Microwave chemistry has expanded over the last two decades due to the enhanced reaction rates achieved for many processes, including organic synthesis, inorganic synthesis and polymerization. Significant time and energy saving can be realized using microwave chemistry, which is important both commercially and for the environment.
One of the most exciting and commercially/technologically significant areas where microwave energy has been demonstrated to influence the kinetics and selectivity is in the synthesis of nanoporous materials, such as zeolites. New nanoporous materials can be created, and the times for their syntheses can be significantly reduced, involving using less energy. By reducing the times by up to over an order of magnitude, continuous production would be possible to replace batch synthesis.
However, the mechanism and engineering for the enhanced rates of these syntheses are unknown. The results from different laboratories are not consistent, and experimental details are sparse. Therefore, more research is required to unlock the mystery behind this “gee-wiz” chemistry. Furthermore, understanding the fundamental processes leading to rate enhancement by microwaves will also enable the optimization of these microwave heated reactions.
In this work, the formation of SAPO-11 (and AlPO-11), silicalite and NaY zeolites under microwave heating was investigated and the influence of various microwave reactor engineering parameters was studied. Microwaves enhanced the SAPO-11 synthesis by two orders of magnitude over the conventional synthesis. Both nucleation and growth steps were enhanced by the presence of microwaves. Fast microwave heating was not solely responsible for this enhancement. This indicates that non-thermal interactions of material with microwaves are present for this synthesis.
Many microwave reactor engineering parameters were identified as possibly influencing the microwave synthesis of SAPO-11 (and AlPO-11). These factors are precursor volume, reaction temperature, reactor size, stirring, applicator type and microwave frequency. Among those, the reaction temperature had the greatest influence on this SAPO-11 (and AlPO-11) synthesis. Increasing the reaction temperature decreased the nucleation time and increased the growth rate. The crystallization growth rate in the microwave synthesis showed higher activation energy (1.5 times) compared to the conventional synthesis. However, the pre-exponential factor increased by 8 orders of magnitude in the microwave synthesis. Nucleation rate also showed an increase in the activation energy (3.6 times) and an increase in the pre-exponential factor (10 orders of magnitude) by using microwave heating. This substantial increase in the pre-exponential factor could be the reason behind this microwave synthesis enhancement.
High temperature, stirred synthesis, large vessel and using multimode field distribution oven found to be the optimum reaction conditions for microwave synthesis of SAPO-11 (and AlPO-11).
Thermal variations within SAPO-11, silicalite and NaY synthesis solutions were measured using a reaction vessel with multiple fiberoptic temperature probes. NaY synthesis solution has the shortest microwave penetration depth among these zeolite synthesis solutions which led to great thermal variations between the region near the wall (high temperature) and the center (low temperature) when placed in a vessel with diameter 20 times larger than its penetration depth. Increasing these thermal variations led to a decrease in the nucleation time and thus enhanced this NaY microwave synthesis.
Microwave power delivery mode (pulsed vs. continuous) effect on the synthesis of the three zeolites mentioned above was investigated. Pulsing the microwave power required less average power to maintain the synthesis reaction temperature compared to continuous delivery mode. No effect of using pulsed compared to continuous microwave power delivery was found on the nucleation time and the crystal growth for these zeolite syntheses. However, pulsed microwave power delivery produced smaller particles in the case of SAPO-11.
The effect of simultaneous cooling effect on the microwave synthesis of SAPO-11 and silicalite was studied. Increasing the amount of power delivered to the SAPO-11 synthesis while maintaining the reaction temperature fixed using the simultaneous cooling, decreased the nucleation time and increased the growth rate. Smaller particles were formed at high power. Silicalite showed no change in the nucleation time, crystal growth and/or the morphology. This indicates that there is no universal pattern among the microwave synthesis of zeolites. What could be an important factor for one synthesis is not necessarily important for another, and is likely dependant on the dielectric properties and the reaction mechanism.
Key words: Zeolite, SAPO-11, silicalite, NaY, microwave synthesis, nucleation, crystal growth, frequency, reactor engineering, overheating, temperature distribution, pulsing power delivery, simultaneous cooling.