Change in the Stable Crystal Phase of Tetra-n-butylammonium

Mar 22, 2013 - ... (MH21 Research Consortium) planned by the Ministry of Economy, ..... Hydrates (ICGH 2011), Edinburg, Scotland, U.K., 17–21 July, ...
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Change in the Stable Crystal Phase of Tetra‑n‑butylammonium Bromide (TBAB) Hydrates Enclosing Xenon Yusuke Jin* and Jiro Nagao* Production Technology Team, Methane Hydrate Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 2-17-2-1, Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan S Supporting Information *

ABSTRACT: This article reports the inversion of the stable phase of tetra-n-butyl ammonium bromide (TBAB) hydrates in the xenon (Xe)−TBAB−water system. In the TBAB solution (mass fraction of TBAB in the water solution, w = 0.40) under atmospheric pressure, the TBAB−water system preferentially forms the TBAB·26H2O hydrate. Through in situ Raman spectral analysis, the stable crystal phase of the TBAB hydrate enclosing Xe changed from TBAB·26H2O to TBAB·38H2O during temperature ramping from 283.6 to 287.4 K at 0.3 MPa. No lattice vibration shift was observed for the TBAB·26H2O hydrate enclosing Xe, whereas the lattice vibration of the TBAB·38H2O hydrate enclosing Xe shifted to a lower wavenumber. When conducting temperature ramping in a pressure vessel, it was observed that phase transformation of the TBAB hydrates enclosing Xe occurred at a pressure and temperature higher than 0.2 MPa and 287 K, respectively.

1. INTRODUCTION Clathrate hydrates are crystalline solids comprising polyhedral structures of hydrogen-bonded water molecules, which are stabilized by nonbonded guest molecules. Gas hydrates and peralkyl ammonium salt hydrates are examples of hydrates.1 Tetra-n-butylammonium bromide (TBAB) is a peralkyl ammonium salt that can form a semiclathrate hydrate with water molecules. TBAB semiclathrate hydrates (TBAB hydrates) mainly have five hydrate structures with different hydration numbers (i.e., 24, 26, 32, 36, and 38) under atmospheric conditions.2,3 Nevertheless, the TBAB−water system prefers to form two hydrates having hydration numbers of 26 and 38, namely, TBAB·26H2O and TBAB·38H2O, respectively.4 The crystal space groups of TBAB·26H2O and TBAB·38H2O hydrates are P4/mmm and Pmma, respectively.2,3 In TBAB hydrates, a tetra-n-butyl ammonium cation is located at the center of four cages. TBAB hydrates have empty cages (pentagonal dodecahedron cages, 512).3,5 Similar to gas clathrate hydrates, the 512 cages of TBAB hydrates can encage gas species such as H2, H2S, N2, CH4, and CO2,6−18 whose diameters are approximately 0.27, 0.46, 0.41, 0.44, and 0.51 nm, respectively. TBAB hydrates can store gas molecules under lower pressure than gas hydrates. In a CH4−TBAB−water system that has a 0.40 mass fraction of TBAB in a water solution (TBAB solution (w = 0.40)), for example, the equilibrium pressure at 289 K is ∼2 MPa; otherwise, a CH4 hydrate dissociates under 14 MPa.19 Therefore, TBAB hydrates show potential as a new gas storage medium. Recently, Jin et al.20 reported that TBAB hydrates could also enclose xenon (Xe), which has a 0.46 nm diameter. To use TBAB hydrates as a gas storage medium, it is important to know the phase stability of gas−TBAB−water systems © 2013 American Chemical Society

enclosing various gas molecules. Knowledge of phase behaviors will enable the design of suitable gas storage systems using hydrates. In a TBAB solution (w = 0.18), the stable crystal structure of the TBAB hydrate changes.4,6 At lower concentrations (w < 0.18), the TBAB·38H2O hydrate is the preferable phase. In contrast, at higher concentrations (w > 0.18), the TBAB·26H2O hydrate predominantly forms at atmospheric pressure. Oyama et al.4 observed that the phase stability line of each TBAB hydrate crossed in the TBAB solution (w = 0.18). In a TBAB solution (w = 0.40), which shows a stoichiometric TBAB concentration for the TBAB·26H2O hydrate, the dissociation temperatures of the TBAB·26H2O and TBAB·38H2O hydrates were 285.1 and 282.1 K, respectively, at atmospheric pressure. Even if the TBAB hydrate encloses guest molecules in gas− TBAB−water systems, the preferable crystal phase is generally regarded to be the TBAB·26H2O hydrate in TBAB solutions (w > 0.18). The Raman spectra of the TBAB hydrate enclosing CH4 in a CH4−TBAB−water system (w = 0.40) exhibited features of a TBAB·26H2O hydrate.15 However, the preferable hydrate crystal in a Xe−TBAB−water system (w = 0.20) was not TBAB·26H2O but TBAB·38H2O.20 The TBAB·26H2O hydrate can predominantly form at higher concentrations (w > 0.18). Furthermore, a CO2−TBAB−water system (w = 0.40) shows a different crystal structure of the TBAB hydrate enclosing CO2 at different temperatures under similar pressure conditions.15,21 Chazallon21 demonstrated that the TBAB·38H2O hydrate was the preferable crystal phase under CO2 Received: October 22, 2012 Revised: March 21, 2013 Published: March 22, 2013 6924

