Coal Science

32 Proton and Carbon-13 NMR of Coal Derivatives and Other Carbonaceous Materials H. L. RETCOFSKY and R. A. FRIEDEL

Downloaded by UNIV LAVAL on December 15, 2015 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0055.ch032

Pittsburgh Coal Research Center, Bureau of Mines, U. S. of the Interior, Pittsburgh, Pa.

Complementary

use

of

Department

proton

and

carbon-13

nuclear magnetic resonance techniques has been applied to studying the mean structural units present in coal derivatives and other carbonaceous materials. The method requires using quantitative data from both proton and C

13

spectra. Although

PMR intensity measurements are known to be quite reliable,

the

corresponding

C

measurements

have been explored very little.

Intensity-concen-

13

tration calibration curves for a series of alkyl aromatics and hydroaromatic compounds suggest that quantitative C

13

data can be used with rea-

sonable assurance of accuracy. The technique has been applied to several coal derivatives and chemically reduced coal derivatives.

J h e literature contains several papers on applying high-resolution proton n u clear magnetic resonance to studying both hydrogen and carbon distributions in coal and other carbonaceous materials (1,2,3,4,5,7,8,10,11,13,14,15). The structural parameters generally derived from the spectral data include the aromaticity, degree of aromatic ring substitution, and average size of the condensed ring system (2,3,7,14) although more recent work has been concerned with the aliphatic hydrogen distribution (10). In all material published to date, however, assumptions generally were made concerning one or more of these parameters before the others could be calculated. A s one example, aliphatic branching indices were necessarily estimated before aromaticity calculations could be made for vacuum distillation products from coal (8). W e propose to eliminate this and a number of other assumptions by complementing and sometimes replacing the proton N M R data with i n 503

In Coal Science; Given, P.; Advances in Chemistry; American Chemical Society: Washington, DC, 1966.

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COAL SCIENCE

formation gained from another N M R technique—namely, carbon-13 spectra of the materials. T o perform the C experiments we rely upon N M R signals arising from only naturally occurring carbon-13 in the material. The low natural abundance, ~ 1.1%, along with its other unfavorable characteristics, does not allow the use of conventional N M R techniques such as those used to detect proton signals. Unlike hydrogen N M R signals, C signals are difficult to detect and possess unsymmetrical line shapes which make absolute intensity measurements impossible. The technique, at present, is limited in sensitivity and requires the use of neat liquids or highly concentrated solutions as samples. Later we w i l l discuss methods for improving sensitivity. W e wish, now, to report what we believe is the first successful attempt to derive quantitative data from C N M R spectra and in particular to discuss application of the method to simple organic mixtures, to a liquid coal derivative, to two coal derivatives having desirable solubility characteristics, and to a crude o i l . 1 3


1 3

1 3

Experimental A l l spectra were obtained on a Varian high-resolution instrument equipped with 60 mc./sec. and 15.085 mc./sec. radio-frequency units for the proton and carbon-13 studies, respectively. Magnetic fields of about 14.1 k g . were employed. Proton spectra were obtained using conventional operating techniques—i.e., observing the absorption mode under slow-passage conditions. The C spectra, however, were of the rapid-passage dispersion mode type previously described by Lauterbur (13). A constant sweep rate and transmitter power level were used when operating at the lower frequency. The double resonance experiments were performed using an N M R Specialties SD-60 spin decoupler. 1 3

Proton

Spectra

Proton N M R spectra of coal derivatives generally give rise to either broad peaks or complicated multiplets which can be easily divided into band envelopes. F o r example, the H spectrum of a coal-hydrogenation asphaltene (4) consists of three peaks, two of which overlap. A broad peak at lowest field is caused by protons in aromatic and phenolic systems, whereas two higher field peaks are caused by protons bonded to carbons situated a to aromatic rings and those bonded to other nonaromatic carbons, respectively. The ratios of these spectral areas are the same as the ratios of the hydrogens in each of these three hydrogen classes. This accounts for one of the most important characteristics of proton N M R spectra—namely, no calibration data are necessary. 1

Carbon-13

Spectra

The most valuable information one should be able to obtain from the carbon spectrum of a coal derivative is the ratio of aromatic to nonaromatic carbon atoms. W e know of no other direct method by which this value can be obtained. Since the literature pertinent to C N M R is sparse, we wish to 1 3


RETCOFSKY AND FRIEDEL

Proton and Carbon-13 NMR

505


32.

CHEMICAL SHIFT, ppm from C S 2 Figure I.

C

1 3

spectra of aromatic

compounds

discuss the spectra i n more detail. Figure 1 presents some illustrative spectra of pure organic compounds. I n spectra of alkylbenzenes, as well as i n spectra of other materials, the aromatic carbon signals are found at l o w field, w e l l separated from signals caused by saturated carbon atoms. Olefinic atoms exhibit signals i n the same general region as aromatic carbons, triply bonded


COAL SCIENCE

506

carbons have their signals m i d w a y between those caused b y saturated carbons and those caused b y aromatic carbons, while carbonyl signals span a region below aromatic signals extending to very l o w magnetic fields (14). These chemical shifts are referred to carbon disulfide, whose C signal is taken arbitrarily as 0 p.p.m. In general it can be said that these chemical shifts depend on electron density, electronic excitation energies, and the bond multiplicities of the carbons under study ( 1 5 ) . Intensity measurements using rapid-passage dispersion mode spectra have been little explored to date. It became necessary, therefore, to study the quantitative aspects of C N M R . O u r early work was limited to measurements on alkyl aromatic a n d hydroaromatic compounds. T h e results of i n tensity measurements on the C spectra of 15 compounds are illustrated graphically i n Figure 2. E a c h point is the average of measurements on at least eight spectra, four of w h i c h were obtained with sweep increasing a n d four 1 3

1

3


1

3

AROMATICITY, f Figure 2.

