U.S. patent application number 10/184473 was filed with the patent office on 2004-01-01 for method for large-scale production of combustion deposited metal-metal oxide catalysts.
This patent application is currently assigned to Conoco Inc.. Invention is credited to Jin, Yaming, Johnston, Carl JR., Wang, Daxiang, Wright, Harold A..
Application Number | 20040002422 10/184473 |
Document ID | / |
Family ID | 29779368 |
Filed Date | 2004-01-01 |
United States Patent
Application |
20040002422 |
Kind Code |
A1 |
Wang, Daxiang ; et
al. |
January 1, 2004 |
Method for large-scale production of combustion deposited
metal-metal oxide catalysts
Abstract
A process and system for producing industrial-scale quantities
of highly dispersed, thermally stable catalysts is disclosed. The
process, which may be continuous production or batch production,
includes mixing together the desired catalyst precursor materials,
a combustible organic material and a solvent; evaporating the
solvent, combusting the catalyst intermediate; and shaping final
catalyst.
Inventors: |
Wang, Daxiang; (Ponca City,
OK) ; Jin, Yaming; (Ponca City, OK) ; Wright,
Harold A.; (Ponca City, OK) ; Johnston, Carl JR.;
(Ponca City, OK) |
Correspondence
Address: |
DAVID W. WESTPHAL
CONOCOPHILLIPS COMPNAY
P.O. BOX 1267
PONCA CITY
OK
74602-1267
US
|
Assignee: |
Conoco Inc.
600 North Dairy Ashford
Houston
TX
77079
|
Family ID: |
29779368 |
Appl. No.: |
10/184473 |
Filed: |
June 27, 2002 |
Current U.S.
Class: |
502/313 ;
422/198; 422/600; 502/324; 502/325 |
Current CPC
Class: |
B01J 23/464 20130101;
B01J 23/63 20130101; B01J 35/006 20130101; B01J 37/03 20130101;
B01J 35/1061 20130101; B01J 37/088 20130101 |
Class at
Publication: |
502/313 ;
502/325; 502/324; 422/198; 422/190 |
International
Class: |
F28D 001/00 |
Claims
What is claimed is:
1. A method of producing a catalyst, the method comprising:
combining in a mixing vessel at least one decomposable precursor
compound of a catalytically active metal or metal oxide,
optionally, at least one decomposable precursor compound of a
refractory metal oxide support, at least one combustible organic
compound, optionally, a liquid mixing agent, such that a mixture is
formed; in an evaporator, evaporating said liquid mixing agent, if
present, and/or a portion of said combustible organic compound to
produce a catalyst intermediate; in a furnace, heating said
catalyst intermediate to the point of autoignition, and allowing
said catalyst intermediate to combust, such that a combustion
product is produced; optionally, calcining said combustion product;
optionally, in a shaping unit, forming said combustion product into
a predetermined shape; and optionally, in an activation unit,
heating said combustion residue under activating conditions, to
provide an activated catalyst.
2. The method of claim 1 comprising at least one step for
automatically performing at least one of the following operations:
adding predetermined amounts of said precursor compound(s),
combustible organic compound and liquid mixing agent, if present,
to a mixing vessel; mixing said precursor compound(s), combustible
organic compound and liquid mixing agent, if present, in said
mixing vessel; introducing said mixture into an evaporator; heating
said liquid mixing agent, if present, and/or a portion of said
combustible organic compound within said evaporator, to produce a
catalyst intermediate; introducing said catalyst intermediate into
a furnace; heating said catalyst intermediate within said furnace
to the point of autoignition; igniting the catalyst intermediate in
the furnace with an igniter; venting combustion exhaust gas from
said furnace; calcining said combustion product; introducing said
combustion product into a shaping unit; forming said combustion
product into a predetermined shape; introducing said combustion
product into an activation unit; heating said combustion residue
under activating conditions, to provide an activated catalyst; and
collecting a final catalyst product.
3. The method of claim 1 wherein said calcining comprises heating
said residue according to a predetermined heating program in an
O.sub.2-containing atmosphere.
4. The method of claim 3 wherein said predetermined heating program
includes heating the combustion residue at rate up to about
10.degree. C./min to a temperature in the range of 300-700.degree.
C.
5. The method of claim 1 wherein said optional calcining comprises
heating the combustion residue to a temperature in the range of
400-1,500.degree. C.
6. The method of claim 1 comprising evaporating said liquid mixing
agent from said mixture prior to said ignition.
7. The method of claim 1 further comprising adding a phase
separation reducing agent to said mixture.
8. The method of claim 1 wherein heating said combustion residue
under activating conditions to provide an activated catalyst
comprises reducing conditions.
