Air Interaction of Cellulose

The EPR intensities of pectin and tobacco chars (with charring temperatures from 250 °C to 600 °C) first increase rapidly and then decrease slowly d...
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Energy & Fuels 2004, 18, 560-568

EPR Investigations of Charring and Char/Air Interaction of Cellulose, Pectin, and Tobacco Ji-Wen Feng, Shaokuan Zheng, and Gary E. Maciel* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received August 13, 2003

A room-temperature EPR (electron paramagnetic resonance) study on the charring (under He flow) and char transformation (by air) of pectin, cellulose, and tobacco is reported. The EPR intensities of pectin and tobacco chars (with charring temperatures from 250 °C to 600 °C) first increase rapidly and then decrease slowly during the room-temperature air exposure, indicating that the air exposure of pectin and tobacco chars involves two different types of chemical processes: the production and annihilation of free radicals. During air exposure, “new” radicals form at nonradical sites of the char surface, not through direct reactions of O2 with “old” free radicals generated by pyrolysis. These new free radicals have larger g-values than the old radicals, implying an oxygen-centered structure for the former. The low-temperature chars of pure cellulose show only an apparently small, slow decrease in radical concentration upon air exposure. For the high-temperature (> 400 °C) chars of cellulose, air exposure is accompanied by a large reduction in apparent radical concentration. Possible free radical structures and processes are considered.

I. Introduction Understanding the pyrolysis, oxidation/combustion, gasification, and charring chemistry1-16 of plant-derived materials is prerequisite to assessing or improving the health and environmental consequences of fires, wood burning, and tobacco smoking. Tobacco is a complex mixture that consists of 6-15% cellulose, 10-15% pectin, roughly 2% lignin, and a variety of other componentssthe exact composition depending on the tobacco part or variety and the growing conditions.17 Cellulose is a polysaccharide made up of glucose monomers and is ubiquitous in nature; it is a primary * Author to whom correspondence should be addressed. Tel: 970491-6480. Fax: 970-491-1801. E-mail: [email protected]. (1) Fitzer, E.; Mueller, K.; Schaefer, W. Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Dekker: New York, 1971; Vol. 7, pp 237-383. (2) McKee, D. W. Chem. Phys. Carbon 1981, 16, 1-118. (3) McKee, D. W. Fuel 1983, 62, 170-175. (4) Wood, J. B.; Sancier, K. M. Catal. Rev.sSci. Eng. 1984, 26 (2), 233-279, and references therein. (5) Kanan, M. P.; Richards, G. N. Fuel 1990, 69, 747-753. (6) Devi, T. G.; Kanan, M. P. Fuel 1998, 77, 1825-1830. (7) Brown, A. L.; Dayton, D. C.; Daily, W. Energy Fuel 2001, 15, 1286-1294. (8) White, J. L.; Conner, B. T.; Perfetti, T. A.; Bombick, B. R.; Avalos, J. T.; Fowler, K. W.; Smith, C. J.; Doolittle, D. J. Food Chem. Toxicol. 2001, 39, 499-505. (9) Blakley, R. L.; Henry, D. D.; Smith, C. J. Food .Chem. Toxicol. 2001, 39, 401-406. (10) Flicker, T. M.; Green, S. A. Anal. Chem. 1998, 70, 2008-2012. (11) Yokoyama, S.; Ogi, T.; Koguchi, K.; Nakamura, E. Chem. Lett. 1983, 151-176. (12) Morterra, C.; Low, M. J. D. Carbon 1985, 23, 335-341. (13) Morterra, C.; Low, M. J. D.; Severdia, A. G. Carbon 1984, 22, 5-12. (14) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; John Wiley & Sons Inc.: New York, 1988; pp 86-193. (15) Bansal, R. C.; Donnet, J. B. Carbon Black; Marcel Dekker Inc.: New York, 1993; pp 67-88. (16) Church, D. F.; Pryor, W. A. Environ. Health Perspect. 1985, 64, 111-126. (17) Bokelman, G. H.; Ryan, W. S., Jr. Beitr. Tabak. Int. 1985, 13, 29-36.

