Titanium Sites Research Articles

New insights into medium-range order around titanium in sol–gel derived silica through isotope difference neutron diffraction and reverse Monte Carlo modelling

Neutron diffraction with isotopic titanium substitution allows the nature of the cation sites to be determined beyond nearest-neighbor distances for the first time in a sol–gel derived titania–silica glass. Reverse Monte Carlo (RMC) modelling of the results shows unequivocally that titanium substitutes for silicon within the homogeneous glass network, existing mostly as TiO 4, with a minority TiO 6 component. The fundamental scientific interest, and wide-ranging technologically significant properties of titania additions to silica, demand an understanding of the precise nature of the titanium site.

Competition between Water and Hydrogen Peroxide at Ti Center in Titanium Zeolites. An ab Initio Study

A combined Car−Parrinello molecular dynamics blue moon sampling approach has been adopted to study the competitive attack of H2O and H2O2 at a tetracoordinated Titanium site in a Ti−zeolite. The results indicate that, although the attack of water to form a trigonal bipyramidal center is thermodynamically more stable, the attack of hydrogen peroxide to form a similar adduct is kinetically favored. In both cases, solvent cooperation is effective in the formation of the adducts. The relevance of such a result in relation to the catalytic properties of Ti−zeolites is discussed.

Ordered mesoporous titanosilicates with better catalytically active titanium sites assembled from preformed titanosilicate precursors with zeolite building units in alkaline media

Mesostructured titanosilicates have been successfully synthesized from the assembly of preformed titanosilicate precursors with CTAB surfactant micelle in both alkaline media (MTS-5) and strongly acidic media (MTS-8). The samples were characterized by powder X-ray diffraction (XRD), nitrogen isotherms, 29Si MAS NMR, IR, and UV–visible spectroscopy. These materials exhibit highly hydrothermal stability, as compared with Ti-MCM-41. Characterization results indicate that these mesostructured titanosilicates of MTS-5 and MTS-8 have TS-1-like titanium species, and that the mesoporous walls contain the primary and secondary building units similar to those in microporous crystal of TS-1 zeolite. In catalytic hydroxylation of 2,3,6-trimethylphenol, MTS-5 and MTS-8 show higher activities in comparison with microporous crystal of TS-1 and mesoporous material of Ti-MCM-41. More importantly, MTS-5 assembled from preformed titanosilicate precursors in alkaline media has higher silica condensation degree, compared with MTS-8 synthesized in strongly acidic media. After heating at 550 °C for 4 h, MTS-5 basically remains its catalytic activities in hydroxylation of phenol and 2,3,6-trimethylphenol, indicating its higher stability of Ti species for calcination. In contrast, calcined MTS-8 shows much lower conversion than as-synthesized MTS-8.

Diffuse Reflectance Laser Flash Photolysis Study of Titanium-Containing Zeolites

A series of porous titanosilicates (Ti-preferrierite, Ti-ferrierite, Ti/ITQ-6, Ti/ITQ-2, and silylated Ti/MCM-41), all of them active as alkene epoxidation catalysts, have been submitted to diffuse reflectance laser flash photolysis. The transient spectra in the microsecond time domain exhibit characteristic absorption bands at 350 nm (sharp) and 450−800 nm (featureless) attributable to triplet ligand-to-metal charge-transfer state and ejected electrons, respectively. It is proposed that the relative intensity of these two bands is a reflection of the presence of isolated, tetrahedral Ti atoms versus Ti aggregates, the former being considered the most active titanium sites in olefin epoxidation. Our study shows the applicability of laser flash photolysis to the characterization of titanium atoms in titanosilicates.

Influence of Ta Substitution on Structural and Dilatometric Behaviour of PCT Ceramics

Structural and thermal properties of PCT ceramics are studied by the substitution of Ta in the titanium site. Samples with compositional formula Pb0.76Ca0.24Mn0.02Ti0.98−5x/4TaxO3 with the variation in x from 0 to 0.04 in steps of 0.01, were prepared by the conventional dry ceramic method. Single-phase formation was confirmed from the X-ray analysis and all the samples show tetragonal structure. Lattice parameters ‘c’ and ‘a’ were found to decrease and increase with substitutional parameter x, respectively. Tetragonality (c/a) decreases with increasing Ta content. Dilatometric study was done to analyze the shrinkage behaviour of green and sintered samples.

