8443
2008, 112, 8443–8446 Published on Web 06/27/2008
Spectroscopic Observation of Critical Guest Concentration Appearing in tert-Butyl Alcohol Clathrate Hydrate Youngjune Park,† Minjun Cha,† Woongchul Shin,† Huen Lee,*,† and John A. Ripmeester*,‡ Department of Chemical and Biomolecular Engineering, Korea AdVanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea, and Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex DriVe, Ottawa, Ontario, K1N 0R6, Canada ReceiVed: May 20, 2008; ReVised Manuscript ReceiVed: June 09, 2008
The tert-butyl alcohol (TBA) is the most hydrophobic of the simple alcohol and by itself does not form a clathrate hydrate with water. A genuine clathrate hydrate is synthesized by exposing a gaseous guest to solid TBA + H2O powders. Here, we examine three consecutive spectroscopic approaches of (1) the occurrence of a “free” OH stretching band (ν(OH)) signal of TBA molecules representing an absence of hydrogen bonding between the host water and guest TBA, (2) a tuning effect for creating fresh cages via the rearrangement of the host-water lattice, and finally (3) the existence of a critical guest concentration (CGC) that appears only when the TBA concentration is dilute. The present findings from this simple three-step approach can be extended to other alcoholic guest species with the specific modifications to provide the new insights into inclusion chemistry. Introduction Simple monohydroxy alcohol compounds play a number of distinct roles in the formation of a clathrate hydrate, as the hydrophobic-hydrophilic balance shifts as the size of the alkyl group increases. Methanol is strictly a hydrate inhibitor,1,2 and ethanol is known to form clathrate hydrates of several types at low temperatures.3,4 Recently, a double hydrate of isopropanol and CH4 was reported as being of the sII type (cubic, Fd3m) clathrate hydrate.5 tert-Butyl alcohol (TBA) is the most hydrophobic of the simple alcohols. Due to its size, it can be expected to form a stable clathrate hydrate if other stabilizing species are present. In particular, TBA appears to be fully miscible with water at any concentration and it typically exhibits complex phase behavior.6–11 Moreover, TBA solutions are known to show various anomalies related to their thermodynamic and physicochemical properties;12 thus, many studies have been performed regarding the structure and phase characteristics of TBA in a liquid state.13–16 It has been suggested that, at a specific concentration, the TBA solution promotes structures of water molecules that can be considered cagelike.14,17,18 Mootz et al. analyzed the crystal structures of solid TBA + H2O using X-ray diffraction and confirmed the formation of heptahydrate (TBA · 7H2O, orthorhombic, Pnma) and dihydrate (TBA · 2H2O, monoclinic, P21) structures.9 However, these structures are not considered to be true clathrate hydrates, as the TBA molecules are hydrogen-bonded with the host water framework.19 From an inclusion chemistry viewpoint, it might be interesting to confirm whether other types of cages can be created by * To whom correspondence should be addressed. Phone: 82-42-8693917(H.L.); 613-993-2011 (J.A.R.). Fax: 82-42-869-3910 (H.L.); 613-9987833 (J.A.R.). E-mail:
[email protected] (H.L.); John.Ripmeester@ nrc-cnrc.gc.ca (J.A.R.). † Korea Advanced Institute of Science and Technology. ‡ National Research Council of Canada.
