Effect of Molecular Nitrogen on Multiple Hydrogen Occupancy in

Article pubs.acs.org/JPCC

Effect of Molecular Nitrogen on Multiple Hydrogen Occupancy in Clathrate Hydrates Seongmin Park, Dong-Yeun Koh, Hyery Kang, Jae W. Lee,* and Huen Lee* Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ABSTRACT: Multiple H2 occupancy in confined cages has been explored for the purpose of enhancing storage capacity. Furthermore, balancing the formation pressure with high storage capacity is one of the most significant factors. Here, we demonstrate the use of binary (LGM + N2) hydrates to capture hydrogen clusters under relatively mild conditions, even observing double H2 occupancy in small cages. The cage occupancy and structures of hydrates were identified by the Raman spectroscopic analysis and highresolution powder diffraction. The reaction product of binary (LGM + N2) hydrates with H2 molecules suggests the possibility of multiple H2 occupancy in both small (512) and large (51264) cages at relatively low pressures. Also, the lattice parameter decreases with an increase in H2 occupancy. The unique and abnormal role of N2 as a preoccupied coguest significantly affects the H2 population in a crystalline hydrate matrix and further lowers the pressure for structure stabilization.

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INTRODUCTION Clathrate hydrates, or gas hydrates, are icelike crystalline compounds formed by a combination of water and guest molecules and are of considerable interest to researchers, particularly with respect to potential gas storage applications.1,2 Since the first report of pure hydrogen hydrates (structure II),3 hydrogen clathrate hydrates have received widespread attention because of their advantages of environmentally friendly features, low cost, and comparatively high storage efficiency.4−6 The loading of multiple H2 molecules into both small (512) and large (51264) cages under moderate conditions is a key factor for the use of clathrate hydrates for hydrogen storage media. The full storage capacity of pure hydrogen hydrates is estimated to be 5.01 wt %, in the case of double occupancy in small cages and quadruple-H2 clusters in large cages.7 However, the formation of pure hydrogen hydrates typically requires a very high pressure (∼2000 bar) and low temperatures.8,9 The commonly used direct and efficient method for reducing the formation pressure is to adopt binary clathrate hydrates of which large cages are occupied by other large guest molecules.10 Many recent studies have suggested the inclusion of thermodynamic promoters, such as tetrahydrofuran (THF), quaternary ammonium salts, and other cyclic ethers, to stabilize H2 molecules in the confined cages of clathrate hydrates.11,12 It is generally understood, however, that the promoter molecule itself occupies a certain type of hydrate cage (e.g., 51264 cages in sII), thus resulting in a significant decrease in overall storage capacity. Recently, Lee et al. suggested a tuning phenomenon occurring at a THF concentration much lower than the corresponding stoichiometric concentration, which can lead to an increase in H2 storage capacity by allowing H2 molecules to occupy large cages instead of THF molecules.13 This tuning © XXXX American Chemical Society

phenomenon has been observed for sII hydrates formed with various guest molecules, including CH4, CO2, and H2.14,15 In a previous study, Lu et al. developed a novel method of reacting H2 gas with N2 hydrate to explore the potential realization of H2 multiple cage occupancy.16 The idea was to substitute the large N2 molecule in N2 hydrate with the smaller H2 molecule; this allowed multiple H2 molecules to be loaded into the clathrate cages. However, the reaction product obtained from pure N2 hydrate was not homogeneous with various H2 occupancies and included a great quantity of the ice phase. In this study, binary clathrate hydrates with a small amount of large guest molecules (LGM), tetrahydrofuran (THF) and pyrrolidine (PRD), were synthesized and characterized by Raman spectroscopy and high-resolution powder diffraction (HRPD). THF forms a stable sII hydrate with water at atmospheric pressure, while the PRD structures sII hydrates with suitable gas molecules.17 We acquired a microscopic spectrum showing cage occupancy according to the increase in H2 pressure and compared the cage occupancies between the reaction product and nonreaction product. Also, we obtained the lattice parameter for each sample to analyze the correlation of lattice constant with cage occupancy. The main aim of this study was to show multiple cage occupancy of H2 molecules at relatively low pressures combined with tuning phenomena and reaction with binary nitrogen hydrate. Received: June 19, 2014 Revised: August 5, 2014

