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 H2O 2 (30%) and titanium-substituted molecular sieves (TS-1 and Al-free Ti-Beta ) was investigated to understand the role of the zeolite structure, the cry stal size, the external Surface of the zeolite, and the nature of the solve nt on the product selectivity. Comparing Al-free Ti-Beta and TS-1 samples w ith Similar pore lengths, the activity as well as the ratio of catechol to hydroquinone was significantly higher for Al-free Ti-Beta, showing diffusio nal 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 i nertization of the external surface, using cycles of low-temperature chemic al vapor deposition (CVD) of tetraethoxysilane followed by high-temperature calcination. Consecutive CVD cycles led to a slight increase of the select ivity toward hydroquinone for all tested solvents as well as a reduction of the coke formation in methanol and water. The ratio of hydroquinone to cat echol, however, did not change much, indicating that catechol must also be formed inside the pore structure. A kinetic analysis of the reaction data w ith the parent and surface-inertized TS-1 revealed that the role of the ext ernal surface in terms of both activity and selectivity is significant and dependent on the solvent used. A reaction mechanism consistent with the obs erved enhanced selectivity for hydroquinone in protic solvents is proposed. (C) 2001 Academic Press.