Nanodiamond-catalyzed fluorination process for a halopyridine

Abstract

Nanodiamond-Catalyzed Fluorination Process for a Halopyridine. The present invention relates to a process for preparing a fluoropyridine comprising the fluorination of a halopyridine with hydrogen fluoride in the gas phase and in the presence of nanodiamonds, at least a portion of whose surface contains sp2 carbon atoms and hydrogenated and / or oxygenated surface functional groups, thereby forming a compound of formula (I) and an acid selected from HCl, HBr, and mixtures thereof. Figure for the abstract: None

Description

Title of the invention: Nanodiamond-catalyzed fluorination process for a halopyridine

[0001] The present invention relates to a process for preparing fluoropyridines from a bromo- or chloropyridine.

[0002] 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 pharmaceutical products 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, fluoropyridinic motifs are present in molecules such as fluoroxypyr, with formula (X), used as a herbicide, or alatrofloxacin, with formula (XI), used as an antibiotic.

[0003] [Chem.l]

[0004] [Chem.2]

[0005] On the other hand, very few fluorinated organic compounds are present in nature. It is therefore necessary to develop appropriate synthetic methods.

[0006] To date, the catalyzed fluorination of chlorinated molecules to form the corresponding fluorinated compounds is only applied industrially to the fluorination of molecules non-functionalized aliphatics leading to the formation of chlorofluoroalkanes (CFCs) and their substitutes, i.e. hydrofluorocarbons (HFCs) and more recently hydrofluoroolefins (HFOs).

[0007] On the other hand, to the inventors' knowledge, the literature does not report the synthesis of fluorinated aromatics by a catalyzed process.

[0008] Indeed, at present, 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).

[0009] [Chem.3] Fluoroded iazotatton (Platform 11) HALEX reaction (Track 2)

[0010] Scheme 1: Synthesis routes of fluorinated aromatic compounds according to the prior art

[0011] These two reactions present constraints and limitations such as the generation of large volumes of non-recoverable saline effluents (KC1, trifluoroborate salts), the toxicity of the reagents (anilines) and the lack of selectivity and reactivity of certain positions.

[0012] There is therefore a need to prepare fluorinated aromatic compounds, in particular fluoropyridines, which do not have these disadvantages.

[0013] To this end, the invention relates to a process for preparing a fluoropyridine of formula (I):

[0014] [Chem.4] H

[0015] in which R1 represents H or F,

[0016] comprising the fluorination of a halopyridine of formula (II):

[0017] [Chem.5]

[0018] in which:

[0019] - R2 represents Cl or Br,

[0020] - R3 represents H, Cl or Br,

[0021] by hydrogen fluoride in the gas phase and in the presence of nanodiamond having at least part of the surface containing sp2 hybridized carbon atoms, by which a compound of formula (I) and an acid chosen from HCl, HBr and a mixture thereof are formed.

[0022] 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 a 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.

[0023] The process uses a halopyridine of formula (II) as a starting material.

[0024] Preferably, in formula (II), and in the formulas described below in the embodiments when technically possible, R2 represents Cl and / or R3 represents H or Cl, preferably H.

[0025] The halopyridine used in the process has, for example, one of the formulas (lia) to (Ilf) following:

[0026] [Chem. 6] (B)

[0027] [Chem.7]

[0028] [Chem. 8]

[0029] [Chem.9]

[0030]

[0031] R 3 [Chem. 10] [Chem. 11] (I©

[0032]

[0033]

[0034] The halopyridine can comprise two halogens, that is, in formula (II) or one of the formulas (Ia) to (Ilf), R3 is chosen from Cl and Br. The process will then lead to a fluoropyridine of formula (I) in which R1 represents F. The different embodiments of the process starting from halopyridines of formulas (Ia) to (Ilf) in which R3 is chosen from Cl and Br are described below. In one embodiment, the process involves the fluorination of a halopyridine of formula (lia) in which R3 is chosen from Cl and Br, and leads to a fluoropyridine of the following formula (la): [Chem. 12]

[0035]

[0036] In one embodiment, the process involves the fluorination of a halopyridine of formula (Ilb) in which R3 is chosen from Cl and Br, and leads to a fluoropyridine of the following formula (Ib): [Chem. 13]

