Energy & Fuels 1992,6, 28-34
28
CRAMPS Determined Proton Aromaticities of Australian Coals: A Comparison with Dipolar Dephasing John V. Hanna,* Anthony M. Vassallo, and Michael A. Wilson CSIRO Division of Coal and Energy Technology, PO Box 136, North Ryde, N S W 2113, Australia Received August 19, 1991
The proton (H,) and carbon aromaticities (f,) of a range of Australian coals and a few coals from other sources have been measured by high-resolution solid-state nuclear magnetic resonance spectroscopy. Combined rotation and multiple pulse spectroscopy (CRAMPS) was used to measure proton aromaticities while cross polarization with magic angle spinning (CPMAS) was used in carbon aromaticity measurement. Whereas reasonable correlations (correlation coefficient r = 0.85) were found between fa and atomic hydrogen to carbon ratios (H/C), proton aromaticities exhibited a greatly smaller dependence upon H/C which predominantly grouped around H, = 0.3 (r = 0.62). It is proposed that, for the coal suite studied here, dealkylation with subsequent protonation of the aromatic rings competes with aryl-aryl ring cross-linking involving loss of aromatic hydrogen without loss of alkyl groups. This information could not be elucidated from previous studies (Anal. Chem. 1984, 56, 933-943) in which dipolar dephasing using a single fixed delay was employed to ascertain the fraction of aromatic carbon which is protonated and indirectly related to H,. It is further concluded that inertinite and vitrinite fractions from the Australian coals examined here are not too dissimilar in structure at least with regard to their proton and carbon distributions as determined by high-resolution lH and 13Csolid-state NMR. A comparison of H, calculated by direct and indirect (dipolar dephasing) methods indicated that reasonable agreement can only be obtained when dephasing data was fitted to a full Gaussian-exponential decay function.
Introduction A measure of the distributions of carbon and hydrogen between aromatic and aliphatic structures is probably the most important structural parameter in coal analysis, after elemental composition. The measurement of carbon aromaticity (fa) is now routinely carried out by 13Csolid-state NMR and several reviews now exist1+ which discuss the successes and limitations of the CPMAS and Bloch decay techniques. 13C solid-state NMR has also been used to indirectly estimate the proton aromaticity by the dipolar dephasing t e c h n i q ~ e . ~ - ~This * ~ , ~approach detects the fraction of aromatic carbons with proton substituents (f,"") by exploiting differential relaxation characteristics of protonated and substituted aromatic carbon. Since the proton aromaticity (H,)is related to faapHby7 Ha, = (c/H)f,"lHfa (1) where C/H is the organic carbon to hydrogen atomic ratio, Ha, can be easily calculated in coals containing low amounts of mineral hydrogen. However, there are significant errors in calculating f 2 H and fa and a more appropriate method would be to measure H, directly. The measurement of proton distributions in coals directly from 'H solid-state NMR spectra has developed rather slowly. Although the CRAMPS technique has been documented for over 15 yearsa it has not been considered routine and requires considerable experience and caution in its application. The advent of modern spectrometer design has invoked more frequent application; however, a judicious choice of pulse parameters, spectral offset, spinning speed, and multipulse sequence is still required.&13 Thus, compared with 13CNMR relatively few 'H studies have been reported&27even though the instrument time needed for spectral acquisition is extremely
