FIELD OF THE INVENTION

The present invention relates to a method for producing a porous boron nitride material, a porous boron nitride material and uses of said porous boron nitride material.

BACKGROUND

Owing to their high surface area, large porosity and rich surface functionalities, porous materials (e.g. zeolites, activated carbon, and metal-organic frameworks) can be used in a range of applications from catalysis to gas separation, and storage. Processing these materials from powders to robust densified structures (e.g. pellets, beads, and monoliths) represents a necessary step towards their industrial deployment. Indeed, powders suffer from limited mass transfer and poor mechanical strength and they are difficult to handle at scale. The large amount of interparticle space in powders also causes low bulk density and low volumetric adsorption/storage capacity, which in turn, leads to an increased footprint.

Shaping and densifying powders into mechanically robust structures can be done as follows: (i) mechanical compaction, (ii) application of binders, or (iii) growth on monolithic supports. These approaches usually result in decreased performance due to pore collapse from high-pressure, pore blockage from binders, or ‘dead’ volume from the support. Recently, Tian et al. (Nat. Mater. 2018, 77, 174, the entire contents of which are incorporated by reference herein) developed a methodology towards the densification of metal organic frameworks via a sol-gel method without using high-pressure compaction or binders, providing scope for the development of monolithic porous materials. However, the advances of producing other robust porous materials is still lacking, particularly for ceramics.

Amorphous boron nitride (BN) represents a relatively new addition to the porous materials family, having use in molecular separation, catalysis, and drug delivery. Like other porous materials, identifying a way to shape and densify BN without compromising the porosity is needed. BN aerogel, the most common form of structured BN, has been synthesised using various methods such as direct chemical reaction, carbonaceous template assisted method, or molecular substitution. Yet, all structured BN aerogels exhibit low density and mechanical strength, preventing their upscaling for practical adsorption applications. Sintering, another method to synthesise structured BN, usually leads to a low surface area, albeit with high mechanical strength. With the current forms of structured BN, a tradeoff persists between mechanical stability, porosity, density and adsorption kinetics.

It is therefore desirable to provide an improved production technique and/or improved porous boron nitride material, and/or otherwise to obviate and/or mitigate one or more of the disadvantages with known production techniques and/or porous boron nitride materials, whether identified herein or otherwise.

SUMMARY

According to a first aspect of the present invention there is provided a method for producing a porous boron nitride material, the method comprising: providing a porous resin comprising a crosslinked nitrogen-containing polymer, a foaming agent, and a boron-containing compound; and heating the resin under an atmosphere comprising a nitrogen-containing gas to cause thermal degradation of the resin and form the porous boron nitride material.

The porous boron nitride material may be a monolithic porous boron nitride material. The porous boron nitride material may have a length of at least about 0.1 mm, optionally at least about 0.5 mm, optionally at least about 1 mm in one direction. For example, the porous boron nitride material may have a height, width and/or depth that is at least about 0.1 mm; optionally at least about 0.5 mm; optionally at least about 1 mm. The porous boron nitride material may be an amorphous and/or turbostratic porous boron nitride material.

The porous boron nitride material may be substantially carbon-free. The porous boron nitride material may have a specific surface area of about 900 m ² /g or more; optionally about 1100 m ² /g or more; optionally about 1300 m ² /g or more; optionally about 1400 m ² /g or more; optionally about 1500 m ² /g or more. The porous boron nitride material may have a volumetric specific surface area of about 100 m ² /cm ³ or more; optionally about 300 m ² /cm ³ or more; optionally about 350 m ² /cm ³ or more; optionally about 400 m ² /cm ³ or more; optionally about 450 m ² /cm ³ or more; optionally about 470 m ² / cm ³ or more. The porous boron nitride material may have a total pore volume of about 0.4 cm ³ /g or more; optionally about 0.5 cm ³ /g or more; optionally about 0.6 cm ³ /g or more; optionally about 0.7 cm ³ /g or more. The porous boron nitride material may have a total pore volume of up to about 10 cm ³ /g; optionally up to about 8 cm ³ /g; optionally up to about 6 cm ³ /g; optionally up to about 4 cm ³ /g; optionally up to about 2 cm ³ /g. The porous boron nitride material may have a total micropore volume of about 0.2 cm ³ /g or more; optionally about 0.3 cm ³ /g or more; optionally about 0.4 cm ³ /g or more; optionally about 0.5 cm ³ /g or more. The porous boron nitride material may have a bulk density is 0.2 g/cm ³ or more; optionally about 0.3 g/cm ³ or more. The porous boron nitride material may have a hardness of about 5 MPa or more; optionally about 10 MPa or more; optionally about 30 MPa or more; optionally about 50 MPa or more; optionally about 60 MPa or more.

The porous resin may have a hardness of about 10 MPa or more; optionally about 50 MPa or more; optionally about 100 MPa or more; optionally about 500 MPa or more optionally about 1 GPa or more. The porous resin may have a total pore volume of about 0.5 cm ³ /g or more; optionally about 0.6 cm ³ /g or more; optionally about 0.7 cm ³ /g or more; and/or a total micropore volume of about 0.3 cm ³ /g or more; optionally about 0.4 cm ³ /g or more; optionally about 0.5 cm.

The crosslinked nitrogen-containing polymer may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon and hydrogen atoms. The crosslinked nitrogen- containing polymer may comprise single, double, or triple carbon-nitrogen bonds; optionally the crosslinked nitrogen-containing polymer may comprise a -C-N-C-N- motif (wherein each C-N bond is a single, double, or triple bond); optionally a -C-N=C-N- motif. The polymer may comprise a triazine ring, preferably a 1,3,5-triazine ring. The crosslinked nitrogen- containing polymer may be a melamine-formaldehyde polymer, a urea-formaldehyde polymer, a melamine-urea-formaldehyde polymer, ora biuret-urea-formaldehyde; optionally a melamine-formaldehyde polymer.

The crosslinked nitrogen-containing polymer may comprise at least one polymerised monomer, wherein the at least one monomer is a nitrogen-containing organic compound. The crosslinked nitrogen-containing polymer may comprise a polymerised first monomer and a polymerised second monomer (and optionally one or more, e.g. one, further polymerised monomer), wherein the first monomer is a nitrogen-containing organic compound.

The porous resin may further comprise unreacted monomer and, optionally, partially reacted monomer.

The porous resin may be formed from a mixture comprising at least one monomer, a foaming agent and a boron-containing compound, wherein at least one monomer is a nitrogen-containing organic compound. The porous resin may be formed from a mixture comprising a first monomer, a second monomer (and optionally one or more further monomers, e.g. a third monomer), a foaming agent and a boron-containing compound, wherein the first monomer is a nitrogen-containing organic compound.

The first monomer (or the at least one monomer) may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon, oxygen and/or hydrogen atoms, optionally nitrogen, carbon, and hydrogen atoms. The first monomer may be selected from melamine, biuret and urea.

The second monomer may be an oxygen-containing organic compound. The second monomer may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon, oxygen and/or hydrogen atoms, optionally oxygen, carbon, and hydrogen atoms. The second monomer may be an aldehyde. The second monomer may be selected from formaldehyde and glyoxal.

The mixture may further comprise a third monomer. The third monomer may be a nitrogen- containing organic compound. The third monomer may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon, oxygen and/or hydrogen atoms, optionally nitrogen, carbon, and hydrogen atoms. The third monomer may be selected from melamine, biuret and urea (provided that the third monomer is not the same as the second monomer).

The foaming agent may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon, and hydrogen atoms. The foaming agent may be selected from dicyandiamide, cyanamide, 5- aminotetrazole, guanidinium carbonate, or a combination thereof. The foaming agent may be dicyandiamide.

The boron-containing compound may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, boron, oxygen and/or hydrogen atoms; optionally boron, oxygen and/or hydrogen atoms. The boron-containing compound may be boric acid, boron oxide, ammonia borane, boron, or a derivative thereof, or a combination thereof. The boron-containing compound may be a derivative of boric acid.

The nitrogen-containing gas may comprise ammonia. The nitrogen-containing gas may comprise a mixture of ammonia and an inert gas, such as nitrogen. The heating according to the first aspect may be to a temperature of at least about 700 °C; optionally at least about 800 °C. The heating may be to a temperature of below about 2000 °C. The heating may be to a temperature of between about 800 °C and about 1200 °C or between about 900 °C to about 1100 °C. The heating may be to about 1000 °C. The heating may be to or above a temperature sufficient to cause oxidation of elemental carbon during said heating. The heating may be to a temperature below the crystallisation temperature of boron nitride. The heating may be for at least about 1 minute, for at least about 30 minutes, or at least about 90 minutes; optionally at least about 120 minutes; optionally at least about 180 minutes.

Each of the crosslinked nitrogen-containing polymer, the boron-containing compound and the foaming agent in the mixture may consist of nitrogen atoms, carbon atoms, boron atoms, hydrogen atoms, and/or other elements which form gaseous products as a result of the heating. The gaseous products may be selected from carbon monoxide, carbon dioxide, nitrous oxide, water, nitrogen, ammonia and isocyanic acid.

The method according to the first aspect may further comprise the step of preparing the porous resin from a mixture comprising at least one monomer, a foaming agent and a boron- containing compound, wherein at least one monomer is a nitrogen-containing organic compound. The preparation of the porous resin may involve contacting the at least one monomer (optionally in the presence of a base), the foaming agent, and the boron- containing compound for a sufficient time to form the porous resin. The base may be an alkali metal base such as sodium hydroxide. The contacting may comprise heating. The heating may be to at least about 50 °C, optionally at least about 70 °C, optionally at least about 80 °C. The heating may be to at most about 300 °C. The heating may be for at least about 5 minutes, optionally at least about 30 minutes, optionally at least about 1 hour, optionally at least about 5 hours; optionally at least about 12 hours, optionally at least about 15 hours. The contacting may be at room temperature.

The preparation of the porous resin may involve: a) contacting a first and second monomer to form a mixture; b) adding the foaming agent to that mixture; and c) adding the boron- containing compound, wherein the first monomer is a nitrogen-containing organic compound. The contacting may comprise heating. The heating may be to at least about 50 °C, optionally at least about 70 °C, optionally at least about 80 °C. The heating may be to at most about 300 °C. The heating may be for at least about 5 minutes, optionally at least about 30 minutes, optionally at least about 1 hour, optionally at least about 5 hours; optionally at least about 12 hours, optionally at least about 15 hours. The contacting may be at room temperature.

The first monomer may be melamine, the second monomer may be formaldehyde, the foaming agent may be dicyandiamide, and the boron-containing compound may be boric acid, and the ratio of dicyandiamide/melamine by weight may be 0.3 to 0.9, optionally 0.4 to 0.8.

The molar ratio of the first monomer to the foaming agent may be selected to provide a predetermined total pore volume, and/or micropore volume and/or bulk density and/or hardness in the porous boron nitride material. The molar ratio of the first monomer to the foaming agent may be about 0.1 :1 to about 10:1; optionally about 0.5:1 to about 5:1; optionally about 0.8: 1 to about 3: 1.

The molar ratio of the second monomer to the first monomer may be at least about 1:1 ; optionally about 1:1 to about 10:1 , optionally about 1.5:1 to about 5:1 ; optionally about 2:1 to about 4: 1 ; optionally about 1 :3. The molar ratio of the boron-containing compound to the first monomer may be at least about 1:10, optionally at least about 1:6, optionally about 1:6 to about 10: 1 ; optionally about 1 :6 to about 6: 1 ; optionally about 1 :3 to about 3: 1 ; optionally about 1:1.

