Methane Conversion

1. Field of the Invention

This invention relates to synthesis of hydrocarbons from a methane source. A particular application of this invention is a method for converting natural gas to more readily transportable material.

2. Description of the Prior Art

A major source of methane is natural gas. Other sources of methane have been considered for fuel supply, e.g., the methane present in coal deposits or formed during mining operations. Relatively small amounts of methane are also produced in various petroleum processes.

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.

Large-scale use of natural gas often requires a sophisticated and extensive pipeline system. Liquefaction has also been employed as a transportation means, but processes for liquefying, transporting, and revaporizing natural gas are complex, energy-intensive, and require extensive safety precautions. Transport of natural gas has been a continuing problem in the exploration of natural gas resources. It would be extremely valuable to be able to convert methane (e.g., natural gas) to more easily handleable, or transportable, products. Moreover, direct conversion to olefins such as ethylene or propylene would be extremely valuable to the chemical industry.

In addition to its use as fuel, methane is used for the production of halogenated products (e.g., methyl chloride, methylene chloride, chloroform and carbon tetrachloride). Methane has also been used as a feedstock for producing acetylene by electric-arc or partial-oxidation processes. Electric-arc processes are operated commercially in Europe. In partial-oxidation processes, a feed mixture of oxygen and methane (the methane may contain other, additional hydrocarbons) are preheated to about 540.degree. C. and ignited in a burner. Representative processes of this type are disclosed in U.S. Pat. Nos. 2,679,544; 3,234,300; and 3,244,765. Partial oxidation produces significant quantities of CO, CO.sub.2 and H.sub.2, yielding a dilute acetylene-containing gas and thereby making acetylene recovery difficult.

The largest, non-fuel use of methane is in the production of ammonia and methanol (and formaldehyde). The first, methane conversion, step of these processes is the production of a synthesis gas (CO+H.sub.2) by reforming of methane in the presence of steam over, for example, a nickel catalyst. Typical reformers are tubular furnaces heated with natural gas, the temperature being maintained at 900.degree. C. and the pressure at about 225 atmospheres.

Pyrolytic or dehydrogenative conversion of methane or natural gas to C.sub.2 + hydrocarbons has previously been proposed. The conversion requires high temperatures (greater than about 1000.degree. C.) and is characterized by the formation of by-product hydrogen. The patent literature contains a number of proposals to catalyze pyrolytic reactions, allowing conversion at lower temperatures. See, for example, U.S. Pat. Nos. 1,656,813; 1,687,890; 1,851,726; 1,863,212; 1,922,918; 1,945,960; 1,958,648; 1,986,238 and 1,988,873. U.S. Pat. No. 2,436,595 discloses and claims a catalytic, dehydrogenative methane-conversion process which employs fluidized beds of heterogeneous catalysts comprising an oxide or other compound of the metals of group VI or VIII.

Including oxygen in a methane feed for conversion over metal oxide catalysts has been proposed. Margolis, L. Ya., Adv. Catal. 14, 429 (1963). and Andtushkevich, T. V., et al, Kinet. Katal. 6, 860 (1965) studied oxygen/methane cofeed over different metal oxides. They report the formation of methanol, formaldehyde, carbon monoxide and carbon dioxide from methane/oxygen feeds. Higher hydrocarbons are either not formed or are converted much faster than methane.

It has been found that methane may be converted to higher hydrocarbon products by contacting a methane-containing gas with an oxide of tin at temperatures within the range of about 500.degree. to 1000.degree. C. Hydrocarbons produced include lower alkanes, lower olefins, and aromatics. The oxide of tin is reduced by the methane contact and is easily reoxidizable by contact with an oxygen-containing gas. A preferred oxide of tin is SnO.sub.2.

The present process is distinguished from previously known pyrolytic methane conversion processes by the use of a reducible oxides of tin to synthesize higher hydrocarbons from methane with coproduction of water, rather than hydrogen.

The present process is also distinguished from previously suggested methane conversion processes which rely primarily on interactions between methane and at least one of nickel and the noble metals, such as rhodium, palladium, silver, osmium, iridium, platinum and gold. An example of this type of process is disclosed in U.S. Pat. No. 4,205,194. The present process does not require that methane be contacted with one or more of nickel and such noble metals and compounds thereof.