Aromaticity

0

calibration curve for a C spectrometer

1

3

NMR

with it decreasing. T h e l i q u i d samples were r u n neat, while the solid ones were r u n as saturated carbon disulfide solutions. Plotted along the abscissa is the quantity w h i c h w e designate /«, the fraction of total carbons i n the m a terial w h i c h are present as aromatic carbons. T h e ordinate, /«', refers to the fraction of the total area under the N M R curve assignable to aromatic carbon resonances. These 15 points were fitted to a second degree equation b y the


32.

RETCOFSKY AND FRIEDEL Table I.


Test of Least Squares Fit of C " NMR Calibration Data

Compound


507

1-Methylnaphthalene Toluene 2,3-Dimethylnaphthalene p-Xylene 0- Xylene Mesitylene Tetralin 1,2,3,4-Tetramethylbenzene terf-Butylbenzene 1- Methyl-4- tert-butylbenzene 1,3,5-Triethylbenzene Hexamethylbenzene Octahydrophenanthrene 1.3.5-Triisopropylbenzene 2.4.6-Tri-tert-amylphenol

Aromaticity C

Known 0.090 0.857 0.833 0.750 0.750 0.667 0.600 0.600 0.600 0.545 0.500 0.500 0.429 0.400 0.286

NMR 0.913 0.850 0.791 0.768 0.775 0.692 0.599 0.610 0.525 0.522 0.548 0.476 0.452 0.411 0.295 1 S

Av.

Dev. +.004 —.007 —.042 -f.018 -f.025 -f.025 —.001 +.010 —.075 —.023 +.048 —.024 +.023 +.011 +.009 ±0.023

method of least squares; the solid line shown is a plot of the calculated equa tion. T h e dotted line would result if the measured areas were directly pro portional to the carbon distribution. It should be pointed out that the scatter of these points does not result from errors i n reproducibility but is caused by characteristics of the nuclear signals involved. T h e differences between the known aromaticity values a n d those cal* culated from the least squares equation are given i n Table I. T h e mean value of the deviations, ± 2 . 3 % absolute, indicates the accuracy of the method, although i n reality it only tests the fit of the calculated equation. T h e devia tions were surprising i n that they were smaller than anticipated. Figure 3 indicates how well this aromaticity calibration curve is repro duced by C measurements on a series of benzene-cyclohexane blends. Here the dotted line is the calibration curve shown previously (Figure 2 ) , while the solid line passes through the experimental points for the blends. T h e dif ference between the two curves is about 2 % . 1

Interpreting

3

the Data

It is appropriate now to re-emphasize just what reasonably quantitative data one can derive from the proton and C spectra. In this early work we assume that atoms other than carbon a n d hydrogen are present i n only negligible amounts. T h e atomic hydrogen to carbon ratios can be evaluated from the elemental analyses of the materials a n d can be used to normalize the hydrogen and carbon N M R measurements so that they a d d to unity. T h e proton spectrum gives, after this minor modification, three items of informa tion: hnr, the fraction of total atoms i n the material which is present as hydro gen atoms directly bonded to aromatic carbons; ha, the fraction bonded to carbons situated a to aromatic rings; and hβ the fraction bonded to other nonaromatic carbons. T h e carbon spectrum i n conjunction w i t h the aro maticity calibration curve yields Car, the fraction of total atoms i n the material 1

3

i



508

COAL SCIENCE

C

Figure 3.

1

3

NMR intensity data for blends

benzene-cyclohexane

present as aromatic carbons, a n d c . a i , the fraction present as nonaromatic carbons. It is unfortunate that signals arising from saturated carbons located a to aromatic rings cannot be separated from signals caused by aliphatic car bons further removed from the rings. It w i l l be shown later that this fact prevents precise calculations of certain structural parameters. K n o w i n g these five quantities one can proceed to develop equations which w i l l yield informa tion as to the structure of an average molecule of the material under study. Three of the more important structural parameters which can be derived from the H a n d C data are the aromaticity, /·, degree of aromatic ring substitution, σ, and the atomic hydrogen to carbon ratio for the hypothetical 1

1

3

unsubstituted aromatic material,

T

n

e

I a s t

parameter indicates the

size of the condensed aromatic ring system. T h e first parameter, /a, can be calculated utilizing only C spectral data and the calibration curve (Figure 1 ) . It is important to note that no assumptions about the structure of the material need be made before /« is calculated. The other two parameters can be evaluated from complementary use of the C a n d proton data. I n order to calculate these precisely, one needs to know the number of aliphatic carbons located a to aromatic rings a n d the number of those further removed. Unfortunately, as pointed out before, there is considerable overlap between these signals; the best one can do is to evalu ate the total aliphatic carbon content, c.««. B y complementing the C data 1

1

3

3

1


3

32.

RETCOFSKY AND

FRIEDEL


509

w i t h proton data, upper and lower limits can be placed on ca and c£, and thereby on σ and [ ç - ] . m .

Since any carbon atom i n an aliphatic grouping

can bond to 0, 1, 2, or 3 hydrogen atoms, the m i n i m u m values for ca and c£ w i l l be Λα and fc/3, respectively. This corresponds to a molecule containing no saturate groups other than methyl groups. T h e maximum ca value occurs when cfi is m i n i m i z e d a n d vice-versa; therefore:


£ < « s[-nr] and hfi < CP < Γ

ha

Ί

It can be shown that: ha

hp

3_

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