Although methane is abundant, its relative inertness has limited its utility in conversion processes for producing higher-value hydrocarbon. For example, oxidative coupling methods generally involve highly exothermic and potentially hazardous methane combustion reactions, frequently require expensive oxygen generation facilities, and produce large quantities of environmentally sensitive carbon oxides. In addition, non-oxidative methane aromatization is equilibrium-limited, and temperatures ≧about 800° C. are needed for methane conversions greater than a few percent.
To obviate this problem, catalytic processes have been proposed for co-converting methane and one or more co-reactants to higher hydrocarbon, such as aromatics. For example, U.S. Pat. No. 5,936,135 discloses reacting methane at a temperature in the range of 300° C. to 600° C. with (i) a C2-10 olefin and/or (ii) a C2-10 paraffin in the presence of a bifunctional pentasil zeolite catalyst, having strong dehydrogenation and acid sites, to produce aromatics. The preferred mole ratio of olefin and/or higher paraffin to methane and/or ethane in the feed ranges from about 0.2 to about 2.0.
Other processes utilize organic oxygenate as a co-reactant for the non-oxidative methane conversion to produce higher hydrocarbon, including aromatics. For example, U.S. Pat. No. 7,022,888 discloses a process for the non-oxidative conversion of methane simultaneously with the conversion of an organic oxygenate, represented by a general formula: CnH2n+1OCmH2m+1, wherein C, H and O are carbon, hydrogen and oxygen, respectively; n is an integer having a value between 1 and 4; and m is an integer having a value between zero and 4. The methane and oxygenate are converted to C2+ hydrocarbon, particularly to gasoline range C6-C10 hydrocarbon and hydrogen, using a bifunctional pentasil zeolite catalyst, having strong acid and dehydrogenation functions, at a temperature below 700° C.
However, since the co-reactants employed in the processes of the '135 and '888 patents are themselves valuable commodities, there is interest in developing alternative routes for the conversion of methane into aromatics and particularly routes that allow more methane to be incorporated into the aromatic product, and that operates over a broad molar ratio range of methane to co-reactant in the feed.