Temperature distributions within zeolite precursor solutions in the presence of microwaves.

While microwave enhancement of chemical syntheses has been demonstrated for a broad variety of chemical reactions, there is no accepted universal mechanism. Is the enhancement due to more efficient heating, to overheating, to nonuniform heating, or to nonthermal effects? Analyses are complicated due to the often significant spatial and temporal temperature variations in microwave reactor systems, particularly within microwave ovens. To address this, we employ multiple fiber-optic temperature probes throughout a cylindrical reactor with a focus on zeolite synthesis solutions being the dielectric medium. First, we vary the modes of power delivery (pulsed versus continuous) to quantify differences in local temperatures within a reaction vessel with water being the dielectric medium. The temperature distribution at steady state in the center of the water increased by 10 degrees C in pulsed delivery mode compared to the temperature distribution obtained in continuous delivery mode at the same average power. Then, we measured the temperature distributions for several zeolite synthesis solutions (NaY, silicalite, and SAPO-11) and water under microwave heating to investigate the temperature variations within these dielectric media. These measured temperature variations were found to be significant, depending on the dielectric permittivities of the reaction medium and their changes with temperature. Temperature profiles also depend on the microwave delivery mode and reactor configuration, i.e., the microwave reactor engineering. NaY synthesis solution exhibited the smallest penetration depth (2.6 mm at room temperature and 2.45 GHz); as a result, the solution temperature near the wall increased by 65 degrees C over the target temperature when the temperature at the center of the solution was targeted to 60 degrees C. To demonstrate the effect of overheating on zeolite synthesis, we synthesized NaY zeolite at 95 degrees C by controlling the temperature of the reaction near the wall, close to the penetration depth, and in the center away from the penetration depth. Controlling the center temperature results in greater overheating and consequently reduced nucleation time by 80 min, from 130 to 50 min.