Showing papers by "Vadim G. Kessler published in 2006"
TL;DR: In this article, the authors summarized recent literature data and presented new experimental data on the mechanisms of chemical modification, hydrolysis and polycondensation of the alkoxides and demonstrates possibilities to approach new classes of materials, exploiting these mechanisms.
Abstract: This paper summarizes recent literature data and presents new experimental data on the mechanisms of chemical modification, hydrolysis and polycondensation of the alkoxides and demonstrates possibilities to approach new classes of materials, exploiting these mechanisms. Low reactivity of silicon alkoxides is improved by either basic catalysis exploiting an SN2 mechanism or acidic catalysis facilitating a proton-assisted SN1 mechanism as well as by modification with chelating ligands. Metal alkoxides are much stronger Lewis bases compared to silicon alkoxides and the acidity of water is strong enough to achieve their rapid hydrolysis via proton-assisted SN1 pathway even in the absence of additional catalysts. Introduction of the modifying chelating ligands is leading generally to increased charge distribution in the precursor molecules. Modifying chelating ligands are also appreciably smaller than the alkoxide ligands they replace. The modification with chelating ligands is thus facilitating the kinetics of hydrolysis and polycondensation. The size and shape of the primary particles formed in sol-gel treatment of metal alkoxides are defined not by kinetic factors in their hydrolysis and polycondensation but by the interactions on the phase boundary, which is in its turn directed by the ligand properties. The products of the fast hydrolysis and condensation sequence consist of micelles templated by self-assembly of ligands (mainly oxo-species). This concept provides explanations for commonly observed material properties and allows for the development of new strategies for the preparation of materials. We discuss the formation of inverted micelles, obtained by the appropriate choice of solvents, which allows for the formation of hollow spheres. The modifying β-diketonate ligands act as the surfactant and form an interface between the hollow sphere and the solvent. Retention of ligands inside the gel particles is possible only if ligands possessing both chelating and bridging properties are applied. Application of such ligands, for example, diethanolamine, permits to prepare new transition metal oxide based microporous membranes.
189 citations
TL;DR: In this article, the same authors proposed a multilayer polysilicon zirconia-titania composite membrane made from powders calcined at 400 °C with type I nitrogen sorption behavior.
Abstract: Microporous zirconia-titania composite membranes have been fabricated by sol-gel processing using diethanolamine-modified precursor solutions. Microporous materials made from powders calcined at 400 °C show type I nitrogen sorption behavior. Supported ~0.1 µm thick membranes (see figure) exhibit molecular-sieving properties and are expected to have great potential for separation and reaction applications under harsh conditions.
62 citations
TL;DR: In this article, uniform nanocrystalline mesoporous mixed cobalt-nickel spinel phases displaying unimodal pores in the 7-12 nm range and relatively high specific surface areas up to 83 m2/g were prepared by a low temperature synthesis approach in which the desired metal oxide stoichiometries were introduced on a molecular level by reacting heterometallic alkoxide precursors in the presence of supramolecular liquid crystalline phases.
Abstract: Uniform nanocrystalline mesoporous mixed cobalt–nickel spinel phases displaying unimodal pores in the 7–12 nm range and relatively high specific surface areas up to 83 m2/g were prepared by a novel low temperature synthesis approach in which the desired metal oxide stoichiometries were introduced on a molecular level by reacting heterometallic alkoxide precursors in the presence of supramolecular liquid crystalline phases. The resultant phases, composed mainly of ca. 8–11 nm uniform cobalt–nickel spinel nanoparticles, were highly promising as low-temperature hydrocarbon combustion catalysts investigated in a model reaction of propane oxidation. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)
49 citations
TL;DR: The compounds obtained from isopropoxide precursors showed a higher stability than those with n-propoxide ligands or a combination of both types and the unusually low solubility and volatility of MIV(thd)4 has been shown to be due to close packing and strong van der Waals interactions in the crystal structures of these compounds.
