Rates and mechanisms of conversion of ice nanocrystals to hydrates of HCl and HBr: acid diffusion in the ionic hydrates

This FTIR study focuses on solid-state chemistry associated with formation and interconversion of the ionic HX (X = Cl, Br) hydrates. Kinetic data are reported for conversions of ice nanocrystal arrays exposed to the saturation pressure of the acids in the 110 approximately 125 K range. The product is amorphous acid dihydrate in the case of HBr, and amorphous monohydrate for HCl. The rate-determining step is identified as HX diffusion through the hydrate product crust toward the interfacial reaction zone, rather than diffusion through ice, as commonly believed. Slowing of the conversion process is thus observed with increasing thickness of the crust. The diffusion coefficient (D(e)) and activation energy values for HX diffusion through the hydrates were evaluated with the help of the shrinking-core model. Hydrate crystallization occurs as a separate step, upon heating above 130 K. Subsequently, rates of reversible transitions between crystal di- and monohydrates were observed upon exposure to acid vapor and acid evacuation. In conversion from di- to monohydrate, the rate slows after fast formation of several layers; subsequently, diffusion through the product crust appears to be the rate-controlling step. The activation energy for HBr diffusion through crystal dihydrate is found to be significantly higher than that for the amorphous analogue. Conjecture is offered for a molecular mechanism of HX transport through the crystal hydrate, based on (i) spectroscopic/computational evidence for the presence of molecular HX bonded to X(-) in each of the ionic hydrate phases and (ii) the relative E(a) values found for HBr and HCl diffusion. Monte Carlo modeling suggests acid transport to the reaction zone along boundaries between "nanocrystallites" generated by multiple hydrate nucleation events at the particle surfaces. The reverse conversion, of crystalline monohydrate particles to the dihydrate phase, as well as dihydrate to trihydrate, displays nearly constant rate throughout the particle conversion; suggesting desorption of HX from the particle surface as the rate-limiting factor. Like for D(e), the activation energies for desorption were found to be approximately 20% greater for HCl than HBr for related hydrate phases.     

Temperature dependence of particle-particle adherence forces in ice and clathrate hydrates

Particle-particle pulloff adherence forces were measured as a function of temperature in the ice/n-decane/ice and tetrahydrofuran (THF) hydrate/n-decane/THF hydrate systems using a newly developed micromechanical testing technique. Experiments using approximately 200 microm radius particles were performed at atmospheric pressure over the temperature range 263-275 K. The ice and hydrate particles displayed very similar behavior. While the measured adherence forces had significant variation, the shapes of the cumulative force distribution curves were similar among the different sets of experiments. The measured adherence forces distributions shifted to lower force values as the temperature was decreased from the solid melting temperature. The observed forces and trends were explained by the capillary cohesion of rough surfaces, with the capillary bridging liquid being stabilized below its freezing point by the negative curvature of the bridging liquid/n-decane interface.     

CS-II binary clathrate hydrates at pressures of up to 15 kbar

The pressure dependence of the decomposition temperatures of binary clathrate hydrates of tetra-hydrofuran with xenon and methane as well as of chloroform and carbon tetrachloride clathrates with xenon has been studied. The absence of phase transitions at pressures from 1 to 15,000 bar indicates that the structure of all the hydrates remains constant (CS-II). The decomposition temperatures of the binary hydrates of tetrahydrofuran and carbon tetrachloride with xenon at 15 kbar (above 124ℴC) are exceedingly high for polyhedral clathrate hydrates because the guest molecules are highly complementary to the cavities of the clathrate lattice. The paper also considers the packing density effect in the crystal structure of hydrates on the behavior of the latter at elevated pressure. Translated fromZhurnal Strukturnoi Khimii, Vol. 41, No. 3, pp. 582-589, May–June, 2000.     

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.     

