Anaerobic oxidative coupling

The anaerobic oxidative coupling of methane to ethylene was studied by ARCO Chemical [70b]. The reaction occurs at a very high temperature of 850-900 °C with a Li/B/Mn/Mg/0-Si02 catalyst, a conversion of 22% was achieved with selectivity to C2 compounds of about 60% under anaerobic conditions. Monsanto [71] studied... 

FIGURE 9.3 Monsanto anaerobic oxidative coupling of toluene. 

Tests on pig gut contents using molecular probes to detect the presence of (aerobic) ammonia oxidizers proved negative. Recently, the anaerobic oxidation of ammonia coupled to nitrate reduction has been demonstrated in... 

Organic Carbon Oxidation Microbially Coupled to Reduction of Fe(III)(hydr)oxide. More and more evidence is accumulating that bacteria can grow anaerobically by coupling organic carbon oxidation to the dissimilatory reduction of iron(III) oxides (Nealson, 1982 Arnold et al., 1986 Lovely and Philips, 1988 Nealson and Myers, 1990). 

Lovley DR, Phillips EJP (1988) Novel mode of microbial energy metabolism organic carbon oxidation coupled to dissimilatory reduction of iron or manganese. App Environ Microbio 54 1472-1480 Lovley DR, Stolz JF, Nord Jr GL, Phillips EJP (1987) Anaerobic production of magnetite by a dissimilatoiy iron-reducing microorganism. Nature 330 252-254... 

A few examples of chemoautolithotrophic processes have been mentioned in this chapter, namely anaerobic methane oxidation coupled to sulfate reduction and the ones listed in Table 12.2 involving manganese, iron, and nitrogen. Another example are the microbial metabolisms that rely on sulfide oxidation. Since sulfide oxidation is a source of electrons, it is a likely source of energy that could be driving denitrification, and manganese and iron reduction where organic matter is scarce. 

Following consumption of dissolved O2, the thermodynamically favored electron acceptor is nitrate (N03-). Nitrate reduction can be coupled to anaerobic oxidation of metal sulfides (Appelo and Postma, 1999), which may include arsenic-rich phases. The release of sorbed arsenic may also be coupled to the reduction of Mn(IV) (oxy)(hydr)oxides, such as birnessite CS-MnCb) (Scott and Morgan, 1995). The electrostatic bond between the sorbed arsenic and the host mineral is dramatically weakened by an overall decrease of net positive charge so that surface-complexed arsenic could dissolve. However, arsenic liberated by these redox reactions may reprecipitate as a mixed As(III)-Mn(II) solid phase (Toumassat et al., 2002) or resorb as surface complexes by iron (oxy)(hydr)oxides (McArthur et al., 2004). The most widespread arsenic occurrence in natural waters probably results from reduction of iron (oxy)(hydr)oxides under anoxic conditions, which are commonly associated with rapid sediment accumulation and burial (Smedley and Kinniburgh, 2002). In anoxic alluvial aquifers, iron is commonly the dominant redox-sensitive solute with concentrations as high as 30 mg L-1 (Smedley and Kinniburgh, 2002). However, the reduction of As(V) to As(III) may lag behind Fe(III) reduction (Islam et al., 2004). 

Thamdrup, B., and Dalsgaard, T. (2002) Production of N2 through anaerobic ammonium oxidation coupled to nitrate reduction in marine sediments. Appl. Environ. Microbiol. 68, 1312-1318. 

Figure 6.3 (Right) Schematic depiction of anammox cell showing the anammoxozome and nucleoid. (Left) Postulated pathway of anaerobic ammonium oxidation coupled to the ana-mmoxosome membrane resulting in a proton motive force and ATP synthesis via membrane-bound ATPases. HH, hydrazine hydrolase HZO, hydrazine oxidizing enzyme NIR, nitrite reductase. (Redrawn from van Niftrik etal., 2004 and Kuypers et al., 2006).

Thamdrup, B., and Dalsgaard, T. (2002). Production of N-2 through anaerobic ammonium oxidation coupled to nitrate reduction in marine sediments. Appl. Environ. Microbiol. 68, 1312-1318. Thamdrup, B., Dalsgaard, T., Jensen, M. M., UUoa, O., Farias, L., and Escribano, R. (2006). Anaerobic ammonium oxidation in the oxygen-deficient waters off northern Chile. Limnol. Oceanogr. 51, 2145-2156. 

Nauhaus K., Boetius A., Kruger M., and Widdel E. (2002) In vitro demonstration of anaerobic oxidation of methane coupled to sulfate reduction in sediment from a marine gas hydrate area. Environ. Microbiol. 4, 296—305. 

Three primary mechanisms have been offered to explain anaerobic CH4 oxidation in marine sediments. The process may be carried out by a single organism that couples methanotrophy to S04 reduction. Anaerobic methane oxidation coupled to sulfate reduction is thermodynamically... 

Alternative pathways for denitrification have been proposed that do not involve the classical enzyme systems, but their existence has only recently been confirmed or investigated in detail. In the absence of O2, a number of elements and compounds have the potential to oxidize NH , including Mn(II), Mn02, NO, NO2 and NO2. Because anaerobic oxidation of NH directly to N2 bypasses the multiple oxidation steps required for coupled nitrification-denitrification... 

Even more recent was the first report of anaerobic Ee(II) oxidation coupled to NOJ reduction (Straub et al., 1996 Hafenbradl et al, 1996) ... 

The conversion of glucose to lactic acid has an overall AG of — 52,000 cal/mole. In an anaerobic cell, this conversion is coupled to the synthesis of 2 moles of ATP per mole of glucose, (a) Calculate the AG of the overall coupled reaction. (b) Calculate the efficiency of energy conservation in the anaerobic cell, (c) At the same efficiency, how many moles of ATp per mole of glucose could be obtained in an aerobic organism in which glucose is completely oxidized to CO2 and H2O (AG = —686,000 cal/mole) (d) Calculate the AG for the overall oxidation coupled to ATP synthesis. 

Under anaerobic conditions, the trimer 21 was formed, presumably by an aldol reaction.37 Treatment with hydrochloric acid in ethanol exposed to the air gave the dimer. This had led to the suggestion that under aerobic conditions, the trimer is converted to phaeomelanic pigments, which may be formed by aldol-type condensation and not by oxidative coupling as previously proposed.38-40... 

The catalytic oxidation of toluene over metal oxides to benzaldehyde and benzoic acid are well-known industrial processes and minor amounts of coupling products are detected among the products. It is shown that the oxidative coupling of toluene is favoured under anaerobic conditions and that metal oxides of the groups III to V of the periodic table catalyse methyl-methyl coupling [1]. Pb/Li/MgO is chosen for this study because it is known as a selective catalyst for the oxidative methylation of toluene with methane [2-4] and as good benzyl radical producer [1]. 

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