U.S. patent application number 11/028077 was filed with the patent office on 2005-05-26 for lanthanide-promoted rhodium catalysts and process for producing synthesis gas.
This patent application is currently assigned to ConocoPhillips Company. Invention is credited to Allison, Joe D., Hu, Baili, Minahan, David M., Niu, Tianyan, Ramani, Sriram, Ricketson, Kevin L., Straguzzi, Gloria I., Swinney, Larry D., Wang, Daxiang, Wright, Harold A..
Application Number | 20050112047 11/028077 |
Document ID | / |
Family ID | 22861903 |
Filed Date | 2005-05-26 |
United States Patent
Application |
20050112047 |
Kind Code |
A1 |
Allison, Joe D. ; et
al. |
May 26, 2005 |
Lanthanide-promoted rhodium catalysts and process for producing
synthesis gas
Abstract
Lanthanide-promoted rhodium-containing supported catalysts that
are active for catalyzing the net partial oxidation of methane to
CO and H.sub.2 are disclosed, along with their manner of making and
high efficiency processes for producing synthesis gas employing the
new catalysts. A preferred catalyst comprises highly dispersed,
high surface area rhodium on a granular zirconia support with an
intermediate coating of a lanthanide metal and/or oxide thereof and
is thermally conditioned during catalyst preparation. In a
preferred syngas production process a stream of methane-containing
gas and O.sub.2 is passed over a thermally conditioned, high
surface area Rh/Sm/zirconia granular catalyst in a short contact
time reactor to produce a mixture of carbon monoxide and
hydrogen.
Inventors: |
Allison, Joe D.; (Ponca
City, OK) ; Swinney, Larry D.; (Stillwater, OK)
; Niu, Tianyan; (Katy, TX) ; Ricketson, Kevin
L.; (Ponca City, OK) ; Wang, Daxiang; (Ponca
City, OK) ; Ramani, Sriram; (Ponca City, OK) ;
Straguzzi, Gloria I.; (Ponca City, OK) ; Minahan,
David M.; (Stillwater, OK) ; Wright, Harold A.;
(Ponca City, OK) ; Hu, Baili; (Miami, FL) |
Correspondence
Address: |
DAVID W. WESTPHAL
CONOCOPHILLIPS COMPANY - I.P. Legal
P.O. BOX 1267
PONONCA CITY
OK
74602-1267
US
|
Assignee: |
ConocoPhillips Company
Houston
MA
|
Family ID: |
22861903 |
Appl. No.: |
11/028077 |
Filed: |
January 3, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11028077 |
Jan 3, 2005 |
|
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09946305 |
Sep 5, 2001 |
|
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60229595 |
Sep 5, 2000 |
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Current U.S.
Class: |
423/418.2 ;
252/373; 423/651; 502/302; 502/325 |
Current CPC
Class: |
C01B 2203/1041 20130101;
B01J 37/0205 20130101; C01B 3/386 20130101; C01B 2203/1023
20130101; C01B 2203/1241 20130101; C01B 2203/1064 20130101; Y02P
20/52 20151101; C01B 2203/1094 20130101; C01B 2203/0261 20130101;
C01B 3/40 20130101; C01B 2203/1017 20130101; B01J 23/63 20130101;
C01B 2203/1082 20130101; B01J 35/04 20130101 |
Class at
Publication: |
423/418.2 ;
252/373; 423/651; 502/325; 502/302 |
International
Class: |
B01J 023/00; C10K
001/00; C07C 001/02; C01B 031/18; C01B 031/24; C01B 003/26 |
Claims
What is claimed is:
1. A method of partially oxidizing a reactant gas mixture
comprising a light hydrocarbon and oxygen to form a product mixture
containing carbon monoxide and hydrogen, the method comprising: in
a reactor, passing said reactant gas mixture over a highly
dispersed, high surface area rhodium based catalyst structure such
that the reactant gas mixture is exposed to a significant portion
of the rhodium, said catalyst structure characterized by having a
metal surface area of at least 1.25 square meters of metal per gram
of catalyst structure, such that a product mixture containing
carbon monoxide and hydrogen is formed.
2. The method of claim 1 wherein said catalyst structure is
characterized by having a metal surface area of at least 1.5 square
meters of metal per gram of catalyst structure.
3. The method of claim 1 wherein said catalyst structure is
characterized by having a metal surface area of at least 2.0 square
meters of metal per gram of catalyst structure.
4. The method of claim 1 wherein the rhodium surface area of said
catalyst is at least 1.25 square meters of rhodium per gram of
catalyst structure.
5. A catalyst structure having catalytic activity in a partial
oxidation reaction process, wherein the catalyst structure
comprises: a refractory support; and highly dispersed, high surface
area rhodium disposed on said refractory support, said catalyst
structure characterized in that the catalyst structure has a metal
surface area of at least about 1.25 square meters of metal per gram
of catalyst structure.
6. The catalyst structure according to claim 5, wherein the metal
surface area is at least about 1.5 square meters of metal per gram
of catalyst structure.
7. The catalyst structure according to claim 6, wherein the metal
surface area is at least about two square meters of metal per gram
of catalyst structure.
8. The catalyst structure according to claim 5 further including a
lanthanide or lanthanide oxide disposed between said rhodium and
said refractory support.
9. The catalyst structure according to claim 5 wherein the rhodium
and lanthanide are present on the catalyst support in a ratio of
rhodium to lanthanide in the range of about 0.5 to about 2.
10. The catalyst structure of claim 9 wherein the rhodium and
lanthanide are present on the catalyst support in a ratio of
rhodium metal to lanthanide metal in the range of about 0.5 to
about 2 and the rhodium comprises a majority of the metal surface
area.
11. The catalyst structure according to claim 5 wherein the
lanthanide is one of praseodymium, samarium, and ytterbium.
12. The catalyst structure of claim 10 wherein the lanthanide is
samarium.
13. The catalyst structure of claim 5, wherein the refractory
support comprises a metal oxide wherein the metal has an atomic
number less than 58.
14. A method of making a high metal surface area catalyst structure
having catalytic activity in a partial oxidation reaction process,
the method comprises: selecting a refractory support; applying
rhodium and a lanthanide on said refractory support in such manner
as to form a catalyst structure having a metal surface area of at
least about 1.25 square meters of metal per gram of catalyst
structure.
15. The method of claim 14, wherein said step of applying rhodium
and lanthanide comprises: making a solution comprising a
decomposable rhodium precursor compound and a separate solution
comprising a decomposable lanthanide precursor compound, applying
said solutions in separate steps to a refractory support, and
stabilizing at least the first applied said lanthanide or rhodium
on the refractory support prior to application of the second
solution.
16. The method of claim 15, wherein the step of stabilizing the
first applied said lanthanide or rhodium comprises thermally
conditioning the refractory support with the first rhodium or
lanthanide compound thereon, and wherein the method further
comprises a calcining step after the second solution has been
applied to the refractory support.
17. The method of claim 15, wherein the lanthanide is chosen from
the group consisting of praseodymium, samarium, and ytterbium, and
the lanthanide solution is applied to the support prior to the
application of the rhodium solution.
18. The method of claim 14, wherein the step of selecting the
refractory support comprises selecting a refractory support
containing a metal oxide, the metal of which having an atomic
number less than 58 and wherein the lanthanide is praseodymium,
samarium or ytterbium.
19. The method of claim 14, wherein the metal surface area is at
least about 1.5 square meters of metal per gram of the catalyst
structure.
20. The method of claim 14, wherein the metal surface area is at
least about two square meters of metal per gram of the catalyst
structure.
21. The method of claim 14, wherein the step of applying rhodium
and lanthanide further comprises applying the rhodium and
lanthanide so as to form a catalyst structure having a ratio of
rhodium to lanthanide of between about 0.5 and about 2.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 09/946,305 filed Sep. 5, 2001, which claims the benefit of
U.S. Provisional Patent Application No. 60/229,595 filed Sep. 5,
2000, the disclosures of which are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention generally relates to processes for the
catalytic partial oxidation of hydrocarbons (e.g., natural gas) to
produce a mixture of carbon monoxide and hydrogen ("synthesis gas"
or "syngas").
[0004] 2. Description of Related Art
[0005] The quantities of methane, the main component of natural
gas, are available in many areas of the world, and natural gas is
predicted to outlast oil reserves by a significant margin. However,
most natural gas is situated in areas that are geographically
remote from population and industrial centers. The costs of
compression, transportation, and storage make its use economically
unattractive.
[0006] To improve the economics of natural gas use, much research
has focused on methane as a starting material for the production of
higher hydrocarbons and hydrocarbon liquids. The conversion of
methane to hydrocarbons is typically carried out in two steps. In
the first step, methane is reformed with water to produce carbon
monoxide and hydrogen (i.e., synthesis gas or syngas). In a second
step, the syngas intermediate is converted to higher hydrocarbon
products by processes such as the Fischer-Tropsch Synthesis. For
example, fuels with boiling points in the middle distillate range,
such as kerosene and diesel fuel, and hydrocarbon waxes may be
produced from the synthesis gas.
[0007] Current industrial use of methane as a chemical feedstock
proceeds by the initial conversion of methane to carbon monoxide
and hydrogen by either steam reforming, which is the most
widespread process, or by dry reforming or by autothermal
reforming. Steam reforming currently is the major process used
commercially for the conversion of methane to synthesis gas,
proceeding according to Equation 1.
CH.sub.4+H.sub.2OCO+3H.sub.2 (1)
[0008] Although steam reforming has been practiced for over five
decades, efforts to improve the energy efficiency and reduce the
capital investment required for this technology continue. For many
industrial applications, the 3:1 ratio of H.sub.2:CO products is
problematic, and the typically large steam reforming plants are not
practical to set up at remote sites of natural gas formations.
[0009] Methane residence times in steam reforming are on the order
of 0.5-1 second, whereas for heterogeneously catalyzed partial
oxidation, the residence time is on the order of a few
milliseconds. For the same production capacity, syngas facilities
for the partial oxidation of methane can be far smaller, and less
expensive, than facilities based on steam reforming. A recent
report (M. Fichtner et al., Ind. Eng. Chem. Res. (2001)
40:3475-3483) states that for efficient syngas production, the use
of elevated operation pressures of about 2.5 MPa is required. Those
authors describe a partial oxidation process in which the
exothermic complete oxidation of methane is coupled with the
subsequent endothermic reforming reactions (water and CO.sub.2
decomposition). This type of process can also be referred to as
autothermal reforming or ATR, especially when steam is co-fed with
the methane. Certain microstructured rhodium honeycomb catalysts
are employed which have the advantage of a smaller pressure drop
than beds or porous solids (foams) and which resist the reaction
heat of the total oxidation reaction taking place at the catalyst
inlet.
[0010] The catalytic partial oxidation ("CPOX") or direct partial
oxidation of hydrocarbons (e.g., natural gas or methane) to syngas
has also been described in the literature. In catalytic partial
oxidation, natural gas is mixed with air, oxygen-enriched air, or
oxygen, and introduced to a catalyst at elevated temperature and
pressure. The partial oxidation of methane yields a syngas mixture
with a H.sub.2:CO ratio of 2:1, as shown in Equation 2.
