Structural characterization of Huadian oil shale kerogen by using 13C DP/MAS NMR

Quantitative C-13 direct polarization/magic angle spinning (DP/MAS) solid-state nuclear magnetic resonance (SSNMR) was used to characterize type I kerogen isolated from Huadian oil shale. The DP/MAS results showed that this kerogen was highly aliphatic and its aromaticity (f a) was as low as 20.23%. The average aliphatic carbon chain length (Cn), average aromatic cluster size (C) and substitute degree of aromatic rings (s) were calculated. The NMR-derived H/C and O/C atomic ratios (R H/C and R O/C) obtained by DP were in agreement with the corresponding results of ultimate analysis, indicating the accuracy of DP for quantification. Besides, using varying contact times cross polarization (CP) spectra were obtained at the same MAS frequency as the DP spectrum. Regardless of contact time, the aromaticities derived from CP were much lower than that from DP. Consequently, the R H/C value from CP was significantly higher than that of ultimate analysis. The contribution of spinning sidebands could be ignored with the MAS frequency up to 10 kHz. It is concluded that DP with a high MAS frequency is necessary for gaining quantitative structural information about kerogen, especially for its molecular modeling.