Ionic Clathrate Hydrates Research Articles

Introduction of Cyclic Structures into Phosphonium Salts As Potential Guest Substances for Ionic Clathrate Hydrates

An ionic clathrate hydrate composed of tri-n-butylbenzyl-phosphonium chloride has been investigated from the viewpoint of phase equilibrium (temperature-composition) relations in comparison with an ionic clathrate hydrate with tri-n-butyl(cyclohexylmethyl)phosphonium bromide. The highest dissociation temperature of the ionic clathrate hydrates composed of tri-n-butylbenzylphosphonium chloride and tri-n-butyl(cyclohexyl-methyl)phosphonium bromide were 274.4 ± 0.1 K and 276.0 ± 0.1 K, respectively at the atmospheric pressure. The dissociation enthalpy of tri-n-butylbenzyl-phosphonium chloride ionic clathrate hydrate was 192 ± 3 kJ·kg−1. These results allow us to suggest that controlling steric structures in the guest compounds might be one of the options for tuning the equilibrium temperatures of ionic clathrate hydrates.

Fluoride-incorporated ionic clathrate hydrates

Fluoride-incorporated ionic clathrate hydrates

Effect of introducing a cyclobutylmethyl group into an onium cation on the thermodynamic properties of ionic clathrate hydrates

Cyclic hydrocarbon groups in phosphonium cations can be one of the options for tuning the equilibrium temperature of ionic clathrate hydrates.

Ionic Clathrate Hydrates of Tetraalkylammonium/phosphonium Salts: Structures, Properties, Some Applications, and Perspectives

Ionic clathrate hydrates (ICHs) belong to one of the groups of a wide class of clathrate hydrates. In the crystal structures of ICHs, cation–anion pairs of guest molecules, typically tetraalkylammonium/phosphonium salts, are included into the host framework of hydrogen-bonded water molecules. Because of their suitable phase change temperature and latent heat, ICHs are considered as promising phase change materials for application in cold energy storage and transportation. The presence of vacant cavities in ICHs structures, which can incorporate small gas molecules such as, for example, H2, CH4, CO2, N2, H2S, determines the possibility of their use for the separation and storage of gases. The formation of these double ICHs occurs under much milder pressure and temperature conditions compared to the pure gas hydrates of the same gases. Studying of the structural characteristics and thermophysical properties of ICHs and double ICHs with gases is the necessary basis for their effective applications in different practical ends. This work reviews information related to the new details and features of the structures of ICHs and double ICHs with gases revealed over the last decade or a little more as well as some thermophysical data such as hydrate numbers, temperatures and enthalpies of dissociation. This review focuses only on ICHs of tetrabutyl-, tetraisoamylammonium (TBA, TiAA), and tetrabutylphosphonium (TBP) salts, which are the most common representatives of this class of compounds. One of the sections is devoted to ICHs of TBA and TiAA with polymeric anions such as cross-linked polyacrylates. The purpose of the present paper is to review the studies that have been relatively little addressed in previous works.

Hydroxide-Enhanced Superexchange Magnetic Couplings in Ionic Clathrate Hydrates

The guest-driven magnetic properties of clathrate hydrates (CHs) has a promising application prospect as a new type of icy magnetic material, but it meets more difficulties for practical applicatio...

Theoretical investigation of thermodynamic properties of tetrabutylammonium bromide ionic clathrate hydrate

In the framework of the lattice dynamics method, the densities of the phonon states of tetrabutylammonium bromide ionic clathrate hydrate have been found. The dynamic stability of the hydrate is shown for a number of temperatures. Using the developed statistical-thermodynamic theory, the free energy curves have been constructed for a wide range of temperatures at different unit cell volumes and, based on the calculated data, the lines of thermal expansion of the hydrate have been constructed.

