U.S. patent application number 09/254500 was filed with the patent office on 2001-08-02 for fluorination catalyst and process.
Invention is credited to RAMSBOTTOM, GRAMHAM, SCOTT, JOHN DAVID, WATSON, MICHAEL JOHN.
Application Number | 20010011061 09/254500 |
Document ID | / |
Family ID | 27268464 |
Filed Date | 2001-08-02 |
United States Patent
Application |
20010011061 |
Kind Code |
A1 |
SCOTT, JOHN DAVID ; et
al. |
August 2, 2001 |
FLUORINATION CATALYST AND PROCESS
Abstract
A chromia-based fluorination catalyst in which the chromia is at
least partially crystalline and which may contain a zinc or a
compound thereof, the production of the catalyst by sintering
amorphous chromia and its use in fluorination processes.
Inventors: |
SCOTT, JOHN DAVID;
(CUDDINGTON, GB) ; WATSON, MICHAEL JOHN;
(EAGLESCLIFFE, GB) ; RAMSBOTTOM, GRAMHAM;
(ECCLESTON, GB) |
Correspondence
Address: |
ANDREW G. KOLOMAYETS
COOK, ALEX, MCFARRON, MANZO CUMMINGS &
MEHLER, LTD.
200 WEST ADAMS STREET- SUITE 2850
CHICAGO
IL
60606
US
|
Family ID: |
27268464 |
Appl. No.: |
09/254500 |
Filed: |
March 8, 1999 |
PCT Filed: |
September 4, 1997 |
PCT NO: |
PCT/GB97/02372 |
Current U.S.
Class: |
502/305 ;
502/306; 502/307; 502/319 |
Current CPC
Class: |
C07C 17/206 20130101;
C07C 17/206 20130101; C07C 19/08 20130101; B01J 23/26 20130101 |
Class at
Publication: |
502/305 ;
502/306; 502/307; 502/319 |
International
Class: |
B01J 023/26 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 10, 1996 |
GB |
9618857.8 |
Sep 10, 1996 |
GB |
9618858.6 |
Sep 10, 1996 |
GB |
9618865.1 |
Claims
1. An improved chromia-based fluorination catalyst wherein the
chromia is at least partially crystalline.
2. A catalyst according to claim 1 wherein the chromia exhibits an
apparent degree of crystallinity as represented by alpha chromia
type crystals of greater than 8%.
3. A catalyst as claimed in claim 1 or claim 2 in which the chromia
exhibits an apparent degree of crystallinity of greater than 20% by
weight.
4. A catalyst as claimed in any preceding claim in which the
chromia exhibits an apparent degree of crystallinity of less than
50% by weight.
5. A catalyst as claimed in any preceding claim which comprises
zinc or a compound of zinc in an amount of less than about 3% by
weight of the catalyst.
6. A catalyst as claimed in claim 5 in which the zinc or compound
of zinc is present in an amount of 0.1 to 2% by weight of the
catalyst
7. A catalyst as claimed in any preceding claim which comprises
zinc or a compound of zinc wherein the catalyst is produced by
inducing crystallinity in chromia and subsequently introducing zinc
or a compound of zinc into the crystallised chromia by impregnation
with a solution of a soluble zinc salt.
8. A catalyst according to any preceding claim in which the degree
of crystallinity in the chromia is controlled so as to result in a
catalyst having a surface area greater than about 20
m.sup.2/gm.
9. A chromium-based fluorination catalyst comprising from 0.1 to 2%
by weight of zinc or a compound of zinc wherein the chromia is at
least partially crystalline and exhibits an apparent degree of
crystallinity as represented by alpha chromia type crystals of
greater than 8% and less than 50% by weight and wherein the
catalyst has a surface area greater than about 20 m.sup.2/gm.
10. A chromium based fluorination catalyst comprising a blend of a
catalyst as claimed in any preceding claim with a non-crystalline
chromia catalyst.
11. A catalyst as claimed in claim 10 in which the amount of the
non-crystalline chromia catalyst is from about 10% to 60% by weight
of the blended catalyst.
