U.S. patent application number 10/298920 was filed with the patent office on 2003-06-26 for catalysts with high cobalt surface area.
This patent application is currently assigned to Johnson Matthey PLC. Invention is credited to Gray, Gavin, Kelly, Gordon J., Lok, Martinus C..
Application Number | 20030119668 10/298920 |
Document ID | / |
Family ID | 26244312 |
Filed Date | 2003-06-26 |
United States Patent
Application |
20030119668 |
Kind Code |
A1 |
Lok, Martinus C. ; et
al. |
June 26, 2003 |
Catalysts with high cobalt surface area
Abstract
A catalyst or precursor thereto comprising cobalt and/or a
cobalt compound on a transition alumina support having a total
cobalt content of at least 41% by weight and a cobalt surface area,
after reduction, greater than 25 m.sup.2 per gram of total cobalt.
The catalyst or precursor may be made by slurrying a transition
alumina powder having a pore volume of at least 0.7 ml/g with an
aqueous cobalt ammine carbonate complex and heating the slurry to
decompose the complex.
Inventors: |
Lok, Martinus C.;
(Guisborough, GB) ; Kelly, Gordon J.; (Darlington,
GB) ; Gray, Gavin; (Billingham, GB) |
Correspondence
Address: |
PILLSBURY WINTHROP, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
Johnson Matthey PLC
London
GB
|
Family ID: |
26244312 |
Appl. No.: |
10/298920 |
Filed: |
November 19, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10298920 |
Nov 19, 2002 |
|
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PCT/GB01/01811 |
Apr 23, 2001 |
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Current U.S.
Class: |
502/332 |
Current CPC
Class: |
B01J 35/0053 20130101;
C07C 209/36 20130101; C10G 2/332 20130101; C07C 209/48 20130101;
B01J 35/1042 20130101; B01J 37/031 20130101; B01J 35/002 20130101;
C07C 2521/04 20130101; B01J 23/75 20130101; B01J 21/04 20130101;
B01J 35/1061 20130101; B01J 35/10 20130101; C07C 209/32 20130101;
C07C 211/46 20130101; C07C 1/0445 20130101; C07C 211/46 20130101;
C07C 209/36 20130101; C07C 211/46 20130101; C07C 2523/75 20130101;
C07C 209/32 20130101; B01J 35/023 20130101; C07C 209/48 20130101;
C10G 45/00 20130101 |
Class at
Publication: |
502/332 |
International
Class: |
B01J 023/75 |
Foreign Application Data
Date |
Code |
Application Number |
May 19, 2000 |
GB |
0012087.3 |
Aug 7, 2000 |
GB |
0019182.5 |
Claims
1. A particulate catalyst precursor comprising cobalt compounds
supported on a transition alumina support and having a total cobalt
content of 20 to 85% by weight, a pore volume above 0.3 ml/g, and a
surface-weighted mean diameter D[3,2] in the range 1 .mu.m to 200
.mu.m and which, upon reduction, has a cobalt surface area of at
least 25 m.sup.2 per gram of total cobalt.
2. A catalyst precursor according to claim 1 having a total cobalt
content above 40% by weight.
3. A catalyst precursor according to claim 1 having a pore volume
above 0.5 ml/g.
4. A catalyst precursor according to claim 1 having a
surface-weighted mean diameter D[3,2] below 20 .mu.m.
5. A catalyst precursor according to claim 1 having an average pore
diameter of at least 8 nm.
6. A catalyst precursor according to claim 1 wherein the cobalt is
present as cobalt oxides.
7. A catalyst precursor according to claim 1 having a total cobalt
content above 50% by weight and wherein the alumina is a gamma
alumina.
8. A particulate catalyst comprising cobalt supported on a
transition alumina support and having a total cobalt content of 20
to 85% by weight, a pore volume above 0.3 ml/g, a surface-weighted
mean diameter D[3,2] in the range 1 .mu.m to 200 .mu.m, and a
cobalt surface area of at least 25 m.sup.2 per gram of total
cobalt.
9. A catalyst according to claim 8 having a total cobalt content
above 40% by weight.
10. A catalyst according to claim 8 having a pore volume above 0.5
ml/g.
11. A catalyst according to claim 8 having a surface-weighted mean
diameter D[3,2] below 20 .mu.m.
12. A catalyst according to claim 8 having an average pore diameter
of at least 8 nm.
13. A catalyst according to claim 8 having a total cobalt content
above 50% by weight and wherein the alumina is a gamma alumina.
14. A catalyst according to claim 8 wherein at least 50% of the
cobalt is present as metallic cobalt.
15. A method of making a catalyst precursor containing 20 to 85% by
weight of total cobalt comprising slurrying a transition alumina
powder having a pore volume above 0.7 ml/g with an aqueous solution
of a cobalt ammine complex, heating the slurry to cause the cobalt
ammine complex to decompose with the deposition of an insoluble
cobalt compound, filtering the solid residue from the aqueous
medium and drying the solid residue.
16. A method according to claim 15 wherein the slurry is heated for
a period of not more than 200 minutes to cause the cobalt ammine
complex to decompose.
