Energy & Fuels 2009, 23, 1551–1554
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Radicals from the Atmospheric Pressure Pyrolysis and Oxidative Pyrolysis of Hydroquinone, Catechol, and Phenol Julian Adounkpe,† Lavrent Khachatryan,*,† Barry Dellinger,† and Mariana Ghosh‡ Department of Chemistry, Louisiana State UniVersity, Baton Rouge, Louisiana 70803 and British American Tobacco, Group R&D Centre, Southampton, SO15 8TL, United Kingdom ReceiVed December 3, 2008. ReVised Manuscript ReceiVed January 7, 2009
The burning of tobacco creates various types of free radicals that have been reported to be biologically active. Some radicals are transient but can initiate catalytic cycles that generate other free radicals. Other radicals are environmentally persistent and can exist in total particulate matter (TPM) for extended periods. In spite of their importance, little is known concerning the precursors of these radicals or under what pyrolysis/ combustion conditions they are formed. We performed studies of the formation of radicals from the gas-phase pyrolysis and oxidative pyrolysis of hydroquinone (HQ) and catechol (CT) between 750 and 1000 °C and phenol from 500 to 1000 °C. The initial electron paramagnetic resonance (EPR) spectra were complex, indicating the presence of multiple radicals. Using matrix annealing and microwave power saturation techniques, phenoxyl, cyclopentadienyl, and peroxyl radicals were identifiable, but only cyclopentadienyl radicals were stable above 750 °C.
Introduction Various types of free radicals have been identified in cigarette smoke, including semiquinones, phenoxyls, methoxyl, and other oxy radicals.1-4 The presence of semiquinone-type radicals in cigarette smoke and their ability to induce oxidative stress is well documented in the literature.1,5 Catechol (CT), hydroquinone (HQ), and structurally similar derivatives have been proposed as progenitors of semiquinone-type radicals and implicated in the toxicity of cigarette smoke.1,5,6 HQ, CT, and phenol are found in tobacco smoke and probably formed from partial decomposition of constituents of tobacco including lignin, pectin, and cellulose.5 Phenols have been observed to be formed from decomposition of high molecular weight tobacco constituents at 700 °C.7 Catechols have been observed from the pyrolysis of lignin from 600 to 800 °C and pyrolyis of tobacco from 650 to 750 °C.8 Phenols were formed at temperatures up to 1000 °C from the pyrolysis of lignin, pectin, cellulose, and pigment.9 * To whom correspondence should be addressed. E-mail: Lkhach1@ lsu.edu. † Louisiana State University. ‡ British American Tobacco. (1) Pryor, W. A.; Hales, B. J.; Premovic, P. I.; Church, D. F. Science 1983, 220, 425. (2) Maskos, Z.; Khachatryan, L.; Dellinger, B. Energy Fuels 2008, 22, 1027. (3) Valavanidis, A.; Haralambous, E. Redox Rep. 2001, 6, 161. (4) Bartalis, J.; Chan, W. G.; Wooten, J. B. Anal. Chem. 2007, 79, 5103– 5106. (5) Cigarette Smoke and Oxidation Stress. Halliwell, B. B., Poulsen, H. E., Eds.; Springer-Verlag: Berlin, Heidelberg, 2006. (6) Dellinger, B.; Lomnicki, S.; Khachatryan, L.; Maskos, Z.; Hall, R.; Adounkpe, J.; McFerrin, C.; Truong, H. ScienceDirect, Proceedings of the Combustion Institute 2007, 31, 521. (7) Schlotzhauer, W. S.; Martin, R. M.; Snook, M. E.; Williamson, R. E. J. Agric. Food Chem. 1982, 30, 372. (8) Carmella, S. G.; Hecht, S. S.; Tso, T. C.; Hoffman, D. J. Agric. Food Chem. 1984, 32, 267. (9) Sharma, R. K.; Wooten, J. B.; Baliga, V. L.; Lin, X.; Chan, W. G.; Hajaligol, M. R. Fuel 2004, 83, 1469.
