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Catalysis Today 227 (2014) 26–32

Contents lists available at ScienceDirect

Catalysis Today

j o ur na l ho me page: www.elsev ier .com/ locate /ca t tod

odification of textural and acidic properties of -SVR zeolitey desilication

artin Kubu ∗, Maksym Opanasenko, Mariya Shamzy. Heyrovsky Institute of Physical Chemistry of the ASCR, v. v. i., Dolejskova 3, CZ-182 23 Prague 8, Czech Republic

r t i c l e i n f o

rticle history:eceived 14 June 2013eceived in revised form 16 October 2013ccepted 25 November 2013vailable online 28 January 2014

eywords:SVR zeolite

a b s t r a c t

Desilication conditions (concentration of NaOH and TPAOH solution, duration of the treatment) of -SVR zeolite were varied and related to the textural and acidic properties of the obtained materials. Thetreatment of -SVR zeolite with NaOH solution led to the formation of mesopores and also the increasingof the concentration of surface acid sites, accessible for bulky molecules (e.g. 2,6-ditertbutyl-pyridine).The volume of mesopores as well as the concentration of accessible acid centres in desilicated materialsdepends mainly on the concentration of the alkaline solution. The increase in the pH of the treatmentresulted in moderate decrease of both micropore volume and Brønsted acid sites concentration, while

esilicationierarchical materialsextural and acidic properties

the volume of mesopores increased significantly. Independently of the duration and pH of the treatmentwith NaOH solution, all desilicated -SVR zeolites are characterized by broad pore-size distribution in therange of 5–20 nm with the maximum around 14 nm. At fixed pH of the treatment with TPAOH, the volumeof mesopores and their average size decreased, while the Si/Al ratio and the concentration of Brønstedacid sites in desilicated materials increased which indicates the inhibiting effect of TPA+ cations on the

zeol

extraction of Si from -SVR

. Introduction

Zeolites, crystalline aluminosilicates with ordered networks oficropores, are utilized in numerous large-scale chemical tech-

ologies as heterogeneous catalysts due to their unique propertiesuch as high surface area, high thermal stability, adjustable aciditynd shape-selectivity [1,2]. Catalytic applications of zeolites cover

wide range of reactions, from oil refining, petrochemistry, finehemical synthesis to biomass upgrade and separation processes3–6]. Despite that, zeolites generally suffer from intracrystallineiffusion limitations because of the molecular dimensions oficropores. The size of zeolite pores (0.3–1.0 nm) also limits the

ccessibility of the active sites located in the channels for bulkyeactants [7], which may negatively impact their catalytic perfor-ance for transformation of large molecules.In relation to this issue, big hopes were connected with the

esoporous molecular sieves, firstly synthesized in 1992. However,ow thermal and hydrothermal stability as well as the low acidityf mesoporous molecular sieves strongly limit their application inatalysis [8–10].

At the same time, great efforts have been undertaken to obtaineolites possessing micropores with the diameter higher than.85 nm (extra-large pores), which are promising for catalytic

∗ Corresponding author.E-mail address: martin.kubu@jh-inst.cas.cz (M. Kubu).

920-5861/$ – see front matter © 2014 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.cattod.2013.11.063

ite.© 2014 Elsevier B.V. All rights reserved.

transformation of bulky molecules [11–18]. However the synthesisof such zeolites requires specially prepared templates. Recently,it was shown that a lamellar MFI zeolite is formed when using“gemini-type” poly(quaternary ammonium)surfactants composedof a long-chain alkyl group (C18–C22) and several quaternaryammonium groups spaced by a C6 alkyl linkage [19].

Significant effort has been devoted to develop the approachesfor the introduction of mesopores in zeolites to combine the perfectproperties of both micro- and mesoporous materials. The creationof new materials, including zeolite nanosheets, by post-synthesismodification of layered materials was proposed. However, theapplication of this strategy is limited to several zeolite struc-tures including MCM-22 precursor [20], pre-ferrierite [21] andgermanosilicate UTL zeolite [22,23].

