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Microporous and Mesoporous Materials 99 (2007) 23–29

Studies on the porosity of SiO2-aerogel inverse opalssynthesised in supercritical CO2

Albertina Cabanas a,*, Eduardo Enciso a, M. Carmen Carbajo b, Marıa J. Torralvo b,Concepcion Pando a, Juan Antonio R. Renuncio a

a Departamento de Quımica-Fısica I, Universidad Complutense de Madrid, 28040 Madrid, Spainb Departamento de Quımica Inorganica, Universidad Complutense de Madrid, 28040 Madrid, Spain

Received 27 April 2006; received in revised form 18 July 2006; accepted 2 August 2006Available online 31 October 2006

Abstract

A new technique to produce highly porous SiO2-aerogels inverse opals in supercritical carbon dioxide (scCO2) is being developed atour laboratory. Polystyrene latex particles decorated with methacrylic acid or a mixture of methacrylic and itaconic acid groups are orga-nized in three-dimensional 3D-latex arrays and used as templates. The polymeric template is reacted with tetraethylorthosilicate (TEOS)or tetramethylorthosilicate (TMOS) dissolved in scCO2 at 40 �C and 85 bar. The reaction is catalysed by organic acids attached to thelatex particle surface. SEM and TEM images show that, upon calcination of the template, highly porous materials replicating the struc-ture of the original template are obtained. N2-adsorption isotherms reveal the presence of a large porosity located in the macropore wall.In all the cases, the maxima in the pore size distribution appear in the mesopore range. In this paper we investigate ways of controllingthe porosity of the material by changing the precursors, the catalyst and/or its concentration. SEM, TEM and N2-adsorption data showthat, at the reaction conditions, reactivity of TEOS and TMOS on the templates is similar, yielding materials of comparable surface area.In the same way, the porosity of the aerogels obtained with templates impregnated using benzosulfonic (BSA) acid and p-toluene sulfonicacid is the same. Lower yields were obtained using monochloroacetic acid. The effect of the catalyst concentration is more important; asthe concentration of BSA in the template increases, lower values are obtained for the maximum in the pore size distribution.� 2006 Elsevier Inc. All rights reserved.

Keywords: Inverse opals; Supercritical fluids; SiO2-aerogels; Porous materials; Gas adsorption

1. Introduction

Porous materials have many applications in catalysis,chromatography, separation, biomaterials, microelectron-ics and photonic materials [1,2]. Most of these applicationsrequire a uniform pore size as well as a regular ordering ofthe pores. In order to achieve a structured pore system, dif-ferent methods have been applied in the literature [3]. Thetemplating technique is one of the most popular due to thevariety of pores sizes and structures which can be achievedby changing the template, thus pore size can be tuned from

1387-1811/$ - see front matter � 2006 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2006.08.030

* Corresponding author. Tel.: +34 91 3944200; fax: +34 91 3944135.E-mail address: a.cabanas@quim.ucm.es (A. Cabanas).URL: http://www.ucm.es/info/leffs (A. Cabanas).

the micropore to the macropore range. We are interested inthe synthesis of structured macroporous materials to beused as separation membranes, supports for catalysts andphotonic band gap materials. These materials can be easilyproduced using colloidal crystal templates such as mono-disperse polymer latex or SiO2 spheres.

A new method to produce structured macroporousmaterials using supercritical fluids has been recently devel-oped at our laboratory [4,5]. The method involves theimpregnation and reaction of inorganic precursors dis-solved in supercritical CO2 (scCO2) on the surface ofwell-ordered colloidal crystal templates. Monodispersepolystyrene (PS) spheres decorated with different hydro-philic groups on their surface are mixed with smallamounts of organic acids and are organized in 3D-ordered

T

reactorP

CO

2

CO2

T

reactorP

CO

2

CO2

Safetyvalve

Template Precursor: TEOS or TMOS

H2O

Thermostaticbath

ISCO pump

Fig. 1. Experimental set-up used to infuse and react TEOS or TMOS intothe 3D-latex array templates.

