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Colloids and Surfaces A: Physicochem. Eng. Aspects 300 (2007) 129–135

Hierarchical aluminosilicate macrochannels with structured mesoporouswalls: Towards a single catalyst for multistep reactions

Alexandre Leonard a,1, Bao-Lian Su a,∗a Laboratoire de Chimie des Materiaux Inorganiques (CMI), University of Namur (FUNDP), 61, rue de Bruxelles, 5000 Namur, Belgium

Received 19 June 2006; received in revised form 3 November 2006; accepted 6 November 2006Available online 15 November 2006

bstract

Two novel and simple synthesis pathways, one surfactant-assisted and one template-free, have recently been developed for the formation ofluminosilicate macrochannels with mesoporous walls. These thermally stable materials are made of straight tubular macrochannels separatedy wormhole-like mesopores. This paper gives a comparative study of the materials obtained by both ways in order to determine the role playedy the surfactant. High specific surface area materials with homogeneous tubular macrochannels are spontaneously formed without needing anyemplating procedure. The use of a non-ionic surfactant favors the incorporation of Al atoms in tetrahedral sites and also controls the homogeneity

f the mesopores. The formation of the macrochannels is believed to result from the spontaneous and strong release of alcohol molecules due toigh hydrolysis and condensation rates of the inorganic sources, which creates a hydrodynamic flow, thus channels of the solvent. The mesoporesre generated by interparticular voids. 2006 Elsevier B.V. All rights reserved.

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eywords: Macro-mesoporous; Hierarchical; Aluminosilicate; Catalyst; Forma

. Introduction

Zeolites and mesoporous materials have been the center ofonsiderable attention for the past decade due to their importantpplication perspectives in various fields such as air sepa-ation, waste-water treatment, ion-sensing, molecular sieving,iomolecule encapsulation, acid or redox catalysis and crack-ng of petrol feedstock [1–6]. The interest of such materialsrises not only from their various compositions (aluminosili-ates, aluminophosphates, titanosilicates, vanadosilicates, . . .),hich can be easily tuned depending on the desired application

acid or redox catalysis, for example), but also from their textu-al characteristics (uniform and tuneable openings at molecularize accompanied by a high accessible surface area) [7,8].

Nevertheless, the openings of zeolites and mesoporous mate-ials remain confined at the nanometre level. The increase infficiency of catalytic processes with lowest energy and crude

∗ Corresponding author. Tel.: +32 81 72 45 31; fax: +32 81 72 54 14.E-mail address: bao-lian.su@fundp.ac.be (B.-L. Su).

1 Charge de Recherches, Fonds National de la Recherche Scientifique, 5 rue’Egmont, B-1000, Bruxelles, Belgium.

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927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2006.11.015

echanism

aterials consumption and environmental-friendly industrialrocesses might however require the concretisation of the “one-ot reactor” concept [9,10], permitting more than one catalyticeaction steps to be performed within one single structure. Suchone-pot reactor could be achieved by developing simple and

apid synthesis procedures of hierarchical materials that con-ain individually adjustable interconnected macro- and meso-or micro-) porous arrays [11–14].

In our human body, capillary vessels ensure an optimal oxy-en distribution to the target cells, but a sufficient transports achieved only thanks to the large arteries. In catalysis, theresence of macropores could account for a better mass trans-er to the active sites dispersed in the micro- and mesopores,specially when large molecules are implied (e.g. polymers oriomolecules) or in viscous systems. Promoting catalytic activ-ty thus involves an increase in inter-channel accessibility [15].n FCC catalysis, hierarchical composites are made of a mainomponent, a USY zeolite mixed with a macroporous matrix,sually amorphous silica, alumina or silicoalumina with clay

16,17]. The bulky precursors are pre-cracked in the macrop-rous array prior to the oriented cracking and fine rearrangementf gas–oil molecules within the microporous cages of USY. Anntermediate cracking step can further be carried out if the zeo-

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30 A. Leonard, B.-L. Su / Colloids and Surfaces A

ite is steam-treated to introduce mesopores. It would howevere more desirable to directly prepare such hierarchical mate-ials instead of mixing artificially two or three materials withefined preformed pore sizes. Tuneable macropores separatedy mesoporous walls, which in turn can also be interconnectedy a microporous array could lead to high performance struc-ures perfectly adapted to the desired application, as for exampleatalytic cracking but also oriented fine chemistry synthesesmedicines, polymer precursors, etc.) or photocatalytic degra-ation of wastes.

