Oxidation reaction cycle

The only step in the overall oxidation reaction cycle which is endothermic is step 2, which involves the direct insertion of oxygen into the C-H bond of benzene. This is costly since it requires the loss of aromaticity in the benzene ring. All other steps in the cycle are exothermic. Furthermore, matrix effects are absent in this reaction. The main role of the lattice appears to be to stabilize Fe + and prevent over-oxidation of N2O decomposing Fe " " oxyhydroxy dicationic clusters. The overall result is that the rate-limiting step for phenol formation is the rate of desorption of phenol. The relative concentration of the different sites varies with Fe loading, as illustrated in Fig. 4.29b. Whereas the rate of phenol formation increases steeply with the Fe content, when the Fe concentration is low, at higher Fe content N2O decomposition increases, but phenol production is constant. 

Fig. 5.3. Schematic illustration of the auto-oxidation reaction cycle. The cycle can be interrupted by radical scavengers reacting with peroxy radicals and by hydroperoxide decomposers forming inert reaction products with hydroperoxides...

Process Safety Considerations. Unit optimization studies combined with dynamic simulations of the process may identify operating conditions that are unsafe regarding fire safety, equipment damage potential, and operating sensitivity. Several instances of fires and deflagrations in ethylene oxide production units have been reported in the past (160). These incidents have occurred in both the reaction cycle and ethylene oxide refining areas. Therefore, ethylene oxide units should always be designed to prevent the formation of explosive gas mixtures. 

The oxidation of HC and CO must proceed in balance with the reduction of NO by CO, HC, or H2. For the NO removal reaction, a reductant is required. First NO is adsorbed on the catalyst surface and dissociates forming N2 which leaves the surface, but the O atoms remain. CO is required to remove the O atoms to complete the reaction cycle (53). 

Lipoic acid exists as a mixture of two structures a closed-ring disulfide form and an open-chain reduced form (Figure 18.33). Oxidation-reduction cycles interconvert these two species. As is the case for biotin, lipoic acid does not often occur free in nature, but rather is covalently attached in amide linkage with lysine residues on enzymes. The enzyme that catalyzes the formation of the lipoamide nk.2Lg c requires ATP and produces lipoamide-enzyme conjugates, AMP, and pyrophosphate as products of the reaction. 

The net reaction accomplished by the TCA cycle, as follows, shows two molecules of COg, one ATP, and four reduced coenzymes produced per acetate group oxidized. The cycle is exergonic, with a net AG° for one pass around the cycle of approximately —40 kj/mol. Table 20.1 compares the AG° values for the individual reactions with the overall AG° for the net reaction. 

The third reaction of this cycle is the oxidation of the hydroxyl group at the /3-position to produce a /3-ketoacyl-CoA derivative. This second oxidation reaction is catalyzed by L-hydroxyacyl-CoA dehydrogenase, an enzyme that requires NAD as a coenzyme. NADH produced in this reaction represents metabolic energy. Each NADH produced in mitochondria by this reaction drives the synthesis of 2.5 molecules of ATP in the electron transport pathway. L-Hydroxyacyl-... 

Metals are most active when they first deposit on the catalyst. With time, they lose their initial effectiveness through continuous oxidation-reduction cycles. On average, about one third of the nickel on the equilibrium catalyst will have the activity to promote dehydrogenation reactions. 

When the Plutonium Project was established early in 1942, for the purpose of producing plutonium via the nuclear chain reaction in uranium in sufficient quantities for its use as a nuclear explosive, we were given the challenge of developing a chemical method for separating and isolating it from the uranium and fission products. We had already conceived the principle of the oxidation-reduction cycle, which became the basis for such a separations process. This principle applied to any process involving the use of a substance which carried plutonium in one of its oxidation states but not in another. By use of this... 