* To whom correspondence should be addressed. 0881-O624/92 /2506-0028$O3.00/0
short once the experiment has been configured correctly. In this study the proton aromaticities of a suite of (1)Miknis, F.P.NMR studies of solid fowil fuels. Magn. Reson. Reu. 1982. 7.87-121. (2) Davidson, R. M. Nuclear Magnetic Resonance Studies of Coal; IEA Coal Research: London, 1976. (3)Axelson, D. E. Solid State NMR of Fossil Fuels: Multiscience: Montreal, 1985. (4)Wilson, M. A. NMR Techniques and Applications in Geochemistry and Soil Chemistry; Pergamon Press: Oxford, U.K., 1987. (5)Wershaw, R. L.; Mikita, M. A. NMR of Humic Substances and
Coal-Techniques, Problems and Solutions; Lewis Publishers Inc.: London, 1987. (6)Snape, C. E.; Axelson, D. E.; Botto, R. E.; Delpuech, J. J.; Tekely, P.; Gerstein, B. C.; Puski, M.; Maciel, G. E.; Wilson, M. A. Fuel 1989,68, 547-560. (7)Wilson, M. A.; Alemany, L. B.; Pugmire, R. J.; Woolfenden, R. J.; Given, P. H.; Grant, D. M.; Karas, J. Anal. Chem. 1984,56, 933-943. (8)Gerstein, B. C.; Pembleton, R. G.; Wilson, R. P.; Ryan, C. M. J. Chem. Phys. 1977,66,361-362. (9)Ryan, L. M.; Taylor, R. E.; Paff, A. J.; Gerstein, B. C. J. Chem. Phys. 1980,72,508-515. (10)Gerstein, B. C. Philos. Trans. R. SOC. London, A 1981,299, 521-546. (11)Maciel, G. E.; Bronnimann, C. E.; Hawkins, B. L. Adu. Magn. Reson. 1990,14,125-150. (12)Bronnimann, C. E.;Maciel, G. E. Org. Geochem. 1989, 14, 189-192. (13)Bronnimann, C. E.; Hawkins, B. L.; Zhang, M.; Maciel, G. E. Anal. Chem. 1988,60,1743-1750. (14)Hoffmen, W.; Schaller, T., Michel, D. Fuel 1990,69,810-812. (15)Jurkiewicz, A.,Bronnimann, C. E., Maciel, G. E. Fuel 1990.69, 804-809. (16)Panek, P., Scheler, G., Neiser, J. Fuel 1990,69,813-817. (17)Jurkiewicz, A.; Wind, R. A.; Maciel, G. E. Fuel 1990,69,830-833. (18)Jurkiewicz, A.; Bronnimann, C. E.; Maciel, G. E. Fuel 1989,68, 872-876. (19)Rosenberger, H.; Scheler, G.; Kunstner, E. Fuel 1988,67,508-514. (20)Derbyshire, F.;Marzec, A.; Schulten, H. R.; Wilson, M. A.; Davis, A,; Tekely, P.; Delpuech, J. J.; Jurkiewicz, A.; Bronnimann, C. E.; Wind, R. A.; Maciel, G. E.; Narayan, R.; Bartle, K.; Snape, C. Fuel 1989,68, 1091-1106. (21)Gerstein, B. C. In Analytical Methods for Coal and Coal Products; Karr, C., Ed.; Academic Press: New York, 1980; Vol. 111, pp 425-444.
0 1992 American Chemical Society
Proton Aromaticities of Australian Coals
Energy & Fuels, Vol. 6, No. 1, 1992 29
Table I. Petrographic and Elemental Composition of Coals and Maceral Concentrates maceral composition, % elemental composition, % (dmmf) coal (concentrate) sourcec vitrinite inertinite liptinite C H N S 0 (diff) A 88 9 3' 78.3 5.4 2.9 0.4 13.0 Wambo 68 2" 71.1 A 4.7 0.8 30 0.2 23.2 Callide 70 A 00 89.7 30 4.8 2.5 2.7 0.3 Bulli (inertinite) 53 A 1" 4.9 46 89.8 2.4 0.3 Bulli (intermediate) 2.6 13 1" A 89.3 5.5 3.8 0.4 86 1.0 Bulli (vitrinite) 12 83 5" 76.5 4.0 A 1.4 0.4 17.7 Blair Athol (inertinite) 12 3" 85 76.0 5.3 A 2.2 0.2 16.3 Blair Athol (vitrinite) 78 7' A 15 79.5 4.9 1.8 0.8 13.0 Duncan (inertinite) 1' 10 89 76.3 5.4 A 1.3 0.9 13.8 Duncan (vitrinite) 82 trace' 89.3 A 5.5 3.8 1.0 18 Wongawilli (inertinite) 0.4 A 11 88.2 5.1 89 trace' 2.6 0.4 3.7 Wongawilli (vitrinite) 85.8 4.4 1.7 9 A 83 8" 0.2 7.9 Bayswater (inertinite) 96 3 A trace 83.0 5.3 2.0 0.5 9.2 Bayswater (vitrinite) 10 A 5" 81.2 5.3 2.1 0.5 10.9 85 Liddell (vitrinite) 18 2" 85.9 5.3 1.5 4.1 80 us 3.2 Lower Kittanning 22 5.3 1.4 2.5 55 G 23" 