Sulfur site-selective

The structural effects can also be observed experimentally. Again we illustrate it for the methane activation over Ni surfaces. Figure 4.16 shows that the stepped Ni(211) surface has a considerably lower barrier for dissociation than the flat (111) surface. This has been confirmed in an elegant set of experiments [59]. It turns out that sulfur atoms preferentially adsorb at steps. In this way sulfur can be used to titrate the step sites, and since it also increases the barrier for methane activation considerably (see Figure 4.16), it can be used to block the step sites selectively. Figure 4.18 shows experimental data for the carbon uptake of a Ni(14 13 13) surface... 

The reduction of functional groups with sulfurated horohydrides has been reviewed by Lalancette et al. The reagent is particularly useful for selective reductions. Thus selective reduction of a nitro, an oxime, or a nitrile group in the presence of an ester group is possible. In the case of steroidal ketones, the carbonyl group at C is the most reactive site selective reduction of a C -carbonyl group in the presence of other carbonyls at C, I, C,2, C,7, or Cjo is possible. The alcohol obtained is the equatorial isomer. Aldehydes can be reduced selectively in the presence of a carhonyl group if the molar ratio of NaBHjS, is kept at a suitable value. 

Reaction of ketene dithioacetals 260 with thioamides gave 5-substituted 2-methyl(phenyl)-6-methylthio-4-thioxopyrimidine (262) (92H1573). The reaction mechanism involves the addition of the thioamide to the ketene dithioacetal 260 to afford 2-cyano-3-methylthio-3-thioamido-propenoni-trile or -propenoate intermediate, which cyclizes in situ by site-selective nucleophilic attack of the sulfur on the cyano group to give 6-imino-l,3-thiazine 261, which rearranged to 262 (Scheme 84). 

Regioselective (site-selective) functionalization of unsaturated haloge-nated nitrogen, oxygen, and sulfur heterocycles by Pd-catalyzed cross-couplings and direct arylation processes 07CSR1036. 

The sulfur-containing receptors have little affinity for hard cations but can form extractable host-guest complexes with soft cations such as Ag+, Hg +, T1+, Pd +, and Pt +. Silver is found to interact strongly with sulfur [138, 139]. It was even possible to isolate 1 1 host-guest silver and mercury complexes in a large sulfur-containing macrocycle a linear HgC molecule is hosted in the cavity, but in the 1 2 complex a second mercury atom is externally attached to the ruthenium atom [140-142]. Palladium and platinum are also selectively complexed by polythia-coronands [143-148]. Some X-ray diffraction studies on palladium [147, 148] and platinum [143, 145] complexes established that the coordination of the metal occurs by interaction with the sulfur sites. 

The following example of hydrocarbon catalysis illustrates the consequences of competitive adsorption for the order of the reaction, and the optimum conditions under which the reaction is carried out. The selectivity of the transition-metal catalyzed conversion of n-hexane to i-hexane versus the hydrogenolysis to smaller hydrocarbons depends strongly on the structure of the surface. Hydrogenolysis requires an ensemble of several atoms. When sites become blocked by inert atoms, such as carbon or sulfur, the selectivity for reactions maintaining chain length is significantly enhanced. 

Structural effects can also be observed experimentally. Again, we illustrate this for the methane activation over Ni surfaces. As expected from the rf-band model, the Ni(l 11) surface has a significantly higher barrier for methane dissociation compared to the stepped Ni(211) surface (see Fig. 8.18). Because sulfur preferentially adsorbs at the step edges on Ni surfaces and since sulfur destabilizes the activated methane complex even more than on the Ni(lll) as seen in Figure 8.18, one can experimentally use atomic sulfur to selectively block the more active step sites. 

The selection of a process can be complex, requiring carehil evaluation of the many variables for each appHcation. The hemihydrate process is energy efficient, but this may not be an overriding consideration when energy is readily available from an on-site sulfuric acid plant. The energy balance in the total on-site complex may be the determining factor. 

A selective poison is one that binds to the catalyst surface in such a way that it blocks the catalytic sites for one kind of reaction but not those for another. Selective poisons are used to control the selectivity of a catalyst. For example, nickel catalysts supported on alumina are used for selective removal of acetjiene impurities in olefin streams (58). The catalyst is treated with a continuous feed stream containing sulfur to poison it to an exacdy controlled degree that does not affect the activity for conversion of acetylene to ethylene but does poison the activity for ethylene hydrogenation to ethane. Thus the acetylene is removed and the valuable olefin is not converted. 

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

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