Elucidation of the local structure of active titanium(iv) sites on silica-based phase-boundary catalysts for alkene epoxidation with aqueous hydrogen peroxide

Structural and functional aspects of active titanium sites for phase boundary catalytic (PBC) epoxidation of 1-octene with hydrogen peroxide (H2O2) were investigated in detail using X-ray absorption fine structure (XAFS) analysis and ultraviolet and visible-light diffuse reflectance (UV-Vis-DR) spectroscopy. By analysis of the Ti K-edge X-ray absorption near edge fine structure (XANES) spectra of several titanium-loaded silica catalysts, the ratio of the amount of 4-coordinated titanium oxide (Ttet) to that of a 6-coordinated one (Toct) was determined. Monotonic increments of catalytic activity for epoxide production and efficiency of H2O2 utilization with the ratio Ttet/(Ttet + Toct) revealed that the highly active catalysts mainly include Ttet but not Toct. UV-Vis-DR spectra of samples with external surfaces partially covered with alkylsilyl groups indicated that there are at least two kinds of Ttet sites with different ligands. One site anchors an acidic hydroxyl (Ttet(OH)), giving absorption centered at the wavelength of ca. 230 nm and the other, exhibiting an absorption peak at the wavelength of ca. 210 nm, is directly attached to an alkylsilyl group (Ttet(OSiR)) formed via reaction of an acidic hydroxy of Ttet(OH) with an alkylsilane reagent. Since the catalysts have both an alkylsilyl-grafted hydrophobic surface and a hydroxy-terminated hydrophilic surface, it is postulated that the former is located on both hydrophilic (Ttet(OH,w)) and hydrophobic (Ttet(OH,o)) surfaces and the latter, Ttet(OSiR), exists only on the hydrophobic surface. From analyses of catalytic activities of several catalysts with different distributions of these Ttet sites, both Ttet(OH,o) and Ttet(OSiR) sites on the hydrophobic surface were proved to be active, while Ttet(OH,w) on the hydrophilic surface did not work for the present PBC system. Among the sites on the hydrophobic surface, moreover, it was found that a Ttet(OSiR) site acted as a more effective site for selective epoxidation when compared with Ttet(OH,o), which induced ring-opening of epoxide to give a by-product, 1,2-octanediol.

Ordered mesoporous titanosilicates with catalytically stable and active four-coordinated titanium sites

The stable ordered mesoporous titanosilicate (Ti-JLU-20) has been successfully synthesized from an assembly of mixed surfactants (fluorocarbon and triblock copolymer surfactants) with preformed titanosilicate zeolite precursors at high temperature (180-220 degrees C), and catalytic tests show that Ti-JLU-20 has highly stable and active four-coordinated titanium sites in oxidations.

Phase transitions in KTP isostructures: correlation between structure and Tc in germanium-doped RbTiOPO4.

Crystals of germanium-doped rubidium titanyl phosphate, Rb(2)(Ti)(Ge(0.121)Ti(0.879))O(2)(PO(4))(2) (GeRTP#1) and Rb(2)(Ge(0.125)Ti(0.875))(Ge(0.225)Ti(0.775))O(2)(PO(4))(2) (GeRTP#2), have been structurally characterized from X-ray diffraction data at room temperature. In addition, a third structure, Rb(2)(TiO)(2)(PO(4))(2) (RTP), has been reinvestigated. The exchange of titanium for germanium results in a less distorted octahedral coordination around the two crystallographically independent titanium sites. Additionally, rubidium split-cation positions have been found in these doped RTP crystals. Dielectric measurements show that the phase-transition temperature, T(c), decreases with increasing germanium concentration, and a direct correlation between the room-temperature split of the rubidium cations and T(c) has been discovered. General trends regarding the relationship between the room-temperature structures of KTP-like compounds and their T(c) values are discussed.

