METHOD FOR THE OXIDATION OF BORN NITRIDE NANOTULES

Description

[0001] The present invention relates to the field of boron nitrides, and in particular to boron nitride nanotubes, also known as BNNTs. Specifically, the invention provides a method for the mild oxidation of these nanotubes. In a second aspect, the invention proposes the use of an oxidizing composition for oxidizing boron nitride nanotubes.

[0002] Among the nanomaterials known to those skilled in the art, very few possess strong thermal conductivity combined with dielectric properties. Boron nitride nanotubes are one-dimensional nanomaterials that meet these criteria. They have numerous potential applications, for example, in composite materials designed for heat dissipation. They are also known to provide significant mechanical reinforcement to certain matrices.

[0003] BNNT nanomaterials are of great technological interest, but they are sometimes difficult to disperse in a matrix due to their tendency to aggregate. Several studies have shown that functionalizing these nanomaterials can facilitate their dispersion in matrices by promoting the interaction between the matrix and the chemically modified surface of the BNNTs. In particular, it is of great interest to oxidize the BNNTs and generate hydroxyl groups on their surface. This allows, on the one hand, for the modification of the surface energy of the BNNTs and consequently alters their chemical and electrostatic interaction with their environment. On the other hand, BNNTs with hydroxyl groups form very interesting intermediates for more advanced functionalization, for example, by silane grafting.

[0004] However, BNNTs are difficult to oxidize due to their very low reactivity. A few methods have yielded promising results, such as nitric or perchloric acid treatments, plasma treatments, or ultrasonic-activated hydrolysis. But the reaction conditions and reagents used are particularly corrosive, harmful to the environment, and lead to significant alterations in the nanotube structure by destroying their hexagonal core. For example, a nanotube measuring 2 micrometers in length becomes only 1 micrometer long after oxidation in a highly reactive medium. This consequently alters their characteristic properties.

[0005] The document ZHENG ZHUOYUAN ET AL: “Surface modification of hexagonal boron nitride nanomaterials: a review” JOURNAL OF MATERIAL SCIENCE, KLUWER ACADEMIC PUBLISHERS, DORDRECHT, vol. 53, no. 1, 23 August 2017, pages 66-99, XP036337756, ISSN: 0022-2461, DOI: 10.1007 / S10853-017-1472-0 describes various treatments using acids, hydrogen peroxide or covalent functionalization in the presence of radicals to oxidize BNNTs and introduce hydroxyl groups.

[0006] The document LIN SHUQIONG ET AL.: “Covalent derivatization of boron nitride nanotubes with peroxides and their application in polycarbonate composites” NEW JOURNAL OF CHEMISTRY, vol. 41, no. 15, June 19, 2017, pages 7571-7577, XP093021219, ISSN: 1144-0546, DOI: 10.1039 / C7NJ00193B describes oxidized BNNTs comprising 3% hydroxyl groups.

[0007] Outside the field of BNNTs, it is known that the use of NHPI in combination with cobalt compounds such as cobalt tetraamide phthalocyanine tetraamide (see LI FEI ET AL: “Synergistic Catalytic effect of N-Hydroxyphthalimide / Cobalt Tetraamide Phtalocyanine and its application for aerobic oxidation of hydrocarbons and alcohols”, CATALYSIS LETTERS, vol. 151, no. 1, June 13, 2020, pages 17-26, XP037338394, ISSN: 1011-372X, DOI: 10.1007 / S10562-020-03283-Z) or cobalt acetate (see HABIBI D ET AL: “Efficient catalytic systems based on cobalt for oxidation of ethylbenzene, cyclohexene and oximes in the presence of N-hydroxyphthalimide”, APPLIED CATALYSIS A: GENERAL, ELSEVIER, AMSTERDAM, NL, vol. 466, July 5, 2013, pages 282-292, XP028695540, ISSN: 0926-860X, DOI: 10.1016 / J.APCATA.2013.06.045) can be considered for the oxidation of organic compounds such as hydrocarbons, alcohols or aromatic compounds.