10.1021/jp804447r CCC: $40.75
introducing gaseous guest molecules such as CH4, potentially leading to the structural stabilization of hydrates formed via lattice transformation. In the present work, tert-butyl alcohol is utilized in an exploration of inclusion phenomena. The study also seeks to verify the possible occurrence of a tuning mechanism at TBA concentrations that are much lower than a stoichiometric concentration, where enclathrated TBA molecules in large cages are pulled out and small CH4 molecules are enclosed instead.20,21 Accordingly, we examine several key physicochemical characteristics of CH4 + TBA clathrate hydrates through thermodynamic and spectroscopic analyses, as the findings are expected to be useful for a better understanding of host-guest interactions and other areas within inclusion chemistry. Experimental Section CH4 gas was purchased from Praxair Technology (Danbury, CT) and had a stated purity of 99.95 mol %. tert-Butyl alcohol (TBA, C4H10O, min. 99.5 mol %) was supplied by SigmaAldrich, Inc. The water of ultrahigh purity was supplied by Merck (Germany). To measure phase equilibria of the CH4 + TBA + H2O mixture, we followed the formal procedures commonly adopted in the hydrate community. At first, the TBA solution was introduced into the equilibrium cell having a magnetic mechanical stirrer. After being pressurized to a desired pressure with CH4, the equilibrium cell was slowly cooled. When the abrupt pressure drop due to CH4 inclusion was detected, the cell was slowly heated. The equilibrium pressures and temperatures were determined by checking the routine PT trajectory consisting of hydrate formation and dissociation stages, and particularly careful observation was given to possible detection of double pressure drops due to two coexisting hydrates. To synthesize the hydrate samples for the spectroscopic analysis, the TBA solutions were frozen at 243 K at least 1 day 2008 American Chemical Society
8444 J. Phys. Chem. B, Vol. 112, No. 29, 2008
Letters
Figure 1. Schematic diagrams of (a) monoclinic dihydrate (TBA · 2H2O, space group P21), (b) orthorhombic heptahydrate (TBA · 7H2O, space group Pnma), (c) entrapment of CH4 and TBA molecules in small (sII-S, 512) and large (sII-L, 51264) cages in sII clathrate hydrate (space group Fd3m). Blue, O; green, C; red, O in TBA; hydrogen atoms are omitted.
and then ground to fine powders (∼200 µm). The powdered crystalline samples were placed in precooled cells having 150 cm3 volumes and, then, exposed to CH4 gas, maintaining 120 bar and 263 K for 2 days to reach the equilibrium conversion. For the spectroscopic analysis, 13C solid-state HPDEC MAS NMR (Bruker AVANCE 400 MHz solid-state NMR spectrometer), Raman (Horiba Jobin Yvon LabRAM HR UV/vis/NIR high resolution dispersive Raman microscope), and powder X-ray diffractometer (PXRD, Rigaku D/Max-RB Geigerflex diffractometer) were used to explore cage dynamics and characteristics. 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 ≈5 kHz, and the measurement temperature was fixed at 223 K. A pulse length of 2 µs and pulse repetition delay of 10 s under proton decoupling was 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 300 K, was used as an external chemical shift reference. The Raman spectrometer, LabRAM HR UV/vis/NIR, with a CCD detector cooled by liquid nitrogen was used. The excitation source was an Ar-ion laser emitting a 514.53 nm line. The laser intensity was typically 30 mW. The PXRD pattern was recorded at 123 K with graphite monochromatized Cu KR radiation (λ ) 1.5406 Å) in the θ/2θ scan mode. The PXRD experiments were performed in step mode with a fixed time of 5 s and an increment of 0.01° in the 2θ range 5-70°. The recorded PXRD patterns were calculated by using the Checkcell program.22 Results and Discussion Unlike in the case of well-known hydrate structures such as sI (cubic, Pm3n), sII (cubic, Fd3m), and sH (hexagonal, P6/ mmm),23 solid TBA + H2O does not incorporate empty small cages that can accommodate gaseous guest molecules; hence, new enclosing cages need to be created that allow the feasible inclusionofadditionalsmallguests.Tothisend,thepressure-temperature (PT) trace was measured to observe CH4 inclusion phenomena
