Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A ~ROCESS FOR THE PREPARATION
OF FISCHER-TROPSCH
CATALYSTS
The invention relates to a process for the preparation
of Fischer-Tropsch catalysts.
The preparation of hydrocarbons from a mixture of car
bon monoxide and hydrogen by contacting this mixture at
elevated temperature and pressure with a catalyst is re-
ferred to in the literature as the hydrocarbon synthesis
according to Fischer-Tropsch. Catalysts which are often
used for this purpose contain one or more metals from the
iron group together with one or more promoters and sometimes
a carrier material. The preparation of the Fischer-Tropsch
catalysts can, in principle be carried out in three ways,
viz. by precipitation, by melting or by impregnation. The
preparation of the catalysts by precipitation briefly con-
sists in an aqueous solution of a salt of a metal from the
iron group, to which, if desired, a salt of a promoter and
a carrier material may have been added, being rendered
alkaline, resulting in the precipitation of the catalyst.
One or more promoters and a carrier material may be added
to this precipitate. The preparation of the catalysts by
melting is effected, for instance, for iron catalysts by
fusing iron oxide with one or more promoter oxides. Both
tne precipitation route and the melting route are not very
attractive procedures for the preparation of the Fischer-
Tropsch catalysts, since their reproducibility is small.
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ll~O9V9
The precipitation route has the additional disadvantage
that it takes up a great deal of time, whilst the melting
route requires much energy. Moreover, the catalytic pro-
perties of the catalysts prepared by melting and by preci-
5 pitation, particularly the activity and stability are un-
satisfactory. A much more attractive procedure for the pre-
paration of the Fischer-Tropsch catalysts is the impregna-
tion route. It is easy to carry out, gives well reproducible
results and, as a rule, leads to catalysts with high activi-
ty and stability. The impregnation route briefly consistsin a porous carrier being impregnated with one or more
aqueous solutions of salts of one or more metals from the
iron group and of one or more promoters, followed by
drying, calcining and reducing of the composite. Many
elements, such as alkali metals, alkaline-earth metals,
metals from group ~I B, Ti, Zr, Th, V, Mn and Cu, are
eligible promoters for the catalysts prepared by impregna-
tion. As the carrier materials for the catalysts prepared
by impregnation amorphous as well as crystalline materials,
may be used. Suitable carriers are, inter alia, silica,
alumina, zirconia, thoria, boria and combinations thereof,
such as silica-alumina and silica-magnesia and furtAer
zeolites, such as mordenite, faujasite and zeolite-omega.
The Applicant has carried out an extensive investiga-
tion concerning the preparation of hydrocarbons from H2~C0
mixtures with an ~2~C0 molar ratio smaller than 1.0, using
Fischer-Tropsch catalysts prepared by impregnation. In the
above-mentioned conversion the behaviour of these catalysts
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was found to be greatly dependent on the ~ollowing factors:
1. the nature o~ the metal from the iron group and the
load used,
2. the nature of the promoter and the load used,
5 3. the nature o~ the carrier, and
4. the temperature treatment used.
Catalysts prepared by impregnation were found to have
- a very high activity and a very high stability for the con-
Yersion of H2/C0 mixtures with an H2~C0 molar ratio smaller
than 1.0, if they contain 10-40 pbw iron and 0.25-10 pbw
chromium per 100 pbw silica and have been reduced at a tem-
perature of 350-750C. These are novel catalysts.
The present patent application therefore relates to a
process for the preparation of novel catalysts, in which
catalysts containing 10-40 pbw iron and 0.25-10 pbw chromium
per 100 pbw silica are prepared by impregnating a silica
carrier with one or more aqueous solutions of salts of iron
and of chromium, followed by drying the composite, calcining
it and reducing it at a temperature of 350-750C. The
patent application further relates to the use of these
catalysts for the preparation of hydrocarbons with an H2~C0
mixture with an H2tC0 molar ratio smaller than 1.0 as the
starting material.
~hen for the above-mentioned application use is made
of catalysts pxepared according to the invention, preferred
catalysts are those cor.taining 20-35 pb~ iron and 0.5-5 pbw
chromium per 100 pbw silica. Further, preference is given
to catalysts which contain, in addition to iron and chromium
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a selectivity promoter. Suitable selectivity promoters are
the alkali metals, in particular potassium. In addition to
iron and chromium it is preferred in the catalyst prepara-
tion according to the invention to incorporate 1-5 pbw
5 potassium per lOQ pbw silica into the catalyst by impregna-
tion.
In the preparation of the catalysts the metal salts can
be deposited on the carrier in one or more steps. Between
the impregnation steps the material is dried and optionally,
10 calcined. Impregnation in more than one step may be necessa-
ry for the preparation of catalysts with a high metal load.
The metal salts may be deposited on the carrier separately
or together from one solution. An attractive method of de-
positing the metal salts on the carrier is the dry impreg-
15 nation technique, according to which a carrier is contactedwith an aqueous solution of the salts concerned, which
aqueous solution has a volume which is substantially the
same as the pore volume of the carrier. Sorption of the
aqueous solution by the carrier can be facilitated by
heating the mixture. If this method is chosen for the
prepration of catalysts with a high metal load, it may be
necessary to carry out more than one dry impregnation and
to dry the material between the separate impregnation steps
and, optionally, to calcine it. The calcination is prefer-
ably carried out at a temperature of 350-700C. The catal~st
preparation is finished with a reduction. This reduction is
carried out at a temperature of 350-700C with a hydrogenous
gas, e.g. a mixture of hydrogen and nitrogen. The reduction
is preferably carried out at a temperature of 350-500C.
