Xie, Shuibo (Albany, CA, US)
Wright, Harold A. (Ponca City, OK, US)
Jin, Yaming (Ponca City, OK, US)
Wang, Daxiang (Ponca City, OK, US)
Fjare, Kristi A. (Ponca City, OK, US)
Minahan, David M. (Stillwater, OK, US)
Ortego, Beatrice C. (Ponca City, OK, US)
Simon, David E. (Bartlesville, OK, US)
2. The catalyst according to claim 1 wherein the active ingredient comprises a metal selected from the group consisting of rhodium, iridium, ruthenium, oxides thereof, and combinations thereof.
3. The catalyst according to claim 1 wherein the active ingredient comprises metallic rhodium, rhodium oxide, or combination thereof.
4. The catalyst according to claim 3 wherein the catalyst comprises between about 0.5 wt % and about 10 wt % of rhodium.
5. The catalyst according to claim 3 wherein the catalyst comprises between about 0.5 wt % and about 6 wt % of rhodium.
6. The catalyst according to claim 1 wherein the support contains less than 25 wt % of alpha, gamma and theta alumina combined.
7. The catalyst according to claim 1 wherein the support contains less than 10 wt % of alpha, gamma and theta alumina combined.
8. The catalyst according to claim 1 wherein the support comprises less than 6 wt % of alpha, gamma and theta alumina combined.
9. The catalyst according to claim 1 wherein the support is essentially free of alpha, gamma and theta alumina.
10. The catalyst according to claim 1 wherein the support comprises a rare earth content greater than the stoichiometric rare earth content of the corresponding rare earth hexaaluminate structure but lower than the stoichiometric rare earth content of the corresponding rare earth aluminate of perovskite structure, exclusive of said stoichiometric rare earth contents.
11. The catalyst according to claim 1 wherein the rare earth-lean aluminate comprises a hexaaluminate structure.
12. The catalyst according to claim 10 wherein the catalyst comprises between about 50 wt % and about 96 wt % of the rare earth hexaaluminate based on the total weight of the catalyst.
13. The catalyst according to claim 10 wherein the catalyst comprises between about 60 wt % and about 90 wt % of the rare earth hexaaluminate based on the total weight of the catalyst.
14. The catalyst according to claim 1 wherein both rare earth aluminates comprise the same rare earth metal selected from the group consisting of lanthanum, neodymium, praseodymium, cerium, samarium, and combinations thereof.
15. The catalyst according to claim 1 wherein both rare earth aluminates comprise lanthanum.
16. The catalyst according to claim 15 wherein the catalyst comprises between 19.2 wt % and 65 wt % of lanthanum based on the total weight of the catalyst, exclusive of endpoints.
17. The catalyst according to claim 15 wherein the catalyst comprises between 20 wt % and 30 wt % of lanthanum based on the total weight of the catalyst, inclusive of endpoints.
18. The catalyst according to claim 1 wherein the rare earth-rich aluminate comprises a perovskite structure.
19. The catalyst according to claim 18 wherein the catalyst comprises between about 0.5 wt % and about 20 wt % of the rare earth aluminate perovskite based on the total weight of the catalyst.
20. The catalyst according to claim 18 wherein the catalyst comprises between about 2 and about 15 wt % of the rare earth aluminate perovskite based on the total weight of the catalyst.
21. The catalyst according to claim 1 wherein the catalyst comprises between about 50 wt % and about 90 wt % of the rare earth-lean aluminate of a hexaaluminate structure based on the total weight of the catalyst.
22. The catalyst according to claim 1 wherein the catalyst comprises between about 65 wt % and about 90 wt % of the rare earth-lean aluminate of a hexaaluminate structure based on the total weight of the catalyst.
23. The catalyst according to claim 1 wherein the rare earth-rich aluminate comprises a rare earth metal selected from the group consisting of lanthanum, neodymium, praseodymium, cerium, samarium, and combinations thereof.
24. The catalyst according to claim 1 wherein the rare earth-rich aluminate comprises lanthanum.
25. The catalyst according to claim 1 wherein the rare earth metal in the catalyst is applied by a surface deposition of a solution of a rare earth metal precursor onto discrete structures of an aluminum-containing precursor material selected from the group consisting of one or more transition aluminas, boehmite, pseudo-boehmite, and combinations thereof, and then calcined at a temperature sufficient to convert the aluminum atoms from the aluminum-containing precursor material to at least two rare-earth aluminates of different aluminum to rare earth metal molar ratios.
26. The catalyst according to claim 1 wherein the rare earth-rich aluminate is predominantly located in an outer layer covering an inner core comprising the rare earth-lean aluminate.
27. The catalyst according to claim 1 wherein the discrete structures of the support comprise: an outer layer comprising the rare earth-rich aluminate with a molar ratio of aluminum to rare earth metal between 1:2 and 2:1, and an inner core comprising the rare earth-lean aluminate with a molar ratio of aluminum to rare earth metal greater than 5:1, wherein the outer layer is essentially free of an alumina phase.
28. The catalyst according to claim 27 wherein the outer layer covers completely the inner core.
29. The catalyst according to claim 27 wherein the outer layer comprises the outer 10% of the catalyst particle as measured from the outer surface of the discrete structures and radiating inward to the center of the discrete structures.
30. The catalyst according to claim 27 wherein the outer layer comprises the outer 6% of the catalyst particle as measured from the outer surface of the particulate catalyst and radiating inward to the center of the particulate catalyst.
31. The catalyst according to claim 27 wherein the outer layer comprises the outer 4% of the catalyst particle as measured from the outer surface of the particulate catalyst and radiating inward to the center of the particulate catalyst.
32. The catalyst according to claim 27 wherein the inner core further comprises alpha-alumina.
33. The catalyst according to claim 27 wherein the active ingredient is located within the outer layer and the inner core.
34. The catalyst according to claim 1 wherein the catalyst exhibits a daily deactivation rate in hydrocarbon conversion of 1% or less for the first 10 days of use under conditions suitable for catalytic partial oxidation of one or more light hydrocarbons at a super atmospheric pressure greater than 200 kPa.
35. The catalyst according to claim 1 wherein the catalyst exhibits a daily deactivation rate in CO selectivity or in hydrogen selectivity of 1% or less for the first 10 days of use under conditions suitable for catalytic partial oxidation of one or more light hydrocarbons.
36. A method for making synthesis gas comprising: converting a gaseous hydrocarbon stream and an oxygen-containing stream over a partial oxidation catalyst, to make a product stream comprising CO and H2, wherein said partial oxidation catalyst includes an active ingredient comprising a metal selected from the group consisting of rhodium, iridium, platinum, palladium, ruthenium, and combinations thereof; and a support in the form of discrete structures, said support comprising a rare earth-lean aluminate having a molar ratio of aluminum to rare-earth metal greater than 5:1, and a rare earth-rich aluminate having a molar ratio of aluminum to rare-earth metal greater than 5:1.
37. The method according to claim 36 wherein both rare earth aluminates comprise the same rare earth metal selected from the group consisting of lanthanum, neodymium, praseodymium, cerium, samarium, and combinations thereof.
38. The method according to claim 36 wherein both rare earth aluminates comprise lanthanum.
39. The method according to claim 38 wherein the catalyst comprises between 19.2 wt % and 65 wt % of lanthanum based on the total weight of the catalyst, exclusive of endpoints.
40. The method according to claim 38 wherein the catalyst comprises between 20 wt % and 30 wt % of lanthanum based on the total weight of the catalyst, inclusive of endpoints.
41. The method according to claim 36 wherein the rare earth-rich aluminate comprises a perovskite structure.
42. The method according to claim 36 wherein the rare earth-lean aluminate comprises a hexaaluminate structure.
43. The method according to claim 36 wherein the catalyst further contains less than 25 wt % alpha-alumina.
44. The method according to claim 36 wherein the rare earth-rich aluminate is predominantly located in an outer layer covering an inner core comprising the rare earth-lean aluminate.
45. The method according to claim 44 wherein the active ingredient is located within the outer layer and the inner core.
46. The method according to claim 36 wherein the gaseous hydrocarbon stream comprises methane.
47. The method according to claim 46 wherein the gaseous hydrocarbon stream is at a super atmospheric pressure of about 700 kPa or greater, and further wherein the catalyst exhibits a CO selectivity of about 85% or greater, a hydrogen selectivity of about 85% or greater and a methane conversion of about 85% or greater after 10 days on line under conditions suitable for catalytic partial oxidation of one or more light hydrocarbons.
48. The method according to claim 46 wherein the catalyst exhibits a carbon dioxide selectivity of about 5% or less.
49. The method according to claim 46 wherein the catalyst exhibits a C2+ selectivity of about 1% or less.
50. The method according to claim 36 wherein the catalyst exhibits less than about a 1% daily deactivation rate in hydrocarbon conversion, or in CO selectivity, or in hydrogen selectivity over the first 10 days of use under conditions suitable for catalytic partial oxidation of said hydrocarbon.
51. The catalyst according to claim 36 wherein the catalyst exhibits less than about a 0.5% daily deactivation rate in hydrocarbon conversion or in CO selectivity, or in hydrogen selectivity, over the first 10 days of use under conditions suitable for catalytic partial oxidation of said hydrocarbon.
52. The method of claim 36 wherein at least a portion of the product stream comprising CO and H2 is further converted to synthesized hydrocarbons, wherein said synthesized hydrocarbons comprise at least in part components of transportation fuels.
53. A method for making a thermally stable supported syngas catalyst suitable for long-term operation in a partial oxidation reactor at high pressure and temperature, said method comprising the following steps: impregnating a solution of a rare earth metal-containing compound onto an aluminum-containing precursor in the form of discrete structures; drying the impregnated aluminum-containing precursor; calcining at a temperature of about 1,100° C. or higher in a manner effective so as to react the aluminum-containing precursor with at least a fraction of said rare earth metal to form a support comprising a rare earth-rich aluminate, a rare earth-lean aluminate, and less than 25 wt % of alumina, wherein the rare earth-rich aluminate has a molar ratio of aluminum to rare earth metal less than 5:1, and the rare earth-lean aluminate has a molar ratio of aluminum to rare earth metal greater than 5:1; depositing an active ingredient compound onto said support, wherein the active ingredient comprises a metal selected from the group consisting of rhodium, iridium, platinum, palladium, ruthenium, oxides thereof, and combinations thereof, calcining and reducing the deposited support so as to form an activated catalyst, and heat treating the activated catalyst in an inert atmosphere at a temperature of at least about 1,100° C. to obtain the thermally stable supported syngas catalyst.
54. The method of claim 53 further comprising heat treating the activated catalyst in an inert atmosphere at a temperature of from about 1250° C. to about 1600° C.
55. The method of claim 53 wherein the aluminum-containing precursor comprises a transition alumina selected from the group consisting of gamma-alumina, delta-alumina, chi-alumina, rho-alumina, kappa-alumina, eta-alumina, theta-alumina, and combinations thereof.
56. The method of claim 53 wherein the aluminum-containing precursor comprises mostly gamma-alumina.
57. The method of claim 53 wherein calcining is done at a temperature between 1,100° C. and 1,600° C.
58. The method of claim 53 wherein calcining is done at a temperature between 1,300° C. and 1,500° C.
59. The method of claim 53 wherein the rare earth metal is selected from the group consisting of lanthanum, neodymium, praseodymium, samarium, cerium and combinations thereof.
