Microporous and Mesoporous Materials 144 (2011) 162–170

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Microporous and Mesoporous Materials

journal homepage: www.elsevier .com/locate /micromeso

Comparison of dealumination of zeolites beta, mordenite and ZSM-5by treatment with acid under microwave irradiation

María Dolores González, Yolanda Cesteros ⇑, Pilar SalagreDepartament de Química Física i Inorgànica, Universitat Rovira i Virgili, C/Marcel�lí Domingo s/n, 43007 Tarragona, Spain

a r t i c l e i n f o

Article history:Received 9 February 2011Received in revised form 31 March 2011Accepted 8 April 2011Available online 14 April 2011

Keywords:MicrowavesDealuminationBetaZSM-5Mordenite

1387-1811/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.micromeso.2011.04.009

⇑ Corresponding author. Tel.: +34 977558785; fax:E-mail address: yolanda.cesteros@urv.cat (Y. Ceste

a b s t r a c t

Commercial mordenite, beta and ZSM-5 zeolites were partially dealuminated in HCl medium in autoclaveby conventional heating or under microwave irradiation at 373 K for 15 min. The extent of dealuminationwas function of the zeolite structure (beta > mordenite > ZSM-5), and the heating method used. Micro-waves led to faster dealumination than conventional heating for the three zeolites. Besides, the use ofmicrowaves affected the surface and acidic properties of the resulting dealuminated samples.

� 2011 Elsevier Inc. All rights reserved.

1. Introduction

Zeolites are well known microporous materials widely used ascatalysts in petrochemical industry due to their large surface areas,shape selectivity, and controllable acidity [1,2]. Dealumination ofzeolites are useful to reduce acid site concentration, improve ther-mal stability, and modify pore structure [1–3]. There are severalfactors that influence zeolites dealumination, such as the zeolitesynthesis conditions, its structure type, and the dealuminationtreatment method.

During crystallisation, there is a preferred Si/Al ratio in theframework of every structure type. When the Si/Al ratio of the syn-thesis gel deviates from the ideal ratio, extraframework aluminiumspecies (EFAl) could be formed, or the distribution of the alumi-nium atoms in the framework could be inhomogeneous [1]. Thiscan affect its behaviour towards dealumination.

Different zeolite structure types, such as beta, mordenite,ZSM-5, ferrierite, zeolite Y, zeolite 4A, are known to exhibit verydifferent behaviour with respect to dealumination [4–7]. Thearrangement and size of the pores influence the accessibility ofthe aluminium atoms in the framework. Zeolite beta, for example,has a three-dimensional 12-ring pore system (straight channels ofdiameter 6.6 � 6.7 Å and sinusoidal channels of diameter5.6 � 5.6 Å) and, because of this property, its framework is veryflexible. Zeolite mordenite has a one-dimensional pore system

ll rights reserved.

+34 977559563.ros).

with main channels of diameter 6.7 � 7.0 ÅA0

and compressedchannels of diameter 2.6 � 5.7 Å whereas ZSM-5 has a three-dimensional 10-ring pore system with channels of diameter5.1 � 5.5 ÅA

0

. Both these structures are less flexible than beta, andconsequently, it is more difficult to dealuminate them. In addi-tion, zeolite beta crystallises with many stacking faults [8] whilemordenite samples, although less frequently, may also havestructurally related stacking faults [9]. Stacking faults increasethe probability of the presence of defect sites in the framework.Also, the number of the T-atoms in four-rings may have an influ-ence on the stability towards dealumination because the tensionin the smaller rings is larger. Thus, a zeolite is easier to dealumi-nate as many aluminium atoms has in an environment with ten-sion [4].

A considerable number of zeolite dealumination techniqueshave been developed. We found dealumination studies by treat-ment of zeolites with steam or SiCl4 vapour at elevated tempera-tures or treatment with (NH4)SiF6, mineral acids (i.e., HCl, HNO3),organic acids (i.e., acetic acid, oxalic acid), F2, chelating agents(i.e., EDTA), etc. [4–7,10–16]. Conventional heating is used whenapplying temperature during dealumination.

