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Graphitization of organic compounds—I. Enhanced graphitization by copyrolysis with aryne precursors
1968, Vol. 6, pp. 765-770.
carbon
Pergamon Press.
Printed in Great Britain
GRAPHITIZATION OF ORGANIC COMPOUNDS-I. ENHANCED GRAPHITIZATION BY COPYROLYSIS WITH ARYNE PRECURSORS LAWRENCE Airco Speer Research Laboratory,
G. ISAACS Niagara Falls, New York 14302
(Received 12 April 1968) Abstract-The graphitization of fluorene and carbazole, as measured by X-ray data on heattreated chars obtained from these compounds, is enhanced by carbonizing in admixture with phthalic anhydride, phthalimide, or pyromellitic dianhydride. The chars prepared by pyrolysis of the mixtures were more graphitic than those prepared from either pure compound; this is the fint known report of such a phenomenon. It is suggested that the improved graphitization is a result of reaction of arynes, generated by heating the anhydrides or imide, with fluorene and carbazole. 1. INTRODUCTION particularly coal-tar
those
pitch,
carbon
and
graphite
research
standing
the
pyrolytic coal-tar
structures industry.
chemistry
pitch,“)
chars. known from
work
aryne
shows
aromatic
precursors
graphitizable
either
the
constituents
reactions
which
that
non-graphitizing
more
particularly
many
or lead to readily
This
normally
con-
of
starting
compounds
enhance
graphitizable copyrolysis
compounds yields than
cokes
those
material. lacking
of with
which
prepared
Arynes two
are
by pyrolysis
Phthalic
anhydride,
of phthalic and
other
The
latter
compounds
and are components having
been
anhydride. compounds
which were predicted to be aryne precursors, were pyrolyzed in admixture with fluorene,
do not graphitize
of coal-tar
of the 127 compounds
which
identified
pitch;
well
in fact 20
MCNEIL(“) lists as
in coal-tar
pitch
are
derivatives of these two compounds. Considerable improvement in graphitization of fluorene and carbazole, and
as measured
crystallite
resulted
from
by the interlayer
spacing
size of the heat-treated
chars,
copyrolysis
with
the
aryne
compounds
were
precursors.
adjacent
atoms; i.e. aromatic diradicals. The simplest aryne, benzyne, C,H,, was conclusively identified by BERRY et al.@) in 1960. It is conveniently prepared
H
to under-
little work has been reported
on in situ pyrolytic graphitization
in
to the
Although
devoted
and
of
occur
of interest
has been
reactions
compounds,
which
is a subject
siderable
are
and with carbazole,
GFUPHITIZATION of organic
THE
2. EXPERIMENTAL Commercially used
available
without
fluorene,
further
which
anol-water. Carbonizations clave,
(High under
Pa.), was placed
purification
was recrystallized were
conducted
except
for
from methin an auto-
Pressure Equipment Co., Erie, argon. The organic material in
the
autoclave
(mixtures
were
thoroughly blended beforehand), the system was flushed and pressurized to 600 psi with 765 c*RBON 6/6-B
766
LAWRENCE G. ISAACS
argon, and the autoclave heated at 150°C/hr until the contents reached 600°C. The resultant carbons were calcined in a resistance tube furnace under an argon atmosphere at 1300°C and graphitized in a graphite resistance furnace (Richard D. Brew and Co., Inc., Concord, N.H.). A heating schedule of 400”C/hr was used to attain a temperature of 2800°C which was maintained for 1 hr. In order to ascertain if the X-ray properties of the final graphites were particularly sensitive to minor changes in heating rates or dwelltime at 28OO”C, a few experiments were run in which these conditions were purposely varied. These are shown in Table 1, where the graphite properties are compared to those prepared at the conditions given above. It is readily apparent that these changes had little or no effect on graphite properties, and that the graphitization enhancement which is reported below cannot be ascribed to minor variations in heating rates or dwell-time at 2800°C. Further, the duplication of the d-spacing values in these experiments indicates that graphitization temperature attainment was also reproducible, since other workerso) found d-spacings of fluorene graphitized at 3000°C to be considerably lower, 3.38A.
