Energy & Fuels 1988,2, 267-272
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Simultaneous Thermogravimetric-Mass Spectrometric Studies of the Thermal Decomposition of Biopolymers. 1. Avicel Cellulose in the Presence and Absence of Catalysts Gabor Varhegyit and Michael J. Antal, Jr.* Department of Mechanical Engineering and the Hawaii Natural Energy Institute, University of Hawaii, Honolulu, Hawaii 96822
Tamas Szekely, Ferenc Till, and Emma Jakab Research Laboratory for Inorganic Chemistry, Hungarian Academy of Sciences, Budapest 1502, Hungary Received September 28, 1987. Revised Manuscript Received December 23, 1987 The course of pyrolysis of a pure microcrystalline cellulose powder (Avicel) was followed by simultaneous mass spectrometry-thermogravimetry a t a low heating rate (10 OC/min) in argon. A thermal pretreatment of 2 h at 260 "C had no significant influence on the subsequent decomposition, contradicting the findings of some earlier workers. The addition of inorganic salts (MgC12, NaC1, FeS04, and ZnC12)to the cellulose led to dramatic changes in the product distribution and the overall course of the decomposition process. Some of these changes were interpreted to be due to physical influences of the inorganic salts.
Introduction A wealth of knowledge exists concerning the thermal decomposition of cellulose and various lignocellulosic materials. Since detailed reviews are available,l+ only a brief overview of the field will be presented here. In recent years considerable attention has been given to the reaction network governing the solid-phase decomposition of cellulose and other biopolymers. The seminal work of Broido e t aL7-16and Patai et al.lB-18first pointed to the roles of competing condensation, depolymerization (unzipping), and fragmentation pathways in the solid-phase pyrolysis of cellulose. Shafiiadeh et and later And et al.%% confirmed much of this work and added new insighh into the phenomena. Basch and Lewin gave special attention to the influences of cellulose crystallinityz7 and orientati~n*~Oon the course of pyrolysis. Their finding that crystalline cellulose preferentially unzips to form monomer levoglucosan (1,6-andro-~-~-glucopyranose) is of critical importance to the results of this research. Questions concerning the underlying mechanism of each step in the reaction network remain unanswered. Strong arguments asserting the roles of both heterolytic and homolytic bond scissions exist in the literature, and no consensus has been reached on this matter.4.5 The effects of the various inorganic additives have also been extensively studied.'+ In a recent series of studies Shafiiadeh et al.31and Sekiguchi and Shafi~zadeh~~ examined the combustion properties of cellulose and cellulosic chars with and without inorganic additives. Using thermogravimetry, they followed the thermal decomposition of cellulose and inveatigated the resulting chars by infrared spectroscopy, l8C NMR spectroscopy, and oxidative thermal analysis. Details of the high-temperature, fast pyrolysis of the cellulose and lignocellulosic materials have been reported recently in a thorough paper by Evans and Milne.= They employed temperature above 500 "C and heating rates exceeding 1800 OC/min in a sophisticated molecular-beam *Towhom all correspondence should be addressed. 'Visiting scientist from the Hungarian Academy of Sciences.
