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Atmospheric pyrolysis of carbohydrates with thermogravimetric and mass spectrometric analyses
Journal of Analytical and Applied Pyrolysis, 8 (1985) 41-48 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
41
ATMOSPHERIC PYROLYSIS OF CARBOHYDRATES WITH THERMOGRAVIMETRIC AND MASS SPECTROMETRIC ANALYSES
ATTILA
E. PAVLATH
* and KAY S. GREGORSKI
Western Regional Research Center, U.S. Department CA 94710 (U.S.A.)
of Agriculture,
800 Buchanan, Berkeley,
SUMMARY The pyrolysis of glucose, maltose, cellobiose, amylose and cellulose was studied by thermogravimetry between 250-4OO’C at atmospheric pressure in helium. The thermogravimetric analyzer was coupled with a mass spectrometer and the evolving gases were continuously monitored. Between 2.6 and 2.8 moles of water were obtained for every hexose unit. For amylose and cellulose the water formation was in one step; for the other carbohydrates, varying amounts of water formed in two different steps. The scans were evaluated by computer and the concentrations of the possible components were plotted versus temperature. The correlation of these curves with the weight loss curve throws new light on carbohydrate pyrolysis reactions.
INTRODUCTION
Carbohydrate pyrolysis, especially that of cellulose, has been studied for many years. The aim for the latter was mostly to determine the factors involved in flame retardation. Only during the past 10-15 years was attention paid to the possibility of using carbohydrates as sources for chemical intermediates. The energy crisis initiated various studies on wood, but limited studies are available on the basic reactions involved in the pyrolysis of various pure carbohydrates. Even in those cases the studies were carried out in vacuum to prevent secondary reactions. Madorsky et al. [l] found that 70% of cellulose was volatilized in vacuum pyrolysis yielding 27% water, 6% CO,, 2% CO and 65% volatile tarry material. When the reaction was carried out in nitrogen atmosphere, the tarry material included more products with a lower boiling point, indicating secondary pyrolysis. Shafizadeh and Fu [2] reported the formation of 28.1% levoglucosan in vacuum, but only 3.6% at atmospheric pressure with increased amounts of more’ volatile materials. Shafizadeh and Lai [3] undertook an intensive study of the pyrolysis of i4C-labeled levoglucosan to throw light on the bond fracturing and thus the reaction mechanism. For example they report the formation of furfural for
42
which 2/3 of the product included the original first carbon atom and only l/3 included the sixth carbon atom. In case of CO, 34.3% came from the first carbon atom and only 6.3% from the sixth. From these and similar data, they proposed various reaction mechanisms. Gardiner [4] studied the vacuum determining the pyrolysis of various mono-, di-, tri- and polysaccharides, major components. While the first step appears to be in each case the internal loss of a mole of water and the formation of levoglucosan-like structures, some differences were observed in the quantities of lesser components. The most recent and extensive review of the pyrolysis of cellulose by Antal [5] summarizes the present state of art in this field. The major problem in the atmospheric pyrolysis of carbohydrate is the multitude of simultaneous and consecutive reactions occurring. The study of such a complex system represents serious difficulties. Baker [6] studied the pyrolysis of tobacco and attempted to separate the reactions through the visual curve-fitting of the CO and CO, evolution curves. In our previous studies [7-91, we reported an empirical curve-fitting of the first derivatives of thermogravimetric curves obtained during the pyrolysis of carbohydrates with and without catalyst. Such analysis provided a relative basis for comparing the reactivity of various mono-, oligo- and polysaccharides. In this paper, we wish to report the successful coupling of a mass spectrometer with the thermogravimetric unit, which provided projections for the possible formation of various volatile C,-C, compounds.
