U.S. patent number RE37,406 [Application Number 09/133,422] was granted by the patent office on 2001-10-09 for surface supported cobalt catalysts, process utilizing these catalysts for the preparation of hydrocarbons from synthesis gas and process for the preparation of said catalysts.
This patent grant is currently assigned to Exxon Research and Engineering Co.. Invention is credited to Kym B. Arcuri, William C. Behrmann, Charles H. Mauldin.
United States Patent |
RE37,406 |
Behrmann , et al. |
October 9, 2001 |
**Please see images for:
( Certificate of Correction ) ** |
Surface supported cobalt catalysts, process utilizing these
catalysts for the preparation of hydrocarbons from synthesis gas
and process for the preparation of said catalysts
Abstract
A supported particulate cobalt catalyst is formed by dispersing
cobalt, alone or with a metal promoter, particularly rhenium, as a
thin catalytically active film upon a particulate support,
especially a silica or titania support. This catalyst can be used
to convert an admixture of carbon monoxide and hydrogen to a
distillate fuel constituted principally of an admixture of linear
paraffins and olefins, particularly a C.sub.10+ distillate, at high
productivity, with low methane selectivity. A process is also
disclosed for the preparation of these catalysts.
Inventors: |
Behrmann; William C. (Baton
Rouge, LA), Arcuri; Kym B. (Baton Rouge, LA), Mauldin;
Charles H. (Baton Rouge, LA) |
Assignee: |
Exxon Research and Engineering
Co. (Florham Park, NJ)
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Family
ID: |
27567808 |
Appl.
No.: |
09/133,422 |
Filed: |
August 13, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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243436 |
May 13, 1994 |
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032916 |
Mar 18, 1993 |
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881935 |
May 11, 1992 |
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667993 |
Mar 12, 1991 |
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310258 |
Feb 13, 1989 |
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072517 |
Jul 13, 1987 |
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046649 |
May 7, 1987 |
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Reissue of: |
377293 |
Jan 24, 1995 |
05545674 |
Aug 13, 1996 |
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Current U.S.
Class: |
518/715 |
Current CPC
Class: |
B01J
23/75 (20130101); B01J 23/83 (20130101); B01J
23/8896 (20130101); B01J 23/8913 (20130101); B01J
37/0221 (20130101); B01J 37/0232 (20130101); C07C
1/0435 (20130101); C07C 1/0445 (20130101); B01J
35/008 (20130101); C07C 2521/06 (20130101); C07C
2523/10 (20130101); C07C 2523/12 (20130101); C07C
2523/36 (20130101); C07C 2523/46 (20130101); C07C
2523/75 (20130101); C07C 2523/83 (20130101); C07C
2523/889 (20130101) |
Current International
Class: |
B01J
23/75 (20060101); B01J 23/83 (20060101); B01J
23/889 (20060101); B01J 23/76 (20060101); B01J
23/89 (20060101); B01J 37/02 (20060101); B01J
37/00 (20060101); C07C 1/04 (20060101); C07C
1/00 (20060101); C07C 027/06 () |
Field of
Search: |
;518/715 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Everson, et al., "Fischer-Tropsch Reaction Studies with Supported
Ruthenium Catalysts I. Product Distributions at Moderate Pressures
and Catalyst Deactivation," Journal of Catalysis 53, 186-197
(1978). .
Dixit, et al., "Kinetics of the Fischer-Tropsch Synthesis,"
Industrial Engineering Chemical Process Des. Dev. 1983, 22, pp.
1-9..
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Primary Examiner: Stockton; Laura L.
Attorney, Agent or Firm: Simon; Jay
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is .Iadd.RE of U.S. application Ser. No.
08/377,293 filed Jan. 24, 1995 abn., which is .Iaddend.a
continuation of U.S. application Ser. No. 08/243,436 filed May 13,
1994, .Iadd.which is a continuation of U.S. application Ser. No.
08/032,916 Mar. 18, 1993 ABN.Iaddend., which is a
continuation-in-part of U.S. Ser. No. 881,935, filed May 11, 1992,
which was a Rule 60 Continuation of U.S. Ser. No. 667,993, filed
Mar. 12, 1991, which was a continuation-in-part of U.S. Ser. No.
310,258, filed Feb. 13, 1989, which was a continuation-in-part of
U.S. Ser. No. 072,517, filed Jul. 13, 1987, which was a
continuation-in-part of U.S. Ser. No. 046,649, filed May 7, 1987
all abandoned.
Claims
What is claimed is:
1. A process useful for the conversion of synthesis gas to liquid
hydrocarbons and less than about 10 mole % methane which comprises
contacting at reaction conditions a feed comprised of carbon
monoxide and hydrogen, in H.sub.2 :CO molar ratio equal to or
greater than about 0.5:1 at total pressure equal to or greater than
about 80 psig, over a catalyst composition having a productivity of
at least 150 hr.sup.-1 at 200.degree. C. and which comprises cobalt
dispersed as a catalytically active layer upon the outer surface of
an inorganic oxide support of a thickness of less than about 200
microns, with the loading of cobalt at least about 0.04 g/cc in
said catalytically active layer, calculated as metallic cobalt per
packed bulk volume of catalyst.
2. The process of claim 1 wherein the molar ratio of H.sub.2 :CO
ranges from about 1.7:1 to about 2.5:1.
3. The process of claim 2 wherein the total pressure of the
reaction ranges from about 140 psig to about 400 psig.
4. The process of claim 1 wherein the reaction conditions are
defined within ranges as follows:
5. The process of claim 1 wherein the catalytically active surface
layer of the catalyst is of average thickness ranging from about 5
microns to about 200 microns with the cobalt loading ranging being
at least about 0.04 g/cc in said catalytically active surface
layer.
6. The process of claim 1 wherein the catalyst further comprises
rhenium rhenium constitutes part of the catalytically active
surface layer of the catalyst.
7. The process of claim 1 wherein the catalyst further comprises
hafnium which constitutes part of the catalytically active surface
layer of the catalyst.
8. The process of claim 1 wherein the support is comprised of
silica or titania.
9. A process for the conversion of synthesis gas to C.sub.10+
hydrocarbons which comprises contacting at reaction conditions a
feed comprised of an admixture of carbon monoxide and hydrogen, in
H.sub.2 :CO molar ratio equal to or greater than about 1.71 at
total pressure equal to or greater than about 80 psig, over a
catalyst composition which comprises cobalt dispersed as a
catalytically active layer upon the outer surface of a silica or
titania containing support, said active layer being of a thickness
of less than 200 microns, and with sufficient cobalt loading to
produce a productivity of at least about 150 hr.sup.-1 at
200.degree. C. and convert to methane less than 10 mole percent of
the carbon monoxide converted.
10. The process of claim 9 wherein the support is comprised
predominantly of silica.
11. The process of claim 1 wherein said process is a slurry-bed
synthesis gas conversion process and said catalyst having a
particle size diameter of about 10 microns to about 1 mm..Iadd.
12. The process of claim 1 wherein said liquid hydrocarbon
comprises a high quality distillate fuel..Iaddend..Iadd.
13. The process of claim 1 wherein said liquid hydrocarbon
comprises C.sub.10+ hydrocarbons..Iaddend..Iadd.
14. The process of claim 13 wherein said C.sub.10+ hydrocarbons
comprise C.sub.10+ linear paraffins..Iaddend..Iadd.