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pressure conditions in a CO2−TBAB−water system (w = 0.40). This change for a preferable TBAB hydrate crystal would be an intriguing phenomenon when discussing host−guest interactions in clathrate hydrates. This study aims to understand the phase stability of a TBAB hydrate enclosing Xe in a stoichiometric TBAB solution (w = 0.40) for the TBAB·26H2O hydrate. The study discusses the stability of two TBAB hydrates (TBAB·26H2O and TBAB·38H2O) in a Xe−TBAB−water system (w = 0.40) and the change of the stable crystal phase of the TBAB hydrate enclosing Xe. In addition, through Raman spectroscopy, it reveals the contrast between the lattice vibration of TBAB·26H2O and TBAB·38H2O hydrates with and without enclosed Xe.

which can establish a reference temperature of 273.15 K by using an ice−water slurry. The temperature measurements were reproducible within ±0.02 K. Therefore, the expanded uncertainty of the obtained equilibrium temperature was estimated to be ±0.1 K, with a confidence level of ∼95%, considering the uncertainty in controlling the sample temperature. The pressure of the inner vessel was reproducible within ±0.005 MPa using a pressure transducer (AP-13S, Keyence, Japan), with a confidence level of ∼95%. The measured pT data were collected using a data logger (midi LOGGER GL200, Graphtec, Japan). Materials. Reagent Plus grade TBAB of 99% purity (SigmaAldrich), water purified by deionization and distillation (>18 MΩcm), and research-grade Xe gas of purity 99.995% (Air Water, Osaka, Japan) were used. All materials were used without further purification.

2. EXPERIMENTAL METHODS Raman Spectroscopy. Raman spectra were collected using a Raman spectrometer (LabRAM HR-800, Horiba Japan) equipped with a 2400 grooves/mm grating, a thermoelectrically cooled CCD detector (operating temperature of ∼203 K), and a high-pressure Raman cell. The CCD detector size was 2048 × 512 pixels. The laser source was a 532 nm line (torus 532, Laser Quantum). This configuration allowed the Raman shifts to be collected at a spectral resolution of ∼0.2 cm−1/pixel. The Raman shifts of samples were observed through a super long working distance objective lens, Olympus SLMPLN 20× (numerical aperture of 0.25: laser spot diameter of ∼3 μm). The output laser power was 150 mW controlled by a power unit (mpc 3000, Laser Quantum). The laser was then reduced to 1/100 using a neutral density filter. The Raman shifts of samples were calibrated using Si emission lines (520.6 cm−1). A schematic of the high-pressure Raman cell is shown in Figure S1 of the Supporting Information (SI). It is a vacuum insulation system that has sapphire windows (thicknesses of outer and inner window are 1 and 3 mm, respectively) and a diaphragm pump that prevents thermal heat flow from the surrounding area to the sample. The cell temperature is maintained within ±0.1 K by circulating liquid N2 and is measured using a calibrated PT-100 platinum resistance thermometer (PT-102, Lake Shore Cryotronics, USA) and a temperature monitor (model 211, Lake Shore Cryotronics, USA). The operating temperature range and maximum operating pressure are 173− 298 K and 5 MPa, respectively. Gas is pressurized from an inlet, and the pressure is measured by a pressure transducer (205-2, Setra System, USA). The TBAB aqueous solution (w = 0.40) was charged in a sample cup composed of Al with an inner diameter of 8 mm and a height of 2 mm. The sample cup containing the sample was loaded into the sample room of the high-pressure Raman cell. First, air was eliminated from the high-pressure Raman cell by a vacuum pump. Next, it was pressurized with Xe gas at a designated pressure, and the TBAB solution was gradually cooled at steps of 0.5 K until a hydrate formed. Measurement of the pT Curve. To collect the TBAB solution−TBAB hydrate−vapor (L−H−V) three-phase equilibrium data for Xe−TBAB−water systems (w = 0.40), a pressure vessel with an optical window and mechanical stirrer was used, and it measured the system pressure by setting the temperature step to 0.2 K. Details of the measurement apparatus and method are described in a previous paper.20 Here the sample temperature was maintained within ±0.1 K by a circulating thermostat. For precision temperature measurements, the thermocouple was connected to a cold-junction compensation device (Zerocon, ZC-114, Coper Electronics),