CH.sub.4+1/2O.sub.2.fwdarw.CO+2H.sub.2 (2)
[0011] This ratio is more useful than the H.sub.2:CO ratio from
steam reforming for the downstream conversion of the syngas to
chemicals such as methanol or to fuels. The CPOX reaction is
exothermic, while the steam reforming reaction is strongly
endothermic. Furthermore, oxidation reactions are typically much
faster than reforming reactions. This allows the use of much
smaller reactors for catalytic partial oxidation processes that is
possible in a conventional steam reforming process.
[0012] While its use is currently limited as an industrial process,
the direct partial oxidation or CPOX of methane has recently
attracted much attention due to its inherent advantages, such as
the fact that due to the significant heat that is released during
the process, there is no requirement for the continuous input of
heat in order to maintain the reaction, in contrast to steam
reforming processes. An attempt to overcome some of the
disadvantages and costs typical of steam reforming by production of
synthesis gas via the catalytic partial oxidation of methane is
described in European Patent No. 303,438. According to that method,
certain high surface area monoliths coated with metals or metal
oxides that are active as oxidation catalysts, e.g., Pd, Pt, Rh,
Ir, Os, Ru, Ni, Cr, Co, Ce, La, and mixtures thereof, are employed
as catalysts. Other suggested coating metals are noble metals and
metals of groups IA, IIA, III, IV, VB, VIB, or VIIB of the periodic
table of the elements.
[0013] U.S. Pat. No. 5,149,464 describes a method for selectively
converting methane to syngas at 650-950.degree. C. by contacting a
methane/oxygen mixture with a solid catalyst which is a d-block
transition metal on a refractory support, an oxide of a d-block
transition metal, or a compound of the formula
M.sub.xM'.sub.yO.sub.z wherein M' is a d-block transition metal and
M is Mg, B, Al, Ga, Si, Ti, Zr, Hf or a lanthanide. U.S. Pat. No.
5,500,149 describes the combination of dry reforming and partial
oxidation of methane, in the presence of added CO.sub.2 to enhance
the selectivity and degree of conversion to synthesis gas. U.S.
Pat. No. 5,431,855 demonstrates the catalytic conversion of
mixtures of CO.sub.2, O.sub.2 and CH.sub.4 to synthesis gas over
selected alumina supported transition metal catalysts. Maximum CO
yield reported was 89% at a gas hourly space velocity (GHSV) of
1.5.times.10.sup.4 hr.sup.-1, temperature of 1,050.degree. K and
pressure of 100 kPa. The addition of CO.sub.2 tends to reduce the
H.sub.2:CO ratio of the synthesis gas, however.
[0014] For successful commercial scale operation a catalytic
partial oxidation process must be able to achieve a high conversion
of the methane feedstock at high gas hourly space velocities, and
the selectivity of the process to the desired products of carbon
monoxide and hydrogen must be high. Dietz III and Schmidt
(Catalysis Letters (1995) 33:15-29) describe the effects of 1.4-6
atmospheres pressure on methane conversion and product
selectivities in the direct oxidation of methane over a Rh-coated
foam monolith. The selectivities of catalytic partial oxidation to
the desired products, carbon monoxide and hydrogen, are controlled
by several factors. One of the most important of these factors is
the choice of catalyst composition. In most of the existing syngas
production processes it is difficult to select a catalyst that will
be economical for large scale industrial use, yet will provide the
desired level of activity and selectivity for CO and H.sub.2 and
demonstrate long on-stream life. Moreover, such high conversion and
selectivity levels must be achieved without detrimental effects to
the catalyst, such as the formation of carbon deposits ("coke") on
the catalyst, which severely reduces catalyst performance.
Accordingly, substantial effort in this field continues to be
devoted to the development of catalysts allowing commercial
performance without coke formation. Also, in order to overcome the
relatively high pressure drop associated with gas flow through a
fixed bed of catalyst particles, and to make possible the operation
of the reactor at high gas space velocities, various types of
structures for supporting the active catalyst in the reaction zone
have been proposed. For example, U.S. Pat. No. 5,510,056 discloses
a monolithic support such as a ceramic foam or fixed catalyst
system having a specified tortuosity and number of interstitial
pores that is said to allow operation at high gas space velocity.
Catalysts used in that process include ruthenium, rhodium,
palladium, osmium, iridium, and platinum. Data are presented in
that patent for a ceramic foam supported rhodium catalyst at a
rhodium loading of from 0.5-5.0 wt %.
[0015] U.S. Pat. No. 5,648,582 also discloses a process for the
catalytic partial oxidation of a feed gas mixture consisting
essentially of methane. The methane-containing feed gas mixture and
an oxygen-containing gas are passed over an alumina foam supported
metal catalyst at space velocities of 120,000 hr..sup.-1 to
12,000,000 hr..sup.-1 The catalytic metals exemplified are rhodium
and platinum, at a loading of about 10 wt %.
[0016] Vernon, D. F. et al. (Catalysis Letters 6:181-186 (1990))
describe the partial oxidation of methane to synthesis gas using
various transition metal catalysts such as Pd, Pt, Ru or Ni on
alumina, or certain transition metal oxides including
Pr.sub.2Ru.sub.2O.sub.7 and Eu.sub.2Ir.sub.2O.sub.7, under a range
of conditions.
[0017] U.S. Pat. No. 5,447,705 discloses a catalyst for the partial
oxidation of methane having a perovskite crystalline structure and
the general composition: Ln.sub.xA.sub.1-yB.sub.yO.sub.3, wherein
Ln is a lanthanide and A and B are different metals chosen from
Group IVb, Vb, VIIb, VIIb or VIII of the Periodic Table of the
Elements.
[0018] K. L. Hohn and L. D. Schmidt (Applied Catalysis A: General
(2001) 211:53-68) describe the effect of space velocity on the
partial oxidation of methane using two types of catalyst support
geometries. Synthesis gas production by certain rhodium coated
monoliths and spheres is discussed, and it is suggested that
differences in heat transfer within the two support geometries may
play a major role in the different results in catalytic performance
observed between spheres and monoliths at increased space velocity.
Factors other than chemistry, such as mass and heat transfer within
the catalyst region, appear to be important at high flow rates.
[0019] PCT Patent Application Publication No. WO 93/01130 describes
another catalyst for the production of carbon monoxide from
methane. The catalyst is composed of Pd, Pt, Rh or Ir on a pure
lanthanide oxide, which may be carried on a ceramic support,
preferably zirconia. Pd on Sm.sub.2O.sub.3 gives relatively low
selectivity for either CO or CO.sub.2, compared to the
selectivities reported for the other compositions evaluated in that
study. The methane conversion process is performed with supplied
heat, the feed gases comprise very low amount of O.sub.2, and very
low amounts of H.sub.2 are produced as a byproduct of the
process.
[0020] A. T. Ashcroft et al. (Nature 344:319-321 (1990)) describe
the selective oxidation of methane to synthesis gas using
ruthenium-lanthanide containing catalysts. The reaction was carried
out at a gas hourly space velocity (GHSV) of 4.times.10.sup.4
hr.sup.-1 and normal atmospheric pressure. A nitrogen diluent was
employed to enhance activity and selectivity.
[0021] Lapszewicz et al. (proceedings of the Symposium on Chemistry
and Characterization of Supported Metal Catalysts presented before
the Division of Petroleum Chemistry, Inc. 206.sup.th National
Meeting, American Chemical Society, Chicago, Ill., (Aug. 22-27,
1993) pp. 815-818) describe the use of certain Rh catalysts on pure
Sm.sub.2O.sub.3 and Pt group metals on MgO for catalyzing the
partial oxidation of natural gas to syngas. That report focuses on
CH.sub.4 conversion to carbon monoxide, which reaches a maximum
level of 80% using 0.5% Rh on Sm.sub.2O.sub.3 as the catalyst.
[0022] Ruckenstein and Wang (Appl. Catal., A (2000), 198:33-41)
describe certain MgO supported Rh catalysts which, at 750.degree.
C. and 1 atm, provided a conversion >80% and selectivities of
95-96% to CO and 96-98% to H.sub.2, at the high space velocity of
7.2.times.10.sup.5 mL/g.sup.-1 h.sup.-1, with very high stability.
Those authors report that there was no significant deactivation of
the catalyst for up to 96 h of reaction. The strong interactions
between rhodium and magnesium oxides are suggested to be
responsible for the high stability of the catalyst. In today's
syngas production processes, productivity typically falls off when
the process is operated at superatmospheric pressure.
[0023] Another potential disadvantage of many of the existing
catalytic hydrocarbon conversion methods is the need to include
steam in the feed mixture to suppress coke formation on the
catalyst. Typically, the ratio of steam to methane, or other light
hydrocarbon, in the feed gas must be maintained at 1:1 or greater.
The volume of gaseous H.sub.2O significantly reduces the available
reactor space for the production of synthesis gas. Another
disadvantage of using steam in the production of syngas is that
steam increases the production of CO.sub.2, which is carbon that is
lost to the process of making CO product. Other existing methods
have the potential drawback of requiring the input of a CO.sub.2
stream in order to enhance the yield and selectivity of CO and
H.sub.2 products. Another drawback of some existing processes is
that the catalysts that are employed often result in the production
of significant quantities of carbon dioxide, steam, and C.sub.2+
hydrocarbons. This often renders the product gas mixture
unsuitable, for example, for feeding directly into a
Fischer-Tropsch type catalytic system for further processing into
higher hydrocarbon products. Moreover, for efficient syngas
production, the use of elevated operation pressures is necessary in
order to ensure the direct transition to a downstream process, such
as a Fischer-Tropsch process, without the need for intermediate
compression.
[0024] At the present time, none of the known processes appear
capable of sufficiently high space-time yields. Typically, partial
oxidation reactor operation under pressure is problematic because
of shifts in equilibrium, undesirable secondary reactions, coking
and catalyst instability. Another problem frequently encountered is
loss of noble metals due to catalyst instability at higher
operating temperatures. Although advancement has been made toward
providing higher levels of conversion of reactant gases and better
selectivities for CO and H.sub.2 reaction products, problems still
remain with finding sufficiently stable and long-lived catalysts
capable of conversion rates that are attractive for large scale
industrial use. Accordingly, a continuing need exists for better
processes and catalysts for the production of synthesis gas,
particularly from methane or methane containing feeds. In such
improved processes the catalysts would be stable at high
temperatures and resist coking. They would also retain a high level
of conversion activity and selectivity to carbon monoxide and
hydrogen under conditions of high gas space velocity and elevated
pressures for long periods of time on-stream.
SUMMARY OF THE INVENTION
[0025] The present invention provides a process and catalysts that
overcome many of the problems associated with existing processes
and catalysts and for the first time, make possible the high
space-time yields that are necessary for a commercially feasible
syngas production facility. A process of preparing synthesis gas
using supported lanthanide-promoted rhodium catalysts for the
catalytic partial oxidation (CPOX) of methane or natural gas is
disclosed. One advantage of the new catalysts employed in the
process is that they demonstrate a high level of activity and
selectivity to carbon monoxide and hydrogen under conditions of
high gas hourly space velocity, elevated pressure and high
temperature. The new catalyst structures contain increased surface
area catalytic materials, which overcome some of the drawbacks of
previous rhodium-based catalysts, to provide higher conversion and
syngas selectivity. In addition, the use of a family of lanthanide
elements that show superior activity for syngas generation under a
variety of operating conditions, and at lower temperatures than
that reported in earlier work is demonstrated. Also these new
catalysts have been demonstrated to operate successfully at
pressures above atmospheric pressure for longer periods of time
onstream, over multi-day syngas production runs, without coking.
