Gas hydrate formation in the system C2H6–H2–H2O at pressures up to 250 MPa

Decomposition curves of gas hydrates formed in the ethane–hydrogen–water system were studied in the pressure interval 2–250 MPa. Gas hydrates synthesized at low (up to 5 MPa) pressures were also studied with use of X-ray powder diffraction and Raman spectroscopy. It was shown that ethane–hydrogen mixtures with hydrogen contents 0–30 mol.% form cubic structure I gas hydrates. Higher hydrogen concentration most probably results in appearance of another hydrate phase. We speculate that the gas mixtures with the hydrogen content above 60 mol.% form cubic structure II double hydrate of hydrogen and ethane at temperatures below ≈280 K and pressures above 25 MPa.     

Solid–liquid equilibria in the NaCl–SrCl2–H2O system at 288.15 K

The phase equilibria in the ternary system NaCl–SrCl₂–H₂O at 288.15 K were studied with the isothermal equilibrium solution method. The phase diagram and refractive index diagram were plotted for this system at 288.15 K. The phase diagram contains one invariant solubility point, two univariant solubility curves, and two crystallization fields of NaCl and SrCl₂ · 6H₂O. The refractive indices of the equilibrium solution change regularly with w(NaCl) increase. The calculated refractive index data are in good agreement with the experimental data. Combining the experimental solubility data of the ternary system, the Pitzer binary parameters for NaCl at 288.15 K and SrCl₂ at 298.15 K, the Pitzer mixing parameters θ_{Na, Sr}, Ψ_{Na, Sr, Cl} and the solubility equilibrium constants Ksp of solid phases existing in the ternary system at 288.15 K were fitted using the Pitzer and Harvie-Weare (HW) models. The mean activity coefficients of sodium chloride and strontium chloride, and the solubilities for the ternary system at 288.15 K were presented. A comparison between the calculated and measured solubilities shows that the predicted data agree well with the experimental results.     

Phase Equilibria in the System NH4Al(SO4)2–H2SO4–H2O

Equilibrium solubility curves of the ammonium aluminium sulphate in aqueous solutions of sulphuric acid have been calculated using checked literature data and our own measurements. The concentration of sulphuric acid ranged from 0 to 23 mass%, temperature range between 20 and 60°C has been extrapolated up to 75°C by means of a thermodynamically based correlation method. The solubility correlation as well as the hydration analysis implied a possible destructuralization of solutions at higher acid concentrations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Interaction parameters and (solid + liquid) equilibria calculation for KCl–H2O–HCl–C2H5OH,K2SO4–H2O–H2SO4 and K2SO4–H2O–C2H5OH mixed solvent-electrolyte systems

This work deals with the prediction and experimental measurements of the (solid + liquid) equilibrium (SLE) in acid medium for industrial purposes. Specific systems including KCl–ethanol–water–HCl and K₂SO₄–water–H₂SO₄ were analyzed. At first, a critical discussion of SLE calculations was given, based on the well-known UNIQUAC extended and LIQUAC models. Two new proposals were derived, considering the explicit necessity of a new reference state for SLE calculations for the studied (solvents + acid) mixtures. The solubility of KCl in water–ethanol–HCl mixed solvents was measured in the temperature range of 300.15 to 315.15 K using an analytical gravimetric method. These results combined with some other experimental data reported in the open literature let us to propose a set of parameters for the new models. They included the interaction parameters between ethanol and the H⁺ ion. The prediction capability of the new models, for calculations in acid medium, was illustrated. Experimentally, it was observed that the (K₂SO₄ + water + H₂SO₄) system presented the unusual behavior of increasing K₂SO₄ solubility with an increase in the sulfuric acid concentration. This was accurately predicted by the newly proposed models.     

