Microwaves and sorption on oxides: Surface temperature and adsorption selectivity investigation
Microwave heating is not the same as conventional heating, and it is believed that this difference, the "microwave effect," may be interpreted to be due to selective local heating. The temperature at the surface where sorption occurs is "effectively" greater than the measured solid or gas temperature. In these studies, measurements of the amount adsorbed as functions of the partial pressure of a specific adsorbate in the presence of microwave irradiation were related to conventional adsorption isotherms. Equating the adsorbate pressure required to achieve a specific coverage (an isostere) in the presence of microwave irradiation to the amount adsorbed for a conventional isotherm allowed for an estimate of the "effective" surface temperature in the presence of microwaves. It was found that the effective surface temperature increased when using adsorbates having a significantly higher permittivity or when increasing the microwave power. The implication of this change in the surface energy for specific species in the presence of microwaves is discussed.
It was hypothesized that the adsorption selectivity in the presence of microwaves is primarily dependent on the permittivity of the adsorbates, while selectivity is dependent on the heat of adsorption under conventional heating. Sorption experiments were carried out using a flow based dual-component adsorption system measuring changes in the amount adsorbed with conventional heating and using microwave heating at 2.45 and 5.8 GHz. The adsorption selectivity as a function of microwave frequency was examined for a case in which the adsorbates have an opposite dependence of permittivity with frequency (isopropanol had a greater permittivity than acetone at 2.45 GHz, and acetone had a greater permittivity than isopropanol at 5.8 GHz). It was found that microwave energy could influence sorption differently than conventional heating. Differences in the adsorption selectivity were not as great as expected based on the bulk liquid permittivities due to the miscibility of the components. The permittivities of the adsorbates in the adsorbed phase at low surface coverage may be different than that of their respective bulk liquids. The smaller than expected change in adsorption selectivity with microwave frequency might also be attributed to the miscibility of acetone and isopropanol.