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Reproduced from New J. Chem., 2023, 47, 231-237 with permission from the Royal Society of Chemistry., https://doi.org/10.1039/d2nj04361k, Ionic clathrate hydrates (ICHs) have been applied to thermal storage materials, which can be utilized as unusual thermal energy sources. In the present study, we developed a novel ICH including tri-n-butyl(cyclobutylmethyl)phosphonium bromide (P444(1c4)-Br) as an unprecedented guest species. The highest equilibrium temperature and the dissociation enthalpy of P444(1c4)-Br ICH were 279.5 ± 0.1 K and 202 ± 2 J g−1, respectively, at the mole fraction of 0.0255 ± 0.0008. The highest equilibrium temperature of P444(1c4)-Br ICH was slightly lower than that of the tetra-n-butylphosphonium bromide (P4444-Br) ICH. The dissociation enthalpy of P444(1c4)-Br ICH was larger than that of other ICHs with similar highest equilibrium temperatures. Cyclic hydrocarbon groups, together with normal, branched, saturated, and unsaturated hydrocarbon chains, can be one of the options for tuning the equilibrium temperature of ICHs.

Azuma S., Shimada J., Tsunashima K., et al. Equilibrium Phase Relations and Dissociation Enthalpies of Tri- n-butylalkenylphosphonium Bromide Semiclathrate Hydrates. Journal of Chemical and Engineering Data, 67, 6, 1415 - 1420, May 25, 2022, © 2022 American Chemical Society. https://doi.org/10.1021/acs.jced.2c00146., Semiclathrate hydrates (SCHs) have received attention as thermal storage materials because of their equilibrium temperature and relatively large dissociation enthalpy. The alkyl length of quaternary onium cations significantly affects the equilibrium temperature and dissociation enthalpy of quaternary onium salt SCHs. To investigate the effects of the double bond introduced in onium cations, in the present study, the thermodynamic stability of tri-n-butylalkenylphosphonium bromide (P444(alkenyl)-Br: alkenyl = 3-butenyl (3═1) or 4-pentenyl (4═1)) SCH was investigated. The equilibrium temperatures and the dissociation enthalpies were determined to be (277.5 ± 0.1) K and (199 ± 2) J/g for P444(3═1)-Br SCH, and (274.8 ± 0.1) K and (188 ± 2) J/g for P444(4═1)-Br SCH. The equilibrium temperatures of P444(3═1)-Br SCH and P444(4═1)-Br SCH were lower than those of the corresponding normal alkyl SCHs (P4444-Br and P4445-Br), although the equilibrium temperature of tri-n-butylallylphosphonium bromide SCH was higher than that of the P4443-Br SCH.

The addition of silver(ii) oxide effectively diminishes the degree of supercooling in the tetrahydrofuran hydrate formation.

Jin Shimada, Moe Yamada, Atsushi Tani et al. Thermodynamic Properties of Tetra-n-butylphosphonium Dicarboxylate Semiclathrate Hydrates. Journal of Chemical & Engineering Data, 67 (1), 67-73, January 13, © 2022 American Chemical Society. https://doi.org/10.1021/acs.jced.1c00741, Semiclathrate hydrate (SCH) is one of the phase change materials suitable for cold energy storage. Thermodynamic properties of SCHs, such as an equilibrium temperature and the dissociation enthalpy, depend on the size and shape of the guest substances. In the present study, to reveal the effects of steric conformations of the guest anions on the thermodynamic properties of SCHs, tetra-n-butylphosphonium dicarboxylate (TBP-DC) SCHs, where the anion was oxalate (TBP-Oxa), malonate (TBP-Mal), succinate (TBP-Suc), glutarate (TBP-Glu), maleate (TBP-Male), or fumarate (TBP-Fum), were investigated. TBP-Oxa, -Mal, -Suc, and -Fum SCHs had similar equilibrium temperatures, whereas the equilibrium temperatures of TBP-Glu and -Male SCHs were higher. This suggests that the size and conformation of glutarate and maleate anions are appropriate for the cage structures of SCHs. Moreover, we compared the equilibrium temperatures of TBP-Suc, -Male, and -Fum SCHs because TBP-Suc, -Male, and -Fum have similar anion structures. The equilibrium temperature of TBP-Suc SCH was similar to that of TBP-Fum SCH, whereas TBP-Male SCH showed a higher equilibrium temperature. This result implies that the succinate anion is accommodated in the trans conformation, similar to the fumarate anion, in the hydrate cages.

