Sulfur catalytic poison

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. 

Catalytic transfer hydrogenation (entries 2 and 3 below) can be used to cleave benzyl esters in some compounds that contain sulfur, a poison for hydrogenolysis catalysts. 

Intrinsic to interpreting catalytic poisoning and promotion in terms of electronic effects is the inference that adsorption of an electropositive impurity should moderate or compensate for the effects of an electronegative impurity. Recent experiments have shown this to be true in the case of CO2 methanation where the adsorption of sulfur decreases the rate of methane formation significantly. The adsorption of potassium in the presence of sulfur indicates that the potassium can neutralize the effects of sulfur. 

Fuel sulfur and phosphorus have been identified in contributing to problems with catalytic converter efficiency. Fuel sulfur does poison converter catalysts, but not as severely as lead. 

The modern methanol synthesis catalyst consists of copper, zinc oxide, and alumina. Copper metal is seen as the catalytically active phase, and ZnO as the promoter. It is well known that the interaction between the two components is essential for achieving a high activity, but the nature of the promoting effect is still a matter of debate. Loss of activity is caused by sintering of the Cu crystallites, and, if the feed gas contains impurities such as chlorine and sulfur, by poisoning. 

Saturation of a carbohydrate double bond is almost always carried out by catalytic hydrogenation over a noble metal. The reaction takes place at the surface of the metal catalyst that absorbs both hydrogen and the organic molecule. The metal is often deposited onto a support, typically charcoal. Palladium is by far the most commonly used metal for catalytic hydrogenation of olefins. In special cases, more active (and more expensive) platinum and rhodium catalysts can also be used [154]. All these noble metal catalysts are deactivated by sulfur, except when sulfur is in the highest oxidation state (sulfuric and sulfonic acids/esters). The lower oxidation state sulfur compounds are almost always catalytic poisons for the metal catalyst and even minute traces may inhibit the hydrogenation very strongly [154]. Sometimes Raney nickel can... 

Sulfur is present in petroleum as sulfides, thiophenes, benzothiophenes, and dibenzothiophenes. In most cases, the presence of sulfur is detrimental to the processing because sulfur can act as catalytic poisons during processing. 

Few examples of catalytic hydrogenation of partially saturated thiophene derivatives are known, because sulfur normally poisons the catalyst. Nevertheless, hydrogenations are sometimes possible when a larger amount of catalyst is used. One example, with a high degree of diastereoselectivity, is hydrogenation of the biotin precursor l18. 

Oudar et al. (112) have studied the influence of sulfur on the hydrogenation of 1,3-butadiene and H2-D2 equilibration over Pt(110). The rates decayed linearly with sulfur coverage, so that each sulfur atom poisons one dissociation site for hydrogen without influencing the activation energies or mechanism. The authors established the first isotherm for sulfur adsorption under actual catalytic reaction conditions. The adsorbed hydrocarbons influenced the equilibrium coverage of sulfur on the Pt surface. The thermodynamics of adsorbed sulfur on several metal single crystal surfaces have been presented by Bernard et al. (114). 

However, in some cases partial catalyst poisoning is desired, for example to lower the catalyst activity or to influence the selectivity. A well-known example is the addition of ppm quantities of H2S in catalytic reforming with nickel catalysts. Compared to platinum, nickel has a higher hydrogenolysis activity, which leads to formation of gases and coke. Sulfur selectively poisons the most active hydrogenolysis centers and thus drastically influences the selectivity towards the desired isomerization reactions. 

Rare earth compounds are also used in numerous catalytic reactions in petrochemical industry. One example is the use of rare earth salts to stabilize zeolites used for the catalytic cracking of crude oils to gasoline. Rare earth doping increases the activity of these zeolites with the consequence of higher gasoline yields. In addition, these rare earth-modified catalysts have found expanded application as a consequence of the refineries use of residual or heavy crude oils which contain high levels of nickel, vanadium, and sulfur which attack zeolites and reduce their activity rare earths are more resistant to these catalytic poisons. ... 

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