24 A. Cabanas et al. / Microporous and Mesoporous Materials 99 (2007) 23–29

arrays by solvent evaporation, membrane filtration, or cen-trifugation [6,7]. The arrays are then used as templates andceramic precursors are infused and reacted into the voidscreated by the polymer particles. After the reaction, thetemplate is removed by calcination or dissolution. Infusionof reactants into the template in the liquid-phase has beenpreviously carried out in the literature [8–10]. The noveltyof our method consists in carrying out the infusion andreaction process in a supercritical fluid. Reactions areperformed in supercritical CO2 since it is cheap, non-toxicand has relatively low critical temperature and pressure(Tc = 31 �C, Pc = 73.8 bar) [11]. Furthermore many metalalkoxides (ceramic precursor) dissolve in scCO2 at moder-ate pressure and temperature [12]. The low viscosity, highdiffusivity relative to liquids and very low surface tensionof scCO2 promote infiltration in complex geometries andmitigate mass transfer limitations common to liquid-phaseprocesses. CO2 is a gas at ambient pressure and it is elimi-nated completely upon depressurization. When using poly-meric templates, attention should be paid to the stability ofthe template, as CO2 is quite soluble in many polymers,thus decreasing the glass transition temperature (Tg) ofthe polymer [13]. This effect can produce aggregation andeven coalescence of the polymer particles [14].

Up to date we have shown the synthesis of SiO2-aerogelinverse opals in scCO2 using tetraethylorthosilicate (TEOS)[4,5]. Because of the good transport properties of the super-critical fluids, materials with exceptional porosity wereobtained. The effect of the latex composition on the poros-ity of the material was further studied. In this paper weinvestigate ways of controlling the porosity of the mate-rial by changing the precursors, the catalyst and/or itsconcentration.

2. Experimental

For the synthesis of the polymeric templates, styrene,methacrylic acid and itaconic acid (reagent grade) fromSigma–Aldrich and distilled water were used. Tetraethyl-orthosilicate (99+%), tetramethylorthosilicate (99+%),benzosufonic acid (99+%) and p-toluenesulfonic acidmonohydrate (98.5+%) were obtained from Aldrich.Monochloroacetic acid was supplied by Probus (purum).CO2 (purity >99.99%) was supplied by Air Liquide. Allchemicals were used as received.

Monosized polystyrene (PS) latex particles microspheresdecorated with methacrylic acid (MA) groups or a mixtureof MA and itaconic acid (IA) groups were copolymerisedin a surfactant free emulsion in water following a proce-dure previously described [7,9,10]. The molar ratio ofstyrene (S) to the monomers MA and IA was 17:1 (PS-MA), and 50:1:2 (PS-IA-MA) . Suspensions were dialysedfor four weeks against water to eliminate residual products.A small amount of monochloroacetic acid (ClAcH), benzo-sulfonic acid (BSA) or p-toluenesulfonic acid (pTSA) wasadded to the suspensions in varying concentrations and3D-latex arrays were prepared by centrifugation or evapo-

ration. Templates were dried in air at room temperatureand at 45 �C in an oven. 3D-latex arrays formed by parti-cles P200 nm appeared iridescent to the visible lightreflecting the 3D ordering of the templates. The degree ofordering in the template was also characterized using N2-adsorption [7].