81.6 9.2 Hagan 14' 28 86.0 5.7 1.7 2.0 4.6 58 UK swallow wood 4' 73.7 5.7 18.4 us 70 26 1.3 0.9 PSOC139 7a.b 8b 85.0 5.4 1.7 0.9 6.9 us 85b Pittsburgh No.8 46 2 87.6 5.2 2.0 5.3 52 A 0.4 Tahmoor 0 3.8 45 A 55 89.4 4.8 1.8 0.4 Coalcliff 42 3 84.8 4.8 A 2.2 0.6 7.6 55 South Blackwater 6.0 2.1 0.9 8.7 69 29 A 3 82.3 Moura D seam 10.2 44 A 0 88.2 5.0 1.6 0.4 56 Westcliff 85.7 5.2 1.8 6.8 65 26 A 10 0.5 Greta (Ellalong) 14.2 77 18 5 A 82.8 5.2 2.1 0.4 Young Wallsend 9 80.9 6.0 2.1 0.4 10.6 A 84 8 Liddell-I1 A 97 2 0 81.7 5.2 2.0 0.8 10.3 Lithgow (vitrinite) 10.7 3 2 82.2 5.0 1.6 0.5 A 92 Katoomba (vitrinite) 2 81.9 5.2 1.7 0.5 10.7 97 A 0 Newvale (vitrinite) 1 87.0 A 2 95 Curragh (bright) 5.2 N/D N/D N/D 15 1 87.3 83 A Curragh (bright banded) 5.2 N/D N/D N/D 42 87.9 1 55 A Curragh (banded) 4.9 N/D N/D N/D 1 44 55 A 88.2 Curragh (dull banded) 4.6 N/D N/D N/D 2 1 88.1 97 A Curragh (dull) 4.4 N/D N/D N/D 89.2 87 11 0 A Curragh (fusain) 3.7 N/D N/D N/D 18 4 83.6 74 A Bowens Road (bright banded) 5.4 N/D N/D N/D 30 3 83.5 61 A Bowens Road (banded) 5.3 N/D N/D N/D 49 41 5 83.2 A Bowens Road (dull banded) 5.1 N/D N/D N/D 71 2 83.2 19 A Bowens Road (dull) 4.7 N/D N/D N/D 9 84 3 A 80.2 Bayswater (clarain) 5.5 N/D N/D N/D 19 4 A 80.2 74 Bayswater (banded) 5.4 N/D N/D N/D 79 8 80.4 9 A Bayswater (durain) 4.7 N/D N/D N/D 1 2 97 A 79.9 Walloon (bright) 5.5 N/D N/D N/D 2 73.5 5 A 58 Walloon (dull) 7.4 N/D N/D N/D 4 13 82.4 76 A Wards River (bright banded) 5.9 N/D N/D N/D 7 81.9 60 25 A Wards River (banded) 5.9 N/D N/D N/D 44 6 39 80.2 A Wards River (dull banded) 6.0 N/D N/D N/D 9 2 84.1 88 A Avons (bright banded) 5.4 N/D N/D N/D 22 84.1 3 A 66 Avons (dull banded) 5.5 N/D N/D N/D 4 1 81.2 89 A Ulan (bright) 5.3 N/D N/D N/D 82.3 A Ulan (banded) 5.5 N/D N/D N/D N/D N/D N/D 83.1 21 64 6 A Ulan (dull) 5.2 N/D N/D N/D 6 0 91.5 93 A Neb0 west (bright) 3.5 N/D N/D N/D 89.4 47 A 0 48 Neb0 west (dull) 3.5 N/D N/D N/D ~
"Expressed on a mineral matter free basis, otherwise difference from 100% is mineral matter. bFrom ref 43. 'A = Australia; G = German; UK = British; US = United States.
Australian coals and selected American and European coals are measured, with subsequent comparison made to proton aromaticities derived by dipolar dephasing, carbon aro(22) Gerstein, B. C.; Ryan, L. M.; Murphy, F. D. In Coal Structure;
Gorbaty, M. L., Ouchi, K., Eds.; Adv. Chem. Ser. American Chemical Society: Washington, DC, 1981; Vol. 192, pp 15-22. (23) Rosenberger, H.; Scheler, G.; Rentrop, K. H. Z.Chem. 1983,23, 24-28. (24) Schmiers, J.; Rosenberger, H.; Scheler, G. Z . Chem. 1982, 22, 424-430. (25) Rosenberger, H.; Scheler, G . Erdol Kohle-Erdegas Petrochem. 1986, 36, 48. (26) Botto, R. E.; Hayatsu, R.; Carrado, K. A.; Winans, R. E. Int. Conf.
Coal Sci. Proc. 1989, 1-4. (27) Vassallo, A. M.; Hanna, J. V.; Wilson, M. A.; Lockhart, N. C. Energy Fuels 1991, 5 , 643-647.
maticities, and other derived parameters. Throughout this paper the contentious arguments pertaining to quantitative aspects of both lH and 13C techniques are addressed.
Experimental Section Many of these coals have been described else~here;~.' however, a list of their elemental and petrographic compositions is given in Table 1. Solid-state 'H NMR spectra were obtained on a Bruker spectrometer operating at a 1H frequency of400.13 M& using the technique. All spectra were acquired with employing a lH 9oo length of 1.5 ~s the BR-24 and a Pulse dead time interval of 2.5 W This w a combined with (28) Burum, D. P.; Rhim, W.-K. J. Chem. Phys. 1979, 71, 944-956.
Hanna et al.
