Incorporation of Ammonium Fluoride and Methanol in Carbon Dioxide Clathrate Hydrates and Their Significance for Hydrate-Based Gas Separation

Methanol is known as one of few small molecules that cannot stabilize a solid clathrate hydrate host lattice as a guest molecule in a simple hydrate phase. Recently, it was discovered that water-NH4F solutions can form clathrates consisting of solid solutions of water and NH4F, which have the same structure as the canonical clathrate hydrates. These doped phases were found to be able to incorporate strongly hydrophilic guests such as methanol. As the next step in testing the utility of these novel hydrates, we prepared NH4F-doped clathrates with simple CO2 and binary CO2 + methanol guest molecules and characterized these by powder X-ray diffraction (PXRD), Raman spectroscopy, and molecular dynamics (MD) simulations. From the PXRD analysis, it was confirmed that CO2 interacts more strongly with the NH4F-doped 5(12) cages than the 5(12) cages without dopants. The MD simulations supported the PXRD results by demonstrating a strong interaction between the O atom of CO2 and the dopant NH4+ in the small cages. The incorporation of methanol into the CO2 + methanol clathrates was confirmed by PXRD analysis. With low concentrations of methanol, this guest shows a preference for the 5(12) cages and may serve as a site blocker for the 5(12) cage that normally would be occupied by small molecules such as CH4 and N-2 in hydrate-based gas separation (HGBS) processes. Phase boundary conditions for hydrate stability in CO2-NH4F-CH3OH- water were obtained, and it was determined that a solution of 5 mol % NH4F and 2.2 mol % CH3OH is a reasonable choice for operating an HBGS process. The present findings provide insight into the potential of the NH4F-doped hydrate lattice, aided by quantities of catalytic methanol, for use in HBGS processes.

Automated summary

It was found that water-NH4F solutions can form clathrates consisting of solid solutions of water and NH4F, which have the same structure as the canonical clathrate hydrates, and can incorporate different guest molecules such as methanol. The research provides insight into the potential of NH4F-doped hydrate lattice, aided by catalytic methanol, for use in hydrate-based gas separation processes.