U.S. patent application number 09/742873 was filed with the patent office on 2001-06-14 for fischer-tropsch activity for "non-promoted" cobalt-on-alumina catalysts.
This patent application is currently assigned to Energy International Corporation. Invention is credited to Goodwin, James G., Oukaci, Rachid, Singleton, Alan H..
Application Number | 20010003787 09/742873 |
Document ID | / |
Family ID | 26775214 |
Filed Date | 2001-06-14 |
United States Patent
Application |
20010003787 |
Kind Code |
A1 |
Singleton, Alan H. ; et
al. |
June 14, 2001 |
Fischer-tropsch activity for "non-promoted" cobalt-on-alumina
catalysts
Abstract
Cobalt catalysts, and processes employing these inventive
catalysts, for hydrocarbon synthesis. The inventive catalyst
comprises cobalt on an alumina support and is not promoted with any
noble or near noble metals. In one aspect of the invention, the
alumina support preferably includes a dopant in an amount effective
for increasing the activity of the inventive catalyst. The dopant
is preferably a titanium dopant. In another aspect of the
invention, the cobalt catalyst is preferably reduced in the
presence of hydrogen at a water vapor partial pressure effective to
increase the activity of the cobalt catalyst for hydrocarbon
synthesis. The water vapor partial pressure is preferably in the
range of from 0 to about 0.1 atmospheres.
Inventors: |
Singleton, Alan H.; (Baden,
PA) ; Oukaci, Rachid; (Gibsonia, PA) ;
Goodwin, James G.; (Cranberry Township, PA) |
Correspondence
Address: |
Dennis D. Brown
Fellers, Snider, Blankenship,
Bailey & Tippens, P.C.
321 S. Boston Ave., Suite 800
Tulsa
OK
74103-3318
US
|
Assignee: |
Energy International
Corporation
135 William Pitt Way
Pittsburgh
PA
15238
|
Family ID: |
26775214 |
Appl. No.: |
09/742873 |
Filed: |
December 20, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09742873 |
Dec 20, 2000 |
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09320327 |
May 26, 1999 |
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6191066 |
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60086846 |
May 27, 1998 |
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Current U.S.
Class: |
585/700 ;
518/700; 518/715; 585/733 |
Current CPC
Class: |
B01J 23/75 20130101;
B01J 23/462 20130101; C07C 2521/04 20130101; C07C 2523/75 20130101;
B01J 23/78 20130101; B01J 23/83 20130101; B01J 23/8913 20130101;
C07C 1/043 20130101; C10G 2/332 20130101 |
Class at
Publication: |
585/700 ;
585/733; 518/700; 518/715 |
International
Class: |
C07C 002/00; C07C
001/00; C07C 027/00; C07C 027/06 |
Goverment Interests
[0002] The government of the United States of America has rights to
this invention pursuant to Contract No. DE-AC22-92 PC9208 awarded
by the U.S. Department of energy.
Claims
What is claimed is:
1. A cobalt catalyst for hydrocarbon synthesis, said cobalt
catalyst comprising cobalt supported on a .gamma.-alumina support
wherein: said cobalt catalyst is not promoted with any noble metals
and is not promoted with any near noble metals; and said
.gamma.-alumina support includes a dopant in an amount effective
for increasing the activity of said cobalt catalyst for said
hydrocarbon synthesis.
2. The cobalt catalyst of claim 1 wherein said dopant is a titanium
dopant.
3. The cobalt catalyst of claim 2 wherein said amount of said
titanium dopant is an amount effective to render said cobalt
catalyst at least 60% as active as a promoted catalyst which is
identical to said cobalt catalyst except that said promoted
catalyst is promoted with ruthenium in a ruthenium to cobalt weight
ratio of 1:40.
4. The cobalt catalyst of claim 3 wherein said amount of said
titanium dopant is an amount effective to render said cobalt
catalyst at least 70% as active as said promoted catalyst.
5. The cobalt catalyst of claim 3 wherein said amount of said
titanium dopant is an amount effective to render said cobalt
catalyst at least 80% as active as said promoted catalyst.
6. The cobalt catalyst of claim 2 wherein said amount of said
titanium dopant, expressed as elemental titanium, is at least 500
ppm by weight of the total weight of said .gamma.-alumina
support.
7. The cobalt catalyst of claim 2 wherein said amount of said
titanium dopant, expressed as elemental titanium, is in the range
of from about 800 to about 2000 ppm by weight of the total weight
of said .gamma.-alumina support.
8. The cobalt catalyst of claim 2 wherein said amount of said
titanium dopant, expressed as elemental titanium, is about 1000 ppm
by weight of the total weight of said .gamma.-alumina support.
9. The cobalt catalyst of claim 2 wherein said cobalt catalyst is
an activated catalyst which has been reduced in the presence of
hydrogen and at a water vapor partial pressure effective to
increase said activity of said cobalt catalyst.
10. The cobalt catalyst of claim 9 wherein said water vapor partial
pressure is in the range of from 0 to about 0.1 atmospheres.
11. The cobalt catalyst of claim 2 wherein said cobalt catalyst is
promoted with at least one of potassium and lanthana.
12. The cobalt catalyst of claim 1 wherein: said .gamma.-alumina
support is produced from synthetic boehmite and said dopant is
added to said .gamma.-alumina support prior to the crystallization
of said synthetic boehmite.
13. A process for hydrocarbon synthesis comprising the step of
reacting a synthesis gas in the presence of a cobalt catalyst
wherein: said cobalt catalyst comprises cobalt supported on a
.gamma.-alumina support; said cobalt catalyst is not promoted with
any noble metals and is not promoted with any near noble metals;
and said .gamma.-alumina support includes a dopant in an amount
effective for increasing the activity of said cobalt catalyst for
said hydrocarbon synthesis.
14. The process of claim 13 wherein said dopant is a titanium
dopant.
15. The process of claim 14 wherein said amount of said titanium
dopant is an amount effective to render said cobalt catalyst at
least 60% as active, for said hydrocarbon synthesis, as a promoted
catalyst which is identical to said cobalt catalyst except that
said promoted catalyst is promoted with ruthenium in ruthenium to
cobalt weight ratio of 1:40.
16. The process of claim 15 wherein said amount of said titanium
dopant is an amount effective to render said cobalt catalyst at
least 70% as active as said promoted catalyst.
17. The process of claim 15 wherein said amount of said titanium
dopant is an amount effective to render said cobalt catalyst at
least 80% as active as said promoted catalyst.
18. The process of claim 14 wherein said amount of said titanium
dopant, expressed as elemental titanium, is at least 500 ppm by
weight of the total weight of said .gamma.-alumina support.
19. The process of claim 14 wherein said amount of said titanium
dopant, expressed as elemental titanium, is in the range of from
about 800 to about 2000 ppm by weight of the total weight of said
.gamma.-alumina support.
20. The process of claim 14 wherein said amount of said titanium
dopant, expressed as elemental titanium, is about 1000 ppm by
weight of the total weight of said .gamma.-alumina support.
21. The process of claim 14 further comprising the step, prior to
said step of reacting, of activating said cobalt catalyst by
reducing said cobalt catalyst in the presence of hydrogen and at a
water vapor partial pressure effective to increase said activity of
said cobalt catalyst.
