Organic polyhexahydrotriazine (PHT) aerogels, use thereof, and methods of preparing and recycling these

Abstract

The present invention relates to novel organic aerogels of polyhexahydrotriazines (PHTs) prepared from formaldehyde and one or more aromatic polyfunctional amines, said aerogel having a specific surface area of more than 50 m2 / gram, a porosity of more than 80 %, and a bulk density of less than 300 kg / m3. The present invention moreover relates to a method of preparing these aerogels and to the use of such aerogels. Moreover, the invention relates to the use of a polyhexahydrotriazine (PHT) prepared from formaldehyde and one or more aromatic polyfunctional amines for the preparation of aerogels and to a method of recycling the aerogels.

Description

[0001] TITLE Organic polyhexahydrotriazine (PHT) aerogels, use thereof, and methods of preparing and recycling these.

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to an aerogel of a polyhexahydrotriazine (PHT), a method of preparing such aerogels and the use thereof, to a method of recycling said aerogels and to the use of a polyhexahydrotriazine (PHT) for the preparation of an aerogel.

[0004] BACKGROUND

[0005] The excessive consumption of energy by humans has surged its highest levels so far, with projections indicating a further 14 % increase by the year 2050. Notably, the heating and cooling of residential and office spaces, amounts to approximately 48 % of the global building energy consumption. The associated environmental issue, such as unprecedented CO2emission and global warming alerts an urgent need to mitigate the problem. Hence, effective thermal insulation techniques are aspired to curb the continuous rise in energy consumption. Aerogels, listed among the top ten emerging technologies of chemistry by IUPAC, are considered suitable solutions for effective thermal insulation.

[0006] Organic aerogels, with their unique features such as low density, large specific surface area, and high porosity, exhibit very low thermal conductivity values. They outperform current commercially available products, such as polystyrene foams, polyurethane foams, and glass / rock wools in terms of thermal insulation performance.

[0007] Organic aerogels are thus an intriguing class of highly porous and ultralight materials which have found widespread applications in thermal insulation, energy storage, and chemical absorption. Organic aerogels are typically made up of crosslinked polymers, which offer high versatility in chemical design. Prior art organic aerogels are fully cross-linked polymeric networks, that pose environmental concerns as they are typically made from fossil-based feedstock and the recycling back to their original monomers is virtually impossible. Their covalent bonds are considered irreversible and non-recyclable, and thus they pose environmental concerns at their end-of-life cycle. Hence, developing strategies to recycle these materials without compromising their performance represents a formidable challenge.

[0008] Examples of known, commercially available organic aerogels are the SLENTITE® aerogels from BASF, as for example disclosed in WO2015 / 144675 A1 and WO20 17 / 050679 A1.

[0009] To overcome these obstacles poly-imine aerogels were prepared and published by the present inventors in for example Wang et al., Advanced Materials, 2022, 220903 (title: closed-loop recyclable high-performance poly-imine aerogels derived from bio-based resources) and Wang et al, Advanced Functional Materials, 2024, 2314447 (title: aerogel-to-sol-to-aerogel (ASA) process for recycling, repairing, reprogramming of high-performance organic aerogels). Even though these poly-imines-based aerogels solve several of the drawbacks of prior art organic aerogels, there remain several points of improvement such as a system that can use less complex monomers, less complex overall chemistry, that is more easy to recycle and that has an intrinsic hydrophobicity and hence does not require an additional hydrophobization step, which is required for the commercially available aerogels cited above as well as for the poly- imine-based aerogels.

[0010] Polyhexahydrotriazine (PHT) chemistry is currently used in designing high performance recyclable polymer thermoset materials. For example US 9.243.107 B2 discloses a general synthesis of PHT in the form of compact thermosets. There is no indication given on the formation of aerogels or how to achieve this, let alone reversible aerogels. Prior art methods for preparing degradable polyhemiaminal aerogels, for example as disclosed in Li et al., Ind. Eng. Chem. Res. 2017, 56 (24), 6508-6514, require excessive amounts of paraformaldehyde and the obtained structures (hemiaminal) formed in this process lead to decreased thermal and mechanical properties of the products. It is clear that a new design strategy is required to create recyclable high-performance aerogels in a more efficient and sustainable manner. OBJECTS

[0011] It is an object of the present invention to provide an improved organic aerogel. In particular, it is an object to provide an organic aerogel that can be prepared from easily accessible (e.g., commercially available) monomers. In addition, it is an object to provide organic aerogel that can be recycled easily, in other words that can be depolymerized under a specific set of conditions to achieve recyclability of the monomers used. In addition, it is a specific object to provide an organic aerogel that has intrinsic hydrophobicity.

[0012] STATEMENT OF THE INVENTION

[0013] In a first aspect, the present invention relates to an aerogel of a polyhexahydrotriazine (PHT) prepared from formaldehyde and one or more aromatic polyfunctional amines, said aerogel having a specific surface area of more than 50 m2 / gram, a porosity of more than 80 %, and a bulk density of less than 300 kg / m3.

[0014] In a second aspect, the present invention relates to a method of preparing an aerogel according to (or as defined in) any one of the preceding claims, said method comprising the steps of: a) forming a solution of the formaldehyde in a solvent and forming a solution of the one or more aromatic polyfunctional amines in a solvent; b) mixing the solutions in a ratio such that there is a substantially equimolar ratio between formaldehyde molecules and amino groups of the polyfunctional amines, and allowing gelation for a certain period of time for an organogel to be formed; c) optionally, the organogel formed in step b) is allowed to age for a certain period of time for a cured organogel to be formed d) optionally, a solvent exchange is carried out to exchange the solvent(s) of step a) for a second solvent; e) supercritical drying, preferably by CO2, of the organogel formed in step b), c), or d) to obtain the aerogel.

[0015] In a third aspect, the invention relates to the use of a polyhexahydrotriazine (PHT) prepared from formaldehyde and one or more aromatic polyfunctional amines for the preparation of aerogels having a specific surface area of more than 50 m2 / gram, a porosity of more than 80 %, and a bulk density of less than 300 kg / m3.

[0016] In a fourth aspect, the invention relates to a method of recycling the aerogel according to the invention, comprising treating the aerogel with an aqueous acid solution having a pH of less than 4, preferably less than 2, more preferably less than 1 , to decompose the polyhexahydrotriazine (PHT) into the one or more aromatic polyfunctional amines.

[0017] In a fifth aspect, the invention relates to a method of recycling the aerogel according to the invention said method comprising the steps of: i) treating the aerogel with a solution comprising an excess, such as at least 1.1 or at least 1.2 or at least 1.4, preferably ranging from 1.5 to 6, of one or more aromatic polyfunctional amines to depolymerize the polyhexahydrotriazine (PHT) into a mixture of soluble oligomers having amino- end-groups; and ii) adding formaldehyde to the mixture obtained in step i) to form a polyhexhydrotriazine; and iii) optionally, the organogel formed in step b) is allowed to age for a certain period of time for a cured organogel to be formed; and iv) optionally, a solvent exchange is carried out to exchange the solvent(s) of step a) for a second solvent; and v) supercritical drying, preferably by CO2, of the organogel formed in step ii), iii), or iv) to obtain the aerogel.

[0018] In a sixth aspect, the invention relates to the use of an aerogel according to the invention or prepared according to the invention for thermal insulation, energy storage, and chemical absorption.

[0019] Corresponding embodiments of the aerogel are also applicable for the methods and use according to the present invention.

[0020] The present inventors have invented and prepared a set of polyhexahydrotriazine (PHT) aerogels containing reversible chemical bonds which can selectively be cleaved on demand. The resulting PHT aerogels were found to exhibit low shrinkage, high porosity, large specific surface area, low thermal conductivity, as well as pronounced thermal stability and intrinsic hydrophobicity. More importantly, the aerogels show excellent recyclability under acidic conditions with high monomer recovery yields and purities. This approach allows for preparation of fresh aerogels from the retrieved building blocks, thus demonstrating efficient closed-loop recycling. Furthermore, aerogels according to the invention can be depolymerized using an excess of aromatic amine into soluble oligomers which can be reused for the synthesis of fresh aerogels. These high-performance and recyclable polyhexahydrotriazine aerogels allow for the preparation of advanced and sustainable super-insulating materials.

[0021] DETAILED DESCRIPTION

[0022] The present invention is elucidated below with a detailed description.

[0023] LIST OF DEFINITIONS

[0024] The following definitions are used in the present description and claims to define the stated subject matter. Other terms not cited below are meant to have the generally accepted meaning in the field.