EXPERIMENTAL
*
Perkin-Elmer TGS-2 and DSC-2 systems equipped with a microprocessorcontrolled programmer were used in our studies with a heating rate of lO”C/min. The pyrolytic chamber was swept with helium at a rate of 30 ml/mm. The flow was analyzed in a modified Finnigan 3000 mass spectrometer using a General Electric dimethylsilicon rubber membrane. Generally, a 10-s scan was used in the range of lo-135 a.m.u. The data were evaluated on a Tektronix 4052 equipped with auxiliary memory using a program written in BASIC. This allowed not only the plotting of the intensity of each mass unit versus time (and temperature), but also the plotting of the relative intensity of possible compounds against the same variables. The thermogravimetry-mass spectrometry (TG-MS) system was calibrated with model compounds, such as NaHCO, for water, in order to enable the quantitation of the signal. The system also included the optional use of a Varian 1520 gas chromato* Reference to a company and/or product named by the Department is only for purposes of information and does not imply approval or recommendation of the product to the exclusion of others which may also be suitable.
43
graph. The gaseous flow could be diverted to go through a Tenax column at room temperature before entering to the mass spectrometer. Nearly all organic materials were adsorbed on the column and they were desorbed at the end of the pyrolysis. The chemicals were commercially available analytical grade glucose, maltose, cellobiose, amylose and cellulose, In order to minimize the effect of heat transfer, the sample sizes were in the range of 0.9-1.2 mg.
RESULTS
AND DISCUSSION
Theoretically, one could write two extreme reactions for the pyrolytic reactions of carbohydrates as illustrated with a simple hexose. The complete dehydration, frequently described as carbonization, would result in carbon and water, leaving a maximum of 40% residue: CSHi206 + 6 C + 6 H,O The other possibility is the complete volatilization without which would be expected from the well-known gasification higher temperature: C,H,,O,
+ 3 CH, + 3 CO,
(1) any residue, of carbon at
(2)
The thermogravimetric study of a wide variety of carbohydrates [7] indicated that 30-45% of residue was left behind after heating the samples to 360°C in nitrogen at atmospheric pressure. However, the reaction was not a simple carbonization of the carbohydrates, since further heating of the residue to 450°C provided additional volatile materials and the final amount of residue was 23-32%. Later it was found [S] that the amount of residue could be altered both upward and downward by the application of various inorganic catalysts. The curves of weight loss rate for mono- and disaccharides show two distinct peaks, while those for polymeric carbohydrates exhibit only one peak [9]. Fig. 1 compares the curves for glucose, cellobiose and cellulose, showing that the distance between the two peaks is less for cellobiose than glucose. Various depolymerized cellulose samples were also studied. The peak positions shifted slightly to higher temperature; but even at a degree of polymerization of 25 only one peak was obtained. It is not known at what point the two peaks become one. Similar results were obtained during the determination of the heat of pyrolysis by differential scanning calorimetry (DSC). The peak positions both for TG and DSC are listed in Table 1. The coupling of the mass spectrometer with the thermogravimetric analyzer allowed not only the determination of the amount of water formed in these pyrolytic reactions but also the comparison of the rates for the loss of weight and the generation of water. The rate of water formation for the five
44
CELLtLosE
CELLOBIOSE
Fig. 1. Thermogravimetric curves (rate of weight loss) of cellulose, cellobiose and glucose in nitrogen atmosphere, heating rate 2O”C/min.
carbohydrates studied shows significant similarity to the weight loss curves. However, while the peak positions are the same within the experimental errors, the shapes of the curves are different. The total amount of water evolved in these reactions is 2.6-2.8 moles per hexose unit; but in those cases when the water formation exhibits two distinct peaks, the ratio of water in these two ranges differs widely, as shown in Table 2.
TABLE
1
Peak positions (“C) obtained by thermogravimetry (TG) and differential scanning calorimetry (DSC) Compound
Glucose Maltose Cellobiose Amylose Cellulose
1st range *
2nd range *
TG
DSC
TG
DSC
241 267 261
230 256 252 **
332 338 334 313 349
331 330 330 316 353
l The first range ends and the second one starts around 280-300°C or the lowest point between two peaks. **The closeness of the melting point of cellobiose makes the determination of this peak difficult.