Abstract: The modification of different zirconium propoxide and hafnium propoxide precursors with 2,2,6,6,-tetramethyl-3,5-heptanedione (Hthd) was investigated by characterization of the isolated modified species. The complexes [Zr(OnPr)3(thd)]2, [Zr(OnPr)(OiPr)2(thd)]2, Zr(OiPr)(thd)3, [Hf(OnPr)3(thd)]2, and Hf(OiPr)(thd)3 were isolated and characterized. The structure of the n-propoxide analogue of Zr(OiPr)(thd)3 could not be refined, but its existence was clearly demonstrated by XRD and 1H NMR. The modification of the propoxide precursors involves mono- and trisubstituted intermediate compounds and does not involve a disubstituted compound; thus, the commercial product that is claimed to be “Zr(OiPr)2(thd)2” and is most commonly used for the MOCVD preparation of ZrO2 does not exist. No evidence was found for the presence of such a compound in either zirconium- or hafnium-based systems. Formation of the dimeric hydroxo-di-thd-substituted complex, [Hf(OH)(OiPr)(thd)2]2, which could be isolated only for hafnium-based systems, occurs on microhydrolysis. All heteroleptic intermediates are eventually transformed to the thermodynamically stable Zr(thd)4 or Hf(thd)4. The compounds obtained from isopropoxide precursors showed a higher stability than those with n-propoxide ligands or a combination of both types. In addition, it is important to note that residual alcohol facilitates the transformation and strongly enhances its rate. The unusually low solubility and volatility of MIV(thd)4 has been shown to be due to close packing and strong van der Waals interactions in the crystal structures of these compounds.
43 citations
TL;DR: The coordination chemistry of solvated Ag(i) and Au(I) ions has been studied in some of the most strong electron-pair donor solvents, liquid and aqueous ammonia, and the P donor solVents triethyl, tri-n-butyl, and triphenyl phosphite and tri- n- butylphosphine.
Abstract: The coordination chemistry of solvated Ag(I) and Au(I) ions has been studied in some of the most strong electron-pair donor solvents, liquid and aqueous ammonia, and the P donor solvents triethyl, tri-n-butyl, and triphenyl phosphite and tri-n-butylphosphine. The solvated Ag(I) ions have been characterized in solution by means of extended X-ray absorption fine structure (EXAFS), Raman, and (107)Ag NMR spectroscopy and the solid solvates by means of thermogravimetry and EXAFS and Raman spectroscopy. The Ag(I) ion is two- and three-coordinated in aqueous and liquid ammonia solutions with mean Ag-N bond distances of 2.15(1) and 2.26(1) A, respectively. The crystal structure of [Ag(NH3)3]ClO4.0.47 NH3 (1) reveals a regular trigonal-coplanar coordination around the Ag(I) ion with Ag-N bond distances of 2.263(6) A and a Ag...Ag distance of 3.278(2) A separating the complexes. The decomposition products of 1 have been analyzed, and one of them, [Ag(NH3)2]ClO4, has been structurally characterized by means of EXAFS, showing [Ag(NH3)2] units connected into chains by double O bridges from perchlorate ions; the Ag...Ag distance is 3.01(1) A. The linear bisamminegold(I) complex, [Au(NH3)2]+, is predominant in both liquid and aqueous ammonia solutions, as well as in solid [Au(NH3)2]BF4, with Au-N bond distances of 2.022(5), 2.025(5), and 2.026(7) A, respectively. The solvated Ag(I) ions are three-coordinated, most probably in triangular fashion, in the P donor solvents with mean Ag-P bond distances of 2.48-2.53 A. The Au(I) ions are three-coordinated in triethyl phosphite and tri-n-butylphosphine solutions with mean Au-P bond distances of 2.37(1) and 2.40(1) A, respectively.
32 citations
TL;DR: In this paper, a simple, one-stage, efficient synthetic method for the growth of carbon sausages filled with in situ formed tungsten oxide nanorods (CSTON), without any external catalyst, was reported.
Abstract: In this article, we report a simple, one-stage, efficient synthetic method for the growth of carbon sausages filled with in situ formed tungsten oxide nanorods (CSTON), without any external catalyst. The thermal dissociation of tungsten(VI) isopropoxide, 5% w/v in isopropanol at 700 °C in a closed Swagelok cell in an inert atmosphere yielded a CSTON product. The product is characterized by SEM, TEM, EDAX, C, H, N, S analysis, TGA, Raman spectroscopy, and HR-TEM. The assignment of the structure of the sausages as a carbon shell, with the central part being the WOx, is based on SAEDS spectra. The same reaction was also conducted at a lower temperature closer to the temperatures of solvothermal reactions, at 350 °C. The reaction yielded WOx nanorods with a diameter ranging from 10 nm to 50 nm and a length of up to 6 μm.
