Allokiric solutions based on clathrate hydrates in tetrabutylammonium fluoride-ammonium fluoride-water system

Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 5, pp. 88–95, September–October, 1992.     

Double clathrate hydrates of cross-linked tetrabutylammonium polyacrylate and noble gases at high pressures

Temperatures of hydrate decomposition were measured by means of the differential thermal analysis at a pressures up to 800–900 Mpa in the systems: cross-linked tetrabutylammonium polyacrylate–water and cross-linked tetrabutylammonium polyacrylate–water–noble gas (He, Ne, Ar, Kr, Xe). The effect of the deformation of D- cavities of the hydrates on the temperature of their decomposition is discussed on the basis of the experimental data.     

Structural transformation due to Co-host inclusion in ionic clathrate hydrates

The present findings on the co-host role in restructuring the host water framework might provide important information on tuning the cage dimensions via lattice distortion and promoting the total number of cages via structural transformation. This co-host-induced structural modification can improve the physicochemical properties of ionized clathrate hydrates, particularly given that the host framework is able to function as a pathway to deliver protons or electrons.     

Cubic structure II double clathrate hydrates with tetra(n-propyl)ammonium fluoride

A double clathrate hydrate with the composition THF·0.5(n-Pr)₄NF·16H₂O and cubic structure II (CS-II,a=17.67 Å) has been obtained. Its experimental density is 1.053±0.001 g/cm³; its melting point is 8.1°C, i.e. 3.1°C higher than that of the THF·17H₂O hydrate. The double hydrates of acetone, 1,4-dioxan, trimethyleneoxide and 1,3-dioxolane with (n-Pr)₄NF have melting points of –14.8, –5.5, –2.6 and –9.6°C, respectively. With pressure increase up to 6 kbar the melting points of the double hydrates increase monotonously in contrast to common CS-II hydrates. The friability of the structure of the hydrates (the packing coefficient) and their sensitivity to pressure (dT/dP) are compared.The results of this work have been reported at the International Seminar on Inclusion Compounds, Jaszowiec (Poland), 24–26th September 1987.     

Thermodynamic studies of clathrate hydrates

Thermodynamic studies of clathrate hydrates, mainly of structures I and II, are considered in this review which is based on 147 references. There are two main subjects. The first is the host lattice energy and the guest-host interaction energy, both of these quantities being related to the enthalpy of dissociation and composition of the hydrates. The second subject concerns orientational ordering phenomena occurring in both host and guest, as reflected in the low temperature heat capacity. The classical theoretical treatment of clathrate formation has been reconsidered on the basis of recent experimental results. Particular emphasis has been given to orientational ordering since this topic is undoubtedly central to clarifying the nature of clathrate hydrates.Ausgehend von 147 Literaturangaben wurden in diesem Review thermodynamische Untersuchungen von Klathrathydraten hauptsächlich der Struktur I und II betrachtet. Es gibt zwei Hauptaugenmerke. Als erstes die Wirtsgitterenergie und die Gast-Wirt-Wechselwirkungsenergie, beide bezogen auf die Dissoziationsenthalpie und die Bildungsenthalpie der Hydrate. Das zweite Hauptaugenmerk betrifft Orientierungs-Konditionierungserscheinungen sowohl in Wirt als auch Gast, wie in den Wärmekapazitäten bei niedrigen Temperaturen widergespiegelt wird. Auf der Basis jüngster experimenteller Ergebnisse wurde die klassische theoretische Betrachtung über die Bildung von Klathraten überprüft. Der Orientierung-Konditionierung wurde besonderer Nachdruck verliehen, da dies zweifellos eine entscheidende Rolle bei der Klärung der Natur der Klathrathydrate spielt. 147 I II. . «» « — », . «» « », . . , .

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Contribution No. 155 from the Chemical Thermodynamics Laboratory.     

Anhydrous tetrabutylammonium fluoride

Tetrabutylammonium fluoride (TBAF) is prepared at low temperature by nucleophilic aromatic substitution of hexafluorobenzene with tetrabutylammonium cyanide. Adventitious water is scavenged during this synthesis by the generated hexacyanobenzene, which readily adds water under basic conditions. Contrary to expectations, TBAF is stable to Hofmann elimination in polar aprotic solvents under anhydrous conditions. Added hydroxylic solvents are shown to catalyze the decomposition of TBAF and to catalyze proton exchange with DMSO. The synthetic utility of this salt is described briefly.     

