Title of Invention	A PROCESS FOR THE SYNTHESIS OF DIFLUOROMETHANE BY GAS-PHASE CATALYTIC FLUORINATION
Abstract	Accordingly, the present invention relates to a process for the synthesis of difluoromethane by gas-phase catalytic fluorination of methylene chloride by means of anhydrous hydrofluoric acid, which operation is carried out in the presence of 0.1 to 5 moles of oxygen per 100 moles of methylene chloride, at a temperature of between 330 and 450°C and with a bulk or supported chromium catalyst.

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The present invention relates to a process for the synthesis of difluoromethane by gas-phase catalytic fluorination. Difluoromethane, known under the designation of F32, is considered of no danger to the ozone layer. It is therefore particularly preferred for the replacement of CFCs. As a mixture with other hydrofluoroalkanes such as 1,1,1-trifluoroethane (F143a), 1,1,1,2-tetrafluoroethane (F134a) or pentafluoroethane (F125) it is proposed to use F32 to replace in particular F22 (chlorodifluoromethane) and F502 (azeotropic mixture of F22 and of chloropentafluoroethane) in refrigeration, air conditioning and other applications. There are various known processes for the synthesis of F32. The hydrogenolysis of F12 (dichlorodifluoromethane) or of F22 (Japanese Patent JP 60-01731 and European Patent Specification EP 508 660) has the disadvantage of being generally not very selective and of giving worthless methane as by¬product. It has recently been proposed to produce F32 by fluorination of bis(fluoromethyl) ether (European Patent Specification EP 518 506). It is also known to produce F32 by fluorination of methylene chloride (F30) using anhydrous HF. Many patents describe this reaction, claiming the use of catalysts such as Cr2O3, CrF3, A1F3, Cr/carbon, Ni/AIF, etc. The difficulty in this reaction lies in the stability of the catalyst, which tends either to coke rapidly or to crystallize. The problem becomes very tricky if it is intended to combine a high space time yield and a good selectivity while maintaining good stability of the catalyst. To reduce this deactivation it has been proposed to employ specific catalysts such as a mechanical mixture of alumina and chromium oxide (British Patent Specification GB 821 211). This specification gives an example for the fluorination of methylene chloride, but the F32 space time yields obtained on this catalyst are low ( More generally, during fluorination reactions, it is very often envisaged to inject oxygen or air continuously to lengthen the lifetime of the catalysts. Thus, Japanese patent JP 51-82206 describes the use of 0.001 to 1 % of oxygen to maintain the activity of a catalyst containing chiefly chromium oxide and optionally other metal oxides. It is indicated that the use of more than 1 % of oxygen results in the appearance of secondary reactions and it is therefore recommended to employ preferably from 0.005 to 0.1 % of oxygen. In this specification the fluorination reactions are carried out between 100 and 500°C and preferably between 250 and 350CC. In addition, it is stated there that, starting at 200°C, the catalyst activity is maintained by the introduction of oxygen. Although this specification mentions, among the reactions, the fluorination of CC14/ CHC13, CH2C12, CC13F, C2C16, C2C14 and CaH3Cl3, the examples refer only to the fluorination of perhalogenated saturated materials (CC14 and CaCl3F3) . It is known, however, that the reactivity of perhalogenated molecules is very different from that of the hydrogenated materials. The hydrogenated materials, such as F133a (1-chloro-2,2,2-trifluoroethane) are sensitive to elimination reactions (loss of HC1 or of HF) and to chlorination reactions, which result in the formation of worthless by-products. As French patent FR 2 433 500 shows, the introduction of oxygen at the reaction temperature (generally higher than that employed for fluorinating perhalogenated molecules) can then result in a drop in selectivity. Chromium oxide, well known as a fluorination catalyst, is also a good catalyst for the oxidation of HC1 (US patents 4 803 065 and 4 822 589) . The oxygen introduced during the fluorination reaction reacts with the HC1 formed to produce chlorine by the Deacon reaction. This chlorine can then easily cause a chlorination of the hydrogenated materials present in the reaction mixture. In the case of fluorination of F133a in the presence of oxygen, products of the F120 series (CaHClnFs.n) are thus chiefly formed. Besides the formation of chlorine, this Deacon reaction also produces water which, because of corrosion problems, is particularly undesirable in a fluorination process. To overcome this disadvantage it has been proposed to employ some chromium-based