Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HYDROGENATION CATALYST AND PROCESS.
This invention relates to a catalyst, in particular a hydrogenolysis (the
replacement of halogen by hydrogen in saturated molecules) and/or hydrogenation
(addition of hydrogen across ethylenic double-bonds) catalyst, (hereafter referred
to collectively as "hydrogenation") and to hydrogenation processes employing thecatalyst, in particular the hydrogenation of halofluorocarbons and
hydrohalofluorocarbons to produce hydrofluoroalkanes, for example the
hydrogenation of dichlorodifluoromethane and chlorodifluoromethane to produce
difluoromethane, the hydrogenation of chloropentafluoroetbane to produce
pentafluoroethane and the hydrogenation of dichlorotetrafluoroethane and
chlorotetrafluoroethane to produce tetrafluoroethane and especially
1, 1, 1,2-tetrafluoroethane.
In recent years there has been increasing international concern that
chlorofluorocarbons, which are used on a large scale around the world, may be
cl~m~ging the earth's protective ozone layer and there is now in place international
agreement to ensure that their m~mlf~cture and use is restricted and eventually
completely phased out. 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
damaging effect on the ozone layer. One approach in the search for suitable
replacements has centred on fluorocarbons which do not contain chlorine but
which contain hydrogen. Hydrofluorocarbons such as difluoromethane, also
known as HFA 32, pentafluoroethane, also knowll as HFA 125 and
1,1,1,2-tetrafluoroethane, also known as HFA 134a, are of interest as
replacements, in particular as replacements in refrigeration, air-conditioning and
other applications.
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Many processes have been proposed for the production of
hydrofluorocarbons and hydrochlorofluorocarbons, including the catalysed gas
phase hydrofluorination of halocarbons or hydrohalocarbons and the catalysed
hydrogenation of halocarbons or hydrohalocarbons which contain fluorine. The
catalysed hydrogenation of chlorofluorocarbons and hydrohalofluorocarbons by
reaction with hydrogen in the presence of a hydrogenation catalyst is consideredto be a potentially commercially attractive process. Hydrogenation of
dichlorotetrafluoroethane and chlorotetrafluoroethane to 1,1,1,2-tetrafluoroethane
is described in, for example, UK Patent No. 1,578,933, the hydrogenation of
chloropentafluoroethane to pentafluoroethane is described in, for example
Japanese Laid-Open Patent Application No. J4-29941 and the hydrogenation of
chlorodifluoromethane to difluoromethane is described in, for example, European
Patent Application No. 0 508 660.
Much attention has focused on improving the catalyst which is employed in
the hydrogenation process, for example to increase the activity of the catalyst
and/or to increase the selectivity with which the desired HFA product is
produced. Thus, improved catalysts for use in the hydrogenation or
hydrogenolysis of chlorofluorocarbons and hydrochlorofluorocarbons have been
disclosed, inter alia, in International Patent ~pplication Nos. WO90/08748
WO92/12113, WO94/11328, US Patent No. 5,136,113, European Patent
Application No. 0 347 830 and Japanese Kokai No. J4-179322.
The present invention is based on the discovery that the activity and
particularly the selectivity of palladium catalysts is improved by incorporating a
controlled amount of platinum in the catalysts.
According to the present invention there is provided a hydrogenation
catalyst which comprises palladium and pl~tinl~m in a ratio by weight of 2: 1 to500:1 carried on a support.
The palladium and platinum components may be used alone or in
combination with other metals, for example other Group VIII metals such as
nickel, Group lB metals such as Ag and Au or even other metals.
The metals are carried on a suitable support, for example alumina,
fluorinated alumina, silica, silicon carbide or carbon but in particular alumina or
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carbon. We particularly prefer to employ a carbon carrier where the carbon has ahigh surface area, eg greater than 200 m2/g. Activated carbon supports with low
inorganic impurity levels are particularly suitable.
The loading of the metal on the support material may be dependent at least
to some extent on the particular metal catalyst/support combination being used
and the difficulty of the hydrogenation reaction. However the % w/w of combined
palladium and platinum to support is typically from about 0.1% w/w to about 40%
w/w, and preferably from about 0.5% w/w to about 20% w/w, more preferably
from about 2.0% w/w to about 20% w/w and especially from about 5% w/w to
about 15% w/w.
