Energy & Fuels 1989, 3, 187-193
187
13C Solid-state NMR of Argonne Premium Coals Mark S. Solum,t Ronald J. Pugmire,*$$and David M. Grant*>? Departments of Chemistry and Fuels Engineering, University of Utah, Salt Lake City, Utah 84112 Received August 9, 1988. Revised Manuscript Received December 1 , 1988
Eight Argonne Premium Coal samples and three other oxidized coals have been investigated by the 13C CP/MAS NMR technique. Spin-lattice relaxation, variable contact time, and dipolar-dephasing experiments were performed on each coal. The proton spin-lattice relaxation time, TIH,the proton spin-lattice relaxation in the rotating frame time, TlpH,and the cross-polarization time, TCH, are reported along with the dipolar-dephasing time constants, TGand TL, for both the aromatic and aliphatic regions of the 11coals. These data, together with normal CP/MAS integrations over selected chemical shift ranges, have been used to derive 12 parameters relating to the carbon skeletal structure, including the aromaticity. From the mole fraction of bridgehead or inner carbons as measured by NMR, the aromatic cluster size may be estimated for each coal.
Introduction In the past decade 13Csolid-state NMR spectroscopy, because of its nondestructive nature and unique capabilities, has been used in the structural analysis of solid fossil fuel samples.lP2 By the use of cro~s-polarization~-~ (CP), m a g i c - a n g l e - ~ p i n n i n g(MAS), ~ ? ~ ~ ~and dipolar-decoupling techniqueslOJ1a direct measurement of the relative number of aromatic and nonaromatic carbons is possible.12 The aromaticity, fa, has been reported for whole coals, macerals, soil, and other fossil fuel related material^.'^-'^ Other researchers have also used dipolar-dephasing (DD) techniques1&22along with the normal CP/MAS integrations over selected chemical shift ranges to subdivide fa values into the amount of protonated and nonprotonated carb o n ~ . ~ ~ - ~ ~ The quantitative accuracy of these aromaticity values has been open to question for two reasons. First, competing effects in the spin dynamics of cross-polarization do not polarize all spins a t the same rate and differences in the spin lattice relaxation in the rotating frame may alter the signals such that they do not achieve full polarization a t the same contact time. This problem can be minimized by determining the aromaticity from variable contact time experiments. Second, the presence of paramagnetic centers may render some carbon resonances invisible. Recent spin-counting experiments13J5J6 have shown that as little as 26% of the carbons can be seen in some macerals whereas in other samples essentially all carbons are detected. Although paramagnetic centers are associated with aromatic carbon,28 the signal from all carbons within 8-11 A may be obscured.29 Wilson30 has discussed the reliability of quantitative results in geochemical samples and concludes that quantitative conditions are frequently obtained. Sethi et al.31have demonstrated that it is possible to obtain a quantitative representation of carbon types in anthracite coals. Nonetheless, the question still remains whether the carbons observed will always be representative of the whole sample. In this paper 13Csolid-state CP/MAS NMR spectroscopy is used to determine the carbon skeletal structure of the eight coals from Argonne National Laboratory's Premium Coal Sample Program and three oxidized coals. The eight Argonne coals are North Dakota Beulah-Zap (lignite),
*
Department of Chemistry. Department of Fuels Engineering. 0887-0624/89/2503-0187$01.50/0
Wyoming Wyodak (subbituminous), Blind Canyon seam (high-volatile bituminous), Illinois No. 6 (high-volatile bituminous), Stockton seam (high-volatile bituminous), Pittsburgh No. 8 (high-volatile bituminous), Pennsylvania Upper Freeport (medium-volatile bituminous), and Poc(1) Axelson, D. E. Solid State Nuclear Magnetic Resonance of Fossil Fuels; Multiscience: Montreal, Canada, 1985. (2) Davidson, R. M. Nuclear Magnetic Resonance Studies of Coal; IEA Coal Research London, 1986. (3) Hartmann, S. R.; Hahn, E. L. Phys. Rev. 1962, 128, 2042. (4) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1972, 56, 1776. (5) Mehring, M. In High Resolution NMR Spectroscopy in Solids: Diehl, P., Fluck, E., Kosfeld, R., Eds.; NMR-Basic Principles and Progress 11; Springer Verlag: New York, 1976. (6) Alemany, L. B.; Grant, D. M.; Pugmire, R. J.; Alger, T. D.; Zilm, K. W. J. Am. Chem. SOC.1983, 105, 2133. (7) Alemany, L. B.; Grant, D. M.; Pugmire, R. J.; Alger, T. D.; Zilm, K. W. J. Am. Chem. SOC.1983, 105, 2142. (8) Andrew, E. R.; Bradbury, A.; Eades, R. G. Nature 1959,183,1802. (9) Lowe, I. J. Phys. Reu. Lett. 1959, 2, 285. (10)Schaefer, J.; Stejskal, E. 0.; Buchdahl, R. Macromolecules 1975, 8, 291. (11) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1973, 59, 569. (12) VanderHart, D. L.; Retcofsky, H. L. Fuel 1976, 55, 202. (13) Botto, R. E.; Wilson, R.; Winans, R. E. Energy Fuels 1987,1,173. (14) Sullivan, M. J.; Maciel, G. E. Anal. Chem. 1982, 54, 1615. (15) Vassallo, A. M.; Wilson, M. A.; Collin, P.; Oades, J. M.; Waters, A. G.; Malcolm, R. L. Anal. Chem. 1987,59,558. (16) Hagaman, E. W.; Chambers, R. R.; Woody, M. C. Anal. Chem. 1986, 58, 381. (17) Vassallo, A. M.; Wilson, M. A.; Edwards, J. H. Fuel 1987,66,622. (18) Alla, M.; Lippmaa, E. Chem. Phys. Lett. 1976, 37, 260. (19) Opella, S. J.; Frey, M. H. J. Am. Chem. SOC.1979, 101, 5854. (20) Murphy, P. D.; Cassady, T. J.; Gerstein, B. C. Fuel 1982,61,1233. (21) Murphy, P. D.; Gerstein, B. C.; Weinberg, V. L.; Yen, T. F. Anal. Chem. 1982,54, 522. (22) Alemany, L. B.; Grant, D. M.; Alger, T. D.; Pugmire, R. J. J.Am. Chem. SOC.1983, 105,6697. (23) Alemany, L. B.; Grant, D. M.; Pugmire, R. J.; Stock, L. M. Fuel 1984. 