ARTICLE pubs.acs.org/JPCC
Water-Soluble Structure H Clathrate Hydrate Formers Woongchul Shin,† Seongmin Park,† Dong-Yeun Koh,† Jiwoong Seol,† Hyeyoon Ro,† and Huen Lee*,†,‡ †
Department of Chemical and Biomolecular Engineering (BK21 Program) and ‡Graduate School of EEWS, KAIST, Daejeon, 305-701, Korea
bS Supporting Information ABSTRACT: Hexamethyleneimine, 1-methylpiperidine, 2-methylpiperidine, 3-methylpiperidine, and 4-methylpiperidine as isomers of C6H13N were revealed as new sH clathrate hydrate forming molecules. They show fully soluble characteristics to water, whereas already known sH formers such as methylcyclohexane and 2,2-dimethylbutane (neohexane) are immiscible or very slightly soluble to water. The L H V equilibrium P T behavior of these new sH clathrate hydrates shows a tendency to shift to much milder conditions than already known ones. We particularly note that 1-methylpiperidine appears to be the best for promotion. To verify the distribution of CH4 molecules and crystal structure of clathrate hydrates, 600 MHz solid-state NMR, Raman spectroscopy, and XRD pattern analysis were conducted. These noticeable properties of new formers are expected to open new research fields to the hydrate community and contribute to hydrate-based technological applications with high energy efficiency.
’ INTRODUCTION Clathrate hydrates have been steadily receiving much attention because of their notable applications involving natural gas hydrate production, energy gas storage, carbon capture and sequestration, and synthesis and fabrication of versatile energy devices.1,2 These ice-like materials either naturally occur in marine gas hydrate sediments as crystalline structures of sI, sII, and sH1,2 or can be artificially formed as many other structures by enclosing polar/nonpolar, ionic/nonionic, and hydrophilic/hydrophobic guest molecules.3,4 Among these familes, sH hydrates, which were first discovered by Ripmeester et al.,5 have stimulated research explorations because of their distinctive physical and chemical characteristics over others.6 First, sH hydrates can lower the LW LHC H V or I LHC H V equilibrium point compared to pure methane (sI) hydrate by adding a small amount of organic guest molecules. Second, sH hydrates can store much greater amounts of gaseous moelcules than sII hydrates, because only 1 large organic molecule is needed per unit cell as compared with 8 molecules for sII hydrate. Thus, the gas storage capacity of sH hydrate increased over 25% compared to sII hydrate. In this regard, recent studies have focused more on the physical behaviors of sH hydrates through approaches such as tuning methane content,7 storing hydrogen gas,8 finding organic compounds that form sH hydrates,9,10 molecular behavior of large guest molecule,11 and determining the hydrate phase equilibrium conditions.12 Among these research fileds, finding the effective sH hydrate formers is the most essential and fundamental task to achieve other goals. Although many sH hydrate formers such as methylcyclopentane (MCP), methylcyclohexane (MCH), 2,2-dimethylbutane (NH), and tert-butylmethylether (TBME) have been suggested, all of these compounds are either r 2011 American Chemical Society
not miscible or very slightly miscible in water. Recently, both the Ripmeester and Ohmura groups proposed water-soluble sH formers, although their solubility appears to be higher than that of previously reported formers, having limited solubility far less than the sH stoichiometric concentration (2.9 mol %).10,13 This unfavorable solubility might hinder the hydrate production rate, the gas storage efficiency and capacity, and further practical applications. Unfortunately, fully soluble sH formers in water have not been discovered yet, in spite of their strong interest in hydrate fields. In this study, we suggest 5 new sH hydrate formers which are fully soluble to water and interpret their molecular behavior and L H V equilibrium condition.
’ EXPERIMENTAL SECTION Materials. CH4 with 99.95 mol % purity was purchased from Special Gas, Inc. Hexamethyleneimine (>99%) was supplied by Aldrich, Inc. Methyl-substituted piperidines were supplied by Tokyo Chemical Industry Co. Water of ultrahigh purity was obtained from a Millipore purification unit. Sample Preparation. Liquid solution of water + LGM (large guest molecule) was frozen in atmospheric pressure roughly at 250 K, and the frozen mixture was ground with a 200 μm sieve in liquid nitrogen. Second, the powdered sample was pressurized with 50 bar of methane gas and matured for 5 days at 263.15 K. Experimental Measurements. The PXRD patterns were recorded at 113.15 K on a Rigaku Geigerflex diffractometer Received: June 9, 2011 Revised: August 1, 2011 Published: August 17, 2011 18885
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Figure 2. L + H + V phase equilibrium conditions of (various sH former + CH4) clathrate hydrate: 2-methylTHF (black dash line), MCH (red dash line), 2,2 -dimethylbutane (blue dash line), hexamethyleneimine (b), 1-methylpiperidine (9), 2-methylpiperidine (2), 3-methylpiperidine (), 4-methylpiperidine (), and pure CH4 (().
