Radical in coal

The stable free radicals in coal can adversely affect the 13C signal intensity. However, the quantitative reliability of the method can be greatly improved by prior treatment of coal with chemicals such as samarium(II) iodide (Stock et al., 1988) to selectively reduce the organic free radicals. Thus, this potential error that has influenced the quantitative aspects of the method can be reduced by the use of a novel pretreatment technique as well as an appropriate standard for measurement of chemical shifts. 

Free Radicals in Macerals. Electron spin resonance (ESR) has been used to study carbon free radicals in coals, and to some extent, separated macerals. The technique provides information on radical density and the environment of the radicals. The resonance position, termed the g-value, is dependent on the structure of the molecule which contains the free electron. The line width is also sensitive to the environment of the unpaired electron. In an early study, Kroger (71) reported that the spin concentration varied between maceral groups with liptinite < vitrinite inertinite. For this limited set of samples the spin concentration increases with rank for liptinites and vitrinites and decreases for the micrinite samples. On the other hand, van Krevelen (72) found the same general results except... 

Petrakis, L., Grandy, D. W. Free Radicals in Coals and Synthetic Fuels, Elsevier, Amsterdam 1983 ... 

Figure 4. TEMPOL ESR spin label studies in Wyodak coal (a) coal radicals prior to spin label addition (b) ESR spectrum immediately after TEMPOL addition, showing carbon radical in coal and the narrow triplet spin label spectrum (the chemical form of the TEMPOL is indicated in the inset) (c) the narrow TEMPOL spectrum broadens with time.

Nature of the Free Radicals in Coals, Pyrolyzed Coals, Solvent-Refined Coal, and Coal Liquefaction Products... 

Characterization of the stable radicals present in coals, pyrolyzed coals, and liquefaction and other coal conversion products is a necessary first step toward elucidating the role of radicals in coal liquefaction. Electron... 

The material presented herein was meant to provide an indication of the utility of ESR and ENDOR spectrometries as probes to characterize the free radicals in coals and coal-derived materials. The results suggest that, in addition to their use in characterization studies, the techniques may have potential in helping to elucidate the mechanism of coal liquefaction. They further suggest that coals may be heat-treated directly in the cavity of an ESR spectrometer with the assurance that reliable data will be obtained provided simple precautionary measures are taken to avoid unwanted effects. Similar in situ ESR measurement of coals under liquefaction conditions remains a possibility, although considerable... 

The observations of an ESR signal due to stable free radicals in coals were independently reported in 1954 by Uebersfeld et al. and Ingram et al. Since then, it has been a challenge to understand the nature of these free radicals, their role in the chemical processes during coal conversion, and their relationship to the structure and rank of the coals. 

The ESR spectra of the free radicals in coal either consist of a single line spectra of width AH 6 Oe or a two-line overlapping spectrum with a narrow component of AH... 

In retrospect, attempts at such correlations are somewhat flawed because of the significant effects of ash present in the coals on the free radicals. For these reasons, as noted earlier. Blind Canyon coal is the most suitable American coal for understanding the role of ESR free radicals in coal conversion processes. 

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