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A laboratory-scale simulated countercurrent moving bed chromatographic reactor (SCMCR) for the direct, homogeneous partial oxidation of methane to methanol has been constructed and tested. Reaction conditions were evaluated from independent experiments with a single-pass tubular reactor. Separation was effected by gas−liquid partition chromatography with 10% Carbowax on Supelcoport. At the optimal reaction conditions of 477 °C, 100 atm, and feed methane-to-oxygen ratio of 16, the methane conversion in the SCMCR was 50%, the methanol selectivity was 50%, and the methanol yield was 25%. Factors affecting methane conversion were investigated. Possible ways to improve the methane conversion, and hence the methanol yield, are discussed.

An ab initio molecular orbital study of the unimolecular elimination of HCl and Cl from the CH2ClO radical is reported. Geometry optimizations were carried out at the HF/6-31G(d), MP2(full)/6-31G(d), and MP2(full)/6-31G(d,p) levels, and total energies were calculated using G2 and G2(MP2) theories. The zero-point-energy-corrected energy barrier for HCl elimination is predicted to be 8 kcal/mol, and for Cl elimination it is predicted to be 10.5 kcal/mol. RRKM models for both unimolecular reactions were made from the ab initio vibrational frequencies (scaled), moments of inertia and barrier heights. The RRKM predicted thermal rate coefficients for HCl elimination are in good agreement with experimental data taken over the temperature range 265−306 K and the pressure range 5−35 Torr (Wu, F.; Carr, R. W. J. Phys. Chem. A 2001, 105, 1423.) when the barrier height is adjusted to 8.5 kcal/mol. Because of the low energy barrier and, to a lesser extent, the small size of the reactant, the falloff curve is very broad, and the high pressure limit is predicted to be closely approached only when pressures of 10⁷ Torr and above are reached. RRKM calculations of the rate coefficient for Cl elimination predict that this reaction is negligible compared with HCl elimination at moderate temperatures, but should be taken into account in high-temperature reactions. G2 theory predicts the 298 K enthalpy of formation of CH2ClO to be −6.6 kcal mol^-1.

An experimental and mathematical modeling study of a simulated countercurrent moving bed chromatographic reactor for oxidative coupling of methane to ethane and ethylene is reported. The experiments were done in a laboratory-scale three-section simulated moving bed reactor. Operating conditions were deduced from independent experiments in a microcatalytic fixed-bed reactor and from adsorber dynamics studies. With a YBa2Zr3O9.5 catalyst and activated charcoal adsorbent, the sum of the ethane and ethylene yields at optimum simulated moving bed operating conditions was 45%. Numerical simulations of the reactor performance were done and compared with experimental results.

The equilibrium constant for the reaction of trimethylgallium and ammonia has been determined at 40−100 °C via direct pressure measurements. The reaction enthalpy found from experimental data (−15.2 ± 1.5 kcal/mol) is in good agreement with the value obtained from ab initio calculations (−15.9 kcal/mol) using a series of Dunning's basis sets. Presented data should be useful for modeling of gallium nitride chemical vapor deposition reactors as well as for further theoretical analysis of thermodynamic and kinetic processes in gallium chemistry.

The kinetics of the title reactions were investigated at 5−35 Torr total pressure and 265−306 K by UV flash photolysis and time-resolved mass spectrometry. The CH2ClO radical was generated by photolysis of NOCl in the presence of CH3Cl, O2, and NO. Atomic chlorine from NOCl photodissociation abstracted H from CH3Cl and initiated oxidation of CH2Cl radicals, leading to formation of CH2ClO. The CH2ClO radical reacted with both O2 and NO, or underwent unimolecular elimination of HCl. The kinetics were determined from growth rates of HC(O)Cl and HCl. The rate coefficient of the CH2ClO + O2 reaction was found to be independent of pressure. The temperature dependence can be expressed as:  k(CH2ClO + O2) = (2.0 ± 0.7) × 10^-12 exp[−(934 ± 128)/T] cm³ molecule^-1 s^-1. The rate coefficients for the CH2ClO + NO reaction were found to be independent of both pressure and temperature, with the value k(CH2ClO + NO) = (2.7 ± 0.6) × 10^-12 cm³ molecule^-1 s^-1, obtained by averaging all rate coefficients determined over the experimental range of pressure and temperature. The rate coefficient for unimolecular elimination of HCl from CH2ClO was found to be pressure dependent, and well into the falloff. The unimolecular rate coefficient at 10 Torr can be expressed as (7.7 ± 2.3) × 10⁹ exp[−(4803 ± 722)/T] s^-1.