Moreover, in a preferred embodiment, such contacting is carried out in the substantial absence of catalytically effective nickel and the noble metals and compounds thereof to minimize the deleterious catalytic effects of such metals and compounds thereof. For example, at the conditions, e.g., temperatures, useful for the contacting step of the present invention, these metals when contacted with methane tend to promote coke formation, and the metal oxides when contacted with methane tend to promote formation of combustion products (CO.sub.x) rather than the desired hydrocarbons. The term "catalytically effective" is used herein to identify that quantity of one or more of nickel and the noble metals and compounds thereof which when present substantially changes the distribution of products obtained in the contacting step of this invention relative to such contacting in the absence of such metals and compounds thereof.

This invention is further described in the following description of its presently preferred embodiments.

FIG. 1 is a plot of methane conversion and hydrocarbon product selectivity vs. time for the results reported in Example 2.

FIG. 2 is a plot of methane conversion vs. time for the results reported in Examples 2 and 3.

Reducible oxides of tin can be supplied from a variety of known sources. The term "reducible" is used to identify those oxides of tin which are reduced by contact with methane at temperatures in the range of about 500.degree. to 1000.degree. C. Among the reducible oxides of tin, those containing major amount of SnO.sub.2 are preferred.

The reducible oxide of tin is preferably provided as solid particles. It may be supported by, or diluted with, a conventional support material such as silica, alumina, titania, zirconia, and the like, and combinations thereof. A presently preferred support material is silica.

The term "oxide of tin" includes: (1) one or more tin oxides and/or (2) one or more oxygen-containing compounds of tin, provided that such oxides and compounds have the capability of performing to produce higher hydrocarbon products as set forth herein. The reducible oxides of tin may be employed alone or in combination with reducible oxides of other metals (see co-pending U.S. application Ser. Nos. 522,925; 522,942; 522,905; 522,877; 522,876; and 522,906) and/or other oxidative synthesizing agents. Oxidative synthesizing agents are compositions comprising at least one oxide of at least one metal, which composition, when contacted with methane at a temperature selected within the range of about 500.degree. to 1000.degree. C., produces C.sub.2 + hydrocarbon products, coproduct water, and a composition comprising a reduced metal oxide.

Solids may be formed in conventional manner using techniques well known to persons skilled in the art. For example, supported solids may be prepared by conventional methods such as adsorption, impregnation, precipitation, coprecipitation, or dry-mixing.

A suitable method is to impregnate the support with solutions of a tin compound. Some examples of tin compounds are the acetate, acetylacetonate, oxide, carbide, carbonate, hydroxide, formate, oxalate, nitrate, phosphate, sulfate, sulfide, tartrate, fluoride, chloride, bromide or iodide. Such compounds of tin may be dissolved in water or other solvent and the solutions are combined with the support and then evaporated to dryness. Preferably, aqueous solutions are employed and water-soluble forms of tin are used. In some cases, the solutions may have acids and/or bases added to them to facilitate dissolution of the precursors of the oxide of tin. For example, acids such as hydrochloric or nitric acid or bases such as ammonium hydroxide may be used as desired. The dried solids may then be screened or otherwise processed to form the desired shape, size, or other physical form of the finished solids. Finally, the solids are prepared for use by calcination at high temperatures for a period of time in accordance with conventional practice in this art. For example, the solids are placed in an oven or kiln, or in a tube through which oxygen (e.g., air or oxygen diluted with other gases) is passed, at an elevated temperature (e.g., about 300.degree. to 1200.degree. C., preferably about 500.degree. to 1100.degree. C.).

The foregoing description regarding preparation of solids in a form suitable for the synthesis of hydrocarbons from a methane source is merely illustrative of many possible preparative methods, although it is a particularly suitable method and is preferred.

Tin loadings on supported solids may be within the range of about 1 to 50 wt. % Sn (calculated as elemental metal).

In addition to methane, the feedstock employed in the method of this invention may contain other hydrocarbon or nonhydrocarbon components, although the methane content should be within the range of about 40 to 100 vol. %, preferably from about 80 to 100 vol. %, more preferably from about 90 to 100 vol. %.

Operating temperatures for the contacting of methane-containing gas and a reducible oxide of tin are in the range of about 500.degree. to 1000.degree. C., preferably with the range of about 600.degree. to 900.degree. C.

Operating pressures for the methane contacting step are not critical to the presently claimed invention. However, both general system pressure and the partial pressure of methane have been found to effect overall results. Copending, concurrently filed U.S. application Ser. No. 522,935 discloses a method wherein an oxidative synthesizing agent comprising a reducible metal oxide is contacted with methane, the claimed improvement residing in the use of elevated pressures to promote the formation of C.sub.3 + hydrocarbon products.

