Article pubs.acs.org/JPCC
Gas Hydrate in Crystalline-Swelled Clay: The Effect of Pore Dimension on Hydrate Formation and Phase Equilibria Daeok Kim,† Yun-Ho Ahn,† Se-Joon Kim,‡ Joo Yong Lee,‡ Jeahyoung Lee,‡ Young-ju Seo,*,‡ and Huen Lee*,† †
Department of Chemical and Biomolecular Engineering (BK21+ program), KAIST, 373-1 Guseong-dong, Yuseong-gu, Daejeon, Republic of Korea ‡ Korea Institute of Geoscience and Mineral Resources (KIGAM), 124, Gwahak-ro, Yuseong-gu, Daejeon, Republic of Korea S Supporting Information *
ABSTRACT: Understanding of gas hydrates, for example, phase equilibria, in reservoir sediments is one of the important issues in the fields of energy production and CO 2 sequestration via CH4 hydrate exploitation and CO2 hydrate formation. The composition of water in clay can change their physical properties, which influences the state in which gas hydrates exist in clays. We investigated gas hydrates in crystalline-swelled Na-montmorillonite (CS-mon) having water content of 0.8 nm. Accordingly, structuring a complete hydrogenbonded water cage in the interlayer appears to be almost physically impossible, as shown in Figure 2. For verification, CS-mon samples containing CH4 hydrate, CO2 hydrate, and hexagonal ice were prepared, and their structural changes were observed by LT-XRD at temperatures elevated from 93 to 203 K, as presented in Figure 3. If a solid water structure formed in the interlayer, the interlayer distance would change according to the water structure and vice versa. This is because the density of water differs depending on its structure: empty sI gas hydrate without a guest molecule (0.79 g/mL), hexagonal ice (0.91 g/mL), and water (1.0 g/mL). In addition, the volume of the interlayer space would change with the transformation of the solid water structure, which should be reflected in the change of interlayer spacing. From Figure 3 we confirm that the CH4 and CO2 hydrates and hexagonal ice peaks disappeared at around 153, 173, and 193 K, respectively. During the structural transition of water there was no shift of the Na-mon (001) peak, indicating that the interlayer spacing was not influenced by the water structure. We therefore conclude that the water structure detected by XRD was not inside but rather outside of the interlayer. In this stage, we are interested in the sites where gas hydrates form in CS-mon. The pore analysis of Na-mon in Figure 4 reveals that the clay contains pores whose size ranges from 0.6 to a few hundred nanometers. The micropores whose size span from 0.7 to 1.8 nm are observed but their pore volume was 7.8 vol % (0.003 cm3 g−1) of total pore volume (0.041 cm3 g−1). Considering the fact that Ar cannot be intercalated in clay interlayer due to weak affinity between Ar molecules and the interlayer, the inside of the clay interlayer cannot be explored by means of Ar gas, which is confirmed from the surface area measured by Ar adsorption. Na-mon reveals only 9.7 m2 g−1 of surface area in microporous region. Therefore, the observed micropores should be attributed to the voids made of the irregular stacking of clay flakes, where Ar can be adsorbed. Excluding the micropores (50 nm) are observed in Na-mon, wherein adsorbed water can transform to hexagonal ice or gas hydrate and their properties are influenced by porosity. After identifying gas hydrates formed in noninterlamellar voids of Na-mon, we performed phase equilibrium measurements of CO2 and CH4 hydrates in CSmon. Contrary to the bulk water system, the influence of the surrounding environment on the properties of the gas hydrate becomes more significant in the confined space. Studies of gas hydrates in porous media have revealed a change of the hydrate phase equilibria behavior, which is ascribed to an inhibition effect by a capillary effect and a decrease in water activity.17−20,22,23 Figure 5 presents the P−T trace curves of CO2 and CH4 hydrates in CS-mon. During the measurements, two noticeable phenomena that cannot occur in bulk water were observed: (1) The CO2 pressure must recover the initial value after a cycle of 22150
DOI: 10.1021/acs.jpcc.5b03229 J. Phys. Chem. C 2015, 119, 22148−22153
Article
The Journal of Physical Chemistry C
Figure 4. Pore size distribution of Na-motmorillonite in (a) micoporous region and (b) in meso/macroporous region with cumulative pore volume.
Figure 5. Pressure and temperature trace curve of (a) CO2 hydrate and (b) CH4 hydrate in crystalline-swelled Na-montmorillonite.
cooling and heating in the P−T trace curve, whereas that in CSmon shows a noticeable discrepancy between the initial and final pressures. (2) The appearance of two distinct inhibited and promoted hydrate-equilibrium points. These two phenomena indicate that CO2 and CH4, respectively, exist in three and two different states in CS clays sediment: CO2 (dissolution in interlayer, inhibited, and promoted gas hydrate) and CH4 (inhibited and promoted gas hydrate). In this paper inhibited and promoted phases indicate the gas hydrate phase with higher and lower equilibrium pressure than that of bulk gas hydrate at the same temperature, respectively. In the following we discuss how CO2 and CH4 behave in the water-swelled interlayer of CS-mon in relation, with the appearance of a CO2 pressure gap in the P−T trace. Contrary to CH4, a relatively larger amount of CO2 can be stored in the water-swelled clay interlayer, causing the unrecovered pressure in the P−T cycle in Figure 5a, which is in accordance with the molecular dynamic simulation result by Jin et al.21 showing higher CO2 storage amount than CH4 in clay interlayers containing water. In addition, this unrecovered CO2 pressure was also documented in our previous reports on phase equilibria of gas hydrates in interlayered graphene oxide and metal organic frameworks due to the storage of CO2 in waterfilled spaces of less than a few nanometers; this is not a result of metastability and slow kinetics of gas dissolution.22,23 The interesting thing is that CO2 can be stored in the water-filled interlayer without gas hydrate formation. The CO2 adsorption result showed that CS-mon can store a considerable amount of CO2 even at 283 K without CO2 hydrate formation, and the storage amount increased as a function of pressure in Figure S1.
Furthermore, the stored CO2 seems to be stable judging from the fact that CO2 is not emitted after end of P−T trace measurement, even after temperature increases to 325 K in Figure S2. The unrecovery of CO2 pressure at the high temperature means that CO2 is strongly stabilized in the water swelled interlayer of clay instead of dissolution in liquid water; otherwise, the CO2 pressure gap should be removed at high temperature. This can be explained by the strong interaction of CO2 with the surface of clay. Botan et al. revealed that CO2 molecules stay close to the clay surface and an oxygen of CO2 molecule is located on the center of hexagonal cavity composed of oxygens in clay layer, and the presence of bulky CO2 molecules in swelled interlayer retards the dynamics of all mobile species.7 It is noteworthy that the appearance of a pressure gap in the P−T trace of the gas hydrate in porous materials depends strongly on the pore size and the interaction between the gas and water-filled pores. Specifically, we revealed that this pressure gap appears when the pore size is
