Processes for the conversion of lower molecular weight alkanes such as methane to higher molecular weight hydrocarbons which have greater value are sought. One of the proposals for the conversion of lower molecular weight alkanes is by oxidative coupling For instance, G. E. Keller and M. M. Bhasin disclose in Journal of Catalysis, Volume 73, pages 9 to 19 (1982) that methane can be converted to, e.g., ethylene The publication by Keller, et al., has preceded the advent of substantial patent and open literature disclosures by numerous researchers pertaining to processes for the oxidative coupling of lower alkanes and catalysts for such processes.
In order for an oxidative coupling process to be commercially attractive, the process should be capable of providing a good rate of conversion of the lower alkanes with high selectivity to the sought higher molecular weight hydrocarbons. Since conversion and selectivity can be enhanced by catalysts, catalytic processes have been the thrust of work done by researchers in oxidative coupling.
Two general types of oxidative coupling processes are the sequential, or pulsed, processes and the cofeed processes. The sequential processes are characterized by alternately cycling an oxygen-containing gas and an alkane-containing gas for contact with a catalyst. These processes typically provide high selectivities to higher hydrocarbon but suffer from operational complexities in cycling the catalyst environment and in the tendency of the processes to produce less desirable, higher molecular weight products and to have carbonaceous deposits form on the catalyst, thereby necessitating regeneration. Thus, from an operational standpoint, cofeed processes, i.e., processes in which oxygen-containing material and alkane are simultaneously fed to the reaction zone containing the catalyst, are more desirable.
In order for cofeed processes to be commercially attractive, especially for the production of large volume commodity chemicals such as ethylene and ethane (C.sub.2 's), not only must the conversion of alkane be high and the selectivity to higher hydrocarbons as opposed to combustion products such as carbon dioxide and carbon monoxide be high, but also, the catalyst must exhibit a relatively long lifetime with the high performance. Moreover, because of the value of ethylene, processes in which the C.sub.2 's produced are rich in ethylene are sought.
Numerous catalysts have been proposed by researchers for oxidative coupling processes. These catalyst have included catalysts containing alkali and/or alkaline earth metals. The alkali and alkaline earth metals have been suggested as being in the oxide, carbonate and halide forms. Other components such as rhenium, tungsten, copper, bismith, lead, tin, iron, nickel, zinc, indium, vanadium, palladium, platinum, iridium, uranium, osmium, rhodium, zirconium, titanium, lanthanum, aluminum, chromium, cobalt, beryllium, germanium, antimony, gallium, manganese, yttrium, cerium, praseodymium (and other rare earth oxides), scandium, molybdenum, thallium, thorium, cadmium, boron, among other components, have also been suggested for use in oxidative coupling catalysts. See, for instance, U.S. Pat. Nos. 4,450,310; 4,443,646; 4,499,324; 4,443,645; 4,443,648; 4,172,810; 4,205,194; 4,239,658; 4,523,050; 4,443,647; 4,499,323; 4,443,644; 4,444,984; 4,695,668; 4,704,487; 4,777,313; 4,780,449; International Patent Publication WO 86/07351, European Patent Application Publications Nos. 189079 (1986); 206042 (1986); 206044 (1986), and 177327 (1985), Australian Patent No. 52925 (1986), Moriyama, et al., "Oxidative Dimerization of Methane Over Promoted MgO, Important Factors," Chem. Soc. Japan, Chem. Lett., 1165 (1986), and Emesh, et al., "Oxidative Coupling of Methane over the Oxides of Groups IIIA, IVA and VA Metals," J. Phys. Chem, Vol. 90, 4785 (1986).
