Stress-Induced Molecular Sieving and Anisotropic Diffusion of CH₄/CO₂ in Deformed Nanoporous Coal Matrix

Understanding the coupling between mechanical deformation and interfacial gas dynamics in disordered nanoporous media is fundamental to subsurface energy applications. To decouple these effects, this study integrated Molecular Dynamics (MD) and Grand Canonical Monte Carlo (GCMC) simulations to investigate the response of a coal matrix under three distinct stress paths (lateral pressure coefficients K = 0.5, 1.0, and 1.5). Structural analysis identified a critical "stress-induced molecular sieving" mechanism: high confinement stress triggers a structural reorganization, disproportionately collapsing larger micropores accessible to CH₄ (>3.8 Å) while preserving smaller pores accessible only to CO₂ (3.3-3.8 Å). This selective pore destruction leads to a decoupled adsorption response; while absolute storage capacities for both gases decline with increasing stress, the CO₂/CH₄ selectivity exhibits an anomalous surge in high-stress regimes due to the preferential exclusion of methane. Furthermore, kinetic analysis reveals that stress anisotropy acts as a directional filter, causing significant diffusion anisotropy that severely inhibits gas mobility along the maximum principal stress vector. These findings elucidate that anisotropic mechanical stress functions as an active modulator of the physicochemical selectivity of disordered carbon matrices. We demonstrate that high-stress environments can paradoxically enhance the CO₂/CH₄ separation efficiency through a stress-induced molecular sieving mechanism. These insights provide a fundamental basis for understanding stress-mediated adsorption and transport dynamics in deformable nanoporous media, with implications extending from subsurface energy storage to the design of stress-responsive adsorbents.