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Incorporation of ammonium fluoride into clathrate hydrate lattices and its significance in inhibiting hydrate formation†
Received 26th February 2015, Accepted 17th April 2015
Seongmin Park,a Dongwook Lim,a Yongwon Seo*b and Huen Lee*a
DOI: 10.1039/c5cc01705j www.rsc.org/chemcomm
The stability of hydrate frameworks is influenced by guest molecules capable of hydrogen bonding with surrounding water molecules. Four remarkable features from the ammonium fluoride incorporation into a crystalline hydrate matrix provide important information on the thermodynamic stability, formation kinetics, structural characteristics, and molecular behavior in clathrate hydrate systems.
Clathrate hydrates have received rather massive attention during the past few decades from both science and engineering fields,1 particularly with respect to gas storage applications,2,3 using CH4 hydrates as a potential energy source,4,5 and deep ocean CO2 sequestration.6,7 Nevertheless, the unexpected formation of gas hydrates during production, transportation, and processing in the oil and gas industry can result in pipeline blockage, which can lead to safety hazards as well as significant financial loss.8–11 Therefore, the petroleum industries are continuously increasing industrial investment and research on this area. Recent structural and computational studies have revealed that some guest molecules form strong hydrogen bonds with the water framework of cubic structure I (sI), cubic structure II (sII) and hexagonal structure H (sH).12–14 These three most common structures exist for gas hydrates predominantly depending on the molecular size of the accommodated guest species.15 Additionally, there have been a few attempts to modify hydrate lattices,16–18 such as by incorporating HF and amino acids into the hydrate crystal lattice or NH3 inclusion as a co-host in ionic clathrate hydrates that contain hydrophobic cations or anions in a counterion incorporated water framework. Recently, Shin et al.19 attempted to form a clathrate hydrate with a solid solution of ammonium fluoride (NH4F) in ice using Xe as a help gas, a
Department of Chemical and Biomolecular Engineering (BK21+ program), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea. E-mail: [email protected]; Fax: +82-42-350-3910; Tel: +82-42-350-3917 b School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea. E-mail: [email protected] † Electronic supplementary information (ESI) available: Hydrate formation kinetics and additional HRPD patterns. See DOI: 10.1039/c5cc01705j
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although an NH4F ion pair was reported to replace two water molecules in some quaternary ammonium salt hydrates.20 Pure NH4F is isostructural with hexagonal ice (ice Ih) and is known to form a solid solution over a limited range of compositions (up to B19 mol% NH4F) with ice-Ih, as well as a monohydrate.21–24 Each NH4F unit replaces two water molecules in the solid solutions and also a canonical clathrate hydrate lattice.19,23 Both NH4+ and F are isoelectronic with water, and thus there is no net change in the number of hydrogen bonds. It has been reported that the stability of hydrate frameworks is influenced by guest molecules capable of hydrogen bonding with surrounding water molecules, such as alcohols, which are wellknown inhibitors of hydrate formation.25,26 In this regard, the presence of strongly hydrogen bonding chemicals in the framework during hydrate formation may also disrupt hydrogen bonds among water molecules and would affect hydrate phase equilibria. As the first attempt, we measured three-phase (hydrate + aqueous liquid + vapor, HLV) equilibrium conditions of NH4F + CH4 hydrates at various concentrations to investigate the thermodynamic stability in relation to a clathrate hydrate incorporated with NH4F. In addition, the formation kinetics of the NH4F + CH4 hydrate was also investigated to evaluate the kinetic inhibition effect. Moreover, structural characteristics and molecular behavior were identified by using high resolution powder diffraction (HRPD) analysis, solid-state 