Investigation of oxygen functional groups in low rank coal

摘要

The distribution of the organic oxygen content of coals among the principal oxygen containing functional groups typically is determined by a combination of chemical and spectroscopic methods (1,2) and results in a classification scheme such as % carboxyl, % hydroxyl, % carbonyl, and % ether. A notable subdivision in this classification scheme is the differentiation of phenols in a coal on the basis of their ortho-substitution pattern (3). Apart from this distinction, the further classification of oxygen into functional group subsets is virtually nonexistent. This paper presents initial experiments that indicate a fuller characterization of oxygen distribution in low rank coal is possible. The experimental approach couples selective chemical perturbation and solid state NMR analysis of the material, specifically, the fluorination of Argonne Premium Coal (number sign)8, North Dakota lignite, and spectroscopic examination by high resolution solid state (sup 19)F NMR (4). The fluorination reagent is diethylaminosulfur trifluoride (DAST), (Et)(sub 2)NSF(sub 3), which promotes a rich slate of oxygen functional group interconversions that introduce fluorine into the coal matrix (5). The virtual absence of this element in coals make (sup 19)F an attractive NMR nuclei for this application (6). The present experiments use direct detection of the (sup 19)F nucleus under conditions of proton ((sup 1)H) heteronuclear dipolar decoupling and magic angle spinning (MAS). The ca 300 ppm range of (sup 19)F chemical shifts in common carbon-fluorine bonding configurations and high (sup 19)F nuclear sensitivity permit the identification of unique and chemically dilute functional groups in the coal milieu. The unique detection of aromatic and aliphatic carboxylic acids and primary and secondary alcohols provide examples of the exquisite functional group detail that is revealed by this combination of techniques.