbonate solution with ((CH.sub.3).sub.3SiCl).

(6) FIG. 5 the proposed chemical net-reactions of all-in-one sol-gel process.

(7) FIG. 6 shows the X-ray diffraction (XRD) spectra of silica aerogel samples A1, A2 and A3.

(8) FIG. 7 shows the high-resolution transmission electron microscopy (HRTEM) image of silica aerogel sample A2 and the corresponding fast Fourier transform pattern of regions that match the pattern expected for the (222) and (220) lattice planes of crystalline NaCl presented in the inset.

(9) FIG. 8 shows scanning transmission electron microscopy (STEM) images and the corresponding energy-dispersive X-ray spectroscopy (EDX) mapping images: (a) silica aerogel sample A1; (b) silica aerogel sample A2; silica aerogel sample A3.

(10) FIG. 9 shows the thermal conductivity of silica aerogel sample B for range of temperatures.

(11) FIG. 10 shows SEM images of the Dawsonite aerogels of Example 6.

DETAILED DESCRIPTION

(12) An aerogel is a porous solid. It can be characterised as being comprised of a microporous solid in which the dispersed phase is a gas. An aerogel is so-called because they are usually made by displacing the liquid in a gel (a gel being a liquid dispersed in a solid) with a gas.

(13) In the context of gels and aerogels, the term wash typically refers to a process in which the gel/aerogel is placed in a liquid and left in the liquid for a predetermined length of time. The liquid is removed (e.g. by pipette) when each wash step is completed.

(14) The aerogels of the invention may be used as thermal insulators, e.g. in the context of power generation or in construction or in reducing heat loss from industrial processes or vehicles. The aerogels of the invention may be used as absorbent materials, e.g. in the clean-up of oil spills. The aerogels of the invention may be used as catalyst supports.

(15) The term C.sub.x-C.sub.y-alkyl in this specification refers to a branched or linear hydrocarbon chain having from x to y carbons. The alkyl groups are typically unsubstituted. Exemplary alkyl groups include methyl, ethyl, n-propyl, iso-propyl, n-butyl, isobutyl etc. Likewise, the term C.sub.x-C.sub.y-alcohol refers to a molecule having the formula C.sub.x-C.sub.y-alkyl-OH. Exemplary alcohols include methanol, ethanol, n-propanol, iso-propanol etc.

(16) Throughout the description and claims of this specification, the words comprise and contain and variations of them mean including but not limited to, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

(17) Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

(18) The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

EXAMPLES

Example 1

Multi-Step Silica Aerogel Preparation Using TEOS as a Source of Silicon

(19) Silica Gel Preparation

(20) All materials and solvents were purchased from Sigma-Aldrich and used without any further purification. Silica gels were prepared by the hydrolysis of the precursor tetraethoxysilane (TEOS, 98%), ethanol (99.5%) and de-ionic (DI) water with a molar ratio as TEOS:ethanol:water=2:38:39. In order to speed gelation 34 ml of prepared precursor with 1 ml of catalyst (catalyst was a mixture of ammonium hydroxide (28-30%), ammonium fluoride (98%) and water with a molar ratio as NH.sub.4OH:NH.sub.4F:H.sub.2O=8:1:111) were used. In approximate 5 minutes, silica gels were removed from the casting mould and then washed and aged with 500 ml of ethanol for 24 hours.

(21) Drying Process

(22) After 24 hours of aging ethanol (solvent) was replaced by a mixture of 500 ml DI water and 22 g sodium bicarbonate 99.7%). Silica gels were subsequently soaked in the bicarbonate solution while stirred for 24 hours. At the end of solvent exchange the silica gels were placed out of sodium bicarbonate solution and trimethylchlorosilane 97%, TMCS) (6 ml/8 ml/10 ml) was slowly poured onto the top of the gels which enabled bubbling of the generated carbon dioxide from inside of the gels. 2 minutes later after a big burst of CO.sub.2 generated by bubbling, ethanol was added as protecting solvent from drying in air, but however still some more CO.sub.2 gas was released from the gels for the following 24 hours. Finally the gels were dried at 60 C. and at ambient pressure with ethanol for 24 hours to obtain the silica aerogels.

