Metal carbide catalysts and process for producing synthesis gas

Details Metal carbide catalysts and process for producing synthesis gas Metal carbide catalysts and process for producing synthesis gas

2000-10-16

2002-10-08

Langel, Wayne A. (Department: 1754)

Compositions

Gaseous compositions

Carbon-oxide and hydrogen containing

C502S177000

Reexamination Certificate

active

06461539

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to catalysts and processes for producing synthesis gas (i.e., a gas mixture containing CO and H
2
). More particularly, the invention relates to mixed and/or promoted metal carbide catalysts and their manner of making, and to processes employing such catalysts for the production of synthesis gas.
2. Description of Related Art
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or “syngas”). In a second step, the syngas is converted to hydrocarbons, for example, using the Fischer-Tropsch process to provide fuels that boil in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widespread process, or by dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, proceeding according to Equation 1.
CH
4
+H
2
O→CO+3H
2
  (1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue.
The catalytic partial oxidation of hydrocarbons, e.g., natural gas or methane to syngas is also a process known in the art. While currently limited as an industrial process, partial oxidation has recently attracted much attention due to significant inherent advantages, such as the fact that significant heat is released during the process, in contrast to steam reforming processes.
In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H
2
:CO ratio of 2:1, as shown in Equation 2.
CH
4
+½O
2
→CO+2H
2
  (2)
This ratio is more useful than the H
2
:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol and to fuels. The partial oxidation is also exothermic, while the steam reforming reaction is strongly endothermic. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes. The syngas in turn may be converted to hydrocarbon products, for example, fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes by processes such as the Fischer-Tropsch Synthesis.
The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors, but one of the most important of these factors is the choice of catalyst composition. Difficulties have arisen in the prior art in making such a choice economical. Typically, catalyst compositions have included precious metals and/or rare earths. The large volumes of expensive catalysts needed by prior art catalytic partial oxidation processes have placed these processes generally outside the limits of economic justification.
For successful operation at commercial scale, the catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, and the selectivity of the process to the desired products of carbon monoxide and hydrogen must be high. Such high conversion and selectivity must be achieved without detrimental effects to the catalyst, such as the formation of carbon deposits (“coke”) on the catalyst, which severely reduces catalyst performance. Accordingly, substantial effort has been devoted in the art to the development of catalysts allowing commercial performance without coke formation.
A number of process regimes have been described in the art for the production of syngas via catalyzed partial oxidation reactions. The noble metals, which typically serve as the best catalysts for the partial oxidation of methane, are scarce and expensive. The widely used, less expensive, nickel-based catalysts have the disadvantage of promoting coke formation on the catalyst during the reaction, which results in loss of catalytic activity. Over the past two decades the transition metal carbides and nitrides have been shown to exhibit catalytic properties similar to the precious metals. A. P. E. York et al., (
Stud. Surf. Sci. Catal.
(1997), 110 (3rd World Congress on Oxidation Catalysis, 1997), 711-720.) disclose the use of molybdenum and tungsten carbides as catalysts for the partial oxidation of methane to syngas. The reaction was done at temperatures of 1073° K and 1173° K and pressures of 4.0 bar (400 kPa) and 8.7 bar (870 kPa) using air at a GHSV of 5.2×10
3
h
−1
. When the reaction was carried out at atmospheric pressure, catalyst deactivation occurred. Binary and ternary metal carbides of Mo or W and Group V metals are also disclosed.
Claridge et al. (
J. Catalysis
180:85-100 (1998)) have described high-surface-area molybdenum carbide catalysts and tungsten carbide catalysts for conversion of methane to synthesis gas via steam reforming, dry reforming or partial oxidation processes. Maintaining elevated pressure during the conversion process stabilized the carbide and deterred catalyst deactivation.
U.S. Pat. No. 4,325,843 (Slaugh et al.) describes a process for making a supported tungsten carbide composition for use as a catalyst. The process includes impregnating an oxidic support material with a solution of a tungsten salt, converting the tungsten to a nitride and treating the supported tungsten nitride with a carbiding gas mixture.
U.S. Pat. No. 4,325,842 (Slaugh et al.) describes a process for preparing a supported molybdenum carbide catalyst by impregnating a porous support with a solution of hexamolybdenum dodecachloride, drying, and heating in a carbiding atmosphere. U.S. Pat. No. 4,326,992 (Slaugh et al.) describes another process for preparing a supported molybdenum carbide catalyst. In this process an ammonium hydroxide solution of molybdic acid is applied to a porous support, dried and heated in a carbiding atmosphere. U.S. Pat. No. 5,338,716 (Triplett et al.) discloses a supported non-oxide metal carbide-containing catalyst that includes an oxide support, a passivating layer, and a non-oxide metal ceramic catalytic component such as tungsten carbide or molybdenum carbide, or another Group VI metal carbide or nitride.
U.S. Pat. Nos. 5,451,557 and 5,573,991 (Sherif) disclose other processes for forming a metal carbide catalyst such as tungsten carbide or another Group VIB transition metal carbide. U.S. Pat. No. 4,331,544 (Takaya et al.) describes a catalyst for catalyzing the synthesis of methane from CO and H
2
. This catalyst comprises a nickel-molybdenum alloy and a molybdenum carbide supported on a porous carrier. Other metal carbide catalysts are disclosed in U.S. Pat. No. 4,219,445 (Finch), U.S. Pat. No. 1,930,716 (Jaeger), and U.S. Pat. No. 4,271,041 (Boudart et al.).
There is a continuing need for better catalysts for catalyzing the partial oxidation of methane, which are capable of high conversion of reactant gas and high selectivity of CO and H
2
reaction products.
BRIEF SUMMARY OF THE INVENTION
The present invent

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