butylammonium Bromide Hydrates Enclosing Krypton

Nov 10, 2015 - ABSTRACT: The phase equilibrium conditions for krypton. (Kr)−tetra-n-butylammonium bromide (TBAB)−water systems were determined usi...
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Phase Transition of Tetra‑n‑butylammonium Bromide Hydrates Enclosing Krypton 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 S Supporting Information *

ABSTRACT: The phase equilibrium conditions for krypton (Kr)−tetra-n-butylammonium bromide (TBAB)−water systems were determined using an isochoric method. The pressure and temperature ranges were (0.06 to 1.0) MPa and (280 to 290) K, respectively, and TBAB solutions had TBAB molar fractions, xTBAB, of 0.0062, 0.0138, 0.0234, and 0.0359. A second order transition of the TBAB hydrate was observed in all the Kr− TBAB−water systems. In the region at lower pressure than the phase transition point, the Kr−TBAB−water systems with low concentration (xTBAB = 0.0062 and 0.0138) and high concentration (xTBAB = 0.0234 and 0.0359) prefer to form TBAB·38H2O and TBAB·26H2O hydrates, respectively. However, a new TBAB hydrate was observed as a stable crystal structure in the higher pressure regions. Raman spectrum of the new TBAB hydrate shows band shapes remarkably similar to that of pure TBAB·38H2O with the crystalline space group Pmma in the frequency ranges of the lattice for C−C stretching, C−H bending, the C−H stretching bands of the −CH2 groups of TBA+ molecules, and the O−H stretching modes of water molecules, excluding the C−H stretching bands of the CH3 groups of TBA+ molecules.

1. INTRODUCTION

solutions with xTBAB < 0.0121, whereas TBAB·26H2O hydrate is the preferred crystal structure for xTBAB > 0.0121.24 Xenon (Xe)−TBAB−water and CO2−TBAB−water systems (xTBAB = 0.0359) show a stable phase change from TBAB· 26H2O to TBAB·38H2O hydrate, even though the TBAB solution (xTBAB = 0.0359) is at stoichiometric concentration for TBAB·26H2O hydrate at atmospheric conditions.25,26 However, no crystal transition has been observed in the CH4− TBAB−water system (xTBAB = 0.0359).9,16,19 If their diameters are compared, then Xe, CO2, and CH4 molecules have diameters of approximately (0.46, 0.51, and 0.44) nm, respectively; thus, CH4 molecules are smaller than Xe and CO2 molecules. Interactions between guest and water molecules in clathrate structures are affected not only by the diameter but also by the shape of the guest molecules in the water frame cages.27 Therefore, noble gases with spherical shape are valuable molecules to understand clathrate hydrate systems. Insight into noble gas−TBAB−water systems would be helpful for designing gas storage systems using gas−TBAB−water systems. Therefore, the noble gas (argon (Ar), krypton (Kr), and Xe)− TBAB−water systems are already reported.25,28−30 In the reported noble gas−TBAB−water systems, only Xe−TBAB−

Peralkyl ammonium salts are quaternary ammonium salts that form clathrate hydrates (hereafter hydrates), which are crystalline compounds with polyhedral structures of hydrogen-bonded water frameworks.1,2 Note that peralkyl ammonium salt hydrates form under atmospheric conditions.1 Tetran-butylammonium bromide (TBAB) molecules are a type of peralkyl ammonium salt and TBAB−water systems are of considerable interest as liquid−solid phase change materials for thermal energy storage.3−7 Moreover, they can store gas molecules as guest molecules in water frame cages and can be stable at lower pressure and higher temperature conditions than the equilibrium conditions of pure hydrates enclosing the same guest.8−19 Hence, semiclathrate hydrates like TBAB hydrates are attractive for industrial uses such as gas storage and separation media.13,18,20−22 The TBAB−water system primarily forms five TBAB hydrates with different hydration numbers of 24, 26, 32, 36, and 38.23 TBAB hydrates formed under atmospheric conditions possess mainly two structures: TBAB·26H2O and TBAB· 38H2O.24 The stoichiometric TBAB molar fractions, xTBAB, for TBAB·26H2O and TBAB·38H2O hydrates are 0.0359 and 0.0256, respectively. The thermodynamic phase stability of each TBAB hydrate depends on the TBAB concentration. Under atmospheric conditions, TBAB·38H2O hydrates are thermodynamically more stable than TBAB·26H2O hydrates in TBAB © XXXX American Chemical Society

