华北晚古生代煤的交叉极化/魔角自旋碳-13核磁共振研究

CP/MAS C-13 NMR STUDIES OF LATE PALEOZOIC COALS FROM NORTH CHINA

用固体高分辨^(13)C核磁共振技术,研究不同变质程度的天然煤样和人工模拟煤样的化学结构演化规律,揭示了煤化过程中煤的结构演变控制着烃类生成的机理。

During coalification, the chemical structure of coal is dramatically altered.In order to establish a geochemical scheme for coalification pathways and to recon-struct the key structural transformations of coaly materials, a series of Late Palaeozoiccoals from North China and some synthetically coalified samples were analyzed by meansof C-13 cross-polarization, magic angle spinning (CP/MAS) NMR spectra. Besidesobservable changes in carbon aromaticity encountered in coals of different rank, the datarevealed also significant changes in aromatic as well as aliphatic structural components. At the early metamorphic stage (lower than 1.28% R_(max)~o), the skeletal carbon struc-ture shows relatively low aromatization and contuiris a relatively high amount of sub-stituents. Methylenic and methinic groups are the dominant aliphatic components. With theincrease of coal rank and, especially, in the range of major oxygen decrease, these compo--nents exhibit relatively high mobility and their amount is gradually reduced. At the medium metamorphic stage (from 1.28 to 2.69% R_(max)~o), there is a rapidincrease in aromaticity. This is accompanied by a significant reduction in number and sizeof aliphatic components. At the late metamorphic stage (higher than 2.65% R_(max)~o), aliphatic carbon groupsalmost have disappeared, except for a small number of methyl and its adjacent methylenegroups. The quantitative variations in the abundancy of the aromatic and aliphatic compo--nents tend to be quite moderate. We suggest that the predominant geochemical mechanismin the late metamorphic stuge has changed from aromatization into ring condensation. The changes in the geochemical structure of coal obviously result in differences ofyield and composition of hydrocarbons with increasing coalification. The three struturalevolutionary stages correspond with the generation stages of oil, condensate+wet gas andlate catagenetic methane respectively.