that occur in solid TBA + H2O in an effort to assess the possible formation of new cages via structural transformation. For reference, the PT stability was checked, and a sudden pressure drop was observed that confirmed the direct entrapment of gas molecules accompanying the dimensional change in the host lattice (Supporting Information Figure S1). This result implies that TBA and CH4 function as a hydrate stabilizer and a help gas, respectively, exhibiting a clathrate hydrate forming pattern that is quite similar to that of typical sH,23 although its guest-host network is apparently dissimilar to that of other liquid guests such as tetrahydrofuran (THF), which readily forms a sII clathrate hydrate with the remaining small cages empty. Regarding the aforementioned thermodynamic stability, it was conjectured that the CH4 molecules would become entrapped in the newly formed cages. Two distinct crystalline structures of solid TBA + H2O are shown in Figure 1a and b. However, the exposure of CH4 to solid TBA + H2O induces a structural transition, eventually forming a sII clathrate hydrate (Figure 1c and Supporting Information Figure S2). To obtain a more thorough understanding of this process, the 13C solid-state NMR spectra were measured. As shown in Figure 2a, the NMR spectra of systems with a low TBA concentration show two distinctive peaks, that are allocated to methyl groups in TBA molecules (28.6 and 30.5 ppm), whereas pure solid TBA shows two peaks, one at 31.8 ppm and the other at 32.0 ppm. At 5.0 and 11.0 mol % of TBA + H2O, two completely different structures of dihydrate and heptahydrate exist according to the 13C solidstate NMR and PXRD patterns, but neither form of solid TBA + H2O contains empty cages for small guests to enter (Figure 1). In a system with 33.0 mol % of TBA + H2O, only a dihydrate peak (30.5 ppm) was observed, and the heptahydrate peak completely disappeared. 33.0 mol % TBA is considered to be the composition that represents the phase boundary of a TBA dihydrate. Thus, the peak at 28.6 ppm clearly originates from heptahydrate, while the peaks at 30.5 ppm represent the dihydrate. It is apparent that, above 33.0 mol % TBA, the dihydrate coexists with the excess pure TBA phase. The TBA + H2O system is known to have one stable vertical equilibrium
Letters
J. Phys. Chem. B, Vol. 112, No. 29, 2008 8445
Figure 3. Raman spectra of 5.0 mol % solid TBA + H2O (black) and CH4 + TBA (5.0 mol %) clathrate hydrates (red) at 123 K.
Figure 2. 13C solid-state HPDEC MAS NMR spectra of solid TBA + H2O and CH4 + TBA clathrate hydrates ranging from 5.0 to 90 mol % of TBA at 223 K. (a) Methyl groups in solid TBA + H2O, (b) methyl groups in CH4 + TBA clathrate hydrates, (c) CH4 in 512 cages of CH4 + TBA clathrate hydrates.
line of T-x phase diagram at 33.0 mol % of TBA, where the dihydrate forms with no heptahydrate.8 On the other hand, at around 12.5 mol % of TBA, the solid phases become metastable,9,11 where the heptahydrate and dehydrate coexist. Even in the quite dilute TBA state below 12.5 mol %, two different hydrates also coexist. To examine the direct effect of an addition of gaseous CH4 on the structural transformation, solid TBA + H2O was exposed to pressurized CH4, and 13C solid-state NMR was employed to confirm CH4 occupancy in newly created cages (Figure 2b and c). Characteristic peaks (-4.7 ppm) representing CH4 enclosed in 512 cages of a sII (sII-S) clathrate hydrate were found at concentrations below 70 mol % (Figure 2c).20 The PXRD pattern of sII clathrate hydrate (Supporting Information Figure S2) and characteristic peaks of