Since organic free radicals are generally considered to be very reactive and highly unstable, the discovery that they can exist for extended periods of time when associated with cigarette smoke10,11 (and other types of airborne particulate matter12,13) is surprising. Thus, identification of the exact nature of the radicals, their origin, and the reason for their stability are important issues to understand. Using the technique of lowtemperature matrix isolation EPR (LTMI-EPR) we previously performed studies of the thermal degradation of hydroquinone,14 catechol,15 and phenol16 under controlled pyrolysis conditions at low pressure (0.1-0.01 Torr). We observed formation of semiquinone radicals from the low-pressure pyrolysis of HQ and CT at temperatures below 700 °C, formation of phenoxyl radicals from pyrolysis of phenol above 700 °C, and formation of cyclopentadienyl (CPD) radicals from all three compounds above 700 °C. Low-pressure conditions were used in our previous studies to help ensure that the radical concentration was maintained in the apparatus such that they could be easily analyzed by EPR. However, the naturally occurring question was whether these radicals were stable enough to survive at atmospheric pressure. In this publication, we report the use of LTMI-EPR to analyze for free radicals produced from the atmospheric pressure pyrolysis and oxidative pyrolysis of HQ, CT, and phenol in an (10) Pryor, W. A.; Terauchi, K.; Davis, W. H. EnViron. Health Perspect. 1976, 16, 161. (11) Pryor, W. A.; Prier, D. G.; Church, D. F. EnViron. Health Perspect. 1983, 47, 345. (12) Dellinger, B.; Pryor, W. A.; Cueto, R.; Squadrito, G. L.; Hedge, V.; Deutsch, W. A. Chem. Res. Toxicol. 2001, 14, 1371. (13) Dellinger, B.; Pryor, W. A.; Ceuto, R.; Squadrito, G. L.; Hedge, V.; Deutsch, W. A. Chem. Res. Toxicol. 2001, 14, 1371. (14) Adounkpe, J.; Khachatryan, L.; Dellinger, B. Energy Fuels 2008, 22, 2986. (15) Khachatryan, L.; Adounkpe, J.; Maskos, Z.; Dellinger, B. EnViron. Sci. Technol. 2006, 40, 5071. (16) Khachatryan, L.; Adounkpe, J.; Dellinger, B. J. Phys. Chem., A 2008, 112, 481.
10.1021/ef801055h CCC: $40.75 2009 American Chemical Society Published on Web 02/27/2009
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Figure 2. EPR spectra from the atmospheric pressure pyrolysis of hydroquinone, phenol, and catechol in a carbon dioxide matrix at selected temperatures. All three spectra are virtually identical to the spectrum of cyclopentadienyl radical reported in the literature.
calibration programs on a Window-based PC [http://us.brukerbiospin.com/brukerepr/winepr.html]. Figure 1. Experimental configuration for low-temperature matrix isolation electron paramagnetic resonance (LTMI-EPR) studies under atmospheric pressure conditions.