3D-latex arrays were infused and reacted with siliconalkoxide precursors such as TEOS or tetramethylorthosili-cate (TMOS) dissolved in scCO2. The experiments werecarried out in a ca. 70 mL custom-made stainless-steelhigh-pressure reactor in the batch mode following a proce-dure previously described [5]. A diagram showing theexperimental set-up is shown in Fig. 1. The reactor wasloaded with three vials containing: (1) Pieces of 3D-latexarrays (1–5 mm thick), (2) TEOS or TMOS in large excessand (3) excess H2O. The reactor was placed into a thermo-static bath (PolyScience) and was filled with CO2 using ahigh-pressure syringe pump at the reaction temperature(Isco, Inc. Model 260D). The pressure was measured usinga pressure transducer (Druck Ltd.). A safety valve (Swage-lok) was fitted to the reactor. All experiments were per-formed at 40 �C and 85 bar. At these conditions, themorphology of the polymeric template was not altered.After 0.5–4 h, depending on the experiment, the reactorwas depressurised through a needle valve in 0.5–2 h. TEOSor TMOS reacted on the template, which was removed bycalcination in air at 500 �C for 3 h. The weight percentageremaining after calcination was determined by weightdifference.

Materials were characterized using scanning electronmicroscopy (SEM), transmission electron microscopy (TEM)and N2-adsorption experiments. SEM images were takenon a JEOL-6400 electron microscope working at 20 kVand a JEOL JSM-6330 F electron microscope working at15 kV. Samples were gold coated prior to analysis. TEMimages were obtained on a JEOL 2000FX electron micro-scope operating at 200 kV equipped with a double tilting(±45�) sample holder. Samples were dispersed in water or1-butanol over copper grids and dried in air. N2-adsorption–desorption isotherms at �196 �C were obtained usingan ASAP-2020 equipment from Micromeritics. Prior to

A. Cabanas et al. / Microporous and Mesoporous Materials 99 (2007) 23–29 25

adsorption measurements, samples were out-gassed at110 �C for 2 h. Isotherms were analysed using the BETequation. Pore size distributions were calculated using theBarrett, Joyner and Halenda (BJH) method for a cylindri-cal pore model [15].

3. Results and discussion

TEOS and TMOS have been previously used as SiO2

sources in scCO2. At low temperature, the decompositionof the silicon alkoxide requires the use of a catalyst [16–18], whilst TEOS can be decomposed at 120 �C withoutany catalyst [19,20]. The reaction seems to proceed througha traditional sol–gel mechanism involving hydrolysis andcondensation steps and is favoured by the presence ofwater [5,18].

PS-MA and PS-MA-IA 3D-latex array templates withBSA acid adsorbed at the surface of the particles werereacted with TEOS or TMOS dissolved in scCO2 at 40 �Cand 85 bar for 2–4 h. After the reaction, the material wascalcined and the polymeric template removed, yieldingthe inverse replica of the template. Materials producedusing the PS-MA-IA template formed by particles of diam-eter 350 nm rendered shinny light-blue coloured powders.Materials produced using the PS-MA template composedof smaller particles 170 nm were dull white in appearance.The colour is a manifestation of the periodicity of the mate-rial at the light scale. SiO2 loading after calcination wasbetween 4–11wt% with respect to the composite material(before calcination). SEM images of the templates andtheir inverse replicas are shown in Fig. 2.

PS-MA particles (170 nm diameter) were impregnatedwith a solution of BSA 0.0070 M (containing 2.951 g ofsolid) and assembled into 3D-latex arrays by centrifuga-tion. A SEM picture of the PS-MA 3D-latex array templateused is shown in Fig. 2(a). Picture shows that the templatepresents regions of face centered cubic (fcc) packingtogether with other less ordered regions. The SiO2 inverseopal obtained after infusion of the template with TEOSat 40 �C and 85 bar and further calcination is shown inFig. 2(b–c). After impregnation and calcination, a materialwith voids replicating the structure of the template wasobtained.

The reaction was also carried out using TMOS and thesame template. The reaction yield was evaluated as the per-centage of remaining mass after calcination of the templateinfused with the precursor in CO2. Slightly larger reactionsyields were observed for the samples obtained usingTMOS, probably due to the higher reactivity of this alkox-ide [21]. SEM pictures of the material obtained usingTMOS are shown in Fig. 2(d–e). A few SiO2 particlesappear on the surface of the material, probably due tothe incipient reaction of TMOS in the gas phase at theseconditions. Other than that, the material obtained usingTMOS was very similar to that obtained using TEOS atthe same conditions. TEM micrographs of the samplesobtained using both reactants are compared in Fig. 3.