The improved stability also manifests itself in terms of more
constant reactor exit temperatures and product gas
compositions.
[0026] In accordance with certain embodiments of the present
invention a method or process of converting methane or natural gas
and O.sub.2 to a product gas mixture containing CO and H.sub.2,
preferably in a molar ratio of about 2:1H.sub.2:CO, is provided.
The process comprises mixing a methane-containing feedstock and an
O.sub.2 containing feedstock to provide a reactant gas mixture
feedstock. Natural gas, or other light hydrocarbons having from 2
to 5 carbon atoms, and mixtures thereof, may also serve as
satisfactory feedstocks. The O.sub.2 containing feedstock may be
pure oxygen gas, or may be air or O.sub.2-enriched air. The
reactant gas mixture may also include incidental or non-reactive
species, in lesser amounts than the primary hydrocarbon and oxygen
components. Some such species are H.sub.2, CO, N.sub.2, NOx,
CO.sub.2, N.sub.2O, Ar, SO.sub.2 and H.sub.2S, as can exist
normally in natural gas deposits. Additionally, in some instances,
it may be desirable to include nitrogen gas in the reactant gas
mixture to act as a diluent. Nitrogen can be present by addition to
the reactant gas mixture or can be present because it was not
separated from the air that supplies the oxygen gas. The reactant
gas mixture is fed into a reactor where it comes into contact with
a catalytically effective amount of a lanthanide-containing or
lanthanide-coated rhodium-containing catalyst structure, catalyst
or catalyst system. Preferably the lanthanide is Pr, Nd, Pm, Sm,
Eu, Gd, Tb, Dy, Ho, Er, Tm or Yb, more preferably Pr, Sm, and Yb.
Advantageously, certain preferred embodiments of the process are
capable of operating at superatmospheric reactant gas pressures
(preferably in excess of 2 atmospheres or about 200 kPa) to
efficiently produce synthesis gas.
[0027] In accordance with certain embodiments of the present
invention, a method of partially oxidizing a reactant gas mixture
comprising a light hydrocarbon and oxygen to form a product mixture
containing carbon monoxide and hydrogen is provided. This method
comprises, in a reactor, passing the reactant gas mixture over a
highly dispersed, high surface area rhodium based catalyst
structure such that the reactant gas mixture is exposed to a
significant portion of the rhodium. The catalyst structure employed
in the reactor is characterized by having a high metal surface area
(i.e., at least 1.25 square meters of metal per gram of catalyst
structure), preferably at least 1.5 m 1 g, and more preferably at
least 2 m 1 g. Preferably the metal is rhodium and the rhodium
surface area at least 1.25 square meters of metal per gram of
supported catalyst, preferably at least 1.5 m.sup.2/g, and more
preferably at least 2 m.sup.2/g. The term "highly dispersed
rhodium" refers to a catalyst in which a limited amount of rhodium
is spread out over the high surface area catalyst surfaces such
that the availability of rhodium for contacting the reactant gas is
enhanced.
[0028] According to certain preferred embodiments of the present
invention, a highly productive process for partially oxidizing a
reactant gas mixture comprising methane and oxygen to form
synthesis gas comprising carbon monoxide and hydrogen is provided.
This process comprises passing the reactant gas mixture over a high
surface area catalyst structure in a reactor under process
conditions that include maintaining a molar ratio of methane to
oxygen ratio in the range of about 1.5:1 to about 3.3:1, the gas
hourly space velocity is maintained in excess of about 20,000
hr.sup.-1, the reactant gas mixture is maintained at a pressure in
excess of about two atmospheres and at a preheat temperature of
between about 30.degree. C. and 750.degree. C. Under these process
conditions within the reactor, the high surface area catalyst
structure causes the partial oxidation of the methane to proceed at
high productivity, i.e., with at least 85% methane conversion, 85%
selectivity to carbon monoxide and 85% selectivity to hydrogen. In
preferred embodiments, the productivity is at least 90% methane
conversion, 90% selectivity to carbon monoxide, and 90% selectivity
to hydrogen, more preferably at least 95% methane conversion, 95%
selectivity to carbon monoxide and 95% selectivity to hydrogen. In
preferred embodiments the catalyst used for producing synthesis gas
comprises about 0.005 to 25 wt % Rh, preferably 0.05 to 25 wt % Rh,
and about 0.005 to 25 wt % of a lanthanide element (i.e., La, Ce,
Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) in the form
of the metal and/or metal oxide coating a refractory monolith or
coating a plurality of distinct or discrete structures or
particulates. Weight percents (wt %) refer to the weight of rhodium
or lanthanide metal relative to the total weight of the catalyst
and support. In some embodiments, the lanthanide is preferably
other than lanthanum or cerium. The more preferred compositions
contain 0.5-10 wt % Rh and 0.5-10 wt % Sm on a refractory support.
In certain preferred embodiments the ratio of rhodium to lanthanide
is in the range of about 0.5-2. The terms "distinct" or "discrete"
structures or units, as used herein, refer to supports in the form
of divided materials such as granules, beads, pills, pellets,
cylinders, trilobes, extrudates, spheres or other rounded shapes,
or another manufactured configuration. Alternatively, the divided
material may be in the form of irregularly shaped particles.
Preferably at least a majority (i.e., >50%) of the particles or
distinct structures have a maximum characteristic length (i.e.,
longest dimension) of less than six millimeters, preferably less
than three millimeters. The term "monolith" as used herein is any
singular piece of material of continuous manufacture such as solid
pieces of metal or metal oxide or foam materials or honeycomb
structures. In some embodiments, two or more catalyst monoliths are
stacked in the catalyst zone of the reactor. In any case, the new
Rh-Lanthanide catalyst systems or catalyst beds have sufficient
porosity, or sufficiently low resistance to gas flow, to permit a
stream of said reactant gas mixture to pass over the catalyst at a
gas hourly space velocity (GHSV) of at least about 20,000
hr.sup.-1, which corresponds to a weight hourly space velocity
(WHSV) of about 200 hr.sup.-1, when the reactor is operated to
produce synthesis gas. Preferably the reactor is operated at a
reactant gas pressure greater than 2 atmospheres, which is
advantageous for optimizing syngas production space-time
yields.
[0029] In some embodiments, the reactant gas mixture is preheated
to about 30.degree. C.-750.degree. C. before contacting the
catalyst. The preheated feed gases pass through the catalytic
materials to the point at which the partial oxidation reaction
initiates. An overall or net catalytic partial oxidation (CPOX)
reaction ensues, and the reaction conditions are maintained to
promote continuation of the process, which preferably is sustained
autothermally.
[0030] For the purposes of this disclosure, the term "net partial
oxidation reaction" means that the partial oxidation reaction shown
in Reaction 2, above, predominates. However, other reactions such
as steam reforming (see Reaction 1), dry reforming (Reaction 3)
and/or water-gas shift (Reaction 4) may also occur to a lesser
extent.
CH.sub.4+CO.sub.22CO+2H.sub.2 (3)
CO+H.sub.2OCO.sub.2+H.sub.2 (4)
[0031] The relative amounts of the CO and H.sub.2 in the reaction
product mixture resulting from the catalytic net partial oxidation
of the methane, or natural gas, and oxygen feed mixture are about
2:1H.sub.2:CO, similar to the stoichiometric amounts produced in
the partial oxidation reaction of Reaction 2.
[0032] As used herein, the term "autothermal" means that after
initiation of the partial oxidation reaction, no additional or
external heat must be supplied to the catalyst in order for the
production of synthesis gas to continue. Under autothermal reaction
conditions the feed is partially oxidized and the heat produced by
that exothermic reaction drives the continued net partial oxidation
reaction. Consequently, under autothermal process conditions there
is no external heat source required. The net partial oxidation
reaction conditions are promoted by optimizing the concentrations
of hydrocarbon and O.sub.2 in the reactant gas mixture, preferably
within the range of about a 1.5:1 to about 3.3:1 ratio of
carbon:O.sub.2 by weight. In some embodiments, steam may also be
added to produce extra hydrogen and to control the outlet
temperature. The ratio of steam to carbon by weight ranges from 0
to 1. The carbon:O.sub.2 ratio is the most important variable for
maintaining the autothermal reaction and the desired product
selectivities. Pressure, residence time, amount of feed preheat and
amount of nitrogen dilution, if used, also affect the reaction
products. The process also includes maintaining a catalyst
residence time of no more than about 10 milliseconds for the
reactant gas mixture. This is accomplished by passing the reactant
gas mixture over, or through the porous structure of the catalyst
system at a gas hourly space velocity of about 20,000-100,000,000
hr.sup.-1, preferably about 100,000-25,000,000 hr.sup.-1. This
range of preferred gas hourly space velocities corresponds to a
weight hourly space velocity of 1,000 to 25,000 hr.sup.-1. In
preferred embodiments of the process, the catalyst system catalyzes
the net partial oxidation of at least 90% of a methane feedstock to
CO and H.sub.2 with a selectivity for CO and H.sub.2 products of at
least about 90% CO and 90% H.sub.2.
[0033] In certain embodiments of the process, the step of
maintaining net partial oxidation reaction promoting conditions
includes keeping the temperature of the reactant gas mixture at
about 30.degree. C.-75.degree. C..degree. C. and keeping the
temperature of the catalyst at about 600-2,000.degree. C.,
preferably between about 600-1,600.degree. C., by self-sustaining
reaction. In some embodiments, the process includes maintaining the
reactant gas mixture at a pressure of about 100-32,000 kPa (about
1-32 atmospheres), preferably about 200-10,000 kPa (about 2-10
atmospheres), while contacting the catalyst.
[0034] In some embodiments, the process comprises mixing a
methane-containing feedstock and an O.sub.2-containing feedstock
together in a carbon:O.sub.2 ratio of about 1.5:1 to about 3.3:1,
preferably about 1.7:1 to about 2.1:1, and more preferably about
2:1). Preferably the methane-containing feedstock is at least 80%
methane, more preferably at least 90%.
[0035] According to certain embodiments of the present invention, a
method of converting a light hydrocarbon and O.sub.2 to a product
mixture containing CO and H.sub.2 is provided. The process includes
forming a reactant gas mixture comprising a light hydrocarbon
containing gas and an O.sub.2 containing gas, and, in a reactor,
passing the reactant gas mixture over a refractory supported
rhodium-lanthanide catalyst prepared by sequentially applying a
rhodium precursor, such as a rhodium salt, to a lanthanide and/or
lanthanide oxide precursor, such as a lanthanide salt, to the
support and stabilizing the catalyst. The term "refractory support"
refers to any material that is mechanically stable to the high
temperatures of a catalytic partial oxidation reaction, which is
typically 500.degree. C.-1,600.degree. C., but may be as high as
2000.degree. C. Suitable refractory support materials include
zirconia, magnesium stabilized zirconia, zirconia stabilized
alumina, yttrium stabilized zirconia, calcium stabilized zirconia,
alumina, cordierite, titania, silica, magnesia, niobia, vanadia and
the like. Preferably the alumina component is alpha-alumina.
Stabilizing includes thermally conditioning the catalyst.