component of plants and the basis of most paper products, as well as cotton and flax. Pectin is a celluloselike polysaccharide in which some of the -CH2OH moieties of cellulose are replaced by carboxylic acid groups, -CO2H. This paper considers the pyrolysis and char/air interaction behaviors of cellulose, pectin, and tobacco from the point of view of observing free radicals by electron paramagnetic resonance (EPR).18,19 It is generally recognized that free radical chemistry is involved in the formation, oxidation, and gasification of chars. By directly observing at least some (the more stable) of the free radicals, one would expect to be able to enhance the fundamental understanding of these processes. Although a substantial number of papers, many of them presenting EPR results,20-28 have dealt with cellulose chars, 20-29 the EPR-based papers on tobacco have largely focused on tar in smoke and have involved spin trapping,16,30-33 and there are no EPRbased reports on pectin chars. Tobacco chars have, from the EPR perspective, been largely neglected. This paper is intended to fill that void and includes cellulose and pectin as important tobacco (18) Wertz, J. E. Chem. Rev. 1955, 55, 829-955. (19) Atherton, N. M. Principles of Electron Spin Resonance; Ellis Horwood Inc.: New York, 1993. (20) Lewis, I. C.; Singer, L. S. J. Phys. Chem. 1981, 85, 354-360. (21) Lewis, I. C.; Singer, L. S. Chem. Phys. Carbon 1981, 17, 1-88. (22) Singer, L. S. Proc. Conf. Carbon, 5th; University Park, PA, 1961, 1963; 2, pp 37-64. (23) DeGroot, W. F.; Shafizadeh, F. Carbon 1983, 21, 61-67. (24) Bradbury, A. G. W.; Shafizadeh, F. Carbon 1980, 18, 109-116 (25) Jackson, C.; Wynne-Jones, W. F. K. Carbon 1964, 2, 227-237. (26) Milsch, B.; Windsch, W.; Henzelmann, H. Carbon 1968, 6, 807812. (27) Armstrong, J. W.; Jackson, C.; Marsh, H. Carbon 1964, 2, 239252. (28) Wind, R. A.; Li, L.; Maciel, G. E.; Wooten, J. B. Appl. Magn. Reson. 1993, 5, 161-176. (29) Bradbury, A. G. W.; Sakai, Y.; Shafizadeh, F. J. Appl. Polym. Sci. 1979, 23, 3271-3280.

10.1021/ef0301497 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/28/2004

Charring and Char Oxidation of Cellulose, Pectin, and Tobacco

constituents with, unlike lignin, well-defined chemical structures. The EPR spectra of amorphous solids, because of the typical broad, largely featureless resonances obtained, seldom lend themselves to definitive structural interpretations, unlike the case of liquidsample EPR, in which sharp resonances often display hyperfine coupling,30-33 liquid sample NMR, or even solid-state NMR. Nevertheless, it is hoped that EPR results will be useful in conjunction with patterns of data emerging from other types of experiments to help establish consistent models for these complicated systems. The oxidation of carbonaceous materials, including cellulose char,12,13,23,24,27 has been the subject of numerous experimental studies.6,12-15,23,24,27,34-40 In the presence of O2 (e.g., in combustion) the formation and decomposition of surface oxidation products are believed to be two important processes occurring during the gasification of materials such as biomass. The formation of surface oxidation products (often termed “oxygen chemisorption”) is significant in controlling the gasification and combustion of such a solid. Many investigations of oxygen chemisorption on carbonaceous materials have been made by a variety of direct or indirect methods.14,15 The main surface functional groups were found to be carboxyl groups, phenolic groups, lactone moieties, and quinone-like carbonyls,14,15 but detailed mechanisms for surface oxidation reactions are not clear. Published reports of O2 oxidation of cellulose chars encompass a range of confusing, and sometimes contradictory, results,12,13,22,23,27 e.g., on the degree to which the relevant processes are easily reversible. The work described in this paper investigated the pyrolysis of cellulose, pectin, and tobacco and the behaviors of their chars upon exposure to air. II. Experimental Section Avicel cellulose obtained from FMC International, citrus pectin obtained from Hercules, and raw tobacco powder from Philip Morris Co., Inc., were used. Pectin contains 2 wt % Na. These cellulose, tobacco, or pectin samples were each charred for 1 h at various high temperatures (250 to 600 °C) in a flowing helium atmosphere (flow rate: 100 mL/min) and then cooled to room temperature under flowing helium; the chars thus obtained were transferred to an EPR tube under N2 without any air exposure. In-situ EPR measurement of air exposure effects was carried out by exposing a char to air at room temperature. X-band EPR measurements were made at room temperature using a Bruker EMX-200 spectrometer with dual cavities. (30) Pryor, W. A. Free Radical Biol. Med. 1992, 13, 659-676. (31) Zang, L. Y.; Stone, K.; Pryor, W. A. Free Radical Biol. Med. 1995, 19, 161-167. (32) Stone, K.; Pryor, W. A. In Lung cancer: Principles and Practice; Pass, H. I., Mitchell, J. B., Johnson, D. H., Turrisi, A. T., Eds.; Lippincott-Raven Publishers: Philadelphia, 1996; pp 323-328. (33) Pryor, W. A.; Stone, K.; Zang, L. Y.; Bermudez, E. Chem. Res. Toxicol. 1998, 11, 441-448. (34) Boyer, S. J.; Clarkson, R. B. Colloids Surf. A: Physicochem. Eng. Aspects 1994, 82, 217-224. (35) Lewis, I. C.; Singer, L. S. J. Phys. Chem. 1981, 85, 354-360. (36) Austen, D. E. G.; Ingram, D. J. E. Chem. Ind. (London) 1956, 981-982. (37) de Ruiter, E.; Tschamler, H. Brennstoff-Chemie 1959, 40, 4143; 1961, 42, 311-312. (38) Dack, S. W.; Hobday, M. D.; Smith, T. D.; Pilbrow, J. R. Fuel 1984, 63, 39-42. (39) Singer, L. S.; Spry, W. J.; Smith, W. H. Proc. 3rd Carbon Conf.; Pergamon: New York, Oxford, 1959; pp 121-128. (40) Ingram, D. J. E. Free Radicals As Studied by Electron Spin Resonance; Academic Press: New York, 1958; pp 210-212.

