Titanium silicate materials

Compositions of the synthesis gel and other physical characteristics of titanium silicate materials obtained in various synthesis methodologies are listed in Table Cl. 

Chapman, D.M., and Roe, A.L. (1990) Synthesis, characterization and crystal chemistry of microporous titanium-silicate materials. Zeolites, 10, 730. 

In green oxidation reactions, zeolite TS-1 is the typical catalyst. Since the size of its channels ranges from 5 to 6 A, TS-1 can be used as the catalyst only for benzene and phenol conversion. However, ordered mesoporous titanium silicate materials have pores large enough for the catalytic reactions of bulkier molecules, and this is very important for the production of fine chemicals. For example, for the oxidation reaction of terpineol, Ti-MCM-41 performs much better than do microporous titanium silicate molecular sieves as a catalyst.1-291... 

Kuznicki, S.M., Trush, K.A., Allen, F.M., Levine, S.M., Hamil, M.M., Hayhurst, D.T., and Mansom, M. (1992) Synthesis and adsorptive properties of titanium silicate molecular sieves, in Synthesis of Microporous Materials, Molecular Sieves, vol. 1 (eds M.L. Ocelli, and H.E. Robson), Van Nostrand Reinhold, New York,... 

Titanium containing pure-silica ZSM-5 (TS-1) materials are synthesized using different methods. The activity of the titanium containing catalysts for the oxidation of alkanes, alkenes and phenol at temperatures below 100 °C using aqueous H2O2 as oxidant is reported. The relationships between the physicochemical and catalytic properties of these titanium silicates are discussed. The effects of added duminum and sodium on the catalytic activity of TS-1 are described. The addition of sodium during the synthesis of TS-1 is detrimental to the catalytic activity while sodium incorporation into preformed TS-1 is not. The framework substitution of aluminum for silicon appears to decrease the amount of framework titanium. 

In the present work the synthesis of highly dispersed niobium or titanium containing mesoporous molecular sieves catalyst by direct grafting of different niobium and titanium compounds is reported. Grafting is achieved by anchoring the desired compounds on the surface hydroxyl groups located on the inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Catalytic activity was evaluated in the liquid phase epoxidation of a-pinene with hydrogen peroxide as oxidant and the results are compared with widely studied titanium silicalites. The emphasis is directed mainly on catalytic applications of niobium or titanium anchored material to add a more detailed view on their structural physicochemical properties. 

Another important point is that, when prepared from pure raw materials, titanium silicates do not have an appreciable acidic character, as demonstrated by the high yields that can be obtained even in applications with acid-sensitive products like propylene oxide. In contrast, mixed oxides of titanium and silicon have been described as being strongly acidic (Tanabe et al., 1981), The reasons for the difference are not clear and deserve further attention. 

As will be made clear in Section III, it is possible that finely dispersed Ti02 is present in titanium silicates. The observed catalytic activity would then be attributed to a combination of the two materials. It is therefore useful to review the chemistry of high-surface-area Ti02-Si02 and its catalytic behavior. 

Octahedral coordination of Tiiv is also present in the titanium silicates ETS-4 and ETS-10. The structure of these materials is reported to be similar to that of zorite, and they can be described as microporous crystals with uniform pores similar in dimensions to classical small- and large-pore zeolites. In ETS-4 and ETS-10, there are two monovalent cations or one divalent cation for each Tilv ion (Kuznicki, 1989, 1990 Kuznicki et al., 1991a, 1991b, 1991c, 1993 Deeba et al., 1994). A recent report of the synthesis of ETS-10 with tetramethyl-ammonium chloride indicates a ratio of monovalent cations to Tilv of 1.6 (Valtchev et al., 1994). The acidic properties of these materials have not been reported. A material modified by the addition of Al3+ has been obtained, ETAS-10, which, after exchange with NH4 salts, exhibits acidic properties but these are due to the presence of Al3+ and not to the Tilv (Deeba et al., 1994). 

The first discovered member of the group of crystalline microporous materials made of oxides of titanium and silicon is titanium silicalite-1 (TS-1). TS-1 has attracted much interest for its unique catalytic properties it is also of interest by virtue of the proposal that Tiiv assumes tetrahedral coordination in substituting for SiIV in framework positions of crystalline silica, as stated above. To clarify this point, many detailed studies of the TS-1 structure have been carried out. An outcome of the work was the discovery of new crystalline microporous titanium silicates. 

The synthesis of these titanium-substituted zeolites has been described to occur by a secondary synthesis process involving the reaction of [NH4]2TiF6 with the preformed corresponding zeolite (Section IV.G). The chemical and physicochemical properties described are not sufficient to establish the presence of Tiiv ions in framework positions. The titanium concentrations reported are much higher than the maximum values observed in titanium silicates for which isomorphous substitution has been demonstrated. The possible presence of Ti02 particles has not been investigated. No indication of the properties of these materials as catalysts in reactions typical of titanium silicates has been provided. It is therefore very doubtful that real isomorphous substitution has been obtained (Skeels et al., 1989 Skeels, 1993). 