60. The method of claim 53 wherein both rare earth aluminates comprises lanthanum.
61. The method of claim 53 wherein the solution of rare earth metal comprises more than one rare-earth metal.
62. The method of claim 53 wherein the rare earth-lean aluminate comprises a hexaaluminate structure, a beta-aluminate structure, or combinations thereof.
63. The method of claim 53 wherein the rare earth-rich aluminate comprises a perovskite structure.
64. The method of claim 53 wherein the rare earth-lean aluminate comprises a lanthanum hexaaluminate, and wherein the rare earth-rich aluminate comprises a lanthanum aluminate perovskite.
65. The method of claim 53 wherein the support comprises a rare earth content greater than the stoichiometric rare earth content of the corresponding rare earth hexaaluminate structure but lower than the stoichiometric rare earth content of the corresponding rare earth aluminate perovskite, exclusive of said stoichiometric rare earth contents.
66. The method of claim 53 wherein the catalyst further comprises less than 15 wt % alumina.
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is a continuation-in-part application of a non-provisional application Ser. No. 10/706,645 filed Nov. 12, 2003, entitled “Stabilized Alumina Supports, Catalysts made therefrom, and Their Use in Partial Oxidation,” which claims the benefit to U.S. Provisional Application Ser. No. 60/425,381 filed Nov. 11, 2002, entitled “Novel Syngas Catalysts and Their Method of Use,” U.S. Provisional Application Ser. No. 60/425,383 filed Nov. 11, 2002, entitled “Improved Supports for High Surface Area Catalysts” and U.S. Provisional Application Ser. No. 60/501,185 filed Sep. 8, 2003, entitled “Stabilized Alumina Supports, Catalysts Made Therefrom, And Their Use in Partial Oxidation,” and which are hereby incorporated by reference in their entirety herein for all purposes.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
TECHNICAL FIELD OF THE INVENTION
The present invention generally relates to catalyst supports having high thermal stability in ultra high temperature conditions, and supported catalysts made therefrom having very low deactivation rate when subjected to high temperature and high pressure catalytic conversion. The present invention particularly relates to processes for making synthesis gas via the catalytic partial oxidation of light hydrocarbons (e.g., methane or natural gas).
BACKGROUND OF THE INVENTION
It is well known that the efficiency of supported catalyst systems is often related to the surface area on the support. This is especially true for systems using precious metal catalysts or other expensive catalysts. The greater the surface area, the more catalytic material is exposed to the reactants and the less time and catalytic material is needed to maintain a high rate of productivity.
Alumina (Al 2 O 3 ) is a well-known support for many catalyst systems. It is also well known that alumina has a number of crystalline phases such as alpha-alumina (often noted as α-alumina or α-Al 2 O 3 ), gamma-alumina (often noted as γ-alumina or γ-Al 2 O 3 ) as well as a myriad of alumina polymorphs. One of the properties of gamma-alumina is that it has a very high surface area. This is commonly believed to be because the aluminum and oxygen molecules are in a crystalline structure or form that is not very densely packed. Gamma-Al 2 O 3 is a particularly important inorganic oxide refractory of widespread technological importance in the field of catalysis, often serving as a catalyst support. Gamma-Al 2 O 3 is an exceptionally good choice for catalytic applications because of a defect spinel crystal lattice that imparts to it a structure that is both open and capable of high surface area. Moreover, the defect spinel structure has vacant cation sites giving the gamma-alumina some unique properties. Gamma-alumina constitutes a part of the series known as the activated, transition aluminas, so-called because it is one of a series of aluminas that can undergo transition to different polymorphs. Santos et al. (Materials Research, 2000, vol. 3 (4), pp. 104-114) disclosed the different standard transition aluminas using Electron Microscopy studies, whereas Zhou et al. (Acta Cryst., 1991, vol. B47, pp. 617-630) and Cai et al. (Phys. Rev. Lett., 2002, vol. 89, pp. 235501) described the mechanism of the transformation of gamma-alumina to theta-alumina.
The oxides of aluminum and the corresponding hydrates, can be classified according to the arrangement of the crystal lattice with γ-Al 2 O 3 being part of the γ series by virtue of a cubic close packed (ccp) arrangement of oxygen groups. Some transitions within a series are known, for example, low-temperature dehydration of an alumina trihydrate (gibbsite, γ-Al(OH) 3 ) at 100° C. provides an alumina monohydrate (boehmite, γ-AlO(OH)). Continued dehydration at temperatures below 450° C. in the γ series leads to the transformation from boehmite to the completely dehydrated γ-Al 2 O 3 . Further heating may result in a slow and continuous loss of surface area and a slow conversion to other polymorphs of alumina having much lower surface areas. Higher temperature treatment ultimately provides α-Al 2 O 3 , a denser, harder oxide of aluminum often used in abrasives and refractories. Unfortunately, when gamma-alumina is heated to high temperatures, the structure of the atoms collapses such that the surface area decreases substantially. The most dense crystalline form of alumina is alpha-alumina. Thus, alpha-alumina has the lowest surface area, but is the most stable at high temperatures. The structure of alpha-alumina is less well suited to certain catalytic applications, such as in the Fischer-Tropsch process because of a closed crystal lattice, which imparts a relatively low surface area to the catalyst particles.
Alumina is ubiquitous as supports and/or catalysts for many heterogeneous catalytic processes. Some of these catalytic processes occur under conditions of high temperature, high pressure and/or high water vapor pressure. The prolonged exposure to high temperature typically exceeding 1,000° C., combined with a significant amount of oxygen and sometimes steam can result in catalyst deactivation by support sintering. The sintering of alumina has been widely reported in the literature (see for example Thevenin et al, Applied Catalysis A: General, 2001, vol. 212, pp. 189-197), and the phase transformation due to an increase in operating temperature is usually accompanied by a sharp decrease in surface area. In order to prevent this deactivation phenomenon, various attempts have been made to stabilize the alumina support against thermal deactivation (see Beguin et al., Journal of Catalysts, 1991, vol. 127, pp. 595-604; Chen et al., Applied Catalysis A: General, 2001, vol. 205, pp. 159-172).
The research focusing on the thermal stabilization of alumina led to the development of high temperature-resistant materials such as hexaaluminates (Matsuda et al., 8 th International Congress on Catalysis Proceedings, Berlin, 1984, vol. 4, pp. 879-889; Machida et al., Chemistry Letters, 1987, vol. 5, pp. 767-770) and the investigation of other potential oxide materials such as perovskites, spinels, and garnets, which have been examined with respect to both the thermal stability and catalytic performance.
Hexaaluminate structures have been shown to be effective structures for combustion catalysts because they provide excellent thermal stability and a higher surface area than alpha-alumina. Of particular interest, Arai and coworkers in Japan have developed hexaaluminates and substituted hexaluminates as combustion catalysts (Arai & Machida, Catalysis Today, 1991, vol. 10, pp. 81-95), and showed that the most promising stabilizer for combustion catalysts was barium (Arai & Machida, Applied Catalysis A: General, 1996, vol. 138, pp. 161-176). The investigation of the hexaaluminate material for the use of combustion has been described for example in Machida et al. (Journal of Catalysis, 1990, vol. 123, pp. 477-485) and in Groppi et al. (Applied Catalysis A: general, 1993, vol. 104, pp. 101-108). Machida et al. (Journal of American Ceramic Society, 1988, vol. 71, pp. 1142-1147) discovered that the crystal growth of one type of hexaaluminates, beta-alumina, also known as magnetoplumbite, was quite slow and anisotropic, and they proposed that its anisotropic growth may be the reason why the hexaaluminate can retain a large surface area at elevated temperatures. Arai and Machida (Catalysis Today, 1991, vol. 10, pp. 81-95) also disclosed that the thermal resistance of hexaaluminates seems to be quite dependent on the preparation procedures, primarily due to the difference of formation mechanism of hexaaluminates in various procedures. Kato et al. (Journal of American Ceramic Society, 1987, vol. 71(7), pp. C157-C159) disclosed a co-precipitation method to prepare mixtures of lanthanum and aluminum precursors, which resulted in formation of lanthanum beta-alumina structures with high surface area.
Destabilization of the support is not the sole cause of catalyst deactivation at high temperature. Stabilizing the catalytically active species on a thermally stable support is also needed. When an active species is supported on an oxide support, solid state reactions between the active species and the oxide support can take place at high temperature, creating some instability. That is why Machida et al. (Journal of Catalysis, 1989, vol. 120, pp. 377-386) proposed the introduction of cations of active species through direct substitution in the lattice site of hexaaluminates in order to suppress the deterioration originating from the solid state reaction between the active species and the oxide support. These cation-substituted hexaaluminates showed excellent surface area retention and high catalytic activity (see the hexaaluminate examples with Sr, La, Mn combinations in Machida et al., Journal of Catalysis, 1990, vol. 123, pp. 477-485). Therefore, the preparation procedure for high temperature catalysts is critical for thermal stability and acceptable surface area.
It has long been a desire in the catalyst support arts to have a form of alumina that has high surface area like gamma-alumina and stability at high temperature like alpha-alumina. Such a catalyst support would have many uses.
One such use is in the production of synthesis gas in a catalytic partial oxidation reactor. Synthesis gas is primarily a mixture of hydrogen and carbon monoxide and can be made from the partial burning of light hydrocarbons with oxygen. The hydrocarbons, such as methane or ethane are mixed with oxygen or oxygen containing gas and heated. When the mixture comes in contact with an active catalyst material at a temperature above an initiation temperature, the reactants quickly react generating synthesis gas and a lot of heat. This very fast reaction requires only milliseconds of contact of the reactant gases with the catalyst. The combination of high exothermicity and very fast reaction time causes reactor temperatures to exceed 800° C., often going above 1,000° C. and even sometimes going above 1,200° C. Since catalysts used in the partial oxidation of hydrocarbons are typically supported, the support should be able to sustain this high thermal condition during long-term operation. In other words, a stable catalyst support which retains most of its surface area while enduring very high temperature, is desirable for long catalyst life.
The reaction pathway for partial oxidation of methane to synthesis gas is still being debated. Two alternate pathways have been proposed (Dissanayake et al., J. Catal., 1991, vol. 132, pp. 117; Jin et al., Appl. Catal., 2000, vol. 201, pp. 71; Heitne et al., Catal. Today, 1995, vol. 24, pp. 211).
These two pathways have come to be known as the combustion-reforming mechanism (Scheme 1) and the direct partial oxidation mechanism (Scheme 2). In Scheme 1, methane is completely oxidized to CO 2 and water, and CO is a result of the reforming of water and CO 2 with the residual methane. In Scheme 2, methane is pyrolyzed over the catalyst to produce CO directly without the pre-formation of CO 2 .
Weng, et al. (The Chemical Record, 2002, vol. 2, pp. 102-113) reported in situ Fourier transform infrared (FTIR) studies of the catalytic partial oxidation (CPOX) mechanism of methane over rhodium and ruthenium based catalysts supported on silica and alumina. They specifically studied the influence of the catalyst pretreatment conditions and their relationship with the concentration of oxygen species on the surface of the catalysts under reaction conditions. They concluded that a) the CPOX mechanism, whether based on Scheme 2 (i.e., -direct oxidation) or based on Scheme 1 (combustion/reforming), is determined by the amount of O 2− on the catalyst surface; b) an oxidized catalyst, such as Rh 2 O 3 , promotes the combustion/reforming mechanism (Scheme 1), whereas rhodium in the reduced state will promote the direct pathway (Scheme 2); c) rhodium on gamma-alumina under normal feed conditions of methane to molecular oxygen ratio in the feed will contain mostly oxidized Rh, even if rhodium was pre-reduced; d) the reducibility of rhodium is greatly affected by the support; and e) a lower reduction peak temperature, as measured by temperature-programmed reduction (TPR), indicates a weaker Rh—O bond.