Clathrate hydrate as well as ice has been spotlighted as promising materials to investigate the properties of hydrogen-bonded networks formed by the water molecules. In the semiclathrate hydrate (SCH, a kind of clathrate hydrate) consisting of host water molecules and guest quaternary onium salts, the anion takes part in the hydrogen-bonded networks with the water molecules. In the present study, we measured the electrical conductivity and electrical relaxation time in the single-crystalline tetra-n-butylammonium bromide (TBAB) SCH by electrochemical impedance spectroscopy. The results clearly revealed that the conduction carrier was proton. The electrical relaxation time of proton conduction in TBAB SCH was similar to the reorientation time of the water molecules in TBAB SCH estimated by the results of nuclear magnetic resonance.

As a guest molecule of urea clathrate, a long-chain normal alkane and its derivative with low substituents in methanol solutions have been reported. To investigate the role of methanol in the urea clathrate formation, in the present study, we used propane (C3H8), propylene (C3H6), ethane (C2H6), and methane (CH4) as guest molecules. Raman spectra and powder X-ray diffraction profiles revealed that, regardless of the existence of methanol, the C3H8, C3H6, and C2H6 molecules are enclathrated into urea clathrates with a hexagonal structure, whereas there is no urea clathrate formation enclathrating CH4. The pressurization of the urea clathrates including C2H6 and C3H8 reveals that no pressure-induced structural phase transition occurs at pressures up to 200 MPa. In spite of the guest molecule much shorter than the lattice constant of the c-axis of the hexagonal channel structure, the urea clathrates have a fairly rigid structure against the compression. Methanol as an auxiliary solution is not always necessary for the urea clathrate formation. Methanol plays a role in decreasing the activation energy of the urea clathrate formation, although it makes urea clathrate thermodynamically unstable due to the high solubility of urea in methanol.

Phase equilibrium (temperature–composition) relations of tetra-n-butylphosphonium formate (TBP-For), acetate (TBP-Ace), and lactate (TBP-Lac) semiclathrate hydrate systems have been measured. The highest equilibrium temperatures of TBP-For, TBP-Ace, and TBP-Lac semiclathrate hydrates were 280.9, 284.6, and 283.8 K at the atmospheric pressure, respectively, where the composition of tetra-n-butylphosphonium carboxylate was approximately 0.035 ± 0.001 (mole fraction) in every system. The dissociation enthalpies of tetra-n-butylphosphonium carboxylate semiclathrate hydrates were measured by differential scanning calorimetry. The dissociation enthalpies of TBP-For, TBP-Ace, and TBP-Lac semiclathrate hydrates were (187 ± 3), (193 ± 3), and (177 ± 3) J·g–1, respectively.

A large degree of supercooling is necessary to form the tetra-n-butyl ammonium bromide (TBAB) semiclathrate hydrate. One of the effective methods to suppress supercooling is the utilization of a kind of hysteresis called the “memory effect”. Generally, the memory effect reduces supercooling and/or the induction period in clathrate hydrate formation. In the present study, we have observed the solution structure of TBAB aqueous solutions in the formation and decomposition processes of TBAB semiclathrate hydrate by scanning electron microscopy (SEM) with the freeze-fracture replica method. The relationship between the memory effect and the solution structures of the TBAB supercooled aqueous solution was elucidated. The memory effect in the TBAB semiclathrate hydrate reformation is derived from the existence of residual clusters of 10–20 nm in the TBAB aqueous solution at 286.2 K, which is 1.0 K higher than the equilibrium decomposition temperature, even after the TBAB semiclathrate hydrate was completely decomposed.

Semiclathrate hydrate (SCH) is one of the phase change materials suitable for cold storage. The thermodynamic properties of SCHs, such as an equilibrium temperature and a dissociation enthalpy, depend on the size and shape of guest substances. In this study, to reveal the effect of cation size and shape on the thermodynamic properties, tri-n-butylalkylphosphonium bromide (P444R-Br) SCHs, where the alkyl group was n-propyl (R = 3), n-butyl (R = 4), n-pentyl (R = 5), i-butyl (R = i-4), i-pentyl (R = i-5), or allyl (R = Al)), were investigated. The branched alkyl groups (R = i-4 or i-5) raised the equilibrium temperature, whereas the shorter alkyl groups (R = 3 or Al) lowered one. Except for P4445-Br and P444(Al)-Br SCHs, the other P444R-Br SCHs had the same orthorhombic structure. Among the orthorhombic systems in the present study, the semiclathrate hydrate with a higher equilibrium temperature had a larger dissociation enthalpy.

