Silver homogeneous reactions

Eranen, K., Lindfors, L.E., Klingdted, F. et al. (2003) Continuous reduction of NO with octane over a silver/alumina catalyst in oxygen-rich exhaust gases combined heterogeneous and surface-mediated homogeneous reactions, J. Catal. 219, 25. 

The reaction of alkyl halides with silver nitrate constitutes an extremely useful method for the synthesis of high purity nitrate esters on a laboratory scale. ° The driving force for these reactions is the formation of the insoluble silver halide. Reactions have been conducted under homogenous and heterogeneous conditions. For the latter a solution of the alkyl halide in an inert solvent like benzene or ether is stirred with finely powdered silver nitrate. However, this method has been outdated and reactions are now commonly conducted under homogeneous conditions using acetonitrile as solvent. 

Some physical development may result from a selective deposition upon the latent image nuclei of silver formed in the homogeneous reaction between silver ions and developing agent. Under the usual conditions of development, however, catalysis of the actual reduction of silver ions is the important factor. 

In addition to water, a variety of organic liquids, including amines, carboxylic acids, and hydrocarbons, have been used as solvents in the study of the homogeneous reactions of hydrogen with metal salts. In general, there is more uncertainty about the nature of the species present in such systems than in aqueous solution and, correspondingly, it is usually more difficult to elucidate the reaction mechanisms in detail. The most extensive solvent effect studies have been made on cupric, cuprous, and silver salts. A number of the more important results are considered below. 

Another modification employs silver benzoate catalyst in a homogeneous reaction medium containing the alcohol and triethylamine. ... 

It is believed that the Kp data for the three homogeneous reactions (A, B, and C) are more reliable than those for the heterogeneous reaction (D) due to the need for a machine calibration constant which appears in the equilibrium expression for the latter. Ehlert et al. (2) determined this constant from vaporization experiments performed with silver contained in their Knudsen cell. Further support for this belief is provided by the large positive drift that arises in the 3rd law analysis of these Kp s. Also, it Is felt that the results otained from the flame-spectrophotometric studies (4, are somewhat more... 

The oxidation and adsorption of ethylene oxide on a silver catalyst is stronger in the presence of acetaldehyde, and the former hinders the formation of carbon dioxide from acetaldehyde. The latter fact is probably due to interaction between acetaldehyde and ethylene oxide at the surface. A conjugated oxidation of this kind is often encountered with homogeneous reactions. 

Epoxidation of ethylene by dioxygen takes place on heterogeneous silver catalysts at elevated temparatures [99-101]. There is one unconfirmed claim that a homogeneous reaction can be carried out [102]. 

Defect relaxation times for homogeneous reactions in solids can be calculated essentially by the methods of homogeneous chemical kinetics [2, 3]. For the sake of illustration, let us consider more closely the equilibration of Frenkel defects in silver bromide following a sudden change in temperature. 

By introducing numerical values for silver bromide into eq. (6-5), we find that even at room temperature the relaxation time for the equilibration of Frenkel defects is of the order of milliseconds and less. Because of the paucity of available data [9], only such rough estimates can be made. Furthermore, it should be remembered that these estimates are dependent upon the assumption that the defect reaction is, in fact, a homogeneous reaction. 

Hashimoto found that low-temperature activation of mannosyl chlorides with silver triflate in the presence of alcohols leads to varying ratios of a- and P-linked mannosides [45, 126, 127]. A study of the factors controlling stereoselectivity in silver triflate-mediated mannosylation has been published [128]. Activation of O-benzyl-protected mannosyl fluorides by a combination of Sn(OTf)2 and La(C104)3 has been reported by Shibasaki to afford mixtures of a- and P-mannosides [129]. The large proportions of a-mannosides formed in the homogeneous reactions outlined in this section indicate the involvement of oxocarbonium-ions enabling the anomeric effect to dictate the steric outcome. Because the stereoselectivity of these approaches is crucially dependent on the reacting partners and conditions their scope remains limited. 

