Investigation of Mechanisms of Polycyclic Aromatic Hydrocarbons

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Environ. Sci. Technol. 2008, 42, 2175–2180

Investigation of Mechanisms of Polycyclic Aromatic Hydrocarbons (PAHs) Initiated from the Thermal Degradation of Styrene Butadiene Rubber (SBR) in N2 Atmosphere EILHANN KWON AND MARCO J. CASTALDI* Department of Earth and Environmental Engineering (HKSM), Columbia University in the City of New York, New York, New York 10027

Received October 19, 2007. Revised manuscript received January 8, 2008. Accepted January 15, 2008.

This study has been carried out to characterize the thermal decomposition of styrene-butadiene rubber (SBR), using thermogravimetric analysis (TGA) coupled to online GC/MS, and to investigate the formation and ultimate fate of chemical species produced during gasification of SBR. A preliminary mechanistic understanding has been developed to explain the formation and relationship of light hydrocarbons (C1–C4), substituted aromatics, and polycyclic aromatic hydrocarbons (PAHs) during the decomposition of SBR in a N2 atmosphere. Identification and absolute concentrations of over 50 major and minor species (from hydrogen to benzo[ghi]perylene) have been established, and the measurements have been carried out between 300 and 500 at 10 °C/min heating rate in a N2 atmosphere. The concentration of styrene reached 120 PPMV and the concentration of other substituted aromatics, such as toluene and ethyl benzene reached 20 and 5 PPMV, respectively. These measurements indicate PAH formation at a relatively lower temperature as compared to conventional fuel, such as coal and diesel. The PAH sequence is not simply the constructing of larger PAHs from smaller ones to achieve the complex polymer structures. It is possible to generate large PAH molecules while circumventing the typical construction pathway.

Introduction The disposal of used automobile tires has given rise to many environmental and economic issues. For example, in the United States, 750 million to 2 billion used tires are currently stockpiled, with their number increasing at a rate of 280 million per year (1) . An ideal system for disposal would permit material and energy recovery in an environmentally benign way. Tires are composed of significant amounts of carbon. Energy recovery should be the desired end use for scrap tires. Cement kilns are already using scrap tires as a fuel cofeed. In addition, tire-derived fuel (TDF, whole or shredded tires) is utilized as a supplemental or dedicated fuel by some pulp and paper mills, electric utilities, and dedicated tire-to-energy facilities. TDF contains 32 to 37 MJ/ kg, which is a higher energy content than most types of coal * Corresponding author tel: 212-854-6390; fax: 212-854-7081; e-mail: [email protected]; mailing address: S.W. Mudd #926B, 500 West 120th Street, New York, NY 10027. 10.1021/es7026532 CCC: $40.75