Takeshi Sugahara and Hironobu Machida. Dissociation and Nucleation of Tetra-n-butyl Ammonium Bromide Semi-Clathrate Hydrates at High Pressures. Journal of Chemical & Engineering Data, 62 (9), 2721–2725, September 14, © 2017 American Chemical Society. https://doi.org/10.1021/acs.jced.7b00115, The equilibrium pressure-temperature relations of the tetra-n-butyl ammonium bromide (TBAB) semiclathrate hydrate were measured at pressures of up to 80 MPa by high-pressure differential scanning calorimetry. As a pressurizing medium, tetrafluoromethane (CF4), which cannot occupy any hydrate cages in the TBAB semiclathrate hydrate at the present experimental pressures, was used. The dissociation temperature of tetragonal TBAB semiclathrate hydrate (TBAB·26H2O) increases with the increase in pressure, whereas the dissociation enthalpy is (192 ± 3) J·g-1 and almost constant at pressures of up to 80 MPa. The temperature difference between formation and dissociation at the same pressure, that is, the maximum allowable degree of supercooling, is (17.7 ± 0.7) K and independent of the pressure.

Takeshi Sugahara. Four-phase equilibrium relations including clathrate hydrate phase in a ternary system of xenon, benzene, and water. Journal of Chemical & Engineering Data, 61 (12), 4057–4060, December 8, © 2016 American Chemical Society. https://doi.org/10.1021/acs.jced.6b00541, Thermodynamic stability boundaries of xenon (Xe) + benzene mixed hydrate system were measured in a temperature range of (274.1 to 284.4) K and pressure range up to 0.5 MPa. The four-phase equilibrium curve of structure-II hydrate (HsII), aqueous (Lw), C6H6-rich liquid (LHC), and gas (G) phases in the Xe+benzene mixed hydrate system intersects with the three-phase equilibrium curve (HsI+Lw+G) in the structure-I simple Xe hydrate system. The slope of HsII+Lw+LHC+G in the Xe +benzene mixed hydrate system changes around (283.00 ± 0.05) K and (0.420 ± 0.005) MPa. At temperatures above 283.00 K, the structure-II Xe+benzene mixed hydrate collapses and instead, the structure-I simple Xe hydrate is formed.

Thermodynamic stability boundaries of the mixed hydrate composed of methane and fluorocyclopentane, chlorocyclopentane, or bromocyclopentane were investigated in a temperature range of (274.17 to 306.19) K and pressures up to 9.93 MPa under the four-phase (hydrate, aqueous, monohalogenated cyclopentane-rich liquid, and gas phases) equilibrium conditions. Also, the crystal structure of the methane + chlorocyclopentane hydrate was determined by means of powder X-ray diffractometry. The each boundary is laid at a pressure lower and a temperature higher than that of the simple methane hydrate. Especially, the methane + fluorocyclopentane hydrate system has the lowest equilibrium pressure among three mixed hydrate systems investigated in the present study. The powder X-ray diffraction patterns reveal that a guest species larger than chrolocyclopentane and smaller than bromocyclopentane is at the border between structure-II and structure-H hydrate formers.

Tomohiro Tanabe, Takeshi Sugahara, Kazuma Kitamura et al. High-Pressure Phase Equilibria of Tertiary-Butylamine Hydrates with and without Hydrogen, Journal of Chemical & Engineering Data, 60 (2), 222–227, February 12, © 2015 American Chemical Society. https://doi.org/10.1021/je5003016, Thermodynamic stability boundaries of the simple tertiary-butylamine (t-BA) hydrate and t-BA+hydrogen (H2) mixed hydrate were investigated at a pressure up to approximately 100 MPa. All experimental results from the phase equilibrium measurement, in situ Raman spectroscopy, and powder X-ray diffraction analysis arrive at the single conclusion that the t-BA hydrates, under pressurization with H2, are transformed from the structure VI simple t-BA hydrate into the structure II t-BA+H2 mixed hydrate. The phase transition point on the hydrate stability boundary in the mother aqueous solutions with the t-BA mole fractions (xt-BA) of 0.056 and 0.093 is located at (2.35 MPa, 267.39 K) and (25.3 MPa, 274.19 K), respectively. On the other hand, in the case of the pressurization by decreasing the sample volume instead of supplying H2, the simple t-BA hydrate retains the structure VI at pressures up to 112 MPa on the thermodynamic stability boundary.

Large-cage occupancy of hydrogen (H 2 ) in the H 2 +tetrahydrofuran (THF) hydrates has been ever reported only in the ex situ spectroscopic measurements with the quenched hydrate sample. In the present study, to verify the proof-of-concept for the large-cage occupancy of H 2 competitive with THF, in situ Raman spectra of H 2 in the H 2 +THF mixed hydrates were detected at 74.3 MPa and 265 K without the quenching procedure. The Raman spectra reveal that H 2 molecules occupy a part of large cages of the H 2 +THF mixed hydrates at a thermodynamically unstable region of simple H 2 hydrate.