9. The method of claim 1 wherein heating said combustion residue
under activating conditions to provide an activated catalyst
comprises oxidizing conditions.
10. The method of claim 1 wherein said catalytically active metal
or metal oxide comprises at least one transition metal or metal
oxide chosen from the group consisting of Rh, Ru, Pd, Pt, Au, Ag,
Os and Ir, and oxides thereof.
11. The method of claim 10 wherein said at least one transition
metal or metal oxide is chosen from the group consisting of Co, Ni,
Mn, V and Mo, and oxides thereof.
12. The method of claim 1 wherein said at least one decomposable
precursor compound of a refractory metal oxide support comprises at
least one metal chosen from the group consisting of Mg, Ca, Al and
Si.
13. The method of claim 1 wherein said metal or metal oxide
comprises a rare earth metal or metal oxide chosen from the group
consisting of La, Yb, Sm, Ce and oxides thereof.
14. The method of claim 1 wherein said combustible organic compound
is chosen from the group consisting of amines, hydrazides, urea and
glycol.
15. The method of claim 1 comprising producing a catalyst
continuously.
16. The method of claim 1 comprising producing catalyst
intermittently or semi-continuously.
17. A catalyst comprising the product of the process of claim
1.
18. An system for producing a catalyst, the apparatus comprising:
means for mixing said predetermined amounts of said at least one
precursor compound, said at least one combustible organic compound
and said liquid mixing agent, if present; means for adding said
predetermined amount of said at least one precursor compound, said
at least one combustible organic compound and said liquid mixing
agent, if present, to said mixing means; means for evaporating said
liquid mixing agent, if present, and/or a portion of said
combustible organic compound, to produce a catalyst intermediate;
means for introducing said mixture into said evaporating means;
means for heating said catalyst intermediate to the point of
autoignition, such that a combustion product is produced;
optionally, means for calcining said combustion product;
optionally, means for shaping said catalyst; and optionally, means
for activating said catalyst.
19. The apparatus of claim 18 comprising means for filtering
combustion furnace exhaust.
20. The apparatus of claim 18 wherein said shaping means comprises
means for sizing said catalyst.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention generally relates to the production of
oxide supported highly dispersed metal or mixed oxide catalysts by
combusting a mixture of catalytic and support precursor
compounds.
[0003] 2. Description of Related Art
[0004] In the pursuit of ultra-fine, high surface area catalysts
for treatment of automobile exhaust, a combustion method for
preparing .alpha.-alumina supported Pt, Pd, Ag and Au metal
particles was devised (Bera et al. J. Mater. Chem. (1999)
9:1801-1805). In that study, combustion of aqueous redox mixtures
containing metal salts and urea yield nearly spherical metal
particles of uniform size (i.e., 7, 12, 20 and 15 nm, respectively)
dispersed on alumina. The catalysts are active for catalyzing the
complete oxidation of CO and NO. The same catalysts are also shown
to be active for catalyzing the complete oxidation of CH.sub.4 and
C.sub.3H.sub.6 to CO.sub.2 (P. Bera et al. Phys. Chem. Chem. Phys.
(2000) 2:373-378). The homogeneously dispersed nanosize metal
particles prepared by a single step combustion method provide the
active sites for catalysis.
[0005] The synthesis of certain CeO.sub.2 supported Pt and Pd
catalysts using an aqueous solution of catalyst precursors and
oxalyldihydrazide has also been described (P. Bera et al. J.
Catalysis (2000) 196:293-301). The solution is heated in an open
vessel until dehydrated and surface ignition of the residue occurs.
The resulting product, an ionic dispersion of Pt or Pd on
CeO.sub.2, was active for catalyzing nitrous oxide reduction,
oxidation of carbon monoxide, and the complete combustion of
hydrocarbons, all of which is applicable to cleaning up automobile
exhaust. A similar combustion technique has also been used to
prepare certain Cu/CeO.sub.2 catalysts in which Cu.sup.2+ is
dispersed on the surface of the CeO.sub.2 (P. Bera et al. J.
Catalysis (1999) 186:36-44) as <100 .ANG. crystallites. That
catalyst was active for catalyzing the reduction of NO by NH.sub.3,
CO reduction by NH.sub.3, and hydrocarbon oxidation by NO.
[0006] U.S. Pat. No. 6,013,313 (Nunan et al.) describes a method of
making compositions with improved homogeneity on the atomic,
nanometer or sub micron scale, by coating or impregnating a support
with a mixture of component precursors, an organic reagent and
solvent. The solvent is evaporated and the organic reagent is
decomposed. The organic reagent causes a viscous, molasses-like gel
or rigid film or matrix to form when solvent is removed during
drying. Some suggested organic reagents are sugars, carboxylic
acids, amino acids and esters. This method is said to be useful for
making ceramics, supports, catalysts and various other products.
