Poisons molecular size

Figure 3a is a schematic of the functionalized zeolite beta. Figure 3b plotted the catalytic conversion of HEX and PYC over 6 A zeolites as a function of time. For sulfonated zeolite (Z-S03H), more than 60 % HEX was converted in 4 hours, and nearly complete conversion was observed over 12 hours. On the other hand, PYC, which has a large molecular size and cannot enter the microporosity, showed less than 8 % conversion over extended reaction time with same Z-S03H as catalyst. Both HEX and PYC were also reacted over pure zeolite beta (Z), and the TMMPS functionalized zeolite (Z-SH) before it was treated with H202. Pure zeolite and Z-SH showed low catalytic activity, and only a small fraction of either HEX or PYC was converted. Further evidence of the size selectivity is provided when amines of different sizes are used to poison (neutralize) the acid sites (19). As shown in Figure 3c, the... 

An area of increasing interest is the selective complexation of Sn2+ and more particularly Pb2+ for the treatment of heavy metal poisoning. Molecular mechanics has been extensively applied to the problem of metal ion selectivity (see Chapter 8) but there have been few studies of lead or tin complexes. The fit of Sn2+ to 18-crown-6 has been considered12811, as has the size selectivity of tetraazamacrocycles with respect to Pb2+ binding131. The binding of Pb2+ to porphyrin-1 has been modeled, though in this case the point of interest was the structural deformations caused by the metal cation11901. 

O Sullivan G, Mohammed N, Foran P, Lawrence G, Dolly O (1999) Rescue of exocytosis in botulinum toxin A-poisoned chromaffin cells by expression of cleavage-resistant SNAP-25. Identification of the minimal essential C-terminal residues. J Biol Chem 274 36897-904 Oberg SG, Kelly RB (1976) The mechanism of beta-bungarotoxin action. I. modification of transmitter release at the neuromuscular junction. J Neurobiol 7 129 11 Ohishi I, Sugii S, Sakaguchi G (1977) Oral toxicities of Clostridium botulinum toxins in response to molecular size. Infect Immun 16 107-9... 

For the characterization of the nature of surface sites, probe molecules are needed. In the following, however, we confine ourselves to those probe molecules that can principally be used as poisons under catalytic conditions. Thus, for example, the indicator molecules usually used for the titration of surface acidity and basicity will not be treated. The conditions under which these measurements are carried out differ greatly from those applied during actual catalysis, and the molecular size of the probe molecules is unfortunately usually very large.2 These methods have been reviewed very recently by Tanabe (20) and by Fomi (42). 

It is concluded from the foregoing considerations that pyridine may successfully be applied as a specific poison, provided the possible pitfalls are carefully kept in mind. The lower basicity of pyridine as compared to ammonia renders its chemisorption more selective. However, its basicity is in most cases still much higher than that of the commonly used reactants, so that one is usually able to determine an upper limit for the number of active sites by pyridine poisoning (239). On the other hand, the hardness of reactants or reaction products may be comparable with that of pyridine [e.g., dehydration of alcohols (47)] the poison will then be partially displaced. The molecular size of pyridine may bring about difficulties, since it restricts the accessibility of pyridine to narrow pores or even the approach to an adsorption site (214). In favorable cases, however, steric effects may be utilized to improve the specificity of poisoning (35, 36,241). 

At low temperatures characteristic of conditions for hydrogenation of organic compounds, sulfur adsorption may occur associatively, in which case the toxicity of the poison is dependent upon molecular size, structure, and strength of adsorption. 

One argument for a lower limit may be the specificity that is required - drugs need to act selectively on their target molecules in order to be clinically useful. There are numerous examples of low-molecular weight poisons - probably the better part of the periodic table is poisonous. There are, however, interesting exceptions to these molecular size rules of thumb. One is lithium another popular example is shown in Figure 1.3. 

The kinetic model reproduces satisfectorily experimental results. Deactivation experiments seems to indicate that the mechanism of deactivation changes with the nature of the contaminant used. When a strong poison for active acidic sites like pyridine is used, the catalyst gets totally deactivated when its concentration is over 250 ppm. In this case, the deactivation is fester than with CS2, hut not as fest as an acid base reaction should be. The behavior can be explained assiuning that the pyridine reaction with acidic sites is a diffesion controlled phenomenon enhanced by its molecular size, which is very near to the zeolite pore size. The presence of a mixed mechanism of deactivation and inhibition is also evident. 

It will be seen that the toxicity increases with the molecular size of the poison. Further, in spite of the obviously far greater specific surface of the 0.05 g. of kieselguhr-supported nickel compared with the surface of the 0.05 g. of unsupported platinum (which is reflected in the different order of the values of a, i.e. 10 and 10 , respectively, in the platinum and... 

The geometrical effect of poisons. This poisoning effect relates to the molecular size and geometrical structure of the poison. Similarly, take sulfides as an example, their poisoning effect on nickel or platinum hydrogenation catalysts have the following trends ... 

The compositional and structural complexity of these systems is their principal advantage. It is this feature which allows surface properties to be tuned in order to optimize selectivity and activity with respect to a specific reaction. At the same time, complexity is the reason of the fact that at a molecular level, an understanding of reaction kinetics at heterogeneous and porous interfaces is difficult to achieve. Consequently, the reaction kinetics on their surfaces depend sensitively on a number of structural and chemical factors including the particle size and structure, the support and the presence of poisons and promoters. 

While the catalysts and conditions are very different than in ZN (water is a severe poison for ZN but is a product in FT), both produce linear polymers, and the molecular weight distributions are very similar because they are controlled by relative rate coefficients of propagation kp and termination k. If these are nearly independent of molecular weight one obtains the Schultz Flory size distribution in both processes. 

Beyer and coworkers later extended these reactions to platinum clusters Ptn and have demonstrated that similar reaction sequences for the oxidation of carbon monoxide can occur with larger clusters [70]. In addition, they were able to demonstrate poisoning effects as a function of surface coverage and cluster size. A related sequence for Pt anions was proposed by Shi and Ervin who employed molecular oxygen rather than N2O as the oxidant [71]. Further, the group of Bohme has screened the mononuclear cations of almost the entire transition metal block for this particular kind of oxidation catalysis [72,73]. Another catalytic system has been proposed by Waters et al. in which a dimolybdate anion cluster brings about the oxidation of methanol to formaldehyde with nitromethane, however, a rather unusual terminal oxidant was employed [74]. 

Mo et al [41] show that the poisoning power of a nitrogen aromatic (polar) compound i primarily determined by a balance between its heaviness or size and basicity. The former may be measured by molecular weight, the latter by proton affinity. 

Thus, complex high-area catalysts are typically not the best for fundamental investigations at the atomic or molecular level. Although many broadly important characteristics of heterogeneous catalysis, such as metal particle size effects, support effects, metal—support interaction, and the influence of the promoters and poisons... 

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