(23) Salt Removal

(24) If desired, the produced sodium chloride can be removed by washing with water three times for 24 hours in total before the gel was heated, or the same procedure can be performed on the silica aerogel formed after the gel had been dried.

Example 2

One Pot Silica Aerogel Preparation Using Water Glass as a Source of Silicon

(25) 3 ml of trimethylchlorosilane (TMCS) was added in to 5 ml of water glass (sodium silicate: Na.sub.2O 10.6%, SiO.sub.2 26.5%). A thin layer of solid immediately formed on the surface. Then 5 ml of sodium bicarbonate solution was added in, and the solid layer was subsequently broken by using a glass rod. With stirring, the gel was formed in seconds, and aged for approximately 2 hours. The produced sodium chloride was removed by washing the gel with water three times. The gel was dried at 60 C. and under ambient pressure for 24 hours to obtain the silica aerogels.

Example 3

Characterisation and Discussion

(26) Characterisations

(27) FEI XL30 ESEM-FEG (Environmental Scanning Electron Microscope-Field Emission Gun) was used to image the samples in high vacuum mode and 10 KeV accelerating voltage at Newcastle University. Before the SEM imaging, the samples were coated with gold to increase electrical conductivity. Coulter SA 3100 Surface Area and Pore Size Analyzers were used to measure BET (Brunauer-Emmet-Teller) surface area at the Newcastle University. The PANalytical X'Pert Pro Multipurpose Diffractometer (MPD) is used for X-ray powder diffraction (XRPD) analysis at the Newcastle University. The High resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) experiments were carried out in a Tecnai F30 300 keV microscope at the Materials Science Centre, University of Manchester. The samples for HRTEM and STEM were prepared with ultrasonication of silica aerogels in de-ionised water for a long time until there were no large pieces of aerogel seen by eye.

(28) Results and Discussion

(29) According to the different amounts of TMCS added, the silica aerogels prepared in Example 1 by adding 6 ml, 8 ml and 10 ml of TMCS and are named as A1, A2 and A3, respectively. FIG. 1 shows photograph images of all three samples. With the increase of amount of TMCS the transparency of samples is reduced. FIG. 2 shows SEM images of all three samples. It is clear that silica aerogel samples exhibit highly porous structures. Samples A1 and A2 have larger porous size than sample A3 due to the largest surface modification by TMCS.

(30) In table 1, the density and BET surface area of silica aerogels is reported. The density of samples was calculated from ratio of weight to volume. The density rises with the increase of amount of TMCS is due to the formation of NaCl crystals in pores of silica aerogel (see discussion below). Aerogel sample A2 has the highest surface area and sample A3 has the lowest surface area. This BET measurements are consistent with the porous structures observed with the SEM images.

(31) TABLE-US-00001 TABLE 1 Density and BET surface area of silica aerogels Samples Density (g/cm.sup.3) BET Surface Area (m.sup.2/g) Silica aerogel (A1) 0.32 555 Silica aerogel (A2) 0.49 585 Silica aerogel (A3) 0.63 470

(32) Capillarity is the primary mechanism when silica gels undergo the drying process. Referring Young-Laplace equation, if the porous shape in aerogels is assumed to be cylinder, the pressure of capillarity in those pores (P) satisfies a relation with surface tension of solvent (), contact angle between solvent and surface (), and radius of surface (R) of pores:
P=2 cos /R(Young-Laplace equation)

(33) In conventional APD method, solvent in pores directly evaporates from the surface when gels are heated. Various kinds of low-surface-tension (LST) solvents have been investigated in the past and successful synthesis of silica aerogels have been reported. For instance, in 2012, Jung (Jung et al., The properties of silica aerogels hybridized with SiO.sub.2 nanoparticles by ambient pressure drying. Ceramics International, 2012, 38, S105-S108) fabricated five silica aerogels by conventional APD method with hexane solvent which gave a BET surface area results ranging from 531 m.sup.2/g to 772 m.sup.2/g, and densities ranging from 0.27 g/cm.sup.3 to 0.35 g/cm.sup.3. These aerogels have very similar properties as silica aerogels do in our work, so by comparison of the properties sodium bicarbonate solution lowers capillarity as effectively as conventional organic solvents used in APD method.