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EXPERIMENTAL DETAILS Materials. H2 gas and N2 gas at a purity of 99.995 mol % were purchased from the Special Gas Co. (Daejeon, Korea). High-purity distilled water was obtained from a Millipore purification unit. Tetrahydrofuran (anhydrous, ≥99.9%, inhibitor free) was purchased from Sigma-Aldrich, Inc., and pyrrolidine (>0.99 mole fraction purity) was supplied by Aldrich. Sample Preparation. An aqueous solution of water and a 1.0 mol % LGM mixture (which is far lower than the corresponding stoichiometric concentration of 5.56 mol %) was frozen at atmospheric pressure and 248 K. The frozen solution was subsequently ground into a fine powder with a 200 μm sieve in liquid nitrogen. After being ground, the powdered sample was placed in a high-pressure cell (18 mL sample capacity) and was pressurized with nitrogen gas up to 200 bar at 243 K. After being pressurized with N2 gas, the high-pressure cell was maintained at 243 K. After 4 days, the high-pressure cell was cooled in liquid nitrogen and the pressure was slowly released. Finally, the cell was stored in a 243 K cooling bath, and H2 gas was charged into the pressure cell up to 150, 250, and 350 bar. The H2 gas was allowed to react with the N2 hydrate for 4 days. After hydrate samples had been synthesized, the high-pressure cell was cooled in liquid nitrogen for a few seconds and the pressure subsequently released. The reaction products then were collected and stored in liquid nitrogen. Experimental Measurements. For Raman measurements, a Horiba Jobin-Yvon LabRAM HR UV/vis/NIR highresolution dispersive Raman microscope equipped with an electrically cooled (203 K) CCD detector was used. The samples were kept at 77.0 K during the measurements. The excitation source was an Ar ion laser emitting a 514.53 nm line. The laser intensity was typically 30 mW. The HRPD patterns of the samples were recorded at 80 K using the Pohang Synchrotron of the Pohang accelerator laboratory (λ = 1.54720 Å) in the θ/2θ scan mode. The experiments were conducted in step mode with a fixed time of 2 s at a step size of 0.005° for 2θ = 5.0−145.5°. The obtained patterns were indexed using Checkcell.18

Figure 1. Raman spectra corresponding to the H2 vibrons in the (THF + H2) hydrates and reaction product of (THF + N2) hydrates with H2 at different pressures. All spectra were recorded at 77 K at ambient pressure: 1H2/small cage (S1), 2−4H2/large cage (L2−L4), 2H2/small cage (S2).

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RESULTS AND DISCUSSION In the (THF + H2) hydrate, Raman peaks corresponding to single H2 molecules in 512 cages were observed at a pressure of 160 bar and the double occupancy of H2 clusters in 51264 cages began to appear at a pressure of ∼400 bar. With a further increase in pressure, Raman peaks of triple- and quadruple-H2 clusters were detected at ∼550 and ∼700 bar, respectively.19 Note that conflicting arguments pertaining to experimental and simulation results have previously been raised regarding double H2 occupancy in the 512 cage.20−23 Figures 1 and 2 show Raman peaks corresponding to H2 vibrons in hydrate samples with χTHF and χPRD values of 0.01, respectively. The Raman spectrum difference between (THF + H2) and ((THF + N2) + H2) hydrates makes it possible to clearly identify multiple H2 occupancy in both 512 cages and 51264 cages even at relatively low pressures (Figure 1). Multiple peaks per cage are observed because of the multiple clusters and two spin isomers of H2 represented by ortho and para. The peak assignments were made with the recent Raman spectroscopic studies of hydrogen clathrate.23,24 The lowfrequency end around 4120−4125 cm−1 of the experimental spectra is assigned to singly occupied H2 in 512 cages (S1), and

Figure 2. Raman spectra corresponding to the H2 vibrons in the (PRD + H2) hydrates and reaction product of (PRD + N2) hydrates with H2 at different pressures. All spectra were recorded at 77 K at ambient pressure: 1H2/small cage (S1), 2−4H2/large cage (L2−L4), 2H2/small cage (S2).

the high-frequency peaks at 4125−4150 cm−1 are derived from doubly, triply, and quadruply occupied 51264 cages (L2−L4). At a high formation pressure [p(H2)] of 350 bar, the pure binary (THF + H2) hydrate evolves only single H2 peaks of 4121.6 and 4127.1 cm−1 in the 512 cage. However, note that for ((THF + N2) + H2) hydrate analyzed from a p(H2) of 150 bar to a p(H2) of 350 bar two other peaks were additionally detected at 4153.5 and 4159.7 cm−1. Wang et al. suggested that Raman peaks of double H2 occupancy in the 512 cage of sII hydrate can be detected in the range of 4140−4180 cm−1.23 On the basis of previous theoretical and experimental analyses, we can determine that two peaks at 4153.5 and 4159.7 cm−1 originate from double H2 occupancy in a 512 cage. Furthermore, peaks denoting two- to four-H2 clusters in 51264 cages appeared with B