A weaker Rh—O bond would lead to easier removal of the surface oxygen, and therefore the lower TPR temperature peak. During normal operating conditions, a weaker Rh—O bond should promote reduced rhodium on the surface, which would favor a direct pathway. In turn, this would lead to lower catalyst surface temperatures, which should slow the alumina phase transformation to ultimately alpha-Al 2 O 3 (also slowing deactivation).
Roh et al. (Chemistry Letters, 2001, vol. 7, pp. 666-667) reported that nickel based partial oxidation catalyst based on theta-alumina had high activity as well as high stability, and they ascribed the excellent performance of these catalyst to the combination of the strong interactions between nickel and theta-alumina and the coexistence of reduced and oxidized nickel species. Liu et al. (Korean J. Chem. Eng., 2002, vol. 19, pp. 742-748) have also shown that a protective layer between Ce—ZrO2 and theta-alumina is formed to suppress the formation of nickel-aluminate spinel structures, which would result in catalyst deactivation. Moreover Miao et al. (Appl. Catal. A, 1997, vol. 154, pp. 17-27) indicated that the modification with an alkali metal (Li, Na, K) oxide and a rare earth metal (La, Ce, Y, Sm) oxide improved the ability of a nickel catalyst on alumina to suppress carbon deposition over the catalyst during partial oxidation of methane. Therefore, the type of support used and the catalytic metal-support interactions are major factors in the catalyst stability and can have an effect on the reaction mechanism.
In addition to the selection and careful preparation of the support, catalyst composition also plays an important role in catalyst activity in catalytic partial oxidation of light hydrocarbons and selectivity towards to the desired products. Noble metals typically serve as the best catalysts for the partial oxidation of methane. Noble metals are however scarce and expensive, making their use economically challenging especially when the stability of the catalyst is questionable. One of the better known noble metal catalysts for catalytic partial oxidation comprises rhodium. Rhodium-based syngas catalysts deactivate very fast due to sintering of both catalyst support and/or metal particles. Prevention of any of these undesirable phenomena is well-sought after in the art of catalytic partial oxidation processes, particularly for successful and economical operation at commercial scale.
It would therefore be highly desirable to create a thermally-stable high surface area support with a metal from Groups 8, 9, or 10 of the Periodic Table of the Elements (based on the new WUPAC notation, which is used throughout the present specification), particularly with rhodium, loaded onto said support for highly productive long lifetime catalysts for the syngas production, specifically via partial oxidation.
SUMMARY OF THE INVENTION
The current invention addresses the stability and durability of catalyst supports and catalysts made therefrom for use in reactors operating at very high temperatures. Particularly, the present invention relates to a high surface area aluminum-based support comprising a transition alumina phase and at least one stabilizing agent. The transition alumina phase preferably comprises theta-alumina and may contain any other alumina phases comprised between low-temperature gamma-alumina and high-temperature stable alpha-alumina. The transition alumina phase preferably comprises mainly a theta-alumina phase. The alumina support preferably may further comprise alpha-alumina, but is preferably substantially free of gamma-alumina. The stabilizing agent comprises at least one element from Groups 1-14 of the Periodic Table of Elements, and is preferably selected from the group consisting of rare earth metals, alkali earth metals and transition metals. The inventive support also is thermally stable at temperatures above 800° C.
The present invention also relates to a thermally stable aluminum-based material, which is suitable as a catalyst support for high temperature reactions. The thermally stable aluminum-based material includes a rare earth aluminate comprising at least one rare earth metal, wherein the rare earth aluminate has a molar ratio of aluminum to rare earth metal (Al:Ln) greater than 5:1. The rare earth aluminate with an Al:Ln greater than 5:1 preferably comprises a lanthanide metal selected form the group consisting of lanthanum, praseodymium, cerium, neodymium, samarium, and combinations thereof. In preferred embodiments, the rare earth aluminate comprises a hexaaluminate-like structure or a beta-alumina-like structure, which comprises an Al:Ln between 11:1 and 14:1.
The present invention further relates to a thermally stable aluminum-based catalyst support, wherein the thermally stable aluminum-based catalyst support comprises an aluminum oxide phase selected from the group consisting of alpha-alumina, theta-alumina, or combinations thereof; and a rare earth aluminate comprising a rare earth metal, wherein the alumina-like rare earth aluminate has a molar ratio of aluminum to rare earth metal greater than 5:1. The rare earth aluminate with a high molar ratio of aluminum to rare earth metal comprises from 100 wt % of the support and more preferably less than 100 wt % down to as little as 1 wt % of the material weight in the catalyst support. In preferred embodiments, the thermally stable support comprises between about 1 wt % and about 50 wt % of said rare earth aluminate. In other embodiments, the thermally stable aluminum-based catalyst support could comprise between 40 wt % and 100 wt % of rare earth aluminate; and in some cases, the support is a rare earth aluminate or a mixture of rare earth aluminates with a molar ratio of aluminum to rare earth metal greater than 5:1. The thermally stable catalyst support could contain between about 1 wt % and about 20 wt % of rare earth metal; preferably between about 1 wt % and about 10 wt % of rare earth metal. The rare earth aluminate preferably comprises lanthanum, praseodymium, cerium, neodymium, samarium, or combinations thereof. In preferred embodiments, the rare earth aluminate comprises a hexaaluminate-like structure, a beta-alumina like structure, or combinations thereof In these preferred embodiments, the thermally stable catalyst support comprises at least one rare earth aluminate with an aluminum-to-rare earth molar ratio between 11:1 and 14:1; and at least one aluminum oxide phase selected from alpha-alumina, theta-alumina, or combinations thereof. The thermally stable aluminum-based material may further comprise a transition alumina, such as delta-alumina, eta-alumina, kappa-alumina, chi-alumina, rho-alumina, kappa-alumina, or any combinations thereof, but is preferably substantially free of gamma-alumina.
The method for making a high surface area aluminum-based support includes applying at least one stabilizing agent to an aluminum-containing precursor following by heat treatment, wherein the heat treatment conditions are selected such that a portion of the aluminum-containing precursor is transformed to a transition alumina and optionally to alpha-alumina, wherein the transition alumina comprises delta-alumina, eta-alumina, kappa-alumina, chi-alumina, rho-alumina kappa-alumina, or any combinations thereof The heat treatment can also be effective in transforming another portion of the aluminum-containing precursor to an aluminate comprising at least a portion of said stabilizing agent, and wherein the resulting support is preferably substantially free of gamma-alumina. The stabilizing agent preferably comprises a rare earth metal. The stabilizing agent preferably includes a lanthanide metal selected from the group consisting of lanthanum, cerium, neodymium, praseodymium, and samarium, but may further include any element from Groups 1-14 of the Periodic Table (new IUPAC notation) such as an alkali metal, an alkali earth metal, a second rare earth metal, or a transition metal. The aluminum-containing precursor comprises at least one material selected from the group consisting of an oxide of aluminum, a salt of aluminum, an alkoxide of aluminum, a hydroxide of aluminum, and combinations thereof.
The present invention also includes a method for making a thermally stable aluminum-based catalyst support suitable for use in a high temperature reaction. This method includes applying at least one rare earth metal compound to an aluminum-containing precursor; and treating by heat the applied precursor, wherein the heat treatment conditions are selected such that at least a portion of the aluminum-containing precursor is transformed to an aluminate comprising at least a portion of said rare earth metal, and wherein the rare earth aluminate comprises an aluminum-to-rare earth metal molar ratio greater than 5:1. The heat treatment is performed in a manner effective to obtain about 1 wt % and 100 wt % of said rare earth aluminate in the thermally stable catalyst support; preferably more than 1 wt % but less than 100 wt % of said rare earth aluminate. In some embodiments, the heat treatment is performed in a manner effective to obtain between about 1 wt % and about 50 wt % of said rare earth aluminate in the thermally stable support. In other embodiments, the heat treatment is performed in a manner effective to obtain between 40 wt % and 100 wt % of rare earth aluminate in the thermally stable catalyst support. In preferred embodiments, the heat treatment is performed in a manner effective to obtain between 50 wt % and 95 wt %, preferably between 60 wt % and 90 wt %, of rare earth aluminate in the thermally stable catalyst support. In some alternate embodiments, the heat treatment is performed in a manner effective to transform all of the aluminum-containing precursor to at least one rare earth aluminate with an aluminum-to-rare earth metal molar ratio greater than 5:1. In other embodiments, the heat treatment is performed in a manner effective to transform all of the aluminum-containing precursor to one rare earth-lean aluminate with an aluminum-to-rare earth metal molar ratio greater than 5:1 and one rare earth-rich aluminate with an aluminum-to-rare earth metal molar ratio less than 5:1. The rare earth-lean and -rich aluminates preferably contain at least one common rare earth. The application and heating steps preferably employ an impregnation technique and calcination in an oxidizing atmosphere, respectively. Additionally, the heat treatment step is effective to transform another portion of said aluminum-containing precursor to an aluminum oxide phase comprising alpha-alumina, a transition alumina, or combinations thereof, wherein the transition alumina comprises delta-alumina, eta-alumina, kappa-alumina, chi-alumina, rho-alumina, kappa-alumina, theta-alumina, or any combinations thereof. The transition alumina comprises preferably theta-alumina. Additionally or alternatively, the heat treatment step is effective to transform a portion of rare earth-containing precursor to a rare earth oxide phase.
The invention further includes a catalyst comprising a catalytically active metal selected from the group consisting of rhodium (Rh), ruthenium (Ru), iridium (kr), platinum (Pt), palladium (Pd), and rhenium (Re), on a thermally stabilized support wherein the thermally stabilized support comprises theta-alumina, a rare earth aluminate with an aluminum to rare earth metal molar ratio greater than 5:1, or combinations thereof.
More particularly, the invention includes a catalyst comprising a catalytically active metal selected from the group consisting of rhodium (Rh), ruthenium (Ru), iridium (fr), platinum (Pt), palladium (Pd), and rhenium (Re), on a thermally stabilized support wherein the thermally stabilized support comprises between about 1 wt % and 100 wt % of a rare earth aluminate with an aluminum to rare earth metal molar ratio greater than 5:1; preferably more than 1 wt % but less than 100 wt % of said rare earth aluminate with an aluminum to rare earth metal molar ratio greater than 5:1; preferably more than 50 wt % but less than 95 wt % of said rare earth aluminate with an aluminum to rare earth metal molar ratio greater than 5:1.
A more specific embodiment of the invention relates to a partial oxidation catalyst with an active ingredient selected from the group consisting of rhodium, iridium, and ruthenium; and an optional promoter loaded onto a thermally stable support, wherein said support includes an alumina phase selected from the group consisting of alpha-alumina, theta-alumina, or any combinations thereof; and between about 1 wt % and about 50 wt % of a rare earth aluminate with a molar ratio of aluminum to said rare earth metal greater than 5:1. In other embodiments, the thermally stable aluminum-based catalyst support could comprise more than 40 wt % of rare earth aluminate and less than 100 wt % of rare earth aluminate.