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Figure 1. EPR line shape for two typical pectin chars (low and high charring temperatures). See text for the definitions of terms. Except for saturation experiments, a microwave power of 0.2 mW was employed to avoid power saturation effects. The concentrations and Lande g-values of the detected free radicals were calculated by comparison with a sample of 2,2-diphenylpicrylhydrazyl (DPPH), for which the g-value was taken as 2.0036.18

III. Results 1. General EPR Features of the Charring of Pectin, Cellulose, and Tobacco (no air exposure). (a) Line Shape and Line Width. Unexposed chars of pectin, of cellulose, or of tobacco show similar overall EPR behaviors, in terms of line shape, line width, g-value, and variation in radical concentration with variation in the pyrolysis temperature or pyrolysis time. Details of these features follow. ESR line shapes of cellulose chars have been examined in some detail previously; however, this approach has not provided simple, direct interpretations in term of structure,24-26 because the generally broad EPR signals (e.g., see Figure 1) of these amorphous solids do not display splittings due to the hyperfine couplings that are commonly seen in the EPR spectra of liquid samples. EPR spectra (microwave power absorption) of chars prepared with charring temperatures T > 400 °C show an approximately Lorentzian line shape (Figure 1), while the spectra of chars prepared at lower temperatures (T e 400 °C) cannot be described by only a single Lorentzian or single Gaussian component. To characterize the line shape variation with charring temperature via simulation, we used a linear combination of two components, Lorentzian and Gaussian, as detailed in eq 1,

f(x) ) a{c(1/{1 + [(x - x0)/b]2}) + (1 - c)exp{-0.5[(x - x0)/d]2}} (1) where c ) the fraction of a Lorentzian component, 1 c ) the fraction of a Gaussian component, and b (or d) ) the line width parameter of a Lorentzian (or Gauss-

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Figure 2. Variations of EPR parameters with charring temperature for pectin char (no air exposure).

ian) component. All of our experimental curves can be fitted well by this model (for example, see Figure 1). Variation with charring temperature of the Lorentzian component fraction, c, thus obtained for pectin chars is shown in Figure 2a. From this figure, it is clear that the Gaussian component is a major contribution to the spectra of low-temperature chars. The Lorentzian component fraction c increases steadily with increasing charring temperature below 450 °C, e. g., from c ) 0.32 for a 250 °C/1 h pectin char to c ) 0.98 (almost pure Lorentzian line shape) for the corresponding 500 °C/1 h char. For low charring temperatures (e400 °C), the EPR peak-to-peak line width is about 5 to 8 G and it does not change much with charring temperature. But with further increasing of the charring temperature, the peak-to-peak EPR line width (from the derivative of the power absorption spectrum) decreases first and then increases dramatically above 500 °C. A minimum of 1.4 G in the peak-to-peak line width appears around a charring temperature of 500 °C for the pectin char (Figure 2). From 500 °C to 600 °C, the EPR line width increases from 1.4 G to about 50 G. (b) Free Radical Concentration and g-Value. The magnitudes of the free radical concentrations of chars observed in this study range up to a maximum between 1020 and 2 × 1020 spins/g. This range is qualitatively consistent with values that have been reported previously for chars, including cellulose chars.23,25-28 The dependences of free radical concentration and g-value on pyrolysis temperature for a pectin char are shown in Figure 2a, where one sees that both of these parameters are very sensitive to pyrolysis temperature. The free radical concentration increases as the temperature is increased from 250 to 500 °C, and then decreases with increased charring temperature. A maximum spin concentration of 1.9 × 1020 spins/g can be found at ∼500 °C, which corresponds to the temperature of minimum line width. The g-value of the char decreases from