Published on Web 02/13/2008

 2008 American Chemical Society

(2). A considerable amount of research focusing on converting energy using the thermal degradation of scrap tires has been done (1, 3–6), but investigation of the thermal degradation mechanisms and levels of pollutants derived from the thermal degradation of scrap tires has not been extensively carried out. Tires are composed of various rubbers, such as natural rubber (NR), butyl rubber (BR), and styrene butadiene rubber (SBR) (3, 7). The most commonly used synthetic rubber in tire industries is SBR, containing 25 wt% styrene. In addition to the rubber compounds, tires contain reinforcing fillers, fiber, extenders, and vulcanizing agents. This heterogeneity results in a variety of coupled degradation pathways that lead to a poor understanding of the decomposition mechanisms and the ultimate production and fate of emissions during the combustion or gasification process. Of particular concern is the formation of substituted aromatics and polycyclic aromatic hydrocarbons (PAHs) due to their adverse health effects. Many PAHs are known to be toxic, mutagenic, and carcinogenic, and their contamination potential is of great environmental concern (8, 9). The major source of PAH emission is from the products of incomplete combustion (10). Over the past years, there has been a great amount of interest in the mechanism of PAH and soot formation in combustion, and considerable progress has been made in our quantitative understanding of these processes. A PAH formation mechanism has been postulated from Frenklach and co-workers (11–13) for acetylene combustion. Similarly a hydrogen abstractionacetylene addition (HACA) scheme was introduced by Bittner and Howard (14–17). Marinov (18) has postulated propargyl consumption by H atoms as an important reaction step that limits aromatic and PAH growth (19). To date, there is very little understanding of PAH formation mechanisms for alternative solid fuels. There have been recent studies where PAH has been identified from the combustion and pyrolysis of waste tires (20–22), as well as characterization of the combustion process and the nature of the gaseous and solid emissions resulting from the burning of coal with waste tire derived fuels (23). In addition, mass spectrometric analysis of the gases evolved during the pyrolysis of NR, SBR, and other synthetic rubbers has been carried out with a TGA (24) and tubular flow reactors (25). The focus of this study is to quantify the release of substituted aromatics and PAH from SBR, which is the main constituent of tires. The molecular structure of SBR consists of a butadiene backbone supporting an aromatic ligand, which serves to accelerate PAH formation (26, 27). Benzene derivatives (substituted aromatics) serve as precursors of PAHs and give insight into the thermal degradation mechanism as well. This includes a more complete description of the benzene derivative formation pathways resulting from the combination of fragments of styrene and butadiene backbone from SBR. This study has identified and quantified 37 benzene derivatives, with structural isomers (4, 7, 16, 5, and 5 structural isomers of benzene derivatives having a molecular weight of 106, 120, 134, 148, and 162, respectively) from the thermal decomposition of SBR. To our knowledge, there has not been a detailed quantitative investigation into the mechanism of decomposition of SBR that has identified PAHs as emissions and their formation pathway. The identification and quantification has been determined experimentally using GC/MS coupled to a TGA unit. This study supplies information on the identities and concentrations of hazardous air pollutants consisting of 50 major and minor species. This information provides new VOL. 42, NO. 6, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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data for the development and validation of detailed reaction mechanisms describing thermal degradation and PAH origin and fate. A knowledge of the concentrations of these gaseous emissions can enable the formation of a possible strategy to mitigate the levels of these chemical species.

Experimental Section All experiments were done using a Netzsch STA 409 PC/4/H TGA unit capable of simultaneous TGA and differential thermal analysis (DTA) measurements. The apparatus was computer controlled using Netzsch software called Proteus for continuous data acquisition and control. The temperature ramp rates were provided as input to the software that then controlled a furnace to achieve the heating rates used for these tests. Those heating rates were 10 and 20 °C/min over a temperature range from ambient to 1000 °C. In addition, all data were digitally recorded and S-type thermocouple readings were compared simultaneously to target temperature and time. The gases used for the experiments were ultrahigh purity and purchased through T.W. Smith (New York). The flow rates were set using Aalborg thermal mass flow controllers (GFCS-010378) certified by Aalborg Inc. The TGA apparatus had two inlet ports, one for a protective gas of pure nitrogen that shrouded the balance mechanism from any heat or effluent gases and was set at a flow rate of 20 mL/min. The second port was used for introducing the gas mixture of interest to provide the desired atmosphere during the experiment. The flow rate of the atmosphere inlet was 80 mL/min; this combined with 20 mL/min protective gas flow to yield a 100 mL/min total flow past the test sample that was maintained for all experiments done. The effluent of the TGA was sent to either a micro-GC (Agilent 3000) or a GC/ MS (Agilent 9890/5973) using a valving system for identification and quantification of chemical species from the TGA. The lag time of the sample from the TGA furnace to the injection block was calculated to be less than 3 s, based on a 5 mL volume transfer line. The injection block of the GC/ MS contained both a 10-port valve and 6-port valve assembly (Valco Custom Valves, Houston, TX) The sampling system, that included transfer lines coupled to a vacuum pump, was maintained near 300 °C to minimize the condensation and/ or adsorption of PAHs onto its surfaces (28). This temperature was determined based on the vapor pressure of expected PAHs. The GC was equipped with a capillary column (0.25 mm × 30 m HP-5MS), which was directly interfaced to a quadrupole mass spectrometer. Identification of species was accomplished by matching both the gas chromatographic retention times to those of the pure components and the mass spectral fragmentation patterns to those species found in standard MS libraries. Permanent gases and C1 and C2 chemical species were determined using a Carboxen 1010 (Supleco 25467) connected to the TCD. To obtain C4 species a micro-GC (Agilent 3000) was used. Species concentrations were determined by multilevel calibrations using a Restek PAH standard (Lot A03448), a Sigma Aldrich aromatic standard (PIANO Aromatic Lot 2102), and a Japanese indoor air standards mixture (Lot 4M7537-U). The high-purity SBR test sample was purchased from Sigma-Aldrich Chemicals (St. Louis, MO) in a granular form ranging in size between 180 and 200 mesh. Initial test sample weights were typically about 10 mg and all samples came from that same chemical batch. The reported concentrations are diluted since there was a considerable flow of purge and protective gas in the TGA and the exact starting masses of the samples varied slightly. Therefore, all concentrations have been normalized to ppmV/10 mg. Prior to each test, a baseline run was done for each sample mixture. This baseline was 2176