[0025] “aerogel” as used in the present description means: a material derived from a gel that has been dried with a moderate impact (shrinkage) on the solid network, regardless of the drying approach used. The pore liquid is replaced by a gas. An aerogel is able to withstands the drying without significant changes in the gel's structure. The most suitable drying technique is supercritical drying using supercritical CO2. Another drying technique for gels, ambient pressure drying during which the capillary tensions cause an irreversible shrinkage of the gel and a collapse of its pores resulting in xerogels. Yet another drying technique for gels, freeze-drying leads to changes to the structure as the growth of solvent crystals (most commonly water) leads to the fragmentation of the samples, resulting in cryogels.

[0026] “organogel” as used in the present description means: a type of gel in which a liquid is immobilized by a three-dimensional network formed by the selfassembly of low molecular weight compounds or polymers.

[0027] “recycling" as used in the present description means: recycling of an aerogel material back into the same starting materials (closed-loop recycling), or into soluble oligomers for use in the preparation of the same or different aerogel materials. This system can create a continuous cycle where the material can be repeatedly reused, reducing the need for new raw materials and minimizing waste. “closed-loop recycling” as used in the present description means: recycling of a material back into the same starting materials, original virgin aerogels, without significant loss of material quality.

[0028] “Polyhexahydrotriazine (PHT)” as used in the present description means: a type of polymer that is characterized by its repeating units containing hexahydrotriazine rings. These rings are six-membered heterocyclic structures composed of three carbon atoms and three nitrogen atoms in a 1 ,3,5-hexahydro-1 ,3,5- triazine configuration.

[0029] “alkyl” as used in the present description means: a group having the general formula CxH2x+1, such as methyl, ethyl, propyl (e.g. n-propyl), butyl (e.g. n- butyl), pentyl, hexyl, heptyl, octyl, nonyl, decyl including all isomers.

[0030] Aerogel

[0031] In a first aspect, the present invention relates to an aerogel of a polyhexahydrotriazine (PHT) prepared from formaldehyde and one or more aromatic polyfunctional amines, said aerogel having a specific surface area of more than 50 m2 / gram, a porosity of more than 80 %, and a bulk density of less than 300 kg / m3.

[0032] The present aerogels have a delicate nanostructure as verified by the low bulk density, high porosity, large surface area and low thermal conductivity. These aerogels also show great thermal resistance and mechanical properties. In addition, some of these aerogels also have intrinsic hydrophobicity without prior modifications.

[0033] The preparation of a polymeric PHT network involve the condensation reaction between aromatic polyfunctional amines (e.g., diamines) and (para)formaldehyde.

[0034] Formaldehyde

[0035] The formaldehyde used in the present aerogel and method may be formaldehyde or paraformaldehyde or any other source of formaldehyde.

[0036] Aromatic polyfunctional amine

[0037] In an embodiment, the aromatic polyfunctional amine is an aromatic amine having 2, 3, 4, 5, 6 or even more amine group and / or wherein the aromatic polyfunctional amine comprises at least 1 , preferably at least 2, at least 3, at least 4 or even more aromatic rings.

[0038] In an embodiment, the aromatic polyfunctional amine is selected from amines from the following formulas: wherein Xi is selected from the group consisting of -O-, -S-, -CH2-, -C(=O)-, -(CH2)n-, -(CH=CH)m-, -(C=C)p-, -C(=CH2)-, -C(R1)(R2)-, -C(R3)(R4)CH2CH(R5)-, -D7- -(C(R6)=C(R7)r-, -NR8-, -(p- or m- or o-phenyl)q-, -N-(p-phenyl), -D5-p-phenyl-D5-, -O- D6-O- , -S-D6-S-, -S(=O)2-D6-S(=O)2-, -(CH2)pNHC(=O)CH2CH2C(=O)NH(CH2)p- , - CH2NHC(=O)-p-phenyl-C(=O)NHCH2-,-CH2NHC(=O)-p-phenyl-CH2-p-phenyl- C(=O)NHCH2-, -CH2NHC(=O)NHCH2-, cyclohexyl, -C(CH3)2-, -CH(phenyl)-, - N(phenyl)- or selected from any of the structures below:

[0039] Including all possible constitutional isomers

[0040] wherein A1-A4, B1-B4, C1-C4, D1-D4, E1-E4, and F1-F4are each independently selected from H, or C1-C6 alkyl or C1-C6 alkoxy and -CF3, preferably H or methyl or ethyl, more preferably H; or wherein two A2, two B1, two C1, two D1, two E1, or two F1moieties are connected via a bond or via a methylene spacer; wherein R1and R2are each independently selected from H, methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, -CF3, cyclohexyl, phenyl, or wherein R1and R2and the carbon atom to which they are attached form a cycloalkyl, such as a cyclobutyl, cyclopentyl, cyclohexyl or cyclohexyl group; preferably R1and R2are H or R1is methyl and R2is ethyl; wherein R3, R4, R6and R7are each independently selected from the group consisting of H, or C1-C6 alkyl, preferably R3= R4=methyl and R6= R7= methyl; wherein R5is a allyl group (-CH2-CH=CH2-) or an C1-C6 alkyl group; wherein D5is selected from the group consisting of -CH2-, -C(CH3)2-, -C(=O)-, -C(phenyl)2- , -O-, or the structure below; wherein D6is selected from the group consisting of -CH2C(CH3)2CH2-, -p- phenyl-, ethyl, n-propyl, n-butyl, -p-phenyl-D7-p-phenyl; wherein D7is selected from the group consisting of -CH2-, -C(CH2)2-, -C(=O)-, -O-, -S-, -C(CF3)2-, -S(=O)2-, -CH(cyclohexyl)-or the structure below: wherein n is an integer between 1 and 10, preferably 2, 3, or 6; wherein m is an integer between 1 and 10, preferably 1 or 2; wherein p is 0 or 1 ; wherein q is an integer between 1 and 10, preferably 1 ; wherein r is an integer between 1 and 10, preferably 1 ; wherein R8is selected from the group consisting of H or phenyl; wherein Y1 is selected from the group consisting of -P(-)2, -N(-)2, -CH(-)2, - C(OH)(-)2, 1 ,3,5-phenyl, 1 ,2,4-phenyl, 1 ,3,5-triazine, (1 ,3,5-Triazine-2,4,6- triyl)trisoxy I , benzene ring that is substituted on 4 of the 6 positions wherein Z1 is selected from the group consisting of C, CH2=CH2, or from the below structures: wherein W1 is selected from the group consisting of a benzene ring that is substituted on 5 of the 6 positions, (-)2-CH-C(-)3wherein Vi is selected from the group consisting of a benzene ring, or from the below structures: or wherein the aromatic amine is a compound according to the formulas VII-

[0041] XXIX below or the skeletal isomers thereof:

[0042] wherein R1- R4are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and C1-C6 alkoxy, and wherein n and m are each independently 0 or an integer between 1 and 20; wherein R1- R8are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and C1-C6 alkoxy, and wherein n and m are each independently 0 or an integer between 1 and 20; and wherein X is selected from wherein R1- R4are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and or C1-C6 alkoxy and wherein n is 0 or an integer between 1 and 20; wherein R1- R8are each independently selected from the group consisting of H, and / or C1-C6 alkyl, and / or C1-C6 alkoxy, and wherein n is 0 or an integer between 1 and 20; and wherein X is as disclosed above for Formula VIII wherein R1- R4are each independently selected from the group consisting of H, or / and

[0043] C1-C6 alkyl, or / and C1-C6 alkoxy, and wherein n is 0 or an integer between 1 and 20; wherein R1- R8are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and C1-C6 alkoxy, and wherein n is 0 or an integer between 1 and 20; and wherein X is as disclosed above for Formula VIII

[0044] wherein R1- R4are each independently selected from the group consisting of H, or / and

[0045] C1-C6 alkyl, or / and C1-C6 alkoxy, and wherein n is 0 or an integer between 1 and 20; and wherein X is as disclosed above for Formula VIII wherein R1- R8are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and C1-C6 alkoxy, and wherein n is 0 or an integer between 1 and 20; and wherein

[0046] wherein R1- R4are each independently selected from the group consisting of H, or / and

[0047] C1-C6 alkyl, or / and C1-C6 alkoxy, and wherein n is 0 or an integer between 1 and 20; wherein R1- R8are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and C1-C6 alkoxy, and wherein n is 0 or an integer between 1 and 20; and wherein

[0048] wherein R1- R4are each independently selected from the group consisting of H, or / and