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Figure 3. (a) HRPD patterns of the (THF + H2) hydrates and reaction product of (THF + N2) hydrates with H2 recorded at ambient pressure and 80 K. (b) Enlarged region from 26.5° to 34.0°. Asterisks denote the reflections from the hexagonal ice-Ih phase.

samples containing PRD show a trend similar to that with binary (THF + H2) hydrate samples (Figure 2). The PRD is known to crystallize into a cubic sII hydrate and show tuning behavior with help gas, such as methane.17 Both single H2 occupancy in 512 cages and double H2 occupancy in 51264 cages are observed in a (PRD + H2) hydrate formed at a pressure of 350 bar. The PRD is not a non-clathrate hydrate former (that is, the PRD cannot form the structure II hydrate without a help gas), requiring double H2 occupancy in 51264 cages for structural stabilization, which can be achieved at a relatively low pressure, in contrast with the binary (THF + H2) hydrate. Also, two other peaks representing double occupancy of H2 in 512 cages were detected at 4154.7 and 4160.8 cm−1 for hydrate samples of ((PRD + N2) + H2) measured from a low pressure [p(H2) = 150 bar] to a high pressure [p(H2) = 350 bar]. With an increase in H2 pressure, double, triple, and quadruple H2

an increased H2 pressure. At a pressure of 350 bar, double, triple, and quadruple H2 occupancy in 51264 cages was observed at 4130.5, 4138.0, and 4144.5 cm−1, respectively. We also observed that two peaks representing double occupancy in a 512 cage were shifted toward lower frequencies (that is, from 4155.7 and 4161.8 cm−1 to 4153.5 and 4159.7 cm−1, respectively), which results from increasing H2 occupancy in 51264 cages. Here, it should be noted that H2 molecules trapped in ice pores are identified at 4170.0−4180.0 cm−1 in Raman spectra, which is ∼20.0 cm−1 higher than the two peaks. The maximal amount of H2 storage in this present study is ∼2.24 wt %, which shows the double occupancy of H2 molecules in the 512 cages and quadruple occupancy in the 51264 cage at 1.0 mol % THF. However, the remaining N2 molecules in hydrate cages with the (H2 + THF) hydrate would lower the H2 storage capacity. Meanwhile, we find that the Raman spectra of hydrate C

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Figure 4. HRPD patterns of the (PRD + H2) hydrates and reaction product of (PRD + N2) hydrates with H2 recorded at ambient pressure and 80 K. Asterisks denote the reflections from the hexagonal ice-Ih phase.

Table 1. Changes in the Lattice Parameter According to Cage Occupancy cage occupancy

(THF + H2) hydrate at p(H2) = 350 reaction product of (THF + N2) hydrate with H2 at p(H2) = 150 reaction product of (THF + N2) hydrate with H2 at p(H2) = 350

lattice parameter (Å)

large cage

small cage

17.183 ± 0.001 17.172 ± 0.001

0 0

1 2

17.162 ± 0.001

4

2

cage occupancy

(PRD + H2) hydrate at p(H2) = 150 reaction product of (PRD + N2) hydrate with H2(150) at p(H2) = 150 reaction product of (PRD + N2) hydrate with H2(350) at p(H2) = 150

occupancy in 51264 cages was detected at 4130.3, 4136.8, and 4143.6 cm−1, respectively. Along with the THF hydrate system, two peaks representing double occupancy in the 512 cage were also shifted to lower frequencies (that is, from 4156.1 and 4162.0 cm−1 to 4154.7 and 4160.8 cm−1, respectively), resulting from an increase in the cage occupancy in the 51264 cage. To identify the crystal structures of hydrate samples, HRPD patterns of the samples were collected (Figures 3 and 4). After the HRPD patterns had been indexed and analyzed, all the hydrate samples were confirmed to be structure II cubic Fd3̅m, which is in good agreement with previously reported values.1 Most of the peaks except hexagonal ice peaks (denoted with an asterisk) were slightly shifted to higher diffraction angles depending on the degree of cage occupancy (Figure 3b). The lattice parameters for each sample are 17.183, 17.172, and 17.162 Å, respectively (see Table 1). These values tend to decrease with an increase in cage occupancy. A pure hydrogen hydrate with doubly occupied 512 cages and quadruply occupied 51264 cages by clusters of H2 molecules was indexed as a facecentered cubic unit cell of a = 17.047 ± 0.010 Å.8 The value of the lattice parameter is smaller than that of a common (THF + H2) hydrate. The increase in H2 occupancy may be linked to a decrease in occupancies of THF and/or N2, of which the molecular size is much larger than that of H2. Accordingly, one can speculate that the lattice parameters of H2 hydrates strongly depend on their cage occupancy and decrease with multiplicity of H2 clusters in both 512 and 51264 cages. Similarly, the lattice parameters for the (PRD + H2) hydrate and reaction product of

