ES2952209T3 - Method for manufacturing a fuel pellet - Google Patents

Abstract

A method of preparing an alcogel for use as a fuel source includes the steps of hydrolyzing at least one tetraalkoxysilane with water in the presence of a solvent and a catalyst, wherein the solvent includes at least one alcohol and wherein the molar ratio (R) of water to alkoxide is greater than 4. The alcogel can then be stored in a tightly closed container to minimize evaporation of the alcohol, and used as a fuel source, for example for camping use. (Automatic translation with Google Translate, without legal value)

Description

DESCRIPTION

Method for manufacturing a fuel pellet

The present application relates to a method for preparing an inorganic alcogel for use as a fuel source and to a fuel pellet (for example, for a camping stove) that can be obtained from said method.

Fuel pellets (particularly solid fuel pellets) are used in portable solid fuel camping stoves for boiling water and preparing food and are distinct from liquid fuels such as gasoline, propane, and alcohols that require special burners. Fuel pellets find use in camping stoves for several reasons. Often, natural fuel (e.g. wood) is not available, is not flammable, or there are no tools available to process the wood into a usable form. Furthermore, a fuel source is often required that can be provided in an optimal size and that can be ignited quickly and reliably regardless of ambient conditions without the need for additional tools and appliances. Solid and gel fuels are two general categories of non-natural fuels other than liquid fuels that can be rationed or processed into fuel pellets.

The most common solid fuel pellets are based on hexamine (IUPAC name: 1,3,5,7-tetraazatricyclo[3.3.1.13,7]decane). Hexamine has a moderately high heat of combustion (30 MJ/kg), but has numerous disadvantages, including:

• Hexamine is a tertiary amine and therefore the tablets have a strong and unpleasant odor.

• Hexamine contains nitrogen and (in addition to carbon dioxide) its combustion additionally creates formaldehyde, ammonia, nitrogen oxide or hydrogen cyanide, or a combination of these compounds.

• Ingestion of hexamine can cause nausea, vomiting, gastrointestinal disorders, and kidney damage.

• When burned, fuel produces harmful vapors, requiring foods to be cooked in tightly closed containers.

• Burnt tablets leave a dark, sticky residue on the bottom of pots.

Often, so-called gel mass or gel fuel products are considered solid fuels, since they do not require special burners like liquid fuels. In some cases, these are not gels, but viscous polymeric alcohol solutions or thixotropic fluids, as described, for example, in US 4157242 A (Energy & Mineral Research). According to the IUPAc definition, a gel is a non-fluid colloidal network or polymeric network that is expanded throughout its volume by a fluid. Viscous or thixotropic liquids that are not gels by the correct definition can be called gel mass products to differentiate them from true gels. In these products, alcohol serves as fuel and other components are present as thickening agents. There are numerous different and distinct materials in a general category of "gel." These can be inorganic, organic, cross-linkable and non-cross-linkable, based on nanoparticles or polymers, based on biopolymers or synthetic polymers, hydrogels or alcogels, and these materials can be prepared with methods ranging from simple mixing to synthesis. complex chemistry, giving rise to different chemical, mechanical, ion exchange and rheological properties for different applications.

Gel mass fuels based on alkyl cellulose have been proposed as gelling agents of organic biopolymers for alcohol (see, for example, the description in GB 2500062 A) and also true gels that do not exhibit fluidity at room temperature (see in document WO 2016/055814, which describes a gel based on cross-linkable synthetic organic polymer poly(acrylic acid) that is chemically cross-linked with poly(ethylene glycol dimethacrylate). These are examples of products in which an organic biopolymer or a polymer is used synthetic organic as a thickening agent for alcohol or as a true gel structure when cross-linked. Such materials are called "organic alcogels" in which an organic polymer network (with a hydrocarbon backbone) is filled throughout its volume with a alcohol.In cases where an organic biopolymer or a synthetic organic polymer is used as a thickening or gelling agent for alcohol, the gelling agent is partially combustible or decomposes during the combustion of the gel, which, in the latter case, does not result in clean burning and may release toxic fumes.

Typically, gel mass fuels contain a significant amount of water, which is a necessary cosolvent for the polymer thickener and alcohol (typically ethanol). As the heat of vaporization of water is quite high (2.257 MJ/kg), the water additive remaining in the alcogel reduces the total heat of combustion of the alcogel since part of the energy that is released during combustion is consumed in the evaporation of water.

