Microporous and Mesoporous Materials 145 (2011) 124–130

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Microporous and Mesoporous Materials

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One-pot synthesis of mesoporous [Al]-SBA-16 and acidity characterizationby CO adsorption

Jean Marcel R. Gallo a,b, Chiara Bisio b, Leonardo Marchese b,⇑, Heloise O. Pastore a,⇑a Grupo de Peneiras Moleculares Micro e Mesoporosas, Instituto de Química, UNICAMP, Rua Monteiro Lobato, 270, Cidade Universitária Zeferino Vaz, CEP 13084-971,Campinas-SP, Brazilb Dipartimento di Scienze e Tecnologie Avanzate and Centro Interdisciplinare Nano-SiSTeMI, Università del Piemonte Orientale ‘‘A. Avogadro’’, Viale T. Michel 11,I-15121 Alessandria, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 February 2011Received in revised form 3 May 2011Accepted 4 May 2011Available online 11 May 2011

Keywords:[Al]-SBA-16pH-adjusting methodAcid sitesMesostructured silicaCO adsorption

1387-1811/$ - see front matter � 2011 Elsevier Inc. Adoi:10.1016/j.micromeso.2011.05.005

⇑ Corresponding authors. Tel.: +55 19 35213095;Pastore), tel.: +39 0131 360262; fax: +39 0131 36025

E-mail addresses: leonardo.marchese@mfn.unipmiqm.unicamp.br (H.O. Pastore).

We report here on the first one-pot synthesis of mesoporous silica SBA-16 containing aluminum. Thismaterial was successfully synthesised using poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) Pluronic F-127 (PEO106PPO70PEO106) as structure directing agent, butanol as co-sur-factant and specifically adjusting the pH during the synthesis (i.e. adopting the so called ‘‘pH-adjustingmethod’’). The structural and porosity properties of the novel [Al]-SBA-16 were studied by X-ray diffrac-tion, 27Al-MAS–NMR and N2 adsorption at 77 K. The acidity was monitored by infrared spectroscopy ofCO adsorption at 100 K.

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1. Introduction

In recent years, environmental and economic considerationshave raised strong interest in redesigning commercially importantprocesses so that the use of harmful substances and the generationof toxic waste could be avoided or diminished. In this respect, thereis no doubt that heterogeneous catalysis plays a key role in thedevelopment of environmentally benign processes in petrochemis-try and in the production of chemicals, for instance by substitutingliquid acid catalysts by solid materials. Zeolites, in particular, haveattracted strong attention as solid acid catalysts, but also as basicand redox catalysts. However, zeolites present severe limitationswhen large reactant molecules are involved. This is especially truein liquid-phase systems, as it is frequently the case in the synthesisof fine chemicals, due to the fact that mass transfer limitations aresevere in microporous frameworks. Attempts to improve the diffu-sion of reactants to the catalytic sites have so far focused onincreasing the zeolite pore sizes, on decreasing zeolite crystal size,or on providing an additional micropore system within the micro-porous crystals. An important line of research has focused on theextension of the pore sizes into the mesopore range, allowing lar-

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fax: +55 19 35213023 (H.O.0 (L. Marchese).

n.it (L. Marchese), gpmmm@

ger molecules to enter the pore system, to be processed there andto leave the pore system again [1,2].

Mesoporous silicas with various structural order (e.g., hexago-nal, cubic, lamellar) of pores or channels and macroscopic mor-phologies (powders, fibers, thin films, and monoliths) havealready been synthesised [3]. The diverse properties of these mate-rials originate from the ability to control sizes and periodic order ofchannels or pores and particles morphology, while a precise tailor-ing of the internal pore surface or channel compositions influencesadsorption, diffusion, photo-response, or other properties [3].

For catalytic applications the mesoporous supporting materialsshould be chosen considering the kind of properties that they areexpected to have, such as surface reactivity properties and poressize and shape for better diffusion of the reactants/products. Fur-thermore, the cost, ease and reliability of the synthesis procedureshould be taken into account. For the application as support formetal nanoparticles high surface areas and organized mesoporosi-ties are necessary in order to guarantee a homogeneous nanoparti-cle distribution throughout the support.