lattice parameter (Å)

large cage

small cage

17.323 ± 0.001 17.291 ± 0.001

2 3

1 2

17.285 ± 0.001

4

2

(PRD + N2) hydrate with H2 decrease from 17.323 to 17.285 Å according to an increase in cage occupancy. The values of the lattice parameters and cage occupancies obtained in this study are summarized in Table 1. Figure 5 shows the Raman spectra of the N−N stretching vibration of nitrogen molecules both in cages of hydrates and in other states (i.e., gas and liquid).25 The spectra reveal the possibility that part of the N2 hydrate remains unreacted. Before the inclusion of H2 gas, the N2 molecules could be stabilized in hydrate cages by forming (THF + N2) and (PRD + N2) hydrates.26 Subsequently, the injected H2 molecules substitute for the N2 molecules, eventually releasing N2 from hydrate cages. Nevertheless, intact N2 molecules in hydrate cages were observed at 2325.3 cm−1. To examine the impact of N2 molecules enclathrated in hydrate cages on H2 occupancy, mixed (LGM + N2 + H2) hydrate samples were formed and analyzed via Raman spectroscopy. Figure 6 shows the Raman spectra of hydrate samples formed with N2/H2 (50 mol %:50 mol %) mixed gas at 350 bar. Only one Raman peak was observed, at 2322.4 cm−1, for nitrogen hydrates (Figure 6a), while the ((LGM + N2) + H2) hydrates showed additional peaks representing fluid N2 from LN2. There results indicate that the H2 molecules dominantly occupy hydrate cages. As shown in Figure 6b, the Raman spectra represent doubly and triply occupied H2 molecules in 51264 cages, which suggests quite different Raman features compared with those of ((LGM + N2) + H2) hydrates. The two peaks representing doubly occupied H2 molecules in 512 cages were not detected, while D

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CONCLUSIONS

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AUTHOR INFORMATION

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The storage capacity of energy gases, such as hydrogen and methane, might be significantly improved in clathrate hydrates by achieving either single target−guest occupation in most confined cages or multiple target−guest occupation in each cage. A previously suggested tuning phenomenon can be a useful tool for singly filling both small and large cages and, in particular, excluding large guest promoters with simultaneous inclusion of small gaseous molecules. In this study, we introduced a new concept that uses N2 to establish optimal cage dimensions, which facilitates multiple H2 occupancy in 512 and 51264 cages at a relatively low pressure. Interestingly, the hydrate samples having double occupancy of 512 cages and quadruple occupancy of 51264 cages showed the smallest lattice parameter. During the reaction with hydrogen, (LGM + N2) hydrate would gradually release N2 and trap the H2 molecules. It is similar to the mechanisms of replacement of CH4 hydrate by CO2 molecules.27−29 We can simply conclude that the double occupancy in 512 cages occurs via displacement of N2 by H2, while the multiple occupancy in 51264 cages results from combined effects of both N2/H2 displacement and remaining preoccupied N2 molecules. At this stage, we recognize that the guest−guest and guest−host dynamic behavior is too complex to draw any clear conclusions, thus requiring deeper insights. Nonetheless, these outcomes are expected to provide contributions to the understanding of the fundamental inclusion phenomena, including guest occupancy and distribution of H2 and CH4 in clathrate hydrate materials.

Figure 5. Raman spectra showing N−N stretching vibrations of the product of the reaction of (THF + N2) hydrates with H2 (red) and (PRD + N2) hydrates with H2 (blue).

additional peaks indicating double and triple occupancy of H2 molecules in 51264 cages were observed around 4129.0−4139.0 cm−1. Compared with a pure binary (LGM + H2) hydrate, such abnormal multiple-H2 occupancy in 51264 cages at a relatively low pressure may arise from enclathrated N2 molecules with H2 molecules in hydrate cages. On the basis of this experimental result, it can be inferred that double occupancy in a 512 cage is a phenomenon that occurs from displacement of N2 by H2 and multiple occupancy in a 51264 cage is created by combination of N2 displacement and the remaining N2 molecules in a hydrate cage.

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interests.

Figure 6. Raman spectra showing (a) N−N stretching modes and (b) H−H vibrons of the (LGM + H2 + N2) mixed hydrate: 1H2/small cage (1s), 2H2/large cage (2L), 3H2/large cage (3L). E

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ACKNOWLEDGMENTS This research was funded by a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MEST) (2010-0029176) and by the WCU program (31-2008-000-10055-0), funded by the Ministry of Education and Science & Technology. It was also supported by the Ministry of Knowledge Economy through the “Recovery/ Production of Natural Gas Hydrate using Swapping Technique” project (KIGAM-Gas Hydrate R&D Organization).

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