Quantitation in 13C NMR Spectroscopy of Carbonaceous Solids

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Quantitation in C NMR Spectroscopy of Carbonaceous Solids 1

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Robert A. Wind , Gary E . Maciel , and Robert E . Botto Downloaded by 80.82.77.83 on May 18, 2017 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch001

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Chemagnetics, Inc., 2555 Midpoint Drive, Fort Collins, CO 80525 Department of Chemistry, Colorado State University, Fort Collins, CO 80523 Chemistry Division, Argonne National Laboratory, Argonne, IL 60439

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This chapter provides an overview of the fundamental issues concerning quantitation in C NMR spectroscopy of carbonaceous solids. General factors governing quantitation in solid-state C NMR spectroscopy (such as sample heterogeneity, the presence of unpaired electron spins, interference of proton decoupling by molecular motion, magic-angle spinning (MAS) effects, and implementation of the proper recycle-delaytime)are discussed together with those factors that play a major role in cross-polarization (CP) experiments (Hartmann—Hahn match, proton spin-locking cross-polarization spin dynamics, and interference of cross-polarization from MAS). Technical aspects and requirements of the solid-state C NMR experiment are outlined, and effective strategies to obtain the most reliable results are presented. 13

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THE POTENTIAL USE OF SOLID-STATE C NMR spectroscopy for the characterization of carbonaceous solids such as coal, the determination of the various functional groups, and the study of fossil-fuel conversion processes has been recognized for more than three decades. Numerous investigations have been reported in the literature, and several reviews have appeared (1-6). With the advent of H decoupling (7), cross-polar1

0065-2393/93/0229-0003$07.00/0 © 1993 American Chemical Society

Botto and Sanada; Magnetic Resonance of Carbonaceous Solids Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

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MAGNETIC RESONANCE OF CARBONACEOUS SOLIDS

ization (CP) (S), and magic-angle spinning (MAS) (9), which have made it possible to obtain high-resolution C N M R spectra in solids in a relatively short measuring time, C P - M A S techniques have been applied extensively in coal research (2-6, 10-16). In fact, C C P - M A S N M R spectroscopy has become one of the most widely used methods to investigate coal. However, concern has been growing over the past decade about the quantitativeness of C P - M A S spectroscopy for coal analyses. In the very first C CP N M R study of coal, VanderHart and Retcofsky (10) estimated that only ~50% of the total carbon spins could be detected via this technique. Since then the issue of quantitation in N M R analyses of coal and related materials has been the topic of much debate (10, 12, 15, 17-25). Although the quantity of observable carbons for coals and their individual organic constituents (macérais) have been shown to vary widely, the general consensus is that for reasons that can be related to both specific coal properties and the applied N M R techniques, a substantial fraction of the carbons is not observed. In principle, two N M R techniques can be used to measure C spectra in coal; these are the CP experiment mentioned and the conventional single-pulse (SP) experiment, consisting of the simple 90° pulse-acquisition-recycle-delay sequence. Figure 1 shows the radio frequency (rf) pulse sequences employed in SP and CP, as well as the time constants and time delays involved in both experiments. In this chapter, sources that can limit the quantitative information in C N M R spectroscopy in coal will be reviewed, and possible remedies will be given to improve the situation. We shall discriminate between limiting factors that play a role in solidstate C N M R spectroscopy generally, and limiting factors that occur when CP is employed. 1 3

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Downloaded by 80.82.77.83 on May 18, 2017 | http://pubs.acs.org Publication Date: December 9, 1992 | doi: 10.1021/ba-1993-0229.ch001

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General Factors Governing Quantitation in Solid-State NMR Spectroscopy 1 3

In both SP and CP it is assumed that, during the C signal acquisition, the protons are decoupled from the carbons via irradiation with a strong rf field at the proton Larmor frequency, and the MAS is applied to remove the chemical-shift anisotropics. The following factors can influence the quantitativeness of the C spectra: 1 3

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sample heterogeneity unpaired electrons interference from molecular motions on * H decoupling magic-angle spinning recycle delay


C


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A major concern is the selection of the appropriate pulse sequence, either SP or CP, to obtain the most quantitative C spectra. Comparing the two methods from previous work found in the literature, we conclude that the SP technique is by far preferable for the quantitative characterization of carbonaceous solids, including coals. In fact, the time has come to abandon CP altogether for quantitative studies. However, this technique may be used for a qualitative survey when sensitivity is an issue or when more detailed knowledge of the coal structure is desired, for example, to confirm the presence of different molecular structures that are in low abundance or to determine the distribution of carbon-proton distances and relaxation times for various carbons and protons. Useful CP results can be achieved by applying the variable-r and variable-^ experiments described herein. However, no rigorous analysis can describe in detail all 1 3

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processes occurring during CP, so that qualitative information can be expected at best. The main disadvantage of the SP technique is the long recycle delay required between successive pulses. However, for lower rank coals (typi cally with Ο > 10%) the C Zeeman relaxation time, T , is rather short (on the order of several seconds) (26) and T in higher-rank coals can be decreased by exposing the coal to air. This step, together with the pos sibility of using large-volume rotors, makes the SP experiment very feasi ble. The remaining factor limiting the quantitativeness in SP experiments is the broadening effects due to the unpaired electrons. However, espe cially for lower rank coals in which the electron concentrations are typi cally lower than about 3 χ 10 c m , the maximum intensity loss to be expected is probably not more than 10-15%. In samples in which the presence of free radicals poses a problem, the judicious application of SP methods combined with pretreatment of the coal with an appropriate radi cal quenching agent, such as samarium iodide (28), can be used effectively for improving the quantitative reliability of the measurements. Finally, in other Η - 0 polarization-transfer techniques, part of the problems arising in CP can be circumvented. These other approaches are polarization transfer via the nuclear solid effect (NSE) (63, 64) and polari zation transfer via rotating-frame (RF) D N P - C P (65, 66). In both instances, dephasing of the protons via T processes is avoided. These experiments are discussed in greater detail in Chapter 10. 1 3

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Acknowledgments R. E . Botto acknowledges support of Division of Chemical Sciences, U.S. tract No. W-31-109-ENG-38. R. nowledge support of Fossil Energy, Contract No. DE-AC22-89PC8840.

the Office of Basic Energy Sciences, Department of Energy, under Con A Wind and G. E . Maciel ack U.S. Department of Energy, under

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RECEIVED for review February 25, 1991. ACCEPTED revised manuscript September 17, 1991.


Quantitation in 13C NMR Spectroscopy of Carbonaceous Solids

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