30 Energy & Fuels, Vol. 6,No. 1, 1992
1
A
LIDDELL
I
C
Chemical shift, ti (ppm)
Figure 1. Typical 'H CRAMPS spectrum of Liddell coal and its deconvolution into aliphatic and aromatic contributions. magic angle spinning at 2.5 KHz using a 7-mm double air bearing rotor system. A repetition rate of 10 s was used for all samples allowing for full 'H relaxation and probe recovery, with 32 transients being averaged for each free induction decay which comprised 128 data points. 'H and 13C chemical shifts were externally referenced to tetramethylsilane. As all samples were dried prior to analysis, 'H signals from water usually seen at -5 ppm12-14 were negligible. Line-shape deconvolution of 'H CRAMPS spectra into aliphatic and aromatic bands was achieved by using a subroutine of the SPECTRACALC software package (Galactic Industries Corp., Salem NH)on an IBM 486 compatible computer. 'H CRAMPS spectra were fitted to two Gaussian line shapes by using a leastrsquares minimization algorithm. A typical deconvoluted spectrum is illustrated in Figure 1. 13C NMR spectra were obtained on a Bruker CXP 90 spectrometer operating at a 13C frequency of 22.5 MHz. A 90° pulse of 4 was used in conjunction with magic angle spinning speeds of 4 kHz. Recycle delays of 1 s and cross polarization times of 1 ms were employed unless otherwise stated. 13C aromaticities vis) were measured directly by integrating the 1 W 1 7 0 ppm (aromatic) spinning sideband corrected spectral region relative to the total spectrum. None of the coals examined in this study possessed carboxyl species in any detectable concentrations by either 13C or 'H NMR techniques. The 13C dipolar dephasing technique employed a cross polarization preparation of 13C magnetization prior to a 180° refocusing pulse between two delay periods in which the decoupler was gated 0ff.'Jtm Two types of dipolar dephasing experiments were utilized. In the first a single 40 ps (total)dephasing time (t) was used, with this spectrum ratioed to that obtained at a dipolar dephasing time of 0 ps. If T i s the total signal intensity at t = 0 ps, A is the sp2-hybridized carbon signal intensity at t = 0 ps, and N is the measured sp2-hybridized carbon signal intensity at t = 40 ps, then P the signal from protonated aromatic carbon can be found from4J fa
= A / T = (N faaM
+ P)/T
(2)
= P/ A
(3)
This method assumes that the loss of nonprotonated aromatic signal intensity after 40 CLS is negligible while the loas of protonated aromatic signal is complete. Both these assumptions could be a considerable source of error. The second method involves extrapolation of the nonprotonated signal intensity at 240 pa back to a dipolar dephasing time of zero. It is argued that signal decay 240 ps is of exponential character described by eq 4, where ZBo is the signal intensity a t time zero from nonprotonated carbon specie^.^-^ ZBo can be found from the intercept of a plot of In
~~~~
0
20
40
60 80 100 120 140 160 180 Dipolar dephasing time (NsI
200
Figure 2. Typical dipolar dephasing plot for coal (Walloon dull banded). and protonated faH = fadfa. A significant reduction in errors can be achieved by fitting all the data from t = 0 to t = n to eq 5 3 7
z = ZA0 exp
(*)
2T2A'2
+ ZB0 exp(
6)
(5)
with Gaussian relaxation behavior characterized by the time constant Tu' expected from protonated carbon species in the time domain 530 ~ s . Full dipolar dephasing studies of the selected coal suite were fitted to eq 5 via a least-squares Simplex algorithm within the SIMFIT program (Bruker Abacus Library) yielding values of ZAo, ZBO,Tu',and Tm' (see Table 111). Subsequent calculations of proton aromaticities (Ha), carbon aromaticities cf,), and fractions of protonated aromatic carbon were performed using eq 1,with resultant values collated in Table 11. A typical SIMFIT simulation of a dipolar dephasii data set to eq 5 is given in Figure 2. (faapH)
Results and Discussion Problems of Aromaticity Quantitation of Coals by NMR. The measurement of aromaticity, fa, is in itself subject to considerable sources of error as outlined in detail elsewhere.6 Motional frequencies close t o t h e 'H decoupling of magic angle spinning (MAS)frequencies will tend t o obstruct the coherent averaging imposed by these techniques so that lines remain intrinsically broad and virtually unobservable over conventional frequency sweep widths of 13C spectra. In addition, the cross polarization (CP)technique normally used to measure fa is a kinetic measurement in which signal intensity is (to a first approximation) a compromise between two opposing rates, namely t h e r a t e of cross polarization (TCH) and t h e r a t e of 'H relaxation in the rot a t i n g f r a m e (Tlp(H));t h u s , signal intensities are not proportional to the amount of carbon present except in the limiting case where TCH 0 and Tl,(H), Tl,(C) m. A heterogeneous material like coal can have a variety of TCH's and Tlp(H)'scorresponding to different components; and hence at a n y given contact time they m a y n o t be detected quantitatively without correction for the differing spin
-
-
ZBo) is protonated carbon and nonprotonated carbon (ZAo measured, the fraction of aromatic carbon which is protonated (fad) can be calculated as ZAO/(ZAO ZBO). Alternatively, the data can be expressed as the fraction of totalcarbon which is aromatic
dynamics. T h i s dilemma is accentuated by t h e presence of stable free radicals and other paramagnetic species in the coal which m a y provide rapid mechanisms for relaxation,6b0 i n some cases resulting i n exceedingly broad resonances that will n o t be digitized sufficiently f a s t enough for proper detection. T h e s e problems naturally extend t o the dipolar dephasing experiment as well. Of course, fa could be measured by single pulse (Bloch decay)
(29)Alemany, L.B.; Grant, D. M.; Alger, T. D.; Pugmire, R. J. J.Am. Chem. Soc. 1983,105, 6697-6704.
(30)Maciel, G.E.;Bronnimann, C. E.; Jurkiewicz, A.; Wind, R. A.; Pan, V. H. Fuel 1991,70,925-930.
