Methane 26 Oxidation

Methane oxidation was the first reaction studied using the AIMS system. The goal was to operate the microreactor under fuel-lean conditions with millisecond contact times (Williams et al, 2005) while achieving high degrees of conversion for O2, which was the limiting reactant. 

High temperatures caused the resistance of the temperature sensor to irreversibly decrease to a lower value. Hence, temperatrues much higher than 600 C produce a false resistance reading close to 600 C because of the change in platinum structrue. 

4 Methane oxidation. In early work Sandler investigated the behaviour of a calcia-stabilized ziiconia (CSZ) cell with platinum and silver electrodes in the presence of methane-oxygen mixtures. The results indicated that the cell could be used as a methane detector device. Haaland also investigated the behaviour of platinum, silver and gold electrodes in the presence of methane-oxygen mixtures 

However, at normal temperature and pressure, methane is a gas therefore, it is difficult to transport. 

At present, the Fischer-Tropsch process is employed in industry to convert standard natural gas into synthetic gasoline, diesel, or jet fuel [65], It involves a series of chemical reactions that lead to a variety of hydrocarbons  

The reactions are catalyzed by transition metals (cobalt, iron, and ruthenium) on high-surface-area silica, alumina, or zeolite supports. However, the exact chemical identity of the catalysts is unknown, and their characterization presents challenges as these transformations are carried out under very harsh reaction conditions. Typically, the Fischer-Tropsch process is operated in the temperature range of 150°C-300°C and in the pressure range of one to several tens of atmospheres [66], Thus, the entire process is costly and inefficient and even produces waste [67]. Hence, development of more economical and sustainable strategies for the gas-to-liquid conversion of methane is highly desirable. 

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The process can be operated in two modes co-fed and redox. The co-fed mode employs addition of O2 to the methane/natural gas feed and subsequent conversion over a metal oxide catalyst. The redox mode requires the oxidant to be from the lattice oxygen of a reducible metal oxide in the reactor bed. After methane oxidation has consumed nearly all the lattice oxygen, the reduced metal oxide is reoxidized using an air stream. Both methods have processing advantages and disadvantages. In all cases, however, the process is mn to maximize production of the more desired ethylene product. 

Methane oxidations occur only by intermediate and high temperature mechanisms and have been reported not to support cool flames (104,105). However, others have reported that cool flames do occur in methane oxidation, even at temperatures >400 ° C (93,94,106,107). Since methyl radicals caimot participate in reactions 23 or 24, some other mechanism must be operative to achieve the quenching observed in methane cool flames. It has been proposed that the interaction of formaldehyde and its products with radicals decreases their concentrations and inhibits the whole oxidation process (93). 

The reported characteristics of methane oxidation at high pressures are interesting. As expected,the reaction can be conducted at lower temperatures eg, 262°C at 334 MPa (3300 atm) (100). However, the cool flame phenomenon is observed even under these conditions. At high pressures. 

Ethane. Ethane VPO occurs at lower temperatures than methane oxidation but requires higher temperatures than the higher hydrocarbons (121). This is a transition case with mixed characteristics. Low temperature VPO, cool flames, oscillations, and a NTC region do occur. At low temperatures and pressures, the main products are formaldehyde, acetaldehyde (HCHOiCH CHO ca 5) (121—123), and carbon monoxide. These products arise mainly through ethylperoxy and ethoxy radicals (see eqs. 2 and 12—16 and Fig. 1). 

Aldehydes are important products at all pressures, but at low pressures, acids are not. Carbon monoxide is an important low pressure product and declines with increasing pressure as acids increase. This is evidence for competition between reaction sequence 18—20 and reaction 21. Increasing pressure favors retention of the parent carbon skeleton, in concordance with the reversibiUty of reaction 2. Propylene becomes an insignificant product as the pressure is increased and the temperature is lowered. Both acetone and isopropyl alcohol initially increase as pressure is raised, but acetone passes through a maximum. This increase in the alcohoLcarbonyl ratio is similar to the response of the methanoLformaldehyde ratio when pressure is increased in methane oxidation. 

The overall process for producing a 1 1 CO to ratio by partial methane oxidation and the water gas shift reaction is represented by equation 5. 

R. Hicks and co-workers, Structure Sensitivity of Methane Oxidation overl latinum and Palladium J. Catal, 280—306 (1990). 

The influence of Zn-deposition on Cu(lll) surfaces on methanol synthesis by hydrogenation of CO2 shows that Zn creates sites stabilizing the formate intermediate and thus promotes the hydrogenation process [2.44]. Further publications deal with methane oxidation by various layered rock-salt-type oxides [2.45], poisoning of vana-dia in VOx/Ti02 by K2O, leading to lower reduction capability of the vanadia, because of the formation of [2.46], and interaction of SO2 with Cu, CU2O, and CuO to show the temperature-dependence of SO2 absorption or sulfide formation [2.47]. 

