Titanium silicalites ammonia

Figure 1. Variations with coverage of the differential heats of ammonia adsorption on titanium-silicalites with different titanium contents (in pmol/g).

The first step operates in the liquid phase with ammonia and H2O2 as the reactants and titanium-silicalite (TS-1) as the catalyst. TS-1 is a zeolite, developed by Eni, having a structure that belongs to the same MEI family as ZSM-5, but in which A1 is absent (acid sites are detrimental for selectivity) and substituted by tetravalent Ti ions, which can activate H2O2 and give selective reactions of oxidation (Eigure 2.33 see also Chapter 6 on propene oxide for further aspects). 

Titanium silicalite has been reeognized as an efficient redox catalyst in a number of industrial processes. Enichem has a process in which Ti silicalite catalyzed the conversion of eyelohexanone by ammonia and hydrogen peroxide to cyclohexnone oxime . The meehanism appears to involve the Ti 0x0 species. A number of proeedures are available from literature to make the Ti silicalite catalyst and it will be easy to incorporate eleetron mediators such as Ru or Pd into the silicalite matrix. The performance of such composite... 

Hydroxylamine can be prepared from ammonia and hydrogen peroxide using a titanium silicalite catalyst in 83% yield.68 The by-product acetic acid could be recycled to acetic anhydride by pyrolyzing part of it to ketene. 

ACIDITY STUDIES ON TITANIUM SILICALITES-1 (TS-1) BY AMMONIA ADSORPTION USING MICROCALORIMETRY... 

It is known that cyclohexanone oxime can be formed from the ketone, ammonia, H2O2 and tungstate [192]. However, it has been found recently that titanium silicalite, TS-1, also catalyses this reaction [193]. As a heterogeneous system, this is of great interest for improving manufacture, and a pilot plant has been announced [194]. It is also disclosed that, in the absence of ketone, hydroxylamine is formed from H2O2 and ammonia... 

An alternative route to cyclohexanone oxime developed in Italy by Enichem is shown in the following reaction. Cyclohexanone oxime is produced by the ammoxidation of cyclohexanone with ammonia and aqueous hydrogen peroxide in the presence of a solid, recyclable catalyst, titanium silicalite (TS-1). This reaction step eliminates approximately one-third of total salt formation. However, the oxime is still converted to caprolactam through the conventional route (Beckmann rearrangement), catalyzed by stoichiometric amounts of sulfuric acid, and produces ammonium sulfate salt. Therefore, this alternative process still leaves something to be desired. 

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). 

In consistence with the direct observation of the differential heats and isotherms, it can be noticed in Table 1 that the initial heat of ammonia adsorption is strongly affected by the presence of a small amount of titanium. Qjnj increases from 77 kJ/mol for pure silicalite up to about 200 kJ/mol for only 0.26 wt% Ti. Then for increasing titanium content, has a slight tendency to decrease. 

Due to the valency IV of titanium, its substitution for Si does not cause any charge unbalance. Accordingly, no Brpnsted acidity is formed on Ti silicalite, as deduced, for example, by the lack of ammonia protonation [73], although an enhancement of the acidity of the silanols does possibly occur (Figure 9.4). Some debate exists on whether framework Ti" + ions can actually act as Lewis acid sites at the gas-solid interface. According to Manoilova et al. [258] CO... 

The ODH of propane over titanium and vanadium containing zeolites and nonzeolitic catalysts revealed that Ti-silicalite was the most active. The addition of water caused an increase in selectivity, probably due to a competitive adsorption on the active sites. The reaction is proposed to occur on the outer surface of the Ti-silicalite crystallites on Lewis acid sites, and a sulfation of the catalyst, which increases the acidity of these sites, results in a further increase of the catalytic activity. The maximum conversion obtained was 17% with a propene selectivity of up to 74% [65]. Comparison of propane oxidation and ammoxidation over Co-zeolites shows an increase in conversion and propene selectivity during ammoxidation. For a conversion of 14%, 40% propene selectivity was obtained with ammonia, whereas, at 10% conversion the propene selectivity was only 12% with oxygen. The increase in activity and selectivity can be due to the formation of basic sites via ammonia adsorption [38]. 

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