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Anion Effect on Surface Density of Silanolate Groups in As-Synthesized Mesoporous Silicas Ali-Reza Badiei, Sylvie Cantournet, Michel Morin, and Laurent Bonneviot* Department of Chemistry, CERPIC, Laval University, G1K 7P4, Sainte-Foy, Que´ bec, Canada Received August 31, 1998. In Final Form: October 21, 1998 Anions present in the synthesis gel (X ) F-, Cl-, Br-, NO3-, and SO42-) partly remain in the channels of mesoporous templated silicas prepared in basic media. Their lyotropic character and their concentration control the silanolate group (tSiO-) density at the surfactant-inorganic interface through the electrical charge neutrality, implicating the positive charges of the templating self-assembled surfactants, S+. Accordingly, the well-accepted {S+ I-} generic pathway should be better referred as to {S+, mX-, (1 - m)I-}.
The recent discovery of aluminosilicate mesoporous molecular sieves in the early nineties1,2 has opened a very fast growing field of novel inorganic materials with a welldefined pore size distribution in the nanoscale range from 2 to 30 nm.3,4 Crystal morphologies,5 absorbants for depollution,6,7 tailored catalysts,7-11 and new material designs12 based on these systems have already been investigated in many laboratories around the world.4 The pore size is controlled using self-assembled surfactant with ad hoc hydrophobic chain lengths. If one uses neutral * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Kresge, C. T.; Leonowicz, M. E.; Vartuli, J. C.; Roth, W. J.; Beck, J. S. Nature 1992, 359, 710. 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.; Scheppard, E. W.; McCullen, C. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (2) Yanagisawa, T.; Shimizu, K.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680. Inagaki, S.; Koiwai, A.; Suzuki, N.; Fukushima, Y.; Kuroda, K. Bull. Chem. Soc. Jpn. 1996, 69, 1449. (3) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (4) Mesoporous Molecular Sieves 1998, Proceedings of the 1st International Symposium, Baltimore, 1998. In Studies in Surface Science and Catalysis; Bonneviot, L., Be´land, F., Danumah, C., Kaliaguine, S., Eds.; Elsevier: Amsterdam, 1998; Vol. 117. (5) Firouzi, A.; Kumar, D.; Bull, L. M.; Besier, T.; Sieger P.; Huo, Q.; Walker, S. A.; Zasadzinski, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138. Yang, H.; Coombs, N.; Ozin, G. A. Nature 1997, 386, 692. Ozin, G. A.; Kresge, C. T.; Yang, H. In ref 4, p 119. Di Renzo, F.; Testa, F.; Chen, D. C.; Cambon, H.; Galarneau, A.; Plee, D.; Fajula, F. Microporous Mesoporous Mater., accepted. (6) Mercier, L.; Pinnavaia, T. J. Adv. Mater. 1997, 9, 500. (7) Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923. (8) Corma, A. Chem. Rev. 1997, 97, 2373. Chen, C. Y.; Li, H. X.; Corma, A.; Kumar, D. In ref 4, p 201. Kloestra, K. R.; van Bekkum, H.; Jansen, J. C. J. Chem. Soc., Chem. Commun. 1997, 2281. Gunnewegh, E. A.; Gopie, S. S.; van Bekkum, H. J. Mol. Catal. 1996, 106, 151. (9) Brunel, D.; Clauvel, A.; Fajula, F.; Di Renzo, F. Stud. Surf. Sci. Catal. 1995, 97, 173. Clauvel, A.; Renard, G.; Brunel, D. J. Org. Chem. 1997, 62, 749. Sutra, P.; Brunel, D. J. Chem. Soc., Chem. Commun. 1996, 2485. Derrien, A.; Renard, G.; Brunel, D. In ref 4, p 445. Van Rhijn, W. M.; De Vos, D. E.; Sels, B. F.; Bossaert, W. D.; Jacobs, P. A. J. Chem. Soc., Chem. Commun. 1998, 317. (10) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature 1995, 378, 159. Anwander, R.; Roesky, R. J. Chem. Soc., Dalton Trans. 1997, 137. (11) Gontier, S.; Tuel, A. Appl. Catal., A: General 1996, 143, 125. Zhang, W.; Fro¨ba, M.; Wang, J.; Pinnavaia, J. T. J. Am. Chem. Soc. 1996, 118, 9164. Sayari, A. Chem. Mater. 1996, 8, 1840. Antonelli, D. M.; Ying, J. Y. Angew. Chem., Int. Ed. Engl. 1996, 35, 426. Trong On, D.; Kapoor, M. P.; Joshi, P. N.; Bonneviot, L.; Kaliaguine, S. Catal. Lett. 1997, 44, 171. (12) See, for example, for conducting fibers and carbon films: Wu, C.-G.; Bein, T. Science 1994, 264, 1757; 1994, 266, 1013.