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FIGURE 1. TGA and DTA curves for the SBR sample in a N2 atmosphere at various heating rates. then used to subtract the changes of sample carrier system due to the buoyancy changes of the gas mixture as temperatures were changed. To ensure reproducibility and precision, three sets of experiments for each individual condition have been carried out and averaged. The typical experimental error was within ( 2%, however error bars are not shown in an effort to keep the plots clear.

Results and Discussion Figure 1 depicts representative data obtained from the TGA experiments with the SBR sample. The lines correspond to the left axis indicating weight loss fraction. The data points correspond to the right axis which indicates the rate of change of mass with time, i.e., the differential thermogravimetric analysis (DTA). The thermal degradation rate and final masses were in agreement with those of Oh et al. (29) and Chen et al. (30) who investigated SBR degradation in both a pure nitrogen and air atmosphere. As shown in Figure 1, four experiments were done varying the heating rate from 10 to 40 °C/min over a temperature range from ambient to 1000 °C. Figure 1 shows that temperatures below 300 °C are not hot enough to initiate reactions. All samples were completely consumed above 600 °C. Therefore, subsequent data are presented up to temperatures of 600 °C for the 10 °C/min heating rate. The shift in the decomposition curves and maxima in the DTA curves were due to the longer times for reaction associated with slower temperature ramp rates. However, all final mass conversions reached 99.9% or greater during the duration of the tests. Post-test inspection of the crucible did not show any sign of residual material which is consistent with previous findings (31). Previous work done by the authors has attempted to explain variations in degradation mechanisms corresponding to various oxygen reaction atmospheres and hydrogen spiked atmospheres (27). The chemical structure of the SBR molecule has been assembled using the SPARTAN interface to the MOPAC program. This simulation has been done to get preliminary information including electron density, lowest unoccupied molecular orbital (LUMO), and highest occupied molecular orbital (HOMO) energy levels using equilibrium geometry based on a Hartree–Fock approximation using the 3-21G basis set. Here the HOMO and LUMO are used to help understand reactivity, and the energy difference between two orbitals was used as an indication for the likely first reactions to occur during decomposition. Figure 2 shows the chemical structure of a unit of the SBR polymer molecule, with an optimized equilibrium state geometry showing the LUMO and HOMO electron densities. This can be thought of as one possible transitional state of the SBR structure during decay. Inspection of Figure 2 enables

FIGURE 2. Chemical structure of SBR; 25% styrene, 75% butadiene linked copolymer. FIGURE 4. Concentration profile of styrene with SBR thermogram in N2.