H 2O 2-based selective oxidations over titaniumsilicates of SBA-15 type

Hexagonally packed mesostructured titanosilicates of the SBA-15 type have been prepared in moderately acidic aqueous media starting from sodium disilicate Na 2Si 2O 5, TiOSO 4, and olefin oxide co-polymer of the Pluronic type (P123) as a template. Structure–texture parameters of the synthesised materials were characterised by using X-ray diffraction and nitrogen adsorption techniques. The titanium state in the silicate matrix was characterised by DR-UV, IR, Raman spectroscopy, and XPS. The catalytic properties of the new titaniumsilicates were tested in oxidation reactions with aqueous H 2O 2 using methyl phenyl sulfide and 2,3,6-trimethylphenol as model substrates and compared with the catalytic properties of TS-1 and Ti-MMM (titaniumsilicate of the MCM-41 type). The structure of the Ti-SBA-15 materials, by contrast to Ti-MMM catalysts, was proved to be hydrolytically stable towards aqueous H 2O 2. The catalytic activity of Ti-SBA-15 is considerably lower than that of Ti-MMM, most likely due to a lower titanium dispersion and higher wall thickness of the former material, which makes part of titanium sites unavailable for the reactants.

Performance of a membrane-catalyst for photocatalytic oxidation of volatile organic compounds

The performance of a hybrid Sil-1/nanostructured anatase TiO 2 membrane-catalyst and a catalytic titanium silicalite-1 (TS-1) membrane was evaluated for gas-phase photocatalytic oxidation (PCO) of trichloroethylene. The membrane-catalyst outperformed the catalytic plate coated with a similar amount of nanostructured TiO 2 catalyst. However, the activity of the catalytic TS-membrane for PCO is low due to the insufficient number of active titanium sites in the TS-1 zeolite.

ETS‐10 as a photocatalyst

ETS‐10 is a microporous titanosilicate zeolite with a framework containing linear Ti‐ O ‐Ti‐ O‐ chains. This paper describes an investigation of the photoreactivity of ETS‐10 with the particular objective of evaluating the potential of this novel material as a zeolitic photocatalyst. The photoreactivity and photocatalytic activity of ETS‐10 are strongly influenced by defects in the structure. A relatively defect free material catalyses photo‐polymerisation of ethene, and in the presence of oxygen catalyses the partial oxidation of ethene to acetic acid and acetaldehyde, which remain strongly adsorbed in the pores. A more defective material is photoreduced when irradiated in the presence of adsorbed ethene, and catalyses the complete oxidation of ethene to carbon dioxide and water in the presence of oxygen. These differences are attributed to differences in the concentrations of exposed titanium sites associated with defects in the ETS‐10 structure.

Role and effect of supercritical fluid extraction of template on the Ti(IV) active sites of Ti-MCM-41

Supercritical fluid (supercritical carbon dioxide mixed with methanol) has been used as an extraction medium for the removal of template for a series of titanosilicates. Mesoporous titanosilicates Ti-MCM-41 with variable Si/Ti ratio (200–25) have been prepared by hydrothermal method. The location and coordination of titanium in the calcined and extracted titanosilicate mesoporous molecular sieve was studied by FTIR, UV–Vis spectroscopy, photoluminescence spectra, 29Si NMR, combined with XRD and TG/DTA. Up to 97% template can be removed by extraction and the extracted material are hydrophobic in character. Removal of template by supercritical fluid extraction have no effect what so ever on the elemental composition of Ti-MCM-41 as well as it improves the structural ordering which was confirmed by XRD. Unequivocal evidences are presented to confirm the presence of same chemical environment of active titanium site after extraction, as present in as-synthesized material. It was also reported that, the entire extracted solid exhibited catalytic activity superior to that of the calcined product for epoxidation of cyclohexene because of the hydrophobic character and unchanged active framework titanium site.

