13C1H Cross-polarization nuclear magnetic resonance spectra of macerals from coal

l 3 C-l H Cross-polarization nuclear magnetic resonance spectra of macerals from coal Herbert L. Retcofsky”

and David L. VanderHartt

*U.S. Department of Energy, Pittsburgh Energy Research Center, 4800 Forbes Avenue, Pittsburgh, Pennsylvania 15213, USA tlJ.S. Department of Commerce, National Bureau of Standards, Washington, D. C. 20234, USA (Received 18 November 1977)

The carbon aromaticities (fa) of vitrinite, exinite, micrinite and fusinite from a high-volatile A bituminous coal have been determined by 13C-’ H cross-polarization nuclear magnetic resonance spectrometry. Values of f, for the four macerals were found to decrease in the order: fusinite > micrinite = vitrinite > exinite. Estimates of the average ring size using the fa value and the elemental composition of each maceral indicated that the fusinite contained the largest polynuclear condensed aromatic ring system (> 5 rings) whereas the mean structural unit of the vitrinite contains 3-4 condensed rings.

It has recently been shown ‘,* that reliable carbon aromaticities (& values) of bituminous and anthracitic coals can be determined using the I%-IH cross-polarization nuclcarmagnetic resonance technique developed by Pines, Gibby and Waugh3. The quantitative reliability of the technique (hereafter referred to as CP n.m.r.) was established by comparing CP n.m.r. f, values for soluble materials derived from coal with their ‘known’ values. The ‘known’ values are essentially unambi uous ones obtained by more conventional high-resolution 1s C n.m.r. studies of the same materials in solution. CP n.m.r. studies of a series of vitrains ranging in rank from peat to anthracite gave added support to the classical views that most coals are highly aromatic substances and that the aromaticities of coals increase with increasing rank. The purpose of this paper is to report the results of a similar investigation of the chemical structure of mace& from coal. PREPARATION

AND CHEMICAL COMPOSITION

The maceral concentrates were prepared by centrifugal separation in salt solutions of adjusted specific gravity. The samples of coal used to obtain the concentrates were selected from a column of Hernshaw bed high-volatile A bituminous coal from Boone County, West Virginia, USA. For comparative purposes, a second sample of vitrinite was ob-

Table 7

Chemical

analyses and carbon aromaticities Proximate

Maceral

Petrographic purity f%)

Moisture

Ash

tained by hand removal of a relatively wide homogeneous band from the coal column. The macerals selected for investigation were vitrinite, micrinite, fusinite and exinite. The first three are thought to have originated from woody and cortical tissues from ancient plants. Exinite has its origin in plant material other than woody tissues, i.e. the fossilized remains of spore exines, cuticles, plant resins and waxes, and algal bodies. Details of the separation procedure as well as the chemical composition, helium density, surface area, indices of refraction, and X-ray scattering intensities for each petrographic concentrate were published previously4. Petrographic purity and proximate and ultimate analyses of the concentrates are given in Table 1. In agreement with the earlier work of Dormans, Huntjens and van Krevelen’ on macerals from coals of similar rank, the exinite sample was found to be richer in hydrogen and poorer in oxygen than the vitrinite. The micrinite however was found to be poorer in hydrogen and richer in oxygen than the vitrinite. The fusinite sample exhibited the highest carbon content and the lowest hydrogen content of all the mace&; this is probably a result of the unusual thermal processes generally associated with fusinization6. CHEMICAL STRUCTURE

STUDIES

The CP n.m.r. spectra of the macerals are reproduced in Figure 1. Each spectrum consists of a non-aromatic peak

of macerals from high-volatile

analysis (%I -.--

A bituminous Ultimate

coal, Hernshaw

bed, West Virginia,

USA

analysis f% maf)

-

-___

Volatile matter

Carbon

Hydrogen

Oxygen

Nitrogen

Sulphur

f,

33.5 32.9 28.4 53.2 12.8

85.8 85.2 85.9 86.2 91.5

5.3 5.4 4.8 6.5 3.2

6.6 7.2 8.0 5.5 4.3

1.6 1.6

0.7

0.85 0.85 0.85 0.66 0.93-0.96

Vitrinitea Vitriniteb Micrinite Exinite Fusinite

n.d. 94 87 86 96

1.9 0.6 1.8 0.9 0.9

1 .4 1.8

7.9 3.1 3.7

0.6 0.6

0.7

0.7

1.1 0.6

0.4

a Hand selected from uniform bands f~ Obtained by gravity separation

0016-2361/78/5707-0421801.00 0 1978 IPC Business Press

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Cross-polarization n.m.r. qxctra of macerals: H. L. Retcofsky and D. L. VanderHart

near 160 ppm the resonance of which extends from 100 to 200 ppm and is superimposed on a much broader, asymmetric peak with its maximum at a lower field. The latter is assigned to aromatic carbons; its pronounced asymmetry is principally the result of the large chemical-shift anisotropy expected for such carbons. Values off, were obtained by using lineshapes for the aromatic and non-aromatic components as deduced from linear combinations of the exinite and vitrinite spectra in the manner described previously172. Use of this deconvolution technique was only partially successful for the fusinite because of the unusually low field at which the aromatic resonance maximum occurred; for this reason, maximum and minimum f,values are reported for this sample. Recent work ‘,* has shown that many of the problems associated with deconvolution of CP n.m.r. spectra of coals can be overcome by utilizing the CP technique in conjunction with magic-angle spinning techniques’. The f,values obtained during the present investigation are given in Table 1. The fusinite exhibits the highest aromaticity whereas the exinite is the least aromatic of the macerals examined. The two vitrinites and the micrinite have essentially identical aromaticities, intermediate between those of exinite and fusinite.