The enclathration of 2,3,3,3-tetrafluoropropene (HFO-1234yf) has been investigated. In the binary system of HFO-1234yf+H2O, simple HFO-1234yf hydrate is not formed even at 50 MPa and 274.15 K. The addition of methane (CH4) as a help molecule results in the structure-II CH4+HFO-1234yf mixed-gas hydrate formation. With difluoromethane (HFC-32), instead of CH4, no HFC-32+HFO-1234yf mixed-gas hydrate is formed. Isothermal phase equilibria for the CH4+HFO-1234yf+H2O ternary system were measured at 279.15 K. Based on the extrapolation of two three-phase {(hydrate, aqueous, and gas phases) and (aqueous, HFO-1234yf-rich liquid, and gas phases)} equilibrium curves, the quadruple point of hydrate, aqueous, HFO-1234yf-rich liquid, and gas phases was determined.

The three-phase (hydrate, aqueous, and HFC-134a-rich liquid phases) equilibrium relations of the 1,1,1,2-tetrafluoroethane (HFC-134a) + water binary system were measured in a pressure range up to 293 MPa. The HFC-134a hydrate has a structural phase transition point from structure-II to structure-I at (65 ± 1) MPa and (282.2 ± 0.1) K, where the slope of the three-phase equilibrium curve is drastically changed. The Raman spectra of the HFC-134a molecule in the structure-I HFC-134a hydrate phase indicate that HFC-134a molecule occupies only large cage of structure-I at a pressure up to 293 MPa.

Isothermal phase equilibrium relations of the CO2 + 1,1,1,2-tetrafluoroethane (HFC-134a) and CO2 + 1,1-difluoroethane (HFC-152a) mixed-gas hydrate systems were measured at 279.15 K. The isothermal phase equilibria of the CO2 + HFC-134a mixed-gas hydrate system exhibit the heterogeneous azeotropic-like behavior derived from the structural phase transition of the mixed-gas hydrate. The CO2 + HFC-152a mixed-gas hydrate system shows neither a structural phase transition nor azeotropic-like behavior.

High-pressure phase equilibrium relations of the 1,1-difluoroethane (HFC-152a) + water binary system were investigated in a temperature range of (275.03 to 319.30) K and a pressure range up to 370 MPa. Four three-phase coexisting curves of hydrate + aqueous + gas phases, hydrate + HFC-152a-rich liquid + gas phases, hydrate + aqueous + HFC-152a-rich liquid phases, and aqueous + HFC-152a-rich liquid + gas phases originate from the quadruple point of hydrate + aqueous + HFC-152a-rich liquid HFC-152a + gas phases located at (288.05 ± 0.15) K and (0.44 ± 0.01) MPa. The structure of HFC-152a hydrate remains structure I (s-I) in the pressure range up to 370 MPa. Raman spectra of the HFC-152a molecule in the HFC-152a hydrate indicate that the HFC-152a molecules occupy only large cages of s-I HFC-152a hydrate in the presence of completely vacant small cages at a pressure up to 370 MPa.

Methane+ethane+1,1-dimethylcyclohexane+water system was investigated by using Raman spectroscopy and isothermal phase equilibrium measurements under four-phase (gas+aqueous+large guest species+hydrate phases) equilibrium conditions at 288.15 K. The results suggest that three kinds of hydrate structures emerge at 288.15 K in the methane+ethane+1,1-dimethylcyclohexane+water system. The hydrate structure for this system changed from structure-H to structure-I via structure-II with increase in the mole ratio of ethane to methane.

The present paper reports isothermal three-phase equilibria (hydrate + aqueous + gas phases) for the carbon dioxide (CO2) + ethane (C2H6) mixed-gas hydrate system at (274.15, 279.15, and 284.15) K and the carbon dioxide (CO2) + tetrafluoromethane (CF4) mixed-gas hydrate system at (274.15 and 279.15) K. None of the equilibrium pressure−composition curves of hydrate, aqueous, and gas phases shows discontinuity in the gradient. Both CO2 + C2H6 and CO2 + CF4 mixed-gas hydrates are the structure-I hydrate under the present experimental conditions, which is supported by the results of in situ Raman spectroscopy.

