to True Clathrate Hydrates Induced by CH4 Enclathration - American

Jul 16, 2012 - strong electronegativity so as to blur some of the distinction between the true ..... Gaussian 03; Gaussian, Inc.: Wallingford, CT,. 20...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/JPCC

Structure Transition from Semi- to True Clathrate Hydrates Induced by CH4 Enclathration Woongchul Shin,† Seongmin Park,† Jong-Won Lee,‡ Yongwon Seo,§ Dong-Yeun Koh,† Jiwoong Seol,† and Huen Lee*,†,∥ †

Department of Chemical and Biomolecular Engineering (BK21 Program), KAIST, 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, Korea ‡ Department of Environmental Engineering, Kongju National University, 275 Budae-dong, Cheonan, Chungnam 331-717, Korea § School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan 689-798, Korea ∥ Graduate School of EEWS, KAIST, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea ABSTRACT: Diethylamine and n-propylamine, known as semiclathrate hydrate formers, are found to show structural transition when a help gas, CH4, was introduced. The diethylamine·8.67H2O semiclathrate hydrates (orthorhombic Pbcn) were changed to sH type (hexagonal P6/mmm) true clathrate hydrates, while the n-propylamine·6.5H2O semiclathrate hydrates (monoclinic P21/n) turned into sII clathrate hydrates (cubic Fd3m). Irregularly distorted voids in the semiclathrate hydrate phases were transformed to conventional ones after changing their structures to gas hydrate phases. The different shape of large voids in the semiclathrate hydrates changed to the typical shape of sH or sII large voids, and pentagonal dodecahedra were formed so as to capture CH4 molecules. Transition pattern and molecular behavior from semiclathrate hydrate to true clathrate hydrates were analyzed with PXRD, NMR, and Raman methods. In addition, the liquid mixture−CH4 hydrates−CH4 vapor (L−H−V) thermodynamic equilibrium conditions were measured.

■

INTRODUCTION The crystal structure formed by accommodation of guest molecules into hydrogen-bonded water frameworks is called clathrate hydrate. Most of the abilities to form various clathrate hydrates are attributed to a special property of the water molecules. As in ice, each water molecule can donate and accept two hydrogen bonds, which makes possible the packing of polyhedral voids. There are many different types of polyhedral void, which contain different combinations of faces. Regardless of the combination, all water molecules are coordinated tetrahedrally because the water molecule has two hydrogen donors and two hydrogen acceptors.1 Although there are features common to the various clathrate hydrates due to the existence of water molecules, differences are affected depending on the chemical nature of guest species such as guest size, polarity, and hydrogen bonding capability.2,3 True clathrate hydrates have been regarded as having no chemical interaction between host and guest molecules.2 Wellknown gas hydrates belong to this category. The gas hydrates can form three crystal structures, known as sI, sII, and sH depending on the molecular sizes of the guest species and the existence of coguest molecules. Small guest species like CH4 or C2H6 form the sI hydrates by stabilizing cavity structure having proper ratios of guest molecules to host cavities, while larger © 2012 American Chemical Society

hydrocarbons with three or more carbon atoms or tetrahydrofuran form the sII hydrates because they only fit into the large cavity of the sII.4−7 It should be noted that the sH hydrate can be formed only when both an organic chemical like methylcyclohexane or hexamethyleneimine and a help gas like CH4 occupy proper cavities to stabilize the entire crystal structure.8 In addition, there are unusual true clathrate hydrates such as bromine hydrate (sIII).9 Meanwhile, some chemical compounds can act as both host and guest molecules at the same time; these are called the semiclathrate hydrate formers. Except for tert-butylamine which forms sVI clathrate hydrate, many alkylamine compounds form semiclathrate hydrates. In the semiclathrate hydrates, the functional groups of guest molecules form hydrogen bonds with water molecules to make up the host framework.4 Then, the cavities are occupied by the hydrophobic alkyl chain of the guest molecules. In general, even identical guest molecules may have different crystal structures when forming semiclathrate hydrates depending on their hydration number. In addition, significant cavity distortion, formation of irregular cavities, and Received: April 18, 2012 Revised: June 19, 2012 Published: July 16, 2012 16352

