Microwave Synthesis of Zeolites: Effect of Power Delivery - The

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Microwave Synthesis of Zeolites: Effect of Power Delivery Murad Gharibeh,† Geoffrey A. Tompsett,† K. Sigfrid Yngvesson,‡ and W. Curtis Conner*,† Department of Chemical Engineering, 159 Goessmann Lab, UniVersity of Massachusetts, Amherst, Massachusetts 01003, and Department of Electrical and Computer Engineering, 201 Marcus Hall, UniVersity of Massachusetts, Amherst, Massachusetts 01003 ReceiVed: January 14, 2009; ReVised Manuscript ReceiVed: April 28, 2009

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 µm for continuous versus 2.49 µm 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 ) 0.4259x2 - 0.2776x + 0.8517). Lower microwave power produced larger crystals but required longer reaction time to complete crystallization (3.77 µm at 65 W compared to 2.04 µm at 210 W). Conversely, silicalite synthesis at 150 °C was found to be independent of the magnitude of the applied microwave power. 1. Introduction Zeolites are regular microporous materials that have important industrial applications such as catalysts1-7 and molecular sieves.8 The synthesis of zeolites using conventional heating techniques is typically achieved in a batchwise fashion and can take days for complete reaction.9 Microwave synthesis has been shown to enhance dramatically the reaction rate of many zeolites and mesoporous materials synthesis.10 The microwave synthesis of zeolites and mesoporous materials was reviewed by Cundy11 and more recently by Tompsett et al.10 Previously, we have reported the effects of reactor engineering properties on the microwave synthesis of silicalite,10,12 NaY,13 and SAPO-1114,15 zeolites. Pulsed versus continuous microwave power delivery effect has been studied by few workers primarily on organic syntheses.16-22 Early work by Kamarzsin et al.16 in 1985 indicated that the thermochemical properties of epoxy resins depended on the pulse frequency of the microwave heating. Later, Fu et al.19 reported that continuous-power microwave curing of epoxy resins produced noticeably higher reaction rates compared to pulsed-power microwave curing. However, this did not prove the existence of a “nonthermal” effect of microwave heating. Zhu et al.21 showed that for the emulsion polymerization of methyl methacrylate using pulsed microwave irradiation in a waveguide reactor a significant increase in the reaction conversion occurred compared to conventional heating * Corresponding author. E-mail: [email protected]. Fax: +1-413545-1647. † Department of Chemical Engineering. ‡ Department of Electrical and Computer Engineering.

(85.8% vs 53% at 20 min). Further, the microwave pulse frequency had no effect on the polymerization rate under those conditions. Few reports exist in the literature on the effect of pulsed microwave synthesis of inorganic materials such as zeolites.23-25 Typically, the synthesis is carried out using a microwave oven operating under a duty cycle pulsing the microwaves. However, using a waveguide allows the microwave field to be controlled and well understood due to the monomode cavity size compared to an oven, which typically contains a complex multimode field distribution. We have reported the use of pulsed and continuous microwave power delivery during synthesis of NaA zeolite.24 The relative amounts of the products determined from the in situ WAXS experiments are similar when either pulsed or continuous microwave heating is applied to the reactor while maintaining the same synthesis temperature. Gharibeh et al. recently reported that different temperature distributions in water (about 10 °C increase) were obtained using pulsed microwave power delivery compared to that under continuous delivery at the same average microwave power.26 Li and Yang reported that using pulsed microwave power delivery for the synthesis of NaA zeolite membranes produced spherical particle morphology while using continuous microwave power delivery produced cubic morphology.27 Simultaneous cooling during microwave synthesis has been suggested to result in higher product yields and new pathways that were previously unattainable.28 However, recent systematic studies on simultaneous cooling with microwave synthesis have shown no reaction rate enhancement for organic reactions.29-31 Greater energy is used at higher microwave powers with insignificant change in yield. For example, Hosseini et al.31

10.1021/jp900400d CCC: $40.75  2009 American Chemical Society Published on Web 06/10/2009