Liquid–liquid equilibria in the system H3PO4–KCl–H2O–tri-n-butyl phosphate: experiments and modelling

The liquid–liquid equilibria of the system H₃PO₄–KCl–H₂O–TBP was studied experimentally in the concentration range 0–6 mol/kg. The obtained data were modelled using the Pitzer equation for the aqueous phase and the Sergievskii–Dannus relationship for the organic phase. A fairly good agreement was observed between the model and the experimental data.     

Reactivity of the Ge–H,Sn–H,P–H,and Se–H Bonds in Radical Abstraction Reactions

The experimental data for the liquid- and gas-phase reactions of atoms and radicals with organoelement compounds R _{n – 1}E–H

Kinetics of the Reaction of 2-Chloro-quinoxaline with Hydroxide Ion in ACN–H2O and DMSO–H2O Binary Solvent Mixtures

The kinetics of alkaline hydrolysis of 2-chloroquinoxaline (QCl) with hydroxide ion was investigated spectrophotometrically at different percentages of aqueous–organic solvent mixtures with acetonitrile (10–60% v/v) and with dimethylesulphoxide (10–80%) over the temperature range from 25 to 45 °C. The reaction was performed under pseudo first order conditions with respect to 2-chloroquinoxaline (QCl). An increase in the percentage of organic solvent (v/v) has different effects on the reaction rate constants, presumably due to hydrogen bond donor and acceptor differences of the media and other solvatochromic parameters. The data were discussed in terms of the Kamelt-Taft parameter and E _T(30). A nonlinear relation between the logarithm of the rate constant and reciprocal of the dielectric constant suggests the presence of selective solvation by the polar water molecules. Activation parameters ΔH ^#, ΔS ^# and ΔG ^# were determined and discussed.     

Thermodynamic Model for SnO2(cr) and SnO2(am) Solubility in the Aqueous Na+–H+–OH−–Cl−–H2O System

The solubility of SnO₂(cassiterite) was studied at 23±2?°C as a function of time (7 to 49 days) and pH (0 to 14.5). Steady state concentrations were reached in <7 days. The data were interpreted using the SIT model. The data show that SnO₂(cassiterite) is the stable phase at pH values of 10 K ⁰ value of ?64.39±0.30 for the reaction (SnO₂(cassiterite) +2H₂O?Sn⁴⁺+4OH^?) and values of 1.86±0.30, ≤?0.62, ?9.20±0.34, and ?20.28±0.34 for the reaction ( $\mathrm{Sn}^{4+} + n\mathrm{H}_{2}\mathrm{O} \rightleftarrows \mathrm{Sn}(\mathrm{OH})_{n}^{4 - n} + n\mathrm{H}^{+}$ ) with values of “n” equal to 1, 4, 5, and 6 respectively. These thermodynamic hydrolysis constants were used to reinterpret the extensive literature data for SnO₂(am) solubility, which provided a log?₁₀ K ⁰ value of ?61.80±0.29 for the reaction (SnO₂(am)+2H₂O?Sn⁴⁺+4OH^?). SnO₂(cassiterite) is unstable under highly alkaline conditions (NaOH concentrations >0.003 mol?dm^?3) and transforms to a double salt of SnO₂ and NaOH. Although additional well-focused studies will be required for confirmation, the experimental data in the highly alkaline region (0.003 to 3.5 mol?dm^?3 NaOH) can be well described with log?₁₀ K ⁰ of ?5.29±0.35 for the reaction Na₂Sn(OH)₆(s)?Na₂Sn(OH)₆(aq).     

C–H...O and C–H...N interactions in RNA structures

Small molecule studies indicate that C–H...X interactions (X: O,N) constitute weak H-bonds. We have performed a comprehensive analysis of their occurrence and geometry in RNA structures. Here, we report on statistical properties of the total set of interactions identified and discuss selected motifs. The distance/angle distribution of all interactions exhibits an excluded region where the allowed C–H...X angle range increases with an increasing H...X distance. The preferred short C–H...X interactions in RNA are backbone-backbone contacts between neighbour nucleotides. Distance/angle distributions generated for various interaction types can be used for error recognition and modelling. The axial C2′(H)...O4′ and C5′(H)...O2′ interactions connect two backbone segments and form a seven-membered ring that is specific for RNA. An AA base pair with one standard H-bond and one C–H...N interaction has been identified in various structures. Despite the occurrence of short C–H...X contacts their free energy contribution to RNA stability remains to be assessed. Received: 17 May 1998 / Accepted: 4 August 1998 / Published online: 2 November 1998     