dx.doi.org/10.1021/jp303719r | J. Phys. Chem. C 2012, 116, 16352−16357

The Journal of Physical Chemistry C

Article

unit. Diethylamine and n-propylamine were purchased from Sigma-Aldrich, Inc., and were used without further purification. In order to prepare the hydrate samples, various concentrations of aqueous alkylamine solutions were frozen at atmospheric pressure and 250 K. Then, frozen solutions were ground into fine powders with a 200 μm sieve in liquid nitrogen in order to promote a fast reaction. After grinding, the powdered sample was placed in a high-pressure cell (20 mL of sample capacity) and was pressurized with methane gas up to 10 MPa. After pressurizing with CH4 gas, the high-pressure cell was maintained at 243 K for 3 days. When the sample preparation was finished, the high-pressure cell was cooled in liquid nitrogen and the pressure was slowly released before collecting hydrate samples and analyzing them with a series of microscopic methods. A Varian (UnityNOVA600) 600 MHz solid-state NMR spectrometer was used. The powdered (∼200 μm) samples were placed in a 3.2 mm o.d. zirconia rotor loaded into a variable temperature probe. All 13C NMR spectra were recorded at a Larmor frequency of 100.6 MHz with MAS approximately at 10 kHz; the measurement temperature was fixed at 183 K. A pulse length of 2 μs and a pulse repetition delay of 10 s under proton decoupling were used with a radio frequency field strength of 50 kHz, corresponding to a 5 μs 90° pulse. The downfield carbon resonance peak of hexamethyl benzene (HMB), assigned a chemical shift of 17.3 ppm at 298 K, was used as an external chemical shift reference. To obtain CH4 signals of a higher intensity, a gaseous mixture of 13CH4 and 12CH4 was used. For Raman measurements, a Horiba Jobin-Yvon LabRAM HR UV/vis/NIR high resolution dispersive Raman microscope was used in which a CCD detector had been equipped. This device was cooled by liquid nitrogen. The samples were kept at 93 K during the measurements. The excitation source was an Ar-ion laser emitting a 514.53 nm line. The laser intensity was typically 30 mW. The PXRD patterns were recorded at 123 K on a Rigaku Geigerflex diffractometer (D/Max-RB) using graphite-monochromatized Cu Kα radiation (λ = 0.15406 nm) in the θ/2θ scan mode. The PXRD experiments were carried out in step mode with a fixed time of 3 s and a step size of 0.02° for 2θ = 5−45°. The equilibrium pressures and temperatures were determined by checking the routine PT trajectory, which consisted of hydrate formation and dissociation stages. The cooling rate was 0.5 K/h, and the heating rate was 0.1 K/h. A four-wire type PT-100 Ω (±0.05% accuracy of full scale) and a PMP4070 device from Druck Inc. were used as temperature and pressure sensing devices.

