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

Abstract

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 8CH3F@46H2O), and the CH2F2 hydrate has two stable phases (the large-cage fully occupied phase 6CH2F2@46H2O and the large- and small-cage fully occupied phase 8CH2F2@46H2O). The CHF3 hydrate has one stable phase (the large-cage fully occupied phase 6CHF3@46H2O), and the CF4 hydrate has one stable phase (the large-cage fully occupied phase 6CF4@46H2O). 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 13C 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.

Thermodynamic and kinetic study of fluorinated gas hydrates for water purification

“…The molecular size of HFC125a is larger than that of HFC134a molecule due to the increased number of F atoms in the guest molecule. This result is attributed to the larger Raman shift of the CC and CH stretching vibrations of the HFC125a molecule, which is also confirmed by Uchida et al [ 31 ] and Li et al [ 32 ] For both HFC134a and HFC125a hydrates in pure water and NaCl solutions, the intensities of the bands increased during the growth of the hydrates (Figures 7 and 8). In addition, there was no change in the positions of the peaks with increasing NaCl concentration (Figure 9).…”

Proposing Crystallographic Theory to Deparameterize Hydrate Phase Description: Physical Insights and Validation

“…A few studies have leveraged quantum mechanical calculations to evaluate hydrate phase properties , and gain insights into their stability. , The observations obtained from such simulations are then used to reduce the parametric dependencies of the phase equilibrium estimates. Others choose to directly use these quantum mechanical calculations and directly simulate the phase equilibrium conditions of different hydrate structures. – However, these methodologies remain convoluted and highly computationally expensive, making the regression-based models more suitable for use.…”