Structural Transition Induced by CH4 Enclathration and Cage

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Structural Transition Induced by CH4 Enclathration and Cage Expansion with Large Guest Molecules Occurring in Amine Hydrate Systems Yeobum Youn, Jiwoong Seol,†,* Minjun Cha,‡ Yun-Ho Ahn, and Huen Lee* Department of Chemical and Biomolecular Engineering (BK21 Program) and Graduate School of EEWS, KAIST, 335 Gwahangno, Yuseong-gu, Daejeon 305-701, Republic of Korea ABSTRACT: Two isomers of C2H7N, dimethylamine (DMA) and ethylamine (EA), are known to be clathrate hydrate formers by themselves. Here we introduced methane gas as a secondary guest into both dimethylamine and ethylamine clathrate hydrates and identified their structural transitions using powder X-ray diffraction (PXRD) and solid-state NMR spectroscopy. We observed the structural transitions of amine clathrate hydrates from expanded structure I (cubic Pm3n) to structure II (cubic Fd3m). In addition, from experimental results obtained through neutron powder diffraction (NPD) and PXRD, we found that neither temperature nor pressure affected the hydrate structural transition. Raman spectroscopy was used to identify the structural transition occurring in these amine clathrate hydrate systems. In addition, we measured the hydrate equilibrium conditions for amine−water−methane hydrates. The DMA and EA act as hydrate inhibitors in DMA/EA + H2O + CH4 hydrate systems compared with pure methane hydrate over our experimental pressure and temperature ranges.

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INTRODUCTION Generally, the structures of simple clathrate hydrates are mainly classified as structure I (sI), structure II (sII), or structure H (sH).1,2 Recently, more complex structural and physicochemical properties of ionic clathrate hydrates have been discovered. Because of the ionic interaction between host and guest molecules, ionic clathrate hydrates show many unique features such as cohost inclusion, improved thermal stability, and high ionic conductivity.3−7 Exploiting these properties, we have suggested a new concept for hydrate applications and have manufactured a hydrogen gas sensor based on a clathrate hydrate detailed in our previous work.8 More recently, Shin et al.9 and Seol et al.10 revealed that cobalt complexes and sodium cations can be stabilized in isolated hydrate cages; they reported improved conductivity due to metal enclathration. However, there are still many unknown interactions and properties of simple clathrate hydrates. Many types of nonpolar and hydrophobic large guest molecules have been the subject of considerable study by previous researchers. Nowadays, however, it is important to note that guest molecules containing polar and hydrophilic functional groups have also been reported because of their unique properties. These guest molecules have stronger affinities for water than those of other hydrocarbons. For example, some guests can be incorporated in a water host framework because of the strong interactions between their O− H groups and the water molecules. Cha et al.11 studied the crystal structure of hydrates with isomers of C5H11OH and © 2014 American Chemical Society

CH4; they suggested that some amyl alcohols with sizes larger than sII-L cages can be used to construct sII hydrates that incorporate a hydroxyl group into the host framework. Shin et al.12 first suggested that several fully water-soluble guest molecules, such as hexamethylenimine and isomers of methylpiperidine having N−H groups, can form sH hydrates. Shin et al.2 and Lee et al.13 also investigated a particular type of hydrate, the so-called “semiclathrate hydrate”, which is formed with alkylamine guests and can be transformed into a “true clathrate hydrate” in the presence of external CH4 gas. With guest molecules containing polar or hydrophilic functional groups, we are convinced that there is a lot of potential to develop unique properties or structure patterns of clathrate hydrates. These unique properties will be induced by the strong interactions of guest molecules with water molecules; of course, this will be possible even though that strong interaction is much weaker than the normal ion−water interaction. From these viewpoints, here we report the phenomena of clathrate hydrate formation and structural transitions of dimethylamine (DMA) and ethylamine (EA) with methane as a secondary guest. We also used microscopic analysis to investigate the structural transitions and measured the threephase equilibrium conditions in order to reveal the effects of dimethylamine and ethylamine. While these two alkylamines Received: February 18, 2014 Accepted: April 28, 2014 Published: May 19, 2014 2004

dx.doi.org/10.1021/je500167n | J. Chem. Eng. Data 2014, 59, 2004−2012

Journal of Chemical & Engineering Data

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To measure the neutron powder diffraction (NPD) data, we made an EA solution with deuterated water (D2O) to prevent incoherent scattering from hydrogen atoms. Deuterated methane (CD4) was used to pressurize the EA hydrate. After hydrate formation, the hydrate sample was put into an NPD sample holder (thin vanadium cylinder with a volume of 3 mL) and immersed in liquid nitrogen. The NPD experiments were performed on a high-resolution powder diffractometer (λ = 0.1834333 nm) installed at the horizontal channel ST2 of the 30 MW “HANARO” reactor of the Korea Atomic Energy Research Institute (KAERI). Low-temperature NPD measurements were obtained using a closed-cycle refrigerator with a cooling power of 0.5 W and a Si diode sensor installed on the neutron beam. We set a step size of 0.05° for 2θ = 0° to 160°. Quantum-chemistry calculations with the Gaussian 03W program14 were performed to determine the sizes of the molecules. We used density functional theory with the B3LYP functional and the 6-31+G basis set. All of the molecule sizes in this paper were calculated as the sum of the longest distance from the center of one end atom to the center of the next end atom as determined using Gaussian 03W and the van der Waals diameter of hydrogen (0.24 nm).

were found individually to make clathrate structures with water, they transformed from the sI phase to the sII phase in the presence of external gaseous CH4. We also first identified the “free NH bond” signals with Raman spectroscopy, which were detected in all of the samples regardless of the alkylamine mole fraction and structure. This confirmed the formation of “true” (or “genuine”) clathrate hydrates without hydrogen-bonding interactions between the amine guests and the water hosts. Furthermore, these two amine molecules were found to act as inhibitors of CH4 hydrate in our experimental pressure and temperature range.

