Metastability of Ethane Clathrate Hydrate Induced by [Co(NH3)6]3+

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Metastability of Ethane Clathrate Hydrate Induced by [Co(NH3)6]3þ Complex Woongchul Shin, Kyuchul Shin,† Jiwoong Seol, Dong-Yeun Koh, Seongmin Park, and Huen Lee* Department of Chemical and Biomolecular Engineering (BK21 program) and Gradute School of EEWS, KAIST, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of South Korea

bS Supporting Information ABSTRACT: The metal complex of [Co(NH3)6]3þ is introduced to C2H6 hydrate to confirm its possible inclusion in hydrogen-bonded water cages and the occurrence of metastable structure. The 13C NMR spectra of C2H6 þ ([Co(NH3)6]Cl3 þ 6NaOH in D2O) hydrate confirmed a new peak at 6.5 ppm matching with C2H6 in sII-L cages. The retarded appearance of metastable sII phase is due to brine rejection of the cobalt complex occurring during solution freezing. The anions of OH- and F- were found to be incorporated in the host water cage framework, providing proton-deficient sites. The ionic conductivity of the frozen [Co(NH3)6]3þ solution increased up to 20-fold after ethane hydrate formation, implying the incorporation of F- into the host lattice. A notable finding of this work is that the metastability occurs only when the cobalt complex is in the presence of anions such as OH- and F-.

’ INTRODUCTION The metastability phenomenon of clathrate hydrates is of interest to researchers because of the formation/dissociation behavior due to distinctive structural transitions, complex phase behavior by molecular reorientation, and practical process development for gas hydrate production.1-6 For the exploration of metastable structure patterns, ethane-related clathrate hydrates have been a focal point of active research. An ethane mixture containing methane is known to form either a structure-I (sI) of 6(51262) 3 2(512) 3 46H2O or a structure-II (sII) of 8(51264) 3 16(512) 3 136H2O, or both, depending on the guest composition.7,8 Sloan and co-workers first reported through their spectroscopic evidence that sI/sII hydrate structures can appear as metastable phases in the composition region of sII or sI hydrate formation.1,2 Interestingly, Murshed and Kuhs found a considerably slow structural transition (up to 158 days) of metastable sII to a thermodynamically stable sI hydrate for mixed methaneethane hydrates near the structural phase boundary composition.3 Ripmeester and co-workers observed the coexistence of two different methane hydrate phases under moderate temperature and pressure conditions and suggested the meaningful concept of kinetic and thermodynamic products.4 Single guests of methane, xenon, and carbon dioxide are also known to form metastable sII phases in the initial stage of hydrate formation.4-6 The occurrence of a metastable hydrate phase is not only of scientific interest in the field of structural chemistry, but also has a significant effect on hydrate crystal nucleation and growth as well as natural gas hydrate formation/dissociation. Similar metastability issues might arise with more complex hydrate systems r 2011 American Chemical Society

mixed with salts or clay minerals, making clear explanations extremely challenging. The ion effect on metastable hydrate structures has not yet been well examined, although several studies noted above presented reliable spectroscopic evidence of the metastability of hydrate systems having nonionic guests. Among various types of ionic coordination compounds, considering ion size and NMR sensitivity, we selected a hexamminecobalt(III) complex and examined its effect on the appearance of metastable hydrates. All of the hydrate samples were identified by neutron powder diffraction (NPD), solid-state 13C NMR, and Raman spectroscopy.

’ EXPERIMENTAL SECTION Materials. Ethane gas (C2H6) with 99.5 mol % purity was purchased from Special Gas, Inc. Deuterium oxide (D2O, 99.96 atom % D) and Hexamminecobalt(III) chloride([Co(NH3)6]Cl3, 99%) were supplied by Aldrich, Inc. Sodium hydroxide (NaOH, 99.998% metals basis) and sodium fluoride (NaF, g99%) were supplied by Sigma-Aldrich, Inc. Sample Preparation. [Co(NH3)6]Cl3 solution (5 wt %) with D2O was used to minimize incoherent scattering of the NPD pattern. Several fold moles of bases (versus mole of [Co(NH3)6]Cl3) were dissolved in the former solution. Directly after full dissolution, the solution was frozen drop by drop at 77 K Received: October 1, 2010 Revised: December 8, 2010 Published: January 6, 2011 2558