Ionic Clathrate Hydrates of Tetra-n-Butylammonium Nitrate (TBANO3) and Mixed TBA(NO3,OH): Novel Superstructures of Tetragonal Structure I

The structures of an ionic clathrate hydrate of tetra-n-butylammonium nitrate TBANO3·26.7H2O and mixed TBA(NO3)1−x(OH)x·(31.5 − x)H2O hydrate obtained from a mixture of tetra-n-butylammonium nitrate and tetra-n-butylammonium hydroxide are determined by single crystal X-ray diffraction. These structures are new types of superstructures based on classical tetragonal structure I with four- and twofold unit cells volumes for TBANO3·26.7H2O and TBA(NO3)1−x(OH)x·(31.5 − x)H2O respectively. The superstructures form as a result of ordering in the arrangement of multicompartment cavities of the water-anionic framework. A TBA cation is displaced from the center of combined cavities due to the anisotropic environment of the anions, which leads to an additional symmetry reduction. The modes of the nitrate anion incorporation into the water framework of ionic clathrate hydrates are determined for the first time.

Enhanced performance on CO2 adsorption and release induced by structural transition that occurred in TBAB·26H2O hydrates

The applications of ionic clathrate hydrates have greatly improved the efficiency and the conditions required for hydrate-based CO2 capture, but high energy input for hydrate growth and complicated treatment of hydrate slurry still hinder their commercial use. Here we chose TBAB·26H2O hydrate particles to adsorb CO2 molecules instead of TBAB solutions below 2 MPa and release them at ambient pressure. Results showed that the TBAB·26H2O hydrate could adsorb CO2 without induction time and enhance the gas storage capacity by structural transition, especially under high pressure. By using in situ Raman, CO2 molecules were found to fill the empty cages in TBAB·26H2O hydrate first, the formed nCO2·TBAB·26H2O hydrate then converted to nCO2·TBAB·38H2O and TBAB·21/3H2O hydrates at 2 MPa. Macroscopic measurements revealed that around 20 volume of CO2 in standard state could be adsorbed by 1 volume of TBAB·26H2O hydrate sample at 1 MPa, but this volume ratio could reach 67 v/v at 2 MPa where structural change was thought to take place. The pressurized CO2 trapped in hydrate phase was assumed to destroy the structure of TBAB·26H2O hydrate easily, and force the water molecules to form a structure that more compatible with CO2 molecules. This may explain why nCO2·TBAB·26H2O hydrates barely grow from TBAB solutions when pressurized CO2 is injected. In the CO2 release process, the nCO2·TBAB·38H2O and TBAB·21/3H2O hydrates quickly transformed back to nCO2·TBAB·26H2O hydrates and 60–80% of the captured CO2 could be released. Combing with their excellent gas selectivity, TBAB·26H2O hydrate particles would be an ideal material for hydrate-based CO2 capture.

Clathrate nanocage reactor for the decomposition of greenhouse gas

The atmospheric level of nitrous oxide (N2O), a representative greenhouse gas, is increasing, and the decomposition of N2O generally requires an energy-intensive reaction. This study introduces a nano-sized cage of ionic clathrate hydrate as a nanoreactor to decompose N2O. Ionic species, (Me)4N+ and OH−, are injected to boost electron transfer in the activated water-framework of ionic clathrate hydrate. Molecular N2O is captured in empty cages and decomposed to non-greenhouse gases of nitrogen and oxygen by the transferred electrons from the methyl radical decay. This suggests that the clathrate hydrate can be used for both the sequestration and treatment of greenhouse gases.

Kinetic Measurements on CO2 Hydrate Formation in the Presence of Tetra-n-butyl Ammonium Bromide

Ionic clathrate hydrates are promising materials for hydrate-based CO2 capture due to their mild forming conditions. In this work, hydrate crystallizations from different TBAB solutions were measur...

Structural and physicochemical studies of hydration of crosslinked TBA polyacrylates with different substitution degrees of H+ ions with TBA+ ions

ABSTRACTX‐ray powder diffraction studies and physicochemical studies have been carried out of the hydrate phases formed in binary water systems with crosslinked tetrabutylammonium (TBA) polyacrylates (n = 1%) in mixed [x(C4H9)4N + (100−x)H] form with low degrees of substitution of proton ions of the carboxylic groups in poly(acrylic acid) for TBA cations x = 40%, 30%, and 20%. As was shown by structural studies, the clathrate hydrate is formed in the water system with the polyacrylate in the mixed form with x = 40%. The structure is analogous to that of earlier studied hydrates formed by crosslinked polyacrylates in mixed [x(C4H9)4N + (100−x)H] form with higher values of x = ∼100%, 80%, and 60% and can be related to the tetragonal structure I, characteristic of the ionic clathrate hydrates of TBA salts with monomeric anions. Decomposition temperature and fusion enthalpy of the studied hydrate were determined using differential thermal analysis and calorimetric methods. It was revealed that the further decrease of x led to the destruction of clathrate hydrate framework. According to the results of X‐ray powder diffraction research, the phase of ice is crystallized instead of the hydrate phase in water systems with the polyacrylates with x = 30% and 20%. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46209.

