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Thermogravimetric analysis of glycol lignin fractions obtained by sequential solvent extraction
Short Thermogravimetric sequential solvent Ronald W. Thring*,
Communications
analysis of glycol extraction
lignin
fractions
obtained
by
E. Chornet and Ralph P. Overendt
Department of Chemical Engineering. University of Sherbrooke, Sherbrooke, Quebec, Canada J 1K 2R 1 tNational Renewable Energy Laboratory, Golden, CO, USA (Received 3 September 1992; revised 9 February 7993)
Data are presented on the thermal behaviour and devolatilization in a thermal analyser.
of a solvolytic lignin and its fractions during combustion
(Keywords: glycol lignin; thermogrnvimetric analysis; fractionation)
Glycol lignin was isolated from the hardwood Populus deltoides by organosolv delignification using ethylene glycol as solvent, followed by precipitation of the acid-insoluble lignin by dilute acidification with aqueous hydrochloric acid’. Fractionation of this lignin into four distinct fractions was achieved by sequential extraction with organic solvents of increasing dissolving power for lignin’. The apparent average molecular weights (Table 1) increase with increasing fraction number. The increase in polydispersity (MJM, or d) with fraction number suggests that Fr-1 and Fr-2 are each dominated by material of similar low molecular weight, whereas Fr-3 and Fr-4 consist of a mixture of medium- and high-molecular-weight material. The carbon content of the fractions is essentially the same, suggesting that all the fractions have a similar degree of cross-linking due to formation of carbon-carbon bonds caused by condensation reactions. A highly cross-linked lignin is generally considered to be thermally more stable. Also, there is a rather significant difference in the hydrogen content of Fr-1 and Fr-2 compared with Fr-3 and Fr-4. In coal studies, a low hydrogen content corresponds to a high degree of aromaticity and cross-linking. The same trend is observed for the lignin fractions in this work. Fr-1, in particular, is known to contain a high proportion of saturated aliphatic side-chain structures, whereas the Fr-3 and Fr-4 mainly comprise aromatic structures (guaiacyl and syringyl units). In addition, it has been demonstrated by 13C n.m.r. that Fr-3 and Fr-4 also contain very small amounts of hemicellulose?, which somewhat increase the hydrogen content in these fractions. Thermogravimetric analyses were performed on 20mg samples at a heating rate of lO”Cmin-’ in flowing nitrogen. * Present address: Department of Chemical Engineering, University of New Brunswick, Fredericton,
NB, Canada
E3B 5A3
0016-2361/93/09/1355-02 0 1993 Butterworth-Heinemann
Ltd.
Before analysis, samples were freezedried at - 60°C and 0.1 mmHg for 2 d and thereafter stored over P,05 in a desiccator. To the authors’ knowledge, the only other study of thermal stability of molecular-weight fractions from lignins has been conducted on industrially produced kraft lignin3. Figure 1 and Table 2 show the results of thermogravimetric analysis of the glycol lignin and its fractions. The total weight loss up to 200°C was < 5% for the unfractionated glycol lignin, N 10% for
fractions 2 to 4, and 25% for Fr-1. For kraft lignin and its fractions3, the weight losses up to 200°C were all < 5%. In both studies, the weight loss may be attributed not only to the presence of some volatile components but also to
thermal degradation of facile bonds in terminal carboxyl and hydroxymethylene groups in the lignin macromolecule. Thermal degradation of lignin in the 300400°C range involves rupture of inter-unit linkages4. As the temperature increases, secondary decomposition reactions occur, leading to some recondensation of aromatic rings?. Also, cleavage of methyl-aryl ether bonds in kraft lignin reportedly reaches a maximum just below 400”C4. In this study, degradation of glycol lignin and its fractions can be observed to reach a maximum rate at a lower temperature, N 350°C. Table 2 shows that all residue weight values increase with increasing fraction number in the 300-500°C pyrolysis temperature range, except for Fr-4, which
Elemental composition, molecular weight averages” and molecular weight distribution of original and fractionated glycol lignin
Table 1
Initial Fr-1
Elemental analysis (wt%) 61.1 C 7.3 H
Fr-2
Fr-3
N Ob
0.1 31.5
61.9 1.3 0.2 30.6
MW M” KvlW,
1155 742 1.6
2788 1368 2.0
Fr-4
lignin
62.2 6.0 0.4 31.4
60.9 6.1 0.4 32.6
61.3 6.0 0.2 32.5
10959 2038 5.4
17429 2406 1.2
3874 1210 3.2
’ Relative values, related to polystyrene standards, obtained by gel permeation chromatography of acetylated samples *By difference
Table 2
Residual lignin at various temperatures Residue (wt%) at temperature (“C)
Sample
300
400
500
600
800
Fr-1 Fr-2 Fr-3 Fr-4 Original
63.6 78.4 80.2 82.0 85.7
32.1 53.4 59.0 51.5 50.6
25.6 46.0 41.9 41.6 38.7