15. The process of claim 13 comprising the further step of
producing a middle distillate fuel from said C.sub.10+
hydrocarbons..Iaddend..Iadd.
16. The process of claim 15 wherein said middle distillate fuel
comprises a diesel fuel..Iaddend..Iadd.
17. The process of claim 15 wherein said middle distillate fuel
comprises a C.sub.10 -C.sub.20 product..Iaddend..Iadd.
18. The process of claim 15 wherein said middle distillate fuel
comprises a jet fuel..Iaddend..Iadd.
19. The process of claim 13 including upgrading said C.sub.10+
hydrocarbons..Iaddend..Iadd.
20. The process of claim 19 including upgrading said C.sub.10+
hydrocarbons to a diesel fuel..Iaddend..Iadd.
21. The process of claim 19 including upgrading said C.sub.10+
hydrocarbons to a jet fuel..Iaddend..Iadd.
22. The process of claim 9 wherein said C.sub.10+ hydrocarbons
comprise C.sub.10+ linear paraffins..Iaddend..Iadd.
23. The process of claim 9 comprising the further step of producing
a middle distillate fuel from said C.sub.10+
hydrocarbons..Iaddend..Iadd.
24. The process of claim 23 wherein said middle distillate fuel
comprises a diesel fuel..Iaddend..Iadd.
25. The process of claim 23 wherein said middle distillate fuel
comprises a jet fuel..Iaddend..Iadd.
26. The process of claim 23 wherein said middle distillate fuel
comprises a C.sub.10 -C.sub.20 product..Iaddend..Iadd.
27. The process of claim 9 including upgrading said C.sub.10+
hydrocarbons..Iaddend..Iadd.
28. The process of claim 27 including upgrading said C.sub.10+
hydrocarbons to a diesel fuel..Iaddend..Iadd.
29. The process of claim 27 including upgrading said C.sub.10+
hydrocarbons to a jet fuel..Iaddend.
Description
BACKGROUND AND PROBLEMS
1. Field of the Invention
This invention relates to catalyst compositions, process wherein
these compositions are used for the preparation of liquid
hydrocarbons from synthesis gas, and process for the preparation of
said catalysts. In particular, it relates to catalysts, and process
wherein C.sub.10+ distillate fuels, and other valuable products,
are prepared by reaction of carbon monoxide and hydrogen over
cobalt catalysts wherein the metal is dispersed as a thin film on
the outside surface of a particulate carrier, or support,
especially a titania carrier, or support.
2. The Prior Art
Particulate catalysts, as is well known, are normally formed by
dispersing catalytically active metals, or the compounds thereof
upon carriers, or supports. Generally, in making catalysts the
objective is to disperse the catalytically active material as
uniformly as possible throughout a particulate porous support, this
providing a uniformity of catalytically active sites from the
center of a particle outwardly.
Catalysts have also been formed by dispersing catalytically active
materials upon dense support particles; particles impervious to
penetration by the catalytically active materials. Ceramic or metal
cores have been selected to provide better heat transfer
characteristics, albeit generally the impervious dense cores of the
catalyst particles overconcentrates the catalytically active sites
within a reduced reactor space and lessens the effectiveness of the
catalyst. Sometimes, even in forming catalysts from porous support
particles greater amounts of the catalytic materials are
concentrated near the surface of the particles simply because of
the inherent difficulty of obtaining more uniform dispersions of
the catalytic materials throughout the porous support particles.
For example, a catalytic component may have such strong affinity
for the support surface that it tends to attach to the most
immediately accessible surface and cannot be easily displaced and
transported to a more central location within the particle.
Catalyst dispersion aids, or agents are for this reason often used
to overcome this effect and obtain better and more uniform
dispersion of the catalytically active material throughout the
catalyst particles.
Fischer-Tropsch synthesis for the production of hydrocarbons from
carbon monoxide and hydrogen is now well known, and described in
the technical and patent literature. The earlier Fischer-Tropsch
catalysts were constituted for the most part of non-noble metals
dispersed throughout a porous inorganic oxide support. The Group
VIII non-noble metals, iron, cobalt, and nickel have been widely
used in Fischer-Tropsch reactions, and these metals have been
promoted with various other metals, and supported in various ways
on various substrates, principally alumina. Most commercial
experience, however, has been based on cobalt and iron catalysts.
The first commercial Fischer-Tropsch operation utilized a cobalt
catalyst, though later more active iron catalysts were also
commercialized. The cobalt and iron catalysts were formed by
compositing the metal throughout an inorganic oxide support. An
important advance in Fischer-Tropsch catalysts occurred with the
use of nickel-thoria on kieselguhr in the early thirties. This
catalyst was followed within a year by the corresponding cobalt
catalyst, 100 Co:18 ThO.sub.2 :100 kieselguhr, parts by weight, and
over the next few years by catalysts constituted of 100 Co:18
ThO.sub.2 :200 kieselguhr and 100 Co:5 ThO.sub.2 :8 MgO:200
kieselguhr, respectively. These early cobalt catalysts, however,
are of generally low activity necessitating a multiple staged
process, as well as low synthesis gas throughput. The iron
catalysts, on the other hand, are not really suitable for natural
gas conversion due to the high degree of water gas shift activity
possessed by iron catalysts. Thus, more of the synthesis gas is
converted to carbon dioxide in accordance with the equation:H.sub.2
+2CO.fwdarw.(CH.sub.2).sub.x +CO.sub.2 ; with too little of the
synthesis gas being converted to hydrocarbons and water as in the
more desirable reaction, represented by the equation: 2H.sub.2
+CO.fwdarw.(CH.sub.2).sub.x +H.sub.2 O.
U.S. Pat. No. 4,542,122 by Payne et al, which issued Sep. 17, 1985,
describes improved cobalt catalyst compositions useful for the
preparation of liquid hydrocarbons from synthesis gas. These
catalyst compositions are characterized, in particular, as
cobalt-titania or thoria promoted cobalt-titania, wherein cobalt,
or cobalt and thoria, is composited or dispersed upon titania, or
titania-containing support, especially a high rutile content
titania. U.S. Pat. No. 4,568,663 by Mauldin, which issued Feb. 4,
1986, also discloses cobalt-titania catalysts to which rhenium is
added to improve catalyst activity, and regeneration stability.
These catalysts have performed admirably well in conducting
Fischer-Tropsch reactions, and in contrast to earlier cobalt
catalysts provide high liquid hydrocarbon selectivities, with
relatively low methane formation.
Recent European Publication 1 178 008 (base on Application No.
85201546.0, filed: Sep. 25, 1985) and European Publication 0 174
696 (based on Application No. 852011412.5, filed: May 5, 1985),
having priority dates Apr. 4, 1984 NL 8403021 and 13.09.84 NL
8402807, respectively, also disclose cobalt catalysts as well as a
process for the preparation of such catalysts by immersion of a
porous carrier once or repetitively within a solution containing a
cobalt compound. The cobalt is dispersed over the porous carrier to
satisfy the relation .SIGMA.V.sub.p /.SIGMA.V.sub.c.ltoreq.0.85 and
.SIGMA.V.sub.p /.SIGMA.V.sub.c.ltoreq.0.55, respectively, where
.SIGMA.V.sub.c represents the total volume of the catalyst
particles and .SIGMA.V.sub.p the peel volumes present in the
catalyst particles, the catalyst particles being regarded as
constituted of a kernel surrounded by a peel. The kernal is further
defined as one of such shape that at every point of the kernal
perimeter the shortest distance (d) to the perimeter of the peel is
the same, d being equal for all particles under consideration, and
having been chosen such that the quantity of cobalt present in
.SIGMA.V.sub.p is 90% of the quantity of cobalt present in
.SIGMA.V.sub.c. These particular catalysts, it is disclosed, show
higher C.sub.5+ selectivities than catalysts otherwise similar
except that the cobalt component thereof is homogeneously
distributed, or uniformly dispersed, throughout the carrier.