FIGURE 3. Concentration profile of H2, ethylene, and n-butane from SBR thermal degradation in a N2 atmosphere. one to characterize the SBR structure as having a butadiene base with an aromatic ligand and one can use the electron density distribution as a guide to the likely first reactions that will occur. Relative bond dissociation energies indicate that the likely first scission reactions are between the ligand carbon and the butadiene base R-carbon. This suggests that the first reactions during the thermal breakdown of SBR will be the release of an aromatic radical species. Figure 3 shows the concentration (reported as PPMV) of ethylene, n-butane, and hydrogen for a 20 °C/min heating rate. As shown in Figure 3, the evolution of n-butane is produced in significant quantities compared to the other species. The concentration reaches a maximum at 400 °C, which is consistent with the maxima of the DTA curves shown in Figure 1, and then decreases to a constant value. This decrease in concentration occurs simultaneously with an increase in ethylene production. Using the structure in Figure 2 as a guide, it suggests that an n-butane species would be released into the gas phase during the ligand scission. As the temperature is increased in the TGA, any butane formed is likely cracked to release ethylene and hydrogen. Other C4 species including 1,3-butadiene and butene were also observed, but at very low levels and thus a trend could not be found. The concentration of hydrogen is significantly lower and begins to form after n-butane concentration begins to decline. This is consistent with the hydrogenation mechanism (i.e., unsaturated carbon bonds are reduced by attachment of a hydrogen atom to each carbon), that is, as less butane is formed there is more free hydrogen produced during the thermal breakdown of SBR. The eventual decrease in hydrogen, butane, and other species near 480 °C and above is a consequence of the SBR sample being exhausted. The concentration profile of styrene is shown in Figure 4 with the thermogram of SBR at 10 °C/min in a N2 atmosphere; its maximum concentration reached a value of 120 PPMV at 400 °C which corresponds to the same temperature as the DTA maximum in Figure 1.

FIGURE 5. Concentration profiles of benzene and phenyl-C1–2 from SBR thermal degradation in N2. In Figure 5, simple benzene derivatives, with the exception of styrene which is the major component principally derived from the styrene backbone, were identified and quantified from the effluent of a TGA unit coupled to a GC/MS heated at a rate of 10 °C/min in a pure nitrogen atmosphere due to minimizing the residence time during transferring sample into GC/MS; thus, as shown in Figure 4, the maximum concentration of styrene was observed in the same temperature as the DTA maximum. The resulting concentrations were then plotted. Among the substituted aromatics, the concentrations of toluene and ethyl benzene are dominant. Significant amounts of toluene and ethyl benzene were observed (24) from cleavage of the styrene backbone followed by hydrogenation. Due to a lower heating rate, the gasification reactions are slower. Therefore any detectable concentrations of these generated chemical species are diluted; for example, the four most prevalent gaseous species (C4, styrene, toluene, ethylbenzene) in the thermal treatment of SBR accounted for over 90% percent of total output PPMV. The concentration profiles of benzene, toluene, and ethyl benzene suggest relative bond dissociation energetics driving the decomposition of styrene at any given temperature. For example, the concentration of toluene is higher than that of ethyl benzene in that bond dissociation from styrene to form toluene occurs spontaneously. This is in contrast to the less energetically favored bond dissociation associated with the hydrogenation of the styrene base to generate ethyl benzene. The thermal degradation of ethyl benzene easily undergoes dissociation to form toluene. As evidenced in Figures 4 and 5, the concentration of toluene was observed to be lower than that of styrene by nearly a factor 5. This experimental evidence confirms the assumption that the initial decomposition reaction is the VOL. 42, NO. 6, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Concentration profiles of phenyl-C2–3 from SBR thermal degradation in N2.

FIGURE 7. Concentration profile of phenyl-C5–6 from SBR thermal degradation in N2. ligand breaking from the SBR. The concentration of benzene quantified with GC/MS is apparently lower than that of toluene and ethyl benzene either because bond dissociation from toluene and ethyl benzene to form benzene is insignificant or because the rates of gas phase addition reactions to transform other benzene derivatives is relatively fast. One interesting feature from the measurements was the detection of biphenyl with a concentration exceeding that of toluene. Considering the amount of biphenyl, the gas phase reaction derived from styrene may have occurred. Thus, monitoring the formation of benzene derivatives with structural isomers characterizes the thermal degradation mechanism by show2178

