International Journal of Chemical Kinetics最新文献

Hydrogen sulfide generated during hydrotreatment of sour crude oil fractions could be absorbed into aqueous ammonium hydroxide to produce ammonium sulfide. This ammonium sulfide can then be utilized to produce commercially valuable aromatic amino compounds by reducing the corresponding nitro compounds. In this work, the reduction of m-iodonitrobenzene (m-INB) to m-iodoaniline (m-IA) was performed by aqueous ammonium sulfide using a phase transfer catalyst, tetrabutylammonium bromide (TBAB). The study scrutinized the influences of various parameters such as concentrations of TBAB and m-INB, as well as initial sulfide and ammonia concentrations, on the rate of reaction of m-INB. An 11-fold increase in reaction rate was obtained with only 0.09 kmol of catalyst TBAB per cubic meter of the organic phase. The selectivity of m-IA was found to be100%. The reaction was found to be kinetically controlled with an activation energy of 40.0 kJ/mol. The rate of reaction of m-INB was observed to be directly proportional to the concentrations of m-INB, initial sulfide, and catalyst. A pseudo-first order kinetic model was developed to correlate the conversion versus time data and an excellent agreement between observed and predicted reaction rates was obtained. The present work has very high commercial importance as it could be a viable alternative to the traditional Claus process to arrest H₂S released by petroleum refineries.

Rate coefficients, k(T), for the gas-phase Cl atom reaction with hexamethyldisiloxane ((CH₃)₃SiOSi(CH₃)₃, L₂), k₁; octamethyltrisiloxane ([(CH₃)₃SiO]₂Si(CH₃)₂, L₃), k₂; decamethyltetrasiloxane ((CH₃)₃SiO[Si(CH₃)₂O]₂Si(CH₃)₃, L₄, k₃; dodecamethylpentasiloxane ((CH₃)₃SiO[Si(CH₃)₂O]₃Si(CH₃)₃, L₅, k₄; hexamethylcyclotrisiloxane ([-Si(CH₃)₂O-]₃, D₃), k₅; octamethylcyclotetrasiloxane ([-Si(CH₃)₂O-]₄, D₄), k₆; decamethylcyclopentasiloxane ([-Si(CH₃)₂O-]₅, D₅, k₇), and dodecamethylcyclohexasiloxane ([-Si(CH₃)₂O-]₆, D₆, k₈) were measured over a range of temperature (273–363 K) using a pulsed laser photolysis (PLP) – resonance fluorescence (RF) technique. The obtained k(296 K) and Arrhenius expressions with 2σ uncertainties including estimated systematic errors are (in units of 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹):

The cyclic permethyl siloxanes (cyclic PMS) were found to be less reactive than the analogous linear permethyl siloxane (linear PMS) with an equal number of CH₃- groups. Both linear and cyclic compounds show a linear relationship between the measured rate coefficient and the number of CH₃- groups in the molecule. A structure–activity relationship (SAR) is presented that reproduces the experimental data to within ∼10% at all temperatures. For [Cl] ≈ 10⁴ atom cm⁻³, an approximate free troposphere abundance, the PMS loss due to Cl atom reaction leads to relatively short estimated lifetimes of 7, 6, 5, 4, 20, 10, 7, and 5 days for L₂, L₃, L₄, L₅, D₃, D₄, D₅, and D₆, respectively. Therefore, the PMSs included in this study are classified as atmospherically very short-lived substances and Cl atom reaction represents a significant loss process.

Rate coefficients, k(T), for the gas-phase OH radical reaction with decamethyltetrasiloxane ((CH₃)₃SiO[Si(CH₃)₂O]₂Si(CH₃)₃, L₄, k₁), dodecamethylpentasiloxane ((CH₃)₃SiO[Si(CH₃)₂O]₃Si(CH₃)₃, L₅, k₂), and decamethylcyclopentasiloxane ([–Si(CH₃)₂O–]₅, D₅, k₃), and dodecamethylcyclohexasiloxane ([–Si(CH₃)₂O–]₆, D₆, k₄) were measured using a pulsed laser photolysis—laser induced fluorescence absolute method over the temperature range 270–370 K. The obtained room temperature rate coefficients, with quoted 2σ absolute uncertainties, and fitted temperature dependence are (cm⁻³ molecule⁻¹ s⁻¹):

The 2σ absolute rate coefficient uncertainty, for all compounds included in this study, is conservatively estimated to be ∼10% over the entire temperature range. The cyclic permethylsiloxanes were found to be less reactive than the analogous linear compound, while both linear and cyclic compounds show increasing reactivity with increasing number of CH₃- groups. A structure activity relationship (SAR) parameterization for the permethylsiloxanes is presented. The estimated atmospheric lifetimes due to OH reaction for L₄, L₅, D₅, and D₆ are 5.2, 4.4, 6.8, and 5.2 days, respectively.

To extend the rule-based approach for hydrogen abstraction reactions from oxygenated compounds, a systematic investigation was performed to examine the reactivity of gas-phase hydrogen abstraction reactions from alkyl groups (methyl and ethyl groups) bound to oxygen atoms in five types of oxygenated compounds (alcohols, ethers, formate esters, acetate esters, and carbonate esters) by H atoms and HO₂ radicals comprehensively considering rotational conformers. Quantum chemical calculations were conducted at the CBS-QB3 level for stationary points. Rate constants were determined employing conventional transition state theory (TST). For hydrogen abstraction reactions by H, the rotational conformer distribution partition function was employed to approximate partition functions, owing to the similarity in vibrational energy-level structures among conformers. In hydrogen abstraction reactions by HO₂, the vibrational structures of transition-state (TS) conformers varied significantly due to the hydrogen bonding, leading to an inappropriate evaluation of rate constants when using the lowest-energy conformer as a representative. Therefore, the rate constants were calculated by the multi-structural TST. It was revealed that the differences in functional groups containing O atoms mainly affect the bond dissociation energies of the C–H bonds and the activation energies of hydrogen abstraction reactions only when the C atoms are adjacent to the O atoms. Additionally, it was found that hydrogen bonds formed in the TSs show minor effect on rate parameters for the overall rate constants, apart from the reduction of the pre-exponential factors for the H-abstraction reactions from the methylene position of ethyl groups. The comparison with the rate constants from previous studies showed reasonable results, indicating that the rate constants in this study, which thoroughly consider rotational conformers, can be the current best estimates.