[0049] C1-C6 alkyl, or / and or C1-C6 alkoxy, and wherein n is 0 or an integer between 1 and 20; wherein R1- R8are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and or C1-C6 alkoxy and wherein n is 0 or an integer between 1 and 20; and

[0050] wherein R1- R8are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and or C1-C6 alkoxy, and wherein n is 0 or an integer between 1 and 20; and Y is as discussed above for Formula XVIII and X is as discussed above for formula VIII, wherein R1- R7, and X are each independently selected from the group consisting of

[0051] H, or / and C1-C6 alkyl, or / and or C1-C6 alkoxy or phenyl wherein R1- R8, and X is each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and or C1-C6 alkoxy, and phenyl, and Y is as discussed above for Formula XVIII;

[0052] wherein R1- R8are each independently selected from the group consisting of H, or / and

[0053] C1-C6 alkyl, or / and or C1-C6 alkoxy, and wherein n is 0 or an integer between 1 and 20; and Y is as discussed above for Formula XVIII and X is as discussed above for formula VIII, wherein R1- Rs are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and or C1-C6 alkoxy, wherein R1- Rs are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and or C1-C6 alkoxy, wherein R1- R6are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and or C1-C6 alkoxy, wherein R1- R10are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and or C1-C6 alkoxy, wherein R1- R9are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and or C1-C6 alkoxy, wherein R1- R12are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and or C1-C6 alkoxy,

[0054] wherein R1- R4are each independently selected from the group consisting of H, or / and C1-C6 alkyl, or / and or C1-C6 alkoxy, and wherein G1 - G4 is selected from the group consisting of hydroxyl, thiol, H, (CH2)n-, (CH=CH)m-, (C=C)p-, -N-(p-phenyl), ((CH2)n-O)m-, cyclohexyl, phenyl, NH2(phenyl)-.

[0055] More specifically, the following amines can be used.

[0056]

[0057]

[0058]

[0059]

[0060]

[0061]

[0062]

[0063] The more preferable amines are these with CAS number: 101-80-4, 101-77-9, 15499- 84-0, 2716-10-1 , 2479-47-2, 13080-86-9, 13080-88-1 , 5981-09-9, 118727-34-7, 3283- 07-6, 60532-63-0. The most preferable are amines with CAS number: 13080-86-9, 118727-34-7, 60532-63-0.

[0064] Features of aerogel

[0065] In an embodiment, the aerogel has a specific surface area of more than 75 m2 / gram, preferably more than 100 m2 / gram, more preferably more than 150 m2 / gram, even more preferably more than 200 m2 / gram, most preferably more than 250 m2 / gram. In an embodiment, the aerogel has a specific surface area of between 75 and 1400 m2 / gram, such as between 100 and 1000 m2 / gram, preferably between 150 and 800 m2 / gram, more preferably between 200 and 700 m2 / gram, such as between 250 and 500 m2 / gram.

[0066] The specific surface area of the aerogels were analysed by Brunauer-Emmett-Teller (BET) analyser (TriStar II Plus). Before measurement, the samples were outgassed at 80 °C for 2 h under vacuum condition. Nitrogen grade 5.0 and Helium grade 4.6 were chosen to measure physisorption isotherm.

[0067] Without wishing to be bound by a particular theory, the present inventors believe that the specific surface area can be tuned by the selection of the aromatic polyfunctional amine(s).

[0068] In an embodiment, the aerogel has a porosity of more than 85 %, more preferably more than 90 %, such as between 80 and 98 %, or between 85 and 95 % or between 90 and 95 %. The porosity of the aerogels were measured by gas pycnometer (AccuPyc II 1345) using Helium grade 4.6. 10 data points were taken with 10 equilibrium cycles. Without wishing to be bound by a particular theory, the present inventors believe that the porosity is directly linked to the density. Porosity (Φ) is defined as the pore-volume fraction or the fraction of nonsolid material. Porosity is a volume ratio and thus dimensionless, and is usually reported as a fraction or percent.

[0069] In an embodiment, the aerogel has bulk density of less than 250 kg / m3, preferably less than 200 kg / m3, more preferably less than 150 kg / m3, the most preferably less than 100 kg / m3, such as between 30 and 150 kg / m3, more preferable between 50 and 100 kg / m3Bulk density is measured by dividing the mass of a dry sample by its volume, including the volume of both the solids and the pores within the material. Typically, a sample is weighed and then placed in a cylindrical container of which the volume is determined by using a digital calliper.

[0070] The unit kg / m3is the same as the unit g / l that is also often used. Without wishing to be bound by a particular theory, the present inventors believe that the bulk density can be tuned by the initial concentration of the amine and formaldehyde in the sol forming the gel. Bulk density, also called apparent density, is defined as the mass of material divided by the bulk volume (viz. the total volume of the mass, including pore volume).

[0071] In an embodiment, the aerogel has a linear shrinkage of less than 25 %, preferably less than 20 %, more preferably less than 15 %, even more preferably less than 10 % and the most preferable less than 5 %. This is calculating by measuring the diameter of the organogel prior to supercritical drying (SCD) and the diameter of the aerogel after SCD. Without wishing to be bound by a particular theory, the present inventors believe that the linear shrinkage may be influenced by several factors. One of the main influencing factor is considered to be the type of amine used, other factors are gel formation time and temperature and solvent used. The more stable gel is formed, the less the linear shrinkage is upon drying. The linear shrinkage is the change in linear dimension of a test specimen due to drying from a saturated condition to an equilibrium weight and length under specified accelerated drying conditions.

[0072] In an embodiment, the aerogel has a contact angle of more than 90 °, preferably more than 95 °, more preferably more than 100 °, most preferably more than 105 °. Without wishing to be bound by a particular theory, the present inventors believe that the contact angle can be tuned by the selection of the aromatic amine. Contact angle is a measure for the hydrophobicity of the aerogel. The hydrophobicity of the aerogels was studied by a contact angle analyzer (Data-Physics OCA30) at relative humidity of 40 %. The more hydrophobic, the larger the angle will be.

[0073] In an embodiment, the aerogel has a decomposition temperature at 5 % weight loss (TD(5 %)) of more than 200 °C, preferably more than 250 °C, more preferably more than 280 °C, most preferably more than 320 °C. The thermal properties of the aerogels were measured by TGA 550 (TA Instruments) under a nitrogen atmosphere at the heating rate of 10 °C min-1 from 40 to 795 °C.

[0074] In an embodiment, the aerogel has a thermal conductivity (λ) that is less than 25 mW / m / K, preferably less than 23 mW / m / K, more preferably less than 21 mW / m / K, even more preferably less than 20 mW / m / K, most preferably less than 19 mW / m / K, such as between 10 and 19 mW / m / K.

[0075] The thermal conductivity was determined in accordance with ASTM C518 using a heat flow meter (Thermtest Inc., HFM-25) at 20 °C. Prior to the measurement, the machine was calibrated with EPS 1450E as reference material.

[0076] Without wishing to be bound by a particular theory, the present inventors believe that the thermal conductivity is a trade-off between the specific surface area which is preferably high and the bulk density which is preferably low. An optimal is desired. An optimum has been be achieved between these values. The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by λ (lambda). Heat transfer occurs at a lower rate in materials of low thermal conductivity, so for insulation materials as the present invention, low thermal conductivity is desirable. Many factors play a role in this parameter.

[0077] Method

[0078] In a second aspect, the present invention relates to a method of preparing an aerogel according to any one of the preceding claims, said method comprising the steps of: a) forming a solution of the formaldehyde in a solvent and forming a solution of the one or more aromatic polyfunctional amines in a solvent; b) mixing the solutions in a ratio such that there is a substantially equimolar ratio between formaldehyde molecules and amino groups of the polyfunctional amines, and allowing gelation for a certain period of time for an organogel to be formed; c) optionally, the organogel formed in step b) is allowed to age for a certain period of time for a cured organogel to be formed d) optionally, one or more solvent exchange step(s) is (are) carried out to exchange the solvent(s) of step a) for a second solvent or mixture of solvents; e) supercritical drying, preferably by CO2, of the gel formed to obtain the aerogel.

[0079] The present inventors suggest a catalyst-free sol-gel process that allows the use of commercially available aromatic amines and (para)formaldehyde followed by supercritical drying and by this method the inventors have successfully prepared the PHT aerogels according to the first aspect of the invention.