The present invention can be more specifically seen as a support, process and catalyst for a partial oxidation reaction, wherein the support comprises a rare earth aluminate having a molar ratio of aluminum to rare earth metal greater than 5:1, and wherein the rare earth aluminate preferably comprises an element selected from the group consisting of lanthanum, cerium, praseodymium, samarium, and neodymium. The support may comprise between 1 wt % and 100 wt % of the rare earth aluminate. In preferred embodiments, the thermally stable support comprises between about 1 wt % and about 50 wt % of said rare earth aluminate. In other embodiments, the thermally stable aluminum-based catalyst support could comprise between 40 wt % and 100 wt % of the rare earth aluminate; and in some alternate embodiments, the support is a rare earth aluminate or a mixture of rare earth aluminates with an aluminum to rare earth metal molar ratio greater than 5:1. The supported catalyst comprises at least one catalytically active metal selected from the group consisting of rhodium, ruthenium, iridium, platinum, palladium, and rhenium, preferably selected from the group consisting of rhodium, iridium, and ruthenium, and optionally the catalyst can also comprise a promoter.
More particularly, the invention relates to processes for the catalytic partial oxidation of light hydrocarbons (e.g., methane or natural gas) to produce primarily synthesis gas and the use of such supported catalysts to make carbon monoxide and hydrogen under conditions of high gas hourly space velocity, elevated pressure and high temperature.
The process for making synthesis gas comprises converting a gaseous hydrocarbon stream and an oxygen-containing stream over a partial oxidation catalyst, to make a product stream comprising CO and H 2 , wherein said partial oxidation catalyst includes an active ingredient comprising rhodium, iridium, platinum, palladium, ruthenium, or combinations thereof, and a support comprising a rare earth aluminate, said rare earth aluminate having a molar ratio of aluminum to rare earth metal greater than 5:1. The support could comprise between about 1 wt % and 100 wt % of said rare earth aluminate, preferably between about 1 wt % and about 50 wt % of said rare earth aluminate. In other embodiments, the support could comprise between 40 wt % and 100 wt % of the rare earth aluminate; and in some alternate embodiments, the support is a rare earth aluminate or a mixture of rare earth aluminates with an molar ratio of aluminum to rare earth metal greater than 5:1. The rare earth metal is selected from the group consisting of lanthanum, neodymium, praseodymium, cerium, and combinations thereof, and the support could comprise between about 1 wt % and about 20 wt % of the rare earth metal, but preferably between about 1 wt % and about 10 wt % of the rare earth metal. The support may further comprise an aluminum oxide such as alpha-alumina, a transition alumina, or combinations thereof, wherein the transition alumina comprises delta-alumina, eta-alumina, kappa-alumina, chi-alumina, rho-alumina, kappa-alumina, theta-alumina, or any combinations thereof. The transition alumina comprises preferably theta-alumina. The support may further comprise an oxide of said rare earth metal and/or an aluminate of said rare earth aluminate with a low aluminum to rare earth metal molar ratio, such as below 2:1.
The present invention further relates to catalysts and processes for the conversion of gaseous light hydrocarbons for producing a hydrocarbon product, comprising primarily hydrocarbons with 5 carbons atoms or more (C 5+ ).
In one embodiment, needs in the art are addressed by a high temperature stable syngas catalyst. The catalyst comprises an active ingredient comprising a metal selected from the group consisting of rhodium, iridium, platinum, palladium, ruthenium, oxides thereof, and combinations thereof. The active ingredient is supported on a catalyst support comprising a rare earth-rich aluminate with a molar ratio of aluminum to rare earth metal less than 5:1; and a rare earth-lean aluminate with a molar ratio of aluminum to rare earth metal greater than 5:1. The support is in the form of discrete structures.
In another embodiment, needs in the art are addressed by a method for making synthesis gas. The method comprises converting a gaseous hydrocarbon stream and an oxygen-containing stream over a partial oxidation catalyst, to make a product stream comprising CO and H 2 . The partial oxidation catalyst includes an active ingredient comprising a metal selected from the group consisting of rhodium, iridium, platinum, palladium, ruthenium, and combinations thereof. The method further comprises a support in the form of discrete structures, said support comprising a rare earth-lean aluminate having a molar ratio of aluminum to rare-earth metal greater than 5:1, and a rare earth-rich aluminate having a molar ratio of aluminum to rare-earth metal greater than 5:1.
Another embodiment addresses needs in the art by a method for making a thermally stable supported syngas catalyst suitable for long-term operation in a partial oxidation reactor at high pressure and temperature. The method comprises impregnating a solution of a rare earth metal-containing compound onto an aluminum-containing precursor in the form of discrete structures. The method further comprises drying the impregnated aluminum-containing precursor. In addition, the method comprises calcining at a temperature of about 1,100° C. or higher in a manner effective so as to react the aluminum-containing precursor with at least a fraction of said rare earth metal to form a support comprising a rare earth-rich aluminate, a rare earth-lean aluminate, and less than 25 wt % of alumina, wherein the rare earth-rich aluminate has a molar ratio of aluminum to rare earth metal less than 5:1, and the rare earth-lean aluminate has a molar ratio of aluminum to rare earth metal greater than 5:1. Moreover, the method comprises depositing an active ingredient compound onto said support, wherein the active ingredient comprises a metal selected from the group consisting of rhodium, iridium, platinum, palladium, ruthenium, oxides thereof, and combinations thereof, calcining and reducing the deposited support so as to form an activated catalyst, and heat treating the activated catalyst in an inert atmosphere at a temperature of at least about 1,100° C. to obtain the thermally stable supported syngas catalyst.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more detailed understanding of the preferred embodiments, reference is made to the accompanying drawings, wherein:
FIG. 1 represents the temperature programmed reduction (TPR) profile of a catalyst comprising mainly theta-alumina according to this invention;
FIGS. 2 a, 2 b and 2 c represent the XRD analysis of materials comprising various loadings of lanthanum applied to gamma-alumina and calcined at different temperatures;
FIGS. 3 a and 3 b represent the effect of lanthanum loadings on the resulting surface area and pore volume (respectively) of catalyst supports made at two different calcinations temperatures;
FIG. 4 represents the performance data for synthesis gas production from a catalyst made according to a preferred embodiment of the invention;
FIGS. 5 a - 5 d illustrate the improved performance (hydrocarbon conversion, the hydrogen selectivity, CO selectivity, and exit temperature) of a partial oxidation process employing 4% Rh catalysts according to the present invention compared to catalysts supported on alpha-alumina at a pressure of 90 psig (about 722 kPa);
FIGS. 6 a - 6 d illustrate the improved performance (hydrocarbon conversion, the hydrogen selectivity, CO selectivity, and exit temperature) of a partial oxidation process employing 2% Rh catalysts according to the present invention compared to catalysts supported on alpha-alumina at a pressure of 90 psig (about 722 kPa); and
FIG. 7 illustrates the improved performance (hydrocarbon conversion, the hydrogen seletivity, CO selectivity) of a large-scale partial oxidation process employing 4% Rh catalysts according to the present invention at a pressure of 180 psig (about 1340 kPa).
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention is based on the surprising discovery that a supported rhodium-based catalyst supported on an aluminum-based matrix modified with a lanthanum compound showed excellent performance with conversion and selectivities above 90%, and a sustainable activity over more than 300 hours on line while in contact with natural gas and molecular oxygen under suitable conditions for catalytic partial oxidation, namely at high temperatures and at high pressure. It was found that this catalyst initially comprised about 65% theta-alumina phase, some small amount of alpha-alumina (10%), but was free of gamma-alumina. In addition, the catalyst comprised a good portion of lanthanum aluminum mixed oxide compounds (La—Al—O) with a hexaaluminate-like structure (18%). This hexaaluminate-like structure comprised the majority of the lanthanum. Moreover, this catalyst showed a low reduction peak temperature in a TPR analysis (shown in FIG. 1), much lower than similar catalysts which comprised supports with less theta-alumina phase, more gamma-alumina, minimal amount of rare earth aluminates, and substantially almost no alpha-alumina, or for similar catalysts which comprised supports of mainly alpha-alumina.
As described in Weng et al. (The Chemical Record, 2002, vol. 2, pp. 101-113), it is believed that a low TPR peak temperature is an indication of a loose Rh—O bond, thereby favoring the formation of reduced rhodium on the surface of the catalyst, which in turn favors the direct mechanism of partial oxidation (Scheme 2). The direct mechanism generates a lot less heat (the heat of CH 4 +½O 2 reaction is −6.6 kcal/mol) whereas the combustion reaction in Scheme 1 generates much more heat (as the heat of CH 4 +2O 2 is −191.3 kcal/mol). Therefore, the direct mechanism should produce a cooler catalyst surface temperature. Without wishing to be bound to this theory, the Applicant believes that the presence of a theta-alumina phase might increase oxygen mobility, increases the fraction of rhodium in reduced state, increases the conversion of methane (and other light hydrocarbons) via the direct mechanism and thereby reduces the catalyst surface temperature. It is expected that a cooler catalyst surface temperature prevents or minimizes the formation of carbonaceous deposit on the catalyst surface, which is one of the sources of catalyst deactivation. Another source of catalyst deactivation is the phase transformation of alumina to ultimately alpha-alumina and concurring support disintegration, surface cracking and/or loss of surface area. Therefore, a cooler catalyst surface temperature should also slow the rate of the phase transformation of alumina, which is thermodynamically favored by increase in temperature.
Modifying alumina (Al 2 O 3 ) with some rare earth metals has been proven to be effective in stabilizing the surface area of modified Al 2 O 3 . Doping a gamma-alumina (γ-Al 2 O 3 ) with certain metal oxides such as for example lanthanum oxide (La 2 O 3 ) inhibits or retards the phase transformation of gamma-alumina phase to theta-alumina (θ-Al 2 O 3 ) phase and eventually to alpha-alumina (α-Al 2 O 3 ) phase and thus stabilizes the surface area and pore structure of the alumina material even at high calcination temperatures above 1,000° C. Not only doping the surface of gamma-alumina (γ-Al 2 O 3 ) can stabilize the surface structure of aluminum oxide (Al 2 O 3 ) and thus delay the phase transformation to alpha alumina, but also it can slow down the sintering at high temperatures. The driving force for sintering is the minimization of surface free energy, and thus thermodynamically, sintering is an irreversible process in which a free energy decrease is brought about by a decrease in surface area. Sintering is usually initiated on the particle surface at elevated temperatures, in a range where surface atoms become mobile and where diffusional mass transport is appreciable. The formation of Ln-Al—O mixed oxide compounds could inhibit the surface diffusion of species responsible for sintering, and thereby may be one of the key stabilization factors on an alumina surface at high temperatures.
