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In-situ monitoring for mesophase formation processes of various pitches by means of high-temperature 13C-NMR
Carbon Prmtcd
Vol. 29. No. 7. pp. 943-947. I” Great Britain.
1991 CopyrIght
0008-6223191 $3.00 + .OO 0 1991 Pergamon Press plc
IN-SITU MONITORING FOR MESOPHASE FORMATION PROCESSES OF VARIOUS PITCHES BY MEANS OF HIGH-TEMPERATURE 13C-NMR KIYOSHI AZAMI, SHUNICHI YAMAMOTO, TETSURO YOKONO* and Yuzo SANADA* Nippon Oil Company, Ltd., Central Technical Research Laboratory, 8, Chidori-cho, Naka-ku, Yokohama 231, Japan *Faculty of Engineering, Hokkaido University, N14, W8, Kita-ku, Sapporo 060, Japan (Received 1 June 1990; accepted in revised form 29 August 1990)
Abstract-High-temperature heating processes for coal-tar-derived pitch (pitch A), petroleum-derived pitch (pitch B), and hydrogenated petroleum-derived pitch (pitch C) have been studied by in-situ hightemperature ‘“C-NMR. Mesophase formation processes have been successfully monitored for all three kinds of pitches. The times required for appearance of mesophase in pitches A and B are similar and shorter than that in pitch C at 430°C. Time intervals from mesophase appearance to finishing mesophase transformation in pitches B and C are nearly the same and shorter than that in pitch A at 430°C. There may be a critical region of aliphatic carbon content for mesophase appearance of petroleum-derived pitch. Completion of mesophase development correlates to the line-width of characteristic peak of 180 ppm assigned as carbon in stacked aromatic layer molecules. Aliphatic carbons of molecules in mesophase seem to cause an increase of mesophase mobility and a narrowing of the characteristic peak for mesophase around 180 ppm.
tube. This paper presents the results of in-situ monitoring of the mesophase formation process using the new technique of “C-NMR, and the relationship between structural characteristics of pitch and its behavior during the mesophase formation process.
1. INTRODUCTION Several in-situ monitoring methods for mesophase formation have been reported by means of the hotstage polarization microscope[l-31, high-temperature ‘H-NMR[4], and the ESR spin probe method[4, 51. The methods have revealed some important aspects of mesophase formation. However, the informations obtained still remain qualitative over the range of the entire mesophase formation process. Nishizawa et a[.[61 have reported that a characteristic peak due to aromatic carbon in mesophase appears around 180 ppm in “C-NMR spectrum of mesophase pitch in a liquid state at high temperature, while aromatic carbon in regular pitch in a liquid or dissolved state shows a peak in the vicinity of 130 ppm. The chemical shift of the aromatic peak to a low magnetic field is well known at the transition from isotropic to nematic in liquid crystal[7]. The new peak appearing at around 180 ppm is assigned as carbon in stacked lamellae in liquid state mesophase pitch. Yokono et al.[S] have shown a correlation between mesophase content and the peak intensity around 180 ppm in mesophase pitch. They have also reported that the addition of solvent causes a decrease in the 180 ppm peak intensity. Monitoring of the transformation from isotropic to nematic phase (so-called mesophase) in pitch using in-situ high-temperature “C-NMR is generally not easy, because of the low sensitivity of “C-NMR in comparison to that of ‘H-NMR. To overcome this problem in measurement, the authors performed insitu measurements of the ‘C-NMR spectra during the high-temperature heating process, using a high magnetic field NMR with a large diameter sample
2. EXPERIMENTAL 2.1 Sample Coal-tar-derived (pitch A), petroleum-derived (pitch B), and hydrogenated petroleum-derived (pitch C) pitches were selected. Table 1 shows the characteristics of the samples used in this experiment. 2.2 High-temperature “C-NMR High-temperature ‘X-NMR measurements were carried out using a Bruker MSL-300 FT-NMR with high-temperature probe (“C resonance frequency of 75 MHz). The magnetic field used is 7.0 T. A “CNMR spectrum of 10% ethylbenzen in CDCl, was measured at room temperature to calibrate chemical shifts of peaks in high-temperature 13C-NMR spectra. A large sample tube was used to get a high signalto-noise ratio. A pitch sample (~500 mg) was put into an NMR tube of 10 mm diameter. Glass wool was softly packed into the sample tube at the position of about 3 cm from the bottom to keep an insulating zone of about 1 cm thickness. Samples were heated at the rate of 5”Cimin up to 430°C and then held at 430°C under a nitrogen gas flow. At temperatures above 2Oo”C, FIDs (Free Induction Decays) were accumulated for four minutes, and then the tuning and the necessary preparations for measurement 943
