Surfactant Templated Sulfonic Acid Functionalized Silica

frameworks containing uniform pore channels with diameters ranging from 2 to 10 nm. ...... to play a significant role in the greening of fine and ...
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Langmuir 2004, 20, 3632-3640

Surfactant Templated Sulfonic Acid Functionalized Silica Microspheres as New Efficient Ion Exchangers and Electrode Modifiers Vellaichamy Ganesan and Alain Walcarius* Laboratoire de Chimie Physique et Microbiologie pour l’Environnement, Unite´ Mixte de Recherche UMR 7564, CNRSsUniversite´ Henri Poincare´ Nancy I, 405, rue de Vandoeuvre, F-54600 Villers-les-Nancy, France Received December 19, 2003. In Final Form: February 11, 2004 Porous sulfonic acid functionalized silica spheres have been prepared by oxidation of thiol-functionalized mesoporous silica samples obtained by co-condensation of (mercaptopropyl)trimethoxysilane and tetraethoxysilane in the presence of cetyltrimethylammonium as a template. The physicochemical characteristics of the resulting ion exchangers have been analyzed by various techniques and discussed with respect to the amount of functional groups in the materials. Their ion exchange behavior was then studied from batch experiments (determination of cation exchange capacities) and by electrochemistry at carbon paste electrodes modified with these solids. In particular, ion exchange voltammetry applied to two model electroactive cations, Cu2+ and Ru(NH3)63+, has pointed out the key role played by the content of organofunctional groups in the materials (which strongly affects their structure and porosity) on their performance as electrode modifiers for preconcentration of target analytes prior to electrochemical detection.

1. Introduction Since the discovery of micelle-templated silicas by the Mobil research group,1,2 the ability to manipulate structures of porous solids on a nanometer scale in a controlled way proved to be so important to the research community.3,4 These materials consist of high surface area (typically in the 700-1400 m2 g-1 range) ordered silica frameworks containing uniform pore channels with diameters ranging from 2 to 10 nm. Of special interest is the functionalization of these mesostructures by the inclusion of chemical groups into their frameworks, which can be achieved either by postgrafting of organofunctional derivatives onto the silica surfaces or by direct incorporation of the organic moiety via the co-condensation route.5-8 These nanostructured organic-inorganic composite materials are currently the object of a tremendous amount of research, because they combine in a single solid both the attractive properties of a mechanically stable inorganic backbone and the specific chemical reactivity of the organofunctional groups. They found applications in several fields, including heterogeneous catalysis,9-12 separation sciences,13-16 and some others.17-19 Most properties of these new high-tech materials are dependent on their * To whom correspondence may be addressed. Fax: (+33) 3 83 27 54 44. E-mail: [email protected]. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) de Soler-Illia, G. J.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. Rev. 2002, 102, 4093. (4) Schueth, F. Angew. Chem., Int. Ed. 2003, 42, 3604. (5) Moller, K.; Bein, T. Chem. Mater. 1998, 10, 2950. (6) Stein, A.; Melde, B. J.; Schroden, R. C. Adv. Mater. 2000, 12, 1403. (7) Sayari, A.; Hamoudi, S. Chem. Mater. 2001, 13, 3151. (8) Wight, A. P.; Davis, M. E. Chem. Rev. 2002, 102, 3589. (9) Corma, A. Top. Catal. 1998, 4, 249. (10) Brunel, D.; Blanc, A. C.; Galarneau, A.; Fajula, F. Catal. Today 2002, 73, 139. (11) De Vos, D. E.; Dams, M.; Sels, B. F.; Jacobs, P. A. Chem. Rev. 2002, 102, 3615.

