Energy & Fuels 1991,5, 767-769 100
Table 111. Electron Spin Concentrations of the Argonne Premium Coals electron s d n concentration x We,spinsh THF, acid acid original coal0 dried demineral. washed washed SmIl 1.07 0.92 Upper Freeport 1.08 1.17 0.99 1.03 0.54 wyodak 1.00 1.05 Illinois No. 6 0.86 0.86 0.66 1.25 1.14 Pittsburgh No. 8 1.30 1.79 1.37 Pocahontas No. 3 1.76
Utah West Virginia Beula Zap
1.85 1.61 1.10
1.01
1.79 1.64 1.05
Table IV. Arrronne Premium Coals. f.
f. coal0
Upper Freeport Wyodak Illinois No. 6 Pittsburgh No. 8 Pocahontas No. 3
Utah West Virginia Beula Zap
THF, acid acid original demineral. washed washed Sm12 0.83 0.66 0.72 0.75 0.89 0.68 0.71 0.74
0.83 0.66
0.66
0.83 0.66 0.72 0.89
0.74
0.74
00 6
EO
70
BO
2
1.64 1.56 0.56
All coals were dried under vacuum at 80 O C for 18 h. Each experiment was repeated six times for the native Argonne premium coals, three times on the identical sample, and three times on separate samples from the same ampule. The values are reproducible to i l 5 % . Relative radical concentrations were determined by double integration of the ESR spectra.
0.83 0.71 0.73 0.74 0.89 0.70 0.71 0.78
All coals were dried under vacuum at 80 O C for 18 h.
We also investigated the removal of the organic spin density. The reactions with alkali metals are effective17 but reduce diamagnetic compounds in the coal. The reaction with lithium aluminum hydride appears to be successful with the Wyodak coal except for the fact that carbonyl compounds are reduced. Samarium(I1) iodide appears to be the most appropriate reagent.'0J1J8 The solid-state 13C NMR spectra of the eight samarium diiodide treated Argonne Premium Coals were recorded with both CP and Bloch decay pulse sequences (Tables 11,111, and IV). Although significant reductions were measured in the radical concentrations of the reduced coals, many radicals persist after treatment with samarium(I1) diiodide. By testing higher reduction potential lanthanides, multiple samarium(I1) reduction cycles, and the reduction reactions of known radicals, solvent-refined coal, and fluid energy milled coal, we conclude that the accessibility of the samarium reagent to the paramagnetic sites within coal is extremely important and perhaps limiting and that certain stable highly delocalized radicals probably resist reduction even in homogeneous reactions. The changes in the g value are consistent with selective removal of heteroatom radicals: in a typical experiment the apparent g value changed from 2.004 to 2.003. This may account for the special effectivenessof the reagent with Wyodak coal. The extent of the improvement that can be realized is illustrated in Figure 1.l0 In summary, CP experiments cannot be relied upon to provide accurate solid-state NMR information for many coal samples. We have established that the Bloch decay (17) Dubera, S.;Wachowka, H.M.; Wieckowski, A. B. Fuel 1987,66, 1069-1072. (18) Girard, P.; Namy, J. L.; Kagan, H. B. J. Am. Chem. SOC.1980, 102,2693-2698.
767
-
0 5
0.5
[e], electron spins x
1 6
per gram of coal
Figure 1. Plot of electron spin concentrations versus percentage of observable carbon atoms in the Argonne Premium Coals before and after reduction. The direction of the arrow indicates the effect of reduction on the magnetic resonance characteristics. A negative slope indicates a net decrease in radical concentration and increase in observable carbon content. T h e length of the line indicates the effectiveness of the reduction on the magnetic resonance spectra. indicates the native dried coal and 0 indicates the dried reduced coal.
experiments on coals that have been treated with samarium(I1) iodide to reduce the spin density yield more accurate % Cobvalues of fa and reveal additional previously unobserved resonances in the aromatic region. Acknowledgment. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S.Department of Energy, under contract No. W-31-109-ENG-38, Registry No. Sm13, 32248-43-4. John V. Muntean, Leon M. Stock* Department of Chemistry The University of Chicago Chicago, Illinois 60637 Chemistry Division Argonne National Laboratory Argonne, Illinois 60439 Received March 7, 1991 Revised Manuscript Received June 17, 1991
Solid-state 13CNMR Spectroscopy of Pocahontas No. 3 Coal
Sir: We propose a structural model of Pocahontas No. 3 coal (APCS 5) based upon new solid-state NMR evidence and other experimental results. This low-volatile bituminous coal, C100H67.601.08N~.~~S~.~1Cb.~~, contains 89% vitrinite, 10% inertinite, and 1% liptinite and was collected in Buchanan County, VA in 1986.' We have adopted the water content measured by Finseth and his co-workers2 to compute the elemental composition. Solid-state 13C NMR spectroscopy has been used to measure the aromaticity, to determine the number of protonated and nonprotonated carbon atoms, and to estimate the average aromatic cluster sizes3 The CP pulse sequence is deficient in the quantitative polarization of remote and fast motion carbon nuclei, especially those (1) Vorres, K. S. Energy Fuels 1990, 4 , 420-426. (2) Finseth, D. Prepr. Pap.-Am. Chem. Soc., Diu.Fuel Chem. 1987, 32(4), 260-265. (3) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989,3, 187-193.
