Fluorination Of Oxides Research Articles

Structure and Magnetic Properties of the n = 3 Ruddlesden-Popper Oxyfluoride La0.5Sr3.5Fe3O7.5F2.6.

Ruddlesden-Popper (RP) compounds of the general formula (AX)(ABX3)n with their unique sequence of perovskite-like (ABX3) and rock-salt-like units (AX) promise applications in diverse fields such as catalysis and superconductivity. Fluorination of RP oxides often leads to dramatic changes in the material properties, caused by differences in the atomic and electronic structure. While current research focuses on fluorination of n = 1 type RP oxides (A2BO4), n = 3 RP oxyfluorides have remained elusive. We present the synthesis of the first iron-based n = 3 RP oxyfluoride, La0.5Sr3.5Fe3O7.5F2.6, which was obtained from an oxide precursor by topochemical fluorination with poly(vinylidene fluoride). Joint Rietveld refinements of neutron and powder X-ray diffraction data were used to determine the crystal structure. Best results were obtained in the space group Pbca (No. 61) with a = 5.5374(1) Å, b = 5.5441(1) Å, and c = 29.2541(2) Å. The effect of the aliovalent incorporation of fluoride ions is particularly evident with respect to changes in structure and magnetic properties. The magnetic behavior was studied using field- and temperature-dependent magnetization measurements, Mößbauer spectroscopy, and neutron diffraction. Additional magnetic Bragg reflections observed in the room-temperature neutron data were successfully refined in the space group Pbca (61.1.497 in Opechowski-Guccione notation), indicating a G-type antiferromagnetic ordering with a surprisingly high Néel temperature above 300 K. This strong increase of TN by several hundred Kelvin compared to the parent oxide is particularly remarkable.

Development of SF6 as a Formal Electrophilic Fluorinating Reagent for Photocatalyzed Oxidative Fluorination of Phosphine Oxides.

Organophosphorus-fluorine compounds are of significant utility across biology, pharmacy, and chemical synthesis. Here, we introduce a photocatalyzed oxidative-fluorination approach employing SF6 as a formal electrophilic fluorinating reagent. It offers an innovative pathway to forge P(O)-F bonds. Notably, sulfur hexafluoride plays a dual role as both the oxidant and the fluorinating reagent under mild conditions in this transformation. Meanwhile, this method contributes to environmental sustainability by consuming a notorious greenhouse gas, underscoring the ecological benefits of our approach.

Regulation of oxygen vacancy within oxide pyrochlores by F-doping to boost oxygen-evolution activity

The development of cost-effective and durable oxygen evolution reaction (OER) catalysts is an essential prerequisite for hydrogen production through proton exchange membrane water electrolysis. Herein, a series of F-doped pyrochlore yttrium ruthenate Y2Ru2O7−δ-xFx (YRuOF) are designed via low-temperature fluorination of precursor oxides with polyvinylidene fluoride. Both X-ray photoelectron spectroscopy and transmission electron microscopy results confirm that F is doped into the pyrochlore crystal lattice, inducing the oxygen vacancies (VO). Importantly, the OER catalytic activity is positively correlated with VO concentration. The density functional calculation (DFT) reveals that the existence of VO enhances the metal-oxygen covalency of pyrochlore, which is conducive to the lattice oxygen evolution mechanism, hence greatly accelerating the OER kinetic. Consequently, the optimized YRuOF catalyst exhibits a greatly enhanced OER activity with an overpotential of ~235 mV at 10 mA cm−2 in comparison with that on undoped pyrochlore (~296 mV). Furthermore, such a F-doped YRuOF catalyst has a very good OER stability. This study provides an ideal perspective to develop highly efficient and durable YRuOF for the oxygen evolution reaction.

A Generalized Method for High-Speed Fluorination of Metal Oxides by Spark Plasma Sintering Yields Ta3 O7 F and TaO2 F with High Photocatalytic Activity for Oxygen Evolution from Water.

A general method to carry out the fluorination of metal oxides with poly(tetrafluoroethylene) (PTFE, Teflon) waste by spark plasma sintering (SPS) on a minute scale with Teflon is reported. The potential of this new approach is highlighted by the following results. i) The tantalum oxyfluorides Ta3 O7 F and TaO2 F are obtained from plastic scrap without using toxic or caustic chemicals for fluorination. ii) Short reaction times (minutes rather than days) reduce the process time the energy costs by almost three orders of magnitude. iii) The oxyfluorides Ta3 O7 F and TaO2 F are produced in gram amounts of nanoparticles. Their synthesis can be upscaled to the kg range with industrial sintering equipment. iv) SPS processing changes the catalytic properties: while conventionally prepared Ta3 O7 F and TaO2 F show little catalytic activity, SPS-prepared Ta3 O7 F and TaO2 F exhibit high activity for photocatalytic oxygen evolution, reaching photoconversion efficiencies up to 24.7% and applied bias to photoconversion values of 0.86%. This study shows that the materials properties are dictated by the processing which poses new challenges to understand and predict the underlying factors.