K. AZAMIet al.
944
Table 1. Elemental analvsis and softening noint of Ditches Elements1 analysis(wt%) Pitch
sample
Abxil-tarderived) B (pahhm+eriv~) C%hymtipetroleum-derived)
C
H
N
94.4 93.3 926
3.6 6.2 6.5
l.2
softening point CC) 90 :
qmade from pitch B ?JYhydrogenation.
were carried out within two minutes. This cycle of testing was repeated at intervals of 30°C (six minutes) up to 430°C. After reaching 430°C FIDs were accumulated for ten minutes and this measurement was repeated until the transformation from isotropic phase to mesophase was finished. All FIDs were obtained with a 45” pulse width of 5 psec, a pulse repetition time of two seconds, and proton complete decoupling. All spectra were obtained through Fourier transformation of accumulated FIDs, with a line broadening factor of 100 Hz. 2.3 Solution NMR Solution NMR is a routine method and gives feasible data on distribution of various types of bonded carbon. A solution of pitch (-1 g) in CDCIS (==3 ml) was put in an NMR sample tube of 10 mm diameter. A small quantity of tetramethylsilane was added into the solution to calibrate chemical shifts for pitch. Solution NMR spectra were obtained using a JEOL FX-100 FT-NMR to calibrate the aliphatic carbon content from high-temperature ‘C-NMR spectra. Conditions of the above high-temperature ‘)C-NMR signal acquisition were chosen as optimal for sensitivity. The calibration curve for aliphatic carbon content in the sample is shown in Fig. 1. This was obtained from the plots of relation between the al-
iphatic peak areas in spectra from quantitative and sensitivity-stressed measurements using various petroleum pitches. The quantitative measurements were made with an approximately 30” pulse width of 3 psec, a pulse repetition time of 10 seconds, and a proton decoupling of non-NOE (Nuclear Overhauser Effect) mode. The sensitivity-stressed measurements were made with almost the same conditions as high-temperature 13C-NMR. The number of signals accumulated was 4000-6000 for both the quantitative and sensitivity-stressed measurements. 2.4 Solid state high resolution CPIMAS “C-NMR A powder sample (-220 mg) was packed in a sample container of 7 mm outer diameter and 18 mm length. CP/MAS (Cross Polarization /Magic Angle Spinnig) 13C-NMR spectra were obtained using Bruker AM-250 FT-NMR with a rotating rate of 4.2 KHz, a contact time of 2.5 psec, a repetition time of 3 seconds, an accumulation of 512 times, and a line broadening factor of 160 Hz. Chemical shifts of peaks for the sample were calibrated using glysine as an external reference. 3. RESULTS AND DISCUSSION
The temperature dependence of 13C-NMR spectra for pitches A, B, and C during the heating step from 380°C to 430°C and the holding step at 430°C are shown in Figs. 2, 3, and 4, respectively. Spectra at the temperatures below 380°C are almost the same as those of 380-400°C. Chemical shifts of various aromatic hydrocarbons are mostly around 110-150 ppm in a liquid or solution state[9]. A peak around 130 ppm (peak I) in the spectra of pitches corresponds to these chemical shifts, that is, a characteristic peak for isotropic phase. A peak around 180 ppm (peak M) corresponds to a characteristic peak for mesophase as described above. It was clearly observed that all the pitches gave peak M. The intensity of peak M relative to that of peak I increased while the peak I decreased and eventually disappeared. Thus, it is clear that the process from mesophase nucleation to bulk mesophase formation was accurately monitored. 3.1 Characteristics of mesophase formation process for three pitches Pitch A. The appearance of peak M was recog-
‘- E
44% If
0
20
Aliphatic
40 carbon
quantitative
60 content
80 determined
measurement
100 by
(%I
Fig. 1. A calibration curve for determining an aliphatic carbon content using high-temperature W-NMR spectra.
