CHEMICAL STRUCTURAL CHANGES IN KEROGEN FROM BITUMINOUS COAL IN RESPONSE TO DIKE INTRUSIONS AS INVESTIGATED BY ADVANCE 13C SOLID-STATE NMR

摘要

Coal maturation due to igneous intrusions in the form of dikes or sills offers the opportunity to study steep petrographic and geochemical gradients in coal seams over relatively short distances. Changes in optical, physical and chemical properties of coal seams around magmatic intrusions in many localities worldwide have been extensively described (1). However, less is known about the chemical structural evolution of coal near dikes. Solid-state ~(13)C NMR spectroscopy is a powerful tool frequently used in the analysis of complex natural organic matter such as coal. The most commonly-used solid-state NMR technique for coal characterization is ~(3)C cross-polarization magic-angle spinning (CP/MAS) that has significantly advanced our understanding of coal chemical structures (2). However, the structural complexity of coal generally limits CP/MAS spectra to two broad bands that represent aromatic and aliphatic carbons. Such generalized information provides limited opportunity for the assignments of specific functional groups within the two bands. In the past years, we have developed, modified and adopted many advanced solid-state NMR techniques for the detailed, systematic characterization of complex organic matter (3-7). Routine ~(13)C solid-state NMR spectra consist of broad and heavily overlapped bands in which functional groups cannot be clearly distinguished. In contrast, our advanced solid-state NMR techniques can selectively retain certain peaks and eliminate others, clearly revealing specific functional groups. Further, we use ~1H-~(13)C heteronuclear correlation NMR (2D HETCOR) to detect connectivities or proximities of different functional groups. These advanced NMR techniques are well suited for the detailed characterization of coal.