surfactants such as nonionic primary amines or polyalkene oxides, pore size tailoring occurs through dispersive forces and hydrogen bonds providing wormlike structures.13 However, when electrostatic forces are at stake, long range order can be achieved.14 In MCM-41 type materials, the hexagonal array of mesopores is templated using cation surfactants, S+ (usually, quaternary amines such as cetyltrimethylammonium, CTMA+). Their positive charges are counterbalanced by the negative charges of the inorganic surface, denoted as I- according to the {S+I-} pathway, where I- stands for the silanolate group, tSiO-, in silica. Consistently, the removal of the template from the inorganic mold can be performed using NaCl,15 NH4Cl,16 HCl,17 NH4NO3,17,18 or cobalt complexes18 according to a cation-exchange process. Recently, chemical analyses revealed that the surfactant counteranions, Br-, can be partly retained in the as-synthesized templated mesoporous silicas (MTSs) prepared in basic media.18 Knowing that there is an anion effect on the long range order of MTS,19,20 it is an important matter to check (i) whether other anions could also be retained and (ii) to what extent this affects the solid characteristics. To investigate these points, a series of MTSs prepared according to the {S+I-} pathway have been prepared in the presence of different anions using various concentrations. The as-synthesized forms were analyzed after careful washing treatments and investigated using titration of the silanolate groups, XRD, FT-IR, and UV-visible techniques. The mesoporous silicas were synthesized according to the literature21 using a gel of molar composition 100 SiO2, 8.6 Na2O, 4.4 (TMA)2O, 30 CTMAX, 6330 H2O (TMA ) tretramethylammonium) and referred to as MTS-X (X ) Cl-, Br-, NO3-, 1/2SO42-). A clear gel was obtained after 10 min of stirring a mixture containing fumed silica (Cab(13) Tanev, P. T.; Pinnavaia, J. T. Science 1995, 267, 865. Bagshaw, S. A.; Prouzet, E.; Pinnavaia, J. T. Science 1995, 269, 1242. (14) Huo, Q.; Margolese, D. I.; Ciesla, U.; Demuth, D. G.; Feng, P.; Gier, T. E.; Sieger, P.; Firouzi, A.; Chmelka, B. F.; Schu¨th, F.; Stucky, G. D. Chem. Mater. 1994, 6, 1176. Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317. Stucky, G. D.; Zhao, D.; Yang, P.; Lukens, W.; Melosh, N.; Chmelka, B. F. In ref 4, p 1. (15) Tuel, A.; Gontier, S. Chem. Mater. 1996, 8, 114. (16) Echchahed, B.; Moen., A.; Nicholson, D.; Bonneviot, L. Chem. Mater. 1997, 9, 1716. (17) Whitehurst, D. D. U.S. Patent 5,143,879, 1992. (18) Badiei, A.-R.; Bonneviot, L. Inorg. Chem. 1998, 37, 4142. Echchahed, B.; Badiei, A.-R.; Be´land, F.; Bonneviot, L. In ref 4, p 559. Be´land, F.; Echchahed, B.; Badiei, A.-R.; Bonneviot, L. In ref 4, p 567. (19) Edler, K. J.; White, J. W. Chem. Mater. 1997, 9, 1226. (20) Ryoo, R.; Jun, S. J. Chem. Phys. B 1997, 101, 317. (21) Reddy, K. M.; Song, C. Catal. Lett. 1996, 36, 103.
