Structures and syntheses of framework triuranyl diarsenate hydrates

Two homeotypic hydrated uranyl arsenates, (UO₂)[(UO₂)(AsO₄)]₂(H₂O)₄, UAs4, and (UO₂)[(UO₂)(AsO₄)]₂(H₂O)₅, UAs5 were synthesized by hydrothermal methods. Intensity data were collected at room temperature using MoKα X-radiation and a CCD-based area detector. Their crystal structures were solved by direct methods and refined by full-matrix least-squares techniques on the basis of F² to agreement indices (UAs4, UAs5) wR₂=0.116, 0.060, for all data, and R₁=0.046, 0.033, calculated for 3176, 5306 unique observed reflections (|F_o|>4σ_F) respectively. UAs4 is monoclinic, space group P2₁/c, Z=4, a=11.238(1), b=7.152(1), c=21.941(2)Å, β=104.576(2)°, V=1706.8(1)ų, D_calc=4.51 g/cm³. UAs5 is orthorhombic, space group Pca2₁, Z=4, a=20.133(2), b=11.695(1), c=7.154(1)Å, V=1684.4(1)ų, D_calc=4.65 g/cm³. Both structures contain sheets of arsenate tetrahedra and uranyl pentagonal bipyramids, with composition [(UO₂)(AsO₄)]¹⁻ and the uranophane sheet anion-topology. The sheets are connected by a uranyl pentagonal bipyramid in the interlayer that shares corners with an arsenate tetrahedron on each of two adjacent sheets, resulting in open-frameworks with isolated H₂O groups in the larger cavities of the structures. The uranyl arsenate sheet in UAs4 is relatively planar, and is topologically identical with the uranyl phosphate sheet in (UO₂)[(UO₂)(PO₄)]₂(H₂O)₄. The uranyl arsenate sheet in UAs5 is the same geometrical isomer as in UAs4, but is highly corrugated, exhibiting approximately right angle bends of the sheet after every second uranyl arsenate chain repeat.     

The conversion process of hydrocarbon hydrates into CO2 hydrates and vice versa: thermodynamic considerations

Microscopy, confocal Raman spectroscopy and powder X-ray diffraction (PXRD) were used for in situ investigations of the CO(2)-hydrocarbon exchange process in gas hydrates and its driving forces. The study comprises the exposure of simple structure I CH(4) hydrate and mixed structure II CH(4)-C(2)H(6) and CH(4)-C(3)H(8) hydrates to gaseous CO(2) as well as the reverse reaction, i.e., the conversion of CO(2)-rich structure I hydrate into structure II mixed hydrate. In the case of CH(4)-C(3)H(8) hydrates, a conversion in the presence of gaseous CO(2) from a supposedly more stable structure II hydrate to a less stable structure I CO(2)-rich hydrate was observed. PXRD data show that the reverse process requires longer initiation times, and structural changes seem to be less complete. Generally, the exchange process can be described as a decomposition and reformation process, in terms of a rearrangement of molecules, and is primarily induced by the chemical potential gradient between hydrate phase and the provided gas phase. The results show furthermore the dependency of the conversion rate on the surface area of the hydrate phase, the thermodynamic stability of the original and resulting hydrate phase, as well as the mobility of guest molecules and formation kinetics of the resulting hydrate phase.     

Hydration of ions and formation of crystal hydrates

Effects of electrostatic and entropic factors on the dependence between the type of hydration of ions and their ability to form crystal hydrates from water solutions are discussed. It is shown that in the case of the single-charged ions because of the insignificant electrostatic interaction the crystal hydrates are formed mainly by the salts containing ions with different type of hydration. In the case of poly-charged ions the increase in the electrostatic interaction leads to formation of crystal hydrates mainly by the salts containing ions with the positive type of hydration. It was established also that the formation of salt crystal hydrates is influenced not only by the electrostatic and entropic factors, but also by the spatial arrangement and the electronic density distribution of ions.     

Mechanisms of aliphatic hydrocarbon formation during co-pyrolysis of coal and cotton stalk

Pyrolysis experiments were carried out in a tubular furnace. The characteristics of pyrolysis tar were analyzed by GC/MS. The results indicated that the aliphatic hydrocarbon yield derived from co-pyrolysis tar of cotton stalk and Shenmu coal was obviously higher than that of Shenmu coal pyrolysis under optimum condition. Moreover, microcrystalline cellulose was selected as a model compound and the copyrolysis tar of microcrystalline cellulose and Shenmu coal was analyzed for comparison. Base on the experimental results, it was indicated that the alkyl radicals generated from pyrolysis were converted to aliphatic hydrocarbons by radical reactions. Furthermore, the mechanisms of aliphatic hydrocarbon formation were discussed during co-pyrolysis of cotton stalk and Shenmu coal.     

