Insights into the structure of mixed CO₂/CH₄in gas hydrates

Local structure and distortions of mixed methane-carbon dioxide hydrates

“…Consistent with former experimental and computational results, the CH 4 molecule in both the MD and experimental distributions in Fig. 4a-f, m-r, respectively, orients with a spherical distribution in both the large and small cages 7,18 . Previously, the CO 2 molecule was shown to orient in an oblate shape parallel to the hexagonal face of the large hydrate cage, along the longer axis 7,10,24,35 .…”

“…A range of cage filling results have been predicted and observed for hydrates synthesized with CH 4 or CO 2 as the feed gas under varying pressure-temperature conditions and synthesis procedures. At 10 K, despite the larger size of the CO 2 molecule, hydrate with CO 2 as a guest has a smaller lattice parameter and CO 2 will preferentially fill the large cage over CH 4 3,7 . Local structure analysis of the guest-host RDFs in Fig.…”

“…Local structure analysis of the guest-host RDFs in Fig. 3 lead to an understanding of how the guest molecules in different compositions interact with the host and affect cage filling, formation, stability, and decomposition behavior 3,7,18 . RDFs corresponding to the CO 2 molecule center (the central C) for pure CO 2 and mixed CH 4 -CO 2 hydrates are plotted in the top row of Fig.…”

“…4a-f, m-r, respectively, orients with a spherical distribution in both the large and small cages 7,18 . Previously, the CO 2 molecule was shown to orient in an oblate shape parallel to the hexagonal face of the large hydrate cage, along the longer axis 7,10,24,35 . Neutron diffraction experiments showed this orientation in both pure CO 2 and mixed CH 4 -CO 2 hydrate, but MD simulations showed that this only held for distributions in the pure CO 2 hydrate.…”

“…This structure is made up of eight cages: two small dodecahedral and six large tetrakaidekahedral cages, which can occlude up to one guest molecule 3 . The average crystal structure of sI CH 4 and CO 2 hydrate has been well characterized with diffraction [6][7][8][9][10][11] , spectroscopy [12][13][14][15] , MD simulations [16][17][18][19][20][21][22][23] , and density functional theory calculations 11,[24][25][26] , which suggest a high degree of disorder within the crystal. This disorder is evident in the crystallographic model of sI CH 4 hydrate which requires four partially occupied proton positions for each H 2 O molecule and many positions to represent the unbonded, freely rotating CH 4 or partially constrained CO 2 molecules 27 .…”