Thermodynamic and crystallographic properties depending on hydration numbers in tetra-n-butylammonium chloride semiclathrate hydrates

doi:10.1016/j.jct.2019.106004

The Journal of Chemical Thermodynamics

Volume 142, January 2020, 106004

https://doi.org/10.1016/j.jct.2019.106004 Get rights and content

Highlights

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Thermodynamic and Crystallographic Properties of TBAC hydrates were studied.
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Tetragonal crystal structures slightly varied with hydration numbers.
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The dissociation enthalpies per TBAC gradually varied with the hydration numbers.
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All hydrate crystals dissociated at approximately the same temperature.

Abstract

Tetra-n-butylammonium chloride (TBAC) semiclathrate hydrate crystals with hydration numbers n = (26.4–33.2) were prepared from TBAC aqueous solutions over the range of TBAC mole fractions (0.0060–0.0500). The thermodynamic and crystallographic properties of the TBAC hydrates were then investigated via ion chromatography, powder X-ray diffraction (PXRD), and differential scanning calorimetry. The crystal system of the TBAC hydrates with n = (26.4–33.2) was tetragonal, while the crystal structures (PXRD patterns) slightly varied with the hydration numbers. The dissociation enthalpies per guest mole (TBAC) gradually varied with the hydration numbers, from ~149 kJ mol⁻¹ at n = 26.4 to ~187 kJ mol⁻¹ at n = (33.1–33.2). These results suggest that the enthalpies per TBAC increased with greater hydrogen-bond formation in the TBAC hydrate crystals. The dissociation temperatures and dissociation enthalpies per H₂O of the TBAC hydrates suggest that the hydrate crystals were most thermally stable at around n = 30 and slightly unstable with higher/lower hydration numbers.

Introduction

Quaternary ammonium salts, such as tetra-n-butylammonium (TBA) salts, can form clathrate hydrates with hydrogen-bonded hydrate cages (such clathrate hydrates are commonly known as “semiclathrate hydrates”) [1], [2], [3]: The guest cations (TBA⁺) are enclathrated over several hydrate cages, and the anions (e.g., F⁻, Cl⁻, Br⁻) are included in the host water lattice with the displacement of water molecules [2], [3]. Semiclathrate hydrates are then formed by cooling the aqueous solutions; they have relatively high latent heat in solid (hydrate)-liquid phase changes, which occur within the range of (273–301) K at atmospheric pressure [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. These characteristics show that semiclathrate hydrates have potential as phase-change materials for thermal/cold energy storage systems [9], [10], [11], [12], [13], [14], [15], [16]. In addition, they can capture small gas molecules such as CO₂, CH₄, H₂, and N₂ in empty cages under lower pressure conditions than those required for the corresponding pure gas hydrates [8], [17]. Fundamental and applied studies for gas storage/separate materials have been performed [8], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30].

The most common crystal systems of TBA salt hydrates are tetragonal, cubic, and orthorhombic structures [2], [31], [32]. TBA chloride (TBAC) forms tetragonal hydrate crystals with three types of hydration numbers (n = 24, 30, and 32), and the space group is P4₂/m [7], [33]. Those hydration numbers were obtained via chemical titration with Hg(NO₃)₂ using diphenylcarbazone/tetraphenylborate as an indicator and potentiometric titration [7], [33]. The dissociation temperatures of TBAC hydrates at atmospheric pressure reported in the previous studies are (287.9–288.1) K at n = 24, (288.2–288.4) K at n = 30, and (288.2–288.9) K at n = 32 [5], [6], [7], [33], [34]. The dissociation enthalpies per guest mole (Δ_dH_TBAC) reported in the previous studies are Δ_dH_TBAC = ~128 kJ mol⁻¹ at n = 24, Δ_dH_TBAC = (157–174) kJ mol⁻¹ at n = 30, and Δ_dH_TBAC = ~179 kJ mol⁻¹ at n = 32 [5], [6], [19], [33].

The thermodynamic properties of TBAC hydrates depend on their crystal structures (i.e., hydration numbers). It has been suggested that hydrate crystals prepared from a TBAC solution of the same mole fraction have different hydration numbers depending on the experimental conditions [33]. Additionally, it has been reported that the hydration numbers in tetragonal TBA bromide (TBAB) hydrate crystals are changeable and vary depending on the mole fractions of TBAB aqueous solutions [35]. It has been suggested that crystals with variable hydration numbers are also formed in TBAC hydrate systems. The thermodynamic and crystallographic properties of TBAC hydrates with variable hydration numbers are important parameters for understanding the crystals’ characteristics, as well as their industrial applications, such as thermal/cold energy storage and gas storage/separate materials. In this study, quantitative evaluations of the relationship between the crystal structures and dissociation enthalpies of TBAC hydrates with a wide range of hydration numbers are reported.