nized in the spectrum at the heating condition for 10 to 20 minutes at 430°C. The intensity of the peak increased with the holding time, and became larger than that of peak I for 130 to 140 minutes. After this step peak I remained for a while, but finally disappeared for 200 to 210 minutes. Pitch B. Peak M was observed for 10 to 20 minutes at 430°C. Then the intensity of the peak increased with the increase of time. Meanwhile, the intensity of peak I decreased and finally disappeared for 80 to 90 minutes. Pitch C. The intensity of aliphatic carbon dis-
945
Monitoring for mesophase formation by “C-NMR
k
j 160-170tin
380
h
300 200 100 0 -100
300 200 100 0 -100
308 200 100 0 -100
300 200 100 0 -1OOwm
Fig. 2. The temperature dependence of “C-NMR spectra for the pitch A (coal-tar-derived pitch) during the heating process from 380°C to 430°C and holding process at 430°C.
tinctly declined just after the holding process at 430°C began, and then decreased to a level less than that of noise peaks at 50 to 60 minutes. Just after this phenomenon, peak M appeared in the spectrum of 70 to 80 minutes. Then the intensity of this peak increased. On the other hand, peak I became weak and finally disappeared around 160 to 170 minutes. 3.2 Comparison of characteristics for mesophase formation process Table 2 shows the time required for mesophase Ta, the time required for finishing appearance, mesophase transformation (equivalent to the time
300 200 loo
0 -100
300 200 100
0 -1OOwm
Fig. 3. The temperature dependence of “C-NMR spectra for the pitch B (petroleum-derived pitch) during the heating process from 380°C to 430°C and holding process at 430°C.
required for disappearance of peak I), Tb, and the time interval from mesophase appearance (Ta) to finishing mesophase transformation (Tb), AT. All data in Table 2 were obtained from a series of the spectra of in-situ “C-NMR. 3.3 Change of aliphatic carbon content on heating and mesophase transformation Table 3 shows the aliphatic carbon content obtained from spectra accumulated for one hour at 250°C and that of the holding process at 430°C. The appearance of mesophase for pitch A is thought to be relatively early because this pitch has little aliphatic carbon and has a high aromaticity. On the other hand, pitch C has a rather large amount of aliphatic carbon. The molecules in this pitch require relatively longer heating time than pitch A in order to become the lamellar molecules, which are to be involved in mesophase. The mesophase appearances in pitches B and C did not occur until the aliphatic carbon content (carbon aliphaticity) reached less than 6% to 10%. Namely, it is thought that an aliphatic carbon content of 6% to 10% may be regarded as a critical region for the mesophase appearance of petroleum-derived pitch. We have reported(41 that there may be a critical region of aromaticity for isotropic-to-mesophase transformation of petroleumderived pitch in terms of hydrogen. The present study has revealed such a critical region in terms of carbon as the main constituent of pitch molecules. Meanwhile, untreated coal-tar-derived pitch already has an aliphatic carbon content of less than 1%. Namely, this pitch is thought to have the necessary planarity of molecule to form a mesophase. However, it is thought that molecules in coal-tar-derived pitch have not yet developed to a size necessary to
K. AZAMIet al.
946
380
aLjt,,_
h
410 B
& 430°C
O-l On-in
lO--2Orrin
20-30min
& &I
300
200
100
0 -100
300
200
100
0 -100
300
200
100
0 -1OOwm
Fig. 4. The temperature dependence of “C-NMR spectra for the pitch C (hydrogenated petroleumderived pitch) during the heating process from 380°C to 430°C and holding process at 430°C.
