Synthesis, characterization and thermal-property measurements of ionic semi-clathrate hydrates formed with tetrabutylphosphonium chloride and tetrabutylammonium acrylate

Abstract

This paper reports an experimental study on the formation of the two new semi-clathrate hydrates with tetrabutylphosphonium chloride (TBPC) and tetrabutylammonium acrylate (TBAAc). The hydrate formation was demonstrated by the measurements of temperature-composition phase diagrams and dissociation heat of the hydrates, visual observations of the hydrate crystals, and single-crystal X-ray diffraction analyses. The highest equilibrium temperature for the TBPC system was 10.3 °C at w_TBPC = 0.36, where w_TBPC denotes the mass fraction of TBPC, (or the mole fraction of TBPC, x_TBPC = 0.034). The TBAAc system was 18.2 °C at w_TBAAc = 0.36, where w_TBAAc is the mass fraction of TBAAc, (or the mole fraction of TBAAc, x_TBAAc = 0.031). The greatest dissociation heat for TBPC system was 194 kJ kg⁻¹ at w_TBPC = 0.37 and the TBAAc system was 195 kJ kg⁻¹ at w_TBAAc = 0.33. For visual observations of the hydrate crystals, the major morphology in both systems was a columnar shape, but hexagonal plate crystals were observed at w_TBPC = 0.10 in the TBPC system. It was also confirmed that the hydrate crystals grown at higher subcooling are finer than those at lower subcooling. The crystallographic structure of TBPC hydrate formed at w_TBPC = 0.36 was identified to be tetragonal with 12.5 × 23.7 × 23.7 Å lattice parameters by the single-crystal X-ray diffraction analysis. Similarly, the crystallographic structure of TBAAc hydrate formed at w_TBAAc = 0.36 was tetragonal with 12.2 × 33.1 × 33.1 Å lattice parameters. The above findings indicate that TBPC and TBAAc hydrates are promising for applications in hydrate based technologies, such as cool energy storage, gas storage and gas separation.

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Introduction

Clathrate hydrates, or hydrates, are ice-like crystalline compounds consisting of host water molecules hydrogen-bonded to form cages that enclose different guest molecules within. There are three crystal structures that are commonly formed depending on the nature of the guest molecules: structure I with space group Pm3n, structure II with space group Fd3m, and structure H with space group P6/mmm. Clathrate hydrates have unique physical properties, such as large gas-storage capacity and large heat of formation/dissociation. These properties have been studied for applications such as storage-media for natural gas¹ and hydrogen,² ground/ocean sequestration of carbon dioxide,^3–7 development of heat pump/refrigeration system⁸ utilizing the heat for hydrate formation/dissociation. Besides, hydrates have the physical properties to preferentially incorporate specific guest molecules in the hydrate cages from a gas mixture. This property may be applied for gas separation technology,⁹ which has recently attracted much attention. However, one of the greatest obstacles to realize this hydrate technology is high pressure required for the hydrate formation. The breakthrough on this problem may be the usage of semi-clathrate hydrates formed with ionic guest substances. These hydrates have the property of being formed around ambient temperature under atmospheric pressure. There are many reports that the equilibrium pressure for hydrate formation is remarkably reduced by the use of the guests that form the ionic semi-clathrate hydrates.^10–12 Additionally these hydrates present no fire hazard and have no risks to the environment and to human health. Because of the above properties, the semi-clathrate hydrates have been investigated for applications in hydrate based technologies, such as cool energy storage,¹³ the separation and storage for hydrogen,¹⁴ CO₂,¹⁵ methane,¹⁶ and H₂S.¹⁷