Suitable porous carriers are disclosed as silica, alumina, or
silica-alumina, and of these silica is preferred. Zirconium,
titanium, chromium and ruthenium are disclosed as preferred of a
broader group of promoters. Albeit these catalysts may provide
better selectivities in synthesis gas conversion reactions
vis-a-vis catalysts otherwise similar except the cobalt is
uniformly dispersed throughout the carrier, like other cobalt
catalysts disclosed in the prior art, the intrinsic activities of
these catalysts are too low as a consequence of which higher
temperatures are required to obtain a productivity which is
desirable for commercial operations. Higher temperature operation,
however, leads to a corresponding increase in the methane
selectivity and a decrease in the production of the more valuable
liquid hydrocarbons.
Productivity, which is defined as the standard volumes of carbon
monoxide converted/volume catalyst/hour, is, of course, the life
blood of a commercial operation. High productivities are essential
in achieving commercially viable operations. However, it is also
essential the high productivity be achieved without high methane
formation, for methane production results in lower production of
liquid hydrocarbons. Accordingly, an important and necessary
objective in the production and development of catalysts is to
produce catalysts which are capable of high productivity, combined
with low methane selectivity.
Despite improvements, there nonetheless remains a need for
catalysts capable of increased productivity, without increased
methane selectivity. There is, in particular, a need to provide
further improved catalysts, and process for the use of these
catalysts in synthesis gas conversion reactions, to provided
further increased liquid hydrocarbon selectivity, especially
C.sub.10+ liquid hydrocarbon selectivity, with further reduced
methane formation.
3. Objects
It is, accordingly, the primary objective of this invention to fill
this and other needs.
It is, in particular, an object of this invention to provide
further improved, novel supported cobalt catalyst compositions, and
process utilizing such compositions for the conversion of synthesis
gas at high productivity, and low methane selectivity, to high
quality distillate fuels characterized generally as C.sub.10+
linear paraffins and olefins.
A further and more particular object is to provide novel, supported
cobalt catalyst compositions, both promoted and unpromoted which
approach, or meet the activity, selectivity and productivity of
powdered catalysts but yet are of a size acceptable for commercial
synthesis gas conversion operation.
A further object is to provide a process utilizing such catalyst
compositions for the production from synthesis gas to C.sub.10+
linear paraffins and olefins, at high productivity with decreased
methane selectivity.
Yet another object i s to provide a process for the preparation of
such catalysts.
4. The Invention
These objects and others are achieved in accordance with this
invention embodying a supported particulate cobalt catalyst formed
by dispersing the cobalt as a thin catalytically active film upon
the surface of a particulate support, preferably silica or titania
or titania-containing support, especially one wherein the
rutile:anatase ratio of a titania support is at least about 3:2.
This catalyst can be used to produce, by contact and reaction at
reaction conditions with an admixture of carbon monoxide and
hydrogen, a distillate fuel constituted principally of an admixture
of linear paraffins and olefins, particularly a C.sub.10+
distillate, preferably C.sub.20+, at high productivity, with low
methane selectivity. This product can be further refined and
upgraded to high quality fuels, and other products such as mogas,
diesel fuel and jet fuel, especially premium middle distillate
fuels of carbon numbers ranging from about C.sub.10 to about
C.sub.20.
In accordance with this invention the catalytically active cobalt
component is dispersed and supported upon a particulate refractory
inorganic oxide carrier, or support as a thin catalytically active
surface layer, ranging in thickness from less than about 200
microns, preferably about 5-200 microns, with the loading of the
cobalt, expressed as the weight metallic cobalt per packed bulk
volume of catalyst, being sufficient to achieve the productivity
required for viable commercial operations, e.g., a productivity in
excess of about 150 hr.sup.-1 at 200.degree. C. The cobalt loading
that achieves this result is at least about 0.04 grams (g) per
cubic centimeter (cc), preferably at least about 0.05 g/cc in the
rim also referred to as the thin catalytically active surface layer
or film. Higher levels of cobalt tend to increase the productivity
further and an upper limit of cobalt loading is a function of
cobalt cost, diminishing increases in productivity with increases
in cobalt, and ease of depositing cobalt. A suitable range may be
from about 0.04 g/cc to about 0.7 g/cc, preferably 0.05 g/cc to
about 0.7 g/cc in the rim, and more preferably 0.05 g/cc to 0.09
g/cc. Suitable supports are, e.g., silica, silica-alumina and
alumina; and silica or titania or titania-containing support is
preferred, especially a titania wherein the rutile:anatase ratio is
at least about 3:2. The support makes up the predominant portion of
the catalyst and is at least about 50 wt % thereof, preferably at
least about 75 wt % thereof. The feature of a high cobalt metal
loading in a thin catalytically active layer located at the surface
of the particles, while cobalt is substantially excluded from the
inner surface of the particles (the catalyst core should have
<0.04 g/cc of active cobalt), is essential in optimizing the
activity, selectivity and productivity of the catalyst in producing
liquid hydrocarbons from synthesis gas, while minimizing methane
formation.
Metals such as rhenium, zirconium, hafnium, cerium, thorium and
uranium, or the compounds thereof, can be added to cobalt to
increase the activity and regenerability of the catalyst. Thus, the
thin catalytically active layers, rims, or films, formed on the
surface of the support particles, especially the titania or titania
containing support particles, can include in addition to a
catalytically active amount of cobalt, any one or more of rhenium,
zirconium, hafnium, cerium, uranium, and thorium, or admixtures of
these with each other or with other metals or compounds thereof.
Preferred thin catalytically active layers, rims, or films,
supported on a support, notably a titania or a titania-containing
support, thus include cobalt-rhenium, cobalt-zirconium,
cobalt-hafnium, cobalt-cerium, cobalt-uranium, and cobalt-thorium,
with or without the additional presence of other metals or
compounds thereof.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a plot of methane yield in the hydrocarbon synthesis
product against productivity, vol. CO converted/vol. catalyst/hr.
The closed circles denote uniformly impregnated spheres; the open
circles denote spheres impregnated only in the outer layer, the
parenthetical number, indicating the thickness of the outer layer
or rim.
FIG. 2 is a similar plot of methane yield in the hydrocarbon
synthesis product against productivity. The closed circle denotes a
uniformly impregnated sphere, the open circle a rim or outer layer
impregnated sphere with rim thickness indicated parenthetically;
the trianglesydenoting data from EPA 178008.
FIG. 3 shows high resolution imaging and EDS analysis of a
cross-sectional view of a microtomed sample revealing the presence
of an outer layer, approximately 0.5 .mu.m thick, and rich in Al,
Co and Re on catalyst pellets of .sup.- 10 to 100 micron sizes.
Imaging and compositional analysis showed that the outermost 50 nm
of this layer was highly enriched in Al, Co and Re.