[0008] One of the aims of the present invention is to overcome at least one of the aforementioned drawbacks. To this end, the present invention proposes a process for the oxidation of boron nitride nanotubes (BNNTs) comprising the steps of: preparation of a reaction mixture comprising: ∘ the dispersion of boron nitride nanotubes (BNNTs) in an organic solvent consisting of a monocarboxylic acid comprising 1 to 5 carbon atoms, and in particular acetic acid, ∘ the addition of N-hydroxyphthalimide (NHPI); ∘ the addition of a cobalt derivative selected from cobalt acetylacetonate Co(C5H7O2)2.4H2O, cobalt acetate Co(C2H3O2).4H2O, and cobalt chloride CoCl2; and ∘ the injection of a gas including oxygen, and heating of the reaction mixture to a temperature between 70°C and 100°C for a period between 1h and 200h, in particular between 1h to 100h and for example between 4h to 60h, so as to obtain oxidized boron nitride nanotubes (BNNT-OH) with an oxidation rate between 0.01 and 5% and in particular between 0.1 and 2%.

[0009] The process of the present invention advantageously utilizes mild oxidation conditions that do not destroy the nanotubes and are compatible with industrial constraints. The oxidation process yield is greater than 90%.

[0010] A catalytic system composed of N-hydroxyphthalimide and a cobalt derivative in the presence of air has already been described as leading to the oxidation of organic compounds, particularly alkanes or alcohols. However, nothing suggested that BNNTs would oxidize with this system operating via a radical pathway; BNNTs are inorganic materials, very stable against oxidants, and have a hexagonal structure very different from those of the organic molecules previously used.

[0011] According to one possibility, the preparation of the reaction mixture further includes the addition of a manganese derivative chosen from Mn(OAc) 2 , Mn(acac) 2 , MnCl 2 .

[0012] An increase in reaction kinetics is observed in the presence of manganese derivatives in addition to cobalt derivatives. The catalytic system is more efficient, which reduces the reaction time and therefore lowers the process cost by requiring less reactor time.

[0013] According to one provision, NHPI is added to the reaction mixture so as to achieve a proportion between 0.001% and 25% molar, in particular between 0.01% and 15% molar and for example between 1 and 10% molar.

[0014] According to one possibility, NHPI is added in a mass ratio of between 0.1 and 2 times the mass of BNNTs, for example a mass ratio of 1.3.

[0015] According to one possibility, the cobalt derivative is added to the reaction mixture so as to achieve a weight proportion of between 10 ppm and 1% by weight, specifically between 100 and 5000 ppm by weight.

[0016] According to one variant, the manganese derivative is added to the reaction mixture so as to achieve a weight proportion of between 10 ppm and 1% by weight, specifically between 100 and 5000 ppm by weight.

[0017] According to one possibility, the injection of the gas including oxygen is carried out with a flow rate between 1x10 -6< m 3< / s / (Liter of solvent) and 1x10 -4< m 3< / s / (Liter of solvent), for example between 2.10 -6< m 3< / s / (Liter of solvent) and 8.10 -5< m 3< / s / (Liter of solvent).

[0018] In practice, the process is carried out under a pressure between 10⁵ and 10⁷ Pa, in particular from 10⁵ to 10⁶ Pa and for example from 10⁵ Pa to 2.10⁵ Pa.

[0019] According to one provision, the process includes, after heating the reaction mixture, and in particular cooling the reaction mixture, filtering the oxidized BNNTs (BNNT-OH) followed by rinsing with water, in particular deionized water.

[0020] According to one possibility, the reaction mixture is kept under agitation, in particular under magnetic stirring, at least part of the duration of the oxidation reaction.

