Method for fluorination of a halopyridine catalyzed by nanodiamonds

Abstract

The present invention relates to a method for preparing a fluoropyridine that comprises fluorinating a halopyridine with hydrogen fluoride in the gas phase and in the presence of nanodiamonds, at least part of the surface of which contains sp2 carbon atoms and hydrogenated and / or oxygenated surface functions, such that there is formed a compound of formula (I) and an acid chosen from HCl, HBr and a mixture thereof.

Description

[0001] DESCRIPTION

[0002] TITLE: Nanodiamond-catalyzed fluorination process for a halopyridine

[0003] The present invention relates to a process for preparing fluoropyridines from a bromo- or chloropyridine.

[0004] Fluorinated molecules have become indispensable in the chemical industry thanks to their exceptional physicochemical characteristics (lipophilicity, dipole moment, pKa, reactivity). Present in more than 25% of pharmaceuticals and 40% of agrochemicals, fluorine plays a key role in organic synthesis. Recent decades have seen a large number of new fluorinated synthons with one or more fluorine atoms or groups (e.g., -CF3 and -CHF2) used in the synthesis of increasingly complex molecules. For example, fluoropyridine motifs are present in molecules such as fluoroxypyr (X), used as a herbicide, and alatrofloxacin (XI), used as an antibiotic.

[0005] [Chem 1]

[0006] [Chem 2]

[0007] In contrast, very few fluorinated organic compounds are found in nature. It is therefore necessary to develop appropriate synthetic methods. To date, the catalyzed fluorination of chlorinated molecules to form the corresponding fluorinated compounds is only applied industrially to the fluorination of non-functionalized aliphatic molecules leading to the formation of chlorofluoroalkanes (CFCs) and their substitutes, i.e. hydrofluorocarbons (HFCs) and more recently hydrofluoroolefins (HFOs).

[0008] However, to the inventors' knowledge, the literature does not report the synthesis of fluorinated aromatics by a catalyzed process.

[0009] Indeed, currently, fluorinated aromatic molecules are prepared by multi-step syntheses, which are not very selective and lead to significant salt discharges. This represents a major drawback from an environmental point of view. More specifically, the industrial production of fluorinated aromatic synthons mainly involves two non-catalytic liquid-phase synthesis routes: the fluorodeiazotation of anilines (Balz-Schiemann, Scheme 1, Route 1) and the Cl / F exchange reaction of chlorinated aromatic substrates (HALEX reaction, Scheme 1, Route 2). [Chem 3]

[0010] Flu orod ed iaz otati on

[0011] HALEX reaction

[0012] (Track 2)

[0013] Diagram 1: Synthesis routes of fluorinated aromatic compounds according to the prior art

[0014] These two reactions present constraints and limitations such as the generation of large volumes of unusable saline effluents (KCl, trifluoroborate salts), the toxicity of the reagents (anilines), and the lack of selectivity and reactivity at certain positions. Therefore, there is a need to prepare fluorinated aromatic compounds, particularly fluoropyridines, which do not have these drawbacks.

[0015] To this end, the invention relates to a process for preparing a fluoropyridine of formula (I):

[0016] [Chem 4] in which R 1 represents H or F, comprising the fluorination of a halopyridine of formula (II):

[0017] [Chem 5] in which:

[0018] R 2 represents Cl or Br,

[0019] R 3represents H, Cl or Br, by hydrogen fluoride in the gas phase and in the presence of a nanodiamond, at least part of whose surface contains sp hybridized carbon atoms 2 , by which a compound of formula (I) and an acid chosen from HCl, HBr and a mixture thereof are formed.

[0020] The process according to the invention allows the preparation of fluoropyridine from a bromo- or chloropyridine by a gas-phase fluorination reaction in the presence of a nanodiamond as a solid catalyst and hydrogen fluoride as the fluorinating agent. Since the process is carried out in the gas phase, it is less expensive than a liquid-phase process and does not require the management of liquid effluents.

[0021] The process uses a halopyridine of formula (II) as a starting material.

