Reaction-selective poisoning

Poisoning is operationally defined. Often catalysts beheved to be permanently poisoned can be regenerated (5) (see Catalysts, regeneration). A species may be a poison ia some reactions, but not ia others, depending on its adsorption strength relative to that of other species competing for catalytic sites (24), and the temperature of the system. Catalysis poisons have been classified according to chemical species, types of reactions poisoned, and selectivity for active catalyst sites (24). 

One promising extension of this approach Is surface modification by additives and their Influence on reaction kinetics. Catalyst activity and stability under process conditions can be dramatically affected by Impurities In the feed streams ( ). Impurities (promoters) are often added to the feed Intentionally In order to selectively enhance a particular reaction channel (.9) as well as to Increase the catalyst s resistance to poisons. The selectivity and/or poison tolerance of a catalyst can often times be Improved by alloying with other metals (8,10). Although the effects of Impurities or of alloying are well recognized In catalyst formulation and utilization, little Is known about the fundamental mechanisms by which these surface modifications alter catalytic chemistry. 

Figure 22.1. Schematic representation of use of insoluble poisons to selectively terminate catalysis from leached palladium species in couphng reactions.

In order to demonstrate the selective effect of pore-mouth poisoning, it is instructive to consider the two limiting cases of reaction conditions corresponding to large and small values of the Thiele modulus for the poisoned reaction. For the case of active catalysts with small pores, the arguments of all the hyperbolic tangent terms in equation 12.3.124 will become unity and... 

In selective poisoning or selective inhibition, a poison retards the rate of one catalysed reaction more than that of another or it may retard only one of the reactions. For example, there are poisons which retard the hydrogenation of olefins much more than the hydrogenation of acetylenes or dienes. Also, traces of sulphur compounds appear selectively to inhibit hydro-genolysis of hydrocarbons during catalytic reforming. 

This book deals with four major areas of selectivity stereoselectivity clusters, alloys, and poisoning shape selectivity and reaction pathway control. An overview of the book and reviews of each of the four major areas are included as introductory chapters. Each review is followed by individual contributions by attendees of the symposium. 

This review encompasses the general area of selectivity in catalysis as well as the four major specific areas discussed in this book Stereoselectivity Clusters, Alloys and Poisoning Shape Selectivity and Reaction Pathway Control. Examples are taken from the literature for each of these four areas of recent articles that focus on selectivity in catalytic reactions. Specific reviews of the four areas listed above can be found in the overview chapters by D. Forster and coworkers, K. J. Klabunde, M. E. Davis and coworkers and H. C. Foley and M. Klein. 

This review is an overview of recent literature research articles that deal with selectivity in catalysis. Four specific areas including stereoselectivity clusters, alloys and poisoning shape selectivity and reaction pathway control will be discussed. This review is not meant to be a complete discussion of these areas. It represents a small fraction of the research presently underway and a very minor fraction of the available literature in this subject. The order of topics will follow the four major areas oudined above, however, there is no particular order for the articles discussed in each section. 

The above literature review gives a comparison of different ways to control selectivity for both homogeneous and heterogeneous catalytic reactions. There are several common features for the four areas of stereoselectivity metal clusters, alloys and poisoning shape selectivity and reaction pathway control. In fact, many times more than one of these areas may be involved in a catalytic system. Some common features for all of these areas include precise control of the structural and compositional properties of the catalysts. This paper serves as an overview for the other manuscripts in this book. Specific review chapters on each of the four areas can be found in reviews that follow by D. Forster et al., K. J. Klabunde et al., M. E. Davis et al., and H. C. Foley and M. Klein et al. 

The catalyst is rapidly poisoned by vanadium and nickel, which deactivate the catalyst permanently and lower reaction selectivity. 

Catalyst poisons and inhibitors are usually added inadvertently to the reaction mixture by the use of impure solvents or substrates. Promoters, however, are generally added deliberately to enhance catalyst activity and/or reaction selectivity. 

The nature of the surface site where acetylene hydrogenation occurs has been discussed extensively. It was proposed that two types of active sites are present on a Pd/Al203 catalyst. l One type promotes the hydrogenation of both triple and double bonds, whereas the other catalyzes only double-bond hydrogenation. This latter site can be poisoned by the addition of carbon monoxide to the reaction mixture as evidenced by the marked increase in reaction selectivity observed in the hydrogenation of acetylene in the presence of carbon monoxide.32.33 other modifiers can presumably act in the same way. 