22.9 42.1 40.5 36.5 33.4
21.0 39.1 34.3 32.1 29.1
lignin
Fuel 1993 Volume 72 Number 9
1355
Short Communications
l
Glycol
o
Fr-1
A
Fr-2
0
Fr-3
x
Fr-4
Lignin
25
I
1
I
I
I
I
I
100200300400500600700800
I
> 1
Temperature,“C Figure 1 Thermogravimetric analysis of glycol lignin and its fractions
is generally lower than the corresponding residue weight values for Fr-2 and Fr-3. This is most likely due to the presence of hemicelluloses in Fr-4. Hemicelluloses are known to degrade at a much faster rate than lignins in the 200-300°C range6. Above 5OO”C,decomposition proceeds at a slow rate for the original lignin and its fractions. However, as seen in Figure I,
1356
the decomposition rate is slowest for Fr-1 and highest for Fr-3. This suggests that Fr-3 contains a higher proportion of facile bonds than Fr-I which are susceptible to cleavage to form volatile degradation products under pyrolytic conditions. Of all the fractions, Fr-3 is known to contain the highest amount of methoxyl groups2, which are cleaved exten-
Fuel 1993 Volume 72 Number 9
sively at these temperatures. Fr-1, on the other hand, forms a very stable residue, probably owing to recondensation and cross-linking reactions after initially exhibiting the most rapid degradation at lower temperature. As seen from Table 2, the involatile residue at 500°C increases with increasing molecular weight from fractions 1 to 3. This suggests an increase in thermal stability with molecular weight and may reflect an increasing degree of branching and condensation in the initial samples. As seen, the medium-molecular-weight fraction (Fr-2) exhibits the highest thermal stability. It should be noted that, of the high-molecular-weight fractions, this fraction has the lowest polydispersity d (see Table I). It has been suggested that high-molecular-weight fractions of low polydispersity may show better thermal stability than fractions of higher polydispersity3.
REFERENCES 1
Thring, R. W., Chornet, E. and Overend,
2
Thring, R. W., Chornet, E., Bouchard, J., Vidal, P. F. and Overend, R. P. Ind.
R. P. Biomass 1990, 23, 289 Eng. Chem. Res. 1991,30,232
3
4 5 6
Yoshida, H., Mlirck, R., Kringstad, K. P. and Hatakeyama, H. Holzforschung 1987,41, 171 Fenner, R. and Lephardt, J. .I. Agric. Food Chem. 1981,29,846 Oren, M. J., Nassar, M. M. and MacKay, G. D. M. Can. J. Spectrosc. 1984, 29, 10 Beall, F. C. WoodandFiber 1969,1,215
Communications
analysis of glycol extraction
lignin
fractions
obtained
by
E. Chornet and Ralph P. Overendt
Department of Chemical Engineering. University of Sherbrooke, Sherbrooke, Quebec, Canada J 1K 2R 1 tNational Renewable Energy Laboratory, Golden, CO, USA (Received 3 September 1992; revised 9 February 7993)
Data are presented on the thermal behaviour and devolatilization in a thermal analyser.
of a solvolytic lignin and its fractions during combustion
(Keywords: glycol lignin; thermogrnvimetric analysis; fractionation)
Glycol lignin was isolated from the hardwood Populus deltoides by organosolv delignification using ethylene glycol as solvent, followed by precipitation of the acid-insoluble lignin by dilute acidification with aqueous hydrochloric acid’. Fractionation of this lignin into four distinct fractions was achieved by sequential extraction with organic solvents of increasing dissolving power for lignin’. The apparent average molecular weights (Table 1) increase with increasing fraction number. The increase in polydispersity (MJM, or d) with fraction number suggests that Fr-1 and Fr-2 are each dominated by material of similar low molecular weight, whereas Fr-3 and Fr-4 consist of a mixture of medium- and high-molecular-weight material. The carbon content of the fractions is essentially the same, suggesting that all the fractions have a similar degree of cross-linking due to formation of carbon-carbon bonds caused by condensation reactions. A highly cross-linked lignin is generally considered to be thermally more stable. Also, there is a rather significant difference in the hydrogen content of Fr-1 and Fr-2 compared with Fr-3 and Fr-4. In coal studies, a low hydrogen content corresponds to a high degree of aromaticity and cross-linking. The same trend is observed for the lignin fractions in this work. Fr-1, in particular, is known to contain a high proportion of saturated aliphatic side-chain structures, whereas the Fr-3 and Fr-4 mainly comprise aromatic structures (guaiacyl and syringyl units). In addition, it has been demonstrated by 13C n.m.r. that Fr-3 and Fr-4 also contain very small amounts of hemicellulose?, which somewhat increase the hydrogen content in these fractions. Thermogravimetric analyses were performed on 20mg samples at a heating rate of lO”Cmin-’ in flowing nitrogen. * Present address: Department of Chemical Engineering, University of New Brunswick, Fredericton,
NB, Canada
E3B 5A3
0016-2361/93/09/1355-02 0 1993 Butterworth-Heinemann
Ltd.