13C NMR, and Raman spectroscopy. Here, four remarkable features of NH4F incorporation into the hydrate crystal lattice are observed: (1) a superior thermodynamic inhibition effect for CH4 hydrates; (2) a kinetic inhibition effect by prolonging the total delay time before the catastrophic growth process; (3) enhancement of structural conversion from ice to hydrate; and (4) variation of the cage occupancy ratio depending on lattice parameters. To investigate the thermodynamic effect of ammonium fluoride (NH4F) on the hydrate-phase equilibria, the three-phase (HLV) equilibria were measured using CH4 as the guest species. Unexpectedly, the thermodynamic inhibition effect of NH4F is better than that of methanol, which is considered to be the most effective thermodynamic hydrate inhibitor (THI) based on weight percent.27,28
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The kinetics of hydrate formation is much less understood than the thermodynamics. Recent studies suggest that the hydrate formation process in the presence of a kinetic hydrate inhibitor (KHI) may be separated into three main stages: (1) nucleation time, (2) slow growth period, and (3) a catastrophic fast growth period.30–32 Fig. 1b shows the CH4 gas uptake, as monitored by the pressure drop and temperature records during hydrate formation in the case of a 5.0 mol% NH4F, DT (subcooling) = 10.0 K system. When the temperature was set to a value below the hydrate-forming temperature, the pressure gradually dropped due to gas consumption by hydrate formation. The time to the sudden pressure drop after the nucleation time is called the slow growth period. A rapid, almost vertical pressure drop due to catastrophic hydrate formation then occurred at around 40 hours. NH4F showed kinetic inhibition effects compared with the pure CH4 hydrate and methanol containing system subcooled by 10.0 K (Fig. 1c and Fig. S1, ESI†). In contrast with NH4F, recent experimental studies showed that methanol has a promotional kinetic effect on the formation of gas hydrates from ice.33,34 This is an important finding as the slow growth caused by NH4F incorporation can be beneficial to manage the hydrate formation kinetics. In order to identify the crystal structures of the hydrate samples, high-resolution powder diffraction (HRPD) patterns of the samples were collected (Fig. 2a), and they revealed two important aspects. After indexing and analyzing the HRPD patterns, all the hydrate samples were confirmed to be structure I cubic Pm3n, whereas the lattice parameter of CH4 hydrate incorporated with NH4F decreased depending on the NH4F
Fig. 1 (a) Three phase (hydrate + aqueous liquid + vapor, HLV) equilibria of CH4 hydrate in the presence of NH4F and methanol (b) typical temperature and pressure records during hydrate formation (system with 5.0 mol% NH4F, T = 270.2 K, DT (sub-cooling) = 10.0 K) (c) pure CH4 hydrate (system with T = 275.6 K, DT (sub-cooling) = 10.0 K).
As can be seen in Fig. 1a, the hydrate-phase equilibrium curve with 5.0 mol% (9.8 wt%) showed a decrease of the dissociation temperature by 4.7–5.1 K at a given pressure compared with that for the pure CH4 hydrate. In addition, adding more NH4F to the aqueous phase up to concentrations of 10.0 mol% (18.6 wt%) and 15.0 mol% (26.6 wt%) shifted the hydrate-phase equilibrium curve further to the inhibition region, which is about 10.3–10.4 K and 14.4–14.7 K, respectively. In order to compare the thermodynamic inhibition effect by methanol on the threephase equilibria of the CH4 hydrate with NH4F, methanol was used at a concentration of 26.6 wt% and consequently showed a low inhibition effect at the same concentration. Furthermore, NH4F was found to inhibit the hydrate system more than a well-known salt, sodium chloride (NaCl),29 which produces both an anion and a cation upon dissolution.
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Fig. 2 (a) HRPD patterns of the NH4F + CH4 hydrates and pure CH4 hydrate recorded at ambient pressure and 80 K. Blue arrows indicate reflections from the hexagonal ice-Ih phase. (b) Schematic illustration of NH4F incorporation into the clathrate hydrate lattice.