Figure 1. (a) Chemical structures of fully water-soluble sH clathrate hydrate formers. (b) XRD patterns of 2.9 mol % hexamethyleneimine + CH4 hydrate (a = 12.24 ( 0.005 Å, c = 10.06 ( 0.003 Å). (c) XRD patterns of 2.9 mol % 1-methylpiperidine + CH4 hydrates (a = 12.25 ( 0.003 Å, c = 10.07 ( 0.004 Å).
(D/Max-RB) by using graphite-monochromatized Cu Kα1 radiation (λ = 1.5406 Å) in the θ/2θ scan mode. The XRD experiments were carried out in step mode with a fixed time of 3s and a step size of 0.02° for 2θ = 10 50°. The obtained patterns were calculated by the Checkcell program. A Varian (UnityNOVA600) 600 MHz solid-state NMR spectrometer was used. The powdered (∼200 μm) samples were placed in a 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 at ∼10 kHz, and the measurement
temperature was fixed at 183.15 K. A pulse length of 2 μs and pulse repetition delay of 10 s under proton decoupling were used with a radiofrequency field strength of 50 kHz, corresponding to a 5 μs 90° pulse. The downfield carbon resonance peak of adamantine, assigned a chemical shift of 38.3 ppm at 298.15 K was used as an external chemical shift reference. To obtain a higher intensity of CH4 signals, 13CH4 gas was used. 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 93.15 K during measurements. To minimize hydrate dissociation during XRD, NMR, and Raman analysis, especially with regard to XRD and NMR, which were fulfilled after several hours, low temperature is needed as possible. We synthesized the samples at the desired temperatures, but we set the temperature in the spectroscopic instruments to a quite low temperature since any serious effects were not given on the samples during experiments. The equilibrium pressures and temperature were determined by checking the routine PT trajectory consisting of hydrate formation and dissociation stages. The cooling rate was 0.5 K/h, and the heating rate was 0.1 K/h. A 4-wire type PT-100Ω ((0.05% accuracy of full scale) and PMP4070 from Druck Inc. were used as temperature- and pressure-sensing devices.
’ RESULTS AND DISCUSSION We tested five isomers of C6H13N (shown in Figure 1a) as potential sH forming compounds with full solubility in water. The hexamethyleneimine has a heptagonal ring-type structure which is composed of six carbon atoms and a nitrogen atom, while methyl-substituted piperidines have a hexagonal ring and a methyl group according to each position. For structure identification and cell parameters of new sH hydrates, we assessed their XRD patterns at 113.15 K. Among five sH hydrates, the XRD patterns of hexamethyleneimine- and 1-methylpiperidine-containing samples are presented as Figure 1b and 1c. XRD patterns and corresponding cell parameters of the other samples are available in the Supporting Information. All samples were made 18886
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Figure 3. Molecular 3D structure and the longest diameter of sH formers (nitrogen, blue atom; carbon, dark gray; hydrogen, white; longest diameter, cyan): (a) Hexamethyleneimine, (b) 1-methylpiperidine, (c) 2-methylpiperidine, (d) 3-methylpiperidine, and (e) 4-methylpiperidine.
