- Email: [email protected]
CH4 enclathration in tetra-iso-amyl ammonium bromide (TiAAB) semiclathrate and its significance for natural gas storage
Chemical Engineering Journal 330 (2017) 1160–1165
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
CH4 enclathration in tetra-iso-amyl ammonium bromide (TiAAB) semiclathrate and its significance for natural gas storage ⁎
Soyoung Kima, Ki-Sub Kimb, , Yongwon Seoa, a b
MARK
⁎
School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, Ulsan 44919, Republic of Korea School of Chemical and Materials Engineering, Korea National University of Transportation, Chungbuk 27469, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Natural gas Gas storage Semiclathrate Tetra-iso-amyl ammonium bromide
Tetra-iso-amyl ammonium bromide (TiAAB), one of the quaternary ammonium salts, was investigated as a semiclathrate former for its potential application in natural gas storage. The dissociation equilibrium temperature of pure TiAAB semiclathrate was the highest at a stoichiometric concentration (3.7 mol%) of TiAAB·26.0H2O. The dissociation enthalpy of pure TiAAB (3.7 mol%) semiclathrate was 253.0 ± 0.5 J/g using a differential scanning calorimeter (DSC). The thermodynamic stability of the CH4 + TiAAB semiclathrate was significantly higher than that of the CH4 gas hydrate. The CH4 + TiAAB (3.7 mol%) semiclathrate was thermodynamically stable at temperatures higher than 300 K for the pressure range of 1.5–8.5 MPa. 13C NMR and in situ Raman spectroscopy revealed that CH4 molecules were enclathrated in the small cages of the TiAAB semiclathrate and a CH4 inclusion did not affect the semiclathrate structure. The overall experimental results clearly indicate that the TiAAB semiclathrate can trap CH4 molecules at favorable conditions and thus it has a potential for natural gas storage.
1. Introduction Clathrate hydrates are solid inclusion compounds formed by the hydrogen bonding of water molecules at high pressure and low temperature conditions [1]. The host water molecules form cage frameworks of clathrate hydrates, whereas the small guest molecules are incorporated in the cages. Clathrate hydrates can be generally classified into two types of true clathrates and semiclathrates, although the strict distinction between true clathrates and semiclathrates has recently been blurred [2–4]. In true clathrates, there are no chemical interactions between the host and guest molecules, and gas hydrates belong to the true clathrates. However, in semiclathrates, there are chemical or ionic interactions between the host and guest molecules, and quaternary ammonium salts (QASs) are representative semiclathrate formers [5–11]. In QAS semiclathrates, quaternary ammonium cations are enclathrated into the partly broken large cages of the semiclathrate structure whereas anions such as F−, Cl−, and Br− are incorporated into the host frameworks [7–11]. The notable features of QAS semiclathrates are that they are thermodynamically stable under atmospheric pressure at the temperature ranges far above the freezing point of water and small dodecahedral (512) cages of the semiclathrates are left vacant, which can be used for encaging small-sized gas molecules [12–18].
⁎
Corresponding authors. E-mail addresses: [email protected] (K.-S. Kim), [email protected] (Y. Seo).
http://dx.doi.org/10.1016/j.cej.2017.08.054 Received 7 July 2017; Received in revised form 11 August 2017; Accepted 11 August 2017 Available online 12 August 2017 1385-8947/ © 2017 Elsevier B.V. All rights reserved.
Clathrate hydrates have many technological applications in energy and environmental fields such as CO2 capture and sequestration, gas storage and transportation, and desalination [19–28]. However, one of the main drawbacks of these clathrate-based technologies is that clathrate hydrates require high pressures and low temperatures to form the cage structure and maintain their thermodynamic stability, which results in high energy requirements. In particular, a significant pressure reduction at any given temperature must be achieved for natural gas storage and transportation using clathrate hydrate formation. Concerted efforts have been made to reduce hydrate equilibrium pressure for natural gas by adding various thermodynamic promoters. Tetrahydrofuran (THF) and cyclopentane (CP) are well-known thermodynamic promoters which can significantly lower the pressure required to form clathrate hydrates from natural gas [29–31]. However, both THF and CP are very volatile and toxic, which indicates that there is a possibility of some loss after repeated use and an unwanted mixing of promoters and retrieved natural gas in the vapor phase after dissociation, and there is also concern about their harmful effect on the human body and corrosion problems. However, QASs are non-volatile and nontoxic and form semiclathrates with water molecules under atmospheric pressure. QAS semiclathrates with guest gases offer a higher thermodynamic stability than other gas hydrates with well-known thermodynamic promoters like THF and CP [32].
Chemical Engineering Journal 330 (2017) 1160–1165
S. Kim et al.
of the semiclathrate dissociation curve and the thermal contraction or expansion curve at each pressure condition.
Semiclathrates formed from tetra-n-butyl ammonium (TBA) salts, such as tetra-n-butyl ammonium bromide (TBAB), chloride (TBAC), and fluoride (TBAF), have been extensively examined for CO2 capture and gas storage [13–17,32–38]. However, tetra-iso-amyl ammonium (TiAA) salts like tetra-iso-amyl ammonium bromide (TiAAB) and fluoride (TiAAF) have rarely been studied, even though TiAA salt semiclathrates have been reported to have significantly higher thermodynamic stability than other QAS semiclathrates [9,39,40]. In this study, TiAAB was selected as a semiclathrate former for its potential application in natural gas storage at mild pressure and temperature conditions. The enclathration of CH4 molecules in TiAAB semiclathrates was investigate with a primary focus on thermodynamic and spectroscopic aspects. The dissociation equilibrium temperatures of pure TiAAB semiclathrates were measured using a high pressure microdifferential scanning calorimeter (HP μ-DSC). The phase equilibria of CH4 + TiAAB semiclathrates were measured using a conventional isochoric method. The dissociation enthalpy of the TiAAB semiclathrate was also measured using a HP μ-DSC. In addition, the confirmation of CH4 enclathration in the clathrate lattices was performed via 13C NMR and in situ Raman spectroscopy.
2.3. High-pressure micro-differential scanning calorimeter A high-pressure micro-differential scanning calorimeter (HP μ-DSC VII evo, Setaram Inc., France) was adopted to measure both dissociation enthalpies (ΔHd) and dissociation equilibrium temperatures of the pure TiAAB semiclathrates. The HP μ-DSC device consists of a reference cell and a sample cell, with an operating pressure range of 0–40 MPa and an operating temperature range of 228.15–393.15 K. The HP µ-DSC had a resolution of 0.02 μW with a temperature deviation of ± 0.2 K. For the ΔHd measurements, the DSC dynamic method with continuous heating was used in this study. The sample cell was charged with approximately 6.5 mg of TiAAB solution, whereas the reference cell was left empty. Then, the cells were inserted into the furnace. A multi-cycle mode of cooling and heating was applied to the cells in order to enhance the conversion of the TiAAB solution to the TiAAB semiclathrate because there was no agitation in the liquid phase in the sample cell. The HP μ-DSC cells were initially cooled to 253 K at a cooling rate of 1.0 K/min. Through repeated cycles of cooling and heating, the TiAAB semiclathrate gradually increased and the complete conversion was confirmed with the disappearance of further formation and dissociation peaks for the pure TiAAB semiclathrate from the heat flow curve. Then, the temperature was increased to 320 K with a scanning rate of 0.5 K/min for the dissociation of the TiAAB semiclathrate. The dissociation enthalpies of the TiAAB semiclathrates were obtained through the integration of an endothermic heat flow curve. For the DSC stepwise experiments, after charging of the TiAAB solution into the cells and repeated cycles of cooling and heating, the cells were heated to a temperature which is 1.0 K lower than the expected dissociation equilibrium temperature in 1.0 K increments with 3 h intervals. The approximate dissociation equilibrium temperature of TiAAB semiclathrates can be estimated from the onset temperature of the endothermic heat flow curve obtained from the DSC dynamic experiments. In order to determine an accurate dissociation equilibrium point of TiAAB semiclathrates, the temperature was then increased in 0.1 K increments with a 3 h isothermal duration at each step until the endothermic heat flow due to semiclathrate dissociation completely disappeared.
