Dynamics of the enzymatic oxidation of methane

Kinetic isotope effect data for the oxidation of deuterium-substituted methane molecules with methane monooxygenase (MMO) are analyzed in the framework of a multistep nonradical mechanism. New evidence is obtained in favor of the hypothesis of the intermediate formation of a complex containing pentacoordinated carbon. A kinetic scheme whose first step involves two hydrogen molecules of the substrate being oxidized is considered. For coincidence between the calculated and experimental distributions of the oxidation products of partially deuterated methane, the formation of the intermediate complex containing pentacoordinated carbon must be reversible and the rate of the back decomposition of this complex must be substantially higher than the rate of its formation (w _?1 ? w ₁). The experimental distribution of the products of deuterated methane (CH₃D, CH₂D₂, and CHD₃) hydroxylation with MMO, which could not earlier be explained within the widely accepted oxygen rebound mechanism, is quantitatively explained for the first time in terms of the dynamics of a nonradical mechanism using parameters having a simple physical meaning and plausible values.     

Reactivity of vibrationally excited methane molecules in reactions with chlorine atoms

At 148–298 K, the rate constant for the reaction of methane molecules excited into bending vibration with atomic chlorine does not exceed by more than 30 times the corresponding constant for methane in thermal equilibrium. Consequently, at low temperatures and thermal equilibrium the reaction of methane with atomic chlorine proceeds through the vibrational ground state of methane.

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, 148–298°K , , 30 , . , .

     

Homogeneous high-temperature oxidation of methane

The kinetics of homogeneous deep oxidation of methane in lean mixtures (up to 2 vol % CH₄ in air) in ceramic tubes and fixed beds of ceramic spheres was studied. Experiments with the homogeneous reaction have shown that the methane oxidation occurs via a consecutive scheme through CO formation. The reaction rate of CH₄ oxidation was found to depend upon the equivalent pass diameter with a significant reaction inhibition in packing of small tubes and spheres, reflecting the influence of mass transfer on the radical-chain termination at the ceramic surfaces. It was also found that CO oxidation practically does not depend upon the mass exchange conditions, but it is visibly inhibited by methane. Recommended kinetic equations and their parameters are presented.     

On the mechanism of thermal oxidation of methane

Using the method of freezing radicals in conjunction with ESR spectroscopic measurements, the kinetics of the thermal oxidation of methane has been studied under atmospheric pressure depending on the temperature, composition of the mixture, and nature of the surface of the reaction vessel. It has been shown that in a reactor treated with boric acid, the intermediates methylhydroperoxide and hydrogen peroxide are responsible for chain branching. It has been established that the leading active centers of the reaction are the HO₂ radicals, while chain branching occurs as a result of the decomposition of peroxy compounds—methylhydroperoxide and hydrogen peroxide. In reactors treated with potassium bromide, the concentrations of radicals and peroxy compounds were found to be lower than the sensitivity of the method of measurement. Computations were performed for the scheme of methane oxidation at 738 K for a reactor treated with boric acid. Satisfactory agreement was found between the experimental and computed kinetic curves of accumulation of main intermediates CH₂O, H₂O₂, CH₃OOH. The influence of their addition on the kinetics of the reaction has been considered. It has been shown that the addition of formaldehyde does not lead to chain branching, however; it contributes to the formation of those peroxy compounds that bring about chain branching. Mathematical modeling confirmed conclusions made on the basis of experimental data concerning the nature of the leading active centers and the products that are responsible for the degenerate chain branching.     

Study of the interaction of hydrogen with palladium-containing catalysts of methane oxidation

The reaction of palladium-containing catalysts PdCl₂/SiO₂ [ratio of the components 0.3:100 (molar)] with H₂ (300°C) after methane oxidation leads to the formation of RH: methane, ethane, propane, n-butane, and isobutane. The sources of RH are PdC_x — products of incomplete oxidation of CH₄ — as well as organic impurities in silica gel. Additions of NaCl (NaCl:SiO₂ = 7:100) triple the yield of RH. Catalysts with additions of H₂SO₄ (H₂SO₄:SiO₂ = 40.100) are not hydrogenated to RH.L. M. Litvinenko Institute of Physico-Organic Chemistry and Coal Tar Chemistry, National Academy of Sciences of Ukraine, 70 R. Lyuksemburg Street, Donetsk 340114, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 3, pp. 177–181, May–June, 1996. Original article submitted July 20, 1995.     