Received: October 1, 2015 Accepted: November 3, 2015

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water system shows the crystal transition.25 In this study, we obtained the TBAB solution−TBAB hydrate−vapor (L−H−V) three-phase equilibrium pressure−temperature (pT) data for the Kr−TBAB−water system in the TBAB solution range xTBAB = 0.0062 to 0.0359. Because Kr has a smaller diameter of 0.40 nm compared to CH4 and can form gas hydrates with H2O molecules,31 it is expected to be captured in the TBAB hydrate structures. Knowledge about the equilibrium in the Kr− TBAB−water system might help to capture the ever-increasing radioactive 85Kr (half-life: 10.76 years) that is emitted from nuclear power plants and nuclear fuel reprocessing plants into atmosphere.32 Using the measured equilibrium pressure− temperature conditions and crystallographic analysis, we report that the Kr−TBAB−water system shows structural phase transitions similar to those seen in Xe−TBAB−water and CO2−TBAB−water systems.

from the change in slope during the increase in pressure. Figure 1 shows examples of determination of the equilibrium pT

2. EXPERIMENTAL SECTION Materials. ReagentPlus grade TBAB with 99 % purity (Sigma-Aldrich Co., Inc.) and research grade Kr gas with 99.995 % purity (Takachiho Chemical Industrial Co., Ltd., Japan) were used. Water was purified by ultrafiltration, reverse osmosis, deionization, and distillation (> 18 MΩ·cm). All of these materials were used without further purification. Experimental Apparatus. The pressure vessel, in which the inner volume was approximately 120 cm3, was made of SUS316 stainless steel and equipped with a fin for stirring the solution. The sample temperature (T) was reproducible within ± 0.02 K by connecting a cold-junction compensation device, which can establish a reference temperature of 273.15 K by using an ice−water slurry. When the uncertainty in controlling the sample temperature is considered, the expanded uncertainty of the obtained equilibrium T was estimated to be ± 0.1 K with a confidence level of ∼95 %. The uncertainty of the pressure (p) measurements was ± 0.005 MPa with a confidence level of ∼ 95 %. Further details are given in a previous study.17 Equilibrium Pressure−Temperature Measurement. We measured the L−H−V three-phase equilibrium pT data for the Kr−TBAB−water system. We obtained the data for four TBAB solutions: xTBAB = 0.0062, 0.0138, 0.0234, and 0.0359. Initially, TBAB aqueous solution with a designated concentration (70 cm3) was poured into the pressure vessel. After air was eliminated from the vessel using a vacuum pump, the vessel was pressurized to 1 MPa by Kr. Subsequently, the gases in the vessel were removed using a vacuum pump. This airelimination procedure was performed twice. Finally, we pressurized the vessel with Kr under designated pressure conditions at room temperature. The vessel temperature was decreased to nucleate hydrate crystals with stirring at approximately 1000 r/min. After hydrate crystal formation was confirmed by a distinct pressure decrease and direct visual observation through the optical windows, the system temperature was increased in increments of 0.1 K to 0.5 K. We increased the system temperature in increments of 0.5 K in the pressure region lower than the initial pressure. When the system pressure increased because of hydrate dissociation and became close to the initial pressure, the system temperature was increased in increments of 0.1 K. At each temperature step, the sample temperature was maintained for (8 to 24) h until the increased system pressure remained constant. After complete hydrate dissociation, the system pressure increased with a gentle slope against the temperature increment. Consequently, the hydrate phase equilibrium pT conditions can be determined

Figure 1. Scheme of determining equilibrium pT conditions in the Kr−TBAB−water system (xTBAB = 0.0359) at (a) 289.45 K and 0.961 MPa and (b) 286.25 K and 0.217 MPa: open circles, pT conditions measured by increase in temperature; closed circles, L−H−V equilibrium pT conditions.

