Molecular insights into methane hydrate dissociation under confinement in a hydrophilic silica nanopore

Abstract

Understanding gas hydrate behaviour under confinement is crucial to the development of strategies to efficiently extract methane from hydrate reservoirs. Thus, we have performed molecular dynamics simulations of methane hydrate dissociation inside a hydrophilic silica slit nanopore (representing the pores present in naturally occurring hydrate reservoirs) in the canonical ensemble at 290, 300, 305, and 310 K. Methane hydrate dissociates at lower temperatures under confinement than in bulk. Hydrate dissociation under confinement proceeds in a shrinking core manner showing an increased dissociation rate in the confined system compared to the bulk system where the dissociation is layer-by-layer only. Under confinement, the observed Arrhenius-type behaviour of the methane hydrate dissociation rate (in the initial 5 ns) with temperature leads to a value of the activation energy of dissociation (i.e., 46.885 kJ/mol) to be twice the hydrogen bond energy. In contrast to the confined system, the activation energy of dissociation in the bulk system is higher (i.e., 56.928 kJ/mol). The hydrophobic methane nanobubble formed after the dissociation tends to adhere to the hydrophilic silica substrate and there is an ordered bound water layer on the hydrophilic silica surface underneath the methane nanobubble, with the water molecules in this bound water layer region ordered in a square lattice arrangement unlike the random orientation of water molecules in the bound water layer at other regions on the hydroxylated silica surface. This ordered arrangement of the bound water molecules underneath the nanobubble maximizes the hydrogen bonding between bound water molecules and the surface hydroxyl groups (i.e., one water molecule is associated with a pair of hydroxyl groups). Our study, thus brings this detailed molecular-level structural insight into the complex interactions that exist among methane, water, and the hydrophilic silica surface under confinement for the first-time, to the best of our knowledge.