Nowadays, microwave irradiation is being applied for the dry,synthesis, and cation-exchange of zeolites [17–19]. The use ofmicrowaves considerably decreases the preparation times, withthe subsequent energy saving, and modifies the samples proper-ties. Therefore, microwave syntheses constitute valuable processesin Green Chemistry. There are only two references about the use ofmicrowaves for dealumination of zeolites [20,21].

Table 1Characterization of samples by XRF, XRD, and FTIR techniques.

Samples Si/Al(XRF)

Crystallinitya

(%)Unit cell volumea

(Å3)IR bands(cm�1)b

t1 t2

M 6.5 100 2791 1068 629MA 11.2 73 2737 1091 641MMW 15.8 70 2713 1084 635B 10.0 100 – 1068 629BA 110.8 62 – 1091 641BMW 121.9 69 – 1084 635BA5 84.7 87 – 1094 626BAMW5 98.9 78 – 1096 632Z 20.0 100 5209 1063 797ZA 21.3 99 5125 1096 797ZMW 22.4 100 5192 1097 797

M.D. González et al. / Microporous and Mesoporous Materials 144 (2011) 162–170 163

In a previous paper, we observed that the use of microwaves formordenite dealumination in acid medium resulted in faster dealu-mination than when using conventional heating. The sampletreated by autoclaving under microwave irradiation at shorter time(15 min), presented active acid centres with medium strength, andlower amounts of strong Lewis acid sites than the rest of samples[20].

The present work aims to extend the investigation of the use ofmicrowaves during dealumination in HCl medium in autoclave toother two zeolite structures: ZSM-5 and Beta. Dealuminationexperiments were performed at short treatment time (15 min) tocompare with the mordenite dealumination results, some of whichhave been also included here. Zeolites were also dealuminated inacid medium by conventional heating in autoclave under identicalconditions for comparison.

a Calculated from XRD patterns.b Frequencies of the main asymmetric stretch (t1), and the main symmetric

stretch (t2) due to the T–O bond (T = Si, Al).

2. Experimental

2.1. Preparation of dealuminated samples

Na-Mordenite (Zeolyst, Si/Al = 6.5, CBV 10A Lot No. 1822-50),Na-Beta (Zeochem, Si/Al = 10, PB Lot No. 6000186), and Na-ZSM-5 (Zeochem, Si/Al = 20, PZ-2/40 Lot No. 6002827-01), designatedas M, B and Z, respectively, were treated with HCl 6 M undermicrowave irradiation (Milestone ETHOS-TOUCH CONTROLequipped with a temperature controller) at 373 K for 15 min (sam-ples MMW, BMW and ZMW, respectively), and in the case of Beta,also for 5 min (sample BMW5). Autoclaves were magnetic stirredand the rotor turned on while the microwave equipment wasworking in order to maximize the homogeneity of heating and toavoid local hotspots. Besides, these zeolites were also acid-treatedby autoclaving in a conventional oven at the same temperature andtimes (samples MA, BA, ZA, and BA5). Then, all samples werewashed several times with deionized water, and dried in an ovenovernight.

-300-1000200400f1 (ppm)

-300-1000200400f1 (ppm)

c

a

Fig. 1. 27Al NMR spectra of samples: (a)

2.2. Elemental analyses

Elemental analyses of the samples were obtained with a PhilipsPW-2400 sequential XRF analyser with Phiplips Super Q software.All measures were made in triplicate.

2.3. X-ray diffraction (XRD)

Powder X-ray diffraction patterns of the samples were obtainedwith a Siemens D5000 diffractometer using nickel-filtered Cu Karadiation. Samples were dusted on double-sided sticky tape andmounted on glass microscope slides. The patterns were recordedover a range of 2h angles from 5� to 40� and crystalline phases wereidentified using the joint committee on powder diffraction stan-dards (JCPDS) files (43-0171, 48-0074, 37-359 corresponds to

-300-1000200400f1 (ppm)

d

-300-1000200400f1 (ppm)

b

MA, (b) MMW, (c) ZA and (d) ZMW.