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EXPERIMENTAL PROCEDURES DMA (0.40 mass fraction in H2O, Sigma-Aldrich), EA (0.70 mass fraction in H2O, JUNSEI), and 13CH4 (0.99 mole fraction, Cambridge Isotope Laboratories) were purchased and used without further treatment. High-purity CH4 (minimum 0.9995 mole fraction) was supplied by the Special Gas Corporation (Republic of Korea). Ultrahigh-purity distilled water was obtained from a Millipore purification unit. First, we mixed the DMA or EA solution with additional water to make DMA/EA solutions with various amine mole fractions (x = 0.010, 0.020, 0.0286, 0.0556, 0.070, and 0.1154). Next, 10 g of each solution was frozen at 203 K and ground with a 200 μm sieve in liquid nitrogen. The powder was packed in a high-pressure resistance cell (inner volume of 20 mL) and pressurized with CH4 gas up to the desired pressure. 13CH4 was mixed into the input gas at a mole fraction of about 0.1 with respect to the total injected CH4 to obtain clear 13C NMR signals. The reactor was placed in a constant-temperature bath (243 K) for 3 days. During this period, methane gas was continuously recharged to maintain the initial pressure. The sample was finally reground in liquid nitrogen before the spectroscopic measurements. A Varian (UnityNOVA600) 600 MHz solid-state NMR spectrometer was used. The powder samples were put into a zirconia rotor with a 3.2 mm o.d., and the rotor was loaded into a low-temperature probe. 13C NMR spectra were recorded at a Larmor frequency of 100.6 MHz with a magic-angle spinning (MAS) speed of about 10 kHz at 183 K. A pulse length of 2 μs and a pulse repetition delay of 10 s under proton decoupling were used with a radiofrequency field strength of 50 kHz, corresponding to a 5 μs 90° pulse. The downfield carbon resonance peak of hexamethylbenzene, assigned a chemical shift of 17.3 ppm at 298 K, was used as an external chemical shift reference. For Raman analysis, we used a HORIBA Jobin Yvon LabRAM HR UV/vis/NIR high-resolution dispersive Raman microscope equipped with a CCD detector. The source of the beam was an Ar laser with a wavelength of 514.53 nm and an intensity of 30 mW. All of the Raman spectra were measured at 93 K and atmospheric pressure. We obtained the powder X-ray diffraction (PXRD) patterns using a Rigaku Geigerflex diffractometer (D/Max-RB) at 93 K. Graphite-monochromatized Cu Kα radiation (λ = 0.15406 nm) in the θ/2θ mode was used. The step mode was a fixed time of 3 s and a step size of 0.02° for 2θ = 5° to 55°. To measure the phase equilibria of the samples of various mole fractions, a pressure−temperature tracer was used. The cooling rate was 1 K·h−1, and the heating rate was 0.1 K·h−1. A four-wire-type PT-100Ω device (with a full-scale accuracy of ± 0.05 %) and a PMP4070 device from Druck Inc. (with a full scale accuracy of ± 0.04 %) were used to measure the temperature and pressure, respectively.

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RESULTS AND DISCUSSION PXRD Pattern Analysis. The molecular size of hydrate formers has been revealed to be one of the key factors determining the hydrate structure. Generally, it is known that molecules with diameters between 0.45 nm and 0.60 nm form sI hydrates. On the other hand, sII and sH hydrates are formed by the enclathration of larger molecules with diameters over 0.60 nm and 0.70 nm, respectively.1,15 In this respect, one might expect that a hydrate structure formed with an arbitrary guest molecule could be predicted simply on the basis of the size of the guest. However, several exceptions have also been reported. Cha et al.11 suggested that 3-methyl-1-butanol can occupy the large cage of the 3-methyl-1-butanol + CH4 hydrate (sII) even though the molecular size of 3-methyl-1-butanol (0.904 nm) is larger than 0.70 nm. Using Raman and NMR spectroscopy, Cha et al.11 revealed that both the hydroxyl group of the guest and the host water molecules can contribute to the hydrate framework. As can be seen in Figure 1, the molecular sizes of the two alkylamines, DMA and EA, are about 0.657 nm and 0.629 nm,

Figure 1. Molecular shapes and sizes of (a) dimethylamine and (b) ethylamine optimized and calculated using Gaussian 03W. Gray, black, and white balls represent carbon, nitrogen, and hydrogen, respectively.