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The Journal of Physical Chemistry C (liquid nitrogen) to avoid the possible occurrence of chemical reactions between the ligands of cobalt and bases. The crystallized samples were then ground to a fine powder (∼200 μm) under liquid nitrogen temperature. Roughly 4 g of the powder was placed in a pressurized cell with a volume of 20 cm3 and liquefied ethane was then fully injected into the cell. The cell was kept at 220 K during each hydrate formation period. At this temperature, the vapor pressure of liquid ethane is about 5.5 bar. The procedure above was repeated for the sample of each hydrate formation period (4 days, 7 days, 2 weeks, and 1 month) and of each experimental measurement (NPD, NMR, and Raman), in order to identify the time-dependent hydrate formation of cobalt complex þ ethane systems. Experimental Measurements. A Bruker AVANCE 400-MHz solid-state NMR spectrometer was used. The powdered samples were placed in a 4 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 at ∼7 kHz, and the measurement temperature was fixed at 203 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 adamantane, assigned a chemical shift of 38.3 ppm at 298 K, was used as an external chemical shift reference. For Raman measurements, the Horiba Jobin Yvon LabRAM HR UV/vis/NIR high resolution dispersive Raman microscope was used in which a CCD detector is equipped and cooled by liquid nitrogen. Samples were kept at 93 K during measurements. The excitation source was an Ar-ion laser emitting a 514.53 nm line. The laser intensity was typically 30 mW. To obtain the NPD patterns of hydrate samples enclosing cobalt complexes, a high resolution powder diffractiometer (HRPD, λ = 1.8342 Å), installed at horizontal channel ST2 of the 30MW reactor “HANARO” of the Korea Atomic Energy Research Institute (KAERI). The samples were packed into thin vanadium cylinders, and each cylinder was mounted in a closed cycle refrigerator (CCR). For each measurement, approximately 2 g of sample was used. The resolution of the diffractometer was variable as any of the four Soller collimators with angular divergences R1 = 200 . The divergence R2 = 300 of the second collimator, and the divergence R3 = 100 of the third collimator were fixed. Neutron diffraction measurements were performed at 150 K and atmospheric pressure using a CCR with cooling power of 0.5 w and a Si-diode sensor installed on the neutron beam. The obtained patterns were calculated by the Checkcell program.7 The bulk resistance (Rb) was determined with complex impedance diagrams of σ = L/Rb A, where L (10 mm) and A (2  8 mm) are the distance between two electrodes and the cross-sectional area of the samples, respectively. The CVs were also measured by a Solartron 1260 impedance/gain-phase analyzer and a 1287 electrochemical interface.

’ RESULTS AND DISCUSSION It is thought that the hydrate structure is not affected by the addition of inorganic salts such as NaOH or NaCl due salt exclusion, otherwise known as “brine rejection”, from a frozen solution occurring within a nanosecond scale.10 First, we examined ethane hydrates synthesized in dissimilar salt types of NaCl, NaOH, and [Co(NH3)6]Cl3. The three ethane-based hydrates show a 13C NMR peak at the chemical shift of δ = 7.7 ppm, which corresponds to a sI-L C2H6 signal (Figure 1a). No noticeable

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Figure 1. 13C NMR spectra of ethane hydrate with different chemical composition. (a) single salt composition Black line: [Co(NH3)6]Cl3 5 wt %; Red line: NaOH 5 wt %; Blue line: NaCl 10 wt % (dissolved in D2O, respectively). (b) [Co(NH3)6]Cl3(5 wt % dissolved in D2O) þ NaOH.

structure transformation was observed during 10 days and there was no appearance of a metastable phase. Unlike some kinds of organic salts such as quaternary ammonium salts that form various hydrate structures with or without secondary guests, most of inorganic salts except specific acids such as HPF6, HBF4, and HClO4 are known to not participate in constructing polyhedral hydrate cages, thus leading to simply inhibition of hydrate formation.11,12 In particular, due to ionic interactions between host and guest ions, the ionic clathrate hydrates exhibit many peculiar features such as metal ion encagement, cohost inclusion, and high thermal stability for gas storage. Figure 1a indicates that a single ionic guest fails to exhibit the unexpected structure formation, only maintaining pure gas hydrate for a long period of time. As a subsequent attempt, we synthesized a hydrate comprising both [Co(NH3)6]3þ and OH- by the addition of [Co(NH3)6]Cl3 and NaOH. Figure 1b represents the 13C NMR spectra of the C2H6 þ ([Co(NH3)6]Cl3 þ 6NaOH in D2O) hydrate. A new peak at 6.5 ppm is confirmed to match with C2H6 in sII-L cages. In the initial stage (∼4 days), most of the hydrate phase existed as sI and only a slight amount of sII phase was detected (Figure 1b, black). A few days later, the sII-L C2H6 signal significantly increased (Figure 1b, red). The peak area ratio of C2H6 (sI/sII) 2559

