Crystal Phase Conditions of Semiclathrate Hydrates in Nitrogen–Tetra

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Crystal Phase Conditions of Semiclathrate Hydrates in Nitrogen− Tetra‑n‑butylammonium Bromide−Water Systems below 1 MPa Yusuke Jin,*,† Masato Kida,†,‡ and Jiro Nagao† †

Methane Hydrate Production Technology Research Group, Research Institute of Energy Frontier, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan ‡ School of Earth, Energy and Environmental Engineering, Faculty of Engineering, Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507, Japan

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S Supporting Information *

ABSTRACT: The phase equilibrium pressure−temperature conditions in nitrogen (N2)−tetra-n-butylammonium bromide (TBAB)−water systems (molar fractions of TBAB in aqueous solution, xTBAB = 0.0062, 0.0138, 0.0234, 0.0359, and 0.0529) under 1 MPa were examined using an isochoric method, and the crystal structures were assessed using the Raman spectroscopic technique. N2−TBAB−water systems with low xTBAB (0.0062 and 0.0138) formed TBAB hydrate crystals, in contrast to the pure TBAB·38H2O hydrate, which is the preferred crystal structure under atmospheric conditions. The TBAB hydrate crystals in low xTBAB showed a symmetric N−N vibrational peak of enclosed N2 molecules at 2324.3 cm−1. N2−TBAB−water systems with high xTBAB (0.0234, 0.0359, and 0.0529) formed TBAB hydrates, similar to the pure TBAB·26H2O hydrate. The Raman spectrum in high xTBAB shows an asymmetric N−N vibrational peak that can be deconvoluted into two peaks, one at 2324.5 cm−1 and the other at 2324.1 cm−1. These results suggest that there are two non-equivalent cages in the TBAB hydrate structure at high xTBAB. Although other experimental techniques are required to identify these cage structures, our Raman results may provide insights into understanding TBAB hydrate structures.

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INTRODUCTION Technologies involving the separation and capture of greenhouse gases, such as carbon dioxide (CO2), are essential for sustainable growth. Gas clathrate hydrates (hereafter referred to as gas hydrates) are promising candidates for the separation and capture of gas molecules, including CO2, because their polyhedral, hydrogen-bonded water frameworks can trap these molecules.1 To capture gas molecules using a gas hydrate, high-pressure and low-temperature conditions are required. Semiclathrate hydrates from peralkyl ammonium salts and water also form a polyhedral water framework. However, in contrast to the gas hydrate system mentioned above, semiclathrate hydrates can form polyhedral water frameworks under atmospheric conditions. Semiclathrate hydrates have empty cages that can capture gas molecules at lower pressures and higher temperatures, compared with equilibrium gas hydrate systems.2−8 Thus, the ability of semiclathrates to capture greenhouse gases under relatively mild temperature and pressure conditions is a significant advantage toward the industrial utilization of these materials. Tetra-n-butylammonium bromide (TBAB) is a peralkyl ammonium salt that forms semiclathrate hydrate (hereafter referred to as hydrate) compounds with water molecules.9 The TBAB−water system can form several hydrate structures with different hydration numbers.10 Among these, the two with hydration numbers 26 and 38 are the preferred structures.11 © XXXX American Chemical Society

The TBAB hydrate with the hydration number 38 (hereafter referred to as TBAB·38H2O hydrate) is an orthorhombic (Pmma) crystal structure in which the TBA+ molecules are located in a super-cage composed of two tetrakaidecahedron (51262, T) and two pentakaidecahedron (51263, P) cages.12 Furthermore, vacant dodecahedral (512, D) cages are present in the orthorhombic TBAB·38H2O hydrate structure. The D cage, with an effective radius of 0.4 nm, can enclose CO2, methane (CH4), and hydrogen sulfide (H2S).1 Recently, single-crystal X-ray diffraction (SXRD) studies have revealed gas selectivity in TBAB·38H2O hydrates and the existence of a distorted D (henceforth referred to as D′) cage for TBAB· 38H2O hydrates enclosing guest molecules.13−15 Here, in the xenon (Xe)−TBAB−water and krypton (Kr)− TBAB−water systems, the crystals formed at around 0.1 MPa show the same crystal structure as that of pure TBAB−water systems. However, the Xe−TBAB−water and Kr−TBAB− water systems show crystal phase transition from pure TBAB hydrate structures with increasing pressure levels.16,17 Muromachi et al.14 reported that N2−TBAB−water systems with a low TBAB concentration form orthorhombic TBAB·(38H2O or 38.1H2O) hydrates, with cages distorted by enclosing N2 Received: March 6, 2019 Accepted: May 10, 2019