Similarly, U.S. Pat. No. 6,326,329 (Nunan et al.) also describes an
improved impregnation method using certain organic additives to
help disperse active catalytic components. Catalysts that are
useful for converting exhaust from internal combustion engines are
disclosed.
[0007] Although significant advances have been made in the
development of catalysts having highly dispersed catalytically
active sites, there continues to be a need for a method of
producing large quantities of highly dispersed catalysts that are
suitable for industrial scale commercial use, particularly for fast
reaction processes that require high space-time yields.
SUMMARY OF PREFERRED EMBODIMENTS
[0008] A new production system and method for synthesizing large
amounts of thermally stable catalysts with highly dispersed active
components are provided in accordance with the present invention.
The method generally employs forming a uniform mixture of catalyst
precursor materials and a combustible organic compound, and then
combusting the mixture. The combustible organic compound preferably
serves as both a liquid medium for mixing the precursor compounds
and as a fuel, preferably auto-ignitable, for combustion.
Alternatively, an additional liquid solvent may be included to
facilitate obtaining a homogeneous mixture of the precursor
materials and the combustible organic material. Advantageously, the
support and the supported system are formed simultaneously in a
single combustion step, to form an improved supported system in
contrast to conventional multiphase processes that employ coating
or impregnating a preformed support. Through the combustion
process, the active catalytic components become anchored into the
metal oxide support with a high degree of dispersion to provide
fine particle, high surface area catalysts that overcome many of
the drawbacks of conventional catalysts. The high surface area
together with the high metal dispersion enhance the presence of
active sites, which is especially desirable for fast catalytic
reactions. Also, anchoring the active phase onto the surface of
thermally stable metal oxide supporting materials can deter or
prevent the active sites from sintering. As a result, ultrafine
high surface area catalyst is obtained.
[0009] In accordance with certain embodiments of the present
invention, a method of producing a catalyst is provided. The method
includes combining in a mixing vessel (a) at least one decomposable
precursor compound of a catalytically active metal or metal oxide,
(b) optionally, at least one decomposable precursor compound of a
refractory metal oxide support, (c) at least one combustible
organic compound, such that a mixture is formed. In certain
embodiments a liquid mixing agent is included. The method also
includes, in an evaporator, evaporating said liquid mixing agent,
if present, and/or a portion of said combustible organic compound
to produce a catalyst intermediate, and in a furnace, heating said
catalyst intermediate to the point of autoignition, and allowing
said catalyst intermediate to combust, such that a combustion
product is produced. In some embodiments the method also includes
calcining said combustion product. In some embodiments, the method
also includes, in a shaping unit, forming said combustion product
into a predetermined shape. In some embodiments the method also
includes, in an activation unit, heating said combustion residue
under activating conditions, to provide an activated catalyst.
[0010] In certain embodiments, the method comprises at least one
step for automatically performing at least one of the following
operations: (a) adding predetermined amounts of said precursor
compound(s), combustible organic compound and liquid mixing agent,
if present, to a mixing vessel; (b) mixing said precursor
compound(s), combustible organic compound and liquid mixing agent,
if present, in said mixing vessel; (c) introducing said mixture
into an evaporator; (d) heating said liquid mixing agent, if
present, and/or a portion of said combustible organic compound
within said evaporator, to produce a catalyst intermediate; (e)
introducing said catalyst intermediate into a furnace; (f) heating
said catalyst intermediate within said furnace to the point of
autoignition; (g) igniting the catalyst intermediate in the furnace
with a igniter; (h) venting combustion exhaust gas from said
furnace; (i) calcining said combustion product; (j) introducing
said combustion product into a shaping unit; (k) forming said
combustion product into a predetermined shape; (l) introducing said
combustion product into an activation unit; (m) heating said
combustion residue under activating conditions (e.g., reducing or
oxidizing conditions), to provide an activated catalyst; and (n)
collecting a final catalyst product. In certain embodiments, the
above-mentioned calcining comprises heating said residue according
to a predetermined heating program in an O.sub.2-containing
atmosphere. For example, in some embodiments the predetermined
heating program includes heating the combustion residue at rate up
to about 10.degree. C./min to a temperature in the range of
300-700.degree. C., and in some embodiments calcining comprises
heating the combustion residue to a temperature in the range of
400-1,500.degree. C. In certain embodiments, the method also
includes evaporating said liquid mixing agent from said mixture
prior to said ignition. A phase separation reducing agent is added
to said mixture in some embodiments.
[0011] In accordance with certain embodiments of the present
invention, a method of producing a catalyst containing at least one
catalytically active transition metal or metal oxide is provided.