[0036] The catalyst employed in the method is preferably prepared
by sequentially applying a lanthanide precursor and a rhodium
precursor to a refractory support and thermally conditioning the
catalyst during catalyst preparation. "Thermally conditioning"
means that when the catalyst is being constructed (e.g., after the
lanthanide precursor is applied to the refractory support and/or
after the rhodium precursor is applied to the lanthanide and/or
lanthanide oxide), it is subjected to two or more heat treatments
which yield a more stable and long lived catalyst for use in the
CPOX reactor. Each heat treatment includes calcining the catalyst,
or an intermediate stage of the catalyst, according to a defined
heating and cooling program. Preferably the final heat treatment
includes heating at a temperature that approaches or approximates
the operating temperature of the CPOX reactor. It is also
preferable to apply the lanthanide or lanthanide oxide precursor
compound to a refractory support first, followed by a programmed
heat treatment, to further enhance catalyst stability when used
onstream in a CPOX reactor. Although less preferred, the lanthanide
may instead be applied over the rhodium, or the rhodium and
lanthanide precursor compounds may be mixed together and applied to
a refractory support, followed by one or more thermally
conditioning treatments.
[0037] In certain embodiments, thermally conditioning comprises
heating the catalyst at a predetermined heating rate up to a first
temperature and then heating said catalyst at a predetermined
heating rate from the first temperature to a second temperature. In
some embodiments of the catalyst preparation method, the thermally
conditioning also includes holding the catalyst, at the first and
second temperatures for predetermined periods of time. In some
embodiments, the first temperature is about 125-325.degree. C. and
the second temperature is about 300 to 900.degree. C., preferably
about 500-700.degree. C. In some embodiments the heating rate is
about 1-10.degree. C./min, preferably 3-5.degree. C./min and the
dwell time at that temperature is about 120-360 min, or more,
preferably about 180 min.
[0038] In some embodiments, thermally conditioning the catalyst
includes heat treating the catalyst between the sequential
applications of lanthanide and/or lanthanide oxide precursor
compound and rhodium precursor compound to said support, i.e.,
treating an intermediate-stage catalyst. In some embodiments, the
catalyst preparation method also includes reducing the catalyst at
a predetermined temperature in a reducing atmosphere. The resulting
Rh-lanthanide containing catalyst is characterized by its enhanced
activity for catalyzing the partial oxidation of light hydrocarbons
such as methane, compared to other rhodium-based catalysts.
[0039] In certain embodiments of the syngas production process, the
reactor is operated at the above-described process conditions to
favor autothermal catalytic partial oxidation of the hydrocarbon
feed and to optimize the yield and selectivity of the desired CO
and H.sub.2 products.
[0040] In accordance with other embodiments of the present
invention, a catalyst is provided that is active for catalyzing the
net partial oxidation of methane to CO and H.sub.2 and possesses
enhanced stability on stream in a short contact time reactor. The
catalyst comprises rhodium and at least one lanthanide or
lanthanide oxide, preferably carried on a refractory support, or is
formed as a self supporting structure or plurality of structures
suitable for use in the catalyst zone of a short contact time
reactor to produce synthesis gas.
[0041] In some embodiments of the process and catalyst of the
present invention, the catalyst system also comprises a support
which is magnesium stabilized zirconia, zirconia stabilized
alumina, yttrium stabilized zirconia, calcium stabilized zirconia,
alumina, cordierite, zirconia, titania, silica, magnesia, niobia
and vanadia or the like. In certain preferred embodiments the
catalyst about 0.005 to 25 wt % Rh and about 0.005 to 25 wt % of a
lanthanide and/or lanthanide oxide deposited on a porous refractory
support, especially PSZ, alpha-alumina or zirconia. In certain
preferred embodiments the lanthanide is samarium. In certain
embodiments Rh and a lanthanide metal and/or lanthanide oxide are
deposited on a monolith support that contains about 45-80 pores per
linear inch. In other preferred embodiments the catalyst and
support comprise a plurality of distinct or discrete structures or
particulates, characterized as described above.
[0042] In some embodiments the catalyst comprises about 0.05-25 wt
% Rh and about 0.1-25 wt % lanthanide and/or lanthanide oxide,
preferably about 0.5-10 wt % Rh and 0.5-10 wt % lanthanide and/or
lanthanide oxide (wt % lanthanide based on total weight of the
supported catalyst). In preferred embodiments the lanthanide is
deposited intermediate the support and a Rh layer. In some
embodiments, the catalyst system comprises about 0.5-10 wt % Rh
over a layer of about 0.5-10 wt % lanthanide, preferably samarium,
ytterbium or praseodymium, and oxides thereof, more preferably
samarium and/or samarium oxide, deposited on a PSZ or alumina
monolith, or, more preferably, on alpha-alumina or zirconia
granules having the size characteristics described above. In other
embodiments, Rh is deposited between the monolith support and the
lanthanide and/or lanthanide oxide layer. In still other
embodiments, a mixture of lanthanide and Rh is deposited on the
support. In any case, the catalyst is preferably subjected to one
or more thermally conditioning treatments during catalyst
construction, as previously described, to yield a more pressure
tolerant, high temperature resistant and longer lived catalyst
system than is presently available in conventional syngas or
catalytic partial oxidation catalysts. These and other embodiments,
features and advantages of the present invention will become
apparent with reference to the following description and
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] FIG. 1 is a graph showing the carbon conversion activity
(y-axis) at various reactant gas pressures .alpha.-axis) obtained
using 3/8", 5/8" and {fraction (15/16)}" long Rh--Sm-containing
monolith catalysts tested under similar conditions. R is the ratio
of O.sub.2:natural gas (by mass).
[0044] FIG. 2 is a graph showing the carbon conversion activities
of the catalyst of FIG. 1 at various weight hourly space velocities
(grams CH.sub.4/grams catalyst/hr.).
[0045] FIG. 3 is a graph showing catalyst performance over a
two-day syngas production run for one catalyst containing 4.52 wt %
Rh and 4.13 wt % Sm.sub.2O.sub.3 supported on ZrO.sub.2 granules of
size 35-50 mesh.
[0046] FIG. 4 is a graph showing catalyst performance over a
two-day syngas production run for a catalyst similar to the one
used in FIG. 3.
[0047] FIG. 5 is a graph showing catalyst performance over an
approximately 24 hr syngas production run for one catalyst
containing 6% Rh and 4% Sm supported on an 80 ppi PSZ monolith
under similar process conditions to those employed in FIGS. 3 and
4.
[0048] FIGS. 6A and 6B are performance graphs for different lots of
catalyst containing 6% Rh and 4% Sm supported on 35-50 mesh
zirconia granules.
[0049] FIG. 7 is a performance graph for a catalyst containing 6.12
wt % Rh and 4.5 wt % Sm.sub.2O.sub.3 supported on 35-50 mesh
Al.sub.2O.sub.3 granules.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0050] New Rh-lanthanide based syngas catalysts are preferably
prepared by impregnating or washcoating the catalytically active
components onto a refractory porous ceramic monolith carrier or
support. "Lanthanide" refers to a rare earth element La, Ce, Pr,
Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb. Suitable supports
include partially stabilized zirconia (PSZ) foam (stabilized with
Mg, Ca or Y), or foams of .alpha.-alumina, corderite, titania,
mullite, Zr-stabilized .alpha.-alumina, or mixtures thereof. The
term "partially stabilized zirconia" (PSZ) refers to the well-known
practice of adding stabilizing oxides, such as MgO, CaO, or
Y.sub.2O.sub.3, into the ZrO.sub.2 structure in a sufficient to
form a solid solution or a mixture of ZrO.sub.2 in different
phases. The resulting material has higher resistance to phase
transformation during heating and cooling compared to pure
ZrO.sub.2. A preferred laboratory-scale ceramic monolith support is
porous PSZ foam with approximately 6,400 channels per square inch
(80 pores per linear inch). Preferred foams for use in the
preparation of the catalyst include those having from 30 to 150
pores per inch (12 to 60 pores per centimeter). The monolith can be
cylindrical overall, with a diameter corresponding to the inside
diameter of the reactor tube. Alternatively, other refractory foam
and non-foam catalyst supports can serve as satisfactory supports
for the Rh-lanthanide containing catalysts. The catalyst
precursors, including Rh and lanthanide salts, with or without a
ceramic support composition, may be extruded to prepare a
three-dimensional form or structure such as a honeycomb, foam,
other suitable tortuous-path structure, and treated as described in
the following Examples. The catalyst can be structured as, or
supported on, a refractory oxide "honeycomb" straight channel
extrudate or monolith, made of cordierite or mullite, or other
configuration having longitudinal channels or passageways
permitting high space velocities with a minimal pressure drop. Such
configurations are known in the art and described, for example, in
Structured Catalysts and Reactors, A. Cybulski and J. A. Moulijn
(Eds.), Marcel Dekker, Inc., 1998, p. 599-615 (Ch. 21, X. Xu and J.
A. Moulijn, "Transformation of a Structured Carrier into Structured
Catalyst").
[0051] Other preferred Rh-Ln catalysts are formed as granules,
particles, pellets, beads, spheres, cylinders, trilobes or other
manufactured shapes, or the Rh-Ln catalytic components are applied
to inert refractory materials such as zirconia, .alpha.-alumina,
cordierite, titania, mullite, zirconia-stabilized .alpha.-alumina,
MgO stabilized zirconia, MgO stabilized alumina, and niobia, or
mixtures thereof, in the form of particles, pellets, beads,
spheres, trilobes, granules or the like. Preferably the support
materials are pre-shaped as granules, spheres, pellets, or other
geometry that provides satisfactory engineering performance, before
application of the catalytic materials. A lanthanide oxide support
formed into a porous refractory three-dimensional structure is a
highly preferred support material for rhodium. Without wishing to
be bound by any particular theory, the coke-reducing effects of the
new catalyst compositions may occur due to formation of a
rhodium-lanthanide alloy or solid solution. Combining a lanthanide
component with rhodium changes the melting properties of the
rhodium, keeping metallic rhodium in place, and also keeping the
rhodium dispersed in the oxide phase. The strong interaction
between rhodium and the lanthanide, made possible by the enhanced
dispersion of rhodium on the lanthanide and/or the refractory
support, contributes to catalyst stability. This results in a
higher melting point for the catalyst and deters deactivation of
the catalyst on stream. Accordingly, a "stability-enhanced"
catalyst, which has been thermally conditioned during its
construction, is more pressure tolerant (to at least 2 atmospheres
operating pressure), high temperature resistant (up to at least
1,500.degree. C.) and longer lived (reduced coking onstream) than a
typical syngas catalyst. The following examples are intended to
illustrate but not limit the present invention.
EXAMPLES
Examples 1, 2 and 3: Rh/Sm on PSZ Monoliths
[0052] An aqueous solution of Sm(NO.sub.3).sub.3.6H.sub.2O was
added dropwise to saturate a PSZ monolith. Suitable PSZ monoliths
about 10 or 15 mm long and 12 mm diameter are commercially
available from well known sources. The monolith was situated on a
Teflon.RTM. plate residing on a warm (75.degree. C.) hotplate. The
entire Sm salt solution was gradually added to the monolith,
allowing the water to evaporate between saturations. The dried
monolith was then calcined in air (static or flowing) according to
the following program: heat from room temperature (RT) to about
125.degree. C. at a rate of about 5.degree. C./min, dwell at that
temperature for about 60 min (extra drying step); heat from about
125.degree. C. to about 400-900.degree. C., preferably about
600.degree. C., at a rate of about 1-10.degree. C./min, preferably
about 5.degree. C./min, dwell at that temperature for about 120-360
min, or more, preferably about 240 min.