[0021] According to a second aspect, the invention proposes the use of an oxidizing composition, the oxidizing composition comprising: of N-hydroxyphthalimide (NHPI), an organic solvent consisting of a monocarboxylic acid comprising 1 to 5 carbon atoms, and in particular acetic acid, a gas comprising oxygen injected into the organic solvent and a cobalt derivative chosen from cobalt acetylacetonate Co(Acac) 2 , cobalt acetate Co(OAc) 2 , and cobalt chloride CoCl 2 , to oxidize boron nitride nanotubes.

[0022] According to one variant, the oxidizing composition used to oxidize boron nitride nanotubes further includes a manganese derivative chosen from Mn(OAc) 2 , Mn(acac) 2 , MnCl 2 .

[0023] According to other features, the BNNT oxidation process of the invention comprises one or more of the following optional features considered alone or in combination: • The oxidation process is carried out within a pressure range from approximately 1 x 10⁵ Pa to 1 x 10⁷ Pa, specifically from approximately 1 x 10⁵ Pa to 1 x 10⁶ Pa, and for example, from approximately 1 x 10⁵ Pa to 2 x 10⁵ Pa. • The oxygen-containing gas is injected from compressed air containing between 15 and 25% vol. oxygen. • The oxygen-containing gas is injected from oxygen-enriched air, specifically containing between 15 and 80% vol. oxygen. • The oxygen-containing gas is injected from a neutral gas such as N₂ or Ar, compressed and enriched with oxygen in a proportion of between 1 and 40% vol. • The oxygen-containing gas is injected by bubbling through the solvent. ∘ Bubble is achieved by using a bubbler consisting of a gas source including pre-compressed oxygen, a gas flow control valve and a dip tube immersed in the solvent.• The reaction is carried out under pressure, and the oxygen-containing gas is injected into the head of the reactor receiving the reaction mixture. • The reaction is carried out within a pressure range of approximately 1 x 10⁵ Pa to 1 x 10⁷ Pa, and the oxygen-containing gas is injected into the reaction mixture.

[0024] Other features and advantages will become apparent upon reading the detailed description below, of a non-exhaustive example of implementation, made with reference to the attached figures in which: [ Fig.1 ] illustrates the oxidation reaction of boron nitride nanotubes according to an embodiment of the invention. Fig. 2 ] is a diagram illustrating the weight curve of the product obtained according to one embodiment of the oxidation process as a function of temperature [ Fig.3 ] illustrates the absorbance of BNNTs subjected to the reaction medium of the invention without a catalytic device. Fig. 4 ] illustrates an enlargement of region A of the figure 3 . [ Fig. 5 ] illustrates an enlargement of region B of the figure 3 . [ Fig. 6 ] illustrates the absorbance of BNNT-OH obtained according to the embodiment of the invention with a catalytic device. Fig. 7 ] illustrates an enlargement of region A of the figure 6 . [ Fig. 8 ] illustrates an enlargement of region B of the figure 6 . [ Fig. 9 ] illustrates an XPS spectrum of BNNTs subjected to the reaction medium of the invention without a catalytic device. Fig. 10 ] illustrates an XPS spectrum of BNNT-OH obtained according to the embodiment of the invention with a catalytic device.

[0025] As illustrated on the figure 1The present invention proposes a mild, aerobic catalytic oxidation process for boron nitride nanotubes (BNNTs) using NHPI and a cobalt derivative that allows the formation of hydroxyl bonds. The process consists of preparing a reaction mixture comprising a stirred dispersion of BNNT in an organic solvent, such as acetic acid, and the addition of a catalytic amount of NHPI and a cobalt(II) or cobalt(III) derivative. In a possible but unillustrated application, a catalytic amount of a manganese derivative is also added to the dispersion. Oxygen is supplied by injecting a compressed gas containing oxygen into the solvent. This injection can be achieved, in particular, by regular bubbling. The gas can be compressed air, enriched or not with O2, or a neutral gas, such as argon or nitrogen enriched with oxygen.

