Gas separation method using tetra-n-butyl ammonium bromide semi-clathrate hydrate

We used tetra-n-butyl ammonium bromide semi-clathrate hydrate, hereafter TBAB hydrate, as a tool for separating gases from binary mixed gas systems of methane+ethane, methane+propane, methane+hydrogen sulfide, methane+nitrogen, and carbon dioxide+hydrogen sulfide by growing the hydrate from 10 wt% TBAB aqueous solutions in a pressure vessel. TBAB hydrate has empty dodecahedral cages in the pure system. We found that small molecules such as methane, nitrogen, and hydrogen sulfide were selectively encaged during TBAB hydrate formation, probably because the dodecahedral cages are too small to incorporate large molecules such as ethane and propane. Furthermore, hydrogen sulfide was more readily encaged compared to the other small molecules; we argue that this was due to the very high solubility of hydrogen sulfide in water. We propose that gases with small molecular size and high solubility in water can be effectively separated using TBAB semi-clathrate hydrate.