conditions in the Kr−TBAB−water system (xTBAB = 0.0359). Changes in the slope were observed at 289.45 K and 286.25 K in Figure 1a and b, respectively. In these experimental runs, we identified the points (289.45 K and 0.961 MPa) and (286.25 K and 0.217 MPa) as the L−H−V three-phase equilibrium pT conditions in the Kr−TBAB−water system (xTBAB = 0.0359). The entire pT trajectories of Figure 1a and b, including the cooling process, are shown in Figures S1a and S1b in the Supporting Information. Crystallographic Analysis. To distinguish between the hydrate structures formed in each Kr−TBAB−water system, Raman spectra of samples obtained from the Kr−TBAB−water systems were measured using a Raman spectrometer (LabRAM HR-800, Horiba Ltd.) with a laser line of 532 nm (Torus 532, Laser Quantum) and a low temperature stage (HFS600E-P, Linkam Scientific Instruments). The detailed setups and method were described in a previous study.25,33 In TBAB hydrates, a tetra-n-butyl ammonium cation (TBA+) molecule is encaged and located at the center of four water-framed cages. The water framework influences the C−H and C−C vibrations of the CH3− and −CH2− groups of the TBA+ molecule. Because the influences are different in each TBAB hydrate structure, the crystal structures of samples can be determined using Raman spectra of the C−H (2800 to 3050 cm−1) and C− C vibrations (950 to 1500 cm−1).11 In addition, lattice vibration (around 200 cm−1) and O−H vibration (3050 to 3500 cm−1) of the samples were measured. Samples were prepared using a B

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system30 and the L−H−V three-phase equilibrium pT data for the pure Kr−water system34 are plotted in Figure 2. For comparison, dissociation temperature data for the pure TBAB· 26H2O and TBAB·38H2O hydrates in the TBAB−water system (xTBAB = 0.0062, 0.0138, 0.0234, and 0.0359) are also plotted in Figure 2. The obtained data are summarized in Table 1. In the negative pressure range (p < 0.1 MPa), the initial system pressure was decreased using a vacuum pump. Moreover, negative pressure values were observed during hydrate formation under the initial low pressure conditions. In the experimental pressure ranges, the L−H−V three-phase equilibrium pT conditions of the Kr−TBAB−water systems (xTBAB = 0.0062 and 0.0138) agree with the literature data, excluding the equilibrium point at approximately 281 K.30 The L−H−V three-phase equilibrium pT conditions of all the Kr− TBAB−water systems in this study are milder than those of the pure gas−water systems (sII Kr hydrates). This is similar to the results seen in other gas−TBAB−water systems.9,11,17,25,35−37 The equilibrium temperature values in the Kr−TBAB−water systems (xTBAB = 0.0062, 0.0138, 0.0234, and 0.0359) shifted to the high temperature region with increase in TBAB concentration under the same system pressure conditions. For example, in the Kr−TBAB−water system (xTBAB = 0.0138), the pressure reduction and temperature increase compared to pure Kr hydrates are 10 MPa and 30 K, respectively, at ∼289 K.34,38 At 0.1 MPa, the TBAB hydrates formed in the Kr− TBAB−water system show a higher equilibrium temperature than the dissociation temperature for pure TBAB hydrates expected in each TBAB solution. This high-temperature shift of the dissociation condition could originate from cage stabilization by enclosing guest molecules in the vacant cages of the TBAB hydrates. This tendency is similar to the results seen in other gas−TBAB−water systems.14,17,25 Furthermore, all of the Kr−TBAB−water systems show discontinuous equilibrium pT curves. The inflection points, Pinf, for each Kr−TBAB−water system (xTBAB = 0.0062, 0.0138, 0.0234, and 0.0359) are approximately (0.2, 0.2, 0.2, and 0.3) MPa. This inflection in the equilibrium pT curve suggests that a structural change of the TBAB hydrate crystals is preferred in the experimental system. From the Clausius−Clapeyron equation, the enthalpy of dissociation, ΔH, can be estimated in each system: xTBAB = 0.0062 is approximately (431 and 194) kJ·mol−1gas in the region at low and high temperature compared

pressure vessel for equilibrium measurements and crystals were obtained from the crystal−solution mixture in the vessel by filtration in a cold room at 278 K. The separated crystals were quenched in liquid nitrogen to preserve the hydrate samples for ex situ analysis and atmospheric treatment. After the crystals were grained, Raman spectra of the sample crystals were collected at 223 K using a superlong working distance objective lens (Olympus SLMPLN 50× ; numerical aperture of 0.35; laser spot diameter of < 2 μm).

