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The study of 1-ethylnaphthalene pyrolysis in a flow reactor
J. Anal. Appl. Pyrolysis 86 (2009) 287–292
Contents lists available at ScienceDirect
Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
The study of 1-ethylnaphthalene pyrolysis in a flow reactor Jun Yang a, Mingming Lu b,*, Ming Chai b a b
Trinity Consultants, 12770 Merit Drive, Suite 900, Dallas, TX 75251, United States Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 January 2008 Accepted 25 July 2009 Available online 3 August 2009
Ethylnaphthalenes (ENs) and other alkylated naphthalenes exist in a variety of fuels such as coal and petroleum, and their decomposition is closely related to soot and polycyclic aromatic hydrocarbons (PAHs) formation in fuel combustion. In this paper, 1-EN pyrolysis was studied experimentally in a quartz tube reactor in the temperature range of 600–850 8C. The products quantified include 1vinylnaphthalene (VN), 1-methylnaphthane (MN), acenaphthalene (AceN), naphthalene, 2-MN, 2-VN, and acenaphthene, with 1-VN as the most abundant. Ethane, ethylene and methane have also been identified as gaseous products. The results suggested that the reactivity of 1-EN is higher than that of the MNs. Possible product formation pathways are discussed, which gives rise to hypothesized reaction mechanisms. There are three decomposition routes for 1-EN: hydrogen abstraction from the ethyl group forming naphthylethyl radicals (both primary and benzylic) and then forming 1-VN, homolytic cleavage of the bond between the a- and b-carbons forming 1-MN, and hydrogen displacement of the ethyl group forming naphthalene. ß 2009 Elsevier B.V. All rights reserved.
Keywords: 1-Ethylnaphthalene Pyrolysis Formation mechanisms Methylnaphthanes Vinylnaphthalene
1. Introduction Naphthalene and its several alkylated homologues are the most abundant polycyclic aromatic constituents in coal tars and the higher distillation fractions of crude oil, such as diesel fuel, aviation fuel and heating oil [1,2]. These compounds usually form soot more easily in combustion than aliphatic hydrocarbons due to the formation of resonance stabilized radicals. Soot and PAHs are of health and environmental concerns due to their respirable sizes and the carcinogenicity of some PAHs. Sooting is also undesirable due to the loss of fuel efficiency. Compared with the number of studies on naphthalene [3–7] and methylnaphthalene (MN) [7–11] pyrolysis, the study on the pyrolysis of ethylnaphthalenes (ENs) has been inadequate. Studies on MNs have indicated that in addition to the formation of naphthylmethyl radical, hydrogen displacement of the methyl side chain can also contribute to PAH and soot formation [11]. The methyl displacement of the side chain can even be favored at higher temperatures over that of benzylic radical formation. In order to better understand the reactivity of the side chain, it is
Abbreviations: AceN, acenaphthalene; DMN, dimethylnaphthalene; EN, ethylnaphthalene; FTIR, Fourier transform infrared; GC-FID, gas chromatography-flame ionization detector; GC-MS, gas chromatography-mass spectrometry; MN, methylnaphthane; PAHs, polycyclic aromatic hydrocarbons; VN, vinylnaphthalene. * Corresponding author at: Department of Civil and Environmental Engineering, PO Box 210071, University of Cincinnati, Cincinnati, OH 45221, United States. Tel.: +1 513 5560996; fax: +1 513 5562599. E-mail address: [email protected] (M. Lu). 0165-2370/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2009.07.007
necessary to study ENs, which have a longer side chain than MNs and may potentially lead to more complex product formation mechanisms. Beltrame et al. [12,13] studied the hydrodealkylation of 1 and 2EN from 550–710 8C at elevated pressures (2.9–44.6 atm). The main products of 2-EN hydrodealkylation were naphthalene, 2-MN and 2VN, whose total yields accounted for more than 96% of naphthalene ring input (the author’s terminology to account for product yields) [12]. The rapidly decreasing yield of 2-VN at pressures higher than 20 atm served as an indication that it is the product of dehydrogenation. The identified gaseous products were methane, ethane, and ethylene in the order of abundance. In the subsequent study of 1-EN hydrodealkylation, naphthalene, 1-MN, acenaphthene and AceN were identified as main products accounting for more than 97% of naphthalene ring input. The pathways of 1-EN forming 1-MN, naphthalene and AceN were proposed in the kinetic study, and kinetic parameters were obtained from experimental data [13]. The reactivity of the ethyl side chain has been observed in this study, however, the high concentrations of hydrogen and high pressure are not always typical in most combustion applications. The study of 1EN pyrolysis at atmospheric pressure is still essential in understanding its reactivity in practical combustion conditions. This study will also help with the understanding of byproduct formation from 2-ENs, based on the prior studies of 1- and 2-MNs [11]. The experimental study of 1-EN pyrolysis is presented in this paper. Only one EN isomer is studied as our earlier work on 1 and 2MNs indicated that the governing thermal decomposition mechanisms of the two MNs are similar [11]. Pyrolytic products have been identified and their possible formation mechanisms are discussed.
