Energy & Fuels 1991,5, 643-647
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High-ResolutionSolid-state Proton NMR Spectroscopy of Density Fractions from Callide Coal A. M. Vassallo,* J. V. Hanna, M. A. Wilson, and N. C. Lockhart CSIRO Division of Coal and Energy Technology, P.O.Box 136,N. Ryde, NSW 2113,Australia Received January 28, 1991. Revised Manuscript Received May 20,1991
Solid-state high-resolution 'H spectra of density fractions from Callide coal have been obtained. These spectra reveal that proton aromaticity (H,)changes from 0.06 for the 1.20-1.25 g cm+ fraction up to 0.76 for the 1.44-1.45 g cm-3 fraction. These values, in conjunction with the atomic H/C ratio, have been used to measure the change in degree of aromatic carbon substitution with density. The fraction of aromatic carbon substitution changes from -0.9 for the lightest fraction to -0.35 for the heaviest fraction, demonstrating that the more dense coal fractions have less aromatic substitution than lighter fractions. Additionally, the NMR measured aliphatic/aromatic proton ratio was compared to the same ratio determined by FT'IR spectroscopy. It is shown that the ratio of aromatic to aliphatic protons determined by solid-state NMR compared to the same ratio determined by infrared spectroscopy varies from -6.5 for low-density fractions to a value of -1.5 for high-density fractions.
Introduction The heterogeneous and complex nature of coal has utilized, and driven the development of, many sophisticated spectroscopic techniques. In particular, solid-state 13Cnuclear magnetic resonance spectroscopy (NMR) has been instrumental in developing our understanding of the type of carbon found in coal. This has allowed more quantitative and detailed studies to be carried out on the formation, classification, and utilization of coal and coal products. Although extremely useful in its own right, 13C NMR can only be used inferentially to study the proton and heteroatom functionality of a substance. The development of high-resolution solid-state proton NMR,'"' and its recent availability on commercial instrumentation, now allows the investigation of the proton distribution in coals in a similar manner to that accomplished on carbon over the past 10-15 For example, the proton An aromaticity of coals can now be determined dire~tly.~ immediate application of proton aromaticity measurementa is in the determination of the degree of aromatic substitution. In the past, coal proton aromaticity has been estimated by using a number of different techniques. Among the earliest was solution NMR measurements of solvent extracts,1°-12where it was assumed that the chemical structure of a solvent extract was representative of the coal. This may be the case for those coals which have a high degree of solubility but is unlikely to hold for high-rank coals or low vitrinite coals. Infrared spectroscopy has also been used for proton aromaticity and aromaticlaliphatic proton ratio These techniques have (1) Gerntein, B. C.; Pembleton, R. G.; Wilson, R. P.; Ryan, C. M. J. Chem. Phys. 1977,66, 361-362. (2) Ryan, L. M.; Taylor, R. E.; Paff, A. J.; Gerstein, B. C. J. Chem. Phys. 1980, 72,608-516. (3) Gembin, B. C. Phib8. %M. R. SOC.tondon A 1981,299,521-546. (4) Maciel, G. E.; Bronnimann, C. E.; Hawkins, B. L. Adu. Magn. Reson. ISSO,14,125-150. (5) Bronnimann, C. E.; Maciel, G.E. Org. Geochem. 1989,14,189-192. (6) Bronnimann, C. E.; Hawkins, B. L.; Zhang, M.; Maciel, G. E. Anal. Chem. 1988,60, 1743-1750. (7) Hoffmann, W.; Schaller, T.; Michel, D. Fuel 1990, 69, 810-812. (8) Jurkiewicz, A.; Bronnimann, C. E.; Maciel, G. E. Fuel 1990, 69, 804-809. (9) Panek, P.; Scheler, G.; Neiser, J. Fuel 1990,69, 813-817. (10)Retcofsky, H. L. Appl. Spectroac. 1977, 31(2), 116-121.