A particularly preferred catalyst is one wherein the cobalt, or the
cobalt and a promoter, is dispersed as a thin catalytically active
film upon titania. TiO.sub.2, or a titania-containing carrier, or
support, in which the titania has a rutile::anatase weight ratio of
at least about 3:2, as determined by ASTM D 3720-78; Standard Test
Method for Ratio of Anatase to Rutile In Titanium Dioxide Pigments
By Use of X-Ray Diffraction. Generally, the catalyst is one wherein
the titania has a rutile:anatase ratio ranging at least about 3:2
to about 100:1, or greater, and more preferably from about 4:1 to
about 100:1, or greater. Where any one of rhenium, zirconium,
hafnium, cerium, thorium, or uranium metals, respectively, is added
to the cobalt as a promoter to form the thin catalytically active
film, the metal is added to the cobalt in concentration sufficient
to provide a weight ratio of cobalt:metal promoter ranging from
about 30:1 to about 2:1, preferably from about 20:1 to about 5:1.
Rhenium and hafnium are the preferred promoter metals, rhenium
being more effective in promoting improved activity maintenance on
an absolute basis, with hafnium being more effective on a
cost-effectiveness basis. These catalyst compositions, it has been
found, produce at high productivity, with low methane selectivity,
a product which is predominantly C.sub.10+ linear paraffins and
olefins, with very little oxygenates. These catalysts also provide
high activity, high selectivity and high activity maintenance in
the conversion of carbon monoxide and hydrogen to distillate
fuels.
The cobalt catalysts of this invention, as contrasted with (i)
cobalt catalysts, the cobalt portion of which is uniformly
distributed throughout the support particles or (ii) cobalt
catalysts having a relatively thick surface layer of cobalt on the
support particles, have proven especially useful for the
preparation of liquid hydrocarbons from synthesis gas at high
productivities, with low methane formation. In contrast with the
catalysts of this invention, the prior art catalysts are found to
have lower activity, and especially poorer selectivity due to
severe diffusion limitations. These catalysts (i) and (ii), supra,
at high productivities, produce altogether too much methane. As
productivity is increased to produce greater conversion of the
carbon monoxide to hydrocarbons, increased amounts of methane are
concurrently produced. It was thus found that increased
productivity with these catalysts could only be obtained at the
cost of increased methane formation. This result occurs, it is
believed, because the carbon monoxide and hydrogen reactants all
too slowly diffuse through the pores of the particulate catalyst
which becomes filled with a liquid product, this resulting in
underutilization of the catalytically active sites located within
the interior of the particles. Both hydrogen and carbon monoxide
must thus diffuse through the product-liquid filled pores, but
hydrogen diffuses through the pores at a greater rate of speed than
the carbon monoxide. Since both the hydrogen and the carbon
monoxide are reacting at the catalytic sites at an equivalent rate,
a high H.sub.2 /CO ratio is created in the interior of the particle
which leads to high methane formation. As the rate of reaction is
increased, e.g., by incorporating higher intrinsic activity or by
operating at higher temperature, the catalyst becomes more limited
by the rate of diffusion of the reactants through the pores.
Selectivities are especially poor under the conditions of high
productivity. Thus, the catalyst used during a Fischer-Tropsch
hydrocarbon synthesis reaction is one the pores of which become
filled with the product liquid. When the CO and H.sub.2 are passed
over the bed of catalyst and consumed at a rate which is faster
than the rate of diffusion, H.sub.2 progresses to the interior of
the particle to a much greater extent than the CO, leaving the
interior of the particles rich in H.sub.2, and deficient in CO. The
formation of methane within the particle interior is thus favored
due to the abnormally high H.sub.2 /CO ratio; an unfavorable result
since CH.sub.4 is not a desirable product. The extent to which
selectivity is debited depends on the magnitude of the difference
between the rate of diffusion and the rate of reaction, i.e., the
productivity.
The catalyst of this invention is thus one wherein essentially all
of the active cobalt is deposited on the surface of the support
particles, notably the titania or titania-containing support
particles, while cobalt is substantially excluded from the inner
surface of the particles. The surface film of cobalt must be very
thin and contain an adequate loading of cobalt to maximize reaction
of the hydrogen and carbon monoxide at the surface of the catalytic
particle. The surface film of cobalt as stated thus ranges
generally from about 5 microns to about 200 microns, preferably
from about 40 microns to about 200 microns, with cobalt loadings of
at least about 0.04 g/cc in the catalyst rim, preferably at least
about 0.05 g/cc, more preferably ranging from about 0.04 g/cc to
about 0.7 g/cc, still more preferably from about 0.05 g/cc to about
0.7 g/cc, and even more preferably ranging from about 0.05 g/cc to
about 0.09 g/cc, calculated as metallic cobalt per pack bulk volume
of catalyst. The promoter metal to be effective must also be
contained within the surface film of cobalt. If extended into the
interior of the particle outside the cobalt film the promoter metal
will have little promotional effect, if any. The metal promoter
should thus also be concentrated within the cobalt film at the
surface of the catalyst, with the weight ratio of cobalt:metal
promoter, as suggested, ranging from about 30:1 to about 2:1,
preferably from about 20:1 to 5:1. The thickness of the surface
metal film can be conveniently measured by an Electron Probe
Analyzer, e.g., one such as produced by the JEOL Company, Model No.
JXA-50A. Cross-sections of the catalyst particles of this invention
measured via use of this instrument show very high peaks, or
shoulders, at the edges of the particle across the line of sweep
representative of cobalt concentration, with little or no cobalt
showing within the particle interior. The edge, or "rim" of the
"radially impregnated catalyst" will thus contain essentially all
of the cobalt added to the catalyst. The thickness of the film, or
rim, is unrelated to the absolute size, or shape of the support
particles. Virtually any size particle can be employed as is
normally employed to effect catalyst reactions of this type, the
diameter of the particle ranging generally from about >10
microns to about 10 mm. For example, for a fixed-bed process,
particle size may range from about 1 mm to 10 mm and for
fluidized-bed processes such as slurry-bed, ebulating-bed and
fluid-bed processes from about >10 microns to about 1 mm. In a
fixed-bed process, particle size is dictated by pressure
considerations. To reduce diffusion limitation effects even
further, particle diameters are preferably less than 2 mm. The
particles can be of virtually any shape, e.g., as is normally
employed to effect reactions for this type, viz., as beads or
spheres, extrudates, saddles or the like. By concentrating the
catalytic metal, or metals, on the extreme outer surface of the
particles, the normal diffusion limitation of the catalyst can be
overcome. This new catalyst is more active in its function of
bringing about a reaction between the CO and H.sub.2. The catalyst
because of its having the thin layer of catalytically active metal
on its surface is in effect found to behave more ideally,
approaching, in fact, the behavior of a powdered catalyst which is
not diffusion limited. However, unlike in the use of powdered
catalysts the flow of the reactants through the catalyst bed,
because of the larger particle size of the catalyst, is virtually
unimpeded. Higher productivity, with lower methane selectivity is
the result; a result of considerable commercial consequence. At
productivities greater than 150 hr.sup.-1 (standard volumes of
carbon monoxide converted per volume of catalyst per hour), notably
from about 150 hr.sup.-1, preferably above about 200 hr.sup.-1, at
200.degree. C. less than 15 mole percent of the carbon monoxide
converted is converted to methane, preferably less than 10 mole %
converted to methane.