45 TABLE
2
Water formation
during pyrolysis
Compound
1st range
Glucose Maltose Cellobiose Amylose Cellulose
1.7 2.0 1.2
of carbohydrates
2nd range
l
l The first range ends and the second between two peaks.
(moles of H,O/hexose
Total
l
1.0 0.8 1.4 2.6 2.7 one starts
units)
2.7 2.8 2.6 2.6 2.7 around
280-300°C
or the lowest
point
The carbonization of carbohydrates based on eqn. 1 would result in 6 moles of water for glucose. This would approach the value of 5 as the degree of polymerization increases. It is evident that just as the amount of residue is less than the theoretical 40%, the amount of water is also considerably lower. The difference is made up with the formation of various organic compounds. The mass spectrometric studies also provided information on the rate of the formation of these organic materials. When this is compared with the water formation and with the weight loss observed during the same period, the carbohydrate pyrolysis appears in a different light. In Table 3 the weight and water loss during pyrolysis are given. In the first range of the pyrolysis the weight loss is mostly water, but at higher temperature the pyrolysis results in the formation of a wide variety of organic compounds. Table 4 shows the ratio of moles of water evolved to the moles of carbohydrates gasified in the two temperature ranges. From Table 2 and 4 it appears that the reaction in the first range is the elimination of 1.2-2.0 moles of water with the formation of a minimal amount of organic materials. The mass spectrometric studies indicated this amount, in most cases, to be at the borderline of the experimental errors. Curiously, it was only with
TABLE
3
Water and total weight loss during pyrolysis Compound
Glucose Maltose Cellobiose Amylose Cellulose *The first range ends between two peaks.
(in I%of initial weight) 2nd range *
1st range * Water
Total
Water
Total
17 21 12
18 23 14
10 8 15 30 29
60 50 64 70 80
and the second
one starts
around
280-300°C
or the lowest
point
46 TABLE 4 Ratio of moles of water evolved and moles of carbohydrate
gasified
Compound
1st range *
2nd range *
Total
Glucose Maltose Cellobiose Amylose Cellulose
9.5 8.7 8.6
1.7 1.6 2.2 3.7 3.4
3.5 3.8 3.0 3.7 3.4
* The first range ends and the second between two peaks.
one starts
around
280-300°C
or the lowest
points
cellobiose when the computerized evaluation disclosed the probable presence of furfural in detectable, but still minimal, quantity in the lower temperature range. The pyrolysis of glucose in vacuum supposed to yield levoglucosan with the loss of 1 mole of water. The same type of reaction would yield only 0.5 mole of water with disaccharides such as maltose or cellobiose. Since the amount of water lost in the lower temperature range is higher without losing a considerable amount of organic fragments, we have to suppose that at atmospheric pressures in nitrogen or helium further dehydration takes place. Whether this dehydration occurs within the hexose units or in some other way, such as aromatization, is uncertain at this time. Some assumptions, however, can be made by the correlation of the heat of reaction determine by DSC and the rate of water formation. For the five carbohydrates studied, endothermic reactions were observed with values in the range lo-170 Cal/g. These values were converted to heat of reaction, kcal per mol of water evolved and are shown in Table 5. While water can evolve in various ways, the major pathways may be hypothesized if the carbon-carbon chain the hexose unit does not fracture: -&&-I-OH I
2-c&H-OH
-+ -6 = dH + H,o I
- 10-12 kcal I
+ -CH-CH-0-CH-CH-
(3)
I
+ H,o
- 4-5 kcal
(4
In the first range, only a minimal amount of volatile organic material is generated. Thus the original carbon-carbon chain is not likely to have suffered major scissions. Therefore, the value listed in Table 5 point toward two different dehydration possibilities: ether bond formation for maltose or double bond formation for cellobiose. In the case of glucose, it appears to be closer to ether formation, perhaps levoglucosan; but both types of reactions may occur. The higher temperature range of the pyrolysis provides a very complex reaction mixture. The DSC data in Table 5 indicate again a major difference
TABLE
5
Heat of reaction
per mole of water evolved (kcal)
Compound
1st range *
2nd range
Glucose Maltose Cellobiose Amylose Cellulose
5.2 2.6 12.8
7.7 3.0 3.4 4.0 9.9
l The first range ends and the second between two peaks.
