Ji-Ho Yoon

Thermodynamic Stability, Spectroscopic Identification, and Gas Storage Capacity of CO2–CH4–N2 Mixture Gas Hydrates: Implications for Landfill Gas Hydrates

Landfill gas (LFG), which is primarily composed of CH(4), CO(2), and N(2), is produced from the anaerobic digestion of organic materials. To investigate the feasibility of the storage and transportation of LFG via the formation of hydrate, we observed the phase equilibrium behavior of CO(2)-CH(4)-N(2) mixture hydrates. When the specific molar ratio of CO(2)/CH(4) was 40/55, the equilibrium dissociation pressures were gradually shifted to higher pressures and lower temperatures as the mole fraction of N(2) increased. X-ray diffraction revealed that the CO(2)-CH(4)-N(2) mixture hydrate prepared from the CO(2)/CH(4)/N(2) (40/55/5) gas mixture formed a structure I clathrate hydrate. A combination of Raman and solid-state (13)C NMR measurements provided detailed information regarding the cage occupancy of gas molecules trapped in the hydrate frameworks. The gas storage capacity of LFG hydrates was estimated from the experimental results for the hydrate formations under two-phase equilibrium conditions. We also confirmed that trace amounts of nonmethane organic compounds do not affect the cage occupancy of gas molecules or the thermodynamic stability of LFG hydrates.

Gas-phase synthesis and characterization of CH4-loaded hydroquinone clathrates.

A CH(4)-loaded hydroquinone (HQ) clathrate was synthesized via a gas-phase reaction using the alpha-form of crystalline HQ and CH(4) gas at 12 MPa and room temperature. Solid-state (13)C cross-polarization/magic angle spinning (CP/MAS) NMR and Raman spectroscopic measurements confirm the incorporation of CH(4) molecules into the cages of the HQ clathrate framework. The chemical analysis indicates that about 69% of the cages are filled by CH(4) molecules, that is, 0.69 CH(4) per three HQ molecules. Rietveld refinement using synchrotron X-ray powder diffraction (XRD) data shows that the CH(4)-loaded HQ clathrate adopts the beta-form of HQ clathrate in a hexagonal space group R3 with lattice parameters of a = 16.6191 A and c = 5.5038 A. Time-resolved synchrotron XRD and quadrupole mass spectroscopic measurements show that the CH(4)-loaded HQ clathrate is stable up to 368 K and gradually transforms to the alpha-form by releasing the confined CH(4) gases between 368-378 K. Using solid-state (13)C CP/MAS NMR, the reaction kinetics between the alpha-form HQ and CH(4) gas is qualitatively described in terms of the particle size of the crystalline HQ.

Separation of CO2 from flue gases using hydroquinone clathrate compounds

Hydroquinone (HQ) samples reacting with (CO2+N2) gas mixtures with various compositions at pressures ranging from 10 to 50 bar are analyzed using spectroscopic methods and an elemental analyzer. The results indicate that while both CO2 and N2 can react with HQ to form clathrate compounds, CO2 has higher selectivity than N2. In particular, at an operating pressure of 20 bar or greater, the CO2 content in the clathrate compound is 85mol% or higher regardless of the feed gas composition. Moreover, if a two-step clathrate-based process is adapted, CO2 at a rate of 93 mol% or higher can be recovered from flue gases. Thus, the clathrate compound described here can be used as a CO2 separation/recovery medium for CO2 in flue gases.

Hydrogen Molecules Trapped in Interstitial Host Channels of α‐Hydroquinone

Easy come, easy go: Hydroquinone forms a channel structure of cages with hydrogen-bonded hexagons. These may provide an ideal route for the fast inclusion and facile release of hydrogen molecules (see figure), which can lead to reversible hydrogen storage under mild conditions.

Equilibrium, Kinetics, and Spectroscopic Studies of SF6 Hydrate in NaCl Electrolyte Solution

Many studies have focused on desalination via hydrate formation; however, for their potential application, knowledge pertaining to thermodynamic stability, formation kinetics, and guest occupation behavior in clathrate hydrates needs to be determined. Herein, the phase equilibria of SF6 hydrates in the presence of NaCl solutions (0, 2, 4, and 10 wt %) were monitored in the temperature range of 277-286 K and under pressures of up to 1.4 MPa. The formation kinetics of SF6 hydrates in the presence of NaCl solutions (0, 2, and 4 wt %) was also investigated. Gas consumption curves of SF6 hydrates showed that a pure SF6 hydrate system allowed fast hydrate growth as well as high conversion yield, whereas SF6 hydrate in the presence of NaCl solutions showed retarded hydrate growth rate as well as low conversion yield. In addition, structural identification of SF6 hydrates with and without NaCl solutions was performed using spectroscopic tools such as Raman spectroscopy and X-ray diffraction. The Raman spectrometer was also used to evaluate the temperature-dependent release behavior of guest molecules in SF6 and SF6 + 4 wt % NaCl hydrates. The results indicate that whereas SF6 hydrate starts to decompose at around 240 K, the escape of SF6 molecules in SF6 + 4 wt % NaCl hydrate is initiated rapidly at around 205 K. The results of this study can provide a better understanding of guest-host interaction in electrolyte-containing systems.

