Understanding effect of structure and stability on transformation of CH4 hydrate to CO2 hydrate

doi:10.1016/j.cplett.2016.02.004

Chemical Physics Letters

Volume 648, 16 March 2016, Pages 75-80

https://doi.org/10.1016/j.cplett.2016.02.004 Get rights and content

Highlights

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CH₄ molecules are likely to reside in 5¹² cages rather than 5¹²6² cages.
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CO₂ molecules are likely to reside in 5¹²6² cages rather than 5¹² cages.
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CO₂ has smaller interaction energy than CH₄ in the same cage.
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The entropy dominates the transformation of CH₄ hydrate into CO₂ hydrate.
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In the same cage CO₂ molecules move much slowly than CH₄ molecules.

Abstract

Understanding the transformation process of CH₄ hydrate to CO₂ hydrate is crucial to develop the CH₄ single bond CO₂ replacement technique for CH₄ production and CO₂ sequestration. Ab initio calculations show that the transformation will slightly distort the host lattice and decrease the binding strength of guest molecules, but it is a thermodynamically spontaneous process dominated by the entropic contribution. Moreover, ab initio molecular dynamics simulations suggest that the dynamics of the host lattice is independent on the guest molecules, while CO₂ in hydrate exhibits slower translational and rotational motion than CH₄ in hydrate.

Graphical abstract

Introduction

Clathrate hydrates are nonstoichiometric crystalline compounds in which small guest molecules are encaged by hydrogen-bonded water cages [1], [2], [3]. Depending on the size and shape of guest molecules, three common crystalline structures of clathrate hydrates have been uncovered and addressed as structure-I (sI, cubic, space group $P m \bar{3} n$ ), structure-II (sII, cubic, $F d \bar{3} m$ ) and structure-H (sH, hexagonal, P6/mmm) [4]. Clathrate hydrates typically form at the ambient temperature (less than 300 K) and moderate pressures (more than 0.6 MPa). They are abundant in Earth's permafrost and subsea sediments [5], [6], and also have been found in other planetary bodies [7], [8]. The most conservative estimates suggest that the amount of carbon in hydrates is equivalent to twice that of other fossil fuels combined [9], [10], and the energy density in hydrates is approximately as the same as that of a compressed gas, that is, one volume of hydrate will dissociate to 180 volumes of gas. Therefore, clathrate hydrates have been evaluated both technically and economically as a potential energy resource and attract increasing worldwide interest [11], [12], [13].

The replacement of CH₄ with CO₂ in clathrate hydrates has been regarded as a promising method for natural gas production [14], [15], [16], [17], [18], [19], which also provides a way to storage CO₂ for a long term [20], [21]. By injecting CO₂ to CH₄ hydrate deposit, CH₄ release from hydrates can be facilitated while the formation of CO₂ hydrates can be largely increased. To analyze this complex replacement phenomenon, it is necessary to understand the stability and physical-chemical properties of CH₄ and CO₂ hydrates. Actually, CH₄ and CO₂ both are likely to form sI structure [4], [22], which consists of 46 water molecules in one unit cell, forming two dodecahedron (12 pentagonal faces; 5¹²) and six tetradecahedron (12 pentagonal and 2 hexagonal faces; 5¹²6²) cages. In addition, researchers have verified that CH₄ single bond CO₂ mixed hydrates can be formed during the replacement process [23], [24], but little attention is paid to the structure and stability of CH₄CO₂ mixed hydrates. Hence, investigating the properties of CH₄CO₂ mixed hydrates could benefit CH₄ production as well as CO₂ sequestration.

