Solid and liquid phase study of alkylated pitches by 1H and 13C n.m.r.

Solid and liquid phase study pitches by ‘H and 13C n.m.r. Hocine

Sfihi,

Pierre Tougne

and Andre

Pierre

of alkylated

Legrand

Laboratoire de Physique Quantique, U.A. 421. ESPCI, 70 Rue Vauquelin Cedex 05, France (Received 22 February 1989; revised 24 May 7989)

The structural determination of heat-treated and alkylated studies shows a good correlation of the aromaticity factor carbons in both solid and liquid phases. However, there is a given by elemental analysis and that deduced from solid-state result from the large size of the polyaromatic units.

75237 Paris

pitches by 13C and ‘H n,m.r. spectroscopic and the fraction of non-protonated aromatic

great deal of difference between the C/H ratio l3 CP/MAS/DD n.m.r.. This difference could

(Keywords: pitch; carbon-13 n.m.r.; n.m.r.)

Coal tar pitches are currently used as binders by the aluminium and graphite industries to produce electrodes. These products are complex mixtures of thousands of molecules. They are obtained by distillation of coke oven coal tars. The carbon yield and the density required by the users are achieved by using a complementary thermal treatment. The objective of this work is to compare structural parameters measured by solid-state and liquid n.m.r. before and after such a heat treatment.

EXPERIMENTAL

Samples Four pitches obtained from the same coal tar were studied. Some of their characteristics are listed in Table I.

Sample B results from the heat treatment of pitch Al. Samples TS’ (A) and TS’ (B) were obtained after a reducing alkylation of the toluene insoluble fractions of pitches A and B respectively2-4. This alkylation consists of grafting one or more butyl groups on the pitch molecule. Only samples TS’ (A) and TS’ (B) could be totally solubilized and were therefore studied in both liquid and solid phases. These two samples were solubilized in deuterated tetrachloro-l,1,2,2-ethane (C,D,Cl,) (around 700 mg of pitch for 2 ml of solvent). Dioxane was added to the solution as an internal reference. To obtain a good Table 1 Structural parameters protonated fl, aromatic carbons Pitches

phase correction in the 13C n.m.r. observation5, carbon disulphide (CS,) was also added to the solution. N.m.r. spectroscopy 13C and ‘H n.m.r. measurements in the liquid phase were made at 50.30 and 200.13 MHz, respectively, on a Bruker AC 200 spectrometer. The frequency sweep width (SW) was 2600 Hz for lH and 119OOHz for 13C. To reduce the duration between each accumulation, the pulse angle was 45” for lH and 65” for 13C. The delay between each accumulation was 3.2 s for ‘H and 300 s for 13C, and the number of accumulations was 8 for the first spins and in the range 1000-2ooO for the second spins. In the case of ’ 3C, inversed gated decoupling pulse sequences, (without nuclear Overhauser effect (NOE)), were used6. Selective probes of diameter 5mm for ‘H and 10mm for 13C were used. The solid-state 13C n.m.r. measurements were made at 25.14 MHz in a static field of 2.1 T with a Bruker CXP 100 spectrometer, using 13C--lH cross-polarization combined with magic sample spinning and dipolar dephasing (CP/MAS/DD) 7,B. For all the measurements, we used a double bearing probe. A rotor consisting of an alumina barrel was used to hold the sample. The sample rotation was z 4 kHz. Only one value (2 ms) of contact time, TCP, was used. The number of accumulations was 1440 and the delay between each accumulation was 5s. Other details of the experiments have been given elsewhere’.

of the different samples determined by 13C solid state n.m.r.: fractions;

non-mobile

CiH

s,

fi;l”; and mobile fi

aliphatic

fx

carbons

aromaticity fractions

S”AI

factor f.; non-protonated

flj’;

fl;;”

_G

and

(A)

1.89-l .64”

0.96

0.47-0.45b

0.53~Slb

-

-

(B)

2xF1.72”

0.96

0.5W.48”

0.50-0.4s*

_

_

TS’ (A)

0.93-0.92”

0.62

0.63UX39b

0.37-0.23b

0.77-0.29*

0.23-0.09*

TS’ (B)

1.03-l”

0.65

0.65-0.42b

0.35-0.23b

0.8 1-0.28b

0.19-0.07b

“Values deduced from CP/MAS/DD ‘jC n.m.r. measurements bValues per carbon atom of the sample, Equation (1’) In the case of samples (A) and (B), the proportion of aliphatic carbons 0016-2361/89/l 1137&05$3.00 0 1989 Butterworth & Co. (Publishers)

1376

Ltd.