Microwave Synthesis of Zeolites studied (S)-proline-catalyzed asymmetric Mannich- and aldoltype reactions using microwave heating, and the observed rate enhancements were a consequence of the increased temperatures attained by microwave dielectric heating and were not related to the presence of the microwave field. Zeolite syntheses have been shown to be sensitive to various microwave reaction parameters.13,15 The effect of microwave power with simultaneous cooling of different zeolites syntheses is therefore of interest. Bonaccorsi and Proverbio32 studied the effect of microwave power on a continuous synthesis of LTA zeolite. They found that using 1260 W at a residence time of 13 min and 0.2% seeding in a coiled pipe reactor results in producing hydroxysodalite, while using 900 W under the same conditions produced LTA zeolite. Furthermore, the LTA zeolite particle size distribution decreased from a median of 1.36 µm at 360 W and residence time of 29 min with 0.2% seeding to 0.91 µm at 900 W and residence time of 13 min and 0.2% seeding. However, they did not report the reaction temperature for each case in this study. In this paper, we investigate the effect of microwave power delivery mode on the microwave synthesis of zeolites, SAPO11, silicalite, and NaY. The relationship between the nucleation time and crystallization rate and two types of microwave heating modes, namely, the use of pulsing versus continuous microwave power delivery, is determined. Second, the effect of varying the microwave power magnitude and hence field intensity, while maintaining the same reaction temperature for different zeolite syntheses, is presented.

J. Phys. Chem. B, Vol. 113, No. 26, 2009 8931 TABLE 1: Microwave Waveguide System and Reaction Parameters system manufacturer description cavity volume frequency (GHz) maximum microwave power magnetron irradiation modus microwave power delivery modes reactor type temperature and pressure measurement

program

reaction temp. (°C) quantity of reactant cooling

2. Experimental Section 2.1. Zeolite Precursor Solutions. SAPO-11 precursor gels were prepared as described previously.14 A precursor solution was prepared by a mixture of two organic structure directing agents prepared by combining an aqueous solution consisting of 11.53 g of 85 wt % orthophosphoric acid and 22.0 g of water with 6.9 g of a hydrated aluminum oxide (a pseudoboehmite, 74.2 wt % Al2O3, 25.8 wt % H2O) and stirring until homogeneous. We then added a mixture of 1.3 g of a fumed silica (92.8 wt % SiO2, 7.2 wt % H2O) in 32.46 g of an aqueous solution of 40.0 wt % tetra-n-butylammonium hydroxide (TBAOH) to this mixture. This mixture was stirred until homogeneous, and then 5.10 g of di-n-propylamine (Pr2NH) was added with stirring until again homogeneous. The composition of the final reaction mixture in molar oxide ratios was: 1.0Pr2NH:0.5(TBA)2O:Al2O3: P2O5:0.4SiO2:50H2O. NaY zeolite was synthesized from aged aluminosilicate gels. Colliodal silica, Ludox AS-40 (Dupont 40 wt %, 7.76 g), was weighed into a Nalgene vessel. The proportion of water was weighed (11.76 g), 10 g of which was added to the Ludox and stirred for 4-5 min. To this solution, sodium aluminate (RiedelDe Haen, 1.23 g) was added and continued to stir for another 4-5 min, until the sodium aluminate dissolved. A small portion of the weighed NaOH (98%, Merck, 1.44 g) was then added followed by stirring until the portion was dissolved before adding more until all NaOH (1.44 g) was dissolved. Finally, the remaining water was added. The suspension was then aged for 48 h before reaction. The NaY precursor employed was a thick white gel with a composition of 8SiO2:Al2O3:4Na2O: 140H2O. This precursor gel was distributed to a reaction ACV (CEM) vessel and reacted at 100 °C. Silicalite precursor solutions were prepared by mixing 25 g of tetraethylorthosilicate (TEOS, Aldrich) with 26.5 g of 1 M tetrapropylammonium hydroxide (TPAOH, Aldrich) solution in