Computational simulation technique has been proved to be one of the most effective methods to explore the microscopic information of the hydrates [25], [26], [27]. By considering the electronic interactions through van der Waals density functional theory, Román-Pérez et al. [28] have suggested that both CO₂ and CH₄ have larger interaction energies in 5¹²6² cage than that in 5¹² cage, and CH₄ always has a larger value than CO₂ in the same cage. Whereas, Srivastava and Sastry [29] tested the impact of different functionals and basis sets on interaction energy and demonstrated that CO₂ is energetically favorable to reside in the 5¹² cages. Yi and coworkers [21] have recently investigated the growth mechanism of CH₄ single bond CO₂ mixed hydrate using molecular dynamics simulations and found that CO₂ molecules are preferably encaged into 5¹²6² cages. Moreover, through classical molecular dynamics simulations, Geng et al. [23] have pointed out that CH₄CO₂ mixed hydrate is more stable than pure CH₄ hydrate or CO₂ hydrate. Hence, in this work, we firstly performed ab initio calculations to optimize CH₄ hydrate, CH₄ single bond CO₂ mixed hydrate and CO₂ hydrate. Structural and energetic characteristics of hydrates were compared to explore the possibility of transformation of CH₄ hydrate to CH₄CO₂ mixed hydrate and CO₂ hydrate. In addition, we carried out ab initio molecular dynamics simulations to observe the structural and thermal fluctuations of hydrates in a more realistic environment. The translational and rotational motions of molecules in hydrates were analyzed to evaluate the kinetic stability of hydrates. Through this work, we could gain a better insight of the transformation of CH₄ hydrate to CO₂ hydrate, which would helpful to understand the CH₄ single bond CO₂ replacement mechanism.

Section snippets

Computational details

The crystal structure of sI clathrate hydrate was taken from Lenz's data [30]. In pure CH₄ hydrate and pure CO₂ hydrate, the 5¹² and 5¹²6² cages were fully filled with CH₄ and CO₂ molecules, respectively. For CH₄ single bond CO₂ mixed hydrate, two structures were considered: 5¹² cages were fully filled with CO₂ molecules and 5¹²6² cages were fully filled with CH₄ molecules, denoting as Mixed-I; 5¹² cages were fully filled with CH₄ molecules and 5¹²6² cages were fully filled with CO₂ molecules, denoting as

Optimized structure and thermodynamic stability

Figure 1 shows the optimized structures of CH₄ hydrate, Mixed-I hydrate, Mixed-II hydrate and CO₂ hydrate. The host lattice of four studied hydrates is nearly same, and CH₄ and CO₂ molecules are basically resided in the center of the clathrate cages. Table 1 shows that the averaged hydrogen bond length of CH₄ hydrate is ∼2.699 Å, which matches Martos-Villa's results [36]. When CO₂ molecules are encaged by 5¹² or/and 5¹²6² cages, the corresponding hydrates have a slightly increased hydrogen bond

Conclusions

Ab initio calculations and simulations were performed to investigate structure and stability of CH₄ hydrate, CH₄ single bond CO₂ mixed hydrate and CO₂ hydrate, and their relationship to the transformation of CH₄ hydrate into CO₂ hydrate. The results show that CO₂ molecules encaged by 5¹² cages can induce the distortion of host lattice, while 5¹²6² cages are nicely suitable to accommodate CO₂ molecules. In both 5¹² and 5¹²6² cages, CO₂ molecules always have smaller interaction energies than CH₄ molecules

Acknowledgements

This work was supported by grants from Natural Science Foundation of Shandong Province (BS2015NJ007), National Natural Science Foundation of China (11504133 and 1137412), and National Basic Research Program of China (2015CB251200).

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Understanding effect of structure and stability on transformation of CH4 hydrate to CO2 hydrate

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Computational details

Optimized structure and thermodynamic stability

Conclusions

Acknowledgements

Planet. Space Sci.

Fuel Process. Technol.

J. Nat. Gas Sci. Eng.

J. Nat. Gas Sci. Eng.

Energy Convers. Manage.

Energy Convers. Manage.

Fluid Phase Equilib.

J. Energy Chem.

Comput. Theor. Chem.

Energy Convers. Manage.

J. Mol. Graph. Model.

Chem. Phys. Lett.

J. Chem. Phys.

J. Chem. Phys.

Phys. Rev. Lett.

Nature

Energies

Rev. Geophys.

Nature

Understanding effect of structure and stability on transformation of CH₄ hydrate to CO₂ hydrate