This invention relates to a process for the production of difluoromethane from formaldehyde and hydrogen fluoride. Several methods for the production of difluoromethane are known but many of these methods involve the use of chlorine-containing starting materials, for example chlorodifluoromethane and dichloromethane , and the production of chlorine-containing by-products, for example chlorofluoromethane and fluorodichloromethane .

A chlorine-free process for the production of difluoromethane is also known. In US 3,377,394, there is disclosed a process for the production of difluoromethane and methyl fluoride by contacting formaldehyde with hydrogen fluoride at an elevated temperature in the range from about 100°C to about 650°C in the presence of a fluorine-containing inorganic acid, a metal fluoride, a metal oxide or a metal chromite. However, the highest yield of difluoromethane reported from this reaction is 4.2Z, the major product being methyl fluoride.

Recently, a process for the production of difluoromethane has been disclosed in published European Patent Application No. 0 518 506 in which bis ( fluoromethyl ) ether is heated in the vapour phase to elevated temperature. It is also disclosed in this document that bis ( fluoromethyl )ether may itself be produced by contacting formaldehyde with hydrogen fluoride and separating the bis (fluoromethyl )ether from the by-product water produced, thus providing a two-step process for the production of difluoromethane in which formaldehyde and hydrogen fluoride are contacted to produce bis ( fluoromethyl ) ether and water, the bis ( fluoromethyl ) ether is separated from unreacted starting material and by-product water and the

bis ( fluoromethyl )ether is then heated to elevated temperature in the vapour phase.

According to the present invention there is provided a process for the production of difluoromethane which comprises contacting formaldehyde with hydrogen fluoride in the presence of a catalyst and a chemical dehydrating agent at a temperature below 150°C. The catalyst and the chemical dehydrating agent may be the same or different chemical compounds. In a preferred embodiment described hereafter, the catalyst and chemical dehydrating agent are the same chemical compound.

A substantial benefit of the process of the invention resides in effecting the process in a single vessel to which formaldehyde, hydrogen fluoride, the catalyst and the dehydrating agent are charged, and from which a product stream comprising difluoromethane is withdrawn. Significantly higher yields of difluoromethane are achieved than have previously been achieved from the one step process as disclosed in US 3,377,394. The process may be operated so that, based upon the amount of formaldehyde charged to the vessel, difluoromethane is produced with a yield of at least 30Z. Yields of difluoromethane greater than 40Z with difluoromethane selectivities greater than 80Z have been achieved in practice .

The process is preferably effected in the liquid phase under conditions whereby the volatile difluoromethane product distills from the reaction medium. In the liquid phase process, formaldehyde is in solution in liquid hydrogen fluoride.

The formaldehyde may be provided in any of its known forms, for example in one of its polymeric

forms, paraformaldehyde or trioxane, or in its monomeric form which may be provided, for example, from a process stream in which it has been freshly made, for example by oxidation of methanol . Accordingly, whenever used herein, the term "formaldehyde" is to be understood as including not only the monomer but also the various polymeric forms and also formaldehyde in the form of aqueous solutions, commonly known as formalin. In general, a polymeric form of formaldehyde such as paraformaldehyde or trioxane is preferred. Where an aqueous solution of formaldehyde is employed, it is preferably as concentrated as possible.

The molar ratio of formaldehyde to hydrogen fluoride may vary considerably, for example in the range about 1:0.5 to 1:50 but in general a stoichiometric excess of hydrogen fluoride is preferred. Typically, the molar ratio of formaldehyde to hydrogen fluoride will be in the range about 1:2 to about 1:10.

Any suitable catalyst may be employed in the process of the invention, such as for example the catalysts described in US 3,377,394. However, we generally prefer to employ a Lewis acid catalyst. Particularly suitable Lewis acids for use in the process of the invention contain fluoride as the ligand, since where ligands other than fluoride are present, in particular halides other than fluoride, e.g. chlorides, many undesirable by-products may be produced. However ligands other than fluoride, for example halide other than fluoride, alkoxide, etc, may result in the production of hydrofluorocarbons and may be employed if desired. Preferred Lewis acids include the fluorides of Group III, IV and V

elements, for example AIF ₃ , BF3, SnF_ _> , SiF , TiF_ _> , NbF ₅ and SbF ₅ .

We particularly prefer to employ Lewis acids in which the central cation has a charge/radius (expressed in Angstroms) ratio of at least 5.0 and preferably at least 6.0. We especially prefer to employ SbFs, BF3, NbFs and/or TiFή in the process of the invention. BF3 ^~ is the most preferred catalyst.

