Silver-catalyzed oxidative

There are many ways to produce acetaldehyde. Historically, it was produced either hy the silver-catalyzed oxidation or hy the chromium activated copper-catalyzed dehydrogenation of ethanol. Currently, acetaldehyde is obtained from ethylene hy using a homogeneous catalyst (Wacker catalyst). The catalyst allows the reaction to occur at much lower temperatures (typically 130°) than those used for the oxidation or the dehydrogenation of ethanol (approximately 500°C for the oxidation and 250°C for the dehydrogenation). 

Propylene Oxide. Unlike ethylene, propylene cannot be selectively transformed to propylene oxide by silver-catalyzed oxidation. Instead, indirect oxidations (the peracid and the hydroperoxide routes) are employed.912-915... 

Kestenbaum, H., Lange de Olivera, A., Schmidt, W., Schuth, F., Ehrfeld, W., Gebauer K., Lowe, H., Richter T., Silver-catalyzed oxidation of ethylene to ethylene oxide in a microreaction system, Ind. Eng. Chem. Res. 2000, 41, 710-719. 

Although the main routes to propylene oxide formation are not based on direct catalytic oxidation of propylene, the direct epox-idation of propylene on silver would be financially preferable if high yield and selectivity to propylene oxide could be achieved. Similarly to ethylene oxidation on silver part of the undesirable byproduct CO2 comes from the secondary oxidation of propylene oxide (2,3). The kinetics of the secondary silver catalyzed oxidation of propylene oxide to CO2 and H2O have been studied by very few investigators (2). 

A simple Langmuir-Hinshelwood model explains quantitatively the steady-state behavior (4) but it fails to explain the oscillatory phenomena that were observed. The origin of the limit cycles is not clear. Rate oscillations have not been reported previously for silver catalyzed oxidations. Oxidation of ethylene, propylene and ethylene oxide on the same silver surface and under the same temperature, space velocity and air-fuel ratio conditions did not give rise to oscillations. It thus appears that the oscillations are related specifically to the nature of chemisorbed propylene oxide. This is also supported by the lack of any correlation between the limits of oscillatory behavior and the surface oxygen activity as opposed to the isothermal oscillations of the platinum catalyzed ethylene oxidation where the SEP measurements showed that periodic phenomena occur only between specific values of the surface oxygen activity (6,9). 

Eigure 36. Silver-catalyzed oxidation of activated C—H bonds. 

Silver can mediate oxidation reactions and has shown unique reactivity. In a few cases, namely, nitrene-, carbene-, and silylene-transfer reactions, novel reactivity was found with homogeneous silver catalysts. Some of these reactions are uniquely facilitated by silver, never having been reported with other metals. While ligand-supported silver catalysts were extensively utilized in enantioselective syntheses as Lewis acids, disappointingly few cases were reported with oxidation reactions. Silver-catalyzed oxidation reactions are still underrepresented. Silver-based catalysts are cheaper and less toxic versus other precious metal catalysts. With the input of additional effort, this field will undoubtedly give more promising results. 

The nucleophilic radicals formed by the silver-catalyzed, oxidative decarboxylation of carboxylic acids by peroxydisulfate ions attack protonated imidazoles mainly at the 2-... 

The third reaction of the sequence is the silver-catalyzed oxidation of color developing agent which leads to dye formation by coupling as in normal color development. This process allows a lowering of the coated silver level but still produces a full-density color image (Figure 16). Typically the coated silver level is 10 % of that required for normal color development, which means that 90 % of the dye image is formed by the RX amplification reaction. 

Nickel and Liu [79] have studied the silver-catalyzed oxidation of A, A -dimethyl-/7-phenylenediamine by Co(NH3)5Cl3 by means of a silver colloid and a rotating silver electrode. They conclude that the reaction can be quantitatively explained by means of the theory of mixture potential and mixture current of Wagner and Traud... 

Imidazoles normally undergo free-radical reactions at the 2-position. For example, homolytic free-radical alkylation of histidines and histamines yields 2,3-disubstituted histidines and histamines. In these reactions, the free radical was generated via silver-catalyzed oxidative decarboxylation of acids with peroxydisulfate 433 (Scheme 103) <2001BML1133>. 

Radical substitution reactions include alkylations and arylations in the main. Nucleophilic radicals produced by the silver-catalyzed oxidative decarboxylation of carboxylic acids (by peroxydisulfate ion) attack proton-ated azoles at the most electron-deficient sites.Thus, imidazole and 1-alkylimidazoles are methylated exclusively at C-2 in rather low yields. The use of isopropyl and t-butyl radicals gives improved yields, but benzyl and acyl radicals tend to dimerize rather than substitute the... 

The classical oxidation example is that of SO2 to SO3, which is described in more detail below. The second class is the partial oxidation of hydrocarbons in which the desired product selectivity is significantly sensitive to both temperature and the oxygen concentration. More specifically, yield losses to carbon oxides, which result from sequential or parallel side reactions, undermine the goal of achieving high product selectivity at a reasonable hydrocarbon conversion per pass. Most hydrocarbon partial oxidation reactions have this feature. Two illustrative commercial processes include the silver-catalyzed oxidation of methanol to formaldehyde and the catalytic oxidation of monomethylforma-mide to methyl isocyanate. 

Important commercial reactions that are carried out in multistage adiabatic reactors include 1) the oxidation of SO2 to SO3 over a vanadium pentoxide catalyst, which is the key step in sulfuric acid manufacture, and 2) the silver-catalyzed oxidation of methanol to... 

The silver-catalyzed oxidation of ethylene was first reported in patents by Lefort in 1931 and 1935 [320,321] which, interestingly, also described the effectiveness of gold, iron, and copper promoters. These are elements that have been rediscovered in recent studies [322,323] (see Chapters 8 and 9). Due to the importance of the commercial process, which has seen phenomenal growth since 1940, the mechanism of the reaction has been investigated extensively. 

The regioselective C3-alkenylation of thiophene-2-carboxylic acids was achieved via rhodium/silver-catalyzed oxidative coupling, accompanied by decarboxylation (13JOC7216). The catalyst can also be used for ort/zo-alkenylation of benzoic acids. 

Silver can adsorb oxygen in different forms, for example, as atomic, molecular, and subsurface oxygen. Different theories exist about the reactivity of adsorbed oxygen in the silver-catalyzed oxidation of ethylene. In one theory, only molecular oxygen reacts with ethylene, in another theory only atomic oxygen is converted with ethylene. In this example, the reaction via molecular oxygen is presented in detail. 

Nagy, A.J., Mestl, G., and Schlogl, R. The role of subsurface oxygen in the silver-catalyzed, oxidative coupling of methane. J. Catal 1999,188, 58-68. 

Scheme 40 Silver-catalyzed oxidative cyclization of olefinic acid to S-lactones...

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