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DOI: 10.1021/acs.jced.9b00210 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. Materials Used in This Work chemicals

CAS RN

MW (kg·kmol−1)

tetrabutylammonium bromide nitrogen

1643-19-2

322.37

7727-37-9

14.01

water

7732-18-5

18.01

supplier

purity (%)

Sigma-Aldrich Japan Fine Products Corp., Japan

comments

≥99.0 (mass fraction) 99.995 (molar fraction) purified by ultrafiltration, reverse osmosis, deionization, and distillation

with lower pressure than the initial one. When the increase in pressure caused by hydrate dissociation during temperature raise was approximately close to the initial pressure, the temperature of the system was increased in increments of 0.1 K. The temperature of the sample was maintained for 12−24 h at each temperature increment. When the hydrate crystals in the system were completely dissociated, the increase in the pressure of the system became gentle. The phase equilibrium pT conditions were determined from the slope of the increase in pressure. The reliability of our experimental set-ups and procedures has been shown in previous reports.17 Spectroscopic Analysis. Raman spectra were collected using a Raman spectrometer (LabRAM HR-800, HORIBA Ltd) with a 2400 grooves/mm grating, a thermoelectrically cooled CCD detector (size: 2048 × 512 pixels), and a superlong working-distance objective lens (Olympus SLMPLN 20×, Olympus; numerical aperture of 0.25). The laser line was 532 nm (Torus 532, Laser Quantum). The sample temperatures during the spectral measurements were maintained at 223 K using a low-temperature stage (HFS600E-P, Linkam Scientific Instruments). The Raman shift was calibrated using the Si emission line (520.6 cm−1) for every measurement. The spectra of the N−N vibration modes of N2 were deconvoluted into separate peaks using the commercial PeakFit v4.12 multiple-peak-fitting program. TBAB hydrates enclosing N2 for the Raman analysis were synthesized from pure orthorhombic TBAB·38H2O hydrates and pure tetragonal TBAB·26H2O hydrates without N2. To synthesize orthorhombic TBAB hydrates enclosing N2, we pressurized pure orthorhombic TBAB·38H2O hydrates at 282 K with N2 at pressures of 0.8 MPa. Tetragonal TBAB hydrates enclosing N2 were prepared at 284 K by pressurizing pure tetragonal TBAB·26H2O hydrates with N2 at 0.9 MPa. Here, the pure orthorhombic TBAB·38H2O hydrates and pure tetragonal TBAB·26H2O hydrates were prepared from 32 and 40 wt % TBAB solutions (nearly stoichiometric concentrations for each crystal structure), and their crystal structures were confirmed using Raman spectroscopy.

above 1 MPa, whereas no crystal system changes. However, the conditions of the structural transition in N2−TBAB−water systems are unclear and few reports on N2−TBAB−water systems with high TBAB concentrations are found in the literature. Raman studies of semiclathrate hydrates are useful in investigating guest molecules and provide information on cage structures, once the Raman peaks of guest molecules are sensitive to their surroundings.18−21 In the Raman spectra of TBAB hydrates, C−C and C−H vibrations are largely attributable to the butyl groups in TBA+.22 In this work, we report the phase equilibrium pressure− temperature (pT) conditions in the N2−TBAB−water systems with five different TBAB concentrations below 1 MPa. In Japan, industrial systems that use a gas pressure >1 MPa must operate in compliance with strict regulations, under the HighPressure Gas Safety Act of Japan. Therefore, the data established under low-pressure conditions are required to promote the industrial use of clathrate hydrates. Additionally, we measured the Raman spectra of crystals formed in the N2− TBAB−water systems in order to understand their crystal structures. As no Raman peaks are observed in the N−N vibration region (2300−2350 cm−1), N−N vibrations of N2 molecules captured in TBAB hydrates would be useful in the discussion of which cages are able to enclose guest molecules.