Kinetic simulation of the enzymatic oxidation of methane

The kinetics of methane oxidation by methane monooxygenase is simulated numerically. Literature data on the distribution of products of the oxidation of deuterated methane CH_{4 ? n} D _n (CH₃D, CH₂D₂, and CHD₃), as well as on the kinetic isotope effect in the competitive oxidation of CH₄ and CD₄ by methane monooxygenase, are analyzed in the framework of a nonradical multistep mechanism. Kinetic schemes whose first step involves two hydrogen atoms of the oxidation substrate are considered. The kinetic models suggested for methane oxidation are in good agreement with experimental data.     

Kinetics of catalytic oxidation of methane

A kinetic model for methane oxidation in sulfur dioxidecontaining gaseous mixtures at 640–830 °C with Al₂O₃ as catalyst is suggested.     

Partial oxidation of methane over phosphate

Partial oxidation of methane over unpromoted and promoted iron phosphate catalysts was investigated in the temperature range of 873–953 K in a flow reactor. Using XRD and IR techniques, the bulk and surface structure of the given catalysts have been studied. The modification functions of lanthanum phosphate for partial oxidation of methane were also examined.

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873–953 . - -. .

     

Simulation of methane oxidation on Pt

The authors present a generic model of CH4 oxidation on Pt with the emphasis on the role of surface-oxide formation. The latter process is treated in terms of the theory of first-order phase transitions. The corresponding Monte Carlo simulations indicate that the surface-oxide formation may result in stepwise features in the reaction kinetics. Specifically, with increasing CH4 pressure and/or decreasing O2 pressure, the model predicts a sharp transition from a low-reactive state with the surface completely covered by oxide to a high-reactive state with the surface covered by chemisorbed oxygen. In the former case, the reaction is first order in CH4 and zero order in O2. In the latter case, both reaction orders are positive. All these findings help in interpreting available experiments.     

Investigation of the dehydrogenation of ethylene with the aid of deuterated compounds

Conclusions An investigation of the thermal conversions of mixtures of C₂H₄+C₂D₄ at 750–1000° permitted us to establish that at temperatures above 850°, the predominant process of formation of H₂ in the pyrolysis of C₂H₂ is the reaction of atomic hydrogen with ethylene. The rate constant of this reaction was measured in the interval 820–940°, and the Arrhenius dependence was found.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 564–570, March, 1974.The author would like to express his gratitude to Yu. V. Maksimov for his interest in the work and his useful discussions.     

Quantum chemical study of the reaction mechanism of ozone and methane with fluorine and chlorine atoms

Ab initio calculations of the potential energy surface for the F + O₃ and Cl + O₃ reactions have been performed using the G3 and G3MP2 methods, which optimize the geometry configuration of reactants, products, intermediates, and transition states. The results show that fluorine atoms react with ozone as violently as chlorine atoms. At the same time, we have studied the reaction mechanisms of F atoms and Cl atoms with methane. It is found that fluorine atoms prefer to react with methane and chlorine atoms with ozone when there is competition between ozone and methane. Therefore, we can reasonably explain why chlorine atoms play the main role of reactants depleting ozone, while the more active fluorine atoms deplete less ozone. © 2002 Wiley Periodicals, Inc.; DOI 10.1002/qua.10119     

Investigation of thermal degradation of polystyrene with the aid of thermal analysis

When the DTA of polystyrene is carried out in air in a platinum sample holder, an anomalously high endothermic effect is observed. This effect was found to be related to gas-phase catalytic styrene oxidation occurring on the surface of both operating and reference platinum sample holders. Methodological recommendations are given concerning the DTA procedure for organic substances, making it possible to avoid this type of anomaly.