Electrochemical behaviour of lanthanum fluoride in molten fluorides

The electrochemical behaviour of lanthanum fluoride dissolved in molten lithium fluoride and in eutectic mixture LiF-CaF₂ was investigated by cyclic voltammetry and laboratory electrolysis. The cyclic voltammetry experiments were carried out at 900°C and 800°C, respectively, in a graphite crucible (counter electrode). Several types of working electrodes (Mo, W, Ni and Cu) were used. Ni/Ni(II) was used as a reference electrode. Laboratory electrolysis was carried out in the system LiF-CaF₂-LaF₃ at 800°C in galvanostatic (j _c = −0.21 A cm⁻²) and potentiostatic (E = 0.87 V) regimes. In both cases, nickel served as the cathode and graphite as the anode. It was found that no new separate reduction peak occurred on the molybdenum or tungsten electrodes in the investigated systems. When copper or nickel electrodes were used, new peaks corresponding to the reduction of lanthanum(III) to lanthanum metal appeared. This can be explained by the formation of alloys or intermetallic compounds of lanthanum with copper or nickel. X-ray microanalysis showed that lanthanum was electrodeposited together with calcium under formation of intermetallic compounds with the electrode materials in the galvanostatic regime. In the potentiostatic regime, mainly lanthanum was deposited, which enabled its separation.     

CH4 reforming with CO2 for syngas production over La2O3 promoted Ni catalysts supported on mesoporous nanostructured γ-Al2O3

Nanostructured γ-Al₂O₃ with high surface area and mesoporous structure was synthesized by sol-gel method and employed as catalyst support for nickel catalysts in methane reforming with carbon dioxide. The prepared samples were characterized by XRD, N₂ adsorption-desorption, TPR, TPO, TPH, NH₃-TPD and SEM techniques. The BET analysis showed a high surface area of 204 m²·g⁻¹ and a narrow pore-size distribution centered at a diameter of 5.5 nm for catalyst support. The BET results revealed that addition of lanthanum oxide to aluminum oxide decreased the specific surface area. In addition, TPR results showed that addition of lanthanum oxide increased the reducibility of nickel catalyst. The catalytic evaluation results showed an increase in methane conversion with increasing lanthanum oxide to 3 mol% and further increase in lanthanum content decreased the catalytic activity. TPO analysis revealed that the coke deposition decreased with increasing lanthanum oxide to 3 mol%. SEM and TPH analyses confirmed the formation of whisker type carbon over the spent catalysts. Addition of steam and O₂ to dry reforming feed increased the methane conversion and led to carbon free operation in combined processes.     

Stereochemistry of reactions in the 1,2-dimethylsilacyclopentane ring system. II. Stereoselective transformation

A number of stereoselective reactions of 1-substituted-1,2-dimethylsilacyclopentanes are described. Reactions of the silyl chloride (II) with ZnF₂ and with alcohols catalyzed by amines are stereoselective as a result of rapid isomerization of II. Alcoholysis of silicon hydride (I) catalyzed by transition metals is apparently an inversion reaction regardless of the nature of the catalyst, but can appear to be stereoselective because of isomerization of alkoxysilane product. Reduction of silyl fluoride (IV) by lithium aluminum hydride is nonstereoselective, a result which is proposed to arise through rapid isomerization of intermediates with expanded coordination.     

A thermal study of some lanthanide trifluoromethyl sulfonates

Lanthanide trifluoromethyl sulfonates, M(SO₃CF₃)₃ · 9H₂O, have been prepared and characterized by analysis, optical properties and thermal behaviour. Dehydration proceeded in two steps for the lanthanum, cerium, praseodymium and neodymium salts and in three steps for other members of the lanthanide series. Thermal decomposition to lanthanide fluoride, carbonyl fluoride and sulfur dioxide occurred at temperatures greater than 400°C. This decomposition has been shown to be a two step process.     

