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Absorption of iodine by coal and lignite
Carbon. 1976, Vol. 14, pp. 91-95.
Pergamon Press.
Printed in Great Britain
ABSORPTION OF IODINE BY COAL AND LIGNITE-t S. ARONSON, A. SCHWEBEL and G. SINENSKYS BrooklynCollege,CityUniversityof NewYork,Brooklyn,NY 11210,U.S.A. (Received 5 January 1976) Abstract-The vapor pressure of iodine over mixtures of iodine and various coals has been measuredat temperatures of 65-9X. Ligniteand bituminouscoalsexhibitsimilarbehaviorin their absorption of iodine whereas the behavior of anthracite coal is different. A region of constant vapor pressure occurs in the reaction between iodine and the bituminous coals and lignite. Complex formation between the iodine and coal is postulated.
1. INTEODUCTION
iodine powder with coal in definite weight ratios ranging from an iodine to coal ratio of 1: 10 to a ratio of 16: 1. The coals were used in their as-received state with the exception of the anthracite which was pulverized before use to pass through aNo. 50 sieve. The absorption of iodine by a natural graphite powder, SP-1 spectroscopic graphite, obtained from the National Carbon Company, was studied for comparison purposes. The various mixtures were sealed in glass tubes and were heated at 90°C for at least 48 hr. Iodine vapor pressures over the various mixtures were measured calorimetrically using a Cary-14 Spectrophotometer. Measurements over a range of temperatures were made using special teflon-stoppered quartz cells with compartments through which hot water could be circulated. The technique has been described elsewhere [6].
The structure of coal is complex and heterogeneous [l-3]. It is generally accepted that a major portion of the carbon in coal is part of a network of polynuclear aromatic compounds. Most of the information about the chemical nature of coal has been obtained by degradative processes such as oxidation or volatilization which have altered the original structure. Iodine reacts reversibly with polynuclear aromatic hydrocarbons to form complexes in which several molecules of iodine are attached to each hydrocarbon molecule[C 61. The iodine can be removed by slight heating of the complex with the restoration of the hydrocarbon to its original form. It seemed evident, therefore, that iodine absorption might be used as a nondestructive technique for obtaining information about the quantity and types of aromatic compounds present in coal. The absorption of iodine by anthracite and bituminous coal and lignite was, therefore, investigated.
3. RESULTS ANDDISCUSSION
2. EXPERIMENTAL The types of coal used in this study are shown in Table 1 together with analytical data on their chemical constituents. The chemical analyses were performed by the Schwarzkopf Microanalytical Laboratory, Woodside, N. Y. The middle section of Table 1 gives information on the composition of the samples as they were received from the suppliers. The iodine absorption work was done on the as-received samples. A calculation of the weight percent of carbon, hydrogen and oxygen was also made on the basis of ash-free and moisture-free conditions since these data reflect more accurately the rank of the various coals. As one proceeds from higher to lower ranked coals, the carbon content is expected to decrease and the hydrogen and oxygen wntents are expected to increase. The atomic ratio of hydrogen to carbon is also a significant measure of coal rank[7]. This parameter consistently increased with decreasing coal rank for our samples. The coal-iodine samples were prepared by mixing tA portionof the work described in this paper was presented at the Twelfth Biennial Conference on Carbon, Pittsburgh, Pa., 28 July 1975and has been published in abstract form in the Proceedings of the Conference, page 61. SMr. Sinensky performed this work as a participant in the National Science Foundation Undergraduate ResearchProject at BrooklynCollege in the spring and summer of 1974. 93 CAR Vol. I4 No. 2-A
Vapor pressure measurements were made at temperatures of 55-95°C on approximately fifty different mixtures of iodine with anthracite and bituminous coals, lignite and graphite. Some typical plots of the natural logarithm of iodine pressure against reciprocal temperature are shown in Fig. 1. A value for the heat of absorption of iodine vapor by the solid can be calculated from the slope of each curve in Fig. 1. The values obtained were in the range of -11 to -15 kcal/mole IZ. The iodine vapor pressures over the samples at 70°C were selected for presentation in Figs. 2 and 3. Figure 2 surveys the whole range of iodine to coal weight ratios whereas Fig. 3 concentrates on the region up to an iodine to coal ratio of three. Some of the salient features of the data in Figs. 2 and 3 are the following: (1) The saturation vapor pressure of iodine over the samples at 70°C is approximately 9.5 torr. At these vapor pressures, solid iodine is, no doubt, present. Since the literature value for the vapor pressure of iodine at 70°C is 8.2 torr[8], it is likely that experimental errors in our technique resulted in values of the vapor pressure which are approximately 15% high. (2) The behavior of the graphite and that of the anthracite coal are significantly different from the behavior of the other substances. Graphite requires relatively little iodine to reach saturation. Anthracite absorbs more iodine than graphite before saturation occurs. The curves for anthracite and graphite are similar in that, up to the point of
S.
