Physical Properties and Characterization of the Binary Clathrate Hydrate with Methane+1,1,1,3,3-Pentafluoropropane (HFC-245fa) + Water

M Kodera and T Matsueda and RV Belosludov and RK Zhdanov and VR Belosludov and S Takeya and S Alavi and R Ohmura, JOURNAL OF PHYSICAL CHEMISTRY C, 124, 20736-20745 (2020).

DOI: 10.1021/acs.jpcc.0c05565

This paper reports the thermodynamic and crystallographic properties of the binary clathrate hydrate with methane + 1,1,1,3,3-pentafluoropropane (HFC-245fa) by phase equilibrium measurements, powder X-ray diffraction (PXRD) measurements, and first-principles and thermodynamic calculations. The four-phase (methane gas + water liquid + HFC-245fa liquid + hydrate) equilibrium conditions are lower in pressure by approximately 2-3 MPa than those of the methane + trans-1,3,3,3-tetrafluoropropene HFO-1234ze(E) hydrates at each temperature from 274.5 to 282.17 K, in spite of the similar molecular structure of HFC-245fa and HFO-1234ze(E). The crystallographic structure of the hydrate with methane + HFC-245fa was identified to be structure II by the PXRD measurement; although because of its molecular length, HFC-245fa would be expected to be encapsulated into a 5(12)6(8) cage of structure H hydrate. First-principles calculations revealed that these thermodynamic and crystallographic phenomena are the result of two physical factors. First, the total HFC-245fa-water interactions in the cages are stronger than the HFO-1234ze(E)-water interactions due to the presence of more fluorine atoms at the two edge groups in the HFC-245fa molecule and the resulting higher dipole moment of HFC-245fa, and second, the HFC-245fa-HFC-245fa interaction is more greatly enhanced by the presence of methane compared to the HFO-1234ze(E)-HFO-1234ze(E) interactions.

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