The ratio of nitrogen atoms in the first monomer to boron atoms in the boron-containing compound may be at least about 1:1 , optionally about 1:1 to about 20:1, optionally about 1 : 1 to about 10:1, optionally about 6: 1.

According to a second aspect of the invention, there is provided a porous boron nitride material obtainable by the method described herein. Also provided is a porous boron nitride material obtained by the method described herein.

The porous boron nitride material according to the second aspect may be substantially carbon-free. The porous boron nitride material may have a specific surface area of about 900 m ² /g or more; optionally about 1100 m ² /g or more; optionally about 1300 m ² /g or more; optionally about 1400 m ² /g or more; optionally about 1500 m ² /g or more. The porous boron nitride material may have a volumetric surface area of about 100 m ² /cm ³ or more; optionally about 300 m ² /cm ³ or more; optionally about 350 m ² /cm ³ or more; optionally about 400 m ² /cm ³ or more; optionally about 450 m ² /cm ³ or more; optionally about 470 m ² / cm ³ or more. The porous boron nitride material may have a total pore volume of about 0.4 cm ³ /g or more; optionally about 0.5 cm ³ /g or more; optionally about 0.6 cm ³ /g or more; optionally about 0.7 cm ³ /g or more. The porous boron nitride material may have a total pore volume of up to about 10 cm ³ /g; optionally up to about 8 cm ³ /g; optionally up to about 6 cm ³ /g; optionally up to about 4 cm ³ /g; optionally up to about 2 cm ³ /g. The porous boron nitride material may have a total micropore volume of about 0.2 cm ³ /g or more; optionally about 0.3 cm ³ /g or more; optionally about 0.4 cm ³ /g or more; optionally about 0.5 cm ³ /g or more. The porous boron nitride material may have a bulk density of 0.2 g/cm ³ or more; optionally about 0.3 g/cm ³ or more. The porous boron nitride material may have a hardness of about 5 MPa or more; optionally about 10 MPa or more; optionally about 30 MPa or more; optionally about 50 MPa or more; optionally about 60 MPa or more.

According to a third aspect, the present invention provides a monolithic porous boron nitride material, wherein the porous boron nitride material has a volumetric surface area of about 350 m ² /cm ³ or more; optionally about 400 m ² /cm ³ or more; optionally about 450 m ² /cm ³ or more; optionally about 470 m ² / cm ³ or more.

The monolithic porous boron nitride material according to the third aspect may have a specific surface area of about 1100 m ² /g or more; optionally about 1300 m ² /g or more; optionally about 1400 m ² /g or more; optionally about 1500 m ² /g or more. The monolithic porous boron nitride material may have a total micropore volume of about 0.5 cm ³ /g or more and/or a total pore volume of about 0.7 cm ³ /g or more and/or a bulk density of 0.2 g/cm ³ or more. The monolithic porous boron nitride material may have a hardness of about 10 MPa or more; optionally about 30 MPa or more; optionally about 50 MPa or more; optionally about 60 MPa or more. The monolithic boron nitride material may be an amorphous and/or turbostratic porous boron nitride material.

According to a fourth aspect, the invention provides a porous resin comprising a crosslinked nitrogen-containing polymer, a foaming agent, and a boron-containing compound.

The porous resin may have a hardness of about 10 MPa or more; optionally about 50 MPa or more; optionally about 100 MPa or more; optionally about 500 MPa or more; optionally about 1 GPa or more. The porous resin may have a total pore volume of about 0.5 cm ³ /g or more; optionally about 0.6 cm ³ /g or more; optionally about 0.7 cm ³ /g or more; and/or a total micropore volume of about 0.3 cm ³ /g or more; optionally about 0.4 cm ³ /g or more; optionally about 0.5 cm ³ /g or more. The crosslinked nitrogen-containing polymer may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon and hydrogen atoms. The crosslinked nitrogen- containing polymer may comprise single, double, or triple carbon-nitrogen bonds; optionally the crosslinked nitrogen-containing polymer may comprise a -C-N-C-N- motif (wherein each C-N bond is a single, double, or triple bond); optionally a -C-N=C-N- motif. The polymer may comprise a triazine ring, preferably a 1,3,5-triazine ring. The crosslinked nitrogen- containing polymer may be a melamine-formaldehyde polymer, a urea-formaldehyde polymer, a melamine-urea-formaldehyde polymer, ora biuret-urea-formaldehyde; optionally a melamine-formaldehyde polymer.

The crosslinked nitrogen-containing polymer may comprise at least one polymerised monomer, wherein the at least one monomer is a nitrogen-containing organic compound. The crosslinked nitrogen-containing polymer may comprise a polymerised first monomer and a polymerised second monomer (and optionally one or more, e.g. one, further polymerised monomer), wherein the first monomer is a nitrogen-containing organic compound. The porous resin may further comprise unreacted monomer and, optionally, partially reacted monomer.

The porous resin may be formed from a mixture comprising at least one monomer, a foaming agent and a boron-containing compound, wherein at least one monomer is a nitrogen-containing organic compound. The porous resin may be formed from a mixture comprising a first monomer, a second monomer (and optionally one or more further monomers, e.g. a third monomer), a foaming agent and a boron-containing compound, wherein the first monomer is a nitrogen-containing organic compound.

The first monomer (or the at least one monomer) may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon, oxygen and/or hydrogen atoms, optionally nitrogen, carbon, and hydrogen atoms. The first monomer may be selected from melamine, biuret and urea.

The second monomer may be an oxygen-containing organic compound. The second monomer may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon, oxygen and/or hydrogen atoms, optionally oxygen, carbon, and hydrogen atoms. The second monomer may be an aldehyde. The second monomer may be selected from formaldehyde and glyoxal. The mixture may further comprise a third monomer. The third monomer may be a nitrogen- containing organic compound. The third monomer may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon, oxygen and/or hydrogen atoms, optionally nitrogen, carbon, and hydrogen atoms. The third monomer may be selected from melamine, biuret and urea (provided that the third monomer is not the same as the second monomer).

The foaming agent may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon, oxygen and/or hydrogen atoms, optionally nitrogen, carbon, and hydrogen atoms. The foaming agent may be selected from dicyandiamide, cyanamide, 5- aminotetrazole, guanidinium carbonate, or a combination thereof. The foaming agent may be dicyandiamide.

The boron-containing compound may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, boron, oxygen and/or hydrogen atoms; optionally boron, oxygen and/or hydrogen atoms. The boron-containing compound may be boric acid, boron oxide, ammonia borane, boron, or a derivative thereof, or a combination thereof. The boron-containing compound may be a derivative of boric acid.

The first monomer may be melamine, the second monomer may be formaldehyde, the foaming agent may be dicyandiamide, and the boron-containing compound may be boric acid, and the ratio of dicyandiamide/melamine by weight may be 0.3 to 0.9, optionally 0.4 to 0.8. The molar ratio of the first monomer to the foaming agent may be selected to provide a predetermined total pore volume, and/or micropore volume and/or bulk density and/or hardness in the porous boron nitride material. The molar ratio of the first monomer to the foaming agent may be about 0.1 :1 to about 10:1; optionally about 0.5:1 to about 5:1; optionally about 0.8:1 to about 3:1. The molar ratio of the second monomer to the first monomer may be at least about 1:1 ; optionally about 1:1 to about 10:1 , optionally about 1.5: 1 to about 5: 1 ; optionally about 2: 1 to about 4: 1 ; optionally about 1 :3. The molar ratio of the boron-containing compound to the first monomer may be at least about 1:10, optionally at least about 1 :6, optionally about 1 :6 to about 10: 1 ; optionally about 1 :6 to about 6:1 ; optionally about 1 :3 to about 3:1; optionally about 1 :1. The ratio of nitrogen atoms in the first monomer to boron atoms in the boron-containing compound may be at least about 1 :1 , optionally about 1 : 1 to about 20: 1 , optionally about 1 : 1 to about 10:1, optionally about 6:1. According to a fifth aspect, the invention provides a method for preparing the porous resin according to the fourth aspect, comprising contacting the at least one monomer (optionally in the presence of a base), the foaming agent, and the boron-containing compound for a sufficient time to form the porous resin; wherein the at least one monomer is a nitrogen- containing organic compound.

The base may be an alkali metal base such as sodium hydroxide. The contacting may comprise heating. The heating may be to at least about 50 °C, optionally at least about 70 °C, optionally at least about 80 °C. The heating may be to at most about 300 °C. The heating may be for at least about 5 minutes, optionally at least about 30 minutes, optionally at least about 1 hour, optionally at least about 5 hours; optionally at least about 12 hours, optionally at least about 15 hours. The contacting may be at room temperature.

The preparation of the porous resin may involve: a) contacting a first and second monomer to form a mixture; b) adding the foaming agent to that mixture; and c) adding the boron- containing compound, wherein the first monomer is a nitrogen-containing organic compound. The contacting may comprise heating. The heating may be to at least about 50 °C, optionally at least about 70 °C, optionally at least about 80 °C. The heating may be to at most about 300 °C. The heating may be for at least about 5 minutes, optionally at least about 30 minutes, optionally at least about 1 hour, optionally at least about 5 hours; optionally at least about 12 hours, optionally at least about 15 hours. The contacting may be at room temperature.

According to a sixth aspect of the present invention, there is provided a use of a porous boron nitride material described herein in separation of a mixture comprising two or more components, for example, first and second gaseous components or separation of a mixture comprising two or more components, for example, first and second liquid components.

According to a seventh aspect of the present invention, there is provided a method for separating a mixture of gasses, the method comprising exposing a mixture comprising a first gaseous component and a second gaseous component to a porous boron nitride material described herein.

The first and second gaseous components may each be independently selected from nitrogen (N2), carbon dioxide (CO2), methane (ChU) and hydrogen (H2), optionally nitrogen (N2), carbon dioxide (CO2) and methane (ChU). The mixture may be at a pressure elevated above about 100 kPa during said exposing; optionally above about 250 kPa during said exposing; optionally above about 500 kPa during said exposing; optionally above about 1000 kPa during said exposing; optionally above about 1500 kPa during said exposing; optionally above about 2000 kPa during said exposing. The mixture may be at a temperature at or below about 40 °C during said exposing; optionally at or below about 25 °C during said exposing; optionally at or below about 10 °C during said exposing.

According to an eighth aspect of the present invention, there is provided a method for separating a mixture of a first liquid component and a second liquid component, the method comprising exposing a mixture comprising said first liquid component and said second liquid component to a porous boron nitride material described herein.

The first liquid component may be substantially immiscible with said second liquid component. The first liquid component may be a hydrocarbon; optionally oil. The second liquid component may be water.

According to a ninth aspect of the present invention, there is provided a use of a porous boron nitride material described herein as a support for one or more catalysts (e.g. catalyst or catalysts) and/or one or more catalyst promoters.

According to a tenth aspect of the present invention, there is provided a use of a porous boron nitride material described herein in the storage or transport of a gas or a mixture of gases, non-limiting illustrative examples of which may be selected from ammonia, carbon oxide and/or oxides, hydrogen, methane, natural gas, produced gas, associated gas or oxygen.

According to an eleventh aspect of the present invention, there is provided a use of a porous boron nitride material described herein as an additive to liquid formulations to improve heat transfer.

According to a twelfth aspect of the present invention, there is provided a method or product substantially as described herein with reference to the examples.