0887-0624/88/2502-0267$01.50/0
mass spectrometer apparatus. In this paper we investigate the thermal decomposition (1)Milne, T. In Biomass Gasification: Principles and Technology; Reed, T. B., Ed.; Noyes Data: Park Ridge, NJ, 1981;pp 91-118. (2)Shafizadeh, F. J. Anal. Appl. Pyrolysis 1982,3,283-305. (3)Shafiiadeh, F. In Cellulose Chemistry and i t s Applications; Nevell, T. P., Zeronian, S. H., Eds.; Wiley: New York, 1985; pp 266-290. (4)Antal, M. J., Jr. In Aduances in Solar Energy; Boer, K. W., Duffie, J. A., Eds.; American Solar Energy Society: Newark, DE,1982;Vol. 1, pp 61-111. (5)Antal, M. J., Jr. In Advances in Solar Energy; Boer, K. W., Duffier, J. A., Eds.; Plenum: New York, 1985;Vol. 2, pp 175-255. (6)Nguyen, T.;Zavarian, E.; Barral, E. M. J. Macromol. Sci., Reu. Macromol. Chem. 1981,(220,1-65. (7)Kilzer, F. J.; Broido, A. Pyrodynamics 1966,2,151-163. (8)Broido, A.; and Weinstein, M. In Therm. Anal., Proc. Znt. Conf., 3rd, 1971 Weidemann, H. G., Ed.; Birkhiiuser Verlag: Basel, Switzerland, 1971;pp 285-296. (9)Broido, A. In Thermal Uses and Properties of Carbohydrates and Lignins; Shafizadeh, F., Sarkanen, K. V., Tillmann, D.A., Eds.; Academic: New York, 1976,19-35. (10)Broido, A.; Kilzer, F. J. Fire Res. Abstr. Reu. 1963,5, 157-161. (11)Broido, A.; Javier-Son, A. C.; Ouano, A. C.; Barrall, E. M. J.Appl. Polym. Sci. 1973,17,3627-3635. (12)Broido, A.; Yow, H. J. Appl. Polym. Sci. 1977,21, 1677-1685. (13)Weinstein, M.; Broido, A. Combust. Sci. Technol. 1970, 1, 287-292. (14)Broido, A.; Weinstein, M. Combust. Sci. Technol. 1970, 1, 279-285. (15)Broido, A.; Nelson, M. A. Combust. Flame 1976,24, 263-268. (16)Halpern, Y.; Patai, S. Zsr. J. Chem. 1969,7,673-683. (17)Halpern, Y.; Patai, S. Zsr. J. Chem. 1969,7,691-696. (18)Patai, S.; Halpern, Y. Zsr. J. Chem. 1970,8,655-662. (19)Shafzadeh, F.; Bradbury, A. G. W. J. Appl. Polym. Sci. 1979,23, 1431-1442. (20)Shafizadeh, F. Adu. Carbohydr. Chem. 1968,23,419-474. (21)Shafizadeh, F.; Fu, Y. L. Carbohydr. Res. 1973,29, 113-122. (22)Shafiizadeh, F.;Furneaux, R. H.; Cochran, T. G.; Scholl, J. P.; Sakai, Y. J. Appl. Polym. Sci. 1979,23,3525-3539. (23)Shafizadeh, F.; Cochran, T.; Sakai, Y. AZChE Symp. Ser. 184, 1979,75 (NO. 184), 24-34. (24)Bradbury, A. G. W.; Sakai, Y.; Sahfizadeh, F. J.Appl. Polym. Sci. 1979,23,3271-3280. (25)Mok, W.; Antal, M. J., Jr. Thermochim. Acta 1983,68,165-186. (26)Hopkins, M.;Antal, M. J., Jr.; Kay, J. J. Appl. Polym. Sci. 1984, 29,2163-2175. (27)Basch, A,; Lewin, M. J.Polym. Sci. 1973,11,3071-3093. (28)Basch, A.; Lewin, M. J. Polym. Sci. 1973,21, 3095-3101. (29)Basch, A.; Lewin, M. J . Polym. Sci. 1974,12,2053-2063. (30)Basch, A,; Lewin, M. J.Polym. Sci., Polym. Lett. Ed. 1976,13, 493-499. (31)Shafiizadeh,F.;Bradbury, A. G. W.; DeGroot, W. F.; Aanerud, T. W. 2nd. Eng. Chem. Prod. Res. Deu. 1982,21,97-101.
0 1988 American Chemical Society
268 Energy & Fuels, Vol. 2, No. 3, 1988
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Figure 1. DTG (-) and MS intensity (0) curves (in arbitrary units) of calcium oxalate monohydrate used to check and calibrate the instrument. The first, second, and third peaks belong to the evolution of water, carbon monoxide, and carbon dioxide, respectively.
of cellulose at moderate temperatures using a well-defmed, low heating rate (10 OC/min). The pyrolytic weight loss was measured by a thermobalance while the mass spectra of the low molecular weight products were simultaneously recorded as a function of temperature. The tar (syrup) fraction was not detected by the mass spectrometric apparatus. A comparison of the weight loss rate (DTG) with the mass spectrometric data can offer new insights into the relative importance of the different chemical processes occuring during the various stages of the decomposition. Results reported in this paper complement those of Milne and whose data were obtained at higher heating rates. In this paper we emphasize studies of the influence of temperature-time history and catalysts on the course of cellulose pyrolysis. The goal of these studies was to gain insight into our ability to influence the relative roles of the various pathways in the reaction network during pyrolysis. Only by influencing the reaction network can improvements be made in reaction specificity towards desirable products (such as charcoal or syrup). Following a description of the apparatus and experimental procedures, we report our findings concerning the influence of thermal pretreatments, sample environment, and various inorganic catalysts on the course of cellulose pyrolysis.