one starts
around
280-3OO’C
l
or the lowest
point
.
between LX-and P-glucosidic bonding in case of amylose and cellulose when all reactions were condensed in one range, but with the other three carbohydrates the picture is inconclusive. There is still a considerable amount of water formation, but this is the point where most of the organic materials are detected by the mass spectrometer. In this complex mixture, no positive identification and quantitative analysis of the components can be made. However, certain general conclusions can be made as to the nature of the possible products and to their relative ratio to each other. Carbon monoxide and carbon dioxide were detected during cellulose pyrolysis and, in somewhat lesser amount, during the pyrolysis of glucose. Their presence in the case of the other three carbohydrates is very minimal, if detected at all. Ethane and ethylene are present in the pyrolytic product of cellulose. However, whereas glucose and cellobiose yielded detectable amounts, maltose and amylose did not produce measurable quantities. Alcohols were mostly absent or highly uncertain; only methanol was detected in low quantity in the case of cellulose and cellobiose. Acids were suggested in somewhat higher quantities than alcohols, but only formic acid and pyruvic acid were above the detectable amounts. The former was only present in glucose pyrolysis, while the latter was only absent in case of the maltose pyrolysis. The computerized library search based on the mass spectrometric data indicated the possible presence of a large number of volatile aldehydes and ketones. Among the aldehydes, this included formaldehyde, acetaldehyde, propionaldehyde, acrolein, crotonaldehyde and tiglaldehyde. Among the ketones the most likely candidates were acetone, methyl ethyl ketone, methyl ally1 ketone and 5-hexen-2-one. Whereas cellulose and cellobiose showed good probability for each of these compounds, for the other carbohydrates mostly acetaldehyde, propionaldehyde, methyl ethyl ketone and 5-hexen-2one were suggested within the confidence limit. Finally, furan and furfural were likely pyrolytic components in the case of the five carbohydrates, but other furan derivatives also appeared to be present, mostly during the pyrolysis of cellobiose.
The tentative identification of the components from such a complex product mixture on the basis of mass spectrometry only has to be viewed with great caution. The incorporation of a gas chromatograph in our combined TG-MS system provides opportunity for the separation and the identification of the products. So far, furfural and methylfurfural have been positively identified.
CONCLUSION
Our pyrolytic study of carbohydrates at atmospheric pressure confirms the complex nature of the reaction. Water is the most evident product, but the reaction is not a complete carbonization. In contrast with the vacuum pyrolysis, where levoglucosan appears to be the major organic product, a wide variety of compounds were formed during our studies. Where two distinct ranges of reaction were observed, the first step is almost exclusively dehydration; but the amount of water is more than one mole per hexose unit indicating, even under these conditions, more than just levoglucosan formation. While the exact reactions are not known yet and additional studies are needed, it appears certain that there is a major difference between the pyrolysis of (Y-and /%glucoside bonding.