[0053] An aqueous solution of RhCl.sub.3.xH.sub.2O was added
dropwise to saturate the Sm coated PSZ monolith, prepared as
described in the above paragraph. The Rh salt solution was
gradually added to the monolith, allowing the water to evaporate
between saturations. The dried monolith was then calcined in air
flowing at about 0.1-1 NLPM (normal liters per minute), or more,
but preferably about 0.4 NLPM, according to the following program:
heating from room temperature (RT) to about 125.degree. C. at a
rate of increase of about 5.degree. C./min, dwell for 60 min at
about 125.degree. C. (extra drying step); heat from about
125.degree. C. to about 400-900.degree. C., preferably about
600.degree. C. at a rate of increase of about 1 to 10.degree.
C./min, preferably about 5.degree. C./min, dwell for about 120 to
360 min, or more, preferably about 240 min at that temperature.
[0054] This final calcined Rh/Sm/PSZ monolith catalyst was then
reduced in flowing H.sub.2 (or H.sub.21N.sub.2 mixture) at a flow
rate of about 0.1-1 NLPM, or more, preferably about 0.6 NLPM, while
applying heat according to the following program: heat from room
temperature (RT) to about 125.degree. C. at a rate of temperature
increase of 5.degree. C./min, dwell for about 30 min at that
temperature (extra drying step); heat from about 125.degree. C. to
about 300 to 900.degree. C., preferably about 400.degree. C., at a
rate of increase of about 1 to 10.degree. C./min, preferably about
5.degree. C./min, dwell at that temperature for about 60-360 min,
or more, preferably about 180 min. The concentrations of the Sm and
Rh solutions and the amounts loaded onto the PSZ monolith were
chosen so as to provide the final wt % of each that is stated in
Tables 1-3.
Example 4
Sm/Rh on PSZ Monolith
[0055] The order of addition of the Sm and Rh metal solutions to
the PSZ monolith described above was reversed to produce a
representative Sm/Rh/PSZ monolith catalyst in which the rhodium is
in closest contact with the PSZ monolith and the samarium coat
overlies the rhodium layer. The concentrations of the Sm and Rh
solutions and the amounts loaded onto the PSZ monolith were chosen
so as to provide the final wt % of each that is stated in Table
4.
[0056] Alternatively, the aqueous solution may contain salts of
both Sm and Rh which are capable of decomposing when heated to form
the respective metal and/or metal oxide, and the ceramic monolith
is loaded in a single step, as described for the Rh solution in
Example 1, to provide a satisfactory monolith catalyst for syngas
production.
[0057] Although samarium was employed in the foregoing examples, it
should be understood that the inventors have found that other
lanthanides also perform satisfactorily. Samarium is considered by
the inventors to be representative of the other lanthanide
elements, including lanthanum, cerium, praseodymium, neodymium,
promethium, europium, gadolinium, terbium, dysprosium, holmium,
erbium, thulium and ytterbium. Accordingly, the decomposable salts
of other lanthanides may be substituted in the methods described
herein, and, in many cases, will provide monolith catalysts of
comparable activity to the rhodium and samarium-containing
exemplary catalysts for catalyzing the net partial oxidation of
methane in a short-contact-time reactor to produce syngas.
Examples 5 and 6
Rh on PSZ Monolith (Comparative Examples)
[0058] An aqueous solution of RhCl.sub.3. xH.sub.2O was added
dropwise to saturate a PSZ monolith. The monolith is situated on a
Teflon.RTM. plate residing on a warm (75.degree. C.) hotplate. The
entire Rh salt solution is added to the monolith over time,
allowing the water to evaporate between saturations. The dried
monolith is then calcined in air (flowing at 0.4 NLPM, range 0.1 to
1 or more NLPM) while applying heat according to the following
program: heat from room temperature (RT) to 125.degree. C. at a
rate of temperature increase of about 5.degree. C./min, dwell for
60 min at that temperature (extra drying step); heat from about
125.degree. C. to about 400 to 900.degree. C., preferably about
600.degree. C., at a rate of increase of about 1 to 10.degree.
C./min, preferably about 5.degree. C./min, dwell at that
temperature for about 120 to 360 min., or more, preferably about
240 min. This final calcined catalyst is then reduced in flowing
H.sub.2 (or a H.sub.2/N.sub.2 mixture) at a flow rate of about 0.1
to 1 NLPM, or more, preferably 0.6 NLPM, according to the following
program: increase the heat from room temperature (RT) to about
125.degree. C. at a rate of increase of 5.degree. C./min, dwell at
that temperature for about 30 min (extra drying step); heat from
about 300-900.degree. C., preferably about 400.degree. C., at a
rate of increase of about 1 to 10.degree. C./min, preferably about
5.degree. C./min, dwell at that temperature for about 60 to 360
min., or more, preferably about 180 min.
[0059] Each of the samarium-containing monolith catalysts of
Examples 1-4 and the comparative Rh/PSZ monolith catalysts of
Examples 5-6 were evaluated in a reduced scale syngas production
reactor, as described in the section entitled "Test Procedure." The
composition and dimensions of the catalysts are summarized in Table
1 and the results of the tests on those samples are shown in Tables
2-4.
[0060] Test Procedure-1
[0061] The partial oxidation reactions were carried out in a
conventional flow apparatus using a 44 mm O.D..times.38 mm I.D.
quartz insert embedded inside a refractory-lined steel vessel. The
quartz insert contained a catalyst bed containing at least one
porous monolith catalyst (.about.37 mm O.D..times.10-15 mm high)
held between two foam disks. The upper disk typically consisted of
65-ppi partially-stabilized zirconia and the bottom disk typically
consisted of 30-ppi zirconia-toughened alumina. Preheating the
methane or natural gas that flowed through the catalyst bed
provided the heat needed to start the reaction. Oxygen was mixed
with the methane or natural gas immediately before the mixture
entered the catalyst bed. The methane or natural gas was spiked
with propane, or another combustable gas, as needed to initiate the
partial oxidation reaction, then the propane was removed as soon as
the reaction initiated. Once the reaction was initiated, it
proceeded autothermally. Two Type K thermocouples with ceramic
sheaths were used to measure catalyst inlet and outlet
temperatures. The molar ratio of CH.sub.4 to O.sub.2 was generally
about 2:1, however the relative amounts of the gases, the catalyst
inlet temperature and the reactant gas pressure could be varied by
the operator according to the parameters being evaluated (see the
following Tables). The product gas mixture was analyzed for
CH.sub.4, O.sub.2, CO, H.sub.2, CO.sub.2 and N.sub.2 using a gas
chromatograph equipped with a thermal conductivity detector. A gas
chromatograph equipped with a flame ionization detector analyzed
the gas mixture for CH.sub.4, C.sub.2H.sub.6, C.sub.2H.sub.4 and
C.sub.2H.sub.2. The CH.sub.4 conversion levels and the CO and
H.sub.2 product selectivities obtained for each catalyst monolith
evaluated in this test system are considered predictive of the
conversion and selectivities that will be obtained when the same
catalyst is employed in a commercial scale short contact time
reactor under similar conditions of reactant concentrations,
temperature, reactant gas pressure and space velocity.
1TABLE 1 Composition of Monolith Catalysts PSZ Monolith
Sm(NO.sub.3).sub.3.6H.sub.2O Sm-PSZ RhCl.sub.3.xH.sub.2O Dimensions
Weight Porosity Weight Sm Weight Weight Rh Ex. (D .times. L, mm)
(grams) (ppi) (grams) (wt %) (grams) (grams) (wt %) 1 38 .times. 14
14.2351 80 2.1092 5.01 15.0968 1.3365 4.09 2 38/10 15.3349 80
2.2755 5.01 16.2987 1.4425 4.01 3a 38/10 12.9296 80 1.9135 5.00
13.7263 1.2317 4.06 3b 38/14 16.9431 80 2.5028 4.99 17.9945 1.4205
3.57 4 38/10 8.56 80 0.4565 2.05 NM 0.5979 3.48 5 37 .times. 15
8.5652 80 NA NA NA 1.0278 5.31 6 38/10 8.8560 80 NA NA NA 1.09 6.05
NA = not applicable; NM = not measured; D = diameter; L = length;
ppi = pores per linear inch
[0062]
2TABLE 2 Comparison of Rh/Sm/PSZ Catalysts to Rh/PSZ Catalysts
Metals Size Content (%) Pressure Temp. CH.sub.4 Selectivity GHSV
Ex. (D .times. L mm) Rh Sm (PSIG) (.degree. C.) Conv. CO H.sub.2
(.times.10.sup.6 hr.sup.-1) 1 38 .times. 14 4.09 5.01 45 1021 93.2
95.6 88.7 1.67 60 1048 91.3 95.0 89.0 2.09 75 1037 88.8 94.3 86.7
2.44 5 37 .times. 15 5.31 45 1142 70.0 93.7 66.1 1.93 60 1138 73.2
93.1 62.7 1.51 75 1127 71.4 92.9 61.3 1.83 2 38 .times. 10 4.01
5.01 25 1135 82 91.4 87.5 1.57 60 1150 79 89.7 79.8 2.74 75 1150 78
88.2 77.2 3.23 6 38 .times. 10 6.05 45 1160 70.6 93.3 72.9 1.86 60
1153 73.3 93.1 70.8 2.56 75 1158 72.9 93.0 71.1 2.95 GHSV = gas
hourly space velocity
[0063]
3TABLE 3 Catalytic Activity of a Combined Rh/Sm/PSZ Stack Metals
Content Size (%) Pressure Temp. CH.sub.4 Selectivity GHSV Ex. 3 (d
.times. l mm) Rh Sm (PSIG) (.degree. C.) Conv. CO H.sub.2
.times.10.sup.6 25 946 93.0 94.5 87.3 0.69 45 990 91.4 94.9 87.8
1.02 3a 38 .times. 10 4.06 5.00 60 1009 90.4 94.8 87.6 1.17 3b
{close oversize brace} 38 .times. 14 {close oversize brace} 3.57
{close oversize brace} 4.99 {close oversize brace} 75 1045 90.5
94.4 85.1 1.45 90 NR 91.6 95.1 85.0 1.73 105 NR 91.4 95.0 84.9 1.88
Note: NR = Not reported
[0064]
4TABLE 4 Catalytic Activity of Sm/Rh/PSZ Metals Size Content (%)
Pressure Temp. CH.sub.4 Selectivity GHSV Ex. (d .times. l mm) Rh Sm
(PSIG) (.degree. C.) Conv. CO H.sub.2 .times.10.sup.6 4 38 .times.
