Molecular characterization of Dachengzi oil shale kerogen by multidimensional solid-state nuclear magnetic resonance spectroscopy

Abstract

Solid-state nuclear magnetic resonance (SSNMR) techniques of 13C multiple cross polarization (multiCP), 13C multiCP with recoupled dipolar dephasing (multiCP/DD) and two-dimensional (2D) 1H–13C heteronuclear correlation (HETCOR) were used to characterize Dachengzi oil shale kerogen. Specific structural information was obtained from the quantitative multiCP and multiCP/DD spectra. The protonated aromatic signals distribute widely in the aromatic region and account for 24% of the total aromatic signals. The strong methylene peak in multiCP/DD spectrum indicates the long methylene chain structures. The 2D HETCOR spectra show that oxygen atoms play an important role in the connectivities between different groups. The strong peaks at the 1H/13C chemical shifts of 5/131 and 1.5/131 ppm indicate the high abundance of (–O–CH2)-aromatic and alkyl-substituted aromatic moieties. The connectivities were validated by calculating the chemical shifts of a series of relevant molecular models using quantum chemistry methods. The 13C signal assignment was completed and improved with the multidimensional SSNMR spectra. The results from SSNMR are in agreement with the fast pyrolysis study, and the appearance of (–O–CH2)-aromatic moieties indicates that the methylbenzene homologues in the pyrolysis products can originate from the cleavage of the C–O bonds in (–O–CH2)-aromatic moieties in addition to the β-scission reaction of linear alkylbenzene moieties.