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concentration, compared with the pure CH4 hydrate. Most of the peaks except hexagonal ice peaks (denoted by blue arrows) were slightly shifted to higher diffraction angle values as the concentration of NH4F increased, as shown in Fig. S2 (ESI†) (enlarged region from 24.01 to 31.01). The lattice parameter calculations after indexing the HRPD pattern for each sample are a = 11.875 Å for pure CH4 hydrate, a = 11.841 Å for NH4F 5.0 mol%, a = 11.805 Å for NH4F 10.0 mol%, and a = 11.782 Å for NH4F 15.0 mol%, respectively. Furthermore, a solid solution including 15.0 mol% NH4F is isostructural with pure ice-Ih in the space group P63/mmc, whereas the lattice parameter is different (Fig. S3, ESI†). Hydrogen bonding in the two solid phases is comparable, with slightly different hydrogen bond lengths (N–H F, 2.71 Å and O–H O, 2.75 Å). Similarly, the lattice constants of the clathrate hydrate composed of N and F substituting for some O in the host framework tend to decrease in proportion to the degree of NH4F incorporation (Fig. 2b). Second, structural conversion from ice to hydrate increases along with NH4F concentration. In general, the gas hydrate samples made from pure ice powders show a large amount of the ice-Ih phase in the X-ray diffraction patterns. But adding more NH4F to the aqueous phase tends to decrease the ice phase amount. It was identified that PVP, a well known KHI, promotes water conversion to hydrate.35 It prevents occluded water by slowing the growth rate and changing the crystal morphology. Besides, methanol and ammonia showed almost complete conversion to hydrate with little residual ice.34 Bjerrum defects in the cages caused by hydrogen bonds with methanol molecules facilitate the entry of methane molecules into the ice lattice. Similarly, it is speculated that rearrangement of randomly distributed ions in the framework influences the hydrate crystal morphology and lattice defects, resulting in an increase of structural conversion, although the rearrangement processes in the hydrogen-bonded network systems is not fully understood.36 The crystal structure and molecular behaviors were also confirmed by solid-state 13C NMR and Raman spectroscopy. As shown in Fig. 3, the atomic signals at around 3.9 ppm and 6.2 ppm correspond to CH4 molecules in small cages (512, S) and large cages (51262, L) of the sI hydrates, respectively. The CH4 signals in the hydrate cage gradually broaden and the chemical shift increases along with the NH4F concentration. This indicates that the CH4 molecule resides in a cavity composed of different numbers and combinations of ions replacing water molecules, giving rise to a change in the local environment. The general trend of increasing shifts reflects the decrease in the lattice parameter. Since the van der Waals radius decreases in the order N (1.55 Å) 4 O (1.52 Å) 4 F (1.47 Å), lower shifts arise from cages with excess F and the higher shifts arise from cages with excess NH4+, although individual peaks are not resolved.19 Also, the relative area of the NMR peaks represents large cage and small cage populations of the hydrate. The occupancy of CH4 in sI-L is almost complete, while it varies in the sI-S from 0.75 to 0.99.37 The peak area ratio of sI-L to sI-S divided by three gives the real cage occupancy ratio (yL,CH4/yS,CH4),38 because there are three times as many large cages compared to small cages in the sI unit cell, as the sI consists of two small cages (512) and six large cages (51262) per unit cell. The cage occupancy ratio of the CH4 hydrate is 1.11, 1.24, 1.30, and 1.42 at a
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Fig. 3 (a) Solid-state 13C NMR spectra of CH4 hydrate with various NH4F concentrations. Peak deconvolution and area calculation along with NH4F concentration (b) 0 mol% (c) 5 mol% (d) 10 mol% and (e) 15 mol%.
concentration of 0, 5, 10, and 15 mol% NH4F, respectively (Table S1, ESI†). The cage occupancy ratio (yL,CH4/yS,CH4) increases as the NH4F concentration increases, which is caused by the decrease of the lattice parameter and cage distortion resulting in a decrease of small cage occupancy. Peak integration for each area was performed using OriginPro 8 (software). Fig. 4 shows the Raman spectra of a solid solution and CH4 hydrate containing 10.0 and 15.0 mol% NH4F in the region of 1500–2500 cm 1. Signals appearing at 1736 cm 1 are assigned to a symmetric bending mode n2 (E) of NH4+. Additionally, strong bands in the Raman spectrum appearing at around 2226 and 1980 cm 1 for the samples cannot be due to fundamentals and are ascribed to combinations involving n2 and n4 (asymmetric bending mode, T2) with the unobserved librational modes (nL) based on numerous Raman studies.39–42 It was reported that the average hydrogen bond strength in the sI clathrate is weaker than that in ice-Ih.43 The half-width of the line is also considerably broader in the clathrate than in ice, due to the much larger distribution of hydrogen bond lengths.
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Notes and references
Fig. 4 Raman spectra of solid solution and CH4 hydrate containing 10.0 and 15.0 mol% NH4F in the region of 1500–2500 cm 1.