Stability and reactivity of methane clathrate hydrates: insights from density functional theory

The sI methane clathrate hydrate consists of methane gas molecules encapsulated as dodecahedron (5(12)CH(4)) and tetrakaidecahedron (5(12)6(2)CH(4)) water cages. The characterization of the stability of these cages is crucial to an understanding of the mechanism of their formation. In the present work, we perform calculations using density functional theory to calculate interaction energies, free energies, and reactivity indices of these cages. The contributions from polarization functions to interaction energies is more than diffuse functions from Pople basis sets, though both functions from the correlation-consistent basis sets contribute significantly to interaction energies. The interaction energies and free energies show that the formation of the 5(12)CH(4) cage (from the 5(12) cage) is more favored compared to the 5(12)6(2)CH(4) cage (from the 5(12)6(2) cage). The pressure-dependent study shows a spontaneous formation of the 5(12)CH(4) cage at 273 K (P ≥ 77 bar) and the 5(12)6(2)CH(4) cage (P = 100 bar). The reactivity of the 5(12)CH(4) cage is similar to that of the 5(12) cage, but the 5(12)6(2)CH(4) cage is more reactive than the 5(12)6(2) cage.     

Micrometeorites in Antarctic ice detected by Ir: estimation of 120k year old accretion rate

The accretion rate of micrometeorites (MMs) was estimated from Ir contents in snow around Dome Fuji Station and ice shards obtained during ice core drilling at Dome Fuji Station, Antarctica. The snow and ice shards were melted and filtered from the residues. Although MMs were not found on filters, we tried to detect them from the residues as Ir peaks determined by instrumental neutron activation analysis (INAA). Although Ir is very rare in the earth’s crust, its concentration is high in extraterrestrial matter (e.g., chondrites). Trace amounts of Ir can be easily detected by INAA, because the cross section of Ir is relatively large (e.g., 309 barn). The accretion rates were estimated for 120k year ago, 5k year ago and at present, as (3.2 ± 0.9) × 10² t/year (8.6 ± 0.18) × 10³ t/year and (1.3 ± 0.10) × 10³ t/year, respectively. These estimates were comparable to those of previous studies, however the rate of 120k year ago was approximately an order of magnitudes lower than the others.     

Instant conversion of air to a clathrate hydrate: CO(2) hydrates directly from moist air and moist CO(2)(g)

The rapid conversion of vapor mixtures containing the gases CO(2), H(2)S, and HCN to clathrate hydrates was reported recently. The novel method is based on the pulsing of warm vapor mixtures, including a carrier gas, into a cold condensation chamber. With cooling, the vapors, which also include ～1% water and either tetrahydrofuran or trimethylene oxide as a catalyst, nucleate aqueous solution nanodroplets that, on a millisecond time scale, crystallize as hydrate nanoparticles that consume 100% of the water. Humid air approximates the content of mixtures used successfully in the vapor-to-hydrate conversions. FTIR spectra are examined for gas hydrates formed directly from air and air enriched with CO(2), as well as hydrate particles for which CO(2)(g) serves as both guest and aerosol medium. In each instance all of the water in the condensed phase converts to a clathrate hydrate. The subsecond ether-catalyzed formation of the hydrates near 230 K requires only a few percent of the CO(2) pressure used in conventional processes that yield fractional amounts of gas hydrates on an hour time scale in the same temperature range.     

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.     

Two-dimensional water and ice layers: neutron diffraction studies at 278, 263, and 20 k

Neutron diffraction elucidates the structures of two-dimensional (2D) water layers (278 K) or 2D ice layers confined in an organic slit-shaped nanospace. The two-dimensional ice phases reported here consist of individual eight-membered rings or folded-chain segments (263 K) and condensed twelve-membered irregular rings (20 K). This is quite different from bulk or other 2D ice structures; the latter usually form hexagonal honeycomb lattices. Both low-temperature structures typically feature water molecules which are surrounded by two or three other water molecules. Neutron diffraction and thermochemical studies indicate a liquid-solid-phase transition around 277 K for two-dimensional D2O layers. A further solid-solid-phase transition occurs between 263 and 20 K.     