be stacked. This may be the reason why carbon aliphaticity is not a criterion for mesophase appearance of coal-tar-derived pitch. 3.4 Line-width of characteristic peak (peak M) for mesophase and mesophase formation process
Comparing the line-widths of peak M for three pitches, pitch A has very wide line-widths, for example, half-widths of 3-4 KHz during the holding process between 50 to 100 minutes at 430°C. On the other hand, pitches B and C have half-widths of 1.52.2 KHz during the holding process of 20 to 70 minutes at 430°C and 1.2-1.7 KHz during the holding process of 110 to 160 minutes at 430°C respectively. Generally, the narrower the line-width is, the higher the mobility of the molecules becomes. The molecules in pitch A are relatively rigid in mesophase already formed. In contrast with pitch A, mesophase-composing molecules in pitches B and C have a high mobility, and rearrangements are rather easy within the mesophase. This is the reason why the mesophase in these petroleum-derived pitches grows rapidly. Because aliphatic carbon is responsible for mesophase appearance as mentioned above, the role of
aliphatic carbon in mesophase has to be taken into account. The aliphatic carbon content in the mesophase pitches obtained from pitches A and B were determined using high resolution solid state r3CNMR spectra*. Figure 5 shows spectra for coal-tarderived and petroleum-derived mesophase pitches. Petroleum-derived mesophase pitch shows about 5% of aliphatic carbon, while aliphatic peaks cannot be observed for coal-tar-derived mesophase pitch. From the results we can draw a schematic model as illustrated in Fig. 6. Molecules in coal-tar-derived mesophase pitch are planar with low mobility and interact with strong intermolecular force. On the other hand, molecules in petroleum-derived mesophase pitch are rather mobile and bend due to the contribution of aliphatic portions. This difference in mobility is thought to be reflected in the mesophase formation rate.
Table 3. Aliphatic carbon content for pitches Aliphatio carbon content (%) Pitoh sample
430°C 25oV 0-1Omin lO-2Omin 20-3Omin
Table 2. The time required for mesophase appearance, Ta, the time required for finishing mesophase transformation, Tb, and the time interval from mesophase appearance (Ta) to the end of mesophase transformation (Tb), AT PdCll
TZI
Tb
AT
sample
0llid
(tin)
(Din)
2ow210
lm-200
-4
lo-20
B
lo-20
8c- 90
Ia- 80
C
7iH30
16+170
8cHOO
A B C
_b
_b
_b
8 22
_b
_b
12
6
aobtainod from spectra accumulated b undeteoted.
for 1 hour.
*13C-NMR spectra were obtained using the CP/MAS method, which enables one to assign peaks using the data base obtained from isotropic solution spectra.
947
Monitoring for mesophase formation by “C-NMR
n
(S) Spinningside band of the peak arolnd 130ppm
A (Coal-tar-derived
(E.G.) Signal
:‘x::::-h-
of san-~lecontainer
Coal-tar-derived Aromatic
250
200
150
100
Fig. 6. A structural
Aliphatic 50
0
model for mesophase-composing molecules.
wm
3. Aliphatic carbon in mesophase causes looseness of molecular planarity, increase of mesophase mobility, and narrowing of the characteristic peak around 180 ppm for mesophase.
B (Petroleum-derived mesophase pitch)
Petrolem-derived
Ii
REFERENCES Aromatic
250
200
150
100
1. D. W. Hoover, A. Davis, A. J. Perrotta, and W. Spackman, Absts. 14th Biennial Conf. on Carbon, (1979), p.
Alichatic
,
50
0
393.
ppm
Fig. 5. High resolution solid state ‘%-NMR spectra.
2. A. J. Perrotta, J. P. McCullough, and H. Beuther, Absts. 16th Biennial Cot@ on Carbon, (1983), p. 76. S. Uemura, T. Hirose, H. Takashima, 0. Kato, and M. Harakawa, Abst. 16th Biennial Conf. on Carbon, (1983),
4. CONCLUSIONS 1. In-situ monitoring for mesophase formation process has been successfully accomplished using high-temperature V-NMR. 2. The mesophase appearance and the time required for finishing mesophase transformation are determined. The wider the line-width of the characteristic peak for mesophase around 180 ppm, the longer the time interval from mesophase appearance to completion of mesophase transformation.
p. 78.
4. K. Azami. T. Yokono. Y. Sanada. and S. Uemura. Carbon 27, lj7 (1989).
5. K. Shibata, H. Kakiyama, Y. Sanada, and J. Sohma, Fuel 57, 572 (1978). 6. T. Nishizawa and M. Sawa, Proc. 14th Annual Meeting of Carbon Material Sot. Japan lA1.5 (1987). 7. K. Hayamizu, M. Yanagisawa, and 0. Yamamoto, Chem. Phys. Letters 127, 566 (1986).
8. T. Yokono, N. Takahashi, T. Kaneko, and Y. Sanada, Fuel 69, 796 (1990). 9. See, for example, F. W. Wehrli and T. Wirthlin, Interpretation of Carbon-13 NMR Spectra, Heyden & Son Ltd., London, a supplement chart (1978).