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J. Yang et al. / J. Anal. Appl. Pyrolysis 86 (2009) 287–292
Fig. 1. Schematic diagram of the experimental setup.
2. Experimental methods The experimental setup and analytical methods have been described elsewhere and the necessary details are briefly provided here for clarity purposes [11,14]. Experiments for 1-EN pyrolysis were conducted in an isothermal reactor at atmospheric pressure which has been used in MN pyrolysis [11]. The setup is illustrated in Fig. 1, which consists of a vaporizer, a quartz tube reactor (17 mm inner diameter and effective length of 24 inches or 0.61 m) and a sample collection assembly. During an experiment, 30 ml of 1-EN (Aldrich) was continuously injected into the preheated glass vaporizer by a syringe pump and the reactant input lasted for 4 min. The vaporizer was heated to around 200 8C to ensure gradual vaporization of the reactant without decomposition. The vapor was carried by prepurified helium (99.995%, Wright Brothers Inc.) into the reactor. Compressed room temperature air was applied outside the quartz tube at the exit end to cool the tube quickly and quenched further reactions. The products were collected by a dual trap impinger filled with dichloromethane (DCM) and put in ice bath. A small piece of glass fiber (GF) filter was also used before the impinger to collect soot, and light gaseous products were collected by Tedlar (by SKC) sample bags. The experiment temperatures ranged from 600 to 850 8C with 50 8C increments. The axial temperature distributions of the reactor were measured with a K-type thermocouple probe (Omega molded quick connect probe KQSUP-14G-12) from 700 to 900 8C at nonreacting conditions and shown in Fig. 2. The temperatures in the reaction zone, i.e. the middle 61 cm (24 in.) of the reactor, are consistently within 5 8C of the set point, and the temperature in the
isothermal zone is referred to as the reaction temperature throughout the paper. The residence time in the reactor was fixed at approximately 1.0 s for all temperatures. The input reactant concentrations and gas flow rates are listed in Table 1. Each run lasts approximately 5 min, with 4-min reaction and 1 min purging. For the purpose of quantity assurance, multiple runs were performed for each experimental condition. Products were quantified by a Varian (Saturn 2200) gas chromatography-mass spectrometry (GC-MS) with a capillary column (CP-Sil 8 CB Low Bleed/MS, 30 m 0.25 mm 0.25 mm). Product yields are reported as percent of input on carbon basis. The purity of the reactant has been quantified as 98% 1-EN, with 1.94% 2-EN and 0.06% 1-(2propyenyl)-naphthalene. The separation of the EN isomers was achieved by using a DB-FFAP column as indicated in literature [15]. 3. Results and discussion 3.1. Experiment results Byproducts were identified from the total ion chromatograph (TIC) with the aid of individual chemical standards. Fig. 3 provides an example of the TIC of product distribution from 1-EN pyrolysis
Table 1 Experimental conditions. Parameters
Helium flow rate (nlpma) Reactant input concentration (ppm) a
Fig. 2. Temperature profiles in the quartz tubular reactor: atmospheric pressure and helium as carrier gas.
Temperature (8C) 600
650
700
750
800
850
2.8 416
2.65 439
2.52 462
2.4 485
2.28 510
2.18 534
nlpm: normal liters per minute at 20 8C, 1 atm.
Fig. 3. Total ion chromatogram of 1-EN pyrolysis at 850 8C.
J. Yang et al. / J. Anal. Appl. Pyrolysis 86 (2009) 287–292
Fig. 4. The overall temperature series of 1-EN pyrolysis. ‘‘1-EN’’: the quantity of unconsumed 1-EN; ‘‘PAH’’: the total yields of all the aromatic products identified; ‘‘Recovery’’: the total recovery, which is the sum of ‘‘1-EN’’ and ‘‘PAH’’.