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EXPERIMENTAL METHODS Sample Preparation. TBAB (99% purity; Sigma-Aldrich) and N2 gas (99.995% purity; Japan Fine Products Corp., Japan) were used. The water was purified by ultrafiltration, reverse osmosis, deionization, and distillation. The detailed information on these materials is presented in Table 1. Equilibrium Pressure−Temperature Measurement. We measured the phase equilibrium pT data in the N2− TBAB−water systems with five different TBAB solutions (molar fractions of TBAB in aqueous solution, xTBAB = 0.0062, 0.0138, 0.0234, 0.0359, and 0.0529) by an isochoric method, using a pressure vessel equipped with a stirring fin.7 The uncertainty of the sample temperature (T) was estimated to be ±0.1 K, with a confidence level of approximately 95%. The uncertainty of the pressure (p) measurements was estimated as ±0.005 MPa, with a confidence level of approximately 95%. The reliability of our experimental set-up has already been reported in a previous manuscript.7 First, TBAB solutions with a designated concentration were poured into the pressure vessel. Before the phase equilibrium pT measurement, gas purges using a vacuum pump and gas pressurization with N2 at 1 MPa were performed three times at 288 K. Subsequently, the vessel was pressurized under designated pressure conditions with N2 and then chilled to initiate hydrate formation. Later, the hydrate crystal formation was confirmed by a distinct decrease in pressure and increase in temperature, using steps from 0.1 to 0.5 K. We increased the temperature of the system in increments of 0.5 K in the region

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RESULTS AND DISCUSSION

Figure 1 shows the equilibrium pT conditions obtained in the N2−TBAB−water systems (xTBAB = 0.0062, 0.0138, 0.0234, 0.0359, and 0.0529). Additionally, literature data in TBAB− water and N2−TBAB−water systems are plotted in Figure 1.11,14 The equilibrium pT conditions in the N2−TBAB−water system are found in a region of lower pressure and higher temperature (milder pT region) than those in the pure N2− water system.23 Furthermore, the phase equilibrium pT conditions shift to an even milder pT region, with xTBAB increasing from 0.0062 to 0.0359. The phase equilibrium pT conditions of xTBAB = 0.0529 appear more unstable than those of xTBAB = 0.0359. This TBAB concentration dependence in the N2−TBAB−water systems is a common tendency in other B

DOI: 10.1021/acs.jced.9b00210 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 2 shows the Raman spectra of the C−H stretching vibrations of TBA+ molecules (2850−3050 cm−1) and the O−

Figure 1. Phase equilibrium pT conditions in the N2−TBAB−water systems (xTBAB = 0.0062, 0.0138, 0.0234, 0.0359, and 0.0529). Green circles: xTBAB = 0.0062 (TBAB/water = 1:160) in this study. Blue circles: xTBAB = 0.0138 (TBAB/water = 1:71.5) in this study. Purple circles: xTBAB = 0.0234 (TBAB/water = 1:41.7) in this study. Red circles: xTBAB = 0.0359 (TBAB/water = 1:26.8) in this study. Black circles: xTBAB = 0.0529 (TBAB/water = 1: 17.9) in this study. Opened green squares: xTBAB = 0.0062 (Muromachi et al.). Opened blue squares: xTBAB = 0.0138 (Muromachi et al.). Opened black squares: xTBAB = 0.0256 (Muromachi et al.). Opened diamonds: TBAB·26H2O in xTBAB = 0.0062 (green), 0.0138 (blue), 0.0234 (purple), and 0.0359 (red).11 Opened triangles: TBAB·38H2O in xTBAB = 0.0062 (green), 0.0138 (blue), 0.0234 (purple), and 0.0359 (red).11 Black line: N2− water system.23