respectively. These values are somewhat smaller than that of propane (0.673 nm), which is one of the most representative sII hydrate formers. In this respect, several previous studies revealed that these two alkylamines construct sI hydrates.16 However, the structures of the binary clathrate hydrates formed in the presence of external CH4 gas have not been identified. 2005



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First, we rechecked the pure hydrates of the two alkylamines. The experimental PXRD patterns of DMA·7.7 H2O [mole fraction (x) = 0.1154] and EA·7.7 H2O (x = 0.1154) are shown in Figure 2, where they are compared with that of pure

Figure 3. PXRD patterns of pure CH4 hydrate (sI, a = 1.195 nm), DMA (x = 0.1154) + CH4 hydrate (sI, a = 1.215 nm), DMA (x = 0.070) + CH4 hydrate (sI, a = 1.22 nm; sII, a = 1.73 nm), and DMA (x = 0.0286) + CH4 hydrate (sII, a = 1.73 nm). DMA (x = 0.070) + CH4 hydrate consists of sII and expanded sI phases. Gray dashed lines represent the peaks of hexagonal ice (Ih). The low-angle-shifted main peaks of the sI phase are assigned with the arrows. Instead, DMA (x = 0.0286) + CH4 hydrate was revealed to be sII with a small amount of sI phase. The calculated powder diffraction peaks are indicated below each pattern (|, sI hydrate; ▲, sII hydrate).

Figure 2. PXRD patterns of CH4 + H2O, EA (x = 0.1154) + H2O, and DMA (x = 0.1154) + H2O hydrates measured at 93 K. Dashed lines represent the peaks of hexagonal ice (Ih). The low-angle-shifted main peaks of the sI phase induced by the lattice expansion are assigned with the arrows. The calculated powder diffraction peaks are indicated below each pattern.

0.1154) + CH4, EA·17H2O (x = 0.0556) + CH4, and EA· 34H2O (x = 0.0286) + CH4 are shown in Figure 4. These

methane hydrate (hereafter, MH) (sI hydrate, lattice parameter a = 1.195 nm). The arrows show the shifts of the peaks with expansion of the sI hydrate. The lattice parameter was calculated by matching with Diamond 3.2.17 The pure DMA and EA hydrates were sI hydrates with lattice parameters of 1.217 nm and 1.250 nm, respectively, which are slightly larger than that of a general sI hydrate such as pure CH4 hydrate. Moreover, it was clear that the bigger the guest molecule was, the larger the lattice would be. We first calculated the greatest distance from oxygen to oxygen of hydrate cages using Diamond 3.2 and then subtracted the van der Waals diameter of oxygen (0.28 nm). The greatest length of a 51262 cage in a pure MH (sI) with a lattice parameter of 1.195 nm was calculated and found to be 0.639 nm. The length values increased to 0.681 nm and 0.656 nm when the lattice parameter of the sI hydrate was expanded to 1.250 nm and 1.217 nm, respectively. This phenomenon seems to be a reasonable expansion compared with the diameters of DMA and EA. The 51262 cage has a distorted spherical shape, and DMA and EA have horizontally elongated shapes. Therefore, we think that the guest molecule could fit into the expanded cage. Park et al. 18 suggested the thermally driven lattice expansion phenomena of several sII hydrates such as THF + H2 and THF + O2 hydrates. They reported that the lattice parameters of these sII hydrates were enlarged from 1.710 nm to 1.732 nm (about 1.4 % increase) when the temperature was increased from 30 K to 273 K. However, the lattice parameter of DMA + H2O hydrate was expanded up to about 4.6 % relative to normal sI hydrate (pure MH). Thus, we can conclude that the effect of large guest molecules is significantly dominant relative to the effect of temperature on the cage expansion. Next, we measured the crystal structures of DMA and EA hydrates formed in the presence of external CH4 gas. Three different mole fractions of DMA/EA were selected to observe structural transitions according to the guest mole fraction. The PXRD patterns of DMA·7.7H2O (x = 0.1154) + CH4, DMA· 13.3H2O (x = 0.070) + CH4, and DMA·34H2O (x = 0.0286) + CH4 are shown in Figure 3, and those of EA·7.7H2O (x =

Figure 4. PXRD patterns of pure CH4 hydrate, EA (x = 0.1154) + CH4 hydrate, EA (x = 0.0556) + CH4 hydrate, and EA (x = 0.0286) + CH4 hydrate. Gray dashed lines represent the peaks of hexagonal ice (Ih). One main peak of an unexpanded sI phase [Miller indices (123)] is assigned with the black dotted line.

patterns are compared with that of pure MH. All of the hydrates were formed over 3 days at 243 K under 10 MPa CH4. In Figures 3 and 4, black and gray (hkl) indices show the planes of the sI hydrate and sII hydrate, respectively, and the gray dashed lines indicate the PXRD peaks of hexagonal ice; the black dotted line in Figure 4indicates the sI hydrate with a = 1.195 nm. Also, the calculated powder diffraction peaks are indicated below each pattern in Figure 3 (|, sI hydrate; ▲, sII hydrate). As shown in Figure 3, DMA + CH4 hydrates were transformed from sI to sII (lattice parameter a = 1.73 nm) 2006



