Claims
- 1. A process for synthesizing one or more hydrocarbons having at least two carbon atoms by the direct partial oxidation of methane, said process comprising contacting a mixture of methane and oxygen with a spinel oxide catalyst under sufficient conversion conditions, said spinel oxide including or being combined with an alkali metal, wherein said spinel oxide is of the formula AB.sub.2 O.sub.4, where A is Li, Mg, Na, Ca, V, Mo, Mn, Fe, Co, Ni, Cu, Zn, Ge, Cd or Sn and B is Na, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, Rh, Ag or In, where A and B are different elements, and wherein said conversion conditions include a temperature of from about 300.degree. C. to about 1200.degree. C. and a reactant partial pressure of from about 0.1 atm to about 30 atm.
- 2. A process according to claim 1, wherein A or B is Mn.
- 3. A process according to claim 1, wherein said spinel oxide is MgMn.sub.2 O.sub.4 or CaMn.sub.2 O.sub.4.
- 4. A process according to claim 1, wherein said mixture of methane and oxygen has a volume ratio of methane to oxygen of 0.1-100:1.
- 5. A process according to claim 1, wherein said mixture of methane and oxygen is provided by a mixture of natural gas and air.
- 6. A process according to claim 1, wherein said conversion conditions include a Weight Hourly Space Velocity of from 0.01 to 150 hr.sup.-1.
- 7. A process according to claim 4, wherein said conversion conditions include a temperature of from 600.degree. C. to 900.degree. C., a reactant partial pressure of from 0.5 to 5 atm and a Weight Hourly Space Velocity of from 0.1 to 50 hr.sup.-1.
- 8. A process according to claim 7, wherein said mixture of methane and oxygen has a volume ratio of methane to oxygen of 1-10:1.
- 9. A process according to claim 3, wherein said alkali metal is lithium.
- 10. A process according to claim 3, wherein said alkali metal is sodium.
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of copending U.S. application Ser. No. 459,221, filed Dec. 29, 1989, now U.S. Pat. No. 5,025,109 the entire disclosure of which is expressly incorporated herein by reference.
There is provided a catalyst and a process for the direct partial oxidation of methane with oxygen, whereby hydrocarbons having at least two carbon atoms are produced. The catalyst used in this reaction comprises a spinel oxide, such as MgMn.sub.2 O.sub.4 or CaMn.sub.2 O.sub.4, modified with an alkali metal, such as lithium or sodium.
Natural gas is an abundant fossil fuel resource. Recent estimates places worldwide natural gas reserves at about 35.times.10.sup.14 standard cubic feet corresponding to the energy equivalent of about 637 billion barrels of oil.
The composition of natural gas at the wellhead varies but the major hydrocarbon present is methane. For example the methane content of natural gas may vary within the range of from about 40 to 95 vol %. Other constituents of natural gas may include ethane, propane, butanes, pentane (and heavier hydrocarbons), hydrogen sulfide, carbon dioxide, helium and nitrogen.
Natural gas is classified as dry or wet depending upon the amount of condensable hydrocarbons contained in it. Condensable hydrocarbons generally comprise C.sub.3 + hydrocarbons although some ethane may be included. Gas conditioning is required to alter the composition of wellhead gas, processing facilities usually being located in or near the production fields. Conventional processing of wellhead natural gas yields processed natural gas containing at least a major amount of methane.
Processed natural gas, consisting essentially of methane, (typically 85-95 volume percent) may be directly used as clean burning gaseous fuel for industrial heat and power plants, for production of electricity, and to fire kilns in the cement and steel industries. It is also useful as a chemicals feedstock, but large-scale use for this purpose is largely limited to conversion to synthesis gas which in turn is used for the manufacture of methanol and ammonia. It is notable that for the foregoing uses no significant refining is required except for those instances in which the wellhead-produced gas is sour, i.e., it contains excessive amounts of hydrogen sulfide. Natural gas, however, has essentially no value as a portable fuel at the present time. In liquid form, it has a density of 0.415 and a boiling point of minus 162.degree. C. Thus, it is not readily adaptable to transport as a liquid except for marine transport in very large tanks with a low surface to volume ratio, in which unique instance the cargo itself acts as refrigerant, and the volatilized methane serves as fuel to power the transport vessel. Large-scale use of natural gas often requires a sophisticated and extensive pipeline system.
A significant portion of the known natural gas reserves is associated with fields found in remote, difficulty accessible regions. For many of these remote fields, pipelining to bring the gas to potential users is not economically feasible.
Indirectly converting methane to methanol by steam-reforming to produce synthesis gas as a first step, followed by catalytic synthesis of methanol is a well-known process. The Mobil Oil Process, developed in the last decade provides an effective means for catalytically converting methanol to gasoline, e.g. as described in U.S. Pat. No. 3,894,107 to Butter et al. Although the market for gasoline is huge compared with the market for methanol, and although this process is currently used in New Zealand, it is complex and its viability appears to be limited to situations in which the cost for supplying an alternate source of gasoline is exceptionally high. There evidently remains a need for other ways to convert natural gas to higher valued and/or more readily transportable products.