attempt to understand the influence of pressure on the yield and nature of radicals. Experimental Section Formation of radicals from the atmospheric pressure pyrolysis and oxidative pyrolysis of hydroquinone and catechol (between 750 and 1000 °C) (HQ and CT) and phenol (between 400 and 1000 °C) at atmospheric conditions was studied using the LTMI-EPR technique.17,18 Each reactant was introduced into a thermoelectrically heated, fused silica reactor at the desired temperature. The details of the LTMI EPR technique can be found elsewhere.18,14 Carbon dioxide was used as a carrier gas, and the residence time in the high-temperature zone was 2.0 s. Carbon dioxide is inert under these reaction conditions and does not affect radical formation. Our experimental reaction time of 2 s corresponds to the duration of a standard puff when smoking a cigarette.19 The carbon dioxide carrier was introduced through a sample vaporizer at atmospheric pressure containing HQ, CT, or phenol thermoregulated at 50 °C for CT, 70 °C for HQ, and room temperature for phenol. The gas flow (saturated by vapors of HQ, CT, or Phenol) exited the vaporizer and entered the pyrolysis zone (cf. Figure 1). To reduce the pressure in the transfer line from the reactor to the EPR coldfinger, the reaction products were continuously sampled through a ∼100 µm orifice pumped down to ∼0.1 Torr on the downstream side of the orifice. The effluent was then deposited on a coldfinger at 77 K within the microwave cavity of the EPR spectrometer. All EPR spectra were recorded on a Bruker EMX-20/2.7 EPR spectrometer with dual cavities, X-band; modulation and microwave frequencies were 100 kHz and 9.516 GHz, respectively. The typical parameters were as follows: sweep width 200 G, EPR microwave power of 0.1-20 mW, and modulation amplitude e 4 G. Time constant and sweep time were varied. Values of g factors were calculated using Bruker’s WINEPR program, which is a comprehensive line of software, allowing control of the Bruker EPR spectrometer, data acquisition, automation routines, tuning, and (17) Khachatryan, L.; Niazyan, O.; Manashyan, A.; Vedeeneev, V.; Teiltelboim, M. Int. J. Chem. Kinet. 1982, 14, 1231. (18) Khachatryan, L.; Adounkpe, J.; Maskos, M.; Dellinger, B. EnViron. Sci. Technol. 2006, 40, 5071. (19) Baker, R. R. Smoke ChemistryDavis, D. D., Nielsen, M. T., Eds.; Tobacco Production, Chemistry and Technology Blackwell Science: Oxford, 1999; Chapter 12, p 398.
Results and Discussion The initially observed spectra were complex, and the identities of the radicals were not easily assigned. As a result, several different techniques needed to be used to simplify the spectra in order to identify the radicals.16 Thermal annealing of the matrix by slight warming above 77 K results in diffusion within the matrix and reaction of the more reactive and mobile radicals. Small reactive radicals such as alkyl radicals are expected to be destroyed using the annealing technique. Variation of the EPR microwave power can also be used to simplify spectra. Each radical has its own microwave susceptibility, and the intensity of spectra of selected radicals can be reduced by reducing the microwave power or saturating the transition by increasing the microwave power. Formation of Cyclopentadienyl Radical from the Atmospheric Pressure Pyrolysis of Phenol, Hydroquinone, and Catechol. The cleanest, simplest spectra were obtained above 750 °C. Representative EPR spectra from the high-temperature pyrolysis of HQ, CT, and phenol over the temperature range of 750-1000 °C are presented in Figure 2. In these experiments, gradual warming of the Dewar was employed to improve resolution through annealing of the matrix and annihilation of mobile or very reactive radicals. The dominant radical formed at high temperature for all three compounds was cyclopentadienyl (CPD) radical. The annealed EPR spectra with six lines, hyperfine splitting constant 6.0 G, g ) 2.0050, and ∆Hp-p for individual lines ≈ 3 G were readily assignable based on favorable comparison to the literature and theoretical calculations.14-16,20,21 Formation of Cyclopentadienyl and Phenoxyl Radical from Atmospheric Pressure Pyrolysis of Phenol at e700 °C. Because phenol is one of the major products from HQ or CT pyrolysis,22 special care was taken to determine if phenoxyl radical was formed. In contrast to the spectra obtained above 750 °C, at lower temperatures the spectra were more complex, indicating the presence of multiple radicals (cf. Figure 3). The unannealed 700 °C spectrum is complex. On the basis of our previous publication,16 it is likely a mixture of CPD radicals (existence of multiple equidistance lines at ∼6 G), (20) Liebling, G. R.; McConnell, H. M. J. Chem. Phys. 1965, 42, 3931. (21) Kira, M.; Watanabe, M.; Sakurai, H. J. Am. Chem. Soc. 1980, 102, 5202. (22) Hieu, T.; Slawo, L.; Dellinger, B. Chemosphere 2008, 73, 629.