mixed catalysts which make it possible to restrict the Deacon reaction. Thus, European patent EP 546 883 shows that, in the case of bulk catalyst, the addition of a metal such as nickel allows the oxidation of HC1 to be partially inhibited. A similar phenomenon is observed on Ni-Cr/AlFj mixed catalysts (European Patent Specification EP 486 333 and published International Application WO 93/25507) . With a similar objective in view, European patent EP 328 127 proposes carrying out the fluorination of Fl33a to F134a on a catalyst containing no chromium. The recommended solids contain at least one metal chosen from cobalt, manganese, nickel, palladium, silver, ruthenium and aluminium. Recently, after having shown that in the case of the reaction of fluorination of methylene chloride in the presence of oxygen the chromium catalysts were not very selective (formation of F22 and of halogenated ethane derivatives), Japanese patent JP 5-339179 also described the use of catalysts devoid of chromium, which are specific to the synthesis of F32. These catalysts, such as CoCl2/AlFj or NiCl2/AlF3, are highly selective and their stability is increased by additives chosen from the rare earths {La, Ce) or alkaline-earth elements (Mg, Ca, Sr). The lifetimes obtained in the presence of oxygen are considerable {150 days), but the space time yields of F32 are very low { In trials of fluorination of methylene chloride, with a shorter contact time, aimed at increasing the space time yield of F32, we have been surprised to find that, in contrast to what the abovementioned patent specifications lead one to expect, usual fluorination catalysts such as Ni/A1F3 or Ni-Cr/AlFj are not stable, even in the presence of oxygen. On the other hand, it has now been found that there is a temperature range in which a catalyst based on pure chromium (without the addition of another metal oxide) can produce, in the presence of oxygen, with an excellent stability, F32 by gas-phase fluorination of methylene chloride, without any significant loss of selectivity. In fact, without an explanation being possible, it has first of all been surprising to find that the Deacon reaction is practically nonexistent, even when the methylene chloride fluorination reaction is carried out in the presence of large quantities of oxygen (3 mol%) between 250 and 450°C on chromium oxide. The by-products originating from chlorination reactions are very minor in quantity. In addition, the absence of a Deacon reaction makes it possible to limit the generation of water in the reactor, and this limits the corrosion phenomena. The fluorination of methylene chloride is therefore a very special reaction, differing from fluorination reactions such as those of F133a, CaCl«, F123 or F124. In contradiction to what might be expected from the prior art, it is therefore possible to employ a chromium-based catalyst to carry out this fluorination reaction in the presence of oxygen, without decrease in the selectivity of the reaction. It is therefore unnecessary to employ special additives in order to increase its selectivity; the elimination of the additives employed in the mixed catalysts enables the manufacture of the catalyst to be simplified and thereby its cost to be reduced. The use of a chromium-based (bulk or supported) catalyst makes it possible furthermore to reach very high F32 space time yields. In addition, it has been surprising to find that, among the fluorination catalysts which have been tested, only the catalysts (bulk or supported) in which the active phase contains only chromium are capable of limiting coke formation at the temperature of fluorination of methylene chloride. It has also been found that only a narrow temperature range enables the catalyst activity to be maintained efficaciously. Below 330° the introduction of oxygen does not enable the coke formation to be slowed down and the catalyst gradually becomes deactivated. On the other hand, the temperature which is higher than 400°C can result in crystallization of the solid, entailing a decrease in its activity. In summary, it has been found that, in order to prepare F32 in a high space time yield, in a stable and selective manner, it is necessary to combine an introduction of oxygen, a bulk or supported chromium-based catalyst and a restricted temperature range. Accordingly the present invention provides a process for the manufacture of F32 by gas-phase catalytic fluorination of methylene chloride (F30) by means of anhydrous hydrofluoric acid, which operation is carried out in the presence of 0.1 to 5 moles of oxygen per 100 moles of F30, at a temperature of between 330 and 450°C and with a bulk or supported chromium catalyst. The precursor employed for preparing the chromium catalyst according to the