460
Energy & Fuels 1992,6,46&468
Characterization of the Argonne Premium Coals by Using 'H and 13CNMR and FT-IR Spectroscopies Luisita dela Rosa, Marek Pruski, David Lang, and Bernard Gerstein* Institute for Physical Research and Technologyt and Department of Chemistry, Iowa State University, Ames, Iowa 50011
Peter Solomon Advanced Fuel Research, Inc., East Hartford, Connecticut 06118 Received December 19,1991. Revised Manuscript Received May 1, 1992 Argonne premium coals were characterized by using Fourier transform infrared spectroscopyand a variety of transient techniques in solid-state 'H and 13CNMR spectroscopy. The NMR and FT-IR spectra were used to infer chemical functionalities of hydrogen and carbon in the premium coals and the results compared. The quantitative validity of the two techniques was also evaluated. Hydrogen aromaticities of the premium coals as measured by NMR are higher than the corresponding values obtained from FT-IR.Carbon aromaticities measured by NMR for the bituminous coals correlated well with the FT-IR values, but for the younger coals discrepancies of over 15% were found. NMR results were also compared with those reported for the same coals by others. The aromatic and aliphatic hydrogen-to-carbon ratios were evaluated and found to be in good correlation with the coals' rank. Introduction The introduction of line-narrowingmethods in 'H and 13Cnuclear magnetic resonance spectroscopy (NMR) in solids extended the application of NMR to the study of fossil A technique used in 'H NMR that is capable of removing line broadening caused by magnetic dipolar interactions between nuclei and by chemical shift anisotropy is combined rotation and multiple pulse spectroscopy (CRAMPS).4ts Magic-angle spinning (MAS)6in concert with high-power proton decoupling and cross polarization (CP)'!* is a routine technique in 13C NMR. CRAMPS and CP/MAS experiments have allowed the direct measurement of hydrogen and carbon functionalities in solid organic compounds and have been applied in the study of coals for over 10 years. Infrared spectroscopy (IR)is another powerful analytical tool used to obtain information on coals since the pioneering work of Brown?Jo Advances in instrumentation, in particular the introductionof Fourier transform infrared spectroscopy (FT-IR), widened ita applications." The question of the quantitative reliability of NMR and FT-IR in the study of coals has been discussed in the literature since their introduction. A potential source of error in 'H NMR might be the inability to observe resonances that are shifted or broadened because of interactions of observed nuclei with unpaired electrons. However, our experiences show that >95% of hydrogen is usually detectable in coals.12 With CRAMPS, the relative intensities of various peaks in the spectra are reliable to within 2-390 under optimal experimental condition^.'^ The main deficiency of this technique when applied to coals is still ita low level of resolution because of broad distributions of chemical shifts. The reliability of 13CNMR intensities in the CP/MAS spectra of coale haa generated a decade-old debate.'"ls A significant fraction (often as much as 50%) of carbon may remain undetected by using CP/MAS because of a broad
distribution of relaxation constants involved in the polarization process and the possible interference of rapid sample spinning with that process. Strategies devised to circumvent this problem include variable-contact-time (vct) CP and single-pulse (Bloch decay) 13CMAS experiments. Most researchers agree that, under appropriate experimental conditions, CP spectra representing a distribution of carbon chemical functionalities in coals can be obtained with an accuracy of a few percent.14 "Appropriate conditions" include the use of low static magnetic fields, where slow MAS suffices to eliminate sidebandsand reduce the interference of spinning with the CP process, and the use of variable polarization transfer times. The FT-IR spectra, which for coals are collections of several overlapping bands, cannot be used directly for quantitative analysis. In order to obtain such information, (1) Davidson, R. M. Nuclear Magnetic Resonance Studies of Coal; ICTIS/TR32; IEA Coal Research London, 1986. (2) Gerstein, B. C. In Analytical Methods for Coal Coal Products; Karr, C., Ed.; Academic Press: New York, 1979; Vol. 111. (3) Gerstein, B. C.; Murphy, P. D.; Ryan, L. M. In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982. (4) Gerstein, B. C. Philos. Trans. R. Soc. London 1981, A299, 521. (5) Gerstein, B. C.; Dybowski, C. R. Transient Techniques in NMR of Solids: An Introduction to Theory and Practice; Academic Press: New York, 1985. (6) Andrew, E. R. Philos. Trans. R. SOC.London 1981, A299, 505. (7) Pines, A.; Gibby, M. G.; Waugh, J. J. Chem. Phys. 1972,56,1776. (8)Pines, A.; Gibby, M. G.; Weugh, J. J. Chem. Phys. 1972,59, 569. (9) Brown, J. K. J. Chem. SOC.1966,744. (10) Brown, J. K.; Ladner, W. R. Fuel 1960,39, 87. (11) Solomon, P. R.; Hamblen, D. G.; Carangelo, R. M. In Coal and Coal Products, Analytical Characterization Techniques; Fuller, E. L., Ed.; American Chemical Society: Washington, DC, 1982. (12) Dela Rosa. L.: Pruski. M.: Gerstein. B. C. In Technioues in Magnetic Resonance for Carbonaceous Solids; Botto, R., Sanida, Y., Ma.; Adv. Chem. Ser. No. 229; American Chemical Society: Washington, DC, in press. (13) Bronnimann, C. E.; Hawks, B. L.; Zhang, M.; Maciel, G. E. Anal. Chem. 1988,60, 1743. (14) Snape, C. E.; Axelson, D. E.; Botto, R. E.; Delpuech, J. J.; Tekely, P.; Gerstein, B. C.; Pruski, M.; Maciel, G. E.; Wilson, M. A. Fuel 1989, 68. 5470.
'Supported by the US. Department of Energy (Basic Energy Sciences Program, Chemical Science Division), under Contract No. W-7405-eng-82.