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with decreasing mole fraction of DMA. We can clearly observe the main peaks of the sI phase in the diffraction patterns of DMA·7.7H2O (x = 0.1154) + CH4 and DMA·13.3H2O (x = 0.070) + CH4, which were shifted to lower angles relative to those of pure MH. Furthermore, sII hydrate coexisted with the expanded sI hydrate in DMA·13.3H2O + CH4. The lattice parameters of the DMA·13.3 H2O + CH4 hydrate were measured and found to be a = 1.22 nm for the sI phase and a = 1.73 nm for the sII phase. The lattice parameter of the sII phase was almost the same as that previously reported for sII hydrates since the size of sII-L is large enough to include the DMA molecule. However, the lattice of the sI phase was found to be somewhat expanded by about 2.1 % relative to pure CH4 hydrate; this expansion of the lattice was induced by the larger size of DMA molecules. In the DMA·34H2O (x = 0.0286) + CH4 hydrate, most of the hydrate was revealed to be sII phase with a small amount of sI phase. At this low mole fraction of DMA, the expanded sI hydrate disappeared and normal sI appeared. Similar tendencies were observed in EA hydrate systems. A somewhat enlarged lattice parameter was measured for the EA· 7.5H2O (x = 0.1154) + CH4 hydrate (sI, a = 1.21 nm). The low-angle-shifted peaks can be clearly seen in Figure 4. With decreasing mole fraction of EA, the majority of the hydrate gradually converted to sII hydrate. Moreover, the change in the lattice parameter of the sI phase is denoted in the PXRD pattern. We clearly found that the expanded sI phase disappears and the normal sI phase appears with decreasing mole fraction of EA. Along with the sII (a = 1.73 nm) phase, EA (x = 0.0556) + CH4 was revealed to contain coexisting expanded sI (a = 1.21 nm) and normal sI (a = 1.195 nm) phases; however, the expanded sI phase was rarely observed in the EA (x = 0.0286) + CH4 hydrate. Figures 3 and 4 reflect the fact of an overall trend of structure transition from sI to sII according to the decreasing mole fraction of DMA or EA and simultaneously the increasing mole fraction of CH4. Actually, in a previous study, the structural transition of cyclopropane/tetrafluoromethane + H2O hydrates (sI) to the sII hydrates by incorporation of gaseous CH4 guest was reported.19,20 This transformation seems to have arisen from an increase of the amount of gas storage in the hydrate phase. In this respect, the structural transition of DMA/EA + H2O hydrate from an sI hydrate to an sII hydrate might have been induced by the increased amount of gas in the hydrate phase. In Figure 3, the DMA·34H2O (x = 0.0286) + CH4 hydrate is almost pure sII phase. We additionally investigated PXRD patterns for a DMA mole fraction lower than 0.0286 to identify the structure transition trends at very low mole fraction. The PXRD patterns of DMA·34H2O (x = 0.0286) + CH4 (sII hydrate), DMA·49H2O (x = 0.020) + CH4 hydrate, and DMA· 99H2O (x = 0.010) + CH4 hydrate are shown in Figure 5. The gray and black dotted lines represent the main peaks of the unexpanded sI (a = 1.195 nm) and sII (a = 1.73 nm) phases, respectively. As we were able to anticipate, there was no expanded sI phase at DMA mole fractions lower than 0.0286. On the other hand, unexpanded sI phase was found to coexist with a measurable amount of sII phase, and this coexistence was maintained even at a very low mole fraction of DMA (x = 0.010). The existence of unexpanded sI phase is thought to have been induced by the reaction between pure methane and excess water molecules. To confirm the effect of the pressure of the external CH4 gas, we also measured the PXRD patterns of DMA (x = 0.070) +

Figure 5. PXRD patterns of pure CH4 hydrate (sI, a = 1.195 nm), DMA (x = 0.0286) + CH4 hydrate (sII, a = 1.73 nm), DMA (x = 0.0200) + CH4 hydrate (sI, a = 1.195 nm; sII, a = 1.73 nm), and DMA (x = 0.0100) + CH4 hydrate (sI, a = 1.195 nm; sII, a = 1.73 nm). The main peaks of the unexpanded sI (a = 1.195 nm) and sII (a = 1.73 nm) phases are assigned with gray and black dotted lines, respectively.

H2O + CH4 with changing pressure from 0.81 MPa to 10.0 MPa. All of the hydrates were formed at 243 K and were maintained for 3 days. We checked the equilibrium P−T conditions of DMA (x = 0.070) + H2O + CH4 and found that the dissociation pressure of the hydrate was 0.56 MPa at 262 K. It was also clearly confirmed by the direct release test that noticeable amounts of methane gas were entrapped in all of the hydrate samples, even for the sample formed under 0.81 MPa CH4 at 243 K. As shown in Figure 6, identical levels of the sII

Figure 6. PXRD patterns of DMA (x = 0.070) + CH4 hydrates formed at various pressures of external CH4 (10 MPa, 6.8 MPa, 3.7 MPa, and 0.81 MPa). All of the samples were revealed to have the same PXRD pattern corresponding to the sII and expanded sI phases regardless of the external pressure.