63. 513. (24) Wilson, M. A.; Collin, P. J.; Pugmire, R. J.; Grant, D. M. Fuel 1982.. 61. .959.~ ~ (2'5)Wilson, M. A.; Pugmire, R. J.; Karas, J.; Alemany, L. B.; Woolfenden, W. R.; Grant, D. M.; Given, P. H. Anal. Chem. 1984, 56, 933. (26) Soderquist,A.; Burton, D. J.; Pugmire, R. J.; Beeler, A. J.; Grant, D. M.; Durand, B.; Huk, A. Y. Energy Fuels 1987, 1, 50. (27) Derenne, S.; Largeau, C.; Casadervall, E.; Laupretre, F. Fuel 1987, 66 - - -- . -- , lnR6 (28) Wind, R. A.; Duijvestijn, M. J.; van der Lugt, C.; Smidt, J.; Vriend, H. Fuel 1987, 66, 876. (29) Muntean, J. V.; Stock, L. M.; Botto, R. E. Energy Fuels 1988,2,
-"-.
1 nR
(30) Wilson, M. A. NMR Techniques and Applications in Geochemistry and Soil Chemistry; Pergamon Press: Oxford, England, 1987. (31) Sethi, N. K.; Pugmire, R. J.; Facelli, J. C.; Grant, D. M. Anal. Chem. 1988, 60, 1574.
0 1989 American Chemical Society
188 Energy & Fuels, Vol. 3, No. 2, 1989
Solum et al.
ahontas No. 3 (low-volatile bituminous). The three oxidized coals are North Dakota Zap (lignite), Montana Rosebud (subituminous), and Illinois No. 6 (high-volatile bituminous). These three oxidized coals were included in the study for comparison to the fresh Argonne coals and because their structural data is used elsewhere in a study of the relationship between coal structure and coal devol a t i l i ~ a t i o n . ~Twelve ~ I3C structural parameters, determined from a variety of experiments (i.e. dipolar-dephasing techniques, variable contact time experiments, and integrations of the NMR spectra over selected chemical shift ranges), are reported for each coal. These structural parameters are then used to estimate the aromatic cluster size and the extent of bridging between clusters for each coal. In addition, three relaxation times: TcH, the time constant for polarization transfer in a CP experiment; TIH, the proton spin lattice relaxation time; and TIPH, the spin lattice relaxation time in the rotating frame are reported for both the aromatic and aliphatic regions of each coal. Eight of the coals used in this study were obtained from the Premium Coal Sample Program at Argonne National Laboratory. All of the Argonne coals were of -100 mesh size except the Wyodak subbituminous coal, which was -20 mesh. All sample vials were thoroughly shaken, opened in a nitrogen glovebag and placed in a boron nitride T-barrel rotor with a Kel-F cap as described by Jiang et alas The rotor was spun under dry nitrogen gas at 4.0-4.2 kHz. The three oxidized coals were used in a pyrolysis s t u d g 4 and were supplied to us by Advanced Fuel Research (AFR). These coals were placed in the rotor and spun a t 4 kHz with air. The elemental analyses for the AFR coals have been given,% and preliminary elemental analyses of the Argonne coals have been p~blished.~~ All CPIMAS spectra were obtained on a Bruker CXP-100 spectrometer with a 13Cfrequency of 25.15 MHz. A 1K FID was acquired with quadrature detection and zero fiied to 8K. Spectral widths of either 12 or 20 kHz were used. The radio frequency fields were ycBlc = ~ H B=~40 HkHz. The dipolar dephasing and TIHexperiments used a contact time of 2.5 ms while delay times were 4-5TIH for all experiments, except for the Argonne Zap dipolar-dephasing experiments, which were run with a contact time of 1.0 ms to increase the signal-to-noise ratio. The proton spin-lattice relaxation times, TIH,were measured by indirect detection using the modified inversion recovery method described previously.'J4 In this method a 18Oo-~-9O0pulse sequence is applied in the proton channel followed by a cross-polarization (CP) sequen~e,4.~ which transfers polarization from the remaining 'H magnetization to the 13Cspins whose signal is then observed under proton decoupling. The magnetization recovery is then fit to the following three-parameter equation:
+ M,)e-T/TIH
(1)
where a is a scaling parameter for incomplete inversion and is usually about 0.8, M , is the equilibrium magnetization and TIH is the proton spin-lattice relaxation time. The above equation was used separately for the aromatic and aliphatic regions of the spectra. The time constants, Tc, and TIPH were measured by using a standard CP sequence while the contact time, T ~ was , varied. The carbon magnetization was fit to the following three-parameter equation:
M(r,.J = Mo(e-rdTl~H - e-rsp/Tm)
7
0
1
0
30
60
TIME ( p s )
90
120
Figure 1. Fit of the dipolar-dephasing data for the aliphatic region of Rosebud. Notice the strong dipolar oscillations between 30 and 90 ps.