In some embodiments the transition metal or metal oxide is Rh, Ru,
Pd, Pt, Au, Ag, Os or Ir, or an oxide thereof, and in some
embodiments the catalyst contains Co, Ni, Mn, V or Mo, or an oxide
thereof. In certain embodiments, the method includes decomposing at
least one decomposable precursor compound of a refractory metal
oxide wherein the metal is Mg, Ca, Al or Si. In some embodiments
the method includes decomposing at least one decomposable precursor
compound of a rare earth metal or metal oxide chosen from the group
consisting of La, Yb, Sm, Ce and oxides thereof. In some
embodiments the method includes combusting a combustible organic
compound chosen from amines, hydrazides, urea and glycol.
[0012] In accordance with certain embodiments of the present
invention, a catalyst is provided which is produced by an
above-described method. In certain embodiments, the catalyst
comprises a dispersion of nanometer diameter range particles, more
preferably 2 to 20 nm in diameter, of said metal or metal oxide
deposited on said support. In some embodiments it comprises a
monolith structure and in some embodiments it comprises a divided
or particulate structure. In some embodiments the particulate
structure comprises a group of regularly or irregularly shaped
units such as particles, granules, beads, pills, pellets,
cylinders, trilobes, extrudates, spheres or other rounded shapes.
In certain embodiments each unit has a diameter or longest
characteristic dimension of about {fraction (1/100)}" to 1/4"
(about 25 mm to 630 mm), more preferably about 50 microns to 6
mm.
[0013] A representative catalyst prepared by a hereindescribed
combustion method is active, selective and stable for producing
synthesis gas from methane or natural gas and oxidation by a
catalytic partial oxidation process. 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
[0014] FIG. 1 is a schematic illustration of the catalyst
production system in accordance with certain embodiments of the
present invention.
[0015] FIG. 2 is a graph showing the pore surface area over the
pore diameter range of a Rh/Al.sub.2O.sub.3 catalyst prepared in
accordance with an embodiment of the present invention.
[0016] FIG. 3 is a graph showing the pore volume over the pore
diameter range of the same catalyst as in FIG. 2.
[0017] FIGS. 4(a) and (b) are transmission electron micrographs
(TEMs) of a representative fresh Rh/Al.sub.2O.sub.3 sample showing
the general morphology and Rh dispersion in the catalyst.
[0018] FIGS. 5(a) and (b) are transmission electron micrographs of
a spent Rh/Al.sub.2O.sub.3 catalyst, in which (a) is from the top
portion of the catalyst bed, and (b) is from the bottom
portion.
[0019] FIGS. 6(a) and (b) are high resolution transmission electron
microscopy (HRTEM) images of the samples shown in FIGS. 4(a) and
(b), respectively.
[0020] FIG. 7 shows the XRD pattern of a representative fresh
Rh/Al.sub.2O.sub.3 catalyst.
[0021] FIG. 8 shows the XRD patterns of a representative fresh
Rh/CeO.sub.2 catalyst.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0022] An apparatus or system employed for making the new catalysts
generally includes a mixer 10, evaporator 20, combustion furnace
40, shaping unit 60 and activation unit 70, each unit adapted for
receiving a process feed from the preceding unit, as schematically
illustrated in FIG. 1. Ancillary equipment includes a steam heater
30 in communication with evaporator 20 and a filtered vent 50
connected to furnace 40.
[0023] General Procedure for Large-Scale Catalyst Preparation.
[0024] The process or method of preparing the catalyst, adapted to
the large scale typically required for producing large quantities
of industrial-scale chemical reactions, generally includes:
[0025] Mixing. Dissolving and/or suspending the selected catalytic
component or a chemical precursor thereof (e.g., thermally
decomposable metal salts and/or mixed metal oxides), in a suitable
solvent, such as water or alcohol. A selected support material or
chemical precursor thereof, such as one or more metal salt or metal
oxide may be likewise dissolved or suspended and combined with the
catalytic component. To this combined solution or suspension is
added a combustible material (such as amines, organic nitrides,
hydrazides, urea, glycol and the like), optionally with more
solvent, to form a thick suspension or a paste-like mixture. These
dissolving and combining steps are carried out in a single mixing
vessel 10, stirred with mechanical agitation or other mixing
mechanism (e.g., passing gas through the suspension).
Alternatively, the dissolving and suspending of the catalyst
precursor components could be performed in a series of separate
vessels, stepwise, and then combined in mixer 10. In one or more of
the dissolving steps, the pH value of the solution may be adjusted
to improve the solubility of the material, if necessary. The
precursor compounds (e.g., thermally decomposable metal salts) of
the desired metals/metal oxides, a combustible organic compound and
a small amount of a liquid mixing agent, preferably water, are
combined to form a uniform mixture of precursor compounds in the
combustible reagent.
