Production of difluoromethane

Abstract: A process for the production of difluoromethane which comprises reacting a compound of formula XYCF₂ wherein X and Y are each H, Cl or Br but at least one of X and Y is an atom other than hydrogen, and in particular chlorodifluoromethane, with hydrogen at elevated temperature in the presence of a hydrogenation catalyst, in particular a catalyst comprising palladium carried on an active carbon support.

Description: This invention relates to a process for the production of difluoromethane. In recent years chlorofluorocarbons, which are used on a large scale around the world, have been perceived as having an adverse effect on the ozone layer and/or as contributing to global warming. Chlorofluorocarbons are used, for example, as refrigerants, as foam blowing agents, as cleaning solvents and as propellants for aerosol sprays in which the variety of applications is virtually unlimited. Consequently, much effort is being devoted to finding suitable replacements for chlorofluorocarbons which will perform satisfactorily in the many applications in which chlorofluorocarbons are used but which will not have the aforementioned environmentally harmful effects. One approach in the search for suitable replacements has centred on fluorocarbons which do not contain chlorine but which may contain hydrogen. The hydrofluorocarbon difluoromethane, also known as HFA 32, is of interest as one such replacement, in particular as a replacement in refrigeration, air-conditioning and other applications. According to the present invention there is provided a process for the production of difluoromethane which comprises reacting a compound of formula XYCF 2 wherein X and Y are each H, Cl or Br but at least one of X and Y is an atom other than hydrogen, with hydrogen at a temperature in the range 225°C to 400°C and at a pressure in the range from 2 to 60 bar in the presence of a hydrogenation catalyst comprising palladium carried on an active carbon support wherein the loading of palladium is in the range 8 to 20 % by weight. The process may be conveniently effected by feeding a stream comprising the compound of formula XYCF 2 and hydrogen, as a combined or as separate streams through a vessel containing the hydrogenation catalyst. The starting compounds of formula XYCF 2 are dichlorodifluoromethane, dibromodifluoromethane, chlorobromodifluoromethane, chlorodifluoromethane and bromodifluoromethane. Mixtures of the above compounds may be employed. Usually the compound of formula XYCF 2 will be a chlorinated difluoromethane and chlorodifluoromethane is the preferred starting compound. Where the compound of formula XYCF 2 is chlorodifluoromethane, we have found that the yield of difluoromethane is dependent upon the catalyst composition, temperature and pressure, and that these parameters may be selected to give surprisingly high yields of difluoromethane, and in particular, substantially higher yields of difluoromethane than the yields which have previously been achieved by hydrogenation of chlorine-containing difluoromethanes. The temperature, pressure and catalyst composition are such that difluoromethane is produced with a yield of at least 20%. Preferably the yield is at least 25%, more preferably at least 30% and especially at least 35%. Hydrogenation catalysts are in themselves well known. Examples of hydrogenation catalysts include nickel or metals of Group VIIIa of the Periodic Table, for example platinum, ruthenium, osmium, iridium and rhodium, or oxides or salts thereof. In use, a compound of such a metal is reduced at least in part to the metal. One very useful metal which can be employed is palladium. The present invention employs a catalyst which comprises palladium on an active carbons support. Furthermore, the catalyst may comprise more than one metal in addition to palladium which may be carried on a suitable support. The other metal is preferably a more active hydrogenation catalyst than palladium and the other metal serves to hydrogenate any chlorofluoromethane which may be produced by the process, and which is a toxic impurity, to methane. We have found that the provision of a metal other than palladium, in addition to palladium, may reduce the level of chlorofluoromethane in the product stream without significantly reducing the overall yield of difluoromethane. Thus, the provision of a more active hydrogenation catalyst than palladium, in addition to palladium, acts to purify the product stream by hydrogenation of the toxic impurity, chlorofluoromethane, to methane. The catalyst comprises palladium and may also