Friedel-crafts acylation reactions using heterogeneous catalysts stimulated by microwave radiation

Abstract

Ion exchanged zeolites, ion exchanged clays, sulphated zirconia and silica supported phosphotungstic acid were prepared. Characterisation of the catalysts using pyridine adsorption revealed the catalysts contained both Brønsted and Lewis acid sites. When the catalytic activities for the acylation of toluene and anisole were studied there was found to be a correlation between the Brønsted acidity and the catalytic activity.
The reactivity of the catalysts is dependant upon both the concentration and the chain length of the carboxylic acid. An optimum chain length being in the range Cm-C12. An aromatic:carboxylic molar ratio of 30: I was demonstrated to be preferable to either 300:1 or 3:1.
The activation energies for the acylation of toluene were determined to be in the range 165-359 ki mol' and those for anisole 67-196 kJ mor 1 .
Microwave experiments were performed in both batch and flow reactors. Under both reactor configurations microwave stimulation gave consistent increases in catalyst activity. Attempts were made to correlate this with the material dielectric properties, unfortunately, no correlation was found.
Under the high E-field conditions Of the single mode (flow reactor) only the zeolites proved to be sufficiently robust as to provide any meaningful data. These catalysts displayed an increased selectivity to the ortho-isomer which was maximised when hexanoic acid was used as the acylating agent. The selectivity to the ortho-isomer increased with increasing microwave power.