Several researchers have proposed the use of alkali or alkaline earth metals in the form of halides (e.g., chloride, bromide or iodide) in oxidative coupling catalysts. Australian Patent No. 52925 discloses the use of supported calcium chloride, barium bromide, potassium iodide, lithium chloride, cesium chloride, among others for catalysts to oxidatively couple methane to ethane and ethylene. The catalysts are supported. The patent application states at page 5
"The chloride, bromide and iodide catalysts are preferably employed on a support consisting of pumice-stone, silica gel, kieselghur, precipitated silica and/or oxides of the alkaline earth elements and/or aluminum oxide, silicon dioxide, zinc oxide, titanium dioxide, zirconium dioxide and/or silicon carbide. Of the oxides of alkaline earth elements which are employed as a support, magnesium oxide and calcium oxide are preferred." PA0 "Unsupported MgO exhibits poor catalytic properties for oxidative coupling of methane while CaO is highly reactive for the reaction. Both of them are comparably active and selective for the reaction when they are impregnated with chloride of magnesium or calcium. However, MgCl.sub.2 on other supports such as TiO.sub.2 or SiO.sub.2, exhibit much poorer catalytic functions than CaO or MgO supported ones." PA0 "a composition consisting essentially of: (1) at least one Group IIA metal, (2) titanium, (3) oxygen and, optionally, (4) at least one material selected from the group consisting of halogen ions . . . said at least one Group IIA metal being present in an amount in excess of any amount present in electrically neutral compounds of said at least one Group IIA metal, said titanium and oxygen" (p. 4)
The patentees disclose feeding hydrogen halide to the reaction zone. None of the reported examples use the addition of hydrogen halide. Also, the patentees did not report the time on stream of the catalyst when the conversion values are determined. Metal halide catalysts often decompose upon initiation of the process. This produces an initial unsteady state operation in which exceptionally high selectivities to C.sub.2 's and high ethylene to ethane ratios occur. These exceptionally high selectivities and ratios have been found by us often to be fleeting. See also the corresponding German patent application 3,503,664. Included within the examples are illustrations of the use of, e.g., CaCl.sub.2 on pumice, CaCl.sub.2 -MgCl.sub.2 on pumice, BaBr on pumice, CaI.sub.2 on pumice, BaBr.sub.2 on calcium oxide, CaBr.sub.2 on magnesia,-BaBr.sub.2 on silicon carbide, KBr on titanium dioxide, and BaBr.sub.2 on zinc oxide.
The deactivation of oxidation coupling catalysts through the loss of components via vaporization of halide compounds under the high temperatures of oxidative coupling has been opined by Fujimoto, et al., "Oxidative Coupling of Methane with Alkaline Earth Halide Catalysts Supported on Alkaline Earth Oxide", Chem. Soc. Japan, Chem. Lett., 2157 (1987). Catalysts exemplified include MgCl.sub.2 on titania, MgF.sub.2 on magnesia, MgCl.sub.2 on magnesia, CaCl.sub.2 on magnesia, MgCl.sub.2 on calcium oxide, CaF.sub.2 on calcium oxide, CaBr.sub.2 on calcium oxide, MgCl.sub.2 on silica, MgBr.sub.2 on magnesia, CaBr on magnesia, MgBr.sub.2 on calcium oxide and CaCl.sub.2 on calcium oxide. The authors conclude
European Patent Application 210 383 (1986) discloses the addition of gas phase material containing halogen component such as chlorine, methyl chloride and methyl dichloride. Enhanced selectivities are reported when the halogen component is present. The catalysts include those containing one or more of alkali and alkaline earth metal, alkali metal with lanthanide oxide, zinc oxide, titanium oxide or zirconium oxide, and others. In the examples on page 28, a comparison is made between a Li.sub.2 O/MgO (with and without halide addition) to a Li.sub.2 O/TiO.sub.2 (with and without halide addition). In both, the ratio of ethylene to ethane was enhanced by the addition of halide (to a greater extent with the Li.sub.2 O/TiO.sub.2 catalyst), but only the Li.sub.2 O/MgO catalyst appeared to have increased in activity with the presence of halide.
U.S. Pat. No. 4,654,460 discloses the addition of a halogen-containing material either in the catalyst or via a feed gas in an oxidative coupling process. The catalyst contains one or more alkali metal and alkaline earth metal components. Although no working examples are provided illustrating the effect of halide additions, conversions of methane are said to be increased with halides and selectivities to higher hydrocarbons, particularly ethylene, improved. See also, Burch, et al., "Role of Chlorine in Improving Selectivity in the Oxidative Coupling of Methane to Ethylene", Appl. Catal., Vol. 46, 69 (1989) and "The Importance of Heterogeneous and Homogeneous Reactions in Oxidative Coupling of Methane Over Chloride Promoted Oxide Catalysts," Catal. Lett., vol. 2, 249 (1989), who propose mechanistic possibilities for the effect of halide in oxidative coupling of methane, and Minachev, et al., "Oxidative Condensations of Methane--a New Pathway to the Synthesis of Ethane, Ethylene, and Other Hydrocarbons", Russ. Chem. Rev., Vol. 57, 221 (1988).