TABLE
~.VARIATION
Interlayer spacings (d) and crystallite size (L,) measurements were determined by X-ray diffraction on a General Electric XRD-5 Diffractometer. Samples were ground to minus 200 mesh, mixed with minus 200 mesh potassium chloride, and irradiated. The difference between the measured and known (222) 2-theta values of potassium chloride was used to correct the (004) or (002) carbon reflections to their true P-theta values from which interlayer spacings were calculated by a standard procedure.@) The crystallite sizes were calculated from the (002) reflections by the equation of Scherrer.“) Most of the 2800”C-treated materials did not give well defined (004) reflection peaks so interlayer spacings were calculated from the (002) reflections, except for those materials with d-spacings of about 3.39A or less. 3. RESULTS AND DISCUSSION Interlayer spacing and crystallite size data of graphites prepared from mixtures of phthalic anhydride with fluorene and carbazole are shown in Figs. 1 and 2. The graphitization enhancement of these mixtures, compared to the compounds from which the mixtures were is readily apparent. prepared, Somewhat
OF CARBONIZATION
AND
GRAPHITIZATION
CONDITIONS
Compound
Experimental conditions
d(A)
L.(A)
Fluorene Pluorene Fluorene Phthalic anhydride Phthalic anhydride
Standard JO”C/hrcarbonization 75”C/hrcarbonization, 3 hr at 2800°C Standard 75°C/hr carbonization
3.42 3.43 3.43 3.47 3.47
91 78 85 28 29
TABLE
2.
COMPARI~ONOFGRAPHITSZATIONENI~ANCR?~RNTOPPI~TI~ALICANI~YDRIDE
(PA)
AND
PYROMELLITICDIANHYDRIDE
Mixture 80% 80% 50% 50%
fluorene, 20% PA fluorene, 20% PMDA carbazole, 50% PA carbazole, 50% PMDA
(PMDA)
Interlayer spacing (A) 3.461 3.393 3.437 3.430
Crystallite size (A) 202 203 69 95
3.42
\
0
0
20 FliTHALANHYDR&
3.36J
8
3.38
CAFBAZOLE \ \ -s-c-
\FLUORENE
\
00
%
4’
80
/
I
01 I
0’
I
/.
/
N/j
1’
/I
l
/
/
/ I
!
/
loo
,//@
A %t.,
20
40
60
\ \
80
z-I_
i
I I \ I I I I I I I I I I I
\
t \ I \
PHTHALIC ANHYDRIDE, %
cm@azcn_E
/ I@ ,‘FLUORENE
/
I\ I
\ \ ‘,
FIG. 2. Crystallite size (LJ of carbons prepared by copyrolysis of phthalic anhydride with fluorene and carbazole.
00
(Y
so-
loo
_
$ 9 8 150-
a ~250 N UY w 5200-
300-
350 -
l
lP
400-
I’
FIG. 1. Interlayer spacing of carbons prepared by copyrolysis of phthalic anhydride with fluorene and carbazole.
z
I- 3.40
ii
6
g P
a 3.44
3.46
\
1
1
450 -
500
768
LAWRENCE G. ISAACS
greater improvement was found upon substituting pyromellitic dianhydride for phthalic anhydride, as shown in Table 2. Phthalimide in a 50 per cent admixture with fluorene gave results identical to those obtained with phthalic anhydride at the same concentration. The graphite properties of anthracene, a compound which graphitizes well,(*) were not abased when it was copyrolyzed with 50 per cent phthalic anhydride. Both materials had interlayer spacings of 3.365 A after heat treatment at 2800°C. As mentioned above, phthalic anhydride yields benzyne on pyrolysis:
alo
c@
= A-o
I (+co+co2
c\\o
This reactive chemical species reacts with aromatic compounds as illustrated below for benzene.@)
Insertion
reactions analogous to those shown above occur prior to carbonization yielding compounds with more aromatic rings, e.g. benzofluorene and benzocarbazole, which graphitize better than the parent compounds. Since identical results were obtained with phthalic anhydride and phthalimide, the latter also probably yields benzyne :
= >H--a c//0
l
1
I I+HKO+CO
0
C\\0
The slight superiority in graphitization enhancement of pyromellitic dianhydride compared to phthalic anhydride may be due to generation of a second aryne, (which can undergo further ring-addition reactions), from the former compound. This can take place only if the first addition is 1,4-; 1,2-addition results in elimination of the second anhydride
I. 2-addition
H I,4-addition
The products were formed, in the above instance, at ratios of insertion: l,2-addition: 1,4-addition of 3 :4: 1. The latter two structures undergo rearomatization upon heating :
group, analogous to elimination of acetylene in the reaction of benzyne with benzene. The final product of 1,2-addition is the same as that expected from 1,2- or 1,4-addition of
Analogous reactions have been shown to occu wi& bemyne and other aromatic com-
benzyne; since the improvement in graphitization enhancement of pyromellitic dianhydride compared to phthalic anhydride was slight, it is assumed that most aryne addition to fluorene and carbazole occurs 1,2.