Experimental Section
M ( T )= M , - ( a M ,
60
magnetization and the above equation was again used separately for the aromatic and aliphatic regions of the spectra. Depending on the recycle time needed to allow the spins to return to equilibrium, 13 or 25 different contact times ranging from 10 ps to 25 ms were used for each sample. The dipolar-dephasing pulse sequence has been described previously,22and the version used here has 180° refocusing pulses in both the carbon and proton channels. In the dipolar-dephasing experiment, the carbon magnetization is created by a cross-polarization sequence. All radio frequency fields are then turned off for a time, T, and the signals from carbons that are strongly coupled to protons rapidly dephase. The proton radio frequency is then turned on, and the remaining signals, from weakly coupled carbons, are observed under proton decoupling. There were 24 interupted decoupling delays in this experiment ranging from 0 to 200 ps for the aromatic region while the aliphatic region used 20 interupted decoupling times from 0 to 120 ws. A typical decay pattern, showing dipolar oscillations, is given in Figure 1for the aliphatic region of the Rosebud sample. The decay of the carbon magnetization was fit to the following four-parameter equation.20
M ( r ) = MoLe-r/TL+ MOGe4.5(r/Td2
(3)
In eq 3 TLand MOL are the Lorentzian decay constant and initial magnetization, respectively, for the weakly coupled spins, TGp d M a are the Gaussian decay constant and initial magnetization, respectively, for the strongly coupled spins, and the sum of M a and M o is~unity for each spectral region, aromatic and aliphatic. By use of the M,'s from the variable contact time experiment, along with dipolar dephasing initial magnetizations and integrations of selected chemical shift ranges, twelve structural parameters were determined for the eleven coals. The first two parameters, f a and fd are the fraction of aromatic and aliphatic carbons, respectively. The aromatic region is taken for chemical shift values greater than 90 ppm. These two parameters (including sidebands for the aromatic region) were determined by fitting the respective integrated intensities in a variable contact time experiment and using the Mo values determined from eq 2 as follows:
(2)
(4)
Mo is a scaling parameter related to the amount of sample
The aromaticity, fa, can be subdivided into two regions as follows: (32)Grant, D. M.; Pugmire, R. J.; Fletcher, T. H.; Kerstein, A. R. Energy Fuels, preceding paper in this issue. (33) Jiang, Y. J.; Woolfenden, W. R.; Alderman, D. W.; Mayne, C. L.; Pugmire, R. J.; Grant, D. M. Reo. Sci. Instrum. 1987, 58, 755. (34) Serio, M. A.; Hamblen, D. G.; Markham, J. R.; Solomon, P. R. Energy Fuels 1987,1 , 138. (35)Vorres, K. S. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1987, 32(4), 221.
fa
= fa,
+ fac
(5)
where f," represents the amount of carbonyl carbon and includes that part of the aromatic band downfield from 165 ppm and where fat, the corrected aromaticity, measures the fraction of total carbon that is sp2hybridized and present in aromatic rings. To determine the value of fac, the fraction of the total aromatic integral
Energy & Fuels, Vol. 3, No. 2, 1989 189
13C NMR of Argonne Premium Coals
Table I. Relaxation Parameters for t h e Aromatic Spins of t h e Argonne Premium Coalsn TIH,ms coalb TL, PS TG,PS TlPH,ms MOL TCH,PS 22 f 1 0.52 f 0.02 239 f 20 132 f 26 8f1 7 f l North Dakota (L) 240 f 30 18 f 2 0.70 f 0.04 8 f l 345 f 57 10 f 1 Wyodak (SB) 24 f 1 0.66 f 0.02 271 f 25 43 f 2 9 f l 334 f 40 Blind Canyon (HVB) 20 f 1 267 f 18 0.64 f 0.02 10 f 1 61 f 2 379 f 34 Illinois No. 6 (HVB) 21 f 1 276 f 11 0.63 f 0.01 554 f 66 15 f 2 193 f 6 Pittsburgh No. 8 (HVB) 293 f 23 20 f 1 0.64 f 0.02 243 f 30 10 f 1 186 f 7 Stockton seam (HVB) 243 f 18 22 f 1 0.65 f 0.02 16 i 2 342 f 53 490 i 22 Upper Freeport (MVB) 243 f 12 0.62 f 0.01 20 f 1 15 f 2 1132 f 74 229 f 34 Pocahontas (LVB) 5 f 1 33 f 2 13 f 2
