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Solid State Sciences 5 (2003) 1303–1308www.elsevier.com/locate/sssc

Preparation of micron-sized spherical particles of mesoporous silifrom a triblock copolymer surfactant, usable as a stationary phas

for liquid chromatography

Monica Mesaa, Ligia Sierraa, Betty Lópeza, Alejandro Ramireza, Jean-Louis Guthb,∗

a Instituto de Química, Universidad de Antioquia, A.A 1226 Medellín, Colombiab Laboratoire de matériaux minéraux, ENSCMu, Université de Haute Alsace, 68093 Mulhouse cedex, France

Received 25 February 2003; received in revised form 11 June 2003; accepted 4 July 2003

Abstract

Spherical particles (∅ > 3 µm, with 5–10 nm pore size) of SBA15-type mesoporous silica, usable as stationary phase forwere prepared with tetraethoxysilane (TEOS) as silica source, triblock copolymer Pluronic P123 (EO20PO70EO20) as surfactant S0 andcetyltrimethylammonium bromide (CTMABr) as co-surfactant S+. The synthesis mechanism involves a surfactant-silica speciesassembly process in acidic medium under quiescent conditions, where S0H3O+X−I+ and S+X−I+ interactions occur. The syntheses wecarried out using two procedures: (a) with one heating step and (b) with two heating steps. Synthesis conditions, such as thtemperature and acidity of the reaction mixture and the temperature and duration of the reaction, influence the porous characwell as the morphology and size of the particles. Spherical particles were obtained with one heating step under conditions that wsurfactants-silicate interactions such as high dilution, high temperature and low acidity. The modification of the micelle volumetemperature and acidity allowed the adjustment of the pore size. 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Keywords: Mesoporous silica; Micron-sized spherical particles; Triblock copolymer surfactant; Pluronic P123; HPLC stationary phases

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1. Introduction

Since the discovery of silica with organized mesoporstructures, many applications of these materials wereposed and developed [1,2]. Among them, their use astionary phases in high performance liquid chromatogra(HPLC) in place of the precipitated silica [3] is a promisifield for further investigations. High specific surface arregular and adjustable mesopores, presence of silanol gallowing their use in normal or reverse phase HPLC, areteresting properties of these solids. But until now, only sohaving a pore size between 2 and 4 nm and in general cotuted of agglomerated fine particles were studied [4,5].obtaining of spherical particles with sizes larger than 2–3was an appreciated improvement [6–8].

* Corresponding author.

E-mail address: jl.guth@uha.fr (J.-L. Guth).

1293-2558/$ – see front matter 2003 Éditions scientifiques et médicales Eldoi:10.1016/S1293-2558(03)00185-7

s

-

The pore size was dramatically increased until 30when the classical molecular surfactants were replacenonionic triblock copolymer like EOxPOyEOx [9,10]. Thesyntheses were carried out in acidic media where the inactions of the surfactant S0 with the cationic silica I+ aremediated by an ionic pair S0H+X−I+. This kind of interac-tion, which is rather weak, allows the preparation of sowith variable shapes (fibers, films, gyroids, spheres) byadjustment of the synthesis parameters and the additioco-solvent or co-surfactant like cetyltrimetylammonium bmide (CTMABr), which interacts with the silica accordingthe S+X−I+ mechanism [6,11].

The aim of this study was to adapt the method ofpreparation of SBA15-type mesoporous silica [10] wPluronic P123 (EO20PO70EO20) in order to obtain sphericaparticles with a size between 3 and 12 µm and a pdiameter adjustable between 5 and 10 nm, which can be

as stationary phases in normal or reverse HPLC.

sevier SAS. All rights reserved.

1304 M. Mesa et al. / Solid State Sciences 5 (2003) 1303–1308

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2. Experimental

2.1. Reactants

Tetraethoxysilane (TEOS 98%, Aldrich). Poly(ethyleoxide)-poly(propylene oxide)-poly(ethylene oxide) triblocopolymer EO20PO70EO20 (Pluronic P123,Maver = 5800,BASF). Cetyltrimethylammoniumbromide (CTMABr, 99%Acros). Hydrochloric acid (36%, analytical grade, MercDeionized water.