pounds. (lo) The most obvious explanation of the graphitization enhancement reported here is that
GRAPHITIZATION
OF ORGANIC COMPOUNDS-I
The carbonization of anthracene, as measured by the X-ray properties of the 2800°C treated materials, was unaffected by the presence of 50 per cent phthalic anhydride. KLANDERMAN’~” has examined the reaction of benzyne with anthracene and found a predominance of 9,10-addition over 1,2- or I,4 of 3O:l.
w 9,10-odduct
Such an adduct is expected to readily rearomatize back to anthracene upon heating; it is therefore not surprising that the final product
769
should be a carbon similar to that obtained from pure anthracene. 4. CONCLUSIONS Aromatic anhydrides which are known to yield arynes upon pyrolysis,(a) effectively promote the graphitization of fluorene and carbazole when mixtures of the two classes of compounds are carbonized. The graphitization enhancement is concentration dependent and is most pronounced at about 50 wt. per cent of phthallic anhydride with either fluorene or carbazole. It is suggested that the enhancement is due to formation of benzo-tluorene or -carbazole derivatives; i.e. the compounds which actually carbonize are no longer 6-5-6-membered ring systems, but 6-6-5-6 or 6-6-5-6-6 or similar systems.
LAWRENCE G. ISAACS
770
Acknowledgments-The author is indebted to many of his associates in the Research Department at Airco Speer. Particular thanks are due to Dr. D. L. BIEDERMANfor many fruitful discussions, to Mr. RICHARD R. FOR~EYwho performed much of the experimental work, and to the Chemical and Physical Measurements Group who performed the X-ray measurements. REFERENCES 1. Improved Graphite Materials for High-Temperature Aerospace Use: Vol. III. Further Research and Development for Improved Graphite Materials. Air Force Materials Laboratory, Research and Technology Division, WrightPatterson Air Force Base, Ohio, Report MLTDR-64-125, Vol. III (1965). 2. BERRY R. S., STOKESG. N. and STILESR. M., J. Amer. Chem. Sot. 82, 5240 (1960). 3. FIELDSE. K. and MEYER~ONS., Chem. Commun. 474 (1965). 4. MCNEIL D., In Bituminous Materials: Asphalts, Tars and Pitches, Vol. III, Coal Tars and Pitches,
5.
6. 7.
8. 9. 10.
11.
(ARNOLDJ. HOIBERT,ed.), pp. 159-192. Interscience, New York (1966). Improved Graphite Materials for High Temperature Aerospace Use: Vol. III. Further Research and Development for Improved Graphite Materials, p. 194. Air Force Materials Laboratory, Research and Technology Division, Wright-Patterson Air Force Base, Ohio, Report ML-TDR-64-125, Vol. III (1965). Measurement of lattice spacing in nuclear graphite. ASTM Designation C 558-65 T (1965). KLUG H. P. and ALEXANDERL. E., X-Ray Di$rction Procedures, pp. 491-538. John Wiley, New York (1954). WALKER P. L., JR. and WEINSTEINA., Carbon 5, 13 (1967). MILLER R. G. and STILESM., J. Amer. Chem. Sot. 85, 1798 (1963). FIELDSE. K. and MEYER~ONS., J. Amer. Chem. Sot. 88, 338 (1966); J. Org. Chem. 31, 3307 (1966); Chem. and Ind. 1230 (1966). KLANDER~N B. H., J. Amer. Chem. Sot. 87, 4649 (1965).
carbon
Pergamon Press.