S-band circular waveguide with 2.45 GHz generator Sairem Corp. France monomode circular waveguide 178 mm (height) × 76 mm (diameter) ) 0.81 L 2.45 300 W one magnetron monomode fixed frequency, continuous wave and pulsed 1 s on; 2 s off and 1 s on; 3 s off 33 mm ID ACV vessel (CEM Corp.) fiberoptic probe in a sapphire thermowell was used to measure the temperature while no pressure measurements were taken 8 min ramp, up to 120 min hold (SAPO-11) 35 min ramp, up to 110 min hold (Silicalite) 3 min ramp, up to 135 min hold (NaY) 160 °C (SAPO-11), 150 °C (Silicalite), 100 °C (NaY) 15 g (SAPO-11 and silicalite), 12 g (NaY) flow of N2 gas cooled by liquid N2 or an ice bath

water and 48.75 g of deionized water. These solutions were stirred for 4 h at room temperature. Nalgene polypropylene bottles were used to mix and store the solutions. 2.2. Microwave Reactor. The reaction precursor for SAPO11 was crystallized at 160 °C under autogenous pressure for different reaction times in a polytetraflouroethylene ACV vessel (CEM Corp.). Silicalite precursor was reacted at 150 °C and NaY at 100 °C. The waveguide system used to study the effect of microwave power on the synthesis consisted of a 2.45 GHz Sairem generator, with a maximum microwave power output of 300 W, and a WR284 S-band waveguide, as described in Table 1. The waveguide setup is described in detail in a previous publication.33 Pulsing experiments were undertaken using continuous microwave power during the temperature ramp period and pulsed microwave power during the hold times unless otherwise stated. A T1 fiber-optic temperature probe by “Neoptix” was used to measure the temperature of the zeolite synthesis solution heated by microwaves in the waveguide. This fiber-optic temperature probe was connected to a “Neoptix-ReFlex” temperature readout which was connected to an A/D Measurement Computing USB-1208LS data acquisition board. This A/D board was interfaced with a PC computer through a USB cable. LabVIEW (National Instruments Inc.) software was used to record the temperature data with time and to control the microwave power produced by the Sairem generator. Two pulsed microwave power delivery sequences were investigated: 1 s on; 2 s off and 1 s on; 3 s off at the same reaction temperatures. Simultaneous cooling of the reaction vessel during microwave heating was achieved by introducing a stream of cold nitrogen gas through the waveguide. The nitrogen gas was first passed

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Figure 1. Crystallization curves for SAPO-11 prepared by microwave synthesis at 160 °C using different microwave power delivery modes in a circular microwave waveguide. Continuous, pulsed: 1 s on; 2 s off and pulsed: 1 s on; 3 s off. Average steady state applied microwave power during hold time is shown.

Figure 2. SEM micrographs of SAPO-11 prepared by microwave synthesis at 160 °C using different microwave power delivery modes in a circular waveguide: (a) continuous, (b) pulsed: 1 s on; 2 s off, and (c) pulsed: 1 s on; 3 s off. Magnification ) ×2000. Scale bar ) 10 µm.

through a heat exchanger immersed in a liquid nitrogen or ice bath, then into the waveguide. 2.3. Characterization. Crystallinity of the synthesized zeolite powders was determined using X-ray diffraction. A Philips X’Pert Pro diffractometer equipped with an X’Celerator detector was used to obtain X-ray patterns. An accelerating voltage of 45 keV was used at 40 A. Patterns were obtained at a scan

speed of 0.1°(2θ)/s. To create crystallization curves for SAPO11, the average relative peak intensity of the hkl ) 020 and 002 peaks was ratioed against the respective peak intensities for synthesis powders at full crystallization. All X-ray patterns were matched to a standard diffraction pattern for aluminophosphate hydrate ICDD #00-043-0563. While, for NaY, the average relative peak intensity of the hkl ) 111 and hkl ) 220