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ing how small fragments from the SBR backbone react with a benzene ring derived from the SBR base. However, ring structure formation from aliphatic compounds by a Diels–Alder reaction should be considered because the concentrations of n-butane and butene, and their subsequent dehydrogenation species, butene and 1, 3-butadiene, respectively, are higher than others. A Diels– Alder reaction is a thermal cylcoaddition reaction between a conjugated diene and an alkene (dienophile) to form cyclohexene. The reaction can be enhanced if a substituted diene with electron-donating groups or a substituted alkene with electron-withdrawing groups is involved. The identification of cyclohexene as an intermediate has been done qualitatively with the GC/MS and its concentration based on area response has not been as apparent when compared to the concentration of benzene. Thus, cyclization by a Diels–Alder reaction is not the dominant reaction during decomposition of SBR. This observation differs from those of previous studies (32–34). In the case of pentyl- and hexyl-benzene, having a molecular weight of 148 and 162, respectively, it is not easy to confirm whether these chemical species were derived from gas phase addition reactions of fragmentation from SBR base or decomposition pathways of the solid styrene base (that is, the chemical species may have been dissociated directly from the SBR base followed by hydrogenation reactions). However, benzene derivatives such as 1,2,4-trimethyl benzene and o,m,p-xylene provide evidence of aromatic ligand scission, i.e., bond breakage between the aliphatic backbone and the substituted aromatics. In this case, the substituted aromatic (considered the ligand) appears to have been formed by gas phase addition reactions. This is further corroborated by the detection of multiple fused ring PAH structures, which can only occur as a result of gas phase addition of the aromatics species. More detailed explanation for gas phase addition under a 10 °C/min heating rate in a pure nitrogen atmosphere is present in Figure 6. All chemical species plotted in Figure 6 are structural isomers having a molecular weight of 120, except p-xylene. Structural isomers of benzene derivatives provide not only evidence of a gas phase addition reaction but also elucidation of hierarchical growing steps of benzene derivatives. The concentration of p-xylene is relatively higher and it begins to form before the 1-methyl-3-ethyl benzene, 1,2,4trimethyl benzene, and n-propyl benzene concentrations begin to increase. This is consistent with gas phase addition reactions. For example, p-xylene may have been consumed as a precursor to form 1-methyl-3-ethyl benzene, 1, 2, 4-trimethyl benzene, and n-propyl benzene. In addition, concentration of m-xylene and o-xylene is relatively negligible by a factor of 10 compared to that of p-xylene. The concentration of 1-methyl-4-ethyl benzene shows a pattern similar to that of p-xylene. Chemical species such as p-xylene and 1-methyl-4-ethyl benzene were observed at a relatively lower temperature, which may have attributed to a steric stability. As evidenced in Figure 6, benzene derivatives (except for p-xylene and 1-methyl-4-ethyl benzene) are formed from gas phase addition reactions, since they are not observed until nearly 400 °C, where there is enough energy to also decompose the butadiene base. Even though Figures 5 and 6 show major species among the substituted aromatic compounds, other minor substituted aromatic compounds were also observed. Because the production of the major and minor substituted aromatics appeared concurrently, a clear trend between the dependence of one to the other could not be discerned. For example, tert-butyl benzene and secbutyl benzene were not observed until nearly 400 °C which was consistent with a gas phase reaction. Moreover, like

FIGURE 8. SBR breakdown mechanisms. p-xylene and 1-methyl-4-ethyl benzene, 1-methyl-4-isopropyl benzene was observed more than 1-methyl-2-isopropyl benzene and 1-methyl-3-isopropyl benzene owing to a steric stability. As mentioned before, the origin and fate of pentyl benzene and hexyl benzene is ambiguous in terms of gas phase addition reactions in that these benzene derivatives have the possibility to have originated directly from the breakdown of the SBR base followed by hydrogenation of the fragment. Thus, concentrations of the isomers of the benzene derivative having a molecular weight of 162 such as hexyl benzene, 1,2,4-triethyl benzene, and 1,3,5-trimethyl benzene are present in Figure 7. Additionally, concentration of n-pentyl benzene was plotted in Figure 7. The concentration of pentyl benzene reaches a peak value of approximately 1.1 PPMV and is compared to the other chemical species plotted in Figure 7. 1,2,4-Triethyl benzene, 1,3,4-triethyl benzene, and pentyl benzene were observed to form at relatively high temperatures. However, all benzene derivatives presented in Figure 7 were increasing in the high temperature regime where the maximum mass was significantly depleted. This behavior provides experimental evidence for the production of the triethyl benzene derivatives via gas phase reactions. As evidenced in Figures 3-7 above, the overall thermal degradation mechanism is postulated in Figure 8. Based on structural isomers of benzene derivatives shown in Figures 3-7, substituted aromatics were mainly generated by gas phase addition reactions of the styrene and fragmentations of the butadiene in the SBR. This leads to a diversity of benzene derivatives. For example, o,m,p-xylene and 1,2,4trimethyl benzene were generated by methyl radical addition on toluene. 1,2,4-Triethyl benzene and 1,3,5-triethyl benzene were derived from ethyl benzene, by means of gas phase addition reactions. Figures 3–7 suggest the following decomposition steps for SBR. Figure 8 indicates possible growth pathways resulting in the formation of benzene derivatives that serve as precursors to PAH formation. First, there is breakage between the ligand and butadiene backbone that results in hydrogen liberation. The backbone can then continue to be hydrogenated to produce a mixture of various C4 species that were observed in the GC analysis. The styrene ligand can undergo various additions, cleavage, and condensation reactions leading to the substituted PAHs observed in the TGA effluent. This condition can lead to the generation of PAHs at relatively lower temperatures as compared to conventional fuels, such as coal and diesel. For example, since tires are waste material and have been used only recently as a fuel/ energy source it is possible that new processes coming online

