Methanol methane conversion

Direct Methane Conversion, Methanol Fuel Cell, and Chemical Recycling of Carbon Dioxide... 

In addition to these principal commercial uses of molybdenum catalysts, there is great research interest in molybdenum oxides, often supported on siHca, ie, MoO —Si02, as partial oxidation catalysts for such processes as methane-to-methanol or methane-to-formaldehyde (80). Both O2 and N2O have been used as oxidants, and photochemical activation of the MoO catalyst has been reported (81). The research is driven by the increased use of natural gas as a feedstock for Hquid fuels and chemicals (82). Various heteropolymolybdates (83), MoO.-containing ultrastable Y-zeoHtes (84), and certain mixed metal molybdates, eg, MnMoO Ee2(MoO)2, photoactivated CuMoO, and ZnMoO, have also been studied as partial oxidation catalysts for methane conversion to methanol or formaldehyde (80) and for the oxidation of C-4-hydrocarbons to maleic anhydride (85). Heteropolymolybdates have also been shown to effect ethylene (qv) conversion to acetaldehyde (qv) in a possible replacement for the Wacker process. 

Investigation of direct conversion of methane to transportation fiiels has been an ongoing effort at PETC for over 10 years. One of our current areas of research is the conversion of methane to methanol, under mild conditions, using li t, water, and a semiconductor photocatalyst. Research in our laboratory is directed toward ad ting the chemistry developed for photolysis of water to that of methane conversion. The reaction sequence of interest uses visible light, a doped tungsten oxide photocatalyst and an electron transfer molecule to produce a hydroxyl i cal. Hydroxyl t cal can then react with a methane molecule to produce a methyl radical. In the preferred reaction pathway, the methyl radical then reacts with an additional wata- molecule to produce methanol and hydrogen. 

Figure 5 shows the results of a typical photocatalytic methane conversion experiment. Methane conversions are -4% with hydrogen and metlmol as the main products of reaction. Note that after the UV lamp is turned off, the detected flow of methanol... 

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... 

Other metals capable of electrophilic substitution of C-H bonds are salts of palladium and, environmentally unattractive, mercury. Methane conversion to methanol esters have been reported for both of them [29], Electrophilic attack at arenes followed by C-H activation is more facile, for all three metals. The method for making mercury-aryl involves reaction of mercury diacetate and arenes at high temperatures and long reaction times to give aryl-mercury(II) acetate as the product it was described as an electrophilic aromatic substitution rather than a C-H activation [30],... 

Gasoline has many advantages over methanol, but conversion to H2 requires temperatures in excess of 650°C and produces greater amounts of CO, methane (CH4), and possibly coke. 

The CH-activation of alkanes and especially of methane and their catalytic conversion to alcohols is one of the major challenges for chemists. Methane as the major part of natural gas is currently the cheapest source of hydrocarbons and the need for methanol will increase in the near future. Methane conversion to methanol would make a conveniently transportable fuel and also a new carbon source for the chemical industry. 

According to another important and promising technology, hydrocarbons are produced from methanol, which, in turn, is synthesized from synthesis gas. Called the methanol-to-gasoline process, it was practiced on a commercial scale and its practical feasibility was demonstrated. Alternative routes to eliminate the costly step of synthesis gas production may use direct methane conversion through intermediate monosubstituted methane derivatives. An economic evaluation of different methane transformation processes can be found in a 1993 review.1... 

The current two-step industrial route for the synthesis of methanol, from coal or methane to synthesis gas and then from synthesis gas to methanol, has certain drawbacks. The economic viability of the whole process depends on the first step, which is highly endothermic. Thus a substantial amount of the carbon source is burned to provide the heat for the reaction. It would be highly desirable, therefore, to replace this technology with a technically simpler, single-step process. This could be the direct partial oxidation of methane to methanol, allowing an excellent way to utilize the vast natural-gas resources. Although various catalysts, some with reasonable selectivity, have been found to catalyze this reaction (see Sections 9.1.1 and 9.6.1), the very low methane conversion does not make this process economically feasible at present. 