[0026] The resulting reaction mixture is heated with stirring to a temperature in the range of approximately 70 to 100°C, the temperature being chosen to be at or below the boiling point of the solvent. This temperature is maintained for the duration required for oxidation, for example, from 1 to 200 hours, depending on the heating temperature, the oxygen content, the amount of catalysts used, and the composition of the catalytic system (Mn derivatives and / or Co derivatives).

[0027] Once oxidation is complete, the oxidized boron nitride nanotubes (BNNT-OH) are dispersed in the solvent. They are recovered by filtration, particularly after cooling to room temperature, and rinsed repeatedly with water, including deionized water, to obtain BNNT-OH of sufficient purity for use or further functionalization. The oxidation level of BNNT-OH, measured by thermogravimetric analysis (TGA), ranges from 0.01% to 5%. 1- Sample example :

[0028] An example of a control implementation, in which BNNT is processed under the conditions of the process of the invention but without a catalytic device, is first carried out for comparison with the process of the invention using the catalytic device.

[0029] BNNT (200 mg, purchased from BNNano) is placed in a flask containing 150 mL of acetic acid under magnetic stirring. A plunger is introduced, ensuring regular air bubbling of approximately 1.3 x 10⁻⁵ m³ / s / (L of solvent).

[0030] The mixture is heated to 100°C for 60 hours.

[0031] After cooling, the residual solid is filtered and washed 4 times with 200 mL of deionized water.

[0032] It is then analyzed by IR spectroscopy. No spectral change is observed compared to the starting nanomaterial.

[0033] 2- Example of implementation of the inventionThe catalytic system, including the following, is now described in detail: BNNT (200 mg, purchased from BNNano) is placed in a flask containing 150 mL of acetic acid, 260 mg of NHPI, 7 mg of cobalt as Co(OAc)₂·4H₂O, and 7 mg of manganese as Mn(OAc)₂·4H₂O, under magnetic stirring. A plunger is introduced, ensuring regular air bubbling at approximately 1.3 x 10⁻⁵ m³ / s / (L of solvent).

[0034] The mixture is heated to 100°C for 60 hours.

[0035] After cooling, the solid is filtered and washed with 4 times 200 mL of deionized water.

[0036] It is then analyzed by TGA (STA 449 - Netzsch - ramp 30-800°C at 5 K / min). As illustrated in the figure 2 , a mass loss is observed between 240°C and 500°C for BNNT treated with the catalytic system (BNNT (I) on the figure 2 ) unlike BNNT treated without a catalytic device (BNNT (0) on the figure 2). This mass loss is attributed to the loss of hydroxyl functions, as already reported in the literature (Diez-Pascual et al., RSC Adv., 2016, 6, 79507).

[0037] The solids resulting from the two BNNT reactions, treated with or without a catalytic system, are also analyzed by infrared spectroscopy (spectrometric analysis on an FTIR instrument: Vertex 70 - Bruker Optics - KBr transmission method). figures 3 to 5 illustrate the result of the measurement carried out on the treated BNNT without the catalytic system and the figures 6 to 8These figures illustrate the results of measurements performed on BNNT treated with the catalytic system (BNNT-OH). In this latter case only, a peak in region A is visible at approximately 3400–3500 cm⁻¹. This peak is attributed to the formation of hydroxyl groups, which is corroborated by the presence of a vibration band in region B at approximately 950 cm⁻¹, characteristic of the B-OH bond. Neither of these peaks appears in measurements performed on BNNT treated without the catalytic device.

[0038] These analyses demonstrate the formation of hydroxyl bonds on BNNTs, de facto reflecting their oxidation according to the process described in this example of an embodiment of the invention.

