In Situ High-Temperature Electron Paramagnetic Resonance (EPR

Energy & Fuels 2005, 19, 1201-1210

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In Situ High-Temperature Electron Paramagnetic Resonance (EPR) Investigation of the Charring of Cellulose and Cellulose/Na2CO3 Mixtures and the O2-Induced and H2O-Induced Behaviors of These Chars Shaokuan Zheng, Ji-Wen Feng, and Gary E. Maciel* Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received September 10, 2004. Revised Manuscript Received January 24, 2005

In situ high-temperature electron paramagnetic resonance (EPR) has been used to study the pyrolysis (under a helium atmosphere) of cellulose and cellulose/Na2CO3 mixtures, as well as changes in the resulting chars under exposure to O2 and H2O after quenching to room temperature. It has been found that the addition of Na2CO3 not only dramatically changes the pyrolysis behavior, but also has substantial effects on the subsequent exposure behaviors of the resulting chars to O2 and H2O. The presence of Na2CO3 substantially narrows the EPR lines of cellulose chars at all charring temperatures used in this study. For low-temperature (e350 °C) pure-cellulose chars and for cellulose/Na2CO3 mixture chars prepared at all but the highest temperature (550 °C) of this study, short-term (e120 min) exposure to O2 has no obvious effect on the spin concentration and the line width; however, decreases in the spin concentration and increases in the line width are observed for high-temperature (g400 °C) pure-cellulose chars. Effects of H2O introduction into the He/O2 gas stream are most dramatic for lower-temperature cellulose/Na2CO3 chars and for higher-temperature chars of pure cellulose. The results are discussed in terms of a complex array of possible free-radical reactions.

I. Introduction Numerous electron paramagnetic resonance (EPR) studies have been reported during the last fifty years on chars produced from a variety of plant materials, including cellulose, wood, pectin, tobacco, and other organic material.1-20 That researchssome of it from this * Author to whom correspondence should be addressed. Telephone: 970-491-6480. Fax: 970-491-1801. E-mail address: maciel@ lamar.colostate.edu. (1) Church, D. F.; Pryor, W. A. Environ. Health Perspect. 1985, 64, 111-126. (2) Lewis, I. C.; Singer, L. S. J. Phys. Chem. 1981, 85, 354-360. (3) Lewis, I. C.; Singer, L. S. Chem. Phys. Carbon 1981, 17, 1-88. (4) Singer, L. S. In Proceedings of the Fifth Conference on Carbon, University Park, PA, 1961; Macmillan: New York, 1962; Vol. 2, pp 37-64. (5) DeGroot, W. F.; Shafizadeh, F. Carbon 1983, 21, 61-67. (6) Bradbury, A. G. W.; Shafizadeh, F. Carbon 1980, 18, 109-116. (7) Jackson, C.; Wynne-Jones, W. F. K. Carbon 1964, 2, 227-237. (8) Armstrong, J. W.; Jackson, C.; Marsh, H. Carbon 1964, 2, 239252. (9) Milsch, B.; Windsch, W.; Henzelmann, H. Carbon 1968, 6, 807812. (10) Wind, R. A.; Li, L.; Maciel, G. E.; Wooten, J. B. Appl. Magn. Reson. 1993, 5, 161-176. (11) Bradbury, A. G. W.; Sakai, Y.; Shafizadeh, F. J. Appl. Polym. Sci. 1979, 23, 3271-3280. (12) Pryor, W. A. Free Radical Biol. Med. 1992, 13, 659-676. (13) Zang, L. Y.; Stone, K.; Pryor, W. A. Free Radical Biol. Med. 1995, 19, 161-167. (14) 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. (15) Pryor, W. A.; Stone, K.; Zang, L. Y.; Bermudez, E. Chem. Res. Toxicol. 1998, 11, 441-448. (16) Sancier, K. M. Fuel 1983, 62, 331-335. (17) Sancier, K. M. Fuel 1984, 63, 679-685. (18) Pastor, R. C.; Hoskins, R. H. J. Chem. Phys. 1960, 32, 264269.