2.2. Synthesis procedure

P123 and CTMABr surfactants were dissolved in aciwater at 10◦C under mild agitation. Then, TEOS was slowadded into the solution under strong magnetic stirringroom temperature (see Table 1 for the gel compositionsynthesis parameters). Two polycondensation procedwere used: (a) with one heating step at a temperabetween 40 and 95◦C, during 3–5 days and (b) with twheating steps involving an initial step at 40◦C (2 days) and asecond step at 95◦C (4 days). The two steps were carriout without stirring. In the sample description (TableI or II represents one or two heating steps, respectivand the Arabic number represents the final temperaturthe two heating steps procedure, the temperature of thestep was always 40◦C). To be used in HPLC, the solidwere dispersed, before filtration and washing, by ultrasostirring during 10 min and left under sedimentation ttimes during 3 h (height of the suspension in the bea

10 cm) to remove the particles with size smaller than

t

3 µm. The materials, dried at 60◦C, were calcined in aiat a heating rate of 1.5◦C min−1 from 25 to 500◦C andheld at this temperature during 3 h in order to remthe surfactants. The yield in silica was∼ 97%. Sampleswithout CTMABr were synthesized following the samprocedure.

2.3. Characterization of the materials

2.3.1. MicroscopyThe samples were examined with a scanning elec

microscope (Philips XL30) in order to determine the shand size of the particles.

2.3.2. Nitrogen adsorption isothermsThe calcined samples were activated at 250◦C and the

isotherms were measured with an ASAP2010 instrum(Micromeritics). The specific surface area was evaluawith the BET method, the pore volume was figured out wthe BJH method and the pore diameter was estimated avalue corresponding to the maximum in the BJH pore sdistribution plot.

2.3.3. X-ray diffractionThe XRD patterns were recorded in the 2θ range from 1

to 10◦ with a Philips PW1130 diffractometer and the Cr-Kα1

radiation.

Table 1Synthesis conditions and results of the characterization of the SBA15-type materials synthesized with one (I) and two (II) heating steps

Sample H2O/TEOS HCl/H2O T (◦C) (days) XRDa Morphologyb Adsorptionc

a0 (nm) t (nm) Shape Size (µm) A (m2 g−1) d (nm) v (cm3 g−1)

EI-70 245 0.007 70 (4) 10.3 5.1 P 4–16 849 5.2 0.80AI-40 245 0.014 40 (3) 9.9 5.9 P� S n.d. 701 4.0 0.51AI-70 245 0.014 70 (3) 10.1 4.6 P 3–8 790 5.5 0.58AI-95 245 0.014 95 (3) 10.4 1.9 P 3–6 760 8.5 1.00DI-95 495 0.005 95 (5) 10.8 4.5 S 3–8 944 6.3 1.09BI-40 495 0.007 40 (5) 9.8 5.8 P∼ S 2–16 827 4.0 0.57BI-40∗d 495 0.007 40 (5) 10.5 4.7 n.d.s < 2 640 6.0 0.46

Se 2–4BI-70 495 0.007 70 (5) 10.4 5.9 P> S 3–12 844 4.5 0.58BI-70∗d 495 0.007 70 (5) 11.5 4.3 n.d.s < 2 643 7.0 0.49

Se 2–4BI-95 495 0.007 95 (5) 10.8 4.3 S 2–10 947 6.5 1.18CI-95 495 0.028 95 (5) 11.0 2.5 P 1–3 771 9.0 1.21AII-95 245 0.014 40 (2) 95 (4) 9.7 3.1 P> S 4–8 863 6.6 0.92DII-95 495 0.005 40 (2) 95 (4) 10.0 3.9 P> S 3–12 811 6.0 0.78FII-95 495 0.014 40 (2) 95 (4) 10.1 2.5 P� S 2–8 828 7.6 0.91CII-95 495 0.028 40 (2) 95 (4) 9.9 2.9 P� S 2–6 832 7.8 0.96

a XRD of calcined materials:a0: unit cell parameter;t : pore wall thickness (t = a0 − d).b SEM micrographs: S: sphere, P: polyhedron and/or spheroid, n.d.s: nondefined shape.c A: BET specific surface area;d : pore diameter (from BJH pore size distribution plot);v: pore volume (BJH method).d Samples prepared without CTMABr.e Very low amount.

M. Mesa et al. / Solid State Sciences 5 (2003) 1303–1308 1305

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2.4. Evaluation of SBA15-type mesoporous silica asstationary phase for HPLC

2.4.1. Preparation of SBA15-type mesoporous silica forreverse HPLC

After sedimentation, calcination and previous actition at 150◦C, sample DII-95 was silylated first witoctyldimethylchlorosilane (ODMCS), in presence of triethlamine at 120◦C, using toluene as the solvent and encapped with trimethylchlorosilane (TMCS) at 60◦C. Thesolid was recovered by filtration, washed 3 times withsolvent and 3 times with dry methanol. The material wthen dried overnight and heated at 150◦C to remove thephysically adsorbed molecules.

2.4.2. HPLC measurementsA stainless-steel column (150×4.6 mm) was packed with

a column-packing apparatus (Touzart-Matignon) by meof the slurry technique (2–2.5 g of sample dried at 120◦Cand dispersed by ultrasonic stirring in 23 ml of CCl4).Methanol was used to push the slurry into the column.packing pressure was 4500 psig.

