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 5¹² cages than the 5¹² 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 5¹² cages and may serve as a site blocker for the 5¹² cage that normally would be occupied by small molecules such as CH₄ and N₂ 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.