The first synthesis of an ordered mesoporous material was de-scribed in the patent literature in 1971 [4,5]. However, due to alack of complete analysis, the remarkable features of this productwere not recognized. In 1992, a similar material was obtained byscientists at Mobil Oil Corporation and named M41S family ofmolecular sieves [6]. The remarkable features of these novel typesof silicas opened up a whole field of research. The synthesis of

J.M.R. Gallo et al. / Microporous and Mesoporous Materials 145 (2011) 124–130 125

these materials represents a breakthrough in materials science,since it proposes the use of a quaternary ammonium surfactantwith a long carbon chain (C8–C16) as template/structure directingagent for the formation of periodic mesoporos silica and alumino-silicates [6–8]. The syntheses of MCM-41 with hexagonal array ofchannels, MCM-48 with cubic pore organization and MCM-50, alamellar phase, were reported, and these three materials composethe M41S family.

After the discovery of M41S materials, many other mesoporoussilicas were reported, but none of them had such a great impact asthe mesoporous SBA (Santa Barbara) family in 1998. The principalmaterials of this family are the hexagonal SBA-15 and the cubicSBA-16 silicas [9,10]. These mesoporous solids are synthesised inacidic medium (pH < 1, that is below the isoelectric point of silica)using the nonionic triblock polymers (polyethylene glycol)(poly-propylene glycol)(polyethylene glycol) as templates/structuredirecting agent and tetraetoxysilane as silica source. At the pH ofreaction, silica is present as a cationic species ((OH)3Si(OH2)+, I+)and the surfactant is protonated on the polyethylene glycol blocks,generating positive charges (S0H+). In this way, the interaction be-tween the template and the inorganic ions is mediated by the acidcounterion (X�) forming long range Coulombic interactions(S0H+)(X�I+). The main differences between M41S and SBA familiesthe materials are that the latter present: (i) micropores that linkthe mesopores, (ii) larger pore sizes and wall thicknesses; (iii)higher hydrothermal stability, (iv) use of biodegradable templateand (v) cheaper and easier synthesis procedure.

Due to their characteristics, the application of SBA mesoporousmaterials as catalysts became potentially interesting. For acid catal-ysis however, it is necessary to introduce aluminum into the SBAsilica networks. The procedure for such modification, that was con-sidered easy for the M41S family materials, is a challenge for SBAmaterials: in the pH of synthesis (<1) aluminum ions are presentas aqua species that do not condense with the growing of the silicamatrix, in contrast with the oxo species formed under alkalineconditions, such as in the case of the [Al]-MCM-41 synthesis [11].Thus, to insert aluminum on SBA materials it would be necessaryto use post-synthesis methods, which require additional prepara-tion steps and a further calcination process after ion exchange,leading to irregularly distributed aluminum sites [12] and produc-ing a high amount of extra-framework aluminum [11]. Bookmarknot defined. In 2004, two groups reported alternatives to the directintroduction of aluminum in SBA-15 silica amorphous matrix: Liand co-workers [12]. Bookmark not defined. suggested a pre-hydrolysis of silicon and aluminum sources while Xiao and co-workers [11] proposed an adjustment of pH during the synthesispreparation. Both methods were successful in directly obtainingthe [Al]-SBA-15; the structural and acid characteristics of the mate-rials prepared with these methods were recently studied by us [13].

Since the synthesis of SBA-15 and SBA-16 differ only by the nat-ure of the surfactant and the reaction temperature, it was supposedthat the methods used for the direct synthesis of [Al]-SBA-15 couldbe adapted to prepare [Al]-SBA-16. Klimova and co-workers [14]tried to synthesize aluminum-containing SBA-16 using the pre-hydrolysis method and the obtained material presented low struc-tural order at long range (evidenced by X-ray diffraction) andincorporated a very low amount of aluminum (Si/Al ratio of 254)into the silica matrix. Furthermore, ammonia and pyridine adsorp-tion showed no presence of Brønsted acid sites. The pH-adjustingmethod was successfully used by us for the preparation of[Al]-SBA-16, [15] and the XRD pattern of the obtained materialwas similar to that obtained by Klimova. However, in our casealmost all aluminum was incorporated in the silica matrix, and aSi/Al ratio of 20 was obtained for a gel prepared with a ratio of15. CO adsorption at 100 K revealed the presence of medium andstrong acid sites in the sample.