TEM projections along the [111] direction in the fcc pack-ing show that the reaction occurs only on the particle sur-face and that both tetrahedral and octahedral holes in thestructure are empty (see small holes in the pictures). Con-sidering the size of the template before calcination andmeasuring the distance between centres of adjacent mac-ropores in the TEM pictures, the shrinkage of the structureupon condensation and calcinations is in both cases 17–18%. We have previously shown that the shrinkage inmaterials synthesised in scCO2 is smaller than the shrink-age observed in materials infused in the liquid-phase [5].The high solubility of the alcohol (by-product) in scCO2

probably favours hydrolysis and condensation reactionsand reduces shrinkage of the network.

N2-adsorption isotherms of these samples are comparedin Fig. 4. Isotherms correspond to type IV, showing abroad adsorption–desorption loop [22]. Surface areas ofthe materials according to the BET model (SBET) are 594and 551 m2/g, for the materials reacted with TEOS andTMOS, respectively. SBET of the materials produced inscCO2 are much larger than those reported by otherauthors for samples prepared from the infusion and reac-tion of TEOS and ethanol liquid mixtures into similar3D-latex array templates (ranging from 173 to 231 m2/g)[8]. The materials produced in scCO2 are aerogels of largesurface area, whilst the materials produced by sol–gel reac-tions in a liquid-phase and air-drying are xerogels of lowersurface area [21]. The method here proposed yields struc-tured aerogels materials templated by the 3D-latex arrays.

Inspection of the N2-adsorption isotherms reveals adrop in the desorption branch at a relative pressure of ca.

0.95 followed by a more pronounced one in the relativepressure range 0.85–0.45 for both samples. The drop at0.95 relative pressure can be related to the evaporationthrough the triangular windows formed by three neigh-bouring hollow spheres. The second drop extends up to0.45 relative pressures thus suggesting a broad distributionof mesopores in the wall, according to the patterns ofhydrophilic components on the particle surface.

The adsorption isotherms were studied using the BJHmethod for a cylindrical pore model [15]. Due to the limi-tations associated to the unknown pore system geometry,no other method was applied. Analysis of the adsorptionbranch gave a monotone decreasing pore size distributionwithout special features. Maxima corresponding to the tet-rahedral and octahedral holes of the fcc lattice were notcaptured by the pore distribution curve, thus suggestingthat the filling of those cavities is blurred by the adsorptionin the macroporous walls. Analysis of the desorptionbranch was more informative and pore size distributionswith maxima in the mesopore range were obtained. Sizedistributions calculated from the desorption branch arepresented in the inset of Fig. 4. Distribution curves inboth cases present maxima at ca. 6 and 5 nm for TEOSand TMOS, respectively. The evaporation of the gasadsorbed in the material takes place through the smallestpores, therefore the macropore walls must be formed by

Fig. 2. SEM of 3D-latex array templates: (a) PS-MA and (f) PS-IA-MA, and the periodic macroporous SiO2 obtained after the reaction with a siliconalkoxide in scCO2 at 40 �C and 85 bar and elimination of the template: (b, c) TEOS on PS-MA 0.0070 M BSA, (d, e) TMOS on PS-MA 0.0070 M BSA, (g,h) TMOS on PS-MA-IA 0.0017 M BSA, (i, j) TMOS on PS-MA-IA 0.020 M BSA. Scale bars = (a–b, f–g) 6 lm, (c–d, h–j) 1 lm and (e) 100 nm.

