synthesis of lsx zeolite by microwave heating

Synthesis of LSX zeolite by microwave heating

M.D. Romero*, J.M. Gomez, G. Ovejero, A. RodrıguezDepartment of Chemical Engineering, Facultad de Quımicas,

Universidad Complutense de Madrid, 28040 Madrid, Spain

Received 22 December 2002; received in revised form 30 October 2003; accepted 30 October 2003

Abstract

Low silica X (LSX) zeolite was synthesised by traditional and microwave heating. In both syntheses the X

zeolite showed high crystallinity and purity, low silicon/aluminium molar ratio and a potassium content per unit

cell of around 7 wt.%. Microwave heating decreased the time of synthesis respect to the traditional heating under

the same conditions. The reduction of the synthesis time was due to uniformity of the heating stage by

microwaves. Microwave heating decreased the nucleation and crystallisation time. Basic catalytic properties were

checked in the toluene alkylation with methanol. There were no differences in the catalytic activity for toluene

alkylation between both LSX zeolites.

# 2003 Elsevier Ltd. All rights reserved.

Keywords: A. Microporous materials; B. Crystal growth; C. Electron microscopy; C. X-ray diffraction; D. Catalytic properties

1. Introduction

In comparison with the broad application of acidic zeolites as solid catalysts in chemical technologymuch less attention has been paid to basic microporous materials. Unfortunately, the number of shape-selective base catalysts in industry is very limited and no industrial reactions are currently performedover basic zeolites [1]. Therefore, the preparation, characterisation and catalytic investigation ofzeolites with basic properties amenable for application in industrial reactions are of interest.

Basicity in the framework of base zeolites is linked to the partial negative charge on the oxygenatoms. This partial negative charge is due to the isomorphous substitution of silicon by aluminium inthe zeolite framework (Scheme 1). The basicity of the zeolites or the partial negative charge can beincreased with the lowering of the silicon/aluminium molar ratio and/or lowering the electronegativityof the cations that counter-balance the negative charges (Scheme 2) [2]. Other characteristic of the basicsites in base zeolites is that the oxygen atoms are fixed in the framework and just are accessible in

Materials Research Bulletin 39 (2004) 389–400

* Corresponding author. Tel.: þ34-913944115; fax: þ34-913944114.

E-mail address: [email protected] (M.D. Romero).

0025-5408/$ – see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2003.10.018

opened frameworks (12R windows). Among the zeolites that present suitable properties for use inheterogeneous basic catalysis is the low silica X zeolite (LSX). This zeolite has got low silicon/aluminium molar ratio, high ion-exchange capacity and FAU framework with 12R windows (7.4 A).

Synthesis of the LSX zeolite has been reported in several patents and papers [3–7]. In these reports,the synthesis was carried out via the hydrothermal method where the heat transfer is from an oil bath oran electrical oven to the reagents (‘‘traditional heating’’). Recently, several reports about synthesis ofzeolites employing others sources of heating such as microwave radiation have been presented; nonehowever have considered the synthesis of LSX zeolite [8–12]. The use of microwave radiation in thesynthesis of zeolites offers advantages over the conventional synthesis including: a higher heating rate,due to volumetric heating, resulting in homogeneous nucleation, fast dissolution of precipitated gels,and eventually, a shorter crystallisation time [13]. Furthermore, it is a clean and economical heatingsystem [14]. The energy transformation from microwave irradiation may occur through differentmechanisms, though usually are through ionic conduction and dipole rotation. In the synthesis ofzeolites, ionic conduction can be affected by the presence of alkali metals (Naþ, Kþ) and dipolerotation by the presence of water molecules [15]. The effect of microwave radiation absorption in thesynthesis could be due to the different mechanisms of heat transfer to the reagents or some specificactivating effect (non-thermal) on the reagent molecules [14]. The action of microwave heating is stillunder debate.

In this work, the effect of microwave irradiation in the synthesis of X zeolite has been studied. The‘‘fast’’ preparation of LSX zeolites via syntheses employing ‘‘traditional’’ heating and microwaveirradiation is reported. These materials were employed as catalysts in the alkylation of toluene in orderto understand their acid–base properties.

