Promotor ions

Schuit and Gates (3) proposed a model for the oxidic catalyst, in which the promotor ions lay just below the molybdate layer. 

It is expected that the activator system in the oxidic catalyst has a structure from which the right configuration is obtained on sulfiding. We feel that the promotor ions have to be... 

A picture has been formed of the way in which the promotor ions are built in the M0O3-AI2O3 system. The neutralization of the Brdnsted acid sites, as originally present in M0O3-AI2O3 systems by the cobalt ions for the catalysts calcined at low temperatures ( 500°C) indicates that the cobalt ions are present on the catalyst surface. The liberation of these sites in catalysts calcined at high temperatures ( 650°C) and the observation of the characteristic reflectance spectrum of C0AI2O4 show that the cobalt ions enter the alumina lattice. However the interaction between cobalt and molybdenum, as indicated by the second Lewis band remains present. This leads to the conclusion that the cobalt ions are present in the surface layers of the alumina lattice. 

The manner and rate of decomposition of hypohalous acids (and hypohalite ions) in solution are much influenced by the concentration, pH and temperature of the solutions, by the presence or absence of salts which can act as catalysts, promotors or activators, and by light quanta. The main competing modes of decomposition are ... 

Komiyama et al. have described [65] enhanced hydrolysis of ApA by La(III) ion in presence of the tetrapyridine ligand 44. The complexation was followed by H NMR spectroscopy. Besides a 1/1 (La(DI)/L) complex, a 2/1 species is formed in the presence of excess metal. Both the 1/1 and 2/1 complexes are efficient promotors of ApA cleavage the monomer complex being ca. 7, the dimer ca. 70-fold more reactive than the metal ion alone under the same conditions (pH = 7.2, T = 323 K). 

For many reactions, especially carbonium-ion type reactions, the zeolites and the amorphous silica-aluminas have common properties. The activation energies of the processes with both types of compounds change insignificantly, and both compounds have similar responses to poisons and promotors (1, 2). In general the zeolites are far more active than the amorphous catalysts, but ion exchange and other modifications can produce changes in zeolite activity which are more important than the differences between the activities of the amorphous and zeolitic catalysts ... 

The Bronsted acid can then react with both the ECH promotor and the THF monomer to form the dialkyl oxonium ions shown. Either of these can react with THF to produce the propagating trialkyl oxonium species. But Saegusa et al., argue that the ECH species will undergo... 

One final class of initiators seems to belong in this group. The use of the Lewis acid/orthoester combination reported by Meerwein (3) is similar to the Lewis acid/promotor systems just discussed, in that the tertiary oxonium ion is generated in situ from the reactants. In this case, however, one first needs to consider the preparation of the "tertiary... 

The principle of the Lewis acid catalyzed rearrangements of hydrocarbons is well documented 4,81. Lewis acids react with a promotor deliberately added or present as an impurity in the reaction mixture to form carbonium ions which initiate intermolecular hydride transfers involving the hydrocarbon. These hydride transfers appear to be fairly unselective processes. While the expected tertiary > secondary > primary selectivity order is observed, the differences are significantly reduced relative to typical carbonium ion reactions. Possibly this is due to a hydride transfer mechanism which involves a pentaco-ordinate carbon transition state in which charge development on carbon would be minimized 38dh... 

The molybdate surface layer in the molybdenum-alumina samples is characterized by the presence of BrGnsted acid sites ( 1545 cm- ) and one type of strong Lewis acid sites (1622 cm l). Cobalt or nickel ions are brought on this surface on impregnation of the promotor. The absence of BrtSnsted acid sites is observed for both cobalt and nickel impregnated catalysts, calcined at the lower temperatures (400-500°C). Also a second Lewis band is observed at 1612 cnrl.The reflection spectra of these catalysts indicate that no cobalt or nickel aluminate phase has been formed at these temperatures. This indicates that the cobalt and nickel ions are still present on the catalyst surface and neutralize the Brdnsted acid sites of the molybdate layer. These configurations will be called "cobalt molybdate" and "nickel molybdate" and are shown schematically in Figure 11a. 

In this paper, we will review the chemical behaviour of transition metal oxides which is of crucial importance for heterogeneous catalysis, adhesion and many technological applications. Among them, MgO(lOO) is the simplest surface, with a square unit-cell containing two ions with opposite charges titanium oxides represent another important class of systems used for their catalytic properties either directly as catalyst or indirectly as support for other catalysts (metals such as Ni, Rh for the Fischer-Tropsch reaction or V2O5 for the reduction of NOx) or as promotors[l]. The most stable surface for rutile is the (110) face. 

The potassium is deposited on the surface from a beam of K+ ions evaporated from a zeolite getter source and directed toward the crystal. Potassium coverages, monitored by the intensity of the potassium Auger feature, up to 0.1 monolayer are investigated. The procedures for the dissociation probability measurements are as described previously. We find that there is no effect of potassium on the dissociation probability of methane and therefore no effect on the rate of the steam reforming reaction is expected. This result is consistent with the use of potassium as an inhibitor of catalyst deactivation rather than a rate promotor of the commercial steam reforming reaction. 

The using of y0-ketoester, ammonia, and alkynone as substrates, with Brousted acid (AcOH) or Lewis acid (ZnBr2) or Amberlyst 15 ion exchange resin as the promotor, to synthesis 2,3,6-trisubstituted or 2,3,4,6-tetrasubstituted pyridines was also reported [67]. Good yields and total regiocontrol can be obtained. 

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