with 2.9 mol % (stoichiometric concentration of sH) of LGMs. All of the hydrates were found to be sH hexagonal P6/mmm. The cell parameters of hexamethyleneimine + CH4 hydrate are a = 12.24 ( 0.005 Å and c = 10.06 ( 0.003 Å. The cell parameters of 1-methylpiperidine showed similar values, a = 12.25 ( 0.003 Å and c = 10.07 ( 0.004 Å. The collected XRD patterns were indexed using the Checkcell program.14 These cell parameters are well matched with reported values.1,9 11 The XRD patterns and cell parameters of 2-methylpiperidine, 3-methylpiperidine, and 4-methylpiperidine hydrate are available in the Supporting Information. Subsequently, we attempted to check the equilibrium of L + H + V by measuring the pressure and temperature condition. Here, we use L to denote the homogeneous mixture of hexamethyleneimine + water and methyl-substituted piperidine + water. Also, the equilibrium point of pure CH4 hydrate was checked to confirm our experimental setup. The experimental apparatus and procedure were tested with a well-defined system of pure CH4 hydrate ((), and we found that both agreed well with ref 1 (see Figure 2). The Lw + LHC + H + V four-phase equilibrium data patterns of already known sH hydrate formers are shown10,15,16 together in Figure 2 for comparison. For these experiments, we used a nearly stoichiometric concentration of sH formers. The 2-methyltetrahydrofuran (2-methylTHF) hydrate (black dash line) forming sH has a relatively high solubility of 15 g/100 mL at 298 K, 1.74 mol %,10 and provides the lowest degree of hydrate formation. For MCH (red dashed line) and 2,2dimethylbutane (NH) (blue dashed line) the equilibrium condition is shifted to more favorable conditions, although its solubility in water is much poorer than that of 2-methylTHF. However, the five tested new sH formers show a promotion effect compared to MCH and NH except for 4-methylpiperidine. Amomg the five compounds, 1-methylpiperidine (9) (1-MPD) can act as the most powerful sH hydrate former. Concisely, using these materials, almost pure sH hydrates can be made under higher temperature and lower pressure environments. Also, the optimum structure and molecular size of hexamethyleneimine and methyl-substituted piperidines were calculated by Gaussian 03. The B3LYP method with the 6-311++G(d, p) basis set was
Figure 4. (a) 13C MAS NMR spectra of (water-soluble sH formers + CH4) clathrate hydrates (sI-S, 4.1 ppm; sII-S, 4.05 ppm; sH-S, 4.3 ppm; sH-M, 4.6 ppm; sI-L, 6.5 ppm). (b) Raman spectra of (watersoluble sH formers + CH4) clathrate hydrates (sI-S, 2905 cm 1; sII-S, 2904 cm 1; sI-L, 2916 cm 1; sII-L, 2915 cm 1; sH, 2913 cm 1).
used for the calculations. The calculated values were 7.63 Å for hexamethyleneimine and 8.14 Å for 1-methylpiperidine. The ratios of molecular diameters to cage diameter were 0.869 for hexamethyleneimine and 0.927 for 1-methylpiperidine. However, we are not convinced that the cell parameters are directly affected by the molecular diameter. The molecular diameters of 2-methylpiperidine and 3-methylpiperidine were 8.22 and 8.36 Å, respectively. We note that the smaller 2-methylpiperidine exhibits the larger cell parameter when compared with 3-methylpiperidine. The molecular 3D structure may have a more significant affect than the calculated diameter value. The molecular diameter of 4-methylpiperidine is only 8.03 Å, the second smallest one, but the cell parameter is much larger than others. Hexamethyleneimine has an almost spherical shape because the methyl group is not substituted. Accordingly, the molecular diameter cannot vary with atom positions. Also, 1-, 2-, and 3-methylpiperidine show symmetry structure when the longest diameter is used as an axis though which the distortion may occur. We conjecture that 4-methylpiperidine may locate with difficulty at the center of cage, which causes cell parameter extension and thermodynamic instability in comparison with other formers. The optimized molecular structure and diameter 18887
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Figure 5. 13C NMR spectra of the hydrate of hexamethyleneimine + CH4 mixed at various ratios.
are shown in Figure 3.The 13C NMR spectra of the tested samples, presented in Figure 4a, were obtained to identify their hydrate structure and analyze the distribution of CH4 molecules over the cages. Pure methane (sI, no large guest molecules), tetrahydrofuran, and methylcyclohexane hydrates were also analyzed for comparison. All of the help gas were CH4. Tetrahydrofuran (THF) and methylcyclohexane (MCH) are wellknown compounds as sII and sH hydrate former. All of the tested samples have 2.9 mol % of organic compounds. As seen in this figure, LGMs containing samples have two peaks at 4.3 and 4.6 ppm, which are assigned to CH4 in small (512, sH-S) and medium (435663, sH-M) cages of sH. An additional peak at 6.5 ppm of MCH hydrate, of course, represents CH4 captured in large cages (51262) of sI. The area ratio of sH-S/sH-M is roughly equal to 1.5, which indicates full occupancy of CH4 in sH hydrate phase. The detailed peak deconvolution graph and area ratio values are available in the Supporting Information. Although much higher pressure than the equilibrium pressure of sI methane hydrate was used, no CH4 in sI phase was observed in the hydrate composed of new hydrate formers as shown in the NMR spectra. However, we note that the MCH hydrate contains a considerable amount of sI phase together with sH phase. This undesirable sI phase occurrence is closely related to the