12. A catalyst as claimed in claim 10 or claim 11 in which the
non-crystalline chromia catalyst itself contains an
activity-promoting amount of a divalent metal selected from zinc,
cobalt, nickel and magnesium.
13. A process for producing a catalyst as claimed in any one of
claims 1 to 9 which includes the step of sintering an essentially
non-crystalline chromia catalyst or precursor thereof at elevated
temperature
14. A process as claimed in claim 13 in which the step of sintering
the catalyst is performed under conditions whereby the apparent
degree of crystallinity induced in the chromia is controlled to
between 8% and 50% by weight.
15. A process as claimed in claim 13 or claim 14 in which
crystallinity is induced in the chromia and subsequently
introducing zinc or a compound of zinc into the crystallised
chromia by impregnation with a solution of a soluble zinc salt.
16. A process for producing a fluorinated hydrocarbon which
comprises reacting a halogenated hydrocarbon with hydrogen fluoride
in the vapour phase at elevated temperature in the presence of a
catalyst as claimed in claim 1.
17. A process as claimed in claim 16 for producing
1,1,1,2-tetrafluoroetha- ne by reacting
1,1,1-trifluoro-2-chloroethane with hydrogen fluoride.
Description
[0001] This invention relates to a fluorination catalyst and the
production and use thereof and particularly to an improved
fluorination catalyst based on chromia, a process for producing the
catalyst and a fluorination process using the catalyst.
[0002] Fluorination processes comprising reaction of a starting
material with hydrogen fluoride to introduce one or more fluorine
atoms into the starting material are well known and are used
extensively in industry. Vapour phase processes in which the
starting material and hydrogen fluoride are reacted in the vapour
phase at elevated temperature are common and such processes usually
employ a fluorination catalyst which often is a catalyst comprising
or based on chromia which has been subjected to a pretreatment with
hydrogen fluoride to provide the working catalyst. It is generally
accepted that chromium oxide catalysts of high surface area and
wherein the chromium is present as chromium (III) have high initial
activity and that such active chromia catalysts are in an amorphous
or essentially amorphous state. A recent development in chromia
catalysts is a catalyst of enhanced activity produced by
incorporating an activity-promoting amount of a divalent metal
oxide such as an oxide of zinc, nickel or cobalt, especially zinc,
in the catalyst, the oxide or at least the chromia remaining in the
essentially amorphous state and having a large surface area.
Catalysts containing other divalent metal oxides such as magnesia
have also been proposed.
[0003] When used in the production of hydrofluorocarbons [HFCs],
the known chromia catalysts and especially those promoted by a
divalent metal such as zinc have a high initial activity and can
result in high conversions and high selectivities. They suffer from
a progressive reduction in activity due to deposition of coke on
the catalyst but they can be regenerated a number of times by
heating in an oxygen-containing atmosphere such as air or a mixture
of air with hydrogen fluoride and have a reasonable and generally
acceptable lifetime. However, the catalysts suffer the disadvantage
that they are not particularly robust, especially in respect of
chemical robustness and are deteriorated under the conditions of
use and especially when subjected to high temperatures in the
presence of hydrogen fluoride so that their lifetime leaves
something to be desired.
[0004] The present invention is based on the discovery that the
robustness of chromia-based catalysts and hence their useful
working lifetimes is increased by inducing or introducing
crystallinity and preferably a controlled degree of crystallinity
into the chromia. Moreover, the initial activity of the catalysts
can be slightly but significantly enhanced, without a reduction in
selectivity, by introducing an activity-promoting amount of zinc or
a compound of zinc into the catalyst.
[0005] According to the first aspect of the invention there is
provided an improved chromia-based fluorination catalyst wherein
the chromia is at least partially crystalline.
[0006] Preferably, the chromia exhibits an apparent degree of
crystallinity as represented by alpha chromia type crystals greater
than 8%, preferably greater than 20%, and less than 50% by
weight.
[0007] Introducing crystallinity into the chromia results in a
decrease in the surface area of the catalyst and too high a degree
of crystallinity results in an unacceptably low surface area, for
example below 20 m.sup.2/gm. The degree of crystallinity in the
catalyst can be controlled so as to result in a catalyst having a
surface area greater than about 20 m.sup.2/gm, preferably from
about 30 to about 70 m.sup.2/gm.