[0022] Preferably, in formula (II), and in the formulas described below in the embodiments where technically feasible, R 2 represents Cl and / or R 3 represents H or Cl, preferably H. The halopyridine used in the process has, for example, one of the following formulas (lia) to (llf):

[0023] [Chem 6] [Chem 7]

[0024] [Chem 8]

[0025] [Chem 9]

[0026] [Chem 10]

[0027] [Chem 11]

[0028] The halopyridine can comprise two halogens, that is, in formula (II) or any of the formulas (Ha) to (Ilf), R 3 is chosen from Cl and Br. The process will then lead to a fluoropyridine of formula (I) in which R 1represents F. The different embodiments of the process starting from halopyridines of formulas (lia) to (llf) in which R 3 is chosen from Cl and Br are described below.

[0029] In one embodiment, the process involves the fluorination of a halopyridine of formula (Ha) in which R 3 is chosen from Cl and Br, and leads to a fluoropyridine of the following formula (la):

[0030] [Chem 12]

[0031] In one embodiment, the process involves the fluorination of a halopyridine of formula (Hb) in which R 3 is chosen from Cl and Br, and leads to a fluoropyridine with the following formula (Ib):

[0032] [Chem 13]

[0033] In one embodiment, the process involves the fluorination of a halopyridine of formula (Ile) in which R 3is chosen from Cl and Br, and leads to a fluoropyridine of the following formula:

[0034] [Chem 14]

[0035] In one embodiment, the process involves the fluorination of a halopyridine of formula (I Id) in which R 3 is chosen from Cl and Br, and leads to a fluoropyridine of the following formula (Id): [Chem 15]

[0036] In one embodiment, the process involves the fluorination of a halopyridine of formula (Ile) in which R 3 is chosen from Cl and Br, and leads to a fluoropyridine of the following formula: [Chem 16]

[0037] In one embodiment, the process involves the fluorination of a halopyridine of formula (llf) in which R 3 is chosen from Cl and Br, and leads to a fluoropyridine of the following formula (If): [Chem 17]

[0038] Preferably, the halopyridine used as a starting material in the process contains only a single halogen. Thus, in formula (II), R 3 represents H. The process then leads to a fluoropyridine of formula (I) in which R 1 represents H. The different embodiments of the process starting from halopyridines of formulas (Ha) to (llf) in which R 3 are described below.

[0039] In one embodiment, the process involves the fluorination of a halopyridine of formula (lia), (llb) or (llf) in which R 3 is H, that is to say a halopyridine with the following formula (I Ig):

[0040] [Chem 18] in which R 2 is Cl or Br, and the process leads to a fluoropyridine with the following formula (Ig):

[0041] [Chem 19]

[0042] In one embodiment, the process involves the fluorination of a halopyridine of formula (Ile) or (lld) in which R 3 is H, that is to say a halopyridine of the following formula (II h):

[0043] [Chem 20] in which R 2 is Cl or Br, and the process leads to a fluoropyridine of the following formula (Ih): [Chem 21]

[0044] In one embodiment, the process involves the fluorination of a halopyridine of formula (Ile) in which R 3 is H, that is to say a halopyridine of the following formula (I Ih): [Chem 22] in which R 2 is Cl or Br, and leads to a fluoropyridine of the following formula (li): [Chem 23]

[0045] The process uses hydrogen fluoride (HF) as the fluorinating agent. In terms of atom economy, HF is the best fluorinating agent, as fluorine represents 95% by weight of the reagent.

[0046] Preferably, the molar ratio of hydrogen fluoride to halopyridine is greater than or equal to 1 / 1 when R 3 represents H (the fluorination is then a monofluorination by substitution of R 2 by F), and greater than or equal to 2 / 1 when R 3 represents Cl or Br (the fluorination is then a difluoriation by substitution of R 2 and the R 3 by F). However, the fluorination yields are improved when the molar ratio of hydrogen fluoride to halopyridine is between 5 / 1 and 10 / 1. Indeed, the inventors observed that the activity was lower when hydrogen fluoride is used in large excess.