0SS7-0624/92/2506-0460~03.00/00 1992 American Chemical Society
Argonne Premium Coals
Energy & Fuels, Vol. 6, No. 4, 1992 461
Table I. MAF Ultimate Analysis of Argonne Premium Coal Sample Program Coals" source seam rank AR %HzO %C %H WN %S I O avformuIa/1000C LVB 0.65 91.05 0.50 501 PocahontaeNo.3,VA 4.44 1.33 2.47 C1&BlNlzSnO14 MVB 1.13 85.50 101 Upper Freeport, PA 4.70 1.55 0.74 7.51 C1&N1&0,,, HVB 1.65 83.20 401 Pittsburgh No. 8, PA 5.32 1.64 0.89 8.83 C1&762N1,S4061 HVB 701 Lewiston-Stockton, WV 2.42 82.58 5.25 1.56 0.65 9.83 Cl,,&,&lsSsOse 601 Blind Canyon, UT HVB 4.63 80.69 5.76 1.57 0.37 11.58 C1,,&~1N17S20100 301 Illinois No. 6, IL HVB 7.97 77.67 5.00 2.38 1.37 13.51 Cl,,&,~15S110M SB 202 Wyodak-Anderson, WY 28.09 75.01 5.35 1.12 0.47 18.02 Cl&~&lsSzOls, 801 BeulahZap,ND L 32.24 72.94 4.83 1.15 0.70 20.34 C1&,&&01M a Abbreviations used in this table: LVB = low volatile bituminous; MVB = medium volatile bituminous; HVB = high volatile bituminous; SB = subbituminous; L = lignite; AR = as received; MAF = moisture and ash free; % are in weight percent.
ID
a detailed knowledge of the variation of the extinction coefficients for different bands in the spectra and the application of sophisticated curve-resolving methods are needed." The overlapping of bands in the IR spectra, the dependence of extinction coefficientsof spectral bands on coal rank, and baseline artifacts pose severe quantitation problems in the IR ~ o r k . " - ' ~ The heterogeneous nature of coal makes it unreasonable to seek information on specifc molecular constituents and their distribution. Instead, average structural parameters of coals, of which hydrogen and carbon aromaticities are mast important, can be measured by spectroscopic methods. This paper summarizes the results of a series of studies using FT-IRand a variety of transient techniques in 'H and 13C NMR to infer average chemical functionalities of the coals in the Argonne Premium Coal Sample Program.*O NMR and FT-IR data are compared. Also, the NMR results are compared with those reported by others on the same set of coals.
Experimental Section Samples of the premium coals were obtained from Argonne National Laboratory, Argonne, IL. The number, seam, rank, percent water, moisture-and-ash-free(MAF) ultimate analyses," and the average molecular formula per 1OOO carbon atoms of these coals are listed in Table I. Samples prepared by three different methods have been studied: virgin (as-received),air-dried, and vacuum-dried. The virgin samples were prepared by transferring coals from the original ampulea to 5mm NMR tubes (for proton relaxation studies) or to rotors (for CRAMPS and CP/MAS measurements) under a He atmosphere. Air-dried samples, prepared from coals that were stored at ambient conditions for several month after opening the original ampules, were studied without further treatment. Vacuum-dried samples were obtained from virgin coals that were evacuated under a static vacuum of 1 mTorr at 100 O C for 3 h and sealed. Most of the NMR spectra were obtained at room temperature by using a homebuilt spectrometer operating at the Larmor frequencieswo of 100.06 MHz for lH and 25.15 MHz for 13c.
Values of longitudinal relaxation time constants Tlof hydrogen for virgin and air-dried coals were determined by using inversion recovery. For each coal, 200 delays were used with values incremented between 2 and 2000 ms. Valuea of longitudinal relaxation time constants in the rotating frame T of hydrogen for virgin and air-dried samples were measurdat a radio frequency (rf) field of 50 kHz by using a standard spin-lock sequence. For each coal, 15-30 different values of the locking pulse length, varying from 0.01 to 10 ms, were used. Ten team at a dwell of 0.6 pa were wllected for each free induction decay observed. 'H CRAMPS experimentswith virgin and vacuum-dried samples were performed at wo of 300 MHz with a Bruker MSL 300 spectrometer. Since the application of estended pulse sequences like the BR-2421does not lead to enhanced resolution in coals, (17) Solomon, P. R. Fuel 1981, 60, 3. (18)Solomon, P. R.; Carangelo, R. M. Fuel 1988,67,949. (19) Solomon, P. R.; Carangelo, R. M. Fuel 1982,61,663. (20) Vorres, K. S. Energy Fuels 1990,4, 420.
the simpler MREV-€ia with a 90°, pulse of 1.5 pa and a cycle time of 42 pa was used. Each spectrum was a result of 8-36 scans with 20 s between scans. Spinning speeds v, of 3.0-3.5 kHz satisfied the condition 6 < v, < l / r c , in which 6 is the chemical shift anisotropy and re the cycle time for the homonuclear decoupling pulse sequence.6 13CCP/MAS spectra were collected for air-dried and virgin coals. Air-dried samples were measured by using a double-tuned singlecoil probe equipped with an air-driven windmill-type MAS system. Approximately 200 mg of coal was put into the Kel-F rotor which was spun at 4-5 kHz. The virgin coals were sealed in 5-mm NMR tubes as described above, and the samples were spun at 4-5 kHz in a doubleresonanceprobe with an Andrew-type M A S system. The static Hartmanr-Hahn condition was established at 50 kHz with adamantane, and the proton rf field was kept at the same level during the high-power lH-13C decoupling. CP contact times were varied from 0.05 to 15 ms in the vct experiments. Typically, 5000-20 OOO scam were acquired by using quadrature detection to obtain a single spectrum. IR spectra of the premium coals were obtained in digital form on a Nicolet Model 7199 FT-IRspectrometer. The coal samples were further ground in a Wig-L-Bug for 20 min. KBr pelleta of the coals were prepared by mixing 1 mg of dry, finely ground coal with 300 mg KBr. The 13-mm pellets were pressed in an evacuated die at a pressure of 20000 lb for 1 min and dried at 110 O C for 24 h to remove water. Calibration of the spectra of the coals and corrections for particle scattering and mineral contents were made as described pre~iously.'~ Listed below are some abbreviations that will be used in the following sections: " MAF weight percent concentration of aliphatic carbon au determined by FT-IR MAF weight percent concentration of aromatic carbon au determined by FT-IR MAF weight percent concentration of carbon au determined by ultimate analysis carbon aromaticity as determined by '3c CP/MAS fraction of carbonyl plus carboxyl carbon as measured by 13C CP/MAS carbon aromaticity as determined by FT-IR fa(IR)corrected for carbonyl plus carboxyl intensities hydrogen aromaticity as determined by 'H CRAMPS Ha corrected for hydroxyl hydrogen intensities hydrogen aromaticity including hydroxyl hydrogen intensities aa determined by FT-IR hydrogen aromaticity as determined by FT-IR M A F weight percent concentration of aliphatic hydrogen au determined by FT-IR MAF weight percent concentration of aromatic hydrogen as determined by FT-IR MAF weight percent concentration of hydroxyl hydrogen as determined by FT-IR MAF weight percent concentration of hydrogen as determined by FT-IR atomic hydrogen-to-carbon ratio of the coal sample aliphatic hydrogen-to-carbon ratio of the coal aamPle ~~
~
(21) Burum, D.; Rhim, W. K. J. Magn. Reson. 1979,34, 241. (22) Mansfield, P. Philos. Tram. R. Soe. London 1981, A299, 479.
