Hydrogen peroxide-titanium silicalite

An extremely versatile catalyst for a variety of synthetically useful oxidations with aqueous hydrogen peroxide is obtained by isomorphous substitution of Si by Ti in molecular sieve materials such as silicalite (the all-silica analogue of zeolite ZSM-5) and zeolite beta. Titanium(IV) silicalite (TS-1), developed by Enichem (Notari, 1988), was the progenitor of this class of materials, which have become known as redox molecular sieves (Arends et al., 1997). 

Oxidations of various organic substrates with aqueous hydrogen peroxide have been reported on titanium containing derivatives of silicalite-1, denoted as Titanium-Silicalite-1 or TS-1 [93-97]. Examples of reactions which are catalyzed by TS-1 with high H2O2 yields and product selectivities are listed in Table 6. The oxidations are generally carried out at atmospheric pressure and at temperatures ranging from 273 to 373 K. 

Miscellaneous Oxidations. Titanium silicalites (TSs) are molecular sieves that incorporate titanium in the framework. They are able to perform oxygenation of various hydrocarbons under mild conditions by hydrogen peroxide.184,185... 

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. 

In classical processes cyclohexanone is converted to the corresponding oxime by reaction with hydroxylamine (see Fig. 2.27). The oxime subsequently affords caprolactam via the Beckmann rearrangement with sulphuric or phosphoric acid. Alternatively, in a more recent development, not yet commercialized, a mixture of cyclohexanone, ammonia and hydrogen peroxide is directly converted to cyclohexanone oxime over a titanium(IV)-silicalite (TS-1) catalyst. This route is more direct than the classical route and reduces the amount of salt formation but it involves the use of a more expensive oxidant (H2O2 rather than O2). 

The advantage of the above two methods are high yields of epoxides, and the titanium silicalite catalyst is not deactivated or poisoned by the contaminants in the crude oxidation mixture. Hence, the processes are commercially attractive. The in situ hydrogen peroxide generation based on the AO process from either the anthraquinone/anthrahydroquinone or ketone/alcohol redox couples has also been used for the following synthetic reactions ... 

Whilst a whole host of redox zeolites have been prepared and employed with hydrogen peroxide for the oxidation of organic substrates, by far the most studied is a titanium-containing silicalite known as TS-1. For this reason, the bulk of this section will discuss this material in terms of its preparation, oxidation ability with aqueous hydrogen peroxide and the mechanistic pathways believed to be functioning during the oxidation of organic substrates. 

The discovery of titanium silicalite-1 (TS1) by Enichem scientists (20-22) and its commercial use as a catalyst for a variety of selective oxidations with aqueous hydrogen peroxide under mild conditions (Figure 1.3) constituted a major breakthrough in oxidation catalysis. 

The epoxidation of propylene with hydrogen peroxide has been implemented to pilot plant scale (2,000 t/a) by EniChem. The reaction is catalyzed by titanium silicalite-1 (TS-1). Although TS-1 has similar composition to the Ti/Si02 developed by Shell for the epoxidation of propylene with organic hydroperoxides ca. 2.5 wt% Ti, as Ti02) the reactivity is quite different. TS-1 prefers H2O2 and protic solvents, and is almost inactive with hydroperoxides or in apolar media. The reverse holds for Ti/Si02. The reason is found in the different... 

More promising from an industrial perspective, however, is the separation of the oxidation zone from the aqueous one effected by the catalytic material itself, through the selective adsorption of the reagents. The introduction of Titanium Silicalite-1 (TS-1), in which the hydrophobic properties of the pores protect the active sites from the inhibition of the external aqueous medium, was a demonstration of the concept. The catalyst, the substrate and the aqueous soluhon of hydrogen peroxide can, in this case, be mixed together, with a great simplification of the process and also a reduction of the hazards. Three commercial processes. 

The discovery in the early 80 s of titanium silicalites [62-64] opened the new application perspective of zeolitic materials as oxidation catalysts. Several reactions of partial oxidation of organic reactants using dilute solutions of hydrogen peroxide could for the first time be performed selectively in very mild conditions. Other elements inserted in the lattice of silicalites have since been shown to have similarly interesting catalytic properties including, vanadium, zirconium, chromium and more recently tin and arsenic [65]. Titanium silicalites with both MFI (TS-1) and MEL (TS-2) structures have however been the object of more attention and they still seem to display unmatched properties. Indeed some of these reactions like the oxyfunctionalization of alkanes [66-69] by H2O2 are not activated by other Ti containing catalysts (with the exception of Ti-Al-Beta [70]). The same situation... 

The primary use for the titanium silicalites is as shape selective catalysts for hydrogen peroxide oxidations. " Propylene is converted to propylene oxide at greater than 98% selectivity and 99% peroxide conversion at 50°C over TS-1. 2,97 Butadiene is oxidized to the monoepoxide (Eqn. 10.26), also in high selectivity, and primary alcohols are oxidized to the aldehydes in all cases with selectivites greater than 80%.97... 

The Oxirane process is a mature technology that has stood the test of time. Both ARCO and Shell have been successfully operating for more than two decades. More recently a heterogeneous titanium-substituted silicalite (TS-1) catalyst was developed by Enichem [43, 44]. In contrast to the Shell Ti /Si02 catalyst, TS-1 has a hydrophobic surface and is a remarkably effective catalyst for a variety of liquid-phase oxidations with 30 % aqueous hydrogen peroxide, including epoxidation [44]. It has been commercialized for the hydroxylation of phenol to... 

M. G. Clerici, P. Ingallina, Epoxidation of lower olefins with hydrogen peroxide and titanium silicalite, J. Catal. 140 (1), 71-83 (1993). 

Balducci, L., Bianchi, D., Bortolo, R., D Aloisio, R., Ricci, M., Tassinari, R. and Ungarelli, R. (2003) Direct oxidation of benzene to phenol with hydrogen peroxide over a modified titanium silicalite. Angew. Chem. Int. Ed., 115, 5087-5090. 

G. F. Tliiele, E. Roland, Propylene epoxidation with hydrogen peroxide and titanium silicalite catalyst Activitv, deactivation, and regeneration of the catalyst, /. Mol. Catal. A Chem. 117(1997)351. 

G. Belussi, A. Carati, M. G. Clerici, G. Maddinelli, R. MiUini, Reactions of titanium silicalite with protic molecules and hydrogen peroxide, J. Catal. 133 (1992) 220. 

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