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

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journal contribution
posted on 15.09.2020, 12:04 by Masamichi Kodera, Tomoyuki Matsueda, Rodion V. Belosludov, Ravil K. Zhdanov, Vladimir R. Belosludov, Satoshi Takeya, Saman Alavi, Ryo Ohmura
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 51268 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.