Indeed, the traditional synthesis of SBA-16 often suffers fromlow reproducibility and reliability, [16] which can be responsiblefor the variable aluminum incorporation into the solid. RecentlyKleitz et al. [17] developed an easy and reproducible synthesis ofSBA-16 using butanol as co-surfactant. The authors stated thatthe addition of butanol emerges as an effective method for increas-ing the phase domains for ordered cage-like mesoporous silicas(SBA-16, in this case) in the syntheses employing triblock copoly-mers as structure-directing species.

Due to the low flexibility of synthesis conditions and the diffi-culty of surface modification, SBA-16 is generally less used thanSBA-15.Therefore, the purpose of this work is to develop a newmethod to introduce aluminum into the SBA-16 structure and thusenlarge the possibilities of applications. In this work it is proposed,for the first time, a one-pot synthesis of well ordered [Al]-SBA-16where butanol was used as co-surfactant and the pH-adjustingmethod as a tool to introduce aluminum in the silica matrix. The acidsites of the sample have been studied by CO adsorption at 100 K.

2. Experimental

2.1. Reagents

Tetraethylorthosilicate (TEOS), 98%, Alfa Aesar, poly(ethyleneglycol)-block-poly(propylene glycol)-block-poly(ethylene glycol)Pluronic F-127 (PEO106PPO70PEO106), Aldrich, aluminum sulfateoctadecahydrate, 98%, Aldrich, concentrated hydrochloric acid,37%, Synth, concentrated ammonium hydroxide solution, Synthand n-butanol, 99.8%, Aldrich were used without further treatments.

2.2. Synthesis of SBA-16 using butanol as co-surfactant (SBA-16-But)

The synthesis was carried out based on previous papers [18,19].In a 250.0 mL round-bottomed flask, 4.00 g of Pluronic F127, 8.30 gof concentrated HCl and 12.0 mL of n-butanol were dissolved in190.0 mL of distilled water at 318 K. Then, 20.00 g of TEOS wereadded and the mixture was stirred for 24 h at 318 K. After this,an Allihn Condenser was coupled to the round-bottomed flaskand the mixture was stirred for 24 h at 363 K. The gel molar ratiowas 1 SiO2: 3.21 � 10�3 Pluronic F127: 1.69 butanol: 0.88 HCl:112.98 H2O. Finally, the solid was filtered and washed with 4 L ofdistilled water and dried in air.

2.3. Synthesis of [Al]-SBA-16 with butanol as co-surfactant ([Al]-SBA-16-But)

In a 250 mL round-bottomed flask, 4.00 g of Pluronic F127,8.30 g of concentrated HCl and 12.0 mL of n-butanol were dis-solved in 190.0 mL of distilled water at 318 K. Then, 20.00 g ofTEOS were added and the mixture was stirred for 3 h at 318 K afterwhich 2.090 g of aluminum sulfate octadecahydrate (Al2(SO4)318-H2O (Si/Al = 15)) were added. The stirring was maintained for21 h at the same temperature as the step before. After this, thepH was adjusted to 7.5 with concentrated ammonium hydroxideand the reaction mixture was stirred for 24 h at 363 K. The gel mo-lar ratio was 1 SiO2: 3.21 � 10�3 Pluronic F127: 1.69 butanol: 0.88HCl: 112.98 H2O: 0.033 Al2O5. Finally, the solid was filtered andwashed with 4 L of water and dried in air.

2.4. Synthesis of [Al]-SBA-16 without butanol as co-surfactant

The sample was synthesised as reported in the literature [15]. Ina 250 mL two-necked round-bottomed flask, 4.00 g of PluronicF127 and 125.0 mL of an aqueous 2 molL�1 HCl solution wereadded and stirred for 15 h at room temperature. A clear solution

1 2 3 4 5

(c)

8x

8x (310)

(211)

(220)

(200)(110)

(b)Inte

nsit

y / a

.u.

2θ / °

5000 Cps

(a)

Fig. 1. X-ray diffractogram of the (a) SBA-16-But (b) [Al]-SBA-16-But (c) [Al]-SBA-16.

126 J.M.R. Gallo et al. / Microporous and Mesoporous Materials 145 (2011) 124–130

was obtained, heated to 301–303 K and 8.50 g of TEOS were added.The mixture was stirred for 4 h and 0.888 g of Al2(SO4)318H2O(Si/Al = 15) were added followed by 20 h of stirring. The whitedispersion was transferred to an autoclave for hydrothermaltreatment at 353 K for 48 h. Then, the cold mixture was transferredto a beaker and, under stirring; the pH was adjusted to 7.5 withconcentrated ammonium hydroxide solution, followed by another48 h period of hydrothermal treatment at 353 K. The gel molarratio was 1 SiO2: 7.54 � 10�3 Pluronic F127: 6.13 HCl: 188.42H2O: 0.033 Al2O5. The solid obtained was filtered, washed withplenty of distilled water and dried in air.

