Thermodynamic and Spectroscopic Identification of Methane Inclusion

Oct 18, 2013 - The crystal structure and lattice parameter were analyzed by the profile-matching mode in the Rietveld refinement by using the Fullprof...
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Thermodynamic and Spectroscopic Identification of Methane Inclusion in the Binary Heterocyclic Compound Hydrates Minjun Cha,† Huen Lee,‡ and Jae W. Lee*,†,‡ †

Chemical Engineering Department, The City College of New York, Steinman Hall, 140th Street and Convent Avenue, New York, New York 10031, United States ‡ Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: Two kinds of heterocyclic organic compounds, a five-membered ring with the chemical formula of C4H4NH (pyrrole, PRL) and a six-membered ring with the chemical formula of C5H5N (pyridine, PRD), are introduced into clathrate hydrate structures as a coguest with methane (CH4) gas. The results from powder X-ray diffraction of the binary (pyrrole + CH4) and (pyridine + CH4) clathrate hydrates showed that the lattice size of the former is smaller than that of the latter but the crystal structure of both binary hydrates is identified as a cubic Fd3m structure II hydrate. Raman spectroscopy also provided the clear evidence of CH4 and each aromatic ring compound occupying in the small and large cavities of structure II hydrates. The thermodynamic behaviors of the two binary systems were compared with those of the pure methane system to identify the role of the two aromatic compounds in the clathrate hydrate system by using high-pressure micro-differential scanning microcalorimetry. It is striking that the two heterocyclic compounds containing a part of hydrate inhibiting functional groups have a promoting effect on the hydrate formation of the methane−water system.

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INTRODUCTION Clathrate hydrates (gas hydrates) are nonstoichiometric crystalline compounds in which small guest molecules such as methane, ethane, nitrogen, and cyclopentane (CP) are captured in the host of polyhedral cavities formed by the complex networks of hydrogen-bonded water molecules.1−3 Recently, clathrate hydrates have received much attention due to their potential applications such as carbon dioxide capture and sequestration (CCS),4,5 gas separation,6 and gas storage.1−3 However, hydrates cause nuisances in the transportation of hydrocarbon resources due to the hydrate plug formation in the pipelines.3,7 Therefore, there are intensive studies about the prevention or inhibition of gas hydrate formation and the adhesion property of hydrate particles inside the pipeline.8−20 Recently, a series of polymers and carboxylic acids were used to identify the kinetic inhibition performance of gas hydrates and to measure the cohesive force of the hydrate particle.19,20 Reyes et al.19 identified that some polymers such as copolymers of 2-isopropenyl-2-oxazoline with N-isopropylmethacrylamide are useful for kinetically inhibiting hydrate formation. Moreover, it was observed that the addition of some carboxylic acids into hydrate systems can lower the hydrate cohesive force.20 Both polymers and carboxylic acids have heterocyclic aromatic ring groups in their structures, and heterocyclic aromatic ring compounds are an important raw material or a precursor for synthesizing some polymers. Accordingly, the application of polymers into hydrate inhibition can accompany hydrate formation induced by the remaining heterocyclic compounds © 2013 American Chemical Society

such as basic precursors or raw materials in the inhibiting additives. At this stage, it is critical to determine whether the existence of the remaining basic aromatic ring compounds in the additive chemicals can affect the formation of gas hydrate in the presence of light gases. As a front task, we investigate the possibility of hydrate formation using the two new potential hydrate formers of heterocyclic organic compounds of hydrophobic C4H4NH (pyrrole, PRL) and hydrophilic C5H5N (pyridine, PRD) with CH4 gas as a coguest. Their crystal structures and new guest distributions will be identified through spectroscopic observations using Raman and powder X-ray diffraction. High-pressure micro-differential scanning calorimetry (MicroDSC) will be used to determine the hydrate phase equilibrium conditions for the binary (pyrrole + CH4) and (pyridine + CH4) clathrate hydrates.