The thermodynamic stability and hydrogen occupancy for the hydrogen+trimethylamine mixed semi-clathrate hydrate system were investigated by means of phase equilibrium (pressure–temperature) measurements and Raman spectroscopic analyses. The hydrogen molecule gradually advanced to occupy the empty small cage of trimethylamine hydrate in proportion to pressure increase. Almost all small cages were filled up with the hydrogen molecules at about 80 MPa. Isothermal Raman spectroscopic analysis showed that the absorption-rate of hydrogen to the pre-treated trimethylamine hydrate was comparable to those of tetrahydrofuran hydrate. Only one hydrogen molecule was enclosed with one small cage at the equilibrium state in pre-treated trimethylamine hydrate.

The storage capacity of hydrogen in the tetrahydrothiophene and furan hydrates was investigated by means of pressure–volume–temperature measurement. The hydrogen–absorption rate of tetrahydrothiophene and furan hydrates is much larger than that of tetrahydrofuran hydrate in spite of same crystal structure (structure-II). The storage amount of hydrogen at 275.1 K is about 1.2 mol (hydrogen)/mol (tetrahydrothiophene or furan hydrate) (∼0.6 mass%) at 41.5 MPa, which is coincident with that of tetrahydrofuran hydrate.

Four-phase equilibrium relations (gas + water + methylcyclohexanol + gas hydrate phases) have been measured in methane + methylcyclohexanol + water systems. We have discovered five new guest species of the methane structure-H hydrate: 1-methylcyclohexanol; cis-2-methylcyclohexanol; trans-2-methylcyclohexanol; cis-4-methylcyclohexanol; trans-4-methylcyclohexanol. Decomposition pressures of these methane-structure-H hydrates increase in this order under isothermal conditions.

The storage capacity of hydrogen in tetrahydrofuran hydrate was investigated by means of pressure–volume–temperature ( p – V – T ) measurement and Raman spectroscopic analysis. We carried out two measurement strategies using Raman spectroscopic analysis. One was isothermal pressure-swing absorption using tetrahydrofuran hydrate at 277.15 K, and the other was the preparation of a single crystal of hydrogen+tetrahydrofuran mixed gas hydrate from compressed hydrogen and tetrahydrofuran aqueous solutions along the stability boundary. The storage amount of hydrogen at 277.15 K obtained from the p – V – T measurement is about 1.6 mol (hydrogen)/mol (tetrahydrofuran) (about 0.8 mass%) at 70 MPa, and isothermal Raman spectroscopic measurement reveals that it reaches the maximum value of 2.0 mol (hydrogen)/mol (tetrahydrofuran) at about 85 MPa. These results agree well with those for a single crystal of hydrogen+tetrahydrofuran hydrate.

Thermodynamic stability and hydrogen occupancy on the hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate have been investigated by means of phase equilibrium (pressure–temperature) measurements and Raman spectroscopic analyses for two mole fractions, 0.018 and 0.034 (stoichiometric for the cubic structure) of tetra-n-butyl ammonium fluoride aqueous solutions. In the case of higher concentration (0.034), the stability boundary curve of hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate locates at about 23 K higher temperature than that of hydrogen+tetrahydrofuran mixed gas hydrate. The storage capacity of hydrogen in the cubic structure for the hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate is smaller than that of hydrogen+tetrahydrofuran mixed gas hydrate. In the case of hydrate prepared from the lower concentration (0.018) of aqueous solution, the Raman spectra and phase behavior reveal that the cubic structure of semi-clathrate hydrate is changed to a different one at about 9 MPa and 299.2 K. The new structure can entrap larger amount of hydrogen than the cubic one. The stability boundary curve of hydrogen+tetra-n-butyl ammonium fluoride semi-clathrate hydrate obtained in the aqueous solution of lower mole fraction (0.018) is shifted to slightly low-temperature or high-pressure side from that of higher mole fraction (0.034).

Phase equilibrium (pressure–temperature) relations of the hydrogen ++ tetra-n-butyl ammonium bromide mixed gas hydrate system have been measured for various concentrations of tetra-n-butyl ammonium bromide aqueous solutions. The three-phase equilibrium curves obtained in the present study are shifted to the low-temperature or high-pressure side from that of the stoichiometric tetra-n -butyl ammonium bromide solution. Each three-phase equilibrium curve of hydrogen ++ tetra-n-butyl ammonium bromide mixed gas hydrate converges at the atmospheric equilibrium point of the pure tetra-n -butyl ammonium bromide hydrate for the mother aqueous solution of same mole fraction. The hydrate-cage occupancy of hydrogen in the hydrogen ++ tetra-n-butyl ammonium bromide mixed gas hydrate is in variable in a range of tetra-n-butyl ammonium bromide mole fraction from 0.006 to 0.070 in the aqueous solution. Hydrogen is entrapped only in the small cages of tetra-n-butyl ammonium bromide hydrates.