[0037]

[0038] In one embodiment, the process involves the fluorination of a halopyridine of formula (Ile) in which R3 is chosen from Cl and Br, and leads to a fluoropyridine of the following formula (le): [Chem. 14]

[0039] O- In one embodiment, the process involves the fluorination of a halopyridine of formula (Ild) in which R3 is chosen from Cl and Br, and leads to a fluoropyridine of the following formula (Id):

[0040] [Chem. 15]

[0041] In one embodiment, the process carries out the fluorination of a halopyridine of formula (Ile) in which R3 is chosen from Cl and Br, and leads to a fluoropyridine of the following formula (le):

[0042] [Chem. 16]

[0043] In one embodiment, the process carries out the fluorination of a halopyridine of formula (Ilf) in which R3 is chosen from Cl and Br, and leads to a fluoropyridine of the following formula (If):

[0044] [Chem. 17] F (If).

[0045] Preferably, the halopyridine used as a starting material in the process contains only one halogen. Thus, in formula (II), R3 represents H. The process then leads to a fluoropyridine of formula (I) in which R1 represents H. The various embodiments of the process starting from halopyridines of formulas (Ia) to (If) in which R3 is H are described below.

[0046] In one embodiment, the process involves the fluorination of a halopyridine of formula (Ia), (Ilb) or (Ilf) in which R3 is H, i.e. a halopyridine of the following formula (Ilg):

[0047] [Chem. 18] 7 FC

[0048]

[0049]

[0050] in which R2 is Cl or Br, and the process leads to a fluoropyridine of the following formula (Ig): [Chem. 19]

[0051]

[0052] In one embodiment, the process involves the fluorination of a halopyridine of formula (Ile) or (Ild) in which R3 is H, i.e. a halopyridine of formula (Ilh) as follows: [Chem. 20]

[0053]

[0054]

[0055] in which R2 is Cl or Br, and the process leads to a fluoropyridine of the following formula (Ih): [Chem.21]

[0056]

[0057]

[0058] In one embodiment, the process involves the fluorination of a halopyridine of formula (Ile) in which R3 is H, i.e. a halopyridine of the following formula (Ilh): [Chem. 22]

[0059] in which R2 is Cl or Br,

[0060] and leads to a fluoropyridine of the following formula (li):

[0061] [Chem.23]

[0062] The process uses hydrogen fluoride (HF) as a fluorinating agent. In terms of atom economy, HF is the best fluorinating agent, since fluorine represents 95% by weight of the reagent.

[0063] Preferably, the molar ratio of hydrogen fluoride to halopyridine is greater than or equal to 1:1 when R3 represents H (the fluorination is then a monofluoration by substitution of R2 with F), and greater than or equal to 2:1 when R3 represents Cl or Br (the fluorination is then a difluoration by substitution of both R2 and R3 with 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 was used in large excess.

[0064] The process can be carried out in an atmosphere comprising, in addition to hydrogen fluoride, one or more other inert gases, typically N2.

[0065] The process uses nanodiamond(s) as a fluorination catalyst. This nanodiamond is specific, 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. This nanodiamond is produced by a detonation process, the primary source of sp2 carbon. Furthermore, due to their size, the formation of an sp2 layer on the 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 use in pyrotechnics" by V. Pichot et al., L'Actualité Chimique, April 2009, No. 329, pp. 8-13.

[0066] 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, and most preferably from 1 to 10 nm. These average diameters correspond to those of the primary nanodiamond particles. In general, the process is carried out with several nanodiamonds. These particles can be in the form of agglomerates, which generally have an average size measured by HRMET of less than 1.0 pm. Preferably, the specific surface area of ​​the nanodiamond, as measured by BET, is from 10 to 1000 m² / g, preferably from 100 to 500 m² / g, and preferably from 150 to 300 m² / 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.

[0067] The core of the nanodiamond contains sp3 hybridized carbon atoms. Part of the surface of the nanodiamond contains sp2 hybridized carbon atoms. The inventors have observed that the removal of the sp2 hybridized carbon atoms on the surface or their transformation into sp3 hybridized carbon atoms, for example by treating the nanodiamond with F2, leads to a significant drop in its catalytic activity for fluorination.

