Another possible alternative for the selective conversion of methane to methanol proceeds through the intermediate catalytic formation of methyl chloride or bromide (CH3Cl, CH3Br), which are then hydrolyzed to methanol (or dimethyl ether) with the byproduct HCl or HBr being reoxidized [197, 198].
The chlorination of paraffins, discovered by Dumas in 1840, is the oldest known substitution reaction and is practiced on a large scale in industry. It is usually a free radical process initiated either thermally or photochemically (by heat or light). The major problem with free radical reactions, also experienced in oxidative methane conversion, is the lack of selectivity. In the case of methane chlorination, all four chloromethanes (CH3Cl, CH2Cl2, CHCl3 and CCl4) are generally obtained.
As methyl chloride is chlorinated more rapidly than methane itself under free radical conditions, a high ratio of methane to chlorine of at least 10:1 is required to obtain methyl chloride as the main desired product. Chlorination of methane has also been reported over catalysts such as active carbon, Kieselguhr, pumice, alumina, kaoline, silica gel and bauxite, but showed limited selectivity due to the free radical nature of the reaction. In industry, the strongly exothermic chlorination reaction is generally conducted in the absence of a catalyst at 400-450 °C without external heating under slightly elevated pressures.
In the 1970s, it was observed by Olah et al. that chlorination of methane with superacidic SbF5/Cl2 at low temperature gives methyl chloride in high selectivity with only small amounts of methylene chloride and no chloroform or carbon tetrachloride [199,200]. The high selectivity obtained in the electrophilic reaction contrasted strongly with the low selectivity for radical chlorinations. As, however, the conversions were low, no adaptation of the reaction for practical use was made.
In extending the electrophilic halogenation of methane to catalytic heterogeneous gas-phase reactions during the 1980s, it was shown that methyl chloride could be formed with high selectivity and acceptable yields . The catalytic monohalogenation (chlorination or bromination) of methane was achieved over either solid superacids such as TaF5/Nafion-H, SbF5/graphite, supported metals such as Pt/Al2O3 and Pd/BaSO4, or supported oxychloride and oxyfluoride such as ZrOF2/Al2O3 and GaOxCly/Al2O3. The reactions were carried out at tempera tures between 180 and 250 °C, giving 10 to 60% conversion with selectivities to methyl chloride (bromide) generally exceeding 90%. Methylene halides (CH2Cl2, CH2Br2) were the only higher halogenated methanes formed and no haloforms (CHCl3, CHBr3) or carbon tetrahalides (CCl4, CBr4) were observed.
In the halogenation of methane, hydrogen halides (HCl or HBr) are formed as equimolar byproducts. Similarly hydrolysis of methyl halides also gives hydrogen halides. Their recycling is essential to utilize halogens only as a catalytic agent in the overall conversion of methane into methanol. The oxidation of hydrogen chloride to chlorine is industrially possible (Deacon process, Kellogg's improved Kel-Chlor process, Mitsui Toatsu Chemicals MT Chlor process), but it remains technologically difficult. An improved way to effect the reaction was also found by Benson and co-workers [201, 202]. In contrast, HBr is readily oxidized to bromine by air. Its reoxidation and continuous recycling thus is more easily achieved.
Combination of the selective monohalogenation (preferentially bromination) of methane with subsequent catalytic hydrolysis to methanol/dimethyl ether offers was shown by Olah and co-workers to be an attractive alternative route to the preparation of methyl alcohol without syn-gas. Methyl halides and/or methyl alcohol/dimethyl ether obtained directly from methane also offer a way to convert methane via zeolite or bifunctional acid-base-catalyzed condensation into ethylene and propylene, starting materials for synthetic hydrocarbons and their products (see Chapter 13). Related processes were developed by a research group at U.C. Santa Barbara  and Dow Chemicals .
In order to produce methanol from methane in a single step, combining halo-genation, hydrolysis and reoxidation of hydrogen halide byproduct using a Br2/ H2O/O2 or Cl2/H2O/O2 mixture can also be achieved, although to date the conversions remain modest.
Iodine was recently found by Periana to be a suitable catalyst for the selective conversion of methane in oleum . At about 200 °C, methyl hydrogen sulfate was formed with up to 45% yield and 90% selectivity, and subsequently hydro-lyzed to methanol. The intermediate formation of methyl iodide, followed by conversion to methyl hydrogen sulfate and reoxidation of HI to I2 can be assumed, not unlike the chlorine- and bromine-catalyzed conversions described above.
The use of iodine for the conversion of methane to methanol through methyl iodide is consequently also of interest.
Methanol production from fossil fuels, as long as they are readily available, will remain a major source of methanol (Fig. 12.6), improved methods of methane conversion without going through syn-gas are developed and will be used. Besides natural gas, methane hydrates and other sources will also be utilized. Biological production of methanol and chemical reductive recycling of CO2 into methanol (vide infra) will, however, increasingly become important.
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