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Figure 3. EPR saturation behavior of pectin chars (no air exposure).

2.0037 at 250 °C to 2.0027 at 600 °C. The latter value is only slightly larger than the free-electron g-value, 2.0023. Similar results were also obtained in our experiments on cellulose chars and tobacco chars (not shown here) and in previous studies of cellulose chars24,25,27,28 (for which maximum spin concentration and minimum EPR line widths are found in the 550 °C to 700 °C rangeshigher than obtained in this work on pectin char). (c) Microwave Power Saturation. Microwave power saturation has been examined for cellulose, pectin, and tobacco chars. Typical results for pectin char are shown in Figure 3. It is evident that a low-temperature char (for example, the 300 °C/1 h pectin char) shows saturation behavior, i.e., a nonlinear intensity dependence on P1/2, where P is the microwave power. As seen in Figure 3, lowering the pyrolysis temperature gives rise to a lower saturation power. Over the range of microwave powers employed in these saturation experiments, the EPR line width does not change with microwave power. Saturation behavior is not observed for high-temperature pectin chars (Figure 3a) over the entire experimental microwave power range available for these experiments. Similar behaviors have also been found in our experiments on cellulose and tobacco chars. 2. EPR Investigations on the Air Exposure of Cellulose, Pectin, and Tobacco Chars. Figures 4-8 summarize variations with charring temperature of EPR parameters observed upon air exposure for pectin, cellulose, and tobacco chars. Pectin and tobacco chars show very different air-exposure EPR behaviors from those of corresponding cellulose chars. Chars produced in 1 h from pectin and tobacco at pyrolysis temperatures from 250 °C to 500 °C show a pronounced initial increase in free radical concentration when exposed to air at room temperature (Figures 4 and 5), indicating that free radicals are formed as intermediates during

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Figure 4. Effects of air exposure on EPR characteristics of 250 °C/1 h, 350 °C/1 h, and 400 °C/1 h pectin chars.

Figure 6. Variations of spin concentrations during initial air exposure period for various pectin chars.

Figure 5. Effects of air exposure time on EPR characteristics of 450 °C/1 h, 500 °C/1 h, and 550 °C/1 h pectin chars.

air exposure of these chars. Correspondingly, the peakto-peak EPR line width and g-value also increase rapidly. On further air exposure, the free radical concentration decreases slowly over a very long airexposure period, accompanied by a slow and small decrease in line width, but the g-value does not change during this slow process. The air exposure pattern of 550 °C/1 h pectin char is somewhat different (Figure 5); the free radical concentration decreases slightly over a short initial airexposure period (∼1 h), then increases substantially during a relatively long period of further air exposure, followed by a decrease. Corresponding to the increase in radical concentration, the peak-to-peak EPR line

Figure 7. Variations of EPR parameters with air exposure time for Burley tobacco char.

width decreases markedly, from 10 to 3.4 G. The information in Figures 4a and 5a is replotted in Figure 6 to show the variation in radical concentration during short-term air-exposure. From Figure 6, we see that the higher the pyrolysis temperature, the more rapid is the

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Figure 8. Variations of EPR parameters with air exposure time for cellulose chars.