Several methods have already been developed in this aim andave led to macro-mesoporous structured SiO2, Al2O3, Nb2O5,iO2, ZrO2, . . . [12,18–23]. The simplest and most employedathway makes use of a dual templating procedure (surfac-ant plus micron-sized polymer spheres). This synthesis is quiteedious and the calcination to remove the organic templates isften accompanied by a collapse of the structure, which sets aarrier to the diversity in compositions that can be achieved.ther preparation pathways have been developed making usef filamentous bacteria, vesicle formation, emulsions or multi-urfactant templating for creating porosity but homogeneousierarchical structures seem to be difficult to obtain [20,24,25].

We described the one-pot synthesis of hierarchical macro-esoporous aluminosilicates by using a single surfactant for

oth pore systems creation [9,26,27]. Later on, we proposed aew synthesis scheme based on a spontaneous formation mecha-ism leading to tubular macrochannels separated by mesoporousalls [10,28–30]. The aim of this paper is to describe and to com-are both the obtained materials in order to shed some light onhe formation mechanism and to determine the influence of theon-ionic surfactant on the final characteristics.

. Experimental

.1. Synthesis

All chemicals were purchased from Aldrich. Surfactant-emplated syntheses were carried out as follows: a 10 wt.%

icellar solution was prepared by dissolving the surfac-ant [C16(EO)10 (Brij 56)] in 60 ml of an aqueous solutionpH 2, 7 or 10, twice-distilled water, acidified or basifiedith H2SO4 or NaOH, respectively) at 70 ◦C under stirring.fter homogenization, 2.41 g of alumina [aluminum tri-sec-utoxide, C12H27AlO3] and 5.96 g of silica [tetramethoxysilane,4H12O4Si] sources were successively added to reach a surfac-

ant/inorganics molar ratio of 0.2 and a Si/Al ratio of 4. Thebtained medium was stirred for 1 h at 70 ◦C before being trans-erred to Teflon cartridges sealed in stainless steel autoclavesnd heated for 1 day at 80 ◦C. The final products were recoveredfter ethanol extraction with a soxhlet apparatus during 30 h andalcination under nitrogen and then air atmosphere at 550 ◦C for8 h in order to remove all the surfactant and impurities.

The template-free synthesis pathway is similar except that the

icellar solutions were replaced by 60 ml of acidic, neutral or

lkaline aqueous solutions. The respective amounts of aluminand silica sources employed were the same as described abovend the gels were either heated in autoclaves (1 day at 80 ◦C)

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sicochem. Eng. Aspects 300 (2007) 129–135

r directly dried at 50 ◦C. As there is no surfactant present, thextraction step was skipped.

.2. Characterization

To assess the structure, powder XRD measurements werearried out on a Philips PW1820 with Cu K� radiation. Theransmission electron micrographs were taken using a 100 kVhilips Tecnaı microscope with sample powders embedded inn epoxy resin and sectioned with an ultramicrotome (thick-ess of the slices: 30–40 nm). The obtained thin films wereupported on copper grids. The morphology as well as theacroporous array was observed with a Philips XL-20 scanning

lectron microscope (SEM) with conventional sample prepara-ion and imaging techniques. Nitrogen adsorption–desorptionsotherms were collected at −196 ◦C over a wide relativeressure range from 0.01 to 0.995 with a volumetric adsorp-ion analyzer ASAP 2010 or TRISTAR 3000 manufacturedy Micromeritics. The samples were degassed overnight underacuum before nitrogen adsorption measurements. The poreiameter and the pore size distribution were determined byhe BJH method [31]. The environment of Al atoms was stud-ed by means of 27Al MAS NMR with a Bruker Avance 500pectrometer.

. Results and discussion

From the structural point of view, no distinction can be madeetween the samples prepared in the presence or absence ofurfactant. Indeed, for a starting pH of 2, the XRD spectra onlyhow one broad band located between 2θ = 20◦ and 30◦, indicat-ng a globally amorphous framework (Fig. 1). If pH is howeverncreased, some weak reflections can be detected, suggestinghe appearance of a crystalline phase within the material (Fig. 1,rrows). This can be assigned to boehmite AlOOH accordingo literature [32]. Higher starting pH values thus favor the for-

ation of small crystalline domains inside the structure or tombedded crystals within the amorphous framework, as sepa-ate crystals could not be seen by microscopy. This observation isery interesting since crystalline structures could show a higherhermal resistance than their amorphous counterparts.