There are many studies on the guests for the semi-clathrate hydrate formation. These hydrates were first discovered by Fowler et al.¹⁸ Then, it was found by X-ray structural analysis that these hydrates were clathrate-like compounds.¹⁹ In semi-clathrate hydrates, the guests compose the part of the water host lattice and occupy cages. Among these ionic semi-clathrate hydrates, the compounds studied most thoroughly are that of tetrabutyl ammonium (Bu₄N) and tetraisoamyl ammonium (iAm₄N) salts.²⁰ Especially, for Bu₄N and iAm₄N halides, there are many reports on the crystallographic structures and thermophysical properties.²¹ It was also reported by Nakayama that a number of the ionic hydrates are formed from organic-salt guest molecules, such as Bu₄N carboxylates and iAm₄N alkanedioates.²² In addition to the quaternary ammonium salts, quaternary phosphonium salts were known to be the guests, but the experimental information is quite limited, i.e., hydrates with Bu₄P fluoride, Bu₄P bromide and iAm₄P bromide were exclusively reported.²⁰

Besides the quaternary ammonium and phosphonium salts, some polymers are known to be guest molecules. Nakayama reported the first clathrate hydrates of Bu₄N and iAm₄N polyacrylates (PA).²³ He determined the temperature-composition phase diagrams of Bu₄NPA-H₂O and iAm₄NPA-H₂O, as well as the approximate composition of the hydrates as Bu₄NPA·30H₂O and iAm₄NPA·42H₂O.²³ It was found by Bogatyrev that similar hydrates could be formed inside the grains of cross-linked polyacrylic acid resin.²⁴ Thermodynamic and X-ray studies of these hydrates were reported.^25–27 The complete crystallographic structure of Bu₄NPA·40H₂O was determined by single-crystal X-ray diffraction measurement.²⁸ In these studies, propionate was employed as the monomer type of the polyacrylates. However, hydrate formation with acrylate that can be viewed as another monomer type of the polyacrylates was not investigated.

As reviewed above, numbers of the semi-clathrate hydrates formed from various guest substances have been identified. Many researches for the semi-clathrate hydrates with quarternary ammonium and phosphonium salts have been done so far. However, the semi-clathrate has a rich diversity of physical properties due to their variety of crystal structure and thermal properties. Further investigation to identify new guests is necessary to develop the potential for hydrate-based technologies by checking the list of guests carefully and systematically. In this study, the formation of two new semi-clathrate hydrates with tetrabutylphosphonium chloride (TBPC) and tetrabutylammonium acrylate (TBAAc) was demonstrated by measurements of the temperature-composition phase diagrams and dissociation heat of the hydrates, visual observations of the hydrate crystals, and single-crystal X-ray diffraction analyses.

Experimental

Materials

The liquid water used in the experiments was laboratory-made distilled liquid water. TBPC aqueous solution was prepared with a solid reagent of TBPC (96%, Aldrich Chemical Co.). TBAAc aqueous solution was obtained by neutralizing the tetrabutylammonium hydroxide solution (40% solution in water, Aldrich Chemical Co.) with acrylic acid (98% solution, Wako Pure Chemical Industries, Ltd.).

(a) Temperature-composition phase diagram.
Experimental apparatus. Fig. 1 illustrates a schematic of the apparatus used in the observation growth and dissociation of the hydrate crystals formed in TBPC aqueous solution or TBAAc aqueous solution. A glass test tube (external diameter 10 mm, bore diameter 8 mm, height 90 mm) was filled with TBPC aqueous solution or TBAAc aqueous solution. Hydrate crystals formed in the test tube were observed with a microscope. A CMOS camera was attached to the microscope and it acquired digital images of the samples. The temperature was controlled by a chiller (Tokyo Rikakikai Co., CTP-3000). The system temperature was measured by a platinum resistance temperature detector inserted in the glass container with the uncertainty of ±0.1 K.

Fig. 1 Schematic diagram of the apparatus for hydrate crystal growth.

Experimental procedures. The temperature of the system was first set at 1.0 °C. When the temperature of the system became constant, seed crystals of TBPC or TBAAc hydrate (about 5 mg), which were formed separately in advance, were dropped in each aqueous solution. This procedure artificially induced nucleation and growth of the hydrate in the test cell. The temperature of the system was kept constant for 24 h to allow for the complete growth of hydrate inside the test tube. T was then incrementally increased in steps of 0.1 °C. At each temperature step, T was maintained for several hours. By repetition of this incremental temperature increase, the hydrate was dissociated. The equilibrium temperature was determined as the one immediately before the complete dissociation of the hydrate was visually confirmed. This procedure was repeated under different compositions.