at 850 8C, with all the major products labeled. Some of the unlabelled peaks are from the column bleed. Fig. 4 presents the temperature series of 1-EN pyrolysis. Average values are used for each data point, and the error bars represent the actual maximum and minimum values. The carbon recovery rate is nearly 100% throughout the temperature range, which suggests that the products have been effectively collected
289
and identified by the experimental setup. At 600 8C, more than 90% of 1-EN remained unreacted, while almost all was decomposed at 850 8C. This indicates that the selected temperature range is appropriate for studying 1-EN pyrolysis. Soot is not observed except in negligible amounts at 850 8C. This may be due to the lower temperature range for 1-EN pyrolysis compared with that for MNs (800–1000 8C) [11], which is an indication that 1-EN may be more reactive than the MNs. Fig. 5 presents the major product yields. The identified products (yields greater than 1%) include 1-VN, 1-MN, AceN, naphthalene, 2MN, 2-VN, and acenaphthene. The most abundant product is 1-VN and its yield decreased slightly when the temperature is higher than 750 8C. The second most abundant is acenaphthene at temperatures below 700 8C and changed to 1-MN at higher temperatures. The minor products (with less than 1% yield) identified include indene, 1,7-dimethylnaphthalene (DMN), 1,6DMN, 1,4-DMN, 1,5-DMN, fluorene, phenanthrene, and benzofulvene. Most of these minor products were only found at temperatures higher than 750 8C. Minor quantities of gaseous products have also been quantified. The gas sample at 800 8C was analyzed using a gas chromatography-flame ionization detector (GC-FID) by a colleague at the Ohio University. The instrument was equipped with a cryogenic pre-concentration system. Ethane and ethylene were identified and each accounted for approximately 1.8% carbon input. However, the system was not setup for methane analysis. Methane concentrations at 700 and 850 8C were measured by a multiple gas analyzer based on Fourier transform infrared (FTIR) spectroscopy and the yields were 0.4% and 3.6% respectively. 3.2. Postulated reaction mechanisms
(1)
(2)
(3)
(4)
(5)
(6) Fig. 5. Major products in 1-EN pyrolysis.
J. Yang et al. / J. Anal. Appl. Pyrolysis 86 (2009) 287–292
290 Table 2 Heats of reaction (kJ mol
1
) for the reactant consumption pathways in 1-EN pyrolysis.
Reaction
Calculated Ref. [22]
1
2
3
4
413.62
341.31 355.7
322.56 304.7
417.87
(7)
(8)
The initiation of 1-EN pyrolysis can occur via four possible pathways as described by reactions (1)–(4). Reactions (1) and (2) involve the dissociation of the C-H bond at the primary C and benzylic locations. Reactions (3) and (4) are resultant from the C–C bond scission at the benzylic site and the aromatic ring
5
6 22.376
7 94.69
8 117.31
45.38
respectively. One study on ethylbenzene pyrolysis [16] suggested that the C–H homolysis is similar to reaction (2) as the major reaction mechanism, while reaction (3) was regarded as the major pathway by other studies [17–20]. As the reactions proceed, the radical pools build up, and these initiation steps give way to hydrogen abstraction reactions (5)–(8). The heats of reaction of the above pathways were estimated and listed in Table 2, together with published thermodynamic data [21] and the NIST Chemistry WebBook (http://webbook.nist.gov/ chemistry/), as an indication of the relative importance of these steps in product formation. Our calculated results indicated that both reactions (2) and (3) are favored energetically due to the formation of resonance stabilized radicals, with (3) being slightly more important. These results qualitatively agree with McMillen and Golden [22]. Reaction (4) is the least favored as it disturbs the aromatic structure. It should be noticed that in reaction (8) the
Fig. 6. Proposed reaction mechanisms of 1-EN pyrolysis.
J. Yang et al. / J. Anal. Appl. Pyrolysis 86 (2009) 287–292
formation of naphthalene and ethyl radical is shown instead of naphthyl radical and ethane since the former is approximately 3 kJ mol 1 lower in heat of reaction than the latter. Fig. 6 describes the hypothesized reaction mechanisms of 1-EN pyrolysis based on experimental results and the discussion above. Hydrogen abstraction from the ethyl group results in naphthylethyl radicals at the benzylic and primary C locations. Both of these steps result in the formation of 1-VN, with the hydrogen abstraction at the benzylic site being favored. Hydrogen displacement of the ethyl group gives rise to naphthalene, and the homolytic cleavage of the ethyl group forms 1-MN. These results are in agreement with the pathways postulated for ethylbenzene oxidation [23]. The yields of 1-VN increase with temperature until 750 8C, and its decrease can be the result of isomerization to form 2-VN or formation of naphthalene by hydrogen displacement, which is analogous to that of styrene decomposition [23]. In addition, the cyclodehydrogenation of 1-VN gives rise to AceN. Similarly, acenaphthene can be resultant from the cyclodehydrogenation of the naphthylethyl radical and further dehydrogenation produces AceN. The hypothesized pathway of acenaphthene and AceN formation is in agreement with the study on hydrodealkylation of ENs [13], and is also supported by the experimental results in Fig. 5. Also, the addition of C2H2 to naphthalene or naphthyl radical can be another possible route of AceN formation, as described in 1-MN pyrolysis [11]. Due to the higher activation energy involved in the C–C homolysis of the ethyl group, it tends to be favored at higher temperatures, which is evidenced by the temperature series of 1-MN in Fig. 5. Naphthalene can be produced via multiple pathways, such as hydrogen displacement of the side chain from 1-EN and 1-VN or as a product of 1-MN pyrolysis as observed in our 1-MN study. However, hydrogen displacement of the methyl group on 1-MN should not be the major route for naphthalene formation in this study as discussed below. The maximum possible yield of naphthalene from the 1-MN pathway is estimated as the product of 1-MN formed in 1-EN pyrolysis and the conversion factor of 1-MN to naphthalene obtained from our earlier study [11]. The calculated maximum yield of 1-MN is shown in Fig. 7, with the assumption that 1-EN pyrolysis (the consumed 1-EN) only resulted in 1-VN, acenaphthene and 1-MN, while the other products are secondary products from these three compounds.