22. The process of claim 22 wherein said water vapor partial
pressure is in the range of from 0 to about 0.1 atmospheres.
23. The process of claim 13 wherein said step of reacting is
conducted in a slurry bubble column reactor.
24. The process of claim 13 wherein said hydrocarbon synthesis is a
Fischer-Tropsch synthesis process.
25. The process of claim 13 wherein: said .gamma.-alumina support
is produced from synthetic boehmite; and said dopant is added to
said .gamma.-alumina support prior to the crystallization of said
boehmite.
26. A cobalt catalyst for hydrocarbon synthesis comprising cobalt
supported on a .gamma.-alumina support, wherein: said cobalt
catalyst is not promoted with any noble metals and is not promoted
with any near noble metals; and said cobalt catalyst has been
reduced in the presence of hydrogen at a water vapor partial
pressure effective to increase the activity of said cobalt catalyst
for said hydrocarbon synthesis.
27. The cobalt catalyst of claim 26 wherein said water vapor
partial pressure is in the range of from 0 to about 0.1
atmospheres.
28. The cobalt catalyst of claim 26 wherein said .gamma.-alumina
support includes an amount of a titanium dopant of at least 500 ppm
by weight, expressed as elemental titanium, of the total weight of
said .gamma.-alumina support.
29. The cobalt catalyst of claim 28 wherein said amount of said
titanium dopant, expressed as elemental titanium, is in the range
from about 800 to about 2000 ppm by weight of the total weight of
said .gamma.-alumina support.
30. The cobalt catalyst of claim 28 wherein: said .gamma.-alumina
support is produced from synthetic boehmite; and said dopant is
added to said .gamma.-alumina support prior to the crystallization
of said synthetic boehmite.
31. A process for hydrocarbon synthesis comprising the steps of:
(a) reducing a cobalt catalyst in the presence of hydrogen and at a
water vapor partial pressure effective to increase the activity of
said cobalt catalyst for said hydrocarbon synthesis, said cobalt
catalyst comprising cobalt supported on a .gamma.-alumina support;
and (b) reacting a synthesis gas in the presence of said cobalt
catalyst, wherein said cobalt catalyst is not promoted with any
noble metals and is not promoted with any near noble metals.
32. The process of claim 31 wherein: step (a) is conducted in a
fixed bed vessel; and step (b) is conducted in a slurry bubble
column reactor.
33. The process of claim 31 wherein said water vapor partial
pressure is in the range of from 0 to about 0.1 atmospheres.
34. The process of claim 31 wherein said .gamma.-alumina support
includes an amount of a titanium dopant of at least 500 ppm by
weight, expressed as elemental titanium, of the total weight of
said .gamma.-alumina support.
35. The process of claim 34 wherein said amount of said titanium
dopant, expressed as elemental titanium, is in the range of from
about 800 to about 2000 ppm by weight of the total weight of said
.gamma.-alumina support.
36. The process of claim 31 wherein said hydrocarbon synthesis is a
Fischer-Tropsch synthesis process.
37. A method of improving the activity of a cobalt catalyst for
hydrocarbon synthesis, wherein said cobalt catalyst has an alumina
support and said cobalt catalyst is not promoted with any noble
metals and is not promoted with any near noble metals, said method
comprising the step of including in said support a titanium dopant
in an amount, expressed as elemental titanium, of at least 500 ppm
by weight of the total weight of said alumina support.
38. The method of claim 37 wherein said amount, expressed as
elemental titanium, of said titanium dopant is in the range of from
about 800 to about 2000 ppm by weight of the total weight of said
alumina support.
39. The method of claim 37 further comprising the step of reducing
said cobalt catalyst in hydrogen at a water vapor partial pressure
in the range of from 0 to about 0.1 atmospheres.
Description
[0001] This application claims the benefit of U.S. provisional
patent application Ser. No. 60/086,846, filed May 27, 1998.
BACKGROUND OF THE INVENTION
[0003] 1. Technical Field
[0004] The present invention relates to systems and processes for
conducting hydrocarbon synthesis and to cobalt-on-alumina catalysts
employed in such processes.
[0005] 2. Background
[0006] In Fischer-Tropsch processes, a synthesis gas ("syngas")
comprising carbon oxide(s) and hydrogen is reacted in the presence
of a Fischer-Tropsch catalyst to produce liquid hydrocarbons.
Certain advanced cobalt catalysts have proven to be very effective
for Fischer-Tropsch synthesis. However, for these catalysts,
extensive promotion with noble and/or near noble metals has been
required in order to enhance the reducibility of the cobalt to an
extent sufficient to achieve acceptable Fischer-Tropsch conversion
activities. Due in significant part to the cost of obtaining and
adding such promoters, these cobalt catalysts have typically been
quite expensive. Thus, a need presently exists for a means of
significantly reducing the cost of cobalt catalysts useful for
Fischer-Tropsch synthesis while maintaining activity levels which
are at least comparable to those heretofore obtained by promoting
such catalysts with noble metals.
[0007] The "syngas" employed in Fischer-Tropsch processes can be
produced, for example, during coal gasification. Processes are also
well known for obtaining syngas from other hydrocarbons, including
natural gas. U.S. Pat. No. 4,423,265 to Chu et al. notes that the
major processes for producing syngas depend either upon (a) the
partial combustion of the hydrocarbon fuel with an
oxygen-containing gas, (b) the reaction of a hydrocarbon fuel with
steam, or (c) a combination of these two reactions. U.S. Pat. No.
5,324,335 to Benham et al. explains the two primary methods (i.e.,
steam reforming and partial oxidation) for producing syngas from
methane. The Encyclopedia of Chemical Technology, Second Edition,
Volume 10, pages 3553-433 (1966), Interscience Publishers, New
York, N.Y. and Third Edition, Volume 11, pages 410-446 (1980), John
Wiley and Sons, New York, N.Y. is said by Chu et al. to contain an
excellent summary of gas manufacture, including the manufacture of
synthesis gas.
[0008] It has long been recognized that syngas can be converted to
liquid hydrocarbons by the catalytic hydrogenation of carbon
monoxide. The general chemistry of the Fischer-Tropsch synthesis
process is as follows:
CO+2H.sub.2.fwdarw.(--CH.sub.2--)+H.sub.2O (1)
2CO+H.sub.2.fwdarw.(--CH.sub.2--)+CO.sub.2 (2)
[0009] The types and amounts of reaction products, i.e., the
lengths of carbon chains, obtained via Fischer-Tropsch synthesis
can vary depending upon process kinetics and choice of
catalyst.
[0010] Many attempts at providing effective catalysts for
selectively converting syngas to liquid hydrocarbons have been
disclosed. U.S. Pat. No. 5,248,701 to Soled et al., presents an
over-view of relevant prior art. The two most popular types of
catalysts heretofore used in Fischer-Tropsch synthesis have been
iron-based catalysts and cobalt-based catalysts. U.S. Pat. No.
5,324,335 to Benham et al. discusses the fact that iron-based
catalysts, due to their high water gas shift activity, favor the
overall reaction shown in (2) above, while cobalt-based catalysts
tend to favor reaction scheme (1).
[0011] The current practice is to support the catalytic components
on porous, inorganic refractory oxides. Particularly preferred
supports have included silica, alumina, silica-alumina, and
titania. In addition, other refractory oxides from Groups III, IV,
V, VI and VIII have been used as catalyst supports.