The advantages of organic alcogel and gel mass fuels compared to hexamine-based products are generally cleaner combustion processes, lower toxicity, and a more pleasant odor when burned. However, the viscosity of polymer solutions can depend on temperature, so gel mass fuels can become more fluid under certain conditions, which can lead to security issues. Additionally, the organic polymer gelling agent decomposes or partially burns, leaving residue and resulting in potentially harmful combustion products.

US 2006/051627 describes alcogels as fuel compositions made from methanol as a solvent and tetramethoxysilane as a gel-forming agent.

US 2008/0095928 A1 (GM Global Tech Operations Inc) describes a coating that includes a silica-based material having pendant functional groups. In a specific example, it describes a method for preparing a sol-gel from one part by weight of 1 N aqueous hydrochloric acid that is added to two parts of N-(triethoxysilylpropyl) urea (50% in methanol) plus one part of tetramethoxysilane.

This method is not suitable for use in the production of a fuel pellet for several reasons:

(i) N-(triethoxysilylpropyl) urea as a precursor contains the propylurea functional group that is hydrolytically stable and is retained within the material. If an alcogel were prepared from that precursor, combustion of that gel would result in toxic combustion fumes due to the nitrogen contained in the propylurea group. (ii) Methanol, which is toxic, is used as a solvent.

(iii) 1N aqueous HCl is used as catalyst. This results in a very low pH and very rapid hydrolysis reactions of the precursor alkoxides, but at this concentration of acid the sol is stable and no gel formation occurs and no alcogel, or gel formation time, is obtained. It would be so long that it would be commercially irrelevant.

(iv) The ratio of water to alkoxide in the specific example is 3.787. This is quite low and is likely to cause soot to form if the product is burned.

JP 2004022393 A (TDK Corp.) describes a sol that is obtained from silica and/or an alkoxysilane and an alkoxysilane containing an alkoxyphosphoryl alkyl group. A gel is obtained from the sun, and the gel is heat treated at a temperature of > 100 °C and < 300 °C. Through hydrolysis, a proton-conducting electrolyte is obtained that is stable in a wet state and durable in fuel cell operation.

Document CN 105037733 A (Wuhan University) describes a phosphate-siloxane high-temperature proton exchange membrane with hydrolytic stability and a preparation method thereof.

Other examples of fuel gel compositions are described in GB 2401274 A (Barrett), WO 2016/055814 A1 (Air Global) and GB 2031018 A (Twente).

The present invention seeks to provide a method of preparing an improved fuel pellet that seeks to mitigate at least some of the above disadvantages.

The invention is defined in the attached claims.

In a first aspect, a method is provided for preparing an alcogel for use as a fuel source, which includes the steps of hydrolyzing at least one tetraalkoxysilane with water in the presence of a solvent and a catalyst, wherein the solvent includes at least one alcohol, and wherein the molar ratio (R) of water to tetraalkoxysilane is 5 to 7. The embodiment of the present application is that the hydrolysis of at least one tetraalkoxysilane under the defined conditions results in an alcogel that can be used as a source made out of fuel. Ideally, no alkoxysilanes other than tetraalkoxysilanes are present. It has been found that mono/di/trialkoxysilanes do not form a suitable alcogel.

In preferred modalities:

• Tetraalkoxysilane can be a combination of tetraethoxysilane (TEOS) and tetrapropoxysilane (TPOS).

• The solvent/fuel is limited to ethanol, denatured ethanol, 1-propanol, 1,6-hexanediol or any combination thereof.

• The volume ratio of solvent to tetraalkoxysilane greater than 1:1, preferably greater than 2 and most preferably 2.3

• The molar ratio of water to tetraalkoxysilane is in the range of 5-7 (preferably equal to 6)

• The pH of the resulting gel is approximately 7. Regardless of whether this value is kept constant throughout the synthesis by using basic catalysis with small concentrations of catalysts or whether the hydrolysis is carried out at a low pH and then the pH is raised. pH close to 7 for gel formation (which otherwise would not happen or would take months at low pH)

• The alcogel that is prepared is enclosed in a hermetically closed container to prevent the evaporation of the alcohol and the consequent loss of combustibility in the transformation of the alcogel into dry amorphous oxide.