(b) Dissociation heat.
Experimental apparatus and procedures. The dissociation heat was measured using a differential scanning calorimeter (DSC-60, Shimadzu Corporation) with the uncertainty of ±5 kJ/kg.

The mass of the test cell (often called the test pan) was measured by an electronic balance. Then, the aqueous solution of the guest substance (13–14 mg) was injected in the cell by a pipette. The cell containing the solution was measured again to accurately measure the mass of the aqueous solution. This cell was sealed with a round plate with a diameter of 4 mm. The test cell temperature during the measurement was controlled. First, the system was cooled to −40 °C at −10 °C min⁻¹ to form the hydrate. Then the temperature was raised to −5.0 °C at 5.0 °C min⁻¹. From −5.0 °C, the temperature was raised constantly by 0.5 °C min⁻¹ until the hydrate was fully dissociated.

(c) Crystal morphology and structure of the hydrates.
Experimental apparatus. For the observation of crystal morphologies, the same apparatus as the experiment of temperature-composition phase diagram was used. For single crystal structure analysis, X-ray diffraction data were collected with Mo Ka radiation (λ = 0.711 Å) using a CCD diffractometer (Bruker AXS, Model SMART APEX) at 153 K. The structure was solved by direct methods using the SHELXTL.
Experimental procedures. The temperature of the system was set at a prescribed level in the range 1.0–20 °C. When the temperature of the system became constant, a seed of the hydrate crystal (about 5 mg) that was formed separately in advance was dropped in the aqueous solution. The instance the seed crystal is dropped was set as the starting time for crystal growth (t = 0). The crystal morphology of TBPC hydrate and TBAAc hydrate was visually observed.

Results

(a) Phase diagram

The dissociation experiments for TBPC hydrate were done in the mass fraction range from 0.051 to 0.51, and the results are summarized in Table 1 and Fig. 2 in the form of a T_eq–w_TBPC diagram, where T_eq is the equilibrium temperature of the hydrate and w_TBPC is the mass fraction of TBPC in TBPC aqueous solution. x_TBPC is the mole fraction of TBPC in TBPC aqueous solution.

Fig. 2 Relationship between the equilibrium temperature and the mass fraction in TBPC system.

Table 1 Dissociation temperature of TBPC hydrate

w TBPC	x TBPC	T eq/°C
0.051	3.3 × 10^−3	3.4
0.10	6.8 × 10^−3	5.2
0.12	8.3 × 10^−3	7.0
0.17	0.012	8.4
0.19	0.014	8.8
0.22	0.017	9.3
0.26	0.021	9.9
0.30	0.026	10.2
0.34	0.030	10.2
0.35	0.032	10.2
0.36	0.034	10.3
0.37	0.035	10.2
0.40	0.039	10.0
0.45	0.048	9.7
0.51	0.060	9.5

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At w_TBPC = 0.051, the equilibrium temperature was 3.4 °C. In the mass fraction range from 0.051 to 0.26, the equilibrium temperature increased with increasing mass fraction. The equilibrium temperature at w_TBPC = 0.26 was 9.9 °C. In the mass fraction range from 0.30 to 0.37, the change in the equilibrium temperature levelled off, and then reached the highest equilibrium temperature of 10.3 °C at w_TBPC = 0.36. At w_TBPC > 0.36, the equilibrium temperature decreased with increasing mass fraction, and the equilibrium temperature at w_TBPC = 0.51 was 9.5 °C.

Similar dissociation experiments for TBAAc hydrate were done in the mass fraction range from 0.10 to 0.43, and the results are summarized in Table 2 and Fig. 3 in the form of T_eq–w_TBAAc diagram, where w_TBAAc is the mass fraction of TBAAc in the TBAAc aqueous solution_.x_TBAAc is the mole fraction of TBAAc in the TBAAc aqueous solution.