Z versus t k 40 ps. If the initial signal intensity at t = 0 from
+
+
~~
~
Proton Aromaticities of Australian Coals
Energy &Fuels, Vol. 6, No. 1, 1992 31
Table 11. NMR Derived Parameters for Coals and Maceral Concentrates H/C fa f,"a(C) HJC) H,W) 0.83 0.64 0.50 0.39 0.30 0.79 0.82 0.38 0.64 0.80 0.31 0.65 0.84 0.34 0.74 0.68 0.32 0.73 0.27 0.68 0.33 0.83 0.67 0.74 0.79 0.39 0.85 0.55 0.33 0.39 0.74 0.83 0.69 0.76 0.31 0.62 0.27 0.73 0.77 0.69 0.29 0.38 0.78 0.68 0.74 0.79 0.34 0.78 0.67 0.40 0.29 0.80 0.64 0.09 0.93 0.52 0.76 0.72 0.30 0.72 0.34 0.76 0.64 0.80 0.26 0.28 0.69 0.76 0.23 0.87 0.66 0.68 0.79 0.30 0.27 0.73 0.74 0.80 0.69 0.26 0.89 0.69 0.27 0.76 0.70 0.27 0.73 0.73 0.31 0.76 0.71 0.33 0.72 0.78 0.35 0.71 0.77 0.37 0.67 0.80 0.33 0.62 0.85 0.28 0.60 0.81 0.31 0.50 0.84 0.27 0.77 0.73 0.27 0.76 0.75 0.28 0.73 0.76 0.32 0.81 0.68 0.40 0.82 0.71 0.29 0.81 0.68 0.31 0.74 0.70 0.29 0.82 0.69 0.30 0.52 0.15 1.2 0.69 0.26 0.86 0.69 0.28 0.86 0.69 0.25 0.90 0.72 0.77 0.32 0.73 0.30 0.78 0.72 0.78 0.31 0.70 0.80 0.30 0.74 0.70 0.30 0.94 0.71 0.46 0.46 0.94 0.74
coal (concentrate) Wambo (coal) Callide (coal) Bulli (inertinite) Bulli (intermediate) Bulli (vitrinite) Blair Athol (inertinite) Blair Athol (vitrinite) Duncan (inertinite) Duncan (vitrinite) Wongawilli (inertinite) Wongawilli (vitrinite) Bayswater (inertinite) Bayswater (vitrinite) Liddell (vitrinite) Lower Kittanning Hagan Swdlowood PSOC139 Pittsburgh No.8 Tahmoor Coalcliff South Blackwater Moura D seam Westcliff Greta (Ellalong) Young Wallsend Liddell-I1 Lithgow (vitrinite) Katoomba (vitrinite) Newvale (vitrinite) Curragh (bright) Curragh (bright banded) Curragh (banded) Curragh (dull banded) Curragh (dull) Curragh (fusain) Bowens Road (bright banded) Bowens Road (banded) Bowens Road (dull banded) Bowens Road (dull) Bayswater (chain) Bayswater (banded) Bayswater (Durain) Walloon (bright) Walloon (dull) Wards River (bright banded) Wards River (banded) Wards River (dull banded) Avons (bright banded) Avons (dull banded) Ulan (bright) Ulan (banded) Ulan (dull) Neb0 west (bright) Neb0 west (dull)
f,",H(H) 0.40 0.37 0.25 0.26 0.35 0.29 0.41 0.37 0.51 0.35 0.28 0.23 0.32 0.44 0.32 0.47 0.36 0.16 0.32 0.32 0.21 0.25 0.30 0.26 0.27 0.30 0.35 0.29 0.31 0.35 0.32 0.34 0.28 0.21 0.23 0.16 0.29 0.28 0.31 0.34 0.33 0.37 0.28 0.36 0.35 0.32 0.35 0.32 0.34 0.32 0.34 0.34 0.32 0.35 0.36
faH(H) 0.25 0.30 0.20 0.22 0.24 0.20 0.27 0.29 0.28 0.29 0.21 0.17 0.22 0.30 0.25 0.31 0.23 0.08 0.23 0.24 0.17 0.19 0.20 0.21 0.20 0.21 0.24 0.20 0.23 0.25 0.25 0.26 0.22 0.18 0.17 0.13 0.21 0.21 0.24 0.28 0.23 0.25 0.21 0.25 0.18 0.22 0.24 0.22 0.24 0.23 0.24 0.25 0.22 0.32 0.34
Established using eq 5, otherwise from eq 3. HJC) is the fraction of protons which are protonated calculated from dipolar dephasing data; HJH) as H,(C) but measured directly through protons. Likewise f,"a(C) refers to carbon measurements and f,","(H) and f,H(H) refer to proton measurements. Measured with a contact time of 3 ms, value obtained a t 1 ms = 0.49. Measured with a contact time of 3 ms, value obtained at 1 ms = 0.48.
