Article pubs.acs.org/jced
Phase Equilibria of CO2 and CH4 Hydrates in Intergranular Meso/ Macro Pores of MIL-53 Metal Organic Framework Daeok Kim,†,‡ Yun-Ho Ahn,‡ and Huen Lee*,†,‡ †
Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Department of Chemical and Biomolecular Engineering (BK21+ program), Korea Advanced Institute of Science and Technology (KAIST), Daejeon, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea S Supporting Information *
ABSTRACT: The formation of gas hydrates in porous media is expected to bring out beneficial properties for gas storage and separation. Appropriate combined use of both gas hydrate and highly porous metal organic frameworks (MOFs) can be useful for achieving advances in the field of gas storage and separation. This makes understanding the behavior of gas hydrates in the confining pores of MOF crucial. The formation and phase equilibria of CO2 and CH4 hydrates in MOF were investigated using MIL-53 MOF through low-temperature synchrotron high-resolution powder diffraction (HRPD) and P−T traces. MIL-53 has both intrinsic micropores and intergranular meso/macropores. Gas hydrate forms in meso/ macropores, and its thermodynamic behavior is relatively inhibited compared to its behavior in bulk phase due to reduced water activity. However, a strong CO2 dissolution appeared instead of gas hydrates in the intrinsic micropores of MIL-53. This led to a notable phenomenon in which the cooling and heating lines in the P−T trace curves of CO2 hydrate did not intersect near the dissociation point of CO2 hydrate.
1. INTRODUCTION Gas hydrates form structures of polyhedral cages in which gas (guest) molecules are contained under suitable temperature and pressure conditions. The facile inclusion of gaseous guest molecules in water cages has created considerable interest in various areas of research: methane hydrate recovery as a new energy source,1−3 greenhouse gas sequestration via hydrate formation,4−6 energy gas (H2, CH4) storage,7−10 and gas separation.11−14 Recent studies have been focusing on the distinctive behavior of gas hydrates in porous media. In terms of methane hydrate production and CO2 sequestration, the physicochemical properties of CH4 and CO2 hydrates within surrounding porous media have been explored using porous clays and silica materials.15−19 For example, the effect of confined clay interlayers on cage occupancy in methane hydrate15 and replacement efficiency between CO2 and N216 were explored in a close examination of intercalation phenomenon. In addition, the formation and dissociation behavior of gas hydrates in nanosized porous media were investigated using silica gel.20−26 These behaviors occurred because the already distinct competition was enhanced between dissimilar guest molecules for occupancy in hydrate cages confined in porous media. This led to the gas hydrates’ potential use for CO2 separation.27−30 As a result, there has been research on gas hydrates using various porous media. However, there were not many results © XXXX American Chemical Society
obtained using porous metal organic framework (MOF) materials. MOFs, in which metal-ion clusters are linked by organic molecules, are highly porous materials. Linkage occurs by means of strong bonds between metal-containing units and organic linkers (e.g., linkage between a metal ion and a carboxylate attached to an organic molecule).31 By modifying the types of metal precursors and organic linkers, a large variety of porous structures can be synthesized with diverse pore characteristics such as surface area, pore size, pore shape, and porosity.32 To shed light on the bright features of MOFs, they have been extensively studied and applied to gas storage and separation. A notable point in MOF research is that the presence of water can tune and enhance the gas storage and separation characteristics of MOFs.33−37 One more interesting thing is that a gas hydrate structure can form in a MOF and can enhance the gas storage capability of the MOF. Specifically, Mu et al. reported that the methane storage capacity of one kind of MOF (ZIF-8) can be increased to 45% with the formation of methane hydrate in the ZIF-8.38 Such a significant increase in gas sorption raises the possibility of two contributions: the Received: April 7, 2015 Accepted: June 5, 2015
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DOI: 10.1021/acs.jced.5b00322 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
conducted in a state where pores are filled with water (i.e., water saturation (WS)). This procedure is done not only to provide enough water to form the hydrate structure but also to elucidate the influence of the pore dimensions on the gas hydrate’s properties. In order to prepare WS MIL-53, an amount of water slightly in excess of the total pore volume of the MOF was placed in a vacuum oven. The remaining air was rapidly evacuated by a vacuum pump and the system was kept closed. Afterward, the water that had vaporized from the liquid water was diffused into the pores of the MOF. The pore structure was saturated within 3 days. Throughout this procedure, the MIL-53 water absorbed corresponded to the total pore volume. This was confirmed by measuring the changes in weight of the MIL-53 before and after the water sorption. HRPD analysis was conducted using the synchrotron at the Pohang Accelerator Laboratory (λ = 1.54950 Å). The WS MOF samples exposed to gas were collected by rapid quenching using liquid nitrogen. After quenching, residual gas in the reactor was vented. The recovered samples were finely ground to particles smaller than 200 μm at the temperature of liquid nitrogen (77 K). The samples were then placed in precooled sample loaders. The pressure−temperature traces of gas hydrates were measured as follows by using the facility described in Figure S1. The high-pressure cell was equipped with a four-wire type Pt-100Ω probe for temperature sensing and with a pressure transducer (Druck, PMP5073) with an accuracy of ±0.2% in the range of 0−25 MPa. The temperature was maintained with a circulating bath (Jeio Tech., RW-2040G) that had a temperature stability of ±0.05 K. Then, the pressure and temperature that were measured using this apparatus were also calibrated using a thermometer and a pressure gauge calibrated by the Korea Research Institute of Standards and Science. Regarding errors that could have occurred during the measuring and analyzing of data, good uncertainties of equilibrium temperature (±0.1 K) and pressure (±0.02 MPa) were achieved. The pressure of the injected gas was controlled by a syringe pump connected to both a gas cylinder and a highpressure cell. To measure the P−T traces, WS MIL-53 was placed in the reactor, and the remaining air was flushed with the injection gas. We followed the usual method to measure P−T traces (i.e., isochoric stepwise cooling and heating procedures at rates of −1 K/h and 0.1 K/h, respectively). During measurement, the temperature and pressure were recorded by a dataacquisition system every 20 s. A phase equilibrium point was determined from the recorded P−T trace curve by using the procedure described in Figure S2. For SEM and XPS analysis, MIL-53 powder was evacuated at 100 °C for 6 h and cooled down to room temperature. The as-prepared sample was also loaded on the preattached carbon tape on a sample loader without further treatment. An SEM image was obtained using a Magellan400 of FEI Company (acceleration voltage: 500 V; current: 6.3 pA; working distance: 3.0 mm). XPS analysis was performed with a multipurpose XPS (Sigma Probe, Thermo VG Scientific, X-ray Source: monochromatic Al Kα).
inclusion of gas in the gas hydrates’ water cages and the inclusion of gas in the intrinsic micropores of MOFs. These two different mechanisms for molecular capture might indicate a new strategy for gas storage and separation, which could lead to more complex patterns from mutual interactions. The first task for this research was to confirm whether it is possible to form gas hydrates in MOFs; the second, by means of P−T traces of phase equilibria, was to determine the conditions favoring hydrate formation. Herein, we investigated the formation and phase equilibria of CO2 and CH4 hydrates in porous MIL-53 MOF in relation with pore size. The framework of MIL-53 is built by connecting infinite chains of aluminum-hydroxide and benzene-1,4-dicarboxylates, as shown in Figure 1. The successive connection of metal ions
Figure 1. Schematic illustration of MIL-53 metal organic framework.
with organic molecules leads to highly porous solid structures with micropores of
