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Oxidation of carbonaceous matter—I
@X%6223/8313.W+.XI 0 1983PergamonPress Ltd.
Co&on Vol. 21,No. 6, pp. 559-564.1983 Printedin Great Britain.
OXIDATION OF CARBONACEOUS MATTER-I ELEMENTAL ANALYSIS (C, H, 0) AND IR SPECTROMETRY D. JOSEPHand A. OBERLIN Laboratoire Marcel Mathieu, ER 131du CNRS, UER Sciences, 45046-OrleansCedex, France (Received 14 September 1982) Abstract-Products of various elemental composition (1.56> (H/C) at > 0.45 and 0 <(O/C) at < 0.45)were oxidized under an air flow at various temperatures (from 150to 280°C)and for various times (from half an hour to one week). When plotted in a Van Krevelen diagram (H/C vs O/C), their elemental composition follows an oxidation path, the slope of which depends only on the original (O/H) atomic ratio and on the oxidizing temperature. Oxidation reactions have an “apparent activation energy” of 2040 kcal/mole. Cross-linking may be due either to ether bonding or to hvdroeen bonding as indicated bv, IR soectrometrv. The final product, named “oxychar”, has a constant composition .
(H/C,‘o/c =0.5y.
1. INTRODUCTION Oxidation of carbonaceous matter occurs very commonly in nature and is frequently used in manufacturing proces-
ses. The influence of the oxygen content on the properties of various materials during the carbonization and graphitization processes [ l] is generally accepted. Coking coals, for instance, lose their coking quality when exposed to air[2]. Oxygen rich precursors after heat-treatment produce non graphitizing carbons (hard carbons) and oxygen poor ones produce “soft” graphitizing carbons[l]. Transmission electron microscopy (TEM) [3,4] and X-ray diffraction techniques[l, 5,6] have shown that their specific textural arrangements are different. Up to now hard and soft carbons were considered as entirely different materials. The present work (Part I) is devoted to the study of the changes in elemental composition (C, H, 0) of various materials resulting from an oxidizing treatment under an air flow at various temperatures (from 150to 280°C)and for various periods of time. Some samples were analyzed also by infrared spectrometry to study the changes in functional group content. To study materials with various elemental compositions, a series of samples was chosen with the (H/C) atomic ratio ranging from 0.45 to 1.56 and (O/C) from nearly 0 to 0.45. The samples were either natural materials such as sporopollenin, lignite and kuckersite, or industrial products such as an asphalt, an insoluble fraction extracted from a heat treated pitch using anthracene-oil and a purified mesophase pitch. A second series of samples was obtained by heat-treating some of the initial raw products (sporopollenin and kuckersite) at various temperatures to make the elemental composition of their pyrolysis residues similar to that of the other products before the oxidizing treatment. This series was prepared to compare the behaviour of raw materials to that of heat-treated products of similar elemental composition when oxidized under the same conditions. tsingle mesophase sphere Garnier, Cerchar, France.
extraction
was carried
out by Mme
Whether oxidized or not, all the samples, pyrolysis residues included, were carbonized at 1000°C(Part II) and examined by transmission electron microscopy to determine the extent of their molecular orientation and to estimate their homogeneity. Only some selected samples were then heat-treated to 2900°C under an argon flow and examined by TEM and X-rays to determine their microtextural arrangement and their ability to graphitize. 2. EXPERIMENTAL 2.1 Sampling of the initial materials
2.1.1 Raw materials. Six carbonaceous materials differing in origin and in elemental composition were chosen: lignite (H/C = 1.13, O/C = 0.45) sampled from an Oligocene formation near Cologne (West Germany); sporopollenin (H/C = 1.56, O/C = 0.35) made of outer cuticles of Lycopodium clavatum spores; kuckersite (H/C = 1.37, O/C = 0.14), a kerogen extracted from the Ordovician Basin of Estonia (USSR); an asphalt extracted from a light Arabian Aramco oil (H/C = 1.06,0/C = 0.004); an anthracene-oil insoluble fraction (H/C = 0.45, O/C = 0.04) extracted from a heat treated coal-tar-pitch; and non coalesced mesophase spheres (H/C = 0.45, O/C = 0.02), also extractedi from a heat treated coal-tar-pitch. 2.1.2 Pyrolysis residues. Figures 1 and 2 show the van Krevelen diagrams [9] and the DTA curves for sporopollenin and kuckersite when heat-treated under a nitrogen flow with a 4°C min-’ heating rate[7,8]. Samples A to G were obtained by quenching at various steps of the DTA curves. They refer to different thermal events: the softening stage (points C and D for sporopollenin and point A for kuckersite), the point of maximum tar production (points D and E for sporopollenin, point B for kuckersite) and the semi-coke stage (points F and G for sporopollenin, points C, D, E, F and G for kuckersite). 2.2 Oxidizing treatment Fine-grained samples were deposited in thin layers in a petri dish and held inside a ventilated furnace for increasing periods of time. The asphalt was first dissolved in toluene and then dried in a petri dish before oxidation. 559
D. JOSEPH and A. 0a~a~1~
Fig. 1. van Krevelen dia~r~ and DTA curve showing representative points for pyrolysis residues of sporopoiIenin.