[0008] According to a further aspect of the invention, there is
provided an improved zinc-promoted chromia fluorination catalyst
wherein the chromia is at least partially crystalline and the
catalyst comprises zinc or a compound of zinc in an amount of less
than about 3% by weight of the catalyst.
[0009] In a further aspect of the invention there is provided an
improved zinc-promoted chromia-based fluorination catalyst wherein
the chromia is at least partially crystalline produced by inducing
crystallinity in chromia and subsequently introducing zinc or a
compound of zinc into the crystallised chromia by impregnation with
a solution of a soluble zinc salt. The catalyst preferably contains
from 0.1% to about 2% by weight of zinc or a compound of zinc
depending upon the degree of crystallinity induced in the
chromia.
[0010] Inducing crystallinity in the chromia results in a decrease
in the surface area of the catalyst and a very high a degree of
crystallinity results in a very low surface area, for example below
10 m.sup.2/gm. The degree of crystallinity in the catalyst of the
invention can be controlled such that the catalyst has a surface
area greater than about 20 m.sup.2/gm, preferably from about 30 to
about 70 m.sup.2/gm.
[0011] Suitably, the catalyst according to the first aspect of the
invention contains zinc or a compound of zinc. A catalyst according
to the invention may contain an activity-promoting amount of a
divalent metal such as cobalt, magnesium or nickel or a compound
thereof in addition to or instead of zinc or a zinc compound.
Nevertheless, the preferred metal is zinc and in this case the
amount of the zinc is important since it is known that zinc can act
as a catalyst poison if present in too large an amount. We have
found that whilst the activity-promoting amount of zinc in
catalysts wherein the chromia is amorphous is generally greater
than about 2% by weight and usually greater than about 5% by weight
depending upon the method of production of the catalyst, the
activity promoting amount of zinc in the partially crystallised
catalysts of the invention should generally be less than about 2%
by weight, preferably no greater than about 1% by weight.
[0012] According to a preferred embodiment of the invention there
is provided a chromium-based fluorination catalyst comprising from
0.1 to 2% by weight of zinc or a compound of zinc wherein the
chromia is at least partially crystalline. The catalyst preferably
has an apparent degree of crystallinity as represented by alpha
chromia type crystals of from about 8% to about 50% and has a
surface area greater than about 20 m.sup.2/gm.
[0013] If present, the amount of divalent metal other than zinc in
the catalyst, whether the divalent metal be an activity promotor or
not, is not critical since such metals are not generally regarded
as catalyst poisons even if present in large amounts. The amount of
such metals may vary over a wide range up to 50% by weight or even
higher of the catalyst, although the amount will usually be in the
range from about 5% to about 25% by weight.
[0014] The apparent degree of crystallinity or the degree of
crystallinity induced in the chromia is determined by X-ray
diffraction analysis using the standard NIST [National Institute of
Standards and Technology] technique and comparing the result with
that obtained by analysis of a pure alpha chromia standard prepared
by sintering chromia at 1223 K in air for 24 hours (100%
crystallinity). The catalysts do not have a true alpha chromia
structure so that the % degree of crystallinity determined by
comparison with the results for pure alpha chromia is not a true %
degree of crystallinity and therefor is referred to herein as the
"apparent degree of crystallinity". Morover, since the catalyst
structure is not true alpha chromia so that the X-ray diffraction
peak tends to be slightly distorted, the apparent degree of
crystallinity is expressed herein as being represented by "alpha
chromia type crystals".
[0015] The apparent degree of crystallinity as represented by alpha
chromia type crystals is determined by measuring the integrated
area of the 104 peak of both the catalyst sample and the pure alpha
chromia standard (at ca. 33.6 .degree. 20 for Cu K radiation)
between 32.5 and 35.0 .degree. 20, subtracting the background to
provide corrected integrated areas and then ratioing the corrected
area for the catalyst sample to the corrected area for the standard
sample.
[0016] The catalyst exhibits an X-ray diffraction peak at a spacing
of lattice planes from 2.65 to 2.7 of half maximum peak width less
than 0.8 degrees.