The formation of highly thermal stable La—Al—O mixed oxide compounds such as those of hexaaluminate-type structure should also ultimately help maintain a relatively high surface area. However, it is not clear from the literature that the formation of lanthanum aluminates with hexaaluminate-like or beta-alumina structures from an alumina precursor modified with lanthanum would explain an improved thermal stability of this catalyst. Beguin et al (1991) in fact disclosed that the formation of lanthanum beta-alumina structures was associated with the loss of the stabilizing effect of lanthanum on an alumina-based material; and therefore showed that the formation of lanthanum beta-alumina structures was detrimental to the stabilization effect associated with the modification of alumina by lanthanum. Oudet et al (Applied Catalysis, 1991, vol. 75, pp. 119-132) attributed the stabilization of alumina by lanthanum to the nucleation of a cubic lanthanum aluminum oxide structure (LaAlO 3 ) on the surface of the alumina support, which inhibits the surface diffusion of species responsible for sintering.
As for the method of preparation, Schaper et al. (Applied Catalysis, 1983, vol. 7, pp. 211-220) who studied the influence of addition of lanthanum (0-5 mol % La 2 O 3 ) on the thermal stability of gamma-alumina between 800 and 1,100° C., did not observe the formation of lanthanum hexaaluminate even though they observed a retardation in the sintering of gamma-alumina by the presence of perovskite-type lanthanum aluminate (LaAlO 3 ). The discrepancy between the formation of lanthanum hexaaluminate structures in Kato et al. (1987) and the absence of lanthanum hexaaluminate structures in Schaper et al (1983) is most likely attributed to the differences of the preparation method. Kato et al. mentioned that, with the impregnation technique, the higher concentration of lanthanum at the surface layer of the alumina phase probably tends to favor the formation of a lanthanum aluminate with a low aluminum-to-lanthanum ratio. However, according to this invention, lanthanum aluminates with a high aluminum-to-lanthanum ratio were being formed using an impregnation technique. It was quite unexpected, first to find that lanthanum hexaaluminate-like structures were formed in a catalyst support made by an impregnation technique on a lanthanum precursor on a gamma-alumina, and that, second, the presence of lanthanum hexaaluminate-like structures in a catalyst support did result in a more stable performance of the catalyst made therefrom. Therefore, this invention relates to a catalyst support, which comprises a rare earth aluminate with a high aluminum-to-rare earth molar ratio, and to catalysts made therefrom used in high temperature environments which show unexpected good thermal stability and have a greater surface area than those catalysts supported on alpha-alumina under similar operating conditions.
Herein will be described in detail, specific embodiments of the present invention, with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein. The present invention is susceptible to embodiments of different forms or order and should not be interpreted to be limited to the specifically expressed methods or compositions or applications contained herein. In particular, various embodiments of the present invention provide a number of different combinations of features to generate high surface area supports for high temperature applications, which also comprise very good thermal stability.
Supports
The thermally stable supports according to this invention can have different forms such as monolith or particulate or have discrete or distinct structures. 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. 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, pastilles, 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 support is preferably in discrete structures, and particulates are more preferred.
Thermally Stable Catalyst Support Comprising a Rare earth Aluminate with Al:Ln>5:1
This invention relates to a thermally stable aluminum-based support comprising a rare earth aluminate with a high aluminum-to-rare earth molar ratio. The aluminum-to-rare earth molar ratio (Al:Ln) is greater than 5:1; preferably greater than about 10; and more preferably between about 11:1 and about 14:1. Preferably the thermally stable aluminum-based contains at least one rare earth aluminate selected from a rare earth hexaaluminate-like structure and/or a rare earth beta-alumina-like structure.
The thermally stable aluminum-based support may comprise between 1 wt % to 100 wt % of the rare earth aluminate with a high Al:Ln ratio. In preferred embodiments, the thermally stable support comprises between about 1 wt % and about 50 wt % of said rare earth aluminate; more preferably between about 5 wt % and about 45 wt % of the rare earth aluminate; and still more preferably between about 10 wt % and about 40 wt % of the rare earth aluminate. In other embodiments, the thermally stable aluminum-based catalyst support could comprise between 40 wt % and 100 wt % of the rare earth aluminate; and in some alternate embodiments, one or more rare earth aluminates with high aluminum-to-rare earth molar ratios (greater than 5:1) comprises 100 wt % of the support. The support in the catalyst could comprise between about 1 wt % and 100 wt % of said rare earth aluminate. In preferred embodiments, the support in the catalyst comprises between about 1 wt % and about 50 wt % of said rare earth aluminate. In other embodiments, the support in the catalyst could comprise more than 40 wt % of rare earth aluminate, i.e., between 40 wt % and 100 wt % of rare earth aluminate; and in some cases, the support is a rare earth aluminate or a mixture of rare earth aluminates with a molar ratio of aluminum to rare earth metal greater than 5:1. It should be readily appreciated that there are preferences within the 1 wt %-100 wt % range for the rare earth aluminate content of the support depending on the desired properties of the support.
The support should contain between about 1 wt % and about 20 wt % of rare earth metal; preferably between about 1 wt % and about 10 wt % of rare earth metal. The rare earth aluminate preferably comprises a hexaaluminate-like structure, a beta-aluminate-like structure, or combinations thereof, such as a lanthanum hexaaluminate or a lanthanum beta-alumina. The rare earth aluminate comprises a rare earth metal selected from the group consisting of lanthanum, neodymium, praseodymium, and combinations thereof. In preferred embodiments, the rare earth aluminate comprises preferably La, and optionally Sm.
It is envisioned that the rare earth aluminate with a high Al:Ln molar ratio could comprise different species of aluminates with varying Al:Ln molar ratios, as long as the different ratios are all greater than 5:1; or that the rare earth aluminate could comprise combinations of different rare earth aluminates of similar structure but comprising different rare earth metals. It should be appreciated that the rare earth aluminate could comprise any combinations of these features. For example, the support could comprise one rare earth aluminate with a Al:Ln ratio of 11:1 and an aluminate of the same rare earth metal with a higher Al:Ln ratio of 12:1. In another example, the support could comprise aluminates of two or more rare earth metals all with an Al:Ln ratio of 11:1.
The thermally stable aluminum-based support could comprise between about 1 wt % and about 20 wt % of the rare earth metal; but preferably between about 1 wt % and about 10 wt %; more preferably between about 2 wt % and about 8 wt %; and still more preferably between about 4 wt % and about 8 wt %.
This rare earth metal content corresponds to rare earth oxide loading between about 1.2 wt % and about 23 wt % of the rare earth oxide; preferably between about 1.2 wt % and about 12 wt %; more preferably between about 2.4 wt % and about 9.4 wt %; and still more preferably between about 4.7 wt % and about 9.4 wt %. This rare earth metal weight content also corresponds to rare earth oxide molar content between about 0.3 mol % and about 7 mol % of the rare earth oxide; preferably between about 0.3 mol % and about 3.5 mol % of the rare earth oxide; more preferably between about 0.6 mol % and about 2.6 mol %; and still more preferably between about 1.2 mol % and about 2.6 mol %. The rare earth oxide molar content is calculated as the ratio of the number of moles of rare earth oxide over the total number of moles of rare earth oxide and aluminum oxide.
The selection of the rare earth loading on the support is dependent on the desirable range of the surface area of the support. There seems to be an optimum range of loadings for which the surface area is maximized as illustrated in FIGS. 3 a and 3 b. Beyond that range, thermal stability can still be achieved, but the support would have a lower surface area.
The thermally stable aluminum-based support may also comprise an oxide of a rare earth metal. For example, the rare earth aluminate with a high Al:Ln ratio might comprise only a fraction of the loaded (or applied) rare earth metal, and the other fraction of the loaded rare earth metal may form a rare earth metal oxide.
The thermally stable aluminum-based support may also comprise other rare earth aluminate structures with a low aluminum-to-rare earth metal molar ratio lower than 5:1, such as perovskite structures, monoclinic structures, or garnet structures with typically Al:Ln ratios less than 2:1. Due to the low Al:Ln molar ratio of aluminum to rare earth metal, these other rare earth aluminates can be denoted herein as a “rare earth-rich aluminate”, wherein the rare earth-lean aluminate comprises a molar ratio of aluminum to rare earth metal (Al:Ln) less than 5:1; preferably comprises an Al:Ln less than 2:1. In contrast, the rare earth aluminate comprising a higher molar ratio of aluminum to rare earth metal can be denoted herein as a “rare earth-lean aluminate”, wherein the rare earth-lean aluminate comprises a molar ratio of aluminum to rare earth metal (Al:Ln) greater than 5:1; preferably comprises a molar ratio of Al:Ln between 11:1 and 14:1.
According to another embodiment of this invention, the thermally stable catalyst support further comprises an alumina phase selected from the group consisting of alpha-alumina, theta-alumina or any combinations thereof. The rare earth aluminate with a high Al:Ln molar ratio and the alumina phase could be intimately mixed, or the rare earth aluminate could coat the alumina phase partially or completely. A surface layer comprising said rare earth aluminate with a high Al:Ln molar ratio preferably covers either partially or completely the alumina phase surface; with a complete coverage being more preferred. Therefore a person skilled in the art could select a method of preparation to achieve a well-mixed rare earth aluminate and alumina combination, such as via a sol-gel method or a co-precipitation method, or to achieve a coating of rare earth aluminate over the alumina surface, such as via impregnation or chemical vapor deposition. For the later techniques, which result in a coating of rare earth aluminate over the alumina surface, the rare earth loading should be selected such that a desired coating is achieved. For example, one can estimate the necessary amount of rare earth aluminate to completely cover the surface of the support precursor by one monolayer of said rare earth aluminate.
In preferred embodiments, the thermally stable catalyst support comprises a rare earth hexaaluminate structure, a rare earth beta-alumina structure, or combinations thereof.
The rare earth aluminate could comprise a chemical formula of LnAl y O z , wherein Al and O represent aluminum atoms and oxygen atoms respectively; Ln comprises lanthanum, neodymium, praseodymium, cerium, or combinations thereof; y is between 11 and 14; and z is between 18 and 23.
The rare earth aluminate could comprise a chemical formula of (Ln 2 O 3 ).y(Al 2 O 3 ), where Ln comprises one rare earth metal chosen from lanthanum, neodymium, praseodymium, cerium, or combinations thereof; and y is between 11 and 14.
In addition to comprising a rare earth metal, the rare earth aluminate may further comprise an element from Groups 1-17 of the Periodic Table; particularly preferred, the rare earth aluminate may further comprise nickel, magnesium, barium, potassium, sodium, manganese, a second rare earth metal (such as samarium), or any combinations thereof.
The rare earth aluminate preferably could comprise a chemical formula characterized by MAl y O z wherein Al and O represent aluminum atoms and oxygen atoms respectively; y=11-14; z=18-23; and wherein M preferably comprises at least one rare earth metal selected from lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), or combinations thereof. M could also comprise two or more elements from Groups 1-17 of the Periodic Table, with at least one of them being a rare earth metal. The other element is selected from Groups 1-14, and preferably comprises nickel, magnesium, barium, potassium, sodium, manganese, a second rare earth metal (such as samarium), or any combinations thereof. In preferred embodiments, M comprises preferably La, and optionally Sm. In some embodiments, M comprises both La and Sm.
In more preferred embodiments, the rare earth aluminate comprises a lanthanum hexaaluminate. The lanthanum hexaaluminates have a chemical formula of (La 2 O 3 ).y(Al 2 O 3 ), where La represents lanthanum, and y is between 11 and 14.
The thermally stable support may further comprise an oxide of said rare earth metal, said rare earth oxide consisting essentially of rare earth metal atoms and oxygen atoms. The oxide of said rare earth metal (Ln) preferably has a chemical formula of Ln 2 O 3 . It should be appreciated that in some cases, the combination of both rare earth aluminates and rare earth oxides in the catalyst support might be desirable to improve support stability.
