Molecular dynamics study of methane hydrate formation at a water/methane interface

We present molecular dynamics simulation results of a liquid water/methane interface, with and without an oligomer of poly(methylaminoethylmethacrylate), PMAEMA. PMAEMA is an active component of a commercial low dosage hydrate inhibitor (LDHI). Simulations were performed in the constant NPT ensemble at temperatures of 220, 235, 240, 245, and 250 K and a pressure of 300 bar. The simulations show the onset of methane hydrate growth within 30 ns for temperatures below 245 K in the methane/water systems; at 240 K there is an induction period of ca. 20 ns, but at lower temperatures growth commences immediately. The simulations were analyzed to calculate hydrate content, the propensity for hydrogen bond formation, and how these were affected by both temperature and the presence of the LDHI. As expected, both the hydrogen bond number and hydrate content decreased with increasing temperature, though little difference was observed between the lowest two temperatures considered. In the presence of PMAEMA, the temperature below which sustained hydrate growth occurred was observed to decrease. Some of the implications for the role of PMAEMA in LDHIs are discussed.     

The promoting effect of energetic activation of a methane molecular-beam in the direct catalytic partial oxidation of methane

Energetic activation of a methane molecular-beam promoted remarkably the direct catalytic partial oxidation on Pt and Rh foils, in particular, hydrogen formation was dramatically enhanced.     

Challenges for the utilization of methane as a chemical feedstock

The abundance of methane has led to a strong interest to use methane as a feedstock in the chemical industry. One of the main challenges is the initial activation of the methane molecule. This has resulted in the development of several different approaches to utilize methane, some more developed than others. In this work the current status of the different approaches is discussed and the main issues for industrial utilization described. A special focus of this work is the status of catalyst development.     

Electron ionization of methane: the dissociation of the methane monocation and dication

Time-of-flight mass spectrometry and two-dimensional coincidence techniques have been used to determine, for the first time, the relative precursor-specific partial ionization cross sections following electron-methane collisions. Precursor-specific partial ionization cross sections quantify the contribution of single, double, and higher levels of ionization to the partial ionization cross section for forming a specific ion (e.g. CH(+)) following electron ionization of methane. Cross sections are presented for the formation of H(+), H(2)(+), C(+), CH(+), CH(2)(+), and CH(3)(+), relative to CH(4)(+), at ionizing electron energies from 30 to 200 eV. We can also reduce our dataset to derive the relative partial ionization cross sections for the electron ionization of methane, for comparison with earlier measurements. These relative partial ionization cross sections are in good agreement with recent determinations. However, we find that there is significant disagreement between our partial ionization cross sections and those derived from earlier studies. Inspection of the values of our precursor-specific partial ionization cross sections shows that this disagreement is due to the inefficient collection of energetic fragment ions in the earlier work. Our coincidence experiments also show that the lower energy electronic states of CH(4)(2+) populated by electron double ionization of CH(4) at 55 eV are the same (ground (3)T(1), first excited (1)E(1)) as those populated by 40.8 eV photoionization. The (3)T(1) state dissociating to form CH(3)(+) + H(+) and CH(2)(+) + H(2)(+) and the (1)E(1) to form CH(2)(+) + H(+) and CH(+) + H(+). At this electron energy, we also observe population of the first excited triplet state of CH(4)(2+) ((3)T(2)) which dissociates to both CH(2)(+) + H(+) + H and CH(+) + H(+) + H(2).     

Equation of state of a model methane clathrate cage

We investigate the behavior of a model methane clathrate cage under high hydrostatic pressures. The methane clathrate cage consists of 20 water molecules forming 12 pentagonal faces, with a methane molecule positioned at the cage center. The clathrate compound is located inside a fullerene-type arrangement of 180 He atoms to simulate an isotropic pressure. Different pressures are simulated by decreasing the radius of the He array. The minimal energy of the total system for each configuration is calculated by using density functional theory. The variation of the energy with the volume of the imprisoned clathrate cage leads to the proposal of a (cold) equation of state in the pressure range [0,60] GPa. The elastic parameters of the state equation are found in agreement with equivalent quantities measured on clathrates in their sI conformation. Special attention is given to the distribution of the confined atoms and the eventual symmetry lost from the clathrate cage with the pressure, as the clathrate cage constitutes a basic structural unit of the crystal. Finally, the strengths and limitations of the model are discussed.     

On the structure of protonated methane

Results of a Fourier transform-ion cyclotron resonance study are reported concerning the reactivity of protonated perdeuteromethane and deuteronated methane, generated under varying pressure conditions in an external chemical ionization ion source, toward ammonia. The competition between proton and deuteron transfer from both protonated perdeuteromethane and deuteronated methane to ammonia exhibits chemically distinguishable hydrogens. The chemical behavior of protonated methane appears to be compatible with the theoretically predicted stable structure with C_S symmetry, involving a three-center two-electron bond associating two hydrogens and the carbon atom. Interconversion of this structure due to exchange between one of these hydrogens and one of the three remaining hydrogens appears to be a fast process that is induced by interactions with the chemical ionization gas.     