The proportions of palladium and platinum present may vary within a wide
range, although we prefer a catalyst in which there is at least twice as much
palladium as platinum. We particularly prefer to employ palladium and platinum
in the ratio by weight from about 2:1 to about 500:1, and more preferably from
about 3:1 to about 100:1. A preferred catalyst comprises from about 0.5% to
20%, in particular from about 5 to 15% w/w palladium and from about 0.05% to
about 5%, in particular from about 0.1% to about 4% by weight platinum,
especially when supported on an active carbon. Overall the amount of platinum isusually in the range from about 0.01% w/w to about 10% w/w ofthe catalyst.
Many methods of preparation of supported metal catalysts are known to
those skilled in the art, for example as described in the prior art documents
referred to on page 2 of this specification and the process of the invention may be
utilised with a supported noble metal catalyst prepared by any such method. A
typical method involves impregnating the support with an aqueous solution of a
soluble salt of the metal, for example a halide, and thereafter drying the catalyst.
The improved catalyst of the invention is particularly useful in the
hydrogenation or hydrogenolysis of a halofluorocarbon or hydrohalofluorocarbon
with hydrogen at elevated temperature and according to a second aspect of the
invention there is provided a process for the production of a hydrofluorocarbon
which comprises contacting a halofluorocarbon or hydrohalofluorocarbon with
hydrogen at elevated temperature in the presence of the improved hydrogenation
catalyst according to the invention
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Usually the halofluorocarbons and hydrohalofluorocarbons used as starting
materials comprise at least one atom of chlorine or bromine and generally will be
chlorofluorocarbons or hydrochlorofluorocarbons. The chlorofluorocarbon or
hydrochlorofluorocarbon will typically comprise 1, 2 or 3 carbon atoms although
it may comprise more than 3, say up to 6, carbon atoms. The
(hydro)halofluorocarbon may be unsaturated or saturated, cyclic or acyclic and
straight chain or branched chain, although the (hydro)halofluorocarbon will
usually be a straight chain saturated acyclic compound, that is a linear
(hydro)halofluoroalkane.
Particularly useful hydrogenation reactions in which a hydrogenation
catalyst according to the invention may be employed include (a) the hydrogenation
of a haloethane, in particular a chloroethane, having 4 fluorine atoms, for example
1,1-dichlorotetrafluoroethane, 1,2-dichlorotetrafluoroethane and
chlorotetrafluoroethane to chlorotetrafluoroethane and/or tetrafluoroethane, in
particular l, 1,1,2-tetrafluoroethane and (b) the hydrogenation of a compound offormula CF2XY where X and Y are independently Cl, Br or H (but not both H) to
difluoromethane .
Conditions for effecting these hydrogenation reactions are described, for
example, in UK Patent Specification No. 1,~78,933 and European Patent
Application No. 0 508 660 respectively, the disclosures of which are incorporated
herein by reference.
An important feature of successful hydrogenation catalysts is a high
hydrogenolysis activity for carbon - chlorine and/or carbon - bromine bonds but a
low hydrogenolysis rate for carbon-fluorine bonds, thus avoiding the loss of
product by removal of fluorine atoms.
The catalyst of the invention is particularly advantageously employed in the
production of difluoromethane where it has a marked effect on improving the
selectivity with which difuoromethane is produced by reducing the level of over
reaction.
According to a preferred embodiment of the second aspect of the invention
there is provided a process for the production of difluoromethane which
comprises reacting a compound of formula XYCF2 wherein X and Y are each H,
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Cl or Br but at least one of X and Y is an atom other than hydrogen, with
hydrogen at elevated temperature in the presence of a hydrogenation catalyst
according to the invention.
The process may be conveniently effected by feeding a stream comprising
the compound of formula XYCF2 and hydrogen, as a combined or as separate
streams through a vessel containing the hydrogenation catalyst.
The starting compounds of formula XYCF2 are dichlorodifluoromethane,
dibromodifluoromethane, chlorobromodifluoromethane, chlorodifluoromethane
and bromodifluoromethane. Mixtures of the above compounds may be employed.
Usually the compound of formula XYCF2 will be a chlorinated difluoromethane
and chlorodifluoromethane is the preferred starting compound.
The proportion of hydrogen to starting compound of formula XYCF2 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 XYCF2 is chlorodifluoromethane it is preferred to employ
between 1 and 2 moles of hydrogen per mole of chlorodifluoromethane.
Atmospheric or superatmospheric pressures, for example up to about 60
barg may be employed. We have found that operation of the process of the
invention at superatmospheric pressure substantially increases the selecti~ity of
the process towards the production of difluoromethane. The process is preferablyoperated at a pressure in the range from about 2 bar to about 60 bar and more
preferably from about 2 bar to about 30 bar, especially 5 bar to 30 bar.
