Calorimetric and Structural Studies of Tetrabutylammonium Chloride Ionic Clathrate Hydrates

Guest size effects on a robust structure of semiclathrate hydrates and their thermophysical properties

The ability to tune the pore size, shape, and functionality of semiclathrate hydrates, host-guest materials formed from aqueous solutions of ionic guest materials and water, makes them attractive materials for thermal storage and gas storage applications. The flexibility of semi-clathrate hydrates and their guest-molecule-dependent reactions produce these unexpected and desirable properties. As an ionic guest, tetra-n-butylammonium cation is known for best-fit in hydrogen-bonded water structures. Few investigations have been conducted for other cations, while there are numerous candidates. Relationships between the molecular structures of ionic guest substances and their hydrate structure and relevant thermodynamic properties are yet to be understood. In this study, the semiclathrate hydrates formed with two variations of tetra-n-butylammonium chloride (N4444Cl) that are n-propyl, tri-n-butylammonium chloride (N3444Cl) and tri-n-butyl, n-pentylammonium chloride (N4445Cl) were investigated. Structure analyses found that both salts formed Jeffrey's type III tetragonal hydrate structure which is the same as that of tetra-n-butylammonium chloride hydrate, although their lattice parameters are significantly different. The present data found that this hydrate structure can cover a wide range of melting temperature compared to the other two main semiclathrate structures. The present N4445Cl hydrate is an example in which its melting temperature was adjusted to be suitable for air conditioning, i.e., ∼282 K, compared to that of the N4444Cl hydrate, the melting temperature of which is slightly too high for this purpose. The results provide insight that the thermal properties of the tetragonal P42/m hydrate structure can be widely tuned by ionic guests for various practical requirements.

Abnormal structural transformation of tetra-n-butyl ammonium chloride + Xe clathrates and its significance for clathrate-based Xe capture and storage

Tetra-n-butyl ammonium chloride (TBAC) is a semi-clathrate former that can be used for clathrate-based gas capture and storage since TBAC semi-clathrate has vacant small cages available for entrapping gas molecules under mild conditions. In this study, the phase equilibria and structural information of TBAC + Xe + water systems were experimentally investigated at two different TBAC concentrations (1.0 and 3.3 mol%). The slopes of the three-phase (clathrate [H] - liquid [L] - vapor [V]) equilibrium lines for the TBAC + Xe + water systems altered at one or two points as the pressure and temperature changed, which indicates that this slope change might be caused by the structural transformation of the clathrates that were formed. The powder X-ray diffraction (PXRD) patterns, in situ Raman spectra, and ¹²⁹Xe nuclear magnetic resonance (NMR) spectra demonstrated that the clathrate structure of the TBAC + Xe + water systems changed from tetragonal (P4₂/m) or orthorhombic (Pmma) to cubic (Pm3̄n) as the pressure increased. Surprisingly, in the higher-pressure region, TBAC acted as a thermodynamic inhibitor without being enclathrated in the clathrate lattices. The thermodynamic and structural information of the TBAC + Xe clathrates will be helpful for conceptualizing and designing the clathrate-based noble gas or radioactive gas capture and storage process.

Intermolecular Interaction of Tetrabutylammonium and Tetrabutylphosphonium Salt Hydrates by Low-Frequency Raman Observation

Semi-clathrate hydrates are attractive heat storage materials because the equilibrium temperatures, located above 0 °C in most cases, can be changed by selecting guest cations and anions. The equilibrium temperatures are influenced by the size and hydrophilicity of guest ions, hydration number, crystal structure, and so on. This indicates that intermolecular and/or interionic interaction in the semi-clathrate hydrates may be related to the variation of the equilibrium temperatures. Therefore, intermolecular and/or interionic interaction in semi-clathrate hydrates with quaternary onium salts was directly observed using low-frequency Raman spectroscopy, a type of terahertz spectroscopy. The results show that Raman peak positions were mostly correlated with the equilibrium temperatures: in the semi-clathrate hydrates with higher equilibrium temperatures, Raman peaks around 65 cm⁻¹ appeared at a higher wavenumber and the other Raman peaks at around 200 cm⁻¹ appeared at a lower wavenumber. Low-frequency Raman observation is a valuable tool with which to study the equilibrium temperatures in semi-clathrate hydrates.

Structural CO2 capture preference of semiclathrate hydrate formed with tetra-n-butylammonium chloride

CO2 capture preference of semiclathrate hydrate analyzed by single crystal XRD. Asymmetrically distorted cages preferentially capture CO2.