3. RESULTS AND DISCUSSION Phase Equilibrium pT Conditions. Figure 2 shows the obtained L−H−V three-phase equilibrium pT data for the Kr−

Figure 2. Equilibrium pressure−temperature conditions of the Kr− TBAB−water, Kr hydrate, and TBAB hydrate systems: circles, Kr− TBAB−water system (this study) [black, xTBAB = 0.0062; blue, xTBAB = 0.0138; red, xTBAB = 0.0234; green, xTBAB = 0.0359]; crosses, Kr− TBAB−water system (Babaee et al.30) [black, xTBAB = 0.0062; blue, xTBAB = 0.0138; Squares, pure Kr−water system (Sugahara et al.34)]; diamonds, TBAB·26H2O hydrate system (Oyama et al.24) [black, xTBAB = 0.0062; blue, xTBAB = 0.0138; red, xTBAB = 0.0234; green, xTBAB = 0.0359]; plus, TBAB·38H2O hydrate system (Oyama et al.24) [black, xTBAB = 0.0062; blue, xTBAB = 0.0138; red, xTBAB = 0.0234; green, xTBAB = 0.0359].

TBAB−water system (xTBAB = 0.0062, 0.0138, 0.0234, and 0.0359) in the pressure range of approximately (0.06 to 1.0) MPa. Moreover, the literature data for the Kr−TBAB−water

Table 1. Equilibrium Pressure−Temperature Conditions of the Kr−TBAB−Water Systems xTBAB = 0.0062 T

a

K 280.65 281.05 281.65 281.95 (phase change) 282.85 283.55 284.25 284.95 285.85 286.85

xTBAB = 0.0138 p

b

MPa 0.089 0.113 0.162 0.199 0.294 0.380 0.472 0.578 0.744 0.939

a

T

K 282.75 283.05 283.35 283.65 (phase change) 284.25 285.15 286.15 287.25 288.15

xTBAB = 0.0234 p

b

a

T

MPa

K

0.066 0.085 0.109 0.134

285.15 285.25 285.35 (phase change) 286.15 287.15 287.85 288.35 289.05 289.45 289.75

0.190 0.279 0.411 0.599 0.761

xTBAB = 0.0359 b

p

MPa 0.106 0.140 0.199 0.311 0.446 0.548 0.649 0.797 0.889 0.963

a

T

K 285.95 286.15 286.25 286.45 (phase change) 286.85 287.55 288.15 288.95 289.45

pb MPa 0.106 0.162 0.217 0.313 0.417 0.541 0.666 0.834 0.961

a Expanded uncertainties in 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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to the Pinf; xTBAB = 0. 0138 is approximately (530 and 240) kJ· mol−1gas in the lower and higher Pinf regions (Pinf = 0.2 MPa), respectively; xTBAB = 0.0234 is approximately (2100 and 230) kJ·mol−1gas in the lower and higher Pinf regions (Pinf = 0.2 MPa), respectively; and xTBAB = 0.0359 is approximately (1500 and 230) kJ·mol−1gas in the lower and higher Pinf regions (Pinf = 0.3 MPa), respectively. The ΔH values for each system in the lower p region were widely distributed from approximately (400 to 2100) kJ·mol−1gas, whereas the ΔH values of each system in the higher p region were approximately (200 to 240) kJ·mol−1gas. Because of changes in the ΔH values in the lower and higher p regions than Pinf, the stable crystal phase in each system will change at the Pinf. The ΔH values are closely related to cage occupation in which guest molecules occupy vacant cages in the hydrate structure.31 A decrease in cage occupation tends to increase the ΔH values. In all the Kr−TBAB−water systems, the TBAB hydrate structure in the lower Pinf region would have many vacant cages. Because of changes in the crystal structure, the TBAB hydrate in the higher Pinf region would enclose more Kr molecules in vacant cages than that in the lower Pinf region. Crystal Characterization. Raman spectra of C−H vibration and O−H vibration modes of TBAB hydrates can be used to distinguish between crystal structures with different hydration numbers.11,15,25 Figures 3 to 5 show Raman spectra

Figure 4. Raman spectra of C−C stretching and C−H bending C−H vibration modes of TBA+ molecules. (a) Pure TBAB·26H2O hydrate. (b) Pure TBAB·38H2O hydrate. (c) to (j) Kr−TBAB−water system: (c) and (d) xTBAB = 0.0062; (e) and (f) xTBAB = 0.0138; (g) and (h) xTBAB = 0.0234; (i) and (j) xTBAB = 0.0359. Blue (labeled “L”) and red (labeled “H”) spectra were collected at the lower and higher Pinf, respectively. All spectra were obtained at 223 K.