Some Lewis acids, for example NbFs and TiF_ _> may be generated in situ, for example by employing the corresponding halides other than fluorides, for example chlorides, or oxides and a source of fluoride, for example hydrogen fluoride. They may also be generated in situ by employing the metal itself and a source of fluoride, especially hydrogen fluoride .

Any suitable chemical dehydrating agent stable to hydrogen fluoride may be employed in the process of the invention although, most conveniently BF3 is employed as the dehydrating agent. The proportion of dehydrating agent employed may be measured ^" against whichever of formaldehyde or hydrogen fluoride is present in the smaller mole fraction. Typically a molar stoichiometric excess of hydrogen fluoride is employed and in this case the molar ratio of dehydrating agent to formaldehyde will usually be at least 0.5:1, and preferably at least 1:1, especially at least 1.5:1. Where however formaldehyde is employed in molar excess over hydrogen fluoride, the amount of dehydrating agent may be measured against the amount of hydrogen fluoride present, so that usually in this case there will be a molar ratio of dehydrating agent to hydrogen fluoride of at least 0.5:1, preferably at least 1:1. Furthermore where water is introduced to the process together with the

reactants, for example where an aqueous solution of formaldehyde is employed, much larger quantities of the dehydrating agent may be employed.

In a particularly preferred embodiment of the invention BF3 is employed as both the catalyst and the dehydrating agent. In this case the molar ratio of BF3 to formaldehyde will usually be at least 0.6:1 and preferably at least 1:1.

Preferably, formaldehyde will be contacted with hydrogen fluoride and the catalyst /dehydrating agent then added to the mixture.

The process is effected under conditions of temperature and pressure such that the hydrogen fluoride is in the liquid phase and preferably the volatile difluoromethane product distills as a vapour from the vessel in which the reaction is effected.

We have found that temperatures below 150°C, preferably below 120°C and more preferably below 100°C tend to favour the selective production of difluoromethane . Preferably the temperature is in the' range from about 0°C to about 120°C, more preferably in the range from about 20°C to about 100°C and especially in the range from about 20°C to about 80°C. The process may be conducted at atmospheric, subatmospheric or superatmospheric pressure although superatmospheric pressures, say up to 40 bar are typically employed. Where the reaction is effected in pressure equipment, for example an autoclave, autogenous pressure is conveniently employed.

The reaction may be conducted in suitable pressure equipment such as an autoclave or in a liquid phase reaction vessel.

The process may be operated as a batch process but is preferably operated as a continuous process in

which formaldehyde, hydrogen fluoride and the catalyst/dehydrating agent are continuously fed to the reaction vessel and the volatile products are continuously withdrawn from the vessel.

The dehydrating agent/water complex by-product of the process may be drained from the vessel and the dehydrating agent may be separated from the water and recycled. Thus for ^" example, where the dehydrating ^" agent is" BF3 , the BF3 may be recovered from the BF3/water complex as is described, for example, in US Patent 3,329,586.

Difluoromethane may be separated from other volatile products of the reaction by conventional techniques, for example distillation.

The invention is illustrated but not limited by the following examples.

EXAMPLE 1.

0.09 mole of trioxane (0.27 mole of formaldehyde), 0.99 mole of hydrogen fluoride and 0.41 mole of BF3 were charged to a 70ml Hastelloy autoclave at room temperature. The autoclave was sealed and heated to 30°C for 16 hours. The maximum pressure observed over this period was 38 bar. After 16 hours, the volatile products were distilled from the autoclave and analysed by Gas Chromatography. The volatile products comprised 86.9Z difluoromethane and 5.2Z methyl fluoride. The conversion of formaldehyde was 73.7Z, and the yield of difluoromethane , based on the formaldehyde converted, was 47. Z.

EXAMPLE 2.

The procedure of example 1 was repeated except that 0.06 mole of trioxane (0.18 mole of formaldehyde), 0.82 mole of hydrogen fluoride and 0.21 mole of BF ₃ were charged to the autoclave and the autoclave was heated to 47°C. The maximum pressure observed was 25 bar. The volatile products comprised 81.0Z difluoromethane and 16.7Z methyl fluoride. The conversion of formaldehyde was 48.5Z, and the yield of difluoromethane , based on the formaldehyde converted, was 33.1Z.

EXAMPLE 3.

The procedure of example 1 was repeated except that 0.27 moles of paraformaldehyde , 1.11 mole of hydrogen fluoride and 0.44 moles of BF3 were charged to the autoclave and the autoclave was heated to 32°C. The volatile products comprised 85.8Z difluoromethane and the yield of difluoromethane , based on the formaldehyde converted, was 23.5Z.