Unexpected Behavior of Helium as Guest Gas in sII Binary Hydrates

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Unexpected Behavior of Helium as Guest Gas in sII Binary Hydrates Nikolaos I. Papadimitriou,† Ioannis N. Tsimpanogiannis,*,†,# Athanassios K. Stubos,† Angel Martin,‡ Laura J. Rovetto,‡ and Cor J. Peters‡ †

Environmental Research Laboratory, National Center for Scientific Research “Demokritos”, 15310 Agia Paraskevi, Greece, and Laboratory of Process Equipment, Department of Process and Energy, Faculty of Mechanical Maritime and Materials Engineering, Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands

‡

ABSTRACT We perform grand canonical Monte Carlo simulations for the sII He-THF binary hydrate and report for the first time the possibility for the large cavity (51264) to be occupied simultaneously by two different types of molecules (i.e., He, THF). The small cavities (512) are found to be capable of enclathrating up to three He molecules, a novel behavior, as well. The current findings have significant implications for the gas storage in hydrates with promoters. SECTION Energy Conversion and Storage

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lathrate hydrates are nonstoichiometric, crystalline, inclusion materials that are composed of hydrogenbonded water molecules. The water molecules form a lattice that contains cavities (cages) that can be stabilized by the enclathration of small guest molecules.1,2 Hydrates have attracted considerable scientific interest due to their involvement in a variety of natural or industrial processes, including flow assurance of oil and gas pipelines,3 energy production from unconventional sources,4 capture and sequestration of “greenhouse” gases,5 and separation,6 storage, and transport7,8 of “energy-carrier” gases. Hydrates are also considered a potential geohazard and can affect the global climate.9 Until recently, it has been believed that small gas molecules, such as helium (He), hydrogen (H2), and neon (Ne), could not form hydrates because their size is not large enough to stabilize any hydrate cavity belonging to the well-known cubic structures I and II (sI, sII) and the hexagonal structure H (sH) hydrates.1 However, recently, pure H2 hydrate (sII) was experimentally synthesized and was found to be stable at high pressures (e.g., 200 MPa at 280 K). On the other hand, it has been known that He can be trapped in the interstices of some ice structures. In particular, it was demonstrated experimentally that He can be trapped in the interstices of hexagonal ice Ih10-12 and ice II.13-15 Lattice dynamics calculations for He in ice Ic were performed by Belosludov et al.16 and for He in ice II by Dong et al.,17 and molecular dynamic simulations of ice II filled with He were performed by Malenkov and Zheligovskaya.18 Udachin et al.19 were the first to report experiments on binary hydrates of helium and hydrogen with tetrahydrofuran (THF). THF as a single guest molecule forms cubic structure II (sII) hydrates at ambient pressure and temperatures up to 277.6 K. The unit cell of the sII hydrate (Fd3m space group) consists of 136 water molecules that form two types of cavities, the small, consisting of 12 pentagonal faces (512), and the large that is formed by 12 pentagons and 4 hexagons (51264).

r 2010 American Chemical Society

There are 16 small and 8 large cavities in the unit cell of the sII hydrate. THF has been also used as a hydrate promoter, essentially lowering significantly the hydrate formation pressure for certain gases.8 THF is an effective hydrate former that is encaged only in the large cavities (it is too large to fit in the small ones) of the cubic structure II, leaving the small cavities available for He. Additional experimental works providing hydrate phase equilibrium data for the sII He-THF binary hydrates were reported by Larionov et al.20 and Yeon et al.21 In this Letter, we report grand canonical Monte Carlo (GCMC) simulations for the sII binary He-THF hydrate performed along the P-T hydrate equilibrium curve reported by Larionov et al.20 Herein, we report for the first time that for higher pressures, He and THF can occupy simultaneously the large cavities. Small cavities are found capable of enclathrating up to three He molecules, which is also a novel behavior. Earlier, Udachin et al.19 reported the double occupancy of the small cavities of the H2-THF binary hydrate at 700 MPa. The phenomenon of multiple occupancy of hydrate cages by the same molecules (e.g., N222 and Ar23) was discovered recently and since then has attracted the attention of the scientific community. Cage occupancy by multiple guest molecules has a significant impact in terms of storage capacity of hydrate materials with promoters. The fixed geometry of the hydrate lattice allows its formation to be described as a process of gas storage in a solid material. GCMC simulations were performed to determine the distribution of He atoms in the cavities of the sII binary He-THF hydrate. In this work, we have performed GCMC simulations following the algorithm proposed by

