Nano-molecular response of bituminous coal to ScCO₂ at different stage: Experiments and ReaxFF-MD/DFT insights

The interaction between supercritical CO₂ (ScCO₂) and coal alters the physicochemical properties of the coal in a staged manner. This study quantitatively investigates the nano-molecular structural response of bituminous to ScCO₂ exposure using a combination of experiments and ReaxFF-MD/DFT simulations. The results show that:In the swelling stage, the coal matrix expanded and early radical formation occurred, leading to a reduction in intramolecular pore volume from 0.0326 to 0.0318 cm³/g, while intermolecular pores increased from 0.0119 to 0.0145 cm³/g, Car-Car from 145 to 149, Cal-H from 103 to 92, weak van der Waals and hydrogen bonds were cleaved. In the dissolution stage, aliphatic chains degraded and oxygen-containing groups formed, intramolecular and intermolecular pores expanded to 0.0334 and 0.0165 cm³/g, respectively, increasing Car–Car bonds to 150, and Cal-H to 89, electron density recovered with the development of aromatic conjugation and polar groups In the rearrangement stage, radicals recombined into new aromatics, compressing intermolecular pores to 0.0160 cm³/g, while intramolecular pores increased to 0.0346 cm³/g, reducing Car–Car bonds to 142 and Cal-H to 84, electrostatic potential strengthened, indicating molecular stabilization.This work provides a novel, stage-specific, and quantitatively supported mechanism of bituminous evolution under ScCO₂ conditions, offering theoretical insight into molecular-scale optimization strategies for CO₂-ECBM.