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C NMR Spectroscopy of Pyridine and Alkylpyridines Sorbed onto Coal
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Anthony M. Vassallo Division of Coal Technology, Commonwealth Scientific and Industrial Research Organization, P.O. Box 136, North Ryde, New South Wales, 2113, Australia Heterocyclic compounds such as pyridine and alkylpyridines sorbed onto coals can be examined by cross-polarization (CP) nuclear magnetic resonance (NMR) techniques in which signalsfromboth the coal and the pyridine are seen. The addition of one methyl group to pyridine has little effect on the intensity of the observed spectrum, but two methyl groups show a decreased signal intensity of the pyridine carbons, especially for the 2,5-substitution pattern. Larger alkyl groups result in a much-reduced signal intensity for the sorbed molecule. In the case of pyridine sorption of the coals studied, those coals with carbon contents greater than 85% showed the weakest pyridine signals in the CP spectrum. A proposed model for the pyridine bonding is one in which surface-immobilized molecules are bonded to a number of additional pyridine molecules. These bonded molecules constitute the bulk of the CP pyridine signal observed in the spectrum. V ^ O A L - S O L V E N T INTERACTIONS A R E IMPORTANT in studies of the macromolecular structure of coal (1). In particular, these interactions are evident when certain solvents (e.g., pyridine) cause swelling of the coal structure. Those solvents that cause a large degree of swelling disrupt hydrogen bonding in the coal and allow the macromolecular network to expand to the limits allowed by the covalent cross-links (2). The degree of 0065-2393/93/0229-0201$06.00/0 © 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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solvent swelling is not easily related to any one parameter of a solvent such as polarity because many highly polar solvents do not appreciably swell coal. For polymers, maximum swelling occurs when the solubility parameter, 5, of the solvent is equal to that of the polymer, and this fact has been applied to coal, although there are many limitations (3). Pyridine bonds to the coal surface through the lone pair of electrons on the nitrogen atom, probably to acidic sites such as carboxylic and hydroxyl groups. Indeed, pyridine has been used to titrate hydroxyl groups in coals (low in carboxylic acid groups), and it was suggested (I) that one pyridine molecule bonds to each hydroxyl group. Alkylation of hydroxyl groups prevents pyridine bonding but does not affect the degree of solvent swelling (4) because the amount of swelling is a consequence of the extent of covalent cross-linking once hydrogen-bonded interactions are removed. Pyridine bonded to the coal surface is greatly immobilized compared to free (liquid) pyridine. As a consequence of this immobilization, solidstate N M R techniques can be used to probe this bound pyridine. These techniques have been applied to pyridine bound to silica-alumina (5, 90 98 93
vit
b
nd 8 6 nd nd 1
0
tip
62 47 53 50 43 35
pyd (wt%)
d
c
a
v
N O T E : A l l values are given as percents unless otherwise indicated. C, H , N , S, and Ο values were determined on a dry, ash-free basis, and percent Ο was calculated by difference. R (max) is vitrinite reflectance. ^vit is percent vitrinite. lip is percent liptinite. nd means not determined.
Morwell Rosewood Myuna Liddell Wongawilli Tongarra
Coal
Table I. Ultimate and Pétrographie Analyses of Coals
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crushed (