A chromatograph from Waters with pump model 600 acontroller, differential refractometer model 410, photodioarray detector model 993 and software Milenum 2000used for the HPLC test. This was carried out with a mixtof aromatic compounds. HPLC grade reagent methanol/2Osolution was used as mobile phase.

3. Results and discussion

3.1. Application of SBA15-type mesoporous silica in HPLC

For the use of mesoporous silica as stationary HPphases, spherical particles with an average diameter o6 µm are required to have appropriate column efficiewith reasonable pressure drop and analysis time. Dutheir large pore diameter, SBA15-type mesoporous silare promising materials for such an application but until nthey only have been produced as spherical particles of aup to 1 µm [10].

According to the chromatographic evaluation thathave done with the materials of this study having thequired morphology and size, they can be used for norHPLC, although the microporosity in their walls is responble of a notable but symmetrical peak broadness whenare compared with MCM41-type mesoporous silica. Afsilylation of the SBA15-type materials for their use asverse phases in HPLC, we have found that the micropoity is screened probably by the insertion of the hydrocarchains in the micropores and we had good results for

application, as the one shown in Fig. 1.

Fig. 1. Reverse-phase chromatogram from a column packed with Dmaterial. Solute: a mixture comprising (a) phenol, (b) benzene, (c) tolu(d) naphthalene, (e) anthracene, (f) benzopyrene. Conditions of elu1 ml min−1 with methanol:water (70%:30%) during 4 min; linear increafrom 1 to 1.5 ml min−1 in 2 min with methanol:water (90%:10%1.5 ml min−1 with methanol:water (90%:10%) to the end.

3.2. Effects of the synthesis conditions on the morphologyand porosity of SBA15-type silica

Zhao et al. [10] prepared spheres of SBA15 with a sup to 1 µm by using CTMABr as co-surfactant in a gwith the following molar composition: 1 TEOS; 0.012 P120.020 CTMABr; 3.2HCl; 114H2O and under stirring duringthe first step of synthesis. As higher dilution will reduthe supersaturation and the nucleation rate, larger sphcan be expected. Table 1 reports the values of the syntparameters used to transform a gel with the following mocomposition 1 TEOS; 0.012 P123; 0.020 CTMABr;xHCl;yH2O with 1.2< x < 13.65 and 245< y < 495.

According to the synthesis procedure, we make thesumption that the meso-structural formation mechanismvolves the presence of mixed (S0H3O+X−)(S+X−) micelleswhere S0 represents the nonionic Pluronic P123 surfacand S+ the cationic CTMABr surfactant. X− correspondsmainly to Cl−, which is present in relative large amoucompared to Br−. The interactions of these micelles withe positively charged silicate species can be describe[(S0H3O+X−)(S+X−)]I+. They occur at the early stagethe synthesis, before the temperature has reached thevalue. Further condensation of silica, as the synthesisand temperature increase, leads to the formation of mporous silica particles. Factors such as the temperaturedilution and the pH of the reaction mixture determine, awill be shown below, the particle morphology and the porcharacteristics of the final mesoporous materials. Thesetors affect probably the association of the surfactants (S0 orS+) with the X− counter-ion and the interaction of the mix(S0H3O+X−)(S+X−) micelles with the silicate species (I+),as well as the silicate condensation, under quiescent cotions [12].

3.2.1. Morphology of the particlesAs it can be seen from the results of the experime

with one heating step (Table 1), high dilution, low acidand low temperature favor the increase of the particle

(compare BI-40, BI-70 and BI-95). This is related to the

1306 M. Mesa et al. / Solid State Sciences 5 (2003) 1303–1308

Fig. 2. Scanning electron micrographs of (a) BI-95, (b) AI-70, (c) DII-95, (d) BI-70∗ (sample prepared without CTMABr).

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decrease of the nucleation rate (lower supersaturationpolycondensation rate). The spherical shape, which isindication of less regular structures (see Fig. 1 inSupplementary Material File), is favored by high dilutiolow acidity and high temperature (compare Fig. 2a with 2Such conditions generate more defects during the formaof the particles as it was shown by Sierra et al. [At low acidity, slow polycondensation rate of the silicspecies, combined with a shortening of the lifetime ofmicelles at high temperature and dilution, leads to poor sassembling. Moreover, the interactions between the PlurP123 surfactant and the silica are weakened by a loprotonation degree of the silica and the surfactant.

In the experiments with two heating steps, the tempture of the first step (40◦C) will determine to a large extenthe characteristics of the final material, which can onlymodulated with the two other synthesis parameters: diluand acidity. This is why it is not possible to synthesize wthat procedure particles exhibiting only the spherical shaThe predominant shape is always gyrohedral and polyhe(Fig. 2c). There is also almost no growth of the particles ding the second heating step.

