Design of Thermophysical Properties of Semiclathrate Hydrates

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Design of thermophysical properties of semiclathrate hydrates formed by tetra-n-butylammonium hydroxybutyrate Sanehiro Muromachi, and Satoshi Takeya Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05028 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018

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Industrial & Engineering Chemistry Research

Design of thermophysical properties of semiclathrate hydrates

formed

by

tetra-n-butylammonium

hydroxybutyrate Sanehiro Muromachi†*, Satoshi Takeya# †

Research Institute of Energy Frontier (RIEF), National Institute of Advanced Industrial Science

and Technology (AIST), 16-1 Onogawa, Tsukuba, 305-8569, Japan #

National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial

Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, 305-8565, Japan

KEYWORDS semiclathrate hydrate, melting temperature, tetra-n-butylammonium, 3-hydroxybutyrate, DSC, crystal structure

ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

Semiclathrate hydrates form from aqueous solution of ionic guest substance such as tetran-butylammonium (TBA) and tetra-n-butylphosphonium (TBP) salts. Thermophysical properties of these materials are varied by the ionic guest cations and anions. Investigation of the modification of the semiclathrate hydrates by ionic guest substances are needed to design and improve properties of semiclathrate hydrates for applications such as cool energy storage and for gas separation and storage. Here we report synthesis of semiclathrate hydrates of tetra-nbutylammonium 3-hydroxybutyrate (TBA 3HB) and measurements for melting temperatures and a heat of fusion. We performed phase equilibrium measurements under optical observation for crystal morphology. A heat of fusion was measured by differential scanning calorimetry (DSC) measurements, and a crystal structure of TBA 3HB hydrate was identified to be tetragonal by single crystal X-ray diffraction (XRD) measurements. The results showed that the highest melting temperature was 282.2 K and the heat of fusion was 172 kJ kg−1. We compared the presently obtained data with similar semiclathrate hydrates reported in the literature. The position of OH group in butyrate varied the melting temperature, but had little influence on the heat of fusion. It was found that substitution of carboxylates by hydrophilic OH group in TBA carboxylates can be used for designings thermophysical properties of TBA carboxylate hydrates.


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1 Introduction Semiclathrate hydrates are host-guest materials in which water and ionic guest substances cooperatively form cage-like hydrogen-bonded network1,2. Tetra-n-butylammonium (TBA) salts and tetra-n-butylphosphonium (TBP) salts are widely used for ionic guest substances, because their butyl chains well fit to and support the cage-like network1–6. Melting temperatures of semiclathrate hydrates range between about 270–300 K. The semiclathrate hydrates are also possible to incorporate small gas molecules7–17 such as CH4, CO2, H2, N2 under milder temperatures than the temperatures at which the semiclathrate hydrates form without the gas. Based on these unique and various thermophysical properties which are not available for gas hydrates and commonly known hexagonal ice, a lot of industrial applications of semiclathrate hydrates regarding cool energy storage18–22, gas separation and storage7–17, 23–35 are suggested Semiclathrate hydrates have basic four structures2: Cubic structure of space group Pm3n, tetragonal structure of space group P42/mmm, tetragonal structure of space group P42/m, orthorhombic structure of space group Pbmm. In the semiclathrate hydrate structures, TBA and TBP cations are incorporated in a cage which are formed by combination of four cages1–6. A counter anion of TBA or TBP cations usually makes bonds with water molecules composing the hydrogen bonding network. There are various ionic guest anions including halogens, i.e., F−, Cl−, Br−, and organic acids, e.g., carboxylates20,22,36–37. In the case of TBA carboxylate anions, the melting temperature of the semiclathrate hydrate varies due to the length of straight carbon chain36–37. The melting temperatures of TBA carboxylates gradually increase from formate to propionate, and reach the highest temperature with TBA propionate and TBA butyrate. The melting temperature of TBA valerate hydrate is slightly lower than that of TBA butyrate hydrate. This tendency clearly shows that the melting temperatures of semiclathrate hydrates are affected