ARONSON et al.
Table 1. Chemical analysis of coal samples As-received samples (weight percent) moisture Anthracite ( Pennsylvmia)
*
c
12.4
79.9
N .a
S
1.1
.5
c 92.4
9 2.7
0 1.7
low volatile bituminous
0.6
3.9
86.9
1.3
.6
91.0
4.4
1.3
Eagle (West Virginia)
mediumvolatile bituminous
0.5
7.8
80.1
1.1
.a
87.4
5.2
3.8
Hernshaw (west Virginia)
high volatile bituminous
0.9
3.2
al.0
1.5
.7
84.5
5.4
6.1
14.6
5.9
55.8
1.1
1.2
70.2
4.9
21.0
6.6
-
6.0
4
7.2
-
6.4
-
,
5.6-
4.6
-
4.0
-
3.2
3
f :: g
2
2.70
2.60
2.90
3.00
3.10
lO’/T(“K)
Fig. 1. Iodine vapor pressures over coal-iodine mixtures: The letters E, & L and P refer respectively to Eagle, Hemshaw, lignite and Pocahontas. The two numbers refer to the weight ratio of iodine to coal.
1 0
j
ANTHRACITE
1
2.60
0
GRAPHITE
/
0 :
and ash-free (weight percent)
Pocahontas No.3 (West Virginia)
Lignite (North Dakota)
;; 2
Moisture-free
2
I 4
I 6
I 6
1
10
I 12
1
14
I 16
Fii. 2. Iodine vapor pressures over iodinscoal mixtures: I, SP-1 graphite; 0, anthracite; A, lignite; X, Pocahontas; Cl, Eagle; 0, Hemshaw.
saturation, the iodine vapor pressure increases monotonically with increasing iodine content. (3) The three bituminous coals and lignite show remarkably similar behavior. A horizontal step occurs from an iodine to coal weight ratio of approximately 0.5-2 for all four sub-
Fig. 3. Iodine vapor pressures over iodintioal mixtures (expanded scale): I, SP-1 graphite; 0, anthracite; A, lignite; X, Pocahontas; 0, Eagle; 0, Hemshaw.