invention is preferably a chromium oxide, hydroxide, halide, acetate or nitrate. In the case of a solid catalyst a partially fluorinated, chromium-based solid with a large surface is preferably chosen, which may optionally contain inert components such as alumina or graphite in order to increase its thermal stability and its robustness. The catalyst may also be obtained by deposition of a chromium derivative on an inert support such as alumina or partially fluorinated alumina. The mass content of deposited chromium will then be preferably below 20%. The oxygen may be introduced pure or diluted in an inert gas such as nitrogen. An 02/CH2Cl2 molar ratio between 0.5 and 3 % is preferably employed. The HF/CH2CI2 molar ratio may vary within wide limits. It is generally between 1.5 and 10, preferably between 2 and 5. As indicated above, the reaction must be carried out at a temperature of between 330DC and 450°C. However, it is preferable to work at a temperature of between 350°C and 400°C, in order to obtain a high space time yield without risking deactivation of the catalyst due to crystallization. The contact time, defined as the ratio of the total flow rate of the reactants (measured in the conditions of reaction) to the catalyst volume, may vary within wide limits and is generally between 0.01 and 10 seconds. In practice it is preferable to work with contact times of between 0.05 and 5 seconds. The reaction may be carried out at atmospheric pressure or at a higher pressure. A pressure of between 1 and 20 bars absolute is preferably chosen. Accordingly, the present invention provides a process for the synthesis of difluoromethane by gas-phase catalytic fluorination of methylene chloride by means of anhydrous hydrofluoric acid, which operation is carried out in the presence of 0.1 to 5 moles of oxygen per 100 moles of methylene chloride, at a temperature of between 330 and 450°C and with a bulk or supported chromium catalyst. Our copending Indian patent application No,1073/MAS/96 relates to a process for the manufacture of difluoromethane. The following examples illustrate the invention without limiting it. CATALYST PREPARATION AND ACTIVATION Bulk catalyst (A) A bulk chromium oxide which has a specific surface of 209 m2/g and a pore volume (4 nm For this purpose the chromium oxide is first dried at 200°C and then treated with an NS/HF mixture at 200°C. When the initial exothermicity has subsided, the temperature is raised to 380°C. The catalyst is then kept at 380°C for 18 hours under a stream of pure anhydrous HF. The activated catalyst (A) has the following physicochemical properties: Fluorine weight content : 27 % Chromium weight content : 53 % Volume o£ the pores with a radius of between 4 nm and 63 [ua : 0.13 ml/g BET surface : 101 m2/g Supported catalysts (B), (C) and (D) In a rotary evaporator are placed 250 ml of partially fluorinated alumina (containing, in all, 83 mass% of aluminium fluoride and 16 % of alumina), obtained beforehand by the fluorination of alumina at about 300°C with the aid of nitrogen and hydrofluoric acid. This fluorinated support has the following physicochemical characteristics before impregnation: form : beads 1-2 mm in diameter apparent density : 0.57 g/ml BET surface : 67 m2/g pore volume : 0.72 ml/g (in the case of pores with a radius of between 4 nm and 63 μm) . An aqueous solution (solution 1) containing the desired metal precursors is prepared separately, using the quantities shown in Table I. In the case of the catalysts (B) and (D) , which are prepared from Cr03, the impregnation is carried out in a methanolic medium, so as to reduce the chromium to the oxidation state III. To do this, the aqueous solution containing the chromium and a methanolic solution (solution 2) are added simultaneously onto the support, with stirring. The impregnation is carried out over 45 minutes, at ambient temperature and atmospheric pressure, on the support which is being stirred. The catalyst is then dried for 4 hours under a nitrogen stream, in a fluidized bed, at about 110°C. The catalyst is then charged into a reactor made of Inconel 600 and activated in a stationary bed with a nitrogen/HF mixture in accordance with the procedure described in European patent EP 0 486 333. Table II, which follows, shows the chemical composition of the catalysts thus activated. example 1 4 ml of prefluorinated chromium oxide (catalyst A) are charged into a tubular reactor made of Inconel 600, with an internal diameter of 1 cm and a volume of 40 ml. In a first stage, HF and air are introduced at respective flow rates of 0.68 mol/h and 0.03 mol/h. Methylene chloride, vaporized in a preheater, the temperature of which is set at 150°C, is introduced next in gaseous form into the reactor at a flow rate of 