All features of each of the first aspect of the present invention as described above can be applied to the second to twelfth aspects of the present invention mutatis mutandis. BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows a schematic representation of monolithic porous boron nitride ( _m BN) synthesis via the formation of a melamine-formaldehyde resin (MF resin).

Figure 2 shows a comparison of absolute methane uptake at 298 K in powder boron nitride (BN) and _mp BN: a) gravimetric uptake; b) volumetric uptake.

Figure 3 shows optical images of the MF resin with different amounts of foaming agent, dicyandiamide (DCD) (a-e), and the derived porous BN samples (a’-e’). DCD/melamine weight ratio of (a) and (a’) 0; (b) and (b’) 0.24; (c) and (o’) 0.48; (d) and (d’) 0.72; (e) and (e’) 0.96 were used. Scale bar = 1 cm.

Figure 4 shows the N2 sorption isotherm at 77 K for BN synthesized under N2.

Figure 5 shows N2 sorption isotherms of _m BN at 77 K, a) pyrolysed for 3 h at different temperatures; b) pyrolysed at 1000 °C for increasing durations.

Figure 6 shows (a) X-ray diffraction (XRD) pattern; (b) Fourier-transform infrared (FTIR) spectrum; (c) scanning electron microscopy (SEM) image; and (d) X-ray photon spectroscopy (XPS) relative atomic percentages for _m BN.

Figure 7 shows (a) Brunauer-Emmett-Teller (BET) specific surface area loss as derived from N2 sorption at 77 K; (b) XRD patterns of powder BN; and (c) XRD patterns of _m BN.

Figure 8 shows N2 adsorption isotherms at 77 K after moisture exposure for (a) structured BN; and (b) powder BN at various timepoints.

Figure 9 shows N2 adsorption isotherms at 77 K for _m BN and powder BN in a) semi- logarithmic scale; and b) linear scale.

Figure 10 shows pore size distribution of _m BN as derived from N2 adsorption isotherms at 77 K.

Figure 11 shows a comparison of absolute methane uptake at 298 K between powder BN and _m BN: a) gravimetric uptake; b) volumetric uptake. Figure 12 shows a comparison of adsorption kinetics of N2 uptake at 77 K and 2.2 ^c 10 ^-6 bar between powder BN and _m BN: (a) linear scale, (b) log scale.

Figure 13 shows the surface composition of samples obtained from the pyrolysis of MF resin under NH ₃ atmosphere at different temperatures, as determined by high resolution XPS spectra: (a) B 1s; (b) N 1s; (c) C 1s; and (d) XPS relative atomic percentage of the samples.

Figure 14 shows FTIR of samples obtained from the pyrolysis of MF resin under NH ₃ at different temperatures.

Figure 15 shows the proposed structural evolution of _mp BN using MF resin as a precursor and NH ₃ as the pyrolysis atmosphere.

DEFINITIONS

The term “monolithic material” (e.g. monolithic porous boron nitride material) may be understood to be a material that has macroscopic dimensions. For example, “monolithic material” may refer to a material having a length of at least about 0.1 mm, optionally at least about 0.5 mm, optionally at least about 1 mm in one direction. For example, “monolithic material” may refer to a material having a height, width, and/or depth that is at least about 0.1 mm, optionally at least about 0.5 mm, optionally at least about 1 mm. Preferably, a “monolithic material” may refer to a material having a height, width and depth that is at least about 0.1 mm, optionally at least about 0.5 mm, optionally at least about 1 mm. Monolithic material may have a predominantly homogeneous and continuous microstructure which does not exhibit any structural components distinguishable by optical microscopy.

The term “resin” may be understood to refer to a polymer, or mixture of polymers, that optionally comprises partially polymerised materials, unreacted monomers, other non polymeric compounds and/or components.

The term “amorphous material” (e.g. amorphous porous boron nitride material) may be understood to be a material with predominantly amorphous character. Such a material does not have long-range crystalline order (i.e. the bulk properties of the material are substantially non-crystalline), although parts of the material may exist in a crystalline form (i.e. short- range order may exist). Crystallinity may be assessed using X-ray diffraction, with broad peaks and/or low intensities indicating lower or poorer crystallinity than narrow peaks and/or higher intensities.

The term “turbostratic material” (e.g. turbostratic porous boron nitride material) may be understood to be a material with partial crystalline character in which the planes (e.g. basal planes) of the crystalline structure are out-of-alignment.

The term “boron-containing compound” relates to a compound comprising at least one boron atom in its molecular structure.

The term “nitrogen-containing organic compound” relates to a compound comprising at least one nitrogen atom and at least one carbon atom in its molecular structure.

The term “oxygen-containing organic compound” relates to a compound comprising at least one oxygen atom and at least one carbon atom in its molecular structure.

The term “thermal degradation” may be understood to mean the breaking down of a compound upon exposure to heat into components which do not recombine on cooling. Thermal degradation may take place by a number of pathways, such as pyrolysis, oxidation, etc.

The term “room temperature” may be understood to mean about 20 °C.

The term “micropore” may refer to pores having a diameter less than about 2 nm. The term “mesopore” may refer to pores having a diameter from about 2 to 50 nm. The term “macropore” may refer to pores having a diameter of more than 50 nm.

The term “porous boron nitride material” may refer to boron nitride material having one or more pores, for example a plurality of pores. Porous boron nitride materials may have a high surface area where the majority of the surface area is a result of the pore(s). For example, a porous boron nitride material may refer to a material having a specific surface area of greater than 0 m ² /g, preferably greater than 1 m ² /g, preferably greater than 10 m ² /g, preferably greater than 100 m ² /g. The term “porous boron nitride material” may refer to boron nitride material having a total pore volume of greater than 0 cm ³ /g, preferably greater than 0.01 cm ³ /g, preferably greater than 0.1 cm ³ /g. The term “porous resin” may refer to a resin having one or more pores, for example, a plurality of pores. For example, a porous resin may comprise one or more macropores or channels. The number of pores may be higher but this is not essential. Where the resin comprises one or more macropores (or a single large macroporous channel), the specific or volumetric surface area may be low (e.g. <0.1 m ² /g or <0.1 m ² /cm ³ , respectively). The resin may have a total pore volume of greater than 0 cm ³ /g, preferably greater than 0.01 cm ³ /g, preferably greater than 0.1 cm ³ /g. The resin is sufficiently porous such that porous boron nitride is obtained according to the method described herein.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of these words, for example "comprising" and "comprises", mean "including but not limited to", and are not intended to (and do not) exclude other components. In any of the embodiment described herein, reference to “comprising” also encompasses “consisting essentially of”.

It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling within the scope of the invention. Each feature disclosed in this specification, unless stated otherwise, may be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

All of the features disclosed in this specification may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. In particular, the preferred features of the invention are applicable to all aspects of the invention and may be used in any combination. Likewise, features described in non- essential combinations may be used separately (not in combination).

It will be appreciated that many of the features described above, particularly of the preferred embodiments, are inventive in their own right and not just as part of an embodiment of the present invention. Independent protection may be sought for these features in addition to or alternative to any invention presently claimed.

DETAILED DESCRIPTION

Described herein is a method for producing a porous boron nitride material (for example, a monolithic porous boron nitride material). The method may be useful for the production of porous boron nitride materials having useful properties in terms of porosity, mechanical stability, bulk density, and/or structure at the macroscale. In particular, it has been unexpectedly found that the porosity, the mechanical stability and/or structure of the material can be tunable (i.e. selectively adjustable) based on the components of the resin.

According to a first aspect of the present invention there is provided a method for producing a porous boron nitride material (for example, a monolithic porous boron nitride material), the method comprising: providing a porous resin comprising a crosslinked nitrogen-containing polymer, a foaming agent, and a boron-containing compound; and heating the resin under an atmosphere comprising a nitrogen-containing gas to cause thermal degradation of the resin and form the porous boron nitride material.

The crosslinked nitrogen-containing polymer, the foaming agent, and the boron-containing compound may be referred to generally as components of the resin.

The materials produced by the methods of the present invention may have porosity characteristics which have not been achievable to date. In particular, the materials of the invention may have novel characteristics in terms of total pore volume, micropore volume and/or mesopore volume levels. Moreover, the materials produced by the methods of the present invention may have a combination of characteristics which have not been achievable to date, such as their porosity, mechanical stability, and/or structure at the macroscale.

The method of the present invention does not rely on any post-processing step (e.g. mechanical compaction, application of binders, growth on monolithic supports) to provide the porous boron nitride material (e.g. monolithic porous boron nitride). As a result, the present invention may offer methods for the production of porous boron nitride materials (in particular, structured materials such as monolithic boron nitride) having improved performance in comparison to porous boron nitride materials prepared from known methods using post-processing steps.

Accordingly, in some embodiments, the method does not involve a post-processing step.

The method may also be useful for producing a porous boron nitride material which is substantially free of carbon impurities.

The porous boron nitride material produced by the methods of the present invention may be a structured material, such as a monolithic material. As used herein, a structured material may refer to a material that is robust and/or densified, for example compared to a powder. For example, a structured material may refer to a material having macroscopic dimensions. For example, a structured material may be a material having a length of at least about 0.1 mm, optionally at least about 0.5 mm, optionally at least about 1 mm in one direction. For example, a structured material may be a material having a height, width, and/or depth that is at least about 0.1 mm, optionally at least about 0.5 mm, optionally at least about 1 mm. Preferably, a structured material may be a material having a height, width and depth that is at least about 0.1 mm, optionally at least about 0.5 mm, optionally at least about 1 mm. Structured (e.g. monolithic) porous boron nitride materials are denser and more robust than powdered porous boron nitride. Advantageously, structured porous boron nitride may have a higher bulk density and/or volumetric adsorption/storage capacity than powdered porous boron nitride, resulting in a reduced footprint.

The porous boron nitride material may be an amorphous and/or turbostratic porous boron nitride material (optionally monolithic), such as an amorphous porous boron nitride material.

The porous boron nitride may comprise nitrogen atoms from the nitrogen-containing polymer and boron atoms from the boron-containing compound, and optionally nitrogen atoms from the nitrogen-containing gas.

The crosslinked nitrogen-containing polymer may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon and hydrogen atoms.

The crosslinked nitrogen-containing polymer may contain one or more single, double, or triple carbon-nitrogen bonds, for example, a -C-N-C-N- motif (wherein each C-N bond may be a single, double, or triple bond, where valence permits). For example, the polymer may contain a -C-N=C-N- motif.

The polymer may contain heteroaromatic rings having the motif -C-N=C-N-. For example, the polymer may contain a triazine ring, preferably a 1,3,5-triazine ring. The polymer may contain the following motif: The polymer may contain heptazine moieties, for example derived from melon.

The crosslinked nitrogen-containing polymer may be a melamine-based polymer, a formaldehyde-based polymer, a urea-based polymer or a biuret-based polymer, or a combination thereof. The crosslinked nitrogen-containing polymer may be a melamine- formaldehyde polymer, a urea-formaldehyde polymer, a melamine-urea-formaldehyde polymer, or a biuret-urea-formaldehyde; optionally the crosslinked nitrogen-containing polymer may be a melamine-formaldehyde polymer.

The crosslinked nitrogen-containing polymer may comprise at least one polymerised monomer, wherein the at least one monomer is a nitrogen-containing organic compound. For example, the crosslinked nitrogen-containing polymer may comprise a polymerised first monomer and a polymerised second monomer (and optionally one or more, e.g. one, further polymerised monomer), wherein the first monomer is a nitrogen-containing organic compound.