Zapc (L) Rosebudc (SB) Illinois No. 6e (HVB)
6f1 6 f l 7 f l
130 f 14 180 i 21 195 f 17
220 f 24 129 f 15 277 f 22
27 f 2 20 f 3 25 f 1
0.64 f 0.04 0.70 f 0.05 0.64 f 0.02
0.48 f 0.02 0.30 f 0.04 0.34 f 0.02 0.36 f 0.02 0.37 f 0.01 0.36 f 0.02 0.35 f 0.02 0.38 f 0.01 0.36 f 0.03 0.30 f 0.05 0.36 f 0.02
= 1 marginal standard deviation. *L = lignite, SB = subbituminous, HVB = high-volatile bituminous, MVB = medium-volatile bituminous, and LVB = low-volatile bituminous. An oxidized sample obtained from Advanced Fuel Research. Table 11. Relaxation Parameters for the Aliphatic Spins of t h e Argonne Premium Coals' coal TIH,ms TCH,PS TlPH,ms TL, TG,PS MOL 0.37 f 0.11 17 f 1 6 f l 54 f 14 11 f 2 40 f 4 North Dakota (L) 0.28 f 0.09 107 f 55 17 f 1 74 f 13 7f1 Wyodak (SB) 13 f 2 0.38 f 0.04 69 f 8 17 f 1 9f1 60 f 3 65 f 5 Blind Canyon (HVB) 16 f 1 0.33 f 0.09 11 f 1 46 f 10 68 f 4 69 f 9 Illinois No. 6 (HVB) 0.52 f 0.06 15 f 1 12 f 1 43 f 4 192 f 7 111 f 10 Pittsburgh No. 8 (HVB) 0.46 f 0.12 54 f 12 16 f 1 9f1 53 f 6 Stockton seam (HVB) 214 f 10 0.52 f 0.12 49 f 9 14 f 1 60 f 9 14 f 2 Upper Freeport (MVB) 474 f 23 0.45 f 0.08 80 f 18 16 f 1 57 f 12 11 f 2 Pocahontas (LVB) 1131 f 194 8 f l 45 f 5 33 f 1
Zapb (L) Roseburdb (SB) Illinois No. 6* (HVB)
5 f l 5 f l 6 f l
34 f 4 62 f 6 45 f 4
60 f 11 56 f 11 74 f 11
17 f 1 18 f 1 16 f 1
0.37 f 0.07 0.47 f 0.10 0.40 f 0.05
MOG 0.63 f 0.10 0.72 f 0.09 0.62 f 0.04 0.67 f 0.08 0.48 f 0.05 0.54 f 0.11 0.48 f 0.11 0.55 f 0.08
0.63 f 0.06 0.53 f 0.09 0.60 f 0.05
f = 1 marginal standard deviation. bAn oxidized sample obtained from Advanced Fuel Research.
downfield from 165 ppm in a normal CP/MAS experiment (taken with a 2.5 ms contact time) is multiplied by the aromaticity, fa, as calculated above from variable contact experiments to obtain fac.
I>166
= fa-
fac
I>90
All other structural parameters that depend on chemical shift ranges are treated in a manner similar to f:. This normalization is based upon the assumption that the aromatic region has a single effective T I P H . For the Argonne Zap sample, f: was determined from a variable contact time experiment by using an appropriate ME. This was done to compensate for reduced signals from the carbonyls that are not polarized fully with the 1-ms contact time used with this coal. By the use of the percentage of Gaussian magnetization of the aromatic region (90-165 ppm only) obtained from the dipolardephasing experiment, f ,may be subdivided into protonated, faH, and nonprotonated, f a d, carbons by
= falMOG
faH
(7) The nonprotonated aromatic carbons are subdivided into three groups depending upon their chemical shifts. The phenolics or phenolic esters, f:, fall in the chemical shift range 150-165 ppm. The alkylated aromatic carbons, f:, have chemical shifts in the range from 135 to 150 ppm. Finally, the fraction of bridgehead carbons, faB, present in the sample is obtained by subtracting fap and f / from-faN. The a h h a t i c Dortion of the sDectrum can also be subdivided using MoGobtained from the aliphatic region of the dipolar-dephasing experiment. The first class of aliphatic carbons, f d H , contains CH and CH2 groups and is calculated as fdH
= fdMoG
(8)
and the s u m of CH3 and/or nonprotonated aliphatic carbon, f d * , is given by fal*
= fd
- fdH
The last subdivision for the aliphatic carbons is for those carbons bonded to oxygen, fdo, and includes resonances from the chemical shift range 50-90 ppm.