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The catalysts prepared according to the invention are
pre-eminently suitable for the preparation of hydrocarbons
from an H2/C0 mixture with an H2/C0 molar ratio smaller than
l.Q. Such H2~CO mixtures can very suitably be prepared by
steam gasification of a carbon-containing material. Examples
of such materials are brown coal, anthracite, coke, crude
mineral oil and fractions thereof and oils produced from tar
sànd and bituminous shale. The steam gasification is prefer-
ably carried out at a temperature of from 900 to 1500C and
a pressure of from 10 to 50 bar.
The preparation of hydrocarbons from an H2~C0 mixture
with an H2~C0 molar ratio smaller than 1.0, using a Fischer-
Tropsch catalyst according to the invention is preferably
carried out at a temperature of from 200 to 350C and in
partilcular of from 250 to 350c, a pressure of from 10 to 70
bar a~nd in particular of from 20 to 50 bar and a space
velocity of from 500 to 5000 and in particular of from 500
to 2500 Nl gas/1 catalyst/h. The hydrocarbon preparation
according to the invention can very suitably be carried out
by conducting the feed in upward or downward direction
through a vertically mounted reactor in which a fixed or a
moving bed of the catalyst concerned is present.
The invention will now be explained with reference to
the following example.
Example
Six catal~sts (A-C and 1-3~ were prepared and tested
for the hydrocarbon synthesis according to Fischer-Tropsch.
The preparation of t~.e catalysts was effected b~ impregnating
114U~09
a silica or alumina carrier with aqueous solutions contain-
îng one or more of the following salts: iron nitrate, chro-
mium nitrate and potassium nitrate. In all impregnations
the dry impregnation technique was used. The reduction of
the catalysts was carried out at atmospheric pressure with
an H2tN2 mixture in a volume ratio of 3:1 at a superficial
gas rate of 1.6 m~s. Further details about the preparation
of the individual catalysts are given below.
Catalyst A
This catalyst was prepared by impregnating a silica
carrier first with a solution of KNO3, followed by drying
at 120C and calcining for two hours at 400C, and then
impregnating with a solution of Fe(N03)3 and Cr(N03)3,
followed by drying at 120C, calcining for two hours at
500C and reduction at 280C.
Catalyst 1
The preparation of this catalyst was performed in sub-
stantially the same way as the preparation of catalyst A,
the difference being that the reduction was carried out at
400C.
Catalyst B
The preparation of this catalyst was performed in sub-
stantially the same way as the preparation of catalyst A,
the differences being that alumina was used as the carrier
and that the reduction was carried out at 400C.
Catalyst C
The preparation of this catalyst was performed in sub-
stantially the same way as the preparation of catalyst
91)9
A, the differences be,ing that in the second impregnation a
solution was used which did not contain chromium and that
the reduction was carried out at 400C.
Catalyst 2
This catalyst was prepared by impregnating a silica
carrier with a solution of Fe(NO3~3, Cr(NO3~3 and KN03,
followed by drying at 120C, calcining for two hours at
500C and reduction at 40oc.
Catalyst 3
The preparation of this catalyst was performed in sub-
stantially the same way as the preparation of catalyst A,
the differences being that in the first impregnation a
solution with a higher concentration of K was used, that in
the second impregnation a solution with higher concentrations
of Fe and Cr was used, and that the reduction was carried
out at 400C. The composition of the catalysts is shown in
Table A.
Table A
Cat. No. Composition expressed in pbw,
Fe Cr K SiO2 2 3
A 25 1 2 100 ---
B 25 1 2 --- 100
C 25 - 2 100 ---
1 25 1 2 100 ---
2 25 1 2 100 ---
3 35 1.5 2.75 100 ---
~.,
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The testing of the catalysts A-C and 1-3 ~or the
hydrocarban synthesis according to Fischer-Tro~sch with
synthesis gas with an H2tC0 molar ratio of 0.5 as the
starting material was performed in a 250-ml reactor which
contained a catalyst bed with a volume of 50 ml. The experi-
ments were carried out at a temperature of 280C, a pressure
of 30 bar and a space velocity of 1000 Nl.l l.h 1.
The results of these experiments are shown in Table B.
Table B
Exp. No. Cat. No. Conversion of the synthesis gas,
a~ter 25 h after 500 h
1 A 79
2 B 72
3 C . 69
4 1 91 90
2 90 89
6 3 91 89
After run hour 500 experiment 4 was continued for
another 500 hours with catalyst 1. By gradually increasing
the reaction temperature the conversion of the synthesis
gas was maintained at 90~. At run hour 1000 the temperature
was 290C.
Of the experiments mentioned in Table B only experi-
ments 4-6 were carried out with catalysts prepared according
to the invention. In these experiments the catalysts showed
both.a very high activity and a very high stability. The
e~periments 1-3 were carried out with catalysts which
114V909
outside the scope o~ the inYentiOn. They have been included
in the patent application for comparison.
Catalyst A had been reduced at too low a temperature.
Catalyst B did not contain silica. Catalyst C did not
contain chromium. The results of experiments 1-3 show that
these catalysts had a low activity.