-200

200

100

0

-100

Values off, for macerals from coals have also been estimated by the graphical densimetric method of van Krevelen, Chermin and Schuyer” and by analyses of proton second moments derived from broadline n.m.r. spectra. The most extensive list off, values was published by Dormans, Huntjens and van Krevelen’. Using the graphical densimetric method, these authors calculated f,values for macerals from a variety of coals representing nearly all ranks. They concluded that coal as a whole is highly aromatic, its aromaticity increasing more or less steadily with rank. Their results indicate that exinites are considerably less aromatic than the corresponding vitrinites, whereas micrinites are more highly aromatic than both the corresponding exinites and vitrinites. Fusinites are the most highly aromatic of the four macerals studied and their final composition is virtually reached during the early stages of coalification. A much more limited number of macerals have been examined by broadline proton magnetic resonance spectrometry1’-‘3. Vitrinite, exinite, and micrinite from a single coal were investigated by Richards and York” and by Tschamler and de Ruiter’?. Ladner and Stacey13 reported studies of vitrinites from six coals, exinites from five coals, and fusinites from three coals. The broadline ‘H n.m.r. results, taken collectively, also indicate that the aromaticities of macerals from the same coal decrease in the order fusinite > micrinite > vitrinite > exinite in agreement with the deductions based on the graphical densimetric results. A summary off, values obtained by the two methods for mace& from coals comparable in carbon content to the Hernshaw coal is given in Table 2; the CP n.m.r. results for the mace& from Hernshaw coal are included. The CP n.m.r. results also show that fusinite is the most highly aromatic maceral from bituminous coals. In contrast to the graphical densimetric and broadline n.m.r. results, which suggest that micrinites are more highly aromatic than the corresponding vitrinites, the CP n.m.r. studies indicate essentially identical aromaticities for these two macerals from Hernshaw coal. Although the reasons for this difference are not immediately obvious it should be noted that interpretation of the CP n.m.r. spectra does not depend as heavily on the use of data from model compounds as do the other two methods. It is conceivable, of course, that the micrinite from Hernshaw coal is atypical when compared to micrinites from other coals. An estimate of the relative sizes of the polynuclear aromatic ring systems in the macerals from Hemshaw coal can be made using the data of Table 1. If one assumes that the non-aromatic carbons are predominantly methylene, i.e. HalifC’ali= 2, and that the oxygen and half the non-aromatic carbons are directly bonded to aromatic rings, the atomic H/C ratio for the hypothetical unsubstituted aromatic nuclei (H&Car) can be calculated using the equation:

300

S, , ppm from CS, ‘%I-1 H cross-polarization nuclear magnetic Figure 1 spectra of macerals from Hernshaw hvAb coal

Table 2

Carbon aromaticities

Coal Markham Main Unspecified Hernshaw Typical mean values ‘American bright coal’ Dinnington Main a Ref. 13;bRef.

422

FUEL,

of macerals from coals as determined

bv various methods

Carbon in vitrinite (%, maf 1

Vitrinite

Exinite

Micrinite

Fusinite

Method

82.3 83.9 85.8 85.0 84.5 85.1

0.55-066 0.77 0.85 0.84 0.89-0.90 0.66-0.74

0.40-0.46 9.62 0.66 0.75 0.77-0.79 0.41-0.53

0.89 0.85 0.92 0.93-0.94 -

0.92 0.93-0.96 1 .o -

Broadline Broadline CP n.m.r.c Graphical Broadline Broadline

12;cPresentwork;dRef.

1978,

resonance

Vol 57, July

fa

5;eRef.

11

‘H n.m.r.a ‘H n.m.r.6 densimetricd ‘H n.m.r.e ‘H n.m.r.a

Cross-polarization n.m.r. spectra of macerals: H. L. Retcofsky and D. L. VanderHart

H,,&‘,, = (H - 3Cali/2 + 0)/C,,. The subscripts ali and ar are used to designate atoms in aliphatic and aromatic groupings, respectively. For the vitrinite, this calculation yields a value of 0.68 which indicates that the mean structural unit consists of a basic 3-4 ring condensed polynuclear aromatic system. Although the assumptions invoked in this calculation are not unreasonable for vitrinite from a bituminous coal, there are few available data to either support or refute their extension to the other mace&. The results of the calculations suggest that the average size of the polynuclear condensed aromatic systems in the macerals decrease in the order: fusinitc (>5 rings) > micrinite > exinite > vitrinite. Other investigators5~*4,‘5 are in agreement with the above sequence except for the reversal of the exinite and vitrinite values. Since exinite is thought to contain aliphatic chains in addition to hydroaromatic structures”, it is conceivable that the assumption that half the aliphatic carbons are alpha to aromatic rings is too high. This would result in a low value of H,,u/C,, and thus overestimate the actual number of rings per condensed aromatic unit.

1

VanderHart, VanderHart,

9 10 11 12

13 14 15

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2

8

D. L. and Retcofsky, D. L. and Retcofsky,

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