In general terms, the present invention describes a method of preparing an inorganic alcogel, derived from sol-gel, based on the SiO ² structure as a fuel pellet. It has been discovered that alcogels based of silica (non-hydrocarbon backbone) burn and can also be burned safely in the ambient atmosphere at a rate suitable for use as a clean-burning fuel pellet in portable solid fuel camping stoves. Advantageously, the fuel pellet of the invention does not produce smoke, harmful chemicals or odors and produces only a small amount of pure non-sticky silica in the form of airgel powder as a combustion residue that can be easily disposed of without recycling problems.

These airgel powders have excellent properties in terms of density, average pore size and specific surface area and therefore have value in their own right. This is possibly the simplest route to prepare silica airgel powders. A sol-gel hydrolytic process involving acid- or base-catalyzed hydrolysis of metal and silicon alkoxides is a well-known and established field of chemistry with decades of detailed scientific research and thousands of scientific articles. There are a large number of known precursors in the form of silicon and metal alkoxides, as well as known processing steps of dip, spin and spray coating that were established decades ago. Despite this, innovation in the field of sol-gel chemistry has been continuous and significant.

Novel developments in the field of sol-gel chemistry can be achieved by relying on these known general principles and the addition of new non-trivial processing steps to obtain structures and devices with novel and useful morphology or functionality. To illustrate this, from the general knowledge that is known of the hydrolysis and polycondensation reactions catalyzed by acids or bases of silicon alkoxides it does not trivially deduce, for example, that silica airgel can be obtained. This requires complex processing steps beyond the hydrolysis and polycondensation reactions of the silica precursors. By adding new processing steps and making new compositions, there is great freedom to use hydrolysis and polycondensation reactions under different conditions and in combination with other chemical reaction mechanisms and different processing steps to prepare new sols, nanoparticles, microspheres, foams, hydrogels and alcogels, phase separated microcomposites, fibers, xerogel monoliths/powders, etc. with a plethora of new properties and applications.

The two most common silicon compounds used in the sol-gel hydrolytic process to obtain silica-based materials are tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS), whose hydrolysis results in methanol and ethanol as production products. reaction, respectively. The hydrolysis reaction is typically accelerated by the use of an acidic or basic catalyst. TMOS is one of the few alkoxides that can undergo hydrolysis without using a catalyst. However, in the case of TEOS, two types of catalysts are commonly used: ammonium hydroxide and ammonium fluoride. The TMOS and TEOS compounds contain 4 alkoxy groups, capable of undergoing hydrolysis. Hydrolysis creates silanol groups (Si-OH) that can undergo a polycondensation reaction with other Si-OH groups, resulting in water as a reaction product:

Si-OH HO-Si ^ Si-O-Si H ² O (1)

Alternatively, silanol groups can react with alkoxy groups (RO-Si), resulting in an alcohol (ROH) as a reaction product:

Si-OH RO-Si ^ Si-O-Si ROH (2)

It depends on the initial ratio of water to alkoxide (R) whether reaction (1) or (2) dominates. If the molar ratio of water to alkoxide increases above 2, this results in more and more water as a reaction product during silanol polycondensation. At R=2, the condensation reaction that produces water is statistically minimal and the polycondensation reaction between the hydroxy and alkoxy groups dominates, resulting in alcohol as a reaction product. At R of about 4, the condensation reaction that produces water dominates. This creates 4 moles of water for every mole of silicon alkoxide that is retained within the gel.

An alcogel forms when growing chains of nanoparticles or oligomeric species reach the length of the reaction vessel, causing a loss of fluidity through polycondensation and thus immobilizing the solvent. For the preparation of aerogels from alcogel, it is necessary to perform a solvent exchange several times, after removing the solvent under supercritical conditions in order to avoid the collapse of the pore structure due to capillary forces. Under normal conditions, gel cracking occurs as a result of compressive stresses due to the existence of a meniscus at the liquid-vapor interface as solvents evaporate from the gel pores, resulting in capillary pressure. differential and gel contraction. This leads to the formation of a dense (and typically cracked) amorphous oxide material called xerogel. Therefore, the preparation of an alcogel, as well as its stable storage without evaporation of alcohol from the pores of the gel, inevitably requires that the alcogel be enclosed in a hermetically closed container. Otherwise, the alcogel is transformed into an amorphous oxide. For example, sun spin coating does not result in an alcogel as a final product, as the alcohol evaporates from the film. Any sol processing procedure that allows evaporation of alcohol does not result in an alcogel.