A substantial mesoporosity of zeolite crystals can be obtainedby using carbon templating approach where carbon particles areincluded in the synthesis of the zeolite, thereby leaving pores inthe zeolite matrix after combustion [24,25]. Although in this casethe optimization of the synthesis parameters is usually requiredto obtain the material with an appropriate crystallinity. Recently,microwave-assisted hydrothermal method has been reported notonly to provide distinct advantages over the conventional synthe-sis (e.g. rapid heating, homogeneous nucleation, supersaturation

by the rapid dissolution of precipitated gels, shorter crystallizationtime) but also is an efficient way for mesopore generation by desil-ication [26,27]. Direct synthesis of carbon-templating mesoporousZSM-5 using microwave heating was shown by Park et al. [25].

sis Today 227 (2014) 26–32 27

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At the same time, mesopores in zeolite crystals can also be eas-ly created by post-synthesis treatments resulting in dealuminatione.g. steaming, acid leaching or their combination [28]) or in desil-cation (e.g. base leaching [8]). While removal of aluminium fromhe framework results in decreasing concentration of acid centresn zeolites, silicon extraction is known as an effective approacho create transport mesopores in various zeolites by preferentialxtraction of framework Si, preserving the Al environment and theelated acidic properties.

Unfortunately, the major drawback of desilication remained inimited applicability to zeolites with Si/Al ratio 25–50 in the frame-

ork [29], until Verboekend has shown the controlled formationf mesopores in high-silica zeolites through pore-directing agentsPDA, e.g. Al(OH)4

–, TPA+) [30]. Since that time, desilication meth-ds were intensively investigated to generate mesopores in zeolitesf various chemical compositions and structure types (e.g. MFI [31],

[32], IFR [33]).Recently synthesized high silica -SVR zeolite is a member

f the medium pore zeolite family. It possesses 3-dimensional0-ring pore system with ordered silicon vacancies – structureefects in tetrahedron surrounding four hydroxyl groups [34].he aluminosilicate -SVR was active in alkylation and transalky-ation reactions of aromatic hydrocarbons, isomerization of C4–C7ydrocarbons and olefins to aromatics [35]. The development of

ntracrystalline mesopores within -SVR crystals upon frameworkilicon extraction may open new perspectives in the application ofhis zeolite in catalysis.

In this work, we present a detailed study of the influence of theesilication conditions (e.g. NaOH and TPAOH concentration, dura-ion of the treatment) of zeolite -SVR (Si/Al = 41) on the structural,extural and acidic properties of the formed micro/mesoporous

aterials to identify the crucial parameters for the formation ofntracrystalline mesopores, while preserving the original acidity ofhe zeolite.

. Experimental part

.1. 2.1 Synthesis ofexamethylene-1,6-bis-(N-methyl-N-pyrrolidinium) hydroxide

3.5 g of N-methylpyrrolidine (97%, Aldrich) was dissolved in0 ml of acetone. Then 4.9 g of 1,6-dibromohexane (98%, Aldrich)as added, and the resulting solution was stirred for 3 days at room

emperature. The formed solid product was recovered by filtration,ashed with diethyl ether and dried. Hexamethylene-1,6-bis-(N-ethyl-N-pyrrolidinium) bromide was converted into hydroxide

orm by ion exchange with AG1-X8 (Bio-Rad) resin. The success-ul synthesis of the structure-directing agent (SDA) was confirmedy 1H NMR spectroscopy after dissolution in methanol-d4 (Fig. 1).

signals, attributed to differently shielded hydrogen atoms in theDA molecule, were found. 1H NMR (300 MHz, CD3OD): � 1.51 (m,H), 1.88 (m, 4H), 2.24 (m, 8H), 3.12 (s, 6H), 3.44 (m, 4H), 3.60 (t,H). The signal at 3.34 ppm (marked with asterisk) belongs to theolvent.