Printed in Great Britain
GRAPHITIZATION OF ORGANIC COMPOUNDS-I. ENHANCED GRAPHITIZATION BY COPYROLYSIS WITH ARYNE PRECURSORS LAWRENCE Airco Speer Research Laboratory,
G. ISAACS Niagara Falls, New York 14302
(Received 12 April 1968) Abstract-The graphitization of fluorene and carbazole, as measured by X-ray data on heattreated chars obtained from these compounds, is enhanced by carbonizing in admixture with phthalic anhydride, phthalimide, or pyromellitic dianhydride. The chars prepared by pyrolysis of the mixtures were more graphitic than those prepared from either pure compound; this is the fint known report of such a phenomenon. It is suggested that the improved graphitization is a result of reaction of arynes, generated by heating the anhydrides or imide, with fluorene and carbazole. 1. INTRODUCTION particularly coal-tar
those
pitch,
carbon
and
graphite
research
standing
the
pyrolytic coal-tar
structures industry.
chemistry
pitch,“)
chars. known from
work
aryne
shows
aromatic
precursors
graphitizable
either
the
constituents
reactions
which
that
non-graphitizing
more
particularly
many
or lead to readily
This
normally
con-
of
starting
compounds
enhance
graphitizable copyrolysis
compounds yields than
cokes
those
material. lacking
of with
which
prepared
Arynes two
are
by pyrolysis
Phthalic
anhydride,
of phthalic and
other
The
latter
compounds
and are components having
been
anhydride. compounds
which were predicted to be aryne precursors, were pyrolyzed in admixture with fluorene,
do not graphitize
of coal-tar
of the 127 compounds
which
identified
pitch;
well
in fact 20
MCNEIL(“) lists as
in coal-tar
pitch
are
derivatives of these two compounds. Considerable improvement in graphitization of fluorene and carbazole, and
as measured
crystallite
resulted
from
by the interlayer
spacing
size of the heat-treated
chars,
copyrolysis
with
the
aryne
compounds
were
precursors.
adjacent
atoms; i.e. aromatic diradicals. The simplest aryne, benzyne, C,H,, was conclusively identified by BERRY et al.@) in 1960. It is conveniently prepared
H
to under-
little work has been reported
on in situ pyrolytic graphitization
in
to the
Although
devoted
and
of
occur
of interest
has been
reactions
compounds,
which
is a subject
siderable
are
and with carbazole,
GFUPHITIZATION of organic
THE
2. EXPERIMENTAL Commercially used
available
without
fluorene,
further
which
anol-water. Carbonizations clave,
(High under
Pa.), was placed
purification
was recrystallized were
conducted
except
for
from methin an auto-
Pressure Equipment Co., Erie, argon. The organic material in
the
autoclave
(mixtures
were
thoroughly blended beforehand), the system was flushed and pressurized to 600 psi with 765 c*RBON 6/6-B
766
LAWRENCE G. ISAACS
argon, and the autoclave heated at 150°C/hr until the contents reached 600°C. The resultant carbons were calcined in a resistance tube furnace under an argon atmosphere at 1300°C and graphitized in a graphite resistance furnace (Richard D. Brew and Co., Inc., Concord, N.H.). A heating schedule of 400”C/hr was used to attain a temperature of 2800°C which was maintained for 1 hr. In order to ascertain if the X-ray properties of the final graphites were particularly sensitive to minor changes in heating rates or dwelltime at 28OO”C, a few experiments were run in which these conditions were purposely varied. These are shown in Table 1, where the graphite properties are compared to those prepared at the conditions given above. It is readily apparent that these changes had little or no effect on graphite properties, and that the graphitization enhancement which is reported below cannot be ascribed to minor variations in heating rates or dwell-time at 2800°C. Further, the duplication of the d-spacing values in these experiments indicates that graphitization temperature attainment was also reproducible, since other workerso) found d-spacings of fluorene graphitized at 3000°C to be considerably lower, 3.38A.
