Titanium-silicate catalyst

P 4] For preliminary investigations a small-scale micro structured reactor was used to perform gas-phase epoxidation of propylene to propylene oxide with evaporated H202. As catalyst titanium silicate (TS-1) was used. 

Enichem made one of the most important steps forward in the development of general heterogeneous oxidation catalysts in the early 1990s with the commercialization of titanium silicate (TS-1) catalysts. TS-1 has a structure similar to ZSM-5 in which the aluminium has been replaced by titanium it is prepared by reaction of tetraethylorthosilicate and tetra-ethylorthotitanate in the presence of an organic base such as tetrapropy-lammonium hydroxide. This catalyst is especially useful for oxidation reactions using hydrogen peroxide (Scheme 4.11), from which the only byproduct is water, clean production of hydroquinone being one of the possibilities. 

In earlier work, Bhaumik and Kumar (1995) have reported that the use of two liquid phases in the oxidation of hydrophobic organic substances with aqueous H2O2 using titanium silicate as the catalyst not only enhances the rate of oxidation but also improves selectivity for species like toluene, anisole, and benzyl alcohol. For a single liquid phase acetonitrile was u.sed a solvent. The solid-liquid system gives high ortho selectivity. Thus, in the case of anisole the ratios of o to p for. solid-liquid and solid-liquid-liquid system were 2.22 1 and 0.35 1, respectively. 

Acid catalysis by titanium silicate molecular sieves another area characterized by recent major progress. Whereas only two categories of acid-catalyzed reactions (the Beckmann rearrangement and MTBE synthesis) were included in the review by Notari in 1996 (33), the list has grown significantly since then. In view of the presence of weak Lewis acid sites on the surfaces of these catalysts, they can be used for reactions that require such weak acidity. 

SCHEME 72. Oxidation of a-hydroxy-containing monoterpenes using titanium silicate catalysts... 

Besides a variety of other methods, phenols can be prepared by metal-catalyzed oxidation of aromatic compounds with hydrogen peroxide. Often, however, the selectivity of this reaction is rather poor since phenol is more reactive toward oxidation than benzene itself, and substantial overoxidation occurs. In 1990/91 Kumar and coworkers reported on the hydroxylation of some aromatic compounds using titanium silicate TS-2 as catalyst and hydrogen peroxide as oxygen donor (equation 72) . Conversions ranged from 54% to 81% with substituted aromatic compounds being mainly transformed into the ortho-and para-products. With benzene as substrate, phenol as the monohydroxylated product... 

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. 

The activity data confirm that an IR absorption band at 960 cm" is a necessary condition for titanium silicates to be active for the selective oxidation of hydrocarbons with aqueous H2O2 as suggested by Huybrechts et al. (9). However, this band is not a sufficient condition for predicting the activity of the TS-1 catalyst. Although TS-l(B) and TS-l(C) show intensities for the 960 cm- band similar to TS-1 (A), their activities are different First of all, the reaction data reveal that TS-1 (A) is much more active than TS-l(B) for phenol hydroxylation, while both samples show similar activity for n-octane oxidation and 1-hexene epoxidation. Therefore, the presence of the IR band at 960 cm-i in TS-1 catalysts may correlate with the activities for the oxidation of n-octane and the epoxidation of 1-hexene but not for phenol hydroxylation. However, note that the amorphous Ti02-Si02 also has an IR absorption band at 960 cm- and it does not activate either substrate. 

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). 

Titanium silicates catalyze a large number of different reactions. The literature of many of the reactions is characterized by discrepancies in the results reported by different groups. Very likely, these discrepancies are consequences of inconsistencies in catalyst preparation and structure, and in reaction conditions. Issues related to catalyst structure are briefly reviewed below, followed by a review of specific reactions. 

Catalysts of such small dimensions can hardly be handled in practice. A procedure for binding 0.1-0.3-//m particles into agglomerates of 20-30 pm that can be easily handled while maintaining the same catalytic properties has been a key step in the development of industrial processes using titanium silicates (Bellussi et al, 1986). 

Titanium silicates are catalysts for various oxidation reactions. Most of those investigated have been carried out with H202 and hydroperoxides as the oxidants, and a few have been investigated with ozone and oxygen as oxidants. 

Many different oxidation reactions with peroxides and H202 as the oxidants and titanium silicates as the catalysts have been reported. 

It is reasonable to consider that in titanium silicate-catalyzed reactions the oxidizing species also acts as an electrophile. The different order of reactivity of the C4 olefins in the presence of titanium silicates relative to that observed with soluble catalysts must therefore arise from the fact that alkyl substitution at the double bond is responsible not only for inductive effects, but also for increases in the size and the steric requirements of the molecules. Since the rates of diffusion of the different butenes cannot be the cause of the different reaction rates, a restricted transition-state selectivity must be operating. 

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). 

Many titanium silicates are active for this reaction, Ti-beta and Ti-HMS being the most active. The results demonstrate that in catalysis by TS-1 and by medium-pore zeolites, the reaction is limited by diffusion. TS-48, which has been found to be inactive in other oxidation reactions, is an active catalyst for the oxidation of aniline (Gontier et al, 1994 Sonawane et al., 1994). 

Tertiary amines are oxidized to the corresponding nitrogen oxides. Tosyl hydrazones of ketones and aldehydes and imines are oxidized to the corresponding carbonyl compounds. Reactions have been carried out with small molecules and also with molecules that would not diffuse into the pore structure of the titanium silicates. As in the case of C—C bond cleavage, it is possible that these reactions take place on the outer surface of the catalyst crystals. 

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. 

The initial coordination of reactants has indeed been proposed to explain the selective oxidation of alkenes in the presence of saturated hydrocarbons. It was argued that, owing to the hydrophobic nature of titanium silicates, the concentration of both hydrocarbons inside the catalyst pores is relatively high and hence the alkenes must coordinate to TiIv. Consequently, the titanium peroxo complex will be formed almost exclusively on Tilv centers that already have an alkene in their coordination sphere, and will therefore oxidize this alkene rather than an alkane which may be present in the catalyst (Huybrechts et al., 1992). Objections to this proposal are based on the fact that the intrinsically higher reactivity of alkenes with respect to saturated hydrocarbons is sufficient to account for the selectivity observed (Clerici et al., 1992). But coordination around the titanium center of an alcohol molecule, particularly methanol, is nevertheless proposed to explain the formation of acidic species, as was previously discussed. In summary, coordination around Tiiv could play a more important role than it does in solution chemistry as a consequence of the hydrophobicity of the environment where the reactions take place. 

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. 

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