Methanol kinetic parameters

The first set of experiments was conducted in methanol. The substrate concentration was varied from 15 to 50 mM at a 200 pM concentration of 1 for the determination of kinetic parameters for the transformation of 8 into 9. The catalytic rate constant was determined to be 0.04 min and the Michael constant was determined to be 40 mM at 30°C. The rate constant is comparable to those reported for other dinuclear Cu(ll) complexes with a comparable Cu -Cu distance of 3.5 A, but about one magnitude lower than those observed for complexes with a shorter intermetallic distances (12-14), e.g. 2.9 A (kcat = 0.21 min ) (12) or 3.075 A (kcat = 0.32 min (13). The rate constant Aion for the spontaneous (imcatalyzed) oxidation of 8 into 9 was determined to be 6 x 10" min and corresponds to the oxidation without catalyst under otherwise identical conditions. The rate acceleration (Arca/Aion) deduced from these values is 60,000-fold. 

The thermal decomposition of some 3,5-disubstituted-l,2,4-thiadiazoles has been studied and some nonisothermal kinetic parameters have been reported <1986MI239>. Polarographic measurements of a series of methylated 5-amino-l,2,4-thiadiazoles show that thiadiazoles are not reducible in methanolic lithium chloride solution, while thiadiazolines are uniformily reduced at 0.5 = — 1.6 0.02 V. This technique has been used to assign structures to compounds which may exist theoretically as either thiadiazoles or thiadiazolines <1984CHEC(6)463>. The photoelectron spectrum for 1,2,4-thiadiazole has been published <1996CHEC-II(4)307>. 

The steady state experiments showed that the two separate phases and the mixture are not very different in activity, give approximately the same product distributions, and have similar kinetic parameters. The reaction is about. 5 order in methanol, nearly zero order in oxygen, and has an apparent activation energy of 18-20 kcal/mol. These kinetic parameters are similar to those previously reported (9,10), but often ferric molybdate was regcirded to be the major catalytically active phase, with the excess molybdenum trioxide serving for mechanical properties and increased surface area (10,11,12). 

The kinetic parameters for the oxidation of a series of alcohols by ALD are shown in Table 4.1 (74). Methanol and ethylene glycol are toxic because of their oxidation products (formaldehyde and formic acid for methanol and a series of intermediates leading to oxalic acid for ethylene glycol), and the fact that their affinity for ALD is lower than that for ethanol can be used for the treatment of ingestion of these agents. Treatment of such patients with ethanol inhibits the oxidation of methanol and ethylene glycol (competitive inhibition) and shifts more of the clearance to renal clearance thus decreasing toxicity. ALD is also inhibited by 4-methylpyrazole. 

The kinetic parameters for the reactions of both methanol and ethanol listed in Tables VIII-XI show some interesting features. First, the frequency factors for the decomposition of the alkoxide intermediates to form the aldehydes were observed to be within an order of magnitude of 10 sec as is expected from simple transition state theory. The activation energy for the transfer of the hydrogen atoms from the alkoxide to the surface was... 

Table 4.2 Kinetic Parameters for Methanol Exchange of First-Row Divalent Transition Metal Ions, M(CH30H)i+ at 25 °C Ref. 26...

Additional information about the reactivity was obtained by determining the kinetic parameters during methanol oxidation for vanadia, molybdena, rhenia, and chromia on different oxide supports. For all these systems the activation energy is approximately the same, 18-22 kcal/mol. The activation energy corresponds to that expected for the breaking of the C-H bond of a surface methoxide intermediate, and should be... 

Aroyl esters of anthracene-9-methanol are photolysed in methanol to give products consistent with the anthracene-9-methyl cation as an intermediate.41 Rate constants for the solvolyses of secondary alkyl tosylates in fluorinated solvents were analysed in terms of the possible involvement of very short-lived carbocation-tosylate ion pair intermediates.42 The effect of added electrolytes on the rate of solvolysis of cumyl chloride and its -methyl derivative was studied in 90% aqueous acetone and 80% aqueous DMSO, with the results revealing a combination of a special salt effect and a mass law effect.43 Kinetic parameters obtained for the solvolysis of (8) (R1 = R2 = Me and R1 = Ar, R2 = H) show that there is substantial n, n participation in the transition state [e.g. (9). 44... 

Kinetic Parameters for Comparison of the Elimination-Addition and the Addition-Elimination Routes with MeO- ion in Methanol... 

Understanding the oxidation mechanism is important. Impedance spectroscopy was recently used to study methanol electrooxidation, and kinetic parameters can be deduced from impedance spectra. Figure 6.58 shows an equivalent circuit that was developed for methanol oxidation on a Pt electrode, but which is common for all electrochemical reactions. In this circuit, a constant phase element was used rather than a double-layer capacitance, since a CPE is more realistic than a simple capacitor in representing the capacitive behaviour. 

Combining the cyclic voltammogram, which provides the number of surface atoms, with fitting of the time-dependent impedance spectra obtained after adding methanol to the sulphuric solution (using Equations 6.41 and 6.44) produces the kinetic parameters of methanol oxidation. Figure 6.62 shows these values at different potentials. 