In addition, there is an intensity difference in the Raman spectra of ice Ih and sI hydrates at the intermolecular hydrogen bonded stretching band region.44 Consequently, the Raman spectrum of the CH4 hydrate incorporated with NH4F into the host framework shows distinct relative peak intensity and Raman shifts compared with a solid solution of ice – NH4F. NH4F incorporation induced structural modification showed improved thermodynamic inhibition effects of CH4 hydrate even compared with methanol at the same concentration level, while it also has a kinetic effect by prolonging the total delay time before a catastrophic growth process, in contrast with methanol. The thermodynamic and kinetic inhibition effects of NH4F are caused by the interaction between two ions (NH4+ and F ) and water molecules by hydrogen bonding. Accordingly, the use of NH4F as the most effective hydrate inhibitor is expected to open a new phase in the flow assurance field. Furthermore, we found that incorporating NH4F into the framework enhances the conversion from ice to hydrate. It is speculated that rearrangement of randomly distributed ions in the framework influences the hydrate crystal morphology and lattice defects, resulting in an increase of the structural conversion rate. The present findings on the co-host role in the thermodynamic stability and lattice shrinkage via restructuring the host water framework might provide important information on crystallization behavior of clathrate hydrates. This research was supported by a grant from the National Research Foundation Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (MSIP) (No. 2014-049237) and by the Ministry of Trade, Industry & Energy (MOTIE) through the ‘‘Recovery of Natural Gas Hydrate in Deep-Sea Sediments Using Carbon Dioxide and Nitrogen Injection’’ project [KIGAMGas Hydrate R & D Organization]. HRPD experiments at PLS (Beamline 9B) were supported by POSTECH.
8764 | Chem. Commun., 2015, 51, 8761--8764
1 A. K. Sum, C. A. Koh and E. D. Sloan, Ind. Eng. Chem. Res., 2009, 48, 7457–7465. 2 W. Hao, J. Wang, S. Fan and W. Hao, Energy Convers. Manage., 2008, 49, 2546–2553. 3 X. Lang, S. Fan and Y. Wang, J. Nat. Gas Chem., 2010, 19, 203–209. 4 S.-Y. Lee and G. D. Holder, Fuel Process. Technol., 2001, 71, 181–186. 5 Y. F. Makogon, J. Nat. Gas Sci. Eng., 2010, 2, 49–59. 6 H. Teng, A. Yamasaki, M.-K. Chun and H. Lee, Energy, 1997, 22, 1111–1117. 7 S. Lee, L. Liang, D. Riestenberg, O. R. West, C. Tsouris and E. Adams, Environ. Sci. Technol., 2003, 37, 3701–3708. 8 E. D. Sloan, Fluid Phase Equilib., 2005, 228, 67–74. 9 M. A. Kelland, Energy Fuels, 2006, 20(3), 825–847. 10 E. D. Sloan, C. Koh, A. Sum, A. L. Ballard, J. Creek, M. Eaton, J. Lachance, N. McMullen, T. Palermo, G. Shoup and L. Talley, Natural Gas Hydrates in Flow Assurance, Gulf Professional Publishing, Houston, TX, 2011. 11 A. Perrin, O. M. Musa and J. W. Steed, Chem. Soc. Rev., 2013, 42, 1996–2015. 12 S. Alavi, R. Susilo and J. A. Ripmeester, J. Chem. Phys., 2009, 130, 174501. 13 S. Alavi, K. Udachin and J. A. Ripmeester, Chem. – Eur. J., 2010, 16, 1017–1025. 14 R. Susilo, S. Alavi, I. L. Moudrakovski, P. Englezos and J. A. Ripmeester, ChemPhysChem, 2009, 10, 824–829. 15 E. D. Sloan and C. A. Koh, Clathrate Hydrates of Natural Gases, CRC Press, Taylor & Francis Group, New York, 3rd edn, 2008. 16 D. Mootz, E.