Microwave Synthesis of Zeolites

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TABLE 2: Nucleation Times and Crystallization Rates for the Microwave Synthesis of SAPO-11 at 160 °C Using Microwave Power Delivery (Pulsed and Continuous) in a Circular Waveguide Reactor and ACV Vessela delivery mode

nucleation time (min)

crystallization rate (min-1)

power at steady stateb (W)

yield (g)

particle size (µm)

estimatedc number of particles × 1010

continuous pulsed: 1 s on; 2 s off pulsed: 1 s on; 3 s off

17 17 17

0.014 0.013 0.015

65 62 55

1.64 1.44 1.08

3.77 ( 0.55 2.70 ( 0.20 2.49 ( 0.24

2.9 6.9 6.7

a 8 minute ramp time. Yields, particle size, and estimated numbers at 0.95 relative crystallinity. b The average microwave power fed by the generator. c Assuming the same density (∼2 g · cm-3) for all samples and spherical particles.

Figure 3. Percent yield curves for silicalite prepared by microwave synthesis at 150 °C using different microwave power delivery modes in a circular microwave waveguide; continuous, pulsed: 1 s on; 2 s off, and pulsed: 1 s on; 3 s off, and pulsed: 1 s on; 2 s off during both ramp and hold times. Average steady state applied microwave power during hold time is shown.

TABLE 3: Nucleation Times and Crystallization Rates for the Microwave Synthesis of Silicalite at 150 °C Using Microwave Power Delivery (Pulsed and Continuous) in a Circular Waveguide Reactor and ACV Vessela delivery mode continuous pulsed: 1 s on; 2 s off pulsed: 1 s on; 3 s off pulsed during ramp and hold times, 1 s on; 2 s off


percent yield rate (min-1)


yield (g)

particle size (µm)


40 40 40 40

0.035 0.028 0.030 0.028

45 33 30 33

0.94 0.91 0.90 0.90

0.70 ( 0.06 0.73 ( 0.05 0.72 ( 0.07 0.73 ( 0.05

2.91 2.48 2.55 2.45

Yields, particle size, and estimated numbers at 75% yield. Ramp time of 35 min to 150 °C. b The average microwave power fed by the generator. c 96 particles measured. Assumed spherical shape and density of silicalite 1.8 g · cm-3. a

were used and the X-ray patterns were matched to standard diffraction pattern for faujasite (ICDD # 01-076-0108). To obtain the crystallization (yield) curves for silicalite, the actual dry yields were ratioed against the theoretical yield for SiO2(TPA) zeolite (with density 1.8 g · cm-3 and 15 g of precursor used). The crystallization rate was calculated as the slope of the linear region of the crystallization curve between 0.2 and 0.8 relative crystallinity. Nucleation time is defined as the reaction time required before 1% relative crystallinity is observed by XRD. The nucleation rate is the reciprocal of the nucleation time.13 Multiple points were obtained for many of the experimental points plotted on the crystallization curves, and a repeatability of >95% relative crystallinity was calculated. The morphology and particle size of the SAPO-11, silicalite, and NaY powders were determined using Scanning Electron Microscopy (SEM). A JEOL X-Vision 6320FXV FESEM was used. Samples were prepared by dispersing a small quantity of powder on an aluminum stub using a droplet of ethanol,

followed by gold sputter coating. Particle size was determined from a statistical average of a sample population of 96 crystals using ImageJ (NIH, USA) software. 3. Results and Discussion The effect of the microwave power delivery mode was studied using three zeolite syntheses, SAPO-11, silicalite, and NaY, as described above. This study was divided into two parts, continuous versus pulsed microwave power delivery and the magnitude of the delivered microwave power. 3.1. Continuous versus Pulsed Microwave Power Delivery. SAPO-11 was synthesized at 160 °C using the waveguide reactor with the microwave power controlled using a PC computer and LabVIEW (National Instruments Inc.) software to remotely operate the Sairem generator. The microwave power delivery was varied using pulsing of 1 s on; 2 s off and 1 s on; 3 s off, throughout the hold time period only. Figure 1 shows

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Figure 4. SEM micrographs of silicalite prepared using microwave heating with (a) continuous microwave power, (b) 1 s on; 2 s off, (c) pulsed: 1 s on; 3 s off, and (d) pulsed: 1 s on; 2 s off during ramp and hold times. Reaction at 150 °C with 35 min ramp time. Scale bar ) 1 µm, ×10000 magnification.