In this study, ascorbic acid (AA) and cysteine (Cys) were used as homogeneous potassium persulfate (S₂O₈²⁻) activators. The efficiency of the S₂O₈²⁻/AA and S₂O₈²⁻/Cys systems was investigated to generate sulfate radicals (SO₄^−•) for the oxidation of sulfathiazole (STZ). The presence of AA and Cys displayed a promoting effect on the activation of S₂O₈²⁻. The results indicated that the STZ/S₂O₈²⁻ redox reaction followed pseudo-first order kinetics with respect to STZ concentrations. The oxidative degradation of STZ is accelerated by temperature, dose of S₂O₈²⁻, AA, Cys, and pH with S₂O₈²⁻/AA and/or S₂O₈²⁻/Cys systems. The degradation rates of STZ followed the order S₂O₈²⁻/AA > S₂O₈²⁻/Cys > S₂O₈²⁻ under similar experimental conditions. The presence of SO₄^−• and HO^• were tested with two radical scavengers, tertiary butanol (TBA) and ethanol, in which HO^• was mainly responsible for STZ degradation at higher pH. In summary, S₂O₈²⁻/AA and S₂O₈²⁻/Cys systems might provide a potentially useful technique for remediation of water contaminants.

Palladium-catalyzed carbonylation of allyl alcohol to 3-butenoic acid has been investigated. A significant effect of halide promoters, p-tolylsulfonic acid (TsOH), water, solvents, and PPh₃ concentration activity and selectivity has been studied. Detailed kinetics of this reaction was investigated in a temperature range of 363–383 K. The influence of parameters such as stirring speed, allyl alcohol, catalyst, benzyltriethylammonium chloride (BTEAC), TsOH concentrations, and CO partial pressures on the activity and selectivity has been studied. An empirical rate equation was suggested and found to be fairly consistent with observed rate data. In addition, the activation energy and kinetic parameters were evaluated.

Dialkyl carbonates (DACs) own an environmentally friendly synthesis route, making them potential candidates as alternative fuels. However, for DACs to be widely accepted as an alternative fuel, a comprehensive understanding of their combustion behavior is essential. Dipropyl carbonate (DPrC) represents a transition from short-chain to mid-chain carbonates, understanding its combustion behaviors holds significance in unraveling the combustion chemistry of carbonates. In this study, the oxidation of DPrC was investigated with the initial fuel mole fraction of 0.5% at three equivalence ratios of 0.5, 1.0, and 2.0 within a temperature range of 550–1100 K in a jet-stirred reactor for the first time. Gas chromatography was utilized for the quantitative detection of reactants, intermediates, and products. A detailed DPrC mechanism was first developed, and good agreements between measurements and simulations were obtained. A notable negative temperature coefficient (NTC) behavior was first observed in the oxidation of DACs. Such NTC phenomenon occurred at fuel-lean conditions in the temperature range of 620–660 K, while only a weak low-temperature consumption was observed at the stoichiometric condition. Kinetic modeling studies showed that this unique low-temperature chemistry of DPrC can be attributed to the differences in the RO₂ isomerization reactions between DPrC and short-chain DACs. The RO₂ isomerization via a six-member ring transition state could happen in DPrC oxidation but not in dimethyl carbonate and diethyl carbonate oxidation, due to the different fuel molecular structure. Therefore, the subsequent reaction pathways via QOOH → O₂QOOH → HO₂Q = O + OH → OQ = O + OH were promoted and two OH radicals were released in this process. Moreover, it is conceivable that mid or long-chain DACs could also exhibit an NTC phenomenon due to the increased potential for RO₂ isomerization via a six- or seven-member ring transition state, thereby increasing the likelihood of RO₂ isomerization occurrence.

Detailed chemical kinetic mechanisms are necessary for resolving many important chemical processes. As the chemistry of smaller molecules has become better grounded and quantum chemistry calculations have become cheaper, kineticists have become interested in constructing progressively larger kinetic mechanisms to model increasingly complex chemical processes. These large kinetic mechanisms prove incredibly difficult to refine and time-consuming to interpret. Traditional sensitivity analysis on a large mechanism can range from inconvenient to practically impossible without special techniques to reduce the computational cost. We first present a new time-local sensitivity analysis we term transitory sensitivity analysis. Transitory sensitivity analysis is demonstrated in an example to accurately identify traditionally sensitive reactions at an 18,000x speed up over traditional sensitivities. By fusing transitory sensitivity analysis with more traditional time-local branching, pathway, and cluster analyses, we develop an algorithm for efficient automatic mechanism analysis. This automatic mechanism analysis at a time point is able to identify the reactions a target is most sensitive to using transitory sensitivity analysis and then propose hypotheses why the reaction might be sensitive using branching, pathway, and cluster analyses. We implement these algorithms within the reaction mechanism simulator (RMS) package, which enables us to report the automatic mechanism analysis results in highly readable text formats and in molecular flux diagrams.