-J. Oellers and M. Wiebcke, J. Am. Chem. Soc., 1987, 109, 1200–1202. 17 K. Shin, S. Choi, J.-H. Cha and H. Lee, J. Am. Chem. Soc., 2008, 130, 7180–7181. 18 J.-H. Sa, G.-H. Kwak, B. Lee, D. Ahn and K.-H. Lee, Phys. Chem. Chem. Phys., 2014, 16, 26730–26734. 19 K. Shin, I. L. Moudrakovski, M. D. Davari, S. Alavi, C. I. Ratcliffe and J. A. Ripmeester, CrystEngComm, 2014, 16, 7209–7217. 20 L. S. Aladko and Y. A. Dyadin, J. Struct. Chem., 1994, 35, 859–864. 21 R. Brill and S. Zaromb, Nature, 1954, 173, 316–317. 22 A. K. Lyashchenko and G. G. Malenkov, J. Struct. Chem., 1969, 10, 616–617. 23 C. G. van Beek, J. Overeem, J. R. Ruble and B. M. Craven, Can. J. Chem., 1992, 33, 694–701. 24 L. C. Labowitz and E. F. Westrum, J. Phys. Chem., 1961, 65, 408–414. 25 S. Alavi, S. Takeya, R. Ohmura, T. K. Woo and J. A. Ripmeester, J. Chem. Phys., 2010, 133, 074505. 26 S. Park, H. Ro, J.-W. Lee, H. Kang and H. Lee, Energy Fuels, 2013, 27, 6581–6586. 27 H.-J. Ng and D. B. Robinson, Fluid Phase Equilib., 1985, 21, 145–155. 28 T. Nakamura, T. Makino, T. Sugahara and K. Ohgaki, Chem. Eng. Sci., 2003, 58, 269–273. 29 R. Kobayashi, H. J. Withrow, G. B. Williams and D. L. Katz, Proc., Annu. Conv., Nat. Gasoline Assoc. Am., Tech. Pap., 1951, 27–31. 30 T. M. Svartaas, M. A. Kelland and L. Dybvik, Ann. N. Y. Acad. Sci., 2000, 912, 744–752. 31 H. Ajiro, Y. Takemoto, M. Akashi, P. C. Chua and M. A. Kelland, Energy Fuels, 2010, 24, 6400–6410. 32 M. Cha, K. Shin, Y. Seo, J.-Y. Shin and S.-P. Kang, J. Phys. Chem. A, 2013, 117, 13988–13995. 33 S. Bobev and K. T. Tait, Am. Mineral., 2004, 89, 1208–1214. 34 G. McLaurin, K. Shin, S. Alavi and J. A. Ripmeester, Angew. Chem., Int. Ed., 2014, 53, 10429–10433. 35 A. A. Khokhar, J. S. Gudmundsson and E. D. Sloan, Fluid Phase Equilib., 1998, 150–151, 383–392. 36 K. Koga and H. Tanaka, J. Chem. Phys., 1996, 104, 263–272. 37 P. Kumar, H. C. Das, K. Anbazhagan, H. Lu and J. A. Ripmeester, Proceeding of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, Canada, 2008. 38 H. Lu, I. Moudrakovski, M. Riedel, G. Spence, R. Durrisa, J. Ripmeester, F. Wright and S. Dallimore, J. Geophys. Res., 2005, 110, B10204. 39 H. F. Shurvell, R. J. C. Brown, P. M. Fredericks and L. Rintoul, J. Raman Spectrosc., 2001, 32, 219–226. 40 A. Alavi, R. M. Lynden-Bell and R. J. C. Brown, J. Raman Spectrosc., 2001, 32, 996–999. 41 J. R. Durig and D. J. Antion, Appl. Spectrosc., 1970, 24, 16–20. 42 R. C. Plumb and D. F. Hornig, J. Chem. Phys., 1955, 23, 947–953. 43 C. A. Tulk, J. A. Ripmeester and D. D. Klug, Ann. N. Y. Acad. Sci., 2000, 912, 859–872. 44 J. M. Schicks, J. Erzinger and M. A. Ziemann, Spectrochim. Acta, Part A, 2005, 61, 2399–2403.
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Incorporation of ammonium fluoride into clathrate hydrate lattices and its significance in inhibiting hydrate formation†
Received 26th February 2015, Accepted 17th April 2015
Seongmin Park,a Dongwook Lim,a Yongwon Seo*b and Huen Lee*a
DOI: 10.1039/c5cc01705j www.rsc.org/chemcomm
The stability of hydrate frameworks is influenced by guest molecules capable of hydrogen bonding with surrounding water molecules. Four remarkable features from the ammonium fluoride incorporation into a crystalline hydrate matrix provide important information on the thermodynamic stability, formation kinetics, structural characteristics, and molecular behavior in clathrate hydrate systems.
