INDEX catalytic sites

The fully protonated H-ZSM-5 had a constraint index of 10.8 in agreement with the literature values. The modified Na, H-ZSM-5 had a constraint index of 1.1 which is typical of a larger pore zeolite with a pore diameter greater than 6A. The modified zeolite reacts with the substrates primarily at external catalytic sites and in a non-discriminating manner. Size and shape selectivity are not major factors since few catalytic sites are located internally. 

Prior to solving the structure for SSZ-31, the catalytic conversion of hydrocarbons provided information about the pore structure such as the constraint index that was determined to be between 0.9 and 1.0 (45, 46). Additionally, the conversion of m-xylene over SSZ-31 resulted in a para/ortho selectivity of <1 consistent with a ID channel-type zeolite (47). The acidic NCL-1 has also been found to catalyze the Fries rearrangement of phenyl acetate (48). The nature of the acid sites has recently been evaluated using pyridine and ammonia adsorption (49). Both Br0nsted and Lewis acid sites are observed where Fourier transform-infrared (FT IR) spectra show the hydroxyl groups associated with the Brpnsted acid sites are at 3628 and 3598 cm-1. The SSZ-31 structure has also been modified with platinum metal and found to be a good reforming catalyst. 

At 24 °C and 15-60 bar ethylene, [Rh(Me)(0H)(H20)Cn] catalyzed the slow polymerization of ethylene [4], Propylene, methyl acrylate and methyl methacrylate did not react. After 90 days under 60 bar CH2=CH2 (the pressure was held constant throughout) the product was low molecular weight polyethylene with Mw =5100 and a polydispersity index of 1.6. This is certainly not a practical catalyst for ethylene polymerization (TOP 1 in a day), nevertheless the formation and further reactions of the various intermediates can be followed conveniently which may provide ideas for further catalyst design. For example, during such investigations it was established, that only the monohydroxo-monoaqua complex was a catalyst for this reaction, both [Rh(Me)3Cn] and [Rh(Me)(H20)2Cn] were found completely ineffective. The lack of catalytic activity of [Rh(Me)3Cn] is understandable since there is no free coordination site for ethylene. Such a coordination site can be provided by water dissociation from [Rh(Me)(OH)(H20)Cn] and [Rh(Me)(H20)2Cn] and the rate of this exchange is probably the lowest step of the overall reaction.The hydroxy ligand facilitates the dissociation of H2O and this leads to a slow catalysis of ethene polymerization. 

In summary, the low-index faces of lanthanum oxide are quite unreactive, and the few very basic sites on which CO chemisorbs are located at defect positions (probably corners). The influence of oxygen vacancies on the chemistry of the C0/La203 system is not known, although oxygen vacancies have repeatedly been considered to be essential for promoting the catalytic activity 416, 417). 

The catalytic efficiency of an enzyme is indicated by its kcatIKM value, the value combining the effectiveness of both the productive substrate binding and the subsequent conversion of substrate molecules into product (Copeland, 2000). This value is the apparent second-order rate constant for enzyme action under conditions in which the binding site of the enzyme is largely unoccupied by substrate. The kcatIKM value is the index for comparing the relative rates of cleavage of alternative, competing substrates. The KM is the Michaelis constant, an apparent dissociation constant and hence a measure of substrate affinity. This value equals the concentration of substrate needed to reach half maximum velocity of the enzyme reaction. 

However, this assumption is not necessarily justified. Even for a well-faceted nanoparticle there are a number of nonequivalent adsorption sites. For example, in addition to the low-index facets, the palladium nanoparticle exhibits edges and interface sites as well as defects (steps, kinks) that are not present on a Pd(l 1 1) or Pd(lOO) surface. The overall catalytic performance will depend on the contributions of the various sites, and the activities of these sites may differ strongly from each other. Of course, one can argue that stepped/kinked high-index single-crystal surfaces (Fig. 2) would be better models (64,65), but this approach still does not mimic the complex situation on a metal nanoparticle. For example, the diffusion-coupled interplay of molecules adsorbed on different facets of a nanoparticle (66) or the size-dependent electronic structure of a metal nanoparticle cannot be represented by a single crystal with dimensions of centimeters (67). It is also shown below that some properties are merely determined by the finite size or volume of nanoparticles (68). Consequently, the properties of a metal nanoparticle are not simply a superposition of the properties of its individual surface facets. 

Modification of ZSM-5 zeolite can result in improved liquid yields and a doubling of the isoparaffin index of the liqnid fuels which indicates higher liquid quality compared with the parent ZSM-5 zeolite. The high catalytic activity of modified ZSM-5 was explained by its nniqne acidic properties with a sharp increase of the number and strength of weak acid sites and a decrease of strong acid sites [13]. 

The increase in isotacticity seems to be essentially connected to the decrease of the initial rate, as practically no change in the isotacticity index with polymerization time was detected. Moreover, while the atactic productivity decreases monotonically with the EB/TEA ratio in both systems, the isotactic productivity has a more complex behavior with the binary catalyst it remains almost unchanged up to EB/TEA s 0.25 and then falls, whereas with the ternary catalyst it increases up to EB/TEA 0.2 and then rapidly drops. On the grounds of these results, Spitz suggested that the reversible adsorption on the catalytic surface of the TEA EB complex (which is supposed to be very fast) changes the non specific centers into stereospedfic, though less active, centers, while the slower adsorption of free EB reversibly poisons both types of sites. The differences between the binary and the ternary catalysts would arise mainly from the presence, in the latter, of a larger number of potential stereospecific sites. 

G8I and C3U/G8A mutations are relatively benign. Whereas the relatively isos-teric C3U and G8A mutations lead to considerably reduced catalytic rates, the G8I [34,78] and C3U/G8A [73] mutations affect the rate by less than an order of magnitude. The C3U/G8A double mutation and G8I single mutation simulations indicate that the hydrogen-bond network retains the overall base positions relative to the wild-type simulation and suggest that these two mutations do not significantly alter any of the active site indexes that would affect activity relative to the wild-type simulation (Table 4). 

Many pesticides are esters or amides that can be activated or inactivated by hydrolysis. The enzymes that catalyze the hydrolysis of pesticides that are esters or amides are esterases and amidases. These enzymes have the amino acid serine or cysteine in the active site. The catalytic process involves a transient acylation of the OH or SH group in serin or cystein. The organo-phosphorus and carbamate insecticides acylate OH groups irreversibly and thus inhibit a number of hydrolases, although many phosphorylated or carbamoylated esterases are deacylated very quickly, and so serve as hydrolytic enzymes for these compounds. An enzyme called arylesterase splits paraoxon into 4-nitrophenol and diethyl-phosphate. This enzyme has cysteine in the active site and is inhibited by mercury(ll) salts. Arylesterase is present in human plasma and is important to reduce the toxicity of paraoxon that nevertheless is very toxic. A paraoxon-splitting enzyme is also abundant in earthworms and probably contributes to paraoxon s low earthworm toxicity. Malathion has low mammalian toxicity because a carboxyl esterase that can use malathion as a substrate is abundant in the mammalian liver. It is not present in insects, and this is the reason for the favorable selectivity index of this pesticide. 

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