The Effect of Stirring, Heating Rate, and Power Delivery on Silicalite Microwave Synthesis

Microwave zeolite synthesis has been shown to greatly reduce crystallization time. Although, fast efficient syntheses and quality crystalline products are widely reported(1), the mechanism leading to the formation of zeolites under microwave heating is not fully understood. Cundy(1) has reviewed the literature and concludes that the likely processes specific to microwave synthesis of zeolites are namely, rapid heating, superheating and selective heating. Conflicting conclusions on the mechanism for the enhanced reaction rates in the literature are likely due to the many differences in experimental procedure which are often not specified. These include, poor temperature measurement, differences in reactor types, amounts of precursor gel used, oven type and power delivery. Conner et al.(2) have shown that many factors should be taken into consideration with the microwave synthesis of zeolites, namely the reactor geometry, amount of zeolite precursor, the dielectric properties and the field distribution in the liquid. Bonaccorsi and Proverbio(3) studied the effect of stirring on NaA zeolite solution during microwave heating, and reported that stirring during reaction gave no significant change in reaction rate of secondary products formed. Romero et al.(4) recently reported the effect of rapid stirring on the conventional synthesis of low silica X zeolite. These workers found that reaction rates similar or greater than that of microwave synthesis could be achieved, suggesting the enhanced effect is due predominantly to a rapid heating rate for this zeolite. Silicalite precursor solutions, prepared as previously described(2), were stirred using a magnetic bead during reaction at 175°C for 10 minutes and the products were compared with those with no simultaneous stirring. As shown in Table 1, yields of 20±2% and crystal size of 1.2 ±0.1 μm (c-axis) were observed in both stirred and non-stirred samples. For silicalite, under the conditions employed, we see no evidence of increased yields crystal size. Table 1. Stirred and nonstirred silicalite solution with microwave heating at 175°C for 10 min. Sample Average yield %yield c x a x b axis (μm) average of 96 crystals No. crystals x10 Non stirred 0.357±0.004 22±0.2 1.19±0.11 x 0.97±0.09 x 0.49±0.06 1.2 stirred 0.293±0.014 18±0.9 1.22±0.02 x 0.97±0.03 x 0.53±0.03 0.9 Koegler et al(5) showed that that microwave heating of silicalite precursor solution to 150°C could be achieved in seconds compared to around 2 hours for a conventionally heating vessel to reach thermal equilibrium. We have studied the effect of rate of temperature increase i.e. ramp rate, on the microwave synthesis of silicalite zeolite. With decreasing the ramp rate below 5°C/min to a reaction temperature of 175°C, and holding for 10 minutes at 175°C, the yield increases to ~90% however, the crystal size decreases to 0.50±0.04 μm in length, shown in Table 2. A slow ramp rate to reaction temperature, (which is typically found with conventional synthesis using steel bombs in an autoclave), produced a high yield of uniform crystals, indicating even and rapid nucleation. For a similar time (10 min) in a conventional oven, negligible zeolite yield is produced. While, a slow heating rate of 1.25°C/min and 10 min. hold time at 175°C, an order of magnitude more crystals were formed. To determine whether the total time in the microwave field was important to the number and type of crystals formed, a short (2 minute) ramp and hold time of 128 min. giving a total of 130 min in the microwave field (the same as 120 min ramp and 10 min hold) was performed. From Figure 1(c), the SEM micrograph, shows the silicalite crystals are much larger at ca. 3 μm length, however, they are coated with a fine amorphous powder. A longer hold time at the reaction temperature (175°C) produces greater crystal growth, however, secondary smaller particles are also produced. Microwave heating can give enhanced reaction rate for silicalite even with a slow heating rate. Table 2. Non-stirred silicalite solution heated with microwaves at 175°C for 10 min after varying ramp times. Sample Ramp rate Average yield (g) %yield Crystal size c x a x b axis (μm) 97 random crystals No. crystals x10 20mL 600 W ramp 2 min 10 min hold 75°C/min 0.357±0.00 4 22±0.2 1.19±0.11 x 0.97±0.09 x 0.49±0.06 1.2 20 mL 600W, ramp 30 min 10 min hold 5°C/min 1.4200 86 1.27±0.09 x 1.04±0.09 x 0.49±0.05 1.05 20mL 600W ramp 120 min 10 min hold 1.25°C/mi n 1.4524 88 0.50+0.04 x 0.46+0.04 x 0.24+0.03 Small amount of twinning 12.5 20 mL, 600W, ramp 2 min 128 min hold 75°C/min, 1.0781 66 3.28+0.21 x 1.84+0.16 x 0.90+0.09 Plus <40 nm particles 0.09 (neglecting small particles)