291
In 1-MN pyrolysis [11], the yield of naphthalene is less than 2.4% at 800 8C and reaches 5.3% at 850 8C. Therefore, the maximum possible yield of naphthalene, if exclusively from the 1-MN pathway, is 0.87% at 800 8C and 2.7% at 850 8C, which are much lower than the actual experimental values (4.0% at 800 8C and rises to 8.7% at 850 8C). One possible pathway of 2-MN formation in 1-EN pyrolysis is the isomerization process of 1-MN [11]. However, the higher yield of 2-MN from 1-EN pyrolysis (1.1% at 800 8C and 5.5% at 850 8C) than that from 1-MN pyrolysis (0.8% at 800 8C and 2.3% at 850 8C) indicates that other pathways also exist. Another possible pathway may be from methyl addition to naphthalene, which agrees with the study of Mimura et al. [6]. The 2-ethylnaphthalene quantity decreased from 2% at 600 8C to 0.1% at 850 8C. This trend does not support the isomerization pathway from 1-EN. The decomposition of 2-EN can result in the formation of 2-VN, as indicated by the temperature series (Fig. 5, bottom panel). One postulation is that the isomerization process, the H addition and intramolecular alkyl shift, as described in our 1methylnaphthalene pyrolysis, has a much higher energy barrier than other reactions. Our future work on 1-propylnaphthalene is underway to further investigate the isomerization process. 4. Conclusion The products of 1-EN pyrolysis suggest the following reaction pathways: hydrogen abstraction from the ethyl group results in naphthylethyl radicals at benzylic and primary locations, both lead to the formation of 1-VN, with the benzylic route being favored; hydrogen displacement of the ethyl group results in naphthalene; and the homolytic cleavage of the ethyl group gives rise to 1-MN. The relative abundance of these products is also an indication of the relative importance of these pathways. The reactivity of 1-EN is higher than that of MNs, as evidenced by the lower reaction temperatures, and the higher reactant consumption/product formation at the same temperature. Unlike in the MN pyrolysis, the isomerization of 1-EN to 2-EN is not observed. Acknowledgements The authors would like to thank Dr. Valerie Young, from the Department of Chemical Engineering, Ohio University, and Dr. SanMou Jeng, from the Department of Aerospace Engineering, University of Cincinnati for the detection of gaseous products. The support from the University Research Council of the University of Cincinnati is gratefully acknowledged. References
Fig. 7. The estimated maximum yield of 1-MN. ‘‘residual 1-EN’’: yields of unconsumed 1-EN; ‘‘consumed 1-EN’’: quantity of consumed 1-EN, which is 1‘‘residual 1-EN’’; ‘‘1-VN + AceN’’: the sum of 1-VN and AceN; ‘‘maximum 1-MN’’: the estimated maximum possible yield of 1-MN in 1-EN pyrolysis.
[1] J.J. Schauer, M.J. Kleeman, G.R. Cass, B.R.T. Simoneit, Environ. Sci. Technol. 33 (1999) 1578–1587. [2] M. Bernabei, R. Reda, R. Galiero, G. Bocchinfuso, J. Chromatogr. A 985 (2003) 197– 203. [3] P.A. Tesner, S.V. Shurupov, Combust. Sci. Technol. 126 (1997) 139–151. [4] M. Lu, J.A. Mulholland, Chemosphere 55 (2004) 605–610. [5] G.M. Badger, S.D. Jolad, T.M. Spotswood, Aust. J. Chem. 17 (1964) 771–777. [6] K. Mimura, T. Madono, S. Toyama, K. Sugitani, R. Sugisaki, S. Iwamatsu, S. Murata, J. Anal. Appl. Pyrolysis 72 (2004) 273–278. [7] V.V. Platonov, L.N. Ivleva, A.B. Vol-Epshtein, T.V. Kasimtseva, O.A. Klyavina, Khimiya Tverdogo Topliva (Solid Fuel Chem.), 20 (1986) 74–82. [8] W. Lijinsky, C.R. Raha, J. Org. Chem. 26 (1961) 3566–3568. [9] C.R. Shaddix, K. Brezinsky, I. Glassman, Combust. Flame 108 (1997) 139–157. [10] C.R. Shaddix, K. Brezinsky, I. Glassman, Oxidation of 1-Methylnaphthalene, The Combustion Institute, 1992. [11] J. Yang, M. Lu, Environ. Sci. Technol. 39 (2005) 3077–3082. [12] P. Beltrame, P. Carnltl, B. Maronglu, L. Mura, V. Solinas, S. Mori, Ind. Eng. Chem. Process Des. Dev. 18 (1979) 338–342. [13] P. Beltrame, P. Carnltl, B. Maronglu, L. Mura, V. Solinas, S. Mori, Ind. Eng. Chem. Process Des. Dev. 20 (1981) 379–384. [14] J. Yang, M. Lu, The Preliminary Results of 1-Ethylnaphthalene Pyrolysis, Joint Session Meeting of the Combustion Institute, Philadelphia, PA, 2005.