De-Linker-Enabled Exceptional Volumetric Acetylene Storage Capacity and Benchmark C2H2/C2H4 and C2H2/CO2 Separations in Metal–Organic Frameworks

An ideal adsorbent for separation requires optimizing both storage capacity and selectivity, but maximizing both or achieving a desired balance remain challenging. Herein, a de-linker strategy is proposed to address this issue for metal–organic frameworks (MOFs). Broadly speaking, the de-linker idea targets a class of materials that may be viewed as being intermediate between zeolites and MOFs. Its feasibility is shown here by a series of ultra-microporous MOFs (SNNU-98-M, M=Mn, Co, Ni, Zn). SNNU-98 exhibit high volumetric C₂H₂ uptake capacity under low and ambient pressures (175.3 cm³ cm⁻³ @ 0.1 bar, 222.9 cm³ cm⁻³ @ 1 bar, 298 K), as well as extraordinary selectivity (2405.7 for C₂H₂/C₂H₄, 22.7 for C₂H₂/CO₂). Remarkably, SNNU-98-Mn can efficiently separate C₂H₂ from C₂H₂/CO₂ and C₂H₂/C₂H₄ mixtures with a benchmark C₂H₂/C₂H₄ (1/99) breakthrough time of 2325 min g⁻¹, and produce 99.9999 % C₂H₄ with a productivity up to 64.6 mmol g⁻¹, surpassing values of reported MOF adsorbents.     

Pd–Fe/TiO2 catalysts for phenol degradation with in situ generated H2O2

This study deals with the degradation of phenol over Pd–Fe/TiO₂ catalysts at mild conditions in the presence of in situ generated H₂O₂ from oxygen and formic acid. This catalytic system demonstrated interesting ability to oxidize phenol by Fenton process in a one-pot reaction without the addition of ferrous ion. Lower Pd content catalysts, despite producing a higher hydrogen peroxide amount for bulk purposes, did not reach the same efficiency as the 5Pd–5Fe catalyst in phenol degradation. A close interaction between Pd and iron oxide species is necessary to obtain high active catalysts. These results highlight the advantage of in situ generation of H₂O₂, for oxidation reactions with respect to conventional Fenton process.     

Solid-Phase Surface Adsorption in Ag–Au|F–, H2O and Ag–Au|ClO–4, H2O Systems: Allowing for the Solid-Solution Nonideality

Integral and differential (with respect to the composition) isotherms of changes in the interfacial free energy (_m– ), the charge density q, and the surface composition X _Au of alloys equilibrated with an aqueous surface-inactive electrolyte are obtained in terms of a finite-thickness interfacial layer, with use of concentration dependences of activity coefficients of components of a polycrystalline binary alloy. Using ac measurements of the double-layer parameters, it is stated that the surface-active component in the Ag–Au|F^–, H₂O and Ag–Au|ClO^– ₄, H₂O systems at 298 K is gold. The Ag–Au solid solution shows negative deviations from Raoult's law, except for the compositions X _Au 0.04 and X _Au 0.80, where the solid solution properties approach those of an ideal solution.     

Polymorphic effects at the eutectic melting in the H2O–glycine system

Solid mixtures of ice with three glycine polymorphs were heated up to the eutectic melting and the DSC has detected the eutectic temperatures of ?2.8 °C for α-, ?3.6 °C for β-, and ?2.8 °C for γ-glycine. DSC peaks of the eutectic melting are rather strange in shape, indicating unclear processes in the solutions. Accurate DSC measurements of extremely small samples can probably provide us with the physicochemical tool for the investigation of polymorphic differences among different solutions. This may be important in relation to different bioavailability of solutions prepared from different polymorphs. Eutectic temperature of ?4.7 °C in water–glycine system allows us to suggest that the unknown polymorph of glycine exists.     