Figure 5. Crystallization curves for NaY prepared by microwave synthesis at 100 °C using different microwave power delivery modes in a circular microwave waveguide. Continuous, pulsed: 1 s on; 2 s off and pulsed: 1 s on; 3 s off. Average steady state applied microwave power during hold time is shown.

the crystallization curves for SAPO-11 prepared by microwave synthesis at 160 °C using different microwave power delivery in a circular microwave waveguide. Pulsing the microwave power delivery has no significant effect on the nucleation time (17 min) or the crystallization rate (∼0.01 min-1). The SEM micrographs of the SAPO-11 powders synthesized using the three different microwave power delivery modes (at 0.95 relative

crystallinity) are shown in Figure 2. The SEM micrographs show that crystals of approximately twice the size are produced using continuous microwave heating compared to pulsed. The particle size was determined from measurements from the SEM micrographs, and the number of particles was estimated from the yield (Table 2). Pulsing the microwave power to maintain the reaction temperature (160 °C) produced a lower yield (1.08



TABLE 4: Nucleation Times and Crystallization Rates for the Microwave Synthesis of NaY at 100 °C Using Microwave Power Delivery (Pulsed and Continuous) in a Circular Waveguide Reactor and ACV Vessela delivery mode




yield (g)

particle size (µm)

continuous pulsed: 1 s on; 2 s off pulsed: 1 s on; 3 s off

60 60 60

0.024 0.026 0.025

21 15 13

2.08 2.05 2.11

∼0.1-0.7 ∼0.1-0.7 ∼0.1-0.8

a Yields, particle size, and estimated numbers at 0.99 relative crystallinity. Ramp time of 3 min to 100 °C. b The average microwave power fed by the generator.

Figure 6. SEM micrographs of zeolite NaY prepared by microwave synthesis at 100 °C using different microwave power delivery modes in a circular waveguide: (a) continuous, (b) pulsed 1 s on; 2 s off, and (c) pulsed 1 s on; 3 s off. Scale bar ) 1 µm, ×7500 magnification.

g versus 1.64 g) of smaller crystals (2.49 µm versus 3.77 µm) compared to continuous microwave power in the same reaction time (∼100 min) utilizing less (∆10 W) average microwave power. However, more numerous particles were produced using pulsing indicating that although the nucleation time is the same a greater number of crystals are nucleated with pulsed microwave power (6.7 × 1010 for pulsed 1 s on; 3 s off, compared to 2.9 × 1010 for continuous microwave power). This may be related to the higher instantaneous microwave power delivery and consequently higher microwave electric field during the pulse. The effect of pulsing versus continuous microwave power was also assessed on the synthesis of silicalite at 150 °C. Similarly, to the synthesis of SAPO-11, silicalite displays no significant change in the nucleation time and crystal growth rate between using continuous or pulsed microwave power delivery (see Figure 3, Table 3). However, there is a reduction in the steady state microwave power usage with pulsing, as with the synthesis of SAPO-11. The reaction temperature can be maintained by using microwave power applied only a fraction of the time, hence less energy. As shown in Table 3, the yield decreases slightly with the use of pulsed delivery; however, the average particle size and number are within error. This is

consistent with the fact that the nucleation times are the same for all microwave power delivery modes, even with using pulsed delivery during both the ramp and hold times. Figure 4 shows the SEM micrographs of the silicalite crystals obtained after the same relative percent yield. The same hexagonal “pill box” shape and crystal size is observed for all cases, with little twinning observed. Significant energy savings can be made using pulsed microwave heating compared to continuous microwave power delivery, with little effect on the crystal morphology, size, or number for silicalite synthesis. A third zeolite synthesis (NaY) was also studied to determine the influence of microwave power delivery mode on the nucleation time and crystallization rate. Synthesis was carried out at 100 °C using 12 g of the precursor gel. Figure 5 shows the crystallization curve for this synthesis using continuous and two-pulsed microwave power delivery sequences as used for the SAPO-11 and silicalite syntheses. There is no significant difference observed for the crystallization curves between continuous and pulsed. The nucleation times, crystallization rates, and particle sizes are summarized in Table 4. Pulsing the microwave power does not affect either the nucleation time or the crystallization rate. The particle sizes and morphology

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Figure 7. Crystallization curves for SAPO-11 prepared by microwave synthesis at 160 °C using different steady state applied microwave power in a monomode circular waveguide reactor and ACV vessel.