The lower than expected yields can be explained by the nature of methane oxidation to methanol in these bacteria. This reaction, catalysed by methane mono-oxygenase, is a net consumer of reducing equivalents (NADH), which would otherwise be directed to ATP generation and biosynthesis. In simple terms the oxidation of methane to methanol consumes energy, lowering the yield. 

D. Eng, and M. Stoukides, Catalytic and Electrocatalytic Methane Oxidation with Solid Oxide Membranes, Catalysis Reviews - Science and Engineering 33, 375-412 (1991). 

This linear variation in catalytic activation energy with potential and work function is quite noteworthy and, as we will see in the next sections and in Chapters 5 and 6, is intimately linked to the corresponding linear variation of heats of chemisorption with potential and work function. More specifically we will see that the linear decrease in the activation energies of ethylene and methane oxidation is due to the concomitant linear decrease in the heat of chemisorption of oxygen with increasing catalyst potential and work function. 

O.A. Mar ina, V.A. Sobyanin, V.D. Belyaev, and V.N. Parmon, The effect of electrochemical pumping of oxygen on catalytic behaviour of metal electrodes in methane oxidation, in New Aspects of Spillover Effect in Catalysis for Development of Highly Active Catalysts, Stud. Surf. Sci. Catal. 77 (T. Inui, K. Fujimoto, T. Uchijima,... 

Methane oxidation and partial oxidation, electrochemical promotion of, 308 dimerization, 470 reforming, 410 Methanol dehydrogenation electrochemical promotion of, 403 selectivity modification, 404 Methanol oxidation electrochemical promotion of 398 selectivity modification, 400 Microscopy... 

Hydrogen Photosynthetic bacteria, methane oxidation A few years... 

Eng W, Palumbo AV, Sriharan S, et al. 1991. Methanol suppression of trichloroethylene degradation by Methylosinus trichosporium (OB3b) and methane-oxidizing mixed cultures. Appl Biochem Biotechnol 28/29 887-899. 

Jones RD, RY Morita (1983) Methane oxidation by Nitrosococcus oceanus and Nitrosomonas europaea. Appl Environ Microbiol 45 401-410. 

Ward BB (1987) Kinetic studies on ammonia and methane oxidation by Nitrosococcus oceanus. Arch Microbiol 147 126-133. 

Little CD, AV Palumbo, SE Herbes, ME Lidstrom, RL Tyndall, PJ Gilmour (1988) Trichloroethylene biodegradation by a methane-oxidizing bacterium. Appl Environ Microbiol 54 951-956. 

Girguis PR, AE Cozen, EF Delong (2005) Growth and population dynamics of anaerobic methane-oxidizing archaea and sulfate-reducing bacteria in a continuous-flow reactor. Appl Environ Microbiol 71 3725-3733. 

Hallam SJ, PR Girguis, CM Preston, PM Richardson, EF DeLong (2003) Identification of methyl coenzyme M reductase A (merA) genes associated with methane-oxidizing archaea. Appl Environ Microbiol 69 5483-5491. 

Orphan VL, CH Hpuse, K-U Hinrichs, KD McKeegan, EE DeLong (2002) Multiple archaeal groups mediate methane oxidation in anoxic cold seep sediments. Proc Natl Acad USA 99 7663-7668. 

BIOMIMETIC FEATURES OF FeZSM-5 ZEOLITE 3.1. a-Oxygcn methane oxidation... 

In order to identify the product, we used a procedure of its extraction from the surface similar to that used in the case of a-oxygen benzene oxidation [18]. For this purpose, a number of single-tum-over runs in the room temperature methane oxidation were carried out according to the following scheme ... 

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

From the results discussed above as well as from the literature data [5-10,12-14] it follows that an important role of O2 in the SCR process is to convert NO into NOj. The latter then initiates methane oxidation into CO, and is itself reduced into NO and N2. Both NO, and O2 may participate in CH4 oxidation (Fig. 1B) and the ratio between the rates of these competitive oxidation reactions will be critical for the selectivity of the SCR process. Hence, the absolute rates of CH4 oxidation by Oj were compared with those occurring in the SCR process. The rates of these reactions were determined under different reaction conditions (using the... 

Partial methane oxidation comprises very high rates so that high space-time yields can be achieved (see original citations in [3]). Residence times are in the range of a few milliseconds. Based on this and other information, it is believed that syngas facilities can be far smaller and less costly in investment than reforming plants. Industrial partial oxidation plants are on the market, as e.g. provided by the Syntroleum Corporation (Tulsa, OK, USA). Requirements for such processes are operation at elevated pressure, to meet the downstream process requirements, and autothermal operation. 

Figure 3.43 Conversion rates and product selectivity of partial methane oxidation as a function of the catalyst temperature. Experimental data (points) and calculated thermodynamic values (lines) [112].

Hoffmann, C., Schmidt, H.W. and Schuth, F. (2001) A multipurpose parallelized 49-channel reactor for the screening of catalysts methane oxidation as the example reaction. J. Catal., 198, 348. 

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