[0012] As mentioned above, the prevailing practice has been to also
add promoters to the supported catalysts. Promoters have typically
included noble metals, such as ruthenium, and near noble metals.
Promoters are known to increase the activity of the catalyst,
sometimes rendering the catalyst three to four times as active as
its unpromoted counterpart. Unfortunately, effective promoter
materials are typically quite costly both to obtain and to add to
the catalyst.
[0013] Contemporary cobalt catalysts are typically prepared by
impregnating the support with the catalytic material. As described
in U.S. Pat. No. 5,252,613 to Chang et al., a typical catalyst
preparation may involve impregnation, by incipient wetness or other
known techniques, of, for example, a cobalt nitrate salt onto a
titania, silica or alumina support, optionally followed or preceded
by impregnation with a promoter material. Excess liquid is then
removed and the catalyst precursor is dried. Following drying, or
as a continuation thereof, the catalyst is calcined to convert the
salt or compound to its corresponding oxide(s). The oxide is then
reduced by treatment with hydrogen, or a hydrogen-containing gas,
for a period of time sufficient to substantially reduce the oxide
to the elemental or catalytic form of the metal. U.S. Pat. No.
5,498,638 to Long points to U.S. Pat. Nos. 4,673,993, 4,717,702,
4,477,595, 4,663,305, 4,822,824, 5,036,032, 5,140,050, and
5,292,705 as disclosing well known catalyst preparation
techniques.
[0014] Fischer-Tropsch synthesis has heretofore been primarily
conducted in fixed bed reactors, gas-solid reactors, and
gas-entrained fluidized bed reactors, fixed bed reactors being the
most utilized. U.S. Pat. No. 4,670,472 to Dyer et al. provides a
bibliography of several references describing these systems.
[0015] Recently, however, considerable efforts have been directed
toward conducting Fischer-Tropsch synthesis in three-phase (i.e.,
solid, liquid, and gas/vapor) reactors. One such system is the
slurry bubble column reactor (SBCR). In a SBCR, catalyst particles
are slurried in liquid hydrocarbons within a reactor chamber,
typically a tall column. Syngas is then introduced at the bottom of
the column through a distributor plate, which produces small gas
bubbles. The gas bubbles migrate up and through the column, causing
a beneficial turbulence, while reacting in the presence of the
catalyst to produce liquid and gaseous hydrocarbon products.
Gaseous products are captured at the top of the SBCR, while liquid
products are recovered through a filter which separates the liquid
hydrocarbons from the catalyst fines. U.S. Pat. Nos. 4,684,756,
4,788,222, 5,157,054, 5,348,982, and 5,527,473 reference this type
of system and provide citations to pertinent patent and literature
art.
[0016] It is recognized that conducting Fischer-Tropsch synthesis
using a SBCR system could provide significant advantages over the
reaction systems commonly employed heretofore. As noted by Rice et
al. in U.S. Pat. No. 4,788,222, the potential benefits of a slurry
process over a fixed bed process include better control of the
exothermic heat produced by the Fischer-Tropsch reactions as well
as better maintenance of catalyst activity by allowing continuous
recycling, recovery and rejuvenation procedures to be implemented.
U.S. Pat. Nos. 5,157,054, 5,348,982, and 5,527,473 also discuss
advantages of the SBCR process. However, the slurry bubble column
process has been expensive to operate, owing in part to the
significant catalyst costs required.
SUMMARY OF THE INVENTION
[0017] The present invention provides "nonpromoted"
cobalt-on-alumina catalysts unexpectedly and surprisingly having
conversion activities at least comparable to those of the best
promoted formulations. The inventive catalysts also exhibit
superior product selectivity characteristics and are particularly
effective for use in SBCR processes and other three-phase reaction
systems. This remarkable discovery significantly decreases the cost
of the Fischer-Tropsch conversion process, as the more expensive
promoters need not be utilized to achieve acceptable results.
[0018] In one aspect, the present invention provides a cobalt
catalyst for hydrocarbon synthesis. The cobalt catalyst comprises
cobalt supported on a .gamma.-alumina support. The catalyst is not
promoted with any noble metals and is not promoted with any near
noble metals. However, the .gamma.-alumina support includes a
dopant in an amount effective for increasing the activity of the
catalyst for hydrocarbon synthesis. The dopant is preferably a
titanium dopant.
[0019] In another aspect, the present invention provides a process
for hydrocarbon synthesis comprising the step of reacting a
synthesis gas in the presence of a cobalt catalyst. The cobalt
catalyst comprises cobalt supported on a .gamma.-alumina support.
The cobalt catalyst is not promoted with any noble metals and is
not promoted with any near noble metals. However, the
.gamma.-alumina support includes a dopant in an amount effective
for increasing the activity of the cobalt catalyst for hydrocarbon
synthesis. The dopant is preferably a titanium dopant.
[0020] In yet another aspect, the present invention provides a
cobalt catalyst for hydrocarbon synthesis, wherein the cobalt
catalyst comprises cobalt supported on a .gamma.-alumina support.
The cobalt catalyst is not promoted with any noble metals and is
not promoted with any near noble metals. However, the cobalt
catalyst has been reduced in the presence of hydrogen at a water
vapor partial pressure effective to increase the activity of the
cobalt catalyst for hydrocarbon synthesis. The water vapor partial
pressure is preferably in the range of from 0 to about 0.1
atmospheres.
[0021] In yet another aspect, the present invention provides a
process for hydrocarbon synthesis comprising the steps of: (a)
reducing a cobalt catalyst in the presence of hydrogen and at a
water vapor partial pressure effective to increase the activity of
the cobalt catalyst for hydrocarbon synthesis and (b) reacting a
synthesis gas in the presence of the cobalt catalyst. The cobalt
catalyst is not promoted with any noble metals and is not promoted
with any near noble metals.
[0022] In yet another aspect, the present invention provides a
method of improving the activity of a cobalt catalyst for
hydrocarbon synthesis, wherein the cobalt catalyst has an alumina
support. The cobalt catalyst is not promoted with any noble metals
and is not promoted with any near noble metals. However, the
alumina support includes a titanium dopant in an amount, expressed
as elemental titanium, of at least 500 ppm by weight of the total
weight of the alumina support.
[0023] Further objects, features and advantages of the present
invention will be apparent upon examining the accompanying drawings
and upon reading the following description of the preferred
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 provides a graph comparing the effect of titanium
dopant concentrations on the activities of ruthenium-promoted
catalysts and "nonpromoted" catalysts for Fischer-Tropsch synthesis
processes conducted in an SBCR. Each test involved 15-25 grams of
catalyst which was sieved to 400-150 mesh, calcined, and then
reduced/activated outside of the SBCR system. Each Fischer-Tropsch
reaction test was conducted at 450 psig and 230.degree. C. using a
synthesis gas flow rate of 15 liters per minute. The synthesis gas
was diluted with 60% nitrogen and had a H.sub.2:CO ratio of 2.