Pyrolysis of Hydroquinone, Catechol, and Phenol
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Figure 3. EPR spectra from the atmospheric pressure pyrolysis of phenol in a carbon dioxide matrix at pyrolysis temperatures of 700 °C unannealed (A), 700 °C annealed (B), 550 °C unannealed (C), and 500 °C unannealed (D).
Figure 4. EPR spectra from the atmospheric pressure oxidative pyrolysis of phenol in a carrier gas of 1000 ppm O2 in N2 at 700 °C. Spectrum A converts to spectrum B and subsequently spectrum C and D by annealing of the matrix. (The asterisks in spectrum A mark the peaks that are removed by annealing). Spectrum E (red line) overlaid on spectrum D is an EPR spectrum of pure phenoxyl radical detected at 77 K.16 * DI/N value is the double integrated (DI) intensity of the EPR spectrum that has been normalized (N) to account for the conversion time, receiver gain, number of data points and sweep width [http://www.bruker-biospin.com/winepr.html?&L)0].
phenoxyl radicals (peaks a and b),16 and traces of secondary/ unknown radicals produced by hydrogen addition to the unsaturated molecules (shoulder on the left-hand side denoted by an asterisk).20,23 Annealing of the matrix converts the complex spectrum to one that is easily identifiable as CPD (spectrum B). Lowering the pyrolysis temperature to 550 °C results a weak spectrum also identifiable as CPD (spectrum C). Traces of CPD radicals were identifiable down to 500 °C (spectrum D). In contrast to our previous low-pressure studies in which CPD radicals were identified as low as 400 °C,16 no radicals were detected at 400 °C at atmospheric pressure. The somewhat surprising formation at 400 °C was previously attributed to wall-catalyzed reactions which may be more limited at atmospheric pressure due to reduction of the diffusion rate. Formation of Phenoxyl, Cyclopentadienyl, and Peroxyl Radicals from the Atmospheric Pressure, Oxidative Pyrolysis of Phenol. It is known that in a cigarette the rate of combustion of the tobacco is controlled by the rate of mass transfer of oxygen to the tobacco surface and that the burning is occurring at zones with different percentage of oxygen, from ∼21% close to the border of tobacco paper down to nearly 0% in the interior of the pyrolysis and distillation zones.19 Thus, there is ample opportunity for oxidative pyrolysis of volatile hydrocarbons in an environment of a few percentage of oxygen. Consequently, we performed additional atmospheric pressure experiments in a reaction atmosphere of 1000 ppm O2 in N2. The spectrum obtained at 700 °C was typical of spectra obtained (23) Ohnishi, S.-I.; Nitta, I. J. Chem. Phys. 1963, 39, 2848.
from 550 to 700 °C. The complex EPR spectrum obtained at 700 °C is depicted in Figure 4, spectrum A. The spectrum is an anisotropic, inhomogeneously broadened24,25 mixture of radicals spectrum with a very high apparent g value of 2.0100. By annealing the matrix, spectrum A converts to spectrum B with a g value of 2.0090 with the disappearance of the two small peaks (with a line splitting of ∼6 G) from the right side of spectrum A, which we believe corresponds to CPD radicals. Further annealing leads to simplification of the spectra and decreasing of the g value from 2.0100 (spectrum A) to 2.0062 for spectrum D (black line). The overlaid red spectrum E is that of pure phenoxyl radical generated photochemically,16 clearly indicating that the residual spectrum is that of phenoxyl radical. Phenoxyl radicals were identified during the multiple annealing procedure, while CPD radicals disappeared after the first annealing as the DI/N decreased from 1.284 to 1.032 (cf. Figure 4). From the DI/N values of spectra A (DI/N ) 1.284) through E (DI/N ) 0.10) it is clear that the concentration of phenoxyl radicals is much higher than CPD radicals. The higher concentration of phenoxyl radical than CPD radicals under oxidative pyrolysis conditions is in contrast to the results under pyrolytic conditions where CPD is dominant. This is because (24) Sahlin, M.; Graslund, A.; Ehrenberg, A. J. Magn. Reson. 1986, 67, 135. (25) Zayachuka, D.; Polyhacha, Y.; Slynkob, E.; Khandozhkoc, O.; Rudowiczd, C. Physica B 2002, 322, 270.
