Selective poisoning

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. 

Figure 10.13. Vanadia wall deposits in a power plant firing Orimulsion fuel catalyze the premature oxidation of SO2 in heat exchangers. Note that potassium enhances the undesired conversion while a selective poison diminishes the effect to some extent.

The nature of such sites seems consistent with the behavior shown In the pyridine and lutldlne poisoning experiments. The acidic nature of the reduced metal sites which hold the nitrogen bases seems established. Difference In the extent to which the exposed metal cation Is accessible to the nitrogen atom of the organic nitrogen base could explain the selective poisoning seen with different substituted pyrldlnes. Presumably the cations In an active pair are somewhat less accessible than most exposed Co and Mo cations, which, because they normally hold two MO molecules, are probably exposed In Incomplete tetrahedral sites. 

The aim of our work is to study, under adequate operating conditions, the dehydrofluorination reaction of CF3CH2CI so as to determine the nature of the sites involved in the 6uorination and the dehydrofluorination of CF3CH2CI. Thus a selective poisoning of dehydrofluorination sites would allow to increase the selectivity for the fluorination reactions. 

An elegant complementary test to mercury poison is the use of dienes as selective poisons for homogeneous catalysts, due to their strong coordination to metal centres yielding inert catalytic complexes. In addition, their interaction with metal surfaces is weak. If the presence of diene (dienemetal =1 1) inhibits the catalytic process and Hg test does not, homogeneity can be strongly supported. 

Often poisoning described by curve 1 in Fig. 3.37 is referred to as nonselective poisoning, whereas the deactivation according to curve 3, which implies disproportionately large deactivation, is called selective poisoning. Explanations of. selective poisoning are ... 

In summary, the total oxidation of propylene to C02 occurred at a higher rate than partial oxidation to propylene oxide and acetone total and partial oxidations occurred in parallel pathways. The existence of the parallel reaction pathways over Rh/Al203 suggest that the selective poisoning of total oxidation sites could be a promising approach to obtain high selectivity toward PO under high propylene conversion. 

Selectivity Poisons. These poisons decrease the selectivity of the catalyst for the main reaction. In many cases impurities in the feed-stream will adsorb on the catalyst surface and then act as catalysts for undesirable side re-... 

This situation is termed pore-mouth poisoning. As poisoning proceeds the inactive shell thickens and, under extreme conditions, the rate of the catalytic reaction may become limited by the rate of diffusion past the poisoned pore mouths. The apparent activation energy of the reaction under these extreme conditions will be typical of the temperature dependence of diffusion coefficients. If the catalyst and reaction conditions in question are characterized by a low effectiveness factor, one may find that poisoning only a small fraction of the surface gives rise to a disproportionate drop in activity. In a sense one observes a form of selective poisoning. 

Crabtree described the use of dibenzo[a,e]cyclooctatetraene, a potent selective poison of homogeneous hydrogenation catalysts, as a tool to distinguish between homogeneous and heterogeneous catalysis in the hydrogenation of hexene with a range of catalysts [24]. 

Selective poisoning is widely employed in catalytic processes. 

In order to increase the selectivity in diene hydrogenation, low-temperature basic additives and the use of less polar solvents may help. In special cases, treatment of the catalysts with the salts of heavy metals (Zn, Cd, Pb) can be the method used to modify the activity and selectivity53. Rh and Ir catalysts could be selectively poisoned with CO-containing hydrogen, in order to saturate 1,3-butadiene to 1-butene without isomerization54. 

The silica-alumina surface is still more strongly acidic than the alumina surface. The acidity is less sensitive to poisoning by water. There has been much discussion whether the acidity of silica-alumina is caused by Bronsted or by Lewis acid sites. This matter has not been. settled definitely, although there is evidence that both types of acidity are present. This would explain the observation that the catalytic efficiency in different reactions may be selectively poisoned by different reagents. 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

Selective poisoning of one and/or another function by added poisons 118-119) or by self-deactivation 112a) as well as the following fact ... 

The catalyst surface contains both reducing and basic sites and experiments involving selective poisoning of the former by m-dinitrobenzene demonstrated the high probability of a SET mechanism. The same group also concluded that an SET mechanism was probably involved for Michael, Wittig-Horner, and Claisen-Schmidt condensations in the presence of the same catalyst [139,140]. 

Seignette salt See potassium sodium tartrate. sen yet, s6lt selective inhibition See selective poisoning. sl lek tlv. In a bish-an selective poisoning chem Retardation of the rate of one catalyzed reaction more than that of another by the use of a catalyst poison. Also known as selective inhibition. si lek-tiv poiz-an-it) ... 

In the case of alkenes, 1-pentene reactions were studied over a catalyst with FAU framework (Si/Al2 = 5, ultrastable Y zeoHte in H-form USHY) in order to establish the relation between acid strength and selectivity [25]. Both fresh and selectively poisoned catalysts were used for the reactivity studies and later characterized by ammonia temperature programmed desorption (TPD). It was determined that for alkene reactions, cracking and hydride transfer required the strongest acidity. Skeletal isomerization required moderate acidity, whereas double-bond isomerization required weak acidity. Also an apparent correlation was established between the molecular weight of the hard coke and the strength of the acid sites that led to coking. 

EXAFS data indicated that tin was only surrounded by four platinum atoms at the same distance of 0.276 nm (Scheme 2.40). This result clearly indicates that tin is located on the metal surface and not in the bulk. For example, in a bulk Pt3Sn alloy tin is surrounded by 12 platinum atoms while in a surface alloy on a platinum bulk it is surrounded by six platinum atoms only. Note that such tin adatoms are always obtained when low amounts of tin are deposited on the metal. This is probably because tetrabutyl tin coordinates first on the metal atoms of the faces, which are the most hydrogenolyzing, rather than corner or edges for which the alkyl ligands remain coordinated to the tin. This fact will be very important in catalysis since it explains selective poisoning of metal particles (see below). 

Adatoms (a very small amount), a situation corresponding to tin complexes having reacted with the most hydrogenolyzing sites. Those sites are thus selectively poisoned (see below). 

Several examples showing the effects of adatoms on activity and selectivity of a given catalytic reaction were observed. In most cases, this effect can be rationalized as a selective poisoning of undesirable sites. Usually, the presence of adatoms leads to a simultaneous decrease of the global activity and to a significant increase of selectivities in favor of the desired products. We describe here two examples. 

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