Interaction-mediated modulation of methane storage in primary amine hydrates

Abstract

Clathrate hydrates are promising media for energy-efficient CH4 storage, yet their capacity under practical conditions remains limited. In this study, we examined how primary amine promoters modulate structural and thermodynamic behavior in structure II (sII) hydrates. Formation experiments under typical conditions (7.2 MPa and 283.2 K) revealed that all tested primary amines significantly enhanced CH4 uptake. High-resolution powder diffraction (HRPD) and 13C solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR) analyses indicate that this enhancement is primarily associated with increased CH₄ occupancy in small cages (sIIsingle bondS). While water-to-hydrate conversion may vary due to the stochastic nature of hydrate formation, HRPD data reveal no pronounced differences in residual ice content among the primary amine systems within experimental resolution, supporting an occupancy-dominated interpretation. Density functional calculations revealed strong host–guest interactions and pronounced electron redistribution in most amine systems, consistent with significant lattice distortion of sII large cages (sII-L). In contrast, tert-butylamine stabilized sII-L, thereby limiting distortion, weakening host–guest interactions, and reducing CH4 uptake, whereas the other amines induced lattice destabilization that promoted occupation. Together, these contrasting behaviors establish a mechanistic framework in which molecular geometry governs lattice distortion, electronic redistribution, and cage-specific CH4 occupancy. By elevating geometry from a structural descriptor to a design principle, this work provides a blueprint for engineering next-generation hydrate promoters that deliver methane storage capacities beyond conventional limits under moderate conditions.