various cavity combinations are often inevitable in the semiclathrate hydrates due to the inherent chemistry of the guest species. Traditionally, the most distinguishable difference of the semiclathrate hydrates from the “true” clathrate hydrates like the gas hydrate was regarded whether directional guest− host interactions exist or not. However, recent investigations reported that some guests can form hydrogen bonding with water molecules so as to cause Bjerrum-type defects which widely affects different dynamics in the clathrate structures.2,3 In particular, chemical compounds having a nitrogen or oxygen atom in the functional group (for example, ethers or amines) are found to show such guest−host interactions due to their strong electronegativity so as to blur some of the distinction between the true and semiclathrates.10 In this regard, in addition to the chemical nature of guest species itself, the guest−host interactions should be taken into account in order to explain different behaviors of the clathrate hydrate such as its stability, the formation kinetics, or inhibition mechanism. Unlike the true and semiclathrate hydrates, ionic clathrate hydrates can be formed by the enclathration of ions into water voids and the incorporation of counterions into the host frameworks. Some acids such as HClO4 and HPF6 are wellknown ionic clathrate hydrate formers.1,11,12 The thing that most easily distinguishes the ionic clathrate hydrates from other clathrate hydrates is that the ionic clathrate hydrates are stabilized by ionic interaction between guest and host molecules, and have an electric charge at guest and host molecules. However, a more complex type of clathrate hydrates exists such as quaternary ammonium salt hydrates. In the case of tetra-butyl ammonium salt hydrates, the anions combined with the hydrogen-bonded water framework cause relatively small changes in topology, while large cavities experience merging and distortion so as to form “super” cavities that can hold cations that are much larger than the cavity diameter.1,13 The tetra-butyl ammonium cation is located at the center of four cages (partially broken two 51262 and 51263).13 Therefore, the quaternary ammonium salts can be considered as semi ionic clathrate hydrates. In this fused form, hydrophobic interaction between water molecules and the alkyl chain of cations also exists in addition to ionic interaction. Even though some structural changes of the clathrate hydrate were reported previously, such investigations were performed in limited systems containing amine compounds. Moreover, such structural changes were not analyzed sufficiently to explain the guest−host interactions. In this study, we report structure transition from semi- to true clathrate hydrates induced by adding a help gas. Such structural changes were analyzed by means of microscopic methods for clathrate hydrate samples formed from diethylamine (DEA) and n-propylamine (nPA) with CH4 as a help gas. In addition, we tried to explain such structural transition in terms of guest−host interactions and molecular behaviors. Experimental results obtained can provide useful information on guest−host behaviors in the clathrate hydrate. In addition to microscopic analysis for the transformed hydrate samples, three-phase (L−H−V) equilibrium conditions were also measured in order to identify thermodynamic effects of the guest species.

■

RESULT AND DISCUSSION The DEA·8.67H2O semiclathrate hydrate is composed of 18and 17-hedral cavities. The 18-hedral cavities (51266) form a two-dimensional network by sharing faces, and the network is linked with 17-hedral cavities whose structure is irregular due to the existence of quadrilateral faces.14 The 17-hedral cavities are distorted and asymmetric, whereas the 18-hedral cavities are well organized and symmetric. A nitrogen atom of the amine group of DEA participates in forming the framework of cavities by hydrogen bonds with water molecules because of its strong electronegativity. The nPA·6.5H2O semiclathrate hydrate has a more complex combination of cavities. Four types of cavities distorted irregularly combine with one another to form a large convex polyhedron. The amine group of nPA is incorporated into the host lattice of large cavities, and empty 11-hedral

■

EXPERIMENTAL PROCEDURE CH4 of 99.95 mol % purity was purchased from Special Gas, Inc. 13CH4 was purchased from Cambridge Isotope Inc. High purity distilled water was obtained from a Millipore purification 16353

dx.doi.org/10.1021/jp303719r | J. Phys. Chem. C 2012, 116, 16352−16357

The Journal of Physical Chemistry C

Article

cavities (425861) are also formed.15 Both DEA·8.67H2O and nPA·6.5H2O semiclathrate hydrates showed no pentagonal dodecahedra (512), which are the most common void in the crystal structures of gas hydrates. The experimental powder Xray diffraction (PXRD) patterns of DEA·8.67H2O and nPA·6.5H2O semiclathrate hydrates are presented in Figure 1.