2.5. Calcination

Before characterization, the prepared samples were calcinedusing the same methodology. Powders were heated from roomtemperature to 823 K at 1 K min�1 under dry nitrogen and main-tained at this temperature for 10 h under dry oxygen flow.

2.6. Characterization methods

X-ray diffractograms were obtained on a Thermo ARL X-ray dif-fractometer, operating with CuKa X-ray radiation (X-ray generatorcurrent and voltage set at 40 mA and 45 kV).

Nuclear magnetic resonance spectra of 27Al with magic-anglespinning conditions were performed in a Bruker Advance III 500spectrometer. The spectra were recorded by using 1.0 ls single-pulse excitation (1H high power decoupling), corresponding to aflip angle of p/20 in order to ensure selective excitation of the cen-tral transition. A recycle delay of 0.5 s was used for all experiments.The spinning frequency was 15 kHz. Acid aqueous solution of alu-minum nitrate (1 mol L�1) was used as external reference.

FTIR analysis was carried out using a Bruker Equinox 55 spec-trophotometer equipped with a pyroelectric detector (DTGS type)using a resolution of 4 cm�1. Self-supporting pellets (made witha mechanical press at 5 ton cm�2) with density between 10 and20 mg cm�2 were placed into an IR cell equipped with KBr win-dows permanently attached to a vacuum line (residual pressure:1 � 10�6 mbar; 1 mbar = 100 Pa), allowing all treatments andadsorption–desorption experiments to be carried out in situ. Beforegas adsorption experiments, samples were outgassed at 673 K at aheating rate of 10 K min�1 for 2 h. Carbon monoxide was adsorbedat 100 K with a maximum pressure of 64 mbar, which was main-tained up to the system equilibrium (30 min). Then the pressurewas progressively reduced at 100 K down to 10�4 mbar.

Nitrogen physisorption measurements were carried out at 77 Kin the relative pressure range of 10�6 to 1 P/P0 using a QuantachromeAutosorb 1MP/TCD instrument. Prior to the analysis the sampleswere outgassed (residual pressure p < 10�7 mbar) at 523 K for15 h. Specific surface areas were determined by using Brunauer–Emmett–Teller (BET) equation. Pore size distributions were ob-tained by applying the NLDFT method for cylindrical pores. Themicroporous surface area and volume were calculated by t-plot.

The quantification of aluminum was carried out by inductivelycoupled plasma-mass spectrometry (ICP-MS) by ITECON s.r.l. labo-ratory (Nizza Monferrato (AT), Italy).

Transmission electron microscopy (TEM) images were per-formed with a JEOL 3010-UHR operating at 300 kV. Samples weredispersed in isopropanol by ultrasound and a drop of the suspen-sion was deposited on a copper grid with a lacey carbon film.

3. Results and discussion

A typical X-ray diffractogram of SBA-16 is expected to presentup to seven peaks at 2h < 3�, corresponding to an Im–3m cubicstructure [9]. These peaks are related to the Miller indexes (1 1 0)

(most intense peak), (2 0 0), (2 1 1), (2 2 0), (3 1 0), (2 2 2) and(3 1 2) whose reflections appears at positions of X,

p2X,

p3X,p

4X,p

5X,p

6X andp

7X. It is very often the case that just the firsttwo peaks are clearly observed [20,21]. The X-ray diffractograms ofthe SBA-16-But, [Al]-SBA-16-But and [Al]-SBA-16 are shown inFig. 1.

The X-ray diffractogram of the SBA-16-But sample (Fig. 1, curvea) shows reflections typical of cubic structure, indexed as (1 1 0),(2 0 0), (2 2 0) and (3 1 0) planes. The [Al]-SBA-16-But diffractionpattern (Fig. 1, curve b) contains the first five peaks expected toSBA-16 material, revealing that the structural order is maintainedafter aluminum insertion. The X-rays diffractogram of Al-contain-ing SBA-16 prepared in the absence of n-butanol as co-surfactant([Al]-SBA-16, Fig. 1, curve c), reveals only the first peak assignedto the (1 1 0) plane. Due the low structural order observed byXRD, the [Al]-SBA-16 sample can be formally considered SBA-16-related-aluminosilicate because it has been synthesised througha typical procedure to obtain SBA-16 materials [15].