Clathrate hydrates have received rather massive attention during the past few decades from both science and engineering fields,1 particularly with respect to gas storage applications,2,3 using CH4 hydrates as a potential energy source,4,5 and deep ocean CO2 sequestration.6,7 Nevertheless, the unexpected formation of gas hydrates during production, transportation, and processing in the oil and gas industry can result in pipeline blockage, which can lead to safety hazards as well as significant financial loss.8–11 Therefore, the petroleum industries are continuously increasing industrial investment and research on this area. Recent structural and computational studies have revealed that some guest molecules form strong hydrogen bonds with the water framework of cubic structure I (sI), cubic structure II (sII) and hexagonal structure H (sH).12–14 These three most common structures exist for gas hydrates predominantly depending on the molecular size of the accommodated guest species.15 Additionally, there have been a few attempts to modify hydrate lattices,16–18 such as by incorporating HF and amino acids into the hydrate crystal lattice or NH3 inclusion as a co-host in ionic clathrate hydrates that contain hydrophobic cations or anions in a counterion incorporated water framework. Recently, Shin et al.19 attempted to form a clathrate hydrate with a solid solution of ammonium fluoride (NH4F) in ice using Xe as a help gas, a
Department of Chemical and Biomolecular Engineering (BK21+ program), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea. E-mail: [email protected]; Fax: +82-42-350-3910; Tel: +82-42-350-3917 b School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea. E-mail: [email protected] † Electronic supplementary information (ESI) available: Hydrate formation kinetics and additional HRPD patterns. See DOI: 10.1039/c5cc01705j
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although an NH4F ion pair was reported to replace two water molecules in some quaternary ammonium salt hydrates.20 Pure NH4F is isostructural with hexagonal ice (ice Ih) and is known to form a solid solution over a limited range of compositions (up to B19 mol% NH4F) with ice-Ih, as well as a monohydrate.21–24 Each NH4F unit replaces two water molecules in the solid solutions and also a canonical clathrate hydrate lattice.19,23 Both NH4+ and F are isoelectronic with water, and thus there is no net change in the number of hydrogen bonds. It has been reported that the stability of hydrate frameworks is influenced by guest molecules capable of hydrogen bonding with surrounding water molecules, such as alcohols, which are wellknown inhibitors of hydrate formation.25,26 In this regard, the presence of strongly hydrogen bonding chemicals in the framework during hydrate formation may also disrupt hydrogen bonds among water molecules and would affect hydrate phase equilibria. As the first attempt, we measured three-phase (hydrate + aqueous liquid + vapor, HLV) equilibrium conditions of NH4F + CH4 hydrates at various concentrations to investigate the thermodynamic stability in relation to a clathrate hydrate incorporated with NH4F. In addition, the formation kinetics of the NH4F + CH4 hydrate was also investigated to evaluate the kinetic inhibition effect. Moreover, structural characteristics and molecular behavior were identified by using high resolution powder diffraction (HRPD) analysis, solid-state 13C NMR, and Raman spectroscopy. Here, four remarkable features of NH4F incorporation into the hydrate crystal lattice are observed: (1) a superior thermodynamic inhibition effect for CH4 hydrates; (2) a kinetic inhibition effect by prolonging the total delay time before the catastrophic growth process; (3) enhancement of structural conversion from ice to hydrate; and (4) variation of the cage occupancy ratio depending on lattice parameters. To investigate the thermodynamic effect of ammonium fluoride (NH4F) on the hydrate-phase equilibria, the three-phase (HLV) equilibria were measured using CH4 as the guest species. Unexpectedly, the thermodynamic inhibition effect of NH4F is better than that of methanol, which is considered to be the most effective thermodynamic hydrate inhibitor (THI) based on weight percent.27,28
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The kinetics of hydrate formation is much less understood than the thermodynamics. Recent studies suggest that the hydrate formation process in the presence of a kinetic hydrate inhibitor (KHI) may be separated into three main stages: (1) nucleation time, (2) slow growth period, and (3) a catastrophic fast growth period.30–32 Fig. 1b shows the CH4 gas uptake, as monitored by the pressure drop and temperature records during hydrate formation in the case of a 5.0 mol% NH4F, DT (subcooling) = 10.0 K system. When the temperature was set to a value below the hydrate-forming temperature, the pressure gradually dropped due to gas consumption by hydrate formation. The time to the sudden pressure drop after the nucleation time is called the slow growth period. A rapid, almost vertical pressure drop due to catastrophic hydrate formation then occurred at around 40 hours. NH4F showed kinetic inhibition effects compared with the pure CH4 hydrate and methanol containing system subcooled by 10.0 K (Fig. 1c and Fig. S1, ESI†). In contrast with NH4F, recent experimental studies showed that methanol has a promotional kinetic effect on the formation of gas hydrates from ice.33,34 This is an important finding as the slow growth caused by NH4F incorporation can be beneficial to manage the hydrate formation kinetics. In order to identify the crystal structures of the hydrate samples, high-resolution powder diffraction (HRPD) patterns of the samples were collected (Fig. 2a), and they revealed two important aspects. After indexing and analyzing the HRPD patterns, all the hydrate samples were confirmed to be structure I cubic Pm3n, whereas the lattice parameter of CH4 hydrate incorporated with NH4F decreased depending on the NH4F
Fig. 1 (a) Three phase (hydrate + aqueous liquid + vapor, HLV) equilibria of CH4 hydrate in the presence of NH4F and methanol (b) typical temperature and pressure records during hydrate formation (system with 5.0 mol% NH4F, T = 270.2 K, DT (sub-cooling) = 10.0 K) (c) pure CH4 hydrate (system with T = 275.6 K, DT (sub-cooling) = 10.0 K).