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EXPERIMENTAL DETAILS Materials. Deionized (DI) water was obtained from a Millipore Direct-Q unit with a resistivity of 18 MΩ cm−1. Methane gas was purchased from Special Gas (Daejeon, Republic of Korea) with a minimum purity of 99.95 mol %. Pyridine (a minimum purity of 99.8 mol %) and pyrrole (a Received: July 31, 2013 Revised: October 17, 2013 Published: October 18, 2013 23515

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DSCVII, SETARAM) was used for this study. The highpressure cell of the MicroDSC with a volume of 0.33 cm3 is made of hastelloy, and it can operate up to 400 bar and at a temperature range from 233.15 to 393.15 K. The MicroDSC was calibrated by n-decane (274.51 K), water (273.15 K), and naphthalene (353.38 K) with a temperature precision of 0.02 K. The sample cell was charged with 0.04 mL of DI water and 0.01 mL of pyridine or pyrrole (slightly in excess of 5.8 mol %). The temperature, pressure, and heat flow of the sample cell are collected by the SETSOFT software. The onset temperature from the endothermic signal is evaluated as the equilibrium conditions for the binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrates. The sample cell was pressurized with CH4 gas up to a desired pressure. The sample and reference cells were initially held at 288.15 K for 10 min, and then, the cells were cooled to 243.15 K at a cooling rate of 3 K/min. After reaching 243.15 K, the cells were held at 243.15 K for 5 min. The cells were heated to 263.15 K with a heating rate of 1 K/min and were held at this temperature for 30 min. From 263.15 to 298.15 K, the heating rate was changed into 0.5 K/min.

minimum purity of 98 mol %) were supplied from SigmaAldrich (Figure 1).

Figure 1. Structures and end-to-end distances of pyridine and pyrrole calculated by Gaussian 03.1,26

Experimental Methods. To synthesize the binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrate, the pyridine or pyrrole solution (5.8 mol %, slightly in excess of the stoichiometric amount of structure II hydrate) was introduced into a high-pressure vessel (with an effective volume of 100 mL) equipped with a mechanical stirrer. Then, the pressure vessel was immersed into an alcohol bath (Jeio Tech., RW2025) under ambient temperature (293.15 K). The pressure vessel was pressurized up to 80 bar by supplying CH4 gas. Before pressurizing the vessel, the pressure vessel was flushed at least three times with CH4 gas to remove any residual air. Before decreasing the temperature to 274.15 K, we hold the experimental setup at 288.15 K for 6 h to confirm the saturation of methane in the system. Normally, the pressure drop in this procedure indicates less than 0.2 bar, so we can neglect the change of methane solubility in pyridine and pyrrole because this pressure drop is due to the temperature stabilization of the reactor system and the pressure drop due to the hydrate formation exceeds 30 bar. After the system temperature and pressure were stabilized, the alcohol bath temperature was gradually decreased to 274.15 K at a rate of 1.0 K/h. The hydrate formation was identified by a sudden pressure drop. When the hydrate conversion was completed, the pressure vessel was immersed into liquid nitrogen to prevent partial hydrate dissociation during the sampling period. The formed hydrate samples were finely powdered in liquid nitrogen. The crystal structure of the binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrates was determined by powder X-ray diffraction (PXRD; RIGAKU D/MAX-2500). The PXRD patterns were collected at 93.15 K, and the graphitemonochromatized Cu Kα1 radiation with a wavelength of 1.5406 Å at a generator voltage of 40 kV and current of 300 mA was used as a light source. During the measurements, the θ/2θ scan mode with a fixed time of 2 s and a step size of 0.02° for 2θ = 5−42° for each hydrate sample was used. The Raman experiments (Horiba Jobin Yvon) utilized an Ar ion laser emitting a 514.53 nm line as an excitation source by cooling a CCD detector with liquid nitrogen. The typical intensity of the laser was about 25 mW. The THMS600G model (Linkam stage) was used to cool the samples to 93.15 K in the Raman analysis. The schematic diagram of the experimental setup to measure the hydrate phase equilibrium conditions for the binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrates is illustrated in the Supporting Information (Figure S1). Highpressure micro-differential scanning calorimetry (Micro-