Hydrates for Cold Storage: Formation Characteristics, Stability, and Promoters

The potential of hydrates formed from R141b (CH3CCl2F), trimethylolethane (TME), and tetra-n-butylammonium bromide/tetra-n-butylammonium chloride (TBAB/TBAC) to be used as working substances for cold storage was investigated to provide a solution for unbalanced energy grids. In this study, the characteristics of hydrate formation, crystal morphology of hydrates, and the stability of hydrate in cyclic formation under 0.1 MPa and at 5 °C were carried out. It found that the ice had a positive effect on the hydrate formation under same conditions. Upon the addition of the ice cube, the induction time of R141b, TME, and TBAB/TBAC hydrates decreased markedly, and significantly high formation rates were obtained. Under magnetic stirring, the rate at which TBAB/TBAC formed hydrates was significantly lower than that when ice was used. In microscopic experiments, it was observed that the TBAB/TBAC mixture formed hydrates with more nucleation sites and compact structures, which may increase the hydrate formation rate. In the multiple cycle formation of TBAB/TBAC hydrates, the induction time gradually decreased with the increasing number of formation cycles and finally stabilized, which indicated the potential of the TBAB/TBAC hydrates for application in cold storage owing to their good durability and short process time for heat absorption and release.

Computation of the Dissociation Temperature of TBAB Semiclathrate in an Aqueous Solution Using Molecular Simulations

Quaternary ammonium salts such as tetra-n-butyl ammonium bromide (TBAB) are known to form semiclathrate hydrates. Since they form at much milder conditions compared to gas hydrates, they have evoked much interest in development of new technologies for gas storage and gas separations. In this work, we present a method to compute the phase equilibrium of TBAB semiclathrate. The TBAB molecule is modeled using OPLS-AA and GAFF force fields and the results from simulations are compared with experimental data to determine the ability of the force fields to accurately predict the semiclathrate phase equilibria. It was observed that the OPLS-AA force field outperforms the GAFF force field in predicting the experimental phase equilibrium data.

CO2 Capture from Flue Gas Based on Tetra-n-butylammonium Fluoride Hydrates at Near Ambient Temperature

Semiclathrate hydrates of tetra-n-butylammonium fluoride (TBAF) are potential CO₂ capture media because they can capture CO₂ at near ambient temperature under moderate pressure such as below 1 MPa. In addition to other semiclathrate hydrates, CO₂ capture properties of TBAF hydrates may vary with formation conditions such as aqueous composition and pressure because of their complex hydrate structures. In this study, we investigated CO₂ capture properties of TBAF hydrates for simulated flue gas, that is, CO₂ + N₂ gas, by the gas separation test with three different parameters for each pressure and aqueous composition of TBAF in mass fraction (w _TBAF). The CO₂ capture amount in TBAF hydrates with w _TBAF = 0.10 was smaller than that obtained with w _TBAF = 0.20 and 0.30. The results found that gas pressure greatly changed the CO₂ capture amount in TBAF hydrates, and the aqueous composition highly affected CO₂ selectivity. The crystal morphology and single-crystal structure analyses suggested that polymorphism of TBAF hydrates with congruent aqueous solution may lower both the CO₂ capture amount and selectivity. Our present results proposed that an aqueous solution with w _TBAF = 0.20 is advantageous for the CO₂ capture from flue gas compared to near congruent solutions of TBAF hydrates (w _TBAF = 0.30) and dilute solution (w _TBAF = 0.10).

X-Ray attenuation and image contrast in the X-ray computed tomography of clathrate hydrates depending on guest species

In this study, X-ray imaging of inclusion compounds encapsulating various guest species was investigated based on the calculation of X-ray attenuation coefficients.

Structure-driven CO2 selectivity and gas capacity of ionic clathrate hydrates

Ionic clathrate hydrates can selectively capture small gas molecules such as CO₂, N₂, CH₄ and H₂. We investigated CO₂ + N₂ mixed gas separation properties of ionic clathrate hydrates formed with tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium chloride (TBAC), tetra-n-butylphosphonium bromide (TBPB) and tetra-n-butylphosphonium chloride (TBPC). The results showed that CO₂ selectivity of TBAC hydrates was remarkably higher than those of the other hydrates despite less gas capacity of TBAC hydrates. The TBAB hydrates also showed irregularly high CO₂ selectivity at a low pressure. X-ray diffraction and Raman spectroscopic analyses clarified that TBAC stably formed the tetragonal hydrate structure, and TBPB and TBPC formed the orthorhombic hydrate structure. The TBAB hydrates showed polymorphic phases which may consist of the both orthorhombic and tetragonal hydrate structures. These results showed that the tetragonal hydrate captured CO₂ more efficiently than the orthorhombic hydrate, while the orthorhombic hydrate has the largest gas capacity among the basic four structures of ionic clathrate hydrates. The present study suggests new potential for improving gas capacity and selectivity of ionic clathrate hydrates by choosing suitable ionic guest substances for guest gas components.

Investigation of kinetics of tetrabutylammonium chloride (TBAC) + CH4 semiclathrate hydrate formation

Different from structures of TBAC hydrate, (TBAC + CH₄) hydrates were formed with hexagonal or tetragonal structure under different w.