methods but this is extremely time consuming since 13C spin-lattice relaxation times (Tl(C)) must be known prior to fa measurement, with values of Tl(C) for coals being generally l0ngl9~9~ (10-30 8 ) . In addition, the paramagnetic/free radical problem may still hinder an accurate intensity distribution from being obtained. For one coal used here (Liddell)fa was determined by both Bloch decay and by cross polarization and both techniques were found to be in agreement. For other coals fa measurements were made only by the cross polarization technique. Values of TCHand Tl,(H) for a wide range of coals have been reported el~ewherel-~ and are comparable to values obtained for this selected suite, being of the order of 50-300 ps and 4-8 ms, respectively. It was noted that aliphatic
and aromatic TcH's of the selected coals differed by a factor of -6 while aliphatic and aromatic Tl,(H)'s were generally within 20% of each other; both TCH and Tl,(H) kinetic data exhibited multicomponent characteristics throughout this coal suite. As an example, a comparison of T C H data for Walloon (low rank) and Neb0 bright coals (high rank) is given in Figure 3. The data can be fitted to a single TCH and Tl,(H) function as described a b ~ v e , l -but ~ best fits are obtained for the two component TCH of eq 6, with T C H=~25-35 ps and TCHZ = 530-550 ps. The f a values z = [Z*0(1 - exp(-t/TcH1) + ZBO) x (1- e x p ( - t / T c ~ ~ ) )exp(-t/Tl,(H)) I (6) determined for these coals appear to be contact time in-
Hanna et al.
32 Energy & Fuels, Vol. 6, No. 1, 1992 1
1
0
1
08
08
04
12
14
atomic H/C
Figure 4. Variation of carbon and proton aromaticity with atomic hydrogen to carbon ratio (H/C). Scheme I. Schematic Diagram To Represent Coalification in Bituminous Gymnosperm Derived Coals’ la)
U
2Ro R
OH
HO /
OH
OH
-+
$R
OH OH
t I
0
0.3
0.6
0.S
1.2 1.5 1.6 2.1 Contact time (ms)
l
l
1
2.4
2.7
3.0
Figure 3. Multicomponent TcHbehavior for (a) Walloon dull banded and (b) Nebo bright coals. Tl,(H) data was aLS0 obtained but is not shown.
dependent for contact times 21 ms,4 and thus the variable nature of TCHand Tl,(H) alluded to above appears not to affect these measurements. Indeed, Gerstein et aL3’ have shown with exhaustive variable rf field and contact time studies on Pittsburgh No. 8 coal that the quality of the Hartmann-Hahn match and subsequent fa measurement is reasonably insensitive to these parameters. This is only true for spectral acquisition using sufficiently low static Bo fields invoking relatively slow spinning speeds for sideband elimination and alleviation of magic angle spinning/cross polarization dynamics interference. The fa values determined in this study have been attained within these limits. Coalification. Bearing in mind the possible sources of error previously discussed, plots of fa versus ?% C or atomic H/C ratio (see Figure 4) for coals or maceral concentrates clearly indicate an increase in fa with increasing % C or decreasing H/C content. Scatter in such plots may arise from errors in fa measurements themselves or because the aromatic and aliphatic structures in the coal do not vary systematically. F ” i n a t i o n of the plot of fa versus atomic H/C ratio for this coal suite (Figure 4) clearly indicates that a good correlation of fa with atomic H/C exists (r = 0.85); however, results from the ‘H CRAMPS experiment simultaneously suggest a poor correlation of H, with atomic H/C (r = 0.62). Indeed only two coals possessing the lowest and highest H/C ratios reflect any trend, with the large majority of the suite tending to group around H, = 0.3. It is subsequently concluded that, unlike fa, H, is a poor rank parameter and that as the carbon content of a particular coal increases H, does not vary proportionally to establish a direct structural relationship between ‘H and (31)Pruski, M.; dela Rosa, L.;Gerstein, B. C. Energy Fuels 1990,4, 160-165.
2 OH
(C)
OH
RooHQ R
H
OH
OH
OH
OH
OH
‘The position of substituents is only meant to be a guide.
