Heterogeneous silver catalysis

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. 

Barbosa et al. [41] studied the heterogeneous catalysis of this reaction in the classical solvent of 80 vol.% (55.2 mol.%) EtOH + H20 at 25°C, using a pH-stat technique to measure the rate. Silver, silver bromide, silver sulphide, mercury(I) bromide, and mercury(II) sulphide were all found to be good catalysts (cf. Fig. 12). With AgBr the catalytic rate was not affected by... 

Barteau, M.A. Madix, R. J. The Surface Reactivity of Silver Oxidation Reactions, The Chemical Physics of Solid and Heterogeneous Catalysis, ESPC, vol. 4, 1982, pp. 96-142. 

Isolation and identification of surface-bonded acetone enolate on Ni(l 11) surfaces show that metal enolate complexes are key intermediates in carbon-carbon bond-forming reactions in both organometaUic chemistry and heterogeneous catalysis. Based on studies on powdered samples of defined surface structure and composition, most of the results were reported for acetone condensation over transition-metal oxide catalysts, as surface intermediate in industrially important processes. With the exception of a preoxidized silver surface, all other metal single-crystal surfaces have suggested that the main adsorption occurs via oxygen lone-pair electrons or di-a bonding of both the carbonyl C and O atoms. 

A different approach to control the stereoselectivity of glycosylations is through the use of heterogeneous catalysis. Catalysts such as silver silicate were developed for this purpose [46]. Reactions of glycosyl halides on the surface of silver silicate are thought to proceed by a concerted mechanism providing, for example, (3-D-mannopyranosides 44 from a-D-mannosyl bromide 43 (Scheme 4.7). 

Coadsorption phenomena in heterogeneous catalysis and surface chemistry quite commonly consider competitive effects between two reactants on a metal surface [240,344]. Also cooperative mutual interaction in the adsorption behavior of two molecules has been reported [240]. Recently, this latter phenomenon was found to be very pronounced on small gas-phase metal cluster ions too [351-354]. This is mainly due to the fact that the metal cluster reactivity is often strongly charge state dependent and that an adsorbed molecule can effectively influence the metal cluster electronic structure by, e.g., charge transfer effects. This changed electronic complex structure in turn might foster (or also inhibit) adsorption and reaction of further reactant molecules that would otherwise not be possible. An example of cooperative adsorption effects on small free silver cluster ions identified in an ion trap experiment will be presented in the following. 

In the book, the section on homogeneous catalysis covers soft Pt(II) Lewis acid catalysts, methyltrioxorhenium, polyoxometallates, oxaziridinium salts, and N-hydroxyphthalimide. The section on heterogeneous catalysis describes supported silver and gold catalysts, as well as heterogenized Ti catalysts, and polymer-supported metal complexes. The section on phase-transfer catalysis describes several new approaches to the utilization of polyoxometallates. The section on biomimetic catalysis covers nonheme Fe catalysts and a theoretical description of the mechanism on porphyrins. 

Key Words Ethylene oxide, Ethylene, Epoxidation, Silver, Cl promotion, Cs promotion. Promotion, Selectivity, Oxametallacycle, Adsorption, Desorption, Chemisorption, Activation energy, Ag-O bond. Reaction mechanism, Oxidation, Cyclisation, Heterogeneous catalysis, Selective oxidation, Eletrophilic oxygen. Nucleophilic oxygen. Subsurface O atoms, Ag/a-A Oj catalyst. 2008 Elsevier B.V. 

Heterogeneous catalysis is activated when the catalyst slides against itself or other materials, e.g. ceramics. Oxidation reactions of hydrogen, carbon monoxide and methane were demonstrated as being enhanced by rubbing platinum, palladium and silver, respectively [29-31], and the reduction of carbon dioxide is enhanced by the rubbing of iron oxide [32],... 

Silver(i) is another catalyst for aquation. Unfortunately, a kinetic study of its catalytic effect on aquation of [CoI(NH8)5l + is complicated by heterogeneous catalysis by the silver iodide produced. Indeed this topic of heterogeneous catalysis of aquation of cobalt(iii) complexes has recently been reviewed and extended by studies of the kinetic effects of a range of diverse solids, e.g. mercury(ii) sulphide, silver bromide, and platinum metal. ... 

Homogeneous and heterogeneous catalysis of bismethoxycarbonylcarbene addition to olefins and the copper- and silver-catalysed decompositions of dimethyl diazomalonate all result in stereospecific cyclopropanation, although three-membered ring formation is accompained by significant amounts of allyic C—H insertions pro-... 

A few years ago the kinetics of heterogeneous catalysis of the aquation of the [Co(NHa)5Br] + cation were investigated - some of the catalysts employed [e.g. mercury(n) and silver(i) compounds] were closely related to the metal-ion catalysts discussed earlier in this section. A recent reinvestigation of some of these systems shows that part of the catalytic effect of these solids can be ascribed to redox processes operating in parallel to simple catalysed aquations. ... 

The search for a new epoxidation method that would be appropriate for organic synthesis should also, preferably, opt for a catalytic process. Industry has shown the way. It resorts to catalysis for epoxidations of olefins into key intermediates, such as ethylene oxide and propylene oxide. The former is prepared from ethylene and dioxygen with silver oxide supported on alumina as the catalyst, at 270°C (15-16). The latter is prepared from propylene and an alkyl hydroperoxide, with homogeneous catalysis by molybdenum comp e ts( 17) or better (with respect both to conversion and to selectivity) with an heterogeneous Ti(IV) catalyst (18), Mixtures of ethylene and propylene can be epoxidized too (19) by ten-butylhydroperoxide (20) (hereafter referred to as TBHP). 

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