As can be seen in Fig. 1a, the hydrate-phase equilibrium curve with 5.0 mol% (9.8 wt%) showed a decrease of the dissociation temperature by 4.7–5.1 K at a given pressure compared with that for the pure CH4 hydrate. In addition, adding more NH4F to the aqueous phase up to concentrations of 10.0 mol% (18.6 wt%) and 15.0 mol% (26.6 wt%) shifted the hydrate-phase equilibrium curve further to the inhibition region, which is about 10.3–10.4 K and 14.4–14.7 K, respectively. In order to compare the thermodynamic inhibition effect by methanol on the threephase equilibria of the CH4 hydrate with NH4F, methanol was used at a concentration of 26.6 wt% and consequently showed a low inhibition effect at the same concentration. Furthermore, NH4F was found to inhibit the hydrate system more than a well-known salt, sodium chloride (NaCl),29 which produces both an anion and a cation upon dissolution.
8762 | Chem. Commun., 2015, 51, 8761--8764
Fig. 2 (a) HRPD patterns of the NH4F + CH4 hydrates and pure CH4 hydrate recorded at ambient pressure and 80 K. Blue arrows indicate reflections from the hexagonal ice-Ih phase. (b) Schematic illustration of NH4F incorporation into the clathrate hydrate lattice.
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concentration, compared with the pure CH4 hydrate. Most of the peaks except hexagonal ice peaks (denoted by blue arrows) were slightly shifted to higher diffraction angle values as the concentration of NH4F increased, as shown in Fig. S2 (ESI†) (enlarged region from 24.01 to 31.01). The lattice parameter calculations after indexing the HRPD pattern for each sample are a = 11.875 Å for pure CH4 hydrate, a = 11.841 Å for NH4F 5.0 mol%, a = 11.805 Å for NH4F 10.0 mol%, and a = 11.782 Å for NH4F 15.0 mol%, respectively. Furthermore, a solid solution including 15.0 mol% NH4F is isostructural with pure ice-Ih in the space group P63/mmc, whereas the lattice parameter is different (Fig. S3, ESI†). Hydrogen bonding in the two solid phases is comparable, with slightly different hydrogen bond lengths (N–H F, 2.71 Å and O–H O, 2.75 Å). Similarly, the lattice constants of the clathrate hydrate composed of N and F substituting for some O in the host framework tend to decrease in proportion to the degree of NH4F incorporation (Fig. 2b). Second, structural conversion from ice to hydrate increases along with NH4F concentration. In general, the gas hydrate samples made from pure ice powders show a large amount of the ice-Ih phase in the X-ray diffraction patterns. But adding more NH4F to the aqueous phase tends to decrease the ice phase amount. It was identified that PVP, a well known KHI, promotes water conversion to hydrate.35 It prevents occluded water by slowing the growth rate and changing the crystal morphology. Besides, methanol and ammonia showed almost complete conversion to hydrate with little residual ice.34 Bjerrum defects in the cages caused by hydrogen bonds with methanol molecules facilitate the entry of methane molecules into the ice lattice. Similarly, it is speculated that rearrangement of randomly distributed ions in the framework influences the hydrate crystal morphology and lattice defects, resulting in an increase of structural conversion, although the rearrangement processes in the hydrogen-bonded network systems is not fully understood.36 The crystal structure and molecular behaviors were also confirmed by solid-state 13C NMR and Raman spectroscopy. As shown in Fig. 3, the atomic signals at around 3.9 ppm and 6.2 ppm correspond to CH4 molecules in small cages (512, S) and large cages (51262, L) of the sI hydrates, respectively. The CH4 signals in the hydrate cage gradually broaden and the chemical shift increases along with the NH4F concentration. This indicates that the CH4 molecule resides in a cavity composed of different numbers and combinations of ions replacing water molecules, giving rise to a change in the local environment. The general trend of increasing shifts reflects the decrease in the lattice parameter. Since the van der Waals radius decreases in the order N (1.55 Å) 4 O (1.52 Å) 4 F (1.47 Å), lower shifts arise from cages with excess F and the higher shifts arise from cages with excess NH4+, although individual peaks are not resolved.19 Also, the relative area of the NMR peaks represents large cage and small cage populations of the hydrate. The occupancy of CH4 in sI-L is almost complete, while it varies in the sI-S from 0.75 to 0.99.37 The peak area ratio of sI-L to sI-S divided by three gives the real cage occupancy ratio (yL,CH4/yS,CH4),38 because there are three times as many large cages compared to small cages in the sI unit cell, as the sI consists of two small cages (512) and six large cages (51262) per unit cell. The cage occupancy ratio of the CH4 hydrate is 1.11, 1.24, 1.30, and 1.42 at a
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Fig. 3 (a) Solid-state 13C NMR spectra of CH4 hydrate with various NH4F concentrations. Peak deconvolution and area calculation along with NH4F concentration (b) 0 mol% (c) 5 mol% (d) 10 mol% and (e) 15 mol%.