Figure 3. Raman spectra of C−H vibration mode of TBA+ molecules and O−H stretching mode of water molecules. (a) Pure TBAB·26H2O hydrate. (b) Pure TBAB·38H2O hydrate. (c) to (j) Kr−TBAB−water system: (c) and (d) xTBAB = 0.0062; (e) and (f) xTBAB = 0.0138; (g) and (h) xTBAB = 0.0234; (i) and (j) xTBAB = 0.0359. Blue (labeled “L”) and red (labeled “H”) spectra were collected at the lower and higher Pinf, respectively. All spectra were obtained at 223 K.

Figure 5. Raman spectra of lattice vibration modes of water and TBA+ molecules. (a) Pure TBAB·26H2O hydrate. (b) Pure TBAB·38H2O hydrate. (c) to (j) Kr−TBAB−water system: (c) and (d) xTBAB = 0.0062; (e) to (f) xTBAB = 0.0138; (g) and (h) xTBAB = 0.0234; (i) and (j) xTBAB = 0.0359. Blue (labeled “L”) and red (labeled “H”) spectra were collected at the lower and higher Pinf, respectively. All spectra were obtained at 223 K.

of TBAB hydrates sampled in each Kr−TBAB−water system. Samples in each Kr−TBAB−water system were synthesized near the higher and lower equilibrium pT conditions than Pinf. The C−C stretching, C−H bending and C−H stretching vibration modes of the TBA+ molecule are primarily observed at approximately (950 to 1400), (1400 to 1500), and (2800 to 3050) cm−1, respectively.39 The O−H stretching vibration modes of H2O molecules are observed at approximately (3050 to 3500) cm−1.40 Moreover, lattice vibration bands are indicators of crystal structure. The Raman peak at around

200 cm −1 is assigned to the lattice vibration band (intermolecular hydrogen stretching band of H2O molecule, O−O vibrational band) of the TBAB hydrate crystal structure.25,26 Moreover, the peaks at around 260 cm−1 are related to the TBA+ molecules. Spectra a and b in Figures 3 to 5 show the C−H and O−H stretching, C−H bending, and C−C stretching lattice vibration modes of the TBAB·26H2O and D