3. RESULTS AND DISCUSSION Figure 1 shows examples of a TBAB hydrate crystal formed in the Xe−TBAB−water system (w = 0.40). Figure 1a shows the

Figure 1. TBAB hydrate crystals grown in the Xe−TBAB−water system (w = 0.40) at 0.3 MPa. (a) 283.6 K. (b) 287.4 K. Crystal in panel b formed at 10 h after the TBAB hydrate in panel a completely dissociated at 287.4 K. Scale bar indicates 100 μm.

TBAB hydrate at 283.6 K and 0.3 MPa that was prepared at approximately 280 K and 0.3 MPa in the high-pressure Raman cell shown in Figure S1 in the SI. The TBAB hydrate crystal existed until ∼287 K and then completely dissociated at 287.4 K during temperature ramping under a constant pressure of 0.3 MPa. At 10 h after hydrate dissociation, a new crystalline solid was observed at 287.4 K and 0.3 MPa (Figure 1b). The Raman spectra of TBAB hydrates enclosing Xe displayed in Figure 1 are shown in Figure 2. The blue and red lines indicate the Raman shifts at a pressure of 0.3 MPa and temperatures of 283.6 and 287.4 K, respectively. A Raman shift in the low-wavenumber region, namely, the lattice vibration region, is shown in Figure 2a. A band observed at ∼200 cm−1 was assigned to an intermolecular hydrogen-stretching band of H2O molecules (O−O vibrational band). A peak at 260 cm−1 was attributed to a Raman shift related to TBAB molecules because a similar peak was observed for the TBAB powder. In Figure 2b, complex composite Raman bands observed in the wavenumber region from 2800 to 3050 cm−1 were assigned to the C−H stretching bands of TBAB, and broad bands observed in the wavenumber region from 3050 to 3500 cm−1 were assigned to the O−H stretching bands of H2O. It was observed that the Raman spectra of each crystalline solid under 0.3 MPa differed. In the lattice vibration region, the O−O vibrational band changed from 190 to 200 cm−1 at 283.6 and 287.4 K, respectively. In the high-wavenumber region, the shape of the C−H and O−H stretching bands also changed. The difference 6925

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room at 278 K. Through Raman spectral analysis, the stable crystal phase of the TBAB hydrate enclosing Xe changed from TBAB·26H2O to TBAB·38H2O during temperature ramping from 283.6 to 287.4 K at 0.3 MPa. The TBAB hydrate (Figure 1b) completely dissociated above ∼288.5 K. At 0.18 MPa, no new crystal formation was observed after complete dissociation at 287 K. Figure 3 shows the pT conditions of the Xe−TBAB−water system (w = 0.40) measured using a pressure vessel and a high-

Figure 3. Equilibrium pressure−temperature conditions of Xe− TBAB−water and Xe hydrate systems. Blue circle, TBAB·26H2O hydrate in the Xe−TBAB−water system (w = 0.40); red circle, TBAB·38H2O hydrate in the Xe−TBAB−water system (w = 0.40); square, the Xe−TBAB−water system (w = 0.10);20 open square, the Xe−TBAB−water system (w = 0.20);20 and solid line, Xe hydrate.22 The uncertainties of equilibrium temperature and pressure were estimated to be ±0.1 K and ±0.005 MPa, respectively, with a 95% confidence level. Blue and red open circles, the pT points of the Raman spectra as shown in Figure 2.

Figure 2. Raman spectra of TBAB hydrates in the Xe−TBAB−water system (w = 0.40) at 0.3 MPa. (a) Lattice vibration region. (b) C−H and O−H stretching regions related to TBA+ and H2O molecules, respectively.