10 3.48 2.05 45 1009 84.8 93.6 85.8 3.81 60 1009 85 93.3 86.0 5.20
75 1139 84.7 93.2 85.5 6.03
[0065] From the comparative data shown in Table 2, it can be seen
that the overall level of CH.sub.4 conversion is about 17 to 23%
higher for the {fraction (5/8)} inch (15 mm.times.12 mm) Rh/Sm/PSZ
catalyst of Example 1 vs. the Rh/PSZ catalyst of Example 5, despite
having 25% more Rh on the latter catalyst. The observed CO
selectivity is approximately 2% higher for Rh/Sm-PSZ, and the
H.sub.2 selectivity is 20 to 25% higher for Rh/Sm-PSZ. The catalyst
temperature is lower for Rh/Sm-PSZ
[0066] Comparing the catalytic activity of the shorter (i.e.,
{fraction (3/8)} inch; about 10 mm.times.12 mm) Rh/Sm/PSZ monolith
catalyst of Example 2 to a similar length Rh/PSZ monolith catalyst
(Example 6), it can be seen in Table 2 that the overall CH.sub.4
conversion is approximately from 5 to 12% higher for Rh/Sm-PSZ vs.
Rh/PSZ, despite having 50% more Rh on the latter catalyst. CO
selectivity is approximately 2 to 5% lower for the Rh/Sm/PSZ
catalyst. H.sub.2 selectivity is 6 to 15% higher for Rh/Sm/PSZ. The
catalyst temperature is similar for both catalysts.
[0067] Table shows the effect of length on the reaction using two
stacked monoliths. The catalyst of Example 3 is a combination of
two separate 12 mm diameter Rh/Sm/PSZ monoliths about 3/8" and 5/8"
(10 mm (3a) and about 15 mm long (3b) which were stacked in the
reactor tube. An overall catalyst bed length of about {fraction
(15/16)}" (24 mm) was obtained. When this combination catalyst bed
was evaluated in the test reactor, it was observed that the
additional monolith length provided some unexpected advantages in
the net partial oxidation of methane to syngas. In this case, the
overall CH.sub.4 conversion was consistently high (i.e., more than
91%) over wide range of pressures (25 to 105 psig), compared to the
value obtained for either the 10 mm or 15 mm Rh/Sm/PSZ monolith of
Examples 1 or 2 (Table 2). CO selectivity was also approximately 2%
higher. H.sub.2 selectivity was 20 to 25% higher for the longer
Rh/Sm/PSZ catalyst bed, and the catalyst temperature was lower for
the catalyst monolith of Example 3. The graph in FIG. 1 shows the
higher methane conversion levels obtained with a {fraction
(15/16)}" or a 5/8" monolith catalyst than with a shorter (3/8")
monolith catalyst when evaluated at reactant gas pressures of 20,
40, 60, 75 and 90 psig. Even after correcting the data for the
greater total amount of Sm and Rh contained in the 15 mm and the 24
mm beds, the conversion level is significantly higher for the
longer monoliths, as demonstrated in FIG. 2. This data suggests
that improved performance synthesis gas catalysts are obtained by
preparing longer monolith catalysts, at least for syngas production
in the reduced scale reactors.
[0068] FIG. 2 is a graph of the same data as in FIG. 1, except that
it presents the carbon conversion activity of each monolith
catalyst relative to the corresponding weight hourly space
velocity. Weight hourly space velocities (WHSV) ranged from about
6000 to 26,000 grams CH.sub.4 fed to the reactor per gram of
monolith catalyst per hour. In these tests the ratio of
CH.sub.4:O.sub.2 in the reactant gas mixture was adjusted slightly
for the 5/8" and the {fraction (15/16)}" monoliths (i.e., from
R=1.05 for the 3/8" monolith to R=1.10 for the 3/8" and {fraction
(15/16)}" monoliths, where R is the ratio of oxygen to natural gas,
by mass), in order to keep the run temperature in each case at no
more than 1,150.degree. C.
[0069] Table 4 shows the effects that reversing the order of metal
addition to the PSZ monolith has on CH.sub.4 conversion, product
selectivity and run temperature. The monolith catalyst of Example 4
was prepared by first loading Rh on the PSZ monolith, followed by
Sm. Note that CH.sub.4 conversion and selectivities are high for
both CO and H.sub.2. Also, the catalyst bed runs at a lower
temperature compared to the monolith loaded with only Rh
(Comparative Example 6) or the corresponding Rh/Sm/PSZ monolith
catalyst of Example 2.
Examples 7-9
Rh/Sm on PSZ Monolith
[0070] 0.4734 g Sm(NO.sub.3).sub.3.6H.sub.2O (Aldrich) was
dissolved in sufficient water to form an aqueous solution. A PSZ
monolith about 15 mm long and 12 mm diameter was situated on a
Teflon.RTM. plate resting on a warm (75.degree. C.) hotplate.
Satisfactory PSZ monoliths can be obtained commercially (Vesuvius
Hi-Tech Ceramics, NY or Porvair Advanced Materials Inc., NC). The
entire Sm salt solution was gradually added to the monolith,
allowing the water to evaporate between saturations. The dried
monolith was then calcined in air (static or flowing) according to
the following program: heat from room temperature (RT) to about
125.degree. C. at a rate of about 3.degree. C./min, dwell at that
temperature for about 60 min; heat from about 125.degree. C. to
about 400-900.degree. C., preferably about 500.degree. C., at a
rate of about 1-10.degree. C./min, preferably about 5.degree.
C./min, dwell at that temperature for about 120-360 min, or more,
preferably about 180 min.
[0071] An aqueous solution of RhCl.sub.3.xH.sub.2O was added
dropwise to saturate the Sm-coated PSZ monolith, prepared as
described in the above paragraph. The Rh salt solution was
gradually added to the monolith, allowing the water to evaporate
between saturations. The dried monolith was then calcined in air,
according to the program described above.
[0072] This final calcined Rh/Sm/PSZ catalyst system was then
reduced in flowing H.sub.2 (or H.sub.2/N.sub.2 mixture) at a flow
rate of about 0.1-1 NLPM, or more, preferably about 0.6 NLPM, while
applying heat according to the following program: heat from room
temperature (RT) to about 125.degree. C. at a rate of temperature
increase of 3.degree. C./min, dwell for about 30 min at that
temperature; heat from about 125.degree. C. to about 300 to
900.degree. C., preferably about 500.degree. C., at a rate of
increase of about 1 to 10.degree. C./min, preferably about
3.degree. C./min, dwell at that temperature for about 60-360 min,
or more, preferably about 180 min. The concentrations of the Sm and
Rh solutions and the amounts loaded onto the PSZ monolith were
chosen so as to provide the final wt % of each metal stated in
Table 5.
Example 10
Rh/Yb on PSZ Monolith
[0073] Rh/Yb/PSZ was prepared according to the procedure used for
Examples 7-9, using aqueous solutions of Yb(NO.sub.3).sub.3 and
RhCl.sub.3.
Example 11
Rh/Pr on PSZ Monolith
[0074] Rh/Pr/PSZ was prepared according to the procedure used for
Examples 7-9, using aqueous solutions of Pr(NO.sub.3).sub.3 and
RhCl.sub.3.
[0075] Each of the catalysts of Examples 7-11 was evaluated in a
reduced scale syngas production reactor as described in the section
entitled "Test Procedure-2." The composition and dimensions of the
catalysts are summarized in Table 5 and the results of the tests on
those samples are shown in Table 6.
[0076] Test Procedure-2
[0077] The catalysts were evaluated for their ability to catalyze
the partial oxidation reaction in a conventional flow apparatus
using a quartz reactor with a length of 12 inches, an outside
diameter of 19 mm and an inside diameter of 13 mm. Ceramic foam
pieces of 99% Al.sub.2O.sub.3 (12 mm outside diameter.times.5 mm
thick, with 45 pores per linear inch) were placed before and after
the catalyst as radiation shields. The inlet radiation shield also
aided in uniform distribution of the feed gases. An
Inconel-sheathed, single point K-type (Chromel/Alumel) thermocouple
was placed axially inside the reactor, touching the top (inlet)
face of the radiation shield. A high temperature S-Type (Pt/Pt 10%
Rh) bare-wire thermocouple was positioned axially touching the
bottom face of the catalyst, and was used to indicate the reaction
temperature. The catalyst and the two radiation shields were
tightly sealed against the inside walls of the quartz reactor by
wrapping the shields radially with a high purity (99.5%) alumina
paper. A 600-watt band heater set at 90% electrical output was
placed around the quartz tube, providing heat to light off the
reaction and preheat the feed gases. The bottom of the band heater
corresponded to the top of the upper radiation shield.
[0078] In addition to the thermocouples placed above and below the
catalyst, the reactor also contained two axially positioned,
triple-point thermocouples, one before and another after the
catalyst. These triple-point thermocouples were used to determine
the temperature profiles of the reactants and products that were
subjected to preheating and quenching, respectively.
[0079] The runs were conducted at a CH.sub.4:O.sub.2 molar ratio of
1.75:1-2:1 with a combined flow rate of about 3.8-7.7 SLPM
(standard liters per minute), corresponding to a gas hourly space
velocity of 192,300-384,600 hr.sup.-1 and at a pressure of 5 psig
(136 kPa). The reactor effluent was analyzed gas chromatograph
equipped with a thermal conductivity detector. The data reported in
6 and 8 are obtained after at least 6 hours on stream at the
specified conditions.
5TABLE 5 Composition of Catalysts Dimensions Porosity Lanthanide Rh
Ex. (D .times. L, mm) Support (ppi) (wt %) (wt %) 7 12 .times. 15
PSZ (Mg) 80 5% Sm 2% 8 12 .times. 15 PSZ (Mg) 80 5% Sm 3% 9 12
.times. 15 PSZ (Mg) 80 5% Sm 4% 10 12 .times. 15 PSZ (Mg) 80 5% Yb
4% 11 12 .times. 15 PSZ (Mg) 80 5% Pr 4% D = diameter; L = length;
PSZ (Mg) denotes that the zirconia is partially stabilized with Mg;
ppi = pores per linear inch; wt % = nominal wt %
[0080]
6TABLE 6 Performance Data for Rh/Lanthanide Catalysts Metals Flow
Selectivity Content (%) CH.sub.4:O.sub.2 rate GHSV Temp. CH.sub.4
(%) Ex. Rh Ln ratio (SLPM) (hr.sup.-1) (.degree. C.) Conv. CO
H.sub.2 7 2 5 (Sm) 1.75:1 3.868 192,300 806 93.9 94.8 91.8 1.75:1
7.873 391,400 822 93.0 95.3 91.0 8 3 5 (Sm) 1.75:1 3.870 192,400
765 93.6 95.3 90.7 1.75:1 7.864 391,000 819 94.4 96.6 90.3 9 4 5
(Sm) 1.75:1 3.870 192,400 810 96.6 96.3 91.6 1.75:1 7.716 383,600
825 95.5 96.7 91.1 10 4 5 (Yb) 1.75:1 3.866 192,200 760 93.9 94.9
91.5 1.75:1 7.866 391,100 760 94.9 95.6 91.7 11 4 5 (Pr) 1.75:1
3.87 192,400 751 93.6 95.0 91.1 1.75:1 7.876 391,600 798 94.6 95.6
91.4
Example 12
Rh/Sm on PSZ Monolith
[0081] Sm was added first to the support, followed by the addition
of Rh using the sequential impregnation procedure described above.
The catalyst was dried and calcined using the conditions described
for Examples 7-9.
Example 13
Rh--Sm on PSZ Monolith
[0082] Both Rh and Sm were added simultaneously using the
co-impregnation procedure, by mixing the Rh and Sm-containing
solutions and then adding to the PSZ monolith, using the conditions
described above.
