Thermodynamic Properties and Crystallographic Characterization of Semiclathrate Hydrates Formed with Tetra-n-butylammonium Glycolate.

Semiclathrate hydrates are a crystalline host-guest material, which forms with water and ionic substances such as tetra-n-butylammonium (TBA) salts. Various anions can be used as a counter anion to the TBA cation, and they can modify thermodynamic properties of the semiclathrate hydrates, which are critical for applications, for example, cold energy storage and gas separation. In this study, the semiclathrate hydrates of the TBA glycolate were newly synthesized. Measurements for melting temperatures and a heat of fusion and a crystal structure analysis were performed. In comparison with the other similar materials, such as acetates, propionates, lactates, and hydroxybutyrates, the glycolate greatly changed the melting temperature and the heat of fusion. The preliminarily determined crystal structure showed that the glycolate anion builds a relatively porous structure compared to the previously reported hydrates formed with hydroxycarboxylates. The present study showed that substitution of a hydrophobic group by a hydrophilic group is an effective method to control the thermodynamic properties as well as to improve environmental, biological, and chemical properties.

Introduction

Semiclathrate hydrates are crystalline host–guest compounds formed from water and ionic/nonionic substances such as ammonium salts and amines. As the semiclathrate hydrates are highly stable even at around 300 K at atmospheric pressure, these materials can be used for cold energy storage.[4−8] In addition, the semiclathrate hydrates can capture gas under moderate temperature and pressure conditions.[9−17] From these unique properties, many industrial applications are proposed so far.[18−22] Semiclathrate hydrates are similar compounds to gas hydrates formed from water and small gases, for example, methane, carbon dioxide, and hydrogen.[1,2] The major difference between these materials is that the semiclathrate hydrates form from water and ionic substances with/without gas. The ionic substances are stably incorporated in the cage-like network, whereas they also make hydrogen bonds to cage water molecules. Therefore, the ionic substances are regarded as both hosts and guests for the hydrates. Tetra-n-butylammonium (TBA) and tetra-n-butylphosphonium (TBP) salts are widely used ionic substances to form the semiclathrate hydrates.[3−8] In the structure of TBA and TBP salt hydrates, the TBA or TBP cation occupies a four-cage fused-cage, for example, three tetrakaidecahedral cages plus one pentakaidecahedral cage, to incorporate each carbon chain into one of the four cages.[1−4,7,8] The rest of the structure is composed of pentagonal dodecahedral cages made by water and the counter anion of the TBA or TBP cation. The dodecahedral cage can incorporate small gases such as CH4, CO2, N2, and H2.[10,17,23,24] There are various options for counter anions for these cations, and the thermodynamic properties of the semiclathrate hydrates are significantly changed by the anions.

Melting temperatures of the TBA and TBP halide hydrates simply decrease from fluoride (or hydroxide) to bromide.[8] In most cases, the halide anions replace a water molecule in the hydrogen bond network.[4,7] Carboxylates also form semiclathrate hydrates with the aid of TBA or TBP cations.[5,25−31] Compared to the halide anions, the tendency on melting temperature with carboxylates is quite different. For linear carboxylates between formate and valerate, propionate and butyrate have the maximal melting temperature. This means that a change in the carbon chain length can control the melting temperature of TBA carboxylate hydrates. On the basis of their crystal structures,[26−29] the carboxy group of the anions makes hydrogen bonds with the cage water molecules, and the hydrophobic carbon chain is oriented toward the cage center. In addition to the carboxylates, hydroxycarboxylates can also form semiclathrate hydrates.[32−34] By adding a hydroxy group to carboxylates, it is possible to improve the environmental, chemical, and biological properties such as stench and biodegradability.[32] In the case of lactate anions, it was found that the carboxy group bonded to the cage water, and the hydroxy group additionally bonded to the water. As a result, the lactate anion was restricted on the cage wall compared to the propionate anion.[28] In our previous studies, two hydroxybutyrates, that is, 2-hydroxybutyrate (2HB)[33] and 3-hydroxybutyrate (3HB),[34] were also subjected to hydrate formation. The melting temperatures of these hydrates significantly differ: 285.3 K for 2HB and 282.2 K for 3HB, whereas the heats of fusion were similar. These results show that a position of the hydroxy group in the anion significantly changes the thermodynamic properties of semiclathrate hydrates.

