Effect of heat treatment on structural-chemical transformations in magnesium hydrosilicate [Mg<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>5</Subscript>(OH)<Subscript>4</Subscript>] nanotubes

The structural-chemical transformations in nanotubular magnesium hydrosilicate of composition Mg₃Si₂O₅(OH)₄ (chrysotile) obtained by hydrothermal synthesis were studied in heating to 1200°C.     

Hydrothermal synthesis of nanotubular Co-Mg hydrosilicates with the chrysotile structure

Nanotubes of the chrysotile structure were obtained under hydrothermal conditions in the series of hydrosilicates of the Mg₃Si₂O₅(OH)₄-Co₃Si₂O₅(OH)₄ system. Substitution of cobalt ions for magnesium ions affects the conditions of formation, morphology, and size of the nanotubes. The formation of nanotubes on hydrothermal treatment of the starting mixtures of cobalt metasilicate and magnesium, cobalt, and, silicon oxides and hydroxides occurs in stages, and the stage of nanotube formation is always preceded by the formation of a lammellar structure.     

Interaction of Mg3Si2O5(OH)4 nanotubes with potassium hydroxide

Interaction of Mg₃Si₂O₅(OH)₄ nanotubes with aqueous solutions of potassium hydroxide in various temperature-time treatment modes was studied.     

Hydrothermal synthesis of nanotubular Mg-Fe hydrosilicate

Magnesium iron hydrosilicate nanotubes with a chrysotile ((Mg,Fe)₃Si₂O₅(OH)₄) structure have been synthesized hydrothermally at t = 250–450°C and p = 30–100 MPa. In the hydrothermal synthesis of (Mg,Fe)₃Si₂O₅(OH)₄ chrysotile, part of the Fe²⁺ ions oxidize to Fe³⁺ and are incorporated into the octahedron and tetrahedron layers of the chrysotile structure. The limiting iron content of chrysotile has been determined up to which cylindrically rolled layers can form to yield nanotubes. The hydrothermal treatment of precursors richer in FeO yields platelike hydrosilicates. The iron ions present in the starting components affect the synthesis parameters, morphology, size, optical properties, and thermal stability of the nanotubes.     

Structure, morphology, and thermal properties of nanocomposites based on polyamido imide and hydrosilicate nanotubes

New nanocomposites based on heat-resistant poly[(diphenyl oxide)amido-N-phenylphthalimide] with Mg₃Si₂O₅(OH)₄ hydrosilicate nanoparticles of tubular structure were prepared. The structure, morphology, and thermal properties of the nanocomposites were studied in relation to the content of hydrosilicate nanotubes.     

Study on the thermal decomposition of chrysotile asbestos

This article reports the possibility of detoxification of chrysotile asbestos through a low temperature heating and grinding treatment. The effect of thermal treatment at different temperatures in the range from 500 to 725 °C for 3 h on raw natural asbestos was characterized by thermal analysis, X-ray diffraction, and scanning electron microscopy. It was found that an isothermal treatment at 650 °C caused the complete dehydroxylation of chrysotile Mg₃Si₂O₅(OH)₄. Transformation of the dehydroxylated phase to forsterite Mg₂SiO₄ was obtained by heat treatment in the range 650–725 °C. The study of microstructure changes of heated asbestos show the destruction of characteristic fibers of chrysotile and formation of strips of forsterite. It is easily milled to pulverulent-shape material by mechanical milling in vibratory mill.     

Structure of aqueous dispersions of Mg<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>5</Subscript>(OH)<Subscript>4</Subscript> nanotubes

Behavior of hydrosilicate nanotubes of composition Mg₃Si₂O₅(OH)₄ in aqueous dispersions was studied. Experiments devoted to observation of steady-state and relaxation electro-optical effects demonstrated that, irrespective of their initial concentration in water, nanotubes form stable bulk aggregates with weak anisotropy, rather than a homogeneous system of separate suspended nanosize cylinders.     