dela Rosa et al.
462 Energy h Fuels, VoE. 6, No. 4,1992
ID 501
source seam Pocahontas No. 3
101
Upper Freeport
401
Pittsburgh No. 8
701
Lewiston-Stockton
601
Blind Canyon
301
Illinois No. 6
202
Wyodak-Anderson
801
Beulah Zap
'Deviations for
(H/C),
Table 11. Summary of 'H T Iand T, of the Premium Coalso preparation Tl', Tl', T?", state a ma ms ms B air-dried 0.95 41 345 251 0.49 virein 0.87 186 1064 659 0.65 350 256 0.59 0.82 116 airrdried 621 419 0.77 virgin 0.86 140 147 113 0.58 0.83 53 air-dried 0.79 387 263 virgin 0.70 150 172 98 0.53 0.40 76 air-dried 0.70 256 140 virgin 0.50 96 132 40 0.50 0.19 34 air-dried 111 58 0.71 0.41 44 virgin 40 0.51 0.28 33 94 air-dried 133 64 0.71 0.38 48 virgin 4.4 0.58 3.1 14 0.38 air-dried 4.9 0.45 16 0.46 3.1 virgin 3.1 0.49 1.9 12 0.46 air-dried 7.0 0.58 virgin 0.41 4.3 16
Tlpl, ms 0.60 1.96 0.47 1.66 0.72 1.70 0.44 0.76 0.44 1.16
Tl,.', ma 1.05 4.46 1.03 5.43 1.46 5.58 0.86 2.14 0.82 3.19 1.52 4.08 0.93 1.30 0.50 0.51
TI;, ma 4.8 14.9 6.0 17.3 6.0 14.9 5.5 9.7 5.5 11.4 6.6 12.7 4.8 6.8 3.9 4.6
0.84 1.54 0.44 0.77 0.27 0.23
T1and T1,range from 3% to 25%; for a and 8, 3% to 20%.
aromatic hydrogen-to-carbon ratio of the coal sample
Results and Discussion Hydrogen Relaxation Studies. A number of pap e r presented ~ ~ ~ results ~ ~ on measurements of proton relaxation time constants in coals. In this work, relaxation of protons in the premium coals was measured in order to establish appropriate delays for the 'H CRAMPS and 13C CP/MAS experiments. Also, the results will be compared with those presented earlier in the literature, and the possible relaxation mechanisms will be discussed. lH T1Relaxation. The data indicate that a distribution of TI exists in the coals studied. To limit the number of parameters, only a two-exponential function was used to analyze the measured decays M(7) = M(-)[l - 2({1 - a)exp(-7/Tls)
,
0
'
1
'
200
,
.
,
.
400
,
.
,
.
600
. ' 800 ' iobo .
I
.
DELAY TIME (MS)
""1 4000
+ a exp(-7/Tl1))I + C (1)
where M ( r ) is the magnetization at time 7 after the 180" pulse, 1 - a and a are the fractions of the magnetization characterized by the short-and long-time constanta T," and T:, and C is a constant. This procedure resulted in reasonable fits, as illustrated by the data and fit for Pittsburgh No. 8 coal in Figure la. A mean relaxation time constant TIavdefined by (TIav)-'= ((1- a)/TlS)+ (a/T,')
(2)
was evaluated in order to simplify analysis and to make comparisons with other values in the literature. See Table I1 for results. Table I1 shows that the nonexponentiality of the TI relaxation in the studied coals increased with decreasing coal rank. The Tlavvalues obtained for the virgin samples are in good agreement with the Tl obtained by Solum et (23) Gembin, B. C.; Chow, C.; Pembleton, R. G.; Wilson, R. C. J.Phys. Chem. 1977,81,565. (24) Yokono, T.; Sanada, Y. Fuel 1978,57, 334. (25) Yokono, T.; Miyazawa, K.; Sanada, Y.; Marsh, H. Fuel 1979,58, 896. (26) Lynch, L. J.; Webster, D. S. J . Magn. Reson. 1980, 40, 259.
(27) Wind, R. A.; Duijveatijn, M. J.; van der Lugt, C.; Smidt, J.; Vriend, J. Fuel 1987, 66, 876. (28) Wind, R. A.; Jurkiewicz, A.; Maciel, G. A. Fuel 1489, 68, 1189. (29) Solum,M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989,
3, 187.
(30) Barton, W. A.; Lynch, L. J. Energy Fuels 1989,3,402.