17. A method according to claim 15 wherein the alumina powder has a
surface-weighted mean diameter D[3,2] in the range 1 .mu.m to 200
.mu.m.
18. A method according to claim 15 wherein the transition alumina
powder has a surface-weighted mean diameter D[3,2] below 20
.mu.m.
19. A method according to claim 15, wherein the transition alumina
powder has an average pore diameter of at least 10 nm.
Description
[0001] This invention relates to catalysts and in particular to
catalysts containing cobalt which are suitable for use in
hydrogenation reactions.
[0002] Catalysts comprising cobalt on a support such as silica or
alumina are known in the art for hydrogenation reactions, e.g. for
the hydrogenation of aldehydes and nitriles and for the preparation
of hydrocarbons from synthesis gas via the Fischer-Tropsch
reaction.
[0003] In comparison with other catalytic metals such as copper and
nickel used for hydrogenation reactions, cobalt is a relatively
expensive and so, to obtain the optimum activity, it is desirable
that as much as possible of the cobalt present is in an active form
accessible to the reactants. For hydrogenation reactions, the
active form of the cobalt is elemental cobalt although in the
active catalyst only some, rather than all, of the cobalt is
normally reduced to the elemental form. Hence a useful measure is
the exposed surface area of elemental cobalt per g of total cobalt
present. Except where expressly indicated, as used herein, total
cobalt contents are expressed as parts by weight of cobalt
(calculated as cobalt metal, whether the cobalt is actually present
as the metal or is in a combined form, e.g. as cobalt oxides) per
100 parts by weight of the catalyst or precursor thereto.
[0004] Cobalt catalysts on different carriers are disclosed in
"Stoichiometries of H.sub.2 and CO Adsorptions on cobalt", Journal
of Catalysis 85, pages 63-77 (1984) at page 67, table 1. From the
total maximum H.sub.2 uptake, it is possible to calculate the
cobalt surface area per gram of catalyst and the cobalt surface
area per gram of cobalt. It can be seen from this reference that
while the cobalt surface area per gram of total cobalt ranges
between 6 and 65 m.sup.2/g for cobalt on silica catalysts, for
cobalt on transition alumina catalysts, the cobalt surface area per
gram of total cobalt ranges only between 15 and 26 m.sup.2/g.
However for some applications it is desirable to use alumina,
rather than silica, as the support.
[0005] It has been proposed in EP 0 029 675 to make catalysts
comprising 25 to 70% by weight, based upon the weight of the
calcined and reduced catalyst, of a metal such as nickel and/or
cobalt by coprecipitating the metal, together with aluminium, in
the presence of porous particles such as gamma alumina particles.
It is stated that cobalt-containing catalysts preferably contain 25
to 60% by weight of cobalt and that cobalt-containing catalysts may
have cobalt surface areas, as determined by hydrogen chemisorption,
in the range 5 to 20 m.sup.2 per gram of catalyst. While a catalyst
containing 25% by weight of cobalt and having a cobalt surface area
of 20 m.sup.2 per gram of catalyst, i.e. combining the extremities
of the specified ranges, would have a cobalt surface area of 80
m.sup.2 per gram of total cobalt, there is no suggestion that
catalysts having such high cobalt surface areas per gram of total
cobalt can in fact be made by the specified route. We have found
that catalysts made by the procedure of Example 1 of EP 0 029 675
containing 26.4%, 37.0% and 52.1% total cobalt, based upon the
weight of the unreduced catalyst, (corresponding to cobalt contents
of 29%, 43% and 64% respectively in the reduced catalyst if it is
assumed that all the cobalt in the reduced catalyst is present in
the elemental form) have, upon reduction, cobalt surface areas of
8.2, 8.1 and 12 5 m per g of unreduced catalyst, corresponding to
cobalt surface areas of 31, 22 and 24 m.sup.2/g total cobalt
respectively. This is indicative that the procedure of that
reference does not provide a route to the production of catalysts
having a high cobalt content that at the same time have a high
cobalt surface area per g total cobalt
[0006] U.S. Pat. No. 5,874,381 describes a cobalt on alumina
catalyst which contains between 3 and 40% by weight of cobalt and
which has a relatively high cobalt surface area of above 30
m.sup.2/g of total cobalt
[0007] As indicated above, the dispersion of the cobalt on the
carrier is important since it is the surface of the cobalt of the
catalyst which is active. Therefore it is beneficial to maximise
the surface area of the metal which is present so as to produce a
catalyst which has a high cobalt surface area per unit mass of
total cobalt. It may be expected that the dispersion of the cobalt
on the catalyst would be maximised at relatively low loadings of
cobalt and that, as the amount of cobalt contained in the catalyst
is increased, the surface area per gram of cobalt would decrease
because the cobalt becomes more difficult to disperse on the
support.