Fig. 3 Relationship between the equilibrium temperature and the mass fraction in TBAAc system.

Table 2 Dissociation temperature of TBPC hydrate

w TBAAc	x TBAAc	T eq/°C
0.10	6.1 × 10^−3	9.9
0.12	7.5 × 10^−3	11.0
0.15	9.7 × 10^−3	13.7
0.18	0.012	14.4
0.21	0.014	14.9
0.23	0.016	16.3
0.26	0.019	16.9
0.29	0.022	17.5
0.32	0.026	17.8
0.34	0.028	18.0
0.36	0.031	18.2
0.39	0.034	17.9
0.41	0.038	17.8
0.43	0.040	17.5

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The obtained T_eq–w_TBAAc is qualitatively similar to that in the TBPC system. At w_TBAAc = 0.10, the equilibrium temperature was 9.9 °C. In the mass fraction range from 0.10 to 0.29, the equilibrium temperature increased with increasing mass fraction. The equilibrium temperature at w_TBAAc = 0.29 was 17.5 °C. Then, in the mass fraction range from 0.32 to 0.41, the change in the equilibrium temperature was hardly seen, and the highest equilibrium temperature was 18.2 °C at w_TBAAc = 0.36. At w_TBAAc > 0.36, the equilibrium temperature decreased with increasing mass fraction, and the equilibrium temperature at w_TBAAc = 0.43 was 17.5 °C.

In these dissociation experiments, two kinds of dissociation behavior were observed. For the TBPC system, hydrate crystals, which were formed in the mass fraction range from 0.35 to 0.37, were dissociated as soon as they reached at the equilibrium temperature. Hydrate crystals formed in the mass fraction range from 0.051 to 0.26 and the mass fraction 0.51 were gradually dissociated by increasing the temperature up to the equilibrium temperature. This result suggests that the formation and co-existence of the hydrates, each having different dissociation temperature. For TBAAc system, a similar dissociation behavior was observed.

(b) Dissociation heat

Table 3 and 4 present the results of the DSC measurements of dissociation heat of TBPC and TBAAc hydrates formed at different mass fractions, where ΔH_d is the dissociation heat of hydrates. Hydration number corresponding to each mass fraction is included in these tables. For TBPC hydrate, the greatest dissociation heat was 194 kJ kg⁻¹ at w_TBPC = 0.37. For TBAAc, the greatest dissociation heat was 195 kJ kg⁻¹ at w_TBAAc = 0.33.

Table 3 Dissociation heat of TBPC hydrate

w TBPC	ΔHd/kJ kg^−1
0.30	193
0.34	186
0.35	183
0.37	194

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Table 4 Dissociation heat of TBAAc hydrate

w TBAAc	ΔHd/kJ kg^−1
0.30	179
0.33	195

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The DSC heating curve in the TBPC system at w_TBPC = 0.30 is shown in Fig. 4(a), where three peaks were confirmed. Their peaks were observed, from the left, at 0 °C, 6.5 °C, and 10 °C, and the peak seen at 10 °C was the greatest. The leftmost peak should correspond to ice melting and the other two peaks are considered due to the dissociation of the hydrates. The hydrate dissociated at 6.5 °C is metastable phase and the hydrate dissociated at 10 °C is stable phase. Thus the DSC measurement in the present study indicated that TBPC hydrate has at least two types of crystal structure and it is known that tetraalkyl salt hydrates have 2–4 kinds of crystallographic structure.²⁰ In fact, two types of crystal morphologies were observed as described in the next section. Fig. 4(b) indicates the typical DSC heating curve in TBPC system observed in the range of w_TBPC = 0.34, 0.35, and 0.37. In this DSC thermograph, a single peak was observed and the existence of ice was not identified. Also for the DSC heating curve in TBAAc system at w_TBAAc = 0.30 and 0.33, a single peak was observed and the existence of ice was not identified, and a typical result is shown in Fig. 5.