[0080] In a preferred embodiment of the second aspect, the present invention relates to a method of preparing an aerogel according to any one of the preceding claims, said method comprising the steps of: a) forming a solution of the formaldehyde in a solvent, preferably dimethylformamide (DMF), dimethylsulfoxide (DMSO), dimethyl lactamide (DML), n-methyl-2-pyrrolidone (NMP), and dimethylacetamide (DMAc), or a mixture thereof, at a temperature of above 75 °C and forming a solution of one or more aromatic polyfunctional amines in the same solvent, or different solvent, or solvent mixture; b) mixing the solutions in a ratio such that there is a substantially equimolar ratio between formaldehyde molecules and amino groups of the polyfunctional amines, and allowing gelation for at least 0.01 hour at a temperature of above 75 °C for an organogel to be formed; c) the organogel formed in step b) is allowed to age for at least 1 hour for a cured organogel to be formed d) one or more solvent exchange step(s) is (are) carried out to exchange the solvent(s) of step a) for a second solvent or mixture of solvents, preferably comprising ethanol, or water, or acetone or methyl ethyl ketone; e) supercritical drying, preferably by CO2, of the gel formed to obtain the aerogel.

[0081] Step a)

[0082] In the first step of this method two solutions (sols) are formed of the two starting materials, namely the formaldehyde and the amine. Suitable solvents for use in the preparation of the solutions in step a) according to the present method are all solvents listed in the Handbook of Organic Solvent Properties (Smallwood, I. Smallwood Handbook of Organic Solvent Properties. Arnold: London, 1996). In an embodiment, the solvent used in step a) for the formaldehyde and the solvent used in step a) for the amine are each independently selected from the group consisting of dimethylformamide, dimethylacetamide, dimethylsulfoxide, dimethyl lactamide, n-methyl-2-pyrrolidone, methanol, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, pentyl acetate, hexyl acetate, acetic anhydride, cyclohexane, ethylbenzene, n-octane, diethyl ether, methyl-t-butyl ether, methyl ethyl ketone, 3- pentanone, 1 ,4-dioxane, pentane, acetone, methylcyclohexane, tetrahydropyran, triethylamine, toluene, acetone, isopropanol, benzene, ethanol, 1-propanol, 1-butanol, tetrahydrofuran, dimethyllactamide (DML), 2-methyltertrahydrofuran, 3-pentanone, tetrahydropyran, pentyl acetate, hexyl acetate, acetic anhydride, and methylcyclobenzene, and a mixture of two or more thereof. In addition, aqueous solutions of organic salt and inorganic salt with a concentration ranging from 0.1 M to 10M may also be used (see Lide, D. R. (Ed.). (2004). CRC handbook of chemistry and physics (Vol. 85). CRC press).

[0083] In a preferred embodiments, in step a) as solvents are used dimethylformamide, dimethylacetamide, dimethylsulfoxide, dimethyl lactamide, and n-methyl-2- pyrrolidone, methanol, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, pentyl acetate, hexyl acetate, acetic anhydride, cyclohexane, ethylbenzene, n-octane, diethyl ether, methyl-t-butyl ether, methyl ethyl ketone, 3-pentanone, 1 ,4-dioxane, pentane, acetone, methylcyclohexane, tetrahydropyran, triethylamine, toluene, acetone, isopropanol, benzene, ethanol, 1-propanol, 1-butanol, tetrahydrofuran, more preferably, acetone, methylethylketone, ethanol, methanol, ethyl acetate, tetrahydrofuran, and mixtures thereof.

[0084] The most preferable solvents for use in step a) are dimethylformamide (DMF), dimethylsulfoxide (DMSO), dimethyl lactamide (DML), n-methyl-2-pyrrolidone (NMP), and dimethylacetamide (DMAc), and mixtures thereof. In an embodiment the solvent used for forming a solution of the formaldehyde and the solvent used for forming a solution of the one or more aromatic polyfunctional amines are the same solvent or the same mixture of solvents. When mixtures of solvents are used any ratio of mixing may be used.

[0085] In an embodiment, the solvent for the solution of the formaldehyde and the solvent for the solution of the one or more aromatic polyfunctional amines is selected from the group consisting of N-methyl-2-pyrrolidone (NMP), N,N-dimethyl lactamide (DML), dimethylformamide (DMF), dimethylsulfoxide (DMSO) and dimethylacetamide

[0086] (DMAc).

[0087] The amine and the paraformaldehyde can both be considered as monomers in the reaction. The monomers are preferably dissolved in different / separate solution, most preferably in NMP. The solutions may be prepared at elevated temperature, such as above 50 °C, or above 75 °C, such as at 100 °C.

[0088] Step b)

[0089] In the second step of this methods the two sols are mixed to provide a sol comprising both components and then a gel is formed via a so-called sol-gel process.

[0090] The gelation can be initiated by mixing the solution at elevated temperature, such as above 50 °C, or above 75 °C, such as at 100 °C. Step b) is carried out for a time sufficient to allow gelation of the organogel. This depends on the concentration of the monomers, the solvent, and the temperature. This amount of time may be at least 0.01 hour, such as between 0.01 h to 24 hours, for example at least 0.5 hour, at least 1 hour, at least 2 hours, or at least 4 hours or 8 hours.

[0091] Polyfunctional amines are used in the present invention in order to arrive at a polymeric gel structure. The formaldehyde will couple two amine-groups of two different molecules thereby forming a reversible bond between these two molecules, forming a polymer.

[0092] It is preferred that the formaldehyde and the amines are added in an substantially equimolar ratio. With this is meant that the ratio between the solutions is such that there is a substantial equimolar ratio between formaldehyde molecules / groups (CH2=O) and amino (NH2-) groups, so that for each formaldehyde molecule there is one amino group, such as in a ratio of between 1.5:1 and 1 : 1.5, such as between 1.2:1 to 1 :1.2, such as between 1.1 :1 and 1.1 : 1 , more preferably in a ratio of between 1 .05: 1 and 1 :1.05 or 1 :1. The present inventors observed that the substantially equivalent ratio of (para)formaldehyde to amine groups will lead to full trimerization without side reaction or unwanted intermediate within a reasonable time frame (e.g. within 8 hours). In case of excess of formaldehyde the ring closure condensation is not complete and a hemiaminal is formed which is undesirable because it will not form a gel, which is required by the present invention to arrive at an aerogel having the required properties. In case of excess of the amine, there will be more linearly polymer structures formed, not the desired PHT structures.

[0093] In an embodiment, the invention is related to an aerogel that is prepared with a substantially equimolar ratio between formaldehyde molecules and amino groups of the polyfunctional amines, preferably with a ratio of between 1.5: 1 and 1 :1.5, such as between 1.2: 1 to 1 : 1.2, such as between 1.1 :1 and 1.1 :1 , more preferably with a ratio of between 1.05:1 and 1 :1.05 or 1 :1.

[0094] The present inventors have confirmed using NMR that full trimerization occurs and that the structure is not a hemiaminal structure, and that is confirmed by the excellent thermal stability found for the present aerogels. Without wishing to be bound to a particular theory, the present inventors believe that the intermediate structure (hemiaminal) during the amine trimerization results in low thermal stability with a decomposition temperature lower than 200 °C.

[0095] For each amino group, one molecule of formaldehyde is required. So in case of a diamine, there are two amine groups (-NH2) present and the molar amount of formaldehyde should be approx, twice the molar amount of the diamine. In case of a triamine, there are three amine groups present and the molar amount of formaldehyde should be approx, three times the molar amount of the triamine, etcetera.

[0096] Step c) - optional

[0097] After the gelation, an optional but preferred step of aging of the organogel formed is carried out. This aging allows a more stable organogel to be formed. The aging can be carried out at temperature between 15 and 150 °C, or between 20 and 120 °C, or between 40 and 100 °C, or between 60 and 80 °C and the most preferable at ambient temperature, namely at 20 °C. In an embodiment step c) is carried out for a period of time of between 1 and 48 hours, preferable between 1 and 24 hours, more preferable between 2 and 18 hours, even more preferably between 3 and 12 hours, most preferably between 3 and 8 hours, or at least 6 hours.

[0098] Step d) - optional

[0099] Once a stable organogel is formed, which can be visibly observed, an optional step, step d) may be carried out, being a solvent exchange using a solvent or solvent mixture that is different from the solvent(s) used in step a).

[0100] When NMP, DML, DMF, DMSO, or DMAc are used as solvent in step a), it is preferred to exchange this solvent for ethanol, or stepwise using aqueous sodium hydroxide solution, water and ethanol to completely remove residual NMP, DML, DMF, DMSO, or DMAc prior to supercritical drying.

[0101] In case the gelation step is performed in a solvent that can be directly subjected to supercritical drying (such as acetone, ethanol, or ethyl acetate) this step d) is superfluous. Any of the solvents cited above for step (a) that is suitable for supercritical drying can be used in this step.