Figure 2. PXRD patterns of CH4 hydrates made from (a) DEA 5.56 mol % solution and (b) nPA 5.56 mol % solution.

chemical shifts of CH4 molecules (at −4.3 ppm for CH4 molecules in small (512) cavities and at −4.6 ppm for CH4 in medium (435663) cavities), the crystal structure of the DEA semiclathrate hydrate was changed into the sH hydrate. In addition, large cavity (51268) occupation of DEA molecules (incorporated into the host lattice in the semiclathrate hydrate) in the sH hydrate also supports such structural transition. At a concentration of 3 mol % DEA, a considerable amount of the sI hydrate phase (pure CH4 hydrate) was observed in addition to the sH phase, even though the stoichiometric concentration of the true sH hydrate is 2.9 mol %. Also, the DEA is a highly miscible compound in water, which guarantees the formation of a homogeneous mixture during solution preparation. The sI hydrate formation is due to the experimental procedure of CH4 hydrate formation. The CH4 hydrate was formed from the freezing solid mixture, not liquid mixture; this could lead to sI phase formation during the polyhedral change and recombination of voids. Namely, during structure transition, small amounts of DEA can be excluded, and then excess water molecules that do not react with DEA form the sI phase of CH4 hydrate. As shown in our previous work,8 a stoichiometric concentration of the liquid mixture formed pure sH. Such a structural transition during hydrate formation can be supported by looking at the ethyl peak of DEA, which has migrated, as can be seen in Figure 3b. The chemical shift of DEA β-carbon at 14.5 ppm for the semiclathrate hydrate was found to shift to 15.5 ppm for the CH4 hydrate, which means the atomic

Figure 1. PXRD patterns of (a) DEA·8.67H2O and (b) nPA·6.5H2O semiclathrate hydrates.

Calculated lattice parameters of orthorhombic (Pbcn) DEA·8.67H2O were found to be a = 13.44 Å, b = 11.77 Å, and c = 27.91 Å, while those of monoclinic (P21/n) nPA·6.5H2O were found to be a = 12.43 Å, b = 20.73 Å, c = 17.28 Å, and β = 89.3°. These semiclathrate hydrates are found to show structural transition after pressurization of CH4. In order to verify the crystal structures and the calculated cell parameters of the hydrate samples, PXRD measurements were conducted for the transformed CH4 gas hydrates. As shown in Figure 2, the DEA + CH4 hydrate was found to be a hexagonal P6/mmm structure (a = 12.12 Å and c = 10.07 Å), while the nPA + CH4 hydrate was analyzed and found to be a cubic Fd3m whose cell parameter is 17.25 Å. These results show good agreement with the previous reference4 and potently support the idea of transformation from semi- to true clathrate hydrate. The 13C NMR spectra of DEA + CH4 and nPA + CH4 hydrates are shown in Figures 3 and 4. The solid-state 13C NMR spectra of CH4 hydrates made from various concentrations of DEA solution (from stoichiometric 10.3 to 0.5 mol %) are shown in Figure 3a. As indicated by the observed 16354

dx.doi.org/10.1021/jp303719r | J. Phys. Chem. C 2012, 116, 16352−16357

The Journal of Physical Chemistry C

Article

Figure 3. 13C NMR spectra of (a) DEA + CH4 hydrates with various DEA concentrations and (b) alkyl group of DEA from DEA semiclathrate and DEA + CH4 hydrate.

Figure 4. 13C NMR spectra of (a) nPA + CH4 hydrates with various nPA concentrations and (b) alkyl group of nPA from nPA semiclathrate and nPA + CH4 hydrate.

the chemical shift at −8.0 ppm indicates that CH4 molecules also occupy large cavities (51264) of the sII hydrate during structure transition even at the stoichiometric concentration of 5.56 mol % for the sII hydrate. Indeed, substantial amounts of CH4 in sII-L were detected at 5.56 mol % of stoichiometric concentration of methane hydrate spectra (Figure 4a), which means part of the nPA molecules could be excluded during the phase transition. Recombination and transformation of cavities could lead to exclusion of nPA, which similarly occurred in the DEA containing system. If the experimental starting material is changed from semiclathrate hydrate (solid powder form) to liquid mixture, there is no sII-L peak at 5.56 mol % stoichiometric concentration, as was found in our previous study.18 Similar to the DEA system, the chemical shift of γcarbon of nPA is found to move. In addition, the chemical shifts from α-carbons of both amine compounds are also migrated slightly (less than 0.5 ppm).

