The Crystallization of Methane Hydrates within Nanoconfined Spaces

Abstract Natural gas hydrates (NGHs) represent a highly promising clean energy source, and clarifying their crystallization mechanisms within nanoconfined spaces is vital for hydrate-based energy exploitation, storage, and transportation. Herein, molecular dynamics (MD) simulations were performed to establish a three-phase system comprising methane hydrate, liquid water, and methane gas via the direct phase coexistence technique. The growth and stability characteristics of methane hydrates in unconfined environments against those within graphene slit pores of varying dimensions were systematically compared. Among them, the graphene slit hole model is an idealized nanochannel composed of graphene layers that are placed parallel to each other and have a certain spacing. This ideal model is commonly used to represent the complex internal structure of porous carbon. The results indicate that graphene confinement significantly shortens the induction period of hydrate crystal growth, substantially advancing the onset of the rapid growth phase compared to the bulk system. Driven by wall hydrophobicity and geometric constraints, the hydrates exhibit a distinct growth pattern characterized by interfacial wetting and bulk crystallization. This confinement reduces the lattice order parameter (F4), keeping the cage structures in a quasi-equilibrium state of dynamic formation and decomposition. Furthermore, evaluating the impacts of pore geometry reveals that increasing the pore length yields sufficient reaction space and a stable growth front, markedly enhancing crystalline integrity and thermodynamic stability. Conversely, narrowing the pore width hinders the restructuring of the water hydrogen-bond network and restricts methane diffusion due to severe confinement effects, which suppresses crystal nucleus growth or even triggers spontaneous decomposition. Overall, this work clarifies the molecular-level mechanisms by which confinement dimensions regulate methane hydrate crystallization kinetics, providing theoretical insights for designing high-performance porous materials for hydrate-based gas storage and transport.