The reaction is suitably carried out in the vapour phase at a temperature
which is at least about 1 50~C and not greater than about 500~C, usually from
about 225~C to about 400~C, and preferably from about 240~C to about 360~C.
The most preferred temperature is dependent upon the pressure at which the
process is operated; at atmospheric pressure, we prefer to operate the process at a
temperature in the range from about 220~C to about 320~C whereas at a pressure
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of about 7. 5 barg, we prefer to employ temperatures in the range from about
260~C to about 380~C.
Contact times are usually in the range I to 60 seconds, especially 5 to 30
seconds when the reaction is carried out in the vapour phase.
In the process according to this preferred embodiment of the second aspect
of the invention any unreacted hydrogen and other starting material, together with
any organic by-products, may be recycled.
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The invention is illustrated but not limited by the following examples.
A. CATALYST PREPARATION.
A sample of carbon support (supplied by Norrit) with an approximate
surface area area of 800 sq.m/g was crushed and sieved to generate particles in
the size range 1.0-1.2 mm. 50 cm3 - 60 cm3 ofthe crushed carbon was then
washed in distilled water and the water drained through a no.4 sinter funnel. The
washed carbon was then transferred to a Buchner flask . The target weight of
palladium chloride or mixed metal chlorides were then dissolved in the minimum
volume of warmed concentrated hydrochloric acid and the solution was added to
the carbon particles in the Buchner flask. A further 200 cm3 of distilled water was
then added to the flask and the slurry was evaporated to dryness on a rotary
evaporator using an oil bath temperature of 120~C. The catalyst was finished by
heating the granules in a vacuum oven at 150~C for approximately 16 hours.
Alternatively, the above catalyst preparation was repeated at a larger scale. In this
scaled-up preparation, the same method was employed with 300 cm3 of carbon
support and the particle diameter was increased to 3 mm.
B. CATALYSTS TESTING IN HYDROGENOLYSIS PROCESSES.
50 cm3 of the 1. lmm or 3 mm catalyst to be tested was charged to 1 .25cm Inconel
reactor and heated to 300~C in a nitrogen stream of 300 cm3/min. After the
catalyst had been dried for 2 hours, the nitrogen flow was replaced with a stream
of mixed reactant gases. In the following comparitive set of examples a mixed
reactant flow rate of 180 cm3/min was employed with an hydrogen:HCFC 22
molar ratio of 2:1. The reactor vent gases were passed into a water scrubber to
remove the acid products and then analysed using conventional gas
chromatographic analysis. The catalyst performance was determined for reactor
temperatures of 300 - 380~C.
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EXAMPLE 1.
Eight catalysts were prepared (on small or large scale as indicated in the Tables)
and tested, and the results of analysis for % conversion of chlorodifluoromethane
and % selectivity to difluoromethane are shown in Tables 1, 2 and 3. The pl~tin~lm
promoted palladium hydrogenation catalyst* was found to have the highest
reaction selectivity and to demonstrate significant advantage over a range of other
palladium catalyts containing Cu, Ni, Ru, or Rh.
TABLE 1.
Conv Catalysts
Results10%Pd 10%Pd * 10%Pd 10%Pd 10%Pd 10%Pd
Temp(oC) 1.8%Pt 5%Ni 1%Ni 1%Cu
(l.lmrn) (l.lmm) (l.lmm) (1.1 mm) (3.0mm) (3.0mrn)
300 80 79 65 74 68
310 86
32089.6 83.2
330 83.7 81.9 96 83
34096.7 88.5
350 93.8 86.699.3 95
36099.9 99.5 89.299.7
370 97.9
38099.9 99.9 98.7 99.9 98
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TABLE 2.
Selectivity Catalysts
Results 10%Pd 10%Pd * 10%Pd 10%Pd 10%Pd 10%Pd
Temp(oC) 1.8%Pt 5%Ni 1%Ni 1%Cu
(1.1 mm) (1.1 mm) (1.1 mm) (1.1 mm) (3.0mm) (3.0mm)
300 85 93.7 85.4 88 87.2
310 91.3
320 87.8 92.9
330 87.1 85 90.8 87
340 88.7 92.9
350 87.5 86 88.6 84
360 86 91.7 89.6 90
370 89.6
380 85.3 91.3 86.9 88.6 87
TABLE 3.