Scheme 1. Partial negative charge due to the isomorphous substitution of silicon by aluminium in the zeolite framework.

Scheme 2. Increasing of the partial negative charge on the oxygen atoms due to the substitution of sodium cation by cesium

cation.

390 M.D. Romero et al. / Materials Research Bulletin 39 (2004) 389–400

2. Experimental

2.1. Materials

Sodium silicate (Na2SiO3), sodium and potassium hydroxide (NaOH and KOH) from Merck andsodium aluminate (NaAlO2) from Carlo Erba were employed in the synthesis of the X zeolites . Waterwas purified (18.2 MO) using a Milli Q plus 185 instrument. Commercial X zeolite was obtained fromAldrich.

The chemicals used for the catalytic alkylations were toluene, methanol, ethylbenzene and o-xylenefrom Merck. Styrene and p-xylene were obtained from Fluka.

2.2. Synthesis

Syntheses of LSX zeolites were carried out according to the method of Kuhl [5] with somemodifications. Molar ratios in the mixture gel were SiO2/Al2O3 ¼ 2:2, (Na2O þ K2O)/SiO2 ¼ 3:25,Na2O/ðNa2O þ K2OÞ ¼ 0:77 and H2O/ðNa2O þ K2OÞ ¼ 17. Sodium aluminate was dissolved in waterand then the sodium and potassium hydroxides were added to the solution. Finally, sodium silicate wasslowly added to the NaAlO2–NaOH–KOH solution. Three zeolites, NaKX-323, NaKX-343 and NaKX-373, were prepared at 323, 343 and 373 K, respectively. Aging and crystallisation stages were carriedout at the same temperature. The solid product was filtered, washed with 0.01 M NaOH aqueoussolution to avoid protonation and dried at 373 K overnight.

Synthesis of LSX zeolite was carried out employing two heat sources: oil thermostatic bath andmicrowave heating, hereafter referred to as ‘‘traditional’’ and ‘‘microwave’’ heating, respectively.

The time of synthesis was recorded from the time the reactor was placed in the oil bath atthe synthesis temperature or, alternatively from when the heat program of the microwave oven wasstarted.

2.3. Installation

The syntheses were carried out in two different installations depending on the kind of heating. Theinstallation for the ‘‘traditional’’ heating was built entirely of glass. This comprised of a 1 L, five-necked round bottomed and cylindrical reactor equipped with reflux condenser, thermocouple,mechanical stirrer, inlet pipe for air and outlet pipe to take the samples. This reactor was placed in anoil thermostatic bath. The equipment for the microwave heating was a microwave oven MLS-1200MEGA (Milestone) with control of the temperature and with continuous microwave radiation (250 W,2450 MHz). The reactor was an autoclave made in tetrafluoromethaxil (TFM).

2.4. Characterization

BET surface areas of the zeolites were determined using a MICROMERITICS ASAP-2010instrument. X-ray diffraction (XRD) patterns were recorded on a PHILIPS diffractometer (X’PERTMPD) with Cu Ka radiation and Ni filter. The scanning range of 2y was set between 58 and 508 with astep size of 0.18. Chemical composition was determined by means of X-ray fluorescence (XRF) in aPHILIPS PW-1480 instrument. 27Al and 29Si MAS NMR spectra were obtained on a Varian VXR-300

M.D. Romero et al. / Materials Research Bulletin 39 (2004) 389–400 391

spectrometer equipped with a Jacobsen probe. Scanning electron micrographs were performed in theLuis Bru Electronic Microscopy Centre with a microscope JEOL, JSM-6400 model.

2.5. Crystallinity

Crystallinity (XC) was calculated from equation 1 according to the method developed by Hermansand Weidinger [16], and applied by Costa et al. [17] in studies of X and Y zeolites. In this method, thecrystallinity is calculated from the ratio between the intensity of the diffraction peak at 2y of 26.658 (IC)and the intensity of the amorphous halo at 2y of 28.58 (IA) at different times of synthesis. Linear fit(Eq. (2)) of theses values (IC and IA) allows I100C (100% crystalline) and I100A (100% amorphous)values to be obtained and consequently the crystallinity of the X zeolite (XC) using Eq. (1).