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RESULTS AND DISCUSSION Both molecular mass and motion of guest molecules in the hydrate cavities could determine the cavity size and the corresponding structure of hydrates. However, the molecular size of a hydrate former has been recognized as a key factor in determining the structure of clathrate hydrates.1,21,22 A molecular size above 7.5 Å is too large to be captured into the small cages of structure II (sII) hydrate.1,23−25 The molecular size of pyridine and pyrrole was determined through the calculation of optimum molecular structures by using Gaussian 03.26 The B3LYP method with the 6-311 ++ G (d,p) basis set was used for this calculation. Pyrrole or pyridine alone has not been experimentally proved to form any structure of sI, sII, and sH hydrates and semiclathrate hydrates. For the pyridine case, it formed a different structure from the clathrate structures and was identified as the planar water structure in a form of pyridine trihydrate using XRD.27 For the pyrrole case, ab initio calculations showed that pyrrole molecules may be stable in large cages of sII hydrates in terms of total energy minimization.28 Together with methane, this study first attempts to investigate whether they can form clathrate hydrates and what structural category they belong to. Figure 1 clearly shows the calculated structures and molecular size of pyridine (7.291 Å) and pyrrole (6.743 Å), implying that both sII and sH hydrates are possible with CH4 gas except for sI hydrates.1 In order to verify the crystal structure of the binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrates, PXRD measurements were conducted at 93.15 K. Two patterns of the binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrates were recognized through the profile-matching mode in the Rietveld refinement package as a part of the Fullprof software.29 The resulting diffraction patterns have a small amount of the reflection signals from hexagonal ice (H2O, ice Ih). The background signal was automatically calculated through the WinPLOTR in the Fullprof software, and the following parameters were refined: zero shift, lattice parameters, and peak profiles (pseudo-Voigt function). Figure 2 indicates the PXRD patterns of the binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrates. The red points represent real signals 23516

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systems, they can be encaged into only large cages of sII hydrate with a limited orientation of their molecular structure.1 Raman spectroscopy was used in order to identify the unique guest inclusion of both CH4 and heterocyclic aromatic ring compounds (pyridine and pyrrole) in small and large cages of structure II hydrates. The Raman spectra from the frozen aqueous solution of 5.8 mol % pyridine (black) and the binary (pyridine + CH4) clathrate hydrate sample (red) have been represented in Figures 3 and 4. Figure 3A clearly shows the

Figure 2. Powder X-ray diffraction patterns of the binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrates. The crystal structure and lattice parameter were analyzed by the profile-matching mode in the Rietveld refinement by using the Fullprof software. The red points indicate real signals from the binary (pyridine + CH4) and (pyrrole + CH4) hydrate samples, while the black lines represent the calculated diffraction patterns. Bragg’s signals from the hydrate phase peaks and hexagonal ice phase peaks are represented as top tick marks and bottom tick marks. Figure 3. Raman spectra of the binary (pyridine + CH4) clathrate hydrate (red) and the mixture of pyridine + H2O (black): (A) C−H vibrational region and (B) in-plane ring bend region.

