Develop a model based on elementary reaction kinetics, pathways and thermodynamic properties which includes microscopic reversibility to understand and describe the photochemical oxidation of aromatic hydrocarbons (benzene, toluene, xylene, phenol, etc.) under atmospheric conditions. The reaction mechanism will be validated against experimental data in the literature.
At present there is insufficient kinetic data and no mechanism for modeling atmospheric reactions of aromatic compounds in photochemical oxidation or other reaction systems. Development and validation of a mechanism will enable models pertaining to photochemical smog, air-shed transport and oxidation processes, to incorporate aromatic species. It will also provide an understanding of atmospheric reactions and product formation rates on aromatic moieties.
Reaction mechanisms utilize elementary kinetic parameters coupled with microscopic reversibility and include calculation of steady state levels of active intermediates. Pressure dependent and chemical activation reaction analysis are included in the reaction kinetic parameter calculations. Thermodynamic properties and transition state parameters are determined via literature evaluation, high level ab initio or density functional molecular orbital methods and group additivity. Fundamental principles of thermochemical kinetics and detailed balance are applied to all reactions. Models are validated against experimental data.
Extension of Thermodynamic Property Database Ð aromatics up through styrene. Thermodynamic properties: enthalpies, entropies and heat capacities for cyclic hydrocarbons, alkyl peroxides and alkyl trioxides have been validated, improved, or investigated. Improved thermodynamic properties developed for key reaction adducts: hydroxyl cyclohexadienyl, hydroxyl cyclohexadiene peroxy radical and corresponding radicals with methyl substitution. Butadiene, styrene and similar conjugated olefins are shown to have secondary vinylic sites with a relatively weak CÑH bond (ca 101 kcal/mole). This CÑH is subject to abstraction by hydroxyl radical. Calculation estimates on Ea abstraction are in process.
Development of a detailed reaction mechanism for toluene atmospheric oxidation continues. The mechanism includes ca 100 elementary reactions and 40 species. Predictions of the model are in good agreement with experimental results on product yields (experimental data in parenthesis): benzaldehyde 9.1% (10%), o-cresol 20.1% (20%), p-cresol 4.4% (ca. 4%), m-cresol 0.9% (ca. 1%), nitro-toluene 1.8% (1.8%), glyoxal 14.3% (10 - 15%), methyl glyoxal 14.1% (10 - 15%). Ab initio calculations on the formation mechanism for HO2 radical + phenol (from benzene oxidation) or HO2 plus methylphenol (from toluene) show the likely path as direct HO2 elimination from the aromatic- HO-O2 adduct, with O2 abstraction from the OH-adduct also a possible path. An initial mechanism for styrene is developed; work on validation and reaction path kinetics continues. Detailed reaction paths and kinetic parameters for reactions of OH with ethylene and with vinyl chloride (chloroethylene) are developed and parameters compare well with experiment.
Two computer codes are under development. SMCPS, (Statistical Mechanics Ð Heat Capacity (Cp) and Entropy (S)) for Thermodynamic Property interpretation of output from ab initio and density functional calculations has been developed and is under improvement. THERMKIN is the second, it is for calculation of rate constants from ab initio data.
Comparison of our model to experimental data indicates: (i) there still exists significant uncertainties and controversies in the experimental data for the benzene + OH + O2 reaction system; (ii) careful interpretation of experimental data is necessary; (iii) the detailed, elementary reaction model, including the reverse reaction rate constants, are of value; (iv) correct thermodynamic data on important reaction adducts are necessary.
The original research plan is maintained and research progress is on schedule. The toluene mechanism is being written up, and continues to undergo comparison with experimental data. A mechanism of styrene oxidation is initiated.
Takahiro Yamada, Chiung-ju Chen