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The new TBAB hydrates show O−H stretching, C−C stretching, C−H bending, and lattice vibration modes similar to TBAB·38H2O hydrates. Nevertheless, the C−H stretching bands of the crystals in the higher Pinf region differ from those of the TBAB·38H2O hydrate, as shown in Figure 3b, c, and e. In detail, the C−H stretching bands of the −CH2− groups of the new hydrate show no change compared to TBAB·38H2O hydrate in the frequency ranges (2850 to 2860 and 2900 to 2950) cm−1, whereas those of the CH3 groups of the new hydrate change in the frequency ranges (2860 to 2900 and 2950 to 3030) cm−1. The change in only the CH3 groups indicates that the interaction between the CH3 groups of the TBA+ molecules and the water framework differs from that in pure TBAB·38H2O hydrates. Chazallon et al. showed a TBAB hydrate with a new crystal phase in CO2−TBAB−water systems (xTBAB = 0.0029).26 Recently, Muromachi et al. reported that TBAB·38H2O hydrates can be synthesized from TBAB solution (xTBAB = 0.0062), and CO2 showed not only the Pmma phase but also an Imma structure.41 Both the Pmma and Imma TBAB·38H2O hydrate structures have dodecahedral (512) cages to enclose guest molecules. Nevertheless, there are highly distorted 512 cages in the Imma TBAB·38H2O hydrate structure. The CH3 groups of TBA+ molecules in Imma TBAB· 38H2O coordinate around the distorted 512 cages, unlike in the case of Pmma TBAB·38H2O. Considering the difference in the C−H stretching modes between the pure (Pmma) TBAB· 38H2O hydrate and the new crystal hydrate, the hydrate crystals obtained in the higher Pinf region are considered to be Imma TBAB·38H2O hydrates. The Kr−TBAB−water systems used in this study show a second-order phase transition, as shown in Figure 2. Unfortunately, the powder X-ray diffraction patterns expected for each crystal structure cannot reveal a sharp distinction between Pmma and Imma TBAB·38H2O hydrates, as shown in Figure S2 in Supporting Information.41,42 Moreover, because the Br atom of TBAB molecule and Kr molecule have high Xray attenuation coefficient values, it is difficult to determine the crystal structure using the powder X-ray diffraction method. Here the phase transition (equilibrium pT condition) in the CO2 −TBAB−water system (xTBAB = 0.0029) shows a continuous curve. Although the diameters or shapes of the guest molecules would lead to phase transition behavior, gas− TBAB−water systems would show a phase transition regardless of molecular size and shape, considering the results seen in three gas−TBAB−water systems (Kr, Xe, and CO2). Therefore, other gas (CH4 and Ar)−TBAB−water systems would show a phase transition. However, because there are few spectral characterizations in the CH4−TBAB−water and Ar−TBAB− water systems, the phase transition is not confirmed yet. To discuss phase transitions in TBAB hydrate systems, Raman spectroscopy may be advantageous.

TBAB·38H2O hydrates. Because the vibration modes of the water framework and those of the TBA+ molecules differ, each TBAB hydrate can be distinguished through its Raman spectrum. In the Kr−TBAB−water system (xTBAB = 0.0359), the C−H, C−C, and O−H vibration bands of the crystal samples obtained in the lower Pinf region (Figures 3i and 4i) were similar to those of the pure TBAB·26H2O hydrates (Figures 3a and 4a). Consequently, hydrate crystals formed in the lower Pinf region would be TBAB·26H2O hydrates from the C−H, C−C, and O−H vibration modes. This result shown from the Raman spectra would be proper, considering that the TBAB−water system (xTBAB = 0.0359) favorably forms TBAB·26H2O hydrate crystals under atmospheric pressure conditions. In the lattice vibration region of Figure 5, Raman peaks related to TBA+ of TBAB·26H2O hydrates without and with Kr molecules are positioned at almost the same position, that is, 261 cm−1. However, the O−O vibrational band of the TBAB hydrates shifted from (193 to 197) cm−1 due to the capture of Kr molecules (Figure 5a and i). This higher frequency shift of the O−O vibrational band shows that Kr in vacant cages changed the strength of the hydrogen bonds in the TBAB·26H2O hydrate structure.40 Moreover, TBAB·38H2O hydrates with/ without Xe enclosed show a frequency shift of the O−O vibrational band.25 In the case of the higher Pinf region, the Raman spectra shown in Figures 3j, 4j, and 5j are altered compared with hydrates formed in the lower Pinf region. The C−H stretching bands of TBA+ in the TBAB hydrates formed in the higher Pinf region show a more split band shape than the bands of the TBAB·26H2O hydrates at lower Pinf (Figure 3i). In the two Kr−TBAB−water systems with lower TBAB concentrations (xTBAB = 0.0062 and 0.0138), changes in the Raman spectra were observed between crystals in the lower and higher Pinf region. In particular, C−H stretching modes in the higher Pinf region (Figure 3d and f) show completely different shapes from those in the lower Pinf region (Figure 3c and e) for the same Kr−TBAB−water system (xTBAB = 0.0062 and 0.0138). The Raman spectra of crystals in the lower Pinf region (spectra c and e in Figures 3 to 5) had almost the same profiles as those of the pure TBAB·38H2O hydrates without guest molecules (spectrum b in Figures 3 to 5). However, the Raman spectra in the higher Pinf region were similar for the same Kr− TBAB−water system (xTBAB = 0.0234 and 0.0359); however, those spectra differed significantly from both the TBAB·26H2O and TBAB·38H2O hydrates. Comparison of the Raman spectra shown in Figures 3 to 5 indicates that the stable crystals in the higher Pinf region in the Kr−TBAB−water system may be hydrates with a new crystal structure. Jin and Nagao25 demonstrated that a second-order phase transition from TBAB·26H2O to TBAB·38H2O hydrates occurs in the Xe−TBAB−water system (xTBAB = 0.0359) and revealed that TBAB·38H2O hydrates are the favorable crystal phase in the higher Pinf region rather than TBAB·26H2O. Moreover, in the CO2−TBAB−water system (xTBAB = 0.0359), the Raman spectra show that the TBAB·38H2O hydrate is a favorable crystal phase and the TBAB·26H2O hydrate is a metastable phase in the high-pressure/temperature region.26 Therefore, a phase transformation of the hydrates from TBAB· 26H2O/TBAB·38H2O to another crystal structure is thought to occur in the higher Pinf region in the Kr−TBAB−water system, considering the Xe−TBAB−water and CO2−TBAB−water systems.