concentration of 0, 5, 10, and 15 mol% NH4F, respectively (Table S1, ESI†). The cage occupancy ratio (yL,CH4/yS,CH4) increases as the NH4F concentration increases, which is caused by the decrease of the lattice parameter and cage distortion resulting in a decrease of small cage occupancy. Peak integration for each area was performed using OriginPro 8 (software). Fig. 4 shows the Raman spectra of a solid solution and CH4 hydrate containing 10.0 and 15.0 mol% NH4F in the region of 1500–2500 cm 1. Signals appearing at 1736 cm 1 are assigned to a symmetric bending mode n2 (E) of NH4+. Additionally, strong bands in the Raman spectrum appearing at around 2226 and 1980 cm 1 for the samples cannot be due to fundamentals and are ascribed to combinations involving n2 and n4 (asymmetric bending mode, T2) with the unobserved librational modes (nL) based on numerous Raman studies.39–42 It was reported that the average hydrogen bond strength in the sI clathrate is weaker than that in ice-Ih.43 The half-width of the line is also considerably broader in the clathrate than in ice, due to the much larger distribution of hydrogen bond lengths.
Chem. Commun., 2015, 51, 8761--8764 | 8763
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Notes and references
Fig. 4 Raman spectra of solid solution and CH4 hydrate containing 10.0 and 15.0 mol% NH4F in the region of 1500–2500 cm 1.
In addition, there is an intensity difference in the Raman spectra of ice Ih and sI hydrates at the intermolecular hydrogen bonded stretching band region.44 Consequently, the Raman spectrum of the CH4 hydrate incorporated with NH4F into the host framework shows distinct relative peak intensity and Raman shifts compared with a solid solution of ice – NH4F. NH4F incorporation induced structural modification showed improved thermodynamic inhibition effects of CH4 hydrate even compared with methanol at the same concentration level, while it also has a kinetic effect by prolonging the total delay time before a catastrophic growth process, in contrast with methanol. The thermodynamic and kinetic inhibition effects of NH4F are caused by the interaction between two ions (NH4+ and F ) and water molecules by hydrogen bonding. Accordingly, the use of NH4F as the most effective hydrate inhibitor is expected to open a new phase in the flow assurance field. Furthermore, we found that incorporating NH4F into the framework enhances the conversion from ice to hydrate. It is speculated that rearrangement of randomly distributed ions in the framework influences the hydrate crystal morphology and lattice defects, resulting in an increase of the structural conversion rate. The present findings on the co-host role in the thermodynamic stability and lattice shrinkage via restructuring the host water framework might provide important information on crystallization behavior of clathrate hydrates. This research was supported by a grant from the National Research Foundation Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (MSIP) (No. 2014-049237) and by the Ministry of Trade, Industry & Energy (MOTIE) through the ‘‘Recovery of Natural Gas Hydrate in Deep-Sea Sediments Using Carbon Dioxide and Nitrogen Injection’’ project [KIGAMGas Hydrate R & D Organization]. HRPD experiments at PLS (Beamline 9B) were supported by POSTECH.
8764 | Chem. Commun., 2015, 51, 8761--8764
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