The crosslinked nitrogen-containing polymer may comprise three polymerised monomers, wherein at least one monomer is a nitrogen-containing organic compound. For example, the crosslinked nitrogen-containing polymer may comprise a polymerised first monomer, a polymerised second monomer, and a polymerised third monomer, wherein the first monomer is a nitrogen-containing organic compound.

Suitable monomers include, for example, melamine, formaldehyde, biuret, glyoxal and urea and combinations thereof.

The porous resin may further comprise unreacted monomer. The porous resin may further comprise partially reacted monomer (i.e. partially polymerised materials). For example, if a monomer is melamine (e.g. the first monomer is melamine), then the porous resin may contain melamine itself, melamine that has polymerised at one amine moiety, and/or melamine that has polymerised at two amine moieties (whereas fully reacted melamine monomer has polymerised at all three amine moieties).

The porous resin may consist essentially of the crosslinked nitrogen-containing polymer, foaming agent, and boron-containing compound. As used herein, the phrase “consist essentially of” as applied to a designated component is used herein to denote that the designated component is present, and that one or more specific further components can be present, as long as those further components do not materially affect the essential characteristic(s) of the designated component. As applied to a porous resin comprising the crosslinked nitrogen-containing polymer, foaming agent, and boron-containing compound, for example, it will be appreciated that the “essential characteristic” of that resin is to enable the formation of a porous boron nitride material. If the resin is to “consist essentially of” such components, then the mixture should not comprise further components which may negatively impact such formation.

In the event that the method of the invention is for forming a material which is substantially free of carbon impurities and in which method the resin consists essentially of the components listed above, the resin should not comprise further components which may yield a material which is not substantially free of carbon impurities.

Suitably, the term “consist essentially of” may be interpreted such that the subject is primarily composed of a designated component (or components; i.e. there is a majority of that component(s)). Suitably, the subject comprises greater than or equal to about 85% of the designated component(s), such as greater than or equal to about 90%, such as greater than or equal to about 95%, such as greater than or equal to about 98%, such as greater than or equal to about 99%, such as about 100% (i.e. the subject consists of the designated component(s)).

The components present in the resin may be selected such that, collectively, they thermally degrade to form boron nitride and release gaseous by-products. Each of the crosslinked nitrogen-containing polymer, the foaming agent and the boron-containing compound may consist of nitrogen atoms, carbon atoms, boron atoms, hydrogen atoms and other elements which form gaseous products as a result of said heating. All boron and nitrogen atoms present in the resin may be either incorporated into the porous boron nitride material or evolve as gasses as a result of said heating, and any non-boron and non-nitrogen atoms may be evolved as gasses. Small amounts of carbon and oxygen atoms may be incorporated into the porous boron nitride material. Herein it is to be understood that atoms not being incorporated into the material may react with other species and thereafter form gaseous products. By way of example, carbon atoms present in the crosslinked nitrogen- containing polymer and/or the foaming agent and/or the boron-containing compound may react with oxygen (e.g. ambient oxygen or oxygen present in the resin) and thereafter form gaseous carbon dioxide. The gaseous products may be selected from carbon monoxide, carbon dioxide, nitrous oxide, water, nitrogen, ammonia and isocyanic acid (HNCO). It will be appreciated that selecting components in this way may yield a porous boron nitride material consisting essentially of boron, nitrogen and optionally oxygen atoms and/or carbon atoms, meaning that impurities (i.e. non boron, nitrogen and optionally oxygen and/or carbon atoms) are not present in the material (since other atoms are evolved in gaseous form). Such embodiments may therefore provide the advantage that there is no need to wash the material in order to remove impurities, thus providing a more straightforward and/or economical method. Certain impurities are not removable by washing in any case. As a result, it will be appreciated that selecting components in this way may yield a purer material than is hitherto known or achievable with techniques known in the art.

Each component may be present in the resin in at least about 0.1 w/w%. The resin may contain at least about 10, 20, 30, 40, 50, 60, 70, or80w/w% crosslinked nitrogen-containing polymer. The resin may contain at least about 5, 10, 15, 20, 25, or 30 w/w% boron- containing compound. The resin may contain at least about 1, 2, 5, 10, or 15 w/w% foaming agent. The w/w% of each component refers to the weight of that component relative to the total weight of the components in the resin.

The porous resin may be formed from a mixture comprising at least one monomer, a foaming agent and a boron-containing compound, wherein the at least one monomer is a nitrogen-containing organic compound. The mixture may comprise at least two monomers, wherein at least one monomer is a nitrogen-containing organic compound. For example, the porous resin may be formed from a mixture comprising a first monomer, a second monomer (and optionally one or more further monomers, e.g. a third monomer), a foaming agent and a boron-containing compound, wherein the first monomer is a nitrogen-containing organic compound. The mixture may comprise three monomers, wherein at least one monomer is a nitrogen-containing organic compound (e.g. a first monomer, a second monomer, and a third monomer, wherein the first monomer is a nitrogen-containing organic compound).

The at least one monomer (e.g. the first monomer) may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally may consist of nitrogen, carbon, oxygen and/or hydrogen atoms, optionally may consist of nitrogen, carbon, and hydrogen atoms. The first monomer may be selected from melamine, biuret and urea.

The second monomer (where present) may be an oxygen-containing organic compound. The second monomer may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon, oxygen and/or hydrogen atoms, optionally oxygen, carbon, and hydrogen atoms. The second monomer may be an aldehyde (e.g. an organic compound comprising an aldehyde moiety). The second monomer may be selected from formaldehyde and glyoxal.

The third monomer (where present) may be a nitrogen-containing organic compound. The third monomer may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally may consist of nitrogen, carbon, oxygen and/or hydrogen atoms, optionally may consist of nitrogen, carbon, and hydrogen atoms. The third monomer may be selected from melamine, biuret and urea (provided that the third monomer is not the same as the second monomer).

The presence of the foaming agent may result in the formation of pores in the crosslinked polymer and the resulting resin. The foaming agent may be referred to as a porogen, where a porogen is an agent used to make pores. During the thermal degradation, some foaming agent may evaporate from the resin.

The foaming agent may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, carbon, oxygen and/or hydrogen atoms, optionally nitrogen, carbon, and hydrogen atoms. The foaming agent may be selected from dicyandiamide, cyanamide, 5- aminotetrazole, guanidinium carbonate, or a combination thereof; optionally the foaming agent is dicyandiamide.

The boron-containing compound may consist of nitrogen, boron, carbon, oxygen and/or hydrogen atoms; optionally nitrogen, boron, oxygen and/or hydrogen atoms; optionally boron, oxygen and/or hydrogen atoms. The boron-containing compound may be boric acid, boron oxide (e.g. boron trioxide), ammonia borane, boron, or a derivative thereof, or a combination thereof.

The boron-containing compound acts as a boron source for the boron nitride formation. The boron-containing compound used to form the porous resin may also act as a curing agent. A curing agent refers to an agent that induces polymerisation. Advantageously, the boron- containing compound used to form the porous resin induces polymerisation of the at least one monomer, resulting in formation of the crosslinked nitrogen-containing polymer. The boron-containing compound used to form the porous resin may preferably be boric acid, boron oxide (e.g. boron trioxide), ammonia borane, boron, or a combination thereof. Following formation of the resin, the boron-containing compound present in the resin may be the same as the boron-containing compound used to form the resin or it may be a derivative of the boron-containing compound used to form the resin (e.g. a derivative of boric acid, boron oxide, ammonia borane, or boron). A derivative of boric acid or boron oxide may refer to a reaction product derived from two or more boric acid and/or boron oxide molecules (e.g. a dimer or trimer). For example, the derivative may be:

A derivative of boron may be boric acid, boron oxide, or an intermediate of empirical formula H _x BO _y wherein x is 0-3 and y is 0-3. A derivative of ammonia borane may be a dehydrogenated derivative of ammonia borane, for example having empirical formula BNH _Z wherein z is less than 6. This derivative can be considered a hydrogen bonded ammonia borane.

The resin may comprise a plurality of boron-containing compounds (e.g. a number of derivatives of boric acid and/or boron oxide, and optionally boric acid and/or boron oxide). The balance of boric acid and/or boron oxide to its derivatives present in the resin may change depending on the level of polymerisation of the resin.

The resin is heated under an atmosphere comprising a nitrogen-containing gas. Preferably, the nitrogen-containing gas comprises a reactive nitrogen gas. The term “reactive nitrogen gas”, as used herein, refers to a gas that may react with the resin during the heating step (e.g. ammonia). For example, the reactive nitrogen gas may provide a source of nitrogen to react with the boron-containing compound (e.g. boric acid or boron oxide) to contribute boron nitride formation. The nitrogen-containing gas may comprise a mixture of a reactive nitrogen gas (e.g. ammonia) and an inert gas (e.g. N2).

Advantageously, the use of a reactive nitrogen gas, such as ammonia, may contribute to formation of the porous boron nitride and/or may improve the surface area of the porous boron nitride.

The porous resin may be mechanically stable. For example, the porous resin may be sufficiently mechanically stable to prevent collapse of the macrostructure of the resin upon thermal degradation (e.g. pyrolysis). For example, the porous resin may have a hardness of about 10 MPa or more; optionally about 50 MPa or more; optionally about 100 MPa or more; optionally about 500 MPa or more; optionally about 1 GPa or more. Hardness may be measured using the Vickers method (for example, using a Struers Duramin -1/-2 Micro- Vickers hardness tester and measuring the sample at room temperature and atmospheric pressure, i.e. 1 atm or 101.325 kPa). The Vickers method is outlined on page 12 of Yovanovich, Micro and Macro Hardness Measurements, Correlations, and Contact Models, 44th AIAA Aerospace Sciences Meeting and Exhibit, Session: TP-10: Conduction and Convection Heat Transfer, 9-12 January 2006, AIAA 2006-979, p. 1-28, (DOI: 10.2514/6.2006-979); the entire contents of which are incorporated by reference herein.

The mechanical stability may depend on the crosslink density of the nitrogen-containing polymer. Crosslink density refers to the number of crosslinks per unit volume in the polymer network. For example, it may refer to the density of chains or segments that connect two parts of the polymer network.

The at least one monomer (e.g. the first and second monomers, e.g. the first, second and third monomers) may advantageously be selected to result in a polymer having the desired crosslink density.

Without wishing to be bound by theory, it is understood that during boron nitride formation, a crosslinked structure is maintained in the resin. This high bonding density allows mechanical robustness to be preserved in the final boron nitride product.

The porous resin may have a total pore volume of about 0.5 cm ³ /g or more; optionally about 0.6 cm ³ /g or more; optionally about 0.7 cm ³ /g or more; and/or a total micropore volume of about 0.3 cm ³ /g or more; optionally about 0.4 cm ³ /g or more; optionally about 0.5 cm ³ /g or more.

The porous resin may comprise one or more macropores (or “channels”). The macropore(s) may have a diameter of at least about 0.05 pm, at least about 0.5 pm, at least about 1 pm, at least about 5 pm, or at least about 10 pm. Where the resin comprises mostly macropores (or a single large macroporous channel), the specific or volumetric surface area may be low (e.g. <0.1 m ² /g or < 0.1 m ² /cm ³ , respectively).