Example 14
Rh/Sm on ZrO.sub.2 Granules
[0083] (a.) Rh/Sm catalyst was prepared on ZrO.sub.2 granules of
35-50 mesh size (0.3 mm to 0.5 mm). Satisfactory ZrO.sub.2 granules
can be obtained commercially (Sud-Chemie, Louisville, Ky.). The
synthesis procedure is as described for Examples 7-9, except the Sm
and Rh precursor solutions were added to the support granules in a
crucible, followed by drying on the hotplate at about 75.degree. C.
for 2 hours with frequent mixing, followed by calcination at the
same conditions as for Examples 7-9. Sm was coated first, followed
by Rh, using the sequential impregnation method. The final catalyst
was obtained after reduction at the conditions described for
Examples 7-9 to provide a catalyst containing 5.8% Rh and 4.1% Sm
on ZrO.sub.2 granules.
[0084] (b.) Alternatively, the following procedure was followed:
0.4734 g Sm(NO.sub.3).sub.3.5H.sub.2O (Aldrich) was dissolved in
sufficient water to form an aqueous solution. The ZrO.sub.2
granules were immersed into the solution for wet impregnation, then
allowed to dry on a hotplate. The impregnated granules were
calcined in air according to the following schedule: 5.degree.
C./min ramp to 325.degree. C., hold at 325.degree. C. for 1 h,
5.degree. C./min ramp to 700.degree. C., hold at 700.degree. C. for
2 h, cool down to room temperature. 0.5839 g RhCl.sub.3.xH.sub.2O
(Aldrich) was dissolved in sufficient water to form an aqueous
solution. The calcined Sm-containing granules were immersed into
the rhodium solution for wet impregnation, then allowed to dry on a
hotplate. The Rh impregnated granules were then calcined in air
according to the following schedule: 5.degree. C./min ramp to
325.degree. C., hold at 325.degree. C. for 1 h, 5.degree. C./min
ramp to 700.degree. C., hold at 700.degree. C. for 2 h, cool down
to room temperature. This material was then reduced at 500.degree.
C. for 3 h under a stream of 300 mL/min H.sub.2 and 300 mL/min
N.sub.2 to provide a catalyst containing 6% Rh and 5% Sm supported
on ZrO.sub.2 granules.
[0085] (c.) A similar procedure was employed to yield a catalyst
with a final loading of 4.52% Rh and 4.13% Sm (in the form of
Sm.sub.2O.sub.3) supported on 35-50 mesh ZrO.sub.2 granules.
Instead of using zirconia granules or spheres, the support could be
MgO modified zirconia, MgO, alpha-alumina, titania, niobia, silica,
or a wide range of other materials that are capable of serving as a
refractory support. The granule or spheres range in size from 50
microns to 6 mm in diameter (i.e., about 120 mesh, or even smaller,
to about {fraction (1/4)} inch). Preferably the particles are no
more than 3 mm in their longest characteristic dimension, or range
from about 80 mesh (0.18 millimeters) to about {fraction (1/8)}
inch, and more preferably about 35-50 mesh. The term "mesh" refers
to a standard sieve opening in a screen through which the material
will pass, as described in the Tyler Standard Screen Scale (C. J.
Geankoplis, TRANSPORT PROCESSES AND UNIT OPERATIONS, Allyn and
Bacon, Inc., Boston, Mass., p. 837), hereby incorporated herein by
reference. Preferably the support materials are pre-shaped as
granules, spheres, pellets, or other geometry that provides
satisfactory engineering performance, before application of the
catalytic materials.
[0086] It is preferred that the BET surface area of the blank
(unimpregnated) granules is higher than that of a corresponding
monolith. The BET surface area of blank 35-50 mesh ZrO.sub.2
granules is about 35 m.sup.2/g, and that of a blank (80 ppi) PSZ
monolith is about 0.609 m 2/g. With similar active catalyst
material loading, granule supported catalysts have higher metal
dispersion than corresponding monolith catalysts, as shown for
representative catalysts in Table 7. The metal surface area of the
catalyst is determined by measuring the dissociation of H.sub.2 on
the surface of the metal. A Micromeritics ASAP 2010 automatic
analyzer system is used, employing H.sub.2 as a probe molecule. The
ASAP 2010 system uses a flowing gas technique for sample
preparation to ensure complete reduction of reducible oxides on the
surface of the sample. A gas such as hydrogen flows through the
heated sample bed, reducing the oxides on the sample (such as
platinum oxide) to the active metal (pure platinum). Since only the
active metal phase responds to the chemisorbate (hydrogen in the
present case), it is possible to measure the active surface area
and metal dispersion independently of the substrate or inactive
components. The analyzer uses the static volumetric technique to
attain precise dosing of the chemisorbate and rigorously
equilibrates the sample. The first analysis measures both strong
and weak sorption data in combination. A repeat analysis measures
only the weak (reversible) uptake of the probe molecule by the
sample supports and the active metal. As many as 1000 data points
can be collected with each point being fully equilibrated.
[0087] Prior to the measurement of the metal surface area the
sample is pre-treated. The first step is to pretreat the sample in
He for 1 hr at 100.degree. C. The sample is then heated to
350.degree. C. in He for 1 hr. These steps clean the surface prior
to measurement.
[0088] Next the sample is evacuated to sub-atmospheric pressure to
remove all previously adsorbed or chemisorbed species. The sample
is then oxidized in a 10% oxygen/helium gas at 350.degree. C. for
30 minutes to remove any possible organics that are on the
surface.
[0089] The sample is then reduced at 500.degree. C. for 3 hours in
pure hydrogen gas. This reduces any reducible metal oxide to the
active metal phase. The sample is then evacuated using a vacuum
pump at 450.degree. C. for 2 hours. The sample is then cooled to
35.degree. C. prior to the measurement. The sample is then ready
for measurement of the metal surface.
[0090] From the measurement of the volume of H.sub.2 uptake during
the measurement step, it is possible to determine the metal surface
area per gram of catalyst structure by the following equation.
MSA=(V)(A)(S)(a)/22400/m
[0091] where MSA is the metal surface are in m.sup.2/gram of
catalyst structure;
[0092] V is the volume of adsorbed gas at Standard Temperature and
Pressure in ml.;
[0093] A is the Avogadro constant;
[0094] S is the stoichiometric factor (2 for H.sub.2
chemisorption);
[0095] m is the sample weight in grams; and
[0096] a is the metal cross sectional area.
[0097] As shown in Table 7, in which the metal in the equation is
rhodium, the lanthanide content helps to increase metal dispersion
on a given support.
7TABLE 7 Dispersion of Active Material on the Support Type of
Support 3-D Monolith Granules Granules Catalyst Composition 6.5%
Rh/4.53% Sm 5.42% Rh/3.73% Sm on 4.98% Rh on 80 ppi PSZ 35-50 mesh
ZrO.sub.2 on 35-50 mesh ZrO.sub.2 Metal dispersion - rhodium 9.0%
15.1% 3.6% Metal Surface Area - Sample 2.43 m.sup.2/g 3.3 m.sup.2/g
0.71 m.sup.2/g (m.sup.2/g catalyst structure) Metal Surface Area -
39.8 m.sup.2/g 66.5 m.sup.2/g 15.38 m.sup.2/g (m.sup.2/g metal)
Example 15
Rh on ZrO.sub.2 Granules
[0098] A catalyst containing 6% Rh loaded on 35-50 mesh ZrO.sub.2
granules was prepared as described in Example 14b, except that Sm
was omitted. A 0.4 mL sample was evaluated in a pilot-scale syngas
production reactor as described in Test Procedure-3, below.
Example 16
6.12% Rh/4.5% Sm on Alumina Granules
[0099] A catalyst containing 6.12% Rh loaded on 35-50 mesh alumina
granules was prepared as described in Example 14b, except that
alumina granules were substituted for the zirconia granules. A 1.2
mL sample was evaluated in a pilot-scale syngas production reactor
as described in Test Procedure-2, above.
[0100] Catalysts 12-14 and 16 were evaluated using Test
Procedure-2. The composition and dimensions of catalysts 12-16 are
summarized in Table 8 and the results of the tests on those samples
are shown in Table 9.
[0101] Test Procedure-3
[0102] The partial oxidation reactions were carried out in a
conventional flow apparatus using a 19 mm O.D..times.13 mm I.D.
quartz insert embedded inside a refractory-lined steel vessel. The
quartz insert contained a catalyst system containing at least one
porous monolith catalyst (about 12 mm O.D..times.15 mm high) held
between two foam disks. In the case of the granule-supported
catalysts (Ex. 14), the catalyst was packed between the two foam
disks. The upper disk typically consisted of 65-ppi PSZ and the
bottom disk typically consisted of 30-ppi zirconia-toughened
alumina. Preheating the methane or natural gas that flowed through
the catalyst system provided the heat needed to start the reaction.
Oxygen was mixed with the methane or natural gas immediately before
the mixture entered the catalyst system. The methane or natural gas
was spiked with propane as needed to initiate the partial oxidation
reaction, then the propane was removed as soon as the reaction
commenced. Once the partial oxidation reaction commenced, the
reaction proceeded autothermally. Two Type K thermocouples with
ceramic sheaths were used to measure catalyst inlet and outlet
temperatures. The molar ratio of CH.sub.4 to O.sub.2 was generally
about 2:1, however the relative amounts of the gases, the catalyst
inlet temperature and the reactant gas pressure could be varied by
the operator according to the parameters being evaluated (see Table
9). The product gas mixture was analyzed for CH.sub.4, O.sub.2, CO,
H.sub.2, CO.sub.2 and N.sub.2 using a gas chromatograph equipped
with a thermal conductivity detector. A gas chromotograph equipped
with flame ionization detector analyzed the gas mixture for
CH.sub.4, C.sub.2H.sub.6, C.sub.2H.sub.4 and C.sub.2H.sub.2. The
CH.sub.4 conversion levels and the CO and H.sub.2 product
selectivities obtained for each catalyst evaluated in this test
system are considered predictive of the conversion and
selectivities that will be obtained when the same catalyst is
employed in a commercial scale short contact time reactor under
similar conditions of reactant concentrations, temperature,
reactant gas pressure and space velocity.
8TABLE 8 Composition of Catalysts Monolith Dimensions Porosity
Lanthanide Rh Ex. (D .times. L, mm) Support (ppi) (wt %) (wt %) 12
12 .times. 15 PSZ (Mg) 80 4.0% Sm 6.2% 13 12 .times. 15 PSZ (Mg) 80
5.3% Sm 4.9% 14 35-50 mesh ZrO.sub.2 N/A (a) 4.1% Sm 5.8% 1.2 mL
(b) 5% Sm 6% (c) 4.13% Sm 4.52% 15 35-50 mesh ZrO.sub.2 N/A -- 6%
.4 mL 16 35-50 mesh Al.sub.2O.sub.3 N/A 4.5% Sm 6.12% 1.2 mL D =
diameter; L = length; PSZ (Mg) denotes that the zirconia is
partially stabilized with Mg; ppi = pores per linear inch
[0103]
9TABLE 9 Performance Data for Rh/Lanthanide Catalysts Metals
Content (%) NG:O.sub.2 Pressure GHSV Temp. CH.sub.4 Selectivity Ex.