In addition, according to one embodiment, there is an expectation that a less acidic surface layer may encourage the formation of more uniform crystallites of a catalytically active metal resulting in smaller metal crystallite sizes. The catalysts made from these thermally stable catalyst supports of the present invention are expected to have excellent stability, high activity and extended catalyst lifetimes, while maintaining desirable selectivity (e.g., hydrogen and CO selectivities), pore structure and particle size.
This rare earth modified support with enhanced thermal stability, which comprises a rare earth aluminate with a high Al:Ln molar ratio, has an initial minimum BET surface area of about 2 m 2 /g, preferably greater than about 5 m 2 /g, more preferably greater than about 7 m 2 /g, but no more than about 30 m 2 /g.
According to another embodiment of this invention, the thermally stable catalyst support comprises a rare earth-rich aluminate (e.g., with a Al:Ln molar ratio less than 5:1) and a rare earth-lean aluminate (e.g., with a low Al:Ln molar ratio greater than 5:1). The rare earth-rich aluminate and the rare earth-lean aluminate preferably comprise at least one rare earth metal in common. In alternate embodiments, the rare earth-rich aluminate and the rare earth-lean aluminate comprise different rare earth metals. The rare earth-rich aluminate may comprise a perovskite structure, a monoclinic structure, a garnet structure, or any combination of two or more thereof; preferably a perovskite structure. The rare earth-rich aluminate may have a low Al:Ln molar ratio from 1:2 to 5:1; preferably from 1:2 to 2:1; more preferably from 1:2 to 5:3; most preferably at about 1:1. The rare earth-rich aluminate of a perovskite structure preferably comprises at least one rare earth element selected form the group consisting of lanthanum (La), cerium (Ce), praesodynium (Pr), neodynium (Nd), and any combinations of two or more thereof; more preferably comprises at least one rare earth element selected form the group consisting of La, Pr, Nd, and any combinations of two or more thereof. The rare earth-lean aluminate may comprise a hexaaluminate structure, a beta-alumina structure, or combinations thereof, preferably a hexaaluminate structure. The rare earth-lean aluminate may have a high Al:Ln molar ratio greater than 5:1; preferably from 11:1 to 14:1. The rare earth-rich and rare earth-lean aluminates could be intimately mixed. Alternatively, the rare earth-rich aluminate could coat the rare earth-lean aluminate either partially or completely. The thermally stable catalyst support may further comprise an alumina phase selected from the group consisting of alpha-alumina, theta-alumina and combinations thereof. The thermally stable catalyst support, which is in the form of discrete structures (e.g., particle, particulate, bead, sphere, trilobe, pill, pellet, and the like), may contain an inner core and a surface layer which covers either partially or completely said inner core for the discrete structures, with a complete coverage being preferred, and wherein the surface layer comprises the rare earth-rich aluminate, and further wherein the inner core of the discrete structures comprises the rare earth-lean aluminate with a high Al:Ln molar ratio. The inner core of the discrete structures may further comprise an alumina phase. But, preferably, the surface layer which comprises the rare earth-rich aluminate is essentially free of alumina, such as alpha-alumina, theta alumina, or gamma-alumina. Therefore a person skilled in the art could select a method of preparation to achieve well-mixed rare earth aluminates combinations, or a well-mixed rare earth aluminates/alumina combination such as via bulk preparation methods like a sol-gel method or a co-precipitation method, or to achieve a coating of a rare earth-rich aluminate over an inner core comprising a rare earth-lean aluminate and optionally an alumina phase (e.g., alpha-alumina), such as via surface deposition methods like impregnation or chemical vapor deposition. In this embodiment, the thermally stable catalyst support preferably comprises a lanthanide content ranging from that of a rare earth-rich aluminate of a perovskite structure (with Al:Ln molar ratio of about 1:1) and that of a rare earth-lean aluminate of a hexaaluminate structure (e.g., with Al:Ln molar ratio of about 11:1 to about 14:1), wherein the range of rare earth content disclosed herein is exclusive of the endpoints. When the rare earth-rich aluminate and the rare earth-lean aluminate both comprise La, the thermally stable catalyst support preferably comprises a La content ranging from 19.2 wt % to 65 wt %, exclusive of endpoints. In other preferred embodiments, the thermally stable catalyst support comprises a La content ranging from 19.3 wt % to 64 wt %, or ranging from 19.5 wt % to 50 wt %, or ranging from 19.8 wt % to 40 wt %, or ranging from 20 wt % to 30 wt %, inclusive of endpoints and all intermediate values of these ranges. All ranges disclosed herein are combinable (e.g., ranges from 19.3 wt % to 64 wt. % desired, and about 20 wt. % to about 30 wt. %, are inclusive of the endpoints and all intermediate values of the ranges, e.g., “about 19.5 wt. % to about 30 wt. %, or about 20 wt. % to about 64 wt. %”, etc.).
High Surface Area Catalyst Support Comprising at Least Theta-Alumina
In another embodiment, a high surface area catalyst support is obtained by heat treatment of an alumina precursor with a stabilizing agent. The high surface area alumina support comprises a transition alumina comprising at least one alumina polymorph between gamma-alumina and alpha-alumina, but excluding gamma-alumina and alpha-alumina. The transition alumina preferably comprises theta-alumina and is preferably substantially free of gamma-alumina. The high surface area alumina support may further comprise alpha-alumina and/or an aluminate of said stabilizing agent. The stabilizing agent comprises at least one element selected from the group consisting of boron, silicon, gallium, selenium, rare earth metals, transition metals, and alkali earth metals, preferably selected from the group consisting of boron (B), silicon (Si), gallium (Ga), selenium (Se), calcium (Ca), zirconium (Zr), iron, (Fe), cobalt (Co), manganese (Mn), magnesium (Mg), and the rare earth elements, i.e., scandium (Sc), ytrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Th), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). More preferably the stabilizing agent comprises La, Sm, Nd, Pr, Ce, Eu, Yb, Si, Ce, Mg, Ca, Mn, Co, Fe, Zr, or any combinations thereof. Most preferably, the stabilizing agent comprises La, Sm, Nd, Pr, Ce, Eu, Yb, Si, Mg, Co, or any combinations thereof. In addition, promoters may be applied to the stabilized support. Such deposited promoters may also maintain an improved dispersion on active species during catalyst preparation.
According to one embodiment of the present invention, a high surface area alumina comprising mostly theta-alumina, which is modified with a rare earth metal and/or a rare earth metal oxide, serves as an improved support for synthesis gas production catalysts used in reactors operating at high-pressure and high-temperature. The catalyst support thus obtained tends to be more resistant to phase deterioration under highly thermal conditions than gamma-alumina, and yet provide greater surface area than alpha-alumina. This thermally stable catalyst support is porous and is suitable for use in high temperature environments. This surface area is typically higher that alpha-alumina, and its thermal stability greater than gamma-alumina. It has a surface area greater than 2 meters square per gram (m 2 /g), preferably between about 5 m 2 /g and 100 m 2 /g, more preferably between about 20 m 2 /g and 80 m 2 /g.
One stabilized alumina support according to one embodiment of this invention preferably comprises, when fresh, at least 50% theta-alumina phase, preferably between about 60% and 75% theta-alumina; not more than about 20% alpha-alumina, and is preferably substantially free of gamma-alumina, i.e., less than about 5% gamma-alumina. In addition, the support may comprise between about 1 wt % and about 50 wt % of a rare earth aluminate with a molar ratio of aluminum to rare earth metal greater than 5:1.
Catalysts
The present invention pertains to catalysts comprising one catalytically active metal on high surface area alumina supports or thermally stabilized aluminum-based supports, wherein the catalysts are active for the conversion of light hydrocarbons to synthesis gas. In particular, the current invention addresses the stability and durability of catalyst supports and catalysts made therefrom for use in catalytic partial oxidation reactors operating at high temperatures and pressures.
Catalysts Based on High Surface Area Supports Comprising at Least Theta-Alumina
According to one embodiment of the present invention, an alumina support comprising mostly theta-alumina, which is modified with one rare earth oxide, serves as an improved support for synthesis gas production catalysts used in reactors operating at high-pressure and high-temperature. The catalyst support thus obtained tends to be more resistant to phase deterioration under highly thermal conditions than gamma-alumina. The presence of mostly theta-alumina may result in a weaker R—O bond, where R is the catalytically active metal. The weaker R—O bond should lead to easier removal of the surface oxygen, and therefore a lower TPR temperature peak. During normal operating conditions, a weaker R—O bond would promote reduced active metal on the surface, which would favor a direct oxidation pathway (Scheme 2). In turn, this would lead to lower catalyst surface temperatures, which will slow the phase transformation of alumina to alpha-alumina (also slows deactivation).
Moreover, interactions between catalytically active metal and the alumina support are affected by the presence of the rare earth oxide. Without wishing to be bound to a particular theory, it is believed that the active metal-support interaction in catalysts supported on rare earth modified alumina, for example La 2 O 3 -modified Al 2 O 3 is stronger than that in the similar catalysts supported on unmodified Al 2 O 3 , and that this strong metal-support interaction in La 2 O 3 -modified Al 2 O 3 supported catalysts might be another reason for the unusually high catalyst stability.
The present invention also relates to improved catalyst compositions using a stabilized alumina support, as well as methods of making and using them, wherein the stabilized alumina support comprises a transition alumina phase (excluding gamma-alumina) between the low-temperature transition gamma-alumina and the high-temperature stable alpha-alumina, wherein the transition alumina is preferably theta-alumina, but could comprise low amounts of other transition alumina phases. In addition, the stabilized alumina may comprise rare earth aluminates. The catalyst is supported on a stabilized alumina with an initial minimum BET surface area of 2 m 2 /g, preferably greater than 5 m 2 /g, more preferably greater than 10 m 2 /g, but no more than 30 m 2 /g, after high temperature treatment or calcination. Preferably the stabilized alumina is modified with compounds of lanthanide metals, such as for example, compounds of lanthanum, samarium, praseodymium, cerium, or neodymium. Without wishing to be bound to a particular theory, it is believed that the metal-support interaction in catalysts supported on for example La 2 O 3 -modified Al 2 O 3 is stronger than that in the catalyst supported on unmodified Al 2 O 3 , and that this strong metal-support interaction in La 2 O 3 -modified Al 2 O 3 supported catalysts might be responsible for the unusually high catalyst stability.