Fig. 4 (a) The DSC heating curve in the TBPC system at w_TBPC = 0.301; (b) the DSC heating curve in TBPC system at w_TBPC = 0.368.

Fig. 5 The DSC heating curve in TBAAc system at w_TBAAc = 0.300.

(c) Crystal morphology and structure of the semi-clathrate hydrates

A TBPC aqueous solution at w_TBPC = 0.36 and 0.10 and TBAAc aqueous solution at w_TBAAc = 0.36 and 0.12 were used in these experiments. Here we define the subcooling temperature ΔT_sub = T_eq − T_ex as the index of the driving force for the crystal growth, where T_eq is the equilibrium temperature of the hydrate at each mass fraction and T_ex is the system temperature. The results obtained with TBPC system at w_TBPC = 0.36 and TBAAc system at w_TBAAc = 0.36 were classified based on ΔT_sub.

Fig. 6 shows a sequence of images of TBPC hydrate crystals growing in the aqueous solution at w_TBPC = 0.36, T_ex = 5.5 °C (ΔT_sub = 4.8 K). As seen in the images, hydrate crystals grow to radiate from the seed crystal. The thickness of the crystal becomes larger with elapsed time. In this system, apparently, the solution was mostly converted into the hydrate after approximately 1 h. After this time, no noticeable further hydrate crystal growth was observed. Fig. 7 shows a sequence of images of TBPC hydrate crystals growing in the aqueous solution at w_TBPC = 0.10, T_ex = 1.0 °C (ΔT_sub = 4.2 K). In contrast with the TBPC system at w_TBPC = 0.36, the majority of the crystal morphology in this system was the hexagonal plate which is 0.5 mm on a side, and the columnar-shape crystal observed in the TBPC system at w_TBPC = 0.36 was hardly observed. The lateral growth the plate-like crystals was observed, but their thickening was not noticeable. It took two days or longer for the completion of the hydrate growth. These results obtained in the TBPC system at w_TBPC = 0.36 and 0.10 indicate that there are at least two different types of crystal structure with different hydration numbers within this mass fraction range. The similar result was previously reported in TBAF (tetrabutylammonium fluoride) system, where it was indicated that columnar-shape and hexagonal-shape hydrate crystals were of tetragonal and cubic structures by single-crystal X-ray diffraction measurements.²⁹ Here, in this study, single-crystal X-ray diffraction analysis showed that a tetragonal cell of 12.5 × 23.7 × 23.7 Å for columnar-shape TBPC hydrate crystal formed at w_TBPC = 0.36.

Fig. 6 Sequential videographs of TBPC hydrate crystal growth at w_TBPC = 0.363, T_ex = 5.5 °C.

Fig. 7 Sequential videographs of TBPC hydrate crystal growth at w_TBPC =0.101, T_ex = 1.0 °C.

Fig. 8 and 9 show the sequential images of TBAAc hydrate crystals growing in the aqueous solution: w_TBAAc = 0.36, T_ex = 14.3 °C (ΔT_sub = 3.9 K) for Fig. 8 and w_TBAAc = 0.12, T_ex = 6.8 °C (ΔT_sub = 4.2 K) for Fig. 9. As recognized in the comparison of Fig. 8 and 9, both crystal morphologies were columnar-shape and the growth behavior was similar to that for the TBPC system at w_TBPC = 0.36. However, as for the time required for the completion of the hydrate growth, it was approximately 35 min at w_TBAAc = 0.36, while it was 25 h at w_TBAAc = 0.12. The single-crystal X-ray diffraction analysis showed that a tetragonal cell of 12.2 × 33.1 × 33.1 Å for a columnar-shape TBAAc hydrate crystal formed at w_TBAAc = 0.36, in this study. Further study is necessary for structure identification of these systems, and is in progress. On the other hand, it should be emphasized that the composition of the solution influences the hydrate crystal growth rate but may not influence the crystal structure formed. These results support the experimental results by means of DSC.

