Poisoning of the active sites

The use of solid catalysts and especially zeolites in Fine Chemical synthesis introduces another complication with respect to homogeneous reactions. There is always a progressive decrease of the catalyst activity with increasing reaction time.1191 In some reactions, this deactivation can be due to irreversible chemical transformation of the zeolite catalyst, e.g. reactions with acid reactants causing dealumination and sometimes collapse of the framework. However, in most cases, deactivation results from poisoning of the active sites by the desired reaction... 

Inhibition (poisoning) of the active sites by impurities, feed components (reactants, solvent) and reaction products. 

Very few data are available on the deactivation and recycling of catalysts other than zeolites. The main factor is probably the poisoning of the active sites by side-products of the reaction which have a very high affinity toward adsorption on those sites. When using exchanged materials, one should, furthermore, be very careful about possible leaching. 

The catalytic tests of selective poisoning of the active sites were carried out in the conditions previously described, but without solvent. The solvent effect will not be presented here. The poison was added to the reagent before introducing the catalyst. We verified the effect of the poison on the reagents in absence of the catalyst. Benzoic acid does not catalyze the reaction, but in presence of tripropylamine, 17 % of heptanal are converted after 5 hours and jasminaldehyde is obtained with 28 % of selectivity. 

In the current trace shown in Figure IB, there is an initial peak, followed by a plateau. The reactivity of the surface drops quickly after the polymerization has started. The reduced reactivity may be either due to poisoning of the active sites or to a reduced initiator diffusion coefficient inside the newly formed gel. Note, however, that the current is not necessarily an indicator of polymer growth, but rather an indicator of initiator decomposition. 

Poisoning of the active sites originates from either permanent or reversible adsorption of species present in the feed at the active sites of the catalyst. One typical poison in the field of fuel processing is carbon monoxide, which blocks precious metal sites such as platinum. 

In the alkylation of trimethylbenzene with methanol over H-ZSM-5, the 1,2,3,4-isomer fraction in tetramethylbenzenes are very high (to 98%). The selectivity for the isomer is further improved by selective dealumination of the external surface of the zeolite crystals or by selective poisoning of the active sites on the external surface. 

CBs, like OPs, act as inhibitors of ChE. They are treated as substrates by the enzyme and carbamylate the serine of the active site (Figure 10.8). Speaking generally, car-bamylated AChE reactivates more rapidly than phosphorylated AChE. After aging has occurred, phosphorylation of the enzyme is effectively irreversible (see Section 10.2.4). Carbamylated AChE reactivates when preparations are diluted with water, a process that is accelerated in the presence of acetylcholine, which competes as a substrate. Thus, the measurement of AChE inhibition is complicated by the fact that reactivation occurs during the course of the assay. Carbamylated AChE is not reactivated by PAM and related compounds that are used as antidotes to OP poisoning (see Box 10.1). 

Figure 17.15 Reactions of the active site of [NiFe]-hydrogenases with small molecules that can poison Pt active sites, showing likely structures for the product species. Inhihition of many [NiFe]-hydrogenases hy O2, CO, and sulhdes is reversible, while hydrogenases from Ralstonia even oxidize H2 in the presence of these molecules (for a review, see Vincent et al. [2(X)7]).

Intrinsic Activity Poisons. These poisons decrease the activity of the catalyst for the primary chemical reaction by virtue of their direct electronic or chemical influence on the catalyst surface or active sites. The mechanism appears to be one that involves coverage of the active sites by poison molecules, removing the possibility that these sites can subsequently adsorb reactant species. Common examples of this type of poisoning are the actions of compounds of elements of the groups Vb and VIb (N, P, As, Sb, O, S, Se, Te) on metallic catalysts. 

Another type of inhibitor combines with the enzyme at a site which is often different from the substrate-binding site and as a result will inhibit the formation of the product by the breakdown of the normal enzyme-substrate complex. Such non-competitive inhibition is not reversed by the addition of excess substrate and generally the inhibitor shows no structural similarity to the substrate. Kinetic studies reveal a reduced value for the maximum activity of the enzyme but an unaltered value for the Michaelis constant (Figure 8.7). There are many examples of non-competitive inhibitors, many of which are regarded as poisons because of the crucial role of the inhibited enzyme. Cyanide ions, for instance, inhibit any enzyme in which either an iron or copper ion is part of the active site or prosthetic group, e.g. cytochrome c oxidase (EC 1.9.3.1). 

Mevinphos inactivates cholinesterase by phosphorylation of the active site of the enzyme to form the dimethylphosphoryl enzyme. Over the following 24—48 hours, there is a process, called aging, of conversion to the monomethylphosphoryl enzyme. Aging is of clinical interest in the treatment of poisoning because cholinesterase reactivators such as pralidoxime (2-PAM, Protopam) chloride are ineffective after aging has occurred. 

TiAl-Beta catalyst ion-exchanged with quaternary ammonium salts.1253 This is due to the selective poisoning of the acid sites without suppressing the oxidation activity of Ti sites. 

In spite of much effort, the nature of the active sites on acid—base inorganic catalysts is still not completely understood. However, the work on this problem has shown how complicated the surface structure may be and that several types of active centres may be simultaneously present on the surface the question is then which type plays the major role in a particular reaction. Also, the catalytic activity may be influenced to a large extent by impurities present in the feed (catalytic poisons) or by-products of the reaction. The last point is often not taken into account and it will be discussed specially in Sect. 1.2.6. First, the models of surface sites on the most important and best-studied catalysts will be described. 

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