From the XRD patterns in Fig. 1, the unit cell can be calculatedby a0 = (

p2)k/2senh), where a0 is the unit cell parameter, k is the

wavelength for the CuKa X-ray radiation and h is the angle relatedto the (1 1 0) plane XRD peak. The results were 16.4, 17.8, and16.0 nm for SBA-16-But, [Al]-SBA-16-But and [Al]-SBA-16, respec-tively. Comparing the aluminum containing sample, it is clear thatthe use of butanol as co-surfactant led to a material with higherunit cell parameter.

The quantification of aluminum by ICP-MS showed that the Si/Al molar ratio is 14 and 20, respectively for [Al]-SBA-16-But and[Al]-SBA-16. It is suggested that n-butanol acts not only as an effi-cient co-surfactant to increase the organization of SBA-16 but alsothat its presence leads to an increase of the aluminum loading intothe structure.

The aluminum containing materials were also characterized bytransmission electron microscopy (TEM), as shown in Fig. 2. TheTEM images confirmed the XRD results, indicating that the [Al]-SBA-16-But (Fig. 2A) sample is a more organized structure thanthe [Al]-SBA-16 (Fig. 2B).

The textural properties and porosity of the materials preparedin this work were studied by nitrogen physisorption at 77 K(Fig. 3). The adsorption branch of the isotherms is characterizedby two significant steps: (i) the formation of the nitrogen mono-layer and filling of the micropores at P/P0 < 0.1; (ii) the capillarycondensation on the mesopores at 0.4 < P/P0 < 0.8. Another adsorp-tion step is observed for the [Al]-SBA-16 isotherm at P/P0 > 0.7 due

Fig. 2. TEM images for (A) [Al]-SBA-16-But and (B) [Al]-SBA-16. Image (B) was reported before on Ref. [15].

J.M.R. Gallo et al. / Microporous and Mesoporous Materials 145 (2011) 124–130 127

to filling of large nonstructural mesopores formed on the materialsaggregate interstices.

Pure silica SBA-16-But (Fig. 3, curve a) displayed a Type-IV iso-therm typical of mesoporous materials with a H2 hysteresis loop.This kind of hysteresis loop, usually attributed to pore blockingor systems in which the entrances of the pores are narrower thanthe internal part, is typical of SBA-16 and confirms that the mesop-ores are centered in mesocavities [9,22].

The isotherm obtained for the [Al]-SBA-16-But (Fig. 3, curveb) is similar to that obtained for SBA-16-But, but with a moreaccentuated capillary condensation on mesopores. This is proba-bly due to the higher pore volume of the aluminum containingmaterial.

[Al]-SBA-16 isotherm (Fig. 3, curve c) is also a Type-IV, however,the capillary condensation is broad and stretches from 0.4 to 1 P/P0,which indicates a wide pore size distribution due to a large numberof pore families. The hysteresis loop seems to be formed of morethan one contribution: (i) at P/P0 > 0.9, a H3 type hysteresis loopis present and revels the existence of non-structural slit poresprobably formed by the aggregation of particles; (ii) at P/P0 < 0.9it is difficult to define if there are just one H1 type hysteresis, or

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

600

700

(c)

(a)

(b)

Volu

me

/ cm

3 g-1

Relative Pressure / PP0

Fig. 3. Nitrogen physisorption isotherms obtained at 77 K of (a) SBA-16-But, (b)[Al]-SBA-16-But and (c) [Al]-SBA-16.

the overlap of two H1 hysteresis loops (one at 0.8 < P/P0 < 0.9 andthe other at 0.5 < P/P0 < 0.8). The isotherm obtained for [Al]-SBA-16 indicates a large and heterogeneous pore size distribution.

For the calculation of the pore diameters of the samples NLDFTmethod was used. For SBA-16-But and [Al]-SBA-16-But isothermsthat present a H2-type hysteresis loop, NLDFT was applied on theadsorption branch, while it was applied on the desorption branchof the [Al]-SBA-16 isotherm, since it presents a H3-type hysteresisloop [23]. The pore size distribution diagrams are show in Fig. 4.