2. Experimental section 2.1. Materials and apparatus The CH4 with a purity of 99.95% was supplied from Gasvally Co. (Republic of Korea). Tetra-iso-amyl ammonium bromide (TiAAB) was synthesized using Menschutkin’s reaction [41]. Tri-iso-amyl amine (Sigma-Aldrich, ≧95%, USA) and iso-amyl bromine (Sigma-Aldrich, 96%, USA) were refluxed in acetonitrile (Junsei, 99.5%, Japan) under nitrogen (Daesung Industrial Gases Co., Ltd, 99.999%, Republic of Korea) for 72 h. The solvent was evaporated by heating in a vacuum, and the crystals were then dissolved in ethyl acetate (Samchun, 99.5%, Republic of Korea). TiAAB was recrystallized for 24 h at 275 K. The crystals were then filtered from the solution and the process was repeated twice. The crystalline mass was dried in a vacuum oven (M.O.tech, USA) at 313 K. 1H NMR (D2O) spectrum consisted of the following peaks: 0.85 ∼ 0.87 (m, 24H), 1.46–1.49 (t, 8H), 1.53–1.59 (m, 4H), 3.12–3.16 (t, 8H). Deionized doubly distilled water was used for the experiment. The high pressure vessel used to measure the semiclathrate phase equilibria was made from 316 stainless steel with an inner volume of 50 cm3. An impeller-type mechanical stirrer was used to vigorously agitate the solutions inside the cell to accelerate the semiclathrate formation and dissociation. The equilibrium cell was immersed in a circulator (RW-2025G, Jeio Tech, Republic of Korea) to control the temperature. A thermocouple ( ± 0.1 K accuracy) was used to measure the cell temperature (calibrated using an ASTM 63C thermometer, H-B Instrument Company, USA). A pressure transducer (S-10, WIKA, Germany) was used to measure the cell pressure (calibrated using a Heise Bourdon tube pressure gauge, CMM-140830, 0–20.0 MPa, Ashcroft, Inc. USA). The uncertainties associated with the temperature and pressure measurements were 0.1 K and 0.02 MPa, respectively.
2.4.
13
C NMR and in situ Raman spectroscopic analyses
A Bruker 400 MHz solid-state FT-NMR spectrometer, which belongs to the Korea Basic Science Institute (KBSI), was used to directly confirm the enclathration of CH4 in the semiclathrate phase. For NMR analysis, semiclathrate samples collected from the high pressure cell were finely powdered in a liquid nitrogen vessel and were then placed in a 4 mm o.d. Zr rotor that was inserted in a variable-temperature probe at 243 K. 13 C NMR spectra were collected at a Larmor frequency of 100.6 MHz with magic angle spinning (MAS) between 2 and 4 kHz. The pulse length of 2 μs and pulse repetition delay of 10 s under proton decoupling were employed when a radio frequency field strength of 50 kHz corresponding to 5 μs 90° pulses was used. The formation process of the pure TiAAB and CH4 + TiAAB semiclathrates was monitored using an in situ fiber-coupled Raman spectrometer (SP550, Horiba, France) with a multichannel air cooled CCD detector and 1800 grooves/mm grating. A fiber-optic Raman probe that was installed in the high pressure reactor provided time-dependent Raman spectra of the pure TiAAB and CH4 + TiAAB semiclathrates. The reactor for the Raman measurement was enclosed in a water jacket whose temperature was controlled by an external circulator (RW2025G, Jeio Tech, Republic of Korea). A more detailed description of the experimental methods and procedure were provided in our previous papers [15,16,32,42].
2.2. Semiclathrate phase equilibrium measurements An isochoric method with step heating and cooling was used to determine the semiclathrate phase equilibria of the CH4 + TiAAB + water system. The cell was charged with 25 cm3 of TiAAB aqueous solution and then pressurized with the CH4 gas to the desired pressure. The temperature was lowered at 1.0 K/h to induce semiclathrate formation below the expected equilibrium temperature. After a significant pressure decrease was observed, which indicated the formation of a semiclathrate with CH4, the cell was slowly heated at 0.1 K/90 min for the dissociation. The three-phase (semiclathrate (H)–liquid water (LW)–vapor (V)) equilibrium point was determined by the intersection 1161
Chemical Engineering Journal 330 (2017) 1160–1165
S. Kim et al.
3. Results and discussion 3.1. Thermodynamic stability of TiAAB semiclathrates In this study, a HP μ-DSC was used to determine accurate equilibrium dissociation temperatures of pure TiAAB semiclathrates formed at three different concentrations (1.0, 3.7, and 5.0 mol% TiAAB). The HP μ-DSC enabled us to measure dissociation enthalpies of semiclathrates as well as to determine equilibrium dissociation temperatures [43]. The dynamic method, which uses a continuous increase of temperature, is less time consuming and useful for measuring both the dissociation enthalpy and the equilibrium dissociation temperature at the same time. However, the dynamic method can sometimes generate inherent uncertainty in determining an accurate equilibrium dissociation temperature from the endothermic heat flow curves when broad and progressive peaks appear during heating [43,44]. In order to overcome this inaccuracy of the equilibrium point determination using the dynamic method, the stepwise method, which uses stepwise temperature increases during heating, was used in this study. The temperature increment and isothermal duration at each step were 0.1 K and 3 h, respectively. The heat flow changes and temperature profiles during stepwise heating for the pure TiAAB (3.7 mol%) semiclathrate under atmospheric pressure was depicted in Fig. 1. The figure shows that the isothermal duration at each temperature step is long enough for the heat flow signal to return to the baseline. For each temperature step, a large endothermic peak caused by dissociation of the TiAAB semiclathrate was observed until the semiclathrate dissociation was completely terminated. The point where the large endothermic peaks due to the semiclathrate dissociation disappeared and accordingly, the calorimetric signal change caused only by the temperature increment appeared was determined to be the equilibrium dissociation temperature of the TiAAB semiclathrate at a specified TiAAB concentration. Through the DSC stepwise method, the dissociation equilibrium temperatures of pure TiAAB semiclathrates were 302.8, 303.6, and 303.4 K for 1.0, 3.7, and 5.0 mol% TiAAB solutions, respectively, under atmospheric pressure. The equilibrium dissociation temperatures of pure TiAAB semiclathrates measured by the DSC stepwise method along with those by the DSC dynamic method under atmospheric pressure are presented in Fig. 2. The results obtained in this study were in good agreement with the value reported in the literature (3.7 mol% TiAAB solution) [9]. The
Fig. 2. Equilibrium dissociation temperatures of pure TiAAB semiclathrates.
slight discrepancy in the equilibrium dissociation temperatures between the DSC stepwise method and the DSC dynamics method can be attributed to the uncertainty involved in determining the onset temperature from the endothermic heat flow curve of the DSC dynamic method. It is generally accepted that the DSC stepwise method offers more accurate equilibrium dissociation temperatures for gas hydrate or semiclathrate systems than the DSC dynamic method. The experimental results clearly indicate that the DSC can give the reliable and accurate equilibrium dissociation temperatures of pure TiAAB semiclathrates. In addition, the maximum dissociation equilibrium temperature was observed at the stoichiometric concentration (3.7 mol%) of TiAAB·26.0 H2O. In order to measure the dissociation enthalpy of pure TiAAB (3.7 mol%) semiclathrate, the dynamic method with continuous heating was used and the resulting DSC thermogram is depicted in Fig. 3. The TiAAB (3.7 mol%) semiclathrate exhibited only one endothermic peak without an ice melting peak during the semiclathrate dissociation, which indicates that all water molecules were involved in TiAAB semiclathrate formation at the stoichiometric concentration of 3.7 mol% (TiAAB·26.0H2O). The dissociation enthalpy of TiAAB (3.7 mol%) semiclathrate was found to be 253.0 ± 0.5 J/g. Three-phase (semiclathrate (H)–liquid water (Lw)–vapor (V)) equilibria of the CH4 + TiAAB (3.7 mol%) + water system are depicted in Fig. 4 along with those of the CH4 + water system [45], and the overall experimental results are summarized in Table 1. The phase equilibrium results clearly demonstrated that the thermodynamic stability of the CH4 + TiAAB semiclathrate was significantly higher than that of the pure CH4 hydrate. For example, the dissociation equilibrium temperature of the CH4+TiAAB (3.7 mol%) semiclathrate is approximately 37 K higher than that of the pure CH4 hydrate at 3.0 MPa, which indicates that the inclusion of TiAAB and subsequent semiclathrate formation resulted in significant thermodynamic stabilization of the system. It is speculated from Figs. 2 and 4 that even above the room temperature CH4 molecules can be enclathrated into the cages of TiAAB semiclathrates at around atmospheric pressure even though the experimental measurements of the semiclathrate phase equilibria were conducted over a wide pressure range (1.5–8.5 MPa). As seen from Fig. 4, the pure CH4+water system requires equilibrium pressure as high as 2.7 MPa even at 273 K for gas hydrate formation, which could be a serious limitation to application of gas hydrate-based CH4 storage
Fig. 1. Heat flow changes during stepwise heating for the TiAAB (3.7 mol%) semiclathrate under atmospheric pressure.
1162
Chemical Engineering Journal 330 (2017) 1160–1165
S. Kim et al.
Fig. 3. Dissociation thermogram of the TiAAB (3.7 mol%) semiclathrate under atmospheric pressure.
Fig. 5. Time-dependent in situ Raman spectra of semiclathrates formed from (a) TiAAB (3.7 mol%) + water system under atmospheric pressure and ΔT = 5.0 K and (b) CH4 + TiAAB (3.7 mol%) + water system at 4.0 MPa and ΔT = 5.0 K.