structural and chemical composition as well as on the dynamical properties inside the blends. A crucial parameter in connection to their practical uses is the accessibility to the active centers and mass transfer rates inside the porous framework,20 for which definite advantage of ordered mesoporous structures over amorphous gels has been recently demonstrated.21-23 During the past half decade, several investigations were carried out on thiol-functionalized nanostructured materials, which were obtained either by grafting mercaptopropyl groups onto the pore walls of mesoporous silicas or by co-condensation of mercaptopropyltrimethoxysilane and a tetraalkoxysilane precursors, most often applied to the sorption of mercury(II) species from dilute aqueous solutions.22-29 These mercaptopropyl chains covalently (12) Macquarrie, D. In Handbook of Green Chemistry and Technology; Clark, J., Macquarrie, D., Eds.; Blackwell Science Ltd.: Oxford, U.K., 2002; pp 120-149. (13) Liu, J.; Fryxell, G. E.; Mattigod, S.; Zemanian, T. S.; Shin, Y.; Wang, L.-Q. Stud. Surf. Sci. Catal. 2000, 129, 729. (14) Kisler, J. M.; Dahler, A.; Stevens, G. W.; O’Connor, A. J. Microporous Mesoporous Mater. 2001, 44-45, 769. (15) Bonelli, B.; Bruzzoniti, M. C.; Garrone, E.; Mentasti, E.; Onida, B.; Sarzanini, C.; Serafino, V.; Tarasco, E. Chromatographia 2002, 56, S189. (16) Ma, Y.; Qi, L.; Ma, J.; Wu, Y.; Liu, O.; Cheng, H. Colloids Surf., A 2003, 229, 1. (17) Scott, B. J.; Wirnsberger, G.; Stucky, G. D. Chem. Mater. 2001, 13, 3140. (18) Walcarius, A. Chem. Mater. 2001, 13, 3351. (19) Clark, J. H. Acc. Chem. Res. 2002, 35, 791. (20) Karger, J.; Freude, D. Chem. Eng. Technol. 2002, 25, 769. (21) Walcarius, A.; Etienne, M.; Bessie`re, J. Chem. Mater. 2002, 14, 2757. (22) Walcarius, A.; Etienne, M.; Lebeau, B. Chem. Mater. 2003, 15, 2161. (23) Walcarius, A.; Delacote, C. Chem. Mater. 2003, 15, 4181. (24) Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Liu, J.; Kemmer, K. M. Science 1997, 276, 923. (25) Mercier, L.; Pinnavaia, T. J. Adv. Mater. 1997, 9, 500. (26) Brown, J.; Mercier, L.; Pinnavaia, T. J. Chem. Commun. 1999, 69. (27) Brown, J.; Richer, R.; Mercier, L. Microporous Mesoporous Mater. 2000, 37, 41. (28) Bibby, A.; Mercier, L. Chem. Mater. 2002, 14, 1591. (29) Etienne, M.; Sayen, S.; Lebeau, B.; Walcarius A. Stud. Surf. Sci. Catal. 2002, 141, 615.