phase were observed from low to high pressures. Thus, we conclude that there is no specific pressure at which the structural transition occurs in the DMA (x = 0.070) + H2O + CH4 hydrate over the pressure range from 0.81 MPa to 10.0 MPa. Eventually, it was possible to expect the structural transition trends according to the mole fraction of DMA or EA shown in Figures 3 to 5 to occur under any pressure of external CH4 gas. NPD Pattern Analysis with Changing Temperature. We measured the NPD patterns over the temperature range from 10 K to 220 K to determine changes in the crystal structure according to temperature. We analyzed two samples, EA (x = 0.0286) + D2O and EA (x = 0.0286) + D2O + CD4, with increasing temperature from 10 K to 220 K followed by decreasing temperature from 220 K to 20 K. As shown in Figure 7a, the EA + D2O sample showed an sI structure over 2007



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hydrate more clearly. As shown in Figure 8, two distinct peaks at −4.2 ppm and −4.7 ppm were detected for the DMA (x =

Figure 8. Solid-state 13C MAS NMR spectra of DMA (x = 0.070) + CH4, DMA (x = 0.040) + CH4, and DMA (x = 0.020) + CH4 hydrates. As assigned with the black dotted lines, the peaks at −4.2 ppm, −4.3 ppm, −4.7 ppm, and −6.4 ppm represent chemical shifts of 13 C of CH4 entrapped in sII-S, sI-S, expanded sI-S, and sI-L cages, respectively.

0.070) + H2O + CH4 hydrate. As reported in previous works in the literature, these two peaks represent the CH4 entrapped in small and medium cages of the sH hydrate, respectively.12 However, we never found any noticeable peaks of the sH phase in the PXRD measurements. Moreover, the chemical shift at −4.7 ppm represents a slightly more shielded condition of 13C in the CH4 molecule than that entrapped in an unexpanded sI-S cage (at −4.4 ppm). Therefore, we can expect that the peak at −4.7 ppm is induced by CH4 in a 512 cage of expanded sI hydrate. We calculated the diameter of each cage from the lattice parameters using Diamond 3.2, and the results are summarized in Table 1. For the DMA (x = 0.040) + H2O +

Figure 7. Temperature dependence of the NPD patterns of (a) EA (x = 0.0286) + D2O hydrate without external CH4 at temperatures increasing from 10 K to 220 K and (b) EA (x = 0.0286) + D2O + CD4 hydrate obtained by increasing the temperature from 10 K to 220 K followed by decreasing it to 20 K again. In (b), the black and light-gray patterns correspond to the sII and sI hydrates, respectively. Labels: *, hexagonal ice; †, sI hydrate; ‡, sII hydrate.

Table 1. Greatest Length (d) for Each Cage with the Given Lattice Parameter (a) of sI Hydrate As Calculated Using Diamond 3.2

the entire temperature range. There was no structural transition with changing temperature. However, the EA + D2O + CD4 sample showed a significant structure transformation at about 200 K and atmospheric pressure. The sample was initially an sII structure in the low-temperature range (10 K to 190 K); however, it suddenly transformed into an sI structure at 220 K (Figure 7b). This result suggests that the CD4 guest molecules started to release above 190 K and that eventually no gaseous guest molecules existed in the host framework. Thus, EA + D2O + CD4 (sII) was completely transformed to EA + D2O (sI) between 190 K and 220 K. Park et al.18 previously reported that sII hydrates including gaseous guests start to dissociate at about 150 K for THF-d8 (x = 0.0556) + H2O + H2 (or D2) and 200 K for THF-d8 (x = 0.0556) + H2O + N2 (or O2) during NPD measurements. The gas-free hydrate (sI) was not reconverted to sII hydrate even when the temperature was again decreased to 20 K. Thus, we can conclude that an sII hydrate with EA guests cannot be constructed without gaseous CH4 as a secondary guest. 13 C NMR Analysis. As determined using PXRD analyses (Figure 3), DMA + H2O + CH4 hydrates were revealed to be a mixture of sII and expanded sI phases according to the mole fraction of DMA above x = 0.0286. We also cross-checked the solid-state 13C MAS NMR spectra (at 183 K and atmospheric pressure) of DMA (x = 0.020, 0.040, and 0.070) + H2O + CH4 to identify the coexistence of the sII hydrate and expanded sI

a/nm

cage type

d/nm

1.195

51262 512 51262 512 51262 512

0.639 0.511 0.658 0.527 0.681 0.547

1.220 1.250

CH4 hydrate, the peaks at −4.2 ppm (sII-S) and −4.7 ppm (expanded sI-S) were also clearly detected, and furthermore, peaks at −6.7 ppm (sI-L) and −8.1 ppm (sII-L) started to appear. However, for the lower mole fraction of DMA (x = 0.020), the peak from CH4 in the expanded sI-S phase was not clearly detected, although the peak at −6.7 ppm was found to increase. Unfortunately, however, in the case of the hydrate for the lower mole fraction of DMA (i.e., x = 0.020), we could not obtain clear signals of CH4 in the range from −4 ppm to −5 ppm. This might be mainly induced by low portions of the sI and sII phases incorporating DMA molecules compared with the pure CH4 hydrate phase, according to the lower mole fraction of DMA. However, it was obviously found that the amount of CH4 in large cages of pure sI methane hydrate increased with decreasing DMA mole fraction; this was somewhat consistent with the results shown in Figure 5. 2008