Fig, 2. van Kreveien diagram and DTA curve showing representative points for pyrolysis residues of kuckersjte.
d
Fig. 3. van Krevelen diagrams s~owjngoxidation pathat ZOO”C,(a) lignite,
s~oropol~enia,(c) kuckersite, (d)aspha~t, anthracen~-oii insoluble fraction fanthr. ins.) and mesophase, (e) summary of the data, (f) coals from (9).
561
Oxidation of carbonaceous matter-1
the horizontal part of the curve. The evolution of sporopollenin is more complex (stars in Fig. 3b). During the first hour only hydrogen is released after which up, to 10hr, oxidation products of the raw materials were heat-treated there is a simultaneous loss of hydrogen and an increase in following the experimental procedures described by M. the oxygen content. Finally, there is only a slow increase in Villey[7,8]. A given quantity of material (150 mg) was oxygen content. The oxidation path for kuckersite (black deposited inside a graphite crucible and heat-treated under a nitrogen flow using a4”C min-’ heating rate up to 1000°C. circles in Fig. 3c) can be divided into two parts. First hydrogen is released, the oxygen content increasing (to This temperature was maintained for 15 min, the material 20-30 hr), followed by only a slow increase in the oxygen was then cooled slowly, ground and deposited on a grid for content, as observed for sporopollenin and lignite. The TEM observation. evolution of asphalt (empty squares in Fig. 3d) follows a 2.3.2 Graphitization. After carbonization, some samples were heat-treated under an argon flow up to 29OO”C, similar path, but the horizontal part (increase in the oxygen content) is reached after 70 hr, i.e. after a longer period of using a 20°C min-’ heating rate. They were then prepared time than for the other materials. The reactions of the for TEM observation and X-ray analysis. anthracene-oil insoluble fraction (crosses in Fig. 3d) and mesophase (empty circles in Fig. 3d) occur much more 3. RESULTS slowly, their (O/C) becoming only 0.1 after 164hr of 3.1 Changes in elemental composition 3.1.1 Oxidation of raw materials. The elemental analysis oxidation. The anthracene-oil insoluble fraction seems to follow a strictly horizontal path whereas mesophase data for the raw initial products are plotted on a van spheres show a slightly inclined slope. Figure 3(e) shows Krevelen diagram (Figs. 3a, b, c, d). These samples were oxidized at 200°C for increasing periods of time and their that after a very long time, all the samples reach the same subsequent elemental analysis data are plotted on the same ultimate state (point 0), except the mesophase which diagram. Figure 3(e) brings all these data together for probably reaches an ultimate composition point near the comparison. Figure 3(f) reproduced from (9) shows the point 0’ for coals as seen in Fig. 3(f) (9). results obtained by van Krevelen for coals. From the above data (Fig. 3e), it can be concluded that The process of oxidation of lignite (black triangles in Fig. the oxidation process corresponds either to an increase in 3a) can be divided into two parts. During the first part (up to the oxygen content or a loss of hydrogen or both. It can be 8 hr), a fast release of hydrogen is mostly observed, after concluded that the slope of the oxidation path differs which the oxygen content increases slowly, as shown by notably between products and depends on the elemental 2.3 Heat-treatment conditions 2.3.1 Carbonization stage. The pyrolysis residues and
d l
00 _A_---
a
--)--
Fig. 4. (a and b): A(H/C)t A(OIC)plottedversus the logarithm of time, (c and d): (O/H)plotted versus the logarithm of time.