environment of DEA β-carbon has been deshielded after introduction of CH4 gas. Figure 4 shows 13C NMR spectra for nPA + CH4 hydrate samples prepared at various concentrations. While DEA semiclathrate hydrate turned into sH gas hydrates after CH4 pressurization, nPA semiclathrate hydrates transformed into sII type gas hydrates. Even though the nPA·6.5H2O semiclathrate hydrate has empty 425861 cavities sufficiently large to accommodate CH4 molecules,16 the crystal structure of the nPA semiclathrate hydrate was found to be transformed to that of the true sII hydrate after CH 4 pressurization. Such transformation was not observed for the other clathrate hydrate prepared with per-alkylonium salt, which remained in its original structure even after CH4 pressurization.17 The chemical shift at −4.2 ppm can be assigned to the CH4 signal captured in the small cavities of the sII hydrate. As the concentration of nPA decreases, the amount of pure CH4 hydrate (sI) made from excess water is found to increase, as observed for the DEA hydrate system. In addition, 16355

dx.doi.org/10.1021/jp303719r | J. Phys. Chem. C 2012, 116, 16352−16357

The Journal of Physical Chemistry C

Article

among all of the gas hydrate voids regardless of their absolute size.4 Transformation to sH or sII with help of CH4 could be understood by the molecular conformation of DEA or nPA. It can be said that DEA takes a gauche conformation rather than an all-trans conformation so as to stably fit into hydrate cavities, as shown for other larger guests like n-pentane or n-hexane.20 The molecular conformation of butane or 1,2-dichloroethane which have similar molecular structures and chain lengths with nPA proposed reliable insight that nPA also existed as a gauche form in sII-L. Ripmeester’s group reported transformation of butane or 1,2-dichloroethane from trans to gauche configuration in sII clathrate hydrate with the help of D2S.21,22 Such a conformation can be supported by the calculated lattice parameters of the samples. As calculated, the lattice parameters showed similar values to those in previous literature, which means no significant cage distortions during accommodating DEA or nPA. However, it should be noted that, although both DEA and nPA are captured in the large cavities of the sH and sII hydrate by taking a gauche conformation, their molecular orientation is limited due to their sizes. Finally, the three-phase equilibria of the liquid mixture−CH4 hydrate−CH4 vapor (L−H−V) were measured to investigate the thermodynamic effect on hydrate formation. As shown in the phase equilibria (Figure 7), DEA−CH4 clathrate hydrate is

The experimental measurements conducted with Raman spectroscopy indicate sH formation for the DEA + CH4 hydrate and sII formation for the nPA + CH4 hydrate. A Raman peak at 2912 cm−1 can be assigned to the C−H signal of the CH4 molecules in the small and medium cavities of the sH hydrate, while the two Raman peaks at 2905 and 2915 cm−1 indicate enclathrated CH4 molecules in large and small cavities of the sII hydrate, respectively (Figure 5).

Figure 5. Raman spectra of DEA 5.56 mol % + CH4 gas hydrate (black line) and nPA 5.56 mol % + CH4 gas hydrate (red line). Deconvolution of nPA 5.56 mol % + CH4 gas hydrate (green dashed line).

