Sanehiro Muromachi

Guest-induced symmetry lowering of an ionic clathrate material for carbon capture

We report a new lattice structure of the ionic clathrate hydrate of tetra-n-butylammonium bromide induced by guest CO2 molecules, which is found to provide high CO2 storage capacity. The structure was characterized by a set of methods, including single crystal X-ray diffraction, NMR, and MD simulations.

Characterization of tetra-n-butylphosphonium bromide semiclathrate hydrate by crystal structure analysis

We report the crystal structure analysis of the semiclathrate hydrate of tetra-n-butylphosphonium bromide (TBPB), which is a candidate material for refrigeration and gas-capture technologies. Refinement of the single crystal X-ray diffraction measurements revealed that the found structure of the TBPB hydrate has an orthorhombic structure, with the space group Pmma, and unit cell parameters a = 21.065(5), b = 12.657(3) and c = 11.992(3) A. The chemical formula is TBPB·38H2O. The TBP ion is accommodated in a combined cage that consists of two tetrakaidecahedra and two pentakaidecahedra. The structure features three dodecahedral cages for each TBPB molecule that may accommodate small gas molecules (e.g., CH4, CO2 and N2). The structure determined in this work is compared in detail with that of a similar hydrate, tetra-n-butylammonium bromide (TBAB) hydrate. In contrast to the TBAB hydrates, the most stable structure of the TBPB hydrate is not tetragonal but orthorhombic. Since C–P has a longer bond length than C–N, the TBP ion was packed tightly by the combined cage, having complex disorder. The relative comparison of the atom positions showed that the difference in the bond lengths of the two cations is counteracted by the displacement of water molecules in the TBPB hydrate lattice.

Selective occupancy of methane by cage symmetry in TBAB ionic clathrate hydrate

Methane trapped in the two distinct dodecahedral cages of the ionic clathrate hydrate of TBAB was studied by single crystal XRD and MD simulation. The relative CH4 occupancies over the cage types were opposite to those of CO2, which illustrates the interplay between the cage symmetry and guest shape and dynamics, and thus the gas selectivity.

Bulk phase behavior of tetra- n -butylammonium bromide hydrates formed with carbon dioxide or methane gas

We report the bulk phase behavior of ionic clathrate hydrates of tetra-n-butylammonium bromide (TBAB) formed with a common guest substance: CO2 or CH4. We formed the bulk samples by a classical mixing reactor for gas hydrates, and measured them by the powder X-ray diffraction (PXRD). PXRD patterns of the TBAB+(CO2 or CH4) hydrates formed with 0.32 of TBAB mass fraction in the aqueous phase were obtained. They are consistent with the orthorhombic hydrate (Shimada et al., Acta Crystallogr. 2005; Muromachi et al., Chem. Commun. 2014), but not identical with the other stable phase, i.e., the tetragonal TBAB hydrate (Rodionova et al., J. Phys. Chem. B 2013). When the aqueous solutions are under the substantial pressure of CO2 or CH4 gas, TBAB is likely to form the orthorhombic Pmma and/or Imma phases. A question for the bulk orthorhombic TBAB hydrate phase about the scarce gas incorporation is newly proposed.

Increasing molecular O3 storage capacity in a clathrate hydrate

This paper reports an experimental study to further increase the ozone storage capacity in a clathrate hydrate and to better understand the relationship between the gas phase O3 concentration and the O3 storage capacity in the hydrate. We performed experiments with the O3 + O2 + CO2 feed gas with an increased O3 fraction in the gas phase exceeding that covered by a preceding study. To accurately specify the thermodynamic conditions to form the hydrate, we first measured the three-phase (gas + liquid + hydrate) equilibrium conditions for the (O3 + O2 + CO2 + H2O) and (O2 + CO2 + H2O) systems. The phase equilibrium data cover the temperature range from 272 to 277 K, corresponding to pressures from 1.6 to 3.1 MPa, for each of the two different (O3 + O2)-to-CO2 or O2-to-CO2 molar ratios in the feed gas, which are approximately 4 : 6 and 5 : 5, respectively. The O3 fraction in the gas phase was ∼0.025. Based on the equilibrium data, we prepared crystal samples of the O3 + O2 + CO2 hydrates at a system pressure of 3.0 MPa and a temperature of 272 K. The highest O3 storage capacity in the hydrates was measured to be 2.15 mass% which is 2.36 times higher than the highest past record of 0.91 mass%. The results also show that the dominant factor to control the O3 storage capacity in the hydrates is the O3 mole fraction in the gas phase in contact with the hydrates.

Thermodynamic stabilization of semiclathrate hydrates by hydrophilic group

Introducing hydrophilic groups into carboxylates is a way to modify semiclathrate hydrate frameworks and change the properties of the hydrates. In this study, we report the characterization of semiclathrate hydrates formed by tetra-n-butylammonium (TBA) 2-hydroxybutyrate (2HB). In addition, TBA lactate and the TBA 2HB salt formed stable hydrate crystals, which basically had rectangular columnar shapes. We performed equilibrium measurements and calorimetry. The melting temperature and fusion heat of the TBA 2HB hydrate crystals were 285.3 K and 177 kJ kg−1, respectively. A comparison with other carboxylate anions showed that the substitution of hydrogen atom at the 2-position in the carbon chain by a hydrophilic hydroxy group stabilizes the hydrates more than that by hydrophobic methyl group, which is the case for alcohols in clathrate hydrates. The phase equilibrium data for a number of semiclathrate hydrates were compared. A rough trend of temperature depending on type of guest anions was observed, but it is unclear if there are other correlating factors.

Structure-driven CO 2 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.