between the C−H stretching bands at ∼2980 cm−1 was remarkable (highlighted by a gray square). The spectral changes shown in Figure 2 indicate that the crystal structural transformation of the TBAB hydrates occurred during the temperature-ramping process under the same pressure condition. The change in the Raman spectra (Figure 2) enables distinguishing between the crystal structural features of the TBAB hydrates. In the TBAB·26H2O hydrate, a tetra-nbutyl ammonium cation (TBA+) is located at the center of three tetrakaidecahedrons and one pentakaidecahedron.6 However, in the TBAB·38H2O hydrate, TBA+ is located at the center of two tetrakaidecahedrons and two pentakaidecahedrons.3 Because the environment surrounding the TBA+ differs between the TBAB·26H2O and TBAB·38H2O hydrates, the C−H stretching bands can be distinguished between the two TBAB hydrates. To characterize the crystalline structures of the TBAB hydrates, we also show the Raman spectra of pure TBAB·26H2O and TBAB·38H2O hydrates without enclosing gas molecules in Figure 2. Hashimoto et al.8 also identified the crystal structure of TBAB hydrates by using Raman shift regions around 2800−3500 cm−1, where the difference of the C−H stretching Raman shift in the TBAB·26H2O and TBAB·38H2O hydrates can be observed. From a spectral comparison in the high-wavenumber region, the TBAB hydrate crystals were identified for TBAB·26H2O and TBAB·38H2O hydrates (Figure 1a,b, respectively). Furthermore, the O−O vibrational band (Figure 2a) enabled identifying the crystal structure of the TBAB hydrates. Powder X-ray diffraction (PXRD) profiles of the crystal samples formed at 287.2 K and 0.29 MPa also showed a pattern similar to that obtained from the TBAB·38H2O hydrates (Figure S2c in the Supporting Informaton). The PXRD samples were yielded from a crystal solution mixture in the pressure vessel by filtration in a cold

pressure Raman cell. The pT conditions of the TBAB hydrate enclosing Xe in the Xe−TBAB−water system (w = 0.10 and 0.20) and the Xe hydrate were also plotted.20,22 The pT conditions under which the TBAB hydrates were characterized by the Raman spectra in Figure 2 are plotted in Figure 3. The blue and red open circles plotted in Figure 3 indicate the existence of the TBAB·26H2O and TBAB·38H2O hydrates, respectively. Below 287.05 K, the pressure of the system increased as shown by the blue circles in Figure 3. As the temperature increased from 287.05 K, a pressure decrease from ∼0.250 to 0.222 MPa was observed at ∼287.2 K. After the pressure drop, the pressure again increased along not the blue but the red circles in Figure 3. Considering the characterization of the crystal structure by the Raman spectra and PXRD patterns (Figure S2 in the SI), the blue and red circles show the equilibrium pT conditions for TBAB·26H2O and TBAB·38H2O, respectively. Their equilibrium pT conditions are summarized in Table S1 of the SI. Here to measure the equilibrium pT conditions of the TBAB·38H2O hydrate below 287 K, after the equilibrium pT conditions of the TBAB·38H2O hydrate were obtained in the temperature range from 287.15 to 287.95 K, we gradually cooled the system temperature to 286 K at steps of 0.2 K to form only the TBAB·38H2O hydrate. Then, the equilibrium pT conditions were measured by increasing the temperature. Because the obtained pT data overlapped the equilibrium pT conditions of the TBAB·38H2O hydrate (red circles at above 287 K), we concluded that the obtained pT data below 287 K showed equilibrium conditions of the TBAB·38H2O hydrate. 6926

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equilibrium pT conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

As shown in Figure 3, the equilibrium pT curves of the two TBAB hydrates enclosing Xe in the Xe−TBAB−water system (w = 0.40) crossed at approximately 287 K and 0.2 MPa. At atmospheric pressure, the stable crystal phase of the TBAB solution (w = 0.40) is preferentially a TBAB·26H2O hydrate.4,6 Otherwise, the TBAB·38H2O hydrate is the stable phase under pT conditions higher than the cross point (287 K and 0.2 MPa). Guest molecules enclosed in the vacant 512 cages of the TBAB hydrate affect the stabilization of the entire hydrate. In Figure 2a, the lattice vibration peak of the H2O molecules in the TBAB·26H2O hydrates with and without enclosed Xe remained at ∼188.4 cm−1. However, in the TBAB·38H2O hydrate, the lattice vibration peak shifted from 203.8 to 199.8 cm−1 owing to the enclosed Xe molecules. The lattice vibration mode of the hydrates suggests the bending and stretching of hydrogen bonds in the H2O structure.23 Therefore, the Raman shift of the lattice vibration in the TBAB·38H2O hydrate shows that the Xe gas trapped in the 512 cages affected the H2O framework in the TBAB hydrate crystal. Xe did not affect the TBAB·26H2O hydrate. The diameters of Xe and CO2 are larger than those of CH4. The diameter ratio of the guest molecule and the cavity of the 512 cage for Xe and CO2 are 0.898 and 1.00, respectively. The ratio in the case of CH4 is ∼0.855. A high ratio of guest/cavity would cause distortion of the H2O framework. Owing to the enclathrating Xe in the 512 cage, pT stability of TBAB·38H2O hydrates may be changed by some interaction between the guest molecules and the host H2O framework. The phase stabilization effect of TBAB·38H2O hydrates enclosing Xe would cause the change of the stable crystal phase exhibited above 287 K and 0.2 MPa, as shown in Figure 3.

