Effect of Ammonium Fluoride Doping on Nitrogen, Oxygen, and Methane Clathrate Hydrates

The van der Waals diameters of N2, O2, and CH4 are almost identical; however, they have different electrostatic charge distributions, and their preferred hydrate structures are different. The O2 and N2 molecules form structure II (sII) clathrate hydrates under moderate pressure conditions up to 1 kbar, while the CH4 molecule forms a structure I (sI) clathrate under these pressure conditions. In this work, we investigated the effect of NH4F doping on N2, O2, and CH4 hydrates with powder X-ray diffraction (PXRD) measurement. From the PXRD pattern analyses, the lattice parameter decreased for all three hydrates as the concentration of NH4F doping in the framework increased. The sizes and electrostatic charge distributions within hydrate cages were "tuned"by the NH4F doping, and the transition of the preferred clathrate structure of N2 hydrate from sII to sI occurred at doping concentrations greater than 5 mol %. This transition was not observed in O2 and CH4 hydrates. The findings in this work reveal that the guest-host van der Waals and electrostatic interactions can be adjusted by the NH4F doping to the host framework and suggest that the crystal engineering of the hydrate lattice can be an alternative to improve hydrate-based gas separation technologies.