Abstract
When ultrasonic irradiation is applied to chemical reactions, the reaction rates are often increased. The application of ultrasonics, therefore provides an alternative mode of acceleration for reactions which is complementery to conventional methods. The current interest in this novel technique stems from its use as an alternative to conventional energy source such as heat, light and pressure as a method of modifying chemical reactivity. One example of the use of ultrasonic irradiation for the enhancement of reactivity is via the effects of particle size reduction in heterogeneous reactions. The important benefit which arises from particle size reduction and simultaneous surface activation is the exposure of large surface areas of the solid to the solution with the consequent possibility of greater chemical reactivity or catalytic efficiency.The effects of changing various reaction parameters (e.g. ultrasonic power and frequency, solvent, reaction temperature, vessel geometry, reaction time and type of bubbled gas) are discussed for a number of model reactions.
(1) The reduction in particle size of metal and inorganic powders
The reduction of particle size for copper bronze (60µ), nickel (65µ), potassium carbonate (70µ) and aluminium oxide (70µ) was found to increase with increasing ultrasonic power and lower ultrasonic frequency (20kHz > 40kHz > 60kHz). It was also found that particle size reduction increases with decreasing solvent vapour pressure, decreasing temperature and when a monatomic gas is bubbled into the system. Reaction vessel geometry also has an important influence on the rate of particle size reduction.
Investigations were carried out to study the kinetic effects of changes in the above parameters on various catalytic reactions (e.g. Simmons-smith cyclopropanation of styrene, hydrogenation of 1-octene and the Michael addition of diethylmalonate to chalcone) and to rationalise the above observations to predict the optimum conditions for maximum sonochemical efficiency in a heterogeneous reaction.
(2) Simmons-Smith Cyclopropanation of Styrene
In the Simmons-Smith cyclopropanation of styrene it was observed that the highest rates and yields were obtained with increasing ultrasonic power, low frequency (20kHz > 40kHz > 60kHz), low vapour pressure solvents, low temperature during the pre-sonication of the zinc catalyst (25°C), high temperature during the sonicated cyclopropanation stage of the reaction (90°C).
(3) Hydrogenation of Oct-l-ene
The hydrogenation of Oct-1-ene using a pre-sonicated 3µ nickel catalyst gave results comparable to those of Raney Nickel, Palladium on Carbon and Rieke Nickel under conventional conditions. It was also observed that the highest yields and rates using a pre-sonicated 3µ nickel catalyst were produced at low temperature, increasing ultrasonic power and mechanical stirring. Hydrogenation yields using pre-sonicated 3µ nickel were found to be better than pre-sonicated submicron nickel.
(4) Michael Addition of Di-ethylmalonate to Chalcone
Studies on the Michael Addition of diethylmalonate to chalcone generated the best yields using increasing ultrasonic power, low ultrasonic frequency (20kHz > 40kHz > 60kHz)) and low temperature. Reaction vessel geometry also had an important influence on the Michael Addition yield.
(5) Terephthalic Acid Dosimetry Method
We have also investigated the effect of irradiation frequency (20 > 40 > 60kHz) on radical production during the sonolysis of water using hydroxyterephthalate as a fluorescence monitor. This has shown that higher frequencies are more effective in free radical generation. Free radical production is also enhanced using increasing ultrasonic power, low temperature and specific vessel geometry (conical flask > round bottom flask).
| Date of Award | 1992 |
|---|---|
| Original language | English |
| Supervisor | Timothy Mason (Supervisor) & J. Phillip Lorimer (Supervisor) |