FIGURE 9. Concentration profiles of two-, three-, and four-ring PAHs from the thermal degradation in SBR in a N2 environment. that do not finely shred the tires can lead to conditions where tire parts are gasified at lower temperatures than the bulk gas measurement would otherwise indicate. The concentration profiles of representative two-, three-, and four-ring PAHs such as naphthalene, anthracene, phenanthrene, and pyrene are presented in Figure 9. Qualitatively substituted PAHs such as 1,2-dimethyl naphthalene and 1,3-dimethyl naphthalene were identified with GC/MS without calibration. This provides further evidence of gas phase reactions where the SBR ligands as well as the butadiene fragments combine to produce higher order PAHs. Besides the presented PAH species, benzo[ghi]perylene, dibenz[a,h]anthracene, benzo[k]fluoranthene, and chrysene were observed at concentrations of 0.5, 0.2, 0.2, and 0.2 PPMV, respectively. As shown in Figure 9, two-, three-, and four-ring PAHs, such as anthracene (3-ring), phenanthrene (3-ring), and pyrene (4-ring) concentrations are compared to that of naphthalene (2-ring). During PAH evolution, the concentration of naphthalene should be dominant in the lower temperature range. Our work shows that the concentration of pyrene reached the maximum concentration in the lower temperature range. Furthermore, the concentration of anthracene showed higher concentrations than naphthalene does. Thus, it can be postulated that highly concentrated substituted aromatic compounds recombine through another mechanism that does not include naphthalene as an intermediate species. This most likely involves cleavage of the butadiene base with the aromatic ligand still attached. The PAH formation sequence does not simply involve the VOL. 42, NO. 6, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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building of larger PAH’s from smaller units to construct the complex polymer structure. It is possible to generate large PAH molecules by alternative reaction pathways that circumvent the typical addition mechanisms. Observed benzene derivatives played an important role as a precursor to PAH formation and these accelerated formation rates occurred in a relatively lower temperature regime and at higher PAH concentrations. PAH formation from the thermal degradation of SBR (waste tire) in a nonoxidizing environment would be inevitable. Thus, maintaining low concentrations of benzene derivatives during thermal treatment of SBR is the most feasible way to prevent PAH formation. Oxidation by combustion processes should be helpful in mitigating levels of benzene derivatives and subsequent PAH concentrations. During the thermal decomposition of SBR, combustion processes are occurring simultaneously with pyrolysis processes. Regarding gas phase addition reactions to form benzene derivatives, combustion processes with secondary air addition can enable the opportunity to control the production of these chemical species that serve as precursors to PAH formation. In summary, our work clarified the PAH formation mechanisms from the thermal degradation of SBR and its justification was done with the concentration profiles of substituted aromatic compounds by means of gas phase reactions; in addition, the pathway to form various substituted aromatic compounds has been suggested. Lastly, our work showed that highly concentrated substituted aromatic compounds recombine through another mechanism that does not include naphthalene as an intermediate species.

Acknowledgments We gratefully acknowledge the Waste-to-Energy Research Technology Council at Columbia University for partial support of this project.

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