Kinetic parameters for alcohol exchange have been deduced from H and studies on Co(ii), (296) Ni(ii), (297) and Mn(ii) (298) systems. The complex [Co(CH30H)5py] (296) provides a clear example of the operation of the trans effect in a labile octahedral complex, in that the exchange rate constants for cis and trans methanol sites with respect to pyridine are 410s and 1200s respectively. 

A dramatic solvent effect in the thermolysis of tetramethyldioxetane, which followed the isokinetic relationship A/7 = /3A5 for a variety of solvents, formed the basis for the postulation of the concerted mechanism. However, it was shortly thereafter reported that the dramatic solvent effect in methanol was the result of catalysis by transition-metal ion impurities. In the presence of metal-ion complexing agents such as EDTA or Chelex 100, the menacing catalysis could be suppressed. That utmost care must be taken in measuring reliable kinetic parameters in 1,2-dioxetane decomposition cannot be overemphasized. 

G. Maria and O. Muntean, Model Reduction and Kinetic Parameters Identification for the Methanol Conversion to Olefins, Chem. Eng. Sci. 42 (1987) 1451-1460. 

NAr reactions of azide ion proceed more readily in dipolar aprotic solvents, the use of which is thus preferred widi substrates of low reactivity. The solvent differences are well illustrated by a comparison of rates and derived kinetic parameters for reaction of/ -fluoro- and p-iodonitrobenzene in a range of solvents. This shows rate ratios of the order of 10 -10 for reactions in dipolar aprotic solvents compared with methanol (section III.A.2). 

Other steps used in the model assume that the heterogeneous conversion of methane is limited to the gas-phase availability of oxygen, O2 adsorption is fast relative to the rate of methane conversion, and heat and mass transports are fast relative to the reaction rates. Calculations for the above model were conducted for a batch reactor using some kinetic parameters available for the oxidative coupling of methane over sodium-promoted CaO. The results of the computer simulation performed for methane dimerization at 800 °C can be found in Figure 7. It is seen that the major products of the reaction are ethane, ethylene, and CO. The formation of methanol and formaldehyde decreases as the contact time increases. 

The starting-point and basic-level principles of models, which can be defined as comprehensive or inclusive, must be the following the combinatorial approach for the compilation of a kinetic scheme and the use of independent values of kinetic parameters. The latter means the flat refusal to use any parameter optimization algorithms based on adjustment of the whole model or its blocks to some selected experimental data. These two basic principles are already discussed above in Sections II.A and II.B with reference to the GRI-Mech (combinatorial approach) and the methane-to-methanol oxidation model developed by Vedeneev and co-authors (use of independent kinetic parameters). However, we could not find any example of consistent employment of both principles in conjunction. 

For many cases Eq. (3.1) serves as a good approximation. However, one must keep in mind that the more correct form of mass action equation must contain activity values, which in the general case differ from concentrations (or, in other words, activity coefficients are not equal to 1). Deviations of activities from concentrations are most pronounced at relatively low temperatures and high pressures, i.e., when properties of the reaction system display a pronounced difference from those of an ideal gas. Uncertainty of kinetic simulations can therefore increase if values of kinetic parameters obtained at low pressures are used to model high-pressure processes in the framework of Eq. (3.1). Among processes of interest announced in this work, at least one—oxidation of methane-to-methanol—severely needs high pressures, at which the non-ideality of the reaction system can in principle manifest itself. 

Methanol and higher oxygenate syntheses follow different mechanistic and kinetic patterns over the various catalysts discussed here. Each such pattern is regular, however, and can be modeled with a few kinetic parameters based on fundamental mechanistic steps involved in the C-H, C-C, and C-0 bond forming reactions. Alkali co-catalysts play an important role by promoting... 

Fig. 5.48. Fluorescence-time curve for the photoreaction of carbol in oxygen free methanolic solution. The symbols represents the points of measurements for two wavelength of observation. (—) is the theoretically calculated curve to determine the kinetic parameters. Irradiation took place at 254 nm. Intensity /rt=1.37 x I0 Einstein cm" s ...

The experimental data were fitted employing the integrated form of Eq. (5). Figure 5 shows that the quality of fits was excellent when using this model to describe the poisoning effect of an acid compound on the side-chain alkylation of toluene with methanol. The kinetics parameters obtained Ifom this model were also summarized in Table 2. 

Kinetic parameters have been established for solvolysis of the pentacyanofer-rate(III) derivative [Fe(CN)5(N02)] . For aquation, which is acid-catalyzed at pH <5, A//= = 43kJmol-, =-80 J K" mol", and A =+2 cm" mol". Intrinsic and solvational contributions are presumably closely balanced in the case of A Rate constants for solvolysis of [Fe(CN)5(N02)] in water, methanol, dimethyl sulfoxide, and dimethylformamide correspond with the electron-donating abilities of the respective solvents. Activation volumes for the nonaqueous solvents, between +20 and +27 cm" mor reflect the dissociative nature of these solvoly-ses. " Rate constants for dissociation of the [Fe(CN)5(2,6-Me2pyrazine)]" anion in binary aqueous solvents containing methanol, acetone, or acetonitrile correlate well with acceptor numbers for the respective media, though with a very different... 

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