10.1021/la0364082 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/13/2004

Functionalized Silica Microspheres

bonded to the ordered silica framework can be oxidized into sulfonic acid moieties, creating thus a mesoporous solid acid ion exchanger.30 A straightforward way to achieve this goal is the use of hydrogen peroxide as the oxidizing agent in acidic medium, as first reported in the case of ordered mesoporous materials by the groups of Jacobs31 and Stein.30 This procedure was then largely applied to produce sulfonic acid bearing mesoporous silicas that were mainly exploited for catalytic purposes (strong acid catalysts),30-45 because they combine in a single solid a thermally and mechanically stable inorganic backbone and the superacidic group -SO3H, despite the generally observed remaining thiol groups due to incomplete oxidation of the organosulfur species.30,31,34,37,41-45 The direct synthesis of ordered mesoporous silicas containing sulfonic acid groups (i.e., by adding H2O2 in the starting medium) was also reported.34,46,47 Notwithstanding these numerous works, it could be rather surprising that none of them was dealing with the ion exchange properties of such new materials, except, of course, the determination of their cation exchange capacity by titration.30,34,37,38,45 To the best of our knowledge, the only related investigation was reported in a short communication by Ogawa et al.48 who highlighted the advantageous effect of electrostatic interactions between sulfonic acid groups and [Ru(bpy)3]2+ cations to promote the accumulation of these positively charged species in mesoporous silicas, in comparison to the lower maximum amounts of adsorbed [Ru(bpy)3]2+ in unmodified silicas or corresponding aluminosilicates. It should be reminded here that the cation exchange capacity of pure silica originating from the weakly acid silanol groups49 is rather low, resulting, e.g., in poor binding efficiency of heavy metal ions even when using high surface area mesoporous silicas,50 which often requires the incorporation of AlO4- centers in the framework to increase the overall ion exchange capacity.51 (30) Lim, M. H.; Blanford, C. F.; Stein, A. Chem. Mater. 1998, 10, 467. (31) Van Rhijn, W. M.; De Vos, D. E.; Sels, B. F.; Bossaert, W. D.; Jacobs, P. A. Chem. Commun. 1998, 317. (32) Van Rhijn, W. M.; De Vos, D. E.; Bossaert, W. D.; Bullen, J.; Wouters, B.; Grobet, P.; Jacobs, P. A. Stud. Surf. Sci. Catal. 1998, 117, 183. (33) Bossaert, W. D.; De Vos, D. E.; Van Rhijn, W. M.; Bullen, J.; Grobet, P.; Jacobs, P. A. J. Catal. 1999, 182, 156. (34) Margolese, D.; Melero, J. A.; Christiansen, S. C.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 2000, 12, 2448. (35) Diaz, I.; Marquez-Alvarez, C.; Mohino, F.; Perez-Pariente, J.; Sastre, E. J. Catal. 2000, 193, 283. (36) Diaz, I.; Marquez-Alvarez, C.; Mohino, F.; Perez-Pariente, J.; Sastre, E. J. Catal. 2000, 193, 295. (37) Clark, J. H.; Elings, S.; Wilson, K. C. R. Acad. Sci. Paris, Ser. IIc 2000, 3, 399. (38) Diaz, I.; Mohino, F.; Perez-Pariente, J.; Sastre, E. Appl. Catal., A 2001, 205, 19. (39) Diaz, I.; Mohino, F.; Sastre, E.; Perez-Pariente, J. Stud. Surf. Sci. Catal. 2001, 135, 1383. (40) Mohino, F.; Diaz, I.; Perez-Pariente, J.; Sastre, E. Stud. Surf. Sci. Catal. 2002, 142B, 1275. (41) Wilson, K.; Lee, A. F.; Macquarrie, D. J.; Clark, J. H. Appl. Catal., A 2002, 228, 127. (42) Shen, J. G. C.; Herman, R. G.; Klier, K. J. Phys. Chem. B 2002, 106, 9975. (43) Dijs, I. J.; van Ochten, H. L. F.; van Walree, C. A.; Geus, J. W.; Jenneskens, L. W. J. Mol. Catal. A: Chem. 2002, 188, 209. (44) Mikhailenko, S.; Desplantier-Giscard, D.; Danumah, C.; Kaliaguine, S. Microporous Mesoporous Mater. 2002, 52, 29. (45) Hamoudi, S.; Kaliaguine, S. Microporous Mesoporous Mater. 2003, 59, 195. (46) Melero, J. A.; Stucky, G. D.; van Grieken, R.; Morales, G. J. Mater. Chem. 2002, 12, 1664. (47) van Grieken, R.; Melero, J. A.; Morales, G. J. Stud. Surf. Sci. Catal. 2002, 142B, 1181. (48) Ogawa, M.; Kuroda, K.; Nakamura, T. Chem. Lett. 2002, 31, 632. (49) Despas, C.; Walcarius, A.; Bessie`re, J. Langmuir 1999, 15, 3186. (50) Walcarius, A.; Bessie`re, J. Chem. Mater. 1999, 11, 3009.