[0102] Preferably, the solvent(s) used is step a) is exchanged in this step by methanol, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, pentyl acetate, hexyl acetate, acetic anhydride, cyclohexane, ethylbenzene, n-octane, diethyl ether, methyl-t-butyl ether, methyl ethyl ketone, 3-pentanone, 1 ,4-dioxane, pentane, methylcyclohexane, tetrahydropyran, triethylamine, toluene, acetone, isopropanol, benzene, ethanol, 1- propanol, 1 -butanol, tetrahydrofuran. In addition, a mixture of one or more of these solvent(s) or aqueous sodium hydroxide may also be used. When aqueous sodium hydroxide is used, at least another, additional, solvent exchange step is preferably carried out using a pure solvent.

[0103] This step of solvent exchange can be carried out multiple times, each time with the same or a different solvent, or mixture of solvents. In an embodiment, step d) is carried out, preferably wherein the solvent is selected from the group consisting of acetone, methylethylketone, 3-pentanone, ethanol, methanol, ethyl acetate, tetrahydrofuran, and a combination of two or more thereof.

[0104] Step e)

[0105] The third (or fourth) step in the method is a drying step, viz supercritical drying (SCD) to arrive at a stable aerogel with the desired properties.

[0106] SCD allows for the removal of the solvent inside the organogel without disrupting its polymer structure, thereby forming an aerogel according to the invention.

[0107] In an embodiment, drying was carried out using liquid carbon dioxide (CO2) (for example grade 2.7; purity > 99.7 %) as exchange agent for the supercritical drying process (SCD).

[0108] In an embodiment, the SCD is carried out at a pressure of between 40 and 250 bar, such as 100 bar. In an embodiment, the SCD is carried out at a temperature of between 40 and 80 °C. The drying process can encompass one or more steps, such as 3, 4, or 5 steps, of pressurizing, venting, and waiting. The venting process and the waiting steps can each independently take between 5 and 60 minutes.

[0109] When freeze drying is used a cryogel - instead of an aerogel - will be formed which has a much higher shrinkage then desirable by the present invention.

[0110] Use of novel PHTs for preparation of aerogels

[0111] In a third aspect, the present invention relates to the use of a polyhexahydrotriazine (PHT) prepared from formaldehyde and one or more aromatic polyfunctional amines for the preparation of aerogels having a specific surface area of more than 50 m2 / gram, a porosity of more than 80 %, and a bulk density of less than 300 kg / m3.

[0112] Method of recycling aerogels

[0113] In a fourth aspect, the present invention relates to a method of recycling the aerogel according to any of claims 1 to 10, comprising treating the aerogel with an aqueous acid solution having a pH of less than 4, preferably less than 2, more preferably less than 1 , to decompose the polyhexahydrotriazine (PHT) into the one or more aromatic polyfunctional amines.

[0114] The aqueous acid solution comprises an acid, that may be selected from the acid in aqueous solution mentioned in Lide, D. R. (Ed.). (2004). CRC handbook of chemistry and physics (Vol. 85) CRC press.

[0115] In an embodiment, an aqueous solution of sulphuric acid, hydrochloric acid, citric acid, nitric acid, phosphoric acid, para-toluene sulfonic acid, and oxalic acid, more preferably hydrochloric, phosphoric and sulphuric acid are used or a combination thereof. The most preferable acid is sulphuric acid. The aqueous acid solution may combined with organic solvents to enhance the depolymerization process. The solvents selected are listed in the Handbook of Organic Solvent Properties, preferably, tetrahydrofuran, ethanol, methanol, and acetone, more preferably, tetrahydrofuran and ethanol.

[0116] Closed-loop recycling (see below) of these highly cross-linked polymeric networks was achieved by a method using acidic conditions to induce the selective cleavage of methylene bonds within the hexahydrotriazine rings. The high yielding recovery (up to 90 %) of pure amine monomers allows for the preparation of fresh aerogels once again, showcasing the full potential of the inventive circular materials.

[0117] The recycling of the aerogels of the present invention using another approach is shown in the examples, which disclose aerogel to sol to aerogel (ASA) recycling. In an aspect, the invention relates to a method of recycling the aerogel according to the invention said method comprising the steps of: i) treating the aerogel with a solution comprising an excess, such as at least 1.1 or at least 1.2 or at least 1.4, preferably ranging from 1.5 to 6, of one or more aromatic polyfunctional amines to depolymerize the polyhexahydrotriazine (PHT) into a mixture of soluble oligomers having amino- end-groups; and ii) adding formaldehyde to the mixture obtained in step i) to form a polyhexhydrotriazine; and iii) optionally, the organogel formed in step b) is allowed to age for a certain period of time for a cured organogel to be formed; and iv) optionally, one or more solvent exchange step (s) is(are) carried out to exchange the solvent(s) of steps i) and ii) for a second solvent or mixture of solvents; and v) supercritical drying, preferably by CO2, of the organogel formed in step ii), iii), or iv) to obtain the aerogel.

[0118] In step i) the excess amine is calculated as the molar ratio of extra amine groups (NH2) compared to the amine groups (-NH2groups) of amine used when preparing the original aerogel. Preferably, the amine used in step i) is the same as the amine used to prepare the aerogel and / or a different amine, or mixture of amines.

[0119] Use of aerogels

[0120] In a sixth aspect, the present invention relates to the use of these aerogels, for example in the field of thermal insulation (e.g., as super-insulating materials), in the field of energy storage, catalysis and for chemical absorption.

[0121] Other aspects of the invention

[0122] In a specific aspect of the present invention, the invention is related to novel compounds and to non-commercially available compounds, that are selected from the group consisting of the compounds discussed above for Formulas VII to XXII, and XXIX. These compounds have been prepared by the present inventors. These amines may be used in the aerogel and method according to the present invention but may also have other uses.

[0123] In a specific aspect of the present invention, the invention is related to a PHT prepared from formaldehyde and an amine selected from the group consisting of the compounds disclosed above under “novel compounds”.

[0124] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. The scope of the present invention is defined by the appended claims. One or more of the objects of the invention are achieved by the appended claims.

[0125] EXAMPLES

[0126] The present invention is further elucidated based on the Examples below which are illustrative only and not considered limiting to the present invention.

[0127] The components used in the present examples are numbered as components a1-a12.

[0128] • Component a1 :2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP)

[0129] • Component a2:4,4'-oxydianiline (ODA)

[0130] • Component a3:a,a'-bis(4-aminophenyl)-1 ,4-diisopropylbenzene (BAPDB)

[0131] • Component a4:9,9-bis(4-aminophenyl)fluorene (FDA)

[0132] • Component a5:4, 4', 4"-triaminotriphenylamine (TAPA)

[0133] • Component a6:N,N-bis(4-aminobenzyl)succinimide (BABS)

[0134] • Component a7: paraformaldehyde (PFA) as the formaldehyde (CHO)

[0135] • Component a8:N-methylpyrrolidone (NMP) - solvent

[0136] • Component a9:N,N-dimethylformamide (DMF) - solvent

[0137] • Component a10:dimethylacetamide (DMAc) - solvent

[0138] • Component a11 :dimethylsulfoxide (DMSO) - solvent

[0139] • Component a12:N,N-dimethyl lactamide (DML) - solvent

[0140] Examples 1-12 below show the synthesis and characterization of 12 different aerogels. Table 1 below shows an overview of the formulation thereof. Table 2 below shows the characteristics of these aerogels. Examples 13-15 show the recyclability of the process. Table 1. Overview of formulation of triazine aerogels and types and amounts (in gram) of components

[0141] Example 1 - synthesis and characterization of aerogel 1

[0142] 1.72 g of the component a1 BAPP was dissolved at 100 °C in 11.3 g of NMP in a polypropylene vial. 0.25 g of the component a7 PFA was dissolved in 11.2 g of NMP in a second polypropylene vial. The two solutions were mixed. This gave a clear, low- viscosity mixture. The mixture was allowed to stand at 100 °C for 8 hour until gelation. Afterwards, the organogel was sealed and let aging 24 h under ambient condition to effect curing. The gel was subsequently taken from the polypropylene vial and was solvent-exchanged with 300 mL 0.1 M sodium hydroxide aqueous solution for 2 times, then with 300 mL distilled water for 2 times, and finally with 300 mL pure ethanol for 2 times.