.2. Synthesis of -SVR zeolite

The synthesis of parent -SVR zeolite was performed accord-ng to the Ref. [36]. The starting gel had the followingomposition: 40 SiO2:1 AlO1.5:6 SDA(OH)2:1200 H2O. In par-icular, 4.2 g of deionised water, 5.3 g Ludox LS-30 (30%

iO2), 11.9 ml of 0.33 M hexamethylene-1,6-bis-(N-methyl-N-yrrolidinium) hydroxide solution and 1.67 g of 15% Al(NO3)3olution were mixed and stirred for 30 min. The resulting fluid gelas charged into 25 ml Teflon-lined autoclave and heated at 160 ◦C

Fig. 1. 1H NMR spectrum of hexamethylene-1,6-bis-(N-methyl-N-pyrrolidinium)bromide.

for 15 days under agitation (∼25 rpm). The solid product was sepa-rated by filtration, washed with distilled water and dried overnightat 95 ◦C. The SDA was removed by calcination. Products were heatedfrom room temperature to 120 ◦C at a rate of 1 ◦C/min, and the tem-perature was maintained for 2 h. The next step involved increasingtemperature up to 540 ◦C for 5 h at a rate of 1 ◦C/min. Finally, thetemperature was increased up to 580 ◦C at a rate of 1 ◦C/min andthe product was kept at this temperature for 5 h.

2.3. Desilication

Desilication procedure was based on Refs. [31,37]. Treatmentswere performed in 10 cm3 glass flasks. The alkaline solution (5 cm3)was stirred and heated to 65 ◦C, after which the zeolite sample(0.167 g) was added. The resulting mixture was left to react underreflux for different periods of time (Table 1) followed by quenching.The solid product was centrifugated and extensively washed outwith distilled water to reach neutral pH and then dried overnightin an oven at 60 ◦C. In the case of TPAOH presence in the alkalinesolution, calcination at 550 ◦C for 5 h (heating rate 5 ◦C/min) wasapplied after the treatment to remove occluded TPA+ species. Na+

forms of -SVR samples were converted to NH4+ form by three-fold

treatment with 1.0 M NH4NO3 solution at room temperature for3 h.

The samples, treated with NaOH solution, are designatedaccording with the expression: -SVR/NaOH conc. (M)/time (min).When treated with the mixture of NaOH and TPAOH, the samplesare designated as -SVR/NaOH conc. (M) + TPAOH conc. (M)/time(min).

2.4. Characterization

1H NMR spectrum of the organic SDA was recorded on a Var-ian Mercury 300 spectrometer at 300.0 MHz in CD3OD solutionsat 25 ◦C. Chemical shifts (�/ppm) are given relative to residualCHD2OD signals (�H 3.34 ppm).

The crystallinity of all samples under investigation was checkedby X-ray powder diffraction (XRD) using a Bruker AXS-D8 Advancediffractometer with a graphite monochromator and a position sen-sitive detector Våntec-1 using CuK� radiation in Bragg–Brentanogeometry.

The size and shape of zeolite crystals were examined by scan-ning electron microscopy (SEM, JEOL JSM-5500LV microscope). For

the measurement crystals were coated with a thin platinum layerby sputtering in vacuum chamber of a BAL-TECSCD-050.

Nitrogen adsorption/desorption isotherms were measured on aMicromeritics GEMINI II 2370 volumetric Surface Area Analyzer at

28 M. Kubu et al. / Catalysis Today 227 (2014) 26–32

Table 1Experimental conditions and respective textural properties.

Sample Treatment conditions BET (m2/g) Vmic (cm3/g) Vmeso (cm3/g) Vtot (cm3/g) �wc (%) Si/Ala

pH min

-SVR/0/0 (parent) – – 468.0 0.17 – 0.17 – 41-SVR/0.02/15 12.3 15 389.0 0.14 0.14 0.33 6.0 39-SVR/0.02/30 30 403.2 0.14 0.16 0.33 10.6 36-SVR/0.02/60 60 393.8 0.14 0.18 0.34 7.9 n.d.b

-SVR/0.05/30 12.7 30 391.1 0.12 0.21 0.34 9.1 n.d.-SVR/0.10/30 13.0 30 404.8 0.12 0.21 0.35 13.4 n.d.-SVR/0.20/15 13.3 15 448.4 0.10 0.41 0.52 48.6 22-SVR/0.20/30 30 451.2 0.10 0.49 0.59 52.0 18-SVR/0.05 + 0.05/30 12.9 30 363.7 0.11 0.20 0.33 11.1 n.d.-SVR/0.05 + 0.15/30 13.2 30 445.8 0.12 0.18 0.39 19.5 35

a Chemical analysis.