from the binary (pyridine + CH4) and (pyrrole + CH4) hydrates samples while the black lines indicate the calculated diffraction patterns through the profile-matching mode in the Rietveld refinement. Bragg’s signals from the hydrate phase peaks and hexagonal ice phase peaks represent as top tick marks and bottom tick marks in Figure 2A,B. The profile-matching results through the Rietveld refinement indicate that both binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrates have the same cubic Fd3m structure with a lattice parameter of a = 17.233 42 (913) Å for the former and with a lattice parameter of a = 17.092 71 (522) Å for latter, respectively. Udachin et al.30 reported that the guest size or volume was the most important factor for determining the lattice parameter and the volume of a hydrate unit cell. Consistent with this observation, the molecular size of pyrrole is smaller than that of pyridine from the result of the simulation (Figure 1) and so is the lattice parameter of the binary (pyrrole + CH4) clathrate hydrate system. Because the ratio of the molecular size to the cavity diameter (c.a. 6.7 Å1 for the large cage of sII hydrate) is about one in both pyridine and pyrrole

Raman signal at 2914 cm−1, assigning the enclathration of CH4 molecules in small cages of sII hydrate. In addition, there is a shoulder peak around 2904 cm−1 indicating the enclathration of a small amount of CH4 molecules in large cages of sII hydrate. But due to the CH4 enclathration, it is evident that pyridine does not completely occupy the large cage.1 The Raman signals from the in-plane ring bend of pyridine molecules around 610 cm−1 for A1 symmetry, 650 cm−1 for B2 symmetry, 990 and 1010 cm−1 for A1 symmetry, and 1030 cm−1 for A1 symmetry are shown in Figures 3B and 4A.31 The pyridine is a highly miscible compound in water, implying that the conformation state of pyridine in the frozen solution and in large cages of sII hydrate is completely different. Therefore, we can observe the peak intensity inversion around the 610 and 1000 cm−1 region and the peak shift from 991 to 996 cm−1 indicating the in-plane ring bend of pyridine with A1 symmetry. 23517

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the binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrates are available in Figures S2 and S3 of the Supporting Information. Unlike pyridine having a high miscibility with water (single phase in 5.8 mol % of the pyridine−water solution), pyrrole has an extremely low miscibility with water (two phases in 5.8 mol % of the pyrrole−water solution), which causes to exhibit new signals in Raman spectra as shown in Figure 5. Figure 5A shows

Figure 5. Raman spectra of binary (pyrrole + CH4) clathrate hydrate (red) and the mixture of pyrrole + H2O (black): (A) C−H, O−H, and N−H vibrational region and (B) C−H vibrational region. Figure 4. Raman spectra of binary (pyridine + CH4) clathrate hydrate (red) and the mixture of pyridine + H2O (black): (A) in-plane ring bend region and (B and C) ring stretch and C−H bend region.

the Raman signals from the C−H vibration of pyrrole molecules around 3054 and 3067 cm−1 for B1 symmetry and 3160 cm−1 for A1 symmetry. In addition, the N−H vibrational signal from the enclathrated pyrrole around 3492 cm−1 with A1 symmetry can be also identified. However, similar to the binary (pyridine + CH4) clathrate hydrate sample, the signal from the enclathrated CH4 molecules in both small and large cages of sII hydrate can be observed at 2904 cm−1 for large cages and 2914 cm−1 for small cages (Figure 5B).1 This also implies that there is a small partition of CH4 in the large cages of sII hydrate. Parts A and B of Figure 6 clearly represent Raman signals from the C−H and C−N stretch of pyrrole molecules around 1381 cm−1 for A1 symmetry and the N−H deformation of pyrrole molecules around 1145 cm−1 for B1 symmetry. After the enclathration of CH4 in small cages of structure II hydrates, we can identify new signals at 1150 and 1393 cm−1 indicating the blue shift of the peaks in Figure 6.32

These peak intensity inversion and shift indicate the restricted conformation of pyridine in the hydrate cage and the enclathration of pyridine in large cavities of sII hydrate. We can also observe the similar patterns of the Raman signals from the ring stretch and CH bend of pyridine molecules around 1220 cm−1 for A1 symmetry, 1580 cm−1 for B2 symmetry, and 1590 cm−1 for A1 symmetry (Figure 4B,C). Figure 4B clearly represents the peak intensity inversion occurring in the three signals from the ring stretch and CH bend, and there is a little bit shift of Raman signal from 1224 to 1221 cm−1. New Raman signals from 1587 and 1603 cm−1 in Figure 4C also indicate the guest inclusion behaviors of pyridine in the hydrate cages.31 The full range Raman spectra of 23518