[0047] The process can be implemented in an atmosphere comprising, in addition to hydrogen fluoride, one or more other inert gases, typically N2.

[0048] The process uses nanodiamond(s) as a fluorination catalyst. This nanodiamond is unique firstly because of its nanometric size, which gives it a large specific surface area, and secondly because at least part of its surface contains sp² hybridized carbon atoms 2 This nanodiamond is produced by a detonation process, the primary source of sp² carbon. 2 Furthermore, due to their size, the formation of a sp layer 2 surface is predicted. Such nanodiamonds are commercially available. They can be prepared by detonation, as described for example in the article "Nanodiamonds produced by detonation: Their synthesis and their use in pyrotechnics" by V. Pichot et al., l'actualité chimique, April 2009, no. 329, p. 8-13.

[0049] The nanodiamond has an average diameter, as measured by high-resolution transmission electron microscopy (HRMET), of less than 1.0 pm, specifically from 0.1 to 150 nm, preferably from 0.5 to 50 nm, and most preferably from 1 to 10 nm. These average diameters correspond to those of the primary nanodiamond particles. Generally, the process is carried out with several nanodiamonds. These particles can be in the form of agglomerates, which typically have an average size, as measured by HRMET, of less than 1.0 pm. Preferably, the specific surface area of ​​the nanodiamond, as measured by BET, is 10 to 1000 m². 2 / g, preferably from 100 to 500 m 2 / g, preferably from 150 to 300 m 2 / g. Typically, nanodiamonds have hexagonal symmetry, which can be determined by the Fast Fourier Transform (FFT) method. This symmetry is consistent with a structure derived from face-centered cubic (fcc) structure.

[0050] The core of the nanodiamond contains sp hybridized carbon atoms 3 Part of the nanodiamond's surface contains sp² hybridized carbon atoms 2 The inventors observed that the elimination of sp hybridized carbon atoms 2 on the surface or their transformation into sp hybridized carbon atoms 3 For example, treating the nanodiamond with F2 leads to a significant drop in its catalytic activity for fluorination.

[0051] The presence of these sp hybridized carbon atoms 2This can be demonstrated by various methods. For example, various groups can be observed on the surface of the nanodiamond, notably involving hydrogen atoms, predominantly in the form of OH and CH, acid (COOH) groups, ester groups, and mixtures thereof. The adsorption of carbon monoxide (CO) on the nanodiamond's surface can also be monitored by infrared spectroscopy. The nanodiamond's ability to adsorb CO reveals the presence of sp² hybridized carbon atoms. 2 to its surface.

[0052] A nanodiamond synthesis process other than detonation can be used as long as the aforementioned physicochemical properties are present.

[0053] Preferably, the nanodiamond has not undergone any pretreatment prior to the fluorination reaction. Pretreatment means any treatment of the nanodiamond before its use as a catalyst in the fluorination process according to the invention. For example, the nanodiamond has not been exposed to an inorganic acid or a metal such as magnesium. Pretreatment does not include processes prior to the preparation of the nanodiamond. For example, the purification of detonation soot that may follow the preparation of the nanodiamond by detonation, or the separation of diamond particle agglomerates, are not pretreatments within the meaning of the application. One or both of these steps may take place even when the reaction is carried out with a nanodiamond that has not undergone any pretreatment prior to the fluorination reaction.

[0054] Alternatively, the nanodiamond may undergo one or more pretreatments. The nanodiamond may have been pretreated with an inorganic acid, or with several inorganic acids used in a mixture or sequentially. The inorganic acid may be chosen, for example, from hydrochloric acid, chromic acid, sulfuric acid, nitric acid, or a mixture thereof. For example, the nanodiamond may have been pretreated with hydrochloric acid or a mixture of acids, and then with nitric acid. The process may thus include, prior to fluorination, one or more pretreatments of the nanodiamond with one or more inorganic acids.

