Guest Gas Enclathration in Semiclathrates of Tetra-n-butylammonium Bromide: Stability Condition and Spectroscopic Analysis

Abstract

In this study, guest gas enclathration behavior in semiclathrates of tetra-n-butylammonium bromide (TBAB) was closely investigated through phase equilibrium measurement and spectroscopic analysis. The three-phase equilibria of semiclathrate (H), liquid water (L W), and vapor (V) for the ternary CH 4 + TBAB + water and CO 2 + TBAB + water mixtures with various TBAB concentrations were experimentally measured to determine the stability conditions of the double TBAB semiclathrates. Equilibrium dissociation temperatures for pure TBAB semiclathrate were also measured at the same concentrations under atmospheric conditions. The dissociation temperature and dissociation enthalpy of pure TBAB semiclathrate were confirmed by differential scanning calorimetry. The experimental results showed that the double CH 4 (or CO 2) + TBAB semiclathrates yielded greatly enhanced thermal stability when compared with pure CH 4 (or CO 2) hydrate. The highest stabilization effect was observed at the stoichiometric concentration of pure TBAB semiclathrate, which is 3.7 mol %. From the NMR and Raman spectroscopic studies, it was found that the guest gases (CH 4 and CO 2) were enclathrated in the double semiclathrates. In particular, from the cage-dependent 13C NMR chemical shift, it was confirmed that CH 4 molecules were captured in the 5 12 cages of the double semiclathrates.