Precursor formation

The first controversial point in this mechanism is the nature of the reaction planes where the precursor formation and the ET reaction take place. Samec assumed that the ET step occurs across an ion-free layer composed of oriented solvent molecules [1]. By contrast, Girault and Schiffrin considered a mixed solvent region where electrochemical potentials are dependent on the position of the reactants at the interface [60]. From a general perspective, the phenomenological ET rate constant can be expressed in terms of... 

Gibbs energy of electron transfer (AGe ) and the work terms for precursor formation (Wp) and successor dissociation (ws),... 

Similar to the soot formation in the high temperature range, PM precursor formation at intermediate temperatures is also influenced by the paraffinic molecular structure. It was proposed that the soot formation yield decreases in the order cycloparaffin > 2-branched iso-paraffin > 1-branched iso-paraffin > normal... 

A general difficulty encountered in kinetic studies of outer-sphere electron-transfer processes concerns the separation of the precursor formation constant (K) and the electron-transfer rate constant (kKT) in the reactions outlined above. In the majority of cases, precursor formation is a diffusion controlled step, followed by rate-determining electron transfer. In the presence of an excess of Red, the rate expression is given by... 

The photochemistry of ru-vinyl-ortho-qui nodi methanes is typical of trienes in which at least one of the two C—C bonds is frozen in the s-cis conformation competing electro-cyclic ring closure to regenerate the precursor, formation of benzobicyclo[3.1.0]hex-2-enes and [1,5]-H shifts to arylallenes. The only triene photoproduct which is not generally... 

Following the conceptual idea introduced by Milliken [68], Takahashi and Glassman [53] have shown, with appropriate assumptions, that, at a fixed temperature, i/c could correlate with the number of C—C bonds in the fuel and that a plot of the log ipc versus number of C—C bonds should give a straight line. This parameter, number of C—C bonds, serves as a measure of both the size of the fuel molecule and the C/H ratio. In pyrolysis, since the activation energies of hydrocarbon fuels vary only slightly, molecular size increases the radical pool size. This increase can be regarded as an increase in the Arrhenius pre-exponential factor for the overall rate coefficient and hence in the pyrolysis and precursor formation rates so that the C/H ratio determines the OH concentration [12]. The 4>c versus C—C bond plot is shown in Fig. 8.14. When these... 

Coke precursor formation and zeolite deactivation mechanistic insights from hexamethylbenzene conversion./. Catal, 215, 30-44. 

Scheme 10 Living polymerization of W-heterocyclic carbene (NHC) precursor, formation of (u-(triethoxysilyl)-telechelic oligomer and immobilization on silica...

Figure 14.8 Simplified scheme for the transfer of the first electron from a reductant R to a NAC (adapted from Eberson, 1987). Panels (a) and (b) show free energy profiles of reactions where the actual electron transfer (a) or other steps such as precursor formation (b) are rate determining. Note that the subscript 1 is used to denote transfer of one electron to the NAC.

We should emphasize that we expect Eq. 14-36 to hold only if the actual electron transfer is rate limiting. If other steps in the reaction sequence are partially or fully rate limiting (e.g., precursor formation, Fig. 14.86), other factors have to be taken into account for evaluating and/or interpreting reduction rates (see below). 

Reaction of Cytochrome cIinn with Bis(ferrozine)copper(II) Knowledge of the redox properties of cytochrome c was an encouragement to initiate a kinetics investigation of the reduction of an unusual copper(II) complex species by cyt c11. Ferrozine (5,6-bis(4-sulphonatophenyl)-3-(2-pyridyl)-1,2.4-triazine)286 (see Scheme 7.1), a ligand that had come to prominence as a sensitive spectrophotometric probe for the presence of aqua-Fe(II),19c,287 forms a bis complex with Cu(II) that is square pyramidal, with a water molecule in a fifth axial position, whereas the bis-ferrozine complex of Cu(I) is tetrahedral.286 These geometries are based primarily upon analysis of the UV/visible spectrum. Both complexes are anionic, as for the tris-oxalato complex of cobalt in reaction with cytochrome c (Section 7.3.3.4), the expectation is that the two partners will bind sufficiently strongly in the precursor complex to allow separation of the precursor formation constant from the electron transfer rate constant, from the empirical kinetic data. 

