Reactive sites poisoning

One form of biological poisoning mirrors the effect of lead on a catalytic converter. The activity of enzymes is destroyed if an alien substrate attaches too strongly to the reactive site, for then the site is blocked and made unavailable to the true substrate (Fig. 13.36). As a result, the chain of biochemical reactions in the cell stops, and the cell dies. The action of nerve gases is believed to stem from their ability to block the enzyme-controlled reactions that allow impulses to travel through nerves. Arsenic, that favorite of fictional poisoners, acts in a similar way. After ingestion as As(V) in the form of arsenate ions (As04J ), it is reduced to As(III), which binds to enzymes and inhibits their action. 

More recent studies have generally concluded that the inhibiting influence of Mg2+ results from difficulties in rapid dehydration of the Mg2+ ion, or from crystal poisoning by adsorption of Mg2+ at reactive sites. Mucci and Morse (1983) found that the log of the rate constant was a linear function of the solution Mg2+ to Ca2+ ratio and that the empirical reaction order increased from 3.07 to 3.70 as the Mg2+ to Ca2+ increased from 1 to 10.3. 

For a homogeneous catalyst, a reactive site on a molecule may be similarly poisoned by the binding of a substance directly to the site or nearby either in a permanent or reversible way. The properties of the site are significantly altered to prevent catalytic action. 

FIGURE 20 Effect of partial reduction by CO of the more reactive sites, followed by selective poisoning of the reduced sites by CO adsorption at 25 °C, on the activity (above) and polymer melt index (below). 

Enzyme inhibitors are often poisonous. For example, diisopropyl-fluorophosphate is a nerve poison because the enzyme acetylcholinesterase has a reactive site serine. Chymotrypsin and acetylcholinesterase are both members of the class of enzymes known as serine esterases, which are all inhibited by diisopropylfluorophosphate. 

Figure 20 A metal containing MIP catalyst with a selective (left) and an unselective reaction site (right). Upon addition of a catalyst poison , the less selective but more reactive site is occupied.

The reaction dimension Q may be the same as the fractal dimension D of the reactive surface or it may be different. If 2 < there may screening or poisoning of reactive sites. A value of Q larger than D usually means that the reaction occurs only in the micropores. The value of Q is not a constant and may change during the course of the reaction. 

Poisoning is a deactivation pathway in which at least one component of the reaction mixture adsorbs in a very strong - often irreversible - manner to the catalytic active center (Figure 2.3.6a). Kinetically speaking, the number and concentration of catalytic sites for this process reduces over time. In cases in which the catalytic material is characterized by different catalytic centers of different reactivity the poisoning process can be selective for one sort of center. By selective poisoning... 

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). 

The Co system is more reactive as well as much more selective than the Ni and Rh catalyst systems (Table XVII). The best systems allow almost 100% conversion with almost 100% yield of c -l,4-hexadiene. The best of the Ni and Rh systems known so far are still far from such amazing selectivity. The tremendous difference between the Ni system and the Co or Fe system must be linked to the difference in the nature of the coordination structures of the complexes, i.e., hexacoordinated (octahedral complexes) in the case of Co and Fe and tetra- or penta-coordinated (square planar or square pyramidal) complexes in the case of Ni. The larger number of coordination sites allows the Co and Fe complex to utilize chelating phosphines which are more effective than monodentate phosphines for controlling the selectivity discussed here. These same ligands are poison for the Ni (and Rh) catalyst system, as shown earlier. 

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. 

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