Fig. 8 Sequential videographs of TBAAc hydrate crystal growth at w_TBAAc = 0.362, T_ex = 14.3 °C.

Fig. 9 Sequential videographs of TBAAc hydrate crystal growth at w_TBAAc = 0.120, T_ex = 6.8 °C.

Fig. 10 The morphology of the hydrate crystals formed in TBPC aqueous solution at w_TBPC = 0.363 based on subcooling ΔT_sub.

Finally, Fig. 10 and 11 show the comparison of the morphology of the hydrate crystals formed in TBPC aqueous solution at w_TBPC = 0.36 and TBAAc aqueous solution at w_TBAAc = 0.36. These images were arranged according to the subcooling ΔT_sub. Both crystal morphologies were columnar or needle shape and the diameter decreased with increasing the subcooling. In the TBPC system, at ΔT_sub = 1.7 K to 2.9 K, the shape of TBPC hydrate crystals was typically columnar with the thickness between 0.1 mm to 0.3 mm. At ΔT_sub = 4.8 K, needle shape crystals of the thickness less than 0.1 mm were observed. Similarly, in TBAAc system, at ΔT_sub = 1.7 K to 3.2 K, the shape of TBAAc hydrate crystals was columnar with the thickness between 0.2 mm to 0.3 mm. At ΔT_sub = 4.9 K, needle shape crystals of thickness less than 0.1 mm were observed. From these figures, it is evident that the morphology of the hydrate crystals significantly varies depending on the subcooling. The hydrate crystals grown at higher subcooling are finer than those at lower subcooling.

Fig. 11 The morphology of the hydrate crystals formed in TBAAc aqueous solution at w_TBAAc = 0.362 based on subcooling ΔT_sub.

Discussion

For the TBPC system, we compare the effect of the anion on the hydrate formation. In Fig. 12, the equilibrium temperatures of the phosphonium hydrates are plotted against the anion radius. Here we used the value of the highest equilibrium temperature 10.3 °C for TBPC hydrate. For comparison, the results of Bu₄N⁺ (F⁻, Cl⁻, Br⁻) and Bu₄P⁺ (Br⁻) by Aladko et al.²⁰ are also shown. The equilibrium temperatures of Bu₄P halide systems were lower by 3 K to 5 K than those of Bu₄N halide systems. The decrease in the anion radius causes the increase of the hydrate equilibrium temperature. Aladko et al. explained that the stability of the hydrate is mainly governed by the distortion effect of anion on the hydrogen-bonded water framework in the hydrate.²¹ By extrapolating the data for TBPC and TBPB (tetrabutylphosphonium bromide) hydrates, the equilibrium temperature of TBPF (tetrabutylammonium fluoride) hydrate is estimated to be approximately 15 °C.

Fig. 12 Equilibrium temperatures as a function of anion radius for the hydrates of Bu₄N⁺ (F⁻, Cl⁻, Br⁻) (line 1) and Bu₄P⁺ (Cl⁻, Br⁻) (line 2).

For the TBAAc system, the equilibrium temperature at w_TBAAc = 0.36 was the highest (18.2 °C), which is close to that of Bu₄N propionate hydrate reported by Nakayama et al.²² Additionally the value is higher than that of Bu₄N polyacrylate. Table 5 indicates that the equilibrium temperature of Bu₄N polyacrylate hydrate depends on the degree of polymerization (that is the length of the polymer chain).

Table 5 Equilibrium temperature of tetrabutylammonium polyacrylate hydrate according to polymerization