Synthesis of a macrobicycle incorporating the tris(pyrazolyl)methane ligand

Steric crowding of the 3-position of tris(pyrazolyl)borate and -methane ligands has produced tetrahedral metal complexes with controlled reactivity. As an alternative, we propose to incorporate the tris(pyrazolyl)methane chelate in a macrobicyclic structure in order to create a cavity with well-defined dimensions and shape. Acid-catalyzed equilibration of excess of the new pyrazole 3-(1H-pyrazol-3-yl)benzenemethanethiol acetate with HC(3,5-Me(2)pz)(3) followed by hydrolysis affords a functionalized tris(pyrazolyl)methane, which reacts with 1,3,5-tris(bromomethyl)benzene in K(2)CO(3)/DMF to give the title compound. [structure: see text]     

Synthesis of methane in nanotube channels by a flash

Inorganic synthesis of organic molecules is a significant step for the primordial life. Generally, inorganic synthesis of methane necessitates, in addition to catalyst, a high-temperature and high-pressure environment. Here we will show that such an environment could be locally and instantly realized in the channels of single-walled carbon nanotubes (SWNTs) even under room temperature and ultrahigh vacuum conditions just by a visible-light flash, owing to the ultra-photothermal effect of nanomaterials. As a result, methane signals were definitely detected by using a quadrupole mass spectrometer and an optical fiber spectrometer. The mechanisms were interpreted as Fischer-Tropsch synthesis. Our results provide an alternative explanation of abiogenic methane origin.     

On the structure of liquid methane

An analytical expression for the structure factor, S(k), obtained by treating the square-well potential as a perturbation on the hard-core in the mean spherical model approximation, has been used in computing the structure factor of liquid CH₄.     

Determination of the charge distribution of methane by a method of density constraints

The molecular charge distribution of methane is expressed in terms of an orthonormal set of molecular orbitals which are determined solely by imposing a set of constraints on the derived one-electron charge density, the constraints being that the charge density reproduce the experimental expectation values of a set of one-electron operators. The molecular orbitals are expanded in terms of an SCF set of atomic orbitals on carbon and a single 1s STO on each hydrogen. The derived charge distribution is found to be equal to the SCF Hartree-Fock distribution in its prediction of one-electron expectation values. The energy, as determined by the associated wave function, is –40.156 a.u. This energy value is comparable to that obtained in a SCF LCAO MO calculation with a similar basis set and is 0.048 a.u. above the best calculated value of the Hartree-Fock limit.

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Zusammenfassung Die molekulare Ladungsverteilung des Methans wird mit Hilfe eines orthogonalen Satzes von Molekülorbitalen ausgedrückt, die lediglich durch eine Reihe von Nebenbedingungen an die Einelektronen-Ladungsdichte bestimmt sind. Die Nebenbedingungen bestehen darin, da\ die Ladungsdichte die experimentellen Erwartungswerte eines Satzes von Einelektronen-Operatoren reproduzieren soll. Die Molekülorbitale werden nach einem Satz von atomaren SCF-Orbitalen am Kohlenstoff sowie einem einzigen 1s STO an jedem Wasserstoffatom entwickelt. Man findet, da\ die erhaltene Ladungsverteilung der SCF-Hartree-Fock-Verteilung bezüglich der Bestimmung von Einelektronen-Erwartungswerten gleichkommt. Die Energie, die aus der zugehörigen Wellenfunktion bestimmt wird, ist –40,156 a.u. Dieser Energiewert ist mit demjenigen, der in SCF LCAO MO-Berechnungen mit einem Ähnlichen Basissatz bestimmt wird, vergleichbar und liegt 0,048 a.u. über dem besten berechneten Wert der Hartree-Fock-Grenze.

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Résumé La distribution de charge moléculaire du méthane est exprimée au moyen d'un ensemble orthonormé d'orbitales moléculaires déterminées uniquement à l'aide d'une série de contraintes sur les densités de charge obtenues, à savoir que ces densités reproduisent les valeurs expérimentales des valeurs moyennes de différents opérateurs monoélectroniques. Les orbitales moléculaires sont développées en orbitales atomiques SCF sur le carbone et en orbitale de Slater 1s sur chaque hydrogène. La distribution de charge obtenue est trouvée égale à la distribution SCF Hartree-Fock dans les prévisions des valeurs moyennes d'opérateurs monoélectroniques. L'énergie, déterminée à l'aide de la fonction d'onde associée, est –40.156 u.a. Cette valeur de l'énergie est comparable à celle obtenue dans un calcul SCF LCAO MO avec une base similaire et est située à 0,048 u.a. au dessus de la meilleure évaluation de la limite Hartree-Fock.