Lin

(Cou

nts)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

2-Theta - Scale6 10 20 30

abc

def

ghi

Fig. 2. XRD patterns of the samples: (a) M, (b) MA, (c) MMW, (d) B, (e) BA, (f) BMW, (g) Z, (h) ZA and (i) ZMW.

Table 2Characterization of samples by nitrogen physisorption.

Samples BET area(m2/g)

Micropore area/non-microporearea ratio

Pore volume(cm3/g)

M 303 8.9 0.059MA 376 6.1 0.096MMW 419 5.9 0.110B 573 1.8 0.228BA 554 1.8 0.244BMW 451 1.5 0.242BA5 527 1.6 0.322BAMW5 507 1.5 0.359Z 300 2.4 0.063ZA 306 2.0 0.072ZMW 300 1.7 0.073

164 M.D. González et al. / Microporous and Mesoporous Materials 144 (2011) 162–170

mordenite, beta and ZSM-5, respectively). For mordenite, cellparameters were calculated from (2 0 0), (0 2 0) and (2 0 2) peaks,and for ZSM-5 from (2 0 0), (0 2 0), (0 0 2) and (�1 0 3) peaks, usinga matching profile with WIN FIT 1.2 software. Crystallinity of themodified mordenites was determined by comparing the sum ofthe peak areas of (1 5 0), (2 0 2), (3 5 0) and (4 0 2) (22–32� 2h)with respect to commercial Na-mordenite. Crystallinity of themodified ZSM-5 samples was calculated using the (0 5 1) peakintensity compared with the parent zeolite sample. The integratedintensity of the signal at 2h = 22.4� was used to evaluate the crys-tallinity of beta samples.

2.4. FTIR

Infrared spectra were recorded on a Bruker-Equinox-55 FTIRspectrometer. The spectra were acquired by accumulating 32 scansat 4 cm�1 resolution in the range of 400–4000 cm�1. Samples wereprepared by mixing the powdered solids with pressed KBr disks ina ratio of 5:95 and dried in an oven overnight.

2.5. Nitrogen physisorption

BET areas were calculated from the nitrogen adsorption iso-therms at 77 K using a Micromeritics ASAP 2000 surface analyserand a value of 0.164 nm2 for the cross-section of the nitrogen mol-ecule. Samples were pretreated in vacuum at 573 K for 6 h. Poresize distribution of micropores and meso-macropores were deter-mined from isotherms using the Horvath–Kawazoe method andthe BJH method, respectively.

2.6. Scanning electron microscopy (SEM)

This technique was used to observe the morphology andparticle sizes of the samples. Experiments were performed on ascanning electron microscope, JEOL JSM6400, operating at acceler-ating voltage of 25 kV and work distances of 10 mm, and magnifi-cations of 10,000�.

2.7. 1H MAS NMR and 27Al MAS NMR

1H NMR and 27Al NMR spectra were obtained with a VarianMercury Vx 400 MHz with a probe of 7 mm CPMAS at a frequencyof 400 MHz by spinning at 5 kHz. The pulse duration was 2 ls and

the delay time was 5 s. The chemical shift reference was trimethylsilil-3 propionic acid d4-2,2,3,3 sodium salt for 1H NMR, and highpurity aluminium nitrate for 27Al NMR.

2.8. Catalytic activity determination

Isomerization of styrene oxide, and styrene oxide ring-openingreactions were carried out in the liquid phase at atmospheric pres-sure at room temperature. Catalytic experiments were performedusing 20 ml of solvent (toluene or ethanol, respectively), 0.8 g ofcatalyst (for mordenite catalysts) or 0.4 g (for beta and ZSM-5 cat-alysts), and 0.48 ml of styrene oxide. A lower catalyst amount wasused when testing beta and ZSM-5 catalysts to decrease theirconversion values below 100%, observing better the differences inthe selectivity values. The reaction products, taken at 3 h ofreaction, were analysed by GC on a Shimadzu GC-2010 instrumentequipped with a 30 m capillary column DB-1 coated withphenylmethylsilicon and a FID detector.

