Site competition

Chapter 10 begins a more detailed treatment of heterogeneous reactors. This chapter continues the use of pseudohomogeneous models for steady-state, packed-bed reactors, but derives expressions for the reaction rate that reflect the underlying kinetics of surface-catalyzed reactions. The kinetic models are site-competition models that apply to a variety of catalytic systems, including the enzymatic reactions treated in Chapter 12. Here in Chapter 10, the example system is a solid-catalyzed gas reaction that is typical of the traditional chemical industry. A few important examples are listed here ... 

Some deactivation processes are reversible. Deactivation by physical adsorption occurs whenever there is a gas-phase impurity that is below its critical point. It can be reversed by eliminating the impurity from the feed stream. This form of deactivation is better modeled using a site-competition model that includes the impurities—e.g., any of Equations (10.18)-(10.21)— rather than using the effectiveness factor. Water may be included in the reaction mixture so that the water-gas shift reaction will minimize the formation of coke. Off-line decoking can be... 

A strict kinetic limitation based on the gas-phase reactant can be modeled using a variable value for h although experience shows that a first order rate expressions with n=l often provides an excellent fit to experimental data regardless of the underlying reaction mechanism. A site-competition model such as Equation (10.12) can also be used. 

Kinetic analysis based on the Langmuir-Hinshelwood model was performed on the assumption that ethylene and water vapor molecules were adsorbed on the same active site competitively [2]. We assumed then that overall photocatalytic decomposition rate was controlled by the surface reaction of adsorbed ethylene. Under the water vapor concentration from 10,200 to 28,300ppm, and the ethylene concentration from 30 to 100 ppm, the reaction rate equation can be represented by Eq.(l), based on the fitting procedure of 1/r vs. 1/ Ccm ... 

Goldberg (2002) found no evidence of any competition in sorption of arsenate and arsenite on Al or Fe-oxides and montmorillonite, but only a small and apparent competitive effect of equimolar arsenate on arsenite sorption on kaolinite and illite. The minor competitive effect in this study was due to the small concentrations of arsenic which is very low for saturation site. Competition for sorption sites is evident by increasing the surface coverage of the sorbents. Arsenate prevents arsenite sorption on metal oxides when the surfaces of the sorbents are saturated by the anions (Jain and Loeppert 2000 Violante and Pigna 2002). 

Using linear regression, it is possible to estimate the protonation constants of the Fe(II) complexes of siderophore complexes where the redox potentials have been measured over a range of pH values (59). This also explains the variation in reversibility of reduction as the pH changes, as the stability of the ferro-siderophore complex is much lower than the ferric complex, and the increased lability of ligand exchange and increased binding site competition from H+ may result in dissociation of the complex before the iron center can be reoxidized. 

As mentioned above, we planned to obtain optically pure styrenyl ethers through Zr-catalyzed kinetic resolution [5] subsequent metal-catalyzed rearrangement would afford optically pure chromenes. However, as shown in Scheme 11, the recovered starting material (40) was obtained with <10% ee (at 60% conversion) upon treatment with 10 mol% (,R)-(EBTHI)Zr-binol (3b) and five equivalents of EtMgCl (70°C, THF). We conjectured that, since the (EBT-HI)Zr-catalyzed reaction provides efficient resolution only when asymmetric alkylation occurs at the cyclic alkene site, competitive reaction at the styrenyl terminal olefin renders the resolution process ineffective. Analysis of the H NMR spectrum of the unpurified reaction mixture supported this contention. Indeed, as shown in Scheme 11, catalytic resolution of disubstituted styrene 49... 

The mechanisms which underlie enzyme inhibition are described more fully in Chapter 3. Suffice to say here that reversible inhibitors which block the active site are called competitive whilst those which prevent release of the product of the reaction are non-competitive. By preventing the true substrate accessing the active site, competitive inhibitors increase Km (designated by or K PParent). A non-competitive inhibitor decreases V mprime symbol ( ) here to imply physiological as it does for energy change. 