Among the linear carboxylates, the propionate is a unique ionic substance that induces the cubic structure of the semiclathrate hydrate probably because of the suitably sized carbon chain to be incorporated in the dodecahedral cage.[28] The cubic structure results from the good symmetric hydration of the anion in the dodecahedral cage. Glycolate is the anion for which a hydroxy group replaces a methyl group in the propionate. When a hydroxy group substitutes a methyl group, the hydration of the anion would become different, which may also affect the thermodynamic properties of the semiclathrate hydrates. In order to develop a crystal engineering technique on the semiclathrate hydrates, modification of the anions by a hydroxy group needs to be understood. In this study, we report thermodynamic properties and a crystal structure analysis of TBA glycolate hydrates. The melting temperatures and the heat of fusion were measured and a crystal structure of the TBA glycolate hydrate was analyzed.

Experimental Section

Materials

The used water was made from deionized water that was sterilized by ultraviolet and filtrated by activated carbon and a hollow fiber. The electrical resistivity of the water was >18.2 MΩ. TBA glycolate was synthesized by the neutralization of TBA hydroxide (Tokyo Chemical Industry, 42.9 mass % in water) and glycol acid (Aldrich, >99%). The obtained aqueous solution was refined by crystallization. The water contained in the melt solution was evaporated to gravimetrically determine the salt concentration. The content of the solution was further confirmed by 1H and 13C nuclear magnetic resonance (NMR) measurements by an NMR spectrometer (Bruker Biospin, AVANCE 500). For the differential scanning calorimetry (DSC) measurements, c-hexane (Wako Pure Chemical Industries, Ltd., 99.8%) is used.

Melting Temperature Measurements

The melting temperature measurements were performed by optical observation of the crystals during stepwise temperature increase as we used in the previous studies.[32,33] We prepared about 2 g of the aqueous solution samples in a glass tube. Mole fraction (x) and mass fraction (w) of the TBA glycolate were gravimetrically determined with the aid of an electronic balance (AUW220D, Shimadzu Co., Kyoto, Japan). The tubes were set in a temperature-controlled bath. Once the bath was cooled down sufficiently to form hydrates, the system temperature was increased with a step of 0.2 or 0.1 K. The system temperature (T) was measured by a thermometer (NRHS1-0, Chino Co. Ltd., Tokyo, Japan) and a bridge (F201B, Chino, Co. Ltd., Tokyo, Japan). Hydrate crystals were observed by a microscope and a charge-coupled device (CCD) camera. At each temperature step, the hydrate crystals were equilibrated until their appearances had not changed. The melting temperature was determined to be the temperature one step before the temperature at which all hydrate crystals in a tube melted.

DSC Measurements

DSC (DSC-60, Shimadzu Co., Kyoto, Japan) was used for heats of fusion of the hydrates. An aqueous solution of TBA glycolate was supplied in an aluminum cell, the volume of which was approximately 20 μL. The cell was sealed by an aluminum lid. As a reference, we measured a heat of fusion of c-hexane, which was reported to be 31.84 kJ/kg in the literature.[35] DSC peaks were measured under nitrogen gas flow. Once the cell was cooled down to 250 K approximately, a heat of hydrate formation was observed. As the sample occasionally contains ice and metastable hydrate phases, we melted them at around a melting temperature of a stable hydrate phase. Subsequently, we gently cooled the sample to ∼270 K. This process enables to grow a stable hydrate phase selectively in the cell. The system temperature was increased with a rate of 2 K/min, and we obtained a heat of fusion of the hydrate from a DSC peak area. The measurement uncertainty of the heat of fusion was 6 kJ/kg.

Crystal Structure Analysis

Single crystals of the TBA glycolate hydrates were formed from the aqueous solution with x = 0.0293 at T = 278.2 K. The subcooling temperature was 2.7 K, which enables to grow thick crystals. After a sufficient number of suitably sized crystals grew, the crystals were separated from the residual aqueous solution. The crystals were preserved in a freezer at 260 K. In the followed processes, the crystals were kept under this temperature to avoid melting. A crystal was chosen and mounted on a single-crystal X-ray diffractometer (Smart APEX CCD, Bruker AXS). The measurement temperature of the X-ray diffraction (XRD) was 123 K. The X-ray source was Mo Kα (wave length: 0.7107 Å). The structure was solved and refined by Shelx.[36] A summary of the present crystal structure analysis is provided in the Supporting Information.

Results and Discussion

The melting temperatures for aqueous solutions of the TBA glycolate with 11 different compositions were measured. The results are shown in Figure and Table . At dilute compositions, x = 0.0027 and 0.0062, the TBA glycolate depressed the freezing point of ice. As at x = 0.0098, solids in the aqueous solution melted over 273 K, it is suggested that the TBA glycolate hydrates are rather more stable than ice at x ≳ 0.0098. As x increased the melting temperature increased, and the highest temperature of 280.9 K was observed at x = 0.0293 and 0.0314. At these compositions the number of water molecules to one TBA glycolate in the aqueous solutions (Nwater) was 31–33, which is likely to be the hydration number of the most stable hydrate phase.

