Influence of Pore and Crystal Size of Crystalline Titanosilicates on Phenol Hydroxylation in Different Solvents

The hydroxylation of phenol into catechol and hydroquinone with aqueous H2O2 (30%) and titanium-substituted molecular sieves (TS-1 and Al-free Ti-Beta) was investigated to understand the role of the zeolite structure, the crystal size, the external surface of the zeolite, and the nature of the solvent on the product selectivity. Comparing Al-free Ti-Beta and TS-1 samples with similar pore lengths, the activity as well as the ratio of catechol to hydroquinone was significantly higher for Al-free Ti-Beta, showing diffusional constraints for the conversion of phenol and geometric constraints for the formation of catechol. The diffusional constraints in the conversion of phenol were confirmed by using small crystallites of TS-1. The role of the external surface of TS-1 in the phenol hydroxylation was investigated by inertization of the external surface, using cycles of low-temperature chemical vapor deposition (CVD) of tetraethoxysilane followed by high-temperature calcination. Consecutive CVD cycles led to a slight increase of the selectivity toward hydroquinone for all tested solvents as well as a reduction of the coke formation in methanol and water. The ratio of hydroquinone to catechol, however, did not change much, indicating that catechol must also be formed inside the pore structure. A kinetic analysis of the reaction data with the parent and surface-inertized TS-1 revealed that the role of the external surface in terms of both activity and selectivity is significant and dependent on the solvent used. A reaction mechanism consistent with the observed enhanced selectivity for hydroquinone in protic solvents is proposed.