Active centers nature

Active centers, nature of, 10 96 Active site, 27 210-221 in catalysts, 17 103-104, 34 1 for olefin chemisorption, 17 108-113 dual-site concept, 27 210 electrical conductivity, 27 216, 217 ESCA, 27 218, 219 ESR, 27 214-216 infrared spectroscopy, 27 213, 214 model, 27 219-221 molybdena catalyst, 27 304-306 Mdssbauer spectroscopy, 27 217, 218 nonuniform distribution, transport-limited pellets, 39 288-291... 

Total concentration of active centers involved in a catalytic reaction Constant in Koble equation Unoccupied active center Natural logarithm Mole ratio of to A in the feed CbqICai,... 

Acetylene, catalytic oxidation of, for oxygen manufacture, 3, 107 Acid base catalysis, and molecular structure, 4, 151 Acidic catalysis, ff, 241 Active centers, nature of, 10, 96 Adsorbed molecules, infreued spectra of, 10, 1... 

The reaction medium plays a very important role in all ionic polymerizations. Likewise, the nature of the ionic partner to the active center-called the counterion or gegenion-has a large effect also. This is true because the nature of the counterion, the polarity of the solvent, and the possibility of specific solvent-ion interactions determines the average distance of separation between the ions in solution. It is not difficult to visualize a whole spectrum of possibilities, from completely separated ions to an ion pair of partially solvated ions to an ion pair of unsolvated ions. The distance between the centers of the ions is different in... 

The main conclusion we wish to draw from this line of development is that the difference between Ej and E could vary widely, depending on the nature o the active center. 

Cerium is one of the most widely used activators, which improve the working characteristics of many scintillators. Determination of the valence state of cerium in single crystals of alkaline and rare-earth borates allows to establish the nature of activator centers for purposeful influence on the scintillation efficiency of the matrix. 

It seems reasonable that an enzyme which used poraaminobenzoic acid as a substrate might be deceived by sulfanilamide. The two compounds are very similar in size and shape and in many chemical properties. To explain the success of sulfanilamide, it is proposed that the amide can form an enzyme-substrate complex that uses up the active centers normally occupied by the natural substrate. 

Usually fairly high concentrations of such a drug are needed for effective control of an infection because the inhibitor (the false substrate) should occupy as many active centers as possible, and also because the natural substrate will probably have a greater affinity for the enzyme. Thus the equilibrium must be influenced and, by using a high concentration of the false substrate, the false substrate-enzyme complex can be made to predominate. The bacteria, deprived of a normal metabolic process, cannot grow and multiply. Now the body s defense mechanisms can take over and destroy them. 

Acyloins (a-hydroxy ketones) are formed enzymatically by a mechanism similar to the classical benzoin condensation. The enzymes that can catalyze reactions of this type arc thiamine dependent. In this sense, the cofactor thiamine pyrophosphate may be regarded as a natural- equivalent of the cyanide catalyst needed for the umpolung step in benzoin condensations. Thus, a suitable carbonyl compound (a -synthon) reacts with thiamine pyrophosphate to form an enzyme-substrate complex that subsequently cleaves to the corresponding a-carbanion (d1-synthon). The latter adds to a carbonyl group resulting in an a-hydroxy ketone after elimination of thiamine pyrophosphate. Stereoselectivity of the addition step (i.e., addition to the Stand Re-face of the carbonyl group, respectively) is achieved by adjustment of a preferred active center conformation. A detailed discussion of the mechanisms involved in thiamine-dependent enzymes, as well as a comparison of the structural similarities, is found in references 1 -4. 

When combined with the isolation and reactivity studies of the patterned aminosilica (7), the increased activity of the patterned catalysts provide further evidence that the patterning technique developed allows for the synthesis of aminosilicas which behave like isolated, single-site materials (although a true single site nature has not been proven). As the olefin polymerization catalysts supported by the patterned materials show a marked improvement over those materials supported on traditional aminosilicas, these patterned materials should be able to improve supported small molecular catalysis as well. Future improvements in catalysis with immobilized molecular active sites could be realized if this methodology is adopted to prepare new catalysts with isolated, well-defined, single-site active centers. 

Although the exact nature of the active center in polymerizations of butadiene with these Ba-Mg-Al catalysts is not known, we believe that the preference for trans-1,4 addition is a direct consequence of two aspects of this polymerization system, namely (1) the formation of a specific organobarium structure in a highly complexed state with Mg and A1 species, and (2) the association of the polybutadiene chain end with a dipositive barium counterion which is highly electropositive. 

The binding of a substrate to its active center was first postulated by E. Fisher in 1894 using the lock and key mechanism which states that the enzyme interacts with its substrate like a lock and a key, respectively, i.e. the substrate has a matching shape to fit into the active site. This theory assumed that the structure of the catalyst was completely rigid and could not explain why the macromolecule was able to catalyze reactions involving large substrates and not those with small ones, or why they could convert non natural compounds with different structural properties to the substrate. 

A wide variation exists for the number of active centers of oxidation in polymer samples. This reflects the statistical nature of the catalyst residues in polymer particles. On the contrary, the spreading rate coefficient b is approximately constant for the studied samples of PP. The coefficient a is probably sensitive to the morphology of the particles. 

The action pattern and the specificity of enzymes acting on homo-polymeric substrates are determined by the nature of the active center.134-137 Hence, it may be assumed that the active centers of enzymes exhibiting the aforementioned action-patterns are different. 

Catalysts were prepared by incipient wetness impregnation of commercial supports using cobalt nitrate as a precursor. Metallic cobalt species were active centers in the ethanol steam reforming. Over 90% EtOH conversion achieved. Nature of support influences the type of byproduct formation. Ethylene, methane and CO are formed over Co supported on A1203, Si02 and MgO, respectively... 

The nature of the adsorbed species can be inferred from the usual chemical parameters, i.e. chemical shifts, linewidths and relaxation times. These latter allow the study of the mobility on the surfaces. As an analytical tool, C-NMR spectroscopy can also be used to determine the concentration of reactants or products as a function of time and hence kinetic constants can easily be determined. As a conclusion, a rather complete kinetic study can be carried out involving the nature of interaction between the admolecule and the surface and eventually the nature of the surface active centers. One can finally arrive at the proposition of a reaction mechanism. 

It is important to note that adsorption does not necessarily lead to a catalytic reaction but the surface catalyzed reactions always occur through adsorption. In their catalytic action, the surfaces are specific in nature. Ni and Cu surfaces are very good catalysts for hydrogenation processes. The physical nature of a surface also influences its catalytic efficiency. Those atoms, which are at the peaks, edges etc. have high residual fields and are likely to have greater adsorption capacity. Taylor (1925) postulated that the adsorption and subsequent reaction takes place preferentially on certain parts of the surface, which are called active centers. The active center may constitute a small portion only of the total surface. Moreover, all active centers where adsorption occurs are not always catalytically effective. 

At this point, it is important to mention that, in spite of the great variety of active centers (molecules, ions in solids, color centers, etc.), it can be demonstrated that only 32 point symmetry groups exist in nature. These 32 point symmetry groups (denoted by the so-called Schoenflies symbols) are listed in Table 7.1. The group order and... 

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