26 A. Cabanas et al. / Microporous and Mesoporous Materials 99 (2007) 23–29

mesoporous SiO2. This can be also inferred from the TEMpictures in Fig. 3. The higher reactivity of TMOS couldexplain the slightly smaller value of the maximum in thepore size distribution. The cumulative pore volume forthe samples produced using TMOS and TEOS were 1.35and 1.42 cm3/g, respectively.

In an effort to control the porosity of the material, dif-ferent organic acids such as pTSA and ClAcH were usedas catalysts. Materials obtained using pTSA and BSA atthe same concentration were very similar due to the similaracid strength. Comparatively, SiO2 loading after calcina-tion of templates impregnated with ClAcH at larger con-centration and reacted with TEOS in CO2 for up to 18 hwas only 3–4%. ClAcH is a weaker acid and did notdecompose the Si alkoxides very efficiently. At the sameacid concentration, reaction with TMOS for 2 h yielded,however, SiO2 loadings as high as 8%.

Further experiments were also performed at differentreaction times. PS-MA templates were impregnated withpTSA or BSA and assembled into 3D-ordered arrays byevaporation of 10 mL of a solution containing 0.64 g oflatex and 0.0014 M in the acid (0.022 mM acid per gramof latex). The templates were further reacted with TMOSand the template was removed by calcination. Two setsof experiments with varying reaction times were performed:(a) 30 min heating followed by 30 min depressurization and(b) 2 h heating followed by 1 h depressurization. Compar-ison of the weight gain after calcination in both casesrevealed a smaller yield for the shortest time. This effectwas more pronounced when the template was impregnatedusing pTSA.

We have also studied the effect of the catalyst concentra-tion in the ceramic material. 350 nm PS-MA-IA particleswere impregnated with BSA at different concentrations

Fig. 3. TEM of a periodic macroporous SiO2 obtained after infusion of silicon alkoxides in scCO2 on a PS-MA template after calcination along the [111]direction: (a) TEOS and (b) TMOS. Scale bars = 50 nm.

SBET(TEOS) = 594 m2/gSBET(TMOS) = 551 m2/g

ABS 0.0070 M

P/Po

PS-MA

dV/d

D/

Ads

orbe

d Q

uant

ity (c

m3

STP/

g)

Diameter/ nm

Fig. 4. Adsorption–desorption isotherms of macroporous SiO2 producedin scCO2 with PS-MA using: (s) TEOS and (h) TMOS. Inset shows thepore size distribution obtained from the desorption branch of eachisotherm.

A. Cabanas et al. / Microporous and Mesoporous Materials 99 (2007) 23–29 27

and assembled in 3D-latex arrays. Fig. 2(f) shows an SEMimage of the PS-MA-IA template. Fig. 2(g–j) and Fig. 5show SEM and TEM pictures, respectively, of the materi-als obtained from the reaction of TMOS in scCO2 at40 �C and 85 bar using this template. Fig. 2(g–h) and5(a–c) show the material obtained using a PS-MA-IA tem-plate prepared by evaporation of 20 mL of a 0.0017 MBSA solution containing 8.3 g of latex (0.004 mM BSAper gram of latex). Fig. 2(i–j) and 5(d–f) show the materialobtained using a PS-IA-MA template ordered by centrifu-gation from 75 mL of a 0.020 M BSA solution containing6.43 g of latex (0.23 mM BSA per gram of latex) and elim-ination of the supernatant. To refer to these templates, wewill use their acid concentrations: 0.0017 M and 0.020 MBSA. Although the reported acid concentration of the tem-