include one or more other group VIIIa metals, for example ruthenium, rhodium or nickel, or other metals, for example silver or chromium, carried on a suitable support. A particularly preferred catalyst comprises palladium and nickel, since we have found that a catalyst comprising palladium and nickel has a profound effect in reducing the amount of the toxic impurity chlorofluoromethane which is present in the product stream. The palladium and nickel are suitably carried on an active carbon support. Each may be carried on the same support, for example an active carbon or on different active carbon supports. Where they are carried on the same support, they may be present as an alloy, if desired. We have also found that with an active carbon support, the overall yield of difluoromethane is profoundly influenced by the particular active carbon which is employed. Whilst it is believed that an important characteristic of the active carbon in determining its effectiveness is the surface area of the carbon, it is clear that other factors are also important including for example acidity, bulk density, the presence of impregnants, the nature of the carbon which is activated and its method of manufacture. Certain active carbons have a demonstrable effect upon the selectivity of the catalyst towards the production of difluoromethane. One type of active carbon which we have found particularly preferably is active carbons which have been manufactured by extrusion. The catalyst/support combination comprises palladium supported on an active carbon, the % w/w Pd to active carbon is usually from 8.0% w/w to 20% w/w, and especially from 8% w/w to 15% w/w. In the case where a mixed metal catalyst is employed the proportions of the metals present may vary within a wide range, provided that the catalyst is based on palladium. Generally we prefer to employ a catalyst in which there is at least twice as much palladium present as other metals. Where the catalyst comprises a mixture of palladium and nickel, we prefer to employ palladium and nickel in the ratio from 2:1 to 500:1, and more preferably from 5:1 to 100:1. A preferred catalyst comprises from 8 to 20% w/w palladium and from about 0.05% to 5%, in particular from 0.1% to 2% by weight nickel, supported on an active carbon. Overall the amount of the metal which is a more active hydrogenation catalyst than palladium, is usually in the range from 0.01% w/w to 5% w/w. The proportion of hydrogen to starting compound of formula XYCF 2 may be varied considerably. Usually at least the stoichiometric amount of hydrogen is employed to replace the chlorine and/or bromine atom(s), and considerably greater than stoichiometric amounts, for example 4 or more moles of hydrogen per mole of starting compound may be employed. Where X and Y are each chlorine or bromine, it is preferred to employ at least two moles of hydrogen (the stoichiometric amount) per mole of starting compound. Where the starting compound of formula XYCF 2 is chlorodifluoromethane it is preferred to employ between 1 and 2 moles of hydrogen per mole of chlorodifluoromethane. We have found that operation of the process of the invention at superatmospheric pressure substantially increases the selectivity of the process towards the production of difluoromethane. The process is operated at a pressure in the range from 2 bar to 60 bar and more preferably from 2 bar to 30 bar, especially 5 bar to 30 bar. The reaction is suitably carried out in the vapour phase at a temperature which is in the range from 225°C to 400°C, and preferably from 240°C to 360°C. The most preferred temperature is dependent upon the pressure at which the process is operated; at a pressure of 7.5 bar, we prefer to employ temperatures in the range from 270°C to 360°C. Contact times are usually in the range 1 to 60 seconds, especially 5 to 30 seconds, when the reaction is carried out in the vapour phase. In the present process, any unreacted hydrogen and other starting material, together with any organic by-products, may be recycled. The difluoromethane product may be contaminated with small amounts of by-products, for example chlorofluoromethane, chloromethane, fluoromethane and methane. Many of these by-products may be removed by conventional means, for example by fractional distillation. However, one by-product which may be produced is chlorofluoromethane which must be removed or at least reduced to extremely low levels, for example below 10ppm, because it is toxic. Although chlorofluoromethane has a different boiling point to that of difluoromethane, so that they may be separated to a high degree