laboratory19,20shas included examination of the effects of inorganic additives on charring. Essentially all of the prior EPR/plant-materials studies, which are intended to characterize the roles(s) of free radicals in charring, have been based on room-temperature EPR measurements on samples that were generated at higher temperature and then temperature-quenched. Recently, the effects of inorganic additives on the pyrolysis of biomass (such as cellulose, wood, agricultural residues, and municipal solid wastes) and the oxidation of the resulting chars have received much attention.5,16,17,19-31 Alkalimetal carbonates or bicarbonates are reported to have the following effects in the pyrolysis of cellulose and the gasification of chars: (i) a lower decomposition temper(19) Feng, J. W.; Zheng, S. K.; Maciel, G. E. Energy Fuels 2004, 18, 560-568. (20) Feng, J. W.; Zheng, S. K.; Maciel, G. E. Energy Fuels 2004, 18, 1049-1065. (21) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; Wiley: New York, 1988; pp 86-193. (22) Kannan, M. P.; Richards, G. N. Fuel 1990, 69, 747-753. (23) Devi, T. G.; Kannan, M. P. Fuel 1998, 77, 1825-1830. (24) Yokoyama, S. Y.; Ogi, T.; Koguchi, K.; Nakamura, E. Chem. Lett. 1983, 151-176. (25) Morterra, C.; Low, M. J. D. Carbon 1985, 23, 335-341. (26) Yamashita, Y.; Ouchi, K. Carbon 1982, 20, 41-45; 47-53; 5558. (27) Boyer, S. J.; Clarkson; R. B. Colloids Surf. A 1994, 82, 217224. (28) Wood, B. J.; Sancier, K. M. Catal. Rev.-Sci. Eng. 1984, 26 (2), 233-279 and references therein. (29) Sekiguchi, Y.; Shafizadeh, F. J. Appl. Polym. Sci. 1984, 29, 1267-1286. (30) Degroot, W. F.; Kannan, M. P.; Richards, G. N.; Theander, O. J. Agric. Food Chem. 1990, 38 (1), 320-323. (31) Kannan, M. P.; Richards, G. N. Fuel 1990, 69 (8), 999-1006.

10.1021/ef040084n CCC: $30.25 © 2005 American Chemical Society Published on Web 04/30/2005

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ature of biomaterials;25 (ii) an enhancement of dehydration and cross-linking, which reduces the production of tar and volatile byproducts and increases the carbon yield and porosity;25-27 (iii) changes in the chemical functionalities of the resulting carbonaceous materials;25,28 and (iv) a lower gasification temperature and enhancement of the gasification rate of cellulose chars.22-26,28 Although extensive studies have been conducted to understand how inorganic additives interact with carbonaceous materials, details about these interactions have not been clear. In work that has been reported previously,19,20 we used room-temperature EPR measurementsssome cases coupled with solid-state nuclear magnetic resonance (NMR)sto study the temperature-quenched pyrolysis products of cellulose, pectin, tobacco, and cellulose mixtures with various inorganic catalysts, and the behaviors of these chars upon exposure to air, O2, or H2O. The behavior of cellulose chars during exposure to air at room temperature has been determined to be strongly dependent on the identities of any inorganic additives and on the charring temperature. The presence of H2O was determined to have an important effect on the free-radical chemistry, as monitored by EPR. In the research described in this report, an in situ high-temperature approach, rather than the common temperature-quench strategy, was used in EPR experiments to further investigate (i) the pyrolysis of purecellulose and cellulose/Na2CO3 mixtures and (ii) the effects of O2 exposure and H2O exposure on the resulting chars. In undertaking this study, as has been discussed previously,19,20 we recognize that the broad EPR peaks typically observed with char samples do not usually yield the detailed chemical/structural information obtained via chemical shifts in NMR experiments on diamagnetic samples or via spectral splittings observed in NMR or EPR experiments on liquid samples. One also must realize that EPR signals, arising from modestly stable species with unpaired electrons (free radicals), may account for only (at most) a few percent of the mass in a char at any point in time3,32 and might or might not be involved (e.g., as intermediates) in the dominant chemistry that occurs in char formation or treatment. There have been numerous explanations or hypotheses implicating free radicals in charring.3,6,32,33 In any case, one cannot hope to have a complete understanding of char chemistry without detailed information on the potentially important free radicals involved. II. Experimental Section Avicel cellulose was obtained from FMC International. A cellulose/Na2CO3 mixture with 6.7 wt % Na2CO3 was prepared as follows: 1 g of cellulose powder was dispersed in 40 mL of dilute aqueous Na2CO3 solution with a concentration of 1.85 g/L, and then this mixture was air-dried while being stirred for 24 h at room temperature. X-band EPR measurements were made at various temperatures, using a Bruker model EMX-200 spectrometer with a high-temperature EPR probe (model ER 4114HT). Because this is a single-cavity configuration, accurate measurements (32) Fitzer, E.; Mueller, K.; Schaefer, W. Chem. Phys. Carbon 1971, 7, 237-383. (33) De Ruiter, E.; Tschamler, H. Brennst.-Chem. 1959, 40, 4143; 1961, 42, 311-312.