0SS7-0624/91/2505-0167~02.50f 0 0 1991 American Chemical Society
768 Energy & Fuels, Vol. 5, No. 5, 1991
Communications 1ooc
80 -
40 20 60
'
0 0
100 200 300 Interrupted Decoupllng tlme (mlcroseconds)
Figure 2. Carbon magnetization vs the interrupted decoupling time for both cross-polarizationand Bloch decay pulse sequences: (-)cross-polarization (0) Bloch decay. The CP data were taken from ref 3. Bioch decay data were fitted to eq 1 and error bars are indicated by size of datum points. Pocahontas Coal 89.5 SP2
n
0.0
89.5
7.8
2.3
0.3
0.1
Carbonyl
Aromatic
CH,
CH,
CH
C
2.3
A59.5 30 5:3
8.5
i4.5
Ethylated Phenolic Methylated Bridgehead and others and others Aromatic Aromatic 1
,
180
,
1
120
1
,
1
60
1
,
8.7 Biaryl
I
Figure 3. Carbon atom distribution of Pocahontas No.3 coal. Errors for the nonprotonated carbon atoms are estimated to be
0
*3. Chemical Shift (ppm)
Figure 1. The 25MHz Bloch decay, interrupted decoupling NMR spectra of the Pocahontas No. 3 coal. The magic angle sample spinning rate was 4.0 kHz. Proton decoupling field was 67 kHz. The intense off-scale resonance at 3.50 ppm is tetrakis(trimethylsily1)silane. The carbon nuclei represented in (i) are those most weakly coupled to hydrogen atoms. (a) 0 ps, (b) 15 ps, (c) 25 fis, (d) 35 P S , (e) 50 w,(0 75 PS,(g) 125 MI,(h) 200 W S , (i) 300 ps associated with paramagneti~m.~Therefore, we adopted Bloch decay procedures and established that 82 f4% of the carbon atoms in the coal could be observed and that the fa value was 0.89 f 0.01.6 The traditional Bloch decay sequence was revised to measure the relaxation properties of carbon nuclei by dipolar dephasing. The results are displayed in Figure 1. Several features are immediately discernible. First, the high-field side of the aromatic 13C resonance band decreased as the delay increased. Second, the low-field side of this band shows virtually no change even after 300 ps. Solum and co-workers3 pointed out that dipolar dephasing observations could be accommodated by eq l. M(t) = MoLe-'/TL+ Moce~~6('/TG)*
(1)
The two experimental variables are t, the interrupted decoupling time, and M ( t ) ,the magnetization as a function the magnetization of time. The four parameters are MOL, at t = 0 for Lorentzian decay of weakly coupled carbon nuclei, Ma,the magnetization at t = 0 for Gaussian decay of strongly coupled carbon nuclei, and TL and Tcare the decay constants. (4)Axelson, D. E. Solid State Nuclear Magnetic Resonance of Fossil Fuels: An Experimental Approach; Multiecience: Canada, 1985. ( 5 ) Muntean, J. V.;Stock, L. M. Energy Fuels, this issue.
Our Bloch decay observations for short delay times, t = 0 to 75 ps, are in excellent agreement with the prior CP results of Solum and co-workers, but there are significant differences for longer time delays (Figure 2). These new experiments establish that of 100 observable carbon atoms in this coal, 11are sp3,59 are sp2and nonprotonated, and 30 are sp2and protonated. These nonprotonated carbon atoms may be further distinguished by their chemical shifts. The Bloch decay spectrum with a 125-ps interrupted decoupling time was employed for integrating these specific regions. Virtually all carbon atoms that are strongly coupled to hydrogen nuclei have disappeared at this time, but the weakly coupled carbon atoms are still intense. The remaining resonances in the 165-150 ppm region can be ascribed to carbon nuclei adjacent to heteronuclei as discussed by other investigator^.^^^^^ The characteristics of the low-volatilebituminous Pocahontas No. 3 coal allow us to assign the resonances from 150 to 129 ppm. One region, from 138 to 129 ppm, encompasses the range of chemical shifts that are anticipated for the methylated aromatic carbon atoms. In addition to nonmethyl alkylated aromatic carbon atoms, the region from 150 to 138 ppm contains the carbon atoms that link biaryla and the bridgehead carbon atoms of indanes. The assignment of biaryl structures, which apparently has not been considered by previous workers, is especially relevant for the higher rank coals. More specifically, Pocahontas No. 3 coal appears to be uniquely (perhaps not for a high-rank coal) deficient in bridging methylene groups. Consequently, biaryl linkages become essential cross-links in the assembly of the macromolecule as discussed subsequently. In the absence of protonated aromatic carbon (6) Spectroscopic Analysis of Coal Liquids; Kershaw, J. R., Ed.; Elsewer Science: New York, 1989, and references therein.