Phase equilibria (pressure−temperature relations) of the H2 + tetrahydrofuran mixed gas hydrate system have been measured for various concentrations of tetrahydrofuran aqueous solutions. The three-phase equilibrium lines obtained in the present study are shifted to the low-temperature or high-pressure side from that of the stoichiometric THF solution. Each three-phase equilibrium line of H2 + tetrahydrofuran hydrate converges at the three-phase equilibrium line of the pure tetrahydrofuran hydrate. At the cross point on the lines, the tetrahydrofuran concentration of mother aqueous solution agrees with each other. The Raman spectra of H2 and tetrahydrofuran for the H2 + tetrahydrofuran mixed gas hydrate do not change with the variation of tetrahydrofuran mole fraction from 0.010 to 0.130 in the aqueous solution.

Phase equilibrium curves of H2+H2+ tetrahydrofuran and H2+H2+ tetra-n-butyl ammonium bromide mixed gas hydrates were measured in a pressure range from 0.1 to 13.6 MPa. Each three-phase equilibrium curve converges at the maximum temperature point of pure tetrahydrofuran and tetra-n-butyl ammonium bromide hydrates, respectively. The difference of maximum temperatures is about 8 K, that is, the equilibrium curve of H2+H2+ tetra-n -butyl ammonium bromide mixed gas hydrate shifts to the high-temperature side from that of H2+H2+ tetrahydrofuran mixed gas hydrate. It is directly confirmed by use of Raman spectroscopy that H2H2 is enclathrated in the hydrate cages by adding a small amount of tetrahydrofuran or tetra-n -butyl ammonium bromide. In both mixed hydrates, H2H2 is enclathrated in only the small cage while tetrahydrofuran or tetra-n-butyl ammonium bromide occupies the large cages of each mixed hydrate.

The SF6 hydrate system has been investigated by use of a laser Raman spectroscopic analysis in a temperature range from 274 K to 313 K and pressure up to 155 MPa. Two quadruple points have been det...

Four mixtures of 1,1-, cis -1,2-, trans -1,2-, and cis -1,4-dimethylcyclohexanes (hereafter abbreviated DMCH) including H 2 O and Xe have been investigated in a temperature range over 274.5 K and a pressure range up to 2.7 MPa. The 1,1-DMCH and cis -1,2-DMCH generate the structure-H hydrate in the temperature range up to 295.2 and 280.2 K, respectively. Especially, very large depression of equilibrium pressure has been observed in the structure-H 1,1-DMCH hydrate system. On the other hand, neither trans -1,2-DMCH nor cis -1,4-DMCH generates the structure-H hydrate in the present temperature range. It is an important finding that the cis -1,4-DMCH does not generate the structure-H hydrate in the presence of Xe, while the mixture of cis -1,4-DMCH and methane generates the structure-H hydrate.

The thermodynamic stability boundaries for two noble gas (Xe and Kr) hydrates have been investigated in a pressure range up to 445 MPa and a temperature range of (324.4 to 345.3) K for the Xe hydrate system and (274.4 to 320.0) K for the Kr hydrate system. The Raman spectrum of the intermolecular O−O stretching vibration mode has been evaluated for each hydrate single crystal along with the three-phase coexisting curve. The slope change on the boundary curve and the pressure dependence on the Raman shift for the Kr hydrate crystal reveal that a structural phase-transition point exists at 414 MPa and 319.20 K, whereas the Xe hydrate crystal of structure I does not exhibit any phase transition over the region studied.

The three-phase coexisting curve of CF4 hydrate + water + fluid CF4 and laser Raman spectroscopic analysis for the CF4 hydrate system have been investigated in a temperature range of 276 to 317 K and at pressures up to 454 MPa. The present experimental results show that the CF4 molecule is able to occupy both the S- and M-cages in the high-pressure region (above 70 MPa) similar to the cyclopropane hydrate system. The pressure independence of the intermolecular O−O vibration reveals that the CF4 hydrate cage is hardly shrunk by pressurization.

The four-phase coexistence curves for the structure-H hydrates of methylcyclohexane and cis-1,2-dimethylcyclohexane in the presence of methane are measured in the temperature range 274.09–288.29K and pressure range 1.42–8.89MPa. Very large pressure reductions from the pure methane hydrate are observed by forming structure-H hydrates. The present investigation on the trans-1,2-dimethylcyclohexane system reveals that the limit of the largest-cage occupancy for the structure-H hydrate is laid between the 1,2-dimethylcyclohexane stereo-isomers.