I
0
10 20 3'0 DELAY TIME (MS)
I
40
Figure 1. Hydrogen relaxation data and nonlinear least-squares fit for virgin and air-dried Pittsburgh No. 8 coal in the (a) laboratory and (b) rotating frame of reference.
aLB using indirect detection of hydrogen by '3c CP/MAS. This agreement seems to indicate that the CP experiments did not significantly prevent observation of the carbon nuclei associated with hydrogen nuclei that are subject to fast relaxation. Note that the Tlavvalues reported here for the virgin samples are lower than the corresponding values measured for the premium coals by Wind et al.28 at two Larmor frequencies of 60 and 187 MHz. While the sample in this work were studied without degassing, Wind worked on virgin samples that were vacuum-dried for 24 h, which sufficed to remove physisorbed water and oxygen. Although the mechanisms of longitudinal relaxation in coals are not well understood, it seems reasonable to suppose that the nonexponential relaxation in coals is associated with lack of facile spin diffusion between domains of differing TI. These domains, which could correspond to different macerals, can vary with respect to concentrations and mobilities of hydrogen nuclei and to concentrations of paramagnetic centers.30 Values of Tlincreased with increasing coal rank. No correlation was found between TI and the concentration of unpaired electrons as measured by electron spin reso-
Argonne Premium Coals
Energy & Fuels, Vol. 6, No. 4, 1992 463
nance spectroscopy.12 However, T1 decreased upon airdrying the ooals as illustrated by Tlavfor the air-dried coals from Table 11. This effect may be explained by the increase in the concentration of oxygen associated with exposing the samples to air. The above results agree with the earlier conclusions of Wind et al.28that the presence of chemisorbed paramagnetic oxygen not removed by vacuum-pumping or unpaired electrons residing in heteroatomsm provides a relaxation mechanism that dominates other mechanisms, especially in the lower-rank coals. In higher-rank coals, the lH-'H dipolar coupling and the interactions of the 'H nuclei with unpaired electrons in aromatic clusters are the primary relaxation mechanisms. 'H T, Relaxation. The data collected from proton spin-lock experiments were fitted to an equation of the form M P ( d = M p ( 0 ) [ ( l- 8) exp(-7/T1ps)
+ B exp(-.r/T1,')1 (3)
to calculate values of Tlp. MP(7)is the magnetization in the rotating frame at time T after the spin-locking pulse and 1 - /3 and @ are the fractions of the magnetization characterized by the time constants Tl; and Tl,'. Figure l b illustrates the nonlinear least-squares fit for the data obtained for Pittsburgh No. 8 coal. An equation similar to eq 2 was used to calculate TIF. Results are shown in Table 11. Generally, Tlprelaxation follows a similar trend to that found for T1.Although close examination of the data indicated the existence of a distribution of Tlpvalues for the coals studied, satisfactory fits were still obtained by using eq 3. For virgin coals, the values of T1F increased with increasing rank from 0.5 to 5 ms with TlpS ranging from 0.23 to 2 ms and Tlpl ranging from 4.6 to 17 ms from lowest to highest ranking coals. For air-dried coals, the Tlpdid not depend strongly on coal rank and were reduced by a factor of 3-5 relative to the results on the virgin coals for all samples except for lower rank coals. Note that a significant portion of hydrogen nuclei in coals exhibited very rapid Tlprelaxation. Values of Tip" of 0.3-0.8 ma with correspondingfractions of -50% were found for air-dried coals. This result means that a large fraction of hydrogen nuclei cannot contribute to carbon polarization in the 13C CP/MAS experiments at longer contact times. Good agreement between the Tl data presented for virgin and air-dried coals and the values obtained from the analysis of the vct 13CCP/IdAS spectra of the same coals (discussed later) supporta the last statement. Note that there is no significant difference between TlPrelaxation times for aliphatic and aromatic protons.
-
!fl
Quantitation of Hydrogen and Carbon Functionalities The results reported below for the premium coals were obtained from 'H CRAMPS, vct 13CCP/MAS, and FT-IR experiments. The NMR and FT-IRresults are compared and their quantitative reliability discussed. 'HCRAMPS and Hydrogen Functionalitiee. The CRAMPS spectra obtained for the virgin and vacuumdried premium coals are presented in Figure 2. The partially overlapping bands centered at -2.0 and at -7.5 ppm are assigned to aliphatic and aromatic hydrogen, respectively. The peak centered at -7.5 ppm also includes resonances from hydrogen in hydroxyl groups. For coals with low water content in the "as-received" state, the spectra of dried and virgin samples are similar. The presence of physisorbed water (-5 ppm downfield from TMS) in the virgin samples of Illinois No. 6, Wyodak and
yr-7
J 1
Beulah Zap 20
15
10 5 0 PPM FROM TMS (a)
-5
20
15
10
5
0
.5
PPM FROM TMS @)
Figure 2. 'H CRAMPS spectra of the premium coals: (a) virgin and (b) vacuum-dried samples.
Beulah coals is strongly manifested in their CRAMPS spectra. These results agreed with those of single-pulse 'H NMR experiments, in which sharp peaks with widths ranging from 7 to 11 ppm located at a chemical shift identical to that of liquid water were observed.I2 For Illinois No. 6 coal, only a small peak at -5 ppm could be observed in its CRAMPS spectrum, but for Wyodak and Beulah coals the water peak obscured resolution such that aliphatic and aromatic bands could not be distinguished. Thus the spectra of the vacuum-dried samples must be examined in order to determine hydrogen functionalities in coals when using CRAMPS. For the computer-aided fitting of the spectra, a superposition of two Lorentzian lines was assumed: F(w) = M,/[(w - w d 2 + + M d / [ ( o- wJ2 + (AwJ21 (4) where M,, Md,w,, and wd are the amplitudes and positions of the aromatic and aliphatic peaks, respectively, while Aw, and bod are the widths at half-height of those peaks. Subsequently, hydrogen aromaticities Ha were calculated by using the equation Ha = (MUAw,) / (M&, + M&d) (5) The observed shifts and line widths for aliphatic and aromatic peaks and Ha are listed in Table 111. A plot of Ha against MAF weight percent oxygen is presented in Figure 3. Values of the hydrogen aromaticities for the bituminous premium coals3' measured by CRAMPS at 187 MHz using BR-24 are also indicated in Figure 3. These two sets of data track to within the eatimated experimental errors associated with both determinations. 13C CP/MAS and Carbon Functionalities. The CP/MAS spectra were measured for virgin and air-dried coals. Typical spectra of air-dried coals were taken with (31) Shin, S. C.; Baldwin, R. M.; Miller, R. L. Energy Fueb 1989,3, 193.
464 Energy & Fuels, Vol. 6, No. 4, 1992
ID 501 101 401 701 601 301 202
801
dela Rosa et al.