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Figure 3. DTG curves of Avicel cellulose at 10 OC/min in a standard nonisothermal experiment (dark solid lie), after a heat treatment of 2 h at 260 OC (a), and in a closed crucible with a pinhole on the top (light solid line).
Experimental Section Samples and Sample Preparation. Pure microcrystalline cellulose (Avicel PH-105 obtained from FMC corporation) was used as a substrate. Its average particle size, degree of polymerization, crystallinity, and ash content were specified to be 30 pm, 240 units, 92%, and 40 ppm (respectively). In the catalytic experiments, the molar ratio of the catalyst to the monomer unit of the cellulose chains was about 0.01. It is known13 that the catalyst will not be uniformly distributed in the cellulose matrix and that its concentration in the vicinity of active end groups on the crystallite surface is likely to be critical. Unfortunately, measurements of the microscopicdistribution of the catalysts were beyond the scope of our research. The catalysts (ZnC12,FeSO,, MgC12and NaC1,) were analytically pure reagents dissolved in water and absorbed by the cellulose samples. The resultant gel-like pastes were thoroughly stirred, dried at room temperature in a desiccator, and used for experiments the next day. Measurement and Data Processing. The experimental apparatus consisted of a Perkin Elmer TGS-2 thermobalance A minconnected to a Balzers QMG-511 mass spectr~meter.~~ (32) Sekiguchi, Y.;Shafzadeh, F. J. Appl. Polym. Sci. 1984, 29, 1267-1286. (33) Evans, R. J.; Milne, T. A. Energy Fuels 1987,1, 123-137. (34) Szekely, T.;Till, F.;Varhegyi, G. In Therm. A d . Proc. Znt. Conf., 6th, 1980 Hemmmger, W., Ed.;Birkhauser Verlag: Basel, Switzerland 1980; Vol. 2, pp 365-370.
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Figure 4. Comparison of the DTG curves of untreated (dark solid line), MgC1,-treated ( O ) , and NaC1-treated (light solid line) samples.
icomputer (PDP-11/34) was used to control the maas spectrometer and the acquisition of the measured data.% The figures containing numerically generated DTG curves and mass spectrometric intensities after subtraction of the background level were plotted by a digital plotter. The low-intensitycurves were magnified for better view. (Usually the highest points of the curves were scaled to an equal height to observe differencesbetween the shape and temperature domain of the curves. Because of this scaling pro(36) Varhegyi, G.;Till, F.; Szekely, T. Thermochim. Acta 1986,102, 115-124.
Energy & Fuels, Vol. 2, No. 3, 1988 269
Thermal Decomposition of Biopolymers
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Figure 5. Comparison of the DTG (dark solid line), water (a), carbon monoxide (A),and carbon dioxide (light solid line) evolution curves of NaC1-treated cellulose.
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Figure 6. Comparison of the DTG curves of untreated (dark solid line), FeS04-treated (o), and ZnClz-treated (light solid line) cellulose.
