Abstract
Owing to a stable and porous cage structure, natural gas hydrates can store abundant methane and serve as a potentially natural gas resource. However, the microscopic mechanism of how hydrate crystalline grows has not been fully explored, especially for the structure containing different guest molecules. Hence, we adopt density functional theory (DFT) to investigate the fusion process of structure I hydrates with CH4/C2H6 guest molecules from mono-cages to triple-cages. We find that the volume of guest molecules affects the stabilities of large (51262, L) and small (512, s) cages, which are prone to capture C2H6 and CH4, respectively. Mixed double cages (small cage and large cage) with the mixed guest molecules have the highest stability and fusion energy. The triangular triple cages exhibit superior stability because of the three shared faces, and the triangular mixed triple cages (large-small-large) structure with the mixed guest molecules shows the highest stability and fusion energy in the triple-cage fusion process. These results can provide theoretical insights into the growth mechanism of hydrates with other mono/mixed guest molecules for further development and application of these substances.
Highlights
The large number of holes in the structure is conducive to the storage of guest molecules, such as hydrogen and carbon dioxide [15,16,17], which provides an effective pathway for exhaust gas capture
The structural configurations of small and large cages with CH4 and C2 H6 are displayed in Figure 1, in which the guest molecules occupy the center of dodecahedron and tetrakaidekahedral water cage structures after optimization
The guest molecules play an important role in supporting the host water cages
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The most common type of hydrates is structure I (sI), which concludes two small cages (owing to 12 pentagonal faces, denoted by 512 , s) and six large cages (owing to 12 pentagonal faces and 2 hexagonal, denoted by 512 62 , L) per unit cell formed at a low temperature and high pressure [4,5]. Experimental data shows that sI will be formed by the existence of C2 H6 while sII can only be discovered when the concentration of C2 H6 lies in between 2% and 22% [21], which indicates that C2 H6 is essential for the formation of sI hydrates This lacks micro-mechanism study on the stability of sI hydrates with. Pure CH4 , pure C2 H6 , and mixed CH4 /C2 H6 were selected as guest molecules to explore the fusion process from mono-cages to tri-cages in sI hydrates. The formation micro-mechanism of hydrates with mixed CH4 /C2 H6 guest molecules is investigated as well, which could provide theoretical guidance for actual hydrate mining
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