[0143] The organogel monolith was transferred to a 940 ml autoclave. The autoclave was filled with ethanol having a purity of >99 % so that the monolith was completely covered by ethanol and subsequently closed. The monolith was dried in a stream of CO2, for 3 hours. The pressure (in the drying system) was in the range from 95 to 105 bar; the temperature was 60 °C. At the end, the pressure in the system was reduced in a controlled manner to atmospheric pressure over a period of about 45 minutes at a temperature of 60 °C. The autoclave was opened and the dried aerogel was taken out.

[0144] More detailed information regarding this process is as follows. The high pressure extraction / drying units “HP-DE200” is utilized as the drying setup. It comprises one autoclave, provided by Eurotechnica, with a maximum working temperature of 100 °C and allowable operation pressure of 220 bar. The autoclave includes a thermowell with a NiCr-Ni thermocouple to measure the internal temperature during the process. Two venting tubes are also attached to the autoclave to extract the covering solvent and depressurization. Apart from the autoclave, the supercritical drying system employed in this work consists of two thermal baths (Selecta, UNITRONIC 200) for two heat exchanger, a mechanical pump (provided by Maximator), a check valve, five needle valves and a CO2 bottle.

[0145] Gels are first introduced into the autoclave and covered with the solvent used for gel formation. This was done to avoid premature solvent evaporation that could lead to a higher shrinkage. Then, CO2 is gradually pressurized up to 100 bar, extracting the solvent from the inside of the wet gel pores. The heat exchanger 2 maintains the autoclave at constant temperature of 60 °C. Once supercritical conditions are achieved, the solvent is extracted from the gels in the autoclave. The supercritical CO2 enriched with extracted solvent is vented out by releasing the autoclave. During this process, the pressure is maintained above with constant fresh CO2 input. The venting process takes around 10 to 15 min and the autoclave will be closed to reach further extraction. Three cycles of extraction were further conducted with waiting interval of 30 min each. Finally, when the aerogel pores are completely free of solvent, pressure is slowly released to atmospheric pressure through the metering valve for 45 min. The aerogel as obtained had a bulk density of 98 kg / m3, a porosity of 92 %, and a specific surface area of 78 m2g-1. The thermal conductivity was 20.1 mW / mK at 20° C.

[0146] Example 2- synthesis and characterization of aerogel 2

[0147] 2.29 g of the component a1 BAPP was dissolved at 100 °C in 11.3 g of NMP in a polypropylene vial. 0.34 g of the component a7 PFA was dissolved in 11.2 g of NMP in a second polypropylene vial. The two solutions were mixed. This gave a clear, low- viscosity mixture. The mixture was allowed to stand at 100 °C for 8 hour until gelation. Afterwards, the organogel was sealed and let aging 24 h under ambient condition to effect curing. The gel was subsequently taken from the polypropylene vial and was initially solvent-exchanged with 300 mL 0.1 M sodium hydroxide aqueous solution for 2 times, then with 300 mL distilled water for 2 times, and finally with 300 mL pure ethanol for 2 times.

[0148] In a manner corresponding to example 1 , the gel was subsequently taken from the container and dried by solvent extraction with supercritical CO2in an autoclave. The aerogel obtained had a bulk density of 145 kg / m3, a porosity of 88 %, and a specific surface area of 129 m2g-1. The thermal conductivity was 18.9 mW / mK at 20° C.

[0149] Example 3 - synthesis and characterization of aerogel 3

[0150] 2.86 g of the component a1 BAPP was dissolved at 100 °C in 11.3 g of NMP in a polypropylene vial. 0.42 g of the component a7 PFA was dissolved in 11.2 g of NMP in a second polypropylene vial. The two solutions were mixed. This gave a clear, low- viscosity mixture. The mixture was allowed to stand at 100 °C for 8 hour until gelation. Afterwards, the organogel was sealed and let aging 24 h under ambient condition to effect curing. The gel was subsequently taken from the polypropylene vial and was initially solvent-exchanged with 300 mL 0.1 M sodium hydroxide aqueous solution for 2 times, then with 300 mL distilled water for 2 times, and finally with 300 mL pure ethanol for 2 times.

[0151] In a manner corresponding to example 1 , the gel was subsequently taken from the container and dried by solvent extraction with supercritical CO2in an autoclave. The aerogel obtained had a bulk density of 189 kg / m3, a porosity of 88 %, and a specific surface area of 162 m2g-1. The thermal conductivity was 20.9 mW / mK at 20° C.

[0152] Example 4 - synthesis and characterization of aerogel 4

[0153] 2.02 g of the component a2 ODA was dissolved at 100 °C in 11.3 g of NMP in a polypropylene vial. 0.61 g of the component a7 PFA was dissolved in 11.2 g of NMP in a second polypropylene vial. The two solutions were mixed. This gave a clear, low- viscosity mixture. The mixture was allowed to stand at 100 °C for 8 hour until gelation. Afterwards, the organogel was sealed and let aging 24 h under ambient condition to effect curing. The gel was subsequently taken from the polypropylene vial and was solvent-exchanged with 300 mL 0.1 M sodium hydroxide aqueous solution for 2 times, then with 300 mL distilled water for 2 times, and finally with 300 mL pure ethanol for 2 times.

[0154] In a manner corresponding to example 1 , the gel was subsequently taken from the container and dried by solvent extraction with supercritical CO2in an autoclave. The aerogel obtained had a bulk density of 147 kg / m3, a porosity of 89 %, and a specific surface area of 57 m2g-1. The thermal conductivity was 23.8 mW / mK at 20° C.

[0155] Example 5 - synthesis and characterization of aerogel 5

[0156] 2.46 g of the component a3 BAPDB were dissolved at 100 °C in 11.3 g of DMF in a polypropylene vial. 0.43 g of the component a7 PFA was dissolved in 11 .2 g of DMF in a second polypropylene vial. The two solutions were mixed. This gave a clear, low- viscosity mixture. The mixture was allowed to stand at 100 °C for 30 min until gelation. Afterwards, the organogel was sealed and let aging 24 h under ambient condition to effect curing. The gel was subsequently taken from the polypropylene vial and was solvent-exchanged with 300 mL pure ethanol for 3 times. In a manner corresponding to example 1 , the gel was subsequently taken from the glass beaker and dried by solvent extraction with supercritical CO2in an autoclave. The aerogel obtained had a bulk density of 129 kg / m3, a porosity of 88 %, and a specific surface area of 91 m2g-1. The thermal conductivity was 22.9 mW / mK at 20° C.

[0157] Example 6 - synthesis and characterization of aerogel 6

[0158] 2.46 g of the component a3 BAPDB were dissolved at 100 °C in 11 .3 g of DMAc in a polypropylene vial. 0.43 g of the component a7 PFA was dissolved in 11 .2 g of DMAc in a second polypropylene vial. The two solutions were mixed. This gave a clear, low- viscosity mixture. The mixture was allowed to stand at 100 °C for 30 min until gelation. Afterwards, the organogel was sealed and let aging 24 h under ambient condition to effect curing. The gel was subsequently taken from the polypropylene vial and was solvent-exchanged with 300 mL pure ethanol for 3 times.

[0159] In a manner corresponding to example 1 , the gel was subsequently taken from the glass beaker and dried by solvent extraction with supercritical CO2in an autoclave. The aerogel obtained had a bulk density of 155 kg / m3, a porosity of 86 %, and a specific surface area of 77 m2g-1. The thermal conductivity was 21.9 mW / mK at 20° C.

[0160] Example 7 - synthesis and characterization of aerogel 7

[0161] 2.24 g of the component a4 FDA were dissolved at 100 °C in 11.3 g of NMP in a polypropylene vial. 0.77 g of the component a7 PFA was dissolved in 11.2 g of NMP in a second polypropylene vial. The two solutions were mixed. This gave a clear, low- viscosity mixture. The mixture was allowed to stand at 100 °C for 20 hour until gelation. Afterwards, the organogel was sealed and let aging 24 h under ambient condition to effect curing. The gel was subsequently taken from the polypropylene vial and was solvent-exchanged with 300 mL pure ethanol for 3 times.

[0162] In a manner corresponding to example 1 , the organogel was subsequently taken from the glass beaker and dried by solvent extraction with supercritical CO2in an autoclave. The aerogel obtained had a bulk density of 124 kg / m3, a porosity of 90 %, and a specific surface area of 537 m2g-1. The thermal conductivity was 20.3 mW / mK at 20° C. Example 8 - synthesis and characterization of aerogel 8

[0163] 2.45 g of the component a4 FDA were dissolved at 100 °C in 11.3 g of DMF in a polypropylene vial. 0.42 g of the component a7 PFA was dissolved in 11 .2 g of DMF in a second polypropylene vial. The two solutions were mixed. This gave a clear, low- viscosity mixture. The mixture was allowed to stand at 100 °C for 20 hour until gelation. Afterwards, the organogel was sealed and let aging 24 h under ambient condition to effect curing. The gel was subsequently taken from the polypropylene vial and was solvent-exchanged with 300 mL pure ethanol for 3 times.