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for the desilication seems to be even more crucial parameterinfluencing the framework density of the samples. While desilica-tion with 0.02 and 0.05 M NaOH solutions (samples -SVR/0.02/30and -SVR/0.05/30) did not lead to any essential changes in the

b Not determined.c Weight reduction after desilication [m(parent) − m(desilicated)]/m(parent) × 10

iquid nitrogen temperature (−196 ◦C) to determine surface area,ore volume and pore size distribution. Prior to the sorption mea-urements, all samples were degassed on a Micromeritics FlowPrep60 instrument under helium at 300 ◦C (heating rate 10 ◦C/min) for

h.The chemical composition of the samples was determined

y elemental analysis. For this purpose 0.2–0.3 g of zeolite waseated at 70 ◦C with 5–7 ml of 10 M NaOH in a platinum cup.fter the total dissolution of the sample, 10–15 ml of concen-

rated HCl was added until pH became 0.6–0.7. Then acid solutionas heated at 80 ◦C for 20 min to coagulate the precipitated

iO2 × xH2O. The precipitate was recovered by filtration on ash-ess filter, washed out with 1 M HCl, and subsequently with hot

ater for complete removal of AlCl3. Precipitated SiO2 × xH2O wasried at 90 ◦C, calcined at 1000 ◦C until the constant mass, andeighed.

Al contents were determined by back-complexation titrationsing the following procedure. 4.0 ml of 0.10 M EDTA was addedo the analyzed solution and warmed to boiling, then cooled downnd neutralized with NH3·H2O (25% solution) until pH = 3.0 waseached. Then 20 ml of acetate buffer solution (pH = 6.0) was addedo adjust the pH. This solution was boiled for 5 min to ensure com-lete complexation of the Al3+ cations and cooling down to theoom temperature. The excess of EDTA was titrated with 0.001 MnSO4 in the presence of xylenol orange until the colour changerom yellow to violet.

Concentration of Lewis (cL) and Brønsted (cB) acid sites wasetermined after adsorption of pyridine (PYR) by FTIR spectroscopyn Nicolet Protégé 460 Magna with a transmission DTGS and MTC/Aetector. Zeolites were pressed into self supporting wafers with aensity of 8.0–12 mg/cm2 and activated in situ at 450 ◦C overnight.yridine adsorption was carried out at 150 ◦C for 20 min at par-ial pressure 600–800 Pa, followed by desorption for 20 min. Beforedsorption pyridine was degassed by freezing and thawing cycles.ll spectra were recorded with a resolution of 4 cm−1 by collecting28 scans for a single spectrum at room temperature. Spectra wereecalculated on wafer density of 10 mg/cm2. Concentration of cLnd cB were evaluated from the integral intensities of bands at454 cm−1 (cL) and at 1545 cm−1 (cB) using extinction coefficients,(L) = 2.22 cm/�mol, and ε(B) = 1.67 cm/�mol [38].

A relatively large probe molecule 2,6-di-tert-butyl-pyridineDTBP) was used to determine the accessibility of acid sites withinrepared zeolites [39]. The adsorption of DTBP took place at 150 ◦Cnd at equilibrium probe vapour pressure with the zeolite wafer

or 15 min. Desorption proceeded at the same temperature for 1 hollowed by collection of spectra at room temperature. Extinctionoefficients for pyridine [38] were used for the quantitative analysisvaluation of cB.

3. Results and discussion

3.1. The structure of desilicated zeolites

-SVR/0/0 zeolite was prepared with Si/Al ratio of 41, which wasshown to be optimal [29] for introduction of mesopores by applyingtreatment with NaOH solutions. Different experimental techniqueshave been applied to characterize in detail structural properties of-SVR zeolites under investigation.

The X-ray diffraction pattern of the parent -SVR zeolite (Fig. 2C,-SVR/0/0) matches well with that one reported in the literature[34,40]. Fig. 2A–C shows XRD patterns of parent and all desilicated-SVR zeolites. It can be seen, that the samples subjected to alkalinetreatment in different conditions still display sharp diffraction linesbetween 5◦ and 40◦ at the characteristic 2-theta positions, whichproves the preservation of the long-range crystal ordering duringthe post-synthesis treatments under experimental conditions.