[0008] For some applications it is desirable to employ catalysts
having a high loading of active material in order to minimise the
amount of support. However under some conditions it is possible
that the cobalt species may react with the alumina support to form
a cobalt aluminate which is difficult to reduce. Indeed, formation
of some cobalt aluminate may be desirable in order to provide a key
for bonding the cobalt species to the alumina. However, because of
the difficulty in reducing the cobalt aluminate, the cobalt
aluminate formation decreases the amount of cobalt available for
reduction to active elemental cobalt. By using a composition having
a high loading of cobalt, a greater proportion of the cobalt
species is available for reduction to the active elemental cobalt,
even if a significant amount of the support alumina reacts with the
cobalt species to form cobalt aluminate. Indeed, even if all the
support alumina reacts with the cobalt species to form cobalt
aluminate, a calcined catalyst precursor having a total cobalt
content above about 41% by weight inevitably contains some cobalt
in a form that is not so combined.
[0009] The aforementioned U.S. Pat. No. 5,874,381 suggests and
exemplifies the production of the catalysts by impregnation of
shaped transition alumina particles, e.g. extrudates, with a
solution of cobalt ammine carbonate, followed by removal of the
excess solution and heating to decompose the cobalt ammine
carbonate. However we have found that it is difficult to obtain
materials with a high cobalt content by this method. This reference
also suggests, but does not exemplify, an alternative procedure
wherein a slurry of transition alumina in a solution of the cobalt
ammine carbonate is heated to cause the cobalt to precipitate as a
hydroxycarbonate.
[0010] We have found that if the transition alumina has a
relatively high pore volume, above 0.7 ml/g, preferably above 0.75
ml/g, then it is possible to achieve high cobalt loadings, and the
resultant catalysts, upon reduction, have a relatively high cobalt
surface are per gram of total cobalt. Preferably the transition
alumina has a pore volume in the range 0.7 to 1.2 ml/g.
[0011] Thus we have now found that compositions containing more
than 40% cobalt by weight may be made which, upon reduction, have a
cobalt surface area of greater than 25 m.sup.2/g of total
cobalt
[0012] Accordingly the invention provides a catalyst or precursor
thereto, comprising a cobalt species on a transition alumina
support characterised in that the catalyst, or precursor, has a
total cobalt content of at least 41% by weight and that, after
reduction, has a cobalt surface area greater than 25 m.sup.2/g of
total cobalt
[0013] The term "cobalt species" is used broadly to include both
elemental cobalt and cobalt in combined form, eg. as compounds such
as cobalt oxides and cobalt hydroxycarbonates The catalyst in its
reduced form is useful for catalysing hydrogenation reactions. The
catalyst may, however, be provided as a precursor wherein the
cobalt is present as one or more compounds such as oxides or
hydroxy carbonates reducible to elemental cobalt In this form, the
material may be a catalyst precursor and may be treated to reduce
the cobalt compounds to metallic cobalt or the material may itself
be a catalyst and used as supplied, eg. for oxidation reactions The
cobalt surface area figures used herein apply to the material after
reduction, but the invention is not limited to the provision of
reduced catalyst
[0014] As indicated above, the compositions of the invention may be
made by the procedure of the aforesaid U.S. Pat. No. 5,874,381 by
utilising a transition alumina having a relatively large pore
volume It will be appreciated that the use of a large pore volume
alumina may also be beneficial when making products having a
smaller cobalt content.
[0015] Accordingly we also provide a method of making a
cobalt/alumina catalyst or precursor thereto containing 5 to 85% by
weight of total cobalt comprising slurrying a transition alumina
powder having a pore volume above 0.7 ml/g with an aqueous solution
of a cobalt ammine complex, heating the slurry to cause the cobalt
ammine complex to decompose with the deposition of an insoluble
cobalt compound, filtering the solid residue from the aqueous
medium, drying and, optionally calcining, the solid residue.
[0016] The transition alumina may be of the gamma-alumina group,
for example a eta-alumina or chi-alumina. These materials may be
formed by calcination of aluminium hydroxides at 400 to 750.degree.
C. and generally have a BET surface area in the range 150 to 400
m.sup.2/g. Alternatively, the transition alumina may be of the
delta-alumina group which includes the high temperature forms such
as delta- and theta-aluminas which may be formed by heating a gamma
group alumina to a temperature above about 800.degree. C. The
delta-group aluminas generally have a BET surface area in the range
50 to 150 m.sup.2/g. The transition aluminas contain less than 0.5
mole of water per mole of Al.sub.2O.sub.3, the actual amount of
water depending on the temperature to which they have been
heated.
