Chemical Composition and Structural Characteristics of Oil Shales and Their Kerogens Using Fourier Transform Infrared (FTIR) Spectroscopy and Solid-State ¹³C Nuclear Magnetic Resonance (NMR)

Abstract

The chemical composition and chemical structure of selected oil shales and the kerogens isolated from them were studied by solid-state 13C nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy with curve-fitting analysis, and the changes during the removal of the inorganic matrix were also investigated. The 13C NMR results indicate that the oil shales and kerogens are mainly composed of aliphatic carbon (≥55%). FTIR analysis indicates that aliphatic hydrocarbons mainly contain the methylene group, mostly forming long straight chains and a small amount of branched chains, and the hydroxy groups mainly contain OH–OH and OH–O bonds (≥65%). Both the 13C NMR and FTIR analyses show that the acid treatment improved the hydrocarbon-generating ability in kerogen. Furthermore, the curve-fitting analysis indicates that the HCl and HF treatments slightly affected the aliphatic and hydroxyl structures, but significantly affected the oxygen-containing and aromatic structures in the oil shales. The oxygen-containing groups in the oil shales are mainly the C–OH group, followed by the C–O, O═C–O, and C═O groups, in descending order. After the acid treatment, the main oxygen-containing groups were the C–OH and C–O groups, because of the hydrolysis, substitution, and ion exchange. The 13C NMR results indicate that the acid treatment not only decreased the extent of aromatic ring condensation (fused aromatic rings), but also decreased the number of condensed aromatic rings.