CH4 enclosed in sII-S strongly support that an additionally observed peak at 32.4 ppm comes from the TBA molecules entrapped in the 51264 cages. It is important to note that, at high TBA concentrations, the CH4 occupancy becomes weak primarily because the water required to build the cage framework is absent. As shown in Figure 2a and b, the relative TBA intensities between TBA · 2H2O and TBA · 7H2O change considerably before and after the enclathration of CH4. Before CH4 inclusion, the area ratios between pure TBA · 2H2O and TBA · 7H2O are 0.3-0.7 for 11.0 mol % TBA and 0.4-0.6 for 5.0 mol % TBA. After CH4 inclusion, these area ratios are changed to 0.15-0.7 (with 0.15 for sII) and 0.05-0.6 (with 0.35 for sII), respectively. This also implies that most of the TBA · 2H2O as well as a small portion of the TBA · 7H2O are converted to a sII clathrate hydrate. To verify the existence of the true clathrate of the CH4 + TBA hydrate and to crosscheck for the clear occupation of TBA in the sII clathrate hydrate, Raman experiments were performed. The results of these experiments are shown in Figure 3. While solid TBA + H2O has a hydrogen-bonded OH band signal, the CH4 + TBA clathrate hydrate shows a “free” OH stretching band (ν(OH)) signal of TBA molecules around 3610 cm-1.24 This result signifies that the CH4 + TBA hydrate is clathrate, as it demonstrates an absence of hydrogen bonding between the host water and the guest TBA. Thus far, we have endeavored to address the unique structural transformation of the TBA clathrate hydrate, providing spectroscopic evidence of a “free” OH state in the cage framework. In fact, a more interesting aspect involves checking for the occurrence of a tuning mechanism; investigation of the CGC as it pertains to diluted TBA concentrations remains to be completed in an effort to confirm the tuning details. The eutectic point is 5.7 mol % TBA for a solid TBA + H2O system; below this concentration, an ice phase is expected to first appear. A large portion of the ice phase can, of course, readily react with pressurized CH4, creating the sI clathrate hydrate. In an earlier study, Kim et al. observed unique structural transitions and tuning phenomena in the CH4 + tert-butylamine clathrate hydrate from sVI (pure) to sII (double) by verifying CH4 occupation in sII-S through 13C MAS NMR.20 As shown in Figure 4a, after lowering the TBA concentration, additional peaks (-8.3 ppm) of CH4 in 51264 large cages of the sII clathrate hydrate were observed. In addition, the ice phase was converted into a sI clathrate hydrate. The cage occupancy ratio, of θCH4sII-L/θCH4sII-S was obtained from the NMR spectra shown in
8446 J. Phys. Chem. B, Vol. 112, No. 29, 2008
Letters for the large cage, their dimensions quite differ. Thus, the sII-L more significantly loses stability for sustaining its cage framework than the sII-S, leading to an abrupt decrease of cage occupancy ratio. The experimentally determined CGC values appeared to be quite different depending on a liquid guest component that participates in the formation of the binary clathrate hydrate. Only a few CGC values have been found so far, and thus, more extensive works might be needed to draw any generalized picture. Furthermore, molecular characteristics such as guest-guest and guest-host framework interactions, guest dynamics, structural dimensions, and so on must be well defined to more clearly reveal the real nature of tuning and CGC. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the National Research Laboratory Program funded by the Ministry of Education, Science, and Technology (No. R0A-2005-00010074-0(2007)) and partially supported by the Brain Korea 21 Project. Supporting Information Available: Phase equilibria, PXRD results. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 4. (a) 13C solid-state HPDEC MAS NMR spectra of CH4 + TBA clathrate hydrates ranging from 5.0 to 0.05 mol % of TBA at 223 K (CH4 in small cage of sI (sI-S), -4.3 ppm; large cage of sI (sI-L), -6.8 ppm). (b) CGC patterns of CH4 + TBA clathrate hydrates.