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by the counter anions of TBA cation. All of these structures have small dodecahedral cages which are to be occupied by small gas molecules. Both cations and anions of the ionic guest substances affect properties of the dodecahedral cages regarding number, size and shape4–6. Flexibility of the structures of semiclathrate hydrates are driven by ionic guest substances and also gases29,32. Investigation of the modification of the semiclathrate hydrates by ionic guest substances are needed to design and improve properties of semiclathrate hydrates both for cool energy storage and for gas separation and storage technologies. Recently, we reported thermophysical properties of semiclathrate hydrates formed with TBA hydroxycarboxylates38–39. Hydroxycarboxylates have OH group in addition to carboxy group that may arrange hydrogen bonding network of the semiclathrate hydrate structures as shown in conceptual Fig. 1. The melting temperatures of TBA lactate hydrates are lower than that of TBA propionate, while the OH group still supported the hydrogen bonding network38. Moreover, comparison between TBA 2-hydroxybutylate (2HB) hydrate and TBA 2methylbutyrate (2MB) hydrate indicated that the substitution by a hydrophilic OH group and a hydrophobic CH3 group in butyrate caused ~5 K difference in melting temperature between their semiclathrate hydrates39.


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Fig. 1 Concept of designing semiclathrate hydrates with hydroxybutyrates. In this study we report melting temperatures and a heat of fusion of semiclathrate hydrates of TBA 3-hydroxybutyrate (3HB). We performed phase equilibrium measurements under optical observation for crystal morphology. A heat of fusion was measured by the differential scanning calorimetry (DSC) measurements, and the crystal structure of TBA 3HB hydrates was identified by single crystal X-ray diffraction (XRD) measurements. These results showed that the TBA 3HB hydrates had 282.2 K of melting temperature and 172 kJ kg−1 of heat of fusion. The TBA 3HB formed stable semiclathrate hydrates and their crystal shapes were rectangular columnar and cubic. The crystal structure of the TBA 3HB hydrate was identified to be the tetragonal hydrate structure. Comparison with similar semiclathrate hydrates reported in the literature found that substitution by hydrophilic OH group in carboxylates can widely modify thermophysical properties of TBA carboxylate hydrates.


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2 Experimental 2.1 Materials The raw materials used in this study are tabulated in Table 1. We synthesized the TBA 3HB by titration of 3-hydroxybutyric acid by tetra-n-butylammonium hydroxide. The reagents were mixed with equimolar amounts and kept reacted for two days under a stirred condition. The obtained aqueous solution was crystalized at ~276 K and. The crystals were filtrated under this temperature. TBA 3HB salt was obtained from drying the aqueous solution melted from the crystals by an evaporator.

2.2 Melting temperature measurements and morphology observation The melting temperature was measured by the same setup with that used in our previous studies38–39 (Fig. 2). The apparatus mainly consisted of a water bath, a cooling water circulator (CTP-3000, EYELA Co. Ltd., Tokyo, Japan), a platinum resistance thermometer (Sensor: NRHS1-0; Bridge: F201B, Chino Co. Ltd., Tokyo, Japan), a microscope lens (VZM-200i, Edmund Optics Inc.) and a CCD camera (STC-MC152USB, Sentech Co. Ltd.). We recorded temperature data during the experiments. By mixing TBA 3HB salt and water, aqueous solution samples having different concentrations were gravimetrically prepared with the aid of an electronic balance (AUW220D, Shimadzu Co., Kyoto, Japan). The sample solutions were injected into glass tubes, and set in the water bath. The system temperature was cooled down to form hydrate crystals. After the hydrate crystals were sufficiently grown, the system temperature was increased stepwise. These crystals were observed through the microscope. The melting


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temperature was determined to be the temperature one step before the step at which these crystals melted completely. To quickly obtain the melting temperature, we employed 0.2 or 0.5 K of the temperature step depending on the aqueous concentration. Uncertainties for aqueous concentration of salt were 0.002 in mole fraction (x) and 0.04 in mass fraction (w). The measurement uncertainty of temperature (T) was 0.1 K with 95% reliability. Uncertainty with 95% reliability for obtained melting temperature was positively extended to be −0.1 K to +0.3 or +0.6 K depending on the used temperature step, because from a low temperature we measured the highest temperature at which the hydrates did not melt.

