Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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THE PRESENT INVENTION relates to the manuf-
acture of ca-talysts.
Recent improvements in catalyst manufacture
have included the use of a technique known as
"sputtering". Sputtering usually involves driving
an~atom1c or ionic dispersion of a catalytic metal
- component or components at a support surface so as
to attach onto the support surface a major prop-
- ortion of the atoms of the catalytic material which
. . ,
lO 1mpinge on the surface. When deposition of isolated
particles on the surface occurs, enhanced activi-ty
is expected because of their high dispersion.,
However, even if "islands" of deposited material ,
are formed, activity is expected to be improved
15 because of the greater accessibility of the cataly-
tically active phase on the surface of the support.
, Sputtering of the catalytic material may be
achieved by bombarding a source of the material in
the neighbourhood of the support with energetic ions
20-or electrons. The ions may be provided by an ion
beam~from an accelerator, ion ,separator or ion gun.
Electrons may be provided by an electrical discharge
-to a cathode of the metal or alloy which it is
desired to deposit. The condltions of atmosphere
25 and selectivity of ions or electrons permitted to
bombard the catalytic material are chosen so that the
metal atoms reach the support, if desired, without
undue agglomeration, and so that the codeposition of
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unwanted material on the support is avoided.
Another technique which may be suitable in
- certain cases is the evaporation of a metal from a
heated surface, for example from a filament either
coated with or consisting of the metal or alloy.
Ion and electron bombardment methods have the
advantage that the metal source remains relatively
cool during the sputtering process; therefore there
is less danger of contamination, for example by
-10 evaporating the mechanical holder of the metal sample.
Furthermore 9 it is recognised that ion and electron
bombardment permits control, for example of the
- energy, of the ejected (sputt;ered) atom and such
control may be useful since in some cases the energy
15 of the sputtered atom may be important in determining
the structure of the catalytic material.
- According to the present invention a method
of preparing a supported catalyst comprises:-
1. sputtering a material which is catalytic,
or which is a component of a catalytic system
on to the surface of particles which are
- comp~tible with the material, and
2, consolidating the thus sputtered particles
into aggregated form.
Preferably, before sputtering the particles
h~ve a diameter in the range 100~ to 0.1 mm, more
preferably in the range 1000~ to 50~.
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The particles are preferably robust and of low
porosity. For con~enience in the sputtering
procedure it is envisaged that the shape of the
par-ticles will be nearly spherical, for example
rounded granules. Although it is more dif~icult to
coat fibrils (i.e. particles of elongated aspect)
uniformly by present sputtering techniques, it is
not intended that use of these particles be excl-
.
uded from the method of this invention. The
10 particles may comprise refractory non-metallic
material~ especially oxidic material and suitably
comprise an oxide such as magnesia, alumi~a,
silica, and mixtures of these with each other or
with other refraGtory oxides. They may be in
15 highly refractory forms such as fused magnesia,
highly calcined alumina, spinels and hydraulic
cement. Optlonally, the particles themselves may
possess catalytic activity, either by themselves or
in combination with the catalytic material deposited
20 on them.
Optionally the sputtering is effected
epitaxially by which it ls to be understood that a
preferred surface structure o~ the deposited
catalytic material is obtained. It is well known
25 that for some reactions, the crystalline habit and
size of catalytically active materials can affect
the activity and selectivity of the catalyst. How-
ever it has n~ hitherto been possible to optimise
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- performance of commercial catalysts by exploiting
epitaxial crystal growth because there was no
! convenient method of preparing an ep1taxial
catalyst having the required high totàl sùrface
. 5 area per unit weight in.a practical form. We
~ ~ believe it is possible, using the method of this
invention, to produce a high area epitaxial
catalyst by consolidating small particles carrying
.
~ epitaxially grown active material. It is believed
10 that a.preferred surface structure of catalytic
material can be obtained with a suitable amorphous
substrate by means of carefully controlled
. . sput-tering conditionsj for example by using
appropriate energies of the material being sputt.ered
or by exposure during sputtering to species capable
of affecting the crystalline habit of the deposited
catalytic material. More preferably~ a crystalline
su~strate may be used, having well defined
crystalline faces on which sputtered material is
deposited in catalytically favourable orientations.
The method of consolidation used will depend
.to some extent on the type and composi-tion of the
catalyst and on the use to which the catalyst will
be put. Sultably, however, the catalyst may be
consolidated by a method selected from compaction,
pelleting, extrusion, and spray dry~ing of a slurry.
The aim, in chosing the method of consolidation, is
to obtain a consolidated catalyst which is an
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aggregate of a suitable size and sufficiently
mechanically robust for use in catalytic reactlon.