[0026] Evaporating. After thoroughly mixing the ingredients, the
resulting mixture (which may have the appearance and consistence of
a clear solution or a thick paste) is fed into evaporator 20, where
the catalyst mixture (preferably a semi-solid paste) is heated up
in air sufficiently to evaporate the solvent. Preferably the
temperature of the mixture is ramped or gradually increased until
the mixture is concentrated. If necessary in order to avoid phase
separation during this stage, the pH of the solution is adjusted by
adding a suitable phase separation preventing agent such as nitric
acid. Preferably heat is provided to the evaporator by a steam
heater 30 which warms the catalyst intermediate inside the
evaporator (e.g., up to about 100.degree. C. in the case of aqueous
solvent), resulting in the evolution of solvent from the
evaporator. As used herein, the term "about" or "approximately,"
when preceding a numerical value, has its usual meaning and also
includes the range of normal measurement variations that is
customary with laboratory instruments that are commonly used in
this field of endeavor (e.g., weight, temperature or pressure
measuring devices), preferably within .+-.10% of the stated
numerical value.
[0027] Combusting. The resulting concentrated catalyst intermediate
is moved into a combustion furnace 40 where the temperature of the
catalyst intermediate is ramped at a rate of about 10.degree.
C./min up to the autoignition point of the mixture (e.g.,
200-500.degree. C.), or the mixture can be ignited with any other
suitable type of energy input, such as sparks, torch, igniters and
the like. It is preferable that the ignition and combustion are
carried out in the presence of air or O.sub.2, but it could also be
done anaerobically.
[0028] When anaerobical conditions are used, the following
exothermic redox reactions may promote the formation of
intermediates of the active catalysts.
M(NO.sub.3).sub.a+H.sub.mC.sub.nMO.sub.b+H.sub.2O+N.sub.2+CO.sub.2+CO
(1)
M(NO.sub.3).sub.a+H.sub.mC.sub.nO.sub.xMO.sub.b+H.sub.2O+CO.sub.2+CO
(2)
M(NO.sub.3).sub.a+H.sub.mC.sub.nN.sub.xMO.sub.b+H.sub.2O+CO.sub.2+CO+N.sub-
.2 (3)
[0029] When air and or oxygen is used, the following exothermic
combustion reactions (1), (2) and or (3) may occur to provide the
high temperature for the solid reaction (2) of the catalyst
precursors to form the active catalysts or its intermediates.
H.sub.mC.sub.n+O.sub.2 H.sub.2O+CO.sub.2+CO (4)
H.sub.mC.sub.nO.sub.x+O.sub.2H.sub.2O+CO.sub.2+CO (5)
H.sub.mC.sub.nN.sub.x+O.sub.2H.sub.2O+CO.sub.2+CO+NO.sub.x (6)
Metal saltMetal oxide (7)
[0030] In the above reactions, H.sub.mC.sub.n,
H.sub.mC.sub.nO.sub.x, and H.sub.mC.sub.nN.sub.x represent
hydrocarbons, oxygenates and nitrogen-containing hydrocarbons.
M(NO.sub.3).sub.a represents the metal nitrate precursor for sample
catalysts. NO.sub.x represents nitrogen oxides. It should be noted
that the above reactions (in which stoichiometric amounts of the
are reactants and products are represented by "a," "b", "m," "n,"
and "x") are representative and may not reflect the actual
stoichiometry in some situations, as the exact stoichiometry will
change with the oxygen to fuel ratios.
[0031] Upon ignition of the mixture the strong exothermic oxidation
reaction of the organic compound quickly (e.g., within a second)
heats the mixture to above 1,000.degree. C. During the combustion,
the organic compound is burnt and the metal precursor compounds
decompose to form the corresponding metal oxides or metals. The
combustion process is so fast that the compositional uniformity of
the mixture before the dispersion is preserved in the resultant
mixed metal/metal oxide material. The type of organic compound, its
concentration in the mixture, the temperature ramping rate, as well
as the environmental temperature and other factors, all have
influence on the maximum flame temperature and hence the properties
(e.g., phase structure, dispersion and stability) of the final
product. For example, by increasing the content of flammable
organic compound, the flame temperature can be increased, which
increases the stability of the final catalyst but may decrease its
surface area. Therefore, the above parameters can be varied and
optimized based on the desired catalytic performance of the final
catalyst.
[0032] Preferably, the heat of ignition is supplied by passing a
hot O.sub.2-containing gas over the catalyst intermediate in the
furnace, such as hot air or pure oxygen. The catalyst intermediate
ignites and burns, typically producing a fluffy powder. Preferably,
the resulting material is calcined in air following the combustion
step, typically at about 300-700.degree. C., to burn off any
flammable residues. Gases evolved from the catalyst during
combustion or calcining are preferably passed through a filtered
vent 50 before release into the atmosphere.
