Methanol, production methane oxidation

A topic of current interest is that of methane activation to give ethane or selected oxidation products such as methanol or formaldehyde. Oxide catalysts are used, and there may be mechanistic connections with the Fischer-Tropsch system (see Ref. 285). 

The production of synthesis gas from natural gas and coal is the basis of the 33 000000 tpa methanol production and is also used in the production of ammonia. After removal of sulfur impurities, methane and water are reacted over a nickel oxide on calcium aluminate catalyst at 730 °C and 30 bar pressure. The reaction is highly endothermic (210 kJmol ) (Equation 6.6). 

After peroxide injection, conversion of methane increases fix)m -4% to -10%, methanol production increases 17 fold, and carbon dioxide increases 5 fold, along with modest increases in hydrogen and carbon monoxide. Introduction of hydroxyl radicals to the reactor leads to a greater fi action of product going to methanol as evidenced by methane conversion increasing 2.5 times, whereas methanol production increases 17 times. The increase in carbon dioxide is fiom "deep" oxidation of... 

NMR analysis of product methanol isotops and KIE values of CH2D2 methane oxidation with... 

In the presence of metal catalysts, hydrogen peroxide oxidations proceed in improved yields. The most common catalyst is an iron(II) salt which produces the well-known Fenton system or reagent. Dimethyl sulphoxide is oxidized to the sulphone using this system although a range of unwanted side-products such as methanol and methane are produced Diphenyl sulphoxide does not react using this reagent due to its insolubility and in all cases some iron(III) is formed by other side-reactions. 

Semiconductor electrodes seem to be attractive and promising materials for carbon dioxide reduction to highly reduced products such as methanol and methane, in contrast to many metal electrodes at which formic acid or CO is the major reduction product. This potential utility of semiconductor materials is due to their band structure (especially the conduction band level, where multielectron transfer may be achieved)76 and chemical properties (e.g., C02 is well known to adsorb onto metal oxides and/ or noble metal-doped metal oxides to become more active states77-81). Recently, several reports dealing with C02 reduction at n-type semiconductors in the dark have appeared, as described below. 

According to Lunsford, most of the observations on methane oxidation over oxide catalysts may be interpreted in terms of methyl radical chemistry.41 Most experimental data support the role of surface O- ions in the formation of methyl radicals. The latter are transformed by reductive addition to methoxide ions, which decompose to formaldehyde or react with water to form methanol. Methyl radicals may desorb to the gas phase and participate in free-radical reactions to yield non-selective oxidation products. 

Methanol is a desired end product in partial methane oxidation, but some methanol is consumed by reaction with the radical pool,... 

It is of interest to assess the process potential of methanol production by a direct partial oxidation of methane. This way the steam reformer and the shift reactor can be saved, and the catalytic methanol reactor can be replaced by a noncatalytic partial oxidation reactor. It is estimated that direct partial oxidation is competitive if a conversion of methane of at least 5.5% can be obtained with a methanol selectivity of at least 80%. 

Beside methanol and formaldehyde, the oxidation of methane may be directed to another route, leading to the formation of its condensation products, for example, ethane, ethylene and benzene. This route may provide an alternative way for the chemical use of natural sources of methane. Here, various catalysts were also tested using both 02 and N20 as the oxidants [22], The general picture observed by most authors was similar to that with methane oxidation to oxygenates. The conversion of methane was always higher with 02 than with N20. However, the selectivity to the coupling products showed an opposite trend. 

Figure 4.10 Temperature dependencies of reaction product yields and selectivity at methane oxidation molar ratio CH4 25% H202 = 1 1, t= 1.2s (1 methanol 2 CO + C02 3 formaldehyde 4 selectivity by formaldehyde and 5 total methane conversion).

It also follows from these experiments that the formation of formaldehyde in low amounts is explained by the presence of methanol in the initial mixture, because under current conditions H202 is mostly consumed for its deep oxidation. Similar results were obtained at methanol oxidation with hydrogen peroxide in the absence of methane (Table 4.3, tests 9-11). Comparison of these data with the results of methane oxidation with hydrogen peroxide under identical conditions (tests 1 and 2) indicates qualitative and quantitative differences in reaction products. 

Studies aimed at creating methods for direct methane oxidation to methanol are not implemented in industry because of high pressure, relatively low yields of target products and great amounts of side products. 

The curve of 02 accumulation shows that short contact times, at which the methane oxidation rate is low, are enough for complete H202 dissociation. However, as observed from shapes of 02 and CH3OH accumulation curves, methanol yield increases synchronously with 02 yield decrease, and from the moment r = 10.2 s both curves stabilize. Such stabilization and synchronization of catalase and monooxygenase reaction product yields is the experimental proof of their interaction, displayed by chemical conjugation. The existence of the stabilization zone of 02 and CH3OH yields is associated with full H202 dissociation. 

Activation and conversion of small molecules photocatalysis. Production of H2 from protons, N2 fixation for ammonia production, C02 fixation for organics, sulphide (H2S and Na2S) oxidation, methane activation for methanol production, etc. 

Photochemical reduction of C02 was also achieved in the presence of the p-type semiconductor (copper oxide) or silicon carbide electrodes [97]. Irradiation of this system generates methanol and methane as the main products in the case of CuO electrode whereas hydrogen (with efficiency about 80%), methanol (16%), methane, and carbon monoxide in the case of SiC electrode. Also Ti02/CuO systems appeared relatively efficient (up to 19.2% quantum yield) in photocatalytic C02 to CH3OH reduction [98]. 

With adjustment of the steam/methane ratio, the reactor can produce a synthesis gas with CO/H2 = 1/2, the stoichiometric proportions needed for methanol production. This mixture at approximately 200 atm pressure is fed to the methanol unit where the reaction then proceeds at 350°C. Per pass conversions range from 30 to 50 over the catalyst— typically a supported copper oxide with a zinc, chromium, or manganese oxide promoter 3... 

Zhang, Q., He, D., Li, J., Xu, B., Liang, Y., Zhu, Q. (2002). Comparatively high yield methanol production from gas phase partial oxidation of methane. Appl Catalysis A General 224, 201-207. 

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