Table 111. Parameters Calculated from the CRAMPS Spectra of the Premium Coals and the Hydrogen CRAMPS Aromaticity aliphatic hydrogen aromatic hydrogen source seam preparation state shift, ppm width, ppm shift, ppm width, ppm Pocahontas No. 3 dry 2.5 2.8 7.8 3.1 virgin 2.3 2.4 7.5 3.0 Upper Freeport dry 2.4 2.2 7.6 2.6 virgin 2.1 2.2 7.6 2.0 Pittsburgh No. 8 dry 2.4 2.2 7.9 2.3 virgin 2.3 2.0 7.8 1.8 Lewiston-Stockton 2.8 2.3 8.3 2.5 dfY wrgin 2.4 2.3 7.9 2.3 Blind Canyon dry 2.2 1.8 8.0 2.4 virgin 2.2 1.8 7.9 2.4 Illinois No. 6 dry 2.4 1.8 7.8 2.8 virgin 2.0 1.7 7.5 2.4 Wyodak-Anderson dry 2.3 1.5 7.5 2.8 virginb 1.8 1.6 7.3 2.4 Beulah Zap dry 2.2 1.7 7.1 2.9 virginb 2.3 1.7 7.1 1.9
H. 0.57 0.46 0.40 0.39 0.37 0.47 0.47 0.46
"Band centers and band width were determined to f O . l ppm; deviations for hydrogen aromaticity values range from *0.01 for coal 501 to a0.05 for coal 801. bThe fitting of these spectra were done with the use of the parameters for the corresponding dry coal spectra. 1 .
n
I
0 . 0 1
U
0
0
0".
0
0
0
0 0.. 2
. i : .
4 10
15
20
25
360
MAF PERCENT OXYGEN
Figure 3. Values of Haof the premium coals measured by lH CRAMPS at 300 MHz using MREV-8 (0) and at 187 MHz using BR-24s1 ( 0 )plotted against M A F weight percent oxygen.
a contact time of 1.5 me are shown in Figure 4. The heterogeneous nature of coal precludes the observation of sharp resonances in the spectra obtained by CP/MAS. A CP/MAS spectrum consists of two distinct but rather broad (20-40 ppm wide) peaks representing a distribution of various aliphatic and aromatic bond types and functionalities. With appropriate signal-to-noise ratios, some fine structures are observed in both aromatic and aliphatic peaks. In the aliphatic region an intense and clearly separated resonance peak at -20 ppm can be assigned to methyl groups. Another distinct peak at -33 ppm originates from methylene resonances. At least three resonancee can be found in the aromatic region. Benzene-like protonated carbon resonates at -128 ppm; carbon bonded to another carbon is found at -145 ppm; and carbon bonded to oxygen absorbs at 156 ppm. In some cases, as in the spectrum of Wyodak coal, peaks at 180-220 ppm which represent carbon nuclei in carboxyl and carbonyl >c-Ogroups are observed. The evaluation of carbon aromaticity fa from the vct CP/MAS experimental4required the spectrum to be di-
-
240
120
0
-120
PPM FROM TMS 13C CP/MAS spectra of the air-dried premium coals taken with a contact time of 1.5 ms.
Figure 4.
vided into aliphatic and aromatic regions, upfield and downfield from 80 ppm, respectively. The areas M ( T ~ ~ ) ~ (i = al, ar) of both peaks were calculated as functions of contact time rep. and the initial magnetizations Mowere extracted by using the equation32
M(T~,)'= Moexp(-rcp/TlPHi)[1- e x p ( - A ~ , , / T ~ ~ ~(6) )] Here A = 1- Tmi/T," in which Tmi is the time constant is the aliphatic or arofor polarization transfer and TIPHi matic proton Tip. In coals where measurable intensities of >C-0 were observed, fractions of > C = O carbon f, were also determined. Values of fa are calculated from the ratio (7) f a = MtY/(MtY + M f ) where included the >C=O carbon intensities. The results of vct CP/MAS experiments are shown in Table IV, where fa, TIPHd, and TlpHervalues, as well as f,, (32) Mehring, M. Principles of High Resolution NMR in Solids; Springer-Verlag: New York, 1983.
Argonne Premium Coals
Energy & Fuels, Vol. 6, No. 4, 1992 468
Table IV. IF CP/MAS VCT Experimental Parameters and Calculated Carbon Aromaticity Values of the Premium Coals ID source seam preparation state Tlpd,ms TI^^, ms f a (40.01) fJ1.5): (40.01) f, (iO.01) 501 Pocahontas No. 3 air-dried 3.1 4.6 0.86 0.87 0.0 virgin 11.0 12.2 0.83 0.0 101 Upper Freeport air-dried 3.8 4.5 0.80 0.79 0.0 401
Pittsburgh No. 8
701
Lewiston-Stockton
601
Blind Canyon
301
Illinois No. 6
202
Wyodak- Anderson
801
Beulah Zap
"romaticity
virgin air-dried virgin air-dried virgin air-dried virgin air-dried virgin air-dried virgin
air-dried virgin
12.9 3.8 10.4 2.9 7.1 4.1 8.8 4.3 11.2 2.9 7.4 2.4 6.2
15.1 5.2 12.2 4.0 7.8 3.9 9.6 5.1 11.7 3.5 8.0 3.7 7.7
0.77 0.71 0.72 0.73 0.72 0.63 0.64 0.70 0.70 0.60 0.65 0.58 0.66
0.0 0.0 0.0 0.0 0.0 0.01 0.01 0.0 0.0 0.06 0.05 0.08 0.05
0.72 0.74 0.62 0.69 0.64 0.63
calculated from spectra taken with a single contact time of 1.5 ms.