Fig. 4. Semiclathrate phase equilibria of the CH4 + TiAAB (3.7 mol%) + water system.
to actual processes. However, the equilibrium pressure required for semiclathrate formation from the CH4+TiAAB (3.7 mol%) + water system was significantly lower at a specified temperature, demonstrating a great potential of TiAAB semiclathrates for a new gas storage medium.
Table 1 Semiclathrate phase equilibrium data of the CH4 + TiAAB (3.7 mol%) + water system. CH4 + TiAAB (3.7 mol%) semiclathrate
3.2. In situ Raman and
T/K
P/MPa
309.1 311.8 313.3 314.2 314.7
1.56 3.29 5.01 7.01 8.49
13
C NMR analyses
Fig. 5(a) shows the time-dependent in situ Raman spectra collected during the conversion of TiAAB (3.7 mol%) solution to pure TiAAB semiclathrate under atmospheric pressure in a wavenumber range of 2860–3000 cm−1. The intensities of Raman peaks gradually increased as the pure TiAAB semiclathrate formation proceeded. The gradual growth of each Raman peak indicates the gradual formation of pure 1163
Chemical Engineering Journal 330 (2017) 1160–1165
S. Kim et al.
Fig. 6. Raman spectra of CH4 hydrate, pure TiAAB (3.7 mol%) semiclathrate, and CH4 + TiAAB (3.7 mol%) semiclathrate.
Fig. 7. 13C MAS NMR spectra of the pure CH4 hydrate and the CH4 + TiAAB (3.7 mol%) semiclathrate.
TiAAB semiclathrate. Fig. 5(b) shows the time-dependent in situ Raman spectra of the CH4+TiAAB (3.7 mol%) semiclathrate at 4.0 MPa and ΔT = 5.0 K. As in the pure TiAAB semiclathrate, all the Raman peaks from the CH4 + TiAAB (3.7 mol%) semiclathrate grew gradually over time. The positions of the Raman peaks from the CH4 + TiAAB semiclathrate were the same as those from the pure TiAAB semiclathrate except for one Raman peak that appeared at 2914 cm−1. The Raman peak at 2914 cm−1 corresponds to CH4 molecules captured in the small cages of the TiAAB semiclathrate. The time-dependent in situ Raman spectra clearly demonstrated that CH4 molecules were enclathrated in the lattices of the TiAAB semiclathrate and the Raman peak from enclathrated CH4 molecules grew and changed along with the peaks from TiAA cations enclathrated in the semiclathrate. Fig. 6 shows the Raman spectra for the CH4 hydrate, pure TiAAB (3.7 mol%) semiclathrate, and CH4+TiAAB (3.7 mol%) semiclathrate, which were obtained after gas hydrate or semiclathrate formation was completed. The Raman spectrum of the CH4+TiAAB (3.7 mol%) semiclathrate was distinguishable from that of the pure TiAAB (3.7 mol %) due to an additional peak appearing at 2914 cm−1. However, the positions and shapes of Raman peaks from the CH4+TiAAB semiclathrate were the same as those from the pure TiAAB semiclathrate except for the peak at 2914 cm−1 that corresponds to enclathrated CH4 molecules. This indicated that the inclusion of CH4 molecules in the TiAAB semiclathrate did not affect the structure of the semiclathrate. Raman spectra depicted in Figs. 5 and 6 clearly demonstrated that CH4 molecules were captured in the cages of semiclathrate without altering the structure of the TiAAB semiclathrate. The cage-dependent resonance peaks of 13C NMR spectroscopy for CH4 molecules enclathrated in different cages of gas hydrates can be effectively used to confirm the hydrate structure and to determine the types of hydrate cages that are occupied by guest molecules [46]. The 13 C MAS NMR spectra of the pure CH4 hydrate and the CH4 + TiAAB (3.7 mol%) semiclathrate are shown in Fig. 7. The pure CH4 hydrate (sI) exhibited two resonance peaks at −4.3 and −6.6 ppm that can be
assigned to CH4 molecules enclathrated in the small (512) and large (51262) cages of sI hydrate, respectively. The CH4+TiAAB (3.7 mol%) semiclathrate showed several resonance peaks in a wide range of chemical shift that are attributable to enclathrated TiAA cations as well as one peak at −4.3 ppm that is attributable to captured CH4 molecules. As indicated by previous researchers, semiclathrates formed from tetraiso-amyl ammonium or tetra-n-butyl ammonium salts possess vacant dodecahedral (512) cages that have a potential for capturing small-sized gas molecules [40,47]. Therefore, Fig. 7 clearly demonstrated that CH4 molecules were captured in the small (512) cages of TiAAB semiclathrate. This study clearly confirmed the enclathration of CH4 in the TiAAB semiclathrate by thermodynamic and spectroscopic approaches. However, more studies on kinetics, gas uptakes, and cage occupancy of the CH4+TiAAB semiclathrates should be conducted in the near future to assess the feasibility of actual application of TiAAB semiclathrate in natural gas storage and transportation. 4. Conclusions The enclathration of CH4 molecules in TiAAB semiclathrates was investigated primarily focusing on thermodynamic and spectroscopic aspects. Pure TiAAB semiclathrate was the most thermodynamically stable at a stoichiometric concentration (3.7 mol%) of TiAAB·26.0H2O. The dissociation enthalpy of TiAAB·26.0H2O was measured to be 253.0 ± 0.5 J/g using a HP μ-DSC. The CH4 + TiAAB (3.7 mol%) semiclathrate demonstrated significantly higher thermodynamic stability than CH4 gas hydrate and maintained its thermodynamic stability even at temperatures higher than 300 K for a pressure range of 1.5–8.5 MPa. The enclathration of CH4 molecules in the TiAAB semiclathrate lattices was clearly confirmed by 13C NMR and Raman spectroscopy. The overall experimental results provide a better understanding of the thermodynamic stability, cage-specific guest 1164
Chemical Engineering Journal 330 (2017) 1160–1165
S. Kim et al.
[22] P. Babu, P. Linga, R. Kumar, P. Englezos, A review of the hydrate based gas separation (HBGS) process for carbon dioxide pre-combustion capture, Energy 85 (2015) 261–279. [23] H. Lee, Y. Seo, Y.T. Seo, I.L. Moudrakovski, J.A. Ripmeester, Recovering methane from solid methane hydrate with carbon dioxide, Angew. Chem. Int. Ed. 42 (2003) 5048–5051. [24] H. Tajima, A. Yamasaki, F. Kiyono, Process design of a new injection method of liquid CO2 at the intermediate depths in the ocean using a static mixer, Fuel Process. Technol. 86 (2005) 1667–1678. [25] Y. Lee, S. Lee, J. Lee, Y. Seo, Structure identification and dissociation enthalpy measurements of the CO2 + N2 hydrates for their application to CO2 capture and storage, Chem. Eng. J. 246 (2014) 20–26. [26] D.-Y. Koh, H. Kang, J.-W. Lee, Y. Park, S.-J. Kim, J. Lee, J.Y. Lee, H. Lee, Energyefficient natural gas hydrate production using gas exchange, Appl. Energy 162 (2016) 114–130. [27] K.C. Kang, P. Linga, K.-N. Park, S.-J. Choi, J.D. Lee, Seawater desalination by gas hydrate process and removal characteristics of dissolved ions (Na+, K+, Mg2+, Ca2+, B3+, Cl–, SO42−), Desalination 353 (2014) 84–90. [28] J.-H. Cha, Y. Seol, Increasing gas hydrate formation temperature for desalination of high salinity produced water with secondary guests, ACS Sustain. Chem. Eng. 1 (2013) 1218–1224. [29] S. Hashimoto, S. Murayama, T. Sugahara, K. Ohgaki, Phase equilibria for H2 + CO2 + tetrahydrofuran + water mixtures containing gas hydrates, J. Chem. Eng. Data 51 (2006) 1884–1886. [30] J. Zhang, J.W. Lee, Equilibrium of hydrogen + cyclopentane and carbon dioxide + cyclopentane binary hydrates, J. Chem. Eng. Data 54 (2008) 659–661. [31] Y.T. Seo, S.P. Kang, H. Lee, Experimental determination and thermodynamic modeling of methane and nitrogen hydrates in the presence of THF, propylene oxide, 1,4-dioxane and acetone, Fluid Phase Equilibr. 