by conventional means, for example by distillation, such conventional processes do not sufficiently reduce the levels of the chlorofluoromethane impurity. We have found that the chlorofluoromethane and other chlorine containing impurities may be removed from the difluoromethane product by contacting the impure difluoromethane with an active carbon. According to a further embodiment of the invention there is provided a process as defined in claim 1 wherein chlorocarbon impurities are removed from the difluoromethane by contacting the impure difluoromethane with an active carbon. A preferred embodiment is characterised by the removal of chlorofluoromethane from difluoromethane by contacting the impure difluoromethane with an active carbon. The invention is illustrated, but not limited, by the following examples. COMPARATIVE EXAMPLE A A catalyst comprising 8.2% palladium on Norit RX3 extrudate active carbon (supplied by Norit) was employed as the catalyst, and the reactor was constructed of Inconel. The catalyst was charged into a heat resistant glass tube reactor of 1 inch internal diameter surrounded by an electric furnace. Hydrogen and chlorodifluoromethane were passed through the heated tube, the catalyst bed being maintained at an essentially constant temperature. Six runs were carried out at the temperatures shown in Table 1 below. The flow rates of hydrogen and chlorodifluoromethane were 60ml/minute and 30ml/minute respectively. The reactor exit gas was diluted with 300ml/minute nitrogen and passed through a scrubbing tower of soda lime to remove acid gases. The scrubbed exit gas was then analysed by gas chromatography. The relative amounts of the only organic products detected in the scrubbed exit gas, based upon gash chromatagram peak areas only, were as shown in Table A. The composition of the organic product after scrubbing was as shown in Table 1 Product. %v/v. Temperature/°C. 135 165 199 216 247 260 CH 2 F 2 . 0.3 2.1 11.7 22.4 50.3 57.0 CF 3 H. 0.0 0.0 0.02 0.04 0.2 0.3 CH 4 . 0.02 0.14 1.04 2.81 13.7 18.7 CH 2 FCl. 0.0 0.0 0.02 0.05 0.16 0.19 % Conversion. of CF 2 ClH. 0.36 2.25 12.8 25.3 64.5 76.6 % difluoromethane selectivity. 92.4 93.8 91.5 88.5 78.0 74.3 EXAMPLES 1 The procedure of Comparative example A was repeated except that the reaction was conducted at a pressure of 7.5 bar. The composition of the organic product after scrubbing was as shown in Table 1. Product. %v/v. Temperature/°C. 290 315 330 CH 2 F 2 . 24.63 31.05 29.79 CF 3 H. 0.09 0.22 0.27 CH 4 . 1.76 2.50 2.64 CH 2 FCl. 0.06 0.06 0.07 % Conversion. of CF 2 ClH. 26.58 33.90 32.85 % difluoromethane selectivity. 92.7 92.8 90.8

Claims: A process for the production of difluoromethane which comprises reacting a compound of formula XYCF 2 wherein X and Y are each H, Cl or Br but at least one of X and Y is an atom other than hydrogen, with hydrogen at a temperature in the range 225°C to 400°C and at a pressure in the range from 2 to 60 bar in the presence of a hydrogenation catalyst comprising palladium carried on an active carbon support wherein the loading of palladium is in the range 8 to 20% by weight. A process as claimed in claim 1 wherein the compound of formula XYCF 2 is chlorodifluoromethane. A process as claimed in claim 1 or claim 2 which comprises feeding a stream comprising the compound of formula XYCF 2 and hydrogen through a vessel containing the catalyst. A process as claimed in any preceding claim wherein the loading of palladium on the support is in the range from 8% to 15% by weight. A process as claimed in any preceding claim wherein the catalyst comprises one or more metals in addition to palladium carried on an active carbon support. A process as claimed in claim 5 wherein the catalyst comprises a Group VIIIa metal in addition to palladium. A process as claimed in claim 6 wherein the catalyst comprises palladium and nickel carried on an active carbon support. A process as claimed in claim 7 wherein the proportion by weight of palladium and nickel in the catalyst is in the range from 2:1 to 500:1. A process as claimed in any one of claims 1 to 8 wherein the molar proportion of hydrogen to compound of formula XYCF 2 is in the range from 1:1 to 4:1. A process as claimed in any preceding claim wherein the impure difluoromethane product stream is contacted with an active carbon whereby to remove chlorine-containing impurities.

Inventors: MOORE GEOFFREY JAMES, O'KELL JENNY, MOORE, GEOFFREY JAMES, O'KELL, JENNY
Applicants: ICI PLC, IMPERIAL CHEMICAL INDUSTRIES PLC
Application year: 1992
Main classification: C07C17
IPC codes: C07C17, C07B61, B01J23, C07C19