Vol. 29. No. 7. pp. 943-947. I” Great Britain.
1991 CopyrIght
0008-6223191 $3.00 + .OO 0 1991 Pergamon Press plc
IN-SITU MONITORING FOR MESOPHASE FORMATION PROCESSES OF VARIOUS PITCHES BY MEANS OF HIGH-TEMPERATURE 13C-NMR KIYOSHI AZAMI, SHUNICHI YAMAMOTO, TETSURO YOKONO* and Yuzo SANADA* Nippon Oil Company, Ltd., Central Technical Research Laboratory, 8, Chidori-cho, Naka-ku, Yokohama 231, Japan *Faculty of Engineering, Hokkaido University, N14, W8, Kita-ku, Sapporo 060, Japan (Received 1 June 1990; accepted in revised form 29 August 1990)
Abstract-High-temperature heating processes for coal-tar-derived pitch (pitch A), petroleum-derived pitch (pitch B), and hydrogenated petroleum-derived pitch (pitch C) have been studied by in-situ hightemperature ‘“C-NMR. Mesophase formation processes have been successfully monitored for all three kinds of pitches. The times required for appearance of mesophase in pitches A and B are similar and shorter than that in pitch C at 430°C. Time intervals from mesophase appearance to finishing mesophase transformation in pitches B and C are nearly the same and shorter than that in pitch A at 430°C. There may be a critical region of aliphatic carbon content for mesophase appearance of petroleum-derived pitch. Completion of mesophase development correlates to the line-width of characteristic peak of 180 ppm assigned as carbon in stacked aromatic layer molecules. Aliphatic carbons of molecules in mesophase seem to cause an increase of mesophase mobility and a narrowing of the characteristic peak for mesophase around 180 ppm.
tube. This paper presents the results of in-situ monitoring of the mesophase formation process using the new technique of “C-NMR, and the relationship between structural characteristics of pitch and its behavior during the mesophase formation process.
1. INTRODUCTION Several in-situ monitoring methods for mesophase formation have been reported by means of the hotstage polarization microscope[l-31, high-temperature ‘H-NMR[4], and the ESR spin probe method[4, 51. The methods have revealed some important aspects of mesophase formation. However, the informations obtained still remain qualitative over the range of the entire mesophase formation process. Nishizawa et a[.[61 have reported that a characteristic peak due to aromatic carbon in mesophase appears around 180 ppm in “C-NMR spectrum of mesophase pitch in a liquid state at high temperature, while aromatic carbon in regular pitch in a liquid or dissolved state shows a peak in the vicinity of 130 ppm. The chemical shift of the aromatic peak to a low magnetic field is well known at the transition from isotropic to nematic in liquid crystal[7]. The new peak appearing at around 180 ppm is assigned as carbon in stacked lamellae in liquid state mesophase pitch. Yokono et al.[S] have shown a correlation between mesophase content and the peak intensity around 180 ppm in mesophase pitch. They have also reported that the addition of solvent causes a decrease in the 180 ppm peak intensity. Monitoring of the transformation from isotropic to nematic phase (so-called mesophase) in pitch using in-situ high-temperature “C-NMR is generally not easy, because of the low sensitivity of “C-NMR in comparison to that of ‘H-NMR. To overcome this problem in measurement, the authors performed insitu measurements of the ‘C-NMR spectra during the high-temperature heating process, using a high magnetic field NMR with a large diameter sample
2. EXPERIMENTAL 2.1 Sample Coal-tar-derived (pitch A), petroleum-derived (pitch B), and hydrogenated petroleum-derived (pitch C) pitches were selected. Table 1 shows the characteristics of the samples used in this experiment. 2.2 High-temperature “C-NMR High-temperature ‘X-NMR measurements were carried out using a Bruker MSL-300 FT-NMR with high-temperature probe (“C resonance frequency of 75 MHz). The magnetic field used is 7.0 T. A “CNMR spectrum of 10% ethylbenzen in CDCl, was measured at room temperature to calibrate chemical shifts of peaks in high-temperature 13C-NMR spectra. A large sample tube was used to get a high signalto-noise ratio. A pitch sample (~500 mg) was put into an NMR tube of 10 mm diameter. Glass wool was softly packed into the sample tube at the position of about 3 cm from the bottom to keep an insulating zone of about 1 cm thickness. Samples were heated at the rate of 5”Cimin up to 430°C and then held at 430°C under a nitrogen gas flow. At temperatures above 2Oo”C, FIDs (Free Induction Decays) were accumulated for four minutes, and then the tuning and the necessary preparations for measurement 943