Modifiers have also been used to influence the selectivity of vapor-phase partial hydrogenations of benzene. The presence of ethylene glycol increased reaction selectivity with a ruthenium black catalyst from 7% to 41% while the turnover frequency (TOF) decreased from 31 to 3. Pyridine also increased selectivity in the short term, but prolonged use poisoned the catalyst. Passivating a ruthenium black catalyst with caprolactam not only stabilized the catalyst toward deactivation but also increased reaction selectivity from 7% to 20%. 

SELECTIVITY POISONS The Selectivity of a solid surface for catalyzing one reaction with respect to another is not well understood. However, it is known that some materials in the reactant stream will adsorb on the surface and then catalyze other undesirable reactions, thus lowering the selectivity. The very small quantities of nickel, copper, iron, etc., in petroleum stocks may act as poisons in this way. When such stocks are cracked, the metals deposit on the catalyst and act as dehydrogenation catalysts. This results in increased yields of hydrogen and coke and lower yields of gasoline. 

Burguet et al. investigated the catalyst decay accompanying the reaction of cyclohexanone oxime over ultrastable H-Y zeolite [58]. The basic compounds present during the reaction i. e. oxime, -caprolactam, methylpyridine, 5-cyano-pent-l-ene, hydroxylamine, and aniline were considered to be the catalyst poisons. Hydroxylamine is more basic than the other products and might be more poisonous. Hydroxylamine selectivity decreased with temperature, which could explain qualitatively the apparent decrease in the deactivation constant (k with increasing temperature. 

These could refer to the main and poisoning reactions, or to reforming or cracking reactions with different compounds in a multicomponent feed stream. Type II selectivity refers to parallel reactions with a single reactant ... 

It is generally considered that surface carbon is a poison for many reactions. Indeed, in the strict sense this is usually true (that is, as carbon builds up on a surface and total activity goes down). However, in this paper we give some examples of surface reactivity which show that carbon can have a very positive role to play in manipulating reaction selectivity, so much so that it can result in higher activity to desired products. 

The D-USY catalyst presented high cracking activity. The results indicate that both nickel and vanadium poison zeolite activity sites. But vanadium poisons preferentially zeolite activities sites for the cracking reaction. Selectivity Sc2-c5 is 49.8 for D-USY, decreased to 42,6 for 1-Ni catalyst and with vanadium adding (from 8Ni-lV to INi-lV catalysts) Sc2-c5 decreases. For the 1-V catalyst selectivity Sc2-c5 is 27,9, 40% lower than D-USY catalyst. This result also confirms a nickel vanadium interaction. The isomerization reaction also increases on 1-V catalyst. This behavior can be related to vanadium acid species formation [8]. 

The results of Eq. 5.2.d-10 indicate the obvious result that when both first-order parallel reactions are equally poisoned, the selectivity is not affected, although the conversion would be. The more interesting case of non-first-order parallel reactions would be much more difficult to solve. Figure S.2.d-2 illustrates the results for several types of poisoning situations ... 

If the selectivity of the MIP catalyst is the main objective, the partial poisoning of active centers might be a way to improve the performance of the system. The imprinting procedure generates a statistical distribution of selective and less selective reactions centers. Studies indicate that the least selective sites are the most reactive [27]. The reaction of an MIP catalyst with sub-stoichiometric amounts of a catalyst poison under kinetic control should, therefore, result in a less active but more selective MIP catalyst. As a poisoning reaction, the covalent modification of functional groups or the irreversible complexation of a metal center could be employed (Fig. 20). 

To move through the sizing process we consider various reactants, top RHS of Fig. 6.1 and select possible reaction routes from the chemistry, understand competing and unwanted side reactions, select the phases and decide if a catalyst should be used. As illustrated on the RHS of Fig. 6.1, having a catalyst introduces questions of selectivity, activity, size, porosity, life, contaminants and poisons that interfere with the catalysf s function and temperature limitations. Our choice of reaction route also sets the heat release from the reaction highly exothermic or... 

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