Heterogeneous Catalysis by Flame-Made Nanoparticles

In search of highly active nanoparticles for catalytic applications, flame aerosol technology turns out to be a promising method for its capacity to consistently produce highly pure nanoparticles. Starting with the production of simple oxides such as silica and titania in the forties, flame aerosol technology now facilitates production of nanoparticles in megaton quantities. Fundamental research on particle formation in the gas phase and the influence of process parameters and reactor design on final product properties have advanced the field to a stage where multicomponent materials can be produced. Flame production of vanadia/titania catalysts for NOx removal from exhaust gases by selective reduction by ammonia affords enhanced activity at low temperature when compared to corresponding materials prepared by wet-chemical methods. Furthermore, flame synthesis of titania/silica yields catalysts for the epoxidation of olefins with high accessibility of the active titanium sites and improved selectivity. Spectroscopic studies revealed that the same active sites as in the wet-phase derived catalysts are produced by flame synthesis but at a higher content. The high temperature during the preparation favors the formation of well-defined species on a hydrophobic surface. The flame synthesis of heterogeneous catalysts is brought from laboratory scale to pilot scale production at up to 500 g/h to make the novel materials suitable for industrial performance tests.

Artificial charge-modulationin atomic-scale perovskite titanate superlattices.

The nature and length scales of charge screening in complex oxides are fundamental to a wide range of systems, spanning ceramic voltage-dependent resistors (varistors), oxide tunnel junctions and charge ordering in mixed-valence compounds. There are wide variations in the degree of charge disproportionation, length scale, and orientation in the mixed-valence compounds: these have been the subject of intense theoretical study, but little is known about the microscopic electronic structure. Here we have fabricated an idealized structure to examine these issues by growing atomically abrupt layers of LaTi(3+)O(3) embedded in SrTi(4+)O(3). Using an atomic-scale electron beam, we have observed the spatial distribution of the extra electron on the titanium sites. This distribution results in metallic conductivity, even though the superlattice structure is based on two insulators. Despite the chemical abruptness of the interfaces, we find that a minimum thickness of five LaTiO(3) layers is required for the centre titanium site to recover bulk-like electronic properties. This represents a framework within which the short-length-scale electronic response can be probed and incorporated in thin-film oxide heterostructures.

Spectroscopy of Amorphous and Crystalline Titania−Silica Materials

Both amorphous and crystalline silica doped with Ti(IV) ions have properties ranging from selective chemical catalysis to useful thermal and optical characteristics. These properties are related to the distribution of titanium sites in the silica lattice. It has been proposed that the Ti(IV) isomorphically substitutes for Si in the lattice in an essentially tetrahedral geometry. In this study we compare the electronic and vibrational spectroscopic characteristics of a series of both dense and porous amorphous titania−silica materials with crystalline titania−silicalite (TS-1). All of the spectroscopic properties are referenced to titania−cristobalite, which contains titanium in a rigorous crystallographic environment that deviates only slightly from tetrahedral. It is found that the amorphous materials have spectroscopic properties that are extremely close to Ti-cristobalite while TS-1 deviates significantly.

In situ XANES study on TiO2–SiO2 aerogels and flame made materials

The structures of TiO2–SiO2 based aerogels and flame made particles, used as epoxidation catalysts, are compared by X-ray absorption near edge structure (XANES) spectroscopy and in situ XANES during dehydration of the calcined catalyst samples. The results are interpreted in terms of the titanium species present in the catalyst and the catalytic performance in the test reaction, the epoxidation of 2-cyclohexene-1-ol by tert-butylhydroperoxide (THBP) at 90 °C. XANES shows that tetrahedral titanium sites are the favoured sites at low TiO2 concentrations (1–3.2 wt.% in this study). The catalytic performance (turnover frequency, selectivity) of such aerogels or flame made materials with low TiO2 content is much better than that of corresponding samples with high TiO2 content. In aerogels, regardless of the TiO2 content, the amount of titanium with tetrahedral coordination increases significantly after dehydration at elevated temperature. In contrast, the pre-edge height and its position in the XANES spectra of the flame made catalyst samples is comparable, even without dehydration, to that of TS-1, which contains titanium in tetrahedral coordination. Thus the extent of tetrahedral Ti is intrinsically high in flame made catalysts and increases only slightly during the drying process. This behaviour can be attributed to the hydrophobic character of the flame made materials and ameliorates the catalytic performance, i.e. the selectivity.