189 (2001) 99–110. [32] S. Kim, S.D. Choi, Y. Seo, CO2 capture from flue gas using clathrate formation in the presence of thermodynamic promoters, Energy 118 (2017) 950–956. [33] K. Sato, H. Tokutomi, R. Ohmura, Phase equilibrium of ionic semiclathrate hydrates formed with tetrabutylammonium bromide and tetrabutylammonium chloride, Fluid Phase Equilibr. 337 (2013) 115–118. [34] X.-S. Li, C.-G. Xu, Z.-Y. Chen, H.-J. Wu, Tetra-n-butyl ammonium bromide semiclathrate hydrate process for post-combustion capture of carbon dioxide in the presence of dodecyl trimethyl ammonium chloride, Energy 35 (2010) 3902–3908. [35] A.H. Mohammadi, A. Eslamimanesh, V. Belandria, D. Richon, P. Naidoo, D. Ramjugernath, Phase equilibrium measurements for semi-clathrate hydrates of the (CO2 + N2 + tetra-n-butylammonium bromide) aqueous solution system, J. Chem. Thermodyn. 46 (2012) 57–61. [36] A. Eslamimanesh, A.H. Mohammadi, D. Richon, Thermodynamic modeling of phase equilibria of semi-clathrate hydrates of CO2, CH4, or N2 + tetra-nbutylammonium bromide aqueous solution, Chem. Eng. Sci. 81 (2012) 319–328. [37] D. Zhong, Y. Ye, C. Yang, Y. Bian, K. Ding, Experimental investigation of methane separation from low-concentration coal mine gas (CH4/N2/O2) by tetra-n-butyl ammonium bromide semiclathrate hydrate crystallization, Indus. Eng. Chem. Res. 51 (2012) 14806–14813. [38] H. Hashimoto, T. Yamaguchi, T. Kinoshita, S. Muromachi, Gas separation of flue gas by tetra-n-butylammonium bromide hydrates under moderate pressure conditions, Energy 129 (2017) 292–298. [39] A. Manakov, T. Rodionova, I. Terekhova, V. Komarov, A. Burdin, A. Sizikov, Structural and physico-chemical studies of ionic clathrate hydrates of tetrabutyland tetraisoamylammonium salts, in: Proceedings of the 7th International Conference on Gas Hydrates, Edinburgh, Scotland, UK, 2011. [40] A. Majumdar, B. Maini, P.R. Bishnoi, M.A. Clarke, Three-phase equilibrium conditions of TiAAB semiclathrates formed from N2, CO2, and their mixtures, J. Chem. Eng. Data 57 (2012) 2322–2327. [41] N. Menschutkin, Beiträgen zur Kenntnis der Affinitätskoeffizienten der Alkylhaloide und der organischen Amine, Z. Phys. Chem. 5 (1890) 589–600. [42] S. Kim, I.-H. Baek, J.-K. You, Y. Seo, Phase equilibria, dissociation enthalpies, and Raman spectroscopic analyses of N2 + tetra-n-butyl ammonium chloride (TBAC), Fluid Phase Equilbr. 413 (2016) 86–91. [43] W. Lin, D. Dalmazzone, W. Fürst, A. Delahaye, L. Fournaison, P. Clain, Accurate DSC measurement of the phase transition temperature in the TBPB-water system, J. Chem. Thermodyn. 61 (2013) 132–137. [44] D. Lee, Y. Lee, W. Choi, S. Lee, Y. Seo, Accurate measurement of phase equilibria and dissociation enthalpies of HFC-134a hydrates in the presence of NaCl for potential application in desalination, Korean J. Chem. Eng. 33 (2016) 1245–1430. [45] S. Adisasmito, R.J. Frank III, E.D. Sloan Jr, Hydrates of carbon dioxide and methane mixtures, J. Chem. Eng. Data 36 (1991) 68–71. [46] J.A. Ripmeester, C.I. Ratcliffe, On the contributions of NMR spectroscopy to clathrate science, J. Struct. Chem. 40 (1999) 654–662. [47] W. Wang, B.O. Carter, C.L. Bray, A. Steiner, J. Bacsa, J.T.A. Jones, C. Cropper, Y.Z. Khimyak, D.J. Adams, A.I. Cooper, Reversible methane storage in a polymersupported semi-clathrate hydrate at ambient temperature and pressure, Chem. Mater. 21 (2009) 3810–3815.
distribution, and gas capturing ability of the TiAAB semiclathrates for their potential applications to natural gas storage and transportation. Acknowledgements This research was supported by the Mid-career Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2017R1A2B4005455) and also by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Education (2016R1D1A1A02937037). This work was also supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (NRF-2015R1A5A7037825). References [1] E.D. Sloan, C.A. Koh, Clathrate Hydrates of Natural Gases, third ed., Taylor & Francis, Boca Raton, 2007. [2] S. Alavi, R. Susilo, J. Ripmeester, Linking microscopic guest properties to macroscopic observables in clathrate hydrates: guest-host hydrogen bonding, J. Chem. Phys. 130 (2009) 174501. [3] S. Alavi, S. Takeya, R. Ohmura, T.K. Woo, J.A. Ripmeester, Hydrogen-bonding alcohol-water interactions in binary ethanol, 1-propanol, and 2-propanol + methane structure II clathrate hydrates, J. Chem. Phys. 133 (2010) 074505. [4] W. Shin, S. Park, J.-W. Lee, Y. Seo, D.-Y. Koh, J. Seol, H. Lee, Structural transition from semi- to true clathrate hydrates induced by CH4 enclathration, J. Phys. Chem. C 116 (2012) 16352–16357. [5] D. Fowler, W. Loebenstein, D. Pall, C.A. Kraus, Some unusual hydrates of quaternary ammonium salts, J. Am. Chem. Soc. 62 (1940) 1140–1142. [6] R. McMullan, G.A. Jeffrey, Hydrates of the tetra n-butyl and tetra i-amyl quaternary ammonium salts, J. Chem. Phys. 31 (1959) 1231–1234. [7] A. Dyadin Yu, I.S. Terekhova, T.M. Polyanskaya, L.S. Aladko, Clathrate hydrates of tetrabutylammonium fluoride and oxalate, J. Struct. Chem. 17 (1977) 566–571. [8] T. Rodionova, V. Komarov, G. Villevald, L. Aladko, T. Karpova, A. Manakov, Calorimetric and structural studies of tetrabutylammonium chloride ionic clathrate hydrates, J. Phys. Chem. B 114 (2010) 11838–11846. [9] L. Aladko, Y.A. Dyadin, T. Rodionova, I. Terekhova, Clathrate hydrates of tetrabutylammonium and tetraisoamylammonium halides, J. Struct. Chem. 43 (2002) 990–994. [10] J. Lipkowski, V.Y. Komarov, T.V. Rodionova, Y.A. Dyadin, L.S. Aladko, The structure of tetrabutylammonium bromide hydrate (C4H9)4NBr·21/3H2O, J. Supramol. Chem. 2 (2002) 435–439. [11] V.Y. Komarov, T. Rodionova, I. Terekhova, N. Kuratieva, The cubic superstructure-I of tetrabutylammonium fluoride (C4H9)4NF·29.7H2O clathrate hydrate, J. Incl. Phenom. Macrocycl. Chem. 59 (2007) 11–15. [12] W. Shimada, M. Shiro, H. Kondo, S. Takeya, H. Oyama, T. Ebinuma, H. Narita, Tetra-n-butylammonium bromide-water (1/38), Acta Cryst. C 61 (2005) 065–066. [13] M. Arjmandi, A. Chapoy, B. Tohidi, Equilibrium data of hydrogen, methane, nitrogen, carbon dioxide, and natural gas in semi-clathrate hydrates of tetrabutyl ammonium bromide, J. Chem. Eng. Data 52 (2007) 2153–2158. [14] S. Muromachi, K.A. Udachin, K. Shin, S. Alavi, I.L. Moudrakovski, R. Ohmura, J.A. Ripmeester, Guest-induced symmetry lowering of an ionic clathrate material for carbon capture, Chem. Commun. 50 (2014) 11476–11479. [15] S. Kim, I.-H. Baek, J.-K. You, Y. Seo, Guest gas enclathration in tetra-n-butyl ammonium chloride (TBAC) semiclathrates: potential application to natural gas storage and CO2 capture, Appl. Energy 140 (2015) 107–112. [16] S. Kim, Y. Seo, Semiclathrate-based CO2 capture from flue gas mixtures: an experimental approach with thermodynamic and Raman spectroscopic analyses, Appl. Energy 154 (2015) 987–994. [17] S. Lee, Y. Lee, S. Park, Y. Kim, J.D. Lee, Y. Seo, Thermodynamic and spectroscopic identification of guest gas enclathration in the double tetra-nbutylammonium fluoride semiclathrates, J. Phys. Chem. B 116 (2012) 9075–9081. [18] S. Lee, Y. Lee, S. Park, Y. Seo, Phase equilibria of semiclathrate hydrate for nitrogen in the presence of tetra-n-butylammonium bromide and fluoride, J. Chem. Eng. Data 55 (2010) 5883–5886. [19] A.A. Khokhar, J.S. Gudmundsson, E.D. Sloan, Gas storage in structure H hydrates, Fluid Phase Equilibr. 150–151 (1998) 383–392. [20] H. Dashti, L.Z. Yew, X. Lou, Recent advances in gas hydrate-based CO2 capture, J. Nat. Gas Sci. Eng. 23 (2015) 195–207. [21] A. Eslamimanesh, A.H. Mohammadi, D. Richon, P. Naidoo, D. Ramjugernath, Application of gas hydrate formation in separation processes: a review of experimental studies, J. Chem. Thermodyn. 46 (2012) 62–71.
