CA1067881A - Catalysts prepared by sputtering and agglomeration - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method of preparing a supported catalyst which 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 compatible with the material, and 2. consolidating the thus sputtered particles into aggregated form.
This method reduces or avoids certain disadvantages of conventionally prepared catalysts in which some of the metal can be trapped in deep pores in the conventional catalyst particle and thereby be unavail-able for catalysis or induce poor selectivity in the catalysed reaction. The method is applicable to a wider range of substrates that is appropriate to conventional catalysts such as are loaded, for example by impregnation techniques. The catalysts 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 effluents and car exhaust treatment systems.

Description

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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 H.27885 ~067l~8~

- 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 H.278~5 ~7~8~

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 H.27885 ~,~78~3~

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 - H.27885 ~0~'78~
.

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 _ 8 --H.27885 ~1~7~8~L

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.

.
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 H.27~85 .

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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~

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 - 11 ~

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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 H.27885 ~i7~

.
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 ~ H.27885 ~0~'7~

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.

~ , ` ' ' .

Claims

The embodiments of the invention in which an exclusive privilege and property are defined as follows:

1. A method of preparing a supported catalyst which 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 compatible with the material, and 2. consolidating the thus sputtered particles into aggregated form.

2. A method as claimed in Claim 1 in which the particles have a porosity which is less than 0.4 ml/gm, 3. A method as claimed in Claim 1 in which the particles comprise magnesia, alumina, or silica or mixtures of these oxides with each other or with another refractory oxide.

4. A method as claimed in Claim 1 in which the sputtering is effected epitaxially.

5. A method as claimed in Claim 1 in which consolidation of the catalyst is effected by a method selected from compaction, pelleting, extrusion, and spray drying of a slurry.

6. A method as claimed in Claim 1 in which the sputtered material is consolidated with a second component, the second component being selected from other sputtered particles, non-sputtered particles and mixtures of these.

7. A method as claimed in claim 1 for preparing a supported catalyst which comprises 1. sputtering a material which is catalytic, or which is a component of a catalytic system on to the surface of particles comprising a refractory oxide selected from magnesia, silica, and alumina and mixtures of these with each other or with another refractory oxide, the particles being compatible with the catalytic material and having a porosity of less than 0.4 ml/gm., and 2. consolidating the thus sputtered particles into aggregated form by a method selected from compaction, pelleting, extrusion, and spray drying of a slurry.

8. A supported catalyst which comprises an aggregate of consolidated particles, the surface of each particle of the aggregate being at least partially covered with sputtered material which is catalytic or which is a component of a catalytic system.

9. A process for the conversion of a hydrocarbon which comprises effecting conversion of a hydrocarbon feedstock under conditions which include contact with a catalyst as claimed in Claim 8 or with a catalyst which has been prepared by a method as claimed in

Claim 1.