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Figure 9. Raman spectra analysis with (A, B) DMA hydrates and (C, D) EA hydrates. The regions of C−H bond stretching are expanded in (B) and (D). The traces labeled (a) are for DMA or EA + H2O hydrate without external CH4. Traces labeled (b), (c), and (d) lines correspond to DMA or EA (x = 0.1154, 0.0556, and 0.0286) + CH4 respectively. The gray dashed rectangles in (A) and (C) show the peaks of N−H stretching modes of DMA and EA, respectively.

vibrational signals of EA molecules were not overlapped with those of the CH4 molecules, we were able to more clearly detect the CH4 peaks at 2905 and 2913 cm−1. Furthermore, the distinct peaks at 3333 cm−1 for the DMA hydrate system and at 3308 cm−1 and 3365 cm−1 for the EA hydrate system were observed in all of the samples (gray dashed rectangules in Figure 9A,C). According to previous research,22,23 the peaks correspond to N−H stretching (3333 cm−1) of DMA and symmetric (3308 cm−1) and antisymmetric (3365 cm−1) stretching of EA with good consistency. On the other hand, Ujike and Tominaga24 reported Raman spectra of the NH3 monomer, NH3−NH3 dimer, and NH3−H2O complex. On the basis of both calculated values and experimental data, they suggested a low-frequency shift and peak broadening of the N−H signal when the NH3 monomers take part in the NH3−H2O complex. Thus, we can conclude that the sharp signals around 3350 cm−1 in our experiments represent the non-hydrogen-bonded N−H stretching modes (so-called “free NH bonds”). That confirms the formation of “true” (or “genuine”) clathrate hydrates without hydrogenbonding interactions between guest molecules (DMA and EA) and the water host framework, regardless of the guest mole fraction and structure. True clathrate hydrates incorporating guest molecules containing polar groups such as O−H and N− H were also confirmed in several previous studies.11,12,25−27 However, to the best of our knowledge, the direct spectroscopic signals of free NH bonds were first found in our present experiments. Phase Equilibria. We measured the L−H−V phase equilibria of the DMA + H2O + CH4 hydrate and the EA + H2O + CH4 hydrate. Figure 10 shows the results of the L−H− V phase equilibria measurements for (a) the DMA + H2O + CH4 hydrate and (b) the EA + H2O + CH4 hydrate with

More detailed quantitative approaches to analyze the relative amounts of each phase (sI, expanded sI, and sII) as functions of the mole fraction of DMA should be studied in further work. Raman Spectroscopy Analysis. We measured the Raman spectra of DMA + H2O and EA + H2O hydrates with various mole fractions of alkylamine and the presence of CH4. The C− H stretches of DMA at 2800 cm−1, 2957 cm−1, and 2994 cm−1 in the Raman spectra as determined by Durig et al.21 correspond to the peaks at 2787 cm−1 (symmetric), 2954 cm−1 (antisymmetric) and 2980 cm−1 (antisymmetric), respectively, in our experimental spectra. According to previous research, the Raman peak at 2905 cm−1 represents CH4 in a 51262 or 51264 (sI-L or sII-L) cage, and the peak at 2913 cm−1 represents CH4 in a 512 cage (sI-S or sII-S).1 As shown in Figure 9A,B, two clear peaks at 2903 cm−1 and 2913 cm−1 can be observed for DMA (x = 0.0556 and 0.0286) + H2O + CH4 hydrate [traces (c) and (d), respectively]. Unfortunately, because of the high intensity of the C−H vibration signal of the DMA molecules, we could not clearly distinguish the CH4 peak at 2903 cm−1 for the DMA (x = 0.1154) + H2O + CH4 hydrate. However, the peak of CH4 in a 512 cage was clearly observed at 2913 cm−1. Furthermore, we observed that a significant quantity of gas bubbles were released when we put the DMA (x = 0.1154) + H2O + CH4 hydrate into water at room temperature. From these results, we can conclude without doubt that the CH4 molecules were entrapped even in the DMA (x = 0.1154) + H2O + CH4 hydrate to form the sI phase. For EA + H2O + CH4 hydrates (Figure 9C,D), almost the same tendencies as found for the DMA + H2O + CH4 hydrates were observed. Of course, we also observed a significant quantity of gas bubbles from the EA + H2O + CH4, as was the case with the DMA hydrate. Moreover, because the C−H 2009



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Table 2. Pressure−Temperature Equilibrium Data for Dimethylamine and Ethylamine with Methane as a Secondary Guest x 0.1154

0.0556

0.0286

0.1154

Figure 10. Pressure−temperature diagrams for (a) DMA and (b) EA with methane as a secondary guest: ▲, x = 0.1154; ●, x = 0.0556; ■, x = 0.0286. In both panels, the ▼ line indicates the equilibrium conditions of pure MH.