Polyhedral clathrate hydrates of a strong base: Phase relations and crystal structures in the system tetramethylammonium hydroxide-water

The tetramethylammonium hydroxide-water system has been studied by low-temperature differential thermal analysis and X-ray powder diffraction. The melting diagram was constructed for concentrations between 66.7 and 100 mol% H₂O. It shows the existence and stability ranges of as many as eight crystalline hydrate phases:- and-Me₄NOH·2H₂O (phase transition at –85°C, decomposition atca. 105°C), Me₄NOH·4 H₂O (melting point 44°C, incongruent), and-Me₄NOH·5 H₂O (phase transition at 42°C, melting point 68°C, congruent),- and-Me₄NOH·7.5 H₂O (phase transition at 6°C, melting point 16°C, incongruent), and Me₄NOH·10 H₂O (melting point –20°C, incongruent). The structures of all these phases, except the already known one of-Me₄NOH·5 H₂O, were determined from single-crystal MoK diffractometer data. The decahydrate and the high-temperature forms of the 7.5-hydrate and the pentahydrate are genuinepolyhedral clathrate hydrates, the first ones reported of a strong base. Their mostly novel three-dimensional anionic host structures, formed by the hydrogen-bonded OH^– ions and H₂O molecules, arefour-connected throughout, in spite of their proton deficiency which is apparently leveled by disorder. Disorder also affects the enclosed cationic Me₄N⁺ guest species. Like the low-temperature form of the pentahydrate, that of the 7.5-hydrate has a clathrate-related, but not fully polyhedral structure, some of the oxygen atoms being three-connected only. The tetrahydrate presents the rare case of both a hydrogen bond of the type OH^–...OH₂ and a (deprotonated) water-channel structure. This is fully ordered and apart from that can be derived from the polyhedral one of the-pentahydrate just by removing the appropriate number of water molecules from certain positions. The structures of- and-Me₄NOH·2 H₂O contain identical one-dimensionalspiro chains [HO^–(HOH)_/42] with the hydroxide protonnot participating in the hydrogen bonding. The Me₄N⁺ ion is ordered in the and disordered in the phase.Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82076 (66 pages).Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena.     

Phase equilibria of clathrate hydrates of methane + carbon dioxide: New experimental data and predictions

In this communication, we report experimental dissociation conditions for region clathrate hydrates of methane + carbon dioxide in gas–liquid water–hydrate (G–Lw–H) equilibrium. The temperature and pressure conditions are in the range of (279.1–289.9) K and (2.96–13.06) MPa, respectively. Concentrations of carbon dioxide in the feed gas are also varied. An isochoric pressure-search method was used to perform the measurements. The dissociation data generated in this work along with the literature data are compared with the predictions of a thermodynamic model and a previously reported empirical equation. A discussion is made on the deviations between the experimental and predicted data.     

Towards 'bio-based' Nylon: conversion of gamma-valerolactone to methyl pentenoate under catalytic distillation conditions

Methyl pentenoate, a promising Nylon intermediate, is produced in >95% yield via the transesterification of gamma-valerolactone, a bio-based intermediate, under catalytic distillation conditions.     

One-pot catalytic conversion of cellulose and of woody biomass solids to liquid fuels

Efficient methodologies for converting biomass solids to liquid fuels have the potential to reduce dependence on imported petroleum while easing the atmospheric carbon dioxide burden. Here, we report quantitative catalytic conversions of wood and cellulosic solids to liquid and gaseous products in a single stage reactor operating at 300-320 °C and 160-220 bar. Little or no char is formed during this process. The reaction medium is supercritical methanol (sc-MeOH) and the catalyst, a copper-doped porous metal oxide, is composed of earth-abundant materials. The major liquid product is a mixture of C(2)-C(6) aliphatic alcohols and methylated derivatives thereof that are, in principle, suitable for applications as liquid fuels.     