[0055] Alternatively, or following the inorganic acid pretreatment(s), the nanodiamond may have been pretreated by impregnation with a metal or metallic salt, possibly followed by calcination. The metal is chosen, for example, from chromium, barium, lanthanum, zinc, magnesium, or a mixture thereof, preferably magnesium. The metallic salt is preferably a salt of one of these metals or a mixture thereof. Examples of magnesium salts include magnesium chloride, magnesium nitrate, magnesium acetate, magnesium carbonate, or a mixture thereof. This impregnation with the metallic salt may be followed by calcination, which removes the counterion of the metal ion (for example, removing the nitrate when magnesium nitrate has been used). This pretreatment preferably leads to a nanodiamond comprising 0.5 to 20% by weight, preferably 1 to 15% by weight of metal.

[0056] The fluorination reaction in this process is generally carried out at atmospheric pressure (1 bar), but it can also be performed under pressure. It is typically carried out at a temperature of 250 to 400°C, preferably 300 to 375°C when the pressure is atmospheric, although these temperatures can be lower if the pressure is increased.

[0057] The process generally involves the recovery of the fluoropyridine of formula (I).

[0058] The fluorination reaction in this process leads to the formation of an acid chosen from HCl, HBr, and a mixture thereof. This acid is the only byproduct of the reaction and is itself valuable. Advantageously, the process is environmentally efficient, particularly because it does not produce saline discharge, unlike liquid-phase fluorination processes.

[0059] If R 2 is Cl and R 3is H or Cl, then the acid formed during fluoridation is HCl. If R 2 is Br and R 3 is H or Br, then the acid formed during fluoridation is HBr. If R 2 is Cl and R 3 is Br, or if R 3 is Cl and R 2 If Br is present, then the acid formed during fluoridation is a mixture of HBr and HCl. The process may include recovering the acid selected from HCl, HBr, and a mixture thereof. This acid can then be used for other purposes.

[0060] The following examples illustrate the invention. EXAMPLES

[0061] Catalyst preparation

[0062] Table 1 below provides a list of the chemicals used, [table 1]

[0063] The synthesis of unsupported (bulk) MgF2 was obtained from trifluoroacetic acid (TFA) following the protocol described in Astruc et al. Applied Catalysis A: General 453 (2013) 20-27.

[0064] Partially fluorinated alumina was prepared from commercial alumina (Sigma-Aldrich®) which was fluorinated for 1 h at 350°C under a flow of HF diluted in nitrogen.

[0065] The supported magnesium-based catalysts were prepared as follows. Magnesium was impregnated onto partially fluorinated alumina (“AIF3”) (commercial), onto commercial activated carbon (AC35), onto commercial nanodiamonds with a particle size of 4 to 5 nm (ND4) and onto fluorinated nanodiamonds (F-ND4) prepared at the Clermont Ferrand Institute of Chemistry (ICCF) from a treatment of ND4 with pure F2 for 12h at 450°C.

[0066] Activated carbon (AC35), nanodiamond (ND4), and fluorinated nanodiamond (F-ND4) were successively pretreated with a 0.3 mol / L HCl solution. -1 for 1 hour at 25°C, then after washing, with a 5 mol / L HNCh solution -1either for 1 hour at 25°C (AC35pt), or for 5 hours at 90°C (AC35pt5h, ND4pt5h, F-ND4pt5h) followed by drying for 12 hours at 120°C, in order to wash and functionalize the surface and thus facilitate the subsequent impregnation of magnesium.

[0067] The catalysts were prepared by dry impregnation of the support (fluorinated alumina, activated carbon, nanodiamond (ND4)) with a magnesium nitrate solution, taking into account the porosity of the support. The solution concentration was adjusted to deposit Mg contents of 2 or 10% by weight. After drying overnight in an oven at 110°C, the catalyst was calcined for 4 h at 500°C under nitrogen (100 mL / min). 1 ).

[0068] Specific surface areas of nanodiamond catalysts

[0069] Table 2 below provides the specific surface areas (ss) BET of the nanodiamond catalysts, according to the pretreatments carried out.