I 0%Pd 3%Ru 10%Pd3%Rh
Temp(oC) Conv(%)¦Sel(%) Temp(oC) Conv(%)¦Sel(%) ¦
300 30082.5 91.1
310 310
320 84 91.9 320
330 330
340 94 91.5 34098.4 91.4
350 350
360 99.5 90.8 36099.9 90 3
370 370
380 99.7 90.5 38099.99 89
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EXAMPLE 2.
Two catalysts were prepared for a series of comparative studies. Catalyst
"A" comprised a 10%w/w Pd metal on carbon catalyst and catalyst "B" comprised
a 10% Pd plus 1.8%Pt catalyst on the same Norrit carbon support. The catalysts
were each prepared by dissolving the metal halides in the minimum volume of
hydrochloric acid and impregnating the 1-1.2mm granules of the carbon with the
resultant solution. The slurry was agitated and evaporated to dryness. The
catalyst was then degassed under a flow of nitrogen at 120~C for 16 hours to
produce the finished catalyst.
50 cm3 of catalyst "A" was then charged to a l/2" Inconel reactor and heated
to 300~C in a 300 ml/min feed of nitrogen for a 2 hour period prior to testing.
The test feed mixture was a 2: 1 molar feed ratio of hydrogen:CFC 115, which wasfed at a flow rate of 180 ml/min to the conditioned catalyst. The catalyst was
stabilised under these feed conditions for approximately two hours and then
cooled under reaction conditions to approximately 200~C. Conversion of CFC
115 and selectivity to HFC 125 were measured and are shown in Table 4. At
measured temperatures of 199~C, 213~C and 236~C, the 115 conversion was
found to increase from approximately 66% to 84% and finally 97%, see examples
A1, A2 and A3. The initial reaction selectivity was found to be 99.77 but fell to
99.59 with increasing feed conversion.
The above catalyst testing procedure was repeated with the Pt promoted
catalyst "B". The results are presented in examples Bl, B2 and B3 in Table 4.
The Pt promoted catalyst was found to be more active (operating at lower
temperatures for a given feed conversion) and more selective (producing lower
levels of by-products) than the base palladium catalyst.
TABLE 4
~C Temp ¦ % Conv ¦ % Sel ¦ 115 ¦125 ¦ 134a ¦ 143a ¦ c2H6
10% Pd on Carbon
FY~mrle
Al 199 66.49 99.77 33.51 66.34 0.03 0.12 0
A2 213 84.01 99.65 15.99 83.71 0.05 0.24 0
A3 236 97.24 99.59 2.76 96.85 0.07 0.33 0
10% Pd + 1.8% Pt on Carbon
Bl192 65.27 99.81 34.73 65.15 0.04 0.08 0
B2209 83.25 99.76 16.75 83.04 0.05 0.16 0
B3207 98.8 99.71 1.2 98.51 0.05 0.220.02
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EXAMPLE 3.
The test procedure described in Example 2 using the two catalysts "A" and
"B" and the stated flow rates was repeated but employing a mixture of 114 and
1 14a rather than 115. The results are shown in Table 5. Using catalyst "A", 114conversions of 81.6-88.7% were achieved at 194-204~C, compared with 173~C for
the pl~tin~lm promoted catalyst. The 114a conversions were 100% for both
catalysts, however the level of over-hydrogenolysis products were reduced for
the Pt containing catalysts. Thus the platinum promoted catalyst was more activeand more selectie than the base palladium catalyst.
TABLE 5
Temp CF2CI-CF2CICF2CI-CHF2 CHF2-C}IF2 CF3-CFC12 CF3-CHFCI CF3-CH2F CF3-CB CB-CB
Deg C 114 124a 134 114 124 134a 143a170
10% Pd on C~rbon
E~l~
Ex. A4 19418.36 34.51 2.55 0 2.5 37.274.82 0
A5 204 11324039 4.7 0 1.94 36.16 5.440.02
10%Pd+ 1.8%PtonCubon
Ex~B4 173 14.55 37.76 2.83 0 0.86- 39.514.46 0
B5 173 15.02 37.44 2.77 0 0.87 39.464.41 0
TempDeg C %Feed Conversions % Over-Hydrogenolysis
114¦ 114a
10% Pd on Carbon
Ex. A4 194 81.6 100 4.82
A5 204 88.7 100 5.46
10% Pd + 1.8% Pt on Carbon
Ex. B4 173 85.5 100 4.46
B5 173 85 100 4.41