Fig. 2 Experimental setup for melting temperature measurements for TBA 3HB semiclathrate hydrates.


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2.3 DSC and XRD measurements We formed single crystals both for DSC and XRD measurements. The aqueous solution with x = 0.028 (w = 0.35) which was used for the melting temperature measurements was crystalized at 279 K. The crystals were separated from the residual liquid in the tube, and kept under below 260 K. A sample used for the DSC measurements was solution from the melted single crystals. About 12 mg of the sample solution was injected into an aluminum sealing pan of which volume is 24 µL. The mass of the injected sample was measured by an electronic balance (AUW220D, Shimadzu Co., Kyoto, Japan). The sample was loaded to a DSC apparatus (DSC-60, Shimadzu Co., Kyoto, Japan). The sample was cooled by cold nitrogen gas and heated by a controlled heater with 2 K min−1 of the temperature increasing rate. To avoid polymorphic phase formation from the sample solution, we employed the annealing process40. To calibrate the DSC apparatus, we used c-hexane for the reference of which heat of fusion was reported to be 31.84 kJ kg−1 41. The measurement uncertainties with 95% reliability for heat of fusion was 6 kJ kg−1 as determined in our previous work39. For the single crystal XRD measurements, a single crystal with the size of 0.15 × 0.2 × 0.2 mm was selected under cold nitrogen atmosphere below 250 K, and it was mounted on the diffractometer (Smart APEX CCD, Bruker AXS). The diffraction data were collected at 123 K with MoKα (λ = 0.7170 Å) and solved by SHELX program42.


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3 Results and Discussion 3.1 Melting temperature and crystal morphology The results of melting temperature measurements for TBA 3HB hydrates are shown in Table 2 and Fig. 3. The melting temperatures ranged from 272–282 K. At the lowest two concentrations x = 0.003 and 0.006 (w = 0.05 and 0.10), the solid melted below the freezing point of water. At x = 0.003 (w = 0.05) any angular crystal was not observed and round crystals which were likely to be ice were observed (Fig. 4 (a)). Therefore, TBA 3HB simply depressed the freezing point of water at such low concentration. At x = 0.006 (w = 0.10) columnar crystals were observed (Fig. 4 (b)), which indicates formation of TBA 3HB hydrates. At denser concentrations, the obtained melting temperatures were over the freezing point of water. The melting temperature raised with increase of the salt concentration. At x = 0.017 (w = 0.25) rounded crystals were observed in addition to the columnar crystals (Fig. 4 (c)). At x = 0.028 and 0.034 (w = 0.35 and 0.40), thin columnar crystals also formed, and the highest melting temperatures were obtained, i.e., 282.2 K. This columnar crystal may be the most stable crystals for TBA 3HB hydrates (Fig. 4 (d) and (e)). The melting temperature with x = 0.041 (w = 0.45) decreased from the highest melting temperature. At this concentration the crystal morphology was different from the others, that is, cubic crystals with sharp corners (Fig. 4 (f)). From the observed morphology, the TBA 3HB hydrates may have at least two or three phases in the present measurement range of x.


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Fig. 3 Results for melting temperature measurements and comparison with the literature data. Here, x and T denote salt concentration in aqueous phase in mole fraction and temperature, respectively. ◇, TBA 3HB hydrates; △, TBA 2HB hydrates; ◆, TBA 3MB hydrates; ▲, TBA 2MB hydrates; ●, TBA butyrate hydrates; ■, TBA valerate hydrates. The error bars show uncertainties with 95% reliability for the temperature. Insets show the chemical structures of the ionic guest anions.