The adjustment of the size and shape of the partiwas not possible without the presence of the co-surfacCTMABr. In the case of its absence the majority of the pticles are very small with nondefined shape (Fig. 2d), whindicates that this co-surfactant is the one that determ

mainly the morphology of mesoporous silica obtained by

l

the method followed in this work. It seems thus thatparticle morphology, obtained in the reaction betweenmixed surfactant system (S0/S+) and the silicate species,more determined by the S+X−I+ interactions than by thS0H3O+X−I+ interactions.

3.2.2. Porosity of the materialsFor the same composition of the synthesis gel, as

the case for the series BI and for the series AI (Tablethe pore diameter and volume increase with the temperain the one heating step experiments. This effect canexplained by the normal thermal expansion of the miceand by an increase in the hydrophobicity of the EO chaof the S0 surfactant, which increases the hydrophomicelle volume [9,12]. But it is also probable that thigher temperature improves the polycondensation deof the silica in the walls, which limits the shrinking othe structure during the removal of the surfactantscalcination. A smaller change in the unit cell parametera0

(�a0 = a0,noncalcined− a0,calcined) is observed for sampleprepared at high temperature, for instance, the�a0 for AI-95and AI-70 are 0.79 and 1.11 nm, respectively. A decreasthe pore wall thickness (figured out from XRD and nitrogadsorption data) is observed for the AI-type samples wthe temperature increases from 40 to 95◦C. These effectsare in agreement with results of the literature for this kind

mesoporous materials [13,14].

M. Mesa et al. / Solid State Sciences 5 (2003) 1303–1308 1307

Fig. 3. Pore size distribution of (a) BI-40, (b) AI-70, (c) CI-95 and (d) DII-95.

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Moreover, for the same dilution degree of the synthegel, the pore diameter and volume increase as the acincreases (see samples DI-95, BI-95, CI-95 and samples95, FII-95, CII-95). Protonation of the EO chains by H3O+,and therefore also the cationic charge density, increwith the acidity of the reaction mixture. This leadsrepulsions between these charges, which are minimizethe stretching of the chains and the increase of the volthey occupy. The decrease of the pore wall thickness at lopH can also be related to stronger micelle-silica interactiwhich favors the incorporation of the surfactants, increasthe number of the mesopores.

The pore diameters and the porous volume of the samprepared by the two heating steps procedure are smthan those measured in samples obtained under thesynthesis conditions but using the one heating step proce(compare AI-95 with AII-95 and CI-95 with CII-95). Likefor the morphology, the temperature of the first heatstep will determine the porosity characteristics of the fimaterial: at 40◦C, the thermal expansion of the micellesless than at 95◦C, and the hydrophilicity of the EO chainis higher than at 95◦C (the hydrophobic micelle volumethus lower). The micelles occupy a smaller volume at 40◦Cin the two heating steps procedure than at 95◦C in the oneheating step procedure.

The comparison of the pore size distribution in the sa

ples prepared with the one or two heating steps procedures

ree

shows that the distribution is broader in all samples prepat 70◦C than in those prepared at 40 or 95◦C (Fig. 3). Sucha behavior cannot be explained with the preceding conerations. We make the assumption that around that temature there is a transition in the structure or compositiothe mixed micelle, which produces a less regular organtion of the materials. This point will be clarified by furthwork (analysis of the mixed micelle composition as a fution of temperature, study by NMR of the conformation amobility of the micelle inside the mesopores).

4. Conclusions

The morphology (particle shape and size) of SBAtype mesoporous silica could be adjusted to the descharacteristics (spheres,∅ > 3 µm and pore size 5–10 nmfor the use as a stationary phase in HPLC. The bconditions are:

(a) high dilution (H2O/TEOS> 245),(b) low acidity (HCl/H2O = 0.014),(c) high temperature (95◦C),(d) one heating step procedure.

The first two conditions decrease the nucleation rate (

,supersaturation and polycondensation rate). The first three

1308 M. Mesa et al. / Solid State Sciences 5 (2003) 1303–1308

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conditions generate defects, which limit the formationgyrohedral and polyhedral (nonspherical) shaped partiThrough the adjustment of the temperature and the aciwhich modify the micelle volume (thermal expansion, moification of the hydrophobic micelle volume and of the coformation of the EO chains), the pore size could be vabetween 4 and 9 nm. The one heating step procedure isuseful to change the characteristics of the materials thantwo heating steps procedure where the characteristics artermined to a large extent by the temperature of the first s

Acknowledgements

We thank COLCIENCIAS and Universidad de Antioqu(UdeA) for the financial support (Project Colciencias-Ud1115-05-10126).

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