Ferromagnetism in talc-like cobalt-phyllosilicates

The synthetic layered 1:1 phyllosilicate, Co_2.3Al_0.7Si_1.3Al_0.7O₅(OH)₄, and 2:1 phyllosilicates Co₃Si₄O₁₀(OH)_0.7F_1.3, Co₃Si₄O₁₀(OH)_1.2F_0.8(Co(CH₃COO)₂)_0.24, Co₃Si₄O₁₀(OH)_1.3F_0.7 (TAEAH)_0.12 and Co₃Si₄O₁₀(OH)_1.3F_0.7 (TAEAH)_0.24 have been obtained under hydrothermal conditions at ca. 200 °C (TAEA : (tris-aminoethyl)-amine). The compounds are characterized by interplanar distances of 7.0, 9.5, 11.1, 13.3 and 14.3 Å, respectively. They show long range magnetic ordering below 16 K, characterized by positive Weiss constants, imaginary component in the ac-susceptibility, spontaneous magnetisation in small applied field and coercive fields in the range 400 to 700 Oe. The magnetisation in field of 3 Tesla classifies them as 3D ferromagnet.     

Hydrothermal Synthesis and Characterization of Nb3O7(OH)

Triniobium hydroxide heptaoxide, Nb₃O₇(OH), was prepared hydrothermally by treating niobic acid or triniobium chloride heptaoxide with 3.0 mol/dm³ sulfuric acid at 250–350°C and 15 MPa. The hydroxide oxide was isomorphous with the low-pressure form of triniobium fluoride heptaoxide which is built up of 3 X ∞ blocks of the ReO₃ structure with crystallographic shear in one dimension. When heated in air, Nb₃O₇(OH) dehydrated up to 460°C to give poorly crystallized Nb₂O₅, which, on further heating, changed slowly into a less ordered precursor of M? Nb₂O₅(¹). Hydrothermal treatment of Nb₃O₇(OH) with pure water at 400–500°C afforded P? and R? Nb₂O₅; the conversion of Nb₃O₇(OH) is explained in terms of the close structural relation among these three forms.     

Preparation of magnetic nanocrystalline Mn0.5Mg0.5Fe2O4 and kinetics of thermal decomposition of precursor

The spinel Mn_0.5Mg_0.5Fe₂O₄ was obtained via calcining Mn_0.5Mg_0.5Fe₂(C₂O₄)₃·5H₂O above 400 °C in air. The precursor and its calcined products were characterized by thermogravimetry and differential scanning calorimetry, Fourier transform FT-IR, X-ray powder diffraction, scanning electron microscopy, energy dispersive X-ray spectrometer, and vibrating sample magnetometer. The results showed that Mn_0.5Mg_0.5Fe₂O₄ obtained at 600 °C had a specific saturation magnetization of 46.2 emu g^–1. The thermal decomposition of Mn_0.5Mg_0.5Fe₂(C₂O₄)₃·5H₂O below 450 °C experienced two steps which involved, at first, the dehydration of five water molecules and then decomposition of Mn_0.5Mg_0.5Fe₂(C₂O₄)₃ into spinel Mn_0.5Mg_0.5Fe₂O₄ in air. Based on Starink equation, the values of the activation energies associated with the thermal decomposition of Mn_0.5Mg_0.5Fe₂(C₂O₄)₃·5H₂O were determined.     

Calorimetric determination of the enthalpy of formation of partheite,a calcium zeolite

A thermochemical study of partheite of composition (Ca_1.96Mg_0.04Na_0.01K_0.01) · [(Al_4.04Fe _0.01 ³⁺ )Si_3.95O_14.97(OH)_2.03] · 4.2H₂O, a natural calcium zeolite extracted from gabbro pegmatites of the Denezhkin Kamen’ deposit (North Ural, Russia), was performed. The enthalpies of formation of partheite from the constituent oxides, (Δ_f H°_ox(298.15 K) = ?359 ± 21), and elements, (Δ_f H°_el(298.15 K) = ?10108 ± 21), were determined by means of high-temperature in-melt-dissolution calorimetry. On the basis of the experimental data obtained, the enthalpy of formation of partheite of theoretical composition Ca₂[Al₄Si₄O₁₅(OH)₂] · 4H₂O from the elements was evaluated, ?10052 ± 21 kJ/mol.     

Thermal decomposition of hydrotalcites with variable cationic ratios

Thermal analysis complimented with evolved gas mass spectrometry has been applied to hydrotalcites containing carbonate prepared by coprecipitation and with varying divalent/trivalent cation ratios. The resulting materials were characterised by XRD, and TG/DTG to determine the stability of the hydrotalcites synthesised. Hydrotalcites of formula Mg₄(Fe,Al)₂(OH)₁₂(CO₃)·4H₂O, Mg₆(Fe,Al)₂(OH)₁₆(CO₃)·5H₂O, and Mg₈(Fe,Al)₂(OH)₂₀(CO₃)·8H₂O were formed by intercalation with the carbonate anion as a function of the divalent/trivalent cationic ratio. XRD showed slight variations in the d-spacing between the hydrotalcites. The thermal decomposition of carbonate hydrotalcites consists of two decomposition steps between 300 and 400°C, attributed to the simultaneous dehydroxylation and decarbonation of the hydrotalcite lattice. Water loss ascribed to dehydroxylation occurs in two decomposition steps, where the first step is due to the partial dehydroxylation of the lattice, while the second step is due to the loss of water interacting with the interlayer anions. Dehydroxylation results in the collapse of the hydrotalcite structure to that of its corresponding metal oxides and spinels, including MgO, MgAl₂O₄, and MgFeAlO₄.     