Probing Structural Transition and Guest Dynamics of Hydroquinone Clathrates by Temperature-Dependent Terahertz Time-Domain Spectroscopy

The structural transition from hydroquinone clathrates to crystalline α-form hydroquinone was observed up to the range of 3 THz frequency as a function of temperatures. We found that all three hydroquinone clathrates, CO(2)-, CH(4)-, and CO(2)/CH(4)-loaded hydroquinone clathrates, transform into the α-form hydroquinone at around 102 ± 7 °C. The resonance peak of the CO(2)-loaded hydroquinone clathrate at 2.15 THz decreases with increasing temperature, indicating that CO(2) guest molecules are readily released from the host framework prior to the structural transformation. This reveals that the hydroquinone clathrates may transform into the stable α-form hydroquinone via the metastable form of guest-free clathrate, which depends on guest molecules enclathrated in the cages of the host frameworks. A strong resonance of the α-form hydroquinone at 1.18 THz gradually shifts to the low frequency with increasing temperature and shifts back to the high frequency with decreasing temperature.

Structural transformation and guest dynamics of methanol-loaded hydroquinone clathrate observed by temperature-dependent Raman spectroscopy.

The structural transformations and guest dynamics of methanol-loaded β-form hydroquinone (HQ) clathrate were investigated using temperature-dependent Raman spectroscopy. Methanol-loaded β-form HQ clathrate was obtained by recrystallization and characterized by elemental analysis, synchrotron X-ray diffraction, solid-state (13)C NMR spectroscopy, and Raman spectroscopy. Temperature-dependent Raman spectra of methanol-loaded β-form HQ clathrate were measured in the temperature range 300-412 K at increments of 4 K. Although no significant changes were evident in the temperature range 300-376 K, abrupt changes in the relative intensity and shape of the Raman bands were observed between 380 and 412 K indicating the structural transition from methanol-loaded β-form HQ clathrate to pure α-form HQ. Methanol molecules were gradually released from the β-form HQ clathrate in the range 364-380 K. Upon returning to ambient conditions, the crystal structure of the HQ sample remained identical to that of pure α-form HQ. Therefore, the temperature-induced structural transition of methanol-loaded HQ clathrate is completely irreversible and α-form HQ is more stable at ambient conditions.

Selective Encaging of N2O in N2O–N2 Binary Gas Hydrates via Hydrate-Based Gas Separation

The crystal structure and guest inclusion behaviors of nitrous oxide-nitrogen (N2O-N2) binary gas hydrates formed from N2O/N2 gas mixtures are determined through spectroscopic analysis. Powder X-ray diffraction results indicate that the crystal structure of all the N2O-N2 binary gas hydrates is identified as the structure I (sI) hydrate. Raman spectra for the N2O-N2 binary gas hydrate formed from N2O/N2 (80/20, 60/40, 40/60 mol %) gas mixtures reveal that N2O molecules occupy both large and small cages of the sI hydrate. In contrast, there is a single Raman band of N2O molecules for the N2O-N2 binary gas hydrate formed from the N2O/N2 (20/80 mol %) gas mixture, indicating that N2O molecules are trapped in only large cages of the sI hydrate. From temperature-dependent Raman spectra and the Predictive Soave-Redlich-Kwong (PSRK) model calculation, we confirm the self-preservation of N2O-N2 binary gas hydrates in the temperature range of 210-270 K. Both the experimental measurements and the PSRK model calculations demonstrate the preferential occupation of N2O molecules rather than N2 molecules in the hydrate cages, leading to a possible process for separating N2O from gas mixtures via hydrate formation. The phase equilibrium conditions, pseudo-pressure-composition (P-x) diagram, and gas storage capacity of N2O-N2 binary gas hydrates are discussed in detail.

Spectroscopic investigation, cage occupancy, and gas storage capacity of hydroquinone clathrates formed with H2S-N2 and COS-N2 binary gas mixtures

The objective of this investigation was to determine whether hydroquinone (HQ) can form clathrate compounds with two sulfides (hydrogen sulfide (H2S) and carbonyl sulfide (COS)) at their diluted concentrations. Hydroquinone samples obtained at ambient temperature and at two pressures (40 and 80 bar) for binary gas mixtures consisting of H2S-N2 and COS-N2, were analyzed using solid-state 13C NMR and Raman spectroscopy. An elemental analyzer was also used to obtain quantitative information regarding the kind and amount of gas captured in the solid samples. Results show that H2S can be concentrated within the solid clathrate from H2S-containing gas, while COS is little captured after reaction with the COS-containing gas. This suggests that the HQ clathrate can be used to remove H2S, and that selective separation can be achieved when two sulfides of H2S and COS coexist. On the basis of the calculated cage occupancies of the gas components in the solid clathrate, the enclathration preference of the gas components used in this research was found to be the order of H2S>N2>COS.

Hydroquinone clathrates by temperature-dependent terahertz time-domain spectroscopy

Using Terahertz Time-Domain Spectroscopy (THz-TDS), we measured the structural transition from hydroquinone (HQ) clathrates having the storage properties to crystalline a-form hydroquinone as a function of temperatures. We found that all three HQ clathrates, CO2-, CH4-, and CO2/CH4-loaded HQ clathrates, have the different vibrational lines and transform into the a-form HQ at around 102±7°C. The resonance peak of the CO2-loaded HQ clathrate at 2.15 THz decreases with increasing temperature, indicating that CO2 guest molecules are readily released from the host framework prior to the structural transformation. This reveals that the HQ clathrates may transform into the stable a-form HQ via the metastable form of guest-free clathrate, which depends on guest molecules enclathrated in the cages of the host frameworks.