[0068] The presence of these sp2 hybridized carbon atoms 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, as well as acid or ester functional groups. The adsorption of carbon monoxide by infrared spectroscopy on the surface of the nanodiamond can also be monitored. The nanodiamond's ability to adsorb CO reveals the presence of sp2 hybridized carbon atoms on its surface.

[0069] A nanodiamond synthesis process other than detonation may be used provided that the aforementioned physicochemical properties are present.

[0070] Preferably, the nanodiamond has not undergone any pretreatment prior to the fluorination reaction. Pretreatment means 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 brought into contact with an inorganic acid or with 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.

[0071] Alternatively, the nanodiamond may undergo one or more pretreatment(s).

[0072] The nanodiamond may have been pretreated with an inorganic acid, or with several inorganic acids used in a mixture or successively. The inorganic acid is 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 comprise, prior to fluoridation, one or more pretreatments of the nanodiamond with one or more inorganic acids.

[0073] Alternatively, or following the inorganic acid pretreatment(s), the nanodiamond may have been pretreated by impregnation with a metal or a metallic salt, optionally followed by calcination. The metal is, for example, chosen 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..

[0074] The fluorination reaction of the process is generally carried out at atmospheric pressure (1 bar) but it can be performed under pressure. It is generally carried out at a temperature of 250 to 400°C, preferably 300 to 375°C when the pressure is atmospheric, it being understood that these temperatures can be lower if the pressure is increased.

[0075] The process generally includes the recovery of the fluoropyridine of formula (I).

[0076] The fluorination reaction of the process leads to the formation of an acid selected from HCl, HBr, and a mixture thereof. This acid is the only by-product of the reaction and is itself recoverable. Advantageously, the process is environmentally efficient, particularly because it does not produce saline discharge, unlike liquid-phase fluorination processes.

[0077] If R2 is Cl and R3 is H or Cl, then the acid formed during fluoridation is HCl. If R2 is Br and R3 is H or Br, then the acid formed during fluoridation is HBr. If R2 is Cl and R3 is Br, or if R3 is Cl and R2 is Br, 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.

[0078] The following examples illustrate the invention. EXAMPLES Catalyst preparation

[0079] Table 1 below provides a list of the chemicals used.

[0080] [Tables 1] Products Formula Supplier Purity (%) Partially fluorinated alumina "A1F3" prepared - Magnesium nitrate (hexahydrate) Mg(NO3)2.6H2O Sigma-Aldrich® 98 Activated carbon AC35 - Merck - Nanodiamond (ND4) - ACS materials - Fluorinated nanodiamond (F-ND4) - ACS materials + treatment with F2 (T>450 °C) -

[0081] Table 1: Chemicals used for the preparation of catalysts.

[0082] 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.

[0083] Partially fluorinated alumina was prepared from commercial alumina (Sigma-Aldrich6) which was fluorinated for 1h at 350°C under a flow of HF diluted in nitrogen.

[0084] The supported magnesium-based catalysts were prepared as follows. Magnesium was impregnated onto a partially fluorinated alumina (“A1F3”) (commercial), onto a 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 with F2pur for 12h at 450°C of ND4.

[0085] Activated carbon (AC35), nanodiamond (ND4) and fluorinated nanodiamond (F-ND4) were pretreated successively with a 0.3 mol.L 1 HCl solution for 1h at 25°C and then after washing, with a 5 mol.L 1 HNO3 solution either for 1h at 25°C (AC35pt), or for 5h at 90°C (AC35pt5h, ND4pt5h, F-ND4pt5h) followed by drying for 12h at 120°C, so as to wash and functionalize the surface and thus facilitate the subsequent impregnation of magnesium.

[0086] 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 concentration of the solution is adjusted to deposit Mg contents of 2 or 10% by weight. After drying overnight in an oven at 110°C, the catalyst is calcined for 4 hours at 500°C under nitrogen (100 mL / min). Specific surface areas of the nanodiamond catalysts

[0087] Table 2 below provides the specific surface areas (ss) BET of the nanodiamond catalysts, according to the pretreatments carried out.

