Note: Descriptions are shown in the official language in which they were submitted.
~1 69957
~ W09S/0662S PCT/GB94/01~8
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PRODUCTION OF DIFLUOROMETHANE.
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 100C to about
650C 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
bistfluoromethyl)ether and water, the
bis(fluoromethyl)ether is separated from unreacted
starting material and by-product water and the
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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 150C. 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 ve~sel, difluoromethane
is produced with a yield of at least 30Z. Yields of
difluoromethane greater than 40~ with difluoromethane
selectivities greater than 80~ 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
3~ known forms, for e~ample in one of its polymeric
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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 .
S 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 l:0.5 to l: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 l:lO.
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 li~ands 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
esample 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
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elements, for example AlF3, BF3, SnF4, SiF4, TiF4,
NbFs and SbFs.
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 6Ø We especially prefer to
employ SbFs, BF3, NbFs and/or TiF4 i~ the process of
the invention. BF3 is the most preferred catalyst.
Some Lewis acids, for example NbFs and TiF4 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 b-y 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
Z 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 escess 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 howe~er 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 proce8s together with the
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reactants, for example where an aqueous solution of
formaldehyde is employed, much lsrger ~uantities 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:l
and preferably at least l:l.
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 150C,
preferably below 120C and more preferably below
100C tend to favour the selective production of
difluoromethane. Preferably the temperature is in the~
range from about 0C to about 120C, more p`referably
in the range from about Z0C to about 100C and
especially in the range from about 20C to about
80C.
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 esample 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
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which formaldehyde, hydrogen fluoride and the
catalystldehydrating 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 e~amples.
EXA~PLE 1.
0.09 mole of trio~ane (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 30C for 16 hours. The maximum
pre~sure 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 47.4Z.
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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 BF3 were charged to the autoclave and
the autoclave was heated to 47C. The maximum
pressure obser~ed was 25 bar. The volatile products
comprised 81.02 difluoromethane and 16.7Z methyl
fluoride. The conversion of formaldehyde was 48.52,
and the yield of difluoromethane, based on the
formaldehyde converted, was 33.12.
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 cha~ged
to the autoclave and the autoclave was heated to
32C. The volatile products comprised 85.82
difluoromethane and the yield of difluoromethane,
based on the formaldehyde converted, was 23.52.
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