13C distributions. As the coal rank increases several mechanisms depicting a change in aromatic hydrogen content are feasible. Scheme I illustrates four proposed structural changes from a polymerized lignin precursor. In path a alkyl groups (R)are lost with no change in aromatic hydrogen. However, because both aliphatic hydrogen and carbon are lost both proton and carbon aromaticity increases. In path b carbon aromaticity increases and proton aromaticity increases a t a rate depending on the nature of R. If R = CH,, the rate is faster than if R = CHz,since in the latter case only two diphatic hydrogens are lost per aromatic hydrogen lost. With path c H, will increase a t the fastest rate because the loss of aliphatic hydrogen results in an increased aromatic hydrogen content. In addition to pathways a-c, the loss of hydrogen from hydroaromatic rings should also generate aromatic hydrogen, as depicted in path d. Subsequently,all of the proposed pathways of Scheme I do not adequately relate to the H, invariant behavior with rank depicted in Figure
Proton Aromaticities of Australian Coals
Energy & Fuels, Vol. 6, No. 1, 1992 33
3 a -
**
:8
,
'1 06 06
4
02 052
056
06
068
064
072
076
08
084
'a
Figure 5. Variation in aliphatic H/C with aromaticity. 4. The only structural alteration that can conceivably
decrease the fraction of aromatic hydrogen, or in combination with the other mechanisms make it fairly rank invariant, is an independent loss of aromatic hydrogen during coalification; i.e., the aromatic rings themselves can polymerize without the loss of aliphatic substituent groups. It is reasonable to expect simultaneous alteration of the aliphatic and aromatic hydrogen distributions of these coals to occur with increasing rank. A suitable guide to this change is the aliphatic H/C ratio. Once fa and f>H have been determined it is possible to calculate the aliphatic atomic H/C ratio, i.e., the molar ratio of hydrogen to carbon in aliphatic structures of coal, which can act as an indicator of aliphatic change with varying coal rank. Assuming initially that the coal is composed of purely hydrocarbon structwes, the molar distribution of hydrogen between aliphatic and aromatic sites is given by (7) but since
H - bcod _
(8) Cdi 1- f a For a real coal system H/Ccodshould be corrected for a phenolic hydrogen presence. Several studies have approximated a value of -60% of the total oxygen content to be of a phenolic n a t ~ r e , even ~ ~ bthough ~ phenolic content is rank d e ~ e n d e n t . ~ Thus, ~ P ~ a corrected value for H/Cdi is given by
where O/C is the atomic oxygen to carbon ratio. This formula calculates the molar ratio of hydrogen to carbon for aliphatic structures assuming 60% of the oxygen is in phenolic or alcoholic structures and that other heteroatoms are nonprotonated. The calculated values of H/Cdi plotted against fa are shown in Figure 5. I t is
evident that as coal rank increases, the aliphatic H/C ratio increases, approaching 2.5 for the highest rank coal, clearly demonstrating that the average aliphatic structure for this coal equates to CH3CH2. This trend is also obtained if all the oxygen is assumed to be phenolic. These results are in general agreement with our earlier work using dipolar dephasing which concluded that methyl species become the more predominant aliphatic structures with increasing coal rank.' Before continuing we should also express the cautionary note that errors in H, directly measured by CRAMPS also exist, although they are probably not so severe as those associated with f a measurements. Both motional averaging at the multipulse cycle frequency and paramagnetic line broadening sources of error may influence observed H, values, and there is the added complication of deconvoluting 'H line shapes into two components and assigning them as purely aliphatic or aromatic contributions. It was found that many of the higher rank coals deconvoluted into a significantly broader aliphatic resonance, the reason for which is unclear. It is possible that the effective chemical shift dispersion of the aliphatic structures in high rank coals may be greater; alternatively, resonance offset broadening may be enhanced by the more rigid, stronger dipolar coupled structures found in higher rank coals. Proton resonances from unexchanged phenols and residual water not removed by drying may contribute to the downfield region of the aliphatic resonance, thus giving the impression of broadening, but this is expected to be more of a problem in lower rank coals because of the higher phenolic ~ o n t e n t . ~Deuterium ~-~~ exchange or derivatization may be an avenue for investigation of this problem, although it has been found that phenols are prone to deuterium exchange with aromatic ring^.^'-^^ A number of interesting additional conclusions can be drawn from the data obtained from different lithotypes of the same coal. These hand picked coal fractions show quite different petrographic compositions (Table I). For Curragh coal the samples with 95%, 8390,55%, 44%, 11%,and 2% vitrinite were studied, and it is clear that although f a drops with decreasing vitrinite and increasing inertinite content the trend is small and of the order of only 0.08. Elsewhere7we have recognized that differences in fa between inertinite and vitrinite concentrates of Australian coals are small, whereas those of many Northem Hemisphere coals are large, probably because of the different botanical origins of the various coals. Major deposits of coal in Australia were formed from plant types not located in the Northern H e m i ~ p h e r e .Much ~ ~ of the fusinite and semifusinite in the coals of the Northern Hemisphere are a kind of charcoal formed in forest fires. The macerals given the same names in Australia (and also South Africa) are thought to derive from some kind of low-temperature oxidation35or freeze-drying cycle.40 We note here that for Curragh coal there is a small decrease in H, with inertinite content, but for other coals, e.g., Bowens Road, there is an increase and for others no change at all. The small differences in both proton and carbon aromaticity between vitrinite and inertinite maceral concentrates studied here and isolated from the same mine ~
(32) Brown, J. K.; Ladner, W. R. Fuel 1960,39, 87-96. (33) Brown, J. K.;Ladner, W. R.; Sheppard, N. Fuel 1960,39,79-86. (34) Snyder, R. M.; Painter, P. C.; Havens, J. R.; Koenig, J. L. Appl. Spectrosc. 1983, 37, 497-501. (35) Teichmuller, M. In Stach's Textbook of Coal Petrology, 3rd ed.; Stach, E., Taylor, G. H., Mackowsky, M.-Th., Chandra, D., Teichmuller, M., Teichmuller, R., Eds.; Gebruder, Borntraeger: Berlin, 1982; pp 19C-191 and 272-279.
(36) Yanab, R.F.;Abdel-Baset, Z.; Given, P. H. Geochim. Cosmochim. Acta 1979, 43, 281-287. (37) Massicot, J.; Zonszajn, F. Bull. SOC.Chim. Fr. 1967, 2206-2209. (38) Ingold, C.K. Structure and Mechanism in Organic Chemistry; Cornel1 University Press: Ithaca, NY, 1969; p 303. (39) Bressel, U.; Katritzky, A. R.; Lea, J. R. J. Chem. SOC. ( B ) 1971, 11-16. (40) Taylor, G.H.; Liu, S. Y.; Diessel, C. F. K. Int. J. Coal Geol. 1989, 11, 1-22.
