Effects of ultrasound emitter type and power on a heterogeneous reaction

In the course of our current interest in designing appropriate reactors for sonochemistry, we have investigated a heterogeneous reaction which is highly improved by ultrasound, with the aim of trying to understand the mechanism of the reaction rate enhancement and the influence of the sonication parameters.

The chosen model reaction is a Michaël reaction: the addition of ethyl malonate to chalcone in toluene under solid-liquid phase transfer conditions. This reaction is very sensitive to sonic irradiation: the initial reaction rate is increased by a factor of 10 under sonication.

In order to separate the physical and chemical effects of ultrasound, we have studied the disruption of the solid catalyst (KOH) in toluene and compared the particle sizes with the values obtained during the reaction with and without ultrasound. It can be seen that sonication prevents particle agglomeration as the reaction proceeds.

The effect of the generator power has been studied using a cuphorn. The reaction yield is dramatically increased near the cavitation power threshold and then remains nearly constant. Finally, different sonication systems have been compared, the cleaning bath giving the poorest results.