(34) In FIG. 3 is presented a schematic diagram of the mechanisms of a) the conventional APD method with LST solvent and b) the APD method with sodium bicarbonate solution. FIG. 4 is the proposed reaction mechanism with the sodium bicarbonate solution with TMCS by APD method. It is known that when TMCS reacts with water and/or hydroxyl group on the surface of the silica gel the hydrogen chloride is formed. Once the HCl is formed it reacts with sodium bicarbonate solution and produces sodium chloride, carbon dioxide gas in the pores of the silica gel and additional water molecules as by products. Moreover water, as a by-product of reaction, reacts with remaining TMCS to further continue producing hydrogen chloride. This is a self-driving process allowing carbon dioxide to be released from the within the silica gel while hydrogen chloride slowly diffuses into the bulk of the gel.

(35) A control experiment of the reactions between solid sodium bicarbonate (powder form), pure TMCS (100%) and water vapour from air was conducted. Any one of the three components would activate the net-reaction with either of the other two, as proposed in FIG. 4. Because it is hard to control the presence of water in ambient environment, the water molecules for this experiment are provided by the air. After 6 seconds after initial addition of TMCS to sodium bicarbonate powder, the visible chain reaction starts due to time needed for the water molecules from the air to diffuse to the TMCS with sodium bicarbonate, as it has been proposed in FIG. 4. After another 7 seconds, the amount of carbon dioxide generated reaches a maximum.

(36) When carbon dioxide forms in the middle of a pore of the gel, the region filled by gas has conquered the capillarity because the CO.sub.2 gas is uncompressible and therefore the pore diameter is increased. From the Young-Laplace equation (above) capillarity is reduced by enlarging radius of the pore (which gives the same result as the reducing the surface tension). This is how this proposed method, successfully fabricates aerogels without commonly used LST solvents. Sodium chloride which is the by-product of this process is more environmentally friendly than HCl. In FIG. 6 the X-ray diffraction (XRD) results of samples A1, A2 and A3 confirm crystalline planes (200), (220) and (222) of NaCl and presence of amorphous SiO.sub.2 in these silica aerogels.

(37) In order to prove that the NaCl is presented within the silica aerogel and is not produced just on the surface, HRTEM and STEM measurements of the same sample are obtained. FIG. 7 shows HRTEM images of silica aerogel. The two insets of FIG. 7 show the Fourier transforms of regions that match the pattern expected for the (222) and (220) lattice planes of the NaCl crystals. In FIGS. 8 (a), (b) and (c) STEM images and the corresponding EDX mapping images of samples A1, A2 and A3, respectively, are presented. The first column of FIG. 8 shows high angle annular dark field (HAADF) images. The throughout porous structure of aerogel has been confirmed by HAADF images (sample A1, the first row). Other 4 columns show elemental distribution maps, obtained via EDX, of the silica aerogels, namely Si, O, Na and Cl. It is clear that Na and Cl (compare the 4th and the 5th row in FIG. 8) have also spatial correlation with Si and O (see the 2nd and the 3rd row of FIG. 8) and also show porosity (silica aerogel). In particular it is important to notice that the material for HRTEM/STEM was ultrasonicated in DI water for long time so any surface NaCl would dissolve in the water. The origin of the Na and Cl in FIG. 8 therefore must originate from the bulk of silica aerogel. Therefore the STEM images and HRTEM images prove that the sodium chloride is formed by the reaction of sodium bicarbonate and generated hydrogen chloride in pores of silica aerogels.