[0033] Shaping. The resulting material is then moved into shaping
unit 60, where it is pressed, crushed or sieved to form the
catalyst into the desired 3-D configuration, such as granules of a
defined mesh.
[0034] Activating. From shaping unit 60 the catalyst may,
optionally, be moved to activation unit 70, if further activation
is necessary.
[0035] Well known commercially available mixing vessels,
evaporators, combustion furnaces, shaping apparatus and activation
units may be employed for constructing the above-described system,
without substantial modification. Suitable mixers, furnaces,
shapers, and so forth have been described in the literature. For
example, by Stiles and Koch (CATALYST MANUFACTURE, 2nd ed., Marcel
Dekker, Inc., New York (1995)). With appropriate modifications that
are within the engineering skill of the artisan, the
above-described equipment and procedure can be employed as a
continuous or semi-continuous flow process from one unit to
another, with continuous or intermittent output of the catalyst,
instead of in a discontinuous or stepwise fashion with batch output
of catalyst. For certain applications of use, one mode of
production might be desired over another. In either case, this
method and processing arrangement provides for the production of
catalyst in sufficient quantities to be commercially feasible for
industrial scale catalytic operations. The catalysts prepared by
this combustion-based process are physically distinct from those
prepared by conventional methods such as precipitation,
impregnation or washcoating and which employ conventional thermal
decomposition techniques.
[0036] By choosing appropriate catalytic components, the
above-described procedure can be used to prepare highly dispersed
and thermally stable catalysts for use in a wide variety of
industrial processes in which catalysts having these qualities are
desirable. One example of a suitable application is the catalytic
partial oxidation of methane to produce synthesis gas ("syngas").
In this case a supported catalyst prepared as described herein
demonstrates enhanced conversion of methane, high selectivity for
CO and H.sub.2 products, and longer on stream life compared to a
catalyst of similar chemical composition prepared by conventional
methods.
EXAMPLE 1
[0037] Rh/Alumina
[0038] A laboratory scale quantity of catalyst containing 4 wt. %
Rh in Al.sub.2O.sub.3 was prepared by combustion synthesis, as
follows: 0.651 g RhCl.sub.3..times.H.sub.2O (Aldrich), and 56.5 g
Al(NO.sub.3).sub.3.9H.su- b.2O (Aldrich) were mixed and dissolved
in about 50 ml deionized water. The weight percent (wt. %) of Rh is
based on the total weight of the catalyst, including the support.
33.8 g oxalic dihydrazide (Aldrich) was added to the above solution
to form a paste. This paste was stirred to uniform and then divided
into four 100 ml porcelain evaporating dishes. The dishes
containing the redox mixture were heated up on a hot plate by
ramping the temperature at about 10.degree. C./min to ignition
temperature. Initially, the solution boiled and dehydrated. At
around 240.degree. C., the paste became a uniform, clear, yellowish
solution. At the point of complete dehydration, the mixture
ignited, burnt and yielded a fluffy solid product. This product is
collected and calcined at 400.degree. C. in air for 4 hours. The
powder product was pressed, crushed and sieved to form 20-40 mesh
granules to facilitate the catalytic performance test for syngas
production.
[0039] Active catalyst was obtained by reducing the calcined sample
in flowing H.sub.2/N.sub.2 (50/50 vol. %) at total flow rate of 300
ml/min for 2 hours while heated at 500.degree. C. prior to
evaluation of its physical characteristics and catalytic activity.
To demonstrate the thermal stability of this catalyst, a portion
(about 2 grams) of this reduced catalyst was further calcined at
1,000.degree. C. in flowing air (50 ml/min) for 2 hours. The
calcined sample was characterized with TEM analysis, as described
below. In another, similar preparation RhCL.sub.3..times.H.sub.2O
was added after the formation of paste containing
Al(NO.sub.3).sub.3 and oxalic dihydrazide. The resulting catalyst
had similar properties to that prepared as described above, as
indicated by transmission electron micrographs and the x-ray
diffraction patterns of the catalysts.
EXAMPLE 2
[0040] Rh/CeO.sub.2
[0041] Using a procedure similar to that used in Example 1, but
substituting CeO.sub.2 for Al.sub.2O.sub.3, a sample containing 4
wt. % Rh carried on CeO.sub.2 was obtained. 0.4066 g
RhCl.sub.3..times.H.sub.2O (Aldrich), 12.110 g Cerium (III) nitrate
hexahydrate (Ce(NO.sub.3).sub.3.6H.sub.2O, Aldrich) and 6.1 g
oxalic dihydrazide (Aldrich), 50 ml deionized water were made into
a uniform paste and heated to combust, as described in Example 1.
