A study of silicon tetrafluoride reduction with hydrogen in radiofrequency discharge

The process of plasma-chemical conversion of silicon tetrafluoride in a radiofrequency (13.56 MHz) discharge has been studied. The dependence of the yield of silicon on the specific energy input and the H₂/SiF₄ molar ratio has been examined in the pressure range of 0.1–0.3 Tort. The maximum yield of silicon is 60%, and the optimal specific energy consumption is 44.6 MJ per mole of Si. A mechanism has been proposed for the plasma-assisted reduction of silicon tetrafluoride under the given experimental conditions.     

Plasma-Chemical Treatment of Process Gases with Low-Concentration Fluorine-Containing Components

The problems of recycling of the gases with low concentration of fluorine-containing components have been considered. The model of their plasma-chemical treatment with the heat recovery of effluent gases is proposed. The process thermodynamics for various compounds of reacting gases is considered. The heat- and mass transfer processes are simulated in the plasma-chemical reactor. Temperature and velocity profiles of treated gas flows have been obtained within the recovery zone, mixing area with the plasma jet as well as in the reactor working zone. The compliance of the resulting distributions and the temperature range needed for the full conversion is demonstrated. The experiments on the plasma-chemical treatment of carbon tetrafluoride and nitrogen trifluoride have been conducted. The resulting degrees of CF₄ and NF₃ decomposition turned out to be above 90% providing more than double reduction of specific energy cost as compared with the available facilities of similar purposes. An approach is proposed to reduce nitrogen oxide content in effluent gases at reactor outlet and to decrease in future the specific energy cost of conversion of halogen-containing gases.     

Reactions of water vapor or molecular hydrogen with trichloroethylene in a microwave plasma reactor

The reaction of trichloroethylene (C₂HCl₃) with water vapor or molecular hydrogen has been studied in a low-pressure [ca. 5 Torr (0.67 kPa)] microwave plasma tubular flow reactor. The experimental apparatus included feed introduction systems, a microwave plasma reactor, and full product analysis by flame ionization and thermal-conductivity gas chromatography, mass spectrometry, and specific ion or pH detection for hydrogen chloride [HCl]. Conversions of C₂HCl₃ in the range 50 to almost 100% are achieved. Product analyses indicate conversion to HCl, some light hydrocarbons, nonparent chlorocarbons, and soot C_(s). For the H₂O case, carbon monoxide and trace carbon dioxide were produced in place of some light hydrocarbons and C_(s). At least 85 mole % of chlorine (Cl) from the converted parent C₂HCl₃ forms thermodynamically stable HCl at parent conversions of 80% or more. The remaining chlorine was present as nonparent chlorocarbons. Preliminary kinetic analyses were performed. The global reaction in the plasma was found to follow one-half-order kinetic dependence on each of C₂HCl₃ and H₂O or H₂. Elementary plasma reaction mechanisms are presented to account for C₂HCl₃ conversion and the observed product distribution.     

Reactions of Difluorosilylene with Amines,Phosphines and Halomethanes The First Evidence of the Insertion of Difluorosilylene into Tetrafluorosilane

The reactions of SiF₂/SiF₄ with amines, phosphines and various halomethanes were studied. The results show that in the presence of strong Lewis base (Et₃N, HNEt₂, PR₃, PR₂Cl), SiF₂ inserts initially into SiF₄ to form Si₂F₆, which was followed by subsequent reactions leading to products containing SiF₃ groups. This is the first report of the insertion of SiF₂ into SiF₄. When the reactants were a weaker base (such as RPCl₂, CX₃Br, X=F, Cl), insertion of SiF₂ into P-Cl and C-Br bonds became predominant.     

THF: An Efficient Electron Donor in Continuous Flow Radical Cyclization Photocatalyzed by Graphitic Carbon Nitride

Mesoporous graphitic carbon nitride (mpg‐C₃N₄) was found to be an efficient heterogeneous photocatalyst for the metal‐free radical cyclization of 2‐bromo‐1,3‐dicarbonyl compounds. Reactions leading to functionalized cyclopentanes proceed under mild conditions and can be conducted in a continuous flow photoreactor. Compared to the batch reaction, the use of a continuous flow reactor resulted in a significant reduction in reaction time (complete conversion of 0.04 mmol of substrate in a batch was achieved after 4 h, whereas in a flow reactor the same amount of substrate was fully converted into a product within 40 min). Mechanistic studies of the reaction showed that THF plays not only the role of solvent, but is also a crucial hydrogen and electron donor.     