Substantial formation of hydrates and hemiacetals from pyridinium ketones

Pyridinium ketones have been found to exist as hydrates and hemiacetals in considerable amount in aqueous and alcoholic solutions, respectively. The relative position of the pyridinium positive charge has a large effect on the equilibrium constants. The polar substituent constants, σ∗, of the pyridinium group substituted at different positions can be estimated from the hydration constants.     

Kinetics and stability of CH4-CO2 mixed gas hydrates during formation and long-term storage.

The formation of CH4-CO2 mixed gas hydrates was observed by measuring the change of vapor-phase composition using gas chromatography and Raman spectroscopy. Preferential consumption of carbon dioxide molecules was found during hydrate formation, which agreed well with thermodynamic calculations. Both Raman spectroscopic analysis and the thermodynamic calculation indicated that the kinetics of this mixed gas hydrate system was controlled by the competition of both molecules to be enclathrated into the hydrate cages. However, the methane molecules were preferentially crystallized in the early stages of hydrate formation when the initial methane concentration was much less than that of carbon dioxide. According to the Roman spectra, pure methane hydrates first formed under this condition. This unique phenomenon suggested that methane molecules play important roles in the hydrate formation process. These mixed gas hydrates were stored at atmospheric pressure and 190 K for over two months to examine the stability of the encaged gases. During storage, CO2 was preferentially released. According to our thermodynamic analysis, this CO2 release was due to the instability of CO2 in the hydrate structure under the storage conditions.     

Energetics of framework formation in cubic clathrate hydrates

Geometrical parameters and energies of CS-III (143d) and CS-IV (Im3m) gas hydrate frameworks are calculated. For the four cubic framework, the dependences of the energies on the sizes of large cavities are compared. Due to this, energetically favorable regions of each framework have been isolated on the abscissa. The CS-I and CS-II frameworks show satisfactory agreement between the sizes of guest molecules and the structures of clathrate hydrates. The experimental lattice parameters of the hydrates vary within narrower ranges than the values calculated for the empty frameworks. An explanation is given for these contradictory results was well as for the discrepancies between the guest sizes and the boundaries between the energetically favorable regions of CS-III and CS-IV. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 3, pp. 494–500, May–June, 1995. Translated by L. Smolina     

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.     

Structures of uranyl peroxide hydrates: a first-principles study of studtite and metastudtite

The structures of the only known minerals containing peroxide, namely studtite [(UO(2))O(2)(H(2)O)(4)] and metastudtite [(UO(2))O(2)(H(2)O)(2)], have been investigated using density functional theory. The structure of metastudtite crystallizing in the orthorhombic space group Pnma (Z = 4) is reported for the first time at the atomic level and the computed lattice parameters, a = 8.45, b = 8.72, c = 6.75 ?, demonstrate that the unit cell of metastudtite is larger than previously reported dimensions (Z = 2) derived from experimental X-ray powder diffraction data.     

Structures of the covalent hydrates and products of acidic hydrolysis of rheumycin and fervenulin

It is shown that pyrimido[5,4-e]-1,2,4-triazinediones are hydrated at the N(₄)-C(_4a) bond in aqueous acidic media. The equilibrium constants of these processes were measured by PMR spectroscopy. The structure of the covalent adduct of fervenulin was established by x-ray diffraction analysis. Formic, 5-diazo-3-methylbarbituric, and methylparabanic acids were identified among the products of destruction of the hydrates in acidic media.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1654–1660, December, 1988.     

Clathrate hydrates of hexagonal structure III at high pressures: Structures and phase diagrams

It was shown in this work that the clathrate hydrates of Hexagonal Structure III, formed in the ternary systems 1-methylpiperazine-help gas-water and iso-amyl alcohol-help gas-water are stable in a wide range of pressures. The decomposition curves of these hydrates were studied for the first time up to the pressures 1 GPa. Ar, Kr, Xe and CH₄ were utilized as the help gases. In a number of the systems studied, high pressure phases were revealed that presumably form due to the distortion of the corresponding low pressure hydrate structures.     