3. Results and discussion

We observed dealumination for all the acid-treated zeolitessince they showed higher framework Si/Al ratio, lower cell vol-umes, and a shift to higher values of the IR bands assigned to sym-metric and asymmetric stretching of the T–O bond (T = Si, Al) thantheir corresponding commercial ones (Table 1). The increase of thestrength of the T–O bond when the Al content decreases was ex-

00.020.040.06

0.080.1

0.12

10 100 1000

Pore

vol

ume

(cc/

g)

Pore diameter (Å)

M MA MMW

0

0.1

0.2

0.3

0.4

0.5

10 100 1000

Pore

vol

ume

(cc/

g)

Pore diameter (Å)

B BA BMW

0

0.05

0.1

0.15

10 100 1000

Pore

vol

ume

(cc/

g)

Pore diameter (Å)

Z ZA ZMW

0

0.03

0.06

0.09

5 10 15 20

Por

e vo

lum

e (c

c/g)

Pore diameter (A)

0

0.01

0.02

0.03

0.04

0.05

0.06

5 10 15 20

Pore

vol

ume

(cc/

g)

Pore Diameter (A)

0

0.001

0.002

0.003

0.004

0.005

0.006

5 10 15 20

Por

e V

olum

e (c

c/g)

Pore Diameter (A)

Fig. 3. Micropore and mesopore size distribution graphics for all samples.

M.D. González et al. / Microporous and Mesoporous Materials 144 (2011) 162–170 165

plained by the fact that Si–O bond is shorter than the Al–O bond,and Al has lower electronegativity than Si [22]. The extent of dea-lumination was function of the zeolite structure and the heatingmethod used. Thus, beta zeolite was easier to dealuminate thanmordenite, whereas dealumination of ZSM-5 was very low. Thisorder can be related to the flexibility of each zeolite framework,and the accessibility of the aluminium atoms depending on the

Fig. 4. Scanning electron micrographs of s

Fig. 5. Scanning electron micrographs of

pores arrangement and sizes, according to the results reported byother authors [4,8,9]. Interestingly, the samples treated with acidunder microwave irradiation showed higher dealumination thanthe samples dealuminated in a conventional oven. In the case ofBeta zeolite, where very high Si/Al ratio values were obtained afterdealumination for 15 min, this effect was confirmed by dealumi-nating commercial beta at shorter heating time (5 min) since,

amples: (a) M, (b) MA and (c) MMW.

samples: (a) B, (b) BA and (c) BMW.

Fig. 6. Scanning electron micrographs of samples: (a) Z, (b) ZA and (c) ZMW.

024681216f1 (ppm)

0.0

0.5

1.0

1.5

2.0

2.5

3.04.58

-5051015f1 (ppm)

0.0

0.5

1.0

1.5

2.0

2.5

3.05.11

-10-505101520f1 (ppm)

0.0

0.5

1.0

1.5

2.0

2.5

3.05.07

a

b

c

Fig. 7. 1H NMR spectra of samples: (a) M, (b) MA and (c) MMW.

166 M.D. González et al. / Microporous and Mesoporous Materials 144 (2011) 162–170

again, the Si/Al ratio of the microwaved sample (BMW5) was high-er than the Si/Al ratio of the sample conventionally heated (BA5)(Table 1).

27Al NMR spectra of commercial zeolites (not shown here),showed tetrahedral Al for commercial mordenite, tetrahedral Alfor commercial ZSM-5, and both tetrahedral Al and octahedral Al(this later in low amount) for commercial beta zeolite. The peakscorresponding to tetrahedral and octahedral aluminium appeararound 50 ppm and 0 ppm, respectively. The presence ofoctahedral Al in commercial Beta can be attributed to extraframe-work aluminium species or to aluminium coordinated in defectsites taking into account the characteristic stacking faults of thiszeolite structure [8,23]. 27Al NMR spectra of modified mordeniteand modified ZSM-5 samples showed octahedral Al, in higher rel-ative amounts for mordenite samples, confirming dealumination(Fig. 1). For acid-treated ZSM-5 samples, we can conclude thatalthough some aluminium was extracted from the frameworkupon acid treatment, dealumination was not very efficient. It isimportant to note the higher relative amounts of octahedral alu-minium for the samples treated under microwave irradiation(ZMW and MMW) with respect to those dealuminated under auto-clave at the same conditions (ZA and MA). For beta zeolites (notshown here), we observed a considerably decrease both in the tet-rahedral and octahedral Al when compared with commercial Betadue to the high dealumination underwent.