Fluorination of niobium compounds with fluorine for fluoride volatility method

Niobium is one of the fission products contained in spent nuclear fuel. Since niobium pentafluoride (NbF5) has high volatility, it is considered that niobium volatilizes with uranium hexafluoride when applying the fluoride volatility method, which is a promising pyro-reprocessing method. In this study, fluorination behavior characteristics of niobium compounds, such as reaction temperature, volatility, and reaction path, were investigated by thermogravimetric and differential thermal analyses and X-ray diffraction analysis. The target compounds were niobium metal, the niobium oxides NbO, Nb2O3, NbO2 and Nb2O5, and niobium oxyfluoride (NbO2F). All the niobium compounds reacted exothermically and were volatilized completely by the reaction with F2. It was considered that niobium volatilized as NbF5. The fluorination reactions started respectively at 180, 200, 300, and 300 °C for niobium metal, NbO, NbO2 and Nb2O5. In the fluorination of niobium oxides, the intermediate product NbO2F was also fluorinated above 300 °C and volatilized completely. Nb2O3, which seemed to be a mixture of NbO and NbO2, reacted with F2 as described by the summation of the fluorination reactions of NbO and NbO2. The reaction mechanism for the fluorination of niobium compounds obtained in this study is applicable to evaluation of the niobium transfer phenomena in the reprocessing process of the fluoride volatility method.

Effect of fluoropolymer composition on topochemical synthesis of SrMnO3−δFγ oxyfluoride films

We report the synthesis of $\mathrm{SrMn}{\mathrm{O}}_{3\text{\ensuremath{-}}\ensuremath{\delta}}{\mathrm{F}}_{\ensuremath{\gamma}}$ perovskite oxyfluoride thin films using a vapor transport method to fluorinate as-grown $\mathrm{SrMn}{\mathrm{O}}_{2.5}$ epitaxial thin films. The influence of the fluoropolymer, which acts as a fluorine vapor source, was investigated by utilizing polyvinyl fluoride (PVF), polyvinylidene difluoride (PVDF), and polytetrafluoroethylene (PTFE) in the reaction. The same process was carried out with polyethylene to isolate the role of carbon in the vapor transport process. The F distribution was probed by x-ray photoemission spectroscopy, which confirmed the incorporation of F into the films and revealed higher F concentrations in films exposed to PVF and PVDF compared to PTFE. The $c$-axis parameter expands after fluorination, a result consistent with density functional theory calculations that attribute the volume expansion to elongated Mn-F bonds compared to shorter Mn-O bonds. Using x-ray absorption spectroscopy, we show that the fluorination process reduces the nominal Mn oxidation state suggesting that F substitutes on O sites in the lattice as opposed to filling anion vacancy sites, a finding further supported by calculated formation energies of different F site occupancies. These results provide insights into topochemical fluorination of perovskite oxides, which should enable future synthesis and design efforts focused on oxyfluoride heterostructures.

Reactions of antimony compounds with fluorine gas by thermogravimetric and differential thermal analyses and X-ray diffraction analysis

Antimony is one of the key fission products in the reprocessing of spent nuclear fuel by the fluoride volatility method because of the high volatility of antimony fluorides. Since the fluorination reaction of antimony compounds is not well understood, the reaction behavior of antimony compounds with F2 was investigated by thermogravimetric and differential thermal analyses and X-ray diffraction analysis in this study. The target antimony compounds were antimony metal, SbF3, Sb2O3, Sb2O4, and Sb2O5. The fluorination reaction of antimony metal started at 150 °C, and the fluorination product was SbF3. SbF3 volatilized completely by the reaction with F2 above 190 °C; it was considered that volatile SbF5 was formed by the reaction with F2. In the series of fluorination of the antimony oxides, Sb2O3, Sb2O4, and Sb2O5, they started to react with F2 and volatilize at 330 °C as SbF5. During the fluorination of Sb2O3, Sb2O4 was formed temporarily in the course of the reaction. Oxygen released from the fluorination reaction of a part of Sb2O3 would oxidize remained Sb2O3 to Sb2O4. The reaction mechanism for the fluorination of antimony compounds obtained in this study is applicable to evaluate the transfer of antimony in the reprocessing process of the fluoride volatility method.