3.1.1. The effect of the concentration of NaOH solutionThe decrease in the intensity of the characteristic diffraction

lines with increasing concentration of NaOH solution (from 0.02to 0.20 M) at the same time (Fig. 2A) indicates the progressivedecreasing of the framework density during leaching of theframework Si atoms. The concentration of alkaline solution used

Fig. 2. XRD patterns of -SVR zeolites. The effect of NaOH concentration at the sametime of the treatment (A), the time of the treatment at the same NaOH concentration(B) and the concentration of the pore-directing agent (C) compared to the parentzeolite.

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ntensities of the characteristic diffraction lines (Fig. 2A), thencrease in the concentration of NaOH up to 0.1 M (-SVR/0.10/30)nd especially 0.2 M (-SVR/0.20/30) led to a significant drop in thentensity of corresponding diffraction lines.

.1.2. The effect of the duration of the treatmentThe duration of the treatment (at the same concentration of the

lkaline solution) had smaller impact on the structural changes pro-eeded during the silicon extraction from the framework, whenompared with previously discussed effect of the concentration ofaOH solution (at the same time). No changes are seen in the inten-

ities of the typical diffraction lines for desilication performed with.02 M NaOH after 15 and 30 min (-SVR/0.02/15 and -SVR/0.02/30),hile significant decrease was observed when the treatment wasrolong to 60 min (Fig. 2B). The substantial decrease in the inten-ity of the diffraction lines after 30 min for the treatment with 0.2 MaOH (-SVR/0.20/15 � -SVR/0.20/30) supports the idea that theoncentration of the alkaline solution is the more crucial parameternfluencing the framework density (Fig. 2B).

.1.3. The effect of the pore-directing agent (PDA)It should be noted, that both desilicated samples obtained

uring the treatment of -SVR/0/0 with TPAOH + NaOH solu-ion, i.e. -SVR/0.05 + 0.05/30 and especially -SVR/0.05 + 0.15/30[TPA+]/[OH–] = 0.75, [OH−] = 0.20 M), showed more intensiveiffraction lines (Fig. 2C) in comparison to -SVR/0.20/30 sample,reated exclusively with 0.20 M NaOH solution. It may be due tohe stabilization effect of relatively high polarizable TPA+ cations,hich were shown to bind to the zeolite surface in alkaline mediumroviding a protective layer on the zeolites external surface and acts “pore-growth moderators” in the desilication process [30,31,41].

.2. The textural properties of desilicated zeolites

Nitrogen isotherms provided valuable information on the text-ral properties of desilicated samples when compared with thearent one. Obtained isotherms agree well with the structuralhanges determining by means of XRD. In contrast to parentSVR/0/0 zeolite (Fig. 3C) which exhibits type I isotherm char-cteristic for microporous solids, all desilicated samples showedombined type I and type IV isotherm, being typical for hierarchi-al micro/mesoporous materials [8]. Total pore volume of parentSVR/0/0 is 0.17 cm3/g and no presence of mesopores was observedTable 1).

According to the SEM images, the micrographs of the par-nt and desilicated zeolites represent quite small crystals (ca.

�m × 8 �m × 0.5 �m, Fig. 4). Therefore, adsorption of nitrogen in

ig. 3. Nitrogen adsorption (©) and desorption (�) isotherms. The effect of NaOHoncentration at the same time of the treatment (A), time of the treatment at theame NaOH concentration (B) and the concentration of the pore-directing agent (C)ompared to the parent zeolite.

ay 227 (2014) 26–32 29

the range of p/p0 = 0.8–1.0 can be explained by filling of intercrys-talline pores.

The results of the chemical analysis indicate decreasing Si/Alratio in the desilicated samples with increasing duration of thetreatment and/or concentration of NaOH solution in the followingsequence (see Table 1):

-SVR/0/0 (Si/Al = 41) > -SVR/0.02/15 (Si/Al = 39) > -SVR/0.02/30(Si/Al = 36) > -SVR/0.20/15 (Si/Al = 22) > -SVR/0.20/30 (Si/Al = 18),which is due to the progressive selective extraction of Si atomsfrom the zeolite framework.