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[15] A. Shimoyama, M. Kozono, H. Mita, S. Nomoto, Geochem. J. 35 (2001) 365–375. [16] L. Brouwer, W. Muller-Markgraf, J. Troe, ‘‘Identification of Primary Reaction Products in the Thermal Decomposition of Aromatic Hydrocarbons’’, The 20th Symposium of the Combustion Institute, 1984, 799–806. [17] H.G. Davis, Int. J. Chem. Kinet. 15 (1983) 469–474. [18] D.A. Robaugh, S.E. Stein, Int. J. Chem. Kinet. 13 (1981) 445–462.
[19] K.M. Pamidimukkala, R.D. Kern, Int. J. Chem. Kinet. 18 (1986) 1341–1353. [20] L.J. Mizerka, J.H. Kiefer, Int. J. Chem. Kinet. 18 (1986) 363–378. [21] H. Curran, C. Wu, N. Marinov, W.J. Pitz, C.K. Westbrook, A. Burcat, J. Phys. Chem. Ref. Data 29 (2000) 463–517. [22] D.F. McMillen, D.M. Golden, Ann. Rev. Phys. Chem. 33 (1982) 493–532. [23] T.A. Litzinger, K. Brezinsky, I. Glassman, Combust. Flame 63 (1986) 251–267.
Contents lists available at ScienceDirect
Journal of Analytical and Applied Pyrolysis journal homepage: www.elsevier.com/locate/jaap
The study of 1-ethylnaphthalene pyrolysis in a flow reactor Jun Yang a, Mingming Lu b,*, Ming Chai b a b
Trinity Consultants, 12770 Merit Drive, Suite 900, Dallas, TX 75251, United States Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Received 18 January 2008 Accepted 25 July 2009 Available online 3 August 2009
Ethylnaphthalenes (ENs) and other alkylated naphthalenes exist in a variety of fuels such as coal and petroleum, and their decomposition is closely related to soot and polycyclic aromatic hydrocarbons (PAHs) formation in fuel combustion. In this paper, 1-EN pyrolysis was studied experimentally in a quartz tube reactor in the temperature range of 600–850 8C. The products quantified include 1vinylnaphthalene (VN), 1-methylnaphthane (MN), acenaphthalene (AceN), naphthalene, 2-MN, 2-VN, and acenaphthene, with 1-VN as the most abundant. Ethane, ethylene and methane have also been identified as gaseous products. The results suggested that the reactivity of 1-EN is higher than that of the MNs. Possible product formation pathways are discussed, which gives rise to hypothesized reaction mechanisms. There are three decomposition routes for 1-EN: hydrogen abstraction from the ethyl group forming naphthylethyl radicals (both primary and benzylic) and then forming 1-VN, homolytic cleavage of the bond between the a- and b-carbons forming 1-MN, and hydrogen displacement of the ethyl group forming naphthalene. ß 2009 Elsevier B.V. All rights reserved.
Keywords: 1-Ethylnaphthalene Pyrolysis Formation mechanisms Methylnaphthanes Vinylnaphthalene
1. Introduction Naphthalene and its several alkylated homologues are the most abundant polycyclic aromatic constituents in coal tars and the higher distillation fractions of crude oil, such as diesel fuel, aviation fuel and heating oil [1,2]. These compounds usually form soot more easily in combustion than aliphatic hydrocarbons due to the formation of resonance stabilized radicals. Soot and PAHs are of health and environmental concerns due to their respirable sizes and the carcinogenicity of some PAHs. Sooting is also undesirable due to the loss of fuel efficiency. Compared with the number of studies on naphthalene [3–7] and methylnaphthalene (MN) [7–11] pyrolysis, the study on the pyrolysis of ethylnaphthalenes (ENs) has been inadequate. Studies on MNs have indicated that in addition to the formation of naphthylmethyl radical, hydrogen displacement of the methyl side chain can also contribute to PAH and soot formation [11]. The methyl displacement of the side chain can even be favored at higher temperatures over that of benzylic radical formation. In order to better understand the reactivity of the side chain, it is
Abbreviations: AceN, acenaphthalene; DMN, dimethylnaphthalene; EN, ethylnaphthalene; FTIR, Fourier transform infrared; GC-FID, gas chromatography-flame ionization detector; GC-MS, gas chromatography-mass spectrometry; MN, methylnaphthane; PAHs, polycyclic aromatic hydrocarbons; VN, vinylnaphthalene. * Corresponding author at: Department of Civil and Environmental Engineering, PO Box 210071, University of Cincinnati, Cincinnati, OH 45221, United States. Tel.: +1 513 5560996; fax: +1 513 5562599. E-mail address: [email protected] (M. Lu). 0165-2370/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2009.07.007
