NH₃ as simple clathrate-hydrate catalyst: Experiment and theory

The catalytic action of NH₃ within the all-vapor approach for instant clathrate hydrate (CH) formation is studied using both FTIR spectroscopy and ab initio molecular dynamics simulations. A unique property of NH₃, namely, the rapid abundant penetration and occupation of the water network, creates defects, particularly Bjerrum D-defects, in the hydrate frame that are generally stabilized by guest NH₃ molecules in the cages. Furthermore, insertion of NH₃ seriously disturbs the hydrate network where the guest NH₃ molecules also make fluxional H-bonds with the host water molecules. These defects strongly facilitate a sub-second formation of the simple NH₃ s-II gas hydrate at 160 K. FTIR spectra of aerosols of the NH₃ s-II CH have been measured, and the displacement of both small and large cage NH₃ guests by CO₂ and tetrahydrofuran is examined.