D. JOSEPH and A. OBERLIN
562
Table 1. Durationof the oxidationstages END OF STAGE 2
END OF STAGE 1
PRODUCT
Lignite
6,5*
Sporopollenin
10,5*
1,5*
Kuckersite
2,s
20*
Asphalt
18
70*
not
Anthracene-oilInsoluble Hesophase
*Stage limits “an Krevelen
at
reached 164
b
-
which can diagrams
be
detected
also
in
the
composition of the initial material. The ultimate point of the oxidation path is the same for lignite, sporopollenin, kuckersite and probably for both asphalt and the anthracene-oil insoluble fraction (H/C = O/C - OS). This last peculiar product is called “oxychar”, in analogy with the u&mate oxidation residue of coals, called “oxycoal” (9). The rate of reaction aIso depends on the elemental composition of the intiai material. The Iarge differences in the reaction rates and the fact that hydrogen loss seems to be equivalent to an increase in oxygen content suggest that the following parameters should be taken into account. First the oxidation time, t and second either the value A(H/C) t A(O/C) or the atomic ratio (O/H). These two parameters were chosen because they were consistent with the fact that a hydrogen loss has the same effect as an increase in oxygen content. They can be plotted versus the logarithm of time, and the curves can be compared(Fig. 4a, b, c, d). Three stages of oxidation can then be recognized for all the materials, except for the mesophase and the anthracene-oil insolubles (their reaction rate is too low). The last stage is more visible for sporopollenin and kuckersite in Fig. 4(a) whereas sporopollenin clearly shows the three stages in Fig. 4(c). Similarly, the three stages are more obvious in Fig. 4(b) than in Fig. 4(d) for asphalt. The end of each stage, which is not always clear in a van Krevelen diagram (Figs. 3a, b, c, d), is emphazized by Figs. 4(a, b, c, d). Lengths of time can be evaluated and are given in hours in Table 1. Figures 4(a, b, c, d) also show that the curves for all the materials have the same slope for each of the stages. The slope is Iow for the first and the third stages and steep for the second. If the slopes for the mesophase and the anthracene oil insolubles are compared to those of the other curves, it can be inferred that only the first stage is present for those materials. The (O/H) ratio appears to be the most suitable parameter for characterizing oxidation, since it represents the elemental composition of the material before and during oxidation. On the other hand, A(H/C) t A(O/C) is a parameter independent of the initial elemental composition of the material chosen. In addition to the standard oxidizing treatment at 2OO”C, lignite and sporopollenin were oxidized at 180, 220 and 25O”C,kuckersite at 180 and 250°C and asphalt at 250°C.
Fig. 5. van Krevelen diagrams showing the oxidation paths at different temperatures for, (a) lignite, (b) sporopollenin, (cc) kuckersite.
The results are plotted on a van Krevelen diagram in Figs. S(a, b, c) (except for asphalt for which the reaction rate is so low that it does not show any significant change from 200 to 25O’C).The van Krevelen diagrams of all the materials exhibit two ch~cte~stics. When the temperature of the oxidizing ~eatment is increased, the slope of the second part of the curve (second stage described above in Fig. 4) increases, and the length of the stage is shortened. For the high temperature, oxidation thus corresponds to a higher degree of dehydrogenation. The rate of reaction for all stages increases as temperature increases. If the reaction is assumed to be first order, the “apparent activation energy” can be roughly estimated by using the Arrhenius equation. At all stages, all the materials have about the same “20-40 kcal mole-’ apparent activation energy” which is the same rangeas that found for coals[P].
563
Oxidation of carbonaceous matter-l
I
Fig. 6. van Krevelen diagram showing oxidation paths of various pyrolysis residues of sporopollenin at 200°C.
0.)
0.1
0.1
Fig. 8. van Krevelen diagram showing the superposition of the oxidation paths (a) of sporopollenin (b) to raw kuckersite; of sporopollenin F to kuckersite E and raw asphalt.
a
H
‘cl”’
I
1.5
Raw kuckersite
u”)
b
1110 cm-’ ,co,
6
[Sporopollcnlnl 110.
Fig. 7. van Krevelen diagram showing oxidation paths of various pyrolysis residues of kuckersite at 200°C.
e
’
3.1.2 Oxidation of pyrolysis residues. As shown in Fig. 6 for sporopollenin and in Fig. 7 for kuckersite, the oxidation paths for the residues of pyrolysis ABC, etc.. . follow the same trend as for the raw materials. The further away from the original sample the representative point of each pyrolysis residue before oxidation is taken, i.e. the higher the pyrolysis temperature, the lower the slope of the representative oxidation line. At the same time the reaction rate decreases. These results are quite similar to those obtained in 3.1.1. They suggest that the slope of the oxidation path is closely related to the elemental composition of the sample before oxidation. If so, the slope of the oxidation path can be expected to be the same for all the samples having the same elemental composition, whatever their origin or their heat-treatment temperature before oxidation. This is supported by Fig. 8, for instance, which shows that sporopollenin heat-treated to the point B, when oxidized, follows a path which can be superimposed on that of oxidized kuckersite. Kuckersite heat-treated to E, and sporopollenin heat-treated to F have about the same elemental analysis and the same oxidation line, which is also the same for asphalt. 3.2 Changes in functional groups Some complementary information is derived from infrared spectrometry. Figures 9(a, b, c) show the changes in
____--___----
,“--,’
100.
