Stability of CH₄, CO₂, and H₂S in two-dimensional clathrate hydrates

Highlights

• The thermodynamic stability of pure hydrates is H₂S > CH₄ > CO₂.

• The dynamic stability of pure hydrates follows the order of CH₄ > H₂S > CO₂.

• The bond polarity of guests plays a key role in the hydrate stability.

• The thermodynamic stability of mixed hydrates is in between pure hydrates.

Abstract

The stability of CH₄, CO₂, and H₂S in four two-dimensional hydrates, denoted as 2D-I, 2D-II, 2D-III, and 2D-IV, was investigated by the first-principle calculations and molecular dynamics simulations. The result shows that two-dimensional CH₄ hydrates are thermodynamically and dynamically stable. CO₂ can only form the 2D-I and 2D-II hydrates, while these structures are dynamically unstable. Four two-dimensional H₂S hydrates are thermodynamically stable, but only the 2D-I structure is dynamically stable. Further, simulation result suggests that the stability of the two-dimensional hydrate can be increased at low temperatures or in the confined environments (for example, nano-slits). As for the 2D-I binary hydrate, its thermodynamic stability is in between two corresponding pure hydrates, and only the CH₄-H₂S binary hydrate is dynamically stable. The CH₄-CO₂-H₂S ternary hydrate is dynamically unstable because of the overflow of CO₂.

Introduction

Clathrate hydrates are non-stoichiometric crystalline inclusion compounds that form when gas molecules encounter water molecules under high-pressure and low-temperature conditions [1], [2]. Hydrates have attracted significant industrial and scientific interest such as the clean energy source [3], gas separation [4], CO₂ sequestration [5], equipment blocking [6], seawater desalination [7], etc. There are three common hydrate structures: structure I (sI), structure II (sII), and structure H (sH) [8], [9], [10]. The unit cell of the sI hydrate contains 46 water molecules forming two different kinds of cages, two dodecahedral cages (denoted by 5¹², comprising 12 pentagonal faces), and six tetrakaidecahedral cages (denoted by 5¹²6², comprising 12 pentagonal and 2 hexagonal faces). In the unit cell of the sII hydrate, 136 water molecules form sixteen 5¹² cages and eight hexacaidecahedral cages (denoted by 5¹²6⁴, comprising 12 pentagonal and 4 hexagonal faces). The unit cell of the sH hydrate includes 34 water molecules, which is composed of three 5¹² cages, two irregular-dodecahedral cages (denoted by 4³5⁶6³, comprising 3 square, 6 pentagonal, and 3 hexagonal faces), and one icosahedral cage (denoted by 5¹²6⁸, comprising 12 pentagonal and 8 hexagonal faces).

In addition, other hydrate structures have been proposed for the energy-demanding applications (e.g. gas storage and transportation). Jeffrey et al labeled the sIII–sVII hydrates that have been experimentally verified, with the exception of sIV and sV [11], [12], [13]. Bai et al carried out the classical molecular dynamics simulations of spontaneous formation of guest-free monolayer hydrate within hydrophobic nano-slit at low temperature [14]. Qian et al predicted two novel hydrogen hydrates (Ih-C₀ and C₃) with ab initio variable-composition evolutionary simulations [15]. Nguyen and Molinero investigated the thermodynamic stability of TS and HS-I hydrate polymorphs by using molecular dynamics simulations with the coarse-grained models [16]. Zhao et al reported the classical molecular dynamics simulation evidence for a new family of two-dimensional hydrates with C₂H₆, C₂H₄, C₃H₄, CO₂, and H₂ [17]. Recently, we have investigated the structure and stability of four two-dimensional hydrogen hydrates via the density functional theory [18]. Investigation of new hydrate structures can enrich the hydrate family, but also can enhance our understanding of the hydrate formation in the special environments such as the solid surface, the nano-porous medium, and the complex temperature-pressure condition. However, there are still scarce reports that focus on the possibility of new hydrate structures.

Many naturally occurring reservoir fluids contain CH₄ together with significant concentration of CO₂ and H₂S [19], [20], and it is necessary to know the formation condition, composition, and structure of mixed hydrates containing these gas molecules. Actually, CH₄, CO₂ and H₂S can form the sI hydrate by itself at the ambient temperature, but also can form the mixed sI hydrate. CO₂ is potentially incorporated in the large 5¹²6² cages, whereas CH₄ and H₂S tend to occupy the small 5¹² cages [21]. However, in the nanoscale pore-throat structures of the reservoir, the hydrogen-bonding network of water molecules is considerably disrupted, and we thus wonder whether there is any other type of the hydrate. In order to address this question, we performed first principle calculations and molecular dynamics simulations of the two-dimensional hydrates for CH₄, CO₂, and H₂S. The corresponding hydrate cage is denoted as m^xn^y, where x and y represent the number of m- and n-membered faces on the cages [22]. The structural characteristics and the stability of the hydrates have been analyzed, which may provide a new strategy for simultaneous removal of CO₂ and H₂S from natural gas.