The pore sizes distribution for the SBA-16-But (Fig. 4, curve a)displays a family of micropores with maximum at 0.9 nm and afamily of mesopores with maximum at 5.9 nm. These resultsare usual for the SBA-16 structure and were reported elsewhere[17]. The distribution for [Al]-SBA-16-But (Fig. 4, curve b) alsoshowed two families of pores: the same family as SBA-16 at0.9 nm in a lower amount and another one probably correspond-ing to that at 5.9 nm for SBA-16-But now shifted to 7.3 nm in[Al]-SBA-16-But with a larger intensity. A broad family of poreswith sizes between 2 and 6 nm is also present. These data indi-cated that the process of aluminum insertion led to a significantreduction of the microporosity and an increase of the mesoporos-ity, probably through a process of micropores condensation

0 2 4 6 8 10 12 14 16 18 20

5.7

3.6

10 cm3nm-1g

(b)

(a)

(c)

dV

/ a.

u.

Pore diameter / nm

7.3

5.9

0.9

13.9

Fig. 4. Pore size distribution for (a) SBA-16-But, (b) [Al]-SBA-16-But and (c) [Al]-SBA-16.

100 80 60 40 20 0 -20 -40

(a)

Chemical Shift / ppm

(b)

Fig. 5. 27Al-MAS–NMR spectra of (a) [Al]-SBA-16 and (b) [Al]-SBA-16-But.

128 J.M.R. Gallo et al. / Microporous and Mesoporous Materials 145 (2011) 124–130

leading to mesopores. It must be taken in consideration that theinsertion of aluminum required a further 24 h hydrothermaltreatment at pH 7.5, hence the differences observed betweenthe SBA-16-But and [Al]-SBA-16-But can also be due to the ex-tended hydrothermal treatment.

The [Al]-SBA-16 (Fig. 4, curve c) exhibits a family of pores with amaximum at 5.7 nm and another one with sizes between 8 and18 nm that can be attributed to slit pores formed between theinterstices in the agglomerates of particles. No microporosity wasidentified by this calculation method.

The collected physisorption data were also used to calculate thespecific surface areas and the pore volume of the samples (Table 1).The specific surface area was calculated using BET model, the totalpore volume was obtained at 0.9 P/P0 (except for [Al]-SBA-16, inwhich 0.6 P/P0 was used) and the microporous surface area andthe volume were obtained by t-plot method.

In the case of [Al]-SBA-16-But, the insertion of aluminum ledto an increase of the specific surface area (from 600 to980 cm2 g�1) and total volume (from 0.43 to 0.98 cm3 g�1) anda decrease in the microporous area (from 285 to 195 cm2 g�1)and volume (from 0.12 to 0.09 cm3 g�1), if compared to the puresilica SBA-16-But. These results are in agreement with those ob-tained by NLDFT, which suggested that the presence of alumi-num in the silica matrix decreased the microporosity andincreased mesoporosity. An interesting observation is that theSBA-16-But presents almost half of its surface area in microporeswhile in [Al]-SBA-15-But the micropores area is around 20% ofthe total area.

[Al]-SBA-16 presented lower specific surface area and pore vol-umes in comparison to [Al]-SBA-16-But (Table 1). Interestingly, forNLDFT [Al]-SBA-16 did not seem to have micropores, while t-plotmethod revealed a significant amount of them. This should indi-cate that either microporosity is disordered in this material or thatthe formalism of each method allows detection of only certain fea-tures of the solids or not.

Comparing the two aluminum containing samples, it is clearthat the use of butanol as co-surfactant leads to the formation ofa material with richer porosity properties such as presence ofmicropores, larger mesopores, higher surface area and porevolumes.

[Al]-SBA-16-But and [Al]-SBA-16 were also studied by 27Al-MAS–NMR in order to confirm the insertion of aluminum in the sil-ica matrix (Fig. 5).

The [Al]-SBA-16-But 27Al-MAS–NMR spectrum (Fig. 5, curves b)presents peaks at 0 and 54 ppm, attributed, respectively to octahe-dral and tetrahedral coordinated aluminum.

