Effect of Help-Guest Size and Hydrogen Bonding on the Stability of N-Methylpiperidine Structure H Clathrate Hydrate

The phase equilibrium diagram measurements, powder X-ray diffraction, and molecular dynamics simulations are used to characterize the stability, structure, and guest dynamics of the structure H (sH) N-methylpiperidine (NMP) clathrate hydrate with the difluoromethane (HFC-32) help gas. The pressure–temperature phase diagram of the NMP + HFC-32 hydrate is more stable under higher temperature–lower pressure conditions than the simple HFC-32 structure I (sI) hydrate or the sH hydrates of NMP with the methane help-gas guest compound. The hydrate with these investigated guest compounds was more stable, although the HFC-32 help-guest molecules should be comparatively large and more difficult to fit into the small and medium cages of sH hydrate. Molecular dynamics simulations show that the NMP guest molecules form hydrogen bonds with the cage water molecules in the NMP + HFC-32 sH hydrate, while similar hydrogen bonding was not observed for the sH hydrate of NMP with CH₄. The larger size and nonzero dipole moment of the HFC-32 help-gas could lead to tension in the small and medium sH cages, which could weaken the cage structures in the hydrate framework, thus enabling the formation of hydrogen bonding between the NMP molecules in the neighboring cages with the cage water molecules. This is a first case where the combination of larger help-gas molecules and hydrogen bonding of the large-cage guests was observed to help stabilize a sH hydrate phase. The newly found phenomena could be the favorable properties for sophisticated gas-storing materials with the minimum environmental impact.