Grantee Research Project Results
Final Report: Formation of Chlorinated PAHs in the Combustion and Thermal Processing of Chlorine Containing Materials
EPA Grant Number: R826167Title: Formation of Chlorinated PAHs in the Combustion and Thermal Processing of Chlorine Containing Materials
Investigators: Taylor, Philip H.
Institution: University of Dayton
EPA Project Officer: Hahn, Intaek
Project Period: November 24, 1997 through November 23, 2000
Project Amount: $360,366
RFA: Exploratory Research - Environmental Engineering (1997) RFA Text | Recipients Lists
Research Category: Safer Chemicals , Land and Waste Management
Objective:
Chlorine is a leading industrial product, giving rise to the manifold of chloro-organic products in the United States and other industrialized countries. Chlorocarbons are widely used as solvents in syntheses, as cleaning agents, as starting materials, and in polymer, pesticide, and other product manufacturing applications. Several chlorocarbons are persistent in the environment. Therefore, chlorocarbons are present in the atmosphere in both municipal and hazardous material combustion, as well as in destruction/cleanup processes related to the above industrial applications. A better understanding of the pyrolysis and combustion of chlorinated hydrocarbons (CHCs) is thus of considerable practical and fundamental importance because emissions of CHCs can be minimized by primary measures applying the knowledge of the relevant formation pathways.
Impending strict regulations of all combustion and thermal sources under the Clean Air Act amendments, Resources Conservation and Recovery Act (RCRA), and EPA's Combustion Strategy are based on calculated risk and the ability of the source to minimize emissions of harmful air pollutants. Previous research indicates that a potentially complex array of planar, chlorinated polycyclic aromatic hydrocarbons may be formed from thermal processing of chlorine-containing materials. The toxicity and endocrine-disrupting capabilities of these products are a significant concern. There is insufficient information on the nature and origin of these pollutants to make scientifically defensible regulatory decisions. It is the origin and pathways of formation of these chemicals that were the subject of this study.
The goals of this program were two-fold: (1) to construct a laser photolysis/photoionization time-of-flight mass spectrometry (LP/ToFMS) technique to study polyatomic free radical reactions important in combustion and atmospheric chemistry; and (2) to use the LP/ToFMS system and theoretical methods to evaluate molecular growth of CHCs by investigating the rate of the elementary reaction of vinyl radicals with ethylene and the chlorinated ethylenes. These reactions are expected to be of the temperature- and pressure-dependent, chemically activated type; for example:
Figure 1.
After the initial reaction, with the addition of the radical to the double bond of the substrate with formation of an excited adduct, different pathways of further reaction can take place. They include reverse decomposition back to reactants, collisional stabilization, and further reaction via elimination of an atom a to the attack site. The interplay of these reaction channels depends on temperature and pressure. Hence, complex temperature and pressure dependence of the overall rate constants and product branching ratios was expected.
The following sequential hypotheses concerning the rate and mechanism of molecular growth of CHCs that were examined include:
- The weaker carbon-chlorine bond (vs. carbon-hydrogen) may facilitate chemically activated addition/elimination reactions that are faster than their hydrocarbon counterpart. Based on data for C2Cl4 versus C2H4, CHCs may form more aromatic species than their hydrocarbon analogs.
- CHCs and chlorinated radicals are resistant to oxidation, thus making these species more available for molecular growth reactions than their hydrocarbon analogs.
- The relative ease of chlorine displacement will increase the H to Cl ratio in aromatics and PAHs.
- At least three sequential transition states can occur in the overall displacement reaction. The addition step has an initial loose transition state involving attack on and electrons above the molecular plane, followed by a tight transition state as the radical migrates to a carbon and the bond is broken. The third transition state is formed as a Cl or H is eliminated to form the stable molecule.
- If the first transition state is rate determining, chlorine substitution of the substrate may decrease the overall reaction rate and preferentially activate addition of the adjacent carbon by introducing more non-bonding character near the substituted carbon. Chlorine substitution of the radical decreases the reaction rate.
- If the second transition state is rate determining, chlorine substitution can increase the overall reaction rate for addition by weakening the carbon-carbon bond that must be broken in the second transition state.
- Chlorine substitution lowers the energy of the third transition state because the broken carbon-chlorine bond is weaker than a carbon-hydrogen bond. Thus, if the third transition state is rate determining, chlorine substitution can increase the overall reaction rate.
Summary/Accomplishments (Outputs/Outcomes):
Construction of LP/PI-ToFMS System. We completed the construction of a Laser Photolysis/ Photo-Ionization Time of Flight Mass Spectrometer (LP/PI-ToFMS) system. A schematic of the overall system is provided in Figure 1.
Figure 1. Schematic of laser photolysis/photo-ionization time-of-flight mass spectrometer.
The main performance features of the apparatus in its current configuration are the following:
- Homogeneous generation of the reactive intermediates (radicals, atoms, and stable compounds) in the reactor during a period that is short in comparison with the half-lives of subsequent reactions.