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Figure 2. NPD patterns of ethane hydrate. (a) [Co(NH3)6]Cl3 (5 wt % dissolved in D2O) (7-day formation) Black index: Cubic Pm3n (a = 11.805 Å) *: ice (b) [Co(NH3)6]Cl3 (5 wt % dissolved in D2O) þ 6NaOH (7-day formation) Black index: Cubic Pm3n (a = 11.809 Å), red index: Cubic Fd3m (a = 17.050 Å) *: ice.

changed from 5.2 to 0.2, leading to a nearly complete transition of sI to sII phase. The sII hydrate phase gradually recovered to sI, eventually terminating metastability after 1 month (Figure 1b, green and blue). This unexpected structural change was also observed from C-C region of Raman spectroscopy. As shown in Figure S1 of the Supporting Information, the C-C stretching mode of ethane in sII-L suddenly appeared after two weeks, but all of this metastable phase fully transformed to sI again after 1 month. However, a little difference of peak ratio (sI/sII) between Raman and NMR spectra at the intermediate stage of phase transition implies that the appearance of metastable phase in the system is stochastic rather than deterministic. For structure identification of the formed hydrates, we also assessed their NPD patterns. The C2H6 þ [Co(NH3)6]Cl3 hydrate (Figure 2a) is found to be sI cubic Pm3n structure (a = 11.805 Å). In contrast, the C2H6 þ [Co(NH3)6]Cl3 þ NaOH (7-day sample) shows mixed sI þ sII hydrate phases of cubic Pm3n (a = 11.809 Å) and cubic Fd3m (a = 17.050 Å) as presented in Figure 2b. XRD patterns for above samples were also examined to check structures of the ethane hydrates.13 Regarding the metastability observed here, one interesting feature is that the metastable phase appears in the middle of the formation process rather than at the initial stage. Ripmeester and co-workers found that the kinetically favored metastable phases are observed initially and are transformed into the thermodynamically favored phases with elapsing time.4,5 During nucleation

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Figure 3. (a) Raman spectra of samples having different salt composition and reaction time. Black line: [Co(NH3)6]Cl3 (5 wt % dissolved in D2O), Red line: [Co(NH3)6]Cl3 (5 wt %) þ NaOH reacted 1 h, Blue line: [Co(NH3)6]Cl3 (5 wt %) þ 6NaOH reacted 3days, Cyan line: C2H6 hydrate containing [Co(NH3)6]Cl3 (5 wt %) þ NaOH (b) 13C NMR spectra of ethane hydrate: [Co(NH3)6]Cl3 (5 wt % dissolved in D2O) þ NaOH (3-day reaction between cobalt complex and sodium hydroxide).

and growth of the gas hydrate phase, the sII phase has facesharing 512 cages and the plane of growth is not affected by hexagonal rings of 51264 cages; this phase is more kinetically favored than sI phase. Thus, a metastable sII phase is usually induced by small guests such as CH4 or Xe, which can easily form a hydrate nucleus of the 512 cage.14 However, this C2H6 hydrate system containing both [Co(NH3)6]Cl3 and NaOH does not have any components that can occupy 512 cages. At this stage, it remains unclear why only the combined effect of [Co(NH3)6]3þ and OH- causes a structural transition accompanying metastability. To minimize the possible reaction between [Co(NH3)6]3þ and OH- during sample preparation, the solution mixture was first frozen drop by drop at 77 K. The Raman spectra in the region of the Co-N vibrational mode15 (Figure 3a) indicates that the Co-N coordinate bond of [Co(NH3)6]3þ is maintained without the direct participation of OH-. Meanwhile, we attempted to prepare a solution that provided sufficient time for the reaction of [Co(NH3)6]3þ with OH- and use it for the formation of C2H6 hydrates, but failed to observe any metastable states (Figure 3b). While revealing the 2560

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The Journal of Physical Chemistry C precise molecular dynamics underlying this phenomenon is beyond our ability, much endeavor will be devoted to suggesting reasonably concrete answers. Another concern is to determine where [Co(NH3)6]3þ is positioned, either in the cage as a guest

Figure 4. 13C NMR spectra of different base condition containing ethane hydrate. (a) [Co(NH3)6]Cl3 (5 wt % dissolved in D2O) þ 3NaOH (b) [Co(NH3)6]Cl3 (5 wt % dissolved in D2O) þ 6NaF.