FUEL, 1989, Vol 68, November

(0.04 = 1 -f,)

is too low to distinguish

between mobile and non-mobile

carbons

Solid and liquid phase studies of alkylated pitches:

RESULTS AND DISCUSSION 13C solid-state n.m.r. results The 13C CP/MAS n.m.r. spectra of the different samples are shown in Figure 1. As can be expected, the aliphatic absorption band (30 ppm) is more important in the case of TS’ (A) and TS’ (B) than in the parent pitches, ,due to the alkylation process. The aromaticity factors deduced from the integrated intensity of the spectra are listed in Table2 for the different samples. As far as the functionality is concerned, the spectra of the parent and the alkylated pitches have the same shape, qualitatively, which indicates the presence of the same functional groups. However, their relative proportion is different from one sample to another. If we consider the spectra of the samples TS’ (A) and TS’ (B), the intensities of the peaks at 13 and 23 ppm appearing in the aliphatic region are different for both samples. The peak at 13 ppm, representative of the terminal methyl groups, is slightly more intense in the case of TS’ (A) than in the case of TS’ (B), indicating a higher content of -CH,-CH,. In Figure I, it is apparent that the intensity of the shoulder at 136 ppm, characteristic of non-protonated aromatic carbons, is higher in the alkylated pitches than in the parent ones. This result indicates that the parent pitches (A and B) contain more non-protonated aromatic carbons than the alkylated ones.

H. Sfihi et al.

Quantitative estimation of the non-protonatedf,“: and protonated fir aromatic carbon fractions can be obtained by using 13C dipolar dephasing method (CP/MAS/DD)1~g~‘0~“~12. Moreover, in the case of the alkylated pitches, it is also possible to distinguish the non-mobile aliphatic carbons from the mobile ones. Therefore we can estimate their fractions calledf,“f” and fi, respectively. The mobile fraction could correspond to terminal methyl groups. Figure2 shows the spectra obtained at different dephasing times for TS’ (A). As can be expected, the intensities of the aromatic and aliphatic bands decrease with increasing dipolar dephasing time. For the aromatic band, it has been indicated in previous WOrk’,%‘0.11,‘2 that this decrease is due mainly to the relaxation of protonated carbons. The structure appearing in this band below T,,= 50 ps means that the non-protonated aromatic carbons are of at least two types. For the aliphatic band, this decrease is due to non-mobile carbons. Interestingly enough, only the peak corresonding to the terminal CH, groups is still present

I,

1

I

,

I,,

,

1,

I,, 100

150

I,,

,

,

50

I,,

0

fwm

Figure 1 ’ 3C CP/MAS phase (from Ref. 15)

Table 2

Comparison

n.m.r. spectra

of the structural

ofthe different samples in solid

parameters

determined

Figure 2 13C CP/MAS/DD different dephasing time T,,

n.m.r. spectra obtained (from Ref. 15)

for the TS’ (A) at

in solid and liquid phase by 13C n.m.r. ____

Pitches

_L

f”Al

fl,”

TS’ (A)

0.62”

0.59 + 0.03b

0.63 + 0.05”

0.69 f 0.05b

0.37 f0.05”

0.3lf0.05*

TS’ (B)

0.65”

0.59 f 0.03b

0.65 f 0.05”

0.75 f 0.05b

0.35 f0.05”