The rest of the preparation procedure was also carried out as
described in Example 1. The XRD pattern of the resulting
Rh/CeO.sub.2 catalyst is shown in FIG. 8.
EXAMPLE 3
[0042] Large-Scale Production of Rh/Alumina Catalyst
[0043] An 80 kg batch of catalyst containing 4 wt. % Rh in
Al.sub.2O.sub.3 is prepared by combustion synthesis, as
follows:
[0044] Mixing: 6.51 kg RhCl.sub.3..times.H.sub.2O and 565 kg
Al(NO.sub.3).sub.3.9H.sub.2O are mixed and dissolved in about 500L
de-ionized water. 338 kg oxalic dihydrazide is added to the above
solution to form a paste. The sequence of mixing is not critical,
however. For example, the paste may also be formed by putting the
solution of rhodium chloride and aluminum nitrate into the
suspension of oxalic dihydrazide in water. The resulting paste is
stirred to uniform consistency.
[0045] Evaporation: The paste is heated up to 240.degree. C. with
hot nitrogen bubbling through it to evaporate water. While heating
up, the solution will start to boil and dehydrate. At around
240.degree. C., the paste becomes a uniform, clear, yellowish
solution, indicating the evaporation is complete.
[0046] Combusting: After complete dehydration, the mixture is
further heated up in a muffle furnace to about 350.degree. C. in
flowing air to the point of ignition. During combustion, the fast
exothermic reactions results in a high temperature flame to yield a
fluffy solid product. After combustion, the fluffy product is
further calcined in the furnace at 400.degree. C. in air for 4
hours and then collected for further processing.
[0047] Shaping: The calcined material is then pressed with a die at
25,000 psi to form 1/4".times.1/4" cylindrical pellets. The pellets
are then crushed to form granules in different size ranges (such as
20-40 mesh, or 1 mm in diameter), depending on the requirements of
the desired reaction application. One application for the catalyst
is for catalytically converting a light hydrocarbon and oxygen to
synthesis gas.
[0048] Activation: Active catalyst is obtained by reducing the
calcined sample in flowing H.sub.2/N.sub.2 (50/50 vol. %) at total
flow rate of 2,000 liter per hour per-kg of catalyst for 2 hours
while heated at 500.degree. C. prior to application for syngas
production, or another application that requires a reduced
catalyst.
[0049] Surface area and pore structure. The representative
combustion-generated catalysts of Examples 1-2 were in the form of
a fluffy powder. The Rh/alumina catalyst of Example 1 had a high
surface area (27 m.sup.2/g) (BET) and a large pore structure
(meso-pores in the range of 10-100 nm diameter). FIG. 2 shows the
surface area distribution over the pore diameter range of a
representative Rh/alumina catalyst prepared according to Example 1.
FIG. 3 shows the pore volume over the pore diameter range of the
same catalyst, as measured by BJH Desorption. The surface area of
the pores in the range of 1.7-300 nm in diameter was 34 sq m/g, as
measured by BJH Desorption. The average pore diameter (4V/A) was 22
nm. It should be noted that the catalyst sample prepared using the
combustion technique has a unique pore structure, as shown in FIG.
2 and FIG. 3. It has a narrow pore distribution at pore size of
about 3-4 nm which provides the catalyst with high surface area.
This sample also has pores ranging from 4 nm to more than 100 nm.
This unique pore distribution is especially advantageous for
catalysts used in a variety of catalytic processes, e.g., for the
catalytic partial oxidation of light hydrocarbon to produce
syngas.
[0050] One example of how a combustion generated catalyst having
the above-described composition is used is in the production of
synthesis gas through selective partial oxidation (CPOX) of natural
gas in a short contact time reaction process, e.g., less than 100
milliseconds, more preferably less than 10 milliseconds. In this
process, the rate of reaction is typically strongly diffusion
limited, that is, the active sites inside the micropores (i.e.,
<10 nm diameter) of a catalyst are hardly accessible to the
reactant, and thus do not contribute appreciably to the overall
reaction rate. The modified meso/macro pore structure, as is shown
in FIG. 2 and FIG. 3, can decrease this diffusion limit by using
the meso/macro pores with diameter of up to 100 nm as the diffusion
channel for the reactant molecule to make all active sites
accessible to the reactant. This special characteristic partially
explains the high activity of these catalysts, as is shown below.
Although it is preferred to use these catalysts for syngas
production at contact times of less than 100 milliseconds, the
process can also employ contact times longer than 100 milliseconds.
