Ammoximation

In the same spirit DFT studies on peroxo-complexes in titanosilicalite-1 catalyst were performed [3]. This topic was selected since Ti-containing porous silicates exhibited excellent catalytic activities in the oxidation of various organic compounds in the presence of hydrogen peroxide under mild conditions. Catalytic reactions include epoxidation of alkenes, oxidation of alkanes, alcohols, amines, hydroxylation of aromatics, and ammoximation of ketones. The studies comprised detailed analysis of the activated adsorption of hydrogen peroxide with... 

TS-1 is a material that perfectly fits the definition of single-site catalyst discussed in the previous Section. It is an active and selective catalyst in a number of low-temperature oxidation reactions with aqueous H2O2 as the oxidant. Such reactions include phenol hydroxylation [9,17], olefin epoxida-tion [9,10,14,17,40], alkane oxidation [11,17,20], oxidation of ammonia to hydroxylamine [14,17,18], cyclohexanone ammoximation [8,17,18,41], conversion of secondary amines to dialkylhydroxylamines [8,17], and conversion of secondary alcohols to ketones [9,17], (see Fig. 1). Few oxidation reactions with ozone and oxygen as oxidants have been investigated. 

In this section we report a detailed summary of the experimental studies on the interaction of TS-1 with H2O and NH3. The choice of these two molecules is far from random. Interaction with water is important since the catalyst works in aqueous solution (Sect. 2). The interest in the study of NH3 is twofold ammonia is a reactant in the ammoximation of cyclohexanone to give cyclohexanone oxime and it is a stronger base than water, thus allowing a direct comparison between the effects induced by Lewis bases of increasing strength. 

The TS-1 catalysed ammoximation of cyclohexanone with NH3/H2O2 is a new process (Romano et ai, 1990) for the production of cyclohexanone oxime, the precursor of caprolactam. In the existing process, the oxime is produced by reaction of cyclohexanone... 

The ammoximation reaction involves the in situ formation of hydroxylamine via TS-1 catalysed oxidation of NH3 with H2O2. Hence, there are no size restrictions with regard to the ketone substrate, because the reaction of NH2OH with the latter occurs in the bulk solution. For example, TS-1 catalyses the ammoximation of / -hydroxyacetophenone (Le Bars et al., 1996). Beckmann rearrangement of the oxime product (see Fig. 2.18) affords the analgesic paracetamol (4-acetaminophenol). 

In the absence of alcohol solvents, hydrogen peroxide is thought to coordinate with Ti by displacing an Si-O bond as shown in Fig. 6.11. This sets the stage for ammonia interaction to form hydroxylamine and subsequent ammoximation of ketones to oximes.20,56 57... 

The oxidation of NH3 to NH2OH forms the basis of a process for the ammoximation of cyclohexanone to the oxime because the NH2OH formed in solution readily reacts with the ketone (non-catalytically) to give the oxime (231). Table XXX (165) illustrates the conversions and selectivites obtained for a few typical ketones and aldehydes. The ammoximation of aldehydes is faster than that of ketones. The oxime selectivity is also higher. The ammoximation of cyclohexanone by this method offers a more eco-friendly alternative route to the cyclohexanone oxime intermediate for the production of Nylon-6. The current route coproduces large quantities of ammonium sulfate and involves the use of hazardous chemicals such as oleum, halides, and oxides of nitrogen. 

One of the major problems in all the ammoximation processes using aqueous H202 + TS-1 with NH3 is that, under the basic conditions (pH > 10) prevailing during the reaction, some of the lattice Si ions of the zeolite structure in TS-1 are leached into solution, leading to catalyst destruction. This leaching is a common characteristic of all silicates. Innovative catalyst formulations and process modifications are needed to overcome this problem. 

Recently, the Sumitomo Chemical Co., Ltd. developed the vapour-phase Beckmann rearrangement process for the production of 8-caprolactam. In the process, cyclohexanone oxime is rearranged to e-caprolactam by using a zeolite as a catalyst instead of sulfuric acid. EniChem in Italy developed the ammoximation process that involves the direct production of cyclohexanone oxime without producing any ammonium sulfate. The Sumitomo Chemical Co., Ltd. commercialized the combined process of vapour-phase Beckmann rearrangement and ammoximation in 2003 ". 

The TS-l catalyzed hydroxylation of phenol to a 1 1 mixture of catechol and hydroquinone has already been commercialized by Enichem. Another reaction of considerable commercial importance is the above mentioned ammoximation of cyclohexanone to cyclohexanone oxime66, an intermediate in the manufacture of caprolactam. It could form an attractive alternative to the established process that involves a circuitous route via oxidation of ammonia to nitric acid followed by reduction of the latter to hydroxylamine (figure 4). 

There is, as is well known, a close similarity between the crystalline and porous structures of silicalite-1 and silicalite-2. The same similarity therefore exists between TS-1 and TS-2, and it appears logical that they should have very similar catalytic properties. TS-2 has been evaluated as a catalyst for many different reactions, such as Beckmann rearrangement of cyclohexanone oxime with vapor-phase reactants H202 oxidation of phenol, anisole, benzene, toluene, n-hexane, and cyclohexane and ammoximation of cyclohexanone. As described in detail in Section V.C.3, differences that had been claimed between the catalytic properties of TS-1 and those of TS-2 have not been substantiated. Later investigations have shown that, when all the relevant parameters are identical, the catalytic activities of TS-1 and TS-2 are also identical. The small differences in the crystalline structure between the two materials have no influence on their catalytic properties (Tuel et al., 1993a). 

The ammoximation of cyclohexanone with NH3 and 02, which had been investigated with silica catalysts (Armor et al., 1979, 1980, 1982), has been investigated with TS-1 as the catalyst (Dreoni et al., 1991). However, the results have not been considered satisfactory for industrial exploitation, especially when... 

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. 

Table 4.3 Cyclohexanone ammoximation with different catalysts nh3 + h2o2 —nh2oh + h2o...

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]. 

Another pertinent example is provided by the manufacture of caprolactam [135]. Current processes are based on toluene or benzene as feedstock, which can be converted to cyclohexanone via cyclohexane or phenol. More recently, Asahi Chemical [136] developed a new process via ruthenium-catalysed selective hydrogenation to cyclohexene, followed by zeolite-catalysed hydration to cyclo-hexanol and dehydrogenation (Fig. 1.49). The cyclohexanone is then converted to caprolactam via ammoximation with NH3/H202 and zeolite-catalysed Beckmann rearrangement as developed by Sumitomo (see earlier). 

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