Effect of water on methane adsorption on the kaolinite (0 0 1) surface based on molecular simulations

doi:10.1016/j.apsusc.2017.12.239

Applied Surface Science

Volume 439, 1 May 2018, Pages 792-800

https://doi.org/10.1016/j.apsusc.2017.12.239 Get rights and content

Highlights

•
Molecular simulation method was used to address kaolinite swelling.
•
Quantify CH₄ absorption for both dry and moist kaolinite.
•
Effect of water on CH₄ adsorption and kaolinite swelling were investigated.
•
Water molecules are preferably adsorbed on oxygen and hydrogen atoms in kaolinite.
•
Methane shows a tendency of being only adsorbed on oxygen.

Abstract

CH₄ adsorption isotherms of kaolinite with moisture contents ranging from 0 to 5 wt% water, the effects of water on maximum adsorption capacity, kaolinite swelling, and radial distribution function were modelled by the implementing combined Monte Carlo (MC) and molecular dynamics (MD) simulations at 293.15 K (20 °C) and a pressure range of 1–20 MPa. The simulation results showed that the absolute adsorption of CH₄ on both dry and moist kaolinite followed a Langmuir isotherm within the simulated pressure range, and both the adsorption capacity and the rate of CH₄ adsorption decreased with the water content increases. The adsorption isosteric heats of CH₄ on kaolinite decreased linearly with increasing water content, indicating that at higher water contents, the interaction energy between the CH₄ and kaolinite was weaker. The interaction between kaolinite and water dominates and was the main contributing factor to kaolinite clay swelling. Water molecules were preferentially adsorbed onto oxygen and hydrogen atoms in kaolinite, while methane showed a tendency to be adsorbed only onto oxygen. The simulation results of our study provide the quantitative analysis of effect of water on CH₄ adsorption capacity, adsorption rate, and interaction energy from a microscopic perspective. We hope that our study will contribute to the development of strategies for the further exploration of coal bed methane and shale gas.

Graphical abstract

Introduction

Clay minerals have large surface areas (approximately 800 m² g⁻¹) and micropore to mesopore structures that can significantly affect the adsorption properties of porous media such as shale and coal [1], [2]. Kaolinite is one of the most abundant components in clay minerals [3], [4], [5], and understanding the interaction between kaolinite and methane molecules is important for research in the fields of shale gas and coal bed methane.

Some coal bed methane and shale gas reservoirs are water saturated [6], [7], and the clay minerals that have similar silicoaluminate crystallographic layers with Al single bond O octahedra and SiO tetrahedra are hydrophilic [8], [9], [10]. Water molecules can be adsorbed onto the surface of the clay mineral without difficulty, which can decrease the total methane sorption of clay minerals [11]. Hence, preloaded water can substantially decrease the total amount of methane adsorbed onto clay-rich rocks [12]. Ross et al. [13] found that the adsorbed capacities of water-saturated montmorillonite and illite were lower than under dry conditions. Ross and Bustin [12] found that at low pressures (6 MPa), the CH₄ adsorption capacities of illite and montmorillonite were lower than that of kaolinite on a moisture-equilibrated basis but were significantly higher than that of kaolinite under dry conditions. Moreover, a few clay minerals, e.g., montmorillonite clay, are able to further enhance the interaction between clay molecules and methane molecules because of the cation-exchange capacity of montmorillonite clay [14].

Some molecular simulations have been carried out on the adsorption of gas in dry and moist clays. Billemont et al. [15] used grand canonical Monte Carlo (GCMC) simulations to consider the influence of water on methane sorption in porous carbons and observed that water molecules have a higher free energy barrier than methane molecules, which cannot displace the water molecules, and the preloaded water molecules notably decreased the adsorption capacity of methane. Jin et al. [16] also utilized GCMC simulations to research the influence of water on methane sorption in montmorillonite clay. However, to our knowledge, no computational and theoretical research has been carried out on the influence of water content on methane sorption in other clay minerals such as kaolinite and illite. Therefore, in this paper, we used the GCMC and molecular dynamics (MD) methods to research the effect of water on methane adsorption in kaolinite over a pressure range of 1–20 MPa and with pre-adsorbed water contents of 0–5 wt% simulated at 293.15 K. Molecular simulations can reveal the absolute adsorption isotherms, interaction energy, isosteric heat of adsorption, kaolinite swelling, adsorbed phase density, and radial distribution functions between the surface of kaolinite and CH₄. The objectives of this work were to better understand CH₄/water/kaolinite clay interactions and to shed light on the influence of water on the adsorption capacity and kaolinite swelling of CH₄ on kaolinite surfaces at the atomic level. The findings of this study are also expected to shed light on the details of coal bed methane and shale gas adsorption.

Section snippets

Models

We use methane to represent coal bed gas and shale gas because methane is the main component of coal bed gas and shale gas. Here, we mainly consider the case of methane in hydrated kaolinite. Methane is represented by an all-atom model [17], [18]; the C single bond H bond length and the CH bond angle were calculated to be 0.109 nm and 109°28′, respectively, as shown in Fig. 1. Water is represented by a simple point charge model [19]; the HO bond length and the HOH bond angle were calculated to be 0.079 nm

Modeling

To validate the simulated data and to evaluate the rationality of the model, the simulated results were compared with experimental adsorption data from the literature [39]. Our calculated value of the lattice vector a, b, c and the lattice angle α, β, γ of the kaolinite structure are in good agreement with the data from the experiment and calculation in Table 2. The lattice vector a, b of the 4 × 2 × 2 kaolinite super cell model are 2.059 nm and 1.787 nm, which are very close to the results

Conclusion

Molecular simulations, which include the MD and MC methods, were used to study the CH₄ adsorption on dry ad moist kaolinite at the pressure range of 1–20 MPa and a temperature of 293.15 K. Our results illustrate that the absolute adsorption of CH₄ onto kaolinite is reduced in the presence of water and decreases with increasing water content. The absolute adsorption of CH₄ on both dry and moist kaolinite followed the Langmuir isotherm within the simulated pressure range. From the Langmuir

Acknowledgements

This research was supported financially by the National Natural Science Foundation of China (51174141 and 50974093), the Postgraduate Innovation Fund of Shanxi Province (800104-02100675), and the Taiyuan University of Technology Postgraduate Technology Innovation Fund (8004-02020061). The use of the Materials Studio software package, which is supported by the Key Laboratory of Coal Science and Technology of the Ministry of Education and Shanxi Province, is gratefully acknowledged.

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Full Length ArticleEffect of water on methane adsorption on the kaolinite (0 0 1) surface based on molecular simulations

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Models

Modeling

Conclusion

Acknowledgements

Fuel

Int. J. Coal Geol.

Int. J. Rock Mech. Min. Sci.

Mar. Pet. Geol.

Fluid Phase Equilib.

Chem. Phys. Lett.

Appl. Clay Sci.

J Colloid Interface Sci

J. Nat. Gas Sci. Eng.

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

Chem. Phys. Lett.

Appl. Geochem.

Energy

Fuel

J. Unconvention. Oil Gas Resour.

Org. Geochem.

Polymer

J. Mol. Catal. A: Chem.

Appl. Surf. Sci.

Determination of the melting temperature of kaolinite by means of the Z-method

Am. Mineral.

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Clay Miner.

Kaolinite properties, structure and influence of metal retention on pH

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Full Length Article
Effect of water on methane adsorption on the kaolinite (0 0 1) surface based on molecular simulations