TABLE 5: Nucleation Times and Crystallization Rates for the Microwave Synthesis of SAPO-11 at 160 °C Using Different Microwave Power in a Circular Waveguide Reactor and ACV Vessela power at steady stateb (W)

ramp time (min)



yield (g)

particle size (µm)


65 130 210

8 8 8

17 17 13

0.014 0.030 0.066

1.64 1.3 1.11

3.77 ( 0.55 2.84 ( 0.41 2.04 ( 0.41

0.29 0.54 1.25

a Yields, particle size, and estimated numbers at 0.85 relative crystallinity. b The average microwave power fed by the generator. c Assuming the same density (∼2 g · cm-3) for all samples and spherical particles.

between the syntheses were determined at the same relative crystallinity point on the curve (0.99) by obtaining the SEM micrographs of the dried powders (Figure 6). The NaY zeolite samples show the same indistinct morphology with general particle size range of ∼0.1-0.7 µm. Particle size number estimates could not be undertaken in this case since the morphology is too irregular. Note that other microwave reactor engineering procedures and protocols produce uniform particle morphology.34 Similarly to the syntheses of SAPO-11 and silicalite, the average steady state microwave power used could be dramatically reduced with using pulsed microwave power delivery with no effect on the crystallization rate or particle morphology. 3.2. Magnitude of Microwave Power. Although the effect of microwave power delivery mode on the reaction rate has been determined for many reactions as described above, the effect of delivering higher microwave power level while maintaining a fixed reaction temperature via use of simultaneous cooling from an external source has not been studied for zeolite synthesis. Recent studies have indicated that homogeneous organic reactions show no significant rate enhancement with simultaneous cooling.35 Is there a similar effect for heterogeneous reactions, such as zeolite syntheses? The microwave synthesis of SAPO-11 at three steady state microwave power levels was undertaken while maintaining the reaction temperature at 160 °C as measured at the center bottom of the precursor solution. Figure 7 shows the crystallization curves for the microwave synthesis of SAPO-11 at 160 °C at microwave power of 65, 130, and 210 W fed by the microwave generator using simultaneous cooling by nitrogen gas cooled in a liquid nitrogen bath.

As shown in Figure 7, there is little change in the nucleation time (at approximately 17 min), while there is a dramatic increase in the crystal growth rate with increasing microwave power. Table 5 shows the nucleation times and crystallization rates for the three synthesis powers. The crystallization rate is four times greater with a 3-fold increase in the applied microwave power. The SEM micrographs were obtained at the same relative crystallinity (∼0.85) position on the curves, shown in Figure 8. The particles are spherical in shape, and at 210 W the particles are the smallest (2.04 µm) compared to 2.84 µm observed for the intermediate microwave power (130 W) and 3.77 µm for the lowest microwave power at 65 W (see Table 5). This consistent decrease in the particle size and the increase in the particle number with increasing microwave power are in great agreement with the results reported for the pulsed delivery experiments, where significantly more microwave power is delivered in the “on” period, versus continuous delivery modes for this material (Table 2). The high intensity microwave power seems to make smaller particles stable that would not be stable in the low microwave power. This results in decreasing the nucleation time and increasing the growth rate which consequently decrease the particle size and increase the number of particles. Again, this effect can be interpreted as due to higher microwave electric field magnitude at higher input microwave power level. The effect of increasing the applied microwave power with simultaneous cooling was also investigated for silicalite synthesis. The relative percent yield curves for silicalite synthesis at 150 °C and microwave power of 45, 90, and 190 W are shown in Figure 9. Unlike the synthesis of SAPO-



Figure 8. SEM micrographs of SAPO-11 prepared by microwave synthesis using 15 g of precursor, reacted at 160 °C using applied microwave power of (a) 65 W, (b) 130 W, and (c) 210 W, in a circular waveguide at 2.45 GHz. Scale bar ) 10 µm. ×2000 magnification.

