Thermodynamic and Spectroscopic Identification of Guest Gas Enclathration in the Double Tetra-n-butylammonium Fluoride Semiclathrates

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.

Designing the structure and relevant properties of semiclathrate hydrates by partly asymmetric alkylammonium salts

Semiclathrate hydrates are host-guest materials that form from ionic guests and water. There are numerous options for ionic guests, such as quaternary ammonium salts, to tune the functional properties of these materials such as melting temperature, fusion heat, and gas capacity and selectivity. To design these materials, the stabilization mechanism of the side chains of quaternary ammonium salts must be understood based on both thermodynamic and crystallographic properties and relevant host-guest dynamics. In this paper, we studied semiclathrate hydrates formed from n-propyl, tri-n-butylammonium bromide (N₃₄₄₄Br) and tri-n-butyl, n-pentylammonium bromide (N₄₄₄₅Br). Their cation side chains are decremented or incremented from tetra-n-butylammonium (N₄₄₄₄ or TBA), which is one of the best fits for semiclathrate hydrate structures. The use of the widely used tetra-n-butylammonium bromide (N₄₄₄₄Br or TBAB) as an ionic guest, an increment of the carbon chain, i.e., N₄₄₄₅Br, caused disorders in its hydrate structure due to the oversizing of the cation. This suitably oversized cation selectively stabilized the orthorhombic structure, whose hydration number is relatively high. As a result, the fusion heat at the congruent composition of the hydrate phase was higher than that of the widely used N₄₄₄₄Br (TBAB) hydrates. The N₃₄₄₄Br hydrate showed both significantly decreased melting temperature and fusion heat compared to the N₄₄₄₄Br (TBAB) hydrates. The phase behaviour of the N₃₄₄₄Br hydrate was found to be analogous to that of the N₄₄₄₄Br (TBAB) hydrates. It was demonstrated that the semiclathrate hydrate structures and relevant properties can be modified by adjusting the alkyl side chain length of quaternary ammonium salts.

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).

Enhanced CH₄ Recovery Induced via Structural Transformation in the CH₄/CO₂ Replacement That Occurs in sH Hydrates

The CH4/CO2 replacement that occurs in sH hydrates is investigated, with a primary focus on the enhanced CH4 recovery induced via structural transformation with a CO2 injection. In this study, neohexane (NH) is used as a liquid hydrocarbon guest in the sH hydrates. Direct thermodynamic measurements and spectroscopic identification are investigated to reveal the replacement process for recovering CH4 and simultaneously sequestering CO2 in the sH (CH4 + NH) hydrate. The hydrate phase behavior and the (13)C NMR and Raman spectroscopy results of the CH4 + CO2 + NH systems demonstrate that CO2 functions as a coguest of sH hydrates in CH4-rich conditions, and that the structural transition of sH to sI hydrates occurs in CO2-rich conditions. CO2 molecules are found to preferentially occupy the medium 4(3)5(6)6(3) cages of sH hydrates or the large 5(12)6(2) cages of sI hydrates during the replacement. Due to the favorable structural transition and resulting re-establishment of guest distributions, approximately 88% of the CH4 is recoverable from sH (CH4 + NH) hydrates with a CO2 injection. The hydrate dissociation and subsequent reformation caused by the structural transformation of sH to sI is also confirmed using a high-pressure microdifferential scanning calorimeter through the detection of the significant heat flows generated during the replacement.

CO2 capture from simulated fuel gas mixtures using semiclathrate hydrates formed by quaternary ammonium salts

In order to investigate the feasibility of semiclathrate hydrate-based precombustion CO2 capture, thermodynamic, kinetic, and spectroscopic studies were undertaken on the semiclathrate hydrates formed from a fuel gas mixture of H2 (60%) + CO2 (40%) in the presence of quaternary ammonium salts (QASs) such as tetra-n-butylammonium bromide (TBAB) and fluoride (TBAF). The inclusion of QASs demonstrated significantly stabilized hydrate dissociation conditions. This effect was greater for TBAF than TBAB. However, due to the presence of dodecahedral cages that are partially filled with water molecules, TBAF showed a relatively lower gas uptake than TBAB. From the stability condition measurements and compositional analyses, it was found that with only one step of semiclathrate hydrate formation with the fuel gas mixture from the IGCC plants, 95% CO2 can be enriched in the semiclathrate hydrate phase at room temperature. The enclathration of both CO2 and H2 in the cages of the QAS semiclathrate hydrates and the structural transition that results from the inclusion of QASs were confirmed through Raman and (1)H NMR measurements. The experimental results obtained in this study provide the physicochemical background required for understanding selective partitioning and distributions of guest gases in the QAS semiclathrate hydrates and for investigating the feasibility of a semiclathrate hydrate-based precombustion CO2 capture process.

2-Propanol as a co-guest of structure II hydrates in the presence of help gases

The enclathration of 2-propanol (2-PrOH) as a co-guest of structure II (sII) hydrates in the presence of CH4 and CO2 was experimentally verified with a focus on macroscopic phase behaviors and microscopic analytical methods such as powder X-ray diffraction (PXRD) and NMR spectroscopy. 2-PrOH functioned as a hydrate promoter in the CH4 + 2-PrOH systems, whereas it functioned as an apparent hydrate inhibitor in the CO2 + 2-PrOH systems despite the inclusion of 2-PrOH in the hydrate lattices. From the PXRD patterns, both double CH4 + 2-PrOH and double CO2 + 2-PrOH hydrates were identified to be cubic (Fd3m) sII hydrates. From the (13)C NMR spectra, it was found that, at a lower 2-PrOH concentration, the small 5(12) cages of the sII hydrate were occupied by CH4 molecules only, whereas the large 5(12)6(4) cages were shared by CH4 and 2-PrOH molecules. However, at a stoichiometric concentration, the large cages were occupied by 2-PrOH molecules only, and the corresponding chemical formula for this concentration is 1.50CH4·0.98 2-PrOH·17H2O.