Silver methanation activity

Catalysts. The methanation of CO and C02 is catalyzed by metals of Group VIII, by molybdenum (Group VI), and by silver (Group I). These catalysts were identified by Fischer, Tropsch, and Dilthey (18) who studied the methanation properties of various metals at temperatures up to 800°C. They found that methanation activity varied with the metal as follows ruthenium > iridium > rhodium > nickel > cobalt > osmium > platinum > iron > molybdenum > palladium > silver. 

The same catalyst compositions used in the more important methane steam reforming [Eq. (3.1), forward reaction], may be used in methanation, too.222 All Group VIE metals, and molybdenum and silver exhibit methanation activity. Ruthenium is the most active but not very selective since it is a good Fischer-Tropsch catalyst as well. The most widely used metal is nickel usually supported on alumina or in the form of alloys272,276,277 operating in the temperature range of 300-400°C. 

A mild procedure for the appendage of MOM groups to acid-sensitive substrates is illustrated by the protection of the allylic alcohol in Avermectin derivative 259.1 using [(methoxymethyl)thio]-2-pyridine (259.2), silver(I) triflate and sodium acetate in THF at room temperature [Scheme 4.259]. Primary, secondary and tertiary alcohols and phenols are methoxymethylated in good yield though phenols are slower to react. Reagent 259.2 (bp 66 C/0.088 kPa) is easily prepared in 75% yield by the reaction of pyridine-2-thiol with dimethoxy-methane activated by trifluoroborane etherate. 

A silver-ion-exchanged HZSM-5 zeolite sample exhibited prominent catalytic performance in the partial oxidation of CH at temperatures above 573 K, exceeding by far the activity of nonzeolitic Ag/SiO -Al Oj and Ag/SiO catalysts. The spectroscopic studies showed that the simultaneous existence of Ag+ ions and small clusters of Ag particles leads to the partial oxidation of methane. The authors believe that dioxygen activated on small Ag metal clusters elaborates a surface oxide layer and the thus-formed species is decomposed and the oxygen activated in this way on the Ag metal spills over and reacts with methane activated hy the Ag+ ions. This catalyst may be promising in the conversion of natural gas into higher value-added chemicals and fuels. 

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

Interaction of chlorine with methane is explosive at ambient temperature over yellow mercury oxide [1], and mixtures containing above 20 vol% of chlorine are explosive [2], Mixtures of acetylene and chlorine may explode on initiation by sunlight, other UV source, or high temperatures, sometimes very violently [3], Mixtures with ethylene explode on initiation by sunlight, etc., or over mercury, mercury oxide or silver oxide at ambient temperature, or over lead oxide at 100°C [1,4], Interaction with ethane over activated carbon at 350°C has caused explosions, but added carbon dioxide reduces the risk [5], Accidental introduction of gasoline into a cylinder of liquid chlorine caused a slow exothermic reaction which accelerated to detonation. This effect was verified [6], Injection of liquid chlorine into a naphtha-sodium hydroxide mixture (to generate hypochlorite in situ) caused a violent explosion. Several other incidents involving violent reactions of saturated hydrocarbons with chlorine were noted [7],... 

Hibernia A process for making formaldehyde by the partial oxidation of methane by ozonized oxygen. The catalyst is barium peroxide activated with silver oxide. Developed in Germany during World War II but not commercialized. 

Silver-alumina type catalysts are by far the most widely used, especially since they are the main catalytic source in the epoxidation of ethylene. Therefore, they are readily available and already have undergone extensive studies. Many systems have sought to utilize the presence of NO (another harmful environmental species) in gas feeds. In this case, the NO species would be reduced to N2, causing oxidation of the hydrocarbon with the support of the catalyst. Studies have helped to elucidate the active species on the catalyst surface at varying temperatures and species leading to the desired products (31). Results from a recent study point to the active silver species being a [Ag O Al] bound intermediate that leads to N2 formation (32). If the silver is present in nanoparticle form, it is simply believed to be a spectator. Other work showed mixed results on the benefit of silver-based alumina systems for the oxidation of methane and higher hydrocarbons. The effect is dependent on the type of reactor system prepared (33,34). 

In the case of silver-modified manganese systems, recent studies agree that the addition of silver increases the activity of methane oxidation, both in the case of Ag-Mn composite catalysts and Ag modified Mn02 catalysts (35,36). Silver-manganese-lanthanide oxide catalyst systems also were shown to be highly active, and recent studies suggested the reasons for this high activity (37). 

Helferich reported that mercuric cyanide is sometimes superior to silver carbonate as base in the synthesis of glycosides and disaccharides. Thus the condensation of tetra-O-benzoylbromoglucose (1 equiv.) with benzyl alcohol (1 equiv.) in nitro-methane in the presence of mercuric cyanide (1 equiv.) gives tetra-O-benzoylbenzyl-/3-D-glucose in 90% yield. Zorbach used this general procedure, except that 1,2-dichloroethane was used as the solvent, in a successful synthesis of the cardiac-active... 

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],... 

The introduction of the catalyst presents one of the main problems in using MSRs for heterogeneously catalyzed reactions. There are some examples of reactors that are constructed directly from the catalytically active material. Kestenbaum et al. [145] used silver foils for the construction of a microchannel reactor for the partial oxidation of ethene to oxirane. A similar concept was proposed by Fichtner et al. [91,146], These authors used a microstructured rhodium catalyst for the partial oxidation of methane to syngas. This reaction can be considered as a coupling of the exothermic oxidation and the endothermic reforming of methane, which occur at different reaction rates. In such a case, the formation of a pronounced axial temperature profile can be avoided through the use of a material with high thermal conductivity. The reactor... 

A computational study of the activation of alkane (methane, ethane, propane, and butane) C-H bonds by silver carbene homoscor-pionate [Ag=C(H)(C02CH3)(Tp)] and [Ag=C(H)(C02CH3)-(TpBr3)], anal0g0us tx> that reported for copper,549 has been performed with DFT Becke3LYP calculations. 

AgNaZSM-5 catalysts were investigated for the selectively catalytic reduction of NO by methane in the excess of oxygen. It was clearly depicted that the conversion rate of NO to N2 had a linear dependence on the silver loading (4.32 13.64%), which indicated that all silver species in the zeolite were active for the CH4-SCR reaction. The presence of excessive oxygen in the feed gas favored the CH4-SCR reaction. The temperature programmed desorption profiles in He and 2%CH4/He after the co-adsorption of NO and O2 revealed that surface nitrates were formed on silver catalyst, and could be effectively reduced by methane... 

A correlation was observed between the amount of silver on the outer surface and thecatalyticactivityinNOconversiontoN2,upto 10% loading. Fig. 16.2. The optimal size for the clusters on the external surface is such that active intermediate NO3 species are stabihsed at temperatures where CH4 is activated on Ag cations present in the pore system, but before methane combustion on metal clusters takes place. 

Using a zeolite support rather than a non-microporous soUd like alumina can be very fruitful, and the same silver nanoclusters on alumina and H-MFI have very different activity. On zeolite, they catalyse NOx SCR by methane, but they lead to combustion of methane at temperatures as low as 300 °C [28]. 

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