Consolidation of relatively small sputtered
catalyst particles usualiy results in a'catalyst
, 5 which has a high area of active phase when compared
with a catalyst which has been prepared hy sputtering
.
the metal component on to a relatively large
particle, for example a pellet, which is already '
of the desired size for catalysis. This is because
the lat-ter relatively large catalyst particle would
- be expected -to have a lower active (external) surface
area per unit weight of catalyst than both a
consolidated sputtered catalyst made according to this
invention and a typical non-sputtered catalyst
- 15 particle.
Preparation of a catalyst according to the
method of this invention reduces or avoids certain
disadvantages of conventionally prepared catalysts in
~ whLch some of the metal can be trapped in deep pores
:20 in the canventional catalyst particle and thereby be
unavailable for catalysis or induce poor selectivity
in the catalysed reaction. By deep pores are understood'
either those which are inaccessible'to reagents or
those from which praducts cannot rapidly be released
to the process stream. It is believed that deep'
pores will tend to be those which are relatively
narrow and long. In conventional catalysts a
compromise is usually necessary between disadvantages
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which can be associated directly with the
presence of active material in deep pores and
advantages stemming from required physical
properties of the catalyst support. Unfortunately,
such advantages may themselves entail the presence
of deep pores, for example high mechanical strength,
high pore volume and high total area of the catalyst
support. A further common problem in designing
- conventional catalysts is that to attain re~uired
physical characteristics of the support, it may be
necessary to use a material which is not chemically
inert towards process ~eagents or products, or which
requires tedious or costly deactivation treatment.
Catalysts prepared according to -this
invention include aggregates of small particles of
low poroslty, say less than 0.4 ml/gm and in some
cases of zero porosity. Actlve material would be
carried on the exterior of these particles and wouId
not be present in deep pores (if any~ within the
particle. In the aggregate, reagents can readily
- reach the surface of the particles via relatively
large access pores between the particles 7 these
pores probably having mean diameters about 10 to
25% of the particle diameter,
In the method of the present invention,
the surface of the sputtered particles may be
either partially or completely covered with
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catalytic material, as desired. For example,
if it is desired to produce a consolldated
catalyst in which the catalytic material
is highly dispersed, then it will probably not
'~ 5 be necessary to completely cover the particles
with sputtered catalytic material.- On the other
hand, there'may be circumstances in which it is
desirable to completely cover the particles,
perhaps to a thlckness of several atomic layers.
1~ ' The present invention thus provides
, means of avoiding undesirable chemistry which
may occur on the catalyst support. When the
, , substrate is completely covered with active
, material and provided it has n*,been mechanic-
; - 15 ally damaged, e.g. during consolldation, adverse
chemistry catalysed by or involving the support
should be largely or completely avoided.
Furthermore, the process of the present invention
, is applicable to a wider range of substrates than
is appropriate to conventional cataiysts such~as
are loaded, for example by impregnation tech-
nique,s. For example the process of the invention
is applicable to substratesof low por~sity and to
substrates which would be soluble in an
impregnating solution. It is thus more likely
that a suitable inert substrate can be selected
for a given application, offering the possibility'
of preparing catalysts of improved selec-tivity
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even -though the substrate may only be partially
covered by active material.
Optionally, in the method of the present
invention the sputtered material is consolidated in
admixture either with other sputtered particles or
with non-sputtered particles or with a mixture of
sputtered and non-sputtered particles. By this
form of the method of the presen-t invention, it is
- possible to produce a catalyst which is an intimate
10 mixture of different particles having separate
catalytic func-tions. It is believed that the
activ1ty of such catalysts will be enhanced compared
with conventional bifunctional catalysts. If
desired, on the other hand, the non-sputtered
15 particles may be non-catalytic. The relative
amounts of the two (or more) components of the
mixture which is to be consolidated according to
this form of invention may vary within wide limits,
for example ~rom a 5 to 95 ratio at one extreme to
20 a 95 to 5 ratio, by weight, at -the other. The
actual relative amounts will depend to a great extent
on the catalytic properties which are sought in the
consolidated catalyst.
In this form of the invention, in which a
25 two (or more) component mixture is to be consolidated,
the particles of the second component may or may not,
as desired, have the same physical and chemical
- characteristlcs as the particles of the first component.
_ 9 _
, .
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For example 9 the particles of the first component
may be granules o~ a refractory non-metallic oxide,
as hereinbefore described, and the particles of
the second component may be the same or another
refractory oxlde or may be fibrous, or some other
material.
If desired, further catalytic components
- may be loaded on to a consolidate containing
sputtered particles using known methods, for example
10 impregnation with a solution cont~ining a precursor
of the desired adduct.
Another feature of the present inventlon
is a supported catalyst made by the present method.
The catalysts made by the method of the
15 present invention may be used in a wide range of
applications. Some examples of such applications
; are in various hydrocarbon conversion processes, for
example naphtha reforming, isomerisation processes,
hydrogenation processes, treatment of gaseous
20 effluents, and car exhaust treatment systems.