The transformed crystal structure after CH4 pressurization is thought to be related to the molecular size of the amine compounds. The optimum structures and calculated molecular sizes were 8.80 Å for DEA and 7.60 Å for nPA, as shown in Figure 6.19 Considering the average diameters of the sH large cavity (8.78 Å) and the sII large cavity (6.66 Å),20 the ratios of the molecular diameter to the cavity diameter were 1.00 for DEA and 1.14 for nPA, respectively. Although the ratio is generally positioned between 0.85 and 0.95, a value under 0.85 or over 1 may exist depending on the formation conditions (temperature and pressure) and guest compounds. Nonetheless, the value of 1.14 for nPA is significantly large considering the symmetry of sII-L. Unlike sH-L (51268) and sI-L (51262), the sII-L void consisted of 12 pentagonal faces and 4 hexagonal faces, showing a more symmetrical topology, which means that only restricted variation of the cavity diameter is allowed. Indeed, the sII-L void shows the much lowest level of variation

Figure 7. Phase equilibria condition of various liquid solution−CH4 hydrate−CH4 vapor. DEA 5.56 mol % (■); pure water (●); nPA 5.56 mol % (▲); 4-methylpiperidine 2.9 mol % (▼); pyrrolidine 5.56 mol % (◀).

not as stable as pure CH4 hydrate, while nPA−CH4 clathrate hydrate is more stable. For example, at 284 K, a CH4 pressure of 120 bar or more is required to make gas hydrates with DEA, while a CH4 pressure of 70 bar with pure water and 60 bar with

Figure 6. Molecular structure and longest diameter of (a) DEA and (b) nPA. 16356

dx.doi.org/10.1021/jp303719r | J. Phys. Chem. C 2012, 116, 16352−16357

The Journal of Physical Chemistry C

■

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea government (MEST) (No. 2010-0029176, WCU program: 31-2008-000-10055-0). This work was also supported by the Energy Efficiency & Resources Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge Economy of Korea. This work was also supported by the Future Creativity and Innovation project (2012) of the UNIST. The authors would also like to thank the Korea Basic Science Institute (Daegu) for assistance with 600 MHz solid-state NMR.

nPA is sufficient to form gas hydrates. In Figure 7, phase equilibria of pyrrolidine (a cyclic amine compound with four carbon atoms and one nitrogen atom forming sII type hydrate18) and 4-methylpiperidine (a cyclic amine with substituted methyl group and hexagonal ring made up of five carbon atoms and one nitrogen atom forming sH type hydrate8) are also presented for comparison. Cyclic structures in these chemicals help the molecules occupy large cavities more easily because the three-dimensional molecular sizes do not differ significantly and their molecular shape and volume are reasonable to occupy the large cages. However, DEA and nPA have limited molecular orientations due to their linear molecular structures even after being accommodated into hydrate cavities by taking a gauche conformation. For making stable clathrate hydrates, not only guest size but shape or volume is important.23 In other words, both DEA and nPA are inefficient guest species in occupying hydrate cavities. Considering microscopic measurements, much harder conditions (i.e., higher pressure) are required to accommodate DEA having a molecular size comparable with n-pentane into hydrate cavities. In addition, although nPA with relatively smaller size showed milder equilibrium conditions, it is also inefficient when they compete with CH4 to occupy large cavities of the sII hydrate. However, such inefficiency can be useful in some application areas. In particular, DEA requiring higher pressure to form the clathrate hydrate can be used as an inhibitor to prevent the plugging problems in oil and gas pipelines for oil transport or natural gas production.