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or on the hydrogen-bonded water framework with a balanced physical interaction. Notably, given the ionic diameter of [Co(NH3)6]3þ, approximately 6.5 Å, this ionic complex would fit the 51264 cage of sII hydrate well.16 The rapid quenching of [Co(NH3)6]Cl3 and NaOH in liquid nitrogen is likely to induce a salting-out phenomenon by brine rejection. Therefore, in order for [Co(NH3)6]3þ to be enclathrated in cages, the diffusion barrier of ions in the solid phase must be overcome. This again raises the question; why does [Co(NH3)6]Cl3 not form a sII phase in the absence of NaOH? If the presence of NaOH is a necessary precondition, what then is the essential role of OH- in structuring hydrates? For ionic guests, in general, anions should be incorporated into the host lattice to balance the overall charge with the enclathrated cations.9,17,18 C2H6 þ ([Co(NH3)6]Cl3 þ 6NaOH in D2O) produces two anions, OH- and Cl-, and both might be incorporated into the cage framework surrounding [Co(NH3)6]3þ. We thus carried out another NMR peak analysis with a solution containing 3NaOH, reduced to a half of 6NaOH. This reduction of OH- in the solution greatly weakened the peak intensity as shown in Figure 4a. The dominant effect of OH- on the metastable sII appearance over Cl- comes from the direct participation of OH- in forming the cage structure. On the contrary, the Cl- cannot be attached to a hydrogen-bonded water cage. In evaluating the effect of anions on the tetrabutylammonium salt hydrates, Nakayama reported that the thermodynamic stability of the hydrates is inversely proportional to the volume of anions.19 Although Cl- or Br- can stabilize quaternary ammonium salt hydrates having a large cage formed by the union of many partially broken cages, the relatively small 51264 cage cannot endure the distortion due to the incorporation of three Cl-. For further confirmation, we conducted an additional test with a NaF solution having F- anions, with a similar ionic volume to OH-. As anticipated, a strong sII-L peak intensity is shown in Figure 4b, further confirming the discrete metastability pattern. Raman spectra of NaF contating system was also checked and confirmed the metastable sII phase (Supporting Information) Ionic clathrate hydrates generally show relatively higher conductivity than nonionic clathrate hydrates due to the incorporation of ions into the host lattice. Due to the disorder of the proton-deficient anionic host lattice site, the negative charge of OH- is delocalized to the whole host lattice. Even in the electrolyte solutions where clathrate hydrates form, the brine

Figure 5. (a) Impedence diagrams (Cole-Cole plot) of icy powder, [Co(NH3)6]Cl3 (5 wt % dissolved in D2O) þ 6NaF (b) Impedence diagrams of C2H6 hydrate, [Co(NH3)6]Cl3 (5 wt % dissolved in D2O) þ 6NaF. 2561

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Figure 6. Schematic diagram of metastable phase occurrence.

rejection might limit the promotion of ionic conductivity. Thus, as indirect evidence supporting that [Co(NH3)6]3þ can be an encaged guest, the ionic conductivities were measured by checking the bulk resistances of a frozen [Co(NH3)6]Cl3 þ 6NaF solution with/ without ethane. Here, we used NaF as a donor of anions that possibly incorporate into the host, because of the possible reaction of OH- with [Co(NH3)6]3þ during sample freezing. The ionic conductivities were 5.00  10-6 S/cm (1.26  106 Ω) and 1.18  10-4 S/cm (5.31  104 Ω) for the frozen solution and ethaneinduced hydrate, respectively (Figure 5a and 5b). The ionic conductivity of the frozen [Co(NH3)6]3þ solution increased up to 20-fold after ethane hydrate formation, implying the incorporation of F- into the host lattice. However, the size of halide ions could be a key factor to determine the conductivity, in a similar manner to the framework structure and guest species. From the present status of spectroscopic analysis, it is difficult to confirm whether the [Co(NH3)6]3þ cations are definitely included in cages under the presence of F- or OH- with ethane. We cautiously suggest that the incorporation of anions in ionic clathrate hydrate should accompany the enclathration of counter cations, causing the occurrence of metastable sII phase (Figure 6).