0.25 &-0.05b

“Using b Using

13C solid-state n.m.r. measurements 13C and ‘H liquid n.m.r. measurements

FUEL, 1989, Vol 68, November

1377

Solid and liquid phase studies of alkylated pitches: H. Sfihi et al

The quantity of aromatic protons fz is equal to the quantity of protonated aromatic carbons. On the other hand, we can assume that the non-mobile aliphatic carbons correspond mainly to CH, groups in the case of TS’ (A) and TS’ (B). We also know that coal tar pitches are mainly composed of aromatic structures with short side-chains. Therefore, we can choose an arbitrary average value of 2.5 for the number of protons per aliphatic carbons. Thus the quantity of aliphatic protons f!, can be approximated by the following formulae: fi = 2fzf” + 3fjJ for TS’ (A) and TS’ (B)

(3)

and fz = 2.5( 1 -f,) for the parent pitches

(4)

Then by using Equations (l)‘, (2)‘, (3) and (4), the atomic C/H ratio is: C/H = 1/[2f,“f”+ 3fl +flJ C/H = 1/[2.5(1 -f,)+f&] The I

I

180.0

I

I

I

I

I

I

I

120.0

150.0

I

,

90.0

,

,

I 60.0

,

,

I 30.0

I

I

I 0.0

wm

Figure 3 13C CP/MAS/DD n.m.r. spectra of the different samples obtained with a dipolar dephasing time of 70~s

above T,, = 90 ps. Because of the high molecular motion involved in the lH-‘jC dipoledipole interaction, these carbons have a long relaxation time T$, compared with the other aliphatic carbons. The same phenomenon is observed in the other samples. The comparison of the spectra obtained with T,, = 70 ~LSin the different samples (Figure3) shows that the alkylated pitches contain qualitatively more non-protonated aromatic carbons than the parent pitches (all the spectra obtained with T,, = 0.25 ~CLS are normalized to the same scale). From the evolution of the 13C n.m.r. signal intensity as a function of the dipolar dephasing timel, we have deduced the different fractions of carbons. The obtained values are given in Table 1. The different fractions are: fA”P+flr = 1

(1)

fA”f” +fS = 1

(2)

The values confirm the qualitative analysis made above: TS’ (B) contains less CH,-CH, groups than TS’ (A); and TS’ (A) and TS’ (B) have a higher number of non-protonated aromatic carbons than (A) and (B). In recent results obtained with other pitches, we have shown that the non-protonated aromatic carbons are not fully detected’. This result has been correlated to the size of the aromatic domains. Thus, it is interesting to calculate the C/H ratio of these samples. They do not contain aromatic carbon only, therefore we need to normalize the different carbon fractions, fip, fir, fiy and a to the whole quantity of carbon atoms present in the sample, and to estimate the whole quantity of the protons (aromatic and aliphatic) also present in the sample9~“*‘2. Equations (1) and (2) became: fAn,P +fir =f,

(1)’

fA”;”+fZ = 1 -Al

(2)’

where f, = aromaticity factor.

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1989,

Vol 68, November

C/H

values

for TS’ (A) and TS’ (B) (5) for the parent pitches

thus calculated

are compared

(6) in

Table I with those given by elemental analysis. Values

are approximately the same in the case of alkylated pitches, whereas in the case of the parent ones the C/H values deduced from n.m.r. measurements are smaller than those given by elemental analysis. This result might mean that all the aromatic carbons are detected in the case of TS’ (A) and TS’ (B), but not fully in case of the parent pitches. The proportion of the carbons that would not be detected is 10% in sample (A) and 30% in the heat treated sample (B). This proportion is obtained by subtracting the C/H value deduced from 13C CP/MAS/DD n.m.r. measurements from the C/H given by elemental analysis, and divided by this later value. Another phenomenon could be evoked for explaining the lower C/H values deduced CP/MAS/DD n.m.r. measurements. In recent work performed on various aromatic model compounds13, the 13C n.m.r. results show that even if the totality of aromatic carbons were detected after cross polarization, the use of dipolar dephasing systematically leads to lower values of C/H compared with the real ones. This is due to the fact that the non-protonated carbon atoms have two exponential decays. The ’ 3C magnetization corresponding to these carbon atoms decays rapidly with practically the same rate as the protonated carbon atoms (in the range T,,=&50ps). T,,> 50,~s causes slow decay. An explanation of these two exponential decays can be found in the crystallographical structure of the model compound molecule, which consists of two independent crystallographical sets. In one set, non-protonated carbons are weakly coupled with intramolecular protons, and in the other set, they are strongly coupled with intermolecular protons. The extrapolation of the results from T,,=50ps to T,, = 0 does not give good values of C/H. Following this hypothesis, the discrepancy between C/H ratios determined chemically or by n.m.r. that are observed on parent pitches may be interpreted in terms of organization of the aromatic clusters. X-ray studies have shown“j-‘* that not many organic molecules (e.g. anthracene, pyrene, and phthalocianines) are arranged symmetrically in the solid state. This organization gives rise to a noticeable intermolecular dipolar interaction, and consequently to an underestimation of non-protonated aromatic carbons