stances. This region of approximately constant vapor pressure is seen more clearly in Fig. 3 and is the most interesting observation in this study. The horizontal step ends at an 1~to coal ratio of approximately 2.5 for the three grades of bituminous coal and at approximately 2.0 for the lignite. The significance of this region of constant pressure can be better understood if we first consider the manner in which iodine reacts with polynuclear aromatic hydrocarbons [6]. In Table 2 some data on the formation of iodine complexes are given for a number of hydrocarbons. The structure of each hydrocarbon is shown in the second column where each circle represents a benzene ring. The compositions of the iodinehydrocarbon complexes are shown in the third column. The vapor pressure of iodine over each mixture of a hydrocarbon and its complex at 70°C is given in the final column. The molecular ratio of iodine to hydrocarbon in the complexes ranges from one to three. Perylene forms two complexes, Pe(I31.5and Pe(Izh.9.Picene does not form a complex. The heats of formation of the complexes listed in Table 2 are in the same range as the heats of absorption of iodine by coal, -11 to -15 kcal/mole L[6]. A possible explanation for the presence of a region of constant pressure in Fig. 3 is that an important constituent of the bituminous coals and lignite is a particular, solid chase r-m ~~which forms a comnlex 1 with iodine. Comolexation I
95
Absorption of iodine by coal and lignite Table 2. Data on polynuclear aromatic hydrocarbon-iodine charge transfer complexes Hydrocarbon
structure
PYm=(Py)
#
pentacene(Pn~ perylene(
Compounds Present
Pe)
000 00
coronene(Cn)
ovalene( OV) Rubrene(Rb) Picene(Pc)
pY(I2)2.,3+ Py
3.9
pn(12)a.5 + pn
0.6
Pe(Ia)a&! +
1.8
Pe(12)lm5
Pe(I2)1.5 + Pe
0.9
%
BP(12)3e0 + BP
2.2
+S
Cn(Ia)I*o + cn
0.3
#!8
OVER,
+ ov
0.3
Xb(12)le5 + Rb
not
none
not measured
perylene(Pe) 1.12 benzoperylene(BP)
12 P;;ss;w&;ce(Torr) "
35 &
begins at an iodine to coal ratio of 0.5 and all of this phase has become complexed at a ratio above 2. This solid phase does not appear to be one of the hydrocarbons listed in Table 2 since the vapor pressure in the region of the step in Fig. 3, 1.2 torr, does not correspond exactly to the vapor pressure of any of the complexes in Table 2. The possibility is not excluded, however, that one of the complexes in Table 2 is present in coal in modified form. It is unlikely that the horizontal step in Fig. 3 is due to surface adsorption or hlling of pores. Juhola has recently studied the adsorption of iodine on activated carbons[9]. His results indicate that at iodine pressures below that of the saturated vapor, uptake of iodine results from surface adsorption. The step in our data in Fig. 3 occurs at a P/P” value of less than 0.2 which is in the region of monolayer coverage. If we assume a maximum surface area for our coals of 100m*/g[ lo], a monolayer of iodine would correspond to 0.2 g 12/gcoal. The observed absorption is a factor of 10 higher than this amount. In addition, surface adsorption or tilling of pores would not account for the presence of a region of constant vapor pressure or for the similarity of the behavior of the bituminous coals and lignite. The conclusion presented here that an important constituent of the bituminous coals and lignite is a particular, although unidentified, solid phase appears to be at variance with the bulk of evidence in the literature which indicates that coals are highly heterogeneous and complex
measured
substances. Further substantiation is required that complex formation is occurring. A full explanation of the data in Figs. 2 and 3 would also have to account for the considerable absorption of iodine by bituminous coals and lignite in the regions outside the vicinity of the step, and for the interaction of iodine with anthracite coal and graphite. REFERFNCFS
1. Tingey Cl.L. and Morrey J. R., Coal Structure and Reactioity. Battege Pacific Northwest LaboratoriesReport (December 1973).
2. Gould R. F. (Ed.), Coal science, In Advances in Chemistry Series No. 55. American Chemical Society, Washington, D.C. (l96@ 3. Lowry H. H. (Ed.), Chemistry of Coal Unitization, Supplementary Volume. Wiley, New York (1963). 4. Mulhken R. S. and Person W. B., Molecular Complexes. Wiley, New York (1%9). 5. Foster R., Organic Charge Transfer Complexes. Academic Press, London (1%9). 6. Aronson S., Sinensky Cl.,Langsam Y. and Binder M., J. Inorg. Nucl. Chem. 38, 407 (1976). 7. Mazumdar B. K., Fuel 51, 284 (1972). 8. Handbook of Chemistry and Physics, 51st E&I, p. D-168.
ClevelandRubber, Cleveland,Ohio (1971). 9. Juhola A. J., Carbon 13, 437 (1975). 10. Dryden J. G. C., In Chemistry ofCoalUtilization, Supplementary Volume (Edited by H. H. Lowry), p. 232. Wiley, New
York (1%3).