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Fig. 4 Morphology of the semiclathrate hydrates of TBA 3HB. (a) x = 0.003 (w = 0.05) at 272.3 K (b) x = 0.006 (w = 0.10) at 272.7 K (c) x = 0.017 (w = 0.25) at 277.2 K (d) x = 0.022 (w = 0.30) at 279.6 K (e) x = 0.028 (w = 0.35) at 279.6 K (f) x = 0.041 (w = 0.45) at 279.6 K.

Fig. 3 also compares the melting temperatures of TBA 3HB hydrates with the other semiclathrate hydrates of which ionic guest anions have analogous molecular structures to 3HB anion. The TBA 3HB hydrate has a 2 K-lower melting temperature than TBA 3-methylbutyrate (3MB) hydrate, which is opposite relationship to that between the TBA 2HB hydrate and the TBA 2MB hydrate as shown in Fig. 3. As seen in this figure, addition of a OH or a CH3 group to butyrate varies the melting temperature of semiclathrate hydrate depending on the position of additional OH group. The melting temperature of the TBA 3HB hydrates is lower than that of the TBA valerate hydrates, but still higher than that of the TBA 2MB hydrates. This clearly showed that the hydrophobic CH3 group neighboring the carboxy group (in 2MB) obstruct incorporation


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of

the

anion.

Based

on

some

reported

structures

regarding

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carboxylate43–45

and

hydroxycarboxylate38, carboxy group makes bonds with water molecules in a cage of semiclathrate hydrates. Therefore, when an anion has a hydrophobic CH3 group neighboring a carboxy group (2MB in the present case), the CH3 group would become too close to the cage wall and inhibit the anion incorporation in the cage. In contrast, a hydrophilic OH group neighboring carboxy group (2HB in the present case) could help anion incorporation. This effect becomes milder at the next 3-position in butyrate (3MB and 3HB in the present case). The comparison between these isomers suggested that substitution in carboxylates by a OH group can help the anion incorporation when the insertion position is suitable. The aqueous concentration of the TBA salts at which the highest melting temperature was obtained may represent a congruent composition of the most stable crystal in the system36–37. The possible hydration numbers of the semiclathrate hydrates were also shown in Fig. 3 at corresponding x. The hydration number of the TBA 3HB hydrates is likely to be between 29 and 35, and that of the TBA 2HB hydrates is around 30. The hydration number for TBA butyrate hydrates was reported to be around 3036, and those for the semiclathrate hydrates of TBA valerate, TBA 2MB and TBA 3MB were reported to be around 4036. Based on these literature data, it is suggested that the five carbon atoms in the number of mono-carboxylate induce around 40 of the hydration number. Here, we note that TBA 2,2-dimethylpropionate was reported to have unstable phase of which melting temperature was below the freezing point of water36. However, the butyrates substituted with OH group at 2 and 3 positions may induce around 30 of the hydration number, keeping the hydration number of the TBA butyrate hydrate.


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3.2 DSC and XRD measurements Fig. 5 shows our DSC data for the TBA 3HB hydrates formed from the solution from the melted single crystals. As shown in Fig. 5, the DSC curve at the 1st run had subliminal peaks around 276–283 K which would show metastable phases formed in the sample pan. Before the 2nd run, we crystalized the sample again and annealed it by temperature swinging around the melting temperature. The 2nd run showed the singular peak and the heat of fusion of the TBA 3HB hydrate was determined to be 172 kJ kg−1. This is consistent with the heat of fusion of the TBA 2HB hydrate which is 177 kJ kg−1

38

. Therefore, it was suggested that the position of the

OH group in the hydroxybutyrate has little influence on the heat of fusion.

Fig. 5 Results of DSC measurements for semiclathrate hydrates of TBA 3HB. Color: blue, 1st run; red, 2nd run.