[0025] FIG. 2 provides a graph comparing the effects of titanium
dopant concentrations on the activities of ruthenium-promoted
catalysts and "nonpromoted" catalysts for Fischer-Tropsch synthesis
conducted in a fixed bed reactor. In each case, the Fischer-Tropsch
reaction was conducted at a pressure of 1 atmosphere, a temperature
of 220.degree. C., and a H.sub.2/CO ratio of 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] Catalyst Compositions
[0027] The present invention provides supported cobalt catalysts
which are well suited for use in Fischer-Tropsch synthesis. These
catalysts are particularly well suited for use in slurry bubble
column reactor processes. Examples of preferred, general catalyst
compositions provided by the present invention include, but are not
limited to: (a) cobalt, without any noble metal or near noble metal
promoter, preferably supported on a doped .gamma.-alumina, and (b)
cobalt promoted with one or more selectivity promoters (preferably
an alkali promoter and/or a rare earth oxide such as lanthana), but
without a noble metal or near noble metal promoter, and preferably
supported on a doped .gamma.-alumina.
[0028] Preferred catalyst compositions comprise (per 100 parts by
weight of support): from about 10 to about 65 pbw cobalt; from
about 0.1 to about 8 pbw potassium (when present); and from about
0.5 to about 8 pbw lanthana (when present). The catalysts will most
preferably comprise (per 100 parts by weight of support): from
about 17 to about 45 pbw (more preferably from about 20 to about 40
pbw, and most preferably about 30 pbw) cobalt; from about 0.2 to
about 1.0 pbw potassium (when present); and/or from about 0.9 to
about 2.1 pbw lanthana (when present).
[0029] The Catalyst Support
[0030] The support employed in the inventive catalyst will
preferably be .gamma.-alumina. We have determined that, for cobalt
catalysts used in both fixed bed and a slurry bubble column reactor
systems, the particular support employed plays a major role in
influencing overall hydrocarbon production rate (i.e., catalyst
activity) with little or no effect on product selectivity. Catalyst
activities generally rank in the following order:
Al.sub.2O.sub.3>SiO.sub.2>>TiO.sub.2. The source of the
alumina and the pretreatment procedures used also play major roles
in determining the performance of the resulting, cobalt-based,
Fischer-Tropsch catalysts.
[0031] Titania-supported cobalt catalysts, with or without
promoters, were found to have poor Fischer-Tropsch synthesis
properties in both the fixed bed and SBCR systems. Compared to
.gamma.-alumina and silica, titania supports have much lower
surface areas and pore volumes. Thus, they do not readily retain
high cobalt loadings.
[0032] Although silica supports have relatively high surface areas,
silica-supported cobalt catalysts also provided low Fischer-Tropsch
synthesis performance. Silica-supported cobalt catalysts are
unstable in reaction conditions, such as those usually encountered
in Fischer-Tropsch reaction systems, where a significant amount of
water is present. The formation of cobalt-silica compounds under
these conditions is believed to cause this lower performance. To
prevent or at least slow down silicate formation, the silica
surface must typically be coated with oxide promoters, such as
ZrO.sub.2, prior to cobalt impregnation.
[0033] Characteristics and Preparation of Preferred Alumina
Supports
[0034] The catalyst support employed in the present invention is
preferably a .gamma.-alumina support having: a low level of
impurities, especially sulfur (preferably less than 100 ppm
sulfur); a spheroidal shape; an average particle size in the range
of from about 10 to about 150 .mu.m (most preferably from about 20
to about 80 microns); a BET surface area, after calcination, in the
range of from about 200 to about 260 m.sup.2/g; and a porosity in
the range of from about 0.4 to about 1.0 cm.sup.3/g.
[0035] The alumina support is preferably produced from relatively
high purity, synthetic boehmite. As discussed hereinbelow, the
boehmite can be formed from aluminum alkoxide of the type obtained
as a byproduct in the manufacture of synthetic fatty alcohols.
Alternatively, suitable, high-purity boehmite materials can be
formed from aluminum alkoxides produced by alcohol/aluminum metal
reaction processes.
[0036] The aluminum alkoxide is preferably hydrolyzed to produce
high purity, synthetic, monohydrate alumina. Next, this material is
preferably spray-dried to yield highly porous, spherical boehmite
particles of relatively high surface area. The particulate boehmite
material is preferably then sieved to remove fines and large
particles so that a desired particle size range is obtained (most
preferably from about 20 to about 80 microns). The sieved material
is calcined to convert the boehmite particles to a .gamma.-alumina
support material having the desired surface area and porosity. The
boehmite material will preferably be calcined at a temperature of
at least 350.degree. C. (more preferably from about 400.degree. C.
to about 700.degree. C. and most preferably about 500.degree. C.)
for a period of from about 3 to about 24 hours (more preferably
from about 5 to about 16 hours and most preferably about 10 hours).
The desired calcination temperature is preferably reached by slowly
heating the system at a rate of about 0.5-2.0.degree.
C./minute.
[0037] Examples of commercially-supplied boehmite materials
suitable for forming the preferred .gamma.-alumina supports
include, but are in no way limited to, the CATAPAL and PURAL
aluminas supplied by Condea/Vista. As discussed below, commercial
materials of this type are particularly effective when
intentionally produced to have certain targeted titanium "impurity"
levels. Product quality reports for the CATAPAL aluminas indicate
that these products, as presently produced and sold, can have
titania impurity levels varying all the way up to 3000 ppm of
elemental titanium by weight. The PURAL products, on the other
hand, typically have varying titanium impurity levels of up to
about 600 ppm.
[0038] Doping of .gamma.-Alumina Supports
[0039] As shown hereinbelow, we have discovered that the presence
of a controlled amount of dopant (preferably a titanium dopant) in
the .gamma.-alumina support unexpectedly and surprisingly improves
significantly the activities of "nonpromoted", cobalt-on-alumina
Fischer-Tropsch catalysts. As used herein, the term "nonpromoted"
means simply that the catalyst is not promoted with any noble or
near noble metals. The term does not exclude other types of
promoters (e.g., potassium and/or lanthana). The phrase "near noble
metal," as used herein, encompasses rhenium and, although not
practical for use as a promoter, also encompasses technetium.
[0040] The titanium dopant should be present in the .gamma.-alumina
support in an amount, expressed as elemental titanium, of at least
500 (preferably of at least 800) parts per million (ppm) by weight.
The dopant will more preferably be present in the support in an
amount, expressed as elemental titanium, in the range of from about
800 ppm to about 2000 ppm by weight and will most preferably be
present in an amount in the range of from about 1000 to about 2000
ppm. The titanium dopant can be added at substantially any time but
will most preferably be added prior to crystallization of the
boehmite.
[0041] As is well known in the art, one method of producing
synthetic boehmite materials utilizes aluminum alkoxides recovered
as byproducts of certain processes (e.g., the Ziegler Process)
employed for manufacturing synthetic fatty alcohols. The Ziegler
Process typically comprises the steps of: (1) reacting high-purity
alumina powder with ethylene and hydrogen to produce aluminum
triethyl; (2) polymerizing ethylene by contacting it with the
aluminum triethyl, thus forming aluminum alkyls; (3) oxidizing the
aluminum alkyls with air to produce aluminum alkoxides; and (4)
hydrolyzing the aluminum alkoxides to produce alcohols and an
alumina byproduct. The oxidation step of the Ziegler process is
typically catalyzed by an organic titanium compound which is itself
converted to titanium alkoxide. The titanium alkoxide remains with,
and is co-hydrolyzed with, the aluminum alkoxide, thus producing an
alumina byproduct which is incidentally "doped" with a small amount
of titania.