The pore(s) in the porous resin may advantageously act as channel(s) to allow the porogen to escape upon thermal degradation (e.g. pyrolysis). The resin may have a suitable range of pore density such that porous monolithic boron nitride is obtained. In the method according to the first aspect, the resin is heated such that it thermally degrades, boron nitride is formed, and gaseous by-products are released.

The heating may be at or above a temperature sufficient to cause oxidation of elemental carbon during said heating. It will be appreciated that the exact temperature at which elemental carbon would be oxidised would depend on ambient conditions (e.g. pressure of a system in which the method/reaction is taking place). The heating may be at a temperature at or above the oxidation temperature of elemental carbon (e.g. the oxidation temperature under standard pressure of about 100 kPa; 1 bar).

It may be undesirable to have carbon-based impurities in the boron nitride material. By way of example, if a boron nitride material is intended to be used in a high temperature application, carbon-based impurities in the material may be caused to thermally decompose (e.g. oxidise) on heating, generating weaknesses (e.g. structural weaknesses) in the material. Heating at or above the temperatures in the preceding paragraph (and at temperatures discussed below) may be useful to remove such impurities and to ameliorate this issue.

The heating may be below the crystallisation temperature of boron nitride. As is well known in the art, heating a material can cause the material to undergo a transition such that the resultant material becomes more crystalline (relative to the starting material). Heating to a temperature at or above the crystallisation temperature of boron nitride may have a negative impact on the porosity of the material.

The heating may be to a temperature of at least about 700 °C; optionally at least about 800 °C; optionally below about 2000 °C; optionally between about 800 °C and about 1500 °C, optionally between about 800 °C and about 1200 °C, optionally between about 900 °C to about 1100 °C, optionally said heating is at about 1000 °C.

Heating may be at increasing temperature (e.g. ramping), fluctuating temperature, or constant temperature. The heating may be for at least about at least about 1 minute, optionally at least about 5 minutes, optionally at least about 30 minutes, optionally at least about 90 minutes; optionally at least about 120 minutes; optionally at least about 180 minutes. The heating may be achieved by ramping the temperature of the mixture at a rate of about 1 to 20 °C per minute; optionally about 1 to 20 °C per minute; optionally about 5 to 15 °C per minute. Said heating may be achieved by ramping the temperature of the mixture at a rate of about 5 °C or about 10 °C or about 15 °C per minute. Said heating may be achieved by ramping the temperature of the mixture from room temperature.

In embodiments where ramping is involved, the above heating time may refer to the dwell time (i.e. the time that heating is maintained once the ramping is complete). Additionally, in embodiments where ramping is involved, the dwell time may be 0 minutes, i.e. the temperature is not constant.

The method according to the first aspect may further comprise the step of preparing the porous resin from a mixture comprising at least one monomer, a foaming agent and a boron-containing compound, wherein the at least one monomer is a nitrogen-containing organic compound.

The preparation of the porous resin may involve contacting the at least one monomer (optionally in the presence of a base), the foaming agent, and the boron-containing compound for a sufficient time to form the porous resin.

Any suitable base may be used. The base may be a Bronsted base. The base may be an organic or inorganic base, optionally an inorganic base. The base may be an alkali metal base such as sodium hydroxide.

The contacting step may be at room temperature or at elevated temperature. Thus, the contacting may comprise heating. The heating may induce polymerisation (i.e. curing). The heating may be to at least 50 °C, optionally at least 70 °C, optionally at least 80 °C. The heating may be to at most 300 °C. The contacting (e.g. the heating) may be for at least about 5 minutes, optionally at least about 30 minutes, optionally at least about 1 hour, optionally at least about 5 hours; optionally at least about 12 hours, optionally at least about 15 hours.

A skilled person would understand that at elevated temperatures, polymerisation will be accelerated. Therefore, as temperature increases, the necessary heating time will shorten.

The preparation of the porous resin may involve: a) contacting a first and second monomer to form a mixture, and b) adding the foaming agent and the boron-containing compound to that mixture; wherein the first monomer is a nitrogen-containing organic compound. The preparation of the porous resin may involve: a) contacting a first and second monomer to form a mixture, b) adding the foaming agent to that mixture, and c) adding the boron- containing compound to the mixture; wherein the first monomer is a nitrogen-containing organic compound. The contacting step may comprise contacting a third monomer with the first and second monomer. The mixture may be heated once the monomers are contacted. The mixture may be heated once the boron-containing compound has been added. Heating temperatures and times may be as described above in relation to the contacting step.

The boron-containing compound used to form the porous resin may preferably be boric acid, boron oxide (e.g. boron trioxide), ammonia borane, boron, or a combination thereof. The boron-containing compound used to prepare the porous resin may act as a curing agent. Advantageously, the boron-containing compound used to prepare the porous resin induces polymerisation of the monomer(s), resulting in formation of the crosslinked nitrogen-containing polymer.

A skilled person would understand that, where a boron-containing compound acts as a curing agent, heating may not be needed to induce polymerisation. Similarly, where the contacting step comprises heating, the boron-containing compound may not need to act as a curing agent.

It has been unexpectedly found that, by adjusting the molar ratios of the first monomer to the foaming agent, it is possible to achieve desirable porosity and mechanical stability characteristics in the boron nitride material produced by the methods of the present invention (i.e. it is possible to “tune” or selectively adjust the porosity and mechanical stability characteristics of the materials produced). In particular, selection of the molar ratios of the precursors may yield materials having desirable and/or predetermined total pore volume, micropore volume, mesopore volume, bulk density and/or hardness levels.

Pore characteristics desired of a porous boron nitride material may be different depending on the application envisaged. In gas separation, for example, it may be desirable to have pores of a particular size (e.g. a highly microporous material) to enable selective sorption of molecules of one gas over another. Other applications may require different porosity characteristics. In some embodiments, selection of the relative molar ratios of the compounds in the mixture may yield materials having a desired total pore volume, mesopore volume (diameter between about 2 and 50 nm) and/or micropore volume (diameter less than about 2 nm), as well as a desired bulk density and/or hardness. The first monomer may be melamine, the second monomer may be formaldehyde, the foaming agent may be dicyandiamide, and the boron-containing compound may be boric acid, and the ratio of dicyandiamide/melamine by weight may be 0.3 to 0.9, optionally 0.4 to 0.8.

The molar ratio of the first monomer to the foaming agent may be selected to provide a predetermined total pore volume, and/or micropore volume and/or bulk density and/or hardness in the porous boron nitride material. The molar ratio of the first monomer to the second monomer (and optionally the first monomer to the third monomer); and/or the first monomer to the boron-containing compound may be selected to provide a predetermined total pore volume, and/or micropore volume and/or bulk density and/or hardness in the porous boron nitride material.

The molar ratio of the first second monomer to the second first monomer may be at least about 1:1; optionally about 1 : 1 to about 10: 1 , optionally about 1.5:1 to about 5: 1 ; optionally about 2:1 to about 4:1; optionally about 1 :3. The molar ratio of the boron-containing compound first monomer to the first monomer boron-containing compound may be at least about 1 :10, optionally at least about 1 :6, optionally about 1 :6 to about 10:1; optionally about 1 :6 to about 6: 1 ; optionally about 1 :3 to about 3: 1 ; optionally about 1 :1. The molar ratio of the first monomer to the foaming agent may be about 0.1:1 to about 10:1; optionally about 0.5: 1 to about 5: 1 ; optionally about 0.8: 1 to about 3: 1.

The ratio of nitrogen atoms in the first monomer to boron atoms in the boron-containing compound may be at least about 1:1 , optionally about 1:1 to about 20:1, optionally about 1 : 1 to about 10:1, optionally about 6: 1.

Any discussion above relating to the first monomer may also apply to the at least one monomer and vice versa.

According to a second aspect of the invention, there is provided a porous boron nitride (optionally a monolithic porous boron nitride material) obtainable by the method according to the first aspect.

The porous boron nitride material may be substantially carbon-free. As used herein, “substantially carbon-free” may refer to the material comprising less than or equal to about 5% of carbon (such as an atomic percentage, based on the total number of atoms in the material; or a weight percentage, based on the total weight of the material), such as less than or equal to about 2%, such as less than or equal to about 1%, such as less than or equal to about 0.5%, such as less than or equal to about 0.1%, such as about 0%. Suitably, the surface carbon-content of a material can be measured by X-ray photoelectron spectroscopy (XPS), which measures the atomic percentage of elements in a sample. Alternatively, a carbon and oxygen analyser could be used to determine the carbon- and oxygen-content, on the basis of the total weight of a sample. Alternatively, inductively coupled plasma optical emission spectrometry (ICP-OES) could be used to determine the carbon- and oxygen-content.

As mentioned above, the materials obtainable by the method according to the first aspect of the present invention may possess useful properties in terms of porosity, mechanical stability and/or macroscopic structure, while remaining substantially free of impurities (such as carbon impurities). The material may further comprise oxygen as set out above.

The porous boron nitride material may be obtained by the method according to the first aspect.

The porous boron nitride material may be defined in terms of volumetric surface area and/or specific surface area. Volumetric surface area is the surface area per volume (m ² /cm ³ ). Specific surface area is the surface area per weight (m ² /g). This is closely related to both the porosity and the bulk density of the material.

The porous boron nitride material may have a specific surface area of about 900 m ² /g or more as determined by BET (in the procedure generally outlined in Brunauer, S., P.H. Emmett, and E. Teller, Adsorption of gases in multimolecular layers, J. Am. Chem. Soc., 1938. 60(2): p. 309-319); the entire contents of which are incorporated by reference herein); optionally about 1100 m ² /g or more; optionally about 1300 m ² /g or more; optionally about 1400 m ² /g or more; optionally about 1500 m ² /g or more.

The porous boron nitride material may have a volumetric surface area of about 100 m ² /cm ³ ; optionally about 300 m ² /cm ³ or more; optionally about 350 m ² /cm ³ or more; optionally about 400 m ² /cm ³ or more; optionally about 450 m ² /cm ³ or more; optionally about 470 m ² /cm ³ or more. Volumetric surface area may be determined using BET (to obtain the specific surface area), as described herein, and mercury porosimetry (to obtain the density of the material, so that surface area per volume can be obtained). Mercury porosimetry may be carried out using the method set out in Giesche, Handbook of Porous Solids, Wiley-VCH Verlag GmbH, Weinheim, Germany, 2008, pp. 309-351, the entire contents of which are incorporated by reference herein. Mercury porosimetry may be measured using an Micromeritics AutoPore IV 9500 instrument.

As used herein, the BET specific surface area may be determined as follows, with reference to the equations below.

Nitrogen isotherms can be measured using a porosity analyser (e.g. Micromeritics 3Flex). In such an experiment, samples should be degassed overnight at 120 °C and about 20 Pa (about 0.2 mbar), followed by degassing in-situ on the porosity analyser for 4 hours down to about 0.3 Pa (about 0.0030 mbar). Measurements should be taken at -196 °C.