The catalyst of the present invention can be used in fixed-bed,
slurry-bed, ebulating-bed, and fluid-bed processes. Generally the
catalyst will achieve a productivity of at least 150 hr.sup.-1 at
200.degree. C. for fixed-bed processes and at least about 1000
hr.sup.-1 at 200.degree. C. in other than fixed-bed processes
(slurry-bed, ebulating-bed and fluid-bed). The processes are
limited only by the ability to remove excess heat.
In conducting synthesis gas reactions the total pressure upon the
CO and H.sub.2 reaction mixture is generally maintained above about
80 psig, and preferably above about 140 psig. It is generally
desirable to employ carbon monoxide, and hydrogen, in molar ratio
of H.sub.2 :CO above about 0.5:1 and preferably equal to or above
about 1.7:1 to increase the concentration of C.sub.10+ hydrocarbons
in the product. Suitably, the H.sub.2 :CO molar ratio ranges from
about 0.5:1 to about 4:1, and preferably the carbon monoxide and
hydrogen are employed in molar ratio H.sub.2 :CO ranging from about
1.7:1 to about 2.5:1. In general, the reaction is carried out at
gas hourly space velocities ranging from about 100 V/Hr/V to about
5000 V/Hr/V, preferably from about 300 V/Hr/V to about 2000 V/Hr/V,
measured as standard volumes of the gaseous mixture of carbon
monoxide and hydrogen (0.degree. C., 1 Atm.) per hour per volume of
catalyst. The reaction is conducted at temperatures ranging from
about 160.degree. C. to about 290.degree. C., preferably from about
190.degree. C. to about 260.degree. C., and more preferably about
190.degree. C. to about 220.degree. C. Pressures preferably range
from about 80 psig to about 600 psig, more preferably from about
140 psig to about 400 psig. The product generally and preferably
contains 60 percent, or greater, and more preferably 75 percent, or
greater, C.sub.10+ liquid hydrocarbons which boil about 160.degree.
C. (320.degree. F.).
The catalysts employed in the practice of this invention can be
prepared by spray techniques where a dilute solution of a cobalt
compound, along or in admixture with a promoter metal compound, or
compounds, as a spray is repetitively contacted with the hot
support particles, e.g., silica, titania, or titania-containing
support particles. The particulate support is maintained at
temperatures equal to or above about 140.degree. C. when contacted
with the spray, and suitably the temperature of the support ranges
from about 140.degree. C. up to the decomposition temperature of
the cobalt compound, or compounds in admixture therewith;
preferably from about 140.degree. C. to about 190.degree. C. The
cobalt compound employed in the solution can be any organometallic
or inorganic compound which decomposes to give cobalt oxide upon
initial contact or upon calcination, such as cobalt nitrate, cobalt
acetate, cobalt acetylacetonate, cobalt naphthenate, cobalt
carbonyl, or the like. Cobalt nitrate is especially preferred while
cobalt halide and sulfate salts should generally be avoided. The
cobalt salts may be dissolved in a suitable solvent, e.g., water,
organic or hydrocarbon solvent such as acetone, methanol, pentane
or the like. The total amount of solution used should be sufficient
to supply the proper catalyst loading, with the film being built up
by repetitive contacts between the support and the solvent. The
preferred catalyst is one which consists essentially of cobalt, or
cobalt and promoter, dispersed upon the titania, or
titania-containing support, especially a rutile support. Suitably,
the hot support, notably the titania support, is contacted with a
spray which contains from about 0.05 g of cobalt/ml of solution to
about 0.25 g of cobalt/ml of solution, preferably from about 0.10 g
of cobalt/ml of solution to about 0.20 g of cobalt/ml of solution
(plus the compound containing the promoter metal, if desired),
generally from at least about 3 to about 12 contacts, preferably
from about 5 to about 8 contacts, with intervening drying and
calcination steps being required to form surface films of the
required thicknesses. The hot support, in other words, is
spray-contacted in a first cycle which includes the spray contact
per se with subsequent drying and calcination, a second cycle which
includes the spray contact per se with subsequent drying and
calcination, a third spray contact which includes the spray contact
per se with subsequent drying and calcination, etc. to form a film
of the required thickness and composition. The drying steps are
generally conducted at temperatures ranging above about 20.degree.
C., preferably from about 20.degree. C. to about 125.degree. C.,
and the calcination steps at temperatures ranging above about
150.degree. C., preferably from about 150.degree. C. to about
300.degree. C.
A preferred method for preparing the catalysts particles is
described in Preparation of Catalysts IV, 1987, Elsevier
Publishers, Amsterdam, in an article by Arntz and Prescher, p. 137,
et seq.
Silica and titania are preferred supports. Titania is particularly
preferred. It is used as a support, either along or in combination
with other materials for forming a support. The titania used for
the support is preferably one which contains a rutile:anatase ratio
of at least about 3:2, as determined by x-ray diffraction (ASTM D
3720-78). The titania supports preferably contain a rutile:anatase
ratio of from about 3:2 to about 100:1, or greater, more preferably
from about 4:1 to about 100:1, or greater. The surface area of such
forms of titania are less than about 50 m.sup.2 /g. These weight
concentrations of rutile provide generally optimum activity, and
C.sub.10+ hydrocarbon selectivity without significant gas and
CO.sub.2 make.
The prepared catalyst as a final step is dried by heating at a
temperature above about 20.degree. C., preferably between
20.degree. C. and 125.degree. C., in the presence of nitrogen or
oxygen, or both, in an air stream or under vacuum. It is necessary
to activate the catalyst prior to use. Preferably, the catalyst is
contacted with oxygen, air, or other oxygen-containing gas at
temperature sufficient to oxidize the cobalt and convert the cobalt
to Co.sub.3 O.sub.4. Temperatures ranging above about 150.degree.
C., and preferably above about 200.degree. C. are satisfactory to
convert the cobalt to the oxide, but temperatures above about
500.degree. C. are to be avoided unless necessary for regeneration
of a severely deactivated catalyst. Suitably, the oxidation of the
cobalt is achieved at temperatures ranging from about 150.degree.
C. to about 300.degree. C. The metal, or metals, contained on the
catalyst are then reduced. Reduction is performed by contact of the
catalyst, whether or not previously oxidized, with a reducing gas,
suitably with hydrogen or hydrogen-containing gas stream at
temperatures above about 200.degree. C.; preferably above about
250.degree. C. Suitably, the catalyst is reduced at temperatures
ranging from about 200.degree. C. to about 500.degree. C. for
periods ranging from about 0.5 to about 24 hours at pressures
ranging from ambient to about 40 atmospheres. A gas containing
hydrogen and inert components in admixture is satisfactory for use
in carrying out the reduction.
The catalysts of this invention can be regenerated, and reactivated
to restore their initial activity and selectivity after use by
washing the catalyst with a hydrocarbon solvent, or by stripping
with a gas. Preferably the catalyst is stripped with a gas, most
preferably with hydrogen, or a gas which is inert or non-reactive
at stripping conditions such as nitrogen, carbon dioxide, or
methane. The stripping removes the hydrocarbons which are liquid at
reaction conditions. Gas stripping can be performed at
substantially the same temperatures and pressures at which the
reaction is carried out. Pressures can be lower, however, as low as
atmospheric or even a vacuum. Temperatures can thus range from
about 160.degree. C. to about 290.degree. C., preferably from about
190.degree. C. to about 260.degree. C., and pressures from below
atmospheric to about 600 psig, preferably from about 140 psig to
about 400 psig. If it is necessary to remove coke from the
catalyst, the catalyst can be contacted with a dilute
oxygen-containing gas and the coke burned from the catalyst at
controlled temperature below the sintering temperature of the
catalyst. Most of the coke can be readily removed in this way. The
catalyst is then reactivated, reduced, and made ready for use by
treatment with hydrogen or hydrogen-containing gas with a fresh
catalyst.
