In order to minimize the formation of side products and increase the selectivity to methanol, the use of lower reaction temperatures (<250 °C) is preferable. Currently used catalysts for methane oxidation are, however, not sufficiently active at lower temperatures. The development of a new generation of catalysts which could produce selectively methanol directly from methane in high yields at lower temperatures is therefore highly desirable. As discussed, gas-phase reactions have met with limited success and require generally higher temperatures (>400 °C). This has prompted much investigation of liquid-phase reactions operating at more moderate temperatures.
During the 1970s it was first observed by Olah and co-workers that methanol could be obtained through electrophilic oxygenation (i.e., oxygen functionaliza-tion) of methane. When reacted in superacids (acids many million times stronger than concentrated sulfuric acid) at room temperature with hydrogen peroxide (H2O2), methane produces methanol in high selectivity. From the results obtained, it was concluded that the reaction proceeds in strong acids through the insertion of protonated hydrogen peroxide, H3O2+ into the methane C-H bond. The formed methanol in the strong acid system is in its protonated form, CH3OH2+ and is thus protected from further oxidation; this accounts for the high selectivity observed. The use of H2O2 is, however, not suited for the large-scale production of methanol, as is generally the high cost of liquid superacids . Studies are continuing with cheaper peroxides and strong acid systems.
The concept of chemically protecting the methanol formed in the oxidation of methane was successfully further developed by Periana and co-workers, using predominantly metals and metal complex catalysts dissolved in sulfuric acid or oleums . Several of these homogeneous catalytic systems, able to activate the otherwise very unreactive C-H bonds of methane at lower temperatures and with surprisingly high selectivity, have been developed. At temperatures around 200 °C, the conversion of methane to methanol by concentrated sulfuric acid using a HgSO4 catalyst was found to be an efficient reaction. The oxidation of methane produces methyl hydrogen sulfate (CH3OSO3H), which can be hydro-lyzed to methanol in a separate step. At a conversion of 50%, an 85% selectivity to methyl hydrogen sulfate was achieved . To complete the catalytic cycle, Hg+ is reoxidized to Hg2+ by H2SO4. Overall, the process uses one molecule of sulfuric acid for each molecule of methanol produced. SO2 generated during the process can be easily oxidized to SO3 which, upon reaction with H2O, will give H2SO4 that can be recycled.
The cleavage of methyl hydrogen sulfate to methanol and its separation from sulfuric acid media is, however, an energy-consuming process. Furthermore, the use of poisonous mercury makes the process also somewhat unattractive. Interest has therefore shifted towards other less toxic metals. Systems incorporating platinum, iridium, rhodium, palladium, ruthenium and others have been tested for homogeneous methane oxidation. Recently, even gold was found to be capable of catalyzing the oxidation of methane to methanol [195, 196]. However, the best results were obtained with a platinum complex in H2SO4.
With this system, methane was converted into methyl hydrogen sulfate, and subsequently, methanol with yields in excess of 70% and selectivities to methanol over 90% .
Water formed during the reaction accumulates progressively in sulfuric acid, decreasing its acidity. This makes the process problematic, as the activity not only of the platinum but also of mercury and most other systems tested, is rapidly decreasing at lower acidities and can be inhibited by high water content. For this reason and others, the conversion is advantageously conducted in oleum (a mixture of H2SO4 and SO3). SO3 reacts with the water formed to give H2SO4, avoiding a decrease in acidity. The use of a catalyst system exhibiting high activity, even at lower acidities, would be preferable however. A gold-based system using selenic acid as the oxidant has also been developed and shows promise .
In order to replace the high-cost platinum and other noble metal-based catalysts, the development of a less expensive, but still effective and selective catalyst would also be desirable.
Although our understanding of the reaction mechanism for the direct oxidation of methane in homogeneous systems has greatly improved over the years, many key questions have still to be solved. There is also need to develop more active, selective and stable catalysts before a commercial process based on this technology becomes a reality. Today, however, this goal appears increasingly achievable.
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