Three-phase equilibrium (pressure–temperature) relation of hydrate+aqueous+fluid phases for the hydrogen (H 2 )+tetra- n -butyl phosphonium bromide (TBPB)+water ternary system was investigated in a temperature range of 281.90–295.94 K and a pressure range up to 170 MPa. The behavior of the three-phase coexisting curve indicates no structural transition in the present experimental region. The Raman spectra obtained in the H 2 +TBPB mixed semi-clathrate hydrate crystal reveal that H 2 molecule occupies only small cage compartmentally and the TBPB molecule is encaged with a set of other large cages.

Stability boundary for the nitrogen hydrate system is investigated in a temperature range from 285 to 310 K and pressure range up to 440 MPa. The present experimental results show that nitrogen hydrates form structure-II without any solid–solid phase transition in the present experimental conditions. The strong pressure dependence of intermolecular O–O vibration reveals that hydrate cage is shrunk by pressurization.

Laser Raman microspectroscopy and stability boundary observation for the cyclopropane hydrate system were investigated in a pressure range up to 460MPa and temperature range from 278 to 320K. The cyclopropane molecule is one of the largest guest species which build up the structure I hydrate lattice. It has been argued that the cyclopropane hydrate lattice would become stable in the presence of the perfectly vacant small cage. In the pressure range lower than 200MPa, we can confirm the perfect vacancy of small hydrate cages. We have ascertained that the cyclopropane molecule is able to occupy the small hydrate cage in the pressure range higher than 200MPa along with the three-phase coexisting curve. It becomes clear that the degree of small-cage occupancy increases in proportion to pressure without any phase transition in a pressure range up to 460MPa.

Thermodynamical stability boundaries of ethylene hydrate system were investigated in a temperature range from 279 to 328 K and pressure range up to 465 MPa . The three-phase coexisting curve of the ethylene hydrate + saturated water + saturated fluid ethylene shows the characteristic “S-shape” behavior in the pressure–temperature projection. From the fugacity–temperature projection, it can be concluded that such curious behavior is caused by the anomalous critical phenomenon. The laser Raman microprobe spectroscopic analysis reveals that the stretching and bending vibration energies of ethylene entrapped in the hydrate cage, fulfill two different conditions. It has been argued that ethylene occupies only the large cage of structure-I. However, based on the double peaks from the Raman scattering on ethylene hydrate crystals, we give direct evidence that, in spite of ethylene's large van der Waals diameter, it also occupies the small cage.

Thermodynamic stability boundaries of nitrous oxide (N2O) hydrate and Raman spectra of the N2O and host water molecules in the N2O hydrate system were investigated in a temperature range of (275.20 to 298.19) K and a pressure range up to 305 MPa. Two three-phase coexisting curves of (hydrate + aqueous + gas) and (hydrate + aqueous + liquid N2O) originate from the quadruple point of (hydrate + aqueous + liquid N2O + gas) located at (285.15 ± 0.05) K and (4.2 ± 0.1) MPa. The phase behavior of the N2O hydrate system is similar to that of the carbon dioxide (CO2) hydrate system up to 100 MPa, while the stability boundaries of the N2O hydrate system are shifted parallel to the (2 to 3) K higher temperature side than that of CO2 hydrate. Raman peak splitting of the intramolecular vibration mode for the nitrous oxide molecule in the hydrate phase indicates the occupancy of the nitrous oxide molecule in both small and large cages of structure-I, and this observation is also corroborated by the pressure-induced Ra...

The μVT-NEMD method, a combination of the μVT-MC and the boundary-driven NEMD, is used for simulating permeation of pure and mixed gases through slit-shaped carbon membranes with belt-like heterogeneous surfaces under the assumption of local adsorption equilibrium at the membrane entrance. The belt-like heterogeneous surfaces are made by removing carbon atoms of the first layer in a line vertically to the permeate direction. Methane and ethane are used as permeating gases. The values of permeation resistance increase almost exponentially with an increase in the potential barrier; the resistance for ethane is always larger than that for methane. In the case of a binary system, the permselectivity is determined by the adsorption equilibrium at the feed side when the potential barrier is low, while it sharply decreases with increasing potential barrier. This result suggests that the separation mechanism changes from the adsorption equilibrium control to the permeation rate control in the pore. The curve of the molar flux of ethane against temperature has a minimum point where temperature is about 510 K.

