Computational tests of models for kinetic parameters of unimolecular reactions of organophosphorus and organosulfur compounds

Theoretical Calculation of Reaction Rates and Combustion Kinetic Modeling Study of Triethyl Phosphate (TEP)

Triethyl phosphate (TEP) is an organophosphorus compound used as a simulant for highly toxic nerve agents such as sarin GB. A high temperature decomposition pathway during TEP pyrolysis has been proposed previously and takes place via seven concerted elimination reactions. A computational study to investigate the kinetics of these seven reactions was carried out at the CBS-QB3 level of theory. The transition state optimization was done at the B3LYP/6-311G(2d,d,p) theory level, and CanTherm was used to derive the Arrhenius coefficients. The pre-exponential factors of the rate constant of these reactions were found to be up to 50 times lower than the estimated values from the literature. In addition, kinetics of reaction of the trioxidophosphorus radical (PO₃) with H₂ (H₂ + PO₃ → HOPO₂ + H), which is one of the important reactions in predicting CO formation during TEP decomposition, was also investigated computationally at the same theory level. The new kinetic parameters derived from the computational study were used with the TEP kinetic model proposed recently by our group. In addition, an alternative decomposition pathway for TEP decomposition via H-abstraction, radical decomposition, and recombination reactions was added. The proposed mechanism was validated with the literature's experimental data, that is, intermediate CO time-history data from pyrolysis and oxidation experiments and ignition delay times. Fairly good agreement with experiments was obtained for pyrolysis and oxidation CO yield within 1200-1700 K. The model was able to predict the ignition times of the rich TEP mixture (φ = 2) within 25% of the experimental results, while the discrepancies for stoichiometric and rich mixtures were larger. Discussions on results of sensitivity and reaction pathway analysis are presented to identify the important phosphorus reactions and to understand the effect of addition of the alternative TEP decomposition pathway.

Shock Tube/Laser Absorption and Kinetic Modeling Study of Triethyl Phosphate Combustion

Pyrolysis and oxidation of triethyl phosphate (TEP) were performed in the reflected shock region at temperatures of 1462-1673 K and 1213-1508 K, respectively, and at pressures near 1.3 atm. CO concentration time histories during the experiments were measured using laser absorption spectroscopy at 4580.4 nm. Experimental CO yields were compared with model predictions using the detailed organophosphorus compounds (OPC) incineration mechanism from the Lawrence Livermore National Lab (LLNL). The mechanism significantly underpredicts CO yield in TEP pyrolysis. During TEP oxidation, predicted rate of CO formation was significantly slower than the experimental results. Therefore, a new improved kinetic model for TEP combustion was developed, which was built upon the AramcoMech2.0 mechanism for C₀-C₂ chemistry and the existing LLNL submechanism for phosphorus chemistry. Thermochemical data of 40 phosphorus (P)-containing species were reevaluated, either using recently published group values for P-containing species or by quantum chemical calculations (CBS-QB3). The new improved model is in better agreement with the experimental CO time histories within the temperature and pressure conditions tested in this study. Sensitivity analysis was used to identify important reactions affecting CO formation, and future experimental/theoretical studies on kinetic parameters of these reactions were suggested to further improve the model. To the best of our knowledge, this is the first study of TEP kinetics in a shock tube under these conditions and the first time-resolved laser-based species time history data during its pyrolysis and oxidation.