REFERENCES 1 2 3 4 5 6 7 8
9
S.L. Madorski, V.E. Hart and S. Strauss, J. Res. Nat. Bureau Standard, 56 (1956) 343. F. Shafizadeh and Y.L. Fu, Carbohyd. Res., 29 (1973) 113. F. Shafizadeh and Y.Z. Lai, J. Org. Chem., 37 (1972) 278. D. Gardiner, J. Chem. Sot. C, (1966) 1473. M.J. Antal, Advances in Solar Energy, American Solar Energy Society, Boulder, CO, 1983, p. 61. R.R. Baker, Thermochim. Acta, 28 (1979) 45. A.E. Pavlath and K.S. Gregorski in D. Dolhmir (Editor), Proc. II. European Symposium on Thermal Analysis, Aberdeen, Scotland, 1981, Heyden, London, 1981, p. 251. V.G. Randall, M.S. Masri and A.E. Pavlath, in B. Miller (Editor), Thermal Analysis, Proc. VII. Int. Conf. On Thermal Analysis, Kingston, Ontario, Canada, 1982, Wiley, Chichester, 1982, p. 1190. A.E. Pavlath and K.S. Gregorski, in R.P. Overend (Editor), Proc. I. Int. Symp. on Fundamental Biomass Pyrolysis, Estes Park, CO, 1982.
41
ATMOSPHERIC PYROLYSIS OF CARBOHYDRATES WITH THERMOGRAVIMETRIC AND MASS SPECTROMETRIC ANALYSES
ATTILA
E. PAVLATH
* and KAY S. GREGORSKI
Western Regional Research Center, U.S. Department CA 94710 (U.S.A.)
of Agriculture,
800 Buchanan, Berkeley,
SUMMARY The pyrolysis of glucose, maltose, cellobiose, amylose and cellulose was studied by thermogravimetry between 250-4OO’C at atmospheric pressure in helium. The thermogravimetric analyzer was coupled with a mass spectrometer and the evolving gases were continuously monitored. Between 2.6 and 2.8 moles of water were obtained for every hexose unit. For amylose and cellulose the water formation was in one step; for the other carbohydrates, varying amounts of water formed in two different steps. The scans were evaluated by computer and the concentrations of the possible components were plotted versus temperature. The correlation of these curves with the weight loss curve throws new light on carbohydrate pyrolysis reactions.
INTRODUCTION
Carbohydrate pyrolysis, especially that of cellulose, has been studied for many years. The aim for the latter was mostly to determine the factors involved in flame retardation. Only during the past 10-15 years was attention paid to the possibility of using carbohydrates as sources for chemical intermediates. The energy crisis initiated various studies on wood, but limited studies are available on the basic reactions involved in the pyrolysis of various pure carbohydrates. Even in those cases the studies were carried out in vacuum to prevent secondary reactions. Madorsky et al. [l] found that 70% of cellulose was volatilized in vacuum pyrolysis yielding 27% water, 6% CO,, 2% CO and 65% volatile tarry material. When the reaction was carried out in nitrogen atmosphere, the tarry material included more products with a lower boiling point, indicating secondary pyrolysis. Shafizadeh and Fu [2] reported the formation of 28.1% levoglucosan in vacuum, but only 3.6% at atmospheric pressure with increased amounts of more’ volatile materials. Shafizadeh and Lai [3] undertook an intensive study of the pyrolysis of i4C-labeled levoglucosan to throw light on the bond fracturing and thus the reaction mechanism. For example they report the formation of furfural for
42
which 2/3 of the product included the original first carbon atom and only l/3 included the sixth carbon atom. In case of CO, 34.3% came from the first carbon atom and only 6.3% from the sixth. From these and similar data, they proposed various reaction mechanisms. Gardiner [4] studied the vacuum determining the pyrolysis of various mono-, di-, tri- and polysaccharides, major components. While the first step appears to be in each case the internal loss of a mole of water and the formation of levoglucosan-like structures, some differences were observed in the quantities of lesser components. The most recent and extensive review of the pyrolysis of cellulose by Antal [5] summarizes the present state of art in this field. The major problem in the atmospheric pyrolysis of carbohydrate is the multitude of simultaneous and consecutive reactions occurring. The study of such a complex system represents serious difficulties. Baker [6] studied the pyrolysis of tobacco and attempted to separate the reactions through the visual curve-fitting of the CO and CO, evolution curves. In our previous studies [7-91, we reported an empirical curve-fitting of the first derivatives of thermogravimetric curves obtained during the pyrolysis of carbohydrates with and without catalyst. Such analysis provided a relative basis for comparing the reactivity of various mono-, oligo- and polysaccharides. In this paper, we wish to report the successful coupling of a mass spectrometer with the thermogravimetric unit, which provided projections for the possible formation of various volatile C,-C, compounds.