For both preparation pathways, the 27Al NMR spectra showwo peaks located at 0 and 50 ppm corresponding, respectively,o octahedral and tetrahedral Al (Fig. 2). The former can bessigned to extra-framework Al (EFAl) whereas the latter isupposed to correspond to Al atoms located inside the inorganicorous framework with the Si atoms. If the (micellar) solutions neutral or alkaline, the EFAl are predominant, indicating thathe incorporation of Al within the framework is not favored.

hen starting however from an acidic solution, the tetrahedralpecies become more important. For the surfactant-templatedreparation, an intense peak is detected at 54 ppm and a weaknd somewhat broader one at 0 ppm, confirming the presence of

he majority of Al in framework positions. This favored incor-oration at lower pH is also valid for the template-free pathwayut in this case, the six-fold-coordinated Al atoms prevail overhe tetrahedral incorporated species, suggesting that the non-

A. Leonard, B.-L. Su / Colloids and Surfaces A: P

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ig. 1. XRD patterns of aluminosilicate materials prepared in the absence ofurfactant as a function of pH of the starting aqueous solution. The arrowsndicate the reflections attributed to the crystalline phase.

onic surfactant helps the incorporation of Al atoms inside theramework.

The formation of macropores is visualized by SEM (Fig. 3).or each preparation pH of the (micellar) solution, a regular arrayf tubular macrochannels is clearly observed. These macrochan-

els are quite parallel to each other and perpendicular to theonolithic particle’s surface. In contrary to the preparations

arried out in the presence of latex spheres, for instance, theacropores do not appear as successive connected holes but

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ig. 2. 27Al NMR spectra of aluminosilicate hierarchical porous materials prepared a

hysicochem. Eng. Aspects 300 (2007) 129–135 131

ather as continuous homogeneous channels that extend through-ut the whole particle. In the absence of surfactant, the sameypes of macrochannels can be evidenced, but in this case, theiruantity seems to increase with starting pH value, as qualita-ively evaluated by SEM observations (Fig. 3B).

A closer look of the section of these macrochannels is giveny TEM (Fig. 4). The micrographs of the templated sampleslearly show the circular homogeneous section of the chan-els, which have openings ranging from 0.5 to 2 �m (Fig. 4A).he deeper look into the structure of the walls separating theacropores reveals a disordered mesoporous array (Fig. 4B).ifferences appear for the template-free preparations as theiameters of the macropores increase with pH of preparationFig. 5A). At a pH value of 2, the openings are the same as forhe templated procedure but they increase at 3 �m at pH 7 andven up to 5 �m for pH 10. The comparison of both pathwaysnder neutral conditions shows a much better resolved circularection in the absence of templating molecules. Moreover, undereutral and alkaline conditions, a dense corona is also observedt the inner side of the cannels, and becomes more visible withpH increase. Higher magnification views are given in Fig. 5B.he walls separating the macrochannels are again made of a dis-rdered mesoporous array. The dark corona at the inner surfacef the channels also appears as a wormhole-like mesoporoustructure, precluding this to be a crystalline phase (white arrown Fig. 5B). No explanation has however been found yet ded-cated to this observation. Nevertheless, these results open uphe debate concerning the exact role played by the surfactantn the formation of the hierarchical macro-mesoporous array.t is indeed evident that both pore systems can be created inhe absence of any template and that the macrochannels evenossess a better resolved circular cross-section in this case.

The porosity of the prepared materials was assessed byitrogen adsorption–desorption measurements (Fig. 6). Inde-endently of the preparation pathway, if pH increases, thedsorption isotherms tend to type II (i.e. macropores) with broadnd weak pore size distributions. However, for a pH of 2, thesotherms are type IV, characteristic of mesoporous compoundsFig. 6). A capillary condensation step can be seen at relative

ressures of about 0.75, indicating the presence of mesopores.he samples prepared at pH 2 without surfactant also show atrong nitrogen uptake at high relative pressures, being charac-eristic of larger sized openings. The corresponding pore size

t a starting pH of 2 in the presence (A) and in the absence (B) of surfactant.

132 A. Leonard, B.-L. Su / Colloids and Surfaces A: Physicochem. Eng. Aspects 300 (2007) 129–135

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ig. 3. SEM pictures showing the macrochannels of aluminosilicate structures p

istributions are quite broad but have a maximum centered atbout 4 nm. The specific surface area has been calculated byhe BET method giving values comprised between 500 and00 m2/g for all the investigated samples prepared under variousreparation conditions.

The tendency is the same whether a surfactant template is

sed or not and clearly confirms, together with microscopybservations, the porosity at different length scales, thus theierarchical character of our materials. As both preparation path-ays give rise to materials with identical textural characteristics,

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ig. 4. TEM picture of the cross-section of the macrochannels (A) and correspondingluminosilicate macro-mesoporous material.

d at different pH values in the presence (A) and in the absence (B) of surfactant.

he role played by the surfactant in mesopore formation is ques-ionable again.

