| Abstract | 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. |
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