The performance of the catalyst of Example 1, and other
similarly-prepared catalysts, described in U.S. Provisional Patent
Application No. 60/336,472, filed Nov. 2, 2001, entitled
"Combustion Deposited Metal-Metal Oxide Catalysts and Process for
Producing Synthesis Gas", incorporated herein by reference,
establish that certain combustion-generated catalysts can produce
syngas at short contact time with high activity and high
selectivity for CO and H.sub.2 products.
[0051] Metal dispersion and phase structure. When Rh is used as the
precious metal, Rh is highly dispersed in the final catalyst, as
can be seen in FIGS. 4-6. The average metal particle size is 8 nm,
which is much smaller than the Rh crystallites achieved by using a
conventional precipitation or impregnation method. FIGS. 4(a) and
(b) are representative TEM micrographs of Rh/Al.sub.2O.sub.3
catalyst prepared as described in Example 1. FIG. 5 shows
representative TEM micrographs of the spent Rh/Al.sub.2O.sub.3
catalyst sample showing that the general morphology is similar to
the fresh catalyst and Rh is still in highly dispersed form in the
top (a) and bottom (b) portions of the catalyst bed. The catalyst
temperature reached as high as 1,200.degree. C. during these
particular syngas reactions. Comparing the TEM patterns of the
fresh and spent samples, the TEM results shown in FIG. 5 indicate
no sintering of rhodium occurred on the spent catalysts, and
demonstrates the high thermal stability of catalyst samples
generated from combustion preparation.
[0052] FIGS. 6(a) and (b) are high resolution transmission electron
microscopy (HRTEM) images of a representative spent catalyst,
Rh/Al.sub.2O.sub.3, prepared by the combustion method. Again, this
result shows the particle sizes of Rh are in the range of 3-10 nm.
It is also of significance that, on representative spent catalyst
samples, there is no indication of the carbon deposition that is
typically seen on spent catalysts that are prepared using
conventional methods, such as impregnation, precipitation, etc. The
arrows in FIGS. 6(a) and (b) indicate the Rh(111) lattice fringes
corresponding to the (111) planes of Rh metal. Since these fringes
are clearly visible in the TEMs, the absence of graphitic carbon
overlayers on the exposed Rh metal surface of the Rh particles is
apparent. These results clearly demonstrate the superior
carbon-resistant of the syngas catalysts of this invention.
[0053] FIG. 7 shows the XRD pattern of a representative fresh
Rh/Al.sub.2O.sub.3 catalyst sample, prepared as described in
Example 1. The four characteristic Rh diffraction lines, Rh(111),
Rh(200), Rh(220) and Rh(311), are highlighted. Each Rh line,
Rh(111), Rh(200), Rh(220) or Rh(311), corresponds to one specific
set of planes as represented by their Miller indices. The XRD
pattern indicates that alpha alumina is the major crystalline phase
having an average crystal size of 46 nm. This is a major factor in
establishing the high surface area (27 m.sup.2/g) of this catalyst
sample. The estimated Rh crystal size is 8 nm.
[0054] FIG. 8 shows the XRD pattern of freshly prepared
Rh/CeO.sub.2 prepared as described in Example 2. The average
crystal size of CeO.sub.2 is 27 nm. No Rh is seen by XRD in FIG. 8,
and a TEM of the same sample indicated only occasional Rh particles
(not shown).
[0055] It is preferable to size the particles or to press the
powder catalyst obtained in the combustion synthesis into granules,
beads, pills, pellets, cylinders, trilobes, extrudates, spheres or
other rounded shapes, or other suitable shapes. A conventional
catalyst binder material such as alumina, silica, graphite, fatty
acid could be combined with the powder, if desired, to facilitate
pelletization, using standard techniques that are well-known in the
art. 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. According to some
embodiments, the divided catalyst structures have a diameter or
longest characteristic dimension of about {fraction (1/100)}" to
1/4" (about 25 mm to 630 mm). In other embodiments they are in the
range of about 50 microns to 6 mm.
[0056] The combustion generated catalyst powders are also suitable
for combining with an appropriate carrier, such as a base metal
oxide, preferably a refractory base metal oxide, and extruding or
forming the catalyst suspension into a three-dimensional structured
catalyst, such as a foam monolith. Alternatively, the powder
catalyst may be suspended in a suitable carrier and washcoated onto
a preformed honeycomb or other monolith support. The catalyst can
be structured as, or supported on, a refractory oxide "honeycomb"
straight channel extrudate or monolith, 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"), which is hereby incorporated herein by reference.
[0057] While preferred and exemplary 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. For example,
the large-scale production of combustion-generated catalysts will
find application in a variety of catalytic processes other than the
production of syngas, where high surface area, highly dispersed
catalytic materials are advantageous. 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. The disclosures of all
patents, patent applications, and publications cited herein are
hereby incorporated by reference.
* * * * *