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*Tel: +81-11-857-8526. Fax: +81-11-857-8417. E-mail: u-jin@ aist.go.jp (Y.J.) and [email protected] (J.N.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by funding from the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium) planned by the Ministry of Economy, Trade and Industry (METI), Japan. We thank Drs. Y. Konno and M. Kida of the AIST for valuable discussions. They also thank Ms. J. Hayashi of AIST for experimental support.

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4. CONCLUSIONS The in situ Raman spectra of TBAB hydrates enclosing Xe and the pT trajectory during equilibrium conditions of the Xe− TBAB−water system (w = 0.40) were measured. Typically, the TBAB·26H2O hydrate is a stable crystal phase at 283.6 K and 0.3 MPa. However, the stable crystal phase transformed into the TBAB·38H2O hydrate at 287.4 K, despite a stoichiometric TBAB solution for the TBAB·26H2O hydrate. In the Xe− TBAB−water system (w = 0.40), the two equilibrium pressure−temperature (pT) curves were observed to cross at a pT point of approximately 287 K and 0.2 MPa. Here a phase stability change of the TBAB hydrate enclosing Xe was observed in the pT region higher than the crossed pT point. Moreover, the Raman peak assigned as the lattice vibration of the TBAB·26H2O hydrate remained in both cases with and without enclosed Xe. However, the lattice vibration peak of the TBAB·38H2O hydrate enclosing Xe shifted from 203.8 to 199.8 cm−1. This Raman shift change is thought to be attributed to the Xe molecule trapped in the vacant cage. A change in the lattice vibration of O−O revealed a structural change of the H2O framework in the TBAB hydrate crystal. Owing to the enclathrating Xe in the 512 cages, a stability change of the TBAB·38H2O hydrate is believed to occur. As a result, the phase stability of TBAB·26H2O and TBAB·38H2O hydrates with Xe changed, and an inversion of the phase stability would occur at the crossed pT point (287 K and 0.2 MPa).

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AUTHOR INFORMATION

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ASSOCIATED CONTENT

S Supporting Information *

Schematic of high-pressure Raman cell, powder X-ray diffraction patterns of the TBAB hydrates, and summarized 6927

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Tetra-n-butylammonium Bromide Semiclathrates Formed from Synthetic Landfill Gases. J. Chem. Eng. Data 2011, 56, 69−73. (15) Lee, S.; Park, S.; Lee, Y.; Lee, J.; Lee, H.; Seo, Y. Guest Gas Enclathration in Semiclathrates of Tetra-n-butylammonium Bromide: Stability Condition and Spectroscopic Analysis. Langmuir 2011, 27, 10597−10603. (16) Nguyen, H. D.; Fabien, C.; Jean-Michel, H. CO2 Capture by Hydrate Crystallization−A Potential Solution for Gas Emission of Steelmaking Industry. Energy Convers. Manage. 2007, 48, 1313−1322. (17) Lin, W.; Delahaye, A.; Fournaison, L. Phase Equilibrium and Dissociation Enthalpy for Semi-Clathrate Hydrate of CO2 + TBAB. Fluid Phase Equilib. 2008, 264, 220−227. (18) Li, S.; Fan, S.; Wang, J.; Lang, X.; Wang, Y. Semiclathrate Hydrate Phase Equilibria for CO2 in the Presence of Tetra-n-butyl Ammonium Halide (Bromide, Chloride, or Fluoride). J. Chem. Eng. Data 2010, 55, 3212−3215. (19) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: New York, 2007. (20) Jin, Y.; Kida, M.; Nagao, J. Phase Equilibrium Conditions for Clathrate Hydrates of Tetra-n-butylammonium Bromide (TBAB) and Xenon. J. Chem. Eng. Data 2012, 57, 1829−1833. (21) Chazallon, B. CO2 Capture by Hydrate Crystallization: In-Situ Study by Raman Spectroscopy. In Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburg, Scotland, U.K., 17−21 July, 2011. (22) Shimada, N.; Sugahara, K.; Sugahara, T.; Ohgaki, K. Phase Transition from Structure-H to Structure-I in the Methylcyclohexane + Xenon Hydrate System. Fluid Phase Equilib. 2003, 205, 17−23. (23) Eisenberg, D.; Kauzmann, W. The Structure and Properties of Water; Oxford University Press: Oxford, U.K., 1969.

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