XC ¼ IC

I100C

(1)

IC ¼ I100C � I100C

I100A

� �IA (2)

2.6. Catalytic reaction

Alkylation of toluene with methanol was carried out as a catalytic test to examine the acid–baseproperties of the LSX zeolites. The yield was expressed as moles of product/moles of fed methanol.Toluene alkylation yields different products depending on the acid–base character of the catalysts(Scheme 3). Styrene and ethylbenzene corresponding to the side chain alkylation are the main productswhen the reaction is carried out over a basic site. On the other hand, acid sites promote the ringalkylation to give xylenes as reaction products. The catalytic reaction was carried out at atmosphericpressure in a fixed bed reactor under a nitrogen atmosphere. Previously, the zeolites were calcined at723 K in nitrogen stream for 1 h, and cooled to the reaction temperature (698 K). Samples from the

CH3

TOLUENEACID

BASIC

STYRENE ETHYLBENZENE

CH3

CH3

XYLENES

CH CH2 CH2 CH3

Scheme 3. Toluene alkylation.


reaction were analysed by gas chromatography in a Varian 3400GC equipped with a capillary column(60 m) and flame ionisation detector (FID).

Toluene alkylation over X zeolite supplied by Aldrich (silicon/aluminium molar ratio of 1.3) wascarried out to compare the catalytic activity of the as-synthesised zeolites with commercial X zeolite.

3. Results and discussion

Fig. 1 shows the XRD patterns obtained for samples synthesised at different temperatures. X zeolitewas the product of synthesis when the temperature was 323 or 343 K, whereas hydroxysodalite wasformed at 373 K. Fig. 2 shows the evolution of crystallinity for the zeolites prepared at 323 and 343 K.Almost 100% crystallinity of the X zeolite was obtained at 343 K after 15 h. The same degree ofcrystallinity was achieved after 32 h when the temperature was lowered to 323 K. In both zeolites thecrystallinity stabilised even when longer heating times were applied.

Table 1 shows the silicon/aluminium molar ratio and the sodium and potassium content in thezeolites. These results were obtained by XRF analysis and show a low silicon/aluminium molar ratiowith the same amount of sodium and potassium in the X zeolites (NaKX-323 and NaKX-343). TheNaKX-373 zeolite showed different sodium and potassium content since the main phase washydroxysodalite.

Silicon/aluminium molar ratio was in accordance with the 29Si MAS NMR measures as shown inFig. 3. In this analysis only one 29Si resonance peak was observed at �84 ppm (Si(4Al)) with a littleshoulder at �88 ppm (Si(3Al)). The silicon/aluminium molar ratio determined by 29Si MAS NMR [18]was 1.03 for the X zeolites. Likewise, 27Al MAS NMR spectra showed a single resonance peakat 58 ppm assigned to aluminium atoms tetrahedrally coordinated. Therefore, there was not extra-framework aluminium.

5 10 15 20 25 30 35 40 45 50

Hydroxysodalite

323K

343K

373K

Inte

nsi

ty (

u.a

.)

2θ

Fig. 1. DRX pattern of as-synthesised zeolites at different temperatures: 323 K (NaKX-323), 343 K (NaKX-343) and 373 K

(NaKX-373).


Fig. 4A shows the scanning electronmicrographs of the NaKX-343 zeolite. The morphology and thesize in the X zeolites (NaKX-323 and NaKX-343) were comparables. Both zeolites crystallised asspherical particles with similar average particle sizes (4:5 � 0:2 mm).

The reactor material (TFM) for synthesis with microwave irradiation differs from the material of thereactor in the traditional heating (glass). Thus, the influence of the reactor material on the synthesis ofX-zeolite was studied by the ‘‘traditional’’ heating method using a microwave reactor under reactionconditions for the NaKX-343 zeolite. XRD pattern shows (Fig. 5) that the solid product was X zeolitewith high crystallinity and no impurities. The silicon/aluminium molar ratio measured by XRF analysiswas 1.1. Therefore, the material of the reactor had no influence on the synthesis of X zeolite under theseconditions.

