Energy & Fuels 2002, 16, 925-927
Optimization of Sensitivity in Pulsed
925
13C
NMR of Coals
Robert E. Botto Chemistry Division, Argonne National Laboratory, 9700 So. Cass Avenue, Argonne, Illinois 60439 Received November 26, 2001
Pulsed 13C NMR analyses (SPE) were performed on Argonne Premium coals using a modified version of the ring-down elimination (RIDE) pulse sequence that included high-power proton decoupling. Employing RIDE in combination with the use of O2 as a relaxation agent and appropriate considerations of the Ernst-Anderson angle produced high-quality spectra devoid of baseline artifacts in reasonable measuring times. Employing this approach reduced the total acquisition time over the standard procedure of applying a 90° excitation pulse by as much as a factor of 1/17 in some instances. Carbon aromaticity values determined in this manner were similar to those obtained previously by SPE using a 90° pulse and long recycle delay times. Values determined by both methods are generally found to be higher than those by cross polarization.
Introduction More than a decade ago, it was shown that singlepulse excitation (SPE; pulse-acquire-relaxation delay) was clearly superior to cross polarization for solid-state 13C NMR of coals when reliable quantitative information was desired.1,2 Since then, SPE has become the method of choice for coal structure determinations by NMR.3-9 The practice commonly employed in SPE experiments is to use a 90°-pulse angle for excitation of the nuclear spins. This condition, however, requires the implementation of a delay time on the order of five times the longest spin-lattice relaxation time (5T1s) in a sample to reestablish the equilibrium magnetization, which is necessary for quantitative analysis. The experiments simply require long acquisition times in order to achieve adequate signal-to-noise (S/N) levels. Over the years, we have developed a simple experimental protocol for optimizing conditions in solid-state NMR determinations of coals, while retaining a high level of quantitative reliability. I have given lectures on this subject on numerous occasions during the past several years and have been encouraged by many to publish these findings. This short note summarizes a recipe for producing NMR spectra of coals with high signal-to-noise that are devoid of baseline distortions, (1) Botto, R. E.; Wilson, R.; Winans, R. E. Energy Fuels 1987, 1 (2), 173-181. (2) Muntean, J. V. Ph.D. Thesis, University of Chicago, 1990. (3) Franz, J. A.; Garcia, R.; Linehan, J. C.; Love, G. D.; Snape, C. E. Energy Fuels 1992, 6 (5), 598-602 (4) Wiggershaus-Eschert, S.; Wieschenkaemper, I.; Riepe, W. Freennius’ J. Anal. Chem. 1993, 346 (6-9), 805-807. (5) Franz, J. A.; Linehan, J. C. In Magnetic Resonance of Carbonaceous Solids; ACS Advances in Chemistry Series 229; Botto, R. E., Sanada, Y., Eds.; Washington, DC, 1993; Chapter 20. (6) Maroto-Valer, M. M.; Mercedes, M.; Love, G. D.; Snape, C. E. Fuel 1994, 73 (12), 1926-1928. (7) Cerny, J.; Pavlikova, H. Coal Sci. Technol. 1995, 24 (Coal Science, Vol. 1), 111-114 . (8) Maroto-Valer, M. M.; Andresen, J. M.; Snape, C. E. Fuel 1998, 77 (7), 783-785. (9) Kidena, K.; Murata, S.; Artok, L.; Nomura, M. Nippon Enerugi Gakkaishi 1999, 78 (10), 869-876.