[0017] In order to obtain a catalyst that is of practical use, it
is generally desirable that there is some interaction between the
cobalt and the alumina support. Such interaction, which can be
considered to be the formation of a cobalt aluminate, is desirable
to "anchor" the cobalt and to prevent leaching of cobalt in use
and/or coalescence of cobalt particles. Such leaching and/or
coalescence would result in loss of catalytic activity. However as
indicated above, the cobalt aluminate interaction product is
difficult to reduce and so in catalysts having low cobalt contents
a significant proportion of the cobalt may be in an unreducible
form. It is therefore preferred to use such an amount of cobalt
that the catalyst or precursor contains at least 20% by weight of
cobalt
[0018] Catalysts made by the conventional impregnation route
typically impregnate the support material with an aqueous solution
of a cobalt salt, typically cobalt nitrate Such solutions have a
low pH and to obtain some interaction between the cobalt and
support it is desirable to use a reactive alumina, especially
gamma-alumina. However, where the cobalt is deposited via a cobalt
ammine solution, the solutions have a high pH and under such
conditions it is possible to obtain adequate interaction using less
reactive aluminas, for example delta-alumina. If the proportion of
cobalt ammine solution is relatively small so that products having
relatively low cobalt contents are obtained, then, if gamma-alumina
is employed, a relatively large proportion of the cobalt, for
example 40% or more thereof, may interact with the alumina and so
the reduced catalyst obtained from such materials will have a
relatively low cobalt surface area
[0019] However the less reactive aluminas such as delta-aluminas
generally have a lower pore volume than the reactive gamma
aluminas. Consequently it may be difficult to make catalysts based
upon theta-alumina containing large amounts of cobalt. At high
cobalt contents, above 50% by weight, it may therefore be desirable
to employ a large pore volume gamma-alumina despite the greater
reactivity of the alumina. As the cobalt loading increases, the
disadvantages resulting from increased interaction between the
cobalt and the reactive alumina become less significant compared to
the advantages of the ability of obtaining high cobalt loadings by
using high pore volume gamma-aluminas.
[0020] The transition alumina powder generally has a
surface-weighted mean diameter D[3,2] in the range 1 to 200 .mu.m.
In certain applications such as for catalysts intended for use in
slurry reactions, it is advantageous to use very fine particles
which are, on average, preferably less than 20 82 m. eg. 10 .mu.m
or less. For other applications e.g. as a catalyst for reactions
carried out in a fluidised bed, it may be desirable to use larger
particle sizes, preferably in the range 50 to 150 .mu.m. The term
surface-weighted mean diameter D[3,2], otherwise termed the Sauter
mean diameter, is defined by M. Alderliesten in the paper "A
Nomenclature for Mean Particle Diameters"; Anal. Proc., vol 21, May
1984, pages 167-172, and is calculated from the particle size
analysis which may conveniently be effected by laser diffraction
for example using a Malvern Mastersizer.
[0021] It is preferred that the alumina powder has a relatively
large average pore diameter as the use of such aluminas appears to
give catalysts of particularly good selectivity. Preferred aluminas
have an average pore diameter of at least 10 nm, particularly in
the range 15 to 30 nm. [By the term average pore diameter we mean 4
times the pore volume as measured from the desorption branch of the
nitrogen physisorption isotherm at 0.98 relative pressure divided
by the BET surface area]. During the production of the compositions
of the invention, cobalt compounds are deposited in the pores of
the alumina, and so the average core diameter of the composition
will be less than that of the alumina employed, and decreases as
the proportion of cobalt increases. It is preferred that the
catalysts or precursors have an average pore diameter of at least 8
nm, preferably above 15 nm and particularly in the range 15 to 25
nm
[0022] It has been found that the bulk of the cobalt is
precipitated as cobalt compounds within the pores of the transition
alumina and only a small proportion of the cobalt is deposited as a
coating round the alumina particles As a result, irrespective of
the cobalt content of the composition, the particle size of the
compositions of the invention is essentially the same as the
particle size of the transition alumina, and so the compositions of
the invention generally have a surface-weighted mean diameter
D[3,2] in the range 1 to 200 .mu.m, in one embodiment preferably
less than 100 .mu.m and particularly less than 20 .mu.m, eg. 10
.mu.m or less, and in a second embodiment preferably in the range
50 to 150 .mu.m
[0023] On the other hand since the cobalt compounds are primarily
precipitated within the pores of the transition alumina, the pore
volume of the compositions in accordance with the invention will be
less than that of the transition alumina employed, and will tend to
decrease as the cobalt species loading increases Compositions
having a total cobalt content less than 30% by weight preferably
have a pore volume of at least 0.5 ml/g while compositions having a
total cobalt content above 30% by weight, particularly above 40% by
weight, preferably have a pore volume of at least 0 3 ml/g,
particularly at least 0.4 ml/g.
[0024] Accordingly the present invention also provides a
particulate cobalt/transition alumina catalyst, or precursor
thereto, having a total cobalt content of 5 to 85% by weight, a
pore volume above 0.5 ml/g, a cobalt surface area, after reduction,
of at least 25 m.sup.2 per gram of total cobalt, and a
surface-weighted mean diameter D[3,2] in the range 1 .mu.m to 200
.mu.m.
[0025] At high cobalt loadings, above 40% by weight, even if a
large pore volume transition alumina is employed, the amount of
cobalt species deposited within the pores may be such that the pore
volume of the compositions is less than 0 3 ml/g. However, useful
catalysts and precursors can be produced if the surface-weighted
mean diameter D[3,2] of the alumina, and hence the precursor and/or
catalyst, is relatively small, below about 20 .mu.m
[0026] Accordingly the present invention also provides a
particulate cobalt/transition alumina catalyst, or precursor
thereto, having a total cobalt content greater than 40% by weight,
a surface-weighted mean diameter D[3,2] below 20 .mu.m and having,
in the reduced state, a cobalt surface area greater than 25
m.sup.2/g of cobalt
[0027] The compositions preferably contain 41 to 85%, more
preferably 45 to 75%, by weight of cobalt.