Zheng et al. of the g values and the absolute intensities are not straightforward; therefore, g values are not presented here, and only relative intensities are given. A small glass tube was used to conduct a gas (He, He + O2 or He + O2 + H2O) to a specific position ∼25 mm from the bottom of the high-temperature EPR sample tube (quartz, 3 mm in diameter). One function of the flowing gas is to provide an opportunity for the gas constituent(s) to react with the char; the other function is to bring most of the tar and volatile byproducts produced in charring out of the cavity (detection) region, while leaving the char in the sample tube. Because the spectrometer-provided thermocouple used to “set” the sample temperature is located outside the EPR sample, a temperature calibration of the sample in the cavity was performed using a second thermocouple (0.010 in. in diameter, Chomega-Alomega type K, from Omega Engineering Co.) that was inserted into a typical char (350 °C cellulose char) contained in a high-temperature EPR sample tube. Although the characteristics of the 350 °C cellulose char must vary with heating, and although the temperature measured by the second thermocouple varied with its positioning in the EPR tube (see Supporting Information), it was found that a consistent and reproducible calibration was achieved if small samples (e10 mg) were used. To keep the temperature over the entire sample volume as uniform as possible, we used samples that were ∼6 mg in size; in this manner, temperature variation within the sample was kept to (5% of the temperature that was set. A plot of the observed temperature (second thermocouple) versus the set temperature (built-in spectrometer thermocouple) constitutes a useful temperature calibration curve, and one could achieve any desired temperature using the appropriate temperature setting and the calibration curve, without the need for having a thermocouple in the sample during EPR measurements. Details of the temperature calibration are presented in the Supporting Information. (See Figure SI-1.) Separate experiments showed that the presence of a fine thermocouple in the sample during EPR measurements seemed to affect the measured EPR signals, i.e., the chemistry and physics of the sample, under the conditions applied in this work. A low microwave power of 0.2 mW was used to avoid power saturation effects. Before heating, an ∼6 mg sample of pure cellulose or cellulose/Na2CO3, held in the 3-mm (outer diameter, OD) quartz EPR tube, was purged for more than 30 min under a flowing helium atmosphere (50 mL/min). Isothermal charring of the sample was performed at various high temperatures (300-550 °C) under a flowing helium atmosphere (50 mL/min), and the EPR signal was obtained as a function of time during the isothermal pyrolysis period (typically ∼2 h). Following this isothermal pyrolysis period and a sample cooling period under helium flow, the flowing helium was replaced by a flowing He/O2 mixture (50 mL/min; 94% He + 6% O2 mixture). This was followed by a flow period with a H2Osaturated version of the 94%/6% He/O2 gas mixture, then a flow period of the He/O2 gas mixture, and finally a flow period of just helium. One of the key experimental parameters in an EPR instrument, affecting the intensities measured in EPR experiments, is the quality factor (Q) of the microwave cavity. Thus, it is important to assess the possible importance that largely physical variations (in contrast to chemical changes) in the sample might have on the Q value manifested by the cavity, so that measured EPR intensity changes can be allocated properly to chemical variations, e.g., the creation or destruction of radicals. To this end, measurements of Q were made as a function of the temperature (300-550 °C) of the sample area on (i) a sample-less cavity, (ii) the cavity containing a 550 °C char sample, and (iii) silica gel. Although some variation was encountered, it was inconsistent (even in sign) and in no case amounted to more than a 6% change over the 300-550 °C temperature range. Similarly, to assess the effect that mois-