10.1021/la981134h CCC: $15.00 © 1998 American Chemical Society Published on Web 11/13/1998
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Table 1. Chemical Analyses of Some Mesoporous Templated Silicas Synthesized Using the Surfactant CTMA+ and Various Counteranions X (mol %) and Their EtOH Washing Solution chemical titrations of as-synthesized forms
EtOH washing solution
counteranion
SiO-/SiO2a ((0.1)
anion/SiO2a ((0.1)
CTMA+/SiO2a ((0.1)
N/SiO2b ((0.1)
CTMA+/SiO2c ((0.05)
anion/SiO2c ((0.05)
(SO42-, Cl-)d ClNO3Br-
19.9 19.0 16.9 16.6
(0.7, 0.1)d 1.3 2.5 2.5
21.2 20.2 19.5 19.6
21.7 20.9 22.3 20.3
3.7 3.9 4.4 4.1
(0.7, 0.1)d 1.4 2.2 2.3
a Using acid-base, ion chromatography, and bichromate methods. b From elemental analyses. c Analyzed as for a on a solution obtained from 1 mg of MTS washed for 30 min at 47 °C in 50 mL of 95% EtOH. d CTMA(SO4, Cl) prepared from CTMACl22 contains a SO4/Cl mole ratio of 55:45.
O-Sil M-5), sodium silicate (Aldrich), and TMA-silicate (Sachem) in water. The addition of the surfactant (CTMACl, CTMABr [both from Aldrich], CTMANO3,22 or (CTMA)2SO422) was followed by vigorous stirring for 60 min. The so-obtained dense foams were maintained for 24 h at 373 K in polyethylene bottles (125 mL for an equivalent of 3.3 g of SiO2). After cooling at room temperature, the bottles were filled with distilled water, increasing the supernate volume by a factor of ∼3, and shaken vigorously. The filtered solids were washed three times under a buchner with a total volume of 500 mL of distilled water. The solid was dried at room temperature for 24 h. The number of tSiO- groups was obtained from acid-base back-titration. Typically, 250 mg of SiO2, equivalent to as-synthesized MTS (weight loss at 813 K for 10 h), was added to a mixture of 100 mL of 95% EtOH and 10 mL of 0.1 N HCl. After stirring for 1 h, the solid was filtered off and washed with ethanol. The excess of HCl was titrated with 0.1 N NaOH. After evaporation of EtOH and dilution in water, the amount of CTMA+ was analyzed using precipitation from an excess of potassium bichromate that was titrated using the absorbance at λ ) 430 nm.23 The anion concentration was measured using ion chromatography. Transmission FT-IR measurements were performed on samples pressed into pellets (70 mg self-supported wafers containing 1% sample in KBr) using a Bomem spectrometer. The UV-visible reflectance spectra were recorded on a Perkin-Elmer Lambda 5 spectrometer equipped with an Harrick reflectance attachment and reported using the Kubelka-Munk function F(R) ) (1 - R∞)2/2R∞.16 The hexagonal channel ordering of the MTS was checked on the powder XRD patterns obtained from a Philips spectrometer. In the first synthesis series, the nature of the surfactant counteranion is varied while the final pH stays constant at 11.3 ( 0.1 (Table 1). The amounts of silanolate groups, counteranions, and CTMA+ cations obtained from independent analyses are consistent with the electrical charge balance. The elemental analysis of nitrogen matches also the analysis of CTMA+. This shows that there is less than 4 mol % TMA+/CTMA+ in the solid, consistent both with the low ratio (14 mol %) in the starting gel and with the short hydrothermal treatment in comparison with that of previous studies where the conditions were designed to incorporate TMA in the purpose of pore size tailoring.24 The concentration of Na+ ions is also found to be negligible in this type of synthesis (8.6 mol % Na/Si in the gel and (22) CTMANO3 and (CTMA)2SO4 were synthesized respectively according to: Button, C. A.; Romsted, L. S.; Sepulveda, L. J. Phys. Chem. 1980, 84, 2611. Button, C. A.; Gan, L. H.; Moffat, J. R.; Romsted, L. S. J. Phys. Chem. 1981, 85, 4118. (23) Sepulveda, L.; Cabrera, W.; Gamboa, C.; Meyer, M. J. Colloid Interface Sci. 1987, 117, 460. (24) Corma, A.; Kan, Q.; Rey, F. J. Chem. Soc., Chem. Commun. 1998, 579. Corma, A.; Kan, Q.; Navarro, M. T.; Perez-Pariente, J.; Rey, F. Chem. Mater. 1997, 9, 2123.