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CPD is reactive with oxygen while phenoxyl is much less so26(vide infra). Examination of spectrum B, Figure 4, reveals the presence of an additional radical species formed from the oxidative pyrolysis of phenol. The g values of 2.0080 for the center and 2.0031 for the shoulder, total width of 75-80 G, and overall anisotropy resemble that of peroxyl radicals, RO2 · , reported in the literature.17,26-28 A conclusive identification of peroxyl radical was reported in our previous paper on the low-pressure oxidative pyrolysis of HQ.26 Formation of the low molecular weight products can proceed through alkyl radicals that can be converted to the observed alkyl peroxy radicals in the presence of trace quantities of O2. Small olefinic hydrocarbons (e.g., acetylene, 1,3-butadiene, and ethene) are also formed when phenol is oxidized,29 and it is documented that electrophilic addition of O2 can result in the formation of various types of RO2 · .30,31 Although the exact peroxyl radical cannot be identified, there is little doubt that peroxyl was formed. Can aromatic peroxyl radicals such as cyclopentadienyl peroxyl (CPDOO · ) or phenoxy peroxyl radicals be formed? Addition of molecular oxygen to the phenoxy radicals has a high activation barrier of ∼22 kcal/mol32 that would appear to exclude formation of phenoxy peroxyl radicals. Addition of O2 to CPD radical is reported to be barrierless, but due to the large (26) Khachatryan, L.; Adounkpe, J.; Dellinger, B. Energy Fuels 2008, 22, 3810. (27) Carlier, M.; Pauwels, J. P.; Sochet, L.-R. Oxidation Commun. 1984, 6, 141. (28) Svistunenko, D. A.; Patel, R. P.; Wilson, M. T. Free Radical Res. 1996, 24, 269. (29) Brezinsky, K.; Pecullan, M.; Glassman, I. J. Phys. Chem. A 1998, 102, 8614. (30) Benson, S. W. Thermochemical Kinetics. John Wiley & Sons, Inc.: Canada, 1976. (31) Benson, S. W. J. Am. Chem. Soc. 1965, 87, 972. (32) McFerrin, C. A.; Hall, R. W.; Dellinger, B. THEOCHEM 2008, 848, 16.
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positive free energy change for the reaction, the initially formed adduct predominantly dissociates back to reactants.33 In addition, it has been proposed that a small fraction of the CPDOO · that is formed undergoes ring-opening reactions.33 Accordingly, we can rule out formation of cyclopentadienyl peroxyl (CPDOO · ) or phenoxy peroxyl radicals from the oxidative pyrolysis of phenol. Conclusions Other than the observation of formation of phenoxyl at a slightly lower temperature, formation of radicals from the atmospheric pressure pyrolysis of HQ, CT, and phenol was nearly identical to that observed at low pressures using the same model system. This indicates that both cyclopentadienyl and phenoxyl radicals are long-lived in a non-oxygen-containing atmosphere. Due to some reactivity of CPD with O2, the atmospheric pressure oxidative pyrolysis of phenol resulted in a reduced concentration of CPD while that of phenoxyl radical remained. It is frequently suggested that peroxyl radical is formed during the burning of tobacco;3,34,35 however, to our knowledge it has not been previously experimentally observed. Formation of peroxyl radical from the experimental oxidative pyrolysis of phenol (as well as from HQ26) suggests that it could also be present in tobacco smoke. Acknowledgment. The authors gratefully acknowledge the partial support of this research by British American Tobacco (BAT) and the NSF (Grant CTS-0317094). EF801055H (33) Zhong, X.; Bozzelli, J. W. J. Phys. Chem. A 1998, 102, 3537. (34) Pryor, W. A.; Prier, D. G.; Church, D. F. EnViron. Health Perspect. 1983, 47, 345. (35) Flicker, T. M.; Green, S. A. EnViron. Health Perspect. 2001, 109, 765.
