Microwave synthesis of zeolites: effect of power delivery

The effect of microwave power magnitude and pulsing frequency on the synthesis enhancement of zeolites, silicoaluminophosphate SAPO-11, silicalite, and NaY, was studied. Pulsing the microwave power compared to continuous delivery at the same averaged fed microwave power showed no effect on the nucleation and crystallization rates of SAPO-11, silicalite, or NaY. However, SAPO-11 synthesized with continuous microwave power delivery produced larger particles compared to pulsed microwave power with the same reaction time (3.77 microm for continuous versus 2.49 microm for pulsed 1 s on; 3 s off). Further, pulsed microwave power delivery used lower steady state power to maintain the same reaction temperature compared to continuous power delivery (55 W compared to 65 W, respectively). The microwave power used to heat the reaction precursors of SAPO-11 and silicalite was varied by applying cooling gas at various rates while maintaining the reaction temperatures. Significant enhancement of the crystallization rate for SAPO-11 was observed with increasing the fed microwave power (0.014 min(-1) at 65 W, 0.030 min(-1) at 130 W, and 0.066 min(-1) at 210 W), with little effect on the nucleation time. The crystallization rate to microwave power relation was found to obey a power curve (y = 0.4259x(2) - 0.2776x + 0.8517). Lower microwave power produced larger crystals but required longer reaction time to complete crystallization (3.77 microm at 65 W compared to 2.04 microm at 210 W). Conversely, silicalite synthesis at 150 degrees C was found to be independent of the magnitude of the applied microwave power.