Methane oxidation systems

Methane is oxidized under aerobic conditions by a group of bacteria called methanotrophs. These widespread bacteria play an important role in the global cycling of methane. Two types of methane oxidation systems are known, a ubiquitous particulate methane monooxygenase (pMMO) and a cytoplasmic soluble methane monooxygenase (sMMO) found in only a few strains. These enzymes have different catalytic characteristics, and so it is important to know the conditions under which each is expressed. In those strains containing both sMMO and pMMO, the available copper concentration controls which enzyme is expressed. However, the activity of the pMMO is also affected by copper. Data on methane oxidation in natural samples suggest that methanotrophs are not copper-limited in nature and express the pMMO predominantly. 

To understand the role of these bacteria in methane cycling, the methane oxidation system must be studied. In methanotrophs, methane is oxidized to methanol by an enzyme called the methane monooxygenase (MMO) (I), which uses methane, molecular oxygen, and reducing equivalents to produce methanol and water. All known methanotrophs contain a membrane-bound MMO, called the particulate methane monooxygenase (pMMO). The presence of this enzyme system is correlated with the complex internal membrane system found in all known methanotrophs. 

Table III. Predicted Methane Oxidation Systems and K, Values Environmental Implications of Copper Availability...

H2 concentrations measured in a sulfate-containing and actively methane-oxidizing system were sufficiently low as to provide a thermodynamically... 

Proposed mechanism for the Catalytica methane oxidation system ... 

However, the combustion process for methane requires no fewer than 325 individual mechanistic steps (elementary reactions) to be accurately described, rather than the one-step route shown above. As such, incomplete combustion is a common occurrence and ROS are pervasive byproducts of that phenomenon, affecting an engine s fuel efficiency and producing atmospherically detrimental emissions. Moreover, combustion varies with system temperature, as different oxidative pathways become accessible, as well as fuel/oxidizer ratio (equivalence ratio). By examining the representative cases of methane oxidation at high and low temperatures, this phenomenon becomes clearer. 

The difference in H2 selectivity between Pt and Rh can be explained by the relative instability of the OH species on Rh surfaces. For the H2-O2-H2O reaction system on both and Rh, the elementary reaction steps have been identified and reaction rate parameters have been determined using laser induced fluorescence (LIF) to monitor the formation of OH radicals during hydrogen oxidation and water decomposition at high surface temperatures. These results have been fit to a model based on the mechanism (22). From these LIF experiments, it has been demonstrated that the formation of OH by reaction 10b is much less favorable on Rh than on Pt. This explains why Rh catalysts give significantly higher H2 selectivities than Pt catalysts in our methane oxidation experiments. 

The present chapter will primarily focus on oxidation reactions over supported vanadia catalysts because of the widespread applications of these interesting catalytic materials.5 6,22 24 Although this article is limited to well-defined supported vanadia catalysts, the supported vanadia catalysts are model catalyst systems that are also representative of other supported metal oxide catalysts employed in oxidation reactions (e.g., Mo, Cr, Re, etc.).25 26 The key chemical probe reaction to be employed in this chapter will be methanol oxidation to formaldehyde, but other oxidation reactions will also be discussed (methane oxidation to formaldehyde, propane oxidation to propylene, butane oxidation to maleic anhydride, CO oxidation to C02, S02 oxidation to S03 and the selective catalytic reduction of NOx with NH3 to N2 and H20). This chapter will combine the molecular structural and reactivity information of well-defined supported vanadia catalysts in order to develop the molecular structure-reactivity relationships for these oxidation catalysts. The molecular structure-reactivity relationships represent the molecular ingredients required for the molecular engineering of supported metal oxide catalysts. 

As an example of a system with a series of reactions, we may look at methane oxidation under conditions of excess oxygen. Following the carbon atom, this process would typically involve the steps CH4 — CH3 — CH2O — HCO — CO —> CO2. We note that each of these steps may involve a number of parallel elementary reactions, but we assume that they do not affect the oxidation pathway. 

The initiating step in the oxidation of methane is the first abstraction of a hydrogen atom. However, because of the tetrahedral molecular structure with comparatively high C-H bond energies, the methane molecule is extremely stable, and at lower temperatures the initiation step may be rate limiting for the overall conversion. In methane-oxygen systems, the chemistry is generally initiated by reaction of CH4 with O2,... 

Integrated systems fuelled by methane Integrated systems fuelled by methanol Systems running on various fuels Indium tin oxide... 

The results of experiments on the contact time effect on methane oxidation reaction (Figure 4.11) show that formaldehyde yield increases to some extent (39%) with the contact time. Maximal yield of formaldehyde is reached at r = 1.2 s, which is optimal for execution of the reaction at selected parameters. As the contact time exceeds 1.2 s, side products occur in the system CO, C02 and CH3OH concentration of the last compound reaches its maximum at r = 1.4 h. The above results show that methane is oxidized to formaldehyde... 

The reaction of methane oxidation with hydrogen peroxide under pressure was studied on an automated micropilot flow unit with integral reactor based on the standard double reactor OL 105/02 system. The OL 105/02 system is usually used in studies of pressurized homogeneous and heterogeneous processes in gas and liquid [123]. The micropilot unit has two equal reactors of 250 cm3 volume and is equipped with standard metering, recording and control instruments. 

The experimental data on the effect of the contact time on the methane oxidation process (Figure 4.20) show increasing dependence of methane conversion on the contact time (r). Carbon oxide formed in the system is after-oxidized to C02 with the contact time. Therefore,... 

Figure 3. Proposed mechanism for methane oxidation by the Pt(bpym)CI2/H2SO system.

Figure 6. Proposed catalytic cycles for methane oxidation with the Au(l)/(lll)/H2Se04/H2S04 system.

This direct, oxidative condensation of methane to acetic acid in one-pot could be competitive with the current three-step, capital intensive process for the production of acetic acid based on methane reforming to CO, methanol synthesis from CO, and generation of acetic acid by carbonylation of methanol. Key improvements required with the PdS04/H2S04 system, however, will be to develop more stable, faster, and more selective catalysts. Although it is possible sulfuric acid could be utilized industrially as a solvent and oxidant for this reaction, it would be desirable to replace sulfuric acid with a less corrosive material. This chemistry has recently been revisited, verified, and extended by Bell et al., who used Cu(II)/02 as the oxidizing system [22],... 

Methane oxidation can be an important sink in estuaries as well, and is highly dependent on temperature and salinity, with lower oxidation rates at higher salinities. Considerable temporal and spatial variability exists in the sources and sinks of methane, water-to-air fluxes, as well as mechanisms of transport (e.g., ebullition, diffusion, plant mediated) in estuarine systems. 

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