[0017] Preferably the chromium in the catalyst is present as
chromium (III) although a small amount, say up to 10%, of chromium
(VI) may be present as a result of the conditions under which the
chromia is crystallised. As described hereinafter, crystallinity
can be induced in the chromia by sintering the catalyst at elevated
temperature and this may be carried out under an inert atmosphere
or in the presence of air. Catalysts produced by sintering in an
inert atmosphere tend to comprise essentially chromium (III) but
require higher sintering temperatures whilst those produced by
sintering in air tend to contain some chromium (VI) but require
lower sintering temperatures. We prefer to sinter the catalysts
under an atmosphere of air or a mixture of air and nitrogen since
these conditions enable relatively low temperatures of 300.degree.
C. to 450.degree. C. to be employed.
[0018] The catalyst of the invention has excellent activity and
selectivity and has improved chemical robustness leading to a long
working lifetime. However, the catalyst lacks the physical
robustness or toughness associated with amorphous chromia catalysts
and is difficult to handle in practice, for example it is not
readily produced in the form of pellets in which fluorination
catalysts are usually produced and it does not easily withstand
temperature shocks as are often encountered in the operation of
large-scale industrial plants. This problem can be alleviated by
blending the improved partially crystalline catalyst with a
non-crystalline chromia so that the catalyst may comprise
essentially amorphous chromia as well as crystalline chromia. Such
blended catalysts have improved toughness and can be pelleted and
handled without too much difficulty. The amount of the
non-crystalline (essentially amorphous) chromia additive may vary
within wide limits but will usually be from about 10% to 60% by
weight of the blended catalyst. The non-crystalline (essentially
amorphous) chromia may itself contain a divalent metal, for example
an activity promoting amount of a divalent metal such as zinc,
cobalt or nickel.
[0019] The partially crystalline catalyst can be produced by
sintering the corresponding amorphous or essentially
non-crystalline catalyst or chromium hydroxide precursor thereof at
elevated temperature under conditions whereby the apparent degree
of crystallinity induced in the chromia is controlled, for example
to between 8% and 50% by weight and such a process is provided
according to another feature of the invention.
[0020] Such a process in which the crystallised chromia is
subsequently impregnated with zinc or a compound of zinc is also
provides a further aspect of the invention.
[0021] Sintering may be carried out under an inert atmosphere such
as nitrogen gas or in an oxidising atmosphere such as air which may
optionally be diluted with an inert gas such as nitrogen. The
temperature of sintering may be within the range from about
400.degree. to 800.degree. C., preferably from 500.degree. C. to
600.degree. C. in an inert atmosphere and from about 300.degree. C.
to 800.degree. C., preferably from 330.degree. C. to 500.degree. C.
in air. Catalysts produced by sintering in nitrogen contain the
chromium as essentially only chromium (III) whilst those produced
by sintering in air tend to contain some chromium (VI) as well as
chromium (III). As described hereinbefore, we prefer to sinter the
catalyst or precursor thereof in a mixed atmosphere of air and an
inert gas such as nitrogen.
[0022] The crystallisation of chromia is an exothermic reaction and
may be accompanied by a rapid rise in temperature leading to hot
spots or run-away reaction unless the reaction is controlled. For
this reason it is desirable to raise the temperature of the chromia
to the desired sintering temperature and induce crystallisation of
the chromia over a period of several hours, for example from 1 to
50 hours and preferably 4 to 12 hours. We have found that operating
in this way enables us to control the reaction and the degree of
crystallisation induced in the chromia.
[0023] During sintering and crystallisation, the surface area of
the chromia/catalyst is reduced generally from above 100 m.sup.2/gm
to below 100 m.sup.2/gm, for example from 150 m.sup.2/gm to below
70 m.sup.2/gm. We have found that within the range of crystallinity
8% to 50%, the surface area of the catalyst decreases with
increasing crystallinity from about 70 m.sup.2/gm to about 20
m.sup.2/gm. The surface area of the catalyst at any particular
stage of the sintering procedure gives a guide as to the degree of
crystallinity in the chromia and provides an indication of
sufficient sintering. The degree of crystallinity in the catalyst
can be controlled by controlling the sintering conditions.