Substoichiometric precipitation of fluoride with lanthanum

The substoichiometric precipitation of fluoride with lanthanum was studied by using¹⁸F and¹⁴⁰La tracers and it was found that fluoride could be precipitated substoichiometrically with lanthanum and the reaction ratio between fluoride and lanthanum was 3∶1. The pH range at which fluoride can be separated substoichiometrically with lanthanum is between 2 and 8. Barium and indium interfere in the precipitation of fluoride, sodium, copper and manganese, however, not. Fluorosilicate can also be precipitated substoichiometrically by using lanthanum as a precipitant and the reaction ratio between fluorosilicate and lanthanum was 1∶2. This separation was applied for the determination of oxygen in silicon crystals. The concentration of oxygen measured in some silicon crystals was between 5 and 27 ppm and in good agreement with those by non-destructive method and infrared spectrophotometry.     

Beiträge Zur Chemie des Brompentafluorids. 3 [5]. Die Reaktionen von Brompentafluorid und Arylbromtetrafluoriden mit N-Basen,von Arylbromtetrafluoriden mit Fluoridionen und die Fluor-Aryl-Monosubstitution am Hexafluorobromat (V)-Anion

Contributions on the Chemistry of Brominepentafluoride. 3. Reactions of Brominepentafluoride and Arylbrominetetrafluoride with N-Bases, of Arylbrominetetrafluoride with Fluoride Ions, and the Fluorine-Aryl Monosubstitution Reactions on the Hexafluorobromate (V) Anion BrF₅ forms 1 : 1 adducts with pyridines in organic solvents. In BrF₅? N-base adducts the axial Br? F bond is activated for fluorine-aryl substitution reactions. Arylbrominetetrafluorides resulting from fluorine-aryl monosubstitutions also yield 1 : 1 adducts with pyridines; in contrast to BrF₅ they react with fluoride ions under reductive elimination. Fluorine-aryl substitutions on [BrF₆]^? are presented and the influence of the Lewis-acidity of the aryltransfer reagent and its coproduct is discussed.     

Thermal analysis of lanthanum hydroxide

Lanthanum oxide (La₂O₃) is of great interest as catalyst material. When La₂O₃ particles are prepared from lanthanum hydroxide (La(OH)₃) by thermal processes under air, various oxycarbonate phases are formed which are resistant to thermal hydroxylation. This phenomenon has not yet been extensively investigated, even though oxycarbonate phases at the particle surfaces cause a change in lanthanum oxide??s catalytic activity. The carbonate phases formed cannot be detected by means of XRD or REM-EDX investigations due to their detection limits. Thermal analysis, particularly TG-FT-IR, allows not only for the detection of the carbonate phases in La(OH)₃, but also for the tracking of the entire dehydration process from La(OH)₃ via LaOOH to La₂O₃ as well as the correct interpretation of mass changes during the thermal transformations. Pursuant to the investigations here carried out, it was determined that carbonate-free lanthanum hydroxide compounds can only be prepared and stored in a CO₂-free protective gas atmosphere (e.g., argon).     

Some reactions of iron (III) and chromium (III) fluorides with oxides

Reactions of FeF₃ with several oxides at elevated temperatures are described. Reaction products were usually Fe₂O₃ and the fluoride of the other element. With higher valency elements complete fluorine exchange did not always occur and oxyfluorides were formed. Intermediates in the reactions include oxyfluorides and mixed oxides, again only found with higher valency elements. Some shorter studies on the reaction of CrF₃ with oxides are included for comparisons, the reactions observed being generally similar.     

Species formed in the potentiometric titration of fluoride with thorium or lanthanum nitrate and functions suitable for the evaluation of the equivalence points

Calculations by means of SILLÉN's HALTAFALL program have been performed on experimental curves by LINGANE for the potentiometric titration of fluoride with thorium and lanthanum in different media. After adjusting the stability constants for some of the main species formed during the titration a good fit was obtained between the experimental data and the calculated titration curves. In the cases where the solution was buffered with acetate it was necessary to consider the formation of mixed fluoride-acetate complexes. Suitable functions for the evaluation of the equivalence point could be derived from the calculations. If the functions: F₁=(v₀+v)[F^-] and F₃=(v₀+v)[F^-]^-3 are used before and after the equivalence point, respectively, it ought to be possible to determine fluoride very accurately by titration with lanthanum nitrate in an unbuffered solution. A value proportional to [F^-] is obtained from 10 exp (—EF/RT In 10), the Nernst e.m.f. equation.     