The acid and heating conditions used here did not cause drasticchanges in the zeolite structures (Fig. 2), although there was somedecrease in the crystallinity of the mordenite and beta zeolitesafter acid treatment (Table 1). The crystallinity of ZSM-5 samplespractically did not change according to their low dealumination.

Table 2 and Fig. 3 show several characterization resultsobtained from nitrogen physisorption for all samples. The acid-treated mordenites presented higher surface area, lower microporearea/non-micropore area ratios, and higher pore volumes thancommercial mordenite. This can be associated to the loss of alu-minium in the mordenite structure, which results in higher meso-porosity, and therefore, higher surface area. Also, a slight increasein the micropore size was observed. This is in agreement with theresults reported by other authors [7,13,16]. Interestingly, this var-iation was more marked for the microwaved sample. On the otherhand, after dealumination of Beta zeolite, we observed a decreasein the BET surface area accompanied to some variations in themicro-mesoporosity (Fig. 3) which can be attributed to the lossof crystallinity after treatment, as reported by other authors [24].Finally, for acid-treated ZSM-5 samples, slight differences in sur-face and porosity characteristics were detected when comparedwith commercial ZSM-5 due to their very low dealumination.The slight higher decrease in the micropore/non-micropore area

-5051015f1 (ppm)

0

1

2

3

4

5

64.24

-5051015f1 (ppm)

0

1

2

3

4

5

6

4.17

-5051015f1 (ppm)

0

1

2

3

4

5

6

2.22

3.98

c

b

a

-5051015f1 (ppm)

0

1

2

3

4

5

6

2.22

4.06d

Fig. 8. 1H NMR spectra of samples: (a) B, (b) BA, (c) BMW and (d) BMW5.

M.D. González et al. / Microporous and Mesoporous Materials 144 (2011) 162–170 167

ratio observed for the three zeolites when treated under micro-wave irradiation, could be related to their higher dealumination.

Scanning electron microscopy was used to monitor the mor-phologies and sizes of the particles of the acid-treated sampleswith respect to the starting commercial zeolites (Figs. 4–6). Dealu-minated mordenite and beta samples appeared less agglomerated,with less densely packed crystallites, than their correspondingcommercial ones, especially those treated under microwave irradi-ation (Figs. 4 and 5) whereas the micrographs of ZSM-5 sampleswere very similar. (Fig. 6). No significant changes in the particlesizes were observed in any case.

The effect of microwaves on dealumination, when comparedwith conventional heating, can be mainly explained taking intoaccount that chemicals do not interact equally with the commonlyused microwave frequencies for dielectric heating, and conse-quently selective heating may be achieved [17,25]. This may leadto a significantly different temperature regime, caused by micro-wave dielectric heating, being the main contributing factor to theacceleration observed with respect to conventional heating.

1H NMR technique has been postulated as a useful techniqueto determine the presence of different kinds of hydroxyl groupsin zeolites, and therefore, to evaluate their Brønsted aciditystrength [4,26–28]. Figs. 7–9 show the 1H NMR spectra of mord-enite, beta and ZSM-5 samples, respectively. For commercial Na-zeolites (Figs. 7a, 8a and 9a), we observed one peak around4 ppm, which can be associated to free Brønsted protons [27].After acid treatment of mordenite (Fig. 7b and c), one broad peakappeared at higher ppm values (5.0–5.1 ppm) than that of com-mercial mordenite, indicating stronger acidity [29]. This peakcould be attributed to Brønsted protons, formed during dealumi-nation in HCl medium, that are interacting with the zeolite

framework [4,24]. The acidity was slightly higher for the micro-waved sample.