[0028] The compositions of the invention, when in the reduced
state, have a cobalt surface area of at least 25 m.sup.2/g of
cobalt as measured by the H.sub.2 chemisorption technique described
herein Preferably the cobalt surface area is greater than 30, more
preferably at least 40, especially at least 60 m.sup.2/g. The
cobalt surface area tends to decrease as higher loadings of cobalt
are used, but we have found that when the composition contains 50
to 60% by weight total cobalt, the cobalt surface area achievable
is about 80 m.sup.2/g or more
[0029] The cobalt surface area is determined by H.sub.2
chemisorption. The sample (about 0.5 g) is degassed and dried under
vacuum at 120.degree. C. and then reduced by heating to 425.degree.
C. at a rate of 3.degree. C. per minute whilst hydrogen gas is
passed through the sample at a flow rate of 250 ml/min for 18
hours. The sample is then heated under vacuum to 450.degree. C.
over 10 minutes and maintained under those conditions for 2 hours
Following this pre-treatment, the chemisorption analysis is carried
out at 150.degree. C. using pure H.sub.2 gas The full isotherm is
measured up to 800 mm Hg pressure of H.sub.2 and the straight line
portion of the chemisorption isotherm between 300 and 800 mm Hg is
extrapolated to zero pressure to calculate the volume of the gas
(V) which is chemisorbed by the sample. The metal surface area is
then calculated from the following equation
Co surface area=(6
023.times.10.sup.23.times.V.times.SF.times.A)/22414
[0030] where
[0031] V=uptake of H.sub.2 in ml/g
[0032] SF=Stoichiometry factor (assumed 2 for H.sub.2 chemisorption
on Co)
[0033] A=area occupied by one atom of cobalt (assumed 0 0662
nm.sup.2)
[0034] This method of calculating cobalt surface area is described
in the Operators Manual for the Micromeritics ASAP 2000 Chemi
System V 1.00, Appendix C, (Part no 200-42808-01, 18.sup.th January
1991)
[0035] The compositions may be made by slurrying the transition
alumina powder with the appropriate amount of an aqueous solution
of a cobalt ammine complex, eg. the product of dissolving basic
cobalt carbonate in a solution of ammonium carbonate in aqueous
ammonium hydroxide, to give a product of the desired cobalt
content. The solution of the cobalt ammine complex preferably has a
pH in the range 7 to 12. The slurry is then heated, e.g. to a
temperature in the range 60 to 110.degree. C., to cause the cobalt
ammine complex to decompose with the evolution of ammonia and
carbon dioxide and to deposit an insoluble cobalt compound, e.g.
basic cobalt carbonate (cobalt hydroxycarbonate) on the surface,
and in the pores, of the transition alumina. The alumina carrying
the deposited cobalt compound is then filtered from the aqueous
medium and dried. The procedure may be repeated, ie. the dried
product may be re-slurried in a solution of the cobalt ammine
complex, heated, filtered and dried, if required to increase the
cobalt content of the product.
[0036] The time allocated for the precipitation of the cobalt
compound is normally about 30 to 200 minutes; the precipitation is
usually complete after about 60 to 80 minutes, but the heating of
the slurry may be prolonged to include an ageing step. During such
an ageing step, it is believed that some of the cobalt is converted
to cobalt aluminate compounds by reaction with the alumina support.
The cobalt aluminate compounds are beneficial in that they may
promote adhesion between the deposited cobalt compounds and the
alumina support and thereby stabilise the catalyst. However these
cobalt aluminate compounds are not catalytically active and may
reduce the available cobalt surface area somewhat. Therefore it is
necessary to select an appropriate process time to enable the
formation of some limited amount of cobalt aluminate to take place
without significantly reducing the available surface area. We have
found that when the cobalt content is relatively low, eg. up to
about 40% by weight, it is beneficial to use relatively short
process times, eg. by limiting the total heating time, ie. for both
the precipitation and any ageing to 200 minutes or less, preferably
less than 150 minutes As the cobalt content of the catalyst is
increased, the catalysts lose less of their surface area to cobalt
aluminate formation and longer process times may be used, eg. up to
about 350 minutes
[0037] Accordingly the present invention also provides a method of
making a cobalt/alumina catalyst or precursor thereto containing 5
to 40% by weight of total cobalt comprising slurrying a transition
alumina powder with an aqueous solution of a cobalt ammine complex,
heating the slurry for a period of not more than 200 minutes to
cause the cobalt ammine complex to decompose with the deposition of
an insoluble cobalt compound, filtering the solid residue from the
aqueous medium, drying and, optionally calcining, the solid
residue
[0038] For some applications it may be desired to incorporate
modifiers, such as other metals or compounds thereof, into the
catalyst or precursor. This may be effected by impregnating the
dried product with a solution of a compound of the desired modifier
that decomposes to the oxide or elemental form upon heating.