Untersuchungen zur Reaktion zwischen Alkalihexafluorosilicaten und einigen oxidischen Niobverbindungen

Investigations of the Reaction between Alkalihexafluorosilicates and Some Oxidic Niobium Compounds The reactions of Na₂SiF₆ with M-Nb₂O₅ or NaNbO₃ always produce Na₂Nb₂O₅F₂ and the thermolysis product is NaNbO₃. On the other hand various reactions of K₂SiF₆. exist. Initially with M-Nb₂O₅ an intermediate phase of a bronzestructure type is formed, like α-K_0,25NbO_2,25F_0,75, being converted into K₃NbO₂F₄ and another amorphous niobium phase. Moreover K₂NbO₃F crystallizes in a long time reaction. However first of all K₂NaNbO₂F₄ is formed by the reaction of K₂SiF₆ with NaNbO₃ and only if there is an excess of K₂SiF₆ this substance is changed slowly into K₃NbO₂F₄. The thermolysis of K₃NbO₂F₄ can produce such compounds as K₆Nb₂O₇F₂.     

氟硅酸铵改性纳米HZSM-5分子筛催化甲醇制丙烯

在静态条件下, 采用不同浓度的氟硅酸铵溶液对纳米ZSM-5分子筛进行了改性处理. 利用粉末X射线衍射(XRD)、²⁷Al 魔角旋转固体核磁共振(²⁷Al MAS NMR)、X射线荧光光谱(XRF)、X射线光电子能谱(XPS)、N₂ 吸附、透射电镜(TEM)、NH₃程序升温脱附(NH₃-TPD)、吡啶吸附红外光谱(Py-IR)等技术对改性前后纳米ZSM-5分子筛的骨架结构、织构性质、酸性质进行了表征. 并在常压、反应温度为450℃、甲醇质量空速(WHSV)为1 h^-1的条件下, 研究了改性前后纳米HZSM- 的甲醇制丙烯(MTP)催化性能. 结果表明, 合适浓度的氟硅酸铵处 理能够选择性地脱除纳米ZSM-5 分子筛的外表面铝, 从而使得HZSM-5 的酸密度降低, 比表面积和孔容增大, MTP催化性能显著提高. 氟硅酸铵改性后纳米HZSM-5 的丙烯选择性和丙烯/乙烯(P/E)质量比分别由原来的 28.8%和2.6提高到45.1%和8.0, 催化剂寿命增加了近2倍.     

Mass spectrometric study of NF3 plasma etching of silicon

NF₃ plasma etching is used for dry cleaning of reactors after plasma-enhanced chemical vapor deposition of hydrogenated amorphous silicon from SiH₄. The NF₃ plasma chemistry, in a closed isothermal plasma box with silicon coated walls, is analyzed by mass spectrometry of gases. Silicon is etched as SiF₄ by F atoms produced in the NF₃ dissociation into F+NF₂, or 2F+NF. The NF radicals recombine as N₂ +2F whereas the long-lived NF₂ radicals do not react with Si, but recombine as N₂F₄ This is the main limitation (or fluorine conversion into SiF₄. The pressure increase at the end point of etching is attributed to the sudden increase of F atom concentration in the gas phase and the consequent recombination q( F atoms as F₂.     

Effect of Nonthermal Plasma on the Methanation of Carbon Monoxide over Nickel Catalyst

Reduction of carbon monoxide to methane by hydrogen was investigated with a nonthermal plasma reactor in which Ni/alumina catalyst pellets was filled. The effect of reaction temperature, pressure and voltage on the conversion of CO was examined. It was found that the nonthermal plasma significantly enhanced the catalytic conversion of CO. The effect of the nonthermal plasma was especially remarkable at lower temperatures and pressures. At high temperatures, the catalyst itself exhibited very high catalytic activity for the conversion of CO. Since high pressure is unfavorable for creating electrical discharge plasma, the increase in pressure lowered the discharge power, thereby weakening the effect of the nonthermal plasma. With the nonthermal plasma alone, there was no conversion of CO. The reaction products identified by FTIR spectra were CH₄, CO₂ and H₂O. FTIR spectra also showed that CO was converted primarily into CH₄ with high selectivity above 90% at most experimental conditions.     

