Poisoning site-selective

The effect of steaming and of extensive poisoning by alkali metal ions is not limited to Y-type zeolites, as Lago et al. (12) have observed similar phenomena with mildly steamed H-ZSM-5. The activity for hexane cracking increased by about a factor of four upon mild steaming of the catalyst. Selective Cs poisoning indicated that the concentration of a more active site in the steamed sample was only about 6% of the tetrahedral framework aluminum. These sites exhibited a specific activity 45-75 times greater than that of a normal site in H-ZSM-5. 

Poisons (true poisons) are characterized by their propensity to attach very strongly, by a true chemical bond (e.g. covalent) to the surface atoms or ions constituting the catalytically active sites. Poisons act in minute quantities. Typical poisons of metals are S, As, etc. In most cases, activity and/or selectivity cannot be recovered without a drastic change in operating conditions (most often a regeneration). Recovery, if at all, takes place very slowly and/or only partially. 

The increase in isobutene selectivity with time-on-stream is a particular property of the ferrierite. This zeolite has two types of active sites the external sites (on the external surface of the zeolite crystallites) which are non-selective for skeletal isomerization and the internal sites (inside the zeolite pores) which are selective for this reaction (9). The changes observed on the selectivity have been associated with modifications of pore shapes ough coke deposition that favor reactions involving small molecules, such as -butene to isobutene isomerization (8). More recently, it has been reported that a bimolecular mechanism takes place at the non-selective acid sites, while a monomolecular mechanism occurs on the selective sites (10), the coke deposition being necessary in order to poison, block, and modify the non-shape selective acid sites. 

Specific Site Poisoning- Crucial to our understanding of the activity of mixed oxide catalysis or oxides on more refractory supports is the ability to measure the surface area of the active oxide. Two methods come readily to mind selective chemisorption and selective poisoning. In each case the critical question will be the selectivity of the adsorbent and considerable further research is still required. As an example, can N2O decomposition be employed for V+4 analysis as it is for reduced copper ... 

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). 

The phenomenon of catalyst poisoning arises from the preferential adsorption of substances on the catalyst s active site, or the formation of a strong chemical bond or a new compound. These changes will affect the performance of the catalj t, and will not free the catalyst to participate in the adsorption or reaction of the reactants. The catalyst s activity will decline, or even more seriously, completely lose its activity. Because the poison can selectively adhere to different active sites, poisoning may also cause a decline in the catalyst s selectivity. ... 

Xhe presence of CO2 causes various kinetic effects it accelerates the reaction rate, enhances the selectivity, alleviates the chemical equilibrium, suppresses the unwanted total oxidation products, prevents the hot spots on the catalyst surface, poisons the non-selective sites of the catalysts, and the equilibrium yield of styrene dehydrogenation is much higher in the presence of CO2 than in that of steam. 

Thorium oxide and UO2 are rather basic catalysts, though acidic sites seem to participate in the base-catalyzed reactions. Although basicity has not been measured by usual methods, catalytic selectivities and poisoning experiments suggest the existence of basic sites or acid — base pair sites on the surfaces. 

Anotlier important modification metliod is tire passivation of tire external crystallite surface, which may improve perfonnance in shape selective catalysis (see C2.12.7). Treatment of zeolites witli alkoxysilanes, SiCl or silane, and subsequent hydrolysis or poisoning witli bulky bases, organophosphoms compounds and arylsilanes have been used for tliis purjDose [39]. In some cases, tire improved perfonnance was, however, not related to tire masking of unselective active sites on tire outer surface but ratlier to a narrowing of tire pore diameters due to silica deposits. 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

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. 

Each precious metal or base metal oxide has unique characteristics, and the correct metal or combination of metals must be selected for each exhaust control appHcation. The metal loading of the supported metal oxide catalysts is typically much greater than for nobel metals, because of the lower inherent activity pet exposed atom of catalyst. This higher overall metal loading, however, can make the system more tolerant of catalyst poisons. Some compounds can quickly poison the limited sites available on the noble metal catalysts (19). 

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). 

There is a complication in choosing a catalyst for selective reductions of bifunctional molecules, For a function to be reduced, it must undergo an activated adsorption on a catalytic site, and to be reduced selectively it must occupy preferentially most of the active catalyst sites. The rate at which a function is reduced is a product of the rate constant and the fraction of active sites occupied by the adsorbed function. Regardless of how easily a function can be reduced, no reduction of that function will occur if all of the sites are occupied by something else (a poison, solvent, or other function). 

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

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. 

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