Isothermal phase equilibria (pressure−composition in the gas phase) for the quaternary system of H2 + CO2 + tetrahydrofuran + water have been measured in the presence of gas hydrate phase. The lowest three-phase equilibrium pressure is obtained under the condition that the mole fraction of tetrahydrofuran in water is 0.056. The Raman spectroscopy results show that the H2 and CO2 molecules competitively occupy the S-cage of structure-II (i.e., the H2 molecule is enclathrated in the hydrate cages with a small amount of tetrahydrofuran at considerably low pressure).

Phase equilibria for the tetrahydrofuran + water system below atmospheric pressure were investigated in a temperature range from 272 K to 278 K. The three-phase (structure-II hydrate + aqueous solution + gas) equilibrium curve has a maximum temperature at 277.45 ± 0.02 K and a pressure of 4.9 ± 0.1 kPa. At this condition, the equilibrium tetrahydrofuran composition of aqueous solution is equal to the stoichiometric ratio of tetrahydrofuran hydrate (structure-II). The tetrahydrofuran hydrate does not coexist with the gas phase beyond 277.45 K. The four-phase (structure-II hydrate + aqueous solution + ice Ih + gas) equilibrium point exists at 272.06 ± 0.02 K and 1.1 ± 0.1 kPa, and the equilibrium tetrahydrofuran mole fraction of aqueous solution is 0.0106 ± 0.0002.

The structure-H hydrate of 1,1-dimethylcyclohexane (DMCH) helped by methane has been investigated in a temperature range of 274.6–289.3 K and pressure range up to 6.7 MPa. The present results suggest that 1,1-DMCH is a suitable additive which makes a mild-pressure handling of natural-gas hydrate possible.

It has been regarded that the limit of the largest cage occupancy for the structure-H hydrate is between the 1,2-dimethylcyclohexane stereo-isomers, because the cis-isomer is able to generate the structure-H hydrate in the presence of methane while the trans-isomer is not. In the present study, gas hydration of trans-1,2-dimethylcyclohexane in the presence of methane and cis-1,2-dimethylcyclohexane is found from stability boundaries for the structure-H hydrate system.

Four three-phase coexisting curves for the trifluoromethane hydrate system were obtained over a pressure range up to 72 MPa and temperature range from 283 to 302 K. Two invariant points determined in the present study are the critical end point of 299.44 K and 4.85 MPa, and the quadruple point of 292.25 K and 4.07 MPa. The former, where the gas and liquid are critically identical in the presence of water phase, is located very close to the critical point of the trifluoromethane fluid. The latter is the terminal point of four three-phase coexisting curves. The overall enthalpy change of hydration is estimated over the whole temperature region. The important finding is that trifluoromethane hydrate is the border system having a quadruple point (hydrate + two liquids + gas) in a series of critical temperatures of known guest species. It is also identified that xenon hydrate is the opposite border system having no quadruple point.

Hydrogen hydrates with tetrahydrofuran (THF) as a promoter molecule are investigated to probe critical unresolved observations regarding cage occupancy and storage capacity. We adopted a new preparation method, mixing solid powdered THF with ice and pressurizing with hydrogen at 70 MPa and 255 +/- 2 K (these formation conditions are insufficient to form pure hydrogen hydrates). All results from Raman microprobe spectroscopy, powder X-ray diffraction, and gas volumetric analysis show a strong dependence of hydrogen storage capacity on THF composition. Contrary to numerous recent reports that claim it is impossible to store H(2) in large cages with promoters, this work shows that, below a THF mole fraction of 0.01, H(2) molecules can occupy the large cages of the THF+H(2) structure II hydrate. As a result, by manipulating the promoter THF content, the hydrogen storage capacity was increased to approximately 3.4 wt % in the THF+H(2) hydrate system. This study shows the tuning effect may be used and developed for future science and practical applications.

The structure-H hydrate of cis-1,4-dimethylcyclohexane in the presence of methane was investigated in the temperature range 274.1 to 287.9 K and the pressure range 1.62 to 9.13 MPa. cis-1,4-Dimethylcyclohexane in the presence of methane was enclathrated into the largest cage of the structure-H hydrate, while trans-1,4-dimethylcyclohexane was not enclathrated. The present study reveals that cis-1,4-dimethylcyclohexane as well as 1,1- and cis-1,2-dimethylcyclohexanes can generate the structure-H hydrate with the help of methane.

A methane hydrate crystal annealed at maximum pressure remembers the compression in the hydrate S-cage until the higher equilibrium temperature for maximum pressure is reached. The hydrate S-cage-like structure in the aqueous solution also remembers the compression until the so-called activation energy is given by heating. Therefore, the hydrate crystal compressed by the maximum pressure is reformed at the equilibrium temperature for the maximum pressure.