Microwaves have been shown to enhance the reaction rates and selectivities on products for organic¹ and inorganic^2,3 syntheses.  The microwave synthesis of nanoporous materials, including zeolites was recently reviewed by Tompsett et al.³, in which the various mechanisms of reaction rate enhancement were discussed.  The reason for these enhancements has been attributed to both thermal and non-thermal effects.

It is known that the dielectric properties of materials are dependant on the frequency of the applied electromagnetic field.  However, very little work has been reported on the effect of microwave frequency on chemical reaction.  This is likely due to the availability of systems with frequencies other than the standard 2.45 GHz.  Only recently, has work started to appear that considers the effect of frequency.  For example, Conde et al.^4-7 showed that frequency effects play an important role in the catalytic oligomerization of methane.  In 2004, Caponetti et al.⁸ investigated the effect of microwave frequency on the synthesis of CdS. They found that at 12 GHz larger particle size distribution of CdS was formed compared to 18, 8, 2.45 and 2.85 GHz. However, they used different vessel sizes at the different frequencies, which likely influenced the synthesis under microwaves as shown by Conner et al.⁹

Malinger et al.¹⁰ studied the microwave synthesis of manganese oxides using a variable frequency microwave oven.  They found that manganese oxide (OMS-2) prepared at 5.5 GHz showed the highest catalytic conversion rate for the oxidation of 2-thiophenemethanol.  Also, OMS-2 synthesized at high microwave frequency 5.5 GHz, had a different morphology than OMS-2 synthesized at low microwave frequency (2.45 GHz). OMS-2 synthesized at high microwave frequency is composed of both small fibers (<100 nm in length) and fibers of a size typical of OMS-2.

The frequency dependence and the influence of power delivery method on the yield and crystallinity of zeolite synthesis were determined for NaY (FAU) and silicalite (MFI) zeolites.  Two methods of microwave heating were investigated, namely waveguide cavities at fixed frequencies and a variable frequency oven. Waveguides at frequencies at 2.45, 5.8 and 10 GHz were used to heat precursor solutions to reaction temperatures of 100°C and 150°C respectively.  NaY and silicalite showed negligible difference in percent crystallinity over this frequency range however, silicalite showed some decrease in yield at higher frequency.  A Teflon vessel with either 11 mm or 33 mm inside diameter was used for reactions in order to determine the effect of reactor geometry at differing frequencies on the formation of silicalite.  Table 1 shows the results at two frequencies (2.45 and 5.8 GHz) and two reactor geometries.  It can be seen that a higher yield is produced in the 33 mm reactor at both frequencies.  Also, insignificant difference in the yield is observed for the two frequencies for the same reactor.  In this range, it can be concluded that microwave frequency has little effect on the formation of silicalite, however, lower average power was used at 5.8 GHz, likely due to better coupling of the microwaves.  This holds for longer reaction times at the two frequencies.

A variable frequency microwave oven system (Lambda Technologies) was used to syntheses silicalite at fixed frequencies 3 and 5.5 GHz as well as with sweeping frequency mode between 3 and 5.5 GHz.  It was found that higher yields and larger crystals formed for reactions at 175°C for 15 minutes, in the same reaction vessel.

The method of power delivery during microwave heating is dependant on the oven system employed.  Typically, a duty cycle pulse is used to maintain the temperature; however, newer laboratory models operate on a more continuous or cycled power.  The average field intensity and duration on steps of the hydrothermal formation of zeolites will provide evidence of the impotence of the microwave field over purely thermal effects on the reaction processes.

Using the waveguide systems, the effect of power delivery on the synthesis of silicalite was also investigated.  Preliminary results show that silicalite prepared with continuous microwave field at 2.45 GHz produced larger and fewer crystals compared to pulsed experiments, where the ratio of power on and off was 1/3.  The power delivery may effect to the nucleation and growth of silicalite zeolite.  Table 1 Yield of silicalite from microwave synthesis at 150°C for two different frequencies.

Frequency (GHz)	2.45	5.8	2.45	5.8	2.45	5.8
Reaction vessel	11 mm	11 mm	33 mm	33 mm	33 mm	33 mm
Ramp (min)	4	4	4	4	4	4
Hold (min)	40	40	40	40	55	55
Ave Power (W)	68.1	38.3	86.8	33.8	78.6	36.9
Yield	2.6%	3.5%	21.9%	24.6%	34.2%	31.0%
Yield (g)	0.021	0.029	0.180	0.202	0.281	0.255

 

References

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                (3)           Tompsett, G.; Panzarella, B.; Conner, W. C.; Lu, F.; Yngvesson, K. S. Submitted to Review of Scientific Instruments 2006.

                (4)           Conde, L. D.; Marun, C.; Suib, S. L.; Fathi, Z. Journal of Catalysis 2001, 204, 324.

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