The oxidation takes place through the observable intermediate 1 to yield methyl bisulfate, which may be readily hydrolyzed to methanol. At a methane conversion of 50%, an 85% selectivity to methyl bisulfate was achieved. The second molecule of H2S04 reoxidizes Hg+ to Hg2+ completing the catalytic cycle. 

Numerous works on the oxidation of methane to methanol and/or formaldehyde as well as on the oxidative dimerization of methane were reviewed by many authors [22-27]. First, high selectivity of methane oxidation by N20 was reported by Lunsford et al. [28-30], Over a supported Mo oxide [30], the total selectivity to methanol and formaldehyde at low methane conversions attained 100%, although this rapidly dropped as the conversion increased (Table 7.4). High selectivity for this reaction was obtained also with supported vanadium oxide [31]. 

Figure 4.10 shows temperature influence on the process results formaldehyde yield reaches its maximum (about 40%) with temperature raise to 520 °C and total methane conversion increase. Above 520 °C, CO and C02 are detected in reaction products. Their formation rates noticeably increase with temperature. The occurrence of these compounds in the system is explained by sequential formaldehyde transformation to intense degradation products in the high temperature range. After-oxidation of methanol synthesized in the system also contributes to formation of these products. 

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).

Direct one-stage oxidation of methane to methanol is performed by two methods catalytic and thermal. Modernization of the process of methane conversion to methanol using various catalysts is ineffective, because in this case methane conversion is usually below 13%. At the present time, methane conversion to methanol has been raised to 24% [122] however, this is still insufficient for an economic assessment of the process. Hence, thermal... 

Figure 7.26 shows temperature kinetic dependencies of methanol yield possessing a maximum at 180 °C, whereas molecular oxygen yield possesses a minimum. In this experiment, methanol yield reaches 46.5 wt.% at methane conversion of 48 wt.%. Non-target products are CH20 and HCOOH, synthesized in low amounts ( 1.5%). Temperature does not noticeably affect their yield. The process selectivity by methanol is preserved at a level of 97% in the whole temperature interval. 

As follows from the above, at short contact times (below 2.9 s) the monooxygenase activity of the mimic remains low, whereas catalase activity is maximal (molecular oxygen yield exceeds 90 wt.%). Methanol yield and methane conversion increase with contact time up to r = 10 s and then stabilize at a level of 49-50 wt.% with —96% selectivity. Formaldehyde and formic acid are side products, giving total 2.7 wt.% no CO and C02 are detected in gaseous products. 

In an important development, Periana made Shilov-hke chemistry more practically useful with a series of methane conversion catalysts. The first such system involved Hg(n) salts in H2SO4 at 180°, the latter being both a solvent and a mild reoxidant (equation 3). Methane was converted to the methanol ester, methyl bisulfate, MeOSOsH, in which the -OSO3H provides a powerful deactivating group to... 

Other steps used in the model assume that the heterogeneous conversion of methane is limited to the gas-phase availability of oxygen, O2 adsorption is fast relative to the rate of methane conversion, and heat and mass transports are fast relative to the reaction rates. Calculations for the above model were conducted for a batch reactor using some kinetic parameters available for the oxidative coupling of methane over sodium-promoted CaO. The results of the computer simulation performed for methane dimerization at 800 °C can be found in Figure 7. It is seen that the major products of the reaction are ethane, ethylene, and CO. The formation of methanol and formaldehyde decreases as the contact time increases. 

Many other partial oxidations have been studied methane conversion to methanol and formaldehyde. 

This example can be applied to a broad class of catalytic reactions but it is much more obvious for partial oxidation reactions where secondary reactions (total combustion) result in a dramatic decrease of selectivity. This is the case with methanol decomposition and methane conversion, where the intensification of gas-phase catalytic operations in micro- or nanochannels clearly appears. 

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