[0070] [Table 2]

[0071] Table 2: Specific surface areas (ss) of BET of nanodiamond catalysts

[0072] The absence of evolution of the specific surface area of ​​the nanodiamond despite the pretreatments it undergoes also demonstrates the interest of this material as a catalyst and / or catalyst support.

[0073] Indeed, its specific surface area is 257 m² 2 .g' 1 Initially (in the absence of pretreatment), it remains stable regardless of the treatment it undergoes (acid pretreatment, F2 pretreatment, or fluorination reaction with HF). This is explained by its structure, which has a sp² carbon core. 3 (diamond) and the size of the particles (4 to 5 nm), which respectively provide great stability of nanodiamonds under HF, and of its specific surface area.

[0074] Evolution of the chemical functions present on the surface of nanodiamonds during the different treatments

[0075] Initially, untreated nanodiamonds (ND4) mainly exhibit alcohol, ketone, nitrogenous functional groups and sp² carbon islands 2 The nitrogen would originate from the precursors of nanodiamond synthesis during the detonation.

[0076] After pretreatment with HCl and then HNO3 (ND4pt5h), the functional groups are similar to those of untreated nanodiamonds (ND4), with the exception of an increase in the amount of CH and carbon-hydroxyl (C-OH) groups, the appearance of carboxylic acid (-COOH) groups, and oxidized forms of nitrogen (-NO2). After treatment of the (initially untreated) nanodiamonds with HF, few changes in surface functional groups are observed. In contrast, for the pretreated nanodiamonds (ND4pt5h) treated with HF, the amount of sp² carbon is significantly increased. 2 decreases in favor of sp carbons 3 but sp hybridized carbons 2are always present. The carboxylic acid functions -COOH are converted by fluorination into the "COF" function. Fluorinated carbons are identified (CF).

[0077] Conversely, when nanodiamonds are pretreated with F2 (F-ND4) followed by an acid pretreatment (F-ND4pt5h), all surface chemical functions are converted to -CF functions, rendering the surface inert. The resulting nanodiamonds no longer have sp² hybridized carbons. 2 on the surface.

[0078] Impregnation of pre-treated acid-treated nanodiamonds (ND4pt5h) with magnesium (2 or 10 wt%) reveals a rutile-like MgF2 surface structure. Furthermore, this leads to the disappearance of the -COOH groups, suggesting that the magnesium either masks or interacts with these groups during impregnation.

[0079] Fluorination of 2-chloropyridine

[0080] The feasibility of the synthesis of fluoropyridine by a catalyzed reaction using HF as a fluorinating agent was established using 2-chloropyridine (2-CIPy) (Sigma-Aldrich®) as a model molecule (halopyridine of formula (II) in which R 2 represents Cl and R 1 represents H), in order to prepare the 2-fluoropyridine (2-Fpy) (fluoropyridine of formula (I) in which R 2 represents F and R 3 represents H).

[0081] In this case, the only by-product of the reaction is HCl, which is valuable.

[0082] All tests were conducted at atmospheric pressure, at a temperature of 350°C, with a molar ratio of HF / N₂ / 2-CIPy = 6 / 1, 7 / 1. Hydrogen fluoride was therefore in excess. The HF was anhydrous and gaseous, with a purity of 99.9% (Rapid'gaz). The nitrogen was supplied by Air Liquide®.

[0083] Mass MgF2 (test 1) was the catalyst that exhibited the highest activity with a 2-fluoropyridine selectivity of 100%. However, its specific surface area remained low (on the order of 35 m²). 2 .g' 1 ).

[0084] The following tests were carried out on supported catalysts in order to increase the specific surface area and therefore the number of active sites.