Received Date: December 28, 2009 Accepted Date: February 23, 2010 Published on Web Date: March 03, 2010

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DOI: 10.1021/jz9004625 |J. Phys. Chem. Lett. 2010, 1, 1014–1017

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Metropolis et al.24 A detailed description of the simulation technique can be found in Allen and Tildesley.25 Simulations were performed on eight (2 2 2) unit cells of sII hydrate containing a total of 1088 fixed water molecules placed on a rigid lattice, and three-dimensional periodic boundary conditions were applied. The lattice parameter used was 17.31 Å.26 The positions of the oxygen atoms of the water molecules were according to the X-ray diffraction measurements of Yousuf et al.27 The proton configuration, consistent with the Bernal-Fowler “ice-rules”,28 with the minimum dipole moment was selected for the simulations. For the THF molecules, the approach proposed by Alavi et al.29 that includes a geometry optimization based on the AMBER force field30 has been followed. Cornell et al.30 gave the structural details of the optimized THF geometry. The SPC/E model31 was used to describe water molecules. Note that for the case of sII H2 hydrate, the cavity occupancies obtained from GCMC simulations using the SPC/E model32 were in good agreement with those using TIP4P.33 Furthermore, we have also used the TIP4P model for the case of sH hydrates34 and obtained similar results for cavity occupancies with the SPC/E model. Frankcombe and Kroes35 offer a detailed discussion on the various water models used in hydrate simulations. Helium molecules are represented by a Lennard-Jones (LJ) interaction site, with the parameters σ = 2.556 Å and ε = 0.0850 kJ/mol reported by de Boer.36 These LJ potential parameters have been previously used to describe the He-H2O interactions in simulations.16,17,37 Due to their spherical shape and nonpolar nature, no electrostatic interactions were considered for He molecules in our calculations. Electrostatic interactions, however, were taken into account for the interaction between THF and water molecules. Furthermore, quantum effects of He were neglected since our simulations were performed at relatively high temperatures (i.e., 277-289 K). Belosludov et al.16 also neglected quantum effects like tunneling or electronic excitation in their lattice dynamics calculations. Frankcombe and Kroes35 offer additional discussion and references on the need or not to include quantum effects. All molecules were considered to be rigid during the simulations. Sizov and Piotrovskaya38 discussed in detail the use of flexible molecules for the hydrate framework in their study of sI methane hydrate. Additional details of the application of the GCMC method to hydrates can be found in our previous work.34 Simulations are performed at hydrate equilibrium conditions using the P-T phase diagram reported by Larionov et al.20 in the range of 277-289 K corresponding to 1-450 MPa. The cage occupancy by He molecules in the 512 cage of the sII He-THF hydrate is shown in Figure 1. The occupancy ratio (%) of the small cage is presented as a function of the pressure for the cases of an empty cage and single, double, and triple occupancy. The occupancy increases rapidly with pressure, and cavities occupied by up to three He atoms are observed at pressures above 300 MPa. For pressures below 350 MPa, the dominant occupancy for the small cavities is the single occupancy, while double occupancy becomes dominant for pressures above 350 MPa. The fraction of triply occupied cavities is 1% at 290 MPa and reaches 8.5% at 450 MPa. For


Figure 1. Helium occupancy ratio (%) as a function of pressure (and the corresponding hydrate equilibrium temperature) of the small cavities of the binary He-THF hydrate.