A mechanism for the homogeneous gas-phase decomposition of SiHCl3, SiH2Cl2, and SiH3Cl in hydrogen is derived from the results of ab initio molecular-orbital studies. It consists of 39 reversible elementary reactions among 25 species, including pressure-dependent unimolecular decomposition of the chlorinated silanes and secondary chemistry due to reactions of SiH2, SiHCl, and SiCl2 with one another and with the chlorinated silanes. Rate parameters in the mechanism have been calculated based on results of ab initio studies using transition-state theory and unimolecular rate theories. This allows us to construct a reasonably complete mechanism that provides qualitative explanations for several features of dichlorosilane decomposition that have been presented in the literature, including observations on the presence and concentrations of SiCl2, SiHCl, and Si atoms. Several chain reactions in which the chain carriers are divalent silicon species have been identified.

Structures, vibrational frequencies, and energies for the chlorinated disilenes, their dibridged isomers, and the transition states connecting the chlorinated disilenes to the corresponding silylsilylenes are presented. Geometries and frequencies were obtained at the MP2/6-31G(d,p) level, and energetics were calculated at the G2, G2(MP2), MP4/6-31+G(2df,p), and/or MP2/6-31+G(2df,p) levels of theory, depending on the number of chlorine atoms in the molecule. The silylsilylene isomer with the structure HnCl3-nSiSiCl was found to be lowest in energy for all of the chlorinated compounds. The dibridged structures are all significantly higher in energy than the silylsilylene and disilene structures. Barriers for isomerization by H transfer and Cl transfer ranged from 5 to 18 kcal/mol and 9 to 23 kcal/mol above the disilene, respectively. Energies along paths for decomposition of the chlorinated disilenes to pairs of silylenes are presented, confirming that the reverse reactions are barrierless. Finally, energetics of various decomposition products of the Si2HnCl4-n compounds are considered. It is shown that for the species with two or more chlorines, decomposition to a pair of silylenes should be the dominant reaction path based on the energetics of competing paths. The isomerization barriers are much smaller than decomposition barriers, so the isomerization reactions will be fast compared to decomposition.

On the basis of concepts from the mathematical theory of approximation of functions, we propose a method of deriving microcanonical transition state theory rate coefficients, both as a function of the total energy and the total angular momentum, from thermal data, namely, the limiting high-pressure rate coefficients. The method does not require the knowledge of the frequencies and degeneracies of the transition state and is general in that it allows for non-Arrhenius forms of thermal data, but it only applies to reactions possessing an intrinsic energy barrier. It is shown that the derived microcanonical rate coefficient is almost identical to the computed Rice−Ramsperger−Kassel−Marcus (RRKM) microcanonical rate coefficient using explicit frequencies and degeneracies of the transition state, and furthermore, that the difference between the two is uniformly distributed over the entire range of total energy and the entire range of the total angular momentum. Comparison of the microcanonical coefficients from the proposed method with those from a standard nonvariational RRKM calculation is presented for the unimolecular decomposition of the ethyl radical and the unimolecular isomerization of methyl isocyanide. The agreement is shown to be excellent. A theoretical analysis of the fine structure of the microcanonical rate coefficient near the threshold of the reaction is enunicated and the difficulty of extending the method to obtain variational microcanonical rate coefficients is described. We also, briefly, speculate on the possible merits of certain theoretical methods of analysis for coping with the representation of thermal data, whose argument is the temperature which is of semiinfinite range.

The thermochemistry and thermal unimolecular decomposition reactions of the chlorinated disilanes have been characterized using ab initio molecular orbital techniques. Silylene, chlorosilylene, dichlorosilylene, and hydrogen elimination reactions and their reverse insertions were considered. Reactant, product, and transition-state geometries and vibrational frequencies were calculated at the MP2/6-31G(d,p) level. Energetics were obtained at the MP2/6-31+G(2df,p), MP4/6-31+G(2df,p), G2(MP2), and/or G2 levels of theory, depending on the number of chlorine atoms in the molecule. In addition to the expected insertion reactions, direct reaction paths for SiHCl + SiHnCl4-n &rlhar2; SiH2 + SiHn-1Cl5-n and SiHCl + SiHnCl4-n &rlhar2; SiCl2 + SiHn+1Cl3-n were observed, with energetic barriers lying a few kcal/mol above the insertion reactions. To our knowledge, these concerted, two-atom exchange reactions have not previously been observed or predicted. They appear to represent a new type of elementary reaction for these compounds. Heats of formation for the chlorinated disilane reactants and chlorinated silylsilylene products of hydrogen elimination were calculated using isodesmic reactions. Energy barriers and conventional transition state theory rate constants for all of the reactions are presented. These can provide a basis for the construction of a detailed mechanism for the multistep thermal decomposition of the chlorinated silanes, which plays an important role in the chemical vapor deposition of epitaxial silicon from the chlorinated silanes.