typically measured the ratio of aromatic to aromatic plus aliphatic (including hydroaromatic) C-H absorptions and assumed a correction factor for the inequivalence of aromatic and aliphatic extinction coefficients. Unfortunately the extinction coefficients are variable, both within the aliphatic absorptions (i.e., absorptions due to CH2 can be more intense and CH) and from one structure to another (e.g., long-chain CH2 compared with alicyclic CHg21). Nevertheless, the infrared technique has been very valuable in showing trends and differences between coals. More recently, NMR has been used to estimate proton aromaticity, using a technique called dipolar dephasingeE In this technique, the degree of aromatic substitution is measured by using the differential relaxation rates of protonated and substituted aromatic carbon, which when combined with the atomic hydrogen to carbon ratio yields the proton aromaticity. Unfortunately, the experiment takes up to 24 h of instrument time and is subject to significant errors. In practice it is difficult to achieve a reproducibility much better than 10% for coals and related substances. Moreover, cross-polarization techniques must be used to obtain adequate signal to noise ratios and this also introduces additional errors. In this work, we use solid-state proton NMR (abbreviated CRAMPS, combined rotation and multiple pulse spectrometry') to measure the fraction of aromatic protons in density fractions from Callide coal. The degree of aromatic substitution is calculated from the proton aromaticity and other structural parameters and the change (11) Durie, R. A.; Shewchyk, Y.; Sternhell, S . Fuel 1966,45,99-113. (12) Brown, J. K.; Ladner, W. R. Fuel 1960,39,87-96. (13) Brown, J. K. J . Chem. SOC.1956, 744-752. (14) Kuehn, D. W.; Snyder, R. W.; Davis, A.; Painter, P. C. Fuel 1982, 61,682-694. (15) Riesser, B.; Starsinic, M.; Squires, E.; Davis, A.; Painter, P. C. Fuel 1984,63, 1263-1261. (16) Sobkowiak, M.; Reieser, E.; Given, P.; Painter, P. C. Fuel 1984. 63, 1245-1252. (17) Solomon, P. R. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1979,24(4), 184-195. (18) Solomon, P. R. Adu. Chem. Ser. 1981, 192, 95-112. (19) Solomon, P. R.; Hamblen, D. G.;Carangelo, R. M. ACS Symp. Ser. 1982,205,77-131. (20) Solomon, P. R.; Carangelo, R. M. Fuel 1988, 67,949-959. (21) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman and Hall: London. 1975; Vol. 1. (22) Wilson, M. A.; Alemany, L. B.; Woolfenden, R. J.; Given, P. H.; Grant, D. M.; Karae, J. Anal. Chem. 1984,66,933-943.
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644 Energy & Fuels, Vol. 5, No. 5, 1991
Table I. density, g cm-* 1.20-1.25' 1.25-1.3V 1.30-1.35c 1.35-1.4V 1.41-1.44' 1.44-1.45b >1.45b a
Vassallo et al.
Denrity Ranges,and C, H,and Petrographic Analyres of Callide Cool Fractions Ic a W Ha atomic H/C 46 liptinite 9% vitrinite W ntd n/d ntd 78.5 17.5 73.2 7:17 1.18 34.0 52.3 22.2 36.2 72.0 6.04 1.01 73.8 5.60 0.91 18.9 26.8 70.0 3.95 0.68 1.0 4.3 72.3 4.76 0.79 0.8 1.5 0.8 1.6 72.2 3.99 0.66
inertinite 5.2 13.7 41.6 54.3 94.7 97.7 97.6
Dry basis. Isolated from borecore A. e Isolated from borecore B.
with density examined. Additionally, the directly measured (NMR) ratio of the amount of aromatic to aliphatic protons is compared to M'IR measurements of the same ratio.
Experimental Section Callide coal is an Australian coal of Jurassic age, noted for ita high content of the m a c d semifueinite. We have recently studied the density fractions from this coal using solid-state I8C NMR and Fourier transform infrared (FTIR)spectroecopy and thia work is a continuation of that study. The preparation of the density fractions and their '8c NMR and FTIR analyses have been reported.= Briefly, the fractions were prepared by using float/sink methods in dense liquids. Two borecores of Callide coal from the same seam were used to obtain the fractions, one having a large component of Durite and the other a large component of Vitrite. This enabled a reasonable amount, at least 300 mg of even the lowest yielding fractions to be obtained by the density separation technique without using prohibitively large amounts of starting material. Data are presented on five fractions from one borecore (designated b in Table I). The amount of sample recovered for some of the lightest fractions, such as the material that floats at density 1.20 g cm" and the fraction between 1.20 and 1.25 g cm4, was insufficient to be characterized by solid-state NMR, petrography, and element analysis. The density range, m a d d y m , and carbon and hydrogen analyea of the density fractions which could be studied by NMR and used in this work are given in Table I. Solid-state 'H NMR ~pect~lscopy (CRAMPS) wu carried out using a Bruker MSL-400 spectrometer operating at a lH frequency of 400.13MHZ. All spectra were acquired with the MREV-8 pulse sequence using a 'H 90° pulse length of 1.5 ps and a pulse dead time interval of 4 pa (resulting in a total cycle time at 12s of 60 ps). This was combined with a magic angle spinning rate of 3 lrHz in a 7-mmdouble-&-bearing rotor system. A repetition rate of 5 s was used for all samples, with 32 tranaienta being averaged for each free induction decay which comprised 256 data points. 'H chemical shifta were externally referenced to TMS. As the samples were dried prior to analysis, proton signals from water (usually seen at -5 ppm') are expected to be negligible. Curve resolution of the CRAMPS spectra into aliphatic and aromatic bands was carried out by using a subroutine of the ~ C A L software C package (Galactic Industries Corp., Salem, NH)on an IBM PC compatible computer. The spectra were fittad to Gaussian line shapes by using a least-squares minimization algorithm.