Both [Al]-SBA-16 and [Al]-SBA-16-But 27Al-MAS–NMR spectra(Fig. 5, curves a and b) present intense peaks at 54 and 49 ppm,respectively, due to tetrahedral aluminum coordinated to the silicamatrix. For aluminum containing mesoporous silica and zeolites,more often the chemical shift for tetrahedral aluminum is assignedat 53–54 ppm. [11,24]. However lower values have also been re-ported, [25,26] and this may be attributed to a distortion in thealuminum tetrahedral coordination. [26,27] Octahedral aluminum(peak at 4 and 0 ppm, respectively for [Al]-SBA-16 and [Al]-SBA-

Table 1Surface area and porosity properties of SBA-16 and [Al]-SBA-16 samples.

Sample SBETa/m2 g�1 SMicro

b/m2 g�1 VTc/cm3 g�1 VMicro

d/cm3 g�1

SBA-16-But 600 285 0.43 0.12[Al]-SBA-16 465 115 0.38 0.05[Al]-SBA-16-But 980 195 0.98 0.09

a BET Surface Area.b Micropores Area obtained by t-plot.c Total Pore Volume.d Micropores Volume obtained by t-plot.

16-But), in minor amounts, was also observed and can be attrib-uted to extra-framework aluminum or to partially extra-frame-work species that may coordinate water molecules when thesample is exposed to air moisture [28]. The ratio between the areasof the resonance peaks related to tetrahedral and octahedral alumi-num were 3.0 and 2.1, respectively for [Al]-SBA-16 and [Al]-SBA-16-But.

Carbon and nitrogen monoxides are exceptionally good probemolecules; together they map sites and reactivity of several typesof solids, from acidic to metallic functions [29]. The relativestrength of Brønsted acid sites was determined by infrared spec-troscopy of CO adsorbed at 100 K for [Al]-SBA-16-But (Fig. 6 Aand A0) and for [Al]-SBA-16. (Fig. 6 B and B0). [30,31] Fig. 6 C andC0display a comparison between the FTIR spectra of [Al]-SBA-16-But (curve a) and [Al]-SBA-16 (curve b) at the maximum CO pres-sure (64 mbar).

The IR spectra of Al-SBA-16-But treated under vacuum at 673 K(Fig. 6A inset, curve a) is dominated by an intense band at3745 cm�1 slightly asymmetric towards low frequency side and abroad tail in the 3700–3500 cm�1 range. As reported in the litera-ture, the band at 3745 cm�1 is associated to the OH stretchingvibration of isolated silanol groups, [32] whereas a component atlow frequency (ca. 3735 cm�1) is associated to weakly perturbedSi–OH sites, probably located inside micropores of SBA-16 struc-ture [33]. Similar IR features were observed in pure SBA-15 sampletreated in similar conditions [13]. The broad band at 3700–3600 cm�1 is associated to hydrogen bonded silanols [34] superim-posed to the stretching vibration of OH bonded to Al sites, whichare clearly revealed upon CO adsorption (vide infra).

The admission of CO at 100 K on [Al]-SBA-16 samples leads tothe formation of Si–O–H� � �CO adducts, partially converting the sil-anol OH stretching band at 3745 cm�1 (curve b on the Fig. 6A insetand as negative band in Fig. 6 A and B) in an absorption with max-imum at ca. 3660 cm�1. The shift of the 3745 cm�1 band is respon-sible for the formation of two symmetric and sharp bands withpositive and negative components in the 3770–3730 cm�1 range,after subtraction of the spectra of the bare sample. The negativeband is slightly asymmetric toward the low frequency side, dueto the partial consumption of the band at ca. 3735 cm�1 (assignedto weakly perturbed Si–OH sites, [35] probably located on themicropores of the structure) in the original spectra. Other bandsat 3470 cm�1 for both samples (Figure 6 A and B) and at 3430and 3230 cm�1 for [Al]-SBA-16-But (Fig. 6A) are due to the interac-tion between CO and acid sites generated by aluminum insertion.The two bands at 3470 and 3430 cm�1 are attributed to medium

3800 3600 3400 3200 3000

3800 3600 3400 3200 3000

0.05

ba

3745

Abs

orba

nce

(a.u

.)