Pergamon Press.
Printed in Great Britain
ABSORPTION OF IODINE BY COAL AND LIGNITE-t S. ARONSON, A. SCHWEBEL and G. SINENSKYS BrooklynCollege,CityUniversityof NewYork,Brooklyn,NY 11210,U.S.A. (Received 5 January 1976) Abstract-The vapor pressure of iodine over mixtures of iodine and various coals has been measuredat temperatures of 65-9X. Ligniteand bituminouscoalsexhibitsimilarbehaviorin their absorption of iodine whereas the behavior of anthracite coal is different. A region of constant vapor pressure occurs in the reaction between iodine and the bituminous coals and lignite. Complex formation between the iodine and coal is postulated.
1. INTEODUCTION
iodine powder with coal in definite weight ratios ranging from an iodine to coal ratio of 1: 10 to a ratio of 16: 1. The coals were used in their as-received state with the exception of the anthracite which was pulverized before use to pass through aNo. 50 sieve. The absorption of iodine by a natural graphite powder, SP-1 spectroscopic graphite, obtained from the National Carbon Company, was studied for comparison purposes. The various mixtures were sealed in glass tubes and were heated at 90°C for at least 48 hr. Iodine vapor pressures over the various mixtures were measured calorimetrically using a Cary-14 Spectrophotometer. Measurements over a range of temperatures were made using special teflon-stoppered quartz cells with compartments through which hot water could be circulated. The technique has been described elsewhere [6].
The structure of coal is complex and heterogeneous [l-3]. It is generally accepted that a major portion of the carbon in coal is part of a network of polynuclear aromatic compounds. Most of the information about the chemical nature of coal has been obtained by degradative processes such as oxidation or volatilization which have altered the original structure. Iodine reacts reversibly with polynuclear aromatic hydrocarbons to form complexes in which several molecules of iodine are attached to each hydrocarbon molecule[C 61. The iodine can be removed by slight heating of the complex with the restoration of the hydrocarbon to its original form. It seemed evident, therefore, that iodine absorption might be used as a nondestructive technique for obtaining information about the quantity and types of aromatic compounds present in coal. The absorption of iodine by anthracite and bituminous coal and lignite was, therefore, investigated.
3. RESULTS ANDDISCUSSION
2. EXPERIMENTAL The types of coal used in this study are shown in Table 1 together with analytical data on their chemical constituents. The chemical analyses were performed by the Schwarzkopf Microanalytical Laboratory, Woodside, N. Y. The middle section of Table 1 gives information on the composition of the samples as they were received from the suppliers. The iodine absorption work was done on the as-received samples. A calculation of the weight percent of carbon, hydrogen and oxygen was also made on the basis of ash-free and moisture-free conditions since these data reflect more accurately the rank of the various coals. As one proceeds from higher to lower ranked coals, the carbon content is expected to decrease and the hydrogen and oxygen wntents are expected to increase. The atomic ratio of hydrogen to carbon is also a significant measure of coal rank[7]. This parameter consistently increased with decreasing coal rank for our samples. The coal-iodine samples were prepared by mixing tA portionof the work described in this paper was presented at the Twelfth Biennial Conference on Carbon, Pittsburgh, Pa., 28 July 1975and has been published in abstract form in the Proceedings of the Conference, page 61. SMr. Sinensky performed this work as a participant in the National Science Foundation Undergraduate ResearchProject at BrooklynCollege in the spring and summer of 1974. 93 CAR Vol. I4 No. 2-A
Vapor pressure measurements were made at temperatures of 55-95°C on approximately fifty different mixtures of iodine with anthracite and bituminous coals, lignite and graphite. Some typical plots of the natural logarithm of iodine pressure against reciprocal temperature are shown in Fig. 1. A value for the heat of absorption of iodine vapor by the solid can be calculated from the slope of each curve in Fig. 1. The values obtained were in the range of -11 to -15 kcal/mole IZ. The iodine vapor pressures over the samples at 70°C were selected for presentation in Figs. 2 and 3. Figure 2 surveys the whole range of iodine to coal weight ratios whereas Fig. 3 concentrates on the region up to an iodine to coal ratio of three. Some of the salient features of the data in Figs. 2 and 3 are the following: (1) The saturation vapor pressure of iodine over the samples at 70°C is approximately 9.5 torr. At these vapor pressures, solid iodine is, no doubt, present. Since the literature value for the vapor pressure of iodine at 70°C is 8.2 torr[8], it is likely that experimental errors in our technique resulted in values of the vapor pressure which are approximately 15% high. (2) The behavior of the graphite and that of the anthracite coal are significantly different from the behavior of the other substances. Graphite requires relatively little iodine to reach saturation. Anthracite absorbs more iodine than graphite before saturation occurs. The curves for anthracite and graphite are similar in that, up to the point of
S.