1165
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
CH4 enclathration in tetra-iso-amyl ammonium bromide (TiAAB) semiclathrate and its significance for natural gas storage ⁎
Soyoung Kima, Ki-Sub Kimb, , Yongwon Seoa, a b
MARK
⁎
School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, Ulsan 44919, Republic of Korea School of Chemical and Materials Engineering, Korea National University of Transportation, Chungbuk 27469, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Natural gas Gas storage Semiclathrate Tetra-iso-amyl ammonium bromide
Tetra-iso-amyl ammonium bromide (TiAAB), one of the quaternary ammonium salts, was investigated as a semiclathrate former for its potential application in natural gas storage. The dissociation equilibrium temperature of pure TiAAB semiclathrate was the highest at a stoichiometric concentration (3.7 mol%) of TiAAB·26.0H2O. The dissociation enthalpy of pure TiAAB (3.7 mol%) semiclathrate was 253.0 ± 0.5 J/g using a differential scanning calorimeter (DSC). The thermodynamic stability of the CH4 + TiAAB semiclathrate was significantly higher than that of the CH4 gas hydrate. The CH4 + TiAAB (3.7 mol%) semiclathrate was thermodynamically stable at temperatures higher than 300 K for the pressure range of 1.5–8.5 MPa. 13C NMR and in situ Raman spectroscopy revealed that CH4 molecules were enclathrated in the small cages of the TiAAB semiclathrate and a CH4 inclusion did not affect the semiclathrate structure. The overall experimental results clearly indicate that the TiAAB semiclathrate can trap CH4 molecules at favorable conditions and thus it has a potential for natural gas storage.
1. Introduction Clathrate hydrates are solid inclusion compounds formed by the hydrogen bonding of water molecules at high pressure and low temperature conditions [1]. The host water molecules form cage frameworks of clathrate hydrates, whereas the small guest molecules are incorporated in the cages. Clathrate hydrates can be generally classified into two types of true clathrates and semiclathrates, although the strict distinction between true clathrates and semiclathrates has recently been blurred [2–4]. In true clathrates, there are no chemical interactions between the host and guest molecules, and gas hydrates belong to the true clathrates. However, in semiclathrates, there are chemical or ionic interactions between the host and guest molecules, and quaternary ammonium salts (QASs) are representative semiclathrate formers [5–11]. In QAS semiclathrates, quaternary ammonium cations are enclathrated into the partly broken large cages of the semiclathrate structure whereas anions such as F−, Cl−, and Br− are incorporated into the host frameworks [7–11]. The notable features of QAS semiclathrates are that they are thermodynamically stable under atmospheric pressure at the temperature ranges far above the freezing point of water and small dodecahedral (512) cages of the semiclathrates are left vacant, which can be used for encaging small-sized gas molecules [12–18].
⁎
Corresponding authors. E-mail addresses: [email protected] (K.-S. Kim), [email protected] (Y. Seo).
http://dx.doi.org/10.1016/j.cej.2017.08.054 Received 7 July 2017; Received in revised form 11 August 2017; Accepted 11 August 2017 Available online 12 August 2017 1385-8947/ © 2017 Elsevier B.V. All rights reserved.
Clathrate hydrates have many technological applications in energy and environmental fields such as CO2 capture and sequestration, gas storage and transportation, and desalination [19–28]. However, one of the main drawbacks of these clathrate-based technologies is that clathrate hydrates require high pressures and low temperatures to form the cage structure and maintain their thermodynamic stability, which results in high energy requirements. In particular, a significant pressure reduction at any given temperature must be achieved for natural gas storage and transportation using clathrate hydrate formation. Concerted efforts have been made to reduce hydrate equilibrium pressure for natural gas by adding various thermodynamic promoters. Tetrahydrofuran (THF) and cyclopentane (CP) are well-known thermodynamic promoters which can significantly lower the pressure required to form clathrate hydrates from natural gas [29–31]. However, both THF and CP are very volatile and toxic, which indicates that there is a possibility of some loss after repeated use and an unwanted mixing of promoters and retrieved natural gas in the vapor phase after dissociation, and there is also concern about their harmful effect on the human body and corrosion problems. However, QASs are non-volatile and nontoxic and form semiclathrates with water molecules under atmospheric pressure. QAS semiclathrates with guest gases offer a higher thermodynamic stability than other gas hydrates with well-known thermodynamic promoters like THF and CP [32].
Chemical Engineering Journal 330 (2017) 1160–1165
S. Kim et al.
of the semiclathrate dissociation curve and the thermal contraction or expansion curve at each pressure condition.
Semiclathrates formed from tetra-n-butyl ammonium (TBA) salts, such as tetra-n-butyl ammonium bromide (TBAB), chloride (TBAC), and fluoride (TBAF), have been extensively examined for CO2 capture and gas storage [13–17,32–38]. However, tetra-iso-amyl ammonium (TiAA) salts like tetra-iso-amyl ammonium bromide (TiAAB) and fluoride (TiAAF) have rarely been studied, even though TiAA salt semiclathrates have been reported to have significantly higher thermodynamic stability than other QAS semiclathrates [9,39,40]. In this study, TiAAB was selected as a semiclathrate former for its potential application in natural gas storage at mild pressure and temperature conditions. The enclathration of CH4 molecules in TiAAB semiclathrates was investigate with a primary focus on thermodynamic and spectroscopic aspects. The dissociation equilibrium temperatures of pure TiAAB semiclathrates were measured using a high pressure microdifferential scanning calorimeter (HP μ-DSC). The phase equilibria of CH4 + TiAAB semiclathrates were measured using a conventional isochoric method. The dissociation enthalpy of the TiAAB semiclathrate was also measured using a HP μ-DSC. In addition, the confirmation of CH4 enclathration in the clathrate lattices was performed via 13C NMR and in situ Raman spectroscopy.
2.3. High-pressure micro-differential scanning calorimeter A high-pressure micro-differential scanning calorimeter (HP μ-DSC VII evo, Setaram Inc., France) was adopted to measure both dissociation enthalpies (ΔHd) and dissociation equilibrium temperatures of the pure TiAAB semiclathrates. The HP μ-DSC device consists of a reference cell and a sample cell, with an operating pressure range of 0–40 MPa and an operating temperature range of 228.15–393.15 K. The HP µ-DSC had a resolution of 0.02 μW with a temperature deviation of ± 0.2 K. For the ΔHd measurements, the DSC dynamic method with continuous heating was used in this study. The sample cell was charged with approximately 6.5 mg of TiAAB solution, whereas the reference cell was left empty. Then, the cells were inserted into the furnace. A multi-cycle mode of cooling and heating was applied to the cells in order to enhance the conversion of the TiAAB solution to the TiAAB semiclathrate because there was no agitation in the liquid phase in the sample cell. The HP μ-DSC cells were initially cooled to 253 K at a cooling rate of 1.0 K/min. Through repeated cycles of cooling and heating, the TiAAB semiclathrate gradually increased and the complete conversion was confirmed with the disappearance of further formation and dissociation peaks for the pure TiAAB semiclathrate from the heat flow curve. Then, the temperature was increased to 320 K with a scanning rate of 0.5 K/min for the dissociation of the TiAAB semiclathrate. The dissociation enthalpies of the TiAAB semiclathrates were obtained through the integration of an endothermic heat flow curve. For the DSC stepwise experiments, after charging of the TiAAB solution into the cells and repeated cycles of cooling and heating, the cells were heated to a temperature which is 1.0 K lower than the expected dissociation equilibrium temperature in 1.0 K increments with 3 h intervals. The approximate dissociation equilibrium temperature of TiAAB semiclathrates can be estimated from the onset temperature of the endothermic heat flow curve obtained from the DSC dynamic experiments. In order to determine an accurate dissociation equilibrium point of TiAAB semiclathrates, the temperature was then increased in 0.1 K increments with a 3 h isothermal duration at each step until the endothermic heat flow due to semiclathrate dissociation completely disappeared.