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temperature and heat of fusion at 0.5 K/min are the closest to the literature values of 273.15 K and 334.0 J/(g of water). The hydrate equilibrium conditions of pure methane hydrates at pressure between 35 and 74 bar are captured in Figure 7. The equilibrium data of pure CH4 hydrates from the

Figure 7. Equilibrium pressure−temperature conditions for pure CH4 hydrates calculated by CSMGem (black filled squares), pure CH4 hydrates by using the MicroDSC method (red filled circles), binary (THF + CH4)37 hydrates (purple filled triangles), binary (pyridine + CH4) hydrates (blue filled triangles), and binary (pyrrole + CH4) clathrate hydrates (green filled triangles).

calculation (CSMGem)1 and the MicroDSC method have good accordance, as shown in Figure 7. The binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrates are tested through the same method, and the thermodynamic conditions for both clathrate hydrates are also represented in Figure 7. The dissociation temperatures for the binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrates at given pressures are higher than those for pure CH4 hydrate, but lower than those for the binary (THF + CH4)37 clathrate hydrate. Even though pyrrole has the N−H group in the body of the aromatic ring and the N−H group has been known to be a dominant functional group to inhibit the hydrate formation, the presence of pyrrole exhibits a significant promotion effect on the hydrate formation compared to pure methane hydrate formation. It implies that the hydrate inhibiting performance of the N−H group in pyrrole is minimized by the large hydrophobic group in the heterocyclic aromatic ring structure. Moreover, the dissociation temperatures for the binary (pyridine + CH4) hydrate are slightly lower than those for the binary (pyrrole + CH4) hydrate in Figure 7. As the lattice size of guest molecules increases, as shown in Figure 1 (6.743 Å in pyrrole < 7.291 Å in pyridine), the equilibrium temperature becomes lower as reported for other guest molecules.38 The thermodynamic stability of the binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrates can strongly support the hypothesis about more hydrate formation induced by the help of aromatic ring compounds as a hydrate former with CH4 gas. This implies that the polymer-related inhibitors for the prevention of hydrate formation in the pipelines should be carefully chosen in the hydrate risk management approach. In addition, the side products from polymerization processes and raw materials such as pyridine and pyrrole should be removed before the application of polymer-related inhibitors to the

Figure 6. Raman spectra of the binary (pyrrole + CH4) clathrate hydrate (red) and the mixture of pyrrole + H2O (black). (A) C−H and C−N stretch region and (B) N−H deform region.

Through spectroscopic analyses such as PXRD and Raman spectra, we observed the formation of sII hydrate by using the two heterocyclic aromatic ring compounds with CH4 molecules as a help gas and confirmed the unique inclusion of guest molecules in both small and large cages of sII hydrate. This strongly implies that the possible existence of basic aromatic ring compounds in the inhibiting additive such as pyridine and pyrrole may lead to more hydrate formation with CH4 gas. Therefore, we should check the thermodynamic stability of the binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrates because the equilibrium information is currently unavailable. The equilibrium conditions of the binary (pyridine + CH4) and (pyrrole + CH4) clathrate hydrates were measured using MicroDSC. In several previous studies,33−36 the high-pressure DSC technique for measuring the equilibrium conditions for clathrate hydrate systems is less time-consuming compared to the classical PVT techniques and it was concluded that results from PVT and DSC techniques are in good agreement with the literature data.34,35 In this study, we measured the melting temperature and the heat of fusion during the ice melting at a heating rate of 0.5 K/min (273.0475 K for melting temperature and 334.3423 J/(g of water) for the heat of fusion) and 1.0 K/ min (272.9089 K for melting temperature and 317.4242 J/(g of water) for the heat of fusion), respectively. Thus, the heating rate of 0.5 K/min was employed to identify the thermodynamic conditions of the clathrate hydrate systems because the melting 23519