Polymerization	T eq/°C	Ref.
0	18.2	This work
15	14.3	28
2100	11.2	26

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The industrial applications of TBPC hydrate and TBAAc hydrate based on the above thermal properties are discussed. One of the most promising applications may be cool energy storage technology for air-conditioning. In cool energy storage technology, ice is currently, widely used. However, the coefficient of performance (COP) of the refrigerator to form ice cannot be high because the ice formation requires temperatures as low as −4 to −10 °C or even lower. Additionally, there is a great difference between the melting temperature of ice 0 °C and the temperature required for air-conditioning. Generally, a temperature of 5 to 15 °C is sufficient for residential air-conditioning. For air-conditioning used in server rooms housing computers, even higher temperature, 15 to 20 °C, may be sufficient. Thus, TBPC hydrate is suitable as a cool energy storage medium for residential air-conditioning use because the equilibrium temperature is 10.3 °C. TBAAc hydrate is suitable as the material for server room air-conditioning use as the equilibrium temperature is 18.2 °C. We now calculate the COP of the refrigerator by supposing the inverse Carnot cycle. The COP of the inverse Carnot cycle is expressed as ε = T_L / (T_H − T_L) where ε, T_H and T_L are the COP and the temperatures of the high- and low- grade heat sources. T_H can be set at 27 °C, the average atmospheric temperature in the summer season. T_L may be set at the value of T_eq minus 10 °C, and thus 0.3 °C and 8.2 °C for TBPC and TBAAc systems. COP values are deduced to be 10.3 and 15.0 for TBPC and TBAAc systems. These values are 1.5 to 2.0 times greater than the COP for ice system.

Moreover, from the viewpoint of the dissociation heat, which is the important thermal property in cool energy storage technology, these materials are hopeful. The greatest dissociation heat of TBPC hydrate was 194 kJ kg⁻¹ at w_TBPC = 0.37, and that of TBAAc hydrate was 195 kJ kg⁻¹ at w_TBAAc = 0.33. These values are comparable to that of TBAB hydrate that was previously studied as cool storage material of the hydrate (typeA: 193 kJ kg⁻¹, typeB: 199 kJ kg⁻¹).³⁰

The above findings indicate that TBPC and TBAAc hydrates are promising cool energy storage materials. These hydrates also have the potential of other industrial applications such as the capture and storage of hydrogen, CO₂, methane, and H₂S. In these hydrate-based technologies, the hydrate formation at a temperature closer to ordinary room temperature is quite often favorable. In this regard, TBAAc hydrate that has a relatively high equilibrium temperature may be promising.

Conclusions

In this study, the formation of the two new semi-clathrate hydrates with TBPC and TBAAc was demonstrated by the measurements of temperature-composition phase diagrams and dissociation heat of the hydrates, visual observations of the hydrate crystals, and single-crystal X-ray diffraction analyses.

In the measurements of temperature-composition phase diagrams, the highest equilibrium temperature for TBPC system was 10.3 °C at w_TBPC = 0.36, (or x_TBPC = 0.034) and that for TBAAc system was 18.2 °C at w_TBAAc = 0.36, (or x_TBAAc = 0.031). For Bu₄P halide systems, the decrease in anion radius caused the increase in the equilibrium temperature of hydrate. For Bu₄N polyacrylate systems, the equilibrium temperature of Bu₄N polyacrylate system depended on the degree of polymerization (that is the length of the polymer chain). In the measurements of dissociation heat of the hydrates, the greatest dissociation heat for TBPC system was 194 kJ kg⁻¹ at w_TBPC = 0.37 and that for TBAAc system was 195 kJ kg⁻¹ at w_TBAAc = 0.33.

The crystallographic structure of TBPC hydrate formed at w_TBPC = 0.36 was identified to be tetragonal with 12.5 × 23.7 × 23.7 Å lattice parameters by single-crystal X-ray diffraction analysis. Similarly, the crystallographic structure of TBAAc hydrate formed at w_TBAAc = 0.36 was tetragonal with 12.2 × 33.1 × 33.1 Å lattice parameters.

The above findings indicate that TBPC and TBAAc hydrates are promising for the application to the hydrate based technologies, such as cool energy storage, gas storage and gas separation.

Acknowledgements

The study was subsidized by JKA through its promotion funds from KEIRIN RACE and also supported by a Grant-in-Aid for the Global Center of Excellence Program for “Center for Education and Research of Symbiotic, Safe and Secure System Design” from the Ministry of Education, Culture, Sport and Technology in Japan.

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