[0042] Another process for forming synthetic boehmite utilizes
aluminum alkoxide produced by reacting an alcohol with a highly
pure aluminum powder. The aluminum alkoxide is hydrolyzed to
produce an alcohol, which is recycled for use in the alkoxide
formation step, and alumina. Because this process does not involve
an oxidation step, the alumina product typically does not contain
titanium. However, for purposes of the present invention, any
desired amount of titanium dopant can be included in the alumina
product by, for example, adding a titanium alkoxide to, and
co-hydrolyzing the titanium alkoxide with, the aluminum alkoxide.
If desired, the same process can be used to add other dopants such
as, for example, silica, lanthanum, barium, etc.
[0043] Heretofore, support manufacturers and catalyst users have
simply considered titania, if present in the alumina support, to be
a harmless impurity. Of the commercial synthetic boehmite products
presently available in the market, some are produced by the Ziegler
process, others are produced by the above-described aluminum
alkoxide hydrolysis process, and still others are produced by a
combination of these processes wherein the resulting products or
product precursors are blended together. Such products are sold and
used interchangeably, without regard to the amount, if any, of
titania present.
[0044] Thus, the amount of titanium present in commercial
.gamma.-alumina supports can vary from 0 ppm to as high as 3000 ppm
titanium by weight or more. Titanium concentrations can also vary
significantly between different batches of the same commercial
product.
[0045] As depicted in FIG. 1, titania has a significant detrimental
effect on the activities of ruthenium-promoted, cobalt-on-alumina
catalysts employed in slurry bubble column reactors. FIG. 1 shows
the activities (g-HC/kg-cat/hr) of three ruthenium-promoted
catalysts (catalysts 1, 2, and 3) which were produced and tested as
described hereinbelow in Example 1. Catalysts 1, 2, and 3 were
identical in all respects except that: catalyst 3 was formed on a
.gamma.-alumina support found to have a titania concentration,
expressed as titanium, of about 7 ppm by weight; catalyst 2 was
formed on a .gamma.-alumina support found to have a titanium
concentration of about 500 ppm; and catalyst 1 was formed on a
.gamma.-alumina support found to have a titanium concentration of
about 1000 ppm. As the amount of titania in the support increased,
the activities of the catalysts, which began at about 1400 for
catalyst 3, declined to about 1322 for catalyst 2, and to about
1195 for catalyst 1. Thus, any preference in the art as to the
presence of titanium would heretofore have been that no titanium be
included in the .gamma.-alumina support.
[0046] We have discovered, however, that, in contrast to the
detrimental effect of titanium on the activities of "noble
metal-promoted" catalysts employed in slurry bubble column
reactors, the activities of "nonpromoted", cobalt-on-alumina
catalysts, in all Fischer-Tropsch reaction systems, are
unexpectedly and surprisingly improved when controlled amounts of
dopant are present in the catalyst supports. The inventive,
"non-promoted" catalysts have activities at least approaching those
of catalysts promoted with noble metals. Moreover, because they
need not be promoted with noble metals, the inventive catalysts
cost much less to produce. Thus, our invention significantly
reduces the cost of Fischer-Tropsch processes, particularly those
processes conducted in slurry bubble column and other three-phase
reaction systems wherein catalyst attrition losses are high.
[0047] Catalyst Preparation
[0048] Although other methods of preparation can be used, the
catalytic components of the inventive catalysts are preferably
added to the support by totally aqueous impregnation using
appropriate aqueous solution compositions and volumes to achieve
incipient wetness of the support material with the desired
component loading(s). Promoted catalysts are most preferably
prepared by totally aqueous co-impregnation. Examples of typical
promoters include, but are not limited to: metal oxides, such as
oxides of Zr, La, K, and other oxides of elements from Groups IA,
IIA, IVA, VA, and VIA.
[0049] The totally aqueous impregnation of cobalt onto the support,
with or without one or more desired promoters, is preferably
accomplished by the steps of: (a) calcining the alumina support in
the manner described above; (b) impregnating the support with an
aqueous solution of cobalt nitrate, using a sufficient quantity of
the solution to achieve incipient wetness with the desired loading
of cobalt; (c) drying the resulting catalyst precursor for about
5-24 hours at approximately 80-130.degree. C., with moderate
mixing, to remove solvent water and obtain a dried catalyst; and
(d) calcining the dried catalyst in air or nitrogen by slowly
raising the temperature of the system at a rate of about
0.5-2.0.degree. C. per minute to approximately 250-400.degree. C.
and then holding for at least 2 hours to obtain the oxide form of
the catalyst. Multiple impregnation/coimpregnation steps (b) can be
used when higher cobalt loadings are desired.
[0050] Alkali (e.g., potassium) and/or rare earth oxide (e.g.,
lanthana)-promoted catalysts can be prepared, for example, by
dissolving potassium nitrate [KNO.sub.3], lanthana nitrate
[La(NO.sub.3).sub.3.multi- dot.6H.sub.2O], and/or other precursors
in the same solution which contains the cobalt precursor. Alkali
promoters, particularly potassium, can significantly improve
product selectivity and reduce methane production. Moreover, when
employed in proper amounts, the alkali promoters do not
substantially reduce catalyst activity. The addition of a lanthana
(La.sub.2O.sub.3) promoter can enhance the attrition resistance of
the catalyst. The improved attrition resistance provided by the
addition of La.sub.2O.sub.3 is not detrimental to Fischer-Tropsch
activity, or to Fischer-Tropsch selectivity. Preferred alkali and
lanthana concentration ranges are provided hereinabove.
[0051] Catalyst Activation
[0052] To provide optimum performance, it is presently preferred
that the catalyst be activated by reducing the catalyst in a
hydrogen-containing gas by slowly increasing the temperature of the
catalyst, preferably at a rate of about 0.5-2.0.degree. C./minute,
to approximately 250-400.degree. C. (preferably about 350.degree.
C.) and holding at the desired temperature for at least 2 hours.
After reduction, the catalyst is preferably cooled in flowing
nitrogen.
[0053] The reducing gas preferably comprises from about 1% to 100%
by volume of hydrogen, with the remainder (if any) being an inert
gas, typically nitrogen. The reducing gas is preferably delivered
at a rate of about 2-4 (preferably about 3) liters per hour per
gram of catalyst. The reduction procedure is preferably conducted
in a fluidized bed reactor vessel. The reduction procedure is most
preferably conducted at conditions (i.e., temperature, flow rate,
hydrogen concentration, etc.) effective to ensure that a very low
water vapor partial pressure is maintained during the
procedure.
[0054] As shown hereinbelow, this activation procedure unexpectedly
enhances the activity of the inventive "non-promoted" cobalt
catalysts.
[0055] The Fischer-Tropsch Reaction Process
[0056] The catalysts prepared and activated in accordance with the
present invention can be employed in generally any Fischer-Tropsch
synthesis process. Where applicable (e.g., for SBCR systems or
continuous stirred tank reactor (CSTR) systems), the catalyst will
preferably be slurried in a Fischer-Tropsch wax or in a synthetic
fluid (e.g., a C.sub.30 to C.sub.50 range isoparaffin
polyalphaolefin such as that available from Chevron under the name
SYNFLUID) having properties similar to those of Fischer-Tropsch
wax. The catalyst slurry will preferably have a catalyst
concentration in the range of from about 5% to about 40% by weight
based on the total weight of the slurry.