For acid-treated beta samples, we observed one main peak inthe 1H NMR spectra (Fig. 8b–d) with a shift similar to that of com-mercial beta (around 4 ppm). This can be explained by the higherdealumination suffered by these samples. Therefore, there arenot protons interacting with the framework since practically thezeolite framework has not negative charge. Interestingly, the sam-ple treated under microwave irradiation (BMW) showed, in addi-tion to this main peak, a second much less intense peak at2.2 ppm (Fig. 7c) attributed to internal silanols groups [28], whichwere formed during acid dealumination of the framework, as indi-cated in Scheme 1. FTIR (not shown here) confirmed this since anincrease of the silanol band was observed after treatment for thissample. This peak at 2.2 ppm was also pointed out in the 1HNMR spectrum of the sample treated under microwave irradiationat shorter time (BMW5) (Fig. 8d).

Finally, although the acid-treated ZSM-5 samples exhibited verylow dealumination, we observed differences in their acidic proper-ties by 1H MAS NMR (Fig. 9). After treatment, both samples showedone peak at higher shift (around 5 ppm) than its corresponding Na-ZSM-5 sample. Again, this can be attributed to the presence ofBrønsted protons interacting with the framework. This could beexplained by some proton exchange occurred during dealumina-tion due to the high acid medium, as reported by other authors[30]. Interestingly, after treatment under microwave irradiationnew signals appeared between 1.2 and 2.2 ppm which can be asso-ciated to the presence of some external/internal silanol groups(Fig. 9c) [4,28].

All samples were tested as catalysts in two reactions catalysedby different acid sites: the isomerization of styrene oxide to obtain

-5051015f1 (ppm)

0

1

2

3

4

5

6

3.79

0246812f1 (ppm)

0

1

2

3

4

5

64.98

a

b

-5051015f1 (ppm)

0

1

2

3

4

5

61.36

2.27

3.93

5.47

c

Fig. 9. 1H NMR spectra of samples: (a) Z, (b) ZA and (c) ZMW.

Fig. 10. Catalytic activity of all catalysts for the isomerization of styrene oxide.

O Al O

O

OSi

Si

Si

Si

H

+ 3HCl O O

O

O

Si

Si

Si

Si

HHH

H+ AlCl3

Scheme 1. Formation of silanols during dealumination in acidic medium.

168 M.D. González et al. / Microporous and Mesoporous Materials 144 (2011) 162–170

b-phenylacetaldehyde (PA), which is mainly catalysed by Brønstedacid sites, and the styrene oxide ring-opening reaction to give2-ethoxy-2-phenylethanol (EPE), which is catalysed by bothBrønsted and Lewis acid sites but mainly by Lewis acid sites[20,31,32].

Fig. 10 depicts the catalytic activity of all catalysts for thestyrene oxide isomerization reaction in terms of % yield to b-phenylacetaldehyde (PA). Modified mordenite, specially that trea-ted with microwaves, showed higher yields to PA than commercialmordenite due to the presence of Brønsted acid sites with higherstrength appeared after dealumination in HCl medium, as observedby 1H NMR (Fig. 7). All beta catalysts had similar high yield valuesto PA for the styrene oxide isomerization. This agrees with the sim-ilar 1H NMR spectra obtained for these samples. In this case, themicrowaved sample presented slight lower PA yield than the con-ventionally autoclaved one. This can be explained by the slightlower strength of the protons related to its main 1H NMR peak tak-ing into account that silanols groups, obtained in low amounts forthis sample, in zeolites with high density of defect sites, as betazeolites, give very weak acidity [24,28]. Modified ZSM-5 catalystsshowed similar PA yields to that obtained for the commercialone. Interestingly, sample treated with microwaves gave higheryield to PA. This can be attributed to the higher strength of theprotons associated to the main 1H NMR peak.