[0164] In a manner corresponding to example 1 , the organogel was subsequently taken from the glass beaker and dried by solvent extraction with supercritical CO2in an autoclave. The aerogel obtained had a bulk density of 152 kg / m3, a porosity of 88 %, and a specific surface area of 349 m2g-1. The thermal conductivity was 23.2 mW / mK at 20° C.

[0165] Example 9 - synthesis and characterization of aerogel 9

[0166] 2.09 g of the component a4 FDA were dissolved at 100 °C in 11.3 g of DMSO in a polypropylene vial. 0.36 g of the component a7 PFA was dissolved in 11 .2 g of DMSO in a second polypropylene vial. The two solutions were mixed. This gave a clear, low- viscosity mixture. The mixture was allowed to stand at 100 °C for 20 hour until gelation. Afterwards, the organogel was sealed and let aging 24 h under ambient condition to effect curing. The gel was subsequently taken from the polypropylene vial and was solvent-exchanged with 300 mL pure ethanol for 3 times.

[0167] In a manner corresponding to example 1 , the organogel was subsequently taken from the glass beaker and dried by solvent extraction with supercritical CO2in an autoclave. The aerogel obtained had a bulk density of 112 kg / m3, a porosity of 91 %, and a specific surface area of 318 m2g-1. The thermal conductivity was 23.7 mW / mK at 20° C.

[0168] Example 10 - synthesis and characterization of aerogel 10

[0169] 2.19 g of the component a5 TAPA were dissolved at 100 °C in 11.3 g of DMAc in a polypropylene vial. 0.68 g of the component a7 PFA was dissolved in 11 .2 g of DMAc in a second polypropylene vial. The two solutions were mixed. This gave a clear, low- viscosity mixture. The mixture was allowed to stand at 100 °C for 7 min until gelation. Afterwards, the organogel was sealed and let aging 24 h under ambient condition to effect curing. The gel was subsequently taken from the polypropylene vial and was solvent-exchanged with 300 mL pure ethanol for 3 times.

[0170] In a manner corresponding to example 1 , the gel was subsequently taken from the glass beaker and dried by solvent extraction with supercritical CO2in an autoclave. The aerogel obtained had a bulk density of 147 kg / m3, a porosity of 90 %, and a specific surface area of 573 m2g-1. The thermal conductivity was 19.3 mW / mK at 20° C.

[0171] Example 11 - synthesis and characterization of aerogel 11

[0172] 2.18 g of the component a6 BABS were dissolved at 100 °C in 11.3 g of DML in a polypropylene vial. 0.4 g of the component a7 PFA was dissolved in 11.2 g of DML in a second polypropylene vial. The two solutions were mixed. This gave a clear, low- viscosity mixture. The mixture was allowed to stand at 100 °C for 1 hour until gelation. Afterwards, the organogel was sealed and let aging 24 h under ambient condition to effect curing. The cured organogel was subsequently taken from the polypropylene vial and was solvent-exchanged with 300 mL pure ethanol for 3 times.

[0173] In a manner corresponding to example 1 , the organogel was subsequently taken from the glass beaker and dried by solvent extraction with supercritical CO2in an autoclave. The aerogel obtained had a bulk density of 63 kg / m3, a porosity of 96 %, and a specific surface area of 345 m2g-1. The thermal conductivity was 17.2 mW / mK at 20° C.

[0174] Example 12 - synthesis and characterization of aerogel 12

[0175] 2.42 g of the component a6 BABS were dissolved at 100 °C in 11.3 g of DMF in a polypropylene vial. 0.44 g of the component a7 PFA was dissolved in 11 .2 g of DMF in a second polypropylene vial. The two solutions were mixed. This gave a clear, low- viscosity mixture. The mixture was allowed to stand at 100 °C for 1 hour until gelation. Afterwards, the organogel was sealed and let aging 24 h under ambient condition to effect curing. The cured organogel was subsequently taken from the polypropylene vial and was solvent-exchanged with 300 mL pure ethanol for 3 times. In a manner corresponding to example 1 , the organogel was subsequently taken from the glass beaker and dried by solvent extraction with supercritical CO2in an autoclave. The aerogel obtained had a bulk density of 170 kg / m3, a porosity of 86 %, and a specific surface area of 91 m2g-1. The thermal conductivity was 20.1 mW / mK at 20° C.

[0176] These examples clearly shows that aerogels with excellent properties can be obtained and hence one or more of the objects of the invention is achieved. The aerogels show good thermal conductivity (low lambda values), good thermal resistance (high TD(5 %), improved mechanical properties (uniaxial compression testing), and intrinsic hydrophobicity (high contact angle) without prior modifications.

[0177] Table 2. Material properties of the aerogels Example 13 - recycling of aerogel 2 and synthesis of recycled aerogel 2

[0178] 4 g of the aerogels prepared in example 2 were placed into 75 g of 1 M H2SO4 / THF (2 / 1 w / w) solution in a 150 mL vial. The mixture was stirred at 60 °C for 2 h. After completed depolymerization, the light pink precipitated BAPP salt was filtered, and washed with distilled water and acetone, and dried in vacuum oven at 60 °C for overnight. The salt of amine was placed into 10 M NaOH aqueous solution with a stirring bar. The mixture was heated at 100 °C under reflux for 24 h. The off-white precipitated BAPP was filtered and washed with distilled water, and dried in the vacuum oven at 60 °C for overnight (yield: 89 %).

[0179] 2.29 g of the recycled BAPP was dissolved at 100 °C in 11.3 g of NMP in a polypropylene vial. 0.34 g of the component a7 PFA was dissolved in 11.2 g of NMP in a second polypropylene vial. The gelling was initiated by mixing the two components into one vial. The mixture was shaken until a homogeneous solution was obtained. The solution was poured into PP mold with 70 mm diameter and was placed in the 100 °C oven for 4 h until gelation. Afterwards, the organogel was sealed and aged for 24 h under ambient condition. After aging, the gel was subsequently taken from the polypropylene mold and was solvent-exchanged with 300 mL 0.1 M sodium hydroxide aqueous solution for 2 times, then with 300 mL distilled water for 2 times, and finally with 300 mL pure ethanol for 2 times.

[0180] In a manner corresponding to example 1 , the gel was subsequently taken from the container and dried by solvent extraction with supercritical CO2in an autoclave. The aerogel obtained had a bulk density of 160 kg / m3, a porosity of 87 %, and a specific surface area of 128 m2g-1. The thermal conductivity was 19.0 mW / mK at 20° C.

[0181] Table 3. Material properties of pristine and the recycled aerogels This example clearly shows excellent recyclability of the aerogels and hence one or more of the objects of the invention is achieved.

[0182] Example 14 - recycling of aerogel 2 using gel to gel approach

[0183] 0.68 g of the aerogels prepared in example 2 were grinded and placed in a PP container in NMP solvent. The aerogels were added with 1.91 g the component a1 BAPP dissolved in 5.8 g NMP solution. After the addition, the vial was sealed with parafilm and ultrasonicated for 5 h until full dissolution. After full dissolution of the mixture is achieved, the solution was placed in 100 °C oven along with 0.28 g the component a7 PFA dissolved in 16.7 g NMP solution. The gelling was initiated by mixing the two components into one vial. The mixture was shaken until a homogeneous solution was obtained. The solution was poured into PP mold with 70 mm diameter and was placed in the 100 °C oven for 4 h until gelation. Afterwards, the organogel was sealed and aged for 24 h under ambient condition. After aging, the gel was subsequently taken from the polypropylene mold and was solvent-exchanged with 300 mL 0.1 M sodium hydroxide aqueous solution for 2 times, then with 300 mL distilled water for 2 times, and finally with 300 mL pure ethanol for 2 times.

[0184] In a manner corresponding to example 1 , the gel was subsequently taken from the container and dried by solvent extraction with supercritical CO2in an autoclave. The aerogel obtained had a bulk density of 157 kg / m3, a porosity of 88 %, and a specific surface area of 125 m2g-1. The thermal conductivity was 20.6 mW / mK at 20° C.