[0088] [Tables2] Nanodiamond catalyst Pretreatment(s) Mg% wt. ss BET (m2^1) ND4 none 0 257 ND4pt5h pt5h 0 231 Mg / ND4pt5h pt5h then Mg 2 255 Mg / ND4pt5h pt5h then Mg 10 220 Mg / F-ND4pt5h F2 then Mg 2 226

[0089] Table 2: Specific surface areas (ss) of BET nanodiamond catalysts

[0090] 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.

[0091] Indeed, its specific surface area, which is initially 257 m².g⁻¹ (in the absence of pretreatment), remains stable regardless of the treatment it undergoes (pretreatment with acid, with F₂, or fluorination reaction with HF). This is explained by its structure, which has an sp³ carbon core (diamond), and by the particle size (4 to 5 nm), which respectively provide high stability to the nanodiamonds under HF and to its specific surface area.

[0092] Evolution of the chemical functions present on the surface of nanodiamonds during the different treatments

[0093] Initially, untreated nanodiamonds (ND4) mainly exhibit alcohol, ketone, nitrogenous functions and sp2 carbon islands. The nitrogen would come from the nanodiamond synthesis precursors during detonation.

[0094] After pretreatment with HCl then HNO3 (ND4pt5h), the functions are similar to those of unpretreated nanodiamonds (ND4), except for the increase in the quantity of CH and C-OH functions, the appearance of -COOH functions and oxidized forms of nitrogen (-NO2).

[0095] After exposure of the nanodiamonds to HF, few changes in surface functionalities are identified. In contrast, for pretreated nanodiamonds (ND4pt5h) exposed to HF, the amount of sp2 carbon decreases in favor of sp3 carbons, but sp2 hybridized carbons are still present. The carboxylic acid (-COOH) functions are converted by fluorination to "COF" functions. Fluorinated carbons (CF) are identified.

[0096] On the other hand, when nanodiamonds are pre-treated with F2 (F-ND4) followed by an acid pretreatment (F-ND4pt5h), all the surface chemical functions are converted into -CF functions, which renders the surface inert. The resulting nanodiamonds no longer have sp2 hybridized carbons on their surface.

[0097] Impregnation of pre-treated acid-treated nanodiamonds with magnesium (2 or 10 wt%) reveals a rutile-type MgF2 surface structure. Furthermore, this leads to the disappearance of the -COOH groups, suggesting that the magnesium masks or interacts with these groups during impregnation. Fluorination of 2-chloropyridine

[0098] The feasibility of the synthesis of fluoropyridine by a reaction catalyzed using HF as a fluorinating agent was established using 2-chloropyridine (2-ClPy) (Sigma-Aldrich®) as a model molecule (halopyridine of formula (II) in which R2 represents Cl and R1 represents H), in order to prepare 2-fluoropyridine (2-Fpy) (fluoropyridine of formula (I) in which R2 represents F and R3 represents H).

[0099] In this case, the only by-product of the reaction is HCl, which is valuable.

[0100] All tests were carried out at atmospheric pressure, at a temperature of 350°C, with a molar ratio of HF / N2 / 2-ClPy = 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®.

[0101] Mass MgF2 (test 1) is the catalyst that exhibited the greatest activity with a 2-fluoropyridine selectivity of 100%. On the other hand, its specific surface area of ​​the catalyst remains low (on the order of 35 m2.g').

[0102] The following tests were carried out on supported catalysts in order to increase the specific surface area and therefore the number of active sites.

[0103] The fluorination of 2-chloropyridine was studied in the presence of magnesium-based catalysts supported on commercial fluorinated alumina (A1F3) (test 2), commercial activated carbon (AC35) (test 3), and nanodiamonds (ND4 with a particle size of 4 to 5 nm) (tests 6 to 8). More 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).

[0104] 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).

[0105] The results are provided in Table 3 below.