necessary to study ENs, which have a longer side chain than MNs and may potentially lead to more complex product formation mechanisms. Beltrame et al. [12,13] studied the hydrodealkylation of 1 and 2EN from 550–710 8C at elevated pressures (2.9–44.6 atm). The main products of 2-EN hydrodealkylation were naphthalene, 2-MN and 2VN, whose total yields accounted for more than 96% of naphthalene ring input (the author’s terminology to account for product yields) [12]. The rapidly decreasing yield of 2-VN at pressures higher than 20 atm served as an indication that it is the product of dehydrogenation. The identified gaseous products were methane, ethane, and ethylene in the order of abundance. In the subsequent study of 1-EN hydrodealkylation, naphthalene, 1-MN, acenaphthene and AceN were identified as main products accounting for more than 97% of naphthalene ring input. The pathways of 1-EN forming 1-MN, naphthalene and AceN were proposed in the kinetic study, and kinetic parameters were obtained from experimental data [13]. The reactivity of the ethyl side chain has been observed in this study, however, the high concentrations of hydrogen and high pressure are not always typical in most combustion applications. The study of 1EN pyrolysis at atmospheric pressure is still essential in understanding its reactivity in practical combustion conditions. This study will also help with the understanding of byproduct formation from 2-ENs, based on the prior studies of 1- and 2-MNs [11]. The experimental study of 1-EN pyrolysis is presented in this paper. Only one EN isomer is studied as our earlier work on 1 and 2MNs indicated that the governing thermal decomposition mechanisms of the two MNs are similar [11]. Pyrolytic products have been identified and their possible formation mechanisms are discussed.
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J. Yang et al. / J. Anal. Appl. Pyrolysis 86 (2009) 287–292
Fig. 1. Schematic diagram of the experimental setup.
2. Experimental methods The experimental setup and analytical methods have been described elsewhere and the necessary details are briefly provided here for clarity purposes [11,14]. Experiments for 1-EN pyrolysis were conducted in an isothermal reactor at atmospheric pressure which has been used in MN pyrolysis [11]. The setup is illustrated in Fig. 1, which consists of a vaporizer, a quartz tube reactor (17 mm inner diameter and effective length of 24 inches or 0.61 m) and a sample collection assembly. During an experiment, 30 ml of 1-EN (Aldrich) was continuously injected into the preheated glass vaporizer by a syringe pump and the reactant input lasted for 4 min. The vaporizer was heated to around 200 8C to ensure gradual vaporization of the reactant without decomposition. The vapor was carried by prepurified helium (99.995%, Wright Brothers Inc.) into the reactor. Compressed room temperature air was applied outside the quartz tube at the exit end to cool the tube quickly and quenched further reactions. The products were collected by a dual trap impinger filled with dichloromethane (DCM) and put in ice bath. A small piece of glass fiber (GF) filter was also used before the impinger to collect soot, and light gaseous products were collected by Tedlar (by SKC) sample bags. The experiment temperatures ranged from 600 to 850 8C with 50 8C increments. The axial temperature distributions of the reactor were measured with a K-type thermocouple probe (Omega molded quick connect probe KQSUP-14G-12) from 700 to 900 8C at nonreacting conditions and shown in Fig. 2. The temperatures in the reaction zone, i.e. the middle 61 cm (24 in.) of the reactor, are consistently within 5 8C of the set point, and the temperature in the
isothermal zone is referred to as the reaction temperature throughout the paper. The residence time in the reactor was fixed at approximately 1.0 s for all temperatures. The input reactant concentrations and gas flow rates are listed in Table 1. Each run lasts approximately 5 min, with 4-min reaction and 1 min purging. For the purpose of quantity assurance, multiple runs were performed for each experimental condition. Products were quantified by a Varian (Saturn 2200) gas chromatography-mass spectrometry (GC-MS) with a capillary column (CP-Sil 8 CB Low Bleed/MS, 30 m 0.25 mm 0.25 mm). Product yields are reported as percent of input on carbon basis. The purity of the reactant has been quantified as 98% 1-EN, with 1.94% 2-EN and 0.06% 1-(2propyenyl)-naphthalene. The separation of the EN isomers was achieved by using a DB-FFAP column as indicated in literature [15]. 3. Results and discussion 3.1. Experiment results Byproducts were identified from the total ion chromatograph (TIC) with the aid of individual chemical standards. Fig. 3 provides an example of the TIC of product distribution from 1-EN pyrolysis
Table 1 Experimental conditions. Parameters
Helium flow rate (nlpma) Reactant input concentration (ppm) a
Fig. 2. Temperature profiles in the quartz tubular reactor: atmospheric pressure and helium as carrier gas.
