NH₃ as simple clathrate-hydrate catalyst: Experiment and theory

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Clathrate hydrates (CH) are inclusion compounds formed by water cages trapping guest molecules.
• Understanding rapid CH formation mechanisms is crucial for various applications, including gas storage and separation.
• Ammonia (NH3) has been investigated for its potential role in catalyzing CH formation.

Purpose of the Study:

• To investigate the catalytic role of ammonia (NH3) in the rapid formation of clathrate hydrates (CH) using an all-vapor approach.
• To elucidate the molecular mechanisms behind NH3-induced CH formation and guest molecule interactions.
• To explore the displacement of NH3 guests by other molecules like CO2 and tetrahydrofuran.

Main Methods:

• Fourier-transform infrared (FTIR) spectroscopy was employed to analyze the structure and composition of NH3-CH.
• Ab initio molecular dynamics simulations were used to model the interactions between NH3 and water molecules at the atomic level.
• Experimental measurements were conducted on aerosols of NH3 s-II clathrate hydrate.

Main Results:

• Ammonia (NH3) rapidly penetrates and occupies the water network, creating defects (Bjerrum D-defects) that stabilize the hydrate structure.
• NH3 molecules form fluxional hydrogen bonds with host water molecules, significantly disturbing the hydrate network.
• These NH3-induced defects facilitate the sub-second formation of NH3 s-II gas hydrate at 160 K.
• FTIR spectra confirmed the presence of NH3 guests in both small and large cages, and their displacement by CO2 and tetrahydrofuran was observed.

Conclusions:

• Ammonia (NH3) acts as a potent catalyst for rapid clathrate hydrate (CH) formation by destabilizing the water lattice and facilitating guest inclusion.
• The study provides molecular-level insights into the mechanism of NH3-catalyzed CH formation and guest exchange.
• The findings highlight the potential of NH3 in developing efficient processes for gas hydrate synthesis and separation.