Microwave effects on some zeolite catalysed reactions
This research was prompted by the growing body of information accumulating in the literature which refers to the advantages of microwave irradiation as opposed to normal conventional heating. Many of the reports claim that activation by microwave radiation results in enhanced rates and changes in product selectivities. Of special interest were reports that microwave irradiation had beneficial effects on heterogeneous catalysed reactions. Microwave irradiation should be able to activate specifically the actual catalytic sites and hence could be used to affect reactions where the bulk of the catalyst, the reactants and products are transparent to microwaves. Investigation of this effect was conducted on zeolite catalysed reactions, namely the disproportionation of toluene and the alkylation of toluene with methanol. Most of the reports fail to address the problem of accurate temperature measurement within a microwave field. This is of paramount importance and this study describes the development and use of a gas thermometer to overcome this problem. This development has been used to measure the temperature of solvents under thermal and microwave conditions. Using this device it was discovered that polar solvents could attain temperatures in excess of 20°C above their normal boiling point and this phenomenon was investigated further to quantify the effect as a function of the microwave power, the volume of the solvent, the dielectric properties associated with the solvent and the surface of the container used. A comprehensive study of the effect of microwave radiation on disproportionation and alkylation of toluene has been made using hydrogen forms of mordenite and ZSM-5 catalysts. The aim of this reaction is to make paraxylene which is then used in the production of terephthalic (1,4-benzenedicarboxylic) acid. This reaction has been studied under both microwave and thermal conditions at various temperatures. Microwave radiation has shown to have no beneficial effect on the reaction selectivity for the disproportionation and alkylation of toluene. Experiments on disproportionation of toluene under high pressure thermal conditions have shown that partial regeneration of the catalyst can be performed by raising the partial pressure of hydrogen. The study describes in detail the comparison between microwave radiation and normal conventional heating, the influence of carbon laydown on the catalyst under conventional and microwave conditions, and the possibilities of catalyst regeneration. Carbon is deposited on the catalyst and observations have shown that it preferentially absorbs microwaves offering novel techniques for regeneration of the catalyst.