Examples of such modifiers include alkali metals, precious metals,
and transition metals such as rhenium.
[0039] If desired, the product may be calcined in air, eg at a
temperature in the range 200 to 600.degree. C., more preferably 200
to 450.degree. C., to decompose the deposited cobalt compound to
cobalt oxide. Upon reduction of the cobalt oxide, the high cobalt
surface area is generated. Alternatively the deposited cobalt
compound may be directly reduced, i.e. without the need for a
calcination step. The reduction whether or not a preliminary
calcination step is employed, may be effected by heating to a
temperature in the range from about 200.degree. C. to about
600.degree. C. in the presence of hydrogen. In the reduced material
it is preferred that at least 50% of the cobalt is present as
metallic cobalt
[0040] The composition may be supplied in its oxidic state, i.e.
without reducing the cobalt oxides to metallic cobalt. It may be
used as a catalyst in this state for eg. oxidation reactions or it
may be a precursor and reduced to an active catalyst by the
end-user. The composition may alternatively be supplied as a
reduced catalyst which has been passivated, so that the cobalt
metal is protected from deactivation during storage and
transportation.
[0041] Alternatively, in some cases the reduction may be effected
in situ. Thus a precursor comprising the transition alumina and the
unreduced cobalt compound, e.g. oxide, possibly dispersed in a
carrier, may be charged to a hydrogenation reactor with the
material to be hydrogenated and the mixture heated while hydrogen
is sparged through the mixture.
[0042] The catalysts may be used for hydrogenation reactions such
as the hydrogenation of olefinic compounds, e.g. waxes, nitro or
nitrile compounds e.g. the conversion of nitrobenzene to aniline or
the conversion of nitriles to amines. They may also be used for the
hydrogenation of paraffin waxes to remove traces of unsaturation
therein They may also be useful in a wide range of other reactions,
for example the Fischer-Tropsch process, ie where hydrogen and
carbon monoxide are reacted in the presence of the catalyst to form
higher hydrocarbons This may be part of an overall process for the
conversion of natural gas to petroleum compounds wherein the
hydrogen/carbon monoxide gas mixture is a synthesis gas formed by
steam reforming natural gas
[0043] The invention will be further described in the following
experimental examples.
EXAMPLE 1
[0044] A 4 liter aqueous stock solution was made up with 1918 g
ammonia solution (SG 0 89 30% ammonia), 198 g ammonium carbonate.
218 g basic cobalt carbonate and 1877 g demineralised water
[0045] The alumina employed was a transition-alumina of the gamma
alumina type having a surface area of about 145 m.sup.2/g and a
pore volume of about 0 85 ml/g and having a surface-weighted mean
diameter D[3,2] of 2 08 .mu.m, supplied by Sumitomo. The average
pore diameter was about 23 nm.
[0046] The alumina particles and a measured amount of the stock
solution were charged to a stirred vessel equipped with a
condenser. The pH of the aqueous solution was 11.1. The mixture was
heated to boiling while stirring and gentle boiling at about
96.degree. C. was maintained for a period of time, during which the
solution became clear after about 90 min. The total heating time is
shown in the following table. The solid was then filtered off,
washed and then dried in air at 120.degree. C. overnight.
[0047] The resultant catalyst precursor was then reduced by passing
hydrogen through a bed of the catalyst while heating to 430.degree.
C. The surface-weighted mean diameter of the reduced catalyst
particles was similar to that of the transition alumina
employed.
[0048] The cobalt content of the reduced catalyst was calculated
from the measured cobalt content of the unreduced material and the
weight difference between the unreduced material and the reduced
catalyst. The relative amount of alumina and stock solution was
varied to provide compositions having different cobalt contents.
