Dissolution behavior of soluble small molecules in Shendong coal through fractional extraction with two-stage solvents

In order to investigate the effects of the composition, structure, and occurrence modes of small molecules in coal on their dissolution behavior, as well as the changes in the pore structure of coal after their dissolution, Shendong coal is sequentially extracted by petroleum ether and carbon disulfide (CS2) using Soxhlet extraction method. The extracts and the residues are characterized by FTIR and N2 adsorption, respectively. The results show that the curves of cumulative extraction rate versus extraction time can clearly reflect the sequential dissolution stages of small molecules with three occurrence forms (free state, micropore-embedded state, and network-embedded state) in different solvents. CS2 is effective for both aliphatic and aromatic substances in coal due to its strong permeability and high solubility, while petroleum ether mainly acts on aliphatic substances. For the dissolution of network-embedded small molecules, CS2 is more effective than petroleum ether. For free state small molecules, those aliphatic hydrocarbons with long side-chain or long-chain structures are preferentially dissolved by petroleum ether, followed by the dissolution of short side-chain small molecules with CS2. For micropore-embedded state small molecules, CS2 preferentially dissolves polycyclic aromatic hydrocarbons, followed by the co-dissolution of small molecules with polycyclic, bicyclic, and monocyclic structures. For network-embedded state small molecules, petroleum ether cannot effectively dissolve them due to the difficulty in penetration, whereas CS2 firstly dissolves short side-chain structures, and then the dissolution of long side-chain or long-chain aliphatic hydrocarbons occurs after a long extraction time. There is basically no co-dissolution relationship between hydrogen-bonded —OH (—NH) and phenolic structures and aliphatic structures. Hydrogen-bonded —OH (—NH) and phenolic structures can be dissolved only after a large amount of aliphatic structures are dissolved. Due to the existence of multiple effects during the solvent extraction process, the pore size evolution process becomes very complex, but the total pore volume of the residual coal generally increases as the dissolution amount of aliphatic small molecules decreases (the dissolution amount of aromatic small molecules increases).