Hanna et al.
34 Energy & Fuels, Vol. 6, No. 1, 1992 Table 111. DiDolar DeDhasina Data for Selected Coals contact time, ma coal 1 Walloon Dull 230 24 1 Neb0 Bright 234 24 1 Neb0 Dull 267 24 3 Neb0 Bright 258 3 26 Neb0 Dull 240 1 22 Lower Kittanning 201 1 22 Liddell 230 1 25 Hagen 230 1 24 swallow wood 178 1 23 Blair Athol inertinite 190 1 23 Blair Athol vitrinite 269 1 21 Pittsburgh No. 8
0.6
O2
t1
A
full dipolar dephasing
=
40 ps
approximation dipolar dephasing Nebd U l l - L N e b D
brlgh1
/
i/ 0
Netawes!
1
I 06
-
~
/
t
04
05
06
07
08
09
1
fa
Figure 7. Relationship between proton and carbon aromaticity.
1 ,
O6
06
Blair Athol ~ltrinile
JI
02
04
06
08
1
Ha,(dipolar dephasing)
Figure 6. Com arison of proton aromaticity determined via 'H CRAMPS and dipolar dephasing experiments.
k!
suggest the vitrinite and inertinite in these coals are quite close in chemical structure, a result borne out for Bayswater coal by its coal liquefaction characteristic^.^^ Dipolar Dephasing. Apart from direct observation via the CRAMPS experiment H, can also be measured indirectly from fa and fae,H utilizing dipolar dephasing data and eqs 1and 3,4, or 5. The determination of faaHby the most accurate method (eq 5 ) is an extremely time consuming experiment since a sufficient number of data points (215) with adequate signal-to-noise is required to accurately describe the dephasing behavior. For a selection of 10 coals (Table I11 and Figure 6) H, has been measured indirectly using eq 5 in addition to direct detection via 'H CRAMPS measurement. An excellent correlation is obtained for H, measurement from both CRAMPS and accurate dipolar dephasing techniques (r = 0.97) across the complete rank range, as observed by the correlation of these data points with the solid (slope = 1)line describing a perfect 1:l relationship. Longer contact times of 3 ms are required for the high rank, semianthracitic (Nebo) coals to preserve a good correlation of H, measurement by both techniques. The necessity of longer contact times in CPMAS experiments for accurate fa determination of anthracites has been previously For many mid ~~~
~~
(41) Heng, S.; Collin, P. J.; Wilson, M. A. Fuel 1983, 62, 1359-1368. (42) Sethi, N. K.; Pugmire, R. J.; Facelli, J. C.; Grant, D. M. Anal. Chem. 1988,60, 1574-1579. (43) Vorres, K. S. Users Handbook for the Argonne Premium Coal
Sample Program; Chemistry Division, Argonne National Laboratory, Argonne, IL 60439.
and lower rank coals such as Liddell, Walloon, Pittsburgh No. 8, Lower Kittaning, etc. exactly equivalent Havalues were elucidated via measurement from both techniques. Figure 6 also depicts H, measurement of this coal suite by the dipolar dephasing 40 4.18 approximation, which clearly indicates that this method overestimates H, in comparison to values obtained by the CRAMPS and full dipolar dephasing methods, to the extent that this inaccuracy renders this H, data meaningless. Elsewhere' we have shown that there is a correlation between f a and H, determined by dipolar dephasing. This correlation still exists in this work in which H, has been measured directly (P = 0.67, Figure 7) but only if the most extreme ranked coals are included, i.e., if the high rank Nebo coals and the hydrogen-rich Walloon Dull coal (Table 11) are used in the correlation. Probably a range of coals with H, in the vicinity 0.4-0.7 is needed to confirm this trend. Of major note is the observation that coal suites with widely varying fa's tend to have much smaller differences in H, as indicated above, with Figure 7 describing an aggregation of H, values around the 0.3 region. Just as fa may be an underestimate of aromaticity because of rapidly relaxing centers in the c0al,6,~ so may H, be similarly affected. The agreement between H,'s calculated directly or indirectly may suggest that the same proportions of the coal are being observed in both experiments, so that as much care must be taken in drawing quantitative conclusions from H, measurements as has been made with fa measurements. The fact that such good correlations are obtained between dipolar dephasing data (when due care is taken with contact time and curve fitting procedures) and direct lH CRAMPS aromaticity measurements does give confidence to both sets of data and probably means that the qualitative comparisons made by us and in elucidating coalification and coal processing mechanisms via 13Csolid-state NMR techniques are well justified.
Acknowledgment. We thank Judy Bailey (Geology Department, University of Newcastle, NSW), Nigel Russell (CSIRO Division of Exploration Geoscience), and Clive Roberts (Australian Coal Industry Research Laboratories) for providing coal and maceral concentrate samples. The Australian Research Council and the Department of Primary Industries and Energy are thanked for providing partial funding.
