Phase equilibria and guest gas occupancy characteristics of H2-DIOX sII hydrates based on calorimetric and Raman analysis

Hydrogen (H₂) as the most abundant element offers a clean energy solution for a sustainable future. Thermodynamic hydrate promoters can enhance hydrate-based H₂ storage under mild pressure conditions. 1,3-dioxolane (DIOX) as a low-toxicity promoter has attracted attention for its potential to improve H₂ hydrate kinetics. However, the phase equilibria of H₂-DIOX in the presence of DIOX and its thermodynamic promotion mechanism are not fully elaborated and warrant thorough investigation. In this study, the phase equilibria of H₂-DIOX hydrates were measured for DIOX concentrations (C_DIOX) ranging from 2.0 mol% to 5.56 mol%. The equilibrium temperature of H₂-DIOX hydrates shifted rightward by 2.3 K at 15.0 MPa for 5.56 mol% DIOX compared to 2.0 mol% DIOX. The measured thermodynamic data were validated by fitting the H₂-DIOX hydrate phase equilibira using the Clausius–Clapeyron equation. The cage occupancy of H₂ and DIOX in H₂-DIOX sII hydrates was revealed through Raman spectroscopy and DSC thermal analysis. Two types of hydrates (DIOX and H₂-DIOX) were observed for all C_DIOX. Single H₂ molecules were enclathrated in the 5¹² cages of H₂-DIOX hydrates and increasing C_DIOX effectively enhanced DIOX molecules enclathration in the 5¹²6⁴ cages but had limited effect on the H₂ molecules in the 5¹² cages. The findings of this study provide fundametnal thermodynamic data and cage occupancy charateristics for H₂-DIOX sII hydrates below 15.0 MPa. The results provide guidance on the optimal thermodynamic promoter concentrations for future large-scale hydrate-based H₂ storage application.