K. AZAMIet al.
944
Table 1. Elemental analvsis and softening noint of Ditches Elements1 analysis(wt%) Pitch
sample
Abxil-tarderived) B (pahhm+eriv~) C%hymtipetroleum-derived)
C
H
N
94.4 93.3 926
3.6 6.2 6.5
l.2
softening point CC) 90 :
qmade from pitch B ?JYhydrogenation.
were carried out within two minutes. This cycle of testing was repeated at intervals of 30°C (six minutes) up to 430°C. After reaching 430°C FIDs were accumulated for ten minutes and this measurement was repeated until the transformation from isotropic phase to mesophase was finished. All FIDs were obtained with a 45” pulse width of 5 psec, a pulse repetition time of two seconds, and proton complete decoupling. All spectra were obtained through Fourier transformation of accumulated FIDs, with a line broadening factor of 100 Hz. 2.3 Solution NMR Solution NMR is a routine method and gives feasible data on distribution of various types of bonded carbon. A solution of pitch (-1 g) in CDCIS (==3 ml) was put in an NMR sample tube of 10 mm diameter. A small quantity of tetramethylsilane was added into the solution to calibrate chemical shifts for pitch. Solution NMR spectra were obtained using a JEOL FX-100 FT-NMR to calibrate the aliphatic carbon content from high-temperature ‘C-NMR spectra. Conditions of the above high-temperature ‘)C-NMR signal acquisition were chosen as optimal for sensitivity. The calibration curve for aliphatic carbon content in the sample is shown in Fig. 1. This was obtained from the plots of relation between the al-
iphatic peak areas in spectra from quantitative and sensitivity-stressed measurements using various petroleum pitches. The quantitative measurements were made with an approximately 30” pulse width of 3 psec, a pulse repetition time of 10 seconds, and a proton decoupling of non-NOE (Nuclear Overhauser Effect) mode. The sensitivity-stressed measurements were made with almost the same conditions as high-temperature 13C-NMR. The number of signals accumulated was 4000-6000 for both the quantitative and sensitivity-stressed measurements. 2.4 Solid state high resolution CPIMAS “C-NMR A powder sample (-220 mg) was packed in a sample container of 7 mm outer diameter and 18 mm length. CP/MAS (Cross Polarization /Magic Angle Spinnig) 13C-NMR spectra were obtained using Bruker AM-250 FT-NMR with a rotating rate of 4.2 KHz, a contact time of 2.5 psec, a repetition time of 3 seconds, an accumulation of 512 times, and a line broadening factor of 160 Hz. Chemical shifts of peaks for the sample were calibrated using glysine as an external reference. 3. RESULTS AND DISCUSSION
The temperature dependence of 13C-NMR spectra for pitches A, B, and C during the heating step from 380°C to 430°C and the holding step at 430°C are shown in Figs. 2, 3, and 4, respectively. Spectra at the temperatures below 380°C are almost the same as those of 380-400°C. Chemical shifts of various aromatic hydrocarbons are mostly around 110-150 ppm in a liquid or solution state[9]. A peak around 130 ppm (peak I) in the spectra of pitches corresponds to these chemical shifts, that is, a characteristic peak for isotropic phase. A peak around 180 ppm (peak M) corresponds to a characteristic peak for mesophase as described above. It was clearly observed that all the pitches gave peak M. The intensity of peak M relative to that of peak I increased while the peak I decreased and eventually disappeared. Thus, it is clear that the process from mesophase nucleation to bulk mesophase formation was accurately monitored. 3.1 Characteristics of mesophase formation process for three pitches Pitch A. The appearance of peak M was recog-
‘- E
44% If
0
20
Aliphatic
40 carbon
quantitative
60 content
80 determined
measurement
100 by
(%I
Fig. 1. A calibration curve for determining an aliphatic carbon content using high-temperature W-NMR spectra.
