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 hydroxy lation of aromatics (such as benzene, toluene and anisole), along with a re versal 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 ace tone, acetonitrile or methanol) a long induction period was observed, in so lvent-free conditions the induction period was almost absent. The effect of various reaction parameters such as substrate/H2O2 molar ratio, reaction t emperature, catalyst concentration, stirring speed and the dilution level w as 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 f actors responsible for an enhancement in the activity and a change in regio -selectivity are: 1. relative hydrophobic nature and restricted pore dimensions of titanium s ilicate TS-1; and 2. diffusive resistance faced by the substrate with an organic co-solvent i n biphase while such a resistance is minimized in triphase. (C) 1999 Elsevi er Science B.V. All rights reserved.