At the same time, the sample -SVR/0.20/30 (Si/Al = 18, formedduring the treatment of parent sample -SVR/0/0 with 0.2 M NaOHsolution) is characterized by significantly lower Si/Al ratio in com-parison to -SVR/0.05 + 0.15/30 (Si/Al = 35), obtained in the presenceof TPA+ cations ([TPA+]/[OH−] = 0.75, [OH−] = 0.2 M). The last resultproved the inhibiting role TPA+ cations in the process of siliconextraction from the framework of -SVR zeolite and corresponds tothe data, obtained for zeolite MFI [31].

3.2.1. The effect of the concentration of NaOH solutionThe increase of the concentration of NaOH solution from 0.02 to

0.05 and 0.10 M led to the enhancement of mesopore volume from0.16 to 0.21 cm3/g, while the volume of micropores decreased from0.17 (-SVR/0/0) to 0.12 cm3/g (-SVR/0.10/30). At the same time,the treatment of -SVR/0/0 with 0.20 M NaOH solution resulted insignificant increase in the volume of mesopores up to 0.41 cm3/g(-SVR/0.20/15) and 0.49 cm3/g (-SVR/0.20/30), respectively, whilethe volume of micropores decreased by 41% to 0.10 cm3/g (Table 1).These changes are nicely seen from the change of the shape of thenitrogen isotherm (Fig. 3A).

3.2.2. The effect of the duration of the treatmentThe alkaline treatment of -SVR/0/0 with 0.02 M NaOH for

15–60 min resulted in a slight decrease of the micropore volume(from 0.17 to 0.14 cm3/g), while the increase in the volume ofmesopores (0.14–0.18 cm3/g) correlates with the duration of thetreatment (Table 1). However, under the experimental conditionsused, the duration of the treatment had smaller impact on thevolume of mesopores formed, than the concentration of the usedalkaline solution (Fig. 3B).

3.2.3. The effect of the pore-directing agent (PDA)The mesopore volume of -SVR/0.20/30 sample exceeded

almost 3 times (0.49 vs. 0.18 cm3/g, Table 1) the value for-SVR/0.05 + 0.15/30 obtained by the treatment of parent -SVR/0/0 zeolite with mixed NaOH/TPAOH solution for 30 min([TPA+]/[OH−] = 0.75, [OH−] = 0.20 M). These results correspond tothe XRD data obtained for respective samples and prove the stabi-lization effect of TPA+ during desilication.

BJH method [42] used for pore-size distribution (Fig. 5) revealedbroad distribution of mesopores in the range of 5–20 nm withthe maximum around 14 nm for all samples desilicated by usingNaOH solutions of different concentrations independently of thetime of the treatment. For the sample -SVR/0.05 + 0.15/30 (Fig. 5,wine curve) obtained by the treatment of parent sample (-SVR/0/0) with mixed NaOH/TPAOH solution ([TPA+]/[OH−] = 0.75,[OH−] = 0.20 M), the maximum is shifted to smaller mesopore sizeof ca 11 nm. It is in a good agreement with literature data [30].

3.3. The nature and concentration of acid sites

The nature of OH-groups within initial and desilicated -SVR

zeolites was studied by FTIR spectroscopy. Three different bandswere distinguished in the spectrum of parent -SVR/0/0 zeolitein the region of hydroxyl vibrations (Fig. 6A). The most intensebands observed at 3745 and 3724 cm−1 were attributed to external

30 M. Kubu et al. / Catalysis Today 227 (2014) 26–32

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Lewis acid sites. The concentrations of Brønsted and Lewis acid sitescalculated from the integral intensities of the bands at 1546 and1454 cm−1 using extinction coefficients [38] are given in Table 2.

Fig. 4. SEM images of parent and treated -SVR zeolites: (A) -SVR/0/0; (B) -SVR/0

nd internal silanol groups, respectively. The band at 3616 cm−1

resents the bridging hydroxyl groups (Si (OH) Al).After pyridine adsorption, the intensities of the bands assigned

o the silanol groups slightly decreased or remained almostnchanged. On the other hand, the band assigned to bridgingydroxyl groups having the acid character disappeared, evidenc-

ng the accessibility of all Brønsted acid centres for pyridine in the

nvestigated samples.