Therefore, in a preferred embodiment, the alcogel to be prepared is enclosed in a container to minimize the evaporation of the alcohol and the consequent loss of combustibility in the transformation of the alcogel into dry amorphous oxide. Most preferably, the container is hermetically sealed.

Many published aerogel synthesis protocols are known that describe the steps of preparation of silica aerogels by supercritical drying. The preparation of the airgel includes the synthesis of an alcogel as an intermediate step in obtaining silica airgel. This stage is followed by a time-consuming solvent exchange and supercritical drying of the wet gel to obtain an airgel monolith.

An available example of a prior art airgel preparation protocol for TMOS as a precursor is published online at http://www.aerogel.org/?p=1406 as well as a somewhat analogous TEOS-based airgel synthesis protocol. as a precursor at https://www.aerogel.org/?p=1027. The synthesis was reproduced based on these protocols to determine its potential relevance in the context of the present invention. The TMOS airgel preparation protocol uses an R value of 4.146 with 0.0386 M NH ⁴ OH as catalyst and alcohol to alkoxide volume ratio of 2, while the TEOS-based airgel preparation protocol uses an R value of 17.97 with 0.068 M NH ⁴ OH and 0.0203 M NH ⁴ F in aqueous solution as catalysts and volume ratio of alcohol to alkoxide 4.4. The resulting heat of combustion values of these alcohols are approximately 17 MJ/kg and 20.9 MJ/kg respectively. Both values are relatively low. For clarity, it is important to note that these gels are intermediate products for the preparation of aerogels and no prior information is available on their combustion properties. The combustibility of sol-gel derived alcogels is by no means trivially obvious and none of this has been described or theorized in the scientific or patent literature. Furthermore, silica and some other inorganic oxides (e.g., Sb2O ₃ ) are known as flame retardants (DOI: 10.1002/1099-1018(200011 /12) ₂ 4:6<277::AID-FAM746>3.0.CO ;2-A and DOI: 10.1007/978-94-011 -5862-6_32). Additionally, by obtaining an intermediate alcogel in the case of airgel preparation, there is no concern about the toxicity of the solvent, the heat of combustion, the resistance of the gel, the influence of alcohols and catalysts on the possible formation of soot or on the toxicity of the respective combustion products. Instead, the optimization of the properties of the resulting airgel is relevant, in terms of density, homogeneity, pore size distribution and specific surface area obtained. The alcohol used in the hydrolytic sol-gel process serves as a cosolvent for the alkoxide and water and is not treated as a fuel.

The strength of the alcogels mentioned above is 10.62 N and 5.22 N respectively. The strength of the gel samples was determined by measuring the force to failure by pressing a metal cylinder into an alcogel perpendicular to the axis of the cylindrical gel sample. This simulates the real-life situation of lifting the gel sample with your fingers. The gel samples were geometrically identical and 27.3 mm in diameter. Resistance below 5N indicates very low resistance, so it is easy to break the gel just by trying to lift it with your fingers, and resistance above 25N is very good. Sol-gel derived alcogels do not reach their final strength immediately after gelation and, in some cases, weeks or months may be required to reach final strength at room temperature. All samples were aged for at least three days, sufficient for relative comparison.

It is shown that, in the case of a narrow, carefully selected and optimized composition range, a clean-burning (i.e., no soot formation, no toxic fumes) inorganic alcogel with adequate mechanical properties and with heat value of combustion to the level relevant for practical applications. The alkoxides that can be used as precursors in the preparation of sol-gel derived inorganic alcogels that can be used as clean-burning fuel pellets are strongly limited by their chemical structure. Only tetraalkoxysilanes can be used, which contain 4 alkoxy groups (Si-OR) attached to the central silicon atom. Dialkoxysilanes and trialkoxysilanes or any silicon alkoxide in general, containing a hydrolytically stable carbon-silicon bond, are not suitable for several reasons - these alkoxides exhibit very low reaction rates and require high catalyst concentrations, almost universally inducing sols and gels. which are phase separated, result in very weak and poorly cross-linked gels, result in dirty combustion and dirty combustion residues and cause the emanation of silica nanoparticles and microparticles due to the incomplete formation of the 3D structure of SiO ² a molecular level, which represents a serious danger to health. Furthermore, the choice of tetra-alkoxysilanes is also limited to compounds containing only alkoxy groups based on linear or branched hydrocarbons, since only these result in the formation of clean-burning alcohol upon hydrolysis. Only when the requirements mentioned above are met is an inorganic gel structure non-combustible and therefore fundamentally incapable of releasing toxic combustion fumes and the choice of alkoxides that can be used in an alcogel fuel tablet preparation is limited to TMOS, TEOS and tetrapropoxysilane (TPOS). All others are ruled out due to the fundamental reasons mentioned above and of these three, TMOS has very limited potential use.