Conversion of C<Subscript>2</Subscript>-C<Subscript>4</Subscript> alcohols over copper-containing catalysts on carbon and fluorocarbon fibers

Carbon and fluorocarbon fibers were used as carriers for the preparation of copper catalysts from copper oxalate as precursor. The catalytic properties of catalyst were studied in the reaction of the dehydrogenation of C₂-C₄ alcohols by the pulsed microcatalytic method. The effect of the copper content in the catalyst, the reaction temperature on the degree of conversion, and the relation of the reaction channels were studied. The electron microphotographs were obtained, specific surfaces were measured, and X-ray pictures and infrared spectra of catalysts were taken. The activity of the catalysts on the carbon and fluorocarbon fibers in the dehydration-dehydrogenation reactions of C₂-C₄ alcohols was comparatively estimated. It was shown that the selectivity of the products from the dehydrogenation reaction is higher for the Cu-fluorocarbon fiber catalyst.     

Halogen fluorosulfate reactions with fluorocarbons

The scope of the reaction of simple fluorocarbon halides with chlorine fluorosulfate and mixtures of chlorine and bromine fluorosulfate to produce R_fOSO₂F compounds has been investigated. It is shown that in many cases even primary chlorine in -CF₂Cl groups can be replaced by -OSO₂F, Primary bromine or iodine in -CF₂X are more readily replaced. The mechanism of this replacement reaction has been established by the isolation of the metastable iodine III intermediate R_fI(OSO₂F)₂. Neither secondary chlorine nor bromine in -CFX- groups is affected. With the secondary iodide, i-C₃F₇I, the salt [(i-C₃F₇)₂I]⁺ [I(OSO₂F)₄]^? is formed. Furthermore, it has been found that ClOSO₂F is capable of converting fluorocarbon acids or their derivatives into fluorocarbon halides. A combination of these two ClOSO₂F reactions with the known conversion of R_fCF₂OSO₂F to the corresponding fluorocarbon acid offers a novel, high yield chain shortening reaction for the otherwise unreactive fluorocarbon halides according to:

     

Unusual transformations of difluorosilanone F<Subscript>2</Subscript>Si=O

The structure and properties of fluorosilicon polymer (fluorosil) formed by the reaction of phenyltrifluorosilane with aliphatic alcohols have been studied by the methods of IR, ¹⁹F, ²⁹Si NMR spectroscopy, high temperature mass-spectrometry, derivatography and atom emission analysis. Due to its high reactivity, this polymer readily reacts with glass of the reaction vessel extracting the ions of all metals entering into its composition. Fluorosil formed in a quartz, teflon or polypropylene reactors is characterized by low stability and is slowly decomposed to SiF₄ and SiO₂. Apparently, fluorosil is the product of incorporation of SiF⁴ into the SiO₂ matrix.     

Interaction between hydrofluorosilicic acid and 1,10-phenanthroline: Hydrolytic stability of chelate complexes of silicon tetrafluoride with bidentate N-donors

Hexafluorosilicate (LH₂)SiF₆ and the cis-[SiF₄(L)] chelate complex characterized by ¹⁹F NMR are products of reaction between hydrofluorosilicic acid and 1,10-phenanthroline (L). XRD findings show that the structure of (LH₂)SiF₆ is stabilized by NH···F hydrogen bonds (N···F 2.618(4), 2.676(4) Å) and CH···F contacts. The relative resistance of the cis-[SiF₄(L)] complex to hydrolysis is associated with the chelate effect.     

Methane production from synthesis gas using a mixed culture ofR. rubrum M. barkeri,and M. formicicum

The components of synthesis gas, CO, H₂, and CO₂, may be converted into CH₄ biologically through either acetate or H₂/CO₂ as intermediates. Of these two routes, conversion through H₂/CO₂ is preferred. This paper presents results of mixed-culture studies employing the photosynthetic bacteriumR. rubrum for converting CO to CO₂ and H₂ by the water gas shift reaction and two methanogens,M. formicicum andM. barkeri, for converting CO₂ and H₂ into CH₄. Results are presented for triculture operation in two types of reactors, the packed bubble column and the trickle-bed reactor.     