are listed. For high-rank coals,there is agreement to within estimated experimental error between the carbon aromaticities obtained for virgin and air-dried samples, but discrepancies exceeding experimental errors were found between the fa values derived for Wyodak and Beulah coals. These discrepancies may be partly attributed to the difficulties of CP/MAS approach for quantitation in coals (see discussion below). However, the changes in fa can also result from oxidation of surface sites and formation of >C=O and -COOH upon exposure to air.33 This view is supportad by the observed increase in f , for the air-dried low-rank coals. For these reasons we will use the CP/MAS resulta for air-dried coals for comparison with FT-Et and for further discussion. Values of fa for the air-dried premium coals from this work and from the literatureB*% are plotted against MAF weight percent oxygen, and the plot is shown in Figure 5. The plot illustrates that the values of f a derived from CP/MAS spectra measured and reported by different laboratories have a reproducibility of better than 95% for higher-rank coals. The controversy on the quantitative reliability of results from CP/MAS remains. A detailed study of the limitations of CP/MAS conducted in the authors' 1aboratoryl6 for air-dried Pitta gh No. 8 coal indicated that f a obz from a vct CP/MAS experiment tained at w,, of 25.16 agreed within experimental error with the value of 0.70 obtained from a Bloch decay experiment. The same Bloch decay experiment detected 94 (f5%) of all carbon nuclei in the sample. A vct CP/MAS experiment on Blind Canyon coal conducted by Maciel et al.15showed that 87% of all carbon nuclei were accounted for. In Maciel's work, the biphasic behavior of TlpH,as detected by CP/MAS with proton spin lock, was included in the data analysis. In a most recent report, Muntean et al.,= concluded that the CP experiment cannot be relied upon to provide accurate relative intensities for many coal samples, although the CP/MAS and Bloch-decay derived aromaticities presented in their work agreed within experimental error for all the premium coals. The biggest discrepancy reported between the two experiments was for the lowestranked coal (Beulah) where the CP/MAS value was 4% lower. The fractions of carbon detected by Bloch decay
%
(33) Berkowitz, N. An Introduction to Coal Technology; Academic Press: New York, 1979. (34) Botto, R. E.; Axelson, D. E. Prepr. Pup.-Am. Chem. SOC.,Diu. Fuel Chem. 1988,33 (3), 50. (35) Choi, C.; Muntean, J. V.; Thompson, A. R.; Botto, R. E. Energy Fuels 1989,3,528. (36) Muntean, J. V.; Stock, L. M. Energy Fuels 1991,5, 765.
0
5
10
15
20
MAFPERCENTOXYGEN
Figure 5. Values of fa of the premium coals at 25 MHZ calculated from vct CP/MAS (0) and from CP/MAS spectra taken with a single contact time of 1.5 ms ( 0 )plotted against MAF weight percent oxygen as indicator of rank. Also indicated are fa from vct CP/MAS experiments by Solum et aLm (A)and from eingle-contact-timeC P / W experiments by Botto et al." (X) and Choi et al.= (I). "he least-squares fit corresponds to vct CP/MAS data (0).
were consistently higher than those by CP/MAS. In summary, experimental evidence supports the view that reasonable relative intensities can be obtained from CP/MAS spectra for most coals under proper conditions. Significant errors can arise for low-ranking oxygen-rich coals because of most unfavorable cross-polarization dynamics for which CP/MAS may underestimate aromatic intensities and result in reduced values of fa. In such casea, the Bloch decay experiment, preferably with the use of a large MAS system16to reduce the data acquisition time, and minimum recycle times of 100 s would seem to be the method of choice. FT-IRMeasurements. The E t spectra of the premium coals, normalized to absorbance unita/mg of coal and corrected for absorption of mineral matter, are presented in Figure 6. To obtain a quantitative measure of functional group concentrations,Solomon et al." used a curve analysis program to synthesize the IR spectra by adding 45 Gaussian absorption peaks with variable positions, widths, and heights. MAF weight percent concentrations
466 Energy & Fuels, Vol. 6, No. 4,1992
dela Rosa et al.
Table V. MAF Weight Percent Concentration of Hydrogen and Carbon in the Premium Coals As Determined by FT-IR" hydrogen carbon oxygen ID source seam C, Hd Hoh Ht Ct C=Oa H:(ir) H, Cd f,(ir) Ooh Os* 0.06 2.19 4.22 0.52 501 Pocahontas No. 3 1.97 14 77 91 1.92 0.85 1.0 1.2 0.11 2.08 5.62 Upper Freeport 3.43 0.37 23 101 63 86 0.63 0.73 1.8 0.8 3.60 0.16 2.07 5.83 401 Pittsburgh No. 8 0.36 24 59 83 0.86 0.71 2.5 1.9 0.36 701 Lewiston-Stockton 3.48 0.23 2.12 5.83 24 59 83 3.59 0.71 3.8 1.8 4.79 0.16 1.90 601 Blind Canyon 6.85 0.28 33 48 81 8.70 0.59 2.5 4.0 3.41 0.23 2.07 5.71 301 Illinois No. 6 0.36 23 55 78 4.48 0.70 3.8 2.2 0.33 Wyodak-Anderson 3.03 1.73 5.09 202 0.34 21 54 75 23.9 0.72 5.2 5.0 801 BeulahZap 2.02 0.34 1.58 3.94 0.40 13 60 73 24.7 0.82 5.5 5.0 Except carbonyl plus carboxyl: relative peak area in absorbance units x em-'. T' 0 6 T A' L 1
M a A I F 1
HO c
c
mi
io1
H UltlnaIe
c 101 anilyala
c mi
c
(01
c 101
c
ra,
c
101
@ FTlR
0NYR
Figure 7. Comparison of the hydrogen concentrations in the 'HNMR.12 premium coals as determined by elemental analy~is.~
and FT-IR. 701 Lewiston-
'T
: t+
H 09
Ob
0.1
m 0 0.8 a Ob 0.4 0.3
601 Blind
c t
301 Illinois W6
oa
y w
0
Cliol
202 Wyodak-
Anderson
801 Beulah Zap A
ClOl
H Ha
I
I &
WAVENLTMBERS (cm-l)
Figure 6. FT-IR spectra of dry premium coals after the removal of the background from mineral matter.