2. Experimental section 2.1. Materials and apparatus The CH4 with a purity of 99.95% was supplied from Gasvally Co. (Republic of Korea). Tetra-iso-amyl ammonium bromide (TiAAB) was synthesized using Menschutkin’s reaction [41]. Tri-iso-amyl amine (Sigma-Aldrich, ≧95%, USA) and iso-amyl bromine (Sigma-Aldrich, 96%, USA) were refluxed in acetonitrile (Junsei, 99.5%, Japan) under nitrogen (Daesung Industrial Gases Co., Ltd, 99.999%, Republic of Korea) for 72 h. The solvent was evaporated by heating in a vacuum, and the crystals were then dissolved in ethyl acetate (Samchun, 99.5%, Republic of Korea). TiAAB was recrystallized for 24 h at 275 K. The crystals were then filtered from the solution and the process was repeated twice. The crystalline mass was dried in a vacuum oven (M.O.tech, USA) at 313 K. 1H NMR (D2O) spectrum consisted of the following peaks: 0.85 ∼ 0.87 (m, 24H), 1.46–1.49 (t, 8H), 1.53–1.59 (m, 4H), 3.12–3.16 (t, 8H). Deionized doubly distilled water was used for the experiment. The high pressure vessel used to measure the semiclathrate phase equilibria was made from 316 stainless steel with an inner volume of 50 cm3. An impeller-type mechanical stirrer was used to vigorously agitate the solutions inside the cell to accelerate the semiclathrate formation and dissociation. The equilibrium cell was immersed in a circulator (RW-2025G, Jeio Tech, Republic of Korea) to control the temperature. A thermocouple ( ± 0.1 K accuracy) was used to measure the cell temperature (calibrated using an ASTM 63C thermometer, H-B Instrument Company, USA). A pressure transducer (S-10, WIKA, Germany) was used to measure the cell pressure (calibrated using a Heise Bourdon tube pressure gauge, CMM-140830, 0–20.0 MPa, Ashcroft, Inc. USA). The uncertainties associated with the temperature and pressure measurements were 0.1 K and 0.02 MPa, respectively.
2.4.
13
C NMR and in situ Raman spectroscopic analyses
A Bruker 400 MHz solid-state FT-NMR spectrometer, which belongs to the Korea Basic Science Institute (KBSI), was used to directly confirm the enclathration of CH4 in the semiclathrate phase. For NMR analysis, semiclathrate samples collected from the high pressure cell were finely powdered in a liquid nitrogen vessel and were then placed in a 4 mm o.d. Zr rotor that was inserted in a variable-temperature probe at 243 K. 13 C NMR spectra were collected at a Larmor frequency of 100.6 MHz with magic angle spinning (MAS) between 2 and 4 kHz. The pulse length of 2 μs and pulse repetition delay of 10 s under proton decoupling were employed when a radio frequency field strength of 50 kHz corresponding to 5 μs 90° pulses was used. The formation process of the pure TiAAB and CH4 + TiAAB semiclathrates was monitored using an in situ fiber-coupled Raman spectrometer (SP550, Horiba, France) with a multichannel air cooled CCD detector and 1800 grooves/mm grating. A fiber-optic Raman probe that was installed in the high pressure reactor provided time-dependent Raman spectra of the pure TiAAB and CH4 + TiAAB semiclathrates. The reactor for the Raman measurement was enclosed in a water jacket whose temperature was controlled by an external circulator (RW2025G, Jeio Tech, Republic of Korea). A more detailed description of the experimental methods and procedure were provided in our previous papers [15,16,32,42].
2.2. Semiclathrate phase equilibrium measurements An isochoric method with step heating and cooling was used to determine the semiclathrate phase equilibria of the CH4 + TiAAB + water system. The cell was charged with 25 cm3 of TiAAB aqueous solution and then pressurized with the CH4 gas to the desired pressure. The temperature was lowered at 1.0 K/h to induce semiclathrate formation below the expected equilibrium temperature. After a significant pressure decrease was observed, which indicated the formation of a semiclathrate with CH4, the cell was slowly heated at 0.1 K/90 min for the dissociation. The three-phase (semiclathrate (H)–liquid water (LW)–vapor (V)) equilibrium point was determined by the intersection 1161
Chemical Engineering Journal 330 (2017) 1160–1165
S. Kim et al.
3. Results and discussion 3.1. Thermodynamic stability of TiAAB semiclathrates In this study, a HP μ-DSC was used to determine accurate equilibrium dissociation temperatures of pure TiAAB semiclathrates formed at three different concentrations (1.0, 3.7, and 5.0 mol% TiAAB). The HP μ-DSC enabled us to measure dissociation enthalpies of semiclathrates as well as to determine equilibrium dissociation temperatures [43]. The dynamic method, which uses a continuous increase of temperature, is less time consuming and useful for measuring both the dissociation enthalpy and the equilibrium dissociation temperature at the same time. However, the dynamic method can sometimes generate inherent uncertainty in determining an accurate equilibrium dissociation temperature from the endothermic heat flow curves when broad and progressive peaks appear during heating [43,44]. In order to overcome this inaccuracy of the equilibrium point determination using the dynamic method, the stepwise method, which uses stepwise temperature increases during heating, was used in this study. The temperature increment and isothermal duration at each step were 0.1 K and 3 h, respectively. The heat flow changes and temperature profiles during stepwise heating for the pure TiAAB (3.7 mol%) semiclathrate under atmospheric pressure was depicted in Fig. 1. The figure shows that the isothermal duration at each temperature step is long enough for the heat flow signal to return to the baseline. For each temperature step, a large endothermic peak caused by dissociation of the TiAAB semiclathrate was observed until the semiclathrate dissociation was completely terminated. The point where the large endothermic peaks due to the semiclathrate dissociation disappeared and accordingly, the calorimetric signal change caused only by the temperature increment appeared was determined to be the equilibrium dissociation temperature of the TiAAB semiclathrate at a specified TiAAB concentration. Through the DSC stepwise method, the dissociation equilibrium temperatures of pure TiAAB semiclathrates were 302.8, 303.6, and 303.4 K for 1.0, 3.7, and 5.0 mol% TiAAB solutions, respectively, under atmospheric pressure. The equilibrium dissociation temperatures of pure TiAAB semiclathrates measured by the DSC stepwise method along with those by the DSC dynamic method under atmospheric pressure are presented in Fig. 2. The results obtained in this study were in good agreement with the value reported in the literature (3.7 mol% TiAAB solution) [9]. The
Fig. 2. Equilibrium dissociation temperatures of pure TiAAB semiclathrates.
slight discrepancy in the equilibrium dissociation temperatures between the DSC stepwise method and the DSC dynamics method can be attributed to the uncertainty involved in determining the onset temperature from the endothermic heat flow curve of the DSC dynamic method. It is generally accepted that the DSC stepwise method offers more accurate equilibrium dissociation temperatures for gas hydrate or semiclathrate systems than the DSC dynamic method. The experimental results clearly indicate that the DSC can give the reliable and accurate equilibrium dissociation temperatures of pure TiAAB semiclathrates. In addition, the maximum dissociation equilibrium temperature was observed at the stoichiometric concentration (3.7 mol%) of TiAAB·26.0 H2O. In order to measure the dissociation enthalpy of pure TiAAB (3.7 mol%) semiclathrate, the dynamic method with continuous heating was used and the resulting DSC thermogram is depicted in Fig. 3. The TiAAB (3.7 mol%) semiclathrate exhibited only one endothermic peak without an ice melting peak during the semiclathrate dissociation, which indicates that all water molecules were involved in TiAAB semiclathrate formation at the stoichiometric concentration of 3.7 mol% (TiAAB·26.0H2O). The dissociation enthalpy of TiAAB (3.7 mol%) semiclathrate was found to be 253.0 ± 0.5 J/g. Three-phase (semiclathrate (H)–liquid water (Lw)–vapor (V)) equilibria of the CH4 + TiAAB (3.7 mol%) + water system are depicted in Fig. 4 along with those of the CH4 + water system [45], and the overall experimental results are summarized in Table 1. The phase equilibrium results clearly demonstrated that the thermodynamic stability of the CH4 + TiAAB semiclathrate was significantly higher than that of the pure CH4 hydrate. For example, the dissociation equilibrium temperature of the CH4+TiAAB (3.7 mol%) semiclathrate is approximately 37 K higher than that of the pure CH4 hydrate at 3.0 MPa, which indicates that the inclusion of TiAAB and subsequent semiclathrate formation resulted in significant thermodynamic stabilization of the system. It is speculated from Figs. 2 and 4 that even above the room temperature CH4 molecules can be enclathrated into the cages of TiAAB semiclathrates at around atmospheric pressure even though the experimental measurements of the semiclathrate phase equilibria were conducted over a wide pressure range (1.5–8.5 MPa). As seen from Fig. 4, the pure CH4+water system requires equilibrium pressure as high as 2.7 MPa even at 273 K for gas hydrate formation, which could be a serious limitation to application of gas hydrate-based CH4 storage
Fig. 1. Heat flow changes during stepwise heating for the TiAAB (3.7 mol%) semiclathrate under atmospheric pressure.
1162
Chemical Engineering Journal 330 (2017) 1160–1165
S. Kim et al.
Fig. 3. Dissociation thermogram of the TiAAB (3.7 mol%) semiclathrate under atmospheric pressure.
Fig. 5. Time-dependent in situ Raman spectra of semiclathrates formed from (a) TiAAB (3.7 mol%) + water system under atmospheric pressure and ΔT = 5.0 K and (b) CH4 + TiAAB (3.7 mol%) + water system at 4.0 MPa and ΔT = 5.0 K.
Fig. 4. Semiclathrate phase equilibria of the CH4 + TiAAB (3.7 mol%) + water system.
to actual processes. However, the equilibrium pressure required for semiclathrate formation from the CH4+TiAAB (3.7 mol%) + water system was significantly lower at a specified temperature, demonstrating a great potential of TiAAB semiclathrates for a new gas storage medium.
