Energy & Fuels 1993, 7,1148-1149
1148
Observation of 13C Nuclear Overhauser Enhancements for Coals Robert V. Law,Gordon D.Love, and Colin E. Snape* Department of Pure & Applied Chemistry, University of Strathclyde, Glasgow GI IXL,U.K. Received July 13, 1993.Revised Manuscript Received August 31,1993 NMR relaxation measurements, particularly ‘H spinlattice (thermal) relaxation times (TI’S),have been widely used to give an indication of the extent of conformational mobility existing within the macromolecular structure of coa1s.l-3 However, the influence of paramagnetic species on relaxation behavior makes interpretation of the results in terms of “rigid” and “mobile” phases far from easy. An alternative approach that has recently been applied to solid polymers is the use of l3C nuclear Overhauser enhancements ( N O E ’ S ) . ~These are the fractional increases (0)in the observed intensities for particular 13C environments upon saturation of the ‘H absorptions and approach the maximium value of 1.99 (i.e., observed peak intensities are 1 9 or 2.99 times those in normal spectra) for groupsthat rapidly reorientate. In this communication, the first such measurements on a coal are reported and put into a general context by comparison with the values for a cross-linked polymer system. The pulse sequence usedG comprises the traditional Bloch decay or single pulse excitation (SPE) technique with a train of 1H 90° pulses for saturating the ‘H absorptions and a suitably long delay to ensure that complete spin-lattice relaxation of the 13C spins occurs. The 1H 90° pulses were separated by 1ms to limit probe heating effects and can be applied for either the total or a fraction of the recycle delay period. The NOE’s for the Pittsburgh No. 8 Argonne Premium Coal Sample (APCS) before and after treatment with chlorobenzene’ have been determined by comparing the peak intensities in the normal and NOE-enhanced SPE spectra obtained with the same number of scans (800 or 1000). For purposes of comparison, NOE measurements have also been carried out on adamantane, hexamethylbenzene, and a highly cross-linked divinylbenzene/styrene (DVB/S) copolymer. All the SPE experiments were carried out on a Bruker MSLlOO spectrometer with magic-anglespinning at 5 kHz as described previously.8 The spectra of the coal samples were obtained using recycle delays of 60 and 200 s, with the 1H pulse train being applied for varying periods. Dipolar dephased (DD) SPE spectra were obtained in the
+
(1)) Sullivan, M. J.; Szeverenyi, N. M.; Maciel, G. E.; Petrakis, L.; Grandy, D. W. In Magnetic Resonance. Introduction, Aduanced Topics and Applications to Fossil Energy, Petrakis, L., Fraissard, J. P., Eds.; NATO AS1 Series C124; Reidel: Dordrecht, 1984; pp 607-616. (2) ) Wind, R. A.; Duijvestijn, M. J.; van der Lugt, C.; Smidt,J.; Vriend, H. Fuel, 1987,66,876-855. (3) ) Barton, W. A.; Lynch, L. J. Energy Fuels 1989,3,402-411 and references therein. (4) ) Ohta, H.; Ando, I.; Fujishige, S.; Kubota, K. J.Mol. Struc., 1991, 245,391-397. (5) ) Findlay, A.; Harris, R. K. J. Magn. Reson. 1990,87, 605-609. (6) ) White, J. L.; Haw, J. F. J.Am. Chem. SOC.,1990,112,5896-5898. (7) ) McArthur, C. A.; Mackinnon, S.; Hall, P. J.; Snape, C. E. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1993, 38 (2), 565-570 and
references therein. (8) ) Franz, J. A,; Garcia, R.; Linehan, J. C.; Love, G. D.; Snape, C. E. Energy Fuels, 1992, 6, 598-602.