[0085] The fluorination of 2-chloropyridine was studied in the presence of magnesium-based catalysts supported on commercial fluorinated alumina (AIF3) (test 2), commercial activated carbon (AC35) (test 3), and nanodiamonds (ND4 with particle sizes of 4–5 nm) (tests 6–8). Specifically, the catalytic performance of magnesium-based catalysts (2 and 10 wt%) supported on fluorinated alumina, activated carbon, and nanodiamonds was measured to highlight the effect of a larger specific surface area on the properties of the active sites and the fluorination activity. These results are also compared to the unsupported MgF2 catalyst, which is the most active for this reaction (test 1).Furthermore, measurements were carried out with magnesium-free catalysts, namely with nanodiamonds not pretreated with acid or magnesium (test 4), and with nanodiamonds pretreated only with acid, but not with magnesium (test 5).

[0086] The results are provided in Table 3 below.

[0087] [Table 3]

[0088] 350°C, HF / N2 / 2-Clpyridine: 6 / 1, 7 / 1

[0089] ND4: nanodiamonds with an average diameter of 4 to 5 nm; ss: specific surface area; 2-Fpy (%): conversion to 2-fluoropyridine; te: contact time; a) Activity per gram: mmol.h' 1 .g' 1 b) Activity per unit area: mmol.h' 1 .m' 2 , pt5h: pretreatment by HCl 1 h at 25°C then HNO3 5h at 90°C Table 3: activities of various catalysts in the fluorination of 2-chloropyridine by HF in gas phase and conversion of fluorination.

[0090] Increasing the specific surface area by using a supported rather than a bulk catalyst does not necessarily lead to an increase in activity. Indeed, the specific surface area of ​​magnesium supported on activated carbon (commercial AC35, 819 m²) is 819 m². 2 / g, test 3) is much higher than that of the unsupported bulk catalyst, i.e. unsupported MgF2 (MgF2, 35 m 2 / g, test 1), but the activity of the catalyst supported on activated carbon is 4 times lower (commercial AC35, 14 mmol.h -1 . g -1 , trial 3 against MgF2, 55 mmol.h- 1 .g -1 , essay 1).

[0091] The best results are obtained using nanodiamond (ND4) as a support (test 4). Indeed, unlike the other supports (fluorinated alumina alone, activated carbon alone), nanodiamond (ND4) is active on its own, even without the need to add magnesium.

[0092] The activity of the acid-pretreated nanodiamond (ND4pt5h, 65 mmol.h-1.g-1, test 5) is of the same order as that calculated for unsupported MgF2 (55 mmol.h-1.g-1, test 5) -1 . g -1 , trial 1). That of the untreated nanodiamond (ND4, 91 mmol.h -1 . g -1 , test 4) is greater than that of the acid-pretreated nanodiamond (ND4pt5h, 65 mmol.h -1 . g -1 , essay 5).

[0093] The best catalytic activity is measured when 2 wt% magnesium has been impregnated onto the pretreated nanodiamonds (ND4pt5h, 147 mmol.h). -1 . g -1 (test 7). In this case, it is 2.6 times greater than the best activity calculated with the bulk catalyst, i.e., unsupported MgF2 (MgF2, 55 mmol.h). -1 . g -1 , essay 1).

[0094] These results were confirmed by reproducing the experiment (trials 6 and 7).

[0095] The catalytic activity remains constant when 10% by weight of Mg is impregnated on the support (test 8).

[0096] Conversely, it drops when 2% by weight of magnesium (Mg / F-ND4) has been impregnated onto the nanodiamond, which has previously undergone treatment with pure F2 at 520°C, the initial purpose of which was to clean the surface (F-ND4, 25 mmol.h). -1 . g -1 (e.g., test 9). Pretreatment with F2 eliminates the active sites present on the surface of the nanodiamond without changing their BET surface area (reduced to the carbon mass). Their surface no longer has sp² hybridized carbon atoms. 2 and hydrogenated and / or oxygenated functions.

[0097] Evolution of the specific surface area of ​​catalysts

[0098] Table 4 below provides the evolution of the specific surface area measured by BET of the catalysts before and after activation by HF, and their respective activities for the transformation of 2-chloropyridine.