Energy & Fuels, Vol. 5, No. 5, 1991 769
Communications
Figure 4. A 3Wcarbon-atom model of the Pocahontas No. 3 coal with representative structures for the aromatic cluster groups.
atoms, the resonances in the 129-90 ppm region may be assigned to aromatic bridgehead carbon atoms. The results of these assignments are summarized in Figure 3. Several independent lines of evidence support these assignments. First, the elemental composition of the coal places severe restrictions on the possible number of alkylated carbon atoms and virtually requires that the resonances in the 150-138 ppm region be assigned to biaryl structures. Early and recent oxidation studies imply that indanes are present.'^^ Third, the Ru(VII1) oxidation procedure indicates that there are not less than 7.8 methyl, 2.3 methylene, 0.3 methine, and 0.1 quaternary carbon groups per 100 carbon atoms of coal, and that biaryls are present among a host of highly condensed aromatic structures.8 Fourth, C-alkylation reactions imply that fluorenes contribute to the resonances in the upfield region of the NMR spectrum. We adopted the approach of the Utah group3to estimate the average aromatic cluster size. The Bloch decay interrupted decoupling experiments imply that the fraction of bridgehead aromatic carbon atoms is 0.38. Linear catenation implies an average cluster size of about 30 carbon atoms and circular catenation yields about 17 carbon atoms per cluster while the dual model of Solum and co-workers implies about 20 carbon atoms per cluster. Information in the literature suggests that the dual model is more appr~priate.~ This result, in conjunction with the compensating observations that the coal contains 10% inertinite and 18% of the carbon atoms remain undetected, suggests that the average cluster in the dominant vitrinite maceral has about 20 carbon atoms or 5 rings. Many different coal structures are consistent with the distribution of carbon atoms in Figure 3, but the known hydrogen and bridgehead aromatic contents place severe limits on the average aromatic molecules that can be (7) Holly, E. D.;,Montgomery, R. S. Fuel 1966,35,49-56 and subsequent papen in this wries. (8) Stock,L. M.; Wang, S. H.Energy Fuels 1989,3, 533-535.
present in abundance. The solid-state '3c NMR data, the chemical shift results, and the small number of bridging aliphatic groups established by the Ru(VII1) data as well as the detection of oxidation products with biaryl linkages support the conclusion that there are about 9 biaryl carbon atoms per 100 carbon atoms. Whitehurst suggested that coals' chemistry could often be discussed on the basii of single representative structurea for coals of different rank.s One compound that accommodates the available information for Pocahontas No. 3 is dimethyldibenzo[def,mno]chrysene. Clearly, more elaborate structures with many more carbon atoms provide a much better guide to the chemical behavior of the coal, the possible bridging groups and connecting points, the heteroatom distribution, and other features as pointed out by Shinn.lo At present it is impossible to specify the structures of the aromatic components of this coal. Neither the substitution patterns for methylation nor the locations of the connecting links, nor the relative abundances of the aromatic building blocks, can be specified. In the face of these difficulties, we have adopted the suggestion of K. B. Anderson and presented a block representation of the structure that encompasses are present understanding of the available information in Figure 4. Plausible structural components for each aromatic block are also shown. The new features that were established in the course of this investigation are presented in this model, which also accounts for the elemental information, the distribution of carbon atoms in Figure 3, the heteroatom distribution implied by the work of other investigators," the low, 10% volatility of this coal, and the fact that the volatiles have an average molecular weight of 500 but are essentially free of mono-, di-, and tricyclic constituenta;11J2the fact that the coal can be alkylated on fluorenylic positions and solubilized by treatment with base by fragmentation at arylated meso positions;13and the concept advanced by Ouchi and his associates that such high-rank coals have high aromatic character but a low degree of p01ymerization.l~
Acknowledgment. We are indebted to the Coal Chemistry Group at Argonne National Laboratory for many valuable conversations. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S.Department of Energy, under contract No. W-31-109-ENG-38. (9) Whitehuret, D. D. A Primer on the Chemistry and Constitution of
CoaL In Organic Chemutry of Coal;Larsen, J. W., Ed.; ACS Sympoeium
Series 71; American Chemical Society: Washington, DC, 1978; Chapter 1.
(10)Shinn, J. H. Fuel 1984,63, 1187. (11) This issue is discussed by: Muntean, J. V. Ph.D. Dissertation, The University of Chicego, 1990. (12) (a) Wallace, S.; Bartle, K. D. Baxby, M.; Taylor, N.; Majchrowia, B. B.; Yperman, J.; Martens, H.J.; Gelan, J. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1989,34(3), 721-728. (b) George, G. N.; Gorbaty, M. L.; Kelemen, S. R.; Sansone, M. Energy Fuels 1991,5,93-97. (13) Chatterjee, K.; Stock,L. M. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991, 36(2) 481. (14) Ouchi, K.; Hirano, Y.;Makabe, M.; Itoh, H. Fuel 1980,59, 761.
John V. Muntean, Leon M. Stock* Department of Chemistry The University of Chicago Chicago, Illinois 60637 Chemistry Division Argonne National Laboratory Argonne, Illinois 60439 Received March 12, 1991 Revised Manuscript Received June 17, 1991