Zur Bedeutung von V2O3(OH)4(g) für den chemischen Transport von V2O5(s) in Anwesenheit von Wasser. Nachweis des Gasteilchens,thermodynamische Daten und Modellrechnungen

V₂O₃(OH)₄(g), Proof of Existence, Thermochemical Characterization, and Chemical Vapor Transport Calculations for V₂O₅(s) in the Presence of Water By use of the Knudsen-cell mass spectrometry the existence of V₂O₃(OH)₄(g) is shown. For the molecules V₂O₃(OH)₄(g), V₄O₁₀(g), and V₄O₈(g) thermodynamic properties were calculated by known Literatur data. The influence of V₂O₃(OH)₄(g) for chemical vapor transport reactions of V₂O₅(s) with water ist discussed. Δ_BH°(V₂O₃(OH)₄(g), 298) = –1920 kJ · mol^–1 and S°(V₂O₃(OH)₄(g), 298) = 557 J · K^–1 · mol^–1, Δ_BH°(V₄O₁₀(g), 298) = –2865,6 kJ · mol^–1 and S°(V₄O₁₀(g), 298) = 323.7 J · K^–1 · mol^–1, Δ_BH°(V₄O₈(g), 298) = –2465 kJ · mol^–1 and S°(V₄O₈(g), 298) = 360 J · K^–1 · mol^–1.     

Variations in the Electrokinetic Potential of Magnesium Hydrosilicate Fibers upon Treatment with Ammonium Chloride

The effect of the treatment of magnesium hydrosilicate (Mg₃Si₂O₅(OH)₄) fibers with an aqueous 5% ammonium chloride solution at 37?40 and 57?60°C on their electrokinetic potential (ζ potential) is studied. The maximum time of exposure in the NH₄Cl solution was 100 min, while the ζ potential was measured every 20 min. It is shown that the treatment of the initial magnesium hydrosilicate fibers with the NH₄Cl solution leads to a reversal of their surface charge and a rise in the absolute value of the negative charge, which is explained by magnesium leaching out of the surface layer of the fibers. Washing of the treated fibers with distilled water leads again to the sign reversal of the ζ potential. Therewith, the character of the dependences of the fiber ζ potential on the time of the treatment with the 5% NH₄Cl solution at T = 37?40°C is the same before and after washing.     

A Strong-Acid-Resistant [Th6] Cluster-Based Framework for Effectively and Size-Selectively Catalyzing Reductive Amination of Aldehydes with N,N-Dimethylformamide

Utilization of N,N-dimethylformamide (DMF) as an amine source and reductant for synthesizing tertiary amines is a promising way to replace the substrates formaldehyde and dimethylamine, and it is desirable to seek porous acid-resistant catalysts for heterogeneous catalysis of this reaction. Herein, a robust metal–organic framework (MOF) {[Th₆O₄(OH)₄(H₂O)₆(BCP)₃]⋅10 DMF}_n ( 1 ) containing stacked nanocages with a diameter of 1.55 nm was constructed. Compound 1 can maintain its single-crystal structure even kept in air at 400 °C for 3 h, and in DMF or water at 200 °C for 7 days. Density functional theory (DFT) calculations suggested that the high interaction energy between the [Th₆O₄(OH)₄(H₂O)₆]¹²⁺ clusters and ligands was responsible for the excellent stability of 1 . Catalytic investigations revealed that 1 can effectively and size-selectively catalyze the reductive amination of aldehydes with DMF, and it can be reused at least five times without obvious loss in catalytic activity.     