D. A. Annis, N. Nazef, C.-C. Chuang, M. P. Scott, H. M. Nash A general technique to rank protein—ligand binding affinities and determine allosteric versus direct binding site competition in compound mixtures. 

Larkin, D. J., et al., Hydrogen Incorporation in Boron-Doped 6H-SiC CVD Epilayers Produced Using Site-Competition Epitaxy, J. Electron. Mater., Vol. 24, 1995, pp. 289-294. 

The presence of gas-phase water is generally beneficial to the photocatalytic oxidation of aromatic contaminants. In continuous photoreactors, humidity appears to prolong catalyst activity and delay or prevent catalyst deactivation. The effects of humidity on reaction rates, however, appear to vary, depending on the aromatic contaminant concentration and the humidity level. For example. Petal and Ollis [18] examined the continuous photocatalytic oxidation of m-xylene in a powder-layer photoreactor at several different relative humidity levels. The m-xylene photo-oxidation reaction rate was observed to increase for gas-phase water concentrations up to 1000 mg/m (—7% relative humidity). Increasing the humidity level further (up to 5500 mg/m ) produced a gradual decrease in the observed reaction rate, possibly due to increased adsorption-site competition between xylene and water. The reported xylene removal rate for a water concentration of 5500 mg/m was approximately half that seen at 1000 mg/m. ... 

Eliprodil Polyamine site competitive blocker SyntheLabo Lorex Phase III Abandoned (Lees, 1997)... 

Table VII shows that for cesium sorption, both KC1 and N H4 are significant for the two geologic solids studied. The negative values indicate that the presence of either KC1 or lowers sorption. Both appear to be competing with Cs+ ion for sorption sites. Competition between K+ and Cs+ ions for sorption sites on mica-like minerals is well known. However, displacement of Cs+ by hydrazine was surprising since N H, should exist mainly as a neutral species at pH 9-10. A small amount (0.0005M to 0.005M) will be protonated and apparently competes with Cs+. Ammonium ion is known to effectively compete with Cs+ for mineral sorption sites. Hydrazinium ion with a similar molecular structure should also displace Cs+. Since hydrazine will not reduce or complex Cs+, the only possible effects on cesium sorption is to compete for sorption sites or to alter the surface of the solid minerals. No evidence of surface alteration (change in color or texture) was observed. Therefore, it appears that an Eh buffer is not required for Cs+ sorption studies and hydrazine only interferes with the sorption reaction.

Mg2+ influences calcite dissolution rates the same way, but not to the same extent as Ca2+. The inhibition effects of Mg2+ can be described in terms of a Langmuir adsorption isotherm. Sjoberg (1978) found he could model results for the combined influences of Ca2+ and Mg2+ in terms of site competition consistent with ion exchange equilibrium. The inhibition effects of Mg2+ in calcite powder runs increase with increasing Mg2+ concentration and as equilibrium is approached. 

Klier et al. investigated several cases of kinetics in which methanol is formed by a surface reaction between CO and hydrogen adsorbed on the Aox sites competitively or noncompetitively with CO on the Aox sites and hydrogen elsewhere on the surface in each case C02 effects sub (/) and (iii) above were taken into account. In addition, it was found empirically that a small amount of C02 is hydrogenated to methanol at a rate that linearly depended on partial pressure of C02. All kinetic equations that successfully described the C02 effects had the general form... 

Vogel et al. (2008) presented a good overview of the mechanisms of action of protein kinase inhibitor. These inhibitors can act by binding directly in the ATP binding site competitively, (type I inhibitors), but they tend to be less specific because of the shared characteristics of the ATP binding pockets among various kinases. More specificity can be attained with type II inhibitors that can extend into an allosteric site next to the ATP pocket and is only available in the inactive (non-phosphorylated) forms of the enzymes. Imatinib is an example of this, with a 200-fold increased... 

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