Heterogeneous Catalysis by Titanium Silicalite

However, the activity of TS-1 in the oxidation of benzene appeared to be very poor. TS-1 does not perform very well in two-phase systems, so that only solvents able to homogenize the hydrophobic substrate and the aqueous hydrogen peroxide can be used. Even in this case, using solvents such as acetone, acetonitrile or tert-butanol, the selectivity to phenol rapidly dropped at very low benzene conversion, mainly due to the formation of dioxygenated products and tars typically, selectivity was already less than 50% at benzene conversion as low as 5%. Even worse results were obtained using methanol, which was oxidized in competition with benzene to give formaldehyde dimethyl acetal. 

In contrast, the use of sulfolane as solvent allows a conversion of benzene close to 8% while maintaining the selectivity to phenol higher than 80%. Detected byproducts are catechol (7%), hydroquinone (4%), 1,4-benzoquinone (1%) and tars (5%) [19]. 

Even better results are obtained by a post-synthesis treatment of TS-1 with both hydrogen peroxide and ammonium hydrogen fluoride, NH4HF2. Upon such a treatment (H202/F/Ti = 10 2.5 1 60 °C 4h), a substantial amount of titanium (up to 75% of the initial value) is removed. Nevertheless, the crystalline structure of the zeolite remains unchanged and the catalytic activity does not decrease. On the contrary, it actually increases since the turnover frequency of residual titanium atoms rises from 31 to 80 h . Even more importantly, at 8.6% benzene conversion the selectivities, both on benzene and on hydrogen peroxide, also increase from 83 to 94% and from 67 to 83% respectively, with formation of catechol (4%) and hydroquinone (2%) as the only by-products, without any evidence of further oxidation reactions [19]. 

A possible mechanism for the TS-1-catalyzed hydroxylation of phenol with hydrogen peroxide has been proposed [20] it is an ionic mechanism that, with 

Such a complex is not stable enough to be isolated but its occurrence was inferred by the IR spectra of the solutions. Drago s suggestion is confirmed by ah initio calculations. Therefore, the improved selectivity observed upon carrying out benzene oxidation in sulfolane may be due to the formation of this large species, which can not enter the titanium silicalite pores, thus allowing phenol to remain relatively protected against further oxidation. 

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