Effect of Methanol Guests on Thermal Properties of NH4F-Doped THF Clathrate Hydrate

Methanol is a widely known thermodynamic hydrate inhibitor, but recent studies revealed that an ammonium fluoride ion-pair doped into the host clathrate hydrate lattice can stabilize methanol in the hydrate cages. In this work, we examined THF + methanol clathrate formed from 27 mol % NH4F aqueous solution with powder X-ray diffraction (PXRD) and molecular dynamics (MD) simulations to investigate the effect of methanol guest molecules on the thermal properties of ammonium fluoride doped THF clathrate hydrates. The PXRD pattern analyses revealed that the dissociation of THF + methanol clathrate of NH4F/H2O occurs at higher temperature than 260 K (the dissociation temperature of the nondoped THF + methanol clathrate) and that the thermal expansivity of this doped clathrate is ∼67% that of pure THF hydrate in the range of 100 K to 260 K. The MD simulation results demonstrated that the smaller thermal expansivity is due to the strong interactions between methanol guests and host NH4F pairs and revealed that methanol and NH4F complementarily stabilize each other in the doped clathrate system. The findings in this work provide a better understanding of the nature of ion-pair doped clathrate systems and the crystal engineering approach of clathrate hydrates for their applications in gas storage.