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(6) Kang, S.-P.; Lee, H. Recovery of CO2 from Flue Gas using Gas Hydrate: Thermodynamic Verification through Phase Equilibrium Measurements. Environ. Sci. Technol. 2000, 34, 4397−4400. (7) Sloan, E. D. Natural Gas Hydrates in Flow Assurance; Gulf: Houston, TX, USA, 2011. (8) Aman, Z. M.; Olcott, K.; Pfeiffer, K.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Surfactant Adsorption and Interfacial Tension Investigations on Cyclopentane Hydrate. Langmuir 2013, 29, 2676−2682. (9) Aman, Z. M.; Joshi, S.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Micromechanical Cohesion Force Measurements to Determine Cyclopentane Hydrate Interfacial Properties. J. Colloid Interface Sci. 2012, 376, 283−288. (10) Aman, Z. M.; Brown, E. P.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Interfacial Mechanisms Governing Cyclopentane Clathrate Hydrate Adhesion/Cohesion. Phys. Chem. Chem. Phys. 2011, 13, 19796−19806. (11) Aman, Z. M.; Dieker, L. E.; Aspenes, G.; Sum, A. K.; Sloan, E. D.; Koh, C. A. Influence of Model Oil with Surfactants and Amphiphilic Polymers on Cyclopentane Hydrate Adhesion Forces. Energy Fuels 2010, 24, 5441−5445. (12) Ajiro, H.; Takemoto, Y.; Akashi, M.; Chua, P. C.; Kelland, M. A. Study of the Kinetic Hydrate Inhibitor Performance of a Series of Poly-(N-alkyl-N-vinylacetamide)s. Energy Fuels 2010, 24, 6400−6410. (13) Chua, P. C.; Sæbø, M.; Lunde, A.; Kelland, M. A. Dual Kinetic Hydrate Inhibition and Scale Inhibition by Polyaspartamides. Energy Fuels 2011, 25, 5165−5172. (14) Kelland, M. A. History of the Development of Low Dosage Hydrate Inhibitors. Energy Fuels 2006, 20, 825−847. (15) Villano, L. D.; Kelland, M. A. An Investigation into the Laboratory Method for the Evaluation of the Performance of Kinetic Inhibitors using Superheated Gas Hydrates. Chem. Eng. Sci. 2011, 66, 1973−1985. (16) Song, J. H.; Courzis, A.; Lee, J. W. Direct Measurements of Contact Force between Clathrate Hydrates and Water. Langmuir 2010, 26, 9187−9190. (17) Song, J. H.; Courzis, A.; Lee, J. W. Investigation of Macroscopic Interfacial Dynamics between Clathrate Hydrates and Surfactant Solutions. Langmuir 2010, 26, 18119−18124. (18) Cha, M.; Courzis, A.; Lee, J. W. Macroscopic Investigation of Water Volume Effects on Interfacial Dynamic Behaviors between Clathrate Hydrate and Water. Langmuir 2013, 29, 5793−5800. (19) Reyes, F. T.; Malins, E. L.; Becer, C. R.; Kelland, M. A. NonAmide Kinetic Hydrate Inhibitors: Performance of a Series of Polymers of Isopropenyloxazoline on Structure II Gas Hydrates. Energy Fuels 2013, 27, 3154−3160. (20) Aman, Z. M.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Lowering of Clathrate Hydrate Cohesive Forces by Surface Active Carboxylic Acids. Energy Fuels 2012, 26, 5102−5108. (21) Cha, M.; Youn, Y.; Kwon, M.; Shin, K.; Lee, S.; Lee, H. Abnormal Thermal Expansion of Clathrate Hydrates Induced by Asymmetric Guest Molecules. Chem.Asian J. 2012, 7, 122−126. (22) 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, 6897−6899. (23) Ripmeester, J. A.; Ratcliffe, C. I. Xenon-129 NMR Studies of Clathrate Hydrates: New Guests for Structure II and Structure H. J. Phys. Chem. 1990, 94, 8773−8776. (24) Ohmura, R.; Takeya, S.; Uchida, T.; Ikeda, I. Y.; Ebinuma, T.; Narita, H. Clathrate Hydrate Formation in the System Methane + 3Methyl-1-butanol + Water: Equilibrium Data and Crystallographic Structures of Hydrates. Fluid Phase Equilib. 2004, 221, 151−156. (25) Cha, M.; Shin, K.; Lee, H. Spectroscopic Identification of Amyl Alcohol Hydrates through Free OH Observation. J. Phys. Chem. B 2009, 113, 10562−10565. (26) 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.; et al. Gaussian 03, Revision C.02; Gaussian: Wallingford, CT, USA, 2004. (27) Mootz, D.; Wussow, H.-G. Crystal Structures of Pyridine and Pyridine Trihydrate. J. Chem. Phys. 1981, 763, 1517−1522.