Figure 2. Raman spectra of C−H vibrations of TBA+ molecules and O−H vibrations in the hydrate structure. (a−j) Crystal samples in N2−TBAB−water systems: (a,b) xTBAB = 0.0062; (c,d) xTBAB = 0.0138; (e,f) xTBAB = 0.0234; (g,h) xTBAB = 0.0359; (i,j) xTBAB = 0.0529. (k) Pure TBAB·26H2O hydrate. (l) Pure TBAB·38H2O hydrate. The blue and red spectra are obtained from crystals sampled at the lower p (0.2−0.3 MPa) and higher p (0.9 MPa) values, respectively. All spectra were collected at 223 K.

gas−TBAB−water systems.4,24,25 The equilibrium pT data obtained are summarized in Table 2. Examples of the determining phase equilibrium pT conditions are displayed in Figure S1 in the Supporting Information file. The equilibrium pT data in the xTBAB = 0.0062 and 0.0138 show discontinuous curves with inflection points, whereas data in the xTBAB = 0.0234, 0.0359, and 0.0529 show no clear changing points in the experimental pressure range. The inflection points in the xTBAB = 0.0062 and 0.0138 appear at around p = 0.6 MPa. The data obtained in the xTBAB = 0.0062 and 0.0138 at p > 0.6 MPa are in agreement with the literature.14 At p > 1 MPa, a study of SXRD shows that the TBAB hydrates enclosing N2 exhibit orthorhombic TBAB· (38H2O or 38.1H2O) with D′ cages (hereafter referred to as orthorhombic TBAB hydrate (D′)).14 Therefore, orthorhombic TBAB hydrates (D′) would be formed at p > 0.6 MPa. In Kr−TBAB−water systems, inflection points appear, showing crystal-phase transitions.17 In N2−TBAB−water systems, subtle inflection points at p = 3−5 MPa show crystal-phase transition from orthorhombic TBAB·38.1H2O (D′) to orthorhombic TBAB·38H2O (D′).14

H stretching vibrations of water molecules (3050−3500 cm−1). Figure 2k,l show the spectra for pure TBAB·26H2O and TBAB· 38H2O hydrates, respectively. Hereafter, the pure TBAB· (26H2O or 38H2O) hydrate refers to crystals formed under atmospheric conditions. It is evident that the C−H vibrations of TBA+ have significantly different crystal systems with distinct hydration numbers. As the spectral difference is caused by the crystal structure, namely, the water framework around the TBA+ molecules, the C−H vibrations are sensitive to their surroundings of TBA+ in crystal structures, as shown in Figure 2k,l.22 The Raman spectra in the xTBAB = 0.0062 and 0.0138 show similar C−H stretching profiles (Figure 2a−d) despite the

Table 2. Liquid−Hydrate−Vapor, Three-Phase Equilibrium Pressure−Temperature Conditions in the N2−TBAB−Water Systems xTBAB = 0.0062 Ta/K

pb/MPa

278.95 0.171 279.35 0.315 279.65 0.454 (gradient change) 280.25 0.708 280.75 0.941

xTBAB = 0.0138 Ta/K

pb/MPa

281.85 0.116 282.05 0.178 282.25 0.292 (gradient change) 282.65 0.527 282.95 0.689 283.25 0.922

xTBAB = 0.0234

xTBAB = 0.0359

xTBAB = 0.0529

Ta/K

pb/MPa

Ta/K

pb/MPa

Ta/K

pb/MPa

284.75 284.85 284.95 285.05 285.15

0.136 0.330 0.482 0.633 0.889

285.55 285.65 285.85 285.95

0.170 0.307 0.595 0.943

285.35 285.45 285.55 285.75

0.215 0.410 0.608 0.905

a Expanded uncertainties in the temperature measurements were estimated to be ±0.1 K, with a confidence level of ∼95%. bUncertainty in the pressure measurements was ±0.005 MPa, with a confidence level of ∼95%.

