(651a) Microwave Reaction Engineering: Penetration Depth And Overheating

Microwave enhanced synthesis has been reported in the literature for many catalyst materials including zeolites.1,2 Up to 2 orders of magnitude enhancement in reaction rates have been reported, however, the mechanism of this rate enhancement is not well understood. Further, few papers have provided sufficient information on the microwave system and synthesis conditions that were employed, which has likely lead to discrepancies between synthesis from different workers.3 Many factors are important in microwave synthesis and these require optimization for each system being irradiated.2 Factors include the dielectric properties, vessel geometry, reaction volume, field density, irradiation method and frequency. The dielectric permittivity and loss of the irradiated material determine the interaction of microwaves with the material, specifically the depth of penetration of the microwave field. As the loss increases, the depth of penetration decreases as the microwaves are more easily dissipated as heat in the material Hermann et al.4 studied the overheating of microwave zeolite synthesis. These workers found that due to the low penetration of microwaves in the zeolite precursor solutions, overheating at the periphery of the liquid volume gives rise to the enhanced reaction rates observed, compared to conventional heating. Two fiberoptic temperature probes (one at the center and one at the periphery of the vessel) were used to measure the temperature throughout microwave heating. A temperature difference at the periphery, compared to the center, of up to 40˚C was recorded. Temperature variations due to macroscopic ?hot-spots? and/or microscopic molecular energy variations due the ?excess dipolar energy? are a possible mechanism for the rate enhancements. A simple model for the mechanism of microwave reaction enhancement has been proposed by Conner and Tompsett5, based on the variations of the energies of reaction intermediates from differences in temperatures. Originally, few workers measured the temperature of the microwave heated materials and the power or time duration was the only method used. Recently, there has been much development in laboratory based microwave systems which employ thermocouples or fiberoptic probes inserted into the heated materials (for example, reaction solutions).6 However, a single temperature probe situated at the center bottom of the reacting medium is used. Few workers have investigated the temperature profile during microwave heating, likely due to the difficulty in measurements in high power electromagnetic fields.6-10 Simulations of the microwave field and temperature distribution are typically calculated using the dielectric properties of the heated material, in order to understand the herating properties. Conner et al.11 reported the field distribution in a zeolite precursor solution heated in a microwave oven. The field distribution was show to vary greatly depending on the vessel and hence solution geometry. Further. Stenzel et al.9 reported the microwave field and temperature distribution of zeolite solutions. We investigate the temperature profile within zeolite precursor solutions during microwave heating using a multiple fiberoptic setup. Three zeolite precursor gels/solutions of varying penetration depth and ionic composition are employed for the measurements: silicalite, NaY and SAPO-11. The effect of temperature on the dielectric properties of these gels was determined using a Network Analyzer and dielectric probe, up to 80˚C. Microwave field distributions, within the gel heating in a microwave oven, are modeled using HFSS® (Agilent) software together with the experimental dielectric properties. From these field values, further calculations of the temperature distribution and heat transfer will be undertaken for comparison to the experimental data.

Keywords: microwave heating, penetration depth, overheating, zeolite synthesis, field distribution.

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