cedure and the simultaneous display of DTG curves and MS intensities on the same graph, the ordinate values of Figures 1-7 are arbitrary and not displayed.) The performance of the system was regularly checked by measuring the three decomposition step of calcium oxalate monohydrate. The good fit between the DTG curves and the mass spectrometric intensities (see Figure 1)established the reliability of both the weight loss and mass spectrometric measurements. The TG-MS system could not detect some of the tar (syrup) fraction evolved during the thermal decomposition. The gas sampling method is shown in Figure 2. Volatile8 formed during the decomposition were led to the ion source through a quartz capaillary tube. The section of the capillary outside the furnace was heated to 180 "C during the experiment. The section of the capillary inside the furnace could not be heated externally without jeopardizing the temperature program of the experiment. This section received some heat from the furnace via conduction through the ambient gas. The w e of a capillary to sample evolving vapors can be flawed by poor positioning of the inlet or other unintended discrimination. In our case, calibration of the instrument with gas standards, followed by further calibration experiments involving calcium oxalate, evidenced the correct molar ratios of HzO, CO, and COz from the oxalate. In addition, our experiments with Avicel were reproducible to within a few percent deviation. Hence we conclude that the capillary tube provided a representative sample of the uncondensed vapors to the MS. Thermoanalytical Conditions. The standard heating rate employed in these studies was 10 OC/min. The initial sample mass was kept as low as the sensitivity of the mass spectrometric instrument permitted between 0.5 and 2.5 mg. A low sample mass is advantageous to reduce the contamination of the instrument (caused by the tar production and the various polycondensation reactions taking place on the walls and surfaces on the instrument during the reaction) and to eliminate the usual heat- and mass-transfer intrusions encountered in pyrolysis and thermal analysis studies. Although it can be argued that the low sample mass also increases the relative surface area of the particles
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m 7. Comparison of DTG (dark solid line), water (o),carbon
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in contact with the potentially catalyticsurface of the sample pan, numerous experiments in our laboratories over the years have shown the composition of a conventional, open pan to have no effect on the course ,of pyrolysis of Avicel and other common biopolymer materials. An argon atmosphere was used with a gas flow rate of 140 mL/min. Mass Spectra and Their Identification. The mass spectra were measured from m/z 15to 150 in intervals of 30 s (5 OC) with an electron-impact ionization energy of 70 eV. The high-intensity peaks of the carrier gas (m/z 20,36, and 40)were omitted in the scan in order to obtain a high sensitivity. The leas abundant argon isotope, @Ar,was measured to check the sensitivity of the mass spectrometer. After the experiments, the measured peak intensities were analyzed as functions of the reaction time. If the difference between the background level and the highest intensity value was close to the noise level, the given mass spectrometric ion was rejected. (The mathematical criterion for rejection was a signal to noise ratio of less than 20.) The remaining mass spectrometric ions were assigned to compounds or group of compounds reported in the literature to evolve from cellulose at similar temperatures under atmospheric Because the composition of the evolved products strongly depends upon the temperature of the measurements, the wealth of data reported in the literature on the high-temperature pyrolysis of biomass materials could not be used in the present studies. The assignments and the highest values of the corresponding intensities are summarized in Table I. Table I also displays values of the DTG maxima and the char yield at 450 OC. All reported values of intensity and DTG were divided by the initial sample mass. The lack of an intensity value in Table I does not mean that the corresponding compoundswere not produced at all; it means only that either they condensed prior to reaching the ion source or the intensity function was too noisy as a consequence of a high background or a high neighboring peak. In parentheses, Table I shows the ratio of the highest values of the intensity curves and the DTG peak maxima. These data are needed when experiments with sharper and broader DTG curves are compared they help to ascertain whether the relative importance of a given fragment has increased or decreased during the weight loss process. One could use other quantities, such as integrated values, for this purpose. In a TG-MS study, however, the intensity values are most reliable at their maxima, especially when the background level is high or changing or when the intensities are small. The quantities of HzO,CO, and COzwere determined by using sensitivity factors calculated from calcium oxalate calibration experiments.
Results and Discussion Standard Experiment with Pure Cellulose. Avicel cellulose evidences a sharp,well-defined DTG peak (Figure 31, similar to those reported by previous investigators. The (36) Tsuchiya, Y.;Sumi,K.J. Appl. Polym. Sci. 1970,14,2003-2013. (37) Lipska, A. E.; Wodley, F. A. J. Appl. Polym. Sci. 1969, 13, 851-865. (38) Wodley, F. A. J. Appl. Polym. Sci. 1971, 15, 835-851.
Varhegyi et al.
270 Energy & Fuels, Vol. 2, No. 3, 1988 Table I. Assignment a n d Intensity Maxima of the Mass S m c t r o m e t r i c Ionsa max intens (ratio to the DTG peak height) pure cellulose cellulose with catalyst assient std Dreheated with cover MeCL NaCl FeSO, .