The differences in the activity between the three types of zeo-lites for this reaction could be explained by the number of Brønstedacid sites (related to the Si/Al ratio) and their strength togetherwith the higher accessibility of the reactants to the active sites inbeta and ZSM-5 samples because of their three-dimensional porestructure compared with the one-dimensional pore structure ofmordenite.

For the styrene oxide ring-opening reaction, the three types ofcatalysts exhibited very different behaviour (Figs. 11–13). Besides2-ethoxy-2-phenylethanol (EPE) and b-phenylacetaldehyde, otherproducts of this reaction were no identified products of highmolecular weights (condensation products), which are responsiblefor catalyst deactivation, as reported in previous studies [20,31,32].

With respect to mordenite samples (Fig. 11), MMW presentedhigher conversion and higher selectivity to 2-ethoxy-2-phenyleth-anol (EPE) than M and MA. The presence of higher amounts ofLewis (Al extra-framework) and Brønsted acid sites with mediumstrength together with the existence of very low amounts of strongLewis acid sites, which are responsible for catalyst deactivation inthis reaction, can explain these results, as previously reported [20].

On the other hand, modified beta samples had lower conversionand lower selectivity to EPE than commercial Na-beta. This can beexplained by the high elimination of aluminium after acidtreatment, having in mind that the starting commercial Beta had

Fig. 11. Catalytic activity of mordenite catalysts for the styrene oxide ring-openingreaction.

Fig. 12. Catalytic activity of beta catalysts for the styrene oxide ring-openingreaction.

Fig. 13. Catalytic activity of ZSM-5 catalysts for the styrene oxide ring-openingreaction.

M.D. González et al. / Microporous and Mesoporous Materials 144 (2011) 162–170 169

aluminium partially coordinated on defect sites, which can act asLewis acid sites [33–35], favouring the formation of EPE. Thisexplanation also agrees with several characterization resultsreported by other authors who stated that the acid leaching of betazeolite eliminates much faster the Lewis acid sites than theBrønsted ones [23,36].

Finally, modified ZSM-5 samples showed higher conversionthan commercial ZSM-5 due to the presence of Brønsted acid siteswith higher strength, as observed by 1H NMR. The very low dealu-mination and, therefore, low generation of Al extraframeworkjustifies the low selectivity values to EPE observed. The slightlyhigher amount of Al extraframework, observed by 27Al NMR(Fig. 1) for the microwaved sample, agrees with the slightly higherselectivity to EPE obtained for this sample.

4. Conclusions

The extent of dealumination was function of the zeolitestructure and the heating method used. Beta zeolite was easier todealuminate than mordenite whereas dealumination of ZSM-5was very low. This can be related to the flexibility of each zeoliteframework, and the accessibility of the aluminium atoms depend-ing on the pores arrangement and sizes. Microwaves led to fasterdealumination than conventional heating for the three zeolites. Be-sides, the use of microwaves affected the surface and acidic prop-erties of the resulting dealuminated samples.

Mordenite treated with HCl under microwave irradiationshowed higher mesoporosity, higher surface area, and Brønstedacid sites with higher strength than the mordenite treated by con-ventional heating. After acid treatment with microwaves, beta zeo-lite exhibited lower surface area accompanied with somevariations in the micro-mesoporosity, and similar Brønsted aciditythan the conventionally acid-treated beta. However, some silanols,with very weak acidity, were only detected for the microwavedsample. Lastly, ZSM-5 treated in acidic medium with microwaveshad similar surface and porosity characteristics than the conven-tionally heated sample, probably due to the very low dealumina-tion achieved. Interestingly, for the microwaved ZSM-5 sample,some accessible external silanol groups appeared after acid treat-ment, resulting in slightly higher Brønsted acididity. Besides,slightly higher amount of extraframework Al, and therefore,slightly Lewis acidity, was observed for this sample.

Acknowledgments

The authors are grateful for the financial support of the Minis-terio de Ciencia e Innovación and FEDER funds (CTQ2008-04433/PPQ). Dolores González acknowledges Ministerio de Educación yCiencia for a FPU Grant (AP2007-03789).

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