Electrophilic Fluorination of Secondary Phosphine Oxides and Its Application to P-O Bond Construction.

A novel and efficient electrophilic fluorination of secondary phosphine oxides with Selectfluor has been achieved. This transformation provides direct access to phosphoric fluorides in up to 92% yield under mild conditions. In addition, P-O bond construction via a one-pot coupling process of secondary phosphine oxides with water or alcohols in the presence of Selectfluor leads to the formation of phosphinic acids or phosphinates in up to 96% yield.

Fluorination of Rare Earth, Thorium and Uranium Oxides and Phosphates from Monazite: A Theoretical Approach

Monazite is a light rare earth phosphate which is difficult to process when using conventional chemical processes. It is considered to be one of the most important commercial sources of thorium and lanthanides. Using conventional chemicals to process monazite have caused severe environmental damage as has been demonstrated in the Baotou region, China. Monazite is found in combination with other minerals in nature such as bastnaesite and xenotime. The conventional techniques and chemicals used for the processing of monazite are expensive. In order to address this issue, South Africa would like to beneficiate monazite as part of its mineral beneficiation strategy. Doing so competitively would require a new cheaper and more environmentally friendlier process. A new method for the processing of monazite is currently being investigated. It is proposed to feed the monazite is fed into a plasma reactor to crack it. Potentially the cracking will allow the monazite to be more reactive and susceptible to react with less harsh chemicals. The plasma treated monazite is reacted with a fluorinating compound such as ammonium bifluoride. Ammonium bifluoride is used rather than fluorine or hydrogen fluoride as it is less dangerous to handle. The fluorinated rare earth mixture can now be separated using various methods.

Fluoride Processing Technology of Rare Earth Oxides

The article contains the results of studies on the fluorination of rare earth oxides with elemental fluorine, the results of studying the kinetics of the fluorination of a rare earth elements mixture under conditions excluding local overheating and external diffusion inhibition. A serial metallurgical mixture of rare earth oxides of a cerium subgroup manufactured by Sillamäe Mining and Chemical Plant was used as the object of the investigation. The composition and properties of the low-melting fluoride glass for immobilization of solid radioactive and toxic wastes are also given in the article.

Optimal reactor material for fluorinating uranium oxides by hydrogen fluoride gas

The results of an analysis of alloy grades for determining the most corrosion resistant material for building a reactor and hydro-fluorination of uranium oxides are presented. The alloy EP-797 or its improved analog EP-797-VI holds the greatest promise for work in a medium consisting of hydrogen fluoride gas and water vapor at temperatures to 600°C. The use of monel in the fluorination of uranium oxides is problematic because of the relatively low application temperature limit ~500°C as well as operating experience gained in foreign conversion plants.

A thermogravimetric study of the fluorination of zirconium and hafnium oxides with anhydrous hydrogen fluoride gas

Zircon mineral contains about 1–3% hafnium which is an undesirable impurity in the nuclear industry. A novel dry fluorination route was investigated to produce ZrF4 as a possible intermediate for the production of nuclear grade zirconium metal. The process was investigated by the reaction of pure ZrO2 and HfO2 with anhydrous hydrogen fluoride using a thermogravimetric analyzer. The findings indicate successful formation of ZrF4, HfF4, and oxyfluorides identified with X-ray powder diffraction. According to the TGA experiments, the stoichiometric formation of ZrF4 was realized at 525°C whilst that for HfF4 was not attained.

A thermogravimetric study of the fluorination of zirconium and hafnium oxides with fluorine gas

The fluorination of ZrO2 and HfO2 and the understanding of the fundamental chemistry of formation of ZrF4 and HfF4 in a fluorine atmosphere were investigated using a thermobalance. The fluorination of these species was performed to understand the chemical behavior of the tetrafluoride species of these metals. Thermogravimetrical analysis indicated that the formation of ZrF4/HfF4 is via the oxyfluorides. X-ray powder diffraction was used to confirm the results.

High‐Temperature Superconductors Based on Complex Layered Copper Oxyfluorides

AbstractFor Abstract see ChemInform Abstract in Full Text.

Poly(vinylidene fluoride) as a reagent for the synthesis of K 2NiF 4-related inorganic oxide fluorides

In this paper we report an improved route to the synthesis of K 2NiF 4-related inorganic oxide fluorides, such as Sr 2TiO 3F 2 and Ca 2CuO 2F 2 using low-temperature fluorination of precursor oxides with poly(vinylidene fluoride). Use of this fluorinating agent results in high quality samples, without SrF 2 or CaF 2 or other impurities, which are commonly seen for alternative fluorination routes.