EXPERIMENTAL
*
Perkin-Elmer TGS-2 and DSC-2 systems equipped with a microprocessorcontrolled programmer were used in our studies with a heating rate of lO”C/min. The pyrolytic chamber was swept with helium at a rate of 30 ml/mm. The flow was analyzed in a modified Finnigan 3000 mass spectrometer using a General Electric dimethylsilicon rubber membrane. Generally, a 10-s scan was used in the range of lo-135 a.m.u. The data were evaluated on a Tektronix 4052 equipped with auxiliary memory using a program written in BASIC. This allowed not only the plotting of the intensity of each mass unit versus time (and temperature), but also the plotting of the relative intensity of possible compounds against the same variables. The thermogravimetry-mass spectrometry (TG-MS) system was calibrated with model compounds, such as NaHCO, for water, in order to enable the quantitation of the signal. The system also included the optional use of a Varian 1520 gas chromato* Reference to a company and/or product named by the Department is only for purposes of information and does not imply approval or recommendation of the product to the exclusion of others which may also be suitable.
43
graph. The gaseous flow could be diverted to go through a Tenax column at room temperature before entering to the mass spectrometer. Nearly all organic materials were adsorbed on the column and they were desorbed at the end of the pyrolysis. The chemicals were commercially available analytical grade glucose, maltose, cellobiose, amylose and cellulose, In order to minimize the effect of heat transfer, the sample sizes were in the range of 0.9-1.2 mg.
RESULTS
AND DISCUSSION
Theoretically, one could write two extreme reactions for the pyrolytic reactions of carbohydrates as illustrated with a simple hexose. The complete dehydration, frequently described as carbonization, would result in carbon and water, leaving a maximum of 40% residue: CSHi206 + 6 C + 6 H,O The other possibility is the complete volatilization without which would be expected from the well-known gasification higher temperature: C,H,,O,
+ 3 CH, + 3 CO,
(1) any residue, of carbon at
(2)
The thermogravimetric study of a wide variety of carbohydrates [7] indicated that 30-45% of residue was left behind after heating the samples to 360°C in nitrogen at atmospheric pressure. However, the reaction was not a simple carbonization of the carbohydrates, since further heating of the residue to 450°C provided additional volatile materials and the final amount of residue was 23-32%. Later it was found [S] that the amount of residue could be altered both upward and downward by the application of various inorganic catalysts. The curves of weight loss rate for mono- and disaccharides show two distinct peaks, while those for polymeric carbohydrates exhibit only one peak [9]. Fig. 1 compares the curves for glucose, cellobiose and cellulose, showing that the distance between the two peaks is less for cellobiose than glucose. Various depolymerized cellulose samples were also studied. The peak positions shifted slightly to higher temperature; but even at a degree of polymerization of 25 only one peak was obtained. It is not known at what point the two peaks become one. Similar results were obtained during the determination of the heat of pyrolysis by differential scanning calorimetry (DSC). The peak positions both for TG and DSC are listed in Table 1. The coupling of the mass spectrometer with the thermogravimetric analyzer allowed not only the determination of the amount of water formed in these pyrolytic reactions but also the comparison of the rates for the loss of weight and the generation of water. The rate of water formation for the five
44
CELLtLosE
CELLOBIOSE
Fig. 1. Thermogravimetric curves (rate of weight loss) of cellulose, cellobiose and glucose in nitrogen atmosphere, heating rate 2O”C/min.
carbohydrates studied shows significant similarity to the weight loss curves. However, while the peak positions are the same within the experimental errors, the shapes of the curves are different. The total amount of water evolved in these reactions is 2.6-2.8 moles per hexose unit; but in those cases when the water formation exhibits two distinct peaks, the ratio of water in these two ranges differs widely, as shown in Table 2.