nized in the spectrum at the heating condition for 10 to 20 minutes at 430°C. The intensity of the peak increased with the holding time, and became larger than that of peak I for 130 to 140 minutes. After this step peak I remained for a while, but finally disappeared for 200 to 210 minutes. Pitch B. Peak M was observed for 10 to 20 minutes at 430°C. Then the intensity of the peak increased with the increase of time. Meanwhile, the intensity of peak I decreased and finally disappeared for 80 to 90 minutes. Pitch C. The intensity of aliphatic carbon dis-
945
Monitoring for mesophase formation by “C-NMR
k
j 160-170tin
380
h
300 200 100 0 -100
300 200 100 0 -100
308 200 100 0 -100
300 200 100 0 -1OOwm
Fig. 2. The temperature dependence of “C-NMR spectra for the pitch A (coal-tar-derived pitch) during the heating process from 380°C to 430°C and holding process at 430°C.
tinctly declined just after the holding process at 430°C began, and then decreased to a level less than that of noise peaks at 50 to 60 minutes. Just after this phenomenon, peak M appeared in the spectrum of 70 to 80 minutes. Then the intensity of this peak increased. On the other hand, peak I became weak and finally disappeared around 160 to 170 minutes. 3.2 Comparison of characteristics for mesophase formation process Table 2 shows the time required for mesophase Ta, the time required for finishing appearance, mesophase transformation (equivalent to the time
300 200 loo
0 -100
300 200 100
0 -1OOwm
Fig. 3. The temperature dependence of “C-NMR spectra for the pitch B (petroleum-derived pitch) during the heating process from 380°C to 430°C and holding process at 430°C.
required for disappearance of peak I), Tb, and the time interval from mesophase appearance (Ta) to finishing mesophase transformation (Tb), AT. All data in Table 2 were obtained from a series of the spectra of in-situ “C-NMR. 3.3 Change of aliphatic carbon content on heating and mesophase transformation Table 3 shows the aliphatic carbon content obtained from spectra accumulated for one hour at 250°C and that of the holding process at 430°C. The appearance of mesophase for pitch A is thought to be relatively early because this pitch has little aliphatic carbon and has a high aromaticity. On the other hand, pitch C has a rather large amount of aliphatic carbon. The molecules in this pitch require relatively longer heating time than pitch A in order to become the lamellar molecules, which are to be involved in mesophase. The mesophase appearances in pitches B and C did not occur until the aliphatic carbon content (carbon aliphaticity) reached less than 6% to 10%. Namely, it is thought that an aliphatic carbon content of 6% to 10% may be regarded as a critical region for the mesophase appearance of petroleum-derived pitch. We have reported(41 that there may be a critical region of aromaticity for isotropic-to-mesophase transformation of petroleumderived pitch in terms of hydrogen. The present study has revealed such a critical region in terms of carbon as the main constituent of pitch molecules. Meanwhile, untreated coal-tar-derived pitch already has an aliphatic carbon content of less than 1%. Namely, this pitch is thought to have the necessary planarity of molecule to form a mesophase. However, it is thought that molecules in coal-tar-derived pitch have not yet developed to a size necessary to
K. AZAMIet al.
946
380
aLjt,,_
h
410 B
& 430°C
O-l On-in
lO--2Orrin
20-30min
& &I
300
200
100
0 -100
300
200
100
0 -100
300
200
100
0 -1OOwm
Fig. 4. The temperature dependence of “C-NMR spectra for the pitch C (hydrogenated petroleumderived pitch) during the heating process from 380°C to 430°C and holding process at 430°C.
