Grantee Research Project Results

1997 Progress Report: Elementary Reaction Mechanism and Pathways for Atmospheric Reactions of Aromatics - Benzene and Toluene

EPA Grant Number: R824970C005
Subproject: this is subproject number 005 , established and managed by the Center Director under grant R824970
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
Center: Research Consortium on Ozone and Fine Particle Formation in California and in the Northeastern United States
Center Director: Seinfeld, John
Title: Elementary Reaction Mechanism and Pathways for Atmospheric Reactions of Aromatics - Benzene and Toluene
Investigators: Bozzelli, Joseph W. , Lay, Tsan
Institution: New Jersey Institute of Technology
EPA Project Officer: Hahn, Intaek
Project Period: January 1, 1992 through May 31, 1997
Project Period Covered by this Report: January 1, 1996 through May 31, 1997
Project Amount: Refer to main center abstract for funding details.
RFA: Center on Airborne Organics (1993) Recipients Lists
Research Category: Targeted Research

Objective:

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. 

Rationale:  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. 

Approach:  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, group additivity, and ab initio molecular orbital methods.  Fundamental principles of thermochemical kinetics and detailed balance are applied to all reactions.  Models are validated against literature experimental data. 

Status:  Extension of Thermodynamic Property Databas 
Thermodynamic properties: enthalpies, entropies and heat capacities for cyclic hydrocarbons, alkyl peroxides, alkyl trioxides have been validated, improved, or investigated. Significant improvement for key reaction adducts: hydroxyl cyclohexadienyl, hydroxyl cyclhexadiene peroxy radical and these two radical with methyl substitution, are especially important 

Establishment of Toluene Reaction Mechanis 
A detailed reaction mechanism has been built for toluene atmospheric oxidation. The mechanism includes 79 elementary reactions and 30 species. The prediction of kinetic modeling is in good agreement with the experimental results on final 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%). 

Comparison of Available Experimental Data 
We compare recently published data of Koch (1996), of Pagsberg (1996), and of Y. P. Lee (1994), all of which is relevant to the gas phase aromatic chemistry. Koch et al. employed flash photolysis/resonance fluorescence to study the reaction of benzene + OH and benzene-OH + O₂ at tropospheric condition; the [OH] concentration as function of time is monitored. Their data show that the decay of OH in the presence of benzene is bi-exponetial.  This verifiers our model prediction which indicates the dissociation of benzene-OH to benzene + OH occurs and the overall reaction of benzene + OH reaches a pseudo-equilibrium. Compari-sons of our model simulation to the data of Koch et al. are presented in the attached Figure A.  Our model correctly predicts OH profiles in the absence of O₂. When O₂ is present, OH decay are predicted from 0 to 0.2 s and slightly underpredicted thereafter. Koch's data is, however, only in abstract form, and we look to further interpret the data when more detailed information is obtained 

The data of Pagsberg, et al.. are pulse radiolysis experiments (Argon bath) on the OH-initiated oxidation of benzene, and reactions of benzene-OH adduct with NO2.  For the reactions of benzene + OH, phenol is identified as the primary product with relative yield of 25?5%.  They conclude that the reaction: OH + benzene ?> H + phenol, should account for the experimental results, at the rate constant k₅=1.7?10¹¹cm³ mol^-1 s^-1. This rate constant value is significantly higher than the literature data: 1.5?10⁵cm³ mol^-1 s^-1 (Fritz, et al.).  Our QRRK calculation results in k₅=3.4?10⁷ cm³ mol^-1 s^-1, which is between these two values. Another supporting value in Pagsberg's article is the rate constant for benzene-OH + O₂ => products, k=3.0?10¹¹ cm³ mol^-1 s^-1, which is in good agreement with our QRRK calculation: k₇=7.1?10¹¹ cm³ mol^-1 s^-1, and 3 orders of magnitude higher than the literature data: 1.1?10⁸ cm³ mol^-1 s^-1. ³  

The data of Y. P. Lee, et al. is on OH plus benzene using laser-photolysis/laser-induced-fluorescence over 250-500 Torr and 345-385 K.  The rate constants they report for OH + benzene = benzene-OH are: k_f = 1.4?10¹² cm³ mol^-1 s^-1 and k_r=2.9s^-1, compared to our data: k_f = 7.3?10¹¹ cm³ mol^-1 s^-1 and k_r = 0.15 s^-1.  We note the entropy value S?₂₉₈(benzene-OH) they adopted is as low as 75 cal mol^-1 K^-1, compared to our value calculated using the PM3 method: 82.7 cal mol^-1 K^-1 (number of optical isomer assigned as 2)  We further analyze their enthalpies using density functional theory at the B3LYP/6-31G* level to determine the S?₂₉₈(benzene-OH) as 82.2 cal mol^-1 K^-1. We are confident of our entropy data and believe that Lee et al. misinterpreted the entropy of the benzene- OH adduct radical by ca. 7cal mol^-1 K^-1.  This suggests the need to reinterpret their experimental data, and modify their rate constants. 

The comparison of our theoretical work 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 required; (iv) correct thermodynamic data of the primary reaction adduct are necessary. 

Future Plans:  We plan on extending the mechanistic study to oxygenated aromatics such as phenol and cresol, expanding the mechanistic study to alkyl substituted aromatics such at xylenes, ethyl-benzene and styrene, and extending the thermodynamic properties to reaction species of above reaction systems. 

Key Personnel 

Graduate Students:  Takahiro Yamada and Chiung-Ju Chen 
Undergraduate Student:  Satyen S. Amin

Supplemental Keywords:

RFA, Ecosystem Protection/Environmental Exposure & Risk, Air, Scientific Discipline, Waste, Toxics, Fate & Transport, particulate matter, Environmental Chemistry, 33/50, tropospheric ozone, Atmospheric Sciences, Physics, photooxidation, atmospheric chemistry, atmospheric transport, benzene, photochemical smog, Toluene, modeling studies, thermodynamics, particulates, fate and transport, aromatics, chemical kinetics, elementary reaction kinetics, contaminant transport models, ambient air, chemical transport model, atmospheric transformation

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Final

Main Center Abstract and Reports:

R824970    Research Consortium on Ozone and Fine Particle Formation in California and in the Northeastern United States

Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
R824970C001 Chemical Kinetic Modeling of Formation of Products of Incomplete Combustion from Spark-ignition Engines
R824970C002 Combustion Chamber Deposit Effects on Engine Hydrocarbon Emissions
R824970C003 Atmospheric Transformation of Volatile Organic Compounds: Gas-Phase Photooxidation and Gas-to-Particle Conversion
R824970C004 Mathematical Models of the Transport and Fate of Airborne Organics
R824970C005 Elementary Reaction Mechanism and Pathways for Atmospheric Reactions of Aromatics - Benzene and Toluene
R824970C006 Simultaneous Removal of Soot and NOx from the Exhaust of Diesel Powered Vehicles
R824970C007 Modeling Gas-Phase Chemistry and Heterogeneous Reaction of Polycyclic Aromatic Compounds
R824970C008 Fundamental Study on High Temperature Chemistry of Oxygenated Hydrocarbons as Alternate Motor Fuels and Additives
R824970C009 Markers for Emissions from Combustion Sources
R824970C010 Experimental Investigation of the Evolution of the Size and Composition Distribution of Atmospheric Organic Aerosols
R824970C011 Microengineered Mass Spectrometer for in-situ Measurement of Airborne Contaminants