Catalysts Based on Supports Comprising a Rare Earth Aluminate With a Al:Ln>5:1
According to another embodiment of the present invention, an alumina-containing support comprising a rare earth aluminate with an aluminum-to-rare earth metal molar ratio greater than 5:1, serves as an improved support for synthesis gas production catalysts used in reactors operating at high-pressure and high-temperature. The catalyst support thus obtained tends to be more resistant to phase deterioration under highly thermal conditions than gamma-alumina, and offers greater surface area than alpha-alumina. In addition to the presence of an alumina phase (either theta-alumina, alpha-alumina, or both), the presence of rare earth hexaaluminate structures is an indication that a distinct ordered aluminum structure comprising at least one rare earth metal is being formed during the preparation of the catalyst support. The formation of hexaaluminates comprising a rare earth metal during the preparation of the support described herein is believed to be another potential source of stabilization of the support, as the presence of rare earth aluminates most likely also affect the active metal-support interactions. The alumina-containing support could comprise more than 1 wt % but less than 100 wt % of said rare earth aluminate with an aluminum to rare earth metal molar ratio greater than 5:1; preferably more than 50 wt % but less than 95 wt %; more preferably more than 60 wt % but less than 90 wt %. The catalyst support which comprises a rare earth aluminate with a Al:Ln ratio greater than 5:1 may fuirther comprise another phase selected from the group consisting of a rare earth aluminate with a Al:Ln ratio less than 5:1 (e.g., perovskite; monoclinic; garnet); a rare earth oxide; an alumina phase (e.g., alpha, theta, and other transition aluminas), and any combinations of two of more thereof. The catalyst support which comprises a rare earth aluminate with a Al:Ln ratio greater than 5:1 and a rare earth aluminate with a Al:Ln ratio less than 5:1 could have a combined content of rare earth aluminates of 70% or greater; preferably a combined content of rare earth aluminates of 75% or greater; more preferably a combined content of rare earth aluminates of 70% or greater. Additionally, the catalyst support which comprises two rare earth aluminates of different Al:Ln ratios may further comprise less than 25% of any alumina phase.
Catalysts Based on High Surface Area Thermally Stable Supports
This invention also relates to a partial oxidation catalyst comprising an active ingredient selected from the group consisting of rhodium, iridium, platinum, palladium, and ruthenium; an optional promoter; and a support comprising a rare earth aluminate with a molar ratio of aluminum to rare earth metal greater than 5:1. The support in the catalyst could comprise between about 1 wt % and 100 wt % of said rare earth aluminate. A preferred support comprises at least a rare earth hexaaluminate with a Al:Ln ratio between 11:1 and 14:1. Other preferred stabilized support comprises a rare earth aluminate with a Al:Ln ratio greater than 5:1 and another phase selected from the group consisting of a rare earth aluminate with a Al:Ln ratio less than 5:1 (e.g., perovskite; monoclinic; garnet); a rare earth oxide; an alumina phase (e.g., alpha, theta, and other transition aluminas), and any combinations of two of more thereof. The stabilized support in the catalyst may further include an aluminum oxide phase such as comprising theta-alumina, alpha-alumina, or combinations thereof. The stabilized support in the catalyst may include between about 1 wt % and 50 wt % of said rare earth aluminate with a Al:Ln ratio greater than 5:1; or may include between about 50 wt % and 95 wt % of said rare earth aluminate with a Al:Ln ratio greater than 5:1. In alternate embodiments, the stabilized support in the catalyst may include two rare earth aluminates. The combined rare earth aluminates content is about 70 wt % or greater; preferably 75 wt % or greater; preferably 80 wt % or greater. In some other embodiments, the stabilized support in the catalyst may include a rare earth-lean aluminate with a Al:Ln ratio greater than 5:1 and lanthanum. In some embodiments, the support in the catalyst comprises between about 1 wt % and about 50 wt % of said rare earth aluminate. In other embodiments, the support in the catalyst could comprise more than 50 wt % of rare earth aluminate, i.e., between 40 wt % and 100 wt % of rare earth aluminate; and in some cases, the support is a rare earth aluminate or a mixture of rare earth aluminates with a molar ratio of aluminum to rare earth metal greater than 5:1, such as a lanthanum hexaaluminate or a lanthanum beta-alumina. The support could contain between about 1 wt % and about 20 wt % of rare earth metal; preferably between about 1 wt % and about 10 wt % of rare earth metal; alternatively greater than 20 wt % of rare earth metal. In some embodiments, the support comprises a rare earth content greater than 1 wt %, but lower than the stoichiometric content of the corresponding rare earth hexaaluminate structure. In other embodiments, the support comprises a rare earth content greater than the stoichiometric content of the corresponding rare earth hexaaluminate structure but lower than the stoichiometric content of the corresponding rare earth aluminate of perovskite structure, exclusive of said stoichiometric rare earth contents. In yet other embodiments, the support comprises a rare earth content greater than the stoichiometric content of the corresponding rare earth hexaaluminate structure but lower than the stoichiometric content of the corresponding rare earth aluminate monoclinic structure, exclusive of said stoichiometric rare earth contents. In still yet alternate embodiments, the support comprises a rare earth content greater than the stoichiometric content of the corresponding rare earth hexaaluminate structure but lower than the stoichiometric content of the corresponding rare earth aluminate garnet structure, exclusive of said stoichiometric rare earth contents. The rare earth aluminate preferably comprises a hexaaluminate structure, a beta-aluminate structure, or combinations thereof. The rare earth aluminate comprises a rare earth metal selected from the group consisting of lanthanum, neodymium, praseodymium, and combinations thereof. In preferred embodiments, the rare earth aluminate comprises preferably La, and optionally Sm. In some embodiments, the support could contain between about 19.2 wt % and about 65 wt % of lanthanum, exclusive of endpoints; preferably between about 19.4 wt % and about 60 wt % of lanthanum, inclusive of endpoints; more preferably between about 19.8 wt % and about 50 wt % of lanthanum, inclusive of endpoints; still more preferably between about 20 wt % and about 30 wt % of lanthanum, inclusive of endpoints; most preferably between about 20 wt % and about 25 wt % of lanthanum, inclusive of endpoints. In an embodiment, the catalyst comprises between about 50 wt % and about 96 wt % of the rare earth hexaaluminate based on the total weight of the catalyst, alternatively between about 60 wt % and about 90 wt % of the rare earth hexaaluminate.
A particularly preferred embodiment discloses a partial oxidation catalyst comprising an active ingredient selected from the group consisting of rhodium, iridium, rhenium, platinum, palladium, and ruthenium; an optional promoter; and a support comprising an alumina phase selected from the group consisting of alpha-alumina, theta-alumina, or any combinations thereof; and a rare earth aluminate with a molar ratio of aluminum to rare earth metal greater than 5:1, and wherein the support comprises between about 1 wt % and about 50 wt % of said rare earth aluminate. The rare earth aluminate preferably comprises a hexaaluminate-like structure, a beta-aluminate-like structure, or any combinations thereof. The rare earth aluminate comprises a rare earth metal selected from the group consisting of lanthanum, neodymium, cerium, praseodymium, and combinations thereof. In preferred embodiments, the rare earth aluminate comprises preferably La, and optionally Sm.
Another embodiment discloses a partial oxidation catalyst comprising an active ingredient selected from the group consisting of rhodium, iridium, and ruthenium; an optional promoter; and a rare earth aluminate, wherein the rare earth aluminate comprises an Al:Ln molar ratio between 11:1 and 14:1. The rare earth aluminate preferably has a hexaaluminate like structure, a beta-aluminate like structure, or combinations thereof. The rare earth aluminate preferably comprises a rare earth metal selected from the group consisting of lanthanum, neodymium, cerium, praseodymium, and combinations thereof. In preferred embodiments, the rare earth aluminate comprises preferably La, and optionally Sm. The active ingredient and the optional promoter are preferably supported on said rare earth aluminate with a high Al:Ln molar ratio.
Yet another embodiment discloses a partial oxidation catalyst comprising an active ingredient selected from the group consisting of rhodium, iridium, rhenium, platinum, palladium, and ruthenium; an optional promoter; and two rare earth aluminates, wherein a rare earth-rich aluminate comprises an Al:Ln molar ratio between 1:2 and 2:1 and a rare earth-lean aluminate comprises an Al:Ln molar ratio between 11:1 and 14:1. The rare earth-lean aluminate preferably has a hexaaluminate like structure, a beta-aluminate like structure, or combinations thereof. The rare earth-rich aluminate preferably has a perovskite structure. The rare earth aluminates preferably comprise a rare earth metal selected from the group consisting of lanthanum, neodymium, cerium, praseodymium, samarium, and combinations thereof. In preferred embodiments, the rare earth aluminates comprise preferably La, and optionally Sm. The active ingredient and the optional promoter are preferably supported on said rare earth aluminates either in a well-mixed matrix or in a layered arrangement with the support discrete structure, wherein the rare earth-rich aluminate is predominantly located in an outer layer of the support discrete structure (e.g., particle), said outer layer covering an inner core comprising the rare earth-lean aluminate. The catalyst may further comprise alpha-alumina. In the layered arrangement, the inner core could comprise alumina, but preferably, the outer layer is essentially free of alumina. Alternatively or additionally, the catalyst may further comprise a rare earth oxide.
All catalysts according to this invention can be used for producing synthesis gas, and therefore should comprise an active metal selected from the group consisting of metals from Groups 8, 9, or 10 of the Periodic Table, rhenium, tungsten, molybdenum, and any mixtures thereof. Preferably the catalyst used for producing synthesis gas comprises rhodium, ruthenium, iridium, platinum, palladium, rhenium, or any combinations thereof. More preferably the catalyst used for producing synthesis gas comprises rhodium, ruthenium, iridium, or any combinations thereof.
In some embodiments, the active metal may be comprised in an alloy form, preferably a rhodium alloy. Although not wishing the scope of this application to be limited to this particular theory, the Applicants believe that alloying rhodium with other metals appears to sustain the resistance of rhodium catalysts to sintering, and therefore to allow the Rh alloy catalysts to deactivate at a slower rate than syngas catalysts containing only rhodium. Suitable metals for the rhodium alloy generally include but are not limited to metals from Groups 8, 9, or 10 of the Periodic Table, as well as rhenium, tantalum, niobium, molybdenum, tungsten, zirconium and mixtures thereof. The preferred metals for alloying with rhodium are ruthenium, iridium, platinum, palladium, tantalum, niobium, molybdenum, rhenium, tungsten, cobalt, and zirconium, more preferably ruthenium, rhenium, and iridium. In accordance with the present invention, the loading of the active metal in the catalyst is preferably between 0.1 and 50 weight percent of the total catalyst weight (herein wt %).
In a preferred embodiment of the invention, the catalyst comprises rhodium as the active metal. The rhodium content in the catalyst is between about 0.1 wt % to about 20 wt %, preferably from about 0.5 wt % to about 10 wt %, and more preferably from about 0.5 wt % to about 6 wt %. In an embodiment, the rhodium content in the catalyst is between about 4 and about 10 wt. %, alternatively between about 0.1 and about 4 wt. %, and alternatively between about 0.1 and about 2 wt. %. When a rhodium alloy is used, the other metal in the rhodium alloy preferably comprises from about 0.1 wt % to about 20 wt % of the catalyst, preferably from about 0.5 wt % to about 10 wt %, and more preferably from about 0.5 wt % to about 5 wt %. The other metal in the rhodium alloy could be iridium, ruthenium, or rhenium. It is to be understood that all disclosed ranges are inclusive and combinable.
In another embodiment of the invention, the catalyst comprises ruthenium as the active metal. The ruthenium content in the catalyst is between about 0.1 to 15 wt %, preferably from about 1 to about 8 wt %, and more preferably from about 2 to about 5 wt %.
The catalyst structure employed is characterized by having a metal surface area of at least 0.5 square meters of metal per gram of catalyst structure, preferably at least 0.8 m 2 /g. Preferably the metal is rhodium and the rhodium surface area at least 0.5 square meters of rhodium per gram of supported catalyst, preferably at least 0.8 m 2 /g.