Section snippets

Materials and solutions

Research-grade TBAC (≥0.97 mass fraction purity, Sigma-Aldrich Co., LLC.) and distilled water (ADVANTEC®, RFD240FC) were used without further purification (as described in Table 1). The mole fractions of the initial TBAC aqueous solutions (x) were 0.0060, 0.0100, 0.0120, 0.0160, 0.0200, 0.0300, 0.0323 (stoichiometric proportion at n = 30.0 [7]), 0.0360, 0.0400, and 0.0500. The sample concentrations were prepared to include the reagent purity. The aqueous solution samples were measured by an

Results and discussion

Fig. 2 and Table 2 show the hydrate mole fractions and hydration numbers (n) calculated from the Cl⁻ mole fractions determined by both ion chromatography and DSC. The hydrate crystals prepared from x = 0.0323 (the stoichiometric proportion at n = 30.0 [7]) have the same mole fraction as the aqueous solution. The hydration numbers from x = 0.0500 to x = 0.0120 varied from n = 26.4 to n = 33.1, and those from x = (0.0120–0.0060) were approximately constant with n = (33.1–33.2). This means that

Conclusions

The hydration numbers, PXRD patterns, dissociation enthalpies, and dissociation temperatures of TBAC hydrates prepared from x = (0.0060–0.0500) were investigated via ion chromatography, PXRD, and DSC measurements. Hydrate crystals with n = (26.4–33.2) were formed. The crystal system with n = (26.4–33.2) was tetragonal, and the crystal structures of unit cells slightly varied with the hydration numbers. The dissociation enthalpies per mass were approximately constant at Δ_dH_mass = (210–215) J g⁻¹

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to thank Dr. J. Yoneda, Dr. N. Tenma, and the laboratory members at AIST for their help and advice.

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Experimental measurement and thermodynamic modeling of hydrate stability conditions in the methane + cyclopentane + tetra-n‑butyl ammonium chloride (TBAC) aqueous solution system
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The present study investigates hydrate dissociation conditions in methane + cyclopentane (CP) and tetra-n‑butyl ammonium chloride (TBAC) aqueous solution. The experimental work was done using an isochoric vessel with varying compositions of the promoters in the pressure and temperature ranges of (1.42 to 4.90) MPa and (291.2 to 298.9) K, respectively. The methane hydrate dissociation pressure was predicted by the Gibbs minimization method. The findings denote that adding CP to the TBAC-containing system shifts the hydrate dissociation temperature of methane to higher values. The results also show an average increase of 12, 9, 5, and 3 K in hydrate dissociation temperature for 5, 10, 20, and 30 wt% TBAC in aqueous solution within the pressures. These results can be caused by the existence of more dodecahedral cages that are inhabited by methane. Thus, it makes hydrates more stable and increases the temperature of hydrate dissociation. On the other hand, adding TBAC to CP containing system has an inhibitor effect. The methane hydrate dissociation temperature was not changed sensibly when a 5 wt% TBAC was added to CP containing system. When the addition of the TBAC weight percentage to CP containing system is increased to 10, the average hydrate dissociation temperature decreases by 1 K within the pressures. When adding 20 and 30 wt% TBAC to CP containing system an average decrease of 2 K and 3 K in the hydrate dissociation temperature is resulted, respectively, within the pressures. The reduction in the hydrate dissociation temperatures can be attributed to salting-out and ion clustering. The experimental data and modeling results for the hydrate dissociation pressures exhibit an AARD of 2.4 %.
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Citation Excerpt :
The literature have reported the mainly crystal structure of those semi-clathrate hydrate are TBAB·38H2O, TBAB·26H2O, TBAB·24H2O [38,39], TBAF·32.3H2O, TBAF·28.6H2O [40], TBAC·32H2O, TBAC·30H2O, TBAC·24H2O [41], TBPB·38H2O, TBPB·32H2O and TBPB·24H2O [42–44], respectively. As a phase change material (PCM), the formation and dissociation of semi-clathrate hydrate involves heat absorption and heat release, so as to realize cold storage and cold release [45]. Kim et al. [46] proposed that the semi-clathrate hydrate had the potential to be applied to the cold storage system because of its high cold storage density and suitable phase change temperature.
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Ionic Clathrate Hydrates of Tetraalkylammonium/phosphonium Salts: Structures, Properties, Some Applications, and Perspectives
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Thermal properties of tetragonal tetra- n -butyl ammonium bromide + tetra- n -butyl ammonium chloride mixed semiclathrate hydrates based on hydration numbers and guest mole fraction rates
2020, Journal of Applied Physics

View full text

Thermodynamic and crystallographic properties depending on hydration numbers in tetra-n-butylammonium chloride semiclathrate hydrates

Highlights

Abstract

Introduction

Section snippets

Materials and solutions

Results and discussion

Conclusions

Declaration of Competing Interest

Acknowledgements

Mendeleev Commun.

Fluid Phase Equilb.

Fluid Phase Equilib.

J. Chem. Thermodyn.

Fluid Phase Equilib.

J. Chem. Thermodyn.

Chem. Eng. J.

Energy

Appl. Energy

Energy

Chem. Phys.

J. Chem. Thermodyn.

Some unusual hydrates of quaternary ammonium salts

J. Am. Chem. Soc.

Water in crystalline hydrates aqueous solutions of simple nonelectrolytes

Clathrate hydrates of natural gases

Clathrate hydrates of tetrabutylammonium fluoride and oxalate

J. Struct. Chem.

Hydrates of organic compounds. VI. Heats of fusion and of solution of quaternary ammonium halide clathrate hydrates

Bull. Chem. Soc. Jpn.

Hydrates of organic compounds. XI. Determination of the melting point and hydration numbers of the clathrate-like hydrate of tetrabutylammonium chloride by differential scanning calorimetry

Bull. Chem. Soc. Jpn.