[0106] [Tables3] Catalyst test (s) 2-Fp y (%) a) b) Nature Pretreatment(s) of ND4 surface BET (m2^1) Mg (% by weight) 1 (com P) MgF2 (not supported) - 35 0 0.26 28 55+3 1.57+0.08 2 (com P) Mg / Al1F3 - 25 2 0.73 9 11+1 0.44+0.02 3 (com P) Mg / AC35pt5h — 819 2 0.86 13 14+1 0.02+0.01 4 (inv) ND4 none 257 0 0.32 30 91+5 0.35+0.01 5 (inv) ND4pt5h pt5h 231 0 0.26 18 65+3 0.28+0.01 6 (inv) Mg / ND4pt5h pt5h then Mg 255 2 0.13 19 141+7 0.55+0.03 7 (inv) Mg / ND4pt5h pt5h then Mg2 255 2 0.08 13 147+7 0.58+0.03 8 (inv) Mg / ND4pt5h pt5h then Mg 220 10 0.08 13 149+7 0.67+0.03 9 (com P) Mg / F-ND4pt5 h F2 then Mg 226 2 0.77 21 25+1 0.11+0.01

[0107] 350°C, HF / N2 / 2-Clpyridine: 6 / 1.7 / 1

[0108] ND4: nanodiamonds with an average diameter of 4 to 5 nm

[0109] ss: specific surface area

[0110] 2-Fpy (%) : conversion to % to 2-fluoropyridine

[0111] te: contact time

[0112] a) Activity per gram: mmol.h⁻¹.g⁻¹

[0113] b) Activity per unit area: mmol.h'.m2,

[0114] pt5h: pretreatment with HCl 1h at 25°C then HNO3 5h at 90°C

[0115] Table 3: Activities of various catalysts in the fluorination of 2-chloropyridine by HF in the gas phase and conversion of fluorination.

[0116] 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 / g, test 3) is much higher than that of the unsupported bulk catalyst, i.e. unsupported MgF2 (MgF2, 35 m2 / g, test 1), but the activity of the supported catalyst on activated carbon is 4 times lower (commercial AC35, 14 mmol.h '.g1, test 3 versus MgF2, 55 mmol.h '.g ', test 1).

[0117] 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.

[0118] 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'.g', test 1). That of the unpretreated nanodiamond (ND4, 91 mmol.h'.g-1, test 4) is greater than that of the acid-pretreated nanodiamond (ND4pt5h, 65 mmol.h'.g', test 5).

[0119] The best catalytic activity is measured when 2 wt% of magnesium has been impregnated onto the pretreated nanodiamonds (ND4pt5h, 147 mmol.h'.g1, 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'.g1, test 1).

[0120] These results were confirmed by reproducing the experiment (trials 6 and 7).

[0121] The catalytic activity remains constant when 10% by weight of Mg are impregnated on the support (test 8).

[0122] Conversely, it decreases when 2 wt% magnesium (Mg / F-ND4) is 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, test 9). The F2 pretreatment 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 and hydrogenated and / or oxygenated functional groups.

Claims

1. Demands Process for preparing a fluoropyridine of formula (I): [Chem.24] in which R1 represents H or F, comprising the fluorination of a halopyridine of formula (II): [Chem.25]

2.

3. in which R2 represents Cl or Br, and R3 represents H, Cl or Br, by hydrogen fluoride in the gas phase and in the presence of nanodiamond of which at least part of the surface contains sp2 carbon atoms, by which are formed a compound of formula (I) and an acid chosen from HCl, HBr and a mixture thereof. Method according to claim 1, wherein, in formula (I), R1 represents H, and in formula (II), R3 represents H. 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] R2

4.

5.

6.

7.

8.

9.

10. in which R2 represents Br or Cl. Method according to claim 3, wherein, in formula (Ilg), R2 represents Cl. A method according to any one of claims 1 to 4, 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. A method according to any one of claims 1 to 5, carried out at a temperature of 250 to 400°C, preferably 300 to 375°C and at atmospheric pressure. A process according to any one of claims 1 to 6, wherein the nanodiamond has not undergone pretreatment prior to fluoridation. A process according to any one of claims 1 to 6, wherein the nanodiamond has been pretreated with an inorganic acid before fluoridation. A process according to any one of claims 1 to 6 or 8, wherein, prior to fluorination, 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, the metal being in particular selected from chromium, barium, lanthanum, zinc, magnesium or a mixture thereof and preferably being magnesium. A process according to any one of claims 1 to 9, comprising the recovery of the acid selected from HCl, HBr and a mixture thereof.