An ESR study of titanium-silicalite in presence of H 2O 2

The interaction of H 2O 2 with TS-1 has been investigated by the ESR technique. A well manufactured TS-1 has been considered, where the presence of extra-structural titanium can be excluded. The two main signals observed are attributed to oxygen radical species co-ordinated on structural titanium sites, and the relevant effect of water presence in the paramagnetic complex is discussed. A comparison is performed with O 2 −/Ti species in TS-1 obtained by reduction/re-oxidation procedures of Ti(IV). Moreover, some possible structural configurations of the paramagnetic sites are proposed, for two of which ab initio calculation yield g z -values quite close to the experimental ones.

Structural characterization of polycrystalline (Nd,Al)-substituted zirconolite

ABSTRACTZirconolite (formally CaZrTi2O7) is a crystalline phase particularly well adapted to actinide immobilization because of its excellent long-term behavior and its good containment capacity. Most of the French studies on zirconolite deal with minor actinides that are mainly responsible for the long-term radiotoxicity of high-level radioactive wastes. For these kind of studies, trivalent minor actinides (Am3+, Cm3+) can be simulated by a lanthanide ion with an ionic radius similar to that of Nd3+. Thus, several materials having the composition Ca1-xNdxZrTi2-xAlxO7 (0 ≤ x ≤ 0.8) were prepared by solid state reaction. These polycristalline materials were first characterized by X-ray diffraction and scanning electron microscopy associated with energy dispersive X-ray analysis in order to determine the nature of the crystalline phases formed. For low neodymium content (x ≤ 0.1), electron spin resonance of Nd3+ ions revealed that a significant proportion of these ions entered into trace amounts of perovskite. Nevertheless, all Ca1-xNdxZrTi2-xAlxO7 samples with x ≤ 0.6 can be considered as almost single phase zirconolite-2M. Structure refinement by the Rietveld method of Ca0.7Nd0.3ZrTi1.7Al0.3O7 showed that Nd3+ and Al3+ ions mainly entered respectively into the calcium site and into the split five-fold coordinated titanium site. Structural characterization of Ca0.3Nd0.7ZrTi1.3Al0.7O7 and Ca0.2Nd0.8ZrTi1.2Al0.8O7 samples confirmed that these compositions led to the crystallization of almost single phase zirconolite-3O, an orthorhombic polytype of zirconolite, whose structure was also refined by the Rietveld method. Results concerning neodymium location in Ca0.7Nd0.3ZrTi1.7Al0.3O7 and Ca0.3Nd0.7ZrTi1.3Al0.7O7 were qualitatively confirmed by optical absorption spectroscopy at low temperature.

Theoretical investigation of the Ziegler–Natta catalysis in heterogeneous conditions

AbstractWe present ab initio calculations on the Ziegler–Natta catalysis in heterogeneous conditions. Both cluster and periodic models with different basis sets have been used to explain the chemical reactivity of the various species that are involved. In heterogeneous conditions, several parameters contribute to the catalysis: the oxidation state of the titanium atom, the structure of the titanium site, the interaction with the support, MgCl2, and the cocatalyst, AlR3. This article yields to a synthetic view of the catalytic activity. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

Structural Investigation of TS-1: Determination of the True Nonrandom Titanium Framework Substitution and Silicon Vacancy Distribution from Powder Neutron Diffraction Studies Using Isotopes

The first complete investigation of the structure of the selective oxidation catalyst TS-1 is presented. Constant wavelength powder neutron diffraction data collected on isotopically substituted titanium silicalite (TS-1) samples, with a Si:Ti molar ratio of 39:1, have been studied using a combination of single and multiple data set Rietveld analysis exploiting the scattering length contrast between the different titanium isotopes and silicon. This has allowed both the silicon vacancy and titanium site substitution distributions to be determined, which has not been possible previously. Both distributions are found to be nonrandom with the titanium preferentially substituting on 3 of the 12 crystallographically independent framework sitesT8, T10 and T3 (in order of decreasing titanium content)and silicon vacancies on 2 framework sitesT1 and T5. This study illustrates the power of isotopic substitution in powder neutron diffraction experiments to yield enhanced structural information in complex systems.