/+
,_.-
/’
--
/-
I,
1 :,I
6o
‘i
(TTGZ]
m
I’
I? 4oi,, ,,~,__
_-_
--
___-----------
C
‘C-OR
t
_ /’
-. .20
-.
4 ----__------
L
SO
100
1so t ,ho”r*,
Fig. 9. Infrared spectrometry data (a) K 2920(CH aliph.) plotted versus oxidation time, (b) K 1710(CO) plotted versus oxidation time, (c) Ic_~a index (IO) plotted vs oxidation time (for evaluating the frequency of ether functional groups).
564
D. JOSEPH and A. OBERUN
the aliphatic CH groups (2~Ocm-‘), in the carbonylcarboxyl CO groups (1710cm-‘), and in an empirical index Ic-oR, all plotted versus theoxidation time. IC-oRis a rough estimate of the occurrence of ether functional groups and is expressed[lO] by the difference between the oxygen weight percentage and the half value of K1710. K2920 and K1710 evolve steadily for sporopollenin, kuckersite and asphalt, but much more slowly for asphalt. 1~~~ decreases for sporopollenin and kuckersite, and increases for asphalt. By comparing K1710 to IGQR, the oxidation of asphalt can be attributed to a cross-tinking by ether groups, the number of which increases as oxidation progresses. On the contrary, these groups decrease continuously when kuckersite and sporopollenin are oxidized. The crosslinking must thus be attributed to hydrogen bonds. It changes as the carboxyl groups concentrateIll].
a Gt Sporopdlcnin
K 2920cm-1
I
tC!i ahph.)
0 Kuckersite :: Asphalt
80
40,
H welqht
2
4
6
8
,
10
b 0
Spcropollcnin
l UucktrsitC ST;Asphalt
Figures iO(a and b) show the evolution of K1710 and K2920 plotted vs the weight percentage of oxygen and of hydrogen. These figures show that the content in the functional groups tends to be constant near the “oxychar” formation. For instance K1710 of sporopollenin and kuckersite reaches its maximum for oxygen= 38% whereas K2920 reaches its minimum for hydrogen = Z3%. The asphalt oxidation rate is so low that its curve cannot be extrapolated. 4. CONCLUSIONS All the carbonaceous materi~s studied here show three stages of oxidation in the range 1.5>(H/C)>0.5 and 0 > (O/C) > 0.5. In the first only hydrogen is released and the oxygen content does not change, or hydrogen is released and the oxygen content increases. The second step is always a hydrogen loss combined with an increase in oxygen content. Finally, the third step corresponds to a slow increase in the oxygen content only. During the second stage, the slope of the oxidation path decreases as (O/H) of the initial sample decreases. Materials having the same initial (O/H) atomic ratio follow the same oxidation path. Infrared spectrometry data show that as oxidation progresses, the diphatic functions groups decrease as carbonyI-~arboxyl groups increase. Cross-linking is assumed to be due to an ether bonding in asphalt and to hydrogen bonds in sporopoilenin and in kuckersite. For all the materials at any stage of oxidation, the “apparent activation energy” was estimated to be about 20-40 kcal mole-‘. The ultimate product after oxidation is identical for sporopollenin, kuckersite and probably for asphalt and the anthracene oil-insoluble fraction. It is an “oxychar” of elemental composition: (H/C) = 0.5, (O/C) = 0.5, i.e. (O/H) ==1. In the second part of this paper, the elemental composition of some of the raw products and their pyrolysis residues will be correlated to their structure and microtexture acquired during the canonization and graphiti~ation processes. The mechanism of the oxidation process will be discussed. REFEREWES
I. R. E. Franklin, Proc. Roy. Sot. 209, 1% (1951).
80.
40-
0 weqnt I &
10
20
30
4’0
Fig. 10. Infrared spectrometry data. (a) K 2920(CHaliph.)plotted
versus H weightper cent,(b) K 1710(CO)plottedversus 0 weight .. per cent.
2. R. Loison, P. Foch and A. Boyer, IO Le Coke. Masson Paris (1970). 3. A. Oberlin, J. L. Boulmierand M. Viiley, in Kerogen (Edited by B. Durand). Technip, Paris (1979). 4. M. Monthioux, M. Oberlin, A. Oberlin. X. Bourrat and R. Bouiet, Carbon, 24, 167(1982). 5. R. E. Franklin, Acta Cryst. 3, 107(1950). 6. R. E. FrankIin, Acta Cryst. 4, 253 (195I). 7. M. Viiiey, A. Oberlin and A. Combaz, Carbon 17,77 (1979). 8. A. Oberlin, M. Viiley and A. Combat, Carbon 18,347 (1980). 9. D. W. van Kreveien, Coal. Elvesier, Amsterdam (1961). 10. P. Robin, Thesis Doctorate, Louvain (1975). 11. P. G. Rouxhet, M. Viiiey and A. Oberlin, Geochim. Cosmochim. Acta 43, 1705(1979).