Reactions of CF3-enones with ethyl nitroacetate and nitromethane: synthesis of CF3-γ-nitroketones

Reactions of α,β-unsaturated CF₃-ketones with nitromethane and ethyl nitroacetate have been investigated. We found that α,β-unsaturated trifluoromethylketones react with ethyl nitroacetate in the presence of calcinated potassium fluoride to form two classes of 1,4-conjugated addition products: CF₃-γ-nitroketones and 6,6,6-trifluoro-2-nitro-5-oxohexanoates in nearly quantitative yields. The products obtained are precursors for CF₃-pyrrolidine synthesis.     

Atom-economy Synthesis of N-Substituted Carbamate from Urea Derivative and Dimethyl Carbonate Catalyzed by La/SiO_2: Characterization and Activity

A series of silica gel immobilized lanthanum catalysts were prepared for the atom‐economy synthesis of N‐substituted carbamates from urea derivatives and dimethyl carbonate. The La/SiO₂ catalysts with lanthanum loadings varied from 1.3 wt% to 8.5 wt% were characterized by AES, BET, XRD, TEM, FT‐IR, XPS and TPD. According to the characterization, lanthanum species with particle sizes of 5–10 nm on the surface of silica gel were formed. The catalysts were all amorphous and the surface areas were 336.5–530.2 m²/g. NH₃‐TPD analysis showed that all samples exhibited similar acid strength with different acid amounts. FT‐IR measurement indicated that the component of lanthanum species on the catalyst surface were La(OH)₃, LaOOH and hydrated La₂O₃. Also, the peak value of the absolute amount of LaOOH was obtained with 4.3 wt% lanthanum loading. The BET surface area decreased dramatically when the lanthanum loading was above 4.3 wt%. In consideration of the results obtained from the catalytic reactions, it could be concluded that LaOOH was the possible active species and high surface area was important for the high catalytic activity.     

Superionic Character of High-Temperature Conduction in Lanthanum Fluoride

The electroconduction of crystalline lanthanum fluoride is studied in the temperature range from 1320 K to the melting point (1766 K). A markedly pronounced superionic behavior of LaF₃ is established: near mp its conductivity amounts to 42% of that corresponding to a molten state. Thermodynamics of the formation of defects and their concentrations are estimated on the basis of available data on the high-temperature enthalpy of LaF₃ within a model of noninteracting defects. The Schottky and Frenkel mechanisms of ion disordering are considered. A comparative analysis of these two mechanisms supplemented with results on the conductivity favors the Frenkel mechanism of the origin of defects in LaF₃.     

Stereospecific benzylic dehydroxyfluorination reactions using Bio’s TMS-amine additive approach with challenging substrates

Reactions involving the conversion of a benzylic alcohol into a benzylic fluoride using RSF₃ reagents are notoriously difficult to achieve with high stereochemical inversion (S_N2 reaction) due to competing dissociative S_N1 reaction processes. This Letter develops the methodology of Bio et al., and reports that the addition of a preformed 1:TMS-amine 1:RSF₃ (fluorination reagent) complex as the reagent in these reactions significantly suppresses the S_N1 process and promotes a highly stereospecific reaction generating benzylic fluorination products of high %ee.     

Potentiometric behaviour of lanthanum polyalkoxylate complexes in PVC membrane electrodes: effect of plasticizers and applications

Tetraphenylborate salts of lanthanum complexes of nonylphenoxypolyethyleneoxyethanol (Antarox CO-880) and polypropylene glycol-425 (PPG-425) have been prepared and examined for their applicability as sensing species for lanthanum ions by incorporating them in a poly (vinyl chloride) membrane system in the presence of 2-nitrophenyl phenyl ether (NPPE), dioctylphenyl phosphonate or a mixture of both as plasticizers. Almost all of the systems studied showed an excellent response for lanthanum ions in pure solutions with a near Nernstian response of 18–20 mV per decade between 10^?5 and 10^?1 M lanthanum and a static response time of less than 1 min. Among the systems studied, lanthanum(Antarox CO-880)TPB/100% NPPE (TPB=tetraphenylborate) was the best in terms of tolerance to interfering ions. K⁺, Ba²⁺, Pb²⁺ and Al³⁺ were serious interferents, but most other interferents investigated were tolerable at [La³⁺] > 10^?4 M. Plasticizers were found to be decisive in determining the sensor selectivity towards the different metal ions. A lanthanum(Antarox CO-880)TPB/100% NPPE PVC membrane electrode was utilized for the potentiometric titration of fluoride, in sodium fluoride, against lanthanum nitrate solutions. It proved to be successful for fluoride concentrations of ? 10^?3 M.     