4. CONCLUSIONS We measured the L−H−V three-phase equilibrium pressure− temperature (pT) data for the Kr−TBAB−water system (xTBAB = 0.0062, 0.0138, 0.0234, and 0.0359) using an isochoric method. The experimental pressure and temperature range from (0.06 to 1.0) MPa and from (280 to 290) K, respectively. The L−H−V three-phase equilibrium pT conditions for the Kr−TBAB−water systems were at a lower pressure and higher temperature region than those for the pure Kr hydrate, similar to the results seen for other guest−TBAB−water systems. In the experimental pressure and temperature ranges, second E

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order transitions of the L−H−V three-phase equilibrium pT conditions were observed in all of the Kr−TBAB−water systems. Under our experimental conditions, the dissociation enthalpy changed by two to five times in the regions at lower pressure than the phase change point. For example, the Kr− TBAB−water system (xTBAB = 0.0359) shows an extreme enthalpy change from approximately (230 to 1500) kJ·mol−1gas in the regions at higher and lower pressure than the phase change point. In addition to equilibrium measurements, we observed Raman spectra of the TBAB hydrates formed in the lower and higher pressure regions to identify their crystal structures. Raman spectra are obtained in the frequency range of (150 to 300) cm−1 (lattice vibration mode), (950 to 1550) cm−1 (C−C stretching and C−H bending modes), and (2800 to 3500) cm−1 (C−H and O−H stretching modes). In the region at lower pressure than the phase transition point, the Kr−TBAB−water systems with low concentration (xTBAB = 0.0062 and 0.0138) and high concentration (xTBAB = 0.0234 and 0.0359) prefer to form TBAB·38H2O and TBAB·26H2O hydrates, respectively. In the higher pressure regions of all of the Kr−TBAB−water systems (xTBAB = 0.0062, 0.0138, 0.0234, and 0.0359); however, stable crystals showed Raman spectra with new features, neither TBAB·38H2O nor TBAB·26H2O hydrates. The C−H stretching bands of the CH3 group of the TBA+ molecules of the new TBAB hydrate differed from those of both TBAB·38H2O and TBAB·26H2O, whereas the other observed bands showed remarkably similar band shapes to those of pure TBAB·38H2O with the Pmma crystalline space group. Unfortunately, because the bromide atom and Kr molecule have high X-ray attenuation coefficient values, it is difficult to determine the crystal structure using the powder Xray diffraction method. The stable crystal in the region at higher pressure than the phase transition point in the Kr−TBAB− water systems may be orthorhombic Imma TBAB·38H2O with lower symmetry than Pmma TBAB·38H2O, considering studies on CO2−TBAB−water systems and the spectral change of the C−H stretching bands of the CH3 group.

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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.5b00842. Examples of pressure−temperature trajectory including cooling (hydrate formation) process and powder X-ray diffraction patterns expected for Pmma and Imma TBAB· 38H2O hydrates. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Address: 2-17-2-1, Tsukisamu-Higashi, Toyohira-Ku, Sapporo 062-8517, Japan. Tel.: +81-11-8578526. Fax: +81-11-857-8417. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Y. Konno and Dr. M. Oshima of AIST for valuable discussions. REFERENCES

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DOI: 10.1021/acs.jced.5b00842 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jced.5b00842 J. Chem. Eng. Data XXXX, XXX, XXX−XXX