The adsorption of pyridine was accompanied by the appearancef a typical set of adsorption bands in the region 1400–1600 cm−1

ig. 5. BJH desorption (dV/dD pore volume) pore-size distribution of -SVR samples.

; (C) -SVR/0.10/30; (D) -SVR/0.20/30; (E) -SVR/0.20/15; (F) -SVR/0.05 + 0.05/30.

(Fig. 6B). The absorption band around 1546 cm−1 is due to theinteraction of pyridine with Brønsted acid sites, while a new bandaround 1454 cm−1 is characteristic for the pyridine adsorbed on

Fig. 6. IR spectra of -SVR zeolites: (A) region of hydroxyl vibrations; (B) region ofpyridine vibrations before (bold spectra) and after adsorption of pyridine.

M. Kubu et al. / Catalysis Today 227 (2014) 26–32 31

Table 2The concentration of Lewis and Brønsted acid sites within -SVR zeolites, determinedby means of FTIR spectroscopy of adsorbed pyridine and 2,6-di-tert-butylpyridine.

Sample cB (mmol/g) cL (mmol/g)

PYR DTBP PYR

-SVR/0/0 0.127 0.039 0.046-SVR/0.02/15 0.120 0.040 0.081-SVR/0.02/30 0.122 0.054 0.056-SVR/0.20/15 0.109 0.077 0.063

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-SVR/0.20/30 0.107 0.079 0.064-SVR/0.05 + 0.15/30 0.127 0.076 0.023

.3.1. The effect of the concentration of NaOH solution anduration of the treatment

The increase of the concentration of NaOH solution or theuration of the alkaline treatment was accompanied with thenhancement of the intensity of the band of isolated silanolroups at 3745 cm−1 (-SVR/0/0 ≈ -SVR/0.02/30 � -SVR/0.20/15 < -VR/0.20/30), see Fig. 6A, while the intensity of the band at724 cm−1 decreased. This indicates the expected growth of theoncentration of external SiOH groups in the progress of the for-ation of mesopores within the zeolite -SVR.As can be seen, with the increasing concentration of NaOH

olution used for the preparation of desilicated -SVR zeolites, theoncentration of Brønsted acid sites was progressively decreasingi.e. 0.127, 0.120 and 0.109 mmol/g for -SVR/0/0, -SVR/0.02/15nd -SVR/0.20/30, respectively), while the duration of the alka-ine treatment had negligible effect on the amount of Brønstedcid sites in the obtained materials (i.e. 0.120 and 0.122 mmol/gor -SVR/0.02/15 and -SVR/0.02/30, respectively). At the same timehe concentration of Lewis acid sites in the desilicated samples,repared using NaOH solutions of different concentrations, usu-lly exceeded the respective value for parent -SVR/0/0 zeolite.his result may originate from the equilibrium processes, whichake place during desilication: (1) releasing of framework Al atomswhich form either Brønsted or Lewis acid sites), resulting in theecreasing of the amount of acid centres within zeolite; (2) rea-

umination of the surface of the material with Al(OH)4−, which

as shown to accompany with the reinsertion of Al atoms into theramework [29].

.4. The surface acidity

The surface acidity of -SVR zeolites was studied by FTIR spec-roscopy of adsorbed 2,6-di-tert-butylpyridine. The size of the DTBP

olecule is about 0.79 nm [39], which is higher than the diame-er of 10-ring channels in zeolites that makes accessible only acidentres on the outer surface of parent -SVR/0/0 crystals. Indeed,s it can be seen from Fig. 7A, the adsorption of DTBP on thearent -SVR/0/0 zeolite led to only partial disappearance of theand at 3616 cm−1, corresponding to bridging hydroxyl groups.n the region of chemically adsorbed DTBP a number of bands

ere detected. According to the previous work of Corma et al. [39]he bands at 3370, 1616 and 1530 cm−1 can be attributed to theormation of DTBPyH+ ions. The absence of a band at 1545 cm−1

onfirms no dealkylation of the probe molecule used. It can be seen,hat the concentration of accessible (surface) acid sites increasesith the increasing of the intensity of the alkaline treatment

n the following sequence (Table 2): -SVR/0/0 < -SVR/0.02/30 < -VR/0.20/15 < -SVR/0.20/30, which correlates with the increasingf mesopore volume for the respective samples.