low solubility of large guest molecules (LGMs) in water. For example, the solubility of MCH is no more than 0.1 g/L (293 K).18 However, it is interesting to see that the highly water-soluble LGMs of hexamethyleneimine and methyl-substituted piperidines could yield a nearly homogeneous mixture of water + LGM, in contrast to the heterogeneous phase with insoluble LGM like MCH. In the case of using insoluble or slightly soluble LGMs to make sH hydrate, the stirring energy to mix water and LGMs is indispensable during the hydrate formation period. Nevertheless, we cannot avoid the noticeable appearance of sI phases as shown in Figure 4a and 4b (brown curve). Sample preparation is conducted at 263.15 K and 50 bar, where both sI and sH coexisting phases can be easily made. Nevertheless, the suggested formers can form only sH phase because of their high water solubility and homogeneous dispersion in solution. Although the MCH is the well-known sH former, the sI phase can be made together with the sH one because of their pretty low water solubility and undesirably
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remaining excess water. This excess water makes the sI phase under higher pressure conditions than the sI phase equilibrium. In this experiment with MCH hydrate the area ratio of sI-L and (sH-S +sH-M) is determined to be about 0.313 through peak deconvolution and integration. The sI-S peak at 4.1 ppm was apparently overlapped with peaks of sH phases, and thus, the area ratio of sI and sH can be increased up to 0.644. Therefore, at least 30% of sI phase forms together with sH phase. Peak deconvolution and calculation can be seen in the Supporting Information. Subsequently, Raman spectra of the above samples were checked (Figure 4b). A peak at 2913 cm 1 is exactly matched with that of ref 1 representing sH hydrate phase. The MCH hydrate formed sH phase seriously contaminated with sI phase (2905 cm 1), as confirmed in the NMR spectra. From NMR and Raman spectra, it is shown that hexamethyleneimine and methyl-substituted piperidines can act as powerful sH forming molecules. We measured the solubiliies of suggested formers using Nephelometry (NEPHELOstar Galaxy, BMG Labtech Inc.). Nephelometry is a method measuring solubility using laser scattering. Using this equipment, 4:6 and 6:4 (former:water, volume ratio) mixed solutions were analyzed as freely soluble. Although this method shows only the tendency or rough values of solubility, it is generally accepted in solution chemistry.18 Unfortunately, more exact measuring tools and equipment have not equipped. Also, we cannot find the exact value of formers in any other report, paper, or book except MSDS. The specific solubility values of potential sH formers were searched in MSDS.19 It is of interest that hexamethyleneimine and methylsubstituted piperidine can act as free soluble materials with water. Furthermore, already known sH forming materials are far lower than 1 mol % solubility. Here, we need again to recognize that the only 0.1 g of MCH is dissolved in 1 L of water.19 During sH hydrate formation, this insoluble tendency leads to formation of two serious products of either sI CH4 hydrate (in the case of CH4 overpressurized than the equilibrium condition of sI) or not reacted excess ice or solution (in the case of CH4 underpressurized than the equilibrium condition of sI) phase. It is hard to analyze the distinctive characteristics of sH clathrate hydrates using contaminated ones. Of course, it is natural to form a larger amount of sI when the concentration of formers decreases. As shown in Figure 5, at higher concentrations than stoichiometric 2.9 mol % of sH the sI phase did not appear (bottom 2 lines). With decreasing LGM concentration the sI phase significantly increases. The deficiency of LGM can make a limited quantity of sH; therefore, the excess pure ice or water phase remain. Subsequently, the ice or water phase converts to sI phase under pressurizing conditions with CH4 gas. For one test sample of 0.05 mol % HMI, more than 80% of sI was confirmed among the total hydrate phase through deconvolution and stoichiometric calculations. Meanwhile, we tested 1-methylpiperazine (C5H12N2), which has been reported as a soluble sH former with help gases such as Xe, Kr, CH4, and Ar.20 However, we could observe only sI phase without any coexisting sH phase for both 1-methylpiperazine + methane and 2-methylpiperazine + methane samples (formation conditions in these experiments; 2.9 mol %, 263.15 K, and 100 bar). Please refer to the Supporting Information.
’ CONCLUSION We suggested five types of new sH clathrate hydrate formers: hexamethyleneimine, 1-methylpiperidine, 2-methylpiperidine, 3-methylpiperidine, and 4-methylpiperidine. We note that 18888
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’ ASSOCIATED CONTENT
bS
Supporting Information. XRD patterns and their cell parameters, peak deconvolution and calculation, 13C NMR spectra of methyl-substituted piperazine hydrate. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Research Foundation of the Korea government (MEST) (No. 2010-0029176, WCU program: 31-2008-000-10055-0). This research was also supported by the Ministry of Knowledge Economy through “Recovery/Production of Natural Gas Hydrate using Swapping Technique” (KIGAM Gas Hydrate R&D Organization). The authors would also like to thank the Korea Basic Science Institute (Daegu) for assistance with 600 MHz solid-state NMR.
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