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In the present work, we have examined the cation exchange properties of various silica-based organicinorganic hybrid materials containing sulfonic acid groups covalently attached to the solid framework via a propyl chain. They were prepared by co-condensation of mercaptopropyltrimethoxysilane (MPTMS) and tetraethoxysilane (TEOS) precursors in various ratios, in the presence of a cationic surfactant acting as structure-directing agent, according to a procedure leading to the formation of regular spheres of monodisperse size.23,52 Some other silica samples (either amorphous gels or MCM-41 type) were grafted with mercaptopropyl groups and used for comparison purposes. The creation of surface sulfonic acid functionality was performed via thiol oxidation by hydrogen peroxide according to a previously published procedure.30,31,34,37,41-45 The products were first characterized by several physicochemical techniques to point out the effect of this oxidation step on the materials composition, structure, and porosity, and their ion exchange properties were evaluated both from batch experiments and from ion exchange voltammetry. The interest of these solid cation exchangers as new electrode modifiers applied to preconcentration analysis is discussed with respect to the density of functional groups in the material and to the advantages of the ordered porous support (high surface area, controlled pore size, mechanical stability, reduced swelling, or contraction upon exchange, e.g., in comparison to sulfonic acid functionalized organic polymers). 2. Experimental Section Reagents and Materials. All reagents were analytical grade, and solutions were prepared with high-purity water (18 MΩ cm) from a Millipore Milli-Q water purification system. Metal ion solutions were obtained from their nitrate salts purchased from Prolabo (Ca(NO3)2, Cu(NO3)2, NaNO3, and KNO3). Nitric acid, ethanol 95%, cetyltrimethylammoniun bromide (CTAB), ammonia (28%), hydrogen peroxide (35%), sulfuric acid, nitric acid, toluene, and methanol were from Merck. Hexaammineruthenium(III) chloride was obtained from Johnson Matthey. Carbon paste was prepared with high-purity graphite powder (Ultra F, 200 mesh, from Johnson Matthey) and paraffin wax (mp 5860°C, from Fluka). Micelle-templated mesoporous silica samples with different percentage of mercaptopropyl groups (from 0 to 50%) were prepared by co-condensation of tetraethoxysilane (TEOS, >98%, Merck) and mercaptopropyltrimethoxysilane (MPTMS, 95%, Lancaster) in water-ethanol-ammonia solution in the presence of CTAB (see details below).23,52 Silica gel samples were purchased from Merck: the chromatographic grade Geduran SI 60 (G60) and Kieselgel 40 (K40). Synthesis of Thiol-Functionalized Silicas. The typical procedure23 for the synthesis of mercaptopropyl-functionalized mesoporous silica spheres involves the dissolution of 2.4 g of CTAB in a solution of 50 mL of deionized water, 45 mL of ethanol and 13 mL of 28% ammonia. A mixture of MPTMS and TEOS in various ratios (total amount: 18.3 mmol) in 5 mL of ethanol was then added to the surfactant solution, which was stirred for 2 h until a white precipitate was observed. Solid products were filtered off, washed with ethanol, and dried under vacuum ( 20) solids. Mesoporosity was only maintained for samples MPS-5%-SO3H and MPS-10%-SO3H, whose N2 adsorption/desorption isotherms were clearly of type IV. These materials are also the most mesostructurally ordered and are therefore expected to ensure a good accessibility to the active organofunctional centers. The N2 adsorption isotherms for MPS-15%-SO3H and MPS20%-SO3H exhibited a tendency to change from type IV to I, which was already reported for other periodic mesoporous organosilicas63 and constitutes a typical behavior for materials with gradually decreasing pore diameter on the borderline between mesopore and micropore ranges (barrier at 2 nm).64 The mercaptopropylgrafted MCM-41 material behaved similarly as the MPSn%-SO3H solids with low levels of functionalization: the BET surface area of MCM-41-SO3H decreased to 522 m2 g-1, from the 687 m2 g-1 value of MCM-41-SH, and the pore volume from 0.52 to 0.30 cm3 g-1. On the opposite, no significant decrease in the BET surface area was observed upon oxidation of the amorphous mercaptopropyl-functionalized silica gels, which can be explained by their more open structures (average pore size in the 4-6 nm range). In agreement with the synthetic procedure applied to get the MPS-n%-SH materials,23,52 their morphology was clearly spherical (this can be even noticed on the TEM image on Figure 1B depicting a portion of the particle). The oxidation step did not affect the particle morphology so that all the MPS-n%-SO3H samples were still in the form of regular spheres of monodisperse size. Particle size analysis reveals however a slight decrease in the particle size upon oxidation but confirms the narrow distribution centered at mean diameters of 400-500 nm (Table 1). This could be due to mesopore contraction, as suggested (63) Asefa, T.; Kruk, M.; MacLachlan, M. J.; Coombs, N.; Grondey, H.; Jaroniec, M.; Ozin, G. A. J. Am. Chem. Soc. 2001, 123, 8520. (64) Kruk, M.; Jaroniec, M. Chem. Mater. 2001, 13, 3169.

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Figure 3. Variation of sulfur content in sulfonic acid functionalized silica materials, as a function of the relative molar percentage of MPTMS (n%) in the starting sol, as measured from elemental analysis (O, right axis) or by XPS (9, left axis). The insets show the XPS spectra of (a) MPS-15%-SH and MPS15%-SO3H and (b) MPS-40%-SH and MPS-40%-SO3H samples.

by XRD data, and/or by some attrition experienced by the particles in suspension during their oxidation. Interestingly, the MPS-n%-SO3H spheres remained isolated from each other when suspended in aqueous medium, contrary to the nonoxidized MPS-n%-SH materials for which a bimodal distribution of particle size was observed as a result of aggregates made of closely associated particles.23,52 This is due to the more hydrophilic character of the oxidized particles in comparison to the hydrophobic mercaptopropyl-functionalized silica spheres. The sulfur content in the materials was determined by elemental analysis (Table 1). The results indicate a high degree of organosilane incorporation (>90%), as previously reported for thiol-functionalized mesoporous silicas obtained by the co-condensation route,26,34 and that the oxidation step did not result in any significant leaching of the organofunctional groups in the external solution. The increase of the sulfur content, as a function of the relative molar percentage of MPTMS (n%) in the starting sol, can be also monitored by XPS. Figure 3 shows that the contribution of the S2p signal in the XPS spectra follows the same trend as that measured by elemental analysis up to 20% MPTMS, indicating a rather homogeneous distribution of the organofunctional groups in the material. Above this value, the XPS data tend to underestimate the sulfur content, most probably because this surface analysis technique can no longer investigate the internal walls of mesopores due to pore blocking (the BET surface area falls down from this 20% value, see Figure 2). On the other hand, the XPS technique enables one to point out the successful creation of sulfonic acid moieties by oxidation of the thiol groups (insets in Figure 3). Prior to the oxidation step, all the MPS-n%-SH materials exhibited a well-defined S2p line at 163.5 eV, which is compatible with a mercaptopropylsilane.41,42,65 Following oxidation by H2O2, this signal was significantly diminished with a concomitant growing of a new S2p line at 168.5 eV, which is attributed to -(CH2)3-SO3H groups anchored to (65) Horr, T. J.; Arora, P. S. Colloids Surf., A 1997, 126, 113.