In both reactions it is evident that AgOH+ is a more reactive species toward the substrates than Ag + however, both species may be reacting via direct electron transfer. Pathways in which Ag might be considered a reactive species are excluded since a second-order dependence on [Ag ] and an inverse first-order dependence on [Ag ] would be predicted in this situation. The use of silver(n) as a reagent for oxidative cleavage of hydroquinone ethers has been reported. In the apparently homogeneous reaction between Ag and chloro-amines, in the presence of base, the rate is probably promoted by a trace of silver metal and is sensitive to oxygen. 

Propylene oxide is also produced in Hquid-phase homogeneous oxidation reactions using various molybdenum-containing catalysts (209,210), cuprous oxide (211), rhenium compounds (212), or an organomonovalent gold(I) complex (213). Whereas gas-phase oxidation of propylene on silver catalysts results primarily in propylene oxide, water, and carbon dioxide as products, the Hquid-phase oxidation of propylene results in an array of oxidation products, such as propylene oxide, acrolein, propylene glycol, acetone, acetaldehyde, and others. 

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). 

The cyclopropanation of 1-trimethylsilyloxycyclohexene in the present procedure is accomplished by reaction with diiodomethane and diethylzinc in ethyl ether." This modification of the usual Simmons-Smith reaction in which diiodomethane and activated zinc are used has the advantage of being homogeneous and is often more effective for the cyclopropanation of olefins such as enol ethers which polymerize readily. However, in the case of trimethylsilyl enol ethers, the heterogeneous procedures with either zinc-copper couple or zinc-silver couple are also successful. Attempts by the checkers to carry out Part B in benzene or toluene at reflux instead of ethyl ether afforded the trimethylsilyl ether of 2-methylenecyclohexanol, evidently owing to zinc iodide-catalyzed isomerization of the initially formed cyclopropyl ether. The preparation of l-trimethylsilyloxybicyclo[4.1.0]heptane by cyclopropanation with diethylzinc and chloroiodomethane in the presence of oxygen has been reported. "... 

A specific example where heterogeneous supports provide nanoparticle size-control is the immobilization of homogeneous silver nanoparticles on polystyrene [366]. This work was extended later to the development of a one-pot method for the size-selective precipitation of silver nanoparticles on PVP-protected thiol-functionalized silica. During the immobilization of very small silver nanoclusters both the size of the silver nanoclusters and the thickness of the silver layer on the support could be controlled directly by the reaction parameters applied (Fi re 16) [367]. 

The main issue of the book is application of nanosized particles in both homogeneous and heterogeneous catalysis. A variety of reactions catalyzed by metal colloids or supported nanosized metals is discussed. The most intriguing reaction seems to be ethane hydrogenolysis catalyzed by Pt clusters on porous carrier and studied by G. A. Somorjai and his group. Another challenging observation by this group is shape isomerization of Pt metal particles affected by the addition of silver ions. 

Similarly, a catalytic route to indigo was developed by Mitsui Toatsu Chemicals (Inoue et al, 1994) to replace the traditional process, which dates back to the nineteenth century (see earlier), and has a low atom efficiency/high E factor (Fig. 2.15). Indole is prepared by vapour-phase reaction of ethylene glycol with aniline in the presence of a supported silver catalyst. The indole is selectively oxidised to indigo with an alkyl hydroperoxide in the presence of a homogeneous molybdenum catalyst. 

ZnO (suspension) sensitizes the photoreduction of Ag" by xanthene dyes such as uranin and rhodamine B. In this reaction, ZnO plays the role of a medium to facilitate the efficient electron transfer from excited dye molecules to Ag" adsortei on the surface. The electron is transferred into the conduction band of ZnO and from there it reacts with Ag. In homogeneous solution, the transfer of an electron from the excited dye has little driving force as the potential of the Ag /Ag system is —1.8 V (Sect. 2.3). It seems that sufficient binding energy of the silver atom formed is available in the reduction of adsorbed Ag" ions, i.e. the redox potential of the silver couple is more positive under these circumstances. 

---