0 5 10 15 20 25 30 35 40 45 500

20

40

60

80

100

323K

343K

Cri

stall

init

y (

%)

Time (h)

Fig. 2. Growth curves for the X zeolite synthesised with traditional heating at: (~) 323 K (NaKX-323) and (&) 343 K

(NaKX-343).

Table 1

Characteristics of the as-synthesised zeolites

Zeolite Heating Si/Ala (molar) Naa (wt.% u.c.) Ka (wt.% u.c.) BET areab (m2 g�1)

NaKX-323c Traditional 1.07 12.1 6.6 835

NaKX-343d Traditional 1.08 12.0 7.0 830

NaKX-373e Traditional 1.07 16.5 2.9 36

NaKX-343f Microwave 1.09 11.7 6.7 790

NaKX-343g stirring Traditional 1.07 12.2 6.6 833

wt.% u.c.: weight percentage per unit cell.a Composition determined from XRF data.b Calculated from N2 adsorption data.c Sample at 32 h.d Sample at 15 h.e Sample at 4 h.f Sample at 8 h.g 250 r.p.m.


The effect of the heat source was studied by carrying out the synthesis with microwave heatinginstead of traditional heating. The synthesis conditions were the same as those for synthesis of theNaKX-343 zeolite. The product was X zeolite with high crystallinity and purity, low silicon/aluminiummolar ratio (1.09) and potassium content per unit cell around 7 wt.%. The X zeolite in this synthesisshowed the same composition and structural characteristics as that synthesised by traditional heating(Table 1). There was no difference in the final product (X zeolite) when either traditional or microwaveheating were used.

Fig. 6 shows the evolution of crystallinity with time corresponding to zeolites prepared by traditionaland microwave heating. It can be observed that microwave heating reduced the time to synthesise Xzeolite. With this heat source X zeolite with high crystallinity was obtained after 8 h whereas withtraditional heating the same degree of crystallinity was achieved after 13–14 h. The reduction in thesynthesis time could be due to the characteristics of the heat source. In microwave heating, the mixturegel absorbs the microwave radiation directly and uniformly and therefore, the temperature is increasedquickly within the entire bulk [13]. On the other hand, the transfer of heat from an oil bath to themixture gel placed inside the reactor (traditional heating) is by convection and conduction. In theseconditions, the increase in the temperature is slower and less uniform than in the microwave heatingdue to the presence of a temperature gradient.

The time to reach the temperature of synthesis was 30–40 min by traditional heating and 1 min bymicrowave heating. However, the time difference between both kinds of heating to obtain the X zeolitewas around 6 h (Fig. 6). Therefore, the effect of the heating rate in lowering the synthesis time was notimportant and the main influence must be the uniform heating.

The effect of the microwave radiation can be observed in Fig. 7 which shows the crystallisation rate(differential of the crystallinity curve respect to the time) for both kinds of heating. The crystallisationperiod was considered to occur once the crystallisation rate was above that value signalled in Fig. 7(shaded area) corresponding to the rate at 3% of crystallinity. The nucleation period took place beforethe crystallisation when the crystallisation rate was below this value. The time required to start thecrystallisation (nucleation period) was 2.5 h by microwave heating and 5 h by traditional heating.

-65 -70 -75 -80 -85 -90 -95 -100 -105

29

Si MAS NMR

-88

-84

δ ppm

Fig. 3. 29Si MAS NMR spectrum of the NaKX-343 zeolite.


Consequently, the difference of time in the nucleation period between both syntheses was ca. 2.5 h. Onthe other hand, the crystallisation period was ca. 5 h by microwave heating and around 7.5 h bytraditional heating; in this case the time difference was also ca. 2.5 h. Therefore, heating by microwaveirradiation produced an increase in the nucleation and crystallisation rate. The time saved in thesynthesis of X zeolite by microwave heating was around 5.5 h and was due to higher heating (�0.5 htime saved), nucleation (�2.5 h time saved) and crystallisation (�2.5 h time saved) rates.

Fig. 4. SEM micrographs of the NaKX-343 X zeolite synthesised by: (A) traditional heating without stirring, (B) microwave

heating, (C) traditional heating with stirring (250 rpm).