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Figure 1. Electron paramagnetic resonance (EPR)-monitored charring, temperature-quenching, and room-temperature exposure behaviors of 300 °C chars prepared from (A) cellulose and (B) a 6.7% cellulose/Na2CO3 mixture. The exposure gas is indicated. ture changes in the sample might have on the cavity’s Q value, measurements were made before and after the introduction of water vapor (by bubbling the stream of helium through H2O) into a sample of cotton or a molecular sieve. Modest decreases in the value of Q were observed when H2O was introduced (∼12% for 10% H2O on cotton, 19% for 10% H2O on type 3A zeolite (EM Science)). Hence, there may be systematic contributions in the measured EPR intensities of this study, because of variations in Q due to temperature changes or changes in moisture content; however, such contributions do not dominate the large intensity changes shown below.

III. Results The results of the in situ high-temperature EPR measurements are organized below in the following two parts: (A) a focus primarily on isothermal pyrolysis and (B) a focus primarily on subsequent exposure to H2O and/or O2. A. Pyrolysis. The left-most time periods in Figure 1A and 1B show the variations with time of the relative EPR intensity and peak-to-peak line width (δH) of derivative spectra during the isothermal charring of pure cellulose and of cellulose/Na2CO3, respectively, at 300 °C. For the charring of both cellulose/Na2CO3 and pure-cellulose samples, the EPR intensity experiences a gradual increase during the initial pyrolysis period, followed by a slow, small increase, as would be expected from previous reports.19,20 Analogous behaviors are observed in Figures 2-6 for higher pyrolysis temperatures (350-550 °C). Although the characteristics of the high-temperature EPR apparatus did not permit reliable measurement of absolute EPR intensities (each plot is scaled so that its maximum intensity value is arbitrarily set to 1.0), one knows from prior temperature-quench studies that the EPR intensity (or unpaired electron content) reaches a maximum for a pyrolysis

temperature of ∼550 °C, which also corresponds approximately to the temperature of minimum line width.19,20 Variations in peak to peak line width δH during pyrolysis show much more diverse behaviors in Figures 1-6. These plotted δH values might mask complex changes in the line shapes associated with resonance patterns that may consist of several components, both broad and narrow. Unlike some cases that we have observed previously of distinct multicomponent EPR patterns in the spectra of temperature-quenched samples,19,20 we have seen no compelling evidence of more than one component in the EPR spectra of this in situ study (except for a hint of two components in the spectrum of one sample, the 500 °C cellulose/Na2CO3 char). At lower pyrolysis temperatures (300 and 350 °C), there is very little variation in δH for the pure-cellulose chars, but substantial decreases in δH with time, following smaller initial increases, for the cellulose/ Na2CO3 chars. The decrease of δH is much faster for cellulose/Na2CO3 charring at 350 °C than at 300 °C. At 350 °C, the dramatically reduced line width (1.7 G) is achieved within only 70 min; in contrast, for a 300 °C pyrolysis temperature, the cellulose/Na2CO3 line width decreases uniformly over a range of 20-180 min, finally attaining a value of 4.5 G in 180 min. At the higher charring temperature of 400 °C (Figure 3), an initial δH increase, followed by a decrease, is observed in both the pure-cellulose char and the cellulose/Na2CO3 char; the observed δH decrease is small for the former and much more dramatic for the latter. At an even higher charring temperature (g450 °C), the decrease of δH with charring time becomes even more dominant and dramatic, with no initial increase, except for a small, short-term increase for the 450 °C cellulose

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Figure 2. EPR-monitored charring, temperature-quenching, and room-temperature exposure behaviors of 350 °C chars prepared from (A) cellulose and (B) a 6.7% cellulose/Na2CO3 mixture. The exposure gas is indicated.


char. Figures 1-6 suggest that the presence of Na2CO3 in a cellulose/Na2CO3 mixture leads to the occurrence of line narrowing at much lower temperature than that for pure cellulose. During the period in which the char sample is brought from the charring temperature to room temperature under helium flow, each sample experiences a very substantial increase in EPR intensity. Only a very small fraction of this change might be due to variations in Q.