- Sensitive, selective, and quantitative detection of reactive and stable species participating in the studied reaction (at initial concentrations as low as 1012 molecule cm-3).
- Good temporal resolution of species monitored during an experiment (i.e., the ability to study reactions with half-lives as short as 2 ms).
- The ability to study reactions over extended ranges of temperature (295 to 1000 K) and pressure (5-20 torr).
Experimental Studies. To validate the theoretical studies of the elementary reaction of vinyl radicals with ethylene and the chlorinated ethylenes, following the construction of the LP/PI-ToFMS apparatus, a series of experiments were conducted to measure the rate of C2H3 radical reactions with the chloroethylenes. Vinyl bromide and vinyl sulfone were used as C2H3 radical precursors at 193, 248, and 308 nm (Fahr and Stein, 1989). Unfortunately, because each of the substrates absorbs strongly in the ultraviolet, substrate photolysis could not be minimized to an extent sufficient to permit useful quantitative reaction rate or stable product determinations. Repeated attempts to mitigate substrate photolysis using longer wavelength photolysis were unsuccessful due to low quantum yields for C2H3 radical production. Similar problems were observed for trichlorovinyl (C2Cl3) radicals using tetrachloroethylene as the radical precursor.
The remainder of the program focused on comprehensive theoretical calculations summarized in the following paragraphs. The theoretical studies of C2H3 radical reactions with ethylene were compared with experimental measurements in the literature (Fahr and Stein, 1989; NIST database, 2001) to provide an indication of the accuracy of the theoretical studies.
Theoretical Calculations. Kinetic modeling and analysis of reaction pathways requires characterization of potential energy surfaces (PESs). Group additivity (GA) methods (Benson, 1976), semi-empirical molecular orbital (MO) theory, PM3 (Stewart, 1989), and several composite ab initio and DFT calculations, CBS-Q (Ochterski, et al., 1996), G3(MP2) (Curtiss and Redfern, 1999), B3LYP/6-311+G(2df,p)//B3LYP/6-31G(d), and others were used to develop PESs. Even though the GA method and PM3 level of theory gives reasonable values, PESs for all reaction systems are being evaluated using ab initio theory. ab initio calculations consistently give more accurate values than GA method and PM3. All ab initio calculations were performed using the Gaussian 98 computer code (Frisch, et al., 1998) on two computer systems: (1) an Origin 2000 supercomputer at the Ohio Supercomputer Center (OSC); and (2) a recently purchased Compaq XP1000 workstation at UDRI. The new calculation theories are being proposed almost semi-annually, and the best available methods were applied for the theoretical studies of vinyl radical addition to chlorinated ethylenes.
CH2=CH2 + C2H3 → C•H2CH2CH=CH2 → Products | (rxn 3) |
CHC1=CH2 + C2H3 → C•HC1CH2CH=CH2 → Products | (rxn 4) |
CHC1=CH2 + C2H3 → C•H2CHC1CH=CH2 → Products | (rxn 5) |
trans-CHC1=CHC1 + C2H3 → Products | (rxn6) |
CC12=CHC1 + C2H3 → C•C12CHC1CH=CH2 → Products | (rxn7) |
CC12=CHC1 + C2H3 → C•HC1CC12CH=CH2 | (rxn8) |
CC12=CC12 + C2H3 → Products | (trxn 9) |
The Arrhenius rate expressions of major reaction paths in each system are tabulated in Table 1. The major products of each reaction at a temperature of 1500 K are listed in Table 2.
Reaction | A | n | Ea | Log k1500 |
---|---|---|---|---|
C2H4 + C2H6 → C4H6 + H | 2.83E+12 | 0.35 | 12.27 | 11.77 |
CHC1CH2 + C2H3 → CH2CHCHCH2 + C1 | 1.99E+12 | 2.44 | 11.08 | 9.43 |
CHC1CH2 + C2H3 → CHC1CHCHCH2 + H | 4.69E+07 | 1.81 | 10.02 | 11.96 |
CHC1CHC1 + C2H3 → CHC1CHCHCH2 | 2.72E+04 | 2.73 | 3.10 | 12.65 |
CHC1CC12 + C2H3 → CC12CHCHCH2 + C1 | 1.01E+02 | 3.37 | 4.85 | 13.41 |
CHC1CC12 + C2H3 → CHC1CC1CHCH2 + C1 | 8.27E+07 | 1.86 | 7.63 | 12.71 |
C2C14 + C2H3 → CC12CC1CHCH2 + C1 | 6.98E+01 | 3.38 | 4.03 | 11.95 |
Reaction | Major Products |
---|---|
C2H4 + C2H3 | 1,3-butadiene |
CHC1CH2 + C2H3 | 1-chloro-1,3-butadiene |
CHC1CHC1 + C2H3 | 1-chloro-1,3 butadiene |
CHC1CC12 + C2H3 | 1,1- dichloro-1,3-butadiene |
C2C14 + C2H3 | 1,1,2-trichloro-1.3-butadiene |
Theoretical calculations indicate that, with the exception of vinyl chloride, vinylidation to form chlorinated butadiene and Cl atom is the most favored chemically activated displacement channel for all chlorinated ethylene reaction systems. In the case of vinyl chloride, chlorinated butadiene formation via H elimination is faster than the analogous Cl elimination pathway forming 1,3-butadiene because of the lower activation energy in the initial addition reaction. Our calculations further indicate that the first transition state involving C2H3 addition to the p and s electrons above the molecular plane of the substrates is rate limiting. Increasing Cl substitution of the substrate increases the activation energy of the addition reaction and decreases the overall reaction rate to products. Furthermore, Cl substitution tends to activate addition at the adjacent carbon by introducing more non-bonding character near the substituted carbon.
