ARTICLE pubs.acs.org/JPCC
Spectroscopic Observation of Na Cations Entrapped in Small Cages of sII Propane Hydrate Jiwoong Seol, Woongchul Shin, Dong-Yeun Koh, Hyery Kang, Boram Sung, 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: Although numerous studies have been conducted on various ionic clathrate hydrates, in spite of its potential importance the inclusion of metal cations in continuous water-host frameworks has not yet been clearly identified by direct spectroscopic evidence. Here, a key question arises as to whether the small alkali metals such as Na+ can be entrapped because they are considered to be too small to be stabilized in the hydrate cages. In this study, we first suggested spectroscopic evidence for the enclathration of Na+ in a small cage of sII propane hydrate. First, we checked the overall structure of sII propane hydrate incorporated with NaSO3CH3 and NaSO3NH2 with powder XRD and 13C NMR. Next, we revealed the difference of chemical shift of 23Na between Ih and sII hydrate phases with solid-state 23Na MAS NMR as direct evidence of entrapped Na+ in 512 cages. In addition, we also checked 13C MAS NMR of the CH3SO3 anion and found that Na+ cations in a small cage could be stabilized with an enclathrated CH3SO3 anion in a large cage. To the best of our knowledge, it is the first discovery of small alkali cations stabilized in a continuous hydrate phase. Finally, we would like to emphasize that clathrate hydrate including small alkali metals can be designed and synthesized for its potential applications to various types of energy devices using its ionic mobility through the hydrate channel.
’ INTRODUCTION Ionic molecules such as HPF6, HBF4, and alkyl ammonium salts are known to form unique structures of pure ionic clathrate hydrates, where the large ions of PF6, BF4, and alkyl ammonium cations act as guest species enclosed in the vacant cages.13 The nonionic clathrate hydrates are stabilized by van der Waals interaction between a guest molecule and a host framework, while the ionic clathrate hydrates are generated by an ionic interaction between an ionic guest and a surrounding host water framework, depending on the size and valence of the cations or anions as well as the hydration number. A recent study further revealed that atomic hydrogen radicals as well as ionized hydrogen molecules can be stabilized in an icy hydrogen hydrate matrix.4 The properties and reactions of single hydrogen atoms are of interest because of their inherent quantum mechanical behavior; experimentally, they can be generated and stabilized at very low temperature (4 K) by high-energy irradiation of solid molecular hydrogen. Particularly, various structures of water clusters with alkali metal ions or hydronium and water clusters, M+(H2O)n [M+ = Li+, Na+, K+, Rb+, and Cs+] and H3O+(H2O)n, were investigated in several studies.58 Steel et al. reported that all of the alkali metal cations except for Na+ show a tendency to form a distinct magic number at n = 20. Magicnumber clusters are generally characterized by the prominence of their peaks in a mass spectral distribution of intensity versus size. A certain cluster has a particular stability at the magic number. They computationally suggested that the Na+(H2O)20 cluster has intermediate stability between K+(H2O)20 and Cs+(H2O)20, r 2011 American Chemical Society
even if it does not have a magic number. They also predicted that dodecahedral Na+(H2O)20 appears to be more stable than neighboring-sized clusters, which provides the physical background why it can be treated as a magic-number cluster in an energetic sense.7 This result means that the pentadodecahedron (512) structure of Na+(H2O)20 has particular stability in gasphase clathrate. However, the all-water clusters encaging alkali metal cations can exist only as single clusters in the gas phase. Furthermore, in the example of Cs+(H2O)n, although it is the most stable cluster at n ≈ 20, just a few numbers of clusters having n ≈ 20 remained at temperatures above 163 K.5 However, in the present study, we revealed that isolated Na+ cations in 512 cages can be stabilized in a continuous icy matrix at much higher temperature (∼248 K) than 163 K. The inclusion of metal cations in “continuous” water-host frameworks has not yet been definitely identified by direct spectroscopic evidence. A previous result to note is that Mootz et al. confirmed the encaged Cs+, which is known to be the largest atomic cation, via single-crystal diffraction.9 Another interesting study has been done for improving thermal stability and ionic conductivity of clathrate hydrates through incorporation with potassium cations.10 We further note that the guest cations of cesium and potassium were able to occupy a 4356 cage, which is much smaller than a 512 cage. In particular, when intercalated Received: August 4, 2011 Revised: December 6, 2011 Published: December 12, 2011 1439
dx.doi.org/10.1021/jp207450e | J. Phys. Chem. C 2012, 116, 1439–1444
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Overall experimental scheme.
methane hydrate formed in the clay of Na-montmorillonite, the mobile cations of Na+ in interlayers strongly affect methane hydrate formation and methane distributions in cages, but the real inclusion of Na+ in cages was not clearly identified.11,12 Here, a key question arises as to whether the Na+ can be entrapped as it is considered to be too small to be stabilized in the most popular hydrate cages, 512. Accordingly, we attempt to identify for the first time sodium cations included in the 512 cages through 23Na and 13C nuclear magnetic resonance (NMR) spectroscopy, X-ray powder diffraction (XRD), and Raman analysis.
’ EXPERIMENTAL SECTION The 8, 20, and 30 wt % sodium methanesulfonate (sodium mesylate, NaSO3CH3) and sodium sulfamate (NaSO3NH2) in H2O were sufficiently mixed to ionize into sodium cations and mesylate and sulfamate anions, respectively. NaSO3CH3 (98%, Aldrich), NaSO3NH2 (TCI’s guaranteed reagent grade, Tokyo Chemical Industry), NaCl (>99.5%, Aldrich), and NaOH (97%, Aldrich) were used as received. Ultrahigh purity water was obtained from a Millipore purification unit. High-purity C3H8 (99%) and CH4 (99.9%) were supplied by Special Gas Corp. The mesylate and sulfamate anions were confirmed to exist quite stably as free anions, thus excluding their conversion to any other species in the aqueous solution. Next, the droplets (∼2 mm diameter) of aqueous solution were dropped into liquid nitrogen, frozen, and finally ground into powder (