. General discussion

All of the obtained results clearly demonstrate that hierarchi-

al macro-mesoporous aluminosilicates can easily be obtainedy a one-pot synthesis without needing any external templateuch as latex spheres for macroporosity creation. The openingshus necessarily arise from the presence of both inorganic precur-

higher magnification view showing the mesopores (B) of a surfactant-templated

A. Leonard, B.-L. Su / Colloids and Surfaces A: Physicochem. Eng. Aspects 300 (2007) 129–135 133

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ig. 5. TEM pictures showing the cross-section of the macrochannels (A) aluminosilicate macro-mesoporous materials prepared without any template.

ors as well as the co-solvents originating from their hydrolysisnd their polycondensation reactions. These materials are madef homogeneous tubular macrochannels with micrometric sizeseparated by wormhole-like mesoporous walls with openingsf about 4 nm. The hierarchical structures are obtained for theurfactant-mediated pathway for pH values between 2 and 10.n the absence of any template, this synthesis is only possiblen an acidic medium (pH 2) as in neutral and alkaline condi-ions, the mesoporous character vanishes and the macroporosityecomes largely predominant. In both cases, only an acidic start-

ng solution allows for incorporation of Al atoms in tetrahedralositions.

Mann et al. reported an ammonia induced template-free andon-stirring method for the preparation of macroporous titania

ig. 6. Nitrogen adsorption–desorption isotherms and corresponding pore sizeistributions of aluminosilicate hierarchical porous materials prepared at a start-ng pH of 2 in the presence (A) and in the absence (B) of surfactant.

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orresponding higher magnification views showing the mesopores (B) of an

33]. Based on this report, on the above results and on obser-ations made by optical microscopy, we propose the followingossible mechanism (Fig. 7). The addition of both inorganicources into the aqueous (micellar) solution would lead toroplets with the hydrolysis and condensation reactions of thenorganic precursors proceeding inwardly. High hydrolysis andolymerization rates induce the release of alcohol molecules,eading to a SiO2–Al2O3 solid phase in which water/alcoholhannels can form due to the hydrodynamic flow of the solvents.y this way, the solid structure could grow around these chan-els until all of the precursors are used up. If this phenomenons verified, neither micellar aggregates nor solid latex spheresre necessary for macrochannel formation but these would ariserom a phenomenon solely due to the solvent. Entire dropletsave however not been evidenced by SEM. The behavior of thenorganics when added to an aqueous solution was followedy optical microscopy. The initially formed droplets rapidlyncrease in size up to three times their initial volume and thenurst and fragment, probably due to the pressure exerted by thelways larger amounts of produced butanol and methanol. Thisakes place very rapidly, in less than a minute of time. Theseragments would then be those observed by SEM. Bursting couldven be favored by the stirring of the mixture during 1 h afterddition of the Si and Al sources.

The mesoporosity inside the walls separating the macrochan-els arises from the voids existing between the stackingf aluminosilicate nanoparticles [34]. However, as describedbove, the mesopores are more homogeneous in the presencef the non-ionic surfactant. The latter could either help in struc-

uring the assembly of these nanoparticles or it could play a realemplating role as commonly observed in the synthesis of pure

esoporous structured materials. The favored incorporation ofl atoms in tetrahedral positions could result from interactions

134 A. Leonard, B.-L. Su / Colloids and Surfaces A: Physicochem. Eng. Aspects 300 (2007) 129–135

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xisting between the Al precursor and the non-ionic hydrophiliceads of the polyoxyethylene alkyl ether surfactant.

. Conclusions

Two new one-pot synthesis routes for hierarchical macro-esoporous materials have been presented. The first one makes

se of a non-ionic surfactant whereas the second one is a spon-aneous route. Both lead to an aluminosilicate network withierarchical porosity, made of tubular macrochannels separatedy wormhole-like mesoporous walls. As no external templates needed to induce the macro- or mesopores, we proposed aynthesis scheme based on a templating action induced by theydrodynamic flow of the solvents. The presence of the surfac-ant leads to more homogeneous mesopores but does not affecthe macropores and it helps in incorporating the Al atoms inetrahedral framework positions.

We believe that this spontaneous (or surfactant-helped) routeould be suitable for the synthesis of other oxides such as titania,irconia, or mixed oxides, provided an inorganic source thatapidly polymerizes is used. This pathway could reveal very

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the formation of the macrochannels.

recious since no calcination is needed to remove the templates,voiding a breakdown of the structure for the more fragile oxiderameworks.

This kind of hierarchical materials with multi-porosity afterncorporation of different functionalities (acido-basic, redox andhotocatalytic properties) can be used as a single catalyst forultistep reactions. This work can open a new avenue for the

ntegration of multi-porosity and multi-functionality in one solidody.

cknowledgements

A. Leonard thanks the FNRS (Fonds National de la Recherchecientifique, Belgium) for a “Charge de Recherches” fellowship.inancial supports from a European Interreg III program (F.W-.1.5) and the Wallonia region are greatly acknowledged.

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