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The structure of the TBA 3HB hydrate was determined to have a primitive tetragonal cell with unit cell parameters of a = 23.422(3) Å and c = 24.538(7) Å at 123(2) K. As well as the TBA butyrate and valerate, TBA 3HB induced the tetragonal hydrate structure. However, both 3HB and 2HB anions doubled and lowered the tetragonal hydrate structure which usually have the cell parameters of a = 23.52 Å and c = 12.30 Å2. The structure parameters for the TBA 2HB hydrate37 and the TBA 3HB hydrate are similar, but both of the a and c axes in TBA 3HB hydrate were elongated from those of the TBA 2HB hydrate which has a cell with a = 23.3850 Å and c = 24.4019 Å at 123 K. Such structural difference in the axis lengths would also result from coordination structures of ionic guest anions in the hydrate cages. The present results for melting temperature measurements, DSC and XRD provide a piece of knowledge for molecular engineering on semiclathrate hydrates to design their thermophysical properties. By substitution of butyrate anion with a hydrophilic OH group or a hydrophobic CH3 group, melting temperatures were varied by ~5 K. For 2MB and 3MB anions, substitutions by a CH3 group at both 2 and 3 positions in butyrate induced similar hydration numbers to that of the TBA butyrate hydrate, which implies similar structures formed throughout these anions. However, for 2HB and 3HB, the substitution by a OH group scarcely changed the crystal structures and also the heats of fusion, but suppressed the melting temperature by 3 K. Such thermophysical and structural modifications by the substitution with a OH or a CH3 group in butyrate may be caused by coordination of the anions and distortion of host water cages. To further design these materials for potential applications, investigation for new ionic guest substances to understand and improve mechanisms for stabilization/destabilization of the hydrates is necessary.


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4 Conclusion Melting temperatures and a heat of fusion for TBA 3HB hydrates are reported in this study. TBA 3HB formed stable semiclathrate hydrates of which highest temperature was 282.2 K and the heat of fusion was measured to be 172 kJ kg−1. The morphologies of the obtained crystals were basically rectangular columnar, but cubic shapes with/without round corners were also observed. By single crystal XRD measurements, the crystal structure of the TBA 3HB hydrate was identified to be the tetragonal hydrate structure with a doubled unit cell. We compared the TBA 3HB hydrates with the other hydrates of TBA carboxylates and hydroxycarboxylates of which anions have analogous molecular structures. Based on the melting temperature differences between these hydrates, it was found that the position of the OH group significantly affected anion incorporation in the hydrate structures. Although the crystal lattice of the TBA 3HB hydrate was similar to that of the TBA 2HB hydrate, the former was a larger likely due to the coordination of the anion. These two hydrates had similar heats of fusion, while their melting temperatures differ by 3 K. These facts support that the TBA 2HB and 3HB hydrates have similar hydration numbers as well as crystal structures. It was suggested that the positioning of OH group at these two positions varies the melting temperatures without influence on thermodynamic stability of the hydrates.


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AUTHOR INFORMATION Corresponding Author *S.M. Fax : +81-29-861-8706 Tel : +81-29-861-4287 E-mail : [email protected] (S. M.)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol) Funding Sources This work is supported by Grant-in-Aid for Young Scientists (B) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (No. 26820069).

ABBREVIATIONS TBA, tetra-n-butylammonium; TBP, tetra-n-butylphosphonium; 3HB, 3-hydroxybutyrate; 2HB, 2-hydroxybutyrate; 3MB, 3-methylbutyrate; 2MB, 2-methylbutyrate; DSC, Differential Scanning Calorimetry; XRD, X-ray diffraction.


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TABLE 1. Raw materials used in this study. Chemical name

CAS registry number

Source

Purification method

Initial mole fraction purity

Analysis method

Water

7732-18-5

Purified in laboratory from deionized water

Filtration and sterilization

-

Electrical resistivity:

Design of Thermophysical Properties of Semiclathrate Hydrates

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