Enhancement in the reaction rates in the hydroxylation of aromatics over TS-1/H₂O₂ under solvent-free triphase conditions

A significant enhancement (3-10 times) in the reaction rates of the hydroxylation of aromatics (such as benzene, toluene and anisole), along with a reversal in regio-selectivity of the products of substituted benzenes such as toluene and anisole was achieved under solvent-free, triphase conditions (solid-liquid-liquid), catalyzed by TS-1 using dilute H₂O₂, compared to that obtained under conventionally used biphase conditions in the presence of a co-solvent (solid-liquid). While in the presence of a co-solvent (like acetone, acetonitrile or methanol) a long induction period was observed, in solvent-free conditions the induction period was almost absent. The effect of various reaction parameters such as substrate/H₂O₂ molar ratio, reaction temperature, catalyst concentration, stirring speed and the dilution level was also studied. In the case of substituted benzenes (anisole and toluene) para-hydroxy product was predominantly obtained under triphase conditions. However, in biphase the formation of ortho-isomer was preferred. Probable factors responsible for an enhancement in the activity and a change in regio-selectivity are: 1. relative hydrophobic nature and restricted pore dimensions of titanium silicate TS-1; and 2. diffusive resistance faced by the substrate with an organic co-solvent in biphase while such a resistance is minimized in triphase.