Results The proton spin-lattice relaxation times for the 11coals are listed in Tables I and I1 for the aromatic and aliphatic regions, respectively. Using the indirect detection method, it is possible to measure separate time constants for the aromatic and aliphatic protons. As can be seen from Tables I and I1 there is a general increase in TIHas the aromacity increases for the fresh Argonne coals. This correlation of T I Hwith rank has been previously noted.36 Generally, the TIHvalues were found to be the same, within two marginal standard deviations for both the aromatic and aliphatic protons. The two exceptions are the Blind Canyon coal, which has a relatively large amount of the resinite maceral, and the oxidized AFR Illinois No. 6 coal. Since it is known that free-radical concentration increases with rank and it is noted that the TIHvalues also increase, the data suggest that molecular motion plays a more important role in the relaxation than spin diffusion to paramagnetic centers. It should also be noted that the TIHvalues for the fresh Pocahontas and Upper Freeport coals are much longer than those previously reported even for coals with O2and H 2 0 removed.36 In comparing the Zap and Illinois No. 6 coals included in both the fresh Argonne and the oxidized AFR coals, the oxidized coals have the shorter TIHvalues. Cross-polarizationtimes, TcH, are given in Tables I and I1 for the aromatic and aliphatic regions, respectively, for each of the 11 coals. The T C H values range from 34 to 111 ps for the aliphatic carbons and from 130 and 554 ps for the aromatic carbons. Thus, aliphatic carbons polarize 4-6 (36) Sullivan, M. J.; Szeverenyi, N. M.; Maciel, G. E.; Petrakis, L.; Grandy, D. W. Magnetic Resonance. Introduction, Advanced Topics and Applications to Fossel Energy; D. Reidel: Dordrecht, The Netherlands, 1984; p 607.
Solum et al.
190 Energy & Fuels, Vol. 3, No. 2, 1989
Table 111. Carbon Structural Distribution of the Argonne Premium Coalsa coal North Dakota (L) Wyodak (SB) Blind Canyon (HVB) Illinois No. 6 (HVB) Pittsburgh No. 8 (HVB) Stockton seam (HVB) Upper Freeport (MVB) Pocahontas (LVB) Zapb (L) Rosebudb (SB) Illinois No. 6b (HVB)
f$
faH
faN
f,P
faS
faB
fa,
fdH
fa,*
falo
0.00 0.00 0.00 0.00 0.00
0.26 0.17 0.22 0.26 0.27 0.27 0.28 0.33
0.28 0.38 0.42 0.46 0.45 0.48 0.53 0.53
0.06 0.08 0.07 0.06 0.06 0.05 0.04 0.02
0.13 0.14 0.15 0.18 0.17 0.21 0.20 0.17
0.09 0.16 0.20 0.22 0.22 0.22 0.29 0.34
0.39 0.37 0.35 0.28 0.28 0.25 0.19 0.14
0.25 0.27 0.22 0.19 0.13 0.14 0.09 0.08
0.14 0.10 0.13 0.09 0.15 0.11 0.10 0.06
0.12 0.10 0.04 0.05 0.03 0.04 0.02 0.01
0.08 0.10 0.04
0.21 0.16 0.24
0.37 0.37 0.43
0.08 0.07 0.09
0.16 0.14 0.19
0.13 0.16 0.15
0.34 0.37 0.29
0.21 0.20 0.17
0.13 0.17 0.12
0.10 0.08 0.03
fa
fa'
0.61 . 0.63 0.65 0.72 0.72 0.75 0.81 0.86
0.54 0.55 0.64 0.72 0.72 0.75 0.81 0.86
0.07 0.08 0.01
0.66 0.63 0.71
0.58 0.53 0.67
a Fractions of sp2-hybridizedcarbon (error estimate): fa = total carbon (=f0.03); fat = in an aromatic ring (zfO.04); f$ = carbonyl, 6 > 165 ppm (=&0.02);faH = protonated and aromatic (zfO.03); faN = nonprotonated and aromatic (=f0.03); fap = phenolic or phenolic ether, 6 = 150-165 ppm (=f0.02); f: = alkylated atomatic, 6 = 135-150 ppm (zf0.03); faB = aromatic bridgehead (zf0.04). Fraction of sp3-hybridized carbon (error estimate): fa, = total carbon (zf0.02); f d H = CH or CH2 (zf0.02); fa,* = CH3 or nonprotonated (zf0.03); fd0 = bonded to oxygen, 6 = 50-90 ppm (=f0.02). bAn oxidized sample obtained from Advanced Fuel Research.
times faster than aromatic carbons. These values are within the range found previously for other fossil fuels.lJ3J6 The TIPH values for the 11coals are also given in Tables I and I1 separately for the aliphatic and aromatic protons. These aromatic and aliphatic proton values are essentially the same (within 2 marginal standard deviations) for each coal and range from 5 to 14 ms for the aliphatic protons and from 6 to 16 ms for the aromatic protons. Values for the fresh Argonne coals are generally longer than for the corresponding oxidized coals.' The dipolar-dephasing time constants (TL,TG)and initial magnetizations (MOL and Mm) are also given in Table I for the aromatic carbons and in Table I1 for the aliphatic carbons. In the aromatic region the Lorentzian or weakly coupled carbons (i.e., nonprotonated) have decay constants that range from 129 to 293 i s . In this sample set the weakly coupled carbons account for 52-70% of the aromatic carbons. The Gaussian or strongly coupled carbons (i.e., protonated) have decay constants that range from 18 to 27 ps and account for about 30-48% of the aromatic carbons. The decay constants in the aliphatic region vary from 14 to 18 ps for the strongly coupled carbons with Gaussian decay (CH and CH2) and from 43 to 107 ps for the weakly coupled carbons (CH3,nonprotonated carbons, or CH and CH2 carbons experiencing rapid segmental motion) with exponential decay. Generally, there was found to be a greater amount of strongly coupled than weakly coupled spins. The dipolar oscillations in the aliphatic region may cause the percentage of weakly coupled spins to be over estimated, and this value should be considered as an upper limit. All of the decay constants in both regions have values that are similar to those reported for other coals and macerals26and for model compounds.22
Discussion The 12 structural parameters for the 11coals are given in Table I11 and organized by increasing coal rank within each coal set (Argonne or AFR). The followingdiscussion focuses principally on the standard Argonne coals. The fa values of the Argonne coals increase with increasing rank. All fa values for these coals are within the range of values previously found for similar coals.2 For ease of comparison, Figure 2 presents the aromatic carbon, fa., aromatic C-H, and bridgehead populations for each coal. The distribution of oxygen functional groups in the Argonne set is given in Figure 3. Only the lignite and the subbituminous coals have any significant amount of carbonyl carbon, fac. The other types of carbon associated with oxygen, fdo, and to a lesser extent, f:, show a decrease as the rank of the coal increases. This trend is consistent
h
ARGONNECOALS
90 80
Blind Canyon
70
Illinois #6
60 50
0
40
Pittsburgh #8 Stockton
30
20
Upper Freeport
10
0
.