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a silica framework.41-43 This result demonstrates both successful and incomplete oxidation of MPS-n%-SH solids, as common for MPS-n%-SO3H materials obtained by postsynthesis oxidation.30,31,34,37,41-45 As illustrated by the insets in Figure 3 for the MPS-15%-SO3H and MPS-40%SO3H samples, the -SO3H to -SH ratio seems to increase with increasing the organic group content in the material, and this trend is confirmed in the whole series from 5% to 50% MPTMS (Table 1). One could be tempted to attribute this trend to the small analysis depth of XPS associated to the fact that nonoxidized thiol groups would be mainly located deeper in the materials (i.e., less accessible to H2O2), but such an explanation has to be taken with great care as incomplete oxidation was also observed in large pore SBA-15 materials,42 in which all SH groups are expected to be reached by the oxidizing agent, the complete oxidation being only achieved via direct synthesis.34 The oxidation of -SH groups into -SO3H moieties was also evidenced by Raman spectroscopy by the complete disappearance of the characteristic peak of the S-H mode at 2576 cm-1 concomitantly to the growing of a band at 1042 cm-1 attributed to the stretching of the sulfonate group; as previously discussed, this confirms both the successful oxidation and the fact that the remaining nonoxidized thiol groups are Raman invisible (due to the weak dipoles exhibited by S-H groups, which make their modes difficult to be detected by vibrational spectroscopy41). Ion Exchange. Ion exchange capacity of sulfonic acid functionalized (mesoporous) silicas was routinely determined from titration experiments.30,34,37,38,45 This involved most often the equilibration of the solid acid exchanger in an excess NaCl solution, followed by titration of the ion exchanged protons by sodium hydroxide.30,34 We have also applied a similar approach to quantify the extent of thiol oxidation (Table 1). Incomplete oxidation was confirmed for all the samples, in agreement with the XPS data, but the effect of the organic group density in the material was not exactly the same, revealing a discrepancy between the two techniques (and even between the three techniques when considering Raman spectroscopy for which the nonoxidized thiol remains invisible41). The % S oxidized determined by titration was found to increase from MPS5%-SO3H to MPS-20%-SO3H and then decreased at higher loadings, while a continuous increase was observed by XPS (Table 1). We have already evoked above the possible overestimation of the XPS technique for the highly functionalized materials, but the restricted porosity induced by high functionalization levels can also contribute to lowering the accessibility to the active centers and, therefore, leading to underestimation of the -SO3H population in these highly functionalized solids. This is especially true when charged functions are involved in the reaction pathway: while the uniform mesoporous structure of thiol-functionalized MCM-41 materials enabled total accessibility to the binding sites for HgII species22-27 (leading to the formation of neutral -S-HgOH moieties), the protonation of ordered mesoporous polysiloxane-immobilized amine ligands (leading to the positively charged -NH3+ end groups) was restricted to ca. 70-80% of the total capacity.22 Some kinetic limitations can also be encountered due to restricted mass transport in these porous media.21-23 We have evaluated this phenomenon by titrating the MPS-n%-SO3H samples directly by NaOH at various speeds, and found that the rate of addition of the reactant was a crucial parameter affecting the quantitativity of the analysis. For example, titration of MPS-20%-SO3H in aqueous suspension (at 0.75 g L-1) by 10-3 M NaOH led to a maximal value of 1.6 mmol

Langmuir, Vol. 20, No. 9, 2004 3637 Table 2. Ion Exchange Capacities of Sulfonic Acid Functionalized Silica Samples ion exchange capacity for various cations/mequiv g-1 samples