The invention will be more fully understood by reference to the
following examples and demonstrations which present comparative
data illustrating its more salient features.
The catalysts of this invention are disclosed in the following
examples and demonstrations as Catalysts Nos. 14-21, and 24. These
are catalysts which have surface films falling within the required
range of thicknesses, and the surface film contains the required
cobalt metal loadings. It will be observed that Catalysts No. 14-21
were formed by a process wherein a heated particulate TiO.sub.2
substrate was repetitively contacted with a dilute spray solution
containing both the cobalt and rhenium which was deposited as a
thin surface layer, or film, upon the particles. Catalyst 24 was
prepared similarly but with an SiO.sub.2 particle. Catalysts Nos.
14-21 and 24 are contrasted in a series of synthesis gas conversion
runs with Catalysts Nos. 1-8 and 22, catalysts wherein the metals
are uniformly dispersed throughout TiO.sub.2 or SiO.sub.2 (No. 22)
support particles, Catalysts Nos. 9-13, also "rim" catalysts but
catalysts wherein the surface films, or rims, are too thick
(Catalysts Nos. 11-13), however prepared, or do not contain an
adequate cobalt metal loading within the surface film, or rim
(Catalysts Nos. 9 and 23). Catalyst 25 is run at 220.degree. C. for
comparison with Catalyst 24. The higher temperature operation shows
increased productivity but significantly higher methane production.
It is clear that at high productivities the catalysts formed from
the uniformly impregnated TiO.sub.2 and SiO.sub.2 spheres produce
high methane. Moreover, even wherein a film of the catalytic metal
is formed on the surface of the particles, it is essential that the
surface film, or rim of cobalt be very thin and also contain an
adequate loading of cobalt in the film. This is necessary to
maximize reaction of the H.sub.2 and CO at the surface of the
particle wherein the cobalt metal reaction sites are located, while
simultaneously reactions within the catalyst but outside the metal
film or rim are suppressed to maximize productivity, and lower
methane selectivity. The following data thus show that the
catalysts of this invention, i.e., Catalysts Nos. 14-21 and 24 can
be employed at productivities above 150 hr.sup.-1, and indeed at
productivities ranging above 150 hr.sup.-1 to 200 hr.sup.-1, and
greater, to produce no more and even less methane than is produced
by (i) catalysts otherwise similar except that the catalysts
contain a thicker surface film, i.e., Catalysts Nos. 11-13, or (ii)
catalysts which contain an insufficient cobalt metal loading with
in a surface film of otherwise acceptable thinness, i.e., Catalyst
Nos. 9 and 23. The data show that the catalysts of this invention
at productivities ranging above about 150 hr.sup.-1 to about 200
hr.sup.31 1, and greater, at 200.degree. C. can be employed to
produce liquid hydrocarbons at methane levels well below 10 mole
percent with TiO.sub.2 and less than 15 mole percent with
SiO.sub.2.
EXAMPLES 1-8
A series of 21 different catalysts were prepared from titania.
TiO.sub.2, and 3 different catalysts from SiO.sub.2, both supplied
by a catalyst manufacturer in spherical form; the supports having
the following physical properties, to wit:
TiO.sub.2 (1 mm average diameter)
86-95% TiO.sub.2 rutile content (by ASTM D 3720-78 test)
14-17 m.sup.2 /g BET surface area
0.11-0.16 cc/g pore volume (by mercury intrusion).
SiO.sub.2 2.5 mm average diameter 244 m.sup.2 /g BET surface area
0.96 cc/g pore volume (by nitrogen adsorption)
In the catalyst preparations, portions of the TiO.sub.2 spheres
were impregnated with cobalt nitrate and perrhenic acid via several
impregnation techniques as subsequently described. In each
instance, after drying in vacuo at 125.degree.-185.degree. C., the
catalysts were calcined in flowing air at 250.degree.-500.degree.
C. for three hours. A first series of catalysts (Catalyst Nos. 1-8)
were prepared wherein TiO.sub.2 spheres were uniformly impregnated,
and these catalysts then used in a series of base runs (Table 1).
Catalyst Nos. 9-11 (Table 2) and 12-21 (Table 3) were prepared such
that the metals were deposited on the outside surface of the
spheres to provide a shell, film or rim. Catalysts Nos. 22-25 were
prepared with 2.5 mm SiO.sub.2 particles (Table 4). Catalyst No. 22
was a fully impregnated SiO.sub.2 particle while Catalysts 23-25
were rim type catalysts prepared by spraying in a manner similar to
that for Catalyst Nos. 14-21. Catalyst No. 23 had insufficient
cobalt loading to produce adequate productivity. The thicknesses of
the catalyst rim, or outer shell, were determined in each instance
by Electron Microprobe Analysis. Runs were made with these
catalysts each being contacted with synthesis gas at similar
conditions and comparisons (except for Catalyst No. 25 run at
220.degree. C.) then made with those employed to provide the base
runs.
Catalyst Nos. 1-8, described in Table 1, were prepared as uniformly
impregnated catalysts to wit: A series of uniformly impregnated
TiO.sub.2 spheres were prepared by immersing the TiO.sub.2 spheres
in acetone solutions of cobalt nitrate and perrhenic acid,
evaporating off the solutions, and then drying and calcining the
impregnated spheres. The Co and Re loadings, expressed as gms metal
per cc of catalyst on a dry basis, deposited upon each of the
catalysts are given in the second and third columns of Table 1.
Catalysts No. 9-11 were prepared to contain an outer rim or shell.
These catalysts were prepared by a liquid displacement method which
involves first soaking the TiO.sub.2 in a water-immiscible liquid,
draining off the excess liquid, and then dipping the wet spheres
into a concentrated aqueous solution of cobalt nitrate (0.24 g
Co/ml) and perrhenic acid (0.02 g Re/ml). Contact with the metal
salt solution is limited to a very short period of time, during
which the solution displaces the pre-soak liquid from the outer
surface of the support particles. The rim-impregnated catalyst is
quickly blotted on paper towels and dried in a vacuum oven at
140.degree. C. Results are summarized in Table 2. The second column
of Table2 thus identifies the presoak liquid, the third column the
displacement time in minutes, the fourth and fifth columns the g
Co/cc and g Re/cc, respectively, and the sixth column the rim
thickness or thickness of the outer metal shell in microns.
Catalysts Nos. 12-21, described in Table 3, were prepared to have
metal shells or rims by use of a series of spray techniques.
TiO.sub.2 spheres were spread out on a wire screen and preheated in
a vacuum oven at various temperatures. The hot spheres were removed
from the oven, sprayed with a small amount of metal salt solution,
and returned without delay to the oven where drying and partial
decomposition of the cobalt nitrate salt occurred. The spraying
sequence was repeated several times in order to impregnate a thin
outer layer or rim of Co-Re onto the support. Preparative details
are as follows:
Three solutions I, II and III, each constituted of a different
solvent, and having specific concentrations of cobalt nitrate and
perrhenic acid were employed in a series of spraying procedures.