Contacting methane and a reducible oxide of tin to synthesize higher hydrocarbons also produces a reduced oxide of tin and coproduct water. The exact nature of the reduced form of the synthesizing agent is unknown, and so is referred to herein as "reduced oxide of tin". Regeneration of a reducible oxide of tin is readily accomplished by contacting the reduced composition with oxygen (e.g., an oxygen-containing gas such as air) for a period of time sufficient to produce a reducible oxide of tin from at least a portion of the reduced composition. Oxygen contacting temperature are more preferably within the range of about 300.degree. to 1200.degree. C., preferably within the range of about 500.degree. to 1100.degree. C.

A single reactor apparatus containing a fixed bed of solids, for example, may be used with intermittent or pulsed flow of a first gas comprising methane and a second gas comprising oxygen (e.g., oxygen, oxygen diluted with an inert gas, or air, preferably air).

Preferably, the methane contacting step and the oxygen contacting step are performed in physically separate zones with solids recirculating between the two zones. Thus, a suitable method for synthesizing hydrocarbons from a methane source comprises: (a) contacting a gas comprising methane and particles comprising a reducible oxide of tin to form higher hydrocarbon products, coproduct water, and particles comprising a reduced oxide of tin; (b) removing particles comprising a reduced oxide of tin from step (a) and contacting the reduced solids with an oxygen-containing gas to form particles comprising a reducible oxide of tin; and (c) returning particles comprising a reducible oxide of tin formed in step (b) to step (a). Steps (a), (b) and (c) are preferably repeated at least periodically, and more preferably the steps are continuous. Thus, in this more preferred embodiment, solids are continuously cireculated between at least one methane-contact zone and at least one oxygen-contact zone. This more preferred embodiment is disclosed and claimed in copending, concurrently filed U.S. application Ser. No. 522,938.

Particles comprising a reducible oxide of tin may be contacted with methane in fixed, moving, fluidized, ebullating, or entrained beds of solids. Preferably, methane is contacted with a fluidized bed of particles.

Similarly, particles comprising a reduced oxide of tin may be contacted with oxygen in fixed, moving, fluidized, ebullating or entrained beds of solids. Preferably, oxygen is contacted with a fluidized bed of particles comprising a reduced oxide of tin.

The invention is further illustrated by reference to the following examples.

The supported oxides of tin employed in the following examples were made by impregnation of tin tartrate, in an aqueous solution of 7% hydrochloric acid, on Houdry HSC 534 silica, the amount of tin provided being sufficient to yield a solid containing 5 wt. % Sn/SiO.sub.2. The solids were dried at 110.degree. C. for 4 hours and then calcined in air at 700.degree. C. for 16 hours. The solids employed in the runs described in the examples are equilibrated solids, i.e., solids prepared as described above which have been contacted at least once with methane at a temperature of about 700.degree. C. and then reoxidized by being contacted with air at about 700.degree. C.

Runs were made in a quartz tube reactor (12 mm. inside diameter) packed with 10 ml. of solids at about atmospheric pressure. The reactors were brought up to temperature under a flow of nitrogen which was switched to methane at the start of the run. Instantaneous samples of the effluent were taken throughout the runs and analyzed by gas chromatography and gas chromatography mass spectroscopy. A cumulative run sample was collected and analyzed.

Experimental results shown below include conversions and selectivities on a molar basis.

A feed of 100% methane was passed over a bed of solids as described above. Contact zone temperature was 700.degree. C. and the GHSV (gas hourly space velocity) was 600 hrs.sup.-1. Results are reported in Table I below. No carbon was detected on the solid present at the end of the run.

A feed of 100% methane was passed over a bed of solids as described above. Contact zone temperature was 750.degree. C. and the GHSV was 600 hrs.sup.-1. Results are reported in Table II below. No carbon was detected on the solid present at the end of the run. FIG. 1 is a plot of % methane conversion and % selectivity to C.sub.2 + hydrocarbon products vs. run time.

The reduced solid remaining at the end of the run described in Example 2 was regenerated under a flow of air at 750.degree. C. for 30 minutes. The reactor was then flushed with nitrogen. The regenerated, reoxidized solid were then contacted with methane at a temperature of 750.degree. C. and GHSV of 600 hrs..sup.-1. Results are shown in Table III below. No carbon oxides were detected during the oxidation of the reduced solid. FIG. 2 is a plot of methane conversion vs. run time for the combined results of Examples 2 and 3.

The procedure of Example 1 was repeated except that contact zone temperature was 800.degree. C. Results are shown in Table IV below. No carbon was detected on the solid present at the end of the run.