Figure 4a in combination with the thermodynamic equation of van der Waals and Platteeuw. The results were then plotted depending on the TBA concentration, as shown in Figure 4b. The cage occupancy ratio increases continuously until it reaches a maximum point at a specific TBA concentration of 0.5 mol %. This unique concentration was referred to as the critical guest concentration (CGC), which becomes apparent when watersoluble liquid guests are quite dilute in the crystalline clathrate hydrate lattice.20,21 As expected, below the CGC, the occupancy ratio decreased abruptly and eventually disappeared at 0.05 mol % TBA. One plausible reason for why the cage occupancy of CH4 in sII clathrate hydrate decreased with TBA concentration at a concentration smaller than CGC might be traced from structure stability. At the highly dilute TBA state below a certain limited concentration (here, referred to CGC), the sI hydrate becomes quite dominant, as shown in Figure 4a, causing the very small amount of coexisting sII clathrate hydrate to gradually lose its stability. During this process, it might be noted that the small cage dimensions of sI and sII are almost identical, but
(1) Davidson, D. W.; Gough, S. R.; Ripmeester, J. A.; Nakayama, H Can. J. Chem. 1981, 59, 2587–2589. (2) Nakayama, H.; Brouwer, D. H.; Handa, Y. P.; Klug, D. D.; Tse, J. S.; Ratcliffe, C. I.; Zhu, X.; Ripmeester, J. A. Prepr.sDiV. Fuel Chem., Am. Chem. Soc. 1997, 42, 516–521. (3) Potts, A. D.; Davidson, D. W. J. Phys. Chem. 1965, 69, 996–1000. (4) Calvert, L. D.; Srivastava, P. Acta Crystallogr., Sect. A 1969, 25, S131. (5) Ohmura, R.; Takeya, S.; Uchida, T.; Evinuma, T. Ind. Eng. Chem. Res. 2004, 43, 4964–4966. (6) Ott, J. B.; Goates, J. R.; Waite, B. A. J. Chem. Thermodyn. 1979, 11, 739–746. (7) Woznyj, M.; Luedemann, H. D. Z. Naturforsch., A 1985, 40, 693– 698. (8) Kasraian, K.; DeLuca, P. P. Pharm. Res. 1995, 12, 484–490. (9) Mootz, D.; Staben, D. Z. Naturforsch. 1993, 48b, 1325–1330. (10) Murthy, S. S. N. J. Phys. Chem. A 1999, 103, 7927–7937. (11) Takaizumi, K. J. Solution Chem. 2000, 29, 377–388. (12) Wojtkow, D.; Czarnecki, M. A. J. Phys. Chem. A 2005, 109, 8218– 8224. (13) Koga, Y. Can. J. Chem. 1988, 66, 1187–1193. (14) Nishikawa, K.; Iijima, T. J. Phys. Chem. 1990, 94, 6227–6231. (15) Bowron, D. T.; Finney, J. I.; Soper, A. K. J. Phys. Chem. B 1998, 102, 3551–3563. (16) Yoshida, K.; Yamaguchi, T.; Kovalenko, A.; Hirata, F. J. Phys. Chem. B 2002, 106, 5042–5049. (17) Iwasaki, K.; Fujiyama, Y. J. Phys. Chem. 1977, 81, 1908–1912. (18) Iwasaki, K.; Fujiyama, Y. J. Phys. Chem. 1979, 83, 463–468. (19) Jeffery, G. A. Inclusion Compounds; Academic Press: London, 1984; Vol. 1. (20) Kim, D.-Y.; Lee, J.-w.; Seo, Y.-T.; Ripmeester, J. A.; Lee, H. Angew. Chem., Int. Ed. 2005, 44, 7749–7752. (21) Kim, D.-Y.; Park, J.; Lee, J.-w.; Ripmeester, J. A.; Lee, H. J. Am. Chem. Soc. 2006, 128, 15360–15361. (22) Laugier, J.; Bochu, B. Laboratoire des Materiaux et du Ge´nie Physique, Ecole Supe´rieure de Physique de Grenoble. Available at http:// www.ccp14.ac.uk. (23) Sloan, E. D.; Koh, C. Clathrate Hydrates of Natural Gases, 3rd ed.; Taylor & Francis-CRC Press: Boca Raton, FL, 2008. (24) Palombo, F.; Paolantoni, M.; Sassi, P.; Morresi, A.; Cataliotti, R. S. J. Mol. Liq. 2006, 125, 139–146.
JP804447R