Solid and liquid phase studies

content by 13C dipolar dephasing CP/MAS n.m.r.. The more important the differences of the C/H values are, the more important is the cross-linking effect. Note that heat treatment of pitches always improves their aromatic characters, observed through C/H values and other macroscopic effects. On the contrary, the TS’ parts obtained after alkylation do not show such discrepancies possibly because the aromatic molecular portions are not strongly physically coupled. This would be due to the larger amount of aliphatic chains that prevent such association, giving rise to a more disorganized solid than in the parent pitches. Nevertheless, for both kinds of materials this means that large aromatic structures are not present, as demonstrated by electronic microscopy. We must conclude that this second hypothesis may be the right one, instead of the non-detectable carbon theory, for that particular class of materials. ‘H and 13C liquid 7l.m.r. results The 13C liquid n.m.r. spectra obtained for samples TS’ (A) and TS’ (B) are shown in Figured. The peaks appearing at ~67.8, 73.7 and 192.8 ppm correspond to the carbons of dioxane, C,D,Cl, and CS,, respectively. One can also observe the presence of four narrow peaks in a ratio 1:2:2:1 in the aromatic region (12&140ppm) and one peak at 21.3 ppm for TS’ (A). These groups of peaks result from aromatic carbons, and methyl groups of the toluene, respectively. Peaks of narrow width and low intensity (at 71 ppm) correspond to the carbons of poly(ethylene oxide), used in the alkylation process5.

1 200

180

160

140

120

100

80

60

40

20

0

rwm

b

of alkylated

pitches:

H. Sfihi et

al.

a

1

I 10.0

I

I 9.0

I

I 8.0

1

1 7.0

I

1

I

6.0

I 5.0

1

I 4.0

1

I 3.0

I

I _L_t__L_LL__ 2.0 1.0 0.0

wm

I

I

I

10.0

I

I

9.0

I

8.0

I

I,1

7.0

I

I

6.0

5.0

I

I

4.0

L

I

I

3.0

I

2.0

I

I

1.0

I

I,

0.0

mm

Figure 5

‘H liquid n.m.r.

spectra of the samples: TS’ (B)

a, TS’ (A); and b,

The aromaticity factor calculated from the integrated intensity of the signal (by subtracting the contribution of the signal from the toluene carbons), is the same for the two samples. The discrepancy between f, values in solid and liquid phases is due to experimental error (z 10%). Non-protonated aromatic carbon fractions have also been calculated in TS’ (A) and TS’ (B). For this, we used simultaneous ‘H n.m.r. and elemental analysis. The ‘H n.m.r. spectra of TS’ (A) and TS’ (B) have the same shape (Figure5). The peaks appearing at 6 and 3.7 ppm in the two spectra result from the protons of C,H$l, (impurities contained in the solvent C,D,Cl,) and the impurities of the alkylation products (polyethylene glycol), respectively. The set of peaks appearing in the region 7.2-7.4 ppm in the TS’ (A) spectrum correspond to the aromatic protons in toluene. The peak at 2.3 ppm in the same spectrum corresponds to the protons of CH, of the toluene. The fraction of aromatic protons in the pitch is given by: H,, = H,,(pitch)/H,

200

180

160

140

720

100

80

60

40

20

0

wm

Figure 4 TS’ (B)

13C liquid n.m.r.

spectra

of the samples:

a, TS’ (A); and b,

where C,,(pitch)= total number of aromatic carbons in the pitch; H,,(pitch) = total number of aromatic protons in the pitch = H,,(total) - 5H,/3; H,, = total number of aromatic protons (pitch + toluene) = integrated intensity of ‘H aromatic n.m.r. spectra (6.5 - 10 ppm); H, = total number of aliphatic protons (three protons) in the reference molecule (toluene) = integrated intensity of the peak at 2.3ppm in ‘H n.m.r. spectra of TS’ (A); H, = H,,(pitch) + H,,(pitch); H,,(pitch) = total number of aliphatic protons in the pitch.

