Hydroxylation of Aromatics with Hydrogen Peroxide over Titanosilicates with MOR and MFI Structures:  Effect of Ti Peroxo Species on the Diffusion and Hydroxylation Activity

Abstract

The shape selectivity in the liquid-phase hydroxylation of aromatic hydrocarbons with hydrogen peroxide has been studied on two kinds of titanosilicate, the large-pore Ti-M with MOR structure and medium-pore TS-1 with MFI structure, and the liquid-phase diffusion of aromatics into both catalysts was compared in the presence of H2O and H2O2 to clarify the origin of the shape selectivity. The hydroxylation rate on TS-1 decreased monotonically with increasing molecular size in the order benzene > toluene ≫ ethylbenzene > cumene, while Ti−M showed the maximum rate for toluene hydroxylation. The apparent diffusivity of aromatics was lowered roughly by 1 order by adding H2O2 to H2O solvent for both titanosilicates but was not affected for their Ti-free derivatives, mordenite and silicalite-1. The reduction in diffusion rate in the presence of H2O2 was more pronounced for TS-1 and for Ti-rich samples. It is concluded that a bulky Ti peroxo species (Ti−OOH) formed by the interaction of Ti site with H2O2 mainly causes a transition-state shape selectivity in the hydroxylation of bulky aromatics in titanosilicate/H2O2 systems.

Effect of solvents on propylene epoxidation over TS-1 catalyst

“…The effect of the steric factor on selectivity can't be ignored. From literature [10], reactants of propylene epoxidation have no obvious diffusional limitations. The steric effect is mainly caused by the geometrical constraints of the fivemembered ring intermediate inside the pores of TS-1.…”

Phenol Hydroxylation over Alkaline Treated TS-1 Catalysts

“…The TS-1 catalysis involves a five membered ring intermediate generated by reacting titanium peroxy species (Ti-OOH) with the hydroxyl group of solvent molecules [4]. Formation of the bulky five membered ring intermediate reduces the pore volume of TS-1 and, thus, imposes remarkable diffusion resistance of substrates upon TS-1 [5]. As a result, the catalytic performance of TS-1 is strongly affected by the molecular size of substrate and oxidant, nature of solvent, crystal size, pore structure and hydrophobicity [6,7].…”

Synthesis of Hierarchical TS-1 Zeolite from Silanized Seeds

“…This behavior is completely the opposite to that observed when using hydrogen peroxide, confirming the strong limitations that TS-1 zeolite suffers for reactions using organic peroxides as the latter cannot enter into the zeolite micropores, fact that it is well known in the literature. Thus, it has been widely reported that conventional TS-1 samples suffer of strong steric and diffusional limitations, especially when using large substrates as it occurs in phenol hydroxylation [35,36]. In contrast, the catalytic activity of the hierarchical TS-1 samples was several times higher than those obtained with the reference ones.…”