be stacked. This may be the reason why carbon aliphaticity is not a criterion for mesophase appearance of coal-tar-derived pitch. 3.4 Line-width of characteristic peak (peak M) for mesophase and mesophase formation process
Comparing the line-widths of peak M for three pitches, pitch A has very wide line-widths, for example, half-widths of 3-4 KHz during the holding process between 50 to 100 minutes at 430°C. On the other hand, pitches B and C have half-widths of 1.52.2 KHz during the holding process of 20 to 70 minutes at 430°C and 1.2-1.7 KHz during the holding process of 110 to 160 minutes at 430°C respectively. Generally, the narrower the line-width is, the higher the mobility of the molecules becomes. The molecules in pitch A are relatively rigid in mesophase already formed. In contrast with pitch A, mesophase-composing molecules in pitches B and C have a high mobility, and rearrangements are rather easy within the mesophase. This is the reason why the mesophase in these petroleum-derived pitches grows rapidly. Because aliphatic carbon is responsible for mesophase appearance as mentioned above, the role of
aliphatic carbon in mesophase has to be taken into account. The aliphatic carbon content in the mesophase pitches obtained from pitches A and B were determined using high resolution solid state r3CNMR spectra*. Figure 5 shows spectra for coal-tarderived and petroleum-derived mesophase pitches. Petroleum-derived mesophase pitch shows about 5% of aliphatic carbon, while aliphatic peaks cannot be observed for coal-tar-derived mesophase pitch. From the results we can draw a schematic model as illustrated in Fig. 6. Molecules in coal-tar-derived mesophase pitch are planar with low mobility and interact with strong intermolecular force. On the other hand, molecules in petroleum-derived mesophase pitch are rather mobile and bend due to the contribution of aliphatic portions. This difference in mobility is thought to be reflected in the mesophase formation rate.
Table 3. Aliphatic carbon content for pitches Aliphatio carbon content (%) Pitoh sample
430°C 25oV 0-1Omin lO-2Omin 20-3Omin
Table 2. The time required for mesophase appearance, Ta, the time required for finishing mesophase transformation, Tb, and the time interval from mesophase appearance (Ta) to the end of mesophase transformation (Tb), AT PdCll
TZI
Tb
AT
sample
0llid
(tin)
(Din)
2ow210
lm-200
-4
lo-20
B
lo-20
8c- 90
Ia- 80
C
7iH30
16+170
8cHOO
A B C
_b
_b
_b
8 22
_b
_b
12
6
aobtainod from spectra accumulated b undeteoted.
for 1 hour.
*13C-NMR spectra were obtained using the CP/MAS method, which enables one to assign peaks using the data base obtained from isotropic solution spectra.
947
Monitoring for mesophase formation by “C-NMR
n
(S) Spinningside band of the peak arolnd 130ppm
A (Coal-tar-derived
(E.G.) Signal
:‘x::::-h-
of san-~lecontainer
Coal-tar-derived Aromatic
250
200
150
100
Fig. 6. A structural
Aliphatic 50
0
model for mesophase-composing molecules.
wm
3. Aliphatic carbon in mesophase causes looseness of molecular planarity, increase of mesophase mobility, and narrowing of the characteristic peak around 180 ppm for mesophase.
B (Petroleum-derived mesophase pitch)
Petrolem-derived
Ii
REFERENCES Aromatic
250
200
150
100
1. D. W. Hoover, A. Davis, A. J. Perrotta, and W. Spackman, Absts. 14th Biennial Conf. on Carbon, (1979), p.
Alichatic
,
50
0
393.
ppm
Fig. 5. High resolution solid state ‘%-NMR spectra.
2. A. J. Perrotta, J. P. McCullough, and H. Beuther, Absts. 16th Biennial Cot@ on Carbon, (1983), p. 76. S. Uemura, T. Hirose, H. Takashima, 0. Kato, and M. Harakawa, Abst. 16th Biennial Conf. on Carbon, (1983),
4. CONCLUSIONS 1. In-situ monitoring for mesophase formation process has been successfully accomplished using high-temperature V-NMR. 2. The mesophase appearance and the time required for finishing mesophase transformation are determined. The wider the line-width of the characteristic peak for mesophase around 180 ppm, the longer the time interval from mesophase appearance to completion of mesophase transformation.
p. 78.
4. K. Azami. T. Yokono. Y. Sanada. and S. Uemura. Carbon 27, lj7 (1989).
5. K. Shibata, H. Kakiyama, Y. Sanada, and J. Sohma, Fuel 57, 572 (1978). 6. T. Nishizawa and M. Sawa, Proc. 14th Annual Meeting of Carbon Material Sot. Japan lA1.5 (1987). 7. K. Hayamizu, M. Yanagisawa, and 0. Yamamoto, Chem. Phys. Letters 127, 566 (1986).
8. T. Yokono, N. Takahashi, T. Kaneko, and Y. Sanada, Fuel 69, 796 (1990). 9. See, for example, F. W. Wehrli and T. Wirthlin, Interpretation of Carbon-13 NMR Spectra, Heyden & Son Ltd., London, a supplement chart (1978).