Catalytic hydroxylation of phenol by hydrogen peroxide. Kinetic study and comparison between solid acids and titanosilicates

Abstract

Hydroxylation of phenol by hydrogen peroxide over solid acids exhibits an autocatalysis that has never been described in earlier works. The induction period is dependent on the acidity and is reduced by initial addition of dihydroxybenzenes or other electron-transfer agents. A new mechanism, initiated by the slow formation of dihydroxybenzenes in the induction period, should be considered. Comparison of various catalysts shows that the reaction is also dependent on the structure of the solid. Zeolites with too small a porosity are not active, according to a large space demand of the reaction. Catalysis by titanium silicalites does not show such behaviour: the reactivity is low but regular. Thus, our results show that valuable comparison between catalysts cannot be deduced from tests performed by stopping the reaction at a determined time, but that kinetic studies are essential.