[0039] Also illustrated at the figure 9An XPS analysis (VERSAPROBE-II instrument - Physical Electronics) performed on the BNNT treated without the catalytic system shows a peak at approximately 190 eV corresponding to a composition consisting exclusively of 'B-N'. The same XPS analysis performed on the solid after treatment with the catalytic system (BNNT-OH on the Figure 10 ) shows a peak at approximately 190eV illustrating the presence of 'BN' and a peak in the C region illustrating the presence of 'BO' with a rate of approximately 5%.

[0040] These two examples, carried out with or without a catalytic device, show the obtaining of oxidized boron nitride nanotubes by means of the aerobic catalytic oxidation process of the present invention.

[0041] Thus, the present invention provides a gentle oxidation method for hexagonal boron nitride nanotubes. This oxidation leads to the formation of hydroxyl bonds, thereby improving the dispersion properties of the BNNTs. The BNNTs thus obtained can also serve as intermediates for the formation of highly functionalized molecules. The oxidation method is inexpensive and meets industrial criteria; it is carried out using a catalytic system, in a common solvent, and in the presence of naturally occurring oxygen from the air. Furthermore, the reaction temperature remains below approximately 100°C.

Claims

1. A process for oxidizing boron nitride nanotubes (BNNTs) comprising the steps of: - preparing a reaction mixture comprising: ∘ dispersing boron nitride nanotubes (BNNTs) in an organic solvent consisting of a carboxylic monoacid comprising 1 to 5 carbon atoms, ∘ adding N-hydroxyphthalimide (NHPI); ∘ adding a cobalt derivative selected from cobalt acetylacetonate, cobalt acetate and cobalt chloride, and ∘ injecting a gas containing oxygen, and - heating the reaction mixture to a temperature between 70°C and 100°C for a duration between 1 hour and 200 hours, so as to obtain oxidized boron nitride nanotubes (BNNT-OH) with an oxidation rate between 0,01 and 5%.

2. The process for oxidizing boron nitride nanotubes according to claim 1, wherein the preparation of the reaction mixture further comprises adding a manganese derivative selected from Mn(OAc)2, Mn(acac)2, MnCl2.

3. The process for oxidizing boron nitride nanotubes according to claim 2, wherein the manganese derivative is added to the reaction mixture so as to achieve a weight proportion between 10 ppm and 1% by weight.

4. The process for oxidizing boron nitride nanotubes according to any of claims 1 to 3, wherein NHPI is added to the reaction mixture so as to achieve a mass proportion between one-tenth and two times the mass of BNNTs.

5. The process for oxidizing boron nitride nanotubes according to any of claims 1 to 4, wherein the cobalt derivative is added to the reaction mixture so as to achieve a weight proportion between 10 ppm and 1% by weight.

6. The process for oxidizing boron nitride nanotubes according to any of claims 1 to 5, wherein the injection of the gas containing oxygen is carried out at a flow rate between 1 x 10-6 m3 / s / (liter of solvent) and 1 x 10-4 m3 / s / (liter of solvent).

7. The process for oxidizing boron nitride nanotubes according to any of claims 1 to 6, wherein the process is carried out under a pressure between 105 and 107 Pa.

8. The process for oxidizing boron nitride nanotubes according to any of claims 1 to 7, comprising, after heating the reaction mixture, filtering the oxidized BNNT followed by rinsing with water.

9. The process for oxidizing boron nitride nanotubes according to any of claims 1 to 8, wherein the reaction mixture is maintained under stirring for at least part of the duration of the oxidation reaction.

10. A use of an oxidizing composition, the oxidizing composition comprising: - N-hydroxyphthalimide (NHPI), - an organic solvent consisting of a carboxylic monoacid comprising 1 to 5 carbon atoms, - a gas containing oxygen injected into the organic solvent, and - a cobalt derivative selected from cobalt acetylacetonate, cobalt acetate, and cobalt chloride, for oxidizing boron nitride nanotubes.

11. The use of an oxidizing composition according to the preceding claim, wherein the oxidizing composition further comprises a manganese derivative selected from Mn(OAc)2, Mn(acac)2, MnCl2.