[0185] Table 4 below shows an overview of the aerogel prior to recycling (denoted as “pristine”) and the aerogel after recycling (denoted as “ASA"). The thermal behavior of the pristine and recycled aerogel was reviewed by comparing the weight at different temperatures, the results were found to be nearly the same.

[0186] Example 15 - reprogramming of aerogel 2 using gel to gel approach

[0187] 1.17 g of the aerogels prepared in example 2 were grinded and placed in a PP container in NMP solvent. The aerogels were added with 1.41 g the component a4 FDA dissolved in 10.1 g NMP solution. After the addition, the vial was sealed with parafilm and ultrasonicated for 5 h until full dissolution. After full dissolution of the mixture is achieved, the solution was placed in 100 °C oven along with 0.24 g the component a7 PFA dissolved in 12.4 g NMP solution. The gelling was initiated by mixing the two components into one vial. The mixture was shaken until a homogeneous solution was obtained. The solution was poured into PP mold with 70 mm diameter and was placed in the 100 °C oven for 4 h until gelation. Afterwards, the organogel was sealed and aged for 24 h under ambient condition. After aging, the gel was subsequently taken from the polypropylene mold and was solvent-exchanged with 300 mL 0.1 M sodium hydroxide aqueous solution for 2 times, then with 300 mL distilled water for 2 times, and finally with 300 mL pure ethanol for 2 times.

[0188] In a manner corresponding to example 1 , the gel was subsequently taken from the container and dried by solvent extraction with supercritical CO2in an autoclave. The aerogel obtained had a bulk density of 191 kg / m3, a porosity of 85 %, and a specific surface area of 229 m2g-1. The thermal conductivity was 16.1 mW / mK at 20° C.

[0189] Table 4. Material properties of pristine and the recycled (ASA) aerogels

Claims

CLAIMS1. An aerogel of a polyhexahydrotriazine (PHT) prepared from formaldehyde and one or more aromatic polyfunctional amines, said aerogel having a specific surface area of more than 50 m2 / gram, a porosity of more than 80 %, and a bulk density of less than 300 kg / m3.

2. The aerogel according to claim 1 , wherein the aerogel is prepared with a substantially equimolar ratio between formaldehyde molecules and amino groups of the polyfunctional amines, preferably with a ratio of between 1.5:1 and 1 :1.5, such as between 1.2: 1 to 1 :1.2, such as between 1.1 :1 and 1.1 :1 , more preferably with a ratio of between 1 .05: 1 and 1 : 1 .05 or 1 :1.

3. The aerogel according to claim 1 or claim 2, wherein the aromatic polyfunctional amine is an aromatic amine having 2, 3, 4, 5, 6 or even more amine group and / or wherein the aromatic polyfunctional amine comprises at least 1 , preferably at least 2, at least 3, at least 4 or even more aromatic rings.

4. The aerogel according to any one of the preceding claims, wherein the aerogel has a specific surface area of more than 75 m2 / gram, preferably more than 100 m2 / gram, more preferably more than 150 m2 / gram, even more preferably more than 200 m2 / gram, most preferably more than 250 m2 / gram.

5. The aerogel according to any one of the preceding claims, having a porosity of more than 85 %, more preferably more than 90 %, such as between 80 and 98 %, or between 85 and 95 % or between 90 and 95 %.

6. The aerogel according to any one of the preceding claims, having a bulk density of less than 250 kg / m3, preferably less than 200 kg / m3, more preferably less than 150 kg / m3, such as between 30 and 150 kg / m3or between 50 and 100 kg / m3.

7. The aerogel according to any one of the preceding claims, having a linear shrinkage of less than 25 %, preferably less than 20 %, more preferably less than 15 %, even more preferably less than 10 % and the most preferable less than 5 %.

8. The aerogel according to any one of the preceding claims, having a contact angle of more than 90 °, preferably more than 95 °, more preferably more than 100 °, most preferably more than 105 °.

9. The aerogel according to any one of the preceding claims, having a decomposition temperature at 5 % weight loss (TD(5 %)) of more than 200 °C,preferably more than 250 °C, more preferably more than 280 °C, most preferably more than 320 °C.

10. The aerogel according to any one of the preceding claims, having a thermal conductivity (λ) that is less than 25 mW / m / K, preferably less than 23 mW / m / K, more preferably less than 21 mW / m / K, even more preferably less than 20 mW / m / K, most preferably less than 19 mW / m / K, such as between 10 and 19 mW / m / K.

11. A method of preparing an aerogel defined in any one of the preceding claims, said method comprising the steps of: a) forming a solution of the formaldehyde in a solvent and forming a solution of the one or more aromatic polyfunctional amines in a solvent; b) mixing the solutions in a ratio such that there is a substantially equimolar ratio between formaldehyde molecules and amino groups of the polyfunctional amines, and allowing gelation for a certain period of time for an organogel to be formed; c) optionally, the organogel formed in step b) is allowed to age for a certain period of time for a cured organogel to be formed d) optionally, one or more solvent exchange step (s) is(are) carried out to exchange the solvent(s) of step a) for a second solvent or mixture of solvents; e) supercritical drying, preferably by CO2, of the organogel formed in step b), c), or d) to obtain the aerogel.

12. The method according to claim 11 , wherein in step a) the solvent for the solution of the formaldehyde and the solvent for the solution of the one or more aromatic polyfunctional amines is selected from the group consisting of N-methyl-2- pyrrolidone (NMP), N,N-Dimethylforamide (DMF), Dimethylsulfoixde (DMSO), dimethylacetamide (DMAc), and N,N-dimethyl lactamide (DML).

13. The method according to claim 11 or claim 12, wherein in step a) the solutions are formed at a temperature of at least 50 °C, preferably at least 75 °C, more preferably at least 100 °C.

14. The method according to any one of claims 11-13, wherein in step b) the gelation is allowed to take place at a temperature of at least 50 °C, preferably at least 75 °C, more preferably at least 100 °C.

15. The method according to any one of claims 11-14, wherein step b) is carried out for a period of at least 0.01 hour.

16. The method according to any one of claims 11-15, wherein step c) is carried out for a period of time of between 1 and 48 hours, preferable between 1 and 24 hours, more preferable between 2 and 18 hours, even more preferably between 3 and 12 hours, most preferably between 3 and 8 hours, or at least 6 hours.

17. The method according to any one of claims 11-16, wherein step d) is carried out one or more times, preferably wherein the second solvent is selected from the group consisting of methanol, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, pentyl acetate, hexyl acetate, acetic anhydride, cyclohexane, ethylbenzene, n-octane, diethyl ether, methyl-t-butyl ether, methyl ethyl ketone, 3-pentanone, 1 ,4- dioxane, pentane, methylcyclohexane, tetrahydropyran, triethylamine, toluene, acetone, isopropanol, benzene, ethanol, water, 1-propanol, 1-butanol, and tetrahydrofuran a mixture of one or more of these solvent(s) or aqueous sodium hydroxide.

18. A use of a polyhexahydrotriazine (PHT) prepared from formaldehyde and one or more aromatic polyfunctional amines for the preparation of aerogels having a specific surface area of more than 50 m2 / gram, a porosity of more than 80 %, and a bulk density of less than 300 kg / m3.

19. A method of recycling the aerogel according to any of claims 1 - 10, comprising treating the aerogel with an aqueous acid solution having a pH of less than 4, preferably less than 2, more preferably less than 1 , to decompose the polyhexahydrotriazine (PHT) into the one or more aromatic polyfunctional amines.

20. A method of recycling the aerogel according to any of claims 1 - 10, comprising the steps of: i) treating the aerogel with a solution comprising an excess, preferably ranging from 1.5 to 6, one or more aromatic polyfunctional amines to depolymerize the polyhexahydrotriazine (PHT) into a mixture of soluble oligomers; and ii) adding formaldehyde to the mixture obtained in step i) to form a polyhexhydrotriazine; and iii) optionally, the organogel formed in step b) is allowed to age for a certain period of time for a cured organogel to be formed; and iv) optionally, one or more solvent exchange step (s) is(are) carried out to exchange the solvent(s) of steps i) and ii) for a second solvent or mixture of solvents; andv) supercritical drying, preferably by CO2, of the organogel formed in step ii), iii), or iv) to obtain the aerogel.

21. The method according to claim 19, wherein the aqueous acid solution comprises an aqueous solution of sulphuric acid, hydrochloric acid, citric acid, nitric acid, phosphoric acid, para-toluene sulfonic acid, and oxalic acid or a combination thereof.

22. A use of an aerogel according to any of claims 1 - 10 or prepared according to any one of the claims 11-17 for thermal insulation, energy storage, and chemical absorption.

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