Cl- > NO3- > Br- (Figure 4). Consistent with the case of the first series, CTMABr systematically generates less tSiOthan CTMACl. All the XRD spectra of the as-synthesized MTS-X exhibit 100, 110, 200, and 210 peaks characteristic of the P6m hexagonal phase typical of MCM-41 materials.1 The fourth peak is not observed for MTS-(Br, NO3), whose low (25) Doubly degenerate asymmetric stretching normal mode E′ in the D3h group symmetry for NO3-: Nakamoto, K. Infrared and Raman Spectra of inorganic and Coordination Compounds, 4th ed.; Wiley: New York, 1986; p 92. Electronic transitions attributed to π f p* and n f p*, respectively, for NO3-: Strickler, S. J.; Kasha, M. J. Am. Chem. Soc. 1963, 85, 2899. For Br-, the less energetic electron autodetachment, 4p4 5s2 f 4p,5 of Br- gives rise to a Br atom at 179 nm, consistent with a charge-transfer band in solid KBr or in solution occurring at lower energies in the range 190-199.5 nm, as reported earlier: Edwards, A. K.; Cunningham, D. L. Phys. Rev. 1974, A10, 448. Mellor’s Comprehensive Treatrise on Inorganic and Theorical Chemistry, Supplement II; Longmans, Green: London, 1956; p 797.
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Langmuir, Vol. 14, No. 25, 1998 7089
Figure 3. Silanolate group concentration in MTS-Cl as function of amounts of NaCl added to the synthesis gel. Figure 1. FT-IR spectra of MTS-NO3 (particle size range 100-1000 nm, external surface < 5 m2/g), a) as-synthesized, b) washed with ETOH at 47 °C c) silica (nonporous CAb-O-Sil M-5, ∼ 12 nm av. particle size, ∼ 200 m2/g) impregnated with CTMANO3 (equivalent of 2.5 mol % CTMANO3/SiO2 as in MTSNO3) and washed as an MTS after synthesis (solid diluted 10 times for an equivalent of 20 m2/g covered, i.e., more than four times the external surface of an MTS): νd assigned to NO3- 25 and δC-H to CTMA+.16,18
Figure 4. Silanolate group concentration in MTS-(X,Y) versus nature of surfactant counteranion X and anion added to the synthesis gel Y.
Figure 2. UV-visible diffuse reflectance spectra on the left of a) 3% NO3- in MgO (CTMANO3: MgO ) 1:7), b) assynthesized MTS-NO3 (diluted 50 wt % in MgO) and c) as in b and wash with ethanol at 47 °C and on the right of a) solid KBr (30 wt % in MgO), b) MTS-Br (25 wt % in MgO) as-synthesized, c) as in b and wash with water (1 g in 50 mL stirred for 30 min at 47 °C), d) as in c using EtOH instead of water and e) calcined at 540 °C.
crystallinity may explain its low concentration of tSiO(Figures 4 and 5). The main differences reside in the line broadening assigned to a size decrease of the ordered mesophase domains and in the XRD peak surface area providing the percentage of mesostructured material. For MTS-Cl, the fwhm is approximately 0.50° while it drops
down to 0.25° and 0.20° for MTS-Br and MTS-NO3, respectively, indicating an increasing long range order along this series. Adding NaY to CTMACl drastically increases the templating efficiency (higher crystallinity and longer range order) according to the following order, Cl- < F- < Br-. By contrast, addition of salt to CTMABr (already a very good templating agent) improves only slightly the synthesis using NaCl or even lowers its quality using NaF and NaNO3, the latter being the worst. The data show that moving through the series SO42-, Cl-, NO3-, and Br- and increasing the anion concentration decrease the tSiO- concentration in the as-synthesized MTS from 20 to 15% tSiO-/SiO2 mole ratio. However, the variation of the unit cell parameter is too small to explain this difference in terms of a change in the bulk-to-surface ratio, that is, in the wall thickness. Furthermore, the best crystallized MTSs (using Br- or NO3-) contain a lower silanolate concentration. This shows that the silanolate group concentration in the solid is decreasing because of a lower surface density at the surfactant-inorganic interface.