ARONSON et al.
Table 1. Chemical analysis of coal samples As-received samples (weight percent) moisture Anthracite ( Pennsylvmia)
*
c
12.4
79.9
N .a
S
1.1
.5
c 92.4
9 2.7
0 1.7
low volatile bituminous
0.6
3.9
86.9
1.3
.6
91.0
4.4
1.3
Eagle (West Virginia)
mediumvolatile bituminous
0.5
7.8
80.1
1.1
.a
87.4
5.2
3.8
Hernshaw (west Virginia)
high volatile bituminous
0.9
3.2
al.0
1.5
.7
84.5
5.4
6.1
14.6
5.9
55.8
1.1
1.2
70.2
4.9
21.0
6.6
-
6.0
4
7.2
-
6.4
-
,
5.6-
4.6
-
4.0
-
3.2
3
f :: g
2
2.70
2.60
2.90
3.00
3.10
lO’/T(“K)
Fig. 1. Iodine vapor pressures over coal-iodine mixtures: The letters E, & L and P refer respectively to Eagle, Hemshaw, lignite and Pocahontas. The two numbers refer to the weight ratio of iodine to coal.
1 0
j
ANTHRACITE
1
2.60
0
GRAPHITE
/
0 :
and ash-free (weight percent)
Pocahontas No.3 (West Virginia)
Lignite (North Dakota)
;; 2
Moisture-free
2
I 4
I 6
I 6
1
10
I 12
1
14
I 16
Fii. 2. Iodine vapor pressures over iodinscoal mixtures: I, SP-1 graphite; 0, anthracite; A, lignite; X, Pocahontas; Cl, Eagle; 0, Hemshaw.
saturation, the iodine vapor pressure increases monotonically with increasing iodine content. (3) The three bituminous coals and lignite show remarkably similar behavior. A horizontal step occurs from an iodine to coal weight ratio of approximately 0.5-2 for all four sub-
Fig. 3. Iodine vapor pressures over iodintioal mixtures (expanded scale): I, SP-1 graphite; 0, anthracite; A, lignite; X, Pocahontas; 0, Eagle; 0, Hemshaw.