■

REFERENCES

(1) Jeffrey, G. A. Inclusion Compounds; Academic Press: London, 1984; Vol. 1, pp 135−190. (2) Susilo, R.; Alavi, S.; Moudrakovski, I. L.; Englezos, P.; Ripmeester, G. A. ChemPhysChem 2009, 10, 824−829. (3) Alavi, S.; Susilo, R.; Ripmeester, J. A. J. Chem. Phys. 2009, 130, 174501. (4) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2007. (5) Mak, T. C. W.; McMullan, R. K. J. Phys. Chem. 1965, 42, 2732− 2737. (6) Tse, J. S.; Handa, Y. P.; Ratcliffe, C. I.; Powell, B. M. J. Inclusion Phenom. 1986, 4, 235−240. (7) Davidson, D. W.; Gough, S. K.; Gough, S. R.; Handa, Y. P.; Ratcliffe, C. I.; Tse, J. S.; Ripmeester, J. A. J. Inclusion Phenom. 1984, 2, 231−238. (8) Shin, W.; Park, S.; Koh, D. -Y.; Seol, J.; Ro, H.; Lee, H. J. Phys. Chem. C 2011, 115, 18885−18889. (9) Udachin, K. A.; Enright, G. D.; Ratcliffe, C. I.; Ripmeester, J. A. J. Am. Chem. Soc. 1997, 119, 11481−11486. (10) Alavi, S.; Udachin, K.; Ripmeester, J. A. Chem.Eur. J. 2010, 16, 1017−1025. (11) Mootz, D.; Oellers, E. J.; Wiebcke, M. J. Am. Chem. Soc. 1987, 109, 1200−1202. (12) Bode, H.; Teufer, G. Acta Crystallogr. 1955, 8, 611−614. (13) Shin, K.; Cha, J.-H.; Seo, Y.; Lee, H. Chem.Asian J. 2010, 5, 22−34. (14) Jordan, T. H.; Mak, T. C. W. J. Chem. Phys. 1967, 47, 1222− 1228. (15) Brickenkamp, C. S.; Panke, D. J. Phys. Chem. 1973, 58, 5284− 5295. (16) Brickenkamp, C. S.; Panke, D. J. Phys. Chem. 1973, 58, 5284− 5295. (17) Lee, S.; Park, S.; Lee, Y.; Lee, J.; Lee, H.; Seo, Y. Langmuir 2011, 27, 10597−10603. (18) Shin, W.; Park, S.; Ro, H.; Koh, D. -Y.; Seol, J.; Lee, H. J. Chem. Thermodyn. 2012, 44, 20−25. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Mongomery, J. A.; Vreven, T.; Kudin, K. N.; Brunt, J. C.; et al. Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004. (20) Lee, J.-W.; Lu, H.; Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A. Angew. Chem., Int. Ed. 2006, 45, 2456−2459. (21) Garg, S. K.; Davison, D. W.; Gough, S. R.; Ripmeester, J. A. Can. J. Chem. 1979, 57, 635−637. (22) Davidson, D. W.; Garg, S. K.; Gouch, S. R.; Hawkins, R. E.; Ripmeester, J. A. Can. J. Chem. 1977, 55, 3641−3650. (23) Ripmeester, J. A.; Ratcliffe, C. I. J. Phys. Chem. 1990, 94, 8773− 8776.

■

CONCLUSIONS In this study, structural transformations of DEA and nPA semiclathrate hydrates into true clathrate hydrates after pressurization of CH4 were investigated by means of NMR, Raman, and PXRD methods. The DEA semiclathrate hydrate was found to change its crystal structure to sH, while the nPA semiclathrate hydrate, which has empty cavities that are sufficiently large to accommodate CH4 molecules, is transformed into sII. In addition, three-phase (L−H−V) equilibrium conditions were also measured in order to identify thermodynamic effects of the guest species. DEA and nPA have limited molecular orientations due to their linear molecular structures even after being accommodated into hydrate cavities by taking a gauche conformation. Due to their molecular size and shape, they occupied the large cage of sH or sII inefficiently, which led to harder equilibrium conditions. Especially, the equilibrium conditions of the DEA + CH4 hydrates are much higher than those of the pure CH4 hydrates at the specific temperature condition. Such thermodynamic and chemical characteristics of DEA indicate that the chemical can be used as an inhibitor to prevent plugging problems in oil and gas pipelines. This abnormal phase transition is thought to extend to a variety of alkylamine semiclathrate hydrates, enabling the discovery of more precise physicochemical background information in the highly sustainable and eco-friendly research field related to water and ice materials.

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 16357

dx.doi.org/10.1021/jp303719r | J. Phys. Chem. C 2012, 116, 16352−16357