Scanning electronmicrographs of the zeolite synthesised with microwave heating (Fig. 4B) shows theother effect of uniform heating. X zeolite crystallised in spherical particles with the same morphology asthat of traditional heating, but with lower particle size. In this case, the average size was 2:8 � 0:1 mm.

In order to prove that uniform heating via microwaves is the cause of the faster synthesis times, thesynthesis of X zeolite was carried out with traditional heating employing stirring to decrease thetemperature gradient. Synthesis conditions were the same as for NaKX-343 zeolite preparation. In thesesyntheses, the stirring speed was varied in the range 250–750 rpm (Table 2). In all cases, the product

10 15 20 25 30 35 40 45 50

Glass

(Synthesis time 15h)

TFM

(Synthesis time15h)

Inte

nsi

ty (

a.u

.)

2θ (degree)

Fig. 5. XRD pattern of the NaKX-343 X zeolite synthesised in microwave reactor (TFM) and traditional reactor (glass).

0 3 6 9 12 15 18

0

20

40

60

80

100

Traditional heating

Microwave heating

Cry

stal

linit

y (%

)

Time (h)

Fig. 6. Growth curves for the NaKX-343 X zeolite synthesised by: (*) traditional heating and (^) microwave heating.


was the same as with quiescent synthesis: X zeolite without impurities, high crystallinity, low silicon/aluminium molar ratio (1.08) and potassium content per unit cell ca. of 7 wt.% (Table 1).

Table 2 shows the synthesis time due to stirring with respect to synthesis under quiescent conditions(no stirring). Synthesis time decreased as the stirring speed was increased. When the stirring speed was750 rpm it took only 4.5 h to obtain X zeolite with ca. 100% crystallinity. On the other hand, almost 10 hmore was necessary to obtain the same crystallinity in the absence of stirring. In Table 2 it can be seenthat low stirring speed (250 rpm) produced a large reduction in the synthesis time (6 h) with respect to thequiescent synthesis. The temperature during syntheses with stirring was more uniform across the bulkreaction mixtures—the same effect that was obtained with microwave heating. In fact, the time ofsynthesis by traditional heating and a stirring rate of 250 rpm was the same as that obtained by microwaveheating. The main difference between the syntheses with and without stirring was in the morphology ofthe crystals (Fig. 4C). X zeolite synthesised with stirring did not show the spherical particles observed inthe synthesis without stirring (traditional and microwave heating). In this case, crystalline clusterswithout definite shape and smaller particle size than in the synthesis without stirring were formed.

Finally, toluene alkylation with methanol was carried out over the as-synthesised zeolites to studytheir acid–base properties. Fig. 8 shows the yield of products in the toluene alkylation. In this figure,‘‘traditional’’ heating includes the samples synthesised with and without stirring because the results

Fig. 7. Crystallinity rate in the NaKX-343 X zeolite (differential of the crystallinity growth curve). Crystallisation period

starts: in ( ) for the microwave heating and in ( ) for the traditional heating.

Table 2

Influence of the stirring speed in the synthesis time

Stirring speed (rpm) Time (XC ¼ 100%, h) Time lowering (h)

0 14 0

250 8 6

500 5 9

750 4.5 9.5

Microwaves 8 6

Synthesis conditions: NaKX-343 zeolite.


were similar. As-synthesised X zeolites with traditional and microwave heating showed similarcatalytic activities. In both zeolites, the yields of ethylbenzene, styrene and xylenes were comparable.On the other hand, the commercial X zeolite showed more catalytic activity, but the main products werexylenes (acid alkylation) being lower the yield of styrene and ethylbenzene (basic alkylation).

The acid–base character of the zeolite can be determined from the molar ratio between the yield ofside chain alkylation products (ethylbenzene and styrene) and the yield of ring alkylation products(xylenes). This ratio, denominated in this work as BA/AA molar ratio (basic alkylation/acid alkylation),is showed in Fig. 9. If the BA/AA molar ratio is lower than 1 then the acid activity (xylenes)

Fig. 8. Toluene alkylation with methanol: ‘‘traditional’’ heating (NaKX-343), microwave heating (NaKX-343) and

commercial X zeolite. Reaction conditions: reaction time:1 h; W/F:25 g h mol�1; TR:698 K; toluene/methanol molar ratio: 5;

nitrogen/reactants molar ratio: 5.