The formation of clathrate-like hydrates of tetrabutylammonium halogenated carboxylates

The solid-liquid phase diagrams of binary mixtures of tetrabutylammonium halogenated carboxylates with water were examined in order to confirm the formation of clathrate-like hydrates. It was found that, among thirteen carboxylates examined, four carboxylates having CH₂FCOO^–, CHF₂COO^–, CF₃COO^–, and CH₂ClCOO^–, formed a hydrate with hydration numbers around 30 and seven carboxylates having CHCl₂COO^–, CCl₃COO^–, CH₂BrCOO^–, CHBr₂COO^–, CBr₃COO^–, CH₃CHClCOO^–, and CH₃CHBrCOO^– formed a hydrate with hydration numbers around 23. The latter hydrate has not been reported earlier. The melting points of these newly found hydrates were fairly high: they lie between 10 and 16°C. The effect of Cl and Br atoms attached to the carbon atom of the-position of a carboxylate anion both on the type of hydrate formed and on its stability was greatly different from that of a CH₃ group attached to the same position of the carboxylate anion.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

Thermal expansion of structure-H clathrate hydrates

The temperature dependence of the unit cell parameters of two newly identified hexagonal structure clathrate hydrates of hexamethylethane (HME) and 2,2-dimethylbutane (DMB) have been measured by X-ray powder diffraction. The thermal expansion of the two distinct crystallographic axes was found to be inequivalent. However, the coefficients of cubic expansion are comparable to that in the cubic structure I and II hydrates. The larger thermal expansivity in the clathrate hydrates relative to ice is attributed to the weakening of the host lattice due to the internal pressure generated by the rattling motions of the encaged guests.Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

Fluorodenitrations using tetrabutylammonium fluoride

Fluoroaromatics are prepared in good yield under mild conditions using the tetrabutylammonium fluoride promoted fluorodenitration of nitroaromatics.     

Clathrate hydrates of tetrabutylammonium carboxylates and dicarboxylates

Phase diagrams of the binary aqueous systems with tetra-n-butylammonium (TBA) carboxylates ((C₄H₉)₄NC_nH_{2 n+1}CO₂, where n=0÷5) and dicarboxylates ([(C₄H₉)₄N]₂(CH₂)_nC₂O₄, where n=1÷3) including some branched carboxylate anions, have been studied in the field of crystallization of clathrate hydrates. Monocrystals of many hydrates have been prepared and their composition, densities, melting points and X-ray data have been determined. In the set of TBA carboxylate hydrates the stability increases towards TBA propionate or butyrate hydrates (for different structures) and then it decreases as sizes of anions grow, This is explained by additional stabilization of the framework caused by small cavities being occupied until the hydrophobic part of the anion is able to go in the cavity. In the set of hydrates of TBA dicarboxylates the change of the stability is easily accounted for by the modelling of the inclusion of dicarboxylate ions in the cavities, within known structures, different ways of hydrophilic inclusion being taken into account.     

Polymorphism in Br2 clathrate hydrates

The structure and composition of bromine clathrate hydrate has been controversial for more than 170 years due to the large variation of its observed stoichiometries. Several different crystal structures were proposed before 1997 when Udachin et al. (Udachin, K. A.; Enright, G. D.; Ratcliffe, C. I.; Ripmeester, J. A. J. Am. Chem. Soc. 1997, 119, 11481) concluded that Br2 forms only the tetragonal structure (TS-I). We show polymorphism in Br2 clathrate hydrates by identifying two distinct crystal structures through optical microscopy and resonant Raman spectroscopy on single crystals. After growing TS-I crystals from a liquid bromine-water solution, upon dropping the temperature slightly below -7 degrees C, new crystals of cubic morphology form. The new crystals, which have a limited thermal stability range, are assigned to the CS-II structure. The two structures are clearly distinguished by the resonant Raman spectra of the enclathrated Br2, which show long overtone progressions and allow the extraction of accurate vibrational parameters: omega(e) = 321.2 +/- 0.1 cm(-1) and omega(e)x(e) = 0.82 +/- 0.05 cm(-1) in TS-I and omega(e) = 317.5 +/- 0.1 cm(-1) and omega(e)x(e) = 0.70 +/- 0.1 cm(-1) in CS-II. On the basis of structural analysis, the discovery of the CS-II crystals implies stability of a large class of bromine hydrate structures and, therefore, polymorphism.     