An oft-studied reaction that may never have been: direct catalytic conversion of methanol or dimethyl ether to hydrocarbons on the solid acids HZSM-5 or HSAPO-34

Using highly purified reagents and careful tests, we show that methanol and dimethyl ether are apparently unreactive on the two most important methanol-to-hydrocarbon catalysts, HZSM-5 and HSAPO-34. Thus, none of the "direct" mechanisms involving two to four carbon atoms in intermediates such as oxonium ylides, carbenes, carbocations, and free radicals are applicable. Only the "indirect" route (hydrocarbon pool) is an established mechanism for this chemistry. An active catalyst requires a hydrocarbon pool that typically begins with products from organic impurities in the feed, carrier gas, or the solid acid itself. Impurities may also play important roles in other reactions catalyzed by solid acids.     

Water at surfaces and interfaces: From molecules to ice and bulk liquid

The structure and growth of water films on surfaces is reviewed, starting from single molecules to two-dimensional wetting layers, and liquid interfaces. This progression follows the increase in temperature and vapor pressure from a few degrees Kelvin in ultra-high vacuum, where Scanning Tunneling and Atomic Force Microscopies (STM and AFM) provide crystallographic information at the molecular level, to ambient conditions where surface sensitive spectroscopic techniques provide electronic structure information. We show how single molecules bind to metal and non-metal surfaces, their diffusion and aggregation. We examine how water molecules can be manipulated by the STM tip via excitation of vibrational and electronic modes, which trigger molecular diffusion and dissociation. We review also the adsorption and structure of water on non-metal substrates including mica, alkali halides, and others under ambient humid conditions. We finally discuss recent progress in the exploration of the molecular level structure of solid-liquid interfaces, which impact our fundamental understanding of corrosion and electrochemical processes.     

Catalytic conversion of n-pentanol to 1,1-dialkyl ether by aluminium and iron exchanged montmorillonite catalyst

Heterogeneous catalytic conversion of n-pentanol to 1,1-dialkyl ether by Fe³⁺- and Al³⁺-exchanged montmorillonite catalyst was studied under different conditions. The catalyst was first heated in the presence of air or hydrogen at temperatures ranging from 80°C to 300°C and X-ray diffractometry, TGA, cation exchange capacity were performed. The final products were analysed by gas chromatography. It was found that Al³⁺-exchanged ions are more efficient as a catalyst than the corresponding Fe³⁺-exchanged ones under the same conditions.     

Optical study of H2O ice to 120 GPa: dielectric function, molecular polarizability, and equation of state

The refractive index of H2O ice has been measured to 120 GPa at room temperature using reflectivity methods. The refractive index increases significantly with pressure on initial compression and exhibits small changes with pressure at previously identified phase transitions. Pressure dependencies of the molecular polarizability show changing slopes in different pressure regions. A general molar refractivity analysis of this change in slope reveals features at 60 GPa due to the onset of the ice VII-X transition. Band gap closure in H2O ice is constrained by the dispersion data using a single oscillator dielectric model. Sample thickness measurements obtained from interference patterns yield pressure-volume relations in excellent agreement with those measured by x-ray diffraction.     

The thermostability and reactivity of GaP nanocrystals in oxygen: from GaP nanocrystals to Ga2O3 nanocrystals

The thermostability and reactivity of GaP nanocrystals in O(2) were investigated using the thermogravimetric analysis (TGA), differential thermal analysis (DTA), powder X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analysis techniques. alpha-Ga(2)O(3) nanoparticles, nano-hollow-particles, or nanorods and nanotubes can be separately obtained from the oxidation of nanocrystalline GaP at 400 degrees C for 30 min in dry O(2) atmosphere via manipulating different heating rates. Transmission electron microscopy (TEM) and energy-dispersive X-ray spectrometry (EDX) analysis showed that the products were all alpha-Ga(2)O(3) but with different morphologies when different heating rates were applied. The formation mechanisms of the different morphological alpha-Ga(2)O(3) nanocrystals were discussed.