Na+

K+

Cu2+

Ca2+

MPS-5%-SO3H MPS-10%-SO3H MPS-15%-SO3H MPS-20%-SO3H MPS-30%-SO3H MPS-40%-SO3H MPS-50%-SO3H MCM41-SO3H K40-SO3H G60-SO3H

0.2 0.5 0.9 1.6 2.0 2.4 2.8 0.4 0.4 0.6

0.1 0.7 1.4 1.8 2.4 2.9 3.5 0.7 0.9 1.2

0.2 0.6 1.2 1.7 1.9 2.2 3.0 0.7 0.3 0.4

0.2 0.4 0.7 1.3 1.7 2.1 2.5 0.4 0.3 0.5

-SO3H per gram of material, independently on titration speed below 0.5 mL min-1, while only 75% and 15% of this maximal value were measured at higher titration speeds as 1 and 5 mL min-1, respectively. The minimum equilibration time required to reach a steady-state capacity value was estimated to about 1 h. On the other hand, the ion exchange equilibrium may be such that one single batch equilibration is not sufficient to exchange all the protons of the MPS-n%-SO3H samples, even in large excess of counterions in the solution (i.e., in the case of greater or similar affinity of the exchanger for the native cation in comparison to the solution-phase one). This was especially observed for some ion exchange reactions in zeolites,66 for which multiple successive equilibrations with fresh solutions or flow-through experiments have been advocated to determine accurately the ion exchange capacity. Therefore, we have chosen to equilibrate three times successively the MPS-n%-SO3H materials with fresh solutions containing selected cations in large excess, to measure their maximum capacity toward Na+, K+, Ca2+, and Cu2+ ions. The results are summarized in Table 2 and expressed in mequiv g-1 in order to allow easier comparison between monovalent and divalent species. As shown, increasing the density of functional groups in the material resulted in a corresponding increase in the ion exchange capacity. Data values for Na+ were in good agreement with those measured from titration experiments using NaOH (Table 1). Surprisingly, data values for K+ were indicative of significantly larger capacities than for Na+ ions, especially in materials displaying the highest levels of functionalization. This might be explained by the smaller size of the hydrated K+ cations, which permits them to reach more ion exchange sites located deeper in the microporous materials, contrary to what was observed in large pore SBA-15 containing the same sulfonate functions covalently attached to the internal surface of mesopores which were accessible for two cations (Na+ and NMe4+) independently on their size.34 In general, the maximum ion exchange capacity of the materials toward divalent cations (Ca2+ and Cu2+) was slightly lower than that observed with K+, despite their usually higher affinity for the cation exchangers,67 most probably because of unfavorable charge distribution in the material (one M2+ species would require two SO3- sites located closely enough to each other). Note that a divalent ion could theoretically occupy one exchange site only (in the form SO3-, M2+, X-), but this would imply the incorporation of negatively charged counteranions (X-) in the positively charged cation exchanger; this process is expected to require very high anion concentrations (>1 M, as for, i.e., zeolites67), so that one can consider that the (66) Townsend, R. P.; Harjula, R. Mol. Sieves 2002, 3, 1. (67) Helfferich, F. Ion exchange; Dover Publications: New York, 1995.

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Figure 4. Normalized ion-exchange isotherms for the binary Cu2+/H+ exchange obtained with (a) MPS-5%-SO3H and (b) MPS-30%-SO3H materials.

vast majority of the incorporated divalent ions are occupying two exchange sites in the material. Finally, one should mention that the grafted products behaved similarly, except that they were characterized by lower capacities in comparison to the ion exchangers prepared by the co-condensation route. The advantage of this direct synthetic procedure has been commonly recognized for organic-inorganic hybrid materials because the extent of functionalization achievable by the postgrafting process remains limited by the surface area of the starting material. The normalized ion exchange isotherms for the binary Cu2+/H+ exchange were also constructed for two samples (MPS-5%-SO3H and MPS-30%-SO3H) according to a fast electrochemical method developed for studying the ion exchange reactions in zeolites.53 They are depicted in Figure 4, where the good selectivity of the exchanger for Cu2+ is clearly shown. This selectivity was better when increasing the density of sulfonate groups in the materials: for example a selectivity coefficient67 of 15 was calculated for the Cu2+/H+ exchange in MPS-5%-SO3H (at an equivalent fraction of Cu2+ in solution equal to 0.4) while this value increased to about 180 when using the MPS-30%-SO3H sample. This highlights further the interest of using micelle-templated materials with high levels of functionalization. Electrochemistry: Ion Exchange Voltammetry. It is well established that employing an ion exchange material as an electrode modifier constitutes one strategy to enhance concentrations at the electrode surface, giving rise to the ion exchange voltammetry technique.68-71 We have evaluated here the interest of MPS-n%-SO3H materials for that purpose. Cu2+ and Ru(NH3)63+ were used as model electroactive probes, and representative cyclic voltammograms are depicted in Figure 5. They were obtained with the aid of a carbon paste electrode containing the MPS-20%-SO3H sample, by scanning potentials continuously for 20 cycles in solutions containing either Cu2+ (Figure 5A) or Ru(NH3)63+ (Figure 5B) species at a (68) Espenscheid, M. W.; Martin, C. R. Electroanalysis 1989, 1, 93. (69) Wang, J.; Lu, Z. J. Electroanal. Chem. 1989, 266, 287. (70) Wielgos, T.; Fitch, A. Electroanalysis 1990, 2, 449. (71) Ugo, P.; Moretto, L. M. Electroanalysis 1995, 7, 1105.

