Microwave technology in zeolite synthesis

Verified Syntheses of Zeolitic Materials

2nd Revised Edition

Koos Jansen
Laboratory of Applied Organic Chemistry and Catalysis,
Deift University of Technology
Julianalaan 136, 2628 BL, Delft
The Netherlands

Microwaves have a wavelength between 1 millimeter and 1 meter which corresponds to frequencies of 300 GHz and 300 MHz. These frequencies are indicated in the electromagnetic wave spectrum in Fig. 1.

γ-rays		X-rays	UV	Vis	IR microwaves			radiowaves
22	20	18	16	14	12	10	8	6	4	→log Hz

Fig. 1. Electromagnetic spectrum including microwave frequencies between infrared and radio frequencies.

As radar and telecommunication also use the microwave frequency band, only 4 specific frequencies are available for microwave heating. One of them, actually 2.45 GHz, is applied in household microwave ovens.

A microwave has an electric and a magnetic component which are in phase and perpendicular to each other in amplitude. Both wave components are perpendicular to the direction of travel. The heating effect in all types of materials that can interact with microwaves is mainly caused by the electric component. The total polarization (α _t) of the material based on the displacement of electrons, nuclei, permanent dipoles and quadrupoles and charges at interfaces is the sum of the following parameters:

α _t = α _e + α _a + α _d+ α _i: in which

As both the electronic and atomic polarization operate at time scales that are smaller than required for microwave frequency field oscillations, these polarizations do not result in conversion of microwave into heat energy. The time-scale of the orientation of permanent dipoles is, however, comparable to the time scale of microwave oscillations. Thus when the amplitude of the electric field increases, the dipoles align themselves accordingly. Next, as the field amplitude decreases to zero intensity, the dipoles return to their random orientation. The change in orientation in both operations results in an energy conversion from electric into thermal energy. The interfacial polarization contributes to dielectric heating when conducting particles are suspended in a non-conducting phase. This effect is not substantial at microwave frequencies and thus results in a modest contribution to the heating. conduction effects can also contribute to the dielectric heating. Since ions are charged, they accelerate in an electric field. Herein, the electromagnetic energy is converted in kinetic energy which is transferred to neighboring molecules resulting in unordered kinetic energy, actually heat.

The way the above mentioned materials react on microwaves is determined by their dielectric constants. The complex dielectric constant ε can be expressed as:

ε = ε ' - ε '' ; in which:

The real part, or relative permittivity, represents the degree to which an electric field may build up inside a material when exposed to an electric field. The imaginary part, or dielectric loss, is a measure of how much of that field will be convened into heat. The loss angle δ is the phase difference between the electric field and the polarization of the material. The loss angle as formulated below is referred to as the dissipation factor.

tan δ = ε '' / ε '

The dissipation factor, or tan δ is a measure for the materialâs ability to transform electromagnetic energy into heat. The higher the dissipation factor, the better the transformation of microwave energy into heat. In Table 1, dissipation factors are given for relevant materials.It is clear from Table 1 that water, and particularly water with a high Z-value, has a high ability to transform microwave energy into heat. Materials that are not heated by microwaves are, for example, glass and Teflon.

The dielectric constants of the materials also determine the penetration depth, which is defined as the depth into the material where the power is reduced to about 1/3rd of the original intensity.

The penetration depth is formulated as:

D_p ∝ λ√( ε ' / ε ''), in which:

Thus a material with a higher dissipation factor will have a lower penetration depth. The wavelength and hence the frequency also greatly influences the penetration depth. From the above it is clear that the sample size, the penetration depth and heating rate are coupled and can result in a homogeneous or heterogeneous heating of the material.

Thus in case a fast homogeneous heating is preferred, the material, for example a zeolite synthesis mixture, must have a relatively high dissipation factor, a high external surface and small volume. In the case of water this phase must have the form of a thin disk. At the same time one should take into account the microwave frequency applied. Higher frequencies will give smaller penetration depths. In case of extremely fast heating of zeolite syntheses mixtures, boiling point retardation might occur which results in small, but uncontrollable, explosions.

In general it is concluded that microwave effects, particularly for zeolite syntheses, are mainly recognized as converting microwave energy into heat. There are still many contributions in literature stating that microwaves do influence the reaction rate and product distribution on a molecular scale. However, microwave photons that have intrinsically a power not much larger than ~ 10^-5 eV at 2.45 GHz are too small and not applicable for molecular activation as compared to those encountered , for example, in UV photochemistry.

In zeolite syntheses heated with microwaves [1], the inexpensive set-up (~ $1,500), compared to commercially available ovens including autoclave equipment (~ $30,000), comprises of [2]:

The synthesis is carried out as follows: A regular, not viscous synthesis mixture of a zeolite is placed in the Teflon autoclave. The autoclave is closed and the thermocouple connected. An initial heating step is then applied which can be up to 1000 W. In this way the mixture is at theprojected synthesis temperature within tens of seconds. Subsequently the synthesis mixture must be kept at a constant temperature. The minimum oven setting is 100 W which is too high to compensate for the heat losses. Thus a lower power which is needed is accomplished by reducing the amount of radiation at 100 W. This is achieved with a screen of perforated metal installed around the autoclave. At a setting of 100 W and depending on the size of the holes in the perforation, only a few tens of watts will reach the autoclave and thus maintain the synthesis temperature at the projected value.

Crystallization times of zeolites were reduced to two minutes in particular cases. The main advantage of microwave heating compared to conventional heating is the high, even extremely high, up to 170ºC in 20 seconds, controllable heating rates under homogeneous conditions, if the right configurational dimensions are applied.

One aspect, the formation of pure product, is of interest if microwaves are applied as heating technique for zeolite synthesis mixtures. The formation of pure product is related to the heat-up rate which is different in conventional heating compared to microwave heating as given in Tab. 2.

Table 2. Autodaves of 0.5 liters were used. Stainless steel in a preheated hot air oven and Teflon in a microwave oven.

If sources, in particular silicate and aluminate, are not extremely well mixed, and immediately after the mixing step exposed to rapid heat-up with microwaves, it is most likely that a mixed product distribution is observed. The product contains partly zeolite, silicate, aluminate and mixtures thereof, not yet converted into zeolite. [1] This is not observed a) after extensive stirring of the mixture at room temperature and subsequently immediate exposure to microwaves or b) after relatively short stirring and next placed in a preheated hot air oven. Apparently the nutrients need to be mixed on a molecular scale to obtain pure zeolite. This is achieved either through extremely long stirring after which the pure zeolite phase forms immediately when the projected synthesis temperature is rapidly reached with microwaves, or slowly heating up the synthesis mixture in a hot air oven which allows for further mixing upon heating up for a relatively long time before the projected synthesis temperature is reached.

References

[1] P. M. Slangen, Thesis, TU-Delft (1998)
[2] A. Arafat, Thesis, Helwan University, Cairo (1993)

Material	tan δ x 10⁴	microwave frequency (GHz)
		2.45	3.00
Teflon	2.1 10^-4	+
Glass	4.0 10^-4	+
Benzene	14 10^-4	+
Ice	9 10^-4	+
Water	0.157		+
0.1M NaCl	0.240		+
0.5M NaCl	0.625		+
Methanol	0.640		+
Ethanol	0.250		+
Ethylene glycol	1.00		+

Mode of heating	projected synthesis temperature (°C)/time (minutes)
Conventional	100 / 30	170 / 60
Microwave	100 / 0.2	170/ 0.3