Enclathration of CHF3 and C2F6 molecules in gas hydrates for potential application in fluorinated gas (F-gas) separation

In this study, gas hydrate-based fluorinated gas (F-gas) separation is proposed as a novel method to capture F-gases. This study investigates the thermodynamic, structural, and cage filling characteristics of the gas hydrates formed by two representative F-gases (CHF3 and C2F6) in order to verify the feasibility of the F-gas separation using gas hydrate formation. The three-phase (gas hydrate (H) - liquid water (Lw) vapor (V)) equilibria of the pure CHF3 and C2F6 hydrates are measured in order to examine the hydrate formation conditions. The PXRD patterns reveal the structure of the CHF3 hydrate and the C2F6 hydrate as a cubic structure I (sI) and structure II (sII), respectively. The enclathration of CHF3 and C2F6 molecules in each pure CHF3 and C2F6 hydrate is confirmed through C-13 and F-19 NMR analyses. In-situ Raman measurements are used to monitor the growth process of pure CHF3 hydrates, and they reveal the CHF3 molecules trapped in the sI large (5(12)6(2)) cages as well as in the sI small (5(12)) cages. The computational study also demonstrates that CHF3 is encaged in both small (5(12)) and large (5(12)6(2)) cages of the sI hydrate, whereas C2F6 only occupies the large (5(12)6(4)) cages of the sII hydrate. (C) 2016 Elsevier B.V. All rights reserved.