The results are shown in Table 1
1 TABLE 1 Cobalt content (wt %) Cobalt surface area alumina/cobalt
End Total heating unreduced reduced m.sup.2 per gram m.sup.2 per
weight ratio pH time (min) product product reduced product gram
cobalt 1.75 8.56 180 27.2 32.1 27.1 84.4 8.50 210 28.7 33.7 25.9
76.8 8.78 240 28.7 33.3 22.1 66.3 8.55 270 27.9 32.1 19.7 61.4 8.32
300 28.6 32.8 17.7 53.9 8.20 330 29.6 33.6 13.5 40.2 1.40 8.5 180
30.3 36.4 34.9 95.9 8.38 240 32.2 39.1 34.4 87.9 7.2 360 32.5 37.4
15.9 42.5 1.00 8.27 240 37.0 47.3 39.1 82.6 8.12 300 36.7 47.1 36.9
78.4 7.01 360 36.9 44.9 22.8 50.7 0.50 8.90 180 41.6 55.2 41.9 75.9
8.31 240 41.0 54.8 41.7 76.1 7.99 300 46.8 62.3 38.6 61.9 0.35 8.34
180 52.4 70.6 29.0 41.1 8.24 240 53.3 72.8 30.7 42.2 8.36 300 54.6
75.4 33.6 44.6 0.25 8.4 180 56.8 78.5 30.5 38.9 7.9 240 56.5 78.5
23.4 29.8 7.45 300 55.7 77.3 35.7 46.2 0.15 8.13 180 60.6 86.9 21.0
24.2 7.99 240 61.1 87.0 19.8 22.7 7.26 300 61.5 85.9 22.4 26.1
[0049] It is clear from the examples that when the amount of cobalt
is relatively low, ageing may be harmful and shorter processing
times may be preferred in those cases
[0050] The BET surface areas and pore volumes of the unreduced
materials made using the alumina to cobalt weight ratio 1.75 was
determined and are shown in the following Table 2
2TABLE 2 Total heating time Cobalt content BET surface area Pore
volume (min) (wt %) (m.sup.2/g) (ml/g) 180 27.2 142 0.69 210 28.7
142 0.68 240 28.7 160 0.56 270 27.9 170 0.56 300 28.6 172 0.55 330
29.6 179 0.51
EXAMPLE 2
[0051] The activity for the Fischer-Tropsch reaction was assessed
using catalysts of the invention and compared with prior art
catalysts The materials tested were as follows.
[0052] Catalyst A
[0053] A sample of the unreduced material of Example 1 produced
with an alumina to cobalt weight ratio of 1.75 and a total heating
time of 270 min (unreduced cobalt content 27.9% by weight) was
formed into pellets which were then broken up and screened
[0054] Catalyst B
[0055] A sample of the unreduced material of Example 1 produced
with an alumina to cobalt weight ratio of 0 5 and a total heating
time of 240 min (unreduced cobalt content 41% by weight) was formed
into pellets which were then broken up and screened
[0056] Catalyst C1 (Comparative)
[0057] A precursor was made in accordance with the examples of U.S.
Pat. No. 5,874,381 by multi-stage impregnation of trilobal
extrudates of 1.2 mm length and 1 3 mm average diameter made from
gamma-alumina with a cobalt ammine carbonate complex solution
followed by heating to decompose the cobalt ammine carbonate. The
precursor had a cobalt content of 15.1% and upon reduction gave a
catalyst having a cobalt surface area of 79 m.sup.2/g total
cobalt
[0058] Catalyst C2 Comparative)
[0059] A precursor containing 16.7% by weight cobalt was made by
one-step impregnation of trilobal extrudates as used to make
Catalyst C1 with an aqueous cobalt nitrate solution followed by
drying at 120.degree. C. for 15 hours and calcination at
300.degree. C. for 10 hours. Upon reduction the catalyst had a
cobalt surface area of 28.1 m.sup.2/g total cobalt
[0060] Catalyst C3 (Comparative)
[0061] A precursor containing 17 8% by weight cobalt, 0 43%
ruthenium, and 1% lanthanum was made by was made by impregnating
trilobal extrudates as used to make Catalyst C1 with an aqueous
solution of cobalt nitrate, drying at 120.degree. C. for 15 h,
impregnating the dried impregnated extrudates with a solution of
ruthenium acetylacetonate and lanthanum nitrate in a mixture of 2
parts by volume acetone to 1 part by volume ethanol, removing the
organic solvent using a rotary evaporator under vacuum at
25.degree. C. and then calcining the product at 300.degree. C. for
10 h. Upon reduction the catalyst had a cobalt surface area of 43 9
m.sup.2/g total cobalt.
[0062] For Catalysis C 1, C2 and C3, before testing, the
impregnated trilobal extrudates were broken up and the particles
screened.
[0063] For all the following activity tests, particles having a
size in the range 0.25 to 0.42 .mu.m were selected for testing.
[0064] The catalysts were tested using an isothermal reactor of
internal diameter 7 5 mm with a catalyst bed length of 8 cm. The
temperature of the catalyst bed was controlled by external heating
responsive to a thermocouple disposed in the middle of the catalyst
bed 4 cm from the bed inlet.
[0065] 1 g of the precursor particles were mixed with silicon
carbide particles having a similar size as diluent. The volume of
silicon carbide particles used was 2.5 times the volume of the
precursor particles. The mixture was charged to the reactor to form
the catalyst bed and then the precursor was reduced by passing a
stream of hydrogen at atmospheric pressure through the reactor at a
rate of 24 liters (at NTP) per hour while increasing the
temperature from ambient to 120.degree. C. maintaining it at that
temperature of 1 hour, then increasing the temperature to
300.degree. C. at a rate of 100 .degree. C./h and maintaining it at
that temperature for 4 hours. For Catalysts A and B, after the 1
hour at 120.degree. C., the temperature was increased at a rate of
180.degree. C./h to 460.degree. C. (instead of 300.degree. C.), and
maintained at that temperature for 2 hours. [The higher reduction
temperature was used for Catalysts A and B since it is believed
that these catalysts have smaller cobalt-containing crystallites
and these are more difficult to reduce than larger crystallites. It
is believed that reducing the comparative catalysts C1, C2 and C3
at higher temperatures would not show any advantage therefor]
[0066] After reduction, the temperature was decreased to
220.degree. C. and the pressure increased to 20 bar abs. Carbon
monoxide and argon (as an internal analysis standard) were then
incrementally added to the hydrogen and the flow rate adjusted
until the feed gas volume composition was 60% hydrogen, 30% carbon
monoxide and 10% argon and the total flow rate was 14.6
liters/h.
