Coal Rank-Driven Macromolecular Reconstruction Regulating Nanopore Topology and CO2 Transport

Structural evolution driven by coalification is an essential force regulating the development of nanopores in coal matrices and CO2 adsorption behavior. However, bridging the characterization gap from the atomic-scale skeleton to macroscopic pore heterogeneity and elucidating the driving mechanism by which structural evolution governs gas occurrence remain core challenges for gas-solid multiscale interactions under complex geological conditions. In this study, three coal samples representing typical evolutionary stages (lignite, bituminous coal, and anthracite) were selected to establish a multiscale research framework combining experimental characterization and molecular simulation. Atomic-scale three-dimensional macromolecular models were constructed using elemental analysis, 13C NMR, and XPS. Nanopores were characterized by scanning electron microscopy (SEM) and low-temperature (273 K) CO2 adsorption, and pore evolution was quantitatively analyzed on the basis of the Dubinin-Radushkevich (D-R) model and multifractal theory. Furthermore, grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations were employed to unravel the coupled adsorption-diffusion mechanism of CO2 at the molecular scale. The results show that coalification is essentially a synergistic restructuring of the macromolecular skeleton through aromatization and deoxygenation, driving a topological transition of nanopores from "open and connected" to "microporous-dominated, dense, and isolated". With increasing coal rank, micropore volume and specific surface area exhibit a U-shaped evolution, and anthracite develops abundant ultramicropores (<0.7 nm) but with a strong size-exclusion effect due to "bottleneck" pores. The CO2 adsorption capacity follows a U-shaped curve, with the mechanism shifting from the chemical affinity of functional groups in low-rank coals to the micropore confinement-induced "anchoring effect" in high-rank coals, where the adsorption characteristic energy reaches 7.16 kJ/mol. Diffusion dynamics are controlled by the competition between "spatial gain" and "energy trap", and the self-diffusion coefficient D shows a nonmonotonic "high-low-medium" variation. This study reveals, at the molecular scale, the differential regulation of gas-solid interactions by coal rank evolution, providing key physical criteria for deep coalbed CO2 storage and efficient coalbed methane development.