Methane is abundantly available in nature in the form of natural gas, which typically contains about 75% methane by weight. Methane is also produced by other means, such as anaerobic biological processes. Although methane is primarily used as fuel, it is also a valuable starting material for the production of synthesis gas and a number of important higher molecular weight saturated and unsaturated hydrocarbons, such as ethane, ethylene, propylene, acetylene and benzene. These compounds, in turn, are useful starting materials in the production of other commercially important petrochemicals and polymers.
Processes are known for converting methane into higher molecular weight hydrocarbons, including aliphatic and aromatic hydrocarbons, using a technique known as "pyrolysis." Such processes use heat to accomplish pyrolysis chemical reactions without the presence of substantial amounts of free oxygen or oxygen containing gas. Furthermore, diamond films can be synthesized onto solid substrates by pyrolyzing methane in the presence of hydrogen on the heated solid substrate. A general discussion of the high temperature pyrolysis of methane can be found, for example, in chapter 1 of Pyrolysis: Theory and Industrial Practice (Academic Press, 1983), edited by L. Albright, B. Crynes, and W. Corcoran, which is incorporated herein by reference and made a part hereof.
Methane can be converted into higher molecular weight hydrocarbons by a number of other processes which also involve the use of pyrolysis. For example, U.S. Pat. No. 2,488,083 to Gorin describes a process for converting methane into normally liquid hydrocarbons by first converting it into methyl halide and then pyrolytically condensing methyl halide into the desired end products. In this process, lower pyrolysis temperatures are made possible by the use of metal based alumina silica catalysts. Benson U.S. Pat. No. 4,199,533 describes a method for producing higher molecular weight hydrocarbons by reacting methane and chlorine. U.S. Pat. No. 4,714,796 to Senkan describes a two-step process in which methane is first converted into methyl halides, which are then pyrolyzed in the presence of oxygen to obtain higher molecular weight hydrocarbons. U.S. Pat. No. 4,544,747 to Sofranko et al. also describes a process of methane conversion using reducible solid metal oxide catalysts with a halogen promotor. U.S. Pat. No. 4,654,460 to Kimble et al. describes a process for oxidative conversion of methane using a solid contact material comprised of various metals, phosphate radicals, and, optionally, halogen ions and halogen ion containing compounds as promoters. U.S. Pat. No. 4,769,504 to Noceti et al. describes a two-step process for the production of gasoline range hydrocarbons from lower alkanes.
U.S. Pat. No. 4,513,164 to Olah describes a process for methane conversion into gasoline range isoalkane mixtures, cycloalkanes and aromatics containing less than 12 carbon atoms, but no olefins, using superacidic heterogeneous catalysts. This patent also describes a two-step process for selectively converting methane into monosubstituted derivatives using halogens and sulfur by superacidic ionic reaction catalysts and further condensation of monosubstituted derivatives into gasoline range hydrocarbon mixtures but no olefins. It is known that such Friedel-Crafts ionic alkylation processes proceed through ionic reactions rather than free radical reaction pathways resulting in the formation of higher molecular weight normal and isoalkanes.
Processes are also known for converting methane into so-called "synthesis gas", a mixture of carbon monoxide (CO) and hydrogen (H.sub.2) using metal-based heterogeneous catalysts. Synthesis gas can be catalytically converted into formaldehyde, methanol, and other useful hydrocarbons in accordance with known processes.
Methane can also be converted into benzene and other aromatic hydrocarbons by first converting it into methyl halide and then pyrolyzing the resulting methyl halide using solid metal oxide catalysts. Such a process is described, for example, in U.S. Pat. No. 2,320,274to Gorin. U.S. Pat. No. 4,695,663 to Hall et al. describes a process for converting methane-containing hydrocarbons into aromatics in the absence of oxygen using solid aluminosilicate catalyst.
None of the methane conversion processes described above can be used for simultaneously producing both higher molecular weight hydrocarbons and synthesis gas. The advantages of direct natural ga conversion into such products at the well site are significant, because the resulting products are less expensive and less dangerous to transport than natural gas.
In my co-pending application Ser. No. 07/110,248, I described a direct conversion process for simultaneously producing synthesis gas and higher molecular weight hydrocarbons from methane and other hydrocarbons.
Processes are also known for producing higher molecular weight hydrocarbons, such as aromatic and alkyl aromatic hydrocarbons, via Diels-Alder cyclization reactions of lower olefinic and acetylinic hydrocarbons, such as acetylene, vinyl acetylene, butadiene etc. Such cyclization reaction Processes are summarized in chapter 5 of Pyrolysis: Theory and Industrial Practice (Academic Press, 1983), edited by L. Albright, B. Crynes, and W. Corcoran, which is incorporated herein by reference and made a part hereof. The processes of coal liquefaction and gassification is another major source for aromatics.
Processes for producing alkyl benzenes and styrene are also known. In most of these processes, the starting material is benzene or toluene. Ethyl benzene is produced by Friedel-Crafts alkylation of benzene and ethylene using a variety of heterogeneous catalysts and promoters. Styrene monomer is produced by dehydrogenation of ethyl benzene. A general discussion of Friedel-Crafts alkylation and pyrolytic dehydrogenation can be found in chapter 9 of Organic Chemistry (Allyn and Bacon, Inc., 1959, 12th printing, 1965) by Morrison and Boyd, which is incorporated herein by reference and made a part hereof.
U.S. Pat. No. 3,848,012 to Applegath et al. describes a process for continuously producing ethyl benzene by reaction of benzene and ethylene in the liquid phase using small amounts of aluminum chloride as Friedel-Crafts alkylation catalyst. U.S. Pat. No. 3,448,161 to Garcia et al. also describes an alkylation process for producing ethyl benzene by reacting benzene and ethylene and an aluminum chloride catalyst complex.
Simultaneous reactions of natural gas (which comprises a mixture of methane, ethane, and small amounts of propane and other higher normal alkanes) and higher molecular weight products of natural gas conversion processes (such as higher olefins and aromatic hydrocarbons) to produce valuable chemicals, such as ethylene and other olefins, ethyl benzene, xylenes, styrene monomer, etc., would also be desirable, since they provide a potential route for better methane and ethane utilization. Such utilization of natural gas by simultaneously reacting it with methyl or ethyl group containing higher molecular weight hydrocarbons to produce corresponding higher alkanes and olefins would be of further value. By this route, paraffinic, naphthenic and aromatic hydrocarbons produced by direct natural gas conversion processes could be further converted into more useful products by pyrolytic oxidation.