The three solutions are constituted as follows:
Cobalt Nitrate Perrhenic Acid Solution Concentration Concentration
Number g Co/ml g Re/ml Solvent I 0.12 0.01 20% H.sub.2 O 80%
Acetone II 0.12 0.03 H.sub.2 O III 0.12 0.01 Acetone
Five separate procedures, Procedures A, B, C, D and E,
respectively, employing each of these three solutions, were
employed to prepare catalysts, as follows:
A: 30 ml of Solution I added to 50 g TiO.sub.2 spheres in 5
sprayings
B: 30 ml of Solution I added to 50 g TiO.sub.2 spheres in 3
sprayings
C: 25 ml of Solution I added to 50 g TiO.sub.2 spheres in 5
sprayings
D: 50 ml of Solution II added to 100 g TiO.sub.2 spheres in 5
sprayings
E: 25 ml of Solution III added to 50 g TiO.sub.2 spheres in 5
sprayings
Reference is made to Table 3. The procedure employed in spray
coating the respective catalyst is identified in the second column
of said table, and the TiO.sub.2 pre-heat temperature is given in
the third column of said table. The g Co/cc and g Re/cc of each
catalyst is given in Columns 4 and 5, respectively, and the
thickness of the catalyst rim is given in microns in the sixth
column of the table.
The catalysts were diluted, in each instance, with equal volumes of
TiO.sub.2 or SiO.sub.2 spheres to minimize temperature gradients,
and the catalyst mixture then charged into a small fixed bed
reactor unit. In preparation for conducting a run, the catalysts
were activated by reduction with hydrogen at 450.degree. C., at
atmospheric pressure for one hour. Synthesis gas with a composition
of 64% H.sub.2-32 % CO-4% Ne was then converted over the activated
catalyst at 200.degree. C. (or 220.degree. C. for Catalyst No. 25),
280 psig for a test period of at least 20 hours. Gas hourly space
velocities (GHSV) as given in each of the tables, represent the
flow rate at 22.degree. C. and atmospheric pressure passed over the
volume of catalyst, excluding the diluent. Samples of the exit gas
were periodically analyzed by as chromatography to determine the
extent of CO conversion and the selectivity to methane, expressed
as the moles of CH.sub.4 formed per 100 moles of CO converted.
Selectivity to C.sub.4- expressed as the wt % of C.sub.4 - in the
hydrocarbon product, was calculated from the methane selectivity
data using an empirical correlation developed from data obtained in
a small pilot plant. A productivity figure is also given for runs
made with each of these catalysts, productivity being defined as
the product of the values represented by the space velocity, the CO
fraction in the feed and the fraction of the CO converted; the
productivity being the volume CO measured at 22.degree. C. and
atmospheric pressure converted per hour per volume of catalyst.
TABLE 1 UNIFORMLY IMPREGNATED CATALYSTS, AND GAS CONVERSION RUNS
MADE THEREWITH Catalyst Wt. % Wt. % % Co Produc- Mol. % Wt. %
Number g Co/cc g Rc/cc GHSV Conv. tivity CH.sub.4 C.sub.4- 1 0.0392
0.0034 200 67 43 5.4 9.4 2 0.0617 0.0046 750 50 120 10.5 16.9 3
0.1003 0.0080 500 80 128 11.1 17.7 4 0.0743 0.0056 750 64 154 11.5
18.3 5 0.0796 0.0050 750 71 170 13.1 20.7 6 0.1014 0.0084 750 77
185 13.9 21.8 7 0.0925 0.0066 750 77 185 13.9 20.9 8 0.1025 0.0068
1000 65 208 14.7 23.0
TABLE 1 UNIFORMLY IMPREGNATED CATALYSTS, AND GAS CONVERSION RUNS
MADE THEREWITH Catalyst Wt. % Wt. % % Co Produc- Mol. % Wt. %
Number g Co/cc g Rc/cc GHSV Conv. tivity CH.sub.4 C.sub.4- 1 0.0392
0.0034 200 67 43 5.4 9.4 2 0.0617 0.0046 750 50 120 10.5 16.9 3
0.1003 0.0080 500 80 128 11.1 17.7 4 0.0743 0.0056 750 64 154 11.5
18.3 5 0.0796 0.0050 750 71 170 13.1 20.7 6 0.1014 0.0084 750 77
185 13.9 21.8 7 0.0925 0.0066 750 77 185 13.9 20.9 8 0.1025 0.0068
1000 65 208 14.7 23.0
TABLE 3 RIM CATALYSTS PREPARED BY SPRAYING METHOD, AND GAS
CONVERSION RUNS MADE THEREWITH Cat- TiO.sub.2 Rim alysts Pre-Heat
Thickness % Co Mol. % Wt. % Number Procedure Temp. .degree. C. g
Co/cc g Rc/cc Microns GMSV Conv. Productivity CH.sub.4 C.sub.4- 12
A 140 0.0624 0.0050 250 400 85 109 8.9 14.5 13 B 125 0.0818 0.0068
300 750 85 204 11.8 18.8 14 C 140 0.0531 00045 140 800 68 174 8.2
13.3 15 C 140 0.0613 0.0050 150 800 71 182 8.2 13.5 16 C 140 0.0739
0.0070 130 800 81 207 8.7 14.2 17 D 185 0.0507 0.0125 160 800 68
174 9.2 15.0 18 E 185 0.0549 0.0049 90 800 68 174 6.5 11.0 19 C 185
0.0483 0.0043 70 800 64 164 6.7 11.3 20 C 185 0.0474 0.0033 90 800
65 166 7.5 12.5 21 C 185 0.0603 0.0046 60 800 74 189 7.2 12.0
TABLE 3 RIM CATALYSTS PREPARED BY SPRAYING METHOD, AND GAS
CONVERSION RUNS MADE THEREWITH Cat- TiO.sub.2 Rim alysts Pre-Heat
Thickness % Co Mol. % Wt. % Number Procedure Temp. .degree. C. g
Co/cc g Rc/cc Microns GMSV Conv. Productivity CH.sub.4 C.sub.4- 12
A 140 0.0624 0.0050 250 400 85 109 8.9 14.5 13 B 125 0.0818 0.0068
300 750 85 204 11.8 18.8 14 C 140 0.0531 00045 140 800 68 174 8.2
13.3 15 C 140 0.0613 0.0050 150 800 71 182 8.2 13.5 16 C 140 0.0739
0.0070 130 800 81 207 8.7 14.2 17 D 185 0.0507 0.0125 160 800 68
174 9.2 15.0 18 E 185 0.0549 0.0049 90 800 68 174 6.5 11.0 19 C 185
0.0483 0.0043 70 800 64 164 6.7 11.3 20 C 185 0.0474 0.0033 90 800
65 166 7.5 12.5 21 C 185 0.0603 0.0046 60 800 74 189 7.2 12.0
The effectiveness of these catalysts for conducting synthesis gas
reactions is best illustrated by comparison of the methane
selectivity at given productivity with Catalysts 1-8 (Table 1), the
catalysts formed by the uniform impregnation of the metals
throughout the TiO.sub.2 catalyst spheres, and Catalysts 9-11
(Table 2) and 12-21 (Table 3), those catalysts wherein the metals
were deposited as a shell, or rim, upon the outside of the
TiO.sub.2 catalyst spheres. The same type of comparison is then
made between certain of the latter class of catalysts, and others,
which also differ one from another dependent upon the thickness of
the metals-containing rim. These data are best graphically
illustrated for ready, visual comparison. Reference is thus made to
FIG. 1 wherein the methane selectivity produced at given
productivity is plotted for each of the twenty-one catalysts
described by reference to Tables 1-3. A solid black data point is
plotted for each of Catalysts Nos. 1-8, formed from the uniformly
impregnated TiO.sub.2 spheres, and each data point is identified by
catalyst number. An open circle is plotted for each data point
representative of Catalysts Nos. 9-21, each is identified by
catalyst number, and the rim thickness of the catalyst is given.