0.0556

various mole fractions of DMA and EA, respectively. All of the equilibrium data are also summarized in Table 2. The equilibrium conditions of these hydrates were significantly shifted to lower temperature and higher pressure than those of pure MH. Furthermore, as the mole fractions of DMA and EA increased, the equilibrium pressures remarkably increased at the same temperature. In previous research, hydrate formers with an amino group or a hydroxyl group were found to induce an inhibition effect since the former can interfere in the formation of the hydrate structure by initiating hydrogen bonding with water molecules.1 Thus, in this respect, the effect against the activity of water induced by hydrogen bonding between DMA or EA and water molecules is clearly considered to be the most significant factor in the inhibitions observed in our experiments. Recently, several studies also suggested another factor that gives rise to the inhibition phenomenon. Lee et al.28 proposed that the inhibition of 4-methyl-1,3-dioxane + H2O + CH4 hydrate (sH) was mainly induced by the large size of the 4-methyl-1,3dioxane molecule relative to the size of the sH-L cage. More recently, Shin et al.2 also suggested that the equilibrium pressure of diethylamine (DEA) + H2O + CH4 (sH) hydrate is higher than that of pure CH4 hydrate at the same temperature because of the molecular size and shape of DEA. It is also possible that the equilibrium conditions are slightly affected by the sizes of DMA and EA, but the detailed effects of the molecular size should be carefully studied in further work.

0.0286

T/K Dimethylamine 280.72 280.09 279.53 278.86 278.04 277.02 281.77 280.67 279.81 278.54 277.07 283.98 283.64 282.82 281.34 280.10 278.73 Ethylamine 282.74 281.80 280.81 279.80 278.95 284.34 283.57 282.53 281.13 279.80 285.77 284.47 283.75 282.83 280.89 279.44

P/MPa 11.207 10.356 9.385 8.418 7.436 6.656 9.359 8.385 7.332 6.308 5.287 9.803 9.199 8.293 7.185 6.287 5.293 11.962 10.546 9.281 7.722 6.601 11.551 10.409 9.188 7.623 6.458 11.434 10.095 9.019 8.039 6.469 5.417

and Raman spectroscopy. The two alkylamine hydrates, DMA (x = 0.1154) + H2O and EA (x = 0.1154) + H2O, were found in our PXRD results to be expanded sI. However, under external gaseous CH4, those compounds transformed to the sII phase with decreasing mole fractions of DMA and EA. According to the PXRD and NPD data, we also conclude that the structural transition seems to be mainly induced by secondary CH4 guests rather than pressure and temperature conditions within our experimental range. Furthermore, we revealed that the structure of DMA or EA + H2O + CH4 sequentially changes from expanded sI to sII and eventually to unexpanded sI (almost pure CH4 hydrate) with decreasing mole fraction of alkylamine from x = 0.1154 to 0.0100. We also found evidence to support the formation of “true” (or “genuine”) clathrate hydrates without hydrogen-bonding interactions between guest molecules (DMA or EA) and the water host framework for all of the hydrates using Raman spectroscopy. Unfortunately, we could not clearly analyze the quantitative relationship between the relative portions of each phase and the mole fraction of DMA or EA molecules; however, it needs to be studied in further research to reveal the detailed phase behaviors. In that direction, Shin et al.29 reported

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CONCLUSION In this study, we investigated structural transition patterns from sI to sII for two types of alkylamine hydrates formed with various mole fractions of DMA and EA in the presence of external CH4 gas. In order to estimate the structural transitions of the DMA and EA hydrates, we used PXRD, NPD, 13C NMR 2010



Article

(5) Cha, J. H.; Shin, K.; Choi, S.; Lee, S.; Lee, H. Maximized proton conductivity of the HPF6 clathrate hydrate by structural transformation. J. Phys. Chem. C 2008, 112 (35), 13332−13335. (6) Chapoy, A.; Anderson, R.; Tohidi, B. Low-pressure molecular hydrogen storage in semi-clathrate hydrates of quaternary ammonium compounds. J. Am. Chem. Soc. 2007, 129 (4), 746−747. (7) Shin, K.; Choi, S.; Cha, J. H.; Lee, H. Structural transformation due to co-host inclusion in ionic clathrate hydrates. J. Am. Chem. Soc. 2008, 130 (23), 7180−7181. (8) Cha, J. H.; Lee, W.; Lee, H. Hydrogen Gas Sensor Based on Proton-Conducting Clathrate Hydrate. Angew. Chem., Int. Ed. 2009, 48 (46), 8687−8690. (9) Shin, W.; Shin, K.; Seol, J.; Koh, D. Y.; Park, S.; Lee, H. Metastability of Ethane Clathrate Hydrate Induced by [Co(NH3)6]3+ Complex. J. Phys. Chem. C 2011, 115 (5), 2558−2562. (10) Seol, J.; Shin, W.; Koh, D. Y.; Kang, H.; Sung, B.; Lee, H. Spectroscopic Observation of Na Cations Entrapped in Small Cages of sII Propane Hydrate. J. Phys. Chem. C 2012, 116 (1), 1439−1444. (11) Cha, M.; Shin, K.; Lee, H. Spectroscopic Identification of Amyl Alcohol Hydrates through Free OH Observation. J. Phys. Chem. B 2009, 113 (31), 10562−10565. (12) Shin, W.; Park, S.; Koh, D. Y.; Seol, J.; Ro, H.; Lee, H. WaterSoluble Structure H Clathrate Hydrate Formers. J. Phys. Chem. C 2011, 115 (38), 18885−18889. (13) Lee, S.; Lee, Y.; Park, S.; Seo, Y. Structural Transformation of Isopropylamine Semiclathrate Hydrates in the Presence of Methane as a Coguest. J. Phys. Chem. B 2012, 116 (45), 13476−13480. (14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03W, version 6.0; Gaussian, Inc.: Wallingford, CT, 2004. (15) Sloan, E. D., Jr. Fundamental principles and applications of natural gas hydrates. Nature 2003, 426 (6964), 353−359. (16) McMullan, R.; Jordan, T. H.; Jeffrey, G. Polyhedral Clathrate Hydrates. XII. The Crystallographic Data on Hydrates of Ethylamine, Dimethylamine, Trimethylamine, n-Propylamine (Two Forms), isoPropylamine, Diethylamine (Two Forms), and tert-Butylamine. J. Chem. Phys. 1967, 47 (4), 1218−1222. (17) Brandenburg, K.; Putz, H. Diamond, version 3.2; Crystal Impact GbR: Bonn, Germany, 2006. (18) Park, Y.; Choi, Y. N.; Yeon, S. H.; Lee, H. Thermal expansivity of tetrahydrofuran clathrate hydrate with diatomic guest molecules. J. Phys. Chem. B 2008, 112 (23), 6897−6899. (19) Kunita, Y.; Makino, T.; Sugahara, T.; Ohgaki, K. Raman spectroscopic studies on methane plus tetrafluoromethane mixed-gas hydrate system. Fluid Phase Equilib. 2007, 251 (2), 145−148. (20) Makino, T.; Tongu, M.; Sugahara, T.; Ohgaki, K. Hydrate structural transition depending on the composition of methane plus cyclopropane mixed gas hydrate. Fluid Phase Equilib. 2005, 233 (2), 129−133. (21) Durig, J. R.; Griffin, M. G.; Groner, P. Analysis of torsional spectra of molecules with two internal C3v rotors. 3. Far-infrared and gas phase Raman spectra of dimethylamine-d0, -d3, and -d6. J. Phys. Chem. 1977, 81 (6), 554−560.