TABLE
~.VARIATION
Interlayer spacings (d) and crystallite size (L,) measurements were determined by X-ray diffraction on a General Electric XRD-5 Diffractometer. Samples were ground to minus 200 mesh, mixed with minus 200 mesh potassium chloride, and irradiated. The difference between the measured and known (222) 2-theta values of potassium chloride was used to correct the (004) or (002) carbon reflections to their true P-theta values from which interlayer spacings were calculated by a standard procedure.@) The crystallite sizes were calculated from the (002) reflections by the equation of Scherrer.“) Most of the 2800”C-treated materials did not give well defined (004) reflection peaks so interlayer spacings were calculated from the (002) reflections, except for those materials with d-spacings of about 3.39A or less. 3. RESULTS AND DISCUSSION Interlayer spacing and crystallite size data of graphites prepared from mixtures of phthalic anhydride with fluorene and carbazole are shown in Figs. 1 and 2. The graphitization enhancement of these mixtures, compared to the compounds from which the mixtures were is readily apparent. prepared, Somewhat
OF CARBONIZATION
AND
GRAPHITIZATION
CONDITIONS
Compound
Experimental conditions
d(A)
L.(A)
Fluorene Pluorene Fluorene Phthalic anhydride Phthalic anhydride
Standard JO”C/hrcarbonization 75”C/hrcarbonization, 3 hr at 2800°C Standard 75°C/hr carbonization
3.42 3.43 3.43 3.47 3.47
91 78 85 28 29
TABLE
2.
COMPARI~ONOFGRAPHITSZATIONENI~ANCR?~RNTOPPI~TI~ALICANI~YDRIDE
(PA)
AND
PYROMELLITICDIANHYDRIDE
Mixture 80% 80% 50% 50%
fluorene, 20% PA fluorene, 20% PMDA carbazole, 50% PA carbazole, 50% PMDA
(PMDA)
Interlayer spacing (A) 3.461 3.393 3.437 3.430
Crystallite size (A) 202 203 69 95
3.42
\
0
0
20 FliTHALANHYDR&
3.36J
8
3.38
CAFBAZOLE \ \ -s-c-
\FLUORENE
\
00
%
4’
80
/
I
01 I
0’
I
/.
/
N/j
1’
/I
l
/
/
/ I
!
/
loo
,//@
A %t.,
20
40
60
\ \
80
z-I_
i
I I \ I I I I I I I I I I I
\
t \ I \
PHTHALIC ANHYDRIDE, %
cm@azcn_E
/ I@ ,‘FLUORENE
/
I\ I
\ \ ‘,
FIG. 2. Crystallite size (LJ of carbons prepared by copyrolysis of phthalic anhydride with fluorene and carbazole.
00
(Y
so-
loo
_
$ 9 8 150-
a ~250 N UY w 5200-
300-
350 -
l
lP
400-
I’
FIG. 1. Interlayer spacing of carbons prepared by copyrolysis of phthalic anhydride with fluorene and carbazole.
z
I- 3.40
ii
6
g P
a 3.44
3.46
\
1
1
450 -
500
768
LAWRENCE G. ISAACS
greater improvement was found upon substituting pyromellitic dianhydride for phthalic anhydride, as shown in Table 2. Phthalimide in a 50 per cent admixture with fluorene gave results identical to those obtained with phthalic anhydride at the same concentration. The graphite properties of anthracene, a compound which graphitizes well,(*) were not abased when it was copyrolyzed with 50 per cent phthalic anhydride. Both materials had interlayer spacings of 3.365 A after heat treatment at 2800°C. As mentioned above, phthalic anhydride yields benzyne on pyrolysis:
alo
c@
= A-o
I (+co+co2
c\\o
This reactive chemical species reacts with aromatic compounds as illustrated below for benzene.@)
Insertion
reactions analogous to those shown above occur prior to carbonization yielding compounds with more aromatic rings, e.g. benzofluorene and benzocarbazole, which graphitize better than the parent compounds. Since identical results were obtained with phthalic anhydride and phthalimide, the latter also probably yields benzyne :
= >H--a c//0
l
1
I I+HKO+CO
0
C\\0
The slight superiority in graphitization enhancement of pyromellitic dianhydride compared to phthalic anhydride may be due to generation of a second aryne, (which can undergo further ring-addition reactions), from the former compound. This can take place only if the first addition is 1,4-; 1,2-addition results in elimination of the second anhydride
I. 2-addition
H I,4-addition
The products were formed, in the above instance, at ratios of insertion: l,2-addition: 1,4-addition of 3 :4: 1. The latter two structures undergo rearomatization upon heating :
group, analogous to elimination of acetylene in the reaction of benzyne with benzene. The final product of 1,2-addition is the same as that expected from 1,2- or 1,4-addition of
Analogous reactions have been shown to occu wi& bemyne and other aromatic com-
benzyne; since the improvement in graphitization enhancement of pyromellitic dianhydride compared to phthalic anhydride was slight, it is assumed that most aryne addition to fluorene and carbazole occurs 1,2.