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DOI: 10.1021/acs.jced.9b00210 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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inflection points. Unfortunately, no clear spectral evidence was observed at the inflection points. The inflection points at p = 0.6 MPa may originate from a delicate structural change, like orthorhombic TBAB 38.1H2O (D′) to orthorhombic TBAB· 38H2O (D′) at p = 3−5 MPa.14 The Raman spectra in the xTBAB = 0.0062 and 0.0138 reveal different asymmetric C−H stretching modes in the CH3 group of TBA+ molecules (lightblue region in Figure 2) from the pure TBAB·38H2O hydrate (Figure 2l). The differences in the C−H vibrations of the −CH3 group would indicate some type of structural change from pure TBAB·38H2O. As the equilibrium pT curves above the inflection points in the xTBAB = 0.0062 and 0.0138 agree with the literature data14 as shown in Figure 1, the C−H stretching pattern as shown in Figure 2a−d may be a fingerprint of the orthorhombic TBAB hydrate (D′). Other gas−TBAB−water systems with high xTBAB > 0.0234 show phase transitions from TBAB·26H2O hydrate structures to other crystal systems under certain pressure conditions.16,17,26 In the xTBAB = 0.0234, 0.0359, and 0.0529, the Raman spectra (Figure 2e−j) in the C−H stretching profiles are similar to those of the pure TBAB·26H2O hydrate (Figure 2k), and there were no spectral changes caused by pressure changes. Here, the dissociation T (Tdis) at 0.1 MPa of samples of the N2−TBAB−water system (xTBAB = 0.0234, 0.0359, and 0.0529) is slightly increased compared to that of the pure TBAB·26H2O hydrate (opened diamonds in Figure 1), preferred in the TBAB−water systems (xTBAB = 0.0234, 0.0359, and 0.0529). This tendency could originate from the fact that the TBAB hydrate is stabilized by the enclosing guest molecules in the same way as the Kr−TBAB−water systems.17 On the other hand, the Tdis of samples in the N2−TBAB− water systems with lower xTBAB shows lower temperature shifts than those of the pure TBAB·38H2O hydrates (opened triangles in Figure 1). Despite the guest molecules being in the cages, TBAB hydrates enclosing N2 in lower xTBAB are considered to be less unstable than pure TBAB·38H2O hydrates. Figure 3 shows the N−N vibrational modes of the N2 molecules enclosed in two types of TBAB hydrates that would be formed at low xTBAB (0.0062 and 0.0138) and high xTBAB (0.0234, 0.0359, and 0.0529). As no crystallographic evidence exists (e.g., SXRD results), in this study, we named the TBAB hydrates at low and high xTBAB as TBAB hydrates (I) and (II), respectively. Here, the C−H vibration patterns of TBAB hydrates (I) and (II) were almost the same as those at low x TBAB (Figure 2a−d) and high x TBAB (Figure 2e−j), respectively. The peak at approximately 2326 cm−1 is from N2 molecules adsorbed or condensed on the surface of the sample.27 One symmetric N−N peak was observed for TBAB hydrate (I) (Figure 3a). The SXRD result was that TBAB hydrates at low xTBAB only enclose N2 molecules in D cages.14 If the crystal structures of TBAB hydrates in this study are the same as the crystals formed at 2.1 MPa,14 a single symmetric N−N peak at 2324.4 cm−1 would arise from N2 molecules enclosed in D cages. The N−N vibrational peak of TBAB hydrate (II) (Figure 3b) is asymmetric with a shoulder at lower wavenumbers. The N−N vibrational peak can be deconvoluted into two peaks, one at 2324.2 cm−1 and the other at 2323.7 cm−1. The Raman results presented herein indicate that the N2 molecules in the TBAB hydrates formed at high xTBAB would be enclosed in two types of cages. The N−N vibration peak positions in TBAB

Figure 3. N−N vibration modes of N2 molecules enclosed in TBAB hydrate systems. (a) TBAB hydrates in xTBAB = 0.0263 (TBAB/water = 1:38). (b) TBAB hydrates in xTBAB = 0.0359 (TBAB/water = 1:41.7). (c) sI (N2 + CH4) hydrate.21 (d) sII (N2 + C3H8) hydrate.21 The weak peak located at 2325.8 cm−1 in (a,b) is attributed to N2 molecules adsorbed or condensed on the crystal grains.27 Raman spectra of (c,d) are adapted with permission from J. Phys. Chem. C, 2015, 119 (17), 9069−9075. Copyright 2015 American Chemical Society.