the existence of a metastable sII phase within 1 h during the formation of the sH phase using an amine molecule (hexamethylenimine). Thus, it is possible to approach the DMA and EA hydrates in a similar way. It might be a valuable topic of further studies to identify the molecular behavior and formation mechanism to form an expanded sI phase rather than pure sI methane hydrate excluding DMA and EA molecules. We also measured the equilibrium P−T conditions of DMA or EA + H2O + CH4 by regulating the mole fraction of the DMA or EA. These two amine molecules were found to act as inhibitors of CH4 hydrate over our experimental pressure and temperature ranges. Moreover, the inhibition effect was dramatically increased with increasing mole fraction of the amine guest; for example, the equilibrium conditions were ∼6.0 MPa for DMA (x = 0.0286) + H2O + CH4 and ∼10.0 MPa for DMA (x = 0.1154) + H2O + CH4 at 280 K. These inhibiting features of DMA and EA can be used for flow assurances to prevent plugging problems in oil and gas pipelines. Finally, we wish to emphasize that these phenomena of a structural transition and guest occupation of clathrate hydrates formed with neutral but polar-group-containing guest molecules such as amines can be exploited to further investigate the unique background of the host−guest interaction. Moreover, the novel features of the amine hydrate might suggest important clues for designing and utilizing sustainable icy functional devices.

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AUTHOR INFORMATION

Corresponding Authors

*J.S.: E-mail: [email protected]. *H.L.: Tel.: +82-42-350-3917. Fax: +82-42-350-3910. E-mail: [email protected]. Present Addresses †

J.S.: Faculty of Liberal Education, Seoul National University, Kwanak-gu Kwanak-ro 1, Seoul 151-742, Republic of Korea. ‡ M.C.: Department of Chemical Engineering, Colorado School of Mines, 1500 Illinois Street, Golden, CO 80401-1881, USA. Funding

This research was funded by the Ministry of Knowledge Economy through the “Recovery/Production of Natural Gas Hydrate using Swapping Technique” Project [KIGAM − Gas Hydrate R&D Organization]. It was also supported by a grant from the National Research Foundation (NRF) of Korea funded by the Korean Government (MEST) (2010-0029176) and by the WCU Program (31-2008-000-10055-0), funded by the Ministry of Education and Science and Technology. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases; CRC Press: Boca Raton, FL, 2008. (2) Shin, W.; Park, S.; Lee, J.-W.; Seo, Y.; Koh, D.-Y.; Seol, J.; Lee, H. Structure Transition from Semi-to True Clathrate Hydrates Induced by CH4 Enclathration. J. Phys. Chem. C 2012, 116 (31), 16352−16357. (3) Cha, J. H.; Lee, W.; Lee, H. Thermal stability and ionic conductivity of the ionic clathrate hydrates incorporated with potassium hydroxide. J. Mater. Chem. 2009, 19 (36), 6542−6547. (4) Cha, J. H.; Shin, K.; Choi, S.; Lee, H. Ionic conductivity enhancement due to coguest inclusion in the pure ionic clathrate hydrates. J. Phys. Chem. C 2008, 112 (28), 10573−10578. 2011



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2012


Structural Transition Induced by CH4 Enclathration and Cage

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