A method for the synthesis of silica aerogel product

Abstract

The present disclosure provides a novel, single-step method for producing silica-based aerogel products. The process employs ambient pressure drying using a specially selected, highly polar, aprotic solvent that forms an azeotropic mixture with 15–20% water, thereby eliminating conventional aging, solvent exchange, and surface modification steps. This innovative approach yields superhydrophobic silica aerogels with nanoporous structures and thermal insulation performance comparable to aerogels produced by supercritical drying. Optionally, IR opacifiers such as metal oxides or MOFs may be incorporated to further enhance thermal insulation, enabling the production of aerogels in various forms including monoliths, powders, and composite materials. The method is cost-effective, scalable, and energy efficient, providing a robust alternative for high-performance thermal insulation applications.

Description

A METHOD FOR THE SYNTHESIS OF SILICA AEROGEL PRODUCTFIELD OF THE INVENTION

[0001] The present disclosure relates to a method for producing silica-based aerogel products. Specifically, the process employs ambient pressure drying using a specially selected, highly polar, aprotic solvent that forms an azeotropic mixture with 15-20% water, thereby eliminating conventional aging, solvent exchange, and surface modification steps. Further, the method is cost-effective, scalable, and energy efficient, providing a robust alternative for high-performance thermal insulation applications.BACKGROUND OF THE INVENTION

[0002] Background description includes information that may be useful in understanding the present disclosure. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed disclosure, or that any publication specifically or implicitly referenced is prior art.

[0003] Silica aerogels are renowned for their exceptional thermal insulation properties over a wide temperature range, from cryogenic to high-temperature applications. These ultralight, highly porous materials, typically having pore sizes below 100 nm, are conventionally produced via sol-gel processes followed by specialized drying techniques.

[0004] Conventional methods for producing silica aerogel typically involve a sol-gel process followed by specialized drying techniques. A simple solvent evaporation-based drying often leads to shrinkage and collapse of the nanoporous network due to capillary forces. To avoid this, methods such as supercritical drying (SCD) are employed, where the gel is placed in a high-pressure vessel and subjected to conditions above the critical point of the solvent. Alternatively, freeze drying (FD) is used, which requires freezing the gel followed by sublimation of the solvent. Both techniques, however, require multiple processing steps including aging, solvent exchange, and surface modification and the use of expensive equipment and large volumes of solvents.

[0005] Further, conventional processes that involve ambient drying conditions for producing silica aerogel generally involve multiple steps aging, solvent exchange, and surface modification and typically do not incorporate IR opacifiers. The solvents used are primarily water and various alcohols, with nonpolar solvents (e.g., hexane or heptane) required for surface modification. Furthermore, to the best of the knowledge of inventors of the presentdisclosure, none of the reported methods provide for collection and recycling of evaporated solvents.

[0006] Consequently, the conventional methods are multi-step, lengthy, and solvent intensive, and they fail to produce silica aerogel with optimal thermal insulation performance at elevated temperatures.

[0007] In addition, the integration of IR opacifiers such as in-situ generated oxide nanoparticles (e.g., titanium dioxide as described in Indian Patent No. 305898) or carbon (as in Indian Patent No. 290370) is critical to reduce radiative heat transfer at elevated temperatures. Although IR opacifiers have been extensively studied, their incorporation into aerogels produced by conventional methods remains inefficient.

[0008] Moreover, the production of flexible silica aerogel sheets typically involves infiltrating the aerogel into porous fiber mats or blankets or foams, a process that further complicates the manufacturing procedure and increases costs.

[0009] Various approaches to produce silica aerogels have been reported, which use precursors such as sodium silicate, Tetramethyl orthosilicate (TMOS), Tetraethyl orthosilicate (TEOS), and Methyl tetramethoxy silane (MTMS), often coupled with post-gelation processes like solvent exchange and surface modification to impart hydrophobicity. These conventional methods are inherently multi-step, lengthy, and solvent-intensive, and they usually employ drying techniques such as SCD or FD, which require high pressure or low-temperature conditions respectively. Consequently, despite their excellent insulation performance, such products are costly and not readily affordable for large-scale applications.Prior arts

[0010] Patent W02023200402A2 describes a method to produce silica aerogel granules or powders or mixture of these, by ambient drying method where silica gel is prepared using conventional silica precursors in the hydrolytic solvents such as alcoholic solvents, to which surfactant is added. The gel is then surface modified by treating with alkoxide molecules to make the gel hydrophobic. Such gel is then aged. All these process steps are carried out at 50- 65°C which take several hours. Finally gel is dried 150-250 °C. The process claimed is multistep and lengthy process and involves use of additional ingredient i.e. surfactant which requires to wash away and if not done so, it degrades at high temperature and degasses while using at high temperatures. This process does not include IIR opacifier, hence may not be efficient thermal insulator at high temperature. Here alcohol is used as a solvent.