of aliphatic and aromatic hydrogen, hydroxyl hydrogen, and carbon functionalities of the premium coals were calculated by using peak areas from the synthesized spectra and rank-dependent extinction coefficienta. The weight percent concentrations of hydroxyl Hoh, aliphatic Ifd,and aromatic H, hydrogen were estimated directly from absorption bands centered near 3200,2900, and 800 cm-',respectively. These resulta, the total weight percent hydrogen I f t l and the hydrogen aromaticity &'(ir) equals HarIHt are- s in Table V. How accurately the concentration of aliphatic hydrogen is determined depends on the accuracy of the assumed stoichiometry of aliphatic hydrogen to aliphatic carbon. Recent research in quantitative infrared spectsoecopyindicates that Hdand HA can be determined to within &lo%. Although H, can be obtained to an accuracy of *lo% for coals with carbon content above 85 w t %, variations of up to i50% arise for coals with lower carbon concentrations.18 The concentrations of aliphatic carbon Cdwere inferred from the corresponding aliphatic peaks by assuming that the average Stoichiometry for aliphatic material" in coal ia CHleB.The concentration of aromatic carbon C, which carbon, was determined by difference, included all >M
c401 cm1 fl H a W
Call
I H,'
Csol
caoa CBOl
Ill1 Ha'(ir)
Figure 8. Comparison of the hydrogen aromaticities of the premium coals determined by using 'H CRAMPS and FT-IR.
C, = Ct - Cd,where C, is the total weight percent concentration of carbon from ultimate analysis. These resulta and the carbon aromaticity fa(ir) equals C,/C, are listed in Table V. In addition, Table V lbts the relative absorption intensities of >C-0 carbon and the weight percentages of oxygen functionalities that could be calculated from these data. Quantitation of Hydrogen and Carbon Functionalities: Comparison of NMR and FT-IR Results Figure 7 compares the results of total hydrogen concentration obtained in this work by FT-IR with the ultimate analysismand with 'H spin counting by NMR from our earlier work.12 The experimental aspects and the accuracy of quantitation of hydrogen in coals by lH NMR were discuesed in detail in ref 12. Despite the shortcomings of 'H NMR and ultimate analysis procedures, the resulta obtained by using these two methods differed by less than 5 % for all coals except the lowest-ranked coals Wyodak and Beulah. On the other hand, F"I'-IR yielded Ht values that in most cases were higher than and differed from those of the other two methods by 10-20%. Hydrogen aromaticities of the premium coals calculated from the two spectroscopic techniques are presented in Figure 8, where four seta of data are compared. First, the Ha values resulting from CRAMPS are plotted. As mentioned earlier, Ha values are the fractions of integrated intensity in the "aromatic" region of the CRAMPS spectra
Energy & Fuels, Vol. 6, No.4, 1992 461
Argonne Premium Coals I T
2
CWl
ClOl
Clol
c701
CBOl
Caol
cloa
1 .
Caol
Figure 9. Comparisonof the carbon aromaticities of the premium coals determined by I3C CP/MAS and FT-IR.
and thus include resonances from hydroxyl groups. The corresponding hydrogen aromaticities resulting from FTIR are represented by the next column. These values, designated H,(ir), were calculated using the formula Ha($ = (H,+ &)/& where H,, Hob, and Htare taken from Table V. The last two columns in Figure 8 illustrate the Haand H,(ir) values correctad for hydroxyl protons. These revised values are designated H,' and H,'(ir), respectively. Since hydroxyl protons could not be directly measured by CRAMPS, the corrections were made by using the Hoh value derived from FT-IR and the formulas H,' = Ha. [H,/(H, + Hob)] and H,'(ir) = H,/Hv There is a good agreement between hydrogen aromaticities obtained with NMR and IR for all coals. The extent of the contribution of hydroxyl protons increases with decreasing rank and is negligible for high-rank coals. Correcting NMR Ha values for hydroxyl hydrogen concentration obtained from FT-IR seems to be the most accurate method in determining the average hydrogen aromaticities of coal. This procedure combines the CRAMPS results that are independent of any assumptions about the stoichiometry of hydrogen to carbon in coals and the sensitivity of the FT-IR to oxygen functionalities. The carbon aromaticities derived from CP/MAS and FT-IR are compared in Figure 9. As explained earlier, the f,(ir) values were derived by using C, concentrations which included >C=O carbon. Correcting f,(ir) values cannot be made on the basis of FT-IR data alone because of the strong dependence of the extinction coefficients of these functionalities on coal rank. However, the FT-IRderived values can be corrected for >C=O carbon intensities by using the NMR data of Table V and the formula f,'(ir) = f,(ir)uJ(f, + f d ] The . values of f,'(ir) are included in Figure 9. Both spectroacopic techniques gave very similar values of carbon aromaticities for the six bituminous coals, whereas a large discrepancy between fa and fl(ir) still existed for the younger coals. For Wyodak and Beulah coals, the NMR values followed the often reported trend of decreasing aromaticity with decreasing rank (compare with results shown in Figure 10 in the next section), while the FT-IR values were higher by 10% and 25%, respectively. As noted earlier, existing experimental evidence suggmts that for moat coals the CP/MAS technique yields aromaticity values with accuracies of 195%. On the other hand, the I?"-IR-derived values, obtained by assuming an average stoichiometry of for the aliphatic fraction in all coals, tend to overestimate aromaticity values for lower-rank coals.
(WC), and (HE),, as Indicators of Rank Chemical and physical properties of coals vary systematically with rank and can be broadly predicted on the basis of the fixed carbon content and volatile matter or the ultimate analysis of the whole coal. NMR offers an
$ t--,-----
~- - - - - (- - -)includes OH 0
---A-
AROMATIC (-)excludes
OH I
Figure. 10. Aliphatic and aromatic hydrogen-to-carbon ratios for the premium coals as indicators of rank. Table VI. Summary of Average Structural Properties of the Premium Coals ID source seam (H/C)O Hlb fa (H/C)yb (H/C)db 501 Pocahontas No. 3 0.58 0.56 0.86 0.37 1.78 101 Upper Freeport 0.65 0.44 0.80 0.36 1.76 0.76 0.37 0.71 401 Pittsburgh No.8 0.40 1.57 701 Lewiston-Stockton 0.76 0.35 0.73 0.37 1.72 601 Blind Canyon 0.85 0.34 0.63 0.46 1.49 0.77 0.42 0.70 0.47 1.36 301 Illinois No.3 202 Wyodak-Anderson 0.85 0.40 0.60 0.56 1.33 0.79 0.38 0.58 0.52 801 BeulahZap 1.25 Ratios were calculated from ultimate analysis d a h m Values were corrected for hydroxyl hydrogen and/or carbonyl plus carboxyl carbon concentrations.