initial increase in free radical concentration when the charring temperature is below 550 °C. The increase in free radical concentration for the 500 °C/1 h char lasts only several minutes, while the corresponding increase for the 250 °C/1 h char persists for a thousand minutes or longer. When the charring temperature is increased to 600 °C (not shown here) or to 550 °C, the increase in free radical concentration upon air exposure becomes slow again (lasting for about 10 h for both 550 °C and 600 °C pectin chars), and an obvious increase in radical concentration does not occur immediately after air exposure. For low-temperature (250-400 °C) pectin chars, the concentration of “newly” formed free radicals (formed during air exposure) is comparable with, or can be even higher than, that of the “old” free radicals (generated by pyrolysis). These EPR results show that air exposure of pectin chars involves two different types of chemical processes, production and annihilation of free radicals, a general statement that is consistent with earlier observations on organic materials (both in solids and liquid phase).20,21,41 But such production and annihilation processes of free radicals were not clearly revealed by previous EPR investigations on the oxidation of solid carbonaceous materials such as char, carbon black, and graphite.21-23,25-28 The “new” free radicals observed here presumably form at nonradical sites of the char surface, not primarily through a direct reaction of O2 with “old” free radical centers, since the latter process only transforms carbon-centered radicals into peroxy free radicals and does not increase the total concentration of free radicals. To verify that the observed increases in EPR intensity are truly due to the increased free radical concentrations (41) Reference 19, Ch. 6.

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and not to microwave power saturation effects (e.g., the presence of paramagnetic O2, enhancing relaxation of free radicals on the char surface), EPR power saturation experiments of the type summarized in Figure 3 demonstrated that for pectin samples the microwave power of 0.2 mW used in the experiments reported here is well below the power required for saturation. Thus, the airexposure results indicate that the behavior of pectin chars involves, or is accompanied by, formation of “new” free radicals, and that increasing the pyrolysis temperature (say, from 250 °C to 500 °C), enhances the formation of “new” free radicals produced during air exposure at room temperature; but increasing the charring temperature further to 550 °C or above attenuates the formation of these “new” free radicals. Figure 7 shows results of the EPR examination of the air-exposure behaviors of 350 °C/1 h and 550 °C/1 h chars of Burley tobacco. From Figure 7, together with Figures 4 and 5. One sees that the variations of EPR parameters of the tobacco chars during air exposure are similar to that of the corresponding pectin chars, except for different line width behaviors of the 550 °C/1 h tobacco and 550 °C/1 h pectin chars. As seen in Figure 8, the pattern of results of an EPR investigation of the air-exposure behaviors of high- and low-temperature cellulose chars is completely different from that of the corresponding pectin and tobacco chars. For the low-temperature (350 °C/1 h) cellulose char, the EPR signal is largely insensitive to air exposure. In contrast, the EPR of a high-temperature (550 °C/1 h) cellulose char is very sensitive to air exposure, decreasing rapidly during the initial air-exposure period. Correspondingly, the line width of the single EPR peak also increases rapidly with increasing exposure time for a period of about 10 minutes, from 1.4 to 12 G, then levels off to a nearly constant value. The g-value stays nearly constant throughout the entire air-exposure period. These results indicate that the air exposure of cellulose chars involves mainly, or only, the annihilation of “old” free radicals generated by pyrolysis. Figure 9 shows EPR (power) absorption spectra (integrations of the “typical” derivative plots), taken before and after air exposure (for 60 min), of the 400 °C/1 h pectin char; the corresponding differencespectrum is shown in Figure 9b. One can note that the broadening and substantial enhancement of EPR peak intensity due to air exposure occur mainly in the lowfield (high-g) side, implying that the newly formed free radicals have a larger g-value than those produced directly by pyrolysis. From the difference-spectrum, which is mainly due to the contribution of “new” free radicals, a g-value of 2.0040 is obtained. The EPR line shape of the “new” free radicals is accounted for by a mixture of 89% Lorentzian and 11% Gaussian. For all of the pectin chars obtained with pyrolysis temperatures in the range of 250 to 600 °C, the “new” radical g-values obtained by this difference-spectrum method are found to be in range of 2.0030 to 2.0048, which is consistent with those of small, oxygen-containing aromatic radicals, such as those derived from anthracene, 1,2benzpyrene,20 and semiquinones.16 Figure 10 shows results on the reversibility of the EPR-monitored behavior of a 350 °C/1 h pectin char upon air exposure. After the char was exposed to air

Charring and Char Oxidation of Cellulose, Pectin, and Tobacco

Figure 9. (a) EPR spectra taken before and after 60-min air exposure of 400 °C/1 h pectin char and (b) difference-spectrum from (a).

Figure 10. EPR spectra of samples derived from 350 °C/1 h pectin char. (a) Prior to air exposure, (b) after air exposure for 1400 min (maximum radical concentration), (c) after air exposure for 70 days. (d) Sample of (c) after 8.5 h evacuation (