[0067] The conditions were maintained and the gas mixture leaving
the reactor was continuously analysed.
[0068] When steady state conditions had been achieved. Catalysts
C1, C2 and C3 gave carbon monoxide conversions of about 10, 18, and
28% respectively with the product distribution shown in the
following Table 3. However the catalysts of the invention,
Catalysts A and B, gave complete conversion of the carbon monoxide
to methane showing that while the catalysts were a good methanation
catalysts, they were too active for use under those conditions as
Fischer-Tropsch catalysts. Further investigation revealed that,
with Catalyst A, although the temperature of the centre of the
catalyst bed was controlled at 220.degree. C., the initial portion
of the bed had been heated by the exothermic reaction to a
significantly higher temperature, namely about 300.degree. C.
[0069] In the following Table also quoted is the chain growth
probability, .alpha., which is obtained from the equation
W.sub.n/n=(1-.alpha.).sup.2 .alpha..sup.n-1 where W.sub.n is the
weight fraction of products containing n carbon atoms
3 TABLE 3 Catalyst C1 C2 C3 Precursor cobalt content (% wt) 15.1
16.7 17.8 Cobalt surface area (m.sup.2/g cobalt) 79 28 44 Average
CO conversion (%) 10 18 28 Carbon products distribution carbon
dioxide (wt %) 5 3 4 alcohols (wt %) 11 4 5 methane (wt %) 16 13 20
C.sub.2 to C.sub.4 hydrocarbons (wt %) 6 10 15 C.sub.5 to C.sub.12
hydrocarbons (wt %) 44 45 47 C.sub.13 to C.sub.15 hydrocarbons (wt
%) 15 14 6 C.sub.19+ hydrocarbons (wt %) 3 11 3 chain growth
probability, .alpha. 0.77 0.82 0.76
[0070] It is seen that despite its high cobalt surface area,
Catalyst C1, made in accordance with U.S. Pat. No. 5,874,381, has a
poor activity, giving a much lower carbon monoxide conversion than
either Catalysts C2 and C3.
[0071] However, when tested by the same procedure but at a lower
gas flow rate (about 5 liters/h), Catalyst C1 gave a carbon
monoxide conversion of about 25% and a product distribution similar
to that specified in Table 3 for Catalyst C2, but giving a slightly
higher proportion of the higher hydrocarbons (C.sub.13+) at the
expense of the C.sub.5 to C.sub.12 hydrocarbons.
[0072] In order to overcome the apparent overheating of Catalyst A,
the test procedure was repeated using Catalysts A, B and C3 but
moving the thermocouple used to control the bed temperature to a
position 1.5 cm from the bed inlet. Also, after reduction, the
temperature was decreased to 190.degree. C., instead of 220.degree.
C., and the pressure increased to 20 bar abs. Carbon monoxide and
argon (as an internal analysis standard) were then incrementally
added to the hydrogen and the flow rate adjusted until the feed gas
volume composition was 60% hydrogen, 30% carbon monoxide and 10%
argon Then the temperature was increased at a rate of 2.degree.
C./min to the test temperature of 220.degree. C. The total flow
rate employed, the productivity defined as the weight of
hydrocarbons containing 11 or more carbon atoms produced per gram
of catalyst precursor per hour, and the product distribution, are
set out in the following Table 4
4 TABLE 4 Catalyst A B C3 Precursor cobalt content (% wt) 27.9 41
17.8 Cobalt surface area (m.sup.2/g cobalt) 61 81 44 Space velocity
(Nl/g precursor/h) 16 46 67 90 29 61 15 Average CO conversion (%)
59 23 16 14 78 20 22 C.sub.11+ productivity (g/g precursor/h) 1.2
1.2 0.7 0.9 2.9 1.6 -- Carbon products distribution carbon dioxide
(wt %) 7 2 2 2 20 2 2 Alcohols (wt %) 2 3 3 3 2 7 6 Methane (wt %)
20 22 12 10 30 23 20 C.sub.2 to C.sub.4 hydrocarbons (wt %) 12 12 7
6 12 11 16 C.sub.5 to C.sub.12 hydrocarbons (wt %) 33 42 31 29 26
33 40 C.sub.13 to C.sub.18 hydrocarbons (wt %) 13 11 16 15 6 11 9
C.sub.19+ hydrocarbons (wt %) 13 8 29 35 4 13 7 .alpha. - chain
growth probability 0.84 0.81 0.89 0.92 0.78 0.83 0.79
[0073] It is seen that Catalysts A and B in accordance with the
invention are much more active than Catalyst C3 and gave useful
product distributions. Also Catalyst B is more active than Catalyst
A.
* * * * *