Figure 5. Multisweep cyclic voltammograms (20 cycles) recorded in solutions containing either (A) 10-5 M Cu(NO3)2 (in 0.01M HNO3) or (B) 10-5 M Ru(NH3)6Cl3 (in 0.01M NaNO3), with using (a) an unmodified carbon paste electrode and (b) a carbon paste electrode containing the MPS-20%-SO3H sample. Scan rate: 50 mV s-1.

rather low concentration (10-5 M). While only weak signals were observed at the unmodified carbon paste electrode, the presence of the ion exchanger at the electrode surface contributes to increase dramatically the voltammetric peaks that grew up progressively as running multisweep voltammetry. This is clearly due to the accumulation of Cu2+ and Ru(NH3)63+ species at the electrode/solution interface by ion exchange (it is noteworthy that Cu2+ species could react with the remaining silanol groups in the material, but this reaction is negligible in acidic medium,72 i.e., at pH 2 as in Figure 5A, so that one can consider that Cu2+ will act with the ion exchange sites only). As common for electrodes modified with nonconductive ion exchangers,70,73 peak currents are diffusioncontrolled as the electrochemical reaction involves the concomitant transport of the electroactive probe out of the modifier to a conductive surface of the electrode and charge compensation in the material by the solution-phase electrolyte cation(s). The fact that peak currents increased drastically upon continuous cycling indicates thus a very (72) Walcarius, A.; Bessie`re, J. Anal. Chim. Acta 1998, 361, 273. (73) Walcarius, A.; Barbaise, T.; Bessie`re, J. Anal. Chim. Acta 1997, 340, 61.

Functionalized Silica Microspheres

Figure 6. Plot of maximum currents sampled at carbon paste electrodes containing MPS-n%-SO3H materials (stationary values measured by multisweep cyclic voltammetry in conditions as in Figure 5), expressed with respect to the relative molar percentage of MPTMS in the starting sol, which were recorded in solutions containing either (a) 10-5 M Cu(NO3)2 (in 0.01 M HNO3) or (b) 10-5 M Ru(NH3)6Cl3 (in 0.01 M NaNO3). The anodic stripping peak currents were used for Cu2+ because of ill-defined cathodic signals, while the data relative to the Ru(NH3)63+ species were collected from the direct reduction peak. Other conditions are as in Figure 5.

high local concentration of the probe in the ion exchanger as its diffusion rate in the mesoporous structure is expected to be much lower than that in solution:21-23 indeed, peak currents are directly proportional to the analyte concentration, C, and the square root of its diffusion coefficient, D1/2, so that an increase in the current value (i ) kD1/2C)74 associated to a decrease in D value implies inevitably a strong increase of C. The effectiveness of the preconcentration process is directly related to the concentration and accessibility to the ion exchange sites as well as the speed at which the analytes are reaching these centers within the porous material, as previously discussed for HgII binding to thiolfunctionalized mesoporous silicas.23 Therefore, one could expect different behaviors in the MPS-n%-SO3H series on the basis of their variable physicochemical characteristics (Table 1, Figure 2). Some effects of the functionalization level on the performance of MPS-n%-SO3H-modified carbon paste electrodes, applied in ion exchange voltammetry, are illustrated in Figures 6 and 7. The maximum peak currents (stationary values) sampled in multisweep cyclic voltammetry displayed a kind of asymmetric bellshaped variation as a function of the degree of functionnalization, reaching the highest level at 20% for Cu2+ and 10% for Ru(NH3)63+ (Figure 6). The first region (current increase) of the curves corresponds to an increase in the number of ion exchange sites in the material (higher capacity, see Tables 1 and 2) while maintaining good accessibility and fast diffusion rates, whereas the second part (current decrease) is due to more and more restricted (74) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and applications; Wiley: New York, 1980.