the conditions examined, no quadruply occupied cavities were observed. On the basis of our GCMC simulations it is possible to accommodate up to three He molecules in the 512 cavities of the sII binary He-THF hydrate for pressures up to 450 MPa. The observation of doubly/triply occupied small cavities by He atoms is a justified result considering the small size of the He molecule and the very simple interactions with the water molecules. Multiple He occupancy of the small cavities, however, remains to be experimentally confirmed. Next, we compare the results from the GCMC simulations with available experimental data. For the case of the binary He-THF hydrate, Udachin et al.19 reported that the degree of filling of the small cavities at a pressure of 350 MPa was 24.5% (corresponding to He content equal to 0.52 wt %). At 700 MPa, even less He was absorbed in the hydrate. Udachin et al.19 suggested the formation of sI hydrate to explain this behavior. These results seem unrealistically low, given the simplicity of the He molecule and the high pressures at which they were obtained. For the case of 350 MPa, our GCMC results give a cavity occupancy that is significantly higher than the value reported by Udachin et al.19 In particular, we report small cavity occupancy approximately equal to 160% (corresponding to a He content equal to 3.28 wt %). Yeon et al.21 reported the He content of the stoichiometric (5.56 mol %) He-THF binary hydrate (for 278 K and pressures lower than 15 MPa) to be equal to 0.2 wt %. This value corresponds to small cavity occupancy equal to 0.1 (cavity filling 10%). This value is close to the values from our GCMC simulations. In particular, we calculated small cavity occupancies equal to 0.098 (0.206 wt %) and 0.13 (0.275 wt %) for pressures of 10 (277.8 K) and 13.5 MPa (278.0 K), respectively. Additionally, we have compared our GCMC results with occupancies obtained from a correlative method based on the van der Waals-Platteeuw theory (a similar approach to the one developed by Martin and Peters39) and found very good agreement of small cavity occupancies in the pressure range where both methods are applicable. The two methods are completely independent, and there is no feedback between them. Both methods predict a small cavity occupancy of 24.5% for pressures around 27 MPa (instead of 350 MPa given by Udachin et al.19). This work is currently in progress and will be submitted for publication in the future.40

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Figure 2. Average occupancy as a function of pressure (and the corresponding hydrate equilibrium temperature) of the large and small cavities of the binary He-THF hydrate.

Figure 4. Simultaneous occupancy of the large cavity by He (shown in yellow) and THF molecules for the sII binary He-THF binary hydrate.

approaching other molecules render its coexistence with a promoter molecule within the same cavity an energetically favorable configuration at high pressures. Figure 4 illustrates a potential location of the THF molecule and He atom in a large cavity as determined from the lowest-energy configuration during a GCMC run (at 288.55 K and 350 MPa). This finding could serve as guidance to the design of future experiments in order to confirm or refute the possibility of simultaneous cavity occupancy by two different types of molecules. Furthermore, the findings of this study could have a significant impact on the storage of energy-carrier gases in hydrates. In particular, promoters are deemed essential to lower the hydrate formation pressure. However, they occupy the large cavities, reducing therefore, the gas-storage capacity of the binary hydrates. If a promoter is selected appropriately, such that it can also accommodate the enclathration of the guest gas in the large cavities (together with the promoter), it can increase significantly the storage capacity of binary hydrates with promoters. Note, however, that high pressures are required in order to achieve high storage capacity following the approach of this study. An alternative approach, known as “hydrate tuning” has been proposed by Lee et al.41 and recently confirmed by Sugahara et al.42

Figure 3. He-THF occupancy ratio (%) of the large cavities as a function of pressure. Empty circles denote one THF molecule in the cavity. Filled circles denote one He and one THF molecule in the cavity, and triangles denote two He and one THF molecules in the cavity.

Figure 2 shows the average cavity occupancy of the small/ large cavities for the binary He-THF hydrate. The small cavity has an average He occupancy that reaches the value of 1.84 (at 450 MPa). However, the most interesting, and rather unexpected, aspect shown in Figure 2 is concerned with the occupancy of the large cavity of the binary He-THF hydrate. While the large cavity exhibits a constant THF average occupancy of 1.0, namely, all large cavities encage a single THF molecule (consistent with previous observations), at the same time, there is a He average occupancy that increases with pressure up to a value of 0.20 (at 450 MPa). This indicates a simultaneous occupancy of the large cavity by He and THF molecules. This is a very surprising finding since it was never reported previously from either experimental observations or simulations. Recall, however, that until recently, the double occupancy (same type of molecules) of cavities was thought not to be possible,1 and yet, experiments proved otherwise.22 To further illustrate the issue, consider Figure 3. At pressures above 100 MPa, a significant fraction of cavities is simultaneously occupied by THF and He atoms. In particular, this fraction is 2.4% at 200 MPa and reaches 19.5% at 450 MPa, as shown in Figure 3. This is the first report for the possibility of two different types of guests enclathrated in the same hydrate cavity. Probably, the small size of the He atom and the weakness of the repulsive interactions that it develops when


AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. Fax: þ30 2106525004. E-mail: [email protected].

Notes #

Visiting scientist.

ACKNOWLEDGMENT Partial funding by the European Commission

DG Research (contract SES6-2006-518271/ NESSHY) is gratefully acknowledged by the authors.

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Unexpected Behavior of Helium as Guest Gas in sII Binary Hydrates

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