m Jz 15 18 26 27 28 29 30 31 43 44 68 84 96
I
CH3' water C,H,+ CiH? carbon monoxide aldehydes formaldehyde methyl alcohol, glycolaldehyde acetaldehyde, pyruvaldehyde carbon dioxide furan C4H402+
2-furaldehyde
DTG max, %/s char at 450 OC,' wt %
0.9 (2.1) 0.5 (1.3) 84.0 (196.0) 82.0 (205.0) 0.6 (1.5) 0.5 (1.3) 0.7 (1.7) 0.7 (1.7) 16.4 (41.1) 16.1 (37.7) 4.7 (11.8) 5.7 (13.4) 2.5 (6.1) 3.2 (7.4) 0.2 (0.5) 0.3 (0.8) 0.5 (1.2) 0.6 (1.5) 7.9 (19.8) 5.8 (13.5) 0.2 (0.5) 0.3 (0.6) 0.1 (0.3) 0.2 (0.4) 0.2 (0.4) 0.2 (0.5)
1.0 (2.1) 212.0 (472.0) 0.5 (1.2)
0.43 5
0.45 19
0.40 6
29.7 (66.1) 3.9 (8.6) 0.3 (0.7) 29.5 (65.7) 0.4 (0.9) 0.2 (0.5)
I
ZnCL
0.5 (1.3) 70.0 (175.0) 0.2 (0.5) 0.2 (0.4) 12.6 (30.2) 3.0 (7.2) 1.7 (4.2) 0.2 (0.5) 0.4 (0.9) 7.1 (17.0) 0.1 (0.2) 0.05 (0.1) 0.06 (0.2)
2.5 (10.0) 136.0 (523.0) 0.2 (0.7) 0.3 (1.2) 51.4 (198.0) 8.0 (30.9) 2.9 (11.3) 1.0 (3.7) 1.8 (7.0) 21.6 (83.0) 0.02 (0.1) 0.04 (0.2) 0.07 (0.1)
0.2 (0.8) 0.3 (2.4) 97.0 (353.0) 83.0 (397b)
0.42 8
0.26 14
0.28 17
11.2 (40.7) 2.0 (7.1) 1.2 (4.5) 4.5 (16.3)
8.4 (67.0) 2.0 (16.3) 1.2 (9.3) 0.1 (0.7) 0.2 (1.1) 7.4 (60.0) 0.06 (0.5) ... (0.1) 0.04 (0.4) 0.13 28
"+"
The compound names in column 2 refer to the probable source of the ions. The charge sign indicates that the corresponding formula represents a mass spectrometric fragment ion. The intensities are normalized with the initial sample mass and are given in nA/mg of starting material. Listed in parentheses are the ratios of the highest values of the MS intensity curves to the DTG peak heighta (wt %/s). The last two rows contain the DTG peak maxima and the char yields. the ZnCl,-catalyzed experiment, this value refers to the height of the second DTG peak. For the first DTG peak the ratio of the water ion intensity and the DTG peak height was about 900. 'Includes residual weight of catalyst.
quantities of evolved H20, CO, and COz were about 17,4, and 1.5 w t %, respectively. The various aldehydes together were estimated to represent a few weight percent, while the amount of furan and 2-furaldehyde was below 0.1 % These yields are in accord with the 320 and 370 OC data of Tsuchiya and Sumi.3e Slight differences between their data and ours may be due to different experimental conditions: Tsuchiya and Sumi pyrolyzed 1.6-g samples in Pyrex tubes. Although the DTG and MS curves are surprisingly symmetric and almost overlap, some asymmetries and displacements can be discerned. Presumably these reflect the role of different reaction pathways a t different temperatures. For example, the water, furan, and 2-furaldehyde peaks (not displayed) preceded the DTG curve by a few degrees Celsius, indicating that the dehydration reactions make a greater contribution to the weight loss during the early stages of the decomposition. However, this phenomena was far less evident here than in the catalyst experiments. Finally, a few words about the nondetected high molecular weight compounds. In our apparatus the "tar fraction", or "syrup", consisting mainly of monomer, dimer, and trimer type volatile products, could not reach the ion source for the reasons outlined in the Experimental Section. Shafizadeh et aLZ2noted that the formation of tar (syrup) strongly depends on the experimental conditions. For a comparison with the present work, their data a t 350 "C are most relevant. At atmospheric pressure and in vacuum they observed 54% and 70% syrup, respectively. The levoglucosan (1,6-anhydro-~-~-glucopyranose) contents of these syrups were about 30% and 50%, respectively. In our experiment, the amount of the nondetected high molecular weight compounds can be estimated (by difference) to be about 70% by weight, close to that obtained by the vacuum experiments of Shafizadeh et al.22 This refleds the fact that our experimental conditions were chosen to facilitate mass transfer: the sample size was 250 times smaller and a high gas-flow rate was applied. However, as will be outlined later, the amount of the high molecular weight products was considerably less in the catalytic and closed-vessel experiments. Effect of a Thermal Pretreatment. Following the work of Broido and Nelson,15 which employed ordinary
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high DP pure cellulose, we attempted to influence the course of the decomposition of Avicel by a thermal pretreatment of 2 h a t 260 OC, followed by the standard 10 OC/min nonisothermal heating. During the pretreatment, a permanent weight loss was observed a t a slow, nearly constant rate of 0.0015% s-l. The overall weight loss was 11%. Due to the extremely low reaction rates, the mass spectrometric intensities were below the noise level at this temperature. (Only the intensities of HzO, CO, and C02 reached the threshold of detectability.) The DTG and intensity data of the following nonisothermal temperature ramp were normalized to the sample mass remaining after the isothermal pretreatment to facilitate a comparison of these results with those obtained from the standard experiment. As the data of Table I and the DTG curves of Figure 3 show, there were no significant differences. Although the entire decomposition occurred a t a