Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Role of Salts in Phase Transformation of Clathrate Hydrates under Brine Environments Donghoon Shin,† Jong-Won Lee,‡ Yesol Woo,§ Minjun Cha,∥ Yongjae Lee,⊥ Seen Ae Chae,# Sun Ha Kim,# Oc Hee Han,*,#,¶,⊗ and Ji-Ho Yoon*,†,§ †
Department of Energy and Resources Engineering, Korea Maritime and Ocean University, 727 Taejong-ro, Yeongdo-gu, Busan 606-791, Korea ‡ Department of Environmental Engineering, Kongju National University, 56 Gongjudaehak-ro, Gongju-si, Chungnam 31080, Korea § Department of Convergence Study on the Ocean Science and Technology, Ocean Science and Technology (OST) School, Korea Maritime and Ocean University, 727 Taejong-ro, Yeongdo-gu, Busan 606-791, Korea ∥ Department of Energy and Resources Engineering, Kangwon National University, 1 Kangwondaehak-gil, Chuncheon-si, Gangwon 24341, Korea ⊥ Department of Earth System Sciences, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea # Western Seoul Center, Korea Basic Science Institute, 150 Bugahyeon-ro, Seodaemun-gu, Seoul 03759, Korea ¶ Graduate School of Analytical Science & Technology, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Korea ⊗ Department of Chemistry & Nano-Science, College of Natural Sciences, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Korea S Supporting Information *
ABSTRACT: Although ion exclusion is a naturally occurring and commonly observed phenomenon in clathrate hydrates, an understanding for the effect of salt ions on the stability of clathrate hydrates is still unclear. Here we report the first observation of phase transformation of structure I and structure II clathrate hydrates using solid-state 13C, 19F, and 23Na magicangle spinning nuclear magnetic resonance (NMR) spectroscopy, combined with X-ray diffraction and Raman spectroscopy. The phase transformation of clathrate hydrates in salt environments is found to be closely associated with the quadruple point of clathrate hydrate/hydrated salts and the eutectic point of ice/hydrated salts. The formation of the quasibrine layer (QBL) is triggered at temperatures a little lower than the eutectic point, where an increasing salinity and QBL does not affect the stability of clathrate hydrates. However, at temperatures above the eutectic point, all hydrated salts and the QBL melt completely to form brine solutions, destabilizing the clathrate hydrate structures. Temperature-dependent in situ NMR spectroscopy under pressure also allows us to directly detect the quadruple point of clathrate hydrates in salt environments, which has been determined only by visual observations. KEYWORDS: Clathrate hydrate, Salts, Phase transformation, Eutectic transition, Quasi-brine layer
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INTRODUCTION Salt is an effective antifreeze that suppresses the formation of ice by altering the chemical potential of water in aqueous solutions. Similar effects hold for clathrate hydrates, which are ice-like crystalline inclusion compounds formed by the interaction between host water and relatively small guest molecules, leading to changes in the clathrate hydrate stability and its properties.1−3 During the past 50 years, it has been understood that when clathrate hydrates form in brine solutions, ion exclusion occurs in order not to alter the clathrate hydrate structure. However, recent studies have demonstrated that salt ions can be incorporated into the © XXXX American Chemical Society
cages of clathrate hydrate when forming in briny environments.4,5 This implies that the methane amount contained in natural gas hydrate (NGH) deposits on the ocean floor should be re-evaluated, signaling changes in the global warming scenario by release of methane from NGH.3,6 The evidence for ion exclusion behavior is, however, still unclear, and more detailed experiments are required. Received: December 8, 2017 Revised: February 5, 2018 Published: February 14, 2018 A
DOI: 10.1021/acssuschemeng.7b04645 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
was determined using the CheckCell software25 and the GSAS program.26,27 Nuclear Magnetic Resonance (NMR) Spectroscopy. Solidstate 23Na magic angle spinning (MAS) NMR spectra were acquired on a Varian INOVA 600 MHz spectrometer at a 14.1 T magnetic field using a 4 mm zirconia rotor. All hydrate samples were packed in the rotors under a liquid nitrogen environment, and the spectra were acquired at a radio frequency of 158.706 MHz using 45° pulse length of 2.5 μs, 8 s pulse repetition delay time, 10 kHz spinning rate, and an external chemical shift reference of 1 M NaCl aqueous solution. Solidstate 19F MAS and 13C MAS NMR spectra were acquired on a Bruker AVANCE II+ 400 MHz spectrometer at a 9.4 T magnetic field using a 4 mm zirconia rotor. The 19F NMR experiments were performed at a radio frequency of 376.47 MHz with a 2.5 kHz spinning rate, 30° pulse width of length of 2 μs, and 3 s pulse repetition delay time. The central position at −69.5 ppm of 4 resonance peaks for LiAsF6 was used as an external chemical shift reference. The 13C MAS NMR spectra were obtained at a Larmor frequency of 100.6 MHz under proton decoupling with a 7 kHz spinning rate, 30° pulse length of 1.6 μs, and 3 s pulse repetition delay time. The downfield shifted carbon resonance peak of adamantane, appearing at 38.56 ppm relative to tetramethylsilane, was used as an external chemical shift reference. In situ temperature-dependent 19F and 23Na MAS NMR experiments were performed using the Bruker 400 MHz spectrometer with spinning rates of 2.5 and 7 kHz, respectively. Several preliminary tests with fluid R22 and R22 clathrate hydrate samples enabled us to confirm the pressure resistance of the rotor materials up to 10 bar. During the temperature-dependent NMR measurements, a temperature control unit with a resolution of ±0.5 K was used without any modification. At each temperature, the equilibrium conditions were confirmed by invariant NMR signals for 30 min. Raman Spectroscopy. A customized Raman spectrometer emitting a 532 nm line of an Nd:YAG laser (Excelsior) with a maximum power of 150 mW was used. The size of the laser spot incident to the samples was ∼20 μm. The combination of a spectrograph (SpectraPro) and a multichannel air-cooled CCD detector (Princeton Instruments) provided a spectral resolution of ∼1.5 cm−1. Control of the sample temperature during Raman measurements was conducted using a temperature-controlled microscope stage (Linkam) with a stability of