[0011] Patent WO2023137435A1 claims to produce silica aerogel using ion exchanged sodium silicate precursor with addition of anionic and cationic surfactants, urea and cellulosefibers to make a gel at 60 to 90 °C in the period of at least 18 hours. The gel is then washed with water followed by ambient pressure drying and calcination at 300 to 600 °C. The pores in the final product are 1 - 500 micrometer size range and not in nanometer as that of conventional aerogel materials which are the reason to make aerogel superior in thermal insulation performance. Here water is used as a solvent.

[0012] Patent WO2023226710A1 describes a method where sodium silicate is used as a silica precursor and gel is formed in acid catalyst in water. The gel is then surface modified by liquid solvent immersion process where alcohol and alkane solvents are used with the hydrolysing chemicals such as HMDS. Then the gels are dried at 90 - 150 °C. The limitation is that it is a multistep method. The remaining liquid after immersion step goes waste, some water from gel also gets mixed and the composition is not useful to make use of it for next batch. The gel does not involve the presence of IR opacifiers in it. Here water is used as a solvent.

[0013] Thus, there is an unmet need in the art to develop a novel method for the synthesis of silica aerogel products that overcome one or more drawbacks in the prior art.OBJECTIVE OF THE INVENTION

[0014] The principal object of the present disclosure is to overcome the disadvantages of the prior art.

[0015] The primary objective of the present disclosure is to provide a single-step method for producing silica-based aerogel products without the need for aging, solvent exchange, or surface modification steps, while achieving thermal insulation performance at both low and high temperatures.

[0016] Another objective of the present disclosure is to employ a highly polar, aprotic solvent with low vapor pressure and a high boiling point that forms an azeotropic mixture with 15-20% water, thereby reducing pore shrinkage and eliminating the need for solvent exchange by ensuring efficient dissolution of reactants.

[0017] Another objective of the present disclosure is to incorporate novel nanoporous IR pacifier materials into the silica matrix, which serve the dual purpose of suppressing radiative heat transfer and enhancing overall thermal insulation performance due to their porous morphology.

[0018] Another objective of the present disclosure is to achieve inherent hydrophobicity in the silica aerogel by using a single silica precursor that forms robust pore walls capable of withstanding the surface tension during simple drying, thus obviating the need for additional surface modification steps.

[0019] Another objective of the present disclosure is to provide a facile, quick, cost-effective, and low-energy process for producing silica-based aerogel thermal insulation products via ambient pressure drying, thereby eliminating the requirement for expensive high-pressure or low- temperature equipment.

[0020] Another objective of the present disclosure is to enable the production of silica-based aerogel in various forms including monoliths, powders, and flexible sheets by infdtrating the aerogel in porous substrates such as porous fibrous mats and foams, to meet a broad range of thermal insulation applications.

[0021] Another objective of the present disclosure is to produce silica-based aerogel composites that exhibit thermal insulation performance comparable to those produced by conventional supercritical drying methods, while being more scalable and economically viable.SUMMARY OF THE INVENTION

[0022] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description section. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0023] The present disclosure encompasses a method for synthesis of hydrophobic silica aerogel products. The process eliminates the requirement of conventional multi-step procedures such as aging, solvent exchange, and surface modification thereby significantly reducing energy consumption and production costs while yielding aerogels with superior thermal insulation performance comparable to those produced by high-pressure supercritical drying.

[0024] Employing a polar, aprotic solvent (preferably, with a high boiling point and low vapor pressure) forms an azeotropic mixture with water and aids in eliminating the need for aging and solvent exchange. Additionally, the process enables incorporation of IR opacifiers into the silica matrix, resulting in enhanced thermal insulation performance even at high temperatures.