Before analysis, samples were freezedried at - 60°C and 0.1 mmHg for 2 d and thereafter stored over P,05 in a desiccator. To the authors’ knowledge, the only other study of thermal stability of molecular-weight fractions from lignins has been conducted on industrially produced kraft lignin3. Figure 1 and Table 2 show the results of thermogravimetric analysis of the glycol lignin and its fractions. The total weight loss up to 200°C was < 5% for the unfractionated glycol lignin, N 10% for
fractions 2 to 4, and 25% for Fr-1. For kraft lignin and its fractions3, the weight losses up to 200°C were all < 5%. In both studies, the weight loss may be attributed not only to the presence of some volatile components but also to
thermal degradation of facile bonds in terminal carboxyl and hydroxymethylene groups in the lignin macromolecule. Thermal degradation of lignin in the 300400°C range involves rupture of inter-unit linkages4. As the temperature increases, secondary decomposition reactions occur, leading to some recondensation of aromatic rings?. Also, cleavage of methyl-aryl ether bonds in kraft lignin reportedly reaches a maximum just below 400”C4. In this study, degradation of glycol lignin and its fractions can be observed to reach a maximum rate at a lower temperature, N 350°C. Table 2 shows that all residue weight values increase with increasing fraction number in the 300-500°C pyrolysis temperature range, except for Fr-4, which
Elemental composition, molecular weight averages” and molecular weight distribution of original and fractionated glycol lignin
Table 1
Initial Fr-1
Elemental analysis (wt%) 61.1 C 7.3 H
Fr-2
Fr-3
N Ob
0.1 31.5
61.9 1.3 0.2 30.6
MW M” KvlW,
1155 742 1.6
2788 1368 2.0
Fr-4
lignin
62.2 6.0 0.4 31.4
60.9 6.1 0.4 32.6
61.3 6.0 0.2 32.5
10959 2038 5.4
17429 2406 1.2
3874 1210 3.2
’ Relative values, related to polystyrene standards, obtained by gel permeation chromatography of acetylated samples *By difference
Table 2
Residual lignin at various temperatures Residue (wt%) at temperature (“C)
Sample
300
400
500
600
800
Fr-1 Fr-2 Fr-3 Fr-4 Original
63.6 78.4 80.2 82.0 85.7
32.1 53.4 59.0 51.5 50.6
25.6 46.0 41.9 41.6 38.7
22.9 42.1 40.5 36.5 33.4
21.0 39.1 34.3 32.1 29.1
lignin
Fuel 1993 Volume 72 Number 9
1355
Short Communications
l
Glycol
o
Fr-1
A
Fr-2
0
Fr-3
x
Fr-4
Lignin
25
I
1
I
I
I
I
I
100200300400500600700800
I
> 1
Temperature,“C Figure 1 Thermogravimetric analysis of glycol lignin and its fractions
is generally lower than the corresponding residue weight values for Fr-2 and Fr-3. This is most likely due to the presence of hemicelluloses in Fr-4. Hemicelluloses are known to degrade at a much faster rate than lignins in the 200-300°C range6. Above 5OO”C,decomposition proceeds at a slow rate for the original lignin and its fractions. However, as seen in Figure I,
1356
the decomposition rate is slowest for Fr-1 and highest for Fr-3. This suggests that Fr-3 contains a higher proportion of facile bonds than Fr-I which are susceptible to cleavage to form volatile degradation products under pyrolytic conditions. Of all the fractions, Fr-3 is known to contain the highest amount of methoxyl groups2, which are cleaved exten-
Fuel 1993 Volume 72 Number 9
sively at these temperatures. Fr-1, on the other hand, forms a very stable residue, probably owing to recondensation and cross-linking reactions after initially exhibiting the most rapid degradation at lower temperature. As seen from Table 2, the involatile residue at 500°C increases with increasing molecular weight from fractions 1 to 3. This suggests an increase in thermal stability with molecular weight and may reflect an increasing degree of branching and condensation in the initial samples. As seen, the medium-molecular-weight fraction (Fr-2) exhibits the highest thermal stability. It should be noted that, of the high-molecular-weight fractions, this fraction has the lowest polydispersity d (see Table I). It has been suggested that high-molecular-weight fractions of low polydispersity may show better thermal stability than fractions of higher polydispersity3.
REFERENCES 1
Thring, R. W., Chornet, E. and Overend,
2
Thring, R. W., Chornet, E., Bouchard, J., Vidal, P. F. and Overend, R. P. Ind.
R. P. Biomass 1990, 23, 289 Eng. Chem. Res. 1991,30,232
3
4 5 6
Yoshida, H., Mlirck, R., Kringstad, K. P. and Hatakeyama, H. Holzforschung 1987,41, 171 Fenner, R. and Lephardt, J. .I. Agric. Food Chem. 1981,29,846 Oren, M. J., Nassar, M. M. and MacKay, G. D. M. Can. J. Spectrosc. 1984, 29, 10 Beall, F. C. WoodandFiber 1969,1,215