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Figure 5. Selected low angle XRD patterns of (left) MTS-X, (middle) MTS-Cl with NaY added (Y/Cl ) 1 molar ratio) and, (right) as in middle but starting from MTS-Br where X or Y ) (___) NO3-, (.....) Br-, (- - -) Cl-, (-‚-) F-.
The anion competition is also explained on the basis of the nature of the anions. Studies based on force, viscosity, conductivity, and electrophoresis measurements show that an anion exhibits a higher degree of association to the positive headgroup of the self-assembled surfactants for a higher hydrophobic character, referring to the lyotropic series (eq 1).26,27 The hydrophobicity increases with the ion radius, F- < Cl- < Br-. However, the NO3- comparable in size to Cl- behaves more like Br-, according to their association degrees.27 The cohesion forces measured in the internal surfactant bilayers place SO42- between Fand Cl-.26 Taking into account that the interface is actually of {S+, X-, H2O, tSiO-} type instead of {S+, X-, H2O} type in the surfactant studies, the trend SO42- = Cl- < NO3- = Br- obtained here is consistent with an anion competition based on the lyotropic series. This proposal corroborates the observations of Edler and White. They argued that a competition occurs during the silicate polymerization process at the tubule surface.19 The actual proposal pushes the argumentation further in suggesting that this competition is still taking place in the final product according to the following equations:
{CTMA+, X-} a {CTMA+} + {X-}
(1)
{CTMA+, (MTS)tSiO-} a {CTMA+} + {(MTS)tSiO-} (2) {CTMA+, (MTS)tSiO-} + {X-} a {CTMA+, X-} + {(MTS)tSiO-} (3) where eqs 1 and 2, which lead to the competition reaction in eq 3, hold for an anion dissociation mechanism in ionic liquid crystals or at silica surfaces, allowing the anions to move from the Stern layer to the Gouy-Chapman layer (GCL) according to the electrical triple-layer model.28 The (26) Marra, J. J. Phys. Chem. 1986, 90, 2145. (27) Gamboa, C.; Sepulveda, L. J. Colloid Interface Sci. 1986, 113, 566. Gamboa, C.; Rios, H.; Sepulveda, L. J. Phys. Chem. 1989, 93, 5540. (28) Hayes, K. F.; Leckie, J. O. J. Colloid Interface Sci. 1987, 115, 564.
actual variations from this model reside in the absence of a diffuse layer (the third one) and the presence of two Stern layers on each surfactant and inorganic side, both sandwitching a shared GCL. Accordingly, the competitive anion exchange should occur via the GCL. Studies of anion speciation are in progress to further document this point. The anions affect not only the long range order, as shown previously,19 but also the percentage of mesostructured material (Figure 5). In addition, improvement of the synthesis may be obtained by adding NaCl or other sodium salts not only in a postsynthesis hydrothermal treatment20 but also in the starting gel, as shown here. However, the XRD pattern of MTS-(Br, NO3) shows that the addition of too strongly held anions may lead to a bad crystallization process. Conversely, weakly held anions such as F- or Cllead to a poorly structured system consistent with the low self-assembly capacities of CTMA associated with these anions.26 Therefore, anions affect not only the kinetic parameters of the silica polymerization but also the thermodynamic characteristics of MTS, consistent with improvements obtained both using acid or salt postsynthesis treatment19,20 and using mixed surfactants (CTMAcarboxylic acids).29 In conclusion, anions present in the synthesis gel (X ) F-, Cl-, Br-, NO3-, and SO42-) partly remain in the channels of mesoporous templated silicas prepared in basic media. Their lyotropic character and their concentration control the silanolate group (tSiO-) density at the surfactant-inorganic interface, where electrical charge neutrality has to be maintained with the positively charged heads of the templating self-assembled surfactants, S+. Accordingly, the well-accepted {S+, I-} generic pathway should be better referred as to {S+, mX-, (1 - m)I-}. Synthesis modifications based on this novel model are in progress. Acknowledgment. This work was supported by NSERC (Canada) and FCAR (Que´bec). A.-R.B. appreciates very much the scholarship provided by the Iranian government. LA981134H (29) Chen, F. X.; Huang, L.; Li, Q. Chem. Mater. 1997, 9, 2685.