Results and Discussion The solid-state 'H N M R spectra of the density fractions from Callide coal are shown in Figure 1. The peak with chemical shift at -2 ppm is due to aliphatic protons while aromatic protons are centered at -6 ppm. Baseline resolution is not achieved in spectra of coals for a number of reasons, the main ones being the large variety of chemical shifta due to the complex structure and line broadening due to anisotropies and inhomogeneities in the bulk magnetic susceptibility.' Even with unresolved spectra, it is clear that the fraction of protons that are aromatic (23)Vaaeallo, A. M.;Lockhart, N.C.; Hanna, J. V.;Chamberlain, R.; Painter, P. C.; Sobkowiak,M.Energy Fuels 1991,6,477.
1.30-1.35 ( l e i 3
8
6
4
2
0
Chemical Shift (6 ppm) Figure 1. Solid-state 'H NMR spectra (CRAMPS) of density fractions from Callide coal. Table 11. Proton Aromaticity (a,) of Callide Coal Density Fraotions fraction, g cm-s H,, fraction, g cm4 H, 1.20-1.25 0.06 1.41-1.44 0.58 1.25-1.30 0.05 1.44-1.45 0.76 1.35-1.40 0.38
increases with increasing density, in line with the increase in carbon aromaticity as determined earlier.23 Resolution of the aliphatic and aromatic resonances can be simulated by curve-resolving techniques. In complex materials such as coals, the spectrum would be made up of numerous bands arising from H20, CH8, CH2, CH, -OCH2,aromatic CH, and many others. It is not feasible to resolve the spectrum into all these individual components because the chemical shift ranges of these protons are wide and variable. For the purposes of this investigation, only the ratio of aliphatic to aromatic protons is required, and consequently resolution of these bands into their components was not pursued. Indeed the aim of curve resolution here is to obtain the areas of the spectrum due to aliphatic and aromatic protons and not to distinguish the types of protons in each band. In some cases, however, more than one band was required in the aromatic or aliphatic region to adequately fit the spectrum. The application of curve-fitting techniques to spectral deconvolution has been reviewedU and the process has been applied to CRAMPS spectra." Bronimann et al.8 showed that a simple deconvolution technique applied to CRAMPS spectra was much more precise for area measurements than a simple integration. The results of the curve-fitting procedure are shown in Figure 2 for selected spectra. The area measurements (24)
Maddame, W. F.Appl. Spectroec. 1980,34, 246287.
Energy & Fuels, Vol. 5, No.5, 1991 645
Spectra of Density Fractions from Callide Coal
0.9 C)
0.7
,
0 4 / ,
03 125 I
1
1
0
1
6
.
6
1
I
I
4
2
0
Chemical Shift (6 ppm)
Figure 2. Curve resolution of solid-state 'H NMR spectra into aliphatic and aromatic bands for seleded densit fractions of Callide Coat. (a) 1.20-1.25 g cm4, (b) 1.35-1.40 g cmJ,(c) 1.44-1.45 g cm-*.
obtained from the curve-resolved spectra have been used to calculate the proton aromaticity (H,, defined here as the ratio of signal from aromatic protons to the total signal) for each sample. This data are given in Table 11. From the data in Table I1 it can be seen that the proton aromaticity is very low for the lightest fractions, rising to -0.3-0.4 for the mid-range density and up to 0.76for the densest fraction. This change in proton aromaticity with density parallels the change observed in coals on going from low-rank to high-rank coals.6 This is not unexpected in view of the increasing carbon aromaticity observed with increasing density within a ~ o a l ~and ~ with * ~ increasing rank across a suite of coal^.^ Degree of Aromatic Carbon Substitution, Once the proton and carbon aromaticity are known, it becomes feasible to determine the degree of aromatic carbon substitution (1 - fad), where the parameter fad is the fraction of aromatic carbon which is protonated. This can be calculated as
where H/C is the atomic hydrogen to carbon ratio and fa is the carbon aromaticity. Strictly, this formula determines how many aromatic carbon atoms are nonprotonated and not the degree of ring substitution, which may be different if heteroatoms are incorporated into the ring. The inclusion of heteroatoms, such as nitrogen, into the ring does not change the value off but rendera ring substitution inequivalent with 1 - fah. Based on the amount of nitrogen in this coal (