[0025] Accordingly, in an aspect, the present disclosure provides a method for the production of a silica aerogel product, said process comprising the steps of:i. Mixing a silica precursor with polar aprotic solvent (e.g., N,N Dimethylformamide (DMF)) and water in the presence of a catalyst to prepare a mixture;ii. Optionally incorporating an IR opacifier titanium dioxide particles and / or other metal dioxide particles, or metal-organic frameworks (MOFs) particles and / or porous additives, either by in-situ generation or by externally adding into the mixture;iii. Stirring the mixture to form a viscous sol;iv. Alternately soaking the dispersion of metal oxide particles and / or MOF particles in DMF / water / alcohol or combination of them and the silica sol in porous substrates, sequentially or alternately, in single or multiple times to form a gel composite,v. Preparing a gel or gel composite by casting the sol into a mould or by impregnating the sol into a porous substrate; andvi . Drying the gel or gel composite at a temperature ranging from 40to l50 °Cto obtain a silica aerogel product.

[0026] In an aspect of the present invention, the silica precursor comprises one or more alkoxysilanes selected from methyltrimethoxysilane (MTMS), ethyltrimethoxysilane (ETMS), methyltriethoxysilane (MTES), ethyltriethoxysilane (ETES), or a combination thereof.

[0027] In an aspect of the present invention, the polar aprotic solvent is N,N- Dimethylformamide (DMF).

[0028] In an aspect of the present invention, the molar ratio of silica precursor to DMF ranges from 1:4 to 1:25.

[0029] In an aspect of the present invention, the molar ratio of silica precursor to water ranges from 1:2 to 1: 28.

[0030] In an aspect of the present invention, the catalyst is selected from acidic or basic catalysts and the catalyst concentration ranges from 5 x 105to 0.1 moles.

[0031] In an aspect of the present invention, the infrared Opacifier is selected from titanium dioxide nanoparticles, other metal oxides particles, metal-organic frameworks (MOFs) particles, or a combination thereof.

[0032] In an aspect of the present invention, the dispersion of metal oxide particles and / or MOF particles in DMF / water / alcohol or combination of them and the sol are sequentially or alternately, in single or multiple times, is soaked in porous substrates to make a composite.

[0033] In an aspect of the present invention, the infrared opacifier concentration is in the range of 0.5 to 10 wt% relative to the silica content.

[0034] In an aspect of the present invention, the porous substrate is selected from organic, inorganic, or hybrid porous materials including fibrous mats, woven or non-woven sheets, needle-punched fabrics, foams, or blankets.

[0035] In an aspect of the present invention, the drying step is carried out at atmospheric pressure and performed at a temperature in the range of 80°C to 110°C.

[0036] In an aspect of the present invention, the silica aerogel product is obtained in the form of monoliths, powders, sheets, or flexible sheets.

[0037] The advantageous method facilitates efficient condensation reactions, yielding a robust three-dimensional silica network that is highly resistant to pore collapse during ambient pressure drying. Further, the process minimizes energy consumption and manufacturing costs by eliminating the need for high-pressure drying equipment and extensive solvent exchange steps. Still further, the method affords a scalable and environmentally sustainable approach to producing high-performance thermal insulation materials suitable for a wide range of industrial applications.

[0038] Various objects, features, aspects, and advantages of the present disclosure will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like features.BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The accompanying drawing(s) are included to provide a further understanding of the present disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure. The diagrams are for illustration only, which thus is not a limitation of the present disclosure.

[0040] FIG. 1 illustrates Flow chart showing the formation of silica based aerogel products according to embodiments under the invention.

[0041] FIG. 2 illustrates Electron microscopy image of silica based aerogel showing porous network prepared as per description mentioned in Example 1

[0042] FIG. 3 illustrates Photograph of the silica based aerogel flexible sheet formed by the process described in Example 2.

[0043] FIG. 4 illustrates Comparison of thermal conductivity of plane silica aerogel, plane MOF and silica aerogel monolith with MOF addition, measured by transient plane method prepared as per the procedure explained in Example 1 and Example 5 respectively.

[0044] FIG. 5 illustrates the Photograph while spraying the MOF dispersion to form a layer on one side of e-glass fiber mat as described in Example 6.

[0045] FIG. 6 illustrates Schematic of the test set up used and the scheme of the thermal insulation wrapped as described in Example 7.