Co&on Vol. 21,No. 6, pp. 559-564.1983 Printedin Great Britain.
OXIDATION OF CARBONACEOUS MATTER-I ELEMENTAL ANALYSIS (C, H, 0) AND IR SPECTROMETRY D. JOSEPHand A. OBERLIN Laboratoire Marcel Mathieu, ER 131du CNRS, UER Sciences, 45046-OrleansCedex, France (Received 14 September 1982) Abstract-Products of various elemental composition (1.56> (H/C) at > 0.45 and 0 <(O/C) at < 0.45)were oxidized under an air flow at various temperatures (from 150to 280°C)and for various times (from half an hour to one week). When plotted in a Van Krevelen diagram (H/C vs O/C), their elemental composition follows an oxidation path, the slope of which depends only on the original (O/H) atomic ratio and on the oxidizing temperature. Oxidation reactions have an “apparent activation energy” of 2040 kcal/mole. Cross-linking may be due either to ether bonding or to hvdroeen bonding as indicated bv, IR soectrometrv. The final product, named “oxychar”, has a constant composition .
(H/C,‘o/c =0.5y.
1. INTRODUCTION Oxidation of carbonaceous matter occurs very commonly in nature and is frequently used in manufacturing proces-
ses. The influence of the oxygen content on the properties of various materials during the carbonization and graphitization processes [ l] is generally accepted. Coking coals, for instance, lose their coking quality when exposed to air[2]. Oxygen rich precursors after heat-treatment produce non graphitizing carbons (hard carbons) and oxygen poor ones produce “soft” graphitizing carbons[l]. Transmission electron microscopy (TEM) [3,4] and X-ray diffraction techniques[l, 5,6] have shown that their specific textural arrangements are different. Up to now hard and soft carbons were considered as entirely different materials. The present work (Part I) is devoted to the study of the changes in elemental composition (C, H, 0) of various materials resulting from an oxidizing treatment under an air flow at various temperatures (from 150to 280°C)and for various periods of time. Some samples were analyzed also by infrared spectrometry to study the changes in functional group content. To study materials with various elemental compositions, a series of samples was chosen with the (H/C) atomic ratio ranging from 0.45 to 1.56 and (O/C) from nearly 0 to 0.45. The samples were either natural materials such as sporopollenin, lignite and kuckersite, or industrial products such as an asphalt, an insoluble fraction extracted from a heat treated pitch using anthracene-oil and a purified mesophase pitch. A second series of samples was obtained by heat-treating some of the initial raw products (sporopollenin and kuckersite) at various temperatures to make the elemental composition of their pyrolysis residues similar to that of the other products before the oxidizing treatment. This series was prepared to compare the behaviour of raw materials to that of heat-treated products of similar elemental composition when oxidized under the same conditions. tsingle mesophase sphere Garnier, Cerchar, France.
extraction
was carried
out by Mme
Whether oxidized or not, all the samples, pyrolysis residues included, were carbonized at 1000°C(Part II) and examined by transmission electron microscopy to determine the extent of their molecular orientation and to estimate their homogeneity. Only some selected samples were then heat-treated to 2900°C under an argon flow and examined by TEM and X-rays to determine their microtextural arrangement and their ability to graphitize. 2. EXPERIMENTAL 2.1 Sampling of the initial materials
2.1.1 Raw materials. Six carbonaceous materials differing in origin and in elemental composition were chosen: lignite (H/C = 1.13, O/C = 0.45) sampled from an Oligocene formation near Cologne (West Germany); sporopollenin (H/C = 1.56, O/C = 0.35) made of outer cuticles of Lycopodium clavatum spores; kuckersite (H/C = 1.37, O/C = 0.14), a kerogen extracted from the Ordovician Basin of Estonia (USSR); an asphalt extracted from a light Arabian Aramco oil (H/C = 1.06,0/C = 0.004); an anthracene-oil insoluble fraction (H/C = 0.45, O/C = 0.04) extracted from a heat treated coal-tar-pitch; and non coalesced mesophase spheres (H/C = 0.45, O/C = 0.02), also extractedi from a heat treated coal-tar-pitch. 2.1.2 Pyrolysis residues. Figures 1 and 2 show the van Krevelen diagrams [9] and the DTA curves for sporopollenin and kuckersite when heat-treated under a nitrogen flow with a 4°C min-’ heating rate[7,8]. Samples A to G were obtained by quenching at various steps of the DTA curves. They refer to different thermal events: the softening stage (points C and D for sporopollenin and point A for kuckersite), the point of maximum tar production (points D and E for sporopollenin, point B for kuckersite) and the semi-coke stage (points F and G for sporopollenin, points C, D, E, F and G for kuckersite). 2.2 Oxidizing treatment Fine-grained samples were deposited in thin layers in a petri dish and held inside a ventilated furnace for increasing periods of time. The asphalt was first dissolved in toluene and then dried in a petri dish before oxidation. 559
D. JOSEPH and A. 0a~a~1~
Fig. 1. van Krevelen dia~r~ and DTA curve showing representative points for pyrolysis residues of sporopoiIenin.