The behavior of many of these catalysts (i.e., Catalysts 9-13), it
will be observed is somewhat analogous to that of Catalysts Nos.
1-8. Catalysts Nos. 14-21, however, behave quite differently from
either of the other groups of catalysts, i.e., Catalysts Nos. 1-8
or Catalysts 9-13. The methane selectivity is thus relatively low
for Catalysts Nos. 9-12, but at the same time the productivities of
these catalysts are quite low. On the other hand, the
productivities of Catalysts Nos. 2-8 are higher than those of
Catalysts Nos. 9-12, but at the same time these catalysts produce
copious amounts of methane. In striking contrast to either of these
groups of catalysts, Catalysts Nos. 14-21, all of which fall within
the "box" depicted on the figure, provide very high productivities
and, at the same time, low methane selectivities. Catalysts Nos.
14-21 thus differ profoundly from any of Catalysts Nos. 1-13 in
their behavior, and in that the metals components of these
catalysts is packed into a very thin rim, or shell, on the surface
of the TiO.sub.2 support.
These data thus show that a constant temperature as productivity
increases so too does methane selectivity for both the groups of
catalyst represented by Catalysts Nos. 1-8, the uniformly
impregnated catalysts, and Catalysts Nos. 11-13 which have
relatively thick outer shells or rims. Thus, methane selectivity
increases in proportion to the metal loadings when the metals are
dispersed throughout the support or carrier portion of the
catalyst. Methane selectivity also increases in proportion to the
thickness of the catalyst rim. Albeit the methane selectivities
obtained with Catalysts Nos. 9-12 are within acceptable ranges, the
productivities obtained with these catalysts are quite low.
Catalyst No. 13 has adequate productivity but makes significant
methane. Catalyst No. 9, although it has a thin metallic rim and
provides low methane selectivity, its productivity is quite poor
because of an insufficient loading of metals within the rim.
Catalyst Nos. 14-21 which have thin metallic rims and relatively
high metals loadings within the rims, on the other hand, provide
low methane selectivities and high productivities.
The results observed with Catalyst Nos. 1-13 and 22 are consistent
with the onset of a significant diffusion limitation at the higher
productivities, which intensifies as the catalyst become more
active. In sharp contrast, however, catalysts which have cobalt rim
thicknesses of less than about 200 microns, notably from about 20
microns to about 180 microns, can produce at high productivities
(i.e., at least about 150 hr.sup.-1, or even at least about 200
hr.sup.-1) very low methane selectivities. Catalysts with very thin
rims counteract the diffusion problem by limiting reaction to the
outer surface of the catalyst wherein lies the catalytically active
metal components. The catalysts of this invention thus provide a
means of operating at high productivity levels with low methane
selectivities. Methane selectivities are reduced at higher and
higher productivities, as the rim thickness is made smaller and
smaller. When productivity is increased beyond 150 hr.sup.-1,
200.degree. C. to at least about 200 hr.sup.-1 the metals rim
should be no more than about 180 microns thick, and the rim can be
even thinner. This region of operation, the best balance between
activity and selectivity, is represented in FIGS. 1 and 2 by the
area enclosed within the box formed by the dashed lines. FIG. 2
presents the performance of SiO.sub.2 catalysts and Catalyst No. 24
illustrates this invention. The methane yield is slightly higher
than for TiO.sub.2 particles, owing to intrinsic qualities of
TiO.sub.2 catalysts. Nevertheless, the requirements for the best
balance between productivity and selectivity are the same for
SiO.sub.2 catalysts as for TiO.sub.2 catalysts, e.g., adequate
metal loading in a relatively thin rim. Catalyst No. 23, although
it has a thin rim and provides low methane selectivity, its
productivity is quite poor because of an insufficient loading of
metals within the rim. FIG. 2 also includes calculated numbers on
productivity from a EPA 178008 based on a 2.5 mm particle all of
which shown unacceptably high levels of methane yield because of
the relatively thick rims encountered in that case.
These data further show that the catalysts of this invention
(Catalyst Nos. 14-21 and 24) can be readily prepared by the process
of sequentially, or repetitively spraying hot, or preheated support
particles with solutions containing compounds or salts of the
metals. Suitably, the TiO.sub.2 or SiO.sub.2 substrate is preheated
to temperatures of at least about 140.degree. C. to about
185.degree. C., prior to or at the time of contact thereof with the
solution. Higher temperatures can be employed, but temperatures
below about 140.degree. C. do not produce a sufficiently thin rim
of the metals on the catalyst support. The repetitive spraying
technique is shown to be superior to liquid displacement technique
used to prepare Catalyst Nos. 9-10 and 23 wherein only low cobalt
loadings were deposited in a single contact because of the cobalt
concentration limit in the displacing solution. Longer displacement
time increases the metal loading but produces a thicker rim as
shown by Catalyst No. 11 compared with Catalyst No. 10. The spray
technique provides especially good dispersion of the metals as a
thin rim at the outer surface of the support particles by
application of the metals a little at a time by multiple
impregnations. Loading the metals onto the catalysts in this manner
increases the activity of the catalysts and provides higher
productivity.
These reactions can be conducted with the catalysts of the present
invention in any Fischer-Tropsch reactor system, e.g., fixed bed,
fluidized bed, or other reactors, with or without the recycle of
any unconverted gas and/or liquid product. The C.sub.10+ product
that is obtained is an admixture of linear paraffins and olefins
which can be further refined and upgraded to high quality middle
distillate fuels, or such other products as mogas, diesel fuel, jet
fuel and the like. A premium grade middle distillate fuel of carbon
number ranging from about C.sub.10 to about C.sub.20 can also be
produced from the C.sub.10+ hydrocarbon product. The catalyst is
constituted of cobalt supported on a carrier, preferably titania,
and especially cobalt supported on a rutile form of TiO.sub.2 or
rutile-titania-containing support which can contain other materials
such as SiO.sub.2, MgO, ZrO.sub.2, Al.sub.2 O.sub.3. The catalyst
is preferably reduced with a H.sub.2 -containing gas at
start-up.
EXAMPLE 9
About 14,500 lbs of finished catalyst was obtained by double
impregnating a spray-dried support made from Degussa P-25 titania.
The titania was spray-dried with an alumina sol binder. Product
with an average particle size of about 45 microns was made in very
high yield (97%). The support was then converted into the rutile
form by calcination at high temperature in a 30".times.50" rotary
calciner. Impregnation with an aqueous solution of cobalt nitrate
and perrhenic acid was performed batch-wise in a 5 ft.sup.3
V-blender. The nitrate salt was decomposed by calcining the
catalyst in the larger rotary at about 450.degree. C. A second pass
through impregnation and calcination produced the finished
catalyst.
High resolution imaging and EDS analysis of a cross-sectional view
of the above catalyst is shown in FIG. 3.
It is apparent that various modifications and changes can be made
without departing the spirit and scope of the present
invention.
* * * * *