.4.1. The effect of the pore-directing agent (PDA)In contrast to desilicated samples obtained by the treatment

ith NaOH solutions, the sample -SVR/0.05 + 0.15/30 prepared

Fig. 7. IR spectra of -SVR zeolites: (A) region of hydroxyl vibrations; (B) region ofDTBP vibrations before (bold spectra) and after adsorption of DTBP.

with the mixed NaOH/TPAOH solution ([TPA+]/[OH−] = 0.75,[OH–] = 0.20 M) contains significantly lower amount of Lewis acidsites (0.023 mmol/g) in comparison with the parent -SVR/0/0 sam-ple (0.046 mmol/g). At the same time, the concentration of Brønstedacid sites for both -SVR/0.05 + 0.15/30 and SVR/0/0 is 0.127 mmol/g.These results may be connected with the “protective” effect of TPA+

cations on the Brønsted acid sites, which leads to the preferen-tial releasing of Lewis acid centres from zeolite during desilication.The mentioned effect is presumably originate from the strongerbinding of TPA+ to the anions on zeolite surface due to the addi-tional contribution of hydrophobic and van der Waals forces,which do not act in the case of Na+ interaction with the sur-face of zeolite. Since the total concentration of acid centres for-SVR/0.05 + 0.15/30 is lower compared with the parent -SVR/0/0sample, the formation of Lewis acid centres in the progress of rea-lumination in the presence of TPA+ can be hindered. However, thisissue requires further investigation. Moreover, despite compara-bly low volume of the formed mesopores (0.18 cm3/g, Table 1), thesample -SVR/0.05 + 0.15/30 contains unexpectedly high amount ofacid centres, accessible for DTBPy (0.076 mmol/g) if compared with-SVR/0.02/30 (0.054 mmol/g, Table 2), which is characterized byclose mesopore volume (0.16 cm3/g, Table 1). The last result maybe connected with the distinctive features of de-/realuminationprocesses, taking place during the silicon extraction from zeoliteframeworks in the presence of TPAOH and representing a challengeto be investigated in details.

4. Conclusions

In the present work the effect of the concentration of NaOH solu-tion and pore-growth moderator TPAOH as well as the durationof desilication treatment on textural and acidic properties of -SVRzeolite was established.

It was shown, that desilication of -SVR zeolite (Si/Al = 41) with0.02–0.20 M NaOH solutions at 65 ◦C for 15–60 min results information of transport mesopores with broad pore size distri-bution (5–20 nm). While the volume of the formed mesoporesincreased with increasing concentration of NaOH solution reaching0.49 cm3/g after 30 min treatment with 0.20 M NaOH, the meso-

pore size distribution was effected neither by the concentrationof alkaline solution used nor by the duration of desilication. Theincrease in the concentration of TPAOH ([TPA+]/[OH−] = 0–0.75) atfixed pH, resulted in decrease of the volume of formed mesopores

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nd their average size, confirming the pore-moderating role, whichPA+ cations play during desilication of zeolites.

All desilicated -SVR zeolites prepared using NaOH solutionsre characterized by increased concentration of Lewis acid cen-res and higher surface acidity in comparison to parent zeolite. Theoncentration of acid sites accessible for 2,6-di-tert-butylpyridineorrelates with the mesopore volume of desilicated samples, reach-ng the maximal value for–SVR zeolite, treated with 0.20 M NaOHolution for 30 min.

In contrast, desilication of -SVR zeolite (Si/Al = 41) withixed NaOH/TPAOH solution ([TPA+]/[OH−] = 0.75, [OH−] = 0.20 M)

esults in decreasing of the concentration of Lewis acid sites, whilehe increasing of the surface acidity could hardly be related to theolume of formed mesopores. The last result may indicate the spe-ific effect of TPA+ cations on the rearrangement of acid centres ineolites during desilication process, which represents an intriguingssue for further investigation.

cknowledgements

The authors thank the Czech Science Foundation for the support13-17593P) and RNDr. Libor Brabec, CSc. for SEM images.

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