’ CONCLUSIONS We observed the unusual metastability pattern of C2H6 hydrate induced by a cobalt complex. In order for metastable sII to form the original sI, NaOH and NaF had to be added to the cobalt complex solution. The notable finding from this work is that hydroxide and fluoride anions restructure the host water framework via lattice distortion. This anion-induced structural modification can improve the ionic conductivity, particularly given that the host framework is able to function as a pathway to deliver protons or electrons. However, the physicochemical pathway for structure-specific metastability remains quite unclear and thus needs to be carefully explored for a variety of coordination compounds. Furthermore, solid spectroscopic evidence revealing the real positions of complex ions should be provided, as such findings might link closely to the essential features of inclusion chemistry. ’ ASSOCIATED CONTENT

bS

Supporting Information. Raman spectra and XRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org

’ AUTHOR INFORMATION

Present Address †

Current address: National Research Council of Canada, 100 Sussex Drive, Ottawa K1A 0R6, Canada.

’ ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea Grant [WCU Program: 31-2008-000-10055-0, NRL Program: R0A-2005-000-10074-0(2009) and KIGAM: N04100035] funded by the Ministry of Education, Science and Technology (MEST). The authors would also like to thank the Korea Basic Science Institute (Daegu) for assistance with 600 MHz solid-state NMR. ’ REFERENCES (1) Ohno, H.; Strobel, T. A.; Dec, S. F.; Sloan, E. D, Jr.; Koh, C. A. J. Phys. Chem. A. 2009, 113, 1711. (2) Subramanian, S; Ballard, A. L.; Kini, R. A.; Dec, S. F.; Sloan, E. D., Jr. Chem. Eng. Sci. 2000, 55, 5763. (3) Murshed, M. M.; Kuhs, W. F. J. Phys. Chem. B. 2009, 113, 5172. (4) Schicks, J. M.; Ripmeester, J. A. Angew. Chem., Int. Ed. 2004, 43, 3310. (5) Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A. Angew. Chem., Int. Ed. 2001, 40, 3890. (6) Staykova, D. K.; Kuhs, W. F.; Salamatin, A. N.; Hansen, T. J. Phys. Chem. B. 2003, 107, 10299. (7) Sloan, Jr. E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker: New York, 1997. (8) Jeffery, G. A. Inclusion Compounds; Academic Press: London, 1984; Vol. 1; pp 135-190. (9) Laugier, J.; Bochu, B. Laboratoire des Materiaux et du Genie Physique, Ecole Superieure de Physique de Grenoble. Available at http://www.ccp14.ac.uk. (10) Vrbka, L.; Jungwirth, P. Phys. Rev. Lett. 2005, 95, 148501. (11) Mootz, D.; Oellers, E. J.; Wiebcke, M. J. Am. Chem. Soc. 1987, 109, 1200. (12) Cha, J. H.; Shin, K.; Choi, S.; Lee, S.; Lee, H. J. Phys. Chem. C 2008, 112, 13332. (13) We also examined X-ray diffraction patterns of the samples. See the Supporting Information. (14) Walsh, M. R.; Koh, C. A.; Sloan, E. D.; Sum, A. K.; Wu, D. T. Science. 2009, 326, 1095. (15) Chen, Y.; Christensen, D. H.; Nielsen, F.; Pedersen, E. J. Mol. Struct. 1993, 294, 215. (16) Ciesielski, H.; Sterckeman, T.; Santerne, M.; Willery, J. P. Agron. Sustain. Dev. 1997, 17, 1. (17) Shin, K.; Cha, J.; Seo, Y.; Lee, H. Chem. Asian J. 2010, 5, 22. (18) Dyadin, Yu. A.; Udachin, K. A. J. Incl. Phenom. 1984, 2, 61. (19) Nakayama, H. Bull. Chem. Soc. Jpn. 1983, 56, 877.

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*Tel: þ82-42-350-3917; Fax: þ82-42-350-3910; E-mail: h_lee@ kaist.ac.kr. 2562

dx.doi.org/10.1021/jp1094273 |J. Phys. Chem. C 2011, 115, 2558–2562