hydrates (I) and (II), as shown in Figure 3, are summarized in Table 3. The previously published 13C NMR results indicate Table 3. Raman Peaks of N2 Molecules Enclosed in Each Crystal TBAB hydrate (I) prepared from pure TBAB·38H2O

TBAB hydrate (II) prepared from pure TBAB·26H2O

Figure 3a

Figure 3b

2324.40 cm−1 (0.70 cm−1)a

2324.20 cm−1 (0.71 cm−1)a 2323.72 cm−1 (0.67 cm−1)a

a

Values in parenthesis are the full-width at half-maximum of the peaks.

that the CH4 molecules can be enclosed within a type of D cage in a tetragonal TBAB hydrate.28 Therefore, in this study, the N−N vibrational peak, at either 2324.2 or 2323.7 cm−1, might be assigned to N2 molecules enclosed in a D cage of the crystal structure, as predicted by Shimada et al.29 Unfortunately, the information obtained from the Raman technique was insufficient to provide an understanding of the crystal structures formed in N2−TBAB−water systems under 1 MPa. Further experimental techniques are required to identify the crystal and cage structures.

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CONCLUSIONS We investigated the phase stability of TBAB hydrate crystal structures enclosing N2, by measuring the phase equilibrium conditions in N2−TBAB−water systems at different molar fractions (xTBAB = 0.0062, 0.0138, 0.0234, 0.0359, and 0.0529). At low TBAB concentrations (xTBAB = 0.0062 and 0.0138), the structure of hydrate crystals are different from that of the pure TBAB·38H2O, whereas in systems with high TBAB concenD