Rh Ln ratio (psig) (hr.sup.-1) (.degree. C.) Conv. CO H.sub.2 12
6.2 4.0 1.82:1 45 1,014,000 983 89.6 96.2 91.2 (Sm) 13 4.9 5.3
1.73:1 45 1,280,000 1161 79.6 88.8 74.8 (Sm) 1.81:1 90 1,065,000
1156 75.6 88.3 74.5 14a 5.8 4.1 1.70:1 45 2,118,000 1033 90.9 95.8
89.5 (Sm) 1.69:1 90 1,803,000 1050 89.1 94.8 88.9 14b 4.52 4.13
1.82:1 90 1,612,000 944 95 95 95 (Sm) 1.82:1 125 2,253,000 984 95
96 92 15 6 -- 2:1 5.5 1,162,050 728 80 96 80 1,162,050 771 85 98 85
584,250 711 88 98 88 814,950 729 86 98 85 16 6.12 4.5 1.90:1 90
2,203,958 951 91 96 90 (Sm) In Ex. 14, 1.4 grams of Rh/Sm/ZrO.sub.2
granules was used for the test. In Ex. 15, 0.4 mL (i.e., bed length
about 1/8 inch, bed diameter about 1/2 inch) was used for the
tests. Ln = lanthanide; n/a = data not available
[0104] From the data shown in Table 6, as a function of the Rh
loading (Examples 7-9), it can be seen that the CH.sub.4 conversion
and CO and H.sub.2 selectivities are all in the 90+% levels, even
as the Rh loading was decreased from 4% to 2%, indicating that the
lanthanide promoted catalyst system requires smaller Rh loading to
achieve superior performance for syngas generation. The presence of
the lanthanide promoter in the supported catalyst facilitates
light-off of the reaction and these catalysts tend to run at much
lower temperatures than conventional rhodium catalysts. From the
comparative data shown in Table 6 (Examples 9-11), it can be seen
that for Sm, Yb and Pr-promoted Rh catalysts, CH.sub.4 conversion
and CO and H.sub.2 selectivities are all in the 90+% range, and are
not affected when the space velocities are increased from 190,000
hr.sup.-1 to 390,000 hr.sup.-1, indicating the stable performance
of these catalyst systems at short contact time conditions.
[0105] As shown in Table 9, comparing the catalytic activity of the
Rh/Sm/PSZ monolith catalyst (Example 12) to a similar composition
supported on zirconia granular support (Example 14a), it can be
seen that for the approximately 5.8% Rh/4.1% Sm loaded catalyst,
the CH.sub.4 conversion and CO and H.sub.2 selectivities are about
the same, despite doubling the space velocity on the latter
catalyst. This stable performance is believed to be a result of
higher active site density and better heat transfer in the case of
the particulate support form. An increase in reactant gas pressure
from 45 psig to 90 psig (Example 13 and Example 14a) does not make
a significant difference in the CH.sub.4 conversion and CO and
H.sub.2 selectivities, again confirming the efficiency of the new
catalyst systems under short contact time operation.
[0106] Repeated tests of the 4.52% Rh/4.13% Sm loaded ZrO.sub.2
granules (Example 14c) yielded significantly better performance
than a similar PSZ monolith-supported catalyst, as shown in the
performance graphs presented as FIGS. 3-5. FIG. 3 is a graph
showing catalyst performance over a two-day syngas production run
for the composition 4.52 wt % Rh and 4.13 wt % Sm.sub.2O.sub.3
supported on 35-50 mesh ZrO.sub.2 granules (Example 14(c)). In this
test, the pressure changed from 45 to 90 to 125 psig over the
approximately 60 hr period. During the final approximately 20 hours
the run conditions were 125 psig, 1,000.degree. C. and
2.25.times.10 hr.sup.-1 gas hourly space velocity. FIG. 4 is a
graph showing catalyst performance over a two-day syngas production
run for a similar catalyst. The run conditions were 45 and 90 psig.
For the last approximately 10 hrs the conditions were 90 psig,
1,080.degree. C. and 0.969.times.10.sup.6 hr.sup.-1. FIG. 5 is a
graph showing catalyst performance over an approximately 24 hr
syngas production run for a catalyst containing 6% Rh and 4% Sm
supported on an 80 ppi PSZ monolith. Over the course of this test
the pressure changed from 45 to 60 and then to 90 psig, with
temperatures of 997.degree. C., 1,080.degree. C. and 1,152.degree.
C., respectively. The respective flowrates (gas hourly space
velocity) were 1.041.times.10.sup.6 hr.sup.-1, 1.280.times.10.sup.6
hr.sup.-1 and 1.821.times.10.sup.6 hr.sup.-1.
[0107] The on-stream performance of an especially preferred
composition comprises 6% Rh/4% Sm on ZrO.sub.2 granules (Example
14(b)). The results of repeated tests of this catalyst are shown in
FIGS. 6A, 6B. Another especially preferred granule supported
catalyst comprises about 6% Rh/4% Sm on alumina granules (Example
16), the 3-day performance graph presented in FIG. 7.
[0108] The catalysts containing Rh/Sm supported on zirconia or
alumina granules showed reproducibly superior results compared to
their monolithic counterparts under the same testing conditions.
The granule-supported catalysts can run at much higher space
velocity than the similarly loaded monolith supported catalysts,
which increases the productivity of the reactor. Other advantages
of the granular catalysts include their ease of preparation,
compared to monolithic catalysts, and the flexibility they provide
to the user for process control and optimization of the geometry of
the catalyst system in short contact time syngas production
processes operated at superatmospheric pressure, preferably in
excess of about 2 atmospheres. It appears that better dispersion of
the active catalyst material is also achieved using granular
supported catalysts. It is expected that similar granular supports
can be successfully employed with syngas catalyst systems other
than the Rh and lanthanide promoted systems exemplified herein.
[0109] Process of Producing Syngas
[0110] A process for producing synthesis gas employs a
lanthanide-promoted rhodium-based monolith or granular catalyst
that is active in catalyzing the efficient conversion of methane or
natural gas and molecular oxygen to primarily CO and H.sub.2 by a
net catalytic partial oxidation (CPOX) reaction.
[0111] Suitable lanthanide-promoted Rh-based catalysts are prepared
as described in the foregoing examples. Certain preferred catalysts
comprise about 0.05-25 wt % rhodium and about 0.1-25 wt %
lanthanide (based on total weight of the supported catalyst) on a
support of partially stabilized zirconia (PSZ) (i.e., magnesium
stabilized zirconia), zirconia stabilized alumina, yttrium
stabilized zirconia, calcium stabilized zirconia, alumina
(preferably alpha-alumina), cordierite, ZrO.sub.2 or TiO.sub.2.
Some of the more preferred catalyst compositions comprise about 4-6
wt % Rh over a layer of 4-6 wt % lanthanide (Sm, Yb or Pr)
deposited on a PSZ monolith or zirconia granules, especially 5.8 wt
% Rh over 4.1 wt % Sm on zirconia granules.
[0112] Preferably employing a very fast contact (i.e., millisecond
range)/fast quench (i.e., less than one second) reactor assembly, a
feed stream comprising a hydrocarbon feedstock and an
oxygen-containing gas are mixed together and contacted with a
lanthanide-containing catalyst described below. One suitable
reaction regime is a fixed bed reaction regime, in which the
catalyst is retained within a reaction zone in a fixed arrangement.
The preferred catalyst bed length to reactor diameter is
.ltoreq.1/8. The feed stream is contacted with the catalyst in a
reaction zone maintained at autothermal net partial
oxidation-promoting conditions effective to produce an effluent
stream comprising primarily carbon monoxide and hydrogen. The
hydrocarbon feedstock may be any gaseous hydrocarbon having a low
boiling point, such as methane, natural gas, associated gas, or
other sources of light hydrocarbons having from 1 to 5 carbon
atoms. The hydrocarbon feedstock may be a gas arising from
naturally occurring reserves of methane, which contain carbon
dioxide. Preferably, the feed comprises at least about 50% by
volume methane, more preferably at least 75% by volume, and most
preferably at least 85% by volume methane.
[0113] The hydrocarbon feedstock is in the gaseous phase when
contacting the catalyst. The hydrocarbon feedstock is contacted
with the catalyst as a mixture with an O.sub.2 containing gas,
preferably pure oxygen. The hydrocarbon feedstock may be contacted
with the catalyst as a mixture containing steam and/or CO.sub.2
along with a light hydrocarbon gas, as sometimes occurs in natural
gas deposits.
[0114] The methane-containing feed and the O.sub.2 containing gas
are mixed in such amounts to give a carbon (i.e., carbon in
methane) to oxygen (i.e., molecular oxygen) ratio from about 1.5:1
to about 3.3:1, more preferably, from about 1.7:1 to about 2.1:1.
The stoichiometric molar ratio of about 2:1 (CH.sub.4:O.sub.2) is
especially desirable in obtaining the net partial oxidation
reaction products ratio of 2:1H.sub.2:CO. In some situations, such
as when the methane-containing feed is a naturally occurring
methane reserve, carbon dioxide may also be present in the
methane-containing feed without detrimentally affecting the
process. The process is operated at atmospheric or superatmospheric
pressures, the latter being preferred. The pressures may be from
about 100 kPa to about 32,000 kPa (about 1-320 atm), preferably
from about 200 kPa to 10,000 kPa (about 2-100 atm).
[0115] The process is preferably operated at temperatures of from
about 600.degree. C. to about 2,000.degree. C., preferably from
about 600.degree. C. to about 1,600.degree. C. The hydrocarbon
feedstock and the oxygen-containing gas are preferably pre-heated
before contacting with the catalyst.
[0116] The hydrocarbon feedstock and the oxygen-containing gas may
be passed over the catalyst at any of a variety of space
velocities. Space velocities for the process, stated as gas hourly
space velocity (GHSV), are from about 20,000 to about 100,000,000
hr.sup.-1, preferably from about 100,000 to about 25,000,000
hr.sup.-1. Although for ease in comparison with prior art systems
space velocities at standard conditions have been used to describe
the present invention, it is well recognized in the art that
residence time is the inverse of space velocity and that the
disclosure of high space velocities equates to low residence times
on the catalyst. Under these operating conditions a flow rate of
reactant gases is maintained sufficient to ensure a residence time
of no more than 10 milliseconds with respect to each portion of
reactant gas in contact with the catalyst system. The product gas
mixture emerging from the reactor is harvested and may be routed
directly into any of a variety of applications. One such
application for the CO and H.sub.2 product stream is for producing
higher molecular weight hydrocarbon compounds using Fischer-Tropsch
technology.
[0117] While the preferred embodiments of the invention have been
shown and described, modifications thereof can be made by one
skilled in the art without departing from the spirit and teachings
of the invention. The embodiments described herein are exemplary
only, and are not intended to be limiting. Many variations and
modifications of the invention disclosed herein are possible and
are within the scope of the invention. Accordingly, the scope of
protection is not limited by the description set out above, but is
only limited by the claims which follow, that scope including all
equivalents of the subject matter of the claims. The disclosures of
all patents, patent applications and publications cited herein are
incorporated by reference. The discussion of certain references in
the Description of Related Art, above, is not an admission that
they are prior art to the present invention, especially any
references that may have a publication date after the priority date
of this application.
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