Therefore, in a preferred embodiment, the tetraalkoxysilane may be a combination of TEOS and TPOS.

As methanol is poisonous, prior art TMOS-based alcogels as intermediates in obtaining aerogels and with no reported own uses, are not suitable for use as fuel pellets, since the fuel pellets come into contact with skin and possible inhalation of methanol vapors represents a serious health hazard. Furthermore, the TMOS/methanol combination produces a low heat of combustion.

TEOS is generally preferred over TMOS because it produces ethanol as a product of the hydrolysis reaction. On the other hand, the prior art airgel preparation protocol that is based on TEOS uses a large excess of water and produces a mechanically weak alcogel. As the hydrolysis of TEOS is very slow compared to TMOS, the hydrolysis is incomplete in gel formation even in the case of two different catalysts and this leads to phase separation. In the preparation of aerogels, this is solved by using a very high concentration of solvent and a large excess of water. This allows phase separation to be avoided, but a very high solvent concentration results in a weak gel.

Additionally, there is an aspect in the preparation of alcogel by the sol-gel method that has never been considered since there has been no need or knowledge to address that issue scientifically or technologically. Specifically, the combustion of sol-gel derived alcogels produces soot when the flame comes into contact with a cold cooking vessel when using aliphatic alcohols higher than methanol. The amount of soot increases progressively as higher monohydroxyaliphatic alcohols are used, as well as in the case of a higher concentration of basic catalysts, leading to higher pH values, far from 7.

Therefore, it is not possible to apply simple logic to use higher molecular weight alcohols (e.g. ethanol, propanol, etc.) in silica alcohols in order to increase the heat of combustion. These alcohols burn cleanly as liquids, but in silica alcohol the alcohol is contained in micro- and mesopores where the high pressures and temperatures during combustion (as well as the possible catalytic effect of the SO ² skeleton itself) partially convert the alcohols in ethers and other compounds with high flash points, causing incomplete combustion and the formation of soot when the flame cools in a cooking vessel. The same applies to the R value, which, in terms of maximum heat of combustion, should be 2 (since the polycondensation mechanism dominates according to equation #2), but an R value of 2 is not optimal in terms of soot formation.

Therefore, the composition of silica alcohols for use as fuel pellets is a balance, on the one hand, between the toxicity of TMOS/methanol and, on the other hand, with the formation of soot in the case of aliphatic alcohols. higher, it additionally influences the concentration of water and the catalyst both in the heat of combustion and in the formation of soot in a complex way. Due to the strong soot formation, the TEOS-based alcogel with 1-propanol as added fuel can be used as a lighter, for example to light a charcoal or wood fire, but is not suitable for use as a lighter. fuel in camping stoves. However, a preferred composition window can be found that produces silica alcohol with properties suitable for use as a fuel pellet in a camping stove. This is achieved with TEOS-based alcogel, with ethanol as fuel, with a molar ratio of water to tetraalkoxysilane of 5-6 and a reduced amount of catalyst to bring the final pH closer to 7. As the phase separation does not show an influence negative in the alcogel fuel pellet, it is possible to work in a markedly different composition range compared to the TEOS-based airgel preparation. Denatured ethanol, containing denatonium benzoate as a denaturant, can be used as well as 96% ethanol taking into account the water content to obtain a correct molar ratio of water to alkoxide.

A pH value of around 7 is preferred for minimal soot formation during combustion. This requirement is very unconventional in the context of typical sol-gel process parameters that typically require a pH value far from neutrality for hydrolysis to occur. This is especially the case with TEOS. This poses serious limitations to the strategy of using catalysts and their concentrations. There are two possible options to meet this requirement: 1) The use of basic conditions with the use of NaOH, NH ⁴ OH or other base catalyst in a small concentration, sufficient to achieve gel formation on a commercially relevant time scale and in amounts small enough for minimal soot formation or 2) use a low pH (e.g. HCl as a catalyst) for rapid hydrolysis and raise the pH to about 7 before packaging for rapid gel formation by adding a strong base in water solution about an equimolar amount to the acid. The latter strategy thus involves adding water in two separate parts.