Barium-containing catalysts for oxidative coupling have been proposed by, for instance, Yamagata, et al., "Oxidative Coupling of Methane over BaO Mixed with CaO and MgO," Chem. Soc. Japan, Chem. Lett., 81 (1987) (catalysts include BaO/MgO, BaO/Al.sub.2 O.sub.3, BaO/CaO, BaO/ZrO.sub.2 and BaO/TiO.sub.2). The authors conclude that the BaO/MgO and BaO/CaO catalysts are much more effective than BaO on other supports such as titania, zirconia, alumina, silica and ferric oxide. They said that XRD analyses suggest the formation of some mixed compounds, probably the mixed oxides or the mixed carbonates, together with BaCO.sub.3. Aika, et al., "Oxidative Dimerization of Methane over BaCO.sub.3, SrCO.sub.3 and these Catalysts Promoted with Alkali", J. Chem. Soc., Chem. Comm. 1210 (1986) (catalysts exemplified include BaCO.sub.3, BaO, SrCO.sub.3, TiO.sub.2, ZrO.sub.2) relate that TiO.sub.2 and ZrO.sub.2 are not good catalysts. See also, International Patent Application WO 86/7351, (BaO/La.sub.2 O.sub.3, BaO/MgO); U S. Pat. Nos. 4,172,810 (Ba-Cr-Pt/MgAl.sub.2 O.sub.4, sequential process), 4,704,487, 4,704,488, and 4,709,108 (BaO/Al.sub.2 O.sub.3, zirconia, titania components); Nagamoto, et al., "Methane Oxidation over Perovskite-Type Oxide Containing Alkaline Earth Metal", Chem. Soc. Japan, Chem. Lett., 237 (1988), (SrZrO.sub.3, BaTiO.sub.3); Otsuka, et al., "Peroxide Anions as Possible Active Species in Oxidative Coupling of Methane", Chem. Soc. Japan, Chem. Lett., 77 (1987) (Strontium peroxide and barium peroxide mechanisms); Roos, et al., "Selective Oxidation of Methane to Ethane and Ethylene Over Various Oxide catalysts", Catal. Today, Vol. 1, 133 (1987), (Ca/Al.sub.2 O.sub.3); Iwamatsu, et al., "Importance of the Specific Surface Area in Oxidative Dimerization of Methane over Promoted MgO", J. Chem. Soc., Chem. Comm., 19 (1987), (Sr/MgO, Ca/MgO, Ba/MgO); Moriyama, et al., "Oxidative Dimerization of Methane over Promoted MgO. Important Factors", Chem. Soc. Japan, Chem. Lett., 1165 (1986) (Ba/MgO, Sr/MgO, Ca/MgO); Roos, et al., "The Oxidative Coupling of Methane: Catalyst Requirements and Process Conditions", Studies in Surface Science & Catalysis; #36 Methane Conversion Symp., Auckland, New Zealand, April, 1987, 373, (Ba/MgO); and U.S. Pat. No. 4,780,449 (BaO/La.sub.2 O.sub.3, SrO/La.sub.2 O.sub.3).
European Patent Application Publication No. 206,042 (1986) discloses a number of catalysts for methane coupling including,
Matsuhashi, et al., in "Formation of C.sub.3 Hydrocarbons from Methane Catalyzed by Na.sup.+ Doped ZrO.sub.2, " Chem Soc. Japan, Chem. Lett., 585 (1989) report that sodium doped zirconia catalysts in oxidative coupling of methane yielded, at 600.degree. C., propane and propene, and state that the C.sub.2 hydrocarbons produced were mostly ethane.
Fujimoto, et al., in "Selective Oxidative Coupling of Methane Over Supported Alkaline Earth Metal Halide Catalysts", Appl. Catal., vol. 50, 223 (1989) compare the performance of MgCl.sub.2 on MgO, TiO.sub.2 and SiO.sub.2 supports. The TiO.sub.2 and SiO.sub.2 catalysts resulted in relatively poor coupling performances.