slightly lower temperature, the other features remained practically the same. These findings are in accord with the results of Basch and L e ~ i n , ~who ' established that crystalline cellulose primarily unzips to monomers and oligomers during pyrolysis. Evidently thermal pretreatments are unable to influence the course of pyrolysis of a pure crystalline cellulose: the crystallinity seems to direct pyrolysis along a single pathway. Effect of a Closed Reaction Vessel. A small sample (0.5 mg) was hermetically sealed in an aluminum DSC crucible and a pinhole of about 0.2 mm diameter was punctured in the top. This arrangement can affect both the decomposing sample and the evolving vapors: the decomposition occurs in the presence of the vapors, and these vapors spend a longer time a t a higher partial pressure in the hot zone above the sample. As the data displayed in Table I and the curves of Figure 3 show, the DTG curve was only slightly influenced the 5% increase in the peak maximum and the 10 OC shift in temperature are not considerable if we compare them to the dramatic effects of closed sample holders on simple inorganic thermal decomposition p r o c e s ~ e s . ~However, ~ the char yield dramatically increased to 19%. In addition the mass spectrometric intensities belonging to HzO, CO, and C02 ~~
(39) G m , T D .Thermoanalytical Methods of Inuestigation; Academic: New York, 1965.
Thermal Decomposition of Biopolymers increased by factors of 2.5, 2, and 5 (respectively); while the intensities belonging to the various organic compounds evidenced only minor changes. The overall water production was about 30% by weight, which compares well with the 25% reported by Tsuchiya and Sumi= a t 370 "C. The increase of the evolved water can be explained by secondary reactions of the syrups in the vapor phase. The closed vessel prevents the quick escape of the products from the hot zone; thus, these vapors may undergo further decomposition and form additional quantities of H20,CO, and COP The water vapors and the acidic decomposition p r o d u ~ t s ~may ' * ~ also catalyze the solid-phase decomposition of the cellulose, which could account for the slightly lower temperature range of the primary decomposition. Effect of MgC12.The thermal decomposition of cellulose is affected by practically every sort of inorganic impurity.'* These effects have been postulated to derive from the impurity's ionic nature, its Lewis acidity or basicity, or its ability to form various intermediate complexes. To study only one of these influences a t a time, we first compared the effects of two simple neutral salts: MgC1, and NaC1. Presumably these two salts effect the decomposition only through their ionic properties. We also studied the influence of two specific inorganic catalysts: FeS04 and ZnC1,. The use of NaC1, FeS04, and ZnC12 as catalysts for cellulose pyrolysis has been studied by earlier investigator^.^^^^^ We are unaware of any literature concerning the thermal decomposition of MgC12-treatedcellulose. A similar compound, CaCl,, has been reported38 to decrease the syrup production and increase the formation of the low molecular weight products a t 320 "C. The MgClz catalyst did not change the overall reaction (observe the good fit between the DTG curves of the untreated and MgC12-treated cellulose in Figure 4). There were significant changes, however, in the mass spectra. The intensities of ions m l e 26,27,68,84, and 96 decreased by factors of 3-4, intensities a t m l e 29, 30, and 31 were roughly halved while peaks due to H20, CO, and C 0 2 underwent only small changes. This behavior indicates that the processes most important in the weight loss were not changed while either processes leading to minor organic components (aldehydes, ketones, furan, 2-furfuraldehyde, etc.) were suppressed by the presence of MgC12 or these organic products underwent further, secondary decompositions in the presence of the salt. Effect of NaCl. The effects of NaCl on the decomposition of cellulose are well-known.37@The salt increases char formation, alters the thermal decomposition properties, and enhances smoldering combustion. NaCl also influences the thermal decomposition of other carbohydrates. In a recent study, Richardsa showed that 1% NaCl facilitates the thermal degradation of sucrose. He explained this effect by the increase of the dielectric constant of the sucrose melt in the presence of NaCl. In the present experiments, the addition of 1 mol % NaCl resulted in broader DTG curves and an increased char yield. As Figure 4 shows, the thermal decomposition began a t a considerably lower temperature and ended slightly later than that of the untreated sample. Defining the onset of decomposition by a DTG threshold value of 0.005% s-l (safely above the random noise), the difference was about 30 OC. Note that Sekiguchi and S h a f i ~ a d e h ~ ~ also observed this difference in the onset of pyrolysis by using 1 mol % NaC1. In their case, however, the main decomposition occurred about 40 "C higher than the decomposition of the pure cellulose. In the present studies, (40)Richards, G.N. Int. Sugar J. 1986,88, 146148.
