Temperature Distributions within Zeolite Precursor Solutions in the Presence of Microwaves

Abstract

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.

Hydrothermal and microwave synthesis of SAPO (CHA) zeolites on aluminium foams for heat pumping applications

“…Conner et al [30,[32][33][34] in their extensive work on microwave synthesis of zeolites have demonstrated the important influence of several parameters on the final results. Microwave power delivery method [33], reactor vessel size, precursor volume [30,32], temperature distribution in the reacting mixture [34], all these factors play a role during the electromagnetic heating of aqueous solutions which is not immediately derivable from a simple comparison with water behaviour, frequently used as reference material in microwave heating at 2.45 GHz. Dielectric properties of precursor solutions for zeolite syntheses, in fact, are significantly different from those of pure water due to the presence of various ions in solution and each zeolite has its specific characteristics.…”

“…The strongly basic pH of some silico-aluminate zeolite solutions (i.e. LTA, NaY) makes them more susceptible to MW heating (higher dissipation factor vs. water) although the penetration depth of electromagnetic waves at 2.45 GHz decreases significantly in these solutions, from 35-40 mm for water to 4-5 mm for NaY [34]. The consequence is an inhomogeneous heating with hotter areas close to the reactor wall and a coldest core.…”

“…The consequence is an inhomogeneous heating with hotter areas close to the reactor wall and a coldest core. For SAPO syntheses, however, Conner et al [34] have demonstrated that precursor solutions, which generally have lower pH, show larger penetration depth and, hence, a more homogeneous temperature distribution inside the solution. In our experiments, the vessel used in MW syntheses was cylindrical, similar in size to the HY autoclave, having just a smaller diameter (35 mm vs. 38 mm) for the same precursor quantity (50 g).…”

“…Vessel size and dielectric properties of SAPO solutions allow supposing that during MW pulse a series of ''hot spots'' formed within the precursor solution, which were hardly detectable by one temperature probe. These local temperature rises are considered the main reason of the reaction rate enhancement even if some particular effects connected to microwaves irradiation cannot be excluded a priori [30,[32][33][34]. The smaller particle size observed in MW SAPO-34 compared to HY is due to the earlier nucleation and faster growth rate which increase the number of particles decreasing their size [33,35].…”

Theoretical Aspects of Microwave Irradiation Practices

“…All the information presented above about the inherent characteristics of different solvents and how they interact with electromagnetic fields is very important when performing a microwave assisted protocols and just as important as is how to measure the temperature inside the reactor [9,12,[17][18][19]. It is not the objective of this chapter to discuss all the different possibilities given by the microwave instrumentation software for running a microwave experiment.…”