Titanium-catalysed Oxidation Reactions

A serious shortcoming of TS-1, in the context of fine chemicals manufacture, is the restriction to substrates that can be accommodated in the relatively small (5.lx5.5 A2) pores of this molecular sieve, e.g. cyclohexene is not epoxidised. This is not the case, however, with ketone ammoximation which involves in situ formation of hydroxylamine by titanium-catalysed oxidation of NH3 with H202. The NH2OH then reacts with the ketone in the bulk solution, which means that the reaction is, in principle, applicable to any ketone (or aldehyde). Indeed it was applied to the synthesis of the oxime of p-hydroxyacetophenone, which is converted, via Beckmann rearrangement, to the analgesic, paracetamol (Fig. 1.24) [75]. 

Titanium silicate (TS-1) which has a structure similar to the zeolite ZSM-5 has been shown to catalyse a number of synthetically important oxidations with hydrogen peroxide under mild conditions.34 A useful feature of the TS-1 catalyst is its enhanced product selectivity in oxidation reactions, for example, cyclohexane is selectively oxidised to cyclohexanone inside the pores of TS-1. On the external surfaces where there is little steric control cyclohexane is oxidised to the dicarboxylic acid. Spinace and co-workers have shown that these external reactions can be prevented by the addition of an antioxidant such as 2,6-dwert-butyl-4-methylphenol (BHT) but which does not interfere with the internal reactions since it is too bulky to enter the pores of the TS-1.35... 

The oxidation of various hydrocarbons such as n-octane, cyclohexane, toluene, xylenes and trimethyl benzenes over two vanadium silicate molecular sieves, one a medium pore VS-2 and the other, a novei, iarge pore V-NCL-1, in presence of aqueous HjOj has been studied. These reactions were carried out in batch reactors at 358-373 K using acetonitrile as the solvent. The activation of the primary carbon atoms in addition to the preferred secondary ones in n-octane oxidation and oxidation of the methyl substituents in addition to aromatic hydroxyiation of alkyl aromatics distinguish vanadium silicates from titanium silicates. The vanadium silicates are also very active in the secondary oxidation of alcohols to the respective carbonyl compounds. V-NCL-1 is active in the oxidation of bulkier hydrocarbons wherein the medium pore VS-2 shows negligible activity. Thus, vanadium silicate molecular sieves offer the advantage of catalysing selective oxidation reactions in a shape selective manner. 

The titanium silicalite is unique in catalyzing the oxidation of aromatics with H2O2 at high selectivity. For example, phenol is hydroxylated to a mixture of cathecol and hydroquinone. Conversion for H2O2 is 70%, and a selectivity for phenol is 90%. A listing of titanium silicalite catalysed oxidation reactions is shown in Table 1. These results suggest the "restricted transition state selectivity" to be effective because of the peculiar pore structure of the catalyst. 

Photoactivated titanium dioxide will also catalyse the oxidative degradation of organic material in solution. Such photocatalytic oxidation reactions are applied very successfully in treatment and removal of organic pollutants from water and from air. Solar photocatal) ic degradation of pollutants or bacteria in water could also provide a cheap and facile route to clean water in parts of the world where traditional water-purifying infrastructure is not available. 

Since the development of titanium silicate (TS-1) materials as catalysts in the 1980s, heterogeneous titanium-catalysed oxidation reactions utilising aqueous hydrogen peroxide have been used in many effective and versatile reactions such as olefin epoxidation, " alcohol oxidation and phenol hydro)q7lation, which adhere more closely to the principles of green chemistry. 

Oxidation reactions of both acyclic and cyclic secondaiy amines to obtain nitrones catalysed by (trialkanolaminato)titanium(iv) complexes in the presence of cumyl hydroperoxide as oxidant occurred in high yields, even when 1 mol% of catalyst was used. ... 

In the green synthesis of paracetamol p-hydroxyacetophenone is reacted with ammonia and hydrogen peroxide in presence of titanium(IV)-silicate (TS-1) catalyst to give the oxime of p-hydroxyacetophenone. In fact, in the above reaction, ammoximation proceeds via in situ formation of hydroxylamine by titanium-catalysed oxidation of ammonia with in the micropores of... 

Lead oxide reacts violently with numerous metals such as sodium powder (immediate ignition), aluminium (thermite reaction, which is often explosive), zirconium (detonation), titanium, some metalloids, boron (incandescence by heating), boron-silicon or boron-aluminium mixtures (detonation in the last two cases). Finally, silicon gives rise to a violent reaction unless it is combined with aluminium (violent detonation). It also catalyses the explosive decomposition of hydrogen peroxide. 

The most important side reaction in heterogeneously catalysed MPVO reactions is the acid-catalysed aldol condensation. Aldol products are usually observed during the Oppenauer oxidation of alcohols, when a surplus of ketone or aldehyde is used as the oxidizing agent and the solvent. The low amount of by-products formed when Ti-beta was used as the catalyst, demonstrates the advantage of the titanium system over Al-beta. This is probably caused by the much weaker Brpnsted acidity of the solvated titanium site [8] compared with the strong H -acidity of the aluminium site in Al-beta. As we have shown earlier Ti-beta has a high tolerance towards water, which further shows the catalytic potential of Ti-beta in MPVO reactions [9]. 

Highly dispersed titanium oxide species on silica prepared by the sol-gel method catalyse the selective epoxidation of propene by molecular oxygen.59 This is potentially very significant as the new commercial route to propene oxide is based on the reaction of propene with hydrogen peroxide catalysed by a mixed Ti-Si oxide the direct reaction with oxygen has clear advantages. 

Transition-metal-catalysed epoxidations work only on aUylic alcohols, so there is one limitation to the method, but otherwise there are few restrictions on what can be epoxidized enantioselectively. When this reaction was discovered in 1981 it was by far the best asymmetric reaction known. Because of its importance, a lot of work went into discovering exactly how the reaction worked, and the scheme below shows what is believed to be the active complex, formed from two titanium atoms bridged by two tartrate ligands (shown in gold). Each titanium atom retains two of its isoprop oxide ligands, and is coordinated to one of the carbonyl groups of the tartrate ligand. The reaction works best if the titanium and tartrate are left to stir for a while so that these dimers can form cleanly. 

Baldwin et al. (1995) surveyed the effect of various mineral phases on the rate of hydrolysis of the model organophosphate ester p-nitrophenyl phosphate. They found that, normalized for the number of independently determined phosphate adsorption sites, the manganese oxides were most effective in catalysing the hydrolysis reaction, followed by iron and titanium oxides, with a small effect for alumina. No effect was... 

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