Our high-level calculations also indicate that C2H3 addition to the chlorinated ethylenes has a higher molecular growth rate than C2H3 addition to ethylene. Specifically, it was found that C2H3 addition to trans-CHClCHCl has higher molecular growth rate than C2H3 addition to C2Cl4 due to the lower activation energy for the initial addition reaction and the similar exothermicity for the vinylidation process. Additional calculations are underway to determine if this trend can be extended to other molecular growth reaction (e.g., vinyl radical addition to partially chlorinated butadiene).
Reliable extrapolation of laboratory or pilot data to field conditions requires development of sophisticated scale-up models that include detailed reaction sets of pollutant formation and destruction pathways. Although combustor and flow-reactor studies often are used to develop mechanisms and begin assembly of detailed kinetic models, there are usually a manageable number of reactions for which very precise and accurate determinations of the kinetics are required for the overall model to be accurate. In the development of molecular growth components of these models, the results of this research represent a first attempt to quantify the effect of chlorine on the rates of these reactions using advanced experimental and theoretical techniques. The previously published models of the combustion of chlorinated compounds (Taylor, et al., 1994; Taylor, et al., 1996a; Taylor, et al., 1996b; Taylor, et al., 1996c) can now be refined based on the results of this study.
References:
Fahr A, Stein S. Proceedings of the Combustion Institute 1989;22:1023-1029.
NIST Chemical Kinetics Database, Standard Reference Database 17, Version 7.0.
Benson SW. Thermochemical Kinetics. New York, NY: Wiley-Interscience, 1976.
Stewart JJP. Journal of Computational Chemistry 1989;10:209.
Ochterski JW, Petersson GA, Montgomery JJA. Journal of Chemical Physics 1996;104:598.
Curtiss LA, Redfern PC. Journal of Chemical Physics 1999;110:4703 .
Frisch, MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR, Zakrzewski VG, Montgomery JA, Stratmann RE, Burant JC, Dapprich JM, Millan JM, Daniels AD, Kudin KN, Strain MC, Farcas O, Tomasi J, Barone V, Cossi M, Cammi R, Mennucci B, Pomelli C, Adamo C, Clifford S, Ochterski J, Petersson GA, Ayala PY, Cui Q, Morokuma K, Marick DK, Rabuck AD, Raghavachari K, Foresman JB, Cioslowski J, Ortiz JV, Stefanov BB, Liu G, Liashenko A, Piskorz P, Komaromi I, Gomperts R, Martin RL, Fox DJ, Keith T, Al-Laham MA, Peng CY, Nanayakkara A, Gonzalez C, Challacombe M, Gill PMW, Johnson B, Chen W, Replogle ES, Pople JA. Gaussian, Inc.: Pittsburgh, PA, 1998.
Taylor PH, Tirey DA, Rubey WA, Dellinger B. Combustion Science and Technology 1994;101:75-102.
Taylor PH, Tirey DA, Dellinger B. Combustion and Flame 1996a;104:260-271.
Taylor PH, Tirey DA, Dellinger B. Combustion and Flame 1996b;105:486-498.
Taylor PH, Tirey DA, Dellinger B. Combustion and Flame 1996c;106:1-10.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 7 publications | 3 publications in selected types | All 2 journal articles |
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Type | Citation | ||
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Yamada T, Taylor PH. Kinetics of vinyl radical reactions with ethylene, trans-1,2-dichloroethylene, and tetrachloroethylene International Journal of Chemical Kinetics. |
R826167 (Final) |
not available |
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Yamada T, Steiger M, Bozzelli JW, Taylor PH. Reaction pathway analysis for vinyl radical reactions with the chloroethylenes. Physical Chemistry Chemical Physics. |
R826167 (Final) |
not available |
Supplemental Keywords:
combustion chemistry, environmental engineering, exposure, air pollution, polyatomic free radicals, ab initio calculations., RFA, Scientific Discipline, Toxics, Waste, Environmental Chemistry, HAPS, Incineration/Combustion, Environmental Engineering, hydrocarbon, mass spectrometry, hazardous air pollutants, air pollution, chemical contaminants, chlorinated PAHs, PAH, carcinogens, EPA's Combustion Strategy, thermal processing, hydrocarbons, incineration, RCRAProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.