Energy & Fuels, Vol. 2, No. 3, 1988 271 the latter difference was only 5 "C. This difference again reveals how the morphological properties of the sample may modify the course of pyrolysis. (Sekiguchi and Shafizadeh used Whatman CF-11 cellulose.) In another paper, Shafizadeh e t al.32 reported a decrease of the decomposition temperature in air using 0.2 mol % NaCl, but the imposition of oxidative processes on top of pyrolysis processes make it difficult to compare Shafizadeh's findings with our own. The amounts of volatile products detected by the MS considerably increased: we estimate that about 60-70 % of the substrate mass was detected as vapors. The char yield also increased (14%); hence, by difference we estimate the nondetected syrup fraction to be between 15 and 25%. This decrase in syrup formation is almost always observed when cellulose decomposes in the presence of an inorganic salt. In this respect, our results with MgC12 can be considered to be an unusual exception. The increase in the yields of the various volatile species was uneven. As the data in Table I show, the ratio of the intensity maxima and the DTG peak values, characterizing the importance of a given ion in the weight loss processes of different experiments, increased at m l e 15,18,28,29, 31, 43, and 58 by factors varying from 2.5 to 8 and decreased a t m l e 26, 27,68, 84, and 96 by factors varying from 1.5 to 4. A plausible explanation would be to assume that all higher molecular weight products, including even furan, 2-furaldehyde, and the higher aldehydes, underwent further fragmentation in the presence of NaC1. However, other explanations are also possible, as will be outlined later. Concerning the time dependence of the curves, a considerably earlier rise was observed for the H20, CO, and C02 curves (see Figure 5). This behavior indicates that the fragmentation of heavier species to HzO, CO, and C02 is even more pronounced a t the earlier stages of the decomposition. As an explanation, we can assume that those monomer units that were in direct contact with the Na+ or the Cl- ions decomposed earlier by a different chemical mechanism than the majority of the sample. Effect of FeS04. Shafizadeh et reported that in air FeS04 considerably reduces the decomposition temperature and increases the char yield. In our experiment, the DTG peak temperature decreased from 350 to 300 "C by the addition of FeS04 (Figure 6). The char yield and the amount of water evolved both increased. The intensity belonging to CO, divided by the DTG peak height, remained the same as in the reference experiment, while the corresponding values of the other intensities decreased. The shift of a whole DTG peak is unusual in the catalytic decomposition of solids, since it is impossible for a catalyst concentration of 1mol 5% to be in contact with the majority of the decomposing molecules. In the case of a polymer capable of "unzipping" (depolymerization), it is plausible to assume that the FeS04 initiated the unzipping of whole polymer chains and, in this way, influenced the decomposition of the macromolecular segments that were not in direct contact with the catalyst. On the other hand, the increase in the char yield indicates that the FeS04 catalyzed a branch of the reaction network that forms more char. The tailing of the DTG curve a t the higher temperatures is probably due to the slow charring of the residual. Effect of ZnC12.ZnC12 is a well-known dehydration catalyst that increases water production and char yield during the thermal decomposition of c e l l u l ~ s e . ~ In~our .~ experiment, the addition of ZnClz resulted in the appearance of two DTG peaks (Figure 6). The first peak,
272 Energy & Fuels, Vol. 2, No. 3, 1988 a t 300 "C, coincided with the FeS04-catalyzed DTG peak, while the second peak appeared near to the temperature of the DTG peak of the untreated sample. The two peaks were followed by a tailing section believed to be slow charring process of the solid residue. The char yield, (28% a t 450 "C) was the highest observed in this study. As Figure 7 shows, the water curve considerably preceded the DTG curve. All of the other MS intensities were lagging. Hence we conclude that the rising part of the first DTG peak was dominated by a dehydration reaction of the cellulose chains. Probably those parts of the chains were dehydrated that were in the vicinity of the absorbed catalyst ions. The sections of the chains having no direct contact with the catalyst underwent a decomposition which resembled that of the untreated sample. The ratios of the intensity maxima to the DTG peak height show some increases (see the values for m / e 18,28, and 44 in Table I), but these changes could be explained by contributions from the slow charring of the residual of the first DTG peak. Another explanation would be to assume secondary reactions of the volatile products on the surfaces of the char formed during the first DTG peak.