while saving a considerable amount of time in signal averaging. Experimental Section Solid-state 13C NMR spectra were recorded at fields of 2.3 and 4.7 T (25.18 and 50.3 MHz for 13C) on Bruker Instruments spectrometers, models CXP-100 and Avance DSX-200, operating in the pulse Fourier transform mode with quadrature phase detection. The 7-mm sample spinners, having an internal volume of 250 µL, were spun about the “magic” angle at a rotational velocity of 4.0 kHz and 7.0 kHz, respectively. A solid-state version of the ring-down elimination (RIDE) pulse sequence10 that incorporates high-power proton decoupling was employed at the lower static magnetic field. In our experimental protocol, the RIDE method consisted of a pulse scheme of alternating pulses with rotational flip angles of θM degrees and (360 - θM) degrees, where θM is the optimum value determined as the Ernst/Anderson angle,11 t is the experimental recycle delay, and T1 is the longest spin-lattice relaxation time in the sample:
θM ) arc-cos[exp(-t/T1)]
(1)
Alternating the receiver phase by 180° between successive scans cancels the incoherent acoustic noise arising from the coil “ring-down” and adds only coherent signal from the sample to memory. General operating parameters employed were a spectral width of 10 kHz, a pulse width equal to θM where the 90° carbon-pulse width is equivalent to 4.5 µs, a 60-kHz protondecoupling field, an acquisition time of 20 ms, a pulse repetition time of 1-3 s, and a total accumulation of 7000 to 20000 transients. Pertinent acquisition parameters for experiments performed at 50.3 MHz for APCS #1 were a 20°-pulse width, a spectral width of 20 kHz, and a recycle delay of 3 s. A carbon aromaticity value of 0.82 was obtained using these conditions, taking into account the intensities of the spinning sidebands. The 13C spin-lattice relaxation times were measured with the CP T1 pulse sequence described previously.12,13 (10) Belton, P. S.; Cox, I. J.; Harris, R. K. J. Chem. Soc., Faraday Trans. II 1985, 81, 63. (11) Ernst, R. R.; Anderson, W. A. Rev. Sci. Instrum. 1966, 37 (1), 93-102.
10.1021/ef010279k CCC: $22.00 © 2002 American Chemical Society Published on Web 05/14/2002
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Energy & Fuels, Vol. 16, No. 4, 2002
Botto
Table 1. Consequences of NMR Acquisition Parameters on Pulse Angle pulse angle (θM)
relative intensity
repetition time a
90o
1.00 0.94 0.70 0.35 0.17
5.0T1 1.0T1 0.5T1 0.1T1 0.02T1
70o 45o 20o 10o a
T1 ) spin-lattice relaxation time.
Coal samples were subjected to molecular oxygen as a relaxation agent prior to analysis. This was accomplished by placing a powdered 100-mesh sample in a fritted funnel (coarse) fitted at the top with perforated aluminum foil. A slow stream of oxygen was passed through the funnel from the bottom overnight. The samples were collected immediately and carefully placed in O-ring sealed NMR rotors for analyses.
Results and Discussion Intrinsically NMR is a form of nonlinear spectroscopy, both with regard to its response function in amplitude and in time. That is to say that the the NMR output, or cumulative signal response, is nonlinear with respect to the magnitude of the stimulating radio frequency field or repetition time of the experiment. In their seminal paper on pulsed Fourier transform NMR spectroscopy, Ernst and Anderson noticed these response characteristics of protons recorded in solution phase and derived a simple mathematical relationship for optimizing the experimental parameters in a pulsed NMR experiment.11 The consequence of changing the pulse angle θM on the signal intensity and repetition time according to eq 1 (vide supra) is summarized in Table 1. Choosing an optimal value of θM for a given set of sample conditions can result in greater than an order of magnitude savings in time. Solid-state NMR methods became enormously popular as a structural tool in the late 1970s with the advent of cross-polarization (CP)14 as a pulsed method for increasing the sensitivity of the experiment, applied together with magic-angle spinning (MAS).15 Later it was realized by many in the field that SPE methods offered the advantage of being more quantitative, but at the expense of sensitivity. The early work of Ernst and Anderson has gone unnoticed by the scientific community performing solid-state NMR on coals. A survey of the recent literature confirms that this is still valid today. The approach that we have implemented during the past decade has incorporated the Ernst-Anderson angle in a solid-state version of the ring-down elimination (RIDE) pulse sequence. Our initial work was performed at 2.3 T (25.18 MHz carbon frequency). Comparing spectra in Figures 1 and 2 shows the substantial improvement in baseline roll as a result of implementing RIDE without any baseline correction applied. Also evident is the high signal-to-noise (S/N) of spectra in Figure 2. NMR spectra typically were re(12) Tsiao, C.; Botto, R. E. In Magnetic Resonance for Carbonaceous Solids; ACS Advances in Chemistry Series No. 229; Botto, R. E., Sanada, Y., Eds.; Washington, DC, 1993; pp 341-358. (13) Botto, R. E.; Axelson, D. E. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1988, 33 (3), 50-57. (14) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1972, 56, 1776. (15) Stejskal, E. O.; Schaefer, J.; McKay, R. A. J. Magn. Reson. 1977, 25, 569.