[0057] The synthesis gas feed used in the reaction process will
preferably have a CO:H.sub.2 volume ratio of from about 0.5 to
about 3.0 and will preferably have an inert gas (i.e., nitrogen,
argon, or other inert gas) concentration in the range of from 0 to
about 60% by volume based on the total volume of the feed. The
inert gas is preferably nitrogen.
[0058] Prior to initiating the reaction process, the activated
catalyst will most preferably be maintained in an inert atmosphere.
Before adding the catalyst thereto, the slurry fluid will
preferably be purged with nitrogen or other inert gas to remove any
dissolved oxygen. The slurry composition will also preferably be
transferred to the reaction system under an inert atmosphere.
[0059] A particularly preferred SBCR reaction procedure comprises
the steps of: (a) filling the SBCR, under an inert atmosphere, with
the activated catalyst slurry; (b) heating and pressurizing the
SBCR, under an inert atmosphere, to the desired pretreatment
conditions (preferably a temperature in the range of from about
220.degree. C. to about 250.degree. C. and a pressure in the range
of from about 50 to about 500 psig) ; (c) replacing the inert gas
with hydrogen and holding the system at these conditions for from
about 2 to about 20 hours; (d) purging the system with inert gas
and lowering the reaction system temperature, if necessary, to a
point at least about 10.degree. C. below the desired reaction
temperature; (e) carefully replacing the inert gas with the desired
synthesis gas; and (f) heating and pressurizing the reaction
system, as necessary, to a desired operating temperature,
preferably in the range of from about 190.degree. C. to about
300.degree. C., and a desired operating pressure, preferably in the
range of from about 50 to about 900 psig.
[0060] Cobalt Catalysts Without Noble Metal Promotion
[0061] It as long been believed that noble or near noble metal
promotion is necessary to provide cobalt catalysts which are truly
viable for commercial Fischer-Tropsch applications. Heretofore, the
accepted view in the art has been that a nonpromoted
Co/Al.sub.2O.sub.3 catalyst will only be from 50% to less than 25%
as active as an otherwise identical catalyst promoted with one or
more noble metals. U.S. Pat. No. 5,157,054 and other references
discuss the necessity of using ruthenium or other promoters to
obtain acceptable performance. However, the present invention
unexpectedly and surprisingly provides "nonpromoted",
cobalt-on-alumina catalysts having activities at least approaching
these of cobalt catalysts promoted with noble metals. As will be
apparent to those skilled in the art, eliminating the use of noble
metal promoters without significantly decreasing catalyst
performance greatly enhances the cost effectiveness of the
Fischer-Tropsch process, particulary in reaction systems
characterized by higher catalyst attrition losses.
[0062] As depicted in FIGS. 1 and 2, the tests described
hereinbelow amazingly show that the nonpromoted, cobalt-on-alumina
catalysts produced and activated in accordance with the present
invention perform at levels comparable to highly desirable,
ruthenium-promoted catalysts. These results were unexpectedly
obtained primarily through the use of doped .gamma.-alumina
supports. However, The catalyst activation procedure used,
particularly the maintenance of a very low water vapor partial
pressure during the reduction process, also surprisingly enhanced
the activity of the nonpromoted, cobalt-on-alumina catalysts.
[0063] The amount of dopant employed will preferably be an amount
effect to provide a catalyst activity which is at least 60%
(preferably at least 70%, more preferably at least 80%, and most
preferably at least 90%) of that of an otherwise identical catalyst
promoted with ruthenium in a ruthenium to cobalt weight ratio of
1:40. The amount of dopant required to obtain a desired activity
level for any given application can be readily determined.
[0064] As indicated above, the dopant is preferably employed in an
amount of at least 500 ppm (more preferably from about 800 ppm to
about 2000 ppm), based on the total weight of the .gamma.-alumina
support, and is most preferably added prior to the crystallization
of the boehmite precursor.
[0065] Low Water Vapor Partial Pressure
[0066] As to the beneficial results obtained by maintaining a very
low water vapor partial pressure during the reduction process, it
is believed that the presence of water vapor in the activation
system promotes the formation of certain cobalt-alumina compounds
which are very difficult or impossible to reduce. Temperature
programmed reduction studies conducted by Applicants indicate that,
in contrast to nonpromoted catalysts, the presence of noble metal
promoters such as ruthenium appear to enhance the reduction process
in a manner which may counteract the deleterious effects of water
vapor.
[0067] When activating a nonpromoted cobalt catalyst in accordance
with the present invention, the partial pressure of water vapor in
the activation system will preferably be maintained at or below a
level effective for increasing the activity of the catalyst. The
water vapor partial pressures effective for providing such results
can readily be determined for any nonpromoted, cobalt-on-alumina
catalyst. Although the values necessary to obtain the desired
results may vary depending upon the specific catalyst selected, it
is presently preferred that the partial pressure of water vapor in
the activation system be maintained below 0.1 atmospheres.
EXAMPLES
[0068] In the following examples 1-4, certain cobalt-on-alumina
catalysts were prepared and then tested in various Fischer-Tropsch
reaction systems. Before testing, each catalyst was reduced in pure
hydrogen by slowly increasing the temperature of the catalyst, at a
rate of about 1.0.degree. C. per minute, to about 350.degree. C.
and holding at this temperature for 10 hours. The hydrogen was
delivered at a rate of about 3 liters per hour per gram of
catalyst. After reduction, the catalyst was cooled in flowing
nitrogen.
[0069] For slurry bubble column reactor (SBCR) tests conducted in
examples 1-4, the reduction procedure was conducted in a fluidized
bed reactor. After cooling to ambient temperature, the catalyst was
weighed, slurried in SYNFLUID, and then transferred to the SBCR
under an inert atmosphere. The SBCR tests were then conducted at
230.degree. C. and 450 psig using 900 sl/hr of syngas and from 15
to 25 grams of reduced catalyst. The syngas contained 60% nitrogen
and had a H.sub.2/CO ratio of 2. In each case, the SBCR results
cited are those obtained after 24 hours on-stream.
[0070] For fixed bed micro-reactor (FBR) tests, the catalyst was
reduced in-situ using the same procedure just described. Prior to
the introduction of syngas, the reduced catalyst was cooled to
about 10-15.degree. C. below the reaction temperature. The FBR
tests were then conducted under differential conditions (i.e. low
conversion) at atmospheric pressure and 220.degree. C. using from
0.5 to 1.0 grams of catalyst and a H.sub.2/CO ratio of 2. In each
case, the FBR results cited are those obtained after 24 hours
on-stream.
Example 1
Effect of Titania Impurities on the Activities of Ru-Promoted
Catalysts in a Slurry Bubble Column Reactor (SBCR)
[0071] The following ruthenium-promoted, cobalt-on-alumina
catalysts were identically prepared and had identical loadings of
cobalt and ruthenium. The catalysts differed only with respect to
the amounts of titanium "impurity" contained in the .gamma.-alumina
supports. The aluminas were all manufactured by Condea/Vista.