As discussed below, much or all of this increase can be attributed to temperature effects on the Boltzmann distribution of electron spin states in the presence of the static magnetic field of an EPR spectrometer. Variation in the peak-to-peak line width δH during the temperature reduction periods again shows much more diverse behavior than does the signal intensity in Figures 1-6. For pure-cellulose chars, the line width increases somewhat as the temperature decreases for

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all charring temperatures, except for the 300 °C char, in which δH is unchanged during sample cooling. For cellulose/Na2CO3 chars, the line width increases greatly for low-temperature (e400 °C) chars and increases slightly for the 450 °C char, but decreases for hightemperature (500 and 550 °C) chars. B. Exposure to H2O and/or O2. The EPR-observed responses shown in Figures 1-6 for introducing O2 into the flow gas at room temperature exhibit substantial

variety. For the chars prepared from pure cellulose at low temperature (300 and 350 °C), the introduction of O2 has almost no effect on the EPR intensity and line width; for the cellulose char prepared at higher temperature (g400 °C), one observes decreases in intensity and increases in line width, and these effects become stronger as the charring temperature increases. For the corresponding cellulose/Na2CO3 chars, the short-term (e120 min) introduction of O2 produces no substantial

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effects on line width and only small increases in the EPR intensity, except for the 550 °C char, for which the intensity increase is substantial. Perhaps the most interesting effects on the EPR characteristics of the chars that are represented in Figures 1-6 are the effects of introducing H2O vapor into the He + O2 stream (by bubbling the gas through liquid H2O before it flows over the sample). For most of the chars, the introduction of H2O vapor leads to a decrease in EPR intensity (free-radical content), in some cases (especially for cellulose/Na2CO3 systems) preceded by an increase in EPR intensity. In the high-temperature (g450 °C) chars of cellulose, the introduction of H2O seems to cause an abrupt discontinuity in the EPR intensity. Effects of H2O introduction into the gas stream are most dramatic for lower-temperature cellulose/Na2CO3 chars and for higher-temperature chars of pure cellulose. For pure-cellulose chars prepared at g400 °C (and possibly at 350 °C), the introduction of H2O vapor into the flowing He + O2 stream leads to an abrupt discontinuity (sharp decrease, then sharp increase) at the beginning of the H2O-introduction period and then a decrease in EPR intensity (free-radical content), and these phenomena become more marked as the charring temperature increases. The decreasing EPR intensity during the final 80% of this period is general for all charring temperatures and becomes slower as the charring temperature increases. After the initial abrupt increases in δH for cellulose chars prepared at g450 °C, the line width decreases when H2O is introduced; the line width decreases more for higher charring temperatures. For cellulose/Na2CO3 chars prepared at low temperatures (e400 °C), the introduction of H2O leads to EPR intensity increases then decreases; the same behavior is observed for the line width. For cellulose/Na2CO3

chars prepared at higher temperatures (g450 °C), the EPR intensity decreases and the line width increases or remains unchanged with H2O introduction; the decrease of EPR intensity becomes faster as the charring temperature increases. For all chars prepared from cellulose or cellulose/ Na2CO3, the Q value of the EPR cavity decreases with time during the H2O-introduction period (roughly changing from Q ) 3400 to Q ) 3200). For all types of chars in our experiments in which H2O is present, we might interpret a portion of the long-term decrease of EPR intensity in terms of the reduction of the Q value of the EPR cavity apparently caused by the presence of water; however, this does not seem to be responsible for dramatic effects. The removal of H2O vapor from the gas stream, leaving 6% O2 in helium, in some cases seems to partially reverse the H2O-caused depletion of EPR intensity (300 °C cellulose, 350 °C cellulose, 400 °C cellulose, 400 °C cellulose/Na2CO3, and 500 °C cellulose/ Na2CO3), whereas for other samples, more-complex behavior or no change is obtained. Corresponding effects on δH of H2O removal from the gas stream range from almost no change or very small changes for lowertemperature (

In Situ High-Temperature Electron Paramagnetic Resonance (EPR

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