Aromatic C
Aromatic C-H
Bridgehead
I
Pocahontas
I
AROMATIC STRUCTURES
Figure 2. Aromatic carbon structural distribution of the Argonne coals. ARGON\IECOALS
X
12
Wyodak
10
Blind Canyon
8
0
Illinois #6
6
0
Pittsburgh #8
€!
Stockton
4
.
2 Upper Freeport
I
0 Carbonyl C
Aromatic C - 0
Aliphatic C - 0
OXYGENSTRUCTURES
Figure 3. Structural distribution of carbons associated with oxygen in the Argonne coals.
with results from the elemental analysis,35which indicate a decrease in oxygen content as the rank increases. Furthermore, as the rank increases faN gets larger as noted by Wilson et al.24for a large set of coals and coal macerals. The value for faH also increases with rank with the exception that the North Dakota lignite has a value more in the range of the HVB coals. The parameters fas does not show a strong correlation with rank. The aliphatic content fd = fd* + fdH decreases as the rank increases as does fdH. However, this strong correlation is not seen in fd* but depends on the specific coal as is shown in Figure 4. Another parameter that shows a very strong correlation with rank is faB, the amount of bridgehead carbon. As the rank of the coal increases, faB also increases. The mole fraction of aromatic bridgehead carbons, Xb, is then calculated as Xb = faB/fa.. This parameter is important as it can be used to estimate the aromatic cluster size (vida infra).
Energy & Fuels, Vol. 3, No. 2,1989 191
13C NMR of Argonne Premium Coals a
25
%
Blind Canyon
20
0 Illinois
15
0
10
#6
Pittsburgh #8 Stockton
5 Upper Freeport
n Aliph. C.CH3 ALIPHATIC STRUCTURES
Aliph. CH2,CH
Pocahontas
b Circular CatenationC6n2&,
Figure 4. Carbon distribution of the aliphatic region of the Argonne coals.
In order to appreciate the coal structural information that NMR data can provide, it is helpful to consider with a few examples the relationship of Xb to the structures of polycondensed aromatic hydrocarbons (PAH). If sixmembered aromatic rings condense to form PAH only by ortho-fused condensation (e.g., naphthalene, anthracene, tetracene, phenanthrene, chrysene, etc.) the series exhibits linear catenation. Such structures belong to the C4n+2H2n+4 family of PAH, and typical members are given in Figure 5a. The molar ratio H/C equals (n + 2)/(2n + 1) and H/C approaches 0.5 in the limit, as n becomes very large. Two-dimensionalor circular catenation (see Figure 5b) may also be considered for PAH with all other types of condensation falling between the structural limits defined by these two extremes. It is obvious that as the number of rings increases to an infinite size in one dimension the H/C ratio can never be reduced to zero. In the case of circular catenation, (e.g., coronene and circumcoronene), the ratio H/C = l / n approaches zero as the cluster grows. In-order to calculate the aromatic cluster size, only linear catenation and circular catenation need be considered. If xp is the mole fraction of carbon a t a peripheral position, (e.g., a C-H carbon), and Xb is the mole fraction of bridgehead carbons then Xb
= 1 - xp
Figure 5. Two limiting cases of polyaromatic hydrocarbon condensation: (a) primary catenation; (b) circular catenation.
0.8 -
0.7 0.6 -
0.11
(10)
[?Y, , I
,
,
Ii
p,
0.0
and xp
0
= H/C
Consider linear catenation in which it can be shown that H / C = (2n
+ 4)/(4n + 2)
(12)
for a linear series index of n = (C - 2)/4
(13)
where C is the number of carbon atoms in the aromatic cluster. From eq 10-13, the h e a r Xb’ becomes X i = 1/2
-3/c
(14)
A plot of Xb’ vs C is shown as the lower dashed curve in Figure 6. The corresponding quantities for circular catenation are H/C = l / n
(15)
and the circular series index is n =
20
30
40
50
60
Carbons per Cluster Figure 6. Plot of the mole fraction of bridgehead carbons, Xb, vs C where C is the number of carbon atoms per aromatic cluster. The solid curve is for the combined model, the upper dashed curve is for the circular catenation model, and the lower dashed curve is for primary catenation model.