Surface chemistry of porous magnesium fluoride

Porous preparations of magnesium fluoride were obtained from magnesium sulphate and magnesium carbonate and their surface properties have been studied. It has been shown that pure magnesium fluoride has a weak surface acidity of the Lewis type and no protonic sites stronger than H_R?0.82. Base properties of MgF₂ surfaces are more pronounced than acid ones, but the base strength is low. Surfaces of MgF₂ also bear oxidizing and reducing centres capable of forming ion radicals with adsorbed perylene and tetracyanoethylene. The low catalytic activity of magnesium fluoride for acid catalysed reactions suggests the application of MgF₂ as a catalyst support rather than as a catalyst.     

Effect of water in the precursor solution on the catalytic property of LaMnAl11O19 high temperature combustion catalyst

Manganese-substituted lanthanum hexaaluminates were prepared by the sol-gel method. The effect of water content on the catalyst preparation was examined. Large water content caused a rapid decrease in the surface area. When the R (R is the mole ratio of H₂O/ Al(OC₃H₇)₃) is equal to 1.5, the catalyst retains a specific surface area of 13.5 m² g⁻¹ and shows higher activity in CH₄ combustion.     

Etude de la réactivité du fluorure de potassium sur des esters polyhalogénés

Reactions of polyhalogenated esters with potassium fluoride in polar solvents have been studied. Two kinds of reactions are possible depending on the starting compound: the hydrochlorinated estersRCCl₂CH₂CHR′CO₂CH₃ (R=Cl, -CF₃ ; R′=H, -CH₃ give the corresponding α-β unsaturated derivatives. Moreover when using ethylene glycol as a solvent, a transesterification reaction occurs after the dehydrohalogenation. From perhalogenated esters, mainly decarboxylation products are formed; for example in DMSO as a solvent, the fluorination of the telomers Cl-(CFClCF₂)_n-CO₂CH₃ yields the corresponding perfluoroalkenes C_2nF_4n. Some mechanistic interpretations are given in order to explain the formation of the different compounds.     

Autothermal reforming of methane to syngas using co-precipitated Ni−(La2O3) x −(ZrO2)1−x catalyst

Two different catalysts, Ni?(ZrO₂) and Ni?(La₂O₃)_0.1?(ZrO₂)_0.9, were synthesized to use as catalysts in the autothermal reforming of methane. Ni?(ZrO₂) catalyst without lanthanum prepared in this study has a good initial performance, but as the reactions keep progressing, the activities gradually decrease. In contrast, the activities of Ni?(La₂O₃)_0.1?(ZrO₂)_0.9 catalyst have high thermal stability and coke resistance due to the presence of La. Both methane conversion and hydrogen yield of Ni?(La₂O₃)_0.1?(ZrO₂)_0.9 in non-reduced form are better than reduced form and as good as commercial Ni-based catalyst (ICI 57-4).     

The stereochemistry of metal catalysed hydrogen cyanide addition to alkenes

Reactions of both terminal and cyclic alkenes have been shown to occur in a stereospecifically $\text{cis}$ - manner with deutrerium cyanide when a catalyst system based on Pd(DIOP)₂ was used. Reactions of cyclohexene give only the products of equatorial cyanide incorporation.     

Silica supported lanthanum triflate mediated synthesis of 5-substituted-1H tetrazoles

Silica supported lanthanum triflate (Ln(OTf)₃-SiO₂) promoted synthesis of 5-substituted 1H-tetrazoles via [3+2] cycloaddition between aromatic/heteroaromatic nitriles and sodium azide is a high product yielding, facile, and straightforward procedure. Non toxicity, recovery, and reusability for three continuous cycles are the noteworthy features of the currently employed heterogeneous catalyst.