Thermodynamic modeling for clathrate hydrates of ozone

We report a theoretical study to predict the phase-equilibrium properties of ozone-containing clathrate hydrates based on the statistical thermodynamics model developed by van der Waals and Platteeuw. The Patel–Teja–Valderrama equation of state is employed for an accurate estimation of the properties of gas phase ozone. We determined the three parameters of the Kihara intermolecular potential for ozone as a = 6.815 · 10⁻² nm, σ = 2.9909 · 10⁻¹ nm, and ε · k_B⁻¹ = 184.00 K. An infinite set of ε–σ parameters for ozone were determined, reproducing the experimental phase equilibrium pressure–temperature data of the (O₃ + O₂ + CO₂) clathrate hydrate. A unique parameter pair was chosen based on the experimental ozone storage capacity data for the (O₃ + O₂ + CCl₄) hydrate that we reported previously. The prediction with the developed model showed good agreement with the experimental phase equilibrium data within ±2% of the average deviation of the pressure. The Kihara parameters of ozone showed slightly better suitability for the structure-I hydrate than CO₂, which was used as a help guest. Our model suggests the possibility of increasing the ozone storage capacity of clathrate hydrates (∼7% on a mass basis) from the previously reported experimental capacity (∼1%).     

Dynamic and thermodynamic properties of clathrate hydrates

Vibrational spectra and thermodynamic properties of ices and the cubic structure I (CS-I) clathrate hydrate have been studied by the lattice dynamics method. The phonon density of states for the empty hydrate framework and for xenon hydrate have been determined; the vibrational frequencies of the guest molecules in large and small cavities have been found. The stability of the hydrate with respect to the external pressure at low temperatures and its thermodynamic stability at temperatures around 0°C have been studied. It has been found that the empty hydrate framework is unstable in certain temperature and pressure regions. A definite degree of occupation of the large cavities by the guest molecules is necessary for the hydrate to become stable. It has been found that there is a maximum of the critical temperature at which the hydrate exists, which is a function of the external pressure.Dedicated to Dr. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

Stability of rare gas structure H clathrate hydrates

Molecular dynamics simulations are used to study the stability of structure H (sH) clathrate hydrates with the rare gases Ne, Ar, Kr, and Xe. Simulations on a 3 x 3 x 3 sH unit cell replica are performed at ambient pressure at 40 and 100 K temperatures. The small and medium (s+m) cages of the sH unit cell are assigned rare gas guest occupancies of 1 and for large (l) cages guest occupancies of 1-6 are considered. Radial distribution functions for guest pairs with occupancies in the l-l, l-(s+m), and (s+m)-(s+m) cages are presented. The unit cell volumes and configurational energies are studied as a function of large cage occupancy for the rare gases. Free energy calculations are carried out to determine the stability of clathrates for large cage occupancies at 100 K and 1 bar and 20 kbar pressures. These studies show that the most stable argon clathrate has five guests in the large cages. For krypton and xenon the most stable configurations have three and two guests in the large cages, respectively.     

On the thermodynamic stability of hydrogen clathrate hydrates

The cage occupancy of hydrogen clathrate hydrate has been examined by grand canonical Monte Carlo (GCMC) simulations for wide ranges of temperature and pressure. The simulations are carried out with a fixed number of water molecules and a fixed chemical potential of the guest species so that hydrogen molecules can be created or annihilated in the clathrate. Two types of the GCMC simulations are performed; in one the volume of the clathrate is fixed and in the other it is allowed to adjust itself under a preset pressure so as to take account of compression by a hydrostatic pressure and expansion due to multiple cage occupancy. It is found that the smaller cage in structure II is practically incapable of accommodating more than a single guest molecule even at pressures as high as 500 MPa, which agrees with the recent experimental investigations. The larger cage is found to encapsulate at most 4 hydrogen molecules, but its occupancy is dependent significantly on the pressure of hydrogen.