Titanium silicates as efficient catalyst for alkylation and acylation of silyl enol ethers under liquid-phase conditions (original) (raw)
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Production of titanium containing molecular sieves and their application in catalysis
Applied Catalysis A-general, 2001
This paper reviews several important issues related with industrial application of catalysts based on titanium silicalite-1 (TS-1). The catalyst preparation has been discussed, especially considering the most critical parameters for industrial application. Two processes already demonstrated on industrial scale (phenol hydroxylation and cyclohexanone ammoximation), and a process, under development by several industrial groups (propylene epoxidation), have been reported and discussed.
Characterization of Catalysts Based on Titanium Silicalite, TS-1, by Physicochemical Techniques
Journal of Catalysis, 1997
FT-IR and FT-Raman spectroscopy, UV-visible diffuse reflectance spectroscopy (DRS), transmission electron microscopy with X-ray energy dispersion spectrometry (to analyze grains and ultramicrotomed sections), and xenon-129 NMR. Activity and selectivity for the epoxidation of oct-1-ene by phosphate-free 30% aqueous hydrogen peroxide have been investigated. The critical factors governing the activities of the samples with molar ratios Si : Ti ≈ 39 (sample A) and 32 (sample B) are the distribution of titanium within the microporous materials and within the extraframework titania (crystalline anatase or amorphous phase as evidenced by UVvisible DRS, FT-Raman spectroscopy, and electron microscopy). Xenon-129 NMR chemical shifts, δ exp = f(n) (n = number of adsorbed xenon atoms per gram of anhydrous sample), depend dramatically on extraframework impurities and Ti(IV) dispersion. Pure silicalite and nearly perfect TS-1 (sample A) are not differentiated by xenon-129 NMR, a fact which is tentatively attributed to the "atomic dispersion" of the titanium in sample A, as evidenced by X-ray EDS and by the channel dimensions.
Comparison of hydrophilic and hydrophobic silicas as supports for titanium catalysts
Applied Catalysis A: General, 2004
Siloxane bridges on the silica surface react with HCl leading to a more hydrophilic solid, which surprisingly shows a better performance as support for titanium centres than the parent silica. The use of a weaker electrophilic species, such as an acyl chloride, allows the modulation of the support properties in such a way that it can be prepared the best silica-supported titanium catalyst for epoxidation with diluted hydrogen peroxide. This result demonstrates that the hydrophobic character of the solid is not a sine qua non requirement to obtain efficient epoxidation catalysts. Although the reasons for this good behaviour are not fully understood, it seems that the type of surface titanium species is the key factor, and modifications of silanol density with the different treatments control the distribution of titanium species. #
Microporous and Mesoporous Materials, 2008
The epoxidation of functionalised substrates of interest as fine chemicals using mesoporous titanium-containing silicas is here reported and the role of silylation in changing the surface hydrophilic character of these catalysts is investigated. The silylation procedure was carried out on two titanium-grafted silicas with different morphologies. An ordered MCM-41 and a non-ordered commercial mesoporous silica were used as supports. The reactivity of bulky substrates with different characteristics (limonene, a-terpineol, carveol and methyl linoleate) is studied and compared. The effect of silylation is more pronounced on Ti-MCM-41 than with low-surface area Ti-SiO 2 and it is shown that the catalytic performances are strongly dependent on the nature of the reactant. Purely alkenic molecules show better reactivity over silylated catalysts than over non-silylated ones. On the other hand, a hydrophilic environment around the titanium active sites has often a beneficial effect in the epoxidation of richly functionalised substrates.
Oxidative Organic Transformations Catalyzed by Titanium- and Vanadium-Silicate Molecular Sieves
Synlett, 1995
I and TS-2, are found to caiolysc a ialiety of ox~dation rcuclions E n the presence of diluce I1-O2 as oxldant. The reactinns reporled are the hydroxylation of sronlatx compounds, ox~darmn of alcohols, the oxyfilncrionnlisation of alkanes, the eponrdation of alkenes and ally1 alcohols, the cleavage af carbon-carbon 01 carbon-nitrogen double bonds. the oxtdatlon of ammes, the ammonimation of carbobonyl compounds, the ox~dstion or cyclic dlenes and sulfides. The plauaiblc mecharnsm mvolvmg ?he reactive SPecles,e.g.,peroxytitan~um complexes for the above leactiorlr rs described.
Catalytic sites in silica-supported titanium catalysts: silsesquioxane complexes as models
Journal of Catalysis, 2005
The number of Si-O-Ti bonds in silica-supported titanium catalysts is an important parameter that affects activity and selectivity. Although comparisons between different catalysts can be made by infrared spectroscopy, estimation of the contribution of the different species is a difficult task. UV spectroscopy has been used to characterize three different titanium-silsesquioxane complexes. The wavelength in solution is highly dependent on the concentration, and broad bands are obtained by DRUV, at wavelengths that correlate with the number of Si-O-Ti bonds. The deconvolution of the DRUV spectra of silica-supported titanium catalysts allows identification of the three types of tetrahedral (pentacoordinated) species, together with octahedral species. These results correlate well with the known silanol densities of the different silica supports, and the catalytic activity and selectivity for epoxidation are clearly favored by the reduction in octahedral titanium and the increase in the number of tripodal species. (J.M. Fraile). spectively . Although Ti(OSiMe 3 ) 4 has been used as a soluble model for titania-silica mixed oxides [6], the most used models for titanium catalysts have been titanium silsesquioxane complexes. These systems are consid- . Possible surface titanium species on silica.
Angewandte Chemie International Edition, 2001
Titanium centers dispersed on, and supported by, various forms of silica are catalysts with remarkable properties for partial oxidations. The character of the active site varies with the silica support used. Although it has been established that in the best (on the basis of activity per gram of titanium) heterogeneous catalysts the Ti centers are all four-coordinate, [2±4] it is still unclear whether, for optimum performance, the active catalytic species requires the specific configuration of four siloxy groups, or if being partially hydrolyzed to, say, one hydroxy and three siloxy ligands is equally good. Chemical modeling studies by various groups [5±7] have shown that Ti catalysts with fewer than four siloxy groups should also be active as catalysts. These considerations indicate that there is still some residual uncertainty regarding the active site, despite the many experimental and computational studies carried out to resolve it. Indeed, several mechanisms, or variations of one underlying mechanism, might operate depending on the precise configuration of the Ti site. Hence, different types of Ti centers might exhibit the desired characteristics.
Materials Chemistry and Physics, 2008
A highly ordered 2D-hexagonal mesoporous titanium silicate synthesized under moderately basic condition by using a modified synthesis procedure in the presence of a non-ionic surfactant dodecylamine (DDA) as the structure-directing agent (SDA) has been reported. This mesoporous material was characterized by using powder XRD, N 2 adsorption/desorption, TEM, SEM-EDS and spectroscopic studies. UV-vis and 29 Si MAS NMR studies suggested the incorporation of isolated tetrahedral Ti(IV) sites in a highly cross-linked silica framework. N 2 sorption analysis revealed good surface area and narrow pore size distribution. This mesoporous titanium silicate efficiently catalyzed the epoxidation of acrylic acid in acetonitrile solvent in the presence of H 2 O 2 as oxidant with high-yield and excellent selectivity. Oxidation of styrene over this mesoporous titanium silicate has also been studied.