19 citations
TL;DR: In this article, a 2D-centered rectangular, orthorhombic and lamellar mesostructures were obtained by variation of the molar ratios of the metal-surfactant complexes.
Abstract: Iron oxide doped silica films and powders with highly ordered mesostructures of extraordinary temperature stability (up to 1000 °C) and iron loading of up to 6 atom% were produced by the use of novel iron alkoxide-coordinated surfactants. Wormhole-like, 2d-centered rectangular, orthorhombic and lamellar mesostructures were obtained by variation of the molar ratios of the metal–surfactant complexes. The influence of a subsequent heat treatment was studied by GI-SAXS, TEM, XRD, and nitrogen sorption techniques. The molecular composition of the films was studied by XPS and EXAFS.
16 citations
TL;DR: In this paper, the synthesis and structural determination of three new heterometallic molybdenum complexes, one with cobalt and two with nickel and two of these with -diketonate ligands and one with amino alcohol ligands, are presented.
Abstract: The synthesis and structural determination of three new heterometallic molybdenum complexes, one with cobalt and two with nickel and two of these with -diketonate ligands and one with amino alcohol ligands, are presented. The reaction of cobalt acetylacetonate with [MoO(OMe)4] provides [Co2Mo2O2(acac)2(OMe)10] (1) and [MoO(acac)(OMe)3] (4),and the reaction of nickel acetylacetonate with [MoO(OMe)4]provides [Ni2Mo2O2(acac)2(OMe)10] (2) and 4. The reaction of [Ni(ORN)2] (RN = CHMeCH2NMe2) with [MoO(OMe)4] yields [Ni2Mo2O2(ORN)2(OMe)10] (3). The two new oxomolybdenum complexes undergo ether elimination upon storage to give the corresponding dioxo complexes [MoO2(acac)(OMe)]2 (5) and [MoO2(ORN)(OMe)]2 (6). Compounds 3 and 4 could also be obtained from the reaction of stoichiometric amounts of Hacac with [MoO(OMe)4] and [MoO2(OMe)2], respectively. The local structure around the nickel atom in compound 2 in solution and compound 3 in the solid state and in toluene/hexane solution has been determined by means of EXAFS spectroscopy. The complexes are intended to be used as single-source precursors, which are attractive in coatings and for the preparation of mesoporous materials; its application for the synthesis of nickel molybdate by sol-gel processing is therefore reported. The oxide material obtained from 3 displays a uniform grain size and a large surface area.
12 citations
TL;DR: In this paper, a rational synthesis of this compound was achieved by reaction between [NaTa(OMe)6] with four equivalents of CoCl2 in toluene/acetonitrile to give a new crystallographic form [Co2Ta2(OMe 12 (MeCN)4] [CoCl3(MeCN)]2 (2), not containing interstitial solvent molecules.
6 citations
TL;DR: In this article, the fabrication of WO(3) nanorods using an efficient straightforward synthetic technique, without a catalyst, and using a single precursor was reported. And the mechanism of the formation of nonstoichiometric W(18)O(49)-nanorods was supported by the measured analytical data and several control experiments.
Abstract: This article reports on the fabrication of WO(3) nanorods using an efficient straightforward synthetic technique, without a catalyst, and using a single precursor. The thermal dissociation of WO(OMe)(4) at 700 degrees C in a closed Swagelok cell under an air/inert atmosphere yielded W(18)O(49) nanorods. Annealing of W(18)O(49) at 500 degrees C under an air atmosphere led to the formation of pure WO(3) nanorods. The obtained products are characterized by morphological (scanning electron microscopy and transmission electron microscopy), structural (X-ray diffraction analysis, high-resolution scanning electron microscopy, and Raman spectroscopy), and compositional [energy-dispersive X-ray and elemental (C, H, N, S) analysis] measurements. The mechanism of the formation of nonstoichiometric W(18)O(49) nanorods is supported by the measured analytical data and several control experiments.
4 citations
TL;DR: In this paper, the structure of 1 is composed of planar layers of tetragonally distorted cubes altered by the layers of alcohol molecules oriented in the direction perpendicular to the metal hydroxide layers.