     

A non-radical mechanism for methane hydroxylation at the diiron active site of soluble methane monooxygenase

We propose a non‐radical mechanism for the conversion of methane into methanol by soluble methane monooxygenase (sMMO), the active site of which involves a diiron active center. We assume the active site of the MMOH_Q intermediate, exhibiting direct reactivity with the methane substrate, to be a bis(μ‐oxo)diiron(IV ) complex in which one of the iron atoms is coordinatively unsaturated (five‐coordinate). Is it reasonable for such a diiron complex to be formed in the catalytic reaction of sMMO? The answer to this important question is positive from the viewpoint of energetics in density functional theory (DFT) calculations. Our model thus has a vacant coordination site for substrate methane. If MMOH_Q involves a coordinatively unsaturated iron atom at the active center, methane is effectively converted into methanol in the broken‐symmetry singlet state by a non‐radical mechanism; in the first step a methane C? H bond is dissociated via a four‐centered transition state (TS1) resulting in an important intermediate involving a hydroxo ligand and a methyl ligand, and in the second step the binding of the methyl ligand and the hydroxo ligand through a three‐centered transition state (TS2) results in the formation of a methanol complex. This mechanism is essentially identical to that of the methane–methanol conversion by the bare FeO⁺ complex and relevant transition metal–oxo complexes in the gas phase. Neither radical species nor ionic species are involved in this mechanism. We look in detail at kinetic isotope effects (KIEs) for H atom abstraction from methane on the basis of transition state theory with Wigner tunneling corrections.     

Development and testing of a comprehensive chemical mechanism for the oxidation of methane

A comprehensive chemical mechanism to describe the oxidation of methane has been developed, consisting of 351 irreversible reactions of 37 species. The mechanism also accounts for the oxidation kinetics of hydrogen, carbon monoxide, ethane, and ethene in flames and homogeneous ignition systems in a wide concentration range. It has been tested against a variety of experimental measurements of laminar flame velocities, laminar flame species profiles, and ignition delay times. The highest sensitivity reactions of the mechanism are discussed in detail and compared with the same reactions in the GRI, Chevalier, and Konnov mechanisms. Similarities and differences of the four mechanisms are discussed. The mechanism is available on the Internet as a fully documented CHEMKIN data file at the address http://www.chem.leeds.ac.uk/Combustion/Combustion.html . © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 513–538, 2001     

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.     

First determination of volume changes and enthalpies of the high-pressure decomposition reaction of the structure H methane hydrate to the cubic structure I methane hydrate and fluid methane

Pressure-temperature (P-T) conditions of the decomposition reaction of the structure H high-pressure methane hydrate to the cubic structure I methane hydrate and fluid methane were studied with a piston-cylinder apparatus at room temperature. For the first time, volume changes accompanying this reaction were determined. With the use of the Clausius-Clapeyron equation the enthalpies of the decomposition reaction of the structure H high-pressure methane hydrate to the cubic structure I methane hydrate and fluid methane have been calculated.     

A theoretical study of the elimination process of methane from a platinum complex

A theoretical analysis of the dissociative reaction cis-PtH(CH₃)(PX₃)₂ → Pt(PX₃)₂ + CH₄ is presented. Several aspects of the effect on the mechanism of this reductive elimination when the substituent X is changed (X₃  (CH₃)₃, case I and X₃  CH₂Cl(CH₃)₂, case II), are considered and compared with the corresponding experimental facts. This indicates the following: the kinetic barrier for case I is small and even if a concerted mechanism is implied, an almost spontaneous process is strongly indicated. The activation energy is lowered in case II, where the presence of an electronegative substituent makes even easier the performance of a spontaneous process. The study of the reaction pathway was carried out through an Extended Hückel calculation.     

Carbon deposits on a resistive FeCrAl catalyst for the suboxidative pyrolysis of methane

The carbon deposits forming upon the suboxidative pyrolysis of methane on resistive FeCrAl catalysts heated with electric current were studied. The suboxidative pyrolysis of methane was carried out in a flow reactor at the ratio CH₄: O₂ = 15: 1 in a catalyst-coil temperature range of 600–1200°C; a cold reaction mixture (～20°C) was supplied. The morphology and structure of the carbon deposits and changes in the composition and structure of the catalyst were characterized by scanning electron microscopy, transmission electron microscopy with EDX analysis, Raman spectroscopy, and X-ray diffraction analysis. Various forms of carbon deposits, including branched nanotubes, and metal carbides formed by catalyst constituents were detected. It was found that the carbon deposits on the catalyst surface were morphologically different from the deposits on quartz reactor walls. The reasons for these differences were considered.     

Mechanism of the combustion of methane

Conclusions The proposed mechanism of the combustion of methane satisfactorily describes the experimental material on the kinetics of atomic flames.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2191–2196, October, 1971.The authors would like to express their gratitude to A. Ya. Dubovitskii under whose supervision the program was compiled for the electronic computer.