Selective Monohalogenation of Methane over Supported Acid or Platinum Metal Catalysts and Hydrolysis of Methyl Halides over γ-Alumina-Supported Metal Oxide/Hydroxide Catalysts. A Feasible Path for the Oxidative Conversion of Methane into Methyl Alcohol/Dimethyl Ether

The catalytic monohalogenation (chlorination and bromination) of methane was achieved over either supported solid acid (such as FeO_xCl_y/Al₂O₃, TaOF₃/Al₂O₃, NbOF₃/Al₂O₃, ZrOF₂/Al₂O₃, SbOF₃/Al₂O₃, SbF₅/graphite, and Na-Fion-H/TaF₅) or platinum metal (Pt/Al₂O₃and Pd/BaSO₄) catalysts. The reactions were carried out at temperatures between 180 and 250 °C, with GHSV of 50–1400 giving 8-58% conversions with selectivity in methyl chloride (bromide) generally exceeding 90%. Limited methylene halide formation accompanies the reactions, but no formation of haloforms or carbon tetrahalides was observed. The mechanism of the halogenations is considered to involve insertion of a surface-coordinated electrophilic halogen species or electron-deficient metal site into a methane C-H bond involving Five-coordinate intermediate carbonium ion formation, with subsequent cleavage-halogenolysis giving the monohalogenated methane. Catalytic hydrolysis of methyl halides was also studied over γ-alumina-supported metal oxide/hydroxide catalysts, giving mixtures of methyl alcohol and dimethyl ether. Combining the selective monohalogenation of methane with subsequent hydrolysis and oxyhalogenative recycling of byproduct HX allows conversion of methane to methyl alcohol/dimethyl ether, a route which offers an alternative to the presently exclusively used preparation of methyl alcohol via syngas. The preparation of methyl halides and/or methyl alcohol/dimethyl ether directly from methane also offers a way to convert methane via previously described bifunctional acid-base-catalyzed condensation into ethylene and subsequently into homogeneous lower olefins and/or higher hydrocarbons.