Fig, 2. van Kreveien diagram and DTA curve showing representative points for pyrolysis residues of kuckersjte.
d
Fig. 3. van Krevelen diagrams s~owjngoxidation pathat ZOO”C,(a) lignite,
s~oropol~enia,(c) kuckersite, (d)aspha~t, anthracen~-oii insoluble fraction fanthr. ins.) and mesophase, (e) summary of the data, (f) coals from (9).
561
Oxidation of carbonaceous matter-1
the horizontal part of the curve. The evolution of sporopollenin is more complex (stars in Fig. 3b). During the first hour only hydrogen is released after which up, to 10hr, oxidation products of the raw materials were heat-treated there is a simultaneous loss of hydrogen and an increase in following the experimental procedures described by M. the oxygen content. Finally, there is only a slow increase in Villey[7,8]. A given quantity of material (150 mg) was oxygen content. The oxidation path for kuckersite (black deposited inside a graphite crucible and heat-treated under a nitrogen flow using a4”C min-’ heating rate up to 1000°C. circles in Fig. 3c) can be divided into two parts. First hydrogen is released, the oxygen content increasing (to This temperature was maintained for 15 min, the material 20-30 hr), followed by only a slow increase in the oxygen was then cooled slowly, ground and deposited on a grid for content, as observed for sporopollenin and lignite. The TEM observation. evolution of asphalt (empty squares in Fig. 3d) follows a 2.3.2 Graphitization. After carbonization, some samples were heat-treated under an argon flow up to 29OO”C, similar path, but the horizontal part (increase in the oxygen content) is reached after 70 hr, i.e. after a longer period of using a 20°C min-’ heating rate. They were then prepared time than for the other materials. The reactions of the for TEM observation and X-ray analysis. anthracene-oil insoluble fraction (crosses in Fig. 3d) and mesophase (empty circles in Fig. 3d) occur much more 3. RESULTS slowly, their (O/C) becoming only 0.1 after 164hr of 3.1 Changes in elemental composition 3.1.1 Oxidation of raw materials. The elemental analysis oxidation. The anthracene-oil insoluble fraction seems to follow a strictly horizontal path whereas mesophase data for the raw initial products are plotted on a van spheres show a slightly inclined slope. Figure 3(e) shows Krevelen diagram (Figs. 3a, b, c, d). These samples were oxidized at 200°C for increasing periods of time and their that after a very long time, all the samples reach the same subsequent elemental analysis data are plotted on the same ultimate state (point 0), except the mesophase which diagram. Figure 3(e) brings all these data together for probably reaches an ultimate composition point near the comparison. Figure 3(f) reproduced from (9) shows the point 0’ for coals as seen in Fig. 3(f) (9). results obtained by van Krevelen for coals. From the above data (Fig. 3e), it can be concluded that The process of oxidation of lignite (black triangles in Fig. the oxidation process corresponds either to an increase in 3a) can be divided into two parts. During the first part (up to the oxygen content or a loss of hydrogen or both. It can be 8 hr), a fast release of hydrogen is mostly observed, after concluded that the slope of the oxidation path differs which the oxygen content increases slowly, as shown by notably between products and depends on the elemental 2.3 Heat-treatment conditions 2.3.1 Carbonization stage. The pyrolysis residues and
d l
00 _A_---
a
--)--
Fig. 4. (a and b): A(H/C)t A(OIC)plottedversus the logarithm of time, (c and d): (O/H)plotted versus the logarithm of time.