plates assembled by evaporation and centrifugation refersto different acid to template ratios, the catalyst concentra-tion in both templates is very different. SEM and TEM pic-tures of the materials obtained with the PS-MA-IAtemplate using the lowest acid concentration (Figs. 2(g–h)and 5(a–c)) are very similar to those shown in Fig. 2(c–e)and 3 obtained from the reaction of TEOS or TMOS ona PS-MA template. TEM projections along the [11 0],[111] and [100] directions show that during infusion pre-cursor coats the template surface, but does not fill the tet-rahedral and octahedral holes in the fcc packing. Detailedinspection of Fig. 5(b) shows small seams between macrop-ores in the projection along the [111] direction. Surfacecoating can be also observed from the analysis of SEMimages (Fig. 2(g–h)). This feature was previously observedin samples produced using the same template and TEOS [5]and seems to be related to the small shrinkage of the struc-ture after calcination. On the contrary, the materialobtained using the template with the highest acid concen-tration (Fig. 5(d–f)) presents thicker walls with no seamsbetween macropores and partial filling of the tetrahedraland octahedral holes. SEM images of this material(Fig. 2(i–j)) show small regions where excess SiO2 seemsto have blocked the macropores.

N2-adsorption isotherms of these samples are presentedin Fig. 6. Isotherms are similar to those shown in Fig. 4with broad adsorption–desorption hysteresis loops. Calcu-lated SBET for the materials produced with concentrationsof BSA 0.0017 M and 0.020 M are 505 and 473 m2/g,respectively. Larger differences are found when the poresize distributions obtained from the desorption branchare examined. As in the previous examples, the BJHmethod was assumed for a cylindrical pore system. Insetin Fig. 6 shows a well defined maximum at ca. 7 nm forthe PS-IA-MA template with 0.0017 M BSA. The materialobtained using the largest acid concentration, however,

Fig. 5. TEM of a periodic macroporous SiO2 obtained after infusion of TMOS in scCO2 on a PS-IA-MA template after calcination, for differentconcentrations of BSA: (a–c) 0.0017 M and (d–f) 0.020 M. Scale bars = (a, c, e, f) = 100 nm, (b,d) = 50 nm.

Fig. 6. Adsorption–desorption isotherms of macroporous SiO2 producedin scCO2 using TMOS over PS-MA-IA templates with different concen-trations of BSA: (s) 0.0017 M and (h) 0.020 M. Inset shows the pore sizedistribution obtained from the desorption branch of each isotherm.

28 A. Cabanas et al. / Microporous and Mesoporous Materials 99 (2007) 23–29

shows a distribution, which rises monotonously up to3 nm. Increasing the concentration of acid on the particlesurfaces seems to decrease the pore size.

The cumulative pore volumes for the samples producedusing BSA 0.020 M and 0.0017 M were 1.00 and 1.32 cm3/g, respectively. The smaller pore volume calculated for thesample produced with the highest acid concentration is due

to the decrease in the pore size for a similar surface areamaterial.

Adsorption data for the samples prepared using TMOSand the templates: (a) PS-MA 0.0070 M BSA ordered bycentrifugation (Fig. 4), and (b) PS-IA-MA 0.0017 M BSAordered by evaporation (Fig. 5) are very similar, suggestingthat for acid concentrations of similar magnitude theordering technique of the template does not affect theporosity of the sample.

4. Conclusions

A new method to produce ordered macroporous ceramicmaterials by sol–gel reaction of TEOS or TMOS in scCO2

using modified polystyrene 3D-latex array templates is pre-sented. Reactions are carried out at 40 �C and 85 bar. Atthese mild conditions, reaction requires a catalyst which ispreviously impregnated onto the particle surface. Theinverse opals obtained after calcination of the template havebeen fully characterized. Analysis by SEM and TEM showthat the materials replicate the structure of the original tem-plates. In comparison to the process in the liquid-phase, thereaction in scCO2 takes place mostly on the particle surface.N2-adsorption analysis reveals that the materials synthes-ised in scCO2 present a very high surface area (594–470 m2/g) due mostly to the presence of mesopores in themacropore wall. Porosity of the materials produced usingTEOS and TMOS is very similar (surface area and pore size

A. Cabanas et al. / Microporous and Mesoporous Materials 99 (2007) 23–29 29

distributions). In the same way, using BSA or pTSA as acidcatalyst does not make any difference. Using a weaker acidsuch as ClAcH leads to a lower reaction yield. Changing theconcentration of catalyst has a larger effect and the materialobtained using TMOS and the highest BSA concentra-tion presents smaller pore sizes, but there are areas whereexcess SiO2 seems to have blocked partly the macropores.