It should be noted that when C is less than 14 carbons (i.e., benzene through either phenanthrene or anthracene, Xb’ governs the relationship for Xb, but when the number of carbons exceeds 24 (coronene), one may expect Xb” to approximate best the dependence of Xb on C. The hyperbolic tangent function may be used to transfer the dependence between the two limiting functions x i and x{, and the following empirical function relates Xb to 1 - tanh
&/fi
10
c - co
c - co
1) + tanh (7) (7
From eq 10, 11, 15, and 16, the value for Xb” for circular catenation becomes X< = 1 -
fi/&
(17)
A plot of x i ’ vs C is shown as the upper dashed curve in Figure 6.
where Co and m are shifting and scaling parameters, respectively, to be adjusted to give the best fit of the dependence of Xb on C (solid curve in Figure 6). With the use of a simplex minimization routine, optimal values for
192 Energy & Fuels, Vol. 3, No. 2, 1989
Solum et al.
20 18
16 14
u
: e
r
12 lo 8
6 4 2
0 North Dakota
Wyodak
Blind Canyon
Illinois #6
Pittsburgh #8
Stockton
Upper Freeport
Pocahontas
Aromatic Carbons per cluster
Figure 7. Aromatic cluster size of the Argonne coals determined from the combined model.
Co = 19.57 and m = 4.15 were obtained for all of the aromatic hydrocarbonsthat may be projected on to coronene. While we recognize the problems of limiting the fit to those species that project on to coronene, the argument may be made that the degeneracy weighting factors for circular structures will rapidly favor two dimensional growth as one considers PAH above coronene. These are shown in Figure 6. The combined expression given in eq 18 determines the aromatic cluster size in compounds for which Xb is known even when the structure is unknown. For low-rank coals where the cluster size is expected to be 1-3 rings the linear model is the limiting law. At an intermediate range (16-24 carbons) a mix of the two limiting models is appropriate as given by eq 18. For higher rank coals where the cluster size is expected to be relatively large (C > 24), the circular model becomes the appropriate limiting law for coals of rank LVB or greater. Using eq 18, it is possible to estimate the average number of aromatic carbons per cluster, C, from the measured values of Xb for the eleven coals (Table Iv). The Xb values range from a low of 0.17 in the Argonne Zap sample to a high of 0.40 for Pocahontas yielding values for C of 9 and 20 aromatic carbons per cluster, respectively. The values in both Xb and C increase in general for the eight Argonne coals, as the rank of coal increases. In the Argonne Zap lignite sample the aromatic cluster size is nine carbons, corresponding to an average cluster size one carbon short of two fused rings. In the Wyodak (SB) sample the cluster size has increased to a total average size of three rings. The four Argonne HVB coals show an average cluster size of about three to four rings. The Upper Freeport coal has a cluster size of 18 carbons, which for purpose of comparison falls between pyrene (16 carbons) and compound I with 20 carbons. The Pocahontas cluster size with 20 carbons is approximately the size of compound I. These representative clusters are only given as illustrations of the general cluster sizes (Figure 7), and it should be realized that a great diversity of sizes and types of ring structures will be found in all coals. The aromatic cluster sizes reported.here are similar to those reported for vitrinites of similar carbon content.37 If one defines the total number of aromatic ring attachments per 100 carbons as f: + f?,then the number of attachments per cluster may be estimated. This number (37)Van Krevelen, D.W. Coal; Elsevier: Amsterdam, 1981. .
Table IV. Aromatic Cluster Size of the Argonne Coals from the Combined Model” coal North Dakota (L) Wyodak (SB) Blind Canyon (HVB) Illinois No. 6 (HVB) Pittsburgh No. 8 (HVB) Stockton seam (HVB) Upper Freeport (MVB) Pocahontas (LVB)
0.17 0.29 0.31 0.31 0.31 0.29 0.36 0.40
AC/C1 9 14 15 15 15 14 18 20
att/Cl 3.2 5.6 5.2 4.8 4.9 5.2 4.3
MW 277 410 359 316 294 275 302 299
Zapb (L) Rosebudb (SB) Illinois No. 6b (HVB)
0.22 0.30 0.22
11 15 11
4.5 5.8 4.6
339 459 267
Xb
5.0
‘Xb = mole fraction of bridgehead carbons (error = k0.06). AC/C1 = number of aromatic carbons per cluster (error = h3). att/C1 = number of attachments per cluster. MW = total molecular weight of a cluster. bAn oxidized sample obtained from Advanced Fuel Research.