[0046] FIG. 7 illustrates Comparison of thermal insulation performance of near to ambient pressure dried silica aerogel as explained in Example 2 with conventionally used fibrous insulation material, supercritically dried silica aerogel product measured as per ASTM C335 at temperatures from 100 to 600°C as explained in Example 7.DETAILED DESCRIPTION OF THE INVENTION

[0047] The following is a detailed description of embodiments of the disclosure. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.

[0048] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

[0049] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0050] In some embodiments, numbers have been used for quantifying weights, percentages, ratios, and so forth, to describe and claim certain embodiments of the disclosure and are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable.

[0051] The numerical values presented in some embodiments of the disclosure may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0052] Unless the context requires otherwise, throughout the specification which follows, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”

[0053] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.

[0054] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein.

[0055] All methods described herein can be performed in a suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples or exemplary language (e.g., “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.

[0056] Groupings of alternative elements or embodiments of the disclosure disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and / or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified.

[0057] The description that follows, and the embodiments described therein, is provided by way of illustration of an example, or examples, of particular embodiments of the principles and aspects of the present disclosure. These examples are provided for the purposes of explanation, and not of limitation, of those principles and of the disclosure.

[0058] It should also be appreciated that the present disclosure can be implemented in numerous ways, including as a system, a method or a device. In this specification, these implementations, or any other form that the invention may take, may be referred to as processes .In general, the order of the steps of the disclosed processes may be altered within the scope of the invention.

[0059] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.

[0060] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus, if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

[0061] The term “or”, as used herein, is generally employed in its sense including “and / or” unless the content clearly dictates otherwise.Various terms are used herein to the extent a term used is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.Definitions:

[0062] For the purpose of the present invention, “Silica aerogel” refers to a highly porous, low-density material primarily comprising silicon dioxide (SiOz) that exhibits a nanoporous network.

[0063] For the purpose of the present invention, “Hydrophobic silica aerogel” refers to silica aerogel that is rendered water-repellent, typically by incorporating methyl groups or other hydrophobic functionalities.

[0064] For the purpose of the present invention, “Silica precursor” means a silicon alkoxide such as methyl trimethoxysilane (MTMS), tetraethyl orthosilicate (TEOS), or tetramethyl orthosilicate (TMOS), and such other silica precursors as known to or appreciated by a skilled artisan to form the silica network.

[0065] For the purpose of the present invention, “IR opacifier” refers to an additive that reduces radiative heat transfer in the aerogel product, for example, titanium dioxide nanoparticles, metal-organic frameworks (MOFs) and the likes.

[0066] In another embodiment of the present invention, the FIG. 1 illustrates Flow chart showing the formation of silica based aerogel products according to embodiments under the invention.

[0067] In an embodiment, the present disclosure provides a method for the production of a silica aerogel product, said process comprising the steps of:i. Mixing a silica precursor with polar aprotic solvent (e.g., N,N Dimethylformamide (DMF)) and water in the presence of a catalyst to prepare a mixture;ii. Optionally incorporating an IR opacifier titanium dioxide particles, other metal dioxide particles or metal-organic frameworks (MOFs)) particles , either by in-situ generation or by externally adding into the mixture;iii. Stirring the mixture to form a viscous sol;iv. Alternately soaking the dispersion of metal oxide particles and / or MOF particles in DMF / water / alcohol or combination of them and the silica sol in porous substrates, sequentially or alternately, in single or multiple times to form a gel composite,v. Preparing a gel or gel composite by casting the sol into a mould or by impregnating the sol into a porous substrate; andvi . Drying the gel or gel composite at a temperature ranging from 40to l50 °Cto obtain a silica aerogel product.

[0068] In some embodiments, the silica aerogel product is in the form of monoliths, powders, sheets, or flexible sheets.

[0069] In an embodiment the mixing of silica precursor comprising alkoxides of silica is selected from methyl trimethoxisilane (MTMS), ethyl triethoxysilane (ETES), methyl triethoxysilane (MTES), Ethyl trimethoxysilane (ETMS) or their combination thereof. In a preferred embodiment, the silica precursor is methyl trimethoxysilane (MTMS), that is added to a mixture of DMF, water and catalyst(s)

[0070] In an embodiment, the molar ratio of the silica precursor to DMF is maintained between 1:4 and 1:25.