Table 1 Semiclathrate phase equilibrium data of the CH4 + TiAAB (3.7 mol%) + water system. CH4 + TiAAB (3.7 mol%) semiclathrate
3.2. In situ Raman and
T/K
P/MPa
309.1 311.8 313.3 314.2 314.7
1.56 3.29 5.01 7.01 8.49
13
C NMR analyses
Fig. 5(a) shows the time-dependent in situ Raman spectra collected during the conversion of TiAAB (3.7 mol%) solution to pure TiAAB semiclathrate under atmospheric pressure in a wavenumber range of 2860–3000 cm−1. The intensities of Raman peaks gradually increased as the pure TiAAB semiclathrate formation proceeded. The gradual growth of each Raman peak indicates the gradual formation of pure 1163
Chemical Engineering Journal 330 (2017) 1160–1165
S. Kim et al.
Fig. 6. Raman spectra of CH4 hydrate, pure TiAAB (3.7 mol%) semiclathrate, and CH4 + TiAAB (3.7 mol%) semiclathrate.
Fig. 7. 13C MAS NMR spectra of the pure CH4 hydrate and the CH4 + TiAAB (3.7 mol%) semiclathrate.
TiAAB semiclathrate. Fig. 5(b) shows the time-dependent in situ Raman spectra of the CH4+TiAAB (3.7 mol%) semiclathrate at 4.0 MPa and ΔT = 5.0 K. As in the pure TiAAB semiclathrate, all the Raman peaks from the CH4 + TiAAB (3.7 mol%) semiclathrate grew gradually over time. The positions of the Raman peaks from the CH4 + TiAAB semiclathrate were the same as those from the pure TiAAB semiclathrate except for one Raman peak that appeared at 2914 cm−1. The Raman peak at 2914 cm−1 corresponds to CH4 molecules captured in the small cages of the TiAAB semiclathrate. The time-dependent in situ Raman spectra clearly demonstrated that CH4 molecules were enclathrated in the lattices of the TiAAB semiclathrate and the Raman peak from enclathrated CH4 molecules grew and changed along with the peaks from TiAA cations enclathrated in the semiclathrate. Fig. 6 shows the Raman spectra for the CH4 hydrate, pure TiAAB (3.7 mol%) semiclathrate, and CH4+TiAAB (3.7 mol%) semiclathrate, which were obtained after gas hydrate or semiclathrate formation was completed. The Raman spectrum of the CH4+TiAAB (3.7 mol%) semiclathrate was distinguishable from that of the pure TiAAB (3.7 mol %) due to an additional peak appearing at 2914 cm−1. However, the positions and shapes of Raman peaks from the CH4+TiAAB semiclathrate were the same as those from the pure TiAAB semiclathrate except for the peak at 2914 cm−1 that corresponds to enclathrated CH4 molecules. This indicated that the inclusion of CH4 molecules in the TiAAB semiclathrate did not affect the structure of the semiclathrate. Raman spectra depicted in Figs. 5 and 6 clearly demonstrated that CH4 molecules were captured in the cages of semiclathrate without altering the structure of the TiAAB semiclathrate. The cage-dependent resonance peaks of 13C NMR spectroscopy for CH4 molecules enclathrated in different cages of gas hydrates can be effectively used to confirm the hydrate structure and to determine the types of hydrate cages that are occupied by guest molecules [46]. The 13 C MAS NMR spectra of the pure CH4 hydrate and the CH4 + TiAAB (3.7 mol%) semiclathrate are shown in Fig. 7. The pure CH4 hydrate (sI) exhibited two resonance peaks at −4.3 and −6.6 ppm that can be
assigned to CH4 molecules enclathrated in the small (512) and large (51262) cages of sI hydrate, respectively. The CH4+TiAAB (3.7 mol%) semiclathrate showed several resonance peaks in a wide range of chemical shift that are attributable to enclathrated TiAA cations as well as one peak at −4.3 ppm that is attributable to captured CH4 molecules. As indicated by previous researchers, semiclathrates formed from tetraiso-amyl ammonium or tetra-n-butyl ammonium salts possess vacant dodecahedral (512) cages that have a potential for capturing small-sized gas molecules [40,47]. Therefore, Fig. 7 clearly demonstrated that CH4 molecules were captured in the small (512) cages of TiAAB semiclathrate. This study clearly confirmed the enclathration of CH4 in the TiAAB semiclathrate by thermodynamic and spectroscopic approaches. However, more studies on kinetics, gas uptakes, and cage occupancy of the CH4+TiAAB semiclathrates should be conducted in the near future to assess the feasibility of actual application of TiAAB semiclathrate in natural gas storage and transportation. 4. Conclusions The enclathration of CH4 molecules in TiAAB semiclathrates was investigated primarily focusing on thermodynamic and spectroscopic aspects. Pure TiAAB semiclathrate was the most thermodynamically stable at a stoichiometric concentration (3.7 mol%) of TiAAB·26.0H2O. The dissociation enthalpy of TiAAB·26.0H2O was measured to be 253.0 ± 0.5 J/g using a HP μ-DSC. The CH4 + TiAAB (3.7 mol%) semiclathrate demonstrated significantly higher thermodynamic stability than CH4 gas hydrate and maintained its thermodynamic stability even at temperatures higher than 300 K for a pressure range of 1.5–8.5 MPa. The enclathration of CH4 molecules in the TiAAB semiclathrate lattices was clearly confirmed by 13C NMR and Raman spectroscopy. The overall experimental results provide a better understanding of the thermodynamic stability, cage-specific guest 1164
Chemical Engineering Journal 330 (2017) 1160–1165
S. Kim et al.
[22] P. Babu, P. Linga, R. Kumar, P. Englezos, A review of the hydrate based gas separation (HBGS) process for carbon dioxide pre-combustion capture, Energy 85 (2015) 261–279. [23] H. Lee, Y. Seo, Y.T. Seo, I.L. Moudrakovski, J.A. Ripmeester, Recovering methane from solid methane hydrate with carbon dioxide, Angew. Chem. Int. Ed. 42 (2003) 5048–5051. [24] H. Tajima, A. Yamasaki, F. Kiyono, Process design of a new injection method of liquid CO2 at the intermediate depths in the ocean using a static mixer, Fuel Process. Technol. 86 (2005) 1667–1678. [25] Y. Lee, S. Lee, J. Lee, Y. Seo, Structure identification and dissociation enthalpy measurements of the CO2 + N2 hydrates for their application to CO2 capture and storage, Chem. Eng. J. 246 (2014) 20–26. [26] D.-Y. Koh, H. Kang, J.-W. Lee, Y. Park, S.-J. Kim, J. Lee, J.Y. Lee, H. Lee, Energyefficient natural gas hydrate production using gas exchange, Appl. Energy 162 (2016) 114–130. [27] K.C. Kang, P. Linga, K.-N. Park, S.-J. Choi, J.D. Lee, Seawater desalination by gas hydrate process and removal characteristics of dissolved ions (Na+, K+, Mg2+, Ca2+, B3+, Cl–, SO42−), Desalination 353 (2014) 84–90. [28] J.-H. Cha, Y. Seol, Increasing gas hydrate formation temperature for desalination of high salinity produced water with secondary guests, ACS Sustain. Chem. Eng. 1 (2013) 1218–1224. [29] S. Hashimoto, S. Murayama, T. Sugahara, K. Ohgaki, Phase equilibria for H2 + CO2 + tetrahydrofuran + water mixtures containing gas hydrates, J. Chem. Eng. Data 51 (2006) 1884–1886. [30] J. Zhang, J.W. Lee, Equilibrium of hydrogen + cyclopentane and carbon dioxide + cyclopentane binary hydrates, J. Chem. Eng. Data 54 (2008) 659–661. [31] Y.T. Seo, S.P. Kang, H. Lee, Experimental determination and thermodynamic modeling of methane and nitrogen hydrates in the presence of THF, propylene oxide, 1,4-dioxane and acetone, Fluid Phase Equilibr. 189 (2001) 99–110. [32] S. Kim, S.D. Choi, Y. Seo, CO2 capture from flue gas using clathrate formation in the presence of thermodynamic promoters, Energy 118 (2017) 950–956. [33] K. Sato, H. Tokutomi, R. Ohmura, Phase equilibrium of ionic semiclathrate hydrates formed with tetrabutylammonium bromide and tetrabutylammonium chloride, Fluid Phase Equilibr. 