Therefore, in one embodiment, NH ⁴ OH and NH ⁴ F are used as catalysts for the preparation of TEOS-based alcohols with a preferred molarity in aqueous solution in the range of 0.005-0.05 M and 0.003-0. 03 M (most preferably 0.0067 M and 0.00327 M) for NH ⁴ OH and NH ⁴ F respectively. In the second embodiment, the tetraalkoxysilane is hydrolyzed with HCl (preferably to an R value of 4 with 0.02 M HCl) and subsequently an aqueous solution of NaOH is added to raise the pH to about 7 (preferably to the final R value of 6 with 0.04 M NaOH) to achieve gel formation.

In one embodiment, the volume ratio of solvent to tetraalkoxysilane is greater than 1:1, preferably greater than 2 and most preferably 2.3. The solvent may include ethanol, denatured ethanol, 1-propanol, 1,6-hexanediol or any combination thereof and is preferably denatured ethanol.

According to a second aspect of the present invention, there is provided a fuel pellet for a camping stove that includes an alcogel obtainable by a method as defined. above and which is contained in a container. For example, the container may be a paper-based wrapper that is impermeable to alcohols or a layer of paraffin around the alcogel.

According to a third aspect of the invention, a method of generating heat is provided that includes the step of burning an alcogel as prepared by the above method or by removing a fuel pellet from the container as defined above and then burning it.

As well as generating heat, this method produces silica airgel powder as a by-product as a combustion residue. Therefore, this is a technically simple way to produce this product.

In an alternative aspect of the invention, a method is provided for preparing an alcogel for use as a fuel source, which includes the steps of hydrolyzing at least one silicon alkoxide with water in the presence of a solvent, wherein the solvent includes minus one alcohol, and where the molar ratio (R) of water to alkoxide is less than 4.

Various preferred embodiments of the invention and counter-examples (i.e., outside the scope of the invention) will now be described.

Example 1

27.4 ml of TEOS are added to 63 ml of denatured ethanol in a beaker and mixed for 1 minute. Consequently, 13.35 mL of water containing NH ⁴ OH and NH ⁴ F as catalysts (0.0067 molar NH ⁴ OH and 0.00333 molar NH ⁴ F respectively) are added and mixed for 10 minutes. This results in a water to alkoxide molar ratio of 6. The sol is then poured into polypropylene containers and hermetically sealed where the gel formation takes place. The gelation time is approximately 2 hours and 10 min at 22 °C and the gel strength increases and reaches a plateau in ~20 days at 22 °C. Silica alcohol is obtained with a heat of combustion value of 23.4 MJ/kg. Gel formation and aging can be carried out at higher temperatures up to the boiling point of alcohol to exponentially decrease the gelation time and accelerate aging without any drawbacks.

Example 2

27.4 ml of TEOS are added to 63 ml of denatured ethanol in a beaker and mixed for 1 minute. Consequently, 13.35 mL of water containing NH ⁴ OH and NH ⁴ F as catalysts (0.00167 molar of NH ⁴ OH and 0.000833 molar of NH ⁴ F respectively) are added and mixed for 10 minutes. This results in a water to alkoxide molar ratio of 6. The sol is then poured into polypropylene containers and hermetically sealed where the gel formation takes place. The gel time is approximately 57 h at 22 °C and ~7.5 h at 50 °C, which represents an approximate upper limit at which the process is commercially relevant in terms of gel time resulting from the concentration that It is used as catalysts. The resulting silica alcohol has a heat of combustion value of 23.4 MJ/kg, identical to that of Example 1.

Example 3

27.4 ml of TEOS are added to 63 ml of n-propanol in a beaker and mixed for 1 minute. Consequently, 13.25 mL of water containing NH ⁴ OH and NH ⁴ F as catalysts (0.0067 molar of NH ⁴ OH and 0.00333 molar of NH ⁴ F respectively) are added and mixed for 10 minutes. This results in a water to alkoxide molar ratio of 6. The sol is then poured into polypropylene containers and hermetically sealed where the gel formation takes place. The gelation time is ~8 hours at 22 °C and the gel strength continues to increase for more than 7 days. This results in a slightly cloudy silica alcogel due to phase separation during gel formation. An alcogel with a heat of combustion value of 27.5 MJ/kg is obtained.