D. JOSEPH and A. OBERLIN
562
Table 1. Durationof the oxidationstages END OF STAGE 2
END OF STAGE 1
PRODUCT
Lignite
6,5*
Sporopollenin
10,5*
1,5*
Kuckersite
2,s
20*
Asphalt
18
70*
not
Anthracene-oilInsoluble Hesophase
*Stage limits “an Krevelen
at
reached 164
b
-
which can diagrams
be
detected
also
in
the
composition of the initial material. The ultimate point of the oxidation path is the same for lignite, sporopollenin, kuckersite and probably for both asphalt and the anthracene-oil insoluble fraction (H/C = O/C - OS). This last peculiar product is called “oxychar”, in analogy with the u&mate oxidation residue of coals, called “oxycoal” (9). The rate of reaction aIso depends on the elemental composition of the intiai material. The Iarge differences in the reaction rates and the fact that hydrogen loss seems to be equivalent to an increase in oxygen content suggest that the following parameters should be taken into account. First the oxidation time, t and second either the value A(H/C) t A(O/C) or the atomic ratio (O/H). These two parameters were chosen because they were consistent with the fact that a hydrogen loss has the same effect as an increase in oxygen content. They can be plotted versus the logarithm of time, and the curves can be compared(Fig. 4a, b, c, d). Three stages of oxidation can then be recognized for all the materials, except for the mesophase and the anthracene-oil insolubles (their reaction rate is too low). The last stage is more visible for sporopollenin and kuckersite in Fig. 4(a) whereas sporopollenin clearly shows the three stages in Fig. 4(c). Similarly, the three stages are more obvious in Fig. 4(b) than in Fig. 4(d) for asphalt. The end of each stage, which is not always clear in a van Krevelen diagram (Figs. 3a, b, c, d), is emphazized by Figs. 4(a, b, c, d). Lengths of time can be evaluated and are given in hours in Table 1. Figures 4(a, b, c, d) also show that the curves for all the materials have the same slope for each of the stages. The slope is Iow for the first and the third stages and steep for the second. If the slopes for the mesophase and the anthracene oil insolubles are compared to those of the other curves, it can be inferred that only the first stage is present for those materials. The (O/H) ratio appears to be the most suitable parameter for characterizing oxidation, since it represents the elemental composition of the material before and during oxidation. On the other hand, A(H/C) t A(O/C) is a parameter independent of the initial elemental composition of the material chosen. In addition to the standard oxidizing treatment at 2OO”C, lignite and sporopollenin were oxidized at 180, 220 and 25O”C,kuckersite at 180 and 250°C and asphalt at 250°C.
Fig. 5. van Krevelen diagrams showing the oxidation paths at different temperatures for, (a) lignite, (b) sporopollenin, (cc) kuckersite.
The results are plotted on a van Krevelen diagram in Figs. S(a, b, c) (except for asphalt for which the reaction rate is so low that it does not show any significant change from 200 to 25O’C).The van Krevelen diagrams of all the materials exhibit two ch~cte~stics. When the temperature of the oxidizing ~eatment is increased, the slope of the second part of the curve (second stage described above in Fig. 4) increases, and the length of the stage is shortened. For the high temperature, oxidation thus corresponds to a higher degree of dehydrogenation. The rate of reaction for all stages increases as temperature increases. If the reaction is assumed to be first order, the “apparent activation energy” can be roughly estimated by using the Arrhenius equation. At all stages, all the materials have about the same “20-40 kcal mole-’ apparent activation energy” which is the same rangeas that found for coals[P].
563
Oxidation of carbonaceous matter-l
I
Fig. 6. van Krevelen diagram showing oxidation paths of various pyrolysis residues of sporopollenin at 200°C.
0.)
0.1
0.1
Fig. 8. van Krevelen diagram showing the superposition of the oxidation paths (a) of sporopollenin (b) to raw kuckersite; of sporopollenin F to kuckersite E and raw asphalt.
a
H
‘cl”’
I
1.5
Raw kuckersite
u”)
b
1110 cm-’ ,co,
6
[Sporopollcnlnl 110.
Fig. 7. van Krevelen diagram showing oxidation paths of various pyrolysis residues of kuckersite at 200°C.
e
’
3.1.2 Oxidation of pyrolysis residues. As shown in Fig. 6 for sporopollenin and in Fig. 7 for kuckersite, the oxidation paths for the residues of pyrolysis ABC, etc.. . follow the same trend as for the raw materials. The further away from the original sample the representative point of each pyrolysis residue before oxidation is taken, i.e. the higher the pyrolysis temperature, the lower the slope of the representative oxidation line. At the same time the reaction rate decreases. These results are quite similar to those obtained in 3.1.1. They suggest that the slope of the oxidation path is closely related to the elemental composition of the sample before oxidation. If so, the slope of the oxidation path can be expected to be the same for all the samples having the same elemental composition, whatever their origin or their heat-treatment temperature before oxidation. This is supported by Fig. 8, for instance, which shows that sporopollenin heat-treated to the point B, when oxidized, follows a path which can be superimposed on that of oxidized kuckersite. Kuckersite heat-treated to E, and sporopollenin heat-treated to F have about the same elemental analysis and the same oxidation line, which is also the same for asphalt. 3.2 Changes in functional groups Some complementary information is derived from infrared spectrometry. Figures 9(a, b, c) show the changes in
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/+
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--
/-
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1 :,I
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‘i
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--
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-.