Portable remote Raman system for monitoring hydrocarbon, gas hydrates and explosives in the environment

We report our initial efforts to use a small portable Raman system for stand-off detection and identification of various types of organic chemicals including benzene, toluene, ethyl benzene and xylenes (BTEX). Both fiber-optic (FO) coupled and a directly coupled f/2.2 spectrograph with the telescope have been developed and tested. A frequency-doubled Nd:YAG pulsed laser (20 Hz, 532 nm, 35 mJ/pulse) is used as the excitation source. The operational range of the FO coupled Raman system was tested to 66 m, and the directly coupled system was tested to a distance of 120 m. We have also measured remote Raman spectra of compressed methane gas and methane gas hydrate. The usefulness of the remote Raman system for identifying unknown compounds is demonstrated by measuring stand-off spectra of two plastic explosives, e.g. tri-amino tri-nitrobenzene (TATB) and beta-HMX at 10 m stand-off distance. The remote Raman system will be useful for terrestrial applications such as monitoring environmental pollution, in identifying unknown materials in public places in 10s or less, and for detecting hydrocarbon plumes and gas hydrates on planetary surfaces such as Mars.     

Experimental and theoretical investigation of simple gas hydrate formation with or without presence of kinetic inhibitors in a flow mini-loop apparatus

Gas hydrates are ice-like crystalline compounds, which form through a combination of water and suitably sized guest molecules under low temperature and elevated pressure conditions. These solid compounds give rise to problems in the natural gas oil industry because they can plug pipelines and process equipment. Low dosage hydrate inhibitors are a recently developed hydrate control technology, which can be more cost-effective than traditional practices such as methanol and glycols. The kinetics of hydrate growth has been modeled by numerous authors who have measured the gas consumption rate during hydrate formation in batch agitator reactors.     

Characterisation of gas hydrates formation using a new high pressure Micro-DSC

Gas hydrates are solid structures formed from water and gas under low temperature and high pressure conditions. Differential scanning calorimeter, operating under high pressure, is a very useful technique for the determination of the thermodynamic properties and the kinetics of gas hydrate formation. Specific gas tight controlled pressure vessels have to be used to obtain the hydrate formation in complex fluids. Based on the MicroDSC technology, a new High Pressure MicroDSC with a vessel (0.7 cm³) operating up to 400 bars between -45 and 120°C is introduced for this type of research. An example of the use of the HP MicroDSC is given with the formation of gas hydrates in drilling muds. With the increasing number of deep offshore drilling operations, operators and service companies have to solve more and more complex technical challenges. Extreme conditions encountered at these depths require an adaptation of the drilling muds. The range of temperature (down to -1°C) and pressure (up to 400 bars) are favorable conditions to the formation of hydrates. HP MicroDSC is used to determine the thermodynamic properties and kinetics of hydrate formation in mud formulations, particularly in the presence of large amounts of minerals. The technique allows the detection of phase transitions vs. time, temperature and pressure. Using such a technique, dangerous areas of hydrate formation in drilling muds formulations (water-base and oil-base) can be predicted. This revised version was published online in July 2006 with corrections to the Cover Date.     

13C NMR studies of hydrocarbon guests in synthetic structure H gas hydrates: experiment and computation

(13)C NMR chemical shifts were measured for pure (neat) liquids and synthetic binary hydrate samples (with methane help gas) for 2-methylbutane, 2,2-dimethylbutane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane, methylcyclopentane, and methylcyclohexane and ternary structure H (sH) clathrate hydrates of n-pentane and n-hexane with methane and 2,2-dimethylbutane, all of which form sH hydrates. The (13)C chemical shifts of the guest atoms in the hydrate are different from those in the free form, with some carbon atoms shifting specifically upfield. Such changes can be attributed to conformational changes upon fitting the large guest molecules in hydrate cages and/or interactions between the guests and the water molecules of the hydrate cages. In addition, powder X-ray diffraction measurements revealed that for the hexagonal unit cell, the lattice parameter along the a-axis changes with guest hydrate former molecule size and shape (in the range of 0.1 ?) but a much smaller change in the c-axis (in the range of 0.01 ?) is observed. The (13)C NMR chemical shifts for the pure hydrocarbons and all conformers were calculated using the gauge invariant atomic orbital method at the MP2/6-311+G(2d,p) level of theory to quantify the variation of the chemical shifts with the dihedral angles of the guest molecules. Calculated and measured chemical shifts are compared to determine the relative contribution of changes in the conformation and guest-water interactions to the change in chemical shift of the guest upon clathrate hydrate formation. Understanding factors that affect experimental chemical shifts for the enclathrated hydrocarbons will help in assigning spectra for complex hydrates recovered from natural sites.