[0024] The preferred catalysts containing a divalent metal promotor
such as zinc, cobalt or nickel or compounds thereof can be produced
by inducing crystallisation in a chromia catalyst already
containing the divalent metal promotor or by creating the partially
crystalline chromia base catalyst and subsequently impregnating it
with the divalent metal promotor. Any of the known techniques for
producing chromia-based catalysts can be used to produce the
precursor catalyst in which crystallinity is induced.
[0025] If present, the amount of the divalent metal promotor is
known in the art but as discussed hereinbefore in the case of zinc
or a zinc compound the amount generally should be less than is used
in amorphous chromia catalysts. Further, the optimum amount of zinc
promotor to afford an increased initial catalyst activity depends
upon the catalyst preparation method and generally is lower for
catalysts made by impregnation of a pre-crystallised chromia base
than for catalysts made by a route involving coprecipitation of
chromium and zinc salts, for example hydroxides. As a guide, the
optimum amount of zinc in a catalyst made by impregnation of a
crystalline chromia may be about 0.5% by weight whilst for a
catalyst made by the coprecipitation route the optimum amount of
zinc may be about 1% by weight.
[0026] The partially crystalline chromia catalysts of the invention
may be blended with conventional amorphous chromia catalysts in
order to impart physical robustness or toughness to the catalyst
and enable it to be pelleted and handled without serious damage. As
described hereinbefore, the amount of the conventional catalyst
additive may be from about 10% to about 60% or even more of the
blended catalyst.
[0027] The improved catalyst of the invention may be used in any of
the fluorinaton reactions in which chromia-based catalysts are
normally employed. These will usually be reactions of halogenated
and particularly chlorine-containing hydrocarbons with hydrogen
fluoride in the gas phase at elevated temperature. Numerous such
reactions are operated commercially and amongst them may be
mentioned the fluorination of halogenated aliphatic hydrocarbons
containing from 1 to 6 carbon atoms, for example methylene chloride
(to produce difluoro- methane, HFC 32); trichloroethylene (to
produce 1,1,1,2-trifluoro-2,2-dichloroethane, HCFC 133a and
1,1,1,2-tetrafluoroethane, HFC 134a); HCFC 133a (to produce HFC
134a); perchloroethylene (to produce pentafluoroethane, HFC 125;
chlorotetrafluoroethane, HCFC 124; and dichlorotrifluoroethane,
HCFC 123); 1,1,2,2-tetrachloroethane (to produce HFC 134) and
dichlorotrifluoroethane (to produce HFC 125). The catalyst is also
useful in the removal of the impurity chlorodifluoro-ethylene (HCFC
1122) from HFC 134a by reacting the impurity with hydrogen fluoride
to produce HCFC 133a. Processes employing the above starting
materials are used commercially and thus are important but it is to
be understood that the fluorination process according to the
present invention is not limited to use of these starting
materials.
[0028] Included within the invention is a process for fluorinating
halogenated hydrocarbons which comprises reacting the halogenated
hydrocarbon with hydrogen fluoride in the vapour phase at elevated
temperature in the presence of the improved fluorination catalyst
described herein. The conditions such as temperature, pressure,
ratios of reactants and number of reaction steps for carrying out
fluorination reactions using chromia-based catalysts are well known
in the art and are generally applicable to the improved catalyst of
the invention, although the increased activity of the improved
catalyst generally enables lower temperatures or shorter contact
times to be employed than have typically been used hithereto.
[0029] When employed in the production of hydrofluorocarbons
[HFCs], the improved catalysts can suffer deactivation due to
coke/carbon deposition and may require periodic regeneration. The
catalysts can be regenerated as necessary by conventional
regeneration techniques such as heating in air or in a mixed
atmosphere of air and hydrogen fluoride and/or an inert gas. The
improved catalysts afford the advantage that they require
replacement less frequently than conventional chromia-based
catalysts and have a longer active working lifetime.
[0030] The invention is illustrated but in no way limited by the
following examples.
