Hydroxylation, aromatics titanium silicalite

The reaction is carried out using a titanium silicalite-1 (TS-1) zeolite catalyst [30, 122]. This type of catalyst is known to accelerate the selective oxidation of alcohols, epoxidation of alkenes and hydroxylation of aromatics. These reactions have importance for fine-chemical production. 

The direct hydroxylation of benzene and aromatics with a mixture of 02 and H2 have been performed by simultaneously mixing benzene, oxygen and hydrogen in the liquid phase using a very complicated system containing a multi-component catalyst, a solvent and some additives. Besides the possibility of an explosive gas reaction, these hydroxylations gave only very low yields, 0.0014—0.69% of phenol and aromatic alcohols. For example, Pd-containing titanium silicalite zeolites catalyzed... 

Titanium siliealites TS-1 and TS-2 catalyze hydroxylation in the aromatic ring of the monoalkylbenzenes studied to corresponding alkylphenols, using hydrogen peroxide as oxidant. Para-isomers are mainly formed in methanol or ethanol as solvents. In the case of ethyl- and 1-propylbenzenes, the first carbon atom of the aliphatic chain is also oxidised both to alcohols and ketones. As expected, the terminal methyl groups in all hydrocarbons are not oxidised. The probable reasons for this behaviour of titanium silicalites are discussed. 

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. 

Of the above two routes, the titanium silicalite appears the simpler process since the catalyst is heterogeneous. It has recently been found that several metal phosphates (e.g. Zr, Sn) are alternative heterogeneous catalysts for aromatic hydroxylation [136]. These materials have strongly... 

---