Structural elucidation for soluble organic oxygenated compounds in soft and hard coals using advanced extraction methods

•Compared with UE and TD, UHPE takes priority in extracting long-chain compounds.•Toluene dissolves aromatics with larger ring size due to the weakened π-π intermolecular force.•O3 and O4 group compounds might be the major oxygen-containing compounds in middle- and low-rank coals. Two coal samples with different coalification degrees were extracted by ultrasonic extraction (UE), thermal dissolution (TD) at 300 °C, and ultra-high pressure extraction (UHPE) at 300 MPa using three solvents, methanol, toluene and acetone. Orbitrap mass spectrometry (MS) was used to analyze coal extracts and molecular characteristics were further elucidated by in-source collision activated dissociation, a tandems MS method. UHPE exhibited an excellent extraction yield, which is related to the acquirement of compounds with long-chain structures. Unlike UE and TD, more compounds containing 3 to 4 oxygen atoms per molecule were extracted by UHPE method. Oxygen distribution index indicates most oxygen atoms in UHPE extract exist on the side chains of aromatic structures. Therefore, advanced separation method reveals that multiple oxygen-containing functional groups (3 and 4 oxygen atoms) on side chains should be the major structure of organic oxygenates in low-rank coals, which is a new knowledge of coal chemistry.