EXAMPLE 1
[0031] An amorphous chromia catalyst containing 1% by weight of
zinc was prepared by the mixed metal hydroxide precipitation
technique. 4 liters of 1 molar chromium nitrate
[Cr(NO.sub.3).sub.3] solution were added to 12 ml of 4 molar zinc
nitrate [Zn(NO.sub.3).sub.2] solution to form a mixed metal nitrate
solution.
[0032] 740 ml of 0.88 molar ammonia solution was prepared and
stirred using an impeller and sufficient of the mixed metal nitrate
solution was added to it to lower the pH to 7.3 at a temperature of
21.degree. C. The resulting mixed metal hydroxide precipitate was
collected using a flat bed filter and washed with demineralised
water. The washed precipitate was dried in a nitrogen atmosphere
for 12 hours at 150.degree. C. and then calcined under nitrogen gas
at 280.degree. C. for a further 8 hours. The resulting solid was
powdered, mixed with 2% by weight of graphite and formed into
pellets of density 2 gm/cm.sup.3. The catalyst at this stage was
found to be essentially amorphous (non-crystalline) and had a
surface area of 239 m.sup.2/gm determined by the BET nitrogen
absorption method.
[0033] The catalyst pellets were crushed and seived to generate
granules of particle size 0.5-1.5 mm and 4 g of the granules was
charged to a 9 mm internal diameter reaction tube for sintering.
The catalyst was heated at 425.degree. C. for 16 hours in a flow of
18 ml/min of nitrogen mixed with 1 ml/min of air after which time
the air flow was stopped and the catalyst was cooled to room
temperature in the nitrogen flow. The catalyst was then discharged
from the reactor and was found to have an apparent crystallinity of
about 45% with a surface area of 57 m.sup.2/gm measured by the BET
nitrogen absorption method.
[0034] 2 gm of the partially crystalline catalyst was re-charged to
the reactor for conditioning and activity testing. The catalyst was
dried at 300.degree. C. for 30 minutes in a nitrogen flow of 50
ml/min and then was heated at 300.degree. C. in a hydrogen fluoride
flow of 20 ml/min until hydrogen fluoride was detected in the
reactor vent stream. The reactor temperature was increased to
380.degree. C. for 16 hours whilst continuing the flow of hydrogen
fluoride, prior to measurement of the activity of the catalyst.
[0035] The catalyst was cooled to 350.degree. C., still in the flow
of hydrogen fluoride, and then 5 ml/min of
chloro-2,2,2-trifluoroethane [HCFC 133a] was added to the hydrogen
fluoride flow to generate a feed having an HF:HCFC 133a molar ratio
of 4:1. After 2 hours, the catalyst temperature was reduced to
300.degree. C. and the yield of 1,1,1,2-tetrafluoroethane [HFC
134a] at 300.degree. C. was quantified by gas chromatographic
analysis. The yield of HFC 134a at 300.degree. C. was 17.2%
COMPARATIVE EXAMPLE A
[0036] For purposes of comparison, the activity of the unsintered
catalyst was determined. 2 gm of the amorphous catalyst granules
was charged into the reactor and the catalyst was dried,
conditioned and tested by the procedure described above except that
the sintering step at 425.degree. C. was omitted so that the
catalyst remained essentially non-crystalline. The yield of HFC
134a at 300.degree. C. was 7.6%.
COMPARATIVE EXAMPLE B
[0037] For purposes of comparison also, an amorphous chromia
catalyst containing 3% by weight of zinc was prepared as described
in Example 1 using 36 ml of the zinc nitrate solution instead of 12
ml. The resulting catalyst had a surface area of 183 m.sup.2/gm.
The catalyst was granulated and sieved as in Example 1 and 4 gm of
catalyst granules was charged to the reactor for sintering. The
catalyst was heated at 400.degree. C. for 16 hours in a flow of 5
ml/min of air after which time the catalyst was cooled to room
temperature in a nitrogen flow of 18 ml/min. The catalyst was
discharged from the reactor and was found to have an apparent
crystallinity of about 90% with a surface area of 23 m.sup.2/gm.
