New insight into the chemical structures of Huadian kerogen with supercritical ethanolysis: Cleavage of weak bonds to small molecular compounds

Oil shale is a potential alternative fossil energy source. The chemical structure of oil shale kerogen is the basis of its application, presently for retorting to yield shale oil and gas. Supercritical ethanolysis is a kind of chemical extraction which can effectively depolymerize organic matter like coal and lignin through breaking some weak bonds. In this work, new insights into weak bonds (including O-containing, N-containing and S-containing functional groups) of the chemical structures of Huadian kerogen (HDK) were obtained with supercritical ethanolysis, and the resulted products, small molecular compounds (SMCs), were identified. It was found that, after ethanolysis at 375 °C, 87.4% of HDK was converted by breaking the weak bonds to SMCs, including aliphatic acid esters, aliphatic acids, alkanes, alcohols, aromatics, N-containing organic compounds (NCOs), and S-containing organic compounds (SCOs). In the SMCs, there are 52.0% aliphatic acid esters and aliphatic acids with carbon numbers from 4 to 26, 11.4% alkanes with carbon numbers from 14 to 22, 19.1% aromatic compounds with single-ring and double-ring aromatic clusters, 5.4% alcohols, 5.0% NOCs, 1.9% SCOs, and 5.4% biomarkers. FTIR and 13C NMR were used to characterize the changes of HDK and its residues, which further demonstrate that the SMCs are present and connected with weak bonds to solid aromatic clusters insoluble in ethanol. During the pyrolysis of oil shale, these weak bonds are first broken and the above SMCs are produced in molecular or radical states, which are the most compounds from the primary reactions and readily undergo the second reactions to yield oil and gas.