Phase Diagrams and Spectral Characteristics of Hydrofluorocarbon Hydrates: Insights from First-Principles Thermodynamics

Understanding the phase equilibrium conditions of hydrofluorocarbon (HFC) hydrates is significant to reduce greenhouse gas emissions and synthetic hydrates in experiments. Herein, we construct the phase diagrams of hydrates with HFCs (CH3F, CH2F2, CHF3, and CF4) as guest molecules through density functional theory and compare them with that of CH4 hydrate. At temperature and pressure that can be reproduced experimentally, the CH3F hydrate has one stable phase (the fully occupied phase 8CH(3)F@46H(2)O), and the CH2F2 hydrate has two stable phases (the large-cage fully occupied phase 6CH(2)F(2)@ 46H(2)O and the large- and small-cage fully occupied phase 8CH(2)F(2)@46H(2)O). The CHF3 hydrate has one stable phase (the large-cage fully occupied phase 6CHF(3)@46H(2)O), and the CF4 hydrate has one stable phase (the large-cage fully occupied phase 6CF(4)@46H(2)O). The Raman spectra and NMR chemical shifts of guest molecules are also briefly analyzed. With the increasing number of F atoms in guest molecules, the C-H symmetric stretching frequency of guest molecules in cages shows a blueshift, and the C-13 NMR chemical shifts of HFCs molecules in cages shift to the positive direction. Our theoretical research shows the optimal equilibrium conditions of HFCs hydrates as refrigerants and provides theoretical guidance for the formation of HFCs hydrates in experiments.