Example 4

Multi-Step Silica Aerogel Preparation Using Water Glass as a Source of Silicon

(38) Water glass (sodium silicate, Sigma Aldrich: Na.sub.2O 10.6%, SiO.sub.2 26.5%) was diluted with deionised water (1:4 water glass: DI water). Ion exchange was carried out with Amberlite (Strongly Acidic Styrene Type Cation Exchange Resin). The silica sol had a pH in the range of 2-3 after finishing the ion exchange process. The sol was transferred to a beaker and stirred for 5 min. For the gelation step, a solution of both ammonium hydroxide and ammonium fluoride solution was used to modify the pH of the sol to 6. The silica sol was transferred to a plastic mold (50 mm in diameter) for gelation. The gelation was completed within 15 min then the hydrogel was soaked in water for 24 h to age. The water was exchanged with a sodium bicarbonate solution (4.4 g for each 100 ml water) as described in Example 1. After magnetic stirring for 24 h, the hydrogel was moved to an empty petri dish. For surface modification and CO.sub.2 generation step, the hydrogel was soaked in a solution of TMCS (6 ml) in ethanol (150 ml) for 8 h. The hydrogel was rinsed and washed with deionised water several times to remove the salt from the pores, as described above in Example 1. The resultant gas filled porous oxide was soaked in ethanol for 24 h and then dried at 60 C. for a further 24 h. The final silica aerogel had a BET surface area of 678 m.sup.2/g. The aerogel made in this example is silica aerogel sample (B)

Example 5

Thermal Conductivity of Silica Aerogel

(39) Thermal conductivity at room temperature for silica aerogel sample (B) is 0.012 W/m.Math.K. The thermal conductivity measurements in FIG. 9 were obtained with a transient plane source technique (S. E. Gustafsson: Transient plane source techniques for thermal conductivity and thermal diffusivity measurements of solid materials. Rev. Sci. Instrum. 62(3), 797 (1991) using HotDisk Model TPS 2500 S. FIG. 9 shows the thermal conductivity as a function of temperature up to 420K. Circles represent data when the temperature was raised from RT to 420K and Triangles when the starting temperature was 420K and sample was cooled. The thermal conductivity is lower in cooling cycle because during the first heating some of the adsorbed water in pores of the silica aerogel have evaporated leaving the aerogel less humid, which lowers thermal conductivity.

Example 6

Dawsonite AerogelA Precursor to an Alumina Aerogel

(40) Dawsonite, a sodium aluminium hydroxy carbonate NaAl(CO.sub.3)(OH).sub.2, is precursor to alumina. Upon heating to 700 C., Dawsonite can be converted to -Al.sub.2O.sub.3 [Zhanglong Yu, Yajing Lv, Yongmei Chen, and Pingyu Wan, Laboratory Studies on the Preparation Procedures of Alumina Converted from Aluminum Citrate, Ind. Eng. Chem. Res. 2010, 49, 1832-1836]. Our method disclosed in this Example prepares the Dawsonite aerogel.

(41) Preparation of the sol involved stirring a mixture of aluminum sec-butoxide, deionised water, ethyl acetoacetate and ethanol in a molar ratio of 1:0.6:0.58:16, for 45 min at 60 C. Hydrolysis and condensation step was carried out by mixing the sol under stirring with a mixture of methanol, water, acetic acid, and N,N-dimethylformamide (DMF) of weight ratio Sol:MeOH:H.sub.2O:DMF=1:0.2:0.003:0.03. 1 mL of acetic acid was added for each 30 mL of the mixture under magnetic stirring for 30 min at room temperature. Then the homogeneous sol was transferred to airtight boxes and kept for 7 days at room temperature to complete the gelation process. The resultant gel was then soaked in sodium bicarbonate solution (4.4 g for each 100 ml water) under stirring for 24 h. The resultant gel comprising the bicarbonate was put into an ethanol solution containing TMCS (4 mL TMCS in 100 ml ethanol) for 24 h at 21 C. The gel was rinsed with ethanol to remove the unreacted TMCS and then soaked with water and stirred for 8 h for washing. Lastly, the gel is immersing in ethanol for 24 h then dried at room temperature for 72 h and 100 C. for 2 h under ambient pressure.

(42) BET, and Barrett-Joyner-Halenda (BJH) analysis were carried out to measure the asurface area and pore size of the Dawsonite aerogel. Mesoporous Dawsonite of high surface area of 345 m.sup.2/g and average pore diameter of 4 nm was obtained. SEM images (FIG. 10) show the macropores and mesopores structure of Dawsonite aerogels.

(43) An alumina aerogel can be formed from the Dawsonite aerogel produced in this example by taking a mixture of (0.05 mol) of Dawsonite aerogel and (0.05 mol) of NaOH and sintering the mixture in a muffle furnace in the temperature range from 100 to 300 C. for 0.5-3 h. After cooling to room temperature, the sinter can be washed with deionized water. The residue can be vacuum-dried and roasted at 700 C. in the muffle.