337 (2013) 115–118. [34] X.-S. Li, C.-G. Xu, Z.-Y. Chen, H.-J. Wu, Tetra-n-butyl ammonium bromide semiclathrate hydrate process for post-combustion capture of carbon dioxide in the presence of dodecyl trimethyl ammonium chloride, Energy 35 (2010) 3902–3908. [35] A.H. Mohammadi, A. Eslamimanesh, V. Belandria, D. Richon, P. Naidoo, D. Ramjugernath, Phase equilibrium measurements for semi-clathrate hydrates of the (CO2 + N2 + tetra-n-butylammonium bromide) aqueous solution system, J. Chem. Thermodyn. 46 (2012) 57–61. [36] A. Eslamimanesh, A.H. Mohammadi, D. Richon, Thermodynamic modeling of phase equilibria of semi-clathrate hydrates of CO2, CH4, or N2 + tetra-nbutylammonium bromide aqueous solution, Chem. Eng. Sci. 81 (2012) 319–328. [37] D. Zhong, Y. Ye, C. Yang, Y. Bian, K. Ding, Experimental investigation of methane separation from low-concentration coal mine gas (CH4/N2/O2) by tetra-n-butyl ammonium bromide semiclathrate hydrate crystallization, Indus. Eng. Chem. Res. 51 (2012) 14806–14813. [38] H. Hashimoto, T. Yamaguchi, T. Kinoshita, S. Muromachi, Gas separation of flue gas by tetra-n-butylammonium bromide hydrates under moderate pressure conditions, Energy 129 (2017) 292–298. [39] A. Manakov, T. Rodionova, I. Terekhova, V. Komarov, A. Burdin, A. Sizikov, Structural and physico-chemical studies of ionic clathrate hydrates of tetrabutyland tetraisoamylammonium salts, in: Proceedings of the 7th International Conference on Gas Hydrates, Edinburgh, Scotland, UK, 2011. [40] A. Majumdar, B. Maini, P.R. Bishnoi, M.A. Clarke, Three-phase equilibrium conditions of TiAAB semiclathrates formed from N2, CO2, and their mixtures, J. Chem. Eng. Data 57 (2012) 2322–2327. [41] N. Menschutkin, Beiträgen zur Kenntnis der Affinitätskoeffizienten der Alkylhaloide und der organischen Amine, Z. Phys. Chem. 5 (1890) 589–600. [42] S. Kim, I.-H. Baek, J.-K. You, Y. Seo, Phase equilibria, dissociation enthalpies, and Raman spectroscopic analyses of N2 + tetra-n-butyl ammonium chloride (TBAC), Fluid Phase Equilbr. 413 (2016) 86–91. [43] W. Lin, D. Dalmazzone, W. Fürst, A. Delahaye, L. Fournaison, P. Clain, Accurate DSC measurement of the phase transition temperature in the TBPB-water system, J. Chem. Thermodyn. 61 (2013) 132–137. [44] D. Lee, Y. Lee, W. Choi, S. Lee, Y. Seo, Accurate measurement of phase equilibria and dissociation enthalpies of HFC-134a hydrates in the presence of NaCl for potential application in desalination, Korean J. Chem. Eng. 33 (2016) 1245–1430. [45] S. Adisasmito, R.J. Frank III, E.D. Sloan Jr, Hydrates of carbon dioxide and methane mixtures, J. Chem. Eng. Data 36 (1991) 68–71. [46] J.A. Ripmeester, C.I. Ratcliffe, On the contributions of NMR spectroscopy to clathrate science, J. Struct. Chem. 40 (1999) 654–662. [47] W. Wang, B.O. Carter, C.L. Bray, A. Steiner, J. Bacsa, J.T.A. Jones, C. Cropper, Y.Z. Khimyak, D.J. Adams, A.I. Cooper, Reversible methane storage in a polymersupported semi-clathrate hydrate at ambient temperature and pressure, Chem. Mater. 21 (2009) 3810–3815.
distribution, and gas capturing ability of the TiAAB semiclathrates for their potential applications to natural gas storage and transportation. Acknowledgements This research was supported by the Mid-career Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2017R1A2B4005455) and also by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Education (2016R1D1A1A02937037). This work was also supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (NRF-2015R1A5A7037825). References [1] E.D. Sloan, C.A. Koh, Clathrate Hydrates of Natural Gases, third ed., Taylor & Francis, Boca Raton, 2007. [2] S. Alavi, R. Susilo, J. Ripmeester, Linking microscopic guest properties to macroscopic observables in clathrate hydrates: guest-host hydrogen bonding, J. Chem. Phys. 130 (2009) 174501. [3] S. Alavi, S. Takeya, R. Ohmura, T.K. Woo, J.A. Ripmeester, Hydrogen-bonding alcohol-water interactions in binary ethanol, 1-propanol, and 2-propanol + methane structure II clathrate hydrates, J. Chem. Phys. 133 (2010) 074505. [4] W. Shin, S. Park, J.-W. Lee, Y. Seo, D.-Y. Koh, J. Seol, H. Lee, Structural transition from semi- to true clathrate hydrates induced by CH4 enclathration, J. Phys. Chem. C 116 (2012) 16352–16357. [5] D. Fowler, W. Loebenstein, D. Pall, C.A. Kraus, Some unusual hydrates of quaternary ammonium salts, J. Am. Chem. Soc. 62 (1940) 1140–1142. [6] R. McMullan, G.A. Jeffrey, Hydrates of the tetra n-butyl and tetra i-amyl quaternary ammonium salts, J. Chem. Phys. 31 (1959) 1231–1234. [7] A. Dyadin Yu, I.S. Terekhova, T.M. Polyanskaya, L.S. Aladko, Clathrate hydrates of tetrabutylammonium fluoride and oxalate, J. Struct. Chem. 17 (1977) 566–571. [8] T. Rodionova, V. Komarov, G. Villevald, L. Aladko, T. Karpova, A. Manakov, Calorimetric and structural studies of tetrabutylammonium chloride ionic clathrate hydrates, J. Phys. Chem. B 114 (2010) 11838–11846. [9] L. Aladko, Y.A. Dyadin, T. Rodionova, I. Terekhova, Clathrate hydrates of tetrabutylammonium and tetraisoamylammonium halides, J. Struct. Chem. 43 (2002) 990–994. [10] J. Lipkowski, V.Y. Komarov, T.V. Rodionova, Y.A. Dyadin, L.S. Aladko, The structure of tetrabutylammonium bromide hydrate (C4H9)4NBr·21/3H2O, J. Supramol. Chem. 2 (2002) 435–439. [11] V.Y. Komarov, T. Rodionova, I. Terekhova, N. Kuratieva, The cubic superstructure-I of tetrabutylammonium fluoride (C4H9)4NF·29.7H2O clathrate hydrate, J. Incl. Phenom. Macrocycl. Chem. 59 (2007) 11–15. [12] W. Shimada, M. Shiro, H. Kondo, S. Takeya, H. Oyama, T. Ebinuma, H. Narita, Tetra-n-butylammonium bromide-water (1/38), Acta Cryst. C 61 (2005) 065–066. [13] M. Arjmandi, A. Chapoy, B. Tohidi, Equilibrium data of hydrogen, methane, nitrogen, carbon dioxide, and natural gas in semi-clathrate hydrates of tetrabutyl ammonium bromide, J. Chem. Eng. Data 52 (2007) 2153–2158. [14] S. Muromachi, K.A. Udachin, K. Shin, S. Alavi, I.L. Moudrakovski, R. Ohmura, J.A. Ripmeester, Guest-induced symmetry lowering of an ionic clathrate material for carbon capture, Chem. Commun. 50 (2014) 11476–11479. [15] S. Kim, I.-H. Baek, J.-K. You, Y. Seo, Guest gas enclathration in tetra-n-butyl ammonium chloride (TBAC) semiclathrates: potential application to natural gas storage and CO2 capture, Appl. Energy 140 (2015) 107–112. [16] S. Kim, Y. Seo, Semiclathrate-based CO2 capture from flue gas mixtures: an experimental approach with thermodynamic and Raman spectroscopic analyses, Appl. Energy 154 (2015) 987–994. [17] S. Lee, Y. Lee, S. Park, Y. Kim, J.D. Lee, Y. Seo, Thermodynamic and spectroscopic identification of guest gas enclathration in the double tetra-nbutylammonium fluoride semiclathrates, J. Phys. Chem. B 116 (2012) 9075–9081. [18] S. Lee, Y. Lee, S. Park, Y. Seo, Phase equilibria of semiclathrate hydrate for nitrogen in the presence of tetra-n-butylammonium bromide and fluoride, J. Chem. Eng. Data 55 (2010) 5883–5886. [19] A.A. Khokhar, J.S. Gudmundsson, E.D. Sloan, Gas storage in structure H hydrates, Fluid Phase Equilibr. 150–151 (1998) 383–392. [20] H. Dashti, L.Z. Yew, X. Lou, Recent advances in gas hydrate-based CO2 capture, J. Nat. Gas Sci. Eng. 23 (2015) 195–207. [21] A. Eslamimanesh, A.H. Mohammadi, D. Richon, P. Naidoo, D. Ramjugernath, Application of gas hydrate formation in separation processes: a review of experimental studies, J. Chem. Thermodyn. 46 (2012) 62–71.
1165