Metal selective site poisoning

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 same type of reaction of organometallic species can also be used to modify the reactivity of small metal particles. In this way, one can for example selectively poison some metal catalytic sites, one can affect the orientation of incoming substrates prior to reaction at the metal surface, and one can isolate metal atoms or small reactive areas on the metal surface. 

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). 

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. 

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). 

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 importance of catalyst stability is often underestimated not only in academia but also in many sectors of industry, notably in the fine chemicals industry, where high selectivities are the main objective (1). Catalyst deactivation is inevitable, but it can be retarded and some of its consequences avoided (2). Deactivation itself is a complex phenomenon. For instance, active sites might be poisoned by feed impurities, reactants, intermediates and products (3). Other causes of catalyst deactivation are particle sintering, metal and support leaching, attrition and deposition of inactive materials on the catalyst surface (4). Catalyst poisons are usually substances, whose interaction with the active surface sites is very strong and irreversible, whereas inhibitors generally weakly and reversibly adsorb on the catalyst surface. Selective poisons are sometimes used intentionally to adjust the selectivity of a particular reaction (2). 

Studies of the deactivation of ATR catalysts show that the sulfur present in conventional fuels is responsible for rapid deactivation of both Ni-based and noble metal catalysts. At some conditions, sulfur appears to selectively poison the sites responsible for the SR reaction(s). 

The discovery that kink sites in steps are effective in breaking C C bonds in addition to C-H and H H bonds, thereby initiating hydrogenolysis reactions, may also explain the effect of trace impurities or second component metals that introduce selectivity. Since these kink sites have fewer nearest neighbors than step or terrace sites, they are likely to bind impurities or other metal atoms with stronger chemical bonds. Thus, these sites are readily blocked by impurities. As a result selective poisoning of hydrogenolysis may be obtained by minute concentrations o veil-chosen impurities or another metal component. 

Overall, it can be concluded that zeolites, and more specifically MFI, are adequate catalysts for oligomerization of short chain olefins to produce gasoline and even diesel range fuels. Selectivity and catalyst life is strongly dependent on parameters such as crystallite size, Si/Al ratio, and poisoning of external surface sites. The introduction of some metals (Ni) can be helpful. 

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