Figure 9. Relative percent yield curves for the microwave synthesis of silicalite at various steady state applied microwave power levels and the same reaction temperature (150 °C).

11, at various applied microwave power levels with simultaneous cooling, silicalite does not show any rate enhancement with the increase in the applied microwave power at the same reaction temperature (150 °C). The nucleation times and relative percent yield rates for silicalite synthesized at various applied microwave power levels are shown in Table 6. Microwave power has no effect on the nucleation time and little influence on the yield rate in this microwave power range. Synthesis temperature and the ramp

rate appear to be the major contributing factor in silicalite microwave synthesis.36 The SEM micrographs of the silicalite powders prepared above at relative percentage yield of 0.85 are shown in Figure 10. Unlike SAPO-11, the particle size and morphology does not alter significantly with changing the applied microwave power. Furthermore, no significant differences in particle sizes or number were measured for silicalite synthesized using applied microwave power between 45 and 190 W.

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TABLE 6: Nucleation Times and Relative Percent Yield Rates for the Microwave Synthesis of Silicalite at 150 °C Using Different Applied Microwave Power in a Circular Waveguide Reactor and ACV Vessela power at steady stateb (W)


relative percent yield rate (min-1)

yield (g)

particle size (µm)


45 90 190

40 40 40

0.035 0.037 0.032

1.02 1.02 1.00

0.78 ( 0.06 0.75 ( 0.06 0.77 ( 0.08

2.28 2.56 2.32

a Yields, particle size, and estimated numbers at 0.85 relative crystallinity. b The average microwave power fed by the generator. particles measured. Assumed spherical shape and density of silicalite 1.8 g · cm-3.

c

96

Figure 10. SEM micrographs of silicalite prepared using continuous microwave heating with power (a) 45 W, (b) 90 W, and (c) 190 W. Reaction at 150 °C with 35 min ramp time. Scale bar ) 1 µm, ×10 000 magnification.

The synthesis of NaY zeolite at higher microwave power could not be achieved due to the high freezing point of the precursor compared to SAPO-11 or silicalite precursors. The solution would freeze under simultaneous cooling with nitrogen gas passed through a liquid nitrogen bath. The ratio of the microwave power at steady state, based on the microwave power when no cooling is used (the lowest microwave power), is plotted versus the crystallization (or yield) rate ratio (based on the rate of the crystallization (or yield) curve at the lowest microwave power) for SAPO-11 and silicalite synthesized at 160 and 150 °C, respectively (Figure 11). The microwave power ratio and rate ratios are also shown in Table 7. SAPO-11 displays an almost quadratic increase in rate with increase in microwave power, whereas silicalite synthesis is independent of the applied microwave power in this range. For SAPO-11, the equation that fits the rate ratio versus the applied microwave power ratio is given as follows in eq 1.

y ) 0.4259x2 - 0.2776x + 0.8517

(1)

where y is the rate ratio and x is the applied microwave power ratio.

Therefore, the rate is proportional to the square of the microwave power (eq 2).

rate ∝ (applied microwave power)2

(2)

SAPO-11 synthesis under microwave heating shows a high level of sensitivity to the microwave power delivery mode. Larger crystals are formed using continuous microwave power delivery, while use of simultaneous cooling leads to faster crystallization rates. This may be related to the significant rate enhancement observed using microwave heating compared to conventional heating. SAPO-11 synthesis is activated rapidly and spontaneously under these conditions. Furthermore, the crystallization rate shows a nonlinear increase on the microwave input microwave power. In comparison silicalite and NaY syntheses are relatively independent of microwave power delivery modes. The dielectric properties for these silicalite and SAPO-11 synthesis solutions are very similar.26 Therefore, the enhancement observed in the SAPO-11 synthesis with increasing microwave input power is likely due to nonthermal (microwave) interactions. In general, microwave heating enhances the reaction



Figure 11. Crystallization (or yield) rate ratio versus microwave power ratio curve for SAPO-11 prepared at 160 °C and silicalite synthesized at 150 °C by microwave synthesis, using different applied microwave power in a monomode circular waveguide reactor and ACV vessel. The error bars indicate the error in calculating the crystallization rate from the curve fitting of the relative crystallinity and time data as well as the fluctuation of the fed microwave power.