DOI: 10.1021/acs.jced.9b00210 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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(7) 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. (8) Shin, K.; Kim, Y.; Strobel, T. A.; Prasad, P. S. R.; Sugahara, T.; Lee, H.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Tetra-n-Butylammonium Borohydride Semiclathrate: a Hybrid Material for Hydrogen Storage. J. Phys. Chem. A 2009, 113, 6415−6418. (9) Franks, F. Water: A Comprehensive Treatise; Plenum Press: London, 1973; p 2. (10) Aladko, L. S.; Dyadin, Y. A.; Rodionova, T. V.; Terekhova, I. S. Clathrate hydrates of tetrabutylammonium and tetraisoamylammonium halides. J. Struct. Chem. 2002, 43, 990−994. (11) Oyama, H.; Shimada, W.; Ebinuma, T.; Kamata, Y.; Takeya, S.; Uchida, T.; Nagao, J.; Narita, H. Phase diagram, latent heat, and specific heat of TBAB semiclathrate hydrate crystals. Fluid Phase Equilib. 2005, 234, 131−135. (12) Shimada, W.; Shiro, M.; Kondo, H.; Takeya, S.; Oyama, H.; Ebinuma, T.; Narita, H. Tetra-n-butylammonium bromide-water (1/ 38). Acta Crystallogr C 2005, 61, o65−o66. (13) Muromachi, S.; Udachin, K. A.; Alavi, S.; Ohmura, R.; Ripmeester, J. A. Selective occupancy of methane by cage symmetry in TBAB ionic clathrate hydrate. Chem. Commun. 2016, 52, 5621− 5624. (14) Muromachi, S.; Hashimoto, H.; Maekawa, T.; Takeya, S.; Yamamoto, Y. Phase equilibrium and characterization of ionic clathrate hydrates formed with tetra- n -butylammonium bromide and nitrogen gas. Fluid Phase Equilib. 2016, 413, 249−253. (15) Muromachi, S.; Udachin, K. A.; Shin, K.; Alavi, S.; Moudrakovski, I. L.; Ohmura, R.; Ripmeester, J. A. Guest-induced symmetry lowering of an ionic clathrate material for carbon capture. Chem. Commun. 2014, 50, 11476−11479. (16) Jin, Y.; Nagao, J. Change in the Stable Crystal Phase of Tetra-nbutylammonium Bromide (TBAB) Hydrates Enclosing Xenon. J. Phys. Chem. C 2013, 117, 6924−6928. (17) Jin, Y.; Kida, M.; Nagao, J. Phase Transition of Tetra-nbutylammonium Bromide Hydrates Enclosing Krypton. J. Chem. Eng. Data 2016, 61, 679−685. (18) Sum, A. K.; Burruss, R. C.; Sloan, E. D. Measurement of Clathrate Hydrates via Raman Spectroscopy. J. Phys. Chem. B 1997, 101, 7371−7377. (19) Subramanian, S.; Sloan, E. D. Trends in Vibrational Frequencies of Guests Trapped in Clathrate Hydrate Cages. J. Phys. Chem. B 2002, 106, 4348−4355. (20) Ohno, H.; Kida, M.; Sakurai, T.; Iizuka, Y.; Hondoh, T.; Narita, H.; Nagao, J. Symmetric Stretching Vibration of CH4 in Clathrate Hydrate Structures. Chem Phys Chem 2010, 11, 3070−3073. (21) Jin, Y.; Kida, M.; Nagao, J. Structural Characterization of Structure H (sH) Clathrate Hydrates Enclosing Nitrogen and 2,2Dimethylbutane. J. Phys. Chem. C 2015, 119, 9069−9075. (22) Hashimoto, S.; Sugahara, T.; Moritoki, M.; Sato, H.; Ohgaki, K. Thermodynamic stability of hydrogen. Chem. Eng. Sci. 2008, 63, 1092−1097. (23) The Center for Hydrate Research, The Colorado School of Mines, Hydrate Prediction Program CSMGem, 2007, http://hydates. mines.edu/CHR/Software.html (retrieved Apr 22, 2019). (24) Mohammadi, A. H.; Eslamimanesh, A.; Belandria, V.; Richon, D. Phase Equilibria of Semiclathrate Hydrates of CO2, N2, CH4, or H2+ Tetra-n-butylammonium Bromide Aqueous Solution. J. Chem. Eng. Data 2011, 56, 3855−3865. (25) Ye, N.; Zhang, P. Equilibrium Data and Morphology of Tetran-butyl Ammonium Bromide Semiclathrate Hydrate with Carbon Dioxide. J. Chem. Eng. Data 2012, 57, 1557−1562. (26) Chazallon, B.; Ziskind, M.; Carpentier, Y.; Focsa, C. CO2 Capture Using Semi-Clathrates of Quaternary Ammonium Salt: Structure Change Induced by CO2 and N2 Enclathration. J. Phys. Chem. B 2014, 118, 13440−13452. (27) Chazallon, B.; Oancea, A.; Capoen, B.; Focsa, C. Ice mixtures formed by simultaneous condensation of formaldehyde and water: an

trations (xTBAB = 0.0234, 0.0359, and 0.0529), TBAB hydrate crystals formed are similar to the pure TBAB·26H2O structure. Considering the expected temperature of dissociation at 0.1 MPa, TBAB hydrates at low xTBAB would be less unstable than the pure TBAB·38H2O hydrates. TBAB hydrates with high xTBAB would be less stable than pure TBAB·26H2O hydrates. TBAB hydrate crystals formed in systems with low xTBAB show a symmetric N−N vibrational peak of enclosed N2 molecules at 2324.3 cm−1. On the other hand, TBAB hydrate crystals formed at high xTBAB show an asymmetric N−N vibrational peak. The asymmetric N−N vibrational peak can be deconvoluted into two peaks, one at 2324.5 cm−1 and the other at 2324.1 cm−1. Our Raman results suggest the existence of two kinds of vacant cages in TBAB hydrate crystals formed in systems with high xTBAB.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.9b00210.

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Examples of trajectories to determine phase equilibrium pT conditions in N2−TBAB−water systems (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-11-857-8526. Fax: +8111-857-8417. ORCID

Yusuke Jin: 0000-0002-6256-7278 Masato Kida: 0000-0003-0075-9781 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Drs. Y. Konno (the University of Tokyo), M. Oshima (AIST), and H. Haneda (AIST) for their experimental support.

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REFERENCES

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DOI: 10.1021/acs.jced.9b00210 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jced.9b00210 J. Chem. Eng. Data XXXX, XXX, XXX−XXX