Example 4

21.92 ml of TEOS and 3.65 ml of TMOS (molar ratio 9/1) are added to 63 ml of denatured ethanol in a beaker and mixed for 1 minute. Consequently, 13.35 mL of water containing NH ⁴ OH and NH ⁴ F as catalysts (0.0067 molar of NH ⁴ OH and 0.00333 molar of NH ⁴ F respectively) are added and mixed for 10 min. This results in a water to alkoxide molar ratio of 6. The sol is then poured into polypropylene containers and hermetically sealed where the gel formation takes place. Gelation time is approximately 3 h and 10 min at 22 °C.

Example 5

27.4 mL of TEOS is added to 30 mL of 96% ethanol in a beaker and mixed for 1 minute. Consequently, 7.418 mL of water containing 0.05 M HCl as catalyst are added and mixed for 30 min. Consequently, 3.709 mL of a 0.1 M aqueous NaOH solution are added, mixed for 3 min, and then Add 33 mL of 96% ethanol. This results in a water to alkoxide molar ratio of 6.13. The sol is then poured into polypropylene containers and hermetically sealed where the gel formation takes place. Gelation time is ~16 hours at 22°C and forms an optically clear, mechanically strong gel that shrinks ~25% by volume during aging, the equivalent amount of solvent is expelled, typical of acid-catalyzed sols. . The resulting gel strength is >2-3 times higher than that of those prepared using the base catalyst, reaching 30N. The strength of the gel samples was determined by measuring the force to failure by pressing a metal cylinder in an alcogel perpendicular to the axis of the cylindrical gel sample that was placed horizontally on the flat surface. This simulates the real-life situation of lifting the gel sample with your fingers. The gel sample was 27.3 mm in diameter.

Example 6

27.4 ml of TEOS are added to 30 ml of denatured ethanol in a beaker and mixed for 1 minute. Consequently, 11.043 mL of water (effectively R=5) containing 0.02 M HCl as catalyst are added and mixed for 24 min. Accordingly, 0.552 ml of 0.4 M NaOH aqueous solution is added dropwise, mixed for 3 min, and then 33 ml of denatured ethanol is added. The sol is then poured into polypropylene containers and hermetically sealed where the gel formation takes place. Gelation time is ~10 hours at 22°C, forming an optically clear and mechanically strong gel that shrinks ~18% by volume during aging.

Example 7

178.45 ml of tetrapropoxysilane (TPOS) is added to 315 ml of denatured ethanol in a beaker and mixed for 1 minute. Consequently, 66.76 mL of water containing NH ⁴ OH and NH ⁴ F as catalysts (0.0067 molar of NH ⁴ OH and 0.00333 molar of NH ⁴ F respectively) are added and mixed for 10 minutes. This results in a water to alkoxide molar ratio of 6. The sol is then poured into polypropylene containers and hermetically sealed where the gel formation takes place.

Example 8

178.45 ml of tetrapropoxysilane (TPOS) is added to 315 ml of denatured ethanol in a beaker and mixed for 1 minute. Consequently, 55.63 mL of water containing NH ⁴ OH and NH ⁴ F as catalysts (0.0067 molar NH ⁴ OH and 0.00327 molar NH ⁴ F respectively) are added and mixed for 10 minutes. This results in a water to alkoxide molar ratio of 5. The sol is then poured into polypropylene containers and hermetically sealed where the gel formation takes place.

Counter-example 1

24.66 ml of TEOS and 2.74 ml of methyltriethoxysilane are added to 63 ml of denatured ethanol in a glass beaker and mixed for 1 minute. Consequently, 11 mL of water containing NH ⁴ OH and NH ⁴ F as catalysts (0.0067 molar NH ⁴ OH and 0.00333 molar NH ⁴ F respectively) are added and mixed for 10 minutes. This results in a water to alkoxide molar ratio of 4.89. Then the sun is poured into a polypropylene container. No gel forms at 22 °C for more than 2 months and at 50 °C a phase-separated suspension forms that does not maintain its shape. By increasing the R value, the tendency for phase separation increases further.