The following equation can then be used to derive a plot, optionally conducted by the porosity analyser itself:

Where:

V: Volume of gas adsorbed (determined from the isotherm)

V _m : Volume corresponding to the monolayer coverage P: Pressure of gas at equilibrium (determined from the isotherm) Po: Saturation pressure (determined from the isotherm)

C: Constant

The software from the porosity analyser may be used to plot the equation rearranged as:

From this plot V _m and C can be calculated (V _m being the Y-intercept and C being the slope of the curve), and the BET specific surface area may then be calculated as follows:

Where: S _adsorptive : Cross sectional area of the adsorptive

VM: Molar volume of the adsorptive at STP (22,414 cm ³ /mol)

Navogadro: Avogadro constant

The BET specific surface area is calculated considering the pressure range in which the data points fit a linear data fit. The data points must satisfy the following criteria:

• Pressure range: V[P ₀ -P] increases with P/P ₀

• The pressure corresponding to V _m should be within the pressure range selected

• Negative intercepts are not acceptable

The porous boron nitride material may have a total pore volume of about 0.4 cm ³ /g or more; optionally about 0.5 cm ³ /g or more; optionally about 0.6 cm ³ /g or more; optionally about 0.7 cm ³ /g or more. The porous boron nitride material may have a total pore volume of up to about 10 cm ³ /g; optionally up to about 8 cm ³ /g; optionally up to about 6 cm ³ /g; optionally up to about 4 cm ³ /g; optionally up to about 2 cm ³ /g.

The total volume of pores may be calculated from the following equation, using the results of the nitrogen isotherm measurement noted above:

Where:

P _standard : standard pressure (10 ⁵ Pa)

V _adsorbed : volume of N2 adsorbed at P/Po = 0.97 (determined from the isotherm) VM: molar volume of liquid N2 at 77K (34.65 cm ³ /mol)

R: gas constant T: standard temperature

The porous boron nitride material may comprise micropores, mesopores and/or macropores.

The porous boron nitride material may have a total micropore volume of about 0.2 cm ³ /g or more; optionally about 0.3 cm ³ /g or more; optionally about 0.4 cm ³ /g or more; optionally about 0.5 cm ³ /g or more. Micropore volume may be calculated using the Dubinin Radushkevich model (see, for example, S. G. Chen, R. T. Yang, Langmuir 1994, 10, 4244; the entire contents of which are incorporated by reference herein) and is based on the following equations, using results of the nitrogen isotherm measurement noted above: f f r lΫ log(t) = log(« , ) - D log p

V V ^J 0 JJ

Where: n: adsorption capacity at P n _miC : adsorption capacity from the micropores

D: empirical constant

P: equilibrium pressure (determined from the isotherm) Po: saturation pressure (determined from the isotherm)

A plot of log(n) against (log(P/Po)) ² enables a value for n _mic to be derived (from the Y intercept). Here, only the linear range of the plot is used.

Then the micropore volume V _mic may be determined from the following equation:

Where M is the adsorbate molar mass and p the adsorbate density.

The porous boron nitride material may consist of micropores and mesopores, in which case the mesopore volume can be calculated by subtracting the micropore volume from the total pore volume.

The porous boron nitride material may include one or more macropores (or “channels”). The macropore(s) may have a diameter of at least about 0.05 pm, at least about 0.5 pm, at least about 1 pm, at least about 5 pm, or at least about 10 pm.

The porous boron nitride material may have a bulk density of 0.2 g/cm ³ or more; optionally about 0.3 g/cm ³ or more. Bulk density is the ratio of the mass of an uncompressed solid sample and its volume, including the volume of the solid and the interparticle space. Bulk density may be measured using mercury porosimetry, as described herein.

Mechanical stability (or mechanical strength) may be assessed in terms of hardness. The porous boron nitride material may have a hardness of about 5 MPa or more; optionally about 10 MPa or more; optionally about 30 MPa or more; optionally about 50 MPa or more; optionally about 60 MPa or more.

The porous boron nitride material may have a low oxygen content. For example, the porous boron nitride material may comprise at most about 10% oxygen, optionally at most about 8%, optionally at most about 5%, optionally at most about 3%. The porous boron nitride material may have a low carbon content. For example, the porous boron nitride material may comprise at most about 5% carbon, optionally at most about 3%, optionally at most about 2%, optionally at most about 0.5%; optionally at most about 0.1%. The percent may be an atomic percentage, based on the total number of atoms in the material; or a weight percentage, based on the total weight of the material. Suitably, the surface oxygen-content and carbon-content of a material can be measured by X-ray photoelectron spectroscopy (XPS), which measures the atomic percentage of elements in a sample. Alternatively, a carbon and oxygen analyser could be used to determine the carbon- and oxygen-content, on the basis of the total weight of a sample. Alternatively, inductively coupled plasma optical emission spectrometry (ICP-OES) could be used to determine the carbon- and oxygen- content.

The porous boron nitride material may be as described in any other aspect.

According to a third aspect, the present invention provides a monolithic porous boron nitride material, wherein the porous boron nitride material has a volumetric surface area of about 350 m ² /cm ³ or more; optionally about 400 m ² /cm ³ or more; optionally about 450 m ² /cm ³ or more; optionally about 470 m ² / cm ³ or more. The monolithic porous boron nitride material may have a length of at least about 0.1 mm, optionally at least about 0.5 mm, optionally at least about 1 mm in one direction. For example, the porous boron nitride material may have a height, width and/or depth that is at least about 0.1 mm; optionally at least about 0.5 mm; optionally at least about 1 mm.

The monolithic porous boron nitride material may have a specific surface area of about 900 m ² /g or more; optionally about 1100 m ² /g or more; optionally about 1300 m ² /g or more; optionally about 1400 m ² /g or more; optionally about 1500 m ² /g. The monolithic porous boron nitride material may have a total micropore volume of about 0.5 cm ³ /g or more and/or a total pore volume of about 0.7 cm ³ /g or more and/or a bulk density of 0.2 g/cm ³ or more.

The monolithic porous boron nitride material may have a hardness of about 10 MPa or more; optionally about 30 MPa or more; optionally about 50 MPa or more; optionally about 60 MPa or more.

The monolithic porous boron nitride material may be as described in any other aspect. For example, the monolithic porous boron nitride material may have a property (e.g. surface area, total micropore volume, total pore volume, bulk density, hardness) as described herein generally for porous boron nitride material.

A structured (e.g. monolithic) porous boron nitride material, as described herein, may have improved properties compared to a porous boron nitride material reported to date (e.g. powders, aerogels). For example, a structured (e.g. monolithic) porous boron nitride material may have improved hydrolytic stability, higher bulk density (and therefore an increased volumetric surface area), and/or improved adsorption capacity than porous boron nitride materials reported to date.

According to a fourth aspect, the invention provides a porous resin comprising a crosslinked nitrogen-containing polymer, a foaming agent, and a boron-containing compound.

The porous resin may be formed from a mixture comprising at least one monomer, wherein the at least one monomer is a nitrogen-containing organic compound (e.g. a first monomer which is a nitrogen-containing organic compound and a second monomer, and optionally a third monomer), a foaming agent, and a boron-containing compound.

The porous resin, crosslinked nitrogen-containing polymer, foaming agent, boron- containing compound, at least one monomer, first monomer, second monomer and/or third monomer according to the fourth aspect may be as described in any previous aspects.

According to a fifth aspect, the invention provides a method for preparing the porous resin described herein, comprising: contacting at least one monomer (optionally in the presence of a base), the foaming agent, and the boron-containing compound for a sufficient time to form the porous resin; wherein the at least one monomer is a nitrogen-containing organic compound.

The method for preparing the porous resin may be as described in the first aspect. The porous resin, at least one monomer, foaming agent, boron-containing compound, and/or base accord to the fifth aspect may be as described in any previous aspects.

Porous boron nitride materials have broad utility in a wide variety of applications, such as gas separation, liquid purification (such as water treatment) and other liquid separation techniques, air treatment, gas storage, drug delivery and catalysis, and as additives to liquid formulations to improve heat transfer. Additionally, such materials have a particularly high thermal stability (e.g. about 800-1000 °C in air and greater than about 1800 °C, such as greater than about 2000 °C under an inert atmosphere). As a result, porous boron nitride materials may offer a useful substitute for carbonaceous porous materials (e.g. activated carbon) in applications where high temperatures are envisaged. One particular feature denoting suitability in this regard relates to the recyclability of boron nitride materials. In particular, substances sorbed to the boron nitride material can be burned away (e.g. thermally degraded, oxidised, etc., and the products evolved as gasses) by heating (optionally in, for example, an oxidising atmosphere such as air or oxygen), thereby regenerating the boron nitride material for further use. In contrast, carbonaceous materials readily undergo degradative processes (e.g. oxidation) under elevated temperature, and so heat-based recycling/regeneration techniques may be less useful.

According to a sixth aspect of the present invention, there is provided a use of a porous boron nitride material according to the second or third aspects of the present invention in gas separation (e.g. separation of a mixture comprising two or more components, for example, first and second gaseous components) or liquid separation (e.g. separation of a mixture comprising two or more components, for example, first and second liquid components). The gases may be vapours, such that the use is for separation of a mixture comprising two or more, for example first and second vapours. The liquid components may comprise dissolved solids, such that the use is for separation of dissolved solids (e.g. separation of a mixture comprising first and second dissolved solids). According to a seventh aspect of the present invention, there is provided a method for separating a mixture of gasses, the method comprising: exposing a mixture comprising two or more gaseous components, for example a first gaseous component and a second gaseous component, to a porous boron nitride material according to the second or third aspects of the present invention.

The gases may be vapours, such that the method is for separating a mixture of vapours, the method comprising: exposing a mixture comprising two or more vapours, for example a first and second vapour, to a porous boron nitride material according to the second or third aspects of the present invention.

As mentioned above, the porosity (e.g. the total, micro, and/or meso porosity) of the materials of the present invention may be tunable to yield boron nitride materials having desirable porosity characteristics. Materials with certain porosity characteristics may be useful for preferential sorption of one gaseous component over another, meaning that such materials are particularly useful for the separation of mixtures of gasses. In particular, a gaseous component may have a higher affinity for a material with a given micro/mesoporosity, whereas another gaseous component may have a lower affinity for that material.

The first and/or second gaseous components may each independently be selected from nitrogen (N2), carbon dioxide (CO2), hydrogen (H2) methane (ChU), optionally nitrogen (N2), carbon dioxide (CO2) and methane (ChU).

The mixture may be at a pressure elevated above about 100 kPa during said exposing; optionally above about 250 kPa; optionally above about 500 kPa; optionally above about 1000 kPa; optionally above about 1500 kPa; optionally above about 2000 kPa. In certain embodiments, materials may have a higher affinity for one gas over another gas at elevated pressure (relative to a comparative affinity at a lower pressure).

The mixture may have a temperature at or below about 40 °C during said exposing; optionally at or below about 25 °C during said exposing; optionally at or below about 10 °C during said exposing. In certain embodiments, materials may have a higher affinity for one gas over another gas at reduced temperature (relative to the affinity at a higher temperature). According to an eighth aspect of the present invention, there is provided a method for separating a mixture of a first liquid component and a second liquid component, the method comprising: exposing a mixture comprising said first liquid component and said second liquid component to a porous boron nitride material according to the second or third aspects.

The liquid components may comprise dissolved solids, such that the method is for separating a mixture comprising two or more dissolved solids, for example a first and second dissolved solid, the method comprising: exposing a mixture comprising two or more dissolved solids, for example a first and second dissolved solid, to a porous boron nitride material according to the second or third aspects.

As mentioned above in relation to gas affinity, a given liquid component may have a higher affinity for the materials of the invention than another liquid component. Thus, the materials of the present invention may be used for the separation of mixtures of two or more liquid components.

The first liquid component may be substantially immiscible with said second liquid component. As used herein, the term “immiscible” may be understood to mean that a first specified liquid component does not form a homogenous solution upon mixture with a second liquid component. Suitably, immiscible may be understood to mean that a first specified liquid component has a solubility of less than about 500 mg/L in a second specified liquid component (i.e. 500 mg of the first component in one litre of a second component), such as less than about 250 mg/L, such as less than about 100 mg/L, such as less than about 50 mg/L, such as less than about 10 mg/L.