The invention will now be particularly
described in the following Examples.
EXAMPLE 1
.
A low porosity alu~ina (particle diameter 1
25 to 25~) was pre-calcined at 1100C for 12 hours and
then loaded with 0.9% w/w platinum by sputtering.
The resulting solid (11.2 g) was suspended in a
solution of 0.647 kg, hydrated aluminium nitrate in
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0.625 litres distilled water. To the suspension was
slowly added with vigorous stirring 005 litre of a
3M aqueous ammonia solution. The precipitate was
allowed to stand overnigh-t in the supernatant liquor
at ambient temperature. It was then filtered and
- - washed twice with 0.25 litre of ~M aqueous ammonia
solution. A portion of the filter cake was
extruded through a 2 mm. diameter die. The
extrudates were dried in àir at 100C for four
hours and then transferred to a nitrogen-purged
furnace in which the temperature was raised to 500C
at a rate of 100C/hour. The extrudates were held
at 500C for 4 hours and then allowed to cool in a
nitrogen stream.
The resulting solid consisted mainly of
hard extrudates about 1 mm, in diameter. The total
surface area (N2 adsorption, BET method) was l99 m2/g.
- and the pore volume was 0.32 ml/g~
EXAMPLE 2
Catalyst prepared as in Example 1 (5.0 g.)
was mixed with fused alumina chips and loaded into
a reactor used for cyclohexane dehydrogenation. The
reactor was a l" diameter silica tube with a central
thermocouple well. The ~eactor was purged with
ni-trogen while being heated to 350C and the catalyst
was pre-reduced for 1 hour at 350C under a flow of
dry and deoxygenated hydrogen.
The flow of hydrogen was maintained while
a feed of 61 ml/hr. of cyclohexane (containing 0.2
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.
p.p.m. sulphur) was passed through the reactor. The
temperature and pressure were maintained at about
344C and 1 atmosphere respectively, and the liquid
product was collected in a series of traps. The
product was analysed for benzene and the dehydrog-
enating activity of the catalyst was calculated as
the number of micro-moles of benzene produced per
,
second per gramme of platinum in the catalyst.
During three half-hour periods following the start
of the feed, the dehydrogenating activity thus
expressed was 6.1 x 103, 7.1 x 103 and 5.8 x 103
respectively.
For comparison, the experiment was
repeated with a catalyst prepared by impregnating
porous alumina spheres with chloroplatinic acid.
This catalyst (5.30 g), containing 0.34% w/w
platinum was loaded into the reactor. Using similar
procedures and operating conditions as before, the
dehydrogenating activity of this cata~yst in the
first, second and third periods following the start
of the feed was found to be 3.3 x 103, 2.5 x 103
and 2.0 x 103 respectively.
E~AMPLE 3
Catalyst prepared as in Example l (5.01 g.)
was loaded into the reactor used in Example 2. The
experimental procedure followed was identical to
that of Example 2 except that 93 ml/hr of methyl-
cyclohexane was fed instead of cyclohexane. The
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measured dehydrogenating activities in successive
periods of 20 minutes and 15 minutes following 'the
- - start of the feed were respectively 8.2 x 103 and
6.7 x-103 at a temperature of about 354C.
: 5 The temperature of the reactor was then
raised to 500C with the methylcyclohexane.flow.
maintained and the hydrogen pressure kept at about
1 atmosphere. At 500C the dehydrogenating activity
was found to have dropped to 0.9 x 103. These drops
10 in ac-tivity are not unexpected since it is known
- : . that treatment of conventional platinum catalysts
: with hydrocarbons at hlgh temperatures and low
hydrogen pressures tends to lead to deactivation,
. attributable to the formation of carbonaceous
15 deposits. However, the activity of the catalyst
used in this Example was restored as follows.
The reactor was purged with nitrogen at
. 350C and the nitrogen stream was gradually replac.ed
by air. After the catalyst had been exposed to air
20 for l hour at 350C, the temperature was raised to
400C for a further hour. The reactor was again -
purged with nitrogen while the temperature was
returned to 350C. The cat~.lyst was then reduced
in a hydrogen stream at 1 atmosphere and 350C for
25 2 hours. Wh.en methylcyclohexane at a rate of about
99 ml/hour was fed in with the hydrogen stream, the
observed dehydrogenating activity was 10.0 x 103 and
7.3 x 103 in the first two pericds of 15 minutes
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following the start oP the Peed, the reactor
temperature being about 348C.
These Examples illustrate the superior
- . activity of a catalyst made according to the
present invention over one made by a conventional
impregnation method. Moreover, the high activity
.
after oxidation shows that the catalyst of the
present invention is.relatively unimpaired by
oxidation-reduction or by exposure to high
~,mperatureS.
~ , ` ' ' .
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