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Abstract Thermodynamic stabilities of tetra- n -butylphosphonium acetate (TBP-Ace) semiclathrate hydrates in the presence of methane (CH 4), carbon dioxide (CO 2), nitrogen (N 2), or ethane (C 2 H 6) were measured in a pressure range up to approximately 5 MPa. The dissociation temperature of TBP-Ace + CH 4 , TBP-Ace + CO 2 , and TBP-Ace + N 2 semiclathrate hydrates increased drastically with an increase in pressure, which means that CH 4 , CO 2 , and N 2 molecules occupy the vacant cages of the TBP-Ace semiclathrate hydrate. On the other hand, the C 2 H 6 molecules hardly occupied the cages, resulting in small pressure dependence of the dissociation temperature. Raman spectra and powder X-ray diffraction patterns of TBP-Ace + CO 2 semiclathrate hydrate reveal that the phase transition occurs at 1.04 ± 0.04 MPa and 285.88 ± 0.05 K. One of the possible reasons why the phase transition occurs is that the carbonate and/or hydrogen carbonate anions derived from the CO 2 molecules are replaced with some of acetate anions in the TBP-Ace + CO 2 semiclathrate hydrate. Graphical abstract Image 1 Highlights • Thermodynamic stabilities of TBP-Acetate (TBP-Ace) + gas hydrates were measured. • The gases used in the present study are CH4, CO2, N2, and C2H6. • TBP-Ace + CH4 hydrate showed the highest equilibrium temperature. • Raman spectra and PXRD patterns reveal a phase transition of TBP-Ace + CO2 hydrate. • A possible reason of phase transition is the existence of anions derived from CO2. [ABSTRACT FROM AUTHOR]

Abstract This paper reports phase equilibrium (temperature–composition) relations of semiclathrate hydrates formed from tetra- n -butylphosphonium propionate (TBP-Pro) and butyrate (TBP-But) + water systems. Their maximum solid–liquid phase equilibrium temperatures at atmospheric pressure were located at (288.75 ± 0.06) K and the mole fraction x 1 = 0.035 ± 0.001 and (287.01 ± 0.06) K and x 1 = 0.028 ± 0.001, respectively. They showed equilibrium temperatures higher than those of tetra- n -butylphosphonium formate, acetate, and lactate semiclathrate hydrates. The dissociation enthalpies of TBP-Pro and TBP-But semiclathrate hydrates were (190 ± 5) J·g−1 and (204 ± 5) J·g−1, respectively. The temperature difference between formation and dissociation, that is, the maximum allowable degree of supercooling, was (17.7 ± 1.5) K for TBP-Pro semiclathrate hydrate and (15.4 ± 1.4) K for TBP-But one. Graphical abstract Image 1 Highlights • TBP-propionate and -butyrate semiclathrate hydrates were investigated. • Equilibrium phase relations were investigated by DSC measurement. • The highest equilibrium temperature of TBP-propionate hydrate was 288.75 K. • The highest equilibrium temperature of TBP-butyrate hydrate was 287.01 K. • The maximum allowable degree of supercooling in hydrate formation was investigated. [ABSTRACT FROM AUTHOR]

Phase equilibrium (temperature–composition) relations of the tetra- n -butylphosphonium trifluoroacetate (TBP-TFA) + water binary system including solid (semiclathrate hydrate, SCH)-liquid equilibria and liquid-liquid equilibria were investigated. Phase diagram of TBP-TFA SCH exhibited peritectic behavior. The maximum decomposition temperature (peritectic temperature) of TBP-TFA SCH at atmospheric pressure were 275.9 ± 0.1 K. The stoichiometric composition of TBP-TFA SCH was located at the mole fraction of 0.027 ± 0.002. The decomposition enthalpy of TBP-TFA SCH was 199 ± 2 J/g. At temperatures above the lower critical solution temperature (LCST), the liquid-liquid phase separation appeared. Interestingly, the composition at the LCST was 0.026 ± 0.002, which is very similar to the stoichiometric composition of TBP-TFA SCH. It implies that both cluster-based solution structures are similar. [Display omitted] [ABSTRACT FROM AUTHOR]