Thermische Zersetzung von Afwillit Ca3(SiO3OH)2 · 2 H2O

Thermal Decomposition of Afwillite Ca₃(SiO₃OH)₂ · 2 H₂O Using the molybdate method [1], infrared, Raman, and high resolution solid state ¹H and ²⁹Si NMR spectroscopy two intermediate phases can be identified in the process of dehydration of afwillite: At temperatures of 250 to 300°C a non-crystalline phase is formed mainly (about 80 to 90% referred to Si) with a structure similar to mineral killalaite Ca₃OH(Si₂O₆OH) [2] and phase F [3]. Between 300 and 500°C this compound is converted to a non-crystalline kilchoanite-like phase γ-Ca₂SiO₄ · Ca₄Si₃O₁₀ [4], consisting of ordered γ-Ca₂SiO₄ and disordered trisilicate layers. By a side reaction polysilicate (about 10 to 20% referred to Si) is formed.     

Formation of variable-composition iron(III) hydrosilicates with the сhrysotile structure

The process of formation of iron hydrosilicates (Mg²⁺,Fe³⁺)_2–3Si₂O₅(OH)₄ was studied. It was shown that the stage of coprecipitation of magnesium and iron hydroxides in the presence of silica nanoparticles forms poorly crystallized layered Mg–Fe double hydroxides having Fe³⁺ ions in the octahedral sites. Hydrothermal treatment of the mixtures of coprecipitated hydroxides and silica nanoparticles gives rise to layered hydrosilicates, where Fe³⁺ ions occupy both the octahedral (preferentially) and tetrahedral sires. The possibility of the formation and a fairly stable existence of the variable-composition layered hydrosilicate (Mg²⁺,Fe³⁺)_2–3Si₂O₅(OH)₄ was shown to correlate with the stability range of its precursor brucite-like Mg–Fe layered double hydroxide.     

The synthesis and thermochemical study of (Mg,Fe)<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>5</Subscript>(OH)<Subscript>4</Subscript> nanotubes

Nanotubular (Mg,Fe²⁺,Fe³⁺)₃Si₂O₅(OH)₄ hydrosilicates with a chrysotile structure were synthesized under hydrothermal conditions. The phases prepared were studied thermochemically on a high-temperature Tian-Calvet microcalorimeter by solution calorimetry. The standard enthalpies of formation of magnesium-iron nanotubular hydrosilicates were determined. The formation of iron-containing nanotubes was shown to be lass favorable energetically than the formation of magnesium nanotubes.     

Phase equilibria in the ternary system: MgONa2OP2O5: Partial system: Mg3(PO4)2Mg4Na(PO4)3Na4P2O7Mg2P2O7

The partial system Mg₃(PO₄)₂Mg₄Na(PO₄)₃Na₄P₂O₇Mg₂P₂O₇ in the ternary system MgONa₂OP₂O₅ was investigated using thermal and X-ray diffraction analyses and microscopy, and its phase diagram has been determined. In this range of composition, two binary phosphates occur: Mg₄Na(PO₄)₃ and Mg₆Na₈(P₂O₇)₅. The former melts incongruently (at 1155°C) and the latter does congruently (at 808°C). In the partial system of interest, the two sections Mg₄Na(PO₄)₃Mg₂P₂O₇ and Mg₄Na(PO₄)₃Mg₆Na₈(P₂O₇)₅ are studied, and their phase diagrams are established. The partial system is divided into three partial ternary systems in which two ternary eutectics and one ternary peritectic occur.     

Determination of the changes of the basic structures of silica species in dependence on the concentration of sodium chloride by FAB-MS

The chemical species of silica in NaCl solutions of different concentrations were identified by FAB-MS (fast atom bombardment mass spectrometry). The basic structures of silica species, such as cyclic pentamer (Si₅¶(OH)₉O₆ ^–), linear pentamer (Si₅(OH)₁₁O₅ ^–), cyclic hexamer (Si₆(OH)₉O₈ ^–, Si₆(OH)₁₁O₇ ^–) and linear hexamer (Si₆(OH)₁₄O₆ ^–), were identified, in addition to dimer (Si₂(OH)₅O₂ ^–), trimer (Si₃(OH)₇O₃ ^–) and cyclic tetramer (Si₄(OH)₇O₅ ^–). The patterns of changes of the peak intensities of the silicate complexes relative to the dimer with increasing NaCl concentration were classified into two types: that represented by linear silicate complexes and the other by cyclic silicate complexes. The differences in the type of chemical species and their changes according to the NaCl concentration reflect the number of bonds necessary for polymerization and hydrolysis of the silica complexes. The differences between the linear and the cyclic silicate type have some implications on the dissolution mechanism of silicate complexes, the hydration of the molecules and the equilibrium between solubility, hydrolysis, polymerization and the salting-out effect in NaCl solution.