FUEL,

1989,

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Solid and liquid phase studies of alkylated pitches: H. Sfihi et al.

Thus, the fraction of the non-protonated carbons fraction is given by:

aromatic

M. Daubenfeld collaboration.

from

ORKEM

group

for

their

f2 = VH= C!&)& where CpAr=H,,/(C/H) is the number of protonated aromatic carbon atoms contained in the pitch. The values off:: obtained by the second method in samples TS’ (A) and TS’ (B) are given in Table 2. These values are slightly higher than those obtained by 13C CP/MAS/DD n.m.r. However, this difference remains inside the experimental errors (10%).

REFERENCES 1 2 3 4 5 6

Slihi, H., Tougne, P., Legrand, A. P., Couderc, P. and Saint Romain, J. L. Fuel Proc. Technol. 1988, 20, 43 Touirssa, L. PhD Thesis Universite de Metz, France, 1987 Rischner, C., Kirsh, G. and Lauer, J. C. Erdol und Kohl, Erdgas und Petrochimie 1988, 41, 467 Rischner, C., Kirsh, G., Lauer, J. C. and Cagnant, D. Fuel submitted for publication Daubenfeld, PhD Thesis Universitt de Nancy I, France, 1984 Freeman, R.. Hill. H. D. W. and Kaptein, R. J. Magn. Reson. 1972, 7, 327

7 8

CONCLUSION The structural parameters of the alkylated pitches determined by liquid n.m.r. are the same, within experimental error of about 10%. For the parent pitches, the determination of the aromaticity factor alone is not sufficient to distinguish between the heat treated pitch (B) and the non-heat treated pitch (A). The estimation n.m.r. of the C/H ratio from 13C CP/MAS/DD measurements seems to indicate that the aromatic units of (B) are more peri-condensed than those of (A).

9 10

11 12 13

14 15

ACKNOWLEDGEMENTS This work was supported by ORKEM group. The authors thank P. Couderc, J. M. Saint Romain and J.

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16 17 18

Opella, S. J. and Frey, M. H. 1. Am. Chem. Sac. 1979,101,5854 Alemanv. L. B.. Grant. D. M., Alger, T. D. and Pugmire, R. J. J. Am. them. sot. 1983, lOS,669? Wilson, M. A., Pugmire, R. J., Karas, J. ef al. Anal. Chem. 1984, 56,933 Pugmire, R. J., Woolfenden, W. R., Mayne, C. L. et al. in ‘Chemistry and Characterization of Coals Maceral’ (Eds. R. E. Winans and J. C. Crelling), ACS Symp. Ser., 252, p. 80 Theriault. Y. and Axelson. D. E. Fuel 1988.67.62 Stihi, H.,’ Legrand, A. P. ‘and Pregermain, S. .J. Chim. Phys. 1988, 85(7/S), 781 Tougne, P., Sfihi, H. and Legrand, A. P. in ‘Carbon 88’ (Eds. B. McEnaney and T. J. Mays), Newcastle-upon-Tyne, UK, 1988, p. 352 Rischner, C. PhD Thesis Nancy, France, 1986 Sfihi, H., Tougne, P., Legrand, A. P. et al. in ‘Carbon 88’ (Eds. B. McEnaney and T. J. Mays), Newcastle-upon-Tyne, UK, 1988, p. 525 Cruikskank, D. W. J. Acta Cryst. 1957, 10, 504 Robertson, J. M. J. Chem. Phys. 1950,47,47 Ehrlich, H. W. W. Acta Cryst. 1957, 10, 699