A

0.05

32303430

3470

Ab

sorb

ance

/ m

g*c

m-2

Wavenumber / cm-12250 2200 2150 2100 2050

A'

0.052155

2172

2134

Ab

sorb

ance

/ m

g*c

m-2

Wavenumber / cm -1

3800 3600 3400 3200 3000

B0.015

3662

3470

Ab

sorb

ance

/ m

g*c

m-2

Wavenumber / cm-12250 2200 2150 2100 2050

B'

2134

2155

2172A

bso

rban

ce /

mg

*cm

-2

Wavenumber / cm-1

0.005

3800 3600 3400 3200 3000

a

b

C0.05

Ab

sorb

ance

/ m

g*c

m-2

Wavenumber / cm-12250 2200 2150 2100 2050

C'

b

a

0.05

(Ab

sorb

ance

/ m

g*c

m-2)

Wavenumber / cm-1

Fig. 6. FTIR spectra of increasing doses of CO adsorbed at 100 K (PCO(max) = 64 mbar) on (A and A0) [Al]-SBA-16-But and (B and B’) [Al]-SBA-16. (C, C0) comparison between[Al]-SBA-16-But (a) and [Al]-SBA-16 (b). Spectrum of the bare sample before CO adsorption was used as background. INSET: IR spectra of Al-SBA-16 treated under vacuum at673 K before (curve a) and after (curve b) admission of 64 mbar of CO. (B) was reported before on Ref. [15].

J.M.R. Gallo et al. / Microporous and Mesoporous Materials 145 (2011) 124–130 129

strength Brønsted acid sites and are commonly assigned to Al–OHgroups present in mesoporous aluminosilicates [13]. The band at3230 cm�1 is due to a family of strong Brønsted acid sites in whichthe hydroxyl groups are bridged (Al–OH–Si) and are typical ofstrong acid sites in zeolites [26].

At lower wavenumbers, for both Al-SBA-16-But (Fig. 6A0) andAl-SBA-16 (Fig. 6B0) three bands located at 2172, 2156, and2138 cm�1 are attributed, respectively, to the stretching of COhydrogen bonded to the strong Brønsted acid sites generated byaluminum introduction, to the stretching frequency of CO hydro-gen-bonded to Si–OH species and to liquid-like CO in the pores

of SBA-16 structure, respectively. By progressive decreasing COpressure the band at 2172 cm�1 becomes more evident, witnessingthe higher stability of these adducts with respect to CO interactingwith Si–OH groups. The relative concentration of the Brønsted acidsites generated by aluminum introduction can be evaluated by thearea of the peak at 2172 cm�1: [Al]-SBA-16-But has 7.7 times moresites than [Al]-SBA-16.

No band at 2230 cm�1 usually assigned to extra-framework alu-minum oxide, [35] was observed. Thus, the octahedral aluminumspecies found on the 27Al-MAS–NMR are probably due to partiallyextra-framework species.

130 J.M.R. Gallo et al. / Microporous and Mesoporous Materials 145 (2011) 124–130

4. Conclusions

The use of a 3D mesoporous solid as catalyst or catalyst supportor as adsorbent is in general more interesting than 1D or 2D mes-oporous solids (such as the well known MCM-41 and SBA-15), dueto the better diffusion of reactants and lower pore blocking effects.Despite the interesting 3D structure, the catalytic uses of SBA-16are limited due to the lack of syntheses methods that allow thecontrolled insertion of aluminum (or other heteroatoms) into thesilica matrix whilst preserving the ordered pore organization.

In the present work, butanol proved to be an efficient co-surfac-tant for the one-pot synthesis of well organized [Al]-SBA-16. Thismaterial displayed high surface area and pore volumes, highlydesirable features for application on catalysis or as adsorbents.Contrarily, when synthesised in the absence of butanol, a non-or-ganized, mesoporous material is formed with half of the surfacearea of [Al]-SBA-16-But.

Careful CO adsorption monitoring showed the similarity be-tween the nature of the families of surface sites present in both[Al]-SBA-16 synthesised in the presence and in the absence ofbutanol, however these sites are ca. 8 times more abundant inAl-SBA-16-But. The surface acidity of the [Al]-SBA-16-But is com-parable to that of [Al]-SBA-15 or [Al]-MCM-41. This work makesa 3D-mesoporous molecular sieve available for use in acid-cata-lyzed reactions, for instance, as support for large-sized anchoredcatalysts, and for adsorption of organic contaminants or complexedmetal ions.

Acknowledgments

The authors acknowledge the Piedmont Region (Microcell Pro-ject) and the ‘‘Fundação de Amparo à Pesquisa no Estado de SãoPaulo’’, FAPESP, for the financial support to this work. JMRG thanksMinistero dell’Istruzione, dell’Università e della Ricerca for the PhDfellowship.

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