Synthetic strategies for new inorganic oxide fluorides and oxide sulfates

The low temperature fluorination of low dimensional mixed metal oxides can be achieved in a variety of ways to provide new structures and modified electronic characteristics. The potential of these methods for the synthesis of new materials is considered, with particular reference to the fields of copper oxide superconductors and layered manganese oxide colossal magnetoresistance (CMR) phases; in both areas, careful structural and electronic control is necessary for optimised properties. The fluorination mechanisms are quite complex, and this paper describes the synthesis of three new oxide fluorides: La 1.9 Sr 1.1 Cu 2 O 5 F 2 , PbSr 2 YCu 3 O 6 F 4 and La 1.2 Sr 1.8 Mn 2 O 7 F 2 , via processes which involve anion insertion, anion substitution and cation extraction. The use of a related strategy to incorporate more complex anions into oxides is also discussed, and the assimilation of SO 4 2– ions into Bi 2 O 3 to give phases of composition Bi 2 O 3–x (SO 4 ) x is described.

The fluorination of uranium and zirconium oxides with xenon difluoride initiated by the addition of water

At room temperature, XeF 2 crystals do not react to any appreciable extent with UO 2, U 3O 8 or ZrO 2 powders but these mixtures can be induced to react in air without heating by the addition of a drop of water. There appeared to be distinct stages of reaction: (i) an initial, relatively slow reaction in the aqueous phase and (ii) a subsequent violent reaction. Depending upon the conditions employed, the products of reaction were either the metal fluoride or oxyfluoride.

Properties of Chromium (III) Oxides Involved in the Catalytic Gas Phase Fluorination of CF3CH2Cl

The fluorination of chromium oxides reduces the chromium specieson the surface. A low fluorination by HF does not modify the totalamount of reversibly oxidizable chromium sites, but increases thestrength of these sites. Mobile dioxygen (measured by isotopicexchange) was required to obtain a good fluorination activity.These dioxygen species would be exchanged by HF which is probably active in the fluorination of CF3CH2Cl and would prevent the irreversibly catalyst fluorination.

Fluorination of osmium and ruthenium oxides: characterization and crystal structure of osmium dioxide tetrafluoride, OsO 2F 4

Current results obtained on the fluorination of ruthenium and osmium oxides by strong fluorinating agents are presented. The product of the reaction of KrF 2 with OsO 4 in HF solution has been further characterized by 19F NMR spectroscopy and single crystal X-ray diffraction data. According to these results, the compound obtained is osmium dioxide tetrafluoride OsO 2F 4. The origin of the incorrect formulation (OsOF 6) previously given to this compound is discussed. The 19F NMR spectra indicate that there are two distinct pairs of equivalent fluorine atoms in the molecule. The X-ray diffraction data reveal a distorted octahedron of light atoms around the osmium atom, but as yet oxygen and fluorine are not distinguishable. Both methods combine clearly to indicate a distorted cis-octahedral structure. The cis-orientation of oxygen atoms is not in accord with the electron pair repulsion model; the distortions, however, can be interpreted in this manner.

Chemistry on fluorinated surfaces. Brønsted and lewis acid sites

The fluorination of high surface area metal oxides is an important step in the production of heterogeneous catalysts for halogen exchange, e.g. fluorinated γ-alumina or silica. Two situations have been encountered experimentally when the defect spinel γ-alumina is fluorinated at room temperature using a variety of volatile inorganic fluorides : (i) formation of new Brønsted acid surface sites, or (ii) formation of new Brønsted and Lewis sites. Models for both types of behaviour have been developed from an idealised γ-alumina surface by combining the results obtained from several experimental methods. I.r. spectroscopy and [ l8F]-labelling were used to follow the course and extent of the gas-solid reactions, to obtain kinetic data and to determine the lability of surface fluoride. Adsorption-desorption phenomena were studied using [ 14C]- and [ 35S]-labelled molecules and reactions involving but-1-ene or CH 3CCl 3 were used to probe Brønsted and Lewis acid character respectively. Polymerization of a C 2 chloroolefin in the presence of Lewis acid promoted, fluorinated γ-alumina leads to the formation of an organic layer on the surface. The nature of these surfaces, which are also catalytically active, will be discussed in the light of 1H, 13C and 27Al solid state MAS n.m.r. measurements. Support of this work from the SERC and I.C.I. plc, is gratefully acknowledged.