North Dakota
81
Wyodak Blind Canyon
[3 lllinios #6
0
Pittsburgh #8 Stockton Upper Freeport
Attachments
Periph Chains
Bridges & Loops
Pocahontas
Per Cluster
Figure 8. Distribution of attachments on an average aromatic cluster in the Argonne coals as determined from the combined catenation model and the structural parameters.
is shown in the third column of Table IV. That this value remains somewhat constant (see Figure 8) for these coals is not surprising since two competing effects balance each other. As the rank increases, the cluster size grows so there are more places to attach side chains, but the aliphatic content decreases so that fewer attachment sites are utilized. However, the distribution of the types of attachments varies widely. If one assumes that a methyl group is the terminator of each side chain (this assumption neglects OH or aryl groups as chain terminators), one can estimate crudely the number of perpheral chains per
Energy & Fuels 1989, 3, 193-199 cluster. The number of methyl groups varies by almost a factor of 3 among the coals studied. The remaining attachments are of two types; either bridges between aromatic clusters or aliphatic loops on an aromatic ring (hydroaromatic structures). Also, included in Table IV is the total molecular weight of an average cluster (calculated from the elemental analysis and f a , ) . The variation in average cluster molecular weight is a combination of cluster size and elemental composition where the molar O/C ratio is a significant factor in the estimated molecular weight. The diversity of structure observed from NMR data on both aliphatic and aromatic carbon types in these coals confirms the expected complexity in the macromolecular structure. In each instance the NMR data are consistent with the structural trends in coals expected for changes in coal rank. The level of detail derived from these data provides a description of the carbon skeletal backbone of these coals and is useful in coal devolatilization It is anticipated that such structural information will also be valuable in assessing liquefaction processes.
193
Acknowledgment. This work was supported by the National Science Foundation under Cooperative Agreement No. CDR 8522618. Also participating in the funding of this effort, in alphabetical order, was a consortium of organizations that includes Advanced Fuel Research, Inc.; Allison Division (General Motors Corp.); Babcock and Wilcox; Chevron Research Co.; Combustion Engineering, Inc.; Conoco, Inc.; Convex Computer Corp.; Corning Glass Works; Dow Chemical USA; Electric Power Research Institute; Empire State Electric Energy Research Corp.; Foster Wheeler Development Corp.; Gas Research Institute; General Electric Co.; Los Alamos National Laboratory; Morgantown Energy Technology Center (US. Department of Energy); Pittsburgh Energy Technology Center (US. Department of Energy); Pyropower Corp.; Questar Development Corp.; Shell Development Co.; Southern California Edison; the State of Utah; Tennessee Valley Authority; and Utah Power and Light Co. Financial support from Brigham Young University and the University of Utah is also acknowledged.
Correlation of Bituminous Coal Hydroliquefaction Activation Energy with Fundamental Coal Chemical Properties S.-C. Shin, R. M. Baldwin,* and R. L. Miller Chemical Engineering and Petroleum Refining Department, Colorado School of Mines, Golden, Colorado 80401 Received May 9, 1988. Revised Manuscript Received November 28, 1988
The rate and extent of direct coal hydroliquefaction for five bituminous coals from the Argonne Premium Sample Bank have been measured. Data were obtained in batch microautoclave tubing bomb reactors a t three temperatures (375,400,425 "C) and five residence times (3,5,10,15,40 min) in 1-methylnaphthalene vehicle under a hydrogen blanket. Data on rate of conversion of coal to THF and toluene solubles were modeled with a simple reversible rate expression, and activation energies for conversion to each solvent solubility class were determined. Data on carbon and proton distribution in the coals were obtained by 'H NMR combined rotational and multiple-pulse spectroscopy and 13C NMR CPMAS/dipolar dephasing spectroscopy. A strong correlation of activation energy with the aliphatic hydrogen content of the coal was found for conversion to T H F solubles. Activation energies for toluene solubles were found to be highly correlated (R2 > 0.90) with total oxygen and with protonated aliphatic carbon. 13CNMR data indicated that a direct relationship existed between protonated aliphatic carbon and total oxygen for the bituminous coals from the Argonne suite, and this structural feature was found to be very significant in determining coal reactivity. This observation provides direct evidence of the importance of etheric oxygen in cross-linking structures for determination of bituminous coal hydroliquefaction reactivity.
Introduction
The relationship between coal chemical and structural features and coal reactivity has been the subject of considerable research for well over 70 years. These studies date from the original observations bf Bergius' regarding the influence of carbon content on hydroliquefaction yield. Early studies of the influence of coal properties on coal reactivity were focused on an attempt to find a single parameter or group of parameters capable of correlating (1) Bergius, F. J . Gasbeleucht. Verw. Beleuchtungsarten Wasseruer-
sorg. 1912,54.
physical, chemical, and geochemical properties with the degree of COnversion to Some solvent-soluble Products under a fixed set of reaction conditions.2-8 Given et al.+l2 (2) Francis, W. Fuel 1932, 11, 171. (3) Petrick, A. J.; Gaigher, B.; Groenewoud, P. J. Chem. Metall. Min. SOC.S. Afr. 19371 38. (4) Wright, C. C.; Sprunk, G. C. Pa. State Colt., Min. Ind. Exp. Stn. Bull. 1939, No, 28. (5) Fisher, C. H.: Srxunk. G. C.: Eisner. A.: Clarke. L.: Fein, M. L.; Storch, H. H. Fuel 19i0, 19, 132. (6) Shatwell, H. G.; Graham, J. L. Fuel 1925, 4 , 25. (7)Horton, L.;Williams, F. A.; King, J. G. G.B. Dept. Sci. Znd. Res., Fuel Res., Tech. Pap 1936, No. 42.
0887-0624/89/2503-Ol93$01.50/00 1989 American Chemical Society