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Figure 7. Variation of enhancement factors (determined by multisweep cyclic voltammetry) relative to the accumulation of (a) Cu2+ and (b) Ru(NH3)63+ at carbon paste electrodes containing MPS-n%-SO3H materials, as a function of the relative molar percentage of MPTMS in the starting sol. Enhancement factors were calculated as the ratio between peak currents measured at the 20th cycle divided by those of the first one. Other conditions are as in Figures 5 and 6.

mass transport in poorly ordered structures with lower and lower porosity (Figure 2). A similar trend was reported from batch experiments for the uptake of HgII by MPSn%-SH adsorbents.23 A different behavior between Cu2+ and Ru(NH3)63+ was however observed, i.e., the lower currents recorded for Cu2+ with materials displaying rather low contents of organofunctional groups (n ) 5-15%), in comparison to Ru(NH3)63+ (curves a and b on Figure 6). This can be explained by the lack of sensitivity for reduction of Cu2+ on carbon electrodes, especially at low Cu2+ concentration (see Figure 5A), which usually requires the formation of a significant amount of Cu0 to enhance the cathodic process (nucleation overpotential). Such a limitation was not observed for Ru(NH3)63+ species which undergoes fast electron transfer on carbon paste electrodes. Another possible contribution likely to explain the higher current response observed in cyclic voltammetry for Ru(NH3)63+ is that trivalent ruthenium species are expected to show higher affinity toward the exchanger than divalent copper species. Performance of the electrode can be also characterized by enhancement factors defined as the ratio between peak currents observed at the MPS-n%-SO3H-modified electrode divided by those sampled at the corresponding unmodified carbon paste. Typical results are depicted in Figure 7 where the enhancement factors calculated for Cu2+ and Ru(NH3)63+ analytes are plotted as a function of the content of organofunctional groups in the material. Independently on the probe cation, the enhancement factors were found to increase from MPS-5%-SO3H up to MPS-30%-SO3H and then tended to level off for Cu2+ and to decrease rapidly for Ru(NH3)63+. Once again, the increase is explained by the greater capacity of materials containing higher organofunctional group contents and limitations observed for MPS-40%-SO3H and MPS-50%-

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SO3H arise from diffusional restrictions. This phenomenon was much more marked for Ru(NH3)63+ because of its bigger size, hindering somewhat its capacity to reach the ion exchange sites located deeply in the less porous materials, as previously observed for other mass transfer reactions in organically modified silicates.21-23 Anyway, in the best cases, enhancement factors higher than 6 and up to 9 were achieved after 20 successive potential scans, making this new class of materials promising for applications in the sensor field. 4. Conclusions Micelle-templated organosilica spheres have been prepared by co-condensation of MPTMS and TEOS at various molar percentages up to 50%. Sulfonic acid functionalized porous materials were then obtained by chemical oxidation of thiol groups using hydrogen peroxide. Both structural order and porosity of the solids suffered somewhat from this oxidation step, especially at high loading of organofunctional groups (>20%). Despite the incomplete oxidation observed for all the materials, high cation exchange capacities (up to 3.5 mequiv g-1) were achieved. The maximum ion exchange capacity was found to be affected by the nature of the cation (size and charge) because of either restricted accessibility (too low porosity or hidden sites) or unfavorable charge distribution. Anyway, the porous sulfonic acid functionalized silica spheres displayed

Ganesan and Walcarius

good ion exchange properties and their selectivity was found to increase by increasing the functionalization level. When used as electrode modifiers, these solid ion exchangers exhibited attractive features in ion exchange voltammetry for the preconcentration of electroactive cations such as Cu2+ or Ru(NH3)63+. Optimum behavior was reached with materials meeting the best compromise between a rather high concentration of organofunctional groups and keeping a sufficiently high porosity to ensure significant accessibility and high rates of access to the ion exchange sites in the porous framework. We anticipate that these organic-inorganic ion exchangers, combining in a single solid both the mechanical stability of a rigid silica network with the reactivity of the organic groups with long-range structural order, offer promising avenues for applications in solid-liquid extraction by ion exchange or electrochemical sensors. Acknowledgment. This work has been supported by the French Ministry of Foreign Affairs under the form of a postdoctoral fellow for one of us (V.G.). The authors gratefully thank J. Cortot, J. Lambert, and B. Humbert (LCPMEsNancy) for help in ICP, XPS, and Raman experiments and for helpful discussions. We also acknowledge J.-P. Emeraux for recording XRD measurements and J. Ghanbaja for TEM images. LA0364082