Acknowledgements

We gratefully acknowledge the financial support ofUCM (Spain) – project PR1/06-14425-A and CAM (Spain)– project MATERYENER. A.C. thanks MEC (Spain) forits support through a ‘‘Ramon y Cajal’’ contract. We alsothank ‘‘Centro de Microscopıa Electronica’’ at UCM fortechnical assistance.

References

[1] O.D. Velev, T.A. Jede, R.F. Lobo, A.M. Lenhoff, Nature 289 (1997)447.

[2] S. Polarz, B. Smarsly, J. Nanosci. Nanotechnol. 2 (2002) 581.[3] G.A. Ozin, A.C. Arsenault, Nanochemistry, A Chemical Approach to

Nanomaterials, RSC Publishing, 2005.[4] A. Cabanas, E. Enciso, M.C. Carbajo, M.J. Torralvo, C. Pando,

J.A.R. Renuncio, Chem. Commun. (2005) 2618.[5] A. Cabanas, E. Enciso, M.C. Carbajo, M.J. Torralvo, C. Pando,

J.A.R. Renuncio, Chem. Mater. 17 (2005) 6137.[6] A. Stein, R.C. Schroden, Curr. Opin. Solid State Mater. Sci. 5 (2001)

553.

[7] M.C. Carbajo, E. Climent, E. Enciso, M.J. Torralvo, J. Colloid Inter.Sci. 284 (2005) 639.

[8] B.T. Holland, C.F. Blanford, T. Do, A. Stein, Chem. Mater. 11(1999) 795.

[9] M.C. Carbajo, A. Gomez, M.J. Torralvo, E. Enciso, J. Mater. Chem.12 (2002) 2740.

[10] M.C. Carbajo, C. Lopez, A. Gomez, E. Enciso, M.J. Torralvo, J.Mater. Chem. 13 (2003) 2311.

[11] M.A. McHugh, V.J. Krukonis, Supercritical Fluid Extraction:Principles and Practice, Butterworths, Boston, 1986.

[12] O. Sphul, S. Herzog, J. Gross, I. Smirnova, W. Arlt, Ind. Eng. Chem.Res. 43 (2004) 4457.

[13] D.L. Tomasko, H. Li, D. Liu, X. Han, M.J. Wingert, J.L. Lee, K.W.Koelling, Ind. Eng. Chem. Res. 42 (2003) 6431.

[14] A. Cabanas, E. Enciso, M.C. Carbajo, M.J. Torralvo, C. Pando,J.A.R. Renuncio, Langmuir 22 (2006) 8966.

[15] E. Barret, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951)373.

[16] D.A. Loy, E.M. Russick, S.A. Yamanaka, B.M. Baugher, K.J. Shea,Chem. Mater. 9 (1997) 2264.

[17] M. Moner-Girona, A. Roig, M. Benito, E. Molins, J. Mater. Chem.13 (2003) 2066.

[18] R. Sui, A.S. Rizkalla, P.A. Charpentier, J. Phys. Chem. B 108 (2004)11886.

[19] H. Wakayama, H. Itahara, N. Tatsuda, S. Inagaki, Y. Fukushima,Chem. Mater. 13 (2001) 2392.

[20] H. Wakayama, Y. Fukushima, Indus. Eng. Chem. Res. 39 (2000)4641.

[21] C.J. Brinker, G.W. Scherer, Sol–gel science: The Physics andChemistry of Sol–gel Processing, Academic Press Inc, Boston,1990.

[22] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity,Academic Press Inc, London, 1982.