Fig. 9. Yield to side chain/yield to ring (BA/AA) molar ratio in the toluene alkylation with methanol: ‘‘traditional’’ heating

(NaKX-343), microwave heating (NaKX-343) and commercial X zeolite.


predominates in the reaction and when it is greater than 1 the basic activity (styrene and ethylbenzene)predominates. The BA/AA molar ratio for the commercial X zeolite was near 0, showing high acidactivity in toluene alkylation. However, in both zeolites as-synthesised (traditional and microwaveheating) the BA/AA molar ratio was around 0.5. The acid activity remained predominant but their basicproperties were improved with respect to the commercial X zeolite.

4. Conclusions

LSX zeolite can be synthesised at low temperature employing microwave heating in a short period oftime (�8 h) without stirring. The product obtained with microwave heating showed the same structural(crystallinity, silicon/aluminium molar ratio and potassium content) and catalytic properties than onesynthesised with traditional heating but with lower particle size. The effect of the microwave radiationin the synthesis of X zeolite is only thermal due to the more uniform heating. In traditional heating, astirring speed of 250 rpm was necessary to obtain LSX zeolite in the same time as it took by microwaveheating. In this case, crystalline clusters without definite shape were formed whereas in the synthesis bymicrowaves spherical crystalline clusters were obtained.

Acknowledgements

This work was conducted with the financial support of the Research Project CICYT QUI98-0695/97(no. 7610) and a grant from the CAM (Comunidad Autonoma of Madrid). The assistance of Mr. DavidO’ Donoghue, CES Department, University of Limerick, Ireland is gratefully acknowledged.

References

[1] J. Weitkamp, M. Hunger, U. Rymsa, Microporous Mesoporous Mater. 48 (2001) 255–270.

[2] D. Barthomeuf, Catal. Rev.-Sci. Eng. 38 (1996) 521.

[3] G. Kuhl, Zeolites 7 (1987) 451–457.

[4] E.I. Basadella, J.C. Tara, Zeolites 15 (1995) 243–246.

[5] H. Li, J.N. Armor, Microporous Mater. 9 (1997) 51–57.

[6] K. Taizo, Y. Eiji, Japan Patent 10053409 (1998).

[7] S. Yoshinori, F. Hajime, USA Patent 5993773 (1999).

[8] A. Arafat, J.C. Jansen, A.R. Ebaid, H. van Bekkum, Zeolites 13 (1993) 162–165.

[9] J.P. Zhao, C. Cundy, J. Dwyer, Studies in Surface Science and Catalysis, vol. 105, 1997.

[10] Z. Pilter, S. Szabo, M. Hasznos-Nezdei, E. Pallai-Varsanyi, Microporous Mesoporous Mater. 40 (2000) 257–262.

[11] K. Jansen, Verified Synthesis of Zeolitic Marerials, in: H. Robson (Ed.), Elsevier, Amsterdam, 2001.

[12] M.A. Uguina, D.P. Serrano, R. Sanz, E. Castillo, in: M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins (Eds.),

Proceedings of the 12th IZC, vol. 3, 1998, 1971.

[13] C.S. Cundy, Collection Czech. Chem. Comm. 63 (1998) 1699–1723.

[14] A. Fini, A. Breccia, Pure Appl. Chem. 71 (4) (1999) 573–579.

[15] B.I. Whittington, N.B. Milestone, Zeolites, vol. 12, 1992.

[16] W. Hermans, T. Weidinger, Makrom. Chem. 24 (1961) 44.

[17] E. Costa, M.L. Gutierrez, M.A. Uguina, An. Quim. 76 (1980) 276–281.

[18] E. Lippmaan, M. Magui, A. Samosan, G. Engelhardt, M. Tarmak, J. Am. Chem. Soc. 103 (1981) 4992.


synthesis of lsx zeolite by microwave heating

Documents