TABLE
1
Peak positions (“C) obtained by thermogravimetry (TG) and differential scanning calorimetry (DSC) Compound
Glucose Maltose Cellobiose Amylose Cellulose
1st range *
2nd range *
TG
DSC
TG
DSC
241 267 261
230 256 252 **
332 338 334 313 349
331 330 330 316 353
l The first range ends and the second one starts around 280-300°C or the lowest point between two peaks. **The closeness of the melting point of cellobiose makes the determination of this peak difficult.
45 TABLE
2
Water formation
during pyrolysis
Compound
1st range
Glucose Maltose Cellobiose Amylose Cellulose
1.7 2.0 1.2
of carbohydrates
2nd range
l
l The first range ends and the second between two peaks.
(moles of H,O/hexose
Total
l
1.0 0.8 1.4 2.6 2.7 one starts
units)
2.7 2.8 2.6 2.6 2.7 around
280-300°C
or the lowest
point
The carbonization of carbohydrates based on eqn. 1 would result in 6 moles of water for glucose. This would approach the value of 5 as the degree of polymerization increases. It is evident that just as the amount of residue is less than the theoretical 40%, the amount of water is also considerably lower. The difference is made up with the formation of various organic compounds. The mass spectrometric studies also provided information on the rate of the formation of these organic materials. When this is compared with the water formation and with the weight loss observed during the same period, the carbohydrate pyrolysis appears in a different light. In Table 3 the weight and water loss during pyrolysis are given. In the first range of the pyrolysis the weight loss is mostly water, but at higher temperature the pyrolysis results in the formation of a wide variety of organic compounds. Table 4 shows the ratio of moles of water evolved to the moles of carbohydrates gasified in the two temperature ranges. From Table 2 and 4 it appears that the reaction in the first range is the elimination of 1.2-2.0 moles of water with the formation of a minimal amount of organic materials. The mass spectrometric studies indicated this amount, in most cases, to be at the borderline of the experimental errors. Curiously, it was only with
TABLE
3
Water and total weight loss during pyrolysis Compound
Glucose Maltose Cellobiose Amylose Cellulose *The first range ends between two peaks.
(in I%of initial weight) 2nd range *
1st range * Water
Total
Water
Total
17 21 12
18 23 14
10 8 15 30 29
60 50 64 70 80
and the second
one starts
around
280-300°C
or the lowest
point
46 TABLE 4 Ratio of moles of water evolved and moles of carbohydrate
gasified
Compound
1st range *
2nd range *
Total
Glucose Maltose Cellobiose Amylose Cellulose
9.5 8.7 8.6
1.7 1.6 2.2 3.7 3.4
3.5 3.8 3.0 3.7 3.4
* The first range ends and the second between two peaks.