One approach to utilizing the methane in natural gas is to convert it to higher hydrocarbons (e.g. C.sub.2 H.sub.6 ; C.sub.2 H.sub.4 ; C.sub.3 H.sub.8 ;C.sub.3 H.sub.6 . . . ); these have greater value for use in the manufacture of chemicals or liquid fuels. For example, conversion of methane to ethane or ethylene, followed by reaction over a zeolite catalyst can provide a route to gasoline production that entails fewer steps than the indirect route via methanol synthesis described above. Unfortunately, the thermal conversion of methane to ethane is a thermodynamically unfavorable process (.DELTA.G.multidot..gtoreq.+8 kcal/mol CH.sub.4) throughout the range from 300-1500K. The upgrading reactions explored here are oxidative conversions of methane to higher hydrocarbons, as exemplified in the following equations.
While numerous catalysts have been employed in this reaction, there has not been any report regarding application of spinels as catalysts to upgrade methane/natural gas in this manner. The application described here likewise stands in contrast to references citing spinels as catalysts for total combustion (Happel, J.; Hnatow, M.; Bajars, L. "Base Metal Oxide Catalysts"; Marcel Dekker, Inc.: New York, NY, 1977) or as catalysts for oxidative chlorination of methane to methylchloride (Vlasenko, V. M., et al, Kinet. Katal, 1984, 22, 28).
There is provided a process for synthesizing one or more hydrocarbons having at least two carbon atoms by the direct partial oxidation of methane, said process comprising contacting a mixture of methane and oxygen with a catalyst under sufficient conversion conditions, said catalyst comprising a spinel oxide modified with an alkali metal. After this conversion, the one or more hydrocarbons having at least two carbon atoms may be recovered. The catalyst of this process is also provided herein.
Methane is converted to higher hydrocarbons such as ethane and ethylene via reaction with oxygen over a catalyst comprised of a metal oxide having the spinel structure with a portion of alkali metal as a modifier. The presence of alkali-modifier results in an improved selectivity for the production of higher hydrocarbons vs the unmodified spinel. The spinel has the general formula AB.sub.2 O.sub.4 ; examples include manganese-based spinels such as AMn.sub.2 O.sub.4 or MnB.sub.2 O.sub.4. The alkali metal used as a modifier may be Li, Na, K, Rb, or Cs. The alkali metal may be combined with or incorporated into the catalyst with the spinel by a variety of methods such as impregnation or precipitation, or by inclusion during the synthesis of the spinel. The modified material may have a crystal structure analogous to the unmodified starting spinel, but with portions of alkali metal ions incorporated into the crystal lattice of the spinel.
Spinel oxides are a known class of materials which may be naturally occurring or synthetic. These spinels are structural type of oxide having the formula AB.sub.2 O.sub.4 where A and B may be the same elements or different elements; the labels A and B distinguish the lattice sites occupied by the metal ions. The valencies of A and B satisfy the charge balance of the formula, i.e. the sums of charges on A plus 2B equals 8+. Examples of A are Li, Mg, Na, Ca, V, Mo, Mn, Fe, Co, Ni, Cu, Zn, Ge, Cd and Sn and examples of B are Na, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Ge, Zn, Rh, Ag and In. Particular examples of spinel oxides, which are useful in the present process, are those where either A or B is Mn. Particular species of such spinel oxides include MgMn.sub.2 O.sub.4 and CaMn.sub.2 O.sub.4. Spinel oxides have a structure similar to ferrites.
In the practice of the present invention, it is preferred to use a dual flow system, i.e., a system in which the methane and the oxygen or air are kept separate until mixed just prior to being introduced into the reactor. However, if desired, the oxygen and methane may be premixed and stored together prior to the reaction. The preferred dual flow system minimizes the risk of fire or explosion.
The methane feed for the present reaction may be provided by pure methane or by a methane containing gas, e.g., containing at least 50 percent by weight methane. An example of a methane feed is natural gas.
Air may be used instead of oxygen; inert diluents such as nitrogen, argon, helium, steam or CO.sub.2 may also be cofed. The gas comprising the methane may be derived from processed natural gas. In the system, the amount of oxygen is controlled so as to prepare a reaction mixture where the volume ratio of methane to oxygen is within the range of 0.1-100:1, more preferably in the range of 1-50:1, even more preferably in the range of 1-10:1. The operating pressure for the reactants (methane and oxygen) may be within the range of 0.1 to 30 atmospheres, preferably within the range of 0.5-5 atm. The flow rate of the methane feed gas over the catalyst may be expressed as the methane weight flow rate divided by the weight of catalyst, giving the Weight Hourly Space Velocity (WHSV) in units of hr.sup.-1. Preferred WHSV is within the range of 0.01-150 hr.sup.-1, e.g., 0.1-50 hr.sup.-1. The WHSV may be chosen to maximize the selectivity to higher hydrocarbon products, or to maximize the conversion of either methane or oxygen reactant.
The temperature in the reaction zone maybe from about 300.degree. C. to 1200.degree. C., preferably about 500.degree. C. to 1000.degree. C., more preferably from 600.degree. C. to 900.degree. C.
The alkali-modified spinel oxides may contain from about 0.01 wt. % to about 10 wt. % of alkali metal based upon the total weight of the alkali metal plus spinel oxide.
US Referenced Citations (10)
Continuation in Parts (1)
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Number |
Date |
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459221 |
Dec 1989 |
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Patent Grant
5105044