The high-temperature synthesis from strongly alkaline suspensions of salts of Tilv and Silv produces crystalline microporous materials in which Tiiv is present in octahedral coordination. These materials do not exhibit the catalytic properties typical of the other titanium silicates in which TiIV is in tetrahedral coordination (Kuznicki, 1989, 1990 Kuznicki et al., 1991a, 1991b, 1991c, 1993 Deeba et at., 1994). The acidic properties of these materials have been discussed (Section II.B). 

Titanium-containing materials have been synthesized from strongly alkaline suspensions obtained by mixing TiCU with alkali silicate and sodium aluminate solutions. The gel obtained was homogenized and mixed in an autoclave at... 

In some cases, oxidation of double bonds does not stop at the epoxide, but proceeds further to oxidative cleavage of the double bond. It was reported that the reaction of a-methyl styrene with H2O2 in the presence of TS-1 or TS-2 produces a-methyl styrene epoxide (15%), a-methyl styrene diol (10-40%) and acetophenone (40-60%) (Reddy, J. S. et al., 1992). However, results similar to those obtained with titanium silicates were obtained for many other catalysts, such as HZSM-5, H-mordenite, HY, A1203, HGa-silicalite-2, and fumed Si02. These materials have much different properties and differ significantly from titanium silicates thus, the results cast some doubt on the role of the catalyst in this reaction. Furthermore, the oxidation of styrene is reported to proceed with C=C cleavage and formation of benzaldehyde, in contrast to previous reports of the formation of phenylacetaldehyde with 85% selectivity (Neri et al., 1986). 

Because the oxidation of phenol is sensitive to the purity of the titanium silicate catalyst, it has been used as a test reaction to evaluate the purity of the catalytic materials. A standard material called EURO TS-1 has recently been prepared and evaluated in several laboratories (Martens et al., 1993). 

For C5 and higher alkanes, the oxidation at the 2-position is favored over that at the 3- and 4-positions. The efficiency of H202 utilization is influenced by the purity of the titanium silicates pure phases give efficiencies of 80% or more, whereas with materials containing Ti02, almost 30% of the H202 is lost by decomposition to 02 (Huybrechts et al., 1992). 

Investigation of mechanisms of reactions catalyzed by titanium silicates has been limited to oxidation reactions with H202 as the oxidant, as described below. As was previously discussed, elements different from titanium and silicon in the catalyst materials change their properties. Catalytic activity of doubly substituted materials such as Ti-beta, H[Al,Ti]-MFI and -MEL, and H[Fe,Ti]-MFI and -MEL is considered separately because the acidic properties associated with the added element affect the composition of the reaction products. 

In the [TiF5(02)]3 ion, the peroxo group is bonded to Tilv side-on, and therefore this could also be the structure of the complex formed on titanium silicates. However, the possibility of a hydroperoxo species bonded end-on cannot be ruled out, because the side-on structure requires a deeper degree of hydrolysis to give the Ti(OH)2 group, whereas the hydroperoxo can form on a TiOH group, which is more easily obtainable in a material resistant to hydrolysis. The two forms can be represented as follows ... 

The discovery of the new titanium silicates and of their catalytic properties in H2O2 oxidation reactions has had a major impact in catalytic science and its industrial applications. One 10,000 ton/year plant for the production of catechol and hydroquinone has been operating since 1986 with excellent results. Moreover, successful tests conducted on a 12,000-ton/year pilot plant for cyclohexanone ammoximation (Notari, 1993b) could be followed soon by an industrial-size plant that would greatly simplify the synthesis of caprolactam. Both these examples are clear indications of the potentials of the new oxidation chemistry made possible by the new materials. 

The book explores various examples of these important materials, including perovskites, zeolites, mesoporous molecular sieves, silica, alumina, active carbons, carbon nanotubes, titanium dioxide, magnesium oxide, clays, pillared clays, hydrotalcites, alkali metal titanates, titanium silicates, polymers, and coordination polymers. It shows how the materials are used in adsorption, ion conduction, ion exchange, gas separation, membrane reactors, catalysts, catalysts supports, sensors, pollution abatement, detergency, animal nourishment, agriculture, and sustainable energy applications. 

Fluidized-bed chlorination was started in 1950. The titanium raw material (with a particle size similar to that of sand) and petroleum coke (with a mean particle size ca. five times that of the Ti02) are reacted with chlorine and oxygen in a brick-Uned fluidized-bed reactor (c) at 800-1200 °C. The raw materials must be as dry as possible to avoid HCl formation. Since the only losses are those due to dust entrainment the chlorine is 98-100% reacted, and the titanium in the raw material is 95-100% reacted, depending on the reactor design and the gas velocity. Magnesium chloride and calcium chloride can accumulate in the fluidized-bed reactor due to their low volatility. Zirconium silicate also accumulates because it is chlorinated only very slowly at the temperatures used. All the other constituents of the raw materials are volatilized as chlorides in the reaction gases. 

Bauxite is not pure aluminum oxide (called alumina) but also contains the oxides of iron, silicon, and titanium, and various silicate materials. The pure hydrated alumina (A1203 wH20) is obtained by treating the crude bauxite with aqueous sodium hydroxide. Being amphoteric, alumina dissolves in the basic solution ... 

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