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.

Gas separation of flue gas by tetra-n-butylammonium bromide hydrates under moderate pressure conditions

Ionic clathrate hydrates are composed of water and ionic guest substance, which can selectively capture gas under moderate conditions. We performed gas separation experiments with tetra-n-butylammonium bromide (TBAB) widely-used for an ionic guest substance. The experiments in a closed system showed good CO2 gas selectivity of the TBAB hydrates even under the mild conditions: 1 MPa and 282 K. We also performed the gas separation with tetrahydrofuran (THF) which is a guest substance forming the structure II clathrate hydrate. Comparison with THF clearly revealed the better CO2 selectivity of TBAB than that of the structure II clathrate hydrate. We further compared our data with the literature, and found that the condition of low pressure and dense TBAB concentration provided superior CO2 selectivity. Gas separation with continuous gas flow was demonstrated. The hydrate formation behavior was similar to the cases without gas flow. The results showed that controlling the crystal growth temperature is important to capture gases by the TBAB hydrates.

Calorimetric and PXRD studies of ionic clathrate hydrates of tetrabutylammonium carboxylates in binary (C4H9)4NCnH2n+1CO2–H2O (n = 0–3) systems

The crystal structure of the ionic clathrate hydrates of peralkylonium salts consists of hydrogen-bonded water molecules and anions forming host cage-like water-anion lattice, while peralkylonium cations are included into the cages of the lattice as guests. The ionic clathrate hydrates of some peralkylonium salts are considered as potentially applicable in various fields including gas separation, gas storage and transportation, cold storage and transportation. In this work, we report a synthesis and experimental measurements of compositions, melting points, enthalpies of fusion, as well as the results of the PXRD studies of eight ionic clathrate hydrates of tetrabutylammonium carboxylates formed in (C4H9)4NCnH2n+1CO2–H2O (n = 0–3) binary systems. The enthalpies of fusion values of these hydrates are measured for the first time. Three structural types were observed in the studied systems: on the base of tetragonal structure-I, cubic structure-I, and hexagonal structure-I. The data on thermal properties of studied ionic clathrate hydrates indicate that they are promising as phase change materials for cold storage and air-conditioning systems.

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.

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.

Characterization of the ionic clathrate hydrate of tetra-n-butylammonium acrylate

The ionic clathrate hydrate of tetra-n-butylammonium (TBA) acrylate was characterized using single-crystal X-ray diffraction, elemental analysis, and nuclear magnetic resonance (NMR) spectroscopy. The crystal structure of TBA acrylate was Jeffrey’s type III and tetragonal P42/n, with a 33.076(7) × 33.076(7) × 12.170(2) Å3 unit cell. The volume of the unit cell was 13315(5) Å3, which is almost twice that of the ideal structure. The TBA cation was disordered and located in two types of fused cages. Although the acrylate anion was located in a pentagonal dodecahedral cage neighboring the TBA cation, there is a residual acrylate anion that could be around the other TBA cation in the unit cell. Solid-state 13C NMR spectra showed that the TBA cation was clearly disordered at 173 K, but not at 239 K. NMR peaks from the acrylate anion were not observed at either temperature. This is probably because of the strong restriction on the acrylate anion by hydrogen bonding with the lattice water. Some of the characteristics of the anion and cation of the ionic guest incorporated in the hydrate structure have yet to be defined. Further research is needed to clarify complexation of the ionic clathrate hydrate and the ionic guest, and the resulting structure.