Zum Einfluß der Gasphase auf die thermische Zersetzung von K2[SiF6]

Effect of the Gas Phase on the Thermal Decomposition of K₂[SiF₆] K₂SiF₆ produced in usual ways is contaminated by traces of oxygen and protons. These and traces of water fed by gas atmosphere influence the thermal decomposition reaction. To study the influence of the gas phase definite amounts of H₂O and HF were added. The formation of SiF₄ was determined. The development of a SiO₂ phase in presence of H₂O and other experimental results suggest the construction of a layer on the K₂SiF₆ surface, which hinders further SiF₄ being developped. Temperature and linear velocity of the gas influence the length of a zone of decomposition migrating through the solid. This is explained by sorption and reaction behavior of intermediately formed fluorosiloxanes.     

Hexafluoropropylene Oxide–Alcohol: A Convenient System for Silica Dissolution

Abstract

A new method of silica dissolution is described. It involves the formation of a stable SiF₄ · n ROH complex (1, 1a) just from SiO₂ and anhydrous alcoholic HF generated in situ from commercially available hexafluoropropene oxide. Alcoholic SiF₄ complexes can be easily converted to different organosilicon compounds of the type SiF₄L₂ and (LH)₂SiF₆ [L = 1,10-phenantroline (2a), 2,2′-dipyridyl (2b), Me₂SO (2c), pyridine (2d), triethanolamine (3a)]. Different silica-containing compounds can be used in this strategy—silicagel, sand, alumosilicates, and even rice husk.

GRAPHICAL ABSTRACT     

Zum Modifikationswechsel von SiO2 in Gegenwart von Alkali-Fluoriden und -Silicaten

On the Modification Change of SiO₂ in Presence of Alkali Fluorides and Silicates The influence of NaF and Na₂SiF₆ on SiO₂ and sodium silicates was investigated. The SiF₄, developed in this process, influences the reaction as shown by a comparison of experiments in open and closed systems. With low Pressures of SiF₄ the reactions of NaF or Na₂SiF₆ with certain SiO₂ modifications lead to sodium silicates, in presence of a perceptible pressure of SiF₄ to cristobalite and tridymite and also to low-quartz. Sodium silicates react with NaF mainly to Na₂SiO₃. Na₆Si₈O₁₉ could be obtained from Na₂SiO₃ only when NaF was replaced by Na₂SiF₆. In this case additional low-quartz was formed. The formation of tridymite, only possible in the presence of certain foreign ions, is attainable at high temperatures (1150°C) if HF is added together with an alkali fluoride (A = Na, K, Rb, Cs). The amount of fluoride added was decisive for the size of the crystals of tridymite, which were prepared by chemical transport. The impurities of the used low-quartz (Furka) are not sufficient for the formation of tridymite.     

Experimentelle Untersuchungen zum Wolframtransport in fluorhaltigen Systemen

Experimental Investigations on the Tungsten Transport in Systems Containing Fluorine Compounds Information on the chemical processes in tungsten incandescent lamps containing gaseous fluorides are obtained by i.r. and u.v. spectroscopic and mass spectrometric analysis of the gasphase as well as by investigating the deposits at the wall and the changes at the filament. Due to the involvement of the wall in the reaction fluorine carrier gases are converted irreversibly into SiF₄ which also maintains the chemical tungsten transport. In systems containing fluorine and bromine silicon bromide fluorides are formed.     

Untersuchungen zur SiF4-Sorption an Kieselglaspulvern

Studies on the Absorption of SiF₄ at Silica Powders The absorption of SiF₄ at silica powder was investigated in dependence on temperature and pressure. It starts at about 800 K. In the range of 800 K up to 1160 K the amount of SiF₄ absorbed within 24 hours increases with temperature, and at a constant temperature (880 K) with the filling pressure of SiF₄ (2 kPa–30 kPa). Desorption of SiF₄ was proved experimentally at temperature above 1273 K. It is connected with crystallization of silica into β-cristobalite.