0.23 mol/h. The reaction is carried out at atmospheric pressure. The reactor temperature is maintained at 350°C and the contact time in these conditions is 0.3 seconds. The reaction products are then washed, dried and analysed by gas chromatography. The results are brought together in Table III which follows. Measurement of the 02/N2 ratio makes it possible, furthermore, to verify whether the oxygen introduced in the form of air has been consumed. In these reaction conditions 5 % of the oxygen introduced is converted to CO; C03 formation is marginal. Other by¬products (P23 and F40) are in quantities that are smaller than 700 ppm. The Oa/N2 molar ratio at the reactor exit is 0.26, which shows the absence of the Deacon reaction. It is found that these reaction conditions enable a completely stable activity to be maintained with the very high space time yield of F32 (1350 g/h/1) and a selectivity for F31+F32 higher than 99.7 %. COMPARATIVE EXAMPLE 1 The reaction is carried out in the same conditions as Example 1 but at a temperature of 300°C. The flow rate of the reactants is adjusted so as to maintain a contact time of 0.3 seconds. The results are brought together in the following table. The oxygen introduced takes practically no part in the reaction; the quantity of CO/C02 formed is smaller than the detection threshold ( The catalyst has become deactivated by coking and after 321 hours' running contains 2.5 % (mass) of carbon. It is found that a temperature of 300 °C does not allow a stable activity to be maintained on this catalyst. This temperature enables a high ?32 space time yield to be obtained (1200 g/h/1) but is insufficient in order continuously to suppress the formation of coke or of its precursors. Example 2 The methylene chloride fluorination reaction is carried out in the Same conditions as in Example 1 on the supported catalyst B (Cr/AlF3) . The results obtained are brought together in Table V. The oxygen introduced takes very little part in the reaction; the quantity of CO/C03 formed is smaller than the detection threshold { COMPARATIVE EXAMPLES 2 AMD 3 The reaction is carried out in the conditions of Example 1 on supported catalysts C and D (Ni/A1F3 and Ni-Cr/AlFj) which are not in accordance with the present invention. The methylene chloride conversion and the selectivities for P32 and F31 are shown in Table VI which follows; It is found that fluoridation catalysts C and D do not make it possible to attain the lifetime obtained on catalysts A and B which are based on chromium alone (solid or supported) and are deactivated by coking despite the continuous introduction of air and the high temperature. Less than 500 ppm of CO/COa is formed on these catalysts, and no bis(fluoromethyl) ether ( These Comparative Examples 2 and 3 show that it is necessary to have a catalyst containing solely chromium {catalysts A and B) in order that the air introduced may suppress or inhibit coke formation at the temperature of fluorination of methylene chloride. WE CLAIM: 1. A process for the synthesis of difluoromethane by gas-phase catalytic fluorination of methylene chloride by means of anhydrous hydrofluoric acid, which operation is carried out in the presence of 0.1 to 5 moles of oxygen per 100 moles of methylene chloride, at a temperature of between 330 and 450°C and with a bulk or supported chromium catalyst. 2. The process as claimed in claim 1, which the O2/CH2CI2 molar ratio is between 0.5 % and 3 %. 3. The process as claimed in claim 1 or 2, in which the operation is carried out at a temperature of between 350 and 400°C. 4. The process as claimed in any one of the claims 1 to 3, in which a bulk chromium catalyst is employed. 5. The process as claimed in any one of the claims 1 to 3, in which a supported chromium catalyst is employed. 6. The process as claimed in claim 5, the weight content of chromium in the supported chromium catalyst is below 20 %. 7. The process as claimed in any one of the claims 1 to 6, in which the contact time is between 0.01 and 10 seconds. 8. The process as claimed in any one of the claims 1 to 6, in which the contact time is between 0.05 and 5 seconds. 9. The process as claimed in any one of the claims 1 to 8, in which the operation is carried out at a pressure of between 1 and 20 bars absolute. 10. A process for the synthesis of difluoromethane by gas-phase catalytic fluorination, substantially as herein described and exemplified. 9

Documents:

1072-mas-1996 abstract.pdf

1072-mas-1996 claims.pdf

1072-mas-1996 correspondence-others.pdf

1072-mas-1996 correspondence-po.pdf

1072-mas-1996 description (complete).pdf

1072-mas-1996 form-2.pdf

1072-mas-1996 form-26.pdf

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1072-mas-1996 form-6.pdf

1072-mas-1996 petition.pdf