analytical technique by which rank can be determined in terms of *aromaticity". Carbon aromaticity as a rank indicator is tested by plotting the values of f a against MAF weight percent oxygen (% 0) in Figure 5. Weight percent oxygen as a rank parameter was chosen for comparison purposes because of the known correlation between age and oxygen content in the The regression line obtained is fa = -0.015(%0) + 0.88 (8) with a coefficient of correlation r of -0.93. (A perfect correlation corresponds to r equals unity. A negative value corresponds to a negative s l ~ p e ) .The ~ deviation of fa of Blind Canyon coal from the regression line probably stems from the high fraction of sp~rinite,~ a maceral originating from waxy components of plants found in this coal. Combining the fractions Haand fa with the atomic hydrogen-to-carbon ratio H/C from the ultimate analysism or from N M R spin counting experiments,'%lsthe aromatic and aliphatic hydrogen-to-carbon ratios (H/C), and (H/fJd, respectively, can be calculated for the premium coals by using the equations (H/C), = (Ha/fa)(H/C) (gal (9b) (H/C), = [(I - Ha)/(l- fa)l(H/C) (H/C), is the mole fraction of peripheral protonated (37)Attar, A.; Hendrickson, G. G. In Coal Structure; Meyers, R. A., Ed.; Academic Press: New York, 1982. (38)Topping, J. Errors of Obseruation and Their Treatment; The Institute of Physics: London, 1955.
Energy & Fuels 1992,6,46&474
468
carbon atoms in the aromatic clusters, and (H/C), is the stoichiometricratio of hydrogen to carbon in the aliphatic portion of the coals. As a first approximation Hacalculated from CRAMPS and f a from CP/MAS may be used. These parameters for the premium coals are listed in Table VI. Figure 10 shows (H/C), and (H/Cld plotted against %O. The regression lines obtained are (H/C), = 0.018(%0) + 0.29 (loa) (H/C), = 4.048(%0) + 2.0
(lob)
with r of 0.93 and -0.95, respectively. By using H,’ instead of Ha in eq 9a and 1- f a - f , instead of 1- f a in eq 9b, the regression lines changed slightly to (H/C),, = 0.012(%0) + 0.30 (10c) (H/C)d = -0.033(%0)
+ 1.9
with r of 0.90 and -0.92, respectively. Similarly good correlations are obtained for (H/C), and (H/C)d plotted as a function of percent carbon, indicating the usefulness of these two parameters as indicators of coal rank. The decreasingtrend of (H/C), versus rank reflects progressing polycondensation and increasing size of aromatic clusters. This process is accompanied by increasing (H/C), values that approach 2 for high-rank coals. In the previous sections, several structural parameters of coals derived from solid-state NMR and FT-IR were
discussed and results compared. Some of these parameters can be used to infer further important properties of coal such as the “average”aromatic ring size that may be estimated from the parameters measured in this work for whole coal. Each of the coals studied is a mixture of different macerals that in general have different average properties by themselves. As was recently demonstrated by Vassallo et different density fractions from one Callide coal studied by petrographic analysis and CRAMPS, exhibited a diversity of structures and marceral compositions. For example, the lightest fraction with a density of 1.20-1.25 g consisted mostly of liptinite and exhibited a hydrogen aromaticity of 0.06, while the heavieat fraction with a density of 1.45 g consisted of inertinite and had a hydrogen aromaticity of 0.76. The analysis of density fractions of each of the premium coals was beyond the scope of this work, although such a study would significantly increase the level of detailed information derived from NMR and FT-IR on these coals. Acknowledgment. We express our appreciation to David Torgeson of the Physics Department of Iowa State University for the use of his NMR spectrometer for the proton longitudinal relaxation measurements. Registry No. C,7440-44-0; H,1333-74-0. (39)Vassallo, A. M.;Hanna, J. V.; Wilson, M.A.; Lockhart, N. C. Energy Fuels 1991, 5, 643.
Oil and Gas Evolution Kinetics for Oil Shale and Petroleum Source Rocks Determined from Pyrolysis-TQMS Data at Two Heating Rates Robert L. Braun,* Alan K. Burnham, and John G. Reynolds Lawrence Livermore National Laboratory, University of California, Livermore, California 94550 Received November 25, 1991. Revised Manuscript Received April 20, 1992
Seven oil shales and petroleum source rocks were subjected to programmed-temperature pyrolysis at heating rates of 1and 10 OC/min using triple-quadrupole mass spectrometry to monitor volatile compound evolution. Kinetic parameters were determined for evolution of hydrocarbons and various heteroatom species. Normalized cumulative generation of oil, light hydrocarbon gas (C2-C4),methane, carbon dioxide, acetic acid, hydrogen sulfide, and methylthiophene were calculated for generic geologic conditions using kinetic parameters that were lumped and averaged into type I, IIa, and IIb source rock classes. The four heteroatom species are largely generated before oil and the hydrocarbon gases. Using these kinetics to simulate hydrous pyrolysis gives favorable comparison with hydrous pyrolysis measurement of acetic acid generation for several different source rocks.
Introduction A variety of techniques have been used to measure oil and gas generation kinetics for kerogen pyrolysis, but most workers do not report detailed information for individual pyrolysis products. The latter information is important for many reasons: to help establish the origin of activation energy distributions, to provide diagnostics of kerogen structure and depositional conditions, to understand variations of gas/oil ratios during kerogen maturation, to
make complex chemical models needed for realistic calculation of overpressuring, and to define the source terms for components that are involved in important secondary reactions, such as mineral diagenesis. Laboratory pyrolysis at a constant heating rate using triple-quadrupole mass spectrometry (TQMS) as the detection method is particularly well suited for measuring the generation rate of individual pyrolysis products. It provides on-line, time-resolved analysis to follow the ev-
0007-0624/92/2506-0~60~03.oo/o 0 1992 American Chemical Society