[0072] CATALYST 1: (Ru-promoted cobalt catalyst on CATAPAL B
alumina with 20 wt % cobalt and 0.5 wt % ruthenium)
[0073] Preparation Procedure:
[0074] CATAPAL B alumina, supplied by Condea/Vista in the boehmite
form, was calcined at 500.degree. C. for 10 hrs to convert it to
.gamma.-alumina. It was then preserved to 400-170 mesh (i.e., a
particle size range of from more than 38 microns to less than 88
microns) and impregnated using an amount of an aqueous solution of
cobalt nitrate [Co(NO.sub.3).sub.26H.sub.2O] and ruthenium (III)
nitrosylnitrate [Ru(NO)(NO.sub.3).sub.3.multidot.xH.sub.2O]
appropriate to achieve incipient wetness (ca. 1.2 ml/g) with the
desired loadings of Co. and Ru. The catalyst precursor was then
dried in air at 115.degree. C. for 5 hours and calcined in air at
300.degree. C. for 2 hours (with a heating rate of ca. 1.degree.
C./min to 300.degree. C.).
[0075] Reduction Procedure before Reaction:
[0076] The catalyst was reduced in 3000 cc/g/hr of pure hydrogen by
heating at 1.degree. C./min to 350.degree. C. and holding for 10
hrs.
[0077] Each of the following catalysts 2 and 3 were prepared and
reduced in the same manner as catalyst 1. The specific supports
employed in catalysts 2 and 3 were as follows:
[0078] CATALYST 2: PURAL SB support supplied by Condea/Vista.
[0079] CATALYST 3: PURAL SB1 support supplied by Condea/Vista.
[0080] The particular CATAPAL B support material employed in
catalyst 1 was determined to contain an amount of titania
"impurity" of about 1000 ppm by weight (expressed as ppm by weight
of titanium) which was incidentally added as part of the Ziegler
Process prior to the crystallization of the boehmite. In contrast,
the particular PURAL SB support material employed in catalyst 2 had
been formed by a blending process and was found to contain about
500 ppm of titanium. The PURAL SB1 support employed in catalyst 3
was identical to the PURAL SB support except that efforts were made
to prevent the addition of titanium. An elemental analysis showed
that the PURAL SB1 support contained only 7 ppm of titanium. The
crystallite characteristics of the CATAPAL B, PURAL SB and PURAL
SB1 supports were substantially identical.
[0081] FIG. 1 shows the activities (expressed in g-HC/kg-cat/hr)
exhibited by catalysts 1-3 in an SBCR at the end of 24 hours
on-stream. A comparison of catalysts 1-3 illustrates the
detrimental effect of titania on the activities of
ruthenium-promoted, cobalt-on-alumina catalysts. As the amount of
titania in the support increased, activity declined significantly.
Catalyst 3 provided an activity of about 1400 and had selectivities
(% C) of 80.5 for C.sub.5.sup.+ and 8.4 for CH.sub.4. Catalyst 2
provided an activity of about 1322 and had selectivities for
C.sub.5.sup.+ and CH.sub.4 of 81.9 and 8.5, respectively. Catalyst
1 provided an activity of about 1195 and had selectivities of 82.2
(C.sub.5.sup.+) and 8.2 (CH.sub.4).
Example 2
Effect of Titania Doping on the Activities of Non-Promoted
Catalysts in a Slurry Bubble Column Reactor
[0082] The following catalysts 4-6 were respectively identical to
catalysts 1-3, except that catalysts 4-6 did not include any
promoters.
[0083] CATALYST 4 (Non-promoted, alumina supported catalyst with 20
wt % Cobalt)
[0084] Preparation Procedure:
[0085] CATAPAL B alumina, supplied by Condea/Vista in the boehmite
form, was calcined at 500.degree. C. for 10 hrs to convert it to
.gamma.-alumina. It was then presieved to 400-170 mesh (i.e, a
particle size range of from more than 38 microns to less than 88
microns) and impregnated using an amount of an aqueous solution of
Co nitrate [Co(NO.sub.3).sub.2.multidot.6HO] appropriate to achieve
incipient wetness (ca. 1.2 ml/g) with the desired loading of Co.
The catalyst precursor was then dried in air at 115.degree. C. and
calcined in air at 300.degree. C. for 2 hours (heating rate of ca.
1.degree. C./min to 300.degree. C.).
[0086] Reduction Procedure before Reaction:
[0087] The catalyst was reduce in a pure hydrogen flow of 3000
cc/g/hr by heating at 1.degree. C./min to 350.degree. C. and
holding for 10 hours.
[0088] Each of the following catalysts 5 and 6 were prepared and
reduced in the same manner as catalyst 4. The specific supports
employed in catalysts 5 and 6 were as follows:
[0089] CATALYST 5: PURAL SB as described above.
[0090] CATALYST 6: PURAL SB1 as described above.
[0091] Catalysts 4-6 were tested in a slurry bubble column reactor.
Their activities (expressed in g-HC/kg-cat/hr) after 24
hours-on-stream are also shown in FIG. 1. In stark contrast to the
results obtain with ruthenium-promoted catalysts 1-3, the presence
of titania in the .gamma.-alumina support unexpected and
surprisingly improved significantly the activities of the
non-promoted catalysts. Catalyst 6 (7 ppm Ti) provided an activity
of about 606 and had selectivities (% C) of 85.6 (C.sub.5.sup.+)
and 5.2 (CH.sub.4). Catalyst 5 (500 ppm Ti) provided an activity of
about 775 and had selectivities of 84.0 (C.sub.5.sup.+) and 6.2
(CH.sub.4). Catalyst 4 (1000 ppm Ti) provided an activity of about
1032 and had selectivities of 86.5 (C.sub.5.sup.+) and 6.0
(CH.sub.4). Thus, in this SBCR test, the activity of non-promoted
catalyst 4 was 86% of that provided ruthenium-promoted catalyst 1.
Further, the selectivities provided by catalyst 4 were
significantly superior to those of the ruthenium-promoted
catalysts.
Example 3
Effect of Titania Doping on the Activities of Non-Promoted
Catalysts in a Fixed Bed Reactor
[0092] Ruthenium-promoted catalysts 1 and 3 and non-promoted
catalysts 4 and 6 were also tested in a fixed bed micro-reactor
(FBR) under the conditions described above, (atmospheric pressure
and a temperature of 220.degree. C.). FIG. 2 illustrates again the
significant, unexpected, and surprising effect of titania doping on
the activities of the non-promoted cobalt catalysts. While the
activities exhibited by the two ruthenium-promoted catalysts
remained relatively constant (220 and 200 g-HC/kg-cat/h for
Catalysts 1 and 3, respectively) the activity of the titanium-doped
(1000 ppm), non-promoted catalyst 4 was three times higher than
that of non-doped (7 ppm), non-promoted catalyst 6. Moreover,
whereas the activity of non-promoted catalyst 6 (7 ppm Ti) was only
about 25% of that of promoted catalyst 3, non-promoted catalyst 4
(1000 ppm Ti) amazingly provided an activity level which was about
75% of that of catalyst 3.
Example 4
Effect of Titania Doping on the Performances of Ru-Promoted and
Non-Promoted Catalysts in a Continuously Stirred Tank Reactor
(CSTR)
[0093] In order to ascertain whether the SBCR results shown in FIG.