[0071] In an embodiment, the precursor-to-water molar ratio is controlled between 1:2 and 1:28.

[0072] In an embodiment, mixing of silica precursor with polar aprotic solvent is effected at catalyst concentrations in the range of 5 / 105to 0.1 moles that forms a robust three- dimensional silica network.

[0073] In an embodiment, the process optionally incorporates infrared (IR) opacifiers into the mixture that enhances thermal insulation performance at elevated temperatures.

[0074] In an embodiment, the IR opacifiers may be in-situ generated such as titanium dioxide nanoparticles formed by adding a titanium precursor (e.g., titanium isopropoxidediluted in DMF or alcohol) or externally added as a dispersion of metal organic frameworks (MOFs) or a combination of metal oxides and MOFs.

[0075] In an embodiment, soaking is alternately done by the dispersion of metal oxide particles and / or MOF particles in DMF / water / alcohol or combination of them and the silica sol in porous substrates, sequentially or alternately, in single or multiple times to form a gel composite

[0076] In some embodiments, the IR opacifier is selected from metal oxides, metal organic frameworks (MOFs), or their combinations thereof.

[0077] In some embodiments, the step of drying is effected at an atmospheric pressure.

[0078] In an embodiment, the gel or gel composite formed is dried at atmospheric pressure by heating at a temperature ranging from 40 to 150 °C. In a preferred embodiment, drying of the gel or gel composite is effected at a temperature ranging from 80 to 110 °C.

[0079] In an embodiment, the final hydrophobic silica aerogel product is obtained in various forms including monoliths, powders (produced by crushing the dried gel), sheets, or flexible sheets depending on whether the sol is cast into a mould or impregnated into a porous substrate such as various types of organic / inorganic / organo-inorganic / woven / non-woven, needle punched or without it, fibrous mats, blankets, sheets or foams or porous structures of any size, shape and density.

[0080] In an embodiment the additive concentration is adjusted to be between 0.5 and 10 weight percent relative to the silica content.

[0081] The advantageous method eliminates the need for expensive high-pressure drying (e.g., supercritical drying) and reduces the use of solvents and processing steps, thereby yielding a cost-effective, rapid, and scalable production method.

[0082] In some embodiments, silica aerogel products exhibit a nanoporous structure and thermal insulation properties comparable to those achieved by conventional supercritical drying methods.

[0083] The advantageous disclosure provides an environmentally sustainable process by reducing energy consumption and eliminating the need for high-pressure equipment, thus contributing to lower production costs and a reduced carbon footprint.

[0084] The following examples illustrate specific embodiments of the disclosure; these examples are provided for illustrative purposes only and are not intended to limit the scope of the present disclosure.EXAMPLES

[0085] The present disclosure is further explained in the form of the following examples. However, it is to be understood that the following examples are merely illustrative and are not to be taken as limitations upon the scope of the disclosure.Example 1: Preparation of Hydrophobic Silica Aerogel

[0086] In the first step, 0.75 L of DMF, 138 mL of water, 99 mL of 0.5 M NH4F, and 4 mL of NH3 are mixed in a flask under continuous stirring. Subsequently, 99 mL of methyl trimethoxysilane (MTMS) is added while stirring. The resulting sol is transferred into a plastic container, where it gels within 10 minutes. The formed gel is then dried in a well-ventilated oven at 80-100 °C for 6 hours, yielding a highly porous, superhydrophobic silica aerogel. FIG.2 illustrates the electron microscopy image and BET isotherm of the aerogel, showing nitrogen adsorption and the calculated surface area.Example 2: Fabrication of a Flexible Silica Aerogel Sheet

[0087] In the first step, 7.44 L of DMF, 1.36 L of distilled water, 39.2 mL of 0.5 M NH4F, and 58.8 mL of NH3 are mixed in a flask under stirring. Next, 0.98 L of MTMS is added while stirring to form a sol. This sol is then soaked into a 10 mm thick e-glass fiber non-woven blanket measuring 33 cm * 300 cm. Within 15 minutes, the sol impregnated in the fiber blanket converts into a gel composite. Finally, the gel composite is dried at 100 °C for 6 hours in a well-ventilated chamber. The vented vapors were passed through water cooled condenser to collect them in a liquid form, to prevent contamination as well as for reusing after purification. The complete drying results in a flexible silica aerogel sheet. FIG. 3 re-presents a photograph of the flexible sheet thus formed.Example 3: Incorporation of Titanium Dioxide in silica aerogel by in-situ formation