The amorphous and crystalline catalysts were tested as described
above. Using the amorphous catalyst, the yield of HFC 134a at
300.degree. C. was 8.6% and using the crystalline catalyst, the
yield of HFC 134a at 300.degree. C. was only 1.8%.
EXAMPLE 2
[0038] An amorphous chromia catalyst was prepared by the
precipitation technique. Aqueous ammonia solution was added to an
aqueous solution containing chromium to produce a precipitate of
chromium hydroxide. The precipitate was washed with demineralised
water, dried in a nitrogen atmosphere at 150.degree. C. and then
calcined under nitrogen at 280.degree. C. for 8 hours. The
resulting solid was powdered, mixed with 2% by weight of graphite
and formed into pellets. The chromia was found to be essentially
amorphous (non-crystalline) and had a surface area of 176
m.sup.2/gm determined by the BET nitrogen adsorption method.
[0039] The amorphous catalyst pellets were crushed and seived to
generate granules of particle size 0.5-1.4 mm and 50 gm of the
granules was charged to a reaction tube for sintering. The catalyst
was heated at 190.degree. C. in a flow of 20 ml/min nitrogen gas
for 2 hours and then the temperature was raised to 550.degree. C.
at the rate of 20.degree. C./hour and maintained at 550.degree. C.
for 24 hours. The catalyst was then cooled to room temperature in
the nitrogen flow and discharged from the reactor. This base
catalyst was found to have an apparent degree of crystallinity of
about 80% with a surface area of 47 m.sup.2/gm.
[0040] 4.95 gm of the base catalyst was added to 0.96 ml of aqueous
zinc chloride solution (prepared by dissolving 13.54 gm of zinc
chloride in demineralised water to provide 250 ml of solution) and
the mixture was stirred and evaporated to dryness to give an
impregnated chromia catalyst containing 0.5% by weight of zinc.
[0041] 2 gm of the impregnated catalyst was charged to an Inconel
reaction tube for conditioning and activity testing. The catalyst
was dried at 250.degree. C. for 90 minutes in a 50 ml/min flow of
nitrogen gas and was then heated at 300.degree. C. in a 20 ml/min
flow of hydrogen fluoride until hydrogen fluoride was detected in
the reactor vent stream whereupon the temperature was raised to
380.degree. C. for 16 hours whilst the flow of hydrogen fluoride
was maintained.
[0042] After conditioning as above, the catalyst was cooled to
350.degree. C., still in the hydrogen fluoride flow and then 5.8
ml/min of 1-chloro-2,2,2-trifluoroethane [HCFC 133a] was added to
the hydrogen fluoride flow to provide a feed having an HF:HCFC 133a
molar ratio of 3.4:1. After two hours the catalyst temperature was
reduced to about or below 300.degree. C. The yield of
1,2,2,2-tetrafluoroethane [HFC 134a] at 297.degree. C. and
288.degree. C. was measured by gas chromatographic analysis. The
yield of HFC 134a at 297.degree. C. was 17.4% and the yield at
288.degree. C. was 14.1%.
EXAMPLE 3
[0043] Using the impregnation procedure described in Example 2, an
impregnated chromia catalyst containing 1% by weight of zinc was
prepared from 4.90 gm of base catalyst and 1.92 ml of zinc chloride
solution. 2 gm of the catalyst was conditioned and tested as
described in Example 2 with a yield of HFC 134a at 297.degree. C.
of 14% and a yield of HFC 134a at 288.degree. C. of 11.5%.
EXAMPLE 4
[0044] Using the impregnation procedure described in Example 2, an
impregnated chromia catalyst containing 3% by weight of zinc was
produced from 4.69 gm of base catalyst and 5.77 ml of zinc chloride
solution. 2 gm of the catalyst was conditioned and tested as
described in Example 2 with a yield of HFC 134a at 303.degree. C.
of 7.4% and a yield at 292.degree. C. of 6.1%.
COMPARATIVE EXAMPLE C
[0045] For purposes of comparison the activity of the base chromia
catalyst (not impregnated with zinc) was determined using the
conditioning and testing procedure described in Example 2. The
yield of HFC 134a at 301.degree. C. was 15% and at 283.degree. C.
was 6.4%.
* * * * *