W Kallow, H von Dohren, H Kleinkauf. Penicillin biosynthesis energy requirement for tripeptide precursor formation by delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine synthetase from Acremonium chrysogenum. Biochemistry 37 5947-5952, 1998. 

We shall examine in succession the effect of the dispersion of the precursor, calcination temperature in the preparation of the precursor, nature and amount of precursor, formation of compounds between precursor and supports, nature of support, effect of modifiers, and activation conditions. 

As shown in Fig. 2, the catalytic activity of the zeolite prepared by the direct heating method for methanol conversion was higher than that of the zeolite crystallization for 25 days by the standard preparation method. However, deactivation of the catalyst by carbon deposit occurred early in the reaction, just as with the catalyst prepared by the standard method. Differences in crystallite morphology between those prepared by the standard method and the direct heating method would be attributed to the stage of the precursor formation. Therefore, after the precursor formation the rapid heating was adopted as described below. 

Precursor heating method. The gel mixture was maintained at 100°C for 3 days for precursor formation. The precursor with the mother liquor was transferred to autoclaves, and the temperature was raised at a constant rate of 1.7°C,min 1 to 130, 160, 190, and 220°C. The temperature was maintained at each level for 0.5 h. The synthesized materials were also treated in the same manner as the standard preparation method. XRD patterns showed that the zeolites prepared at 190 and 220°C were ZSM-34 however, the zeolite prepared at 220°C contained some sodalite structure. The zeolites crystallized at 130 and 160°C had insufficient XRD intensity of ZSM-34 patterns and showed an activity of only DME formation. When the crystallization temperature was raised to 190°C, DME decreased to ca. 1/10, and C2-C, olefins increased dramatically. However, when the crystallization temperature was raised to 220°C, ethylene formation decreased markedly and DME increased. 

Fig. 3. Schematic of stability regions for a salt AB. 1, Region of precursor formation 2, colloid stability regions 3, boundary line for homogeneous nucleation 4, boundary line for heterogeneous nucleation 5, boundary line of the metastable region (solubility line) 6, region of undersatuartion.

Benzimidazolate betaines dipole moments. 60, 231 and precursors formation, 60, 202-19 nmr spectra. 60, 224-7... 

Vanadium phosphates have been established as selective hydrocarbon oxidation catalysts for more than 40 years. Their primary use commercially has been in the production of maleic anhydride (MA) from n-butane. During this period, improvements in the yield of MA have been sought. Strategies to achieve these improvements have included the addition of secondary metal ions to the catalyst, optimization of the catalyst precursor formation, and intensification of the selective oxidation process through improved reactor technology. The mechanism of the reaction continues to be an active subject of research, and the role of the bulk catalyst structure and an amorphous surface layer are considered here with respect to the various V-P-O phases present. The active site of the catalyst is considered to consist of V and V couples, and their respective incidence and roles are examined in detail here. The complex and extensive nature of the oxidation, which for butane oxidation to MA is a 14-electron transfer process, is of broad importance, particularly in view of the applications of vanadium phosphate catalysts to other processes. A perspective on the future use of vanadium phosphate catalysts is included in this review. 

Baek 1. G., Isobe T., Senna M. Mechanochemical effects on the precursor formation and microwave dielectric characteristics of MgTiO,. Solid State Ionics 1996 90 269-79. 

Liu, S.-Q., Pritchard, G.G., Hardman, M.J., PUone, G.J. (1994). Citrulline production and ethyl carbamate (urethane) precursor formation from arginine degradation by wine lactic acid bacteria Leuconostoc oenos and Lactobacillus buchneri. Am. J. Enol. Vitic., 45, 235-242. 

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