one starts
around
280-300°C
or the lowest
points
cellobiose when the computerized evaluation disclosed the probable presence of furfural in detectable, but still minimal, quantity in the lower temperature range. The pyrolysis of glucose in vacuum supposed to yield levoglucosan with the loss of 1 mole of water. The same type of reaction would yield only 0.5 mole of water with disaccharides such as maltose or cellobiose. Since the amount of water lost in the lower temperature range is higher without losing a considerable amount of organic fragments, we have to suppose that at atmospheric pressures in nitrogen or helium further dehydration takes place. Whether this dehydration occurs within the hexose units or in some other way, such as aromatization, is uncertain at this time. Some assumptions, however, can be made by the correlation of the heat of reaction determine by DSC and the rate of water formation. For the five carbohydrates studied, endothermic reactions were observed with values in the range lo-170 Cal/g. These values were converted to heat of reaction, kcal per mol of water evolved and are shown in Table 5. While water can evolve in various ways, the major pathways may be hypothesized if the carbon-carbon chain the hexose unit does not fracture: -&&-I-OH I
2-c&H-OH
-+ -6 = dH + H,o I
- 10-12 kcal I
+ -CH-CH-0-CH-CH-
(3)
I
+ H,o
- 4-5 kcal
(4
In the first range, only a minimal amount of volatile organic material is generated. Thus the original carbon-carbon chain is not likely to have suffered major scissions. Therefore, the value listed in Table 5 point toward two different dehydration possibilities: ether bond formation for maltose or double bond formation for cellobiose. In the case of glucose, it appears to be closer to ether formation, perhaps levoglucosan; but both types of reactions may occur. The higher temperature range of the pyrolysis provides a very complex reaction mixture. The DSC data in Table 5 indicate again a major difference
TABLE
5
Heat of reaction
per mole of water evolved (kcal)
Compound
1st range *
2nd range
Glucose Maltose Cellobiose Amylose Cellulose
5.2 2.6 12.8
7.7 3.0 3.4 4.0 9.9
l The first range ends and the second between two peaks.
one starts
around
280-3OO’C
l
or the lowest
point
.
between LX-and P-glucosidic bonding in case of amylose and cellulose when all reactions were condensed in one range, but with the other three carbohydrates the picture is inconclusive. There is still a considerable amount of water formation, but this is the point where most of the organic materials are detected by the mass spectrometer. In this complex mixture, no positive identification and quantitative analysis of the components can be made. However, certain general conclusions can be made as to the nature of the possible products and to their relative ratio to each other. Carbon monoxide and carbon dioxide were detected during cellulose pyrolysis and, in somewhat lesser amount, during the pyrolysis of glucose. Their presence in the case of the other three carbohydrates is very minimal, if detected at all. Ethane and ethylene are present in the pyrolytic product of cellulose. However, whereas glucose and cellobiose yielded detectable amounts, maltose and amylose did not produce measurable quantities. Alcohols were mostly absent or highly uncertain; only methanol was detected in low quantity in the case of cellulose and cellobiose. Acids were suggested in somewhat higher quantities than alcohols, but only formic acid and pyruvic acid were above the detectable amounts. The former was only present in glucose pyrolysis, while the latter was only absent in case of the maltose pyrolysis. The computerized library search based on the mass spectrometric data indicated the possible presence of a large number of volatile aldehydes and ketones. Among the aldehydes, this included formaldehyde, acetaldehyde, propionaldehyde, acrolein, crotonaldehyde and tiglaldehyde. Among the ketones the most likely candidates were acetone, methyl ethyl ketone, methyl ally1 ketone and 5-hexen-2-one. Whereas cellulose and cellobiose showed good probability for each of these compounds, for the other carbohydrates mostly acetaldehyde, propionaldehyde, methyl ethyl ketone and 5-hexen-2one were suggested within the confidence limit. Finally, furan and furfural were likely pyrolytic components in the case of the five carbohydrates, but other furan derivatives also appeared to be present, mostly during the pyrolysis of cellobiose.
The tentative identification of the components from such a complex product mixture on the basis of mass spectrometry only has to be viewed with great caution. The incorporation of a gas chromatograph in our combined TG-MS system provides opportunity for the separation and the identification of the products. So far, furfural and methylfurfural have been positively identified.
CONCLUSION
Our pyrolytic study of carbohydrates at atmospheric pressure confirms the complex nature of the reaction. Water is the most evident product, but the reaction is not a complete carbonization. In contrast with the vacuum pyrolysis, where levoglucosan appears to be the major organic product, a wide variety of compounds were formed during our studies. Where two distinct ranges of reaction were observed, the first step is almost exclusively dehydration; but the amount of water is more than one mole per hexose unit indicating, even under these conditions, more than just levoglucosan formation. While the exact reactions are not known yet and additional studies are needed, it appears certain that there is a major difference between the pyrolysis of (Y-and /%glucoside bonding.
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