stances. This region of approximately constant vapor pressure is seen more clearly in Fig. 3 and is the most interesting observation in this study. The horizontal step ends at an 1~to coal ratio of approximately 2.5 for the three grades of bituminous coal and at approximately 2.0 for the lignite. The significance of this region of constant pressure can be better understood if we first consider the manner in which iodine reacts with polynuclear aromatic hydrocarbons [6]. In Table 2 some data on the formation of iodine complexes are given for a number of hydrocarbons. The structure of each hydrocarbon is shown in the second column where each circle represents a benzene ring. The compositions of the iodinehydrocarbon complexes are shown in the third column. The vapor pressure of iodine over each mixture of a hydrocarbon and its complex at 70°C is given in the final column. The molecular ratio of iodine to hydrocarbon in the complexes ranges from one to three. Perylene forms two complexes, Pe(I31.5and Pe(Izh.9.Picene does not form a complex. The heats of formation of the complexes listed in Table 2 are in the same range as the heats of absorption of iodine by coal, -11 to -15 kcal/mole L[6]. A possible explanation for the presence of a region of constant pressure in Fig. 3 is that an important constituent of the bituminous coals and lignite is a particular, solid chase r-m ~~which forms a comnlex 1 with iodine. Comolexation I
95
Absorption of iodine by coal and lignite Table 2. Data on polynuclear aromatic hydrocarbon-iodine charge transfer complexes Hydrocarbon
structure
PYm=(Py)
#
pentacene(Pn~ perylene(
Compounds Present
Pe)
000 00
coronene(Cn)
ovalene( OV) Rubrene(Rb) Picene(Pc)
pY(I2)2.,3+ Py
3.9
pn(12)a.5 + pn
0.6
Pe(Ia)a&! +
1.8
Pe(12)lm5
Pe(I2)1.5 + Pe
0.9
%
BP(12)3e0 + BP
2.2
+S
Cn(Ia)I*o + cn
0.3
#!8
OVER,
+ ov
0.3
Xb(12)le5 + Rb
not
none
not measured
perylene(Pe) 1.12 benzoperylene(BP)
12 P;;ss;w&;ce(Torr) "
35 &
begins at an iodine to coal ratio of 0.5 and all of this phase has become complexed at a ratio above 2. This solid phase does not appear to be one of the hydrocarbons listed in Table 2 since the vapor pressure in the region of the step in Fig. 3, 1.2 torr, does not correspond exactly to the vapor pressure of any of the complexes in Table 2. The possibility is not excluded, however, that one of the complexes in Table 2 is present in coal in modified form. It is unlikely that the horizontal step in Fig. 3 is due to surface adsorption or hlling of pores. Juhola has recently studied the adsorption of iodine on activated carbons[9]. His results indicate that at iodine pressures below that of the saturated vapor, uptake of iodine results from surface adsorption. The step in our data in Fig. 3 occurs at a P/P” value of less than 0.2 which is in the region of monolayer coverage. If we assume a maximum surface area for our coals of 100m*/g[ lo], a monolayer of iodine would correspond to 0.2 g 12/gcoal. The observed absorption is a factor of 10 higher than this amount. In addition, surface adsorption or tilling of pores would not account for the presence of a region of constant vapor pressure or for the similarity of the behavior of the bituminous coals and lignite. The conclusion presented here that an important constituent of the bituminous coals and lignite is a particular, although unidentified, solid phase appears to be at variance with the bulk of evidence in the literature which indicates that coals are highly heterogeneous and complex
measured
substances. Further substantiation is required that complex formation is occurring. A full explanation of the data in Figs. 2 and 3 would also have to account for the considerable absorption of iodine by bituminous coals and lignite in the regions outside the vicinity of the step, and for the interaction of iodine with anthracite coal and graphite. REFERFNCFS
1. Tingey Cl.L. and Morrey J. R., Coal Structure and Reactioity. Battege Pacific Northwest LaboratoriesReport (December 1973).
2. Gould R. F. (Ed.), Coal science, In Advances in Chemistry Series No. 55. American Chemical Society, Washington, D.C. (l96@ 3. Lowry H. H. (Ed.), Chemistry of Coal Unitization, Supplementary Volume. Wiley, New York (1963). 4. Mulhken R. S. and Person W. B., Molecular Complexes. Wiley, New York (1%9). 5. Foster R., Organic Charge Transfer Complexes. Academic Press, London (1%9). 6. Aronson S., Sinensky Cl.,Langsam Y. and Binder M., J. Inorg. Nucl. Chem. 38, 407 (1976). 7. Mazumdar B. K., Fuel 51, 284 (1972). 8. Handbook of Chemistry and Physics, 51st E&I, p. D-168.
ClevelandRubber, Cleveland,Ohio (1971). 9. Juhola A. J., Carbon 13, 437 (1975). 10. Dryden J. G. C., In Chemistry ofCoalUtilization, Supplementary Volume (Edited by H. H. Lowry), p. 232. Wiley, New
York (1%3).