The first liquid component may be a hydrocarbon; optionally oil. Oil may be understood as being a liquid comprising a mixture of hydrocarbons. The first liquid component may be “crude oil” (petroleum), which is a naturally occurring liquid mixture of hydrocarbons, typically extracted from the ground.

The second liquid component may be water.

Porous boron nitride materials are particularly useful in the separation of oil-water mixtures (in which applications oil may be preferentially sorbed to the material, over water) owing to recyclability and hydrophobicity of certain boron nitride materials. In particular, since boron nitride has relatively high thermal resistance as described above, oil sorbed in/on the material can simply be burned away (e.g. thermally degraded, oxidised, etc.), yielding a regenerated material ready for further sorption.

The properties described herein for the porous boron nitride material are advantageous for its use as an adsorbant. For example, good bulk density may be needed due to the limited space for the absorbant in practical use and good mechanical strength may be needed if the material is to be exposed mechanical stress such as operational vibration and compression, weight of packed adsorbent in their applications. Advantageously, the prorous boron nitride materials described herein may demonstrate high bulk density and mechanical strength (or stability), while maintaining high porosity.

According to a ninth aspect of the present invention, there is provided a use of a porous boron nitride material described herein as a support for one or more catalysts and/or catalyst promoters.

According to a tenth aspect of the present invention, there is provided a use of a porous boron nitride material described herein in the storage or transport of a gas or a mixture of gases, non-limiting illustrative examples of which may be selected from ammonia, carbon oxide and/or oxides, hydrogen, methane, natural gas, produced gas, associated gas or oxygen.

According to an eleventh aspect of the present invention, there is provided a use of a porous boron nitride material described herein as an additive to liquid formulations to improve heat transfer.

According to a twelfth aspect of the present invention, there is provided a method or product substantially as described herein with reference to the examples.

Features described above in relation to the first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, tenth, eleventh and/or twelfth aspects of the present invention also represent features of the each other aspect of the present invention (and vice versa) subject to a technical incompatibility that would prevent such a combination of preferred features. Furthermore, it will be evident to the skilled person that advantages set out above in respect of the first, second, third, fourth, fifth, sixth, seventh, eighth, ninth, tenth, eleventh and/or twelfth aspects of the present invention are also offered by each other aspect of the present invention (again and vice versa). EXAMPLES

The following examples are merely illustrative examples of the invention described herein and are not intended to be limiting upon the scope of the invention.

Materials

Melamine (99%), and boric acid (³ 99.5%) were purchased from Sigma-Aldrich; dicyandiamide (99.5%), and formaldehyde solution (37%) were purchased from Fisher. All chemicals were used as received. The gases used in this study, namely NH3 (99.98%), CFU (99.5%), N2 (99.9995%), and Ar (99.999%) were purchased from BOC.

Synthesis of monolithic porous BN ( _mp BN)

A mechanically robust melamine-formaldehyde resin, referred to as MF resin, was used as the precursor to synthesise monolithic porous BN. The overall synthesis procedure is shown in Figure 1 and described below.

Synthesis of MF resin

Formaldehyde (5 mL), NaOH (1 M, 0.15 mL) and melamine (2.08 g) were mixed in a 50 mL round bottom flask and stirred at 90 °C for 1 h. 1.5 g dicyandiamide (DCD) was then added to the transparent solution and stirred for 5 min. Boric acid (1 g) was added to the transparent solution and vigorously stirred for 2 min, followed by a curing step at 80 °C overnight. The collected transparent solid was further dried at 120 °C under vacuum overnight and is referred to as MF resin. The MF resin ‘slab’ broke into small pieces after vacuum treatment.

Synthesis of _mp BN

The MF resin was placed in an alumina boat crucible and transferred to a tubular furnace. The resin was first degassed at room temperature under Ar flow (250 mL/min). Upon completion of the degassing, the sample was heated to 1000 °C (10 °C/min) under pure NH3 gas flow (100 mL/min) and held isothermally for 3 h, unless otherwise specified. The furnace was cooled naturally under NH3 to 600 °C and then to room temperature under Ar. Light yellow monoliths were collected after the synthesis and further dried at 120 °C under vacuum overnight.

Material characterisation

Chemical properties Infrared (IR) spectra were collected using a PerkinElmer Spectrum 100 spectrometer equipped with an attenuated total reflectance accessory. The samples were first ground using an agate mortar and spectra were collected in the range of 650 - 4000 cm ^-1 . X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific K-Alpha ⁺ X-ray photoelectron spectrometer equipped with a MXR3 Al Kcr monochromated X-ray source (h u= 1486.6 eV). The samples were initially ground and mounted onto an XPS sample holder using a small piece of conductive carbon tape. The X-ray gun power was set to 72 W (6 mA and 12 kV). The high-resolution spectra were obtained using 20 eV pass energy and 0.1 eV step size. Thermo Avantage software was used to analyse the data.

Textural properties and morphology

Powder X-ray diffraction (PXRD) patterns were recorded using a PANalytical X’Pert PRO diffractometer using Cu Kcr radiation (l = 1.54178 A) with a step of 0.01 ° at a scanning speed of 10 s per step. An anode voltage of 40 kV and emission current of 20 mA were chosen as the operating conditions. Scanning electron microscope (SEM) images were taken using a Zeiss Auriga microscope with an accelerating voltage of 5 kV. The samples were placed on the carbon tape without grinding and coated with 15 nm of gold. N2 adsorption isotherms were undertaken at 77 K, using a Micromeritics 3Flex instrument. The samples were initially degassed overnight at 120 °C at approximately 0.2 mbar pressure. Prior to the sorption isotherm measurement, the samples were further degassed in-situ for 4 h at 120 °C. The equivalent specific surface areas of the samples were determined using the Brunauer-Emmett-Teller (BET) method (Brunauer, S., P.H. Emmett, and E. Teller, J. Am. Chem. Soc., 1938. 60(2): p. 309-319). The total pore volume was evaluated from the volume of N2 adsorbed at a relative pressure (P/Po) of 0.97. The micropore volume was determined using the Dubinin-Radushkevich model (S. G. Chen, R. T. Yang, Langmuir 1994, 10, 4244). The pore size distribution was derived from the built-in software from Micromeritics, using DFT model for carbon slit shape pores. Rate of adsorption data was logged by intercepting the raw 3Flex data output using PuTTy. Mercury porosimetry was employed up to a final pressure of 2300 bar using an AutoPore IV 9500 instrument from Micromeritics to measure the bulk density. Prior to the analysis, the samples were activated overnight at 120 °C under vacuum.

Moisture stability test

A vial with 20 mL of deionised (Dl) water was placed in a 1 L sealed container until the relative humidity reached >99 %, which was measured by a hygrometer. _m BN was kept in the container for different durations before it was collected and dried at 120 °C under vacuum. Mechanical stability test

The hardness of _m BN was measured using a Struers Duramin -1/-2 Micro-Vickers hardness tester, following the Vickers method. A load of 0.025 kgf (1 kgf = 9.8 N) was applied with a dwell time of 12 s. For each sample, at least 10 indentations were performed at different points. The samples were polished on 2000 grit sandpaper with a Struers TegraPol-31 polisher before the measurement.

Methane adsorption isotherms

High-pressure CH ₄ adsorption isotherms were carried out at 298 K in the pressure range of 0-70 bar using the equipment and method reported in Microporous Mesoporous Mater. 2020, 308, 110537, the contents of which are incorporated by reference In brief, CH4 uptake was measured gravimetrically using a Rubotherm Magnetic Suspension Balance (MSB) with an equilibration time of at least 90 min for each pressure point. The samples were initially degassed overnight at 393 K at approximately 0.2 mbar pressure. Prior to the sorption isotherm measurement, the samples were further degassed in-situ for 12 h at 393 K and a Helium gravimetry experiment was carried out at 298 K to estimate the adsorbent skeletal density.

Bulk density measurement via mercury porosimetry.

The bulk density of both _mp BN and powder BN was measured using mercury porosimetry, a well-established technique that has been used to measure the bulk density of other porous materials. Mercury is a non-wetting liquid that does not intrude into small pores at ambient pressure, facilitating the measurement of bulk volume, which includes both the material volume and the interstitial volume, and thus the bulk density.

In a typical mercury intrusion porosimetry measurement, the sample is filled in a penetrometer, which has a known weight and volume. It should be noted that the sample is not mechanically compressed. After evacuation, the penetrometer is filled with mercury. The mercury will surround the sample at ambient pressure but not enter pores and voids smaller than ca. 6 pm. The weight of mercury is obtained by reweighing the penetrometer and by subtracting the known weights of the empty penetrometer and the sample. The volume of intruded mercury is then computed from the known mercury density (13.5394 g/mL) and the bulk volume of the sample is obtained as the difference between the empty penetrometer volume and the intruded mercury volume. As powder BN and _m BN has the same composition and similar specific surface area, the materials volume of both samples can be exactly same. However, due to the more packed structure of _mp BN (i.e. less interparticle volume), _m BN shows much higher bulk density compared to powder BN.

Methane adsorption isotherms at 298 K.

The data of high-pressure methane uptake was obtained as excess gravimetric adsorption capacity (N _exc ), and was converted into absolute gravimetric capacity (Na _b s) using equation (1):

^abs — ^exc T Pgas^tot where p _gas is the density of the non-adsorbed gas and V _tot is the total pore volume of the adsorbent.

Absolute volumetric adsorption capacity is converted from the absolute gravimetric uptake by multiplying by the bulk density. Bulk density is the ratio of the mass of an uncompressed solid sample and its volume, including the volume of the solid and the interparticle space.

A comparison of absolute methane uptake at 298 K in powder BN and _m BN is provided in Figure 2, in relation to a) gravimetric uptake; and b) volumetric uptake

Results and Discussion

Synthesis of monolithic porous BN ( _mp BN)

A macroporous melamine-formaldehyde (MF) resin was used as the precursor to produce monolithic porous BN, _m BN (Figure 1, wherein A = melamine + formaldehyde; B = dicyandiamide, C = boric acid). To synthesise the resin, dicyandiamide (DCD) was employed as a foaming agent, and boric acid as the curing agent. Various weight ratios of DCD/melamine were tested. The DCD content controlled the pore density in the MF resin and appears to impact the final morphology of the porous BN. Figure 3 provides optical images of the MF resin with different amounts of DCD (a-e), and the derived porous BN samples (a’-e’) (Scale bar = 1 cm). DCD/melamine weight ratios of (a) and (a’) 0; (b) and (b’) 0.24; (c) and (o’) 0.48; (d) and (d’) 0.72; (e) and (e’) 0.96 were used. Overall, a DCD/melamine weight ratio of 0.72 was found to result in a precursor with moderate pore density and high mechanical strength. In this case, the pores act as channels for the porogen to escape while the good crosslink density prevents the collapse of the macrostructure upon pyrolysis. As a result, a monolithic BN sample was obtained (Figure 3c’, d’). Based on the observations above, subsequent efforts were focused on the BN samples obtained using a 0.72 DCD/melamine weight ratio (Figure 3d’). The effects of reaction atmosphere, reaction temperature and duration on the pyrolysis product were explored to identify the optimal pyrolysis conditions. Looking first at the reaction atmosphere, a sample obtained under NH3 was found to exhibit a high BET specific surface area of 1513 m ² /g (Figure 4, which shows the N2 sorption isotherm at 77 K for BN synthesized under N2).