Figure 1

Results for melting temperature measurements. ×, TBA glycolate hydrates (this work); ■, TBA propionate hydrates;[25] □, TBA acetate hydrates;[25] ◇, TBA lactate hydrates;[32] △, TBA 2HB hydrates;[33] ▲, TBA 3HB hydrates.[34] The bars for TBA glycolate hydrates show the measurement uncertainties for temperature.

Table 1

Results for Melting Temperature Measurements in the System of TBA Glycolate + Watera

x	w	Nwater	T/K	U(T)/K
0.0027	0.05	376.2	272.6	+0.1
				–0.1
0.0062	0.10	159.3	272.8	+0.1
				–0.1
0.0098	0.15	101.2	274.6	+0.1
				–0.1
0.0143	0.20	68.9	276.3	+0.1
				–0.1
0.0187	0.25	52.5	278.1	+0.1
				–0.2
0.0221	0.29	44.2	279.9	+0.1
				–0.2
0.0240	0.30	40.7	280.3	+0.1
				–0.2
0.0258	0.32	37.7	280.7	+0.1
				–0.2
0.0293	0.35	33.2	280.9	+0.1
				–0.2
0.0314	0.36	30.9	280.9	+0.1
				–0.2
0.0351	0.39	27.5	280.7	+0.1
				–0.2

Uncertainties of x and w are 0.0006 and 0.01 with 95% reliability, respectively.

In the T–x curve, a peak shoulder is found between x = 0.005 and 0.015, though there is a clear peak top at x ≈ 0.03. The T–x curve of the TBA glycolate hydrates may contain two peaks. This fact suggests that there are two or more phases of the TBA glycolate hydrates. Such T–x curves were also found in TBA bromide hydrates[37] and TBA propionate hydrates.[25] TBA bromide hydrates have two phases, that is, tetragonal and orthorhombic phases, which are stable over the freezing point of ice.[4,37] In Figure , the TBA propionate hydrates have a peak shoulder at around x = 0.01. This is probably due to the several structures of the TBA propionate hydrates.[3]

Figure also compares the present material with several TBA carboxylate hydrates and TBA hydroxycarboxylate hydrates, the anions of which are similar to glycolate. The glycolate is an anion for which the hydroxy group substitutes the methyl group in propionate, or is attached to acetate. As clearly shown by this figure, the TBA glycolate has the lowest melting temperatures among these similar hydrates. The melting temperatures with propionate and glycolate differ by 10 K, whereas both propionate and glycolate have a similar molecular structure and volume. This is interesting because the melting temperature shift with these two anions from TBA acetate hydrates reflects the opposite effects on stabilization on the static structure of the hydrates by the hydrophobic methyl group and hydrophilic hydroxy group.

Figure shows the single crystals formed at x = 0.0293 and T = 278.2 K. The shape of the crystals was rectangular-columnar. The DSC measurements were performed on the TBA glycolate hydrates which was formed from the aqueous solution melted from the single crystals. We collected the DSC curve three times as shown in Figure . Up to ∼275 K, the system temperature was increased with 5 K/min, and for the higher temperature region the rate of 2 K/min was used. The DSC peaks agreed with each other between these runs. The heat of fusion of the TBA glycolate hydrate was determined to be 161 kJ/kg, which is slightly smaller than those of the other TBA hydroxycarboxylate hydrates: 191 kJ/kg with TBA lactate, 177 kJ/kg with TBA 2HB, and 172 kJ/kg with TBA 3HB.

Figure 2

Single crystals of the TBA glycolate hydrate formed from an aqueous solution with x = 0.0293 and T = 278.2 K.

Figure 3

DSC data for TBA glycolate hydrates. □ with blue, first run; □ with red, second run; □ with blank, third run.

In the case of halide anions, the melting temperature linearly decreased from F or OH to Cl and Br depending on the volume of the halide anion.[8] The crystal structure changes from the cubic to the tetragonal and orthorhombic. On the basis of the typical structures of the TBA salt hydrates,[1,2] the cubic structure has the highest density of the dodecahedral cages per the number of water molecules: the ratios of water molecules to the dodecahedral cages are 20 in the cubic structure, 16.4 in the tetragonal structure, and 12.7 in the orthorhombic structure.[38] In the case of linear carboxylate anions, the hydrate formed with propionate has the highest melting temperature[25] with the cubic structure.[28] The hydrates with formate, acetate, butyrate, and valerate, which form the tetragonal structure,[28] have lower melting temperatures than that with propionate. The smallest one, that is, formate, has the lowest melting temperature among them. These two cases for halides and linear carboxylates suggest that the more porous the structure, the lower the melting temperature. The presently obtained melting temperatures with TBA glycolate are the lowest between the reported hydrates with hydroxybutyrates. Considering the relationship between melting temperature and structure, the structure of the TBA glycolate hydrate is likely to be the most porous among them. On the other hand, it is reported that the powder XRD analyses on the TBA carboxylates suggested several structures in the vicinity of the maximal point on the T–x curve.[39,40] Further investigation by single-crystal XRD is needed for the polymorphic behavior of these semiclathrate hydrates.

