In homogeneous gas-phase oxidation, methane is generally reacted with oxygen at high pressures (30 to 200 atm) and high temperatures (200 to 500 °C). Optimum conditions for the selective oxidation to methanol have been extensively in vestigated. It was observed that selectivity to methanol increases with decreasing oxygen concentration in the system. The best result (75-80% selectivity in methanol formation at 8-10% conversion) was achieved under cold flame conditions (450 °C, 65 atm, less than 5% O2 content), using a glass-lined reactor, which seemed to suppress secondary reactions. Most other studies obtained under their best reaction conditions (temperatures of 450-500 °C and pressures of 30-60 atm), selectivities of 30-40% methanol with 5-10% methane conversion . At high pressure, the gas-phase radical reactions are predominant, limiting the expected favorable effects of solid catalysts. Selectivity to methanol can only be moderately influenced by controlling factors affecting the radical reactions such as reactor design and shape and residence time of the reactants.
At about ambient pressure (1 atm), the catalysts can play a crucial role in the partial oxidation of methane with O2. A large number of them, primarily metal oxides and mixed oxides, have been studied. Metals were also tested but they tend to favor complete oxidation. In most cases the reactions were performed at temperatures from 600 to 800 °C, and formaldehyde (HCHO) was obtained as the predominant (and often only) partial oxidation product. Silica itself exhibits a unique activity in the oxidation of methane to formaldehyde. Higher methane conversions were however obtained with silica-supported molybdenum (MoO3) and vanadium (V2O5) oxide catalysts. The yield of formaldehyde produced remained nevertheless in the range of 1 to 5%. An iron molybdate catalyst, Fe2O3 (MoO3)2.25, was found to be most active catalyst for partial oxidation of methane, with a reported formaldehyde yield of 23%. Reaction over a silica-supported PCl3-MoCl5-R4Sn catalyst gave 16% yield. More recently, a high selectivity of 90% to oxygenated compounds (CH3OH and HCHO) was obtained at methane conversion of 20-25% in an excess of steam, using a silica-supported MoO3 catalyst. In most cases, however, such high yields were difficult to reproduce by other research groups, and yields of oxygenates generally do not exceed 2-5%. At the high temperatures used for partial oxidation of methane, methanol formed on the catalyst's surface is rapidly decomposed or oxidized to formaldehyde and/or carbon oxides, which explains its absence from the obtained product mixture.
As methanol is formed together with formaldehyde and formic acid in the oxidation of methane, it has recently been found by Olah and Prakash that this mixture can be further processed in a second treatment step without the need of prior separation, resulting in a substantial increase in methanol content and making the overall process more selective and practical for the production of methanol .
The initial oxidation of methane to a mixture of methanol, formaldehyde and formic acid can utilize any known catalytic oxidation procedure. The formation of CO2, however, is to be kept to a minimum. In order to minimize overoxidation, the overall amounts of oxygenated products are kept at relatively low levels (20-30%), with unreacted methane being recycled.
The subsequent treatment of the formaldehyde and formic acid formed in the oxidation step, without any separation can be carried out in different ways. One is by dimerizing the formed formaldehyde over TiO2 or ZrO2  to give methyl formate. Formaldehyde can also undergo conversion over solid-base catalysts such as CaO and MgO, a variation of the so-called Cannizaro reaction, giving methanol and formic acid. These readily react with each other to form methyl formate.
2HCHO H2° » CH3OH + HC02H CH3OH + HC02H ~H2°- HCOOCH3
The methyl formate obtained can then be catalytically hydrogenated or electroche-mically reduced using suitable electrodes (made from copper, tin, lead, etc.), giving two molecules of methanol with no other byproduct.
Formic acid formed during the oxidation can itself serve as a hydrogen source to be reacted with formaldehyde in aqueous solutions at 250 °C or over suitable catalysts to produce methanol and CO2.
Suitable combinations of these reactions allow the oxidative conversion of methane to methanol with overall high selectivity and yield, considering that CO2 formed as byproduct is to be recycled to methanol (vide infra). As methanol is readily dehydrated to dimethyl ether, the oxidative conversion of methane to methanol is also well suited to produce dimethyl ether for fuel or chemical applications.
Avoiding any additional steps, and producing directly methanol from methane remains a most desirable goal. Catalysts able to activate methane at lower temperatures are much sought after for the direct and selective synthesis of methanol from methane and air (oxygen).
A new development for the oxidation of methane is the use of O2-H2 gas mixtures. With FePO4 as a catalyst, methanol is formed as the main product in the presence of O2-H2 at temperatures below 400 °C . Nitrous oxide (N2O) was also reported to be effective in producing both methanol and formaldehyde in the presence of silica-supported MoO3 and V2O5 catalysts. Iron-containing catalysts also exhibited very unique properties in the partial oxidation of methane. Using the above-mentioned FePO4 catalyst, partial oxidation of methane with N2O, gave methanol with a remarkable selectivity close to 100% in the presence of H2 at 300 °C. The yield is however low, with only 3% methanol obtained at 450 °C. The O- ion formed by the decomposition of N2O at the catalyst's surface was postulated to be the active species initiating the activation process of methane. Even if N2O turns out to be a suitable oxidizing agent, its application on an industrial scale for methanol production is unlikely due to the costs asso ciated with its production and its high activity as a greenhouse gas. Oxygen of the air is, and will remain, the most affordable, ubiquitous - and therefore preferred -oxidizing agent.
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