Silver oxide-supported metal catalysts

In contrast, recent work (4-12) has shown that Raman spectroscopy can be used to study Ti) adsorption on oxides, oxide supported metals and on bulk metals [including an unusual effect sometimes termed "enhanced Raman scattering" wherein signals of the order of 10 - 106 more intense than anticipated have been reported for certain molecules adsorbed on silver], (ii) catalytic processes on zeolites, and (iii) the surface properties of supported molybdenum oxide desulfurization catalysts. Further, the technique is unique in its ability to obtain vibrational data for adsorbed species at the water-solid interface. It is to these topics that we will turn our attention. We will mainly confine our discussion to work since 1977 (including unpublished work from our laboratory) because two early reviews (13,14) have covered work before 1974 and two short recent reviews have discussed work up to 1977 (15,16). 

The key to the success of the oxidation examples cited above is the ability of the catalysts used to exert proper kinetic control on the possible side reactions. Without it, thermodynamically favorable but undesired products such as CO2 and H2O are made instead. Controlling oxidation kinetics to stop at the desired oxygenated products is quite difficult, and has yet to be solved for many other systems. For instance, although many attempts have been made to develop a commercial process for the oxidation of propylene to propylene oxide, both the activity and the selectivity of the systems proposed to date, mostly based on silver catalysts, are still too low to be of industrial interest " propylene oxide is presently manufactured by processes based on chlorohydrin or hydrogen peroxide instead. In spite of these difficulties, though, recent advances in selective liquid phase oxidation of fine chemicals on supported metal catalysts have shown some promise, offering high yields (close to 100%) under mild reaction conditions." ... 

Supported catalysts in which a metal oxide is supported on a different metal oxide find tremendous use in commercial chemical production, petroleum refining, and environmental remediation [7], Table 2.1 lists a number of important industrial reactions that are catalyzed by supported metal oxides. The active phase consists of single metal oxides, metal oxide mixtures, or complex metal oxides. Some supported metal catalysts can, technically, be considered supported metal oxide catalysts if the metal is partially oxidized under reaction conditions, such as alumina-supported silver epoxidation catalysts and palladium-on-stabilized alumina combustion catalysts. Clearly, supported metal oxide compositions are diverse, especially considering that the range of relative amounts of the active phase to the support is large, and that additional metal oxides and modifiers can be introduced. 

Catalyst Selectivity. Selectivity is the property of a catalyst that determines what fraction of a reactant will be converted to a particular product under specified conditions. A catalyst designer must find ways to obtain optimum selectivity from any particular catalyst. For example, in the oxidation of ethylene to ethylene oxide over metallic silver supported on alumina, ethylene is converted both to ethylene oxide and to carbon dioxide and water. In addition, some of the ethylene oxide formed is lost to complete oxidation to carbon dioxide and water. The selectivity to ethylene oxide in this example is defined as the molar fraction of the ethylene converted to ethylene oxide as opposed to carbon dioxide. 

The reaction is carried out over a supported metallic silver catalyst at 250—300°C and 1—2 MPa (10—20 bar). A few parts per million (ppm) of 1,2-dichloroethane are added to the ethylene to inhibit further oxidation to carbon dioxide and water. This results ia chlorine generation, which deactivates the surface of the catalyst. Chem Systems of the United States has developed a process that produces ethylene glycol monoacetate as an iatermediate, which on thermal decomposition yields ethylene oxide [75-21-8]. 

Despite the existing uncertainty about the exact nature of the oxygen species present during reaction several different approaches have been reported which lead to an increase in the selectivity to ethylene oxide. These include alloying silver with other metals (14) and using different catalyst supports and various promoters (6,15). It is also known and industrially proven that the addition of few PPM of chlorinated hydrocarbon "moderators" to the gas feed improves the selectivity to ethylene oxide but decreases the catalyst activity (15). It has also been found recently by Carberry et al that selectivity increases with y-ir-radiation of the catalyst (16). 

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. 

Liquid-phase selective oxidations are normally catalysed homogeneously. A small but significant interest has recently arisen in the use of solid catalysts for liquid-phase oxidation, particularly of alkyl aromatics. Shalya et al. have compared the activity of copper, silver, and gold metals as catalysts for cumene oxidation (Table 2). Silver was found to combine good selectivity for the desired product, cumene hydroperoxide, with an activity similar to that of copper. With supported catalysts, silver is considerably more active than copper, while gold is totally inactive. 

Catalysts of 10 per cent copper-90 per cent silver, and silver gauze were found to be most effective with ethanol oxidation 50 per cent copper-50 per cent silver and 99 per cent silver-1 per cent bismuth were most effective with isopropanol copper wire and silver gauze were most effective with butanol.00 Yields of better than 70 per cent of aldehyde or ketone were obtained under the conditions of operation. The catalyst was supported in an externally heated tube half a meter long and seven millimeters iuside diameter. The air rate was 83 liters (standard conditions) of air per hour and the alcohol rate very nearly 0.8 mols per hour, a ratio of air to alcohol of very nearly mol per mol. After reaction had begun the heat of reaction was sufficient in most cases to maintain the catalyst at red heat. The conversion in each case may be made more efficient by allowing a smaller conversion of alcohol per pass over the catalyst. With these metal catalysts, three times as much ethanol and twice as much isopropanol break-down to carbon dioxide as butanol. Butanol, however, produced a larger amount of unsaturated hydrocarbons than either of the two other alcohols. The ratio of break-down to carbon dioxide is independent of the kind of metal catalyst and even of the oxide mixtures that remained hot. 

To optimize the HP decomposition, different catalysts have been prepared on monolith supports. Two active phases were selected metallic silver and manganese oxide. The monolithic catalysts have been eharacterized and evaluated in order to assess their decomposition efficiency. Properties and preliminary performances of the catalysts are presented. 

Metal clusters in zeolites an intriguing class of catalysts Zeolite-supported transition metal catalysts Stoichiometric and catalytic reactivity of organometallic fragments supported on inorganic oxides Silver clusters and chemistry in zeolites Structure and reactivity of surface species obtained by interaction of organometallic compounds with oxidic surfaces infra-red studies... 

In the kinetic models discussed so far, the catalyst surface was assumed to be uniform and to have constant activity. In practice, catalysts often show a decline in activity with time, because of poisoning or fouling these changes are discussed later. Catalysts may also change activity because of reactions that alter the chemical composition of the surface. One example of this is the supported silver catalyst used for the partial oxidation of ethylene. Kinetic data suggest that the active catalyst is a partial layer of silver oxide and not metallic silver. But a modified form of the Langmuir-Hinshelwood model can still be used to correlate the data, as shown in the following example. 

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