Figure 1. MHz.
13
C SPE spectrum of APCS #3 recorded at 25.18
Figure 2. 25.18 MHz13C SPE-RIDE spectra: APCS #2 (top); APCS #4 (middle); APCS #5 (bottom).
corded overnight. Treatment of the coal samples with O2 prior to analysis was used to reduce sample T1s. For example, T1s of the aromatic carbons (20.9 and 20.5 s, respectively) in APCS #1 and APCS #6, which exhibit the longest carbon relaxation in the suite of Argonne coals,13 were effectively shortened to 11.5 and 6.4 s. On the other hand, relaxation times of carbons in APCS #2 were largely unaffected by prior treatment with O2, presumably because they were shortened already owing
Optimization of Sensitivity in Pulsed
13C
NMR of Coals
to the presence of sizable quantities of paramagnetic minerals distributed through the organic matter.13,16 The use of O2 as a relaxation agent together with selecting optimal pulse conditions produces a sensitivity gain close to a factor of 10 for those samples with the most unfavorable relaxation times. Moreover, spectra of O2-treated samples were found to be identical to those obtained on the pristine coals, thus no apparent reaction of O2 with the Argonne coals was detected by NMR. A recent result on APCS #1 recorded at higher field (carbon frequency of 50.3 MHz) clearly demonstrates this increase in sensitivity; a S/N of ca. 30 for the aromatic carbon signal with a total acquisition time of less than 6 h rivals the sensitivity attained with cross polarization. Finally, there is the question concerning quantitative aspects of the method. In Table 2, carbon aromaticity values determined by SPE-RIDE are compared with average values derived from those reported in the literature.16 Values compare favorably with those obtained previously by SPE; both are generally higher than aromaticity values obtained by CP. Larger devia(16) Silbernagel, B. G.; Botto, R. E. In Magnetic Resonance for Carbonaceous Solids; ACS Advances in Chemistry Series No. 229; Botto, R. E., Sanada, Y., Eds.; Washington, DC, 1993; pp 629-643.
Energy & Fuels, Vol. 16, No. 4, 2002 927 Table 2. Carbon Aromaticity Values (fa) of Argonne Premium Coals coal a
fa (SPE-RIDE)
fa (SPE)b
fa (CP)b
APCS #1 (UF) APCS #2 (WA) APCS #3 (IL6) APCS #4 (PT8) APCS #5 (POC) APCS #6 (BC) APCS #7 (LS) APCS #8 (BZ)
0.84 0.71(0.06 CO2H) 0.74 0.78 0.89 0.70 0.82 0.77(0.04 CO2H)
0.81 0.74 0.73 0.75 0.86 0.66 0.76 0.76
0.82 0.67 0.72 0.73 0.85 0.63 0.78 0.66
a UF ) Upper Freeport; WA ) Wyodak-Anderson; IL6 ) Illinois No. 6; PT8 ) Pittsburgh No. 8; POC ) Pocohontas No. 3; BC ) Blind Canyon; LS ) Lewiston-Stockton; BZ ) Beulah-Zap. b See ref 16.
tions seen for low-rank coals may be due to considerations of the carboxyl carbon content in the aromaticity calculations. Acknowledgment. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences, U. S. Department of Energy, under contract number W-31-109-ENG-38. EF010279K