[0088] In the first step, 18 mL of DMF, 7 mL of distilled water, 0.2 mL of 0.5 M NH4F, and 0.3 mL of NH3 are mixed in a flask under stirring. Then, 0.4 mL of titanium isopropoxide solution in 3 mL ethyl alcohol is added, followed by 5 mL of MTMS while stirring. The resulting sol is transferred into a plastic container, where gelation occurs in 80 minutes. The gel is subsequently dried at 80-100 °C for 6 hours in a well-ventilated oven, producing a highly porous silica aerogel with incorporated TiCL and exhibiting superhydrophobic properties. Example 4: Fabrication of a Flexible Silica Aerogel Sheet with external addition of titanium dioxide particles

[0089] In the first step, 6.25 g of rutile phased TiO2 powder with particle size of average 1 micrometer size was added to 3.77 L of DMF and stirred vigorously till stable dispersion is achieved. Then, to that, 0.695 L of distilled water, 30 mL of 0.5 M NH4F, and 20 mL of NH3are mixed under stirring. Next, 0.496 L of MTMS is added while stirring to form a sol. This sol is then soaked into a 10 mm thick e-glass fiber non-woven blanket measuring 33 cm x 150 cm. Within 10-15 minutes, the sol impregnated in the fiber blanket converts into a gel composite. Finally, the gel composite is dried at 100 °C for 6 hours in a well-ventilated chamber. The vented vapors were passed through water cooled condenser to collect them in a liquid form, to prevent contamination as well as for reusing after purification. The complete drying resulting in a flexible silica aerogel sheet with titanium dioxide particles incorporated into silica aerogel.Example 5: Incorporation of Metal-Organic Framework (MOF)

[0090] In the first step, 18 m of DMF, 7 m of distilled water, 0.2 mb of 0.5 M NFLF, and 0.3 mb of NH3are mixed in a flask under stirring. Then, 0.12 g of a MOF (designated as MOF- Fe-100) is added, followed by 5 mb of MTMS with continuous stirring. The resulting sol is transferred into a plastic container, where gelation occurs in 20 minutes. The gel is dried in a well-ventilated oven at 80-100 °C for 6 hours, yielding a highly porous, superhydrophobic silica aerogel. FIG. 4 compares the thermal conductivity of a plain silica aerogel with that of a silica aerogel monolith containing MOF, as measured by the transient plane method.Example 6: Fabrication of a Flexible Silica Aerogel Sheet with a layered structure of MOF silica aerogel on either side.

[0091] 152 grams of MOF (MIL-100-Fe) was dispersed into 0.51 liters of DMF with vigorous stirring. In another flask, silica sol was prepared by mixing under stirring 1.15 liters of DMF, 0.212 liters of distilled water, 6 ml of NH3 solution and 9 ml of 0.5 molar NH4F aqueous solutionand 0.152 liters of MTMS. A 10 mm thick non-woven e-glass fiber mat measuring 33 cm x 55 cm was taken. The MOF dispersion was sprayed using spraying gun on one side of the e-glass fiber mat, to get uniform layer of MOF on that side till the depth of approximately 2 mm. Then the fiber mat with MOF layer was flipped on other side. The silica sol was poured on the other side of the e-glass fiber mat till the remaining mat is completely filled by it. The sol was converted into gel within 10-15 mins. Thus formed layered fiber mat was then dried in well ventilated chamber. The vented vapours were passed through water cooled condenser to collect them in a liquid form, to prevent contamination as well as for reusing after purification. The complete drying resulting in a flexible silica aerogel sheet with one side layer of MOF and other side layer of silica aerogel. FIG. 5 re-presents the photograph of thus formed layered sheet.Example 7: Thermal Insulation Performance Evaluation