I represented the intrinsic activities of the non-promoted,
cobalt-on-alumina catalysts, ruthenium-promoted Catalyst 1 and
non-promoted Catalyst 4 were tested in a continuously stirred tank
reactor (CSTR). As discussed above, both catalysts were supported
on CATAPAL B alumina containing about 1000 ppm by weight of
titanium. In a CSTR, mass transfer limitations are substantially
negligible such that true intrinsic kinetics can be measured. The
performance of the two cobalt catalysts in the CSTR was evaluated
under reaction conditions substantially similar to those employed
in the SBCR. Amazingly, in accordance with the unexpected and
surprising results obtained in the SBCR and FBR tests, the
activities exhibited by promoted catalyst 1 and nonpromoted
catalyst 4 in the CSTR were, within experimental error,
substantially the same. At 240.degree. C. and 450 psig,
ruthenium-promoted Catalyst 1 and non-promoted Catalyst 4 exhibited
activities of 1390 and 1330 (g-HC/g-cat/h), respectively.
Example 5
Temperature Programmed Reduction Studies of the Effects of Titania
Doping
[0094] It is well known that noble metal promoters enhance the
reducibility of cobalt. The effects of titania on the
reducibilities of non-promoted catalysts 4 and 6 were determined
using temperature programmed reduction (TPR). The results were
compared to TPR results obtained for ruthenium-promoted catalysts 1
and 3.
[0095] In each case, the calcined catalyst was initially dried,
under an argon flow, at 120.degree. C. for 30 minutes to remove
water. In the TPR tests, a 5% H.sub.2/Ar gas mixture was used as
the reducing gas. The reducing gas flow rate was 30 cm.sup.3/min.
During the reduction test, the catalyst was heated to 900.degree.
C. at a rate of 5.degree. C./min. The effluent gas was delivered
through a cooling trap (less than 50.degree. C.) to condense and
collect water generated by the reduction process. The amount of
H.sub.2 consumed by the catalyst was monitored, using a thermal
conductivity detector (TCD), and recorded as a function of
temperature. From this data (and assuming the oxide to be in the
form of Co.sub.3O.sub.4), the total reducibility of each catalyst,
up to a temperature of 900.degree. C., was determined and expressed
as the percent of cobalt completely reduced. The percent
reducibilities of catalysts 1, 3, 4, and 6 at 900.degree. C. are
shown in Table 1. Table 1 also provides the low and high reduction
temperature peaks exhibited by each catalyst.
1TABLE 1 Temperature Programmed Reduction Results Low Temperature
High Temperature Reducibility Catalyst Peak (.degree. C.) Peak
(.degree. C.) (%) Catalyst 1 253 483 98 Catalyst 3 241 471 99
Catalyst 4 238/322 587 99 Catalyst 6 328 584 90
[0096] As expected, both of the ruthenium-promoted catalysts were
almost totally reduced (98+%) under the conditions used in the TPR
experiments. In addition, they reduced at lower temperatures than
their non-promoted analogs. However, a comparison of the results
obtained for the non-promoted catalysts shows, again, that titania
doping provided a significant, beneficial effect. Like the
ruthenium-promoted catalysts, the non-promoted catalyst having a
doped support was completely (99%) reduced. However, the
non-promoted catalyst having a no dopant was only 90% reduced.
[0097] It is believed that certain, hard-to-reduce, cobalt-alumina
compounds can form during reduction, especially when, as is
typically the case, the reduction system has a relatively high
water vapor partial pressure. Noble metal promoters, which allow
cobalt reduction at lower temperature, either help to prevent the
formation of these compounds and/or enhance their reducibility. Our
findings suggest that, for a non-promoted cobalt-on-alumina
catalyst, the presence in the support of a controlled amount of
dopant helps prevent the formation of cobalt-alumina compounds,
thus improving the overall reducibility of the catalyst. This would
explain the significant improvement in Fischer-Tropsch activity
observed for nonpromoted, cobalt-on alumina catalysts having doped
supports.
Example 6
[0098] To test the effects of water vapor partial pressure during
the reduction process, two different batches of the same
nonpromoted, 20 wt % cobalt-on-alumina catalysts (i.e., a 50 g (lab
size) batch and a 220 g batch) were reduced as set forth below and
then tested in an SBCR.
[0099] Test 1: The 50 g (lab size) batch was reduced at 350.degree.
C. for 18 hours with 100% H.sub.2 at a flow rate above 3 l/hr per
gram of catalyst. The SBCR test was started with a charge of 15 g
of the reduced catalyst. The reaction conditions were as follows: a
temperature of 220.degree. C., a pressure of 450 psig, a total gas
flow rate of 900 sl/hr with 60% N.sub.2, and a H.sub.2/CO ratio of
2. The CO conversion was 13.6%, the hydrocarbon productivity
(activity) was 0.66 g HC/g cat./hr, and the CH.sub.4 selectivity
was 3.2%.
[0100] Test 2: The 220 g batch of the same catalyst was reduced at
the same conditions except that a hydrogen flow rate of 1.8 l/hr
per gram of catalyst was used. The SBCR test was started with a 15
g charge of catalyst and was run under the same reaction conditions
as in Test 1. The CO conversion was only 3.4%, with a hydrocarbon
productivity (activity) of just 0.17 g HC/g cat./hr.
[0101] A re-reduction of the catalyst employed in Test 2, using a
smaller batch, did not produce better results. The catalyst was
irreversibly damaged during the first reduction. Additional large
batch reductions (ca. 0.6 - 1 kg) produced similarly inactive or
very low activity catalysts.
[0102] These results suggest that a higher water vapor partial
pressure present during the reduction of the larger batches had a
negative effect on the reducibility of the catalyst.
Example 7
[0103] Following the unsuccessful, attempted reduction of large
batches of the non-promoted Co/Al.sub.2O.sub.3 catalyst, and in
view of the belief that the presence of a relatively high water
vapor partial pressure during the initial stages of the reduction
process was responsible for their low reducibility, it was
suggested that higher reduction temperatures might provide improved
reducibility. Hence, two new lab size batches (ca. 50 g) of the
same unpromoted Co/Al.sub.2O.sub.3 catalyst tested in Example 6
were reduced at 410.degree. C. and 380.degree. C., respectively. In
order to test the effect of high water vapor partial pressure, the
hydrogen stream used for each reduction was saturated with water
vapor by passing it through a saturator, at room temperature,
before use in the reduction system.
[0104] The SBCR tests were again carried out at the same reaction
conditions as described in Example 6. In spite of the different
reduction temperatures used, each batch had a CO conversion
activity of below 1%. The results showed that the high
concentration of water in the hydrogen stream had a drastic effect
on catalyst reducibility and that, under such conditions, the use
of an increased reduction temperature did not improve the
reducibility of the catalyst. The high water vapor partial pressure
is believed to have caused the formation of cobalt-alumina
compounds which were very difficult or impossible to reduce. The
use of a higher reduction temperature seemed to actually promote
the production of such compounds rather than enhance the reduction
process.
[0105] Thus, the present invention is well adapted to carry out the
objects and attain the ends and advantages mentioned above, as well
as those inherent therein. While the invention has been described
with a certain degree of particularity, it is manifest that many
changes may be made without departing from the spirit and scope of
this disclosure. It is understood that the invention is not limited
to the embodiments set forth herein for purposes of
exemplification.
* * * * *