Phase Diagrams of Fullerenol-d–LaCl3–H2O and Fullerenol-d–GdCl3–H2O Systems at 25°С

Russian Journal of Physical Chemistry A - Isothermal phase diagrams of ternary systems fullerenol-d–LaCl3–H2O and fullerenol-d–GdCl3–H2O at 25°C are studied via...     

Solubility and Crystallization in the System MgCl2–MgSO4–H2O at 50 and 75°C

The system MgCl₂–MgSO₄–H₂O has been investigated experimentally and modeled thermodynamically according to the Pitzer method at 50 and 75°C. It was found that, even when seemingly all requirements for reaching the stable thermodynamic equilibrium are fulfilled, the crystallization of higher hydrates as metastable phases is possible, and cannot be avoided in each crystallization field of a stable lower hydrate of magnesium sulfate. Crystallization of MgSO₄ · x H₂O (x = 1, 4, 6) and MgCl₂ · 6 H₂O at 50°C and of MgSO₄ · H₂O and MgCl₂ · 6 H₂O at 75°C as stable phases has been observed. Three metastable crystallization fields of MgSO₄ · x H₂O (x = 4, 6, 7) have been detected at 50°C and two of MgSO₄ · x H₂O (x = 4, 6) at 75°C. The results obtained and the contradictions existing in the literature with respect to the solubility and the crystallizing solid phases are discussed in terms of the crystal structures.     

Recent advances in mechanochemical C–H functionalization reactions

The mechanochemical synthesis has provided a greener alternative to solution-based approaches by eliminating the use of organic solvents and reducing the energy consumption. The C–H functionalization is among the most concise and economical synthetic strategies. The combination of the benefits from these two methods provides new opportunities to further increase the efficiency and sustainability of organic synthesis. In this digest, we aim to provide a brief overview of the recent advances in mechanochemical C–H functionalization reactions.     

Measurements of the Solid–Liquid Equilibria in the Quaternary System NaCl–NaBr–Na2SO4–H2O at 323 K

Phase equilibria of the quaternary NaCl–NaBr–Na₂SO₄–H₂O system at 323 K were studied by the isothermal dissolution equilibrium method. The solubilities of salts and densities of saturated solutions were determined. Solid solutions [Na(Cl, Br)] were found in the experiments. The phase diagram of the quaternary system has no invariant point, but has one univariant curve at the boundary of Na(Cl, Br) and Na₂SO₄ crystallization fields. The experimental results show that an increase of the NaBr concentration is accompanied by an obvious increase of the solution density and the decrease of the solubilities of NaCl and Na₂SO₄.     

Mean activity coefficients in the NaCl–NH4HCO3–H2O system at 293.15–308.15 K

The mean activity coefficients of NaCl in the NaCl–NH₄HCO₃–H₂O system, measured by an electrochemical cell using ion-selective electrodes, were used to calculate the mean activity coefficients of NH₄HCO₃ in the system through a thermodynamic model. The measurements were made at four temperatures from 293.15 to 308.15 K up to 1.8 mol kg⁻¹, at NH₄HCO₃ molality fractions of 0.2 and 0.4. Both sets of values agree with those obtained from Pitzer equation incorporated with chemical reactions. This observation supports the applicability of the existing parameters for calculating mean activity coefficients of this system.     

Experimental Study on Phase Equilibria in Na2B4O7–Na2SO4–H2O and Li2B4O7–Na2B4O7–H2O Aqueous Ternary Systems at 273 K

Russian Journal of Physical Chemistry A - The solubility data for ternary systems Na2SO4–Na2B4O7–H2O and Li2B4O7–Na2B4O7–H2O at 273 K were obtained experimentally by the...