pipelines because a small amount of the binary hydrates provides seeding points for further hydrate plugging.

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CONCLUSION We have introduced the two new kinds of heterocyclic aromatic ring compounds, hydrophobic pyrrole and hydrophilic pyridine, to form sII methane hydrates as coguests. The crystal structure of binary (pyrrole + CH4) and (pyridine + CH4) hydrates is a cubic Fd3m structure II hydrate identified by powder X-ray diffraction. In addition, the encapsulation of CH4 and two aromatic ring compounds in the small and large cages of structure II hydrates was observed in Raman spectra. Calorimetric measurements of the thermodynamic stability of the two binary hydrates showed that the pyrrole binary system even containing the N−H hydrate inhibiting-group has more favorable hydrate formation conditions than pure methane hydrates. The lattice size of the pyrrole binary hydrate is smaller than that of the pyridine binary hydrate, which results in higher equilibrium temperatures in the pyrrole binary system. Both pyridine and pyrrole greatly promote the methane hydrate formation at moderate temperature and pressure conditions. Thus, polymer hydrate inhibitors are carefully applied to the pipeline after the removal of potential monomer hydrate promoters.

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing a schematic diagram for micro-DSC experiments and the full range of Raman spectra and text giving the complete details of ref 26. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-42-350-3940. Fax: +82-42-350-3910. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

The authors are grateful for the financial support from the Korea CCS R & D Center funded by the Ministry of Science, ICT and Future Planning (Grant No. NRF2013M1MA8A1040703), and from ACS-PRF (under Grant No. 51991-ND9). This research was also made possible in part by a grant from BP/GoMRI.

(1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gasese, 3rd ed.; CRC Press: Boca Raton, FL, USA, 2008. (2) Sloan, E. D. Fundamental Principles and Applications of Natural Gas Hydrates. Nature 2003, 426, 353−359. (3) Ripmeester, J. A.; Ratcliffe, C. I.; Udachin, K. A. Encyclopedia of Supramolecular Chemistry; Marcel Dekker: New York, 2004. (4) Lee, H.; Seo, Y.; Seo, T.-T.; Moudrakovski, I. L.; Ripmeester, J. A. Recovering Methane from Solid Methane Hydrate with Carbon Dioxide. Angew. Chem., Int. Ed. 2003, 42, 5048−5051. (5) Park, Y.; Kim, D.-Y.; Lee, J.-W.; Huh, D.-G.; Park, K.-P.; Lee, J.; Lee, H. Sequestering Carbon Dioxide into Complex Structures of Naturally Occurring Gas Hydrates. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 12690−12694. 23520

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp4076564 | J. Phys. Chem. C 2013, 117, 23515−23521