Conclusion Earlier workers have reported molecular weight changes, cross-linking and other transformations in the low-temperature region of cellulose d e c o m p ~ s i t i o n .Following ~~~ a 2-h thermal pretreatment a t 260 OC, we observed no significant change in the TGMS curves during subsequent heating a t 10 OC/min. Hence the reported low-temperature phenomena either do not occur in a pure microcrystalline cellulose, or they have no influence on thermal decomposition at higher temperatures. Closing the sample holder, we observed considerably higher yields of HzO, CO, and C02 without significant changes in the overall DTG curve. This finding indicates that the formation of H20, CO, and C02is mainly due to secondary reactions involving the primary pyrolysis products. The addition of a small amount (1 mol %) of simple inorganic salts resulted in a wide variety of changes. MgC12 did not influence the reactions responsible for the overall weight loss but decreased the production of several minor products by factors of 2-4. NaCl causes some changes in the DTG curves (earlier onset and less sharpness) while increasing the total amount of low molecular weight products by a factor of about 3 and considerably decreasing
Varhegyi et al. the amount of some minor components. FeS04 resulted in a shift of 50 OC in the DTG peak maxima with only a minor influence on the product distribution. ZnClz led to a double DTG peak, the first peak being associated with a dehydration reaction, while the second peak resembled that of the untreated sample. Keeping in mind that only a few percent of the monomer units of the polymer chains may have been in direct contact with the catalyst and presuming the catalyst molecules were immobile during pyrolysis, one can only explain many of the resulting phenomena (e.g. the shift of a whole DTG peak by 50 OC or the increase and decrease of certain components by actors of 2-8) by indirect, long-range effects of the catalysts. In the case of FeS04,this long-range effect may be the initiation of unzipping reactions a t lower temperatures. The first DTG peak of the ZnCl,-treated sample may also be due to some type of chain reaction, probably involving dehydration, catalyzed by the acidity of ZnC1,. (Here the term "acidity" may refer to a Lewis acidity as well as to HCl evolved by the reaction of ZnCl, with water formed by pyrolysis.) However, it is equally possible that these effects result from a change in the physical structure of the cellulose (crystallinity, orientation, diffusion coefficients, etc.), incurred through the addition of the inorganic salts. This possibility is in accord with the findings of earlier workersn-30~"concerning the influence of physical properties on the course of pyrolysis. Recognizing that the addition of these salts should have no influence on free radical chemistry within the cellulose and recalling that each of these salts manifests Lewis acidity and/or basicity, we conclude that the dramatic influence of these salts on the course of pyrolysis supports the viewpoint that heterolytic bond scissions must play some role in the pyrolysis chemistry of cellulose.
Acknowledgment. This work was supported by the Hungarian Academy of Sciences and the National Science Foundation under Grant INT85-04282. We thank Bonnie Thompson and Gerson Sher (NSF) for their continuing interest in this work and the reviewers for their many helpful comments. Registry No. ZnCl,, 7646-85-7; FeS04, 7720-78-7; MgCl,, 7786-30-3; NaCl, 7647-14-5; cellulose, 9004-34-6. (41)Antal, M. J . , Jr. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 366-375.