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L
SO
100
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Fig. 9. Infrared spectrometry data (a) K 2920(CH aliph.) plotted versus oxidation time, (b) K 1710(CO) plotted versus oxidation time, (c) Ic_~a index (IO) plotted vs oxidation time (for evaluating the frequency of ether functional groups).
564
D. JOSEPH and A. OBERUN
the aliphatic CH groups (2~Ocm-‘), in the carbonylcarboxyl CO groups (1710cm-‘), and in an empirical index Ic-oR, all plotted versus theoxidation time. IC-oRis a rough estimate of the occurrence of ether functional groups and is expressed[lO] by the difference between the oxygen weight percentage and the half value of K1710. K2920 and K1710 evolve steadily for sporopollenin, kuckersite and asphalt, but much more slowly for asphalt. 1~~~ decreases for sporopollenin and kuckersite, and increases for asphalt. By comparing K1710 to IGQR, the oxidation of asphalt can be attributed to a cross-tinking by ether groups, the number of which increases as oxidation progresses. On the contrary, these groups decrease continuously when kuckersite and sporopollenin are oxidized. The crosslinking must thus be attributed to hydrogen bonds. It changes as the carboxyl groups concentrateIll].
a Gt Sporopdlcnin
K 2920cm-1
I
tC!i ahph.)
0 Kuckersite :: Asphalt
80
40,
H welqht
2
4
6
8
,
10
b 0
Spcropollcnin
l UucktrsitC ST;Asphalt
Figures iO(a and b) show the evolution of K1710 and K2920 plotted vs the weight percentage of oxygen and of hydrogen. These figures show that the content in the functional groups tends to be constant near the “oxychar” formation. For instance K1710 of sporopollenin and kuckersite reaches its maximum for oxygen= 38% whereas K2920 reaches its minimum for hydrogen = Z3%. The asphalt oxidation rate is so low that its curve cannot be extrapolated. 4. CONCLUSIONS All the carbonaceous materi~s studied here show three stages of oxidation in the range 1.5>(H/C)>0.5 and 0 > (O/C) > 0.5. In the first only hydrogen is released and the oxygen content does not change, or hydrogen is released and the oxygen content increases. The second step is always a hydrogen loss combined with an increase in oxygen content. Finally, the third step corresponds to a slow increase in the oxygen content only. During the second stage, the slope of the oxidation path decreases as (O/H) of the initial sample decreases. Materials having the same initial (O/H) atomic ratio follow the same oxidation path. Infrared spectrometry data show that as oxidation progresses, the diphatic functions groups decrease as carbonyI-~arboxyl groups increase. Cross-linking is assumed to be due to an ether bonding in asphalt and to hydrogen bonds in sporopoilenin and in kuckersite. For all the materials at any stage of oxidation, the “apparent activation energy” was estimated to be about 20-40 kcal mole-‘. The ultimate product after oxidation is identical for sporopollenin, kuckersite and probably for asphalt and the anthracene oil-insoluble fraction. It is an “oxychar” of elemental composition: (H/C) = 0.5, (O/C) = 0.5, i.e. (O/H) ==1. In the second part of this paper, the elemental composition of some of the raw products and their pyrolysis residues will be correlated to their structure and microtexture acquired during the canonization and graphiti~ation processes. The mechanism of the oxidation process will be discussed. REFEREWES
I. R. E. Franklin, Proc. Roy. Sot. 209, 1% (1951).
80.
40-
0 weqnt I &
10
20
30
4’0
Fig. 10. Infrared spectrometry data. (a) K 2920(CHaliph.)plotted
versus H weightper cent,(b) K 1710(CO)plottedversus 0 weight .. per cent.
2. R. Loison, P. Foch and A. Boyer, IO Le Coke. Masson Paris (1970). 3. A. Oberlin, J. L. Boulmierand M. Viiley, in Kerogen (Edited by B. Durand). Technip, Paris (1979). 4. M. Monthioux, M. Oberlin, A. Oberlin. X. Bourrat and R. Bouiet, Carbon, 24, 167(1982). 5. R. E. Franklin, Acta Cryst. 3, 107(1950). 6. R. E. FrankIin, Acta Cryst. 4, 253 (195I). 7. M. Viiiey, A. Oberlin and A. Combaz, Carbon 17,77 (1979). 8. A. Oberlin, M. Viiley and A. Combat, Carbon 18,347 (1980). 9. D. W. van Kreveien, Coal. Elvesier, Amsterdam (1961). 10. P. Robin, Thesis Doctorate, Louvain (1975). 11. P. G. Rouxhet, M. Viiiey and A. Oberlin, Geochim. Cosmochim. Acta 43, 1705(1979).