[0099] [Table 4]

[0100] 1) Mg 2% by weight

[0101] 2) Activated carbon (AC35) and nanodiamonds (ND4) were successively pretreated with a 0.3 mol.L HCl solution -1 for 1 hour at 25°C, followed by washing and treatment with a mol.L HNO35 solution -1 for 5 hours at 90°C. This treatment was followed by drying for 12 hours at 120°C to clean and functionalize the surface, thus facilitating metal impregnation. 2 hours at 350°C under a flow of HF gas (N2 / HF = 1 / 4).

[0102] 3) T=350°C for 1 hour, HF / N2= 4

[0103] 4) Transformation of 2-chloropyridine: T=350°C, HF / N2 / 2CIPy: 6 / 1, 7 / 1

[0104] Table 4: Evolution of the specific surface area of ​​the catalysts before and after activation by HF, and their respective activities for the transformation of 2-chloropyridine.

[0105] These results show that:

[0106] - The specific surfaces of materials based on activated carbon and nanodiamonds have better stability to HF activation compared to Al2O3,

[0107] - The specific surface area of ​​AC35 activated carbon is much larger than that of nanodiamonds, both before and after HF activation.

[0108] - A good specific surface area of ​​the catalyst is an essential, but not sufficient, condition for achieving good catalytic activity in the transformation of 2-chloropyridine. The catalytic activity is better with nanodiamonds, clearly demonstrating that the increased activity is due to the unique surface properties of nanodiamonds, which can be modified depending on the treatment.

Claims

DEMANDS Process for preparing a fluoropyridine of formula (I): [Chem 24] in which R 1 represents H or F, comprising the fluorination of a halopyridine of formula (II): [Chem 25] in which R 2 represents Cl or Br, and R 3 represents H, Cl or Br, by hydrogen fluoride in the gas phase and in the presence of a nanodiamond, at least part of whose surface contains sp carbon atoms 2 , by which a compound of formula (I) and an acid chosen from HCl, HBr and a mixture thereof are formed.

2. A method according to claim 1, wherein, in formula (I), R 1 represents H, and in formula (II), R 3 represents H.

3. A process according to claim 1 or 2, for preparing a fluoropyridine of formula (Ig): [Chem 26] by reaction of a halopyridine with the following formula: [Chem 27] in which R 2 represents Br or Cl.

4. A method according to claim 3, wherein, in formula (I Ig), R 2 represents Cl.

5. A method according to any one of claims 1 to 4, wherein the nanodiamond was prepared by detonation.

6. A method according to any one of claims 1 to 5, wherein the surface of the nanodiamond carries groups selected from OH, CH, acid functions, ester functions or mixtures thereof.

7. A method according to any one of claims 1 to 6, wherein the nanodiamond has a specific surface area such as that measured by BET from 10 to 1000 m 2 / g, preferably from 100 to 500 m 2 / g, preferably from 150 to 300 m 2 / g.

8. A method according to any one of claims 1 to 7, wherein the nanodiamond has an average diameter, as measured by high-resolution transmission electron microscopy (HRMET), of less than 1.0 pm, in particular from 0.1 to 150 nm, preferably from 0.5 to 50 nm, particularly preferably from 1 to 10 nm.

9. A method according to any one of claims 1 to 8, carried out at a temperature of 250 to 400°C, preferably 300 to 375°C and at atmospheric pressure.

10. A method according to any one of claims 1 to 9, wherein the nanodiamond has not undergone pretreatment prior to fluoridation.

11. A process according to any one of claims 1 to 9, wherein the nanodiamond has been pretreated with an inorganic acid before fluoridation.

12. A process according to any one of claims 1 to 9 or 11, wherein, prior to fluoridation, the nanodiamond, optionally pre-treated with an inorganic acid, has been pre-treated by impregnation with a metal or metallic salt, optionally followed by calcination.

13. A method according to claim 12, wherein the metal is selected from chromium, barium, lanthanum, zinc, magnesium or a mixture thereof.

14. A method according to claim 13, wherein the metal is magnesium.

15. A process according to any one of claims 1 to 12, comprising the recovery of the acid selected from HCl, HBr and a mixture thereof.

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