Crystal engineering and characterization of a structure-H ionic clathrate hydrate

Ionic hydrates are known to form numerous clathrate structures in which either the cations or anions sit in cages and the counterions are incorporated into the water framework. Due to the inclusion of the ionic species, such ionic clathrate hydrates not only show many peculiar features such as metal ion encagement and superoxide ion generation, but also exhibit notable physicochemical properties such as outstanding ionic conductivity and thermal stability. Thus, the ionic clathrate hydrates are considered for their potential applicability in various fields, including those that involve solid electrolytes, gas sensors, and energy storage. In this study, we report the design, synthesis, and characterization of the first ionic clathrate hydrate of the hexagonal structure-H (Str.H) crystal type. Diethyl-dimethyl-ammonium hydroxide hydrate was synthesized with CH4 and Xe as help gases, and the crystal structure was identified by powder X-ray diffraction analysis. Further confirmation of the formation of Str.H was obtained from Raman spectroscopy and 13C, 129Xe, and 2H solid-state NMR spectroscopy. From 13C NMR and ab initio calculations, it was shown that the quaternary ion occupies the large cage of Str.H with a conformation different from that in solution, due to constraints imposed by the dimensions of the cage. The H deficiency introduced by substitution of OH– for a water molecule appears, from 129Xe NMR, to be disordered over the framework, and, from 2H NMR, to substantially increase the rate of reorientational mobility of the D atoms in the framework, over that observed for a Str.I hydrate and for ice. The Str.H hydrates are commonly more stable than other structures, thus the present findings on the ionic Str.H clathrate hydrate may offer a new approach for improving the stability of ionic clathrate hydrates for their practical application.

Phase Behavior and Structural Characterization of Ionic Clathrate Hydrate Formed with Tetra-n-butylphosphonium Hydroxide: Discovery of Primitive Crystal Structure

This paper reports phase equilibrium measurements and crystal structure analysis on the ionic clathrate hydrate formed from tetra-n-butylphosphonium hydroxide (TBPOH). Phase equilibrium temperatures were measured in the mole fraction range of TBPOH in aqueous solution from 0.0072 to 0.0416. The highest ionic clathrate hydrate–solution equilibrium temperature was determined to be 290.2 K at a TBPOH mole fraction of 0.0340, which corresponds to the congruent composition. Single-crystal X-ray diffraction measurements were performed on the crystal formed at 288.7 K, and the chemical composition of the TBPOH hydrate crystal was determined to be TBPOH·29.6H2O, which is consistent with the congruent composition obtained by the phase equilibrium measurement. The crystal structure of the TBPOH hydrate has a superstructure identical with Jeffrey’s type I cubic structure, with an I43d space group with a lattice constant of 24.5191(13) A. The TBPOH hydrate structure is compared with the same hydrate structure formed...

Thermodynamic stability conditions, methane enrichment, and gas uptake of ionic clathrate hydrates of mine ventilation air

Abatement of methane from mine ventilation air (MVA) is a significant challenge faced by coal mining industry. A promising method for methane capture from gas mixture is clathrate hydrate formation. In search of suitable and cost-effective low-dosage promoters for hydrate-based methane capture processes, this paper reports the pressure requirement for the hydrate formation of simulated MVA (0.5vol% CH4+99.5vol% air) and its potential for methane extraction, in the presence of tri-n-butyl phosphine oxide (TBPO) or tetra-n-butyl ammonium bromide (TBAB) at three different initial loadings (5wt%, 15wt%, and 26wt%). An isochoric equilibrium step-heating pressure search method was used to measure the hydrate phase equilibrium conditions at the temperature range of (277.61–295.54) K and pressure range of (0.23–19.11) MPa. It was found that at a given initial loading, TBPO was largely more effective than TBAB in reducing the pressure requirement for hydrate formation of MVA. At a given temperature, the equilibrium pressures of the clathrate hydrates were indifferent to the change in the initial loading of TBPO from 5wt% to 26wt%, in contrast to those of TBAB. Gas composition analysis by gas chromatography confirmed that CH4 could be significantly enriched in the ionic clathrate hydrates, and the highest methane enrichment ratio obtained in the present work was 300%, with TBPO at initial loading of 5wt%. At this relatively low loading, within a given period of 5h, TBPO also led to higher gas uptake compared with TBAB. The advantages of TBPO as a promoter of MVA hydrate were discussed.