Temperature (8C) 600
650
700
750
800
850
2.8 416
2.65 439
2.52 462
2.4 485
2.28 510
2.18 534
nlpm: normal liters per minute at 20 8C, 1 atm.
Fig. 3. Total ion chromatogram of 1-EN pyrolysis at 850 8C.
J. Yang et al. / J. Anal. Appl. Pyrolysis 86 (2009) 287–292
Fig. 4. The overall temperature series of 1-EN pyrolysis. ‘‘1-EN’’: the quantity of unconsumed 1-EN; ‘‘PAH’’: the total yields of all the aromatic products identified; ‘‘Recovery’’: the total recovery, which is the sum of ‘‘1-EN’’ and ‘‘PAH’’.
at 850 8C, with all the major products labeled. Some of the unlabelled peaks are from the column bleed. Fig. 4 presents the temperature series of 1-EN pyrolysis. Average values are used for each data point, and the error bars represent the actual maximum and minimum values. The carbon recovery rate is nearly 100% throughout the temperature range, which suggests that the products have been effectively collected
289
and identified by the experimental setup. At 600 8C, more than 90% of 1-EN remained unreacted, while almost all was decomposed at 850 8C. This indicates that the selected temperature range is appropriate for studying 1-EN pyrolysis. Soot is not observed except in negligible amounts at 850 8C. This may be due to the lower temperature range for 1-EN pyrolysis compared with that for MNs (800–1000 8C) [11], which is an indication that 1-EN may be more reactive than the MNs. Fig. 5 presents the major product yields. The identified products (yields greater than 1%) include 1-VN, 1-MN, AceN, naphthalene, 2MN, 2-VN, and acenaphthene. The most abundant product is 1-VN and its yield decreased slightly when the temperature is higher than 750 8C. The second most abundant is acenaphthene at temperatures below 700 8C and changed to 1-MN at higher temperatures. The minor products (with less than 1% yield) identified include indene, 1,7-dimethylnaphthalene (DMN), 1,6DMN, 1,4-DMN, 1,5-DMN, fluorene, phenanthrene, and benzofulvene. Most of these minor products were only found at temperatures higher than 750 8C. Minor quantities of gaseous products have also been quantified. The gas sample at 800 8C was analyzed using a gas chromatography-flame ionization detector (GC-FID) by a colleague at the Ohio University. The instrument was equipped with a cryogenic pre-concentration system. Ethane and ethylene were identified and each accounted for approximately 1.8% carbon input. However, the system was not setup for methane analysis. Methane concentrations at 700 and 850 8C were measured by a multiple gas analyzer based on Fourier transform infrared (FTIR) spectroscopy and the yields were 0.4% and 3.6% respectively. 3.2. Postulated reaction mechanisms
(1)
(2)
(3)
(4)
(5)
(6) Fig. 5. Major products in 1-EN pyrolysis.
J. Yang et al. / J. Anal. Appl. Pyrolysis 86 (2009) 287–292
290 Table 2 Heats of reaction (kJ mol
1
) for the reactant consumption pathways in 1-EN pyrolysis.
Reaction
Calculated Ref. [22]
1
2
3
4
413.62
341.31 355.7
322.56 304.7
417.87
(7)
(8)
The initiation of 1-EN pyrolysis can occur via four possible pathways as described by reactions (1)–(4). Reactions (1) and (2) involve the dissociation of the C-H bond at the primary C and benzylic locations. Reactions (3) and (4) are resultant from the C–C bond scission at the benzylic site and the aromatic ring
5
6 22.376
7 94.69
8 117.31
45.38
respectively. One study on ethylbenzene pyrolysis [16] suggested that the C–H homolysis is similar to reaction (2) as the major reaction mechanism, while reaction (3) was regarded as the major pathway by other studies [17–20]. As the reactions proceed, the radical pools build up, and these initiation steps give way to hydrogen abstraction reactions (5)–(8). The heats of reaction of the above pathways were estimated and listed in Table 2, together with published thermodynamic data [21] and the NIST Chemistry WebBook (http://webbook.nist.gov/ chemistry/), as an indication of the relative importance of these steps in product formation. Our calculated results indicated that both reactions (2) and (3) are favored energetically due to the formation of resonance stabilized radicals, with (3) being slightly more important. These results qualitatively agree with McMillen and Golden [22]. Reaction (4) is the least favored as it disturbs the aromatic structure. It should be noticed that in reaction (8) the
Fig. 6. Proposed reaction mechanisms of 1-EN pyrolysis.