The synthesis under NH ₃ provides an excess of reactive N over a wide range of temperature, which is known to enhance the surface area of porous BN. Consequently, NH ₃ was used as the reaction atmosphere for further tests.

The effects of the reaction temperature and the duration of the isothermal step were also investigated. Keeping the duration at 3 h, the BET specific surface area increased from 1349 m ² /g at 800 °C to 1513 m ² /g at 1000 °C (Figure 5a). Further increasing the reaction temperature to 1200 °C resulted in a decreased BET area (1397 m ² /g), possibly due to a partial crystallization of BN. Furthermore, extending the duration of the isothermal step up to 3 h at 1000 °C enhanced the porosity (Figure 5b). Beyond 3 h, the surface area reached a plateau, pointing towards a completion of the reactions leading to the gases release and porosity formation. To summarise, pyrolysing the MF resin in NH ₃ atmosphere at 1000 °C for 3 h appeared the most effective method in obtaining _m BN and was used in the current work.

Characterisation of monolithic porous BN ( _mp BN)

The features of _mp BN obtained using the optimal conditions determined above were analysed in more detail. Figure 6a shows the XRD pattern of the sample. The two broad peaks corresponding to (002) and (100) planes together with the absence of other reflections point to the presence of turbostratic/amorphous BN. The two main characteristic IR bands of BN at -1360 cm ^-1 (in-plane BN stretching) and -810 cm ^-1 (out-of-plane B-N-B bending) are detected (Figure 6b), confirming the chemical composition of _m BN. The additional small band at -1100 cm ^-1 is attributed to B-0 bending mode. The presence of oxygen is further confirmed by XPS analyses, with a quantity of 3 at% (Figure 6d). The lower oxygen content of _m BN compared to powder BN (3 at% vs 9 at%) leads to a significant improvement of hydrolytic stability, an important feature for industrial application, as shown in Figures 7 and 8. Figure 7 shows structural features of monolithic porous BN ( _m BN) and powder BN after moisture exposure; specifically, (a) BET specific surface area loss as derived from N2 sorption at 77 K; (b) XRD patterns of powder BN; and (c) XRD patterns of _m BN. Figure 8 shows N2 adsorption isotherms at 77 K after moisture exposure for (a) structured BN; and (b) powder BN at various timepoints.

SEM image of _mp BN (Figure 6c) reveals a continuous solid phase with slender cracks. The compact solid phase presents a relatively flat surface similar to other highly dense monolithic porous materials. The morphology is different from that of powders or aerogels, which contain a large amount of interparticle space. The slender cracks with macropores are distributed within the continuous flat surface and may result from the pores in the resin precursor. The pores propagated and enlarged under the stress generated during the synthesis, forming these gully-like cracks.

Textural and gas adsorption properties of monolithic porous BN ( _mp BN) vs powder BN

The porosity of _m BN was evaluated using the N2 adsorption isotherms at 77 K and compared with a highly porous BN powder described in WO 2018/167507 A1 (Table 1, Figure 9). _m BN possesses a similar specific surface area to powder BN. However, different to powder BN which exhibits type IV isotherms, _m BN exhibits a type l(b) isotherm, typical for materials with both micropores and narrow mesopores. The pore size distribution in Figure 10 shows that the majority of the pores in _m BN are < 2.5 nm.

Furthermore, N ₂ sorption in _m BN plateaus for P/Po > 0.5 (Figure 9a). On the contrary, N ₂ adsorption in powder BN increases towards high P/Po values due to N ₂ condensation.

This is due to the compact nature of _m BN, which results in minimal interparticle porosity.

To quantify the compact nature of _m BN, its bulk density was measured using mercury porosimetry, a well-established technique that has been used to measure the bulk density of other porous materials. Mercury is a non-wetting liquid that does not intrude small pores at ambient pressure, which facilitates the measurement of bulk volume and thus the density. Table 1 shows the bulk density of _m BN was 0.31 g/cm ³ , nearly 50 % higher than that of the bulk powder, which leads to a significantly higher volumetric BET area, achieving 473 m ² /cm ³ (vs 315 m ² /cm ³ for powder BN). Structured BN aerogels usually exhibit bulk densities of 0.01-0.02 g/cm ³ , more than one order of magnitude lower than that of _m BN, ultimately leading to a low volumetric BET area (i.e. 0.1 - 14 cm ² /cm ³ ). On the other hand, sintered structured BN typically shows high density but a low BET area (< 450 m ² /g), which results in a moderate volumetric BET area similar to the powder BN. _m BN therefore exhibits the highest volumetric BET area for a BN sample published to date. Table 1. Comparison of specific and volumetric properties of monolithic porous BN ( _m BN) and powder BN. BET area (SBET(9)) , total pore volume (V _to t), micropore volume (Vmicro) obtained from N2 sorption isotherms at 77 K and bulk density (p) and volumetric BET area (SBET (vol)) derived from mercury porosimetry.

SBET (mass) Vtot Vmicro P SBET (VOl)

(m ² /g) (cm ³ /g) (cm ³ /g) (ratio) (m ² /cm ³ )

Monolithic _mp BN 1523 079 056 031 473

Powdered BN 1500 1.14 0.56 0.21 315

Volumetric methane storage capacity can be attractive for on board applications, where space for fuel is always a constraint. In these cases, the volumetric methane uptake is to be considered. Considering the volumetric properties of _m BN, ChU adsorption isotherms were performed at 298 K up to 70 bar. Figure 11a shows that the absolute gravimetric uptake of powder is higher than that of _mp BN, owing to the higher total pore volume. The opposite is true when considering the absolute volumetric adsorption capacity (65 vs 50 (STP) cm ³ /cm ³ at 70 bar) as a result of the higher bulk density of _m BN (Figure 11b).

A higher material density often compromises adsorption kinetics due to the mass transfer limitation. To explore the adsorption kinetics of _m BN, the adsorption equilibration time of N2 adsorption was measured at an extremely low pressure (2.2 ^c 10 ^-6 bar) at 77 K and the results were compared to those obtained for the powder sample (Figure 12, a) linear scale; b) log scale). At very low pressures, differences in kinetics between the two materials are likely to be the most visible, hence the selection of 2.2 ^c 10 ⁶ bar for the measurement. Both samples show very fast adsorption kinetics, reaching equilibrium within 60s. The small difference was mainly attributed to the lower macroporosity in _m BN, which results in a slightly lower diffusivity.

Moisture stability of monolithic porous BN ( _mp BN) and powder BN The hydrolytic instability of porous BN reported to date represents a major barrier to industrial applications. Figures 7 and 8 show the influence of moisture (> 99% humidity) exposure time on the BET area of _m BN and powder BN. Both samples show virtually the same BET areas after the first hour. However, the porosity decreased significantly after 2 h of exposure for powder BN, showing a 54% loss in the surface area. For _m BN, the reduction was of 18%. More importantly, _m BN maintains more than 60% of its original surface area after 8 h exposure, whereas powder BN only retains 15%. A 25% more drop of the surface area is observed for _m BN after further 4 h exposure, at which point powder BN is virtually non- porous.

The structural changes in both samples upon exposure to moisture were analysed using XRD. In the case of powder BN (Figure 7b), the (002) peak related to hexagonal BN increases in intensity and shifts to higher angles after exposure to moisture. This observation agrees with previous studies and indicates a loss of the amorphous portion of BN upon decomposition in water, leaving the residual material with a higher crystallinity (i.e. lower porosity and surface area). _mp BN exhibits a different trend (Figure 7c). The sample does not show any increase in crystallinity before 12 h of exposure. This observation suggests a slower degradation compared to powder BN, consistent with the BET area trend (Figure 7a). The greater hydrolytic stability of _m BN compared to that of powder BN was attributed to its lower content of oxygen atom (3 at% vs 9 at%). Indeed, oxygen sites are susceptible to hydrolysis attack.

Mechanical stability of monolithic porous BN ( _mp BN)

Mechanical stability of adsorbents is an important property in practical applications as materials are always exposed to mechanical stress such as operational vibration and compression, i.e. weight of the packed adsorbent. As a preliminary and quick visual test of the mechanical robustness of _m BN, a calibration weight was first loaded onto the sample. Ca. 0.005 g of _m BN maintained its integral bulk structure under a 200 g calibration weight, pointing to an apparent robustness. The hardness of the sample was then quantitatively evaluated using microindentation. Our results show the hardness of _m BN is 66.4 ± 4.5 MPa, which is comparable to dense binder-free graphene oxide pellets and macroporous nickel foam. Remarkably, the hardness is 1-2 orders of magnitude higher than that reported for BN aerogels, suggesting a much higher resistance to plastic deformation, mainly attributed to the higher density.

Structure and formation mechanism of monolithic porous BN ( _mp BN)

Samples obtained from the pyrolysis of MF resin under NH3 at 800 and 1000 °C were collected. These were then analysed for structural and chemical properties.

XPS was used to identify the chemical structure of the samples. The core level spectra of B1s, N1s and 01s are presented in Figure 13a-c. BN is formed at 800 °C, as evidenced by the peaks at 190.6 eV and 398.2 eV from B1s and N1s, respectively. The majority of carbon atoms are removed at this stage as seen in Figure 13d and as suggested by the yellow- brown colour of the 800 °C sample. Increasing the temperature to 1000 °C boosts carbon and oxygen removal and BN conversion through nitridation, resulting in a porous BN with a bright yellow colour. The formation of BN is supported by the analysis from FTIR shown in

Figure 14.

Without wishing to be bound by theory, it is proposed that the structural evolution from MF resin to _m BN proceeds as shown in Figure 15. First, methylene bridged MF resin bonds chemically to boron oxide via B-C-O. Then, the formation of a melon-based polymer takes place. Carbon atoms are gradually removed while the remaining compounds retain the crosslinked structure via B-O-C and B-O-N bonds. BN is then formed via a nitridation process and eventually the monolithic BN with minimum impurities is produced. Throughout _mp BN formation, a crosslinked structure is maintained. This high bonding density in turns allows mechanical robustness to be preserved in the final product. This structural evolution study paves the way for further engineering of _mp BN at larger scale.

In summary, a monolithic porous BN ( _mp BN) has been developed using a porous melamine- formaldehyde resin (MF resin) as precursor. A suitable range of pore density in the precursor and a NH3 synthesis atmosphere are necessary to obtain porous _mp BN. Compared to BN powders, _mp BN exhibits an improved hydrolytic stability, a 50% higher bulk density and therefore an increased volumetric surface area, than the highest among BN materials reported to date. As a result, _mp BN displays an enhanced volumetric CFU adsorption capacity, while sorption kinetics remains fast. _mp BN shows remarkable mechanical strength (i.e. hardness), being one to two orders of magnitude higher than that of aerogels. It is proposed that the polymer intermediates derive from the resin crosslink with boron atoms, which impart the mechanical stability to _mp BN. The promising properties support the potential of _m BN for practical molecular adsorption applications and the formation mechanism study facilitates the industrial scale production of _m BN.

It will be appreciated that the above description is made byway of example and not limitation of the scope of the appended claims, including any equivalents as included within the scope of the claims. Various modifications are possible and will be readily apparent to the skilled person in the art. Likewise, features of the described embodiments can be combined with any appropriate aspect described above and optional features of any one aspect can be combined with any other appropriate aspect.