The structure of the TBA glycolate hydrate was determined to be a tetragonal cell with 23.4211(9) Å of a lattice and 12.3114(10) Å of c lattice at 123 K by the single-crystal XRD. The present hydrate has a unit cell similar to the TBA lactate hydrate.[32] Although the TBA propionate forms the cubic structure, the glycolate derives the tetragonal structure. The a lattice with glycolate is slightly longer than that with 2HB,[33] but shorter than that with 3HB.[34] It was found that the unit cell with the TBA lactate and glycolate retains the usual size of the tetragonal structure, that is, a = 23.5 Å and c = 12.3 Å approximately ,[2] whereas the unit cells of the hydrates with the two hydroxybutyrates, that is, 2HB[33] and 3HB,[34] are doubled from the usual size.

Further refinement of the structure found the glycolate anion in the water cage, which is highly distorted from regular dodecahedra. Figure shows a pattern of glycolate hydration in the dodecahedral cage found near the TBA cation incorporated in the fused cage composed of three tetrakaidekahedral cages and a pentakaidecahedral cage. A water molecule was found at the cage center with ∼20% of occupancy. Two oxygen atoms in the carboxy group formed a five-membered ring by hydrogen bonding with water molecules. No hydrogen bonds were found between the water of the dodecahedral cage and one of the oxygens of the carboxy group. This suggests that the glycolate plays the role of a bridge to link the network with less hydrogen bonds in the structure, and builds the relatively porous structure. The presently determined structure has Nwater = 34 of the hydration number, which is equivalent to x = 0.029. The highest melting temperature was obtained around this composition; therefore, the structure may be reasonably determined.

Figure 4

Hydration of the glycolate anion in a dodecahedral cage. A five-membered ring made by water and the carboxy group in the glycolate is together shown. Red: oxygen in water, magenta: oxygen in glycolate, green: carbon in glycolate, orange: oxygen in water trapped in the cage. Hydrogen atoms are omitted for clarity.

Figure compares the hydration patterns between the lactate and the glycolate anions. In the case of the lactate,[32] the methyl group in the lactate tends to stay in a wider space available at the cage center. As a result, both of the oxygen atoms in the carboxy group make hydrogen bonds with the dodecahedral cage water. Compared to this case, the glycolate is simply attracted to the cage waters to bridge them. Alternatively, a water molecule places at the cage center where the distance from the cage water was a hydrogen bond length, that is, 2.8–3 Å. This may be because the glycolate does not have any more hydrophobic parts to prevent the dodecahedral cage from becoming dented. This preliminarily determined structure suggests that the anion of which each carbon is modified with hydrophilic group makes more voids, in other words, a porous structure. Such a porous structure may cause a lowering of the melting temperature of the semiclathrate hydrates. That is, the modification of anions by a hydroxy group is a sort of crystal engineering technique on the semiclathrate hydrates for controlling the thermodynamic properties.

Figure 5

Comparison of hydration between lactate and glycolate. (a) Hydration pattern of glycolate (this work). (b) Hydration pattern of lactate.[32] Red: oxygen in water, magenta: oxygen in glycolate, green: carbon in glycolate, orange: oxygen in water trapped in the cage. Hydrogen atoms are omitted for clarity.

The present XRD data were insufficient to refine this structure completely because the structure contains a lot of disorder around the fused cages composed of tetrakaidecahedral cages which incorporate the TBA cations. Further structure analysis with the assistance of spectroscopic methods such as NMR will be needed to reveal the detailed hydration structure of the glycolate anion.

Conclusions

The semiclathrate hydrates of TBA glycolate were formed and subjected to measurements for thermodynamic properties and a crystal structure analysis. The melting temperature of the TBA glycolate hydrate was 280.9 K. The heat of fusion was determined to be 161 kJ/kg. The obtained melting temperatures and the heat of fusion were significantly lower than those of the previously reported semiclathrate hydrates formed with hydroxycarboxylates. The structure of the TBA glycolate hydrate was determined to be of a tetragonal structure as well as those formed with lactate, 2HB and 3HB. The preliminarily determined structure suggested that the porous structure of TBA glycolate hydrates may cause the lowest melting temperatures among the previously reported hydrates formed with hydroxycarboxylates. The present study showed that substitution of a hydrophobic group by a hydrophilic group is an effective method to control thermodynamic properties of semiclathrate hydrates as well as to improve environmental, biological, and chemical properties.