Catalyst compositions may also contain one or more promoters. In some embodiments when one active metal is rhodium, rhenium, ruthenium, palladium, platinum, or iridium, the promoter comprises an element selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lu, preferably Sm, Eu, Pr and Yb. The introduction of a lanthanide oxide, especially Sm 2 O 3 , on the stabilized alumina support surface before deposition of active metal is believed to further enhance the metal-support interaction, and that the active metal also disperses better on the surface of Al 2 O 3 modified with La 2 O 3 and/or Sm 2 O 3 . According to some embodiments with the use of a rhodium alloy, the presence of a promoter metal can be omitted without detriment to the catalyst activity and/or selectivity. It is foreseeable however that, in some alternate embodiments, a promoter could be added to a catalyst material comprising a rhodium alloy.
One embodiment of the present invention is more preferably directed towards syngas catalysts used in partial oxidation reactions and even more preferably used in syngas catalysts that contain solely rhodium or rhodium alloys. However, it should be appreciated that the catalyst compositions according to the present invention are useful for other partial oxidation reactions, which are intended to be within the scope of the present invention.
A preferred embodiment of this invention relates to a partial oxidation catalyst composition. The partial oxidation catalyst comprises an active ingredient selected from the group consisting of rhodium, iridium, platinum, palladium, and ruthenium; an optional promoter; and a support comprising an alumina phase selected from the group consisting of alpha-alumina, theta-alumina or any combinations thereof, and a rare earth aluminate comprising a rare earth metal, wherein the rare earth aluminate has a molar ratio of aluminum to rare earth metal greater than 5:1, and wherein the support comprises between about 1 wt % and about 50 wt % of said rare earth aluminate. The optional promoter comprises an element selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb and Lu, preferably Sm, Eu, Pr and Yb. The preferred promoter comprises samarium.
Methods of Support Preparation
This invention covers several embodiments of means for making catalyst supports disclosed earlier. All method embodiments comprise an application step of at least one stabilizing agent followed by a high temperature treatment. For instance, in an embodiment, a rare earth metal is applied by a surface deposition of a solution of a rare earth metal precursor onto discrete structures of an aluminum-containing precursor material. The aluminum-containing precursor material includes transition aluminas, boehmite, pseudo-boehmite, or combinations thereof. It may be calcined at a temperature sufficient to convert the aluminum atoms from the aluminum-containing precursor material to at least two rare-earth aluminates of different aluminum to rare earth metal molar ratios.
Preferably the stabilizing agent comprises a rare earth metal. The rare earth metal is selected from lanthanum, cerium, praseodymium, neodymium, samarium, or combinations. The aluminum-containing precursor may comprise at least one material selected from the group consisting of an oxide of aluminum, an aluminum salt, a salt of aluminum, an alkoxide of aluminum, a hydroxide of aluminum and any combination thereof. The aluminum-containing precursor comprises an aluminum structure selected from the group consisting of bayerite, gibbsite, boehmite, pseudo-boehmite, bauxite, gamma-alumina, delta-alumina, chi-alumina, rho-alumina, kappa-alumina, eta-alumina, theta-alumina, and any combinations thereof. The aluminum-containing precursor preferably comprises a transition alumina selected from the group consisting of gamma-alumina, delta-alumina, chi-alumina, rho-alumina, kappa-alumina, eta-alumina, theta-alumina, and combinations thereof. In a preferred embodiment, the aluminum-containing precursor comprises mostly gamma-alumina.
The gamma-alumina used as the aluminum-containing precursor in the present method of preparation of the catalyst support possesses a desired profile of physical characteristics with respect to, say, morphology and pore structure. Preferably, the gamma-alumina of the present method possesses a surface area between about 100 m 2 /g and about 300 m 2 /g; more preferably between about 120 m 2 /g and about 300 m 2 /g; but most preferably between about 120 m 2 /g and about 220 m 2 /g. The gamma-alumina as used in the present method further possesses a pore volume of at least about 0.2 ml/g. Any aluminum oxide, which meets these requirements in surface area and pore dimension, is called for the purpose of this patent gamma-alumina.
It should be understood that the aluminum-containing precursor could be pre-treated prior to calcination or application of the stabilizing agent. The pre-treatment could be heating, spray-drying to for example adjust particle sizes, dehydrating, drying, steaming or calcining. When the aluminum-containing precursor comprises an aluminum oxide such as gamma-alumina, steaming can be done at conditions sufficient to transform the aluminum oxide into a hydrated form of said aluminum oxide, such as boehmite or pseudo-boehmite or gibbsite.
The present process for preparing a stabilized alumina support may further comprise steaming the aluminum-containing precursor at conditions sufficient to at least partially transform the aluminum-containing precursor into a boehmite or pseudo-boehmite wherein steaming is defined as subjecting a given material, within the confines of an autoclave or other suitable device, to an atmosphere comprising a saturated or under-saturated water vapor at conditions of elevated temperature and elevated water partial pressure.
In one aspect, the steaming of the modified alumina precursor is preferably performed at a temperature ranging from 150° C. to 500° C., more preferably ranging from 180° C. to 300° C., a most preferably ranging from 200° C. to 250° C.; a water vapor partial pressure preferably ranging from 1 bar to 40 bars, more preferably ranging from 4 bars to 20 bars, and most preferably from 10 bars to 20 bars; and an interval of time preferably from 0.5 hour to 10 hours, and most preferably 0.5 hour to 4 hours. Preferably, under these steaming conditions, the deposited aluminum-containing precursor is at least partially transformed to at least one phase selected from the group boehmite, pseudo-boehmite and the combination thereof. A pseudo-boehmite refers to a monohydrate of alumina having a crystal structure corresponding to that of boehmite but having low cystallinity or ultrafine particle size. Alternatively, the optional steaming of the modified aluminum-containing precursor may comprise same conditions of temperature and time as above, but with a reduced water vapor partial pressure preferably ranging from 1 bar to 5 bar, and more preferably ranging from 2 bars to 4 bars.
The compound or precursor of a stabilizing agent can be in the form of salt, acid, oxide, hydroxide, oxyhydroxide, carbide, and the like. Preferably the compound or precursor of a stabilizing agent is an oxide or a salt (such as carbonate, acetate, nitrate, chloride, or oxalate). The stabilizing agent comprises at least-one element selected from the group consisting of aluminum, boron, silicon, gallium, selenium, rare earth metals, transition metals, alkali earth metals, their corresponding oxides or ions, preferably at least one element selected from the group consisting of B, Si, Ga, Se, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb, Lu, and their corresponding oxides or ions. More preferably, the stabilizing agent comprises La, Pr, Ce, Eu, Yb, Sm, their corresponding oxides, their corresponding ions, or any combinations thereof. Preferably the compound or precursor of the stabilizing agent comprises a nitrate salt or a chloride salt, as for example only La(NO 3 ) 3 , or Al(NO 3 ). It should be understood that more than one stabilizing agent or more than one compound or precursor of a stabilizing agent can be used.
The stabilizing agent can be applied to the aluminum-containing precursor by means of different techniques. For example only, application methods can be spray-drying, impregnation, co-precipitation, chemical vapor deposition, and the like. It should also be understood that any combination of techniques or multiple steps of the same technique could be used to applying a stabilizing agent.
One preferred technique for applying the stabilizing agent is impregnation, particularly incipient wetness impregnation. Generally, a stabilizing agent compound is dissolved in a solvent and a volume corresponding between about 75 and 100% of the total pore volume of a porous aluminum-containing precursor is applied to the aluminum-containing precursor. When the application is done via impregnation, a drying step at temperatures between 80° C. and 150° C. is performed on the modified aluminum-containing precursor prior to calcinations, typically to remove the solvent used in the impregnation solution.
In another embodiment, the modified aluminum-containing precursor is derived from the aluminum-containing precursor by contacting the aluminum-containing precursor with the stabilizing agent so as to form a support material and treating the support material so as to form a hydrothermally stable support. Contacting the modified aluminum-containing precursor with the stabilizing agent preferably includes dispersing the aluminum-containing precursor in a solvent so as to form a sol, adding a compound of the stabilizing agent to the sol, and spray drying the sol so as to form the support material. It should be understood that more than one stabilizing agents or more than one compound or precursors of a stabilizing agent can be added to the sol. Alternatively, one stabilizing agent can be incorporated into the support by means of the aforementioned techniques. Alternatively, two or more stabilizing agents can be incorporated into the support by means of the aforementioned techniques. The preferred stabilizing agent comprises at least one rare earth selected from the group consisting of lanthanum, cerium, praseodymium, and neodymium.
In another embodiment, a method of making a stabilized alumina support further comprises applying at least one promoter to the stabilized alumina support. In some embodiments, the promoter comprises an element selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, preferably Sm, Eu, Pr and Yb. It is believed that the introduction of a lanthanide oxide, especially Sm 2 O 3 , on the stabilized alumina support surface before deposition of active metal seems to further enhance the metal-support interaction, and that the active metal also disperses better on the surface of stabilized support comprising an aluminum oxide and a rare earth aluminate.
Methods of Preparation of High Surface Area Catalyst Support Comprising Theta-Alumina
In particular, the present invention discloses, in one aspect, a method of making a catalyst support comprising calcining an aluminum-comprising precursor in a manner effective for converting at least a portion of the aluminum-comprising precursor to an alumina support comprising a majority of theta-alumina, and substantially free of gamma-alumina. The calcination is preferably performed after an application of a stabilizing agent to the aluminum-comprising precursor, wherein the stabilizing agent preferably comprises a rare earth metal.
In some embodiments, the calcination could be done at a high temperature greater than 800° C., but not greater than 1,300° C. Alternatively, the calcination could be done at a high temperature greater than 1,100° C., but not greater than 1,600° C., preferably between 1,200° C. and 1,500° C., preferably from about 1,250° C. to about 1,600° C., and more preferably between 1,300° C. and 1,500° C.; most preferably at about 1,375-1,425° C. The calcination temperature could be selected based on the highest temperature the catalyst would likely experience in operation, i.e. the catalytic reactor.
When the aluminum-comprising precursor comprises mainly gamma-alumina, the calcination temperature is preferably selected such that it is above the minimum temperature necessary to start the phase transformation from gamma-alumina to another transition alumina phase between the low-temperature metastable transition gamma-alumina and the high-temperature thermodynamically stable alpha-alumina, but below about the minimum temperature necessary to start the phase transformation from said transition alumina to alpha-alumina. The other transition alumina (i.e., which excludes gamma-alumina) is preferably theta-alumina, but could comprise low amounts of other transition alumina phases. In some embodiments, the calcination temperature is selected such that substantially all of the gamma-alumina phase is transformed into other alumina phases, particularly to theta-alumina or a combination of theta-alumina and alpha-alumina. For example, if a good portion of theta-alumina is desired in the support, the calcination following the application step of a rare earth compound to a gamma-alumina, should be performed at a temperature preferably between 800° C. and 1,100° C., more preferably between 900° C. and 1,000° C. Under these conditions of calcination temperatures, it is most likely that the formation of rare earth hexaaluminates would be minimized. The heat treatment is preferably performed, for a time period between 3 to 24 hours.
The calcination can be performed under an oxidizing atmosphere, either statically or under a flow of gas, preferably in static air or under a flow of a gas comprising diatomic oxygen. Stearn, either by itself or in combination with air, can also be used.
The calcination can be done at a pressure between 0 and 500 psia; preferably under atmospheric pressure (about 101 psia), or under