Counter-example 2

24.66 ml of TEOS and 2.74 ml of methyltriethoxysilane are added to 30 ml of denatured ethanol in a beaker and mixed for 1 minute. Consequently, 7.333 mL of water containing HCl as catalyst (0.04 molar) is added and mixed for 20 minutes, followed by the addition of 3.666 mL of a solution of 0.08 M NaOH in water, mixing continues. for 5 min and 33 mL of ethanol are added. This results in a total water to alkoxide molar ratio of 4.89. Then the sun is poured into a polypropylene container. No gel forms at 22°C for more than 2 months and a clear, stable sol is obtained without an appreciable increase in viscosity. Ethanol is added in two parts to increase the rate of hydrolysis; Otherwise, the dilution effect would strongly prevent hydrolysis from occurring, evident from the exothermic nature of the reaction.

Counter-example 3

2.222 mL of phenyltrimethoxysilane is added to 6.312 mL of denatured ethanol in a glass vial and mixed for 1 minute. Consequently, 1.26 mL of water containing NH ⁴ OH and NH ⁴ F as catalysts (0.0067 molar NH ⁴ OH and 0.00333 molar NH ⁴ F respectively) are added and mixed for 20 minutes. This results in a water to alkoxide molar ratio of 6. An opaque sol is obtained after adding water and after 2 days at 22 °C phase-separated waxy material forms at the bottom of the closed reaction vessel.

The same synthesis with R value of 4 and 0.04 M NH ⁴ OH and 0.02 M NH ⁴ F produces a clear sol after 2 minutes of mixture after adding water, but in ~1.5 hours the sol becomes opaque and a similar phase-separated waxy material forms at the bottom of the reaction vessel after 2 days, compared to the R value of 6.

Counter-example 4

1.6 ml of vinyltriethoxy is added to 6.312 ml of denatured ethanol in a glass vial and mixed for 1 minute. Consequently, 0.84 mL of water containing NH ⁴ OH and NH ⁴ F as catalysts (0.04 molar of NH ⁴ OH and 0.02 molar of NH ⁴ F respectively) are added and mixed. This results in a molar ratio of water to alkoxide of 4. In 15 min a transparent sol is obtained, but in 2 days a suspension of separated phases is formed and no alcogel is obtained.

The counter-examples demonstrate that a gel cannot be formed with a trialkoxysilane in a composition range that is necessary to obtain an alcogel fuel pellet.

Furthermore, all optional and preferred features and modifications of the described embodiments and dependent claims may be used in all aspects of the invention taught in the present description. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments, are combinable and interchangeable with each other.

The present application claims priority to the descriptions in UK patent application number 1913244.8

Claims (12)

1. A method for preparing an alcogel for use as a fuel source, which includes the steps of hydrolyzing at least one tetraalkoxysilane with water in the presence of a solvent and a catalyst at a pH above or below pH7, wherein the tetraalkoxysilane is tetraethoxysilane (TEOS), tetrapropoxysilane (TPOS), or any combination thereof, wherein the solvent includes ethanol, denatured ethanol, 1-propanol, 1,6-hexanediol or any combination thereof, and wherein the molar ratio (R) of water to tetraalkoxysilane is 5 to 7, the step of controlling the pH of the resulting alcogel to approximately 7 is included. 2. A method as claimed in claim 1, wherein R is about 6. 3. A method as claimed in any preceding claim, wherein the ratio of solvent to tetraalkoxysilane is greater than 1:1. 4. A method as claimed in any preceding claim, wherein the ratio of solvent to tetraalkoxysilane is greater than 2:1. 5. A method as claimed in any preceding claim, wherein the catalyst is an acidic or basic catalyst. 6. A method as claimed in claim 5, wherein the catalyst is a combination of ammonium hydroxide and ammonium fluoride. 7. A method as claimed in any preceding claim, which includes the additional step of storing the alcogel in a hermetically closed container in order to minimize the evaporation of said at least one alcohol. 8. A method as claimed in any preceding claim, wherein the pH is controlled through the use of acidic and/or basic catalysts. 9. A method as claimed in any preceding claim, wherein the only reactant that is hydrolyzed is a tetraalkoxysilane. 10. A fuel pellet for a camping stove that includes an alcogel that can be obtained by a method as claimed in any preceding claim and that is contained in a hermetically closed container. 11. A fuel pellet as claimed in claim 10, wherein the container is a paper-based wrapper or is a layer of paraffin around the alcogel. 12. A method of generating heat that includes the step of burning an alcogel prepared by a method as claimed in any of claims 1 to 9 or removing a fuel pellet from the container according to claim 10 or 11 and then burning it.