pounds. (lo) The most obvious explanation of the graphitization enhancement reported here is that
GRAPHITIZATION
OF ORGANIC COMPOUNDS-I
The carbonization of anthracene, as measured by the X-ray properties of the 2800°C treated materials, was unaffected by the presence of 50 per cent phthalic anhydride. KLANDERMAN’~” has examined the reaction of benzyne with anthracene and found a predominance of 9,10-addition over 1,2- or I,4 of 3O:l.
w 9,10-odduct
Such an adduct is expected to readily rearomatize back to anthracene upon heating; it is therefore not surprising that the final product
769
should be a carbon similar to that obtained from pure anthracene. 4. CONCLUSIONS Aromatic anhydrides which are known to yield arynes upon pyrolysis,(a) effectively promote the graphitization of fluorene and carbazole when mixtures of the two classes of compounds are carbonized. The graphitization enhancement is concentration dependent and is most pronounced at about 50 wt. per cent of phthallic anhydride with either fluorene or carbazole. It is suggested that the enhancement is due to formation of benzo-tluorene or -carbazole derivatives; i.e. the compounds which actually carbonize are no longer 6-5-6-membered ring systems, but 6-6-5-6 or 6-6-5-6-6 or similar systems.
LAWRENCE G. ISAACS
770
Acknowledgments-The author is indebted to many of his associates in the Research Department at Airco Speer. Particular thanks are due to Dr. D. L. BIEDERMANfor many fruitful discussions, to Mr. RICHARD R. FOR~EYwho performed much of the experimental work, and to the Chemical and Physical Measurements Group who performed the X-ray measurements. REFERENCES 1. Improved Graphite Materials for High-Temperature Aerospace Use: Vol. III. Further Research and Development for Improved Graphite Materials. Air Force Materials Laboratory, Research and Technology Division, WrightPatterson Air Force Base, Ohio, Report MLTDR-64-125, Vol. III (1965). 2. BERRY R. S., STOKESG. N. and STILESR. M., J. Amer. Chem. Sot. 82, 5240 (1960). 3. FIELDSE. K. and MEYER~ONS., Chem. Commun. 474 (1965). 4. MCNEIL D., In Bituminous Materials: Asphalts, Tars and Pitches, Vol. III, Coal Tars and Pitches,
5.
6. 7.
8. 9. 10.
11.
(ARNOLDJ. HOIBERT,ed.), pp. 159-192. Interscience, New York (1966). Improved Graphite Materials for High Temperature Aerospace Use: Vol. III. Further Research and Development for Improved Graphite Materials, p. 194. Air Force Materials Laboratory, Research and Technology Division, Wright-Patterson Air Force Base, Ohio, Report ML-TDR-64-125, Vol. III (1965). Measurement of lattice spacing in nuclear graphite. ASTM Designation C 558-65 T (1965). KLUG H. P. and ALEXANDERL. E., X-Ray Di$rction Procedures, pp. 491-538. John Wiley, New York (1954). WALKER P. L., JR. and WEINSTEINA., Carbon 5, 13 (1967). MILLER R. G. and STILESM., J. Amer. Chem. Sot. 85, 1798 (1963). FIELDSE. K. and MEYER~ONS., J. Amer. Chem. Sot. 88, 338 (1966); J. Org. Chem. 31, 3307 (1966); Chem. and Ind. 1230 (1966). KLANDER~N B. H., J. Amer. Chem. Sot. 87, 4649 (1965).