TABLE 7: Microwave Power Ratio and Crystallization (or Yield) Rate Ratio for Microwave Synthesis of SAPO-11 at 160 °C and Silicalite at 150 °C, Using Different Applied Microwave Power in a Circular Waveguide Reactor and ACV Vessel crystallization (or yield) rate ratiob power ratio at steady statea

SAPO-11

silicalite

1 2 3.2 4.3

1 2 4.4 -

1 1 0.9

a Ratio of generator output microwave power at steady state temperature to the microwave power at steady state for the baseline crystallization (or yield) curve (65 W for SAPO-11 and 45 W for silicalite). b Ratio of the crystallization (or yield) rate at the specific microwave power, to the crystallization (or yield) rate determined for the baseline crystallization (or yield) curve (0.014 min-1 for SAPO-11 and 0.035 min-1 for silicalite).

rate compared to conventional heating for zeolite syntheses but not for all zeolites. SAPO-11 synthesis is particularly sensitive to the microwave field conditions. These differences in the enhancement among zeolite syntheses under microwave heating could be due to the differences in their formation mechanisms. Different rate-controlling steps can be involved in the zeolite formation each of which may have different susceptibility to microwaves. Although microwave energy is not strong enough to break bonds,37,38 small perturbation in energy (or temperature) in the presence of microwaves can change the relative energy state of the reaction intermediates which could lead to enhancements in the microwave synthesis.34 Therefore, the influence of microwave exposure on the rate of zeolite formation will differ for different mechanisms and syntheses. Moreover, the mechanism is dependent on the chemical composition; in this case, SAPO-11 is a silicoaluminophosphate compared to (alumino)silicate; NaY is an (alumino)silicate; and silicalite is a silicate.

4. Conclusions The effect of the applied microwave power level and delivery mode on the synthesis of zeolites SAPO-11, silicalite, and NaY was studied, using a circular waveguide reactor. Pulsing the microwave power at a rate of 1 s on; 2 s off, or 1 s on; 3 s off, compared to continuous microwave power delivery showed no significant effect on the crystallization curve for SAPO-11, silicalite, or NaY syntheses. However, continuous microwave power heated SAPO-11 produced larger particles. Pulsing the applied microwave power has little effect on the crystallization rate compared to other parameters; however, substantial energy savings can be made using this mode of microwave power delivery. Second, the effect of the applied microwave power magnitude on the synthesis rate while maintaining the same reaction temperature was investigated for SAPO-11 and silicalite. The microwave power used to heat the SAPO-11 reaction solution to 160 °C was varied by applying cooling gas at various rates. Significant enhancement of the crystallization rate was observed with increasing the applied microwave power, with little effect on the nucleation time. The crystallization rate to the applied microwave power relation was found to obey a quadratic curve, indicating a nonlinear process. Lowering the applied microwave power produced larger crystals but increased the reaction time. Conversely, silicalite synthesis at 150 °C was found to be independent of the magnitude of the applied microwave power. Pulsing the microwave power during the zeolite synthesis can be used to save energy. However, simultaneous cooling could be a waste of energy if it does not enhance the zeolite synthesis significantly. Acknowledgment. The authors would like to thank NSF for funding this project under NIRT grant CTS-0304217 (Glenn Schrader). We would like to thank the Department of Electrical and Computer Engineering, UMass Amherst, for the waveguide equipment. M.G. would like to thank Karl Hammond for the control software programming.

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Microwave Synthesis of Zeolites: Effect of Power Delivery - The

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