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Publication numberUS20040002422 A1
Publication typeApplication
Application numberUS 10/184,473
Publication dateJan 1, 2004
Filing dateJun 27, 2002
Priority dateJun 27, 2002
Publication number10184473, 184473, US 2004/0002422 A1, US 2004/002422 A1, US 20040002422 A1, US 20040002422A1, US 2004002422 A1, US 2004002422A1, US-A1-20040002422, US-A1-2004002422, US2004/0002422A1, US2004/002422A1, US20040002422 A1, US20040002422A1, US2004002422 A1, US2004002422A1
InventorsDaxiang Wang, Yaming Jin, Harold Wright, Carl Johnston
Original AssigneeConoco Inc.
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Heat resistance, stability; combustion of an organometallic compound; evaporation of solvent, shaping
US 20040002422 A1
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.
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Claims(20)
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 O2-containing atmosphere.
4. The method of claim 3 wherein said predetermined heating program includes heating the combustion residue at rate up to about 10° C./min to a temperature in the range of 300-700° 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° 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 α-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 CH4 and C3H6 to CO2 (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 CeO2 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 CeO2, 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/CeO2 catalysts in which Cu2+ is dispersed on the surface of the CeO2 (P. Bera et al. J. Catalysis (1999) 186:36-44) as <100 Å crystallites. That catalyst was active for catalyzing the reduction of NO by NH3, CO reduction by NH3, 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 O2-containing atmosphere. For example, in some embodiments the predetermined heating program includes heating the combustion residue at rate up to about 10° C./min to a temperature in the range of 300-700° C., and in some embodiments calcining comprises heating the combustion residue to a temperature in the range of 400-1,500° 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 ¼″ (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/Al2O3 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/Al2O3 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/Al2O3 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/Al2O3 catalyst.

[0021]FIG. 8 shows the XRD patterns of a representative fresh Rh/CeO2 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° 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° C./min up to the autoignition point of the mixture (e.g., 200-500° 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 O2, 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(NO3)a+HmCn

MOb+H2O+N2+CO2+CO   (1)

M(NO3)a+HmCnOx

MOb+H2O+CO2+CO   (2)

M(NO3)a+HmCnNx

MOb+H2O+CO2+CO+N2   (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.

HmCn+O2

H2O+CO2+CO   (4)

HmCnOx+O2

H2O+CO2+CO   (5)

HmCnNx+O2

H2O+CO2+CO+NOx   (6)

Metal salt

Metal oxide   (7)

[0030] In the above reactions, HmCn, HmCnOx, and HmCnNx represent hydrocarbons, oxygenates and nitrogen-containing hydrocarbons. M(NO3)a represents the metal nitrate precursor for sample catalysts. NOx 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° 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 O2-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° 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 H2 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 Al2O3 was prepared by combustion synthesis, as follows: 0.651 g RhCl3.×H2O (Aldrich), and 56.5 g Al(NO3)3.9H2O (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° C./min to ignition temperature. Initially, the solution boiled and dehydrated. At around 240° 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° 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 H2/N2 (50/50 vol. %) at total flow rate of 300 ml/min for 2 hours while heated at 500° 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° 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 RhCL3.×H2O was added after the formation of paste containing Al(NO3)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/CeO2

[0041] Using a procedure similar to that used in Example 1, but substituting CeO2 for Al2O3, a sample containing 4 wt. % Rh carried on CeO2 was obtained. 0.4066 g RhCl3.×H2O (Aldrich), 12.110 g Cerium (III) nitrate hexahydrate (Ce(NO3)3.6H2O, 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/CeO2 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 Al2O3 is prepared by combustion synthesis, as follows:

[0044] Mixing: 6.51 kg RhCl3.×H2O and 565 kg Al(NO3)3.9H2O 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° C. with hot nitrogen bubbling through it to evaporate water. While heating up, the solution will start to boil and dehydrate. At around 240° 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° 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° 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 ¼″×¼″ 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 H2/N2 (50/50 vol. %) at total flow rate of 2,000 liter per hour per-kg of catalyst for 2 hours while heated at 500° 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 m2/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 H2 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/Al2O3 catalyst prepared as described in Example 1. FIG. 5 shows representative TEM micrographs of the spent Rh/Al2O3 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° 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/Al2O3, 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/Al2O3 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 m2/g) of this catalyst sample. The estimated Rh crystal size is 8 nm.

[0054]FIG. 8 shows the XRD pattern of freshly prepared Rh/CeO2 prepared as described in Example 2. The average crystal size of CeO2 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 ¼″ (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.

Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US8063063Mar 23, 2007Nov 22, 2011Novartis AgImmunopotentiating compounds
CN1990384BNov 24, 2006Jul 25, 2012奥图泰有限公司Method and apparatus for preparing metal-oxide from metallic compound
Classifications
U.S. Classification502/313, 502/325, 502/324, 422/198, 422/600
International ClassificationB01J23/63, B01J37/08, B01J35/10, B01J23/46, B01J37/03
Cooperative ClassificationB01J23/464, B01J37/03, B01J35/006, B01J35/1061, B01J23/63, B01J37/088
European ClassificationB01J23/63, B01J37/03, B01J37/08B6, B01J23/46D
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Owner name: CONOCO INC., TEXAS
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:WANG, DAXIANG;JIN, YAMING;WRIGHT, HAROLD A.;AND OTHERS;REEL/FRAME:013072/0226;SIGNING DATES FROM 20020617 TO 20020621