Transition metal complexes that are easy to handle and store are usually used for the reaction. The catalytically active species such as Pd(0) and Ni(0) can be generated in situ to enter the reaction cycle. The oxidative addition of aryl-alkenyl halides can occur to these species to generate Pd(II) or Ni(II) complexes. The relative reactivity for aryl-alkenyl halides is RI > ROTf > RBr > RC1 (R = aryl-alkenyl group). Electron-deficient substrates undergo oxidative addition more readily than those electron-rich ones because this step involves the oxidation of the metal and reduction of the organic aryl-alkenyl halides. Usually... 

As an introductory example we take one of the key reactions in cleaning automotive exhaust, the catalytic oxidation of CO on the surface of noble metals such as platinum, palladium and rhodium. To describe the process, we will assume that the metal surface consists of active sites, denoted as We define them properly later on. The catalytic reaction cycle begins with the adsorption of CO and O2 on the surface of platinum, whereby the O2 molecule dissociates into two O atoms (X indicates that the atom or molecule is adsorbed on the surface, i.e. bound to the site ) ... 

Figure I.S. Reaction cycle and potential energy diagram for the catalytic oxidation of CO by O2.

The initiation step in the chain oxidation, reaction (43), is not affected by the presence of oxygen. SO4 radicals formed in (43) give arsenic(fV), reaction (45), initiating the following propagation cycle which leads to the reformation of As(IV)... 

For the majority of redox enzymes, nicotinamide adenine dinucleotide [NAD(H)j and its respective phosphate [NADP(H)] are required. These cofactors are prohibitively expensive if used in stoichiometric amounts. Since it is only the oxidation state of the cofactor that changes during the reaction, it may be regenerated in situ by using a second redox reaction to allow it to re-enter the reaction cycle. Usually in the heterotrophic organism-catalyzed reduction, formate, glucose, and simple alcohols such as ethanol and 2-propanol are used to transform the... 

Iron centers that are more electron-deficient than iron(III) compounds are used for efficient and highly specific oxidation reactions in, for example, heme and nonheme enzymes [166-172]. Most iron(IV)-complexes found in biological reaction cycles possess terminal or bridging 0x0 groups as is known from a large number of structural and spectroscopic investigations. With the exception of iron(IV)-nitrido groups, nonoxo iron(IV) centers very rarely take part in such reactions. 

Physical studies of the hydroxylase have established the structural nature of the diiron core in its three oxidation states, Hox, Hmv, and Hred. Although the active site structures of hydroxylase from M. tri-chosporium OB3b and M. capsulatus (Bath) are similar, some important differences are observed for other features of the two MMO systems. The interactions with the other components, protein B and reductase, vary substantially. More structural information is necessary to understand how each of the components affects the others with respect to its physical properties and role in the hydroxylation mechanism and to reconcile the different properties seen in the two MMO systems. The kinetic behavior of intermediates in the hydroxylation reaction cycle and the physical parameters of intermediate Q appear similar. The reaction of Q with substrate, however, varies. The participation of radical intermediates is better established with the M. triehosporium... 

Based upon the activity and electrochemical experimental results the 5%Pt,l%Bi/C catalyst was chosen for further detailed evaluation. For the catalyst to be effective in industrial applications it is desirable that it should remain active for a number of reaction cycles. The recycle capability of 5%Pt,l%Bi/C was evaluated under realistic conditions for a number of selective oxidation reactions, see Table 3. 

Table 3 indicates that 5%Pt,l%Bi/C is active for three reaction cycles in the selective oxidation of the chosen alcohols. For primary alcohols the use of water as solvent can promote the aldehyde to carboxylic acid reaction (3). This effect is observed in the selective oxidation of 1-octanol where octanoic acid is formed with 97% selectivity in the first cycle dropping to 81% in the third. In the selective oxidation of geraniol only citral is observed as the oxidation product. The presence of the double bond stabilises the aldehyde even in the presence of... 

It has been found (Polyakov et al. 2001c) that when carotenoids are involved in a reaction cycle with the participation of iron as Fe2+, an increase of the total radical yield or a prooxidant effect will occur and will increase with decreasing carotenoid oxidation potential and its scavenging activity. The mechanism of the participating carotenoid is shown in Scheme 9.4 (Polyakov et al. 2001c). 

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