J. Yang et al. / J. Anal. Appl. Pyrolysis 86 (2009) 287–292
formation of naphthalene and ethyl radical is shown instead of naphthyl radical and ethane since the former is approximately 3 kJ mol 1 lower in heat of reaction than the latter. Fig. 6 describes the hypothesized reaction mechanisms of 1-EN pyrolysis based on experimental results and the discussion above. Hydrogen abstraction from the ethyl group results in naphthylethyl radicals at the benzylic and primary C locations. Both of these steps result in the formation of 1-VN, with the hydrogen abstraction at the benzylic site being favored. Hydrogen displacement of the ethyl group gives rise to naphthalene, and the homolytic cleavage of the ethyl group forms 1-MN. These results are in agreement with the pathways postulated for ethylbenzene oxidation [23]. The yields of 1-VN increase with temperature until 750 8C, and its decrease can be the result of isomerization to form 2-VN or formation of naphthalene by hydrogen displacement, which is analogous to that of styrene decomposition [23]. In addition, the cyclodehydrogenation of 1-VN gives rise to AceN. Similarly, acenaphthene can be resultant from the cyclodehydrogenation of the naphthylethyl radical and further dehydrogenation produces AceN. The hypothesized pathway of acenaphthene and AceN formation is in agreement with the study on hydrodealkylation of ENs [13], and is also supported by the experimental results in Fig. 5. Also, the addition of C2H2 to naphthalene or naphthyl radical can be another possible route of AceN formation, as described in 1-MN pyrolysis [11]. Due to the higher activation energy involved in the C–C homolysis of the ethyl group, it tends to be favored at higher temperatures, which is evidenced by the temperature series of 1-MN in Fig. 5. Naphthalene can be produced via multiple pathways, such as hydrogen displacement of the side chain from 1-EN and 1-VN or as a product of 1-MN pyrolysis as observed in our 1-MN study. However, hydrogen displacement of the methyl group on 1-MN should not be the major route for naphthalene formation in this study as discussed below. The maximum possible yield of naphthalene from the 1-MN pathway is estimated as the product of 1-MN formed in 1-EN pyrolysis and the conversion factor of 1-MN to naphthalene obtained from our earlier study [11]. The calculated maximum yield of 1-MN is shown in Fig. 7, with the assumption that 1-EN pyrolysis (the consumed 1-EN) only resulted in 1-VN, acenaphthene and 1-MN, while the other products are secondary products from these three compounds.
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In 1-MN pyrolysis [11], the yield of naphthalene is less than 2.4% at 800 8C and reaches 5.3% at 850 8C. Therefore, the maximum possible yield of naphthalene, if exclusively from the 1-MN pathway, is 0.87% at 800 8C and 2.7% at 850 8C, which are much lower than the actual experimental values (4.0% at 800 8C and rises to 8.7% at 850 8C). One possible pathway of 2-MN formation in 1-EN pyrolysis is the isomerization process of 1-MN [11]. However, the higher yield of 2-MN from 1-EN pyrolysis (1.1% at 800 8C and 5.5% at 850 8C) than that from 1-MN pyrolysis (0.8% at 800 8C and 2.3% at 850 8C) indicates that other pathways also exist. Another possible pathway may be from methyl addition to naphthalene, which agrees with the study of Mimura et al. [6]. The 2-ethylnaphthalene quantity decreased from 2% at 600 8C to 0.1% at 850 8C. This trend does not support the isomerization pathway from 1-EN. The decomposition of 2-EN can result in the formation of 2-VN, as indicated by the temperature series (Fig. 5, bottom panel). One postulation is that the isomerization process, the H addition and intramolecular alkyl shift, as described in our 1methylnaphthalene pyrolysis, has a much higher energy barrier than other reactions. Our future work on 1-propylnaphthalene is underway to further investigate the isomerization process. 4. Conclusion The products of 1-EN pyrolysis suggest the following reaction pathways: hydrogen abstraction from the ethyl group results in naphthylethyl radicals at benzylic and primary locations, both lead to the formation of 1-VN, with the benzylic route being favored; hydrogen displacement of the ethyl group results in naphthalene; and the homolytic cleavage of the ethyl group gives rise to 1-MN. The relative abundance of these products is also an indication of the relative importance of these pathways. The reactivity of 1-EN is higher than that of MNs, as evidenced by the lower reaction temperatures, and the higher reactant consumption/product formation at the same temperature. Unlike in the MN pyrolysis, the isomerization of 1-EN to 2-EN is not observed. Acknowledgements The authors would like to thank Dr. Valerie Young, from the Department of Chemical Engineering, Ohio University, and Dr. SanMou Jeng, from the Department of Aerospace Engineering, University of Cincinnati for the detection of gaseous products. The support from the University Research Council of the University of Cincinnati is gratefully acknowledged. References
Fig. 7. The estimated maximum yield of 1-MN. ‘‘residual 1-EN’’: yields of unconsumed 1-EN; ‘‘consumed 1-EN’’: quantity of consumed 1-EN, which is 1‘‘residual 1-EN’’; ‘‘1-VN + AceN’’: the sum of 1-VN and AceN; ‘‘maximum 1-MN’’: the estimated maximum possible yield of 1-MN in 1-EN pyrolysis.
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