[0092] Thermal insulation performance is measured using a test setup based on ASTM C 335. A 3 -inch diameter steel pipe, equipped with internal heaters to maintain a uniform surface temperature (with variations <1%), is wrapped with three layers of a 10 mm thick thermal insulation sheet produced as per Example 2. For comparison, a plain e-glass fiber mat without aerogel and a silica aerogel product made by the supercritical drying process (as described in our earlier granted patent No. 305898) are also tested. The total thickness of the three layers is 30 mm, with a 0.7 mm thick aluminum foil covering the top. The test sample is 33 cm in length. FIG. 6 illustrates the schematic of the test setup and the insulation wrap configuration, while FIG. 7 presents the temperature measurements on the aluminum foil at steady pipe surface temperatures of 100, 200, 300, 400, 500, and 600 °C.ADVANTAGES OF THE PRESENT INVENTION

[0093] The present disclosure provides several advantages over conventional methods for producing silica-based aerogel products.

[0094] Versatility of Product Forms: The process produces silica aerogel either in pure form or with additives such as metal oxides, organic frameworks, or MOF particles, and can yield products in various forms including monoliths, powders, sheets, flexible sheets, or blocks of any desired size, shape, and density.

[0095] Cost Efficiency: By utilizing ambient pressure drying via normal evaporation rather than expensive supercritical drying (which operates at pressures above 70-80 bar), the disclosure significantly reduces production costs and minimizes the risks associated with high- pressure operations.

[0096] Process Simplification: The elimination of the lengthy aging, solvent exchange and surface modification steps not only speeds up the overall process but also reduces the quantity of solvent required, further lowering operational costs.

[0097] Scalability: The streamlined process is easily scalable for industrial applications, providing a cost-effective and low-risk alternative for the production of high-performance thermal insulation materials.

[0098] Overall, the disclosure offers a robust solution that combines improved product quality with reduced energy consumption and manufacturing costs, thereby enabling wider adoption in various high-temperature and energy-efficient applications.

Claims

We Claim:

1. A method for the synthesis of silica aerogel product, the method comprising the steps of:i. mixing a silica precursor with a polar aprotic solvent and water in the presence of a catalyst to form a mixture;ii. optionally, incorporating one or more infrared (IR) opacifiers into the mixture, either by in-situ generation or external addition;iii. stirring the mixture to obtain a viscous sol;iv. preparing a gel composite by casting the sol into a mould or impregnating it into the porous substrate; andv. drying the gel composite at a temperature ranging from 40°C to 150°C to obtain a silica aerogel product.

2. The method as claimed in claim 1, wherein the silica precursor comprises one or more alkoxysilanes selected from methyltrimethoxysilane (MTMS), ethyltrimethoxysilane (ETMS), methyltriethoxysilane (MTES), ethyltriethoxysilane (ETES), or a combination thereof.

3. The method as claimed in claim 1, wherein the polar aprotic solvent is N,N- Dimethylformamide (DMF).

4. The method as claimed in claim 1 , wherein the molar ratio of silica precursor to amount of DMF ranges from 1:4 to 1:25.

5. The method as claimed in claim 1, wherein the molar ratio of silica precursor to water ranges from 1:2 to 1:28.

6. The method as claimed in claim 1, wherein the catalyst is selected from acidic or basic catalysts and the catalyst concentration ranges from 5 x 105to 0.1 moles.

7. The method as claimed in claim 1, wherein the infrared opacifier is selected from titanium dioxideparticles, other metal oxides particles, metal-organic frameworks (MOFs) particles, or a combination thereof, either formed in-situ or added externally 8. The method as claimed in claim 1, wherein the infrared opacifier concentration is in the range of 0.5 to 10 wt% relative to the silica content.

9. The method as claimed in claim 1 , wherein soaking is alternately done by the dispersion of titanium dioxide particles and / or other metal oxide particles and / or MOF particlesin DMF / water / alcohol or combination of them and the silica sol in porous substrates, sequentially or alternately, in single or multiple times to form a gel composite.

10. The method as claimed in claim 1, wherein the porous substrate is selected from organic, inorganic, or hybrid porous materials including fibrous mats, woven or non- woven sheets, needle-punched fabrics, foams, or blankets.

11. The method as claimed in claim 1, wherein the drying step is carried out at atmospheric pressure and performed at a temperature in the range of 80°C to 110°C.

12. The method as claimed in claim 1, wherein the silica aerogel product obtained is either IR opacified or IR non-opacified and is in the form of monoliths, powders, sheets, or flexible sheets.

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