Table I. ‘43 Nuclear Overhauser Enhancements Determined Using the SPE Techniaue
NOE SaIllDld
CHI, CH
aromatic
CHs
1.89 (CH,) 1.54 (CH)
adamantane
1.13 1.46 0.9F 0.92* 1.5P initial coal 0.26 0.49 1.29 CB-treated coal 0.19 0.37d 0.95e a Protonated aromatic carbon. b Backbone CH, + CH (40ppm). -N(CH& (53ppm). 25-60 ppm. e 5-25 ppm. f HMB = hexamethylbenzene, DVB/S = divinylbenzene/styrene, CB = chlorobenzene. HMB
DVB/S copolymer
Table 11. ‘42TI’Sfor Initial a n d Chlorobenzene-Treated Pittsburgh No. 8 coal
TI’S aromatic aliphatic, CH2 aliphatic, CH,
+ CH
initial coal
treatad coal
ca. 30
ca. 30
13.4 9.1
12.4 9.3
normal manner9J0by switching the lH decoupler off for a period of 40 ps immediately prior to data acquisition. The NOE’s determined for all the samples investigated and the 13C 7’1% for the initial and treated coal are given in Tables I and 11,respectively. Figure 1showsthe normal and NOE-enhanced spectra, and Figure 2 showsthe NOEenhanced spectra from the normal dephasing and corresponding DD experiment all for the initial coal. The most important aspect of the SPE sequence in attaining the full NOE’s is the length of the lH pulse train in relation to the recycle time. For the coal and DVB/S copolymer, the full NOE’s were not reached until the train was applied for the whole of the recycle time. Further, the values for adamantane and hexamethylbenzene differed somewhat from those obtained in other recent communications.q-6 Adamantane, a plastic crystal, gave NOE’s of 1.89 (methylene) and 1.54 (methine), approaching the theoretical maximum of 1.99 and thus demonstrating the efficacy of the pulse sequence as used here. The values of 1.13 for the quaternary and 1.43 for the methyl carbons in hexamethylbenzene using a ‘H 20-8 pulse train were significantly higher than those of 0.54 and 0.76,respectively, reported previously with a relatively low power continuous irradiation.5 As anticipated for the coal samples, the NOE’s for CH3 (0.9-1.3) are greater than those for CH2 plus CH (0.4-0.5) which in turn are greater than the average value for the protonated and nonprotonated aromatic carbon (0.2); the same values for the inital coal were obtained using recycle delays of 60 and 200s. Of direct relevanceto the rotational mobility in coals, the NOESfor the aromatic carbon and (9) ) Love, G. D.; Law, R. V.; Snape, C . E. Energy Fuels, in prase. (10) ) Muntean, J. V.; Stock,L. M. Energy Fuels 1991,5, 767-769.
0887-0624/93/2507-1148$04.00/00 1993 American Chemical Society
Communications
Energy & Fuels, Vol. 7,No. 6,1993 1149
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Figure 2. Dipolar dephased NOE-enhanced (top) and normal dephased NOEenhanced (bottom) SPE apectraof the Pittsburgh No.8 Argonne Premium Coal Sample (lo00 scans, recycle time of 60 s, and line broadening of 30 Hz).
,," Figure 1. NOE-enhanced (top) and normal (bottom) SPE spectra of the Pittsburgh No.8 Argonne Premium Coal Sample (lo00 scans, recycle time of 60 s, and line broadening of 30 Hz). c
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backbone CH2 and CH in the DVB/S copolymer are significantly higher despite the extensive cross-linking. Thus, at first sight, the NOE's for the coal samples (Table I) suggest that rotational mobility is constrained although the values attained might be limited by paramagnetic centers making a significant contribution to the spinlattice relaxation of l3C nuclei. However, the relatively long 13C 2'1's (Table III9 of the coals suggests that the dipolar contribution dominates. While a fraction of the NOE's observed for the coal samples is likely to arise from the solvent-extractable material present, the major contribution is thought to arise from rotationally mobile components in the macromolecular structure. Indeed, chlorobenzenetreatment, which from differential scanning calorimetry (DSC) and other evidence: is known to alter the macromolecular conformation of bituminous coals while removing virtually none
of the extractable phase, gives rise to small but significant decreases in the NOE's (estimated measurement error is ca. A0.02) for both the aromatic and aliphatic carbons in the Pittsburgh No. 8 APCS (Table I). The treatment did not markedly affect the l3C 2'1's (Table 11),suggesting that the influence of paramagnetics on spin-lattice relaxation has not altered markedly although the 'H 2'1% increase? Thus, the reduced NOEs for the treated coal possibly signify a less rotationally-mobile but more relaxed conformation containing more noncovalent cross-links. Clearly, further measurements are required to elucidate more precisely the origin of l3C NOEs in coals, but although the NOE experiments are time consuming, our preliminary results indicate that the approach may prove to be an important source of conformational information for coals. Acknowledgment. The authors acknowledge financial support from (i) the Science and Engineering Research Council (Grant No. GR/F/87264, studentships for G.D.L. and R.V.L.), (ii) British Gas plc (CASE studentship for G.D.L.), (iii) Purolite Ltd. (contribution to studentship for R.V.L.), and (iv) the Britsh Coal Utilisation Research Association (BCURA) in conjunction with the Department of Trade & Industry (DTI). The views expressed are those of the authors, and not necessarily those of BCURA or the DTI.
